[LEU] enkephalin enhances active avoidance conditioning in rats and mice

Abstract

The effects of intraperitoneal (IP) administration of the endogenous opioid peptide, [Leulenkephalin (LE), on avoidance conditioning in rodents were investigated. At a dose of 30 μg/kg (IP), LE enhanced acquisition of a one-way step-through active avoidance response when administered 2 minutes before training to Swiss Webster mice. [Leulenkephalin produced a U-shaped dose-response function because both lower and higher doses of LE did not affect avoidance responding. [Leulenkephalin-induced enhancement of avoidance acquisition was also observed in Sprague-Dawley rats; the intraperitoneal injection of 10 μg/kg LE, administered 5 minutes before training, enhanced acquisition of a jump-up one-way active avoidance response. When administered to Sprague-Dawley rats immediately after training, LE (30 μg/kg IP) enhanced jump-up avoidance responding at test 24 hours after peptide injection. Previously, we found LE to impair acquisition in the same tasks in both rats and 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 modulate learning and memory. © 1994 American College of Neuropsychopharmacology.