Transmitter-Gated Channels


Abstract

Guide to Receptors and Channels

SPH Alexander, A Mathie, JA Peters

British Journal of Pharmacology (2004) 141, S61–S70. doi:10.1038/sj.bjp.0705674

Acetylcholine (nicotinic)

Overview: Nicotinic acetylcholine receptors are members of the cys-loop superfamily of transmitter-gated ion channels that includes the GABAA, strychnine-sensitive glycine and 5-HT3 receptors. All nicotinic receptors are formed as pentamers of subunits. Genes (Ensembl family ID ENSF00000000049) encoding a total of 17 subunits (α1–10, β1–4, δ, ɛ and γ) have been identified. All subunits are of mammalian origin with the exception of α8 (avian). Each subunit possesses 4 TM domains. All α subunits possess two tandem cysteine residues near to the site involved in acetylcholine binding, and subunits not named α lack those tandem cysteines. The acetylcholine-binding site is formed by at least three peptide loops on the α subunit (principal component), and three on the adjacent subunit (complementary component). The determination of a high-resolution (2.7 Å) crystal structure of the acetylcholine-binding protein from Lymnaea stagnalis, a structural homologue of the extracellular binding domain of a nicotinic receptor pentamer, has revealed the binding site in detail (reviewed by Karlin, 2002). Nicotinic receptors at the somatic neuromuscular junction of adult animals have the stoichiometry (α1)2β1ɛδ, whereas an extrajunctional (α1)2β1γδ receptor predominates in embryonic and denervated skeletal muscle. 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, α2β4) are sufficient to form a functional receptor in vitro, but more complex isoforms may exist in vivo. α5 and β3 subunits lack function when expressed pairwise, but participate in the formation of functional hetero-oligomeric receptors (e.g. α4α5αβ2, α6β2β3) when co-expressed with at least two other subunits. 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. The α7, α8, and α9 subunits form functional homo-oligomers, but can also combine with a second α subunit to constitute a hetero-oligomeric assembly (e.g. avian α7α8). 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.

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 (Lukas et al., 1999). 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).

Nomenclatureα1*α2*α3*
Previous namesMuscle-type, muscleAutonomic, ganglionic
Potency order of commonly used(α1)2β1ɛδ: sux > cyt = DMPP > nicα2β2: epi > ana-aα3β2: epi > DMPP = cyt > nic > ACh
agonists > DMPP > nic = cyt > AChα3β4: epi > ana-a >
  α2β4: epi > DMPP = nic = cyt > AChDMPP > cyt = nic > ACh
Selective antagonistsα-bungarotoxin, α-conotoxins GI and MI, pancuroniumα3β2: α-conotoxin MII (also blocks α6β2*) α3β4: α-conotoxin AuIB
Commonly used antagonists(α1)2β1γδ (embryonic): Bgt > pan >α2β2: DHβE (KB=0.9 μm), (+)-Tcα3β2: DHβE (KB = 1.6 μm), (+)-Tc
 (+)-Tc (high-affinity α1/δ binding(KB = 1.4 μm)(KB = 2.4 μm)
 site, low-affinity α/γ site)α24bT4: DHβE (KB = 3.6 μm), (+)-Tcα3β4: DHβE (KB = 19 μm), (+)-Tc
 α(1)2β1ɛδ (adult): Bgt > pan >(+)-Tc(KB = 4.2 μm)(KB =2.2 μm)
Channel blockersGallamineMecamylamine, hexamethonium
Radioligands Kd[3H]/[125I]-α-bungarotoxin[3H]/[125I]-epibatidine (hα2β4, 42 pm;[3H]/[125I]-epibatidine (hα3β2, 7 pm;
  rα2β2, 10 pm; rα2β4, 87 pm),hα3β4, 230 pm; rα3β2, 14 pm; rα3β4,
  [3H]-cytisine300 pm), [3H]-cytisine
Functional characteristicsα(1)2βγδ: PCa/PNa ˜ 0.3α2β2: PCa/PNa ˜ 1.5α3β2: PCa/PNa ˜ 1.5; α3β4:
 α(1)2βɛδ: PCa/PNa ˜ 0.9 PCa/PNa ˜ 1.0, fractional calcium flux = 2.7%
Nomenclatureα4*α6*α7*
Previous namesNeuronal, α-bungarotoxin-insensitiveNeuronal, α-bungarotoxin-sensitive
Selective agonistsα4β2: TC-2559 (Chen et al., 2003), RJR-2403 (Papke et al., 2000), ABT-594 (Donnelly-Roberts et al., 1998)AR-R17779 (Mullen et al., 2000), choline, PASB-OFP (Broad et al., 2002)
Potency order of commonly used agonistsα4β2: epi > ana-a > nic = cyt > DMPP > Ach α4β4: epi > cyt > nic > DMPP ≫ AChrα6hβ4: Ach > cyt > nic > DMPP cα6hβ4: epi > cyt ≥ nic ≥ ACh(α7)5: ana-a > epi > DMAC > OH-GTS-21 = DMPP > cyt > nic = GTS-21 ≥ ACh > cho
Selective antagonistsα6/α3β2β3 chimera: α-conotoxin PIA (Dowell et al., 2003) α6β2*: conotoxin MII (also blocks α3β2)(α7)5: α-bungarotoxin, methyllycaconitine, α-conotoxin ImI
Commonly used antagonistsα4β2: DHβE (KB = 0.1 μm), (+)-Tc (KB = 3.2 μm)cα6hβ4: mec, (+)-Tc, hex rα6hβ4: (+)-Tc(α7)5: Bgt > MLA > (+)-Tc > atr > DHβE
 α4β4: DHβE (KB = 0.01 μm), (+)-Tc (KB=0.2 μm)  
Channel blockersMecamylamine, hexamethonium
Radioligands Kd[3H]/[125I]-epibatidine (hα4β2, 10–33 pm;[3H]-epibatidine (native chick[3H]/[125I]-αbungarotoxin,
 hα4β4, 187 pm; rα4β2, 30 pm; rα4β4,cα6β4*, 35pm)((hα7)5 700–800 pm)
 85 pm), 5-iodo-A-85380 (hα4* 12 pm), [3H]-methyllycaconitine,
 [3H]-cytisine, [3H]-nicotine (native rα7*, 1.9 nm)
Functional characteristicsα4β2: PCa/PNa ˜ 1.5, fractionalPCa/PNa ˜ 6–20, fractional calcium
 calcium flux = 2.6% flux=11.4%
 α4β4: fractional calcium flux = 1.5%  
Nomenclatureα8*(avian)α9*α10*
Previous namesNeuronal, α-bungarotoxin-sensitive
Selective agonists
Potency order of commonly used agonists(α8)5: cyt ˜ nic ≥ ACh > DMPP(α9)5: cho > ACh > sub > carACh
Selective antagonists(α9)5: α-bungarotoxin, strychnine, nicotine, muscarineα10α9: α-bungarotoxin, strychnine, nicotine, muscarine
Commonly used antagonists(α8)5: Bgt > atr ≥ (+)-Tc ≥ str(α9)5: Bgt > MLA > str ˜ tropisetron > (+)-TC > bic ≥ atr ˜ epi > mec > DHβE > cyt > nic > musα10α9: Bgt > tropisetron = str > (+)-Tc > bic = atr > nic > mus
Channel blockers
Radioligands Kd[3H]/[125I]-α-bungarotoxin[3H]/[125I]-α-bungarotoxin
Functional characteristicsPCa/PNa ˜ 80

A firm consensus has yet to emerge concerning the pharmacological profiles at different nACh receptor subtypes. There are differences in profiles for a given receptor subtype across species. Moreover, measures of agonist potencies and efficacies, or antagonist affinities, are confounded by differences in experimental design across studies (oocyte or mammalian cell heterologous expression systems or natural expression; test agonist concentrations; competitive/noncompetitive modes of antagonism; electrophysiological, ion flux, or calcium ion mobilization measurements, etc.). Therefore, provisional and incomplete information about pharmacological rank order potency profiles (no efficacy data) is provided in the table, based largely on data from studies of heterologously expressed, human nACh receptors. The dagger (†) as superscript designates ligands whose rank order placement differs across species and/or experimental design.

