Muscle inflammation induces a rapid increase in calcitonin gene-related peptide (CGRP) mRNA that temporally relates to CGRP immunoreactivity and nociceptive behavior

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.

Neck Pain Disability on Headache Impact and the Association between Sleep Disturbance and Neck Pain in Migraine

Neck pain (NP) is a prevalent symptom among migraine patients, but its disability on headache impact and the contributing factors for comorbid NP are poorly understood. This study aimed to investigate NP disability on the impact of headaches among migraineurs and factors linked to comorbid NP, including sleep-related variables. This cross-sectional study was conducted at a university hospital headache center, for headache patients at their first visits. Included in the study were 295 patients with migraines (217 females; 39.0  ±  10.8 years; 101 chronic migraine). Information on NP, history of physician-diagnosed cervical spine or disc disorders, detailed parameters of headache, and sleep and mood variables were collected. Logistic analysis of the severe impact of headache and contributing factors for NP were performed. NP was present in 153 participants (51.9%) with migraine, with high NP disability observed in 28 patients, and 125 patients had low NP disability. In multivariable analysis, NP disability, medication days per month, severe disability of migraine, and excessive daytime sleepiness were significant predictors for severe impact of headache. Thirty-seven patients with physician-diagnosed cervical spine or disc disorders were excluded from the NP analysis. Higher monthly headache days, female gender, and a high likelihood of obstructive sleep apnea were positively correlated with the presence of NP among migraineurs in multivariable analysis. Overall, the study highlights the potential impact of sleep-related variables and monthly headache days on NP in these patients. The high disability of NP was also associated with severe impact of headache.

A new hypothesis linking oxytocin to menstrual migraine

OBJECTIVE

To highlight the emerging understanding of oxytocin (OT) and oxytocin receptors (OTRs) in modulating menstrual-related migraine (MRM).

BACKGROUND

MRM is highly debilitating and less responsive to therapy, and attacks are of longer duration than nonmenstrually related migraine. A clear understanding of the mechanisms underlying MRM is lacking.

METHODS

We present a narrative literature review on the developing understanding of the role of OT and the OTR in MRM. Literature on MRM on PubMed/MEDLINE database including clinical trials and basic science publications was reviewed using specific keywords.

RESULTS

OT is a cyclically released hypothalamic hormone/neurotransmitter that binds to the OTR resulting in inhibition of trigeminal neuronal excitability that can promote migraine pain including that of MRM. Estrogen regulates OT release as well as expression of the OTR. Coincident with menstruation, levels of both estrogen and OT decrease. Additionally, other serum biochemical factors, including magnesium and cholesterol, which positively modulate the affinity of OT for OTRs, both decrease during menstruation. Thus, during menstruation, multiple menstrually associated factors may lead to decreased circulating OT levels, decreased OT affinity for OTR, and decreased expression of the trigeminal OTR. Consistent with the view of migraine as a threshold disorder, these events may collectively result in decreased inhibition promoting lower thresholds for activation of meningeal trigeminal nociceptors and increasing the likelihood of an MRM attack.

CONCLUSION

Trigeminal OTR may thus be a novel target for the development of MRM therapeutics.

Restless legs-like syndrome as an emergent adverse event of CGRP monoclonal antibodies: A report of two cases

BACKGROUND

One of the advantages of CGRP monoclonal antibodies is their excellent safety and tolerability. However, postmarketing surveillance, is essential to detect potential rare emergent adverse events.

OBJECTIVES

To report two patients who developed restless legs syndrome symptoms after treatment with CGRP antibodies.

METHODS AND RESULTS

Two women with chronic refractory migraine, with no significant medical antecedents, developed typical restless legs syndrome symptoms 1.5 and 4 months after starting erenumab 140 mg, respectively. In case 1 symptoms resolved when erenumab was stopped for two months but reappeared on galcanezumab. In both patients migraine attacks had dramatically decreased and no iron deficiency was found.

CONCLUSIONS

Even though caution is needed before establishing a causal relationship, these cases suggest that restless legs-like symptoms might be an emergent adverse event of CGRP antibodies, regardless of the mechanism of action. We propose that plastic changes in CGRP sensory fibers, which are very abundant in legs, induced by CGRP monoclonal antibodies could be the reason for restless legs syndrome development.

Analysis of the mylohyoid nerve in elderly Japanese cadavers for dental implant surgery

OBJECTIVES

Injury to the mandibular nerve (MN) branches may cause pain and irregular occlusal movement during mastication after mandibular dental treatments. Growing evidence indicates that the calcitonin gene-related peptide (CGRP) plays a key role in the development of peripheral sensitization and the associated enhanced pain, suggesting it may be a sign to ensure a safe and reliable dental implant treatment. Our focus was on the distribution of the MN branches and their communication with the lingual nerve (LN), the localized expression of CGRP, and the identification of a pain area related to the mylohyoid muscle (MM) fascia in the mandibular floor.

MATERIAL AND METHODS

In this study, MM samples from 440 sides of 303 human cadavers aged 61-103 years were examined microscopically and immunohistochemically. These data were further evaluated by the use of principal component analysis.

RESULTS

A complex but weak attachment site was identified for the fascia of the MM. CGRP expression was mainly located in small vessels and was scattered throughout the whole fascia of the MM. Communication between the MN and LN was found in 62.5% (275/440) of the samples. The results from the principal component analysis showed that the positive contributions were from the descending branch in the premolar region (correlation coefficient value R = 0.665), the ascending branch in the molar region (R = 0.709) and the intermediate branch of the digastric branch (R = 0.720) in component 1. In the fascia off the MM, strongly labeled CGRP-positive cells were also found around the blood vessels and the nerve.

