Differential effects of dopamine receptor subtype blockade on performance of rats in a reaction-time paradigm

Pharmacological manipulation of the dopaminergic system with antipsychotic agents disrupts motor behavior. Although most antipsychotic drugs have high affinity for D2 receptors, they also interact with other dopamine receptor subtypes. Therefore, the role of each of these receptor subtypes on motor performance is unclear.

The present study sought to investigate the relative importance of D1, D2, and D3 receptors on performance in a conditioned reaction-time task known to be extremely sensitive to dysfunction of the dopaminergic nigrostriatal pathway.

Rats were trained to release a lever in response to a visual cue within a reaction-time limit to receive a reinforcer (45mg food pellet). After the behavior of the rats had stabilized, the effects of a D1 (A69024), D2 (eticlopride), and D3 (nafadotride) receptor antagonists were assessed.

A-69024 had no effect on performance at any dose tested (0.3, 0.6, and 1.3 mg/kg s.c.). Nafadotride (0.1, 0.3, and 1 mg/kg s.c.) produced only a mild deficit in performance at the highest dose. This deficit was characterized by an increase in the number of delayed responses with a non-significant decrease in the number of premature responses indicative of non-specific sedative effects. In contrast, the D2 receptor antagonist eticlopride (0.005, 0.01, and 0.02 mg/kg s.c.) produced profound deficits in performance as evidenced by a dose-dependent decrease in the number of correct responses. This decrease was accompanied by an increase in the number of delayed responses and a lengthening of the reaction time at the highest doses.

These results provide further evidence that the execution of the reaction-time task is dependent preferentially upon the activation of D2 receptors, but not D1 or D3 receptors.