PACAP-PAC1 receptor inhibition is effective in opioid induced hyperalgesia and medication overuse headache models

Pituitary adenylate cyclase-activating polypeptide plays a role in neuropsychiatric and substance use disorders: sex-specific perspective

The neuropeptide pituitary adenylate cyclase-activating peptide (PACAP) plays a pivotal role in regulating stress, fear, and anxiety responses. Genetic and molecular studies investigating PACAP demonstrate sex-dimorphic characteristics, with females exhibiting increased reactivity of PACAP signaling in neuropsychiatric disorders. Studies expand the role of PACAP to substance use disorders (SUD) by demonstrating modulation of PACAP can lead to neurobiological changes induced by nicotine, ethanol, stimulants and opioids. Given that females with SUD exhibit distinct drug use, relapse, and withdrawal sensitivity relative to males, we hypothesize that the PACAP system contributes to these sex-specific differences. Therefore, we review the role of PACAP in SUD by characterizing the role of PACAP at the molecular, brain regional, and behavioral levels relevant to the addiction cycle. We present literature linking PACAP to neuropsychiatric disorders, which demonstrate the intricate role of PACAP within neuronal signaling and pathways modulating addiction. We hypothesize that females are more particularly susceptible to PACAP-related changes during the intoxication and withdrawal phases of the addiction cycle. Altogether understanding the sex-specific differences in the PACAP system offers a foundation for future studies aimed at developing tailored interventions for addressing SUD.

Activation of δ-opioid receptors blocks allodynia in a model of headache induced by PACAP

BACKGROUND AND PURPOSE

Pituitary adenylate cyclase activating polypeptide (PACAP) is a human migraine trigger that is being targeted for migraine. The δ-opioid receptor (δ-receptor) is a novel target for the treatment of migraine, but its mechanism remains unclear. The goals of this study were to develop a mouse PACAP-headache model using clinically significant doses of PACAP; determine the effects of δ-receptor activation in this model; and investigate the co-expression of δ-receptors, PACAP and PACAP-PAC1 receptor.

EXPERIMENTAL APPROACH

Cephalic allodynia to low doses of acute and chronic PACAP were tested. A triptan (sumatriptan) and a CGRP receptor antagonist (olcegepant) were tested in this model. The δ-receptor agonist SNC80 was tested in PACAP and CGRP-induced headache models. Expression of PACAP, PAC1, CRLR and δ-receptors was determined using in situ hybridisation.

KEY RESULTS

Low doses of PACAP produced dose-dependent acute and chronic cephalic allodynia, blocked by sumatriptan but not by olcegepant. The PAC1 antagonist (M65) did not inhibit CGRP-induced allodynia. There was moderate co-expression of PAC1 and CRLR transcripts in migraine-related regions. SNC80 blocked PACAP- and CGRP-induced allodynia. There was low co-expression of PACAP and δ-receptors in brain regions measured. However, there was high co-expression of PAC1 and δ-receptors in somatosensory cortex, hippocampus and trigeminal nucleus caudalis.

CONCLUSION AND IMPLICATIONS

We developed a translationally significant model of PACAP-induced headache, which was mechanistically distinct from CGRP. Activation of δ-receptors blocked PACAP- and CGRP-induced allodynia, and δ-receptors were highly co-expressed with the PACAP-ergic system. Future studies will examine the functional relationship between δ-receptors and PAC1.

Regulatory T cells require peripheral CCL2-CCR2 signaling to facilitate the resolution of medication overuse headache-related behavioral sensitization

BACKGROUND

Medication overuse headache (MOH) is the most common secondary headache disorder, resulting from chronic and excessive use of medication to treat headaches, for example, sumatriptan. In a recent study, we have shown that the peripheral C-C motif ligand 2 (CCL2), C-C motif chemokine receptor 2 (CCR2) and calcitonin-gene-related peptide (CGRP) signaling pathways interact with each other and play critical roles in the development of chronic migraine-related behavioral and cellular sensitization. In the present study, we investigated whether CCL2-CCR2 and CGRP signaling pathways play a role in the development of sumatriptan overuse-induced sensitization, and whether they are involved in its resolution by the low-dose interleukin-2 (LD-IL-2) treatment.

