Activation of trigeminovascular neurons by glyceryl trinitrate

Molecular Mechanisms of Migraine: Nitric Oxide Synthase and Neuropeptides

Migraine is a common condition with disabling attacks that burdens people in the prime of their working lives. Despite years of research into migraine pathophysiology and therapeutics, much remains to be learned about the mechanisms at play in this complex neurovascular condition. Additionally, there remains a relative paucity of specific and targeted therapies available. Many sufferers remain underserved by currently available broad action preventive strategies, which are also complicated by poor tolerance and adverse effects. The development of preclinical migraine models in the laboratory, and the advances in human experimental migraine provocation, have led to the identification of key molecules likely involved in the molecular circuity of migraine, and have provided novel therapeutic targets. Importantly, the identification that vasoconstriction is neither necessary nor required for headache abortion has changed the landscape of migraine treatment and has broadened the therapy targets for patients with vascular risk factors or vascular disease. These targets include nitric oxide synthase (NOS) and several neuropeptides that are involved in migraine. The ability of NO donors and infusion of some of these peptides into humans to trigger typical migraine-like attacks has supported the development of targeted therapies against these molecules. Some of these, such as those targeting calcitonin gene-related peptide (CGRP), have already reached clinical practice and are displaying a positive outcome in migraineurs for the better by offering targeted efficacy without significant adverse effects. Others, such as those targeting pituitary adenylate cyclase activating polypeptide (PACAP), are showing promise and are likely to enter phase 3 clinical trials in the near future. Understanding these nitrergic and peptidergic mechanisms in migraine and their interactions is likely to lead to further therapeutic strategies for migraine in the future.

Differential actions of indomethacin: clinical relevance in headache

ABSTRACT

Nonsteroidal anti-inflammatory drugs, cyclooxygenase inhibitors, are used routinely in the treatment of primary headache disorders. Indomethacin is unique in its use in the diagnosis and treatment of hemicrania continua and paroxysmal hemicrania. The mechanism of this specific action is not fully understood, although an interaction with nitric oxide (NO) signaling pathways has been suggested. Trigeminovascular neurons were activated by dural electrical stimulation, systemic administration of an NO donor, or local microiontophoresis of L-glutamate. Using electrophysiological techniques, we subsequently recorded the activation of trigeminovascular neurons and their responses to intravenous indomethacin, naproxen, and ibuprofen. Administration of indomethacin (5 mg·kg-1), ibuprofen (30 mg·kg-1), or naproxen (30 mg·kg-1) inhibited dural-evoked firing within the trigeminocervical complex with different temporal profiles. Similarly, both indomethacin and naproxen inhibited L-glutamate-evoked cell firing suggesting a common action. By contrast, only indomethacin was able to inhibit NO-induced firing. The differences in profile of effect of indomethacin may be fundamental to its ability to treat paroxysmal hemicrania and hemicrania continua. The data implicate NO-related signaling as a potential therapeutic approach to these disorders.

Signaling Interaction between Facial and Meningeal Inputs of the Trigeminal System Mediates Peripheral Neurostimulation Analgesia in a Rat Model of Migraine

Peripheral neurostimulation within the trigeminal nerve territory has been used for pain alleviation during migraine attacks, but the mechanistic basis of this non-invasive intervention is still poorly understood. In this study, we investigated the therapeutic role of peripheral stimulation of the trigeminal nerve, which provides homosegmental innervation to intracranial structures, by assessing analgesic effects in a nitroglycerin (NTG)-induced rat model of migraine. As a result of neurogenic inflammatory responses in the trigeminal nervous system, plasma protein extravasation was induced in facial skin by applying noxious stimulation to the dura mater. Noxious chemical stimulation of the dura mater led to protein extravasation in facial cutaneous tissues and caused mechanical sensitivity. Trigeminal ganglion (TG) neurons were double-labeled via retrograde tracing to detect bifurcated axons. Extracellular recordings of wide dynamic range (WDR) neurons in the spinal trigeminal nucleus caudalis (Sp5C) demonstrated the convergence and interaction of inputs from facial tissues and the dura mater. Peripheral neurostimulation of homotopic facial tissues represented segmental pain inhibition on cephalic cutaneous allodynia in the migraine model. The results indicated that facial territories and intracranial structures were directly connected with each other through bifurcated double-labeled neurons in the TG and through second-order WDR neurons. Homotopic stimulation at the C-fiber intensity threshold resulted in much stronger inhibition of analgesia than the same intensity of heterotopic stimulation. These results provide novel evidence for the neurological bases through which peripheral neurostimulation may be effective in treating migraine in clinical practice.

Amplification of glyceryl trinitrate-induced headache features by noxious craniofacial stimuli in pain-free healthy humans

AIM

Glyceryl trinitrate (GTN) provokes an immediate migraine-like headache, followed by a delayed migraine attack in migraineurs. In healthy volunteers, only an immediate, less severe and shorter headache occurs. The presence of an already sensitized nervous system in migraineurs may underlie the more intense and prolonged GTN-evoked headaches. We tested if in healthy humans, application of noxious cutaneous and/or mechanical stimulation within craniofacial region would enhance or prolong GTN-evoked headache.

MATERIALS & METHODS

Noxious stimuli with a capsaicin patch on forehead, a mechanical headband, or both were applied prior to sublingual GTN (0.5 mg) in 20 healthy volunteers. GTN-induced headache characteristics and sensory responsiveness were recorded.

RESULTS

A more intense GTN-evoked headache was produced following application of headband.

CONCLUSION

Noxious mechanical stimulation prior to GTN resulted in a more intense GTN-evoked headache.

The role of the transient receptor potential ankyrin type-1 (TRPA1) channel in migraine pain: evaluation in an animal model

Background

Clinical and experimental studies have pointed to the possible involvement of the transient receptor potential ankyrin type-1 (TRPA1) channels in migraine pain. In this study, we aimed to further investigate the role of these channels in an animal model of migraine using a novel TRPA1 antagonist, ADM_12, as a probe.

Methods

The effects of ADM_12 on nitroglycerin-induced hyperalgesia at the trigeminal level were investigated in male rats using the quantification of nocifensive behavior in the orofacial formalin test. The expression levels of the genes coding for c-Fos, TRPA1, calcitonin gene-related peptide (CGRP) and substance P (SP) in peripheral and central areas relevant for migraine pain were analyzed. CGRP and SP protein immunoreactivity was also evaluated in trigeminal nucleus caudalis (TNC).

