Dopamine receptors in emesis: Molecular mechanisms and potential therapeutic function

Abstract

Dopamine is a member of the catecholamine family and is associated with multiple physiological functions. Together with its five receptor subtypes, dopamine is closely linked to neurological disorders such as schizophrenia, Parkinson’s disease, depression, attention deficit–hyperactivity, and restless leg syndrome. Unfortunately, several dopamine receptor-based agonists used to treat some of these diseases cause nausea and vomiting as impending side-effects. The high degree of cross interactions of dopamine receptor ligands with many other targets including G-protein coupled receptors, transporters, enzymes, and ion-channels, add to the complexity of discovering new targets for the treatment of nausea and vomiting. Using activation status of signaling cascades as mechanism-based biomarkers to foresee drug sensitivity combined with the development of dopamine receptor-based biased agonists may hold great promise and seems as the next step in drug development for the treatment of such multifactorial diseases. In this review, we update the present knowledge on dopamine and dopamine receptors and their potential roles in nausea and vomiting. The pre- and clinical evidence provided in this review supports the implication of both dopamine and dopamine receptor agonists in the incidence of emesis. Besides the conventional dopaminergic antiemetic drugs, potential novel antiemetic targeting emetic protein signaling cascades may offer superior selectivity profile and potency.

1. Introduction

Dopamine is an endogenous monoamine catecholamine involved in multiple physiological functions [1]. Dopamine acts on both central (CNS) and peripheral nervous systems. In the CNS, dopamine operates as a neurotransmitter and signals through multiple distinct pathways to regulate locomotion, cognition, emotion, and neuroendocrine secretion [2,3]. Outside the CNS, dopamine functions primarily as a local chemical messenger. Indeed, dopamine acts as a modulator of cardiovascular function, catecholamine release, hormone secretion, vascular tone, renal function, and gastrointestinal motility. Deficits or surfeits in dopamine tissue levels results in diverse and severe pathological conditions ranging from schizophrenia, Parkinson’s disease, depression, attention deficit–hyperactivity, to restless leg syndrome to name a few. Several dopamine-based agonists developed for the treatment of some of these diseases evoke nausea and vomiting (emesis) as common and distressing adverse-effects. The use of dopamine agonist therapy can also lead to other severe side-effects including impulse control disorders characterized by pathological gambling, compulsive eating, hypersexuality, and compulsive buying. These symptoms have been reported in up to 17% of Parkinson’s disease and restless leg syndrome patients who continually use dopamine receptor agonists [4]. The most effective way to lessen these side-effects is to reduce or discontinue the therapy, which can lead to dopamine agonist syndrome in a subset of patients. The symptoms of dopamine agonist syndrome not only include anxiety, panic attacks, dysphoria, depression, suicidal ideation, drug cravings, but also nausea and vomiting [5].

Nausea and vomiting are important protective defense mechanisms by which humans and animals avoid ingestion and/or digestion of potentially toxic substances. Specifically, vomiting is the act of forceful expulsion of gastrointestinal contents through the mouth. However, severe and chronic vomiting can become detrimental due to significant loss of fluid and ion imbalance as is the case during exposure to cancer chemotherapeutic drugs such as cisplatin [6]. The act of vomiting is usually preceded by retching, where the gastrointestinal tract contents are forced into the esophagus, without the vomitus being expelled. On the other hand, nausea is an unpleasant painless subjective feeling that one will imminently vomit. Nausea and vomiting are often thought to exist on a temporal continuum; however, this is not always the case. There are situations when severe nausea may be present without emesis, and less frequently, where emesis may be present without preceding nausea. Most patients state that nausea is more common, more disabling, feels worse and lasts longer than vomiting. While significant knowledge exists on the neurotransmitter and anatomical basis of vomiting [7], nausea is the neglected symptom and its anatomical neurochemistry remains to be fully defined.

Functional pathophysiology of nausea and vomiting indicate that these processes are controlled by a balance between the gastrointestinal enteric nervous system and the CNS [7] as detailed below and in Fig. 1. Emetogens such as lipid soluble dopamine D₂/D₃ receptor agonists can directly act in the gastrointestinal tract and/or by stimulating the brainstem emetic nuclei [7]. The brainstem cluster of medullary nuclei involved in vomiting are the area postrema, the nucleus of the solitary tract, and the dorsal motor nucleus of the vagus, which are collectively named as the dorsal vagal complex. A more ventrolaterally localized group of cells forming the central pattern generator coordinates the act of vomiting. The peripheral emetic loci include the sentinel epithelial enterochromaffin cells present in the mucosa of gastrointestinal tract as well as the vagus and splanchnic nerves, and other intestinal tissues and nerves. Vagal afferent and efferent nerves mainly transfer information between the gastrointestinal tract and the brainstem at the dorsal vagal complex/central pattern generator sites. The different compartments of the emetic reflex arc are detailed later in Fig. 1.

Fig. 1. Schematic representation of the main emesis - mediating components of the brain-gut axis involved in the vomiting reflex.

The dorsal vagal complex nuclei include the area postrema, nucleus of the solitary tract, and the dorsal motor nucleus of the vagus in the brainstem. The area postrema zone has both fenestrated capillaries and active transport systems, allowing blood-borne chemicals or those secreted from the intestinal mucosa to bypass the brain-blood barrier and directly stimulate the dorsal vagal complex to induce vomiting. The intestinal enterochromaffin cells reside alongside the epithelium lining the lumen of the digestive tract and act as custodian cells in the intestinal mucosa. The afferent and efferent nerves connect the gastrointestinal tract to the CNS dorsal vagal complex vomiting nuclei. Vagal afferent nerves = blue arrows; Vagal efferent nerves = red arrows.

The actions of dopamine including vomiting are mediated by cell membrane-bound dopamine receptors. There are five different but closely related heterotrimeric G protein-coupled dopamine receptors [8–10], divided into two major dopamine receptors families: D₁-like and D₂-like classes of receptors [11,12]. The dopamine D₁-like receptors include dopamine D₁ and D₅ subtypes, whereas dopamine D₂-like receptors include dopamine D₂, D₃, and D₄ subtypes. Behavioral evidence from ferrets, dogs, and least shrews (Cryptotis parva) have convincingly shown that dopamine D₂ and D₃ receptors are associated with an emetic response [13,14]. Conversely, dopamine D₁/D₅, or D₄ receptor-selective agonists lack emetic effects [14]. Despite the pivotal role of the different dopamine D₂-like receptors in vomiting and the continued preclinical research, their corresponding downstream intracellular biochemical emetic signals, are at best scantly defined or remain unknown.

In this review, we focus on dopamine and dopamine receptors and their potential roles in emesis. We describe: 1) dopamine synthesis, storage, and release, 2) dopamine receptors basic structural and genetic organization, distribution, function in the brain and the periphery, and their signal transduction mechanisms, and finally 3) we discuss experimental and clinical evidence for dopamine receptors’ agonists-mediated emesis together with dopamine antagonist-based antiemetic drugs.

2. Dopamine synthesis, storage, and release

Dopamine is synthesized in the cytoplasm from the non-essential amino acid L-tyrosine. A structural based summary of dopamine biosynthesis and breakdown is illustrated in Fig. 2. In brief, tyrosine is converted to L-3,4-dihydroxyphenylalanine (levodopa, L-DOPA) by the tyrosine hydroxylase enzyme using tetrahydrobiopterin, oxygen (O₂), and iron (Fe²⁺) as cofactors in both the CNS and in a non-neuronal dopaminergic system within the gastrointestinal tract. In neurons, L-DOPA is quickly decarboxylated to dopamine by aromatic L-amino acid decarboxylase (AADC) enzyme [15]. This oxidation reaction is a rate-limiting step in the production of both dopamine and other catecholamines. In noradrenergic neurons, L-DOPA is converted by the β-hydrolase enzyme into norepinephrine (NE) with O₂ and L-ascorbic acid as cofactors. In turn, NE can be converted into epinephrine by phenylethanolamine N-methyltransferase (PMNT) with S-adenosyl-L-methionine as the cofactor [16,17].

Fig. 2. Structurally based summary of primary pathways of biosynthesis and metabolism of dopamine.

Dopamine biosynthesis: L-tyrosine is converted to L-DOPA in the presence of the TH. In a second reaction, the enzyme AADC converts L-DOPA into dopamine. Dopamine is converted into norepinephrine by the enzyme DBH, with O₂ and L-ascorbic acid as cofactors. Norepinephrine is converted into epinephrine by the enzyme PMNT with S-adenosyl-L-methionine as the cofactor. Dopamine metabolism: Dopamine is broken down into inactive metabolites by a set of enzymes including MAO and COMT. Several breakdown pathways exist but the major end-product is the HVA, which has unknown biological activity.

Abbreviations: AADC: aromatic L-amino acid decarboxylase; COMT: Catechol-o-methyltransferase; DOPAC:3,4-Dihydroxyphenylacetic acid; L-DOPA: levodopa or L-3,4-dihydroxyphenylalanine; HVA: homovanillic acid; MAO: monoamine oxidase; PMNT: phenylethanolamine N-methyltransferase, TH: tyrosine hydroxylase.

Once formed, dopamine is transferred into secretory vesicles for storage. Following nerve cell stimulation, the vesicles containing dopamine merge with the cytoplasmic membrane to deliver dopamine into the synaptic cleft by exocytosis. In the synaptic cleft, dopamine binds to postsynaptic dopaminergic receptors and or presynaptic dopaminergic D₂ autoreceptors. Auto-receptors are found at both somatodendritic and axonal sites where they regulate the firing patterns of dopamine neurons and control the timing and amount of dopamine released from their terminals in target regions [18]. Most of the free dopamine is moved back into presynaptic neurons by a dopamine active transporter, where it is either recycled back into vesicles for reuse or broken down by monoamine oxidase (MAO) enzymes into inactive metabolites. Notably, MAO-A is found in the gastrointestinal tract, whereas MAO-B is primarily located in the brain. Both isoforms can metabolize dopamine to dihydroxyphenyl acetic acid (DOPAC) in the presence of aldehyde dehydrogenase (ALDH). Any remaining extracellular dopamine either diffuses out of the presynaptic cleft or is broken down by catechol-o-methyltransferase (COMT). COMT is expressed in both the brain and peripheral tissues. Together with MAO, COMT converts dopamine to homovanillic acid (HVA) (Fig. 2), which has no recognized biological activity. Dopamine exerts its physiological effects by binding to and activating cell surface dopamine receptor subtypes.

3. Molecular properties of dopamine receptors

Similarities and differences in gene structure, amino acid composition, and signaling cascades of the dopamine receptors are detailed in the sections below:

3.1. Gene structure and amino acid composition

The genetic structure of dopamine D₁- and D₂-like classes of receptors are differentiated by the presence or the absence of introns in their coding sequences [19]. The genetic organization of the dopamine D₂-like receptors provides the foundation for receptor splice transcript variants. For instance, alternative splicing of an 87-base-pair exon between introns 4 and 5 of the dopamine D₂ receptor yields two major dopamine D₂ receptor variants; dopamine D₂S (D₂-short) and dopamine D₂L (D₂-long) [20,21] carrying distinctive anatomical, physiological, and pharmacological properties. Splice variants for dopamine D₃ and D₄ receptors have been described but are poorly characterized [22,23]. Dopamine D₃ receptor can be split at the third cytoplasmic loop into two fragments (dopamine D₃ trunk and D₃ tail), and the mixture of the two fragments retains the binding and functional activity of the wild type receptor. The dopamine D₄ receptors have three variants dopamine D_4.2, D_4.4 and D₄ receptors [24]. The pharmacological characteristics of dopamine D₄ receptors resemble that of the dopamine D₂ and D₃ receptors. The atypical neuroleptic drug clozapine has a higher affinity for dopamine D₄ than for dopamine D₂ and D₃ receptors [25], which suggests the existence of important characteristics that may separate dopamine D₄ receptor from the other receptors.

Dopamine D₁- and D₂-like class receptors display considerable amino acid sequence conservation within their transmembrane domains [26] and members of each class share a high level of homology in their transmembrane domains. For example, dopamine D₁ and D₅ receptors share an 80% identity in their transmembrane domains. Dopamine D₂ and D₃ receptors have a 75% identity in their transmembrane domains, and dopamine D₂ and D₄ receptors share a 53% identity [8]. Site-directed mutagenesis for catecholamine receptors [27–29], and protein modeling with the adrenergic α₂-, β₂- and dopamine D₂ receptors [30,31], indicate that dopamine agonist binding site is likely located within the hydrophobic transmembrane domains.