CONCLUSIONS

The findings reported in this study indicate that there is a risk of damage when pulling the fascia off the MM at the border of the molar and premolar regions during dental implant surgery.

Ictal neck pain investigated in the interictal state – a search for the origin of pain

Background

Neck pain is reported in more than 50% of migraine patients during migraine attacks and may be an important source to migraine pain.

Objectives

To investigate phenotypical differences between migraine patients with and without ictal neck pain in the interictal phase. Additionally, to prospectively examine the association between pericranial muscle tenderness and the impending migraine attack.

Methods

Migraine patients (n = 100) and controls (n = 46) underwent a semi-structured interview and sensory testing interictally. Pericranial muscle tenderness was determined using total tenderness score and local tenderness score. The occurrence of migraine attacks was then prospectively recorded for the following seven days.

Results

Patients with ictal neck pain had increased tenderness of pericranial neck muscles compared to migraine patients without ( p = 0.023). Ictal neck pain was not associated with migraine localization, tension-type headache, or markers of central sensitization. Prospective data of 84 patients showed that tenderness of trigeminal sensory innervated muscles increased the migraine attack rate ( p = 0.035).

Conclusion

The distinction of migraine patients based on the occurrence of ictal neck pain could indicate migraine subtypes and possible involvement of peripheral tissue in the pathophysiology. Whether treatment responses differ among these groups would be fascinating. Additionally, we found that cephalic muscle tenderness is a risk factor for an impending migraine attack.

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.

Antagonism of Transient Receptor Potential Ankyrin Type-1 Channels as a Potential Target for the Treatment of Trigeminal Neuropathic Pain: Study in an Animal Model

Transient receptor potential ankyrin type-1 (TRPA1) channels are known to actively participate in different pain conditions, including trigeminal neuropathic pain, whose clinical treatment is still unsatisfactory. The aim of this study was to evaluate the involvement of TRPA1 channels by means of the antagonist ADM_12 in trigeminal neuropathic pain, in order to identify possible therapeutic targets. A single treatment of ADM_12 in rats 4 weeks after the chronic constriction injury of the infraorbital nerve (IoN-CCI) significantly reduced the mechanical allodynia induced in the IoN-CCI rats. Additionally, ADM_12 was able to abolish the increased levels of TRPA1, calcitonin gene-related peptide (CGRP), substance P (SP), and cytokines gene expression in trigeminal ganglia, cervical spinal cord, and medulla induced in the IoN-CCI rats. By contrast, no significant differences between groups were seen as regards CGRP and SP protein expression in the pars caudalis of the spinal nucleus of the trigeminal nerve. ADM_12 also reduced TRP vanilloid type-1 (TRPV1) gene expression in the same areas after IoN-CCI. Our findings show the involvement of both TRPA1 and TRPV1 channels in trigeminal neuropathic pain, and in particular, in trigeminal mechanical allodynia. Furthermore, they provide grounds for the use of ADM_12 in the treatment of trigeminal neuropathic pain.

Behavioral characteristics of capsaicin mediated cutaneous, myogenic, and arthrogenic orofacial nociception in rats

OBJECTIVE

To assess changes in orofacial tactile sensitivity and gnawing related to capsaicin-mediated cutaneous, myogenic, and arthrogenic nociception in the rat.

DESIGN

After recovery from anesthesia, orofacial tactile sensitivity and gnawing were assessed using operant testing methods following capsaicin application. Twenty female CD-Hairless rats were tested with bilateral capsaicin cream application to the cheek or with isoflurane anesthesia alone. Following several weeks of recovery, animals (n = 20) received either 10 μL unilateral masseter injections of vehicle, or phosphate buffered saline (PBS) to assess injection sensitization. After several weeks, masseter capsaicin (1.0%) injections (10 μL) were assessed compared to vehicle and PBS (n = 13). Weeks later capsaicin TMJ injections were evaluated. Animals (n = 11) received either 10 μL unilateral TMJ injections of capsaicin solution (1%) or vehicle.

RESULTS

Capsaicin cream to the skin significantly altered gnawing activity (increased puncture time by 248 s (p = 0.0002)) and tactile sensitivity (decreased tolerated bottle distance by 0.980 cm compared to isoflurane only (p = 0.0001)). Similarly, capsaicin masseter injection increased puncture time (339.6 s, p = 0.07) and decreased tolerated bottle distance (1.04 cm, p = 0.005) compared to vehicle. However, intra-articular capsaicin in the TMJ only modified gnawing (increased puncture time by 133 s), with no changes found in tactile sensitivity compared to vehicle.

CONCLUSION

Application of capsaicin to the skin and masseter had similar behavioral effects; however, intra-articular injections to the TMJ only affected gnawing. These data indicate the behavioral changes in rodent models of myogenic and cutaneous pain may be markedly different than models of arthrogenic pain originating from the TMJ.

The localization of calcitonin gene-related peptide in the human trigeminal ganglion and masseter muscle

The localization of calcitonin gene-related peptide (CGRP) is similar to that of a neurotransmitter which indicates masticatory muscle pain in the area of the masseter fascia. CGRP is released from the trigeminal ganglion (TG). The aim of this study was to analyze the distribution of CGRP in the fascia of the masseter muscle (FMM) and TG in a morphometric manner, with respect to the location and density of CGRP-immunopositive reaction fiber (CGRP-IRF). A higher number of the CGRP-IRF were mainly found located around elongated blood vessels and small nerves on the origin side of the middle zone FMM in the O group (presented with occlusion). In the sectional histochemical analysis of the O group, the CGRP-IRF were clearly detected in oval vessels, large elongated vessels and large nerves in contrast with that of the Non-O group (presented with no occlusion) samples. The number of CGRP-immunopositive ganglion cells (CGRP-IPGCs) in the O group mandibular nerve division was higher than that of other divisions. A reduction of the CGRP-IRF numbers were found in the no-loading groups. The characterization of these locations of CGRP-IPGCs can also provide useful data for the understanding of myofascial pain syndrome of the masseter muscle (MM).

