AN ENDOGENOUS OPIATE MECHANISM SEEMS TO BE INVOLVED IN STRESS-INDUCEDANHEDONIA

This study assessed the effect of an uncontrollable stressor on the pr eference for a palatable solution (sucrose 1%), and on the preference for a context associated with a single administration of D-amphetamine (3 mg/kg i.p.) by means of the conditioning place preference test, We also evaluated the effect of prior naloxone (2 mg/kg, i.p.) administr ation on the influence of this stressful stimulus in both tests. Anima ls previously submitted to a 120-min - but not 60-min - restraint peri od showed a selective reduction in the preference for sucrose intake a s compared to unstressed animals. Similarly, an identical restraint ex posure elicited a diminished preference for the place previously paire d with amphetamine. Both stress-induced effects were blocked by prior naloxone administration, These data demonstrate that a highly aversive experience decreased the reinforcing efficacy of sucrose and amphetam ine, suggesting that uncontrollable stress may lead to an impaired cap acity to experience pleasure, which could resemble the anhedonia obser ved in clinical depression. Furthermore, an endogenous opiate mechanis m activated by stress seems to be involved in stress-induced anhedonia since naloxone normalized the reduction of the rewarding induced by b oth reinforcers.