Melatonin Receptors in Peripheral Tissues: A New Area of Melatonin Research

Melatonin is synthesized and secreted by pineal gland, retina, harderian gland, lacrimal gland, and gut. It is a lipid-soluble molecule with ubiquitous distribution and multifarious functions. Thus, along with the better established sites of melatonin action, such as brain, retina, and pituitary, direct melatonin actions on other peripheral tissues should also be considered. The presence of melatonin receptors in peripheral tissues was rightfully hypothesized. Earlier studies on melatonin receptors had limited success. The advent of 2-[125I]iodomelatonin, a labelled melatonin agonist with a specific activity as high as 2,200 Ci/mmol, has allowed the studies of melatonin receptors with picomolar affinity and femtomolar density. Putative melatonin receptors demonstrated by autoradiography and/or radioreceptor assay in gut, kidney, lung, heart, vas deferens, and blood vessels are discussed.