@article {Langmead236, author = {Christopher J. Langmead and Victoria A. H. Fry and Ian T. Forbes and Clive L. Branch and Arthur Christopoulos and Martyn D. Wood and Hugh J. Herdon}, title = {Probing the Molecular Mechanism of Interaction between 4-n-Butyl-1-[4-(2-methylphenyl)-4-oxo-1-butyl]-piperidine (AC-42) and the Muscarinic M1 Receptor: Direct Pharmacological Evidence That AC-42 Is an Allosteric Agonist}, volume = {69}, number = {1}, pages = {236--246}, year = {2006}, doi = {10.1124/mol.105.017814}, publisher = {American Society for Pharmacology and Experimental Therapeutics}, abstract = {4-n-Butyl-1-[4-(2-methylphenyl)-4-oxo-1-butyl]-piperidine hydrogen chloride (AC-42) is a selective agonist of the muscarinic M1 receptor previously suggested to interact with an {\textquotedblleft}ectopic{\textquotedblright} site on this receptor. However, the pharmacological properties of this site (i.e., whether it overlaps to any extent with the classic orthosteric site or represents a novel allosteric site) remain undetermined. In the present study, atropine or pirenzepine significantly inhibited the ability of either carbachol or AC-42 to stimulate inositol phosphate accumulation or intracellular calcium mobilization in Chinese hamster ovary (CHO) cells stably expressing the human M1 receptor. However, the interaction between either of these antagonists and AC-42 was characterized by Schild slopes significantly less than unity. Increasing the concentrations of atropine revealed that the Schild regression was curvilinear, consistent with a negative allosteric interaction. More direct evidence for an allosteric mode of action of AC-42 was obtained in [3H]N-methylscopolamine ([3H]NMS) binding studies, in that both AC-42 and the prototypical modulator gallamine failed to fully inhibit specific [3H]NMS binding in a manner that was quantitatively described by an allosteric model applied to both modulator data sets. Furthermore, AC-42 and gallamine significantly retarded the rate of [3H]NMS dissociation from CHO-hM1 cell membranes, conclusively demonstrating their ability to bind to a topographically distinct site to change M1 receptor conformation. These data provide the first direct evidence that AC-42 is an allosteric agonist that activates M1 receptors in the absence of the orthosteric agonist.}, issn = {0026-895X}, URL = {https://molpharm.aspetjournals.org/content/69/1/236}, eprint = {https://molpharm.aspetjournals.org/content/69/1/236.full.pdf}, journal = {Molecular Pharmacology} }