Perivascular neurotransmitters: Regulation of cerebral blood flow and role in primary headaches

Introduction

Migraine is a frequent primary headache disorder, with higher prevalence in females (15.5%) than males (4.5%), ¹ and associated with clinical symptoms (typical headache, and phonophobia/photophobia/nausea/vomiting that occur over 4–72 h). ² On the contrary, cluster headache is more prevalent in males than females for most age groups. ³ The origin of headache attacks is still under discussion, with particular focus on whether they represent a vascular or neurogenic disorder.^4,5 During the different phases of the migraine attack (premonitory and during attacks), there are variations in cerebral blood flow (CBF), coinciding with changes in brain activity, observed in the somatosensory cortex, brainstem and thalamus.^6,7 In headache-free periods (inter-ictal), elevated CBF in the primary somatosensory cortex has been reported for migraineurs. This effect was positively correlated to headache attack frequency, but also to the number of cutaneous allodynia symptoms. ⁶ During headache attacks, autonomic and sensory ganglia are known to be variably activated and sensitized,^8,9 which gives rise to neurotransmitter release, believed to be important in the migraine pathophysiology.^10,11 In particular, trigeminal ganglion (TG) stimulation in cats resulted in increased CBF and calcitonin gene-related peptide (CGRP) release. ¹² Several signaling molecules are involved in both migraine and innervation of the cerebral vasculature.

The cerebral vasculature is known to be innervated by (i) sympathetic nerves storing adenosine triphosphate (ATP), noradrenaline and neuropeptide Y (NPY), (ii) parasympathetic nerves with acetylcholine, vasoactive intestinal peptide (VIP), peptide histidine methionine, pituitary adenylate cyclase-activating peptide (PACAP) and nitric oxide (NO), and (iii) sensory nerves containing CGRP, PACAP, NO, substance P (SP) and neurokinin A (NKA).^13,14 The extrinsic innervation ends close to the entry of the vasculature into the brain parenchyma; a transition zone in the Virchow-Robin space. Moreover, the intraparenchymal nerve supply to arterioles and microvessels stems primarily from central nervous system (CNS) neurons. ¹⁴ Therefore, it is necessary to discuss migraine in the light of extrinsic and intrinsic neural inputs, and their role in regulation of CBF.

The link between headache, CNS symptoms, intracranial vessel diameter and regulation of blood flow has long been pursued. In general, primary headaches are considered complex polygenic disorders (genetic and environmental influence) with pathophysiological neurovascular alterations.^15–17 Several primary headache theories have been proposed over time, with the purpose of elucidating the underlying cause of headache attacks and associated characteristics. Early studies on involvement of extracranial vasodilation and a link between intracranial vasospasm and cortical spreading depression (CSD) in migraine with aura led to the vascular theory of migraine, originally forwarded by Wolff in the early 1940s.^18,19 Although a pure vascular theory has now lost its role as the leading hypothesis to explain migraine, involvement of extracranial and intracranial vasculature has persisted over time as an etiological or coexisting factor.^20,21 Direct experimental evidence, contrary to the vascular theory, came from a magnetic resonance angiography study showing no/minor alterations in the diameter of meningeal and cerebral arteries in genuine migraine attacks. ²² Nevertheless, this study is not entirely conclusive, as the intracranial middle meningeal artery (MMA) was not measured due to technical limitations. ²³

Today CSD remains one of the possible early links between vascular observations and the neurogenic theory. CSD is a self-propagating wave of neuronal and glial cortical depolarization resulting in biophysical and biochemical alterations such as changes in parenchymal blood flow and ionic concentrations. ²⁴ The aura, a clinical symptom (mostly visual) seen in about 20–30% migraineurs, ²⁵ is supposedly due to CSD.^26,27 It has been suggested that CSD directly activates trigeminal sensory nerve terminals in dura mater/meninges followed by trigeminovascular system activation.^24,28 For decades, activation and sensitization of the trigeminovascular system have been considered a pathophysiological mechanism/consequence behind primary headaches.^29–31 Interestingly, CSD not only affects meningeal arteries, but also cortical cerebral arterioles and CBF. ³² An alternative hypothesis involves activation of CNS regions by CSD which in turn, via thalamus and brainstem, causes activation of the trigeminovascular system with dural inflammation being secondary to the CNS activation. Moreover, during the headache phase in genuine attacks, waves of flow reductions slowly spread over the cortex bilaterally. This pattern resembled CSD even though the subject did not have obvious aura symptoms. ³³ Other studies also report the occurrence of local depression of CBF in migraine. ³⁴ More details on this research topic has been discussed by Pietrobon and Moskowitz^26,27 and by Ayata and Lauritzen. ³⁵

