β-adrenergic receptor control of rat pineal hydroxyindole-o-methyltransferase

Abstract

The nature of the postsynaptic adrenergic receptor on rat pinealocytes which controls hydroxyindole-O-meth-yltransferase (HIOMT) activity was studied. This enzyme is involved in the synthesis of the pineal hormone melatonin. Adrenergic drugs were administered continuously for a 7-day period to rats in which neural stimulation of the pineal gland was blocked by either superior cervical ganglionectomy or exposure to constant light. ιIsoproterenol, a β-adrenergic agonist, prevented the fall in enzyme activity that occurs when neural stimulation is interrupted; d-isoproterenol was ineffective. The potency order of different adrenergic agonists was d, ι-isoproter-enol < ι-norepinephrine < ι-epinephrine. Terbutaline, a selective α2-adrenergic agonist, was ineffective. The selective α1-adrener-gic agonists phenylephrine and methoxamine and the α2-agonist clonidine were also ineffective. High doses of the β-adrenergic blocker propranolol antagonized the effect of isoproterenol and caused a fall in HIOMT activity in normal rats housed under normal diurnal lighting. This in vivo evidence is consistent with the hypothesis that the neural control of pineal HIOMT is mediated via a β-adrenergic receptor. (Endocrinology 113: 348, 1983). © 1983 by The Endocrine Society.