GABAA receptor beta 1, beta 2, and beta 3 subunits: comparisons in DBA/2J and C57BL/6J mice

GABAA receptors in GtoPdb v.2021.3

The GABA_A receptor is a ligand-gated ion channel of the Cys-loop family that includes the nicotinic acetylcholine, 5-HT₃ and strychnine-sensitive glycine receptors. GABA_A receptor-mediated inhibition within the CNS occurs by fast synaptic transmission, sustained tonic inhibition and temporally intermediate events that have been termed 'GABA_A, slow' [45]. GABA_A receptors exist as pentamers of 4TM subunits that form an intrinsic anion selective channel. Sequences of six α, three β, three γ, one δ, three ρ, one ε, one π and one θ GABA_A receptor subunits have been reported in mammals [278, 235, 236, 283]. The π-subunit is restricted to reproductive tissue. Alternatively spliced versions of many subunits exist (e.g. α4- and α6- (both not functional) α5-, β2-, β3- and γ2), along with RNA editing of the α3 subunit [71]. The three ρ-subunits, (ρ1-3) function as either homo- or hetero-oligomeric assemblies [359, 50]. Receptors formed from ρ-subunits, because of their distinctive pharmacology that includes insensitivity to bicuculline, benzodiazepines and barbiturates, have sometimes been termed GABA_C receptors [359], but they are classified as GABA _A receptors by NC-IUPHAR on the basis of structural and functional criteria [16, 235, 236]. Many GABA_A receptor subtypes contain α-, β- and γ-subunits with the likely stoichiometry 2α.2β.1γ [168, 235]. It is thought that the majority of GABA_A receptors harbour a single type of α- and β - subunit variant. The α1β2γ2 hetero-oligomer constitutes the largest population of GABA_A receptors in the CNS, followed by the α2β3γ2 and α3β3γ2 isoforms. Receptors that incorporate the α4- α5-or α 6-subunit, or the β1-, γ1-, γ3-, δ-, ε- and θ-subunits, are less numerous, but they may nonetheless serve important functions. For example, extrasynaptically located receptors that contain α6- and δ-subunits in cerebellar granule cells, or an α4- and δ-subunit in dentate gyrus granule cells and thalamic neurones, mediate a tonic current that is important for neuronal excitability in response to ambient concentrations of GABA [209, 272, 83, 19, 288]. GABA binding occurs at the β+/α- subunit interface and the homologous γ+/α- subunits interface creates the benzodiazepine site. A second site for benzodiazepine binding has recently been postulated to occur at the α+/β- interface ([254]; reviewed by [282]). The particular α-and γ-subunit isoforms exhibit marked effects on recognition and/or efficacy at the benzodiazepine site. Thus, receptors incorporating either α4- or α6-subunits are not recognised by 'classical' benzodiazepines, such as flunitrazepam (but see [356]). The trafficking, cell surface expression, internalisation and function of GABA_A receptors and their subunits are discussed in detail in several recent reviews [52, 140, 188, 316] but one point worthy of note is that receptors incorporating the γ2 subunit (except when associated with α5) cluster at the postsynaptic membrane (but may distribute dynamically between synaptic and extrasynaptic locations), whereas as those incorporating the δ subunit appear to be exclusively extrasynaptic. NC-IUPHAR [16, 235, 3, 2] class the GABA_A receptors according to their subunit structure, pharmacology and receptor function. Currently, eleven native GABA_A receptors are classed as conclusively identified (i.e., α1β2γ2, α1βγ2, α3βγ2, α4βγ2, α4β2δ, α4β3δ, α5βγ2, α6βγ2, α6β2δ, α6β3δ and ρ) with further receptor isoforms occurring with high probability, or only tentatively [235, 236]. It is beyond the scope of this Guide to discuss the pharmacology of individual GABA_A receptor isoforms in detail; such information can be gleaned in the reviews [16, 95, 168, 173, 143, 278, 216, 235, 236] and [9, 10]. Agents that discriminate between α-subunit isoforms are noted in the table and additional agents that demonstrate selectivity between receptor isoforms, for example via β-subunit selectivity, are indicated in the text below. The distinctive agonist and antagonist pharmacology of ρ receptors is summarised in the table and additional aspects are reviewed in [359, 50, 145, 223]. Several high-resolution cryo-electron microscopy structures have been described in which the full-length human α1β3γ2L GABA_A receptor in lipid nanodiscs is bound to the channel-blocker picrotoxin, the competitive antagonist bicuculline, the agonist GABA (γ-aminobutyric acid), and the classical benzodiazepines alprazolam and diazepam [198].

