Benzodiazepine receptor protein identified and visualized in brain tissue by a photoaffinity label

Development of 1,3-thiazole analogues of imidazopyridines as potent positive allosteric modulators of GABAA receptors

Structure-activity relationship studies were conducted in the search for 1,3-thiazole isosteric analogs of imidazopyridine drugs (Zolpidem, Alpidem). Three series of novel γ-aminobutyric acid receptor (GABA_AR) ligands belonging to imidazo[2,1-b]thiazoles, imidazo[2,1-b][1,3,4]thiadiazoles, and benzo[d]imidazo[2,1-b]thiazoles were synthesized and characterized as active agents against GABA_AR benzodiazepine-binding site. In each of these series, potent compounds were discovered using a radioligand competition binding assay. The functional properties of highest-affinity compounds 28 and 37 as GABA_AR positive allosteric modulators (PAMs) were determined by electrophysiological measurements. In vivo studies on zebrafish demonstrated their potential for the further development of anxiolytics. Using the OECD "Fish, Acute Toxicity Test" active compounds were found safe and non-toxic. Structural bases for activity of benzo[d]imidazo[2,1-b]thiazoles were proposed using molecular docking studies. The isosteric replacement of the pyridine nuclei by 1,3-thiazole, 1,3,4-thiadiazole, or 1,3-benzothiazole in the ring-fused imidazole class of GABA_AR PAMs was shown to be promising for the development of novel hypnotics, anxiolytics, anticonvulsants, and sedatives drug-candidates.

GABAA receptor: Positive and negative allosteric modulators

gamma-Aminobutyric acid (GABA)-mediated inhibitory neurotransmission and the gene products involved were discovered during the mid-twentieth century. Historically, myriad existing nervous system drugs act as positive and negative allosteric modulators of these proteins, making GABA a major component of modern neuropharmacology, and suggesting that many potential drugs will be found that share these targets. Although some of these drugs act on proteins involved in synthesis, degradation, and membrane transport of GABA, the GABA receptors Type A (GABAAR) and Type B (GABABR) are the targets of the great majority of GABAergic drugs. This discovery is due in no small part to Professor Norman Bowery. Whereas the topic of GABABR is appropriately emphasized in this special issue, Norman Bowery also made many insights into GABAAR pharmacology, the topic of this article. GABAAR are members of the ligand-gated ion channel receptor superfamily, a chloride channel family of a dozen or more heteropentameric subtypes containing 19 possible different subunits. These subtypes show different brain regional and subcellular localization, age-dependent expression, and potential for plastic changes with experience including drug exposure. Not only are GABAAR the targets of agonist depressants and antagonist convulsants, but most GABAAR drugs act at other (allosteric) binding sites on the GABAAR proteins. Some anxiolytic and sedative drugs, like benzodiazepine and related drugs, act on GABAAR subtype-dependent extracellular domain sites. General anesthetics including alcohols and neurosteroids act at GABAAR subunit-interface trans-membrane sites. Ethanol at high anesthetic doses acts on GABAAR subtype-dependent trans-membrane domain sites. Ethanol at low intoxicating doses acts at GABAAR subtype-dependent extracellular domain sites. Thus GABAAR subtypes possess pharmacologically specific receptor binding sites for a large group of different chemical classes of clinically important neuropharmacological agents. This article is part of the "Special Issue Dedicated to Norman G. Bowery".

Differential cerebellar GABAA receptor expression in mice with mutations in CaV2.1 (P/Q-type) calcium channels

Ataxia is the predominant clinical manifestation of cerebellar dysfunction. Mutations in the human CACNA1A gene, encoding the pore-forming α1 subunit of CaV2.1 (P/Q-type) calcium channels, underlie several neurological disorders, including Episodic Ataxia type 2 and Familial Hemiplegic Migraine type 1 (FHM1). Several mouse mutants exist that harbor mutations in the orthologous Cacna1a gene. The spontaneous Cacna1a mutants Rolling Nagoya (tg(rol)), Tottering (tg) and Leaner (tg(ln)) mice exhibit behavioral motor phenotypes, including ataxia. Transgenic knock-in (KI) mouse strains with the human FHM1 R192Q and S218L missense mutations have been generated. R192Q KI mice are non-ataxic, whereas S218L KI mice display a complex behavioral phenotype that includes cerebellar ataxia. Given the dependence of γ-aminobutyric acid type A (GABAA) receptor subunit functioning on localized calcium currents, and the functional link between GABAergic inhibition and ataxia, we hypothesized that cerebellar GABAA receptor expression is differentially affected in Cacna1a mutants and contributes to the ataxic phenotype. Herein we quantified functional GABAA receptors and pharmacologically dissociated cerebellar GABAA receptors in several Cacna1a mutants. We did not identify differences in the expression of GABAA receptor subunits or in the number of functional GABAA receptors in the non-ataxic R192Q KI strain. In contrast, tg(rol) mice had a ∼15% decrease in the number of functional GABAA receptors, whereas S218L KI mice showed a ∼29% increase. Our data suggest that differential changes in cerebellar GABAA receptor expression profile may contribute to the neurological phenotype of cerebellar ataxia and that targeting GABAA receptors might represent a feasible complementary strategy to treat cerebellar ataxia.

Allosteric ligands and their binding sites define γ-aminobutyric acid (GABA) type A receptor subtypes

GABAA receptors (GABA(A)Rs) mediate rapid inhibitory transmission in the brain. GABA(A)Rs are ligand-gated chloride ion channel proteins and exist in about a dozen or more heteropentameric subtypes exhibiting variable age and brain regional localization and thus participation in differing brain functions and diseases. GABA(A)Rs are also subject to modulation by several chemotypes of allosteric ligands that help define structure and function, including subtype definition. The channel blocker picrotoxin identified a noncompetitive channel blocker site in GABA(A)Rs. This ligand site is located in the transmembrane channel pore, whereas the GABA agonist site is in the extracellular domain at subunit interfaces, a site useful for low energy coupled conformational changes of the functional channel domain. Two classes of pharmacologically important allosteric modulatory ligand binding sites reside in the extracellular domain at modified agonist sites at other subunit interfaces: the benzodiazepine site and the high-affinity, relevant to intoxication, ethanol site. The benzodiazepine site is specific for certain GABA(A)R subtypes, mainly synaptic, while the ethanol site is found at a modified benzodiazepine site on different, extrasynaptic, subtypes. In the transmembrane domain are allosteric modulatory ligand sites for diverse chemotypes of general anesthetics: the volatile and intravenous agents, barbiturates, etomidate, propofol, long-chain alcohols, and neurosteroids. The last are endogenous positive allosteric modulators. X-ray crystal structures of prokaryotic and invertebrate pentameric ligand-gated ion channels, and the mammalian GABA(A)R protein, allow homology modeling of GABA(A)R subtypes with the various ligand sites located to suggest the structure and function of these proteins and their pharmacological modulation.

