CP-93,129, sumatriptan, dihydroergotamine block c-fos expression within rat trigeminal nucleus caudalis caused by chemical stimulation of the meninges

Early treatment with sumatriptan prevents PACAP38-induced migraine: A randomised clinical trial

OBJECTIVE

To determine whether early treatment with sumatriptan can prevent PACAP38-induced migraine attacks.

METHODS

A total of 37 patients with migraine without aura were enrolled between July 2018 to December 2019. All patients received an intravenous infusion of 10 picomole/kg/min of PACAP38 over 20 min followed by an intravenous infusion of 4 mg sumatriptan or placebo over 10 min on two study days in a randomised, double-blind, placebo-controlled, crossover study.

RESULTS

Of 37 patients enrolled, 26 (70.3%) completed the study and were included in analyses. Of the 26 patients, four (15%) developed a PACAP38-induced migraine attack on sumatriptan and 11 patients (42%) on placebo (p = 0.016). There were no differences in area under the curve for headache intensity between sumatriptan (mean AUC 532) and placebo (mean AUC 779) (p = 0.35). Sumatriptan significantly constricted the PACAP38-dilated superficial temporal artery immediately after infusion (T30) compared with infusion of placebo (p < 0.001).Conclusions and relevance: Early treatment with intravenously administered sumatriptan prevented PACAP38-induced migraine. Prevention of migraine attacks was associated with vasoconstriction by sumatriptan in the earliest phases of PACAP provocation. These results suggest that sumatriptan prevents PACAP38-induced migraine by modulation of nociceptive transmission within the trigeminovascular system.Trial Registration: ClinicalTrials.gov (NCT03881644).

α6GABAA Receptor Positive Modulators Alleviate Migraine-like Grimaces in Mice via Compensating GABAergic Deficits in Trigeminal Ganglia

Migraine is caused by hyperactivity of the trigeminovascular system, where trigeminal ganglia (TG) play an important role. This hyperactivity might originate from an underfunctional GABAergic system in TG. To investigate this possibility, we adapted a mouse model of migraine by inducing migraine-like grimaces in male mice via repeated injections of nitroglycerin (NTG, 10 mg/kg, i.p.), once every 2 days, for up to 5 sessions. Migraine-like facial pain scores were measured using the mouse grimace scale. Repeated NTG treatments in mice caused significant increases in migraine-like grimaces that were aborted and prevented by two anti-migraine agents sumatriptan and topiramate, respectively. After 5 sessions of NTG injections, the GABA-synthesizing enzyme, 65-kDa glutamate decarboxylase (GAD65), but not the GABA transporter 1 (GAT1) or the α6 subunit-containing GABAA receptors (α6GABAARs), was downregulated in mouse TG tissues. Taking advantage of the unaffected TG α6GABAAR expression in NTG-treated mice, we demonstrated that an α6GABAAR-selective positive allosteric modulator (PAM), DK-I-56-1, exhibited both abortive and prophylactic effects, comparable to those of sumatriptan and topiramate, respectively, in this migraine-mimicking mouse model. The brain-impermeable furosemide significantly prevented the effects of DK-I-56-1, suggesting its peripheral site of action, likely via preventing α6GABAAR modulation in TG. Results suggest that a decreased GABA synthesis caused by the reduced GAD65 expression in TG contributes to the trigeminovascular activation in this repeated NTG-induced migraine-mimicking model and that the unaltered α6GABAARs in TG are potential targets for migraine treatment. Thus, α6GABAAR-selective PAMs are potential anti-migraine agents for both abortive and preventive therapies.

Trigeminal activation patterns evoked by chemical stimulation of the dura mater in rats

BACKGROUND

Although migraine is one of the most common primary headaches, its therapy is still limited in many cases. The use of animal models is crucial in the development of novel therapeutic strategies, but unfortunately, none of them show all aspects of the disease, therefore, there is a constant need for further improvement in this field. The application of inflammatory agents on the dura mater is a widely accepted method to mimic neurogenic inflammation in rodents, which plays a key role in the pathomechanism of migraine. Complete Freund's Adjuvant (CFA), and a mixture of inflammatory mediators, called inflammatory soup (IS) are often used for this purpose.

METHODS

To examine the activation pattern that is caused by chemical stimulation of dura mater, we applied CFA or IS over the right parietal lobe. After 2 h and 4 h (CFA groups), or 2.5 h and 4 h (IS groups), animals were perfused, and c-Fos immunoreactive cells were counted in the caudal trigeminal nucleus. To explore every pitfall, we examined whether our surgical procedure (anesthetic drug, stereotaxic apparatus, local lidocaine) can alter the results under the same experimental settings. c-Fos labeled cells were counted in the second-order neuron area based on the somatotopic organization of the trigeminal nerve branches.

RESULTS

We could not find any difference between the CFA and physiological saline group neither 2 h, nor 4 h after dural stimulation. IS caused significant difference after both time points between IS treated and control group, and between treated (right) and control (left) side. Stereotaxic frame usage had a substantial effect on the obtained results.

CONCLUSIONS

Counting c-Fos immunoreactive cells based on somatotopic organization of the trigeminal nerve helped to examine the effect of chemical stimulation of dura in a more specific way. As a result, the use of IS over the parietal lobe caused activation in the area of the ophthalmic nerve. To see this effect, the use of lidocaine anesthesia is indispensable. In conclusion, application of IS on the dura mater induces short-term, more robust c-Fos activation than CFA, therefore it might offer a better approach to model acute migraine headache in rodents.

Inflammation induces developmentally regulated sumatriptan inhibition of spinal synaptic transmission

BACKGROUND AND PURPOSE

While triptans are used to treat migraine, there is evidence that they also reduce inflammation-induced pain at the spinal level. The cellular mechanisms underlying this spinal enhancement are unknown. We examined whether inflammation alters sumatriptan modulation of synaptic transmission in the rat spinal dorsal horn.

EXPERIMENTAL APPROACH

Three to four days following intraplantar injection of complete Freund's adjuvant (CFA) or saline, whole cell recordings of evoked glutamatergic EPSCs were made from lumbar lamina I-II dorsal horn neurons in rat spinal slices KEY RESULTS: In 2- to 3-week-old animals, sumatriptan reduced the amplitude of evoked EPSCs and this was greater in slices from CFA, compared to saline-injected rats. In CFA-injected animals, sumatriptan increased the paired pulse ratio of evoked EPSCs and reduced the rate of spontaneous miniature EPSCs. The 5-HT_1B and 5-HT_1D agonists CP9 3129 and PNU109291 both inhibited evoked EPSCs in CFA but not saline-injected rats. By contrast, the 5-HT_1A agonist R(+)-8-OH-DPAT inhibited evoked EPSCs in saline but not CFA-injected rats. In CFA-injected rats, the sumatriptan-induced inhibition of evoked EPSCs was reduced by the 5-HT_1B and 5-HT_1D antagonists NAS181 and BRL-15572. Intriguingly, the difference in sumatriptan inhibition between CFA and saline-injected animals was only observed in animals less than 4 weeks old.

CONCLUSION AND IMPLICATIONS

These findings indicate that inflammation induces a developmentally regulated 5-HT_1B/1D presynaptic inhibition of excitatory transmission into the rat superficial dorsal horn. Thus, triptans could potentially act as spinal analgesic agents for inflammatory pain in the juvenile setting.

