Differential distribution of calcitonin gene-related peptide and its receptor components in the human trigeminal ganglion

Distribution of CGRP and CGRP receptor components in the rat brain

BACKGROUND

Calcitonin gene-related peptide and its receptor, consisting of receptor activity-modifying protein 1 and calcitonin receptor-like receptor, are of considerable interest because of the role they play in migraine and recently developed migraine therapies.

METHODS

To better understand the function of this neuropeptide, we used immunohistochemistry to determine a detailed distribution of calcitonin gene-related peptide, receptor activity-modifying protein 1 and calcitonin receptor-like receptor in the rat brain in a region of 0.5-1.5 mm lateral to the midline. We found calcitonin gene-related peptide immunoreactivity in most of the neurons of the cerebral cortex, hippocampus, cerebellum, thalamic nuclei, hypothalamic nuclei and brainstem nuclei. In contrast, receptor activity-modifying protein 1 and calcitonin receptor-like receptor immunoreactivity were found almost exclusively in the neuronal processes in the investigated regions.

CONCLUSION

Overall, the degree of expression of calcitonin gene-related peptide and calcitonin gene-related peptide receptor components in the central nervous system is astonishingly complex and suggestive of many different brain functions, including a possible role in migraine. However, currently, the presence of calcitonin gene-related peptide and the nature of its receptors throughout the brain is an enigma yet to be solved.

Novel Therapeutic Targets Against Spreading Depression

Migraine is among the most prevalent and disabling neurological diseases in the world. Cortical spreading depression (SD) is an intense wave of neuronal and glial depolarization underlying migraine aura, and a headache trigger, which has been used as an experimental platform for drug screening in migraine. Here, we provide an overview of novel therapeutic targets that show promise to suppress SD, such as acid-sensing ion channels, casein kinase Iδ, P2X7-pannexin 1 complex, and neuromodulation, and outline the experimental models and essential quality measures for rigorous and reproducible efficacy testing.

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.

The role of calcitonin gene-related peptide in peripheral and central pain mechanisms including migraine

Calcitonin gene-related peptide (CGRP) is a 37-amino acid peptide found primarily in the C and Aδ sensory fibers arising from the dorsal root and trigeminal ganglia, as well as the central nervous system. Calcitonin gene-related peptide was found to play important roles in cardiovascular, digestive, and sensory functions. Although the vasodilatory properties of CGRP are well documented, its somatosensory function regarding modulation of neuronal sensitization and of enhanced pain has received considerable attention recently. Growing evidence indicates that CGRP plays a key role in the development of peripheral sensitization and the associated enhanced pain. Calcitonin gene-related peptide is implicated in the development of neurogenic inflammation and it is upregulated in conditions of inflammatory and neuropathic pain. It is most likely that CGRP facilitates nociceptive transmission and contributes to the development and maintenance of a sensitized, hyperresponsive state not only of the primary afferent sensory neurons but also of the second-order pain transmission neurons within the central nervous system, thus contributing to central sensitization as well. The maintenance of a sensitized neuronal condition is believed to be an important factor underlying migraine. Recent successful clinical studies have shown that blocking the function of CGRP can alleviate migraine. However, the mechanisms through which CGRP may contribute to migraine are still not fully understood. We reviewed the role of CGRP in primary afferents, the dorsal root ganglion, and in the trigeminal system as well as its role in peripheral and central sensitization and its potential contribution to pain processing and to migraine.

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.

Pathophysiology of Migraine: A Disorder of Sensory Processing

Plaguing humans for more than two millennia, manifest on every continent studied, and with more than one billion patients having an attack in any year, migraine stands as the sixth most common cause of disability on the planet. The pathophysiology of migraine has emerged from a historical consideration of the "humors" through mid-20th century distraction of the now defunct Vascular Theory to a clear place as a neurological disorder. It could be said there are three questions: why, how, and when? Why: migraine is largely accepted to be an inherited tendency for the brain to lose control of its inputs. How: the now classical trigeminal durovascular afferent pathway has been explored in laboratory and clinic; interrogated with immunohistochemistry to functional brain imaging to offer a roadmap of the attack. When: migraine attacks emerge due to a disorder of brain sensory processing that itself likely cycles, influenced by genetics and the environment. In the first, premonitory, phase that precedes headache, brain stem and diencephalic systems modulating afferent signals, light-photophobia or sound-phonophobia, begin to dysfunction and eventually to evolve to the pain phase and with time the resolution or postdromal phase. Understanding the biology of migraine through careful bench-based research has led to major classes of therapeutics being identified: triptans, serotonin 5-HT1B/1D receptor agonists; gepants, calcitonin gene-related peptide (CGRP) receptor antagonists; ditans, 5-HT1F receptor agonists, CGRP mechanisms monoclonal antibodies; and glurants, mGlu5 modulators; with the promise of more to come. Investment in understanding migraine has been very successful and leaves us at a new dawn, able to transform its impact on a global scale, as well as understand fundamental aspects of human biology.

Topical dura mater application of CFA induces enhanced expression of c-fos and glutamate in rat trigeminal nucleus caudalis: attenuated by KYNA derivate (SZR72)

Background

Migraine is a debilitating neurological disorder where trigeminovascular activation plays a key role. We have previously reported that local application of Complete Freund’s Adjuvant (CFA) onto the dura mater caused activation in rat trigeminal ganglion (TG) which was abolished by a systemic administration of kynurenic acid (KYNA) derivate (SZR72). Here, we hypothesize that this activation may extend to the trigeminal complex in the brainstem and is attenuated by treatment with SZR72.

Methods

Activation in the trigeminal nucleus caudalis (TNC) and the trigeminal tract (Sp5) was achieved by application of CFA onto the dural parietal surface. SZR72 was given intraperitoneally (i.p.), one dose prior CFA deposition and repeatedly daily for 7 days. Immunohistochemical studies were performed for mapping glutamate, c-fos, PACAP, substance P, IL-6, IL-1β and TNFα in the TNC/Sp5 and other regions of the brainstem and at the C₁-C₂ regions of the spinal cord.

Results

We found that CFA increased c-fos and glutamate immunoreactivity in TNC and C₁-C₂ neurons. This effect was mitigated by SZR72. PACAP positive fibers were detected in the fasciculus cuneatus and gracilis. Substance P, TNFα, IL-6 and IL-1β immunopositivity were detected in fibers of Sp5 and neither of these molecules showed any change in immunoreactivity following CFA administration.

