Long-range interactions in neuronal gene expression: evidence from gene targeting in the GABA(A) receptor beta2-alpha6-alpha1-gamma2 subunit gene cluster

Clustering of GABA(A) receptor alpha1, alpha6, beta2, and gamma2 subunit genes on mouse chromosome 11/human chromosome 5 may have functional significance for coordinating expression patterns, but until now there has been no evidence for cross-talk between the genes. However, altering the structure of the alpha6 gene, specifically expressed in the cerebellum, with neomycin gene insertions in two different experiments unexpectedly reduced the expression of the widespread alpha1 and beta2 genes in the forebrain. There were corresponding reductions in the levels of alpha1 and beta2 subunit proteins and in autoradiographic ligand binding densities to GABA(A) receptors in the forebrain of alpha6-/- mice. The gamma2 mRNA level was not changed, nor were beta3 and delta mRNAs. The data suggest that elements in the neo gene may have an influence over long distances in the GABA(A) subunit gene complex on as yet undefined structures coordinating the expression of the alpha1 and beta2 genes.

Synthesis and GABA(A) receptor activity of 6-oxa-analogs of neurosteroids

The 6-oxasteroids 3alpha-hydroxy-6-oxa-5alpha-pregnan-20-one (3) and 3alpha-hydroxy-6-oxa-5beta-pregnan-20-one (4) were obtained from pregnenolone acetate via the corresponding (5alpha or 5beta) 3beta, 20beta-diacetoxy-6-oxa-pregnane. Both steroids showed ca. 100-fold reduced potency for modulating [(3)H]flunitrazepam, [(3)H]muscimol or [(35)S]TBPS binding to the GABA(A) receptor when compared to their natural carbon analogs 3alpha-hydroxy-5alpha-pregnan-20-one (1) and 3alpha-hydroxy-5beta-pregnan-20-one (2).

Colocalization of multiple GABA(A) receptor subtypes with gephyrin at postsynaptic sites

Clustering of gamma aminobutyric acid (GABA)(A) receptors to postsynaptic sites requires the presence of both the gamma2 subunit and gephyrin. Here, we analyzed by double-immunofluorescence staining the colocalization of gephyrin and major GABA(A)-receptor subtypes distinguished by the subunits alpha1, alpha2, alpha3, or gamma2 in adult rat brain. By using confocal laser scanning microscopy, GABA(A)-receptor subunit staining revealed brightly stained clusters that were colocalized with gephyrin-positive clusters of similar size and distribution in several brain regions, including cerebellum, hippocampus, thalamus, and olfactory bulb. In addition, a diffuse staining was observed for GABA(A)-receptor subunits in the neuropil, presumably representing extrasynaptic receptors. Overall, only few gephyrin-positive clusters were not colocalized with GABA(A)-receptor subunit clusters. Electron microscopic analysis in cerebellar cortex confirmed the selective postsynaptic localization of gephyrin. High-resolution images (voxel size, 50 x 50 x 150 nm) were restored with an iterative image deconvolution procedure based on a measured point-spread function to analyze the colocalization between GABA(A)-receptor subunits and gephyrin in individual clusters. This analysis revealed a considerable heterogeneity in the micro-organization of these presumptive GABAergic postsynaptic sites. For instance, whereas gephyrin- and gamma2 subunit-positive clusters largely overlapped in the cerebellar molecular layer, the colocalization was only partial in glomeruli of the granule cell layer, where small gephyrin clusters typically were "embedded" in larger GABA(A)-receptor clusters. These findings show that gephyrin is associated with a majority of GABA(A)-receptor subtypes in brain, and document the usefulness of image deconvolution for analyzing the structural organization of the postsynaptic apparatus by fluorescence microscopy.

GABA(A) receptor assembly. Identification and structure of gamma(2) sequences forming the intersubunit contacts with alpha(1) and beta(3) subunits

GABA(A) receptors are ligand-gated chloride channels composed of five homologous subunits that specifically recognize one another and assemble around an aqueous pore. To identify domains responsible for the specificity of subunit association, we constructed C-terminal truncated gamma(2) subunits, as well as mutated and chimeric fragments. From their ability to interfere with alpha(1)beta(3)gamma(2) receptor assembly and to associate with full-length subunits, we concluded that amino acid sequences gamma(2)-(91-104) and gamma(2)-(83-90) form the sites mediating assembly with alpha(1) and beta(3) subunits, respectively. Neural network-based secondary structure prediction, Monte Carlo optimization, and hydrophobicity analysis led to the conclusion that these sites also form the intersubunit contacts in the completely assembled receptor and provided important information on the benzodiazepine-binding site and structure of GABA(A) receptors.

Functional correlation of GABA(A) receptor alpha subunits expression with the properties of IPSCs in the developing thalamus

GABA(A) receptor alpha1 and alpha2 subunits are expressed differentially with ontogenic period in the brain, but their functional roles are not known. We have recorded GABA(A) receptor-mediated IPSCs from laterodorsal (LD) thalamic relay neurons in slices of rat brain at various postnatal ages and found that decay times of evoked IPSCs and spontaneous miniature IPSCs undergo progressive shortening during the first postnatal month. With a similar time course, expression of transcripts and proteins of GABA(A) receptor alpha2 subunit in LD thalamic region declined, being replaced by those of alpha1 subunit. To further address the causal relationship between alpha subunits and IPSC decay time kinetics, we have overexpressed GABA(A) receptor alpha1 subunit together with green fluorescent protein in LD thalamic neurons in organotypic culture using recombinant Sindbis virus vectors. Miniature IPSCs recorded from the LD thalamic neurons overexpressed with alpha1 subunit had significantly faster decay time compared with control expressed with beta-galactosidase. We conclude that the alpha2-to-alpha1 subunit switch underlies the developmental speeding in the decay time of GABAergic IPSCs.

Identification of amino acid residues of GABA(A) receptor subunits contributing to the formation and affinity of the tert-butylbicyclophosphorothionate binding site

A chimeric GABA(A) receptor subunit was constructed that contained the beta3 sequence from the N-terminus to the first two amino acids of the second transmembrane (TM2) domain. The remaining part of this chimera had the sequence of the alpha1 subunit. On co-expression with alpha1 subunits, this chimera was able to form heterooligomeric channels that were open in the absence of GABA. Picrotoxin and tert-butylbicyclophosphorothionate (TBPS) were able to block these channels with low potency. These channels exhibited high-affinity [3H]muscimol but no high-affinity [35S]TBPS binding sites. Introduction of V251, A252, and L253 of the beta3 subunit into the chimera resulted in the formation of closed channels that could be opened by GABA. The introduction of A252 and L253 of the beta3 subunit into this chimera was sufficient to reconstitute the specific high-affinity [35S]TBPS binding site in receptors composed of the chimera and alpha1 subunits. Replacement of other amino acids of the TM2 region of the chimera with corresponding amino acids of the beta3 subunit modulated the affinity of this [35S]TBPS binding site. Results obtained provide important information on the structure-function relationship of GABA(A) receptors.

Subunit composition and pharmacological characterization of gamma-aminobutyric acid type A receptors in frog pituitary melanotrophs

The frog pars intermedia is composed of a single population of endocrine cells directly innervated by gamma-aminobutyric acid (GABA)ergic nerve terminals. We have previously shown that GABA, acting through GABA(A) receptors, modulates both the electrical and secretory activities of frog pituitary melanotrophs. The aim of the present study was to take advantage of the frog melanotroph model to determine the relationship between the subunit composition and the pharmacological properties of native GABA(A) receptors. Immunohistochemical labeling revealed that in situ and in cell culture, frog melanotrophs were intensely stained with alpha2-, alpha3-, gamma2-, and gamma3-subunit antisera and weakly stained with a gamma1-subunit antiserum. Melanotrophs were also immunolabeled with a monoclonal antibody to the beta2/beta3-subunit. In contrast, frog melanotrophs were not immunoreactive for the alpha1-, alpha5-, and alpha6-isoforms. The effects of allosteric modulators of the GABA(A) receptor on GABA-activated chloride current were tested using the patch-clamp technique. Among the ligands acting at the benzodiazepine-binding site, clonazepam (EC50, 5 x 10(-9) M), diazepam (EC50, 10(-8) M), zolpidem (EC50, 3 x 10(-8) M), and beta-carboline-3-carboxylic acid methyl ester (EC50, 10(-6) M) were found to potentiate the whole cell GABA-evoked current in a dose-dependent manner. Methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (IC50, 3 x 10(-5) M) inhibited the current, whereas Ro15-4513 had no effect. Among the ligands acting at other modulatory sites, etomidate (EC50, 2 x 10(-6) M) enhanced the GABA-evoked current, whereas 4'-chlorodiazepam (IC50, 4 x 10(-7) M), ZnCl2 (IC50, >5 x 10(-5) M), and furosemide (IC50, >3 x 10(-4) M) depressed the response to GABA. PK 11195 did not affect the GABA-evoked current or its inhibition by 4'-chlorodiazepam. The results indicate that the native GABA(A) receptors in frog melanotrophs are formed by combinations of alpha2-, alpha3-, beta2/3-, gamma1-, gamma2-, and gamma3-subunits. The data also demonstrate that clonazepam is the most potent, and zolpidem is the most efficient positive modulator of the native receptors. Among the inhibitors, 4'-chlorodiazepam is the most potent, whereas ZnCl2 is the most efficient negative modulator of the GABA(A) receptors. The present study provides the first correlation between subunit composition and the functional properties of native GABA(A) receptors in nontumoral endocrine cells.

