The importance of nucleus accumbens core and shell dopamine in operant responding in the rat

The involvement of nucleus accumbens dopamine in behavior has become the focus of intense scrutiny. Accumbens dopamine has been identified as playing a role in several different behaviors. It has been suggested that dopamine in the accumbens mediates the reinforcing effects of stimuli such as food, but mounting evidence seems to suggest that the nucleus accumbens serves as an interface between motivation and motor output. Anatomical evidence indicates that the nucleus accumbens is not a homogenous structure, but is instead composed of at least two different subregions, which are known as the core and the shell. At the present time, little work has focused upon the behavioral functions of the two regions, and none has investigated the role of dopamine. For that reason, a series of four experiments was undertaken to investigate the behavioral functions of core and shell dopamine. In the first experiment, rats were trained on a concurrent lever pressing/free-feeding paradigm, and then injected with the neurotoxic agent 6-hydroxydopamine in either the core or the shell of the accumbens to deplete dopamine. In the second and third experiments, rats were trained to lever press on either the fixed-ratio 5 or variable-interval 30 operant schedule, and then underwent the same dopamine depletions as in experiment one. In the final experiment, rats were trained to lever press on the fixed-ratio 5, variable-interval 30, or a variable-interval 30/fixed-ratio 5 chain schedule and then implanted with microdialysis probes in either the core or the shell in order to investigate dopamine release during performance of the tasks. In the dopamine depletion studies, significant behavioral effects were seen only following injection of 6-hydroxydopamine into the nucleus accumbens core. Conversely, in all three experimental microdialysis groups, lever pressing significantly increased DA levels in both accumbens subregions. In addition, greater increases in extracellular dopamine were found in the shell. In conclusion, relatively selective depletion of shell DA has little effect on lever pressing. However, DA release in the shell, as compared to the core, is more responsive to lever pressing on various operant schedules.