Structure and distribution of multiple GABAA receptor subunits with special reference to the cerebellum.
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Biomedical subjects
W Wisden
Publications and source records attributed to W Wisden.
[Detection of GABA(A) receptor mRNA in cochlear tissue. An in situ hybridization study].
Gamma-aminobutyric acid (GABA) and the GABAergic system play an important role in the efferent modulation of cochlear function. We examined surface preparations of guinea pig and mouse cochleae by in situ hybridization using radioactive labelled oligonucleotides for several subunits of the GABAA receptor. Frozen sections of rat and guinea pig brain (cortex, hippocampus, cerebellum) served as controls. In the mouse cochlea the mRNA of the alpha-1 and alpha-5, beta-1 and gamma-1 subunit were detected, while in guinea pig cochlea mRNA of the alpha-1, alpha-4, alpha-5, and gamma-1 subunit of the GABAA receptor were found. Positive signals were located in the regions of the outer hair cells and had a weaker intensity in the inner hair cells. In the brain sections the several subunits were detected in a variable distribution in the cerebellum, hippocampus and cortical regions. Rat specimens exhibited stronger signals than guinea pig brain sections. These investigations have extended previous results of immunocytochemical experiments from our laboratory demonstrating mRNA sequences of GABAA receptor subunits in the mammalian inner ear. Detection of these nucleotide sequences using surface preparations of the cochlea on a molecular level by in situ hybridization supports the importance of GABA as a cochlear neurotransmitter. Furthermore, it can be concluded that the mammalian cochlea is able to express a GABA-dependent neurotransmission system.
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Gamma-aminobutyric acidA-receptor messenger ribonucleic acid (alpha-1 subunit) detection by in situ hybridization.
The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) is most likely involved in the efferent cochlear neurotransmission. In situ hybridization (ISH) results in specific annealing of a labelled nucleic acid probe to complementary sequences in fixed tissue and allows subsequent visualization of the location of the probe. We used the ISH technique to localize messenger ribonucleic acid (mRNA) sequences of the alpha-1 subunit of the GABAA receptor with an S-35 labeled oligonucleotide probe. Experiments were performed in rat and guinea pig brain sections and surface tissue preparations of the guinea pig cochlea. Positive signals were obtained for the alpha-1 probe in cortical and hippocampal regions of the rat brain and had weaker expression in the guinea pig brain. Alpha-1 subunit mRNA was localized in Purkinje cells and in stellate and basket cells of the stratum moleculare in the rat and guinea pig cerebellum. In surface tissue preparations of the guinea pig cochlea mRNA sequences of the alpha-1 subunit were detectable with high signal expression. Positive signals were seen on both sides of the tunnel of Corti, predominantly in the region of the outer hair cells. The results indicate expression of GABAA-receptor mRNA in cochlear tissue, supporting the importance of GABAA receptors in cochlear neurotransmission.
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Molecular biology of glutamate receptors.
The ligand-gated receptors for L-glutamate play a central role in acute neuronal degeneration. Recently cDNAs have been isolated for subunits of several glutamate receptor subtypes. By sequence homology all these subunits clearly belong to one large gene family. Several subfamilies exist and match roughly previously pharmacologically and electrophysiologically defined subtypes of glutamate receptors. Currently four genes (GluR A, B, C and D) are known that code for the AMPA subtypes of glutamate receptors. Recombinant expression of wild type and mutated sequences identified a critical residue in the putative TM2 channel-lining segment that controls Ca2+ ion permeability. The arginine (R) found in GluR B subunits at that position renders AMPA channels impermeable for Ca2+ ions, whereas glutamine (Q) containing GluR A, C and D subunits give rise to Ca2+ permeable channels. RNA editing converts the genomically encoded glutamine codon into the arginine codon found in GluR B cDNAs for the Q/R site. NMDA subtypes of glutamate receptors are formed after coexpression of the NR1 cDNA with a cDNA of the NR2 family. Depending on the member of the NR2 family used, NMDA receptors with different kinetical and pharmacological properties are generated. Common to all channels of these NMDA receptors is a high permeability for Ca2+ ions and a voltage dependent block by Mg2+ ions. All currently known NMDA receptor subunits have an asparagine at the Q/R homologous position. We found that this residue is critical for Mg2+ block and Ca2+ permeability of NMDA receptor channels.