Differential regulation of synaptic GABAA receptors by cAMP-dependent protein kinase in mouse cerebellar and olfactory bulb neurones

1. It has been demonstrated that the regulation of recombinant GABAA receptors by phosphorylation depends on the subunit composition. Here we studied the regulation of synaptic GABAA receptor function by cAMP-dependent protein kinase (PKA) in neurones expressing distinct receptor subtypes. 2. Light microscopic immunocytochemistry revealed that granule cells of the olfactory bulb express only the beta3 as the beta subunit variant, whereas cerebellar stellate and basket cells express only the beta2 as the beta subunit. 3. In cerebellar interneurones, intracellular application of 20 microM microcystin, a protein phosphatase 1/2A inhibitor, prolonged (63 +/- 14 %; mean +/- s.e.m.) the decay time course of miniature IPSCs (mIPSCs) without significantly affecting their amplitude, rise time and frequency. The effect of microcystin could be blocked by co-applying PKA inhibitory peptide (PKA-I, 1 microM). 4. No significant changes in any of the mIPSC parameters could be detected after intracellular application of PKA-I alone or following the inhibition of calcineurin with FK506 (50 nM). 5. In granule cells of the olfactory bulb expressing the beta3 subunit fast and slowly rising mIPSCs were detected, resulting in a bimodal distribution of the 10-90 % rise times, suggesting two distinct populations of events. Fast rising mIPSCs (mIPSCFR) had a 10-90 % rise time of 410 +/- 50 micros, an amplitude of 68 +/- 6 pA, and a weighted decay time constant (tauw) of 15.8 +/- 2.9 ms. In contrast, slowly rising mIPSCs (mIPSCSR) displayed an approximately threefold slower rise time (1.15 +/- 0.12 ms), 57 % smaller amplitude (29 +/- 1.7 pA), but had a tauw (16.8 +/- 3.0 ms) similar to that of the fast events. 6. mIPSCs in olfactory granule cells were not affected by the intracellular perfusion of microcystin. In spite of this, intracellular administration of constitutively active PKA caused a small, gradual, but significant increase (18 +/- 5 %) in the amplitude of the events without changing their time course. 7. These findings demonstrate a cell-type-dependent regulation of synaptic inhibition by protein phosphorylation. Furthermore, our results show that the effect of PKA-mediated phosphorylation on synaptic inhibition depends upon the subunit composition of postsynaptic GABAA receptors.

Dopamine D3 receptor gene polymorphism and response to clozapine in schizophrenic Pakastani patients

The dopamine D3 receptor (DRD3) appears to play an important role in the mediation of antipsychotic drug action. Genetic association of treatment response to the atypical antipsychotic drug clozapine with the DRD3 polymorphism Ser9Gly was investigated in a sample of 32 schizophrenic patients. We found association of treatment response with allele Gly-9 (P=0.0058) and with genotypes consisting of Gly-9 (P=0.033) by this pharmacogenetic approach. A combined analysis with two previous studies (Shaikh et al., Hum. Genet. 97 (1996) 714-719; Malhotra et al., Mol. Psychiatry 3 (1998) 72-75) further substantiates these results (P=0.0041).

The citalopram challenge test in patients with major depression and in healthy controls

Neuroendocrine challenge tests in depressed patients have revealed a blunted hormonal reaction to serotonergic stimuli. In the present study, citalopram was chosen as the serotonergic agent for neuroendocrine stimulation. Compared to earlier challenge agents, citalopram has the advantage of serotonergic selectivity, its application is well tolerated and the possibility of intravenous application reduces pharmacokinetic interference. Sixteen patients suffering from an acute episode of major depression and 16 healthy controls underwent the stimulation procedure with 20 mg of citalopram and placebo. Whereas significant differences in the secretion of prolactin and cortisol between citalopram and placebo challenge were observed in the control group, no differences were found in the group of depressed patients. Comparison of depressed patients and controls showed a significantly blunted prolactin secretion in patients. Differences in cortisol secretion following serotonergic stimulation with citalopram did not become significant. The stimulation procedure was well tolerated in all subjects, although a higher number of side effects was observed in the control group. The amount of side effects did not correlate with the hormone responses. These results are in line with the hypothesis of serotonergic hypofunction in depressed patients. In conclusion, the 20-mg citalopram challenge test is thought to be a promising tool for further investigation of serotonergic function in psychiatric illness.

Attenuated sensitivity to neuroactive steroids in gamma-aminobutyrate type A receptor delta subunit knockout mice

gamma-Aminobutyric acid (GABA) type A receptors mediate fast inhibitory synaptic transmission and have been implicated in responses to sedative/hypnotic agents (including neuroactive steroids), anxiety, and learning and memory. Using gene targeting technology, we generated a strain of mice deficient in the delta subunit of the GABA type A receptors. In vivo testing of various behavioral responses revealed a strikingly selective attenuation of responses to neuroactive steroids, but not to other modulatory drugs. Electrophysiological recordings from hippocampal slices revealed a significantly faster miniature inhibitory postsynaptic current decay time in null mice, with no change in miniature inhibitory postsynaptic current amplitude or frequency. Learning and memory assessed with fear conditioning were normal. These results begin to illuminate the novel contributions of the delta subunit to GABA pharmacology and sedative/hypnotic responses and behavior and provide insights into the physiology of neurosteroids.

Possible association between childhood absence epilepsy and the gene encoding GABRB3

BACKGROUND

Childhood Absence Epilepsy (CAE) is considered to have a predominantly, perhaps exclusively, genetic background. To date, genes responsible for susceptibility to CAE have not been identified. The object of the present study was to test association between CAE and the genes encoding the gamma-aminobutyric acid (GABA) type-A receptor subunits alpha 5 (GABRA5) and beta 3 (GABRB3) located on the long arm of chromosome 15 (15q11-q13).

METHODS

A family-based candidate gene approach was applied: 50 Austrian nuclear families ascertained for the presence of an affected child were investigated. GABRA5 and GABRB3 subunit genes were genotyped using DNA gained from peripheral blood samples by Polymerase Chain Reactions (PCR). Genetic association was tested using a Monte Carlo Version of the multi-allele Transmission-Disequilibrium Test (TDT).

RESULTS

The TDT displayed significant overall association with GABRB3 (p = .0118).

CONCLUSIONS

The present data suggest that the tested polymorphism may be either directly involved in the etiology of CAE or in linkage disequilibrium with disease-predisposing sites.

Composition of the GABA(A) receptors of retinal dopaminergic neurons

Transgenic technology, single-cell RT-PCR, and immunocytochemistry were combined to investigate the composition of the GABA(A) receptors of dopaminergic (interplexiform) amacrine (DA) cells. A mouse line was used in which these neurons were labeled with human placental alkaline phosphatase and could therefore be identified in vitro after dissociation of the retina. We performed single-cell RT-PCR on the isolated cells and showed that (1) DA cells contained the messages for alpha1, alpha3, alpha4, beta1, beta3, gamma1, gamma2(S), and gamma2(L) subunits; (2) this transcript repertory did not change on dissociation of the retina and throughout the time required for cell harvesting; and (3) all DA cells contained the entire transcript repertory. Immunocytochemistry with subunit-specific antibodies showed that all subunits were expressed and appeared homogeneously distributed throughout the cell membrane at a low concentration. In addition, with the exception of alpha4, the subunits formed clusters at the surface of the dendrites and on the inner pole of the cell body. Because of their size, shape, and topographic coincidence with GABAergic endings, the clusters were interpreted as postsynaptic active zones containing GABA(A) receptors. The composition of the synaptic receptors was not uniform: clusters distributed throughout the dendritic tree contained alpha3, beta3, and, less frequently, beta1 subunits, whereas clusters containing the alpha1 subunit were confined to large dendrites. Therefore, DA cells possess at least two types of GABA(A) receptors localized in different synapses. Furthermore, they exhibit multiple extrasynaptic GABA(A) receptors.

