Melatonin synthesis in the greenfrog retina in culture:: I.: Modulation by the light/dark cycle, forskolin and inhibitors of protein synthesis

Melatonin is synthesized in the pineal gland and the retina of vertebrates. Retinal serotonin N-acetyltransferase (NAT) activity and melatonin show a daily rhythm with high levels during the dark phase of the photocycle. In some vertebrates, these retinal NAT and melatonin rhythms are maintained in vitro. The aim of present work is to develop an eyecup culture system for the greenfrog (Rana perezi), suitable to analyze the mechanisms of regulation of melatonin synthesis by simultaneous determination of NAT activity and melatonin release. The R. perezi eyecups released melatonin to the culture medium in a rhythmic manner at least over a 27-h period under photocycle conditions. NAT activity and melatonin rhythms were similar to that observed in vivo under natural environmental conditions. Rana perezi retina exhibits a pronounced photosensitivity in vitro. Forskolin increased up to 2-fold the NAT activity and 4-fold the melatonin production at any lighting conditions. The addition of the translation inhibitor, cycloheximide, to the medium reduced significantly both nocturnal NAT activity and melatonin release, suggesting that de novo protein synthesis is produced daily during darkness. Actinomycin D, a transcription inhibitor, needs a longer time of action, because pre-existing mRNA must be depleted before the inhibition of melatonin release can be observed. The eyecup culture system is highly sensitive to light and chemical factors, which makes it particularly suitable as a model for the neurochemical analysis of melatonin biosynthesis in the retina of Rana perezi.