Roles of M 2 and M 3 muscarinic receptors in cholinergic nerve‐induced contractions in mouse ileum studied with receptor knockout mice

Background and purpose: The functional roles of M 2 and M 3 muscarinic receptors in neurogenic cholinergic contractions in gastrointestinal tracts remain to be elucidated. To address this issue, we studied cholinergic nerve‐induced contractions in the ileum using mutant mice lacking M 2 or M 3 receptor subtypes. Experimental approach: Contractile responses to transmural electrical (TE) stimulation were isometrically recorded in ileal segments from M 2 ‐knockout (KO), M 3 ‐KO, M 2 /M 3 ‐double KO, and wild‐type mice. Key results: TE stimulation at 2‐50 Hz frequency‐dependently evoked a fast, brief contraction followed by a slower, longer one in wild‐type, M 2 ‐KO or M 3 ‐KO mouse preparations. Tetrodotoxin blocked both the initial and later contractions, while atropine only inhibited the initial contractions. The initial cholinergic contractions were significantly greater in wild‐type than M 2 ‐KO or M 3 ‐KO mice; the respective mean amplitudes at 50 Hz were 91, 74 and 68 % of 70mM K + ‐induced contraction. Pretreatment with pertussis toxin blocked the cholinergic contractions in M 3 ‐KO but not in M 2 ‐KO mice. Cholinergic contractions also remained in wild‐type preparations, but their sizes were reduced by 20‐30 % at 10‐50 Hz. In M 2 /M 3 ‐double KO mice, TE stimulation evoked only slow, noncholinergic contractions, which were significantly greater in sizes than in any of the other three mouse strains. Conclusion and Implications: These results demonstrate that M 2 and M 3 receptors participate in mediating cholinergic contractions in mouse ileum with the latter receptors assuming a greater role. Our data also suggest that the lack of both M 2 and M 3 receptors causes upregulation of noncholinergic excitatory innervation of the gut smooth muscle. British Journal of Pharmacology (2006) 149 , 1022–1030. doi: 10.1038/sj.bjp.0706955