The legacy of the benzodiazepine receptor: from flumazenil to enhancing cognition in Down syndrome and social interaction in autism

The study of the psychopharmacology of benzodiazepines continues to provide new insights into diverse brain functions related to vigilance, anxiety, mood, epileptiform activity, schizophrenia, cognitive performance, and autism-related social behavior. In this endeavor, the discovery of the benzodiazepine receptor was a key event, as it supplied the primary benzodiazepine drug-target site, provided the molecular link to the allosteric modulation of GABAA receptors and, following the recognition of GABAA receptor subtypes, furnished the platform for future, more selective drug actions. This review has two parts. In a retrospective first part, it acknowledges the contributions to the field made by my collaborators over the years, initially at Hoffmann-La Roche in Basle and later, in academia, at the University and the ETH of Zurich. In the second part, the new frontier of GABA pharmacology, targeting GABAA receptor subtypes, is reviewed with special focus on nonsedative anxiolytics, antidepressants, analgesics, as well as enhancers of cognition in Down syndrome and attenuators of symptoms of autism spectrum disorders. It is encouraging that a clinical trial has been initiated with a partial inverse agonist acting on α5 GABAA receptors in an attempt to alleviate the cognitive deficits in Down syndrome.

GABAA Receptor Expression in the Forebrain of Ataxic Rolling Nagoya Mice

The human CACNA1A gene encodes the pore-forming α₁ subunit of Ca_V2.1 (P/Q-type) calcium channels and is the locus for several neurological disorders, including episodic ataxia type 2 (EA2), spinocerebellar ataxia type 6 (SCA6) and Familial Hemiplegic Migraine type 1 (FHM1). Several spontaneous mouse Cacna1a mutant strains exist, among them Rolling Nagoya (tg^rol), carrying the R1262G point mutation in the mouse Cacna1a gene. tg^rol mice display a phenotype of severe gait ataxia and motor dysfunction of the hind limbs. At the functional level, the R1262G mutation results in a positive shift of the activation voltage of the Ca_V2.1 channel and reduced current density. γ-Aminobutyric acid type A (GABA_A) receptor subunit expression depends critically on neuronal calcium influx, and GABA_A receptor dysfunction has previously been described for the cerebellum of tg^rol and other ataxic Cacna1a mutant mice. Given the expression pattern of Ca_V2.1, it was hypothesized that calcium dysregulation in tg^rol might affect GABA_A receptor expression in the forebrain. Herein, functional GABA_A receptors in the forebrain of tg^rol mice were quantified and pharmacologically dissociated using [³H] radioligand binding. No gross changes to functional GABA_A receptors were identified. Future cell type-specific analyses are required to identify possible cortical contributions to the psychomotor phenotype of tg^rol mice.

Analysis of γ-aminobutyric acid (GABA) type A receptor subtypes using isosteric and allosteric ligands

The GABAA receptors (GABAARs) play an important role in inhibitory transmission in the brain. The GABAARs could be identified using a medicinal chemistry approach to characterize with a series of chemical structural analogues, some identified in nature, some synthesized, to control the structural conformational rigidity/flexibility so as to define the 'receptor-specific' GABA agonist ligand structure. In addition to the isosteric site ligands, these ligand-gated chloride ion channel proteins exhibited modulation by several chemotypes of allosteric ligands, that help define structure and function. The channel blocker picrotoxin identified a noncompetitive channel blocker site in GABAARs. This ligand site is located in the transmembrane channel pore, whereas the GABA agonist site is in the extracellular domain at subunit interfaces, a site useful for low energy coupled conformational changes of the functional channel domain. Also in the trans-membrane domain are allosteric modulatory ligand sites, mostly positive, for diverse chemotypes with general anesthetic efficacy, namely, the volatile and intravenous agents: barbiturates, etomidate, propofol, long-chain alcohols, and neurosteroids. The last are apparent endogenous positive allosteric modulators of GABAARs. These binding sites depend on the GABAAR heteropentameric subunit composition, i.e., subtypes. Two classes of pharmacologically very important allosteric modulatory ligand binding site reside in the extracellular domain at modified agonist sites at other subunit interfaces: the benzodiazepine site, and the low-dose ethanol site. The benzodiazepine site is specific for certain subunit combination subtypes, mainly synaptically localized. In contrast, the low-dose (high affinity) ethanol site(s) is found at a modified benzodiazepine site on different, extrasynaptic, subtypes.

Characterization of GABA receptors

Described in this unit are ligand-binding assays for GABAA , GABAB , and the homomeric ρ GABAA (formerly GABAC ) receptor recognition sites in brain tissue. Although GABA binding sites are present in peripheral organs, most research is directed toward examining these receptors in the CNS. These assays may also be used to determine the affinity of an unlabeled compound for the GABA binding sites. Excluded from the unit are ligand-binding assays for other components of the GABAA receptor complex, such as the benzodiazepine or ion-channel binding sites.

Relative positioning of diazepam in the benzodiazepine-binding-pocket of GABA receptors

GABA(A) receptors are the major inhibitory neurotransmitter receptors in the brain. Some of them are targets of benzodiazepines that are widely used in clinical practice for their sedative/hypnotic, anxiolytic, muscle relaxant and anticonvulsant effects. In order to rationally separate these different drug actions, we need to understand the interaction of such compounds with the benzodiazepine-binding pocket. With this aim, we mutated residues located in the benzodiazepine-binding site individually to cysteine. These mutated receptors were combined with benzodiazepine site ligands carrying a cysteine reactive group in a defined position. Proximal apposition of reaction partners will lead to a covalent reaction. We describe here such proximity-accelerated chemical coupling reactions of alpha(1)S205C and alpha(1)T206C with a diazepam derivative modified at the C-3 position with a reactive isothiocyanate group (-NCS). We also provide new data that identify alpha(1)H101C and alpha(1)N102C as exclusive sites of the reaction of a diazepam derivative where the -Cl atom is replaced by a -NCS group. Based on these observations we propose a relative positioning of diazepam within the benzodiazepine-binding site of alpha(1)beta(2)gamma(2) receptors.

