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

Identification of brain areas in mice with peak neural activity across the acute and persistent phases of post-traumatic headache

BACKGROUND

Post-traumatic headache is very common after a mild traumatic brain injury. Post-traumatic headache may persist for months to years after an injury in a substantial proportion of people. The pathophysiology underlying post-traumatic headache remains unknown but is likely distinct from other headache disorders. Identification of brain areas activated in acute and persistent phases of post-traumatic headache can provide insights into the underlying circuits mediating headache pain. We used an animal model of mild traumatic brain injury-induced post-traumatic headache and c-fos immunohistochemistry to identify brain regions with peak activity levels across the acute and persistent phases of post-traumatic headache.

METHODS

Male and female C57BL/6 J mice were briefly anesthetized and subjected to a sham procedure or a weight drop closed-head mild traumatic brain injury . Cutaneous allodynia was assessed in the periorbital and hindpaw regions using von Frey filaments. Immunohistochemical c-fos based neural activity mapping was then performed on sections from whole brain across the development of post-traumatic headache (i.e. peak of the acute phase at 2 days post- mild traumatic brain injury), start of the persistent phase (i.e. >14 days post-mild traumatic brain injury) or after provocation with stress (bright light). Brain areas with consistent and peak levels of c-fos expression across mild traumatic brain injury induced post-traumatic headache were identified and included for further analysis.

RESULTS

Following mild traumatic brain injury, periorbital and hindpaw allodynia was observed in both male and female mice. This allodynia was transient and subsided within the first 14 days post-mild traumatic brain injury and is representative of acute post-traumatic headache. After this acute post-traumatic headache phase, exposure of mild traumatic brain injury mice to a bright light stress reinstated periorbital and hindpaw allodynia for several hours - indicative of the development of persistent post-traumatic headache. Acute post-traumatic headache was coincident with an increase in neuronal c-fos labeling in the spinal nucleus of the trigeminal caudalis, primary somatosensory cortex, and the nucleus accumbens. Neuronal activation returned to baseline levels by the persistent post-traumatic headache phase in the spinal nucleus of the trigeminal caudalis and primary somatosensory cortex but remained elevated in the nucleus accumbens. In the persistent post-traumatic headache phase, coincident with allodynia observed following bright light stress, we observed bright light stress-induced c-fos neural activation in the spinal nucleus of the trigeminal caudalis, primary somatosensory cortex, and nucleus accumbens.

CONCLUSION

Examination of mild traumatic brain injury-induced changes in peak c-fos expression revealed brain regions with significantly increased neural activity across the acute and persistent phases of post-traumatic headache. Our findings suggest mild traumatic brain injury-induced post-traumatic headache produces neural activation along pain relevant pathways at time-points matching post-traumatic headache-like pain behaviors. These observations suggest that the spinal nucleus of the trigeminal caudalis, primary somatosensory cortex, and nucleus accumbens may contribute to both the induction and maintenance of post-traumatic headache.

Emodin Interferes With Nitroglycerin-Induced Migraine in Rats Through CGMP-PKG Pathway

The purpose of this research was to explore the effect and mechanism of emodin in interfering with nitroglycerin-induced migraine rats. We carried out behavioral research within 2 h post-nitroglycerin (NTG) injection, and blood samples were collected through the abdominal aorta for measurements of nitric oxide (NO), calcitonin gene-related peptide (CGRP), substance P (SP), tumor necrosis factor (TNF-α) and cyclic guanosine monophosphate (cGMP) levels. Immunohistochemistry was adopted to detect the activation of c-Fos immunoreactive neurons in brain tissues. The number and integrated optical density (IOD) of c-Fos positive cells were measured using Image-Pro Plus. Western blotting was applied to detect the levels of PKG protein in rat brain tissues. The results showed that emodin can alleviate the pain response of migraine rats and significantly reduce the levels of NO, CGRP, SP, TNF-α and cGMP in migraine rats. In addition, emodin can significantly reduce the number of c-Fos positive cells and the IOD value. Moreover, the expression of PKG protein was significantly inhibited by emodin. Therefore, it is inferred that emodin can relieve migraine induced by NTG through the cGMP-PKG pathway, and can be used as a potential botanical medicine for the treatment of migraine.

Long-Term Depression Induced by Optogenetically Driven Nociceptive Inputs to Trigeminal Nucleus Caudalis or Headache Triggers

The peripheral trigeminovascular pathway mediates orofacial and craniofacial pain and projects centrally to the brainstem trigeminal nucleus caudalis (TNc). Sensitization of this pathway is involved in many pain conditions, but little is known about synaptic plasticity at its first central synapse. We have taken advantage of optogenetics to investigate plasticity selectively evoked at synapses of nociceptive primary afferents onto TNc neurons. Based on immunolabeling in the trigeminal ganglia, TRPV1-lineage neurons comprise primarily peptidergic and nonpeptidergic nociceptors. Optical stimulation of channelrhodopsin-expressing axons in the TRPV1/ChR2 mouse in TNc slices thus allowed us to activate a nociceptor-enriched subset of primary afferents. We recorded from lamina I/II neurons in acutely prepared transverse TNc slices, and alternately stimulated two independent afferent pathways, one with light-activated nociceptive afferents and the other with electrically-activated inputs. Low-frequency optical stimulation induced robust long-term depression (LTD) of optically-evoked EPSCs, but not of electrically-evoked EPSCs in the same neurons. Blocking NMDA receptors or nitric oxide synthase strongly attenuated LTD, whereas a cannabinoid receptor 1 antagonist had no effect. The neuropeptide PACAP-38 or the nitric oxide donors nitroglycerin or sodium nitroprusside are pharmacologic triggers of human headache. Bath application of any of these three compounds also persistently depressed optically-evoked EPSCs. Together, our data show that LTD of nociceptive afferent synapses on trigeminal nucleus neurons is elicited when the afferents are activated at frequencies consistent with the development of central sensitization of the trigeminovascular pathway.SIGNIFICANCE STATEMENT Animal models suggest that sensitization of trigeminovascular afferents plays a major role in craniofacial pain syndromes including primary headaches and trigeminal neuralgia, yet little is known about synaptic transmission and plasticity in the brainstem trigeminal nucleus caudalis (TNc). Here we used optogenetics to selectively drive a nociceptor-enriched population of trigeminal afferents while recording from superficial laminae neurons in the TNc. Low-frequency optical stimulation evoked robust long-term depression at TRPV1/ChR2 synapses. Moreover, application of three different headache trigger drugs also depressed TRPV1/ChR2 synapses. Synaptic depression at these primary afferent synapses may represent a newly identified mechanism contributing to central sensitization during headache.

