ABSTRACT
Rats were trained in a step-down inhibitory avoidance task and retrieval was measured during a test session conducted 24 h after training. The ip administration of a low dose of gamma-endorphin (0.2 microng/kg) immediately after training reduced retrieval time from 40.6 to 13.5 s (N = 15). Higher doses of gamma-endorphin given 5 min before testing (1.0 microng/kg) or immediately after training (5 microng/kg) enhanced retrieval time from 38.6 to 300 s (N = 15) and from 40.6 to 104.4 s (N = 15). All of these effects were either after training or before testing had no effect on retrieval. Since at least the amnestic effect of gamma-endorphin is similar to that of beta-endorphin, and gamma-endorphin is a possible metabolite of beta-endorphin by limited proteolysis of the carboxyl-terminal amino acid, it is suggested that at least some effects of beta-endorphin on memory may be mediated by its proteolysis product, gamma-endorphin