Delayed effect of the vasopressin metabolite VP4-8 on the social memory of sexually naive male rats

Abstract.

Rationale: Endogenous vasopressin is involved in the social memory of the male rat and administration of exogenous vasopressin improves social memory. These findings are mainly based on studies using sexually experienced males that were tested in the social recognition test. Objective: The present study was aimed to establish whether the modulation of social memory by vasopressin fragments depends on the sexual experience of the male rat. For this purpose, the social discrimination test was used, since this test is more suitable than the social recognition test for measuring social memory in sexually naive males. Methods: Male rats were tested in the social discrimination test and treated subcutaneously with the vasopressin metabolite [pGlu⁴,Cyt⁶]vasopressin-(4-8) (VP4-8). VP4-8 shares with vasopressin the effects on memory processes but lacks the peripheral effects of vasopressin. Results: VP4-8 (1 µg/kg) acutely improved the social memory of sexually experienced male rats, confirming previous reports. However, in sexually naive males VP4-8 failed to improve social memory in doses ranging from 0.1 µg/kg to 1 µg/kg. Instead, 1 µg/kg VP4-8 or 6 µg/kg desglycinamide-vasopressin were found to exert a delayed effect in sexually naive rats. This delayed effect resulted in an improved social memory 2 days after administration. Conclusions: Vasopressin sensitisation is discussed as a possible underlying 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.