Evidence for involvement of an adrenal catecholamine in the beta-adrenergic inhibition of oxytocin release in lactating rats.

Adrenergic systems exert dual control over the release of oxytocin (OT) in rats, with stimulation of alpha-adrenergic receptors exciting, and stimulation of beta-adrenergic receptors inhibiting, release of this neurohormone. Because suckling stimulation also releases epinephrine from the adrenal medulla, the present experiments tested whether catecholamines of adrenal origin may participate in the adrenergic regulation of OT release during lactation. In two independent experiments, adrenal demedullation of rats in midlactation did not alter the basal plasma levels of OT, but markedly enhanced the suckling-induced release of OT, suggesting an inhibitory action of an adrenal catecholamine. The OT release induced by suckling in both sham-operated and adrenal demedullated rats was prevented by stimulation of beta-adrenergic receptors with isoproterenol. Conversely, blockade of beta-adrenergic receptors with propranolol prevented the inhibitory effects of isoproterenol, and when given alone, mimicked the effects of demedullation to enhance suckling-induced OT release. Stimulation of peripheral alpha-adrenergic receptors with phenylephrine did not affect either basal or suckling-induced OT release, but blockade of alpha-adrenergic receptors with phentolamine also completely prevented the release of OT by suckling. These data support the concept that stimulation of beta-adrenergic receptors inhibits OT secretion, and further suggest that this may be due, at least in part, to an action of an adrenal catecholamine, which may act centrally and/or directly on the neurohypophysis. The present results also provide further evidence that activation of central, but not peripheral, alpha-adrenergic mechanisms is necessary for suckling-induced OT release.