Today, leading scientists advocate that already in the premonitory phase of an acute migraine attack, distinct blood flow alterations within, e.g. hypothalamus-thalamus and the adjacent brainstem (periaqueductal gray) can be observed.^36,37 The proposal of a “migraine generator” originates from Weiller et al. ³⁸ demonstrating activation of brain regions such as the periaqueductal gray, raphe nuclei, and locus coeruleus in the early phase of a migraine attack. Following injection of sumatriptan for acute treatment of migraine, pain subsided but the areas remained activated. After some hours, the pain returned, probably due to disappearance of the triptan from the involved receptors. ³⁸ Thus, evidence suggests that migraine pathophysiology includes dysfunction of subcortical structures such as diencephalic and brainstem nuclei which can modulate the trigeminovascular system and perhaps also participate in CSD generation. ³⁹

Complexity of neurovascular coupling and CBF

Brain homeostasis is known to be controlled by a tight regulation between neuronal function, local blood flow and maintenance of neurovascular coupling.^40,41 More precisely, the CBF is controlled by (i) autoregulation, maintenance of adequate CBF regardless of blood pressure (BP) fluctuations (within some limits),^42,43 (ii) chemical and metabolic factors, e.g. interstitial pH ⁴⁴ and CO₂, ⁴⁵ and (iii) neurogenic factors such as perivascular innervation.^46,47 Regulatory mechanisms of CBF depend on variations in the rate of local brain metabolism. ⁴⁸

The changes of CBF in primary headaches are obviously not as drastic as observed in ischemic stroke. Weiller et al. ³⁸ reported minor changes in cortical blood flow which was significant only if all migraine patients were normalized and grouped (about 5% of resting flow). In migraineurs with aura, a reduction in CBF was observed closely in time to the aura symptoms.^49,50 In the interictal period, Loehrer et al. ⁵¹ reported a higher total and parenchymal CBF in migraineurs compared to controls, suggested to be driven by a higher basilar artery flow. In experimental animals, induction of CSD causes an abrupt rise in CBF within a minute followed by depression of function and flow. ⁵²

Relevant for primary headaches, an association between neurovascular dysfunction and CSD has been proposed.^52,53 It is important to clarify that the intraparenchymal neurovascular system is distinctly different from that of the large cerebral arteries and arterioles located around the brain and leading into the intraparenchymal system.^14,41 The extraparenchymal vasculature (arteries belonging to the circle of Willis, and arteries located on the cortical surface) receives a dense supply of perivascular nerves that originate in cranial ganglia. ⁴⁷ The intraparenchymal neurovascular system is thin and mainly contains pericytes and endothelial cells. Their connections with the local environment come from local neurons and astrocytes, inter alia. They are not supplied by innervation from the cranial ganglia, and ⁴⁷ hence there are two different sites of neuronal regulation of the cerebral circulation.

Extrinsic and intrinsic cerebral vasculature: Neurogenic factors and neurotransmitter release

Internal and external factors and genetic predisposition in primary headaches

Internal and external factors and genetic predisposition are known to affect the susceptibility to develop primary headaches. Therefore, individual variation in regards to response to triggers, neuronal hyperexcitability and vascular regulation might exist. Figure 4 illustrates a proposed hypothesis of migraine development due to variable predispositions from genetic and environmental factors. It is evident when comparing subject B-D (migraineurs), that there can be individual differences in the factors leading to development of migraine, and hence they will probably respond differently to treatment strategies (e.g. medication). Therefore, creation of individual treatment strategies might be beneficial for treating primary headache disorders. OPEN IN VIEWER

