THE CONCISE GUIDE TO PHARMACOLOGY 2019/20: Ion channels

The Concise Guide to PHARMACOLOGY 2019/20 is the fourth in this series of biennial publications. The Concise Guide provides concise overviews of the key properties of nearly 1800 human drug targets with an emphasis on selective pharmacology (where available), plus links to the open access knowledgebase source of drug targets and their ligands (http://www.guidetopharmacology.org/), which provides more detailed views of target and ligand properties. Although the Concise Guide represents approximately 400 pages, the material presented is substantially reduced compared to information and links presented on the website. It provides a permanent, citable, point‐in‐time record that will survive database updates. The full contents of this section can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.14749. Ion channels are one of the six major pharmacological targets into which the Guide is divided, with the others being: G protein‐coupled receptors, nuclear hormone receptors, catalytic receptors, enzymes and transporters. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. The landscape format of the Concise Guide is designed to facilitate comparison of related targets from material contemporary to mid‐2019, and supersedes data presented in the 2017/18, 2015/16 and 2013/14 Concise Guides and previous Guides to Receptors and Channels. It is produced in close conjunction with the International Union of Basic and Clinical Pharmacology Committee on Receptor Nomenclature and Drug Classification (NC‐IUPHAR), therefore, providing official IUPHAR classification and nomenclature for human drug targets, where appropriate.


Comments:
Quantitative data in the table refer to homooligomeric assemblies of the human 5-HT 3 A subunit, or the receptor native to human tissues. Significant changes introduced by co-expression of the 5-HT 3 B subunit are indicated in parenthesis. Although not a selective antagonist, methadone displays multimodal and subunit-dependent antagonism of 5-HT 3 receptors [210]. Similarly, TMB-8, diltiazem, picrotoxin, bilobalide and ginkgolide B are not selective for 5-HT 3 receptors (e.g. [974]). The anti-malarial drugs mefloquine and quinine exert a modestly more potent block of 5-HT 3 A versus 5-HT 3 AB receptor-mediated responses [976]. Known better as a partial agonist of nicotinic acetylcholine α4β2 receptors, varenicline is also an agonist of the 5-HT 3 A receptor [601]. Human [75,668], rat [419], mouse [628], guinea-pig [536] ferret [670] and canine [441] orthologues of the 5-HT 3 A receptor subunit have been cloned that exhibit intraspecies variations in receptor pharmacology. Notably, most ligands dis-play significantly reduced affinities at the guinea-pig 5-HT 3 receptor in comparison with other species. In addition to the agents listed in the table, native and recombinant 5-HT 3 receptors are subject to allosteric modulation by extracellular divalent cations, alcohols, several general anaesthetics and 5-hydroxy-and halidesubstituted indoles (see reviews [758,977,978,1042]).

Acid-sensing (proton-gated) ion channels (ASICs)
Ion channels → Ligand-gated ion channels → Acid-sensing (proton-gated) ion channels (ASICs) Overview: Acid-sensing ion channels (ASICs, nomenclature as agreed by NC-IUPHAR [475]) are members of a Na + channel superfamily that includes the epithelial Na + channel (ENaC), the FMRF-amide activated channel (FaNaC) of invertebrates, the degenerins (DEG) of Caenorhabitis elegans, channels in Drosophila melanogaster and 'orphan' channels that include BLINaC [858] and INaC [872] that have also been named BASICs, for bile acidactivated ion channels [1070]. ASIC subunits contain two TM domains and assemble as homo-or hetero-trimers [44,321,437] to form proton-gated, voltage-insensitive, Na + permeable, channels (reviewed in [339,1067] , has been identified. A fourth mammalian member of the family (ASIC4/SPASIC) does not support a proton-gated channel in heterologous expression systems and is reported to downregulate the expression of ASIC1a and ASIC3 [14,235,338,572]. ASIC channels are primarily expressed in central and peripheral neurons including nociceptors where they participate in neuronal sensitivity to acidosis. They have also been detected in taste receptor cells ( and bone (ASIC1-3). A neurotransmitter-like function of protons has been suggested, involving postsynaptically located ASICs of the CNS in functions such as learning and fear perception [242,516,1150], responses to focal ischemia [1090] and to axonal degeneration in autoimmune inflammation in a mouse model of multiple sclerosis [298], as well as seizures [1151] and pain [98,219,220,229]. Heterologously expressed heteromultimers form ion channels with differences in kinetics, ion selectivity, pH-sensitivity and sensitivity to blockers that resemble some of the native proton activated currents recorded from neurones [42,60,269,575]. Labelled ligands [ 125 I]psalmotoxin 1 (ASIC1a) (pK d 9.7) -Functional Characteristics ASIC1a: γ = 14pS P Na /P K = 5-13, P Na /P Ca =2.5 rapid activation rate (5.8-13.7 ms), rapid inactivation rate (1.2-4 s) @ pH 6.0, slow recovery (5.3-13s) @ pH 7.4 ASIC1b: γ = 19 pS P Na /P K =14.0, P Na P Ca rapid activation rate (9.9 ms), rapid inactivation rate (0.9-1.7 s) @ pH 6.0, slow recovery (4.4-7.7 s) @ pH 7.4 Zn 2+ , protein kinase C and serine proteases (reviewed in [475,1067]). Rapid acidification is required for activation of ASIC1 and ASIC3 due to fast inactivation/desensitization. pEC 50 values for H + -activation of either transient, or sustained, currents mediated by ASIC3 vary in the literature and may reflect species and/or methodological differences [43,204,1035]. The transient ASIC current component is Na + -selective (PNa/PK of about 10) [1035,1109] whereas the sustained current component that is observed with ASIC3 and some ASIC heteromers is non-selective between Na + and K + [204]. The reducing agents dithiothreitol (DTT) and glutathione (GSH) increase ASIC1a currents expressed in CHO cells and ASIC-like currents in sensory ganglia and central neurons [29,169] whereas oxidation, through the formation of intersubunit disulphide bonds, reduces currents mediated by ASIC1a [1134]. ASIC1a is also irreversibly modulated by extracellular serine proteases, such as trypsin, through proteolytic cleavage [1030]. Non-steroidal anti-inflammatory drugs (NSAIDs) are direct inhibitors of ASIC currents (reviewed in [58]). Extracellular Zn 2+ potentiates proton activation of homomeric and heteromeric channels incorporating ASIC2a, but not homomeric ASIC1a or ASIC3 channels [59]. However, removal of contaminating Zn 2+ by chelation reveals a high affinity block of homo-meric ASIC1a and heteromeric ASIC1a+ASIC2 channels by Zn 2+ indicating complex biphasic actions of the divalent [170]. Nitric oxide potentiates submaximal currents activated by H + mediated by ASIC1a, ASIC1b, ASIC2a and ASIC3 [121]. Ammonium ions activate ASIC channels (most likely ASIC1a) in midbrain dopaminergic neurones: that may be relevant to neuronal disorders associated with hyperammonemia [791]. The positive modulation of homomeric, heteromeric and native ASIC channels by the peptide FMRFamide and related substances, such as neuropeptides FF and SF, is reviewed in detail in [1015]. Inflammatory conditions and particular pro-inflammatory mediators such as arachidonic acid induce overexpression of ASIC-encoding genes and enhance ASIC currents [220,624,912]. The sustained current component mediated by ASIC3 is potentiated by hypertonic solutions in a manner that is synergistic with the effect of arachidonic acid [220]. ASIC3 is partially activated by the lipids lysophosphatidylcholine (LPC) and arachidonic acid [629]. Mit-Toxin, which is contained in the venom of the Texas coral snake, activates several ASIC subtypes [98]. Selective activation of ASIC3 by GMQ at a site separate from the proton binding site is potentiated by mild acidosis and reduced extracellular Ca 2+ [1124].

Epithelial sodium channel (ENaC)
Ion channels → Ligand-gated ion channels → Epithelial sodium channel (ENaC) Overview: The epithelial sodium channels (ENaC) are located on the apical membrane of epithelial cells in the distal kidney tubules, lung, respiratory tract, male and female reproductive tracts, sweat and salivary glands, placenta, colon and some other organs [123,246,354]. In these epithelia, ENaC allows flow of Na + ions from the extracellular fluid in the lumen into the epithelial cell. Na + ions are then pumped out of the cytoplasm into the interstitial fluid by the Na + /K + ATPase located on the basolateral membrane [1017]. As Na + is one of the major electrolytes in the extracellular fluid (ECF), osmolarity change initiated by the Na + flow is accompanied by a flow of water accompanying Na + ions [104]. Thus, ENaC has a central role in the regulation of ECF volume and blood pressure, especially via its function in the kidney [475,845]. The expression of ENaC subunits, hence its activity, is regulated by the renin-angotensin-aldosterone system, and other factors that are involved in electrolyte homeostasis [35,749,845]. In the respiratory tract and female reproductive tract large segments of the tracts are covered by multi-ciliated cells. In these cells ENaC has been shown to be located along the entire length of the cilia [263]. Cilial location greatly increases ENaC density per cell surface and allows ENaC to serve as a sensitive regulator of osmolarity of the periciliary fluid throughout the whole depth of the fluid bathing the cilia [263]. In contrast to ENaC, CFTR that is defective in cystic fibrosis is not located on non-cilial cell-surface [263]. Thus, ENaC function is also essential for the clearance of respiratory airways, transport of germ cells, fertilization, implantation and cell migration [263,846]. ENaC has been recently localized in the germinal epithelium of the testis, Sertoli cells, spermatozoa, along the epididymis ducts, and smooth muscle cells [890,891]. Evidence has been provided that rare mutations in ENaC are associated with female infertility [97].