Expression of CGRP in embryonic mouse masseter muscle

Neuropeptide calcitonin gene-related peptide (CGRP) is a mediator of inflammation and head pain that influences the functional vascular blood supply. The CGRP also regulate myoblast and acetylcholine receptors on neuromuscular junctions in development. However, little is known about its appearance and location during mouse masseter muscle (MM) development. We detected the mRNA abundance of CGRP, vascular genesis markers (Vascular endothelial growth factor A (VEGF-A), PECAM (CD31), lymphatic vessel endothelial hyaluronan receptor-1 (LYVE-1)) and embryonic and adult myosin heavy chain (MyHCs) (embryonic, IIa, IIb, and IIx) using real-time RT-PCR during development from the embryonic stage to after birth (E12.5, E14.5, E17.5, E18.5, P0, P1 and P5). We also endeavored to analyze the expression and localization of CGRP in situ hybridization in the developing mouse MM during development from the embryonic stage to after birth (E12.5, E14.5, E17.5, and P1). The antisense probe for CGRP was detected by in situ hybridization at E12.5, E14.5 E17.5 and then no longer detected after birth. The CGRP, CD31, embryonic MyHC abundance levels are highest at E17.5 (p<0.001) and they show a pattern similar to that of the other markers from E12.5 to P5. PCA analysis indicates a specific relation between CGRP and embryonic MyHC, CD31, and LYVE-1 in MM development. Cluster analyses identified the following distinct clusters for mRNA abundance in the MM: cluster 1, P5; cluster 2, E12.5, E14.5, E17.5, E18.5, P0, and P1. The positive correlation between CGRP and embryonic MyHC (Pearson's r>0.65; p<0.01) was analyzed. These data suggested that CGRP may have an influence on embryonic MyHC during mouse MM development. CGRP also affects the angiogenesis markers at embryonic stages.

Prolonged Jaw Opening Promotes Nociception and Enhanced Cytokine Expression

AIMS

To test the hypothesis that prolonged jaw opening, as can occur during routine dental procedures, increases nociceptive sensitivity of the masseter muscle and increases cytokine expression.

METHODS

Sprague-Dawley rats were used to investigate behavioral and cellular changes in response to prolonged jaw opening. A surgical retractor was placed around the maxillary and mandibular incisors, and the jaw was held at near maximal opening for 20 minutes. Head-withdrawal responses to mechanical stimuli applied to the facial skin overlying the left and right masseter muscles were determined following jaw opening. Cytokine levels in the upper cervical spinal cord containing the caudal part of the spinal trigeminal nucleus were evaluated using protein antibody microarrays (n = 3). Statistical analysis was performed using a nonparametric Mann-Whitney U test.

RESULTS

Prolonged jaw opening significantly increased nocifensive head withdrawal to mechanical stimuli at 2 hours, and days 3 and 7 postinduction (P < .05). The increase in nociceptive response resolved after 14 days. Sustained jaw opening also stimulated differential cytokine expression in the trigeminal ganglion and upper cervical spinal cord that persisted 14 days postprocedure (P < .05).

CONCLUSION

These findings provide evidence that near maximal jaw opening can lead to activation and prolonged sensitization of trigeminal neurons that results in nociceptive behavior evoked by stimulation of the masseter muscle, a physiologic event often associated with temporomandibular disorders (TMD). Results from this study may provide a plausible explanation for why some patients develop TMD after routine dental procedures that involve prolonged jaw opening.

New insights into protease-activated receptor 4 signaling pathways in the pathogenesis of inflammation and neuropathic pain: a literature review

Pain is an unpleasant sensory and emotional experience that is commonly associated with actual or potential tissue damage. Despite decades of pain research, many patients continue to suffer from chronic pain that is refractory to current treatments. Accumulating evidence has indicated an important role of protease-activated receptor 4 (PAR4) in the pathogenesis of inflammation and neuropathic pain. Here we reviewed PAR4 expression and activation via intracellular signaling pathways and the role of PAR4 signaling pathways in the development and maintenance of pain. Understanding PAR4 and its corresponding signaling pathways will provide insight to further explore the molecular basis of pain, which will also help to identify new targets for pharmacological intervention for pain relief.

Sensory nerve fibers containing calcitonin gene-related peptide in gastrocnemius, latissimus dorsi and erector spinae muscles and thoracolumbar fascia in mice

Chronic pain is a significant burden and much is attributed to back muscles. Back muscles and their associated fasciae make important and distinct contributions to back pain. Peptidergic nociceptors innervating these structures contribute to central transmission and pain modulation by peripheral and central actions. Plastic changes that augment and prolong pain are exhibited by neurons containing calcitonin gene-related peptide (CGRP) following muscle injury. Subpopulations of neurons containing this peptide have been identified in dorsal root ganglia but the distribution of their fibers in skeletal muscles and associated fasciae has not been fully documented. This study used multiple-labeling immunofluorescence and retrograde axonal tracing to identify dorsal root ganglion cells associated with muscle, and to characterize the distribution and density of their nerve fibers in mouse gastrocnemius and back muscles and in the thoracolumbar fascia. Most nerve fibers in these tissues contained CGRP and two major subpopulations of neurons were found: those containing CGRP and substance P (SP) and those containing CGRP but not SP. Innervation density was three times higher in the thoracolumbar fascia than in muscles of the back. These studies show mouse back and leg muscles are predominantly innervated by neurons containing CGRP, an important modulator of pain signal transmission. There are two distinct populations of neurons containing this peptide and their fibers were three times more densely distributed in the thoracolumbar fascia than back muscles.

