Nucleus accumbens dopamine and rate of responding: Neurochemical and behavioral studies

It has been suggested that accumbens dopamine (DA) is involved in the proce ss of enabling organisms to expend energy in various situations, including foraging, maze running, and leverpressing. Although accumbens DA depletions impair stimulant self-administration, the effects of these depletions on v arious food-reinforced operant schedules are highly variable. Accumbens DA depletions have Little or no effect on total response output in rats respon ding on the simplest schedule of food reinforcement (i.e., the fixed ratio 1). In addition, it has been shown clearly that the effects of accumbens DA depletions do not resemble the effects of extinction or prefeeding to redu ce food motivation. Over the last several years, our laboratory has investi gated the effects of accumbens DA depletions on several schedules, includin g fixed ratio 1, variable interval 30 sec, fixed interval 30 sec, progressi ve ratio, and fixed ratio 4, 5, 16, and 64. These schedules generate very d ifferent rates of responding, ranging from 300 to 3,000 responses per 30 mi n. Regression analyses of all these studies indicates a significant linear relation between control rates of responding and the degree of suppression of responding produced by accumbens DA depletions. Schedules that generate relatively low response rates, such as fixed ratio 1 or variable interval 3 0 sec, are little affected by accumbens DA depletions, yet schedules that g enerate high response rates (e.g., fixed ratio 64) are severely disrupted. Prefeeding shows different patterns of suppression as a function of respons e rate. Microanalysis of the temporal characteristics of leverpressing has shown that accumbens DA depletions produce a response slowing, as measured by the interresponse time; extinction and prefeeding produce a different pa ttern of effects. These results indicate that; accumbens DA depletions do n ot blunt the reinforcing effects of food, but do suppress responding in a r ate-dependent manner. Ln addition, microdialysis studies have shown that ac cumbens DA release is positively correlated with leverpressing response rat e. Accumbens DA appears to be involved in energy expenditure, behavioral ac tivation, or maintenance of high local rates of responding, which are funct ions that represent an area of overlap between motor and motivational proce sses.