Quantitative trait locus for seizure susceptibility on mouse chromosome 5 confirmed with reciprocal congenic strains

Multiple quantitative trait locus (QTL) mapping studies designed to localize seizure susceptibility genes in C57BL/6 (B6, seizure resistant) and DBA/2 (D2, seizure susceptible) mice have detected a significant effect originating from midchromosome 5. To confirm the presence and refine the position of the chromosome 5 QTL for maximal electroshock seizure threshold (MEST), reciprocal congenic strains between B6 and D2 mice were created by a DNA marker-assisted backcross breeding strategy and studied with respect to changes in MEST. A genomic interval delimited by marker D5Mit75 (proximal to the acromere) and D5Mit403 (distal to the acromere) was introgressed for 10 generations. A set of chromosome 5 congenic strains produced by an independent laboratory was also studied. Comparison of MEST between congenic and control (parental genetic background) mice indicates that genes influencing this trait were captured in all strains. Thus, mice from strains having D2 alleles from chromosome 5 on a B6 genetic background exhibit significantly lower MEST compared with control littermates, whereas congenic mice harboring B6 chromosome 5 alleles on a D2 genetic background exhibit significantly higher MEST compared with control littermates. Combining data from all congenic strains, we conclude that the gene(s) underlying the chromosome 5 QTL for MEST resides in the interval between D5Mit108 (26 cM) and D5Mit278 (61 cM). Generation of interval-specific congenic strains from the primary congenic strains described here may be used to achieve high-resolution mapping of the chromosome 5 gene(s) that contributes to the large difference in seizure susceptibility between B6 and D2 mice.

Neuroinformatics: a new tool for studying the brain

BACKGROUND

Central nervous system diseases constitute a major target for drug development. Genes expressed by the nervous system may represent half or more of the mammalian genome, with literally tens of thousands of gene products.

METHODS

Better methods are therefore required to accelerate the pace of mapping gene expression patterns in the mouse brain and to evaluate the progressive phenotypic changes in genetic models of human brain diseases.

CONCLUSIONS

Recent studies of mouse models of Amyotrophic Lateral Sclerosis and Alzheimer's disease illustrate how such data could be used for drug development. Since these two diseases-- especially Alzheimer's Disease-- entail disordered behavior, cognition and emotions, the framework and the methodology described in this article might in the future find applications in research on affective disorders.

From Gene to Behavior and Back Again: New Perspectives on GABAA Receptor Subunit Selectivity of Alcohol Actions1

gamma-Aminobutyric acid A (GABA(A)) receptors are believed to mediate a number of alcohol's behavioral actions. Because the subunit composition of GABA(A) receptors determines receptor pharmacology, behavioral sensitivity to alcohol (ethanol) may depend on which subunits are present (or absent). A number of knockout and/or transgenic mouse models have been developed (alpha1, alpha2, alpha5, alpha6, beta2, beta3, gamma2S, gamma2L, delta) and tested for behavioral sensitivity to ethanol. Here we review the current GABA(A) receptor subunit knockout and transgenic literature for ethanol sensitivity, and integrate these results into those obtained using quantitative trait loci (QTL) analysis and gene expression assays. Converging evidence from these three approaches support the notion that different behavioral actions of ethanol are mediated by specific subunits, and suggest that new drugs that target specific GABA(A subunits may selectively alter some behavioral actions of ethanol without altering others. Current data sets provide stronge)st evidence for a role of alpha1 subunits in ethanol-induced loss of righting reflex and alpha5 subunits in ethanol-stimulated locomotion. Nevertheless, three-way validation is hampered by the incomplete behavioral characterization of many of the mutant mice, and additional subunits are likely to be linked to alcohol actions as behavioral testing progresses.

Altered expression of GABA(A) and GABA(B) receptor subunit mRNAs in the hippocampus after kindling and electrically induced status epilepticus