Neurosteroids: endogenous regulators of the GABAA receptor

GABA(A) (gamma-aminobutyric acid type A) receptors mediate most of the 'fast' synaptic inhibition in the mammalian brain and are targeted by many clinically important drugs. Certain naturally occurring pregnane steroids can potently and specifically enhance GABA(A) receptor function in a nongenomic (direct) manner, and consequently have anxiolytic, analgesic, anticonvulsant, sedative, hypnotic and anaesthetic properties. These steroids not only act as remote endocrine messengers, but also can be synthesized in the brain, where they modify neuronal activity locally by modulating GABA(A) receptor function. Such 'neurosteroids' can influence mood and behaviour in various physiological and pathophysiological situations, and might contribute to the behavioural effects of psychoactive drugs.

Photoaffinity labeling with a neuroactive steroid analogue. 6-azi-pregnanolone labels voltage-dependent anion channel-1 in rat brain

Neuroactive steroids modulate the function of gamma-aminobutyric acid, type A (GABA(A)) receptors in the central nervous system by an unknown mechanism. In this study we have used a novel neuroactive steroid analogue, 3 alpha,5 beta-6-azi-3-hydroxypregnan-20-one (6-AziP), as a photoaffinity labeling reagent to identify neuroactive steroid binding sites in rat brain. 6-AziP is an effective modulator of GABA(A) receptors as evidenced by its ability to inhibit binding of [(35)S]t-butylbicyclophosphorothionate to rat brain membranes and to potentiate GABA-elicited currents in Xenopus oocytes and human endothelial kidney 293 cells expressing GABA(A) receptor subunits (alpha(1)beta(2)gamma(2)). [(3)H]6-AziP produced time- and concentration-dependent photolabeling of protein bands of approximately 35 and 60 kDa in rat brain membranes. The 35-kDa band was half-maximally labeled at a [(3)H]6-AziP concentration of 1.9 microM, whereas the 60-kDa band was labeled at higher concentrations. The photolabeled 35-kDa protein was isolated from rat brain by two-dimensional PAGE and identified as voltage-dependent anion channel-1 (VDAC-1) by both matrix-assisted laser desorption ionization time-of-flight and ESI-tandem mass spectrometry. Monoclonal antibody directed against the N terminus of VDAC-1 immunoprecipitated labeled 35-kDa protein from a lysate of rat brain membranes, confirming that VDAC-1 is the species labeled by [(3)H]6-AziP. The beta(2) and beta(3) subunits of the GABA(A) receptor were co-immunoprecipitated by the VDAC-1 antibody suggesting a physical association between VDAC-1 and GABA(A) receptors in rat brain membranes. These data suggest that neuroactive steroid effects on the GABA(A) receptor may be mediated by binding to an accessory protein, VDAC-1.

Identification of the bovine gamma-aminobutyric acid type A receptor alpha subunit residues photolabeled by the imidazobenzodiazepine [3H]Ro15-4513

Ligands binding to the benzodiazepine-binding site in gamma-aminobutyric acid type A (GABA(A)) receptors may allosterically modulate function. Depending upon the ligand, the coupling can either be positive (flunitrazepam), negative (Ro15-4513), or neutral (flumazenil). Specific amino acid determinants of benzodiazepine binding affinity and/or allosteric coupling have been identified within GABA(A) receptor alpha and gamma subunits that localize the binding site at the subunit interface. Previous photolabeling studies with [(3)H]flunitrazepam identified a primary site of incorporation at alpha(1)His-102, whereas studies with [(3)H]Ro15-4513 suggested incorporation into the alpha(1) subunit at unidentified amino acids C-terminal to alpha(1)His-102. To determine the site(s) of photoincorporation by Ro15-4513, we affinity-purified ( approximately 200-fold) GABA(A) receptor from detergent extracts of bovine cortex, photolabeled it with [(3)H]Ro15-4513, and identified (3)H-labeled amino acids by N-terminal sequence analysis of subunit fragments generated by sequential digestions with a panel of proteases. The patterns of (3)H release seen after each digestion of the labeled fragments determined the number of amino acids between the cleavage site and labeled residue, and the use of sequential proteolytic fragmentation identified patterns of cleavage sites unique to the different alpha subunits. Based upon this radiochemical sequence analysis, [(3)H]Ro15-4513 was found to selectively label the homologous tyrosines alpha(1)Tyr-210, alpha(2)Tyr-209, and alpha(3)Tyr-234, in GABA(A) receptors containing those subunits. These results are discussed in terms of a homology model of the benzodiazepine-binding site based on the molluscan acetylcholine-binding protein structure.

Deduction of amino acid residues in the GABA(A) receptor alpha subunits photoaffinity labeled with the benzodiazepine flunitrazepam

Peptide mapping and microsequencing were used to infer the site of photoaffinity labeling by the gamma-aminobutyric acidA receptor modulator [3H]flunitrazepam. Peptide mapping with and without N-deglycosylation was used to restrict the domain for photoaffinity labeling to residues 74-123 of the bovine alpha1 subunit, in agreement with a previously predicted labeling domain between residues 59-148 based on cyanogen bromide fragmentation. Edman degradation of partially purified photolabeled peptides gave release of 3H counts in the ninth cycle of a tryptic peptide sequence. A second V8/chymotryptic peptide produced an impure sequence with release of 3H counts in the seventh through ninth cycle of sequence. The combined data support those previously reported, i.e., that the primary site for photoaffinity labeling by [3H]flunitrazepam is His102 of the bovine alpha1 subunit. In addition we also detected possible secondary labeling of Pro97.