Why is the therapeutic effect of acute antimigraine drugs delayed? A review of controlled trials and hypotheses about the delay of effect

In randomised controlled trials (RCTs) of oral drug treatment of migraine attacks, efficacy is evaluated after 2 hours. The effect of oral naratriptan 2.5 mg with a maximum blood concentration (Tmax ) at 2 hours increases from 2 to 4 hours in RCTs. To check whether such a delayed effect is also present for other oral antimigraine drugs, we hand-searched the literature for publications on RCTs reporting efficacy. Two triptans, 3 nonsteroidal anti-inflammatory drugs (NSAIDs), a triptan combined with an NSAID and a calcitonin gene-related peptide receptor antagonist were evaluated for their therapeutic gain with determination of time to maximum effect (Emax ). Emax was compared with known Tmax from pharmacokinetic studies to estimate the delay to pain-free. The delay in therapeutic gain varied from 1-2 hours for zolmitriptan 5 mg to 7 hours for naproxen 500 mg. An increase in effect from 2 to 4 hours was observed after eletriptan 40 mg, frovatriptan 2.5 mg and lasmiditan 200 mg, and after rizatriptan 10 mg (Tmax  = 1 h) from 1 to 2 hours. This strongly indicates a general delay of effect in oral antimigraine drugs. A review of 5 possible effects of triptans on the trigemino-vascular system did not yield a simple explanation for the delay. In addition, Emax for triptans probably depends partly on the rise in plasma levels and not only on its maximum. The most likely explanation for the delay in effect is that a complex antimigraine system with more than 1 site of action is involved.

Vascular Contributions to Migraine: Time to Revisit?

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

Migraine Enters the Molecular Era

Migraine headache is the first neurological condition treatable by a drug targeted to a specific receptor binding site. Originally viewed as a disorder of brain blood vessels, migraine may have as its biological basis a disturbance in brain function. Regarding therapy, recent molecular data document that 5-HT1D receptors on primary afferent fibers are coupled to inhibition of neuropeptide release, blockade of neurogenic inflammation, and c-fos expression within the trigeminal nucleus caudalis after noxious meningeal stimulation in experimental animals. The 5-HT1Dα receptor subtype (as opposed to the 5-HT 1Dβ receptor) has emerged as an important therapeutic target aimed at blocking trigeminal nerve fibers without constricting vascular smooth muscle. NEUROSCIENTIST 2:191-200, 1996

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.

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.

The effect of sumatriptan on nitric oxide synthase enzyme production after iatrogenic inflammation in the brain stem of adolescent rats: A randomized, controlled, experimental study

BACKGROUND

Migraine is a common disabling disorder of childhood and adolescence. Despite advances in the understanding of migraine pathophysiology, treatment remains a challenge.

OBJECTIVES

The aims of this study were to investigate the production of nitric oxide synthase (NOS) enzymes in the brain stem of adolescent rats, using an experimental model of migraine, and the effect of sumatriptan pretreatment on the production of the NOS enzymes.

METHODS

Male adolescent (aged ~2 months) Wistar rats were used in the study. The animals were anesthetized using pentobarbital. The trigeminovascular system was stimulated by injecting a proinflammatory molecule, carrageenan, into the cis-terna magna of the anesthetized rats. The animals were divided into 3 groups of equal size: (1) the study group, in which the rats were treated with sumatriptan succinate 2 hours before intracisternal carrageenan injection; (2) the sham group, in which the rats were not administered intracisternal carrageenan injection or sumatriptan pretreatment; and (3) the control group, in which the rats were administered intracisternal carrageenan injection but were not pretreated with sumatriptan. In the control and study groups, the rats were euthanized using ether anesthesia 1 hour after intracisternal carrageenan injection. Rats in the sham group were euthanized 1 hour after intracisternal catheterization. Brain tissue was removed and endothelial NOS (eNOS), neuronal NOS (nNOS), and inducible NOS (iNOS) immunohistochemistry was performed.

RESULTS

Twenty-one rats were randomized into 3 groups of 7. The mean values of the immunolabeling intensities for eNOS, nNOS, and iNOS enzymes in the brain stem were significantly lower in the sham group compared with the control group (P = 0.001, P = 0.002, and P = 0.001, respectively). The mean values of the immunolabeling intensities of eNOS, nNOS, and iNOS in the brain stem were significantly lower in the study group compared with the control group (P = 0.001, P = 0.025, and P = 0.005, respectively).

CONCLUSIONS

In this experimental model of migraine in adolescent rats, intracisternal injection of carrageenan was associated with a significant increase in the production of NOS enzymes in the brain stem. Pretreatment with sumatriptan was associated with a decrease in NOS production.

Increase of capsaicin-induced trigeminal Fos-like immunoreactivity by 5-HT(7) receptors

OBJECTIVE

To explore whether pharmacological stimulation of the 5-hydroxytryptamine(7) (5-HT(7) ) receptor modulates Fos-like immunoreactivity in the trigeminal nucleus caudalis of rats.

BACKGROUND

The serotonin 5-HT(7) receptor was proposed to be involved in migraine pathogenesis and evidence suggests it plays a role in peripheral nociception and hyperalgesia through an action on sensory afferent neurons.

METHODS

The potential activating or sensitizing role of 5-HT(7) receptors on trigeminal sensory neurons, as visualized by Fos-like immunoreactivity in the superficial layers of the trigeminal nucleus caudalis in rats, was investigated using the 5-HT(7) receptor agonist, LP-211, in the absence and the presence of intracisternal capsaicin, respectively. The agonist effect was characterized with the 5-HT(7) receptor antagonist, SB-656104. Male Wistar rats received a subcutaneous injection of LP-211, SB-656104, and SB-656104 + LP-211. They were then anesthetized and prepared to receive an intracisternal injection of capsaicin or its vehicle. Animals were perfused and brains removed; sections of the brain stem from the area postrema to the CI level were obtained and processed for Fos immunohistochemistry.

RESULTS

Capsaicin but not its vehicle induced Fos-like immunoreactivity within laminae I and II of trigeminal nucleus caudalis. Pretreatment with LP-211 had no effect on Fos-like immunoreactivity but strongly increased the response produced by capsaicin; this effect was abolished by SB-656104. Interestingly, capsaicin-induced Fos-like immunoreactivity was abolished by SB-656104 pretreatment thus suggesting involvement of endogenous 5-HT.

CONCLUSIONS

Data suggest that 5-HT(7) receptors increase activation of meningeal trigeminovascular afferents and/or transmission of nociceptive information in the brain stem. This mechanism could be relevant in migraine and its prophylactic treatment.

The role of chemosensitive afferent nerves and TRP ion channels in the pathomechanism of headaches

The involvement of trigeminovascular afferent nerves in the pathomechanism of primary headaches is well established, but a pivotal role of a particular class of primary sensory neurons has not been advocated. This review focuses on the evidence that supports the critical involvement of transient receptor potential (TRP) channels in the pathophysiology of primary headaches, in particular, migraine. Transient receptor potential vanilloid 1 and transient receptor potential ankyrin 1 receptors sensitive to vanilloids and other irritants are localized on chemosensitive afferent nerves, and they are involved in meningeal nociceptive and vascular responses involving neurogenic dural vasodilatation and plasma extravasation. The concept of the trigeminal nocisensor complex is put forward which involves the trigeminal chemosensitive afferent fibers/neurons equipped with specific nocisensor molecules, the elements of the meningeal microcirculatory system, and the dural mast cells. It is suggested that the activation level of this complex may explain some of the specific features of migraine headache. Pharmacological modulation of TRP channel function may offer a novel approach to the management of head pain, in particular, migraine.

