Characteristics of glutamate-evoked temporomandibular joint afferent activity in the rat

Injection of glutamate into the rat temporomandibular joint (TMJ) capsule c an reflexly induce a prolonged increase in the electromyographic (EMG) acti vity of the jaw muscles, however, the characteristics of TMJ afferents acti vated by glutamate have not been investigated. In the present study, we exa mined the effect of glutamate injection into the TMJ capsule on jaw muscle EMG activity and the extracellularly recorded activity of single trigeminal afferents that had receptive fields in the TMJ tissue and antidromically i dentified projections to the brain stem subnucleus caudalis (Vc) in rats of both sexes. Glutamate (0.05-1.0 M, 10 mul) injection into the TMJ capsule evoked EMG activity in a dose-related manner; however, at concentrations of 0.5 and 1.0 M, glutamate-evoked digastric muscle responses were greater in female than in male rats. In experiments where jaw muscle EMG and afferent activity were recorded simultaneously, glutamate (0.5 M, 10 mul) injection into the TMJ capsule evoked activity in the jaw muscles as well as in 27 ( 26 A delta and 1 C-fiber afferent) of 34 trigeminal afferents that could be activated by blunt mechanical stimulation of the TMJ tissue. In these expe riments, glutamate-evoked jaw muscle activity was significantly increased f or 6 min after the glutamate injection, whereas afferent activity was signi ficantly increased only during the first minute after the glutamate injecti on. The glutamate-evoked afferent activity was inversely related to conduct ion velocity and, in afferents with conduction velocities <10 m/s, was sign ificantly greater in female (n = 6) than in male (n = 10) rats. These resul ts suggest that glutamate excites putative nociceptive afferents within the TMJ to a greater degree in female than in male rats. This sex-related diff erence in afferent discharge may, in part, underlie sex-related differences in glutamate-evoked jaw muscle EMG activity.