[Effects of dopamine receptor agonists and antagonists on cocaine-induced behaviors in rats]

In this study, cocaine (5-20 mg/kg), an indirect dopamine agonist, increased locomotor activity and rearing accompanied with head circling and body shaking in a dose-dependent manner. A high dose of cocaine (40 mg/kg), meaning a toxic dose, slightly induced sniffing and licking. Both SCH23390, a D-1 receptor antagonist, and raclopride, a D-2 antagonist, inhibited all behaviors induced by cocaine, suggesting that the behavioral actions of cocaine may involve the activation of D-1 and D-2 receptors. Selective D-2 agonist quinpirole (0.1 and 1.0 mg/kg) inhibited hyperlocomotion induced by cocaine (20 mg/kg), but was replaced by the typical stereotyped behaviors such as sniffing at low dose (0.1 mg/kg), and licking and gnawing at high dose (1.0 mg/kg). SK & F38393, a selective D-1 agonist, in combination of cocaine did not induce these stereotyped behaviors which resulted in synergistic interaction of D-1 and D-2 receptor stimulation. These results suggest that the indirect stimulation of postsynaptic D-2 receptors by cocaine (20 mg/kg) was insufficient to induce stereotyped behaviors. The actions of cocaine on dopamine D-1 receptors seem to be more potent than that on D-2 receptors.