Role of 5-HT(1) receptor subtypes in the modulation of pain and synaptic transmission in rat spinal superficial dorsal horn

BACKGROUND AND PURPOSE

5-HT receptor agonists have variable nociceptive effects within the spinal cord. While there is some evidence for 5-HT(1A) spinally-mediated analgesia, the role of other 5-HT(1) receptor subtypes remains unclear. In the present study, we examined the spinal actions of a range of 5-HT(1) agonists, including sumatriptan, on acute pain, plus their effect on afferent-evoked synaptic transmission onto superficial dorsal horn neurons.

EXPERIMENTAL APPROACH

For in vivo experiments, 5-HT agonists were injected via chronically implanted spinal catheters to examine their effects in acute mechanical and thermal pain assays using a paw pressure analgesymeter and a Hargreave's device. For in vitro experiments, whole-cell patch-clamp recordings of primary afferent-evoked glutamatergic EPSC were made from lamina II neurons in rat lumbar spinal slices.

KEY RESULTS

Intrathecal (i.t.) delivery of the 5-HT(1A) agonist R ± 8-OH-DPAT (30-300 nmol) produced a dose-dependent thermal, but not mechanical, analgesia. Sumatriptan and the 5-HT(1B), 5-HT(1D), 5-HT(1F) agonists CP93129, PNU109291 and LY344864 (100 nmol) had no effect on either acute pain assay. R ± 8-OH-DPAT (1 µM) and sumatriptan (3 µM) both reduced the amplitude of the evoked EPSC. In contrast, CP93129, PNU109291 and LY344864 (0.3-3 µM) had no effect on the evoked EPSC. The actions of both R ± 8-OH-DPAT and sumatriptan were abolished by the 5-HT(1A) antagonist WAY100635 (3 µM).

CONCLUSIONS AND IMPLICATIONS

These findings indicate that the 5-HT(1A) receptor subtype predominantly mediates the acute antinociceptive and cellular actions of 5-HT(1) ligands within the rat superficial dorsal horn.

Tramadol and its enantiomers differentially suppress c-fos-like immunoreactivity in rat brain and spinal cord following acute noxious stimulus

Tramadol hydrochloride, (1RS,2RS)-2-[(dimethylamino)methyl]-1-(3-methoxyphenyl)-cyclohexanol hydrochloride, is an orally-active, centrally-acting analgesic with a putative dual mechanism of action, including an opioid and non-opioid component. The analgesic properties of tramadol and the possible co-existence of dual mechanisms has been postulated to be due to complementary and interactive pharmacologies of its enantiomers. We examined the ability of tramadol, its enantiomers, and morphine as reference to suppress c-fos-like immunoreactivity (c-fos-ir) in rat spinal cord and brain regions following a noxious stimulus (i.p. administration of 3.5% acetic acid). c-fos-ir was measured by immunocytochemistry and the stained cells in each region were counted 2 h after the acetic-acid injection (2:25 h after tramadol or morphine). Equi-analgesic doses of s.c. morphine (10 mg/kg) or tramadol (30 mg/kg) significantly suppressed c-fos-ir in all areas examined, except dorsal central gray of the spinal cord. The enantiomers of tramadol had distinctive patterns of suppression, neither one suppressed c-fos-ir in all of the regions, and hence neither one alone accounted for the suppression produced by the racemate. These findings support differential and complementary effects of tramadol enantiomers in sub-populations of spinal and supraspinal nociceptive neurons, consistent with the proposed antinociceptive interaction between the enantiomers.

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.

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.

5-HT(1F) Receptor agonists: a new treatment option for migraine attacks?

Migraine is a debilitating disorder of the CNS. Although therapeutic options for migraine attacks have tremendously advanced with the development of triptans more than a decade ago, several conditions (such as vascular disease) restrict their use. Moreover, some patients do not respond to triptans and other currently available medications. Therefore, treatment alternatives are needed. Study data show that 5-HT(1F) receptor agonists successfully abort migraine attacks. These data also suggest a favorable vascular side-effect profile of these substances, which could be beneficial for migraine treatment in subjects with cardiac or vascular disease. We discuss the current knowledge of 5-HT(1F) receptor-mediated effects, in part by comparing them to triptans, and we also summarize data from basic research and clinical trials.

Acute treatment of migraine

Acute treatment of migraine has benefited first from major advances in pharmacological science followed in short order, sometimes preceded, by an improved understanding of pathogenesis, especially of headache. This chapter reviews the mechanisms of migraine that provide an understanding of the pharmacology and therapeutic targets for acute migraine medications. General clinical approaches to acute therapy are reviewed, and indices of acceptable acute therapeutic outcomes are discussed. Currently the serotonin (5-HT) 1B/1D agonist group of drugs, triptans, forms the mainstay of acute therapeutic regimens. Other approaches to acute treatment such as simple analgesics, non-steroidal anti-inflammatory drugs (NSAIDs), ergots, and combination medications are reviewed. Finally, the newest acute treatments that are currently exploratory or under clinical investigation are discussed.

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.

Does sumatriptan cross the blood-brain barrier in animals and man?

Sumatriptan, a relatively hydrophilic triptan, based on several animal studies has been regarded to be unable to cross the blood–brain barrier (BBB). In more recent animal studies there are strong indications that sumatriptan to some extent can cross the BBB. The CNS adverse events of sumatriptan in migraine patients and normal volunteers also indicate a more general effect of sumatriptan on CNS indicating that the drug can cross the BBB in man. It has been discussed whether a defect in the BBB during migraine attacks could be responsible for a possible central effect of sumatriptan in migraine. This review suggests that there is no need for a breakdown in the BBB to occur in order to explain a possible central CNS effect of sumatriptan.

Glycine inhibitory dysfunction induces a selectively dynamic, morphine-resistant, and neurokinin 1 receptor- independent mechanical allodynia

Dynamic mechanical allodynia is a widespread and intractable symptom of neuropathic pain for which there is a lack of effective therapy. We recently provided a novel perspective on the mechanisms of this symptom by showing that a simple switch in trigeminal glycine synaptic inhibition can turn touch into pain by unmasking innocuous input to superficial dorsal horn nociceptive specific neurons through a local excitatory, NMDA-dependent neural circuit involving neurons expressing the gamma isoform of protein kinase C. Here, we further investigated the clinical relevance and processing of glycine disinhibition. First, we showed that glycine disinhibition with strychnine selectively induced dynamic but not static mechanical allodynia. The induced allodynia was resistant to morphine. Second, morphine did not prevent the activation of the neural circuit underlying allodynia as shown by study of Fos expression and extracellular-signal regulated kinase phosphorylation in dorsal horn neurons. Third, in contrast to intradermal capsaicin injections, light, dynamic mechanical stimuli applied under disinhibition did not produce neurokinin 1 (NK1) receptor internalization in dorsal horn neurons. Finally, light, dynamic mechanical stimuli applied under disinhibition induced Fos expression only in neurons that did not express NK1 receptor. To summarize, the selectivity and morphine resistance of the glycine-disinhibition paradigm adequately reflect the clinical characteristics of dynamic mechanical allodynia. The present findings thus reveal the involvement of a selective dorsal horn circuit in dynamic mechanical allodynia, which operates through superficial lamina nociceptive-specific neurons that do not bear NK1 receptor and provide an explanation for the differences in the pharmacological sensitivity of neuropathic pain symptoms.

