Delayed upregulation of ATP P2X3 receptors of trigeminal sensory neurons by calcitonin gene-related peptide

Role of neuropeptides in orofacial pain: A literature review

BACKGROUND

Neuropeptides play a critical role in regulating pain and inflammation. Despite accumulating evidence has further uncovered the novel functions and mechanisms of different neuropeptides in orofacial pain sensation and transmission, there is deficient systematic description of neuropeptides' pain modulation in the orofacial region, especially in the trigeminal system.

OBJECTIVES

The present review aims to summarise several key neuropeptides and gain a better understanding of their major regulatory roles in orofacial inflammation and pain.

METHODS

We review and summarise current studies related to calcitonin gene-related peptide (CGRP), substance P (SP), opioid peptide (OP), galanin (GAL) and other neuropeptides' functions and mechanisms as well as promising targets for orofacial pain control.

RESULTS

A number of neuropeptides are clearly expressed in the trigeminal sensory system and have critical functions in the transduction and pathogenesis of orofacial pain. The functions, possible cellular and molecular mechanisms have been introduced and discussed. Neuropeptides and their agonists or antagonists which are widely studied to be potential treatment options of orofacial pain has been evaluated.

CONCLUSIONS

Various neuropeptides play important but distinct (pro-nociceptive or analgesic) roles in orofacial pain with different mechanisms. In summary, CGRP, SP, NPY, NKA, HK-1, VIP mainly play proinflammatory and pro-nociceptive effects while OP, GAL, OXT, OrxA mainly have inhibitory effects on orofacial pain.

Quantifying Peripheral Modulation of Olfaction by Trigeminal Agonists

In the mammalian nose, two chemosensory systems, the trigeminal and the olfactory mediate the detection of volatile chemicals. Most odorants are able to activate the trigeminal system, and vice versa, most trigeminal agonists activate the olfactory system as well. Although these two systems constitute two separate sensory modalities, trigeminal activation modulates the neural representation of an odor. The mechanisms behind the modulation of olfactory response by trigeminal activation are still poorly understood. We addressed this question by looking at the olfactory epithelium (OE), where olfactory sensory neurons (OSNs) and trigeminal sensory fibers co-localize and where the olfactory signal is generated. Our study was conducted in a mouse model. Both sexes, males and females, were included. We characterize the trigeminal activation in response to five different odorants by measuring intracellular Ca2+ changes from primary cultures of trigeminal neurons (TGNs). We also measured responses from mice lacking TRPA1 and TRPV1 channels known to mediate some trigeminal responses. Next, we tested how trigeminal activation affects the olfactory response in the olfactory epithelium using electro-olfactogram (EOG) recordings from wild-type (WT) and TRPA1/V1-knock out (KO) mice. The trigeminal modulation of the olfactory response was determined by measuring responses to the odorant, 2-phenylethanol (PEA), an odorant with little trigeminal potency after stimulation with a trigeminal agonist. Trigeminal agonists induced a decrease in the EOG response to PEA, which depended on the level of TRPA1 and TRPV1 activation induced by the trigeminal agonist. This suggests that trigeminal activation can alter odorant responses even at the earliest stage of the olfactory sensory transduction.SIGNIFICANCE STATEMENT Most odorants reaching the olfactory epithelium (OE) can simultaneously activate olfactory and trigeminal systems. Although these two systems constitute two separate sensory modalities, trigeminal activation can alter odor perception. Here, we analyzed the trigeminal activity induced by different odorants proposing an objective quantification of their trigeminal potency independent from human perception. We show that trigeminal activation by odorants reduces the olfactory response in the olfactory epithelium and that such modulation correlates with the trigeminal potency of the trigeminal agonist. These results show that the trigeminal system impacts the olfactory response from its earliest stage.

Ion Channel Disturbances in Migraine Headache: Exploring the Potential Role of the Kynurenine System in the Context of the Trigeminovascular System

Migraine is a primary headache disorder, which is an enormous burden to the healthcare system. While some aspects of the pathomechanism of migraines remain unknown, the most accepted theory is that activation and sensitization of the trigeminovascular system are essential during migraine attacks. In recent decades, it has been suggested that ion channels may be important participants in the pathogenesis of migraine. Numerous ion channels are expressed in the peripheral and central nervous systems, including the trigeminovascular system, affecting neuron excitability, synaptic energy homeostasis, inflammatory signaling, and pain sensation. Dysfunction of ion channels could result in neuronal excitability and peripheral or central sensitization. This narrative review covers the current understanding of the biological mechanisms leading to activation and sensitization of the trigeminovascular pain pathway, with a focus on recent findings on ion channel activation and modulation. Furthermore, we focus on the kynurenine pathway since this system contains kynurenic acid, which is an endogenous glutamate receptor antagonist substance, and it has a role in migraine pathophysiology.

Maresin-2 inhibits inflammatory and neuropathic trigeminal pain and reduces neuronal activation in the trigeminal ganglion

Pain is a common symptom associated with disorders involving the orofacial structures. Most acute orofacial painful conditions are easily recognized, but the pharmacological treatment may be limited by the adverse events of current available drugs and/or patients' characteristics. In addition, chronic orofacial pain conditions represent clinical challenges both, in terms of diagnostic and treatment. There is growing evidence that specialized pro-resolution lipid mediators (SPMs) present potent analgesic effects, in addition to their well characterized role in the resolution of inflammation. Maresins (MaR-1 and MaR-2) were the last described members of this family, and MaR-2 analgesic action has not yet been reported. Herein the effect of MaR-2 in different orofacial pain models was investigated. MaR-2 (1 or 10 ng) was always delivered via medullary subarachnoid injection, which corresponds to the intrathecal treatment. A single injection of MaR-2 caused a significant reduction of phases I and II of the orofacial formalin test in rats. Repeated injections of MaR-2 prevented the development of facial heat and mechanical hyperalgesia in a model of post-operative pain in rats. In a model of trigeminal neuropathic pain (CCI-ION), repeated MaR-2 injections reversed facial heat and mechanical hyperalgesia in rats and mice. CCI-ION increased c-Fos positive neurons and CGRP⁺ activated (nuclear pNFkB) neurons in the trigeminal ganglion (TG), which were restored to sham levels by MaR-2 repeated treatment. In conclusion, MaR-2 showed potent and long-lasting analgesic effects in inflammatory and neuropathic pain of orofacial origin and the inhibition of CGRP-positive neurons in the TG may account for MaR-2 action.

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.

Quantifying peripheral modulation of olfaction by trigeminal agonists

In the mammalian nose, two chemosensory systems, the trigeminal and the olfactory mediate the detection of volatile chemicals. Most odorants in fact are able to activate the trigeminal system, and vice versa, most trigeminal agonists activate the olfactory system as well. Although these two systems constitute two separate sensory modalities, trigeminal activation modulates the neural representation of an odor. The mechanisms behind the modulation of olfactory response by trigeminal activation are still poorly understood. In this study, we addressed this question by looking at the olfactory epithelium, where olfactory sensory neurons and trigeminal sensory fibers co-localize and where the olfactory signal is generated. We characterize the trigeminal activation in response to five different odorants by measuring intracellular Ca2+ changes from primary cultures of trigeminal neurons (TGNs). We also measured responses from mice lacking TRPA1 and TRPV1 channels known to mediate some trigeminal responses. Next, we tested how trigeminal activation affects the olfactory response in the olfactory epithelium using electro-olfactogram (EOG) recordings from WT and TRPA1/V1-KO mice. The trigeminal modulation of the olfactory response was determined by measuring responses to the odorant, 2-phenylethanol (PEA), an odorant with little trigeminal potency after stimulation with a trigeminal agonist. Trigeminal agonists induced a decrease in the EOG response to PEA, which depended on the level of TRPA1 and TRPV1 activation induced by the trigeminal agonist. This suggests that trigeminal activation can alter odorant responses even at the earliest stage of the olfactory sensory transduction.

