Characterization of the day-night variation of retinal melatonin content in the chick.

By monitoring two time points (one at mid-light and the other at mid-dark), the day-night variation of melatonin content in the retina of 19-day-old chicks was characterized. Melatonin was detected in the retina plus attached pigment epithelium by a specific radioimmunoassay, and its identity was verified by high performance liquid chromatography with electrochemical detection. Melatonin content in the posterior pole of the eye showed a fivefold day-night variation, with high levels during the dark period of diurnal lighting. Light exposure during the dark period lowered the normally high nighttime value; maintenance of darkness during the normal light period did not alter the low melatonin values typical of mid-light. Pineal melatonin content responded similarly to the above lighting manipulations. Neither pinealectomy nor optic nerve transection had an effect on retina-pigment epithelium melatonin or its light-dark rhythm. We next examined the relative contributions of retinal and pineal melatonin to blood levels. Pinealectomy reduced the normally high mid-dark plasma melatonin value by 80%. The addition of bilateral enucleation reduced the mid-dark value by another 9% of control values. The day-night variation of retina-pigment epithelium melatonin was first evident in the embryo 2 days prior to hatching and persisted through adulthood. It was concluded that the chick retina from the latter stages of embryonic development is capable of rhythmically synthesizing melatonin; that retinal melatonin content displays a photically controlled circadian rhythm in phase with, but independent of, the pineal gland; and that the retinal rhythm is not regulated by afferent optic nerve fibers. The pineal gland is the major source of plasma melatonin in the intact chick, with at most a small contribution from the retina.