Time course of scopolamine effect on memory consolidation and forgetting in rats

•Scopolamine impairs memory consolidation on 24h passive avoidance retention task.•Scopolamine improves memory consolidation on 48h passive avoidance retention task.•Scopolamine modifies memory consolidation within first 61/2h after training.•Scopolamine delays forgetting of emotional memory. The effect of scopolamine on the consolidation and forgetting of emotional memory has not been completely elucidated yet. The aim of the present study was to investigate the time course of scopolamine effect on consolidation and forgetting of passive avoidance response. In a first experiment of the present study, we tested the effect of scopolamine (1mg/kg, i.p., immediately after acquisition), on 24h and 48h retention performance of the step-through passive avoidance task, in adult male Wistar rats. On the 24h retested trial, the latency of the passive avoidance response was significantly lower, while on the 48h retested trial it was significantly higher in scopolamine-than in the saline-treated group. In a second experiment, we assessed the 24h time course of scopolamine (1mg/kg) effect on memory consolidation in passive avoidance task. We found that scopolamine administration only within the first six and half hours after acquisition improved memory consolidation in 48h retention performance. Finally, a third experiment was performed on the saline- and scopolamine-treated rats (given immediately after acquisition) that on the 48h retention test did not step through into the dark compartment during the cut-off time. These animals were retested weekly for up to first three months, and after that, every three months until the end of experiment (i.e., 15months after acquisition). The passive avoidance response in the saline treated group lasted up to 6weeks after acquisition, while in the scopolamine treated group 50% of animals conserved the initial level of passive avoidance response until the experiment end point. In conclusion, the present data suggest that (1) improving or impairment effect of scopolamine given in post-training periods depends on delay of retention trial, (2) memory consolidation process could be modify by scopolamine within first six and half hours after training and (3) scopolamine could delay forgetting of emotional memory.