Neuronal hyperexcitability and over-activated neuronal circuits are considered functional abnormalities in migraine and believed to be important in triggering headache/migraine attacks. ¹⁸⁶ More information on multiple internal and external headache triggers (e.g. sleep disturbance, stress, light, odors) can be found in thorough reviews by Ho et al. ¹⁵ and Borkum. ¹⁸⁷ Migraine patients have subjectively reported a link between headache attacks and triggers (e.g. alcohol, food, visual triggers, stress/tiredness, environment), categorized by a principal component analysis. ¹⁸⁸ Migraine triggers have been proposed to lead to fluctuations in neurotransmitter levels and hypersensitivity in specific regions of the brain.^12,15 In humans, short exposure to noise resulted in sensitization in subjects without headache; however, prolonged exposure led to desensitization. For headache patients, the trend was not clear. ¹⁸⁹ The effect of stress in rats included satellite glial cell (SGC) activation and elevated SP expression in the TG. IL-1β and interleukin 1 receptor type 1 (IL-1R1) expression in TG neurons were also elevated. ¹⁹⁰

Development of migraine and cluster headache also seems to be age- and gender-dependent,^3,191,192 e.g. with particular dependence on gender hormones (e.g. rate of estrogen withdrawal in females). ¹⁹³ During the menstrual cycle, fluctuations in steroid hormones seem to be associated with changes in cerebrovascular reactivity of which vasodilation primarily is observed in the right hemisphere. ¹⁹⁴ Regarding the effect of age and sex on cerebrovascular regulation, inconsistency exists between human studies. One study reported that these two factors did not seem to affect cerebrovascular regulation to any major extent, ¹⁹⁵ while another study suggested an effect of age on BP and CBF. ¹⁹⁶ Moreover, neuronal activity and CBF are coupled tightly to brain metabolism.^197,198 Increased brain activity causes increased local energy expenditure and is thus correlated with increased local CBF. For long, it has been discussed which physiological parameter comes first, metabolism or flow? Recent studies suggest that they indeed may be closely coupled.^62,199 Moreover, Gantenbein et al. ²⁰⁰ put forward a hypothesis stating that processing of sensory stimuli in migraine has a higher energy demand for neuronal function compared to that of healthy subjects, possibly linked to the theory of neuronal hyperexcitability.

In terms of genetic migraine predisposition, there are several lines of evidence including rare mutations in known proteins, single nucleotide polymorphisms and microsatellite markers. For the rare condition FHM, there are now at least three subtypes (FHM1-3) reporting gene mutations of calcium voltage-gated channel subunit alpha1 A (CACNA1A), ATPase Na⁺/K⁺ transporting subunit alpha 2 (ATP1A2) and sodium voltage-gated channel alpha subunit 1 (SCN1A). ²⁰¹ The subtypes seem to share the potential to modify glutamate signaling in the CNS, making the neurons more susceptible to depolarize (cortical hyperexcitability). ²⁰² The first mutation (FHM1) was detected in locus 19p13 ²⁰³ which relates to the P/Q calcium channel in the CNS, and affects calcium-dependent facilitation and synaptic plasticity. ²⁰⁴ In a genetic family study, a considerable number of individuals (86%), carrying a mutation in ATP1A2, were diagnosed with FHM, ²⁰⁵ which implies a high effect size.

