Antagonism of apomorphine-induced yawning by SCH 23390: evidence against the autoreceptor hypothesis.

The ability of apomorphine to induce yawning (YWG) in normal and reserpinized rats and its interaction with SCH 23390, a potent and specific D-1 receptor antagonist, was studied. Apomorphine was more potent in inducing YWG in reserpine-pretreated as compared to control rats. SCH 23390, in low doses (0.05 mg/kg SC), was able to significantly reduce the YWG evoked by apomorphine both in control and in reserpine-pretreated rats. The results indicate that D-1 receptors contribute to YWG elicited by apomorphine and contradict the idea that this effect is mediated by DA autoreceptors.