Catecholamine metabolism revisited: From neurochemistry to integrative physiology and pathophysiology

Abstract

After a previous overview published more than 20 years ago, key aspects of catecholamine metabolism remain incompletely understood by physiologists. Meanwhile, the repertoire of assayable catecholamine-related compounds has expanded. Moreover, alterations in catecholamine metabolic patterns associated with physiological changes, drug effects, and pathophysiological states were not covered; and biochemical phenotyping via kinetic models now makes it possible to elucidate physiology and pathophysiology in individuals. Accordingly, this review starts with the inventory of catecholamine-related compounds and the three main peripheral catecholamine systems (sympathoneuronal, adrenomedullary, and autocrine/paracrine). Catecholamine systems contribute to virtually every organ function, behavior, and emotional state, and we describe situations where alterations in those systems exert syndromic pathophysiological effects. We then describe the relationships of catecholamine metabolic patterns to synthesis, storage, release, reuptake, and intra-neuronal and extra-neuronal metabolism of catecholamines; effects of physiological processes and drugs on those patterns; and catecholamine biochemical patterns associated with pathophysiologic states. We provide examples of how detailed, comprehensive knowledge acquired over the past half century has led to computational models for estimating the rates of processes that characterize specific physiological changes and some clinical disorders. The integrative physiological approach presented here has the potential to pinpoint specific functional abnormalities within catecholaminergic cells and thereby rationalize novel, testable treatment and prevention strategies.

Graphical Abstract

I. INTRODUCTION

The catecholamines dopamine, norepinephrine, and epinephrine are among the most intensively studied signaling molecules in physiology. Their roles as neurotransmitters, hormones, and autocrine–paracrine mediators were central to the emergence of modern concepts of neuroeffector transmission, stress responses, and homeostatic regulation.

More than two decades have passed since our prior comprehensive overview of catecholamine metabolism (1). In the interval the field has expanded substantially in both analytical capability and conceptual scope. Several developments have motivated a renewed, integrative synthesis. First, the repertoire of assayable catecholamine-related compounds has grown markedly, encompassing not only the parent amines but also deaminated, O-methylated, sulfoconjugated, and cysteinylated metabolites measurable in plasma, urine, cerebrospinal fluid, and tissue. Some now quantifiable products of intermediary metabolism of catecholamines are toxic and may play pathophysiological roles in diseases (autotoxicity). Moreover, advances in kinetic and isotope-dilution methodologies have enabled detailed estimation of rates of synthesis, release, reuptake, vesicular storage, and intra-neuronal metabolism in humans in vivo, moving the field beyond static concentration measurements and providing physiologically meaningful information about intracellular processes and system-level regulation. Finally, analyses of catecholamine metabolomic patterns can pinpoint specific abnormalities or patterns of abnormalities in catecholaminergic cells, aiding clinical diagnosis and generating pathophysiologically relevant experimental therapeutic hypotheses.

Peripheral catecholaminergic signaling is organized into three major systems—the sympathetic noradrenergic system, the sympathetic adrenergic (adrenomedullary) system, and autocrine–paracrine systems such as the renal DOPA–dopamine axis—each distinguished by cellular origin, modes of release, and regulatory architecture. These systems are inherently integrative and interact dynamically with neuroendocrine, immune, and central neural networks, forming what can be viewed as an “extended autonomic system” that supports both moment-to-moment homeostasis and longer-term adaptive responses (2, 3). Central catecholaminergic systems, although anatomically segregated by projection fields and transmitter phenotypes, are complexly coupled to peripheral catecholamine dynamics through descending neural pathways and neuroendocrine outputs rather than by direct biochemical exchange (4, 5).

Traditional physiological interpretations have often treated catecholamine responses as essentially reactive outputs proportional to the magnitude of a stressor. More recent work emphasizes anticipatory, feed-forward, and history-dependent regulation—captured conceptually by the theory of allostasis (6–9)—without implying discrete anatomical controllers or fixed set points. In this view, stability emerges from distributed, adaptive processes whose parameters can change with age, experience, and disease. Catecholamine metabolomic patterns provide a uniquely powerful window into these processes, because the relevant enzymes and transporters occupy defined intracellular compartments, allowing biochemical signatures to be mapped onto specific functional alterations (10).

We begin this review with an inventory of the catecholamines, their metabolites, and the enzymatic pathways that generate them. We examine catecholamine neuroeffector functions, including local and long-distance feedback regulation, differential receptor signaling, and cotransmission. Building on this foundation, we describe how characteristic metabolomic patterns relate to defined intracellular processes—synthesis, vesicular storage, leakage, reuptake, and metabolism—and how these patterns are modified by physiological states, pharmacological interventions, and pathophysiological conditions. Finally, we illustrate how quantitative integration of these relationships has enabled computational modeling approaches that estimate rates of key intra-neuronal processes (11, 12), providing a framework for biochemical phenotyping with potential implications for prognosis, treatment, and prevention.

This work aims to advance catecholamine neurochemistry by developing a coherent, testable framework linking metabolic patterns to integrative physiological function. Emphasizing pattern recognition, kinetic analysis, and systems-level interpretation, it provides tools to understand how three closely related molecules exert broad and persistent effects on human physiology and disease.

II. CATECHOLAMINES AND THEIR METABOLITES

II.1. Repertoires of catecholamine-related biochemicals

The number of known catecholamine metabolites exceeds by far that of the three parent compounds, dopamine, norepinephrine, and epinephrine (Tables 1 and 2, Figures 1–5). Figures 1 and 2 provide overviews of the main known metabolic pathways. Figures 3–5 separately depict dopamine, norepinephrine, and epinephrine metabolic steps. We refer the reader to these Figures as we review below the enzymes and co-factors responsible for the synthesis and metabolic fate of the catecholamines.

Table 1:

Catecholamines and their metabolites

N	Biochemical	Abbreviation	MW	Precursor	Enzyme	Co-factors	Other	Other
1	3,4-Dihydroxyphenylalanine	DOPA	197.2	Tyr	TH	BH4, Fe+2	Oxygen
2	DOPA-quinone	DOPA-Q	195.2	Tyr	Tyr-ase	Cu+2	Oxygen	In melanocytes
3	3-Methoxytyrosine	3-MTyr	181.2	DOPA	COMT	SAMe
4	5-S-CysteinylDOPA	Cys-DOPA	300.3	DOPA	GST	Glutathione	Cysteine	Dipeptidases
5	Dopamine Leukaminochrome	DA LAC	153.2 151.2	DOPA DA	LAAAD (Spontaneous)	Vit. B6	Reduction of ACM
6	3-Methoxytyramine	3-MT	167.2	DA	COMT	SAMe
7	3-Methoxytyramine-sulfate Aminochrome	3-MT-S ACM	248.3 148.1	3-MT DA	SULT1A3 (Spontaneous)	PAPS	Sulfate Oxygen
8	5-S-Cysteinyldopamine Dopamine-quinone	Cys-DA DA-Q	284.3 151.2	DA DA	GST (Spontaneous)	Glutathione	Cysteine Oxygen	Dipeptidases
9	Dopamine sulfate 5,6-Dihydroxyindole	DA-S DHI	233.2 149.2	DA DA	SULT1A3 (Spontaneous)	PAPS	Sulfate	In gut
10	3,4-dihydroxyphenylacetaldehyde	DOPAL	152.2	DA	MAO	FAD	Oxygen
11	3,4-Dihydroxyphenylacetic acid	DOPAC	168.1	DOPAL	ALDH	NAD+
12	3,4-Dihydroxyphenylethanol DOPAL-quinone	DOPET DOPAL-Q	154.2 151.2	DOPAL DOPAL	AR (Spontaneous)	NADPH Oxygen
13	5-S-CysteinylDOPAC	Cys-DOPAC	271.3	DOPAC	GST		Cysteine	Dipeptidases
14	Homovanillic acid	HVA	182.2	DOPAC	COMT	SAMe
15	Homovanillic acid-sulfate	HVA-S	263.3	HVA	SULT1A3	PAPS	Sulfate
16	Norepinephrine	NE	169.2	DA	DBH	Cu+2, Vit. C	Oxygen	VMAT, ATP
17	3,4-Dihydroxyphenylglycolaldehyde	DOPEGAL	167.2	NE	MAO	FAD	Oxygen
18	3,4-Dihydroxyphenylglycol	DHPG	170.2	DOPEGAL	AR	NADPH
19	3,4-Dihydroxymandelic acid	DHMA	184.2	DOPEGAL	ALDH	NAD+
20	Epinephrine	EPI	183.2	NE	PNMT	SAMe
21	Normetanephrine	NMN	183.2	NE	COMT	SAMe
22	Metanephrine	MN	197.2	EPI	COMT
23	DHPG-sulfate	DHPG-S	263.2	DHPG	SULT1A3	PAPS	Sulfate	In liver
24	Epinephrine sulfate	EPI-S	263.3	EPI	SULT1A3	PAPS	Sulfate	In liver
25	Norepinephrine sulfate	NE-S	249.2	NE	SULT1A3	PAPS	Sulfate	In liver
26	3-Methoxy-4-hydroxyphenylglycol	MHPG	199.2	DHPG	COMT	SAMe
27	MHPG-sulfate	MHPG-S	279.2	MHPG	SULT1A3	PAPS		In liver
28	Vanillylmandelic acid	VMA	196.2	MHPG	ADH, ALDH			In liver
29	Normetanephrine sulfate	NMN-S	263.3	NMN	SULT1A3	PAPS	Sulfate
30	Metanephrine sulfate	MN-S	277.3	MN	SULT1A3	PAPS	Sulfate
31	Homovanillyl alcohol	MOPET	168.2	DOPET	COMT	SAMe

Table 2:

Relationships of catecholamine metabolic patterns with enzymatic reactions and transporters

Biochemical	Abbreviation	Number	TH	LAAAD	COMT	SULT1A3	MAO	ALDH	AR	DBH	Vesicular uptake	Exocytosis	NET (Uptake-1)	OCT3 (Uptake-2)
3,4-Dihydroxyphenylalanine	DOPA	1	(457)	(458)	(459)
DOPA-quinone	DOPA-Q	2
3-Methoxytyrosine	3-MTyr	3	(460)	(458)	(459)
5-S-CysteinylDOPA	Cys-DOPA	4	(460)	(458)	(459)
Dopamine	DA	5	(457)	(458)	(459)	(47)	(241)			(461)	(462)	(306)
3-Methoxytyramine	3-MT	6			(459)
3-Methoxytyramine-sulfate	3-MT-S	7			(459)
5-S-Cysteinyldopamine	Cys-DA	8					(241)
Dopamine sulfate	DA-S	9
3,4-dihydroxyphenylacetaldehyde	DOPAL	10					(241)	(463)
3,4-Dihydroxyphenylacetic acid	DOPAC	11	(464)				(241)	(463)			(462)
3,4-Dihydroxyphenylethanol	DOPET	12					(241)
5-S-CysteinylDOPAC	Cys-DOPAC	13
Homovanillic acid	HVA	14					(241)
Homovanillic acid-sulfate	HVA-S	15
Norepinephrine	NE	16	(457)	(458)	(459)	(308)	(241)			(461)	(462)	(276)	(363)
3,4-Dihydroxyphenylglycolaldehyde	DOPEGAL	17
3,4-Dihydroxyphenylglycol	DHPG	18	(464)								(462)
3,4-Dihydroxymandelic acid	DHMA	19
Epinephrine	EPI	20	(457)		(459)		(241)			(461)
Normetanephrine	NMN	21
Metanephrine	MN	22
DHPG-sulfate	DHPG-S	23
Epinephrine sulfate	EPI-S	24
Norepinephrine sulfate	NE-S	25
3-Methoxy-4-hydroxyphenylglycol	MHPG	26					(241)		(35)	(461)
MHPG-sulfate	MHPG-S	27								(461)
Vanillylmandelic acid	VMA	28					(241)		(35)	(461)

Bold text indicates effects of increased activities and italic text decreased activities of enzymatic reactions or transporters.

Figure 1: Overview of catecholamines and their metabolites.

Numbers in rectangles identify the compounds listed in Table 1. Abbreviations: 3-MT=3-methoxytyramine; 3-MT-S=3-methoxytyramine-sulfate; 3-MTyr=3-methoxytyrosine; ACM=aminochrome; ADH=alcohol dehydrogenase; ALDH=aldehyde dehydrogenase; AR=aldehyde reductase; COMT=catechol-O-methyltransferase; Cys-DOPA=5-S-cysteinylDOPA; Cys-DOPAC=5-S-cysteinylDOPAC; DA=dopamine; DA-Q=dopamine-quinone; DA-S=dopamine sulfate; DBH=dopamine-beta-hydroxylase; DHI=5,6-dihydroxyindole; DHMA=3,4-dihydroxymandelic acid; DHPG=3,4-dihydroxyphenylglycol; DHPG-S=DHPG-sulfate; DOPA=3,4-dihydroxyphenylalanine; DOPA-Q=DOPA-quinone; DOPAC=3,4-dihydroxyphenylacetic acid; DOPAL=3,4-dihydroxyphenylacetaldehyde; DOPAL-Q=DOPAL-quinone; DOPEGAL=3,4-dihydroxyphenylglycolaldehyde; DOPET=3,4-dihydroxyphenylethanol; EPI=epinephrine; EPI-S=epinephrine sulfate; HVA=homovanillic acid; HVA-S=homovanillic acid-sulfate; LAAAD=L-aromatic-amino-acid decarboxylase; LAC=leukaminochrome; MAO=monoamine oxidase; MHPG=3-methoxy-4-hydroxyphenylglycol; MHPG-S=MHPG-sulfate; MN=metanephrine; MN-S=metanephrine sulfate; NE=norepinephrine; NE-S=norepinephrine sulfate; NMN=normetanephrine; NMN-S=normetapnephrine sulfate; PNMT=phenylethanolamine-N-methyltransferase; SULT1A3=phenolsulfotransferase 1A3; TH=tyrosine hydroxylase; Tyr-ase=tyrosinase; VMA=vanillylmandelic acid

Figure 5: Epinephrine metabolic pathways.

Numbers in rectangles identify the compounds listed in Table 1. Red numbers= epinephrine-related; purple numbers=both norepinephrine- and epinephrine-related. Abbreviations: ADH=alcohol dehydrogenase; AR=aldehyde reductase; COMT=catechol-O-methyltransferase; DHMA=3,4-dihydroxymandelic acid; DHPG=3,4-dihydroxyphenylglycol; DOPEGAL=3,4-dihydroxyphenylglycolaldehyde; MAO=monoamine oxidase; MHPG=3-methoxy-4-hydroxyphenylglycol; MN-Sulfate=metanephrine sulfate; NAD+=nicotinamide adenine dinucleotide, oxidized form; PAPS=3’-phosphoadenosine-5’-phosphosulfate; SAMe=S-adenosylmethionine; SULT1A3=phenolsulfotransferase 1A3; VMA=vanillylmandelic acid.

Figure 2: Overview of catecholamine metabolism.

Figure adapted from (13). Arrow thicknesses indicate relative predominances of alternative pathways. Not shown are reactions involving sulfoconjugation or some intermediary metabolites. Abbreviations: 3-MT=3-methoxytyramine; ADH=alcohol dehydrogenase; ALDH=aldehyde dehydrogenase; COMT=catechol-O-methyltransferase; DA=dopamine; DBH=dopamine-beta-hydroxylase; DHMA=3,4-dihydroxymandelic acid; DHPG=3,4-dihydroxyphenylglycol; DOPAC=3,4-dihydroxyphenylacetic acid; DOPET=3,4-dihydroxyphenylethanol; EPI=epinephrine; HVA=homovanillic acid; LAAAD=L-aromatic-amino-acid decarboxylase; MAO=monoamine oxidase; MHPG=3-methoxy-4-hydroxyphenylglycol; MOPET=3-methoxy-4-hydroxyphenylethanol (homovanillyl alcohol); MN=metanephrine; NE=norepinephrine; NMN=normetanephrine; PNMT=phenylethanolamine-N-methyltransferase; VMA=vanillylmandelic acid

Figure 3: Dopamine metabolic pathways.

Numbers in rectangles identify the compounds listed in Table 1. Green numbers=dopamine-related; blue numbers=norepinephrine-related. Abbreviations: ADH=alcohol dehydrogenase; ALDH=aldehyde dehydrogenase; AR=aldehyde reductase; COMT=catechol-O-methyltransferase; Cys-DOPAC=5-S-cysteinylDOPAC; DOPAC=3,4-dihydroxyphenylacetic acid; DOPAL=3,4-dihydroxyphenylacetaldehyde; DOPAL-Q=DOPAL-quinone; DOPET=3,4-dihydroxyphenylethanol; HVA=homovanillic acid; HVA-S=homovanillic acid-sulfate; LAAAD=L-aromatic-amino-acid decarboxylase; MAO=monoamine oxidase; MOPET=3-methoxy-4-hydroxyphenylethanol; NAD+=nicotinamide adenine dinucleotide, oxidized form; NADPH=nicotinamide adenine dinucleotide phosphate, reduced form; PAPS=3’-phosphoadenosine-5’-phosphosulfate; SAMe=S-adenosylmethionine; SULT1A3=phenolsulfotransferase 1A3.