Results

In rats bearing nitroglycerin-induced hyperalgesia, ADM_12 showed an anti-hyperalgesic effect in the second phase of the orofacial formalin test. This effect was associated to a significant inhibition of nitroglycerin-induced increase in c-Fos, TRPA1 and neuropeptides mRNA levels in medulla-pons area, in the cervical spinal cord and in the trigeminal ganglion. No differences between groups were seen as regards CGRP and SP protein expression in the TNC.

Conclusions

These findings support a critical involvement of TRPA1 channels in the pathophysiology of migraine, and show their active role in counteracting hyperalgesia at the trigeminal level.

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.

Parthenolide is the Component of Tanacetum Parthenium that Inhibits Nitroglycerin-Induced Fos Activation: Studies in an Animal Model of Migraine

Tanacetum parthenium (TP) is a member of the Asteracee family long used empirically as a herbal remedy for migraine. So far, however, clinical trials have failed to prove consistently the effectiveness of TP extracts in preventing migraine attacks, probably as a consequence of the uncertainty as regards the active principle. In this study, the biological effects of different TP extracts and purified parthenolide were tested in an animal model of migraine based on the quantification of neuronal activation induced by nitroglycerin. The extract enriched in parthenolide significantly reduced nitroglycerin-induced Fos expression in the nucleus trigeminalis caudalis. Purified parthenolide inhibited nitroglycerin-induced neuronal activation in additional brain nuclei and, significantly, the activity of nuclear factor-κB. These findings strongly suggest that parthenolide is the component responsible for the biological activity of TP as regards its antimigraine effect and provide important information for future controlled clinical trials.

CGRP Release and c-fos Expression within Trigeminal Nucleus Caudalis of the Rat following Glyceryltrinitrate Infusion

Neuropeptide release and the expression of c-fos like immunoreactivity (c-fos LI) within trigeminal nucleus caudalis neurons (TNC) are activation markers of the trigeminal nerve system. Glyceryltrinitrate (GTN) is believed to stimulate the trigeminal nerve system, thereby causing headache. We examined the effects of a 30 min NO-donor infusion on CGRP release in jugular vein blood and c-fos LI within TNC of the rat. GTN (2 and 50 μg/kg/min) or NONOate infusion (25 nmol/kg/min) did not cause any CGRP release during and shortly after infusion, whereas administration of capsaicin resulted in strongly increased CGRP levels. GTN infusion (2 μg/kg/min for 30 min) did not lead to enhanced c-fos LI after 2 h and 4 h, whereas capsaicin infusion caused a time- and dose-dependent expression of c-fos LI within laminae I and II of the TNC. Surprisingly, GTN attenuated capsaicin-induced c-fos expression by 64%. The nitric oxide synthase (NOS) inhibitor L-NAME (5 and 50 mg/kg) reduced capsaicin-induced c-fos LI dose dependently (reduction by 13% and 59%). We conclude that GTN may lead to headaches by mechanisms independent of CGRP release from trigeminal nerve fibres. GTN doses comparable to those used in humans did not activate or sensitize the trigeminal nerve system. Both GTN and L-NAME reduced capsaicin-induced c-fos LI. This is most likely due to a feedback inhibition of nitric oxide synthases, which indicates that the c-fos response to capsaicin within TNC is mediated by NO dependent mechanisms.

Meningeal blood flow is controlled by H2 S-NO crosstalk activating a HNO-TRPA1-CGRP signalling pathway

BACKGROUND AND PURPOSE

Meningeal blood flow is controlled by CGRP released from trigeminal afferents and NO mainly produced in arterial endothelium. The vasodilator effect of NO may be due to the NO-derived compound, nitroxyl (HNO), generated through reaction with endogenous H2 S. We investigated the involvement of HNO in CGRP release and meningeal blood flow.

EXPERIMENTAL APPROACH

Blood flow in exposed dura mater of rats was recorded by laser Doppler flowmetry. CGRP release from the dura mater in the hemisected rat head was quantified using an elisa. NO and H2 S were localized histochemically with specific sensors.

KEY RESULTS

Topical administration of the NO donor diethylamine-NONOate increased meningeal blood flow by 30%. Pretreatment with oxamic acid, an inhibitor of H2 S synthesis, reduced this effect. Administration of Na2 S increased blood flow by 20%, an effect abolished by the CGRP receptor antagonist CGRP8-37 or the TRPA1 channel antagonist HC030031 and reduced when endogenous NO synthesis was blocked. Na2 S dose-dependently increased CGRP release two- to threefold. Co-administration of diethylamine-NONOate facilitated CGRP release, while inhibition of endogenous NO or H2 S synthesis lowered basal CGRP release. NO and H2 S were mainly localized in arterial vessels, HNO additionally in nerve fibre bundles. HNO staining was lost after treatment with L-NMMA and oxamic acid.

CONCLUSIONS AND IMPLICATIONS

NO and H2 S cooperatively increased meningeal blood flow by forming HNO, which activated TRPA1 cation channels in trigeminal fibres, inducing CGRP release. This HNO-TRPA1-CGRP signalling pathway may be relevant to the pathophysiology of headaches.

Abnormal brain activity changes in patients with migraine: a short-term longitudinal study

BACKGROUND AND PURPOSE

Whether or not migraine can cause cumulative brain alterations due to frequent migraine-related nociceptive input in patients is largely unclear. The aim of this study was to characterize longitudinal changes in brain activity between repeated observations within a short time interval in a group of female migraine patients, using resting-state functional magnetic resonance imaging.

METHODS

Nineteen patients and 20 healthy controls (HC) participated in the study. Regional homogeneity (ReHo) and functional interregional connectivity were assessed to determine the focal and global features of brain dysfunction in migraine. The relationship between changes in headache parameters and longitudinal brain alterations were also investigated.

RESULTS

All patients reported that their headache activity increased over time. Abnormal ReHo changes in the patient group relative to the HC were found in the putamen, orbitofrontal cortex, secondary somatosensory cortex, brainstem, and thalamus. Moreover, these brain regions exhibited longitudinal ReHo changes at the 6-week follow-up examination. These headache activity changes were accompanied by disproportionately dysfunctional connectivity in the putamen in the migraine patients, as revealed by functional connectivity analysis, suggesting that the putamen plays an important role in integrating diverse information among other migraine-related brain regions.

CONCLUSIONS

The results obtained in this study suggest that progressive brain aberrations in migraine progress as a result of increased headache attacks.

