[125I]-labelled iodomelatonin binding sites in the duck bursa of Fabricius: binding characteristics and diurnal variation.

Melatonin receptors in membrane preparations of the duck bursa of Fabricius were studied by using [125I]-labelled iodomelatonin as the radioligand. Specific binding of [125I]-labelled iodomelatonin in the membrane preparations of bursa was stable, saturable, reversible and of high affinity. Scatchard analysis of the specific binding revealed an equilibrium dissociation constant (Kd) of 48.5 +/- 7.4 pmol/l and a total number of binding sites (Bmax) of 1.38 +/- 0.12 fmol/mg protein at mid-light. A diurnal study showed that the Bmax of [125I]-labelled iodomelatonin binding sites at mid-light was 94.4% higher (P < 0.05) than that at mid-dark. There was no significant difference in the mid-light and mid-dark Kd values. The Kd value determined by kinetic analysis was 42.0 +/- 9.3 pmol/l at mid-light. The pharmacological characteristics indicated that [125I]-labelled iodomelatonin binding sites are highly specific for melatonin. Our results suggest that the bursa is a target organ of melatonin action.