DIURNAL-VARIATIONS IN [I-125] MELATONIN BINDING BY RAT THYMUS MEMBRANES - EFFECTS OF CONTINUOUS LIGHT EXPOSURE AND PINEALECTOMY

Binding of melatonin by rat thymus membranes exhibited diurnal changes . Binding increased during the daytime and reached maximal values befo re entering the dark period. Then, binding decreased rapidly during th e dark phase. In rats kept in light at night, binding of [I-125]melato nin by membranes was significantly higher than in animals that entered the normal dark period. Neonatal pinealectomy, which suppresses the c ircadian rhythm of melatonin, led to an increase in melatonin binding of 106%. Moreover, in animals maintained under continuous light exposu re, which corresponds to functional pinealectomy, binding of melatonin by thymus membranes also increased in a time-dependent manner. The re sults support the hypothesis of a regulatory role of melatonin in the thymus in which melatonin downregulates its own binding sites.