Candidate Review:

Assembly and Heterogeneity of GABAA Receptors

Katharine N. Gurba

Neuroscience Graduate Program, Vanderbilt University School of Medicine, U1205 Medical Center North, Nashville, TN 37232, USA.
Correspondence e-mail:

Abstract | Full Text | PDF

ABSTRACT | GABAA receptors (GABAARs)are pentameric, ligand-gated chloride channels that mediate the majority offast inhibitory synaptic neurotransmission in the brain.  The receptors areassembled from a repertoire of 19 subunits (α1-6, β1-3, γ1-3,δ, ε, π, and ρ1-3), providing the possibility for vastisoform heterogeneity.  Because the subunit subtypes included in a receptordetermine its physiological and pharmacological properties, identification ofreceptor isoforms has clear clinical relevance.  A large body of literatureindicates that GABAARs do not assemble randomly; rather,incorporation of specific subunits into a receptor is regulated at manylevels.  Each subunit has a characteristic temporal and spatial expressionpattern; however, most neurons express many GABAAR subunits atonce.  Consequently, certain “rules” of assembly must exist to limit receptorheterogeneity.  In this review, we discuss the regulation of GABAARbiogenesis, including limitation of heterogeneity, as well as the specificreceptor isoforms that have been identified in vivo.

The vast majority of inhibitoryneurotransmission in the brain is mediated by γ-aminobutyric acid (GABA). It has been detected in approximately 30% of all synapses1 and acts via ionotropic GABAAreceptors, which mediate fast inhibitory neurotransmission2, and metabotropic GABAB receptors,which mediate slower inhibitory effects3.  GABAA receptors (GABAARs)are chloride channels belonging to the Cys-loop receptor superfamily ofligand-gated ion channels (LGIC), which also includes nicotinic acetylcholinereceptors (nAChR), 5-hydroxytryptamine type 3 receptors (5-HT3), and glycine receptors(GlyR)4.  Like most members of thissuperfamily, GABAARs are pentamers that are assembled from an arrayof homologous subunits.  All subunits share a common structure: each contains alarge, extracellular N-terminal domain, which contains the ligand-binding siteand the eponymous Cys-loop; four α-helical transmembrane domains (M1-4); alarge intracellular loop between the third and fourth transmembrane helices(M3-M4 loop); and a very short, extracellular C-terminal domain5 (Figure 1a).

Nineteen subunits, grouped by sequencehomology into eight subunit families, have been identified for the GABAAreceptor: α1-6, β1-3, γ1-3, δ, ε, π, andρ1-36.  Several of these subunit subtypesalso undergo alternative splicing and/or RNA editing, further increasing thepotential diversity of GABAA receptor isoforms.  Each subunitexhibits a characteristic expression pattern in the brain; however, thesepatterns overlap extensively.  Indeed, a single neuron can express manysubunits simultaneously.  Consequently, many but not all of themathematically-possible GABAAR isoforms could exist somewhere in thebrain.  The most common isoforms, however, are thought to comprise two αsubunits, two β subunits, and one γ or δ subunit7-9 (Figure 1b), though this remains a subject ofvigorous debate.

The large variety of GABAARisoforms exhibit a concomitant variety of physiological properties2.  For instance, most receptorscontaining a γ subunit are located in the synapse, where they mediatephasic inhibition in response to presynaptically-released GABA10.  These receptors have a relativelylow affinity for GABA, activate quickly, desensitize extensively, anddeactivate slowly.  Conversely, receptors containing a δ subunit arelocated outside the synapse, where they mediate tonic inhibition in response tolow concentrations of ambient GABA.  Unsurprisingly, δ-subunit-containingreceptors also differ physiologically; they have a relatively high affinity forGABA, activate slowly, desensitize minimally, and deactivate rapidly11.

Additionally, GABAARs havebeen linked to many diseases and disorders, including epilepsy12-14, insomnia15, anxiety16, depression16, schizophrenia17, alcoholism18, and autism19.  Predictably, then, GABAARsare targeted by numerous drugs, particularly sedatives, anxiolytics, andanticonvulsants; examples include benzodiazepines, zolpidem, etomidate, andpropofol20, 21.  Both the pathology and thepharmacology of GABAARs depend highly upon receptor subunitcomposition – for instance, epilepsy-associated mutations have been identifiedonly in the α1, β3, γ2, and δ subunits, and benzodiazepinesact only at receptor isoforms containing both a γ subunit and certain αsubunits.

Figure 1 |GABAA receptor morphology. a | Structure of a GABAAR subunit.  Cys-loop cysteines marked in orange; transmembrane domains enclosed in cylinders and numbered 1-4. b | Schematic view of most common GABAA isoform  (putative) from the synaptic cleft.  G = GABA binding site; BZ = benzodiazepine binding site.

