The role of brain dopamine in response initiation: effects of haloperidol and regionally specific dopamine depletions on the local rate of instrumental responding.

Two experiments were undertaken to investigate dopaminergic involvement in the local rate of responding on a fixed ratio 5 (FR5) instrumental lever pressing schedule. Rats were trained to press a lever for food reinforcement on a FR5 schedule, and a computer program was used to record the interresponse time (IRT) for each response. The IRT is the time between each lever pressing response, which is equal to the reciprocal of the local response rate. After several weeks of training, rats received i.p. injections of the dopamine antagonist haloperidol (HP; 0.1, 0.2 and 0.4 mg/kg). HP produced a dose-related decline in overall response number. In addition, HP dramatically altered the IRT distribution. HP-treated rats showed a dose-related reduction in the proportion of IRTs with low time values (high local rates of responding), and a corresponding increase in the relative number of IRTs with high time values (low local rates of responding). In the second experiment, the neurotoxic agent 6-hydroxydopamine was injected directly into the nucleus accumbens, medial neostriatum or ventrolateral neostriatum in order to determine the effects of DA depletion on lever pressing performance. Dopamine depletion in all regions significantly reduced lever pressing, and dopamine-depleted rats had substantial changes in their IRT distributions. Rats with dopamine depletions showed significant reductions in the proportion of IRTs with low time values, and increases in the relative number of IRTs with high time values. The greatest reductions in response number and the most pronounced alterations of the IRT distribution were shown by rats with ventrolateral neostriatal dopamine depletions.(ABSTRACT TRUNCATED AT 250 WORDS)