Muscarinic acetylcholine receptors (mAChRs) (nomenclature as agreed by the NC-IUPHAR Subcommittee on Muscarinic Acetylcholine Receptors [13]) are activated by the endogenous agonist acetylcholine. All five (M1-M5) mAChRs are ubiquitously expressed in the human body and are therefore attractive targets for many disorders. Functionally, M₁, M₃, and M₅ mAChRs preferentially couple to G_q/11 proteins, whilst M₂ and M₄ mAChRs predominantly couple to G_i/o proteins. Both agonists and antagonists of mAChRs are clinically approved drugs, including pilocarpine for the treatment of elevated intra-ocular pressure and glaucoma, and atropine for the treatment of bradycardia and poisoning by muscarinic agents such as organophosphates. Of note, it has been observed that mAChRs dimerise reversibly [38] and that dimerisation/oligomerisation can be affected by ligands [61,65].

Atomic structures for all five mAChRs bound to antagonists have been determined [36,100,105,110,118]. Structures of agonist-bound M₁, M₂, M₃, and M₄ mAChRs [51,111,117,119] and β₂-arrestin-bound M₂ mAChR have been reported [98]. These structures show that the orthosteric binding site of this family of receptor is absolutely conserved and, as a consequence, explain why highly selective orthosteric ligand binding to any specific mAChR has been notoriously difficult to achieve. As such, it is common to assess the rank order of affinity for a range of antagonists with limited selectivity (e.g., 4-DAMP, darifenacin, pirenzepine, AFDX384) to identify the involvement of particular subtypes- although caution should be used in the design and interpretation of such experiments due to the lack of absolute ligand subtype selectivity [72]. Some ligands may display selectivity at the level of function (e.g., xanomeline) or binding kinetics (e.g., tiotropium) [84,91,104]. In addition, structures of the M₃ and M₄ mAChR DREADDS (designer receptors exclusively activated by designer drugs) have been reported providing insights into orthosteric ligand selectivity for these chemogenetic tools [119].

Structures of the M₁ and M₂ and M₄ mAChRs in complex with allosteric modulators [51,63,111] have validated numerous pharmacological studies that indicated the presence of a common mAChR allosteric site located at the extracellular entrance to these receptors. In addition, a structure of the M₁ mAChR with muscarinic toxin 7 (MT7) bound to the common allosteric site has provided insight into the extreme subtype selectivity of MT7 [63]. Allosteric ligands proposed to bind to this common allosteric site include gallamine, strychnine, C₇/3-phth, brucine and LY2033298. Additionally, a second allosteric site has been proposed on the mAChRs based on pharmacological analyses of the actions of compounds such as KT 5720, WIN 62,577, WIN 51,708, staurosporine and amiodarone [10,57-58,97]. In the presence of the orthosteric ligand, allosteric modulators can exert positive, negative, or neutral cooperativity with that ligand. Direct receptor activation via an allosteric site has been reported for a number of allosteric ligands of the mAChRs [24,59-60,62,73-74]. ‘Atypical agonists’ are ligands that have been suggested to have bitopic binding modes for at least one subtype whereby the agonist occupies both the orthosteric and allosteric sites [4,47,108]. Several mAChR PET radioligands have been reported, but their utility for subtype selectivity measurements in the human brain has yet to be confirmed [79].

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Subcommittee members: Arthur Christopoulos (Chairperson) NHMRC Senior Principal Research Fellow Drug Discovery Biology Monash Institute of Pharmaceutical Sciences & Department of Pharmacology Monash University 399 Royal Parade Parkville, VIC, 3052 Australia Nigel J. M. Birdsall (Past chairperson) The Francis Crick Institute 1 Midland Road London NW1 1AT UK Sophie Bradley MRC Toxicology Unit Hodgkin Building PO Box 138 University of Leicester Lancaster Road, Leicester LE1 9HN, UK David A. Brown Department of Pharmacology University College London Gower Street London WC1E 6BT UK Frederick Ehlert Department of Pharmacology School of Medicine University of California Irvine CA 92697-4625 USA Christian C. Felder Lilly Research Laboratories Lilly Corporate Center Drop 0510 Indianapolis IN 46285 USA Chris Langmead Drug Discovery Biology Monash Institute of Pharmaceutical Sciences Monash University Parkville VIC 3052 Australia Fred Mitchelson Department of Pharmacology The University of Melbourne Victoria Australia Neil M. Nathanson Department of Pharmacology SJ-30 University of Washington Seattle WA 98195 USA Andrew B. Tobin Department of Cell Physiology and Pharmacology University of Leicester Hodgkin Building Lancaster Road Leicester LE1 9HN UK Celine Valant Analytical & Structural Neuropharmacology Laboratory Drug Discovery Biology Theme Monash Institute of Pharmaceutical Sciences & Department of Pharmacology Monash University 399 Royal Parade Parkville, VIC, 3052 Australia Jurgen Wess Molecular Signaling Section Laboratory of Bioorganic Chemistry, NIDDK National Institutes of Health Building 8A, Room B1A-05 8 Center Drive MSC 0810 Bethesda MD 20892-0810 USA

Other contributors: Noel J. Buckley King's College London Institute of Psychiatry Centre for the Cellular Basis of Behaviour James Black Centre 125 Coldharbour Lane London SE5 9NU UK R.A. John Challiss Department of Molecular and Cell Biology Henry Wellcome Building University of Leicester Leicester LE1 7RH UK Richard M. Eglen PerkinElmer William Street Wellesley Hills MA 02481 USA Rudolf Hammer Nippon Boehringer Ingleheim 3-10-1 Yato Kawanishi-shi Hyogo #666-0193 Japan Heinz J. Kilbinger Pharmakologisches Institut University of Mainz Mainz D-55101 Germany Günter Lambrecht Department of Pharmacology University of Frankfurt Biocentre Niederursel Frankfurt D-60349 Germany Ernst Mutschler Department of Pharmacology University of Frankfurt Biocentre Niederursel Frankfurt D-60349 Germany Roy D. Schwarz Pfizer Global Research and Development Ann Arbor Laboratories Neuroscience Therapeutics 2800 Plymouth Road Ann Arbor MI 48105 USA David Thal Analytical & Structural Neuropharmacology Laboratory Drug Discovery Biology Theme Monash Institute of Pharmaceutical Sciences & Department of Pharmacology Monash University 399 Royal Parade Parkville, VIC, 3052 Australia