Abbreviations: ABT-594, (R)-5-(2-azetidinylmethoxy)-2-chloropyridine; ACh, acetylcholine; ana-a, anatoxin-a; AR-R17779, (—)-spiro[1-azabicyclo[2.2.2]octane-3,5′-oxazolidin-2′-one; atr, atropine; Bgt, α-bungarotoxin; bic, bicuculline; car, carbamylcholine; cho, choline; cyt, cytisine; DHβE, dihydro-β-erythroidine; DMAC, 3-(4)-dimethylaminocinnamylidine anabaseine; DMPP, 1,1-dimethyl-4-phenylpiperazinium; epi, epibatidine; GTS-21, 3-(2,4)-dimethoxybenzylidine anabaseine (DMXB); hex, hexamethenium; mec, mecamylamine; MLA, methyllycaconitine; mus, muscarine; nic, nicotine; OH-GTS-21, 3-(4-hydroxy, 2-methoxy)benzylidine anabaseine; pan, pancuronium; PSAB-OFP, (R)-(—)-5′phenylspiro[1-azabicyclo[2.2.2] octane-3,2′-(3′H)furo[2,3-b]pyridine; RJR-2403, (E)-N-methyl-4-(3-pyridinyl)-3-butene-1-amine; str, strychnine; sub, suberyldicholine; sux, succinylcholine; (+)-Tc, (+)-tubocurarine; TC-2559, (E)-N-methyl-4-[3-(5-ethoxypyridin)yl]-3-buten-1-amine 5-iodo-A-85380, 5-iodo-3-(2(S)-azetidinylmethoxy)pyridine

Further Reading:

ASTLES, P.C., BAKER, S.R., BOOT, J.R., BROAD, L.M., DELL, C.P. & KEENAN, M. (2002). Recent progress in the development of subtype selective nicotinic acetylcholine receptor ligands. Curr. Drug Target CNS Neurol. Disord., 4, 337–348.

CORDERO-ERAUSQUIN, M., MARUBIO, L.M., KLINK, R. & CHANGEUX, J.-P. (2000). Nicotinic receptor function: new perspectives from knockout mice. Trends Pharmacol. Sci., 21, 211–217.

CORRINGER, P.J., LE NOVERE, N. & CHANGEUX, J.-P. (2000). Nicotinic receptors at the amino acid level. Annu. Rev. Pharmacol. Toxicol., 40, 431–458.

DWOSKIN, L.P. & CROOKS, P.A. (2001). Competitive neuronal nicotinic receptor antagonists: a new direction for drug discovery. J. Pharmacol. Exp. Ther., 298, 395–402.

KARLIN, A. (2002). Emerging structure of the nicotinic acetylcholine receptors. Nat. Rev. Neurosci., 3, 102–114.

LE NOVERE, N. & CHANGEUX, J.-P. (1999). The ligand-gated ion channel database. Nucleic Acids Res., 27, 340–342. (http://www.pasteur.fr/recherche/banques/LGIC).

LUKAS, R.J., CHANGEUX, J.-P., LE NOVERE, N., ALBUQUERQUE, E.X., BALFOUR, D.J., BERG, D.K., BERTRAND, D., CHIAPPINELLI, V.A., CLARKE, P.B., COLLINS, A.C., DANI, J.A., GRADY, S.R., KELLAR, K.J., LINDSTROM, J.M., MARKS, M.J., QUIK, M., TAYLOR, P.W. & WONNACOTT, S. (1999). International Union of Pharmacology. XX. Current status of the nomenclature for nicotinic acetylcholine receptors and their subunits. Pharmacol. Rev., 51, 397–401.

MCINTOSH, J.M., SANTOS, A.D. & OLIVERA, B.M. (1999). Conus peptides targeted to specific nicotinic acetylcholine receptor subtypes. Ann. Rev. Biochem., 68, 59–88.

SCHMITT, J.D. (2000). Exploring the nature of molecular recognition in nicotinic acetylcholine receptors. Curr. Med. Chem., 7, 749–800.

SKOK, V.I. (2002). Nicotinic acetylcholine receptors in autonomic ganglia. Auton. Neurosci., 97, 1–11.

TAYLOR, P., MALANZ, S., MOLLES, B.E., OSAKA, H. & TSIGELNY, I. (2000). Subunit interface selective toxins as probes of nicotinic acetylcholine receptor structure. Pflügers Arch., 440 (Suppl), R115–R117.

References:

BROAD, L.M. et al. (2002). Eur. J. Pharmacol., 452, 137–144.

CHEN, Y. et al. (2003). Neuropharmacology, 45, 334–344.

DONNELLY-ROBERTS, D.L. et al. (1998). J. Pharmacol. Exp. Ther., 285, 777–786.

DOWELL, C. et al. (2003). J. Neurosci., 25, 8445–8452.

MULLEN, G. et al. (2000). J. Med. Chem., 43, 4045–4050.

PAPKE, R.L. et al. (2000). J. Neurochem., 75, 204–216.

GABAA (γ-aminobutyric acid)

Overview: The GABAA receptor is a transmitter-gated ion channel of the cys-loop family that includes the nicotinic acetylcholine, 5-HT3 and strychnine-sensitive glycine receptors. The receptor exists as a pentamer of 4TM subunits that form an intrinsic anion channel. Sequences of six α, three β, three γ, one δ, three ρ, one ɛ, one π and one θ GABAA receptor subunits (Ensembl family ID ENSF00000000053) have been reported in mammals (Barnard, 2000; Korpi et al., 2002). The π subunit is restricted to reproductive tissue. Alternatively spliced versions of α6- (not functional), α5-, β2-, β3- and γ2-subunits exist (see Barnard, 2000). In addition, three ρ subunits (ρ1–3) function as either homo- or hetero-oligomeric assemblies (Bormann & Feigenspan, 2000; Zhang et al., 2001). Many GABAA receptor subtypes contain α, β and γ subunits with the likely stoichiometry 2α.2β.1γ (Korpi et al., 2002; Fritschy & Brünig, 2003). It is thought that the majority of GABAA receptors harbour a single type of α and β subunit variant. The α1β2γ2 hetero-oligomer constitutes the largest population of GABAA 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, ɛ and θ subunits, are less numerous, but they may nonetheless serve important functions. The α- and β-subunits contribute to the GABA-binding site and both the α- and γ-subunits are required for the benzodiazepine (BDZ) site. The particular α and γ subunit isoforms exhibit marked effects on recognition and/or efficacy at the BDZ site. Thus, receptors incorporating either α4 or α6 subunits are not recognized by ‘classical’ benzodiazepines, such as flunitrazepam. It is beyond the scope of this supplement to discuss the pharmacology of individual receptor isoforms in detail; such information can be gleaned from the reviews by Barnard et al. (1998), Frolund et al. (2002), Korpi et al. (2002) and Krogsgaard-Larsen et al. (2002). Agents that discriminate between α-subunit isoforms are noted in the table and additional agents that demonstrate selectivity between receptor isoforms are indicated in the text below.

The classification of GABAA receptors has been addressed by NC-IUPHAR (Barnard et al., 1998). The proposed scheme utilizes subunit structure and receptor function as the basis for classification. In view of the fact that a benzodiazepine (BDZ)-binding site is not unique to the GABAA receptor, and that certain receptor isoforms (i.e. those incorporating α4- or α6-subunits) are insensitive to classical benzodiazepines, it is recommended that the term ‘GABAA/benzodiazepine receptor complex’ should no longer be used and be replaced by ‘GABAA receptor’. The term benzodiazepine receptor itself is contentious because receptors should generally be named to reflect their endogenous ligand, and many discriminatory ligands acting at this site are generally not benzodiazepines (e.g. zolpidem, an imidazopyridine). Here, the term ‘BDZ site of the GABAA receptor’ is adopted as one of the two alternatives proposed by NC-IUPHAR.