Synaptic control of glycine and GABA(A) receptors and gephyrin expression in cultured motoneurons

We have evaluated the influence of the secretory phenotype of presynaptic boutons on the accumulation of postsynaptic glycine receptors (GlyRs), type A GABA receptors (GABA(A)Rs), and gephyrin clusters. The cellular distribution of these components was analyzed on motoneurons cultured either alone or with glycinergic and/or GABAergic neurons. In motoneurons cultured alone, we observed gephyrin clusters at nonsynaptic sites and in front of cholinergic boutons, whereas glycine and GABA(A) receptors formed nonsynaptic clusters. These receptors are functionally and pharmacologically similar to those found in cultures of all spinal neurons. Motoneurons receiving GABAergic innervation from dorsal root ganglia neurons displayed postsynaptic clusters of gephyrin and GABA(A)Rbeta but not of GlyRalpha/beta subunits. In motoneurons receiving glycinergic and GABAergic innervation from spinal interneurons, gephyrin, GlyRalpha/beta, and GABA(A)Rbeta formed mosaics at synaptic loci. These results indicate that (1) the transmitter phenotype of the presynaptic element determines the postsynaptic accumulation of specific receptors but not of gephyrin and (2) the postsynaptic accumulation of gephyrin alone cannot account for the formation of GlyR-rich microdomains.

Benzodiazepine-mediated regulation of alpha1, alpha2, beta1-3 and gamma2 GABA(A) receptor subunit proteins in the rat brain hippocampus and cortex

Prolonged flurazepam exposure regulates the expression of selected (alpha1, beta2, beta3) GABA(A) receptor subunit messenger RNAs in specific regions of the hippocampus and cortex with a time-course consistent with benzodiazepine tolerance both in vivo and in vitro. In this report, the immunostaining density of six specific GABA(A) receptor subunit (alpha1, beta2, beta1-3 and gamma2) antibodies was measured in the hippocampus and cortex, among other brain areas, in slide-mounted brain sections from flurazepam-treated and control rats using quantitative computer-assisted image analysis techniques. In parallel with the localized reduction in alpha1 and beta3 subunit messenger RNA expression detected in a previous study, relative alpha1 and beta3 subunit antibody immunostaining density was significantly decreased in flurazepam-treated rat hippocampal CA1, CA3 and dentate dendritic regions, and in specific cortical layers. Quantitative western blot analysis showed that beta3 subunit protein levels in crude homogenates of the hippocampal dentate region from flurazepam-treated rats, an area which showed fairly uniform decreases in beta3 subunit immunostaining (16-21%), were reduced to a similar degree (18%). The latter findings provide independent support that relative immunostaining density may provide an accurate estimate of protein levels. Consistent with the absence of the regulation of their respective messenger RNAs immediately after ending flurazepam administration, no changes in the density of alpha2, beta1 or beta2 subunit antibody immunostaining were found in any brain region. gamma2 subunit antibody staining was changed only in the dentate molecular layer. The selective changes in GABA(A) receptor subunit antibody immunostaining density in the hippocampus suggested that a change in the composition of GABA(A) receptors involving specific subunits (alpha1 and beta3) may be one mechanism underlying benzodiazepine anticonvulsant tolerance.

A novel serine kinase with specificity for beta3-subunits is tightly associated with GABA(A) receptors

Tuning of gamma-aminobutyric acid type A (GABA(A)) receptor function via phosphorylation of the receptor potentially allows neurons to modulate their inhibitory input. Several kinases, both of the serine-threonine kinase and the tyrosine kinase families, have been proposed as candidates for such a modulatory role in vivo. However, no GABA(A) receptor-phosphorylating kinase physically associated with the receptor has been identified so far on a molecular level. In this study, we demonstrate a GABA(A) receptor-associated protein serine kinase phosphorylating specifically beta3-subunits of native GABA(A) receptors. The characteristics of this novel kinase clearly distinguish it from enzymatic activities that have been shown so far to phosphorylate the GABA(A) receptor. We putatively identify this protein kinase as the previously described GTAP34 (GABA(A) receptor-tubulin complex-associated protein of molecular mass 34 kDa). Using expressed recombinant fusion proteins, we identify serine 408 as a major target of the phosphorylation reaction, whereas serine 407 is not phosphorylated. This demonstrates the high specificity of the kinase. Phosphorylation of serine 408 is known to result in a decreased receptor function. The direct association of this kinase with the receptor indicates an important physiological role.

A significant part of native gamma-aminobutyric AcidA receptors containing alpha4 subunits do not contain gamma or delta subunits

Using a novel antibody directed against the alpha4 subunit of gamma-aminobutyric acidA (GABAA) receptors, 5% of all [3H]muscimol but only about 2% of all [3H]Ro15-4513 binding sites present in brain membrane extracts could be precipitated. This indicated that part of the alpha4 receptors containing [3H]muscimol binding sites did not contain [3H]Ro15-4513 binding sites. Immunoaffinity purification and Western blot analysis of alpha4 receptors demonstrated that not only alpha1, alpha2, alpha3, beta1, beta2, and beta3 subunits but also gamma1, gamma2, gamma3, and delta subunits can be colocalized with alpha4 subunits in native GABAA receptors. Quantification experiments, however, indicated that only 7, 33, 4, or 7% of all alpha4 receptors contained gamma1, gamma2, gamma3, or delta subunits, respectively. These data not only explain the low percentage of [3H]Ro15-4513 binding sites precipitated by the anti-alpha4 antibody but also indicate that approximately 50% of the alpha4 receptors did not contain gamma1, gamma2, gamma3, or delta subunits. These receptors, thus, either are composed of alpha4 and beta1-3 subunits only, or additionally contain epsilon, pi, or so far unidentified GABAA receptor subunits.

EDPC: a novel high affinity ligand for the benzodiazepine site on rat GABA(A) receptors

Rat recombinant alpha1beta2gamma2 gamma-aminobutyric acid type A (GABAA) receptors were functionally expressed in Xenopus laevis oocytes and analyzed for the action of EDPC (Ethyl 3-(1,3-dithian-2-yl)-1H-pyrrolo[2,3-c]pyridine-5-carboxylate) using electrophysiological techniques. EDPC inhibited GABA currents at low concentrations (IC50 approximately/= 2 nM). The inhibition by 100 nM EDPC could be reversed by 1 microM of the benzodiazepine antagonistflumazenil (Ro 15-1788), indicating a negative allosteric modulation via the benzodiazepine binding site. In line with this conclusion are radioactive ligand binding studies. EDPC inhibited the binding of 2 nM [3H]flunitrazepam to membranes from the cerebellum or the cortex with IC50 values of about 8 and 25 nM, respectively.

GABAA receptor subunit composition and functional properties of Cl- channels with differential sensitivity to zolpidem in embryonic rat hippocampal cells

Using flow cytometry in conjunction with a voltage-sensitive fluorescent indicator dye (oxonol), we have identified and separated embryonic hippocampal cells according to the sensitivity of their functionally expressed GABAA receptors to zolpidem. Immunocytochemical and RT-PCR analysis of sorted zolpidem-sensitive (ZS) and zolpidem-insensitive (ZI) subpopulations identified ZS cells as postmitotic, differentiating neurons expressing alpha2, alpha4, alpha5, beta1, beta2, beta3, gamma1, gamma2, and gamma3 GABAA receptor subunits, whereas the ZI cells were neuroepithelial cells or newly postmitotic neurons, expressing predominantly alpha4, alpha5, beta1, and gamma2 subunits. Fluctuation analyses of macroscopic Cl- currents evoked by GABA revealed three kinetic components of GABAA receptor/Cl- channel activity in both subpopulations. We focused our study on ZI cells, which exhibited a limited number of subunits and functional channels, to directly correlate subunit composition with channel properties. Biophysical analyses of GABA-activated Cl- currents in ZI cells revealed two types of receptor-coupled channel properties: one comprising short-lasting openings, high affinity for GABA, and low sensitivity to diazepam, and the other with long-lasting openings, low affinity for GABA, and high sensitivity to diazepam. Both types of channel activity were found in the same cell. Channel kinetics were well modeled by fitting dwell time distributions to biliganded activation and included two open and five closed states. We propose that short- and long-lasting openings correspond to GABAA receptor/Cl- channels containing alpha4beta1gamma2 and alpha5beta1gamma2 subunits, respectively.

