Identification of 2-[125I] iodomelatonin binding sites in the thymus of mice and its significance.

The melatonin binding sites in membrane preparations of the mouse thymus were demonstrated using 2-[125I] iodomelatonin as a radioligand. The binding sites were stable, saturable, reversible and of high affinity. Studies on specificity of 2-[125I] iodomelatonin binding suggested that the 2-[125I] iodomelatonin binding sites are highly specific for melatonin. These binding sites fulfilled the standard criteria for receptors. Our work suggested that melatonin should have direct regulatory action on immune system mediated through the melatonin binding sites. Studies on the circadian rhythm showed that there existed the circadian rhythm in the binding capacity for 2-[125I] iodomelatonin in the mouse thymus with the peak values at 12:00-16:00 and the trough values between 00:00 and 4:00. The subcellular distribution of 2-[125I] iodomelatonin binding sites in the mouse thymus was in the following descending order: nuclear > mitochondrial > microsomal > cytosolic fraction. There was also an age-related decrease in 2-[125I] iodomelatonin binding in the mouse thymus. This is correlated with the involution of the thymus.