NomenclatureGABAA
Ensembl family IDENSF00000000053
Selective agonists (GABA site)Muscimol, isoguvacine, THIP (gaboxadol), piperidine-4-sulphonic acid (low efficacy at α4 and α6 subunits), isonipecotic acid (α4 and α6 subunit selective via relatively high efficacy)
Selective antagonists (GABA site)Bicuculline, gabazine (SR95531)
Selective agonists (BDZ site)Diazepam (not α4- or α6-subunits), flunitrazepam (not α4- or α6-subunits), zolpidem and zaleplon (α1 subunit selective via high affinity), L838417 (α2, α3 and α5 subunit selective via partial agonist activity),
Selective antagonists (BDZ site)Flumazenil (low affinity for α4- or α6-subunits), L838417 (α1 subunit selective via antagonist activity) ZK93426
Inverse agonists (BDZ site)DMCM, Ro194603, L655708 (α5 selective via high affinity), RY024 (α5 selective via high affinity)
Endogenous allosteric modulators5α-pregnan-3α-ol-20-one (potentiation), Zn2+ (potent inhibition of receptors formed from binary combinations of α and β subunit, incorporation of a γ subunit reduces inhibitory potency, Krishek et al., 1998), extracellular protons (subunit-dependent activity, Krishek et al., 1996)
Channel blockersPicrotoxin, TBPS
Radioligands 
GABA site[3H]-muscimol, [3H]-gabazine (SR95531)
BDZ site[3H]-Flunitrazepam (not α4- or α6-subunits), [3H]-zolpidem (α1 subunit selective), [3H]-L655708 (α5 selective),
 [3H]-Ro154513 [selectively labels α4 and α6 subunit-containing receptors in the presence of a saturating concentration of a non-radioactive ‘classical’ benzodiazepine (e.g. diazepam)], [3H]-CGS8216
Anion channel[35S]-TBPS

The potency and efficacy of many GABA agonists vary between GABAA receptor isoforms (Frolund et al., 2002; Krogsgaard-Larsen et al., 2002). For example, THIP (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 (Brown et al., 2002; Bianchi & MacDonald, 2003). The GABAA 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 (see Lambert et al., 2003). 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 pictrotoxinin site, whereas positive allosteric regulation by such compounds is proposed to occur at a distinct locus. Many intravenous (e.g. etomidate, propofol) and volatile (e.g. halothane, isoflurane) anaesthetics and alcohols also exert a regulatory influence upon GABAA receptor activity. Specific amino-acid residues within GABAA receptor α- and β-subunits that influence allosteric regulation by anaesthetic and nonanaesthetic compounds have been identified (see Belelli et al., 1999; Krazowski et al., 2000; Thompson & Wafford, 2001).

In addition to the agents listed in the table, modulators of GABAA receptor activity that exhibit subunit-dependent activity include: loreclezole, etomidate, tracazolate and mefenamic acid (positive allosteric modulators with selectivity for β2/β3 over β1 subunit-containing receptors, see Korpi et al. (2002); tracazolate (intrinsic efficacy, that is, potentiation, or inhibition, is dependent upon the identity of the γ1–3, delta, or epsilon subunit co-assembled with α1 and β1 subunits (Thompson et al., 2002)); amiloride (selective blockade of receptors containing an α6 subunit (Fisher, 2002)); frusemide (selective blockade of receptors containing an α6 subunit co-assembled with β2/β3, but not β1, subunit (see Korpi et al. (2002)); La3+ (potentiates responses mediated by α1β3γ2L receptors, weakly inhibits α6β3γ2L receptors, and strongly blocks α6β3δ receptors (Saxena et al., 1997)). 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. α4β3δ) may be a consequence of the unusually low efficacy of GABA at this receptor isoform (Bianchi et al., 2003).

A bicuculline- and baclofen-insensitive site has been located in cerebellum using cis-4-aminocrotonic acid. A subpopulation of retinal GABA receptors (activated by trans-4-aminocrotonic acid) assembled from ρ subunits is similarly bicuculline-insensitive and gates Cl- channels that are insensitive to barbiturates and benzodiazepines and selectively blocked by TPMPA. Isoguvacine, THIP and piperidine-4-sulphonic acid do not activate GABAA receptors assembled from ρ subunits. Receptors formed from ρ subunits have often been found to be insensitive to neuroactive steroids (see Bormann, 2000), but relatively high concentrations of such compounds can modulate the activity of the ρ1 subunit in a stereoselective manner, 5α-pregnanes potentiating, and 5β-pregnanes inhibiting, responses elicited by low concentrations of GABA. Although these receptors have sometimes been termed GABAC receptors (see Bormann, 2000; Zhang, 2001), they may represent a subpopulation of GABAA receptors, classed as the GABAA0r subtype, under NC-IUPHAR proposals (Barnard et al., 1998). This suggestion is strengthened by the observation that single amino-acid mutations can impart some features of GABAA receptor pharmacology upon the GABAA0r subtype (Belelli et al., 1999; Walters et al., 2000).

Abbreviations: CGS8216, 2-phenylpyrazolo[4,3-c]quinolin-3(5)-one; DMCM, methyl-6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate; L655708, ethyl(s)-(11,12,13,13a-tetrahydro-7-methoxy-9-oxo)-imidazo[1,5-a]pyrrolo[2,1-c][1,4]benzodiazepine-1-carboxylate; L838417, 7-tert-butyl-3-(2,5-difluoro-phenyl)-6-(2-methyl-2H-[1,2,4]triazol-3-ylmethoxy)-[1,2,4]triazolo[4,3-b]pyridazine; Ro154513, methyl-8-azido-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-α][1,4] benzodiazepine-3-carboxylate; Ro194603, imidazo[1,5-a]1,4-thienodiazepinone; RY024, tert-butyl-8-ethynyl-5,6-dihydro-5-methyl-6-oxo-4H-imidazol[1,5-α][1,4]benzodiazepine-3-carboxylate; SR95531, 2-(3′-carboxy-2′-propyl)-3-amino-6-ρ-methoxyphenylpyridazinium bromide; TBPS, tert-butylbicyclophosphorothionate; TPMPA, (1,2,5,6-tetrahydropyridine-4-yl)methylphosphinic acid; ZK93423, 6-benzyloxy-4-methoxymethy-β-carboline-3-carboxylate ethyl ester; ZK93426, 5-isopropyl-4-methyl-β-carboline-3-carboxylate ethyl ester

Further Reading:

BARNARD, E.A., SKOLNICK, P., OLSEN, R.W., MÖHLER, H., SIEGHART, W., BIGGIO, G., BRAESTRUP, C., BATESON, A.N. & LANGER, S.Z. (1998). International Union of Pharmacology. XV. Subtypes of γ-aminobutyric acidA receptors: classification on the basis of subunit structure and receptor function. Pharmacol. Rev., 50, 291–313.

BARNARD, E.A. (2000). The molecular architecture of GABAA receptors. In: Handbook of Experimental Pharmacology, Pharmacology of GABA and Glycine Neurotransmission, Vol. 150. ed. Möhler, H. pp. 79–100. Berlin: Springer.

BELELLI, D., PISTIS, M., PETERS, J.A. & LAMBERT, J.J. (1999). General anaesthetic action at transmitter-gated inhibitory amino acid receptors. Trends Pharmacol. Sci., 20, 496–502.

BORMANN, J. (2000). The ‘ABC’ of GABA receptors. Trends Pharmacol. Sci., 21, 16–19.

