A potent and selective calcitonin gene-related peptide (CGRP) receptor antagonist, MK-8825, inhibits responses to nociceptive trigeminal activation: Role of CGRP in orofacial pain

Chronic orofacial pain and pharmacological management-a clinical guide

Orofacial pain is a widespread health concern that significantly hinders an individual's capacity to engage in daily activities. This type of pain can be classified into three main categories: nociceptive pain, neuropathic pain, and nociplastic pain. Each category involves different mechanisms and requires specific treatment approaches. For optimal treatment of orofacial pain disorders, a multidisciplinary pain management approach is essential. This approach should integrate both nonpharmacological and pharmacological modalities to address the diverse underlying causes and manifestations of pain. In this review, we focus on the current evidence and advancements in the pharmacological management of chronic orofacial pain. We explored the effectiveness of different medications, their mechanisms of action, and their role within a comprehensive pain management plan.

Pathophysiology and Management of Refractory Trigeminal Neuralgia

PURPOSE OF REVIEW

Discuss the current understanding of the pathophysiology and management of refractory trigeminal neuralgia (TN). This includes a discussion on why TN can recur after microvascular decompression and a discussion on "outside of the box" options when both first- and second-line management strategies have been exhausted.

RECENT FINDINGS

This review discusses second- and third-line oral medication options, botulinum toxin A, repeat microvascular decompression, repeat ablative procedures, internal neurolysis, trigeminal branch blockade, and neuromodulation using TMS or peripheral stimulation. Additional management for chronic neuropathic facial pain such as deep brain stimulation, motor cortex stimulation, and focused ultrasound thalamotomy are also discussed, though evidence in trigeminal neuralgia is limited. Treatment of recurrent TN despite multiple surgeries can be challenging, and multiple minimally invasive and more invasive management options have been reported in small studies and case reports. Further studies are needed to determine an optimal stepwise approach.

Elucidation of neuronal activity in mouse models of temporomandibular joint injury and inflammation by in vivo GCaMP Ca 2+ imaging of intact trigeminal ganglion neurons

ABSTRACT

Patients with temporomandibular disorders (TMDs) typically experience facial pain and discomfort or tenderness in the temporomandibular joint (TMJ), causing disability in daily life. Unfortunately, existing treatments for TMD are not always effective, creating a need for more advanced, mechanism-based therapies. In this study, we used in vivo GCaMP3 Ca 2+ imaging of intact trigeminal ganglia (TG) to characterize functional activity of the TG neurons in vivo, specifically in mouse models of TMJ injury and inflammation. This system allows us to observe neuronal activity in intact anatomical, physiological, and clinical conditions and to assess neuronal function and response to various stimuli. We observed a significant increase in spontaneously and transiently activated neurons responding to mechanical, thermal, and chemical stimuli in the TG of mice with TMJ injection of complete Freund adjuvant or with forced mouth opening (FMO). An inhibitor of the calcitonin gene-related peptide receptor significantly attenuated FMO-induced facial hypersensitivity. In addition, we confirmed the attenuating effect of calcitonin gene-related peptide antagonist on FMO-induced sensitization by in vivo GCaMP3 Ca 2+ imaging of intact TG. Our results contribute to unraveling the role and activity of TG neurons in the TMJ pain, bringing us closer to understanding the pathophysiological processes underlying TMJ pain after TMJ injury. Our study also illustrates the utility of in vivo GCaMP3 Ca 2+ imaging of intact TG for studies aimed at developing more targeted and effective treatments for TMJ pain.

Dietary supplementation with grape seed extract from Vitus vinifera prevents suppression of GABAergic protein expression in female Sprague Dawley trigeminal ganglion in a model of chronic temporomandibular joint disorder

OBJECTIVE

To investigate cellular changes in protein expression in the trigeminal ganglion in an established preclinical chronic model of temporomandibular joint disorder (TMD) in response to grape seed extract (GSE) supplementation based on its beneficial use in preclinical chronic orofacial pain models.

DESIGN

Three experimental conditions included female Sprague-Dawley rats as naïve controls, and animals subjected to neck muscle inflammation and prolonged jaw opening with and without daily supplementation of GSE in the drinking water prior to inflammation. Changes were evaluated in mechanical sensitivity to von Frey filaments and protein expression in the trigeminal ganglion of animals 14 days post jaw opening.

RESULTS

Calcitonin-gene related peptide and protein kinase A, proteins positively associated with peripheral sensitization and enhanced nociception, did not show elevated expression at day 14 in the model compared to naïve or GSE supplemented animals. However, neuronal levels of glutamate decarboxylase (GAD) 65/67, which are enzymes responsible for the synthesis of the inhibitory neurotransmitter GABA that functions to suppress neuronal excitability, were significantly decreased on day 14 post jaw opening. Similarly, a significant decrease in neuronal expression of the GABA receptor subunits GABAB1 and GABAB2, but not GABAA, was observed in the TMD model. Importantly, GSE prevented suppression of GAD 65/67 and GABAB subunits, maintaining levels similar to naïve animals.

CONCLUSION

Results from our study provide evidence of the downregulation of inhibitory GABAergic proteins in trigeminal ganglion neurons in a preclinical chronic TMD model and the benefits of GSE supplementation in preventing their suppression and maintaining normal levels.

Temporomandibular Disorders, Bruxism and Headaches

Temporomandibular disorders (TMDs) and headache disorders are highly prevalent in the population. TMDs can present headache symptoms as a secondary headache and, in addition, be comorbid with primary headache disorders. This overlap has significant clinical implications for which it is essential for the physician to be aware, and they should screen for the potential presence of TMDs in a headache patient. Bruxism is a parafunctional behavior also prevalent in the population which has a role in TMDs and may influence headache symptomatology, but it is still necessary to clarify this relationship.

Expression of CGRP in the Trigeminal Ganglion and Its Effect on the Polarization of Macrophages in Rats with Temporomandibular Arthritis

Calcitonin gene-related peptide (CGRP) is synthesized and secreted by trigeminal ganglion neurons, and is a key neuropeptide involved in pain and immune regulation. This study investigates the expression of CGRP in the trigeminal ganglion (TG) and its regulatory role in the polarization of macrophages in rats with temporomandibular arthritis. A rat model of temporomandibular arthritis was established using CFA. Pain behavior was then observed. Temporomandibular joint (TMJ) and the TG were collected, and immunohistochemistry, immunofluorescence (IF) staining, and RT-qPCR were used to examine the expression of CGRP and macrophage-related factors. To investigate the impact of CGRP on macrophage polarization, both CGRP and its antagonist, CGRP 8-37, were separately administered directly within the TG. Statistical analysis revealed that within 24 h of inducing temporomandibular arthritis using CFA, there was a significant surge in CD86 positive macrophages within the ganglion. These macrophages peaked on the 7th day before beginning their decline. In this context, it's noteworthy that administering CGRP to the trigeminal ganglion can prompt these macrophages to adopt the M2 phenotype. Intriguingly, this study demonstrates that injecting the CGRP receptor antagonist (CGRP 8-37) to the ganglion counteracts this shift towards the M2 phenotype. Supporting these in vivo observations, we found that in vitro, CGRP indeed fosters the M2-type polarization of macrophages. CGRP can facilitate the conversion of macrophages into the M2 phenotype. The phenotypic alterations of macrophages within the TG could be instrumental in initiating and further driving the progression of TMJ disorders.

Method for cryopreservation of trigeminal ganglion for establishing primary cultures of neurons and glia

BACKGROUND

Primary neuronal cultures are used to elucidate cellular and molecular mechanisms involved in disease pathology and modulation by pharmaceuticals and nutraceuticals, and to identify novel therapeutic targets. However, preparation of primary neuronal cultures from rodent embryos is labor-intensive, and it can be difficult to produce high-quality consistent cultures. To overcome these issues, cryopreservation can be used to obtain standardized, high-quality stocks of neuronal cultures.

NEW METHOD

In this study, we present a simplified cryopreservation method for rodent primary trigeminal ganglion neurons and glia from Sprague-Dawley neonates, using a 90:10 (v/v) fetal bovine serum/dimethyl sulfoxide cell freezing medium.