II.1.1. Enzymes

Tyrosine hydroxylase

Tyrosine hydroxylase (TH) is the initial and rate-limiting step in catecholamine biosynthesis (14). The enzyme catalyzes the hydroxylation of L-tyrosine to L-3,4-dihydroxyphenylalanine (DOPA), with tetrahydrobiopterin (BH₄) a required co-factor for the reaction (15). Also necessary are molecular oxygen and ferrous iron. TH activity is controlled by several mechanisms—substrate and co-factor availability, end-product feedback inhibition by catecholamines that bind to the enzyme and reduce activity, and post-translational modification, particularly phosphorylation of serine residues (16–20).

L-aromatic-amino-acid decarboxylase (LAAAD)

L-Aromatic amino acid decarboxylase (LAAAD, also known as DOPA decarboxylase, DDC) catalyzes the decarboxylation of DOPA to form dopamine (21). LAAAD activity depends on pyridoxal-5′-phosphate (PLP), which is vitamin B6 (22).

Monoamine oxidase (MAO)

Monoamine oxidase (MAO) is a flavin adenine dinucleotide (FAD)-dependent mitochondrial enzyme that catalyzes the oxidative deamination of catecholamines and serotonin to aldehydes—3,4-dihydroxyphenylacetaldehyde (DOPAL) from dopamine, 3,4-dihydroxyphenylglycolaldehyde (DOPEGAL) from NE and EPI, and 5-hydroxyindoleacetaldehyde (5-HIAL) from serotonin (23). Two isoforms of MAO, MAO-A and MAO-B, are encoded by distinct genes on the X-chromosome (24, 25). Both isoforms are localized to the outer mitochondrial membrane. MAO-A is relatively abundant in catecholaminergic neurons and peripheral tissues such as the liver and gut; MAO-B is prominent in glial cells and platelets. Sympathetic nerves express mainly MAO-A (26). MAO-A is more active than MAO-B in metabolizing dopamine and NE (27). Because both MAO isoforms deaminate dopamine more efficiently than NE, and since sympathetic nerves express mainly MAO-A, catecholaminergic neurons are especially vulnerable to intra-neuronal dopamine oxidation and downstream catecholaldehyde formation, a key concept in the catecholaldehyde hypothesis of neurodegeneration (28).

Dopamine-beta-hydroxylase (DBH)

Dopamine-beta-hydroxylase (DBH) is copper-dependent monooxygenase that catalyzes the conversion of dopamine to NE (29). Uniquely among catecholamine-synthetic enzymes, DBH is localized to synaptic vesicles and chromaffin granules (30), so its activity depends on prior uptake of dopamine from the cytoplasm. DBH requires molecular oxygen, ascorbate as an electron donor, and copper as an essential co-factor. The enzyme is non-specific and beta-hydroxylates a wide variety of phenylethylamines derivatives including tyramine (forming octopamine) and amphetamine (forming norephedrine).

Phenylethanolamine-N-methyltransferase (PNMT)

Phenylethanolamine-N-methyltransferase (PNMT) catalyzes the conversion of norepinephrine to epinephrine by N-methylating the amine group using S-adenosylmethionine (SAMe) as the methyl donor (31). Since PNMT is a cytosolic enzyme, norepinephrine must be available in the cytosol to be methylated to epinephrine; this is generally thought to require leakage or efflux of norepinephrine from chromaffin storage vesicles, followed by vesicular uptake of the newly formed epinephrine. PNMT expression is strongly regulated by glucocorticoids delivered to the adrenal medulla via the intra-adrenal portal circulation.

Catechol-O-methyltransferase (COMT)

Catechol-O-methyltransferase (COMT) transfers a methyl group from S-adenosyl-methionine (SAMe) onto catecholamines, converting them to O-methylated metabolites (methoxytyramine from dopamine, normetanephrine (NMN) from NE, and metanephrine (MN) from EPI (32). COMT also metabolizes DHPG to form MHPG (33).

Aldehyde dehydrogenase (ALDH)

Aldehyde dehydrogenases (ALDHs) are a family of NAD⁺-dependent enzymes that catalyze the oxidation of aldehydes to their corresponding carboxylic acids. In catecholamine metabolism, ALDH1A1 and ALDH2 play critical roles by converting DOPAL to 3,4-dihydroxyphenylacetic acid (DOPAC) (34). ALDH also catalyzes the conversion of NE to the minor metabolite 3,4-dihydroxymandelic acid (DHMA) (1, 35). ALDHs are localized in both the cytosol (ALDH1A1) and mitochondria (ALDH2) of catecholaminergic neurons.

Aldehyde reductase (AR)

Aldehyde reductase (AR), also known as aldo-keto reductase family 1 member A1 (AKR1A1), is a NADPH-dependent cytosolic enzyme that catalyzes the reduction of a wide range of endogenous and exogenous aldehydes to their corresponding alcohols (36). In catecholamine metabolism, AR converts the DOPAL to 3,4-dihydroxyphenylethanol (DOPET) and 3,4-dihydroxyphenylglycolaldehyde (DOPEGAL) to 3,4-dihydroxyphenylglycol (DHPG). AR is far more efficient in metabolizing DOPEGAL to DHPG than DOPAL to DOPET (36, 37).

Alcohol dehydrogenase (ADH)

Alcohol dehydrogenase (ADH), a zinc-dependent, NAD⁺-requiring oxidoreductase, participates in the conversion of the glycol MHPG to the acid VMA (38, 39), especially in the liver (40, 41). The immediate product of ADH acting on MHPG is an aldehyde that is rapidly metabolized to VMA by ALDH. Analogously, ADH metabolizes the dietary alcohol hydroxytyrosol (synonymous with DOPET) found in olives to DOPAL, which is converted efficiently to DOPAC by ALDH. The 2-step series of ADH and ALDH explains why after eating olives there is a large increase in plasma DOPAC (42). After ethanol ingestion, the same enzymatic series produces acetaldehyde, which is metabolized to acetate.

Phenolsulfotransferase (PST)

Monoamine-preferring phenolsulfotransferase (PST), also known as SULT1A3 (previously termed M-PST), is a cytosolic sulfotransferase enzyme that catalyzes the sulfate conjugation of catecholamines—notably dopamine, NE, and 5-hydroxytryptamine (5-HT, serotonin)—using 3′-phosphoadenosine-5′-phosphosulfate (PAPS) as the universal sulfate donor (43). SULT1A3 is expressed primarily in the intestinal mucosa and brain (44) and little if at all in adrenal medulla. A variety of catecholamine metabolites are sulfoconjugated by SULT1A3 (Figs. 2–4).

Figure 4: Norepinephrine metabolic pathways.

Numbers in rectangles identify the compounds listed in Table 1. Blue numbers= norepinephrine-related; purple numbers=both norepinephrine- and epinephrine-related. Abbreviations: ADH=alcohol dehydrogenase; AR=aldehyde reductase; COMT=catechol-O-methyltransferase; DHMA=3,4-dihydroxymandelic acid; DHPG=3,4-dihydroxyphenylglycol; DOPEGAL=3,4-dihydroxyphenylglycolaldehyde; MAO=monoamine oxidase; MHPG=3-methoxy-4-hydroxyphenylglycol; NAD+=nicotinamide adenine dinucleotide, oxidized form; NMN-Sulfate=normetapnephrine sulfate; PAPS=3’-phosphoadenosine-5’-phosphosulfate; SAMe=S-adenosylmethionine; SULT1A3=phenolsulfotransferase 1A3; VMA=vanillylmandelic acid.

Virtually all of circulating dopamine is conjugated (45, 46). Conjugated dopamine can be deconjugated by arylsulfatase to generate free dopamine, which might play a physiological role in heart failure (47) or a pathophysiological role in pseudopheochromocytoma (48); however, there has been little if any recent research on the clinical significance of deconjugation of DA-S.

II.1.2. Co-factors

Tetrahydrobiopterin

Tetrahydrobiopterin (BH₄) is a pteridine co-factor required for the activity of aromatic amino acid hydroxylases—TH, tryptophan hydroxylase, and phenylalanine hydroxylase—as well as nitric oxide synthases (49). In catecholamine biosynthesis, BH₄ donates electrons to TH for the hydroxylation of L-tyrosine to L-DOPA. BH₄ is synthesized de novo from guanosine triphosphate (GTP) through the sequential actions of GTP cyclohydrolase I (GCH1), 6-pyruvoyltetrahydropterin synthase (PTPS), and sepiapterin reductase (SPR). BH₄ levels are maintained by salvage and regeneration pathways involving pterin-4a-carbinolamine dehydratase (PCD) and dihydropteridine reductase (DHPR), which recycle oxidized pterins back to BH₄ (15).

Pyridoxal phosphate (PRP)

As noted above, PRP is a required co-factor for LAAAD (22).

S-Adenosylmethionine (SAMe)

S-Adenosyl-L-methionine (SAMe) is a universal methyl donor that plays a central role in catecholamine metabolism by providing the methyl group for COMT–mediated inactivation of catecholamines and their metabolites (50). SAMe is an example of a nucleotide-derived co-factor, since it is synthesized from ATP and methionine. COMT transfers a methyl group from SAM to a hydroxyl group on the catechol ring of dopamine, NE, EPI, DOPAC, and DHPG, forming the corresponding O-methylated products such as MHPG and homovanillic acid (HVA). COMT activity consumes significant amounts of SAMe in tissues with high catecholamine turnover, such as liver, kidney, and brain (51). SAMe is also the methyl group donor for N-methylation of norepinephrine to form epinephrine via PNMT.

NAD+

Nicotinamide adenine dinucleotide (NAD⁺) is a redox coenzyme that functions as an electron carrier in metabolic pathways involved in catecholamine metabolism and mitochondrial energy production. Within catecholaminergic neurons, NAD⁺ serves as an essential co-factor for aldehyde dehydrogenases (ALDH1A1 and ALDH2), which oxidize the dopamine metabolite DOPAL to DOPAC (52). ALDH-catalyzed oxidation of DOPAL requires NAD⁺ as an electron acceptor. Since mitochondrial complex 1 generates NAD⁺, blockade of complex 1 by the metabolic toxin rotenone decreases ALDH activity (53) and increases endogenous DOPAL levels (54).

Ascorbate

Ascorbate is required as an electron donor to maintain the active-site copper of DBH in the reduced Cu⁺ state, thereby enabling oxygen activation and beta-hydroxylation of dopamine. Without reduction of copper by ascorbate, O₂ would not be productively bound or reduced, and DBH would remain catalytically inactive despite the presence of substrate and oxygen (55). DBH is the only step in catecholamine synthesis or metabolism with a direct, mechanistic requirement for ascorbate.

Mg⁺⁺

Mg⁺⁺ is required for ATP-dependent reactions and for enzymes that use nucleotide co-factors—notably COMT, where Mg⁺⁺ stabilizes binding of SAMe with the catechol substrate, enabling efficient O-methylation. Mg⁺⁺ is not required for PNMT activity, since the methylation occurs at the terminal amine group of norepinephrine, not at the catechol nucleus.

Fe⁺⁺

Ferrous iron is the essential catalytic metal for TH. Fe⁺⁺, together with tetrahydrobiopterin, enables activation of molecular oxygen and hydroxylation. Loss or oxidation of Fe⁺⁺ to Fe⁺⁺⁺ renders TH inactive. Consequently, by chelating the essential Fe⁺⁺ co-factor, divalent cation–scavenging agents such as ethylenediaminetetraacetic acid (EDTA) decrease TH activity.

Nicotinamide adenine dinucleotide phosphate, reduced form (NADPH)

Nicotinamide adenine dinucleotide phosphate, reduced form (NADPH) serves as a reducing equivalent in catecholamine metabolism. It is required for aldehyde/aldose reductase (AR), which catalyzes the conversion of DOPEGAL to DHPG and DOPAL to DOPET. NADPH also supports cellular redox balance that limits oxidative stress from catecholamine turnover.

Flavin adenine dinucleotide (FAD)

Flavin adenine dinucleotide (FAD) is a tightly bound redox co-factor for MAO. FAD accepts electrons during oxidative deamination of dopamine, norepinephrine, and epinephrine, forming the corresponding aldehydes (e.g., DOPAL) and hydrogen peroxide. Within catecholamine synthesis and metabolism, MAO is the only enzyme that has an obligate, direct requirement for FAD.

III. CATECHOLAMINE NEUROEFFECTOR COUPLING

III.1. Exocytosis

Catecholamine neuroeffector coupling begins with calcium-dependent exocytosis of the contents of vesicles. According to the exocytosis theory, catecholamines are stored at very high concentrations within chromaffin granules and synaptic vesicles and are released when stimulation evokes Ca⁺⁺ influx, which causes fusion of the granule/vesicle with the plasma membrane and discharge of the contents (56). Electrical measurements in single adrenal chromaffin cells have shown discrete increases in membrane capacitance during secretion, indicating quantized vesicle fusion events consistent with exocytosis (57). Exocytotic sites have been visualized by antibody labeling of granule membrane proteins exposed transiently at the cell surface during secretion, further supporting the exocytosis/endocytosis secretory cycle in chromaffin cells (58). Since 2004, confocal and super-resolution stimulated emission depletion (STED) microscopy have demonstrated vesicle fusion (Ω-profiles) (59) and dynamic fusion-pore opening/expansion/constriction/closure (60) in chromaffin cells.

III.2. Reuptake

Most released catecholamines are removed from the extracellular fluid by reuptake (61–63), which is mediated by cell membrane transporters—the cell membrane dopamine transporter (DAT, SLC6A3) (64, 65) and the cell membrane norepinephrine transporter (NET, SLC6A2) (66). Both are high-affinity Na⁺/Cl⁻-dependent plasma membrane transporters that terminate signaling and also recycle the neurotransmitter. The DAT shows the highest affinity and transport capacity for dopamine, with much lower affinity for norepinephrine or epinephrine. Dopamine is a better substrate for the NET than is norepinephrine, which in turn is a better substrate than epinephrine (67), explaining the limited sympathoneural uptake of circulating EPI.

Reuptake is not only a mechanism for terminating catecholamine signaling but also a mechanism for conserving transmitter. In both DAT and NET knockout mice, lack of catecholamine reuptake markedly prolongs extracellular dopamine and norepinephrine signaling while producing profound compensatory changes, including reduced tissue catecholamines stores and altered catecholamines release, indicating that synthesis alone cannot fully replace the recycling function of membrane catecholamine transporters during ongoing monoaminergicneurotransmission (68–70). Analogously, NET blockade by norepinephrine-selective antidepressants or SNRIs prolongs extracellular norepinephrine signaling. NET knockout mice have decreased cardiac tissue concentrations of norepinephrine and DHPG (71).

NET knockout mouse models differ substantially from pharmacological blockade of the NET. There are profound developmental alterations in the former that are not recapitulated in the latter. All known NET blockers readily penetrate the central nervous system and elicit profound sympathoinhbition (72, 73).

III.3. Neuromodulator vs. neurotransmitter

Dopamine and norepinephrine act as central neurotransmitters with prominent neuromodulatory actions. The “neuromodulator” designation reflects the fact that dopamine receptors are metabotropic G protein-coupled receptors, rather than ionotropic receptors mediating fast synaptic transmission.

Regarding neuromodulator vs. neurotransmitter function, it is not just a matter of metabolotrophic vs. ionotrophic receptors but also how close those receptors are to sites of release, as relevant to fast and slow transmission. Neurotransmitters act within nanometers of sites of release (e.g., synaptic transmission) whereas neuromodulators mostly act from hundreds of nanometers to micrometers from sites of release (volume transmission) (74–77). In the central nervous system there may be both synaptic and volume transmission for norepinephrine and dopamine; however, the latter is predominant (78). Presumably levodopa treatment to bypass nigrostriatal dopamine deficiency in Parkinson disease works via general production of dopamine for volume transmission. Similarly, norepinephrine released by sympathetic nerves (at varicosities rather than boutons) functions as both a neurotransmitter and neuromodulator. Since 2004, fast-scan cyclic voltammetry and genetically encoded dopamine/norepinephrine sensors have made it possible to relate phasic and tonic catecholamine dynamics to behavior and circuit activity in vivo (79–82).

III.4. Neuronal sites of release vs. hormonal sites of action

Catecholamines act through neuronal and hormonal pathways that differ fundamentally in sites of release, diffusion, and target specificity. In the neuronal mode, catecholamines are released from nerve terminals or varicosities. This junctional release governs organ-specific responses such as vascular tone, heart rate, and gastrointestinal motility. In contrast, the hormonal mode especially involves secretion of epinephrine from adrenomedullary chromaffin cells into the systemic circulation, reaching distant organs to produce diffuse, longer-lasting effects such as mobilization of energy stores and the “fight-or-flight” response. The two modes are integrated but distinct: neuronal norepinephrine acts locally within milliseconds to seconds, while circulating epinephrine exerts systemic effects lasting seconds to minutes.

Moreover, catecholamine metabolism differs between compartments—neural norepinephrine is largely cleared by reuptake mediated by the NET, whereas circulating catecholamines are inactivated mainly by extra-neuronal uptake followed by COMT and MAO (83–85).

Based on concurrent measurements of plasma norepinephrine levels and blood pressure during sympathetic stimulation or norepinephrine infusion, concentration gradients have been estimated between sites of release and the circulation (86, 87). A 20 mm Hg sympathetically mediated pressor response is associated with about a 560 pg/ml (3.3 nM) concentration of norepinephrine in the average neuroeffector junction. Relationships of estimated rates of norepinephrine release with spillover into the circulation vary depending on the organ. In the kidneys, norepinephrine release is about three times spillover, in skeletal muscle 12 times spillover, and in the heart >20 times spillover. Thus, norepinephrine release greatly but variably exceeds spillover in these organs (88).