BORMANN, J. & FEIGENSPAN, A. (2000). GABAC receptors: structure, function and pharmacology. In: Handbook of Experimental Pharmacology, Pharmacology of GABA and Glycine Neurotransmission, Vol. 150. ed. Möhler, H. pp. 271–296. Berlin: Springer.

CHEBIB, M. & JOHNSTON, G.A. (2000). GABA-activated ligand gated ion channels: medicinal chemistry and molecular biology. J. Med. Chem., 43, 1427–1447.

FRITSCHY, J.M. & BRUNIG, I. (2003). Formation and plasticity of GABAergic synapses: physiological mechanisms and pathophysiological implications. Pharmacol. Ther., 98, 299–323.

FROLUND, B., EBERT, B., KRISTIANSEN, U., LILJEFORS, T. & KROGSGAARD-LARSEN, P. (2002). GABAA receptor ligands and their therapeutic potentials. Curr. Top. Med. Chem., 2, 817–832.

KORPI, E.R., GRUNDER, G. & LUDDENS, H. (2002). Drug interactions at GABAA receptors. Prog. Neurobiol., 67, 113–159.

KRASOWSKI, M.D., HARRIS, R.A. & HARRISON, N.L. (2000). Allosteric modulation of GABAA receptor function by general anaesthetics and alcohols. In: Handbook of Experimental Pharmacology, Pharmacology of GABA and Glycine Neurotransmission, Vol. 150. ed. Möhler, H. pp. 141–172. Berlin: Springer.

KROGSGAARD-LARSEN, P., FROLUND, B., & LILJEFORS, T. (2002). Specific GABAA agonists and partial agonists. Chem. Rec., 2, 419–430.

LAMBERT, J.J., BELELLI, D., PEDEN, D.R., VARDY, A.W. & PETERS, J.A. (2003). Neurosteroid modulation of GABAA receptors. Prog. Neurobiol., 71, 67–80.

MÖHLER, H., FRITSCHY, J.M. & RUDOLPH, U. (2002). A new benzodiazepine pharmacology. J. Pharmacol. Exp. Ther., 300, 2–8.

RUDOLPH, U., CRESTANI, F. & MÖHLER, H. (2001). GABAA receptor subtypes; dissecting their pharmacological functions. Trends Pharmacol. Sci., 22, 188–194.

THOMPSON, S.-A. & WAFFORD, K. (2001). Mechanism of action of general anaesthetics — new information from molecular pharmacology. Curr. Opin. Pharmacol., 1, 78–83.

ZHANG, D., PAN, Z.H., AWOBULUYI, M. & LIPTON, S.A. (2001). Structure and function of GABAC receptors: a comparison of native versus recombinant receptors. Trends Pharmacol. Sci., 22, 121–132.

References:

BELELLI, D. et al. (1999). Br. J. Pharmacol., 127, 601–604.

BIANCHI, M.T. & MACDONALD, R.L. (2003). J. Neurosci., 23, 10934–10943.

BROWN, N. et al. (2002). Br. J. Pharmacol., 136, 965–974.

KRISHEK, B.J. et al. (1996). J. Physiol., 492, 431–443.

KRISHEK, B.J. et al. (1998). J. Physiol., 507, 639–652.

SAXENA, N.C. et al. (1997). Mol. Pharmacol., 51, 328–335.

THOMPSON, S.A. et al. (2002). Mol. Pharmacol., 61, 861–869.

WALTERS, R.J. et al. (2000). Nat. Neurosci., 3, 1274–1281.

Glutamate (ionotropic)

Overview: The ionotropic glutamate receptors comprise members of the NMDA (N-methyl-d-aspartate), AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) and kainate receptor classes, named originally according to their preferred, synthetic, agonist (see Dingledine et al. (1999) for a comprehensive review). Receptor heterogeneity within each class arises from the homo-oligomeric, or hetero-oligomeric, assembly of distinct subunits into cation-selective tetramers. All glutamate receptor subunits have the membrane topology of an extracellular N-terminus, three transmembrane domains (TM1, TM3 and TM4), a channel lining re-entrant ‘p-loop’ (MD2) located between TM1 and TM3 that enters and exits the membrane at its cytoplasmic surface, and an intracellular C-terminus (see Madden, 2002). 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 in the reviews by Dingledine et al. (1999), Yamakura & Shimoji (1999), Jane et al. (2000), Cull-Candy et al. (2001) and Huettner (2003). 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 receptors has been addressed by NC-IUPHAR (Lodge & Dingledine, 2000). The proposed scheme, which recommends a revised nomenclature for ionotropic glutamate receptor subunits, is adopted here. Commonly used alternative appellations are indicated in parenthesis.

NMDA receptors: NMDA receptors assemble as heteromers that may be drawn from GLUN1 (NMDA-R1, NR1, GluRξ1), GLUN2A (NMDA-R2A, NR2A, GluRɛ1), GLUN2B (NMDA-R2B, NR2B, GluRɛ2), GLUN2C (NMDA-R2C, NR2C, GluRɛ3), GLUN2D (NMDA-R2D, NR2D, GluRɛ4), GLUN3A (NMDA-R3A) and GLUN3B (NMDA-R3B) subunits. Alternative splicing can generate eight isoforms (one nonfunctional) of GLUN1 with differing pharmacological properties. Various splice variants of GLUN2B,2C,2D and GLUN3A have also been reported (see Cull-Candy et al., 2001). Activation of NMDA receptors requires the binding of two agonists, glutamate to the GLUN2 subunit and glycine to the GLUN1 subunit. The minimal requirement for efficient functional expressional of NMDA receptors in vitro is a diheteromeric assembly of GLUN1 and at least one GLUN2 subunit variant, most likely in a dimer of dimers arrangement (Madden, 2002). However, more complex triheteromeric assemblies, incorporating multiple subtypes of GLUN2 subunit, or GLUN3 subunits, can be generated in vitro and occur in vivo. The NMDA receptor channel commonly has a high relative permeability to Ca2+ and is blocked, in a voltage-dependent manner, by Mg2+ at resting potential.

NomenclatureNMDA
Ensembl family IDENSF00000000436
Selective agonists (glutamate site)Aspartate, NMDA, D,L(tetrazol-5-yl)glycine, homoquinolinic acid
Selective antagonists (glutamate site)d-AP5, CGS19755, CGP37849, LY233053, d-CCPene (GLUN2A = GLUN2B > GLUN2C = GLUN2D), conantokin-G (GLUN2B > GLUN2D = GLUN2C = GLUN2A)
Selective agonists (glycine site)Glycine, d-serine, (+)-HA966 (partial agonist)
Selective antagonists (glycine site)5,7-Dichlorokynurenate, L689560, L701324, GV196771A
Channel blockersMg2+, dizocilpine (MK801), ketamine, phencyclidine, memantine, amantidine
Radioligands 
Glutamate site[3H]-CPP, [3H]-CGS19755, [3H]-CGP39653
Glycine site[3H]-Glycine, [3H]-L689560, [3H]-MDL105519
Cation channel[3H]-Dizocilpine

In addition to the glutamate and glycine binding sites documented in the table, physiologically important inhibitory modulatory sites exist for Mg2+, Zn2+ and protons (see Dingledine et al., 1999; Yamakura & Shimoji, 1999; Cull-Candy et al., 2001). The receptor is also allosterically modulated, in both positive and negative directions, by endogenous neuroactive steroids in a subunit-dependent manner. For example, pregnenolone sulphate potentiates diheteromeric assemblies of GLUN1/GLUN2A and GLUN1/GLUN2B subunits, but inhibits receptors assembled as GLUN1/GLUN2C, or GLUN1/GLUN2D, heteromers (Malayev et al., 2002). Tonic proton blockade of NMDA receptor function is alleviated by polyamines and the inclusion of exon 5 within GLUN1 subunit splice variants, whereas the noncompetitive antagonist ifenprodil increases the fraction of receptors blocked by protons at ambient concentration. Receptors assembled from GLUN1 and GLUN2C subunits are unusually insensitive to proton blockade. Ifenprodil, its analogue CP101606, haloperidol, felbamate and Ro84304 discriminate between recombinant NMDA receptors assembled from GLUN1 and either GLUN2A, or GLUN2B, subunits by acting as selective, noncompetitive, antagonists of heterooligomers incorporating GLUN2B. 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 GLUN2Bversus GLUN2A, GLUN2D and, to a lesser extent, GLUN2C subunits. Conversely, the voltage-independent component of NMDA receptor inhibition by Zn2+ is most pronounced for receptors that contain the GLUN2Aversus GLUN2B subunit. 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 Mg2+, single-channel conductance and channel deactivation time (Cull-Candy et al., 2001). Incorporation of the GLUN3A subunit into triheteromers containing GLUN1 and GLUN2 subunits is associated with decreased single-channel conductance, reduced permeability to Ca2+ and decreased susceptibility to block by Mg2+. Reduced permeability to Ca2+ has also been observed following the inclusion of GLUN3B in triheteromers.

