Acetylcholine Receptors (Nicotinic)
Nicotinic acetylcholine receptors (nAChRs) constitute a family of ligand-gated channels, originally classified on the basis of their activation by the alkaloid nicotine, with acetylcholine (ACh) being the endogenous ligand. nAChRs are pentameric transmembrane proteins that form cationic channels permeable to Na+, K+, and Ca2+. nAChRs consist of five homologous subunits, from a portfolio of ten α (α1- α10) and four β (β1- β4) subunits cloned from mammalian and avian sources. Each nAChR subunit comprises four transmembrane spanning domains, M1-M4, of which M2 lines the channel. The structure of the muscle-type (from Torpedo electric organs) nAChR has been resolved to 4 Ǻ by electron microscopy, whereas insight into the extracellular domain and agonist binding site of nAChRs has come from the X-ray crystal structure (resolution down to 2.7 Ǻ) of a molluscan ACh binding protein. The latter has provided a template for modelling the binding site of neuronal nAChRs.
α1, β1, γ, δ and ε subunits are only expressed in skeletal muscle, whereas the other subunits are found mainly in neurons (hence “neuronal nicotinic receptors”) but also in other cell types. For example, it has been reported that the α7 subunit is transiently expressed in skeletal muscle during development and in activated macrophages, whereas other subunits are also expressed by various epithelial and endothelial cell types.
The predominant forms of nAChRs found in the brain are α4β2* and putative homomeric α7 nAChRs, where * denotes the possible presence of additional subunits. In dopaminergic neurons for example, there is evidence for multiple nAChR subtypes: α4β2, α4α5β2, α6β2β3, α4α6β2β3 on terminals; α7 nAChR and some of the heteromeric forms on somatodendritic regions. In autonomic ganglia, α3α5β4 and α3α5β2β4 combinations exist, in addition to α7 homomers. The α9 and α10 subunits have a restricted distribution, limited to sensory tissues (principally cochlear hair cells). The α10 subunit is only functional as a heteromer with α9, whereas α9 can form homomeric nAChR. These are the most divergent of the nAChR family and show mixed nicotinic/muscarinic pharmacology, and sensitivity to other drugs including strychnine and 5HT3 ligands.
The ACh binding site resides at the interface between an a subunit (except α5) and the neighboring subunit; thus nAChR heteromers typically have two binding sites for agonists and competitive antagonists, while α7 homomers could have up to five putative binding sites. Most of the potent and selective ligands for nAChRs are natural products, like (-)-Nicotine. They have provided a basis for rational drug design, stimulated by the perception of neuronal nAChRs as therapeutic targets.
The frog alkaloid epibatidine is one of the most potent nicotinic agonists (Ki ~50 pM for binding to a4b2 nAChR) and is 100-fold more potent than morphine as an analgesic. However its interactions with ganglionic and neuromuscular nAChRs result in toxicity at doses close to the analgesic ones. Most synthetic chemistry efforts have focussed on α4β2 or α7 nAChRs. ABT-418, an isoxazole isostere of nicotine, exhibits cognition-enhancing activities and shows activity in adults with attention deficit hyperactivity disorder; ABT-594, an azetidine analog of epibatidine, is a potent analgesic with reduced side effects in animal models, compared to epibatidine; SIB-1508Y is an α4β2-preferring nicotine analog that shows activity in animal models of Parkinson’s disease. The 3-pyridyl ether A-85380 was designed to discriminate central and peripheral nAChRs, and can also distinguish subpopulations of [3H]epibatidine binding sites in brain membranes. The 5-iodo analog (5-Iodo-A-85380) was developed for SPECT imaging and shows improved potency and selectivity compared with the parent compound. GTS-21 is a partial agonist at α7 nAChR, with reported cognition enhancing activity, while AR-R 17779 is a full agonist at α7 nAChRs. SIB-1553A is an α3β4-preferring agent that showed some cognitive enhancing activity in animal models. Various α-conotoxins isolated from the venoms of Conus snail species provide subtype-selective nAChR antagonists and represent important research tools (although their exquisite specificity can result in differences in recognition of homologous nAChR from different species). Certain α-conotoxins have also been mooted as potential analgesics.
In addition to the ACh binding site, nAChRs contain modulatory sites, e.g. for neurosteroids, Ca2+ and certain acetylcholinesterase inhibitors (physostigmine and galantamine). Other binding sites are located within the ion channel, e.g. for anesthetics (such as lidocaine), phencyclidine and MK-801. The anthelmintic, ivermectin, and 5-hydroxy-indole potentiate α7 nAChR (the latter also potentiates 5-HT3 receptors), whereas 17-b-estradiol selectively potentiates α4β2 nAChR. The neurotoxic peptide, Ab1-42, is reported to interact with the α7 nAChR and an association of a-synuclein with α7 subunits has also been proposed. The α7 nAChR plays a role in hippocampal paired pulse inhibition, and defects in this mechanism, in addition to genetic data, have implicated α7 nAChR in the pathophysiology of schizophrenia. Transgenic mice lacking particular subunits or over-expressing mutated subunits, support a role for both α4 and β2 subunits in nicotine dependence and in nociception. Human mutations in either α4 or β2 subunits underlie rare epileptic diseases such as ADNFLE (autosomal dominant nocturnal frontal lobe epilepsy).
The Table below contains accepted modulators and additional information. For a list of additional products, see the "Similar Products" section below.
Footnotes
a) Partial agonists
Abbreviations
A-85380: 3-(2[S]-Azetidinylmethoxy)pyridine
ABT-418: (S)-3-Methyl-5-(1-methyl-2-pyrrolidinyl)isoxazole
ADNFLE: Autosomal dominant nocturnal frontal lobe epilepsy
AR-R 17779: (–)-spiro[1-Azabicyclo[2.2.2]octane-3,5’-oxazolidin-2’-one (4a)
DMAC: 3-(4)-Dimethylaminocinnamylidine anabaseine
DMPP: N,N-Dimethyl-N’-phenyl-piperazinium iodide
GTS-21: [3-(2,4-Dimethoxybenzylidene)-anabaseine
RJR-2403: N-Methyl-4-(3-pyridinyl]-3-buten-1-amine
SIB-1553A: 4-[[2-(1-Methyl-2-pyrrolidinyl)ethyl]thio]phenyl hydrochloride
TMA: Tetramethylammonium
References
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