Identification of the alpha 3-subunit in the GABAA receptor purified from bovine brain

Endogenous phosphorylation of the GABA(A) receptor protein is counteracted by a membrane-associated phosphatase

Incubation of bovine brain membranes with [gamma-33P]ATP phosphorylated mainly a 51-kDa band. Electrophoretic co-migration was observed for 33P- and [3H]flunitrazepam-labeled bands in both membrane fractions and in affinity-purified GABA(A) receptor (GABAA-R) preparations. An alpha-subunit monoclonal antibody adsorbed most of the radiolabeled-band, suggesting that the labeled-membrane polypeptide corresponds to the GABA(A)-R alpha1-subunit, which is the only GABA(A)-R subunit with a molecular weight of 51 kDa. The phosphorylation rate was much faster in membranes than in purified receptor. Dephosphorylation was detected in membranes only. The membrane-bound phosphatase was potently inhibited by vanadate and Zn2+>>Mn2+ , but was insensitive to okadaic acid (a phosphatase 1, 2 and 2B inhibitor), cyclosporin (specific calcineurin inhibitor) and phosphatase-1 inhibitor. Endogenous kinase was activated by divalent cations including calcium (Mg2- > Mn2+ > Ca2+), whilst dephosphorylation did not require the presence of Ca2+ ions. This suggests that at least one membrane-bound phosphatase counteracts the endogenous phosphorylation of the GABA(A)-R: the lack of dephosphorylation in the purified receptor preparation indicates that, in contrast to the endogenous kinase, no phosphatase is closely associated with the receptor protein complex.

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.

Distribution of [3H]zolpidem binding sites in relation to messenger RNA encoding the alpha 1, beta 2 and gamma 2 subunits of GABAA receptors in rat brain

Localization of the messenger RNAs that encode the alpha 1, beta 2 and gamma 2 subunits of GABAA showed a distinct topographic pattern in rat brain which corresponded with [3H]zolpidem binding in most brain regions. The close topographic correspondence between the specific receptor subunits examined and the distribution of [3H]zolpidem binding sites provides support for the hypothesis that this benzodiazepine type 1 selective ligand binds to a GABAA receptor that consists of alpha 1, beta 2 and gamma 2 subunits in the rat brain. Brain regions with relatively high densities of alpha 1, beta 2 and gamma 2 subunits of GABAA and [3H]zolpidem binding included olfactory bulb, medial septum, ventral pallidum, diagonal band, inferior colliculus, substantia nigra pars reticulata and specific layers of the cortex. Two areas with low [3H]zolpidem binding and a virtual absence of these GABAA receptor subunit messenger RNAs were the lateral septum and the striatum. In contrast to the discrete pattern observed for alpha 1 and beta 2 subunit messenger RNAs, the gamma 2 subunit messenger RNA was distributed more diffusely in brain. Only the hippocampus, layer 2 of the piriform cortex and the cerebellum showed a strong concentration of the gamma 2 subunit messenger RNA. It was determined with a polymerase chain reaction assay that both long and short variants of the gamma 2 subunit messenger RNAs were present within several of the brain sites selected for examination. Sites with high densities of [3H]zolpidem binding sites had a greater relative abundance of the gamma 2 long splice variant, compared to the gamma 2 short variant. There were some regions that expressed high levels of alpha 1, beta 2 and gamma 2S subunit messenger RNAs but low [3H]zolpidem binding, suggesting that gamma 2 splice variant expression may modulate high-affinity [3H]zolpidem binding. To determine relationships between in vitro [3H]zolpidem binding and functional sensitivity in vivo, interactions between zolpidem and GABA were assessed in brain regions that contained high and low densities of [3H]zolpidem binding sites. In the medial septum, a brain region with a high concentration of [3H]zolpidem binding sites, iontophoretic application of zolpidem enhanced the inhibitory effect of GABA responses on 70% of the neurons examined. In the lateral septum, which contains very low densities of [3H]zolpidem binding sites, neurons were not sensitive to zolpidem enhancement of GABA-induced inhibition. These electrophysiological results demonstrate a correspondence between the regional distribution of [3H]zolpidem binding in vitro and functional sensitivity to the drug in vivo.

