Possible involvement of differing classes of dopamine D-2 receptors in yawning and stereotypy in rats.

The present experiments were performed to investigate differences in the properties of the dopamine D-2 receptors related to yawning and stereotypy. Subcutaneous injection of talipexole (B-HT 920) (10-250 micrograms/kg) or SND 919 ((S)-2-amino-4,5,6,7-tetrahydro-6-propylamino-benzothiazole) (25-500 micrograms/kg) evoked yawning behavior with bell-shaped responses. However, SK&F 38393 (1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol) (0.5-10 mg/kg SC) did not elicit yawning and decreased yawning responses to low doses of talipexole (25 micrograms/kg SC) or SND 919 (100 micrograms/kg SC). These low but effective doses for inducing yawning of talipexole or SND 919 in combination with SK&F 38393 (0.5-10 mg/kg SC) did not elicit stereotypy. In contrast, yawning behavior was not produced after very high doses of talipexole (500 micrograms/kg SC) or SND 919 (1000 micrograms/kg SC) given alone or in combination with SK&F 38393 (0.5-10 mg/kg SC). These extremely high doses of talipexole or SND 919 evoked slight stereotypy, which was enhanced by the combined treatment with SK&F 38393. The present results suggest that the dopamine D-2 receptors related to yawning are more sensitive to dopamine receptor agonists than those related to stereotypy, and that concurrent stimulation of postsynaptic dopamine D-1 receptors with D-2 receptors reduces the incidence of yawning but enhances that of stereotypy.