METHODS

Mice received daily sumatriptan administration for 12 days. MOH-related behavioral sensitization was assessed by measuring changes of periorbital mechanical thresholds for 3 weeks. CCL2-CCR2 and CGRP signaling pathways were inhibited by targeted gene deletion or with an anti-CCL2 antibody. Ca2+-imaging was used to examine whether repetitive sumatriptan treatment enhances CGRP and pituitary adenylate cyclase-activating polypeptide (PACAP) signaling in trigeminal ganglion (TG) neurons. LD-IL-2 treatment was initiated after the establishment of sumatriptan-induced sensitization. Immunohistochemistry and flow cytometry analyses were used to examine whether CCL2-CCR2 signaling controls regulatory T (Treg) cell proliferation and/or trafficking.

RESULTS

CCL2, CCR2 and CGRPα global KO mice exhibited robust sumatriptan-induced behavioral sensitization comparable to wild-type controls. Antibody neutralization of peripheral CCL2 did not affect sumatriptan-induced behaviors either. Repeated sumatriptan administration did not enhance the strength of CGRP or PACAP signaling in TG neurons. Nevertheless, LD-IL-2 treatment, which facilitated the resolution of sumatriptan-induced sensitization in wild-type and CGRPα KO mice, was completely ineffective in mice with compromised CCL2-CCR2 signaling. In CCL2 KO mice, we observed normal LD-IL-2-induced Treg expansion in peripheral blood, but the increase of Treg cells in dura and TG tissues was significantly reduced in LD-IL-2-treated CCL2 KO mice relative to wild-type controls.

CONCLUSIONS

These results indicate that the endogenous CCL2-CCR2 and CGRP signaling pathways are not involved in sumatriptan-induced behavioral sensitization, suggesting that distinct molecular mechanisms underlie chronic migraine and MOH. On the other hand, peripheral CCL2-CCR2 signaling is required for LD-IL-2 to reverse chronic headache-related sensitization.

Pituitary adenylate cyclase-activating polypeptide signalling as a therapeutic target in migraine

Migraine is a disabling neurological disorder that affects more than one billion people worldwide. The clinical presentation is characterized by recurrent headache attacks, which are often accompanied by photophobia, phonophobia, nausea and vomiting. Although the pathogenesis of migraine remains incompletely understood, mounting evidence suggests that specific signalling molecules are involved in the initiation and modulation of migraine attacks. These signalling molecules include pituitary adenylate cyclase-activating polypeptide (PACAP), a vasoactive peptide that is known to induce migraine attacks when administered by intravenous infusion to people with migraine. Discoveries linking PACAP to migraine pathogenesis have led to the development of drugs that target PACAP signalling, and a phase II trial has provided evidence that a monoclonal antibody against PACAP is effective for migraine prevention. In this Review, we explore the molecular and cellular mechanisms of PACAP signalling, shedding light on its role in the trigeminovascular system and migraine pathogenesis. We then discuss emerging therapeutic strategies that target PACAP signalling for the treatment of migraine and consider the research needed to translate the current knowledge into a treatment for migraine in the clinic.

Potential treatment targets for migraine: emerging options and future prospects

Migraine is a leading cause of disability worldwide. Despite the recent approval of several calcitonin gene-related peptide-targeted therapies, many people with migraine do not achieve satisfactory headache improvement with currently available therapies and there continues to be an unmet need for effective and tolerable migraine-specific treatments. Exploring additional targets that have compelling evidence for their involvement in modulating migraine pathways is therefore imperative. Potential new therapies for migraine include pathways involved in nociception, regulation of homoeostasis, modulation of vasodilation, and reward circuits. Animal and human studies show that these targets are expressed in regions of the CNS and peripheral nervous system that are involved in pain processing, indicating that these targets might be regarded as promising for the discovery of new migraine therapies. Future studies will require assessment of whether targets are suitable for therapeutic modulation, including assessment of specificity, affinity, solubility, stability, efficacy, and safety.