III.5. Adrenoceptors and relevance to functions of specific catecholamines

The recognition beginning in the mid-twentieth century of catecholamine receptor types, subtypes, and intracellular second-messenger systems greatly complicated—but also substantially enriched—understanding of how three simple molecules give rise to myriad adaptive responses that have enhanced survival over aeons.

III.5.1. Evolutionary origins of adrenoceptor multiplicity

It is remarkable that the three endogenous catecholamines signal via so many receptor types and subtypes. This multiplicity can be traced to particular events in evolutionary history—two periods, referred to as R1 and R2, when whole-genome duplication (WGD) occurred in back to back eras (89). With WGD the entire chromosome set was copied, briefly creating a tetraploid organism. WGDs quadrupled the genomic ancestor of catecholamine receptors from a single G-protein-coupled receptor (GPCR), resulting in receptor types and subtypes in alpha- and beta-adrenoceptor and dopamine receptor families.

After the R1 and R2 events four families of receptors emerged—DRD1-like, with dopamine the endogenous ligand and Gs the second messenger; DRD2-like, with dopamine the ligand and Gi/o the second messenger (90); ADRA-like, with norepinephrine or epinephrine the agonist at alpha-adrenoceptors and Gq / Gi/o the second messengers; and ADRB-like, with norepinephrine or epinephrine the agonist at beta-adrenoceptors and Gs the second messenger. Small subsequent duplications and losses led to the current vertebrate repertoire: Dopamine D1, D5 (D1-like) and D2, D3, D4 (D2-like), adrenergic alpha (alpha-1A/B/D and alpha-2A/B/C), and adrenergic beta (beta-1, beta-2, and beta-3) (91, 92).

These evolutionary developments help understand the complexity and tissue specificity of catecholaminergic signaling. Indeed, the same ligands can elicit opposite effects (e.g., augmentation of sympathetically mediated norepinephrine release via beta-adrenoceptors (93) vs. inhibition via alpha-2 adrenoceptors (94)), tissue-specific actions (e.g., beta-3 adrenoceptors in brown fat (95) and bladder detrusor (96); D4 receptors in prefrontal cortex (97)), and fine partitioning of hormonal vs. synaptic vs. extra-synaptic (junctional) signaling (98–101).

III.5.2. Adrenoceptors mediating effects of circulating vs. neuronally released catecholamine

Different adrenoceptor populations mediate effects of circulating vs. neuronally released catecholamine. Neuronally released norepinephrine acts primarily on local alpha-1, alpha-2, and beta-1 adrenoceptors. In contrast, circulating catecholamines—predominantly epinephrine—engage a broader array of adrenergic receptors including beta-2 adrenoceptors on myocardial cells (102) and vascular smooth muscle cells in skeletal muscle (103). Neuronal norepinephrine release is modulated by inhibitory alpha-2 autoreceptors (104, 105) as well as by muscarinic heteroreceptors (106) on sympathetic nerves—enabling refined adjustments compared to hormonal epinephrine.

III.5.3. Receptor down-regulation

Chronic elevations of catecholamines produce adrenoceptor down-regulation and desensitization (107), representing an allostatic adjustment. When alpha- or beta-adrenoceptors are exposed to agonist for a prolonged period, receptor signaling is turned off by a multi-step process involving (a) receptor phosphorylation by GPCR kinases (GRKs) at specific serine/threonine residues on the receptor’s intracellular domains (108); (b) recruitment of β-arrestin by the GRK-phosphorylated receptor binding to β-arrestin, sterically blocking further coupling of the receptor to G proteins (109); and (c) receptor internalization (endocytosis) and recycling back to the cell membrane (resensitization) or down-regulation by degradation (110). Different GRKs and β-arrestin isoforms can yield distinct functional outcomes (111).

Arrestin recruitment to activated GPCRs is now understood to do more than terminate G-protein signaling. Arrestins also act as adaptor/scaffold proteins that can initiate distinct signaling pathways, including ERK/MAPK and other kinase cascades. Thus, receptor–arrestin engagement should be described as a dual-function process mediating both desensitization/internalization and G-protein-independent or arrestin-biased signaling (112, 113).

III.5.4. Denervation supersensitivity

Whereas desensitization has been dissected in exquisite molecular detail, denervation supersensitivity remains a phenomenon with incompletely understood cellular and molecular contributors (114, 115). Some proposed mechanisms are (a) upregulation, i.e., increased transcription or membrane insertion of postsynaptic adrenoceptors; (b) enhanced receptor-effector coupling via increased G-protein availability or adenylate cyclase responsiveness: (c) Loss of local neurotransmitter uptake via the NET or decreased metabolism by MAO//COMT; and (d) changes in resting membrane potential and ion channel sensitivity.

III.6. Chemical coding

Early models of catecholamine signaling were shaped by Dale’s principle, which emphasized a single chemical transmitter released from each neuron. This view was fundamentally revised by Burnstock’s demonstration of purinergic cotransmission, showing that catecholaminergic neurons—particularly sympathetic nerves—corelease adenosine triphosphate (ATP) with norepinephrine and, in some contexts, neuropeptides, thereby generating temporally and functionally distinct components of target-organ responses (116, 117).

III.6.1. Purinergic transmission

Many catecholaminergic neurons, particularly sympathetic post-ganglionic fibers, co-release ATP along with norepinephrine from their synaptic vesicles, forming a key component of purinergic neurotransmission (118). ATP acts as a co-transmitter, mediating fast excitatory junctional potentials and producing rapid contractions of vascular and visceral smooth muscle before the slower, more sustained alpha-adrenoceptor-mediated responses elicited by norepinephrine.

Upon release into the synaptic cleft, ATP activates P2X ligand-gated ion channels on post-synaptic membranes, resulting in depolarization and calcium influx, while P2Y G-protein–coupled receptors contribute to longer-lasting modulatory effects. Since 2004, the co-transmission model proposed by Burnstock and colleagues has established that sympathetic neurotransmission is triphasic, involving a rapid ATP-mediated response, an intermediate norepinephrine-mediated response, and sometimes a slower neuropeptide Y (NPY) component (119).

III.6.2. Chromogranins

Chromogranins are a family of acidic glycoproteins that are major constituents of the soluble matrix of catecholamine storage vesicles in chromaffin cells and sympathetic neurons. Among these, chromogranin A (CgA) is the most abundant and extensively studied (120). CgA binds catecholamines and ATP in the vesicular lumen. Upon exocytotic release, CgA undergoes proteolytic processing to yield several biologically active peptides, including catestatin, which inhibits nicotinic receptor–mediated catecholamine secretion and thereby provides autocrine negative feedback on sympathetically-mediated exocytosis (121).

III.6.3. Endogenous opioids

Endogenous opioids, including enkephalins, endorphins, and dynorphins, are neuropeptides derived from larger precursor proteins—proenkephalin (PENK), proopiomelanocortin (POMC), and prodynorphin (PDYN)—that exert their effects through μ-, δ-, and κ-opioid receptors, all of which are G-protein–coupled receptors. Among these, the enkephalins (Met- and Leu-enkephalin) are particularly relevant to catecholaminergic function. They are colocalized with catecholamines in adrenal chromaffin cells, sympathetic ganglia, and certain central noradrenergic neurons, especially those in the nucleus of the solitary tract, locus ceruleus, and hypothalamus (122, 123).

III.6.4. Neuropeptide Y

Neuropeptide Y (NPY) is a 36–amino acid peptide that serves as a major sympathetic cotransmitter and neuromodulator throughout the central and peripheral nervous systems (124). NPY is co-stored and co-released with norepinephrine from large dense-core vesicles of sympathetic postganglionic neurons and adrenal chromaffin cells and is one of the most abundant neuropeptides in the mammalian brain and autonomic nervous system. Since 2004 it has been recognized that upon severe stress-related sympathetic activation, NPY is released in parallel with norepinephrine but produces longer-lasting effects through its action on Y₁, Y₂, and Y₅ receptors, G-protein–coupled receptors expressed on vascular smooth muscle, sympathetic nerve terminals, and central autonomic neurons (125, 126).

III.7. Cotransmission by location

Sympathetic noradrenergic nerves contain multiple cotransmitters including ATP, NPY, and sometimes enkephalins or galanin stored together with norepinephrine in synaptic vesicles. Upon nerve stimulation, these are coreleased in a frequency- and pattern-dependent manner, producing temporally distinct phases of target-organ response (118).

In adrenomedullary chromaffin cells, catecholamines are stored in large dense-core vesicles with chromogranins, ATP, calcium, and opioid peptides such as enkephalins. Co-released chromogranin-derived peptides like catestatin and vasostatin provide autocrine negative feedback on secretion and cardiovascular tone, while enkephalins inhibit further catecholamine release via opioid receptors (121, 122, 127).

Within central catecholaminergic neurons, including those in the locus ceruleus, hypothalamus, and nucleus of the solitary tract, dopamine or norepinephrine may be coreleased with neuropeptides such as galanin, dynorphin, or somatostatin, as well as with ATP or glutamate.

Collectively, the organization of cotransmission by anatomical locus—peripheral, adrenal, or central—enables catecholaminergic systems to incorporate fast synaptic excitation, intermediate neuromodulation, and slow neuropeptide-mediated plasticity, ensuring flexible and context-dependent regulation of autonomic and behavioral states.

IV. THREE PERIPHERAL CATECHOLAMINERGIC SYSTEMS

The physiological roles of catecholamines in the periphery can be conceptualized as organized into distinct systems that differ in cellular origin, modes of release, patterns of regulation, and functional scope. This section describes three types of peripheral catecholaminergic systems—autocrine/paracrine, hormonal, and neurotransmitter—and examines how their specialized architectures enable coordinated yet differential contributions to homeostasis, stress responses, and allostatic adaptation. We also present here the relatively new concept of the “extended autonomic system” (EAS), which incorporates neuroendocrine, immune/inflammatory, and central components and reflects developments over the past century since Langley’s original formulation of the autonomic nervous system (2). Central catecholaminergic systems are considered here primarily within the context of the EAS.

IV.1. Autocrine-paracrine catecholaminergic systems

Most of the production and metabolism of norepinephrine and dopamine in humans takes place not in the brain or in the autonomic nervous system but in mesenteric organs (41, 128). Non-neuronal cell bodies in mesenteric organs express immunoreactive TH and have detectable TH activity, implying an ability to generate DOPA locally (129). During the course of evolution, autocrine-paracrine catecholaminergic systems probably were superseded by neurotransmitter and hormonal systems (130), but they are still present. Based on human arteriovenous balance and portal sampling, the gastrointestinal tract is the predominant source of circulating and urinary dopamine metabolites (45, 46).

IV.1.1. The renal DOPA/dopamine autocrine-paracrine system and sodium balance

The renal DOPA/DA system can be considered to be the prototype for autocrine-paracrine catecholaminergic systems in the human body (Figure 3). Comparative physiological data suggest that the DOPA/DA system evolved first but was superseded by endocrine and neuroendocrine systems such as the renin-angiotensin-aldosterone, pituitary-adrenocortical, and sympathetic adrenergic systems and by nerve networks culminating in the sympathetic nervous system (130).

In rats (131) and humans (132) most or all of the dopamine excreted in the urine reflects uptake and enzymatic decarboxylation of circulating DOPA. The released dopamine acts on dopaminergic receptors on the same or nearby cells to enhance sodium excretion. Dietary salt loading increases urinary excretion of both DOPA and dopamine (133), and carbidopa, which inhibits LAAAD, decreases urinary sodium excretion (134). Patients with salt-sensitive hypertension have decreased dopamine for a given amount of DOPA excretion (135), consistent with decreased proximal tubular uptake or decreased enzymatic decarboxylation of DOPA.

IV.1.2. Autocrine-paracrine catecholamines and the gut

Over the past several years, numerous studies have begun to address the roles of the gut microbiome and relationships with catecholamines in normal physiology and pathophysiological states (136). Despite the fact that splanchnic organs constitute the main site of production and metabolism of endogenous catecholamines in the body (41, 128), studies to date have not systematically incorporated the autocrine-paracrine actions of locally produced catecholamines on gastrointestinal functions or on the gut microbiome.

Catecholamine metabolism has an important peripheral dimension in the gastrointestinal tract. Substantial dopamine production occurs in human mesenteric organs, and gut luminal catecholamine availability is influenced by the microbiota, supporting autocrine and paracrine actions of catecholamines within the intestine (128, 137). Dopamine can act locally in the gut; for example, luminal dopamine stimulates duodenal bicarbonate secretion through apical D2 receptors (138). From this initial conceptual understanding of over 20 years ago, there has been expansion to biological and clinical relevance. Thus more recent experimental work also indicates that gut–brain vagal signaling can regulate mesolimbic dopamine dynamics and reward-related behavior, linking intestinal signals to central dopaminergic function within the gut–brain axis (139). These findings suggest that intestinal catecholamine metabolism should be considered not only as a peripheral biochemical process but also as a component of microbiota–gut–brain communication. This is a rapidly expanding field of research (139–141).

IV.1.3. Catecholamines and cytokines

Endogenous catecholamines and cytokines interact bi-directionally (142): hormonal epinephrine—the main endogenous agonist at beta-2 adrenoceptors—generally inhibits pro-inflammatory cytokine production, whereas inflammatory cytokines such as IL-1β, IL-6, and TNF-α activate central and peripheral catecholaminergic systems, linking immune and stress responses.

IV.1.4. Catecholamines, the pancreas, and glucose homeostasis

The main hormones regulating glucose levels are insulin, glucagon, and epinephrine. Epinephrine decreases insulin levels (143), and plasma insulin and glucagon levels during stress states reflect the opposing interaction between hyperglycemia and catecholamines (144).

Epinephrine’s roles in hepatic glycogenolysis and gluconeogenesis and in skeletal-muscle glycogenolysis with enhanced glycolytic flux are well established and form a core component of acute glucose homeostasis during stress. In contrast, although dopamine is synthesized locally in multiple peripheral tissues (e.g., gastrointestinal tract, kidney, pancreas), its integrated physiological roles in glucose regulation and intermediary metabolism remain incompletely defined.

IV.2. Catecholamines and neuroendocrine systems

IV.2.1. The Sympathetic Adrenergic System (SAS) and responses to global threats

Walter B. Cannon and a colleague reported in 1911 that emotional distress evokes release of a substance from the adrenal gland into the circulation (145). One may reasonably infer that epinephrine was the first hormone to be described (146). The sympathetic adrenergic system (SAS), in which epinephrine is the main chemical effector, may also be considered to be the first neuroendocrine system (2).

As discussed in more detail below, SAS activation is a highly sensitive indicator of experienced distress (147) and any global threat to homeostasis, even mild acute glucoprivation (148). In contrast, the sympathetic noradrenergic system (SNS) plays major roles in circulatory adjustments during activities of daily living such as orthostasis, mild exercise, the post-prandial state, and altered environmental temperature (149).

IV.2.2. Related neuroendocrine systems

The SNS and SAS interact importantly with a variety of neuroendocrine systems.

HPA axis-catecholamine interactions

In general, stressors that evoke increases in activity of the hypothalamic-pituitary-adrenocortical (HPA) axis also increase SAS outflow as indicated by plasma epinephrine levels (150). Systemic administration of the beta-adrenoceptor agonist isoproterenol tends to inhibit the HPA axis while increasing plasma norepinephrine levels (151).

Renin-angiotensin-aldosterone system (RAAS)-catecholamine interactions

Catecholamines and the RAAS amplify each other. Renal sympathetic activation stimulates beta-adrenoceptors on juxtaglomerular cells, which increases renin release and initiates the angiotensin-to-aldosterone cascade. Classic in vitro work demonstrated a direct beta-adrenergic drive on renin secretion (152). Angiotensin II (AII), in turn, can facilitate sympathetic neurotransmission via stimulatory effects in the brain (153) and possibly by pre-junctional actions that enhance norepinephrine release, although there is insufficient evidence for functional AII receptors on sympathetic nerves (154). Surgical stress results in parallel activation of the SNS and RAAS (155).

Pituitary adenylate cyclase-activating polypeptide (PACAP) and adrenomedullary functions

Pituitary adenylate cyclase-activating polypeptide (PACAP) was first identified in hypothalamus, based on its ability to elevate cyclic AMP in the anterior pituitary. PACAP subsequently was found to be an adrenomedullary neurotransmitter (156). PACAP evokes catecholamine secretion and activation of catecholamine biosynthetic enzymes during stress in mammals (157). Low-frequency splanchnic neural activity evokes adrenomedullary catecholamine secretion mainly through cholinergic mechanisms via nicotinic receptors, whereas high-frequency firing as during stress recruits PACAP-dependent peptidergic signaling that supports both catecholamine secretion and induction or activation of catecholamine-biosynthetic enzymes, thereby coupling release from chromaffin granules to replenishment of catecholamine stores (156, 158–162).

IV.3. Catecholamines and immune/inflammatory systems

IV.3.1. The vagal anti-inflammatory system

The vagal anti-inflammatory system provides an example of bi-directional neuroimmune regulation in which afferent inflammatory signals engage central autonomic circuits that, in turn, modulate peripheral cytokine production through autonomic efferent pathways. The system conceptualized by Tracey (163) entails a catecholaminergic component, in that the efferent pathway includes the splenic nerve (164), and there is no convincing evidence for direct vagal innervation of the spleen. Nicotinic receptors containing the α7 subunit are required for the reflex (165). The reflex as currently understood seems to involve a paradox, because afferent vagal stimulation inhibits activity of brainstem centers mediating sympathetic outflows (166), yet beta-2 adrenoceptor stimulation exerts an inhibitory effect on release of pro-inflammatory cytokines (167). It seems likely that the resolution will come from better understanding of complex interplays within the extended autonomic network.