RESULTS

Cryopreserved trigeminal ganglion cells stored for up to one year in liquid nitrogen exhibited similar neuronal and glial cell morphology to fresh cultures and retained high cell viability. Proteins implicated in inflammation and pain signaling were expressed in agreement with the reported subcellular localization. Additionally, both neurons and glial cells exhibited an increase in intracellular calcium levels in response to a depolarizing stimulus. Cryopreserved cells were also transiently transfected with reporter genes.

COMPARISON WITH EXISTING METHODS

Our method is simple, does not require special reagents or equipment, will save time and money, increase flexibility in study design, and produce consistent cultures.

CONCLUSIONS

This method for the preparation and cryopreservation of trigeminal ganglia results in primary cultures of neurons and glia similar in viability and morphology to fresh preparations that could be utilized for biochemical, cellular, and molecular studies, increase reproducibility, and save laboratory resources.

Elucidation of neuronal activity in mouse models of TMJ injury by in vivo GCaMP Ca 2+ imaging of intact trigeminal ganglion neurons

Patients with temporomandibular disorders (TMD) typically experience facial pain and discomfort or tenderness in the temporomandibular joint (TMJ), causing disability in daily life. Unfortunately, existing treatments for TMD are not always effective, creating a need for more advanced, mechanism-based therapies. In this study, we used in vivo GCaMP3 Ca 2+ imaging of intact trigeminal ganglia (TG) to characterize functional activity of the TG neurons in vivo , specifically in TMJ animal models. This system allows us to observe neuronal activity in intact anatomical, physiological, and clinical conditions and to assess neuronal function and response to various stimuli. We observed a significant increase in spontaneously and transiently activated neurons responding to mechanical, thermal, and chemical stimuli in the TG of forced mouth open (FMO) mice. An inhibitor of the CGRP receptor significantly attenuated FMO-induced facial hypersensitivity. In addition, we confirmed the attenuating effect of CGRP antagonist on FMO-induced sensitization by in vivo GCaMP3 Ca 2+ imaging of intact TG. Our results contribute to unraveling the role and activity of TG neurons in the TMJ pain animal models of TMD, bringing us closer understanding the pathophysiological processes underlying TMD. Our study also illustrates the utility of in vivo GCaMP3 Ca 2+ imaging of intact TG for studies aimed at developing more targeted and effective treatments for TMD.

An Update on Temporomandibular Disorders (TMDs) and Headache

PURPOSE OF REVIEW

To provide an overview and highlight recent updates in temporomandibular disorders (TMDs) and their comorbidity with headache disorders regarding pathophysiology and management.

RECENT FINDINGS

In the last decade, there have been great advancements in the understanding of TMDs and their relationship with neurovascular pains such as headaches. Understanding of TMDs is necessary for the context of its comorbidity with primary headache disorders. The literature regarding management of these comorbidities is scarce but points to combination therapy including pharmacological and non-pharmacological approaches to optimize management. The use of CGRP receptor-targeted monoclonal antibodies or CGRP receptor antagonists should be explored for the management of chronic TMDs. It could also be used as a novel monotherapy or in combination with non-pharmacological approaches for TMDs' comorbidity with headache, particularly migraine. Research is needed to support evidence-based management protocols. A team involving neurology (headache medicine) and dentistry (orofacial pain) is critical for optimal management.

Aging exaggerates pulpal pain sensation by increasing the expression levels of nociceptive neuropeptides and inflammatory cytokines

BACKGROUND

Dental pain is a main clinical problem in the elderly population and its assessment and treatment make special challenges for health care services. However, the age-induced alteration in dental pain perception and the underlying molecular mechanism(s) has not yet been fully clarified.

METHODS

Here, the effect of aging on nociceptive behaviors following inflammatory dental pulp pain was evaluated. Since prostaglandins, nociceptive neuropeptides, and inflammatory cytokines have critical roles in the development of aging as well as pain signaling, the expression levels of COX-2, CGRP, IL-1β, IL-6, TNF-α and its converting enzyme TACE were assessed in the trigeminal ganglion of young and aged rats with dental pain. Dental pulp pain was induced by intradental application of capsaicin (100 μg). The immunofluorescence (COX-2 and CGRP) and western blot techniques were used.

RESULTS

The data showed that aged animals have different pattern of pain. So that, the mean of nociceptive scores was significantly greater in aged rats at 10 and 15 min after capsaicin injection. In aged rats, dental pain was persisting over 7 h, while it was disappeared at 300 min in young rats. Molecular data showed that dental pain significantly increased the expression of COX-2, CGRP, IL-1β, IL-6, TNF-α and TACE in the trigeminal ganglion of the young and aged rats. In addition, the amount of those parameters, except TACE, in capsaicin-treated aged animals were significantly (p < 0.05) greater than those in capsaicin-treated young rats.

CONCLUSION

It seems that the induction of pro-inflammatory cytokines in an acute inflammatory pulpal pain model may contribute, at least in part to the increased nociceptive behaviors and pain perception in aged rats.

Pharmacological Management of Orofacial Pain

Orofacial pain is a category of complex disorders, including musculoskeletal, neuropathic and neurovascular disorders, that greatly affect the quality of life of the patient. These disorders are within the fields of dentistry and medicine and management can be challenging, requiring a referral to an orofacial pain specialist, essential for adequate evaluation, diagnosis, and care. Management is specific to the diagnosis and a treatment plan is developed with diverse pharmacological and non-pharmacological modalities. The pharmacological management of orofacial pain encompasses a vast array of medication classes and approaches. This includes anti-inflammatory drugs, muscle relaxants, anticonvulsants, antidepressants, and anesthetics. In addition, as adjunct therapy, different injections can be integrated into the management plan depending on the diagnosis and needs. These include trigger point injections, temporomandibular joint (TMJ) injections, and neurotoxin injections with botulinum toxin and nerve blocks. Multidisciplinary management is key for optimal care. New and safer therapeutic targets exclusively for the management of orofacial pain disorders are needed to offer better care for this patient population.

Calcitonin Gene-Related Peptide-Mediated Trigeminal Ganglionitis: The Biomolecular Link between Temporomandibular Disorders and Chronic Headaches

A bidirectional causal relationship has been established between temporomandibular disorders (TMDs) and chronic headaches. Recent advances in the neurobiology of chronic pain offer a framework for understanding the comorbidity between these two conditions that might reside in the shared biomolecular mechanisms of peripheral and central sensitization. The initiation of these processes is inflammatory in nature and is most likely mediated by key molecules, including calcitonin gene-related peptide (CGRP). This scoping review proposes that CGRP-mediated neuroinflammation in the trigeminal ganglion may partly explain the biomolecular bidirectional link between TMDs and chronic headaches. Finally, clinical implications of this neuropathologic process are briefly discussed.

Evaluation of Plasma Calcitonin Gene-Related Peptide as a Biomarker for Painful Temporomandibular Disorder and Migraine

Objective

To assess associations of plasma calcitonin gene-related peptide (CGRP) with chronic temporomandibular disorder (TMD) myalgia/arthralgia or frequent/chronic migraine, alone and in combination, and to evaluate relations between the CGRP concentration and clinical, psychological, and somatosensory characteristics of participants.

Methods

The cross-sectional study selected four groups of adult volunteers: healthy controls (HCs), TMD without migraine, migraine without TMD, and TMD with migraine. Each group comprised 20 participants, providing 94% power to detect statistically significant associations with CGRP concentration for either TMD or migraine. TMD and headache were classified according to the Diagnostic Criteria for TMD and the International Classification for Headache Disorders, 3rd edition, respectively. Plasma CGRP was quantified with a validated high-sensitivity electrochemiluminescent Meso Scale Discovery assay. Questionnaires and clinical examinations were used to evaluate characteristics of TMD, headache, psychological distress, and pressure pain sensitivity. Univariate regression models quantified associations of the CGRP concentration with TMD, migraine, and their interaction. Univariate associations of the CGRP concentration with clinical, psychological, and pressure pain characteristics were also assessed.