GABAA receptor subunit polypeptides increase in parallel but exhibit distinct distributions in the developing rat cerebellum

The GABAA receptor, a multisubunit ligand-gated ion channel, plays a central role in cell-cell communication in the developing and adult nervous system. Although the developmental expression of mRNAs encoding many subunit isoforms has been extensively characterized throughout the central nervous system, little is known concerning the relationship between subunit mRNA and polypeptide expression. To address this issue, we examined the developmental expression of the alpha 1, beta 2/3, and gamma 2 subunit polypeptides, subunits that are thought to coassemble in many brain regions. Western blot analysis using subunit-specific antibodies revealed that the levels of these polypeptides in both the cerebral cortex and cerebellum increased severalfold during the second postnatal week. Whereas polypeptide expression in the cerebellum paralleled that of the corresponding subunit mRNAs, increases in beta 2/3 and gamma 2 polypeptide expression in the cerebral cortex occurred in the absence of detectable changes in the mRNA levels. To determine whether the increases in subunit polypeptide expression in the cerebellum were accompanied by changes in distribution, immunohistochemistry was performed. These studies demonstrated that the subunits exhibited different but partially overlapping distributions that remained constant throughout postnatal development. Our findings suggest that although GABAA receptor subunit polypeptide expression may be regulated primarily at the level of the mRNA, additional regulatory mechanisms may play a role. Furthermore, the observation that subunit distribution remains constant in the cell bodies of cerebellar Purkinje neurons, which express the alpha 1, beta 2, beta 3, and gamma 2 subunit mRNAs exclusively, suggests that GABAA receptor subunit composition in this cell population does not change during postnatal maturation.

Bicyclic [b]-heteroannulated pyridazine derivatives--II. Structure-activity relationships in the 6-aryltriazolo-[4,3-b]pyridazine ligands of the benzodiazepine receptor

Electronic parameters (molecular electrostatic potential MEP, charge distribution on the nitrogen atoms, dipole moment mu and ionization potential IP) were calculated by semiempirical quantum chemistry methods for 2 sets (X = H and m-CF3, the syn- and anti-rotamers of the latter being considered separately) of the 6-aryl-3-substituted-triazolo[4,3-b]pyridazine ligands of the benzodiazepine receptors (Figure 1; for X and Y c.f. Table 1). The calculations located the deepest MEP minimum near the = N-N = fragment of the triazole ring (Figure 2). Activity of the investigated compounds (1 microM), expressed as % inhibition of in vitro 3H-diazepam (1.5 nM) binding, revealed a significant dependence on IP, which combined in correlation studies with the hydrophobic constants pi X and pi Y and the Swain-Lupton field constant FY gave a 100% explanation of variance (Equations 1-3). However, extrapolation pointed to a compound with excessive hydrophobicity. The dipole moment orientation, roughly consistent with the C(6)-aryl main molecular axis, was considered as another factor controlling the docking of the investigated triazolopyridazine ligands to the benzodiazepine receptor (Figure 3). A model of the triazolopyridazine-benzodiazepine receptor interaction was proposed (Figure 4).

GABAA receptors in mouse cortical homogenates are phosphorylated by endogenous protein kinase A

Biochemical, molecular, and electrophysiological studies suggest that phosphorylation of beta subunits of the GABAA receptor (GaR) by exogenous protein kinase A inactivates the receptor channels. We have developed a method which for the first time allows the study of GaR phosphorylation in brain tissues by endogenous PKA. Desalted homogenates or crude synaptic membranes from mouse cerebral cortex were incubated with [gamma-32P]ATP and 8-Br-cAMP or chlorophenylthio-cAMP. Extracts from these incubations were immunoprecipitated by polyclonal antibodies against native GaR and analyzed by SDS-gel electrophoresis and autoradiography. In both homogenates and membranes, cAMP-dependent incorporation of 32P was observed for a 57-kDa peptide, and to a lesser extent 51- to 53-kDa peptides. Phosphorylation of affinity-purified GaR by the catalytic subunit of PKA also produced a major 57-kDa phosphopeptide and a minor 51-kDa phosphopeptide. Limited digestion by S. aureus V-8 protease of the 57-kDa phosphopeptide from the desalted homogenates or from purified receptors produced a major 32P-labeled fragment of 11 kDa, suggesting that the phosphorylation site is similar to that shown previously to reduce GaR function. The phosphorylation of GaRs in homogenates was time dependent and blocked by H-89 or protein kinase inhibitor 5-24, specific inhibitors of protein kinase A. Prolonged incubations resulted in dephosphorylation of the 57-kDa phosphoprotein by a microcystin-LR sensitive phosphatase. In cortical homogenates the level of cAMP-dependent phosphorylation of the 57-kDa GaR peptide was more than 5 times that obtained with washed synaptic membranes. However, assays of PKA using the heptamer kemptide as substrate showed that the specific activity in the particulate fraction was 57% that of the homogenate. This suggests that GaRs on synaptic membranes are preferentially phosphorylated by a cytoplasmic form of protein kinase A. By comparing the [3H]flunitrazepam-photolabeled 53-kDa GaR subunit with the 51-57 kDa [32P]peptides from cortical homogenates, the molar ratio of [32P]/[3H] was estimated at 0.43, suggesting that a substantial fraction of the GaR pool is phosphorylated under these conditions.