IV.3.2. Catecholamine-driven “stress leukogram”

Acute elevations of circulating catecholamines can rapidly alter leukocyte trafficking and circulating counts (demargination/redistribution), producing a “stress leukogram” pattern with an increased neutrophil/lymphocyte ratio depending on timing and compartment shifts. Human infusion studies have shown that catecholamine administration increases circulating neutrophils (and alters lymphocyte counts) in vivo (168) and that epinephrine can increase total leukocyte counts with measurable shifts in neutrophil and lymphocyte populations (169). The general view that catecholamine excess is pro-inflammatory is supported by the findings that patients harboring a pheochromocytoma have elevated neutrophil/lymphocyte ratios that are correlated with plasma metanephrine levels and are mitigated by adrenalectomy (170).

In humans, perioperative infusion of the alpha-2 adrenoceptor agonist dexmedetomidine evokes a pattern of biochemical and immune/inflammatory effects. Dexmedetomidine decreases norepinephrine, epinephrine, cortisol, and blood glucose levels. Interleukin-6, tumor necrosis factor-alpha, C-reactive protein levels, and CD8(+) T cells decrease, while interleukin-10, numbers of natural killer cells, B cells, CD4(+) T cells, and ratios of CD4(+):CD8(+) and Th1:Th2 increase (171). These results may be interpreted in terms of sympatholysis attenuating perioperative central stress system activation and associated cytotoxic immune tone with relative restoration or expansion of helper/regulatory adaptive immunity (172, 173).

Immune cells can express catecholamine-synthetic enzymes and generate catecholamines upon activation, providing potentially rich interfaces between catecholaminergic signaling and inflammation (174). The recently proposed theory of the extended autonomic system offers a unifying concept integrating the roles of neurotransmitter, neuroendocrine, and autocrine-paracrine catecholaminergic systems in immune/inflammatory functions (2, 3).

IV.4. Central catecholaminergic systems

Dopamine and norepinephrine act as central neurotransmitters with prominent neuromodulatory actions. The “neuromodulator” designation reflects the fact that dopamine receptors are metabotropic G protein-coupled receptors, rather than ionotropic receptors mediating fast synaptic transmission.

The dopaminergic nigrostriatal system originates in the substantia nigra pars compacta and projects to the striatum, where it modulates motor initiation, coordination, and habit learning. Nigrostriatal dopaminergic neurodegeneration produces the cardinal motor features of Parkinson’s disease. The meso-limbo-cortical system, arising from the ventral tegmental area and projecting to limbic and cortical regions, mediates reward processing, motivation, affect, and cognitive flexibility, and its dysregulation contributes to addiction, depression, and schizophrenia.

The brain’s largest dopaminergic projection system is the nigrostriatal pathway, arising mainly from substantia nigra pars compacta neurons and innervating the dorsal striatum, whereas the adjacent ventral tegmental area gives rise largely to mesolimbic and mesocortical projections to limbic and cortical targets (175–177).

There is massive literature on the nigrostriatal and meso-limbo-cortical dopaminergic systems and the locus ceruleus noradrenergic system. Animal models of acute and chronic stress have noted increases in both dopamine and norepinephrine in the medial prefrontal cortex(178, 179); however, relatively little attention has been paid to dopamine-norepinephrine interactions (180).

Since 2004 it has been recognized that TH-positivity alone does not imply a fully catecholaminergic phenotype. In hypothalamic and developmental contexts, neurons may express only TH and produce L-DOPA, whereas neighboring LAAAD-expressing neurons may convert taken-up L-DOPA to dopamine (181). Weihe and colleagues have distinguished “dopaergic” from classical dopaminergic neurons (182). Therefore, identifying neurons as catecholaminergic generally requires evidence beyond TH immunoreactivity, such as LAAAD and VMAT2 co-expression or direct evidence of catecholamine synthesis, storage, and release.

IV.4.1. Roles of central neural dopamine

Dopamine is a central neurotransmitter that plays key roles in movement, motivation, reward learning, cognition, and neuroendocrine regulation. In the nigrostriatal system, dopamine shapes action selection via basal-ganglia loops (183). In mesolimbic circuits, dopamine neurons encode reward-prediction errors and incentive salience (184, 185). In prefrontal cortex, dopamine modulates working memory and executive control (186). In the tuberoinfundibular pathway, hypothalamic dopamine tonically inhibits prolactin (187). Thus, in the past two decades there has been increasing recognition that dopamine functions in the brain not only as a simple “pleasure chemical” or regulator of locomotion but as a multi-faceted neurotransmitter with prominent neuromodulatory actions, orchestrating motor, motivational, cognitive, emotional, and endocrine domains of central nervous system function.

IV.4.2. Roles of central neural norepinephrine

Most of the norepinephrine in the brain originates from the pontine locus ceruleus (LC). The LC influences virtually all major forebrain and brainstem structures (188). The LC–noradrenergic system plays pivotal roles in arousal, attention, stress responsiveness, and behavioral flexibility, adjusting neuronal excitability and network synchrony in response to salient or novel stimuli (189). Local GABAergic neurons within the LC dynamically fine control norepinephrine release during behavioral state transitions (190).

Epinephrine is scarce in the brain, although C1 neurons in the rostral ventrolateral medulla (RVLM) express phenylethanolamine-N-methyltransferase (PNMT) (191). Normally there is no epinephrine detected in cerebrospinal fluid.

IV.4.3. Sources of sympathetic outflows

Descending pathways to sympathetic preganglionic neurons arise from multiple brain areas (RVLM, PVN, A5, rostral ventromedial medulla, caudal raphe nuclei) but not the locus ceruleus (A6) (192–194).

Compared to studies considering central dopaminergic and noradrenergic systems in isolation, studies integrating the two neuromodulators are relatively scarce (195). Dopamine can be co-released with norepinephrine from central noradrenergic terminals (196), contributing to memory consolidation (197, 198).

IV.4.4. Relationships between central and peripheral catecholaminergic systems

Circulating catecholamines have extremely limited access to most brain parenchyma, demonstrating an effective blood-brain barrier after systemic administration (199, 200). In contrast, L-DOPA crosses the blood–brain barrier via large neutral amino-acid transport systems, providing the mechanistic basis for pharmacologic augmentation of central catecholamine synthesis (201).

The blood-brain barrier for catecholamines is imperfect in the hypothalamus—particularly neuroendocrine regions such as the median eminence—and circumventricular structures such as the area postrema, where fenestrated capillaries and specialized barriers permit exposure to circulating hormones, metabolites, and cytokines. These regions function as sensory and integrative interfaces rather than conduits for bulk equilibration of catecholamines between blood and brain (202). In the opposite direction, kinetic studies using cerebral venous sampling and isotope dilution have noted measurable norepinephrine overflow from the brain into venous plasma; however, this contribution is small relative to whole-body norepinephrine spillover (203, 204). Instead, central catecholaminergic systems mainly influence peripheral catecholamine dynamics indirectly, via descending neural and neuroendocrine pathways regulating sympathetic preganglionic neurons, adrenomedullary secretion, and reflexive autonomic responses. Thus, cerebrospinal fluid and circulating norepinephrine levels are correlated despite the blood-brain and brain-blood barriers for norepinephrine (205, 206). Cerebral venous overflows of dopamine metabolites occur without parallel increases in circulating dopamine (207), and renal proximal tubular dopamine production depends on local precursor handling and decarboxylation rather than central catecholamine synthesis (208, 209).

IV.5. Differential regulation of endogenous catecholaminergic systems

Although the use of terms such as “autonomic failure” and “dysautonomia” suggest single entities, it has become clear in the past two decades that the autonomic nervous system has functionally and neurochemically distinct components (210). Evidence has accumulated for differential noradrenergic vs. adrenergic responses in various situations. The largest SAS responses are seen when the organism encounters stressors that pose a global or metabolic threat, such as glucoprivation, hypotensive hemorrhage, and emotional distress, whereas SNS activation dominates the responses to orthostasis, moderate exercise, and exposure to cold (150). There seems to be at least as good a justification for the concept of coordinated HPA-SAS responses as for coordinated SNS-SAS responses in stress (211). Heterogeneous patterns of SNS and neuroendocrine responses belie the notion of a non-specific response of the body to any demand imposed on it (212, 213). The section on catecholamine metabolic patterns in pathophysiological states elaborates on this theme.

IV.6. Interpreting plasma catecholamine levels: Effects of skin color, age, and sex

In melanocytes, tyrosine is converted to DOPA and then DOPA-quinone via tyrosinase (214). During early development in mice tyrosinase can be a source of dopamine independently of TH (215). Based on comparisons of albino subjects and Black vs Caucasian participants, in humans plasma DOPA is not related to skin pigmentation; however, urinary excretion of DOPA and dopamine are greater in Black Americans than in Caucasian Americans (216). There are no consistent differences in plasma levels or urinary excretion of norepinephrine as a function of skin color under resting conditions.

Plasma L-DOPA tends to decrease with age, whereas plasma norepinephrine increases (217). Higher plasma norepinephrine with aging at least partly reflects reduced norepinephrine clearance (218). Rather than reflecting a uniform increase in sympathetic outflow, aging is associated with heterogeneous changes across different vascular beds (219, 220). After accounting for age-related reductions in clearance, total body epinephrine spillover does not increase with aging (221, 222).

With respect to sex, the literature about plasma norepinephrine and epinephrine has been inconsistent. An early study noted higher plasma norepinephrine in females and epinephrine in males (223). In another study, plasma norepinephrine did not differ by sex or skin color; plasma epinephrine was lower in white females than in other groups (224). In a third study, muscle sympathetic nerve activity was found to increase progressively from young women and young men to older women and older men, but plasma norepinephrine did not differ across these groups (225). Plasma norepinephrine is higher in the luteal phase of the ovulatory cycle, underscoring sex/hormonal influences on “baseline” reference values (226).

In a large reference cohort, men had higher plasma free metanephrine but not normetanephrine than women, and normetanephrine increased with subject age (227).

IV.7. The “extended” autonomic system

The “extended autonomic system” (EAS), a concept introduced within the past two decades (2, 3), encompasses the traditional sympathetic, parasympathetic, and enteric divisions postulated by Langley (228) but also incorporates neuroendocrine and immune/inflammatory systems and the central autonomic network (4). Taken together, these components regulate allostasis and coordinate adaptive responses to a variety of stressors (2, 3).

Catecholaminergic systems exemplify brain–body cross-talk. Catecholamines in the brain modulate autonomic outflows, catecholamines in the periphery act as the chemical messengers of those outflows, and catecholaminergic mechanisms such as norepinephrine release in the spinal dorsal horn modulate afferent input—together forming bi-directional “two-way streets” (229) that integrate central and peripheral regulation of homeostasis and allostasis (6, 7, 230).

V. CATECHOLAMINE EFFECTS IN PATHOPHYSIOLOGICAL STATES

Since catecholaminergic systems contribute to virtually every organ function, behavior, and emotional state, it is expected that in many situations alterations in those systems exert syndromic pathophysiological effects. Examples discussed below are dysregulations due to genomic lesions; stress cardiomyopathy as in takotsubo syndrome; acute and chronic heart failure; autonomic failure syndromes related to cardiac noradrenergic deficiency; pathophysiological consequences of altered functions of the renal DOPA-dopamine autocrine-paracrine system; the catecholamine-driven stress leukogram; and fainting reactions.

V.1. Genetic abnormalities

Genomic variants affecting catecholamine biosynthesis, storage, recycling, or metabolism provide insights about functional consequences of specific lesions within catecholaminergic systems as indicated by particular catecholamine metabolomic patterns.

Multiple genetic abnormalities lead to decreased activity of dopamine-beta-hydroxylase (DBH) and thereby norepinephrine synthesis, even without a variant of the DBH gene itself, by disrupting essential co-factors or intracellular conditions. A shared and prominent clinical consequence in these conditions is neurogenic orthostatic hypotension (nOH), reflecting an impaired ability of postganglionic sympathetic neurons to increase norepinephrine release in response to reduced venous return on standing. Menkes disease (ATP7A deficiency) is a prototypical example. DBH is a copper-dependent enzyme, and ATP7A is required for copper delivery to the secretory pathway. In Menkes disease, defective copper trafficking leads to functional DBH deficiency, with elevated dopamine and reduced norepinephrine and DHPG, resulting in a high DA:NE ratio and impaired sympathetic vasoconstriction (231, 232).

A second example, described relatively recently, involves defects in intra-vesicular ascorbate regeneration, notably cytochrome b561 (CYB561) deficiency (55, 233). Ascorbate is required to maintain DBH copper in its reduced, catalytically active state. Biallelic CYB561 mutations cause selective sympathetic noradrenergic failure, characterized by lifelong, severe nOH and catecholamine profiles consistent with impaired DBH activity despite an intact DBH gene (55, 233).

Genetic deficiency of L-aromatic-amino-acid decarboxylase (LAAAD), coded for by the DDC gene, is associated with decreased production of all three catecholamines as well as of serotonin (234, 235). There is no known disease that manifests with isolated dopamine deficiency. LAAAD deficiency manifests as a drastic, progressive, lethal pediatric disease that presents in infancy with severe hypotonia and hypokinesia, oculogyric crises, dystonia, and global developmental delay, accompanied by prominent autonomic dysfunction including orthostatic hypotension, episodic hypotension or hypothermia, ptosis, nasal congestion, and gastrointestinal dysmotility, reflecting combined central and peripheral monoamine deficiency.

A translationally important example is inherited LAAAD deficiency, which impairs synthesis of catecholamines and serotonin. This disorder has become a target for AAV2-mediated gene therapy. Intraputaminal delivery of eladocagene exuparvovec, an AAV2 vector carrying the human DDC gene, is intended to restore LAAAD expression and striatal dopamine production. Recent clinical studies have reported sustained improvements in motor function after treatment, illustrating both the central role of LAAAD in monoamine biosynthesis and the therapeutic potential of restoring catecholamine synthesis in a defined neuronal target region (236, 237).

Although phenylethanolamine-N-methyltransferase (PNMT) is required for conversion of norepinephrine to epinephrine, and in humans there is a single PNMT gene, no Mendelian disorder has been reported with a characteristic clinical syndrome due to PNMT loss. Evidence for the physiological importance of PNMT comes mainly from PNMT-knockout mouse models, which show stress- and metabolic abnormalities without altered cardiovascular regulation (238). On the other hand, cortisol is trophic for PNMT gene expression. This means that congenital adrenal hyperplasia (the most frequent cause being variation of the CYP21A2 gene) is associated with biochemical evidence of decreased epinephrine production (239). Among the downstream effects is attenuated glucose elevation during high-intensity exercise (240).

The genes encoding the monoamine oxidase (MAO) types MAO-A and MAO-B are located close to each other on the X-chromosome. A family of disorders with different clinical manifestations reflect variants of these genes. The most well established is loss of function mutations of MAO-A (Brunner syndrome (241)), characterized by behavioral abnormalities such as aggressiveness and impulsivity. The extent to which these clinical manifestations reflect altered functions of catecholaminergic systems is incompletely understood, since the main fate of intra-neuronal catecholamines is vesicular uptake, and MAO deficiency builds up other monoamines such as serotonin. An exception is catecholamine-mediated hypertension upon administration of tyramine in the setting of MAO-A deficiency (evoking the “cheese effect”) (13). In combined MAO-A/MAO-B deficiency and Norrie disease (a syndrome resulting from pathogenic variants in the NDP gene (Norrin), located near the MAO genes) there is severe mental retardation, whereas genetic MAO-B deficiency does not entail behavioral abnormalities or mental retardation (13).

V.2. Pheochromocytoma/paraganglioma

In the past two decades it has been recognized that sustained systemic catecholamine overproduction in pheochromocytoma is associated with a hypermetabolic, catabolic phenotype. Patients exhibit weight loss or reduced adiposity compared with appropriate comparators, consistent with a net increase in energy expenditure and lipolysis (242, 243). After curative tumor resection, the catecholamine-excess state resolves, and body weight and basal metabolic rate increase during follow-up, consistent with reversal of the pre-operative hypermetabolic drive (244). The post-operative gain is often interpreted as “unmasking” baseline energy balance once circulating catecholamine levels normalize.

Catecholamine excess in pheochromocytoma is associated with a high prevalence of diabetes/glucose intolerance and insulin resistance, consistent with catecholamine-driven impairment of glucose homeostasis (245). Following tumor resection, the diabetes frequently improves or resolves in a substantial proportion of patients, supporting a causal role of catecholamine excess in the dysglycemic state (246).

Pheochromocytoma can manifest with a dilated cardiomyopathy and congestive heart failure due to high circulating levels of catecholamines. Histopathologic analysis in end-stage cardiomyopathy related to pheochromocytoma shows focal myocarditis and contraction band necrosis. After removal of the tumor and reversal of the hypercatecholaminergic state the heart failure resolves (247), indicating that the circulating catecholamine excess is the cause of the cardiomyopathy.

