Identification and characterization of guanine nucleotide-sensitive melatonin receptors in chicken brain

Specific melatonin receptors present in crude membrane preparations of chicken forebrain were identified by specific binding activity of 2-[125I]iodomelatonin and melatonin-dependent inhibition of forskolin-stimulated adenylate cyclase activity by a pertussis toxin-sensitive mechanism. The membrane-bound melatonin receptors were successfully solubilized with digitonin, and the ligand binding activity was shown to be preserved in solubilized membranes when compared with that of crude membranes. In both fractions, melatonin receptors consisted of two binding components with picomolar and nanomolar affinity. The ligand binding activity of solubilized receptors was decreased after treatment with guanine nucleotides, and guanosine 5'-(gamma-thio)triphosphate (GTP gamma S) induced a transformation of the high affinity receptors to low affinity state. Moreover, [35S]GTP gamma S was indicated to bind to solubilized membranes in a melatonin-dependent fashion. These results demonstrate the existence of functional melatonin receptors--G proteins coupling in digitonin-solubilized membranes of chicken forebrain membranes.