Inhibitory effect of BIBN4096BS on cephalic vasodilatation induced by CGRP or transcranial electrical stimulation in the rat

Effect of CGRP and sumatriptan on the BOLD response in visual cortex

To test the hypothesis that calcitonin gene-related peptide (CGRP) modulates brain activity, we investigated the effect of intravenous CGRP on brain activity in response to a visual stimulus. In addition, we examined if possible alteration in brain activity was reversed by the anti-migraine drug sumatriptan. Eighteen healthy volunteers were randomly allocated to receive CGRP infusion (1.5 μg/min for 20 min) or placebo. In vivo activity in the visual cortex was recorded before, during and after infusion and after 6 mg subcutaneous sumatriptan by functional magnetic resonance imaging (3 T). 77% of the participants reported headache after CGRP. We found no changes in brain activity after CGRP (P = 0.12) or after placebo (P = 0.41). Sumatriptan did not affect brain activity after CGRP (P = 0.71) or after placebo (P = 0.98). Systemic CGRP or sumatriptan has no direct effects on the BOLD activity in visual cortex. This suggests that in healthy volunteers both CGRP and sumatriptan may exert their actions outside of the blood–brain barrier.

Antidromic vasodilatation and the migraine mechanism

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

Effects of ionotropic glutamate receptor antagonists on rat dural artery diameter in an intravital microscopy model

BACKGROUND AND PURPOSE

During migraine, trigeminal nerves may release calcitonin gene-related peptide (CGRP), inducing cranial vasodilatation and central nociception; hence, trigeminal inhibition or blockade of craniovascular CGRP receptors may prevent this vasodilatation and abort migraine headache. Several preclinical studies have shown that glutamate receptor antagonists affect the pathophysiology of migraine. This study investigated whether antagonists of NMDA (ketamine and MK801), AMPA (GYKI52466) and kainate (LY466195) glutamate receptors affected dural vasodilatation induced by alpha-CGRP, capsaicin and periarterial electrical stimulation in rats, using intravital microscopy.

EXPERIMENTAL APPROACH

Male Sprague-Dawley rats were anaesthetized and the overlying bone was thinned to visualize the dural artery. Then, vasodilator responses to exogenous (i.v. alpha-CGRP) and endogenous (released by i.v. capsaicin and periarterial electrical stimulation) CGRP were elicited in the absence or presence of the above antagonists.

KEY RESULTS

alpha-CGRP, capsaicin and periarterial electrical stimulation increased dural artery diameter. Ketamine and MK801 inhibited the vasodilator responses to capsaicin and electrical stimulation, while only ketamine attenuated those to alpha-CGRP. In contrast, GYKI52466 only attenuated the vasodilatation to exogenous alpha-CGRP, while LY466195 did not affect the vasodilator responses to endogenous or exogenous CGRP.

CONCLUSIONS AND IMPLICATIONS

Although GYKI52466 has not been tested clinically, our data suggest that it would not inhibit migraine via vascular mechanisms. Similarly, the antimigraine efficacy of LY466195 seems unrelated to vascular CGRP-mediated pathways and/or receptors. In contrast, the cranial vascular effects of ketamine and MK801 may represent a therapeutic mechanism, although the same mechanism might contribute, peripherally, to cardiovascular side effects.

Calcitonin gene-related peptide receptor antagonists for migraine

IMPORTANCE OF THE FIELD

Migraine is a highly prevalent disabling condition, and the current treatment options are not satisfactory. The role of calcitonin gene-related peptide (CGRP) in migraine pathophysiology is well established. CGRP receptor antagonists address this new target and have the potential to improve therapy for both responders and non-responders to previous options.

AREAS COVERED IN THIS REVIEW

This review describes CGRP, its receptors and their role in the pathophysiology of migraine. CGRP receptor antagonists are a recent development; all reported antagonists are reported in chronological order. The experimental evidence, as well as all clinical trials since the first proof-of-concept study in 2004, is discussed.

WHAT THE READER WILL GAIN

An overview of the CGRP system and why it provides an attractive drug target for headache. The main focus is on the currently presented CGRP receptor antagonists and clinical evidence for this new therapeutic option.

TAKE HOME MESSAGE

CGRP receptor antagonists will provide an additional and valuable therapeutic option for the treatment of headaches.

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.

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

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

CGRP receptor antagonism and migraine

Calcitonin gene-related peptide (CGRP) is expressed throughout the central and peripheral nervous systems, consistent with control of vasodilatation, nociception, motor function, secretion, and olfaction. alphaCGRP is prominently localized in primary spinal afferent C and ADelta fibers of sensory ganglia, and betaCGRP is the main isoform in the enteric nervous system. In the CNS there is a wide distribution of CGRP-containing neurons, with the highest levels occurring in striatum, amygdala, colliculi, and cerebellum. The peripheral projections are involved in neurogenic vasodilatation and inflammation, and central release induces hyperalgesia. CGRP is released from trigeminal nerves in migraine. Trigeminal nerve activation results in antidromic release of CGRP to cause non-endothelium-mediated vasodilatation. At the central synapses in the trigeminal nucleus caudalis, CGRP acts postjunctionally on second-order neurons to transmit pain signals centrally via the brainstem and midbrain to the thalamus and higher cortical pain regions. Recently developed CGRP receptor antagonists are effective at aborting acute migraine attacks. They may act both centrally and peripherally to attenuate signaling within the trigeminovascular pathway.

