On the temporal relationship between throbbing migraine pain and arterial pulse

Pathophysiology of Migraine

OBJECTIVE

This article provides an overview of the current understanding of migraine pathophysiology through insights gained from the extended symptom spectrum of migraine, neuroanatomy, migraine neurochemistry, and therapeutics.

LATEST DEVELOPMENTS

Recent advances in human migraine research, including human experimental migraine models and functional neuroimaging, have provided novel insights into migraine attack initiation, neurochemistry, neuroanatomy, and therapeutic substrates. It has become clear that migraine is a neural disorder, in which a wide range of brain areas and neurochemical systems are implicated, producing a heterogeneous clinical phenotype. Many of these neural pathways are monoaminergic and peptidergic, such as those involving calcitonin gene-related peptide and pituitary adenylate cyclase-activating polypeptide. We are currently witnessing an exciting era in which specific drugs targeting these pathways have shown promise in treating migraine, including some studies suggesting efficacy before headache has even started.

ESSENTIAL POINTS

Migraine is a brain disorder involving both headache and altered sensory, limbic, and homeostatic processing. A complex interplay between neurotransmitter systems, physiologic systems, and pain processing likely occurs. Targeting various therapeutic substrates within these networks provides an exciting avenue for future migraine therapeutics.

Multi-region local field potential signatures and brain coherence alternations in response to nitroglycerin-induced migraine attacks

OBJECTIVE

To decipher the underlying mechanisms of nitroglycerin (NTG)-induced migraine electrophysiologically.

BACKGROUND

Migraine is a recurrent primary headache disorder with moderate to severe disability; however, the pathophysiology is not fully understood. Consequently, safe and effective therapies to alleviate migraine headaches are limited. Local field potential (LFP) recording, as a neurophysiological tool, has been widely utilized to investigate combined neuronal activity.

METHODS

We recorded LFP changes simultaneously from the anterior cingulate cortex, posterior nucleus of the thalamus, trigeminal ganglion, and primary visual cortex after NTG injection in both anesthetized and freely moving rats. Additionally, brain coherence was processed, and light-aversive behavior measurements were implemented.

RESULTS

Significant elevations of LFP powers with various response patterns for the delta, theta, alpha, beta, and gamma bands following NTG injection were detected in both anesthetized and freely moving rats; however, a surge of coherence alternations was exclusively observed in freely moving rats after NTG injection.

CONCLUSION

The multi-region LFP signatures and brain coherence alternations in response to NTG-induced migraine attacks were determined. Furthermore, the results of behavior measurements in the freely moving group indicated that NTG induced the phenomenon of photophobia in our study. All these findings offer novel insights into the interpretation of migraine mechanisms and related treatments.

Migraine drugs

According to recent studies, migraine affects more than 1 billion people worldwide, making it one of the world’s most prevalent diseases. Although this highly debilitating illness has been known since ancient times, the first therapeutic drugs to treat migraine, ergotamine (Gynergen) and dihydroergotamine (Dihydergot), did not appear on the market until 1921 and 1946, respectively. Both drugs originated from Sandoz, the world’s leading pharmaceutical company in ergot alkaloid research at the time. Historically, ergot alkaloids had been primarily used in obstetrics, but with methysergide (1-methyl-lysergic acid 1′-hydroxy-butyl-(2S)-amide), it became apparent that they also held some potential in migraine treatment. Methysergide was the first effective prophylactic drug developed specifically to prevent migraine attacks in 1959. On the basis of significantly improved knowledge of migraine pathophysiology and the discovery of serotonin and its receptors, Glaxo was able to launch sumatriptan in 1992. It was the first member from the class of triptans, which are selective 5-HT1B/1D receptor agonists. Recent innovations in acute and preventive migraine therapy include lasmiditan, a selective 5-HT1F receptor agonist from Eli Lilly, the gepants, which are calcitonin gene-related peptide (CGRP) receptor antagonists discovered at Merck & Co and BMS, and anti-CGRP/receptor monoclonal antibodies from Amgen, Pfizer, Eli Lilly, and others.