Given the prevalence of GABAARexpression, the pathology resulting from receptor malfunction, and thepharmacological dependence upon isoform identity, it is clearly important tounderstand the process of receptor assembly.  Therefore, in this review, wewill examine the generation of GABAAR diversity.  First, we willreview the general processes of receptor biogenesis, after which we willdiscuss the selective oligomerization of GABAAR subunits.  Finally,we will examine the ultimate product of these processes: native GABAAreceptor isoforms.


As with other LGICs, GABAAreceptor subunits are inserted co-translationally into the membrane of theendoplasmic reticulum (ER).  There, they fold and oligomerize in a process thatdepends heavily upon ER-resident chaperones.  The process of receptoroligomerization is slow and inefficient; studies suggest that approximately 70%of subunits are degraded without being incorporated into a pentameric receptor,and receptors do not appear on the cell surface for several hours followingtransfection22.  While in the ER, GABAAreceptor subunits also undergo typical protein modifications, including theearly stages of N-linked glycosylation.  Interestingly, however, N-linkedglycosylation is not required for subsequent forward trafficking, althoughmultiple glycosylation sites have been identified on all subunits23 and glycosylation is necessary forproper assembly and trafficking of other Cys-loop receptors24, 25.  Properly folded and assembledsubunits proceed to the Golgi apparatus, where they undergo furthermodification such as palmitoylation and glycan trimming26.  With the assistance of multipleGABAAR-associated proteins, receptors are then trafficked to theneuronal surface.  They may be inserted directly into their final subcellularlocation (i.e. post-, peri-, or extrasynaptic), or they may diffuse intothat location after membrane insertion27.  Finally, GABAARs undergoconstitutive and activity-dependent endocytosis (both clathrin-dependent andclathrin-independent)28, after which they are recycled to thecell surface or targeted for lysosomal degradation.  Every step of GABAAreceptor assembly and trafficking is regulated by signals within the subunits29 as well as by various associatedproteins30.


After temporal andspatial regulation of subunit expression, the first (and, arguably, the mostimportant) opportunity for a neuron to control what GABAA receptorisoforms it will produce is the process of selective subunit oligomerization. Presumably, a neuron expressing many GABAAR subunit subtypes wouldhave a hierarchical yet flexible assembly mechanism that favors associationbetween certain subunits and, ultimately, directs the incorporation of assemblyintermediates (e.g. dimers, trimers) into full receptors.  Indeed, severalstudies have indicated that, though all subunit combinations can formoligomers, only a subset can form pentamers23.  This is a key distinction becausepentamers are trafficked to the cell surface, but oligomers of lower molecularweight are retained in the ER and subsequently degraded23, 31.  Importantly, some disease-causingmutations appeared to reduce surface expression and function by disrupting theprocess of oligomerization14.

Expression of recombinant subunits inheterologous cells has provided insight into the “rules” governing assembly ofthe most prevalent subunit subtypes.  When expressed individually, α1,β2, and γ2 subunits formed primarily monomers and dimers, as didcombinations of γ2 with either α1 or β2/3.  Conversely,co-expression of α1 and β2/3 subunits, with or without γ2subunits, predominantly yielded pentamers, indicating that the combination ofα and β subunits is necessary and sufficient for complete receptorassembly31, 32.  Interestingly, however, receptorsincluding a third (non-α/β) subunit appear to assemble moreefficiently.  When α, β, and a third subunit (either γ, δ,ε, or π) were co-expressed in heterologous systems, the kineticsignature of αβ receptors could not be detected33-35; furthermore, that signature has beendetected in very few neurons36, 37.  Clearly, both neurons andheterologous cells are capable of selective oligomerization, suggesting theexistence of assembly signals within the subunits themselves. 

Several studies have, in fact,isolated amino acid sequences and individual residues that are important forspecific subunit interactions29, 38. These sequences have beenidentified in the α139-43, α639, β342-45, γ242, 46, and γ347 subunits, primarily in the largeN-terminal domain, though there were some reports of assembly sequences in theM3-M4 loop48, 49.  Although homology modeling based onthe nAChR50 and AChBP51 has provided some insight into thestructural basis of these interactions, it is important to note that thesesequences might not directly contact adjacent subunits; rather, they mightsimply facilitate oligomerization by encouraging proper protein folding.


Most studies mentioned thus far havebeen conducted in heterologous expression systems or in cultured neurons. Because of the great potential for GABAAR heterogeneity, it isnecessary to use such systems to investigate properties of specific subunits (i.e.assembly sequences) and isoforms (i.e. kinetic and pharmacologicalproperties).  Unfortunately, these studies cannot answer a crucial question:what GABAA receptor isoforms actually exist in the brain?  In anattempt to construct a standardized response to that question, theInternational Union of Pharmacology recently established a list of potentialnative GABAAR oligomers6.  These receptor isoforms weredivided into three categories (“identified”, “existence with high probability”,and “tentative”) based on multiple types of evidence.  The authors alsospecified a logical strategy, summarized below, for determining whether or nota receptor isoform exists in vivo.  First, the long list of potentialisoforms can be narrowed based on subunit co-expression patterns, which can beascertained by in situ hybridization and immunoreactivity.  If subunitsare indeed co-expressed in a specific cell type, evidence for association ofthose subunits should then be sought, primarily throughco-immunoprecipitation.  Subunits that associate should be co-expressed inheterologous systems, where electrophysiology can be performed andcharacteristic kinetics and pharmacology can be assessed.  These characteristicproperties can then be sought in neurons.  Finally, knockout animals can becreated and studied for the absence of characteristic physiology andpharmacology associated with isoforms containing the deleted subunit.  The listof “identified” and “high probability” isoforms, along with their localization(regional and subcellular) and basic forms of inhibition (phasic or tonic), ispresented in Table 1

