Nicotinic acetylcholine receptor
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. 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. In the immune system, nAChRs regulate inflammatory processes and signal through distinct intracellular pathways.[1] In insects, the cholinergic system is limited to the central nervous system.[2]
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 receptors but selectively binds to the nicotinic receptors instead.[3][4][5] The muscarinic acetylcholine receptor likewise gets its name from a chemical that selectively attaches to that receptor—muscarine.[6] Acetylcholine itself binds to both muscarinic and nicotinic acetylcholine receptors.[7]
As ionotropic receptors, nAChRs are directly linked to ion channels. New evidence suggests that these receptors can also use second messengers (as metabotropic receptors do) in some cases.[8] Nicotinic acetylcholine receptors are the best-studied of the ionotropic receptors.[3]
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[9] that normally corrects changes in blood pressure by sympathetic and parasympathetic stimulation of the heart.
Channel opening[edit]
Nicotinic AChRs may exist in different interconvertible conformational states. Binding of an agonist stabilizes the open and desensitized states. In normal physiological conditions, the receptor needs exactly two molecules of ACh to open.[19] Opening of the channel allows positively charged ions to move across it; in particular, sodium enters the cell and potassium exits. The net flow of positively charged ions is inward.
The nAChR is a non-selective cation channel, meaning that several different positively charged ions can cross through.[3] It is permeable to Na+ and K+, with some subunit combinations that are also permeable to Ca2+.[4][20][21] The amount of sodium and potassium the channels allow through their pores (their conductance) varies from 50 to 110 pS, with the conductance depending on the specific subunit composition as well as the permeant ion.[22]
Many neuronal nAChRs can affect the release of other neurotransmitters.[5] The channel usually opens rapidly and tends to remain open until the agonist diffuses away, which usually takes about 1 millisecond.[4] AChRs can spontaneously open with no ligands bound or can spontaneously close with ligands bound, and mutations in the channel can shift the likelihood of either event.[23][18] Therefore, ACh binding changes the probability of pore opening, which increases as more ACh binds.
The nAChR is unable to bind ACh when bound to any of the snake venom α-neurotoxins. These α-neurotoxins antagonistically bind tightly and noncovalently to nAChRs of skeletal muscles and in neurons, thereby blocking the action of ACh at the postsynaptic membrane, inhibiting ion flow and leading to paralysis and death. The nAChR contains two binding sites for snake venom neurotoxins. Progress in discovering the dynamics of binding action of these sites has proved difficult, although recent studies using normal mode dynamics[24] have aided in predicting the nature of both the binding mechanisms of snake toxins and of ACh to nAChRs. These studies have shown that a twist-like motion caused by ACh binding is likely responsible for pore opening, and that one or two molecules of α-bungarotoxin (or other long-chain α-neurotoxin) suffice to halt this motion. The toxins seem to lock together neighboring receptor subunits, inhibiting the twist and therefore, the opening motion.[25]
Effects[edit]
The activation of receptors by nicotine modifies the state of neurons through two main mechanisms. On one hand, the movement of cations causes a depolarization of the plasma membrane (which results in an excitatory postsynaptic potential in neurons) leading to the activation of voltage-gated ion channels. On the other hand, the entry of calcium acts, either directly or indirectly, on different intracellular cascades. This leads, for example, to the regulation of activity of some genes or the release of neurotransmitters.
Regulation[edit]
Desensitization[edit]
Ligand-bound desensitization of receptors was first characterized by Katz and Thesleff in the nicotinic acetylcholine receptor.[26]
Prolonged or repeated exposure to a stimulus often results in decreased responsiveness of that receptor toward a stimulus, termed desensitization. nAChR function can be modulated by phosphorylation[27] by the activation of second messenger-dependent protein kinases. PKA[26] and PKC,[28] as well as tyrosine kinases,[29] have been shown to phosphorylate the nAChR resulting in its desensitization. It has been reported that, after prolonged receptor exposure to the agonist, the agonist itself causes an agonist-induced conformational change in the receptor, resulting in receptor desensitization.[30]
Desensitized receptors can revert to a prolonged open state when an agonist is bound in the presence of a positive allosteric modulator, for example PNU-120,596.[31] Also, there is evidence that indicates specific chaperone molecules have regulatory effects on these receptors.[32]
Roles[edit]
The subunits of the nicotinic receptors belong to a multigene family (16 members in humans) and the assembly of combinations of subunits results in a large number of different receptors (for more information see the Ligand-Gated Ion Channel database). These receptors, with highly variable kinetic, electrophysiological and pharmacological properties, respond to nicotine differently, at very different effective concentrations. This functional diversity allows them to take part in two major types of neurotransmission. Classical synaptic transmission (wiring transmission) involves the release of high concentrations of neurotransmitter, acting on immediately neighboring receptors. In contrast, paracrine transmission (volume transmission) involves neurotransmitters released by axon terminals, which then diffuse through the extra-cellular medium until they reach their receptors, which may be distant.[33] Nicotinic receptors can also be found in different synaptic locations; for example the muscle nicotinic receptor always functions post-synaptically. The neuronal forms of the receptor can be found both post-synaptically (involved in classical neurotransmission) and pre-synaptically[34] where they can influence the release of multiple neurotransmitters.
