Beta-endorphin enhancement of retrieval in a three-session paradigm.

Rats were submitted to three consecutive sessions, one session per day, of step-down inhibitory avoidance task (60-Hz, 0.3-mA footshock) or of two-way active avoidance task (25 trials of a 5-s, 1-kHz tone and a 0.4-mA footshock). Animals received intraperitoneal (ip) injections of saline or beta-endorphin (2.0 micrograms/kg) before or after the second session and before the third session. beta-Endorphin given before either the second or the third session improved retention of both tasks, while its administration after the second session had no effect upon performance on both tasks. In Experiment 2, it was shown that rats receiving naloxone (0.2 mg/kg) ip after the first session did not exhibit the enhancement of retrieval by beta-endorphin administration before the second session, so a non-state-dependent improvement of retrieval by the opioid seems to be unlikely. In view of these results we can propose that the presession retrieval enhancing effect of beta-endorphin is due to an endogenous state dependency on the opioid that can be expressed either in the second or in the third session of aversive tasks.