Accumbens dopamine and the regulation of effort in food-seeking behavior: modulation of work output by different ratio or force requirements.

It has been suggested that depletions of accumbens dopamine (DA) make rats more sensitive to work-related response costs. One way of controlling work costs in operant tasks has been to use fixed-ratio (FR) schedules with different ratio requirements. In addition to using ratio requirements to control response costs, investigators also can employ different force requirements. In the present study, different groups of rats were trained on two schedules (FR 1 and FR 5) and weights were placed on the levers 2 days each week. In the FR 5 studies, two different weights were used (32 or 64 g), while three different weights were used in the FR 1 studies (32, 64, or 96 g). After baseline training, rats received intra-accumbens injections of either 6-OHDA to deplete DA, or ascorbate vehicle as the control. The effects of DA depletions were highly schedule-dependent. DA-depleted animals on the FR 5 schedule showed reductions in responding across the different weight conditions. In contrast, DA depletion did not significantly suppress FR 1 responding under any conditions. Addition of weights to the levers reduced responding on the FR 1 and FR 5 schedules, but did not enhance sensitivity to DA depletion. Thus, rats with accumbens DA depletions were sensitive to different ratio requirements, yet they were relatively insensitive to different force requirements within the range tested. These studies indicate that DA depletions make animals sensitive to temporal or rate components of work that greatly influence responding on ratio schedules. Copyright 2003 Elsevier B.V.