More recently, genome-wide association studies (GWAS) have revolutionized the discovery of susceptibility variants for common traits and diseases (low-moderate effect size of markers), including primary headaches.^16,17 The first GWAS paper investigating single nucleotide polymorphisms implicated in primary headaches, and consequently pathological mechanisms, was published in 2010 according to the GWAS catalog. ²⁰⁶ Numerous headache-related single nucleotide polymorphisms and suggested migraine candidate genes (e.g. LDL receptor related protein 1 (LRP1), gap junction protein alpha 1 (GJA1), phosphatase and actin regulator 1 (PHACTR1)) have been identified and are involved in molecular mechanisms of ion homeostasis, oxidative stress and regulation of vascular tone.^207,208 Although the vascular theory is no longer the primary theory on the origin of migraine, GWAS show that several candidate migraine genes are indeed related to the vasculature, based on data from recent meta-analyses.^207,208 Table 1 summarizes selected candidate migraine genes detected by GWAS and their relation to cerebral vasculature in order to reveal potential interconnected biological mechanisms and effects on vascular function. The candidate genes seem to be involved in vascular diseases/disorders (e.g. cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL), arteriovenous malformations (AVMs)), hemodynamics, regulation of transport across the BBB, neurogenesis and gliogenesis amongst others (Table 1). Other genome wide scans have been carried out investigating microsatellite markers in a cluster headache cohort. The hypocretin receptor 2 (HCRTR2) was detected as a candidate cluster headache gene, ²⁰⁹ also detected by other genetic studies.^210,211 HCRTR2 has also been associated with heart failure. The findings included (i) upregulation of HCRTR2 in dilated cardiomyopathy in humans and (ii) dysfunctional diastolic BP in HCRTR2 transcription-disrupted mice. ²¹² In addition, ADCYAP1R1, a PACAP receptor gene believed to be involved in signal transmission in migraine, was detected by GWAS to be associated with the susceptibility to develop cluster headache. ¹⁷ These findings could again give life to the importance of the vasculature in primary headaches, maybe with focus on hemodynamics and regulation of intracerebral flow and the neurovascular unit.

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Table 1.

Selected candidate migraine genes detected by GWAS meta-analyses.

Candidate genes	Gene description	Potential linkage to cerebral vasculature and primary headaches	References
HTRA1* a	HtrA serine peptidase 1	▪ Associated with cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy (CARASIL), a cerebral small vessel disease.	222,260–262
		▪ CARASIL: Reported vascular smooth muscle cell (vSMC) loss, change of small vessels in cerebral write matter and basal ganglia, and extracellular matrix breakdown.
		▪ Migraine with aura has been referred to as a central feature of CADASIL, the autosomal dominant variant.
GJA1*	Gap junction protein alpha 1 (also known as gap junction protein connexin43)	▪ Functions in synaptic transmission and homeostasis of the brain/immune system.	263,264
		▪ Connexin43 (a main astroglial connexin) forming gap junction channels and hemichannels involved in (dys)regulation of BBB transportation in human endothelial cells.
		▪ Detected, especially, in the end-feet b of astrocytes, interacting with the endothelium, implicated in maintenance of BBB integrity.
HEY2* a	Hes related family bHLH transcription factor with YRPW motif 2	▪ HEY2 is expressed in the cerebral cortex and cranial ganglia, and believed to be involved in neuro- and gliogenesis in the developing brain, but also cognitive impairment.	265,266
		▪ HEY regulated genes are involved in neurogenesis, apoptosis and vascular development.
JAG1*	Jagged 1	▪ Associated with cerebral arteriovenous malformations (AVMs) and endothelial differentiation.	267–269
		▪ Lesions in AVMs might be linked to increased blood flow and vessel wall shear stress, hemodynamic factors. These hemodynamic factors might trigger release of angiogenic factors and inflammatory markers.
LRP1** a	Low density lipoprotein receptor-related protein 1	▪ LRP1 is involved in amyloid uptake by vSMCs and possibly cerebral amyloid angiopathy.	270,271
		▪ Cerebral amyloid angiopathy might be implicated in late onset of aura reporting occipital haemorrhage and subcortex haemosiderin deposition.
NRP1*	Neuropilin 1	▪ Implicated in vascular development (e.g. angiogenesis and vascularization) and formation of arteriovenous connections.	272,273
		▪ Knockdown of NRP1 in human brain microvascular endothelial cells affected chemokine levels through TNF-α and IFNγ, inflammatory responses. It also showed that NRP1 is involved in BBB function, (de)myelination of neurons and lymphocyte infiltration.
RNF213*	Ring finger protein 213	▪ Link between RNF213 and intracranial vasculopathies.	274–277
		▪ RNF213 variants affect the susceptibility to develop intracranial atherosclerotic stenosis and moyamoya disease.
		▪ Migraine-like attacks with aura observed in a female patient showing signs of moyamoya disease evaluated by computed tomography and angiograms.
SLC24A3*	Solute carrier family 24 member 3 (also known as NCKX3)	▪ NCKX3 (Na+–Ca2+ exchanger dependent on K+) is believed to be involved in vision and olfaction through its regulatory effects on calcium homeostasis and thus Ca2+ flux.	278,279
		▪ Gene variants associated with variations in systolic blood pressure possibly through altered vascular tone as a response to changes in intracellular Ca2+ concentrations. This implies that NCKX3 might be involved in the vasoconstriction and vasodilation system.
TGFBR2**	Transforming growth factor beta receptor 2,	▪ Implicated in intracerebral haemorrhages, expression of angiogenic factors, BBB integrity and permeability, vessel morphology including vascular branching investigated in conditional TGFBR2 knock-out mice (neural deletion of TGFBR2). This implies potential neurovascular interactions.	280,281
		▪ Gene variants of TGFBR2 suggested being involved in cerebral cavernous malformation (CCM) type 1 in terms of severity of the disease but also phenotypic variability.