GABA A receptors
Ion channels → Ligand-gated ion channels → GABA A receptors Overview: The GABA A receptor is a ligand-gated ion channel of the Cys-loop family that includes the nicotinic acetylcholine, 5-HT 3 and strychnine-sensitive glycine receptors. GABA A receptormediated inhibition within the CNS occurs by fast synaptic transmission, sustained tonic inhibition and temporally intermediate events that have been termed 'GABA A , slow' [132]. GABA A receptors exist as pentamers of 4TM subunits that form an intrinsic anion selective channel. Sequences of six α, three β, three γ, one δ, three ρ, one , one π and one GABA A receptor subunits have been reported in mammals [737,738,900,902]. The π-subunit is restricted to reproductive tissue. Alternatively spliced versions of many subunits exist (e.g. α4and α6-(both not functional) α5-, β2-, β3and γ2), along with RNA editing of the α3 subunit [196]. The three ρ-subunits, (ρ1-3) function as either homoor hetero-oligomeric assemblies [152,1135]. Receptors formed from ρ-subunits, because of their distinctive pharmacology that includes insensitivity to bicuculline, benzodiazepines and barbiturates, have sometimes been termed GABA C receptors [1135], but they are classified as GABA A receptors by NC-IUPHAR on the basis of structural and functional criteria [55,737,738].
Many GABA A receptor subtypes contain α-, βand γ-subunits with the likely stoichiometry 2α.2β.1γ [507,738]. It is thought that the majority of GABA A receptors harbour a single type of αand β -subunit variant. The α1β2γ2 hetero-oligomer constitutes the largest population of GABA A receptors in the CNS, followed by the α2β3γ2 and α3β3γ2 isoforms. Receptors that incorporate the α4-α5-or α6-subunit, or the β1-, γ1-, γ3-, δ-, -and -subunits, are less numerous, but they may nonetheless serve important functions. For example, extrasynaptically located receptors that con-tain α6and δ-subunits in cerebellar granule cells, or an α4and δ-subunit in dentate gyrus granule cells and thalamic neurones, mediate a tonic current that is important for neuronal excitability in response to ambient concentrations of GABA [76,279,671,886,917]. GABA binding occurs at the β+/αsubunit interface and the homologous γ+/αsubunits interface creates the benzodiazepine site. A second site for benzodiazepine binding has recently been postulated to occur at the α+/βinterface ( [824]; reviewed by [901]). The particular α-and γ-subunit isoforms exhibit marked effects on recognition and/or efficacy at the benzodiazepine site. Thus, receptors incorporating either α4or α6-subunits are not recognised by 'classical' benzodiazepines, such as flunitrazepam (but see [1118]). The trafficking, cell surface expression, internalisation and function of GABA A receptors and their subunits are discussed in detail in several recent reviews [164,427,602,1019] Searchable database: http://www.guidetopharmacology.org/index.jsp GABA A receptors S149 Full Contents of ConciseGuide: http://onlinelibrary.wiley.com/doi/10.1111/bph.14749/full but one point worthy of note is that receptors incorporating the γ2 subunit (except when associated with α5) cluster at the postsynaptic membrane (but may distribute dynamically between synaptic and extrasynaptic locations), whereas as those incorporating the d subunit appear to be exclusively extrasynaptic. NC-IUPHAR [55,738] class the GABA A receptors according to their subunit structure, pharmacology and receptor function. Currently, eleven native GABA A receptors are classed as conclusively identified (i.e., α1β2γ2, α1βγ2, α3βγ2, α4βγ2, α4β2δ, α4β3δ, α5βγ2, α6βγ2, α6β2δ, α6β3δ and ρ) with further receptor isoforms occurring with high probability, or only tentatively [737,738]. It is beyond the scope of this Guide to discuss the pharmacology of individual GABA A receptor isoforms in detail; such information can be gleaned in the reviews [55,300,448,507,518,686,737,738,900] and [37,38]. Agents that discriminate between α-subunit isoforms are noted in the table and additional agents that demonstrate selectivity between receptor isoforms, for example via β-subunit selectivity, are indicated in the text below. The distinctive agonist and antagonist pharmacology of ρ receptors is summarised in the table and additional aspects are reviewed in [152,449,702,1135].
Several high-resolution cryo-electron microscopy structures have been described in which the full-length human α1β3γ2L GABA A receptor in lipid nanodiscs is bound to the channel-blocker picrotoxin, the competitive antagonist bicuculline, the agonist GABA (γ-aminobutyric acid), and the classical benzodiazepines alprazolam and diazepam [634]. Comments Zn 2+ is an endogenous allosteric regulator and causes potent inhibition of receptors formed from binary combinations of α and β subunits, incorporation of a δ or γ subunit causes a modest, or pronounced, reduction in inhibitory potency, respectively [517] Zn 2+ is an endogenous allosteric regulator and causes potent inhibition of receptors formed from binary combinations of α and β subunits, incorporation of a δ or γ subunit causes a modest, or pronounced, reduction in inhibitory potency, respectively [517]  Comments Zn 2+ is an endogenous allosteric regulator and causes potent inhibition of receptors formed from binary combinations of α and β subunits, incorporation of a δ or γ subunit causes a modest, or pronounced, reduction in inhibitory potency, respectively [517] Diazepam and flunitrazepam are not active at this subunit. Zn 2+ is an endogenous allosteric regulator and causes potent inhibition of receptors formed from binary combinations of α and β subunits, incorporation of a δ or γ subunit causes a modest, or pronounced, reduction in inhibitory potency, respectively [517]. [  Comments Zn 2+ is an endogenous allosteric regulator and causes potent inhibition of receptors formed from binary combinations of α and β subunits, incorporation of a δ or γ subunit causes a modest, or pronounced, reduction in inhibitory potency, respectively [517] Diazepam and flunitrazepam are not active at this subunit. Zn 2+ is an endogenous allosteric regulator and causes potent inhibition of receptors formed from binary combinations of α and β subunits, incorporation of a δ or γ subunit causes a modest, or pronounced, reduction in inhibitory potency, respectively [517]. [ 469,518]. For example, gaboxadol is a partial agonist at receptors with the subunit composition α4β3γ2, but elicits currents in excess of those evoked by GABA at the α4β3δ receptor where GABA itself is a low efficacy agonist [86,111]. The antagonists bicuculline and gabazine differ in their ability to suppress spontaneous openings of the GABA A receptor, the former being more effective [982]. The presence of the γ subunit within the heterotrimeric complex reduces the potency and efficacy of agonists [941]. The GABA A receptor contains distinct allosteric sites that bind barbiturates and endogenous (e.g., 5α-pregnan-3α-ol-20-one) and synthetic (e.g., alphaxalone) neuroactive steroids in a diastereo-or enantio-selective manner [77,370,399,1010]. Picrotoxinin and TBPS act at an allosteric site within the chloride channel pore to negatively regulate channel activity; negative allosteric regulation by γ-butyrolactone derivatives also involves the picrotoxinin site, whereas positive allosteric regulation by such compounds is proposed to occur at a distinct locus. Many intravenous (e.g., etomidate, propofol) and inhala-tional (e.g., halothane, isoflurane) anaesthetics and alcohols also exert a regulatory influence upon GABA A receptor activity [101,736]. Specific amino acid residues within GABA A receptor αand β-subunits that influence allosteric regulation by anaesthetic and non-anaesthetic compounds have been identified [368,399]. Photoaffinity labelling of distinct amino acid residues within purified GABA A receptors by the etomidate derivative, [ 3 H]azietomidate, has also been demonstrated [561] and this binding subject to positive allosteric regulation by anaesthetic steroids [560]. An array of natural products including flavonoid and terpenoid compounds exert varied actions at GABA A receptors (reviewed in detail in [448]). In addition to the agents listed in the  [1032]]. It should be noted that the apparent selectivity of some positive allosteric modulators (e.g., neurosteroids such as 5α-pregnan-3α-ol-20-one for δ-subunit-containing receptors (e.g., α1β3δ) may be a consequence of the unusually low efficacy of GABA at this receptor isoform [76,86].

Glycine receptors
Ion channels → Ligand-gated ion channels → Glycine receptors Overview: The inhibitory glycine receptor (nomenclature as agreed by the NC-IUPHAR Subcommittee on Glycine Receptors) is a member of the Cys-loop superfamily of transmittergated ion channels that includes the zinc activated channels, GABA A , nicotinic acetylcholine and 5-HT 3 receptors [603]. The receptor is expressed either as a homo-pentamer of α subunits, or a complex now thought to harbour 2α and 3β subunits [82,336], that contain an intrinsic anion channel. Four differentially expressed isoforms of the α-subunit (α1-α4) and one variant of the β-subunit (β1, GLRB, P48167) have been identified by genomic and cDNA cloning. Further diversity originates from alternative splicing of the primary gene transcripts for α1 (α1 INS and α1 del ), α2 (α2A and α2B), α3 (α3S and α3L) and β (β 7) subunits and by mRNA editing of the α2 and α3 subunit [260,647,731]. Both α2 splicing and α3 mRNA editing can produce subunits (i.e., α2B and α3P185L) with enhanced agonist sensitivity. Predominantly, the mature form of the receptor contains α1 (or α3) and β subunits while the immature form is mostly composed of only α2 subunits. RNA transcripts encoding the α4-subunit have not been detected in adult humans. The N-terminal domain of the α-subunit contains both the agonist and strychnine binding sites that consist of several discontinuous regions of amino acids. Inclusion of the β-subunit in the pentameric glycine receptor contributes to agonist binding, reduces single channel conductance and alters pharmacology. The β-subunit also anchors the receptor, via an amphipathic sequence within the large intracellular loop region, to gephyrin. The latter is a cytoskeletal attachment protein that binds to a number of subsynaptic proteins involved in cytoskeletal structure and thus clusters and anchors hetero-oligomeric Selective antagonists nifedipine (when co-expressed with the α1 subunit) (pIC 50 5.9), pregnenolone sulphate (when co-expressed with the α1 subunit) (pK i 5.6), tropisetron (when co-expressed with the α2 subunit) (pK i 5.3), pregnenolone sulphate (when co-expressed with the α2 subunit) (pK i 5), nifedipine (when co-expressed with the α3 subunit) (pIC 50 4.9), bilobalide (when co-expressed with the α2 subunit) (pIC 50 4.3), bilobalide (when co-expressed with the α1 subunit) (pIC 50 3.7), ginkgolide X (when co-expressed with the α1 subunit) (pIC 50 >3.5), ginkgolide X (when co-expressed with the α2 subunit) (pIC 50 >3.5) Channel blockers ginkgolide B (when co-expressed with the α2 subunit) (pIC 50 6.1-6.9), ginkgolide B (when co-expressed with the α1 subunit) (pIC 50 5.6-6.7), ginkgolide B (when co-expressed with the α3 subunit) (pIC 50  Comments: Data in the table refer to homo-oligomeric assemblies of the α-subunit, significant changes introduced by coexpression of the β1 subunit are indicated in parenthesis. Not all glycine receptor ligands are listed within the table, but some that may be useful in distinguishing between glycine receptor isoforms are indicated (see detailed view pages for each subunit: α1, α2, α3, α4, β ). Pregnenolone sulphate, tropisetron and colchicine, for example, although not selective antagonists of glycine receptors, are included for this purpose. Strychnine is a potent and selective competitive glycine receptor antagonist with affinities in the range 5-15 nM. RU5135 demonstrates comparable potency, but additionally blocks GABA A receptors. There are conflicting reports concerning the ability of cannabinoids to inhibit [594], or potentiate and at high concentrations activate [12,217,366,1089,1111] glycine receptors. Nonetheless, cannabinoid analogues may hold promise in distinguishing between glycine receptor subtypes [1111]. In addition, potentiation of glycine receptor activity by cannabinoids has been claimed to contribute to cannabis-induced analgesia relying on Ser296/307 (α1/α3) in M3 [1089]. Several analogues of muscimol and piperidine act as agonists and antagonists of both glycine and GABA A receptors. Picrotoxin acts as an allosteric inhibitor that appears to bind within the pore, and shows strong selectivity towards homomeric receptors. While its components, picrotoxinin and picrotin, have equal potencies at α1 receptors, their potencies at α2 and α3 receptors differ modestly and may allow some distinction between different receptor types [1112]. Binding of picrotoxin within the pore has been demonstrated in the crystal structure of the related C. elegans GluCl Cys-loop receptor [373]. In addition to the compounds listed in the table, numerous agents act as allosteric regulators of glycine receptors (comprehensively reviewed in [537,604,1062,1116]). Zn 2+ acts through distinct binding sites of high-and lowaffinity to allosterically enhance channel function at low (<10 μM) concentrations and inhibits responses at higher concentrations in a subunit selective manner [661]. The effect of Zn 2+ is somewhat mimicked by Ni 2+ . Endogenous Zn 2+ is essential for normal glycinergic neurotransmission mediated by α1 subunitcontaining receptors [383]. Elevation of intracellular Ca 2+ produces fast potentiation of glycine receptor-mediated responses. Dideoxyforskolin (4 μM) and tamoxifen (0.2-5 μM) both potentiate responses to low glycine concentrations (15 μM), but act as inhibitors at higher glycine concentrations (100 μM). Additional modulatory agents that enhance glycine receptor function include inhalational, and several intravenous general anaesthetics (e.g. minaxolone, propofol and pentobarbitone) and certain neurosteroids. Ethanol and higher order n-alcohols also enhance glycine receptor function although whether this occurs by a direct allosteric action at the receptor [633], or through βγ subunits [1113] is debated. Recent crystal structures of the bacterial homologue, GLIC, have identified transmembrane binding pockets for both anaesthetics [724] and alcohols [401]. Solvents inhaled as drugs of abuse (e.g. toluene, 1-1-1-trichloroethane) may act at sites that overlap with those recognising alcohols and volatile anaesthetics to produce potentiation of glycine receptor function. The function of glycine receptors formed as homomeric complexes of α1 or α2 subunits, or hetero-oligomers of α1/β or α2/β subunits, is differentially affected by the 5-HT 3 receptor antagonist tropisetron (ICS 205-930) which may evoke potentiation (which may occur within the femtomolar range at the homomeric glycine α1 receptor), or inhibition, depending upon the subunit composition of the receptor and the concentrations of the modulator and glycine employed. Potentiation and inhibition by tropeines involves different binding modes [621]. Additional tropeines, including atropine, modulate glycine receptor activity.