Melatonin treatment decreases c-fos expression in a headache model induced by capsaicin

The aim of the present work was to analyze c-fos response within the trigeminal nucleus caudalis (TNC) of pinealectomized rats and animals that received intraperitoneal melatonin, after intracisternal infusion of capsaicin, used to induce intracranial trigeminovascular stimulation. Experimental groups consisted of animals that received vehicle solution (saline-ethanol-Tween 80, 8:1:1, diluted 1:50) only (VEI, n=5); animals that received capsaicin solution (200 nM) only (CAP, n=6); animals submitted to pinealectomy (PX, n=5); sham-operated animals (SH, n=5); animals submitted to pinealectomy followed by capsaicin stimulation (200 nM) after 15 days (PX + CAP, n=7); and animals that received capsaicin solution (200 nM) and intraperitoneal melatonin (10 mg/kg) (CAP + MEL, n=5). Control rats, receiving vehicle in the cisterna magna, showed a small number of c-fos-positive cells in the TNC (layer I/II) as well as the sham-operated and pinealectomized rats, when compared to animals stimulated by capsaicin. On the other hand, pinealectomized rats, which received capsaicin, presented the highest number of c-fos-positive cells. Animals receiving capsaicin and melatonin treatment had similar expression of the vehicle group. Our data provide experimental evidence to support the role of melatonin and pineal gland in the pathophysiology of neurovascular headaches.

Sumatriptan inhibits synaptic transmission in the rat midbrain periaqueductal grey

Background

There is evidence to suggest that the midbrain periaqueductal grey (PAG) has a role in migraine and the actions of the anti-migraine drug sumatriptan. In the present study we examined the serotonergic modulation of GABAergic and glutamatergic synaptic transmission in rat midbrain PAG slices in vitro.

Results

Serotonin (5-hydroxytriptamine, 5-HT, IC₅₀ = 142 nM) and the selective serotonin reuptake inhibitor fluoxetine (30 μM) produced a reduction in the amplitude of GABA_A-mediated evoked inhibitory postsynaptic currents (IPSCs) in all PAG neurons which was associated with an increase in the paired-pulse ratio of evoked IPSCs. Real time PCR revealed that 5-HT1A, 5-HT1B, 5-HT1D and 5-HT1F receptor mRNA was present in the PAG. The 5-HT1A, 5-HT1B and 5-HT1D receptor agonists 8-OH-DPAT (3 μM), CP93129 (3 μM) and L694247 (3 μM), but not the 5-HT1F receptor agonist LY344864 (1 – 3 μM) inhibited evoked IPSCs. The 5-HT (1 μM) induced inhibition of evoked IPSCs was abolished by the 5-HT1B antagonist NAS181 (10 μM), but not by the 5-HT1A and 5-HT1D antagonists WAY100135 (3 μM) and BRL15572 (10 μM). Sumatriptan also inhibited evoked IPSCs with an IC₅₀ of 261 nM, and reduced the rate, but not the amplitude of spontaneous miniature IPSCs. The sumatriptan (1 μM) induced inhibition of evoked IPSCs was abolished by NAS181 (10 μM) and BRL15572 (10 μM), together, but not separately. 5-HT (10 μM) and sumatriptan (3 μM) also reduced the amplitude of non-NMDA mediated evoked excitatory postsynaptic currents (EPSCs) in all PAG neurons tested.

Conclusion

These results indicate that sumatriptan inhibits GABAergic and glutamatergic synaptic transmission within the PAG via a 5-HT1B/D receptor mediated reduction in the probability of neurotransmitter release from nerve terminals. These actions overlap those of other analgesics, such as opioids, and provide a mechanism by which centrally acting 5-HT1B and 5-HT1D ligands might lead to novel anti-migraine pharmacotherapies.

Investigation of the immunoreactivities of NOS enzymes and the effect of sumatriptan in adolescent rats using an experimental model of migraine

The aim was to investigate the immunoreactivities for NOS enzymes in frontal cortex and meningeal vessels after chemical stimulation of the subarachnoid space of adolescent rats and the effect of sumatriptan pre-treatment on the immunoreactivities of the NOS enzymes. Male adolescent Wistar rats were used. Rats in group 1 did not taken intracisternal injection. Rats in group 2 were taken intracisternal autologous blood injection, but no sumatriptan pre-treatment. Rats in group 3 were taken intracisternal autologous blood injection, but they were taken sumatriptan pre-treatment. Tissue samples were investigated for the presence of NOS immunoreactivity. The mean values of immunolabeling intensities for NOS enzymes in frontal cortex and meningeal vessels were significantly increased in group 2 compared to group 1. The mean values of immunolabeling intensities for NOS enzymes in frontal cortex and meningeal vessels were significantly reduced in group 3 compared to group 2. These results suggest that, chemical stimulation of the subarachnoid space increased the immunoreactivities of NOS enzymes in the brain of adolescent rats. The increased NOS immunoreactivities could be antagonized by pre-treatment with sumatriptan.

Animal models of migraine headache and aura

PURPOSE OF REVIEW

Over the past 30 years, animal models of migraine have led to the identification of novel drug targets and drug treatments as well as helped to clarify a mechanism for abortive and prophylactic drugs. Animal models have also provided translational knowledge and a framework to think about the impact of hormones, genes, and environmental factors on migraine pathophysiology. Although most acknowledge that these animal models have significant shortcomings, promising new drugs are now being developed and brought to the clinic using these preclinical models. Hence, it is timely to provide a short overview examining the ways in which animal models inform us about underlying migraine mechanisms.

RECENT FINDINGS

First generation migraine models mainly focused on events within pain-generating intracranial tissues, for example, the dura mater and large vessels, as well as their downstream consequences within brain. Upstream events such as cortical spreading depression have also been modeled recently and provide insight into mechanisms of migraine prophylaxis. Mouse mutants expressing human migraine mutations have been genetically engineered to provide an understanding of familial hemiplegic migraine and possibly, by extrapolation, may reflect on the pathophysiology of more common migraine subtypes.

SUMMARY

Animal models of migraine reflect distinct facets of this clinically heterogeneous disorder and contribute to a better understanding of its pathophysiology and pharmacology.

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.

Evaluation of the anti-emetic potential of anti-migraine drugs to prevent resiniferatoxin-induced emesis in Suncus murinus (house musk shrew)

Activation of vanilloid receptors has commonly been used to facilitate neurogenic inflammation and plasma exudation to model components of the pathogenesis of migraine; however, these studies have been performed mainly in species lacking the emetic reflex. In the present studies, therefore, we used Suncus murinus, a species of insectivore capable of emesis, to investigate if the vanilloid receptor agonist resiniferatoxin is capable of modeling the emesis associated with migraine. Resiniferatoxin (100 nmol/kg, s.c.) induced an emetic response that was antagonized significantly (P<0.05) by ruthenium red (1-3 micromol), (2R-trans)-4-[1-[3,5-bis(trifluromethyl)benzoyl]-2-(phenylmethyl)-4-piperidinyl]-N-(2,6-dimethylphenyl)-1-acetamide (S)-hydroxybutanedioate (R116301; 10-100 micromol/kg), and scopolamine (1 micromol/kg), but not by dihydroergotamine (0.3-3 micromol/kg), sumatriptan (1-10 micromol/kg), methysergide (1-10 micromol/kg), tropanyl 3,5-dichlorobenzoate (MDL72222; 3-30 micromol/kg), ondansetron (0.3-3 micromol/kg), metoclopramide (3-30 micromol/kg), domperidone (3-30 micromol/kg), diphenhydramine (1-10 micromol/kg), or indomethacin (3-30 micromol/kg). The failure of a wide range of representative anti-migraine drugs to reduce retching and vomiting limits the use of this model to identify/investigate novel treatments for the emesis (and nausea) associated with migraine attacks in humans. However, the results provide further evidence for the involvement of a novel vanilloid receptor in resiniferatoxin-induced emesis and implicate both tachykinins and acetylcholine in the pathway(s) activated by resiniferatoxin in S. murinus.