Isoforms that have been unequivocallyidentified

Given the widespread distribution ofthe α1β2γ2 GABAAR isoform, it is perhaps unsurprisingthat this isoform is thought to account for up to 60% of all GABAAreceptors in the brain20. Mice lacking either the α1 orβ2 subunit have been generated; in both lines, total GABAARexpression in the brain was reduced by more than 50%52.  A γ2 knockout mouse was foundto lack 94% of all benzodiazepine binding sites53 (recall that the BZ binding site islocated at the interface of an α and a γ subunit; consequently, thisresult indicates that receptors including the γ1 or γ3 subunit mightmake up only 6% of all αβγ receptors).  As indicated in Table1, the other five α subunits can likewise co-assemble with β andγ2 subunits.  Strong evidence for the existence of theseαxβxγ2 receptors is provided by isoform-specific pharmacologyfrom benzodiazepine (BZ) site ligands.  Such ligands include classicbenzodiazepines (i.e. diazepam); imidazobenzodiazepines (i.e.flumazenil and Ro15-4513); and the so-called “Z-drugs” (i.e. zolpidemand zaleplon).

Classic benzodiazepines cannot bindreceptors containing α4 or α6 subunits, and they have much loweraffinity for receptors containing γ1 or γ3 subunits than forreceptors containing γ2 subunits.  Furthermore, through the use oftransgenic mice, the various actions of benzodiazepines have been attributed tospecific α subunit subtypes.  Point mutations conferring diazepaminsensitivity were introduced into the genes of individual α subunits andthe resulting mice were subjected to behavioral tests with and withoutadministration of diazepam54, 74, 75.  Results indicated that the α1subunit mediated the sedative, anterograde amnestic, and some of theanticonvulsant effects of diazepam74, 76; the α2 and α3 subunitsmediated the anxiolytic and muscle-relaxant effects54, 75 and the α5 subunit was involvedin amnestic effects as well as other aspects of learning and memory. Imidazobenzodiazepines, however, bind without regard to α subunitsubtype.  Therefore, receptors that are benzodiazepine-insensitive but imidazobenzodiazepine-sensitivecan be identified as α4βγ2 or α6βγ2 isoforms. Conversely, Z-drugs act with differing potency at BZ-sensitive isoformscontaining α1,2,3, or 5; specifically, they display high potency atα1βγ2 isoforms, lower potency at α2βγ2 andα3βγ2 isoforms, and no action at α5βγ277.  Taken together, thesepharmacological properties allow positive identification ofα1βγ2 and α5βγ2 receptors, as well as tentativeidentification of α(2,3)βγ2 and α(4,6)βγ2receptors; however, expression patterns can differentiate these latter twopairs of isoforms.  Consequently, all αβγ2 isoforms areconsidered to have been identified in vivo.

The aforementioned evidence accountsfor six of the 11 identified native isoforms.  Four of the remaining fiveisoforms contain the δ subunit, which possesses many unusual propertiesthat help to identify δ-subunit-containing isoforms in vivo. First, the δ subunit has been found exclusively in extrasynapticmembranes, where it is incorporated into receptors that have a high affinityfor GABA and mediate a constant, “tonic” current with low amplitude and littledesensitization11, 78.  The pharmacology ofδ-subunit-containing receptors is also very different from that ofγ-subunit-containing receptors.  Though GABA binds to δ-containingisoforms with high affinity, its efficacy is relatively low.  Conversely,ethanol79 and neuroactive steroids80 act strongly atδ-subunit-containing receptors.  Demonstration of these properties invivo56, combined with co-localization,co-immunoprecipitation, and gene deletion studies81, have allowed identification of theδ-subunit-containing receptors listed in Table 155.

Table 1 | GABAAR isoforms likely to exist in vivo.

List of isoforms from reference 6, which also identifies “tentative” isoforms that assembled in heterologous systems (ρ1-3, αβγ1, αβγ3, αβε, αβθ, αβπ, and αxαyβγ2). Also see the following general references: in situ hybridization70; immunohistochemistry71, 72; reviews20, 73.

