title: Structural basis of the subtype selectivity of muscarinic antagonists: a study with chimeric m2/m5 muscarinic receptors.
abstract: The five muscarinic receptors (m1-m5), although structurally closely related, can be distinguished pharmacologically by the use of subtype-selective ligands. Various tricyclic muscarinic antagonists, including the AF-DX derivative AQ-RA 741 and the alkaloid himbacine, for example, have been shown to display up to 200-fold higher affinities for m2 and m4 than for m5 receptors. On the other hand, antagonists such as sila-hexocyclium and the pirenzepine derivative UH-AH 37 exhibit lower affinities for m2 than for m5 and all other muscarinic receptors. To identify receptor epitopes that contribute to the subtype selectivities of these antagonists, we prepared a series of chimeric m2/m5 muscarinic receptors in which regions of the m5 receptor were systematically replaced with the homologous regions of the m2 receptor. AQ-RA 741, himbacine, and sila-hexocyclium bound to the various chimeric receptors, expressed in COS-7 cells, with affinity profiles indicative of multiple receptor domains contributing to the subtype selectivities of these antagonists. On the other hand, the higher affinity of UH-AH 37 for m5 than for m2 receptors appears to be largely dependent on a short stretch of 31 amino acids comprising most of transmembrane region VI and the third extracellular loop, a region that does not contribute to the subtype selectivity of AQ-RA 741 and himbacine. Our data indicate that different receptor epitopes are involved in conferring subtype selectivity on structurally different muscarinic antagonists.
authors.affiliation: National Institute of Neurological Disorders and Stroke, Laboratory of Molecular Biology, Bethesda, Maryland 20892.
authors.forename: J
authors.initials: J
authors.lastname: Wess
authors.forename: D
authors.initials: D
authors.lastname: Gdula
authors.forename: M R
authors.initials: MR
authors.lastname: Brann
mesh_terms: D000470:Alkaloids / Q000378:metabolism; D000595:Amino Acid Sequence; D001570:Benzodiazepinones / Q000378:metabolism; D002678:Chimera / Q000235:genetics*; D003984:Dibenzazepines; D005663:Furans; D006801:Humans; D008969:Molecular Sequence Data; D018727:Muscarinic Antagonists; D019832:N-Methylscopolamine; D009281:Naphthalenes; D010276:Parasympatholytics / Q000378:metabolism; D010879:Piperazines / Q000378:metabolism; D010880:Piperidines / Q000378:metabolism; D011976:Receptors, Muscarinic / Q000235:genetics / Q000378:metabolism; D012602:Scopolamine Derivatives / Q000378:metabolism; D012689:Sequence Homology, Nucleic Acid
chemical_list: D000470:Alkaloids; D001570:Benzodiazepinones; D003984:Dibenzazepines; D005663:Furans; D018727:Muscarinic Antagonists; D009281:Naphthalenes; D010276:Parasympatholytics; D010879:Piperazines; D010880:Piperidines; D011976:Receptors, Muscarinic; D012602:Scopolamine Derivatives; C059904:UH-AH 37; C069054:AQ-RA 741; C061673:silahexocyclium; C048172:himbacine; D019832:N-Methylscopolamine