Effect of the pre-training administration of beta-endorphin, leucine-, methionine- and [des-Tyr1] methionine-enkephalin on the acquisition and retention of a shuttle avoidance response by rats.

1. Intraperitoneal administration of beta-endorphin, leucine-, methionine- and [des-Tyr1] methionine-enkephalin 5 min prior to a 50-trial training session of shuttle avoidance conditioning in rats depressed avoidance performance both in the training session and in a retention test carried out after 24 h. All four peptides therefore depress both acquisition and retention of shuttle avoidance behavior. 2. There were, however, differences in sensitivity to the two effects of the peptides. Whereas the enkephalins depressed both acquisition and retention at the lowest dose tested (0.32 micrograms/kg, 5.6 to 8.0 X 10(-10) moles/kg), beta-endorphin caused anterograde amnesia at a lower dose (0.8 micrograms/kg, 2.3 X 10(-10) moles/kg) than the one that disrupted acquisition (20.0 micrograms/kg = 5.8 X 10(-9) moles/kg). 3. These findings support the suggestion that beta-endorphin may act as an endogenous modulator of memory processes (I. Izquierdo, R.D. Dias, D.O. Souza, M.A. Carrasco, E. Elisabetsky and M.L. Perry, Behavioral Brain Research, 1: 451-468, 1980), but rule out the possibility that the enkephalins may play a similar role.