Capillary Endothelial Na⁺, K⁺, ATPase Transporter Homeostasis and a New Theory for Migraine Pathophysiology

Abstract

Background.—

Cerebrospinal fluid sodium concentration ([Na⁺]_csf) increases during migraine, but the cause of the increase is not known.

Objective.—

Analyze biochemical pathways that influence [Na⁺]_csf to identify mechanisms that are consistent with migraine.

Method.—

We reviewed sodium physiology and biochemistry publications for links to migraine and pain.

Results.—

Increased capillary endothelial cell (CEC) Na⁺, K⁺, -ATPase transporter (NKAT) activity is probably the primary cause of increased [Na⁺]_csf. Physiological fluctuations of all NKAT regulators in blood, many known to be involved in migraine, are monitored by receptors on the luminal wall of brain CECs; signals are then transduced to their abluminal NKATs that alter brain extracellular sodium ([Na⁺]_e) and potassium ([K⁺]_e).

Conclusions.—

We propose a theoretical mechanism for aura and migraine when NKAT activity shifts outside normal limits: (1) CEC NKAT activity below a lower limit increases [K⁺]_e, facilitates cortical spreading depression, and causes aura; (2) CEC NKAT activity above an upper limit elevates [Na⁺]_e, increases neuronal excitability, and causes migraine; (3) migraine-without-aura may arise from CEC NKAT over-activity without requiring a prior decrease in activity and its consequent spreading depression; (4) migraine triggers disturb, and treatments improve, CEC NKAT homeostasis; (5) CEC NKAT-induced regulation of neural and vasomotor excitability coordinates vascular and neuronal activities, and includes occasional pathology from CEC NKAT-induced apoptosis or cerebral infarction.

A major problem in migraine pathophysiology is to understand the basis of symptoms. Migraine affects peripheral and central trigeminovascular pathways and central sensitization appears responsible for allodynia;^1–10 cortical spreading depression (CSD) is associated with migraine aura;^11–13 CSD can activate trigeminovascular pathways;¹³ and a large variety of medications have benefit in treating acute (and, less effectively, chronic) migraine.^14–19 These findings delineate some of the anatomy, physiology, biochemistry, and pharmacology of migraine, but it is not clear what happens in neurons that causes CSD/aura or migraine.

The reason for a decreased CSD threshold among migraineurs is not known, though electrolyte changes during CSD include acute changes in [K⁺]_e and [Na⁺]_e.^20,21 Some insight comes from mutations in 3 different genes identified in the rare familial hemiplegic migraine (FHM):^22–26 CACNA1A (gain of function of a slow Ca²⁺ channel gene), ATP1A2 (loss of function of the α-2 isoform of the Na⁺, K⁺ -ATPase transporter [NKAT] gene), and SCN1A (gain of function of a voltage-gated Na⁺ channel gene). Elevation of extracellular glutamate and/or potassium has been suggested as a common mechanism for how 2 of these distinct genetic loci predispose a person to CSD.²⁷ Alternatively, since these 3 genes have not been found mutated in the common forms of migraine to date,²⁸ and since their dysfunction will influence potassium/sodium homeostasis, we propose that rather than the FHM mutations themselves it is their impact on potassium/sodium homeostasis that may reveal the common link in causing their migraine phenotypes.

We are investigating whether episodically increased neuronal excitability in migraine arises from a disturbance of brain sodium homeostasis, since we had found that [Na⁺]_csf was altered in migraine, whereas Ca²⁺, K⁺, and Mg²⁺ were not.²⁹ [Na⁺]_csf rose significantly during the peak of migraine compared with the non-headache state of migraineurs and controls. This increase is found only in the cerebrospinal fluid (CSF) and not the plasma samples of migraineurs, which argues for a brain source and against a systemic origin. Radioactive Na⁺ distribution studies reveal that though the CSF Na⁺ composition is modified at different points along the neuraxis,^30,31 it is reasonable to assume the [Na⁺]_csf reflects [Na⁺]_e, since equilibration of [Na⁺]_e with lumbar CSF occurs rapidly,^30,32,33 especially in ambulant people. We expect [Na⁺]_e may be increased even more in specific brain regions in migraine, since the volume of CSF and its dispersion to the lumbar site of collection may have diluted the values we measured.

Elevated [Na⁺]_e is important because it can lead to significant physiological effects by increasing neuronal excitability: Hodgkin and Katz³⁴ demonstrated that the action potential rose at a rate roughly proportional to the rise of [Na⁺]_e. When a neuron is at rest, the Na⁺ influx through voltage-gated Na⁺ channels is low, as these channels are usually closed or inactivated. However, the channel gate is displaced when [Na⁺]_e increases.³⁵ Higher [Na⁺]_e speeded recovery from the inactivation state, enabling an earlier action potential and leading to hyperexcitability.³⁵ Higher [Na⁺]_csf caused a sympathetic hyperactivity response (increasing blood pressure and heart rate) through increasing ouabain-like substances and activating the brain renin-angiotensin-aldosterone system.^36,37

We suggest that brain potassium/sodium homeostasis is disturbed in migraine because: (1) [K⁺]_e is increased and [Na⁺]_e is reduced during CSD/aura; (2) all 3 mutations in FHM affect potassium/sodium regulation; (3) [Na⁺]_csf is increased in migraine; and (4) higher [Na⁺]_e increases neuronal excitability. We derive a theory of migraine pathophysiology that may explain migraine symptoms resulting from a compromise in brain potassium/sodium homeostasis.