Results

Among 80 participants enrolled, neither TMD nor migraine was associated with plasma CGRP concentration (P = 0.761 and P = 0.972, respectively). The CGRP concentration (mean ± SD) was similar in all 4 groups: HCs 2.0 ± 0.7 pg/mL, TMD 2.1 ± 0.8 pg/mL, migraine 2.1 ± 0.9 pg/mL, and TMD with migraine 2.2 ± 0.7 pg/mL. CGRP concentration was positively associated with age (P = 0.034) and marginally with body mass index (P = 0.080) but was unrelated to other participant characteristics.

Conclusion

In this well-powered study, interictal plasma concentration of CGRP was a poor biomarker for TMD and migraine.

Sensory Neuron-TRPV4 Modulates Temporomandibular Disorder Pain Via CGRP in Mice

Temporomandibular disorder (TMD) pain that involves inflammation and injury in the temporomandibular joint (TMJ) and/or masticatory muscle is the most common form of orofacial pain. We recently found that transient receptor potential vanilloid-4 (TRPV4) in trigeminal ganglion (TG) neurons is upregulated after TMJ inflammation, and TRPV4 coexpresses with calcitonin gene-related peptide (CGRP) in TMJ-innervating TG neurons. Here, we extended these findings to determine the specific contribution of TRPV4 in TG neurons to TMD pain, and examine whether sensory neuron-TRPV4 modulates TMD pain via CGRP. In mouse models of TMJ inflammation or masseter muscle injury, sensory neuron-Trpv4 conditional knockout (cKO) mice displayed reduced pain. Coexpression of TRPV4 and CGRP in TMJ- or masseter muscle-innervating TG neurons was increased after TMJ inflammation and masseter muscle injury, respectively. Activation of TRPV4-expressing TG neurons triggered secretion of CGRP, which was associated with increased levels of CGRP in peri-TMJ tissues, masseter muscle, spinal trigeminal nucleus, and plasma in both models. Local injection of CGRP into the TMJ or masseter muscle evoked acute pain in naïve mice, while blockade of CGRP receptor attenuated pain in mouse models of TMD. These results suggest that TRPV4 in TG neurons contributes to TMD pain by potentiating CGRP secretion. PERSPECTIVE: This study demonstrates that activation of TRPV4 in TG sensory neurons drives pain by potentiating the release of pain mediator CGRP in mouse models of TMJ inflammation and masseter muscle injury. Targeting TRPV4 and CGRP may be of clinical potential in alleviating TMD pain.

Standardized Centella asiatica (ECa 233) extract decreased pain hypersensitivity development in a male mouse model of chronic inflammatory temporomandibular disorder

Chronic inflammatory temporomandibular disorder (TMD) pain has a high prevalence, and available nonspecific treatments have adverse side effects. ECa 233, a standardized Centella asiatica extract, is highly anti-inflammatory and safe. We investigated its therapeutic effects by injecting complete Freund's adjuvant (CFA) into right temporomandibular joint of mice and administering either ibuprofen or ECa 233 (30, 100, and 300 mg/kg) for 28 days. Inflammatory and nociceptive markers, bone density, and pain hypersensitivity were examined. CFA decreased ipsilateral bone density, suggesting inflammation localization, which ipsilaterally caused immediate calcitonin gene-related peptide elevation in the trigeminal ganglia (TG) and trigeminal subnucleus caudalis (TNC), followed by late increase of NaV1.7 in TG and of p-CREB and activation of microglia in TNC. Contralaterally, only p-CREB and activated microglia in TNC showed delayed increase. Pain hypersensitivity, which developed early ipsilaterally, but late contralaterally, was reduced by ibuprofen and ECa 233 (30 or 100 mg/kg). However, ibuprofen and only 100-mg/kg ECa 233 effectively mitigated marker elevation. This suggests 30-mg/kg ECa 233 was antinociceptive, whereas 100-mg/kg ECa 233 was both anti-inflammatory and antinociceptive. ECa 233 may be alternatively and safely used for treating chronic inflammatory TMD pain, showing an inverted U-shaped dose-response relationship with maximal effect at 100 mg/kg.

The putative role of neuroinflammation in the complex pathophysiology of migraine: From bench to bedside

The implications of neurogenic inflammation and neuroinflammation in the pathophysiology of migraine have been clearly demonstrated in preclinical migraine models involving several sites relevant in the trigemino-vascular system, including dural vessels and trigeminal endings, the trigeminal ganglion, the trigeminal nucleus caudalis as well as central trigeminal pain processing structures. In this context, a relevant role has been attributed over the years to some sensory and parasympathetic neuropeptides, in particular calcitonin gene neuropeptide, vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide. Several preclinical and clinical lines of evidence also support the implication of the potent vasodilator and messenger molecule nitric oxide in migraine pathophysiology. All these molecules are involved in vasodilation of the intracranial vasculature, as well as in the peripheral and central sensitization of the trigeminal system. At meningeal level, the engagement of some immune cells of innate immunity, including mast-cells and dendritic cells, and their mediators, has been observed in preclinical migraine models of neurogenic inflammation in response to sensory neuropeptides release due to trigemino-vascular system activation. In the context of neuroinflammatory events implicated in migraine pathogenesis, also activated glial cells in the peripheral and central structures processing trigeminal nociceptive signals seem to play a relevant role. Finally, cortical spreading depression, the pathophysiological substrate of migraine aura, has been reported to be associated with inflammatory mechanisms such as pro-inflammatory cytokine upregulation and intracellular signalling. Reactive astrocytosis consequent to cortical spreading depression is linked to an upregulation of these inflammatory markers. The present review summarizes current findings on the roles of immune cells and inflammatory responses in the pathophysiology of migraine and their possible exploitation in the view of innovative disease-modifying strategies.

Rodent behavior following a dural inflammation model with anti-CGRP migraine medication treatment

Background

Migraine is a widespread and prevalent disease with a complex pathophysiology, of which neuroinflammation and increased pain sensitivity have been suggested to be involved. Various studies have investigated the presence of different inflammatory markers in migraineurs and investigated the role of inflammation in inflammatory models with complete Freund's adjuvant (CFA) or inflammatory soup added to the dura mater.

Objective

The aim of the current study was to examine whether application of CFA to the dura mater would cause behavioral alterations that are migraine relevant. In addition, we investigated the potential mitigating effects of fremanezumab, a CGRP (calcitonin gene-related peptide) specific antibody, following CFA application.

Methods

Male Sprague-Dawley rats were randomly divided into six groups: fresh (n = 7), fresh + carprofen (n = 6), fresh + anti-CGRP (n = 6), sham (n = 7), CFA (n = 16), CFA + anti-CGRP (n = 8). CFA was applied for 15 min on a 3 × 3 mm clearing of the skull exposing the dura mater of male Sprague-Dawley rats. We applied the Light/Dark box and Open Field test, combined with the electronic von Frey test to evaluate outcomes. Finally, we observed CGRP immunoreactivity in the trigeminal ganglion.

Results

No differences were observed in the Light/Dark box test. The Open Field test detected behavior differences, notably that sham rats spend less time in the central zone, reared less and groomed more than fresh + carprofen rats. The other groups were not significantly different compared to sham rats, indicating that activation of the TGVS is present in sham surgery and cannot be exacerbated by CFA. However, for the allodynia, we observed specific periorbital sensitization, not observed in the sham animals. This could not be mitigated by fremanezumab, although it clearly reduced the amount of CGRP positive fibers.

Conclusion

CFA surgically administered to the dura causes periorbital allodynia and increases CGRP positive fibers in the trigeminal ganglion. Fremanezumab does not reduce periorbital allodynia even though it reduces CGRP positive fibers in the TG. Further work is needed to investigate whether CFA administered to the dura could be used as a non-CGRP inflammatory migraine model.

Exploring the Tryptophan Metabolic Pathways in Migraine-Related Mechanisms

Migraine is a complex neurovascular disorder, which causes intense socioeconomic problems worldwide. The pathophysiology of disease is enigmatic; accordingly, therapy is not sufficient. In recent years, migraine research focused on tryptophan, which is metabolized via two main pathways, the serotonin and kynurenine pathways, both of which produce neuroactive molecules that influence pain processing and stress response by disturbing neural and brain hypersensitivity and by interacting with molecules that control vascular and inflammatory actions. Serotonin has a role in trigeminal pain processing, and melatonin, which is another product of this pathway, also has a role in these processes. One of the end products of the kynurenine pathway is kynurenic acid (KYNA), which can decrease the overexpression of migraine-related neuropeptides in experimental conditions. However, the ability of KYNA to cross the blood-brain barrier is minimal, necessitating the development of synthetic analogs with potentially better pharmacokinetic properties to exploit its therapeutic potential. This review summarizes the main translational and clinical findings on tryptophan metabolism and certain neuropeptides, as well as therapeutic options that may be useful in the prevention and treatment of migraine.

