In a series of experiments rats were trained and subsequently extinguished under different conditions of drug administration. High doses of chlorpromazine (5 mg/kg) or pentobarbital (20 mg/kg) were administered during the first extinction session and the dose was progressively reduced to zero over 10 successive sessions. Extinction was continued until all subjects had consistent near zero responding. When compared to saline control, chlorpromazine had no effect on total extinction responding, while pentobarbital significantly reduced total extinction responding. Subjects receiving both drugs had erratic runs of responding in extinction, whereas subjects receiving saline showed a normal extinction pattern. Subjects receiving pentobarbital during both training and extinction had more total extinction responding than subjects receiving only pentobarbital in extinction.
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