GABAA Receptor Subtypes Expressed in Cerebellar Granule Cells: A Developmental Study

Abstract: The developmental properties of primary rat cerebellar granule cells have been characterised with respect to their expression of GABA_A receptor subtypes using both an immunological approach and radioligand binding assays. At day 1 in culture, the GABA_A receptor α1 subunit was detectable in immunoblots and increased in level up to day 9. The GABA_A receptor α6 subunit was not detectable at day 1; however, at days 3–5, a specific M_r 58,000 anti-α6 1–16 Cys immunoreactive species was present which further increased in level up to 9 days in culture. Similar qualitative results were obtained for the expression of the GABA_A receptor α6 subunit in age-matched rat cerebellar membranes. In parallel studies, it was found that although there was an overall increase in [³H]Ro 15–4513 binding sites with days in culture, the relative contributions of diazepam-sensitive and diazepam-in-sensitive [³H]Ro 15–4513 binding changed. A time-dependent enrichment of the diazepam-insensitive binding site up to a maximum of 74% of total [³H]Ro 15–4513 sites was found. This was concomitant with the appearance of the GABA_A receptor α6 subunit. These results are in agreement with the pharmacology described for α6βγ2 cloned receptors. They suggest a developmentally regulated expression of the GABA_A receptor α6 subunit gene at a time that is correlated in vivo with establishment of neuronal connections.     

Antibodies to the Human γ2 Subunit of the γ-Aminobutyric AcidA/Benzodiazepine Receptor

Abstract: A γ-aminobutyric acid_A (GABA_A) receptor (GABA_AR) γ₂ subunit (short form) was cloned from an adult human cerebral cortex cDNA library in bacteriophage λgt11. The 261-bp intracellular loop (IL) located between M3 and M4 was amplified using the polymerase chain reaction and inserted into the expression vectors λgt11 and pGEX-3X. Both γ-galactosidase (LacZ) and glutathione-S-transferase (GST) fusion proteins containing the γ₂IL were purified, and a rabbit antibody to the LacZ–γ₂IL was made. The antibody reacted with the γ₂IL of both LacZ and GST fusion proteins and immunoprecipitated the GABA_AR/ benzodiazepine receptor (GABA_AR/BZDR) from bovine and rat brain. The antibody reacted in affinity-purified GABA_AR/BZDR immunoblots with a wide peptide band of 44,000–49,000 M_r. Immunoprecipitation studies with the anti-γ₂IL antibody suggest that in the cerebral cortex, 87% of the GABA_ARs with high affinity for benzodiazepines and 70% of the GABA_ARs with high affinity for muscimol contain at least a γ subunit, probably a γ₂. These results indicate that there are [³H]muscimol binding GABA_ARs that do not bind [³H]flunitrazepam with high affinity. Immunoprecipitations with this and other anti-GABA_AR/BZDR antibodies indicate that the most abundant combination of GABA_AR subunits in the cerebral cortex involves α₁, γ₂ (or other γ), and β₂ and/or β₃ subunits. These subunits coexist in >60% of the GABA_AR/BZDRs in the cerebral cortex. The results also show that a considerable proportion (20–25%) of the cerebellar GABA_AR/BZDRs is clonazepam insensitive. At least 74% of these cerebellar receptors, which likely contain α₆, also contain γ₂ (or other γ) subunit(s). The α₁ and β₂ or β₃ subunits are also frequently associated with γ₂ (or other γ) and α₆ in these cerebellar receptors.     

Phenotypic and Genotypic Analysis of Rats with Cerebellar GABAA Receptors Composed from Mutant and Wild-Type α6 Subunits

Abstract: The alcohol-sensitive (ANT) rat line, developed for high behavioral sensitivity to ethanol, also exhibits enhanced sensitivity to benzodiazepines, such as diazepam. The rat line carries a point mutation in the cerebellum-specific γ-aminobutyric acid type A (GABA_A) receptor subunit α6, making their diazepam-insensitive (DIS) receptors sensitive to diazepam. We now report that phenotypes of individual ANT and alcohol-insensitive rats, classified on diazepam sensitivity of cerebellar [³H]Ro 15-4513 binding, correlated well with homozygous wild-type, homozygous mutant, and heterozygous genotypes, although some heterozygotes were biased toward the parental phenotypes. GABA down-modulated DIS [³H]Ro 15-4513 binding in mutant homozygotes but tended to up-modulate it in heterozygotes and wild-type homozygotes. Slopes for GABA inhibition of cerebellar t-butylbicyclophosphoro[³⁵S]thionate binding were larger in mutant than in wild-type homozygotes, with heterozygotes being intermediate. Diazepam displacement of [³H]Ro 15-4513 binding in heterozygotes revealed three components, with their affinities indistinguishable from those in combined wild-type and mutant homozygotes. This lack of interaction in DIS binding between wild-type and mutant α6 subunits was substantiated by experiments on recombinant receptors. The data suggest that the α6 subunit-containing GABA_A receptors in the heterozygotes are formed from individual mutant and wild-type subunits with their relative expression differing from animal to animal.     