Epilepsy may result from altered transmission of the principal inhibitory transmitter GABA in the brain. Using in situ hybridization in two animal models of epileptogenesis, we investigated changes in the expression of nine major GABA(A) receptor subunits (alpha1, alpha2, alpha4, alpha5, beta1-beta3, gamma2 and delta) and of the GABA(B) receptor species GABA(B)R1a, GABA(B)R1b and GABA(B)R2 in 1) hippocampal kindling and 2) epilepsy following electrically-induced status epilepticus (SE). Hippocampal kindling triggers a decrease in seizure threshold without producing spontaneous seizures and hippocampal damage, whereas the SE model is characterized by spontaneous seizures and hippocampal damage. Changes in the expression of GABA(A) and GABA(B) receptor mRNAs were observed in both models, and compared with those seen in other models and in human temporal lobe epilepsy. The most prominent changes were a relatively fast (24 h after kindling and electrically-induced SE) and lasting (7 and 30 days after termination of kindling and SE, respectively) reduction of GABA(A) receptor subunit delta mRNA levels (by 43-78%) in dentate granule cells, accompanied by increases in mRNA levels of all three beta-subunits (by 8-79%) and subunit gamma2 (by 11-43%). Levels of the minor subunit alpha4 were increased by up to 60% in dentate granule cells in both animal models, whereas those of subunit alpha5 were decreased 24 h and 30 days after SE, but not after kindling. In cornu ammonis 3 pyramidal cells, downregulation of subunits alpha2, alpha4, alpha5, and beta1-3 was observed in the ventral hippocampus and of alpha2, alpha5, beta3 and gamma2 in its dorsal extension 24 h after SE. Similar but less pronounced changes were seen in sector cornu ammonis 1. Persistent decreases in subunit alpha2, alpha4 and beta2 transcript levels were presumably related to SE-induced cell loss. GABA(B) receptor expression was characterized by increases in GABA(B)R2 mRNA levels at all intervals after kindling and SE. The observed changes suggest substantial and cell specific rearrangement of GABA receptors. Lasting downregulation of subunits delta and alpha5 in granule cells and transient decreases in subunit alpha2 and beta1-3 mRNA levels in cornu ammonis 3 pyramidal cells are suggestive of impaired GABA(A) receptor-mediated inhibition. Persistent upregulation of subunits beta1-3 and gamma2 of the GABA(A) receptor and of GABA(B)R2 mRNA in granule cells, however, may result in activation of compensatory anticonvulsant mechanisms.

γ-Aminobutyric acid A receptor subunit mutant mice: new perspectives on alcohol actions

gamma-Aminobutyric acid A (GABA(A)) receptors are believed to mediate a number of alcohol's behavioral actions. Because the subunit composition of GABA(A) receptors determines receptor pharmacology, behavioral sensitivity to alcohol (ethanol) may depend on which subunits are present (or absent). A number of knock-out and/or transgenic mouse models have been developed (alpha1, alpha2, alpha5, alpha6, beta2, beta3, gamma2S, gamma2L, delta) and tested for behavioral sensitivity to ethanol. Here we review the current GABA(A) receptor subunit knock-out and transgenic literature for ethanol sensitivity, and integrate these results into those obtained using quantitative trait loci (QTL) analysis and gene expression assays. Converging evidence from these three approaches support the notion that different behavioral actions of ethanol are mediated by specific subunits, and suggest that new drugs that target specific GABA(A) subunits may selectively alter some behavioral actions of ethanol, without altering others. Current data sets provide strongest evidence for a role of alpha1-subunits in ethanol-induced loss of righting reflex, and alpha5-subunits in ethanol-stimulated locomotion. However, three-way validation is hampered by the incomplete behavioral characterization of many of the mutant mice, and additional subunits are likely to be linked to alcohol actions as behavioral testing progresses.

Differences in ionotropic glutamate receptor subunit expression are not responsible for strain-dependent susceptibility to excitotoxin-induced injury

Systemic administration of kainic acid in C57BL/6 and FVB/N mice induces a comparable level of seizure induction yet results in differential susceptibility to seizure-induced cell death. While kainate administration causes severe hippocampal damage in mice of the FVB/N strain, C57BL/6 mice display no demonstrable cell loss or damage. At present, while the cellular mechanisms underlying strain-dependent differences in susceptibility remain unclear, some of this variation is assumed to have a genetic basis. As glutamate receptors are thought to participate in seizure induction and the subsequent neuronal degeneration that ensues, previous studies have proposed that variation in the precise subunit composition of glutamate receptors may result in differential susceptibility to excitotoxic cell death. Thus, we chose to examine the relationship between the cellular distribution and expression of glutamate receptor subunit proteins and cell loss within the hippocampus in mouse strains resistant and susceptible to kainate-induced excitotoxicity. Using semi-quantitative Western blot techniques and immunohistochemistry with the use of antibodies that recognize subunits of the KA (GluR5,6,7), AMPA (GluR1, GluR2, and GluR4), and NMDA (NMDAR1 and NMDAR2A/2B) receptors, we found no significant strain-dependent differences in the expression or distribution of these glutamate receptor subunits in the intact hippocampus. Following kainate administration, expression changes in ionotropic glutamate receptor subunits paralleled the development of susceptibility to cell death in the FVB/N strain only. Strain differences in hippocampal vulnerability to kainate-induced status epilepticus are not due to glutamate receptor protein expression.

