ANHEDONIA OR ANERGIA - EFFECTS OF HALOPERIDOL AND NUCLEUS-ACCUMBENS DOPAMINE DEPLETION ON INSTRUMENTAL RESPONSE SELECTION IN A T-MAZE COST-BENEFIT PROCEDURE

Two experiments were conducted to study the role of dopamine in the performance of a novel cost/benefit procedure. Rats were trained on a T-maze task in which one arm contained a high reinforcement density (4 x 45 mg Bioserve pellets) and the other arm contained a low reinforcement density (2 x 45 mg pellets). Different groups of rats were trained either with unobstructed access to both arms from the start area, or under a condition in which a large vertical barrier (44 cm) was placed in the arm that contained the high density of food reinforcement. In the first experiment, rats trained under each procedure received injections of 0.1 mg/kg haloperidol and tartaric acid vehicle as a control procedure. Analysis of variance indicated that there was a significant effect of the barrier on maze arm choice, a significant effect of haloperidol, and a significant drug x barrier interaction. Haloperidol did not affect arm choice in rats tested without the barrier present, but this drug significantly reduced the number of selections of the arm with high reinforcement density when the barrier was present. In the second experiment, groups of rats were trained as described above, and then received intraaccumbens injections of 6-hydroxydopamine or ascorbate vehicle. Nucleus accumbens dopamine depletions produced by 6-hydroxydopamine decreased the number of selections of the arm with high reinforcement density when the barrier was present, but had no effect on arm choice when the barrier was not present. These results indicate that blockade of dopamine receptors or depletion of accumbens dopamine did not affect the discrimination of different reinforcement densities, nor alter response selection based on reinforcement magnitude. Thus, the present findings are not consistent with the notion that 0.1 mg/kg haloperidol or nucleus accumbens dopamine depletion fundamentally affected the process of food reinforcement. Rather, the present results are consistent with the notion that haloperidol and nucleus accumbens dopamine depletions affected instrumental response selection based upon the kinetic requirements of the instrumental response.