CGRP physiology, pharmacology, and therapeutic targets: migraine and beyond

Calcitonin gene-related peptide (CGRP) is a neuropeptide with diverse physiological functions. Its two isoforms (α and β) are widely expressed throughout the body in sensory neurons as well as in other cell types, such as motor neurons and neuroendocrine cells. CGRP acts via at least two G protein-coupled receptors that form unusual complexes with receptor activity-modifying proteins. These are the CGRP receptor and the AMY1 receptor; in rodents, additional receptors come into play. Although CGRP is known to produce many effects, the precise molecular identity of the receptor(s) that mediates CGRP effects is seldom clear. Despite the many enigmas still in CGRP biology, therapeutics that target the CGRP axis to treat or prevent migraine are a bench-to-bedside success story. This review provides a contextual background on the regulation and sites of CGRP expression and CGRP receptor pharmacology. The physiological actions of CGRP in the nervous system are discussed, along with updates on CGRP actions in the cardiovascular, pulmonary, gastrointestinal, immune, hematopoietic, and reproductive systems and metabolic effects of CGRP in muscle and adipose tissues. We cover how CGRP in these systems is associated with disease states, most notably migraine. In this context, we discuss how CGRP actions in both the peripheral and central nervous systems provide a basis for therapeutic targeting of CGRP in migraine. Finally, we highlight potentially fertile ground for the development of additional therapeutics and combinatorial strategies that could be designed to modulate CGRP signaling for migraine and other diseases.

Calcitonin gene relating peptide inhibitors in combination for migraine treatment: A mini-review

The discovery of calcitonin gene-related peptide (CGRP) and its role in migraine pathophysiology has led to advances in the treatment of migraine. Since 2018, the Food and Drug Administration (FDA) has approved four monoclonal antibody (mab) therapies targeting either the CGRP ligand or receptor and 3 oral small molecule CGRP receptor antagonists. These targeted therapies have been shown to be safe and effective for either preventive or acute treatment of migraine in adults. Given their efficacy and tolerability profile, CGRP inhibitors have revolutionized the approach to migraine treatment. Theoretically, combining therapies within this therapeutic class could lead to more CGRP blockade and, subsequently, improved patient outcomes. There are providers currently combining CGRP therapies in clinical practice. However, limited data are available regarding the efficacy and safety of this practice. This mini-review provides a summary of available data and poses important considerations when combining CGRP therapies for migraine treatment.

Antagonism of CGRP Receptor: Central and Peripheral Mechanisms and Mediators in an Animal Model of Chronic Migraine

Calcitonin-gene-related peptide (CGRP) plays a key role in migraine pathophysiology and more specifically in the mechanisms underlying peripheral and central sensitization. Here, we explored the interaction of CGRP with other pain mediators relevant for neuronal sensitization in an animal model of chronic migraine. Male Sprague-Dawley rats were exposed to nitroglycerin (NTG, 5 mg/kg, i.p.) or vehicle co-administered with the CGRP receptor antagonist olcegepant (2 mg/kg i.p.), or its vehicle, every other day over a 9-day period. Twenty-four hours after the last injection of NTG (or vehicle), behavioral test and ex vivo analysis were performed. Olcegepant attenuated NTG-induced trigeminal hyperalgesia in the second phase of the orofacial formalin test. Interestingly, it also reduced gene expression and protein levels of CGRP, pro-inflammatory cytokines, inflammatory-associated miRNAs (miR-155-5p, miR-382-5p, and miR-34a-5p), and transient receptor potential ankyrin channels in the medulla-pons area, cervical spinal cord, and trigeminal ganglia. Similarly, olcegepant reduced the NTG-induced increase in CGRP and inflammatory cytokines in serum. The findings show that the activation of the CGRP pathway in a migraine animal model was associated to the persistent activation of inflammatory pathways, which was paralleled by a condition of hyperalgesia. These molecular events are relevant for informing us about the mechanisms underlying chronic migraine.

A Female-Specific Role for Calcitonin Gene-Related Peptide (CGRP) in Rodent Pain Models

We aimed to investigate a sexually dimorphic role of calcitonin gene-related peptide (CGRP) in rodent models of pain. Based on findings in migraine where CGRP has a preferential pain-promoting effect in female rodents, we hypothesized that CGRP antagonists and antibodies would attenuate pain sensitization more efficaciously in female than male mice and rats. In hyperalgesic priming induced by activation of interleukin 6 signaling, CGRP receptor antagonists olcegepant and CGRP_8-37 both given intrathecally, blocked, and reversed hyperalgesic priming only in females. A monoclonal antibody against CGRP, given systemically, blocked priming specifically in female rodents but failed to reverse it. In the spared nerve injury model, there was a transient effect of both CGRP antagonists, given intrathecally, on mechanical hypersensitivity in female mice only. Consistent with these findings, intrathecally applied CGRP caused a long-lasting, dose-dependent mechanical hypersensitivity in female mice but more transient effects in males. This CGRP-induced mechanical hypersensitivity was reversed by olcegepant and the KCC2 enhancer CLP257, suggesting a role for anionic plasticity in the dorsal horn in the pain-promoting effects of CGRP in females. In spinal dorsal horn slices, CGRP shifted GABA_A reversal potentials to significantly more positive values, but, again, only in female mice. Therefore, CGRP may regulate KCC2 expression and/or activity downstream of CGRP receptors specifically in females. However, KCC2 hypofunction promotes mechanical pain hypersensitivity in both sexes because CLP257 alleviated hyperalgesic priming in male and female mice. We conclude that CGRP promotes pain plasticity in female rodents but has a limited impact in males.SIGNIFICANCE STATEMENT The majority of patients impacted by chronic pain are women. Mechanistic studies in rodents are creating a clear picture that molecular events promoting chronic pain are different in male and female animals. We sought to build on evidence showing that CGRP is a more potent and efficacious promoter of headache in female than in male rodents. To test this, we used hyperalgesic priming and the spared nerve injury neuropathic pain models in mice. Our findings show a clear sex dimorphism wherein CGRP promotes pain in female but not male mice, likely via a centrally mediated mechanism of action. Our work suggests that CGRP receptor antagonists could be tested for efficacy in women for a broader variety of pain conditions.