New insights into the mechanism of action of hypnotics

Between 1987 and 1989, the different protein subunits that make up the receptor for the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) were identified. These make up the alpha, beta, gamma and delta families, for each of which exist several subtypes. This receptor is the molecular target of modern hypnotic drugs (i.e. benzodiazepines, zopiclone, zolpidem and zaleplon). In the 10 years that have followed this milestone, significant progress has been made in exploring the molecular mechanisms of hypnotic drug action. Receptor subtype specificity of hypnotics has been explained in terms of differential affinity for receptors containing different alpha subunits, which are expressed in different brain regions. Zolpidem and zaleplon bind preferentially to alpha1-containing receptors, whereas benzodiazepines and zopiclone are aspecific. Different sets of subunits are encoded in contiguous 'cassettes' on the genome, and the transcription of each set appears to be regulated coherently. The predominant GABA(A) receptor composition found in the brain is alpha1beta2gamma2, which are all encoded on human chromosome 5. Targeted gene disruption has provided clues to the physiological functions served by GABA(A) receptors containing different subunits. Receptors containing gamma2 appear to have a vital role in maintaining appropriate central inhibition, beta3-containing receptors may also be important determinants of excitability in certain brain regions, whereas a clear role for alpha5-, alpha6- and gamma3-containing receptors has not yet been established by these techniques. Site-directed mutagenesis has indicated that benzodiazepines bind to a cleft on the GABA(A) receptor surface at the interface between the alpha and gamma subunits. Other drugs (flumazenil, zopiclone, zolpidem) also bind to the a subunit, but interact with amino acids in different binding domains to the benzodiazepines. The molecular mechanism of hypnotic dependence has been explored, and seems to involve downregulation of transcription of the normally prevalent alpha1, beta2 and gamma2 subunits, and the reciprocal upregulation of the expression of rarer subunits. Chronic treatment with hypnotic drugs that may have less dependence potential, such as zopiclone and zolpidem, appears to produce more limited change in GABA(A) receptor subunit expression. These ideas will be important both for designing new hypnotic drugs with a better safety/efficacy profile, and for evaluating more appropriate ways of using the drugs available today.

Helix 6 of subdomain III A of human serum albumin is the region primarily photolabeled by ketoprofen, an arylpropionic acid NSAID containing a benzophenone moiety

It is well known that the subdomain III A (site II) of human serum albumin (HSA) binds a variety of endogenous and exogenous substances. However, the nature of the microenvironment of the binding site remains unclear. Ketoprofen (KP), an arylpropionic acid NSAID which contains a benzophenone moiety, was used as a photoaffinity labeling agent to label the binding region. Subsequent CNBr cleavage of the photolabeled HSA revealed that the 11.6 kDa and 9.4 kDa fragments contained most of the incorporated radioactivity. Competition experiments showed that the 11.6 kDa fragment contains the common binding region for site II ligands. This fragment was redigested with Achromobacter lyticus protease I (AP-I) and the amino acid sequence of the photolabeled peptide was determined to be XCTESLVNRR, which corresponds to the sequence 476C-485K of HSA. The complete amino acid sequence of the corresponding AP-I digested HSA peptide encompasses residues 476 to 499, which form helices 5 and 6 of subdomain III A. The HSA-Myr X-ray crystallography data showed that helix 5 is involved to the least extent in ligand binding. A docking model provided further support that helix 6 represents the photolabeled region of KP.

Pharmacology of recombinant gamma-aminobutyric acidA receptors rendered diazepam-insensitive by point-mutated alpha-subunits

Amino acids in the alpha- and gamma-subunits contribute to the benzodiazepine binding site of GABA(A)-receptors. We show that the mutation of a conserved histidine residue in the N-terminal extracellular segment (alpha1H101R, alpha2H101R, alpha3H126R, and alpha5H105R) results not only in diazepam-insensitivity of the respective alphaxbeta2,3gamma2-receptors but also in an increased potentiation of the GABA-induced currents by the partial agonist bretazenil. Furthermore, Ro 15-4513, an inverse agonist at wild-type receptors, acts as an agonist at all mutant receptors. This conserved molecular switch can be exploited to identify the pharmacological significance of specific GABA(A)-receptor subtypes in vivo.

The major site of photoaffinity labeling of the gamma-aminobutyric acid type A receptor by [3H]flunitrazepam is histidine 102 of the alpha subunit

The alpha subunit of the gamma-aminobutyric acid type A (GABA(A)) receptor is known to be photoaffinity labeled by the classical benzodiazepine agonist, [3H]flunitrazepam. To identify the specific site for [3H]flunitrazepam photoincorporation in the receptor subunit, we have subjected photoaffinity labeled GABA(A) receptors from bovine cerebral cortex to specific cleavage with cyanogen bromide and purified the resulting photolabeled peptides by immunoprecipitation with an anti-flunitrazepam polyclonal serum. A major photolabeled peptide component from reversed-phase high performance liquid chromatography of the immunopurified peptides was resolved by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The radioactivity profile indicated that the [3H]flunitrazepam photoaffinity label is covalently associated with a 5.4-kDa peptide. This peptide is glycosylated because treatment with the enzyme, peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase, reduced the molecular mass of the peptide to 3.2 kDa. Direct sequencing of the photolabeled peptide by automated Edman degradation showed that the radioactivity is released in the twelfth cycle. Based on the molecular mass of the peptides that can be generated by cyanogen bromide cleavage of the GABA(A) receptor alpha subunit and the potential sites for asparagine-linked glycosylation, the pattern of release of radioactivity during Edman degradation of the photolabeled peptide was mapped to the known amino acid sequence of the receptor subunit. The major site of photoincorporation by [3H]flunitrazepam on the GABA(A) receptor is shown to be alpha subunit residue His102 (numbering based on bovine alpha 1 sequence).