Soluble guanylyl cyclase is a critical regulator of migraine-associated pain

Background Nitric oxide (NO) has been heavily implicated in migraine. Nitroglycerin is a prototypic NO-donor, and triggers migraine in humans. However, nitroglycerin also induces oxidative/nitrosative stress and is a source of peroxynitrite - factors previously linked with migraine etiology. Soluble guanylyl cyclase (sGC) is the high affinity NO receptor in the body, and the aim of this study was to identify the precise role of sGC in acute and chronic migraine. Methods We developed a novel brain-bioavailable sGC stimulator (VL-102), and tested its hyperalgesic properties in mice. We also determined the effect of VL-102 on c-fos and calcitonin gene related peptide (CGRP) immunoreactivity within the trigeminovascular complex. In addition, we also tested the known sGC inhibitor, ODQ, within the chronic nitroglycerin migraine model. Results VL-102-evoked acute and chronic mechanical cephalic and hind-paw allodynia in a dose-dependent manner, which was blocked by the migraine medications sumatriptan, propranolol, and topiramate. In addition, VL-102 also increased c-fos and CGRP expressing cells within the trigeminovascular complex. Importantly, ODQ completely inhibited acute and chronic hyperalgesia induced by nitroglycerin. ODQ also blocked hyperalgesia already established by chronic nitroglycerin, implicating this pathway in migraine chronicity. Conclusions These results indicate that nitroglycerin causes migraine-related pain through stimulation of the sGC pathway, and that super-activation of this receptor may be an important component for the maintenance of chronic migraine. This work opens the possibility for negative sGC modulators as novel migraine therapies.

Stimulation of rat cranial dura mater with potassium chloride causes CGRP release into the cerebrospinal fluid and increases medullary blood flow

Primary headaches may be accompanied by increased intracranial blood flow induced by the release of the potent vasodilator calcitonin gene-related peptide (CGRP) from activated meningeal afferents. We aimed to record meningeal and medullary blood flow simultaneously and to localize the sites of CGRP release in rodent preparations in vivo and ex vivo. Blood flow in the exposed rat parietal dura mater and the medulla oblongata was recorded by laser Doppler flowmetry, while the dura was stimulated by topical application of 60mM potassium chloride (KCl). Samples of jugular venous plasma and cerebrospinal fluid (CSF) collected from the cisterna magna were analysed for CGRP concentrations using an enzyme immunoassay. In a hemisected rat skull preparation lined with dura mater the CGRP releasing effect of KCl superfusion was examined. Superfusion of the dura mater with KCl decreased meningeal blood flow unless alpha-adrenoceptors were blocked by phentolamine, whereas the medullary blood flow was increased. The same treatment caused increased CGRP concentrations in jugular plasma and CSF and induced significant CGRP release in the hemisected rat skull preparation. Anaesthesia of the trigeminal ganglion by injection of lidocaine reduced increases in medullary blood flow and CGRP concentration in the CSF upon meningeal KCl application. CGRP release evoked by depolarisation of meningeal afferents is accompanied by increased blood flow in the medulla oblongata but not the dura mater. This discrepancy can be explained by the smooth muscle depolarising effect of KCl and the activation of sympathetic vasoconstrictor mechanisms. The medullary blood flow response is most likely mediated by CGRP released from activated central terminals of trigeminal afferents. Increased blood supply of the medulla oblongata and CGRP release into the CSF may also occur in headaches accompanying vigorous activation of meningeal afferents.

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.

Pharmacology of reflex blinks in the rat: a novel model for headache research

Background

Migraineurs are highly sensitive to the nitric oxide donor glyceryl trinitrate which triggers attacks in many sufferers. In animal studies, glyceryl trinitrate increases neuronal activity in the trigeminovascular pathway and elevates neurotransmitter levels in the brainstem. Many migraineurs also display alterations in blink reflexes, known to involve brainstem circuits. We investigated the effect of GTN on evoked blinks in the anaesthetised rat to determine whether such reflexes may prove useful as the basis for a novel animal model to evaluate potential anti-migraine therapeutic agents.

Method

In anaesthetised rats the electromyogram associated with the reflex blink evoked by corneal airpuff was recorded. Rats were infused with glyceryl trinitrate, sumatriptan plus glyceryl trinitrate or vehicle control. Changes in the magnitude of the reflex blink-associated electromyogram following these treatments were measured.

Results

Glyceryl trinitrate potentiated the evoked reflex blink-associated EMG response from 2 h after infusion. That effect was abolished by simultaneous infusion of sumatriptan with glyceryl trinitrate.

Conclusions

These results show that simple skin surface measurements of evoked electromyographic activity in the rat can reliably detect the evoked blink reflex that can be potentiated by nitric oxide donors. This novel model may be an effective tool for evaluating putative anti-migraine therapeutic agents.