The role of TRP ion channels in migraine and headache

Migraine afflicts more than 10% of the general population. Although its mechanism is poorly understood, recent preclinical and clinical evidence has identified calcitonin gene related peptide (CGRP) as a major mediator of migraine pain. CGRP, which is predominantly expressed in a subset of primary sensory neurons, including trigeminal afferents, when released from peripheral terminals of nociceptors, elicits arteriolar vasodilation and mechanical allodynia, a hallmark of migraine attack. Transient receptor potential (TRP) channels include several cationic channels with pleiotropic functions and ubiquitous distribution in various cells and tissues. Some members of the TRP channel family, such as the ankyrin 1 (TRPA1), vanilloid 1 and 4 (TRPV1 and TRPV4, respectively), and TRPM3, are abundantly expressed in primary sensory neurons and are recognized as sensors of chemical-, heat- and mechanical-induced pain, and play a primary role in several models of pain diseases, including inflammatory, neuropathic cancer pain, and migraine pain. In addition, TRP channel stimulation results in CGRP release, which can be activated or sensitized by various endogenous and exogenous stimuli, some of which have been proven to trigger or worsen migraine attacks. Moreover, some antimigraine medications seem to act through TRPA1 antagonism. Here we review the preclinical and clinical evidence that highlights the role of TRP channels, and mainly TRPA1, in migraine pathophysiology and may be proposed as new targets for its treatment.

Excitatory Effects of Calcitonin Gene-Related Peptide (CGRP) on Superficial Sp5C Neurons in Mouse Medullary Slices

The neuromodulator calcitonin gene-related peptide (CGRP) is known to facilitate nociceptive transmission in the superficial laminae of the spinal trigeminal nucleus caudalis (Sp5C). The central effects of CGRP in the Sp5C are very likely to contribute to the activation of central nociceptive pathways leading to attacks of severe headaches like migraine. To examine the potential impacts of CGRP on laminae I/II neurons at cellular and synaptic levels, we performed whole-cell patch-clamp recordings in juvenile mouse brainstem slices. First, we tested the effect of CGRP on cell excitability, focusing on neurons with tonically firing action potentials upon depolarizing current injection. CGRP (100 nM) enhanced tonic discharges together with membrane depolarization, an excitatory effect that was significantly reduced when the fast synaptic transmissions were pharmacologically blocked. However, CGRP at 500 nM was capable of exciting the functionally isolated cells, in a nifedipine-sensitive manner, indicating its direct effect on membrane intrinsic properties. In voltage-clamped cells, 100 nM CGRP effectively increased the frequency of excitatory synaptic inputs, suggesting its preferential presynaptic effect. Both CGRP-induced changes in cell excitability and synaptic drives were prevented by the CGRP receptor inhibitor BIBN 4096BS. Our data provide evidence that CGRP increases neuronal activity in Sp5C superficial laminae by dose-dependently promoting excitatory synaptic drive and directly enhancing cell intrinsic properties. We propose that the combination of such pre- and postsynaptic actions of CGRP might underlie its facilitation in nociceptive transmission in situations like migraine with elevated CGRP levels.

Preclinical studies investigating the neural mechanisms involved in the co-morbidity of migraine and temporomandibular disorders: the role of CGRP

BACKGROUND AND PURPOSE

Temporomandibular disorders (TMD) and migraine can be co-morbid. This can be a significant factor in exacerbating and increasing the prevalence of migraine-like symptoms. However, the underlying mechanisms involved are unknown. Our objective was to investigate these neural mechanisms and the role of CGRP as a key modulator in this co-morbidity.

EXPERIMENTAL APPROACH

We combined experimental approaches using CGRP, which triggers a migraine-like response in patients, with that of masseteric muscle injection of complete Freund's adjuvant (CFA), to model myofascial TMD-like inflammation. Using validated electrophysiological methods to assess each of the above approaches independently or in combination, we examined their effects on the response properties of migraine-like dural-trigeminocervical neurons.

KEY RESULTS

Independently, in ~2/3 of animals (rats) each approach caused delayed migraine-like activation and sensitisation of dural-trigeminocervical neurons. The response to masseteric-CFA was attenuated by a selective CGRP receptor antagonist. The combination approach caused a migraine-like neuronal response in all animals tested, with somatosensory-evoked cranial hypersensitivity significantly exacerbated.

CONCLUSION AND IMPLICATIONS

The data demonstrate a neuronal phenotype that translates to the exacerbated clinical co-morbid phenotype, supporting this combination approach as a relevant model to study the mechanisms involved. It provides a pathophysiological rationale for this exacerbated phenotype, strongly implicating the involvement of CGRP. The results provide support for targeting the CGRP pathway as a novel monotherapy approach for treating this co-morbid condition. This has key implications into our understanding of this co-morbid condition, as well as potentially addressing the major unmet need for novel and effective therapeutic approaches.

Novel Analgesics with Peripheral Targets

A limited number of peripheral targets generate pain. Inflammatory mediators can sensitize these. The review addresses targets acting exclusively or predominantly on sensory neurons, mediators involved in inflammation targeting sensory neurons, and mediators involved in a more general inflammatory process, of which an analgesic effect secondary to an anti-inflammatory effect can be expected. Different approaches to address these systems are discussed, including scavenging proinflammatory mediators, applying anti-inflammatory mediators, and inhibiting proinflammatory or facilitating anti-inflammatory receptors. New approaches are contrasted to established ones; the current stage of progress is mentioned, in particular considering whether there is data from a molecular and cellular level, from animals, or from human trials, including an early stage after a market release. An overview of publication activity is presented, considering a IuPhar/BPS-curated list of targets with restriction to pain-related publications, which was also used to identify topics.

No central action of CGRP antagonising drugs in the GTN mouse model of migraine

Introduction

Clinically, calcitonin gene-related peptide antagonising drugs are recognized as effective in migraine treatment, but their site of action is debated. Only a small fraction of these compounds pass the blood-brain barrier and accesses the central nervous system. Regardless, it has been argued that the central nervous system is the site of action. Here, we test this hypothesis by bypassing the blood-brain barrier through intracerebroventricular injection of calcitonin gene-related peptide antagonising drugs.

Methods

We used the glyceryl trinitrate (GTN) mouse model, which is well validated by its response to specific migraine drugs. The calcitonin gene-related peptide receptor antagonist olcegepant and the calcitonin gene-related peptide monoclonal antibody ALD405 were administered either intraperitoneally or intracerebroventricularly. The outcome measure was cutaneous mechanical allodynia.