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.

Migraine models

Migraine is a high prevalence disorder which affects a significant proportion of the general population, especially women during their central and more productive time of the life, thus causing severe disability. The genetic basis of the disease is unknown and the mechanism is poorly understood. The possibility that following a perturbation in the central nervous system, and particularly in the brainstem, trigeminal neurons become hyperexcitable and produce an uncontrolled release of sensory neuropeptides which eventually results in arterial vasodilatation and neuronal sensitization, has been gaining credit from studies in experimental animals and humans. In particular, experimental and clinical data with antagonists of the calcitonin gene-related peptide (CGRP) propose this molecule and its receptor as a major target for migraine treatment.

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.

Finding New Drug Targets for the Treatment of Migraine Attacks

No new preventive drugs specific to migraine have appeared for the last 20 years and existing acute therapies need improvement. Unfortunately, no animal models can predict the efficacy of new therapies for migraine. Because migraine attacks are fully reversible and can be aborted by therapy, the headache- or migraine-provoking property of naturally occurring signalling molecules can be tested in a human model. This model has predicted efficacy of nitric oxide synthase inhibition and calcitonin gene-related peptide receptor blockade. The pharmaceutical industry should pay more attention to human models, although methods are different from normal target validation.

Origin of pain in migraine: evidence for peripheral sensitisation

Migraine is the most common neurological disorder, and much has been learned about its mechanisms in recent years. However, the origin of painful impulses in the trigeminal nerve is still uncertain. Despite the attention paid recently to the role of central sensitisation in migraine pathophysiology, in our view, neuronal hyperexcitability depends on activation of peripheral nociceptors. Although the onset of a migraine attack might take place in deep-brain structures, some evidence indicates that the headache phase depends on nociceptive input from perivascular sensory nerve terminals. The input from arteries is probably more important than the input from veins. Several studies provide evidence for input from extracranial, dural, and pial arteries but, likewise, there is also evidence against all three of these locations. On balance, afferents are most probably excited in all three territories or the importance of individual territories varies from patient to patient. We suggest that migraine can be explained to patients as a disorder of the brain, and that the headache originates in the sensory fibres that convey pain signals from intracranial and extracranial blood vessels.

The in vivo Effect of VIP, PACAP-38 and PACAP-27 and mRNA Expression of Their Receptors in Rat Middle Meningeal Artery

The parasympathetic nervous system is probably involved in migraine pathogenesis. Its activation releases a mixture of signalling molecules including vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP), which subsequently stimulate VPAC1, VPAC2 and PAC1 receptors. The objective of the present study was to investigate the in vivo effect of VIP, PACAP-27, PACAP-38, the selective VPAC1 agonist ([Lys15, Arg16, Leu27]-VIP(1–7)-GRF(8–27)) and a PAC1 agonist, maxadilan on rat middle meningeal artery (MMA) diameter using the closed cranial window model. Selective antagonists were used for further characterization of the responses. Reverse transcriptase-polymerase chain reaction experiments were also conducted to determine expression of mRNA of PACAP receptors in the MMA. The results showed that VIP, PACAP-38, PACAP-27 and the VPAC1 specific agonist evoked significant dilations with the rank order of potency; VIP = PACAP-38 > PACAP-27 = [Lys15, Arg16, Leu27]-VIP(1–7)-GRF(8–27). Significant inhibition of dilation was only observed for the VPAC1 antagonist PG97–269 on PACAP-38-induced dilation of MMA. The VPAC2 antagonist PG99–465 and PAC1 antagonist PACAP(6–38) did not significantly block VIP- or PACAP-induced dilation. Expression of mRNA of all three receptors was detected in the MMA. In conclusion, the VPAC1 receptor seems to be predominant in mediating MMA dilation. A selective VPAC1 antagonist may be a candidate molecule in the treatment of migraine headache.

Effect of central serotonin depletion on 5-HT receptor-mediated vasomotor responses in the middle meningeal artery of anaesthetized rats

