GABA_A (γ-aminobutyric acid)

Overview: The GABA_A receptor is a ligand-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 ε, one π and one θ GABA_A receptor subunits (Ensembl gene family ID ENSF00000000053) have been reported in mammals (Korpi et al., 2002; Whiting, 2003; Sieghart, 2006; Olsen and Sieghart, 2008; 2009). The π subunit is restricted to reproductive tissue. Alternatively spliced versions of α4 and α6 (both not functional) α5, β2, β3 and γ2 subunits exist. In addition, three ρ subunits, (ρ1–3) function as either homo- or hetero-oligomeric assemblies (Zhang et al., 2001). Receptors formed from ρ subunits, because of their distinctive pharmacology that includes insensitivity to benzodiazepines and barbiturates, have sometimes been termed GABA_C receptors (Zhang, 2001), but they are classified as distinct GABA_A receptors by NC-IUPHAR on the basis of structural and functional criteria (Barnard et al., 1998; Olsen and Sieghart, 2008; 2009). This position is strengthened by the observation that single amino acid mutations can impart some typical features of GABA_A receptor pharmacology upon such receptors (Belelli et al., 1999; Walters et al., 2000). The distinctive agonist and antagonist pharmacology of ρ receptors is summarized in the table, and additional aspects are reviewed by Zhang et al. (2001), Johnston et al. (2003) and Chebib (2004).

Many GABA_A receptor subtypes contain α, β and γ subunits with the likely stoichiometry 2α.2β.1γ (Korpi et al., 2002; Fritschy and Brünig, 2003). It is thought that the majority of GABA_A receptors harbour a single type of α and 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, ε and θ subunits, are less numerous, but they may nonetheless serve important functions. For example, extrasynaptically located receptors that contain α6 and δ subunits in cerebellar granule cells, or an α4 and δ subunit in dentate gyrus granule cells and thalamic neurones, mediate a non-desensitizing tonic current that is important for neuronal excitability in response to ambient concentrations of GABA (see Mody and Pearce, 2004; Semyanov et al., 2004; Farrant and Nusser, 2005). The α and β subunits contribute to the GABA binding site, and both the α and γ subunits are required for the benzodiazepine site. The particular α and γ subunit isoforms exhibit marked effects on recognition and/or efficacy at the benzodiazepine site. Thus, receptors incorporating either α4 or α6 subunits are not recognized by ‘classical’ benzodiazepines, such as flunitrazepam. A variety of proteins that associate with the large intracellular M3–M4 loop of GABA_A receptor subunits influence the trafficking, cell surface expression, internalization and function of the receptor (Chen and Olsen, 2007).

The classification of GABA_A receptors has been addressed by NC-IUPHAR (Barnard et al. 1998; Olsen and Sieghart, 2008). The scheme utilizes subunit structure, pharmacology and receptor function as the basis for classification. Currently, 11 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 (Olsen and Sieghart, 2008; 2009). It is beyond the scope of this supplement to discuss the pharmacology of individual GABA receptor isoforms in detail; such information can be gleaned in the reviews by Barnard et al. (1998), Frolund et al. (2002), Korpi et al. (2002), Krogsgaard-Larsen et al. (2002), Johnston (2005), Sieghart (2006), Möhler (2007) and Olsen and Sieghart (2008; 2009). 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.