INCREASED CEC NKAT ACTIVITY IS THE LIKELY MECHANISM FOR INCREASED [Na⁺]_csf

Routes of sodium transport across the cellular membrane include passage through voltage- or ligand-gated sodium channels, as well as by means of sodium transporters. We review herein the biochemistry of these 3 portals for sodium flux, and deduce that increased CEC NKAT activity is the likely cause of the increase in [Na⁺]_csf. Review of NKAT regulators reveals that most have been implicated in migraine.

Voltage-Gated Sodium Channels.—

While there are 9 different types of voltage-gated sodium channels, only 5 exist in the central nervous system: Na_V1.1, Na_V1.2, Na_V1.3, Na_V1.6, and Na_V1.7.³⁸ Na_V 1.1 function is relevant in migraine since SCN1A mutations have been reported in FHM, as described above. Na_V1.3 has been implicated in the long-term effects of spinal cord injury, which leads to altered regulation of the Na_V1.3 channel, resulting in hyperexcitability and central neuropathic pain.³⁹ Similarly, alteration of sodium channels may be connected to the pain associated with migraine headaches.

Ligand-Gated Sodium Permeability.—

Ionotropic receptors are a group of transmembrane ion channels that are regulated by neurotransmitters.The ion channels are selective to one or more ions, including Na⁺, K⁺, Ca²⁺, or Cl⁻, so they are also responsible for sodium influx. This category includes glutamate (AMPA, kainate, and N-methyl-D-aspartic acid [NMDA]) and 5-HT3 receptors that allow sodium influx on receptor binding.

Sodium Transporters.—

These include (direction of ion flux in relation to the cell):

• The NKAT (3Na⁺ out, 2K⁺ in).

• The Na⁺/Ca²⁺ exchangers⁴⁰ (sodium and calcium exchangers [NCXs] and NCKXs) (3–4 Na⁺, 1Ca²⁺, +/− 1K⁺, either in or out).

• The Na⁺/glutamate symporter (3Na⁺ and 1 glutamate, in).

• The Na⁺/H⁺ antiporter (1Na⁺, 1H⁺, either in or out).

Clearly, all 4 transporter groups could be involved in altering the [Na⁺]_e, either by gain or loss of functions.

We can interpret an increase in [Na⁺]_e during headache in 2 ways:

The first interpretation is that impairment of voltage- or ligand-gated sodium channels in migraineurs, or reduction in their density, would decrease sodium influx. Less sodium entering the cell would result in increased [Na⁺]_e during migraine relative to the non-headache state and to controls. Since action potentials do not substantially change external ion concentrations, decreased function of voltage- or ligand-gated sodium channels will not cause the observed 4 mM increase in [Na⁺]_csf, let alone higher values that may occur at local brain regions. Glutamate is known to increase in CSF in migraine⁴¹ and a glutamate-gated sodium influx would decrease [Na⁺]_e, the opposite effect to that observed during migraine. Perhaps more importantly, while reduced ion flux through the Na⁺ channels might elevate [Na⁺]_e, the increased neuronal excitability in migraine requires more, rather than less, ion flux through the channels. Thus, we exclude sodium channel block from causing increased [Na⁺]_csf and [Na⁺]_e during migraine.

The second interpretation is that a mechanism exists in migraineurs that causes excess sodium to be pumped into the extracellular space. This could result from sustained over-activation of sodium transporters, which are the only proteins capable of pumping sodium against its concentration and electrochemical gradients. The NKAT is the main exporter of Na⁺ and is reported to consume almost half of the brain’s energy.^42,43 The other transporters consume much less energy and are therefore more likely involved in smaller modulations of Na⁺. For example, we suggest that potassium-dependent sodium and calcium exchanger (NCKX) has a role in CSD: the greater rise of [K⁺]_e in ouabain-induced CSD in Ca²⁺ free solution⁴⁴ is most likely from absence of the reverse mode NCKX that would export Na⁺ from the cell and import K⁺ and Ca²⁺ to the cell, if Ca²⁺ had not been removed.

Increased NKAT activity not only increases [Na⁺]_e, but also decreases [K⁺]_e. Thus, if NKAT over-activation contributes to the increased [Na⁺]_e observed during migraine headache, one might expect an associated decrease in [K⁺]_e. Our CSF data did not reveal a significant decrease in [K⁺]_csf, though the overall [Na⁺]_csf : [K⁺]_csf ratio increased.²⁹ We suggest that, during increased NKAT activity, the decrease in [K⁺]_e/csf has been minimized by the strong glial regulation system for [K⁺].⁴⁵

The magnitude of the NKAT role in the brain^42,43 suggests that its increased activity is the most likely cause of the increased [Na⁺]_csf and [Na⁺]_e in migraine, though further studies of [Na⁺]_e and [Na⁺]_csf are required. If NKAT is involved in migraine pathophysiology, then the structural, functional, regional, and cellular heterogeneity of its 3 α-chain subunits, 3 β-chain subunits, and 7 γ-chain subunits is important.⁴⁶ These are summarized in Table 1.