TRPV1 channel-mediated bilateral allodynia induced by unilateral masseter muscle inflammation in rats

Pain in masticatory muscles is among the most prominent symptoms of temperomandibular disorders (TMDs) that have diverse and complex etiology. A common complaint of TMD is that unilateral pain of craniofacial muscle can cause a widespread of bilateral pain sensation, although the underlying mechanism remains unknown. To investigate whether unilateral inflammation of masseter muscle can cause a bilateral allodynia, we generated masseter muscle inflammation induced by unilateral injection of complete Freund’s adjuvant (CFA) in rats, and measured the bilateral head withdrawal threshold at different time points using a von Frey anesthesiometer. After behavioral assessment, both right and left trigeminal ganglia (TRG) were dissected and examined for histopathology and transient receptor potential vanilloid 1 (TRPV1) mRNA expression using quantitative real-time PCR analysis. A significant increase in TRPV1 mRNA expression occurred in TRG ipsilateral to CFA injected masseter muscle, whereas no significant alteration in TRPV1 occurred in the contralateral TRG. Interestingly, central injection of TRPV1 antagonist 5-iodoresiniferatoxin into the hippocampus significantly attenuated the head withdrawal response of both CFA injected and non-CFA injected contralateral masseter muscle. Our findings show that unilateral inflammation of masseter muscle is capable of inducing bilateral allodynia in rats. Upregulation of TRPV1 at the TRG level is due to nociception caused by inflammation, whereas contralateral nocifensive behavior in masticatory muscle nociception is likely mediated by central TRPV1, pointing to the involvement of altered information processing in higher centers.

Bilateral increase in expression and concentration of tachykinin in a unilateral rabbit muscle overuse model that leads to myositis

Background

Tachykinins can have pro-inflammatory as well as healing effects during tissue reorganization and inflammation. Recent studies report an up-regulation in the expression of the substance P (SP)-preferred receptor, the neurokinin-1 receptor, in marked muscle inflammation (myositis). There is, however, only very little information on the expression patterns and levels of tachykinins in this situation.

Methods

The tachykinin system was analyzed using a rabbit experimental model of muscle overuse, whereby unilateral muscle exercise in combination with electrical stimulation led to muscle derangement and myositis in the triceps surae muscle (experimental length 1–6 weeks). Evaluations were made for both parts of the muscle (soleus and gastrocnemius muscles) in experimental and non-experimental (contralateral) sides. Morphologic evaluation, immunohistochemistry, in situ hybridization and enzyme immunoassay (EIA) analyses were applied.

Results

Myositis and muscle derangement occurred focally not only in the experimental side but also in the non-experimental side. In the inflammatory areas (focal myositis areas), there were frequent nerve fibers showing tachykinin-like immunoreactivity and which were parts of nerve fascicles and which were freely dispersed in the tissue. Cells in the inflammatory infiltrates showed tachykinin-like immunoreactivity and tachykinin mRNA expression. Specific immunoreactivity and mRNA expression were noted in blood vessel walls of both sides, especially in focally affected areas. With increasing experimental length, we observed an increase in the degree of immunoreactivity in the vessel walls. The EIA analyses showed that the concentration of tachykinin in the tissue on both sides increased in a time-dependent manner. There was a statistical correlation in the concentration of tachykinin and the level of tachykinin immunoreactivity in the blood vessel walls between experimental and non-experimental sides.

Conclusions

The observations show an up-regulation of the tachykinin system bilaterally during muscle derangement/myositis in response to pronounced unilateral muscle overuse. This up-regulation occurred in inflammatory areas and was related not only to increased tachykinin innervation but also to tachykinin expression in blood vessel walls and inflammatory cells. Importantly, the tachykinin system appears to be an important factor not only ipsilaterally but also contralaterally in these processes.

Marked Effects of Tachykinin in Myositis Both in the Experimental Side and Contralaterally: Studies on NK-1 Receptor Expressions in an Animal Model

Muscle injury and inflammation (myositis) in a rabbit model of an unilateral muscle overuse were examined. It is unknown if the tachykinin system has a functional role in this situation. In this study, therefore, the neurokinin-1 receptor (NK-1R) expression patterns were evaluated. White blood cells, nerve fascicles, fine nerve fibers, and blood vessel walls in myositis areas showed NK-1R immunoreaction. NK-1R mRNA reactions were observable for white blood cells and blood vessel walls of these areas. NK-1R immunoreaction and NK-1R mRNA reactions were also seen for muscle fibers showing degenerative and regenerative features. There were almost no NK-1R immunoreactions in normal muscle tissue. Interestingly, marked NK-1R expressions were seen for myositis areas of both the experimental side and the contralateral nonexperimental side. EIA analyses showed that the concentration of substance P in the muscle tissue was clearly increased bilaterally at the experimental end stage, as compared to the situation for normal muscle tissue. These observations show that the tachykinin system is very much involved in the processes that occur in muscle injury/myositis. The effects can be related to proinflammatory effects and/or tissue repair. The fact that there are also marked NK-1R expressions contralaterally indicate that the tachykinin system has crossover effects.

