Long-term effects of leucine-enkephalin on active avoidance responding in mice

The effects of peripherally administered leucine-enkephalin on the acquisition and retention of active avoidance behavior was studied in mice. Mice received 4 training trials on Day 1, 24 test trials on Day 2, and 10 test trials on Day 5. Leucine-enkephalin impaired the aquisition of avoidance behavior when administered before testing on Day 2. Impairment of retention was demonstrated on Day 2 and Day 5 when leucine-enkephalin was administered immediately after training on Day 1, results indicating the long-term nature of the enkephalin effect. There was no effect of leucine-enkephalin when administered just prior to testing on Day 5. The impairing effect was attenuated but not blocked by naloxone. The pattern of the results from the different tests supports the hypothesis that leucine-enkephalin impairs both acquisition of learning and memory consolidation.