DISTRIBUTION OF MELATONIN IN MAMMALIAN-TISSUES - THE RELATIVE IMPORTANCE OF NUCLEAR VERSUS CYTOSOLIC LOCALIZATION

Besides its presence in the pineal gland, melatonin has been found in a variety of other tissues as well. The indoleamine also has been iden tified in invertebrates including an unicellular organism where it exh ibits a diurnal rhythm. Although melatonin is mainly known for its eff ects on seasonal reproduction and endocrine physiology, there is evide nce showing that this ubiquitously acting hormone is also a potent fre e radical scavenger, thereby providing protection from oxidative attac k to DNA and other biomolecules. Through the years, melatonin was thou ght to be exclusively cytosolic. However, careful examination of some of these pioneering reports revealed a nuclear localization of melaton in in different tissues including the retina and Harderian glands. Usi ng a very sensitive immunocytochemical method, we have also found that melatonin is located in the nucleus of many cells where it may bind t o nuclear components. The use of cell fractionation studies followed b y radioimmunoassay confirmed these results. The administration of exog enous melatonin resulted in a marked increase in the nuclear melatonin content without a concomitant change in the cytosolic fraction. In ad dition to its ability to scavenge free radicals, its location in the n ucleus suggests possible genomic actions.