PRELIMINARY EVIDENCE OF A SYNERGISTIC ALPHA(1)-ADRENOCEPTOR AND BETA(1)-ADRENOCEPTOR REGULATION OF RAT PINEAL HYDROXYINDOLE-O-METHYLTRANSFERASE

The activity of serotonin N-acetyltransferase in the rat pineal is reg ulated 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 appa rently under beta(1)-adrenoceptor control. The present study reports t hat 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 eithe r agent alone, indicating that rat pineal HIOMT is also subject to syn ergistic alpha(1)- and beta(1)-adrenergic regulation. The relationship between alpha(1)-adrenoceptor stimulation, serotonin release, and HIO MT activity is considered, following the observation that nocturnal HI OMT activity is elevated in serotonin-depleted pineal glands. (C) 1995 Academic Press, Inc.