1 It has been hypothesized that craniovascular 5-HT receptors mediating dilatation of cranial vessels undergo sensitization on decreased serotonergic transmission in migraine. This study analysed the effect of chemical lesion of the 5-HT system in the brain with 5,7-dihydroxytryptamine (5,7-DHT) on 5-HT receptor-mediated dilator responses to 5-carboxamidotryptamine (5-CT) in the middle meningeal artery of anaesthetized rats. 5-CT has recently been shown to elicit dilator responses in this cranial vessel via 5-HT(7) receptors and, to a much lesser extent, 5-HT(1B/1D) receptors. 2 Pretreatment with 5,7-DHT produced a drastic and selective decrease of 5-HT levels in the brain (78 +/- 6% and 94 +/- 2% in dorsal raphe and hypothalamic paraventricular nuclei, respectively) compared with controls (1% ascorbic acid). 3 Topical application of 5-CT (1-1000 microm) to exposed dura mater encephali produced concentration-dependent decreases in diastolic blood pressure and dilator responses in the middle meningeal artery that were similar in vehicle- and 5,7-DHT-pretreaed animals. 4 Hypotensive and meningeal dilator responses to 5-CT were unaltered by the 5-HT(1B/1D) receptor antagonist, GR-127935 (1 mg kg(-1), i.v.), but were strongly inhibited by the 5-HT(7) receptor antagonist, SB-269970 (1 mg kg(-1), i.v.), with similar efficacy, in both groups of animals. Treatment with GR-127935 + SB-269970 (1 mg kg(-1), i.v. each), produced a stronger inhibitory effect than individual treatments on hypotensive but not on meningeal responses to 5-CT. Meningeal 5-HT(7) receptor-mediated responses (i.e. in GR-127935-pretreated animals) were unchanged by 5,7-DHT pretreatment. 5 Results suggest that the sensitivity of craniovascular 5-HT(7) receptors mediating dilatation is unaffected by a decrease of 5-HT levels in the brain. A neuronal involvement of 5-HT in migraine seems more likely, therefore.

Improvement of the Closed Cranial Window Model in Rats by Intracarotid Infusion of Signalling Molecules Implicated in Migraine

Intravital microscopy on a closed cranial window allows one to measure change in the diameter of cranial blood vessels after intravenous (i.v.) administration of pharmacodynamic substances. Putative targets being pursued in migraine are large vasodilating peptide molecules such as calcitonin gene-related peptide (CGRP) and pituitary adenylate cyclase polypeptide (PACAP)-38. High i.v. doses are required to study their craniovascular pharmacology. Unfortunately, this leads to a drop in blood pressure (BP) that subsequently causes blood vessels to dilate by autoregulation. Hence it is difficult to decipher what effect is caused by direct receptor agonist interaction or contributed by autoregulation. In the present study we infused substances with an ingenious indwelling catheter in the common carotid artery in rats. Intracarotidly seven-, 12- and 17-fold lower doses of CGRP, PACAP-38 and capsaicin were required, respectively, compared with i.v. infusion to induce the same dilation in dural artery. Dilating intracarotid (i.c.) doses caused no or a minimal fall in BP, whereas equi-responsive i.v. doses caused a marked BP reduction. The CGRP blocking potential of olcegepant was amplified by > 20 times on i.c. infusion. Pial artery responses to CGRP did not change with i.c. infusion, demonstrating that dilations after i.v. CGRP are mediated by autoregulation rather than through specific receptors. We applied CGRP topically, which induced concentration-dependent dural vasodilation, but no effect on pial artery or on BP. In conclusion, this new approach offers an improvement of the existing model by allowing more accurate assessment of effects of pharmaca on the cranial vasculature without inducing significant systemic effects.

CGRP function-blocking antibodies inhibit neurogenic vasodilatation without affecting heart rate or arterial blood pressure in the rat

BACKGROUND AND PURPOSE

Calcitonin gene-related peptide (CGRP) receptor antagonists effectively abort migraine headache and inhibit neurogenic vasodilatation in humans as well as rat models. Monoclonal antibodies typically have long half-lives, and we investigated whether or not function-blocking CGRP antibodies would inhibit neurogenic vasodilatation with a long duration of action and therefore be a possible approach to preventive therapy of migraine. During chronic treatment with anti-CGRP antibodies, we measured cardiovascular function, which might be a safety concern of CGRP inhibition.

EXPERIMENTAL APPROACH

We used two rat blood flow models that measure electrically stimulated vasodilatation in the skin or in the middle meningeal artery (MMA). These vasomotor responses are largely dependent on the neurogenic release of CGRP from sensory afferents. To assess cardiovascular function during chronic systemic anti-CGRP antibody treatment, we measured heart rate and blood pressure in conscious rats.

KEY RESULTS

Treatment with anti-CGRP antibodies inhibited skin vasodilatation or the increase in MMA diameter to a similar magnitude as treatment with CGRP receptor antagonists. Although CGRP antibody treatment had a slower onset of action than the CGRP receptor antagonists, the inhibition was still evident 1 week after dosing. Chronic treatment with anti-CGRP antibodies had no detectable effects on heart rate or blood pressure.

CONCLUSIONS AND IMPLICATIONS

We showed for the first time that anti-CGRP antibodies exert a long lasting inhibition of neurogenic vasodilatation in two different rat models of arterial blood flow. We have provided strong preclinical evidence that anti-CGRP antibody may be a suitable drug candidate for the preventive treatment of migraine.

The blood-brain barrier in migraine treatment

Salient aspects of the anatomy and function of the blood-barrier barrier (BBB) are reviewed in relation to migraine pathophysiology and treatment. The main function of the BBB is to limit the access of circulating substances to the neuropile. Smaller lipophilic substances have some access to the central nervous system by diffusion, whereas other substances can cross the BBB by carrier-mediated influx transport, receptor-mediated transcytosis and absorptive-mediated transcytosis. Studies of drugs relevant to migraine pathophysiology and treatment have been examined with the pressurized arteriography method. The drugs, given both luminally and abluminally, provide important notions regarding antimigraine site of action, probably abluminal to the BBB. The problems with the BBB in animal models designed to study the pathophysiology, acute treatment models and preventive treatments are discussed with special emphasize on the triptans and calcitonin gene-related peptide (CGRP). The human experimental headache model, especially the use of glycerol trinitrate (the nitric oxide model), and experiences with CGRP administrations utilize the systemic administration of the agonists with effects on other vascular beds also. We discuss how this can be related to genuine migraine attacks. Our view is that there exists no clear proof of breakdown or leakage of the BBB during migraine attacks, and that antimigraine drugs need to pass the BBB for efficacy.

