Melatonin effects on thymus steroid receptors in the course of primary antibody responses: significance of circulating glucocorticoid levels.

1. The effects of chronic melatonin treatment on antibody production and thymus glucocorticoid and progestin binding sites in the course of primary immune response were investigated in rats subjected to long-term manipulations of circulating glucocorticoid concentrations. 2. The chronic melatonin treatment enhanced the PFC-ability in control and ADX animals and decreased it in the presence of high circulating corticosterone levels. 3. Melatonin treatment increased the affinity and decreased the density of glucocorticoid and progestin receptors in non-immunized animals, while in SRBC-primed rats it was ineffective. 4. The influence of melatonin was apparent when steroid environment was subject of variations: it decreased the affinity of both types of receptors in ADX animals and increased it in immunized rats which have been subjected simultaneously to corticosterone overdose.