Preclinical neuropharmacology of naratriptan

The basic CNS neuropharmacology of naratriptan is reviewed here. Naratriptan is a second-generation triptan antimigraine drug, developed at a time when CNS activity was thought not to be relevant to its therapeutic effect in migraine. It was, however, developed to be a more lipid-soluble, more readily absorbed and less readily metabolized variant on preexisting triptans and these variations conferred on it a higher CNS profile. Naratriptan is a 5-HT(1B/1D) receptor agonist with a highly selective action on migraine pain and nausea, without significant effect on other pain or even other trigeminal pain. Probable sites of therapeutic action of naratriptan include any or all of: the cranial vasculature; the peripheral terminations of trigeminovascular sensory nerves; the first-order synapses of the trigeminovascular sensory system; the descending pain control system; and the nuclei of the thalamus. Naratriptan may prevent painful dilatation of intracranial vessels or reverse such painful dilatation. Naratriptan can prevent the release of sensory peptides and inhibit painful neurogenic vasodilatation of intracranial blood vessels. At the first order synapse of the trigeminal sensory system, naratriptan can selectively suppress neurotransmission from sensory fibers from dural and vascular tissue, while sparing transmission from other trigeminal fibers, probably through inhibition of neuropeptide transmitter release. In the periaqueductal gray matter and in the nucleus raphe magnus, naratriptan selectively activates inhibitory neurons which project to the trigeminal nucleus and spinal cord and which exert inhibitory influences on trigeminovascular sensory input. Naratriptan has also a therapeutic effect on the nausea of migraine, possibly exerting its action at the level of the nucleus tractus solitarius via the same mechanisms by which it inhibits trigeminovascular nociceptive input. The incidence of naratriptan-induced adverse effects in the CNS is low and it is not an analgesic for pain other than that of vascular headache. In patients receiving selective serotonin uptake inhibitors (SSRIs) naratriptan may cause serotonin syndrome-like behavioral side effects. The mechanism of action involved in the production of behavioral and other CNS side effects of naratriptan is unknown.

Effects of sumatriptan on rat medullary dorsal horn neurons

This study examined the cellular actions of the anti-migraine drug sumatriptan, on neurons in the substantia gelatinosa of the spinal trigeminal nucleus pars caudalis. Sumatriptan inhibited the miniature EPSC (mEPSC) rate in a dose dependent fashion, with an EC(50) of 250 nM. Sumatriptan (3 microM) inhibited the mEPSC rate by 36%, without altering the mEPSC amplitude. This effect was partially reversed by the 5HT(1D) specific antagonist BRL15572 (10 microM). In contrast, the 5HT(1B) agonist CP93129 (10 microm) did not alter the mEPSC rate. Furthermore, sumatriptan (3 microM) decreased the amplitude of electrically evoked EPSCs (eEPSC) by 40%. After incubating the slices in ketanserin (an antagonist which shows selectivity for 5HT(1D) over 5HT(1B) receptors) sumatriptan had little effect on eEPSC amplitude. In control conditions paired stimuli resulted in paired pulse depression (PPD; the ratio eEPSC(2)/eEPSC(1)=0.7+/-0.01), whilst in the presence of sumatriptan the PPD was blocked (ratio eEPSC(2)/eEPSC(1)=0.9+/-0.1). Sumatriptan produced no post-synaptic membrane current and had no significant effect on membrane conductance over a range of membrane potentials (-60 to -130 mV). RT-PCR experiments revealed the presence of mRNA for both 5HT(1D) and 5HT(1B) receptor subtypes in the trigeminal ganglia and subnucleus caudalis. These data suggest that sumatriptan acts pre-synaptically on trigeminal primary afferent central terminals to reduce the probability of release of glutamate, and that this action is mediated through 5HT(1D) receptors.

The anti-migraine agent sumatriptan induces a calcium-dependent discharge in meningeal sensory neurons

The anti-migraine agent sumatriptan constricts cerebral blood vessels, and also blocks neuropeptide release from meningeal sensory neurons. We investigated whether sumatriptan can also affect neuronal discharge, by recording the activity of dural primary afferent neurons during dural application of sumatriptan in anesthetized rats. Sumatriptan (0.24-24 microM) induced a concentration-dependent increase in ongoing discharge. The discharge occurred only in the presence of calcium. The suppressive effect of calcium removal was not a result of a general decrease in excitability, because calcium removal produced a pronounced increase in mechanically evoked discharge. The excitatory effect on dural afferent discharge might underlie the initial worsening of the headache that can occur following sumatriptan administration, prior to the subsequent onset of headache relief.

5-HT(1B/1D) serotonin receptor agonist attenuates nitroglycerin-evoked nitric oxide synthase expression in trigeminal pathway

This study was conducted to investigate the effect of 5-HT(1B/1D) receptor activation on nitroglycerin (NTG)-induced cerebral hyperaemia and neuronal nitric oxide synthase (nNOS) expression in trigeminovascular neurones. NTG (10 mg/kg) was infused intravenously to adult male Wistar rats with or without pretreatment with 5-HT(1B/1D) receptor agonist, sumatriptan (0.4 mg/kg, intravenously). Cortical blood flow and expression of nNOS enzyme in trigeminal ganglia, trigeminal nucleus caudalis and perivascular nerve fibre surrounding superior sagittal sinus were measured. The results showed that pretreatment with sumatriptan could significantly shorten the period of NTG-induced cerebral hyperaemia without compromising the magnitude of hyperaemic peak. Sumatriptan pretreatment also attenuated the NTG-evoked expression of nNOS in all studied areas. Based on these findings, we suggest that 5-HT(1B/1D) receptor has an important role in stabilizing the trigeminovascular system by attenuating the expression of nNOS enzyme, hence reducing nitric oxide production.

Innocuous jaw movements increase c-fos expression in trigeminal sensory nuclei produced by masseter muscle inflammation

Muscle tenderness and pain during movements are prominent symptoms associated with persistent jaw muscle pain. However, there is virtually no information on how trigeminal neurons respond to jaw movements (JM) or muscle palpation in the presence of muscle tissue injury or myositis. In this study, we investigated the effects of innocuous JM in the presence of acute masseteric inflammation on postsynaptic responses in the trigeminal brainstem nuclei by examining the expression of c-fos. In one group of rats, unilateral injections of an inflammatory substance, mustard oil (MO: 20%, 25 microl) were made into a masseter muscle. In another group, controlled and systematic JM were provided following MO injection. Three additional groups of rats were used to control for anesthetic, JM, and injection procedure. MO injected in the masseter muscle induced a high level of Fos protein expression in four principal trigeminal regions: the subnucleus caudalis (Vc), the ventral and dorsal regions of the Vc/Vi (subnucleus interpolaris) transition zone, and the paratrigeminal nucleus (PTN). Movements following MO injection consistently produced a significantly greater level of Fos expression in all these areas, especially in the Vc/Vi transition region and caudal Vc on the ipsilateral side. Importantly, movements also induced a significantly greater level of Fos expression in the caudal Vc on the contralateral side. The present results provide the first documentation that innocuous JM in the presence of muscle inflammation significantly increase the MO-induced c-fos expression in the trigeminal brainstem nuclei, which may explain the greater pain experienced during movement of inflamed or injured muscles.

