THE EFFECTS OF QUIPAZINE AND FLUOXETINE ON EXTINCTION OF A PREVIOUSLY-REINFORCED OPERANT RESPONSE IN RATS

Previous studies have shown that treatments that reduce serotonergic neurotransmission lead to enhanced responding during extinction. To evaluate the generality of this effect, the effects of the serotonin agonists, quipazine and fluoxetine, on responding in extinction were examined. In experiment 1, 72 rats were trained to lever press on a continuous reinforcement schedule for five 30-min sessions. Four sessions of extinction followed; 30-min prior to each, 3 groups (n = 16) received quipazine (0, 1.0, 5.0 mg/kg) and 3 groups (n = 8) received fluoxetine (0, 1.0, 5.0 mg/kg). The 5.0 mg/kg dose of quipazine resulted in a significant reduction in responding on day 1; the lower dose of quipazine and both doses of fluoxetine were without significant effect. In experiment 2, 3 similarly trained groups (n = 8) received either saline or quipazine (5.0 mg/kg) prior to each extinction session; also, 1 quipazine group was injected twice with the 5.0 mg/kg dose, in its home cage, several days before the beginning of extinction. The results of the drug-naive quipazine group replicated those of that group from experiment 1, while the drug-experienced group showed no significant effect of quipazine in extinction. Evidently, prior drug experience modifies the effects of quipazine on behavior. Apart from this drug novelty effect, the lack of significant effect of either quipazine or fluoxetine suggested that the effects of manipulations believed to increase and decrease serotonin functioning on responding in extinction may not be symmetrical. These results may be understood with reference to the hypothesis that serotonin plays a role in tuning out or reducing responsiveness to nonreinforced or irrelevant stimuli.