Structure of α6β3δ GABAA receptors and their lack of ethanol sensitivity

Delta (δ) subunit containing GABA_A receptors are expressed extra‐synaptically and mediate tonic inhibition. In cerebellar granule cells, they often form a receptor together with α₆ subunits. We were interested to determine the architecture of these receptors. We predefined the subunit arrangement of 24 different GABA_A receptor pentamers by subunit concatenation. These receptors (composed of α₆, β₃ and δ subunits) were expressed in Xenopus oocytes and their electrophysiological properties analyzed. Currents elicited in response to GABA were determined in presence and absence of 3α, 21‐dihydroxy‐5α‐pregnan‐20‐one and to 4,5,6,7‐tetrahydroisoxazolo[5,4‐c]‐pyridin‐3‐ol. α₆‐β₃‐α₆/δ receptors showed a substantial response to GABA alone. Three receptors, β₃‐α₆‐δ/α₆‐β₃, α₆‐β₃‐α₆/β₃‐δ and β₃‐δ‐β₃/α₆‐β₃, were only uncovered in the combined presence of the neurosteroid 3α, 21‐dihydroxy‐5α‐pregnan‐20‐one with GABA. All four receptors were activated by 4,5,6,7‐tetrahydroisoxazolo[5,4‐c]‐pyridin‐3‐ol. None of the functional receptors was modulated by physiological concentrations (up to 30 mM) of ethanol. GABA concentration response curves indicated that the δ subunit can contribute to the formation of an agonist site. We conclude from the investigated receptors that the δ subunit can assume multiple positions in a receptor pentamer composed of α₆, β₃ and δ subunits.     

The Subunit Composition of a GABAA/Benzodiazepine Receptor from Rat Cerebellum

Abstract: The pentameric subunit composition of a large population (36%) of the cerebellar granule cell GABA_A receptors that show diazepam (or clonazepam)-insensitive [³H]Ro 15-4513 binding has been determined by immunoprecipitation with subunit-specific antibodies. These receptors have α₆, α₁, γ_2S, γ_2L, and β₂ or β₃ subunits colocalizing in the same receptor complex.     

Bidirectional Regulation of GABAA Receptor α1 and α6 Subunit Expression by a Cyclic AMP-Mediated Signalling Mechanism in Cerebellar Granule Cells in Primary Culture

Abstract: Forskolin treatment of cerebellar granule cells in culture resulted in bidirectional regulation of the expression of GABA_A receptor α1 and α6 subunits. Thus, forskolin applied at 2 days in vitro (DIV) increased expression of the α1 subunit but decreased the expression of the α6 subunit. Values with respect to control cultures, both assayed at 9 DIV by immunoblotting, were 310 ± 48% for α1 and 25 ± 16% for the α6 subunit. Similar effects were evoked following chronic treatment with both dibutyryl cyclic AMP and 3-isobutyl-1-methylxanthine. Dideoxyforskolin had no effect on the level of expression of either the α1 or the α6 GABA_A receptor subunits. The changes in subunit expression were accompanied by a 1.7-fold increase in number of total specific [³H]Ro 15-4513 binding sites expressed by intact cerebellar granule cells. This increase in total binding sites was accommodated by a 2.7-fold increase in number of diazepam-sensitive Ro 15-4513 binding sites in accordance with the observed increase in α1 subunit expression. The number of diazepam-insensitive subtype of binding sites were not significantly changed. These results suggest that GABA_A receptor subtype expression can be differentially regulated by intracellular cyclic AMP concentration.     