FM1-43 Dye Memorizes Piezo1 Activation in the Trigeminal Nociceptive System Implicated in Migraine Pain

It has been proposed that mechanosensitive Piezo1 channels trigger migraine pain in trigeminal nociceptive neurons, but the mechanosensitivity of satellite glial cells (SGCs) supporting neuronal sensitization has not been tested before. Moreover, tools to monitor previous Piezo1 activation are not available. Therefore, by using live calcium imaging with Fluo-4 AM and labeling with FM1-43 dye, we explored a new strategy to identify Piezo channels' activity in mouse trigeminal neurons, SGCs, and isolated meninges. The specific Piezo1 agonist Yoda1 induced calcium transients in both neurons and SGCs, suggesting the functional expression of Piezo1 channels in both types of cells. In Piezo1-transfected HEK cells, FM1-43 produced only a transient fluorescent response, whereas co-application with Yoda1 provided higher transient signals and a remarkable long-lasting FM1-43 'tail response'. A similar Piezo1-related FM1-43 trapping was observed in neurons and SGCs. The non-specific Piezo channel blocker, Gadolinium, inhibited the transient peak, confirming the involvement of Piezo1 receptors. Finally, FM1-43 labeling demonstrated previous activity in meningeal tissues 3.5 h after Yoda1 washout. Our data indicated that trigeminal neurons and SGCs express functional Piezo channels, and their activation provides sustained labeling with FM1-43. This long-lasting labelling can be used to monitor the ongoing and previous activation of Piezo1 channels in the trigeminal nociceptive system, which is implicated in migraine pain.

Role of ATP in migraine mechanisms: focus on P2X3 receptors

Migraine is a major health burden worldwide with complex pathophysiology and multifarious underlying mechanisms. One poorly understood issue concerns the early steps in the generation of migraine pain. To elucidate the basic process of migraine pain further, it seems useful to consider key molecular players that may operate synergistically to evoke headache. While the neuropeptide CGRP is an important contributor, we propose that extracellular ATP (that generally plays a powerful nociceptive role) is also a major component of migraine headache, acting in concert with CGRP to stimulate trigeminal nociceptive neurons. The aim of the present focused review is to highlight the role of ATP activating its P2X3 membrane receptors selectively expressed by sensory neurons including their nerve fiber terminals in the meninges. Specifically, we present data on the homeostasis of ATP and related purines in the trigeminovascular system and in the CNS; the basic properties of ATP signalling at peripheral and central nerve terminals; the characteristics of P2X3 and related receptors in trigeminal neurons; the critical speed and persistence of P2X3 receptor activity; their cohabitation at the so-called meningeal neuro-immune synapse; the identity of certain endogenous agents cooperating with ATP to induce neuronal sensitization in the trigeminal sensory system; the role of P2X3 receptors in familial type migraine; the current state of P2X3 receptor antagonists and their pharmacological perspectives in migraine. It is proposed that the unique kinetic properties of P2X3 receptors activated by ATP offer an interesting translational value to stimulate future studies for innovative treatments of migraine pain.

Association between P2X3 receptors and neuropathic pain: As a potential therapeutic target for therapy

Neuropathic pain is a common clinical symptom of various diseases, and it seriously affects the physical and mental health of patients. Owing to the complex pathological mechanism of neuropathic pain, clinical treatment of pain is challenging. Therefore, there is growing interest among researchers to explore potential therapeutic strategies for neuropathic pain. A large number of studies have shown that development of neuropathic pain is related to nerve conduction and related signaling molecules. P2X3 receptors (P2X3R) are ATP-dependent ion channels that participate in the transmission of neural information and related signaling pathways, sensitize the central nervous system, and play a key role in the development of neuropathic pain. In this paper, we summarized the structure and biological characteristics of the P2X3R gene and discussed the role of P2X3R in the nervous system. Moreover, we outlined the related pathological mechanisms of pain and described the relationship between P2X3R and chronic pain to provide valuable information for development of novel treatment strategies for pain.

Dural Immune Cells, CGRP, and Migraine

Migraine is the most common neurological disorder in the world, affecting 12% of the population. Migraine involves the central nervous system, trigeminal nerves and meninges. Recent advances have shown that targeting calcitonin gene-related peptide (CGRP) through either antibodies or small molecule receptor antagonists is effective at reducing episodic and chronic migraine episodes, but these therapeutics are not effective in all patients. This suggests that migraine does not have a singular molecular cause but is likely due to dysregulated physiology of multiple mechanisms. An often-overlooked part of migraine is the potential involvement of the immune system. Clinical studies have shown that migraine patients may have dysregulation in their immune system, with abnormal plasma cytokine levels either during the attack or at baseline. In addition, those who are immunocompromised appear to be at a higher risk of migraine-like disorders. A recent study showed that migraine caused changes to transcription of immune genes in the blood, even following treatment with sumatriptan. The dura mater is densely packed with macrophages, mast and dendritic cells, and they have been found to associate with meningeal blood vessels and trigeminal afferent endings. Recent work in mice shows activation and morphological changes of these cells in rodents following the migraine trigger cortical spreading depression. Importantly, each of these immune cell types can respond directly to CGRP. Since immune cells make up a large portion of the dura, have functional responses to CGRP, and interact with trigeminal afferents, CGRP actions on the dural immune system are likely to play key roles in migraine.

Beyond CGRP: The calcitonin peptide family as targets for migraine and pain

The CGRP system has emerged as a key pharmacological target for the treatment of migraine. However, some individuals who suffer from migraine have low or no response to anti-CGRP or other treatments, suggesting the need for additional clinical targets. CGRP belongs to the calcitonin family of peptides, which includes calcitonin, amylin, adrenomedullin and adrenomedullin 2. These peptides display a range of pro-nociceptive and anti-nociceptive actions, in primary headache conditions such as migraine. Calcitonin family peptides also show expression at sites relevant to migraine and pain. This suggests that calcitonin family peptides and their receptors, beyond CGRP, may be therapeutically useful in the treatment of migraine and other pain disorders. This review considers the localisation of the calcitonin family in peripheral pain pathways and discusses how they may contribute to migraine and pain. LINKED ARTICLES: This article is part of a themed issue on Advances in Migraine and Headache Therapy (BJP 75th Anniversary). To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v179.3/issuetoc.

Testing the Role of Glutamate NMDA Receptors in Peripheral Trigeminal Nociception Implicated in Migraine Pain

The pro-nociceptive role of glutamate in the CNS in migraine pathophysiology is well established. Glutamate, released from trigeminal afferents, activates second order nociceptive neurons in the brainstem. However, the function of peripheral glutamate receptors in the trigeminovascular system suggested as the origin site for migraine pain, is less known. In the current project, we used calcium imaging and patch clamp recordings from trigeminal ganglion (TG) neurons, immunolabelling, CGRP assay and direct electrophysiological recordings from rat meningeal afferents to investigate the role of glutamate in trigeminal nociception. Glutamate, aspartate, and, to a lesser extent, NMDA under free-magnesium conditions, evoked calcium transients in a fraction of isolated TG neurons, indicating functional expression of NMDA receptors. The fraction of NMDA sensitive neurons was increased by the migraine mediator CGRP. NMDA also activated slowly desensitizing currents in 37% of TG neurons. However, neither glutamate nor NMDA changed the level of extracellular CGRP. TG neurons expressed both GluN2A and GluN2B subunits of NMDA receptors. In addition, after removal of magnesium, NMDA activated persistent spiking activity in a fraction of trigeminal nerve fibers in meninges. Thus, glutamate activates NMDA receptors in somas of TG neurons and their meningeal nerve terminals in magnesium-dependent manner. These findings suggest that peripherally released glutamate can promote excitation of meningeal afferents implicated in generation of migraine pain in conditions of inherited or acquired reduced magnesium blockage of NMDA channels and support the usage of magnesium supplements in migraine.

NGF Enhances CGRP Release Evoked by Capsaicin from Rat Trigeminal Neurons: Differential Inhibition by SNAP-25-Cleaving Proteases

Nerve growth factor (NGF) is known to intensify pain in various ways, so perturbing pertinent effects without negating its essential influences on neuronal functions could help the search for much-needed analgesics. Towards this goal, cultured neurons from neonatal rat trigeminal ganglia—a locus for craniofacial sensory nerves—were used to examine how NGF affects the Ca²⁺-dependent release of a pain mediator, calcitonin gene-related peptide (CGRP), that is triggered by activating a key signal transducer, transient receptor potential vanilloid 1 (TRPV1) with capsaicin (CAP). Measurements utilised neurons fed with or deprived of NGF for 2 days. Acute re-introduction of NGF induced Ca²⁺-dependent CGRP exocytosis that was inhibited by botulinum neurotoxin type A (BoNT/A) or a chimera of/E and/A (/EA), which truncated SNAP-25 (synaptosomal-associated protein with Mr = 25 k) at distinct sites. NGF additionally caused a Ca²⁺-independent enhancement of the neuropeptide release evoked by low concentrations (<100 nM) of CAP, but only marginally increased the peak response to ≥100 nM. Notably, BoNT/A inhibited CGRP exocytosis evoked by low but not high CAP concentrations, whereas/EA effectively reduced responses up to 1 µM CAP and inhibited to a greater extent its enhancement by NGF. In addition to establishing that sensitisation of sensory neurons to CAP by NGF is dependent on SNARE-mediated membrane fusion, insights were gleaned into the differential ability of two regions in the C-terminus of SNAP-25 (181–197 and 198–206) to support CAP-evoked Ca²⁺-dependent exocytosis at different intensities of stimulation.