Neuronal hyperexcitability and vascular regulation

Neurophysiological alterations such as altered neural plasticity, hyperexcitability and lack of habituation to sensory stimuli have been associated with primary headaches.^213–215 Recent research focuses on the epigenome, which is believed to affect the susceptibility to develop migraine, ²¹⁶ with the field being at an emerging state. For example, a stressful environment during childhood has been linked to stress-induced alterations of homeostasis systems and development of migraine later in life ²¹⁷ thereby including the psychological aspect of primary headaches. One postulation is that epigenetic changes, occurring as a response to chronic stress, results in hyperexcitable neurons, ²¹⁸ thereby linking the epigenome to the CSD and trigeminovascular theory. However, methylation patterns have been found to vary considerably both between tissues and genes reported in a study investigating migraine-related genes. ²¹⁹

Focusing on vascular diseases, hypoperfusion in the white matter has been observed in CADASIL patients, ²²⁰ who seem to have prolonged and more frequent CSD ²²¹ possibly due to parenchymal hyperexcitability. ³⁵ Interestingly, a study showed that migraine is present in 75% of CADASIL patients of which many reported aura. The migraine aura seemed to increase the odds of encephalopathy. ²²² Moreover, a microvascular vasoconstrictor anomaly has been associated with CADASIL in noradrenaline and angiotensin pathways, but not with a vasodilator anomaly. ²²³ There are also some gender differences; brain vessels from males seem to be more sensitive to vasoconstrictors (angiotensin II and endothelin) than from females. ²²⁴ Overall, these parameters might predispose individuals to enhanced migraine susceptibility or vice versa.

As a response to neuronal hyperexcitability, one would expect increased neurotransmitter release from perivascular ganglionic nerve endings on the cerebral vasculature. Moreover, neurotransmitters might be released directly into the extracellular space surrounding the neuronal cell bodies in the ganglia, triggered by neuronal depolarization evoking SGC activation through neurotransmitter-receptor binding.^225,226 CGRP evoked release of NO and inflammatory responses in these glial cells, which was blocked by a CGRP receptor antagonist. ²²⁶ The exact functional role of SGCs is still unknown; however, they are believed to be involved in control of the neuronal microenvironment.^190,227 Neuronal excitability has been associated with SGC activation ²²⁷ and various pain syndromes seem to be related to diverse activation states of the glial cells. ²²⁸ The SGCs may thus be an important player in the sensitization process and chronification of migraine, in addition to the phenomena discussed in the previous sections. The SGCs constitute >90% of the total number of cells in the TG, and are linked to neurons via gap junctions (e.g. connexin) implicated in cell-cell signaling.^226,229 Connexin, but also pannexin are involved in neuronal excitability and synaptic plasticity in the CNS forming gap junctions/hemichannels facilitating neuronal transmission.^230,231 Experimental CSD provoked opening of pannexin1 channels, which was suggested to affect activation of the trigeminovascular system. ²³² Ashina et al. ¹⁴⁵ suggested that migraineurs suffer from abnormal neurogenic vascular control enduringly or permanently. Figure 5 gives an overview of possible pathophysiological effects, which occur due to migraine susceptible gene pools and external and internal headache triggers, all resulting in altered CBF. OPEN IN VIEWER

