Calcitonin gene-related peptide receptor antagonists for migraine

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.

Electroacupuncture at Fengchi (GB20) inhibits calcitonin gene-related peptide expression in the trigeminovascular system of a rat model of migraine

Most migraine patients suffer from cutaneous allodynia; however, the underlying mechanisms are unclear. Calcitonin gene-related peptide (CGRP) plays an important role in the pathophysiology of migraine, and it is therefore, a potential therapeutic target for treating the pain. In the present study, a rat model of conscious migraine, induced by repeated electrical stimulation of the superior sagittal sinus, was established and treated with electroacupuncture at Fengchi (GB20) (depth of 2–3 mm, frequency of 2/15 Hz, intensity of 0.5–1.0 mA, 15 minutes/day, for 7 consecutive days). Electroacupuncture at GB20 significantly alleviated the decrease in hind paw and facial withdrawal thresholds and significantly lessened the increase in the levels of CGRP in the trigeminal ganglion, trigeminal nucleus caudalis and ventroposterior medial thalamic nucleus in rats with migraine. No CGRP-positive cells were detected in the trigeminal nucleus caudalis or ventroposterior medial thalamic nucleus by immunofluorescence. Our findings suggest that electroacupuncture treatment ameliorates migraine pain and associated cutaneous allodynia by modulating the trigeminovascular system ascending pathway, at least in part by inhibiting CGRP expression in the trigeminal ganglion.

Trigeminal Pain Molecules, Allodynia, and Photosensitivity Are Pharmacologically and Genetically Modulated in a Model of Traumatic Brain Injury

The pain-signaling molecules, nitric oxide synthase (NOS) and calcitonin gene-related peptide (CGRP), are implicated in the pathophysiology of post-traumatic headache (PTH) as they are for migraine. This study assessed the changes of inducible NOS (iNOS) and its cellular source in the trigeminal pain circuit, as well as the relationship between iNOS and CGRP after controlled cortical impact (CCI) injury in mice. The effects of a CGRP antagonist (MK8825) and sumatriptan on iNOS messenger RNA (mRNA) and protein were compared to vehicle at 2 weeks postinjury. Changes in CGRP levels in the trigeminal nucleus caudalis (TNC) in iNOS knockouts with CCI were compared to wild-type (WT) mice at 3 days and 2 weeks post injury. Trigeminal allodynia and photosensitivity were measured. MK8825 and sumatriptan increased allodynic thresholds in CCI groups compared to vehicle (p < 0.01), whereas iNOS knockouts were not different from WT. Photosensitivity was attenuated in MK8825 mice and iNOS knockouts compared to WT (p < 0.05). MK8825 and sumatriptan reduced levels of iNOS mRNA and iNOS immunoreactivity in the TNC and ganglia (p < 0.01). Differences in iNOS cellular localization were found between the trigeminal ganglia and TNC. Although the knockout of iNOS attenuated CGRP at 3 days (p < 0.05), it did not reduce CGRP at 2 weeks. CGRP immunoreactivity was found in the meningeal layers post-CCI, while negligible in controls. Findings support the importance of interactions between CGRP and iNOS in mediating allodynia, as well as the individual roles in photosensitivity. Mitigating prolonged increases in CGRP may be a promising intervention for treating acute PTH.

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.

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.

Peripheral calcitonin gene-related peptide receptor activation and mechanical sensitization of the joint in rat models of osteoarthritis pain

Objective

To investigate the role of the sensory neuropeptide calcitonin gene-related peptide (CGRP) in peripheral sensitization in experimental models of osteoarthritis (OA) pain.

Methods

Experimental knee OA was induced in rats by intraarticular injection of monosodium iodoacetate (MIA) or by transection of the medial meniscus (MMT). Single-unit recordings of joint-innervating nociceptors were obtained in MIA- and saline-treated rats following administration of CGRP or the CGRP receptor antagonist CGRP 8–37. Effects of CGRP 8–37 were also examined in rats that underwent MMT and sham operations. Protein and messenger RNA (mRNA) levels of CGRP receptor components in the L3–L4 dorsal root ganglion (DRG) were investigated following MIA treatment.

Results

In both the MIA and MMT groups, the mechanical sensitivity of joint nociceptors was enhanced compared to that in the control groups. Exogenous CGRP increased mechanical sensitivity in a greater proportion of joint nociceptors in the MIA-treated rats than in the saline-treated rats. Local blockade of endogenous CGRP by CGRP 8–37 reversed both the MIA- and MMT-induced enhancement of joint nociceptor responses. Joint afferent cell bodies coexpressed the receptor for CGRP, called the calcitonin-like receptor (CLR), and the intracellular accessory CGRP receptor component protein. MIA treatment increased the levels of mRNA for CLR in the L3–L4 DRG and the levels of CLR protein in medium and large joint afferent neurons.

Conclusion

Our findings provide new and compelling evidence implicating a role of CGRP in peripheral sensitization in experimental OA. Our novel finding of CGRP-mediated control of joint nociceptor mechanosensitivity suggests that the CGRP receptor system may be an important target for the modulation of pain during OA. CGRP receptor antagonists recently developed for migraine pain should be investigated for their efficacy against pain in OA.

Calcitonin gene-related peptide receptor antagonists: beyond migraine pain--a possible analgesic strategy for osteoarthritis?

