Activation of peripheral GABAA receptors inhibits temporomandibular joint-evoked jaw muscle activity

We have previously shown that injection of mustard oil or glutamate into rat temporomandibular joint (TMJ) tissues, an experimental model of acute TMJ injury, can reflexly induct a prolonged increase in the activity of both digastric (jaw-opener) and masseter (jaw-closer) muscles. In this study, GABA was applied to the TMJ region by itself or in combination with glutamate, and the magnitude of evoked jaw muscle electromyographic (EMG) activity was measured. Application of GABA alone to the TMJ region did not evoke significant jaw muscle EMG activity when compared with normal saline controls. In contrast, co-application of GABA and glutamate into the TMJ region decreased the magnitude of glutamate-evoked EMG activity. This GABA-mediated inhibition of glutamate-evoked EMG activity followed an inverse dose-response relationship with an estimated median inhibitory dose (ID50) of 0.17 +/- 0.05 (SE) mu mol and 0.031 +/- 0.006 mu mol for the digastric and masseter muscles, respectively. Go-administration of the GABA(A) receptor antagonist bicuculline (0.05 mu mol) but not the GABA(B) receptor antagonist phaclofen (0.05 or 0.15 mu mol) reversed the suppressive actions of GABA, indicating that this action of GABA may be mediated by peripheral GABA(A) receptors located within the TMJ region. Our results suggest that activation of peripheral GABA(A) receptors located within the TMJ region could act to decrease the transmission of nociceptive information.