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 dopami ne (DA) depletions on progressive ratio responding for food reinforcement. In one version of this schedule, ratio requirement increased by one respons e after each reinforcer was obtained (PROG1). In the other version, ratio r equirement increased by five responses after each reinforcer was obtained ( PROG5). For both versions, 60-min sessions were conducted. Accumbens DA dep letions 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 te sting for both the PROG1 and FROGS schedules. However, there were differenc es between the effects of DA depletions on the two schedules in terms of th e 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 testi ng on the PROG1 schedule, rats recovered on this measure by the second week after surgery. In contrast, DA-depleted rats on the FROGS schedule showed a more persistent suppression of the time to complete the last ratio, which lasted through both weeks of postsurgical testing. Performance on schedule s 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 rat io response output. The high work output necessary for responding on the FR OGS schedule may make these animals more sensitive to the effects of accumb ens DA depletions. (C) 1999 Elsevier Science Inc.