CHARACTERIZATION OF MELATONIN BINDING-SITES IN THE EYE OF THE JAPANESE-QUAIL (COTURNIX JAPONICA)

Melatonin binding sites were examined in the quail eye using 2-[I-125] iodomelatonin. Radioreceptor assays indicated similar binding sites in membrane preparations of neural retina (NR) and choroid-retinal pigme nt epithelium (C-RPE) eye components. In both tissues binding of the r adioligand was specific, saturable, and of high affinity [K-d values N R 50.8 +/- 19.5 pM, C-RPE 98.2 +/- 35.4 pM, mean +/- SEM (n = 4)] and low capacity (B-max values NR 12.4 +/- 2.7 fmol/mg protein. C-RPE 21.5 +/- 3.2 fmol/mg protein). Kinetic studies demonstrated that associati on of 2-[I-125]iodomelatonin was rapid and further that this binding w as reversible upon the addition of 1 mu M melatonin. The order of phar macological potencies of various indoles tested in 2-[I-125]iodomelato nin displacement studies was melatonin > 6-chloromelatonin > 6-hydroxy melatonin > N-acetylserotonin much greater than 5-methoxytryptophol > 5-hydroxptryptamine > 5-methoxytryptamine (5-hydroxytryptamine > 5-met hoxytryptophol 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 pha rmacological similarities with those investigated in the retina of oth er species and with those previously characterised in the quail brain. (C) 1995 Academic Press, Inc.