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-acids4,5. 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.