Specific 2-[125I]iodomelatonin binding sites in the duck adrenal gland

Using 2-[125I]iodomelatonin as the radioligand, putative melatonin receptors in the duck adrenal gland were investigated. 2-[125I]Iodomelatonin binding to the membrane preparations of duck adrenals collected at mid-light was specific, rapid, saturable, stable, reversible and of high affinity. Scatchard analyses showed one class of binding sites with an equilibrium dissociation constant of 27.4 +/- 2.9 pmol/l and a maximum number of binding sites of 3.38 +/- 0.26 fmol/mg protein. Binding of 2-[125I]iodomelatonin in different subcellular fractions demonstrated the following descending order of density: mitochondrial > nuclear > microsomal >>> cytosol. Pharmacological studies indicated that these sites were highly specific to melatonin. As 2-[125I]iodomelatonin is a specific agonist of melatonin, it is proposed that the sites studied are adrenal melatonin receptors.