Differential effects of dopaminergic agonists on food-reinforced operant behavior in the long-tailed macaque (Macaca fascicularis).

Dopaminergic involvement in the regulation of operant behavior was examined by monitoring responding after administration of different dopaminergic agonists. A chain schedule of reinforcement was used to classify the effects of these agonists in long-tailed macaques (Macaca fascicularis). The schedule included the following: (1) a 30-s time-out period; (2) a clock-cued 30-s period of differential reinforcement of zero-response rate; (3) a time-in period whose duration depended on response latency; (4) a 2-s reinforcement period in which applesauce was delivered. This cycling schedule maintained a low operant rate and was sensitive to both inhibition (decreases) and excitation (increases) of responding. IV injection of the dopaminergic agonists resulted in the following two basic effects: (1) d-amphetamine and amfonelic acid disrupted performance by response inhibition, which was shown not to be attributable to anorexia; (2) apomorphine and bromocriptine disrupted performance by response excitation. Both effects were reduced by pretreatment with trifluperidol, a dopaminergic antagonist. The results, indicating that dopaminergic systems mediate disruption of food-reinforced operant behavior by altering the frequency of responding, are interpreted in terms of the known neuropharmacological actions of the agonists.