INHIBITORY EFFECTS OF ELECTRICAL-STIMULATION OF VENTROLATERAL ORBITALCORTEX ON THE RAT JAW-OPENING REFLEX

In previous studies, we have shown that electrically or chemically evo ked activation of the ventrolateral orbital cortex (VLO) depresses the rat tail-flick (TF) reflex, and this antinociceptive effect is mediat ed by the periaqueductal gray (PAG). The aim of the present study was to examine whether electrical stimulation of the VLO could inhibit the rat jaw-opening reflex (JOR), and to determine whether electrolytic l esions of the FAG could attenuate this VLO-evoked inhibition. Unilater al electrical stimulation of the VLO significantly depressed the JOR e licited by tooth pulp or facial skin stimuli, with a mean threshold of 30.5 +/- 2.3 mu A (n = 22). Increasing stimulation intensities from 3 0 to 80 mu A resulted in greater reduction of the dEMG amplitude from 22.9 +/- 5.0% to 69.7 +/- 3.7% of the baseline value (P < 0.01, n = 22 ). The inhibitory effect appeared SO ms after the beginning of VLO sti mulation and lasted about 150 ms, as determined by varying the conditi oning-test (C-T) time interval. Unilateral lateral or ventrolateral le sions of the FAG produced only a small attenuation of the VLO-evoked i nhibition of the JOR, but bilateral lesions eliminated this inhibition . These findings suggest that the VLO plays an important role in modul ation of orofacial nociceptive inputs, and provide further support for the hypothesis that the antinociceptive effect of VLO is mediated by FAG leading to activation of a brainstem descending inhibitory system and depression of nociceptive inputs at the trigeminal level. The role played by VLO in pain modulation is discussed in association with the proposed endogenous analgesic system consisting of medullary cord-Sm- VLO-PAG-medullary cord. (C) 1998 Published by Elsevier Science B.V. Al l rights reserved.