Elsevier

Neuroscience

Volume 138, Issue 2, 2006, Pages 549-559
Neuroscience

Neuroanatomy
Muscarinic M2 acetylcholine receptor distribution in the guinea-pig gastrointestinal tract

https://doi.org/10.1016/j.neuroscience.2005.11.021Get rights and content

Abstract

In the enteric nervous system, acetylcholine is the most common neurotransmitter to induce gastrointestinal smooth muscle contractions. Cholinergic signaling is mediated by muscarinic acetylcholine receptors on the surface of smooth muscle cells. Five different muscarinic receptor subtypes (M1–M5) have been identified and characterized, all of which belong to the superfamily of the G-protein-coupled receptor. The muscarinic M2 acetylcholine receptor is the major muscarinic receptor subtype expressed by smooth muscle tissues in the gastrointestinal tract, where it is coexpressed with a smaller population of M3 receptor. In this study, we examined the immunohistochemical distribution of the M2 receptor using a specific antibody in the guinea-pig gastrointestinal tract. M2 receptor-like immunoreactivity was mainly observed as associated with smooth muscle cells in the gastrointestinal tract. M2 receptor-like immunoreactivity in smooth muscle cells was distributed throughout the cell membrane associated with caveolae. In the proximal colon, M2 receptor-like immunoreactivity in the smooth muscle cells was weak. In the small intestine, interstitial cells of Cajal that possessed neurokinin 1 receptor-like immunoreactivity had intense M2 receptor-like immunoreactivity. In the proximal colon, intramuscular and myenteric interstitial cells of Cajal exhibited M2 receptor-like immunoreactivity. These findings indicate that, in the gastrointestinal musculature, M2 receptors are distributed both in the smooth muscle cells and interstitial cells of Cajal, suggesting that the M2 receptor elicits smooth muscle cell contraction and the interstitial cells of Cajal are the sites of innervation by enteric cholinergic neurons.

Section snippets

Experimental procedures

Adult male Hartley guinea-pigs aged 4–6 weeks and weighing 250–400 g (Japan SLC, Hamamatsu, Shizuoka, Japan) were used in this study. All animal experiments were carried out in accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals and the Guidelines for Animal Experiments, University of Fukui Faculty of Medical Sciences. All efforts were made to minimize the number of animals used and their suffering. All guinea-pigs were anesthetized with an i.p.

Results

M2 receptor-like immunoreactivity (M2R-LI) was observed associated with smooth muscle cells in the gastrointestinal tract (Fig. 1, Table 1). The external muscle layers, except the esophagus, were immunoreactive for M2 receptor. The intensity of immunoreactivity was higher in the stomach, small intestine, cecum, distal colon and rectum than in the proximal colon. Distinct immunoreaction was also observed in the muscularis mucosae in the pylorus, colon and rectum.

Discussion

In this study, we obtained evidence that muscarinic M2 acetylcholine receptor-like immunoreactivity (M2R-LI) is distributed in the smooth muscle cells and interstitial cells of Cajal in the musculature of the guinea-pig gastrointestinal tract. These findings are the first evidence of M2 receptor distribution on a morphological level.

Presence of M2 receptor in the smooth muscle tissue is known by immunoprecipitation studies, ligand binding studies and PCR studies (Dorje et al 1991, Eglen et al

Conclusion

In summary, we examined the immunohistochemical distribution of the muscarinic M2 acetylcholine receptor, which is the major muscarinic receptor subtype expressed by smooth muscle tissues in the guinea-pig gastrointestinal tract. M2R-LI was mainly observed as associated with smooth muscle cells in the gastrointestinal tract. M2R-LI in smooth muscle cells was distributed throughout the cell membrane associated with caveolae. Interstitial cells of Cajal in the small intestine DMP and in the

Acknowledgment

We thank Prof. N. W. Bunnett (University of California, San Francisco) for the gift of the anti-neurokinin 1 receptor (#94168). This work was supported by Grant-in-Aid for Scientific Research from Japan Society for Promotion of Science.

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