PACAP-38 but not VIP induces release of CGRP from trigeminal nucleus caudalis via a receptor distinct from the PAC1 receptor

PACAP and its role in primary headaches

Pituitary adenylate cyclase-activating peptide (PACAP) is a neuropeptide implicated in a wide range of functions, such as nociception and in primary headaches. Regarding its localization, PACAP has been observed in the sensory trigeminal ganglion (TG), in the parasympathetic sphenopalatine (SPG) and otic ganglia (OTG), and in the brainstem trigeminocervical complex. Immunohistochemistry has shown PACAP-38 in numerous cell bodies of SPG/OTG, co-stored with vasoactive intestinal peptide (VIP), nitric oxide synthase (NOS) and, to a minor degree, with choline acetyltransferase. PACAP has in addition been found in a subpopulation of calcitonin gene-related peptide (CGRP)-immunoreactive cells in the trigeminal system. The PACAP/VIP receptors (PAC₁, VPAC₁, and VPAC₂) are present in sensory neurons and in vascular smooth muscle related to the trigeminovascular system. It is postulated that PACAP is involved in nociception. In support, abolishment of PACAP synthesis or reception leads to diminished pain responses, whereas systemic PACAP-38 infusion triggers pain behavior in animals and delayed migraine-like attacks in migraine patients without marked vasodilatory effects. In addition, increased plasma levels have been documented in acute migraine attacks and in cluster headache, in accordance with findings in experimental models of trigeminal activation. This suggest that the activation of the trigeminal system may result in elevated venous levels of PACAP, a change that can be reduced when headache is treated. The data presented in this review indicate that PACAP and its receptors may be promising targets for migraine therapeutics.

PACAP and its receptors in cranial arteries and mast cells

Background

In migraineurs pituitary adenylate cyclase activating peptide1–38 (PACAP1–38) is a potent migraine provoking substance and the accompanying long lasting flushing suggests degranulation of mast cells. Infusion of the closely related vasoactive intestinal peptide (VIP) either induces headache or flushing. This implicates the pituitary adenylate cyclase activating peptide type I receptor (PAC1) to be involved in the pathophysiology of PACAP1–38 provoked headaches. Here we review studies characterizing the effects of mainly PACAP but also of VIP on cerebral and meningeal arteries and mast cells.

Discussion

PACAP1–38, PACAP1–27 and VIP dilate cerebral and meningeal arteries from several species including man. In rat cerebral and meningeal arteries the dilation seems to be mediated preferably via vasoactive intestinal peptide receptor type 1 (VPAC1) receptors while, in human, middle meningeal artery dilation induced via vasoactive intestinal peptide receptor type 2 (VPAC2) receptors cannot be ruled out. PACAP1–38 is a strong degranulator of peritoneal and dural mast cells while PACAP1–27 and VIP only have weak effects. More detailed characterization studies suggest that mast cell degranulation is not mediated via the known receptors for PACAP1–38 but rather via a still unknown receptor coupled to phospholipase C.

Conclusion

It is suggested that PACAP1–38 might induce migraine via degranulation of dural mast cells via a yet unknown receptor.

Ghrelin attenuated hyperalgesia induced by chronic nitroglycerin: CGRP and TRPV1 as targets for migraine management

Background

According to the neurovascular theory of migraine, activation of the trigeminovascular system contributes to the development of migraine. This study examined the effects of chronic intraperitoneal ghrelin (150 µg/kg) treatment on the development of chronic migraine induced by intermittent injection of nitroglycerin 10 mg/kg.

Methods

Baseline and post-drug (2 h following nitroglycerin injection) mechanical and thermal sensitivity were assessed by von Frey hair and tail immersion tests, respectively on days 1, 3, 5, 7, 9 and 11. Moreover, we investigated the effect of ghrelin treatment on nitroglycerin-induced aversive behavior by using a two-chamber conditioned place aversion paradigm. At the end of behavioral testing, on day 11, animals were sacrificed and plasma concentration of calcitonin gene-related peptide was measured using a rat-specific enzyme-linked immunosorbent assay kit. Also, real time polymerase chain reaction was used to quantify mRNA expression of calcitonin gene-related peptide and transient receptor potential vanilloid 1 in the trigeminal ganglion.