Table 1.—

The Cellular Locations, Effects of Gene Knockout, Known Human Mutations, and Functions of the 3 α, 3 β, and 5 γ Isoforms of Nkat That Are Found in the Brain

Isoform	Brain/cell location multiple isoforms in same cell	Effect from gene knockout	Human mutations	Function	Refs
α1	Neurons and glia	Homozygous die at blastocyst. Heterozygous normal		Catalytic, ion transport, phos-phorylation, and ATP binding. They differ in Na+, K+, and ATP binding, ouabain and Ca2+ inhibition. Eg, ouabain and ATP affinities are lowest for α1, highest for α2 and 3	106–111
α2	Glial cells mainly, occasional neurons	Homozygous die at birth: ↓ breathing. Heterozygous have ↑ anxiety and ↓ long-term memory	FHM-2		106–109,111
α3	Neurons	Homozygous die at birth. Heterozygous ↓ non-spatial learning and long-term memory	Rapid onset dystonia parkinsonism		106–111
β1	Mainly neurons	Not known		Membrane insertion, protein:protein interactions, tight junctions, cell polarity, and signaling	112,113
β2	Mainly glia	Deficient neural cell migration, die 15 d			112–114
β3	Oligodendroglial cells and optic nerve	Not known			115
FXYD1	Combines with α1–3 in cerebellum. Most abundant in Purkinje cells, and choroid plexus and ependymal cells. In neurons and glia. Small amount in Allan Atlas rat brain*.	↑ NKAT activity in heart		↓ Affinity of NKAT for Na+ and K+	46, 116–118
FXYD2	Moderate amount in Allan Atlas rat brain*.	↑ affinity of NKAT for Na+.	Renal hypo-magnesemia	↓ Affinity of NKAT for Na+	46,119–121
FXYD3	None in Allan Atlas rat brain*.	Not done		↓ Affinity of NKAT for Na+/K+	46
FXYD4	None in Allan Atlas rat brain*.	Not known in brain ↓ Na+ resorption			46
FXYD5	Small amount in Allan Atlas rat brain*.	Not done		↑ NKAT Imax	46
FXYD6	Extensive, abundant expression in Allan Atlas rat brain*. Cerebellum and hippocampus.	Not done		↓ Affinity of NKAT for Na+/K+	46,122
FXYD7	Almost exclusive to brain. Extensive PTMs. Large amount in Allan Atlas rat brain*.	Not done		↓ Affinity of NKAT for K+	46,87,123,124

FHM = familial hemiplegic migraine; NKAT = Na+, K+ -ATPase transporter.

^*www.brain-map.org.

Brain CEC NKAT.—

NKAT activity provides the mechanism for the higher concentration of Na⁺ and lower concentration of K⁺ in CSF and brain extracellular fluid compared with blood. This is achieved because a large excess of blood is delivered to the brain (blood flow is 1000-fold greater than interstitial fluid flow), the tight junctions between brain capillary endothelial cells separate CSF and extracellular fluid from blood, and NKAT is restricted to the abluminal surface of the CECs,⁴⁷ as illustrated in Figure 1. Brain CECs have 500-fold more NKAT than peripheral CECs,⁴⁸ thus brain CECs will respond with greater sensitivity to changes in NKAT regulators than elsewhere. The CEC NKAT-regulated interstitial fluid cations are the main source of [Na⁺]_e in brain tissue. Minimal [Na⁺]_e arises from the neurons and glial cells, since the [Na⁺]_i is normally only 10–15 mM.

Fig 1.—

Illustration of brain capillary endothelium, luminal Na⁺, K⁺ -ATPase transporter (NKAT) regulator receptors, tight junctions, and the differences in flow and [Na⁺]_e and [K⁺]_e between blood and cerebrospinal fluid (CSF). Capillary endothelial cells (CECs) are exposed to circulating NKAT regulators on the blood luminal side, and if they detect a change, they signal to their abluminal NKATs to alter interstitial fluid [Na⁺]_e and [K⁺]_e that modulate neuronal and vasomotor excitability. Not shown in the figure (for simplicity) are the glial cell end-feet that wrap around the CECs and form part of the blood-brain-barrier, and the choroid plexus with its epithelial, abluminal NKAT that produces the majority of the [Na⁺]_csf.

The NKAT on the ventricular/apical surface of the epithelial cells of the choroid plexus⁴⁹ is supplied with its NKAT-rich capillary network, and is considered the primary source of [Na⁺]_csf, though a significant contribution comes from CECs by way of the interstitial fluid.^50–53 Since choroid and CSF are midline, perhaps the more lateralized tissue CEC NKATs, especially those in the cerebral cortex, are better placed to increase the [Na⁺]_e, consistent with the common laterality in migraine.

