Abstract
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.
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