Ionotropic glutamate receptors
Ion channels → Ligand-gated ion channels → Ionotropic glutamate receptors Overview: The ionotropic glutamate receptors comprise members of the NMDA (N-methyl-D-aspartate), AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid) and kainate receptor classes, named originally according to their preferred, synthetic, agonist [226,588,997]. Receptor heterogeneity within each class arises from the homo-oligomeric, or hetero-oligomeric, assembly of distinct subunits into cation-selective tetramers. Each subunit of the tetrameric complex comprises an extracellular amino terminal domain (ATD), an extracellular ligand binding domain (LBD), three transmembrane domains composed of three membrane spans (M1, M3 and M4), a channel lining re-entrant 'p-loop' (M2) located between M1 and M3 and an intracellular carboxy-terminal domain (CTD) [458,523,639,691,997]. The X-ray structure of a homomeric ionotropic glutamate receptor (GluA2 -see below) has recently been solved at 3.6Å resolution [919] and although providing the most complete structural information current available may not representative of the subunit arrangement of, for example, the heteromeric NMDA receptors [466]. It is beyond the scope of this supplement to discuss the pharmacology of individual ionotropic glutamate receptor isoforms in detail; such information can be gleaned from [155,190,226,266,433,434,479,750,751,752,997,1086]. Agents that discriminate between subunit isoforms are, where appropriate, noted in the tables and additional compounds that distinguish between receptor isoforms are indicated in the text below.
The classification of glutamate receptor subunits has been re-addressed by NC-IUPHAR [183]. The scheme developed recommends a nomenclature for ionotropic glutamate receptor subunits that is adopted here. AMPA and Kainate receptors AMPA receptors assemble as homomers, or heteromers, that may be drawn from GluA1, GluA2, GluA3 and GluA4 subunits. Transmembrane AMPA receptor regulatory proteins (TARPs) of class I (i.e. γ2, γ3, γ4 and γ8) act, with variable stoichiometry, as auxiliary subunits to AMPA receptors and influence their trafficking, single channel conductance gating and pharmacology (reviewed in [270,426,663,991]). Functional kainate receptors can be expressed as homomers of GluK1, GluK2 or GluK3 subunits. GluK1-3 subunits are also capable of assembling into heterotetramers (e.g. GluK1/K2; [553,783,795]). Two additional kainate receptor subunits, GluK4 and GluK5, when expressed individu-ally, form high affinity binding sites for kainate, but lack function, but can form heteromers when expressed with GluK1-3 subunits (e.g. GluK2/K5; reviewed in [433,783,795]). Kainate receptors may also exhibit 'metabotropic' functions [553,841]. As found for AMPA receptors, kainate receptors are modulated by auxiliary subunits (Neto proteins, [554,783]). An important function difference between AMPA and kainate receptors is that the latter require extracellular Na+ and Cl-for their activation [105,803]. RNA encoding the GluA2 subunit undergoes extensive RNA editing in which the codon encoding a p-loop glutamine residue (Q) is converted to one encoding arginine (R). This Q/R site strongly influences the biophysical properties of the receptor. Recombinant AMPA receptors lacking RNA edited GluA2 subunits are: (1) permeable to Ca 2+ ; (2) blocked by intracellular polyamines at depolarized potentials causing inward rectification (the latter being reduced by TARPs); (3) blocked by extracellular argiotoxin and Joro spider toxins and (4) demonstrate higher channel conductances than receptors containing the edited form of GluA2 [417,885]. GluK1 and GluK2, but not other kainate receptor subunits, are similarly edited and broadly similar functional characteristics apply to kainate receptors lacking either an RNA edited GluK1, or GluK2, subunit [553,783]. Native AMPA and kainate receptors displaying differential channel conductances, Ca 2+ permeabilites and sensitivity to block by intracellular polyamines have been identified [189,417,583]. GluA1-4 can exist as two variants generated by alternative splicing (termed 'flip' and 'flop') that differ in their desensitization kinetics and their desensitization in the presence of cyclothiazide which stabilises the nondesensitized state. TARPs also stabilise the non-desensitized conformation of AMPA receptors and facilitate the action of cyclothiazide [663]. Splice variants of GluK1-3 also exist which affects their trafficking [553,783].  [426,663]. AMPA is weak partial agonist at GluK1 and at heteromeric assemblies of GluK1/GluK2, GluK1/GluK5 and GluK2/GluK5 [433]. Quinoxalinediones such as CNQX and NBQX show limited selectivity between AMPA and kainate receptors. Tezampanel also has kainate (GluK1) receptor activity as has GYKI53655 (GluK3 and GluK2/GluK3) [433]. ATPO is a potent competitive antagonist of AMPA receptors, has a weaker antagonist action at kainate receptors comprising GluK1 subunits, but is devoid of activity at kainate receptors formed from GluK2 or GluK2/GluK5 subunits. The pharmacological activity of ATPO resides with the (S)-enantiomer. ACET and UBP310 may block GluK3, in addition to GluK1 [39,782].

NMDA receptors
NMDA receptors assemble as obligate heteromers that may be drawn from GluN1, GluN2A, GluN2B, GluN2C, GluN2D, GluN3A and GluN3B subunits. Alternative splicing can generate eight isoforms of GluN1 with differing pharmacological properties. Various splice variants of GluN2B, 2C, 2D and GluN3A have also been reported. Activation of NMDA receptors containing GluN1 and GluN2 subunits requires the binding of two agonists, glutamate to the S1 and S2 regions of the GluN2 subunit and glycine to S1 and S2 regions of the GluN1 subunit [156,265]. The minimal requirement for efficient functional expression of NMDA receptors in vitro is a di-heteromeric assembly of GluN1 and at least one GluN2 subunit variant, as a dimer of heterodimers arrangement in the extracellular domain [303,466,639]. However, more complex tri-heteromeric assemblies, incorporating multiple subtypes of GluN2 subunit, or GluN3 subunits, can be generatedin vitro and occurin vivo. The NMDA receptor channel commonly has a high relative permeability to Ca 2+ and is blocked, in a voltagedependent manner, by Mg 2+ such that at resting potentials the response is substantially inhibited.

Nomenclature
GluN1 GluN2A GluN2B    GluN2D Channel blockers phencyclidine (pIC 50 7.1) [240], ketamine (pIC 50 [155,240,266,524,752,997]. In addition to the glutamate and glycine binding sites documented in the table, physiologically important inhibitory modulatory sites exist for Mg 2+ , Zn 2+ , and protons [190,226,997]. Voltage-independent inhibition by Zn 2+ binding with high affinity within the ATD is highly subunit selective (GluN2A GluN2B > GluN2C ≥ GluN2D; [752,997]). The receptor is also allosterically modulated, in both positive and negative directions, by endogenous neuroactive steroids in a subunit dependent manner [398,622]. Tonic proton blockade of NMDA receptor function is alleviated by polyamines and the inclusion of exon 5 within GluN1 subunit splice variants, whereas the non-competitive antagonists ifenprodil and traxoprodil increase the fraction of receptors blocked by protons at ambient concentration. Inclusion of exon 5 also abolishes potentiation by polyamines and inhibition by Zn 2+ that occurs through binding in the ATD [996]. Ifenprodil, traxoprodil, haloperidol, felbamate and Ro 8-4304 discriminate between recombinant NMDA receptors assembled from GluN1 and either GluN2A, or GluN2B, subunits by acting as selective, non-competitive, antagonists of heterooligomers incorporating GluN2B through a binding site at the ATD GluN1/GluN2B subunit interface [466]. LY233536 is a competitive antagonist that also displays selectivity for GluN2B over GluN2A subunit-containing receptors. Similarly, CGP61594 is a photoaffinity label that interacts selectively with receptors incorporating GluN2B versus GluN2A, GluN2D and, to a lesser extent, GluN2C subunits. TCN 201 and TCN 213 have recently been shown to block GluN2A NMDA receptors selectively by a mechanism that involves allosteric inhibition of glycine binding to the GluN1 site [81,257,353,643]. In addition to influencing the pharmacological profile of the NMDA receptor, the identity of the GluN2 subunit co-assembled with GluN1 is an important determinant of biophysical properties that include sensitivity to block by Mg 2+ , single-channel conductance and maximal open prob-ablity and channel deactivation time [190,265,317]. Incorporation of the GluN3A subunit into tri-heteromers containing GluN1 and GluN2 subunits is associated with decreased single-channel conductance, reduced permeability to Ca 2+ and decreased susceptibility to block by Mg 2+ [140,369]. Reduced permeability to Ca 2+ has also been observed following the inclusion of GluN3B in tri-heteromers. The expression of GluN3A, or GluN3B, with GluN1 alone forms, in Xenopus laevis oocytes, a cation channel with unique properties that include activation by glycine (but not NMDA), lack of permeation by Ca 2+ and resistance to blockade by Mg 2+ and NMDA receptor antagonists [147]. The function of heteromers composed of GluN1 and GluN3A is enhanced by Zn 2+ , or glycine site antagonists, binding to the GluN1 subunit [611]. Zn 2+ also directly activates such complexes. The co-expression of GluN1, GluN3A and GluN3B appears to be required to form glycine-activated receptors in mammalian cell hosts [918].

IP 3 receptors
Ion channels → Ligand-gated ion channels → IP 3 receptor Overview: The inositol 1,4,5-trisphosphate receptors (IP 3 R) are ligand-gated Ca 2+ -release channels on intracellular Ca 2+ store sites (such as the endoplasmic reticulum). They are responsible for the mobilization of intracellular Ca 2+ stores and play an important role in intracellular Ca 2+ signalling in a wide variety of cell types. Three different gene products (types I-III) have been isolated, which assemble as large tetrameric structures. IP 3 Rs are closely associated with certain proteins: calmodulin (CALM1 CALM2 CALM3, P62158) and FKBP (and calcineurin via FKBP). They are phosphorylated by PKA, PKC, PKG and CaMKII. Nomenclature

Nicotinic acetylcholine receptors
Ion channels → Ligand-gated ion channels → Nicotinic acetylcholine receptors Overview: Nicotinic acetylcholine receptors are members of the Cys-loop family of transmitter-gated ion channels that includes the GABA A , strychnine-sensitive glycine and 5-HT 3 receptors [18,659,903,963,1083]. All nicotinic receptors are pentamers in which each of the five subunits contains four α-helical transmembrane domains. Genes encoding a total of 17 subunits (α1-10, β1-4, γ, δ and ) have been identified [459]. All subunits with the exception of α8 (present in avian species) have been identified in mammals. All α subunits possess two tandem cysteine residues near to the site involved in acetylcholine binding, and subunits not named α lack these residues [659]. The orthosteric ligand binding site is formed by residues within at least three peptide domains on the α subunit (principal component), and three on the adjacent subunit (complementary component). nAChRs contain several allosteric modulatory sites. One such site, for positive allosteric modulators (PAMs) and allosteric agonists, has been proposed to reside within an intrasubunit cavity between the four transmembrane domains [318, 1119]; see also [373]). The high resolution crystal structure of the molluscan acetylcholine binding protein, a structural homologue of the extracellular binding domain of a nicotinic receptor pentamer, in complex with several nicotinic receptor ligands (e.g. [142]) and the crystal structure of the extracellular domain of the α1 subunit bound to α-bungarotoxin at 1.94 Å resolution [212], has revealed the orthosteric binding site in detail (reviewed in [145,459,850,903]). Nicotinic receptors at the somatic neuromuscular junction of adult animals have the stoichiometry (α1) 2 β1δ , whereas an extrajunctional (α1) 2 β1γδ receptor predominates in embryonic and denervated skeletal muscle and other pathological states. Other nicotinic receptors are assembled as combinations of α(2-6) and β(2-4) subunits. For α2, α3, α4 and β2 and β4 subunits, pairwise combinations of α and β (e.g. α3β4 and α4β2) are sufficient to form a functional receptor in vitro, but far more complex isoforms may exist in vivo (reviewed in [324,325,659]). There is strong evidence that the pairwise assembly of some α and β subunits can occur with variable stoichiometry [e.g. (α4) 2 (β2) 2 or (α4) 3 (β2) 2 ] which influences the biophysical and pharmacological properties of the receptor [659]. α5 and β3 subunits lack function when expressed alone, or pairwise, but participate in the formation of functional hetero-oligomeric receptors when expressed as a third subunit with another α and β pair [e.g. α4α5αβ2, α4αβ2β3, α5α6β2, see [659] for further examples]. The α6 subunit can form a functional receptor when co-expressed with β4 in vitro, but more efficient expression ensues from incorporation of a third partner, such as β3 [1108]. The α7, α8, and α9 subunits form functional homo-oligomers, but can also combine with a second subunit to constitute a hetero-oligomeric assembly (e.g. α7β2 and α9α10). For functional expression of the α10 subunit, co-assembly with α9 is necessary. The latter, along with the α10 subunit, appears to be largely confined to cochlear and vestibular hair cells. Comprehensive listings of nicotinic receptor subunit combinations identified from recombinant expression systems, or in vivo, are given in [659]. In addition, numerous proteins interact with nicotinic ACh receptors modifying their assembly, trafficking to and from the cell surface, and activation by ACh (reviewed by [32,451,658]).
The nicotinic receptor Subcommittee of NC-IUPHAR has recommended a nomenclature and classification scheme for nicotinic acetylcholine (nACh) receptors based on the subunit composition of known, naturally-and/or heterologously-expressed nACh receptor subtypes [600]. Headings for this table reflect abbreviations designating nACh receptor subtypes based on the predominant α subunit contained in that receptor subtype. An asterisk following the indicated α subunit denotes that other subunits are known to, or may, assemble with the indicated α subunit to form the designated nACh receptor subtype(s). Where subunit stoichiometries within a specific nACh receptor subtype are known, numbers of a particular subunit larger than 1 are indicated by a subscript following the subunit (enclosed in parentheses -see also [183]).