Behavioral testing in rodent models of orofacial neuropathic and inflammatory pain

Orofacial pain conditions are often very debilitating to the patient and difficult to treat. While clinical interest is high, the proportion of studies performed in the orofacial region in laboratory animals is relatively low, compared with other body regions. This is partly due to difficulties in testing freely moving animals and therefore lack of reliable testing methods. Here we present a comprehensive review of the currently used rodent models of inflammatory and neuropathic pain adapted to the orofacial areas, taking into account the difficulties and drawbacks of the existing approaches. We examine the available testing methods and procedures used for assessing the behavioral responses in the face in both mice and rats and provide a summary of some pharmacological agents used in these paradigms to date. The use of these agents in animal models is also compared with outcomes observed in the clinic.

Keratinocyte expression of calcitonin gene-related peptide β: implications for neuropathic and inflammatory pain mechanisms

Calcitonin gene-related peptide (CGRP) is a vasodilatory peptide that has been detected at high levels in the skin, blood, and cerebrospinal fluid (CSF) under a variety of inflammatory and chronic pain conditions, presumably derived from peptidergic C and Aδ innervation. Herein, CGRP immunolabeling (IL) was detected in epidermal keratinocytes at levels that were especially high and widespread in the skin of humans from locations afflicted with postherpetic neuralgia (PHN) and complex region pain syndrome type 1 (CRPS), of monkeys infected with simian immunodeficiency virus, and of rats subjected to L5/L6 spinal nerve ligation, sciatic nerve chronic constriction, and subcutaneous injection of complete Freund's adjuvant. Increased CGRP-IL was also detected in epidermal keratinocytes of transgenic mice with keratin-14 promoter driven overexpression of noggin, an antagonist to BMP-4 signaling. Transcriptome microarray, quantitative Polymerase Chain Reaction (qPCR), and Western blot analyses using laser-captured mouse epidermis from transgenics, monolayer cultures of human and mouse keratinocytes, and multilayer human keratinocyte organotypic cultures, revealed that keratinocytes express predominantly the beta isoform of CGRP. Cutaneous peptidergic innervation has been shown to express predominantly the alpha isoform of CGRP. Keratinocytes also express the cognate CGRP receptor components, Calcitonin receptor-like receptor (CRLR), Receptor activity-modifying protein 1 (RAMP1), CGRP-receptor component protein (RCP) consistent with known observations that CGRP promotes several functional changes in keratinocytes, including proliferation and cytokine production. Our results indicate that keratinocyte-derived CGRPβ may modulate epidermal homeostasis through autocrine/paracrine signaling and may contribute to chronic pain under pathological conditions.

Repeated muscle injury as a presumptive trigger for chronic masticatory muscle pain

skeletal muscles sustain a significant loss of maximal contractile force after injury, but terminally damaged fibers can eventually be replaced by the growth of new muscle (regeneration), with full restoration of contractile force over time. After a second injury, limb muscles exhibit a smaller reduction in maximal force and reduced inflammation compared with that after the initial injury (i.e., repeated bout effect). In contrast, masticatory muscles exhibit diminished regeneration and persistent fibrosis, after a single injury; following a second injury, plasma extravasation is greater than after a single injury and maximal force is decreased more than after the initial injury. Thus, masticatory muscles do not exhibit a repeated bout effect and are instead increasingly damaged by repeated injury. We propose that the impaired ability of masticatory muscles to regenerate contributes to chronic muscle pain by leading to an accumulation of tissue damage, fibrosis, and a persistent elevation and prolonged membrane translocation of nociceptive channels such as P2X(3) as well as enhanced expression of neuropeptides including CGRP within primary afferent neurons. These transformations prime primary afferent neurons for enhanced responsiveness upon subsequent injury thus triggering and/or exacerbating chronic muscle pain.

Role of neuropeptides in skin inflammation and its involvement in diabetic wound healing

IMPORTANCE OF THE FIELD

In 2010, the world prevalence of diabetes is 6.4%, affecting 285 million adults. Diabetic patients are at risk of developing neuropathy and delayed wound healing that can culminate in incurable diabetic foot ulcerations (DFUs) or even foot amputation.

AREAS COVERED IN THIS REVIEW

The contrast between cellular and molecular events of wound healing and diabetic wound healing processes is characterized. Neuropeptides released from the autonomous nervous system and skin cells reveal a major role in the immunity of wound healing. Therefore, the signaling pathways that induce pro/anti-inflammatory cytokines expression and its involvement in diabetic wound healing are discussed. The involvement of neuropeptides in the activation, growth, migration and maturation of skin cells, like keratinocytes, Langerhans cells, macrophages and mast cells, are described.

WHAT THE READER WILL GAIN

This review attempts to address the role of neuropeptides in skin inflammation, focusing on signal transduction, inflammatory mediators and pro/anti-inflammatory function, occurring in each cell type, as well as, its connection with diabetic wound healing.

TAKE HOME MESSAGE

Understanding the role of neuropeptides in the skin, their application on skin wounds could be a potential therapy for skin pathologies, like the problematic and prevalent DFUs.

Light touch induces ERK activation in superficial dorsal horn neurons after inflammation: involvement of spinal astrocytes and JNK signaling in touch-evoked central sensitization and mechanical allodynia

Activation of extracellular signal-regulated kinase (ERK) in spinal cord neurons could serve as a marker for sensitization of dorsal horn neurons in persistent pain. ERK is normally activated by high-threshold noxious stimuli. We investigated how low-threshold mechanical stimuli could activate ERK after complete Freund's adjuvant (CFA)-induced inflammation. Unilateral injection of CFA induced ipsilateral heat hyperalgesia and bilateral mechanical allodynia. CFA-induced ERK activation in ipsilateral dorsal horn neurons declined after 2 days. Interestingly, low-threshold mechanical stimulation given by light touch either on the inflamed paw or the contralateral non-inflamed paw dramatically increased ERK phosphorylation in the dorsal horn ipsilateral to touch stimulation. Notably, light touch induced ERK phosphorylation mainly in superficial neurons in laminae I-IIo. Intrathecal administration of the astroglial toxin L-α-aminoadipate on post-CFA day 2 reversed CFA-induced bilateral mechanical allodynia but not heat hyperalgesia. Furthermore, L-α-aminoadipate, the glial inhibitor fluorocitrate, and a peptide inhibitor of c-Jun N-terminal Kinase all reduced light touch-evoked ERK activation ipsilateral to touch. Collectively, these data suggest that (i) ERK can be activated in superficial dorsal horn neurons by low-threshold mechanical stimulation under pathological condition and (ii) ERK activation by light touch is associated with mechanical allodynia and requires an astrocyte network.