AMPA and kainate receptors: AMPA receptors assemble as homomers, or heteromers, that may be drawn from GLUA1 (GluR1, GluRA, GluR-A, GluR-K1), GLUA2 (GluR2, GluRB, GluR-B, GluR-K2), GLUA3 (GluR3, GluRC, GluR-C, GluR-K3), or GLUA4 (GluR4, GluRD, GluR-D) subunits. Homotetramers formed from GLUA2 subunits express relatively poorly due to their retention within the endoplasmic reticulum (see Bredt & Nicoll, 2003). Functional kainate receptors can be expressed as homomers of GLUK5 (GluR5, GluR-5, EAA3), GLUK6 (GluR6, GluR-6, EAA4), or GLUK7 (GluR7, GluR-7, EAA5) subunits. GLUK5–7 subunits are also capable of assembling into heterotetramers (see Lerma, 2003). Two additional kainate receptor subunits, GLUK1 (KA1, KA-1, EAA1) and GLUK2 (KA2, KA-2, EAA2), when expressed individually, form high-affinity binding sites for kainate, but lack function (see Heuttner, 2003). GLUK1 and GLUK2 can form heteromers when co-expressed with GLUK5–7 subunits (Lerma, 2003). 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 Ca2+; (2) blocked by intracellular polyamines at depolarized potentials causing inward rectification; (3) blocked by extracellular argiotoxin and Joro spider toxins and (4) demonstrate higher channel conductances than receptors containing the edited form of GLUA2 (Seeburg & Hartner, 2003). GLUK5 and GLUK6, but not other kainate receptor subunits, are similarly edited and broadly similar functional characteristics apply to kainate receptors lacking either an RNA-edited GLUK5, or GLUK6, subunit (Lerma, 2003). Native AMPA and kainate receptors displaying differential channel conductances, Ca2+ permeabilites and sensitivity to block by intracellular polyamines have been identified. 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. Splice variants of GLUK5–7 also exist, but their functional significance is unknown (Lerma, 2003).

NomenclatureAMPAKainate
Ensembl family IDENSF00000000118ENSF00000000118
Selective agonistsAMPA, (s)-5-flurowillardiineATPA, LY339434, LY382884, (s)-5-iodowillardiine, (2s,4r)-4-methyl glutamate (SYM2081), domoic acid (except homomeric GLUK7), kainate
Selective antagonistsNBQX, ATPO, LY293558, GYKI53655/LY300168 (active isomer GYKI 53784/LY303070) (noncompetitive)LY294486
Channel blockersIntracellular polyamines, extracellular argiotoxin, extracellular Joro toxin (all subtype selective)Intracellular polyamines (subtype selective)
Radioligands[3H]-AMPA, [3H]-CNQX[3H]-Kainate, [3H-](2s,4r)-4-methyl glutamate]

All AMPA receptors are additionally activated by kainate (and domoate) with relatively low potency (EC50 100 μm). Activation of kainate receptors by AMPA shows subunit dependency (e.g. heteromers containing GLUK6- and GLUK2-subunits are sensitive; homomers assembled from the GLUK6 subunit, or GLUK7 subunit, are insensitive). Quinoxalinediones such as CNQX and NBQX show limited selectivity between AMPA and kainate receptors. LY293558 also has kainate (GLUK5) receptor activity. ATPO, a potent competitive antagonist of AMPA receptors, has a weaker antagonist action at kainate receptors comprising GLUK5 subunits, but is devoid of activity at kainate receptors formed from GLUK6 or GLUK6/GLUK2 subunits. The pharmacological activity of ATPO resides with the (s)-enantiomer. ATPA, LY294486, LY339434, LY382884 and (s)-5-iodowillardiine interact selectively with kainate receptors containing a GLUK5 subunit. (2s,4r)-4-methyl glutamate (SYM2081) is equipotent in activating (and desensitizing) GLUK5 and GLUK6 receptor isoforms and, via the induction of desensitization at low concentrations, has been used as a functional antagonist of kainate receptors. Both (2s,4r)-4-methyl glutamate and LY339434 have agonist activity at NMDA receptors. (2s,4r)-4-methyl glutamate is also an inhibitor of the glutamate transporters EAAT1 and EAAT2.

Abbreviations: AMPA, (rs)-α-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid; APTA, (rs)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl)propionic acid; ATPO, (rs)-2-amino-3-(3-[5-tert-butyl-3-(phosphonomethoxy)-4-isoxazolyl]propionic acid; CGP37849, (rs)-(E)-2-amino-4-methylphosphono-3-pentanoic acid; CGP39653, (rs)-(E)-2-amino-4-propylphosphono-3-pentanoic acid; CGS19755, 4-phosphonomethyl-2-piperidinecarboxylic acid; CNQX, 6-cyano-7-nitroquinoxaline-2,3-dione; CP101606, (1s,2s)-1-4-hydroxyphenyl)-2-(4-hydroxy-4-phenylpiperidino)-1-propanol; CPP, (±)-2-carboxypiperazine-4-yl)propyl-1-phosphonic acid; d-AP5, d (2)-2-amino-5-phosphonopentanoate; d-CCPene, 3-(2-carboxypiperazine-4-yl)-propenyl-1-phosphonic acid; GV196771A, E-4,6-dichloro-3-(2-oxo-1-phenyl-pyrrolidin-3-ylidenemethyl)-1H-indole-2-carboxylic acid; GYKI53655, 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-(3N-methylcarbamate)-2,3-benzodiazepine; also known as LY300168; GYKI53784, (-)1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-4,5-dihydro-3-methylcarbamoyl-2,3-benzodiazepine, also known as LY303070; HA966, 3-amino-1-hydroxypyrrolid-2-one; L689560, trans-2-carboxy-5,7-dichloro-4-phenylaminocarbonylamino-1,2,3,4-tetrahydroquinoline; L701324, 7-chloro-4-hydroxy-3-(3-phenoxy)phenyl-2(H)quinolone; LY233053, cis(1)-4-[(2H-tetrazole-5yl)methyl]piperidine-2-carboxylic acid; LY233536, (rs)-6-(1H-tetrazol-5-ylmethyl)decahydraisoquinoline-3-carboxylic acid; LY293558, 3s,4αr,6r,8αr-6-[2-(1(2)H-tetrazol-5yl)ethyl]-decahydroisoquinoline-3-carboxylate; LY294486, (3rs,4αrs,6sr,8rs)-6-([{(1H-tetrazol-5yl)methyl}oxy]methyl)-1,2,3,4α,5,6,7,8,8α-decahydroisoquinolone-3-carboxylic acid; LY339434, (2s,4r,6E)-2-amino-4-carboxy-7-(2-naphthyl)hept-6-enoic acid; LY382884, (3s, 4αr, 6s, 8αr)-6-((4-carboxyphenyl)methyl-1,2,3,4,4α,5,6,7,8,8α-decahydro isoquinoline-3-carboxylic acid; MDL105519, (E)-3-(2-phenyl-2-carboxyethenyl)-4,6-dichloro-1H-indole-2-carboxylic acid; NBQX, 6-nitro-7-sulfamoyl-benz(f)quinoxaline-2,3-dione; Ro8–4304, 4–3-[4-(4-fluro-phenyl-)3,6-dihydro-2H-pyridin-1-yl]-2-hydroxy-propoxy-benzamide

Further Reading:

BREDT, D.S. & NICOLL, R.A. (2003). AMPA receptor trafficking at excitatory synapses. Neuron, 40, 361–379.

