Effects of sildenafil on long-term retention of an inhibitory avoidance response in mice.
Sildenafil (1, 3, 10, and 30mg/kg, intraperitoneally (i.p.)), a cyclic guanosine monophosphate (cGMP)-specific phosphodiesterase type 5 (PDE5) inhibitor, facilitated retention performance of a one-trial step-through inhibitor avoidance task, when administered to male Swiss mice immediately after training, as indicated by performance on a retention test 48 h later. The dose-response curve was an inverted U in this dose range, although only the dose of 3 mg/kg of sildenafil produced significant effects. Sildenafil did not affect response latencies in mice not given the footshock on the training trial, indicating that the actions of sildenafil on retention were not due to non-specific proactive effects on retention performance. The effects of sildenafil (3 mg/kg, i.p.) were time-dependent, and the administration of sildenafil (3 mg/kg, i.p.) 30 min prior to the retention test did not affect retention in mice given post-training injections of vehicle or sildenafil (3 mg/kg, i.p.). However, the administration of sildenafil (3mg/kg, i.p.) 30 min before training also enhanced retention performace. Further, when mice were trained and received immediate post-training sildenafil (3 mg/kg) and were tested for retention either 1 week or 1 month later, at each retention interval the performance was comparable to that found with a 48-h retention interval. Finally, an enhancement of retention was also observed in female Swiss mice that received sildenafil (3 mg/kg, i.p.) immediately, but not 180min, after training. These findings could indicate that the actions of sildenafil on retention are not sex-dependent. The results suggest that sildenafil influences retention by modulating time-dependent mechanisms involved in memory storage and that the effects are long lasting. A possible participation of the nitric oxide (NO)-guanylyl cyclase-cGMP system also is suggested.