Localization and quantification of high-affinity melatonin binding sites in Rana pipiens retina

Melatonin binding was localized to the inner plexiform layer (IPL) of the frog retina by in vitro autoradiography, using 2-125I-melatonin as the radioligand. Radioreceptor binding assays of frog retinal homogenate demonstrated saturable melatonin binding. Scatchard analysis revealed a single population of binding sites with an apparent dissociation constant (Kd) of 125 pM, with a Bmax of 0.138 fmoles/mg of protein. These results suggest that high-affinity melatonin binding sites are present in the IPL of the frog retina, which may reflect the presence of melatonin receptors in this synaptic layer.