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.

Neurovascular pharmacology of migraine

Migraine is a paroxysmal neurovascular disorder, which affects a significant proportion of the population. Since dilation of cranial blood vessels is likely to be responsible for the headache experienced in migraine, many experimental models for the study of migraine have focussed on this feature. The current review discusses a model that is based on the constriction of carotid arteriovenous anastomoses in anaesthetized pigs, which has during the last decades proven of great value in identifying potential antimigraine drugs acting via a vascular mechanism. Further, the use of human isolated blood vessels in migraine research is discussed. Thirdly, we describe an integrated neurovascular model, where dural vasodilatation in response to trigeminal perivascular nerve stimulation can be studied. Such a model not only allows an in-depth characterization of directly vascularly acting drugs, but also of drugs that are supposed to act via inhibition of vasodilator responses to endogenous neuropeptides, or of drugs that inhibit the release of these neuropeptides. We discuss the use of this model in a study on the influence of female sex hormones on migraine. Finally, the implementation of this model in mice is considered. Such a murine model allows the use of genetically modified animals, which will lead to a better understanding of the ion channel mutations that are found in migraine patients.

Treatment of migraine attacks based on the interaction with the trigemino-cerebrovascular system

Primary headaches such as migraine are among the most prevalent neurological disorders, affecting up to one-fifth of the adult population. The scientific work in the last decade has unraveled much of the pathophysiological background of migraine, which is now considered to be a neurovascular disorder. It has been discovered that the trigemino-cerebrovascular system plays a key role in migraine headache pathophysiology by releasing the potent vasodilator calcitonin gene-related peptide (CGRP). This neuropeptide is released in parallel with the pain and its concentration correlates well with the intensity of the headache. The development of drugs of the triptan class has provided relief for the acute attacks but at the cost of, mainly cardiovascular, side effects. Thus, the intention to improve treatment led to the development of small CGRP receptor antagonists such as olcegepant (BIBN4096BS) and MK-0974 that alleviate the acute migraine attack without acute side events. The purpose of this review is to give a short overview of the pathological background of migraine headache and to illustrate the mechanisms behind the actions of triptans and the promising CGRP receptor blockers.

Ethanol causes neurogenic vasodilation by TRPV1 activation and CGRP release in the trigeminovascular system of the guinea pig

Ethanol stimulating transient receptor potential vanilloid 1 (TRPV1) on primary sensory neurons promotes neurogenic inflammation, including calcitonin gene-related peptide (CGRP)-mediated coronary dilation. Alcoholic beverages trigger migraine attacks and activation of trigeminal neurons plays a role in migraine. We have investigated in guinea pigs whether ethanol by TRPV1 stimulation causes neurogenic inflammation in the trigeminovascular system. Ethanol-evoked release of neuropeptides from slices of dura mater was abolished by Ca(2+) removal, capsaicin pretreatment and the TRPV1 antagonist, capsazepine. Intragastric ethanol increased plasma extravasation in dura mater, an effect abolished by capsazepine and the NK1 receptor antagonist, SR140333, and caused vasodilation around the middle meningeal artery, an effect abolished by capsazepine and the CGRP receptor antagonist, BIBN4096BS. Vasodilation of meningeal vessels by TRPV1 activation and CGRP release may be relevant to the mechanism by which alcohol ingestion triggers migraine attacks.

Novel migraine therapy with calcitonin gene-regulated peptide receptor antagonists

Primary headaches, for example, migraine and cluster headaches represent the most prevalent neurological disorders, affecting up to 15-20% of the adult population. There is a clear association between head pain and the release of calcitonin gene-related peptide (CGRP). In this review the role of CGRP in human cranial circulation is described and the role for specific CGRP antagonism elucidated. It is well known that triptans (5-HT(1B/1D) agonist) alleviate headache in part through normalisation of CGRP levels. The central role of CGRP in migraine pathophysiology has resulted in the development of small-molecule CGRP antagonists with no cardiovascular side effects. Such compounds have high selectivity for human CGRP receptors and are efficacious in the relief of acute migraine attacks. Research indicates that they effect the abluminal side of the blood-brain barrier and that they are not vasoconstrictive, providing a new dimension in therapy.

Female sex hormones and rat dural vasodilatation to CGRP, periarterial electrical stimulation and capsaicin

BACKGROUND

The prevalence of migraine is 2 to 3-fold higher in females than in males, and it is intricately related to the levels of female sex hormones. These hormones may regulate the synthesis and receptor expression of calcitonin gene-related peptide (CGRP), which mediates neurogenic dural vasodilatation and is implicated in migraine pathogenesis.