Conclusion

This is the first study demonstrating that dural application of CFA increases the expression of c-fos and glutamate in TNC neurons. Treatment with the KYNA analogue prevented this expression.

The effect of capsaicin on expression patterns of CGRP in trigeminal ganglion and trigeminal nucleus caudalis following experimental tooth movement in rats

Objectives

The aim of this study was to explore the effect of capsaicin on expression patterns of calcitonin gene-related peptide (CGRP) in the trigeminal ganglion (TG) and trigeminal subnucleus caudalis (Vc) following experimental tooth movement.

Material and Methods

Male Sprague-Dawley rats were used in this study and divided into small-dose capsaicin+force group, large-dose capsaicin+force group, saline+force group, and no force group. Closed coil springs were used to mimic orthodontic forces in all groups except for the no force group, in which springs were inactivated. Capsaicin and saline were injected into periodontal tissues. Rats were euthanized at 0 h, 12 h, 1 d, 3 d, 5 d, and 7 d following experimental tooth movement. Then, TG and Vc were obtained for immunohistochemical staining and western blotting against CGRP.

Results

Immunohistochemical results indicated that CGRP positive neurons were located in the TG, and CGRP immunoreactive fibers were distributed in the Vc. Immunohistochemical semiquantitative analysis and western blotting analysis demonstrated that CGRP expression levels both in TG and Vc were elevated at 12 h, 1 d, 3 d, 5 d, and 7 d in the saline + force group. However, both small-dose and large-dose capsaicin could decrease CGRP expression in TG and Vc at 1 d and 3 d following experimental tooth movement, as compared with the saline + force group.

Conclusions

These results suggest that capsaicin could regulate CGRP expression in TG and Vc following experimental tooth movement in rats.

Substance P/Neurokinin 1 and Trigeminal System: A Possible Link to the Pathogenesis in Sudden Perinatal Deaths

Sudden demise of a healthy fetus or a neonate is a very tragic episode in the life of parents. These deaths have been a mystery since ages but still remain unexplained. This review proposes the involvement of trigeminal nerve, neurotransmitter substance P (SP), and its receptor neurokinin 1 (NK-1R) in regulation of cardiorespiratory control in fetuses and newborns. Anomalies and immaturity of neuroregulatory systems such as trigeminal system in medulla oblongata of brainstem may provide a possible mechanism of sudden perinatal deaths. Vulnerable infants are born with respiratory center immaturity which in combination with any stressor such as cold, hypoxia, and smoking may lead to cessation of breathing and ventilatory response. SP/NK-1R may be involved in regulating the ventilatory control in neonates while it is decreased in fetal and adult life in humans, and any alterations from these may lead to irreversible sleep apnea and fatal breathing, ultimately sudden death. This review summarizes the studies performed to highlight the expression of SP or NK-1R in sudden perinatal deaths and proposes the involvement of trigeminal ganglion along with its nerve and SP/NK-1R expression alteration as one of the possible pathophysiological underlying mechanism. However, further studies are required to explore the role of SP, NK-1R, and trigeminal system in the pathogenesis of sudden infant deaths, sudden intrauterine deaths, stillbirths, and sudden deaths later in human life.

CGRP and its receptors

The calcitonin gene-related peptide (CGRP) neuropeptide system is an important but still evolving target for migraine. A fundamental consideration for all of the current drugs in clinical trials and for ongoing development in this area is the identity, expression pattern, and function of CGRP receptors because this knowledge informs safety and efficacy considerations. In recent years, only the calcitonin receptor-like receptor/receptor activity-modifying protein 1 (RAMP1) complex, known as the CGRP receptor, has generally been considered relevant. However, CGRP is capable of activating multiple receptors and could have more than one endogenous receptor. The recent identification of the CGRP-responsive calcitonin receptor/RAMP1 complex (AMY₁ receptor - amylin subtype 1 receptor) in the trigeminovascular system warrants a deeper consideration of the molecular identity of CGRP receptor(s) involved in the pathophysiology, and thus potential treatment of migraine. This perspective considers some of the issues and implications.

CGRP receptor antagonist activity of olcegepant depends on the signalling pathway measured

Background Calcitonin gene-related peptide (CGRP) is a neuropeptide that acts in the trigeminovascular system and is believed to play an important role in migraine. CGRP activates two receptors that are both present in the trigeminovascular system; the CGRP receptor and the amylin 1 (AMY₁) receptor. CGRP receptor antagonists, including olcegepant (BIBN4096BS) and telcagepant (MK-0974), can treat migraine. This study aimed to determine the effectiveness of these antagonists at blocking CGRP receptor signalling in trigeminal ganglia (TG) neurons and transfected CGRP and AMY₁ receptors in Cos7 cells, to better understand their mechanism of action. Methods CGRP stimulation of four intracellular signalling molecules relevant to pain (cAMP, CREB, p38 and ERK) were examined in rat TG neurons and compared to transfected CGRP and AMY₁ receptors in Cos7 cells. Results In TG neurons, olcegepant displayed signal-specific differences in antagonism of CGRP responses. This effect was also evident in transfected Cos7 cells, where olcegepant blocked CREB phosphorylation more potently than expected at the AMY₁ receptor, suggesting that the affinity of this antagonist can be dependent on the signalling pathway activated. Conclusions CGRP receptor antagonist activity appears to be assay-dependent. Thus, these molecules may not be as selective for the CGRP receptor as commonly reported.

Induction of Migraine-Like Photophobic Behavior in Mice by Both Peripheral and Central CGRP Mechanisms

The neuropeptide calcitonin gene-related peptide (CGRP) is a key player in migraine. Although migraine can be treated using CGRP antagonists that act peripherally, the relevant sites of CGRP action remain unknown. To address the role of CGRP both within and outside the CNS, we used CGRP-induced light-aversive behavior in mice as a measure of migraine-associated photophobia. Peripheral (intraperitoneal) injection of CGRP resulted in light-aversive behavior in wild-type CD1 mice similar to aversion seen previously after central (intracerebroventricular) injection. The phenotype was also observed in C57BL/6J mice, although to a lesser degree and with more variability. After intraperitoneal CGRP, motility was decreased in the dark only, similar to motility changes after intracerebroventricular CGRP. In addition, as with intracerebroventricular CGRP, there was no general increase in anxiety as measured in an open-field assay after intraperitoneal CGRP. Importantly, two clinically effective migraine drugs, the 5-HT1B/D agonist sumatriptan and a CGRP-blocking monoclonal antibody, attenuated the peripheral CGRP-induced light aversion and motility behaviors. To begin to address the mechanism of peripheral CGRP action, we used transgenic CGRP-sensitized mice that have elevated levels of the CGRP receptor hRAMP1 subunit in nervous tissue (nestin/hRAMP1). Surprisingly, sensitivity to low light was not seen after intraperitoneal CGRP injection, but was seen after intracerebroventricular CGRP injection. These results suggest that CGRP can act in both the periphery and the brain by distinct mechanisms and that CGRP actions may be transmitted to the CNS via indirect sensitization of peripheral nerves.