Short and Long Form γ2 Subunits of the GABAA/Benzodiazepine Receptors

Abstract: Three novel antisera to the γ₂ subunit of the γ-aminobutyric acid_A (GABA_A) receptor/benzodiazepine receptor (GABA_AR/BZDR) complex have been made. Anti-γ_2S and anti-γ_2L are specific antibodies to synthetic peptides that recognize the γ_2S (short) and γ_2L (long) forms, respectively, of the γ₂ subunit. An antibody (anti-γ₂IL2) to staphylococcal protein A fusion protein of the large intracellular loop (γ₂IL) located between the putative transmembrane segments M3 and M4 of γ_2S recognizes both γ_2S and γ_2L subunits. The antibodies immunoprecipitated both the solubilized and affinity-purified GABA_AR/BZDR from rat and bovine brain. Immunoblots with membranes from rat brain cerebral cortex as well as with affinity-purified receptor from bovine cortex show that anti-γ_2S and anti-γ_2L recognize peptides of 45,000 and 47,000 M_r, respectively. Immunoprecipitation experiments indicate that γ_2S is more prevalent in hippocampus, whereas γ_2L is more abundant in cerebellum. Intermediate values for each form are found in the cerebral cortex. The results suggest that in the rat brain there is a considerable amount of colocalization of γ_2S and γ_2L in the same receptor complex. In the cerebral cortex, 15% of the BZDRs contain both γ_2S and γ_2L subunits and 41–48% of the γ_2L subunit coexists with γ_2S in the same receptor complex. In cerebellum, in 27% of the clonazepam-sensitive and 39% of the clonazepam-insensitive BZDRs the γ_2S and γ_2L coexist in the same receptor complex. The latter are presumably localized in granule cells and also contain α₆. In addition, almost all (93%) the clonazepam-insensitive BZDRs that contain γ_2L also contain a γ_2S subunit in the same receptor complex. The most likely interpretation of the results is that there is an important population of granule cell receptors that contain α₆, γ_2S, and γ_2L coexisting in the same receptor complex. Nevertheless, 31% of the cerebellar receptors that contain α₆ subunit(s) have neither γ_2S nor γ_2L subunits. There are also species differences with respect to the relative abundance of γ_2S and γ_2L. These results might be relevant for understanding the molecular mechanisms underlying some of the GABA_AR/BZDR-mediated effects of ethanol intoxication involving cerebellar granule cells.     

Effect of Noradrenergic Lesions on Subtypes of α2-Adrenoceptors in Rat Brain

Abstract: The binding of [³H]rauwolscine to α_2A- (also referred to as α_2D-) and α_2C-adrenoceptors in homogenates of rat cerebral cortex was measured by exploiting the selectivity of oxymetazoline for α_2A-adrenoceptors. Inhibition of [³H]rauwolscine binding by oxymetazoline was modeled best assuming binding to two sites (p < 0.001). Competition curves for oxymetazoline were shifted rightward by the addition of GTP (250 µM) but were still fit best by a two-site model (p < 0.001). A concentration of oxymetazoline was calculated that would optimally antagonize [³H]rauwolscine binding (with GTP present) to oxymetazoline-sensitive α_2A-adrenoceptors, minimally inhibiting binding to α_2C-adrenoceptors. Subsequently, [³H]rauwolscine binding to α_2A- and α_2C-adrenoceptors in cortex was examined 3 weeks after destruction of noradrenergic terminals. Binding to α_2C-adrenoceptors was increased significantly after treatment with 6-hydroxydopamine (6-OHDA) compared with vehicle-treated controls, whereas binding to α_2A-adrenoceptors was unchanged. Pretreatment of rats with desipramine before 6-hydroxydopamine, to protect noradrenergic neurons, resulted in no changes in binding to either α_2A- or α_2C-adrenoceptors. Thus, α_2C-adrenoceptors are regulated by changes in synaptic availability of norepinephrine. α_2A-Adrenoceptors are either not regulated by synaptic norepinephrine or are located both post- and presynaptically so that up-regulation of postsynaptic α_2A-adrenoceptors is offset by a loss of presynaptic α_2A-adrenoceptors.     