Primary headaches and sensitization

There are several primary headache disorders; migraine and trigeminal autonomic cephalgias (cluster headache, chronic paroxysmal hemicranias, inter alia) of which all are characterized by pain varying in duration, location and quality with accompanied autonomic and sensory neurological signs/symptoms depending on the diagnosis. ² Cluster headache patients experience autonomic symptoms such as rhinorrhea, lacrimation and abnormal sweating (parasympathetic influence) and miosis and ptosis (sympathetic influence).^2,233 Cranial sensory symptoms include experience of allodynia, and abnormalities in sensory perception of visual, olfactory and auditory stimuli. ²³⁴ This implies involvement of the parasympathetic, sympathetic and sensory nervous systems which form complex functional units implicated in processing of impulses. ²³⁵

During the headache attack phase, ganglia such as the TG and SPG, located outside the BBB, are activated and sensitized. The discovery of the sensitization process was pioneered by Burstein et al. ²³⁶ Basically, upon repeated activation/stimulation (heat, cold or pressure stimuli), the subsequent response to the same stimulus was enhanced, being referred to as sensitization. Sensitization has been associated with release of neurotransmitters, including CGRP, and could be a part of the mechanism behind allodynia. ²³⁶ Despite that the sensory trigeminal system contains several messenger molecules (e.g. CGRP, SP/NKA, PACAP and NO), only CGRP seems to be clearly associated with headache in migraine attacks.^156,237,238 So far, release of CGRP/PACAP has only been positively linked to acute attacks of migraine, or in bouts of cluster headache.^{12,150,152,153,156,157} Moreover, patients with nasal congestion and rhinorrhea show release of VIP, ²³⁷ which is observed in chronic migraine as well. ¹⁴³ Putatively, other molecules such as NO and cytokines are involved.^149,239

Perivascular nerve terminals originating from sensory, parasympathetic and sympathetic ganglia, believed to be involved in primary headaches, have been observed on cerebral arteries and arterioles. ⁴⁷ In addition to the involvement of multiple neurotransmitters in CBF regulation, they are part of: (i) the neurovascular unit in the brain parenchyma, and (ii) larger arteries belonging to the circle of Willis and pial cortex arterioles.

Chronification of pain in primary headaches

Migraine experienced at least 15 days per month for >3 months is considered chronic. Chronic cluster headache, on the other hand, should last for at least a year with or without remission periods (<1 month). ² Chronic headache and headache frequency correlated significantly in adults ²⁴⁰ possibly due to the detrimental effects of pathophysiological changes (e.g. trigeminovascular activation and sensitization) arising during headache attacks. In coherence, chronic migraine seems to increase with age.^241,242 Trigeminovascular activation has also been observed as a response to inflammation, which could be involved in headache progression. ²⁴³ At a cellular level, systemic inflammation of L4/5 DRG, involved in transmission of sensory signal, seems to modify the SGCs. Abnormal growth, formation of new gap junctions and increased ATP sensitivity of SGCs could potentially contribute to pain. ²⁴⁴ Structural and functional modification of SGCs might be connected to cellular stress and inflammation.^190,243,245 Other findings suggest that SGC activation, as a response to inflammation, results in modulated excitability of neurons in TG and DRG.^244,246 We hypothesize that neurogenic inflammation might have a reinforcing effect on neuronal hyperexcitability, a feed-forward loop, probably contributing to development of chronic headache.

The chronic pain state of the brain is believed to result in further structural, functional and chemical modifications. Chronic pain might arise due to impairment of pathways in the CNS, but also of the peripheral nervous system, which collectively manifest phenotypically as altered sensation, emotion and cognition or modulated pain response. ²⁴⁷ These changes are believed to be involved in disease progression from the episodic to chronic state. Therefore, it appears to be important for the future migraine management strategy to treat primary headaches early to avoid chronification bearing in mind that many primary headaches are considered complex polygenic disorders.

