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 acc umbens dopamine (DA) in intrumental lever pressing on a continuous rei nforcement (CRF) schedule. Rats trained to press a lever for food rein forcement on a CRF schedule, and food-deprived control rats, were impl anted with dialysis probes in the nucleus accumbens. The day after imp lantation, rats were tested and dialysis samples were assayed for DA a nd the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC). Performan ce of the lever-pressing task resulted in significant increases in ext racellular levels of DA and DOPAC relative to control rats. The increa ses in extracellular DA were significantly correlated (r = 0.92) with the number of lever press responses committed. In the second experimen t, the neurotoxic agent 6-hydroxydopamine was infused directly into th e nucleus accumbens to investigate the effects of DA depletion on leve r-pressing performance. DA depletion had only a modest effect on the t otal number of lever presses, and there was a significant effect on to tal lever presses only on the first test day (third day postsurgery). Analyses also were performed on responding across the 45-min session b y breaking down the session into three 15-min periods. There was a sig nificant group x time interaction, with DA-depleted rats showing a sig nificant reduction in the numbers of responses in the first 15-min per iod, 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 in creases in rats performing on a CRF schedule, and that DA depletion pr oduces a slowing of initial response rate.