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 pe rformance 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 reinforceme nt density (2 x 45 mg pellets). Different groups of rats were trained either with unobstructed access to both arms from the start area, or u nder a condition in which a large vertical barrier (44 cm) was placed in the arm that contained the high density of food reinforcement. In t he first experiment, rats trained under each procedure received inject ions of 0.1 mg/kg haloperidol and tartaric acid vehicle as a control p rocedure. Analysis of variance indicated that there was a significant effect of the barrier on maze arm choice, a significant effect of halo peridol, 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 w ith high reinforcement density when the barrier was present. In the se cond experiment, groups of rats were trained as described above, and t hen received intraaccumbens injections of 6-hydroxydopamine or ascorba te vehicle. Nucleus accumbens dopamine depletions produced by 6-hydrox ydopamine decreased the number of selections of the arm with high rein forcement density when the barrier was present, but had no effect on a rm choice when the barrier was not present. These results indicate tha t blockade of dopamine receptors or depletion of accumbens dopamine di d not affect the discrimination of different reinforcement densities, nor alter response selection based on reinforcement magnitude. Thus, t he present findings are not consistent with the notion that 0.1 mg/kg haloperidol or nucleus accumbens dopamine depletion fundamentally affe cted the process of food reinforcement. Rather, the present results ar e consistent with the notion that haloperidol and nucleus accumbens do pamine depletions affected instrumental response selection based upon the kinetic requirements of the instrumental response.