Identification of subunits mediating clustering of GABA(A) receptors by rapsyn

Human embryonic kidney 293 cells transfected with alpha1beta1gamma2, alpha1beta2gamma2, alpha1beta3gamma2, alpha1beta1, alpha1beta2, alpha1beta3, beta3gamma2, or beta3 subunits formed gamma-aminobutyric acidA receptors on the cell surface that could be clustered by rapsyn. In contrast, alpha1, beta1, beta2, or gamma2 subunits, or alpha1gamma2 subunit combinations could not be detected on the surface of transfected cells and could not be clustered by rapsyn. Experiments investigating the ability of rapsyn to cluster chimeras consisting of the N-terminus of the beta3 subunit and the remaining part of the alpha1, beta2 or gamma2 subunits indicated that the intracellular domains of beta1, beta2, beta3 or gamma2 subunits, but not those of alpha1 subunits are able to form sites mediating clustering by rapsyn. These results demonstrate that rapsyn has the potential to cluster the majority of GABA(A) receptor subtypes via beta or gamma2 subunits. Further experiments will have to clarify the physiological importance of this observation.

Alterations in the expression of GABAA receptor subunits in cerebellar granule cells after the disruption of the alpha6 subunit gene

Any given subunit of the heteromultimeric type-A gamma-aminobutyric acid (GABA) GABAA receptor may be present in several receptor subtypes expressed by individual neurons. Changes in the expression of a subunit may result in differential changes in the expression of other subunits depending on the subunit composition of the receptor subtype, leading to alterations in neuronal responsiveness to GABA. We used the targeted disruption of the alpha6 subunit gene to test for changes in the expression of other GABAA receptor subunits. Immunoprecipitation and ligand binding experiments indicated that GABAA receptors were reduced by approximately 50% in the cerebellum of alpha6 -/- mice. Western blot experiments indicated that the alpha6 subunit protein completely disappeared from the cerebellum of alpha6 -/- mice, which resulted in the disappearance of the delta subunit from the plasma membrane of granule cells. The amount of beta2, beta3 and gamma2 subunits was reduced by approximately 50%, 20% and 40%, respectively, in the cerebella of alpha6 -/- mice. A comparison of the reduction in the level of alpha1, beta2, beta3, gamma2, or delta-subunit-containing receptors in alpha6 -/- cerebellum with those observed after removal of alpha6-subunit-containing receptors from the cerebella of alpha6 +/+ mice by immuno-affinity chromatography demonstrated the presence of a significantly higher than expected proportion of receptors containing beta3 subunits in alpha6 -/- mice. The receptors containing alpha1, beta2, beta3 and gamma2 subunits were present in the plasma membrane of granule cells of alpha6 -/- mice at both synaptic and extrasynaptic sites, as shown by electron microscopic immunocytochemistry. Despite the changes, the alpha1 subunit content of Golgi-cell-to-granule-cell synapses in alpha6 -/- animals remained unaltered, as did the frequency of alpha1 immunopositive synapses in the glomeruli. Furthermore, no change was apparent in the expression of the alpha1, beta2 and gamma2 subunits in Purkinje cells and interneurons of the molecular layer. These results demonstrate that in alpha6 -/- mice, the cerebellum expresses only half of the number of GABAA receptors present in wild-type animals. Since these animals have no gross motor deficits, synaptic integration in granule cells is apparently maintained by alpha1-subunit-containing receptors with an altered overall subunit composition, and/or by changes in the expression of other ligand and voltage gated channels.

Structure and subunit composition of GABA(A) receptors

GABA(A) receptors are the major inhibitory neurotransmitter receptors in the brain and are the site of action of many clinically important drugs. These receptors are composed of five subunits that can belong to eight different subunit classes. If all GABA(A) receptor subunits could randomly combine with each other, an extremely large number of GABA(A) receptor subtypes with distinct subunit composition and arrangement would be formed. Depending on their subunit composition, these receptors would exhibit distinct pharmacological and electrophysiological properties. Recent evidence, however, indicates that not all subunits can assemble efficiently with each other and form functional homo- or hetero-oligomeric receptors. In addition, the efficiency of formation of hetero-oligomeric assembly intermediates determines the subunit stoichiometry and subunit arrangement for each receptor and thus further reduces the possible heterogeneity of GABA(A) receptors in the brain. Studies investigating the subunit composition of native GABA(A) receptors support this conclusion, but also indicate that receptors composed of one, two, three, four, or five different subunits might exist in the brain. Using a recently established immunodepletion technique, the subunit composition and quantitative importance of native GABA(A) receptor subtypes can be determined. This information, together with studies on the regional, cellular and subcellular distribution of these receptor subtypes, will be the basis for a rational development of drugs that specifically affect the GABAergic system.

Clusters of GABAA receptors on cultured hippocampal cells correlate only partially with functional synapses

We describe a method to label gamma-aminobutyric acid (GABA)A receptors on the surface of living hippocampal neurons in primary culture, and we compare the distribution of receptors with that of active synapses. To visualize GABAA receptors, the affinity-purified antibody beta3(1-13), recognizing the extracellular N-termini of the GABAA receptor beta2- and beta3-subunits, was used in combination with fluorescent secondary antibodies. The beta2- and beta3-subunits belong to the predominant GABAA receptor subunits in the hippocampus. As expected for aggregates of GABAA receptors in the somato-dendritic plasma membrane, a patchy staining pattern similar to that seen by labelling neurons after fixation was obtained. An antiserum recognizing an intracellular epitope of GABAA receptor beta3-subunits did not label the receptors in living neurons. Whole-cell recordings of GABA-evoked Cl - currents were not affected after decorating GABAA receptors with antibody beta3(1-13). Combining the staining of GABAA receptors with the labelling of active presynaptic terminals with the fluorescent dyes FM1-43 or FM4-64, consistently resulted in the detection of GABAA receptor clusters that were not located at active synapses. These amounted to approximately 50% of all labelled GABAA receptor clusters. GABAA receptor clusters that were not associated with active presynaptic terminals partially colocalized with the synaptic vesicle marker protein sv2, while another fraction had no presynaptic counterpart at all. These findings suggest the presence of presynaptically silent GABAergic synapses in cultured hippocampal neurons. They also indicate that for the maintenance of GABAA receptor aggregates, the release of GABA from an opposing active terminal is not essential.

Behavioral effects of tryptophan depletion in seasonal affective disorder associated with the serotonin transporter gene?

There is some evidence that the neurotransmitter serotonin (5-hydroxytryptamine; 5-HT) may be involved in the pathogenesis of seasonal affective disorder (SAD). Short-term tryptophan (TRP) depletion was carried out in 18 drug-free remitted patients who met DSM-IV criteria for SAD. Behavioral effects were measured with the Hamilton Depression Rating Scale (HDRS) both 24 h before and 24 h after TRP depletion. Some of the patients showed behavioral responses such as lowered mood, feelings of guilt, loss of interest, agitation, loss of energy, fatigue, social withdrawal, increased appetite, and carbohydrate craving. It was the aim of our study to investigate whether the genotypes of the serotonin transporter gene were associated with symptoms of transient depressive relapse after TRP depletion. In addition, we matched the SAD patients with healthy control subjects to see if alleles and genotypes of the serotonin transporter gene were associated with SAD. High molecular weight DNA was isolated from peripheral blood leukocytes using standard methods. For the 5-HTT receptor gene, a 17-bp repetitive element of intron 2 was genotyped (variable number tandem repeat, VNTR). Alterations in HDRS scores after TRP depletion showed no significant association with alleles or genotypes of the 5-HTT gene, although heterozygotes showed a trend toward increased HDRS scores. The serotonin transporter is known to play a critical role in the termination of serotonergic neurotransmission by sodium-dependent uptake of 5-HT into the presynaptic neuron. The present study in a small group of SAD patients was unable to demonstrate that the 5-HTT gene plays a role in the pathogenesis of SAD or in short-term depressive relapse after TRP depletion.

