Passive avoidance behavior: opposite effects of oxytocin analogs with agonist and antagonist properties.

Deamino-6-carba-oxytoxin (dC60), a potent oxytocin analog considered to be resistant to some of the physiologically significant enzymic systems, and N-alpha-acetyl-[2-O-methyltyrosine]oxytocin (AMTO), an analog acting as a competitive inhibitor of oxytocin on the rat uterus, were studied in rats trained in a passive avoidance task. Subcutaneous administration of dC60 (5-50 microgram . kg-1) during different phases of the passive avoidance learning paradigm attenuated avoidance latencies; the results indicated that the drug induced state-dependent learning. AMTO (5-20 microgram . kg-1) enhanced avoidance latencies when administered subcutaneously before training trials and/or before retention test trials. This effect occurred in both males and females. The analogs did not influence exploratory behavior in open field. The results suggest that oxytoxin, in contrast to vasopressin, may impair memory processes. However, both analogs failed to influence the passive avoidance response when administered after training. This finding indicates that dC60 and AMTO did not influence the mechanism of memory consolidation whereas vasopressin and oxytoxin had a marked effect.