From ion currents to genomic analysis: recent advances in GABAA receptor research

The gamma-aminobutyric acid type A (GABAA) receptor represents an elementary switching mechanism integral to the functioning of the central nervous system and a locus for the action of many mood- and emotion-altering agents such as benzodiazepines, barbiturates, steroids, and alcohol. Anxiety, sleep disorders, and convulsive disorders have been effectively treated with therapeutic agents that enhance the action of GABA at the GABAA receptor or increase the concentration of GABA in nervous tissue. The GABAA receptor is a multimeric membrane-spanning ligand-gated ion channel that admits chloride upon binding of the neurotransmitter GABA and is modulated by many endogenous and therapeutically important agents. Since GABA is the major inhibitory neurotransmitter in the CNS, modulation of its response has profound implications for brain functioning. The GABAA receptor is virtually the only site of action for the centrally acting benzodiazepines, the most widely prescribed of the anti-anxiety medications. Increasing evidence points to an important role for GABA in epilepsy and various neuropsychiatric disorders. Recent advances in molecular biology and complementary information derived from pharmacology, biochemistry, electrophysiology, anatomy and cell biology, and behavior have led to a phenomenal growth in our understanding of the structure, function, regulation, and evolution of the GABAA receptor. Benzodiazepines, barbiturates, steroids, polyvalent cations, and ethanol act as positive or negative modulators of receptor function. The description of a receptor gene superfamily comprising the subunits of the GABAA, nicotinic acetylcholine, and glycine receptors has led to a new way of thinking about gene expression and receptor assembly in the nervous system. Seventeen genetically distinct subunit subtypes (alpha 1-alpha 6, beta 1-beta 4, gamma 1-gamma 4, delta, p1-p2) and alternatively spliced variants contribute to the molecular architecture of the GABAA receptor. Mysteriously, certain preferred combinations of subunits, most notably the alpha 1 beta 2 gamma 2 arrangement, are widely codistributed, while the expression of other subunits, such as beta 1 or alpha 6, is severely restricted to specific neurons in the hippocampal formation or cerebellar cortex. Nervous tissue has the capacity to exert control over receptor number, allosteric uncoupling, subunit mRNA levels, and posttranslational modifications through cellular signal transduction mechanisms under active investigation. The genomic organization of the GABAA receptor genes suggests that the present abundance of subtypes arose during evolution through the duplication and translocations of a primordial alpha-beta-gamma gene cluster. This review describes these varied aspects of GABAA receptor research with special emphasis on contemporary cellular and molecular discoveries.

GABAA receptor subtypes: ligand binding heterogeneity demonstrated by photoaffinity labeling and autoradiography

Heterogeneity of binding affinities for a variety of ligands was observed for gamma-aminobutyric acid type A (GABAA) receptors in the rat CNS, at both GABA and benzodiazepine recognition sites. Photoaffinity labeling by [3H]flunitrazepam and [3H]muscimol to affinity column-purified receptor proteins was examined by gel electrophoresis in sodium dodecyl sulfate. Anesthetic barbiturates (pentobarbital) and steroids (alphaxalone) both differentially stimulated the incorporation of [3H]flunitrazepam more so into the 51-kDa alpha 1 subunit than into the 53-kDa alpha 2 polypeptide, and incorporation of [3H]muscimol into the 55-kDa beta 2 subunit more so than the 58-kDa beta 3 polypeptide. Binding to these polypeptides was also affected differentially by other allosteric modulators and competitive inhibitors, including the benzodiazepine "type 1" selective ligand CL218,872. Heterogeneity in affinity of this drug for the single 51-kDa alpha 1 polypeptide strongly suggests that type I receptors, like type II, are heterogeneous. In brain sections, the extent of enhancement of [3H]muscimol binding showed significant regional variation, similar for both steroids and barbiturates, and the GABA analogues THIP and taurine inhibited muscimol binding with regional variations in affinity that were almost opposites of each other. Modulation of [3H]flunitrazepam binding by steroids, barbiturates, and THIP significantly varied with regions. Taken together, ligand binding heterogeneity exhibited by photoaffinity labeling and autoradiography demonstrate the existence of multiple pharmacological-binding subtypes resulting from the combination of multiple polypeptide gene products into several oligomeric isoreceptors. Comparison of the regional distribution of binding subtypes with that of different subunit gene products allows the following conclusions about possible subunit compositions of native pharmacological receptor subtypes present in the brain: Benzodiazepine pharmacology of the oligomeric receptor isoforms is dependent on the nature of alpha and subunits other than alpha, GABA-benzodiazepine coupling is dependent on the nature of the alpha subunits, GABA site pharmacology is dependent on the nature of the beta subunits, and several subunits including alpha and beta contribute to the degree of sensitivity to steroids and barbiturates. Finally, the presence of discrete subunits may be necessary but is not sufficient to postulate a defined pharmacological property.

Involvement of a disulfide bond in the binding of flunitrazepam to the benzodiazepine receptor from bovine cerebral cortex

The effects of chemical modification of a disulfide bond(s) (-SS-) or sulfhydryl group(s) (-SH) on the [3H]-flunitrazepam ([3H]FNZ) binding to membrane-bound or immunoprecipitated benzodiazepine (BZD) receptors (BZD-R) from bovine cerebral cortex were examined. Reduction of -SS- with dithiothreitol (DTT) brought about a reversible, time- and dose-dependent inhibition of [3H]FNZ binding to the membrane-bound BZD-R. Alkylation of the membranes with the -SH-modifying reagent iodoacetamide (IAA) or 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) produced a slight inhibition of [3H]FNZ binding in a dose-dependent manner. Scatchard analysis of saturation curves of [3H]FNZ binding in the presence and absence of 5 mM DTT revealed changes in affinity without modification in the maximal binding capacity, thus indicating a competitive mode of interaction. DTT pretreatment of both the membrane-bound and the immunoprecipitated BZD-R led to [3H]FNZ binding inhibition. Consistent with the modification of a binding site is the observation that reduction of -SS- does not bear on the binding affinity, but rather reduces the number of sites. Complete protection from DTT inhibition of [3H]FNZ binding by FNZ (an agonist) or by Ro 15-1788 (an antagonist) suggests the presence of -SS- at, or very close to, the BZD recognition binding site. No protection against IAA or DTNB inhibition was provided by FNZ. Photoaffinity labeling experiments with [3H]FNZ revealed a clear-cut band of 50 kDa in native and alkylated membranes but an extremely weak label in 5 mM DTT/IAA-treated membranes. The present results provide evidence for the participation of a disulfide bond in the recognition binding site of the bovine cerebral cortex BZD-R.