Immunochemical characterization of the beta 2 subunit of the GABAA receptor

To date three beta subunits of the GABAA receptor have been identified in rat brain as a result of cDNA library screening. The beta 2 subunit has been reported to have a wide distribution in rat brain based on in situ hybridization studies quantifying beta 2 mRNA. To study the beta 2 subunit more directly, we have raised a polyclonal antibody to a synthetic peptide representing residues 315-334 of the intracellular loop of the beta 2 subunit. The antibody, which had been affinity-purified, recognized the beta 2 peptide but did not immunolabel homologous beta 1 and beta 3 subunit peptides, indicating that this antibody is specific for the beta 2 subunit of the receptor. In western blots of the purified receptor, the antibody recognized a major diffuse band of 54-58 kDa and exhibited minor labeling of lower-molecular-mass polypeptides. In western blots of cortex homogenate, the antibody exhibited nervous system-specific labeling of a 55-kDa band that comigrated with the 55-kDa band of the purified receptor. Quantitative immunolabeling of this 55-kDa polypeptide permitted direct determination of the relative amounts of the beta 2 subunit in different brain regions. The brainstem contained the highest relative specific activity of the beta 2 subunit, followed by the inferior colliculus, olfactory lobe, and cerebellum. Lower levels of immunolabeling were seen in hypothalamus, hippocampus, thalamus, and cortex.

Radioligands for PET studies of central benzodiazepine receptors and PK (peripheral benzodiazepine) binding sites--current status

The status of the radiochemical development and biological evaluation of radioligands for PET studies of central benzodiazepine (BZ) receptors and the so-called peripheral benzodiazepine binding sites, here discriminated and referred to as PK binding sites, is reviewed against current pharmacological knowledge, indicating those agents with present value and those with future potential. Practical recommendations are given for the preparation of two useful radioligands for PET studies, [N-methyl-11C]flumazenil for central BZ receptors, and [N-methyl-11C]PK 11195 for PK binding sites. Quality assurance and plasma metabolite analysis are also reviewed for these radioligands and practical recommendations are given on methodology for their performance.

Antibodies specific for alpha-subunit subtypes of GABAA receptors reveal brain regional heterogeneity

Antisera were produced in rabbits against synthetic peptides based on subtype-specific regions of the cDNA sequences of the alpha 1, alpha 2, alpha 3, and alpha 4 (also termed alpha 5) subunits of mammalian GABAA receptors. The antigen peptides were chosen from the putative cytoplasmic loop between the proposed third and fourth membrane spanning helices; they were not only subtype-specific sequences, but also their hydrophilicity and predicted secondary structures suggested high potential antigenicity. In all cases, antipeptide antisera recognized on western blots the corresponding alpha-subunit polypeptide of the GABAA receptors purified from bovine brain by benzodiazepine-affinity chromatography, and were able to immunoprecipitate binding activity from detergent-solubilized purified receptors. The four antisera each recognized a unique polypeptide, and only one, in the purified receptor, with alpha 1, alpha 2, alpha 3, and alpha 4 identified at 51, 52, 56, and 57 kDa, respectively. This represents the first identification of the alpha 4 gene product on a gel. Both the relative amount of staining in immunoblots and the fraction of receptor binding that could be immunoprecipitated by saturating concentrations of each of the four subtype-specific antibodies varied in a consistent manner between receptors purified from different brain regions. Thus, cerebral cortex receptor contained all four alpha polypeptides on western blots, and significant activity could be precipitated by all four. Hippocampal receptor lacked alpha 3 immunoreactivity on blotting and by immunoprecipitation; alpha 1 was less, whereas both alpha 2 and alpha 4 were more abundant in hippocampus than in cortex by both techniques. Cerebellum receptor contained only alpha 1 of the four alpha subunits tested, and the anti-alpha 1 antibodies immunoprecipitated > 90% of the binding activity. The variable amounts of staining and immunoprecipitation from the three brain areas by the four antisera demonstrate the presence of heterooligomeric receptor complexes with different alpha-subunit constituents in cortex, hippocampus, and cerebellum. The sum of cortical receptor activity precipitated individually by the four anti-alpha antisera was > 150%, indicating that some heterooligomers are likely to contain more than one class of alpha subtype, although most receptor complexes probably contain only one alpha subtype. These alpha-subunit subtype-specific antibodies should be useful in analyzing structure, function, and localization of GABAA/benzodiazepine receptors in mammalian brain.

