EFFECTS OF DOPAMINE ANTAGONISTS AND ACCUMBENS DOPAMINE DEPLETIONS ON TIME-CONSTRAINED PROGRESSIVE-RATIO PERFORMANCE

Four experiments were conducted to determine the effects of dopamine ( DA) antagonists and DA depletions on progressive-ratio responding for food reinforcement. On this schedule, ratio requirement increased by o ne response after each reinforcer was obtained, and rats were tested i n 30-min sessions. Response rates and highest ratio completed were red uced in a dose-related manner by systemic injections of the D-1 antago nist SCH 23390, and also by the D-2 antagonists haloperidol and raclop ride. Drug-treated rats also showed reductions in time to complete the last ratio, demonstrating that they had stopped responding before the end of the session. DA depletions produced by injections of 6-OHDA di rectly into the nucleus accumbens substantially decreased both the num ber of responses and the highest ratio completed. The deficits in resp onse number and highest ratio completed induced by DA depletions persi sted through the first 3 weeks of postsurgical testing, with some reco very by the fourth week. However, the deficits resulting from dopamine depletions were largely a manifestation of a decrease in response rat e; although time to complete the last ratio was significantly reduced by dopamine depletions in the first few days of testing, rats recovere d on this measure by the fifth day after surgery. Although previous wo rk has shown that performance on several schedules (e.g., continuous, low value ratios, variable interval) is relatively unaffected by accum bens DA depletions, the present data demonstrate that such depletions do produce a substantial and persistent impairment of progressive rati o response output. Rats with accumbens DA depletions appear to have de ficits in maintaining the high work output necessary for responding at large ratio values. The relative sparing of responding on some simple schedules, together with the present progressive ratio results, sugge st that rats with accumbens DA depletions remain directed toward the a cquisition and consumption of food, but they show deficits in work out put for food. (C) 1998 Elsevier Science Inc.