Dihydroergotamine and its metabolite, 8'-hydroxy-dihydroergotamine, as 5-HT1A receptor agonists in the rat brain

1 In addition to stopping migraine attacks, dihydroergotamine (DHE) is an efficient drug for migraine prophylaxis. Whether 5-HT(1A) receptors could contribute to the latter action was assessed by investigating the effects of DHE and its metabolite, 8'-OH-DHE, on these receptors in the rat brain. 2 Membrane binding assays with [(3)H]8-OH-DPAT and [(3)H]WAY 100635 as radioligands showed that both DHE (IC(50)=28-30 nM) and 8'-OH-DHE (IC(50)=8-11 nM) are high-affinity 5-HT(1A) receptor ligands. 3 Both DHE and 8'-OH-DHE enhanced the specific binding of [(35)S]GTP-gamma-S to the dorsal raphe nucleus and the hippocampus in brain sections, but to a lower extent than 5-carboxamido-tryptamine (5-CT) in the latter area. 4 Both DHE (EC(50)=10.9+/-0.3 nM) and 8'-OH-DHE (EC(50)=30.4+/-0.8 nM) inhibited the firing of serotoninergic neurons in the dorsal raphe nucleus within brain stem slices. 5 Intracellular recording showed that 8'-OH-DHE was more potent than DHE to hyperpolarize CA1 pyramidal cells in rat hippocampal slices. 6 Both the stimulatory effects of DHE and 8'-OH-DHE on [(35)S]GTP-gamma-S binding and their electrophysiological effects were completely prevented by the selective 5-HT(1A) receptor antagonist WAY 100635. 7 As expected of 5-HT(1A) receptor partial agonists, DHE and 8'-OH-DHE prevented any subsequent hyperpolarization of CA1 pyramidal cells by 5-HT or 5-CT. 8 Through their actions at 5-HT(1A) auto- (in the dorsal raphe nucleus) and hetero-(notably in the hippocampus) receptors, DHE, and even more its metabolite 8'-OH-DHE, can exert both an inhibitory influence on neuronal excitability and anxiolytic effects which might contribute to their antimigraine prophylactic efficiency.

The antimigraine 5-HT 1B/1D receptor agonists, sumatriptan, zolmitriptan and dihydroergotamine, attenuate pain-related behaviour in a rat model of trigeminal neuropathic pain

1. Peripheral lesion to the trigeminal nerve may induce severe pain states. Several lines of evidence have suggested that the antimigraine effect of the triptans with 5-HT(1B/1D) receptor agonist properties may result from inhibition of nociceptive transmission in the spinal nucleus of the trigeminal nerve by these drugs. On this basis, we have assessed the potential antinociceptive effects of sumatriptan and zolmitriptan, compared to dihydroergotamine (DHE), in a rat model of trigeminal neuropathic pain. 2. Chronic constriction injury was produced by two loose ligatures of the infraorbital nerve on the right side. Responsiveness to von Frey filament stimulation of the vibrissal pad was used to evaluate allodynia. 3. Two weeks after ligatures, rats with a chronic constriction of the right infraorbital nerve displayed bilateral mechanical hyper-responsiveness to von Frey filament stimulation of the vibrissal pad with a mean threshold of 0.38+/-0.04 g on the injured side and of 0.43+/-0.04 g on the contralateral (left) side (versus > or =12.5 g on both sides in the same rats prior to nerve constriction injury). 4. Sumatriptan at a clinically relevant dose (100 microg kg(-1), s.c.) led to a significant reduction of the mechanical allodynia-like behaviour on both the injured and the contralateral sides (peak-effects 6.3+/-1.1 g and 4.4+/-0.7 g, respectively). A more pronounced effect was obtained with zolmitriptan (100 microg kg(-1), s.c.) (peak-effects: 7.4+/-0.9 g and 3.2+/-1.3 g) whereas DHE (50-100 microg kg(-1), i.v.) was less active (peak-effect approximately 1.5 g). 5. Subcutaneous pretreatment with the 5-HT(1B/1D) receptor antagonist, GR 127935 (3 mg kg(-1)), prevented the anti-allodynia-like effects of triptans and DHE. Pretreatment with the 5-HT(1A) receptor antagonist, WAY 100635 (2 mg kg(-1), s.c.), did not alter the effect of triptans but significantly enhanced that of DHE (peak effect 4.3+/-0.5 g). 6. In a rat model of peripheral neuropathic pain, which consisted of a unilateral loose constriction of the sciatic nerve, neither sumatriptan (50-300 microg kg(-1)) nor zolmitriptan (50-300 microg kg(-1)) modified the thresholds for paw withdrawal and vocalization in response to noxious mechanical stimulation. 7. These results support the rationale for exploring the clinical efficacy of brain penetrant 5-HT(1B/1D) receptor agonists as analgesics to reduce certain types of trigeminal neuropathic pain in humans.

5-Hydroxytryptamine(1B/1D) and 5-hydroxytryptamine1F receptors inhibit capsaicin-induced c-fos immunoreactivity within mouse trigeminal nucleus caudalis

In order to investigate the c-fos response within the trigeminal nucleus caudalis (Sp5C) after noxious meningeal stimulation, capsaicin (0.25, 0.5, 1 and 5 nmol) was administered intracisternally in urethane (1 g/kg) and alpha-chloralose (20 mg/kg) anaesthetized male mice. Capsaicin induced a robust and dose-dependent c-fos-like immunoreactivity (c-fos LI) within Sp5C. C-fos LI was observed within laminae I and II of the entire brain stem from the area postrema to C2 level, being maximum at the decussatio pyramidum level. The area postrema, solitary tract, medullary and lateral reticular nuclei were also labelled. The 5-hydroxytryptamine(1B/1D/1F) receptor agonist sumatriptan (0.01, 0.1, 1 and 10 mg/kg), administered intraperitoneally 15 min before capsaicin stimulation (1 nmol), decreased the c-fos response within Sp5C, but not within solitary tract. The novel specific 5-hydroxytryptamine1F agonist LY 344864 (0.1 and 1 mg/kg, i.p.) significantly decreased the c-fos LI within the Sp5C as well. These findings suggest that intracisternally administered capsaicin activates the trigeminovascular system and that the pain neurotransmission can be modulated by 5-hydroxytryptamine(1B/1D/1F) receptors in mice. Thus, the availability of this model in mice, taken together with the possibility of altering the expression of specific genes in this species, may help to investigate further the importance of distinct proteins in the neurotransmission of cephalic pain.

New insights into the molecular actions of serotonergic antimigraine drugs

Migraine is a painful and debilitating neurological disorder that affects approximately 10% of the adult population in Western countries. Sensitization and activation of the trigeminal ganglia nerves that innervate the meningeal blood vessels is believed to play an important role in the initiation and maintenance of migraine pain. In this capacity, release of the neuropeptide calcitonin gene-related peptide (CGRP) and the resultant neurogenic inflammation is thought to underlie the pathophysiology of migraine. Largely due to the success of the serotonin Type 1 migraine drugs such as sumatriptan, migraine pathology and therapy has become a focus of intensive clinical and physiological research during the past decade. The effectiveness of these drugs is thought to be due to their ability to block the stimulated secretion of neuropeptides from trigeminal nerves to break the vicious nociceptive cycle of migraine. A component of this nociceptive cycle involves activation of mitogen-activated protein kinase signaling pathways. Indeed, activation of mitogen-activated protein kinase pathways can increase CGRP neuropeptide synthesis and secretion. Recently, the serotonin Type 1 agonists have been shown to cause a prolonged increase in intracellular Ca(2+) in trigeminal ganglia neurons and an increased phosphatase activity that can repress stimulated CGRP secretion and transcription. Identification of molecular signaling events in migraine pathology and therapy has provided new insight into the pharmacology and signaling mechanisms of sumatriptan and related drugs, and may provide the foundation for development of novel treatments for migraine.

