Acetylcholine receptor

An acetylcholine receptor (abbreviated AChR) is an integral membrane protein that responds to the binding of acetylcholine, a neurotransmitter.

Classification
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
 * 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.

Receptor types
Molecular biology has shown that the nicotinic and muscarinic receptors belong to distinct protein superfamilies.

nAChR
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. The cytoplasm side of the nAChR receptor has rings of high negative charge that determine the specific cation specificity of the receptor and remove the hydration shell often formed by ions in aqueous solution. In the intermediate region of the receptor, within the pore lumen, valine and leucine residues (Val 255 and Leu 251) define a hydrophobic region which the dehydrated ion must pass through.

The nAChR is found at the edges of junctional folds at the neuromuscular junction on the postsynaptic side; it is activated by acetylcholine release across the synapse. The diffusion of Na+ and K+ across the receptor causes depolarization, the end-plate potential, that opens voltage-gated sodium channels, which allows for firing of the action potential and potentially muscular contraction.

mAChR
In contrast, the mAChRs are not ion channels, but belong instead to the superfamily of G-protein-coupled receptors that activate other ionic channels via a second messenger cascade.

Role in health and disease
Nicotinic acetylcholine receptors can be blocked by curare, hexamethonium and toxins present in the venoms of snakes and shellfishes, like α-bungarotoxin. Drugs such as the neuromuscular blocking agents bind reversibly to the nicotinic receptors in the neuromuscular junction and are used routinely in anaesthesia.

Nicotinic receptors are the primary mediator of the effects of nicotine. In myasthenia gravis, the receptor at NMJ is targeted by antibodies, leading to muscle weakness. Muscarinic acetylcholine receptors can be blocked by the drugs atropine and scopolamine.