title: Melatonin MT₁ and MT₂ receptors display different molecular pharmacologies only in the G-protein coupled state.
abstract: OBJECTIVE
  Melatonin receptors have been extensively characterized regarding their affinity and pharmacology, mostly using 2-[(125)I]-melatonin as a radioligand. Although [(3)H]-melatonin has the advantage of corresponding to the endogenous ligand of the receptor, its binding has not been well described.
  
  
  METHODS
  We characterized [(3)H]-melatonin binding to the hMT₁ and hMT₂ receptors expressed in a range of cell lines and obtained new insights into the molecular pharmacology of melatonin receptors.
  
  
  RESULTS
  The binding of [(3)H]-melatonin to the hMT₁ and hMT₂ receptors displayed two sites on the saturation curves. These two binding sites were observed on cell membranes expressing recombinant receptors from various species as well as on whole cells. Furthermore, our GTPγS/NaCl results suggest that these sites on the saturation curves correspond to the G-protein coupled and uncoupled states of the receptors, whose pharmacology was extensively characterized.
  
  
  CONCLUSIONS
  hMT₁ and hMT₂ receptors spontaneously exist in two states when expressed in cell lines; these states can be probed by [(3)H]-melatonin binding. Overall, our results suggest that physiological regulation of the melatonin receptors may result from complex and subtle mechanisms, a small difference in affinity between the active and inactive states of the receptor, and spontaneous coupling to G-proteins.
authors.affiliation: Biotechnologies, Pharmacologie Moléculaire et Cellulaire, Institut de Recherches Servier, Croissy-sur-Seine, France.
authors.forename: Céline
authors.initials: C
authors.lastname: Legros
authors.forename: Séverine
authors.initials: S
authors.lastname: Devavry
authors.forename: Sarah
authors.initials: S
authors.lastname: Caignard
authors.forename: Clémence
authors.initials: C
authors.lastname: Tessier
authors.forename: Philippe
authors.initials: P
authors.lastname: Delagrange
authors.forename: Christine
authors.initials: C
authors.lastname: Ouvry
authors.forename: Jean A
authors.initials: JA
authors.lastname: Boutin
authors.forename: Olivier
authors.initials: O
authors.lastname: Nosjean
mesh_terms: D000818:Animals; D001665:Binding Sites; D016466:CHO Cells; D045744:Cell Line, Tumor; D006224:Cricetinae; D003412:Cricetulus; D004305:Dose-Response Relationship, Drug; D019204:GTP-Binding Proteins / Q000378:metabolism*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D057809:HEK293 Cells; D006801:Humans; D007700:Kinetics; D008024:Ligands; D008550:Melatonin / Q000378:metabolism*; D051379:Mice; D008969:Molecular Sequence Data; D011869:Radioligand Assay; D051381:Rats; D044123:Receptor, Melatonin, MT2 / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D012756:Sheep; D012965:Sodium Chloride / Q000494:pharmacology; D013045:Species Specificity; D014162:Transfection
chemical_list: D008024:Ligands; C573871:MTNR1B protein, human; D044123:Receptor, Melatonin, MT2; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D012965:Sodium Chloride; D019204:GTP-Binding Proteins; D008550:Melatonin
keywords: GTPγS; [3H]-melatonin; coupling state; melatonin; melatonin receptors