Oestrogen-Modulated Increase of Calmodulin-Dependent Protein Kinase II (CamKII) in Rat Spinal Trigeminal Nucleus After Systemic Nitroglycerin

Migraine can be triggered by systemic administration of the nitric oxide (NO) donor nitroglycerin (NTG) and by abrupt falls in plasma oestradiol. Calmodulin-dependent protein kinase II (CamKII) present in superficial dorsal horns is thought to play a role in sensitization of central nociceptors, a phenomen present in migraineurs. We therefore examined in rats the expression of CamKII in the caudal trigeminal nucleus (TNC) after subcutaneous NTG (10 mg/kg) and its modulation by oestrogen. In male rats and in ovariectomized females, after 4 h NTG increased significantly CamKII expression in the superficial layers of TNC, but not in the upper thoracic spinal cord. NTG had no effect on CamKII expression in oestradiol-treated ovariectomized animals. Thus NTG, i.e. NO, selectively enhances CamKII in the rat TNC and oestradiol blocks this effect. These data may help to understand the mechanisms by which NO triggers migraine attacks and oestrogens influence migraine severity.

Valproate ameliorates nitroglycerin-induced migraine in trigeminal nucleus caudalis in rats through inhibition of NF-кB

Background

As a complex nervous system disease, migraine causes severe healthy and social issues worldwide. Valproate (VPA) is a widely used treatment agent against seizures and bipolar disorder, and its function to alleviate damage due to migraine has also been verified in clinical investigations. However, the mechanism underlying the protective effect of VPA against migraine remains poorly revealed. In the current study, the major purpose was to uncover the mechanism which drove VPA to antagonize migraine.

Methods

Nitroglycerin (NTG) was employed to induce a migraine model in rats and the migraine animals were exposed to treatment of VPA of different doses. Thereafter, the levels of indicators related to oxidative stress were measured and used to evaluate the anti-oxidant potential of VPA. The expression of calcitonin gene-related peptide (CGRP) and c-Fos was also quantified with ELISA and immunohistochemistry, respectively. Western blotting and electrophoretic mobility shift assays (EMSA) were conducted to explore the effect of VPA treatment on NF-кB pathway.

Results

NTG induced the activation of oxidative stress and led to migraine in model animals, but pre-treatment with VPA attenuated the damage due to migraine attack in brain tissues. The level of lipid peroxidation was significantly reduced while the prodcution of anti-oxidant factors was restored. Furthermore, expressions of CGRP and c-Fos, which represented the neuronal activation, were also down-regulated by VPA. The results of western blotting and EMSA demonstrated that the above mentioned effect of VPA acted through the inhibition of NF-кB pathway.

Conclusions

Although controversies on the effect of VPA on NF-кB pathway existed, our study revealed an alternative mechanism of VPA in protecting against migraine, which would promote the development of therapeutic strategies of migraine.

Two TRPV1 receptor antagonists are effective in two different experimental models of migraine

Background

The capsaicin and heat responsive ion channel TRPV1 is expressed on trigeminal nociceptive neurons and has been implicated in the pathophysiology of migraine attacks. Here we investigate the efficacy of two TRPV1 channel antagonists in blocking trigeminal activation using two in vivo models of migraine.

Methods

Male Sprague–Dawley rats were used to study the effects of the TRPV1 antagonists JNJ-38893777 and JNJ-17203212 on trigeminal activation. Expression of the immediate early gene c-fos was measured following intracisternal application of inflammatory soup. In a second model, CGRP release into the external jugular vein was determined following injection of capsaicin into the carotid artery.

Results

Inflammatory up-regulation of c-fos in the trigeminal brain stem complex was dose-dependently and significantly reduced by both TRPV1 antagonists. Capsaicin-induced CGRP release was attenuated by JNJ-38893777 only in higher dosage. JNJ-17203212 was effective in all doses and fully abolished CGRP release in a time and dose-dependent manner.

Conclusion

Our results describe two TRPV1 antagonists that are effective in two in vivo models of migraine. These results suggest that TRPV1 may play a role in the pathophysiological mechanisms, which are relevant to migraine.

Mechanisms of glyceryl trinitrate provoked mast cell degranulation

BACKGROUND

Migraine patients develop attacks several hours after intravenous infusion of glyceryl trinitrate. Due to the short half-life of nitric oxide, this delayed migraine cannot be caused by a direct action of nitric oxide derived from glyceryl trinitrate. The involvement of meningeal inflammation and dural mast cell degranulation is supported by the effectiveness of prednisolone on glyceryl trinitrate-induced delayed headache.

METHODS

Using a newly developed rat model mimicking the human glyceryl trinitrate headache model, we have investigated the occurrence of dural mast cell degranulation after a clinically relevant dose of glyceryl trinitrate.

RESULTS

A 6-fold increase in degranulation was observed starting at 2 hours after glyceryl trinitrate infusion. Interestingly, pre-treatment with the effective anti-migraine substances L-nitro-arginine methyl ester and sumatriptan prevented glyceryl trinitrate-induced mast cell degranulation whereas the calcitonin gene-related peptide-receptor antagonist olcegepant and the substance P receptor antagonist L-733,060 did not affect mast cell degranulation. However, topical application of two different nitric oxide donors did not cause mast cell degranulation ex vivo.

CONCLUSIONS

Direct application of an exogenous nitric oxide donor on dural mast cells does not cause mast cell degranulation ex vivo. In vivo application of the nitric oxide donor glyceryl trinitrate leads to a prominent level of degranulation via a yet unknown mechanism. This effect can be completely blocked by inhibition of the endogenous nitric oxide production and by 5-HT1B/1D receptor agonists but is unaffected by calcitonin gene-related peptide and substance P receptor antagonists.

