Pergamon Pharmacology Biochemistry and Behavior, Vol. 50, No. 3, pp. 437-443, 1995 Copyright o 1995 Elsevia Science Ltd Printed in the USA. All rights reserved oo91-3057/95 $9.50 + .oo OO!Jl-3057(94)00294-O The Effects of Nucleus Accumbens Dopamine Depletions on Continuously Reinforced Operant Responding: Contrasts With the Effects of Extinction J. D. SALAMONE,’ P. KURTH, L. D. MCCULLOUGH AND J. D. SOKOLOWSKI Department of Psychology, University of Connecticut, Storm, CT 06269-1020 Received 10 January 1994 SALAMONE, J. D., P. KURTH, L. D. MCCULLOUGH AND J. D. SOKOLOWSKI. The effects ofnucleus uccum- bens dopamine depletions on continuously reinforced operant responding: Contrasts with the effects of extinction. PHARMA- COL BIOCHEM BEHAV 50(3) 437-443, 1995. -Two experiments were undertaken to study the role of nucleus accumbens dopamine (DA) in instrumental lever pressing on a continuous reinforcement schedule (CRF). In the first experiment, the neurotoxic agent Bhydroxydopamine 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. During the test session on day 3 postsurgery, 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-mitt 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 accumbens DA depletions produced a slowing of the local rate of responding as indicated by a significant decrease in high rate (i.e., short-duration IRT) responses and an increase in low rate (i.e., long-duration IRT) responses. In a second experiment, the effects of extinction on CRF performance were investigated. Unlike the effects of nucleus accumbens DA depletion, extinction produced lower levels of responding through- out 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 probably indicative of an extinction “burst.” These results indicate that accumbens DA depletions produce a response slowing that does not closely resemble the effects of extinction. Nucleus accumbens Dopamine Motivation Operant behavior Behavioral activation Reinforcement Extinction Motor CONSIDERABLE evidence indicates that dopamine (DA) has some involvement in the performance of appetitively moti- vated behavior. Nevertheless, there continues to be uncer- tainty about the precise nature of dopaminergic involvement in appetitively motivated instrumental responding. Several studies have shown that systemic administration of DA antag- onists impairs positively reinforced instrumental responses [for reviews see (29,31,33,45)], and it has been suggested that DA systems are important for mediating the reinforcing or “hedonic” effects of rewarding stimuli (45-47). Although DA is present in several different terminal regions, DA in nucleus accumbens has received particular emphasis in regard to the hypothesized involvement of DA in reinforcement processes (7,13). Nucleus accumbens DA has been implicated in a num- ber of behavioral functions related to appetitive motivation (3-6,10,14,18,21,22,26,36). Considerable research has focus- sed upon studies of drug reinforcement processes (7,13,42). Moreover, several studies have shown that accumbens DA release or metabolism is increased during the performance of appetitive tasks (4,19,24,34). ’ To whom requests for reprints should be addressed. 437