The c-Jun N-terminal kinase 1 (JNK1) in spinal astrocytes is required for the maintenance of bilateral mechanical allodynia under a persistent inflammatory pain condition

Peripheral inflammation induces persistent central sensitization characterized by mechanical allodynia and heat hyperalgesia that are mediated by distinct mechanisms. Compared to well-demonstrated mechanisms of heat hyperalgesia, mechanisms underlying the development of mechanical allodynia and contralateral pain are incompletely known. In this study, we investigated the distinct role of spinal JNK in heat hyperalgesia, mechanical allodynia, and contralateral pain in an inflammatory pain model. Intraplantar injection of complete Freund's adjuvant (CFA) induced bilateral mechanical allodynia but unilateral heat hyperalgesia. CFA also induced a bilateral activation (phosphorylation) of JNK in the spinal cord, and the phospho JNK1 (pJNK1) levels were much higher than that of pJNK2. Notably, both pJNK and JNK1 were expressed in GFAP-positive astrocytes. Intrathecal infusion of a selective peptide inhibitor of JNK, D-JNKI-1, starting before inflammation via an osmotic pump, reduced CFA-induced mechanical allodynia in the maintenance phase but had no effect on CFA-induced heat hyperalgesia. A bolus intrathecal injection of D-JNKI-1 or SP600126, a small molecule inhibitor of JNK also reversed mechanical allodynia bilaterally. In contrast, peripheral (intraplantar) administration of D-JNKI-1 reduced the induction of CFA-induced heat hyperalgesia but did not change mechanical allodynia. Finally, CFA-induced bilateral mechanical allodynia was attenuated in mice lacking JNK1 but not JNK2. Taken together, our data suggest that spinal JNK, in particular JNK1 plays an important role in the maintenance of persistent inflammatory pain. Our findings also reveal a unique role of JNK1 and astrocyte network in regulating tactile allodynia and contralateral pain.

Glyceroltrinitrate facilitates stimulated CGRP release but not gene expression of CGRP or its receptor components in rat trigeminal ganglia

Nitric oxide (NO) donors induce delayed headaches in migraineurs. In a corresponding rat model NO donors cause delayed ongoing activity in central trigeminal neurons which process intracranial afferent input. Cellular models indicate that NO may increase the release or production of calcitonin gene-related peptide (CGRP), a key mediator in primary headaches. CGRP release from intact isolated trigeminal ganglia of adult male Wistar rats was investigated in vitro. Exposure to high NO donor concentrations did not affect basal or stimulated CGRP release. After a two hour infusion of the NO donor glyceroltrinitrate (250microg/kg/h), however, inflammatory mediators-induced CGRP release was 80% higher compared to control animals. Administration of the soluble guanylate cyclase inhibitor ODQ or the application of 8Br-cGMP revealed a cGMP-independent mechanism. In four groups of separate experiments total mRNA was extracted from rat trigeminal ganglia up to 6h after glyceroltrinitrate or saline infusion. Gene expression of CGRP and the CGRP-receptor components, receptor activity-modifying protein 1, receptor component protein and calcitonin receptor-like receptor was measured by quantitative RT-PCR. Glyceroltrinitrate infusion did not change mRNA levels of these genes compared to infusion of saline. The present data suggest that prolonged increase in NO levels facilitates stimulated CGRP release from trigeminal ganglion neurons. The underlying mechanism appears to be independent of the cGMP pathway and not to interact with CGRP in the trigeminal ganglion. Delayed headaches induced by NO may change CGRP or CGRP-receptor expression.

alphaCGRP and betaCGRP transcript amount in mouse tissues of various developmental stages and their tissue expression sites

The calcitonin gene-related peptides (CGRP), alphaCGRP and betaCGRP, have been implicated to play various roles in primates and rodent. However, since the expression information has been limited, in the present study, we measured the amount of gene expression in mouse brain, liver, kidney, heart, and testis at embryonic day (E) 14, E17, postnatal day (P) 1, P7, and adult using real-time PCR, and determined the precise localization of alphaCGRP and betaCGRP sense/antisense transcripts in tissues using in situ hybridization. The sense transcripts of alphaCGRP and betaCGRP were found mainly in brain, and their amount profiles were similar in the course of development: one expression peak was observed at E17 and the other at P7. The amounts of alphaCGRP transcripts were greater than those of betaCGRP transcripts in the range between 3.6 and 31 times. In the E17 and P7 brains, the localization pattern of alphaCGRP sense transcripts was similar with that of alphaCGRP antisense transcripts. Fewer transcripts were found in neuroblasts of E17 corpus callosum, and neuroblasts of P7 corpus callosum, olfactory bulb, plexus chorioideus, and ventriculus lateralis than in other brain areas. The localization pattern of betaCGRP sense and antisense transcripts was similar to that for alphaCGRP except that the betaCGRP antisense transcripts showed spot-like localizations. Additionally, the alphaCGRP sense transcript, and betaCGRP sense and antisense transcripts were found in parafollicular cells (C cells) of E17 thyroid lobe. These findings together indicate that alphaCGRP and betaCGRP have their own roles in the ontogenic process.