Special relationship of gamma-aminobutyric acid to the ventromedial nucleus of the hypothalamus during embryonic development

The ventromedial nucleus of the hypothalamus (VMH) is a key nucleus for regulating homeostatic, neuroendocrine, and behavioral functions. We conducted immunocytochemical analyses by using antisera directed against gamma-aminobutyric acid (GABA), its synthetic enzyme glutamic acid decarboxylase (GAD67), GABA-A receptor subunits (alpha2, beta3, epsilon), estrogen receptor-alpha, and Neuropeptide Y (NPY) in the region of the VMH in embryonic mice to identify potential patterning elements for VMH formation. Cells and fibers containing GABA and GAD67 encircled the primordial VMH as early as embryonic day 13 (E13) when the cytoarchitecture of the VMH was not recognizable by Nissl stain. At E16-17 the cytoarchitecture of the VMH became recognizable by Nissl stain as GABAergic fibers invaded the nucleus, continued postnatally, and by adulthood the density of GABAergic fibers was greater inside than outside the VMH. GABA-A receptor subunit expression (beta3 by E13 and alpha2 by E15) within the primordial VMH suggested potential sensitivity to the surrounding GABA signal. Brain slices were used to test whether fibers from distal or proximal sites influenced VMH development. Coronal Vibratome slices were prepared and maintained in vitro for 0-3 days. Nissl stain analyses showed a uniform distribution of cells in the region of the VMH on the day of plating (E15). After 3 days in vitro, cellular aggregation suggesting VMH formation was seen. Nuclear formation in vitro suggests that key factors resided locally within the coronal plane of the slices. It is suggested that either GABA intrinsic to the region nearby the VMH directly influences the development and organization of the VMH, or along with other markers provides an early indicator of pattern determination that precedes the cellular organization of the VMH.

Genetic polymorphisms for drug metabolism (CYP2D6) and tardive dyskinesia in schizophrenia

In the present study, the occurrence of tardive dyskinesia (TD) in chronic schizophrenia patients was investigated in relation to pharmacogenetic polymorphisms. It is known that the metabolism of important neuroleptic drugs is influenced by polymorphisms of the CYP2D6 gene, which encodes the cytochrome P450 enzyme debrisoquine/spartein hydroxylase. Forty-five patients meeting the DSM IV criteria for schizophrenia, chronic course, were recruited. The patients were examined for the mutations CYP2D6*3, CYP2D6*4 and CYP2D6*5. The CYP2D6 genotype distribution in the patient group did not differ from that in healthy Caucasian populations. Tardive dyskinesia was found in 26 patients (57.8%). When comparing patients without CYP2D6 mutations with patients heterozygous for one mutation, we found a higher incidence of TD in the latter (81.3% vs. 46.4%, p = 0.031, multiple regression analysis), which demonstrates a significant influence of the CYP2D6 genotype of the manifestation of TD. As slight differences in the metabolism of drugs in patients heterozygous for CYP2D6 mutations and patients without such mutations are known, we conclude that heterozygous carriers of 2D6 mutated alleles may show an increased susceptibility to developing TD.

GABA(A) receptor alpha1, alpha4, and beta3 subunit mRNA and protein expression in the frontal cortex of human alcoholics

Animal studies have shown that chronic ethanol consumption produces physical dependence upon ethanol and alters gamma-aminobutyric acid-A (GABA(A)) receptor subunit gene expression in brain. Although extensive investigation has been conducted in animal models, relatively little work has been performed directly on human alcoholic brain tissue to determine if there are alterations in GABA(A) receptor gene expression. In this study, GABA(A) receptor alpha1, alpha4, and beta3 subunit mRNA and peptide expression in postmortem frontal cortex from human alcoholics (n = 15) and age- and sex-matched controls (n = 13) were measured by quantitative, competitive reverse transcription polymerase chain reaction and Western blot analysis. GABA(A) receptor beta3 subunit mRNA expression was 35% greater (p < 0.05) in alcoholics, compared with nonalcoholic controls. We found no significant difference in alpha1 and alpha4 subunit mRNA levels between groups. However, there was a trend toward greater (21%) alpha1 subunit mRNA expression. There was no difference in alpha1, alpha4, or beta2/3 subunit peptide levels in frontal cortex between controls and alcoholics. Neither the age of the subjects nor the postmortem interval correlated with mRNA or peptide levels. Blood ethanol content also did not correlate with mRNA or peptide expression in alcoholic samples. These data suggest that GABA(A) receptor adaptations, resulting from prolonged alcohol consumption in human alcoholics, may differ from animal models of alcohol dependence. These differences may be related to the longevity of alcohol exposure in human alcoholics, as well as variability in the dependence/withdrawal state of the human subjects. Therefore, further studies in human postmortem brain tissue are warranted.

Initially expressed early rat embryonic GABA(A) receptor Cl- ion channels exhibit heterogeneous channel properties

We have studied the earliest expression of GABA-induced CI- channels in the rat embryonic dorsal spinal cord (DSC) using in situ hybridization, immunocytochemistry, flow cytometry and electrophysiology. At embryonic day 13 (E13) cells in the dorsal region are still proliferating. In situ hybridization consistently showed transcripts encoding only three GABAA receptor subunits (alpha4, beta1 and gammal); immunocytochemistry both in tissue sections and in acutely isolated cells in suspension demonstrated the expression of the corresponding proteins and also revealed staining for other subunits (alpha2, alpha3, beta3, gamma2). In patch-recordings performed in cells acutely isolated from the dorsal cord, responses to GABA were detected in 356 out of 889 cells. GABA-evoked responses, which often displayed the opening of a few channels, were mediated by CI- ions, were inhibited by bicuculline and picrotoxin, and potentiated by benzodiazepines. Taken together, these observations indicate that CI- channels likely involve GABAA type receptors. Fluctuation analysis revealed channel kinetics consisting of three exponential components (Ts: approximately 1,9 and 90 ms) and a wide variety of inferred unitary conductance values, ranging between 4 and 40 pS. A comparison of these results with observations in other, later embryonic cell types and recombinant receptors suggests that most of the earliest E13 DSC GABAA receptors may include alpha3 subunit. These GABAA receptor Cl- channels may be activated physiologically as both GABA synthesizing enzymes and GABA are present in the E13 dorsal cord.

Subunit composition and quantitative importance of hetero-oligomeric receptors: GABAA receptors containing alpha6 subunits

In cerebellum, GABAA receptors containing alpha6 subunits are expressed exclusively in granule cells. The number of alpha6 receptor subtypes formed in these cells and their subunit composition presently are not known. Immunoaffinity chromatography on alpha6 subunit-specific antibodies indicated that 45% of GABAA receptors in cerebellar extracts contained alpha6 subunits. Western blot analysis demonstrated that alpha1, beta1, beta2, beta3, gamma2, and delta subunits co-purified with alpha6 subunits, suggesting the existence of multiple alpha6 receptor subtypes. These subtypes were identified using a new method based on the one-by-one immunochromatographic elimination of receptors containing the co-purifying subunits in parallel or subsequent experiments. By quantification and Western blot analysis of alpha6 receptors remaining in the extract, the proportion of alpha6 receptors containing the eliminated subunit could be calculated and the subunit composition of the remaining receptors could be determined. Results obtained indicated that alpha6 receptors in cerebellum are composed predominantly of alpha6betaxgamma2 (32%), alpha1alpha6betaxgamma2 (37%), alpha6betaxdelta (14%), or alpha1alpha6betaxdelta (15%) subunits. Other experiments indicated that 10%, 51%, or 21% of alpha6 receptors contained homogeneous beta1, beta2, or beta3 subunits, respectively, whereas two different beta subunits were present in 18% of all alpha6 receptors. The method presented can be used to resolve the total number, subunit composition, and abundancy of GABAA receptor subtypes in the brain and can also be applied to the investigation of other hetero-oligomeric receptors.

