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

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.