OBJECTIVE

To investigate the effects of the female sex steroids, 17beta-estradiol and progesterone, separately and in combination, on dural vasodilatation induced by alphaCGRP, periarterial electrical stimulation and capsaicin in ovariectomized rats, using intravital microscopy.

METHODS

Sprague-Dawley rats were ovariectomized and, 7 days later, subcutaneously implanted with 21-day release pellets of 17beta-estradiol, progesterone, their combination or placebo. On day 19 to 21, the animals were anesthetized, overlying bone thinned to visualize the middle meningeal artery and vasodilator responses to alphaCGRP (10 to 3000 ng kg(-1)), periarterial electrical stimulation (25 to 125 microA) and capsaicin (0.3 to 18 microg kg(-1)) elicited.

RESULTS

There were no significant differences in the vasodilator potency or efficacy of alphaCGRP or capsaicin in the different groups studied. In contrast, the vasodilator response to electrical stimulation was significantly higher in rats treated with 17beta-estradiol (Emax:157 +/- 19%) as compared to those observed after placebo treatment (Emax:93 +/- 11%).

CONCLUSION

Our results show that, in contrast to CGRP- or capsaicin-induced dural vasodilatation, 17beta-estradiol enhanced neurogenic vasodilatation, suggesting increased CGRP release from perivascular nerves. This may be one of the mechanisms through which 17beta-estradiol exacerbates migraine in women.

Effect of two novel CGRP-binding compounds in a closed cranial window rat model

We investigated the in vivo effects of two novel calcitonin gene-related peptide (CGRP) binding molecules in the genuine closed cranial window model in the rat. The RNA-Spiegelmer (NOX-C89) and the monoclonal CGRP antibody are CGRP scavengers and might be used as an alternative to CGRP-receptor antagonists in the treatment of migraine. Rats were anaesthetized and a closed cranial window established. Changes in dural and pial artery diameter and mean arterial blood pressure were measured simultaneously. Infusion of the RNA-Spiegelmer or the CGRP antibody alone had no effect on the arteries or the mean arterial blood pressure. We then used a bolus of 0.3 microg/kg CGRP (n=6) or electrical stimulation (25 V, 5 Hz, 1 ms pulse width and of 10 s of duration) (n=6) to induce dilatation of dural and pial arteries (mediated via CGRP-receptors). Pre-treatment with the RNA-Spiegelmer inhibited CGRP-induced vasodilatation of the dural artery (from 38+/-17% to 7+/-3%) and the pial artery (from 14+/-1% to 3+/-2%) (P<0.05). The RNA-Spiegelmer, however, did not significantly inhibit dilatation induced by electrical stimulation (P>0.05). The CGRP antibody caused a significant reduction of the dural artery diameter caused by intravenous CGRP-infusion (from 23+/-5% to 12+/-3%) (P<0.05), but did not inhibit dilatation caused by electrical stimulation (P>0.05). In conclusion, the CGRP scavengers effectively inhibited the effect of circulating CGRP but do not modify the effect of electrical stimulation and the consequent liberation of CGRP from perivascular sensory nerve fibres.

Inhibitory effect of BIBN4096BS, CGRP(8-37), a CGRP antibody and an RNA-Spiegelmer on CGRP induced vasodilatation in the perfused and non-perfused rat middle cerebral artery

BACKGROUND AND PURPOSE

A new concept for the inhibition of CGRP signalling has been developed by interaction with the CGRP molecule per se by using a CGRP antibody or a CGRP binding RNA-Spiegelmer (NOX-C89). We have compared these CGRP scavengers with two known receptor antagonists (CGRP8-37 and BIBN4096BS) on CGRP-induced relaxations in the rat middle cerebral artery (MCA). Furthermore, the role of the endothelial barrier has been studied.

EXPERIMENTAL APPROACH

We used the luminally perfused MCA in an arteriograph, pressurized to 85 mm Hg and myograph studies of isolated ring segments of the MCA.

KEY RESULTS

In myograph studies and in the perfusion system during abluminal application, alphaCGRP and betaCGRP induced concentration-dependent dilatation of the MCA. Given luminally neither peptide was significantly vasodilator. Adrenomedullin and amylin induced weak dilatations. In myograph experiments, relaxation induced by alphaCGRP was prevented by the four CGRP blockers (CGRP8-37, BIBN4096BS, the CGRP antibody and NOX-C89.). In abluminal perfusion experiments, the relaxant response to alphaCGRP was prevented by these agents to a varying degree. Dilatation induced by abluminal application of alphaCGRP was inhibited by luminal CGRP8-37 but not by luminal BIBN4096BS, CGRP antibody or NOX-C89.

CONCLUSIONS AND IMPLICATIONS

alpha or betaCGRP acted on smooth muscle cell CGRP receptors in rat MCA and were effectively prevented from reaching these receptors by the endothelial barrier. The CGRP blockers significantly inhibited alphaCGRP induced relaxation but were also prevented from reaching the CGRP receptors by the arterial endothelium.