Differential regulation of GABA(A) receptor gene expression by ethanol in the rat hippocampus versus cerebral cortex

Previous research has shown that chronic ethanol consumption dramatically alters GABA(A) receptor alpha1 and alpha4 subunit gene expression in the cerebral cortex and GABA(A) receptor alpha1 and alpha6 subunit gene expression in the cerebellum. However, it is not yet known if chronic ethanol consumption produces similar alterations in GABA(A) receptor gene expression in other brain regions. One brain region of interest is the hippocampus because it has recently been shown that a subset of GABA(A) receptors in the hippocampus is responsive to pharmacologically relevant concentrations of ethanol. Therefore, we directly compared the effects of chronic ethanol consumption on GABA(A) receptor subunit gene expression in the hippocampus and cerebral cortex. Furthermore, we investigated whether the duration of ethanol consumption (14 or 40 days) would influence regulation of GABA(A) receptor gene expression in these two brain regions. Chronic ethanol consumption produced a significant increase in the level of GABA(A) receptor alpha4 subunit peptide in the hippocampus following 40 days but not 14 days. The relative expression of hippocampal GABA(A) receptor alpha1, alpha2, alpha3, beta(2/3), or gamma2 was not altered by either period of chronic ethanol exposure. In marked contrast, chronic ethanol consumption for 40 days significantly increased the relative expression of cerebral cortical GABA(A) receptor alpha4 subunits and significantly decreased the relative expression of cerebral cortical GABA(A) receptor alpha1 subunits. This finding is consistent with previous results following 14 days of chronic ethanol consumption. Hence, chronic ethanol consumption alters GABA(A) receptor gene expression in the hippocampus but in a different manner from that in either the cerebral cortex or the cerebellum. Furthermore, these alterations are dependent on the duration of ethanol exposure.

Segregation of different GABAA receptors to synaptic and extrasynaptic membranes of cerebellar granule cells

Two types of GABAA receptor-mediated inhibition (phasic and tonic) have been described in cerebellar granule cells, although these cells receive GABAergic input only from a single cell type, the Golgi cell. In adult rats, granule cells express six GABAA receptor subunits abundantly (alpha1, alpha6, beta2, beta3, gamma2, and delta), which are coassembled into at least four to six distinct GABAA receptor subtypes. We tested whether a differential distribution of GABAA receptors on the surface of granule cells could play a role in the different forms of inhibition, assuming that phasic inhibition originates from the activation of synaptic receptors, whereas tonic inhibition is provided mainly by extrasynaptic receptors. The alpha1, alpha6, beta2/3, and gamma2 subunits have been found by immunogold localizations to be concentrated in GABAergic Golgi synapses and also are present in the extrasynaptic membrane at a lower concentration. In contrast, immunoparticles for the delta subunit could not be detected in synaptic junctions, although they were abundantly present in the extrasynaptic dendritic and somatic membranes. Gold particles for the alpha6, gamma2, and beta2/3, but not the alpha1 and delta, subunits also were concentrated in some glutamatergic mossy fiber synapses, where their colocalization with AMPA-type glutamate receptors was demonstrated. The exclusive extrasynaptic presence of the delta subunit-containing receptors, together with their kinetic properties, suggests that tonic inhibition could be mediated mainly by extrasynaptic alpha6beta2/3delta receptors, whereas phasic inhibition is attributable to the activation of synaptic alpha1beta2/3gamma2, alpha6beta2/3gamma2, and alpha1alpha6beta2/3gamma2 receptors.

CYP2D6 genotype and phenotyping by determination of dextromethorphan and metabolites in serum of healthy controls and of patients under psychotropic medication

Fourteen drug free healthy volunteers and 22 psychiatric patients under psychotropic medication were phenotyped for their individual CYP2D6 activity using dextromethorphan as a probe drug. A solution containing 20 mg dextromethorphan was administered and blood was taken 60 min later for determination of dextromethorphan and metabolites in serum. For comparison, urine was collected over 8 h after ingestion of 20 mg dextromethorphan in a separate test. The CYP2D6 phenotype was determined from the ratio of dextromethorphan to dextrorphan. For genotyping, mutant alleles of the CYP2D6 gene were identified using allele-specific polymerase chain reactions. Genotyping revealed five poor metabolizers of CYP2D6. The others were extensive metabolizers. The ratio of dextromethorphan to dextrorphan ranged from 0.01-2.53 in serum and from 0.0007-4.252 in urine. Probit analysis of serum ratios revealed a bimodal distribution with an antimode at 0.126. According to this antimode, control subjects exhibited identical phenotypes and genotypes, whereas patients under paroxetine, moclobemide or metoprolol who had been genotyped as extensive metabolizers were poor metabolizer phenotypes. Administration of tricyclic antidepressants did not change the CYP2D6 phenotype. The serum assay was more rapid and more accurate than the standard urine approach. Therefore the determination of dextromethorphan and metabolites in serum could be advantageous to measure individual CYP2D6 activities in vivo and thus optimize the dosing of drugs metabolized by CYP2D6.

GABA(A) receptor subunits in the rat hippocampus III: altered messenger RNA expression in kainic acid-induced epilepsy

Kainic acid-induced seizures in rats represent an established animal model for human temporal lobe epilepsy. The neuropathological sequelae include acute status epilepticus followed by neurodegeneration in the CA1 and CA3 sector of the Ammon's horn and of interneurons in the hilus of the dentate gyrus. After about three weeks spontaneous recurrent seizures become manifest. We investigated changes in messenger RNA expression of 13 GABA(A) receptor subunits in the hippocampus of rats in the initial phase (6 h, 12 h and 24 h) after acute kainic acid-induced status epilepticus and seizure-related neuronal cell damage during and after acquisition of spontaneous recurrent seizures (seven and 30 days after kainic acid injection). In the granule cell layer, initial (after 6 to 12 h) decreases in (alpha2, alpha3, alpha5, beta1, beta3, gamma2 and delta messenger RNAs (by about 25 to 50%) were accompanied by increases (by about 50%) in alpha1, alpha4, and beta2 messages. At later intervals (after seven to 30 days), expression of alpha2, alpha4, beta3 and gamma2 messenger RNAs recovered to control values, with alpha5 and delta messenger RNA still being reduced (by 15 and 40% below control levels, respectively). Concentrations of the transcripts encoding for alpha1, alpha3, beta1, beta2, became markedly enhanced (between 20 and 50% of controls). Within the pyramidal cell layers CA1 and CA3, decreases in alpha2, alpha4, alpha5, beta(1-3) and gamma2 messenger RNAs were detected after seven to 30 days, reflecting pronounced neurodegeneration in these areas. The alpha1 transcript was decreased in CA3 after 24 h and increased to control levels indicating compensatory up-regulation of this message after seven days. Messenger RNAs encoding for alpha3-, gamma1-, and gamma3-subunits were detected at rather low levels, alpha6 was not present in the hippocampus. Our data suggest a fast but transient change in the expression of messenger RNAs encoding for different subunits of the GABA(A) receptor in the granule cell layer of the dentate gyrus. This is followed by a lasting augmentation of messenger RNAs encoding different GABA(A) receptor subunits in the same cell layer indicating long-lasting GABAergic inhibition. Changes within the pyramidal cell layer are mostly determined by concomitant neurodegenerative processes.

GABA(A) receptor subunits in the rat hippocampus I: immunocytochemical distribution of 13 subunits

The GABA(A) receptor is a ligand-operated chloride channel. It has a pentameric structure. In mammalian brain different subunits are recruited from four gene subfamilies. Using immunocytochemistry, we investigated the distribution of the 13 GABA(A) receptor subunits in the hippocampus of the rat. GABA(A) receptor subunits were heterogeneously distributed within different hippocampal subfields. High concentrations of alpha1-, alpha2-, alpha4-, beta3-, gamma2- and delta-immunoreactivities were observed within the molecular layer of the dentate gyrus, representing the dendritic area of the granule cells. In the hippocampus proper, the predominant GABA(A) receptor subunits were alpha1, alpha2, alpha5, beta3 and gamma2 that were located throughout the strata radiatum and oriens of CA1 to CA3. Immunocytochemical staining was there less prominent for alpha4-, beta1-, beta2- gamma3- and delta- subunits. In the hippocampus proper, the beta1 subunit was preferentially located in CA2. The alpha4- and delta-subunits were somewhat more abundant in CA1 than in CA3. Numerous local circuit neurons in the hippocampus proper and the hilus of the dentate gyrus contained alpha1-, beta2-, gamma2- and/or delta-subunits. Alpha3 and gamma1 were present only in minute amounts and no alpha6-IR was detected in the hippocampal formation. The distribution of the GABA(A) receptor subunits indicates the existence of heterogenously constituted GABA(A) receptor complexes within various hippocampal subfields, which may exert different physiological or pharmacological properties upon stimulation by GABA or its agonists.