Effect of memantine on the levels of neuropeptides and microglial cells in the brain regions of rats with neuropathic pain

Neuropathic pain induced by sciatic nerve injury not only causes peripheral dysfunctions but also affects the cortical and subcortical regions of the brain. It is still unknown whether neuropathic pain could relate to behavioral and neurochemical alterations in the central nervous system. This paper deals with the effect of peripheral neuropathic pain on mechanical allodynia, neuropeptide levels, neuropeptide-degrading enzyme activities, and microglial cells in the brain regions of rats by applying chronic constriction injury, a partial sciatic nerve injury. We examined the possible protection effect on the allodynia and changes in levels of neuropeptides and microglial activation in chronic constriction injury of the rat brain by memantine. On 4 days after chronic constriction injury, the induction of mechanical allodynia was suppressed by memantine treatment. Reductions in the substance P in the hypothalamus and somatostatin in the periaqueductal gray of chronic constriction injury rat brain were reversed by memantine. This suggests the role of these neuropeptides in pain information processing in the brain. Immunohistochemical experiments revealed that the expression of CD11b, a marker protein of microglia, was increased in the hypothalamus and periaqueductal gray in the chronic constriction injury rat brain as compared with the controls, and memantine treatment could suppress the activation of microglia, suggesting the involvement of microglia in pain mechanism. The present behavioral, biochemical, and immunohistochemical studies demonstrated that peripheral neuropathic pain affects the neuropeptide levels and microglial activation in the brain regions, and these events described above may play an important role in neuropathic pain pathogenesis.

TNFalpha mechanically sensitizes masseter muscle afferent fibers of male rats

Behavioral evidence in rats indicates that injection of tumor necrosis factor alpha (TNFalpha) into skeletal muscle results in a prolonged mechanical sensitization without gross inflammation. To investigate whether a peripheral mechanism could underlie this effect, in the present study, TNFalpha (1 or 0.1 microg) was injected into the rat masseter muscle to assess its effect on the excitability and mechanical threshold (MT) of muscle nociceptors as well as on inflammation. Expression of TNFR1 (P55 receptors) and TNFR2 (P75 receptors) by the masseter muscle and trigeminal ganglion neurons that innervate that muscle was determined by Western blot and immunohistochemistry, respectively. The Evans blue dye technique was used at the end of the TNFalpha experiments to assess for plasma protein extravasation. In subsequent experiments to confirm the involvement of receptor activation in TNFalpha-induced effects, P55 or P75 receptor antibody was co-injected with TNFalpha. Intramuscular injection of 1 microg TNFalpha did not excite nociceptors but did significantly decrease MT compared with vehicle control. There was no evidence of gross inflammation 3 h after injection of TNFalpha. Co-injection of TNFalpha with P55 or P75 receptor antibodies attenuated TNFalpha-induced mechanical sensitization. P55 and P75 receptors were expressed by 29 and 62% of masseter nociceptors, respectively. These findings indicate that TNFalpha induces mechanical sensitization of masseter nociceptors that is mediated through activation of peripheral P55 and P75 receptors. These results support the hypothesis that a peripheral receptor mechanism could contribute to TNFalpha-induced noninflammatory mechanical sensitization of skeletal muscle previously reported in behaving rats.

Increases in transient receptor potential vanilloid-1 mRNA and protein in primary afferent neurons stimulated by protein kinase C and their possible role in neurogenic inflammation

A recent study by our group demonstrates pharmacologically that the transient receptor potential vanilloid-1 (TRPV(1)) is activated by intradermal injection of capsaicin to initiate neurogenic inflammation by the release of neuropeptides in the periphery. In this study, expression of TRPV(1), phosphorylated protein kinase C (p-PKC), and calcitonin gene-related peptide (CGRP) in dorsal root ganglion (DRG) neurons was visualized by using immunofluorescence, real-time PCR, and Western blots to examine whether increases in TRPV(1) mRNA and protein levels evoked by capsaicin injection are subject to modulation by the activation of PKC and to analyze the role of this process in the pathogenesis of neurogenic inflammation. Capsaicin injection into the hindpaw skin of anesthetized rats evoked increases in the expression of TRPV(1), CGRP and p-PKC in mRNA and/or protein levels and in the number of single labeled TRPV(1), p-PKC, and CGRP neurons in ipsilateral L4-5 DRGs. Coexpressions of TRPV(1) with p-PKC and/or CGRP in DRG neurons were also significantly increased after CAP injection. These evoked expressions at both molecular and cellular levels were significantly inhibited after TRPV(1) receptors were blocked by 5'-iodoresiniferatoxin (5 microg) or PKC was inhibited by chelerythrine chloride (5 microg). Taken together, these results provide evidence that up-regulation of TRPV(1) mRNA and protein levels under inflammatory conditions evoked by capsaicin injection is subject to modulation by the PKC cascade in which increased CGRP level in DRG neurons may be related to the initiation of neurogenic inflammation. Thus, up-regulation of TRPV(1) receptors in DRG neurons seems critical for initiating acute neurogenic inflammation.

Inflammation and neuropeptides: the connection in diabetic wound healing

Abnormal wound healing is a major complication of both type 1 and type 2 diabetes, with nonhealing foot ulcerations leading in the worst cases to lower-limb amputation. Wound healing requires the integration of complex cellular and molecular events in successive phases of inflammation, cell proliferation, cell migration, angiogenesis and re-epithelialisation. A link between wound healing and the nervous system is clinically apparent as peripheral neuropathy is reported in 30-50% of diabetic patients and is the most common and sensitive predictor of foot ulceration. Indeed, a bidirectional connection between the nervous and the immune systems and its role in wound repair has emerged as one of the focal features of the wound-healing dogma. This review provides a broad overview of the mediators of this connection, which include neuropeptides and cytokines released from nerve fibres, immune cells and cutaneous cells. In-depth understanding of the signalling pathways in the neuroimmune axis in diabetic wound healing is vital to the development of successful wound-healing therapies.