Nomenclature	GABAA
Ensembl Gene family ID	ENSF00000000053
Selective agonists (GABA site)	Muscimol (partial agonist at ρ subunits), isoguvacine (partial agonist at ρ subunits), THIP (gaboxadol; antagonist at ρ subunits), piperidine-4-sulphonic acid (low efficacy at α4 and α6 subunits, antagonist at ρ subunits), isonipecotic acid (α4 and α6 subunit-selective via relatively high efficacy, antagonist at ρ subunits), (±)-cis-2-CAMP (ρ subunit-selective), CACA (ρ subunit-selective),
Selective antagonists (GABA site)	Bicuculline (not active at ρ subunits), gabazine (SR95531), TPMPA (ρ subunit-selective), cis- and trans-3-ACPBuPA (ρ subunit-selective)
Selective agonists (benzodiazepine site)	Diazepam (not α4 or α6 subunits), flunitrazepam (not α4 or α6 subunits), zolpidem, zaleplon and indiplon (α1 subunit-selective via high affinity), ocinaplon (α1 subunit-selective as essentially a full agonist vs. partial agonist at α2, α3 and α5 subunit-containing receptors), L838417 (α2, α3 and α5 subunit-selective as a partial agonist vs. antagonist at α1 subunit-containing receptors), Ro154513 (selective for α4 and α6 subunit-containing receptors as an agonist vs. inverse agonist at α1, α2, α3 and α5 subunit-containing receptors), TP003 (selective for α3 subunit-containing receptors as a high-efficacy partial agonist vs. essentially antagonist activity at α1, α2 and α5 subunit-containing receptors), TPA023 (selective for α2 and α3 subunit-containing receptors as a low-efficacy partial agonist vs. essentially antagonist activity at α1 and α5 subunit-containing receptors)
Selective antagonists (benzodiazepine site)	Flumazenil (low affinity for α4 or α6 subunits and partial agonist), ZK93426, L838417 (α1 subunit-selective via antagonist activity vs. partial agonist at α2, α3 and α5 subunit-subunit-containing receptors)
Inverse agonists (benzodiazepine site)	DMCM, Ro194603, α3IA (α3-selective via higher affinity and greater inverse agonist activity vs. α1, α2 and α5 subunit-containing receptors), L655708 (α5-selective via high affinity), RY024 (α5-selective via high affinity), α5IA (α5-selective vs. α1, α2 and α3 subunit-containing receptors via greater inverse agonist efficacy), Ro4938581 (α5-selective vs. α1, α2 and α3 subunit-containing receptors via higher affinity and greater inverse agonist activity)
Endogenous allosteric modulators	5α-pregnan-3α-ol-20-one (potentiation), tetrahydrodeoxycorticosterone (potentiation), Zn2+ (potent inhibition of receptors formed from binary combinations of α and β subunits, incorporation of a γ subunit reduces inhibitory potency, Krishek et al., 1998), extracellular protons (subunit dependent activity, Krishek et al., 1996)
Channel blockers	Picrotoxin, TBPS
Probes GABA site benzodiazepine site	[3H]muscimol, [3H]gabazine (SR95531) [3H]Flunitrazepam (not α4 or α6 subunit), [3H]zolpidem (α1 subunit-selective), [3H]L655708 (α5 subunit-selective), [3H]RY80 (α5 subunit-selective), [3H]Ro154513 [selectively labels α4 and α6 subunit-containing receptors in the presence of a saturating concentration of a ‘classical’ benzodiazepine (e.g. diazepam)], [3H]CGS8216, [11C]flumazenil (PET ligand with low affinity for α4 or α6 subunits), [18F]fluoroethylflumazenil (PET ligand)
Anion channel	[35S]TBPS

The potency and efficacy of many GABA agonists varies between receptor GABA_A 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 and MacDonald, 2003). The presence of the γ subunit within the heterotrimeric complex reduces the potency and efficacy of agonists (Stórustovu and Ebert, 2006). 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 (see Belelli and Lambert 2005; Herd et al., 2007; Hosie et al., 2007; Veleiro and Burton, 2009). 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 volatile (e.g. halothane, isoflurane) anaesthetics and alcohols also exert a regulatory influence upon GABA_A receptor activity (Bonin and Orser, 2008). Specific amino acid residues within GABA_A receptor α and β subunits that influence allosteric regulation by anaesthetic and non-anaesthetic compounds have been identified (see Thompson and Wafford, 2001; Hemmings et al., 2005; Hosie et al., 2007). Photoaffinity labelling of distinct amino acid residues within purified GABA_A receptors by the etomidate derivative, [³H]-azietomidate, has also been demonstrated (Li et al., 2006) and this binding subject to positive allosteric regulation by anaesthetic steroids (Li et al., 2009). An array of natural products including flavonoid and terpenoid compounds exert varied actions at GABA_A receptors (reviewed in detail by Johnston, 2005)

In addition to the agents listed in the table, modulators of GABA_A receptor activity that exhibit subunit dependent activity include: salicylidene salicylhydrazide [negative allosteric modulator-selective for β1 vs. β2, or β3 subunit-containing receptors (Thompson et al., 2004)]; loreclezole, etomidate, tracazolate mefenamic acid, etifoxine, stiripentol [positive allosteric modulators with selectivity for β2/β3 over β1 subunit-containing receptors, see Korpi et al. (2002), Fisher (2009)]; tracazolate [intrinsic efficacy, i.e. potentiation, or inhibition, is dependent upon the identity of the γ1–3, δ or ε subunit co-assembed 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 and α4β3δ receptors (Saxena et al., 1997; Brown et al. (2002)]; ethanol [selectively potentiates responses mediated by α4β3δ and α6β3δ receptors versus receptors in which β2 replaces β3, or γ replaces δ (Wallner et al., 2006, but see also Korpi et al., 2007)]; DS1 and DS2 [selectively potentiate responses mediated by δ subunit-containing receptors (Wafford et al., 2009)]. 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 (Bianchi and MacDonald, 2003).