GABA(A) receptor subunits in the rat hippocampus II: altered distribution in kainic acid-induced temporal lobe epilepsy

Intraperitoneal injection of kainic acid in the rat represents a widely used animal model of human temporal lobe epilepsy. Injection of kainic acid induces acute limbic seizures which are accompanied by seizure-induced brain damage and late spontaneous recurrent seizures. There is considerable evidence for an altered transmission of GABA in human temporal lobe epilepsy and in the kainic acid model. We therefore investigated by immunocytochemistry the distribution of 13 GABA receptor subunits in the hippocampus of rats 12 h, 24 h, and two, seven and 30 days after injection of kainic acid. Within the molecular layer of the dentate gyrus, decreases in alpha2- and delta- and slight increases in alpha1, beta2- and beta3-immunoreactivities were observed at early intervals (12 to 24 h) after kainic acid injection. These changes were succeeded by marked increases in alpha1-, alpha2-, alpha4-, alpha5-, beta1-, beta3-, gamma2- and delta-immunoreactivities in the same area after seven to 30 days. Within the hippocampus proper, changes in expression of GABA(A) receptor subunits were demarcated by considerable neurodegeneration of CA1 and CA3 pyramidal neurons. All subunits present within dendritic areas of CA1 and CA3 were affected. These were alpha1, alpha2, alpha5, beta1-beta3, gamma2 and alpha4 (present only in CA1). Decreases in these subunits were followed by increased expression of alpha2-, alpha5-, beta3-, gamma2- and delta-subunits in the hippocampus proper notably in CA3 at later intervals (up to 30 days). Alpha1-, beta2-, gamma2- and delta-subunits were found in presumed GABA containing interneurons throughout the hippocampus. Their immunoreactivity was augmented after two to seven days. Some alpha4-, gamma3- and delta-immunoreactivity was also found in astrocytes 48 h after kainic acid injection. Our data indicate an impairment of GABA-mediated neurotransmission due to a lasting loss of GABA(A) receptor containing cells after kainic acid-induced seizures. The seizure-induced loss in GABA(A) receptors within the hippocampus may in part be compensated by increased expression of GABA(A) receptor subunits within the molecular layer of the dentate gyrus and in pyramidal cells.

Bidirectional alterations of GABA(A) receptor subunit peptide levels in rat cortex during chronic ethanol consumption and withdrawal

The pharmacological properties of gamma-aminobutyric acidA (GABA(A)) receptors are altered by prolonged exposure to ethanol both in vivo and in vitro. We have shown previously that prolonged ethanol exposure elicits selective alterations in various GABA(A) receptor subunit mRNA levels in rat cerebral cortex. Some of these effects are rapidly reversed during ethanol withdrawal. The present study was conducted to determine the effects of prolonged ethanol exposure (dependence) and ethanol withdrawal on cerebral cortical peptide expression for several subunits. GABA(A) receptor alpha1 subunit peptide levels were decreased by nearly 40%, whereas alpha4 subunit peptide levels were increased by 27% in both ethanol-dependent and withdrawn rats. These changes correlate well with observed alterations in mRNA levels following prolonged ethanol exposure in dependent rats, but do not match the effects on mRNA levels during ethanol withdrawal. Beta2/3 subunit peptide levels increased by approximately 32% in both ethanol-dependent rats and rats undergoing ethanol withdrawal. We observed a 30-60% increase in gamma1 subunit peptide levels in both dependent rats and those undergoing withdrawal, also correlating with the previous report on ethanol-induced alterations in mRNA levels. Peptide levels for gamma2 subunits did not differ from control values in either condition. These findings show that specific alterations in GABA(A) receptor subunit peptide levels are associated with ethanol dependence in rats. GABA(A) receptor subunit peptide expression is more stable than mRNA expression, and mRNA levels are not representative of peptide expression during ethanol withdrawal. These findings are consistent with the suggestion that alterations in GABA(A) receptor gene expression underlie the functional properties of GABA(A) receptors in ethanol-dependent rats and those undergoing ethanol withdrawal.

Stoichiometry and assembly of a recombinant GABAA receptor subtype

GABAA receptors are ligand-gated chloride ion channels that are presumed to be pentamers composed of alpha, beta, and gamma subunits. The subunit stoichiometry, however, is controversial, and the subunit arrangement presently is not known. In this study the ratio of subunits in recombinant alpha1beta3gamma2 receptors was determined in Western blots from the relative signal intensities of antibodies directed against the N terminus or the cytoplasmic loop of different subunits after the relative reactivity of these antibodies had been determined with GABAA receptor subunit chimeras composed of the N-terminal domain of one and the remaining part of the other subunit. Via this method a subunit stoichiometry of two alpha subunits, two beta subunits, and one gamma subunit was derived. Similar experiments investigating the composition of alpha1beta3 receptors expressed on the surface of human embryonic kidney (HEK) 293 cells cotransfected with alpha1 and beta3 subunits resulted in a stoichiometry of two alpha and three beta subunits. Density gradient centrifugation studies indicated that combinations of alpha1beta3gamma2 or alpha1beta3 subunits expressed in HEK 293 cells are able to form pentamers, whereas combinations of alpha1gamma2 or beta3gamma2 subunits predominantly form heterodimers. These results provide valuable information on the mechanism of GABAA receptor assembly and support the conclusion that GABAA receptors are pentamers in which a total of four alternating alpha and beta subunits are connected by a gamma subunit.

Anatomical gradients in proliferation and differentiation of embryonic rat CNS accessed by buoyant density fractionation: alpha 3, beta 3 and gamma 2 GABAA receptor subunit co-expression by post-mitotic neocortical neurons correlates directly with cell buoyancy

Development of the CNS occurs as a complex cascade of pre-programmed events involving distinct phases of cell proliferation and differentiation. Here we show these phases correlate with cells of specific buoyant densities which can be readily accessed by density gradient fractionation. Sprague-Dawley dams were pulse-labelled with bromodeoxyuridine (BrdU) and selected regions of embryonic (E) CNS tissues at E11-22 dissociated with papain into single-cell suspensions. Proliferative cell populations were assessed by anti-BrdU and propidium iodide staining using flow cytometry. Cell differentiation was evaluated using molecular and immunocytochemical probes against mRNAs and antigens differentiating the neuroepithelial, neuronal and glial cell lineages. The results show the emergence of distinctive spatiotemporal changes in BrdU+ populations throughout the CNS during embryonic development, which were followed by corresponding changes in the cellular distributions of antigens distinguishing specific cell types. Fractionation of neocortical cells using discontinuous Percoll gradients revealed that an increasing number of cells increase their buoyancy during corticogenesis. Immunocytochemical and molecular characterization showed that the proliferative and progenitor cell populations are for the most part associated with lower buoyancy or higher specific buoyant densities (> 1.056 g/ml) whereas the post-mitotic, differentiated neurons generally separated into fractions of higher buoyancy or lower specific buoyant densities (< 1.043 g/ml). Immunostaining with antibodies against several GABAA receptor subunits (alpha 3, beta 3, gamma 2) revealed that the highest percent (70-90%) of immunopositive cells could be identified in the most buoyant, differentiating neurons found in the cortical plate/subplate regions, with the lowest percent of the immunopositive cells found in the least buoyant, proliferative and progenitor cell populations originating from the ventricular/subventricular zones. Taken together, these results indicate that buoyant density is a distinguishing characteristic of embryonic CNS cells transforming from primarily proliferative to mainly differentiating, and that fractionation of these cells according to their buoyant densities provides rapid access to the properties of specific cell lineages during the prenatal period of CNS development.

Ligand-gated ion channel subunit partnerships: GABAA receptor alpha6 subunit gene inactivation inhibits delta subunit expression

Cerebellar granule cells express six GABAA receptor subunits abundantly (alpha1, alpha6, beta2, beta3, gamma2, and delta) and assemble various pentameric receptor subtypes with unknown subunit compositions; however, the rules guiding receptor subunit assembly are unclear. Here, removal of intact alpha6 protein from cerebellar granule cells allowed perturbations in other subunit levels to be studied. Exon 8 of the mouse alpha6 subunit gene was disrupted by homologous recombination. In alpha6 -/- granule cells, the delta subunit was selectively degraded as seen by immunoprecipitation, immunocytochemistry, and immunoblot analysis with delta subunit-specific antibodies. The delta subunit mRNA was present at wild-type levels in the mutant granule cells, indicating a post-translational loss of the delta subunit. These results provide genetic evidence for a specific association between the alpha6 and delta subunits. Because in alpha6 -/- neurons the remaining alpha1, beta2/3, and gamma2 subunits cannot rescue the delta subunit, certain potential subunit combinations may not be found in wild-type cells.

