Retrieval effects of both post- and presession beta-endorphin administration in a three-session paradigm

Rats were submitted to three sessions, with a 24-hr interval between, of step-down inhibitory avoidance task using a 60-Hz, 0.3-mA footshock, or of two-way active avoidance task using 25 presentations of a 5-sec. 1-kHz tone and a 0.4-mA footshock. Animals received intraperitoneal injections of either saline or beta-endorphin (2.0 microgram/kg) after the first session, and before the second or the third sessions, in a 2 x 2 x 2 design. beta-Endorphin given before the second or the third sessions improved retention for both tasks, but when administered after the first session, it impaired retention for the second session. The administration of beta-endorphin after the first session prevented the retrieval enhancement by the opioid given before the third session. Rats receiving beta-endorphin both after the first and before the second sessions, whilst showing no retrieval impairment on the second session, also did not show the pre-third session beta-endorphin retrieval enhancing effect. These data suggest that the post-first session exaggeration of the endogenous opioid state by beta-endorphin administered after the first session causes a long-lasting change in retrievability for the active and inhibitory avoidance tasks, as shown by the lack of the retrieval enhancing effect of beta-endorphin given before the third session.