Characterisation of Melatonin Binding Sites in the Eye of the Japanese Quail (Coturnix japonica)

Abstract

Melatonin binding sites were examined in the quail eye using 2-[125I]iodomelatonin. Radioreceptor assays indicated similar binding sites in membrane preparations of neural retina (NR) and choroidretinal pigment epithelium (C-RPE) eye components. In both tissues binding of the radioligand was specific, saturale and of high affinity [Kd values NR 50.8 ± 19.5 pM, C-RPE 98.2 ± 35.4 pM, mean ± SEM (n = 4)] and low capacity (Bmax values NR 12.4 ± 2.7 fmol/mg protein. C-RPE 21.5 ± 3.2 fmol/mg protein). Kinetic studies demonstrated that association of 2-[125I]iodomelatonin was rapid and further that this binding was reversible upon the addition of 1 μM melatonin. The order of pharmacological potencies of various indoles tested in 2-[125I]iodomelatonin displacement studies was melatonin > 6-chloromelatonin > 64-hydroxymelatonin >N-acetylscrotonin ⪢ 5-methoxytryptophol > 5-hydroxytryptamine > 5-methoxytryptamine (5-hydroxytryptamine > 5-methoxytryptophol for C-RPE). Studies with guanine nucleotides indicated that the signal transduction mechanism of the binding site may involve a G-protein linkage. This melatonin binding site displays several pharmacological similarities with those investigated in the retina of other species and with those previously characterised in the quail brain.