Comparative molecular neuroanatomy of cloned GABAA receptor subunits in the rat CNS

gamma-Aminobutyric acidA (GABAA) receptors in the mammalian central nervous system (CNS) are members of a family of ligand-gated ion channels consisting of heterooligomeric glycoprotein complexes in synaptic and extrasynaptic membranes. Although molecular cloning studies have identified 5 subunits (with approximately 40% amino acid homology) and isoforms thereof (approximately 70% homology), namely alpha 1-6, beta 1-4, gamma 1-3, delta, and rho, the subunit composition and stoichiometry of native receptors are not known. The regional distribution and cellular expression of GABAA receptor messenger RNAs (mRNAs) in the rat CNS have now been investigated by in situ hybridization histochemistry with subunit-specific 35S-labelled oligonucleotide probes on adjacent cryostat sections. Whereas alpha 1, beta 2, and gamma 2 transcripts were the most abundant and ubiquitous in the rat brain--correlating with the radioautographic distribution of GABAA receptors revealed by an ionophore ligand--others had a more restricted expression while often being abundant. For example, alpha 2 transcripts were found only in the olfactory bulb, cerebral cortex, caudate putamen, hippocampal formation, and certain lower brain stem nuclei; alpha 3 only in the olfactory bulb and cerebral cortex; alpha 5 in the hippocampal formation; and alpha 6 only in cerebellar granule cells. In addition, beta 1, beta 3, gamma 1, and delta mRNAs were also uniquely expressed in restricted brain regions. Moreover, in the spinal cord, alpha 1-3, beta 2,3, and gamma 2 mRNAs were differently expressed in Rexed layers 2-9, with alpha 2, beta 3, and gamma 2 transcripts most prominent in motoneurons of layer 9. Although differential protein trafficking could lead to the incorporation of some subunits into somatic membranes and others into dendritic membranes, some tentative conclusions as to the probable composition of native proteins in various regions of the CNS may be drawn.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Multiple benzodiazepine receptors: no reason for anxiety

Since the introduction of the benzodiazepines into clinical practice in 1960, these drugs have been widely employed as anxiolytics, sedative/hypnotics and anticonvulsants. In recent years, concern has been expressed about their side-effects, and their use has declined. During this latter period many advances have been made in understanding the molecular mechanisms by which these drugs produce their effects. Adam Doble and Ian Martin review this progress and highlight the possibilities that these advances may hold for the development of more efficacious and specific medicines.

Quantitative immunoprecipitation studies with anti-gamma-aminobutyric acidA receptor gamma 2 1-15 Cys antibodies

Antibodies raised against the synthetic peptide NH2-QKSDDDYEDYASNKTC-COOH (gamma 2 1-15 Cys), which corresponds to the N-terminal amino acid sequence with a C-terminal cysteine of the human gamma 2 subunit of the gamma-aminobutyric acidA (GABAA) receptor, were used to study the quantitative immunoprecipitation of agonist benzodiazepine binding sites from bovine brain. Anti-gamma 2 1-15 Cys antibodies were found to immunoprecipitate specifically in parallel [3H]flunitrazepam- and [3H]muscimol-reversible binding sites in a dose-dependent manner. The maximum percentages of [3H]flunitrazepam binding sites immunoprecipitated from detergent extracts of bovine cerebral cortex, cerebellum, and hippocampus were 68, 77, and 83%, respectively. Immunoprecipitation studies with anti-alpha 1 324-341 antibodies carried out in parallel with anti-gamma 2 1-15 Cys antibodies provided evidence for the promiscuity of the gamma 2 subunit within native GABAA receptors. These results substantiate the association of the gamma 2 polypeptide with native GABAA receptors.

Promiscuity of GABAA-receptor beta 3 subunits as demonstrated by their presence in alpha 1, alpha 2 and alpha 3 subunit-containing receptor subpopulations

Polyclonal antibodies were raised in rabbits against the GABAA-receptor beta 3 subunit peptide sequence, KQSMPREGHGRHMDR-NH2 coupled to keyhole limpet haemocyanin. These anti-beta 3 379-393 antibodies immunoprecipitated in a dose-dependent manner specific benzodiazepine agonist binding sites from Na+ deoxycholate extracts of bovine cerebral cortex. In immunoblots, anti-beta 3 379-393 antibodies recognised two species with Mr 59,900 and Mr 57,200 in all preparations tested, which included crude detergent-solubilised, benzodiazepine affinity chromatography-purified receptor, anti-alpha 1 324-341 antibody, anti-Cys alpha 2 414-424 antibody and anti-Cys alpha 3 454-467 antibody immunoaffinity-purified GABAA-receptor subpopulations. These results provide evidence for the ubiquity and promiscuity of the GABAA-receptor beta 3 subunit.

Separation of alpha 1, alpha 2 and alpha 3 subunits of the GABAA-benzodiazepine receptor complex by immunoaffinity chromatography

Polyclonal antibodies raised to synthetic amino acid sequences of the rat gamma-aminobutyric acid (GABAA) receptor alpha 1-, alpha 2- and alpha 3-subunits selectively recognized single proteins with apparent molecular weight 51 kDa (P51), 53 kDa (P53) and 59 kDa (P59), respectively, in GABAA receptor preparations affinity purified from the brains of 5-10-day-old rats. The antibodies were coupled to Affigel 10, and the resulting immunoaffinity columns were used to isolate these proteins from affinity purified GABAA receptors.