Exploring the roles of neuropeptides in trigeminal neuropathic pain: A systematic review and narrative synthesis of animal studies

OBJECTIVE

This systematic review aims to explore the changes in expression of neuropeptides and/or their receptors following experimental trigeminal neuropathic pain in animals.

DESIGN

MEDLINE, Embase, and Scopus were searched for publications up to 31st March 2021. Study selection and data extraction were completed by two independent reviewers based on the eligibility criteria. The quality of articles was judged based on the Systematic Review Centre for Laboratory Animal Experimentation (SYRCLE) risk-of-bias tool.

RESULTS

A total of 19 studies satisfied the eligibility criteria and were included for narrative synthesis. Methods of trigeminal neuropathic pain induction were nerve ligation, nerve compression/crush, nerve transection and dental pulp injury. Animal behaviours used for pain verification were evoked responses to mechanical and thermal stimuli. Non-evoked behaviours, including vertical exploration, grooming and food consumption, were also employed in some studies. Calcitonin gene-related peptide (CGRP) and substance P were the most frequently reported neuropeptides. Overall, unclear to high risk of bias was identified in the included studies.

CONCLUSIONS

Limited evidence has suggested the pro-nociceptive role of CGRP in trigeminal neuropathic pain. In order to further translational pain research, animal models of trigeminal neuropathic pain and pain validation methods need to be optimised. Complete reporting of future studies based on available guidelines to improve confidence in research is encouraged.

Altered gene expression signatures by calcitonin gene-related peptide promoted mast cell activity in the colon of stress-induced visceral hyperalgesia mice

BACKGROUND

Calcitonin gene-related peptide (CGRP) is possibly involved in recruitment of mucosal mast cells (MCs) in the gut that may be associated with the development of irritable bowel syndrome (IBS), but the role of CGRP on the activation of MCs is still unknown.

METHODS

Using RNA sequencing (RNA-seq), we examined differentially expressed genes (DEGs) in mouse MCs following CGRP treatment. The expression of key genes in colonic MCs and their relationship with CGRP-containing fibers were examined by immunofluorescence in chronic water-avoidance stress (WAS)-induced visceral hyperalgesia mice.

KEY RESULTS

A total of 29 DEGs were found significantly changed with 28 upregulated and 1 downregulated following treatment of MCs with CGRP. Bioinformatics analysis showed that key higher DEGs included those associated with response to corticotropin-releasing hormone (CRH), regulation of transcription, MC activation, and proliferation. These processes are enriched for genes associated with stress-induced MC activation in IBS. Western blot verified changes in representative DEGs (Nr4a3, Crem, Gpr35, FosB, Sphlk1) and real-time cell analysis (RTCA) verified the MC proliferation. The vast majority of colonic MCs nearly CGRP-containing fibers in WAS mice overexpressed only Nr4a3 with little to no FosB, Gpr35, Sphlk1, or Crem expression. Nr4a3 knockdown may attenuate the promotion effect of CGRP on MC viability.

CONCLUSIONS & INFERENCES

Our results suggest that CGRP is a critical regulator of key expressed genes in MC activation. Nr4a3 as a novel regulator of MC function may have an effect on stress-induced visceral hyperalgesia, and this may represent the novel target for drug development.

Increase in trigeminal ganglion neurons that respond to both calcitonin gene-related peptide and pituitary adenylate cyclase-activating polypeptide in mouse models of chronic migraine and posttraumatic headache

A large body of animal and human studies indicates that blocking peripheral calcitonin gene-related peptide (CGRP) and pituitary adenylate cyclase-activating polypeptide (PACAP) signaling pathways may prevent migraine episodes and reduce headache frequency. To investigate whether recurring migraine episodes alter the strength of CGRP and PACAP signaling in trigeminal ganglion (TG) neurons, we compared the number of TG neurons that respond to CGRP and to PACAP (CGRP-R and PACAP-R, respectively) under normal and chronic migraine-like conditions. In a mouse model of chronic migraine, repeated nitroglycerin (NTG) administration significantly increased the number of CGRP-R and PACAP-R neurons in TG but not dorsal root ganglia. In TG neurons that express endogenous αCGRP, repeated NTG led to a 7-fold increase in the number of neurons that respond to both CGRP and PACAP (CGRP-R&PACAP-R). Most of these neurons were unmyelinated C-fiber nociceptors. This suggests that a larger fraction of CGRP signaling in TG nociceptors may be mediated through the autocrine mechanism, and the release of endogenous αCGRP can be enhanced by both CGRP and PACAP signaling pathways under chronic migraine condition. The number of CGRP-R&PACAP-R TG neurons was also increased in a mouse model of posttraumatic headache (PTH). Interestingly, low-dose interleukin-2 treatment, which completely reverses chronic migraine-related and PTH-related behaviors in mouse models, also blocked the increase in both CGRP-R and PACAP-R TG neurons. Together, these results suggest that inhibition of both CGRP and PACAP signaling in TG neurons may be more effective in treating chronic migraine and PTH than targeting individual signaling pathways.

The interaction between P2X3 and TRPV1 in the dorsal root ganglia of adult rats with different pathological pains

Peripheral inflammatory and neuropathic pain are closely related to the activation of purinergic receptor P2X ligand-gated ion channel 3 (P2X3) and transient receptor potential vanilloid 1 (TRPV1), but the interaction between P2X3 and TRPV1 in different types of pathological pain has rarely been reported. In this study, complete Freund’s adjuvant (CFA)-induced inflammatory pain and spared nerve injury (SNI)-induced neuropathic pain models were established in adult rats. The interactions between P2X3 and TRPV1 in the dorsal root ganglion were observed by pharmacological, co-immunoprecipitation, immunofluorescence and whole-cell patch-clamp recording assays. TRPV1 was shown to promote the induction of spontaneous pain caused by P2X3 in the SNI model, but the induction of spontaneous pain behaviour by TRPV1 was not completely dependent on P2X3 in vivo. In both the CFA and SNI models, the activation of peripheral P2X3 enhanced the effect of TRPV1 on spontaneous pain, while the inhibition of peripheral TRPV1 reduced the induction of spontaneous pain by P2X3 in the CFA model. TRPV1 and P2X3 had inhibitory effects on each other in the inflammatory pain model. During neuropathic pain, P2X3 facilitated the function of TRPV1, while TRPV1 had an inhibitory effect on P2X3. These results suggest that the mutual effects of P2X3 and TRPV1 differ in cases of inflammatory and neuropathic pain in rats.

Protective Effects of Hydrogen Sulfide Against the ATP-Induced Meningeal Nociception

We previously showed that extracellular ATP and hydrogen sulfide (H₂S), a recently discovered gasotransmitter, are both triggering the nociceptive firing in trigeminal nociceptors implicated in migraine pain. ATP contributes to meningeal nociception by activating the P2X3 subunit-containing receptors whereas H₂S operates mainly via TRP receptors. However, H₂S was also proposed as a neuroprotective and anti-nociceptive agent. This study aimed to test the effect of H₂S on ATP-mediated nociceptive responses in rat meningeal afferents and trigeminal neurons and on ATP-induced degranulation of dural mast cells. Electrophysiological recording of trigeminal nerve activity in meninges was supplemented by patch-clamp and calcium imaging studies of isolated trigeminal neurons. The H₂S donor NaHS induced a mild activation of afferents and fully suppressed the subsequent ATP-induced firing of meningeal trigeminal nerve fibers. This anti-nociceptive effect of H₂S was specific as an even stronger effect of capsaicin did not abolish the action of ATP. In isolated trigeminal neurons, NaHS decreased the inward currents and calcium transients evoked by activation of ATP-gated P2X3 receptors. Moreover, NaHS prevented ATP-induced P2X7 receptor-mediated degranulation of meningeal mast cells which emerged as triggers of migraine pain. Finally, NaHS decreased the concentration of extracellular ATP in the meningeal preparation. Thus, H₂S exerted the multiple protective actions against the nociceptive effects of ATP. These data highlight the novel pathways to reduce purinergic mechanisms of migraine with pharmacological donors or by stimulation production of endogenous H₂S.

