Temporomandibular joint pain: A critical role for Trpv4 in the trigeminal ganglion

Temporomandibular joint disorder (TMJD) is known for its mastication-associated pain. TMJD is medically relevant because of its prevalence, severity, chronicity, the therapy-refractoriness of its pain, and its largely elusive pathogenesis. Against this background, we sought to investigate the pathogenetic contributions of the calcium-permeable TRPV4 ion channel, robustly expressed in the trigeminal ganglion sensory neurons, to TMJ inflammation and pain behavior. We demonstrate here that TRPV4 is critical for TMJ-inflammation-evoked pain behavior in mice and that trigeminal ganglion pronociceptive changes are TRPV4-dependent. As a quantitative metric, bite force was recorded as evidence of masticatory sensitization, in keeping with human translational studies. In Trpv4^-/- mice with TMJ inflammation, attenuation of bite force was significantly less than in wildtype (WT) mice. Similar effects were seen with systemic application of a specific TRPV4 inhibitor. TMJ inflammation and mandibular bony changes were apparent after injections of complete Freund adjuvant but were remarkably independent of the Trpv4 genotype. It was intriguing that, as a result of TMJ inflammation, WT mice exhibited significant upregulation of TRPV4 and phosphorylated extracellular-signal-regulated kinase (ERK) in TMJ-innervating trigeminal sensory neurons, which were absent in Trpv4^-/- mice. Mice with genetically-impaired MEK/ERK phosphorylation in neurons showed resistance to reduction of bite force similar to that of Trpv4^-/- mice. Thus, TRPV4 is necessary for masticatory sensitization in TMJ inflammation and probably functions upstream of MEK/ERK phosphorylation in trigeminal ganglion sensory neurons in vivo. TRPV4 therefore represents a novel pronociceptive target in TMJ inflammation and should be considered a target of interest in human TMJD.