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Kainate receptor gene expression in the developing rat brain.
Kainate-preferring receptors are a subclass of ionotropic glutamate receptors that might play a role in brain development. The expression of the five known genes encoding kainate receptor subunits (GluR-5, -6, -7, KA-1, and KA-2) was studied by in situ hybridization during pre- and postnatal development of the rat brain. We compared the combined expression patterns of these genes with autoradiography using 3H-kainate in the developing brain from embryonic day 12 (E12) through to adult. Although mRNAs for the receptor subunits (except KA-1) can be detected at stage E12, 3H-kainic acid binding (as an index of receptor protein) is not found at this stage. However, by E14 high-affinity kainate sites are found throughout the gray matter, but particularly in spinal cord, primordial cerebellum, and ventral forebrain structures. All genes undergo a peak in their expression in the late embryonic/early postnatal period. GluR-5 expression during development shows the most interesting features because the changes are qualitative. The GluR-5 gene shows peaks of expression around the period of birth in the sensory cortex (layers II, III, and IV), in CA1 hippocampal interneurons in the stratum oriens, in the septum, and in the thalamus. GluR-6 shows a prenatal expression peak in the cingulate gyrus of the neocortex. KA-1 transcripts appear with the development of the hippocampus and remain largely confined to discrete areas such as the CA3 region, the dentate gyrus, and subiculum. KA-2 transcripts are found throughout the CNS from as early as E12 and remain constant until adulthood. The GluR-5 and GluR-6 genes are coexpressed in multiple peripheral ganglia (e.g., cranial nerve ganglia, dorsal root ganglia, and mural ganglia) at E14.
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Cloning and characterization of the rat 5-HT5B receptor. Evidence that the 5-HT5B receptor couples to a G protein in mammalian cell membranes.
A gene encoding a novel G protein-coupled 5-hydroxytryptamine (5-HT) receptor, termed 5-HT5B, was cloned. The ligand binding profile of this receptor is distinct from that of other cloned 5-HT receptors. The 5-HT5B receptor couples to a G protein in COS1 cell membranes; however, activation of the 5-HT5B receptor does not appear to alter either cAMP accumulation or phosphoinositide turnover in a variety of fibroblast cell lines. In the rat brain, 5-HT5B gene expression occurs predominantly in the medial habenulae and hippocampal CA1 cells of the adult. Little expression is seen during embryonic development.
Amino Acid Sequence↗
The rat delta-1 and delta-2 subunits extend the excitatory amino acid receptor family.
We have characterized a second member (delta-2) of a new class of subunits for the ligand-gated excitatory amino acid receptor superfamily. The sequence of delta-2 exhibits an average identity of 25% and 18.5% to the non-NMDA and NMDA receptor subunits, respectively. The rat delta-2 gene is expressed predominantly in Purkinje cells of the cerebellum whereas only low levels of delta-1 transcripts are found in the adult brain. However, delta-1 gene expression undergoes a pronounced developmental peak, with particularly high mRNA levels in the caudate putamen of late embryonic/early postnatal stages.
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Mammalian ionotropic glutamate receptors.
Exciting new milestones in glutamate receptor (GluR) channel research include the following: the cloning of N-methyl-D-aspartate (NMDA) receptors; delineation of molecular determinants for ion flow through glutamate-gated channels; the discovery that Ca2+ permeability of non-NMDA receptor channels is determined by RNA editing; the construction of antibodies and their use in immunocytochemical localizations of alpha-amino-3-hydroxy-5-methyl isoxazole-4-propionic acid (AMPA) receptor subunits in the rat brain; and the return to prominence of the high-affinity kainate site with the publication of cDNA sequences for subunits (GluR-5, -6, -7; KA-1, -2) constituting subtypes of this site. Major unresolved issues comprise the transmembrane topology and subunit stoichiometries of native receptor channels.
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The differential expression of 16 NMDA and non-NMDA receptor subunits in the rat spinal cord and in periaqueductal gray.
