Nicotinic acetylcholine receptor

Nicotinic acetylcholine receptors, or nAChRs, are ionotropic receptors that form ion channels in cells' plasma membranes. Like the other type of acetylcholine receptors, muscarinic acetylcholine receptors, their opening is triggered by the neurotransmitter acetylcholine, but they are also opened by nicotine (Siegel et al., 1999; Itier and Bertrand, 2001). Their action is inhibited by curare.

Nicotinic acetylcholine receptors are present in many tissues in the body. The neuronal receptors are found in the central nervous system and the peripheral nervous system. The neuromuscular receptors are found in the neuromuscular junctions of somatic muscles; stimulation of these receptors causes muscular contraction.

Structure
Nicotinic receptors, with a molecular weight of about 280 kDa, are made up of five receptor subunits, arranged symmetrically around the central pore. They share similarities with GABAA receptors, glycine receptors, and the type 3 serotonin receptors, which are all therefore classed into the nicotinicoid receptor family, or the signature Cys-loop proteins (Cascio, 2004).

Twelve types of nicotinic receptor subunits, &alpha;2 through 10 and &beta;2 through 4 (Itier and Bertrand, 2001), combine to form pentamers. The subunits are somewhat similar to one another, especially in the hydrophobic regions (Siegel et al., 1999). The muscle form of the nAChR consist of two &alpha; subunits, a &beta;, a &delta; and either a &gamma; or an &epsilon; (Siegel et al., 1999; Itier and Bertrand, 2001; Giniatullin et al., 2005). The neuronal forms are much more heterogeneous, with a wide range of possible subunit combinations.

The sites for binding ACh are on the outside of the &alpha; subunits near their N termini (Siegel et al., 1999). When the agonist binds, the &alpha; subunits become more similar to the other subunits, the channel becomes more symmetrical (Colquhoun and Sivilotti, 2004), and a pore with a diameter of about 0.65 nm opens (Siegel et al., 1999).

Opening the channel
Nicotinic AChRs may exist in different interconvertible conformational states. Binding of nicotine stabilizes the open and desensitised states. Opening of the channel allows positively charged ions, in particular, sodium and calcium, to enter the cell.

The nAChR is permeable to Na+ and K+, with some subunit combinations that are also permeable to Ca2+ (Siegel et al., 1999). The amount of sodium and potassium the channels allow through their pores (their conductance) is about 25 pS (Siegel et al., 1999), but the conductance depends on the actual subunit composition. Interestingly, because some neuronal nAChRs are permeable to Ca2+, they can affect the release of other neurotransmitters (Itier and Bertrand, 2001). The channel usually opens rapidly and tends to remain open until the agonist diffuses away, usually for about 1 millisecond (Siegel et al., 1999). However, AChRs can open sometimes with only one agonist bound and in rare cases with no agonist bound, and they can close spontaneously even when ACh is bound, so ACh binding only creates a probability of pore opening, which increases as more ACh binds (Colquhoun and Sivilotti, 2004).

Effects
This activation of receptors by nicotine modifies the state of neurons through two main mechanisms. On one hand, the movements of cations cause a depolarization of the plasma membrane, which results in an excitation, particularly of neurons, but also by the activation of other voltage-gated ion channels. On the other hand, the entry of calcium acts, either directly or indirectly, on different intracellular cascades leading, for example, to the regulation of the activity of some genes or the release of neurotransmitters.

Roles
The subunits of the nicotinic receptors belong to a multigene family (16 members in human) 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 differently to nicotine, 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 neighbouring receptors. In contrast, paracrine transmission (volume transmission) involves neurotransmitters released by synaptic buttons or varicosities, 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 other neurotranmsitters).

Subunits
To date 17 nAChR subunits have been identified, these are divided into muscle-type and neuronal-type subunits. 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 tribes.


 * Alpha genes: (muscle),  (neuronal),, , , , , , ,
 * Beta genes: (muscle),  (neuronal),, ,
 * Other genes: (delta),  (epsilon),  (gamma)