The Purinergic Receptor P2rx3 is Required for Spiral Ganglion Neuron Branch Refinement during Development

The mammalian cochlea undergoes a highly dynamic process of growth and innervation during development. This process includes spiral ganglion neuron (SGN) branch refinement, a process whereby Type I SGNs undergo a phase of “debranching” before forming unramified synaptic contacts with inner hair cells. Using Sox2^CreERT2 and R26R^tdTomato as a strategy to genetically label individual SGNs in mice of both sexes, we report on both a time course of SGN branch refinement and a role for P2rx3 in this process. P2rx3 is an ionotropic ATP receptor that was recently implicated in outer hair cell spontaneous activity and Type II SGN synapse development (Ceriani et al., 2019), but its function in Type I SGN development is unknown. Here, we demonstrate that P2rx3 is expressed by Type I SGNs and hair cells during developmental periods that coincide with SGN branching refinement. P2rx3 null mice show SGNs with more complex branching patterns on their peripheral synaptic terminals and near their cell bodies around the time of birth. Loss of P2rx3 does not appear to confer general changes in axon outgrowth or hair cell formation, and alterations in branching complexity appear to mostly recover by postnatal day (P)6. However, when we examined the distribution of Type I SGN subtypes using antibodies that bind Calb2, Calb1, and Pou4f1, we found that P2rx3 null mice showed an increased proportion of SGNs that express Calb2. These data suggest P2rx3 may be necessary for normal Type I SGN differentiation in addition to serving a role in branch refinement.

[Pathophysiological role of calcitonin gene-related peptide (CGRP) in migraine and cluster headache]

Calcitonin gene-related peptide (CGRP) is released from trigeminal afferents during migraine and cluster headache attacks and can be detected in the jugular plasma. Infusion of CGRP can induce headache attacks in migraine and cluster patients. Inhibition of the CGRP signal system is therapeutic in migraine and cluster headache. CGRP is a potent dilator of intracranial arteries but does not immediately activate the trigeminal pain system. CGRP may act as a signal molecule between different cells in the trigeminal ganglion and enhances nociceptive transmission in the spinal trigeminal nucleus. Peripheral inhibition of the CGRP system reduces these actions. Outside the trigeminovascular system, CGRP is important for maintaining the perfusion of organs in critical situations, promotes growth and repair functions and is an immunomodulatory factor. These actions should be considered when the CGRP system is suppressed for a long time.

P2X7 receptors exert a permissive effect on the activation of presynaptic AMPA receptors in rat trigeminal caudal nucleus glutamatergic nerve terminals

Background

Purine receptors play roles in peripheral and central sensitization and are associated with migraine headache. We investigated the possibility that ATP plays a permissive role in the activation of AMPA receptors thus inducing Glu release from nerve terminals isolated from the rat trigeminal caudal nucleus (TCN).

Methods

Nerve endings isolated from the rat TCN were loaded with [³H]D-aspartic acid ([³H]D-ASP), layered into thermostated superfusion chambers, and perfused continuously with physiological medium, alone or with various test drugs. Radioactivity was measured to assess [³H]D-ASP release under different experimental conditions.

Results

Synaptosomal [³H]D-ASP spontaneous release was stimulated by ATP and to an even greater extent by the ATP analogue benzoylbenzoylATP (BzATP). The stimulation of [³H]D-ASP basal release by the purinergic agonists was prevented by the selective P2X7 receptor antagonist A438079. AMPA had no effect on basal [³H]D-ASP release, but the release observed when synaptosomes were exposed to AMPA plus a purinoceptor agonist exceeded that observed with ATP or BzATP alone. The selective AMPA receptor antagonist NBQX blocked this “excess” release. Co-exposure to AMPA and BzATP, each at a concentration with no release-stimulating effects, evoked a significant increase in [³H]D-ASP basal release, which was prevented by exposure to a selective AMPA antagonist.

Conclusions

P2X7 receptors expressed on glutamatergic nerve terminals in the rat TCN can mediate Glu release directly and indirectly by facilitating the activation of presynaptic AMPA receptors. The high level of glial ATP that occurs during chronic pain states can promote widespread release of Glu as well as can increase the function of AMPA receptors. In this manner, ATP contributes to the AMPA receptor activation involved in the onset and maintenance of the central sensitization associated with chronic pain.

The role of CGRP receptor antagonist (CGRP8-37) and Endomorphin-1 combination therapy on neuropathic pain alleviation and expression of Sigma-1 receptors and antioxidants in rats

BACKGROUND

Spinal cord injury is one of the most common causes of neuropathic pain which is not responsive to common treatments. Owing to the adverse effects of drugs, it seems that the use of Calcitonin Gene-Related Protein (CGRP) receptor antagonist or Morphine and their combination could be an appropriate strategy for pain alleviation.

METHOD

To achieve the objective, fifty six male Wistar rats were divided into seven groups. CGRP8-37 and Endomorphin-1 alone, and in combinated administration, as bolus and continues dose. Both mechanical and cold allodynia, and mechanical hyperalgesia were evaluated before and also15 and 60 min after injection to indicate the efficacy of the therapies in the acute and chronic circumstances on pain induced by spinal cord compression injury. Sigma-1 receptor experssion, oxidant and antioxidant activity after the seven days of the drug adminestration were evaluated.

RESULT

The results showed that Endomorphin-1and CGRP8-37 injections were able to reduce neuropathic pain after spinal cord compression injury. Compared to Endomorphin-1, or CGRP8-37 monotherapy, combination therapy did not show more attenuating effects on the pain threshold. Compared to the continous administration of Endomorphin-1 alone, and CGRP8-37 alone, the continous combination therapy did not reduce the pain further. Molecular studies disclosed the increased expression of the Sigma1 receptor, in the spinal cord after administration of Endomorphin-1, and CGRP8-37 alone, as well as combination therapy. Although, an increase in GPx and SOD activity, and decrease in MDA activity was observed in the combination therapy.

CONCLUSION

Our results demonstrate that either Endomorphin-1 or CGRP receptor antagonist is able to decrease the neuropathic pain after SCI but combination therapy by a CGRP receptor antagonist and Endomorphin-1 did not make any further reduction in pain sensation.

Targeting CGRP for migraine treatment: mechanisms, antibodies, small molecules, perspectives

INTRODUCTION

Calcitonin Gene-Related Peptide (CGRP) has gradually emerged as a suitable therapeutic target to treat migraine. Considering the social and economic burden of migraine, it is fundamental to optimize the disease management with efficacious and safe treatments. In this scenario, drugs targeting GCRP, monoclonal antibodies (MoAbs) and gepants, represent new therapeutic strategies.

AREAS COVERED

In the present work, the authors aim at appraising the main insights and implications of treatments targeting CGRP by reviewing pathophysiology and clinical information.

EXPERT OPINION

Anti-CGRP MoAbs are the first migraine-specific preventive treatments representing a suitable option especially for difficult-to-treat patients. They can be safely administered for long periods even in association with preventatives acting on different targets. Gepants are a safe alternative to triptans for the acute management of migraine and are currently being tested for prevention, thus representing the first transitional molecules for disease therapy. In the future, it might be possible to adapt the treatment according to patients' characteristics and disease phenotype even combining the two treatments targeting the CGRP pathway.

Modeling a Nociceptive Neuro-Immune Synapse Activated by ATP and 5-HT in Meninges: Novel Clues on Transduction of Chemical Signals Into Persistent or Rhythmic Neuronal Firing

Extracellular ATP and serotonin (5-HT) are powerful triggers of nociceptive firing in the meninges, a process supporting headache and whose cellular mechanisms are incompletely understood. The current study aimed to develop, with the neurosimulator NEURON, a novel approach to explore in silico the molecular determinants of the long-lasting, pulsatile nature of migraine attacks. The present model included ATP and 5-HT release, ATP diffusion and hydrolysis, 5-HT uptake, differential activation of ATP P2X or 5-HT3 receptors, and receptor subtype-specific desensitization. The model also tested the role of branched meningeal fibers with multiple release sites. Spike generation and propagation were simulated using variable contribution by potassium and sodium channels in a multi-compartment fiber environment. Multiple factors appeared important to ensure prolonged nociceptive firing potentially relevant to long-lasting pain. Crucial roles were observed in: (i) co-expression of ATP P2X2 and P2X3 receptor subunits; (ii) intrinsic activation/inactivation properties of sodium Nav1.8 channels; and (iii) temporal and spatial distribution of ATP/5-HT release sites along the branches of trigeminal nerve fibers. Based on these factors we could obtain either persistent activation of nociceptive firing or its periodic bursting mimicking the pulsating nature of pain. In summary, our model proposes a novel tool for the exploration of peripheral nociception to test the contribution of clinically relevant factors to headache including migraine pain.