Diverse arrays of glutamate-gated channels in the spinal cord and associated pathways are partly responsible for sensory input, for altered sensitivity to peripheral stimuli during inflammation, and for generation of motor patterns. The expression of 16 genes, encoding all known subunits for the NMDA receptor (NR1, NR2A to NR2D), AMPA/low-affinity kainate (GluR-A to -D), high-affinity kainate ionotropic receptors (KA-1, -2, GluR-5 to -7) and two orphan receptor subunits (delta-1 and -2) was examined by in situ hybridization in rat lumbar spinal cord, and in the periaqueductal gray. Subunit mRNAs for GluR-A, -B Flip, KA-2, and NR1 were abundant in the dorsal horn, with NR2D lightly expressed. Occasional cells in lamina II contained NR2C mRNA. While the GluR-A gene was preferentially expressed in laminae I and II-outer, GluR-B mRNA was evenly expressed throughout all superficial laminae (I, II-outer, II-inner, and III). Large neurons in laminae IV and V expressed mainly NR1, GluR-C, and to lesser extents the GluR-B, GluR-D, and NR2D genes. Lamina I contained occasional cells expressing the GluR-5 gene, whereas GluR-7 mRNA was present in scattered cells in all superficial laminae. In motor neurons, GluR-B Flip, -C Flip, -D Flip, and NR1 mRNAs were expressed heavily, and those of NR2D and KA-1 weakly. Possibly connected to the RNA editing mechanism, GluR-B was the only subunit whose RNA was concentrated in motor neuron cell nuclei in addition to the cytoplasm. delta-1 and -2 mRNAs were found at low levels throughout the gray matter. NR2A, NR2B, and GluR-6 mRNAs were undetectable. For the periaqueductal gray, prominent mRNAs were GluR-A, -B, and NR1. An en passant observation concerned high levels of NR2C mRNA in the pineal gland.
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A complex mosaic of high-affinity kainate receptors in rat brain.
The significance for CNS function of glutamate-gated cation channels that exhibit high-affinity kainate sites is not understood. Such receptors, which on dorsal root ganglia and in recombinant systems exhibit currents that rapidly desensitize to kainate application, have not been detected electrophysiologically in the brain. However, a comparison of the distribution of mRNAs encoding five glutamate receptor subunits exhibiting high-affinity kainate sites (GluR-5-GluR-7, KA-1, and KA-2) indicates that high-affinity kainate receptors are most likely involved in all central neuronal circuits of the rat brain. The KA-1 mRNA occurs mainly in the CA3 field of the hippocampus and dentate gyrus, with much lower amounts being found in inner cortical layers, cerebellar Purkinje cells, and white matter (e.g., corpus callosum and anterior commissure). The KA-2 gene is widely expressed in many neuronal nuclei including layers II-VI of neocortex, hippocampal pyramidal (CA1-CA3) and dentate granule cells, septal nuclei such as the bed nucleus of the stria terminalis, medial preoptic, suprachiasmatic, and ventral medial hypothalamic nuclei, dorsal raphe, locus coeruleus, and cerebellar granule cells. KA-2 mRNA is also found in the pineal gland. GluR-5 transcripts are in the cingulate and piriform cortex, the subiculum, lateral septal nuclei, anteroventral thalamus, suprachiasmatic nucleus, the tegmental nuclei, pontine nuclei, and Purkinje cells. GluR-6 mRNA is most abundant in cerebellar granule cells, with lower levels in caudate-putamen and the pyramidal cell layers and dentate granule cells of hippocampus. The GluR-7 gene is prominently expressed in the inner neocortical layers and some cells in layer II, subiculum, caudate-putamen, reticular thalamus, ventral medial hypothalamic nucleus, pontine nuclei, and in putative stellate/basket cells in the cerebellum. These findings suggest that a complex mosaic of receptor variants underlies the high-affinity kainate receptor in the vertebrate brain.
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High-affinity kainate and domoate receptors in rat brain.
Mammalian brain expresses receptors which bind the potent neurotoxins, kainate and domoate, with high affinity, and which form a subclass of ionotropic glutamate receptors. A new member of these receptors, expressed in both adult and embryonic CNS is compared in its ligand binding properties to its closely sequence-related homologs.
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Calcium-permeable AMPA-kainate receptors in fusiform cerebellar glial cells.
