Functions of Muscarinic Receptor Subtypes in Gastrointestinal Smooth Muscle: A Review of Studies with Receptor-Knockout Mice

Parasympathetic signalling via muscarinic acetylcholine receptors (mAChRs) regulates gastrointestinal smooth muscle function. In most instances, the mAChR population in smooth muscle consists mainly of M2 and M3 subtypes in a roughly 80% to 20% mixture. Stimulation of these mAChRs triggers a complex array of biochemical and electrical events in the cell via associated G proteins, leading to smooth muscle contraction and facilitating gastrointestinal motility. Major signalling events induced by mAChRs include adenylyl cyclase inhibition, phosphoinositide hydrolysis, intracellular Ca2+ mobilisation, myofilament Ca2+ sensitisation, generation of non-selective cationic and chloride currents, K+ current modulation, inhibition or potentiation of voltage-dependent Ca2+ currents and membrane depolarisation. A lack of ligands with a high degree of receptor subtype selectivity and the frequent contribution of multiple receptor subtypes to responses in the same cell type have hampered studies on the signal transduction mechanisms and functions of individual mAChR subtypes. Therefore, novel strategies such as genetic manipulation are required to elucidate both the contributions of specific AChR subtypes to smooth muscle function and the underlying molecular mechanisms. In this article, we review recent studies on muscarinic function in gastrointestinal smooth muscle using mAChR subtype-knockout mice

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