Osteoarthritis (OA) pain is poorly understood and managed, as current analgesics have only limited efficacy and unwanted side effect profiles. A broader understanding of the pathological mechanisms driving OA joint pain is vital for the development of improved analgesics. Both clinical and preclinical data suggest an association between joint levels of the sensory neuropeptide calcitonin gene-related peptide (CGRP) and pain during OA. Whether a direct causative link exists remains an important unanswered question. Given the recent development of small molecule CGRP receptor antagonists with clinical efficacy against migraine pain, the interrogation of the role of CGRP in OA pain mechanisms is extremely timely. In this article, we provide the background to the importance of CGRP in pain mechanisms and review the emerging clinical and preclinical evidence implicating a role for CGRP in OA pain. We suggest that the CGRP receptor antagonists developed for migraine pain warrant further investigation in OA.

CGRP and migraine: could PACAP play a role too?

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

Sumatriptan inhibition of N-type calcium channel mediated signaling in dural CGRP terminal fibres

The selective 5-HT₁ receptor agonist sumatriptan is an effective therapeutic for migraine pain yet the antimigraine mechanisms of action remain controversial. Pain-responsive fibres containing calcitonin gene-related peptide (CGRP) densely innervating the cranial dura mater are widely believed to be an essential anatomical substrate for the development of migraine pain. 5-HT₁ receptors in the dura colocalize with CGRP fibres in high density and thus provide a possible peripheral site of action for sumatriptan. In the present study, we used high-resolution optical imaging selectively within individual mouse dural CGRP nociceptive fibre terminations and found that application of sumatriptan caused a rapid, reversible dose-dependent inhibition in the amplitude of single action potential evoked Ca²⁺ transients. Pre-application of the 5-HT₁ antagonist GR 127935 or the selective 5-HT(1D) antagonist BRL 15572 prevented inhibition while the selective 5-HT(1B) antagonist SB 224289 did not, suggesting this effect was mediated selectively through the 5-HT(1D) receptor subtype. Sumatriptan inhibition of the action potential evoked Ca²⁺ signaling was mediated selectively through N-type Ca²⁺ channels. Although the T-type Ca²⁺ channel accounted for a greater proportion of the Ca²⁺ signal it did not mediate any of the sumatriptan inhibition. Our findings support a peripheral site of action for sumatriptan in inhibiting the activity of dural pain fibres selectively through a single Ca²⁺ channel subtype. This finding adds to our understanding of the mechanisms that underlie the clinical effectiveness of 5-HT₁ receptor agonists such as sumatriptan and may provide insight for the development of novel peripherally targeted therapeutics for mitigating the pain of migraine.

Calcitonin gene-related peptide in migraine: intersection of peripheral inflammation and central modulation

Over the past two decades, a convergence of basic and clinical evidence has established the neuropeptide calcitonin-gene-related peptide (CGRP) as a key player in migraine. Although CGRP is a recognised neuromodulator of nociception, its mechanism of action in migraine remains elusive. In this review, we present evidence that led us to propose that CGRP is well poised to enhance neurotransmission in migraine by both peripheral and central mechanisms. In the periphery, it is thought that local release of CGRP from the nerve endings of meningeal nociceptors following their initial activation by cortical spreading depression is critical for the induction of vasodilation, plasma protein extravasation, neurogenic inflammation and the consequential sensitisation of meningeal nociceptors. Mechanistically, we propose that CGRP release can give rise to a positive-feedback loop involved in localised increased synthesis and release of CGRP from neurons and a CGRP-like peptide called procalcitonin from trigeminal ganglion glia. Within the brain, the wide distribution of CGRP and CGRP receptors provides numerous possible targets for CGRP to act as a neuromodulator.

Topical capsaicin for pain management: therapeutic potential and mechanisms of action of the new high-concentration capsaicin 8% patch

Topical capsaicin formulations are used for pain management. Safety and modest efficacy of low-concentration capsaicin formulations, which require repeated daily self-administration, are supported by meta-analyses of numerous studies. A high-concentration capsaicin 8% patch (Qutenza™) was recently approved in the EU and USA. A single 60-min application in patients with neuropathic pain produced effective pain relief for up to 12 weeks. Advantages of the high-concentration capsaicin patch include longer duration of effect, patient compliance, and low risk for systemic effects or drug-drug interactions. The mechanism of action of topical capsaicin has been ascribed to depletion of substance P. However, experimental and clinical studies show that depletion of substance P from nociceptors is only a correlate of capsaicin treatment and has little, if any, causative role in pain relief. Rather, topical capsaicin acts in the skin to attenuate cutaneous hypersensitivity and reduce pain by a process best described as 'defunctionalization' of nociceptor fibres. Defunctionalization is due to a number of effects that include temporary loss of membrane potential, inability to transport neurotrophic factors leading to altered phenotype, and reversible retraction of epidermal and dermal nerve fibre terminals. Peripheral neuropathic hypersensitivity is mediated by diverse mechanisms, including altered expression of the capsaicin receptor TRPV1 or other key ion channels in affected or intact adjacent peripheral nociceptive nerve fibres, aberrant re-innervation, and collateral sprouting, all of which are defunctionalized by topical capsaicin. Evidence suggests that the utility of topical capsaicin may extend beyond painful peripheral neuropathies.