P2X receptors and trigeminal neuralgia

There is currently no effective cure for trigeminal neuralgia (TN) - a relatively common disease that causes long-term pain in patients. Previous research has shown that ionotropic ATP signaling through excitatory and calcium-permeable P2X receptor channels plays a critical role in pathological pain generation and maintenance. In this paper, we review several hypotheses on the pathogenic mechanisms underlying TN. We further discuss pathways or agents that can target P2X expression in TN, thereby affecting pain induction and maintenance.

The Emerging Role of Mechanosensitive Piezo Channels in Migraine Pain

Recently discovered mechanosensitive Piezo channels emerged as the main molecular detectors of mechanical forces. The functions of Piezo channels range from detection of touch and pain, to control of the plastic changes in different organs. Recent studies suggested the role of Piezo channels in migraine pain, which is supposed to originate from the trigeminovascular nociceptive system in meninges. Interestingly, migraine pain is associated with such phenomenon as mechanical hypersensitivity, suggesting enhanced mechanotransduction. In the current review, we present the data that propose the implication of Piezo channels in migraine pain, which has a distinctive pulsatile character. These data include: (i) distribution of Piezo channels in the key elements of the trigeminovascular nociceptive system; (ii) the prolonged functional activity of Piezo channels in meningeal afferents providing a mechanistical basis for mechanotransduction in nociceptive nerve terminals; (iii) potential activation of Piezo channels by shear stress and pulsating blood flow; and (iv) modulation of these channels by emerging chemical agonists and modulators, including pro-nociceptive compounds. Achievements in this quickly expanding field should open a new road for efficient control of Piezo-related diseases including migraine and chronic pain.

Update on novel purinergic P2X3 and P2X2/3 receptor antagonists and their potential therapeutic applications

Introduction: Purinergic P2X3-P2X2/3 receptors are placed in nociceptive neurons' strategic location and show unique desensitization properties; hence, they represent an attractive target for many pain-related diseases. Therefore, a broad interest from academic and pharmaceutical scientists has focused on the search for P2X3 and P2X2/3 receptor ligands and has led to the discovery of numerous new selective antagonists. Some of them have been studied in clinical trials for the treatment of pathological conditions such as bladder disorders, gastrointestinal and chronic obstructive pulmonary diseases.Areas covered: This review provides a summary of the patents concerning the discovery of P2X3 and/or P2X2/3 receptor antagonists published between 2015 and 2019 and their potential clinical use. Thus, the structures and biological data of the most representative molecules are reported.Expert opinion: The 2016 publication of the crystallographic structure of the human P2X3 receptor subtype gave an improvement of published patents in 2017. Hence, a great number of small molecules with dual antagonist activity on P2X3-P2X2/3 receptors, a favorable pharmacokinetic profile, and reasonable oral bioavailability was discovered. The most promising compounds are the phenoxy-diaminopyrimidines including gefapixant (AF-219), and the imidazo-pyridines like BLU-5937, which are in phase III and phase II clinical trials, respectively, for refractory chronic cough.

Investigation of distinct molecular pathways in migraine induction using calcitonin gene-related peptide and sildenafil

Objective

Migraine displays clinical heterogeneity of attack features and attack triggers. The question is whether this heterogeneity is explained by distinct intracellular signaling pathways leading to attacks with distinct clinical features. One well-known migraine-inducing pathway is mediated by cyclic adenosine monophosphate and another by cyclic guanosine monophosphate. Calcitonin gene-related peptide triggers migraine via the cyclic adenosine monophosphate pathway and sildenafil via the cyclic guanosine monophosphate pathway. To date, no studies have examined whether migraine induction mediated via the cyclic adenosine monophosphate and cyclic guanosine monophosphate pathways yields similar attacks within the same patients.

Methods

Patients were subjected to migraine induction on two separate days using calcitonin gene-related peptide (1.5 µg/min for 20 minutes) and sildenafil (100 mg) in a double-blind, randomized, double-dummy, cross-over design. Data on headache intensity, characteristics and accompanying symptoms were collected until 24 hours after drug administration.

Results

Thirty-four patients were enrolled and 27 completed both study days. Seventeen patients developed migraine after both study drugs (63%; 95% CI: 42–81). Eight patients developed migraine on one day only (seven after sildenafil and one after calcitonin gene-related peptide). Two patients did not develop migraine on either day. Headache laterality, nausea, photophobia and phonophobia were similar between drugs in 77%, 65%, 100%, and 94%, respectively, of the 17 patients who developed attacks on both days.

Conclusion

A majority of patients developed migraine after both calcitonin gene-related peptide and sildenafil. This supports the hypothesis that the cyclic adenosine monophosphate and cyclic guanosine monophosphate intracellular signaling pathways in migraine induction converge in a common cellular determinator, which ultimately triggers the same attacks. Trial registration: ClinicalTrials.gov Identifier: NCT03143465.

Exploration of purinergic receptors as potential anti-migraine targets using established pre-clinical migraine models

BACKGROUND

The current understanding of mechanisms behind migraine pain has been greatly enhanced with the recent therapies targeting calcitonin gene-related peptide and its receptor. The clinical efficacy of calcitonin gene-related peptide-blocking drugs indicates that, at least in a considerable proportion of patients, calcitonin gene-related peptide is a key molecule in migraine pain. There are several receptors and molecular pathways that can affect the release of and response to calcitonin gene-related peptide. One of these could be purinergic receptors that are involved in nociception, but these are greatly understudied with respect to migraine.

OBJECTIVE

We aimed to explore purinergic receptors as potential anti-migraine targets.

METHODS

We used the human middle meningeal artery as a proxy for the trigeminal system to screen for possible anti-migraine candidates. The human findings were followed by intravital microscopy and calcitonin gene-related peptide release measurements in rodents.

RESULTS

We show that the purinergic P2Y13 receptor fulfills all the features of a potential anti-migraine target. The P2Y13 receptor is expressed in both the human trigeminal ganglion and middle meningeal artery and activation of this receptor causes: a) middle meningeal artery contraction in vitro; b) reduced dural artery dilation following periarterial electrical stimulation in vivo and c) a reduction of CGRP release from both the dura and the trigeminal ganglion in situ. Furthermore, we show that P2X3 receptor activation of the trigeminal ganglion causes calcitonin gene-related peptide release and middle meningeal artery dilation.

CONCLUSION

Both an agonist directed at the P2Y13 receptor and an antagonist of the P2X3 receptor seem to be viable potential anti-migraine therapies.

CGRP and the Trigeminal System in Migraine

OBJECTIVE

The goal of this narrative review is to provide an overview of migraine pathophysiology, with an emphasis on the role of calcitonin gene-related peptide (CGRP) within the context of the trigeminovascular system.

BACKGROUND

Migraine is a prevalent and disabling neurological disease that is characterized in part by intense, throbbing, and unilateral headaches. Despite recent advances in understanding its pathophysiology, migraine still represents an unmet medical need, as it is often underrecognized and undertreated. Although CGRP has been known to play a pivotal role in migraine for the last 2 decades, this has now received more interest spurred by the early clinical successes of drugs that block CGRP signaling in the trigeminovascular system.

DESIGN

This narrative review presents an update on the role of CGRP within the trigeminovascular system. PubMed searches were used to find recent (ie, 2016 to November 2018) published articles presenting new study results. Review articles are also included not as primary references but to bring these to the attention of the reader. Original research is referenced in describing the core of the narrative, and review articles are used to support ancillary points.

RESULTS

The trigeminal ganglion neurons provide the connection between the periphery, stemming from the interface between the primary afferent fibers of the trigeminal ganglion and the meningeal vasculature and the central terminals in the trigeminal nucleus caudalis. The neuropeptide CGRP is abundant in trigeminal ganglion neurons, and is released from the peripheral nerve and central nerve terminals as well as being secreted within the trigeminal ganglion. Release of CGRP from the peripheral terminals initiates a cascade of events that include increased synthesis of nitric oxide and sensitization of the trigeminal nerves. Secreted CGRP in the trigeminal ganglion interacts with adjacent neurons and satellite glial cells to perpetuate peripheral sensitization, and can drive central sensitization of the second-order neurons. A shift in central sensitization from activity-dependent to activity-independent central sensitization may indicate a mechanism driving the progression of episodic migraine to chronic migraine. The pathophysiology of cluster headache is much more obscure than that of migraine, but emerging evidence suggests that it may also involve hypersensitivity of the trigeminovascular system. Ongoing clinical studies with therapies targeted at CGRP will provide additional, valuable insights into the pathophysiology of this disorder.

