[LEU]ENKEPHALIN ENHANCES ACTIVE-AVOIDANCE CONDITIONING IN RATS AND MICE

The effects of intraperitoneal (IP) administration of the endogenous o pioid peptide, [Leu]enkephalin (LE), on avoidance conditioning in rode nts were investigated. At a dose of 30 mu g/kg (IP), LE enhanced acqui sition of a one-way step-through active avoidance response when admini stered 2 minutes before training to Swiss Webster mice. [Leu]enkephali n produced a U-shaped dose-response function because both lower and hi gher doses of LE did not affect avoidance responding. [Leu]enkephalin- induced enhancement of avoidance acquisition was also observed in Spra gue-Dawley rats; the intraperitoneal injection of 10 mu g/kg LE, admin istered 5 minutes before training, enhanced acquisition of a jump-up o ne-way active avoidance response. When administered to Sprague-Dawley rats immediately after training, LE (30 mu g/kg IP) enhanced jump-up a voidance responding at test 24 hours after peptide injection. Previous ly, we found LE to impair acquisition in the same tasks in both rats a nd mice, also at microgram doses, and also in a U-shaped manner. Thus, LE can either enhance or impair learning within the same species and the same task; these findings are in agreement with recent theoretical proposals regarding the nature of compounds, such as LE, that modulat e learning and memory.