Results

Our results indicated that nitroglycerin activated the trigeminovascular system, which was reflected by mechanical and thermal hypersensitivity and elevation of mRNA expression of calcitonin gene-related peptide and transient receptor potential vanilloid-1, as migraine markers, and plasma calcitonin gene-related peptide levels. Moreover, chronic nitroglycerin injection induced conditioned place aversion and body weight loss. Nevertheless, ghrelin modulated nitroglycerin-triggered changes in transient receptor potential vanilloid-1 and calcitonin gene-related peptide expression, and mitigated nitroglycerin-induced hyperalgesia.

Conclusion

These results provide the first convincing evidence that ghrelin has a modulating effect on central sensitization induced by chronic intermittent nitroglycerin, and its antinociceptive effect may be related to a reduction of these factors in the trigeminal ganglion.

First study on the peptidergic innervation of the brain superior sagittal sinus in humans

The superior sagittal sinus (SSS) of the mammalian brain is a pain-sensitive intracranial vessel thought to play a role in the pathogenesis of migraine headaches. Here, we aimed to investigate the presence and the potential co-localization of some neurotransmitters in the human SSS. Immunohistochemical and double-labeling immunofluorescence analyses were applied to paraformaldehyde-fixed, paraffin-embedded, coronal sections of the SSS. Protein extraction and Western blotting technique were performed on the same material to confirm the morphological data. Our results showed nerve fibers clustered mainly in large bundles tracking parallel to the longitudinal axis of the sinus, close in proximity to the vascular endothelium. Smaller fascicles of fibers encircled the vascular lumen in a spiral fashion, extending through the subendothelial connective tissue. Isolated nerve fibers were observed around the openings of bridging veins in the sinus or around small vessels extending into the perisinusal dura. The neurotransmitters calcitonin gene related peptide (CGRP), substance P (SP), neuronal nitric oxide synthase (nNOS), vasoactive intestinal polypeptide (VIP), tyrosine hydroxylase (TH), and neuropeptide Y (NPY) were found in parietal nerve structures, distributed all along the length of the SSS. Overall, CGRP- and TH-containing nerve fibers were the most abundant. Neurotransmitters co-localized in the same fibers in the following pairs: CGRP/SP, CGRP/NOS, CGRP/VIP, and TH/NPY. Western blotting analysis confirmed the presence of such neurosubstances in the SSS wall. Overall our data provide the first evidence of the presence and co-localization of critical neurotransmitters in the SSS of the human brain, thus contributing to a better understanding of the sinus functional role.

Stimulation of rat cranial dura mater with potassium chloride causes CGRP release into the cerebrospinal fluid and increases medullary blood flow

Primary headaches may be accompanied by increased intracranial blood flow induced by the release of the potent vasodilator calcitonin gene-related peptide (CGRP) from activated meningeal afferents. We aimed to record meningeal and medullary blood flow simultaneously and to localize the sites of CGRP release in rodent preparations in vivo and ex vivo. Blood flow in the exposed rat parietal dura mater and the medulla oblongata was recorded by laser Doppler flowmetry, while the dura was stimulated by topical application of 60mM potassium chloride (KCl). Samples of jugular venous plasma and cerebrospinal fluid (CSF) collected from the cisterna magna were analysed for CGRP concentrations using an enzyme immunoassay. In a hemisected rat skull preparation lined with dura mater the CGRP releasing effect of KCl superfusion was examined. Superfusion of the dura mater with KCl decreased meningeal blood flow unless alpha-adrenoceptors were blocked by phentolamine, whereas the medullary blood flow was increased. The same treatment caused increased CGRP concentrations in jugular plasma and CSF and induced significant CGRP release in the hemisected rat skull preparation. Anaesthesia of the trigeminal ganglion by injection of lidocaine reduced increases in medullary blood flow and CGRP concentration in the CSF upon meningeal KCl application. CGRP release evoked by depolarisation of meningeal afferents is accompanied by increased blood flow in the medulla oblongata but not the dura mater. This discrepancy can be explained by the smooth muscle depolarising effect of KCl and the activation of sympathetic vasoconstrictor mechanisms. The medullary blood flow response is most likely mediated by CGRP released from activated central terminals of trigeminal afferents. Increased blood supply of the medulla oblongata and CGRP release into the CSF may also occur in headaches accompanying vigorous activation of meningeal afferents.

