Discrimination of pentobarbital doses and drug mixtures under fixed-ratio and fixed-interval reinforcement schedules

Pigeons were trained to discriminate among 5 mg/kg pentobarbital, 10 mg/kg pentobarbital, and saline, under either fixed-interval (FI) or fixed-ratio (FR) reinforcement schedules. When baseline responding stabilized, a higher percentage of responses occurred on the key that produced the reinforcer under the FR schedule than under the FI schedule. After low doses of pentobarbital, responding shifted from the saline key to the 5 mg/kg pentobarbital key; at higher doses of pentobarbital responding shifted to the 10 mg/kg pentobarbital key under both schedules. After low doses of ethanol and chlordiazepoxide, responding shifted from the saline key to the 5 mg/kg pentobarbital key, but after high doses of these drugs, responding continued to occur on the 5 mg/kg pentobarbital key under both reinforcement schedules. A 5 mg/kg dose of pentobarbital increased responding on the 10 mg/kg pentobarbital key when it was combined with pentobarbital, ethanol or chlordiazepoxide. Phencyclidine and d -amphetamine produced responding largely on the saline key under both reinforcement schedules. Under the FR schedule, pentobarbital dose–response curves were usually quantal, whereas under the FI schedule the pentobarbital dose–response curves usually were graded.