Repression of calcitonin gene-related peptide expression in trigeminal neurons by a Theobroma cacao extract

ETHNOPHARMACOLOGICAL RELEVANCE

Cocoa bean preparations were first used by the ancient Maya and Aztec civilizations of South America to treat a variety of medical ailments involving the cardiovascular, gastrointestinal, and nervous systems. Diets rich in foods containing abundant polyphenols, as found in cocoa, underlie the protective effects reported in chronic inflammatory diseases. Release of calcitonin gene-related peptide (CGRP) from trigeminal nerves promotes inflammation in peripheral tissues and nociception.

AIM OF THE STUDY

To determine whether a methanol extract of Theobroma cacao L. (Sterculiaceae) beans enriched for polyphenols could inhibit CGRP expression, both an in vitro and an in vivo approach was taken.

RESULTS

Treatment of rat trigeminal ganglia cultures with depolarizing stimuli caused a significant increase in CGRP release that was repressed by pretreatment with Theobroma cacao extract. Pretreatment with Theobroma cacao was also shown to block the KCl- and capsaicin-stimulated increases in intracellular calcium. Next, the effects of Theobroma cacao on CGRP levels were determined using an in vivo model of temporomandibular joint (TMJ) inflammation. Capsaicin injection into the TMJ capsule caused an ipsilateral decrease in CGRP levels. Theobroma cacao extract injected into the TMJ capsule 24h prior to capsaicin treatment repressed the stimulatory effects of capsaicin.

CONCLUSIONS

Our results demonstrate that Theobroma cacao extract can repress stimulated CGRP release by a mechanism that likely involves blockage of calcium channel activity. Furthermore, our findings suggest that the beneficial effects of diets rich in cocoa may include suppression of sensory trigeminal nerve activation.

Injection of adjuvant but not acidic saline into craniofacial muscle evokes nociceptive behaviors and neuropeptide expression

Craniofacial muscle pain including muscular temporomandibular disorders accounts for a substantial portion of all pain perceived in the head and neck region. In spite of its high clinical prevalence, the mechanisms of chronic craniofacial muscle pain are not well understood. Injection of acidic saline into rodent hindlimb muscles produces pathologies which resemble muscular pathologies in chronic pain patients. Here we investigated whether analogous transformations occur following repeated injections of acidic saline into the rat masseter muscle. Injection of acidic saline (pH 4) into the masseter muscle transiently lowered i.m. pH to levels comparable to those reported for rodent hindlimb muscles. Nevertheless, repeated unilateral or bilateral injections of acidic saline (pH 4) into the masseter muscle failed to alter nociceptive behavioral responses as occurs in the hindlimb. Changing the pH of injected saline to pH 3.0 or 5.0 also did not evoke nocifensive behavior. Acid sensing ion channel 3 receptors, which are implicated in transformations following acidification of hindlimb muscles, were found on trigeminal ganglion muscle afferent neurons via combined neuronal tracing and immunocytochemistry. In contrast to the acidic saline, injection of complete Freund's adjuvant (CFA) into the masseter muscle induced mechanical allodynia for 3 weeks, thermal hyperalgesia for 1 week and an increase in the number of calcitonin gene-related peptide (CGRP)-immunoreactive muscle afferent neurons in the trigeminal ganglion. Although pH may alter CGRP release in primary afferent neurons, the number of CGRP-muscle afferent neurons did not change following i.m. injection of acidic saline. Further, there was no change in ganglionic iCGRP levels at 1, 4 or 12 days after i.m. injection of acidic saline. While these findings extend our earlier reports that CFA-induced muscle inflammation results in behavioral and neuropeptide changes they further suggest that i.m. acidification in craniofacial muscle evokes different responses than in hindlimb muscle and imply that disparate proton sensing mechanisms underlie these discrepancies.

Downregulation of selective microRNAs in trigeminal ganglion neurons following inflammatory muscle pain

Active regulation of gene expression in the nervous system plays an important role in the development and/or maintenance of inflammatory pain. MicroRNA (miRNA) negatively regulates gene expression via posttranscriptional or transcriptional inhibition of specific genes. To explore the possible involvement of miRNA in gene regulation during inflammatory pain, we injected complete Freund's adjuvant (CFA) unilaterally into the rat masseter muscle and quantified changes in neuron-specific mature miRNAs in the trigeminal ganglion (TG). Real-time reverse-transcription polymerase chain reaction revealed significant, but differential, downregulation of mature miR-10a, -29a, -98, -99a, -124a, -134, and -183 in the ipsilateral mandibular division (V3) of the TG within 4 hr after CFA. In contrast, levels of tested miRNAs did not change significantly in the contralateral V3 or the ipsilateral ophthalmic and maxillary divisions of the TG from inflamed rats, nor in the ipsilateral V3 of saline-injected animals. The downregulated miRNAs recovered differentially to a level equal to or higher than that in naive animals. Full recovery time varied with miRNA species but was at least 4 days. Expression and downregulation of some miRNAs were further confirmed by in situ hybridization of TG neurons that innervate the inflamed muscle. Although neurons of all sizes expressed these miRNAs, their signals varied between neurons. Our results indicate that miRNA species specific to neurons are quickly regulated following inflammatory muscle pain.