The calcitonin gene-related peptide (CGRP) receptor antagonist BIBN4096BS reduces neurogenic increases in dural blood flow

In an in vivo preparation of the exposed rat cranial dura mater electrical field stimulation causes increases in blood flow that are mainly due to the vasodilatory effect of calcitonin gene-related peptide (CGRP) released from meningeal afferents. In this preparation the effect of BIBN4096BS, a non-peptide competitive antagonist of CGRP receptors, was examined. Additionally, in an in vitro preparation of the hemisected rat skull the effect of BIBN4096BS on CGRP release stimulated by activation of meningeal afferents was analysed. Injection of BIBN4096BS at cumulative doses of 300 microg/kg and 900 microg/kg caused dose-dependent inhibition of the electrically evoked blood flow increases. The basal blood flow and vital parameters were not significantly changed by any dose. In the hemisected skull BIBN4096BS at 10(-6) M did not alter the CGRP release evoked by depolarizing K(+) concentrations or antidromic electrical stimulation of the trigeminal ganglion. We conclude that neurogenic increases in dural blood flow are reduced by BIBN4096BS without changing basal vascular parameters. This peripheral effect may be important with regard to CGRP receptor inhibition as an antimigraine strategy.

Impaired vasodilation in response to perivascular nerve stimulation in mesenteric arteries of TRPV1-null mutant mice

BACKGROUND

The role of the transient receptor potential vanilloid type 1 (TRPV1) channels expressed in perivascular sensory nerves in the regulation of vascular reactivity is largely unknown. This study was designed to test the hypothesis that vasodilation induced by electrical field stimulation (EFS) of perivascular sensory nerves is mediated by the TRPV1 via release of sensory neurotransmitters in wild-type (WT) mice, and this effect is abolished in gene-targeted TRPV1-null mutant (TRPV1(-/-)) mice.

METHODS

Isolated mesenteric resistance arteries from WT and TRPV1(-/-)) mice were perfused and pretreated with guanethedine and atropine to block sympathetic and parasympathetic nerve activity, respectively. After precontracting with phenylephrine, changes of vascular diameters induced by EFS were monitored in the absence or presence of the TRPV1 receptor antagonist capsazepine; the calcitonin gene-related peptide (CGRP) receptor antagonist, CGRP8-37; or the substance P (SP) receptor antagonist, RP67580.

RESULTS

EFS-induced vasodilation was significantly reduced in arteries in TRPV1(-/-)) mice when compared to that of WT mice. Capsazepine and CGRP8-37 attenuated vasodilation induced by EFS in WT but not TRPV1(-/-)) mice. In contrast, RP67580 had no effect on the EFS-induced vasodilation in WT or TRPV1(-/-)) mice. The release of CGRP in the face of EFS challenge was significantly increased in both WT and TRPV1(-/-)) arteries, which was attenuated by capsazepine in WT but not TRPV1(-/-)) arteries. Exogenous CGRP caused dose-dependent vasodilation to a similar degree in WT and TRPV1(-/-)) arteries.

CONCLUSIONS

Our data show that in WT mice transmural stimulation of perivascular sensory nerves activates the TRPV1, leading to CGRP release from sensory nerve endings; and blockade of CGRP, but not SP, receptors abolishes TRPV1-mediated vasodilation during EFS. All these effects are impaired in TRPV1(-/-)) mice, indicating that TRPV1 plays a key role in modulating perivascular sensory nerve-mediated vasodilation.

New and future migraine therapy

Modern neuroscience advanced our understanding of putative migraine mechanisms, which led to improved therapeutics. Indeed, mechanism-based acute migraine therapy gained steam in the early 1990s after the introduction of the triptans (5-HT1B,D agonists). Post-triptans, novel targets such as calcitonin gene-related peptide (CGRP) antagonists, inhibitors of excitatory glutamatergic receptors, and nitric oxide synthase (NOS) inhibitors are leading the pack in this exploding field of discovery research. In contrast, novel therapeutic targets for migraine prevention are lacking despite a hugely unmet need. To date, migraine prophylactic drugs are advanced based on expanded indications for already approved pharmaceuticals (e.g., topiramate, valproate, propranolol, and timolol). An improved understanding of the predisposition to an attack, genomic discoveries, valid and reliable biomarkers and surrogates, and predictive preclinical models likely will unravel the neuronal substrates for central hyperexcitability and nociceptive dysmodulation, hopefully leading us to better mechanism-based targets for prevention, and ultimately yielding drugs with optimal therapeutic ratios or indices.

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.

