Effects of phencyclidine, atropine and physostigmine, alone and in combination, on variable-interval performance in the squirrel monkey.

The role played by cholinergic activity in the effects of phencyclidine (PCP) on schedule-controlled responding was studied in three squirrel monkeys trained to respond on a variable-interval (VI) 100 sec schedule of food presentation. A low dose of PCP (0.08 mg/kg IM) produced small increases in rates of responding. Higher doses (0.16--0.64 mg/kg) produced dose-dependent decreases in rates of responding. Atropine (0.05--3.2 mg/kg IM) and physostigmine (0.025--0.20 mg/kg IM) caused only decreases in response rates, the dose-response curve for atropine being particularly flat over a wide range of doses. When atropine was combined with PCP, no significant interaction was obtained. When physostigmine was combined with PCP, a complex interaction was observed. Evidence fo partial antagonism of PCP by physostigmine was obtained only at the highest PCP dose tested. Atropine-physostigmine combinations resulted in response rates suggestive of antagonism.