Although most tumors of neural crest origin are sporadic, a scientifically and clinically important minority of pheochromocytomas develop as part of inherited syndromes. Two of the most well studied are multiple endocrine neoplasia type 2 and von Hippel-Lindau disease (248). The two forms display distinct biochemical and clinical phenotypes (249). MEN 2 patients have more symptoms and a higher incidence of paroxysmal hypertension, which are associated with more prominent increased epinephrine content in the tumor and a high rate of epinephrine synthesis as indicated by plasma metanephrine. Thus, variants of genes controlling catecholamine synthesis link differences in clinical presentation of pheochromocytoma to differential involvement of catecholamines in the tumor.

V.3. Stress cardiomyopathy

Takotsubo syndrome provides a particularly clear clinicopathophysiologic example in which acute stress–linked catecholamine elevations are associated with transient left ventricular dysfunction (“myocardial stunning”) and characteristic wall-motion patterns despite unobstructed coronary arteries. In a foundational clinical study published within the past two decades, patients with stress-related myocardial dysfunction had markedly elevated circulating levels of catecholamines (especially of epinephrine) compared to patients with acute myocardial infarction, supporting catecholamine excess as the proximate driver of the takotsubo phenotype (250).

V.4. Chronic heart failure

In chronic heart failure, persistent sympathetic activation is a core neurohumoral feature: elevated plasma norepinephrine reflects sustained sympathoneural outflow and spillover. Higher circulating norepinephrine is associated with worse clinical status and prognosis, consistent with a contributory role of chronic catecholamine excess in disease progression (251, 252).

V.5. Autonomic failure with postganglionic noradrenergic deficiency

Autonomic failure illustrates the opposite directionality: reduced post-ganglionic sympathetic noradrenergic function yields inadequate norepinephrine mobilization with upright posture and defective reflex vasoconstriction, contributing to neurogenic orthostatic hypotension. Classic physiological studies document blunted plasma norepinephrine responses in autonomic failure during orthostatic challenge, supporting the mechanistic link between noradrenergic failure and orthostatic intolerance (253).

V.6. Renal intrarenal DOPA-dopamine system and hypertension

When sufficiently sensitive methods were introduced for assaying endogenous catecholamines, an early application was to examine whether elevated norepinephrine or epinephrine levels would indicate increased sympathetic nervous or adrenomedullary activity in essential hypertension (254, 255). Augmented depressor responses to clonidine (94) and pressor responses to yohimbine (256) in patients with high norepinephrine levels identified patients in whom hypertension was pathophysiologically linked to increased sympathetic outflow. Based on hypertensive-normotensive differences in plasma norepinephrine being most apparent in relatively young groups (257), the concept has emerged in the past two decades that increased sympathetic noradrenergic activity contributes to the pathogenesis of essential hypertension (258).

The renal DOPA-dopamine autocrine-paracrine system contributes to sodium balance, extracellular fluid volume, and blood pressure. This system participates in the natriuretic response to volume expansion (259). Dopamine generated locally from L-DOPA in proximal tubule tissue can inhibit sodium-transport machinery, including Na⁺/K⁺-ATPase activity under high-transport conditions (260). In humans, pharmacologic inhibition of dopamine formation with carbidopa reduces urinary dopamine excretion and is associated with altered sodium excretion, consistent with renal dopamine acting as an intrarenal natriuretic factor (134). Blocking dopamine generation can abolish renal functional responses to renal dopamine prodrugs, further supporting functional intra-renal dopamine axis (261). In patients with heart failure, levodopa administration evokes a natriuretic response (262, 263).

Evidence in salt-sensitive states supports a “relative renal dopamine deficiency” concept: with salt loading, some patients exhibit an abnormal pattern of urinary DOPA/dopamine handling consistent with impaired intra-renal dopamine generation and an attenuated natriuretic response, a physiology aligned with salt-sensitive hypertension (135, 264).

V.7. Fainting (vasovagal or neurally mediated syncope)

Vasovagal syncope provides an example of differential alterations in SNS and SAS outflows. Sympathoadrenal imbalance, with disproportionate adrenomedullary activation relative to sympathetic noradrenergic responses, can precede hypotension, bradycardia, presyncope, and syncope both during tilt and in spontaneous episodes (265). This imbalance correlates with concurrent peripheral epinephrine-induced, beta-2-adrenoceptor mediated vasodilation and supports a mechanistic role for catecholamine patterning in syncope pathophysiology (265–269).

VI. CATECHOLAMINE METABOLOMIC PATTERNS AS BIOMARKERS OF INTRACELLULAR PROCESSES

Because of the localization of enzymes and transporters in catecholamine-producing cells in different body organs, analysis of metabolomic patterns can provide information about intracellular processes. Figure 7 and Tables 2 and 3 provide overviews that facilitate understanding of some of the catecholamine metabolomic biomarkers and intra-neuronal and extra-neuronal processes. Filled entries indicate relationships supported by empirical data or by kinetic modeling as specified; blank entries indicate that no direct empirical evidence is available or that the relationship has not been evaluated. Model-derived inferences should be interpreted as conditional on the assumptions of the underlying kinetic framework.

Figure 7: Overview of sources of plasma levels of catecholamines and metabolites.

Numbers in rectangles identify the compounds listed in Table 1. Abbreviations: ADH=alcohol dehydrogenase; ALDH=aldehyde dehydrogenase; AR=aldehyde reductase; COMT=catechol-O-methyltransferase; DA=dopamine; DA-Q=dopamine-quinone; DA-S=dopamine sulfate; DBH=dopamine-beta-hydroxylase; DHPG=3,4-dihydroxyphenylglycol; DHPG-S=DHPG-sulfate; DOPAC=3,4-dihydroxyphenylacetic acid; EPI=epinephrine; EPI-S=epinephrine sulfate; MAO=monoamine oxidase; MHPG=3-methoxy-4-hydroxyphenylglycol; MHPG-S=MHPG-sulfate; MN=metanephrine; MN-S=metanephrine sulfate; NE=norepinephrine; NE-S=norepinephrine sulfate; NMN=normetanephrine; NMN-S=normetanephrine sulfate; SULT1A3=phenolsulfotransferase 1A3; VMA=vanillylmandelic acid.

Table 3:

Effects of catecholamines in pathophysiological states

Primary catecholaminergic change	Direction / locus	Dominant physiological mechanism	Resulting physiological or pathophysiological phenotype	References
Pheochromocytoma / paraganglioma (chronic catecholamine excess)	↑ NE/EPI, systemic	Sustained catecholamine-driven thermogenesis and lipolysis	Low BMI and reduced adiposity relative to matched hypertensive controls	(242); (243)
Post-adrenalectomy withdrawal of catecholamine excess (pheochromocytoma)	↓ NE/EPI toward normal	Loss of catecholamine-mediated energy expenditure	Postoperative weight gain and increased adiposity	(244)
Pheochromocytoma-associated glucose intolerance	↑ NE/EPI, systemic	Catecholamine-induced hepatic glucose output and insulin resistance	Apparent type 2 diabetes resolving after tumor resection	(245); (246)
Acute stress cardiomyopathy (Takotsubo syndrome)	Abrupt ↑ circulating catecholamines	Direct catecholamine-mediated myocardial stunning	Transient LV systolic dysfunction with apical ballooning	(250)
Chronic heart failure with sympathetic overactivity	↑ cardiac NE spillover	Catecholamine toxicity to cardiomyocytes	Progressive LV systolic dysfunction and adverse remodeling	(251); (252)
Autonomic failure (postganglionic noradrenergic loss)	↓ NE synthesis and release	Failure of reflex vasoconstriction during orthostasis	Neurogenic orthostatic hypotension	(465)
Renal intrarenal DOPA→dopamine autocrine/paracrine system (physiology)	↑ intrarenal DA synthesis from filtered L-DOPA	Local inhibition of proximal tubular Na+ transport	Salt-induced natriuresis	(260); (133); (261)
Renal intrarenal dopamine deficiency	↓ intrarenal DA synthesis	Loss of intrarenal natriuretic brake	Salt-sensitive hypertension	(264)
Catecholamine-driven stress leukogram / NLR shift	↑ NE/EPI, systemic	Neutrophil demargination and lymphocyte redistribution	Increased neutrophil-to-lymphocyte ratio	(167); (168)
Vasovagal (neurally mediated) syncope	Epinephrine surge with sympathetic noradrenergic withdrawal	Sympathoadrenal imbalance causing vasodilation and hypotension	Fainting with cerebral hypoperfusion	(265)

VI.1. Biosynthesis

Catecholamine metabolomic patterns provide sensitive biochemical indicators of TH and LAAAD activity. Altered activity of either enzyme produces distinct, measurable shifts in plasma, cerebrospinal fluid, or tissue catecholamine and metabolite levels. Reduced TH activity, whether genetic, pharmacologic, or due to co-factor (BH₄ or Fe⁺⁺) deficiency, leads to accumulation of tyrosine, with low levels of DOPA, dopamine, and downstream metabolites (DOPAC, HVA, NE, and MHPG), as seen in autosomal recessive TH deficiency and dopa-responsive dystonia (271, 272).

Conversely, increased TH activity, as in acute distress-associated sympathoadrenal activation, raises DOPA and DOPAC relative to dopamine and norepinephrine, reflecting enhanced cytosolic turnover and oxidative metabolism (1). LAAAD inhibition—pharmacologic (e.g., carbidopa) or genetic—causes accumulation of L-DOPA and depletion of dopamine, DOPAC, and HVA (234). Thus, the ratios DOPA/tyrosine, DA/DOPA, and DOPAC/DA provide metabolomic fingerprints of TH and LAAAD function, offering quantitative insight into catecholamine synthetic flux and neuronal integrity in both experimental and clinical contexts.

VI.2. Catecholamine turnover

Catecholamine turnover represents the continuous loss and replenishment by synthesis. Although catecholamine turnover has often been considered to be driven by catecholamine release with escape of neuronal reuptake, the main determinant of norepinephrine turnover is leakage from vesicular stores, originally proposed by Kopin in 1964 (273). In the human heart about 12% of norepinephrine turnover under resting conditions is from extra-neuronal uptake and metabolism or loss of the transmitter to the circulation, 15% from intra-neuronal metabolism after reuptake, and 73% from intra-neuronal metabolism of norepinephrine leaking from storage vesicles (274).

The large contribution of leakage to catecholamine turnover may seem inconsistent with cellular economy. In fact, this contribution provides an important mechanism for “gearing down” the requirement for increases in catecholamine synthesis to match increases in catecholamine release and thereby provides sympathetic nerves with a capacity for a more extended range of sustainable release rates in response to sustained stress than would otherwise be possible (275).

VI.3. Vesicular storage and dopamine-beta-hydroxylase (DBH)

Vesicular uptake is an energy requiring process that depends on a proton pump. Vesicular monoamine transporters (VMAT1 and VMAT2) transport cytoplasmic monoamines into the vesicles. Vesicular contents leak passively back into the cytoplasm.

Dopamine-beta-hydroxylase (DBH), which catalyzes the conversion of dopamine to norepinephrine, is localized to the vesicles. The enzyme contains, and its activity requires, copper. Decreased vesicular uptake or decreased DBH activity therefore decreases levels of norepinephrine and epinephrine and their metabolites with respect to those of dopamine and its metabolites.

VI.4. Intracellular metabolism

VI.4.1. Monoamine oxidase (MAO)

Monoamine oxidase (MAO) catalyzes the conversion of cytoplasmic dopamine to 3,4-dihydroxyphenylacetaldehyde (DOPAL) and cytoplasmic norepinephrine to 3,4-dihydroxyphenylglycolaldehyde (DOPEGAL).

Differential susceptibilities to monoamine oxidase

It has recently been recognized that dopamine is a better substrate than norepinephrine for both isoforms of monoamine oxidase (MAO), MAO-A and MAO-B (27). Both catecholamines are better substrates for MAO-A than MAO-B.

VI.4.2. Aldehyde dehydrogenase (ALDH) and aldehyde reductase

Differential susceptibilities to aldehyde dehydrogenase & aldehyde/aldose reductase

The product of MAO acting on dopamine is 3,4-dihydroxyphenylacetaldehyde (DOPAL), and the product of MAO acting on norepinephrine and epinephrine is 3,4-dihydroxyphenylglycolaldehyde (DOPEGAL). DOPAL is metabolized mainly by aldehyde dehydrogenase (ALDH) to form the acid 3,4-dihydroxyphenylacetic acid (DOPAC). DOPEGAL is metabolized mainly by aldehyde/aldose reductase (AR) to form the glycol 3,4-dihydroxyphenylglycol (DHPG). This difference explains why the acid, homovanillic acid (HVA), is the main end-product of dopamine metabolism, while the glycol, 3-methoxy-4-hydroxyphenylglycol (MHPG), is the main end-product of norepinephrine (and epinephrine) metabolism. In the liver, MHPG is converted to vanillylmandelic acid (VMA) via alcohol dehydrogenase and then aldehyde dehydrogenase (276).

VI.5. Exocytosis

In humans, the intra-neuronal norepinephrine metabolite DHPG tracks vesicular cycling and reuptake, so when exocytotic release increases, plasma DHPG rises via neuronal uptake of released norepinephrine. Conversely, suppression of release lowers DHPG, demonstrating tight coupling to norepinephrine turnover (277). Plasma DHPG increases during sympathetic stimulation but less steeply than norepinephrine, consistent with plasma DHPG being derived importantly from ongoing leakage of norepinephrine from vesicular stores independently of sympathetically-mediated exocytosis (277–279). Plasma MHPG (an extra-neuronal/whole-body norepinephrine metabolite) also increases (280). DOPAC and HVA primarily are related to dopamine turnover and are less direct readouts of sympathetic noradrenergic activation than DHPG and MHPG (277, 280).

VI.6. Reuptake

Neuronal uptake of dopamine from the extracellular fluid is mediated by the cell membrane dopamine transporter (DAT), and neuronal uptake of norepinephrine is mediated by the cell membrane NE transporter (NET).

Norepinephrine is a better substrate for neuronal uptake (Uptake-1) via the NET than is epinephrine (281), and dopamine is a better substrate than norepinephrine for uptake via the NET (282). The latter helps explain why ¹⁸F-dopamine is an excellent imaging agent for visualizing cardiac sympathetic innervation by positron emission tomography (283).

VI.7. Extracellular uptake and metabolism

Catechol-O-methyltransferase (COMT) converts DOPA to methoxytyrosine (3-MT), dopamine to methoxytyramine (3-MTy), norepinephrine to normetanephrine (NMN), and epinephrine to metanephrine (MN) (32, 284).

Brain catecholaminergic nuclei express relatively little COMT (51, 285–290), whereas most non-neuronal cells do (291–295).

Whether sympathetic nerves express COMT has not been tested directly. Indirect evidence suggests low intra-neuronal COMT activity in cardiac sympathetic nerves, based on far greater entry of DHPG than of MHPG into the venous drainage of the heart (274); however, surgical denervation of the aorta and destruction of sympathetic nerves by 6-hydroxydopamine decrease COMT activity (296).

Different fates of circulating vs. neuronal catecholamines

The organic cation transporter 3 OCT3 (SLC22A3) and plasma membrane monoamine transporter (PMAT, SLC29A4) mediate non-neuronal uptake, which is linked to cytoplasmic degradative enzymes. Neuronal catecholamines are degraded extensively by reuptake via the Uptake-1 process via the NET, followed by intra-neuronal metabolism by monoamine oxidase (MAO), whereas circulating catecholamines are removed extensively by extra-neuronal uptake via the Uptake-2 process (297, 298).

VI.8. Physiological implications of observed patterns

The observation of particular patterns of levels of catecholamines and their metabolites has implications for potential physiological correlates (Table 3). For instance, high levels of dopamine and its metabolites with respect to norepinephrine and its metabolites may indicate low DBH activity, while elevations of DOPA and its metabolites compared to the catecholamines and their metabolites may indicate low LAAAD activity.

Most publications relating catecholamine metabolic patterns to alterations in intracellular processes are based on drug effects (Table 4). One should interpret these relationships with caution, especially for drugs such as MAO and NET inhibitors that readily penetrate the blood-brain barrier and may indirectly affect autonomic outflows.

Table 4:

Catecholamine-related biochemical patterns associated with decreased activities of intracellular processes

Process (decreased activity)	Increased biochemicals (full names)	Increased biochemicals (abbreviations)	Decreased biochemicals (full names)	Decreased biochemicals (abbreviations)
LAAAD	3,4-Dihydroxyphenylalanine; 3-Methoxytyrosine; 5-S-CysteinylDOPA	DOPA; 3-MTyr; Cys-DOPA	Dopamine; Norepinephrine; Epinephrine; Metanephrine	DA; NE; EPI; MN
COMT	5-S-CysteinylDOPA; Dopamine; Norepinephrine; Epinephrine	Cys-DOPA; DA; NE; EPI	3-Methoxytyrosine; 3-Methoxytyramine; 3-Methoxytyramine-sulfate	3-MTyr; 3-MT; 3-MT-S
MAO	Dopamine; 5-S-Cysteinyldopamine	DA; Cys-DA	3,4-dihydroxyphenylacetaldehyde; 3,4-Dihydroxyphenylacetic acid; 3,4-Dihydroxyphenylethanol; Homovanillic acid; 3-Methoxy-4-hydroxyphenylglycol; Vanillylmandelic acid	DOPAL; DOPAC; DOPET; HVA; MHPG; VMA
ALDH	3,4-dihydroxyphenylacetaldehyde	DOPAL	3,4-Dihydroxyphenylacetic acid	DOPAC
AR	Vanillylmandelic acid	VMA	3-Methoxy-4-hydroxyphenylglycol	MHPG
DBH	Dopamine	DA	Norepinephrine; Epinephrine; 3-Methoxy-4-hydroxyphenylglycol; MHPG-sulfate; Vanillylmandelic acid	NE; EPI; MHPG; MHPG-S; VMA
Vesicular uptake	Dopamine; 3,4-Dihydroxyphenylacetic acid	DA; DOPAC	Norepinephrine; 3,4-Dihydroxyphenylglycol	NE; DHPG

VII. CATECHOLAMINE METABOLOMIC PATTERNS AS BIOMARKERS OF PHYSIOLOGICAL STATES

Catecholamine systems participate not only in “fight-or-flight” emergencies to maintain homeostasis, as promulgated in this journal by Walter B. Cannon almost a century ago (299), but also in activities of daily living such as orthostasis, exercise, and meal ingestion. This section describes catecholamine metabolic patterns associated with physiological states and compares homeostasis with the relatively new concept of allostasis as integrative physiological ideas (230). The relevant literature is tabulated in Table 6.