The last isoform that has beenidentified unequivocally in vivo comprises ρ subunits alone.  Thesereceptors, previously classified as GABAC receptors due to theirunique pharmacology, are expressed predominantly in retinal bipolar cells63; however, low levels of ρsubunit transcripts have also been detected in hippocampus82, cerebellum83, amygdala84, and certain brain areas importantfor visual signal processing (superior colliculus, lateral geniculate nucleus,and visual cortex)62, 83 .  Evidence for both homomeric andheteromeric ρ isoforms has been reported85, 86; consequently, the subunit subtypespresent in these receptors remain undefined.

Isoforms that exist with highprobability

Finally, we will briefly discuss theevidence supporting the “existence with high probability” of certain key GABAARisoforms listed in Table 1.  Each of these isoforms assemblesefficiently and has been studied extensively in heterologous systems11, 31, 33, 35, 80, 87-89; moreover, the subunits areco-expressed in vivo70-72.  Indeed, most were not classified as“identified” simply because few animal studies have been conducted.  First,although α1 and γ2 subunits seem to partner most frequently with theβ2 subunit, expression patterns indicate that this cannot always be thecase, because certain areas expressing the α1 and γ2 subunits do notexpress the β2 subunit71.  In these areas, it is quite likelythat α1β3γ2 receptors are formed, as indicated by variouspharmacological properties64.  The evidence supporting theexistence of α5β3γ2 is also extensive; the only reason that itis not considered to be unequivocally identified is that, to date, α5 andβ3 have not been co-immunoprecipitated6.  However, these three subunits havebeen co-localized71, α5 and β3 subunits wereco-depleted in knockout mice6, α5-selective etomidate effectshave been identified90, and electrophysiology indicates thatthis isoform mediates tonic inhibition in the hippocampus66.  Another widely-accepted isoform,α1βδ, clearly assembled in heterologous systems and responded toknown modulators of δ-subunit-containing receptors.  Furthermore, onerecent report identified this isoform in molecular layer interneurons of thehippocampus65. Finally, as previously mentioned,two different αβ isoforms have been identified in rat brain viasequential co-immunoprecipitation37 and electrophysiology36.


GABAA receptors in thebrain are ubiquitous, implicated in many diseases, and highly heterogeneous. Each receptor isoform exhibits unique physiological and pharmacologicalproperties and a characteristic expression pattern.  Consequently, a thoroughunderstanding of GABAAR assembly, trafficking, and function couldyield significant therapeutic advantages, such as isoform-specific drugs thatminimize unwanted side effects.  Currently, only 11 GABAAR isoformshave been conclusively identified in vivo, and the existence of anothersix is considered to be highly probable.  Further study of the assembly,trafficking, and function of these receptors may improve clinical practice, aswill attempts to identify other GABAAR isoforms that occur in thebrain.


1.   Bloom FE and Iversen LL (1971).Localizing 3H-GABA in nerve terminals of rat cerebral cortex by electronmicroscopic autoradiography. Nature. 229 (5287): 628-630.

2.   Farrant M and Nusser Z (2005).Variations on an inhibitory theme: phasic and tonic activation of GABA(A)receptors. Nat Rev Neurosci. 6 (3): 215-229.

3.   Couve A, Moss SJ and Pangalos MN (2000). GABAB receptors: a new paradigm in G protein signaling. Mol Cell Neurosci.16 (4): 296-312.

4.   Connolly CN and Wafford KA (2004). TheCys-loop superfamily of ligand-gated ion channels: the impact of receptorstructure on function. Biochem Soc Trans. 32 (Pt3): 529-534.

5.   Campagna-Slater V and Weaver DF(2007). Molecular modelling of the GABAA ion channel protein. J Mol GraphModel. 25 (5): 721-730.

6.   Olsen RW and Sieghart W (2008).International Union of Pharmacology. LXX. Subtypes of gamma-aminobutyricacid(A) receptors: classification on the basis of subunit composition,pharmacology, and function. Update. Pharmacol Rev. 60 (3):243-260.

7.   Tretter V, Ehya N, Fuchs K andSieghart W (1997). Stoichiometry and assembly of a recombinant GABAA receptorsubtype. J Neurosci. 17 (8): 2728-2737.

8.   Baumann SW, Baur R and Sigel E (2001).Subunit arrangement of gamma-aminobutyric acid type A receptors. J Biol Chem.276 (39): 36275-36280.

9.   Baumann SW, Baur R and Sigel E (2002).Forced subunit assembly in alpha1beta2gamma2 GABAA receptors. Insight into theabsolute arrangement. J Biol Chem. 277 (48): 46020-46025.

10. Mozrzymas JW (2004). Dynamism ofGABA(A) receptor activation shapes the "personality" of inhibitorysynapses. Neuropharmacology. 47 (7): 945-960.

11. Haas KF and Macdonald RL (1999). GABAAreceptor subunit gamma2 and delta subtypes confer unique kinetic properties onrecombinant GABAA receptor currents in mouse fibroblasts. J Physiol. 514( Pt 1): 27-45.

12. Macdonald RL and Kang JQ (2009).Molecular Pathology of Genetic Epilepsies Associated with GABA(A) Receptor SubunitMutations. Epilepsy Curr. 9 (1): 18-23.