Electron microscopic localization of photoaffinity-labelled delta opioid receptors in the neostriatum of the rat

The distribution of delta opioid receptors, selectively labelled in vitro with the photoaffinity probe monoiodo azido-DTLET ([D-Thr2,pN3Phe4, Leu5]enkephaly-Thr6), was analyzed by light and electron microscopic radioautography in sections from rat neostriatum. Preliminary experiments indicated that up to 65% of specific 125I-azido-DTLET binding to rat striatal sections was still detectable following prefixation of the brain with 0.5% glutaraldehyde. These experiments also showed that up to 20-30% of the specifically bound radioactivity was covalently linked following ultraviolet irradiation and was thereby retained in tissue during subsequent postfixation and dehydration steps. Accordingly, the topographic distribution of the covalently attached azido-DTLET molecules was similar to that seen in fresh frozen sections and characteristic of that previously described for delta sites. Light and electron microscopic examination of the label in prefixed, striatal sections irradiated with ultraviolet light revealed that a significant proportion of specifically bound 125I-azido-DTLET molecules was intraneuronal. Specifically, 16% of the labelled binding sites were found in dendrites, 12% in perikarya and 4% in axon terminals. These results suggest that an important proportion of delta opioid binding sites labelled in the neostriatum correspond to receptors that are undergoing synthesis, transport and/or recycling. They also imply that a major fraction of delta sites are associated with intrastriatal neurons, as opposed to afferent axons. Approximately 44% of the labelled binding sites were associated with neuronal plasma membranes. Although most of these were found at the level of axodendritic (20%) and dendrodendritic (7%) appositions, comparison of the labelling incidence of these two compartments with their frequency of occurrence in tissue suggested that delta sites are fairly widely dispersed along neuronal plasma membranes. Only a small proportion (smaller than that of mu or kappa sites labelled in the same region) was associated with synaptic specializations. These results support the concept that delta receptors correspond to molecular entities that are distinct from mu and kappa sites and suggest that delta ligands act primarily nonjunctionally on the plasma membrane of striatal neurons.

m-Sulfonate benzene diazonium chloride: a powerful affinity label for the gamma-aminobutyric acid binding site from rat brain

m-Sulfonate benzene diazonium chloride (MSBD) was used to affinity-label the gamma-aminobutyric acid (GABA) binding site from rat brain membranes. To assess the irreversibility of the labeling reaction, we used an efficient ligand dissociation procedure combined to a rapid [3H]muscimol binding assay, both steps being performed on filter-adsorbed membranes. Inactivation of specific [3H]-muscimol binding sites by MSBD and its prevention by GABA were both time- and concentration-dependent. The time course of MSBD labeling was shortened as the pH of the incubation medium was increased from 6.2 to 8. These data suggest that MSBD can efficiently label the GABA binding site through alkylation of a residue having an apparent dissociation constant around neutrality.

Pharmacological and Electrophysiological Properties of Recombinant GABAA Receptors Comprising the alpha3, beta1 and gamma2 Subunits

To assess the role of subunits for channel function and drug modulation in recombinant GABAA receptors, the alpha3beta1gamma2 subunits and the dual combinations alpha3beta1, beta1gamma2 and alpha3gamma2 were expressed by transfection of human embryonic kidney cells and by RNA injection in Xenopus oocytes (alpha3beta1gamma2 combination). GABA-induced chloride currents were recorded using the whole-cell configuration of the patch-clamp technique (transfected cells) or the voltage-clamp technique (oocytes). The currents recorded from the alpha3beta1gamma2 subunit combination in transfected cells were reduced by bicuculline and picrotoxin, enhanced by flunitrazepam in a flumazenil-sensitive manner and reduced by beta-carboline-3-carboxylic acid methyl ester (beta-CCM). The GABA-induced current was reduced by beta-CCM in all combinations containing the gamma2 subunit, but potentiation by flunitrazepam was only obtained when the gamma2 subunit was coexpressed in the presence of the alpha3 subunit (alpha3beta1gamma2 or alpha3gamma2). The GABA sensitivities of the receptors were similar when the alpha3beta1gamma2 combination was expressed in oocytes (half-maximum effective concentration=240 microM) or in the kidney cell line (270 microM). However, the currents were less potentiated by flunitrazepam in oocytes (129% of controls) than in transfected cells (189%). These results suggest that the alpha3beta1gamma2 subunit combination, which is coexpressed in various brain regions as shown by in situ hybridization histochemistry, may represent a building block of functional GABAA receptors in situ.

The GABAA receptor family in the mammalian brain

The GABAA-benzodiazepine receptor protein from bovine brain was purified by affinity chromatography and the subunit composition examined by gel electrophoresis in sodium dodecyl sulfate. Protein staining revealed a doublet at 51-53 kDa, a band at 55 kDa, and a broad band at 57-59 kDa. The 51 and 53 kDa bands co-migrated with the alpha 1 and alpha 2 gene products identified by Western blotting with subtype-specific antibodies. These two bands were also photoaffinity labeled by [3H]flunitrazepam, as was a breakdown product at 44 kDa. Partial sequencing of proteolytic fragments of these polypeptides yielded sequences found in all alpha clones, and identified the benzodiazepine binding site within residues 8-297 and probably between 106-297 of alpha 1; the 44 kDa and 31 kDa bands yielded fragments containing alpha 3 sequence. The native alpha 3 polypeptide was identified with subtype-specific antibody at 57 kDa overlapping with the two major bands photolabeled with [3H]muscimol at 55 and 58 kDa. Antisera to a beta-selective peptide recognized four bands at 60, 58, 57 and 55 kDa. Thus, one can identify 6-8 distinct polypeptides with the possibility of another 4-6 in purified GABAA receptor proteins, depending on brain region, consistent with the family of gene products suggested by molecular cloning.

