Neuronal mechanisms involved in yawning behavior: Role of <alpha> -melanocyte-stimulating hormone.

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A behavioral study was performed in an attempt to understand the neuronal mechanisms involved in yawning behavior in rats. Subcutaneous injections of low doses of apomorphine, piribedil, 3-PPP and TL-99 evoked yawning. SK&F-38393, a dopamine D-1 receptor agonist, induced neither yawning nor stereotypy. But bromocriptine, a dopamine D-2 receptor agonist, induced yawning for which the dose-response curves showed a bellshaped form. Yawning responses induced by systemic injection of apomorphine, piribedil, 3-PPP or bromocriptine were wholly suppressed after the treatment with sulpiride, a dopamine D-2 receptor antagonist. On the other hand, the apomorphine-induced yawning was increased by dl-propranolol and pindolol which did not induce yawning but not by prazosin. SK&F-38393 in combination with <beta> -adrenoceptor antagonists did not induce yawning. The yawning elicited by apomorphine or piribedil in combination with pindolol was suppressed by spiperone and YM-09151-2, dopamine D-2 recep … More tor antagonists, and scopolamine, a muscarinic receptor antagonist, but not by SCH-23390, a dopamine D-1 receptor antagonist. Pindolol also enhanced the yawning induced by physostigmine and pilocarpine, and the enhanced yawning was inhibited by scopolamine but not by spiperone, YM-09151-2 and SCH-23390. Bilateral injections of apomorphine, piribedil or 3-PPP into the striatum or septum also elicited a marked yawning. Apomorphine induced yawning in adult rats which had been injected during the neonatal period with monosodium glutamate. Moreover, apomorphine-induced yawning was not inhibited by an intraventricular injection of antiserum of <alpha> -melanotropin. The results indicate that the yawning induced by dopaminergic agonists is mediated by dopamine D-2 receptors but not by <alpha> -melanotropinergic systems. <beta> -Adrenoceptor blockade exerts a stimulatory effect in the occurrence of yawning and striatal and septal dopaminergic systems may be also related to the occurrence of yawning behavior. Less