Can vasopressin alone act as an unconditioned stimulus to produce passive avoidance behaviour in rats in a typical...

The claim that vasopressin improves memory has been largely based on results obtained from shock avoidance experiments. In the majority of these studies, "memory" was defined operationally as the hesitation of a rat to enter the darkened compartment of a box in which it had once received an electric foot-shock. A single post-trial injection of arginine vasopressin (AVP) enhances such passive avoidance behaviour. In view of the recent demonstration that AVP has aversive effects, it was argued that vasopressin alone (without giving the rats foot-shock, prior to the peptide) might be a sufficient inhibitory stimulus to produce passive avoidance behaviour in a typical memory experiment. This hypothesis was tested in the present study. The results of these experiments indicate that a behaviourally active dose of AVP (10 micrograms/kg; s.c.) was a sufficient stimulus to produce passive avoidance behaviour in the rats. A small dose of AVP (1.25 micrograms/kg; s.c.) was without effect. However, AVP (10 microgram/kg) was only effective with repeated administration (Experiment 1). This result is in contrast with the post-trial effect of the peptide on inhibitory avoidance behaviour, which is obtained with just one injection in the normal single trial step-through experiment. However, it was found that if the rats were injected with AVP (10 micrograms/kg) and placed in the dark compartment of the apparatus for 20 min, thereby ensuring that the animals made the explicit connection between the aversive effects of the peptide and the dark environment, they displayed avoidance behaviour after a single trial (Experiment 2).(ABSTRACT TRUNCATED AT 250 WORDS)