SIGNIFICANCE STATEMENT

The neuropeptide calcitonin gene-related peptide (CGRP) is a central player in migraine pathogenesis, yet its site(s) of action remains unknown. Some preclinical studies have pointed to central sites in the brain and brainstem. However, a peripheral site of action is indicated by the ability of intravenous CGRP to trigger migraine in humans and the efficacy of CGRP receptor antagonists that evidently do no penetrate the CNS in effective amounts. Resolving this issue is particularly important given recent clinical trials showing that anti-CGRP monoclonal antibodies can reduce and even prevent migraine attacks. In this study, we report that CGRP can act in both the brain and the periphery of the mouse to cause migraine-like photophobia by apparently distinct mechanisms.

Peripheral Sensory Neurons Expressing Melanopsin Respond to Light

The ability of light to cause pain is paradoxical. The retina detects light but is devoid of nociceptors while the trigeminal sensory ganglia (TG) contain nociceptors but not photoreceptors. Melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) are thought to mediate light-induced pain but recent evidence raises the possibility of an alternative light responsive pathway independent of the retina and optic nerve. Here, we show that melanopsin is expressed in both human and mouse TG neurons. In mice, they represent 3% of small TG neurons that are preferentially localized in the ophthalmic branch of the trigeminal nerve and are likely nociceptive C fibers and high-threshold mechanoreceptor Aδ fibers based on a strong size-function association. These isolated neurons respond to blue light stimuli with a delayed onset and sustained firing, similar to the melanopsin-dependent intrinsic photosensitivity observed in ipRGCs. Mice with severe bilateral optic nerve crush exhibit no light-induced responses including behavioral light aversion until treated with nitroglycerin, an inducer of migraine in people and migraine-like symptoms in mice. With nitroglycerin, these same mice with optic nerve crush exhibit significant light aversion. Furthermore, this retained light aversion remains dependent on melanopsin-expressing neurons. Our results demonstrate a novel light-responsive neural function independent of the optic nerve that may originate in the peripheral nervous system to provide the first direct mechanism for an alternative light detection pathway that influences motivated behavior.

Involvement of CGRP receptors in retinal spreading depression

BACKGROUND

Cortical spreading depression (CSD) is a transient propagating excitation of synaptic activity followed by depression, which is implicated in migraine with aura and is regarded as the underlying cause of migraine. Calcitonin-gene related peptide (CGRP) receptors play a crucial role in mediating the magnitude of CSD in rat cortical slice. This study aimed to examine whether CGRP receptors are involved in retinal spreading depression (RSD) in chicks.

METHODS

Western blot was used for detection of calcitonin-receptor like receptor (CALCRL) and intrinsic optical imaging was used for pharmacological investigation.

RESULTS

We found that the key component of CGRP receptor, CALCRL, is expressed in the chick retina. Using an in vitro migraine RSD model, we demonstrated that BIBN4096, a potent antagonist for CGRP receptors, markedly reduced the magnitude of RSD induced by K(+), but also the propagation rate.

CONCLUSIONS

The data suggest that CGRP receptors mediate RSD propagation involving neuronal mechanism and approve that RSD is an efficient in vitro approach for assessing anti-migraine drugs targeting CGRP receptors.

Immunohistochemical localization of the calcitonin gene-related peptide binding site in the primate trigeminovascular system using functional antagonist antibodies

Calcitonin gene-related peptide (CGRP) is a potent vasodilator and a neuromodulator implicated in the pathophysiology of migraine. It binds to the extracellular domains of calcitonin receptor-like receptor (CLR) and receptor activity-modifying protein (RAMP) 1 that together form the CGRP receptor. Antagonist antibodies against CGRP and its binding site at the receptor are clinically effective in preventing migraine attacks. The blood-brain barrier penetration of these antagonist antibodies is limited, suggesting that a potential peripheral site of action is sufficient to prevent migraine attacks. To further understand the sites of CGRP-mediated signaling in migraine, we used immunohistochemical staining with recently developed antagonist antibodies specifically recognizing a fusion protein of the extracellular domains of RAMP1 and CLR that comprise the CGRP binding pocket at the CGRP receptor in monkey and man. We confirmed binding of the antagonist antibodies to human vascular smooth muscle cells (VSMCs) of dural meningeal arteries and neurons in the trigeminal ganglion, both of which are likely sites of action for therapeutic antibodies in migraine patients. We further used one of these antibodies for detailed mapping on cynomolgus monkey tissue and found antagonist antibody binding sites at multiple levels in the trigeminovascular system: in the dura mater VSMCs, in neurons and satellite glial cells in the trigeminal ganglion, and in neurons in the spinal trigeminal nucleus caudalis. These data reinforce and clarify our understanding of CGRP receptor localization in a pattern consistent with a role for CGRP receptors in trigeminal sensitization and migraine pathology.

Calcitonin Gene-Related Peptide Modulates Heat Nociception in the Human Brain - An fMRI Study in Healthy Volunteers

Background

Intravenous infusion of calcitonin-gene-related-peptide (CGRP) provokes headache and migraine in humans. Mechanisms underlying CGRP-induced headache are not fully clarified and it is unknown to what extent CGRP modulates nociceptive processing in the brain. To elucidate this we recorded blood-oxygenation-level-dependent (BOLD) signals in the brain by functional MRI after infusion of CGRP in a double-blind placebo-controlled crossover study of 27 healthy volunteers. BOLD-signals were recorded in response to noxious heat stimuli in the V1-area of the trigeminal nerve. In addition, we measured BOLD-signals after injection of sumatriptan (5-HT_1B/1D antagonist).

Results

Brain activation to noxious heat stimuli following CGRP infusion compared to baseline resulted in increased BOLD-signal in insula and brainstem, and decreased BOLD-signal in the caudate nuclei, thalamus and cingulate cortex. Sumatriptan injection reversed these changes.

Conclusion

The changes in BOLD-signals in the brain after CGRP infusion suggests that systemic CGRP modulates nociceptive transmission in the trigeminal pain pathways in response to noxious heat stimuli.

