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 u nder the FR schedule than under the FI schedule. After low doses of pentoba rbital, 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 chl ordiazepoxide, responding shifted from the saline key to the 5 mg/kg pentob arbital key, but after high doses of these drugs, responding continued to o ccur 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 pentob arbital key when it was combined with pentobarbital, ethanol or chlordiazep oxide, Phencyclidine and D-amphetamine produced responding largely on the s aline key under both reinforcement schedules. Under the FR schedule, pentob arbital dose-response curves were usually quantal, whereas under the Fl sch edule the pentobarbital dose-response curves usually were graded. (C) 2001 Lippincott Williams & Wilkins.