Melatonin in the retina and the harderian gland. Ontogeny, diurnal variations and melatonin treatment

Using highly specific antibodies, melatonin has been identified immunohistologically in the rat retina, and the Harderian gland. The first truly significant amount of retinal melatonin was already detected in the 2 day old pups. The amount of melatonin progressively increased with age reaching adult levels around the 20th day. Diurnal variations with higher night levels of melatonin have been found in the adult in both retina and Harderian gland. Intraperitoneal injection or subcutaneous implantation of melatonin in beeswax (150 μg/rat) resulted in a vast increase in melatonin content in the retina and the Harderian gland of the juvenile and adult rats. No sexual differences have been registered in any experimental group. The concept of melatonin synthesis at peripheral sites independent of pineal production, the involvement of light-dark rhythm in the regulation of pineal and extrapineal melatonin content and the possibility of uptake mechanisms or receptors for melatonin in the retina and Harderian gland are discussed.