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

Although muscarinic M_2 and M_3 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. M_2 and/or M_3 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 M_3R-KO and M_2/M_3R-KO mice but not in M_2R-KO mice. Ranking of colonic propulsion was wild-type = M_2R-KO > M_3R-KO > M_2/M_3R-KO. In vitro, the amplitude of migrating motor complexes in M_2R-KO, M_3R-KO and M_2/M_3R-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 M_2R-KO mice, the concentration-contraction curves shifted to the right and downward. In contrast, carbachol caused non-sustained contraction and relaxation in M_3R-KO mice depending on its concentration. Carbachol did not cause contraction but instead caused relaxation of colonic strips from M_2/M_3R-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 M_2/M_3R-KO mice, McN-A-343 caused only relaxation, which was decreased by tetrodotoxin, pirenzepine and L-NAME. In conclusion, M_1, M_2 and M_3 receptors regulate colonic motility of the mouse. M_2 and M_3 receptors mediate cholinergic contraction, but M_1 receptors on inhibitory nitrergic nerves counteract muscarinic contraction