THE EFFECTS OF NUCLEUS-ACCUMBENS DOPAMINE DEPLETIONS ON CONTINUOUSLY REINFORCED OPERANT RESPONDING - CONTRASTS WITH THE EFFECTS OF EXTINCTION

Two experiments were undertaken to study the role of nucleus accumbens dopamine (DA) in instrumental lever pressing on a continuous reinforc ement schedule (CRF). In the first experiment, the neurotoxic agent 6- hydroxydopamine was infused directly into the nucleus accumbens to inv estigate the effects of DA depletion on lever pressing performance. DA depletion had only a modest effect on the total number of lever press es, and there was a significant effect on total lever presses only on the first test day (third day postsurgery). Analyses also were perform ed on responding across the 45-min session by breaking down the sessio n into three 15-min periods. During the test session on day 3 postsurg ery, there was a significant group x time interaction, with Da-deplete d 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 period within the session. Although control rats showed a within-session decline in responding, the DA-depleted rats did not. In addition, analysis of interresponse times (IRTs) indicated that acc umbens DA depletions produced a slowing of the local rate of respondin g as indicated by a significant decrease in high rate (i.e., short-dur ation IRT) responses and an increase in low rate (i.e., long-duration IRT) responses. In a second experiment, the effects of extinction on C RF performance were investigated. Unlike the effects of nucleus accumb ens DA depletion, extinction produced lower levels of responding throu ghout the entire test session. In contrast to the effects of accumbens DA depletions, analysis of IRTs indicated that extinction produced a significant increase in high-rate responses (low IRTs), which is proba bly indicative of an extinction ''burst.'' These results indicate that accumbens DA depletions produce a response slowing that does not clos ely resemble the effects of extinction.