External stimulus control in a "drug-discrimination" procedure: drug effects and inter-animal variation

Rats (N = 12) were trained to discriminate between two stimulus conditions S1 and S2, consisting of combinations of visual and tactile stimuli [S1: box lighted (L) and wooden floor insert present (W); S2: box dark (D) and grid floor present (G)] in a two-lever discrimination procedure. S1 was continuously present during half of the sessions, and S2 was continuously present during the remaining sessions. Morphine (0.3--6 mg/kg) and haloperidol (0.04--0.08 mg/kg), but not chlorpromazine (1--4 mg/kg) and scopolamine (0.031--0.125 mg/kg), decreased the accuracy of discriminative responding during S1 sessions. None of the drugs significantly affected discrimination accuracy during S2 sessions. After drug testing was completed, reversed combinations of the visual and tactile stimuli were tested (i.e. L + G, and D + W). Inter-animal variation in the control by the component stimuli was observed. The results suggest that "intermediate results" (i.e. equal responding on drug and saline lever) in drug discrimination research, if observed at the highest dose of a drug at which animals still respond, may be interpreted in terms of a drug-induced disruption of discriminative responding. The results further suggest that inter-animal variation in the outcome of drug generalization tests may be partly related to inter-animal variation in the degree of stimulus control by different components of the training drug stimulus.