13. Gallagher MJ, Shen W, Song L andMacdonald RL (2005). Endoplasmic reticulum retention and associated degradationof a GABAA receptor epilepsy mutation that inserts an aspartate in the M3transmembrane segment of the alpha1 subunit. J Biol Chem. 280 (45):37995-38004.

14. Frugier G, Coussen F, Giraud MF, Odessa MF, Emerit MB, Boue-Grabot E and Garret M (2007). A gamma 2(R43Q) mutation, linkedto epilepsy in humans, alters GABAA receptor assembly and modifies subunitcomposition on the cell surface. J Biol Chem. 282 (6): 3819-3828.

The preceding two papers elegantly demonstrate that altered GABAAreceptor biogenesis and subunit composition may lead to epilepsy.

15. Buhr A, Bianchi MT, Baur R, Courtet P,Pignay V, Boulenger JP, Gallati S, Hinkle DJ, Macdonald RL and Sigel E (2002).Functional characterization of the new human GABA(A) receptor mutationbeta3(R192H). Hum Genet. 111 (2): 154-160.

16. Kalueff AV and Nutt DJ (2007). Roleof GABA in anxiety and depression. Depress Anxiety. 24 (7):495-517.

17. Blum BP and Mann JJ (2002). TheGABAergic system in schizophrenia. Int J Neuropsychopharmacol. 5 (2):159-179.

18. Enoch MA (2008). The role of GABA(A)receptors in the development of alcoholism. Pharmacol Biochem Behav. 90(1): 95-104.

19. Fatemi SH, Reutiman TJ, Folsom TD andThuras PD (2009). GABA(A) receptor downregulation in brains of subjects withautism. J Autism Dev Disord. 39 (2): 223-230.

20. Mohler H (2006). GABA(A) receptordiversity and pharmacology. Cell Tissue Res. 326 (2): 505-516.

21. Rudolph U and Antkowiak B (2004).Molecular and neuronal substrates for general anaesthetics. Nat Rev Neurosci.5 (9): 709-720.

22. Green WN and Millar NS (1995).Ion-channel assembly. Trends Neurosci. 18 (6): 280-287.

23. Connolly CN, Krishek BJ, McDonald BJ,Smart TG and Moss SJ (1996). Assembly and cell surface expression ofheteromeric and homomeric gamma-aminobutyric acid type A receptors. J BiolChem. 271 (1): 89-96.

Thisstudy presents one of the first methodical analyses of selective subunitoligomerization and its effects on forward trafficking.

24. Blount P and Merlie JP (1990).Mutational analysis of muscle nicotinic acetylcholine receptor subunitassembly. J Cell Biol. 111 (6 Pt 1): 2613-2622.

25. Ramanathan VK and Hall ZW (1999).Altered glycosylation sites of the delta subunit of the acetylcholine receptor(AChR) reduce alpha delta association and receptor assembly. J Biol Chem.274 (29): 20513-20520.

26. Keller CA, Yuan X, Panzanelli P, MartinML, Alldred M, Sassoe-Pognetto M and Luscher B (2004). The gamma2 subunit ofGABA(A) receptors is a substrate for palmitoylation by GODZ. J Neurosci.24 (26): 5881-5891.

27. Bogdanov Y, Michels G, Armstrong-GoldC, Haydon PG, Lindstrom J, Pangalos M and Moss SJ (2006). Synaptic GABAAreceptors are directly recruited from their extrasynaptic counterparts. EMBOJ. 25 (18): 4381-4389.

28. Kanematsu T, Fujii M, Mizokami A, KittlerJT, Nabekura J, Moss SJ and Hirata M (2007). Phospholipase C-related inactiveprotein is implicated in the constitutive internalization of GABAA receptorsmediated by clathrin and AP2 adaptor complex. J Neurochem. 101 (4):898-905.

29. Sarto-Jackson I and Sieghart W (2008).Assembly of GABA(A) receptors (Review). Mol Membr Biol. 25 (4):302-310.

30. Jacob TC, Moss SJ and Jurd R (2008).GABA(A) receptor trafficking and its role in the dynamic modulation of neuronalinhibition. Nat Rev Neurosci. 9 (5): 331-343.

31. Gorrie GH, Vallis Y, Stephenson A,Whitfield J, Browning B, Smart TG and Moss SJ (1997). Assembly of GABAAreceptors composed of alpha1 and beta2 subunits in both cultured neurons andfibroblasts. J Neurosci. 17 (17): 6587-6596.

32. Klausberger T, Ehya N, Fuchs K, FuchsT, Ebert V, Sarto I and Sieghart W (2001). Detection and binding properties ofGABA(A) receptor assembly intermediates. J Biol Chem. 276 (19):16024-16032.

33. Angelotti TP and Macdonald RL (1993).Assembly of GABAA receptor subunits: alpha 1 beta 1 and alpha 1 beta 1 gamma 2Ssubunits produce unique ion channels with dissimilar single-channel properties.J Neurosci. 13 (4): 1429-1440.

