Photoneural regulation of rat pineal hydroxyindole-O-methyltransferase (HIOMT) messenger ribonucleic acid expression: an analysis of its complex relationship with HIOMT activity.

In the pineal gland, synthesis of melatonin requires O-methylation catalyzed by hydroxyindole-O-methyltransferase (HIOMT; EC 2.1.1.4). We investigated in vivo the molecular mechanisms involved in the regulation of rat pineal HIOMT messenger RNA (mRNA) expression and activity using in situ hybridization and radioenzymatic assay. HIOMT mRNA levels and activity are both detectable during the daytime and display nocturnal increases of 100% and 30%, respectively. These variations are controlled by the endogenous clock, as they persist in constant darkness. The nocturnal increase in HIOMT mRNA mainly results from a beta-adrenergic stimulation of HIOMT gene expression without requiring de novo synthesis of a transcription factor. In contrast, the nocturnal increase in HIOMT activity appears independent of beta1/alpha1-adrenergic stimulation. A light pulse at night abolishes the nighttime increase in HIOMT mRNA, but not HIOMT activity. Constant light application for up to 11 days does not depress HIOMT mRNA levels lower than the daytime levels, but decreases enzyme activity down to 50% of the daytime level. This finding indicates that the nocturnal stimulation of HIOMT gene expression is required for sustaining a basal level of activity over a few days. Our data suggest 1) that HIOMT gene expression is partly regulated by beta1-stimulation; and 2) that HIOMT activity is regulated over the short term by a nonnoradrenergic stimulus and over the long term by noradrenergic stimulation.