Trigeminal nociception-induced cerebral Fos expression in the conscious rat

Little is known about trigeminal nociception-induced cerebral activity and involvement of cerebral structures in pathogenesis of trigeminovascular headaches such as migraine. Neuroimaging has demonstrated cortical, hypothalamic and brainstem activation during the attack and after abolition with sumatriptan. This has led to the conclusion that the dorsal raphe and locus coeruleus may initiate events that generate migraneous headache. Using a conscious rat model of trigeminal nociception and cerebral Fos expression as histochemical markers of neuronal activity, we characterized the pattern of brain activity after noxious trigeminal stimulation with capsaicin (250 and 1000 nm). A significantly increased Fos immunoreactivity was found in the trigeminal nucleus caudalis (layers I and II), the area postrema, the nucleus of the solitary tract, the parvicellular reticular nucleus, the locus coeruleus, the parabrachial nucleus and the raphe nuclei. In addition, the ventrolateral periaqueductal grey, the intralaminar thalamic and various hypothalamic areas, showed an enhanced Fos expression after the intracisternal administration of capsaicin. Other responding areas were the amygdala, the upper lip and forelimb regions of the primary somatosensory cortex, and the insula. Many of these areas participate in (anti)-nociception, although we cannot exclude the possibility that in conscious animals the pain-associated physiological and behavioural responses that are an intrinsic and necessary part of coping with pain have generated the increased Fos expression. Trigeminal stimulation-induced locus coeruleus, dorsal raphe and hypothalamic activation are opposed to a suggested pathogenic role of these nuclei in migraine and cluster headache, respectively.

Nitric oxide potentiates response of trigeminal neurones to dural or facial stimulation in the rat

Infusing glyceryl trinitrate as a donor molecule, we have used electrophysiological and c-fos immunostaining techniques to study the effects of nitric oxide on neurones in the nucleus trigeminalis caudalis. Following infusion of glyceryl trinitrate, responses of neurones to electrical stimulation of periorbital cutaneous afferents were potentiated and threshold for activation of neurones by stimulation of dural afferents was reduced. Expression of c-fos was unchanged by glyceryl trinitrate compared to saline controls. Intradermal injection of capsaicin in the periorbital area increased c-fos expression in nucleus trigeminalis caudalis; this was significantly potentiated by glyceryl trinitrate. These results suggest that, in the anaesthetized rat, glyceryl trinitrate alone may not acutely activate the trigeminovascular system to a significant degree at doses that cause headache and later trigger migraine headache in migraineurs. Nevertheless, it is susceptible to exogenous nitric oxide in that activation of trigeminal neurones through cutaneous or dural pathways is potentiated. This may in some measure underlie the pathogenesis of migraine headache.

Role of opioid receptors in neurogenic dural vasodilation and sensitization of trigeminal neurones in anaesthetized rats

Migraine headache is thought to be caused by a distension of meningeal blood vessels, the activation of trigeminal sensory neurones and the the development of a central sensitization within the trigeminal nucleus caudalis (TNC). It has been proposed that clinically effective 5-HT(1B/1D) agonists act peripherally to inhibit the release of calcitonin gene-related peptide (CGRP) and neurogenic dural vasodilation, and to attenuate nociceptive neurotransmission within the TNC. Since opioids are also effective anti-migraine agents the present studies investigated the role of opioids within the trigemino-vascular system in anaesthetised rats. Electrical stimulation of the dura mater evoked neurogenic dural vasodilation which was significantly inhibited by morphine (1 mg kg(-1)) the selective mu-opioid agonist DAGO (10 microg kg(-1)) and the mixed agonist/antagonist butorphanol (1 mg kg(-1)) but not by the kappa- and delta-opioid agonists (+/-) U50488H (100 microg kg(-1)) and DPDPE (1 mg kg(-1)). Morphine had no effect on CGRP-evoked dural vasodilation. In electrophysiological studies morphine (1 - 10 mg kg(-1)) significantly attenuated brainstem neuronal activity in response to electrical stimulation of the dura by 65% at 10 mg kg(-1). Morphine (3 mg kg(-1)) also inhibited the TNC neuronal sensitization following CGRP-evoked dilation. The present studies have demonstrated that opioids block the nociceptive neurotransmission within the trigeminal nucleus caudalis and in addition inhibit neurogenic dural vasodilation via an action on mu-opioid receptors located on trigeminal sensory fibres innervating dural blood vessels. These peripheral and central actions are similar to those of the 'triptan' 5-HT(1B/1D) agonists and could account for the anti-migraine actions of opioids.

Inhibition of synovial plasma extravasation by preemptive administration of an antiinflammatory irrigation solution in the rat knee

Inflammation and hyperalgesia during surgical procedures are caused by the local release of multiple inflammatory mediators. We used a rat knee joint model of acute inflammation (synovial plasma extravasation) to determine whether preemptive intraarticular irrigation of the antiinflammatory drugs ketoprofen, amitriptyline, or oxymetazoline, alone or in combination, can reduce inflammatory soup-induced plasma extravasation. These three drugs were selected because of their abilities to collectively inhibit the inflammatory effects of biogenic amines, eicosanoid production, and the release of neuropeptides from C-fiber terminals. Synovial perfusion of each one of the three drugs 10 min before, and then in combination with, the inflammatory soup (bradykinin, 5-hydroxytryptamine, and mustard oil) did not reduce plasma extravasation. Similarly, two-drug combinations did not significantly reduce inflammatory soup-induced plasma extravasation. The combination of all three drugs (amitriptyline, ketoprofen, and oxymetazoline) produced a dramatic inhibition of plasma extravasation and was more effective than any of the two-drug combinations. A comparison between the preemptive (10 min before inflammatory soup perfusion) and postinflammatory administration (10 min after inflammatory soup perfusion) showed that the postinflammatory administration of the three-drug solution lost all ability to inhibit inflammatory soup-induced plasma extravasation. We conclude that acute synovial inflammation, which is induced and maintained by multiple mediators, can be substantially inhibited only by the preemptive administration of a drug combination that targets multiple inflammatory mediators.

IMPLICATIONS

Preemptive, intraarticular irrigation of a combination of multiple antiinflammatory drugs is a novel and potentially effective method for reducing the synovial inflammatory response, such as that during arthroscopy. In this study, a three-drug combination infusion was statistically superior to one- or two-drug infusions in a rat model.

Nerve fibers innervating the cranial and spinal meninges: morphology of nerve fiber terminals and their structural integration

Pachymeninx and leptomeninx of cranial cavity and spine are considerably different in their collagenous fiber texture, cellular composition, vascularization, and innervation. The majority of meningeal nerve fibers terminate as free nerve endings whereas encapsulated and lamellated nerve terminals additionally occur in higher vertebrates including man. With respect to nerve fiber classification, arborization pattern, topography, and organization of the microenvironment at the termination site afferent and efferent nerve terminals are differentiated. Only the dura mater and the pial subcompartment of the leptomeninx possess the morphological prerequisites for neurogenic inflammation. In the current review, the results of morphological studies regarding the meningeal innervation including the sites of CSF (cerebrospinal fluid) production and absorption are discussed with emphasis on their structure-function relationships.

