Multiple Pharmacophores for the Selective Activation of Nicotinic 7-Type Acetylcholine Receptors

The activation of heteromeric and homomeric nicotinic acetylcholine receptors was studied in Xenopus laevis oocytes to identify key structures of putative agonist molecules associated with the selective activation of homomeric 7 receptors. We observed that selectivity between 7 and 4 2 was more readily obtained than selectivity between 7 and 3 4. Based on structural comparisons of previously characterized selective and nonselective agonists, we hypothesize at least three chemical motifs exist that, when present in molecules containing an appropriate cationic center, could be associated with the selective activation of 7 receptors. We identify the three distinct structural motifs based on prototypical drugs as the choline motif, the tropane motif, and the benzylidene motif. The choline motif involves the location of an oxygen-containing polar group such as a hydroxyl or carbonyl separated by two carbons from the charged nitrogen. The tropane motif provides 7-selectivity based on the addition of multiple small hydrophobic groups positioned away from the cationic center in specific orientations. We show that this motif can convert the nonselective agonists quinuclidine and ethyltrimethyl-ammonium to the 7-selective analogs methyl-quinuclidine and diethyldimethyl-ammonium, respectively. We have shown previously that the benzylidene group of 3–2,4, dimethoxy-benzylidene anabaseine (GTS-21) converts anabaseine into an 7-selective agonist. The benzylidene motif was also applied to quinuclidine to generate another distinct family of 7-selective agonists. Our results provide insight for the further development of nicotinic therapeutics and will be useful to direct future experiments with protein structure-based modeling and site-directed mutagenesis. The nicotinic acetylcholine receptors of the brain can be broadly divided into two classes: heteromeric -subunit-containing receptors, and homomeric 7-type receptors. Homomeric 7-type receptors have emerged as an exciting potential therapeutic target for several indications, and this has encouraged the development of 7-selective agonists. This path of drug development relies on the consideration of both features that distinguish the heteromeric receptors from the homomeric receptors and features that distinguish selective agonists from nonselective agonists. Both heteromeric, -subunit-containing receptors and homomeric 7-type receptors are pentameric. The heteromeric neuronal nAChR contain at least one or more subunits ( 2– 6) and additional subunits ( 2– 4), with two agonist binding sites located at the interface between and subunits (Dani, 2001). Neuronal nicotinic receptor subunits are classified as such based on sequence homology to the subunits of muscle-type receptors (Heinemann et al., 1990), and essential conserved aspects of muscle and neuronal subunits provide specialized subdomains that contribute to the primary face of an asymmetrical binding site for acetylcholine and other agonists. In contrast, there is structural homology between muscle-type , , and subunits and neuronal 2 and 4 subunits that provide in these subunits the specialized subdomains for the complimentary face of the agonist binding site (Le Novere et al., 2002b). Although the majority of heteromeric receptors in the mammalian brain are believed to contain just 4 and 2 subunits (Flores et al., 1992), minor populations may contain additional subunits in various configurations (Turner and Kellar, 2005). The specializations for forming an agonist binding site seem to be These studies were supported by National Institutes of Health grant GM57481. Article, publication date, and citation information can be found at http://molpharm.aspetjournals.org. doi:10.1124/mol.108.048892. ABBREVIATIONS: nAChR, nicotinic acetylcholine receptor; GTS-21, 3–2,4,dimethoxy-benzylidene anabaseine; TMA, tetramethyl-ammonium; ETMA, ethyltetramethyl-ammonium; dEdMA, diethyldimethylammonium; QN, quinuclidine; MQN, 1-methyl-1-azoniabicyclo[2.2.2]octane iodide; QN-O, quinuclidinone; QN-OH quinuclidinol; EQN, 1-ethyl-1-azoniabicyclo[2.2.2]octane iodide; BQNE, E-3-benzylidene-1-azoniabicyclo[2.2.2]octane chloride; BQNZ, (Z)-3-benzylidene-1-azoniabicyclo[2.2.2]octane chloride; MBQNE, (E)-3-(4-methoxybenzylidene)-1-azoniabicyclo[2.2.2]octane chloride; MBQNZ, (Z)-3-(4-methoxybenzylidene)-1-azoniabicyclo[2.2.2]octane chloride; DME, 1,2-dimethoxy ethane; ACME, cis-1-methyl2,3,3a,4,5,9b,-hexahydro-1H-pyrrolo[2,3-f]quinoline; 5HT3, 5-hydroxytryptamine-3; ACh, acetylcholine; MS222, tricaine methanesulfonate. 0026-895X/08/7406-1496–1511$20.00 MOLECULAR PHARMACOLOGY Vol. 74, No. 6 Copyright © 2008 The American Society for Pharmacology and Experimental Therapeutics 48892/3404497 Mol Pharmacol 74:1496–1511, 2008 Printed in U.S.A. 1496 at U iv of F loida-G ainsville H lth S ci C tr ib on N ovem er 0, 2008 m oharm .aspeurnals.org D ow nladed fom lacking in the muscle 1 and the neuronal 5 and 3 subunits, so these have been identified as “structural subunits” (Gotti et al., 2006). At least two emergent properties are likely to have come from the specialization of the nonsubunits in the agonist binding sites. These two properties are the failure of heteromeric receptors to be activated efficiently by the ACh precursor choline (Papke et al., 1996), and the conversion of the Fig. 1. Structural diversity of selective activators of 7 nAChR. Multiple structural classes of nicotinic agonists are represented. Circled compounds are 7-selective. Compounds in filled gray circles had no significant agonist activity when tested at 300 M on 7, 4 2, or 3 4 receptors (data not shown). 7 nAChR Pharmacophore 1497 at U iv of F loida-G ainsville H lth S ci C tr ib on N ovem er 0, 2008 m oharm .aspeurnals.org D ow nladed fom