Table 6:

Effects of some physiological changes on levels of DOPA, catecholamines, and catecholamine metabolites

Physiol. Change	DOPA	DA	NE	EPI	DOPAC	DHPG	DA Sulfate	NMN	MN	HVA	MHPG	NE Sulfate	References
Orthostasis
		No change	Increase ++				No change					No change	(483)
			Increase ++			Increase +							(276)
	No change	Increase +	Increase ++	Increase +	No change	Increase +							(484)
		Increase +	Increase ++										(306)
Exercise
		Increase ++	Increase ++	Increase +++									(485)
											Increase +		(279)
			Increase +++	Increase ++++								Increase +++	(486)
			Increase ++	Increase ++			No change					Increase +	(310)
			Increase ++	Increase +++								Increase +++	(487)
			Increase +++	Increase ++++								Increase +++	(488)
				Increase +++									(489)
			Increase ++	Increase +++									(277)
			Increase +++	Increase ++++								Increase +++	(490)
	Increase ++		Increase +++	Increase ++++									(491)
		Increase ++	Increase ++	Increase +++									(492)
		Increase ++	Increase ++	Increase ++			Decrease −					Decrease −	(309)
			Increase +++	Increase +++				Increase ++					(493)
			Increase +	Increase ++						Increase ++	Increase ++		(494)
			Increase +++	Increase +++				Increase ++	Increase ++				(495)
Meal
	Increase ++	Increase +					Increase +++						(352)
			Increase ++										(496)
					Increase ++		Increase ++						(497)
	Increase +	Increase ++		Increase +	Increase ++		Increase ++++						(45)
			Increase +	Decrease −									(318)
	Increase +		Increase +	Decrease −			Increase ++						(323)
			Increase +	Increase ++									(317)
												Increase ++	(498)
					Increase ++++	Increase +							(41)
Glucoprivation
			Increase ++	Increase ++++									(499)
			Increase +	Increase +++									(500)
			Increase +	Increase +++									(501)
			Increase ++	Increase ++++									(502)
			Increase ++	Increase +++									(503)
			Increase ++	Increase +++									(504)
			Increase ++	Increase ++++									(505)
			Increase +	Increase +++									(506)
	No change		Increase ++	Increase ++++	No change	Decrease −							(319)
			Increase +	Increase ++++		Decrease −							(320)
			Increase ++	Increase +++									(507)
Altered Environmental Temp.
													(328)
													(331)
													(508)
			Decrease −	No change									(326)

Sustained catecholamine release must be matched by increased synthesis if releasable stores are to be maintained. In sympathetic nerves, Weiner and colleagues showed that nerve stimulation increases norepinephrine synthesis and that cocaine and catecholamine-releasing drugs modify this relationship, supporting functional coupling among release, reuptake, synthesis, and transmitter-store status (338). Analogous stimulus–secretion–synthesis coupling is well established in the adrenal medulla, where stress-level splanchnic firing recruits PACAP-dependent peptidergic signaling to support both catecholamine secretion and catecholamine-biosynthetic enzyme expression (156, 160). In brain catecholaminergic systems, prolonged biosynthetic load also occurs during psychostimulant exposure: d-amphetamine acutely increases striatal catecholamine synthesis, whereas chronic cocaine alters tyrosine hydroxylase expression and activity in dopaminergic cell groups (339–341). These observations support the concept that catecholaminergic neurons and chromaffin cells use activity- and drug-responsive biosynthetic adaptations to limit depletion during prolonged transmitter mobilization.

VII.1. Orthostasis

The challenges posed by prolonged upright posture are comparatively new developments in human evolution (130), since the conversion to bipedalism occurred relatively recently—Homo erectus appeared only about 2 million years ago (342). One may speculate that because of this relatively short timespan, only one physiological system has evolved to adjust to orthostatic stress—the SNS (149).

The integrated response to orthostasis reflects interoception mediated by “low-pressure” baroreceptors especially in the walls of the atria and pulmonary veins and “high-pressure” baroreceptors in the walls of arteries such as in the carotid sinuses. Although there are some differences in terms of responses of different neuroendocrine systems (343), in general orthostasis releases the SNS from baroreceptor restraint. Plasma norepinephrine normally approximately doubles within 5’ of standing up from the supine position (344). Plasma DHPG increases concurrently by a smaller fraction (277). There are also small increases in plasma dopamine, probably reflecting exocytotic co-release with norepinephrine from sympathetic nerves (302). In contrast, plasma levels of DOPA and dopamine metabolites change by little during orthostasis (Table 5).

Table 5:

Effects of some drugs on levels of DOPA, catecholamines, and catecholamine metabolites

Drug	DA	DOPAC	HVA	DA Sulfate	NE	EPI	DHPG	NMN	MN	MHPG	NE Sulfate
AMPT (metyrosine)
(340)
(466)				Decrease −	Decrease −
(467)					Decrease −	Decrease −
Clonidine (alpha-2 agonist)					Decrease − − −		Decrease − −
(468)					Decrease − − −		Decrease − −
(469)					Decrease − − −
(276)					Decrease − − −		Decrease − −
COMT inhibitors
(347)		Increase ++	Decrease −				Increase +++			Decrease −
(346)					Increase ++		Increase ++
(470)	Increase ++				Increase ++	Increase ++
Droxidopa (L-DOPS,oral/infusion)
(471)					Increase ++
(345)					Increase +		Increase ++
(346)					Increase +		Increase +
Levodopa
(352)	Increase +			Increase ++++
(472)	Increase +			Increase ++++
Levodopa+Carbidopa
(352)	Increase +			Increase ++
MAO inhibitors
(357)			Decrease −− −		Increase +	Increase +	Decrease − − −			Decrease − − −
(356)		Decrease − −	Decrease − − −		No change		Decrease − −
(473)	Decrease − −					Decrease − −
(474)							Decrease − − −			Decrease − − −
Neuronal reuptake inhibitors
(475)					Increase ++	Increase +++
(476)						Increase ++
(276)					Increase +		Decrease −
(477)					Increase +		Decrease −
(478)					Increase +++
(479)					Increase ++
Reserpine (VMAT inhibitor)
(360)					Decrease −	No change
(278)					Decrease − −	Increase ++	Decrease − −
Trimethaphan (ganglionic blocker)
(479)					Decrease − −
Tyramine (IV)
(480)					Increase +	No change
(361)					Increase +		Increase ++
Yohimbine (alpha-2 antagonist)
(481)					Increase +++	Increase +
(276)					Increase +++	Increase +	Increase +
(482)					Increase +++	Increase +	Increase +

VII.2. Exercise

During dynamic exercise in humans, plasma catecholamines increase in proportion to workload and intensity. Plasma norepinephrine and epinephrine rise progressively and show larger increases above the individual anaerobic (lactate) threshold compared with below threshold workloads, indicating enhanced activation of both the SNS and SAS as metabolic demand increases (345, 346) (Table 5). Plasma norepinephrine typically increases earlier and to a greater extent than epinephrine at moderate intensities, with epinephrine showing sharp rises as intensity passes the anaerobic threshold, consistent with SAS recruitment.

The literature on effects of exercise on catecholamine metabolites is relatively sparse. Plasma DOPA, O-methylated metabolites, and NE-sulfate increase (Table 5). Plasma DHPG and MHPG increase to a proportionately smaller extent than norepinephrine (278, 280). Plasma dopamine increases, with smaller or no increases in DA-sulfate (305, 312). Figure 8 compares the catecholamine metabolomic pattern during cycling exercise with that during supine rest.

Figure 8: Catecholamine metabolic pattern in cardiac sympathetic nerves with exercise.

Abbreviations: COMT=catechol-O-methyltransferase; Cys-DOPA=5-S-cysteinylDOPA; DA=dopamine; DOPA=3,4-dihydroxyphenylalanine; DOPAC=3,4-dihydroxyphenylacetic acid; MAO=monoamine oxidase; MHPG=3-methoxy-4-hydroxyphenylglycol; NE=norepinephrine; NMN=normetanephrine; TYR=tyrosine; U1=Uptake-1; U2=Uptake-2. Adapted from (11, 274)

VII.3. Meal ingestion

According to a widely held and still persistent concept, the parasympathetic nervous system is the “rest-and-digest” component of the autonomic nervous system, whereas the opposing sympathetic nervous system is the “fight-or-flight” emergency system (347, 348). Perhaps ironically, differential responses of plasma catecholamine levels to meal ingestion belie this notion (Table 5). The sympathetic nervous system can be viewed as containing three sub-systems based on the chemical messenger—the sympathetic noradrenergic system (SNS, with norepinephrine the neurotransmitter involved with adjustments in the distribution of the cardiac output), the sympathetic adrenergic system (SAS, with epinephrine the hormone participating in glucose homeostasis), and the sympathetic cholinergic system (SCS, with acetylcholine the neurotransmitter involved with thermoregulatory sweating) (349). After meal ingestion there is a dissociation between plasma norepinephrine, which increases (350), especially after a high-carbohydrate meal (351, 352), and plasma epinephrine, which decreases (319, 321). The rest-and-digest vs. fight-or-flight dichotomy does not account for this differential catecholaminergic response.

After prolonged (72-hour) fasting, meal ingestion results in large increases in plasma levels of DA-sulfate (46), with only modest changes in free dopamine or DOPA, indicating a dominant role for local extensive sulfoconjugation rather than increased SNS outflow. After prolonged fasting there is still a substantial amount of circulating DA-sulfate, consistent with ongoing endogenous production in the DOPA-dopamine autocrine-paracrine system (45).

VII.4. Glucoprivation

Numerous studies have assessed effects of acute glucoprivic stress evoked by insulin or 2-deoxyglucose on plasma levels of catecholamines. Glucoprivation markedly increases SAS outflow, and this dominates the catecholamine response. There are smaller and regionally heterogeneous increases in SNS outflows (331, 332). The small increases in plasma NE in response to insulin-induced hypoglycemia reflect adrenomedullary output (353, 354).

Prolonged fasting alters activities of a variety of effector systems. Glucose and insulin levels decline, while arterial glucagon increases, and beta-hydroxybutyrate, acetoacetate, and palmitate levels increase markedly, associated with small increases in plasma NE and EPI (320). Norepinephrine spillover in adipose tissue increases, suggesting a contribution of locally increased SNS outflow to the lipolytic response.

VII.5. Thermoregulation

It has long been known that in humans thermoregulation during exposure to high environmental temperature depends on evaporative heat loss (355, 356), which reflects two processes— sweating from activation of the sympathetic cholinergic system (SCS) and cutaneous vasodilation from inhibition of SNS outflow (149, 337, 349, 357). Conversely, cold exposure increases SNS outflows, decreasing cutaneous blood flow. There is relatively little change in SAS outflow unless there is an actual decline in core temperature (334, 335, 358, 359). Plasma EPI changes little during exposure to dry heat but increases during exposure to humid heat (337), a situation where evaporative heat loss is impaired.

Skin surface cooling improves orthostatic tolerance, probably via increased SNS outflows. Plasma NE levels increase (360), as does central venous pressure (361).

VII.6. Homeostasis vs. allostasis

Until relatively recently, effects of physiological changes were viewed as following the stimulus. According to the concept of allostasis (6, 7, 362), physiological changes can occur in anticipation of the stimulus, based on instinct, imprinting, or conditioning (9, 363).

For all five physiological changes described above there can be anticipatory adjustments in advance of the stimulus. For instance, it is common human experience that people eat more from habit than from sensed metabolic need, and they don winter coats before venturing out into the cold. In patients with postural tachycardia syndrome, heart rate can increase as part of a conditioned fear response before head-up tilt table testing (364).

Evidence that “central command” (anticipation/volitional drive at the start of exercise) can elevate plasma catecholamines before or very soon after exercise onset comes from original human studies. In resistance exercise with deliberate pre-effort arousal, plasma NE and EPI increased before the first work set (anticipatory rise), and then increased further during sets (365). Rapid, early increases in plasma NE with the start of dynamic exercise are also well documented, consistent with an immediate feed-forward component (366). For metabolites, simultaneous sampling during graded cycling showed that plasma DHPG rises with sympathetic activation during exercise—though less steeply than NE—supporting very early increased intra-neuronal turnover when exercise begins (278), in advance of metabolic need.

VIII. CATECHOLAMINE METABOLOMIC PATTERNS AS BIOMARKERS OF DRUG EFFECTS

Numerous drugs directly or indirectly affect activities of catecholamine-related processes. Some of the more prominent drugs and their associated biochemical patterns are listed in Table 5 and described below.

VIII.1. Alpha-methyl-para-tyrosine (AMPT)

Alpha-methyl-para-tyrosine (AMPT, Demser) competes with tyrosine for TH. AMPT administration produces a particular pattern of plasma levels of catechols. Plasma DOPA levels decrease (392), as one would expect from TH inhibition. Despite daily repeated AMPT administration, however, plasma DOPA partly recovers (367). Plasma DOPAC and DHPG are persistently severely decreased, while NE is decreased slightly.

VIII.2. Alpha-methyldopa

Alpha-methyldopa (Aldomet) is a centrally acting antihypertensive prodrug that is metabolized in the brain to α-methylnorepinephrine, which activates central α2-adrenergic receptors to decrease sympathetic outflow and lower arterial pressure (393). Biochemically, alpha-methyldopa decreases plasma NE concentrations in parallel with reductions in blood pressure, reflecting diminished noradrenergic neurotransmission; similar patterns of reduced catecholamines have been observed with alpha-2 adrenoceptor agonists (394).

VIII.3. Carbidopa and benserazide

Carbidopa is a catechol that inhibits LAAAD outside the brain. Combining levodopa with carbidopa was revolutionary in the treatment of PD (395), because the limiting side effect of levodopa is nausea and vomiting due to increased dopamine production, and carbidopa attenuates (but does not block) levodopa-induced dopamine production outside the brain. Because existing human data focus almost exclusively on carbidopa given with levodopa, there is little published information documenting effects of carbidopa alone on plasma catecholamines and their metabolites. Carbidopa decreases urinary NE excretion (396). Benserazide is also a peripherally acting LAAAD inhibitor that in some formulations is co-administered with levodopa to reduce peripheral conversion of levodopa to dopamine.

VIII.4. Droxidopa

In contrast with levodopa, which exerts effects mainly on dopamine and dopamine metabolites with little effect on NE and NE metabolites, droxidopa (L-DOPS, 3,4-L-threo-dihydroxyphenylserine, Northera) affects levels of NE and its metabolites with little effect on dopamine and dopamine metabolites. After oral dosing, plasma NE rises modestly while deaminated NE metabolites increase robustly. As one would expect, carbidopa blunts both the increases in NE and DHPG/DHMA, while COMT inhibition with entacapone augments the metabolite responses (373, 376).

VIII.5. COMT inhibition

COMT inhibitors such as entacapone decrease levels of end-products of catecholamine metabolism while building up levels of the deaminated metabolites (372, 373). COMT inhibition is clinically important in Parkinson disease, where peripheral COMT inhibitors reduce peripheral O-methylation of levodopa, thereby increasing the bioavailability of levodopa and prolonging its symptomatic effect. In addition to entacapone and tolcapone, the newer once-daily COMT inhibitor opicapone is now in clinical use as adjunctive therapy for levodopa-treated patients with end-of-dose motor fluctuations. Opicapone reduces daily OFF time and increases ON time without troublesome dyskinesia, supporting continued therapeutic relevance of COMT as a target in catecholamine pharmacology (397).

VIII.6. Levodopa

3,4-Dihydroxyphenylalanine (levodopa, DOPA) is the immediate product of the rate-limiting step in catecholamine biosynthesis—hydroxylation of tyrosine by tyrosine hydroxylase (TH). Clinically, levodopa/carbidopa remains the gold-standard symptomatic therapy for Parkinson disease (PD). In humans, plasma DOPA reflects multiple sources: dietary contributions and intestinal handling, plus production in sympathetic nerves (320, 367). Levodopa administration markedly increases plasma levels of DOPA, Cys-DOPA, dopamine, and dopamine metabolites. Although carbidopa attenuates these effects, the amount of substrate for LAAAD is so large that levodopa/carbidopa increases levels of dopamine and its metabolites (398, 399).

Levodopa produces little or no acute change in plasma levels of NE and only modest or negligible effects on major norepinephrine metabolites (316, 400–402).

The main circulating metabolites of levodopa are 3-O-methyldopa (3-OMD) and its sulfate conjugate, 3-O-methyldopa sulfate (3-OMD-S) (403). Concentrations of 3-OMD average many times the concentration of DOPA.