Results

Mice given olcegepant intraperitoneally + GTN on day 1 had a mean 50% withdrawal threshold of 1.2 g in contrast to mice receiving placebo + GTN, which had a threshold of 0.3 g ( p < 0.001). Similarly, in the ALD405 + GTN group, mice had thresholds of 1.2 g versus 0.2 g in the placebo + GTN group ( p < 0.001). However, both drugs were ineffective when delivered intracerebroventricularly, as control and active groups had identical mechanical sensitivity thresholds, 0.2 g versus 0.1 g and 0.1 g versus 0.1 g for olcegepant and ALD405, respectively ( p > 0.99 in both cases).

Discussion

The site of action of olcegepant and of the monoclonal antibody ALD405 is outside the blood-brain barrier in this mouse model of migraine. It is likely that these results can be generalised to all gepants and all antibodies and that the results are relevant for human migraine.

Insulin-like growth factor-1 inhibits spreading depression-induced trigeminal calcitonin gene related peptide, oxidative stress & neuronal activation in rat

Migraineurs can show brain hyperexcitability and oxidative stress that may promote headache. Since hyperexcitability can enhance oxidative stress which promotes hyperexcitability, ending this feed-back loop may reduce migraine. Neocortical spreading depression, an animal model of migraine begins with hyperexcitability and triggers oxidative stress in the neocortical area involved and in the trigeminal system, which is important to pain pathway nociceptive activation in migraine. Additionally, oxidative stress causes increased trigeminal ganglion calcitonin gene-related peptide release and oxidative stress can reduce spreading depression threshold. Insulin-like growth factor-1 significantly protects against spreading depression in vitro by reducing oxidative stress and it is effective against spreading depression after intranasal delivery to animals. Here, we used adult male rats and extend this work to study the trigeminal system where insulin-like growth factor-1 receptors are highly expressed. Recurrent neocortical spreading depression significantly increased surrogate markers of trigeminal activation - immunostaining for trigeminal ganglion oxidative stress, calcitonin gene related peptide levels and c-fos in the trigeminocervical complex versus sham. These effects were significantly reduced by intranasal delivery of insulin-like growth factor-1 a day before recurrent neocortical spreading depression. Furthermore, intranasal treatment with insulin-like growth factor-1 significantly reduced naïve levels of trigeminal ganglion calcitonin gene related peptide versus sham with no impact on blood glucose levels. Intranasal delivery of insulin-like growth factor-1 not only mitigates neocortical spreading depression, a cause of migraine hyperexcitability modeled in animals, but also when neocortical spreading depression is triggered by supra-threshold stimuli, insulin-like growth factor-1 effectively reduces nociceptive activation in the trigeminal system.

Sex differences in the expression of calcitonin gene-related peptide receptor components in the spinal trigeminal nucleus

BACKGROUND AND PURPOSE

Calcitonin gene-related peptide (CGRP) plays an important role in migraine pathophysiology. CGRP acts primarily by activating a receptor composed of 3 proteins: calcitonin receptor-like receptor (CLR), receptor activity-modifying protein 1 (RAMP1), and receptor component protein (RCP). We tested the hypothesis that sex differences exist in protein levels of two key components of this CGRP receptor: CLR and RCP.

METHODS

We used specific antibodies to assess baseline protein levels of CLR and RCP in the spinal trigeminal nucleus caudalis (SpVc) and upper cervical spinal cord of both male and female rats. We also tested if manipulations that knock-down the expression of RCP in SpVc, using locally-mediated gene transfer of short hairpin RNA (shRNA), ameliorate pain in an animal model of intracranial migraine-like pain induced by chemical noxious stimulation of the meninges. To assess pain, we used tests of ongoing pain (rat face grimace test and freezing behavior) and tests of facial mechanical hypersensitivity and allodynia.

RESULTS

There was no difference in CLR levels between male and female animals (p > 0.11) in SpVc and the upper cervical cord. However, female animals exhibited greater baseline levels of RCP (up to 3-fold higher) compared to males (p < 0.002). The knock-down of RCP expression in SpVc attenuated mechanical facial allodynia induced by chemical noxious stimulation of the meninges, but had little effect on ongoing pain behaviors in female and male animals.

CONCLUSIONS

RCP is an integral component of the CGRP receptor and may play a key role in mediating CGRP induced central sensitization after noxious stimulation of the meninges. RCP expression in the SpVc and upper cervical cord is sexually dimorphic, with higher levels of expression in females. This dimorphism may be related to the increased incidence of migraines in females-a hypothesis that should be tested in the future.

Targeting CGRP via receptor antagonism and antibody neutralisation in two distinct rodent models of migraine-like pain

Introduction

Rodent disease models can play an indispensable role in drug development. Confirming that translationally-relevant disease mechanisms are engaged in such models is a crucial facet of this process. Accordingly, we have validated the role of calcitonin gene-related peptide signaling in a mouse model of glyceryl trinitrate-provoked migraine-like pain and a spontaneous rat model of migraine-like pain by assessing their pharmacological responsiveness to the small molecule calcitonin gene-related peptide receptor antagonist olcegepant, and the humanised monoclonal calcitonin gene-related peptide antibody ALD405.

Methods

Cutaneous sensitivity to hind paw, and periorbital mechanical stimulation were used as surrogate markers of activation of relevant pain pathways in each respective model. Separate experiments were performed to identify the time-course of treatment response to olcegepant (1 mg/kg i.p.) and ALD405 (10 mg/kg i.p.).

Results

Olcegepant and ALD405 significantly alleviated cutaneous mechanical hypersensitivity in both models compared with corresponding control treatments (saline and IgG control antibody respectively). As expected, the duration of anti-nociceptive action obtained with ALD405 was considerably longer than that associated with olcegepant. Surprisingly, in the spontaneous rat model the onset of action of ALD405 occurred within just 4 hours after administration.

Discussion

The current data clearly show that calcitonin gene-related peptide-mediated signaling is critically involved in the manifestation of cutaneous hypersensitivity in distinct rodent models of migraine-like pain and emphasise their translational relevance. Moreover, the unexpected rapidity of onset observed for ALD405 supports i) a probable site of action outside the blood-brain barrier, and ii) a potential clinical utility of specific monoclonal calcitonin gene-related peptide antibodies in the abortive treatment of migraine.

