Yawning is elicited by D2 dopamine agonists but is blocked by the D1 antagonist, SCH 23390.

The subtype of dopamine (DA) receptors mediating the yawning response to DA agonists was determined in rats. Yawning was elicited both by the mixed D1-D2 agonist apomorphine and by the specific D2 agonist LY 171555, but not by the selective D1 agonist SKF 38393. Both apomorphine- and LY 171555-induced yawning were antagonized not only by the selective D2 antagonist sulpiride but, unexpectedly, also by the selective D1 antagonist SCH 23390. The results suggest that DA receptors mediating the yawning response are of the D2 type, and that these receptors are connected with D1 receptors in such a way that the blockade of the latter results in the functional inactivation of the former.