Episodic to chronic headache: Neurogenic inflammation and chronification

It is often argued, based on clinical observations, that a “migraine highway” is present in many patients. In the beginning, occasional attacks occur which, over the years, become frequent episodic migraine with up to 14 migraine headache days per month (according to ICHD-III beta ² ). This may, in some cases, expand even more, sometimes associated with migraine medication overuse.^248,249 The mechanisms involved are still not clear. Vasodilation, release of vasoactive peptides and plasma-protein extravasation constitute factors that are involved in development of inflammatory responses, also referred to as neurogenic inflammation,^76,139,250 which can be triggered by electrical stimulation of ganglia. ⁷⁶ Research shows that PACAP-38 is involved in central sensitization, ¹⁵⁴ neuronal and satellite glial cyclic adenosine monophosphate (cAMP) production in TG, ²⁵¹ and neurogenic inflammation. ²⁵² CGRP and VIP are associated with cAMP accumulation occurring simultaneously with cerebral vasodilation. ¹¹² Sarchielli et al. ¹⁰ showed that cAMP and cGMP release occurred after release of CGRP, NKA and nitrites in migraine attacks measured in internal jugular venous blood. In addition, both CGRP and cAMP induced NO and inducible NOS release from TG glial cells in culture. ²²⁶ However, single nucleotide polymorphisms in the neuronal NOS gene do not seem to increase the risk of migraine. ²⁵³ Moreover, in migraineurs, release of other inflammatory markers such as interleukin (IL) subtypes (e.g. IL-1β, IL-6, IL-10) and TNF have been observed,^239,254 which seems to be most likely in frequent or chronic migraine. A tendency of higher lymphocyte number in the peripheral blood was observed in chronic migraine compared to episodic migraine suggested to be linked to the inflammatory state of the patients. ²⁵⁵ In the calvarial periosteum of chronic migraineurs, gene expression of proinflammatory markers was increased (e.g. IL-6, IL-1 receptor type 2 (IL1R2), C-C motif chemokine ligand 2 (CCL2), C-X3-C motif chemokine ligand 1 (CX3CL1)), whereas gene expression of markers involved in suppression of inflammation was decreased. ²⁵⁶ Some of these markers might affect nociception. In addition, results show that application of IL-1β to the meninges is involved in activation and sensitization of nociceptors, which was not observed to the same degree for IL-6. ²⁵⁷

Summary

Primary headaches are considered cerebral neurovascular disorders of which abnormal brain activity and CBF changes have been observed. CBF changes might occur as a response to pathophysiological changes such as neuronal hyperexcitability and autoregulatory impairment. GWAS studies, investigating the susceptibility to develop migraine, revealed suggestive candidate genes related to vascular diseases/mechanisms. One might postulate that some headache attacks are linked to underlying undiscovered vascular diseases. On the other hand, neuronal abnormalities have an effect on vascular mechanisms and function. Even though the original vascular theory is considered to be secondary in nature, these findings still emphasize the importance of the vasculature in primary headaches.

To summarize, this review gives rise to the following connections between primary headaches and CBF: (i) headache attacks start in the CNS and result in activated and sensitized ganglia and neurotransmitter release (e.g. CGRP, PACAP), (ii) the intracranial cerebral vasculature (large cerebral arteries and arterioles on the surface of the brain) is supplied with perivascular nerve fibers originating from the sensory, sympathetic and parasympathetic nervous systems, (iii) neurovascular activation putatively gives rise to vasoconstrictive/vasodilative phases and altered CBF and metabolism in the affected brain regions, (iv) neurotransmitter release seems to affect cAMP levels followed by inflammatory responses, but also plasma–protein extravasation in certain areas such as the dura mater (if SP is released), and (v) inflammation might reinforce neuronal hyperexcitability, pathological alterations and cellular dysfunctions through a feed-forward loop and thus contribute to chronification from the episodic state.

Finally, from a treatment perspective, it is important to detect and treat primary headache disorders in time in order to avoid chronification. Furthermore, new treatment strategies are required, which take into account that some individuals are genetically predisposed to develop primary headaches while others, primarily, might develop it as a response to environmental stimuli (e.g. stress).