34. Fisher JL and Macdonald RL (1997).Single channel properties of recombinant GABAA receptors containing gamma 2 ordelta subtypes expressed with alpha 1 and beta 3 subtypes in mouse L929 cells. JPhysiol. 505 ( Pt 2): 283-297.

35. Saxena NC and Macdonald RL (1994).Assembly of GABAA receptor subunits: role of the delta subunit. J Neurosci.14 (11 Pt 2): 7077-7086.

36. Mortensen M and Smart TG (2006).Extrasynaptic alphabeta subunit GABAA receptors on rat hippocampal pyramidalneurons. J Physiol. 577 (Pt 3): 841-856.

37. Bencsits E, Ebert V, Tretter V andSieghart W (1999). A significant part of native gamma-aminobutyric AcidAreceptors containing alpha4 subunits do not contain gamma or delta subunits. JBiol Chem. 274 (28): 19613-19616.

38. Bollan K, Robertson LA, Tang H andConnolly CN (2003). Multiple assembly signals in gamma-aminobutyric acid (typeA) receptor subunits combine to drive receptor construction and composition. BiochemSoc Trans. 31 (Pt 4): 875-879.

39. Srinivasan S, Nichols CJ, Lawless GM,Olsen RW and Tobin AJ (1999). Two invariant tryptophans on the alpha1 subunitdefine domains necessary for GABA(A) receptor assembly. J Biol Chem. 274(38): 26633-26638.

40. Taylor PM, Connolly CN, Kittler JT,Gorrie GH, Hosie A, Smart TG and Moss SJ (2000). Identification of residueswithin GABA(A) receptor alpha subunits that mediate specific assembly withreceptor beta subunits. J Neurosci. 20 (4): 1297-1306.

41. Klausberger T, Sarto I, Ehya N, FuchsK, Furtmuller R, Mayer B, Huck S and Sieghart W (2001). Alternate use ofdistinct intersubunit contacts controls GABAA receptor assembly andstoichiometry. J Neurosci. 21 (23): 9124-9133.

42. Sarto I, Wabnegger L, Dogl E andSieghart W (2002). Homologous sites of GABA(A) receptor alpha(1), beta(3) andgamma(2) subunits are important for assembly. Neuropharmacology. 43 (4):482-491.

43. Bollan K, King D, Robertson LA, BrownK, Taylor PM, Moss SJ and Connolly CN (2003). GABA(A) receptor composition isdetermined by distinct assembly signals within alpha and beta subunits. JBiol Chem. 278 (7): 4747-4755.

44. Taylor PM, Thomas P, Gorrie GH,Connolly CN, Smart TG and Moss SJ (1999). Identification of amino acid residueswithin GABA(A) receptor beta subunits that mediate both homomeric andheteromeric receptor expression. J Neurosci. 19 (15): 6360-6371.<

This study identifies specific residues mediating the unusual assembly patterns ofthe β3 subunit, which may promote significant heterogeneity of recombinantreceptors.

45. Ehya N, Sarto I, Wabnegger L andSieghart W (2003). Identification of an amino acid sequence within GABA(A)receptor beta3 subunits that is important for receptor assembly. J Neurochem.84 (1): 127-135.

46. Klausberger T, Fuchs K, Mayer B, Ehya Nand Sieghart W (2000). GABA(A) receptor assembly. Identification and structureof gamma(2) sequences forming the intersubunit contacts with alpha(1) andbeta(3) subunits. J Biol Chem. 275 (12): 8921-8928.

47. Sarto I, Klausberger T, Ehya N, MayerB, Fuchs K and Sieghart W (2002). A novel site on gamma 3 subunits importantfor assembly of GABA(A) receptors. J Biol Chem. 277 (34):30656-30664.

48. Nymann-Andersen J, Sawyer GW and OlsenRW (2002). Interaction between GABAA receptor subunit intracellular loops:implications for higher order complex formation. J Neurochem. 83 (5):1164-1171.

49. Lo WY, Botzolakis EJ, Tang X andMacdonald RL (2008). A conserved Cys-loop receptor aspartate residue in theM3-M4 cytoplasmic loop is required for GABAA receptor assembly. J Biol Chem.283 (44): 29740-29752.

50. Unwin N (2005). Refined structure ofthe nicotinic acetylcholine receptor at 4A resolution. J Mol Biol. 346(4): 967-989.

51. Brejc K, van Dijk WJ, Klaassen RV,Schuurmans M, van Der Oost J, Smit AB and Sixma TK (2001). Crystalstructure of an ACh-binding protein reveals the ligand-binding domain ofnicotinic receptors. Nature. 411 (6835): 269-276.

52. Sur C, Wafford KA, Reynolds DS,Hadingham KL, Bromidge F, Macaulay A, Collinson N, O'Meara G, Howell O, NewmanR, Myers J, Atack JR, Dawson GR, McKernan RM, Whiting PJ and Rosahl TW (2001).Loss of the major GABA(A) receptor subtype in the brain is not lethal in mice. JNeurosci. 21 (10): 3409-3418.