Glutamate-operated ion channels (GluR channels) of the L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-kainate subtype are found in both neurons and glial cells of the central nervous system. These channels are assembled from the GluR-A, -B, -C, and -D subunits; channels containing a GluR-B subunit show an outwardly rectifying current-voltage relation and low calcium permeability, whereas channels lacking the GluR-B subunit are characterized by a doubly rectifying current-voltage relation and high calcium permeability. Most cell types in the central nervous system coexpress several subunits, including GluR-B. However, Bergmann glia in rat cerebellum do not express GluR-B subunit genes. In a subset of cultured cerebellar glial cells, likely derived from Bergmann glial cells. GluR channels exhibit doubly rectifying current-voltage relations and high calcium permeability, whereas GluR channels of cerebellar neurons have low calcium permeability. Thus, differential expression of the GluR-B subunit gene in neurons and glia is one mechanism by which functional properties of native GluR channels are regulated.
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Glutamate receptor expression in the rat retina.
The expression of five genes (GluR A; B; C; D; GluR 5) encoding functional subunits of glutamate receptors was investigated in the rat retina using in situ hybridization with oligonucleotide probes. All five genes are expressed in the retina. All probes label cell bodies in the ganglion cell layer as well as somata in the inner third of the inner nuclear layer (INL), where the amacrine cells are located. In addition GluR 5, B and D, and to a lesser extent also GluR A are found in the middle and outer part of the INL, where bipolar and horizontal cells reside. Different subsets of retinal neurons may thus use glutamate receptors of different subunit composition.
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The third gamma subunit of the gamma-aminobutyric acid type A receptor family.
Cloned cDNAs encoding a member of the gamma-aminobutyric acid type A receptor gamma-subunit class were isolated from rat-brain-mRNA-derived libraries. The gamma 3 mRNA is present in cortex, claustrum, caudate putamen, and some thalamic nuclei, particularly the medial geniculate nucleus, where it is the predominant gamma-subunit transcript. The gamma 3 gene is expressed at very low levels in cerebellum and hippocampus. In coexpression experiments with the alpha 1 and beta 2 subunits, gamma 3 imparts benzodiazepine binding to gamma-aminobutyric acid type A receptors and forms gamma-aminobutyric acid-gated benzodiazepine-modulated chloride channels that exhibit a larger conductance than alpha 1 beta 2 receptor channels. Furthermore, the presence of gamma 3 in place of gamma 2 in alpha 1 beta 2 gamma x receptors generates a marked decrease in the affinity of agonists while leaving the affinity of antagonists or negative modulators largely unaffected.
Amino Acid Sequence↗
The KA-2 subunit of excitatory amino acid receptors shows widespread expression in brain and forms ion channels with distantly related subunits.
A new ionotropic glutamate receptor subunit termed KA-2, cloned from rat brain cDNA, exhibits high affinity for [3H]kainate (KD approximately 15 nM). KA-2 mRNA is widely expressed in embryonic and adult brain. Homomeric KA-2 expression does not generate agonist-sensitive channels, but currents are observed when KA-2 is coexpressed with GluR5 or GluR6 subunits. Specifically, coexpression of GluR5(R) and KA-2 produces channel activity, whereas homomeric expression of either subunit does not. Currents through heteromeric GluR5(Q)/KA-2 channels show more rapid desensitization and different current-voltage relations when compared with GluR5(Q) currents. GluR6/KA-2 channels are gated by AMPA, which fails to gate homomeric GluR6 receptor channels. These results suggest possible in vivo partnership relations for high affinity kainate receptors.
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GABAA receptor channels: from subunits to functional entities.
GABAA receptor channels mediate postsynaptic inhibition. The functional diversity of these receptors rests on differences in subunit composition and on a large repertoire of subunits. Subunit expression patterns in the brain have been found to predict in vivo compositions of GABAA receptors. In addition, molecular determinants underlying the differential binding properties of allosteric ligands to receptor subtypes have been identified.
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The distribution of 13 GABAA receptor subunit mRNAs in the rat brain. I. Telencephalon, diencephalon, mesencephalon.