GABA(A) receptor beta(2) subunit mRNA content is differentially regulated in ethanol-dependent DBA/2J and C57BL/6J mice

Chronic ethanol treatment is known to alter gene expression and function of gamma-aminobutyric acid type-A (GABA(A)) receptors. Here we focus on the beta(2) subunit which is widely expressed in the mammalian brain, and plays a key role in the GABA binding site. Previous studies using rodent models of ethanol dependence show either increased or no change of beta(2) subunit mRNA and peptide content following chronic ethanol administration. In humans, polymorphism at the beta(2) subunit is associated with ethanol dependence in some, but not all, populations. In the present study we measured mRNA content in the cerebellum and cerebral cortex using ethanol-naive and ethanol-dependent DBA/2J and C57BL/6J mice. The DBA/2J strain displays severe ethanol withdrawal severity, while the C57BL/6J strain shows milder withdrawal reactions. RNase protection analysis demonstrated that the DBA/2J strain is more sensitive to ethanol-induced increases in beta(2) subunit mRNA content in the cerebellum, showing significant increases at lower blood ethanol concentrations than C57BL/6J mice. The ethanol-induced regulation in C57BL/6J mice appears to be more complex, with decreases in beta(2) subunit mRNA content at low blood ethanol concentrations, and increases at higher concentrations. These data suggest that differences between C57BL/6J and DBA/2J mice in the degree of physical dependence (withdrawal) on ethanol may be related to differential sensitivity to ethanol regulation of beta(2) subunit expression.

Identification of an amino acid defining the distinct properties of murine beta1 and beta3 subunit-containing GABA(A) receptors

Murine gamma-aminobutyric acid (GABA) type A homomeric receptors made of beta1 subunits are profoundly different, when expressed in Xenopus oocytes, from beta3 homomeric receptors. Application of the intravenous general anesthetic pentobarbital, etomidate, or propofol to beta3 homomeric receptors allows current flow. In contrast, beta1 homomers do not respond to any of these agents. Through construction of chimeric beta1/beta3 receptors, we identified a single amino acid that determines the pharmacological difference between the two beta subunits. When the serine residue present in the wild-type nonresponsive beta1 subunit is replaced by an asparagine found in the same position in the beta3 subunit, the resulting point-mutated beta1S265N forms receptors responsive to intravenous general anesthetics, like the wild-type beta3 subunits. Conversely, after mutation of the wild-type beta3 to beta3N265S, the homomeric receptor loses its ability to respond to these same general anesthetics. Wild-type-to-mutant titration experiments showed that the nonresponsive phenotype is dominant: A single nonresponsive residue within a pentameric receptor is sufficient to render the receptor nonresponsive. In alpha1betax or alpha1betaxgamma2 heteromeric receptors, the same residue manifests as a partial determinant of the degree of potentiation of the GABA-induced current by some general anesthetics. The location of this amino acid at the extracellular end of the second transmembrane segment, its influence in both homomeric and heteromeric receptor function, and its dominant behavior suggest that this residue of the beta subunit is involved in an allosteric modulation of the receptor.

A Sequence-Ready BAC Contig of the GABAA Receptor Gene Cluster Gabrg1–Gabra2–Gabrb1 on Mouse Chromosome 5

