EFFECT OF AN NMDA RECEPTOR ANTAGONIST ON THE WIND-UP OF NEURONS IN THE TRIGEMINAL ORALIS SUBNUCLEUS

Extracellular recordings of convergent neurons of the oralis subnucleu s of the trigeminal sensory complex were performed in paralysed rats u nder halothane-N2O-O-2 anesthesia using glass micropipettes. The effec ts of MK801, a noncompetitive NMDA receptor antagonist, were observed on the increased cell activity (wind-up), triggered by the repetition, at a low frequency (0.66 Hz) and high intensity (3 times the threshol d of C-fiber response), of electrical stimulation of the receptive fie ld. Successive cumulative doses (up to 1 mg/kg) of MK801 i.v. resulted in a dose-dependent decrease in the responses related to C-fiber inpu t (11 cells). A single dose of 1 mg/kg applied in four cells had effec ts similar to the 1 mg/kg dose given cumulatively. Three units were ei ther weakly or not modified by MK801. In a second experiment, recordin gs were performed in 12 cells for 80 min after an injection of a small dose of MK801 (0.15 mg/kg). C input was not significantly modified by the antagonist. The effects of MK801 on the first part of the wind-up response (wind-up proper) peaked between 15 and 50 min and returned t o control values at about 80 min. The effects on the postdischarge fol lowed approximately the same time course. It is concluded that despite being devoid of substantia gelatinosa, the oralis subnucleus contains neurons that display an NMDA receptor-linked wind-up similar to the p henomenon described in the dorsal horn of the spinal cord. (C) 1997 El sevier Science B.V.