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 accumbens dopamine (DA) in instrumental and consummatory responses for food. In vivo microdialysis methods were used to study DA release and metabolism in the nucleus accumbens of behaving rats. Four behavioral conditions were used: performance on a fixed ratio 5 (FR 5) schedule of food reinforcement, consumption of Bioserve food pellets, consumption of laboratory chow, and food deprivation control. Groups of rats that were previously exposed to these conditions were implanted with dialysis probes in the nucleus accumbens and tested the day after implantation. The rats that pressed a lever on a FR 5 schedule showed significant increases in extracellular DA and DA metabolites compared to food-deprived control rats. In further analyses, rats that responded on the FR5 schedule were divided into three groups based upon their response rates. The rats with low response rates did not significantly differ from control rats, whereas rats with medium and high rates of responding showed significant increases in DA release relative to the control group. Rats that received massed presentation of food pellets or laboratory chow consumed large quantities of food, but showed no significant increases 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.