Overview:
The zinc-activated channel (ZAC, nomenclature as agreed by the NC-IUPHAR Subcommittee for the Zinc Activated Channel) is a member of the Cys-loop family that includes the nicotinic ACh, 5-HT 3 , GABA A and strychnine-sensitive glycine receptors [200,400,995]. The channel is likely to exist as a homopentamer of 4TM subunits that form an intrinsic cation selective channel equipermeable to Na + , K + and Cs + , but impermeable to Ca 2+ and Mg 2+ [995]. ZAC displays constitutive activity that can be blocked by tubocurarine and high concentrations of Ca 2+ [995]. Although denoted ZAC, the channel is more potently activated by protons and copper, with greater and lesser efficacy than zinc, respectively [995]. ZAC is present in the human, chimpanzee, dog, cow and opossum genomes, but is functionally absent from mouse, or rat, genomes [200,400].

Voltage-gated ion channels
Ion channels → Voltage-gated ion channels Overview: The voltage-gated ion channels and their structural relatives comprise a superfamily encoded by at least 143 genes in the human genome and are therefore one of the largest superfamilies of signal transduction proteins, following the G protein-coupled receptors and the protein kinases in number [137]. In addition to their prominence in signal transduction, these ion channels are also among the most common drug targets. As for other large protein superfamilies, understanding the molecular re-lationships among family members, developing a unified, rational nomenclature for the ion channel families and subfamilies, and assigning physiological functions and pharmacological significance to each family member has been an important challenge.  [445,586,815], in common with a putative 2TM auxiliary CatSperβ protein [581] and two putative 1TM associated CatSperγ and CatSperδ proteins [172,1044], are restricted to the testis and localised to the principle piece of sperm tail. The novel cross-species CatSper channel inhibitor, RU1968, has been proposed as a useful tool to aid characterisation of native CatSper channels [831]. Two-pore channels (TPCs) are structurally related to CatSpers, Ca V s and Na V s. TPCs have a 2x6TM structure with twice the num-ber of TMs of CatSpers and half that of Ca V s. There are three animal TPCs (TPC1-TPC3). Humans have TPC1 and TPC2, but not TPC3. TPC1 and TPC2 are localized in endosomes and lysosomes [122]. TPC3 is also found on the plasma membrane and forms a voltage-activated, non-inactivating Na + channel [125]. All the three TPCs are Na + -selective under whole-cell or whole-organelle patch clamp recording [126,127,1087]. The channels may also conduct Ca 2+ [674].  [820,830]. The properties of CatSper1 tabulated above are derived from whole cell voltage-clamp recordings comparing currents endogenous to spermatozoa isolated from the corpus epididymis of wild-type andCatsper1 (-/-) mice [488] and also mature human sperm [577,940]. I CatSper is also undetectable in the spermatozoa of Catsper2 (-/-) ,Catsper3 (-/-) , Catsper4 (-/-) , or CatSperδ (-/-) mice, and CatSper 1 associates with CatSper 2, 3, 4, β, γ, and δ [172,581,815]. Moreover, targeted disruption of Catsper1, 2, 3, 4, or δ genes results in an identical phenotype in which spermatozoa fail to exhibit the hyperactive movement (whip-like flagellar beats) necessary for penetration of the egg cumulus and zona pellucida and subsequent fertilization. Such disruptions are associated with a deficit in alkalinization and depolarization-evoked Ca 2+ entry into spermatozoa [135,172,815]. Thus, it is likely that the CatSper pore is formed by a heterotetramer of CatSpers1-4 [815] in association with the auxiliary subunits (β, γ, δ) that are also es-sential for function [172]. CatSper channels are required for the increase in intracellular Ca 2+ concentration in sperm evoked by egg zona pellucida glycoproteins [1087]. Mouse and human sperm swim against the fluid flow and Ca 2+ signaling through CatSper is required for the rheotaxis [657]. In vivo, CatSper1-null spermatozoa cannot ascend the female reproductive tracts efficiently [173,385]. It has been shown that CatSper channels form four linear Ca 2+ signaling domains along the flagella, which orchestrate capacitation-associated tyrosine phosphorylation [173].The driving force for Ca 2+ entry is principally determined by a mildly outwardly rectifying K + channel (KSper) that, like CatSpers, is activated by intracellular alkalinization [695]. Mouse KSper is encoded by mSlo3, a protein detected only in testis [632,695,1132]. In human sperm, such alkalinization may result from the activation of H v 1, a proton channel [578]. Mutations in CatSpers are associated with syndromic and non-syndromic male infertility [374]. In human ejaculated spermatozoa, progesterone (<50 nM) potentiates the CatSper current by a non-genomic mechanism and acts synergistically with intracellular alkalinisation [577,940]. Sperm cells from infertile patients with a deletion in CatSper2 gene lack I CatSper and the progesterone response [913]. In addition, certain prostaglandins (e.g. PGF 1α , PGE 1 ) also potentiate CatSper mediated currents [577,940]. In human sperm, CatSper channels are also activated by various small molecules including endocrine disrupting chemicals (EDC) and proposed as a polymodal sensor [107,107]. TPCs are the major Na + conductance in lysosomes; knocking out TPC1 and TPC2 eliminates the Na + conductance and renders the organelle's membrane potential insensitive to changes in [Na + ] (31). The channels are regulated by luminal pH [126], PI(3,5)P2

Further reading on Voltage-gated ion channels
[1051], intracellular ATP and extracellular amino acids [127]. TPCs are also involved in the NAADP-activated Ca 2+ release from lysosomal Ca 2+ stores [122,674]. Mice lacking TPCs are viable but have phenotypes including compromised lysosomal pH stability, reduced physical endurance [127], resistance to Ebola viral infection [860] and fatty liver [331]. No major human diseaseassociated TPC mutation has been reported. Cyclic nucleotide-regulated channels Ion channels → Voltage-gated ion channels → Cyclic nucleotide-regulated channels

Further reading on CatSper and Two-Pore channels
Overview: Cyclic nucleotide-gated (CNG) channels are responsible for signalling in the primary sensory cells of the vertebrate visual and olfactory systems. CNG channels are voltage-independent cation channels formed as tetramers. Each subunit has 6TM, with the pore-forming domain between TM5 and TM6. CNG channels were first found in rod photoreceptors [287,472], where light signals through rhodopsin and transducin to stimulate phosphodiesterase and reduce intracellular cyclic GMP level. This results in a closure of CNG channels and a reduced 'dark current'. Similar channels were found in the cilia of olfactory neurons [692] and the pineal gland [241]. The cyclic nucleotides bind to a domain in the C terminus of the sub-unit protein: other channels directly binding cyclic nucleotides include HCN, eag and certain plant potassium channels.
A standardised nomenclature for CNG and HCN channels has been proposed by the NC-IUPHAR subcommittee on voltage-gated ion channels [388].

Hyperpolarisation-activated, cyclic nucleotide-gated (HCN)
The hyperpolarisation-activated, cyclic nucleotide-gated (HCN) channels are cation channels that are activated by hyperpolarisation at voltages negative to˜-50 mV. The cyclic nucleotides cyclic AMP and cyclic GMP directly activate the channels and shift the activation curves of HCN channels to more positive voltages, thereby enhancing channel activity. HCN channels underlie pacemaker currents found in many excitable cells including cardiac cells and neurons [225,753]. In native cells, these currents have a variety of names, such as I h , I q andI f . The four known HCN channels have six transmembrane domains and form tetramers.
It is believed that the channels can form heteromers with each other, as has been shown for HCN1 and HCN4 [22]. High resolution structural studies of CNG and HCN channels has provided insight into the the gating processes of these channels [540,563].

Potassium channels
Ion channels → Voltage-gated ion channels → Potassium channels Overview: Activation of potassium channels regulates excitability and can control the shape of the action potential waveform.
They are present in all cells within the body and can influence processes as diverse as cognition, muscle contraction and hormone secretion. Potassium channels are subdivided into families, based on their structural and functional properties. The largest family consists of potassium channels that activated by membrane depolarization, with other families consisting of channels that are either activated by a rise of intracellular calcium ions or are constitutively active. A standardised nomenclature for potassium channels has been proposed by the NC-IUPHAR subcommittees on potassium channels [320,346,520,1064], which has placed cloned channels into groups based on gene family and structure of channels that exhibit 6, 4 or 2 transmembrane domains (TM).

Calcium-and sodium-activated potassium channels
Ion channels → Voltage-gated ion channels → Potassium channels → Calcium-and sodium-activated potassium channels Overview: Calcium-and sodium-activated potassium channels are members of the 6TM family of K channels which comprises the voltage-gated K V subfamilies, including the KCNQ subfamily, the EAG subfamily (which includes herg channels), the Ca 2+ -activated Slo subfamily (actually with 6 or 7TM) and the Ca 2+and Na + -activated SK subfamily (nomenclature as agreed by the NC-IUPHAR Subcommittee on Calcium-and sodiumactivated potassium channels [457]). As for the 2TM fam-ily, the pore-forming a subunits form tetramers and heteromeric channels may be formed within subfamilies (e.g. K V 1.

Inwardly rectifying potassium channels
Ion channels → Voltage-gated ion channels → Potassium channels → Inwardly rectifying potassium channels Overview: The 2TM domain family of K channels are also known as the inward-rectifier K channel family. This family includes the strong inward-rectifier K channels (K ir 2.x) that are constitutively active, the G-protein-activated inward-rectifier K channels (K ir 3.x) and the ATP-sensitive K channels (K ir 6.x, which combine with sulphonylurea receptors (SUR1-3)). The pore-forming α subunits form tetramers, and heteromeric channels may be formed within subfamilies (e.g. K ir 3.2 with K ir 3.3).

