ANXIOLYTIC-LIKE EFFECT INDUCED BY CHRONIC STRESS IS REVERSED BY NALOXONE PRETREATMENT

The present study assesses the influence of different restraint schedu les on behavioral parameters determined by a conflict test, namely the light-dark transitions (LDT) as well as the opiate modulation on the behavioral consequences induced by chronic restraint, Finally, another group of animals that received naloxone (NAL) and/or chronic stress w as either exposed to a single foot shock session or administered a sin gle dose of the beta-carboline FG 7142 (N'-methyl-beta-carboline-3-car boxamide) immediately prior to the LDT test. We observed that a single restraint session (2 h) induced a decrease of LDT and time spent in t he lit box, while chronic restraint (2 h per day for up to 7 days) ind uced a significant increase in both parameters. However, this increasi ng effect was blocked by a NAL administration (2 mg/kg IF) prior to ea ch of the seven restraint events. A single foot shock or FG administra tion produced a clear anxiogenic response, an effect that was absent i n animals previously submitted to chronic stress. In addition, NAL pre treatment abolished the chronic stress-induced attenuating effect on t he behavioral suppression induced after either foot shock or FG admini stration. Therefore, these findings demonstrate that a previous histor y of chronic stress, leading to adaptation, induced an anxiolytic-like effect, and attenuated the behavioral supression produced by acute st ressors. There seems to be an endogenous opiate mechanism involved in the behavioral influence induced by chronic stress.