Delayed effect of the vasopressin metabolite VP4-8 an the social memory ofsexually naive male rats

Rationale: Endogenous vasopressin is involved in the social memory of the m ale rat and administration of exogenous vasopressin improves social memory. These findings are mainly based on studies using sexually experienced male s that were tested in the social recognition test. Objective: The present s tudy was aimed to establish whether the modulation of social memory by vaso pressin fragments depends on the sexual experience of the male rat. For thi s purpose, the social discrimination test was used, since this test is more suitable than the social recognition test for measuring social memory in s exually naive males. Methods. Male rats were tested in the social discrimin ation test and treated subcutaneously with the vasopressin metabolite [pGlu (4),Cyt(6)]vasopressin-(4-8) (VP4-8). VP4-8 shares with vasopressin the eff ects on memory processes but lacks the peripheral effects of vasopressin. R esults: VP4-8 (1 mug/kg) acutely improved the social memory of sexually exp erienced male rats, confirming previous reports. However, in sexually naive males VP4-8 failed to improve social memory in doses ranging from 0.1 mug/ kg to 1 mug/kg. Instead, I mug/kg VP4-8 or 6 mug/kg desglycinamide-vasopres sin were found to exert a delayed effect in sexually naive rats. This delay ed effect resulted in an improved social memory 2 days after administration Conclusions: Vasopressin sensitisation is discussed as a possible underlyi ng mechanism of the observed delayed effect of vasopressin fragments. It is concluded that in male rats sexual experience can influence the modulation of social memory by vasopressin.