Chronic stress attenuation of alpha 2-adrenoceptor reactivity is reversed by naltrexone.

Low doses of clonidine (50-100 micrograms/kg IP) evoke a clear dose-dependent hypoactivity response. Seven daily immobilization sessions prevented the motor activity decrease induced by clonidine. On the contrary, a single stress failed to modify clonidine response. Pretreatment with naltrexone (2 mg/kg IP) fully antagonized the attenuating effect induced by chronic stress on clonidine sedative action. These evidences suggest that chronic but not acute stress reduces the reactivity of alpha 2-adrenoceptors involved in clonidine-induced sedation. In addition, a regulatory mechanism of endogenous opioids seems to participate on alpha 2-adrenoceptors adaptative changes.