Regulators of NKAT (Table 2).—

Table 2.—

Summary of Many of the Regulators of NKAT, Partitioned in Three Classes by Their Site of Modulation: Genetic, Direct, or Signal Transduction

NKAT modulator	Site of modulation by class	Refs
	Genetic class
FHM-1	Gain of function in slow calcium channel. ↑ Ca2+ may modulate by dephosphorylating NKAT.	72,125
FHM-2	Loss of function in α2 of NKAT. ↑ [Na+]i will alter NKAT expression.	22
FHM-3	Gain of function in Na channel. ↑ [Na+]i will alter NKAT expression.	25
Dexamethasone	Increases NKAT via ↑RNA/protein.	126
Aldosterone	Increases NKAT via ↑RNA/protein.	126
TNFα	Down regulates NKAT via NF-κB.	127
Interleukin-1	Increases NKAT localization and activity.	128
Thyroxine	Hyper- and hypothyroidism ↓ cerebellar NKAT. T3 ↑ muscle NKAT.	129,130
	Direct class
Na/K	[Na+]i and [K+]e ↑NKAT by binding to a-chain. Inward Na currents can activate or inactivate NKAT. Resting membrane potential may modulate ion-binding.	101,131–137
Mg	Activates NKAT. Binds α-NKAT. Deprivation activates calcineurin-mediated Ca signaling. Dominant isolated renal Mg loss from γNKAT loss.	101,138–141
ATP and MgATP	Activates, binds α-NKAT.	101
V, Fe, Pb, Cu	Inhibits, binds NKAT.	101
Zn	Inhibits, binds NKAT. Involved in oxygen/glucose deprivation induced CSD.	101,142
Exogenous NKAT inhibitors	Specific inhibitors. Digoxin family.	143–146
Endogenous NKAT inhibitors	Specific endogenous inhibitors. Includes ouabain.	147–153
Phospholipids	Usually activate NKAT, but ↓ PE ↑ NKAT activity.	103,154–156
Cholesterol	↓ NKAT. Lipid membrane fluidity and binds receptor.	157,158
Kinases	Generally inhibit NKAT, eg, PKA/PKC/PKG phosphorylation.	159,160
Phosphatases	Generally activate NKAT.	159
	Signal transduction class
Ca	α1/2 NKAT interacts with NCX to regulate Ca2+. Release of membrane-bound intracellular Ca2+ ↑ NKAT by activating calcineurin-mediated dephosphorylation. Physiological Ca2+ levels do not change α-1, but inhibit α-2 and 3 NKAT.	100,107,161
Dopamine	D1-mediated (D1&5 receptors) via G protein, PKA, and adenyl cyclase, ↑NKAT. D2-mediated (D2,3,4 receptors) inhibits G proteins, via PKC, decreasing adenyl cyclase and ↓ α2 and 3 NKAT.	162–164
Norepinephrine	Activates NKAT via α1 adrenoreceptor, ↑Ca, activating calcineurin and dephosphorylating NKAT. Beta adrenergic receptors ↓ NKAT via cAMP-mediated inhibition of phosphatase PP1.	157,165–168
Serotonin	Agonists ↑the # of NKATs, antagonists inactivate NKAT. Activate 5HT1A in cortex, 5HT6 in cerebellum. Inactivate NKAT in choroid via 5HT2c, DAG and PKC-induced phosphorylation. Activates glial α2 NKAT.	160,169,170
Prostanoids, leukotrienes, and thromboxanes	Through GPCRs, G proteins, camp, and phosphorylation. Also prostaglandin responsive elements in NKAT transcription.	171–173
Nitric oxide	NO donors trigger migraine via cGMP, and blockade of NOS treats migraine with aura.	174
Estrogen	Activate 5HT2AR and SERT, may activate NKAT.	175,176
Glutamate	Activate NKAT via increased [Na+]i by ↑Na influx, and by G protein and PKG.	101,177–179
Cannabinoids	Activate NKAT via CB1 receptors via G proteins. Anandamide NKAT, possibly by inhibiting dopamine and 5HT uptake.	180,181
Hypoxia	Acutely degrades NKAT. 24 hours after hypoxia, ↑ α1 and 2 NKAT activity, via PKMζ. GM1 protects after ischemia.	142,159,182,183
CO	Induces persistent NKAT activation in Purkinje neurons, via G proteins and PKG.	179
Lipid peroxidation	MMA inhibits succinate dehydrogenase, increases TBARS, depresses NKAT in some locations, increases in others. 4HNE ↓ Dopamine uptake and NKAT activity.	179,184–187
Insulin	Increases α1 NKAT in cultured astrocytes via IGF-1R.	126,188,189
Endothelin	↑NKAT in ciliary cells/brain capillary endothelium.	190–192
CGRP	Increases NKAT in depolarized (high [K+]e) muscle.	193,194
Capsaicin	Capsaicin releases CGRP at sensory nerve endings. Capsazepine inactivates NKAT, inhibits ATP hydrolysis.	194,195
Caffeine	Inhibits NKAT via GPCRs Adenosine A1a, A2a, A2b.	196

CGRP = calcitonin G related peptide; CSD = cortical spreading depression; FHM = familial hemiplegic migraine; IGF-1 = insulin growth factor 1; NKAT = Na⁺, K⁺ -ATPase transporter.