5-HT7 Receptors Are Not Involved in Neuropeptide Release in Primary Cultured Rat Trigeminal Ganglion Neurons

Migraine is a common but complex neurological disorder. Its precise mechanisms are not fully understood. Increasing indirect evidence indicates that 5-HT7 receptors may be involved; however, their role remains unknown. Our previous in vivo study showed that selective blockade of 5-HT7 receptors caused decreased serum levels of calcitonin gene-related peptide (CGRP) in the external jugular vein following electrical stimulation of the trigeminal ganglion (TG) in an animal model of migraine. In the present study, we used an in vitro model of cultured TG cells to further investigate whether 5-HT7 receptors are directly responsible for the release of CGRP and substance P from TG neurons. We stimulated rat primary cultured TG neurons with capsaicin or potassium chloride (KCl) to mimic neurogenic inflammation, resulting in release of CGRP and substance P. 5-HT7 receptors were abundantly expressed in TG neurons. Greater than 93 % of 5-HT7 receptor-positive neurons co-expressed CGRP and 56 % co-expressed substance P. Both the capsaicin- and KCl-induced release of CGRP and substance P were unaffected by pretreatment of cultured TG cells with the selective 5-HT7 receptor agonist AS19 and antagonist SB269970. This study demonstrates for the first time that 5-HT7 receptors are abundantly co-expressed with CGRP and substance P in rat primary TG neurons and suggests that they are not responsible for the release of CGRP and substance P from cultured TG neurons evoked by capsaicin or KCl.

Clinical study of cerebrospinal fluid neuropeptides in patients with primary trigeminal neuralgia

OBJECTIVES

To investigate the expression levels of calcitonin gene-related peptide (CGRP), substance P (SP), vasoactive intestinal peptide (VIP), and β-endorphin in the cerebrospinal fluid (CSF) and peripheral blood of patients with primary trigeminal neuralgia (TN).

PATIENTS AND METHODS

We included 20 patients with primary TN who underwent percutaneous radiofrequency thermocoagulation and collected four types of samples from all of them: sample A: CSF samples; sample B: peripheral blood samples; sample C: peripheral blood samples collected one day before the operation; sample D: peripheral blood samples withdrawn one day after the operation. Another 20 CSF samples of patients with nervous system disease or gynecological disease were collected as a control (sample E). Samples A and B were obtained at the same time. We also evaluated the expression of CGRP, SP, β-endorphin, and VIP by visual analog scale (VAS) scores one day before and one day after the operation. In addition, heart rate (HR) at baseline and at the time of sample collection, mean arterial pressure (MAP), and all side effects of the procedure were recorded.

RESULTS

Significance were found concerning about CGRP, SP, β-endorphin, and VIP in TN patients and the controls (P<0.001). The expression of CGRP, SP, and VIP in sample A was higher than that in sample E. However, the β-endorphin level in sample A was lower than that in sample E. There was a positive correlation between sample A and B regarding the expression of CGRP, SP, β-endorphin, and VIP (P<0. 01). There was no relationship between the time of disease onset and the expression of CGRP, SP, β-endorphin, and VIP in sample A and sample B (P>0.05). No difference was detected between the neuropeptides levels in samples B and C (P>0.05). Notably, VAS in sample D was significantly lower than that in sample C (P<0.01). Finally, there was no difference between the intraoperative HR and MAP values in the studied samples.

CONCLUSION

In primary TN patients, the blood levels of CGRP, SP, β-endorphin, and VIP were associated with those in CSF samples. There was a significant difference between the levels of the four neuropeptides in CSF and control samples. Our results also indicated that the levels of neuropeptides in blood samples can be tested for those in CSF. The disease onset and duration exerted insignificant effects on the production and release of CGRP, SP, β-endorphin, and VIP.

Kynurenic acid modulates experimentally induced inflammation in the trigeminal ganglion

Background

The trigeminal ganglion (TG) plays a central role in cranial pain. Administration of complete Freund’s adjuvant (CFA) into the temporomandibular joint (TMJ) elicits activation of TG. Kynurenic acid (KYNA) is an endogenous excitatory amino acid receptor blocker, which may have an anti-inflammatory effect. We hypothesize that KYNA may reduce CFA-induced activation within the TG.

Methods

A local inflammation was induced by administration of CFA into the TMJ in rats. KYNA and kynurenic acid amide 2 (KYNAA2) were intraperitoneally administered. We investigated changes of mitogen-activated protein kinases (MAPKs as ERK1/2, p38 and SAPK/JNK), NF-κB, CaMKII and DREAM, in addition to calcitonin gene-related peptide (CGRP) and its receptor components calcitonin receptor-like receptor (CLR) and receptor activity-modifying protein 1 (RAMP1) in the TG, with immunohistochemistry and Western blot at 2 and 10 days post-CFA injection.

Results

We showed CFA-induces increases in pERK1/2, pp38, CaMKII, NF-κB and DREAM immunohistochemistry after 2 and 10 days. KYNAA2 displayed stronger effects on MAPKs than KYNA. Increased expression of CaMKII, NF-κB and DREAM were found in the neurons. Western blot showed significantly increase in pERK expression at 10 days post-CFA, which decreased after 10 days of KYNA treatment. Two days post-CFA, a significantly increase in pp38 expression was found, which decreased after 2 days of KYNA and KYNAA2 treatment.

Conclusions

The CFA-induced inflammatory model for the TG activation provided a time-related expression of MAPK (pERK1/2, pp38) and NF-κB. It involves both the neuronal and glial activation, which points to possible neuron-glia interactions during this process. The administration of the endogenous NMDA-receptor antagonists, KYNA and its derivative KYNAA2, resulted in the inhibition of the induced signaling system of the TG, which further points the importance of the glutamate receptors in this mechanism.

Electronic supplementary material

The online version of this article (doi:10.1186/s10194-015-0581-x) contains supplementary material, which is available to authorized users.

CGRP mechanism antagonists and migraine management

Migraine is a complex disorder of the brain that is common and highly disabling. As understanding of the neural pathways has advanced, and it has become clear that the vascular hypothesis does not explain the disorder, new therapeutic avenues have arisen. One such target is calcitonin gene-related peptide (CGRP)-based mechanisms. CGRP is found within the trigeminovascular nociceptive system widely from the trigeminal ganglion to second-order and third-order neurons and in regulatory areas in the brainstem. Studies have shown CGRP is released during severe migraine attacks and the reversal of the attack with effective triptan treatment normalizes those levels. CGRP administration triggers migraine in patients, and CGRP receptor antagonists have been shown to abort migraine. Here, we review the current state of CGRP mechanism antagonist therapy as its research and development is increasing in migraine therapeutics. We discuss several recent trials, highlighting the evidence base behind these novel drugs, and their potential future contribution to migraine management.