CONCLUSIONS

CGRP plays an essential role in the pathophysiology of migraine. Treatments that interfere with the functioning of CGRP in the peripheral trigeminal system are effective against migraine. Blocking sensitization of the trigeminal nerve by attenuating CGRP activity in the periphery may be sufficient to block a migraine attack. Additionally, the potential exists that this therapeutic strategy may also alleviate cluster headache as well.

Shared Fate of Meningeal Mast Cells and Sensory Neurons in Migraine

Migraine is a primary headache disorder which has complex neurogenic pathophysiological mechanisms still requiring full elucidation. The sensory nerves and meningeal mast cell couplings in the migraine target tissue are very effective interfaces between the central nervous system and the immune system. These couplings fall into three categories: intimacy, cross-talk and a shared fate. Acting as the immediate call-center of the neuroimmune system, mast cells play fundamental roles in migraine pathophysiology. Considerable evidence shows that neuroinflammation in the meninges is the key element resulting in the sensitization of trigeminal nociceptors. The successive events such as neuropeptide release, vasodilation, plasma protein extravasation, and mast cell degranulation that form the basic characteristics of the inflammation are believed to occur in this persistent pain state. In this regard, mast cells and sensory neurons represent both the target and source of the neuropeptides that play autocrine, paracrine, and neuro-endocrine roles during this inflammatory process. This review intends to contribute to a better understanding of the meningeal mast cell and sensory neuron bi-directional interactions from molecular, cellular, functional points of view. Considering the fact that mast cells play a sine qua non role in expanding the opportunities for targeted new migraine therapies, it is of crucial importance to explore these multi-faceted interactions.

Mechanosensitive meningeal nociception via Piezo channels: Implications for pulsatile pain in migraine?

BACKGROUND

Recent discovery of mechanosensitive Piezo receptors in trigeminal ganglia suggested the novel molecular candidate for generation of migraine pain. However, the contribution of Piezo channels in migraine pathology was not tested yet. Therefore, in this study, we explored a potential involvement of Piezo channels in peripheral trigeminal nociception implicated in generation of migraine pain.

METHODS

We used immunohistochemistry, calcium imaging, calcitonin gene related peptide (CGRP) release assay and electrophysiology in mouse and rat isolated trigeminal neurons and rat hemiskulls to study action of various stimulants of Piezo receptors on migraine-related peripheral nociception.

RESULTS

We found that essential (35%) fraction of isolated rat trigeminal neurons responded to chemical Piezo1 agonist Yoda1 and about a half of Yoda1 positive neurons responded to hypo-osmotic solution (HOS) and a quarter to mechanical stimulation by focused ultrasound (US). In ex vivo hemiskull preparation, Yoda1 and HOS largely activated persistent nociceptive firing in meningeal branches of trigeminal nerve. By using our novel cluster analysis of pain spikes, we demonstrated that 42% of fibers responded to Piezo1 agonist and 20% of trigeminal fibers were activated by Yoda1 and by capsaicin, suggesting expression of Piezo receptors in TRPV1 positive peptidergic nociceptive nerve fibers. Consistent with this, Yoda1 promoted the release of the key migraine mediator CGRP from hemiskull preparation.

CONCLUSION

Taken together, our data suggest the involvement of mechanosensitive Piezo receptors, in particular, Piezo1 subtype in peripheral trigeminal nociception, which provides a new view on mechanotransduction in migraine pathology and suggests novel molecular targets for anti-migraine medicine.

P2X3 receptors are transducers of sensory signals

Peripheral stimuli are transduced by specific receptors expressed by sensory neurons and are further processed in the dorsal horn of spinal cord before to be transmitted to the brain. While relative few receptor subtypes mediate the initial depolarisation of sensory neurons, an impressive number of molecules and ion channels integrate these inputs into coded signals. Soluble mediators and ambient conditions further shape these processes, potentially triggering peripheral and central sensitisation, or sensory downregulation. Extracellular ATP is a major signaling molecule that acts via purinergic receptors and is a powerful modulator of cell communication as well as a neurotransmitter at peripheral/central synapses. In particular, ATP-mediated signals are transduced by P2X3 receptors expressed mainly by peripheral sensory neurons. Recent evidence suggests that P2X3 receptor function not only induces neuron depolarisation and firing with consequent neurotransmitter release, but it also triggers intracellular molecular changes that amplify purinergic signaling with important consequences.

Long-term application of cannabinoids leads to dissociation between changes in cAMP and modulation of GABAA receptors of mouse trigeminal sensory neurons

Antinociception caused by cannabinoids may have a partial peripheral origin in addition to its central site of action. In fact, we have observed that anandamide selectively and reversibly inhibits GABA_A receptors of putative nociceptive neurons of mouse trigeminal sensory ganglia via CB1 receptor activation to inhibit adenylyl cyclase and decrease cAMP with downstream posttranslational alterations. Since cannabinoids are often used chronically, we studied changes in cAMP levels and GABA-mediated currents of trigeminal neurons following 24 h application of anandamide (0.5 μM) or the synthetic cannabinoid WIN 55,212-2 (5 μM). With this protocol GABA responses were similar to control despite persistent fall in cAMP levels. Inhibition by WIN 55,212-2 of GABA effects recovered after 30 min washout and was not associated with changes in CB1 receptor expression, indicating lack of CB1 receptor inactivation and transient loss of negative coupling between CB1 receptors and GABA_A receptors. The phosphodiesterase inhibitor rolipram (100 μM; 24 h) enhanced cAMP levels and GABA-mediated currents, suggesting GABA_A receptors were sensitive to persistent upregulation via cAMP. While the adenylyl cyclase activator forskolin (1-20 μM) facilitated cAMP levels and GABA currents following 30 min application, this action was lost after 24 h in line with the drug limited lifespan. The PKA inhibitor PKI 14-22 (10 μM) increased cAMP without changing GABA currents. These data indicate that modulation of GABA_A receptors by intracellular cAMP could be lost following persistent application of cannabinoids. Thus, these observations provide an insight into the waning antinociceptive effects of these compounds.

CGRP Receptor Signalling Pathways

Calcitonin gene-related peptide (CGRP) is a promiscuous peptide, similar to many other members of the calcitonin family of peptides. The potential of CGRP to act on many different receptors with differing affinities and efficacies makes deciphering the signalling from the CGRP receptor a challenging task for researchers.Although it is not a typical G protein-coupled receptor (GPCR), in that it is composed not just of a GPCR, the CGRP receptor activates many of the same signalling pathways common for other GPCRs. This includes the family of G proteins and a variety of protein kinases and transcription factors. It is now also clear that in addition to the initiation of cell-surface signalling, GPCRs, including the CGRP receptor, also activate distinct signalling pathways as the receptor is trafficking along the endocytic conduit.Given CGRP's characteristic of activating multiple GPCRs, we will first consider the complex of calcitonin receptor-like receptor (CLR) and receptor activity-modifying protein 1 (RAMP1) as the CGRP receptor. We will discuss the discovery of the CGRP receptor components, the molecular mechanisms controlling its internalization and post-endocytic trafficking (recycling and degradation) and the diverse signalling cascades that are elicited by this receptor in model cell lines. We will then discuss CGRP-mediated signalling pathways in primary cells pertinent to migraine including neurons, glial cells and vascular smooth muscle cells.Investigation of all the CGRP- and CGRP receptor-mediated signalling cascades is vital if we are to fully understand CGRP's role in migraine and will no doubt unearth new targets for the treatment of migraine and other CGRP-driven diseases.

Migraine induction with calcitonin gene-related peptide in patients from erenumab trials

BACKGROUND

Migraine prevention with erenumab and migraine induction by calcitonin gene-related peptide (CGRP) both carry notable individual variance. We wanted to explore a possible association between individual efficacy of anti-CGRP treatment and susceptibility to migraine induction by CGRP.

METHODS

Thirteen migraine patients, previously enrolled in erenumab anti-CGRP receptor monoclonal antibody trials, received CGRP in a double-blind, placebo-controlled, randomized cross-over design to investigate their susceptibility to migraine induction. A standardized questionnaire was used to assess the efficacy of previous antibody treatment. The patients were stratified into groups of high responders and poor responders. Primary outcomes were incidence of migraine-like attacks and area under the curve of headache intensity after infusion of CGRP and placebo. All interviews and experiments were performed in laboratories at the Danish Headache Center, Copenhagen, Denmark.

