INVOLVEMENT OF NMDA RECEPTOR MECHANISMS IN JAW ELECTROMYOGRAPHIC ACTIVITY AND PLASMA EXTRAVASATION INDUCED BY INFLAMMATORY IRRITANT APPLICATION TO TEMPOROMANDIBULAR-JOINT REGION OF RATS

The aim of this study was to examine the possible role of N-methyl-D-a spartate (NMDA) receptor mechanisms in responses induced by the small- fibre excitant and inflammatory irritant mustard oil injected into the temporomandibular joint (TMJ) region of rats. The effects of the non- competitive NMDA antagonist MK-801 were tested on the mustard oil-evok ed increases in electromyographic (EMG) activity of the masseter and d igastric muscles and Evans Blue plasma extravasation. Five minutes bef ore the mustard oil injection, MK-801 or its vehicle was administered systemically (i.v.), into the third ventricle (i.c.v.), or locally int o the TMJ region. Compared with control animals receiving vehicle, the rats receiving MK-801 at an i.v. dose of 0.5 mg/kg (n = 5) showed a s ignificant reduction in the incidence and magnitude of EMG responses a s well as in the plasma extravasation evoked by mustard oil; MK-801 at an i.v. dose of 0.1 mg/kg (n = 5) had no significant effect on plasma extravasation or on the incidence and magnitude of EMG responses but did significantly increase the latency of EMG responses. An i.c.v. dos e of 0.1 mg/kg (n = 5) or 0.01 mglkg (n = 5) had no significant effect on plasma extravasation or incidence of EMG responses but did signifi cantly reduce the magnitudes of the masseter EMG response; the 0.01 mg /kg dose also significantly increased the latency of the digastric EMG response. The magnitudes of both the masseter and digastric EMG respo nses were also significantly reduced by MK-801 administered into the T MJ region at a dose of 0.1 mg/kg (n = 5) but not by 0.01 mg/kg (n = 5) ; neither dose significantly affected the incidence of EMG responses o r the plasma extravasation. These data suggest that both central and p eripheral NMDA receptor mechanisms may play an important role in EMG r esponses evoked by the small-fibre excitant and inflammatory irritant mustard oil, but that different neurochemical mechanisms may be involv ed in the plasma extravasation induced by mustard oil.