DRUG DISCRIMINATION UNDER A CONCURRENT FIXED-INTERVAL FIXED-INTERVAL SCHEDULE

Pigeons were trained to discriminate 5.0 mg/kg pentobarbital from sali ne under a concurrent fixed-interval (FI) FI schedule of food presenta tion on which, after pentobarbital administration, responses on one ke y were reinforced with food under an FI 60-s component and responses o n the other key were reinforced under an FI 240-s component. After sal ine administration, the schedule contingencies on the two keys were re versed. After both pentobarbital and saline, pigeons responded more fr equently on the key on which responses had been programmed to produce the reinforcer under the FI 60 component of the concurrent schedule. T he schedule was changed to concurrent FI 150 FI 150 s for drug-substit ution tests. In each bird, increasing doses of pentobarbital, ethanol, and chlordiazepoxide produced increases in the proportion of response s on the key on which responses had been reinforced under the FI 60 co mponent after pentobarbital administration during training sessions. T he proportion of responses on that key was slightly lower for ethanol than for chlordiazepoxide and pentobarbital. At a dose of pentobarbita l higher than the training dose, responding decreased on the key that had been reinforced under the FI 60 component during training sessions . Phencyclidine produced less responding on the key programmed under t he FI 60-s component than did pentobarbital. Methamphetamine produced responding primarily on the key on which responses had been reinforced under the FI 60-s component after saline administration.