Endorphins alter acquisition and consolidation of an inhibitory avoidance response in rats.

Peripheral i.p. administration of 1.0 microgram/kg gamma-endorphin to rats, 30 min prior to training in a 1-trial inhibitory avoidance task, enhanced retention performance measured 72 h later. A much smaller dose of 0.1 microgram/kg beta-endorphin administered immediately following training produced a retention deficit. Both of these endorphin effects were time dependent, since gamma-endorphin given 90 min prior to, and beta-endorphin given 90 min after training produced no effect on retention performance. A wide dose range of either gamma-endorphin (0.1-10 micrograms/kg) administered after training or beta-endorphin (0.1-10 micrograms/kg) administered before training had no effect. These results suggest that the behavioral actions of beta-endorphin and smaller beta-lipotropin sequences such as gamma-endorphin, are distinct. We also found that 0.1-100 micrograms/kg of either alpha-endorphin or Met-enkephalin given before or after training was without effect on retention of the response.