Beta-adrenergic receptor control of rat pineal hydroxyindole-O-methyltransferase

The nature of the postsynaptic adrenergic receptor on rat pinealocytes which controls hydroxyindole-O-methyltransferase (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. l-Isoproterenol, a beta-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,l-isoproterenol greater than l-norepinephrine greater than l-epinephrine. Terbutaline, a selective beta 2-adrenergic agonist, was ineffective. The selective alpha 1-adrenergic agonists phenylephrine and methoxamine and the alpha 2-agonist clonidine were also ineffective. High doses of the beta-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 beta-adrenergic receptor.