GABAA receptor subtypes immunopurified from rat brain with alpha subunit-specific antibodies have unique pharmacological properties

The unique cytoplasmic loop regions of the alpha 1, alpha 2, alpha 3, and alpha 5 subunits of the GABAA receptor were expressed in bacterial and used to produce subunit-specific polyclonal antisera. Antibodies immobilized on protein A-Sepharose were used to isolate naturally occurring alpha-specific populations of GABAA receptors from rat brain that retained the ability to bind [3H]muscimol, [3H]flunitrazepam, [3H]Ro15-1788, and [125I]iodo-clonazepam with high affinity. Pharmacological characterization of these subtypes revealed marked differences between the isolated receptor populations and was generally in agreement with the reported pharmacological profiles of GABAA receptors in cells transiently transfected with alpha 1 beta 1 gamma 2, alpha 2 beta 1 gamma 2, alpha 3 beta 1 gamma 2, and alpha 5 beta 1 gamma 2 combinations of subunits. Additional subtypes were also identified that bind [3H]muscimol but do not bind benzodiazepines with high affinity. The majority of GABAA receptor oligomers contains only a single type of alpha subunit, and we conclude that alpha 1, alpha 2, alpha 3, and alpha 5 subunits exist in vivo in combination with the beta subunit and gamma 2 subunit.

Identification of alpha 1-, alpha 2- and alpha 3-subunit isoforms of the GABAA-benzodiazepine receptor in the rat brain

GABAA receptors purified from the brains of 5- to 10-day-old rats were completely N- and O-deglycosylated using N-glycanase and/or neuraminidase plus O-glycanase. Intact or completely deglycosylated receptors were subjected to SDS-polyacrylamide gel electrophoresis and Western blot analysis. Polyclonal antibodies directed against synthetic aminoacid sequences specific for the GABAA receptor alpha 1-, alpha 2- or alpha 3-subunits each identified an apparently single protein of about 51 kDa, 53 kDa or 59 kDa, respectively, in the intact receptors. In the deglycosylated receptors, however, three different proteins were identified by antibodies directed against the alpha 3-subunit and at least two different proteins were identified by antibodies directed against the alpha 2- or alpha 1-subunit.

N-deglycosylation and immunological identification indicates the existence of beta-subunit isoforms of the rat GABAA receptor

beta 2- and beta 3-subunits of GABAA receptors purified from the brains of 5-10-day-old rats were investigated in the intact or completely N-deglycosylated state using the beta-subunit-specific monoclonal antibody bd-17 and polyclonal antibodies directed against synthetic amino acid sequences specific for the GABAA receptor beta 2- or beta 3-subunits. The present results seem to indicate the existence of two different isoforms of the beta 3-subunit and several different isoforms of the beta 2-subunit of the GABAA receptor which probably are produced by alternative splicing.

Differential expression of GABAA receptor alpha-subunits in rat brain during development

Unique cytoplasmic loop regions of the alpha 1, alpha 2, alpha 3, and alpha 5 subunits of the GABAA receptor have been expressed in E. coli and used to generate polyclonal antisera specific for these subunits. The antibodies identify proteins by SDS-polyacrylamide gel electrophoresis and western blotting of molecular size 51 kDa, 53 kDa, 59 kDa and 55 kDa, respectively, which show differential patterns of expression during development. Whereas the alpha 2 and alpha 3 subunits are present at early stages, the expression of alpha 1 and alpha 3 subunits is low at birth and increases with age. This differential expression could be correlated with previous studies examining the developmental expression of BZ1 and BZ2 benzodiazepine binding sites.

Isolation of type I and type II GABAA-benzodiazepine receptors by immunoaffinity chromatography

Anti-peptide alpha 1 (1-9) and anti-peptide alpha 3 (459-467) antibodies coupled to Affigel-10 were used for the isolation of GABAA receptors containing the alpha 1- or alpha 3-subunit, respectively. Both types of GABAA receptors exhibited a high affinity for [3H]flunitrazepam, and binding of [3H]flunitrazepam was stimulated in the presence of GABA. GABAA receptors eluted from the anti-peptide alpha 1 (1-9) immunoaffinity column exhibited a high affinity and those from the anti-peptide alpha 3 (459-467) columns a low affinity for the type I benzodiazepine receptor-selective ligand Cl 218872, indicating the enrichment of type I and type II GABAA-benzodiazepine receptors, respectively.