Graphical abstract

Migraine without aura

Migraine without aura is the commonest form of migraine in both children and adults. The diagnosis is made by applying the International Classification of Headache Disorders Third Edition subsection for migraine without aura (ICHD-3 subsection 1.1). Attacks in patients with migraine without aura are characterized by their polyphasic presentation (prodrome, headache phase, postdromal phase). The symptomatology of attacks is diverse and heterogeneous, with most common symptoms being photophobia, phonophobia, nausea, vomiting, and aggravation of pain by movement. The clinician and researcher who wants to learn about migraine without aura needs to be able to apply the ICHD-3 criteria with its specific symptomatology to make a correct diagnosis, but also needs to be aware about the plethora of symptoms patients may experience. In this chapter, the reader will explore the clinical phenotypical features of migraine without aura.

Pathophysiology of Migraine

PURPOSE OF REVIEW

This article summarizes the current understanding of the pathophysiology of migraine, including some controversial aspects of the underlying mechanisms of the disorder.

RECENT FINDINGS

Recent functional neuroimaging studies focusing on the nonpainful symptoms of migraine have identified key areas of the central nervous system implicated in the early phases of a migraine attack. Clinical studies of spontaneous and provoked migraine attacks, together with preclinical studies using translational animal models, have led to a better understanding of the disease and the development of disease-specific and targeted therapies.

SUMMARY

Our knowledge of the pathophysiology of migraine has advanced significantly in the past decades. Current evidence supports our understanding of migraine as a complex cyclical brain disorder that likely results from dysfunctional sensory processing and dysregulation of homeostatic mechanisms. This article reviews the underlying mechanisms of the clinical manifestations of each phase of the migraine cycle.

Clinical and vascular responses to propranolol and candesartan in migraine patients: A randomized controlled clinical trial

Background:

Both propranolol and candesartan are prophylactic drugs for migraine, but with unknown mechanisms of action. The objectives of the present study were to investigate these drugs’ effects on arterial wall dynamics and the potential relation between their vascular and clinical effect.

Methods:

The study was based on data from a previously published randomized, placebo-controlled, triple-blinded, double crossover clinical trial comparing the prophylactic effects of candesartan and propranolol in 72 patients. Finapres noninvasive blood pressure curves were analyzed. On the descending limb of the pulse curve, a notch is produced by pulse wave reflection, and its relative height compared to the top of the curve (the notch ratio) was used as a marker of arterial wall stiffness.

Results:

Candesartan decreased the notch ratio from baseline ( p = 0.005), reflecting more compliant arteries and vasodilation, whereas propranolol increased the notch ratio ( p = 0.005), reflecting less compliant arteries and vasoconstriction. There was no difference in baseline notch ratio between clinical responders and nonresponders.

Conclusion:

The drugs are both efficient prophylactic medications, yet they have opposite effects on arterial wall dynamics. This suggests that drug effects other than those on arterial compliance must be responsible for their prophylactic effect in migraine.

Repetitive Transcranial Magnetic Stimulation for Dysesthesia Caused by Subacute Myelo-Optico-Neuropathy: A Case Report

Subacute myelo-optico-neuropathy (SMON) is caused by the ingestion of clioquinol (5-chloro-7-iodo-8-hydroxyquinoline), which is an intestinal antibacterial drug. Patients with SMON typically suffer from abnormal dysesthesia in the lower limbs, which cannot explain the mechanism only in pathology and electrophysiology. Neuromodulation therapies are increasingly being investigated as a means of alleviating abnormal sensory disturbances. We report here the response to repetitive transcranial magnetic stimulation (rTMS) for dysesthesia in a patient with SMON. The patient underwent rTMS treatment once per week for 12 weeks. rTMS was administered at 10 Hz, 90% of the resting motor threshold over the bilateral primary motor cortex foot area, for a total of 1,500 stimuli per day. After the treatment had finished at 12 weeks, the abnormal dysesthesia gradually declined. At first, there were improvements only in the area with a feeling of adherence. Later, this sensation was eliminated. Three months following the application, most of the feeling of adherence had disappeared and the feeling of tightness was slightly reduced. In contrast, the throbbing feeling had not changed during this period. Dysesthesia may indicate a process of central sensitization, which would contribute to chronic neuromuscular dysfunction. This case suggests that rTMS is a promising therapeutic application for dysesthesia.