Pharmacological and biochemical properties of the gamma-aminobutyric acid-benzodiazepine receptor protein from codfish brain

The gamma-aminobutyric acidA (GABAA) receptor of codfish brain has been purified to homogeneity and contains a single polypeptide band of 56 kDa molecular mass. Polyacrylamide gel electrophoresis in sodium dodecyl sulfate (SDS-PAGE) of codfish GABA receptor photoaffinity-labeled by both [3H]flunitrazepam ([3H]Flu) and [3H]muscimol showed a single radioactive peak with molecular mass of 56 kDa, in contrast to the multiple subunits found in other vertebrate species. The codfish receptor, purified using benzodiazepine (BZ, Ro 7-1986/1) affinity chromatography, contains an apparent single band both by isoelectric focussing and on a silver-stained SDS gel. The receptor density and affinity constants for [3H]muscimol and [3H]Flu binding are comparable to those in mammalian brain, and the specific activity (greater than 1,000 pmol/mg of protein) is comparable to that of preparations purified from those sources. The pharmacological specificity of the codfish GABA-BZ receptor is generally similar to that of mammalian brain, including GABA-BZ coupling. The BZ binding exhibits homogeneous kinetic properties resembling those of the mammalian BZ2 receptor type, and shows strong GABA enhancement of [3H]Flu binding and weaker pentobarbital potentiation. This is consistent with other observations of an earlier phylogenetic, as well as ontogenetic, emergence in mammals of the BZ2 receptor subtype than the BZ1. Codfish GABA receptor is postulated to be a homo-oligomer in which the conformation of GABA and BZ recognition sites is very similar to that in the mammalian hetero-oligomeric GABAA receptor. The codfish receptor appears to be encoded by an ancestral gene and indicates an early development of BZ-GABA coupling.

Immunochemical identification of the alpha 1- and alpha 3-subunits of the GABAA-receptor in rat brain

To identify subunit variants of the GABAA-receptor antisera were developed against specific cDNA-derived peptide sequences of the alpha 1- and alpha 3-subunits of rat brain. The alpha 1-subunit antiserum selectively recognized a protein of 50 +/- 1 kDa in rat and bovine GABAA-receptor preparations, while the alpha 3-subunit antiserum interacted with a protein doublet of 59 +/- 2 kDa and 61 +/- 3 kDa. The alpha 1-subunit immunoreactivity resides in a large population of GABAA-receptors as shown by immunoprecipitation of 63 +/- 6% of [3H]flumazenil binding sites with the alpha 1-subunit antiserum. In contrast, only 24 +/- 3% of receptor binding sites were precipitated with the alpha 3-subunit antiserum. Co-precipitation studies suggest that the alpha 1- and alpha 3-subunit immunoreactivities do not share the same receptor population while the gamma 2-subunit immunoreactivity is associated with the alpha 1-subunit immunoreactivity.

Calpain inhibitor I prevents rapid postmortem degradation of benzodiazepine binding proteins: fluorographic and immunological evidence

Endogenous proteolysis of the major central benzodiazepine (BZ) binding protein of 53K occurs rapidly postmortem and leads to a fragment of 47K. To determine indirectly the protease responsible for this proteolysis, membranes of porcine cortex were prepared from homogenates, which were either frozen immediately or left at room temperature for 12 h in the presence or absence of various representative protease inhibitors. Membranes were subsequently photolabeled with [3H]flunitrazepam, and subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography or immunoblotted using an alpha-subunit-specific monoclonal antibody bd-24. Both fluorographs and immunoblots revealed that calpain inhibitor I, Ep-459 (E-64 analogue), and EDTA (greater than or equal to 1 mM) prevent endogenous proteolysis. In future studies one of these inhibitors should be added to receptor preparations. The results indicate that calpain is the responsible protease.

Subunit selectivity and epitope characterization of mAbs directed against the GABAA/benzodiazepine receptor

mAbs bd 17, bd 24, and bd 28 raised against bovine cerebral gamma-aminobutyric acid (GABAA)/benzodiazepine receptors were analyzed for their ability to detect each of 12 GABAA receptor subunits expressed in cultured mammalian cells. Results showed that mAb bd 17 recognizes epitopes on both beta 2 and beta 3 subunits while mAb bd 24 is selective for the alpha 1 subunit of human and bovine, but not of rat origin. The latter antibody reacts with the rat alpha 1 subunit carrying an engineered Leu at position four, documenting the first epitope mapping of a GABAA receptor subunit-specific mAb. In contrast to mAbs bd 17 and bd 24, mAb bd 28 reacts with all GABAA receptor subunits tested but not with a glycine receptor subunit, suggesting the presence of shared epitopes on subunits of GABA-gated chloride channels.

Immunocytochemistry of neurotransmitter receptors

Over the last several years our knowledge of neurotransmitter receptors has increased dramatically as receptor types and subtypes have been identified through the development of selective antagonists, neuropharmacological studies, and radioactive ligand binding studies. At the same time major advances were made in the immunocytochemical localization of neurotransmitters and their related enzymes. However, only recently has immunocytochemistry been used to localize neurotransmitter receptors, and these studies have been limited. Four receptors have been localized in the CNS with immunocytochemistry: the nicotinic acetylcholine receptor, the beta-adrenergic receptor, the GABA/benzodiazepine receptor, and the glycine receptor. Of these the glycine receptor has been the most thoroughly characterized. Glycine receptor immunoreactivity is highly concentrated at postsynaptic sites, and the distribution of immunoreactivity appears to correlate closely with glycinergic neurons. However, immunocytochemical studies done on other receptors suggest such a distribution may not always be the case. Some receptors may not be concentrated at postsynaptic sites, and receptor distribution may not always closely fit the distribution of the respective neurotransmitter. Work is rapidly progressing on the purification of other receptors and on the production of selective antibodies which will allow immunocytochemical studies which address these and other questions.

GABAA-receptors: structural requirements and sites of gene expression in mammalian brain

GABAA-receptors, the major synaptic targets for the neurotransmitter GABA, are gated chloride channels. By their allosteric drug-induced modulation they serve as molecular control elements through which the levels of anxiety, vigilance, muscle tension and epileptiform activity can be regulated. Despite their functional prominence, the structural requirements of fully functional GABAA-receptors are still elusive. Expression of cDNAs coding for the alpha 1- beta 1-subunits of rat brain yielded GABA-gated chloride channels which were modulated by barbiturates but displayed only agonistic responses to ligands of the benzodiazepine receptor. GABAA-receptors with fully functional benzodiazepine receptor sites were formed when the alpha 1- and beta 1-subunits were co-expressed with the gamma 2-subunit of rat brain. These receptors, however, failed to show cooperativity of GABA in gating the channel. In order to determine the subunit repertoire available for receptor assembly in different neuronal populations in vivo, the sites of subunit gene expression were (alpha 1, alpha 2, alpha 3, alpha 5, alpha 6, beta 1, beta 2, beta 3, gamma 2) mapped by in situ hybridization histochemistry in brain sections. The mRNAs of the alpha 1-, beta 1- and gamma 2-subunits were co-localized e.g. in mitral cells of olfactory bulb, pyramidal cells of hippocampus as well as granule cells of dentate gyrus and cerebellum. The lack of colocalization in various other brain areas points to an extensive receptor heterogeneity. The presence of multiple GABAA-receptors in brain may contribute to synaptic plasticity, differential responsiveness of neurons to GABA and to variations in drug profiles.