Spinal sources of noxious visceral and noxious deep somatic afferent drive onto the ventrolateral periaqueductal gray of the rat

Studies utilizing the expression of Fos protein as a marker of neuronal activation have revealed that pain of deep somatic or visceral origin selectively activates the ventrolateral periaqueductal gray (vlPAG). Previous anatomical tracing studies revealed that spinal afferents to the vlPAG arose from the superficial and deep dorsal horn and nucleus of the dorsolateral funiculus at all spinal segmental levels, with approximately 50% of vlPAG-projecting spinal neurons found within the upper cervical spinal cord. This study utilized detection of Fos protein to determine the specific populations of vlPAG-projecting spinal neurons activated by noxious deep somatic or noxious visceral stimulation. Pain of cardiac or peritoneal (i.e., visceral) origin activated neurons in the superficial and deep dorsal horn and nucleus of the dorsolateral funiculus of the thoracic cord, whereas pain of hindlimb (i.e., deep somatic) origin activated neurons in the same laminar regions but in the lumbosacral cord. Each of these deep noxious manipulations also activated neurons in the superficial and deep dorsal horn and nucleus of the dorsolateral funiculus of the upper cervical spinal cord. In a second set of experiments, the combination of retrograde tracing and Fos immunohistochemistry revealed that vlPAG-projecting spinal neurons activated by deep somatic pain were located in both the upper cervical and lumbosacral cord, whereas those activated by visceral pain were restricted to the thoracic spinal cord. Thus pain arising from visceral versus deep somatic body regions influences neural activity within the vlPAG via distinct spinal pathways. The findings also highlight the potential significance of the upper cervical cord in integrating pain arising from deep structures throughout the body.

Magnetic field desensitizes 5-HT(1B) receptor in brain: pharmacological and functional studies

It was previously suggested that exposure to magnetic fields (MFs) could generate dysfunction of the CNS. The physiological manifestations described lead us to postulate that these symptoms might be related to a dysfunction of the serotonergic system and particularly of the 5-HT(1B) receptors. Accordingly, MFs could modify the conformation of these receptors altering their functional activities. In rat brain membrane preparations, we showed that the affinity constant of 5-HT for 5-HT(1B) receptors was modified under exposure to MFs since K(d) varied from 4.7+/-0.5 to 12+/-3 nM in control and exposed (2.5 mT) membranes, respectively. This effect was intensity-dependent (the sigmoidal dose-response curve was characterized by an EI(50) of 662+/-69 microT and a maximal increase of 321+/-13% of the control K(d)), reversible, temperature-dependent and specific to the 5-HT(1B) receptors. Similar results have also been obtained with the human 5-HT(1B) receptors. In parallel assays, the functional activity of 5-HT(1B) receptors was investigated. The capacity of a 5-HT(1B) agonist to inhibit the cAMP production was reduced by 37% (53.7+/-3.5% to 33.7+/-4.1%) following exposure to MFs and the cellular activity of the receptors (inhibition of the synaptosomal release of 5-HT) also was markedly reduced (66.5+/-3.2% to 28.5+/-4.2%). These results clearly show that in in vitro assays, MF specifically interacts with 5-HT(1B) receptors, inducing structural changes of the protein that result in a functional desensitization of the receptors. Thus, in vivo, exposure to MFs may lead to physiological changes, particularly in the field of mood disorders where the 5-HT system is strongly involved.

Acute and chronic craniofacial pain: brainstem mechanisms of nociceptive transmission and neuroplasticity, and their clinical correlates

This paper reviews the recent advances in knowledge of brainstem mechanisms related to craniofacial pain. It also draws attention to their clinical implications, and concludes with a brief overview and suggestions for future research directions. It first describes the general organizational features of the trigeminal brainstem sensory nuclear complex (VBSNC), including its input and output properties and intrinsic characteristics that are commensurate with its strategic role as the major brainstem relay of many types of somatosensory information derived from the face and mouth. The VBSNC plays a crucial role in craniofacial nociceptive transmission, as evidenced by clinical, behavioral, morphological, and electrophysiological data that have been especially derived from studies of the relay of cutaneous nociceptive afferent inputs through the subnucleus caudalis of the VBSNC. The recent literature, however, indicates that some fundamental differences exist in the processing of cutaneous vs. other craniofacial nociceptive inputs to the VBSNC, and that rostral components of the VBSNC may also play important roles in some of these processes. Modulatory mechanisms are also highlighted, including the neurochemical substrate by which nociceptive transmission in the VBSNC can be modulated. In addition, the long-term consequences of peripheral injury and inflammation and, in particular, the neuroplastic changes that can be induced in the VBSNC are emphasized in view of the likely role that central sensitization, as well as peripheral sensitization, can play in acute and chronic pain. The recent findings also provide new insights into craniofacial pain behavior and are particularly relevant to many approaches currently in use for the management of pain and to the development of new diagnostic and therapeutic procedures aimed at manipulating peripheral inputs and central processes underlying nociceptive transmission and its control within the VBSNC.

Intracisternal octreotide does not ameliorate orthodromic trigeminovascular nociception

Octreotide is a long-acting somatostatin analogue that has been effectively used to treat migraine. Octreotide poorly penetrates the blood-brain barrier, but has potential central target sites in the trigeminal nucleus caudalis, which is the primary central relay station for trigeminal nociceptive information in the brain. We studied the effect of intracisternally applied octreotide in a model of trigeminovascular stimulation in the unrestrained rat using intracisternal capsaicin infusion to stimulate intracranial trigeminal nerves. Fos expression in the outer layers of the trigeminal nucleus caudalis (TNC I-II) and behavioural analysis were used to measure the effects of octreotide on capsaicin-induced trigeminovascular activation. Increases of head grooming and scratching behaviour are an indication of octreotide-induced trigeminal activation. However, octreotide did not alter the average capsaicin-induced Fos expression in the TNC I-II and capsaicin sensitive behaviours were not modified by octreotide pretreatment. This argues against a role for central (TNC I-II) somatostatin receptors in the processing of the nociceptive trigeminovascular signals.

Auditory evoked potentials in the assessment of central nervous system effects of antimigraine drugs

Because the "intensity dependence" of cortical auditory evoked potentials (IDAP) is under serotonergic control, it can be used to assess central antimigraine effects of 5HT1B/1D agonists. We measured IDAP before and 2 h after naratriptan (5 mg, n = 19) and zolmitriptan (5 mg, n = 19) in healthy volunteers. IDAP was expressed as the amplitude-stimulus intensity function ("ASF slope"). Naratriptan tended to increase ASF slope (mean difference 0.23 +/- 0.62 microV/10 dB, p = 0.06) while zolmitriptan (0.08 +/- 0.95 microV/10 dB, p = 0.35) did not. We assessed the suitability of IDAP for measuring central antimigraine drug effects using repeatability data (see companion paper). We calculated the trade-off between the size of the expected drug effects (ASF slope difference) and the necessary sample size. Because of poor repeatability 36 to 80 subjects are required to detect ASF slope changes in the 0.25-0.5 microV/10 dB range. These data can be used to design trials using IDAP.