Pre- and postoperative headache in patients with meningioma

Background

Meningiomas are generally slowly growing intracranial tumors. They are often incidentally diagnosed, given that symptoms may be absent even in cases of an enormous tumor size. Headache is a frequent but not consistent symptom. Therefore, we examined the association between structural, biochemical and histochemical tumor parameters with preoperative as well as postoperative occurrence of headache.

Methods

In our study, we prospectively investigated 69 consecutive patients enrolled for meningioma neurosurgery. Anatomical, histological and biochemical parameters were acquired, and headache parameters were registered from the clinical report and from a questionnaire filled by the patients before neurosurgery. The headache was re-evaluated one year after neurosurgery. The study was designed to exploratively investigate whether there is an association of acquired clinical and biological parameters with the occurrence of preoperative and postoperative headache.

Results

Edema diameter and the proliferation marker MIB-1 were negatively associated with the incidence and intensity of preoperative headache, while the content of prostaglandin E2 in the tumor tissue was positively associated with preoperative headache intensity. Headache was more prevalent when the meningioma was located in the area supplied by the ophthalmic trigeminal branch. Compared to preoperative headache levels, an overall reduction was observed one year postoperative, and patients with a larger tumor had a higher headache remission. In parietal and occipital meningiomas and in those with a larger edema, the percentage of the headache remission rate was higher compared to other locations or smaller edema. Multivariable analyses showed an involvement of substance P and prostaglandin E2 in preoperative headache.

Conclusions

The study demonstrates new associations between meningiomas and headache. The postoperative headache outcome in the presented patient sample is encouraging for the performed neurosurgical intervention. These results should be tested in a prospective study that incorporates all patients with meningiomas.

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.

Selective cephalic upregulation of p-ERK, CamKII and p-CREB in response to glyceryl trinitrate infusion

Background A common characteristic of migraine-inducing substances is that they cause headache and no pain in other areas of the body. Few studies have compared pain mechanisms in the trigeminal and spinal systems and, so far, no major differences have been noted. We compared signalling molecules in the trigeminal and spinothalamic system after infusion of the migraine-provoking substance glyceryltrinitrate. Method A catheter was placed in the femoral vein of rats and one week later glyceryltrinitrate 4 µg/kg/min was infused for 20 min. Protein expression in the dura mater, trigeminal ganglion, nucleus caudalis, dorsal root ganglion and the dorsal horn of the thoracic spinal cord was analysed at different time points using western blotting and immunohistochemistry. Results Glyceryltrinitrate caused a threefold increase in expression of phosphorylated extracellular signal-regulated kinases at 30 min in the dura mater and nucleus caudalis ( P < 0.05) and at 2 h in the trigeminal ganglion with very few expressions in the dorsal root ganglion. In the nucleus caudalis, expression of phosphorylated extracellular signal-regulated kinases and Cam KII increased 2.6-fold and 3.2-fold, respectively, at 2 h after glycerytrinitrate infusion ( P < 0.01). p-CREB/ATF-1 upregulation was observed only at 30 min ( P < 0.05) in the nucleus caudalis. None of these markers showed increased expression in the regions of thoracic spinal cord dorsal horn. Conclusion The dura, trigeminal ganglion and nucleus caudalis are activated shortly after glycerytrinitrate infusion with long-lasting expression of phosphorylated extracellular signal-regulated kinases observed in the nucleus caudalis. These activations were not observed at the spinal level.

Targeted CGRP Small Molecule Antagonists for Acute Migraine Therapy

Migraine is a highly prevalent, severe, and disabling neurological condition with a significant unmet need for effective acute therapies. Patients (~50%) are dissatisfied with their currently available therapies. Calcitonin gene-related peptide (CGRP) has emerged as a key neuropeptide involved in the pathophysiology of migraines. As reviewed in this manuscript, a number of small molecule antagonists of the CGRP receptor have been developed for migraine therapy. Incredibly, the majority of the clinical trials conducted have proven positive, demonstrating the importance of this signalling pathway in migraine. Unfortunately, a number of these molecules raised liver toxicity concerns when used daily for as little as 7 days resulting in their discontinuation. Despite the clear safety concerns, clinical trial data suggests that their intermittent use remains a viable and safe alternative, with 2 molecules remaining in clinical development (ubrogepant and rimegepant). Further, these proofs of principle studies identifying CGRP as a viable clinical target have led to the development of several CGRP or CGRP receptor-targeted monoclonal antibodies that continue to show good clinical efficacy.

The role of the brainstem in migraine: Potential brainstem effects of CGRP and CGRP receptor activation in animal models

Background

Migraine is a severe debilitating disorder of the brain that is ranked as the sixth most disabling disorder globally, with respect to disability adjusted life years, and there remains a significant unmet demand for an improved understanding of its underlying mechanisms. In conjunction with perturbed sensory processing, migraine sufferers often present with diverse neurological manifestations (premonitory symptoms) that highlight potential brainstem involvement. Thus, as the field moves away from the view of migraine as a consequence of purely vasodilation to a greater understanding of migraine as a complex brain disorder, it is critical to consider the underlying physiology and pharmacology of key neural networks likely involved.

Discussion

The current review will therefore focus on the available evidence for the brainstem as a key regulator of migraine biology and associated symptoms. We will further discuss the potential role of CGRP in the brainstem and its modulation for migraine therapy, given the emergence of targeted CGRP small molecule and monoclonal antibody therapies.

Conclusion

The brainstem forms a functional unit with several hypothalamic nuclei that are capable of modulating diverse functions including migraine-relevant trigeminal pain processing, appetite and arousal regulatory networks. As such, the brainstem has emerged as a key regulator of migraine and is appropriately considered as a potential therapeutic target. While currently available CGRP targeted therapies have limited blood brain barrier penetrability, the expression of CGRP and its receptors in several key brainstem nuclei and the demonstration of brainstem effects of CGRP modulation highlight the significant potential for the development of CNS penetrant molecules.

Effect of a calcitonin gene-related peptide-binding L-RNA aptamer on neuronal activity in the rat spinal trigeminal nucleus

Background

Calcitonin gene-related peptide (CGRP) plays a major role in the pathogenesis of migraine and other primary headaches. Spinal trigeminal neurons integrate nociceptive afferent input from trigeminal tissues including intracranial afferents, and their activity is thought to reflect facial pain and headache in man. CGRP receptor inhibitors and anti-CGRP antibodies have been demonstrated to be therapeutically effective in migraine. In parallel, CGRP receptor inhibition has been shown to lower spinal trigeminal neuron activity in animal models of meningeal nociception.

