This paper was published as Journal of Pharmacology and Experimental Therapeutics, 2008, 327 (2), pp. 365-374. It is available from http://jpet.aspetjournals.org/content/327/2/365.abstract. Doi: 10.1124/jpet.108.141788Metadata only entryThe M1 muscarinic acetylcholine (mACh) receptor is among a growing number of G protein-coupled receptors that are able to activate multiple signaling cascades. AC-42 (4-n-butyl-1-[4-(2-methylphenyl)-4-oxo-1-butyl] piperidine) is an allosteric agonist that can selectively activate the M1 mACh receptor in the absence of an orthosteric ligand. Allosteric agonists have the potential to stabilize unique receptor conformations, which may in turn cause differential activation of signal transduction pathways. In the present study, we have investigated the signaling pathways activated by AC-42, its analog 77-LH-28-1 (1-[3-(4-butyl-1-piperidinyl)propyl]-3,4-dihydro-2(1H)-quinolinone), and a range of orthosteric muscarinic agonists [oxotremorine-M (oxo-M), arecoline, and pilocarpine] in Chinese hamster ovary cells recombinantly expressing the human M1 mACh receptor. Each agonist was able to activate Gαq/11-dependent signaling, as demonstrated by an increase in guanosine 5′-O-(3-thiotriphosphate) ([35S]GTPγS) binding to Gαq/11 proteins and total [3H]inositol phosphate accumulation assays in intact cells. All three orthosteric agonists caused significant enhancements in [35S]GTPγS binding to Gαi1/2 subunits over basal; however, neither allosteric ligand produced a significant response. In contrast, both orthosteric and allosteric agonists are able to couple to the Gαs/cAMP pathway, enhancing forskolin-stimulated cAMP accumulation. These data provide support for the concept that allosteric and orthosteric mACh receptor agonists both stabilize receptor conformations associated with Gαq/11- and Gαs-dependent signaling; however, AC-42 and 77-LH-28-1, unlike oxo-M, arecoline, and pilocarpine, do not seem to promote M1 mACh receptor-Gαi1/2 coupling, suggesting that allosteric agonists have the potential to activate distinct subsets of downstream effectors
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