RESULTS

Ten high responders and three poor responders were included. CGRP induced migraine-like attacks in ten (77%) patients, whereof two were poor responders, compared to none after placebo (p = 0.002). The area under the curve for headache intensity was greater after CGRP, compared to placebo, at 0-90 min (p = 0.009), and 2-12 h (p = 0.014). The median peak headache intensity score was 5 (5-9) after CGRP, compared to 2 (0-4) after placebo (p = 0.004).

CONCLUSIONS

Patients with an excellent effect of erenumab are highly susceptible to CGRP provocation. If an association is evident, CGRP provocation could prove a biomarker for predicting antibody treatment efficacy.

TRIAL REGISTRATION

Retrospectively registered at clinicaltrials.gov with identifier: NCT03481400 .

Microglia P2X4 receptor contributes to central sensitization following recurrent nitroglycerin stimulation

BACKGROUND

The mechanism underlying migraine chronification remains unclear. Central sensitization may account for this progression. The microglia P2X4 receptor (P2X4R) plays a pivotal role in the central sensitization of inflammatory and neuropathic pain, but there is no information about P2X4R in migraine. Therefore, the aim of this study was to identify the precise role of microglia P2X4R in chronic migraine (CM).

METHODS

We used an animal model with recurrent intermittent administration of nitroglycerin (NTG), which closely mimics CM. NTG-induced basal and acute mechanical hypersensitivity were evaluated using the von Frey filament test. Then, we detected Iba1 immunoreactivity (Iba1-IR) and P2X4R expression in the trigeminal nucleus caudalis (TNC). To understand the effect of microglia and P2X4R on central sensitization of CM, we examined whether minocycline, an inhibitor of microglia activation, and 5-BDBD, a P2X4R antagonist, altered NTG-induced mechanical hyperalgesia. In addition, we also evaluated the effect of 5-BDBD on c-Fos and calcitonin gene-related peptide (CGRP) expression within the TNC.

RESULTS

Chronic intermittent administration of NTG resulted in acute and chronic basal mechanical hyperalgesia, accompanied with microglia activation and upregulation of P2X4R expression. Minocycline significantly decreased basal pain hypersensitivity but did not alter acute NTG-induced hyperalgesia. Minocycline also reduced microglia activation. 5-BDBD completely blocked the basal and acute hyperalgesia induced by NTG. This effect was associated with a significant inhibition of the NTG-induced increase in c-Fos protein and CGRP release in the TNC.

CONCLUSIONS

Our results indicate that blocking microglia activation may have an effect on the prevention of migraine chronification. Moreover, we speculate that the P2X4R may be implicated in the microglia-neuronal signal in the TNC, which contributes to the central sensitization of CM.

ATP-Gated P2X3 Receptors Are Specialised Sensors of the Extracellular Environment

P2X3 receptors are ion channels expressed by autonomic and sensory nerves and specialised in transducing extracellular ATP signals. Structural data, together with functional and biochemical studies, suggest that conformational changes of P2X3 receptors upon agonist binding influence downstream intracellular molecular mechanisms relevant for neuronal responses. Activity of P2X3 receptors is implicated in pain, itch, asthma, cardiovascular dysfunction and other pathologies. The study of these receptors has therefore a large potential in the field of drug development and interdisciplinary efforts could clarify molecular mechanisms controlling P2X3 receptor function in different physiological or pathological contexts.

P2X receptor channels in chronic pain pathways

Chronic pain is a highly prevalent debilitating condition for which treatment options remain limited for many patients. Ionotropic ATP signalling through excitatory and calcium-permeable P2X receptor channels is now rightfully considered as a critical player in pathological pain generation and maintenance; therefore, their selective targeting represents a therapeutic opportunity with promising yet untapped potential. Recent advances in the structural, functional and pharmacological characterization of rodent and human ATP-gated P2X receptor channels have shed brighter light on the role of specific subtypes in the pathophysiology of chronic inflammatory, neuropathic or cancer pain. Here, we will review the contribution of P2X3, P2X4 and P2X7 receptors to chronic pain and discuss the opportunities and challenges associated with the pharmacological manipulation of their function.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

The big CGRP flood - sources, sinks and signalling sites in the trigeminovascular system

BACKGROUND

Calcitonin gene-related peptide (CGRP) has long been a focus of migraine research, since it turned out that inhibition of CGRP or CGRP receptors by antagonists or monoclonal IgG antibodies was therapeutic in frequent and chronic migraine. This contribution deals with the questions, from which sites CGRP is released, where it is drained and where it acts to cause its headache proliferating effects in the trigeminovascular system.

RESULTS

The available literature suggests that the bulk of CGRP is released from trigeminal afferents both in meningeal tissues and at the first synapse in the spinal trigeminal nucleus. CGRP may be drained off into three different compartments, the venous blood plasma, the cerebrospinal fluid and possibly the glymphatic system. CGRP receptors in peripheral tissues are located on arterial vessel walls, mononuclear immune cells and possibly Schwann cells; within the trigeminal ganglion they are located on neurons and glial cells; in the spinal trigeminal nucleus they can be found on central terminals of trigeminal afferents. All these structures are potential signalling sites for CGRP, where CGRP mediates arterial vasodilatation but not direct activation of trigeminal afferents. In the spinal trigeminal nucleus a facilitating effect on synaptic transmission seems likely. In the trigeminal ganglion CGRP is thought to initiate long-term changes including cross-signalling between neurons and glial cells based on gene expression. In this way, CGRP may upregulate the production of receptor proteins and pro-nociceptive molecules.

CONCLUSIONS

CGRP and other big molecules cannot easily pass the blood-brain barrier. These molecules may act in the trigeminal ganglion to influence the production of pronociceptive substances and receptors, which are transported along the central terminals into the spinal trigeminal nucleus. In this way peripherally acting therapeutics can have a central antinociceptive effect.

Pathways of CGRP Release from Primary Sensory Neurons

The benefit reported in a variety of clinical trials by a series of small molecule antagonists for the calcitonin gene-related peptide (CGRP) receptor, or four monoclonal antibodies against the neuropeptide or its receptor, has underscored the release of CGRP from terminals of primary sensory neurons, including trigeminal neurons, as one of the major mechanisms of migraine headaches. A large variety of excitatory ion channels and receptors have been reported to elicit CGRP release, thus proposing these agonists as migraine-provoking agents. On the other side, activators of inhibitory channels and receptors may be regarded as potential antimigraine agents. The knowledge of the intracellular pathways underlying the exocytotic process that results in CGRP secretion or its inhibition is, therefore, of importance for understanding how migraine pain originates and how to treat the disease.

In situ imaging reveals properties of purinergic signalling in trigeminal sensory ganglia in vitro

Chronic pain is supported by sterile inflammation that induces sensitisation of sensory neurons to ambient stimuli including extracellular ATP acting on purinergic P2X receptors. The development of in vitro methods for drug screening would be useful to investigate cell crosstalk and plasticity mechanisms occurring during neuronal sensitisation and sterile neuroinflammation. Thus, we studied, at single-cell level, membrane pore dilation based on the uptake of a fluorescent probe following sustained ATP-gated P2X receptor function in neurons and non-neuronal cells of trigeminal ganglion cultures from wild-type (WT) and R192Q CaV2.1 knock-in (KI) mice, a model of familial hemiplegic migraine type 1 characterised by neuronal sensitisation and higher release of soluble mediators. In WT cultures, pore responses were mainly evoked by ATP rather than benzoyl-ATP (BzATP) and partly inhibited by the P2X antagonist TNP-ATP. P2X7 receptors were expressed in trigeminal ganglia mainly by non-neuronal cells. In contrast, KI cultures showed higher expression of P2X7 receptors, stronger responses to BzATP, an effect largely prevented by prior administration of CaV2.1 blocker ω-agatoxin IVA, small interfering RNA (siRNA)-based silencing of P2X7 receptors or the P2X7 antagonist A-804598. No cell toxicity was detected with the protocols. Calcitonin gene-related peptide (CGRP), a well-known migraine mediator, potentiated BzATP-evoked membrane permeability in WT as well as R192Q KI cultures, demonstrating its modulatory role on trigeminal sensory ganglia. Our results show an advantageous experimental approach to dissect pharmacological properties potentially relevant to chronic pain and suggest that CGRP is a soluble mediator influencing purinergic P2X pore dilation and regulating inflammatory responses.