Peptide heterogeneity of GABAA/benzodiazepine receptors in bovine cerebral cortex and cerebellum

The GABAA/benzodiazepine receptor complex has been purified from both bovine cerebral cortex and cerebellum by immunoaffinity chromatography on immobilized monoclonal antibody 62-3G1. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the purified receptor from either cerebral cortex or cerebellum revealed 3 main bands corresponding to 51,000, 55,000 and 57,000 Mr silver-stained peptides. In addition, a minor band corresponding to a 53,000 Mr peptide was also found. The differences between the two receptor preparations were: (1) that the main silver-stained 55,000 Mr subunit was present in a relative smaller quantity in cerebellum than in cerebral cortex, and (2) when the membrane-bound receptor was photoaffinity-labeled with [3H]flunitrazepam and subsequently immunoaffinity-purified, two photolabeled peptide bands of 51,000 and 57,000 Mr were found in cerebral cortex while only the 51,000 Mr photolabeled peptide was detected in cerebellum following one-dimension sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Peptide maps of the 57,000 Mr [3H]flunitrazepam photoaffinity-labeled peptide indicated that it was composed of two closely migrating photolabeled peptides of 55,000 Mr and 57,000 Mr. Peptide mapping and deglycosylation experiments using the [3H]flunitrazepam photolabeled receptor suggested that the photolabeled peptides commonly present in cerebellum and cerebral cortex are qualitatively similar if not identical. The results suggest that there are subunits of some type(s) of GABAA R/BZDR complex(es) which are more abundant in cerebral cortex than in cerebellum. Photoaffinity labeling with [3H]muscimol showed similar photolabeled peptides in both cerebral cortex and cerebellum: two main peptides of 54,000 and 57,000 Mr were photolabeled with [3H]muscimol to a similar extent in both receptor preparations.(ABSTRACT TRUNCATED AT 250 WORDS)

Peptide subunits of gamma-aminobutyric acidA/benzodiazepine receptors from bovine cerebral cortex

The gamma-aminobutyric acidA/benzodiazepine receptor complexes from bovine cerebral cortex were purified by immunoaffinity chromatography, and the main component peptide subunits were characterized. The peptide band originally thought to be a single beta subunit [57,000 Mr band in sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE)] is composed of at least four different peptides of 54,000-57,000 Mr. Two peptides of 55,000 and 57,000 Mr were recognized by the beta subunit-specific monoclonal antibody 62-3G1. Peptides in the range of 54,000-57,000 Mr were photoaffinity-labeled with [3H]muscimol. A different 57,000 Mr peptide was photoaffinity-labeled by [3H]flunitrazepam, but neither was recognized by the monoclonal antibody 62-3G1 nor photoaffinity-labeled with [3H]muscimol. Some peptides could be identified by their differential mobility shift in SDS-PAGE after treatment with endoglycosidase H. Two additional subunit peptides of 51,000 and 53,000 Mr were also photoaffinity-labeled by [3H]flunitrazepam and reacted with antiserum A. However, the 57,000 Mr peptide that also was photoaffinity-labeled by [3H]flunitrazepam did not react with antiserum A.

Purification of the gamma-aminobutyric acidA/benzodiazepine receptor complex by immunoaffinity chromatography

The bovine gamma-aminobutyric acidA/benzodiazepine receptor complex has been purified by a novel immunoaffinity chromatography method on immobilized monoclonal antibody 62-3G1. Immunopurification of the complex was achieved in a single step with an improved yield over affinity chromatography on the benzodiazepine Ro 7-1986/1. High-resolution sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of the immunoaffinity-purified receptor revealed three major peptide bands of 51,000, 55,000, and 57,000 Mr which were also present in the Ro 7-1986/1 affinity-purified receptor. Peptide mapping, immunoblotting with subunit specific antibodies, and photoaffinity labeling with [3H]flunitrazepam and [3H]muscimol have been used for the identification of receptor subunits, including several which comigrated in a single band in SDS-PAGE.

Taurine acts on a subclass of GABAA receptors in mammalian brain in vitro

Taurine, an inhibitory amino acid, potently acts on a subclass of gamma-aminobutyric acid type A (GABAA) receptors. Taurine competitively inhibits [3H]muscimol binding to purified GABAA receptors with an average IC50 value of 50 microM, and enhances [3H]flunitrazepam binding to GABAA-linked benzodiazepine receptors with an EC50 of about 10 microM and with maximal extent lower than that for GABA. Taurine shows variable affinities (low micromolar to near millimolar) for muscimol binding sites in different brain regions as measured by autoradiography. The taurine-sensitive GABA sites are enriched in dentate gyrus, substantia nigra, cerebellum molecular layer, median thalamic nuclei, and hippocampal field CA3; these areas are also enriched in mRNA for the GABA-binding beta 2 subunit subtype. Taurine shows differential affinities for the multiple muscimol-GABA binding polypeptides present in purified GABAA receptors, notably a higher affinity for a beta 55 than a beta 58 polypeptide; these probably represent beta 2 and beta 3 clones, respectively. This work defines a significant target of taurine's inhibitory activity as some GABAA receptor GABA sites, lending support to the hypothesis that this endogenous substance may have a physiological action.

