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 inf ormation for goal-directed orientation behaviors. Sixteen male Long-Evans r ats, under partial water deprivation, were trained in a plus-maze to find w ater rewards in the respective arms which were lit in pseudo-random sequenc e (training trials). Each day one reward arm was selected to deliver six dr ops of water (at 1 s intervals) the others provided only one drop per visit . After 32 visits, probe trials were intermittently presented among trainin g 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 trai ned rats were given bilateral electrolytic lesions in the Nacc shell, two o thers had unilateral lesions and eight had sham operations with approved pr otocols). Field potentials evoked by fornix stimulation were recorded in le sion electrodes to guide placements. Only the lesioned rats showed signific ant impairments (P < 0.05) in selecting the greater reward on probe trials. However on training trials, lesioned land sham-operated) rats made only ra re errors. While the motivation to drink and the capacity for cue-guided go al-directed orientation behavior was spared, lesioned rats were impaired in learning the location of the larger reward. The accumbens lesions apparent ly impaired integration of position and reward value information, consisten t with anatomical and electrophysiological data showing the convergence of hippocampal, amygdalar, ventral tegmental area (VTA) and prefrontal cortica l inputs there. (C) 2000 Elsevier Science B.V. All rights reserved.