Safety and efficacy of erenumab in patients with trigeminal neuralgia in Denmark: a double-blind, randomised, placebo-controlled, proof-of-concept study

BACKGROUND

Trigeminal neuralgia is a severe facial pain disorder that is difficult to treat. Erenumab, a monoclonal antibody against the calcitonin gene-related peptide (CGRP) receptor, has proven efficacy in migraine. Erenumab modulates sensory processing in peripheral trigeminal pain pathways in mice and was reported to be effective for patients with trigeminal neuralgia in open-label studies. We aimed to evaluate the efficacy of erenumab in patients with trigeminal neuralgia.

METHODS

We did a randomised, double-blind, placebo-controlled trial in adults (aged 18-85 years) with idiopathic or classic trigeminal neuralgia as defined by the 3rd edition of the International Classification of Headache Disorders. The trial was based at the Danish Headache Center, Copenhagen University Hospital. Eligible participants had no clinically significant cerebrovascular or cardiovascular disease, had self-reported pain intensity of at least 4 on the 11-point Numeric Rating Scale (0=no pain, 10=worst pain imaginable), and had at least three daily pain paroxysms. After a 1-week pre-screening period, patients entered a 4-week baseline period. Participants who met pain inclusion criteria at the end of the baseline period were randomly assigned (1:1) to receive subcutaneous injections of either erenumab 140 mg or placebo and entered the 4-week follow-up period. Randomisation was done in blocks of 10 using a computer-generated schedule by a third-party company. Participants and assessors were masked to treatment allocation, and erenumab and placebo were packed in identical prefilled syringes. The primary outcome was the number of responders, defined as patients who had a reduction of at least 30% in mean average daily pain intensity during the follow-up period compared with during the baseline period, analysed in the intention-to-treat population. This trial is registered with the European Union Drug Regulating Authorities Clinical Trials Database, EudraCT number 2019-000848-95.

FINDINGS

We assessed 860 patients for suitability and excluded 741 between Oct 28, 2019, and Sept 13, 2021. 119 participants entered a 1-week pre-screening period and 26 were excluded, 93 participants entered a 4-week baseline period with 13 excluded before randomisation, and 80 participants were randomly assigned to erenumab 140 mg (n=40) or placebo (n=40). There was no difference between groups in the number of responders at 4 weeks in the intention-to-treat population (14 [35%] of 40 with erenumab vs 18 [45%] of 40 with placebo; estimated effect size -10% [95% CI -31 to 11]; p=0·36). 20 (50%) of 40 participants reported adverse events in each group. The most common adverse events were constipation (28%) and headache (10%) in the erenumab group, and headache (13%), constipation (10%), and abdominal pain (10%) in the placebo group.

INTERPRETATION

Erenumab did not reduce pain intensity compared with placebo in patients with trigeminal neuralgia and CGRP probably does not have an important role in paroxysmal pain. Well tolerated, effective treatments in trigeminal neuralgia are still needed.

FUNDING

Novartis.

Could Experimental Inflammation Provide Better Understanding of Migraines?

Migraines constitute a common neurological and headache disorder affecting around 15% of the world’s population. In addition to other mechanisms, neurogenic neuroinflammation has been proposed to play a part in migraine chronification, which includes peripheral and central sensitization. There is therefore considerable evidence suggesting that inflammation in the intracranial meninges could be a key element in addition to calcitonin gene-related peptide (CGRP), leading to sensitization of trigeminal meningeal nociceptors in migraines. There are several studies that have utilized this approach, with a strong focus on using inflammatory animal models. Data from these studies show that the inflammatory process involves sensitization of trigeminovascular afferent nerve terminals. Further, by applying a wide range of different pharmacological interventions, insight has been gained on the pathways involved. Importantly, we discuss how animal models should be used with care and that it is important to evaluate outcomes in the light of migraine pathology.

Calcitonin gene‐related peptide: An intra‐articular therapeutic target for TMJ disorders

Objectives

The goal of this project was to evaluate the role of calcitonin gene‐related peptide (CGRP) in the development of arthritis.

Methods

Herein, we employed somatic mosaic analysis in two different joints by FIV(CGRP) intra‐articular inoculation in the knees or temporomandibular joints (TMJ) of young adult male C57/BL6 mice. FIV(CGRP) is a feline immunodeficiency virus over‐expressing full‐length CGRP. Joint pathology and function were evaluated at the histopathological and behavioral levels. In addition, CGRP signaling was inhibited by intra‐articular inoculation using FIV(CGRP_8‐37), such that the inhibitory peptide CGRP(8‐37) was overexpressed 4 weeks after induction of joint inflammation in the TMJ of IL‐1β^XAT transgenic mouse model. The mice were evaluated for behavior and killed for evaluation of knee and TMJ pathology.

Results

Overexpression of CGRP in the joints of wild‐type mice induced the development of joint anomalies, including meniscal hypertrophy and articular pathology, associated with nocifensive behavior. Intriguingly, overexpression of the CGRP(8‐37) inhibitory peptide in the knee and TMJ of IL‐1β^XAT transgenic mice with joint inflammation resulted in partial amelioration of the attendant joint pathology.

Conclusions

The results of this study suggest that CGRP is sufficient and necessary for the development of joint pathology and may serve as an intra‐articular therapeutic target using gene therapy or monoclonal antibody‐based therapies.

Interplay of Oral, Mandibular, and Facial Disorders and Migraine

PURPOSE OF THE REVIEW

Migraine and other primary headache disorders can be localized in the face resembling facial or dental pain, indicating the influence of the trigeminovascular system in the structures innervated by the maxillary (V2) and mandibulary (V3) branches of the trigeminal nerve. Disorders of oral and craniofacial structures may influence primary headache disorders. In the current article, we review the potential links of this interplay.

RECENT FINDINGS

This interplay may be related to anatomy, with the trigeminal pathway and the involvement of both peripheral and central mechanisms, and the presence of calcitonin gene-related peptide (CGRP), a key mediator in migraine pathophysiology. CGRP is also involved in the pathophysiology of temporomandibular disorders (TMD) and their comorbidity with migraine and is also implicated in dental and periodontal pathology. Inflammatory and pathological processes of these structures and their trigeminal nociceptive pathways may influence the trigeminovascular system and consequently may exacerbate or even potentially trigger migraine.

Temporal characteristics of astrocytic activation in the TNC in a mice model of pain induced by recurrent dural infusion of inflammatory soup

Background

Astrocytic activation might play a significant role in the central sensitization of chronic migraine (CM). However, the temporal characteristics of the astrocytic activation in the trigeminal nucleus caudalis (TNC) and the molecular mechanism under the process remain not fully understood. Therefore, this study aims to investigate the duration and levels change of astrocytic activation and to explore the correlation between astrocytic activation and the levels change of cytokines release.

Methods

We used a mice model induced by recurrent dural infusion of inflammatory soup (IS). The variation with time of IS-induced mechanical thresholds in the periorbital and hind paw plantar regions were evaluated using the von Frey filaments test. We detected the expression profile of glial fibrillary acidic protein (GFAP) in the TNC through immunofluorescence staining and western blot assay. We also investigated the variation with time of the transcriptional levels of GFAP and ionized calcium binding adapter molecule 1 (Iba1) through RNAscope in situ hybridization analysis. Then, we detected the variation with time of cytokines levels in the TNC tissue extraction and serum, including c-c motif chemokine ligand 2 (CCL2), c-c motif chemokine ligand 5 (CCL5), c-c motif chemokine ligand 7 (CCL7), c-c motif chemokine ligand 12 (CCL12), c-x-c motif chemokine ligand 1 (CXCL1), c-x-c motif chemokine ligand 13 (CXCL13), interferon gamma (IFN-γ), tumor necrosis factor alpha (TNF-α), macrophage colony-stimulating factor (M-CSF), interleukin 1beta (IL-1β), interleukin 6 (IL-6), interleukin 10 (IL-10), interleukin 17A (IL-17A).

