Preliminary evidence of a synergistic alpha 1- and beta 1-adrenoceptor regulation of rat pineal hydroxyindole-O-methyltransferase.

The activity of serotonin N-acetyltransferase in the rat pineal is regulated through a synergistic action of norepinephrine on alpha 1- and beta 1-adrenoceptors. Rat pineal hydroxyindole-O-methyltransferase (HIOMT) activity is also responsive to the beta 1-adrenoceptor, while melatonin synthesis and indoleamine release in certain species is apparently under alpha 1-adrenoceptor control. The present study reports that the simultaneous administration of doxazosin (a selective alpha 1-adrenergic antagonist) and metoprolol (a selective beta 1-adrenergic antagonist) reduces peak HIOMT activity significantly more than either agent alone, indicating that rat pineal HIOMT is also subject to synergistic alpha 1- and beta 1-adrenergic regulation. The relationship between alpha 1-adrenoceptor stimulation, serotonin release, and HIOMT activity is considered, following the observation that nocturnal HIOMT activity is elevated in serotonin-depleted pineal glands.