Circadian rhythms of melatonin-synthesizing enzyme activities and melatonin levels in planarians.

In most vertebrates and several insects, melatonin (N-acetyl-5-methoxytryptamine) is synthesized enzymatically from serotonin (5-hydroxytryptamine) by the sequential action of arylalkylamine N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT). In the freshwater planarian Dugesia japonica, which belongs to the most primitive metazoan phylum, activities of NAT and HIOMT, as well as melatonin, were found. The apparent Michaelis constants for substrates of NAT and HIOMT in the planarian were similar to those reported for the mammalian pineal gland and retina. When the planarians were maintained under a 12 h light:12 h dark cycle, the activities of NAT and HIOMT and melatonin levels exhibited a significant diurnal variation, peaking at the mid-dark time. In constant darkness, NAT activity and melatonin levels fluctuated with a circadian (about 24 h) rhythm. These data demonstrate that the planarian synthesizes melatonin through the same pathways as those in most vertebrates and several insects, and that its melatonin synthesis fluctuates in a circadian manner. Thus, it is strongly suggested that the planarian contains a circadian clock controlling melatonin synthesis.