Methods

In a rat model of meningeal nociception, single cell activity of neurons in the spinal trigeminal nucleus with meningeal afferent input was recorded to test a further pharmacological approach, scavenging CGRP with a CGRP-binding l-RNA oligonucleotide, the l-aptamer NOX-C89. Cumulative ascending doses of NOX-C89 were intravenously infused.

Results

Spontaneous activity of spinal trigeminal neurons did not change after 0.05 mg/kg NOX-C89, however, after additional infusion of 0.5 mg/kg and 5 mg/kg NOX-C89, spontaneous activity was dose-dependently reduced. Identical doses of a control l-aptamer had no effect. This pharmacological effect of NOX-C89 was observed 10–25 min after infusion, but no difference was detected in the period 0–5 min. For comparison, the previously investigated CGRP receptor antagonist olcegepant had reduced activity within 5 min after infusion. Alongside the reduced spontaneous activity, after infusion of NOX-C89 the heat-induced neuronal activity was abolished.

Conclusions

Scavenging CGRP by mirror-image RNA aptamers provides further evidence that this approach can be used to control spinal trigeminal activity.

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.

Where are we? The anatomy of the murine cortical meninges revisited for intravital imaging, immunology, and clearance of waste from the brain

Rapid progress is being made in understanding the roles of the cerebral meninges in the maintenance of normal brain function, in immune surveillance, and as a site of disease. Most basic research on the meninges and the neural brain is now done on mice, major attractions being the availability of reporter mice with fluorescent cells, and of a huge range of antibodies useful for immunocytochemistry and the characterization of isolated cells. In addition, two-photon microscopy through the unperforated calvaria allows intravital imaging of the undisturbed meninges with sub-micron resolution. The anatomy of the dorsal meninges of the mouse (and, indeed, of all mammals) differs considerably from that shown in many published diagrams: over cortical convexities, the outer layer, the dura, is usually thicker than the inner layer, the leptomeninx, and both layers are richly vascularized and innervated, and communicate with the lymphatic system. A membrane barrier separates them and, in disease, inflammation can be localized to one layer or the other, so experimentalists must be able to identify the compartment they are studying. Here, we present current knowledge of the functional anatomy of the meninges, particularly as it appears in intravital imaging, and review their role as a gateway between the brain, blood, and lymphatics, drawing on information that is scattered among works on different pathologies.

[Physiology of pain]

Pain research is based broadly on physiological disciplines and its development follows the methodological progress of the era, from classical psychophysiology to electrophysiological investigations at peripheral and central nociceptive systems, single cells and ion channels to modern imaging of nociceptive processing. Physiological pain research in Germany has long been part of an interdisciplinary research network extending beyond all political boundaries, and this situation has continued since molecular techniques started to dominate all biomedical research. Current scientific questions, such as intracellular nociceptive signal mechanisms, interactions with other physiological systems including the immune system, or the genetic basis of epidemic and chronic pain diseases can only be solved interdisciplinary and with international collaboration.

CGRP receptor antagonist activity of olcegepant depends on the signalling pathway measured

Background Calcitonin gene-related peptide (CGRP) is a neuropeptide that acts in the trigeminovascular system and is believed to play an important role in migraine. CGRP activates two receptors that are both present in the trigeminovascular system; the CGRP receptor and the amylin 1 (AMY₁) receptor. CGRP receptor antagonists, including olcegepant (BIBN4096BS) and telcagepant (MK-0974), can treat migraine. This study aimed to determine the effectiveness of these antagonists at blocking CGRP receptor signalling in trigeminal ganglia (TG) neurons and transfected CGRP and AMY₁ receptors in Cos7 cells, to better understand their mechanism of action. Methods CGRP stimulation of four intracellular signalling molecules relevant to pain (cAMP, CREB, p38 and ERK) were examined in rat TG neurons and compared to transfected CGRP and AMY₁ receptors in Cos7 cells. Results In TG neurons, olcegepant displayed signal-specific differences in antagonism of CGRP responses. This effect was also evident in transfected Cos7 cells, where olcegepant blocked CREB phosphorylation more potently than expected at the AMY₁ receptor, suggesting that the affinity of this antagonist can be dependent on the signalling pathway activated. Conclusions CGRP receptor antagonist activity appears to be assay-dependent. Thus, these molecules may not be as selective for the CGRP receptor as commonly reported.

Possible role of calcitonin gene-related peptide in trigeminal modulation of glomerular microcircuits of the rodent olfactory bulb

Chemosensation in the mammalian nose comprises detection of odorants, irritants and pheromones. While the traditional view assigned one distinct sub-system to each stimulus type, recent research has produced a more complex picture. Odorants are not only detected by olfactory sensory neurons but also by the trigeminal system. Irritants, in turn, may have a distinct odor, and some pheromones are detected by the olfactory epithelium. Moreover, it is well established that irritants change odor perception and vice versa. A wealth of psychophysical evidence on olfactory-trigeminal interactions in humans contrasts with a paucity of structural insight. In particular, it is unclear whether the two systems communicate just by sharing stimuli, or whether neuronal connections mediate cross-modal signaling. One connection could exist in the olfactory bulb that performs the primary processing of olfactory signals and receives trigeminal innervation. In the present study, neuroanatomical tracing of the mouse ethmoid system illustrates how peptidergic fibers enter the glomerular layer of the olfactory bulb, where local microcircuits process and filter the afferent signal. Biochemical assays reveal release of calcitonin gene-related peptide from olfactory bulb slices and attenuation of cAMP signaling by the neuropeptide. In the non-stimulated tissue, the neuropeptide specifically inhibited the basal activity of calbindin-expressing periglomerular interneurons, but did not affect the basal activity of neurons expressing calretinin, parvalbumin, or tyrosine hydroxylase, nor the activity of astrocytes. This study represents a first step towards understanding trigeminal neuromodulation of olfactory-bulb microcircuits and provides a working hypothesis for trigeminal inhibition of olfactory signal processing. This article is protected by copyright. All rights reserved.

