CORTISOL DECREASES 2[I-125]IODOMELATONIN BINDING-SITES IN THE DUCK THYMUS

The immunosuppressive effect of chronic glucocorticoid treatment on 2[ I-125]iodomelatonin binding in the duck thymus was studied. Two-week-o ld ducks were injected intraperitoneally with either 1 mg of cortisol per day (experimental group) or an equivalent volume of vehicle (contr ol group) in the middle of the light period for 7 days. 2[I-125]Iodome latonin binding assays were performed on thymic membranes. Cortisol in jection reduced the body weight gain, size of the bursa of Fabricius a nd absolute weights of the primary lymphoid organs but had no effect o n the spleen weights. The relative weights of the spleen were increase d while those of the primary lymphoid organs were unchanged. The densi ty of the thymus 2[I-125]iodomelatonin binding sites was decreased whi le the affinity was not affected. The modulation of the thymic 2[I-125 ]iodomelatonin binding sites by changes in the immune status of the du ck suggests that these binding sites represent physiologically relevan t melatonin receptors and that melatonin exerts its action on the lymp hoid tissues directly. Our findings support the hypothesis that the th ymus is the target site for the immunomodulatory interactions between the pineal melatonin and the adrenal steroids. A possible inhibitory i nfluence of adrenal steroids on the immune-enhancing effect of melaton in is also suggested.