NUCLEAR-LOCALIZATION OF MELATONIN IN DIFFERENT MAMMALIAN-TISSUES - IMMUNOCYTOCHEMICAL AND RADIOIMMUNOSSAY EVIDENCE

Melatonin was detected by an improved immunocytochemical technique in the cell nuclei of most tissues studied including several brain areas, pineal gland, Harderian gland, gut, liver, kidney, and spleen from ro dents and primates. Cryostat sections from tissues fixed in Bouin's fl uid, formalin, or acetone/ethanol were used. The nuclear staining appe ared primarily associated with the chromatin. The nucleoli did not exh ibit a positive reaction. The melatonin antiserum was used in the rang e of 1:500 to 1:5,000. Incubation of the antibody with an excess of me latonin resulted in the complete blockade of nuclear staining. Pretrea tment of the sections with proteinase K (200-1,000 ng/ml) prevented th e positive immunoreaction. In a second aspect of the study, we estimat ed the concentration of melatonin by means of radioimmunoassay in the nuclear fraction of several tissues including cerebral cortex, liver, and gut. The subcutaneous injection of melatonin (500 mug/kg) to rats resulted, after 30 min, in a rapid increase in the nuclear concentrati on of immunoreactive melatonin which varied in a tissue-dependent mann er. However, samples collected 3 h after the injection showed that mel atonin levels had decreased to control values. Pinealectomy in rats re sulted in a clear reduction in the nuclear content of melatonin in the cerebral cortex and liver but not in the gut. The results of these st udies suggest that melatonin may interact with nuclear proteins and th at the indole may have an important function at the nuclear level in a variety of mammalian tissues. (C) 1993 Wiley-Liss, Inc.