Ontogenesis of pineal thyroxine 5′-deiodinase activity and plasma melatonin concentration in the rat

Rat pineal thyroxine 5′-deiodinase (5′-D) activity was detectable by 5 days after birth. Nyctohemeral differences became significant by 10 days of age and gradually reached adult magnitude at 1-2 months, primarily due to a progressive increase in nocturnal enzyme activity. A nyctohemeral difference in N-acetyltransferase (NAT) activity was observed by 5 days postnatally and the maturation of NAT rhythmicity was characterized by both a decrease in diurnal and an increase in nocturnal enzyme activity. The maturation of 5′-D rhythmicity was slower than that of NAT. Isoproterenol stimulated 5′-D activity in 5-day-old rats in which there was no spontaneous nyctohemeral 5′-D rhythm, suggesting that the appearance of rhythmicity in pineal 5′-D may depend on the development of pineal innervation. Melatonin was detectable in plasma by 5 days postnatally and reached adult levels at 21 days. A nyctohemeral difference in plasma melatonin concentration existed from day 5 with the highest value at midnight. The difference between noon and midnight increased approximately 10-fold to the adult amplitude by 15 days of age. There was a transient rise of both diurnal and nocturnal plasma melatonin concentration which peaked at 10 days before dropping severalfold to the adult level for both noon and midnight values by 21 days. Since the rhythms of pineal NAT activity and of plasma melatonin concentration are established before that of pineal 5′-D, it is unlikely that the rhythm of melatonin secretion is initiated by a prior maturation of the pineal 5′-D rhythm.