Guanine Nucleotides Regulate 2-[125I]Iodomelatonin Binding Sites in Chick Retinal Pigment Epithelium but Not in Neuronal Retina

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Abstract
The characteristics of the binding sites labeled by the radioligand 2-[125I]iodomelatonin were compared in chicken neuronal retina and retinal pigment epithelium (RPE). Specific binding of 2-[125I]iodomelatonin in both sites was stable, saturable, reversible, and of high affinity. Scatchard analysis revealed an affinity constant (KD) of 446 +/- 55 pM and a total number of binding sites (Bmax) of 25.4 +/- 2.2 fmol/mg of protein for neuronal retina. For RPE the KD was 34.1 +/- 2.2 pM and the Bmax 59.5 +/- 5.2 fmol/mg of protein. Competition experiments with various melatonin analogues gave the following order of affinities: 2-iodomelatonin greater than 2-chloromelatonin greater than melatonin greater than 6-chloromelatonin greater than 6-hydroxymelatonin greater than N-acetylserotonin greater than 6-methoxyharmalan greater than 5-hydroxytryptamine. Linear regression of log Ki values from neuronal retina and RPE gave a highly significant correlation (r = 0.994, n = 8; p less than 0.001). GTP inhibited specific binding to RPE membranes in a concentration-dependent manner, but not in neuronal retinal membranes. The present results strongly suggest that a single type of melatonin receptor is found in neuronal retina and RPE, and that the site in RPE is coupled to a guanine nucleotide-binding regulatory protein (G protein), but that in neuronal retina is not.