3.2. Dopamine signaling

Dopamine signals through its receptors to induce and regulate a myriad of cellular responses as well as dopamine-associated behaviors and functions [19,32]. Signaling cascades have provided a basis for the understanding of intracellular mechanisms implicated in dopamine-related pathologies. The role of these pathways is well studied in neurodegenerative disease but poorly explored in emesis. In this section, we describe the potential intracellular signaling pathways associated with emetogenic dopamine D_2/3 receptors.

3.2.1. cAMP/PKA signaling

Dopamine receptors have signaling systems that contribute to the definition of each class of receptors, though differences within a class have been reported. For instance, the non-emetic dopamine D₁-like receptors are generally coupled to G_s alpha proteins (Gαs/olf or Gαs/o proteins = adenylyl cyclase stimulator) and adenylyl cyclase to yield higher levels of the second messenger cyclic adenosine monophosphate or cyclic AMP (cAMP), which in turn stimulates the activity of protein kinase A (PKA). Consequently, specific proteins are phosphorylated by PKA [33]. The phosphorylated and activated substrates include dopamine cAMP-regulated phosphoprotein, MW 32 kDa (DARPP-32). When DARPP-32 is phosphorylated on Thr³⁴, it inhibits protein phosphatase 1 (PP1). The inhibition of PP1 results in the phosphorylation and the activation of many downstream physiological effectors, including various neurotransmitter receptors, ion channels and pumps, and transcription factors leading to evoked cellular effects (Fig. 3A). Conversely, dopamine D₂-like receptors, which are coupled to pertussis toxin (PTX)-sensitive Gi/Go proteins (Gαi/o = adenylyl cyclase inhibitor) block adenylyl cyclase and lower the intracellular concentration of cAMP [34], resulting in lower PKA activity [8,35,36] (Fig. 2A). Dopamine D₂-like receptors also activate the protein phosphatase calcineurin, which dephosphorylates DARPP-32 at Thr³⁴.

Fig. 3. Proposed dopamine receptor signaling pathways, some of which may be involved in emesis.

(A) Dopamine D₁- and D₂-receptors mediated cAMP/PKA signaling cascades. Dopamine D₁-like receptors are coupled with Gα/o proteins. Adenylyl cyclase stimulation by GPCRs causes cAMP production, which in turn activates PKA. PKA phosphorylates DARPP-32. DARPP-32 phosphorylated on Thr³⁴ is a potent inhibitor of PP₁, which results in PP₁ targets being phosphorylated. Dopamine D₂-like receptors coupled with Gio proteins block AC and lower the intracellular concentration of cAMP causing PKA inhibition. Additionally, dopamine D₂-like receptors activate the protein phosphatase calcineurin, which dephosphorylates DARPP-32 at Thr³⁴. Like the dopamine D₁ receptors, these depicted dopamine D₂ receptor signaling pathways are not involved in vomiting. Abbreviations: cAMP: cyclic adenosine monophosphate or cyclic AMP; P: phosphorylation of protein; PKA: protein kinase A; GPCRs: G protein-coupled receptors; DARPP-32: dopamine and cyclic AMP-regulated phosphoprotein relative molecular mass 32,000; PP1: protein phosphatase 1. Black solid arrows = emesis; Red arrows = increase or decrease of cAMP; Dashed solid arrows = no emesis.

(B) Dopamine D₂ receptor - mediated ERK activation signaling pathway. ERK protein is an integral part of the Ras-MEK-ERK signaling cascade. Dopamine D₂ receptors can activate ERK by two signaling cascades via: i) the Ras/Raf/MEK1/ERK pathway where dopamine through tyrosine kinase receptors activates Ras, which binds and phosphorylate Raf. The Raf kinase phosphorylates and activates MEK, which in turn activates ERK protein; and ii) Dopamine D₂ receptor can recruit Arrestins (Arr-₂ and -₃) proteins, which terminate G protein signaling, facilitates receptor internalization, and engages non-canonical G protein-independent signaling pathways. Dopamine D₂ receptor - mediated ERK activation may potentially cause emesis. D₁-like receptors regulate a protein complex composed of β-Arr₂/ERK and MEK and induce ERK activation. However, dopamine D₁-like receptors are devoid of emetic activity. Abbreviations: Arr: Arrestin; ERK_1/2: extracellular signal-regulated kinase 1 and 2; Raf: rapidly accelerated fibrosarcoma; Ras: rapidly accelerated sarcoma. Black solid arrows = emesis; Dashed solid arrows = no emesis.

(C) Dopamine D₂-receptors mediated activation of GSK-3/Akt pathway. From current findings one could envisage that direct phosphorylation of Akt following dopamine D₂ receptor stimulation may promote emesis. Dopamine can exert part of its actions by modulating the activity of Akt/GSK-3 proteins acting through various mechanisms. Two mechanisms have an opposite effect on the activity of these two kinases. On one hand, dopamine D₂ receptor activation leading to phosphorylation of GSK-3 causes its deactivation and reduction of emesis. On the other hand, the opposite may take place where activation of dopamine D₂ receptors deactivate Akt by the PP₂A. The inactivation of Akt and formation of an Akt, β-Arr₂, and PP₂A signaling complex facilitate both the inactivation of Akt by the phosphatase and the increase in GSK-3 activity, which may promote emesis. Abbreviations: GSK-3: glycogen synthase kinase-3; Akt/PKB: thymoma viral proto-oncogene; P: phosphorylation of protein; PP₂A: protein phosphatase 2A. Black solid arrows = emesis; Dashed solid arrows = no emesis; Red T lines = inhibition.

(D) PLC signaling pathways. Dopamine receptors can couple to Gαq to activate PLC causing the hydrolysis of PIP₂ into two-second messengers; DAG and IP₃. DAG activates PKC. IP₃ stimulates IP₃ receptor with subsequent release of Ca²⁺ into the cytosol. The released Ca²⁺ regulates CaMKII and protein phosphatase 2B (calcineurin) both of which may contribute to emesis. LTCCs activated by dopamine D₂ receptor agonists such as quinpirole can cause the influx of extracellular Ca²⁺ into the cell. The influx of extracellular Ca²⁺ can also be partly due to evoked intracellular Ca²⁺ release from SER stores via Gα/q-mediated PLC pathway. Overall, the increase cytosolic Ca²⁺ may potentially support the incidence of emesis. Abbreviations: PLC: phospholipase C; DAG: 1,2-diacylglycerol; IP_3: inositol-1,4,5-trisphosphate; PKC: protein kinase C; IP₃R: IP₃ receptors; CaMKII: Ca²⁺/calmodulin-dependent kinase II; LTCC: L-type Ca²⁺ channels; SER: sarcoplasmic and endoplasmic reticulum. Ca²⁺-induced Ca²⁺ release (CICR) is a biological process whereby Ca²⁺ can activate Ca²⁺ release from intracellular Ca²⁺ stores (e.g., endoplasmic reticulum or sarcoplasmic reticulum). Black solid arrows = emesis; Dashed solid arrows = no emesis; Blue arrows = influx of Ca²⁺.

In terms of behavior, the cAMP/PKA signaling pathway in the CNS is well-characterized and has a pivotal role for different physiological responses that are valuable for cell survival, synaptic plasticity, and gene expression [37,38]. The emetic role of cAMP in the PKA pathway has been demonstrated by microinjection of cAMP analogs (e.g., 8-bromo cAMP) and forskolin (an activator of adenylyl cyclase that increases endogenous of cAMP) in the brainstem AP emetic zone, which elevates the electrical activity of local neurons and stimulates vomiting in dogs [38]. Likewise, the administration of 8-chloro cAMP to cancer patients produces nausea and vomiting [39]. The use of phosphodiesterase inhibitors (such as rolipram) boosts cAMP tissue levels, which trigger excessive nausea and vomiting in vomit competent animals and humans [40]. Our laboratory has found a direct association between increased PKA phosphorylation with peak vomit frequency during immediate and delayed phases of vomiting caused by either cisplatin or cyclophosphamide in the least shrew [41–43]. It is important to note that since dopamine D₂-like receptor activation results in decreased cAMP production, vomiting is improbable to occur as a direct effect of dopamine D₂-like stimulation via the cAMP/PKA signaling cascade. We also expect stimulation of cAMP by dopamine D₁/D₅ receptor-selective agonists and subsequent PKA increase is unlikely to elicit an emetic response since dopamine D₁-like receptors are devoid of emetic properties [14].

3.2.2. ERK signaling

Extracellular signal-regulated kinases 1 and 2 (ERK_1/2 or p44^ERKand p42^ERK) are part of the Ras-MEK-ERK signaling cascade which belongs to the mitogen-activated protein kinases (MAPKs), a highly conserved family of serine/threonine protein kinases [44]. Evidence support ERK_1/2 activation through dopamine receptors [45,46]. Indeed, primary mesencephalic neuronal cells from wild-type (WT) mice treated with the dopamine D₂ receptor agonist quinpirole, results in an increase in the number of TH-positive neurons, which coincide with ERK activation in the same neurons [47]. Dopamine D₂ receptor-mediated ERK activation has also been observed in a variety of other cell lines, including in HEK-293, fibroblast-like COS-7, and C6 glioma cells [48,49]. The mechanisms by which the dopamine D₂ receptor stimulates ERKs have not been fully clarified [50]. However, it is well established that the βγ subunits of G-protein downstream of tyrosine kinase receptors (TKRs) and the small G-protein of the proto-oncogene protein P21 (Retrovirus-associated DNA sequences (Ras)) have been implicated in dopamine D₂ receptor activation of ERK [48,49,51] (Fig. 3B). Dopamine D₂ receptors can activate ERK [48] by the typical Ras-associated signaling pathway i.e., Ras/Raf (rapidly accelerated fibrosarcoma)/MEK₁. In this cascade, stimulated Ras binds Raf kinase and carries it to plasma membranes for activation through tyrosine phosphorylation [50]. The Raf kinase phosphorylates and stimulates MEK, which activates ERK [48] (Fig. 3B). Furthermore, ERK activation by G protein-coupled receptors (GPCRs) can be mediated by arrestins (Arrs) [52]. Arrs are a small family of proteins that are important for regulating signal transduction of GPCRs [53,54]. Dopamine D₁ receptors can regulate protein complex formed of β-Arr₂/ERK and MEK and induce ERK activation. Indeed, in transfected cells, β-Arr₂ can support the formation of a MAPK complex composed of β-Arr₂ and the MAPK pathway kinases Raf, MEK, and ERK in response to GPCRs activation [55] resulting in activation of ERK [56]. In dopamine D₂ receptors signaling, the receptors-β-Arr₂ complex can stimulate ERK, but dopamine D₃ receptors cannot except for specific conditions. In fact, using the same experimental conditions, dopamine D₃ receptors could still activate ERK in HEK-293 and COS-7 cell lines when Gαo was co-expressed but not Gαi. Meanwhile, dopamine D₂ receptors can mediate ERK activation by both isoforms Gαi/o proteins and by dopamine D₂ receptors-β-Arr complex [57]. Upon stimulation, activated ERK enters the cell nucleus to modulate its downstream targets and ultimately behavior [58–62].

In terms of behavior, dopamine receptors are frequently associated with the activation of ERK signaling. This is consistent with a study showing that psychostimulants such as amphetamine augment levels of dopamine, locomotor hyperactivity, and activation of ERK in mice [63]. Other effects associated with ERK signaling include: memory formation, survival, and adaption [64,65]. Yet, some studies suggest that activation of the ERK signaling cascade may induce a cytotoxic neuronal response to oxidative stress [66,67], implying that ERK signaling may have an alternate function mediating neurotoxic actions. Previous work from our laboratory has implicated the phosphorylation of ERK_1/2 in the induction of emesis in the brainstem following stimulation of serotonergic 5-hydroxytryptamine 3 receptor (5-HT₃R), which is in line with ERK potentiation of a cytotoxic neural stress response [68]. Also, stimulation of phosphatidylinositol-3-kinase (PI3K)-ERK and protein kinase C (PKC)-ERK signaling pathways by the selective substance P neurokinin type 1 receptor (NK₁R) agonist GR73632, yields an emetic response [69]. Additionally, Ca²⁺ mobilizers such as the selective L-type Ca²⁺ (LTCC) agonist FPL44176, and the sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase (SERCA) inhibitor thapsigargin, are potent emetogens [70, 71]. They respectively evoke vomiting through extracellular and intracellular Ca²+ mobilization via phosphorylation of ERK_1/2 in the least shrew brainstem [71]. Since a role of dopamine D₂-like receptors in ERK phosphorylation during emesis has yet to be established, we speculate that these receptors may promote ERK phosphorylation and subsequent vomiting through either of the above ERK signaling pathways or by other pathways yet to be discovered. Dopamine D₁ receptors activate ERK_1/2 as well, but this activation is unlikely to result in vomiting since dopamine D₁ class of dopamine receptors is devoid of emetic properties.