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

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

Current and novel insights into the neurophysiology of migraine and its implications for therapeutics

Migraine headache and its associated symptoms have plagued humans for two millennia. It is manifest throughout the world, and affects more than 1/6 of the global population. It is the most common brain disorder, and is characterized by moderate to severe unilateral headache that is accompanied by vomiting, nausea, photophobia, phonophobia, and other hypersensitive symptoms of the senses. While there is still a clear lack of understanding of its neurophysiology, it is beginning to be understood, and it seems to suggest migraine is a disorder of brain sensory processing, characterized by a generalized neuronal hyperexcitability. The complex symptomatology of migraine indicates that multiple neuronal systems are involved, including brainstem and diencephalic systems, which function abnormally, resulting in premonitory symptoms, ultimately evolving to affect the dural trigeminovascular system, and the pain phase of migraine. The migraineur also seems to be particularly sensitive to fluctuations in homeostasis, such as sleep, feeding and stress, reflecting the abnormality of functioning in these brainstem and diencephalic systems. Implications for therapeutic development have grown out of our understanding of migraine neurophysiology, leading to major drug classes, such as triptans, calcitonin gene-related peptide receptor antagonists, and 5-HT1F receptor agonists, as well as neuromodulatory approaches, with the promise of more to come. The present review will discuss the current understanding of the neurophysiology of migraine, particularly migraine headache, and novel insights into the complex neural networks responsible for associated neurological symptoms, and how interaction of these networks with migraine pain pathways has implications for the development of novel therapeutics.

CGRP in human models of primary headaches

Objective To review the role of CGRP in human models of primary headaches and to discuss methodological aspects and future directions. Discussion Provocation experiments demonstrated a heterogeneous CGRP migraine response in migraine patients. Conflicting CGRP plasma results in the provocation experiments are likely due to assay variation; therefore, proper validation and standardization of an assay is needed. To what extent CGRP is involved in tension-type headache and cluster headache is unknown. Conclusion Human models of primary headaches have elucidated the role of CGRP in headache pathophysiology and sparked great interest in developing new treatment strategies using CGRP antagonists and antibodies. Future studies applying more refined human experimental models should identify biomarkers of CGRP-induced primary headache and reveal whether CGRP provocation experiments could be used to predict efficacy of CGRP antagonists in migraine patients.

Part II: Biochemical changes after pituitary adenylate cyclase-activating polypeptide-38 infusion in migraine patients

Background Intravenous infusion of pituitary adenylate cyclase-activating polypeptide-38 (PACAP38) provokes migraine attacks in 65-70% of migraine without aura (MO) patients. We investigated whether PACAP38 infusion causes changes in the endogenous production of PACAP38, vasoactive intestinal polypeptide (VIP), calcitonin gene-related peptide (CGRP), tumour necrosis factor alpha (TNFα), S100 calcium binding protein B (S100B), neuron-specific enolase and pituitary hormones in migraine patients. Methods We allocated 32 previously genotyped MO patients to receive intravenous infusion PACAP38 (10 pmol/kg/minute) for 20 minutes and recorded migraine-like attacks. Sixteen of the patients were carriers of the risk allele rs2274316 ( MEF2D), which confers increased risk of MO and may regulate PACAP38 expression, and 16 were non-carriers. We collected blood samples at baseline and 20, 30, 40, 60 and 90 minutes after the start of the infusion. A control group of six healthy volunteers received intravenous saline. Results PACAP38 infusion caused significant changes in plasma concentrations of VIP ( p = 0.026), prolactin ( p = 0.011), S100B ( p < 0.001) and thyroid-stimulating hormone (TSH; p = 0.015), but not CGRP ( p = 0.642) and TNFα ( p = 0.535). We found no difference in measured biochemical variables after PACAP38 infusion in patients who later developed migraine-like attacks compared to those who did not ( p > 0.05). There was no difference in the changes of biochemical variables between patients with and without the MEF2D-associated gene variant ( p > 0.05). Conclusion PACAP38 infusion elevated the plasma levels of VIP, prolactin, S100B and TSH, but not CGRP and TNFα. Development of delayed migraine-like attacks or the presence of the MEF2D gene variant was not associated with pre-ictal changes in plasma levels of neuropeptides, TNFα and pituitary hormones.

