Functional roles of muscarinic M2 and M3 receptors in mouse stomach motility: Studies with muscarinic receptor knockout mice

Functional roles of muscarinic acetylcholine receptors in the regulation of mouse stomach motility were examined using mice genetically lacking muscarinic M2 receptor and/or M3 receptor and their corresponding wild-type (WT) mice. Single application of carbachol (1 nM–30 μM) produced concentration-dependent contraction in antral and fundus strips from muscarinic M2 receptor knockout (M2R-KO) and M3 receptor knockout (M3R-KO) mice but not in those from M2 and M3 receptors double knockout (M2/M3R-KO) mice. A comparison of the concentration–response curves with those for WT mice showed a significant decrease in the negative logarithm of EC50 (pEC50) value (M2R-KO) or amplitude of maximum contraction (M3R-KO) in the muscarinic receptor-deficient mice. The tonic phase of carbachol-induced contraction was decreased in gastric strips from M3R-KO mice. Antagonistic affinity for 4-diphenylacetoxy-N-methyl-piperidine (4-DAMP) or 11-([2-[(diethylamino)methyl]-1-piperdinyl]acetyl)-5,11-dihydro-6H-pyrido[2,3-b][1,4]benzodiazepine-6-one (AF-DX116) indicated that the contractile responses in M2R-KO and M3R-KO mice were mediated by muscarinic M3 and M2 receptors, respectively. Electrical field stimulation (EFS, 0.5–32 Hz) elicited frequency-dependent contraction in physostigmine- and Nω-nitro-l-arginine methylester (l-NAME)-treated fundic and antral strips from M2R-KO and M3R-KO mice, but the cholinergic contractile components decreased significantly compared with those in WT mice. In gastric strips from M2/M3R-KO mice, cholinergic contractions elicited by EFS were not observed but atropine-resistant contractions were more conspicuous than those in gastric strips from WT mice. Gastric emptying in WT mice and that in M2/M3R-KO mice were comparable, suggesting that motor function of the stomach in the KO mice did not differ from that in the WT mice. The results indicate that both muscarinic M2 and M3 receptors but not other subtypes mediate carbachol- or EFS-induced contraction in the mouse stomach but that the contribution of each receptor to concentration–response relationships is distinguishable. Although there was impairment of nerve-mediated cholinergic responses in the stomach of KO mice, gastric emptying in KO mice was the same as that in WT mice probably due to the compensatory enhancement of the non-cholinergic contraction pathway.