Receptor Activity-Modifying Proteins (RAMPs): New Insights and Roles

It is now recognized that G protein-coupled receptors (GPCRs), once considered largely independent functional units, have a far more diverse molecular architecture. Receptor activity-modifying proteins (RAMPs) provide an important example of proteins that interact with GPCRs to modify their function. RAMPs are able to act as pharmacological switches and chaperones, and they can regulate signaling and/or trafficking in a receptor-dependent manner. This review covers recent discoveries in the RAMP field and summarizes the known GPCR partners and functions of RAMPs. We also discuss the first peptide-bound structures of RAMP-GPCR complexes, which give insight into the molecular mechanisms that enable RAMPs to alter the pharmacology and signaling of GPCRs.

The Journey to Establish CGRP as a Migraine Target: A Retrospective View

In this retrospective, Dr. Lars Edvinsson recounts early steps and milestones in our understanding of the neuropeptide calcitonin gene-related peptide (CGRP) in the trigeminovascular system and its role in migraine. The discovery of the presence and function of CGRP and other neuropeptides in the cerebral vasculature and its sensory innervation is described. He relates the seminal finding that CGRP is uniquely released during migraine and the journey to develop blockers of CGRP effects. Now, over 30 years since its discovery, CGRP has become the target for a number of promising novel treatments for migraine patients.

Modulation of inflammatory mediators in the trigeminal ganglion by botulinum neurotoxin type A: an organ culture study

Background

Onabotulinumtoxin type A (BoNT-A) has been found to reduce pain in chronic migraine. The aim of the present study was to ask if BoNT-A can interact directly on sensory mechanisms in the trigeminal ganglion (TG) using an organ culture method.

Methods

To induce inflammation, rat TGs were incubated for 24 hrs with either the mitogen MEK1/2 inhibitor U0126, BoNT-A or NaCl. After this the TGs were prepared for immunohistochemistry. Sections of the TG were then incubated with primary antibodies against CGRP (neuronal transmitter), iNOS (inflammatory marker), IL-1β (Interleukin 1β), SNAP-25 (synaptic vesicle docking protein) or SV2-A (Botulinum toxin receptor element).

Results

We report that CGRP, iNOS, IL-1β, SNAP-25 and SV2-A were observed in fresh TG with a differential distribution. Interestingly, NaCl organ culture of the TG resulted in enhanced expression of CGRP and SNAP-25 in neurons and iNOS in SGCs. Co-incubation with U0126 or BoNT-A retained the increased expression of SNAP-25, while it decreased the IL-1β immunoreactivity in neurons. The iNOS expression in SGCs returned to levels observed in fresh specimens. Moreover, we observed no alteration SV2-A expression in SGCs. Thus, the overall picture is that both U0126 and BoNT-A have the ability to modify the expression of certain molecules in the TG.

Conclusion

We hypothesize that chronic migraine might be associated with some degree of inflammation in the TG that could involve both neurons and SGCs. It is clinically well recognized that treatment with corticosteroids will reduce the symptoms of chronic migraine; however this remedy is associated with long-term side effects. Understanding the mechanisms involved in the expressional alterations may suggest novel ways to modify the changes and indicate novel therapeutics. The results of the present work illustrate one way by which BoNT-A may modify these expressional alterations.

Localization of CGRP receptor components and receptor binding sites in rhesus monkey brainstem: A detailed study using in situ hybridization, immunofluorescence, and autoradiography

Functional imaging studies have revealed that certain brainstem areas are activated during migraine attacks. The neuropeptide calcitonin gene-related peptide (CGRP) is associated with activation of the trigeminovascular system and transmission of nociceptive information and plays a key role in migraine pathophysiology. Therefore, to elucidate the role of CGRP, it is critical to identify the regions within the brainstem that process CGRP signaling. In situ hybridization and immunofluorescence were performed to detect mRNA expression and define cellular localization of calcitonin receptor-like receptor (CLR) and receptor activity-modifying protein 1 (RAMP1), respectively. To define CGRP receptor binding sites, in vitro autoradiography was performed with [(3)H]MK-3207 (a CGRP receptor antagonist). CLR and RAMP1 mRNA and protein expression were detected in the pineal gland, medial mammillary nucleus, median eminence, infundibular stem, periaqueductal gray, area postrema, pontine raphe nucleus, gracile nucleus, spinal trigeminal nucleus, and spinal cord. RAMP1 mRNA expression was also detected in the posterior hypothalamic area, trochlear nucleus, dorsal raphe nucleus, medial lemniscus, pontine nuclei, vagus nerve, inferior olive, abducens nucleus, and motor trigeminal nucleus; protein coexpression of CLR and RAMP1 was observed in these areas via immunofluorescence. [(3)H]MK-3207 showed high binding densities concordant with mRNA and protein expression. The present study suggests that several regions in the brainstem may be involved in CGRP signaling. Interestingly, we found receptor expression and antagonist binding in some areas that are not protected by the blood-brain barrier, which suggests that drugs inhibiting CGRP signaling may not be able to penetrate the central nervous system to antagonize receptors in these brain regions.

Migraine and neuropeptides

Migraine is a common disabling neurovascular primary headache disorder. The pathomechanism is not clear, but extensive preclinical and clinical studies are ongoing. The structural basis of the leading hypothesis is the trigeminovascular system, which includes the trigeminal ganglion, the meningeal vasculature, and the distinct nuclei of the brainstem, the thalamus and the somatosensory cortex. This review covers the effects of sensory (calcitonin gene-related peptide, pituitary adenylate cyclase-activating polypeptide and substance P), sympathetic (neuropeptide Y) and parasympathetic (vasoactive intestinal peptide) migraine-related neuropeptides and the functions of somatostatin, nociceptin and the orexins in the trigeminovascular system. These neuropeptides may take part in neurogenic inflammation (plasma protein extravasation and vasodilatation) of the intracranial vasculature and peripheral and central sensitization of the trigeminal system. The results of human clinical studies are discussed with regard to the alterations in these neuropeptides in the plasma, saliva and cerebrospinal fluid during or between migraine attacks, and the therapeutic possibilities involving migraine-related neuropeptides in the acute and prophylactic treatment of migraine headache are surveyed.

