Muscarinic acetylcholine receptors, or mAChRs, are acetylcholine receptors that form G protein-coupled receptor complexes in the cell membranes of certain neurons and other cells. They play several roles, including acting as the main end-receptor stimulated by acetylcholine released from postganglionic fibers in the parasympathetic nervous system. Muscarinic receptors are so named because they are more sensitive to muscarine than to nicotine. By the use of selective radioactively labeled agonist and antagonist substances, five subtypes of muscarinic receptors have been determined, named M1-M5 (using an upper case M and subscript number). M1, M3, M5 receptors are coupled with Gq proteins, while M2 and M4 receptors are coupled with Gi/o proteins.

M3 receptor: The M3 muscarinic receptors are located at many places in the body. They are located in the smooth muscles of the blood vessels, as well as in the lungs. Because the M3 receptor is Gq-coupled and mediates an increase in intracellular calcium, it typically causes contraction of smooth muscle, such as that observed during bronchoconstriction and bladder voiding. However, with respect to vasculature, activation of M3 on vascular endothelial cells causes increased synthesis of nitric oxide, which diffuses to adjacent vascular smooth muscle cells and causes their relaxation, thereby explaining the paradoxical effect of parasympathomimetics on vascular tone and bronchiolar tone. Indeed, direct stimulation of vascular smooth muscle, M3 mediates vasconstriction in pathologies wherein the vascular endothelium is disrupted. The M3 receptors are also located in many glands, which help to stimulate secretion in, for example, the salivary glands, as well as other glands of the body.

References:
1) K. Shiozaki, E. Iseki, H. Uchiyama, Y. Watanabe, T. Haga, K. Kameyama, T. Yamada, T. Yamamoto, K. Kosaka Alterations of muscarinic acetylcholine receptor subtypes in diffuse Lewy body disease: relation to Alzheimer's's disease J Neurol Neurosurg Psychiatry. 1999 Aug; 67 (2): 209-13.
2) Kazumasa Shiozaki, Eizo Iseki, Hiroaki Hino, Kenji Kosaka Distribution of m1 acetylcholine receptors in the hippocampus of patients with patients and patients with dementia with Lewy bodies-an immunohistochemistry study J Neurol Sci. 2001 Dec 15

Muscarinic acetylcholine receptors, or mAChRs, are acetylcholine receptors that form G protein-coupled receptor complexes in the cell membranes of certain neurons and other cells. They play several roles, including acting as the main end-receptor stimulated by acetylcholine released from postganglionic fibers in the parasympathetic nervous system. Muscarinic receptors are so named because they are more sensitive to muscarine than to nicotine. By the use of selective radioactively labeled agonist and antagonist substances, five subtypes of muscarinic receptors have been determined, named M1-M5 (using an upper case M and subscript number). M1, M3, M5 receptors are coupled with Gq proteins, while M2 and M4 receptors are coupled with Gi/o proteins.

M3 receptor: The M3 muscarinic receptors are located at many places in the body. They are located in the smooth muscles of the blood vessels, as well as in the lungs. Because the M3 receptor is Gq-coupled and mediates an increase in intracellular calcium, it typically causes contraction of smooth muscle, such as that observed during bronchoconstriction and bladder voiding. However, with respect to vasculature, activation of M3 on vascular endothelial cells causes increased synthesis of nitric oxide, which diffuses to adjacent vascular smooth muscle cells and causes their relaxation, thereby explaining the paradoxical effect of parasympathomimetics on vascular tone and bronchiolar tone. Indeed, direct stimulation of vascular smooth muscle, M3 mediates vasconstriction in pathologies wherein the vascular endothelium is disrupted. The M3 receptors are also located in many glands, which help to stimulate secretion in, for example, the salivary glands, as well as other glands of the body.

References:
1) K. Shiozaki, E. Iseki, H. Uchiyama, Y. Watanabe, T. Haga, K. Kameyama, T. Yamada, T. Yamamoto, K. Kosaka Alterations of muscarinic acetylcholine receptor subtypes in diffuse Lewy body disease: relation to Alzheimer's's disease J Neurol Neurosurg Psychiatry. 1999 Aug; 67 (2): 209-13.
2) Kazumasa Shiozaki, Eizo Iseki, Hiroaki Hino, Kenji Kosaka Distribution of m1 acetylcholine receptors in the hippocampus of patients with patients and patients with dementia with Lewy bodies-an immunohistochemistry study J Neurol Sci. 2001 Dec 15