Schedule control of quantal and graded dose-effect curves in a drug-drug-saline discrimination

Pigeons were trained to discriminate among 5 mg/kg pentobarbital, 5 mg/kg morphine, and saline when responding was maintained under fixed-interval (FI) or fixed-ratio (FR) reinforcement schedules. After the discrimination was established, other drugs were substituted for the training drugs. After low doses of pentobarbital and chlordiazepoxide, responding shifted from the saline key to the pentobarbital key under both FR and FI schedules. After low doses of morphine and methadone, responding shifted from the saline key to the morphine key under both reinforcement schedules. After all doses of d-amphetamine, responding occurred largely on the saline key under both schedules. Responding also was confined largely to the saline key after phencyclidine administration under the FR schedule, but under the FI schedule, responding shifted from the saline key to the pentobarbital key at high doses of phencyclidine. When responding was maintained under the FR schedule, the dose-response curves for drugs that generalized to the training drugs were quantal in shape, while under the FI schedule, the dose-response curves for drugs that generalized to the training drugs were graded. These data extend observations that FR schedules generate quantal dose-response curves, and FI schedules generate graded dose-response curves to complex three-key drug discriminations.