Identification and immunohistochemical mapping of GABAA receptor subtypes containing the delta-subunit in rat brain

Synaptic inhibition in brain is mainly mediated via GABAA receptors which display a striking structural heterogeneity. A novel type of GABAA receptor subunit, the delta-subunit, has recently been described based on molecular cloning of its cDNA. To identify the prevalence and distribution of GABAA receptors which contain the delta-subunit protein in situ, polyclonal site-directed antisera were developed against three synthetic peptides derived form the rat delta-subunit cDNA-sequence. All antisera specifically recognized a 54 kDa protein in GABAA receptor preparations. Nearly 30% of the GABAA receptors contained the delta-subunit immunoreactivity and displayed high affinity GABA and high affinity benzodiazepine binding sites as shown by immunoprecipitation. Receptors which contain the delta-subunit were immunohistochemically shown to be restricted to a few brain areas such as the cerebellum, thalamus and dentate gyrus of the hippocampal formation. Thus, those neurons which express GABAA receptors with a delta-subunit have now been visualized and made accessible for a functional analysis of this GABAA receptor subtype 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.

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.

Developmental expression of the GABAA receptor alpha 1 subunit mRNA in the rat brain

Recent studies have suggested that the GABAA, receptor complex, the site of action of the inhibitory neurotransmitter gamma amino-butyric acid (GABAA) and the anxiolytic benzodiazepines, is heterogeneous. Moreover, its composition may change during development. To better understand the molecular basis of receptor heterogeneity, the levels and distribution of the mRNA encoding the alpha 1 receptor subunit were examined in the developing and adult rat brain with quantitative in situ hybridization histochemistry. Our studies demonstrate that alpha 1 subunit mRNA expression changes during ontogeny. At late embryonic stages and in the first postnatal week, low levels of the mRNA were detected in the cortex, inferior colliculus, and hippocampus. The mRNA levels in these regions increased during the second and third postnatal weeks. Furthermore, a dramatic change in the distribution of the alpha 1 subunit mRNA was seen in the second postnatal week when the message first became detectable in the cerebellar cortex. During subsequent development and in the mature brain, the alpha 1 subunit mRNA was most abundant in the cerebellum, olfactory bulb, and inferior colliculus, although the absolute levels of mRNA varied by as much as sixfold in selected brain regions. The mature distribution of alpha 1 subunit mRNA, along with its temporal appearance in the cerebellum, suggests that this subunit is a constituent of the Type 1 benzodiazepine site of the GABAA receptor complex. Furthermore, the onset of alpha 1 subunit mRNA expression in the cerebellar cortex coincides with a period of extensive synapse formation, raising the possibility that synaptic interactions modulate the appearance of this GABAA receptor subunit in the cerebellum.

Cloned GABA receptors are maintained in a stable cell line: allosteric and channel properties

The cloned cDNAs encoding the alpha 1 and beta 1 subunits of the bovine brain GABA(A) receptor have been co-transfected, using a dexamethasone-inducible promoter, into cultured hamster ovary cells, with selection to form a stable cell line. The use, alternatively, of a much stronger constitutive promoter led to cell death consequent upon high receptor density. After induction, the cells contained the alpha 1 and beta 1 mRNAs. The expressed receptors showed the high-affinity binding of [3H]muscimol and of the GABA(A) receptor channel blocker, t-butylphosphorothionate (TBPS), and the characteristic enhancement of the former by a pregnanolone. Their GABA-activated current was potentiated by the barbiturate, pentobarbitone, was reversibly blocked by bicuculline and picrotoxin, but was not enhanced by benzodiazepines. In mouse spinal cord neurons GABA activates channel openings to at least four conductance states (45, 30, 19 and 12 pS) with the 30 pS state being the most frequently observed (main) state. However, the main state of the alpha 1/beta 1 GABA(A) receptor was the 19 pS state. The enhancement of GABA(A) receptor current by barbiturates wa due to prolongation of mean channel lifetime, whereas the reduction of GABA(A) receptor current by picrotoxin was due to reduction of channel opening frequency and mean channel lifetime. Stable cell lines containing subunit combinations of this receptor should provide a powerful tool for the elucidation of its channel features and control mechanisms.

Identification of alpha 2- and alpha 3-subunits of the GABAA-benzodiazepine receptor complex purified from the brains of young rats

Polyclonal antibodies were raised to synthetic amino acid sequences of the bovine GABAA receptor alpha 2- and alpha 3-subunits and purified by affinity chromatography on a column coupled with the respective peptide. Anti-peptide alpha 2(416-424) and anti-peptide alpha 3(459-467) antibodies immunoprecipitated GABAA receptors and recognized a protein of 53 kDa (P53) and 59 kDa (P59), respectively, in Western blots of GABAA receptors purified from the brains of 5-10 day old rats. P53 as well as P59 are specifically photolabeled by [3H]flunitrazepam and are recognized by the alpha-subunit specific monoclonal antibody bd 28.