53. Gunther U, Benson J, Benke D, FritschyJM, Reyes G, Knoflach F, Crestani F, Aguzzi A, Arigoni M, Lang Y and et al.(1995). Benzodiazepine-insensitive mice generated by targeted disruption of thegamma 2 subunit gene of gamma-aminobutyric acid type A receptors. Proc NatlAcad Sci U S A. 92 (17): 7749-7753.

54. Low K, Crestani F, Keist R, Benke D,Brunig I, Benson JA, Fritschy JM, Rulicke T, Bluethmann H, Mohler H and RudolphU (2000). Molecular and neuronal substrate for the selective attenuation ofanxiety. Science. 290 (5489): 131-134.

55. Sur C, Farrar SJ, Kerby J, Whiting PJ,Atack JR and McKernan RM (1999). Preferential coassembly of alpha4 and deltasubunits of the gamma-aminobutyric acidA receptor in rat thalamus. MolPharmacol. 56 (1): 110-115.

56. Jia F, Pignataro L, Schofield CM, YueM, Harrison NL and Goldstein PA (2005). An extrasynaptic GABAA receptormediates tonic inhibition in thalamic VB neurons. J Neurophysiol. 94 (6):4491-4501.

57. Caraiscos VB, Elliott EM, You-Ten KE,Cheng VY, Belelli D, Newell JG, Jackson MF, Lambert JJ, Rosahl TW, Wafford KA,MacDonald JF and Orser BA (2004). Tonic inhibition in mouse hippocampal CA1pyramidal neurons is mediated by alpha5 subunit-containing gamma-aminobutyricacid type A receptors. Proc Natl Acad Sci U S A. 101 (10):3662-3667.

58. Jechlinger M, Pelz R, Tretter V,Klausberger T and Sieghart W (1998). Subunit composition andquantitative importance of hetero-oligomeric receptors: GABAA receptorscontaining alpha6 subunits. J Neurosci. 18 (7): 2449-2457.

59. Nusser Z, Sieghart W and Somogyi P(1998). Segregation of different GABAA receptors to synaptic and extrasynapticmembranes of cerebellar granule cells. J Neurosci. 18 (5):1693-1703.

60. Poltl A, Hauer B, Fuchs K, Tretter Vand Sieghart W (2003). Subunit composition and quantitative importance ofGABA(A) receptor subtypes in the cerebellum of mouse and rat. J Neurochem.87 (6): 1444-1455.

61. Qian H and Dowling JE (1995). GABAA andGABAC receptors on hybrid bass retinal bipolar cells. J Neurophysiol. 74(5): 1920-1928.

62. Zhang D, Pan ZH, Awobuluyi M and LiptonSA (2001). Structure and function of GABA(C) receptors: a comparison of nativeversus recombinant receptors. Trends Pharmacol Sci. 22 (3):121-132.

63. Enz R, Brandstatter JH, Wassle H andBormann J (1996). Immunocytochemical localization of the GABAc receptor rhosubunits in the mammalian retina. J Neurosci. 16 (14): 4479-4490.

64. Jurd R, Arras M, Lambert S, Drexler B,Siegwart R, Crestani F, Zaugg M, Vogt KE, Ledermann B, Antkowiak B and RudolphU (2003). General anesthetic actions in vivo strongly attenuated by a pointmutation in the GABA(A) receptor beta3 subunit. FASEB J. 17 (2):250-252.

65. Glykys J, Peng Z, Chandra D, HomanicsGE, Houser CR and Mody I (2007). A new naturally occurring GABA(A) receptorsubunit partnership with high sensitivity to ethanol. Nat Neurosci. 10(1): 40-48.

66. Sur C, Quirk K, Dewar D, Atack J andMcKernan R (1998). Rat and human hippocampal alpha5 subunit-containinggamma-aminobutyric AcidA receptors have alpha5 beta3 gamma2 pharmacologicalcharacteristics. Mol Pharmacol. 54 (5): 928-933.

67. Li M and De Blas AL (1997). Coexistenceof two beta subunit isoforms in the same gamma-aminobutyric acid type Areceptor. J Biol Chem. 272 (26): 16564-16569.

68. Mossier B, Togel M, Fuchs K andSieghart W (1994). Immunoaffinity purification of gamma-aminobutyric acidA(GABAA) receptors containing gamma 1-subunits. Evidence for the presence of asingle type of gamma-subunit in GABAA receptors. J Biol Chem. 269 (41):25777-25782.

69. Mtchedlishvili Z and Kapur J (2006).High-affinity, slowly desensitizing GABAA receptors mediate tonic inhibition inhippocampal dentate granule cells. Mol Pharmacol. 69 (2):564-575.

70. Laurie DJ, Wisden W and Seeburg PH(1992). The distribution of thirteen GABAA receptor subunit mRNAs in the ratbrain. III. Embryonic and postnatal development. J Neurosci. 12 (11):4151-4172.

