Characterization of 2-[¹²⁵I]-iodomelatonin binding sites in the golden hamster retina by autoradiography

The characterization of 2-[¹²⁵I]-iodomelatonin binding sites was performed in the golden hamster retina using in vitro quantitative autoradiography. The specific binding of the radioligand fulfills all the criteria for binding to a receptor site, being stable, reversible, saturable and of high affinity. 2-[¹²⁵I]-iodomelatonin labels a single class of sites in the sections of whole eyes as well as in isolated retinas with a similar affinity, whereas the total number of receptors was higher in sections of whole eyes than in isolated retinas. Melatonin and related analogues competed for 2-[¹²⁵I]-iodomelatonin binding with the following order of affinities: 2-iodomelatonin > 6-hydroxymelatonin > melatonin > 6-chloromelatonin >>> N-acetyl-5-hydroxytryptamine (NAS) > 5-methoxytryptamine > 5-hydroxytryptamine (serotonin). Micromolar concentrations of GTP and GDP dose-dependently and specifically inhibited agonist binding, suggesting coupling of the binding sites to a Gi protein. These results suggest the participation of melatonin in the regulation of retinal physiology trough activation of melatonin receptor subtypes.