Search PubMed⌕ Search

Biomedical subjects

H Monyer

Publications and source records attributed to H Monyer.

At least 37 records · Page 2Linked to original sources
PubMed1996

GABAA receptor activation attenuates excitotoxicity but exacerbates oxygen-glucose deprivation-induced neuronal injury in vitro.

We examined the effects of GABA receptor stimulation on the neuronal death induced by exogenously added excitatory amino acids or combined oxygen-glucose deprivation in mouse cortical cell cultures. Death induced by exposure to NMDA, AMPA, or kainate was attenuated by addition of GABA or the GABAA receptor agonist, muscimol, but not by the GABAB receptor agonist, baclofen. The antiexcitotoxic effect of GABAA receptor agonists was blocked by bicuculline or picrotoxin. In contrast, GABA or muscimol, but not baclofen, markedly increased the neuronal death induced by oxygen-glucose deprivation. Muscimol potentiation of neuronal death was associated with increased glutamate efflux to the bathing medium, and increased cellular 45Ca2+ accumulation; it was blocked by MK-801, but not NBQX, suggesting mediation by NMDA receptors. Bicuculline only weakly attenuated muscimol potentiation of oxygen-glucose deprivation-induced neuronal death, probably because it itself increased this death. Present results raise a note of caution in the proposed use of GABAA receptor stimulation to limit ischemic brain damage in vivo.

Animals↗
PubMed1996

Expression of AMPA-glutamate receptor B subunit in rat hippocampal GABAergic neurons.

The physiological properties of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type glutamate receptor (GluR) channels are determined by their subunit composition. In particular, the expression of the GluR-B subunit determines the divalent ion selectivity of the channel. We studied the distribution of GluR-B mRNA and protein in adult rat hippocampal GABAergic neurons combining non-radioactive in situ hybridization with immunocytochemistry. The majority of GABAergic hippocampal neurons were GluR-B mRNA-positive, but overall the levels of GluR-B transcript were lower than in pyramidal and granule cells, which showed the highest hybridization signal. The different GluR-B mRNA expression levels in GABAergic versus principal neurons were also observed at the protein level. There was a paucity of GluR-B/C immunoreactivity in the vast majority of somata of the GABAergic neurons studied, which contrasted with the strong expression of GluR-B/C proteins in the hippocampal principal neurons. Our results demonstrate the general low expression of the GluR-B subunit mRNA and GluR-B/C proteins in GABAergic hippocampal neurons. Considering the dominant role of the GluR-B subunit in determining the divalent ion permeability of the receptor, it is likely that most GABAergic neurons express AMPA receptor channels with different calcium permeabilities.

Animals↗
PubMed1995

Relative abundance of subunit mRNAs determines gating and Ca2+ permeability of AMPA receptors in principal neurons and interneurons in rat CNS.

Recording of glutamate-activated currents in membrane patches was combined with RT-PCR-mediated AMPA receptor (AMPAR) subunit mRNA analysis in single identified cells of rat brain slices. Analysis of AMPARs in principal neurons and interneurons of hippocampus and neocortex and in auditory relay neurons and Bergmann glial cells indicates that the GluR-B subunit in its flip version determines formation of receptors with relatively slow gating, whereas the GluR-D subunit promotes assembly of more rapidly gated receptors. The relation between Ca2+ permeability of AMPAR channels and the relative GluR-B mRNA abundance is consistent with the dominance of this subunit in determining the Ca2+ permeability of native receptors. The results suggest that differential expression of GluR-B and GluR-D subunit genes, as well as splicing and editing of their mRNAs, account for the differences in gating and Ca2+ permeability of native AMPAR channels.

Animals↗
PubMed1995

Molecular biology and physiology at the single-cell level.

The functional characterization of the many complex proteins expressed in the CNS is a daunting task. The development of nucleic acid amplification techniques has provided a powerful tool to tackle this challenging enterprise. Recently, these techniques have been successfully used to correlate the functional properties of various CNS proteins with their specific mRNA expression patterns in the brain.

Animals↗
PubMed1995

Differential expression of AMPA receptor subunits in NOS-positive neurons of cortex, striatum, and hippocampus.

