Effect of the D1 receptor agonist SKF 38393 on some behavioural effects of apomorphine in rats.

The dopamine receptor agonist apomorphine in experiments on rats in low doses (0.025-0.2 mg/kg, s.c.) induced yawning which reflected a selective activation of presynaptic dopamine receptors. In high doses (0.25-1.0 mg/kg) apomorphine induced stereotyped sniffing and yawning in consequence of postsynaptic D2 receptor activation. Dopamine D1 receptor agonist SKF 38393 inhibited yawning induced by low doses of apomorphine. The inhibitory effect of SKF 38393 on apomorphine-induced yawning was attenuated by pretreatment with specific D1 receptor antagonist SCH 23390 [2-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine-7-ol). On the other hand however, SKF 38393 potentiated sniffing induced by the high doses of apomorphine without affecting gnawing. These data indicate that D1 receptor activation modulates both pre- and postsynaptic effects of apomorphine in opposite directions.