Results

Recurrent IS infusion resulted in cutaneous allodynia in both the periorbital region and hind paw plantar, ranging from 5 d (after the second IS infusion) to 47 d (28 d after the last infusion) and 5 d to 26 d (7 d after the last infusion), respectively. The protein levels of GFAP and messenger ribonucleic acid (mRNA) levels of GFAP and Iba1 significantly increased and sustained from 20 d to 47 d (1 d to 28 d after the last infusion), which was associated with the temporal characteristics of astrocytic activation in the TNC. The CCL7 levels in the TNC decreased from 20 d to 47 d. But the CCL7 levels in serum only decreased on 20 d (1 d after the last infusion). The CCL12 levels in the TNC decreased on 22 d (3 d after the last infusion) and 33 d (14 d after the last infusion). In serum, the CCL12 levels only decreased on 22 d. The IL-10 levels in the TNC increased on 20 d.

Conclusions

Our results indicate that the astrocytic activation generated and sustained in the IS-induced mice model from 1 d to 28 d after the last infusion and may contribute to the pathology through modulating CCL7, CCL12, and IL-10 release.

Efficacy of Erenumab in the Treatment of Trigeminal Neuralgia: A Retrospective Case Series

Objective

Trigeminal neuralgia (TN) is a chronic, often refractory, pain condition, which adversely affects the lives of patients. Current treatments are only mildly effective. Anti-calcitonin gene-related peptide (CGRP) monoclonal antibodies have been successfully studied in the treatment of migraines. CGRP plays a role in both TN and migraine. It is prudent to attempt CGRP monoclonal antibody therapy in TN. Erenumab, a human anti-CGRP monoclonal antibody medication, modulates CGRP, which is elevated in patients with TN. The primary objective of this study was to evaluate the efficacy of erenumab for patients with TN.

Methods

Retrospective analysis was performed on data collected from 10 patients diagnosed with TN and treated with erenumab for 6 months. Pain was tracked using a numeric pain rating scale (NPRS) from 0 to 10. The effect of erenumab on NPRS after 6 months' time was the primary end point. Secondary end points included side effects to therapy, improvement in headache frequency in those with comorbid migraine, evaluating mood following therapy, and global mood improvement using scale (worse, no change, improved).

Results

Nine of 10 patients (90.0%) reported improvement in pain severity and in global mood improvement. Three patients reported resolution of anxiety and/or depression. Side effects were minimal, with 3 patients reporting constipation, injection site reactions, or both.

Conclusions

Based on these results, erenumab appears to be an efficacious treatment option for patients with refractory TN. Patients experienced improvement in pain, reduced frequency of headache, and improvement in mood. Treatment was well tolerated with only mild side effects reported.

Classification of Evidence

This study provides Class IV evidence that erenumab increases the probability of improved pain control in patients with medication-resistant TN.

Neuroimmune interactions in painful TMD: Mechanisms and treatment implications

The underlying mechanisms and treatment of painful temporomandibular disorders (TMDs) are important but understudied topics in craniofacial research. As a group of musculoskeletal diseases, the onset of painful TMD is proved to be a result of disturbance of multiple systems. Recently, emerging evidence has revealed the involvement of neuroimmune interactions in painful TMD. Inflammatory factors play an important role in peripheral sensitization of temporomandibular joint (TMJ), and neurogenic inflammation in turn enhances TMJs dysfunction in TMD. Furthermore, centralized neuroimmune communications contribute to neuron excitability amplification, leading to pain sensitization, and is also responsible for chronic TMD pain and other CNS symptoms. Therapeutics targeting neuroimmune interactions may shed light on new approaches for treating TMD. In this review, we will discuss the role of neuroimmune interactions in the onset of painful TMD from the peripheral and centralized perspectives, and how understanding this mechanism could provide new treatment options. Insights into the neuroimmune interactions within TMJs and painful TMD would broaden the knowledge of mechanisms and treatments of this multifactorial disease.

Animal Models of Temporomandibular Disorder

Temporomandibular disorders (TMD) are a group of diseases in the oral and maxillofacial region that can manifest as acute or chronic persistent pain, affecting millions of people worldwide. Although hundreds of studies have explored mechanisms and treatments underlying TMD, multiple pathogenic factors and diverse clinical manifestations make it still poorly managed. Appropriate animal models are helpful to study the pathogenesis of TMD and explore effective treatment measures. At present, due to the high cost of obtaining large animals, rodents and rabbits are often used to prepare TMD animal models. Over the past decade, various animal models have been intensively developed to understand neurobiological and molecular mechanisms of TMD, and seek effective treatments. Although these models cannot carry out all clinical features, they are valuable in revealing the mechanisms of TMD and creating curative access. Currently, there are multitudinous animal models of TMD research. They can be constructed in different means and summarized into four ways according to the various causes and symptoms, including chemical induction (intra-articular injection of ovalbumin, collagenase, formalin, vascular endothelial growth factor, intramuscular injection of complete Freund’s adjuvant, etc.), mechanical stress stimulation (passive mouth opening, change of chewing load), surgical operation (partial disc resection, joint disc perforation) and psychological stress induction. Here, we summarize and discuss different approaches of animal models for determining neurophysiological and mechanical mechanisms of TMD and assess their advantages and limitations, respectively.

5-HT3/7 and GABAB receptors mediate inhibition of trigeminal nociception by dietary supplementation of grape seed extract

Background: Temporomandibular joint disorder is a prevalent orofacial pain condition involving sensitization and activation of trigeminal nociceptive neurons. Dietary supplementation with a proanthocyanin-enriched grape seed extract (GSE) was found to inhibit trigeminal nociception in a chronic TMD model. In this study, the cellular mechanisms by which GSE inhibits sustained trigeminal nociception in male and female Sprague Dawley rats were investigated.Methods: Some animals were supplemented with 0.5% GSE dissolved in their water one week prior to neck muscle inflammation induced by injection of complete Freund's adjuvant into the trapezius. To investigate the mechanism of GSE, some animals were injected intracisternally with antagonists of 5-HT3, 5-HT7, GABAA, or GABAB, receptor prior to jaw opening.Results: In males and females, trapezius inflammation prior to jaw opening resulted in sustained mechanical hypersensitivity of trigeminal nociceptors that was significantly inhibited by GSE. Further, GSE beginning 14 days post jaw opening also inhibited trigeminal nociception. Intracisternal injection of antagonists of the 5-HT3/7 and GABAB, but not GABAA receptors reduced the anti-nocifensive effect of GSE in both sexes. Neuronal expression of GABAB protein and mRNA in the spinal cord and trigeminal ganglion were detected.Conclusions: The inhibitory effect of GSE is mediated via activation of 5-HT3/7 receptors and GABAB to enhance central descending inhibitory pain pathways and suppress ongoing trigeminal nociception. Further, our findings support the use of GSE as a dietary supplement in the management of pain associated with TMD and other orofacial pain conditions involving central sensitization and dysfunction of descending pain modulation.

CGRP Receptor Antagonism in COVID-19: Potential Cardiopulmonary Adverse Effects

Recently, the US FDA has authorized a drug repurposing trial with calcitonin gene-related peptide (CGRP) receptor antagonists to reduce lung inflammation in coronavirus 2019 (COVID-19). However, the well-established cardiopulmonary protective effects of CGRP raise concerns about the safety of antagonizing CGRP in COVID-19. Awareness regarding potential cardiopulmonary adverse effects may enable their early detection and prevent illness from worsening.

Preclinical studies investigating the neural mechanisms involved in the co-morbidity of migraine and temporomandibular disorders: the role of CGRP

BACKGROUND AND PURPOSE

Temporomandibular disorders (TMD) and migraine can be co-morbid. This can be a significant factor in exacerbating and increasing the prevalence of migraine-like symptoms. However, the underlying mechanisms involved are unknown. Our objective was to investigate these neural mechanisms and the role of CGRP as a key modulator in this co-morbidity.