Treatment of Chronic Migraine with OnabotulinumtoxinA: Mode of Action, Efficacy and Safety

Background: Chronic migraine is a common, highly disabling, underdiagnosed and undertreated entity of migraine. It affects 0.9%–2.2% of the general adult population. The present paper overviews the preclinical and clinical data regarding the therapeutic effect of onabotulinumtoxinA in chronic migraineurs. Methods: A literature search was conducted in the database of PubMed up to 20 May 2015 for articles related to the pathomechanism of chronic migraine, the mode of action, and the efficacy, safety and tolerability of onabotulinumtoxinA for the preventive treatment of chronic migraine. Results: The pathomechanism of chronic migraine has not been fully elucidated. The mode of action of onabotulinumtoxinA in the treatment of chronic migraine is suggested to be related to the inhibition of the release of calcitonin gene-related peptide and substance P in the trigeminovascular system. Randomized clinical trials demonstrated that long-term onabotulinumtoxinA fixed-site and fixed-dose (155–195 U) intramuscular injection therapy was effective and well tolerated for the prophylactic treatment of chronic migraine. Conclusions: Chronic migraine is a highly devastating entity of migraine. Its exact pathomechanism is unrevealed. Two-third of chronic migraineurs do not receive proper preventive medication. Recent clinical studies revealed that onabotulinumtoxinA was an efficacious and safe treatment for chronic migraine.

CGRP as a neuropeptide in migraine: lessons from mice

Migraine is a neurological disorder that is far more than just a bad headache. A hallmark of migraine is altered sensory perception. A likely contributor to this altered perception is the neuropeptide calcitonin gene-related peptide (CGRP). Over the past decade, CGRP has become firmly established as a key player in migraine. Although the mechanisms and sites of action by which CGRP might trigger migraine remain speculative, recent advances with mouse models provide some hints. This brief review focuses on how CGRP might act as both a central and peripheral neuromodulator to contribute to the migraine-like symptom of light aversive behaviour in mice.

Hydrogen sulfide determines HNO-induced stimulation of trigeminal afferents

Endogenous NO and hydrogen sulfide form HNO, which causes CGRP release via TRPA1 channel activation in sensory nerves. In the present study, stimulation of intact trigeminal afferent neuron preparations with NO donors, Na2S or both was analyzed by measuring CGRP release as an index of mass activation. Combined stimulation was able to activate all parts of the trigeminal system and acted synergistic compared to stimulation with both substances alone. To investigate the contribution of both substances, we varied their ratio and tracked intracellular calcium in isolated neurons. Our results demonstrate that hydrogen sulfide is the rate-limiting factor for HNO formation. CGRP has a key role in migraine pathophysiology and HNO formation at all sites of the trigeminal system should be considered for this novel means of activation.

Meningeal blood flow is controlled by H2 S-NO crosstalk activating a HNO-TRPA1-CGRP signalling pathway

BACKGROUND AND PURPOSE

Meningeal blood flow is controlled by CGRP released from trigeminal afferents and NO mainly produced in arterial endothelium. The vasodilator effect of NO may be due to the NO-derived compound, nitroxyl (HNO), generated through reaction with endogenous H2 S. We investigated the involvement of HNO in CGRP release and meningeal blood flow.

EXPERIMENTAL APPROACH

Blood flow in exposed dura mater of rats was recorded by laser Doppler flowmetry. CGRP release from the dura mater in the hemisected rat head was quantified using an elisa. NO and H2 S were localized histochemically with specific sensors.

KEY RESULTS

Topical administration of the NO donor diethylamine-NONOate increased meningeal blood flow by 30%. Pretreatment with oxamic acid, an inhibitor of H2 S synthesis, reduced this effect. Administration of Na2 S increased blood flow by 20%, an effect abolished by the CGRP receptor antagonist CGRP8-37 or the TRPA1 channel antagonist HC030031 and reduced when endogenous NO synthesis was blocked. Na2 S dose-dependently increased CGRP release two- to threefold. Co-administration of diethylamine-NONOate facilitated CGRP release, while inhibition of endogenous NO or H2 S synthesis lowered basal CGRP release. NO and H2 S were mainly localized in arterial vessels, HNO additionally in nerve fibre bundles. HNO staining was lost after treatment with L-NMMA and oxamic acid.

CONCLUSIONS AND IMPLICATIONS

NO and H2 S cooperatively increased meningeal blood flow by forming HNO, which activated TRPA1 cation channels in trigeminal fibres, inducing CGRP release. This HNO-TRPA1-CGRP signalling pathway may be relevant to the pathophysiology of headaches.

CGRP receptor antagonists and antibodies against CGRP and its receptor in migraine treatment

Recently developed calcitonin gene-related peptide (CGRP) receptor antagonistic molecules have shown promising results in clinical trials for acute treatment of migraine attacks. Drugs from the gepant class of CGRP receptor antagonists are effective and do not cause vasoconstriction, one of the major limitations in the use of triptans. However their use had to be discontinued because of risk of liver toxicity after continuous exposure. As an alternative approach to block CGRP transmission, fully humanized monoclonal antibodies towards CGRP and the CGRP receptor have been developed for treatment of chronic migraine (attacks >15 days/month). Initial results from phase I and II clinical trials have revealed promising results with minimal side effects and significant relief from chronic migraine as compared with placebo. The effectiveness of these various molecules raises the question of where is the target site(s) for antimigraine action. The gepants are small molecules that can partially pass the blood-brain barrier (BBB) and therefore, might have effects in the CNS. However, antibodies are large molecules and have limited possibility to pass the BBB, thus effectively excluding them from having a major site of action within the CNS. It is suggested that the antimigraine site should reside in areas not limited by the BBB such as intra- and extracranial vessels, dural mast cells and the trigeminal system. In order to clarify this topic and surrounding questions, it is important to understand the localization of CGRP and the CGRP receptor components in these possible sites of migraine-related regions and their relation to the BBB.

A second trigeminal CGRP receptor: function and expression of the AMY1 receptor

Objective

The trigeminovascular system plays a central role in migraine, a condition in need of new treatments. The neuropeptide, calcitonin gene-related peptide (CGRP), is proposed as causative in migraine and is the subject of intensive drug discovery efforts. This study explores the expression and functionality of two CGRP receptor candidates in the sensory trigeminal system.

Methods

Receptor expression was determined using Taqman G protein-coupled receptor arrays and immunohistochemistry in trigeminal ganglia (TG) and the spinal trigeminal complex of the brainstem in rat and human. Receptor pharmacology was quantified using sensitive signaling assays in primary rat TG neurons.

