Chronic variable stress induces supersensitivity of central alpha2-adrenoceptors which modulate the jaw-opening reflex in the rat.

In a previous study, we found that the sensitivity of central postsynaptic alpha2-adrenoceptors which modulate, in an inhibitory way, the activity of the jaw-opening reflex (JOR) is reduced after chronic repeated stress (tail pinch) in the rat. The aim of this study was to assess the effects of exposure to a chronic variable stress regime on these adrenoceptors. To do this, the digastric electromyographic responses elicited by orofacial electrical stimulation after the intravenous administration of cumulative doses (x3.3) of the alpha2-adrenoceptor agonist, clonidine (0.1-10000 microgram/kg), were recorded. As expected, in unmanipulated control rats, clonidine inhibited the reflex, in a dose-dependent manner, until abolition (ED50 = 17.3 +/- 2.2 microgram/kg). Single tail pinch did not significantly alter the ability of clonidine to abolish the reflex. However, chronic variable stress led to an enhancement of the inhibitory effect of clonidine on the amplitude of JOR, resulting in a shift to the left of the dose-response curve in comparison with that of the control group (ED50 was reduced by 37%, P = 0.032), without affecting either the estimated maximum effect for the agonist or the slope of the inhibitory function. This in vivo result indicates that chronic variable stress leads to an increased sensitivity of central alpha2-adrenoceptors which modulate JOR, in contrast to the desensitization of these adrenoceptors found after repeated exposure to the same stressor.