Unno T, Matsuyama H, Izumi Y, Yamada M, Wess J, Komori S
Laboratory of Pharmacology, Department of Veterinary Medicine, Faculty of Applied Biological Science, Gifu University, Gifu, Japan.
Br J Pharmacol. 2006 Dec;149(8):1022-30. doi: 10.1038/sj.bjp.0706955. Epub 2006 Nov 13.
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.
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.
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.
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.
M2和M3毒蕈碱受体在胃肠道神经源性胆碱能收缩中的功能作用仍有待阐明。为解决这一问题，我们使用缺乏M2或M3受体亚型的突变小鼠研究了回肠中胆碱能神经诱导的收缩。
在M2基因敲除（KO）、M3基因敲除、M2/M3双基因敲除和野生型小鼠的回肠段中，等长记录对跨壁电刺激（TE）的收缩反应。
在野生型、M2基因敲除或M3基因敲除小鼠标本中，2 - 50Hz的TE刺激频率依赖性地诱发快速、短暂的收缩，随后是较慢、较长的收缩。河豚毒素阻断了初始和后期的收缩，而阿托品仅抑制初始收缩。野生型小鼠的初始胆碱能收缩明显大于M2基因敲除或M3基因敲除小鼠；50Hz时各自的平均幅度分别为70mM K⁺诱导收缩的91%、74%和68%。百日咳毒素预处理阻断了M3基因敲除小鼠的胆碱能收缩，但未阻断M2基因敲除小鼠的。野生型标本中也存在胆碱能收缩，但在10 - 50Hz时其大小减少了20 - 30%。在M2/M3双基因敲除小鼠中，TE刺激仅诱发缓慢的非胆碱能收缩，其大小明显大于其他三种小鼠品系中的任何一种。
这些结果表明，M2和M3受体参与介导小鼠回肠中的胆碱能收缩，其中后者受体起更大作用。我们的数据还表明，M2和M3受体的缺失导致肠道平滑肌非胆碱能兴奋性神经支配上调。