The type-A receptors for the neurotransmitter GABA (γ-aminobutyric acid) are ligand-gated chloride channels that mediate postsynaptic inhibition. The functional diversity of these receptors comes from the use of a large repertoire of subunits encoded by separate genes, as well as from differences in subunit composition of individual receptors. In mammals, a majority of GABAAreceptor subunit genes are located in gene clusters that may be important for their regulated expression and function. We have established a high-resolution physical map of the cluster of genes encoding GABAA receptor subunits α2 (Gabra2), β1 (Gabrb1), and γ1 (Gabrg1) on mouse chromosome 5. Rat cDNA probes and specific sequence probes for all three GABAA receptor subunit genes have been used to initiate the construction of a sequence-ready contig of bacterial artificial chromosomes (BACs) encompassing this cluster. In the process of contig construction clones from 129/Sv and C57BL/6J BAC libraries were isolated. The assembled 1.3-Mb contig, consisting of 45 BACs, gives five- to sixfold coverage over the gene cluster and provides an average resolution of one marker every 32 kb. A number of BAC insert ends were sequenced, generating 30 new sequence tag sites (STS) in addition to 6 Gabr gene-based and 3 expressed sequence tag (EST)-based markers. STSs from, and surrounding, theGabrg1–Gabra2–Gabrb1 gene cluster were mapped in the T31 mouse radiation hybrid panel. The integration of the BAC contig with a map of loci ordered by radiation hybrid mapping suggested the most likely genomic orientation of this cluster on mouse chromosome 5: cen–D5Mit151–Gabrg1–Gabra2–Gabrb1–D5Mit58–tel. This established contig will serve as a template for genomic sequencing and for functional analysis of the GABAA gene cluster on mouse chromosome 5 and the corresponding region on human chromosome 4.

The sequence data described in this paper have been submitted to the GenBank/GSS data libraries under accession nos.AF156490 and AQ589406–AQ589436.

An update on GABAA receptors

Recent advances in molecular biology and complementary information derived from neuropharmacology, biochemistry and behavior have dramatically increased our understanding of various aspects of GABAA receptors. These studies have revealed that the GABAA receptor is derived from various subunits such as alpha1-alpha6, beta1-beta3, gamma1-gamma3, delta, epsilon, pi, and rho1-3. Furthermore, two additional subunits (beta4, gamma4) of GABAA receptors in chick brain, and five isoforms of the rho-subunit in the retina of white perch (Roccus americana) have been identified. Various techniques such as mutation, gene knockout and inhibition of GABAA receptor subunits by antisense oligodeoxynucleotides have been used to establish the physiological/pharmacological significance of the GABAA receptor subunits and their native receptor assemblies in vivo. Radioligand binding to the immunoprecipitated receptors, co-localization studies using immunoaffinity chromatography and immunocytochemistry techniques have been utilized to establish the composition and pharmacology of native GABAA receptor assemblies. Partial agonists of GABAA receptors are being developed as anxiolytics which have fewer and less severe side effects as compared to conventional benzodiazepines because of their lower efficacy and better selectivity for the GABAA receptor subtypes. The subunit requirement of various drugs such as anxiolytics, anticonvulsants, general anesthetics, barbiturates, ethanol and neurosteroids, which are known to elicit at least some of their pharmacological effects via the GABAA receptors, have been investigated during the last few years so as to understand their exact mechanism of action. Furthermore, the molecular determinants of clinically important drug-targets have been investigated. These aspects of GABAA receptors have been discussed in detail in this review article.

GABA(A) receptor alpha4 subunit in DBA/2J and C57BL/6J mice

GABA(A) receptors are chloride channels in the brain activated by binding of gamma-aminobutyric acid (GABA). Several important classes of drugs, including alcohol and certain antiepileptic drugs, modulate the actions of GABA. We report the sequence and expression of alpha4 subunits of GABA(A) receptors in two inbred strains of mice, DBA/2J and C57BL/6J, which differ in susceptibility to seizures and to behavioral effects of alcohol. We find no differences between the two strains in cDNA sequence, or in levels of alpha4 mRNA in whole brains of the two strains at 21 days of age, when DBA/2J are most susceptible to audiogenic seizures. We also describe the pattern of developmental expression and brain regional distribution of this subunit in mice, finding the highest developmental expression at about 14 days of age in whole brains, and the highest regional levels in hippocampus and basal forebrain (including thalamus) in adults.

Drug withdrawal convulsions and susceptibility to convulsants after short-term selective breeding for acute ethanol withdrawal

High Alcohol Withdrawal (HAW) and Low Alcohol Withdrawal (LAW) mice were selectively bred from a foundation population of C57BL6/J (B6) x DBA/2J (D2) F2 intercross progeny for display of intense or mild handling-induced withdrawal convulsions, respectively, following a single injection of a hypnotic dose of ethanol (alcohol; 4 g/kg). The HAW line had significantly greater alcohol withdrawal severity scores compared to the LAW line after only a single generation of selection; the magnitude of the line difference was 8-fold by the fourth selected generation. We tested these lines for severity of withdrawal convulsions following the benzodiazepine, diazepam; the gaseous anesthetic, nitrous oxide; the imidazopyridine, zolpidem and the barbiturate, pentobarbital. In all cases, HAW mice had significantly greater withdrawal severity than mice of the LAW line. These results indicate that some genes influencing withdrawal convulsion severity following ethanol also affect withdrawal from other CNS depressants. D2 mice are more sensitive to a variety of convulsants than B6 mice (and have more severe withdrawal convulsions). We, therefore, tested separate groups of mice of both selectively bred lines for threshold sensitivity to pentylenetetrazol (PTZ), N-methyl-D-aspartate (NMDA) and kainic acid (KA). No line differences were detected. These results indicate that genes influencing severity of withdrawal from several depressant drugs are largely different from those affecting susceptibility to GABAergic or glutamatergic convulsants.