VIII.7. MAO inhibition

Effects of MAO inhibitors on levels of catecholamines and their metabolites are predictable from the central position of MAO in the fate of cytoplasmic dopamine and NE (379). MAO-A inhibition markedly decreases levels of DHPG, MHPG, and HVA, with smaller increases in NE and EPI and no change in DOPA (378). Treatment with MAO-B inhibitors such as selegiline and rasagiline can result in a catecholamine metabolic profile indicating MAO-A inhibition (404, 405).

VIII.8. VMAT inhibition

Reserpine is a classic inhibitor of vesicular uptake of cytoplasmic monoamines. Since NE synthesis takes place in vesicles, reserpine depletes NE stores (387). Reserpine administration transiently increases plasma DHPG levels, due to ongoing passive leakage of NE stores, followed by low NE, DHPG, and MHPG levels when the stores become depleted (279). Plasma EPI levels, however, are increased.

VIII.9. Tyramine

Infusion of the indirectly acting sympathomimetic amine tyramine displaces NE from vesicular stores, increasing plasma DHPG more than plasma NE (389). Because neuronal uptake of tyramine from the extracellular fluid depends on the cell membrane norepinephrine transporter (NET), tyramine infusion testing was used to identify “neuropathic” POTS (406) and POTS related to hypofunctional variation of the SLC6A2 gene that encodes the NET (407). Tyramine infusion also has been used to assess functional effects of cardiac sympathetic denervation (389). It should be noted that some of the research was compromised, in that tyramine in aqueous solution that is stored at temperatures above −70° degrees centigrade undergoes spontaneous oxidation to form dopamine (408, 409).

VIII.10. ALDH inhibition

A variety of drugs inhibit aldehyde dehydrogenase (ALDH), such as disulfiram, (Antabuse), daidzein, and benomyl. The metabolic toxin rotenone inhibits ALDH indirectly, by blocking mitochondrial complex 1. This decreases generation of NAD+, which is a required co-factor for ALDH (53).

VIII.11. Neuronal reuptake inhibition

Inhibition of neuronal uptake prevents the formation of DHPG from exogenous NE and halves the formation of MHPG, indicating that DHPG is derived from NE metabolized intra-neuronally (279). All inhibitors of neuronal reuptake of NE, whether cocaine, tricyclic antidepressants, amphetamines, or “oxetines,” increase NE with respect to DHPG.

VIII.12. Adrenoceptors

Yohimbine is a selective α₂-adrenoceptor antagonist that acts both centrally and peripherally to augment release of NE from sympathetic nerves (410, 411). The imidazoline clonidine exerts opposite effects by stimulating α₂-adrenoceptors in the brain and periphery, inhibiting sympathetic outflows and attenuating NE release for a given amount of nerve traffic (94).

IX. CATECHOLAMINE METABOLOMIC PATTERNS AS BIOMARKERS OF PATHOPHYSIOLOGICAL STATES

Myriad pathophysiological states are associated with alterations in levels of catecholamines and their metabolites. Some of these are listed in Table 7. This section highlights conditions that feature catecholamine metabolomic patterns indicating specific abnormalities in the synthesis, storage, reuptake, or metabolism of catecholamines.

Table 7:

Effects of some pathophysiological changes on levels of DOPA, catecholamines, and catecholamine metabolites

Pathophysiological state	Characteristic catecholamine/metabolite pattern	Key analytes / ratios	References
Monoamine oxidase (MAO)-A deficiency	Markedly ↓ deaminated monoamine metabolites, especially DHPG, preserved NE & EPI, diversion to O-methylation	DHPG ↓↓↓; NMN/DHPG ↑↑; plasma NE ~normal.	(241)
Dopamine-β-hydroxylase (DBH) deficiency	Markedly ↑ DA with very low/absent NE and EPI; ↑ DOPAC with very low DHPG and MHPG; high DA/NE and DOPAC/DHPG ratios	DA, NE, EPI, DOPAC, DHPG; DA/NE, DOPAC/DHPG	(373); (509)
HSAN III (familial dysautonomia)	Low basal plasma NE; blunted NE increase to orthostasis; ↑ DOPAC/DHPG ratio; ↑ EPI during “crises”	Plasma NE (supine/upright); EPI; NE response to tilt	(510); (377); (375); (511)
Menkes disease	↑ DA/NE; ↑ DOPAC/DHPG; ↑ HVA/MHPG; ↑ DOPA	DA, NE, DOPA, DOPAC, DHPG; DA/NE	(376); (231)
PPGL	↑ plasma free normetanephrine/metanephrine; ↑ methoxytyramine (DA tumors)	NMN, MN, 3-MT	(512); (513)
Neuroblastoma	↑ urinary HVA and VMA	HVA, VMA	(514); (515)
Postural tachycardia syndrome (POTS)	↑ cardiac NE spillover; ↑ standing plasma NE in “hyperadrenergic POTS”; normal arterial NE/DHPG); ↑ arterial EPI	Cardiac NE spillover (supine); plasma NE (supine and standing); arterial plasma NE, DHPG, and EPI	(402); (362); (403)
Malignant melanoma	↑ plasma/urine 5-S-cysteinyldopa	Cys-DOPA	(387); (389)
Congestive heart failure	↓ myocardial NE despite ↑ spillover	NE, DHPG	(516); (391); (11)
Neurocardiogenic syncope	Disproportionate ↑ EPI relative to NE (sympathoadrenal imbalance)	EPI/NE	(265); (517)
Takotsubo syndrome	Acute ↑ plasma catecholamines; ↑ cardiac NE spillover	EPI, NE, coronary sinus NE	(250); (518); (519)
Obesity	↑ regional NE spillover; altered clearance; ↓ fasting EPI in some	NE spillover, EPI	(520); (521)
Type 2 diabetes mellitus	Early: ↑ NE activity; Late: ↓ NE/DHPG	NE, DHPG	(522); (523)

Abbreviations and interpretive notes: DA, dopamine; NE, norepinephrine; EPI, epinephrine; DOPA, 3,4-dihydroxyphenylalanine; DOPAC, 3,4-dihydroxyphenylacetic acid; HVA, homovanillic acid; DHPG, 3,4-dihydroxyphenylglycol; MHPG, 3-methoxy-4-hydroxyphenylglycol; NMN, normetanephrine; MN, metanephrine; 3-MT, 3-methoxytyramine; Cys-DOPA, 5-S-cysteinyldopa; DBH, dopamine-β-hydroxylase; SNS, sympathetic noradrenergic system; SAS, sympathetic adrenergic (adrenomedullary) system.

IX.1. Genetic disorders of catecholamine biosynthesis

IX.1.1. Decreased tyrosine hydroxylase (TH) activity

Decreased TH activity can reflect genotypic variants of the enzyme itself or decreased provision of its required co-factor tetrahydrobiopterin (BH₄, Figure 9). Since BH₄ is required also for activity of phenylalanine hydroxylase, tryptophan hydroxylase, and nitric oxide synthase, the biochemical phenotypes for hypofunctional variants of genes required for BH₄ synthesis or recycling differ from those for decreased TH activity alone. BH₄ synthesis/recycling defects manifest with signs of combined monoamine neurotransmitter deficiency (dopamine+norepinephrine+serotonin) and hyperphenylalaninemia (436, 437). The catecholamine metabolic pattern distal to the TH step is the same, with low levels of the catecholamines, their metabolites, and DOPA and its metabolites (438).

Figure 9: Tetrahydrobiopterin (BH₄) synthesis and recycling.

(Left) Tetrahydrobiopterin (BH₄) is synthesized from guanosine triphosphate (GTP) via a three-step pathway involving GTP cyclohydrolase I (GCH1), 6-pyruvoyltetrahydropterin synthase (PTPS), and sepiapterin reductase (SPR). BH₄ is an obligate co-factor for phenylalanine hydroxylase and tyrosine hydroxylase (TH). The latter catalyzes the conversion of tyrosine to 3,4-dihydroxyphenylalanine (DOPA). (Right) During the hydroxylation reaction, BH₄ is oxidized to dihydrobiopterin, which is rapidly reduced back to BH₄ by dihydropteridine reductase (DHPR) using NADH as an electron donor, thereby maintaining intracellular BH₄ availability independently of de novo synthesis. Other abbreviations: GTP=guanosine triphosphate; NADH=reduced nicotinamide adenine dinucleotide; PCD=pterin-4a-carbinolamine dehydratase.

IX.1.2. Decreased L-aromatic-amino-acid decarboxylase (LAAAD) activity

The catecholamine metabolomic phenotype in LAAAD deficiency is distinctive and different from TH deficiency in that concentrations of DOPA and its metabolites (especially 3-O-methyldopa) are built up compared to catecholamines and their metabolites (235, 439). Since LAAAD is required for conversion of 5-hydroxytryptophan to serotonin, levels of 5-hydroxyindoleacetic acid, the main end-product of serotonin metabolism, are low in patients with LAAAD deficiency.

IX.1.3. Decreased norepinephrine biosynthesis

Unlike dopamine, norepinephrine is synthesized within vesicles (or in chromaffin cells in storage granules). The required enzyme is dopamine-beta-hydroxylase (DBH). In addition to DBH deficiency itself, which is very rare (412, 440, 441), several genetic disorders entail the same general pattern of signs and symptoms of norepinephrine deficiency, such as neurogenic orthostatic hypotension. Some of these genetic disorders are familial dysautonomia (hereditary sensory and autonomic neuropathy Type III), deficiency of the type 2 vesicular monoamine transporter encoded by the SLC18A2 gene, mutation of CYB561 (cytochrome b561), which is required for appropriate supply of ascorbate (55, 233), and Menkes disease, discussed in more detail below. These conditions are associated with decreased levels of norepinephrine metabolites such as DHPG compared to dopamine metabolites such as DOPAC (10, 232, 416).

Menkes disease

Menkes disease is an X-linked recessive pediatric disorder caused by defective copper transport due to variants of the copper ATPase gene ATP7A. Because DBH contains, and its activity absolutely requires, copper, Menkes disease patients have low DBH activity. From the concept diagram in Figure 6, decreased DBH activity results in high ratios of DA/NE and their respective metabolites DOPAC/DHPG and HVA/MHPG. Menkes disease patients also have increased DOPA levels, consistent with compensatorily increased TH activity. Given that Menkes disease is transmitted as an X-linked recessive trait, among at-risk newborn males one-half would have the disease and one-half would neither have the disease nor be a carrier.

Figure 6: Relationships of extracellular fluid levels of catecholamines and metabolites to reactions within sympathetic nerves.

Numbers in rectangles identify the compounds listed in Table 1. Abbreviations: ALDH=aldehyde dehydrogenase; AR=aldehyde reductase; COMT=catechol-O-methyltransferase; Cys-DOPA=5-S-cysteinylDOPA; DA=dopamine; DBH=dopamine-beta-hydroxylase; DHPG=3,4-dihydroxyphenylglycol; DOPA=3,4-dihydroxyphenylalanine; DOPAC=3,4-dihydroxyphenylacetic acid; DOPAL=3,4-dihydroxyphenylacetaldehyde; DOPEGAL=3,4-dihydroxyphenylglycolaldehyde; EPI=epinephrine; HVA=homovanillic acid; LAAAD=L-aromatic-amino-acid decarboxylase; MAO=monoamine oxidase; MHPG=3-methoxy-4-hydroxyphenylglycol; NE=norepinephrine.

In the past two decades it has been established that the pattern of increased plasma DOPAC/DHPG, DOPA/DHPG, and DA/NE ratios is perfectly sensitive and specific for diagnosing Menkes disease in at-risk newborns (232, 418). As indicated in Table 6, Menkes disease shares with familial dysautonomia (type 3 hereditary and sensory autonomic neuropathy, HSAN III) and with DBH deficiency a catecholamine metabolic pattern in which the dopamine metabolite DOPAC is built up with respect to the NE metabolite DHPG (415, 416).

IX.1.4. Decreased epinephrine biosynthesis

There is no known human inborn error of metabolism that is caused by isolated PNMT deficiency. Since cortisol is trophic for PNMT, congenital disorders such as 21-hydroxylase deficiency that result in decreased delivery of cortisol to adrenomedullary chromaffin cells are associated with low levels of metanephrine (442).

IX.1.5. Congenital central hypoventilation syndrome (CCHS)

In congenital central hypoventilation syndrome (CCHS) the disease pathogenesis is not from abnormal catecholamine biosynthesis but from heterozygous mutations (most commonly polyalanine repeat expansions) in PHOX2B, which encodes a homeobox transcription factor that is required for development of sympathetic nerves. CCHS does not exhibit a reproducible catecholamine metabolomic signature.

IX.2. Genetic disorders of catecholamine metabolism

IX.2.1. Monoamine oxidase (MAO) deficiency

Genetic MAO-A deficiency and combined MAO-A/B deficiency are associated with a distinctive, abnormal catecholamine metabolic pattern that includes markedly reduced deaminated norepinephrine metabolites (especially DHPG), preservation of levels of the catecholamines themselves, and diversion of metabolism toward O-methylated products, including increased normetanephrine and normetanephrine sulfate (13). In contrast, isolated MAO-B deficiency has no particular catecholamine metabolomic phenotype.

IX.2.2. Catechol-O-methyltransferase (COMT) deficiency

There is no known Mendelian disorder involving catechol-O-methyltransferase (COMT) deficiency, despite the enzyme being encoded by a single gene (COMT) located on the long arm of chromosome 22 at 22q11.21. The frequently studied polymorphism Val158Met (rs4680) (443)is not associated with a known catecholamine metabolic signature; however, plasma metanephrines have not been reported in individuals harboring the Val158Met polymorphism.

IX.3. Neural crest tumors

Neural-crest tumors are characterized by distinctive patterns of catecholamines and metabolites that reflect both tumor biosynthetic activity and downstream metabolism. Pheochromocytomas and paragangliomas (PPGLs) continuously produce catecholamines that are O-methylated within the tumor, and so plasma free metanephrines (normetanephrine and metanephrine) are markedly elevated and provide high diagnostic sensitivity and specificity compared with levels of the parent catecholamines (295, 444).

Elevated plasma methoxytyramine is now recognized as a biomarker of dopamine-producing PPGLs and correlates with metastatic potential (445).

It should be noted that a substantial proportion of PPGLs reflect germline mutations of genes regulating hypoxia signaling (e.g., VHL, endothelial PAS domain protein 1/hypoxia-inducible factor 2A), mitochondrial metabolism and the tricarboxylic acid cycle (e.g., succinate dehydrogenase complex genes, fumarate hydratase), and receptor tyrosine kinase/growth signaling genes (e.g., rearranged during transfection proto-oncogene (RET), neurofibromin 1 (NF1), transmembrane protein 127 (TMEM127), MYC-associated factor X (MAX)) (446, 447).

In neuroblastoma, another neural crest–derived tumor, catecholamine metabolism is also altered with high excretion rates of HVA and VMA in urine reflecting increased catecholamine turnover. LaBrosse et al. were the first to recognize metabolism within the tumors and that there is impaired vesicular storage due to minimal vesicles (448). Since norepinephrine is synthesized in vesicles, dopamine metabolites are the most importantly elevated biomarkers. Recent work supports the supplemental value of plasma 3-methoxytyramine and 3-O-methyldopa for disease detection and monitoring (449, 450).

Malignant melanoma, also derived from neural crest melanocytes, does not classically overproduce catecholamines but shows alterations in melanin-associated catechol metabolites. In melanoma, 5-S-cysteinyldopa (Cys-DOPA)—related to pheomelanin synthesis—is elevated in the plasma and urine of patients with metastatic disease and correlates with tumor burden and progression, making Cys-DOPA a useful biochemical marker alongside imaging and clinical evaluation (425, 426, 451).

Collectively, PPGL biochemical signatures are dominated by elevated metanephrines and methoxytyramine, neuroblastoma by HVA/VMA excess, and melanoma by high Cys-DOPA reflecting melanin pathway activity. The specific metabolite profiles can improve diagnosis, staging, and monitoring.

IX.4. Heart failure

In congestive heart failure (CHF), sympathetic noradrenergic activation initially supports cardiac output but ultimately promotes myocardial remodeling, arrhythmogenesis, and progressive worsening of cardiac dysfunction. It has long been recognized that CHF is associated with myocardial norepinephrine depletion (452). Although one might attribute the depletion to denervation, this would not explain increased entry of endogenous norepinephrine into the cardiac venous drainage (11, 428). Estimated rates of several reactions have indicated that instead, myocardial norepinephrine deficiency in CHF reflects multiple concurrent functional abnormalities in cardiac sympathetic nerves (11) (Figure 10). Increased release of norepinephrine and decreased efficiency of norepinephrine reuptake both contribute to increased cardiac noradrenergic drive in CHF. Decreased norepinephrine store size in the failing heart therefore appears to result not from insufficient TH but from chronically increased norepinephrine turnover and reduced norepinephrine recycling.

Figure 10: Catecholamine metabolic pattern in cardiac sympathetic nerves in congestive heart failure.

Abbreviations: COMT=catechol-O-methyltransferase; Cys-DOPA=5-S-cysteinylDOPA; DA=dopamine; DBH=dopamine-beta-hydroxylase; DOPA=3,4-dihydroxyphenylalanine; DOPAC=3,4-dihydroxyphenylacetic acid; MAO=monoamine oxidase; MHPG=3-methoxy-4-hydroxyphenylglycol; NE=norepinephrine; NMN=normetanephrine; TYR=tyrosine; U1=Uptake-1; U2=Uptake-2. Adapted from (11).

