ENHANCEMENT AND IMPAIRMENT OF MEMORY PROCESSES WITH POST-TRIAL INJECTIONS OF ADRENOCORTICOTROPIC HORMONE

Post-trial injections of ACTH produced both enhancement and impairment of memory processes. Rats were trained in a 1-trial inhibitory (passive) avoidance task and were tested for retention 24 h later. In the 1st experiment animals were trained in the avoidance task using a weak footshock. Immediate post-trial ACTH injections at 2 doses (0.03 or 0.3 IU/animal) enhanced later retention performance as compared to saline-injected control animals. If animals received a higher ACTH dose (3.0 IU/rat), the treatment produced retrograde amnesia; i.e., there was an inverted-U dose-response curve for the effects of ACTH on memory. Both enhancement and impairment of memory processes were time-dependent; the effect on memory decreased as the time after training increased. The findings of a 2nd experiment indicated that retention performance of control animals varied with time of day. Control animals trained and tested in the morning (9-10:00 a.m.) show retention performance significantly poorer than that of animals trained and tested in the afternoon (1-2:00 p.m.). Post-trial injections of ACTH enhanced later retention only in the afternoon conditions. A 3rd experiment examined the interactions between the footshock parameters used in training and the effect, on memory, of ACTH. A single post-trial dose of ACTH (either 3 or 6 IU/rat) enhanced later retention of training with weak footshock and impaired later retention of training with a more intense footshock. ACTH may modulate memory storage processing of recent information. The possibility that other post-trial treatments which facilitate or impair memory processes may act via hormonal mechanisms is suggested.