Phosphodiesterase 3 and 5 and cyclic nucleotide-gated ion channel expression in rat trigeminovascular system

Activation of the trigeminovascular pain signalling system appears involved in migraine pathophysiology. However, the molecular mechanisms are only partially known. Stimulation of cAMP and cGMP production as well as inhibition of their breakdown induce migraine-like headache. Additionally, migraine may be associated with mutations in ion channels. The aim of the present study was to describe the expression of phosphodiesterase 3 (PDE3) and 5 (PDE5) and cyclic nucleotide-gated ion channels (CNG) in cerebral arteries, meninges, and the trigeminal ganglion. mRNA for PDE and CNG was determined in the rat middle cerebral artery, basilar artery, trigeminal ganglion, and dura mater using real-time PCR. PDE and CNG proteins were identified using Western blot. For comparison, rat aorta and mesenteric artery were analysed. PDE3A, PDE3B, and PDE5A mRNA were detected in all tissues examined except for PDE3A mRNA in dura mater and the trigeminal ganglion. PDE5A and PDE3A protein expression was present in both cerebral and peripheral arteries, whereas PDE3B protein was present only in the cerebral arteries. The CNGA4 and B1 subunit mRNAs were detected in cerebral arteries and CNGA2 also in the mesenteric artery. CNGA2 and A3 proteins were found in cerebral arteries and dura and CNGA1, CNGA2 and CNGA3 in the trigeminal ganglion. In conclusion, PDE3A, PDE3B, PDE5A, and five CNG subunits were expressed in several components of the trigeminovascular system of the rat. This suggests that modulation of cAMP and cGMP levels by PDE and activation of CNG may play a role in trigeminovascular pain signalling leading to migraine headache.

The Preclinical Pharmacology of BIBN4096BS, a CGRP Antagonist

CGRP is an important neuropeptide found throughout the cardiovascular system. However, until recently it has been difficult to define its pharmacology or physiological role because of the lack of suitable antagonists. BIBN4096BS is a high-affinity, nonpeptide antagonist that shows much greater selectivity for human CGRP1 receptors compared to any other drug. Its pharmacology has been defined with studies on transfected cells or cell lines endogenously expressing receptors of known composition. These have allowed confirmation that in many human blood vessels, CGRP is working via CGRP1 receptors. However, it also interacts with other CGRP-activated receptors, of unknown composition. In vivo, clinical studies have shown that BIBN4096BS is likely to be useful in the treatment of migraine. It has also been used to define the role of CGRP in phenomena such as plasma extravasation and cardioprotection following ischemia.

The in vivo effect of adrenomedullin on rat dural and pial arteries

Adrenomedullin is related to the calcitonin gene-related peptide (CGRP) family and is present in cerebral blood vessels. It may be involved in migraine mechanisms. We measured the change in dural and pial artery diameter, mean arterial blood pressure and local cerebral blood flow flux (LCBF(Flux)) after intravenous (i.v.) infusion of adrenomedullin. The study was performed in the presence or absence of the CGRP1 (calcitonin-receptor-like-receptor (CALCRL)/receptor activity-modifying protein-1 (RAMP1)) receptor antagonists BIBN4096BS, CGRP-(8-37) and the adrenomedullin receptor antagonist adrenomedullin-(22-52). I.v. infusion of 15 mug kg(-1) adrenomedullin (n=8) induced dilatation of dural (32+/-7.5%) and pial (18+/-5.5%) arteries, a reduction in mean arterial blood pressure (19+/-3%) and an increase in LCBF(Flux) (16+/-8.4%). The duration of the responses was 25 min for the dural artery, while the response of the pial artery lasted for 15 min. The CGRP1-receptor antagonists BIBN4096BS and CGRP-(8-37) and the adrenomedullin receptor antagonist adrenomedullin-(22-52) significantly inhibited the effect of adrenomedullin (n=7, P<0.05 for both arteries) on dural and pial artery diameter and mean arterial blood pressure. No significant inhibition of LCBF(Flux) was found. The antagonist alone had no effect on mean arterial blood pressure or LCBF(Flux). In conclusion, we suggest that adrenomedullin in the rat cranial circulation dilates dural and pial arteries, reduces mean arterial blood pressure and increases LCBF(Flux), probably via a CGRP1-receptor.

Inhibition of stimulated meningeal blood flow by a calcitonin gene-related peptide binding mirror-image RNA oligonucleotide

Calcitonin gene-related peptide (CGRP) released from trigeminal afferents is known to play an important role in the control of intracranial blood flow. In a rat preparation with exposed cranial dura mater, periods of electrical stimulation induce increases in meningeal blood flow. These responses are due to arterial vasodilatation mediated in part by the release of CGRP. In this preparation, the effect of a CGRP-binding mirror-image oligonucleotide (Spiegelmer NOX-C89) was examined. Spiegelmer NOX-C89 applied topically at concentrations between 10(-7) and 10(-5) M to the exposed dura mater led to a dose-dependent inhibition of the electrically evoked blood flow increases. The highest dose reduced the mean increases in flow to 56% of the respective control levels. A nonfunctional control Spiegelmer (not binding to CGRP) was ineffective in changing blood flow increases. Intravenous injection of NOX-C89 (5 mg kg(-1)) reduced the evoked blood flow increases to an average of 65.5% of the control. The basal blood flow was not changed by any of the applied substances. In addition, an ex vivo preparation of the hemisected rat skull was used to determine CGRP release from the cranial dura mater caused by antidromic activation of meningeal afferents. In this model, 10(-6) M of NOX-C89 reduced the evoked CGRP release by about 50%. We conclude that increases in meningeal blood flow due to afferent activation can be reduced by sequestering the released CGRP and thus preventing it from activating vascular CGRP receptors. Moreover, the Spiegelmer NOX-C89 may inhibit CGRP release from meningeal afferents. Therefore, the approach to interfere with the CGRP/CGRP receptor system by binding the CGRP may open a new opportunity for the therapy of diseases that are linked to excessive CGRP release such as some forms of primary headaches.

