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Roles of M₂ and M₄ Muscarinic Receptors in Regulating Acetylcholine Release From Myenteric Neurons of Mouse Ileum

¹Department of Veterinary Pharmacology, Graduate School of Agriculture and Life Science, Osaka Prefecture University, Sakai Osaka; ²Department of Pharmacology, Graduate School of Medicine, Kyoto University, Kyoto; and ³Division of Neuronal Network, Department of Basic Medical Sciences, The Institute of Medical Science, The University of Tokyo, Tokyo, Japan

Submitted 22 September 2004; accepted in final form 30 November 2004

We investigated the subtype of presynaptic muscarinic receptors associated with inhibition of acetylcholine (ACh) release in the mouse small intestine. We measured endogenous ACh released from longitudinal muscle with myenteric plexus (LMMP) preparations obtained from M₁–M₅ receptor knockout (KO) mice. Electrical field stimulation (EFS) increased ACh release in all LMMP preparations obtained from M₁–M₅ receptor single KO mice. The amounts of ACh released in all preparations were equal to that in the wild-type mice. Atropine further increased EFS-induced ACh release in the wild-type mice. Unexpectedly, atropine also increased, to a similar extent, EFS-induced ACh release to the wild-type mice in all M₁–M₅ receptor single KO mice. In M₂ and M₄ receptor double KO mice, the amount of EFS-induced ACh release was equivalent to an atropine-evoked level in the wild-type mouse, and further addition of atropine had no effect. M₂ receptor immunoreactivity was located in both smooth muscle cells and enteric neurons. M₄ receptor immunoreactivity was located in the enteric neurons, being in co-localization with M₂ receptor immunoreactivity. These results indicate that both M₂ and M₄ receptors mediate the muscarinic autoinhibition in ACh release in the LMMP preparation of the mouse ileum, and loss of one of these subtypes can be compensated functionally by a receptor that remained. M₁, M₃, and M₅ receptors do not seem to be involved in this mechanism.

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				T. Takeuchi, K. Tanaka, H. Nakajima, M. Matsui, and Y.-T. Azuma M2 and M3 muscarinic receptors are involved in enteric nerve-mediated contraction of the mouse ileum: findings obtained with muscarinic-receptor knockout mouse Am J Physiol Gastrointest Liver Physiol, January 1, 2007; 292(1): G154 - G164. [Abstract] [Full Text] [PDF]