Modelling headache and migraine and its pharmacological manipulation

Similarities between laboratory animals and humans in anatomy and physiology of the cephalic nociceptive pathways have allowed scientists to create successful models that have significantly contributed to our understanding of headache. They have also been instrumental in the development of novel anti-migraine drugs different from classical pain killers. Nevertheless, modelling the mechanisms underlying primary headache disorders like migraine has been challenging due to limitations in testing the postulated hypotheses in humans. Recent developments in imaging techniques have begun to fill this translational gap. The unambiguous demonstration of cortical spreading depolarization (CSD) during migraine aura in patients has reawakened interest in studying CSD in animals as a noxious brain event that can activate the trigeminovascular system. CSD-based models, including transgenics and optogenetics, may more realistically simulate pain generation in migraine, which is thought to originate within the brain. The realization that behavioural correlates of headache and migrainous symptoms like photophobia can be assessed quantitatively in laboratory animals, has created an opportunity to directly study the headache in intact animals without the confounding effects of anaesthetics. Headache and migraine-like episodes induced by administration of glyceryltrinitrate and CGRP to humans and parallel behavioural and biological changes observed in rodents create interesting possibilities for translational research. Not unexpectedly, species differences and model-specific observations have also led to controversies as well as disappointments in clinical trials, which, in return, has helped us improve the models and advance our understanding of headache. Here, we review commonly used headache and migraine models with an emphasis on recent developments.

Effect of genetic deletion and pharmacological antagonism of P2X7 receptors in a mouse animal model of migraine

BACKGROUND

Purine receptors participate in peripheral and central sensitization and are associated with migraine headache. We investigated the role of P2X7 receptor (P2X7) in a nitroglycerin (NTG)-induced mouse model of migraine.

METHODS

Intraperitoneal NTG injection (15 mg/kg) triggered thermal hyperalgesia in the hindpaws of wild-type C57BL/6J mice, followed by the induction of c-fos in upper cervical spinal cord and trigeminal nucleus caudalis. The effect of genetic deletion of P2X7 and the selective P2X7 antagonist Brilliant Blue G (BBG) were examined on hyperalgesia and c-fos induction.

RESULTS

NTG decreased the paw withdrawal threshold in both wild-type and P2X7 knockout mice. Nevertheless, subacute BBG treatment (50 mg/kg/day i.p.) completely prevented the effect of NTG in wild-type, but not in knockout mice. Whereas P2X7 deficiency differentially affected the expression of c-fos, the average number of fos-immuno-reactive neurons in trigeminal nucleus caudalis, but not in upper cervical spinal cord was lower in BBG-treated wild-type mice after NTG treatment.

CONCLUSIONS

Our results show that P2X7 receptors might participate in the pathogenesis of migraine, although upregulation of other P2X receptors probably compensate for the loss of its action in knockout mice. The data also suggest the therapeutic potential of P2X7 antagonists for the treatment of migraine.

Effects of CGRP receptor antagonism in nitroglycerin-induced hyperalgesia

Background

The release of calcitonin gene-related peptide (CGRP) from trigeminal nerves plays a central role in the pathophysiology of migraine and clinical evidence shows an antimigraine effect for CGRP receptor antagonists. Systemic administration of nitroglycerin (NTG), a nitrovasodilator, consistently provokes spontaneous-like migraine attacks in migraine sufferers; in the rat, systemic NTG induces a condition of hyperalgesia, probably through the activation of cerebral/spinal structures involved in nociceptive transmission.

Aim

The aim of this article is to test the analgesic effect of the CGRP receptor antagonist MK-8825 in two animal models of pain that may be relevant for migraine: the tail flick test and the formalin test performed during NTG-induced hyperalgesia.

Results

MK-8825 showed analgesic activity when administered alone at both the tail flick test and the formalin test. Furthermore, the CGRP antagonist proved effective in counteracting NTG-induced hyperalgesia in both tests. MK-8825 indeed reduced the nociceptive behavior when administered either simultaneously or prior to (30–60 minutes before) NTG.

Conclusion

These data suggest that MK-8825 may represent a potential therapeutic tool for the treatment of migraine.

Effect of sex and estrogens on neuronal activation in an animal model of migraine

OBJECTIVE

In this study, we evaluated the influence of sex and estrogen treatment on nitroglycerin (NTG)-induced neuronal activation in the rat brain.

BACKGROUND

Systemic NTG activates cerebral nuclei of rat involved in nociceptive transmission, as well as in neuroendocrine and autonomic functions. These changes are considered relevant for migraine, since NTG consistently induces spontaneous-like attacks in migraineurs.

METHODS

Intact and castrated male and female rats, and castrated female rats treated with estradiol benzoate (or placebo) were injected with NTG and sacrificed after 4 hours. Rats were perfused, and their brains were processed for Fos protein, a marker of neuronal activation.

RESULTS

Data showed a reduced expression of NTG-induced Fos protein in the paraventricular nucleus (PVH), supraoptic nucleus (SON), and nucleus trigeminalis caudalis (SPVC) of male rats in comparison with female rats. Furthermore, in castrated female rats, NTG-induced neuronal activation was reduced in PVH, SON, central nucleus of the amygdala (AMI), nucleus tractus solitarius (NTS), area postrema (AP), and SPVC, while in castrated male rats Fos expression was reduced uniquely in the SPVC. Chronic administration of estrogens restored Fos protein expression in PVH, SON, AMI, NTS, AP, and SPVC in castrated female rats.

CONCLUSION

These data provide a support for the existence of a sexual dimorphism in NTG-induced neuronal activation, and they prompt a specific model for evaluating and modulating the influence of estrogens upon the cerebral structures implicated in the pathophysiology of migraine.