Migraine Pathophysiology

Migraine is the leading cause of years lost due to disability in individuals aged 15 to 49 years. Much has changed over the last three decades about our understanding of this complex neurological disorder. Various phases of migraine have been characterized and are the focus of this review. The premonitory phase involves bothersome symptoms experienced hours to days before migraine pain. Behavioral changes and functional neuroimaging studies point toward hypothalamic involvement during the premonitory and other migraine phases. Migraine aura is a disruptive, reversible neurological phenomenon that affects up to one-third of all migraineurs, and can overlap with the headache phase. The mechanism responsible for this phase is thought to be cortical spreading depolarization through the cortex. This process leads to temporary disruptions in ion homeostasis and the ensuing neuronal dysfunction. The headache phase involves activation of the trigeminocervical complex. Neuropeptides are implicated in trigeminal activation, and calcitonin gene-related peptide in particular has become a promising target of therapeutic intervention for migraine. The final phase of migraine is the postdrome, the period of time from the resolution of headache symptoms until return to baseline following a migraine. People often report neuropsychiatric, sensory, gastrointestinal, and general symptoms during this time, which can limit activity. Elucidating the neuroanatomical, chemical, and neuroimaging correlates of these migraine phases allows for an improved comprehension of the underlying changes associated with migraine symptomatology and can assist with evaluation of arising therapeutics for migraine management.

Updated Evaluation of IV Dihydroergotamine (DHE) for Refractory Migraine: Patient Selection and Special Considerations

Dihydroergotamine (DHE) is an ergot alkaloid derivative of substances produced by rye fungus. Ergotamine was first used in the field of gynecology and obstetrics, then used for migraine treatment a few years later. DHE was developed as a derivative of ergotamine. DHE, when compared to ergotamine, demonstrates greater alpha-adrenergic antagonist activity, lower arterial vasoconstriction, less dopaminergic agonism, and lower emetic potential. DHE can be delivered via several routes including intravenous (IV), intramuscular (IM), subcutaneous (SC), intranasal (IN), oral, and orally inhaled (although the latter two are not available in the USA and the last remains experimental only). DHE can be used in an outpatient basis in infusion centers, emergency departments, and urgent care centers, as well as inpatient treatment for admitted patients. There are protocols for adults as well as pediatric migraine treatment. DHE and other ergot alkaloids are considered contraindicated in pregnant women as they decrease uterine blood flow and increase uterine muscle contractility predisposing to spontaneous abortion. DHE during lactation is also not recommended as it can lead to gastrointestinal distress and weakness in infants; it can also suppress milk production. Caution should be taken before administering DHE in patients with cardiovascular risk factors. DHE is an older drug with an interesting history, yet it is still clinically useful today for patients with migraine attacks not responsive to triptans, who have a greater burden from migraine, and in refractory migraine.

Step-Headache: A Distinct Symptom of Migraine

Objectives  The diagnosis of migraine depends on various characteristics of headache with their associated constitutional symptoms such as nausea, vomiting, photophobia, and phonophobia. Relatively severe intensity, throbbing character, unilaterality, and aggravation with physical activity are the key features of migraine headache. We did this study to describe a new symptom (step-headache) in migraine in which some patients complained of uncomfortable or painful thump over the head with each footstep during walking or running. Materials and Methods  Self-designed proforma was filled in each clinically diagnosed patient of migraine or tension-type headache in an outpatient clinic setting. The symptom designated here as step-headache was evaluated in 150 patients of migraine including 26 patients with overlapping headache and 244 patients of tension-type headache. Binary logistic regression was used for association analysis of step-headache with subgroups of migraine and with other migraine features. Statistical Analysis  Frequency distributions were expressed as numbers (percentages) or mean ± standard deviation. Binary logistic regression was used for association analysis of step-headache with subgroups of migraine and with other migraine features. Results  Step-headache was experienced by 97 (64.67%) migraine patients with nearly equal distribution among the two clinical subtypes (61.5% for migraine with aura and 65.3% for migraine without aura) but with high prevalence among perimenopausal onset migraine. Of all the patients who had this symptom, 77.32% experienced it during all the migraine attacks. The step-headache was differentiable from throbs of migraine and their exacerbation during physical activity by its synchrony with footsteps and its presence during nonpulsatile headaches or nonheadache phases of migraine. Sensitivity of this symptom was 64.67% while specificity was 100%. Conclusion  Among primary headaches, step-headache is a less well-known but common and distinct symptom of migraine. It has good sensitivity and high specificity for migraine.

