Nicotinic receptors

Nicotinic acetylcholine receptors, or nAChRs, are neuron receptor proteins that respond to the neurotransmitter acetylcholine. Nicotinic receptors also respond to drugs, including the nicotinic receptor agonist nicotine. They are found in the central nervous system of humans, and also play two important roles in the peripheral nervous system: (1) they transmit outgoing signals from the presynaptic to the postsynaptic cells within the sympathetic and parasympathetic nervous system, and (2) they are the receptors found on skeletal muscle that receive acetylcholine released to signal for muscular contraction. Nicotinic receptors are also found in other creatures. In insects, the cholinergic system is limited to the central nervous system.

The nicotinic receptors are considered cholinergic receptors, since they respond to acetylcholine. Nicotinic receptors get their name from nicotine, which does not stimulate the muscarinic acetylcholine receptor, but instead selectively binds to the nicotinic receptor. The muscarinic acetylcholine receptor likewise gets its name from a chemical that selectively attaches to that receptor -- muscarine. Acetylcholine itself binds to both muscarinic and nicotinic acetylcholine receptors.

As ionotropic receptors, nAChRs are directly linked to ion channels and do not use second messengers (as metabotropic receptors do). Nicotinic acetylcholine receptors are the best-studied of the ionotropic receptors.

Since nicotinic receptors help transmit outgoing signals for the sympathetic and parasympathetic systems, nicotinic receptor antagonists such as hexamethonium interfere with the transmission of these signals. Thus, for example, nicotinic receptor antagonists interfere with the baroreflex that normally corrects changes in blood pressure by sympathetic and parasympathetic stimulation of the heart.

Nicotinic receptors, with a molecular mass of 290 kDa, are made up of five subunits, arranged symmetrically around a central pore. Each subunit comprises four transmembrane domains with both the N- and C-terminus located extracellularly. They possess similarities with GABA_A receptors, glycine receptors, and the type 3 serotonin receptors (which are all ionotropic receptors), or the signature Cys-loop proteins.

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