CHRONIC VARIABLE STRESS INDUCES SUPERSENSITIVITY OF CENTRAL ALPHA(2)-ADRENOCEPTORS WHICH MODULATE THE JAW-OPENING REFLEX IN THE RAT

In a previous study, we found that the sensitivity of central postsyna ptic a,alpha(2)-adrenoceptors which modulate, in an inhibitory way, th e activity of the jaw-opening reflex (JOR) is reduced after chronic re peated stress (tail pinch) in the rat. The aim of this study was to as sess the effects of exposure to a chronic variable stress regime on th ese adrenoceptors. To do this, the digastric electromyographic respons es elicited by orofacial electrical stimulation after the intravenous administration of cumulative doses (x 3.3) of the alpha(2)-adrenocepto r agonist, clonidine (0.1-10000 mu g/kg), were recorded. As expected, in unmanipulated control rats, clonidine inhibited the reflex, in a do se-dependent manner, until abolition (ED50 = 17.3 +/- 2.2 mu g/kg). Si ngle tail pinch did not significantly alter the ability of clonidine t o abolish the reflex However, chronic variable stress led to an enhanc ement of the inhibitory effect of clonidine on the amplitude of JOR, r esulting in a shift to the left of the dose-response curve in comparis on with that of the control group (ED,, was reduced by 37%, P = 0.032) , without affecting either the estimated maximum effect for the agonis t or the slope of the inhibitory function. This in vivo result indicat es that chronic variable stress leads to an increased sensitivity of c entral a,alpha(2)-adrenoceptors which modulate JOR, in contrast to the desensitization of these adrenoceptors found after repeated exposure to the same stresser. (C) 1998 Elsevier Science B.V. All rights reserv ed.