The large number of molecular modulators of NKAT (a reflection of its importance in the brain) exert their effects by 3 distinct yet integrated mechanisms: genetically (polymorphism in NKAT or altered genetic regulation); directly (cations, ATP, membrane lipids, direct inhibitors or activators); or indirectly by signal transduction, usually from G-protein coupled receptors (GPCRs). Endothelial cell composition and functions are known to differ throughout the body and within tissues and are far from fully characterized, but receptors for many major NKAT regulators known to be involved in migraine have been demonstrated on brain CECs, including those for estrogen,⁵⁴ serotonin,⁵⁵ lysophospholipids,⁵⁶ and GPCRs. Table 2 provides a framework of reference (a summary, but not exhaustive) for the extensive range of NKAT regulation pathways. We propose that the CEC is in a unique position to sense variation in any of these circulating NKAT regulators from blood, and signal changes to its abluminal NKAT to alter the brain interstitial fluid [Na⁺] and [K⁺]. This alters extracellular cations on the brain side of the CEC tight junctions and affects the excitability of neurons. The neuronal responses are manifest as aura or migraine.

THE CEC NKAT HOMEOSTASIS THEORY, AURA, AND MIGRAINE

We propose a potassium/sodium homeostasis theory for migraine pathophysiology based on the need to unify the known change in [K⁺]_e and [Na⁺]_e reported during CSD^20,21 (that we assume also occurs during migraine aura), the observed increase in [Na⁺]_csf during the peak of acute migraine pain²⁹ (that we assume is accompanied by increased [Na⁺]_e), that all 3 FHM mutations effect [K⁺]_e and [Na⁺]_e, and that increased [Na⁺]_e increases neuronal excitability.

We hypothesize that NKAT regulators in the blood arrive at their receptors on the luminal side of CECs and transduce signals to NKATs on the abluminal CEC wall (Fig. 1). These altered CEC NKATs modify brain interstitial cations, causing symptoms:

Well State (Fig. 2).—

Fig 2.—

This schema represents the principal differences in [Na⁺]_e and [K⁺]_e predicted to occur at neurons for migraineurs in the Well State, during Aura, or during Migraine, based on the capillary endothelial cell Na⁺, K⁺ -ATPase transporter (CEC NKAT) theory. The boxed regions above each axon illustrate the effects from the Na⁺ changes on neuronal excitability: Normal action potentials in the Well State; cortical spreading depression (CSD) in Aura; increased action potential firing frequency in Migraine.

NKATs change the interstitial [Na⁺] and [K⁺] surrounding neurons and vary their excitability within normal homeostatic limits.

In migraine, the excitability shifts outside normal limits.

Aura (Fig. 2).—

When CEC NKAT activity falls below its normal lower limit, interstitial [K⁺]_e rises, depolarizing the neuron, facilitating CSD that manifests clinically as aura. Reduced neuronal and glial NKAT activity along with potassium leak from cells may also contribute to the increased [K⁺]_e at the start of CSD, since they have high [K⁺]_i. Interestingly, decreased brain energy favors CSD⁵⁷ and, because NKAT consumes a large amount of energy, this adds additional support to a role for NKAT in CSD. During CSD, extreme ion shifts occur at the neuron: [K⁺]_e increases from 2.3 to 35 mM and [Na⁺]_e decreases to 75 mM.^20,21 Increases of [K⁺]_e could induce persistent Na⁺ current (I_Nap).⁵⁸ Computer simulation demonstrates that I_Nap and/or NMDA current could cause CSD-like depolarization.⁵⁹ The elevated [K⁺]_e induced depolarization could reverse NCX and NCKX, resulting in Ca²⁺ overloading, thus leading to apoptosis or excitotoxicity⁶⁰ in neurons. CEC-induced vasoconstriction may also cause ischemia by 2 mechanisms: CEC NKATs change vasomotor tone by regulating the [K⁺] in interstitial fluid where it can alter excitability of local vascular smooth muscles.^61–63 CECs may thus exert effects humorally by releasing mediators that include calcitonin G related peptide (CGRP), serotonin, and endothelin, which are altered in the blood during acute migraine.

We propose that repetitive and/or prolonged inhibition of CEC NKAT activity with prolonged increase of [K⁺]_e may cause apoptosis of neurons and micro-infarction from vasoconstriction. We suggest that these neuronal and vasomotor effects are the basis for the increased frequency of hyperintense MRI signals and stroke observed in migraineurs with aura. This could be tested in an animal model by repeated and prolonged CEC NKAT inhibition with MRI and pathological evaluation.

In response to CSD and to avoid apoptosis and cerebral infarction, glial K⁺ re-uptake and increased NKAT activity rapidly restore the normal low [K⁺]_e. Sustaining NKAT inhibition in cultured astrocytes with ouabain increases NKAT activity and leads to NKAT over-expression,⁶⁴ and elevated [Na⁺]_i has been shown to increase NKAT activity.⁶⁵ We propose that if compensation is perfect, the aura ends without migraine. This is uncommon (see Aura Followed by Migraine, below).

The CSD moves at 3–6 mm per minute. Recovery of evoked potentials takes 15–30 minutes and ion redistribution takes at least 30 minutes.⁶⁶ Leão initially described vascular changes in addition to the neuronal depression of CSD⁶⁷ and the propagation of the CSD effect on cortical surface arterioles was recently reported to be separate from that in the brain parenchyma.⁶⁸ The mechanism for propagation of either vascular or neuronal changes is not well understood. Based on the theory that decreased CEC NKAT induces CSD by increasing [K⁺]_e in the interstitial fluid, we hypothesize that the “spreading” may be governed by the flow rates of K⁺ in the interstitial fluid.

