Muscarinic receptor subtypes involved in regulation of colonic motility in mice: Functional studies using muscarinic receptor-deficient mice

Although muscarinic M2 and M3 receptors are known to be important for regulation of gastric and small intestinal motility, muscarinic receptor subtypes regulating colonic function remain to be investigated. The aim of this study was to characterize muscarinic receptors involved in regulation of colonic contractility. M2 and/or M3 receptor knockout (KO) and wild-type mice were used in in vivo (defecation, colonic propulsion) and in vitro (contraction) experiments. Amount of feces was significantly decreased in M3R-KO and M2/M3R-KO mice but not in M2R-KO mice. Ranking of colonic propulsion was wild-type = M2R-KO > M3R-KO > M2/M3R-KO. In vitro, the amplitude of migrating motor complexes in M2R-KO, M3R-KO and M2/M3R-KO mice was significantly lower than that in wild-type mice. Carbachol caused concentration-dependent contraction of the proximal colon and distal colon from wild-type mice. In M2R-KO mice, the concentration-contraction curves shifted to the right and downward. In contrast, carbachol caused non-sustained contraction and relaxation in M3R-KO mice depending on its concentration. Carbachol did not cause contraction but instead caused relaxation of colonic strips from M2/M3R-KO mice. 4-[[[(3-chlorophenyl)amino]carbonyl]oxy]-N,N,N-trimethyl-2-butyn-1-aminium chloride (McN-A-343) caused a non-sustained contraction of colonic strips from wild-type mice, and this contraction was changed to a sustained contraction by tetrodotoxin, pirenzepine and L-nitroarginine methylester (L-NAME). In the colon of M2/M3R-KO mice, McN-A-343 caused only relaxation, which was decreased by tetrodotoxin, pirenzepine and L-NAME. In conclusion, M1, M2 and M3 receptors regulate colonic motility of the mouse. M2 and M3 receptors mediate cholinergic contraction, but M1 receptors on inhibitory nitrergic nerves counteract muscarinic contraction.