Steroid hormone receptors in the thymus: a site of immunomodulatory action of melatonin.

1. Melatonin and glucocorticoids are known to affect the immune response in an opposite mode. The probability for an interaction between these hormones in the thymus gland has been investigated in rats following chronic administration of exogenous melatonin and long-term exposure to variable levels of circulating glucocorticoids. 2. Daily melatonin administration was shown to affect the properties of corticosterone and progestin receptors in the thymus in the presence of normal and increased systemic corticosterone concentrations, but not in adrenalectomized animals. 3. In intact rats melatonin caused a marked increase in the affinity and a decrease in the density of thymic receptors for adrenal steroids. Following corticosterone overdosage, simultaneously with melatonin treatment, a decrease in receptor affinity and a relative increase in the number of binding sites was observed. 4. The results suggest that steroid hormone receptors in the thymus might be considered as a target site for the interaction between melatonin and adrenal steroids in the modulation of the immune response.