Antibodies Specific for GABAA Receptor α Subunits Reveal that Chronic Alcohol Treatment Down-Regulates α-Subunit Expression in Rat Brain Regions

Abstract— Chronic administration of ethanol results in the development of tolerance and dependence. The molecular mechanism underlying these behavioral actions of ethanol is poorly understood. Several lines of evidence have suggested that some of the pharmacological actions of ethanol are mediated via a potentiation of GABAergic transmission. Chronic ethanol administration results in a reduction in the GABA_A receptor-mediated ³⁶Cl^? uptake in cortical synaptoneurosomes and primary cultured neurons. We and others have shown that it also results in a 40-50% reduction in GABA_A receptor α-subunit mRNA levels in the rat cerebral cortex. In the present study, we investigated the expression of α₁, α₂, and α₃ subunits of the GABA_A receptor in the cerebral cortex and the α₁ subunit in the cerebellum by immunoblotting using polyclonal antibodies raised against α₁-, α₂-, and α₃-subunit polypeptides following chronic ethanol treatment. These results reveal that chronic ethanol administration to rats results in a 61 ± 4% reduction in level of the GABA_A receptor α₁subunit (51 kDa), 47 ± 8% reduction in level of the α₂subunit (53 kDa), and 30 ± 7% reduction in level of the α₃subunit (59 kDa) in the cerebral cortex and a 56 ± 5% reduction in content of the α₁ subunit in the cerebellum. In summary, this ethanol-induced reduction in content of the GABA_A receptor α subunits may underlie alterations in the GABA_A receptor function and could be related to cellular adaptation to the functional disturbance caused by ethanol.     

Small N-Terminal Deletion by Splicing in Cerebellar α6 Subunit Abolishes GABAA Receptor Function

Abstract: Sequence variation was found in cDNA coding for the extracellular domain of the rat γ-aminobutyric acid type A (GABA_A) receptor α6 subunit. About 20% of polymerase chain reaction (PCR)-amplified α6 cDNA prepared from rat cerebellar mRNA lacked nucleotides 226–255 as estimated by counting single-stranded phage plaques hybridized specifically to the short (α6S) and long (wild-type) forms of the α6 mRNA. Genomic PCR revealed an intron located upstream of the 30-nucleotide sequence. Both splice forms were detected in the cerebellum by in situ hybridization. Recombinant receptors, resulting from coexpression of the α6S subunit with the GABA_A receptor β2 and γ2 subunits in human embryonic kidney 293 cells, were inactive at binding [³H]muscimol and [³H]Ro 15-4513. In agreement, injection of complementary RNAs encoding the same subunits into Xenopus oocytes produced only weak GABA-induced currents, indistinguishable from those produced by β2γ2 receptors. Therefore, the 10 amino acids encoded by the 30-nucleotide fragment may be essential for the correct assembly or folding of the α6 subunit-containing receptors.     

Human α1β3γ2L gamma‐aminobutyric acid type A receptors: High‐level production and purification in a functional state

Gamma‐aminobutyric acid type A receptors (GABA_ARs) are the most important inhibitory chloride ion channels in the central nervous system and are major targets for a wide variety of drugs. The subunit compositions of GABA_ARs determine their function and pharmacological profile. GABA_ARs are heteropentamers of subunits, and (α1)₂(β3)₂(γ2L)₁ is a common subtype. Biochemical and biophysical studies of GABA_ARs require larger quantities of receptors of defined subunit composition than are currently available. We previously reported high‐level production of active human α1β3 GABA_AR using tetracycline‐inducible stable HEK293 cells. Here we extend the strategy to receptors containing three different subunits. We constructed a stable tetracycline‐inducible HEK293‐TetR cell line expressing human (N)–FLAG–α1β3γ2L–(C)–(GGS)₃GK–1D4 GABA_AR. These cells achieved expression levels of 70–90 pmol [³H]muscimol binding sites/15‐cm plate at a specific activity of 15–30 pmol/mg of membrane protein. Incorporation of the γ2 subunit was confirmed by the ratio of [³H]flunitrazepam to [³H]muscimol binding sites and sensitivity of GABA‐induced currents to benzodiazepines and zinc. The α1β3γ2L GABA_ARs were solubilized in dodecyl‐d ‐maltoside, purified by anti‐FLAG affinity chromatography and reconstituted in CHAPS/asolectin at an overall yield of ～30%. Typical purifications yielded 1.0–1.5 nmoles of [³H]muscimol binding sites/60 plates. Receptors with similar properties could be purified by 1D4 affinity chromatography with lower overall yield. The composition of the purified, reconstituted receptors was confirmed by ligand binding, Western blot, and proteomics. Allosteric interactions between etomidate and [³H]muscimol binding were maintained in the purified state.     

