Role of central melanotropinergic and adrenergic neurons in neuronal mechanisms involved in yawning behavior.

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The present experiments were performed to understand the role of central adrenergic and melanotropinergic neurons in neuronal mechanisms involved in yawning behavior in rats. Yawning was evoked by subcutaneous injections of apomorphine and piribedil, mixed dopamine D_1/D_2-receptor agonists, but not by SK＆F 38393, a dopamine D_1-receptor agonist. The putative dopamine autoreceptor agonists, talipexole and snd 919 also elicited yawning. The yawning induced by these dopamine receptor agonists was increased by pindolol and propranolol which block the central beta-adrenoceptors, but not by the peripheral beta-adrenoceptor antagonists, carteolol and atenolol. these beta-adrenoceptor antagonists given alone did not elicit yawning. Conversely, the yawning was inhibited by salbutamol, a beta-adrenoceptor agonist, without being affected by prazosin, an alpha-adrenoceptor antagonist. The combined administration of SK＆F 38393 and the beta-adrenoceptor antagonists did not induce yawning.The yawnin … More g elicited by either apomorphine or piribedil in combination with pindolol was suppressed by spiperone and YM-09151-2, dopamine D_2-receptor antagonists, and scopolamine, a muscarinic receptor antagonist, but not by SCH 23390, a dopamine D_1-receptor antagonist. Physostigmine or pilocarpine induced yawning, which was also enhanced by pindolol and propranolol. This enhanced yawning was inhibited by scopolamine, but not by spiperone, YM-09151-2 and SCH 23390. On the other hand, the yawning responses induced by talipexole, apomorphine or piribedil were markedly increased without eliciting stereotypy by reserpine and p-chlorophenylalanine. Furthermore, yawning was observed after low doses of talipexole or SND 919, but was not observed after very high doses of the drugs. The high doses of talipexole or SND 919 given alone evoked a slight stereotypy, which was enhanced by the combined treatment with SK＆F 38393. However, low and effective doses for inducing yawning of talipexole or SND 919 in combination with SK＆F 38393 did not elicit stereotypy. The present results suggest that yawning is evoked by stimulation of dopamine D_2-receptors having a high affinity and consequent muscarinic activation, and that the beta-adrenoceptor blockade facilitates the occurrence of yawning induced by dopaminergic and cholinergic agonists. It was also suggested that the yawning induced by dopamine receptor agonists is potentiated by decreases in serotonergic neuron activity and that the stereotypy-related dopamine D_2-receptors not only cooperate with the dopamine D_1-receptors but also be less sensitive than the yawning-related dopamine D_2-receptors. Less