CULL-CANDY, S., BRICKLEY, S. & FARRANT, M. (2001). NMDA receptor subunits, diversity, development and disease. Curr. Opin. Neurobiol., 11, 327–335.

DANYSZ, W. & PARSONS, C.G. (1998). Glycine and N-methyl-D-aspartate receptors: physiological significance and possible therapeutic applications. Pharmacol. Rev., 50, 597–664.

DINGLEDINE, R., BORGES, K., BOWIE, D. & TRAYNELIS, S.F. (1999). The glutamate receptor ion channels. Pharmacol. Rev., 51, 7–61.

HUETTNER, J.E. (2003). Kainate receptors and synaptic transmission. Prog. Neurobiol., 70, 387–407.

JANE, D.E., TSE, H.-W., SKIFTER, D.A., CHRISTIE, J.M. & MONAGHAN, D.T. (2000). Glutamate receptor ion channels: activators and inhibitors. In: Handbook of Experimental Pharmacology, Pharmacology of Ionic Channel Function: Activators and Inhibitors. ed. Endo, M., Kurachi, Y. & Mishina, M. Vol 147, pp. 415–478, Berlin: Springer.

LERMA, J. (2003). Roles and rules of kainate receptors in synaptic transmission. Nat. Rev. Neurosci., 4, 481–495.

LODGE, D. & DINGLEDINE, R. (2000). Ionotropic glutamate receptors. In: The IUPHAR Receptor Compendium. pp. 189–194. IUPHAR Media.

LOFTIS, J.M. & JANOWSKY, A. (2003). The N-methyl-D-aspartate subunit NR2B: localization, functional properties, regulation and clinical implications. Pharmacol. Ther., 97, 55–85.

MADDEN, D.R. (2002). The structure and function of glutamate receptor ion channels. Nat. Rev. Neurosci., 3, 91–101.

MISHINA, M. (2000). Molecular diversity, structure and function of glutamate receptor ion channels. In: Handbook of Experimental Pharmacology, Pharmacology of Ionic Channel Function: Activators and Inhibitors. ed. Endo, M., Kurachi, Y. & Mishina, M. Vol 147, pp. 393–414. Berlin: Springer.

SEEBERG, P.H. & HARTNER, J. (2003). Regulation of ion channel/neurotranmitter receptor function by alternative splicing. Curr. Opin. Neurobiol., 13, 279–283.

YAMAKURA, T. & SHIMOJI, K. (1999). Subunit and site-specific pharmacology of the NMDA receptor. Prog. Neurobiol., 59, 279–298.

References:

MALAYEV, A. et al. (2002). Br. J. Pharmacol., 135, 901–909.

Glycine

Overview: The inhibitory glycine receptor is a member of the cys-loop superfamily of transmitter-gated ion channels that includes the GABAA, nicotinic acetylcholine and 5-HT3 receptors. Structurally and functionally, the glycine receptor is most closely related to the GABAA receptor. The receptor is expressed either as a homo- (α subunit) or hetero- (3α:2β subunits) pentameric assembly containing an intrinsic Cl- channel. Four differentially expressed isoforms of the α-subunit (α1–α4) and one variant of the β-subunit (β1) have been identified by genomic and cDNA cloning. Further diversity originates from alternative splicing of the α1, α2 and α3 subunits. In addition, a rat specific α2 subunit variant (termed α2*) demonstrates greatly reduced affinity towards glycine and strychnine. 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 α4 subunit may be a pseudogene in man and is not tabulated here. 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 reduces single channel conductance and alters pharmacology. It also anchors the receptor, via an amphipathic sequence within the intracellular loop region, to gephyrin, a cytoskeletal attachment protein, that binds to tubulin and thus clusters and anchors hetero-oligomeric receptors to the synapse (see Kneussel & Betz, 2000; Moss & Smart, 2001). There is no NC-IUPHAR recommendation for the classification of glycine receptors. The provisional nomenclature adopted here classifies glycine receptor isoforms according to their α-subunit.

Nomenclatureα1α2α3
Ensembl IDENSG00000145888ENSG00000101958ENSG00000145451
Selective agonists (potency order)Glycine > β-alanine > taurineGlycine > β-alanine > taurineGlycine > β-alanine > taurine
Selective antagonists and modulatorsStrychnine, PMBA, picrotoxin (+βStrychnine, PMBA, picrotoxinStrychnine, picrotoxin (+β weakens
with subunit selectivityweakens block), pregnenolone(+β weakens block), pregnenoloneblock), αEMBTL (+β converts block to
 sulphate (Ki = 1.9 μm; +β = 2.7 μm),sulphate (Ki = 5.5 μm; +β = 10.1 μm),potentiation)
 tropisetron (Ki = 84 μm; +β = 44 μm), colchicine (IC50=324 μm)tropisetron (Ki = 13 μm; +β = 5.4 μm), colchicine (IC50 = 64 μm), DCKA (IC50 = 188 μm) 
Selective potentiatorsαEMBTL (αEMBTL reduces α3-mediated responses)
Channel blockers (IC50)CyanotriphenylborateCyanotriphenylborate 
 (1.3 μm + β = 2.8 μm), BN52021 (270 nm)(> > 20 μm; + β = 7.5 μm) 
Radioligands[3H]-strychnine[3H]-strychnine[3H]-strychnine
Functional characteristicsγ = 86 pS (main state) (+ β = 44 pS)γ = 111 pS (main state) (+ β = 54 pS)γ = 105 pS (main state) (+ β = 48)

Data in the table refer to homo-oligomeric assemblies of the α-subunit; significant changes introduced by co-expression of the β1 subunit (ENSG00000109738) are indicated in parenthesis. Not all glycine receptor ligands are listed within the table, but those that may be useful in distinguishing between glycine receptor isoforms are indicated. Pregnenolone sulphate, tropisetron and colchicine, for example, although not selective antagonists of glycine receptors, are included for this purpose. Strychnine is the most potent and selective competitive glycine receptor antagonist with affinities in the range 5–15 nm. Several analogues of muscimol and piperidine act as agonists and antagonists of both glycine and GABAA receptors. Picrotoxin has been reported to block the chloride channel of glycine receptors (Pribilla et al., 1992) or, by contrast, to act as a competitive antagonist (Lynch et al., 1995). Picrotoxin shows strong selectivity towards homomeric receptors composed of α subunits (Pribilla et al., 1992; Lynch et al., 1995), and its components, picrotoxinin and picrotin, have similar inhibitory potencies. In addition to the compounds listed in the table, numerous agents act as allosteric regulators of glycine receptors (reviewed by Rajendra et al., 1997; Laube et al., 2002). Zn2+ acts through distinct binding sites of high and low affinity to allosterically enhance channel function at low (<10 μm) concentrations and inhibit responses at higher (>50–100 μm) concentrations. The effect of Zn2+ is mimicked by Ni2+. Elevation of intracellular Ca2+ 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 act allosterically to enhance glycine receptor function. 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-HT3 receptor antagonist tropisetron (ICS 205–930), which may evoke potentiation or inhibition depending upon the subunit composition of the receptor and the concentrations of the modulator and glycine employed (Maksay et al., 1999; Supplisson & Chesnoy-Marchais, 2000). Additional tropienes, including atropine, modulate glycine receptor activity.

