Anatomic distribution of reinforcer selective cell firing in the core and shell of the nucleus accumbens

Abstract

We have previously reported that distinct populations of nucleus accumbens (NAc) neurons differentially encode information about goal-directed behaviors for "natural" (food/water) vs. cocaine reinforcement [J Neurosci 20:4255-4266, (2000)]. Here, the anatomic distribution of reinforcer-selective cell firing was examined within the core and shell of the NAc. Rats (n=8) were trained on a multiple schedule for water reinforcement and cocaine (0.33 mg/infusion) self-administration. Next, microelectrode arrays (eight wires/array) were bilaterally positioned in the NAc core and shell, and cell firing was recorded during the multiple schedule. All electrode tip placements were then "marked," and histological reconstruction of each electrode position was completed. Of 93 NAc cells, 44 neurons (47%) exhibited 1 of 4 types of well-documented patterned discharges relative to the water- or cocaine-reinforced response. Of the 44 phasic cells, 39 neurons (89%) displayed differential, nonoverlapping neuronal firing patterns across the two reinforcer conditions (i.e., reinforcer-selective activity). Histological reconstruction of electrode placement revealed that NAc patterned discharges, specific to goal-directed behaviors for water or cocaine, were not limited to one NAc subregion but were evenly distributed and intermixed throughout the core and shell. These findings are discussed with respect to the functional organization of the NAc.