Nucleus accumbens dopamine depletions make animals highly sensitive to high fixed ratio requirements but do not impair primary food reinforcement.

It has been suggested that dopamine in nucleus accumbens is involved in the process of enabling organisms to overcome work-related response costs. One way of controlling work costs with operant schedules is to use fixed ratio schedules with different ratio requirements. In the present study, the effects of nucleus accumbens dopamine depletions were investigated using six schedules: fixed ratio 5, 20, 50, 100, 200, and 300. In the first three schedules the food reinforcement consisted of one 45 mg food pellet per ratio completed. In the remaining schedules the food reinforcement per ratio completed was increased to two pellets for fixed ratio 100, four pellets for fixed ratio 200, and six pellets for fixed ratio 300. All rats were trained extensively prior to surgery, and rats were able to maintain high levels of responding on all schedules up to the fixed ratio 300. After training, rats were injected with either ascorbate vehicle or 6-hydroxydopamine into the nucleus accumbens. Rats were tested post-surgically on each of the schedules, with 3 days of testing per schedule. Rats with nucleus accumbens dopamine depletions exhibited behavioral deficits that were highly dependent upon the ratio value. There were small and transient effects of dopamine depletion on fixed ratio 5 lever pressing, but as the ratio value got larger the impairment became greater. On the fixed ratio 20 and 50 schedules, response rates were partially reduced in dopamine-depleted rats. Responding on the fixed ratio 200 and 300 schedules was severely impaired, and on the last day of fixed ratio 300 testing no dopamine-depleted rats obtained a single reinforcer. These data are consistent with previous reports that accumbens dopamine depletions enhance 'ratio strain', making rats more sensitive to high ratio values. The induction of ratio strain by dopamine depletions does not appear to be related to a loss of appetite, and seems to be relatively independent of the baseline rate of responding and the overall density of food reinforcement across the session. We conclude that dopamine in nucleus accumbens may be important for enabling rats to overcome behavioral constraints such as work-related response costs, and may be critical for the behavioral organization and conditioning processes that enable animals to emit large numbers of responses in the absence of primary reinforcement.