71. Pirker S, Schwarzer C, Wieselthaler A,Sieghart W and Sperk G (2000). GABA(A) receptors: immunocytochemicaldistribution of 13 subunits in the adult rat brain. Neuroscience. 101(4): 815-850.

72. Sieghart W and Sperk G (2002). Subunitcomposition, distribution and function of GABA(A) receptor subtypes. CurrTop Med Chem. 2 (8): 795-816.

73. Fritschy JM and Brunig I (2003).Formation and plasticity of GABAergic synapses: physiological mechanisms andpathophysiological implications. Pharmacol Ther. 98 (3): 299-323.

74. Rudolph U, Crestani F, Benke D, BrunigI, Benson JA, Fritschy JM, Martin JR, Bluethmann H and Mohler H (1999). Benzodiazepineactions mediated by specific gamma-aminobutyric acid(A) receptor subtypes. Nature.401 (6755): 796-800.

75. Morris HV, Dawson GR, Reynolds DS,Atack JR and Stephens DN (2006). Both alpha2 and alpha3 GABAA receptor subtypesmediate the anxiolytic properties of benzodiazepine site ligands in theconditioned emotional response paradigm. Eur J Neurosci. 23 (9):2495-2504.

76. McKernan RM, Rosahl TW, Reynolds DS,Sur C, Wafford KA, Atack JR, Farrar S, Myers J, Cook G, Ferris P, Garrett L,Bristow L, Marshall G, Macaulay A, Brown N, Howell O, Moore KW, Carling RW,Street LJ, Castro JL, Ragan CI, Dawson GR and Whiting PJ (2000). Sedative butnot anxiolytic properties of benzodiazepines are mediated by the GABA(A)receptor alpha1 subtype. Nat Neurosci. 3 (6): 587-592.

77. Winsky-Sommerer R (2009). Role of GABAAreceptors in the physiology and pharmacology of sleep. Eur J Neurosci. 29(9): 1779-1794.

78. Mody I, Glykys J and Wei W (2007). Anew meaning for "Gin & Tonic": tonic inhibition as the target forethanol action in the brain. Alcohol. 41 (3): 145-153.

79. Olsen RW, Hanchar HJ, Meera P andWallner M (2007). GABAA receptor subtypes: the "one glass of wine"receptors. Alcohol. 41 (3): 201-209.

80. Wohlfarth KM, Bianchi MT and MacdonaldRL (2002). Enhanced neurosteroid potentiation of ternary GABA(A) receptorscontaining the delta subunit. J Neurosci. 22 (5): 1541-1549.

81. Jia F, Pignataro L and Harrison NL(2007). GABAA receptors in the thalamus: alpha4 subunit expression and alcoholsensitivity. Alcohol. 41 (3): 177-185.

82. Didelon F, Sciancalepore M, Savic N,Mladinic M, Bradbury A and Cherubini E (2002). gamma-Aminobutyric acidA rhoreceptor subunits in the developing rat hippocampus. J Neurosci Res. 67(6): 739-744.

83. Harvey VL, Duguid IC, Krasel C andStephens GJ (2006). Evidence that GABA rho subunits contribute to functionalionotropic GABA receptors in mouse cerebellar Purkinje cells. J Physiol.577 (Pt 1): 127-139.

84. Fujimura J, Nagano M and Suzuki H(2005). Differentialexpression of GABA(A) receptor subunits in the distinct nuclei of the ratamygdala. Brain Res Mol Brain Res. 138 (1): 17-23.

85. Enz R and Cutting GR (1999). GABACreceptor rho subunits are heterogeneously expressed in the human CNS and formhomo- and heterooligomers with distinct physical properties. Eur J Neurosci.11 (1): 41-50.

86. Pan Y, Ripps H and Qian H (2006).Random assembly of GABA rho1 and rho2 subunits in the formation of heteromericGABA(C) receptors. Cell Mol Neurobiol. 26 (3): 289-305.

87. Feng HJ and Macdonald RL (2004).Multiple actions of propofol on alphabetagamma and alphabetadelta GABAAreceptors. Mol Pharmacol. 66 (6): 1517-1524.

88. Fisher JL (2002). A histidine residuein the extracellular N-terminal domain of the GABA(A) receptor alpha5 subunitregulates sensitivity to inhibition by zinc. Neuropharmacology. 42 (7):922-928.

89. Luddens H and Korpi ER (1995). GABAantagonists differentiate between recombinant GABAA/benzodiazepine receptorsubtypes. J Neurosci. 15 (10): 6957-6962.

90. Cheng VY, Martin LJ, Elliott EM, KimJH, Mount HT, Taverna FA, Roder JC, Macdonald JF, Bhambri A, Collinson N,Wafford KA and Orser BA (2006). Alpha5GABAA receptors mediate the amnestic butnot sedative-hypnotic effects of the general anesthetic etomidate. JNeurosci. 26 (14): 3713-3720.


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