Nucleus accumbens dopamine depletions and time-constrained progressive ratio performance: effects of different ratio requirements.

Two experiments were conducted to determine the effects of accumbens dopamine (DA) depletions on progressive ratio responding for food reinforcement. In one version of this schedule, ratio requirement increased by one response after each reinforcer was obtained (PROG1). In the other version, ratio requirement increased by five responses after each reinforcer was obtained (PROG5). For both versions, 60-min sessions were conducted. Accumbens DA depletions produced by local injections of 6-OHDA substantially decreased the number of responses on both schedules. The deficits in the response number induced by DA depletions persisted through the two weeks of postsurgical testing for both the PROG1 and PROG5 schedules. However, there were differences between the effects of DA depletions on the two schedules in terms of the time to complete the last ratio. Although time to complete the last ratio was significantly reduced by DA depletions only in the first week of testing on the PROG1 schedule, rats recovered on this measure by the second week after surgery. In contrast, DA-depleted rats on the PROG5 schedule showed a more persistent suppression of the time to complete the last ratio, which lasted through both weeks of postsurgical testing. Performance on schedules that generate low baseline rates of responding (e.g., continuous, fixed, and variable interval) is relatively unaffected by accumbens DA depletions; nevertheless, accumbens DA depletions substantially impair progressive ratio response output. The high work output necessary for responding on the PROG5 schedule may make these animals more sensitive to the effects of accumbens DA depletions.