EXPERIMENTAL APPROACH

We combined experimental approaches using CGRP, which triggers a migraine-like response in patients, with that of masseteric muscle injection of complete Freund's adjuvant (CFA), to model myofascial TMD-like inflammation. Using validated electrophysiological methods to assess each of the above approaches independently or in combination, we examined their effects on the response properties of migraine-like dural-trigeminocervical neurons.

KEY RESULTS

Independently, in ~2/3 of animals (rats) each approach caused delayed migraine-like activation and sensitisation of dural-trigeminocervical neurons. The response to masseteric-CFA was attenuated by a selective CGRP receptor antagonist. The combination approach caused a migraine-like neuronal response in all animals tested, with somatosensory-evoked cranial hypersensitivity significantly exacerbated.

CONCLUSION AND IMPLICATIONS

The data demonstrate a neuronal phenotype that translates to the exacerbated clinical co-morbid phenotype, supporting this combination approach as a relevant model to study the mechanisms involved. It provides a pathophysiological rationale for this exacerbated phenotype, strongly implicating the involvement of CGRP. The results provide support for targeting the CGRP pathway as a novel monotherapy approach for treating this co-morbid condition. This has key implications into our understanding of this co-morbid condition, as well as potentially addressing the major unmet need for novel and effective therapeutic approaches.

The effect of natural products in animal models of temporomandibular disorders

Treatment of temporomandibular disorders (TMD) is a challenge for health care professionals. Therefore, new approaches have been investigated, such as the use of natural products.

A Pre-Existing Myogenic Temporomandibular Disorder Increases Trigeminal Calcitonin Gene-Related Peptide and Enhances Nitroglycerin-Induced Hypersensitivity in Mice

Migraine is commonly reported among patients with temporomandibular disorders (TMDs), especially myogenic TMD. The pathophysiologic mechanisms related to the comorbidity of the two conditions remain elusive. In the present study, we combined masseter muscle tendon ligation (MMTL)-produced myogenic TMD with systemic injection of nitroglycerin (NTG)-induced migraine-like hypersensitivity in mice. Facial mechanical allodynia, functional allodynia, and light-aversive behavior were evaluated. Sumatriptan, an FDA-approved medication for migraine, was used to validate migraine-like hypersensitivity. Additionally, we examined the protein level of calcitonin gene-related peptide (CGRP) in the spinal trigeminal nucleus caudalis using immunohistochemistry. We observed that mice with MMTL pretreatment have a prolonged NTG-induced migraine-like hypersensitivity, and MMTL also enabled a non-sensitizing dose of NTG to trigger migraine-like hypersensitivity. Systemic injection of sumatriptan inhibited the MMTL-enhanced migraine-like hypersensitivity. MMTL pretreatment significantly upregulated the protein level of CGRP in the spinal trigeminal nucleus caudalis after NTG injection. Our results indicate that a pre-existing myogenic TMD can upregulate NTG-induced trigeminal CGRP and enhance migraine-like hypersensitivity.

Neuropeptides are associated with pain threshold and bone microstructure in ovariectomized rats

OBJECTIVES

Postmenopausal osteoporosis (PMO) is a metabolic skeletal disorder with impaired bone density and bone quality in postmenopausal women. The aim of the present study was to investigate the correlation between neuropeptides, bone microstructure and pain threshold in ovariectomized (OVX) rats.

METHODS

Female rats were randomly divided into the ovariectomized (OVX) group and the sham surgery (SHAM) group. Bone microstructure and immunocytochemistry for substance P (SP), calcitonin gene-related peptide (CGRP), vasoactive intestinal peptide (VIP) and neuropeptide Y (NPY) in tibial and DRG were performed. Pain threshold was assessed at post-operative 11 weeks. Pearson correlation coefficients were calculated between neuropeptides, bone microstructure and pain threshold.

RESULTS

Significant decreases in bone volume fraction (BV/TV) and trabecular number (Tb. N) but significant increases in trabecular spacing (Tb.Sp) were showed in OVX group. Mechanical pain threshold (MPT) in OVX group was significantly decreased. The MOD values for SP, CGRP and VIP of tibial in OVX group were significantly lower, whereas NPY, NPY1R and NPY2R were significantly higher. And SP, CGRP, VIP, NPY and NPY2R of DRG were significantly increased in OVX group, while NPY1R was significantly decreased. Correlation analysis showed that NPY, Y1R and Y2R in bone were negatively correlated with BV/TV. MPT was negatively correlated with NPY and Y2R in DRG, and positively correlated with Y1R in DRG.

CONCLUSIONS

Our results suggested that SP, CGRP, VIP and NPY were involved in the osteoporotic bone microstructure and mechanical hypersensitivity in OVX rats, indicating the potential to utilize neuropeptides as novel therapeutic targets for PMO.

Cytokines, Masticatory Muscle Inflammation, and Pain: an Update

Cytokines are proteins secreted by diverse types of immune and non-immune cells and play a role in the communication between the immune and nervous systems. Cytokines include lymphokines, monokines, chemokines, interleukins, interferons, colony stimulating factors, and growth factors. They can be both pro- and anti-inflammatory and have autocrine, paracrine, and endocrine activities. These proteins are involved in initiation and persistence of pain, and the progress of hyperalgesia and allodynia, upon stimulating nociceptive sensory neurons, and inducing central sensitization. The objective of this review is to discuss several types of pro- and anti-inflammatory mediators and their relation with inflammatory pain in masticatory muscles.

Antinociceptive Effect of Spirocyclopiperazinium Salt Compound DXL-A-24 and the Underlying Mechanism

The antinociceptive effects of spirocyclopiperazinium salt compound DXL-A-24 on neuropathic pain and chemical-stimulated pain were investigated in this study. After the administration of DXL-A-24, the paw withdrawal latency (PWL) and mechanical withdrawal threshold (MWT) were increased in rats suffering from neuropathic pain (chronic constriction injury, CCI) on days 1, 3, 5, 7 and 14 after surgery, and pain responses were inhibited in mice stimulated with chemicals (formalin or acetic acid). In the analysis of antinociceptive targets, the effect of DXL-A-24 was blocked by a peripheral nicotinic acetylcholine receptor (nAChR) antagonist (hexamethonium, Hex) or α7 nAChR antagonist (methyllycaconitine, MLA) in the formalin test. Meanwhile, the effect of DXL-A-24 was also blocked by a peripheral muscarinic acetylcholine receptor (mAChR) antagonist (atropine methylnitrate, Amn) or M4 mAChR antagonist (tropicamide, TRO). The antinociceptive signalling pathway was explored using molecular biology methods in ipsilateral dorsal root ganglions (DRGs) of CCI rats after the administration of DXL-A-24 for 7 days. Western blot analyses showed that the increased levels of phosphorylation of calcium/calmodulin-dependent protein kinase II alpha (CaMKIIα) and cAMP response element-binding protein (CREB) were eliminated, and the qRT-PCR assay showed that the increase in the expression of Tumor necrosis factor alpha (TNF-α) mRNA was reduced. Meanwhile, immunofluorescence staining revealed that the increase in calcitonin gene related peptide (CGRP) expression was inhibited by the administration of DXL-A-24, and the effect was blocked by MLA or TRO. In conclusion, DXL-A-24 exerts significant antinociceptive effects on neuropathic pain and chemical-stimulated pain. The antinociceptive effect of DXL-A-24 is probably attributed to the activation of peripheral α7 nAChR and M4 mAChR, the subsequent inhibition of the CaMKIIα/CREB signalling pathway, and finally the inhibition of TNF-α and CGRP expression.

Vagal Afferent Processing by the Paratrigeminal Nucleus

The paratrigeminal nucleus is an obscure region in the dorsal lateral medulla, which has been best characterized as a collection of interstitial cells located in the dorsal tip of the spinal trigeminal tract. The paratrigeminal nucleus receives afferent input from the vagus, trigeminal, spinal, and glossopharyngeal nerves, which contribute to its long-known roles in the baroreceptor reflex and nociceptive processing. More recently, studies have shown that this region is also involved in the processing of airway-derived sensory information. Notably, these studies highlight an underappreciated complexity in the neuronal content and circuit connectivity of the paratrigeminal nucleus. However, much remains to be understood about how paratrigeminal processing of vagal afferents is altered in disease. The aim of the present review is to provide an update of the current understanding of vagal afferent processing in the paratrigeminal nucleus and to explore how dysregulation at this site may contribute to vagal sensory neural dysfunction during disease.