3.2.3. Akt/GSK-3 signaling

GSK-3 (glycogen synthase kinase-3) and Akt/PKB (thymoma viral protooncogene, also known as protein kinase B) are serine-threonine kinases that participate in the Akt/GSK-3 signaling cascade. GSK-3 is a downstream target for Akt protein, which can phosphorylate GSK-3 and suppresses its activity. GSK-3 is encoded by two known genes, GSK-3α and GSK-3β. Dopamine can exert part of its actions by modulating the activity of Akt/GSK-3 [72] acting through various mechanisms, two of which have an opposite effect on the activity of these two kinases (Fig. 3C). In the first mechanism, Akt protein activation and subsequent deactivation of GSK-3 by respective phosphorylation occur after the stimulation of the dopamine D₂-like receptors [73–76]. In the second mechanism, the opposite occurs; stimulation of dopamine D₂ receptors leads to a regulated deactivation of Akt by the protein phosphatase 2A (PP₂A), which is known to have a role in mediating Akt inactivation in other biological systems [77]. The inactivation of Akt and the ensuing formation of an Akt, β-Arr₂, and PP₂A signaling complex facilitates the inactivation of Akt by the phosphatase [78], which consequently leads to a reduced phosphorylation and increased activation of GSK-3α and GSK-3β that, in turn, regulate dopamine-associated behaviors [78–80] (Fig. 3C).

The importance of Akt/GSK-3 signaling for dopamine-related behaviors is apparent from the pioneering findings which uncovered the role of this pathway in dopamine receptor signaling. Pharmacological and genetic manipulations that elevate or reduce dopaminergic activity can cause profound effects on locomotor activity in mice and significant alterations in the phosphorylation of Akt and GSK-3 [79]. For instance, pharmacological or genetic inhibition of GSK-3 activity prevents locomotor hyperactivity associated with excessive dopaminergic tone in dopamine transporter knockout or amphetamine-treated mice [79,81]. Mice overexpressing GSK-3β show noticeable locomotor hyperactivity as well [82]. Activation of Akt/GSK-3 signaling by dopamine receptors has also been extensively discussed in cell survival and neuroprotection against oxidative stress, which may indirectly be related to emesis [83–85]. Indeed, Akt which is a downstream effector of PI3K, and the antiapoptotic protein Bcl₂ (B-cell CLL/lymphoma 2) were activated by the dopamine D₂-like receptor agonist bromocriptine in samples from primary cultures obtained from the cerebral cortex of fetal rats (17–19 days of gestation) [83]. These findings imply that dopamine D₂ receptor activation has a vital role in neuroprotection against cytotoxicity with the upregulation of Bcl₂ expression located upstream of Akt protein in the PI3K/Akt/GSK-3 cascade is, at least partially, associated with this effect [83]. It has also been suggested that neurodegeneration may result from dopaminergic agonists due to side-effects of retching and vomiting that are known for high levels of oxidative stress [86] and elevated free radical production [87]. Specific activation of dopamine D₃ receptors boosts the Akt activity associated increase in dendritic arborization in dopaminergic neurons from mouse embryos [88]. Given that reactive oxygen species (ROS) regulation and inhibition can be controlled by the Akt upstream of PI3K protein [89], the role of these proteins in neurodegenerative diseases associated with emesis requires further assessment. Since dopaminergic drugs have other molecular targets and alternative mechanisms such as an action on serotonin receptors [90–92], these additional mechanisms could also contribute to the effect of these drugs on Akt/GSK-3 signaling including in vomiting but a reliable link to emesis has still to be made. More recently preliminary findings from our laboratory indicate that GSK-3 inhibitors such as AR-A014418 and SB216763 protect shrews from vomiting caused by the nonselective dopamine D₂ receptor agonist apomorphine and the more selective dopamine D₂ receptor agonist quinpirole [93] are an initial indication of the direct involvement of Akt/GSK-3 signaling in the mechanism of the induced-emesis.

3.2.4. Phospholipases signaling

Phospholipases (PL) are a diverse group of enzymes that catalyze the hydrolysis of membrane-forming phospholipids. Phospholipase A (PLA), phospholipase C (PLC), and phospholipase D (PLD) form the major groups among which PLC and PLD are the most studied in correlation with dopamine receptors signaling.

Several investigators have shown that dopamine and dopamine D₁-like receptor agonists can stimulate PLC-phosphoinositide hydrolysis in mammalian tissues [94]. These findings have been reproduced in rat [95, 96] and mouse brains, [97], postmortem human brain [98,99], and fresh monkey brain [100]. Interestingly, two studies indicate dopamine and dopamine D₂ receptor agonists inhibit phosphoinositide hydrolysis in striatal slice preparations [101, 102]. However, Undieh [103] and his group were unable to replicate the findings in the absence of kynurenine, scopolamine, and glutathione, all of which were reported to be relevant for showing a dopamine or quinpirole blockade effect [102]. These agents have unpredictable effects on signaling events [100, 104], as in the case of glutathione. Thus, it remains unclear if dopamine D₂ receptor stimulation, on its own, can prevent basal or heterologous agonist-stimulated phosphoinositide hydrolysis. The mechanism by which PLC can participate in phosphatidylinositol 4,5-bisphosphate (PIP₂) metabolism signaling pathways in a calcium (Ca²⁺)-dependent-manner includes several steps [103]. Specifically, dopamine receptors can couple to Gαq to activate PLC leading to the hydrolysis of PIP₂ into two-second messengers; 1,2-diacylglycerol (DAG) and inositol-1,4,5-trisphosphate (IP₃) [105]. As a very hydrophobic molecule, the DAG remains associated with the cytoplasmic membrane, where it activates PKC. Being a water-soluble messenger, the IP₃ travels through the cytosol to the endoplasmic reticulum (ER) membrane where it stimulates Ca²⁺ channel IP₃ receptors (IP₃R) with subsequent release of intracellular stored Ca²⁺ into the cytosol [105–107]. The released Ca²⁺ regulates Ca²⁺-dependent activity of proteins including Ca²⁺/calmodulin-dependent kinase II (CaKMII) and protein phosphatase 2B (calcineurin). The essential components of these interconnected cascades are illustrated in Fig. 3D. Interestingly, not all dopamine receptors can activate Ca²⁺ release since transfected cells expressing dopamine D₁ receptors do not have any effect on intracellular Ca²⁺, whereas the expression of dopamine D₅ receptors transfected in the same cells caused substantial Ca²⁺ mobilization after stimulation. These observations imply that either dopamine D₅ receptors are the major receptors in this signaling cascade in vivo, or the dopamine D₁ receptors need to interact with other proteins to mobilize Ca²⁺, which is consistent with PLC involvement in receptors heterodimerization. In fact, co-activation of dopamine D₁- and D₂-like receptors causes activation of PLC likely through a Gαq-mediated mechanism [108], with a subsequent increase in DAG and IP₃. The activation of Gαq/PLC signaling requires both dopamine D₁ and D₂ receptors since dopamine D₁ or D₂ receptor antagonists or genetic deletion of either dopamine D₁ or D₂ receptors prevents the effect [109]. Besides heterodimerization with dopamine D₁ receptors, dopamine D₂ receptors alone can evoke Ca²⁺ mobilization. Although the Gαi/o subunits of the G-proteins, which are traditionally associated with dopamine D₂ receptor signal transduction, have not been directly connected with the increase of intracellular Ca²⁺, the PTX-sensitive signaling pathways have been associated with the activation of Ca²⁺ signals. In such pathways, Gβγ dimers of PTX-sensitive G-proteins stimulate PLC directly (reviewed in Ref. [110]) or by changing the Gαq pathway, which in turn activates PLC leading to Ca²⁺ signaling [111]. Whether these dopamine D₂ receptors mediate PLC pathways activation in the dorsal vagal complex or gastrointestinal tract have not been explored and requires further consideration.

Behavioral studies show that the activation of the dopamine D₁-like receptor/Gαq/PLC signaling cascade is involved in vacuous jaw movements and intense grooming [104,112] that could be important during the incidence of emesis. The activation of Ca²⁺/CaMKII, a product of PLC pathway, is thought to be a crucial mediator of learning and memory [113]. On the other hand, dysregulation of CaMKII is associated with Alzheimer’s disease, Angelman syndrome, and heart arrhythmia to name a few [113]. Our laboratory has found that the activation of the Ca²⁺/CaMKII cascade promotes vomiting in the least shrew through: 1) activation of substance P neurokinin NK1 receptor, and 2) thapsigargin-evoked Ca²⁺/CaMKII signaling cascade involving substance P release [70]. The potential existence of dopamine D₂-like receptor/PLC/CaMKII pathways mediated emesis has yet to be described.

PLD is a lipid-modifying enzyme that catalyzes the conversion of phosphatidylcholine into phosphatidate and choline. A wide variety of neurotransmitters [114] activates PLD through stimulation of GPCRs via mechanisms not completely understood. The importance of the GPCRs in dopamine receptors mediated activation of PLD has been shown in various studies [115]. Among the GPCRs, dopamine D₂ [116] and D₃ receptor [117] stimulation of PLD [116,118], is not mediated by Gαi/o since the activation is insensitive to PTX [116]. It has been suggested that the stimulation of PLD activity by dopamine D₂ receptors is associated with the presence of a PKCε isoform.

PLDs have been implicated in the pathophysiology of various diseases: cancer, Alzheimer’s disease, and Parkinson’s disease [119]. Hence, several PLD inhibitors have been developed for the treatment of some of these diseases among which halopemide, NOPT (N-[2-(4-oxo-1-phenyl-1, 3,8-triazaspiro[4,5]dec-8-yl)ethyl]-2-naphthalenecarboxamide [120], and NFOT (N-[2-[1-(3-Fluorophenyl)-4-oxo-1,3,-8-triazaspiro[4,5] dec-8-yl]ethyl]-2 naphthalenecarboxamide) have been developed specifically for the inhibition of PLD2 [121,122]. These inhibitors suppress PLD2 activity through various pathways preventing phosphatidic acid production and biological processes mediated by the expression of PLD [122]. Certain inhibitors affect the activity of both PLD1 and PLD2, including Fifi (5-fluoro-2-indolyldes-chlorohalopemide), ML-299 (4-bromo-N-[(1S)-2-[1-(3-fluorophenyl)-4-oxo-1,3,8-triazaspiro[4.5] dec-8-yl]-1-methylethyl]-benzamide), VU-0155056, and VU-0285655–1 [123,124]. Whether these inhibitors would induce vomiting in humans or in vomit competent animals remains unknown.

3.2.5. Ca²⁺ channel signaling

Ionic Ca²⁺ is a highly flexible intracellular messenger, which modulates complex cellular processes including cell activation and neurotransmission. The loss of Ca²⁺ homeostasis is associated with many neurological diseases and neuropsychiatric disorders such as Parkinson’s, Alzheimer’s, and schizophrenia [125]. It is also considered as a possible unifying signal in emesis [126]. Levels of Ca²⁺ concentration in the cell depend on the homeostasis between Ca²⁺ influx and its extrusion from the cell. In resting cells, the cytosolic concentration of Ca²⁺ can be as low as 50–150 nM. The transcellular Ca²⁺ transport mechanism has been described in nerve cells [127, 128]. Typically, the intracellular rise in Ca²⁺ concentration triggers neurotransmitter release. This elevation in Ca²⁺ is attributed to intracellular Ca²⁺ mobilization, which consists of extracellular Ca²⁺ influx and/or Ca²⁺ discharge from intracellular pools [70,129]. Two transport mechanisms are responsible for the entry of Ca²⁺ into the cell [128]. In the passive paracellular pathway, Ca²⁺ enters via tight junctions located between epithelial cells, whereas transepithelially Ca²⁺ enters via active transport. The extracellular Ca²⁺ influx into the cell for transmitter release is carried out by several Ca²⁺ permeable channels coexisting in the plasma membrane [130]. The latter proceeds through a well-controlled sequence of three events: 1) an apical Ca²⁺ entry involving channels such as voltage-dependent Ca²⁺ channels (VDCCs including L-type, N-type, P/Q type, and R-type sometimes classified as intermediate voltage-activated (IVA) channels [131,132 ], and members of the transient receptor potential (TRP) gene family, then 2) a cytosolic diffusion of Ca²⁺ bound to calbindins (CaBPs), and finally 3) a basolateral extrusion of Ca²⁺ through plasma membrane Ca²⁺-ATPase (PMCA) and to a lesser extent by Na+/Ca²⁺ exchanger (NCX) [133,134]. When intracellular Ca²⁺ homeostasis is uncontrolled, emesis can occur. The voltage-gated L-type Ca²⁺ channels (LTCCs), the transient receptor potential vanilloid type 1 (TRPV1) channel and the sarcoplasmic and endoplasmic reticulum (SER) Ca²⁺-ATPase (SERCA) pump are central to emesis, will be discussed in the subsequent sections.

