Melatonin has long been known to be produced by the pineal gland and to influence daily cycles in vertebrates. Melatonin is also produced in the vertebrate retina where it controls functions such as allowing old fragments of light sensitive membrane to be discarded. Melatonin is synthesized from serotonin by rods and cones during the dark, and because it is lipid soluble, it quickly diffuses through the retina. In light, the enzyme converting serotonin to melatonin disappears and serotonin accumulates. It is unknown if all photoreceptors can actively accumulate serotonin and manufacture melatonin or if only green sensitive rods perform these functions. Also unknown is the path information must take from the reception of light by the photoreceptor to melatonin synthesis and from there to photomembrane disposal. Possible paths are: (i) a single transducer such as the green sensitive rods, suppressing all photoreceptors; (ii) each photoreceptor receiving the signal for itself; and (iii) every photoreceptor broadcasting an inhibitory message to all other rods and cones. Radioactive serotonin and electron microscopic autoradiography will be used to determine if all classes of photoreceptors in the frog retina can accumulate serotonin and which colors of light are capable of inhibiting melatonin syntheses. The relation of melatonin synthesis to the periodic disposal of photoreceptor membrane will be investigated.
