REVERSAL OF EXTINCTION BY SCOPOLAMINE

The aim of this experiment was to determine the effects of muscarinic blockade on extinction of passive avoidance conditioning. Rats were tr ained with a foot shock of 2.5, 3.0, or 6.0 mA and were tested for ret ention for 8 weeks (once weekly). Five minutes before the seventh test they were injected with 8 mg/kg scopolamine. The groups that had been trained with 2.5 and 3.0 mA showed extinction, which was reversed by the scopolamine; the overreinforced group (6.0 mA) did not show extinc tion and the scopolamine did not alter the conditioned response. The d ata support the hypothesis that extinction represents the learning of a new response sustained by a set of cholinergic neurons, different fr om that which mediated original passive avoidance learning.