Nucleus accumbens dopamine depletions make rats more sensitive to high ratio requirements but do not impair primary food reinforcement

It has been suggested that nucleus accumbens dopamine is involved in the process of enabling organisms to overcome work-related response costs. One way of controlling work requirements in operant schedules is to use fixed ratio schedules with different ratio requirements. In the present study, the effects of nucleus accumbens dopamine depletions were assessed using four schedules: fixed ratio 1, 4, 16, and 64. Rats with nucleus accumbens dopamine depletions showed behavioral deficits that were highly dependent upon the ratio value; there were no effects of dopamine depletions on fixed ratio 1 lever pressing, but as ratio value got larger, the impairment became much greater. In a separate experiment, pre-feeding to reduce food motivation was shown to produce a different pattern, such that performance on all ratio schedules was substantially impaired. Thus, aspects of food reinforcement that are involved in fixed ratio 1 performance are highly sensitive to food motivation, bur are largely unaffected by nucleus accumbens dopamine depletions. Nevertheless, nucleus accumbens dopamine depletions affected the elasticity of demand for food, and enhanced "ratio strain", i.e. they exacerbated the response-suppressing effects of increasing ratio value. (C) 1999 IBRO. Published by Elsevier Science Ltd.