The association between migraine and physical exercise

BACKGROUND

There is an unmet need of pharmacological and non-pharmacological treatment options for migraine patients. Exercise can be used in the treatment of several pain conditions, including. However, what exact role exercise plays in migraine prevention is unclear. Here, we review the associations between physical exercise and migraine from an epidemiological, therapeutical and pathophysiological perspective.

METHODS

The review was based on a primary literature search on the PubMed using the search terms "migraine and exercise".

RESULTS

Low levels of physical exercise and high frequency of migraine has been reported in several large population-based studies. In experimental studies exercise has been reported as a trigger factor for migraine as well as migraine prophylaxis. Possible mechanisms for how exercise may trigger migraine attacks, include acute release of neuropeptides such as calcitonin gene-related peptide or alternation of hypocretin or lactate metabolism. Mechanisms for migraine prevention by exercise may include increased beta-endorphin, endocannabinoid and brain-derived neurotrophic factor levers in plasma after exercise.

CONCLUSION

In conclusion, it seems that although exercise can trigger migraine attacks, regular exercise may have prophylactic effect on migraine frequency. This is most likely due to an altered migraine triggering threshold in persons who exercise regularly. However, the frequency and intensity of exercise that is required is still an open question, which should be addressed in future studies to delineate an evidence-based exercise program to prevent migraine in sufferers.

Vascular Contributions to Migraine: Time to Revisit?

Migraine is one of the most prevalent and disabling neurovascular disorders worldwide. However, despite the increase in awareness and research, the understanding of migraine pathophysiology and treatment options remain limited. For centuries, migraine was considered to be a vascular disorder. In fact, the throbbing, pulsating quality of the headache is thought to be caused by mechanical changes in vessels. Moreover, the most successful migraine treatments act on the vasculature and induction of migraine can be accomplished with vasoactive agents. However, over the past 20 years, the emphasis has shifted to the neural imbalances associated with migraine, and vascular changes have generally been viewed as an epiphenomenon that is neither sufficient nor necessary to induce migraine. With the clinical success of peripherally-acting antibodies that target calcitonin gene-related peptide (CGRP) and its receptor for preventing migraine, this neurocentric view warrants a critical re-evaluation. This review will highlight the likely importance of the vasculature in migraine.

Characterization of dural afferent neurons innervating cranial blood vessels within the dura in rats

Dural afferent neurons are implicated in primary headaches including migraine. Although a significant portion of primary afferent neurons innervating the dura are myelinated A-type neurons, previous electrophysiological studies have primarily characterized the functional properties of small-sized C-type sensory neurons. Here we show the functional characterization of dural afferent neurons identified with the fluorescent dye DiI. DiI-positive neurons were divided into three types: small-, medium-, and large-sized neurons, based on their diameter, area, and membrane capacitance. The immunoreactivity of NF200, a marker of A-type myelinated neurons, was detected in most large-sized, but it was also present in a limited number of small- and medium-sized DiI-positive neurons. Capsaicin, a transient receptor potential vanilloid 1 agonist, induced the membrane currents in most small- and medium-sized neurons, but not in large-sized DiI-positive neurons. Tetrodotoxin-resistant Na⁺ channels were expressed in almost all types of DiI-positive neurons. Mechanosensitive currents were detected from a majority of large-sized, and to a lesser extent, small- and medium-sized DiI-positive neurons. The results suggest that most dural afferent neurons are nociceptive, e.g., polymodal C-type for small- and medium-sized neurons, and high-threshold nociceptive A-type mechanoreceptors for large-sized neurons. We also found that DiI-positive neurons differed with respect to passive and active membrane properties, and that sumatriptan, a representative drug used for the acute treatment of migraine attack, inhibited voltage-gated Ca²⁺ currents in all types of DiI-positive neurons. The present results showing the nociceptive properties of dural afferent neurons would contribute to understand the pathophysiology of primary headaches.

