Drug discrimination under two concurrent fixed-interval fixed-interval schedules

Pigeons were trained to discriminate 5.0 mg/kg pentobarbital from saline un der a two-key concurrent fixed-interval (FI) 100-s FI 200-s schedule of foo d presentation, and later under a concurrent FI 40-s FT 80-s schedule, in w hich the Fl component with the shorter time requirement reinforced respondi ng on one key after drug administration (pentobarbital-biased key) and on t he other key after saline administration (saline-biased key). After respond ing stabilized under the concurrent Fl 100-s FI 200-s schedule, pigeons ear ned an average of 66% (after pentobarbital) to 68% (after saline) of their reinforcers for responding under the FI 100-s component of thr concurrent s chedule. These birds made an average of 70% of their responses on both the pentobarbital-biased key after the training dose of pentobarbital and the s aline-biased key after saline, After responding stabilized under the concur rent FI 40-s FI 80-s schedule, pigeons earned an average of 67% of their re inforcers for responding under the FI 40 component after both saline and th e training dose of pentobarbital. These birds made an average of 75% of the ir responses on the pentobarbital-biased key after the training dose of pen tobarbital, but only 55% of their responses on the saline-biased key after saline. In test sessions preceded by doses of pentobarbital, chlordiazepoxi de, ethanol, phencyclidine, or methamphetamine, the dose-response curves we re similar under these two concurrent schedules. Pentobarbital, chlordiazep oxide, and ethanol produced dose-dependent increases in responding on the p entobarbital-biased key as the doses increased. For some birds, at the high est doses of these drugs, the dose-response curve turned over. Increasing d oses of phencyclidine produced increased responding on the pentobarbital-bi ased key in some, but not all, birds. After methamphetamine, responding was largely confined to thr saline-biased key. These data show that pigeons ca n perform drug discriminations under concur rent schedules in which the rei nforcement frequency under the schedule components differs only by a factor of two, and that when other drugs are substituted for the training drugs t hey produce dose-response curves similar to the curves produced by these dr ugs under other concurrent interval schedules.