Long Term and Short Term Effects on Retention of a Conditioned Avoidance Response in Rats by Treatment with Long Acting Pitressin and α-MSH

Abstract

ATTENTION has recently been focused on the effect of peptides on conditioned avoidance behaviour. It has been shown that adrenocorticotrophic hormone (ACTH) administered during avoidance learning results in a delay of extinction of a conditioned avoidance response (CAR)^1,2. We found that the rapid extinction of a CAR which occurs after posterior lobectomy or hypophysectomy can be prevented by treatment with ACTH, melanocyte stimulating hormone (α-MSH) or lysine vasopressin given during the extinction period³. Our observations indicate that the effect of ACTH which results in a delay of extinction of the CAR is located in the N-terminal part of the peptide, presumably within the first ten amino-acids^4,5. The effect of lysine vasopressin or pitressin (posterior pituitary extract) administered during the extinction period appeared to be similar to that of ACTH and related peptides³. Accordingly, it was considered of interest to investigate whether the effect of vasopressin and of an ACTH-like peptide on extinction of the CAR involves the same mechanisms despite the structural difference of these two peptides.