NUCLEUS-ACCUMBENS DOPAMINE RELEASE INCREASES DURING INSTRUMENTAL LEVER PRESSING FOR FOOD BUT NOT FREE FOOD-CONSUMPTION

This experiment was undertaken to investigate the role of nucleus accu mbens dopamine (DA) in instrumental and consummatory responses for foo d. In vivo microdialysis methods were used to study DA release and met abolism in the nucleus accumbens of behaving rats. Four behavioral con ditions were used: performance on a fixed ratio 5 (FR 5) schedule of f ood reinforcement, consumption of Bioserve food pellets, consumption o f laboratory chow, and food deprivation control. Groups of rats that w ere previously exposed to these conditions were implanted with dialysi s probes in the nucleus accumbens and tested the day after implantatio n. The rats that pressed a lever on a FR 5 schedule showed significant increases in extracellular DA and DA metabolites compared to food-dep rived control rats. In further analyses, rats that responded on the FR 5 schedule were divided into three groups based upon their response ra tes. The rats with low response rates did not significantly differ fro m control rats, whereas rats with medium and high rates of responding showed significant increases in DA release relative to the control gro up. Rats that received massed presentation of food pellets or laborato ry chow consumed large quantities of food, but showed no significant i ncreases in DA release. This experiment demonstrated that performance of lever pressing behavior is accompanied by an increase in accumbens DA release and metabolism, and that DA release in nucleus accumbens is more closely related to the performance of highly active instrumental responses than it is to consumption of large quantities of food.