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

The characterization of 2-[I-125]-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 bindi ng to a receptor site, being stable, reversible, saturable and of high affi nity. 2-[I-125]-iodomelatonin labels a single class of sites in the section s of whole eyes as well as in isolated retinas with a similar affinity, whe reas the total number of receptors was higher in sections of whole eyes tha n in isolated retinas. Melatonin and related analogues competed for 2-[I-12 5]-iodomelatonin binding with the following order of affinities: 2-iodomela tonin > 6-hydroxymelatonin > melatonin > 6-chloromelatonin > > > N-acetyl-5 -hydroxytryptamine (NAS) > 5-methoxytryptamine > 5-hydroxytryptamine (serot onin). Micromolar concentrations of GTP and GDP dose-dependently and specif ically inhibited agonist binding, suggesting coupling of the binding sites to a Gi protein. These results suggest the participation of melatonin in th e regulation of retinal physiology trough activation of melatonin receptor subtypes.