Headache and sleep: shared pathophysiological mechanisms

OBJECTIVE

The objective of the current article is to review the shared pathophysiological mechanisms which may underlie the clinical association between headaches and sleep disorders.

BACKGROUND

The association between sleep and headache is well documented in terms of clinical phenotypes. Disrupted sleep-wake patterns appear to predispose individuals to headache attacks and increase the risk of chronification, while sleep is one of the longest established abortive strategies. In agreement, narcoleptic patients show an increased prevalence of migraine compared to the general population and specific familial sleep disorders have been identified to be comorbid with migraine with aura.

CONCLUSION

The pathophysiology and pharmacology of headache and sleep disorders involves an array of neural networks which likely underlie their shared clinical association. While it is difficult to differentiate between cause and effect, or simply a spurious relationship the striking brainstem, hypothalamic and thalamic convergence would suggest a bidirectional influence.

CGRP infusion in unanesthetized rats increases expression of c-Fos in the nucleus tractus solitarius and caudal ventrolateral medulla, but not in the trigeminal nucleus caudalis

BACKGROUND AND AIMS

Calcitonin gene-related peptide (CGRP) and glyceryl trinitrate (GTN) infusion in migraineurs provokes headache resembling spontaneous migraine, and CGRP receptor antagonists are effective in the treatment of acute migraine. We hypothesized that CGRP infusion would increase molecular markers of neuronal activation in migraine-relevant tissues of the rat.

METHODS

CGRP was infused intravenously (i.v.) in freely moving rats to circumvent factors like anesthesia, acute surgery and severe hypotension, the three confounding factors for c-Fos expression. The trigeminal nucleus caudalis (TNC) was isolated at different time points after CGRP infusion. The level of c-Fos mRNA and protein expression in TNC were analyzed by qPCR and immunohistochemistry. c-Fos-stained nuclei were also counted in the nucleus tractus solitarius (NTS) and caudal ventrolateral medulla (CVLM), integrative sites in the brain stem for processing cardiovascular signals. We also investigated Zif268 protein expression (another immediate early gene) in TNC. The protein expression of p-ERK, p-CREB and c-Fos was analyzed in dura mater, trigeminal ganglion (TG) and TNC samples using Western blot.

RESULTS

CGRP infusion caused a significant dose-dependent fall in mean arterial blood pressure. No significant activation of c-Fos in the TNC at mRNA and protein levels was observed after CGRP infusion. A significant increase in c-Fos protein was observed in the NTS and CVLM in the brain stem. Zif268 expression in the TNC was also not changed after CGRP infusion. p-ERK was increased in the dura mater 30 minutes after CGRP infusion.

CONCLUSION

CGRP infusion increased the early expression of p-ERK in the dura mater but did not increase c-Fos and Zif268 expression in the TNC. The rats may, thus, differ from migraine patients, in whom infusion of CGRP caused headache and a delayed migraine attack. The rat CGRP infusion model with c-Fos or Zif268 as neuronal pain markers in TNC is unsuitable for antimigraine drug testing.

CGRP and migraine: could PACAP play a role too?

Migraine is a debilitating neurological disorder that affects about 12% of the population. In the past decade, the role of the neuropeptide calcitonin gene-related peptide (CGRP) in migraine has been firmly established by clinical studies. CGRP administration can trigger migraines, and CGRP receptor antagonists ameliorate migraine. In this review, we will describe multifunctional activities of CGRP that could potentially contribute to migraine. These include roles in light aversion, neurogenic inflammation, peripheral and central sensitization of nociceptive pathways, cortical spreading depression, and regulation of nitric oxide production. Yet clearly there will be many other contributing genes that could act in concert with CGRP. One candidate is pituitary adenylate cyclase-activating peptide (PACAP), which shares some of the same actions as CGRP, including the ability to induce migraine in migraineurs and light aversive behavior in rodents. Interestingly, both CGRP and PACAP act on receptors that share an accessory subunit called receptor activity modifying protein-1 (RAMP1). Thus, comparisons between the actions of these two migraine-inducing neuropeptides, CGRP and PACAP, may provide new insights into migraine pathophysiology.

Animal migraine models for drug development: status and future perspectives

Migraine is number seven in WHO's list of all diseases causing disability and the third most costly neurological disorder in Europe. Acute attacks are treatable by highly selective drugs such as the triptans but there is still a huge unmet therapeutic need. Unfortunately, drug development for headache has almost come to a standstill partly because of a lack of valid animal models. Here we review previous models with emphasis on optimal characteristics of a future model. In addition to selection of animal species, the method of induction of migraine-like changes and the method of recording responses elicited by such measures are crucial. The most naturalistic way of inducing attacks is by infusion of endogenous signaling molecules that are known to cause migraine in patients. The most valid response is recording of neural activity in the trigeminal system. The most useful headache related responses are likely to be behavioral, allowing multiple experiments in each individual animal. Distinction is made between acute and prophylactic models and how to validate each of them. Modern insight into neurobiological mechanisms of migraine is so good that it is only a question of resources and efforts that determine when valid models with ability to predict efficacy in migraine will be available.

Pathophysiology of Migraine

Migraine is a serious health problem which impair quality of life. It is the second most common primary headache that affects approximately more than %10 people in general population. Migraine pathophysiology is still unclear. Increasing results of studies suggest to migraine pathophysiology is related with primary neuronal mechanisms. Migraine pain starts in which region of brain and what brain regions are activated in different stages is unenlightened. There is evidences that growing number of studies which using new imaging techniques as positron emission tomography (PET) and functional magnetic resonans imaging (fMRI) show that migraine and cluster headaches are related with neuronal structures and vasodilatation. There are four phases to a migraine. The prodrome phase, aura, the attack, and the postdrome phase. Some datas obtained from last ten years indicate that cortical excitability has increased in interictal phase too. For many years, studies in rodents show trgimenial nerve is activated and it leads to vasodilatation and neurogenic inflammation in the headache phase. Although the majority of patients encountered in clinical practice are migraine without aura or chronic migraine, experimental studies of the migraine pathophysiology are focusing on the aura model which is used cortical spreading depression.