IX.3.1. Takotsubo cardiopathy

Takotsubo (stress) cardiomyopathy is characterized by an acute catecholaminergic surge rather than a distinctive steady-state metabolomic signature. In their seminal report, Wittstein et al. demonstrated that patients presenting with stress cardiomyopathy had markedly elevated plasma catecholamine concentrations—including epinephrine, norepinephrine, DHPG, and dopamine—often several-fold higher than levels observed in comparison patients with Killip class III myocardial infarction (250). This extreme catecholamine excess supports a pathophysiological model of acute sympathoadrenal activation and intense cardiac sympathetic nerve discharge, leading to myocardial stunning via beta-adrenoceptor mediated calcium overload, microvascular dysfunction, and possibly direct catecholamine toxicity (453).

IX.5. Catecholamines and disorders of the “central stress system”

Catecholamines in the brain are integral components of the “central stress system,” interacting with corticotropin-releasing hormone and other mediators to coordinate neural, endocrine, and autonomic responses that maintain homeostasis under threat.

Numerous studies have reported on secondary changes in enzyme phosphorylation, gene expression, proteomic, or epigenetic changes consequent to exposure to distressing stimuli such as immobilization in rats (454–461). Stress-evoked changes in catecholamine and metabolite levels are well documented and occur rapidly, reflecting altered neural firing, secretion, and turnover rather than downstream gene or proteomic programs. These level changes typically normalize quickly after stress cessation (462). Thus, most observed stress-induced alterations in levels of catecholamines and their metabolites represent primary functional activation of catecholaminergic systems, without clear consequences of stress-induced molecular remodeling.

IX.5.1. Postural tachycardia syndrome (POTS)

Postural tachycardia syndrome (POTS) is not associated with a specific overall biochemical signature. Cardiac norepinephrine spillover is increased even during supine rest (423). Plasma norepinephrine normally approximately doubles during orthostasis, and a standing plasma NE concentration > 600 pg/mL has been taken to indicate “hyperadrenergic POTS” (407). This operational criterion does not separate the absolute from the relative magnitude of the orthostatic increase from supine baseline. Analyses of arterial plasma norepinephrine and DHPG do not support a major defect in neuronal uptake, whereas arterial plasma epinephrine is increased, implicating tonic sympathetic adrenergic system activation (424).

IX.5.2. Neurocardiogenic syncope (fainting)

According to the “collapse firing” hypothesis of syncope (463), the sudden cardiovascular collapse arises from a paradoxical afferent signal originating in the underfilled, hypercontractile left ventricle. Under conditions such as prolonged standing or sudden emotional stress, venous return decreases while sympathetic tone remains high, and the resulting small, vigorously contracting heart activates cardiac C-fiber afferents, which project via the vagus to medullary cardiovascular centers. This input triggers reflex withdrawal of sympathetic vasoconstrictor outflow and enhancement of parasympathetic (vagal) activity, producing vasodilation, bradycardia, and hypotension.

A variety of studies have refuted the “collapse-firing” hypothesis. Syncope can occur in patients with denervated (transplanted) hearts (464), echocardiography demonstrates no mechanical ventricular collapse (465), and syncope can occur in individuals who are supine (265). Characteristic sympathetic manifestations—pallor, sweating, pupillary dilation, and piloerection—reflect the effects of high circulating epinephrine and cannot be explained by sudden SNS failure.

Sympathoadrenal imbalance is a state in which SNS activity, reflected by plasma norepinephrine, fails to keep pace with or declines relative to SAS, reflected by plasma epinephrine (265, 466–468). The proximate cause of the syncope may be epinephrine-induced skeletal muscle vasodilation via beta-2 adrenoceptor agonism that is unopposed by SNS activation.

IX.5.3. Post-traumatic stress disorder (PTSD)

Because of the well-known associations between catecholamines and stress, dating back to Selye’s General Adaptation Syndrome (469) and the Chrousos/Gold theory of the central stress system (470), several studies have reported on the involvement of peripheral and central catecholaminergic systems, especially noradrenergic systems, in post-traumatic stress disorder (PTSD). The literature on catecholamine metabolism in PTSD generally supports augmented central noradrenergic and peripheral sympathoadrenal responsiveness.

Early inpatient and combat-veteran studies reported increased urinary norepinephrine and epinephrine excretion in PTSD (471), although not uniformly (472). One study found that urinary norepinephrine and dopamine, but not epinephrine, correlated with PTSD symptom severity in Vietnam combat veterans (473), while a large community-based epidemiologic study noted higher 24-hour urinary excretion of all three catecholamines in individuals with lifetime PTSD than in trauma-exposed individuals without PTSD or nonexposed controls (474).

Data based on plasma levels have indicated enhanced peripheral catecholaminergic responsiveness in PTSD, while under resting conditions there may be no overall abnormality (475, 476). Vietnam veterans with PTSD have been reported to have augmented plasma epinephrine responses to combat-film stress compared to controls (477) and an approximately 30% rise in plasma norepinephrine to combat-related stimuli, with no such change in comparison veterans (478). In the majority—but not all—combat veterans with PTSD intravenous yohimbine evokes panic, associated with elevated MHPG levels (479).

CSF studies have supported the occurrence of central noradrenergic activation in a manner correlated with symptoms in PTSD (480). Treatment with the alpha-1 adrenoceptor antagonist prazosin has been reported to improve PTSD-related nightmares and global PTSD symptoms (481, 482); however, a large VA cooperative trial was negative for distressing dreams and sleep quality (483).

IX.5.4. Attentional deficit/hyperactivity disorder (ADHD)

Catecholamine transporters are major therapeutic targets in ADHD. Methylphenidate occupies the dopamine transporter at clinically relevant doses and increases extracellular dopamine, providing a mechanistic basis for stimulant efficacy (484). Atomoxetine, a nonstimulant ADHD medication, acts primarily as a selective norepinephrine transporter inhibitor and increases extracellular norepinephrine and dopamine in prefrontal cortex, while sparing striatal dopamine to a greater extent than methylphenidate (485). Randomized clinical trials have shown atomoxetine efficacy in children and adults with ADHD, supporting NET blockade as a clinically effective catecholaminergic strategy distinct from classical stimulant treatment (486–488).

IX.5.5. Tardive dyskinesia

VMAT2 is an established therapeutic target in tardive dyskinesia. By inhibiting vesicular monoamine uptake, VMAT2 inhibitors reduce packaging of cytoplasmic monoamines into synaptic vesicles and thereby decrease regulated monoamine release, including dopaminergic transmission in motor circuits. Two selective VMAT2 inhibitors, valbenazine and deutetrabenazine, have shown efficacy in randomized placebo-controlled trials of tardive dyskinesia and are now clinically important examples of targeting vesicular monoamine storage pharmacologically (489–491).

IX.6. Lewy body diseases (LBDs)

The prototype of Lewy body diseases (LBDs) is Parkinson’s disease (PD). Although PD classically is attributed to degenerative loss of dopaminergic neurons in the substantia nigra pars compacta, the proximate neurochemical abnormality underlying its motor manifestations is dopamine depletion within the striatum, particularly the putamen (492). Evidence from imaging, neurochemical, and postmortem studies indicates that the depletion of dopamine in PD in turn reflects not only the loss of nigrostriatal innervation but also functional impairments in surviving dopaminergic terminals such as reduced dopamine synthesis, impaired vesicular storage, and reduced release, together with increased cytosolic dopamine oxidation that contributes to neurotoxicity. Human post-mortem analyses show severe reductions in vesicular dopamine storage capacity in PD compared with controls, consistent with dysfunction in VMAT2-mediated sequestration of dopamine (493). Experimental evidence from animal models demonstrates that reduced vesicular storage of dopamine leads to increased cytosolic dopamine and oxidative stress markers, linking impaired storage with oxidation-related toxicity (494). Additionally, imaging and release studies reveal impairment of dopamine release mechanisms that precede or accompany terminal degeneration in PD models, indicating release dysfunction independent of cell body loss (495). We have referred to functional abnormalities in intact catecholaminergic terminals as the “sick-but-not-dead” phenomenon (496).

VMAT2 function is relevant to catecholamine turnover, because vesicular uptake determines whether newly synthesized or recaptured catecholamines are stored for regulated release or remain in the cytoplasm, where they are vulnerable to spontaneous and enzymatic oxidation. In PD, Borges and colleagues reported impaired platelet dense-granule serotonin storage, uptake, and thrombin-evoked release, suggesting that a systemic secretory-vesicle defect may be detectable outside the nervous system and may have implications for early PD biomarkers as well as pathogenesis (497). In VMAT2-deficient mouse models, reduced vesicular monoamine storage produces marked monoamine depletion and vulnerability to neurodegeneration; notably, VMAT2-low mice show severe loss of striatal dopamine together with cortical and cardiac norepinephrine depletion, supporting the broader concept that impaired vesicular sequestration can affect both central and peripheral catecholaminergic neuronal terminals (493, 498–501).

Computational modeling has revealed multiple functional abnormalities in cardiac sympathetic nerves in Lewy body diseases (12) (Figure 11). These include increased vesicular leakage, attenuated vesicular uptake, decreased TH and LAAAD rates, and decreased neuronal reuptake, reflected by a particular catecholamine metabolomic pattern (red Xs in Figure 11). An analogous approach has been applied recently to identify determinants of putamen dopamine deficiency in PD.

Figure 11: Catecholamine metabolic pattern in cardiac sympathetic nerves in Lewy body diseases.

Numbers in rectangles identify the compounds listed in Table 1. Red X’s indicate abnormal reactions in LBDs. Abbreviations: 3-MT=3-methoxytyramine; ALDH=aldehyde dehydrogenase; AR=aldehyde reductase; DA=dopamine; DBH=Dopamine-beta-hydroxylase; DHPG=3,4-dihydroxyphenylglycol; DOPA=3,4-dihydroxyphenylalanine; DOPAC=3,4-dihydroxyphenylacetic acid; LAAAD=L-aromatic-amino-acid decarboxylase; MAO=monoamine oxidase; NE=norepinephrine; TH=tyrosine hydroxylase.

X. SUMMARY AND PERSPECTIVES

New discoveries and concepts

Over the past two decades in the field of catecholamine metabolism many new discoveries have been made and concepts introduced. Table 8 lists some of these, along with brief explanations of why they matter.

Table 8:

Discoveries and concepts since 2004

Rank	Main discovery/new concept since 2004	Why it matters	Relevant References
1	Real-time, high-resolution catecholamine measurements in vivo	Makes it possible to relate phasic & tonic catecholamine dynamics to behavior & circuit activity in vivo	(79–82)
2	From point-to-point neurotransmission to volume/network neuromodulation	DA & NE not only transmitters at discrete release sites but also spatially diffuse modulators	(444, 445)
3	Transporters & clearance as dynamic regulators of signaling & transmitter conservation	DAT, NET, OCTs, low-affinity uptake systems influence extracellular signal spread & duration, transmitter cross-talk, drug effects, conservation of releasable stores	(71, 446, 447)
4	Catecholamine metabolomic patterns as biomarkers of intracellular processes	Multi-analyte patterns reflect synthesis, vesicular sequestration, leakage, release, reuptake, & metabolism	(1, 450)
5	Kinetic & computational modeling of catecholaminergic systems	Kinetic/computational models link catecholamine patterns to rates of synthesis, vesicular uptake, leakage, reuptake, & metabolism	(450, 451)
6	Catecholaldehyde/autotoxicity concept	Catecholaldehydes are potentially pathogenic products linking cytosolic catecholamine metabolism to dysfunctions, alpha-synuclein, & neurodegeneration	(27, 452–455)
7	Vesicular storage failure as a disease mechanism	Vesicular sequestration determines whether catecholamines are safely stored or remain in the cytoplasm, where they are vulnerable to oxidation & autotoxicity	(442, 443, 456)
8	Sick-but-not-dead catecholaminergic terminals	Functional impairment of surviving catecholaminergic terminals may precede denervation, linking metabolomic patterns, imaging abnormalities, & preclinical disease	(443, 456, 457)
9	Lewy body diseases as multi-process catecholaminergic dyshomeostasis	PD, PAF, & DLB entail not only cell loss disorders but also disorders of synthesis, storage, leakage, reuptake, & metabolism	(448, 450, 451)
10	Catecholamine neuron heterogeneity & circuit-specific function	Optogenetics, chemogenetics, tracing, in vivo recording show DA & NE neurons are heterogeneous projection-specific populations with distinct functions	(444, 445)
11	DA theory beyond reward prediction error	DA signaling encodes not only reward prediction error but also motivation, salience, movement, policy updating, context, & uncertainty	(444, 445)
12	Peripheral catecholamines as autocrine/paracrine regulators & mediators of organ cross-talk	In addition to neurotransmitters & hormones, autocrine/paracrine catecholamines affect metabolism, inflammation, cardiovascular disease, tumor biology, & gut–brain signaling	(1)
13	Improved biochemical & imaging diagnostics	LC-MS/MS, plasma metanephrine/methoxytyramine testing, NET-related imaging, cardiac sympathetic imaging enable precision diagnosis & functional phenotyping	(382, 386, 448, 517, 518)
14	Stimulus–secretion–synthesis coupling	Activity-dependent & cotransmitter-mediated mechanisms maintain stores during stress-level firing /prolonged transmitter mobilization	(157, 160, 519).
15	Pharmacological evolution: transporters, VMAT2, receptor/arrestin signaling, & gene therapy	DAT/NET-targeted treatment strategies, VMAT2 inhibition for tardive dyskinesia, arrestin-biased signaling concepts, & AAV2-mediated restoration of LAAADC activity	(236, 431, 432, 449)
16	Biochemical phenotyping as a bridge to precision physiology	Mechanism-based framework connecting molecules, intracellular reactions, cells, circuits, organs, behavior, & disease, supporting rational diagnosis, treatment, & prevention	(2, 450)

Since the 2004 review, catecholamine biology has evolved from a relatively linear pathway model—synthesis, vesicular release, receptor activation, reuptake, and metabolism—into a multilevel regulatory science linking molecules, cells, circuits, organs, behavior, and disease. Central catecholamines are now understood not only as neurotransmitters but also as volume/network neuromodulators whose spatial and temporal actions shape arousal, salience, motivation, learning, movement, cognition, and adaptive behavior (502, 503). Real-time measurement methods, including genetically encoded dopamine and norepinephrine sensors, have made it possible to relate phasic and tonic catecholamine dynamics to behavior and circuit activity in vivo (79–82). Transporters and low-affinity uptake systems are now viewed not simply as clearance mechanisms but as determinants of signal spread, signal duration, transmitter cross-talk, drug effects, and conservation of releasable stores (71, 504, 505).

In parallel, peripheral catecholamine biology has expanded beyond the classical sympathetic noradrenergic and adrenomedullary systems to include autocrine/paracrine catecholaminergic systems, organ cross-talk, gut–brain interactions, immune/inflammatory modulation, cardiovascular disease, stress cardiomyopathy, and tumor biology. Clinical translation has advanced through refined LC-MS/MS assays, catecholamine-system imaging, and functional phenotyping of autonomic and neurodegenerative disorders (1, 506). Pharmacological advances include transporter-targeting strategies, VMAT2 inhibition, receptor/arrestin signaling concepts, and AAV2-mediated restoration of LAAAD activity in genetic LAAAD deficiency (236, 489, 490, 507).

Catecholamine metabolomic patterns can now be interpreted as biomarkers of intracellular processes, including synthesis, vesicular sequestration, vesicular leakage, release, reuptake, and intra-neuronal metabolism. When coupled with kinetic and computational modeling, these patterns can estimate functional abnormalities that are otherwise inaccessible in humans in vivo (12, 508, 509). These approaches support the concepts of catecholamine autotoxicity, catecholaldehyde-mediated injury, deficient vesicular storage, “sick-but-not-dead” catecholaminergic terminals, and Lewy body diseases as multi-process disorders of catecholamine synthesis, storage, leakage, reuptake, and metabolism rather than denervation alone (1, 28, 493, 500, 501, 506, 510–515). Together, these developments provide a framework for mechanism-based biochemical phenotyping and for rational diagnostic, therapeutic, and preventive strategies in catecholamine-related disorders.

Conclusions

The advances in understanding of catecholamine metabolism extend well beyond the classical view of dopamine, norepinephrine, and epinephrine as simple chemical messengers and their metabolites as expected downstream products. Instead, catecholamine metabolic patterns provide key insights into physiological processes such as biosynthesis, vesicular handling, release, and reuptake.

A key organizing principle is the distinction among three types of peripheral catecholaminergic systems—neurotransmitter, hormonal, and autocrine/paracrine—embedded within an extended autonomic system that incorporates neuroendocrine and immune–inflammatory components. Advances in analytical chemistry now permit comprehensive measurement of catecholamine-related compounds, enabling inferences about specific intracellular processes across physiological states, drug exposures, and pathophysiological states. Conversely, genetic and acquired alterations in activities of these catecholamine systems produce specific clinical syndromes.

From this perspective, catecholamine metabolomic patterns function as biomarkers of intracellular processes. When coupled with quantitative kinetic modeling, these patterns can reveal functional abnormalities such as impaired vesicular sequestration or increased cytosolic oxidation that may not only accompany but precede clinical manifestations of some chronic diseases. Catecholamine metabolomics provides a framework for mechanism-based phenotyping and rationalizing strategies for diagnosis, treatment, and prevention.