Nicotinic acetylcholine receptors, or nAChRs, are receptor polypeptides that respond to the neurotransmitter acetylcholine. Nicotinic receptors also respond to drugs, including the nicotinic receptor agonist nicotine.
They are found in the central and peripheral nervous system, muscle,
and many other tissues of many organisms, including humans. 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. 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. New evidence suggests that these receptors can also use second messengers (as metabotropic receptors do) in some cases. 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.
Structure
Nicotinic receptor structure
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 GABAA receptors, glycine receptors, and the type 3 serotonin receptors (which are all ionotropic receptors), or the signature Cys-loop proteins.
In vertebrates, nicotinic receptors are broadly classified into two subtypes based on their primary sites of expression: muscle-type nicotinic receptors and neuronal-type
nicotinic receptors. In the muscle-type receptors, found at the
neuromuscular junction, receptors are either the embryonic form,
composed of α1, β1, γ, and δ subunits in a 2:1:1:1 ratio, or the adult
form composed of α1, β1, δ, and ε subunits in a 2:1:1:1 ratio.
The neuronal subtypes are various homomeric (all one type of subunit)
or heteromeric (at least one α and one β) combinations of twelve
different nicotinic receptor subunits: α2−α10 and β2−β4. Examples of
the neuronal subtypes include: (α4)3(β2)2, (α4)2(β2)3, (α3)2(β4)3, α4α6β3(β2)2, (α7)5,
and many others. In both muscle-type and neuronal-type receptors, the
subunits are very similar to one another, especially in the hydrophobic regions.
A number of electron microscopy and x-ray crystallography studies
have provided very high resolution structural information for muscle
and neuronal nAChRs and their binding domains.
Binding to the receptor
As with all ligand-gated ion channels, opening of the nAChR channel
pore requires the binding of a chemical messenger. Several different
terms are used to refer to the molecules that bind receptors, such as ligand, agonist, or transmitter. As well as the endogenous agonist acetylcholine, agonists of the nAChR include nicotine, epibatidine, and choline. Nicotinic antagonists that block the receptor include mecamylamine, dihydro-β-erythroidine, and hexamethonium.
In muscle-type nAChRs, the acetylcholine binding sites are
located at the α and either ε or δ subunits interface. In neuronal
nAChRs, the binding site is located at the interface of an α and a β
subunit or between two α subunits in the case of α7 receptors. The
binding site is located in the extracellular domain near the N terminus. When an agonist binds to the site, all present subunits undergo a conformational change and the channel is opened and a pore with a diameter of about 0.65 nm opens.
Opening the channel
Nicotinic AChRs may exist in different interconvertible conformational states. Binding of an agonist stabilises the open and desensitised states. In normal physiological conditions, the receptor needs exactly two molecules of ACh to open. 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. It is permeable to Na+ and K+, with some subunit combinations that are also permeable to Ca2+. The amount of sodium and potassium the channels allow through their pores (their conductance) varies from 50–110 pS, with the conductance depending on the specific subunit composition as well as the permeant ion.
Many neuronal nAChRs can affect the release of other neurotransmitters. The channel usually opens rapidly and tends to remain open until the agonist diffuses away, which usually takes about 1 millisecond.
However, 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. 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 towards discovering the dynamics of binding
action of these sites has proved difficult, although recent studies
using normal mode dynamics
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.
Effects
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 the activity of some genes or the release of neurotransmitters.
Receptor regulation
Receptor desensitisation
Ligand-bound desensitisation of receptors was first characterised by Katz and Thesleff in the nicotinic acetylcholine receptor.
Prolonged or repeated exposure to a stimulus often results in
decreased responsiveness of that receptor toward a stimulus, termed
desensitisation. nAChR function can be modulated by phosphorylation by the activation of second messenger-dependent protein kinases. PKA and PKC, as well as tyrosine kinases,
have been shown to phosphorylate the nAChR resulting in its
desensitisation. 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
desensitisation.
Desensitised receptors can revert to a prolonged open state when an
agonist is bound in the presence of a positive allosteric modulator, for
example PNU-120596. Also, there is evidence that indicates specific chaperone molecules have regulatory effects on these receptors.
Roles
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. 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 synaptic boutons, which then diffuse through the extra-cellular medium until they reach their receptors, which may be distant.
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 where they can influence the release of multiple neurotransmitters.
Subunits
17
vertebrate nAChR subunits have been identified, which are divided into
muscle-type and neuronal-type subunits. However, although an α8
subunit/gene is present in avian species such as the chicken, it is not
present in human or mammalian species.
The nAChR subunits have been divided into 4 subfamilies (I-IV) based on similarities in protein sequence. In addition, subfamily III has been further divided into 3 types.
| Neuronal-type | Muscle-type | ||||
| I | II | III | IV | ||
|---|---|---|---|---|---|
| α9, α10 | α7, α8 | 1 | 2 | 3 | α1, β1, δ, γ, ε |
| α2, α3, α4, α6 | β2, β4 | β3, α5 | |||
