title: Guanine nucleotide-binding protein regulation of melatonin receptors in lizard brain.
abstract: 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.
authors.affiliation: Laboratory of Developmental Chronobiology, Massachusetts General Hospital, Harvard Medical School, Boston 02114.
authors.forename: S A
authors.initials: SA
authors.lastname: Rivkees
authors.forename: L L
authors.initials: LL
authors.lastname: Carlson
authors.forename: S M
authors.initials: SM
authors.lastname: Reppert
mesh_terms: D000663:Amphibians / Q000502:physiology; D000818:Animals; D001921:Brain / Q000502:physiology*; D002462:Cell Membrane / Q000502:physiology; D002850:Chromatography, Gel; D019204:GTP-Binding Proteins / Q000502:physiology*; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate / Q000031:analogs & derivatives / Q000494:pharmacology; D008116:Lizards; D008565:Membrane Proteins / Q000502:physiology; D008970:Molecular Weight; D044094:Receptors, Melatonin; D017981:Receptors, Neurotransmitter / Q000502:physiology*; D012995:Solubility; D013873:Thionucleotides / Q000494:pharmacology
chemical_list: D008565:Membrane Proteins; D044094:Receptors, Melatonin; D017981:Receptors, Neurotransmitter; D013873:Thionucleotides; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate; D019204:GTP-Binding Proteins