β-Lumicolchicine Interacts with the Benzodiazepine Binding Site to Potentiate GABAA Receptor-Mediated Currents

Abstract: An analogue of colchicine,β-lumicolchicine, does not bind tubulin or disrupt microtubules. However, this compound is not pharmacologically completely inactive. β-Lumicolchicine was found to competitively inhibit [³H]flunitrazepam binding and to enhance muscimol-stimulated ³⁶Cr-uptake in mouse cerebral cortical microsacs. It also markedly potentiated GABA responses in Xenopusoocytes expressing human α₁β₂γ_2S, but not α₁β₂, GABA_A receptor subunits; this potentiation was reversed by the benzodiazepine receptor antagonist flumazenil. These results strongly suggest a direct effect of β-Lumicolchicine on the GABA_A receptor/chloride channel complex and caution that it possesses pharmacological effects, despite its inability to disrupt microtubules. Furthermore, β-Lumicolchicine is structurally unrelated to benzodiazepines or quinolines and may provide a novel approach to the synthesis of ligands for this receptor.     

Differential Effects of the Alkylating Agent N-Ethoxycarbonyl-2-Ethoxy-1,2-Dihydroquinoline on Brain α2-Adrenoceptors and I2-Imidazoline Sites In Vitro and In Vivo

Abstract— The alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) is a peptide-coupling agent that is being used to inactivate irreversibly α₂-adrenoceptors and other receptors. The aim of the present study was to assess the in vitro and in vivo effects of EEDQ on the newly discovered brain l₂-imidazoline sites, located mainly in mitochondria. Preincubation of rat cortical membranes with EEDQ (10^?8-10^?5M) markedly decreased (20–90%) the specific binding of the selective antagonist [³H]R821002 to α₂-adrenoceptors without affecting that of [³H]idazoxan (in the presence of adrenaline) to l₂-imidazoline sites. In EEDQ-pretreated membranes (10^?5M, 30 min at 25°c), the density of l₂-imidazoline sites (B_max= 80 ± 4 fmol/mg of protein) was not different from that determined in untreated membranes in the presence of 10^?6M (-)-adrenaline (B_max= 83 ± 4 fmol/mg of protein), and both densities were lower (24%, p < 0.05) than the total native density of [³H]idazoxan binding sites (B_max= 107 ± 6 fmol/mg of protein) (l₂-imidazoline sites plus a₂-adrenoceptors). Treatment of rats with an optimal dose of EEDQ (1.6 mg/kg, i.p., for 2 h to 30 days) reduced maximally at 6 h (by 95 ± 1%) the specific binding of [³H]-R821002 to α₂-adrenoceptors, but also the binding of [³H]idazoxan to l₂-imidazoline sites (by 44 ± 5%). Pretreatment with yohimbine (10 mg/kg, i.p.) fully protected against EEDQ-induced α₂-adrenoceptor inactivation. In contrast, pretreatment with cirazoline (1 mg/kg, i.p.), did not protect against EEDQ-induced inactivation of l₂-imidazoline sites. Treatment with EEDQ (1.6 mg/kg, i.p., for 6 h) did not alter the density of brain monoamine oxidase-A sites labeled by [³H]Ro 41–1049 or that of monoamine oxidase-B sites labeled by [³H]Ro 19–6327 (lazabemide), two relevant mitochondrial markers. Competition experiments with cirazoline against the specific binding of [³H]idazoxan to l₂-imidazoline sites demonstrated the presence of the expected two affinity states for the drug in EEDQ-pretreated membranes as well as in rats treated with EEDQ. The results indicate that EEDQ in vitro is a useful tool for quantitating l₂-imidazoline sites when using [³H]-imidazoline ligands that also recognize α₂-adrenoceptors. In vivo, however, EEDQ is also able to inactivate partially brain l₂-imidazoline sites probably by an indirect mechanism. Key Words: Brain l₂-imidazoline sites—[³H]-Idazoxan—α₂-Adrenoceptors—[³H] R821002—N -Ethoxycarbonyl-2-ethoxy-li2-dihydroquinoline—Monoamine oxidase-A—[³H]Ro 41–1049—Monoamine oxidase-B—[³H]Ro 19–6327.     