Two P domain potassium channels
Ion channels → Voltage-gated ion channels → Potassium channels → Two P domain potassium channels Overview: The 4TM family of K channels mediate many of the background potassium currents observed in native cells. They are open across the physiological voltage-range and are regulated by a wide array of neurotransmitters and biochemical mediators. The pore-forming α-subunit contains two pore loop (P) domains and two subunits assemble to form one ion conduction pathway lined by four P domains. It is important to note that single channels do not have two pores but that each subunit has two P domains in its primary sequence; hence the name two P domain, or K 2P channels (and not two-pore channels). Some of the K 2P subunits can form heterodimers across subfamilies (e.g. K 2P 3.1 with K 2P 9.1). The nomenclature of 4TM K channels in the literature is still a mixture of IUPHAR and common names. The suggested division into subfamilies, described in the More detailed introduction, is based on similarities in both structural and functional properties within subfamilies and this explains the "common abbreviation" nomenclature in the tables below.  [763], isoflurane (studied at 1-5 mM) [763] halothane (studied at 1-10 mM) [538] riluzole (studied at 1-100 μM) [248] Inhibitors -norfluoxetine (pIC 50  Knock-out of the kcnk3 gene leads to a prolonged QT interval in mice [207] and disrupted development of the adrenal cortex [365]. K 2P 3.1 is inhibited by acid pH o with a pK a of 6.4 [591]. K 2P 3 forms heterodimers with K 2P 1 [800] and K 2P 9 [193]. K 2P 4 is activated by membrane stretch [614], and increased temperature (12 to 20-fold between 17 and 40°C [462]) and can heterodimerize with K 2P 2 [93].  [1009], itch [136], and airway disease [326,1068], are available. The pharmacology of most TRP channels has been advanced in recent years. Broad spectrum agents are listed in the tables along with more selective, or recently recognised, ligands that are flagged by the inclusion of a primary reference. See Rubaiy (2019) for a review of pharmacological tools for TRPC1/C4/C5 channels [847]. Most TRP channels are regulated by phosphoinostides such as PtIns(4,5)P 2 although the effects reported are often complex, occasionally contradictory, and likely to be dependent upon experimental conditions, such as intracellular ATP levels (reviewed by [714, 842, 1021]). Such regulation is generally not included in the tables.When thermosensitivity is mentioned, it refers specifically to a high Q10 of gating, often in the range of 10-30, but does not necessarily imply that the channel's function is to act as a 'hot' or 'cold' sensor. In general, the search for TRP activators has led to many claims for temperature sensing, mechanosensation, and lipid sensing. All proteins are of course sensitive to energies of binding, mechanical force, and temperature, but the issue is whether the proposed input is within a physiologically relevant range resulting in a response.

TRPA (ankyrin) family
TRPA1 is the sole mammalian member of this group (reviewed by [307]). TRPA1 activation of sensory neurons contribute to nociception [452,644,933]. Pungent chemicals such as mustard oil (AITC), allicin, and cinnamaldehyde activate TRPA1 by modification of free thiol groups of cysteine side chains, especially those located in its amino terminus [65,381,608,610]. Alkenals with α, β-unsaturated bonds, such as propenal (acrolein), butenal (crotylaldehyde), and 2-pentenal can react with free thiols via Michael addition and can activate TRPA1. However, potency appears to weaken as carbon chain length increases [30,65]. Covalent modification leads to sustained activation of TRPA1. Chemicals including carvacrol, menthol, and local anesthetics reversibly activate TRPA1 by non-covalent binding [467,549,1092,1093]. TRPA1 is not mechanosensitive under physiological conditions, but can be activated by cold temperatures [211,468]. The electron cryo-EM structure of TRPA1 [766] indicates that it is a 6-TM homotetramer. Each subunit of the channel contains two short 'pore helices' pointing into the ion selectivity filter, which is big enough to allow permeation of partially hydrated Ca 2+ ions.  [5,23,72,89,297,490,761,813]) fall into the subgroups outlined below. TRPC2 is a pseudogene in humans. It is generally accepted that all TRPC channels are activated downstream of G q/11 -coupled receptors, or receptor tyrosine kinases (reviewed by [801,998,1084]). A comprehensive listing of G-protein coupled receptors that activate TRPC channels is given in [5]. Hetero-oligomeric complexes of TRPC channels and their association with proteins to form signalling complexes are detailed in [23] and [491]. TRPC channels have frequently been proposed to act as store-operated channels (SOCs) (or compenents of mulimeric complexes that form SOCs), activated by depletion of intracellular calcium stores (reviewed by [23,68,165,166,741,773,805,862,1125]). However, the weight of the evidence is that they are not directly gated by conventional store-operated mechanisms, as established for Stim-gated Orai channels. TRPC channels are not mechanically gated in physiologically relevant ranges of force. All members of the TRPC family are blocked by 2-APB and SKF96365 [357,358]. Activation of TRPC channels by lipids is discussed by [72]. Important progress has been recently made in TRPC pharmacology [100,480,664,847]. TRPC channels regulate a variety of physiological functions and are implicated in many human diseases [73,311,927,1053]. TRPC1/C4/C5 subgroup TRPC1 alone may not form a functional ion channel [223]. TRPC4/C5 may be distinguished from other TRP channels by their potentiation by micromolar concentrations of La 3+ . TRPC2 is a pseudogene in humans, but in other mammals appears to be an ion channel localized to microvilli of the vomeronasal organ. It is required for normal sexual behavior in response to pheromones in mice. It may also function in the main olfactory epithelia in mice [571, 739,740,1117,1120,1123,1155]. TRPC3/C6/C7 subgroup All members are activated by diacylglycerol independent of protein kinase C stimulation [358]. When the photoreceptors are stimulated by light, glutamate release is reduced, and TRPM1 channels are more active, resulting in cell membrane depolarization. Human TRPM1 mutations are associated with congenital stationary night blindness (CSNB), whose patients lack rod function. TRPM1 is also found melanocytes. Isoforms of TRPM1 may present in melanocytes, melanoma, brain, and retina. In melanoma cells, TRPM1 is prevalent in highly dynamic intracellular vesicular structures [416,727]. TRPM3 (reviewed by [730]) exists as multiple splice variants which differ significantly in their biophysical properties. TRPM3 is expressed in somatosensory neurons and may be important in development of heat hyperalgesia during inflammation (see review [972]). TRPM3 is frequently coexpressed with TRPA1 and TRPV1 in these neurons. TRPM3 is expressed in pancreatic beta cells as well as brain, pituitary gland, eye, kidney, and adipose tissue [729,971]. TRPM3 may contribute to the detection of noxious heat [1028]. TRPM2 TRPM2 is activated under conditions of oxidative stress (respiratory burst of phagocytic cells) and ischemic conditions. However, the direct activators are ADPR(P) and calcium. As for many ion channels, PIP 2 must also be present (reviewed by [1102]). Numerous splice variants of TRPM2 exist which differ in their activation mechanisms [243]. The C-terminal domain contains a TRP motif, a coiled-coil region, and an enzymatic NUDT9 homologous domain. TRPM2 appears not to be activated by NAD, NAAD, or NAADP, but is directly activated by ADPRP (adenosine-5'-O-disphosphoribose phosphate) [1003]. TRPM2 is involved in warmth sensation [884], and contributes to neurological diseases [79]. Recent study shows that 2'-deoxy-ADPR is an endogenous TRPM2 superagonist [293]. TRPM4/5 subgroup TRPM4 and TRPM5 have the distinction within all TRP channels of being impermeable to Ca 2+ [1084]. A splice variant of TRPM4 (i.e.TRPM4b) and TRPM5 are molecular candidates for endogenous calcium-activated cation (CAN) channels [341]. TRPM4 is active in the late phase of repolarization of the cardiac ventricular action potential. TRPM4 deletion or knockout enhances beta adrenergic-mediated inotropy [637]. Mutations are associated with conduction defects [428,637,924]. TRPM4 has been shown to be an important regulator of Ca 2+ entry in to mast cells [1011] and dendritic cell migration [53]. TRPM5 in taste receptor cells of the tongue appears essential for the transduction of sweet, amino acid and bitter stimuli [570] TRPM5 contributes to the slow afterdepolarization of layer 5 neurons in mouse prefrontal cortex [550]. Both TRPM4 and TRPM5 are required transduction of taste stimuli [253]. TRPM6/7 subgroup TRPM6 and 7 combine channel and enzymatic activities ('chanzymes'). These channels have the unusual property of permeation by divalent (Ca 2+ , Mg 2+ , Zn 2+ ) and monovalent cations, high single channel conductances, but overall extremely small inward conductance when expressed to the plasma membrane. They are inhibited by internal Mg 2+ at 0.6 mM, around the free level of Mg 2+ in cells. Whether they contribute to Mg 2+ homeostasis is a contentious issue. When either gene is deleted in mice, the result is embryonic lethality. The C-terminal kinase region is cleaved under unknown stimuli, and the kinase phosphorylates nuclear histones. TRPM7 is responsible for oxidant-induced Zn 2+ release from intracellular vesicles [4] and contributes to intestinal mineral absorption essential for postnatal survival [666].

TRPM8
Is a channel activated by cooling and pharmacological agents evoking a 'cool' sensation and participates in the thermosensation of cold temperatures [67,180,221]    Comments: Ca 2+ activates all splice variants of TRPM2, but other activators listed are effective only at the full length isoform [243]. Inhibition of TRPM2 by clotrimazole, miconazole, econazole, flufenamic acid is largely irreversible. Co-application of pregnenolone sulphate and clotrimazole caused TRPM3 currents to acquire an inwardly rectifying component at negative voltages, resulting in a biphasic conductance-voltage relationship. Evidence was presented that the inward current might reflect the permeation of cations through the opening of a non-canonical pore [1027]. TRPM3 activity is impaired in chronic fatigue syn-drome/myalgic encephalomyelitis patients suggesting changes in intracellular Ca 2+ concentration, which may impact natural killer cellular functions [120]. TRPM4 exists as multiple spice variants: data listed are for TRPM4b. The sensitivity of TRPM4b and TRPM5 to activation by [Ca 2+ ] i demonstrates a pronounced and time-dependent reduction following excision of inside-out membrane patches [1005]. The V ù for activation of TRPM4 and TRPM5 demonstrates a pronounced negative shift with increasing temperature. Activation of TRPM8 by depolarization is strongly temperature-dependent via a channel-closing rate that decreases with decreasing temperature. The V ù is shifted in the hyperpolarizing direction both by decreasing temperature and by exogenous agonists, such as (-)-menthol [1020] whereas antagonists produce depolarizing shifts in V ù [684]. The V ù for the native channel is far more positive than that of heterologously expressed TRPM8 [684]. It should be noted that (-)-menthol and structurally related compounds can elicit release of Ca 2+ from the endoplasmic reticulum independent of activation of TRPM8 [612]. Intracellular pH modulates activation of TRPM8 by cold and icilin, but not (-)-menthol [25].

TRPML (mucolipin) family
The TRPML family [188,810,816,1096,1129] consists of three mammalian members (TRPML1-3). TRPML channels are probably restricted to intracellular vesicles and mutations in the gene (MCOLN1) encoding TRPML1 (mucolipin-1) cause the neu-rodegenerative disorder mucolipidosis type IV (MLIV) in man. TRPML1 is a cation selective ion channel that is important for sorting/transport of endosomes in the late endocytotic pathway and specifically, fission from late endosome-lysosome hybrid vesicles and lysosomal exocytosis [863]. TRPML2 and TRPML3 show increased channel activity in low extracellular sodium and are activated by similar small molecules [332]. A naturally occurring gain of function mutation in TRPML3 (i.e. A419P) results in the varitint waddler (Va) mouse phenotype (reviewed by [715,816] conducts Na + > K + > Cs + with maintained current in the presence of Na + , conducts Ca 2+ and Mg 2+ , but not Fe 2+ , impermeable to protons; inwardly rectifying Wild type TRPML3: γ = 59 pS at negative holding potentials with monovalent cations as charge carrier; conducts Na + > K + > Cs + and Ca 2+ (P Ca /P K ∼ = 350), slowly inactivates in the continued presence of Na + within the extracellular (extracytosolic) solution; outwardly rectifying Comments TRPML1 current is potentiated by acidic pH and sphingosine [894].

Nomenclature
Currents have been measured directly from primary cilia and also when expressed on plasma membranes. Primary cilia appear to contain heteromeric TRPP2 + PKD1-L1, underlying a gently outwardly rectifying nonselective conductance (P Ca /P Na 6: PKD1-L1 is a 12 TM protein of unknown topology). Primary cilia heteromeric channels have an inward single channel conductance of 80 pS and an outward single channel conductance of 95 pS. Presumed homomeric TRPP2 channels are gently outwardly rectifying. Single channel conductance is 120 pS inward, 200 pS outward [205]. TRPP2 (PKD2L1) displays calcium dependent activation. Calcium accumulation due to prolonged channel activity may lead to outward-moving Ca 2+ ions to close the channel [206].
The functional characteristics of PKD2L2(PC2L2) has not been established yet.
Comments: Data in the table are extracted from [195,215] and [898]. Broadly similar single channel conductance, mono-and di-valent cation selectivity and sensitivity to blockers are observed for TRPP2 co-expressed with TRPP1 [214]. Ca 2+ , Ba 2+ and Sr 2+ permeate TRPP3, but reduce inward currents carried by Na + . Mg 2+ is largely impermeant and exerts a voltage dependent inhibition that increases with hyperpolarization.