Pathophysiology of Migraine: A Disorder of Sensory Processing

Plaguing humans for more than two millennia, manifest on every continent studied, and with more than one billion patients having an attack in any year, migraine stands as the sixth most common cause of disability on the planet. The pathophysiology of migraine has emerged from a historical consideration of the "humors" through mid-20th century distraction of the now defunct Vascular Theory to a clear place as a neurological disorder. It could be said there are three questions: why, how, and when? Why: migraine is largely accepted to be an inherited tendency for the brain to lose control of its inputs. How: the now classical trigeminal durovascular afferent pathway has been explored in laboratory and clinic; interrogated with immunohistochemistry to functional brain imaging to offer a roadmap of the attack. When: migraine attacks emerge due to a disorder of brain sensory processing that itself likely cycles, influenced by genetics and the environment. In the first, premonitory, phase that precedes headache, brain stem and diencephalic systems modulating afferent signals, light-photophobia or sound-phonophobia, begin to dysfunction and eventually to evolve to the pain phase and with time the resolution or postdromal phase. Understanding the biology of migraine through careful bench-based research has led to major classes of therapeutics being identified: triptans, serotonin 5-HT1B/1D receptor agonists; gepants, calcitonin gene-related peptide (CGRP) receptor antagonists; ditans, 5-HT1F receptor agonists, CGRP mechanisms monoclonal antibodies; and glurants, mGlu5 modulators; with the promise of more to come. Investment in understanding migraine has been very successful and leaves us at a new dawn, able to transform its impact on a global scale, as well as understand fundamental aspects of human biology.

Characterization of a mouse model of headache

Migraine and other primary headache disorders affect a large population and cause debilitating pain. Establishing animal models that display behavioral correlates of long-lasting and ongoing headache, the most common and disabling symptom of migraine, is vital for the elucidation of disease mechanisms and identification of drug targets. We have developed a mouse model of headache, using dural application of capsaicin along with a mixture of inflammatory mediators (IScap) to simulate the induction of a headache episode. This elicited intermittent head-directed wiping and scratching as well as the phosphorylation of c-Jun N-terminal kinase in trigeminal ganglion neurons. Interestingly, dural application of IScap preferentially induced FOS protein expression in the excitatory but not inhibitory cervical/medullary dorsal horn neurons. The duration of IScap-induced behavior and the number of FOS-positive neurons correlated positively in individual mice; both were reduced to the control level by the pretreatment of antimigraine drug sumatriptan. Dural application of CGRP(8-37), the calcitonin gene-related peptide (CGRP) receptor antagonist, also effectively blocked IScap-induced behavior, which suggests that the release of endogenous CGRP in the dura is necessary for IScap-induced nociception. These data suggest that dural IScap-induced nocifensive behavior in mice may be mechanistically related to the ongoing headache in humans. In addition, dural application of IScap increased resting time in female mice. Taken together, we present the first detailed study using dural application of IScap in mice. This headache model can be applied to genetically modified mice to facilitate research on the mechanisms and therapeutic targets for migraine headache.

Treatment of Chronic Migraine with Focus on Botulinum Neurotoxins

Migraine is the most common neurological disorder, and contributes to disability and large healthcare costs in the United States and the world. The treatment of migraine until recently has focused on medications, both abortive and prophylactic, but treatment of chronic migraine has been revolutionized with the introduction of botulinum toxin injection therapy. In this review, we explore the current understanding of migraine pathophysiology, and the evolution of the use of botulinum toxin therapy including proposed pathophysiological mechanisms through animal data. We also discuss the similarities and differences between three injection techniques.

