Lesions of the medial shell of the nucleus accumbens impair rats in finding larger rewards, but spare reward-seeking behavior.

The goal of this study was to help better understand the importance of the nucleus accumbens (Nacc) in the processing of position and reward value information for goal-directed orientation behaviors. Sixteen male Long-Evans rats, under partial water deprivation, were trained in a plus-maze to find water rewards in the respective arms which were lit in pseudo-random sequence (training trials). Each day one reward arm was selected to deliver six drops of water (at 1 s intervals) the others provided only one drop per visit. After 32 visits, probe trials were intermittently presented among training trials. Here, all four arms were lit and offered the previously assigned reward. The rats rapidly learned to go to the highly rewarded arm. Six trained rats were given bilateral electrolytic lesions in the Nacc shell, two others had unilateral lesions and eight had sham operations (with approved protocols). Field potentials evoked by fornix stimulation were recorded in lesion electrodes to guide placements. Only the lesioned rats showed significant impairments (P<0.05) in selecting the greater reward on probe trials. However on training trials, lesioned (and sham-operated) rats made only rare errors. While the motivation to drink and the capacity for cue-guided goal-directed orientation behavior was spared, lesioned rats were impaired in learning the location of the larger reward. The accumbens lesions apparently impaired integration of position and reward value information, consistent with anatomical and electrophysiological data showing the convergence of hippocampal, amygdalar, ventral tegmental area (VTA) and prefrontal cortical inputs there.