Vascular extracellular signal-regulated kinase mediates migraine-related sensitization of meningeal nociceptors

OBJECTIVE

To examine changes in the response properties of meningeal nociceptors that might lead to migraine pain and examine endogenous processes that could play a role in mediating them using a clinically relevant model of migraine triggering, namely infusion of the nitric oxide (NO) donor nitroglycerin (NTG).

METHODS

Single-unit recordings made in the trigeminal ganglion of rats were used to test changes in the activity and mechanosensitivity of meningeal nociceptors in response to administration of the migraine trigger NTG or another NO donor S-nitroso-N-acetyl-DL-penicillamine (SNAP) at doses relevant to the human model of migraine headache. Immunohistochemistry and pharmacological manipulations were used to investigate the possible role of meningeal vascular signaling in mediating the responses of meningeal nociceptors to NO.

RESULTS

Infusion of NTG promoted a delayed and robust increase in the mechanosensitivity of meningeal nociceptors, with a time course resembling the development of the delayed migraine headache. A similar sensitization was elicited by dural application of NTG and SNAP. NTG-evoked delayed meningeal nociceptor sensitization was associated with a robust extracellular signal-regulated kinase (ERK) phosphorylation in meningeal arteries. Pharmacological blockade of meningeal ERK phosphorylation inhibited the development of NTG-evoked delayed meningeal nociceptor sensitization.

INTERPRETATION

The development of delayed mechanical sensitization evoked by the migraine trigger NTG is potentially of great importance as the first finding of a neurophysiological correlate of migraine headache in meningeal nociceptors. The arterial ERK phosphorylation and its involvement in mediating the NTG-evoked delayed sensitization points to an important, yet unappreciated, role of the meningeal vasculature in the genesis of migraine pain.

New Agents for Acute Treatment of Migraine: CGRP Receptor Antagonists, iNOS Inhibitors

OPINION STATEMENT

The treatment of migraine was advanced dramatically with the introduction of triptans in the early 1990s. Despite the substantial improvement in the quality of life that triptans have brought to many migraineurs, a substantial cohort of patients remain highly disabled by attacks and need new therapeutic approaches, which ideally should be quick-acting, have no vasoconstrictor activity, and have a longer duration of action and be better tolerated than current therapies. The calcitonin gene-related peptide (CGRP) receptor antagonists (gepants)-olcegepant (BIBN 4096 BS), telcagepant (MK-0974), MK3207, and BI 44370 TA-are effective in treating acute migraine. They have no vasoconstrictive properties, fewer adverse effects, and may act longer than triptans. Their development has been complicated by liver toxicity issues when used as preventives. Results from studies with BI 44370 TA do not support broad concern about a class effect, and further studies are ongoing in this respect. Many experimental studies and clinical trials suggest that nitric oxide may have a role in the pathophysiology of migraine. Therefore, the inhibition of nitric oxide synthase (NOS) for the acute or prophylactic treatment of migraine offered a feasible approach; as inducible NOS (iNOS) is involved in several pain states, such as inflammatory pain, it appeared to be an attractive target. However, despite high selectivity and potency, the iNOS inhibitor GW274150 was not effective for acute treatment or prophylaxis of migraine, suggesting that iNOS is very unlikely to be a promising target.

Spontaneous trigeminal allodynia in rats: a model of primary headache

Animal models are essential for studying the pathophysiology of headache disorders and as a screening tool for new therapies. Most animal models modify a normal animal in an attempt to mimic migraine symptoms. They require manipulation to activate the trigeminal nerve or dural nociceptors. At best, they are models of secondary headache. No existing model can address the fundamental question: How is a primary headache spontaneously initiated? In the process of obtaining baseline periorbital von Frey thresholds in a wild-type Sprague-Dawley rat, we discovered a rat with spontaneous episodic trigeminal allodynia (manifested by episodically changing periorbital pain threshold). Subsequent mating showed that the trait is inherited. Animals with spontaneous trigeminal allodynia allow us to study the pathophysiology of primary recurrent headache disorders. To validate this as a model for migraine, we tested the effects of clinically proven acute and preventive migraine treatments on spontaneous changes in rat periorbital sensitivity. Sumatriptan, ketorolac, and dihydroergotamine temporarily reversed the low periorbital pain thresholds. Thirty days of chronic valproic acid treatment prevented spontaneous changes in trigeminal allodynia. After discontinuation, the rats returned to their baseline of spontaneous episodic threshold changes. We also tested the effects of known chemical human migraine triggers. On days when the rats did not have allodynia and showed normal periorbital von Frey thresholds, glycerol trinitrate and calcitonin gene related peptide induced significant decreases in the periorbital pain threshold. This model can be used as a predictive model for drug development and for studies of putative biomarkers for headache diagnosis and treatment.