ASICs as therapeutic targets for migraine

Migraine is the most common neurological disorder and one of the most common chronic pain conditions. Despite its prevalence, the pathophysiology leading to migraine is poorly understood and the identification of new therapeutic targets has been slow. Several processes are currently thought to contribute to migraine including altered activity in the hypothalamus, cortical-spreading depression (CSD), and afferent sensory input from the cranial meninges. Decreased extracellular pH and subsequent activation of acid-sensing ion channels (ASICs) may contribute to each of these processes and may thus play a role in migraine pathophysiology. Although few studies have directly examined a role of ASICs in migraine, studies directly examining a connection have generated promising results including efficacy of ASIC blockers in both preclinical migraine models and in human migraine patients. The purpose of this review is to discuss the pathophysiology thought to contribute to migraine and findings that implicate decreased pH and/or ASICs in these events, as well as propose issues to be resolved in future studies of ASICs and migraine. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

[A causative role of vasodilation in migraine? No]

INTRODUCTION

The hypothesis that migraine pain is caused by vasodilation has been challenged by clinical and experimental evidence.

STATE OF ART

The most convincing arguments against the vascular hypothesis come from neuroimaging data. Magnetic resonance imaging studies show that spontaneous migraine attacks are not accompanied by extracranial vasodilation, and by only slight dilation of the intracranial arteries. Pharmacologically-induced migraine attacks also provide further evidence against the role of vasodilation in migraine. Vasodilators such as sildenafil and nitroglycerine trigger attacks without dilation of the middle cerebral artery diameter, whereas VIP (vasoactive intestinal peptide) markedly dilates intra- and extracranial arteries but does not induce migraine attacks. Clinical studies also show a lack of correspondence between the subjective experience of throbbing headache and the arterial pulse. Moreover, many acute anti-migraine agents are not vasoconstrictors.

PERSPECTIVES

Further studies are necessary to clarify the mechanisms of migraine headache generation.

CONCLUSIONS

Contrary to a longstanding and widespread belief, vasodilatation is neither sufficient nor necessary to cause migraine headache and is probably an epiphenomenon.

Towards dynamical network biomarkers in neuromodulation of episodic migraine

Computational methods have complemented experimental and clinical neurosciences and led to improvements in our understanding of the nervous systems in health and disease. In parallel, neuromodulation in form of electric and magnetic stimulation is gaining increasing acceptance in chronic and intractable diseases. In this paper, we firstly explore the relevant state of the art in fusion of both developments towards translational computational neuroscience. Then, we propose a strategy to employ the new theoretical concept of dynamical network biomarkers (DNB) in episodic manifestations of chronic disorders. In particular, as a first example, we introduce the use of computational models in migraine and illustrate on the basis of this example the potential of DNB as early-warning signals for neuromodulation in episodic migraine.

Does throbbing pain have a brain signature?

Pain sometimes has a throbbing, pulsating quality, particularly when it is severe and disabling. We recently challenged the presumption that this throbbing quality is a sensory experience of arterial pulsations, but were unable to offer an alternative explanation for its rhythmic character. Here we report a case study of a woman with a history of daily headache consistent with the diagnosis of chronic migraine, but whose throbbing quality persisted long after the resolution of the headache. This chronic, daily, and persistent throbbing sensation, in the absence of headache pain, prompted closer examination for its neurophysiological correlate. By simultaneously recording the subjective report of the throbbing rhythm, arterial pulse, and high-density electroencephalogram, we found that the subjective throbbing rate (48±1.7beats per minute) and heart rate (68±2beats per minute) were distinct, in accord with our previous observations that the 2 are unrelated. On spectral analysis of the electroencephalogram, we found that the overall amount of activity in the alpha range (8 to 12Hz), or alpha power, increased in association with greater throbbing intensity. In addition, we also found that the rhythmic oscillations of overall alpha power, the so-called modulations of alpha power, coincided with the timing of the throbbing rhythm, and that this synchrony, or coherence, was proportional to the subjective intensity of the throbbing quality. This index case will motivate further studies whose aim is to determine whether modulations of alpha power could more generally represent a neurophysiological correlate of the throbbing quality of pain.

