Effects of peripherally injected vasopressin and des-glycinamide vasopressin on the extinction of a spatial learning task in rats

An elevated eight-arm radial maze was employed to study the effects of neuropeptide administration on the spatial learning abilities of food-deprived rats. Following 18 days of reinforced training, each animal was briefly exposed to the maze with no food available in any of the eight food-cups. Immediately after this preliminary trial, animals were injected with a single subcutaneous dose of either saline, arginine vasopressin (AVP: 1.0 or 5.0 micrograms/kg), or an AVP analog with only weak endocrinological activity, des-gly-arginine vasopressin (DG-AVP: 1.0, 5.0 or 10.0 micrograms/kg). Additional extinction trials were conducted at 2, 4, 6 and 8 h post-injection. These tests consisted of individually placing an animal on the empty maze and recording the number of arms chosen in a 5-min period. In this situation, animals learn that food is no longer present in the maze and, consequently, extinguish responding. Vasopressin potentiated this radial maze extinction behavior while DG-AVP produced behavioral results directionally opposite to those predicted by a memory facilitation hypothesis. In a subsequent experiment, vasopressin had no effects on unconditioned locomotor activity measured 2 and 4 h post-injection. These results suggest that: vasopressin improved the learning that occurred during extinction of conditioned appetitive behaviors, these vasopressin effects on conditioned behavior were independent of any unconditioned, sedative or non-specific actions of the peptide, and peripheral endocrinological responses may be necessary to demonstrate memory-enhancing effects following peripherally administered AVP.