NUCLEUS-ACCUMBENS DOPAMINE DEPLETIONS ALTER RELATIVE RESPONSE ALLOCATION IN A T-MAZE COST BENEFIT TASK/

This experiment was conducted to study the role of nucleus accumbens d opamine in the performance of a novel T-maze cost/benefit procedure. R ats were trained on a T-maze task fdr food reinforcement. Under one of the test conditions, one arm of the maze contained a high reinforceme nt density (4 x 45 mg Bioserve pellets) and the other arm contained a low reinforcement density (2 x 45 mg pellets). A large vertical barrie r (44 cm) was placed in the arm that contained the high density of foo d reinforcement. In the second test condition, a separate group of rat s was trained in the same T-maze, in which there were 4 food pellets i n the arm that was obstructed by the barrier, yet there were no food p ellets in the unobstructed arm. After training rats received intra-acc umbens of injections 6-hydroxydopamine or ascorbate vehicle. Nucleus a ccumbens dopamine depletions substantially decreased the number of sel ections of the obstructed arm with the high reinforcement density when the unobstructed arm also contained 2 food pellets. Dopamine-depleted rats in this condition showed increased selection of the no-barrier a rm as well as decreased entry into the arm that contained the barrier. These effects persisted throughout the 3 weeks of post-surgical testi ng. Nevertheless, when the unobstructed arm contained no food pellets, and the only way to obtain food was to climb the barrier, rats with n ucleus accumbens dopamine depletions showed only a modest effect on se lections of the obstructed arm, which recovered by the second week of testing. Dopamine-depleted rats that were tested with food in the unob structed arm showed significantly fewer barrier crossings than dopamin e-depleted rats that were tested with no food in the unobstructed arm. Thus, the present findings are not consistent with the notion that nu cleus accumbens dopamine depletion rendered the animals unable to clim b the barrier, or set an absolute ceiling on the number of barrier cro ssings the animals could perform. Instead, the present results indicat e that nucleus accumbens dopamine depletions affected the relative all ocation of barrier climbing responses if alternative food sources were available.