Post-training and pretest effects of adrenocorticotropin on retention: the influence of the hour of the day, the training-test interval, and pretest naloxone administration.

Rats received an ip injection of 0.2 microgram/kg of ACTH-(1-39) 1 min after step-down inhibitory avoidance training and/or 5 min prior to retention testing. Experiments were carried out either in the morning or in the afternoon using either a 3- or a 24-h training-test interval. Post-training ACTH induced memory facilitation in the morning and amnesia in the afternoon at both training-test intervals. Pretest ACTH reversed the afternoon amnesic effect, also at both training-test intervals. In addition, pretest ACTH induced a naloxone-reversible memory enhancement, both on its own and in animals treated with a facilitatory post-training dose of ACTH in the morning; this effect was seen only at the 24-h training-test interval. Naloxone had no effect of its own and did not influence the reversal of ACTH-induced amnesia caused by pretest ACTH in the afternoon. The results point to the variety of memory modulatory influences of ACTH, and to some of the factors involved in the elicitation of one or other effect, namely, the presumable basal rate of secretion of endogenous ACTH, and the previous pharmacological history of the animal.