Ontogenesis of hydroxyindole-O-methyltransferase gene expression and activity in the rat pineal gland

Postnatal development of hydroxyindole-O-methyltransferase (HIOMT) mRNA expression, HIOMT activity and melatonin content was investigated in the rat pineal gland from birth to adulthood (62-day old). For each age, animals were sacrificed at two different time-points: midday and midnight. HIOMT mRNA was first detectable one day after birth and maximal diurnal levels were reached at the end of the first week. A 2-fold nocturnal increase appeared significantly 11 days after birth. HIOMT activity was detectable from 5 days of life and significant day/night variations did not appear before 21 days after birth. Appearance of melatonin synthesis and rhythm followed that of HIOMT mRNA. The 10-day delay observed between the appearance of the nocturnal increase in HIOMT activity and expression is discussed in term of differential regulation of both HIOMT mRNA expression and HIOMT activity.