TRPV (vanilloid) family
Members of the TRPV family (reviewed by [1012]) can broadly be divided into the non-selective cation channels, TRPV1-4 and the more calcium selective channels TRPV5 and TRPV6. TRPV1-V4 subfamily TRPV1 is involved in the development of thermal hyperalgesia following inflammation and may contribute to the detection of noxius heat (reviewed by [794,925,954]). Numerous splice variants of TRPV1 have been described, some of which modulate the activity of TRPV1, or act in a dominant negative manner when co-expressed with TRPV1 [883]. The pharmacology of TRPV1 channels is discussed in detail in [343] and [1026]. TRPV2 is probably not a thermosensor in man [756], but has recently been implicated in innate immunity [576]. TRPV3 and TRPV4 are both thermosensitive. There are claims that TRPV4 is also mechanosensitive, but this has not been estab-lished to be within a physiological range in a native environment [129,567]. TRPV5/V6 subfamily TRPV5 and TRPV6 are highly expressed in placenta, bone, and kidney. Under physiological conditions, TRPV5 and TRPV6 are calcium selective channels involved in the absorption and reabsorption of calcium across intestinal and kidney tubule epithelia (reviewed by [202,280,687,1075] -Functional Characteristics γ = 35 pS at -60 mV; 77 pS at + 60 mV, conducts mono and di-valent cations with a selectivity for divalents (P Ca /P Na = 9.6); voltage-and time-dependent outward rectification; potentiated by ethanol; activated/potentiated/upregulated by PKC stimulation; extracellular acidification facilitates activation by PKC; desensitisation inhibited by PKA; inhibited by Ca 2+ / calmodulin; cooling reduces vanilloid-evoked currents; may be tonically active at body temperature Conducts mono-and di-valent cations (P Ca /P Na = 0.9-2.9); dual (inward and outward) rectification; current increases upon repetitive activation by heat; translocates to cell surface in response to IGF-1 to induce a constitutively active conductance, translocates to the cell surface in response to membrane stretch Functional Characteristics γ = 59-78 pS for monovalent ions at negative potentials, conducts mono-and di-valents with high selectivity for divalents (P Ca /P Na > 107); voltage-and time-dependent inward rectification; inhibited by intracellular Ca 2+ promoting fast inactivation and slow downregulation; feedback inhibition by Ca 2+ reduced by calcium binding protein 80-K-H; inhibited by extracellular and intracellular acidosis; upregulated by 1,25-dihydrovitamin D3 γ = 58-79 pS for monovalent ions at negative potentials, conducts mono-and di-valents with high selectivity for divalents (P Ca /P Na > 130); voltage-and time-dependent inward rectification; inhibited by intracellular Ca 2+ promoting fast and slow inactivation; gated by voltage-dependent channel blockade by intracellular Mg 2+ ; slow inactivation due to Ca 2+ -dependent calmodulin binding; phosphorylation by PKC inhibits Ca 2+ -calmodulin binding and slow inactivation; upregulated by 1,25-dihydroxyvitamin D3 Comments: Activation of TRPV1 by depolarisation is strongly temperature-dependent via a channel opening rate that increases with increasing temperature. The V ù is shifted in the hyperpolarizing direction both by increasing temperature and by exogenous agonists [1020]. TRPV3 channel dysfunction caused by genetic gain-of-function mutations is implicated in the pathogenesis of skin inflammation, dermatitis, and chronic itch. In rodents, a sponateous gain-of-function matation of the TRPV3 gene causes the development of skin lesions with pruritus and dermatitis [34,573]. In contrast to other thermoTRP channels, TRPV3 sensitizes rather than desensitizes, upon repeated stimulation with either heat or agonists [175,579,1095]. The sensitivity of TRPV4 to heat, but not 4α-PDD is lost upon patch excision. TRPV4 is activated by anandamide and arachidonic acid following P450 epoxygenase-dependent metabolism to 5,6-epoxyeicosatrienoic acid (reviewed by [719]). Activation of TRPV4 by cell swelling, but not heat, or phorbol esters, is mediated via the formation of epoxyeicosatrieonic acids. Phorbol esters bind directly to TRPV4. Different TRPV4 mutations load to a broad spectrum of dominant skeletal dysplasias [512,840] and spinal muscular atrophies and hereditary motor and sensory neuropathies [41,218]. Similar mutations were also found in patients with Charcot-Marie-Tooth disease type 2C [533]. TRPV5 preferentially conducts Ca 2+ under physiological conditions, but in the absence of extracellular Ca 2+ , conducts monovalent cations. Single channel conductances listed for TRPV5 and TRPV6 were determined in divalent cation-free extracellular solution. Ca 2+ -induced inactivation oc-curs at hyperpolarized potentials when Ca 2+ is present extracellularly. Single channel events cannot be resolved (probably due to greatly reduced conductance) in the presence of extracellular divalent cations. Measurements of P Ca /P Na for TRPV5 and TRPV6 are dependent upon ionic conditions due to anomalous mole fraction behaviour. Blockade of TRPV5 and TRPV6 by extracellular Mg 2+ is voltage-dependent. Intracellular Mg 2+ also exerts a voltage dependent block that is alleviated by hyperpolarization and contributes to the time-dependent activation and deactivation of TRPV6 mediated monovalent cation currents. TRPV5 and TRPV6 differ in their kinetics of Ca 2+ -dependent inactivation and recovery from inactivation. TRPV5 and TRPV6 function as homo-and hetero-tetramers.

Further reading on Transient Receptor Potential channels
Aghazadeh Tabrizi

Voltage-gated calcium channels
Ion channels → Voltage-gated ion channels → Voltage-gated calcium channels Overview: Calcium (Ca 2+ ) channels are voltage-gated ion channels present in the membrane of most excitable cells. The nomenclature for Ca 2+ channels was proposed by [267] and approved by the NC-IUPHAR Subcommittee on Ca 2+ channels [139]. Ca 2+ channels form hetero-oligomeric complexes. The α1 subunit is pore-forming and provides the binding site(s) for practically all agonists and antagonists. The 10 cloned α1-subunits can be grouped into three families: (1) the high-voltage activated dihydropyridine-sensitive (L-type, Ca V 1.x) channels; (2) the highvoltage activated dihydropyridine-insensitive (Ca V 2.x) channels and (3) the low-voltage-activated (T-type, Ca V 3.x) channels. Each α1 subunit has four homologous repeats (I-IV), each repeat having six transmembrane domains and a pore-forming region between transmembrane domains S5 and S6. Gating is thought to be associated with the membrane-spanning S4 segment, which contains highly conserved positive charges. Many of the α1-subunit genes give rise to alternatively spliced products. At least for high-voltage activated channels, it is likely that native channels comprise coassemblies of α1, β and α2-δ subunits. The γ subunits have not been proven to associate with channels other than the α1s skeletal muscle Cav1.1 channel. The α2-δ1 and α2-δ2 subunits bind gabapentin and pregabalin.

Voltage-gated proton channel
Ion channels → Voltage-gated ion channels → Voltage-gated proton channel Overview: The voltage-gated proton channel (provisionally denoted H v 1) is a putative 4TM proton-selective channel gated by membrane depolarization and which is sensitive to the transmembrane pH gradient [131,208,209,827,868]. The structure of H v 1 is homologous to the voltage sensing domain (VSD) of the superfamily of voltage-gated ion channels (i.e. segments S1 to S4) and contains no discernable pore region [827,868]. Proton flux through H v 1 is instead most likely mediated by a water wire completed in a crevice of the protein when the voltage-sensing S4 helix moves in response to a change in transmembrane potential [826,1079]. H v 1 expresses largely as a dimer mediated by intracellular C-terminal coiled-coil interactions [564] but individual promoters nonetheless support gated H + flux via separate conduction pathways [502,547,787,989]. Within dimeric structures, the two protomers do not function independently, but display co-operative interactions during gating resulting in increased voltage sensitivity, but slower activation, of the dimeric,versus monomeric, complexes [322,990]. Functional Characteristics Activated by membrane depolarization mediating macroscopic currents with time-, voltage-and pH-dependence; outwardly rectifying; voltage dependent kinetics with relatively slow current activation sensitive to extracellular pH and temperature, relatively fast deactivation; voltage threshold for current activation determined by pH gradient ( pH = pH o -pH i ) across the membrane

Comments:
The voltage threshold (V thr ) for activation of Hv1 is not fixed but is set by the pH gradient across the membrane such that V thr is positive to the Nernst potential for H + , which ensures that only outwardly directed flux of H + occurs under physiological conditions [131,208,209]. Phosphorylation of H v 1 within the N-terminal domain by PKC enhances the gating of the chan-nel [682]. Tabulated IC 50 values for Zn 2 + and Cd 2+ are for heterologously expressed human and mouse H v 1 [827,868]. Zn 2+ is not a conventional pore blocker, but is coordinated by two, or more, external protonation sites involving histamine residues [827]. Zn 2+ binding may occur at the dimer interface between pairs of histamine residues from both monomers where it may interfere with channel opening [683]. Mouse knockout studies demonstrate that H v 1 participates in charge compensation in granulocytes during the respiratory burst of NADPH oxidasedependent reactive oxygen species production that assists in the clearance of bacterial pathogens [828]. Additional physiological functions of H v 1 are reviewed by [131].

Voltage-gated sodium channels
Ion channels → Voltage-gated ion channels → Voltage-gated sodium channels Overview: Sodium channels are voltage-gated sodium-selective ion channels present in the membrane of most excitable cells. Sodium channels comprise of one pore-forming α subunit, which may be associated with either one or two β subunits [425]. α-Subunits consist of four homologous domains (I-IV), each containing six transmembrane segments (S1-S6) and a pore-forming loop. The positively charged fourth transmembrane segment (S4) acts as a voltage sensor and is involved in channel gating. The crystal structure of the bacterial NavAb channel has revealed a number of novel structural features compared to earlier potassium channel structures including a short selectivity filter with ion selectivity determined by interactions with glutamate side chains [767]. Interestingly, the pore region is penetrated by fatty acyl chains that extend into the central cavity which may allow the entry of small, hydrophobic pore-blocking drugs [767]. Auxiliary β1, β2, β3 and β4 subunits consist of a large extracellular N-terminal domain, a single transmembrane segment and a shorter cytoplasmic domain.
Activation V 0.5 = -24 mV. Fast inactivation (τ = 0.8 ms for peak sodium current).  Comments: Sodium channels are also blocked by local anaesthetic agents, antiarrythmic drugs and antiepileptic drugs. In general, these drugs are not highly selective among channel subtypes. There are two clear functional fingerprints for distinguishing dif-ferent subtypes. These are sensitivity to tetrodotoxin (Na V 1.5, Na V 1.8 and Na V 1.9 are much less sensitive to block) and rate of fast inactivation (Na V 1.8 and particularly Na V 1.9 inactivate more slowly). All sodium channels also have a slow inactivation process that is engaged during long depolarizations (>100 msec) or repetitive trains of stimuli. All sodium channel subtypes are blocked by intracellular QX-314.

Other ion channels
Ion channels → Other ion channels A number of ion channels in the human genome do not fit readily into the classification of either ligand-gated or voltage-gated ion channels. These are identified below.

