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Muscarinic acetylcholine receptors (mAChRs) are acetylcholine receptors that form G protein-coupled receptor complexes in the cell membranes of certain neurons [1] and other cells. They play several roles, including acting as the main end-receptor stimulated by acetylcholine released from postganglionic fibers .
Like the M 1 muscarinic receptor, M 3 receptors are coupled to G proteins of class G q, which upregulate phospholipase C and, therefore, inositol trisphosphate and intracellular calcium as a signalling pathway. [8] The calcium function in vertebrates also involves activation of protein kinase C and its effects.
The muscarinic acetylcholine receptor M 2, also known as the cholinergic receptor, muscarinic 2, is a muscarinic acetylcholine receptor that in humans is encoded by the CHRM2 gene. [5] Multiple alternatively spliced transcript variants have been described for this gene. [5] It is G i-coupled, reducing intracellular levels of cAMP.
The muscarinic acetylcholine receptor M 4, also known as the cholinergic receptor, muscarinic 4 (CHRM4), is a protein that, in humans, is encoded by the CHRM4 gene. [ 5 ] [ 6 ] Function
Muscarinic acetylcholine receptors have a more complex mechanism, and affect target cells over a longer time frame. In mammals, five subtypes of muscarinic receptors have been identified, labeled M1 through M5. All of them function as G protein-coupled receptors, meaning that they exert their effects via a second messenger system.
Nicotinic acetylcholine receptors (nAChR, also known as "ionotropic" acetylcholine receptors) are particularly responsive to nicotine. The nicotine ACh receptor is also a Na +, K + and Ca 2+ ion channel. Muscarinic acetylcholine receptors (mAChR, also known as "metabotropic" acetylcholine receptors) are particularly responsive to muscarine.
The muscarinic acetylcholine receptor M 1, also known as the cholinergic receptor, muscarinic 1, is a muscarinic receptor that in humans is encoded by the CHRM1 gene. [5] It is localized to 11q13. [5] This receptor is found mediating slow EPSP at the ganglion in the postganglionic nerve, [6] is common in exocrine glands and in the CNS. [7] [8]
Muscarinic receptors mediate many of the effects of acetylcholine in the central and peripheral nervous system. The clinical implications of this receptor have not been fully explored; however, stimulation of this receptor is known to effectively decrease cyclic AMP levels and downregulate the activity of protein kinase A (PKA).