Peripheral and central alterations of pituitary adenylate cyclase activating polypeptide-like immunoreactivity in the rat in response to activation of the trigeminovascular system

Pituitary adenylate cyclase activating polypeptide (PACAP) is present in the cranial arteries and trigeminal sensory neurons. We therefore examined the alterations in PACAP-like immunoreactivity (PACAP-LI) in a time-dependent manner in two rat models of trigeminovascular system (TS) activation. In one group chemical stimulation (CS) was performed with i.p. nitroglycerol (NTG), and in the other one the trigeminal ganglia (TRG) were subjected to electrical stimulation (ES). The two biologically active forms, PACAP-38 and PACAP-27, were determined by means of radioimmunoassay (RIA) and mass spectrometry (MS) in the plasma, the cerebrospinal fluid (CSF), the trigeminal nucleus caudalis (TNC), the spinal cord (SC) and the TRG. The tissue concentrations of PACAP-27 were 10 times lower than those of PACAP-38 in the TNC and SC, but about half in the TRG. PACAP-38, but not PACAP-27, was present in the plasma. Neither form could be identified in the CSF. PACAP-38-LI in the plasma, SC and TRG remained unchanged after CS, but it was increased significantly in the TNC 90 and 180 min after NTG injection. In response to ES of the TRG, the level of PACAP-38 in the plasma and the TNC was significantly elevated 90 and 180 min later, but not in the SC or the TRG. The alterations in the levels of PACAP-27 in the tissue homogenates in response to both forms of stimulation were identical to those of PACAP-38. The selective increases in both forms of PACAP in the TNC suggest its important role in the central sensitization involved in migraine-like headache.

A trigeminoreticular pathway: implications in pain

Neurons in the caudalmost ventrolateral medulla (cmVLM) respond to noxious stimulation. We previously have shown most efferent projections from this locus project to areas implicated either in the processing or modulation of pain. Here we show the cmVLM of the rat receives projections from superficial laminae of the medullary dorsal horn (MDH) and has neurons activated with capsaicin injections into the temporalis muscle. Injections of either biotinylated dextran amine (BDA) into the MDH or fluorogold (FG)/fluorescent microbeads into the cmVLM showed projections from lamina I and II of the MDH to the cmVLM. Morphometric analysis showed the retrogradely-labeled neurons were small (area 88.7 µm(2)±3.4) and mostly fusiform in shape. Injections (20-50 µl) of 0.5% capsaicin into the temporalis muscle and subsequent immunohistochemistry for c-Fos showed nuclei labeled in the dorsomedial trigeminocervical complex (TCC), the cmVLM, the lateral medulla, and the internal lateral subnucleus of the parabrachial complex (PBil). Additional labeling with c-Fos was seen in the subnucleus interpolaris of the spinal trigeminal nucleus, the rostral ventrolateral medulla, the superior salivatory nucleus, the rostral ventromedial medulla, and the A1, A5, A7 and subcoeruleus catecholamine areas. Injections of FG into the PBil produced robust label in the lateral medulla and cmVLM while injections of BDA into the lateral medulla showed projections to the PBil. Immunohistochemical experiments to antibodies against substance P, the substance P receptor (NK1), calcitonin gene regulating peptide, leucine enkephalin, VRL1 (TPRV2) receptors and neuropeptide Y showed that these peptides/receptors densely stained the cmVLM. We suggest the MDH- cmVLM projection is important for pain from head and neck areas. We offer a potential new pathway for regulating deep pain via the neurons of the TCC, the cmVLM, the lateral medulla, and the PBil and propose these areas compose a trigeminoreticular pathway, possibly the trigeminal homologue of the spinoreticulothalamic pathway.

The lack of peripheral pathology in migraine headache

This article reviews the baffling problem of the pathophysiology behind a peripheral genesis of migraine pain--or more particularly the baffling problem of its absence. I examine a number of pathophysiological states and the effector mechanisms for these states and find most of them very plausible and that they are all supported by abundant evidence. However, this evidence is mostly indirect; to date the occurrence of any of the presumed pathological states has not been convincingly demonstrated. Furthermore, there is little evidence of increased trigeminal sensory traffic into the central nervous system during a migraine attack. The article also examines a number of observations and experimental programs used to bolster a theory of peripheral pathology and suggests reasons why they may in fact not bolster it. I suggest that a pathology, if one exists, may be in the brain and even that it may not be a pathology at all. Migraine headache might just happen because of random noise in an exquisitely sensitive and complex network. The article suggests an experimental program to resolve these issues.

Effects of heating coagulation of middle meningeal artery on plasma CGRP level and c-fos expression in migraine rat triggered by nitroglycerin

Current theory or hypothesis relevant to migraine indicates that trigeminovascular system plays a pivotal role in the pathophysiology of migraine. Particularly, release of neuropeptide and induction of c-fos like immunoreactivity (c-fos LI) within trigeminal nucleus caudalis neurons are regarded as activation markers of trigeminovascular system. In the present study, we set up a rat model for migraine triggered by nitroglycerin (NTG) and coagulated the middle meningeal artery by heating. Using this model, we determined the plasma calcitonin gene-related peptide (CGRP) level as well as the expression of c-fos in trigeminal nucleus caudalis of rats. We found that NTG led to markedly increase in plasma CGRP level and c-fos expression in trigeminal nucleus caudalis compared with the isotonic saline-treated group (P < 0.05). More importantly, heat coagulation of middle meningeal artery could decrease plasma CGRP level and c-fos expression in trigeminal nucleus caudalis (P < 0.05). Heat coagulation of middle meningeal artery may ameliorate sufferings of rat induced by NTG and play an important role in restraining the release of CGRP as well as the activation of neurons in trigeminal nucleus caudalis in rats following NTG infusion.

Sumatriptan alleviates nitroglycerin-induced mechanical and thermal allodynia in mice

The association between the clinical use of nitroglycerin (NTG) and headache has led to the examination of NTG as a model trigger for migraine and related headache disorders, both in humans and laboratory animals. In this study in mice, we hypothesized that NTG could trigger behavioural and physiological responses that resemble a common manifestation of migraine in humans. We report that animals exhibit a dose-dependent and prolonged NTG-induced thermal and mechanical allodynia, starting 30-60 min after intraperitoneal injection of NTG at 5-10 mg/kg. NTG administration also induced Fos expression, an anatomical marker of neuronal activity in neurons of the trigeminal nucleus caudalis and cervical spinal cord dorsal horn, suggesting that enhanced nociceptive processing within the spinal cord contributes to the increased nociceptive behaviour. Moreover, sumatriptan, a drug with relative specificity for migraine, alleviated the NTG-induced allodynia. We also tested whether NTG reduces the threshold for cortical spreading depression (CSD), an event considered to be the physiological substrate of the migraine aura. We found that the threshold of CSD was unaffected by NTG, suggesting that NTG stimulates migraine mechanisms that are independent of the regulation of cortical excitability.

What have we learnt from triggering migraine?

PURPOSE OF REVIEW

This review presents what we have learnt from triggering migraine.

