Binding of [125I]-Labelled lodomelatonin in the Duck Thymus

[125I]-labelled iodomelatonin binding sites in membrane preparations of duck thymus were studied. The specific binding of [125I]-labelled iodomelatonin in the duck thymus was stable, saturable, reversible and of high affinity. Scatchard analysis of the binding of [125I]-labelled iodomelatonin in the duck thymus collected at midlight had an equilibrium dissociation constant (Kd) of 34.8 ± 9.4 pmol/l and a maximum number of binding sites (Bmax) of 0.98 ± 0.07 fmol/mg protein. Two-point diurnal study demonstrated that the Bmax in samples collected at midlight was 42.0% higher (p 0.05) between the midlight and middark Kd values. Competition inhibition studies showed that only melatonin, 2-iodomela-tonin, 6-chloromelatonin, 6-hydroxymelatonin, N-acetylserotonin, 5-methoxytryptophol, and 5-hydroxytryptamine showed significant inhibition of the [125I]iodomelatonin binding in duck thymus membrane preparations, while the other compounds had no significant inhibition. Our results suggested a direct action of melatonin on the thymus and, thus, the cellular immune system.