The alpha5 subunit of the murine type A GABA receptor

GABA[A] receptors in the brain convert binding of GABA (gamma-aminobutyric acid) to inhibition by chloride currents. Several important classes of drugs, including benzodiazepines and alcohol, modulate these receptors, which have also been implicated in epilepsy. We describe the alpha5 subunit of GABAA receptors in mice, comparing inbred DBA/2J mice, prone to juvenile audiogenic seizures, with seizure resistant C57BL/6J mice. We find no sequence differences between the strains, although there are several interesting amino acid differences from the rat. We also compare the expression of the alpha5 subunit in whole brains of DBA/2J mice to that in C57BL/6J mice at 21 days, the peak of the former's seizure susceptibility, again finding no significant difference. We further describe the pattern of expression of alpha5 mRNA during mouse brain development, with a peak at 3 days after birth, and among five brain regions in the adult mouse, with the highest levels in the hippocampus. Finally, we present preliminary evidence for rare alternative splicing of this subunit's message, in the N-terminal extracellular domain, to give a form not translatable into a functional protein.

Expression of functional GABAA receptors in cholecystokinin-secreting gut neuroendocrine murine STC-1 cells

1. Gastrointestinal neuroendocrine (NE) cells synthesize, store and secrete gamma-aminobutyric acid (GABA). Recently, an autocrine-paracrine function of GABA has been proposed for secretion from NE cells. 2. To search for functional GABAA receptors in NE gut cells, we performed whole-cell and perforated-patch-clamp studies in the intestinal cholecystokinin (CCK)-secreting NE cell line STC-1. 3. Application of GABA evoked currents in STC-1 cells. These effects were mimicked by muscimol, an agonist of GABAA receptors, and blocked by picrotoxin or bicuculline, antagonists of GABAA receptors. The GABA- or muscimol-activated currents reversed near 0 mV, which under the recording conditions used was consistent with the activation of the GABAA receptor-Cl- channel complex. 4. In contrast to the effect on most neurons, GABA as well as muscimol led to a (reversible) depolarization of the membrane potential of STC-1 cells. Membrane depolarization in turn activated voltage-gated Ca2+ channels and increased intracellular Ca2+ concentrations in STC-1 cells. 5. In accordance with the observed membrane depolarization and activation of voltage-gated Ca2+ channels, both GABA and muscimol stimulated Ca2+-dependent CCK release. In contrast, bicuculline inhibited the GABA-induced secretion of CCK. 6. Using the reverse transcription-polymerase chain reaction (RT-PCR), mRNA of the GABAA receptor subunits alpha2, alpha3, alpha5, beta1, beta3 and delta could be detected in STC-1 cells. 7. In summary, we have shown that the CCK-secreting gut NE cell line STC-1 expresses functional GABAA receptors and that GABA stimulates CCK release. Thus, GABA is involved in the fine tuning of CCK secretion from the gut NE cell line STC-1.

The GABA(A) receptor gamma1-subunit in seizure prone (DBA/2) and resistant (C57BL/6) mice

Gamma-aminobutyric acid (GABA)A receptors are the sites of action for many antiepileptic drugs such as benzodiazepines and barbiturates. We report the results of molecular cloning of the gamma1-subunit from seizure prone DBA/2J and resistant C57BL/6J inbred mice, and analyses of nucleotide sequences and expression of the gamma1-subunit messenger RNA (mRNA) in DBA/2 and C57BL/6 inbred mice. The mouse gamma1-subunit complementary DNA (cDNA) shares 98% similarity with that of the rat at the level of amino acid sequence. Northern blot hybridization indicates that the gamma1-subunit mRNA is expressed predominantly in areas other than the cerebral cortex and cerebellum and shows little change with postnatal development. No differences have been found for the subunit between DBA/2 and C57BL/6 mice either for nucleotide sequence or for level of expression of the subunit's mRNA in whole brain by Northern blots at 3 weeks of age.