LTCCs are activated by membrane depolarization and operate as the principal route of Ca²⁺ entry in electrically excitable cells such as neurons and muscle [135,136]. Recently, our laboratory has found direct evidence of Ca²⁺ mobilization as a critical factor in the mediation of emesis. These studies indicate that intraperitoneal administration of the selective LTCC agonist FPL64176 causes vomiting in the least shrew in a dose-dependent manner [130,137]. Pretreatment with corresponding LTCC antagonists amlodipine (0.5–10 mg/kg) or nifedipine (2.5–10 mg/kg), not only attenuate in a dose-dependent and potent manner FPL64176-evoked vomiting, but also suppress vomiting caused by a variety of other emetogens including those caused following intraperitoneal (i.p.) administration of dopamine D_2/3 receptor agonists such as apomorphine (2 mg/kg) or quinpirole (2 mg/kg) [130,137]. These findings indicate that the LTCC agonists may trigger an influx of Ca²⁺ into cells via LTCC opening (Fig. 3D), which then prompts a Ca²⁺ signaling cascade leading to emesis, whereas selective antagonists of LTCCs prevent the extracellular Ca²⁺ influx and the ensuing emesis. The mechanism by which LTCC antagonists prevent emesis in the presence of dopamine D_2/3 receptors agonists warrant further investigation.

TRPV1 is a nonselective cation channel with a preference for Ca²⁺. The presence of TRPV1 on neural membranes as well as in the membrane of intracellular organelles (e.g., mitochondria, ER) [138,139], substantiates a role for this receptor in intracellular Ca²⁺ mobilization and consequent vomiting. The expression of TRPV1 in the brainstem nuclei (area postrema, nucleus of the solitary tract, and dorsal motor nucleus of the vagus) and gastrointestinal tract vomit circuits, further supports a functional role for this receptor in emesis [140,141]. TRPV1 is activated by numerous agonists from natural sources including the pungent component of chili peppers, capsaicin (8-methyl-N-vanillyl-6-nonenamide) and its ultrapotent analog resiniferatoxin from the plant genus Euphorbia [142]. An initial dose-dependent bell-shaped pro-emetic effect (10–1000 μg/kg) was found for resiniferatoxin in ferrets and house musk shrews. Furthermore, a 100 μg/kg (s.c.) resiniferatoxin dose was shown to protect these species from vomiting evoked by various centrally and peripherally acting emetogens including cisplatin. In the least shrew, resiniferatoxin also evokes a dose-dependent and bell-shaped profile of vomiting, whose maximal pro-emetic effects occurred at its 18 μg/kg dose, whereas doses below 5 μg/kg had no significant emetic effect [143]. Recently, our laboratory has demonstrated that ultra-low doses of resiniferatoxin (0, 0.01, 0.025, 0.05, and 0.5 μg/kg) suppress vomiting evoked by apomorphine (2 mg/kg, i.p.) with complete protection occurring at its 0.5 μg/kg (s.c.) dose [144]. Resiniferatoxin was protective against vomiting evoked by the more potent and more selective dopamine D₂ receptor agonist quinpirole, with maximal efficacy at its 5 μg/kg dose as well. Capsazepine, a synthetic antagonist of capsaicin [145], was the first reported competitive antagonist of the TRPV1 receptor. Capsazepine blocks the painful sensation of heat caused by capsaicin, an resiniferatoxin analog, known to activate TRPV1.

Some insight into TRPV1 signaling associated with dopamine and dopamine receptors activation were provided by Chakraborty group [146] who investigated the effect of dopamine and dopamine receptors on the activation of capsaicin. They found that dopamine and SKF 81, 297 (an agonist at dopamine D₁/D₅ receptors), but not quinpirole (an agonist at dopamine D₂ receptors), decrease the activity of TRPV1 channels in dorsal root ganglion neurons. The authors suggested that modulation of TRPV1 channels by dopamine in nociceptive neurons may represent a way for dopamine to modulate incoming noxious stimuli. They further argued against a direct implication of PKC or PKA proteins [146]. However, Naittou and his team [147] have shown that dopamine D₂ receptor antagonists prevent capsaicin-evoked colorectal motility which contrasts with the latter report and supports the notion that phosphorylation by PKC sensitizes TRPV1 channels by increasing the channel open probability [148], while phosphorylation by PKA may reverse the desensitization of TRPV1 channels induced by repeated administration of the agonist [149]. Remarkably, capsaicin activation of TRPV1 on astrocytes yields endogenous ciliary neurotrophic factor (CNTF) in vivo, which prevents the degeneration of dopamine neurons by acting through CNTF receptor alpha (CNTFRα) on dopamine neurons in animal models of Parkinson’s disease. This endogenous neuroprotective system (TRPV1 and CNTF on astrocytes, and CNTFRα on dopamine neurons) could be exploited as a novel beneficial therapeutic target for the treatment of Parkinson’s disease [150]. These findings on TRPV₁ may provide a platform that links dopamine D₂ receptors to pathways involved in vomiting and neuroprotection.

Excess of intracellular Ca²⁺ concentration is counterbalanced by the SERCA pump, which transports and sequesters free surplus cytosolic Ca²⁺ into the lumen of sarcoplasmic/endoplasmic reticulum (SER) for storage [151,152] to help maintain internal cell homeostasis. Intracellular Ca²⁺ release from the SER store into the cytoplasm is achieved via two Ca²⁺ release channels: i) IP₃Rs which are activated by metabotropic receptor-dependent IP₃ production [153], and ii) ryanodine receptors (RyRs) activated by Ca²⁺ via Ca²⁺-induced Ca²⁺ release mechanism, and by a change in membrane voltage [154,155]. Recently, our laboratory has found that the i.p. injection of thapsigargin (0.1 10 mg/kg), a specific and potent inhibitor of SERCA pumps, causes vomiting in the least shrew suggesting that the increase in the cytosolic level of free Ca²⁺ in emetic loci result from SERCA inhibition and subsequent depletion of corresponding SER Ca²⁺ pool stores, leading to an influx of extracellular Ca²⁺ [70]. The evoked vomiting may also be due to SERCA inhibition and luminal ER Ca²⁺ release through RyRs and IP₃Rs in the brainstem. The thapsigargin-induced increase in cytosolic Ca²⁺ resulted in CaMKII activation in the brainstem emetic nuclei. How dopamine D₂-like receptors can evoke vomiting though remains unknown and warrant further exploration. It is possible that dopamine D₂ receptors couple to Gαq to activate PLC and stimulate Ca²⁺ release. Indeed, previous work from our laboratory has shown that GR736322 induced-activation of substance P neurokinin NK1R can stimulate intracellular Ca²⁺ release from SER stores via Gαq-mediated PLC pathway which elicits extracellular Ca²⁺ influx through LTCC and evokes emesis (Fig. 3D). Interestingly, Patel and his group [151] have demonstrated that SERCA participates in dopamine release using fast-scan cyclic voltammetry to monitor stimulated extracellular dopamine concentration ([DA]_o) in midbrain slices. These investigators found that SERCA inhibition by cyclopiazonic acid lowers evoked [DA]_o release in the substantia nigra pars compacta, which implies a functional role for ER Ca²⁺ stores in somatodendritic dopamine release. We speculate that SERCA may indirectly contribute to emesis through the release of dopamine. In fact, thapsigargin has been shown to cause vomiting via the activation of Ca²⁺-CaMKII-ERK1/2 cascade, involved in an evoked increase in substance P content in the least shrew brainstem emetic loci, and the evoked emesis occurred through substance P-induced activation of corresponding neurokinin NK₁ receptors [70].

Collectively, these findings suggest a pivotal role for dopamine and dopamine D₂ receptors in cell signaling cascades in both physiological and pathological states. Further knowledge of the pathways activated/deactivated by dopamine would likely contribute to the identification of crucial specific alterations associated with the pathologic conditions and side-effects of dopamine-related treatments. The use of experimental animal models and the design of drugs capable of selectively altering these pathways should contribute to our understanding of the role of signaling cascades in eliciting emesis.

4. Distribution of dopamine and dopamine receptors in the emetic reflex arc

4.1. Emetic reflex: anatomical components and distribution, and potential function of dopamine and dopamine receptors

4.1.1. Dorsal vagal complex

The key known players of the emetic reflex arc include the dorsal vagal complex, the central pattern generator, the enteric nervous system, the gastrointestinal tract, and the vagus nerve (Fig. 1). The dorsal vagal complex comprises the area postrema, the nucleus of the solitary tract, and the dorsal motor nucleus of the vagus in the brainstem of both non-emetic (e.g., mice and rats) as well as vomit-competent species such as humans and animals (e.g., ferrets, house musk, and least shrews). The area postrema, which is pharmacologically defined as the chemoreceptive trigger zone, is a circumventricular organ with both fenestrated capillaries and active transport systems, which allow blood-borne chemicals absorbed by the gastrointestinal tract or secreted from the gastrointestinal mucosa (e.g., substance P) to bypass the brain-blood barrier and stimulate the dorsal vagal complex to induce vomiting [156,157] (Fig. 1). Likewise, the nucleus of the solitary tract contains large numbers of fenestrated capillaries and lacks a brain-blood barrier and thus permit neurons in both areas access to blood-borne circulating elements including brain-blood barrier penetrating emetogens. The area postrema/chemoreceptive trigger zone and nucleus of the solitary tract have high concentrations of emetic receptors for serotonin 5-HT₃, dopamine D_2/3, substance P, neurokinin NK₁, muscarinic, histamine H1, and opioid (μ), among others [7]. The nucleus of the solitary tract receives emesis-related information both from the central nervous system (area postrema and cerebral cortex), as well as the peripheral emetic loci such as the gastrointestinal tract conveyed by vagal afferents and integrate these signals which eventually result in vomiting. Following the integration of emesis associated central and peripheral signals, the nucleus of the solitary tract neurons project the integrated emetic signals to the dorsal motor nucleus of the vagus (Fig. 1). The dorsal motor nucleus of the vagus sends emetic signals via motor vagal efferent pathways to the gastrointestinal tract and modulates vomiting [7,158,159]. It also sends efferent signals to the enteric nervous system as well as the central pattern generator (described below) that coordinate peristaltic activity and its reversal during emesis [7] (Fig. 1).

Dopamine has been consistently found in the area postrema, nucleus of the solitary tract, and dorsal motor nucleus of the vagus nuclei [160, 161]. Although only 10% of neurons in the nucleus of the solitary tract appear to be dopaminergic, the application of dopamine into the nucleus of the solitary tract resulted in 64% of these being stimulated by dopamine [162]. Vagally correlated catecholaminergic neurons in the dorsal motor nucleus of the vagus are also dopaminergic and display tyrosine hydroxylase immunoreactivity. The dorsal vagal complex harbors a high density of emetogenic dopamine D₂-like receptors (dopamine D₂ and D₃), which are substantially represented in the dorsal vagal complex of the mammalian medulla [163]. Most dopamine D₂ receptors are in the intermediate and medial sub-nuclei of the area postrema, nucleus of the solitary tract, and dorsal motor nucleus of the vagus. Conversely, dopamine D₃ receptors are homogenously expressed in area postrema, nucleus of the solitary tract, and dorsal motor nucleus of the vagus. Dopamine D₄ receptors are almost exclusively found in the intermediate and medial sub-nuclei of the nucleus of the solitary tract, and the dorsal motor nucleus of the vagus [163]. Contrary to dopamine D₂-like receptors, D₁-like dopaminergic receptors have low expression across the emetic loci.