Olvanil acts on transient receptor potential vanilloid channel 1 and cannabinoid receptors to modulate neuronal transmission in the trigeminovascular system

The transient receptor potential vanilloid channel 1 (TRPV1) is a nociceptive transducer located on nociceptive neurons. TRPV1 channels located on peripheral neurons mainly transduce the sense of heat and are also activated by low pH or capsaicin. The role of centrally located TRPV1 channels is not fully understood. Likewise their importance in pain syndromes of central origin, such as migraine, is not known. Experimental data suggest a relationship to migraine. However, experimental studies with TRPV1 receptor antagonists indicate that the receptor may not be a useful target for new acute migraine treatments. Any potential role for the receptor in the chronification of migraine has not been investigated. The present study aimed at analyzing the use of the TRPV1 channel as a target to desensitize trigeminal neurons and thereby inhibit neuronal activity in the trigeminocervical complex. The TRPV1 receptor agonist olvanil was used for desensitization because, as compared with capsaicin, it is non-noxious and lacks capsaicin's pungency and CGRP release potential. We further investigated a possible effect of olvanil on cannabinoid (CB(1)) receptors, as an interaction between both receptor systems has been described previously. The results show that olvanil dose-dependently inhibited spontaneous and stimulus-induced activity within the trigeminocervical complex, whereas it had no effect on CSD susceptibility. We further demonstrated that the inhibiting effect of olvanil is mediated by vanilloid and cannabinoid receptor systems, thereby using the synergistic effects this dual mechanism offers. Curiously, TRPV1 receptor agonism may have anti-nociceptive properties through central mechanisms that would be of considerable interest to elucidate.

Calcitonin gene-related peptide receptors in rat trigeminal ganglion do not control spinal trigeminal activity

Calcitonin gene-related peptide (CGRP) is regarded as a key mediator in the generation of primary headaches. CGRP receptor antagonists reduce migraine pain in clinical trials and spinal trigeminal activity in animal experiments. The site of CGRP receptor inhibition causing these effects is debated. Activation and inhibition of CGRP receptors in the trigeminal ganglion may influence the activity of trigeminal afferents and hence of spinal trigeminal neurons. In anesthetized rats extracellular activity was recorded from neurons with meningeal afferent input in the spinal trigeminal nucleus caudalis. Mechanical stimuli were applied at regular intervals to receptive fields located in the exposed cranial dura mater. α-CGRP (10(-5) M), the CGRP receptor antagonist olcegepant (10(-3) M), or vehicle was injected through the infraorbital canal into the trigeminal ganglion. The injection of volumes caused transient discharges, but vehicle, CGRP, or olcegepant injection was not followed by significant changes in ongoing or mechanically evoked activity. In animals pretreated intravenously with the nitric oxide donor glyceryl trinitrate (GTN, 250 μg/kg) the mechanically evoked activity decreased after injection of CGRP and increased after injection of olcegepant. In conclusion, the activity of spinal trigeminal neurons with meningeal afferent input is normally not controlled by CGRP receptor activation or inhibition in the trigeminal ganglion. CGRP receptors in the trigeminal ganglion may influence neuronal activity evoked by mechanical stimulation of meningeal afferents only after pretreatment with GTN. Since it has previously been shown that olcegepant applied to the cranial dura mater is ineffective, trigeminal activity driven by meningeal afferent input is more likely to be controlled by CGRP receptors located centrally to the trigeminal ganglion.

A naturalistic glyceryl trinitrate infusion migraine model in the rat

BACKGROUND AND AIM

Glyceryl trinitrate (GTN) infusion is a reliable method to provoke migraine-like headaches in humans. Previous studies have simulated this human model in anaesthetized or in awake rodents using GTN doses 10,000 times higher than used in humans. The relevance of such toxicological doses to migraine is not certain. Anaesthesia and low blood pressure caused by high GTN doses both can affect the expression of nociceptive marker c-fos. Therefore, our aim was to simulate the human GTN migraine model in awake rats using a clinically relevant dose.

METHODS

Awake rats were infused with GTN (4 µg/kg/min, for 20 min, i.v.), a dose just 8 times higher than in humans. mRNA and protein expression for c-fos were analysed in the trigeminal vascular system at various time points using RT-PCR and immunohistochemistry, respectively.

RESULTS

A significant upregulation of c-fos mRNA was observed in the trigeminal nucleus caudalis at 30 min and 2 h that was followed by an upregulation of Fos protein in the trigeminal nucleus caudalis at 2 h and 4 h after GTN infusion. Pre-treatment with sumatriptan attenuated the activation of Fos at 4 h, demonstrating the specificity of this model for migraine.

CONCLUSION

We present a validated naturalistic rat model suitable for screening of acute anti-migraine drugs.

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.

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.

The relevance of preclinical research models for the development of antimigraine drugs: focus on 5-HT(1B/1D) and CGRP receptors

Migraine is a complex neurovascular syndrome, causing a unilateral pulsating headache with accompanying symptoms. The past four decades have contributed immensely to our present understanding of migraine pathophysiology and have led to the introduction of specific antimigraine therapies, much to the relief of migraineurs. Pathophysiological factors culminating into migraine headaches have not yet been completely deciphered and, thus, pose an additional challenge for preclinical research in the absence of any direct experimental marker. Migraine provocation experiments in humans use a head-score to evaluate migraine, as articulated by the volunteer, which cannot be applied to laboratory animals. Therefore, basic research focuses on different symptoms and putative mechanisms, one at a time or in combination, to validate the hypotheses. Studies in several species, utilizing different preclinical approaches, have significantly contributed to the two antimigraine principles in therapeutics, namely: 5-HT(1B/1D) receptor agonists (known as triptans) and CGRP receptor antagonists (known as gepants). This review will analyze the preclinical experimental models currently known for the development of these therapeutic principles, which are mainly based on the vascular and/or neurogenic theories of migraine pathogenesis. These include models based on the involvement of cranial vasodilatation and/or the trigeminovascular system in migraine. Clearly, the preclinical strategies should involve both approaches, while incorporating the newer ideas/techniques in order to get better insights into migraine pathophysiology.

