Dual action of post-training naloxone on memory.

Naloxone administered at a dose of 0.2 mg/kg post-training antagonizes the deleterious effect of post-training beta-endorphin administration and prevents the enhancing effect of pretest beta-endorphin administration, on retention of a step-down inhibitory avoidance task in rats. Given at a higher dose (0.4 mg/kg), naloxone caused, in addition to these effects, a pronounced retrograde facilitation which was additive with that caused by post-training ACTH or epinephrine administration. These findings show that post-training naloxone has two different effects or sets of effects, each with a different dose threshold: An interference with beta-endorphin induced state dependency and a true modulatory effect.