4.1.2. Gastrointestinal tract and enteric nervous system

Like the dorsal vagal complex, the gastrointestinal tract is central for the act of vomiting. Enterochromaffin cells residing alongside the epithelium lining the lumen of the digestive tract act as custodian cells in the intestinal mucosa [7] and play a crucial role in gastrointestinal tract regulation, especially intestinal motility and secretion [164] (Fig. 1). Since enteric afferent and efferent nerves do not protrude into the intestinal lumen, enterochromaffin cells act as a form of sensory transduction. Interestingly, the presence of dopamine has been found in gastric mucosa of cats and rabbits [165]. Besides contributing to the vomiting reflex, dopamine modulates several other vital functions in the gastrointestinal tract [166] such as fluid absorption [167], motility [7, 168], blood flow [169], and cytoprotection [170] to name a few. These broad and essential dopamine functions are consistent with mesenteric organs producing nearly half of the dopamine formed in the body [166]. Contrary to some beliefs that suggest tyrosine hydroxylase is found solely in catecholaminergic neurons in the brain, sympathetic nerves, and chromaffin tissue; various molecular biology techniques (RT-PCR, in situ hybridization, and immunohistochemistry) have revealed the presence of dopamine, dopamine receptors, and tyrosine hydroxylase in non-neuronal cells within the basal granulated cells of the mucosal epithelium along the entire extent of the small intestine of gerbils [171, 172], in the denervated rat and human gastric mucosal and parietal cells [173]. Furthermore, the mammalian gut has several populations of amine-containing neurons, which would likely contribute to the control of gastrointestinal motility. These populations include noradrenergic neurons of extrinsic sympathetic origin, which contain dopamine as a metabolic intermediate in the formation of norepinephrine. Although the physiological effects of dopamine in the gut can be mediated through α- or β-adrenoreceptor activation, a dopaminergic receptor-specific response has also been found [174]. In fact, dopaminergic receptors are present in several areas of the gastrointestinal tract [175,176]. Specifically, dopamine D₁-D₅ receptor mRNAs and respective proteins have been observed in rats, mice, and human gastric, duodenal, ileal, and colonic mucosa [173,177,178]. Additionally, dopamine D₁ and D₂ receptors are widely distributed in the cholinergic and catecholaminergic neurons in the dorsal motor nucleus of the vagus of rats, where it has been suggested that they may play a vital role in the regulation of dopamine on the activity of cholinergic and catecholaminergic neurons [179]. This is consistent with the critical role of dorsal motor nucleus of the vagus in the regulation of gastrointestinal tract [180] as the dorsal motor nucleus of the vagus motoneurons project to various parts of the gastrointestinal tract including stomach, lower esophageal sphincter, duodenum, and jejunum. During vomiting, dopamine evokes biphasic effects on the lower esophageal sphincter, with relaxation (a dopamine D₂ receptors-mediated effect) followed by a marked contraction via dopamine D₁ receptors in mammals [181,182]. Intriguingly, transcripts encoding D₄ were confined to the mucosal layer, whereas dopamine D₁, D₃, and D₅ were expressed both in nerve-containing layers of the gut and in the mucosa. The authors argued that transcripts encoding D₄ restriction to the mucosa line with the probability that the enteric D₄ was non-neuronal while both neurons and non-neuronal cells possibly expressed dopamine D₁, D₃, and D₅ in the gut [178]. Recently, dopamine D₄ ligands with improved selectivity for dopamine D₄ receptors against not only dopamine D_1–3,₅ receptors but also other biogenic amine targets have emerged [183]. In fact, the dopamine D₄ receptor is in the spotlight as a novel target for both addiction and Parkinson’s disease, as well as other emerging diseases [183]; however, no direct link to emesis has been found for this receptor yet [14].

Like the dorsal vagal complex and the gastrointestinal tract, the enteric nervous system is pivotal for the act of emesis. Enterochromaffin cells modulate neuron signaling in the enteric nervous system via the secretion of the neurotransmitter serotonin and peptides, which are vital for the act of vomiting [184–186]. Once the transmitters are released, they directly act on the corresponding receptors present in the enteric nervous system plexi since enteric nervous system has the ability to regulate enteric behavior in the absence of CNS input [187]. Distinctively, the enteric plexuses harbor intrinsic primary afferent neurons and interneurons that enable the enteric nervous system to independently mediate integrative responses to local stimuli [188]. The enteric nervous system communicates with the CNS via the parasympathetic (e.g., via the vagus nerve described below) and the sympathetic nervous systems (e.g., via the prevertebral ganglia) [189]. Hence, several of the emetic neurotransmitters secreted by the enteric nervous system are also found in the CNS [190] including 5-HT [191] and substance P [7,192]. There is no direct evidence supporting the existence of dopamine in the enteric nervous system. Although segments of the gastrointestinal tract of mice contains dopamine [193]; it has been difficult to determine whether enteric dopamine is an integral part of intrinsic neurons or because dopamine is the precursor of norepinephrine in the sympathetic innervation [188]. Indirect evidence supporting the existence of enteric dopaminergic neurons in the enteric nervous system are found in: i) pigs [194], ii) the enteric dopamine transporter-immunoreactive nerves [195], iii) transgenic mice lacking dopamine active transporter indicated that the mouse colon may usually contain inhibitory dopaminergic neurons [196], iv) dopamine-containing neurons in the myenteric plexus of the gastrointestinal tract of mice [197], and v) humans ganglion-containing layers of the gut in Parkinson’s disease, which affects dopaminergic neurons [197]. Our laboratory has found that dopamine D₂-like receptor-selective agonists mediated-vomiting increases the expression of c-fos protein in the enteric nervous system of the least shrew [198].

4.1.3. Vagal afferents and efferents

The brain connects with the gut via the vagus nerve. It facilitates autonomic information transfer from the gastrointestinal tract to the CNS and vice versa [199,200] (Fig. 1). Specifically, the vagal afferent nerves relay a considerable amount of sensory information from thoracic and abdominal organs to the CNS circuits regulating gastric function. The cell bodies of origin for this afferent sensory pathway are in the nodosal ganglion near the jugular vein. Independent of their functions or modalities, all vagal afferent neurons use glutamate as the primary neurotransmitter to transfer information to the nucleus of the solitary tract. Neurons of the nucleus of the solitary tract implicated in the gastrointestinal tract regulation, innervate the adjacent dorsal motor nucleus of the vagus. The dorsal motor nucleus of the vagus has pre-ganglionic parasympathetic motoneurons that transmit the precise integrated neuronal response back to the upper gastrointestinal tract via the efferent vagus nerve (reviewed in [159]). Efferent vagus nerve transmits the integrated and coordinated output response to the peripheral organs. Activity within vagal efferent pathways during emetic reflexes results in a large retropulsive wave of intestinal motility accompanied by gastric contraction. It is important to note that nodose ganglionic vagal afferents have dopamine mRNA markers and dopamine D₂ receptors [201]. In the rat nucleus of the solitary tract, the bulk of dopamine D₂ receptors seem to localize post-synaptically with respect to vagal terminals and are located either on ascending glossopharyngeal terminals, descending terminals from higher brain regions, or on neuronal cell bodies within the nucleus of the solitary tract [201]. It is suggested that dopamine D₂ receptors are synthesized in nodose perikarya and axoplasmically transported to the central nerve terminals in the medial sub-nucleus of the nucleus of the solitary tract [201]. There is a close relationship between the state of the peripheral vagus nerve activity and the function of the dopamine system in the brain. In the rat brain structures, chronic impairment of the function of the vagus nerve leads to the inhibition of dopaminergic but not serotoninergic neurons [202].

4.1.4. Vomiting coordinator site: the central pattern generator

Regardless of how emesis is initiated, the motor act of vomiting is coordinated by the central pattern generator previously known as the vomiting center [203] (Fig. 1). The central pattern generator is located in the ventrolateral medulla in the area of the nucleus ambiguous (or retrofacial nucleus in the brainstem), which instigates a coordinated stimulation of appropriate motor nuclei [204–206]. Cerebral, vestibular, area postrema, and gut afferent inputs for nausea and vomiting converge on the nucleus of the solitary tract, which seems to be the logical candidate as a potential final common pathway integrated by central pattern generator for vomiting [207]. This is supported by data showing c-fos immunoreactivity was increased in the central pattern generator in response to pseudo-stimuli induced by vagal afferent stimulation. Interestingly, nodose neurons from the nodose ganglion propel considerably branched afferent fibers in both ascending and descending directions leading to the same neurons innervating a segment within the enteric nervous system and the medulla. In the medulla, branches of vagal afferents innervate the central pattern generator area and possibly the dorsal vagal complex [208,209]. Modulatory inputs are essential components of central pattern generator function: neuromodulators set the parameters of central pattern generator neurons and synapses to render the networks functional. Central pattern generator networks have provided a unique opportunity to study opposing actions of neuromodulators such as dopamine at the level of a single identified neuron or synapse. At several sites, dopamine evokes responses that oppose its overall actions on pyloric neurons and synapses. For example, dopamine hyperpolarizes and silences one neuron, but simultaneously enhances its hyperpolarization-activated inward current, which would depolarize the neuron to resume firing. Dopamine increases spike frequency in several neurons but enhances a slowly activating potassium current that would reduce spike frequency. Dopamine can also activate opposing effects at a single synapse. For instance, it can enhance transmitter release from the pre-synaptic terminal by increasing calcium currents but reduce post-synaptic responsiveness to the released transmitter. How these phenomena play out during emesis are still unexplored [210]. However, it is well known that central pattern generator activities of neural structures produce a complex patterned response, resulting in the action of vomiting [211] (Fig. 1).

Collectively, these findings support the existence of dopamine and dopamine D₂-like receptors throughout the emetic reflex arc, as well as a dopaminergic transmission through brainstem-gastrointestinal-vagal circuits during the process of vomiting.

5. Pharmacological evidence for dopamine-mediated emesis and treatment

Dopaminergic agonists act directly on dopamine receptors mimicking endogenous dopamine [212]. There are two subclasses of dopamine receptors agonists: non-ergoline-derived (non-ergots) and ergoline-(structural skeleton is the alkaloid ergoline; ergots) agonists both of which target dopamine D₂-like receptors (Table 1). In this section, we describe pre- and clinical evidence associated with the activation of emetogenic dopamine D₂-like receptors. We also list dopamine receptor antagonists used with varying degrees of success for suppression of vomiting (Tables 2–4).

Table 1.

Dopamine receptor agonists.

Chemical names, formula, and structure	Affinity	Pre- and clinical utilization	Nausea and vomiting
Non-ergot agonists
Apomorphine  (PubChem CID: 6005)  Trade name: Apokyn  Chemical formula and structure: C17H17NO2	- Ki = 52 and 26 nM for dopamine D2 and D3 receptors, respectively. - Ki = 484 nM for dopamine D1-like receptors [227].	- Used for the treatment of Parkinson’s disease[273] [274], and restless leg syndrome Additional uses are detailed in the text.	- Induces nausea and vomiting.
Quinpirole  (PubChem CID: 54562)  Trade name: Unavailable  Chemical formula and structure: C13H21N3	- Ki = 4.8, ~24, ~30 and 1900 nM at dopamine D2, D3, D4, and D1 receptors, respectively [275].	- Low doses (e.g. 0.2 mg/kg) cause a decrease in offensive behavior without impacting motility or exploratory behaviors. - Higher doses (e.g. 0.8 mg/kg) produce a significant elevation in defense/submission, avoidance/fleeing, and social investigation behaviors [276]. - Repeated treatment re-verses quinpirole-induced sensorimotor gating deficits in rats [277].	- Induces nausea and vomiting.
Non-ergot agonists
Pramipexole  (PubChem CID: 119570)  Trade name: Mirapex  Chemical formula and structure: C10H17N3S	- Ki = 79.5 μM for dopamine D2 receptors [222]. - It has a high selectivity for dopamine D2-like receptors with preferential affinity for dopamine D3 receptor subtype [227]. - Ki = 0.5 nM for dopamine D3 receptors [278].	- Used for the treatment of Parkinson’s disease where new patients are started with pramipexole therapy and L-DOPA is added only as necessary [279]. - Pramipexole decreases leg movements in restless leg syndrome [280].	- Significant incidence of nausea with pramipexole [281].
Rotigotine  (PubChem CID: 59227)  Trade name: Neupro  Chemical formula and structure: C19H25NOS	- Ki = 0.71 nM For dopamine D3 receptor [282]. - Affinities to other dopamine receptors being (Ki in nM): D2 (13.5), D4.2 (3.9), D4.4 (15), D4.7 (5.9), D5 (5.4), and D1 (83) [283].	- Used in the treatment of Parkinson’s disease and restless leg syndrome [284, 285].	- Most frequent (>20%) adverse effects include nausea, and somnolence [286].
Ergot agonists
Bromocriptine  (PubChem CID: 31101)  Trade name: Parlodel  Chemical formula and structure: C32H40BrN5O5	- Ki = 2.96 nM for dopamine D2 - Ki = 5.42 nM for dopamine D3 receptors.	- Used to treat Parkinson’s disease, hyperprolactinaemia, and neuroleptic malignant syndrome.	- Nausea, vomiting, headache, and dizziness [287].
Cabergoline  (PubChem CID: 54746)  Trade name: Dostinex  Chemical formula and structure: C26H37N5 O2	- Ki = 0.95 nM for dopamine D2 receptors. - Ki = 0.79 nM for dopamine D3 receptors. - Ki = 56.2 nM for dopamine D4 receptors. - Ki = 214 nM for dopamine D1 receptors.	- Prescribed for treatment of hyperprolactinemic disorders due to either idiopathic or pituitary adenomas. - Decreases the size and histopathologic grade in endometrial lesions in rats [288]. - Used for motor fluctuations associated with Parkinson’s disease.	- Nausea and vomiting [289].
Pergolide  (PubChem CID: 47811)  Trade name: Permax  Chemical formula and structure: C19H26N2 S	- Ki = 447 nM for dopamine D1/D2 receptors. - Ki = 0.86 nM for dopamine D3 receptor.	- Used for treatment of Parkinson’s disease [290]. FDA withdrew pergolide from the human market due to cardiac complications. - In veterinary, pergolide is the agent of choice for the treatment of common endocrine diseases in older horses and ponies.	- Nausea and vomiting [291].
Partial agonists
Alizapride  (PubChem CID: 135413504)  Trade name: Litican  Chemical formula and structure: C16H21N5O2		- Used for treatment of schizophrenia, bipolar disorder (mania) [292], depression [293], and tic disorders [294].	- Used in the treatment of nausea and vomiting, including postoperative nausea and vomiting.
Aripiprazole:  (PubChem CID: 60795)  Trade names: Abilify  Chemical formula and structure: C23H27Cl2N3O4	- Ki = 0.34 nM for dopamine D2 (partial agonist [295]) - Ki 1.7–5.6 nM for 5- HT1A (partial agonists) - Ki = 0.11–0.36 nM for5-HT2B (inverse agonist) - Ki = 3.4–35 nM 5- HT2A (antagonist).	- Used for bipolar disorder, irritability linked with autism spectrum disorder, schizophrenia, Tourette’s disorder, and as an adjunctive treatmentof major depressive disorder.	- The most frequently cited adverse effects of aripiprazole included nausea and vomiting [296].
Quetiapine  (PubChem CID: 5002)  Trade names: Seroquel  Chemical formula and structure: C21H25N3O2S	- Ki = 428 nM for dopamine D1. - Ki = 626 nM for dopamine D2. - Ki = 38 nM for 5-HT- 2A.	Used to treat schizophrenia or bipolar disorder	- Severe withdrawal syndrome with nausea and vomiting have been reported with quetiapine treatment [297].
Preclamol 3-PPP (N-n-propyl-3-(3-hydroxyphenyl) piperidine)  (PubChem CID: 5311189)  Trade name: Preclamol (INN)  Chemical formula and structure: C14H21NO	- It has substantial affinities for non-dopamine D2 receptors (for example, σ-, α2B- or α2C- adrenoceptors	- It was suggested as a potential clinical utility in the treatment of psychotic disorders, whilst lacking the seriously debilitating motor dysfunctions caused by current neuroleptic therapy. - It was also being investigated for the schizophrenia [298].	- May block vomiting in animal models.
Terguride  (PubChem CID: 443951)  Trade names: Dironyl  Chemical formula and structure:C20H28N4O	Ki = 0.8 nM for dopamine D2L receptors [299].	- Its use in schizophrenia patients is lowered due to its low tolerability [300]. - Co-administration of terguride with L- DOPA may prophylactically decrease L-DOPA-induced abnormal behavioral responses such as dyskinesia [301].	- It may induce nausea and vomiting [302,303].