Triptan-induced enhancement of neuronal nitric oxide synthase in trigeminal ganglion dural afferents underlies increased responsiveness to potential migraine triggers

Migraine is a common neurological disorder often treated with triptans. Triptan overuse can lead to increased frequency of headache in some patients, a phenomenon termed medication overuse headache. Previous preclinical studies have demonstrated that repeated or sustained triptan administration for several days can elicit persistent neural adaptations in trigeminal ganglion cells innervating the dura, prominently characterized by increased labelling of neuronal profiles for calcitonin gene related peptide. Additionally, triptan administration elicited a behavioural syndrome of enhanced sensitivity to surrogate triggers of migraine that was maintained for weeks following discontinuation of drug, a phenomenon termed 'triptan-induced latent sensitization'. Here, we demonstrate that triptan administration elicits a long-lasting increase in identified rat trigeminal dural afferents labelled for neuronal nitric oxide synthase in the trigeminal ganglion. Cutaneous allodynia observed during the period of triptan administration was reversed by NXN-323, a selective inhibitor of neuronal nitric oxide synthase. Additionally, neuronal nitric oxide synthase inhibition prevented environmental stress-induced hypersensitivity in the post-triptan administration period. Co-administration of NXN-323 with sumatriptan over several days prevented the expression of allodynia and enhanced sensitivity to stress observed following latent sensitization, but not the triptan-induced increased labelling of neuronal nitric oxide synthase in dural afferents. Triptan administration thus promotes increased expression of neuronal nitric oxide synthase in dural afferents, which is critical for enhanced sensitivity to environmental stress. These data provide a biological basis for increased frequency of headache following triptans and highlight the potential clinical utility of neuronal nitric oxide synthase inhibition in preventing or treating medication overuse headache.

Animal models of headache: from bedside to bench and back to bedside

In recent years bench-based studies have greatly enhanced our understanding of headache pathophysiology, while facilitating the development of new headache medicines. At present, established animal models of headache utilize activation of pain-producing cranial structures, which for a complex syndrome, such as migraine, leaves many dimensions of the syndrome unstudied. The focus on modeling the central nociceptive mechanisms and the complexity of sensory phenomena that accompany migraine may offer new approaches for the development of new therapeutics. Given the complexity of the primary headaches, multiple approaches and techniques need to be employed. As an example, recently a model for trigeminal autonomic cephalalgias has been tested successfully, while by contrast, a satisfactory model of tension-type headache has been elusive. Moreover, although useful in many regards, migraine models are yet to provide a more complete picture of the disorder.

CGRP receptor antagonist olcegepant (BIBN4096BS) does not prevent glyceryl trinitrate-induced migraine

There is a striking similarity between the migraine-provoking effect of the nitric oxide (NO) donor glyceryl trinitrate (GTN) and that of calcitonin gene-related peptide (CGRP). We tested the hypothesis that NO releases CGRP to cause the delayed migraine attack after GTN.

Methods: In a double-blind-cross-over study, 13 migraine without aura (MO) patients were administered GTN 0.5 µg/kg/minute for 20 minutes and subsequently BIBN4096BS (olcegepant) 10 mg or placebo. Headache scores and development of MO were followed for 24 hours.

Results: MO developed in seven of 13 with olcegepant and in nine of 13 with placebo ( p = 0.68). The headache scores were similar after the two treatments ( p = 0.58). Thus CGRP receptor blockade did not prevent GTN-induced migraine.

Conclusions: The present study indicates that NO does not induce migraine by liberating CGRP. The most likely explanation for our findings is that CGRP has its effect higher than NO in the cascade of events leading to MO attacks.

The neurobiology of migraine

The understanding of migraine has moved well beyond its traditional characterization as a "vascular headache." In considering the basic neurobiology of migraine, it is important to begin with the concept of migraine as not merely a headache, but rather a heterogeneous array of episodic symptoms. Among the array of phenomena experienced by migraine patients are visual disturbances, nausea, cognitive dysfunction, fatigue, and sensitivity to light, sound, smell, and touch. These symptoms may occur independently or in any combination, and in some patients occur even in the absence of headache. The diversity and variability of symptoms experienced by migraine patients belies a complex neurobiology, involving multiple cellular, neurochemical, and neurophysiological processes occurring at multiple neuroanatomical sites. Migraine is a multifaceted neurobiological phenomenon that involves activation of diverse neurochemical and cellular signaling pathways in multiple regions of the brain. Propagated waves of cellular activity in the cortex, possibly involving distinct glial and vascular signaling mechanisms, can occur along with activation of brainstem centers and nociceptive pathways. Whether different brain regions become involved in a linear sequence, or as parallel processes, is uncertain. The modulation of brain signaling by genetic factors, and by sex and sex hormones, provides important clues regarding the fundamental mechanisms by which migraine is initiated and sustained. Each of these mechanisms may represent distinct therapeutic targets for this complex and commonly disabling disorder.

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.

Calcitonin gene-related peptide (CGRP) levels during glyceryl trinitrate (GTN)-induced headache in healthy volunteers

The role of nitric oxide (NO) in migraine has been studied in the experimental glyceryl trinitrate (GTN)-infusion headache model. We hypothesized that GTN-induced headache may activate the trigeminovascular system and be associated with increased levels of sensory neuropeptides, including calcitonin gene-related peptide (CGRP). CGRP, vasoactive intestinal peptide (VIP), neuropeptide Y (NPY) and somatostatin plasma levels were measured before and after placebo/sumatriptan injection and during GTN-induced headache. Following a double-blind randomized cross-over design, 10 healthy volunteers received subcutaneous sumatriptan 6 mg or placebo. This was succeeded by 20 min of GTN (0.12 µg kg−1 min−1) infusion. At baseline no subject reported headache (using verbal rating scale from 0 to 10) and the jugular CGRP-like immunoreactivity (-LI) level was 18.6 ± 2.5 pmol/l. After a 20-min intravenous infusion of GTN 0.12 µg kg−1 min−1, median peak headache intensity was 4 (range 2–6) ( P < 0.05), while jugular CGRP-LI levels were unchanged (19.0 ± 2.8 pmol/l; P > 0.05). There were no changes in VIP-, NPY- or somatostatin-LI. In conclusion, the NO donor GTN appears not to induce headache via immediate CGRP release.