The role of the GABA(A) receptor alpha1 subunit N-terminal extracellular domain in propofol potentiation of chloride current

Propofol (2,6-diisopropylphenol), an intravenous general anesthetic in active clinical use today, potentiates the action of gamma-aminobutyric acid (GABA) at the type-A receptor and also directly induces current in the absence of GABA. We expressed different combinations of murine GABA(A) receptor alpha1, beta3 and gamma2 subunits in Xenopus oocytes to investigate the subunit dependence of propofol potentiation of pentobarbital-induced current. Pentobarbital induces current in all beta3-subunit-containing receptors, whereas current gating by GABA requires the presence of both alpha1 and beta3 subunits. Therefore, pentobarbital rather than GABA was used to induce current in order to separate the subunit dependence of current gating from the subunit dependence of potentiating action of propofol. alpha1beta3gamma2, alpha1beta3, beta3gamma2, or beta3 subunit combinations all responded to pentobarbital in a dose-dependent manner. True potentiation was defined as the current magnitude to simultaneous application of pentobarbital and propofol exceeding the additive responses to individual drug applications. A dose-dependent propofol potentiation of pentobarbital-induced current was observed in oocytes injected with alpha1beta3 or alpha1beta3gamma2 but not in beta3gamma2 or beta3 subunits, suggesting that the alpha1 subunit was necessary for this modulatory action of propofol. Further examination of the propofol potentiation in chimeras between the alpha1 and beta3 subunits showed that the extracellular amino-terminal half of the alpha1 subunit was sufficient to support propofol potentiation. The different requirements of the receptor structure for the agonistic (gating) and the potentiating actions suggest that these two actions of propofol are distinct processes mediated through its action at distinct sites.

Conserved phosphorylation of the intracellular domains of GABA(A) receptor beta2 and beta3 subunits by cAMP-dependent protein kinase, cGMP-dependent protein kinase protein kinase C and Ca2+/calmodulin type II-dependent protein kinase

All mammalian GABA(A) receptor beta subunits contain a conserved consensus site for phosphorylation by a number of serine/threonine protein kinases. This site corresponds to Serine 410 of the beta2 subunit and Serine 409 of the beta3 subunit, each of which lies within the conserved sequence R-R-R-X-S-L-Q-K, where X = A (beta1, beta2 and beta4) or S (beta3). We have analysed the phosphorylation of the beta2 and beta3 subunits of the murine GABA(A) receptor by expressing the large intracellular domains of these subunits as soluble fusion proteins in E. coli. The intracellular domain of the beta2 subunit was phosphorylated to high stoichiometry by both cAMP- and cGMP-dependent protein kinases, protein kinase C and Ca2+/calmodulin type II-dependent protein kinase in vitro. Site-directed mutagenesis identified Serine 410 as the single site within the beta2 subunit phosphorylated by these four protein kinases. Using similar methodologies, Serine 409 of the beta3 subunit was shown to be a substrate for phosphorylation by these protein kinases. Serine 408 was also seen to be phosphorylated by protein kinase C and Serine 383 was phosphorylated by Ca2+/calmodulin type II-dependent protein kinase. Since beta subunits are believed to be essential for robust GABA(A) receptor expression, these results suggest a critical role for conserved phosphorylated amino acids within the beta subunits in coordinating cellular regulation of GABA(A) receptors via multiple protein kinases.

Quantitative trait loci involved in genetic predisposition to acute alcohol withdrawal in mice

Alcohol dependence (alcoholism) is accompanied by evidence of tolerance, withdrawal (physiological dependence), or compulsive behavior related to alcohol use. Studies of strain and individual differences using animal models for acute physiological dependence liability are useful means to identify potential genetic determinants of liability in humans. Behavioral and quantitative trait analyses were conducted using animal models for high risk versus resistance to acute physiological dependence. Using a two-step genetic mapping strategy, loci on mouse chromosomes 1, 4, and 11 were mapped that contain genes that influence alcohol withdrawal severity. In the aggregate, these three risk markers accounted for 68% of the genetic variability in alcohol withdrawal. Candidate genes in proximity to the chromosome 11 locus include genes encoding the alpha1, alpha6, and gamma2 subunits of type-A receptors for the inhibitory neurotransmitter, GABA. In addition, suggestive linkage is indicated for two loci on mouse chromosome 2, one near Gad1 encoding glutamic acid decarboxylase, and the other near the El2 locus which influences the seizure phenotype in the neurological mutant strain El. The present analyses detect and map some of the loci that increase risk to develop physiological dependence and may facilitate identification of genes related to the development of alcoholism. Syntenic conservation between human and mouse chromosomes suggests that human homologs of genes that increase risk for physiological dependence may localize to 1q21-q32, 2q24-q37/11p13, 9p21-p23/1p32-p22.1, and 5q32-q35.