RECENT FINDINGS

Experimental studies have shown that glyceryl trinitrate (GTN), calcitonin gene-related peptide (CGRP), pituitary adenylate cyclase activating polypeptide-38 (PACAP38) and prostaglandin I2 (PGI2) induce migraine-like attacks in migraine suffers indistinguishable from their spontaneous attacks. These studies point to two key pathways to play an important role in migraine pathophysiology: cyclic guanosine monophosphate (cGMP) and cyclic adenosine monophosphate (cAMP). At present, no valid experimental model exists to reproduce aura episodes in migraine with aura patients. Familiar hemiplegic migraine patients seem to be less sensitive to GTN and CGRP provocation compared with common types of migraine. Advances in recent imaging studies suggest neuronal mechanisms to be behind migraine attacks. The experimental headache models have resulted in development and an ongoing search of new migraine targets.

SUMMARY

Human models of migraine offer unique possibilities to study mechanisms responsible for different migraine subtypes and to explore the mechanisms of action of existing and future antimigraine drugs. Adding advanced imaging techniques to the models may lead to a better understanding of the complex events that constitutes a migraine attack and thereby more targeted ways of intervention.

Temporal profile of vascular changes induced by systemic nitroglycerin in the meningeal and cortical districts

BACKGROUND

Clinical studies indicated that nitric oxide (NO) donors cause regional changes in cerebral blood flow (CBF), similar to those reported in spontaneous migraine. Systemic nitroglycerin (NTG), a NO donor, is a well-accepted experimental model of migraine. In this study we have examined the effects of NTG on the meningeal and cortical blood flow in rats.

METHODS

Regional blood flow was monitored in male Sprague-Dawley rats using laser Doppler flowmetry before and after NTG/saline injection over 150 minutes. The effect of pre-treatment with Nω-nitro-L-arginine ester (L-NAME) or 7-nitroindazole (7-NI) on NTG-induced changes on blood flow was also investigated.

RESULTS

In the dura NTG caused a biphasic response represented by an initial decrease in blood flow followed by a significant increase. At variance, in the cortex NTG caused only an increase in blood flow. Pre-treatment with either L-NAME or 7-NI prevented NTG-induced increase in blood flow in both districts, while only L-NAME also prevented NTG-induced decrease in dural blood flow.

CONCLUSION

The present findings provide additional information on the timing of effects of NTG on blood flow at both the meningeal and cortical levels. These effects seem to be related to vasoregulatory mechanisms and/or metabolic activity in response to the synthesis of endogenous NO.

Experimental models of migraine

In vitro studies on animal and human cephalic vessels allow the measurement of second messengers or intracellular calcium concentrations and the evaluation of the role of endogenous neuropeptides in perivascular nerve endings involved in migraine pathophysiology. In addition, in vitro human models allow the assessment of receptorial cranial selectivity and the collection of reliable information regarding the behavior of these vessels in migraine headache. The availability of animal models of migraine has favoured impressive advances in understanding the mechanisms and mediators underlying migraine attacks, as well as the development of new and more specific therapeutic agents. The trigeminovascular system (TVS) has emerged as a critical efferent component, and the mediators of its activity have been identified and characterized, as have some of the receptors involved. The similarity of the trigeminal innervation across species has made it possible to draw conclusions on the neurophysiological responses to electrical or chemical stimulation of the trigeminal fibers. Studies involving substances known to induce migraine-like attacks, i.e., nitric oxide (NO) donors, have provided interesting insights into the central nuclei probably involved in the initiation and repetition of migraine attacks. The neuronal and vascular effects of such substances might yield an increasing body of evidence for a better understanding of the pathophysiology of migraine attacks.

Nitroglycerin sensitises in healthy subjects CNS structures involved in migraine pathophysiology: evidence from a study of nociceptive blink reflexes and visual evoked potentials

Nitroglycerin (NTG), a NO donor, induces an attack in migraine patients approximately 4-6 h after administration. The causative mechanisms are not known, but the long delay leaves room for a central effect, such as a change in neuronal excitability and synaptic transmission of various CNS areas involved in pain and behaviour including trigeminal nucleus caudalis and monoaminergic brain stem nuclei. To explore the central action of NTG, we have studied its effects on amplitude and habituation of the nociceptive blink reflex (nBR) and the visual evoked potential (VEP) before, 1 h and 4 h after administration of NTG (1.2 mg sublingual) or placebo (vehicle sublingual) in two groups of 10 healthy volunteers. We found a significant decrease in nBR pain and reflex thresholds both 1 and 4 h post-NTG. At the 4 h time point R2 latency was shorter (p=0.04) and R2 response area increased (p<0.01) after NTG but not after placebo. Habituation tended to become more pronounced after both NTG and placebo administration. There was a significant amplitude increase in the 5th VEP block (p=0.03) at 1h after NTG and in the 1st block (p=0.04) at 4 h. VEP habituation was replaced by potentiation at both delays after NTG; the change in habituation slope was significant at 1h (p=0.02). There were no significant VEP changes in subjects who received sublingual placebo. In conclusion, we found that in healthy subjects sublingual NTG, but not its vehicle, induces changes in a trigeminal nociceptive reflex and an evoked cortical response which are comparable to those found immediately before and during an attack of migraine. These changes could be relevant for the attack-triggering effect of NTG in migraineurs.

Role of central dopaminergic circuitry in pain processing and nitroglycerin-induced hyperalgesia

Experimental evidence shows that dopaminergic transmission within the basal ganglia is involved in the modulation of nociceptive information. Epidemiological studies show that in some disease states inherent pathophysiological mechanisms that involve degenerative changes (Parkinson's disease; PD) can also impact negatively on other unrelated functional systems (i.e. nociception). Delayed Fos expression in response to nitroglycerin (NTG) administration is a procedure used to identify the neuroanatomical substrates of the migraine condition. In this study, we investigated the influence of dopaminergic nigrostriatal denervation, obtained by intrastriatal injection of 6-hydroxydopamine (6-OHDA), on this response in Sprague-Dawley rats. We also explored the effects on the NTG-induced hyperalgesic response to painful stimuli (formalin and tail-flick tests). Nigrostriatal lesion prevented the neuronal activation typically induced by NTG in sub-cortical areas involved in pain perception, autonomic control and neuroendocrine functions, such as hypothalamic nuclei, periaqueductal grey, parabrachial nucleus and the medullary nuclei. In addition, 6-OHDA-induced lesion inhibited NTG-induced hyperalgesia. Our data show that integrity of central dopaminergic neurotransmission is required for the NTG-induced activation of sub-cortical areas involved in the expression of migraine symptoms, as well as for the hyperalgesic response to painful stimuli elicited by the drug.