Aquaporins
Ion channels → Other ion channels → Aquaporins Overview: Aquaporins and aquaglyceroporins are membrane channels that allow the permeation of water and certain other small solutes across the cell membrane, or in the case of AQP6, AQP11 and AQP12A, intracellular membranes, such as vesicles and the endoplasmic reticulum membrane [514]. Since the isolation and cloning of the first aquaporin (AQP1) [807], 12 additional mammalian members of the family have been identified, although little is known about the functional properties of one of these (AQP12A; Q8IXF9) and it is thus not tabulated. The other 12 aquaporins can be broadly divided into three families: orthodox aquaporins (AQP0,-1,-2,-4,-5, -6 and -8) permeable mainly to water, but for some additional solutes [201]; aquaglyceroporins (AQP3,-7 -9 and -10), additionally permeable to glycerol and for some isoforms urea [493], and superaquaporins (AQP11 and 12) located within cells [422]. Some aquaporins also conduct ammonia and/or H 2 O 2 giving rise to the terms 'ammoniaporins' ('aquaammoniaporins') and 'peroxiporins', respectively. Aquaporins are impermeable to protons and other inorganic and organic cations, with the possible exception of AQP1 [493]. One or more members of this family of proteins have been found to be expressed in almost all tissues of the body [reviewed in Yang (2017) [1104]]. AQPs are involved in numerous processes that include systemic water homeostasis, adipocyte metabolism, brain oedema, cell migration and fluid se-  [493]]. Functional AQPs exist as homotetramers that are the water conducting units wherein individual AQP subunits (each a protomer) have six transmembrane helices and two half helices that con-stitute a seventh 'pseudotransmembrane domain' that surrounds a narrow water conducting channel [514]. In addition to the four pores contributed by the protomers, an additional hydrophobic pore exists within the center of the complex [514] that may mediate the transport of gases (e.g. O 2 , CO 2 , NO) and cations (the central pore is the proposed transport pathway for cations through AQP1) by some AQPs [314,460]. Although numerous small molecule inhibitors of aquaporins, particularly APQ1, have been reported primarily from Xenopus oocyte swelling assays, the activity of most has subsequently been disputed upon retesting using assays of water transport that are less prone to various artifacts [271] and they are therefore excluded from the tables [see Tradtrantip et al. (2017)  Endogenous activator AQP0 is gated by calmodulin [493] cGMP (see comment) -Permeability water (rat single channel permeability 0.25 x 10 -14 cm 3 s -1 ) (Rat) [1106] water (rat single channel permeability 6.0 x 10 -14 cm 3 s -1 ), ammonia, H 2 O 2 [314,1106] water (rat single channel permeability 3.3 x 10 -14 cm 3 s -1 ) [606] Inhibitors Hg 2+ Ag + , Hg 2+ , pCMBS Hg 2+ Comments AQP0 appears permeable to CO 2 [314].
Human, but not mouse, AQP1 appears permeable to CO 2 , probably through the central pore of the tetrameric complex [314]. NO also appears permeable [371]. Permeability to H 2 0 2 has been demonstrated for rat, but not human, AQP1 [87]. Numerous small molecule inhibitors of AQP1 have been proposed, but re-evaluation indicates that they have no significant effect upon water permeability at concentrations in excess of their originally reported IC 50 values [310]. A fifth pore located at the central axis of the tetrameric complex has, controversially, been described as a cation conductance activated by cGMP and phosphorylation by protein kinases A and C. Evidence in support and against this proposal is discussed in detail by Kitchen [87,314,1106] water (rat single channel permeability 24 x 10 -14 cm 3 s -1 [1106] water (rat single channel permeability 5.0 x 10 -14 cm 3 s -1 ), H 2 O 2 [460] water (zero, or very low basal, permeability is enhanced by low pH and in mouse and rat by Hg 2+ ), glycerol, ammonia, urea, anions [ AQP5 may conduct CO 2 [314]. AQP6 is an intracellular channel that localises to acid secreting intercalated cells of the renal collecting ducts. Notably, AQP6 is activated by Hg 2+ and by low pH and is unusually permeable to anions (with the permeability sequence NO 3 ->I ->>Br ->Cl ->>Fl -) as well as water, both through the monomeric pore [493,823]. AQP6 may also conduct CO 2 [314]. Comments AQP7 also transports silicon [310]. Permeability to urea is controversial, but might be explained by differences between mouse and human caused by a pore-lining amino acid residue that differs between species [493].
It is not known if AQP10 is permeable to ammonia. Permeability to silicon has been described [310].

Further reading on Aquaporins
Agre P.

Chloride channels
Ion channels → Other ion channels → Chloride channels Overview: Chloride channels are a functionally and structurally diverse group of anion selective channels involved in processes including the regulation of the excitability of neurones, skeletal, cardiac and smooth muscle, cell volume regulation, transepithelial salt transport, the acidification of internal and extracellular compartments, the cell cycle and apoptosis (reviewed in [251]).
Excluding the transmittergated GABA A and glycine receptors (see separate tables), well characterised chloride channels can be classified as certain members of the voltage-sensitive ClC subfamily, calcium-activated channels, high (maxi) conductance channels, the cystic fibrosis transmembrane conductance regulator (CFTR) and volume regulated channels [1014]. No official recommenda-tion exists regarding the classification of chloride channels. Functional chloride channels that have been cloned from, or characterised within, mammalian tissues are listed with the exception of several classes of intracellular channels (e.g. CLIC) that are reviewed by in [259].

ClC family
Ion channels → Other ion channels → Chloride channels → ClC family Overview: The mammalian ClC family (reviewed in [8,157,251,254,442]) contains 9 members that fall, on the basis of sequence homology, into three groups; ClC-1, ClC-2, hClC-Ka (rClC-K1) and hClC-Kb (rClC-K2); ClC-3 to ClC-5, and ClC-6 and -7. ClC-1 and ClC-2 are plasma membrane chloride channels. ClC-Ka and ClC-Kb are also plasma membrane channels (largely expressed in the kidney and inner ear) when associated with barttin (BSND, Q8WZ55), a 320 amino acid 2TM protein [272]. The localisation of the remaining members of the ClC family is likely to be predominantly intracellular in vivo, although they may traffic to the plasma membrane in overexpression systems. Numerous recent reports indicate that ClC-4, ClC-5, ClC-6 and ClC-7 (and by inference ClC-3) function as Cl -/H + antiporters (secondary active transport), rather than classical Clchannels [328,551,698,790,873]; reviewed in [8,812]). It has recently been reported that the activity of ClC-5 as a Cl -/H + exchanger is important for renal endocytosis [723]. Alternative splicing increases the structural diversity within the ClC family. The crystal structure of two bacterial ClC proteins has been described [255] and a eukaryotic ClC transporter (CmCLC) has recently been described at 3.5 Å resolution [284]. Each ClC subunit, with a complex topology of 18 intramembrane segments, contributes a single pore to a dimeric 'double-barrelled' ClC channel that contains two independently gated pores, confirming the predictions of previous functional and structural investigations (reviewed in [157,254,442,812]). As found for ClC-4, ClC-5, ClC-6 and ClC-7, the prokaryotic ClC homologue (ClC-ec1) and CmCLC function as H + /Cl antiporters, rather than as ion channels [7,284]. The generation of monomers from dimeric ClC-ec1 has firmly established that each ClC subunit is a functional unit for transport and that cross-subunit interaction is not required for Cl -/H + exchange in ClC transporters [837]. Comments: ClC channels display the permeability sequence Cl -> Br -> I -(at physiological pH). ClC-1 has significant opening probability at resting membrane potential, accounting for 75% of the membrane conductance at rest in skeletal muscle, and is important for stabilization of the membrane potential. S-(-)CPP, 9-anthroic acid and niflumic acid act intracellularly and exhibit a strongly voltage-dependent block with strong inhibition at negative voltages and relief of block at depolarized potentials ( [565] and reviewed in [811]). Inhibition of ClC-2 by the peptide GaTx2, from Leiurus quinquestriatus herbareus venom, is likely to occur through inhibition of channel gating, rather than direct open channel blockade [980]. Although ClC-2 can be activated by cell swelling, it does not correspond to the VRAC channel (see below). Alternative potential physiological functions for ClC-2 are reviewed in [798]. Functional expression of human ClC-Ka and ClC-Kb requires the presence of barttin [272,877] reviewed in [276]. The rodent homologue (ClC-K1) of ClC-Ka demonstrates limited expression as a homomer, but its function is enhanced by barttin which increases both channel opening probablility in the physiological range of potentials [272,289,877] reviewed in [276]). ClC-Ka is approximately 5 to 6-fold more sensitive to block by 3-phenyl-CPP and DIDS than ClC-Kb, while newly synthesized benzofuran derivatives showed the same blocking affinity (<10 μM) on both CLC-K isoforms [566]. The biophysical and pharma-cological properties of ClC-3, and the relationship of the protein to the endogenous volume-regulated anion channel(s) VRAC [20,340] are controversial and further complicated by the possibility that ClC-3 may function as both a Cl -/H + exchanger and an ion channel [20,790,1052]. The functional properties tabulated are those most consistent with the close structural relationship between ClC-3, ClC-4 and ClC-5. Activation of heterologously expressed ClC-3 by cell swelling in response to hypotonic solutions is disputed, as are many other aspects of its regulation. Depen-dent upon the predominant extracellular anion (e.g. SCNversus Cl -), CIC-4 can operate in two transport modes: a slippage mode in which behaves as an ion channel and an exchanger mode in which unitary transport rate is 10-fold lower [19]. Similar findings have been made for ClC-5 [1128]. ClC-7 associates with a β subunit, Ostm1, which increases the stability of the former [535] and is essential for its function [551].

CFTR
Ion channels → Other ion channels → Chloride channels → CFTR Overview: CFTR, a 12TM, ABC transporter-type protein, is a cAMP-regulated epithelial cell membrane Clchannel involved in normal fluid transport across various epithelia. Of the 1700 mutations identified in CFTR, the most common is the deletion mutant F508 (a class 2 mutation) which results in impaired trafficking of CFTR and reduces its incorporation into the plasma membrane causing cystic fibrosis (reviewed in [192]). Channels carrying the F508 mutation that do traffic to the plasma membrane demonstrate gating defects. Thus, pharmacological restoration of the function of the F508 mutant would require a compound that embodies 'corrector' (i.e. facilitates folding and trafficking to the cell surface) and 'potentiator' (i.e. promotes opening of channels at the cell surface) activities [192]. In addition to acting as an anion channel per se, CFTR may act as a regulator of several other conductances including inhibition of the epithelial Na channel (ENaC), calcium activated chloride channels (CaCC) and volume regulated anion channel (VRAC), activation of the outwardly rectifying chloride channel (ORCC), and enhancement of the sulpho-nylurea sensitivity of the renal outer medullary potassium channel (ROMK2), (reviewed in [710]). CFTR also regulates TRPV4, which provides the Ca 2+ signal for regulatory volume decrease in airway epithelia [33]. The activities of CFTR and the chloride-bicarbonate exchangers SLC26A3 (DRA) and SLC26A6 (PAT1) are mutually enhanced by a physical association between the regulatory (R) domain of CFTR and the STAS domain of the SCL26 transporters, an effect facilitated by PKA-mediated phosphorylation of the R domain of CFTR [499].   [908]). Corrector compounds that aid the folding of DF508CFTR to increase the amount of protein expressed and potentially delivered to the cell surface include VX-532 (which is also a potentiator), VRT-325, KM11060, Corr-3a and Corr-4a see [1014] for details and structures of Corr-3a and Corr-4a). Inhibition of CFTR by intracellular applica-tion of the peptide GaTx1, from Leiurus quinquestriatus herbareus venom, occurs preferentially for the closed state of the channel [302]. CFTR contains two cytoplasmic nucleotide binding domains (NBDs) that bind ATP. A single open-closing cycle is hypothesised to involve, in sequence: binding of ATP at the Nterminal NBD1, ATP binding to the C-terminal NBD2 leading to the formation of an intramolecular NBD1-NBD2 dimer associated with the open state, and subsequent ATP hydrolysis at NBD2 fa-cilitating dissociation of the dimer and channel closing, and the initiation of a new gating cycle [21,679]. Phosphorylation by PKA at sites within a cytoplasmic regulatory (R) domain facilitates the interaction of the two NBD domains. PKC (and PKGII within intestinal epithelial cells via guanylinstimulated cyclic GMP formation) positively regulate CFTR activity.