The expression patterns of 13 GABAA receptor subunit encoding genes (alpha 1-alpha 6, beta 1-beta 3, gamma 1-gamma 3, delta) were determined in adult rat brain by in situ hybridization. Each mRNA displayed a unique distribution, ranging from ubiquitous (alpha 1 mRNA) to narrowly confined (alpha 6 mRNA was present only in cerebellar granule cells). Some neuronal populations coexpressed large numbers of subunit mRNAs, whereas in others only a few GABAA receptor-specific mRNAs were found. Neocortex, hippocampus, and caudate-putamen displayed complex expression patterns, and these areas probably contain a large diversity of GABAA receptors. In many areas, a consistent coexpression was observed for alpha 1 and beta 2 mRNAs, which often colocalized with gamma 2 mRNA. The alpha 1 beta 2 combination was abundant in olfactory bulb, globus pallidus, inferior colliculus, substantia nigra pars reticulata, globus pallidus, zona incerta, subthalamic nucleus, medial septum, and cerebellum. Colocalization was also apparent for the alpha 2 and beta 3 mRNAs, and these predominated in areas such as amygdala and hypothalamus. The alpha 3 mRNA occurred in layers V and VI of neocortex and in the reticular thalamic nucleus. In much of the forebrain, with the exception of hippocampal pyramidal cells, the alpha 4 and delta transcripts appeared to codistribute. In thalamic nuclei, the only abundant GABAA receptor mRNAs were those of alpha 1, alpha 4, beta 2, and delta. In the medial geniculate thalamic nucleus, alpha 1, alpha 4, beta 2, delta, and gamma 3 mRNAs were the principal GABAA receptor transcripts. The alpha 5 and beta 1 mRNAs generally colocalized and may encode predominantly hippocampal forms of the GABAA receptor. These anatomical observations support the hypothesis that alpha 1 beta 2 gamma 2 receptors are responsible for benzodiazepine I (BZ I) binding, whereas receptors containing alpha 2, alpha 3, and alpha 5 contribute to subtypes of the BZ II site. Based on significant mismatches between alpha 4/delta and gamma mRNAs, we suggest that in vivo, the alpha 4 subunit contributes to GABAA receptors that lack BZ modulation.
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The distribution of 13 GABAA receptor subunit mRNAs in the rat brain. II. Olfactory bulb and cerebellum.
In an effort to determine subunit compositions of in vivo GABAA receptors, the cellular localization of 13 subunit encoding mRNAs (alpha 1-alpha 6, beta 1-beta 3, gamma 2-gamma 3, delta) was determined in the rat olfactory bulb and cerebellum. Cerebellar granule cells expressed significant quantities of alpha 1, alpha 6, beta 2, beta 3, gamma 2, and delta mRNAs. They contained very much lower levels of alpha 4, beta 1, and gamma 3 mRNAs, and the alpha 2, alpha 3, alpha 5, and gamma 1 genes appeared to be silent. Purkinje cells contained only the alpha 1, beta 2, beta 3, and gamma 2 mRNAs. Putative Bergmann glial cells were found to contain the gamma 1 mRNA and possibly the alpha 2 mRNA. In the molecular layer, only the alpha 1, beta 2 and gamma 2 mRNAs were expressed in stellate/basket cells. The alpha 3 probe hybridized weakly to targets in the molecular layer. The inferior olivary nucleus contained significant quantities of alpha 2, alpha 4, and gamma 1 transcripts, with the alpha 1, alpha 3, beta 2, beta 3, and gamma 2 mRNAs also present. In the olfactory bulb, mitral cells were found to express the alpha 1, beta 1, beta 2, beta 3, and gamma 2 mRNAs strongly and the alpha 3 mRNA weakly. Tufted cells contained alpha 1, alpha 3, beta 2, beta 3, and gamma 2 mRNAs and, occasionally, the alpha 2 mRNA. In the internal granule cells the alpha 2, alpha 4, alpha 5, beta 3, and delta mRNAs were all present. Low levels of alpha 3, gamma 1, gamma 2, and gamma 3 mRNAs were also noted in these cells. Periglomerular cells expressed low levels of alpha 2, alpha 3, alpha 4, beta 2, beta 3, gamma 1, gamma 2, and gamma 3 mRNAs. No alpha 6 mRNA was present in the olfactory bulb. Correlations that are general ones from other brain regions are the colocalizations of alpha 1 beta 2, alpha 2 beta 3, and alpha 4 delta mRNAs. In both the olfactory bulb and cerebellum, alpha 1 beta 2 gamma 2 receptor cores are probably employed. The delta-subunit mRNA appears to codistribute with alpha-subunit mRNAs (alpha 4 and alpha 6) associated with GABAA subunits that fail to bind benzodiazepine agonists.
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