Determination of Region-Specific Roles of the M₃ Muscarinic Acetylcholine Receptor in Gastrointestinal Motility

Background: The specific role of the M₃ muscarinic acetylcholine receptor in gastrointestinal motility under physiological conditions is unclear, due to a lack of subtype-selective compounds. Aims: The objective of this study was to determine the region-specific role of the M₃ receptor in gastrointestinal motility. Methods: We developed a novel positive allosteric modulator (PAM) for the M₃ receptor, PAM-369. The effects of PAM-369 on the carbachol-induced contractile response of porcine esophageal smooth muscle and mouse colonic smooth muscle (ex vivo) and on the transit in mouse small intestine and rat colon (in vivo) were examined. Results: PAM-369 selectively potentiated the M₃ receptor under the stimulation of its orthosteric ligands without agonistic or antagonistic activity. Half-maximal effective concentrations of PAM activity for human, mouse, and rat M₃ receptors were 0.253, 0.345, and 0.127 μM, respectively. PAM-369 enhanced carbachol-induced contraction in porcine esophageal smooth muscle and mouse colonic smooth muscle without causing any contractile responses by itself. The oral administration of 30 mg/kg PAM-369 increased the small intestinal transit in both normal motility and loperamide-induced intestinal dysmotility mice but had no effects on the colonic transit, although the M₃ receptor mRNA expression is higher in the colon than in the small intestine. Conclusions: This study provided the first direct evidence that the M₃ receptor has different region-specific roles in the motility function between the small intestine and colon in physiological and pathophysiological contexts. Selective PAMs designed for targeted subtypes of muscarinic receptors are useful for elucidating the subtype-specific function.