New targets in acute migraine treatment

Migraine is a common and highly disabling neurological problem, whose acute treatment was revolutionized by the triptans, serotonin 5-HT1B/1D receptor agonists. Some patients do not respond to triptans, while others are not suitable for them largely because of contraindications based on vascular disease. The exploration of nonvasoconstrictor treatments for acute migraine offers the prospect of dramatic improvements in patient care, as well as important insights into the mechanisms of migraine. Possibilities for such developments include, calcitonin gene-related peptide receptor antagonists, serotonin 5-HT1F and 5-HT1D receptor agonists, glutamate excitatory amino acid receptor antagonists, nitric oxide synthase inhibitors and adenosine A1 receptor agonists. Taken together, the future for migraine and affected patients is bright and promising.

Triptan-induced contractile (5-HT1B receptor) responses in human cerebral and coronary arteries: relationship to clinical effect

Triptans are agonists at 5-HT1B and 5-HT1D (where 5-HT is 5-hydroxytryptamine; serotonin) receptors and cause vasoconstriction of isolated blood vessels. The aim of the present study was to determine vasoconstrictor potency (EC50) of triptans in human coronary and cerebral arteries and to examine whether there was any relationship with the maximal plasma concentrations (Cmax; nM) of the drugs achieved following oral administration of clinically relevant doses to man using values reported in the literature. We also examined the expression of 5-HT1B receptors in atherosclerotic and normal coronary arteries. The vasocontractile responses to sumatriptan, rizatriptan or eletriptan were characterized by in vitro pharmacology. The ratio of Cmax/EC50 was calculated. 5-HT1B and 5-HT1D receptors were visualized by immunohistochemical techniques in coronary arteries. Sumatriptan, rizatriptan and eletriptan were powerful vasoconstrictors in cerebral artery. The rank order of agonist potency was eletriptan=rizatriptan=sumatriptan. In the coronary artery, the triptans were weaker vasoconstrictors. The rank order of potency was similar. In cerebral artery the ratio of Cmax/EC50 was not significantly different from unity, indicating a relationship between these two parameters. In general for the coronary artery, the ratios were significantly less than unity, indicating no direct relationship. Immunohistochemistry showed expression of 5-HT1B receptors in the medial layer, but did not reveal any obvious difference in 5-HT1B receptor expression between normal and atherosclerotic coronary arteries. The results support the notion that triptans are selective vasoconstrictors of cerebral arteries over coronary arteries and that there is a relationship between vasoconstrictor potency in cerebral arteries and clinically relevant plasma levels.

Calcitonin gene-related peptide does not excite or sensitize meningeal nociceptors: Implications for the pathophysiology of migraine

Migraine is among the most common types of pain, but its mechanisms are poorly understood. A growing body of evidence points to a critical role of calcitonin gene-related peptide (CGRP) in the pathophysiology of migraine headache. During migraine, CGRP is thought to be released from peripheral endings of perivascular meningeal nociceptors primary and to promote vasodilatation. A current hypothesis suggests that peripheral CGRP and its related meningeal vasodilatation results in activation and sensitization, leading to the generation of migraine headache. However, direct evidence supporting this idea is lacking. Here, using electrophysiological, extracellular, single-unit recording combined with laser-Doppler flowmetry measurements of dural blood flow (DBF), we examined whether CGRP and meningeal vasodilatation promote activation or sensitization of meningeal nociceptors. Changes in (DBF), ongoing discharge, and responsiveness to mechanical stimulation of the dura were studied after either topical administration or intravenous infusion of rat alpha-CGRP in anesthetized rats. Both topical and systemic administration of CGRP caused a significant increase in dural blood flow; however, neither method of CGRP administration resulted in activation or sensitization of meningeal nociceptors. The results of this study suggest that CGRP effects in the meninges, including meningeal vasodilatation, are not sufficient to activate or sensitize meningeal nociceptors.

Clinical data on the CGRP antagonist BIBN4096BS for treatment of migraine attacks

Basal studies have shown that calcitonin gene-related peptide (CGRP) is a major sensory neuronal messenger in the trigeminovascular system, the pathway conveying intracranial pain. In migraine and cluster headache attacks, CGRP is released in parallel with the pain and successful treatment of the attacks abort both the associated pain and the CGRP release. The search for a potent small molecule CGRP antagonist has been successful and such an agent has been tested in preclinical and clinical studies. The aim of the present study was to examine current knowledge on the clinical pharmacology of systemic BIBN4096BS, which has been shown in man to abort acute migraine attacks as well or better than oral sumatriptan. BIBN4096BS is a specific and potent CGRP receptor antagonist in humans. In safety and tolerability studies the substance is well tolerated with no or only mild side effects. In acute migraine attacks the overall response was 66% with the drug and 27% with placebo. A difference as compared to placebo was seen at 30 min; the response was still rising at 4 h suggesting a long duration of action. At 24 h the pain-free rate was better than that with triptans, suggesting a lower grade of rebound and perhaps even a prophylactic possibility.