Abstract
Chronic temporomandibular joint disorders (TMDs) present with pain in the temporomandibular joint (TMJ) and muscles of mastication. Risk factors for TMD include localized joint/muscle inflammation and estrogen status. This study determined whether mild tissue inflammation and estrogen status influenced the responses of trigeminal ganglion neurons to jaw palpation or jaw movement, 2 key diagnostic features of clinical TMD, in adult rats. Neuronal activity was recorded from male rats, ovariectomized (OvX) female rats, and OvX female rats injected with 17β-estradiol 24 h prior to testing (OvXE). Neurons were tested for responses to deep press over the TMJ region and jaw movement in 3 directions (open, protrusion, lateral) 10 d after intra-TMJ injection of a low dose of complete Freund's adjuvant (CFA) or vehicle (sham). Deep press evoked similar responses in all treatment groups. The response magnitude to jaw opening and protrusion was significantly greater for neurons recorded from OvXE CFA-treated rats than from OvX CFA-treated or OvXE sham rats. The responses to lateral movement of the jaw were similar across all treatment groups. Most neurons (70% to 90%) displayed a static response pattern to jaw movement independent of direction. Estradiol treatment also increased the proportion of neurons that were excited by jaw movement in >1 direction as compared with untreated OvX females or males. These results suggest that mild localized inflammation in the TMJ region during periods of elevated estrogen were sufficient to increase the peripheral driving force for jaw movement-evoked hyperalgesia.