Potentiation by serotonergic inhibition of yawning induced by dopamine receptor agonists in rats.

Low doses of the dopamine D2-receptor agonist, B-HT 920 (25 micrograms/kg, SC), and the dopamine D1/D2-receptor agonists, apomorphine (50 micrograms/kg, SC) and piribedil (1 mg/kg, SC), evoked yawning. However, the dopamine D1-receptor agonist, SK&F 38393 (2 mg/kg, SC), failed to induce yawning. The yawning responses induced by B-HT 920, apomorphine or piribedil were markedly increased without eliciting stereotypy by pretreatment with reserpine (5 mg/kg, IP, 24 hr). These yawning responses were also enhanced by p-chlorophenylalanine (PCPA) (300 mg/kg, IP, 72 hr), but not by alpha-methyl-p-tyrosine (300 mg/kg, IP, 6 hr). The yawning induced by B-HT 920 given alone and in combination with reserpine or PCPA was inhibited by spiperone (0.5 mg/kg, IP) or scopolamine (0.5 mg/kg, IP), but not by SCH 23390 (0.5 mg/kg, IP). The present results suggest that yawning is evoked by stimulation of dopamine D2-receptors having a high affinity and consequent muscarinic activation, and that the yawning induced by dopamine receptor agonists is potentiated by decreases in serotonergic neuron activity.