Glossary

Abbreviations:

3-ACPBuPA

3-amino-cyclopentenylbutylphosphonic acid

(±)-cis-2-CAMP

(±)-cis-2-aminomethylcyclopropane carboxylic acid

α3IA

6-(4-pyridyl)-5-(4-methoxyphenyl)-3-carbomethoxy-1-methyl-1H-pyridin-2-one

α5IA

3-(5-methylisoxazol-3-yl)-6-[(1-methyl-1,2,3-triazol-4-yl)methyloxy]-1,2,4-triazolo[3,4-a]phthalazine

CACA

cis-aminocrotonic acid

CGS8216

2-phenylpyrazolo[4,3-c]quinolin-3(5)-one

DMCM

methy-6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate

DS1

4-chloro-N-[6,8-dibromo-2-(2-thienyl)imidazo[1,2-a]pyridine-3-yl benzamide

DS2

4-chloro-N-[2-(2-thienyl)imidazo[1,2-a]pyridine-3-yl benzamide

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

ethyl-8-azido-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4] benzodiazepine-3-carboxylate

Ro194603

imidazo[1,5-a]1,4-thienodiazepinone

Ro4938581

3-bromo-10-difluoromethyl-9H-imidazo[1,5-a][1,2,4]triazolo[1,5-d][1,4]benzodiazepine

RY024

tert-butyl-8-ethynyl-5,6-dihydro-5-methyl-6-oxo-4H-imidazol[1,5-α][1,4]benzodiazepine-3-carboxylate

RY80

ethyl-8-acetylene-5, 6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5a][1, 4]benzodiazepine-3-carboxylate

SR95531

2-(3′-carboxy-2′-propyl)-3-amino-6-p-methoxyphenylpyridazinium bromide

TBPS

tert-butylbicyclophosphorothionate

TP003

4,2′-difluro-5′-[8-fluro-7-(1-hydroxy-1-methylethyl)imidazo[1,2-á]pyridine-3-yl]biphenyl-2-carbonitrile

TPA023

7-(1,1-dimethylethyl)-6-(2-ethyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(2-fluorphenyl)-1,2,4-triazolo[4,3-b]pyridazine

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

Atack JR (2005). The benzodiazepine binding site of GABA_A receptors as a target for the development of novel anxiolytics. Expert Opin Investig Drugs14: 601–618.

Atack JR (2008). GABA_A receptor subtype-selective efficacy: TPA023, an α2/α3 selective non-sedating anxiolytic and α5IA, an α5 selective cognition enhancer. CNS Neurosci Ther14: 25–35.

Barnard EA, Skolnick P, Olsen RW, Mohler H, Sieghart W, Biggio G et al. (1998). International Union of Pharmacology. XV. Subtypes of γ-aminobutyric acid_A receptors: classification on the basis of subunit structure and receptor function. Pharmacol Rev50: 291–313.

Belelli D, Lambert JJ (2005). Neurosteroids: endogenous regulators of the GABA_A receptor. Nat Rev Neurosci6: 565–575.

Bonin RP, Orser BA (2008). GABA_A receptor subtypes underlying general anesthesia. Pharmacol Biochem Behav90: 105–112.

Bowery NG, Smart TG (2006). GABA and glycine as neurotransmitters: a brief history. Br J Pharmacol147 (Suppl. 1): S109–S119.

Chebib M (2004). GABA_C receptor ion channels. Clin Exp Pharmacol Physiol31: 800–804.

Chen ZW, Olsen RW (2007). GABA_A receptor associated proteins: a key factor regulating GABA_A receptor function. J Neurochem100: 279–294.

D'Hulst C, Atack JR, Kooy RF (2009). The complexity of the GABAA receptor shapes unique pharmacological properties. Drug Discov Today14: 866–875.

Farrant M, Kaila K (2007). The cellular, molecular and ionic basis of GABA_A receptor signalling. Prog Brain Res160: 59–87.

Farrant M, Nusser Z (2005). Variations on an inhibitory theme: phasic and tonic activation of GABA_A receptors. Nat Rev Neurosci6: 215–229.

Fritschy JM, Brunig I (2003). Formation and plasticity of GABAergic synapses: physiological mechanisms and pathophysiological.implications. Pharmacol Ther98: 299–323.

Frolund B, Ebert B, Kristiansen U, Liljefors T, Krogsgaard-Larsen P (2002). GABA_A receptor ligands and their therapeutic potentials. Curr Top Med Chem2: 817–832.

Hanchar HJ, Wallner M, Olsen RW (2004). Alcohol effects on γ-aminobutyric acid type A receptors: are extrasynaptic receptors the answer? Life Sciences76: 1–8.