The influence of gender and sex steroids on craniofacial nociception

Several pain conditions localized to the craniofacial region show a remarkable sex-related difference in their prevalence. These conditions include temporomandibular disorders and burning mouth syndrome as well as tension-type, migraine, and cluster headaches. The mechanisms that underlie sex-related differences in the prevalence of these craniofacial pain conditions remain obscure and likely involve both physiological and psychosocial factors. In terms of physiological factors relevant to the development of headache, direct evidence of sex-related differences in the properties of dural afferent fibers or durally activated second-order trigeminal sensory neurons has yet to be provided. There is, however, evidence for sex-related differences in the response properties of afferent fibers and second-order trigeminal sensory neurons that convey nociceptive input from other craniofacial tissues associated with sex-related differences in chronic pain conditions, such as those that innervate the masseter muscle and temporomandibular joint. Further, modulation of craniofacial nociceptive input by opioidergic receptor mechanisms appears to be dependent on biological sex. Research into mechanisms that may contribute to sex-related differences in trigeminal nociceptive processing has primarily focused on effect of the female sex hormone estrogen, which appears to alter the excitability of trigeminal afferent fibers and sensory neurons to noxious stimulation of craniofacial tissues. This article discusses current knowledge of potential physiological mechanisms that could contribute to sex-related differences in certain craniofacial pain conditions.

Animal models of migraine: looking at the component parts of a complex disorder

Animal models of human disease have been extremely helpful both in advancing the understanding of brain disorders and in developing new therapeutic approaches. Models for studying headache mechanisms, particularly those directed at migraine, have been developed and exploited efficiently in the last decade, leading to better understanding of the potential mechanisms of the disorder and of the action for antimigraine treatments. Model systems employed have focused on the pain-producing cranial structures, the large vessels and dura mater, in order to provide reproducible physiological measures that could be subject to pharmacological exploration. A wide range of methods using both in vivo and in vitro approaches are now employed; these range from manipulation of the mouse genome in order to produce animals with human disease-producing mutations, through sensitive immunohistochemical methods to vascular, neurovascular and electrophysiological studies. No one model system in experimental animals can explain all the features of migraine; however, the systems available have begun to offer ways to dissect migraine's component parts to allow a better understanding of the problem and the development of new treatment strategies.

Prostaglandins, glutamate and nitric oxide synthase mediate nitroglycerin-induced hyperalgesia in the formalin test

Increasing evidence supports a possible role for nitric oxide (NO) in the transmission of pain signals and in the development of central mechanisms of hyperalgesia. Previously, we have shown that nitroglycerin, an NO donor, is able to induce a long-lasting hyperalgesic state in rats. Nitroglycerin-induced hyperalgesia can be detected as an increase in the nociceptive behavior evoked by the formalin test. In the present study we investigated the possible mediators in the nitroglycerin-induced hyperalgesic state. Male Sprague-Dawley rats were injected with nitroglycerin and pretreated with indomethacin, 5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclo-hepten-5,10-imine hydrogen maleate (MK-801) or N(omega)-nitro-L-arginine methyl ester (L-NAME). The results obtained showed that inhibition of prostaglandins or NO synthesis prevents nitroglycerin-induced hyperalgesia in Phase II of the formalin test. A similar inhibitory effect was also observed following pretreatment with the glutamate antagonist MK801. The present findings point to the role of prostaglandins, NO synthesis and glutamate activity in the induction of nitroglycerin-induced hyperalgesia.

Activation of the Transcription Factor NF-κB in the Nucleus Trigeminalis Caudalis in an Animal Model of Migraine

The infusion of nitroglycerin (NTG) induces an inflammatory state in perivascular meningeal tissues of rat via the activation, inter alia, of nuclear factor kappa B (NF-kappaB). This phenomenon has been related to the mechanisms involved in the pathophysiology of migraine, a common neurovascular disorder. In the present study, we sought to elucidate whether NF-kappaB activation might have a role in the determinism of migraine attacks also at the neuronal level. Therefore, we investigated the transcriptional activity of NF-kappaB in the brainstem of rats systemically injected with NTG and killed 4h later. Activation of NF-kappaB in brain areas was detected by means of both the immunohistochemical technique and the Western blot analysis. A significant increase of nuclear immunostaining of p65, an indicator of NF-kappaB activation, was detected in lamina I and II of nucleus trigeminalis caudalis in rats injected with NTG when compared with the control group. Western blot analysis confirmed the activation of the NF-kappaB pathway showing an increase in the optical density of p65 in nuclear extracts of lower brainstem of rats injected with the nitric oxide (NO) donor. The present study contributes to expand on our understanding of the complex mechanisms by which NTG may trigger migraine-like headaches in migraineurs. Furthermore, these findings pave the way to new bio-molecular and pharmacological avenues for the development of innovative migraine therapies.

Comparative analysis of the neuronal activation and cardiovascular effects of nitroglycerin, sodium nitroprusside and l-arginine

In this study, we compare the biological effects, Fos expression and cardiovascular responses induced in the rat, of different nitric oxide modulators (nitroglycerin, sodium nitroprusside and L-arginine). Nitroglycerin and sodium nitroprusside induced a similar pattern of neuronal activation in several areas, which include the paraventricular and supraoptic nuclei of the hypothalamus, central nucleus of the amygdala, parabrachial nucleus, locus coeruleus, ventrolateral medulla and nucleus tractus solitarius. However, only nitroglycerin activated the periaqueductal grey and nucleus trigeminalis caudalis. L-arginine-induced neuronal activation was restricted to the paraventricular and supraoptic nuclei of the hypothalamus. As regards cardiovascular effect, both nitroglycerin and sodium nitroprusside induced moderate hypotension (nitroglycerin: -23.3%, sodium nitroprusside: -24.3%) that lasted 40 min in the case of sodium nitroprusside and 80 min in the case of nitroglycerin. L-arginine did not significantly influence blood pressure. These data suggest that nitroglycerin, sodium nitroprusside and L-arginine are associated with different biological effects on both the central nervous system and the cardiovascular system. Of the NO-related drugs tested in this study, only nitroglycerin confirmed its ability to activate brainstem areas implicated in nociception.