Abbreviations: αEMBTL, α-ethyl,α-methyl-γ-thiobutyrolactone; BN52021, (±)-trans-2,5-bis(3,4,5-trimethoxyphenyl)-1,3-dioxolane; DCKA, dichlorokynurenic acid; PMBA, 3-[2′-phosphonomethyl[1,1′-biphenyl]-3-yl]alanine

Further Reading:

BETZ, H., KUHSE, J., SCHMIEDEN, V., LAUBE, B., KIRSCH, J. & HARVEY R.J. (1999). Structure and functions of inhibitory and excitatory glycine receptors. Ann. N.Y. Acad. Sci., 868, 667–676.

BETZ, H., HARVEY, R.J. & SCHLOSS, P. (2000). Structures, diversity and pharmacology of glycine receptors and transporters. In: Handbook of Experimental Pharmacology, Pharmacology of GABA and Glycine Neurotransmission. ed. Möhler, H. Vol 150, pp. 375–401. Springer: Berlin.

BREITINGER, H.-G. & BECKER, C.-M. (2003). The inhibitory glycine receptor — simple views of a complicated channel. Chem. BioChem., 3, 1042–1052.

LAUBE, B., MAKSAY, G., SCHEMM, R. & BETZ, H. (2002). Modulation of glycine receptor function: a novel approach for therapeutic intervention at inhibitory synapses. Trends Pharmacol. Sci., 23, 519–527.

LEWIS, T.M. & SCHOFIELD, P.R. (1999). Structure-function relationships for the human glycine receptor: insights from hyperekplexia mutations. Ann. N.Y. Acad. Sci., 868, 681–684.

KNEUSSEL, M. & BETZ, H. (2000). Clustering of inhibitory neurotransmitter receptors at developing postsynaptic sites: the membrane activation model. Trends Neurosci., 23, 429–435.

MOSS, S.J. & SMART, T.G. (2001). Constructing inhibitory synapses. Nat. Neurosci. Rev., 2, 240–250.

RAJENDRA, S., LYNCH, J.W. & SCHOFIELD, P.R. (1997). The glycine receptor. Pharmacol. Ther., 73, 121–146.

References:

LYNCH, J.W. et al. (1995). J. Biol. Chem., 270, 13799–13806.

MAKSAY, G. et al. (1999). J. Neurochem., 73, 802–806.

PRIBILLA, I. et al. (1992). EMBO. J., 11, 4305–4311.

SUPPLISSON, S. & CHESNOY-MARCHAIS, D. (2000). Mol. Pharmacol., 58, 763–770.

5-Hydroxytryptamine3 (5-HT3)

Overview: The 5-HT3 receptor (nomenclature as agreed by NC-IUPHAR Subcommittee on 5-hydroxytryptamine (serotonin) receptors (Hoyer et al., 1994) and subsequently revised (Hartig et al., 1996)) is a transmitter-gated ion channel of the cys-loop family that includes the nicotinic acetylcholine, GABAA and strychnine-sensitive glycine receptors. The receptor exists as a pentamer of 4TM subunits that form an intrinsic cation channel. Three 5-HT3 receptor subunits (5-HT3A, 5-HT3B and 5-HT3C) have been cloned, but only homo-oligomeric assemblies of 5-HT3A and hetero-oligomeric assemblies of 5-HT3A and 5-HT3B subunits have been characterised in detail. Putative HTR3D and HTR3E genes have also been described (Niesler et al., 2003), but there is presently no evidence that they encode bona fide 5-HT3 receptor subunits that are functional. The 5-HT3B subunit imparts distinctive biophysical properties upon hetero-oligomeric (5-HT3A/5-HT3B) versus homo-oligomeric (5-HT3A) recombinant receptors (Davies et al., 1999; Dubin et al., 1999; Hanna et al., 2000; Kelley et al., 2003; Stewart et al., 2003), but generally has little effect upon the apparent affinity of agonists, or the affinity of antagonists (Brady et al., 2001; but see Dubin et al., 1999). The diversity of 5-HT3 receptors is increased by alternative splicing of the 5-HT3A subunit.

Nomenclature5-HT3
Former namesM
Ensembl ID5-HT3A ENSG00000166736, 5-HT3B ENSG00000149305
Selective agonists (pEC50)2-Methyl-5-HT (5.3–5.5), 3-chlorophenyl-biguanide (5.4–5.7)
Selective antagonists (pIC50)Granisetron (9.5), ondansetron (9.5), tropisetron (9.2)
Channel blockersDiltiazem, TMB-8
Radioligands[3H]-ramosetron (0.15 nm), [3H]-granisetron (1.2 nm), [3H]-(S)-zacopride (2.0 nm), [3H]-GR65630 (2.6 nm), [3H]-LY278584 (3 nm)
Functional characteristicsγ = 0.4–0.8 pS (+5-HT3B, γ = 16 pS); inwardly rectifying current (+ 5-HT3B, rectification reduced); relative permeability to divalent cations reduced by co-expression of the 5-HT3B subunit

Data in the table refer to homo-oligomeric assemblies of the human 5-HT3A subunit, or the receptor native to human tissues. Significant changes introduced by co-expression of the 5-HT3B subunit are indicated in parenthesis. Human (Belelli et al., 1995; Miyaki et al., 1995), rat (Isenberg et al., 1993), mouse (Maricq et al., 1991), guinea-pig (Lankiewicz et al., 1998) and ferret (Mochizuki et al., 2000) orthologues of the 5-HT3A receptor subunit have been cloned, that exhibit intraspecies variations in receptor pharmacology. Notably, most ligands display significantly reduced affinities at the guinea-pig 5-HT3 receptor in comparison with other species. In addition to the agents listed in the table, native and recombinant 5-HT3 receptors are subject to allosteric modulation by extracellular divalent cations, alcohols, several general anaesthetics and 5-hydroxy and halide-substituted indoles (see the reviews by Lambert et al., 1995; Parker et al., 1996; Peters et al., 1997; Lovinger, 1999).

Abbreviations: GR65630, 3-(5-methyl-1H-imidazol-4-yl)-1-(1-methyl-1H-indol-3-yl)-1-propanone; LY278584, 1-methyl-N-(8-methyl-8-azabicyclo[3.2.1]oct-3-yl)-1H-indazole-3-carboxamide; TMB-8, 8-(diethylamine)octyl-3,4,5-trimethoxybenzoate

Further Reading:

BARNES, N.M. & SHARP, T. (1999). A review of central 5-HT receptors and their function. Neuropharmacology, 38, 1083–1152.

HOYER, D., CLARKE, D.E., FOZARD, J.R., HARTIG, P.R., MARTIN, G.R., MYLECHARANE, E.J., SAXENA, P.R. & HUMPHREY, P.P. (1994). International Union of Pharmacology. VII. Classification of receptors for 5-hydroxytryptamine (serotonin). Pharmacol. Rev., 46, 157–203.

HARTIG, P.R., HOYER, D., HUMPHREY, P.P. & MARTIN, G.R. (1996). Alignment of receptor nomenclature with the human genome: classification of 5-HT1B and 5-HT1D receptor subtypes. Trends Pharmacol. Sci., 17, 103–105.

LOVINGER, D.M. (1999). 5-HT3 receptors and the neural actions of alcohols: an increasingly exciting topic. Neurochem. Int., 35, 125–130.

PARKER, R.M., BENTLEY, K.R. & BARNES, N.M. (1996). Allosteric modulation of 5-HT3 receptors: focus on alcohols and anaesthetic agents. Trends Pharmacol. Sci., 17, 95–99.

LAMBERT, J.J., PETERS, J.A. & HOPE, A.G. (1995). 5-HT3 receptors. In Handbook of Receptors and Ion Channels. Ligand and Voltage-Gated Ion Channels. ed. North, R.A. pp. 117–211. Boca Raton: CRC Press.