Selective Increase of α2A-Adrenoceptor Agonist Binding Sites in Brains of Depressed Suicide Victims

Abstract: The α_2A- and α_2C-adrenoceptor subtypes were evaluated in postmortem brains from suicides with depression (n = 22), suicides with other diagnoses (n = 12), and controls (n = 26). Membrane assays with the antagonist [³H]RX821002 (2-[³H]methoxyidazoxan) suggested the presence of α_2A-adrenoceptors in the frontal cortex and both α_2C-adrenoceptors and α_2A-adrenoceptors in the caudate. The proportions in caudate were similar in controls (α_2A, 86%; α_2C, 14%), depressed suicides (α_2A, 91%; α_2C, 9%), and suicides with other diagnoses (α_2A, 88%; α_2C, 12%). Autoradiography of [³H]RX821002 binding under α_2B/C-adrenoceptor-masking conditions confirmed the similar densities of α_2A-adrenoceptors in the cortex, hippocampus, and striatum from controls and suicides. In the frontal cortex of depressed suicides, competition of [³H]RX821002 binding by (?)-adrenaline revealed a greater proportion (61 ± 9%) of α_2A-adrenoceptors in the high-affinity conformation for agonists than in controls (39 ± 5%). Simultaneous analysis with the agonists [³H]clonidine and [³H]UK14304 and the antagonist [³H]RX821002 in the same depressed suicides confirmed the enhanced α_2A-adrenoceptor density when evaluated by agonist, but not by antagonist, radioligands. The results indicate that depression is associated with a selective increase in the high-affinity conformation of the brain α_2A-adrenoceptors.     

GABAA Receptors Containing the α4-Subunit: Prevalence, Distribution, Pharmacology, and Subunit Architecture In Situ

Abstract: Recombinant GABA_A receptors, expressed from α-, β-, and γ2-subunits, are diazepam-insensitive when the α-subunit is either α4 or α6. In situ, diazepam-insensitive receptors containing the α6-subunit are almost exclusively expressed in the granule cell layer of the cerebellum. However, diazepam-insensitive receptors are also expressed in forebrain areas. Here, we report on the presence of diazepam-insensitive GABA_A receptors in various brain areas containing the α4-subunit. GABA_A receptors immunoprecipitated with a newly developed α4-subunit-specific antiserum displayed a drug binding profile that was indistinguishable from those of α4β2γ2-recombinant receptors and diazepam-insensitive [³H]Ro 15-4513 binding sites in rat brain membranes. In addition, α4-subunit containing receptors and forebrain diazepam-insensitive receptors are present at comparably low abundance in rat brain and exhibit virtually identical patterns of distribution. Analysis of the subunit architecture of α4-subunit containing receptors revealed that the α4-subunit contributes to several receptor subtypes. Depending on the brain region, the α4-subunit can be coassembled with a second type of α4-subunit variant being α1, α2, or α3. The data demonstrate that native receptors containing the α4-subunit are structurally heterogeneous, expressed at very low abundance in the brain, and display the drug binding profile of diazepam-insensitive [³H]Ro 15-4513 binding sites. Pharmacologically, these receptors may contribute to the actions of nonclassical ligands such as Ro 15-4513 and bretazenil.     

GABAA Receptor Subtypes: Ligand Binding Heterogeneity Demonstrated by Photoaffinity Labeling and Autoradiography

Abstract: Heterogeneity of binding affinities for a variety of ligands was observed for γ-aminobutyric acid type A (GABA_A) receptors in the rat CNS, at both GABA and ben-zodiazepine recognition sites. Photoaffinity labeling by [³H]flunitrazepam and [³H]muscimol to affinity column-purified receptor proteins was examined by gel electropho-resis in sodium dodecyl sulfate. Anesthetic barbiturates (pentobarbital) and steroids (alphaxalone) both differentially stimulated the incorporation of [³H]flunitrazepam more so into the 51-kDa α1 subunit than into the 53-kDa aL2 polypeptide, and incorporation of [³H]muscimol into the 55-kDa β2 subunit more so than the 58-kDaβ3 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 α1 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 THlP 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 THlP 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 isofotms is dependent on the nature of α and subunits other than α, GABA-benzodiazepine coupling is dependent on the nature of the α subunits, GABA site pharmacology is dependent on the nature of the β sub-units, and several subunits including α and β 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.     

