An acetylcholine receptor (abbreviated AChR) is an integral membrane protein that responds to the binding of acetylcholine, a neurotransmitter.
Like other transmembrane receptors, acetylcholine receptors are classified according to their "pharmacology," or according to their relative affinities and sensitivities to different molecules. Although all acetylcholine receptors, by definition, respond to acetylcholine, they respond to other molecules as well.
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 Ca2+ ion channel.
Muscarinic acetylcholine receptors (mAChR, also known as "metabotropic" acetylcholine receptors) are particularly responsive to muscarine.
Nicotinic and muscarinic are two main kinds of "cholinergic" receptors.
Molecular biology has shown that the nicotinic and muscarinic receptors belong to distinct protein superfamilies. Nicotinic receptors are of two types: Nm and Nn. Nm is located in the neuromuscular junction which causes the contraction of skeletal muscles by way of end-plate potential (EPPs). Nn causes depolarization in autonomic ganglia resulting in post ganglionic impulse. Nicotinic receptors cause the release of catecholamine from the adrenal medulla, and also site specific excitation or inhibition in brain. Both Nm and Nn are Na+ and Ca2+ channel linked but Nn is also linked with an extra K+ channel.
Nicotinic acetylcholine receptor
The nAChRs are ligand-gated ion channels, and, like other members of the "cys-loop" ligand-gated ion channel superfamily, are composed of five protein subunits symmetrically arranged like staves around a barrel. The subunit composition is highly variable across different tissues. Each subunit contains four regions which span the membrane and consist of approximately 20 amino acids. Region II which sits closest to the pore lumen, forms the pore lining.
Binding of acetylcholine to the N termini of each of the two alpha subunits results in the 15° rotation of all M2 helices.
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Nicotinic acetylcholine receptors, or nAChRs, are receptor polypeptides that respond to the neurotransmitter acetylcholine. Nicotinic receptors also respond to drugs such as the agonist nicotine. They are found in the central and peripheral nervous system, muscle, and many other tissues of many organisms. At the neuromuscular junction they are the primary receptor in muscle for motor nerve-muscle communication that controls muscle contraction.
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.
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