Opposing expression gradients of calcitonin-related polypeptide alpha (Calca/Cgrpα) and tyrosine hydroxylase (Th) in type II afferent neurons of the mouse cochlea

Type II spiral ganglion neurons (SGNs) are small caliber, unmyelinated afferents that extend dendritic arbors hundreds of microns along the cochlear spiral, contacting many outer hair cells (OHCs). Despite these many contacts, type II afferents are insensitive to sound and only weakly depolarized by glutamate release from OHCs. Recent studies suggest that type II afferents may be cochlear nociceptors, and can be excited by ATP released during tissue damage, by analogy to somatic pain-sensing C-fibers. The present work compares the expression patterns among cochlear type II afferents of two genes found in C-fibers: calcitonin-related polypeptide alpha (Calca/Cgrpα), specific to pain-sensing C-fibers, and tyrosine hydroxylase (Th), specific to low-threshold mechanoreceptive C-fibers, which was shown previously to be a selective biomarker of type II versus type I cochlear afferents (Vyas et al., ). Whole-mount cochlear preparations from 3-week- to 2-month-old CGRPα-EGFP (GENSAT) mice showed expression of Cgrpα in a subset of SGNs with type II-like peripheral dendrites extending beneath OHCs. Double labeling with other molecular markers confirmed that the labeled SGNs were neither type I SGNs nor olivocochlear efferents. Cgrpα starts to express in type II SGNs before hearing onset, but the expression level declines in the adult. The expression patterns of Cgrpα and Th formed opposing gradients, with Th being preferentially expressed in apical and Cgrpα in basal type II afferent neurons, indicating heterogeneity among type II afferent neurons. The expression of Th and Cgrpα was not mutually exclusive and co-expression could be observed, most abundantly in the middle cochlear turn.

Cholinergic Nociceptive Mechanisms in Rat Meninges and Trigeminal Ganglia: Potential Implications for Migraine Pain

BACKGROUND

Parasympathetic innervation of meninges and ability of carbachol, acetylcholine (ACh) receptor (AChR) agonist, to induce headaches suggests contribution of cholinergic mechanisms to primary headaches. However, neurochemical mechanisms of cholinergic regulation of peripheral nociception in meninges, origin place for headache, are almost unknown.

METHODS

Using electrophysiology, calcium imaging, immunohistochemistry, and staining of meningeal mast cells, we studied effects of cholinergic agents on peripheral nociception in rat hemiskulls and isolated trigeminal neurons.

RESULTS

Both ACh and carbachol significantly increased nociceptive firing in peripheral terminals of meningeal trigeminal nerves recorded by local suction electrode. Strong nociceptive firing was also induced by nicotine, implying essential role of nicotinic AChRs in control of excitability of trigeminal nerve endings. Nociceptive firing induced by carbachol was reduced by muscarinic antagonist atropine, whereas the action of nicotine was prevented by the nicotinic blocker d-tubocurarine but was insensitive to the TRPA1 antagonist HC-300033. Carbachol but not nicotine induced massive degranulation of meningeal mast cells known to release multiple pro-nociceptive mediators. Enzymes terminating ACh action, acetylcholinesterase (AChE) and butyrylcholinesterase, were revealed in perivascular meningeal nerves. The inhibitor of AChE neostigmine did not change the firing per se but induced nociceptive activity, sensitive to d-tubocurarine, after pretreatment of meninges with the migraine mediator CGRP. This observation suggested the pro-nociceptive action of endogenous ACh in meninges. Both nicotine and carbachol induced intracellular Ca²⁺ transients in trigeminal neurons partially overlapping with expression of capsaicin-sensitive TRPV1 receptors.

CONCLUSION

Trigeminal nerve terminals in meninges, as well as dural mast cells and trigeminal ganglion neurons express a repertoire of pro-nociceptive nicotinic and muscarinic AChRs, which could be activated by the ACh released from parasympathetic nerves. These receptors represent a potential target for novel therapeutic interventions in trigeminal pain and probably in migraine.

CGRP receptor activity in mice with global expression of human receptor activity modifying protein 1

BACKGROUND AND PURPOSE

CGRP is a potent vasodilator and nociceptive neuropeptide linked to migraine. CGRP receptors are heterodimers of receptor activity modifying protein 1 (RAMP1) and either calcitonin receptor-like receptor (CLR; forms canonical CGRP receptor) or calcitonin receptor (CT receptor; forms AMY₁ receptor). The goal of this study was to test whether transgenic mice globally expressing human RAMP1 have increased CGRP receptor activity and whether the receptors are sensitive to human selective antagonist telcagepant.

EXPERIMENTAL APPROACH

cAMP production was measured in primary cultures of aortic smooth muscle and trigeminal ganglia neurons from global hRAMP1 mice and non-transgenic littermates. Functional activity and inhibition were compared with clonal cell lines expressing combinations of CLR or CT receptors with RAMP1.

KEY RESULTS

Cultured smooth muscle from global hRAMP1 mice had a 10-fold greater CGRP-induced cAMP maximal response (Rmax) than non-transgenic littermates, with similar EC₅₀ s. In contrast, cultured trigeminal ganglia from global hRAMP1 mice had a 40-fold leftward shift of the EC₅₀ , with similar Rmax values as littermates. In both hRAMP1 cultures, telcagepant blocked CGRP-induced cAMP production, but was not effective in non-transgenic cultures. IC₅₀ values were closer to those observed for CT receptor/hRAMP1 than CLR/hRAMP1 in clonal cell lines.

CONCLUSIONS AND IMPLICATIONS

Overexpression of hRAMP1 increases CGRP signalling by changing the maximal response or ligand sensitivity, depending on tissue type. Furthermore, telcagepant inhibited transgenic hRAMP1 CGRP receptors, but the degree of inhibition suggests that the transgenic mice are only partially humanized or both canonical CGRP and AMY₁ receptors are functional in trigeminal ganglia neurons and vascular smooth muscle.

2',3'-O-Substituted ATP derivatives as potent antagonists of purinergic P2X3 receptors and potential analgesic agents

Blocking membrane currents evoked by the activation of purinergic P2X3 receptors localized on nociceptive neurons represents a promising strategy for the development of agents useful for the treatment of chronic pain conditions. Among compounds endowed with such antagonistic action, 2',3'-O-(2,4,6-trinitrophenyl)-ATP (TNP-ATP) is an ATP analogue, whose inhibitory activity on P2X receptors has been previously reported. Based on the results of molecular modelling studies performed with homology models of the P2X3 receptor, novel adenosine nucleotide analogues bearing cycloalkyl or arylalkyl substituents replacing the trinitrophenyl moiety of TNP-ATP were designed and synthesized. These new compounds were functionally evaluated on native P2X3 receptors from mouse trigeminal ganglion (TG) sensory neurons using patch clamp recordings under voltage clamp configuration. Our data show that some of these molecules are potent (nanomolar range) and reversible inhibitors of P2X3 receptors, without any apparent effect on trigeminal GABAA and 5-HT3 receptors, whose membrane currents were unaffected by the tested compounds.

Serotonergic mechanisms of trigeminal meningeal nociception: Implications for migraine pain

Serotonergic mechanisms play a central role in migraine pathology. However, the region-specific effects of serotonin (5-HT) mediated via multiple types of receptors in the nociceptive system are poorly understood. Using extracellular and patch-clamp recordings, we studied the action of 5-HT on the excitability of peripheral and central terminals of trigeminal afferents. 5-HT evoked long-lasting TTX-sensitive firing in the peripheral terminals of meningeal afferents, the origin site of migraine pain. Cluster analysis revealed that in majority of nociceptive fibers 5-HT induced either transient or persistent spiking activity with prevailing delta and theta rhythms. The 5-HT3-receptor antagonist MDL-72222 or 5-HT1B/D-receptor antagonist GR127935 largely reduced, but their combination completely prevented the excitatory pro-nociceptive action of 5-HT. The 5-HT3 agonist mCPBG activated spikes in MDL-72222-dependent manner but the 5HT-1 receptor agonist sumatriptan did not affect the nociceptive firing. 5-HT also triggered peripheral CGRP release in meninges, which was blocked by MDL-72222.5-HT evoked fast membrane currents and Ca²⁺ transients in a fraction of trigeminal neurons. Immunohistochemistry showed expression of 5-HT3A receptors in fibers innervating meninges. Endogenous release of 5-HT from degranulated mast cells increased nociceptive firing. Low pH but not histamine strongly activated firing. 5-HT reduced monosynaptic inputs from trigeminal Aδ- and C-afferents to the upper cervical lamina I neurons and this effect was blocked by MDL-72222. Consistent with central inhibitory effect, 5-HT reduced CGRP release in the brainstem slices. In conclusion, 5-HT evokes powerful pro-nociceptive peripheral and anti-nociceptive central effects in trigeminal system transmitting migraine pain.