Glyceryl trinitrate may trigger endogenous nitric oxide production in patients with chronic tension-type headache

Experimental studies in humans have shown that nitric oxide (NO) may play an important role in initiation of primary headaches. It has been proposed that activation of L-arginine-NO pathway and increased endogenous production of NO may be responsible for NO induced headache. NO is synthesized from L-arginine and that reaction also yields citrulline. In the present study we aimed to investigate plasma levels of citrulline and arginine as markers of NO production after infusion of the NO donor, glyceryl trinitrate (GTN). We recruited 16 patients with chronic tension-type headache and 16 healthy controls. The subjects were randomly allocated to receive 0.5 microg/kg/min GTN or placebo over 20 min. Patients were examined on headache free days. Blood samples were collected at baseline and 60 min after start of infusion. Both patients and controls developed stronger immediate headache on the GTN day than on the placebo day (P = 0.008). The headache was more pronounced in patients than in controls (P = 0.02). Plasma levels of citrulline increased significantly 60 min after start of GTN infusion compared to placebo infusion in patients (P = 0.01) but not in controls (P = 0.50). Plasma levels of arginine were unchanged in both patients (P = 0.12) and controls (P = 0.18). We suggest that GTN administration may trigger endogenous production of NO in patients with chronic tension-type headache resulting in activation of perivascular sensory afferents.

Nitroglycerin induces migraine headache and central sensitization phenomena in patients with migraine without aura: a study of laser evoked potentials

In migraineurs nitroglycerin (NTG) induces severe delayed headache, resembling spontaneous migraine attacks. The aim of the present study was to evaluate NTG laser evoked potentials (LEP) features amplitude and pain sensation to laser stimuli during NTG-induced headache. Nine patients were selected. Headache was induced by oral administration of 0.6 mg of NTG; signals were recorded through disk electrodes placed at the vertex and referred to linked earlobes. CO(2)-LEPs delivered by stimulation of the dorsum of both hands and the right and left supraorbital zones were evaluated after the onset of moderate or severe headache resembling spontaneous migraine and at least 72 h after the end of the headache phase. Patients exhibited a significant heat pain threshold reduction and an LEPs amplitude increment during headache when both the supraorbital zones were stimulated. NTG appeared to support a reliable experimental model of migraine, based on the neuronal effects on the integrative-nociceptive structures. The LEPs facilitation during NTG-induced headache may be subtended by a hyperactivity of nociceptive cortex as well as by a failure of pain-inhibitory control.

Nitrergic and glutamatergic neuronal mechanisms at the trigeminovascular first-order synapse

Nitric oxide (NO) donors such as glyceryl trinitrate cause headache, which suggests involvement of NO in trigeminovascular sensory processing. Sensory transmission at first-order synapses is believed to involve glutamate and the question arises as to whether it is also involved in trigeminovascular sensation and whether it might interact with nitrergic mechanisms. We investigated these questions at the first central synapse in the trigeminovascular sensory system of the cat. Neuronal action potentials in the trigeminal nucleus were recorded while the superior sagittal sinus (SSS) or facial receptive field (RF) were stimulated electrically. Drugs, including the neuronal excitant glutamate, were applied to neurons via microiontophoresis. Results were obtained from 152 neurons activated with A-delta latencies by SSS stimulation and by glutamate. The NO donor S-nitrosoglutathione (SNOG, 50 nA) was applied iontophoretically to 41 neurons during SSS stimulation and 13 neurons during pulsatile glutamate ejection. Responses to both modes of stimulation were enhanced by SNOG; the proportion of neurons enhanced was 56% to SSS stimulation and 59% to glutamate. The inhibitor of nitric oxide synthase (NOS), N(omega)-propyl-L-arginine (p-ARG, 50 nA) was applied iontophoretically to 17 neurons during stimulation of SSS and to 10 neurons during pulsatile glutamate ejection. Responses to both stimuli were suppressed by p-ARG: The proportion of neurons suppressed were: to SSS stimulation 59% and to glutamate 80%. Microiontophoretic ejection of eletriptan (50 nA) reversibly suppressed responses of neurons to SSS stimulation, to RF electrical stimulation and to pulsatile iontophoretic application of glutamate. This suppression of responses was antagonised by the concurrent local iontophoretic application of the 5-HT1B/1D receptor antagonist GR127935 or by concurrent iontophoretic application of the selective 5-HT1D receptor antagonist BRL155732. These results suggest that glutamatergic mechanisms are important in sensory transmission in the trigeminovascular system and that they can be modulated by nitrergic and serotonergic mechanisms.

Nitroglycerin enhances cGMP expression in specific neuronal and cerebrovascular structures of the rat brain

Although the involvement of nitric oxide (NO) in mediating pain and neurovascular coupling is well established, the precise mechanisms sustaining these effects are still unclear. Cyclic GMP (cGMP) probably represents the main effector of the biological effects of NO at the vascular and neuronal levels. Nitroglycerin is a NO donor, which easily crosses the blood brain barrier. Several reports have suggested that the study of nitroglycerin effects upon neuronal and cerebrovascular elements is a useful animal model for investigating the pathophysiological mechanisms underlying migraine. In this study, the anatomic distribution of cGMP in the rat brain was evaluated at serial time-points after systemic administration of nitroglycerin or vehicle. The results show an increase in cGMP immunoreactivity in the nucleus trigeminalis caudalis and in the superficial cortical arterioles 2, 3 and 4h after the drug administration. The data obtained sustains the idea that cGMP is an important mediator of nitroglycerin effect in vascular and neuronal structures that are critical elements for the transmission of cephalic pain.

Reliability of the nitroglycerin provocative test in the diagnosis of neurovascular headaches

Nitroglycerin administration provokes spontaneous-like migraine attacks in migraine and cluster headache (CH) patients. Nitroglycerin-induced migraine-like headache has been used as an experimental model of migraine. In this paper, we evaluate the possibility of using the nitroglycerin provocative test (NPT) as a supportive measure in the diagnosis of primary neurovascular headaches by assessing its reliability on a large population and adopting strict criteria for rating the response as positive or negative. Our population consisted of 197 migraineurs, 42 subjects suffering from cluster headache and 53 healthy controls. In migraine without aura, the test sensitivity was 82.1%, specificity 96.2% and accuracy 85.5%, while in subjects suffering from migraine with aura, the reliability of the NPT was less satisfactory (sensitivity 13.6%, specificity 96.2% and accuracy 72%). In CH patients tested during the active phase of the disease the sensitivity was 80.6%, specificity 100% and accuracy 92.9%. NPT is an easy, low-cost and reliable method for supporting the diagnosis of migraine without aura and cluster headache.

