[Behavioral effects of chronic apomorphine, and D-1/D-2 dopamine receptor activities in rats].

The present study was conducted to investigate the effects of chronic treatment with apomorphine on yawning and stereotyped behaviors induced by apomorphine, and catalepic responses induced by haloperidol. Rats received apomorphine (1 mg/kg, sc), a direct dopamine D1/D2 agonist, or vehicle once a day for 21 days. The chronic treatment with apomorphine shifted to the right the dose response curve of yawning to administration of apomorphine, which preferentially activates presynaptic dopamine D-2 receptors (auto receptors) at low doses. Haloperidol (0.5 mg/kg, ip)-induced catalepsy mediated by the inhibition of postsynaptic D-2 receptors was unaltered. A subsequent challenge dose of apomorphine (5 mg/kg, ip) produced oral stereotyped behaviors such as sniffing, licking and biting in the vehicle-treated rats. Chronic apomorphine treatment produced significant enhancement of sniffing alone, which may be behavioral sensitization to apomorphine, and, in contrast, attenuated licking and biting, which depend on D-2 receptor activities. These phenomena lasted for at least 30 days. Sniffing might involve relatively increased stimulation of D-1 receptors, as compared with licking and biting. These results suggest that chronic apomorphine reduces presynaptic dopamine D-2 receptor activity, and as a consequence may induce long-lasting postsynaptic dopamine receptor (mainly D-1 receptor) activation.