Southern Headache Society supplement: the neurobiology of throbbing pain in migraine

Virtually everyone can recall an experience, migraine or not, in which pain had a throbbing, pulsatile quality, particularly in association with intense pain. Its pulsatile character strongly reinforces the common presumption that it coincides with the heartbeat. For migraine, a cerebral vascular origin of the throbbing quality is a central tenet of the prevailing scientific view of migraine pain. However, recent data challenge this perspective, with implications for our understanding of throbbing pain not only for migraine but also for the pathophysiology of throbbing pain in other conditions as well.

Is there a relationship between throbbing pain and arterial pulsations?

Pain can have a throbbing quality, especially when it is severe and disabling. It is widely held that this throbbing quality is a primary sensation of one's own arterial pulsations, arising directly from the activation of localized pain-sensory neurons by closely apposed blood vessels. We examined this presumption more closely by simultaneously recording the subjective report of the throbbing rhythm and the arterial pulse in human subjects of either sex with throbbing dental pain-a prevalent condition whose pulsatile quality is widely regarded a primary sensation. Contrary to the generally accepted view, which would predict a direct correspondence between the two, we found that the throbbing rate (44 bpm ± 3 SEM) was much slower than the arterial pulsation rate (73 bpm ± 2 SEM, p < 0.001), and that the two rhythms exhibited no underlying synchrony. Moreover, the beat-to-beat variation in arterial and throbbing events observed distinct fractal properties, indicating that the physiological mechanisms underlying these rhythmic events are distinct. Confirmation of the generality of this observation in other pain conditions would support an alternative hypothesis that the throbbing quality is not a primary sensation but rather an emergent property, or perception, whose "pacemaker" lies within the CNS. Future studies leading to an improved understanding of the neurobiological basis of clinically relevant pain qualities, such as throbbing, will also enhance our ability to measure and therapeutically target severe and disabling pain.

The role of the neurovascular scalp structures in migraine

AIM

To review reports suggesting a role for neurovascular scalp structures in migraine.

MAIN DATA REPORTED

(A) Scalp periarterial nervous fibres contain all the main peptides and receptors involved in pain. (B) It is possible to interrupt or alleviate migraine pain with a prolonged compression of the main scalp arteries, which decreases blood flow through the pain-sensitized vessels and probably induces a temporary conduction block of periarterial nociceptive fibres. (C) Painful points are present on the scalp arteries of a considerable percentage of migraine sufferers. (D) It is possible to stop or alleviate pain by intervening on nociceptive periarterial fibres, as for example with the injection of lidocaine or 3-5 ml saline, and with percutaneous application of a capsaicin cream.

CONCLUSION

The data reported suggest a role for neurovascular scalp structures in at least some patients with migraine. It would be of interest to find a clinical distinction between patients according to the prevalence of an intracranial or extracranial peripheral pain mechanism. This could lead to more efficacious treatments.

Migraine: possible role of shear-induced platelet aggregation with serotonin release

BACKGROUND

Migraine patients are at an increased risk for stroke, as well as other thromboembolic events. This warrants further study of the role of platelets in a proportion of migraine patients.

OBJECTIVE

To extend the "platelet hypothesis" using literature data and observations made in a rat model of shear stress-induced platelet aggregation. Such aggregation causes release of serotonin, leading to vasoconstriction during sufficiently strong aggregation and to long-lasting vasodilation when aggregation diminishes. This vasodilation also depends on nitric oxide and prostaglandin formation.

RESULTS

A role for platelet aggregation in a number of migraineurs is indicated by reports of an increased platelet activity during attacks and favorable effects of antiplatelet medication. We hypothesize that in those patients, a migraine attack with or without aura may both be caused by a rise in platelet-released plasma serotonin, albeit at different concentration. At high concentrations, serotonin may cause vasoconstriction and, consequently, the neuronal signs of aura, whereas at low concentrations, it may already stimulate perivascular pain fibers and cause vasodilation via local formation of nitric oxide, prostaglandins, and neuropeptides. Platelet aggregation may be unilaterally evoked by elevated shear stress in a stenotic cervico-cranial artery, by reversible vasoconstriction or by other cardiovascular abnormality, eg, a symptomatic patent foramen ovale. This most likely occurs when a migraine trigger has further enhanced platelet aggregability; literature shows that many triggers either stimulate platelets directly or reduce endogenous platelet antagonists like prostacyclin.