PETERS, J.A., HOPE, A.G., SUTHERLAND, L. & LAMBERT, J.J. (1997). Recombinant 5-hydroxytryptamine3 receptors. In: Recombinant Cell Surface Receptors: Focal Point for Therapeutic Intervention. ed. Brown, M.J. pp. 119–154. Austin: R.J. Landes Company.

REEVES, D.C. & LUMMIS, S.C.R. (2002). The molecular basis of the structure and function of the 5-HT3 receptor: a model ligand-gated ion channel. Mol. Membr. Biol., 19, 11–26.

VAN HOOFT, J.A. & YAKEL, J.L. (2003). 5-HT3 receptors in the CNS: 3B or not 3B? Trends Pharmacol. Sci., 24, 157–160.

References:

BELELLI, D. et al. (1995). Mol. Pharmacol., 48, 1054–1062.

BRADY, C.A. et al. (2001). Neuropharmacology, 41, 282–284.

DAVIES, P.A. et al. (1999). Nature, 397, 359–363.

DUBIN, A. et al. (1999). J. Biol. Chem., 274, 30799–30810.

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ISENBERG, K.E. et al. (1993). Neuroreport, 18, 121–124.

KELLEY, S.P. et al. (2003). Nature, 424, 321–324.

LANKIEWICZ, S. et al. (1999). Mol. Pharmacol., 53, 202–212.

MARICQ, A.V. et al. (1991). Science, 254, 432–437.

MIYAKE, A. et al. (1995). Mol. Pharmacol., 48, 407–416.

MOCHIZUKI, S. et al. (2000). Eur. J. Pharmacol., 399, 97–106.

NIESLER, B. et al. (2003). Gene, 310, 101–111.

STEWART, A. et al. (2003). Neuropharmacology, 44, 214–223.

P2X

Overview: P2X receptors (nomenclature as agreed by NC-IUPHAR Subcommittee on P2X Receptors, Khakh et al., 2001) are putative trimeric (Jiang et al., 2003) transmitter-gated channels conducting Na+, K+ and Ca2+, with two putative transmembrane domains, where the endogenous ligand is ATP. The relationship of many of the cloned receptors to endogenously expressed receptors is not yet established. The Nomenclature Subcommittee has recommended that, for P2X receptors, structural criteria should be the initial criteria for nomenclature, where possible. Functional P2X receptors exist as polymeric transmitter-gated channels; the native receptors may occur as homopolymers (e.g. P2X1 in smooth muscle) or heteropolymers (e.g. P2X2:P2X3 in the nodose ganglion). P2X7 receptors have been shown to form functional homopolymers which form pores permeable to low molecular weight solutes (Surprenant et al., 1996).

NomenclatureP2X1P2X2P2X3P2X4
Ensembl IDENSG00000108405ENSG00000177026ENSG00000109991ENSG00000135124
Selective agonistsl-βγ-meATP, αβ-meATPαβ-meATP
Selective antagonistsTNP-ATP (pIC50 8.9, Virginio et al., 1998), Ip5I (pIC50 8.5), NF023 (pIC50 6.7)TNP-ATP (pIC50 8.9, Virginio et al., 1998), A317491 (7.5, Jarvis et al., 2002)
NomenclatureP2X5P2X6P2X7
Other namesP2Z
Ensembl IDENSG00000083454ENSG00000099957ENSG00000089041
Selective antagonistsBrilliant Blue G (pIC50 8.0, Jiang et al., 2000)

Agonists listed show selectivity within recombinant P2X receptors of ca. one order of magnitude. Several P2X receptors (particularly P2X1 and P2X3) may be inhibited by desensitisation using stable agonists (e.g. αβ-meATP); suramin and PPADS are nonselective antagonists at rP2X1–3,5 and hP2X4, but not rP2X4,6,7 (Buell et al., 1996), and can also inhibit ATPase activity (Crack et al., 1994). Ip5I is inactive at rP2X2, an antagonist at rP2X3 (pIC50 5.6), and enhances agonist responses at rP2X4 (King et al., 1999). Antagonist potency of NF023 at recombinant P2X2, P2X3 and P2X5 is two orders of magnitude lower than that at P2X1 receptors (Soto et al., 1999). Some recombinant P2X receptors expressed to high density bind [35S]-ATPγS and [3H]-αβ-meATP, although the latter can also bind to 5′-nucleotidase (Michel et al., 1995).

Abbreviations: A317491, 5-({[3-phenoxybenzyl][(1S)-1,2,3,4-tetrahydro-1-naphthalenyl]amino}carbonyl)-1,2,4-benzenetricarboxylic acid; ATPγS, adenosine 5′-(3-thio) triphosphate; Ip5I, diinosine-5′,5″-pentaphosphate; αβ-meATP, αβ-methylene-adenosine 5′-triphosphate; βγ-meATP, βγ-methylene-adenosine 5′-triphosphate; NF023, 8,8′-(carbonylbis[imino-3,1-phenylene carbonylimino])bis-1,3,5-naphthalenetrisulphonic acid; PPADS, pyridoxalphosphate-6-azophenyl-2′,4′-disulphonate; TNP-ATP, 2′,3′-O-(2,4,6-trinitrophenyl)-ATP

Further Reading:

BURNSTOCK, G. & WILLIAMS, M. (2000). P2 purinergic receptors: modulation of cell function and therapeutic potential. J. Pharmacol. Exp. Ther., 295, 862–869.

BURNSTOCK, G. (2002). Potential therapeutic targets in the rapidly expanding field of purinergic signalling. Clin. Med., 2, 45–53.

JACOBSON, K.A., JARVIS, M.F. & WILLIAMS, M. (2002). Purine and pyrimidine (P2) receptors as drug targets. J. Med. Chem., 45, 4057–4093.

KHAKH, B.S. (2001). Molecular physiology of P2X receptors and ATP signalling at synapses. Nat. Rev. Neurosci., 2, 165–174.

KHAKH, B.S., BURNSTOCK, G., KENNEDY, C., KING, B.F., NORTH, R.A., SÉGUÉLA, P., VOIGT, M. & HUMPHREY, P.P.A. (2001). International Union of Pharmacology. XXIV. Current status of the nomenclature and properties of P2X receptors and their subunits. Pharmacol. Rev., 53, 107–118.

NÖRENBERG, W. & ILLES, P. (2000). Neuronal P2X receptors: localisation and functional properties. Naunyn-Schmiedeberg's Arch. Pharmacol., 362, 324–339.

NORTH, R.A. (2002). Molecular physiology of P2X receptors. Physiol. Rev., 82, 1013–1067.

NORTH, R.A. & SURPRENANT, A. (2000). Pharmacology of cloned P2X receptors. Annu. Rev. Pharmacol. Toxicol., 40, 563–580.

WILLIAMS, M. & JARVIS, M.F. (2000). Purinergic and pyrimidinergic receptors as potential drug targets. Biochem. Pharmacol., 59, 1173–1185.

References:

BUELL, G. et al. (1996). EMBO J., 15, 55–62.

CRACK, B.E. et al. (1994). Br. J. Pharmacol., 113, 1432–1438.

JIANG, L.H. et al. (2000). Mol. Pharmacol., 58, 82–88.

JIANG, L.H. et al. (2003). J. Neurosci., 23, 8903–8910.

KHAKH, B.S. et al. (2001). Pharmacol. Rev., 53, 107–118.

KING, B.F. et al. (1999). Br. J. Pharmacol., 128, 981–988.

MICHEL, A.D. et al. (1995). Br. J. Pharmacol., 115, 767–774.

SOTO, F. et al. (1999). Neuropharmacology, 38, 141–149.

SURPRENANT, A. et al. (1996). Science, 272, 735–738.

VIRGINIO, C. et al. (1998). Mol. Pharmacol., 53, 969–973.

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