TRPA1 and TRPV1 contribute to iodine antiseptics-associated pain and allergy

Iodine antiseptics exhibit superior antimicrobial efficacy and do not cause acquired microbial resistance. However, they are underused in comparison with antibiotics in infection treatments, partly because of their adverse effects such as pain and allergy. The cause of these noxious effects is not fully understood, and no specific molecular targets or mechanisms have been discovered. In this study, we show that iodine antiseptics cause pain and promote allergic contact dermatitis in mouse models, and iodine stimulates a subset of sensory neurons that express TRPA1 and TRPV1 channels. In vivo pharmacological inhibition or genetic ablation of these channels indicates that TRPA1 plays a major role in iodine antiseptics-induced pain and the adjuvant effect of iodine antiseptics on allergic contact dermatitis and that TRPV1 is also involved. We further demonstrate that iodine activates TRPA1 through a redox mechanism but has no direct effects on TRPV1. Our study improves the understanding of the adverse effects of iodine antiseptics and suggests a means to minimize their side effects through local inhibition of TRPA1 and TRPV1 channels.

Localization of CGRP receptor components and receptor binding sites in rhesus monkey brainstem: A detailed study using in situ hybridization, immunofluorescence, and autoradiography

Functional imaging studies have revealed that certain brainstem areas are activated during migraine attacks. The neuropeptide calcitonin gene-related peptide (CGRP) is associated with activation of the trigeminovascular system and transmission of nociceptive information and plays a key role in migraine pathophysiology. Therefore, to elucidate the role of CGRP, it is critical to identify the regions within the brainstem that process CGRP signaling. In situ hybridization and immunofluorescence were performed to detect mRNA expression and define cellular localization of calcitonin receptor-like receptor (CLR) and receptor activity-modifying protein 1 (RAMP1), respectively. To define CGRP receptor binding sites, in vitro autoradiography was performed with [(3)H]MK-3207 (a CGRP receptor antagonist). CLR and RAMP1 mRNA and protein expression were detected in the pineal gland, medial mammillary nucleus, median eminence, infundibular stem, periaqueductal gray, area postrema, pontine raphe nucleus, gracile nucleus, spinal trigeminal nucleus, and spinal cord. RAMP1 mRNA expression was also detected in the posterior hypothalamic area, trochlear nucleus, dorsal raphe nucleus, medial lemniscus, pontine nuclei, vagus nerve, inferior olive, abducens nucleus, and motor trigeminal nucleus; protein coexpression of CLR and RAMP1 was observed in these areas via immunofluorescence. [(3)H]MK-3207 showed high binding densities concordant with mRNA and protein expression. The present study suggests that several regions in the brainstem may be involved in CGRP signaling. Interestingly, we found receptor expression and antagonist binding in some areas that are not protected by the blood-brain barrier, which suggests that drugs inhibiting CGRP signaling may not be able to penetrate the central nervous system to antagonize receptors in these brain regions.

CGRP as a neuropeptide in migraine: lessons from mice

Migraine is a neurological disorder that is far more than just a bad headache. A hallmark of migraine is altered sensory perception. A likely contributor to this altered perception is the neuropeptide calcitonin gene-related peptide (CGRP). Over the past decade, CGRP has become firmly established as a key player in migraine. Although the mechanisms and sites of action by which CGRP might trigger migraine remain speculative, recent advances with mouse models provide some hints. This brief review focuses on how CGRP might act as both a central and peripheral neuromodulator to contribute to the migraine-like symptom of light aversive behaviour in mice.

Hydrogen sulfide determines HNO-induced stimulation of trigeminal afferents

Endogenous NO and hydrogen sulfide form HNO, which causes CGRP release via TRPA1 channel activation in sensory nerves. In the present study, stimulation of intact trigeminal afferent neuron preparations with NO donors, Na2S or both was analyzed by measuring CGRP release as an index of mass activation. Combined stimulation was able to activate all parts of the trigeminal system and acted synergistic compared to stimulation with both substances alone. To investigate the contribution of both substances, we varied their ratio and tracked intracellular calcium in isolated neurons. Our results demonstrate that hydrogen sulfide is the rate-limiting factor for HNO formation. CGRP has a key role in migraine pathophysiology and HNO formation at all sites of the trigeminal system should be considered for this novel means of activation.

Meningeal afferent signaling and the pathophysiology of migraine

Migraine is the most common neurological disorder. Attacks are complex and consist of multiple phases but are most commonly characterized by intense, unilateral, throbbing headache. The pathophysiology contributing to migraine is poorly understood and the disorder is not well managed with currently available therapeutics, often rendering patients disabled during attacks. The mechanisms most likely to contribute to the pain phase of migraine require activation of trigeminal afferent signaling from the cranial meninges and subsequent relay of nociceptive information into the central nervous system in a region of the dorsal brainstem known as the trigeminal nucleus caudalis. Events leading to activation of meningeal afferents are unclear, but nerve endings within this tissue are mechanosensitive and also express a variety of ion channels including acid-sensing ion channels and transient receptor-potential channels. These properties may provide clues into the pathophysiology of migraine by suggesting that decreased extracellular pH and environmental irritant exposure in the meninges contributes to headache. Neuroplasticity is also likely to play a role in migraine given that attacks are triggered by routine events that are typically nonnoxious in healthy patients and clear evidence of sensitization occurs during an attack. Where and how plasticity develops is also not clear but may include events directly on the afferents and/or within the TNC. Among the mediators potentially contributing to plasticity, calcitonin gene-related peptide has received the most attention within the migraine field but other mechanisms may also contribute. Ultimately, greater understanding of the molecules and mechanisms contributing to migraine will undoubtedly lead to better therapeutics and relief for the large number of patients across the globe who suffer from this highly disabling neurological disorder.

Targeting TRP channels for novel migraine therapeutics

Migraine is increasingly understood to be a disorder of the brain. In susceptible individuals, a variety of “triggers” may influence altered central excitability, resulting in the activation and sensitization of trigeminal nociceptive afferents surrounding blood vessels (i.e., the trigeminovascular system), leading to migraine pain. Transient receptor potential (TRP) channels are expressed in a subset of dural afferents, including those containing calcitonin gene related peptide (CGRP). Activation of TRP channels promotes excitation of nociceptive afferent fibers and potentially lead to pain. In addition to pain, allodynia to mechanical and cold stimuli can result from sensitization of both peripheral afferents and of central pain pathways. TRP channels respond to a variety of endogenous conditions including chemical mediators and low pH. These channels can be activated by exogenous stimuli including a wide range of chemical and environmental irritants, some of which have been demonstrated to trigger migraine in humans. Activation of TRP channels can elicit CGRP release, and blocking the effects of CGRP through receptor antagonism or antibody strategies has been demonstrated to be effective in the treatment of migraine. Identification of approaches that can prevent activation of TRP channels provides an additional novel strategy for discovery of migraine therapeutics.