Hemmings HC, Akabas MH, Goldstein PA, Trudell JR, Orser BA, Harrison NL (2005). Emerging molecular mechanisms of general anesthetic action. Trends Pharmacol Sci26: 503–510.

Herd MB, Belelli D, Lambert JJ (2007). Neurosteroid modulation of synaptic and extrasynaptic GABA_A receptors. Pharmacol Ther 116: 20–34.

Hosie AM, Wilkins ME, Smart TG (2007). Neurosteroid binding sites on GABA_A receptors. Pharmacol Ther116: 7–19.

Jacob TC, Moss SJ, Jurd R (2008). GABA_A receptor trafficking and its role in the dynamic modulation of neuronal inhibition. Nat Rev Neurosci9: 331–343.

Johnston GA (2005). GABA_A receptor channel pharmacology. Curr Pharm Des11: 1867–1885.

Johnston GA, Chebib M, Hanrahan JR, Mewett KN (2003). GABA_C receptors as drug targets. Curr Drug Targets CNS Neurol Disord2: 260–268.

Korpi ER, Grunder G, Luddens H (2002). Drug interactions at GABA_A receptors. Prog Neurobiol67: 113–159.

Korpi ER, Debus F, Linden AM, Malecot C, Leppa E, Vekovischeva O et al. (2007). Does ethanol act preferentially via selected brain GABA_A receptor subtypes? The current evidence is ambiguous. Alcohol41:163–176.

Krogsgaard-Larsen P, Frolund B, Liljefors T (2002). Specific GABA_A agonists and partial agonists. Chem Rec2: 419–430.

Mody I, Pearce RA (2004). Diversity of inhibitory neurotransmission through GABA_A receptors. Trends Neurosci27: 569–575.

Möhler H (2006). GABA_A receptors in central nervous system disease: anxiety, epilepsy, and insomnia. J Recept Signal Transduct Res26: 731–740.

Möhler H (2007). Molecular regulation of cognitive functions and developmental plasticity: impact of GABA_A receptors. J Neurochem102: 1–12.

Möhler H, Fritschy JM, Vogt K, Crestani F, Rudolph U (2005). Pathophysiology and pharmacology of GABA_A receptors. Handb Exp Pharmacol169: 225–247.

Olsen RW, Sieghart W (2008). IUPHAR, LXX. Subtypes of γ-aminobutyric acid_A receptors: classification on the basis of subunit composition, pharmacology, and function. Update. Pharmacol Rev60: 243–260.

Olsen RW, Sieghart, W (2009). GABA_A receptors: Subtypes provide diversity of function and pharmacology. Neuropharmacology56: 141–148.

Olsen RW, Chang CS, Li G, Hanchar HJ, Wallner M (2004). Fishing for allosteric sites on GABA_A receptors. Biochem Pharmacol68:1675–1684.

Rudolph U, Möhler H (2004). Analysis of GABA_A receptor function and dissection of the pharmacology of benzodiazepines and general anesthetics through mouse genetics. Annu Rev Pharmacol Toxicol44: 475–498.

Rudolph U, Möhler H (2006). GABA-based therapeutic approaches: GABA_A receptor subtype functions. Curr Opin Pharmacol6: 18–23.

Semyanov A, Walker MC, Kullmann DM, Silver RA (2004). Tonically active GABA_A receptors: modulating gain and maintaining the tone. Trends Neurosci27: 263–269.

Sieghart W (2006). Structure, pharmacology, and function of GABA_A receptor subtypes. Adv Pharmacol54: 231–263.

Thompson S-A, Wafford K (2001). Mechanism of action of general anaesthetics – new information from molecular pharmacology Curr Opin Pharmacol1: 78–83.

Veleiro AS, Burton G (2009). Structure-activity relationships of neuroactive steroids acting on the GABA_A receptor. Curr Med Chem16: 455–472.

Wallner M, Hanchar HJ, Olsen RW (2006). Low dose acute alcohol effects on GABA_A receptor subtypes. Pharmacol Ther112: 513–528.

Whiting PJ (2003). The GABA_A receptor gene family: new opportunities for drug development. Curr Opin Drug Discov Devel6: 648–657.

Zeller A, Jurd R, Lambert S, Arras M, Drexler B, Grashoff C et al. (2008). Inhibitory ligand-gated ion channels as substrates for general anesthetic actions. Handb Exp Pharmacol182: 31–51.

Zhang D, Pan ZH, Awobuluyi M, Lipton SA (2001). Structure and function of GABA_C receptors: a comparison of native versus recombinant receptors. Trends Pharmacol Sci22: 121–132.