Protein kinase C and cAMP-dependent protein kinase phosphorylate the beta subunit of the purified gamma-aminobutyric acid A receptor

A number of recent studies have suggested that phosphorylation of the gamma-aminobutyric acid A (GABAA) receptor could modulate receptor function. Activators of protein kinase C and cAMP-dependent protein kinase have been shown to influence GABAA receptor function. In addition, Sweetnam et al. [Sweetnam, P. M., Lloyd, J., Gallombardo, P., Malison, R. T., Gallager, D. W., Tallman, J. F. & Nestler, E. J. (1988) J. Neurochem. 51, 1274-1284] have reported that a kinase associated with a partially purified preparation of the receptor could phosphorylate the alpha subunit of the receptor. Moreover, Kirkness et al. [Kirkness, E. F., Bovenkerk, C. F., Ueda, T. & Turner, A. J. (1989) Biochem. J. 259, 613-616] have recently shown that cAMP-dependent protein kinase could phosphorylate a muscimol binding polypeptide of the GABAA receptor. To explore the issue further, we have examined the ability of specific kinases to catalyze significant phosphorylation of the GABAA receptor that has been purified to near homogeneity. The GABAA receptor was purified as previously described using benzodiazepine affinity chromatography. The purified receptor possessed no detectable kinase activity. Protein kinase C and cAMP-dependent protein kinase catalyzed the phosphorylation of the beta and alpha subunits of the receptor. However, most of the phosphate incorporation was associated with the beta subunit. Two muscimol binding polypeptides designated beta 58 (Mr 58,000) and beta 56 (Mr 56,000) were present in the preparation. The higher molecular weight polypeptide, beta 58, was phosphorylated specifically by cAMP-dependent protein kinase. beta 56 was phosphorylated specifically by protein kinase C. beta 58 and beta 56 gave distinct patterns in a one-dimensional phosphopeptide analysis. The stoichiometry of phosphorylation (mol of phosphate/mol of muscimol binding) catalyzed by cAMP-dependent protein kinase was 0.52 and that catalyzed by protein kinase C was 0.38. Taken together these data confirm that there are two forms of the beta subunit of the GABAA receptor and suggest that these two forms of the beta subunit are phosphorylated by distinct kinases.

Differential distribution of GABAA receptor mRNAs in bovine cerebellum--localization of alpha 2 mRNA in Bergmann glia layer

Using in situ hybridization histochemistry, we have demonstrated that 3 alpha subunit mRNAs of the GABAA receptor are present in different cell populations of the bovine cerebellum. While the alpha 1 mRNA is the most abundant and is present in granule cells, Purkinje cells and stellate/basket cells, the alpha 2 mRNA appears to be confined to the Bergmann glial cell layer. The alpha 3 mRNA is only expressed in the Golgi cells. This differential distribution of GABAA receptor mRNA subtypes suggests a previously unrecognized complexity of GABAergic transmission in the cerebellum.

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.

Multiplicity of GABAA--benzodiazepine receptors

Binding studies suggest the presence of at least two pharmacologically distinct 'central' benzodiazepine receptors in the brain. Since central benzodiazepine receptors are allosteric modulatory sites on GABAA receptors, this evidence indirectly points to the existence of at least two GABAA receptors. Werner Sieghart describes biochemical studies that have identified several different alpha- and beta-subunits of these receptors, and molecular biological studies in which the genes encoding a variety of different alpha-, beta- and gamma-subunits have been isolated, sequenced and expressed in Xenopus oocytes. These studies all point to the existence of multiple GABAA receptors in the brain.

Type I and type II GABAA-benzodiazepine receptors produced in transfected cells

GABAA (gamma-aminobutyric acid A)-benzodiazepine receptors expressed in mammalian cells and assembled from one of three different alpha subunit variants (alpha 1, alpha 2, or alpha 3) in combination with a beta 1 and a gamma 2 subunit display the pharmacological properties of either type I or type II receptor subtypes. These receptors contain high-affinity binding sites for benzodiazepines. However, CL 218 872, 2-oxoquazepam, and methyl beta-carboline-3-carboxylate (beta-CCM) show a temperature-modulated selectivity for alpha 1 subunit-containing receptors. There were no significant differences in the binding of clonazepam, diazepam, Ro 15-1788, or dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) to all three recombinant receptors. Receptors containing the alpha 3 subunit show greater GABA potentiation of benzodiazepine binding than receptors containing the alpha 1 or alpha 2 subunit, indicating that there are subtypes within the type II class. Thus, diversity in benzodiazepine pharmacology is generated by heterogeneity of the alpha subunit of the GABAA receptor.