CGRP receptor blockade by MK-8825 alleviates allodynia in infraorbital nerve-ligated rats

BACKGROUND

Previous data showed that, in rats, anti-migraine drugs (triptans, olcegepant) significantly reduced mechanical allodynia induced by infraorbital nerve (ION) ligation but not that evoked by sciatic nerve (SN) ligation. Whether this also occurs with MK-8825, a novel anti-migraine drug also acting through CGRP receptor blockade (but chemically unrelated to olcegepant) was tested in the present study, which also investigated possible anti-neuroinflammatory effects of this drug.

METHODS

Adult male Sprague-Dawley rats underwent unilateral chronic constriction injury (CCI) to either the ION or the SN, and mechanical allodynia was assessed 2 weeks later within the ipsilateral vibrissae territory or hindpaw, respectively. Transcripts of neuroinflammatory markers were quantified by real-time quantitative RT-PCR in ipsilateral trigeminal ganglion and spinal trigeminal nucleus in CCI-ION rats.

RESULTS

Acute as well as repeated (for 4 days) administration of MK-8825 (30-100 mg/kg, i.p.) significantly reduced CCI-ION-induced mechanical allodynia but was ineffective in CCI-SN rats. CCI-ION was associated with marked up-regulation of neuronal and glial inflammatory markers (ATF3, IL6, iNOS, COX2) in ipsilateral trigeminal ganglion but not spinal trigeminal nucleus. MK-8825-induced inhibition of iNOS mRNA up-regulation probably underlay its anti-allodynic effect because pharmacological blockade of iNOS by AMT (6 mg/kg, s.c.) mimicked this effect.

CONCLUSIONS

These data further support the idea that CGRP receptor blockade might be a valuable approach to alleviate trigeminal, but not spinal, neuropathic pain through, at least partly, an inhibitory effect on neuropathic pain-associated increase in NO production in trigeminal ganglion.

Calcitonin gene-related peptide (CGRP): a new target for migraine

Migraine is a neurological disorder that manifests as a debilitating headache associated with altered sensory perception. The neuropeptide calcitonin gene-related peptide (CGRP) is now firmly established as a key player in migraine. Clinical trials carried out during the past decade have proved that CGRP receptor antagonists are effective for treating migraine, and antibodies to the receptor and CGRP are currently under investigation. Despite this progress in the clinical arena, the mechanisms by which CGRP triggers migraine remain uncertain. This review discusses mechanisms whereby CGRP enhances sensitivity to sensory input at multiple levels in both the periphery and central nervous system. Future studies on epistatic and epigenetic regulators of CGRP actions are expected to shed further light on CGRP actions in migraine. In conclusion, targeting CGRP represents an approachable therapeutic strategy for migraine.

Calcitonin gene-related peptide: physiology and pathophysiology

Calcitonin gene-related peptide (CGRP) is a 37-amino acid neuropeptide. Discovered 30 years ago, it is produced as a consequence of alternative RNA processing of the calcitonin gene. CGRP has two major forms (α and β). It belongs to a group of peptides that all act on an unusual receptor family. These receptors consist of calcitonin receptor-like receptor (CLR) linked to an essential receptor activity modifying protein (RAMP) that is necessary for full functionality. CGRP is a highly potent vasodilator and, partly as a consequence, possesses protective mechanisms that are important for physiological and pathological conditions involving the cardiovascular system and wound healing. CGRP is primarily released from sensory nerves and thus is implicated in pain pathways. The proven ability of CGRP antagonists to alleviate migraine has been of most interest in terms of drug development, and knowledge to date concerning this potential therapeutic area is discussed. Other areas covered, where there is less information known on CGRP, include arthritis, skin conditions, diabetes, and obesity. It is concluded that CGRP is an important peptide in mammalian biology, but it is too early at present to know if new medicines for disease treatment will emerge from our knowledge concerning this molecule.

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.

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.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Release of CGRP from mouse brainstem slices indicates central inhibitory effect of triptans and kynurenate

BACKGROUND

CGRP is contained in a substantial proportion of unmyelinated trigeminal neurons innervating intracranial tissues. Previously, we have described a hemisected rodent scull preparation and later the intact trigeminal ganglion to measure stimulated CGRP release from trigeminal afferents.

METHODS

Here, we establish a preparation for examining CGRP release from central trigeminal terminals using single fresh slices of the mouse medullary brainstem.

RESULTS

Basal and stimulated amount of CGRP substantially exceeded the detection level. Experiments were designed as matched pairs of at least six brainstem slices per animal. Stimulation with high potassium induced calcium-dependent and reversible CGRP release. Capsaicin stimulation of TRPV1 provoked concentration-dependent CGRP release. The anti-migraine drug naratriptan did not inhibit capsaicin-induced CGRP release from peripheral terminals but inhibited the release from brainstem slices. The glutamate antagonist kynurenate showed a similar pattern of site-specific inhibition of CGRP release.

CONCLUSIONS

As observed earlier for other drugs used in the treatment of migraine this indicates that the central terminals in the spinal trigeminal nucleus may be the main site of action. The preparation allows evaluating the trigeminal brainstem as a pharmacological site of action.

Calcitonin gene-related peptide and migraine with aura: A systematic review

Background

Calcitonin gene-related peptide (CGRP) is a key molecule in migraine pathophysiology. Most studies have focused on CGRP in relation to migraine without aura (MO). About one-third of migraine patients have attacks with aura (MA), and this is a systematic review of the current literature on CGRP and MA.

Methods

We performed a systematic literature search on MEDLINE for reports of CGRP and MA, covering basic science, animal and human studies as well as randomized clinical trials.

Results

The literature search identified 594 citations, of which 38 contained relevant, original data. Plasma levels of CGRP in MA patients are comparable to MO, but CGRP levels varied among studies. A number of animal studies, including knock-ins of familial hemiplegic migraine (FHM) genes, have examined the relationship between CGRP and cortical spreading depression. In patients, CGRP does not trigger migraine in FHM, but is a robust trigger of migraine-like headache both in MA and MO patients. The treatment effect of CGRP antagonists are well proven in the treatment of migraine, but no studies have studied the effect specifically in MA patients.

Conclusion

This systematic review indicates that the role of CGRP in MA is less studied than in MO. Further studies of the importance of CGRP for auras and migraine are needed.