Calcium activated chloride channel
Ion channels → Other ion channels → Chloride channels → Calcium activated chloride channel Overview: Chloride channels activated by intracellular calcium (CaCC) are widely expressed in excitable and non-excitable cells where they perform diverse functions [359]. The molecular nature of CaCC has been uncertain with both CLCA, TWEETY andBEST genes having been considered as likely candidates [251,360,589]. It is now accepted that CLCA expression products are unlikely to form channels per se and probably function as cell adhesion proteins, or are secreted [762]. Similarly, TWEETY gene products do not recapictulate the properties of endogenous CaCC. The bestrophins encoded by genes BEST1-4 have a topology more consistent with ion channels [360] and form chloride channels that are activated by physiological concentrations of Ca 2+ , but whether such activation is direct is not known [360]. However, currents generated by bestrophin over-expression do not resemble native CaCC currents. The evidence for and against bestrophin proteins forming CaCC is critically reviewed by Duran et al. [251]. Recently, a new gene family, TMEM16 (anoctamin) consisting of 10 members (TMEM16A-K; anoctamin 1-10) has been identified and there is firm evidence that some of these members form chloride channels [250,525]. TMEM16A (anoctamin 1; Ano 1) produces Ca 2+ -activated Clcurrents with kinetics similar to native CaCC currents recorded from different cell types [133,839,879,1110]. Knockdown of TMEM16A greatly reduces currents mediated by calcium-activated chloride channels in submandibular gland cells [1110] and smooth muscle cells from pulmonary artery [625]. In TMEM16A (-/-) mice secretion of Ca 2+ -dependent Clsecretion by several epithelia is reduced [746,839]. Alternative splicing regulates the voltage-and Ca 2+ -dependence of TMEM16A and such processing may be tissue-specific manner and thus contribute to functional diversity [286]. There are also reports that TMEM16B (anoctamin 2; Ano 2) supports CaCC activity (e.g. [792]) and in TMEM16B (-/-) mice Ca-activated Clcurrents in the main olfactory epithelium (MOE) and in the vomeronasal organ are virtually absent [88]. Comments: Blockade of I Cl(Ca) by niflumic acid, DIDS and 9-anthroic acid is voltage-dependent whereas block by NPPB is voltage-independent [359]. Extracellular niflumic acid; DCDPC and 9-anthroic acid (but not DIDS) exert a complex effect upon I Cl(Ca) in vascular smooth muscle, enhancing and inhibiting inwardly and outwardly directed currents in a manner dependent upon [Ca 2+ ] i (see [539] for summary). Considerable crossover in pharmacology with large conductance Ca 2+ -activated K + chan-nels also exists (see [329] for overview). Two novel compounds, CaCC inh -A01 and CaCC inh -B01 have recently been identified as blockers of calcium-activated chloride channels in T84 human intestinal epithelial cells [203] for structures). Significantly, other novel compounds totally block currents mediated by TMEM116A, but have only a modest effect upon total current mediated by CaCC native to T84 cells or human bronchial epithelial cells, suggesting that TMEM16A is not the predominant CaCC in such cells [693]. CaMKII modulates CaCC in a tissue dependent manner (reviewed by [359,539]). CaMKII inhibitors block activation of I Cl(Ca) in T84 cells but have no effect in parotid acinar cells. In tracheal and arterial smooth muscle cells, but not portal vein myocytes, inhibition of CaMKII reduces inactivation of I Cl(Ca) . Intracellular Ins(3,4,5,6)P 4 may act as an endogenous negative regulator of CaCC channels activated by Ca 2+ , or CaMKII. Smooth muscle CaCC are also regulated positively by Ca 2+ -dependent phosphatase, calcineurin (see [539] for summary).

Maxi chloride channel
Ion channels → Other ion channels → Chloride channels → Maxi chloride channel Overview: Maxi Clchannels are high conductance, anion selective, channels initially characterised in skeletal muscle and subsequently found in many cell types including neurones, glia, cardiac muscle, lymphocytes, secreting and absorbing epithelia, macula densa cells of the kidney and human placenta syncytiotrophoblasts [855]. The physiological significance of the maxi Clchannel is uncertain, but roles in cell volume regulation and apop-tosis have been claimed. Evidence suggests a role for maxi Clchannels as a conductive pathway in the swelling-induced release of ATP from mouse mammary C127i cells that may be important for autocrine and paracrine signalling by purines [252,854]. A similar channel mediates ATP release from macula densa cells within the thick ascending of the loop of Henle in response to changes in luminal NaCl concentration [78]. A family of human high conductance Clchannels (TTYH1-3) that resemble Maxi Clchannels has been cloned [949], but alternatively, Maxi Clchannels have also been suggested to correspond to the voltagedependent anion channel, VDAC, expressed at the plasma membrane [46,733].

Nomenclature
Maxi Cl - ATP is a voltage dependent permeant blocker of single channel activity (P ATP /P Cl = 0.08-0.1); channel activity increased by patch-excision; channel opening probability (at steady-state) maximal within approximately ± 20 mV of 0 mV, opening probability decreased at more negative and (commonly) positive potentials yielding a bell-shaped curve; channel conductance and opening probability regulated by annexin 6 Comments Maxi Clis also activated by G protein-coupled receptors and cell swelling. Tamoxifen  Comments: Differing ionic conditions may contribute to variable estimates of γ reported in the literature. Inhibition by arachidonic acid (and cis-unsaturated fatty acids) is voltageindependent, occurs at an intracellular site, and involves both channel shut down (K d = 4-5 μM) and a reduction of γ (K d = 13-14 μM). Blockade of channel activity by SITS, DIDS, Gd 3+ and arachidonic acid is paralleled by decreased swelling-induced release of ATP [252,854]. Channel activation by anti-oestrogens in whole cell recordings requires the presence of intracellular nucleotides and is prevented by pre-treatment with 17β-estradiol, bucladesine, or intracellular dialysis with GDPβS [222]. Activation by tamoxifen is suppressed by low concentrations of okadaic acid, suggesting that a dephosphorylation event by protein phosphatase PP2A occurs in the activation pathway [222]. In contrast, 17β-estradiol and tamoxifen appear to directly inhibit the maxi Clchannel of human placenta reconstituted into giant liposomes and recorded in excised patches [836].

Volume regulated chloride channels
Ion channels → Other ion channels → Chloride channels → Volume regulated chloride channels Overview: Volume activated chloride channels (also termed VSOAC, volume-sensitive organic osmolyte/anion channel; VRC, volume regulated channel and VSOR, volume expansion-sensing outwardly rectifying anion channel) participate in regulatory volume decrease (RVD) in response to cell swelling. VRAC may also be important for several other processes including the regulation of membrane excitability, transcellular Cltransport, angiogenesis, cell proliferation, necrosis, apoptosis, glutamate release from astrocytes, insulin (INS, P01308) release from pancreatic β cells and resistance to the anti-cancer drug, cisplatin (reviewed by [80,680,710,735]). VRAC may not be a single entity, but may instead represent a number of different channels that are expressed to a variable extent in different tissues and are differentially activated by cell swelling. In addition to ClC-3 expression products (see above) several former VRAC candidates including MDR1 (ABCB1 P-glycoprotein), Icln, Band 3 anion exchanger and phospholemman are also no longer considered likely to fulfil this function (see reviews [710,867]).

Comments
VRAC is also activated by cell swelling and low intracellular ionic strength. VRAC is also blocked by chromones, extracellular nucleotides and nucleoside analogues Comments: In addition to conducting monovalent anions, in many cell types the activation of VRAC by a hypotonic stimulus can allow the efflux of organic osmolytes such as amino acids and polyols that may contribute to RVD. Other chloride channels In addition to some intracellular chloride channels that are not considered here, plasma membrane channels other than those listed have been functionally described. Many cells and tissues contain outwardly rectifying chloride channels (ORCC) that may correspond to VRAC active under isotonic conditions. A cyclic AMP-activated Clchannel that does not correspond to CFTR has been described in intestinal Paneth cells [1000]. A Cl channel activated by cyclic GMP with a dependence on raised intracellular Ca 2+ has been recorded in various vascular smooth muscle cells types, which has a pharmacology and biophysical characteristics very different from the 'conventional' CaCC [635,796]. It has been proposed that bestrophin-3 (BEST3, Q8N1M1) is an essential component of the cyclic GMP-activated channel [636]. A proton-activated, outwardly rectifying anion channel has also been described [532].

Connexins and Pannexins
Ion channels → Other ion channels → Connexins and Pannexins Overview: Gap junctions are essential for many physiological processes including cardiac and smooth muscle contraction, regulation of neuronal excitability and epithelial electrolyte transport [114,185,274]. Gap junction channels allow the passive diffusion of molecules of up to 1,000 Daltons which can include nutrients, metabolites and second messengers (such as IP 3 ) as well as cations and anions. 21 connexin genes and 3 pannexin genes which are structurally related to the invertebrate innexin genes) code for gap junction proteins in humans. Each connexin gap junc-tion comprises 2 hemichannels or 'connexons' which are themselves formed from 6 connexin molecules. The various connexins have been observed to combine into both homomeric and heteromeric combinations, each of which may exhibit different functional properties. It is also suggested that individual hemichannels formed by a number of different connexins might be functional in at least some cells [372]. Connexins have a common topology, with four α-helical transmembrane domains, two extracellular loops, a cytoplasmic loop, and N-and C-termini located on the cytoplasmic membrane face. In mice, the most abundant connexins in electrical synapses in the brain seem to be Cx36, Cx45 and Cx57 [955]. Mutations in connexin genes are associated with the occurrence of a number of pathologies, such as peripheral neuropathies, cardiovascular diseases and hereditary deafness. The pannexin genes Px1 and Px2 are widely expressed in the mammalian brain [1023]. Like the connexins, at least some of the pannexins can form hemichannels [114,778]

Sodium leak channel, non-selective
Ion channels → Other ion channels → Sodium leak channel, non-selective

Overview:
The sodium leak channel, non selective (NC-IUPHAR tentatively recommends the nomenclature Na Vi 2.1, W.A. Catterall, personal communication) is structurally a member of the family of voltage-gated sodium channel family (Na v 1.1-Na v 1.9) [542,1121]. In contrast to the latter, Na Vi 2.1, is voltage-insensitive (denoted in the subscript 'vi' in the tentative nomenclature) and possesses distinctive ion selectivity and pharmacological properties. Na Vi 2.1, which is insensitive to tetrodotoxin (10 μM), has been proposed to mediate the tetrodotoxin-resistant and voltage-insensitive Na + leak current (I L -Na) observed in many types of neurone [595]. However, whether Na Vi 2.1 is constitutively active has been challenged [951]. Na Vi 2.1 is widely distributed within the central nervous system and is also expressed in the heart and pancreas specifically, in rodents, within the islets of Langerhans [542,595]. Recently, Na Vi 2.1 has been proposed to be a core effector for the action of inhibitory G proteins [789]. Functional Characteristics γ = 27 pS (by fluctuation analysis), P Na /P Cs = 1.3, P K /P Cs = 1.2, P Ca /P Cs = 0.5, linear current voltage-relationship, voltage-independent and non-inactivating Comments: In native and recombinant expression systems Na Vi 2.1 can be activated by stimulation of NK 1 (in hippocampal neurones), neurotensin (in ventral tegmental area neurones) and M3 muscarinic acetylcholine receptors (in MIN6 pancreatic βcells) and in a manner that is independent of signalling through G proteins [596,951]. Pharmacological and molecular biological evidence indicates such modulation to occur though a pathway that involves the activation of Src family tyrosine kinases. It is suggested that Na Vi 2.1 exists as a macromolecular complex with M3 receptors [951] and peptide receptors [596], in the latter instance in association with the protein UNC-80, which recruits Src to the channel complex [596,1045]. By contrast, stimulation of Na vi 2.1 by decreased extracellular Ca 2+ concentration is G protein dependent and involves a Ca 2+ -sensing G protein-coupled receptor and UNC80 which links Na vi 2.1 to the protein UNC79 in the same complex [597]. Na vi 2.1 null mutant mice have severe disturbances in respiratory rhythm and die within 24 hours of birth [595]. Na vi 2.1 heterozygous knockout mice display increased serum sodium concentrations in comparison to wildtype littermates and a role for the channel in osmoregulation has been postulated [905].