Aura Followed by Migraine (Fig. 2).—

We propose that a more typical CEC NKAT response to the inhibition that caused aura is over-activation. This causes NKAT activity to exceed its normal upper limit, analogous to the cellular responses to NKAT inhibition,^64,65 elevating [Na⁺]_e and decreasing [K⁺]_e. The high [Na⁺]_e causes increased neuronal excitability, manifest clinically as migraine (Fig. 2).

Migraine Without Aura (Fig. 2).—

Independent of CSD, we propose that NKAT up-regulators cause CEC NKAT activity to rise above the normal upper limit, elevating [Na⁺]_e and decreasing [K⁺]_e. This causes varying degrees of increased neuronal excitability, manifest clinically as migraine (Fig. 2). It is possible that CSD may occur without obvious symptoms in patients who have migraine-without-aura, in the form of a silent aura.⁶⁹ Genetic analysis may help to separate aura and headache.⁷⁰

Deviation from NKAT Homeostasis.—

Homeostasis is disturbed enough to cause aura or migraine when any regulator (Table 2) or NKAT itself alters CEC NKAT activity outside normal levels. Mechanisms that may alter these regulators include mutations/polymorphisms, variations from the environment (such as diet, medications, exercise, or stress), or alterations in their transport, synthesis, signaling, metabolism, or clearance. The multiple redundancy of this system is understandable to regulate such an important enzyme.

Many molecular circuits known to be involved in the pathophysiology of migraine are regulators of NKAT (Table 2), including serotonin, CGRP, dopamine, estrogens, glutamate, cannabinoids, nitric oxide, noradrenaline, or caffeine. The CECs are known to have serotonin, estrogen, and phospholipid receptors, and GPCRs.^54–56 This theory adds to the understanding of migraine pathophysiology by offering a mechanism for excessive fluctuations of neuronal excitability resulting from deviations in CEC NKAT activity caused by changes in its many regulatory inputs.

THE CEC NKAT HOMEOSTASIS THEORY AND GENERAL MIGRAINE FEATURES

Increased neuronal excitation in trigeminal neurons from increased [Na⁺]_e could cause pain without an external stimulus and release of Substance P and CGRP.⁷¹ CEC NKAT disturbance is not limited to the trigeminal pathway, and when [Na⁺]_e is elevated in any neural pathway, hyperexcitability would be manifest with pathway-specific symptoms. Each hyperexcitable pathway would have its own manifestation, consistent with the many symptoms during migraine. For instance, hyperexcitability of the vestibular pathway would cause nausea and/or ataxia, hyperexcitability of the auditory or visual pathways would cause phonophobia or photophobia, and hyperexcitability of the locus coeruleus would increase vigilance and alarm. Hyperexcitability of neurons that have inhibitory neurotransmission will further add to the variety of functional consequences. The range of possible symptoms are only limited by the neuronal circuits that can be affected by changes in the interstitial [Na⁺] and [K⁺]_e.

We propose that deviation of CEC NKAT activity in migraine from normal limits can be short-lived or extended. As the [K⁺]_e increases beyond tolerable levels during aura, glial re-uptake of [K⁺]_e and increased NKAT activity is required to reduce [K⁺]_e and avoid apoptosis. If there are sufficient intracellular stores of NKAT available for translocation to the cell membrane, this process can be rapid, within seconds. However, if not locally available, the transcription, translation, and translocation of new NKAT protein can take 30–60 minutes.^73,74 Increased expression of CEC NKAT will restore the normal balance of [Na⁺]_e and [K⁺]_e and terminate the CSD. At this time, the migraineur returns to neuronal homeostasis and feels normal. If the required NKAT levels are perfectly attained, then there is no migraine. More often, there is widespread over-expression of the CEC NKAT and [Na⁺]_e increases above the upper normal limits. Diffusely increased [Na⁺]_e causes neuronal excitation that produces symptoms depending on the specific molecular isoforms and the location of the involved CECs, neurons, and vascular smooth muscles. These neuronal and vasomotor effects in migraine rapidly invoke many other cellular reactions, including activation of lipid mediators of inflammation and pain, disruption of oxidation control, etc. It may take hours to return these NKAT-derived activities to normal, a duration conforming to typical migraine.

We propose that when migraineurs are stressed and NKAT activity is close to the upper limit of normal, a single aggravating trigger such as sensory input (eg, sound, light, smell) can unmask the precarious CEC NKAT activity, with increased [Na⁺]_e rapidly causing neuronal hyperexcitability and onset of migraine. In chronic migraine, we propose that chronically elevated CEC NKAT activity and the many reactive consequences of altered neuronal excitability fail to stabilize, and the high [Na⁺]_e sustains neuronal hyperexcitability.