The effect of orofacial complete Freund's adjuvant treatment on the expression of migraine-related molecules

Background

Migraine is a neurovascular primary headache disorder, which causes significant socioeconomic problems worldwide. The pathomechanism of disease is enigmatic, but activation of the trigeminovascular system (TS) appears to be essential during the attack. Migraine research of recent years has focused on neuropeptides, such as calcitonin gene-related peptide (CGRP) and pituitary adenylate cyclase-activating polypeptide 1–38 (PACAP1–38) as potential pathogenic factors and possible therapeutic offensives. The goal of present study was to investigate the simultaneous expression of CGRP and precursor of PACAP1–38 (preproPACAP) in the central region of the TS in a time-dependent manner following TS activation in rats.

Methods

The right whisker pad of rats was injected with 50 μl Complete Freund’s Adjuvant (CFA) or saline. A mechanical allodynia test was performed with von Frey filaments before and after treatment. Transcardial perfusion of the animals was initiated 24, 48, 72 and 120 h after injection, followed by the dissection of the nucleus trigeminus caudalis (TNC). After preparation, the samples were stored at − 80 °C until further use. The relative optical density of CGRP and preproPACAP was analyzed by Western blot. One-way ANOVA and Kruskal-Wallis followed by Tukey post hoc test were used to evaluate the data. Regression analysis was applied to explore the correlation between neuropeptides expression and hyperalgesia.

Results

Orofacial CFA injection resulted in significant CGRP and preproPACAP release in the TNC 24, 48, 72 and 120 h after the treatment. The level of neuropeptides reached its maximum at 72 h after CFA injection, corresponding to the peak of facial allodynia. Negative, linear correlation was detected between the expression level of neuropeptides and value of mechanonociceptive threshold.

Conclusion

This is the first study which suggests that the expression of CGRP and preproPACAP simultaneously increases in the central region of activated TS and it influences the formation of mechanical hyperalgesia. Our results contribute to a better understanding of migraine pathogenesis and thereby to the development of more effective therapeutic approaches.

CGRP and Painful Pathologies Other than Headache

CGRP has long been suspected as a mediator of arthritis pain, although evidence that CGRP directly mediates human musculoskeletal pain remains circumstantial. This chapter describes in depth the evidence surrounding CGRP's association with pain in musculoskeletal disorders and also summarises evidence for CGRP being a direct cause of pain in other conditions. CGRP-immunoreactive nerves are present in musculoskeletal tissues, and CGRP expression is altered in musculoskeletal pain. CGRP modulates musculoskeletal pain through actions both in the periphery and central nervous system. Human observational studies, research on animal arthritis models and the few reported randomised controlled trials in humans of treatments that target CGRP provide the context of CGRP as a possible pain biomarker or mediator in conditions other than migraine.

Studies on a novel regimen for management of orofacial pain and morphine tolerance

Background/purpose

The prevalence of orofacial pain is high but the etiology of orofacial pain is not well understood. Because of clinical treatment is not so effective, it is urgent to explore novel regimens with more effective and less side effects for clinical application.

Materials and methods

Male mice (ICR strain) were injected with capsaicin (10μg/5 μl) in vibrissa pad. Spontaneous orofacial pain in 20 min was recorded after receiving capsaicin to quantify the nociceptive level. Green tea polyphenols (GTP 60 mg/kg), memantine (Mem 10 mg/kg), and GTPm (GTP 30 mg/kg plus Mem 3 mg/kg) were dissolved in 2% carboxymethyl cellulose, which was orally administered to mice twice per day and five times per week consecutively for 2 weeks. TruScan photobeam tracking was used to record changes of behavior and locomotor activities.

Results

GTPm by itself attenuated orofacial pain induced by capsaicin. Moreover, GTPm enhanced morphine analgesic effects, reduced morphine depressant side effects and delayed morphine tolerance. Along with this experiment, GTPm was tested on the hot plate (52 °C)-induced peripheral thermal pain. It was found that both memantine and GTPm reduced morphine-analgesia in hind paw thermal pain.

Conclusion

In this study, GTP (60 mg/kg/day) orally administrated produced a significant analgesic effect on capsaicin–induced orofacial pain. Memantine combined with GTP synergistically not only reduced orofacial pain but also enhanced morphine analgesic effects. Thus, a new regimen of GTPm orally administered twice per day attenuated orofacial pain after consecutive 5 days.

TRPA1 and CGRP antagonists counteract vesicant-induced skin injury and inflammation

The skin is highly sensitive to the chemical warfare agent in mustard gas, sulfur mustard (SM) that initiates a delayed injury response characterized by erythema, inflammation and severe vesication (blistering). Although SM poses a continuing threat, used as recently as in the Syrian conflict, no mechanism-based antidotes against SM are available. Recent studies demonstrated that Transient Receptor Potential Ankyrin 1 (TRPA1), a chemosensory cation channel in sensory nerves innervating the skin, is activated by SM and 2-chloroethyl ethyl sulfide (CEES), an SM analog, in vitro, suggesting it may promote vesicant injury. Here, we investigated the effects of TRPA1 inhibitors, and an inhibitor of Calcitonin Gene Related Peptide (CGRP), a neurogenic inflammatory peptide released upon TRPA1 activation, in a CEES-induced mouse ear vesicant model (CEES-MEVM). TRPA1 inhibitors (HC-030031 and A-967079) and a CGRP inhibitor (MK-8825) reduced skin edema, pro-inflammatory cytokines (IL-1β, CXCL1/KC), MMP-9, a protease implicated in skin damage, and improved histopathological outcomes. These findings suggest that TRPA1 and neurogenic inflammation contribute to the deleterious effects of vesicants in vivo, activated either directly by alkylation, or indirectly, by reactive intermediates or pro-inflammatory mediators. TRPA1 and CGRP inhibitors represent new leads that could be considered for validation and further development in other vesicant injury models.

Enriched Chicken Bone Broth as a Dietary Supplement Reduces Nociception and Sensitization Associated with Prolonged Jaw Opening

AIMS

To test a commercially available enriched chicken bone broth (ECBB) product for its potential anti-inflammatory properties and to evaluate its ability to reduce nociception and expression of protein kinase A (PKA) in a clinically relevant model of temporomandibular disorder (TMD) caused by prolonged jaw opening in rats.

METHODS

The potential of the ECBB and of a homemade broth was investigated using the Folin-Ciocalteu reagent and percent inhibition of cyclooxygenase-2 (COX-2) activity, which was determined using a commercially available kit. Additionally, the effect of ECBB and homemade broth on nocifensive head withdrawal responses to mechanical stimulation in male Sprague-Dawley rats subjected to prolonged jaw opening was evaluated. Differences were considered significant at P < .025. Changes in PKA expression in the medullary dorsal horn region of the spinal trigeminal nucleus associated with prolonged jaw opening were assessed using immunofluorescence, and these changes were considered significant at P < .05. Behavioral data were analyzed by using multiple nonparametric tests, and immunohistochemistry data were analyzed by using one-way analysis of variance with Games-Howell post hoc tests in SPSS software.

RESULTS

ECBB exhibited greater reducing potential and inhibition of COX-2 activity compared to homemade broth. Near maximal jaw opening was sufficient to induce sustained nocifensive responses to mechanical stimuli for 7 days. This increased sensitivity was correlated with elevated levels of the active form of PKA. Importantly, dietary inclusion of ECBB, but not of homemade broth, for 2 weeks prior to jaw opening was sufficient to reduce nocifensive behaviors and PKA expression.

CONCLUSION

Findings from this study provide evidence that ECBB attenuates nociception and expression of the pro-inflammatory protein PKA and thus may be beneficial as a nutraceutical supplement to manage inflammatory pain associated with TMD.

