Guanine nucleotide-binding protein regulation of melatonin receptors in lizard brain

Melatonin receptors were identified and characterized in crude membrane preparations from lizard brain by using 125I-labeled melatonin (125I-Mel), a potent melatonin agonist. 125I-Mel binding sites were saturable; Scatchard analysis revealed high-affinity and lower affinity binding sites, with apparent Kd of 2.3 +/- 1.0 x 10(-11) M and 2.06 +/- 0.43 x 10(-10) M, respectively. Binding was reversible and inhibited by melatonin and closely related analogs but not by serotonin or norepinephrine. Treatment of crude membranes with the nonhydrolyzable GTP analog guanosine 5'-[gamma-thio]triphosphate (GTP[gamma S]), significantly reduced the number of high-affinity receptors and increased the dissociation rate of 125I-Mel from its receptor. Furthermore, GTP[gamma S] treatment of ligand-receptor complexes solubilized by Triton X-100 also led to a rapid dissociation of 125I-Mel from solubilized ligand-receptor complexes. Gel filtration chromatography of solubilized ligand-receptor complexes revealed two major peaks of radio-activity corresponding to Mr greater than 400,000 and Mr ca. 110,000. This elution profile was markedly altered by pretreatment with GTP[gamma S] before solubilization; only the Mr 110,000 peak was present in GTP[gamma S]-pretreated membranes. The results strongly suggest that 125I-Mel binding sites in lizard brain are melatonin receptors, with agonist-promoted guanine nucleotide-binding protein (G protein) coupling and that the apparent molecular size of receptors uncoupled from G proteins is about 110,000.