Aging.—

Migraine lessens with age, as do a number of NKAT parameters: NKAT activity and the neuronal resting membrane potential decrease with age in mice in a ouabain-dependent manner, considered to be due to lipid changes in the membrane.^75,76 The mRNA levels of the NKAT α−3 isoform in neurons are reduced in normal aging^77–79 (considered to increase the risk of Alzheimer’s disease with age⁸⁰). NKAT activity is reduced with aging in synaptosome preparations from female rat brains.⁸¹ In a proteomic study of aging transgenic mice, NKAT levels dropped dramatically with aging.⁸² We propose that these multiple, independent studies of reduced NKAT activity with age are consistent with changes in migraine: (1) Inhibition of an already lowered NKAT activity will make it easier to drop below the lower activity limit, increasing [K⁺]_e and causing spreading depression, consistent with observations that auras by themselves are more common in the elderly; (2) Less NKAT activity will make older people less capable of increasing [Na⁺]_e to levels that cause migraine. This reduced NKAT activity may not be beneficial when faster reactions or cognition are desired from physiological increases in neuronal excitability, but it has the advantage of less migraine.

Diurnal Timing.—

Several studies have suggested that migraine has a circadian rhythm, with the time of onset either in the early morning or afternoon.^83–86 Experimentally, R192Q migraine mice lack the physiological retardation in circadian adaption to phase-advance shifts.⁸⁷ The NKAT theory is consistent with these observations, since its biochemistry has similar rhythms; 2 studies report a diurnal rhythm for NKAT expression that match the migraine onset times, consistent with predisposing migraineurs to attacks starting at these times: NKAT activity in the ventral suprachiasmatic nucleus neurons is diurnally regulated, with activity increasing during the day and decreasing at night.⁸⁸ Inhibition by the NKAT inhibitor strophanthidin decreases at night.⁸⁸ While other analytes also exhibit periodic rhythms, these clinical and NKAT diurnal correlates are consistent with, and may be responsible for, migraine chronovariations.

In humans, one can evaluate whether [Na⁺]_csf has chronobiological variation similar to that of the time of onset of migraine and the diurnal NKAT variation. We found [Na⁺]_csf does have such a rhythm in CSF sampled every 10 minutes for 24 hours from non-headache suffering controls.⁸⁹ To extend this research, it is necessary to obtain less invasive samples than CSF; however, blood taken at the same time as CSF did not reveal changes in [Na⁺]. In searching for an alternative sample source with sodium regulation similar to that in the brain, salivary glands may be informative because they have significant origin from neuro-ectoderm.⁹⁰ We are currently evaluating whether migraineurs have chronobiological variation in saliva [Na⁺] that is similar to the changes in [Na⁺]_csf.

Known chronobiological variations in migraine time of onset, [Na⁺]_csf, and NKAT suggest that chrono-pharmaceutical approaches may help. Specifically, timing of drug administration may be more effective if optimized to coincide with the circadian variations in [Na⁺]_csf and NKAT, with particular attention to the higher risk times in early morning and mid-afternoon.

Triggers.—

Dehydration and over-hydration are both known migraine triggers. Such extreme ionic challenges will directly influence CEC NKAT activity and may lead to migraine. Salt intake is largely controlled by kidney NKAT, and blood plasma [Na⁺] varies much more than in brain or CSF, yet equilibration between plasma and brain/CSF is rapid (minutes).^30,32,33 Thus, it is not surprising that salt intake can trigger headaches.⁹¹ Notably the elevated [Na⁺]_csf we found was not reflected in blood plasma,²⁹ perhaps emphasizing the substantial regulation (at CEC NKATs) that differentiates these body compartments.

Sleep disturbance, a known migraine trigger, is associated with NKAT dysfunction: Sleep deprivation induces overexpression of NKAT in rat brain synaptosomal preparations, and in the locus coeruleus, laterodorsal tegmentum, pedunculo pontine tegmentum, and medial preoptic area.⁹² Rapid eye movement sleep deprivation led to an increased ouabain binding Kd for NKAT in cultured fetal rat telencephalon neurons, which might contribute to the increased excitability in these rats.⁹³ Migraine triggers of stress (eg, by noradrenergic receptors) and anoxia directly affect NKAT function by signal transduction pathways (Table 2).

Diet can trigger migraine in some migraineurs. We propose that any dietary component may influence the NKAT regulators (Table 2) and we single out lipids for specific comment. A reduction in polyunsaturated fatty acids (PUFAs) or increase in saturated fatty acids in membranes inhibit NKAT activity; we suggest that counteracting this inhibition with PUFA supplements may be the basis for the improved migraine symptoms reported from one open and one blinded trial,^94,95 though another double-blinded trial failed to show that PUFA supplements prevented migraine.⁹⁶

Hormonal changes, especially peri-menstrually, are known to trigger more frequent and severe migraine. We propose that 2 mechanisms from the NKAT theory may be responsible: Estrogens activate NKAT by signal transduction (Table 2); and, similar to cholesterol, estrogens influence membrane fluidity-based changes in NKAT activity.

Alterations of many if not all the pathways in Table 2 have been reported in migraine, adding further support for this theory. The fact that NKAT has different expression levels and isoforms in different brain regions would allow for almost endless specific variation in symptoms. This would be consistent with the endless unique aspects of each migraine between and within sufferers that are superimposed on the most characteristic symptoms (pulsatile, unilateral, headache, with nausea, photophobia, and phonophobia).