Brain regional distribution of GABAA receptors exhibiting atypical GABA agonism: Roles of receptor subunits

The major inhibitory neurotransmitter in the brain, γ-aminobutyric acid (GABA), has only partial efficacy at certain subtypes of GABA_A receptors. To characterize these minor receptor populations in rat and mouse brains, we used autoradiographic imaging of t-butylbicyclophosphoro[³⁵S]thionate ([³⁵S]TBPS) binding to GABA_A receptors in brain sections and compared the displacing capacities of 10 mM GABA and 1 mM 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP), a competitive GABA-site agonist. Brains from GABA_A receptor α1, α4, δ, and α4 + δ subunit knockout (KO) mouse lines were used to understand the contribution of these particular receptor subunits to “GABA-insensitive” (GIS) [³⁵S]TBPS binding. THIP displaced more [³⁵S]TBPS binding than GABA in several brain regions, indicating that THIP also inhibited GIS-binding. In these regions, GABA prevented the effect of THIP on GIS-binding. GIS-binding was increased in the cerebellar granule cell layer of δ KO and α4 + δ KO mice, being only slightly diminished in that of α1 KO mice. In the thalamus and some other forebrain regions of wild-type mice, a significant amount of GIS-binding was detected. This GIS-binding was higher in α4 KO mice. However, it was fully abolished in α1 KO mice, indicating that the α1 subunit was obligatory for the GIS-binding in the forebrain.Our results suggest that native GABA_A receptors in brain sections showing reduced displacing capacity of [³⁵S]TBPS binding by GABA (partial agonism) minimally require the assembly of α1 and β subunits in the forebrain and of α6 and β subunits in the cerebellar granule cell layer. These receptors may function as extrasynaptic GABA_A receptors.     

Unsaturated Free Fatty Acids Increase Benzodiazepine Receptor Agonist Binding Depending on the Subunit Composition of the GABAA Receptor Complex

Abstract: It has been shown previously that unsaturated free fatty acids (FFAs) strongly enhance the binding of agonist benzodiazepine receptor ligands and GABA_A receptor ligands in the CNS in vitro. To investigate the selectivity of this effect, recombinant human GABA_A/benzodiazepine receptor complexes formed by different subunit compositions (α_xβ_yγ₂, x = 1, 2, 3, and 5; y = 1, 2, and 3) were expressed using the baculovirus-transfected Sf9 insect cell system. At 10^?4M, unsaturated FFAs, particularly arachidonic (20:4) and docosahexaenoic (22:6) acids, strongly stimulated (>200% of control values) the binding of [³H]flunitrazepam ([³H]FNM) to the α₃β₂γ₂ receptor combination in whole cell preparations. No effect or small increases in levels of unsaturated FFAs on [³H]FNM binding to α₁β_xγ₂ and α₂β_xγ₂ receptor combinations were observed, and weak effects (130% of control values) were detected using the α₅β₂γ₂ receptor combination. The saturated FFAs, stearic and palmitic acids, were without effect on [³H]FNM binding to any combination of receptor complexes. The hydroxylated unsaturated FFAs, ricinoleic and ricinelaidic acids, were shown to decrease the binding of [³H]FNM only if an α₁β₂γ₂ receptor combination was used. Given the heterogeneity of the GABA_A/benzodiazepine receptor subunit distribution in the CNS, the effects of FFAs on the benzodiazepine receptor can be assumed to vary at both cellular and regional levels.     

Conservation of γ-Aminobutyric Acid Type A Receptor α6 Subunit Gene Expression in Cerebellar Granule Cells

Abstract: The γ-aminobutyric acid type A receptor cDNAs encoding the α6 subunit homologues from chicken and goldfish have been cloned and sequenced. These proteins exhibit 83 and 75% identity, respectively, to the rat α6 polypeptide. In situ hybridization has demonstrated that, as in mammals, the avian and teleost fish α6 subunit genes are predominately expressed in cerebellar granule cells. Correspondingly, flunitrazepam-nondisplaceable binding of [³H]Ro 15-4513 (a benzodiazepine partial inverse agonist), which is a major characteristic of γ-aminobutyric acid type A receptors that contain the α6 polypeptide, is also mainly found for cerebellar granule cells of fish and chick. The conservation of this expression pattern suggests that γ-aminobutyric acid type A receptors possessing the α6 subunit are of fundamental importance for cerebellar function and that the corresponding gene regulatory elements, e.g., granule cell-specific enhancers, have also been conserved.