REPEATED TAIL PINCH LEADS TO DESENSITIZATION OF POSTSYNAPTIC ALPHA(2)-ADRENOCEPTORS WHICH MODULATE THE JAW OPENING REFLEX IN THE RAT

There are few in vivo studies which have investigated the modulation o f central postsynaptic alpha(2)-adrenoceptors functionality provoked b y stress. We assessed in the rat the effects of either single or repea ted tail pinch on clonidine-induced inhibition of the jaw-opening refl ex (JOR) via activation of postsynaptic central alpha(2)-adrenoceptors . At the end of each experimental period, the progressive inhibition o f the digastric electromyographic responses elicited by orofacial elec trical stimulation after the IV administration of cumulative doses (x3 .3) of clonidine (0,10000 mu g/kg) was recorded. Single tail pinch did not significantly modify the ability of the agonist to inhibit the JO R, although there was a tendency to decrease the basal amplitude of th e reflex (a 40% reduction) immediately after exposure to the single st ressor. However, the dose-response curve for clonidine-induced inhibit ion of the JOR was clearly shifted to the right in rats exposed to rep eated tail pinch (ED50 was increased by 152%, P < 0.0001) when compare d with the unstressed control group, without affecting the slope of th e inhibitory function and the estimated maximum effect for the agonist . These results show that repeated stress leads to a subsensitivity of the alpha(2)-adrenoceptors which modulate the JOR, suggesting the dev elopment of adaptive mechanisms in postsynaptic alpha(2)-adrenoceptors in response to stress.