Effects of peripherally administered arginine-vasopressin on learning, retention and forgetting in mice.

The effects of peripheral injections of (Arg)-vasopressin were investigated on different stages of memory processes using an appetitive visual discrimination task and a one-trial passive avoidance conditioning in mice. The peptide was administered at one of two doses: 50 micrograms/kg or 25 micrograms/kg. The main effects of vasopressin were observed only for the higher dose. Concerning pre-session vasopressin administration in the visual discrimination task, the effect of the peptide seemed to depend on the level of learning reached at the time of treatment. Indeed, we observed a deleterious effect of vasopressin on learning capacities when the peptide was administered before the first learning session, a bimodal effect (either an improvement or an impairment) on performance when the peptide was administered before the second learning session and an important enhancement of retention performance when the peptide was administered before the retention session, performed 24 days after training. When post-session vasopressin administration was assessed, an improvement of performance was observed indicating a facilitatory effect of vasopressin on consolidation processes. When passive avoidance conditioning was used, an enhancement of retention performance was registered only when the peptide was injected before the retention session at the 50 micrograms/kg dose. No facilitation was observed for the 25 micrograms/kg dose whatever the experimental condition was, i.e. post-learning or pre-retention injection. In order to test eventual non-specific effects of vasopressin, the influence of the peptide on locomotor activity was assessed before the two doses. The results show an important reduction of locomotor activity with the 50 micrograms/kg dose, during 4 h following vasopressin injection. No effect was observed with the 25 micrograms/kg dose. The whole results suggest that vasopressin-induced hypoactivity can directly influence the subsequent learning performance when the treatment was performed in pre-session situations. However, when the level of information is sufficient and beyond the direct effect of the drug, a memory effect may be considered with the 50 micrograms/kg dose independently from the locomotor effect, when the treatment was delivered during consolidation period (post-session) or in long-term retrieval situation (pre-session).