CGRP in the trigeminovascular system: a role for CGRP, adrenomedullin and amylin receptors?

The neuropeptide calcitonin gene-related peptide (CGRP) is reported to play an important role in migraine. It is expressed throughout the trigeminovascular system. Antagonists targeting the CGRP receptor have been developed and have shown efficacy in clinical trials for migraine. However, no CGRP antagonist is yet approved for treating this condition. The molecular composition of the CGRP receptor is unusual because it comprises two subunits; one is a GPCR, the calcitonin receptor-like receptor (CLR). This associates with receptor activity-modifying protein (RAMP) 1 to yield a functional receptor for CGRP. However, RAMP1 also associates with the calcitonin receptor, creating a receptor for the related peptide amylin but this also has high affinity for CGRP. Other combinations of CLR or the calcitonin receptor with RAMPs can also generate receptors that are responsive to CGRP. CGRP potentially modulates an array of signal transduction pathways downstream of activation of these receptors, in a cell type-dependent manner. The physiological significance of these signalling processes remains unclear but may be a potential avenue for refining drug design. This complexity has prompted us to review the signalling and expression of CGRP and related receptors in the trigeminovascular system. This reveals that more than one CGRP responsive receptor may be expressed in key parts of this system and that further work is required to determine their contribution to CGRP physiology and pathophysiology.

LINKED ARTICLES

This article is part of a themed section on Neuropeptides. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.170.issue-7.

The calcitonin gene-related peptide receptor antagonist MK-8825 decreases spinal trigeminal activity during nitroglycerin infusion

Background

Calcitonin gene-related peptide (CGRP) and nitric oxide (NO) are regarded as key mediators in migraine and other primary headaches. Migraineurs respond to infusion of nitroglycerin with delayed headaches, and inhibition of CGRP receptors has been shown to be effective in migraine therapy. In animal experiments nitrovasodilators like nitroglycerin induced increases in spinal trigeminal activity, which were reversed after inhibition of CGRP receptors. In the present study we asked if CGRP receptor inhibition can also prevent spinal trigeminal activity induced by nitroglycerin.

Methods

In isoflurane anaesthetised rats extracellular recordings were made from neurons in the spinal trigeminal nucleus with meningeal afferent input. The non-peptide CGRP receptor inhibitor MK-8825 (5 mg/kg) dissolved in acidic saline (pH 3.3) was slowly infused into rats one hour prior to prolonged glyceryl trinitrate (nitroglycerin) infusion (250 μg/kg/h for two hours).

Results

After infusion of MK-8825 the activity of spinal trigeminal neurons with meningeal afferent input did not increase under continuous nitroglycerin infusion but decreased two hours later below baseline. In contrast, vehicle infusion followed by nitroglycerin was accompanied by a transient increase in activity.

Conclusions

CGRP receptors may be important in an early phase of nitroglycerin-induced central trigeminal activity. This finding may be relevant for nitroglycerin-induced headaches.

Nitric oxide synthase, calcitonin gene-related peptide and NK-1 receptor mechanisms are involved in GTN-induced neuronal activation

BACKGROUND AND AIM

Infusion of glyceryltrinitrate (GTN), a nitric oxide (NO) donor, in awake, freely moving rats closely mimics a universally accepted human model of migraine and responds to sumatriptan treatment. Here we analyse the effect of nitric oxide synthase (NOS) and calcitonin gene-related peptide (CGRP) systems on the GTN-induced neuronal activation in this model.

MATERIALS AND METHODS

The femoral vein was catheterised in rats and GTN was infused (4 µg/kg/min, for 20 minutes, intravenously). Immunohistochemistry was performed to analyse Fos, nNOS and CGRP and Western blot for measuring nNOS protein expression. The effect of olcegepant, L-nitro-arginine methyl ester (L-NAME) and neurokinin (NK)-1 receptor antagonist L-733060 were analysed on Fos activation.

RESULTS

GTN-treated rats showed a significant increase of nNOS and CGRP in dura mater and CGRP in the trigeminal nucleus caudalis (TNC). Upregulation of Fos was observed in TNC four hours after the infusion. This activation was inhibited by pre-treatment with olcegepant. Pre-treatment with L-NAME and L-733060 also significantly inhibited GTN induced Fos expression.

CONCLUSION

The present study indicates that blockers of CGRP, NOS and NK-1 receptors all inhibit GTN induced Fos activation. These findings also predict that pre-treatment with olcegepant may be a better option than post-treatment to study its inhibitory effect in GTN migraine models.

Chemical mediators of migraine: preclinical and clinical observations

Migraine is a neurovascular disorder, and although the pathophysiology of migraine has not been fully delineated, much has been learned in the past 50 years. This knowledge has been accompanied by significant advancements in the way migraine is viewed as a disease process and in the development therapeutic options. In this review, we will focus on 4 mediators (nitric oxide, histamine, serotonin, and calcitonin gene-related peptide) which have significantly advanced our understanding of migraine as a disease entity. For each mediator we begin by reviewing the preclinical data linking it to migraine pathophysiology, first focusing on the vascular mechanisms, then the neuronal mechanisms. The preclinical data are then followed by a review of the clinical data which support each mediator's role in migraine and highlights the pharmacological agents which target these mediators for migraine therapy.

Obesity increases nociceptive activation of the trigeminal system

BACKGROUND

Obesity is a risk factor associated with several pain syndromes. However, the mechanisms underlying the association between obesity and pain are not known. The aim of this study was to test the hypothesis that obesity enhances neuronal responses to nociceptive stimulation within the trigeminal nucleus caudalis (TNC).

METHODS

Male and female C57BL/6J mice were fed a high-fat or regular diet from the time of weaning until 20 weeks of age. We then quantified neuronal activation by measuring Fos immunoreactivity within the TNC in response to a facial injection of a low dose of capsaicin (1 μg/10 μL).

RESULTS

We found that 0.01% capsaicin did not significantly increase Fos immunoreactivity in control mice fed a regular diet. In contrast, this low dose of capsaicin caused a 3.3-fold increase in Fos in the TNC in obese mice (p < 0.001).

CONCLUSIONS

These results support the hypothesis that diet-induced obesity in mice enhances nociceptive processing within the TNC. Diet-induced obesity may be a useful model for mechanistic studies. Future studies will improve our understanding of how obesity may contribute to trigeminal pain by sensitizing the trigeminal nociceptive system.