Electrophysiological characterization of the rat trigeminal caudalis (Vc) neurons following intramuscular injection of capsaicin

Peripheral glutamate receptor and transient receptor potential channel mechanisms of craniofacial muscle pain

Temporomandibular joint disorder is a common chronic craniofacial pain condition, often involving persistent, widespread craniofacial muscle pain. Although the etiology of chronic muscle pain is not well known, sufficient clinical and preclinical information supports a contribution of trigeminal nociceptors to craniofacial muscle pain processing under various experimental and pathological conditions. Here, we review cellular and molecular mechanisms underlying sensitization of muscle nociceptive afferents. In particular, we summarize findings on pronociceptive roles of peripheral glutamate in humans, and we discuss mechanistic contributions of glutamate receptors, including N-methyl-D-aspartate receptors and metabotropic glutamate receptors, which have considerably increased our understanding of peripheral mechanisms of craniofacial muscle pain. Several members of the transient receptor potential (TRP) family, such as transient receptor potential vanilloid 1 (TRPV1) and transient receptor potential ankyrin 1, also play essential roles in the development of spontaneous pain and mechanical hypersensitivity in craniofacial muscles. Furthermore, glutamate receptors and TRP channels functionally and bi-directionally interact to modulate trigeminal nociceptors. Activation of glutamate receptors invokes protein kinase C, which leads to the phosphorylation of TRPV1. Sensitization of TRPV1 by inflammatory mediators and glutamate receptors in combination with endogenous ligands contributes to masseter hyperalgesia. The distinct intracellular signaling pathways through which both receptor systems engage and specific molecular regions of TRPV1 are offered as novel targets for the development of mechanism-based treatment strategies for myogenous craniofacial pain conditions.

Transcriptome analysis of trigeminal ganglia following masseter muscle inflammation in rats

Background

Chronic pain in masticatory muscles is a major medical problem. Although mechanisms underlying persistent pain in masticatory muscles are not fully understood, sensitization of nociceptive primary afferents following muscle inflammation or injury contributes to muscle hyperalgesia. It is well known that craniofacial muscle injury or inflammation induces regulation of multiple genes in trigeminal ganglia, which is associated with muscle hyperalgesia. However, overall transcriptional profiles within trigeminal ganglia following masseter inflammation have not yet been determined. In the present study, we performed RNA sequencing assay in rat trigeminal ganglia to identify transcriptome profiles of genes relevant to hyperalgesia following inflammation of the rat masseter muscle.

Results

Masseter inflammation differentially regulated >3500 genes in trigeminal ganglia. Predominant biological pathways were predicted to be related with activation of resident non-neuronal cells within trigeminal ganglia or recruitment of immune cells. To focus our analysis on the genes more relevant to nociceptors, we selected genes implicated in pain mechanisms, genes enriched in small- to medium-sized sensory neurons, and genes enriched in TRPV1-lineage nociceptors. Among the 2320 candidate genes, 622 genes showed differential expression following masseter inflammation. When the analysis was limited to these candidate genes, pathways related with G protein-coupled signaling and synaptic plasticity were predicted to be enriched. Inspection of individual gene expression changes confirmed the transcriptional changes of multiple nociceptor genes associated with masseter hyperalgesia (e.g., Trpv1, Trpa1, P2rx3, Tac1, and Bdnf) and also suggested a number of novel probable contributors (e.g., Piezo2, Tmem100, and Hdac9).

Conclusion

These findings should further advance our understanding of peripheral mechanisms involved in persistent craniofacial muscle pain conditions and provide a rational basis for identifying novel genes or sets of genes that can be potentially targeted for treating such conditions.

Peripheral metabotropic glutamate receptor subtype 5 contributes to inflammation-induced hypersensitivity of the rat temporomandibular joint

Temporomandibular disorders (TMD) comprise an assortment of clinical conditions characterized by pain in the temporomandibular joint (TMJ). TMD patients have a variety of symptoms, including jaw movement disorder and TMJ pain. Metabotropic glutamate receptor subtype 5 (mGluR5) was reported to be involved in pain processing in several animal models of neuropathic and inflammatory pain. In this study, the head withdrawal threshold and mGluR5 expression were investigated in rats with complete Freund's adjuvant (CFA)-induced TMJ inflammatory pain. CFA injection into the TMJ significantly decreased the mechanical head withdrawal thresholds relative to vehicle injection, and the effects were blocked by pre-injection of 2-methyl-6-(phenylethynyl)-pyridine (MPEP). mGluR5 expression in the trigeminal ganglion was predominantly increased in the CFA-injected group compared with the normal control group. Pretreatment with MPEP, a selective mGluR5 antagonist, reduced mGluR5 expression in the trigeminal ganglion compared with the CFA group, as measured by immunohistochemistry, western blotting, and RT-PCR. Significant differences in the proportion or intensity of mGluR5 expression were found in animals with inflammation versus control animals at the examined time point. These findings indicate a role for peripheral mGluR5 in CFA-induced nociceptive behavior and TMJ inflammation. Peripheral application of mGluR5 antagonists could provide therapeutic benefits for inflammatory TMJ pain.