The GABA-benzodiazepine interaction fifteen years later

The main steps are presented that led to our current understanding of the interaction between benzodiazepine receptor ligands and the GABAA receptor. The benzodiazepine receptor is a modulatory site located on the GABAA receptor-chloride channel complex that has the unique property of being able to mediate positive as well as negative modulation of the chloride channel gating by the GABAA receptor. Some critical issues concerning the structure of the receptor-channel complex remain to be clarified. Research on the benzodiazepine-GABA interaction has led to novel concepts of drug action and receptor function and provides the basis for a whole spectrum of potential drugs with therapeutic utility.

Functional expression and sites of gene transcription of a novel alpha subunit of the GABAA receptor in rat brain

Two alpha subunits of the GABAA receptor in rat brain have been identified by molecular cloning. The deduced polypeptide sequences share major characteristics with other chemically gated ion channel proteins. One polypeptide represents the rat homologue of the alpha 3 subunit previously cloned from bovine brain, while the other polypeptide is a yet known subunit, termed alpha 5. When coexpressed with the beta 1 subunit in Xenopus oocytes the receptors containing the alpha 5 subunit revealed a higher sensitivity to GABA than receptors expressed from alpha 1 + beta 1 subunits or alpha 3 + beta 1 subunits (Ka = 1 microM, 13 microM and 14 microM, respectively). The alpha 5 subunit was expressed only in a few brain areas such as cerebral cortex, hippocampal formation and olfactory bulb granular layer as shown by in situ hybridization histochemistry. Since the mRNA of the alpha 5 subunit was colocalized with the alpha 1 and alpha 3 subunits only in cerebral cortex and in the hippocampal formation the alpha 5 subunit may be part of distinct GABAA receptors in neuronal populations within the olfactory bulb.

Characterization with antibodies of the gamma-aminobutyric acidA/benzodiazepine receptor complex during development of the rat brain

The postnatal development of the gamma-aminobutyric acidA/benzodiazepine receptor (GABAR/BZDR) complex of the rat brain has been investigated using the monoclonal antibody 62-3G1 and the polyclonal rabbit antiserum A, specific for the 57,000 and 51,000 Mr receptor subunits, respectively. Both GABAR and BZDR binding activities co-precipitated during all postnatal ages. Adult rats showed a main 51,000 Mr[3H]flunitrazepam photoaffinity-labeled peptide, whereas newborn rats showed several photolabeled peptides of higher Mr. All the photolabeled peptides could be immunoprecipitated with each antibody regardless of the age of the rats. These results suggest that the physical coupling between the GABAR and the BZDR is already present in newborn animals and it is maintained afterwards during development. Glycosidase and peptidase treatments of the immunoprecipitated GABAR/BZDR complex indicated that all the [3H]flunitrazepam-photolabeled subunits are different peptides, although they seem to conserve a high degree of homology. In addition to the age-dependent heterogeneity, the results also suggest that for each age, there is heterogeneity in the subunit composition of the GABAR/BZDR complex.

Sequence and expression of a novel GABAA receptor alpha subunit

Cloned cDNA encoding the bovine alpha 4 subunit of the GABAA receptor has been isolated. The predicted 51 amino acid long mature protein contains an exceptionally long intracellular domain and shares 53-56% sequence similarity to the previously characterized alpha 1, alpha 2 and alpha 3 subunits. Co-expression of alpha 4 and beta 1 in Xenopus oocytes resulted in the formation of GABA-gated chloride channels with expected pharmacology, although no benzodiazepine potentiation was observed. Northern analysis indicates that a 4 kb alpha 4 mRNA is expressed in the calf cerebellum, cortex and hippocampus but is barely detectable in the rat brain.

Mapping the benzodiazepine photoaffinity-labelling site with sequence-specific gamma-aminobutyric acidA-receptor antibodies

The gamma-aminobutyric acidA (GABAA) receptor purified from adult bovine cerebral cortex was photoaffinity-labelled with the agonist benzodiazepine [3H]flunitrazepam and the radioactivity shown to be coincident with a band with Mr 53,000 that was recognized by three anti-(GABAA receptor alpha 1 subunit sequence)-specific antibodies. Complete and limited CNBr cleavage of the purified photoaffinity-labelled receptor was carried out. The products of this reaction were analysed for radioactivity, for immunoreactivity with anti-[alpha 1-(1-15)-peptide], anti-[alpha 1-(324-341)-peptide] and anti-[alpha 1-(413-429)-peptide] polyclonal antibodies and for carbohydrate by biotinylated concanavalin A lectin overlay. Complete CNBr cleavage gave a radioactive peptide with Mr 10,000-12,000 that was not recognized by the above-mentioned specific antisera. By using the deduced amino acid sequence of the alpha 1 subunit [Schofield, Darlison, Fujita, Burt, Stephenson, Rodriguez, Rhee, Ramachandran, Reale, Glencorse, Seeburg & Barnard (1987) Nature (London) 328, 221-227], it is proposed that the site of the benzodiazepine-agonist photoaffinity-labelling reaction does not lie within the amino acid sequences alpha 1 1-58 and alpha 1 149-429.

Postmortem degradation alters fluorographic labeling patterns and affinities of benzodiazepine binding proteins

To investigate the effect of endogenous proteolysis on the molecular weights of the benzodiazepine binding proteins, brains of trout, chicken, and rat were removed immediately after death and stored at room temperature for various periods of time before they were frozen. Photoaffinity labeling of membranes with [3H]flunitrazepam, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography, revealed proteolytic fragments of 47K in trout, chicken, and rat. The proteolysis set in rapidly after death. Seemingly in parallel with the degradation observed fluorographically, the affinity for [3H]flunitrazepam increased without systematic changes in receptor density. The degradation pattern was not identical to that of the photolabeled trypsinized benzodiazepine binding proteins. The endogenous proteolytic fragments were deglycosylated in two steps. In conclusion, proteolytic effects must be taken into account when interpreting labeling patterns and binding parameters.