Results

mRNA and histological expression analysis in rat and human samples revealed the presence of two CGRP-responsive receptors (AMY₁: calcitonin receptor/receptor activity-modifying protein 1 [RAMP1]) and the CGRP receptor (calcitonin receptor-like receptor/RAMP1). In support of this finding, quantification of agonist and antagonist potencies revealed a dual population of functional CGRP-responsive receptors in primary rat TG neurons.

Interpretation

The unexpected presence of a functional non-canonical CGRP receptor (AMY₁) at neural sites important for craniofacial pain has important implications for targeting the CGRP axis in migraine.

Periaqueductal gray calcitonin gene-related peptide modulates trigeminovascular neurons

Background

Calcitonin gene-related peptide (CGRP) receptor antagonism is an approach to migraine therapy. The locus of action of antimigraine treatment is not resolved. The objective was to investigate CGRP receptors in the ventrolateral periaqueductal gray (vlPAG) involved in the modulation of trigeminovascular nociception by descending influences on neurotransmission.

Methods

The presence of calcitonin receptor-like receptor (CLR) and receptor activity modifying protein 1 (RAMP1), which form functional CGRP receptors, was investigated. CGRP and its receptor antagonists, olcegepant and CGRP ( 8 – 37 ), were microinjected into the vlPAG while changes of neural responses in the trigeminocervical complex (TCC) were monitored.

Results

Immunoreactivity indicated the presence of functional CGRP receptor components in the vlPAG and adjacent mesencephalic trigeminal nucleus. Inhibition of TCC responses to stimulation of dural afferents and ophthalmic cutaneous receptive fields after microinjection of bicuculline into vlPAG indicated a connection between the vlPAG and TCC neurons. CGRP facilitated these TCC responses, whereas olcegepant and CGRP ( 8 – 37 ) decreased them.

Conclusions

CGRP and its receptor antagonists act on neurons in the region of vlPAG to influence nociceptive transmission in the TCC. This suggests CGRP receptor antagonists may act at loci outside of the TCC and reinforces the concept of migraine as a disorder of the brain.

Peripheral scaffolding and signaling pathways in inflammatory pain

Peripheral injury precipitates the release and accumulation of extracellular molecules at the site of injury. Although these molecules exist in various forms, they activate specific receptor classes expressed on primary afferent neurons to mediate cellular and behavioral responses to both nonpainful and painful stimuli. These inflammatory mediators and subsequent receptor-mediated effects exist to warn an organism of future injury, thereby resulting in protection and rehabilitation of the wounded tissue. In this chapter, inflammatory mediators, their target receptor classes, and downstream signaling pathways are identified and discussed within the context of inflammatory hyperalgesia. Furthermore, scaffolding mechanisms that exist to support inflammatory signaling in peripheral afferent neuronal tissues specifically are identified and discussed. Together, the mediators, pathways, and scaffolding mechanisms involved in inflammatory hyperalgesia provide a unique knowledge point from which new therapeutic targets can be understood.

Microglial content-dependent inhibitory effects of calcitonin gene-related peptide (CGRP) on murine retroviral infection of glial cells

C57BL/6 (B6) mice develop peripheral neuropathy post-LP-BM5 infection, a murine model of HIV-1 infection, along with the up-regulation of select spinal cord cytokines. We investigated if calcitonin gene-related peptide (CGRP) contributed to the development of peripheral neuropathy by stimulating glial responses. An increased expression of lumbar spinal cord CGRP was observed in vivo, post-LP-BM5 infection. Consequently, in vitro CGRP co-treatments led to a microglial content-dependent attenuation of viral loads in spinal cord mixed glia infected with selected doses of LP-BM5. This inhibition was neither caused by the loss of glia nor induced via the direct inhibition of LP-BM5 by CGRP.

New treatments for headache

Migraine and cluster headache are primary headache disorders commonly encountered in clinical practice. Despite the profound disability caused by these primary headache disorders, available acute and preventive treatment options are limited. Recent understanding of headache pathophysiology has led to the development of new drug formulations and novel drug targets that are extremely promising. This article will highlight several of the new treatments that are currently under investigation including novel delivery mechanisms of already existing medications, calcitonin gene-related peptide (CGRP) receptor antagonists, antibodies to CGRP and its receptor, serotonin receptor agonists, transient receptor potential vanilloid receptor modulators, orexin receptor antagonists, glial cell modulators, and neuromodulation. If data is supportive, these therapies will be welcome additions to the headache specialist's armamentarium.

Distribution of gastrin-releasing peptide in the rat trigeminal and spinal somatosensory systems

Gastrin-releasing peptide (GRP) has recently been identified as an itch-specific neuropeptide in the spinal sensory system in mice, but there are no reports of the expression and distribution of GRP in the trigeminal sensory system in mammals. We characterized and compared GRP-immunoreactive (ir) neurons in the trigeminal ganglion (TG) with those in the rat spinal dorsal root ganglion (DRG). GRP immunoreactivity was expressed in 12% of TG and 6% of DRG neurons and was restricted to the small- and medium-sized type cells. In both the TG and DRG, many GRP-ir neurons also expressed substance P and calcitonin gene-related peptide, but not isolectin B4 . The different proportions of GRP and transient receptor potential vanilloid 1 double-positive neurons in the TG and DRG imply that itch sensations via the TG and DRG pathways are transmitted through distinct mechanisms. The distribution of the axon terminals of GRP-ir primary afferents and their synaptic connectivity with the rat trigeminal sensory nuclei and spinal dorsal horn were investigated by using light and electron microscopic histochemistry. Although GRP-ir fibers were rarely observed in the trigeminal sensory nucleus principalis, oralis, and interpolaris, they were predominant in the superficial layers of the trigeminal sensory nucleus caudalis (Vc), similar to the spinal dorsal horn. Ultrastructural analysis revealed that GRP-ir terminals contained clear microvesicles and large dense-cored vesicles, and formed asymmetric synaptic contacts with a few dendrites in the Vc and spinal dorsal horn. These results suggest that GRP-dependent orofacial and spinal pruriceptive inputs are processed mainly in the superficial laminae of the Vc and spinal dorsal horn.

Activation of TLR-4 to produce tumour necrosis factor-α in neuropathic pain caused by paclitaxel

BACKGROUND

Neuropathic pain is a common complication of treatment with the anti-neoplastic drug paclitaxel. Animal studies suggest neuroinflammation and transient receptor potential channels TRPA1 and TRPV4 are involved in the pathogenesis of pain in this condition. However, how neuroinflammation and TRPA1 and TRPV4 are linked to cause pain in paclitaxel-treated animals is not known.