European multicentre evaluation of the analytical performance of the Abbott AxSYM Abused Drugs Assays

In accordance with the guidelines of the European Committee for Clinical Laboratory Standards (ECCLS), the performance of the Abbott AxSYM Abused Drugs assays were evaluated and compared with the results provided by the following systems: Syva Emit d.a.u./Roche Cobas Mira S Plus, Abbott TDx and ADx, Syva Emit d.a.u./Syva ETS Plus, Syva Emit II/Hitachi 717 and Roche Abuscreen OnLine/Roche Cobas Mira S Plus. The test analytes, cannabinoids, cocaine metabolites, opiates, benzodiazepines and barbiturates, were each investigated in three laboratories on different systems. The imprecision of all systems in the series and from day to day was good, with CVs less than 5% or 10%, respectively. The AxSYM calibration curves were stable for 3-4 months and none of the systems displayed any shift in the results of the analyses within one day or any faults caused by sample contamination. Within the framework of this study, a total of 1860 urine samples were investigated; 741 results were positive. All results which remained discrepant between AxSYM and the comparison systems after repeated analysis (n = 17) were subjected to further investigation using a reference method, with the exception of one barbiturate and two benzodiazepine samples. An additional test criterion was the practicability of the systems investigated and the versatility of the software. During this evaluation, the results provided by the Abbott AxSYM were excellent and were fully in line with the manufacturer's claims. The reliability of the FPIA technology that has been the subject of frequent investigation was also convincing during this evaluation. The possibility of semi-quantitative determination, the stability of the calibration curves, the ability to process an emergency sample without delay and its high suitability to routine operations are the convincing benefits offered by this system.

Biperiden and haloperidol plasma levels and extrapyramidal side effects in schizophrenic patients

Anticholinergic drugs such as biperiden are used for the treatment of extrapyramidal side effects (EPS) induced by neuroleptics such as haloperidol. The effects of biperiden and haloperidol plasma levels on EPS were studied in 29 chronically ill schizophrenics. The results show relationships between biperiden dose and biperiden plasma levels (BPL), and between BPL and haloperidol plasma levels (HPL). Neither BPL nor HPL seem to influence EPS.

Differential synaptic localization of two major gamma-aminobutyric acid type A receptor alpha subunits on hippocampal pyramidal cells

Hippocampal pyramidal cells, receiving domain specific GABAergic inputs, express up to 10 different subunits of the gamma-aminobutyric acid type A (GABAA) receptor, but only 3 different subunits are needed to form a functional pentameric channel. We have tested the hypothesis that some subunits are selectively located at subsets of GABAergic synapses. The alpha 1 subunit has been found in most GABAergic synapses on all postsynaptic domains of pyramidal cells. In contrast, the alpha 2 subunit was located only in a subset of synapses on the somata and dendrites, but in most synapses on axon initial segments innervated by axo-axonic cells. The results demonstrate that molecular specialization in the composition of postsynaptic GABAA receptor subunits parallels GABAergic cell specialization in targeting synapses to a specific domain of postsynaptic cortical neurons.

Striatal efferents preferentially innervate neurons in the ventral pallidum containing GABAA receptor alpha 1 subunit-like immunoreactivity

The gamma-aminobutyric acid-A receptor consists of several subunits. In this immunohistochemical study we investigated the regional distribution of the alpha 1 and alpha 2 subunits with subunit-specific antibodies in the ventral pallidum, and compared the staining patterns to those of substance P (SP). alpha 1 subunit antigenic sites were found to be localized to pallidal neurons, varicosities, and varicose fibers. alpha 1 immunopositive fibers mainly appeared "tubulus-like" due to the intense staining of the membranes of the long pallidal dendrites. Double labelling of alpha 1 subunit and substance P revealed that alpha 1 subunit-like immunoreactive (IR) dendrites and somata of the pallidal neurons were often invested by SP-IR striatal efferents. Subcellularly, the dendritic and somatic membranes of pallidal neurons were strongly immunopositive for the alpha 1 subunit, whereas only a few axon terminals exhibited alpha 1-IR. alpha 2-IR was restricted to a low number of ventral pallidal neurons. The distributional patterns obtained for the alpha 1 and alpha 2 subunits suggest that striatal efferent neurons directly influence pallidal neurons displaying a distinct GABAA subunit composition, which may be of pharmacological importance since the alpha 1 beta x gamma 2-subunits containing receptors have mainly a benzodiazepine type I pharmacology.

Affinity of various ligands for GABAA receptors containing alpha 4 beta 3 gamma 2, alpha 4 gamma 2, or alpha 1 beta 3 gamma 2 subunits

The potency of 30 benzodiazepine binding site ligands from 14 different structural classes for inhibition of [3H]Ro 15-4513 (ethyl-8-azido-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4] benzodiazepine-3-carboxylate) binding to human embryonic kidney (HEK) 293 cells transiently transfected with alpha 4 beta 3 gamma 2S or alpha 1 beta 3 gamma 2S subunits of GABAA receptors was investigated. Most of these compounds were unable to significantly inhibit [3H]Ro 15-4513 binding to alpha 4 beta 3 gamma 2S receptors under conditions where they potently inhibited binding to alpha 1 beta 3 gamma 2S receptors. Nevertheless, compounds from four different structural classes were identified which exhibited a high affinity for alpha 4 beta 3 gamma 2S receptors. Variation of the structure of these compounds could lead to new ligands selectively interacting with alpha 4 beta 3 gamma 2S receptors. Compounds interacting with alpha 4 beta 3 gamma 2S receptor were also able to inhibit [3H]Ro 15-4513 binding to receptors consisting of alpha 4 gamma 2S subunits with comparable potency. These results support the conclusion that the alpha subunit is a major determinant of the benzodiazepine binding site properties of GABAA receptors containing alpha and gamma subunits.

Interaction of allosteric ligands with GABAA receptors containing one, two, or three different subunits

The presence of allosteric binding sites on recombinant GABAA receptors formed after transfection of human embryonic kidney (HEK) 293 cells with alpha 1-, beta 3-, or gamma 2-subunits, or with various combinations of these subunits, was systematically investigated. From all possible subunit combinations, high affinity [3H]muscimol binding sites were induced in cells transfected with alpha 1 beta 3- or alpha 1 beta 3 gamma 2-subunits only. GABAA receptor associated [3H]flunitrazepam binding sites were induced in cells after transfection with alpha 1 gamma 2- or alpha 1 beta 3, gamma 2-subunits, and [35S]r-butylbicyclophosphorothionate (TBPS) binding sites were found in cells transfected with beta 3-, beta 3 gamma 2-, alpha 1 beta 3-, or alpha 1 beta 3 gamma 2-subunits. Binding of [35S]TBPS could be inhibited by pentobarbital, etazolate, (+)-etomidate, alphaxalone, propofol, chlormethiazole, and 4'-chlorodiazepam (Ro 5-4864) with a potency which differed in cells transfected with beta 3-, beta 3 gamma 2-, alpha 1 beta 3-, or alpha 1 beta 3 gamma 2-subunits. Results obtained indicate that receptors with different subunit composition actually can be formed in HEK cells and exhibit distinct pharmacological properties.

Colocalization of GABA, glycine, and their receptors at synapses in the rat spinal cord

To determine whether GABA and glycine can act as cotransmitters at synapses in the rat spinal cord, we have compared the ultrastructural distribution of GABAA-receptor beta 3 subunit with that of the glycine receptor-associated protein gephyrin and combined this with postembedding detection of GABA and glycine. We also used a dual-immunofluorescence method to confirm that gephyrin was associated with the glycine-receptor alpha 1 subunit throughout the cord. GABAA beta 3-subunit immunoreactivity was restricted primarily to synapses, and at a majority of these synapses the presynaptic axon was GABA-immunoreactive. Many synapses showed both GABAA beta 3 and gephyrin immunoreactivity, and at most of these synapses GABA and glycine were enriched in the presynaptic axon. These results strongly support the idea that cotransmission by GABA and glycine occurs in the spinal cord.