Task-dependent development of tolerance to scopolamine.

Rats were chronically treated with once daily injections of either 0.5 mg/kg scopolamine hydrochloride or isotonic saline for 21 days. When spontaneous locomotor activity or acquisition of active avoidance in a two-way shuttle box were measured at 48 hours after the cessation of chronic treatment, no differences were observed between the two chronically treated groups. Tolerance to scopolamine's locomotor stimulatory effects was evident as the increase in locomotor activity following acute treatment was smaller in the group which had been chronically treated with scopolamine. On the other hand, acutely administered scopolamine facilitated the acquisition of active avoidance responding to an equal degree in both chronically treated groups. The reasons which may account for this task-dependent tolerance development to scopolamine are discussed.