The role of nucleus accumbens dopamine in responding on a continuous reinforcement operant schedule: a neurochemical and behavioral study.

Two experiments were undertaken to investigate the role of nucleus accumbens dopamine (DA) in instrumental lever pressing on a continuous reinforcement (CRF) schedule. Rats trained to press a lever for food reinforcement on a CRF schedule, and food-deprived control rats, were implanted with dialysis probes in the nucleus accumbens. The day after implantation, rats were tested and dialysis samples were assayed for DA and the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC). Performance of the lever-pressing task resulted in significant increases in extracellular levels of DA and DOPAC relative to control rats. The increases in extracellular DA were significantly correlated (r = 0.92) with the number of lever press responses committed. In the second experiment, the neurotoxic agent 6-hydroxydopamine was infused directly into the nucleus accumbens to investigate the effects of DA depletion on lever-pressing performance. DA depletion had only a modest effect on the total number of lever presses, and there was a significant effect on total lever presses only on the first test day (third day postsurgery). Analyses also were performed on responding across the 45-min session by breaking down the session into three 15-min periods. There was a significant group x time interaction, with DA-depleted rats showing a significant reduction in the numbers of responses in the first 15-min period, but no significant effects over the second or third 15 min in the session. This initial slowing of response rate was present across all 5 test days. These results indicate that DA release and metabolism increases in rats performing on a CRF schedule, and that DA depletion produces a slowing of initial response rate.