Treatments.—

We propose that existing or future medication will be successful when treatment re-establishes sodium homeostasis either directly or via signal transduction pathways of CEC NKATs. Non-pharmaceutical treatments, such as stress reduction, sleep hygiene, and avoiding dehydration or over-hydration may benefit by reducing NKAT perturbation. Since the gateway cell is the CEC, blood borne modifiers can affect CEC NKATs via their luminal receptors, thus removing the necessity for passage across the blood-brain-barrier. This may explain the central effects from well-established migraine medications that are known to have poor barrier penetrance, including triptans.

Cohen suggested that prophylactic drugs used to treat migraine do so through their affects on sodium regulation.⁹⁷ Drug mechanisms in vivo are very complex but, based on the NKAT homeostasis theory, we can propose that blocking sodium channels may reduce the neuronal excitability that would otherwise result from excessively high [Na⁺]_e. Prophylaxis takes many weeks before benefit, perhaps consistent with the time required to adjust all NKAT regulatory inputs for optimal balance and to stabilize (not flatten!) the interstitial fluid circulation of fluctuating [Na⁺]_e and [K⁺]_e.

The NKAT homeostasis theory affords useful directions to both evaluate and conceive of new treatment approaches. First, new medications can be evaluated on cell cultures, brain slices, CECs, and small blood vessels, or in vivo for their effect on CEC NKATs and sodium/potassium homeostasis. Second, we may help to personalize medical therapy by defining the most disruptive NKAT regulators (Table 2) in an individual. This could be evaluated by screening endothelial or lymph cells from an individual to identify regulators (Table 2) that most perturb their NKATs (NKAT changes in lymphocytes have been found in migraineurs⁹⁸ and endothelial cell precursors have been identified in peripheral blood⁹⁹). Third, in status migrainosis or severe chronic migraine, perhaps invasive efforts to correct the high [Na⁺]_e and low [K⁺]_e by blood plasma or CSF exchange may be worth considering. Finally, on a precautionary note, we predict the possibility of cellular apoptosis or stroke during aura if potassium homeostasis is not corrected quickly; aggressive efforts are therefore justified to lessen aura severity.

CAVEATS

There is a paucity of direct data at this stage on migraine sodium/potassium homeostasis, and much of this theory is deduced from basic science knowledge of NKAT biochemistry and functions. We propose that the principal caveats are due to the limited knowledge of brain molecular pathophysiology. There has only been our single study to report elevated CSF [Na⁺] and nothing is known about regional differences in brain or CSF [Na⁺]. Details of all molecules involved in migraine, including their isoform and cell distribution, amount and duration of change, and circadian variations are, as yet, poorly defined. As an example, the Allen Mouse Brain Atlas reveals the ubiquitous presence of transcripts of all NKAT isoforms in the brain, but does not reveal specific differential cellular details. We have tabulated many modulators of NKAT, but have not discussed many consequences that result from altered NKAT activity (eg,^46,100–105). Defining the mechanisms of neuronal excitability with regard to sodium/potassium homeostasis requires further research, especially in neurons, glial cells, smooth muscle cells, and CECs. Defining vasomotor effects from altered [K⁺]_e and NKAT-induced humoral vasoactive compounds requires further research. The effects of NKAT disturbance in neuron-to-neuron connections, excitatory vs inhibitory neurotransmission, and the interactions between altered NKAT activity in different brain regions and in surrounding non-neuronal cells are beyond the scope of this analysis.

CONCLUSIONS

To explain the increased [Na⁺]_csf reported in migraine, we propose a theory based on deviations from CEC NKAT homeostasis. Migraineurs have genetic or environmental variations of the myriad regulators of NKATs that predispose them to disturbances in their homeostasis, specifically in CECs. Fluctuations of NKAT activity outside normal limits lead to neuronal and vasomotor effects. Activity below the normal limit increases [K⁺]_e, depolarizes neurons, causing aura.Activity above the normal limit elevates [Na⁺]_e, increases neuronal excitability, causing migraine. The same ionic disturbance from CEC NKATs also affects vasomotor tone by direct or humoral effects on smooth muscles.

This theory is consistent with the known events of migraine. We propose that deviation from normal CEC NKAT is a common pathway in the genesis of the distinct components of aura and migraine. There is much CEC NKAT regulation to define, but testing described herein, both in migraineurs and model systems, will help elucidate NKAT involvement in migraine, and help evaluate whether CEC NKAT modulation could more successfully manage migraine.

Abbreviations:

AMPA

α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid

CEC

capillary endothelial cell

CGRP

calcitonin G related peptide

CSD

cortical spreading depression

CSF

cerebrospinal fluid

FHM

familial hemiplegic migraine

GPCR

G-protein coupled receptor

IGF1

insulin growth factor 1

I_Nap

persistent Na⁺ current

[Na⁺]_csf or [K⁺]_csf

sodium or potassium ion concentration in CSF

[Na⁺]_e or [K⁺]_e

brain tissue extracellular sodium or potassium ion concentration

[Na⁺]_i or [K⁺]_i

intracellular sodium or potassium ion concentration

NCX

sodium and calcium exchanger

NCKX

potassium-dependent sodium and calcium exchanger

NKAT

Na⁺, K⁺ -ATPase transporter

NMDA

N-methyl-D-aspartic acid

PKA

C, G protein kinase A, C, G

PUFA

polyunsaturated fatty acid

SD

spreading depression