Isolation and genomic structure of a human homolog of the yeast periodic tryptophan protein 2 (PWP2) gene mapping to 21q22.3

As part of efforts to identify candidate genes for disease mapping to the 21q22.3 region, we have assembled a 770-kb cosmid and BAC contig containing eight tightly linked markers. These cosmids and BACs were restriction mapped using eight rare cutting enzymes, with the goal of identifying CpG-rich islands. One such island was identified by the clustering of NotI, EagI, SstII, and BssHII sites, and corresponded to the NotI linking clone LJ104 described previously. A 7.6-kb HindIII fragment containing this CpG-rich island was subcloned and partially sequenced. A homology search using the sequence obtained from either side of the NotI site identified an expressed sequence tag with homology to the yeast periodic tryptophan protein 2 (PWP2). Several cDNAs corresponding to the human PWP2 gene were identified and partially sequenced. Northern blot analysis revealed a 3.3-kb transcript that was well expressed in all tissues tested. A cDNA consensus of 3157 bp was obtained, and an open reading frame potentially encoding 919 amino acid residues was identified. The predicted protein shows 42% identity and 57% similarity at the amino acid level to the yeast PWP2 protein, which is a member of the WD-repeat containing superfamily, and potentially encodes a G-protein beta subunit. The PWP2 gene is split into 21 exons, ranging in size from 53 to 516 bp, and spans an estimated 25 kb. The gene is transcribed in a 21cen-->21qter direction, with its 5' end mapping approximately 195 kb proximal to the 5' end of the phosphofructokinase-liver isoform gene. Four single base-pair polymorphisms were identified using single-stranded conformation polymorphism analysis. Possible functions of the protein based on homology to other members of the WD-repeat-containing family are discussed.

Subcellular localization of gamma-aminobutyric acid type A receptors is determined by receptor beta subunits

gamma-aminobutyric acid type A (GABAA) receptors are the major sites of fast synaptic inhibition in the brain. They are constructed from four subunit classes with multiple members: alpha (1-6), beta (1-4), gamma (1-4), and delta (1). The contribution of subunit diversity in determining receptor subcellular targeting was examined in polarized Madin-Darby canine kidney (MDCK) cells. Significant detection of cell surface homomeric receptor expression by a combination of both immunological and electrophysiological methodologies was only found for the beta 3 subunit. Expression of alpha/beta binary combinations resulted in a nonpolarized distribution for alpha 1 beta 1 complexes, but specific basolateral targeting of both alpha 1 beta 2 and alpha 1 beta 3 complexes. The polarized distribution of these alpha/beta complexes was unaffected by the presence of the gamma 2S subunit. Interestingly, delivery of receptors containing the beta 3 subunit to the basolateral domain occurs via the apical surface. These results show that beta subunits can selectively target GABAA receptors to distinct cellular locations. Changes in the spatial and temporal expression of beta-subunit isoforms may therefore provide a mechanism for relocating GABAA receptor function between distinct neuronal domains. Given the critical role of these receptors in mediating synaptic inhibition, the contribution of different beta subunits in GABAA receptor function, may have implications in neuronal development and for receptor localization/clustering.

Assembly and cell surface expression of heteromeric and homomeric gamma-aminobutyric acid type A receptors

The ability of differing subunit combinations of gamma-aminobutyric acid type A (GABAA) receptors produced from murine alpha 1, beta 2, and gamma 2L subunits to form functional cell surface receptors was analyzed in both A293 cells and Xenopus oocytes using a combination of molecular, electrophysiological, biochemical, and morphological approaches. The results revealed that GABAA receptor assembly occurred within the endoplasmic reticulum and involved the interaction with the chaperone molecules immunoglobulin heavy chain binding protein and calnexin. Despite all three subunits possessing the ability to oligomerize with each other, only alpha 1 beta 2 and alpha 1 beta 2 gamma 2L subunit combinations could produce functional surface expression in a process that was not dependent on N-linked glycosylation. Single subunits and the alpha 1 gamma 2L and beta 2 gamma 2L combinations were retained within the endoplasmic reticulum. These results suggest that receptor assembly occurs by defined pathways, which may serve to limit the diversity of GABAA receptors that exist on the surface of neurons.