Effects of posttrial hormone injections on memory processes

This experiment examined the effect on memory of posttrial injections of epinephrine, norepinephrine, ACTH, growth hormone, vasopressin and corticosterone. Rats were trained with a weak footshock (0.7 mA, 0.35 sec) in a one-trial inhibitory (passive) avoidance task. The animals received subcutaneous injections of one of the above hormones or saline immediately after training. On a retention test 24 hr after training, animals which received ACTH (0.03 or 0.3 IU/rat), epinephrine (0.1 mg/kg) or norepinephrine (0.1, 0.3 or 1.0 mg/kg) had retention performance which was significantly better than that of saline control animals. A higher posttrial ACTH dose (3.0 I.U./animal) impaired later retention performance. ACTH (0.3 I.U./animal) and norepinephrine (0.3 mg/kg) injections administered 2 hr after training had no significant effect on retention. Immediate posttrial injections of vasopressin (dose range 0.001–1.0 I.U./animal), growth hormone (0.5–1.0 mg/kg), or corticosterone (0.01–4 mg/kg) did not significantly enhance retention. These findings indicate that epinephrine, norepinephrine, and ACTH injections can enhance memory processes if the hormones are injected shortly after training. Such results are consistent with the view that hormonal consequences of an experience, particularly epinephrine, norepinephrine and ACTH release, may normally have a modulatory influence on memory processes in untreated animals. In addition, it is therefore possible that other posttrial treatments which enhance or impair later retention performance may act through hormonal mechanisms.