Nucleus accumbens dopamine depletions in rats affect relative response allocation in a novel cost/benefit procedure

Rats were tested on days 1, 3, and 5 of a 5-day test week in an operant chamber in which they could either lever press on a fixed-ratio 5 (FR5) schedule to obtain food pellets (Bioserve) or approach and consume lab chow that was also available in the chamber (Teklad Premier). Rats typically pressed at high rates to obtain the food pellets and ate little of the lab chow. On days 2 and 4 of each week lab chow was not concurrently available, and rats could only lever press on the FR5 schedule for pellets to obtain food. Dopamine depletions produced by intraaccumbens injections of the neurotoxic agent 6-hydroxydopamine produced a dramatic decrease in lever pressing and increase in chow consumption on days when lab chow was available. Lever pressing was not significantly reduced in dopamine-depleted rats on days when chow was not available, although there was a significant correlation between lever pressing and accumbens dopamine levels. These results suggest that nucleus accumbens dopamine depletions do not produce a general deficit in food motivation. Moreover, accumbens dopamine depletions do not appear to produce severe deficits in fine motor control that impair the execution of individual motor acts involved in lever pressing. Rather, the present results are consistent with the notion that accumbens dopamine sets constraints upon which food-related response is selected in a particular situation, and that these depletions alter the relative allocation of food-related responses.