Effect of the intraperitoneal and intracerebroventricular administration of ACTH, epinephrine, or beta-endorphin on retrieval of an inhibitory avoidance task in rats.

Rats were trained in a step-down inhibitory avoidance task using a small start platform (5-cm high, 25 X 7 cm) and a low intensity footshock (0.3 mA, 60 Hz). Retrieval of this task was measured on a test session 24 hr after training. The ip administration of ACTH1-24 (25 ng/rat), epinephrine HCl (625 ng/rat), or human beta-endorphin (125 ng/rat) 5 min prior to testing enhanced retrieval. beta-Endorphin was also effective when given by the icv route at a dose of 25 ng/rat. The icv administration of ACTH (5, 25, or 125 ng/rat) or epinephrine (5, 25, 125, or 625 ng/rat) was ineffective. These findings suggest that memory modulation by beta-endorphin at the time of retrieval may be centrally mediated, whereas the influence of ACTH and epinephrine is probably mediated peripherally.