ABSTRACT
N-acetyltransferase (NAT) is the other melatonin synthesizing enzyme which converts serotonin to N-acetylserotonin. A diurnal rhythm with higher retinal melatonin levels at night was observed in chicks maintained under a lighting regimen of 12 h light and 12 h dark. The compensatory increase in retinal melatonin levels after Px raises a possibility that melatonin biosynthesis in the retina is modulated through a negative feedback mechanism. The diurnal rhythm of melatonin in the aqueous humor is remarkably similar to the rhythms of intraocular pressure, aqueous flow, and melatonin content in the ciliary body. Melatonin biosynthesis in the retina is affected most significantly and directly by light and darkness. The persistence of retinal melatonin and NAT rhythms in frogs, chicken, an quail adapted to constant darkness suggests that an endogenous circadian clock regulating retinal melatonin biosynthesis exists. The existence of an intraocular circadian pacemaker for melatonin biosynthesis is an attractive hypothesis.