METHODS

Paclitaxel-induced pain was modelled by IP injection of paclitaxel (16 mg/kg) once a week for 5 weeks. The role of toll-like receptor 4 (TLR-4) in tumour necrosis factor-α (TNF-α) production and the effect of TNF-α on the expression of TRPA1 and TRPV4 were evaluated in vitro and in vivo. TNF-α signalling in dorsal root ganglion (DRG) was blocked by expressing soluble TNF receptor I (TNFsR) from a herpes simplex virus (HSV)-based vector (vTNFsR).

RESULTS

Paclitaxel treatment increased the expression and release of TNF-α in satellite glial cells and increased the expression of TRPA1 and TRPV4 in DRG neurons in animals. In vitro, paclitaxel enhanced the expression and release of TNF-α in enriched primary satellite glial cells, an effect that was blocked by an inhibitor of TLR-4. Direct application of TNF-α to primary DRG neurons in culture up-regulated the expression of TRPA1 and TRPV4. In vivo, vector-mediated TNFsR release from DRG neurons reduced paclitaxel-induced up-regulation of TRPA1 and TRPV4 expression and prevented paclitaxel-induced pain.

CONCLUSION

These results suggest that paclitaxel activation of TLR-4 to cause release of TNF-α from satellite glial cells increases the expression of TRPA1 and TRPV4 in DRG neurons to cause neuropathic pain.

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.

Calcitonin gene-related peptide: physiology and pathophysiology

Calcitonin gene-related peptide (CGRP) is a 37-amino acid neuropeptide. Discovered 30 years ago, it is produced as a consequence of alternative RNA processing of the calcitonin gene. CGRP has two major forms (α and β). It belongs to a group of peptides that all act on an unusual receptor family. These receptors consist of calcitonin receptor-like receptor (CLR) linked to an essential receptor activity modifying protein (RAMP) that is necessary for full functionality. CGRP is a highly potent vasodilator and, partly as a consequence, possesses protective mechanisms that are important for physiological and pathological conditions involving the cardiovascular system and wound healing. CGRP is primarily released from sensory nerves and thus is implicated in pain pathways. The proven ability of CGRP antagonists to alleviate migraine has been of most interest in terms of drug development, and knowledge to date concerning this potential therapeutic area is discussed. Other areas covered, where there is less information known on CGRP, include arthritis, skin conditions, diabetes, and obesity. It is concluded that CGRP is an important peptide in mammalian biology, but it is too early at present to know if new medicines for disease treatment will emerge from our knowledge concerning this molecule.

Role of Ser102 and Ser104 as regulators of cGMP hydrolysis by PDE5A

Background

Phosphodiesterases (PDEs) cleave phosphodiester bonds in cyclic nucleotides and play diverse roles in cell biology. PDE5A is a cytoplasmic phosphodiesterase which specifically degrades cyclic guanosine monophosphate (cGMP), a cell signaling molecule that plays important roles in neuronal signaling and vascular smooth muscle contraction. Inhibition of PDE5A induces headache resembling migraine headaches.

Aim

To test the hypothesis that Ser102 and Ser104 in PDE5A and/or their phosphoserine derivatives 1) regulate the intracellular localization and/or activity of PDE5A, and 2) modulate the interaction between PDE5A and pharmaceutical reagents in clinical or pre-clinical use for migraine headaches and other types of vascular dysfunction.

Methods

Wild type PDE5A or PDE5A with substitution mutations (Ser102Ala, Ser104Ala or Ser102Ala/Ser104Ala) were overexpressed in SK-N-AS neuroblastoma cells as C-terminal fusions with green fluorescent protein. Transfected cells were treated with sildenafil, cilostazol, glyceryl trinitrate, calcitonin gene-related peptide (CGRP) or sumatriptan. PDE5A-GFP fusion proteins were localized in fixed cells by immunofluorescence and PDE activity was quantified in cell extracts by standard in vitro assay using [³H] cGMP.

Results

The intracellular distribution of wild-type, single and double mutant PDE5A was similar and was not altered by exposure to sildenafil, cilostazol, glyceryl trinitrate, calcitonin gene-related peptide (CGRP) or sumatriptan. PDE5 activity was similar for wild type, Ser102Ala and Ser104Ala PDE5A, but activity of the Ser102Ala/Ser104Ala mutant was approximately two-fold higher than wild type. Double mutant Ser102Ala/Ser104Ala migrated as a single band on a native acrylamide gel, while wild-type and single mutant PDE5A migrated as a doublet.

Interpretation

Ser102 and Ser104 may influence the conformational flexibility of PDE5A, which may in turn influence phosphorylation status, allosteric regulation by cGMP or other as yet unknown regulatory mechanisms for PDE5A. PDE5A activation could be important in reversal of migraine-like headache.

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

Objective

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

Methods

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

Results

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

Conclusion

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

Localization of CGRP receptor components, CGRP, and receptor binding sites in human and rhesus cerebellar cortex

The cerebellum is classically considered to be mainly involved in motor processing, but studies have suggested several other functions, including pain processing. Calcitonin-gene-related peptide (CGRP) is a neuropeptide involved in migraine pathology, where there is elevated release of CGRP during migraine attacks and CGRP receptor antagonists have antimigraine efficacy. In the present study, we examined CGRP and CGRP receptor binding sites and protein expression in primate cerebellar cortex. Additionally, mRNA expression of the CGRP receptor components, calcitonin receptor-like receptor (CLR) and receptor activity modifying protein 1 (RAMP1), was examined. In addition, expression of procalcitonin was studied. We observed high [(3)H]MK-3207 (CGRP receptor antagonist) binding densities in the molecular layer of rhesus cerebellar cortex; however, due to the limit of resolution of the autoradiographic image the exact cellular localization could not be determined. Similarly, [(125)I]CGRP binding was observed in the molecular layer and Purkinje cell layer of human cerebellum. CLR and RAMP1 mRNA was expressed within the Purkinje cell layer and some expression was found in the molecular layer. Immunofluorescence revealed expression of CGRP, CLR, and RAMP1 in the Purkinje cells and in cells in the molecular layer. Procalcitonin was found in the same localization. Recent research in the biology of cerebellum indicates that it may have a role in nociception. For the first time we have identified CGRP and CGRP receptor binding sites together with CGRP receptor expression through protein and mRNA localization in primate cerebellar cortex. These results point toward a functional role of CGRP in cerebellum. Further efforts are needed to evaluate this.