Cluster headache

Cluster headache is an excruciating, strictly one-sided pain syndrome with attacks that last between 15 minutes and 180 minutes and that are accompanied by marked ipsilateral cranial autonomic symptoms, such as lacrimation and conjunctival injection. The pain is so severe that female patients describe each attack as worse than childbirth. The past decade has seen remarkable progress in the understanding of the pathophysiological background of cluster headache and has implicated the brain, particularly the hypothalamus, as the generator of both the pain and the autonomic symptoms. Anatomical connections between the hypothalamus and the trigeminovascular system, as well as the parasympathetic nervous system, have also been implicated in cluster headache pathophysiology. The diagnosis of cluster headache involves excluding other primary headaches and secondary headaches and is based primarily on the patient's symptoms. Remarkable progress has been achieved in developing effective treatment options for single cluster attacks and in developing preventive measures, which include pharmacological therapies and neuromodulation.

Central Role of Protein Kinase A in Promoting Trigeminal Nociception in an In Vivo Model of Temporomandibular Disorders

AIMS

To investigate cellular changes in the spinal trigeminal nucleus (STN) and trigeminal ganglion (TG) associated with trigeminal nociception mediated by inflammation in the temporomandibular joint (TMJ).

METHODS

Male Sprague-Dawley rats (n = 86) were utilized to investigate cellular and behavioral responses to prolonged TMJ inflammation caused by bilateral injection of Complete Freund's Adjuvant (CFA) in the TMJ capsules. To investigate the cellular effects of protein kinase A (PKA) in the STN, rats were injected intrathecally with the selective PKA inhibitor KT5720 prior to injection of CFA into both TMJ capsules. Levels of calcitonin gene-related peptide (CGRP), active PKA, and ionized calcium-binding adapter molecule 1 (Iba1) in the STN and expression of phosphorylated extracellular regulated kinases (p-ERK) in the TG were determined with immunohistochemistry (n ≥ 3 experiments per test condition). Nocifensive head withdrawal responses to mechanical stimulation of the cutaneous tissue over the TMJ were monitored following CFA injection in the absence or presence of KT5720 (n = 7). Statistical analysis was performed using parametric analysis of variance (ANOVA) tests.

RESULTS

Intrathecal injection of KT5720 significantly inhibited the stimulatory effect of CFA on levels of CGRP, PKA, and Iba1 in the STN. In addition, administration of KT5720 decreased the average number of CFA-induced nocifensive withdrawal responses to mechanical stimulation and the CFA-mediated increase in p-ERK expression in the ganglion.

CONCLUSION

These findings provide evidence that elevated PKA activity in the STN promotes cellular events temporally associated with trigeminal nociception caused by prolonged TMJ inflammation.

CGRP receptor antagonist MK-8825 attenuates cortical spreading depression induced pain behavior

BACKGROUND AND OBJECTIVE

The present study aimed to investigate the effects of selective calcitonin gene related peptide (CGRP) receptor antagonist (MK-8825) on cortical spreading depression (CSD) induced pain behavior and anxiety in freely-moving rats, and neuronal activation in the correlated anatomical regions.

METHODS

CSD was induced while keeping all meningeal layers and BBB intact and MK-8825 was administered in two different doses. Regional cerebral blood flow (rCBF), arterial pressure and DC shift were recorded. Behavioral studies were conducted in freely-moving rats. Spontaneous behavior, mechanical allodynia, ultrasonic vocalization, and anxiety were evaluated. Immunohistochemistry of c-fos, CGRP, calcitonin receptor like-receptor (CLR) and receptor activity modifying protein 1 (RAMP1) were studied.

RESULTS

MK-8825 did not block DC shifts in the cerebral cortex and accompanied hemodynamic response. CSD significantly induced freezing and grooming behavior in freely-moving rats. MK-8825 reversed increased episodes of freezing, grooming, wet dog shake and head shake behavior. MK-8825 increased CSD-induced reductions in von Frey thresholds, but did not change elevated plus maze results. MK-8825 blocked c-fos induction by CSD in the brainstem trigeminal nucleus caudalis (TNC) and reticular nucleus of thalamus (TRN) but not in the amygdala. Immunofluorescence analysis showed no co-localization of CGRP, CLR or RAMP1 with c-fos positive cells.

CONCLUSION

CGRP receptor antagonist MK-8825 dose dependently attenuated CSD-induced trigeminal nerve mediated pain response without altering CSD waves and accompanied rCBF response. While blocking TNC activation, MK-8825 did not exert any effect on amygdala and anxiety behavior. CGRP receptor antagonists may also modulate thalamo-cortical gating.

Temporomandibular Disorders and Headache

Temporomandibular disorders (TMD) and primary headaches can be perpetual and debilitating musculoskeletal and neurological disorders. The presence of both can affect up to one-sixth of the population at any one time. Initially, TMDs were thought to be predominantly musculoskeletal disorders, and migraine was thought to be solely a cerebrovascular disorder. The further understanding of their pathophysiology has helped to clarify their clinical presentation. This article focuses on the role of the trigeminal system in associating TMD and migraine. By discussing recent descriptions of prevalence, diagnosis, and treatment of headache and TMD, we will further elucidate this relationship.

Animal models of pain and migraine in drug discovery

Preclinical research activities in relation to pain typically involve the 'holy trinity' of nociceptive, inflammatory and neuropathic pain for purposes of target validation and defining target product profiles of novel analgesic compounds. For some reason it seems that headache or migraine are rarely considered as additional entities to explore. Frontline medications used in the treatment of, for example, inflammatory pain, neuropathic pain and migraine (NSAIDs versus pregabalin/duloxetine versus triptans) reveal distinct differences in pathophysiology that partially explain this approach. Nevertheless, for many patients enduring chronic pain, regardless of aetiology, high unmet needs remain. By focusing more on commonalities shared between neuropathic pain and headache disorders such as migraine, drug discovery efforts could be spread more efficiently across a larger indication area. Here, some of the most commonly used models and methods employed within 'pain and migraine' drug development will be presented. Recent advances within these disciplines suggest that, with the addition of a few extra carefully chosen ancillary models and/or endpoints, the relative value in terms of resources used, reciprocal flow of information and net worth of a 'typical' package could be increased substantially for the pain and migraine fields.

Transmission pathways and mediators as the basis for clinical pharmacology of pain

INTRODUCTION

Mediators in pain transmission are the targets of a multitude of different analgesic pharmaceuticals. This review explores the most significant mediators of pain transmission as well as the pharmaceuticals that act on them. Areas covered: The review explores many of the key mediators of pain transmission. In doing so, this review uncovers important areas for further research. It also highlights agents with potential for producing novel analgesics, probes important interactions between pain transmission pathways that could contribute to synergistic analgesia, and emphasizes transmission factors that participate in transforming acute injury into chronic pain. Expert commentary: This review examines current pain research, particularly in the context of identifying novel analgesics, highlighting interactions between analgesic transmission pathways, and discussing factors that may contribute to the development of chronic pain after an acute injury.

Advances in the understanding of cluster headache

INTRODUCTION

Cluster headache is the worst primary headache form; it occurs in paroxysmal excruciatingly severe unilateral head pain attacks usually grouped in cluster periods. The familial occurrence of the disease indicates a genetic component but a gene abnormality is yet to be disclosed. Activation of trigeminal afferents and cranial parasympathetic efferents, the so-called trigemino-parasympathetic reflex, can explain pain and accompanying oculo-facial autonomic phenomena. In particular, pain in cluster headache is attributed, at least in part, to the increased CGRP plasma levels released by activated trigeminal system. Posterior hypothalamus was hypothesized to be the cluster generator activating the trigemino-parasympathetic reflex. Efficacy of monoclonal antibodies against CRGP is under investigation in randomized clinical trials. Areas covered: This paper will focus on main findings contributing to consider cluster headache as a neurovascular disorder with an origin from within the brain. Expert commentary: Accumulated evidence with hypothalamic stimulation in cluster headache patients indicate that posterior hypothalamus terminates rather than triggers the attacks. More extensive studies on the genetics of cluster headache are necessary to disclose anomalies behind the increased familial risk of the disease. Results from ongoing clinical trials in cluster headache sufferers using monoclonal antibodies against CGRP will open soon a new era.