Stimulatory effect of pituitary adenylate cyclase-activating polypeptide 6-38, M65 and vasoactive intestinal polypeptide 6-28 on trigeminal sensory neurons

Pituitary adenylate cyclase-activating polypeptide (PACAP) acts on G protein-coupled receptors: the specific PAC1 and VPAC1/VPAC2 receptors. PACAP6-38 was described as a potent PAC1/VPAC2 antagonist in several models, but recent studies reported its agonistic behaviors proposing novel receptorial mechanisms. Since PACAP in migraine is an important research tool, we investigated the effect of PACAP and its peptide fragments on trigeminal primary sensory neurons. Effect of the peptides was studied with ratiometric Ca-imaging technique using the fluorescent indicator fura-2 AM on primary cultures of rat and mouse trigeminal ganglia (TRGs) neurons. Specificity testing was performed on PAC1, VPAC1 and VPAC2 receptor-expressing cell lines with both fluorescent and radioactive Ca-uptake methods. Slowly increasing intracellular free calcium concentration [Ca(2+)]i was detected after PACAP1-38, PACAP1-27, vasoactive intestinal polypeptide (VIP) and the selective PAC1 receptor agonist maxadilan administration on TRG neurons, but interestingly, PACAP6-38, VIP6-28 and the PAC1 receptor antagonist M65 also caused similar activation. The VPAC2 receptor agonist BAY 55-9837 induced similar activation, while the VPAC1 receptor agonist Ala(11,22,28)VIP had no significant effect on [Ca(2+)]i. It was proven that the Ca(2+)-influx originated from intracellular stores using radioactive calcium-45 uptake experiment and Ca-free solution. On the specific receptor-expressing cell lines the antagonists inhibited the stimulating actions of the respective agonists, but had no effects by themselves. PACAP6-38, M65 and VIP6-28, which were described as antagonists in numerous studies in several model systems, act as agonists on TRG primary sensory neurons. Currently unknown receptors or splice variants linked to distinct signal transduction pathways might explain these differences.

Novel strategies for the treatment of migraine attacks via the CGRP, serotonin, dopamine, PAC1, and NMDA receptors

INTRODUCTION

Migraine is a common, paroxysmal, and disabling primary headache with a high personal and socioeconomic impact. It involves ∼ 16% of the general population. During the years, a number of hypotheses have been put forward concerning the exact pathomechanism, but the final solution is still undiscovered.

AREAS COVERED

Although the origin is enigmatic, parallel therapeutic efforts have been developed. Current attack therapy does not meet the expectations of the patients or the doctors. This article, based on a PubMed search, reviews the novel pharmacological possibilities that influence the peripheral and central sensitization involved in the disease.

EXPERT OPINION

In order to overcome the therapeutic insufficiency, a calcitonin gene-related peptide receptor antagonist without the side-effect of liver transaminase elevation is required. Another therapeutic option is to develop a neurally acting antimigraine agent, such as a serotonin-1F receptor agonist, with low adverse central nervous system events. Development of a potent dopamine receptor antagonist is necessary to diminish the premonitory symptoms of migraine. A further option is to decrease the headache intensity with a pituitary adenylate cyclase-activating polypeptide type 1 receptor blocker which can cross the blood-brain barrier. Finally, synthetic kynurenine analogues are required to block the pain transmission in the activated trigeminal system.

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.