Table 2.

Major phenothiazine used as antiemetics.

Chemical names, formula, and structure	Affinity	Pre- and clinical utilization	Anti-nausea and vomiting
Chlorpromazine  (PubChem CID: 2726)  Trade name: Thorazine  Chemical formula and structure: C17H19ClN2S	- Ki =0.003 μM for dopamine D2 [304]. - Contrary to most drugs of this class, it has a high affinity for dopamine D1 receptors as well.	- Treatment of schizophrenia [305,306].	- Used against nausea and vomiting [307]. - In veterinary medicine, chlorpromazine is utilized as antiemetic agent and boosting effect of the activity of other drugs including anesthetics and sedatives [308].
Fluphenazine  (PubChem CID: 3372)  Trade name: Prolixin  Chemical formula and structure: C22H26F3N3OS	- Ki = 0.2 and 0.11 nM for dopamine D2 and D3 receptors, respectively.	- Effective treatment for symptoms of schizophrenia [309].	- Fluphenazine has a greater antiemetic potency versus triflupromazine [310].
Metopimazine  (PubChem CID: 26388)  Trade name: Vogalène  Chemical formula and structure: C22H27N3O3S2	- Ki = 0.07 nM for dopamine D2 receptors.	- It can potentially be used as an alternative to metoclopramide and domperidone for the treatment of gastroparesis.	- Often used to treat nausea and vomiting [311].
Perphenazine  (PubChem CID: 4748)  Trade name: Trilafon  Chemical formula and structure: C21H26ClN3OS	- Ki = 3.4 nM for dopamine D2L. - Ki = 0.13 nM for dopamine D3.	- Alleviates psychotic symptoms, such as hallucinations and delusions.	- Also used as an antiemetic, which is effective in the prevention of postoperative nausea and vomiting in children and adults without serious adverse effects compared with placebo [312].
Prochlorperazine  (PubChem CID: 4917)  Trade name: Compazine  Chemical formula and structure: C20H24ClN3S	- Ki = 3.4 nM for dopamine D2L.	- Is a potent typical anti- psychotic used to treat acute and chronic mental illnesses [313]. - Specifically used to treat delusions, hallucinations, agitation, disorganized speech, and behavior [314].	- Utilized in antiemetic treatments.
Promethazine  (PubChem CID: 4927)  Trade names: Phenergan  Chemical formula and structure: C17H20N2S	- Ki = 250 nM for dopamine D2 receptors.	- Is a potent synthetic antihistamine, antipsychotic [315,316].	- Is used alone against nausea and vomiting or with dexamethasone, more effective than metoclopramide/dexamethasone in preventing and reducing nausea, epigastric fullness, and reflux in morbidly obese patients undergoing laparoscopic gastric plication [317].
Thiopropazate  (PubChem CID: 6762)  Trade name: Artalan  Chemical formula and structure: C23H28ClN3O2S		- A potent neuroleptic with antipsychotic properties - Has no anti serotonin and hypotensive action and no antihistaminic properties.	- Has a marked general antiemetic effect.
Trifluoperazine  (PubChem CID: 5566)  Trade name: The brand name Stelazine has been discontinued. This medication is obtainable in generic form only.  Chemical formula and structure: C21H24F3N3S		- Used as an anti- psychotic [318, 319].	- Used as an antiemetic [320].
Triflupromazine  (PubChem CID: 5568)  Trade name: Vesprin  Chemical formula and structure: C18H19F3N2S		- Used to treat psychotic disorders [314].	- Used as an antiemetic [321].

Table 4.

Major benzamides used as antiemetics.

Chemical names	Affinity	Pre- and clinical utilization	Anti-nausea and vomiting
Amisulpride  (PubChem CID: 2159)  Trade name: Solian  Chemical formula and structure: C17H27N3O4S	- Ki = 2.8 and 3.2 nM for dopamine D2 and D3 receptors, respectively [335].	- Used as an antipsychotic, antidepressant and to treat bipolar disorder [293].	- Effective treatment for preventing postoperative nausea and vomiting as well as chemotherapy-induced emesis [336].
Bromopride  (PubChem CID: 2446)  Trade name: Bromoprida  Chemical formula and structure: C14H22BrN3O2	- Ki ~14 nM for dopamine D2 receptors [182].	- Was associated with decreased tensile strength and hydroxyproline concentration in colonic anastomoses in rats three days after surgery [337].	- Used as an antiemetic [338].
Clebopride  (PubChem CID: 2780)  Trade name: Clebopride Malate  Chemical formula and structure: C20H24ClN3O2	- Ki ~ 2 nM for dopamine D2 receptors [182].	- Has prokinetic properties and is used for functional gastro intestinal disorders [339].	- Has antiemetic properties [339].
Itopride  (PubChem CID: 3792)  Trade name: Ganaton  Chemical formula and structure: C20H26N2O4	Ki = 3.7 ± 0.8 μM for dopamine D2S receptor [340].	- Used for treatment of dyspepsia, and gastroparesis [341].	- Used for the treatment of nausea and vomiting [341].
(-) Sulpiride  (PubChem CID: 5355)  Trade name: Abilit  Chemical formula and structure: C15H23N3O4S	- Ki = 38 nM for dopamine D2 receptors [342]. - Levosulpride has a Ki = 27–134 nM for dopamine D2 receptors [182].	- Increases dexamethasone responses during the treatment of drug-resistant and metastatic breast cancer [343]. - Used as anti-depressant [293].	- It prevents chemotherapy- induced and post-operative vomiting. -Also used in the treatment of nausea and vomiting during hepatic, biliary and gastroduodenal disorders, functional dyspepsia, motion sickness, and vertigo [344]. - Levo-sulpiride is a substituted benzamide with antiemetic activity [345].
Raclopride  (PubChem CID: 3033769)  Trade names: Not available  Chemical formula and structure: C15H20Cl2N2O3	Ki = 1.1–13 nM for dopamine D2 receptor antagonists in monkeys [342]. - Ki = 2.3–2.5 for dopamine D2 receptor antagonists in rats [342].	- Used in schizophrenia, [346] senile dementia, or transient psychosis after surgery.	- Used against nausea and vomiting.
Remoxipride  (PubChem CID: 54477)  Trade name: Remoxiprida  Chemical formula and structure: C16H23BrN2O3	- Ki = 113 nM for dopamine D2 receptors [342,347].	- Use was restricted in 1993 due to aplastic anemia in some patients. But it is still an appealing tool for neurochemical and behavioral studies [347].	- Used to relieve severe nausea and vomiting.
Tiapride  (PubChem CID: 5467)  Trade name: Delpral  Chemical formula and structure: C15H24N2O4S	- Ki = 114 nM for dopamine D2 receptors [348]. - Lacks affinity for dopamine D1 and D4 receptors, and histamine H1, adrenergic α1, α2 and serotonergic receptor [349].	- Is an atypical neuroleptic and sedative agent [347]. - Used for the treatment of agitation and aggressiveness in elderly patients [349].	- Has sedative as well as antiemetic properties [348].
Trimethobenzamide  (PubChem CID: 5577)  Trade name: Tigan  Chemical formula and structure: C21H28N2O5	Ki = 640 nM for dopamine D2 in neural membranes.		- Was approved for use in the United States in 1974 and is widely used in therapy of nausea, vomiting caused by gastroenteritis, medications, and other illnesses. -It acts centrally to block dopamine D2 receptors, thereby inhibiting the medullary chemoreceptor trigger zone blocking emetic impulses to the vomiting center [315].

5.1. Preclinical studies revealing the role of dopamine and dopamine receptors in vomiting

Experimental studies investigating dopamine and dopamine receptor agonists sites of action, dose-dependency, binding affinity, and potency across species are described in the subsequent sections.

The discovery of the emetic sites of action of dopamine started nearly seven decades ago with a set of pioneering experiments by Wang and Borison using apomorphine [213]. These investigators posed the possibility that the site of emetic action of apomorphine, orally or intravenously injected, was in the brainstem, because they found the ablation of the area postrema/chemoreceptive trigger zone prevented the induced emesis in dogs, whereas vagotomy and abdominal sympathectomy did not alter the response. These findings were later corroborated by new evidence demonstrating that 50 μg/animal intracerebroventricular (i.c. v) or 0.1 mg/kg intravenous (i.v.) injections of the nonselective dopamine D_2/5-HT₃ receptor antagonist metoclopramide in dogs blocked the vomiting produced by subcutaneously injected apomorphine, while vagotomy in combination with splanchnectomy did not [214]. Another report revealed that the CNS penetrable D₂ receptor preferring antagonist sulpiride was more effective when injected via i.v. versus the i.c.v. route against vomiting caused by i.v. administration of apomorphine. But, when apomorphine was administered via i.c.v., sulpiride was similarly effective in preventing vomiting independent of its route of injection [214]. If the drug does not penetrate or barely crosses the blood-brain barrier, the peripheral effects will prevail and conversely in the case of extensive penetration in the brain. Since dopamine is a polar molecule, it cannot enter or leave the brain easily. Thus, the above discussed results indicate that systemic release of endogenous dopamine stimulates peripheral emetic dopamine D₂ receptors in the area postrema zone on the blood side of the brain-blood barrier, whereas endogenous dopamine within the brain activates emetic dopamine D₂ receptors on the cerebrospinal fluid side of the brain-blood barrier. This concept was further validated by four additional studies showing that: 1) surgical disruption of blood flow in the area postrema zone abolished the emetic response to i.c.v. apomorphine, but only transiently interfered with emesis evoked by i.v. apomorphine [214], 2) the prevention of peripheral conversion of L-DOPA to dopamine by carbidopa (a peripheral decarboxylase inhibitor) simultaneously decreased blood dopamine concentration and emesis despite the increase in brain dopamine levels, but vomiting was often not entirely stopped in dogs [215]. Moreover, in the least shrew carbidopa (0, 10, 20 and 40 mg/kg, i.p.) pretreatment dose-dependently and nearly completely prevented vomiting caused by several different doses of L-DOPA (50, 100 and 150 mg/kg, i.p.). In addition, the peripherally acting dopamine D₂ receptor antagonist domperidone can only partially decrease apomorphine-induced vomiting in dogs in doses up to 40 mg/kg, whereas the CNS permeable dopamine D₂ receptor antagonist risperidone fully prevented the emesis at just 10 mg/kg [216], and finally, 4) peripheral injection of direct-acting and selective dopamine D_2/3 receptor agonists trigger emesis and induces c-fos expression in the nucleus of the solitary tract and dorsal motor nucleus of the vagus but not in the area postrema zone of the least shrew [198]. This is consistent with dopamine causing emesis by acting on dopamine D_2/3 receptors localized to neurons whose dendrites stretch out the nucleus of the solitary tract into the area postrema. Further support for a site of action in the nucleus of the solitary tract comes from studies in ferrets showing that thermocautery lesions of the area postrema abolished the emetic response to loperamide [217]. It has been proposed that loperamide induced emesis by acting on opiate receptors located on the dendrites of neurons projecting from the nucleus of the solitary tract into the area postrema [218, 219].