Nitroglycerin headache and nitroglycerin-induced primary headaches from 1846 and onwards: a historical overview and an update

Nitroglycerin (NTG) (glyceryl trinitrate) was synthesized by the Italian chemist Ascanio Sobrero in Paris in 1846. A very unstable explosive, Alfred Nobel while working on explosives, combined it with Kiselguhr and patented it as dynamite in 1867. NTG was introduced in 1879 in medicine in the treatment of angina pectoris by the English doctor William Murrell. NTG-induced headache was quickly recognized as an important adverse event both in the industrial use of NTG, where it was used to produce dynamite, as well as in the use of NTG as drug. This review traces the evolution of our understanding of NTG headache.

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.

Calcitonin gene-related peptide in blood: is it increased in the external jugular vein during migraine and cluster headache? A review

The involvement of calcitonin gene-related peptide (CGRP) in migraine pathophysiological mechanisms is shown by the facts that CGRP can induce migraine and that two CGRP antagonists, olcegepant and telcagepant, are effective in the treatment of migraine attacks. Increase of the neuropeptide CGRP during migraine and cluster headache attacks in the extracerebral circulation as measured in the external jugular vein (EJV) has been regarded as an established fact. Then in 2005, a study, using the migraine patients as their own controls, showed; however, no changes of CGRP in EJV. For migraine there is thus some uncertainty as to whether CGRP is increased in all migraine patients and more research is needed. In contrast, there are three 'positive' studies in cluster headache in which both sumatriptan, O(2) and spontaneous resolution normalized CGRP. The source of an increase of CGRP in EJV is most likely a 'nervous vasodilatory drive' in the extracranial vascular bed. It remains an enigma how the observed increase of CGRP in the EJV fits into the mechanisms of migraine and cluster headache.

Migraine: where and how does the pain originate?

Migraine is a complex neurological disease with a genetic background. Headache is the most prominent and clinically important symptom of migraine but its origin is still enigmatic. Numerous clinical, histochemical, electrophysiological, molecular and genetical approaches form a puzzle of findings that slowly takes shape. The generation of primary headaches like migraine pain seems to be the consequence of multiple pathophysiological changes in meningeal tissues, the trigeminal ganglion, trigeminal brainstem nuclei and descending inhibitory systems, based on specific characteristics of the trigeminovascular system. This contribution reviews the current discussion of where and how the migraine pain may originate and outlines the experimental work to answer these questions.

Episodic dural stimulation in awake rats: a model for recurrent headache

OBJECTIVES

To model, in rats, the development of chronic trigeminal nociceptive hypersensitivity seen in patients with recurrent headache.

BACKGROUND

Pathophysiology studies suggest that patients with recurrent migraine headache experience repeated bouts of dural nociceptor activation. In some patients, the severity and frequency of headache attacks increase over time. Patients with recurrent headache are hypersensitive to nitric oxide donors, such as glyceryl trinitrate (GTN). Current trigeminal pain models do not reflect the repeated episodic nature of dural nociceptor activation in patients with recurrent headache. Repeated nociceptor activation creates long-lasting changes in the periphery and brain due to activity-dependent neuronal plasticity. An animal model of repeated activation of dural nociceptors will facilitate the study of the physiological changes caused by repeated, episodic pain and the factors important for the transition of episodic to chronic migraine.

METHODS

We induced dural inflammation by infusing an inflammatory soup (IS) through a cannula on the dura in awake behaving rats. This was repeated 3 times per week for up to 4 weeks. Periorbital pressure sensory testing was used to monitor the change in trigeminal sensitivity. Rats were challenged with GTN to test the hypothesis that many dural stimulations are required to model the hypersensitivity of migraine patients. Quantitative trigeminal sensory testing and microdialysis in the trigeminal nucleus caudalis (TNC) were used to measure GTN hypersensitivity.

RESULTS

Multiple infusions of IS (>8), over weeks, induced a long-lasting decrease in periorbital pressure thresholds that lasted >3 weeks after the last infusion. In contrast, IS infusion in IS-naive rats and those that received 3 IS infusions produced only short-lasting decreases in periorbital pressure thresholds. Rats that received more than 8 IS infusions showed a marked increase in their neurochemical and behavioral responses to GTN. In these rats, GTN induced a decrease in periorbital von Frey thresholds that lasted >5 hours. In contrast, in rats that received only 3 IS infusions, GTN caused a threshold decrease for 1.5 hour. In vivo microdialysis in the TNC showed that GTN increased extracellular glutamate levels in rats with more than 8 IS infusions to 7.7 times the basal levels. In IS-naive rats and those that received only 3 IS infusions, the extracellular glutamate levels rose to only 1.7 and 1.9 times the basal level, respectively.

CONCLUSIONS

Repeated IS stimulation of the dura produces a chronic state of trigeminal hypersensitivity and potentiates the response to GTN. This hyperresponsiveness outlasts the last IS infusion and is the basis of our rat model of recurrent headache. This model can be used to study the changes in the brain and periphery induced by repeated trigeminovascular nociceptor activation and has the potential to elucidate the mechanisms for the transition of episodic to chronic headache.