Peripheral and central activation of trigeminal pain pathways in migraine: data from experimental animal models

Animal models for migraine have provided substantial advances on the mechanisms and mediators underlying migraine attacks. The neurogenic inflammation model has helped understanding the perivascular mechanisms underlying the pathophysiology of migraine attacks, the receptors involved and the effect of specific antimigraine drugs. The model based on probing the neuronal effects of nitroglycerin--an organic nitrate known to induce spontaneous-like migraine attacks in predisposed subjects--in the rat has provided interesting insights into the neuroanatomic circuits and neuropharmacological mechanisms involved in the initiation and repetition of migraine attacks [corrected].

Nitroglycerin induces hyperalgesia in rats--a time-course study

Nitroglycerin is a nitric oxide (NO) donor which activates nuclei involved in nociceptive transmission following systemic administration. The effect of nitroglycerin on the nociceptive threshold was studied in rats by means of two experimental tests that explore different modalities of pain: the tail-flick test and the formalin test. Nitroglycerin induced a significant reduction in the latency of the tail flick 2 and 4 h after its administration. Similarly, formalin-induced pain-related behaviour increased significantly 2 and 4 h after nitroglycerin administration.

Molecular Mechanisms of Migraine

Migraine is a common complex disorder that affects a large portion of the population and thus incurs a substantial economic burden on society. The disorder is characterized by recurrent headaches that are unilateral and usually accompanied by nausea, vomiting, photophobia, and phonophobia. The range of clinical characteristics is broad and there is evidence of comorbidity with other neurological diseases, complicating both the diagnosis and management of the disorder. Although the class of drugs known as the triptans (serotonin 5-HT(1B/1D) agonists) has been shown to be effective in treating a significant number of patients with migraine, treatment may in the future be further enhanced by identifying drugs that selectively target molecular mechanisms causing susceptibility to the disease.Genetically, migraine is a complex familial disorder in which the severity and susceptibility of individuals is most likely governed by several genes that may be different among families. Identification of the genomic variants involved in genetic predisposition to migraine should facilitate the development of more effective diagnostic and therapeutic applications. Genetic profiling, combined with our knowledge of therapeutic response to drugs, should enable the development of specific, individually-tailored treatment.

Suppression by eletriptan of the activation of trigeminovascular sensory neurons by glyceryl trinitrate

The effect of intracarotid arterial infusions of glyceryl trinitrate (GTN), a substance known to precipitate vascular headache, on the spontaneous activity of trigeminal neurons with craniovascular input was studied in cats. Cats were anaesthetised with alpha-chloralose, immobilised and artificially ventilated. The superior sagittal sinus (SSS) was isolated and stimulated electrically. Facial receptive fields (RF) were also stimulated. Single neurons were recorded from the trigeminal nucleus caudalis with a metal microelectrode equipped with six glass barrels for microiontophoresis. Infusions of GTN were administered via a catheter inserted retrogradely into the common carotid artery through the lingual artery. Infusions of GTN (mean rate 19+/-7, range 5-100 microg kg(-1) min(-1), in a volume of 2 ml min(-1)) increased the spontaneous discharge rate of second-order neurons which received dural and facial sensory input to 429+/-80% of control. Iontophoretic application of the 5-HT(1B/1D) receptor agonist eletriptan (50 nA) at the peak of the response decreased the discharge rate of neurons towards pre-GTN control levels. In the presence of continuous iontophoretic application of the 5-HT(1B/1D) receptor antagonist GR127935, the decrease in discharge rate caused by eletriptan was antagonised. We conclude (1) that GTN activates craniovascular sensory pathways at a site at, or peripheral to, the second-order neuron and that such an action may account for at least the acute-onset headache induced by GTN and (2) that the antimigraine agent eletriptan is able to selectively suppress noxious sensory information from the dura, induced by GTN, via an action at 5-HT(1B/1D) receptors.

Nitric oxide synthase inhibitors can antagonize neurogenic and calcitonin gene-related peptide induced dilation of dural meningeal vessels

1. The detailed pathophysiology of migraine is beginning to be understood and is likely to involve activation of trigeminovascular afferents. 2. Clinically effective anti-migraine compounds are believed to have actions that include peripheral inhibition of calcitonin gene-related peptide (CGRP) release from trigeminal neurones, or preventing dural vessel dilation, or both. CGRP antagonists can block both neurogenic and CGRP-induced dural vessel dilation. 3. Nitric oxide (NO) can induce headache in migraine patients and often triggers a delayed migraine. The initial headache is thought to be caused via a direct action of the NO-cGMP pathway that causes vasodilation by vascular smooth muscle relaxation, while the delayed headache is likely to be a result of triggering trigeminovascular activation. Nitric oxide synthase (NOS) inhibitors are effective in the treatment of acute migraine. 4. The present studies used intravital microscopy to examine the effects of specific NOS inhibitors on neurogenic dural vasodilation (NDV) and CGRP-induced dilation. 5. The non-specific and neuronal NOS (nNOS) inhibitors were able to partially inhibit NDV, while the non-specific and endothelial NOS (eNOS) inhibitors were able to partially inhibit the CGRP induced dilation. 6. There was no effect of the inducible NOS (iNOS) inhibitor. 7. The data suggest that the delayed headache response triggered by NO donors in humans may be due, in part, to increased nNOS activity in the trigeminal system that causes CGRP release and dural vessel dilation. 8. Further, eNOS activity in the endothelium causes NO production and smooth muscle relaxation by direct activation of the NO-cGMP pathway, and may be involved in the initial headache response.

Effects of nimesulide on nitric oxide-induced hyperalgesia in humans--a neurophysiological study

Nonsteroidal anti-inflammatory drugs (NSAIDs) are known to induce analgesia mainly via the inhibition of cyclo-oxygenase. Several reports suggest that chronic pain is mediated by central sensitization, an N-methyl-D-aspartate (NMDA)-mediated phenomenon influenced by cyclo-oxygenase activity and nitric oxide (NO). In this double-blind study, we evaluated the effects of a preferential inhibitor of the inducible isoform of cyclo-oxygenase-2, nimesulide, on the spinal nociceptive flexion reflex (RIII reflex) before and after administration of an NO donor in healthy volunteers. Nimesulide caused a reduction of the RIII reflex area, which persisted after NO donor administration. Conversely, in the placebo group the RIII reflex area significantly increased following the administration of the NO donor. These data suggest a central effect for nimesulide, possibly related to a reduction of nociceptive activity at spinal level.