Dopamine D₂-like receptor agonists evoke vomiting in a dose-dependent manner in emetic-competent species. Indeed, our laboratory has shown that quinpirole dose-dependently increases both the number of shrews vomiting and the mean frequency of emesis which were comparable to that of apomorphine [220]. Dopamine D₃ receptor involvement in vomiting was substantiated by data demonstrating that dopamine D₃-like agonists such as 7-OH-DPAT (7-hydroxy-2-(di-N-propylamino) tetralin) dose-dependently increased the number of shrews vomiting and potentiated the mean vomit frequency [13,220]. Interestingly, combinations of different doses of dopamine D₂ and D₃ preferring antagonists more potently prevented vomiting caused by various dopamine D₂ and D₃ preferring agonists. One of these combinations is U99194A (a dopamine D₃ receptor preferring antagonist) with sulpiride (a dopamine D₂ receptor preferring antagonist) (2 or 4 mg/kg each), which blocked quinpirole-induced vomiting frequency and significantly reduced the number of shrews vomiting, relative to each antagonist tested alone at the same doses. This synergism suggests that quinpirole probably causes emesis via the activation of both dopamine D₂ and D₃ receptor sites [221].

Another important parameter in dopamine D₂-like receptor agonists-mediated vomiting is its binding affinity for the receptor. In preclinical investigations, the inhibition constant (Ki) is the parameter of prime interest. This parameter demonstrates the capacity of a dopamine receptor agonist to displace a selective ligand for a receptor subtype from the receptor; the smaller the value, the higher the affinity of the agonist for the receptor subtype and hence, theoretically, the efficacy of the drug [222]. Dopamine activates dopamine D₁ to D₅ receptors with varying affinities ranging from nanomolar to micromolar range. In general, different subtypes of dopamine receptors vary significantly in their sensitivity to dopamine agonists and antagonists [223,224]. For example, non-ergot agonists such as apomorphine display high affinity for dopaminergic D₂, D₃, D₅, and D₄ receptors and, to a lesser extent, for dopamine D₁ receptor subtypes [216]. Another non-ergot agonist, pramipexole, a dopamine D₃ receptor preferring agonist which binds to these receptors in low doses [225,226] (receptors affinities are summarized in Table 1). Depending on binding conditions, pramipexole affinity to dopamine D₃-versus dopamine D₂-receptors was reported to be 6–95 times higher [225,227]. Ropinirole binding affinity is similar to that of pramipexole, except that it does not have an affinity for α₂-adrenergic receptors [228] (Table 1). The ergot agonists cabergoline has the highest affinities for dopamine D₂ receptor (Ki = 0.61 nM) [222] (Table 1) while pergolide has a high affinity for the dopamine D₂ receptors and moderate affinity for the dopamine D₁ receptors [229] (Table 1). Affinities of cabergoline (Ki = 1.27 nM), lisuride (Ki = 1.08 nM), and pergolide (Ki = 0.86 nM) for the dopamine D₃ receptor subtype were comparable to that of pramipexole with negligible affinity for dopamine D1 receptor family or 5-HT receptors [222]. It has been suggested that the chemical structure of dopamine agonists may account for their greater selectivity for dopamine D₂-like family of receptors [230].

Dopamine receptor agonist potency has also been determined in several species. For example, apomorphine’s (a non-selective dopamine receptor agonist) emetic potency varies across species with the following ED₅₀ potency order: dog > human > ferret, cat = pigeon. Also, 7-(OH) DPAT (a dopamine D₃ receptor preferring agonist) is a more potent emetogenic agent in dogs [231] and ferrets [14], when compared to the least shrew [220]. Based on the results obtained from the three species, the ED₅₀ potency order is: 7-(OH) DPAT < apomorphine < quinpirole. On the other hand, it has been well-established that agonists of dopamine D₁-like and D₄ receptors have no emetic effect in ferrets and dogs. Dopamine D₁/D₅ agonists like SKF1297 and SKF38393 do not induce emesis in ferrets [13], dogs [231], or least shrews [220]. Moreover, their corresponding antagonists do not block apomorphine-evoked vomiting [231].

In sum, findings from animal models have established that dopamine D₂ and/or D₃ receptors are important players in dopamine-induced emesis in vomit-competent species. In contrast, dopamine D₁-like and D₄ receptors are devoid of emetic response. Furthermore, the specificity of the site of the antiemetic action of an antagonist on peripheral or central dopamine D₂/D₃ receptors depends primarily on its pharmacokinetic properties.

5.2. Clinical applications of dopamine receptor agonists and consequent emesis

The dopaminergic system has been implicated in Parkinson’s disease, drug addiction, depression, psychotic depression, ADHD, bipolar disorder, and schizophrenia. Some of the conditions are dominantly marked by low dopamine levels (hypodopaminergia) like in Parkinson’s disease, pituitary tumors (prolactinomas), and restless leg syndrome [232], whereas others exhibit increased dopamine levels (hyperdopaminergia) as in schizophrenia [233]. However, many dopamine-related diseases are complex and feature both ends of the spectrum, frequently associated with pathogenetic implications of other neurotransmitters. The dopaminergic system is considered a major target for drug design applied in the treatment of neurological diseases. The sections below and Table 1 discuss established agonists and new therapies for patients with neurological diseases.

5.2.1. Dopamine and dopamine receptor agonists use for the treatments of neuropsychiatric conditions

The discovery of dopamine and the account of an analytical method for its measurement were trailed by a study of markedly depleted levels of dopamine in the basal ganglia of individuals dying from Parkinson’s disease [234 ]. Unlike dopamine, L-DOPA, a naturally occurring amino acid converted into dopamine in the brain and periphery is used clinically in the management of Parkinson’s disease as it crosses the brain-blood barrier. One of the most common issues in patients treated with L-DOPA is the delayed onset of response following the ingestion of the drug. Indeed, a delay in reaching an “on” state or a full response failure accounted for more than 60% of daily “off” time (no active drug given) among 327 patients with advanced Parkinson’s disease [235]. Moreover, L-DOPA therapy is frequently associated with motor complications leading to the addition of carbidopa, a peripheral decarboxylase inhibitor, to enhance the therapeutic benefit of this indirect-acting dopamine receptor agonist. Therefore, several psychotropic non-ergot and ergot direct-acting agonists were introduced as an adjunct to L-DOPA treatment in patients exhibiting fluctuating motor responses and dyskinesias associated with its chronic use [236,237]. The addition of agonists to these patients’ regimes allows around a 20%–30% reduction in the dose of L-DOPA in practice and leads to improvement in the disabling complications. Dopamine agonists have also been successfully used as monotherapy in de novo patients with the intention of delaying treatment with L-DOPA and subsequently deferring the onset of complications [238]. Apomorphine for example is widely accepted as a suitable therapeutic alternative for Parkinson’s disease [239]. It has also been applied in a variety of other treatments ranging from addiction (i.e., to heroin, alcohol, or cigarettes) to erectile dysfunction [240]. A potential important neuroprotective role for apomorphine in the treatment of Alzheimer’s disease has also been implied since this drug lowers neural cell death [240]. In the 1990s, two other non-ergot dopamine receptors agonists, pramipexole and ropinirole (description in Table 1), were approved in the United States for use in Parkinson’ s disease and restless leg syndrome patients [222]. These drugs have been favored by many clinicians for various reasons, involving a more stable motor response, an improved side-effect profile, and a more desirable dosing schedule [240]. Notably, the early oral ergot derivatives of dopamine agonists encompassing bromocriptine, pergolide, lisuride, and the long-acting cabergoline (described in Table 1) [ 212], are among the most used clinically drugs nowadays. Moreover, bromocriptine is used as an adjunct to L-DOPA therapy in patients experiencing a worsening response to L-DOPA or who suffer fluctuations in response to the drug. Other patients who may benefit from bromocriptine therapy include those with limited clinical response to L-DOPA due to an inability to tolerate higher doses.

5.2.2. Dopamine D_2/3 receptor agonist-mediated emesis

Despite the beneficial actions of dopamine receptor agonists in patients with CNS disorders, nausea and vomiting are subsequent debilitating side-effects that cannot be ignored. Chiefly, emesis may be due to the direct action of dopaminergic D_2/3 receptor agonists or neuroleptic withdrawal syndrome [241] in patients. As detailed in the previous section, L-DOPA-mediated vomiting can be explained by its ability to have both central and peripheral effects. Usually, L-DOPA is combined with carbidopa to prevent it from being converted into dopamine pre-maturely in the bloodstream, allowing more of it to get into the brain. Dopaminergic agonists including non-ergot derivatives (e.g., apomorphine, piribedil, quinagolide, ropinirole, and pramipexole), ergot derivatives (e.g., bromocriptine, lisuride, pergolide, and cabergoline), or other drugs used to treat neurological disorders can also cross the brain-blood barrier and evoke emesis. This is reflected in a meta-analysis of randomized controlled trials evaluating the efficacy of long-acting non-ergot dopamine agonists vs. placebo in Parkinson’s disease. The results demonstrated that the long-acting non-ergot agonists were associated with a higher risk of nausea and vomiting [242]. One systematic review using PUBMED, EMBASE, and clinical trial databases to find placebo-controlled clinical trials of non-ergot ropinirole for restless leg syndrome revealed that ropinirole use led to a higher risk of nausea and vomiting at nearly 50% of all adverse events reported [243]. The authors of the study suggested that as restless leg syndrome was more widely recognized and treated; the prevalence of ropinirole-induced nausea and vomiting could grow substantially, and thus this drug use should be considered as a cause of chronic nausea and vomiting. As with many non-ergot dopamine agonists (Table 1), apomorphine’s common initial side-effects are nausea and vomiting [244]. Indeed, in placebo-controlled trials used for US registration, nausea occurred in almost a third (31%) of patients and led to treatment discontinuation in 3% of patients. Vomiting was reported in 11% of patients and led to discontinuation in 2% of patients [245–247]. Notably, nausea and vomiting are more frequently found in intermittent injections than with continuous infusion therapy in apomorphine-treated patients [248]. Perhaps this could be attributed to a down-regulation of medullary dopamine receptor sensitivity (i.e., tolerance) with the continuous dopaminergic stimulation. The peripheral dopaminergic effects of apomorphine are usually well controlled by the co-administration of an appropriate antiemetic until tolerance emerges (typically 3–6 weeks after initiation of apomorphine therapy). Although, patients who inject apomorphine less than 4 times per day are plausible exceptions to this rule since they have been shown to be more prone to nausea and might require antiemetic prophylaxis for a longer period [234]. Emesis has also been reported during the utilization of ergot-derived dopamine agonists such as bromocriptine. Moreover, clinical research indicates that the partial agonist aripiprazole (described in Table 1) can cause nausea or vomiting in some patients.

Despite the benefits of conventional agonists, there has been a significant decrease in the development of new psychiatric drugs around the world, because of the multiple etiologies related to these diseases. New promising therapies based on biased GPCRs signaling being developed and are described in the following section.