CONCLUSION

New strategies for migraine medication and risk reduction of stroke are suggested.

Antidromic vasodilatation and the migraine mechanism

Despite the fact that an unprecedented series of new discoveries in neurochemistry, neuroimaging, genetics and clinical pharmacology accumulated over the last 20 years has significantly increased our current knowledge, the underlying mechanism of the migraine headache remains elusive. The present review article addresses, from early evidence that emerged at the end of the nineteenth century, the role of ‘antidromic vasodilatation’ as part of the more general phenomenon, currently defined as neurogenic inflammation, in the unique type of pain reported by patients suffering from migraine headaches. The present paper describes distinctive orthodromic and antidromic properties of a subset of somatosensory neurons, the vascular- and neurobiology of peptides contained in these neurons, and the clinical–pharmacological data obtained in recent investigations using provocation tests in experimental animals and human beings. Altogether, previous and recent data underscore that antidromic vasodilatation, originating from the activation of peptidergic somatosensory neurons, cannot yet be discarded as a major contributing mechanism of the throbbing head pain and hyperalgesia of migraine.

Activation of TRPV4 on dural afferents produces headache-related behavior in a preclinical rat model

BACKGROUND

The mechanisms contributing to the pain of migraine are poorly understood although activation of afferent nociceptors in the trigeminovascular system has been proposed as a key event. Prior studies have shown that dural-afferent nociceptors are sensitive to both osmotic and mechanical stimuli. Based on the sensitivity to these stimuli we hypothesized that dural afferents express the osmo/mechano-sensitive channel transient receptor-potential vanilloid 4 (TRPV4).

METHODS

These studies used in vitro patch-clamp electrophysiology of trigeminal neurons retrogradely labeled from the dura to examine the functional expression of TRPV4. Additionally, we used a rat headache model in which facial/hind paw allodynia following dural stimulation is measured to determine whether activation of meningeal TRPV4 produces responses consistent with migraine.

RESULTS

These studies found that 56% and 49% of identified dural afferents generate currents in response to hypotonic solutions and 4α-PDD, respectively. The response to these stimuli indicates that dural afferents express TRPV4. Activation of meningeal TPRV4 using hypotonic solution or 4α-PDD in vivo resulted in both facial and hind paw allodynia that was blocked by the TRPV4 antagonist RN1734.

CONCLUSION

These data indicate that activation of TRPV4 within the meninges produces afferent nociceptive signaling from the head that may contribute to migraine headache.

Peripheral targets of 5-HT(1D) receptor immunoreactive trigeminal ganglion neurons

OBJECTIVE

The aim of the current study was to determine the proportion of trigeminal primary afferent neurons that innervate the intracranial vasculature, and other craniofacial tissues, that are also 5 hydroxy triptamine (5-HT)(1D) receptor immunoreactive.

METHODS

Retrograde tracing and immunohistochemistry was used to identify 5-HT(1D) receptor labeled trigeminal primary afferent neurons that innervate the lacrimal gland (n = 3 animals), nasal mucosa (n = 3 animals), and the intracranial vasculature (middle meningeal artery in the dura [n = 3 animals] and middle cerebral artery [n = 3 animals]).

RESULTS

The percentage of neurons that were 5-HT(1D) receptor immunoreactive was greater for primary afferent neurons innervating the middle meningeal artery (41.8 ± 1%) than those innervating the middle cerebral artery (28.4 ± 0.8%), nasal mucosa (25.6 ± 1%), or lacrimal gland (23.5 ± 3%). For each retrograde labeled population, the 5-HT(1D) receptor immunoreactive cells were among the smallest of the retrograde labeled cells.

CONCLUSIONS

These findings provide a basis for understanding the role of 5-HT(1D) receptor agonists (eg, triptans) in the treatment of primary vascular headaches and suggest that the selectivity of triptans in the treatment of these headaches does not appear to result from specific localization of the 5-HT(1D) receptor to trigeminovascular neurons alone.