Cortisol decreases 2[125I]iodomelatonin binding sites in the duck thymus

The immunosuppressive effect of chronic glucocorticoid treatment on 2[125I]iodomelatonin binding in the duck thymus was studied. Two-week-old ducks were injected intraperitoneally with either 1 mg of cortisol per day (experimental group) or an equivalent volume of vehicle (control group) in the middle of the light period for 7 days. 2[125I]Iodomelatonin binding assays were performed on thymic membranes. Cortisol injection reduced the body weight gain, size of the bursa of Fabricius and absolute weights of the primary lymphoid organs but had no effect on the spleen weights. The relative weights of the spleen were increased while those of the primary lymphoid organs were unchanged. The density of the thymus 2[125I]iodomelatonin binding sites was decreased while the affinity was not affected. The modulation of the thymic 2[125I]iodomelatonin binding sites by changes in the immune status of the duck suggests that these binding sites represent physiologically relevant melatonin receptors and that melatonin exerts its action on the lymphoid tissues directly. Our findings support the hypothesis that the thymus is the target site for the immunomodulatory interactions between the pineal melatonin and the adrenal steroids. A possible inhibitory influence of adrenal steroids on the immuno-enhancing effect of melatonin is also suggested.