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Comparison of fixed‐ratio and tandem schedules of reinforcement in discrimination of nicotine in rats

There have been few comparisons between different schedules of reinforcement for establishing drugs as discriminative stimuli. Fixed‐ratio (FR‐10) and tandem variable‐interval 1‐min FR‐10 schedules have, therefore, been compared directly in a conventional, drug‐saline discrimination paradigm with food reinforcement in rats. The drug used for training was nicotine (0.4 mg/kg s.c., “faded” to 0.1 mg/kg s.c.). The discrimination was acquired rapidly under both schedules, with stimulus control by nicotine being slightly superior under the FR‐10 schedule. In 5‐min extinction tests with nicotine, the FR‐10 schedule gave a clear dose‐response curve with a bar‐selection (quantal) index but with generally poor results when the percentage of drug‐appropriate responding (quantitative index) was calculated. In contrast, the tandem schedule yielded clear dose‐response data with both indices. In tests with (+)‐amphetamine, full generalization was obtained with both FR‐10 and tandem schedules and with both quantitative and quantal indices. Tests for generalization to morphine were negative regardless of the training schedule. The results confirm that orderly data may be obtained with either an FR‐10 or a tandem schedule provided that an appropriate index of discriminative response is employed. The results generally support the validity of current practices, and there will probably not be very marked differences between conclusions depending on which schedule is used.