5.2.3. Potential use of biased GPCRs signaling for the treatment of neurological disorders

Remarkable advances have been made in signaling cascades that are linked with the risks of neurological diseases. Normally, activation of GPCRs engages broad networks of signaling pathways, which for most receptors are mediated by both G-proteins and β-Arrs. Distinctive biological responses are frequently associated with these diverse pathways. D₂-like receptor agonists such as those used to treat schizophrenia and the receptor antagonists activate or prevent the whole signaling network of the receptor. In contrast, “biased” receptor agonists selectively involve some signaling pathways but evade, or even deactivate, other signals mediated by the same receptor. By stabilizing a particular receptor conformation(s) that preferentially stimulate one specific signaling pathway, these biased receptor agonists allow a more targeted modulation of cell function and treatment of disease [249]. Biased ligands selectivity is expected to control biological functions of GPCRs and Arrs in a more precise way, thus yielding new drug molecules with superior efficacy and/or reduced side-effects [250]. Indeed, the introduction of biased receptor agonists has led to the finding of encouraging drug candidates in clinical development like the G-protein-biased μ opioid receptor biased agonist oliceridine (also known as TRV130). Oliceridine causes robust G-protein signaling, with strength and efficacy comparable to morphine, but with far less β-Arr₂ recruitment and receptor internalization. It also demonstrates fewer undesirable effects than morphine. Biased dopamine D₂ receptor-βArr₂ ligands, UNC9975, UN0006, and UNC9994 have been discovered during the exploration of multiple regions of ariprazole [251] (described in Table 1). It was found depending on the cell type, ariprazole (Table 1) may act as a partial receptor agonist at dopamine D₂-Gi/o, and/or a dopamine D₂ receptor-β-Arr₂ pathway [252]. However, these compounds have dopamine D₂ receptor-β-Arr₂ agonistic/antagonistic activity in various cellular assays but show minimal dopamine D₂ receptor-Gi/o agonist activity [251,253]. They also have inhibitory effects on amphetamine-induced hyperlocomotion via the β-Arr₂-mediated pathway with less motoric side-effect in mice. Thus, perhaps ariprazole-derived compounds could spare nausea and vomiting symptoms, compared to their parent drug aripiprazole. However, this will require further investigation.

5.3. Antiemetic use of dopamine receptor antagonists

Emesis can lead to several complications (e.g., water and electrolytes loss and tear of esophageal mucosa) especially if it recurs over a period of hours or days. This prompted a rationale for the development of antiemetic drugs that can prevent the binding of dopamine to dopamine D_2/3 receptor subtypes [254]. Based on the initial findings in animal models, clinical studies have further refined the effectiveness of dopamine receptor-based antiemetics. The dopamine receptor antiemetics prevent vomiting by blocking either the area postrema / chemoreceptive trigger zone or peripheral stimulation of the dorsal vagal complex. Although serotonin 5-HT₃R antagonists became the drug of choice to treat chemotherapy-induced emesis, dopamine receptor antagonists especially dopamine D₂-like receptors antagonists remain effective drugs of choice in other emetic conditions associated with uremia, radiation sickness, and viral gastroenteritis. Contrary to dopamine D₂ receptor antagonism, partial dopamine D₂ receptor agonism (Table 1) is thought to concurrently decrease dopamine activity in overactive dopamine systems and increase its activity in parts of the brain where dopaminergic activity is inadequate. There are three classes of commonly prescribed dopamine receptor antagonists:

5.3.1. Phenothiazines

Phenothiazines are organic compounds related to the thiazine-class of heterocyclic compounds. Phenothiazines are grouped into three groups based on the substituents on nitrogen: (i) aliphatic analogs, which harbor acyclic groups; (ii) piperidines, which contain piperidine-derived groups; (iii) piperazine, which contains piperazine-derived substituents [255,256]. The first neuroleptic phenothiazine prototype; chlorpromazine was discovered in 1950, had many pharmacological effects including; anti-dopaminergic, antihistaminic, antiserotonin, anti-muscarinic, and antiemetic [257,258]. Since the discovery of the importance of chlorpromazine in the treatment of vomiting, numerous phenothiazine derivatives have been tested in search of a compound with a more increased antiemetic potency (Table 2). Currently, phenothiazines are considered as general purpose antiemetics and used against vomiting evoked by uremia, radiation-sickness, viral gastroenteritis, postoperative nausea and vomiting as well as prophylaxis in patients receiving chemotherapy with low emetogenic potential or as salvage antiemetic in patients who have breakthrough emesis. The most significant antiemetic phenothiazines together with their properties are illustrated in Table 2.

5.3.2. Butyrophenones

These compounds have a functional ketone group and are often clinically used to treat various psychiatric diseases. They may be utilized for the prevention of post-operative nausea and vomiting as well. The dual use of butyrophenones is exemplified by neuroleptic drugs like haloperidol, the most widely used classical antipsychotic drug in this class, is a key tranquilizing drug, with substantial antiemetic activity due to its dopamine D₂ receptor antagonist properties [259]. Unfortunately, side-effects [260] such as drowsiness, dysphoria, delayed discharge, extrapyramidal reactions, restlessness, and anxiety after discharge have led to the refusal to use these drugs in the outpatient setting. Another dopamine D₂ receptor antagonist of this class is domperidone that is utilized in functional dyspepsia [261,262], and as a prokinetic and as an antiemetic agent [263]. Unlike haloperidol, domperidone does not produce any adverse neurological symptoms since it has minimal penetration through the brain-blood barrier. Thus, domperidone offers an excellent safety profile for long-term administration orally in the suggested doses [263]. Structures and properties of the commonly used antiemetic butyrophenones are depicted in Table 3.

Table 3.

Major butyrophenones used as antiemetics.

Chemical names, formula, and structure	Affinity	Pre- and clinical utilization	Anti-nausea and vomiting
Benperidol  (PubChem CID: 16363)  Trade name: Anquil  Chemical formula and structure: C22H24FN3O2	- Ki = 0.027 nM for dopamine D2 receptors [322].	- Has highest neuroleptic potency in terms of dopamine D2 receptor blockade [323]. - Best candidate drug interacting with various target proteins participating in Alzheimer’s disease [324].	- Used for the treatment of nausea and vomiting.
Domperidone  (PubChem CID: 3151)  Trade name: Motilium  Chemical formula and structure: C22H24ClN5O2	Ki = 0.3–3.4 nM for dopamine D2-like receptors [182].	- Utilized in functional dyspepsia in adults and children [261].	- Used to treat nausea and vomiting [263].
Droperidol  (PubChem CID: 3168)  Trade name: Inapsin  Chemical formula and structure: C22H22FN3O2	- Ki = 0.25 nM for dopamine D2 receptors [322] with some histamine and serotonin antagonism [325].	- Used in patients with acute psychotic episodes, especially those with agitation or violent behavior [326].	- Is a short-acting neuroleptic drug with enhanced antiemetic effects [293,327].
Haloperidol  (PubChem CID: 11495267)  Trade name: Haldol  Chemical formula and structure: C21H23ClFNO2	- Ki = 2 nM for dopamine D2 receptors. - Ki = 4 nM for dopamine D3 receptors [322].	- Used mainly in the treatment of psychotic conditions [328].	- Used postoperatively and in as antiemetic, with minimal toxicity [329,330].
Melperone  (PubChem CID: 15387)  Trade name: Flubuperone  Chemical formula and structure: C16H22FNO	- Ki = 180 nM for dopamine D2 receptors. -It also has antagonistic activity at 5HT2A receptors [331].	- It has been tried in treatment-resistant cases of schizophrenia with some limited success [332,333]. - It also possesses anxiolytic properties [334].	- Its reactions include nausea and vomiting.

5.3.3. Benzamides

Benzamides are amide of benzoic acid or any of its byproducts; some of its derivatives are pharmaceuticals. Although nowadays benzamides have largely been replaced by 5-HT₃-antagonists, they are still used in post-chemotherapy nausea and vomiting as an adjunct agent for delayed emesis. Benzamide analogs screening revealed that metoclopramide is one of the most effective drug in blocking vomiting [264]. Metoclopramide has been used clinically for the suppression of moderate-to-severe chemotherapy nausea and vomiting. High doses of metoclopramide have been combined with dexamethasone as antiemetic for highly emetogenic chemotherapy. It is also an advantageous antiemetic for Parkinson’s disease patients for whom it was used to prevent L-DOPA-induced emesis [7]. Metoclopramide exerts its antiemetic activity on central [265,266], and peripheral systems [267]. Besides its ability to block dopaminergic receptors, metoclopramide blocks serotonin 5-HT₃ receptors, increases lower esophageal sphincter tone, and enhances gastric and small bowel motility [268]. Another important active substance from this group of benzamides is sulpiride, which is utilized not only as an antiemetic but also as antipsychotic agent. Sulpiride acts primarily as a dopamine D₂-preferring antagonist and lacks effects on norepinephrine, acetylcholine, serotonin, histamine, or gamma-aminobutyric acid (GABA) receptors. The low risk of unfavorable side-effects like extrapyramidal symptoms and torsade de pointes, has restored interest in dopamine D₂ receptor class antagonists [269]. Amisulpride is another such agent that has high and selective affinity for both dopamine D₂ and D₃ receptors and shows evidence of dose-dependent activity at preventing acute chemotherapy nausea and vomiting, especially, nausea [270]. This is consistent with results obtained in a recent randomized trial reporting that 17% of patients treated with amisulpride alone had no emesis and 67% had insignificant nausea [271]. When amisulpride was combined with ondansetron (5-HT₃-receptor antagonist), the efficacy among 83% of the patients treated was increased, relative to ondansetron treatment alone. Since drugs such as 5-HT₃ or NK₁R antagonists are less effective against nausea, the use of amisulpride may prove to be more appropriate. More on common anti-emetic benzamides and their properties are shown in Table 4.

Collectively, these findings underline the availability of several dopamine D₂ receptor antagonists with versatile pharmacological activities as they can be both antiemetic and antipsychotic.

6. Conclusion

Dopamine is involved in a variety of critical physiological functions and as such, it is not surprising that several human disorders have been associated with dopaminergic dysfunctions. Agents that activate the dopamine D₂-like family of receptors elicit vomiting in humans and other vomit competent species. Dopamine D₂-like receptor antagonists are the oldest and most frequently utilized antiemetic agents in chemotherapy-induced nausea and vomiting, especially when serotonin antagonists have failed. However, dopamine antagonists tend to be at best intermediate in antiemetic effectiveness. Some progress has been made in revealing the intracellular signaling pathways that underlie the actions of dopamine accounting for its effect on vomiting. In particular, preliminary findings from our laboratory have pointed to potential development of new potent broad-spectrum classes of antiemetics targeting PI3K intracellular signals and their interaction with the downstream Akt/GSK-3 signaling [93,272]. Based upon these results, potential antiemetic drugs could be developed clinically. Together with partial agonists such as ariprazole, these novel antiemetic targets may contribute to the development of antidopaminergic agents that will provide selective, long-lasting effects, with an optimal safety profile.

Abbreviations:

AADC

aromatic L-amino acid decarboxylase

Akt/PKB

thymoma viral proto-oncogene

ALDH

aldehyde dehydrogenase

Arr

Arrestin

Ca²⁺

calcium

CaMKII

Ca²⁺/calmoduli n-dependent kinase II

cAMP

cyclic adenosine monophosphate

CNS

central nervous system

CNTF

ciliary neurotrophic factor

COMT

catechol-o-methyltransferase

DAG

1,2-diacylglycerol

DARPP-32

dopamine and cyclic AMP-regulated phosphoprotein, 32 kDa

DOPAC

3,4-dihydroxyphenylacetic acid

ER

endoplasmic reticulum

ERK_1/2 or p44^ERK and p42^ERK

extracellular signal-regulated kinase 1 and 2

GABA

gamma aminobutyric acid

GPCRs

G protein-coupled receptors

GRKs

G-protein-coupled receptor kinases

GSK-3

glycogen synthase kinase-3

HIAA

hydroxy indole acetic acid

5-HT₃R

5-hydroxytryptamine 3 receptor

HVA

homovanillic acid

IP₃

inositol triphosphate

IP₃R

IP₃ receptor

L-DOPA

L-3,4-dihydroxyphenylalanine

LTCC

L-type calcium channel

MAO

monoamine oxidase

MAPK

mitogen-activated protein kinase

MEK

mitogen-activated protein kinase (also known as MAP2K, MEK, MAPKK)

mTOR

mammalian target of rapamycin

NE

norepinephrine

NK₁R

neurokinin type 1 receptor

6-OHDA

6-hydroxydopamine

PDGF

platelet-derived growth factor

PDK

phosphoinositide-dependent kinase

PI3K

phosphatidylinositol 3-kinase or phosphoinositide 3 kinase

PIP2

phosphatidylinositol 4, 5-diphosphate

PIP₃

phosphatidylinositol 3,4,5-trisphosphate

PKA

protein kinase A

PKC

protein kinase C

PL

Phospholipase

PLCβ

phospholipase C isoform β

PMNT

phenylethanolamine N-methyltransferase

PP1, PP2A, or PP2B

protein phosphatase 1, 2A or 2B

Raf

rapidly accelerated fibrosarcoma

Ras

retrovirus-associated DNA sequences

ROS

reactive oxygen species

RyR

ryanodine receptors

SER

sarcoplasmic and endoplasmic reticulum

SERCA

sarcoplasmic and endoplasmic reticulum Ca²⁺ ATPase

Th³⁴

phosphorylation of protein at threonine 34 site

TKRs

tyrosine kinase receptors

TRVP1

transient receptor potential vanilloid type 1