AMPA receptor (AMPAR) subunits expression was studied in nitric oxide synthase (NOS)-positive neurons of the adult rat cortex, striatum, and hippocampus, by a double-labeling approach, combining nonradioactive in situ hybridization and immunocytochemistry. The majority of cortical and hippocampal NOS-immunopositive neurons were characterized by a predominant expression of GluR-A and -D mRNA and low or undetectable expression of GluR-B and -C mRNA. In the striatum, the expression profile of AMPAR subunits in NOS-positive neurons differed from that in the other two regions. This is reflected in the overall low expression of all AMPA receptor subunits and the paucity of GluR-D subunit expression that contrasts with the high expression of this subunit in NOS-positive cells in the hippocampus. Double-labeling experiments revealed a substantial correspondence between mRNA and protein levels of AMPAR subunits. Further evidence for the regional diversity of NOS-positive neurons is derived from the expression analysis of glutamate decarboxylase (GAD)-65 and -67 mRNAs. NOS-positive neurons expressed high levels of GAD-65, but not -67 in the cortex, high levels of both forms in the hippocampus, and low or undetectable levels of both mRNAs in the striatum. Despite of these differences, NOS-positive neurons share the common feature of low GluR-B subunit expression, suggesting the presence of AMPAR channels with high Ca2+ permeability, regardless of the regional location. The relative resistance of NOS-positive interneurons in neurodegenerative diseases suggests that glutamate receptor-mediated Ca2+ influx alone does not suffice to explain neuronal vulnerability, and additional factors have thus to be considered.

Animals↗
PubMed1994

Control of kinetic properties of AMPA receptor channels by nuclear RNA editing.

AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor channels mediate the fast component of excitatory postsynaptic currents in the central nervous system. Site-selective nuclear RNA editing controls the calcium permeability of these channels, and RNA editing at a second site is shown here to affect the kinetic aspects of these channels in rat brain. In three of the four AMPA receptor subunits (GluR-B, -C, and -D), intronic elements determine a codon switch (AGA, arginine, to GGA, glycine) in the primary transcripts in a position termed the R/G site, which immediately precedes the alternatively spliced modules "flip" and "flop." The extent of editing at this site progresses with brain development in a manner specific for subunit and splice form, and edited channels possess faster recovery rates from desensitization.

Alternative Splicing↗
PubMed1994

A molecular determinant for submillisecond desensitization in glutamate receptors.

The decay of excitatory postsynaptic currents in central neurons mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors is likely to be shaped either by receptor desensitization or by offset after removal of glutamate from the synaptic cleft. Native AMPA receptors show desensitization time constants of 1 to about 10 milliseconds, but the underlying molecular determinants of these large differences are unknown. Cloned AMPA receptors carrying the "flop" splice variants of glutamate receptor subtype C (GluR-C) and GluR-D are shown to have desensitization time constants of around 1 millisecond, whereas those with the "flip" variants are about four times slower. Cerebellar granule cells switch their expression of GluR-D splice variants from mostly flip forms in early stages to predominantly flop forms in the adult rat brain. These findings suggest that rapid desensitization of AMPA receptors can be regulated by the expression and alternative splicing of GluR-D gene transcripts.

Alternative Splicing↗
PubMed1994

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.

Amino Acid Sequence↗
PubMed1994

Developmental and regional expression in the rat brain and functional properties of four NMDA receptors.

An in situ study of mRNAs encoding NMDA receptor subunits in the developing rat CNS revealed that, at all stages, the NR1 gene is expressed in virtually all neurons, whereas the four NR2 transcripts display distinct expression patterns. NR2B and NR2D mRNAs occur prenatally, whereas NR2A and NR2C mRNAs are first detected near birth. All transcripts except NR2D peak around P20. NR2D mRNA, present mainly in midbrain structures, peaks around P7 and thereafter decreases to adult levels. Postnatally, NR2B and NR2C transcript levels change in opposite directions in the cerebellar internal granule cell layer. In the adult hippocampus, NR2A and NR2B mRNAs are prominent in CA1 and CA3 pyramidal cells, but NR2C and NR2D mRNAs occur in different subsets of interneurons. Recombinant binary NR1-NR2 channels show comparable Ca2+ permeabilities, but marked differences in voltage-dependent Mg2+ block and in offset decay time constants. Thus, the distinct expression profiles and functional properties of NR2 subunits provide a basis for NMDA channel heterogeneity in the brain.

Animals↗
PubMed1994

NMDA receptor channels: subunit-specific potentiation by reducing agents.

Sulfhydryl redox agents affect NMDA receptor activity. We investigated a putative redox site in four recombinant NMDA receptors. In 293 cells expressing NR1-NR2A channels dithiothreitol (DTT) rapidly potentiated L-glutamate-activated whole-cell currents and decreased the time course of desensitization and deactivation. Part of the current potentiation (reversible component) and all kinetic changes reversed upon washout. The remaining potentiation (persistent component) was abolished by an oxidizing agent. The N-terminal 370 residues of NR2A mediate the reversible component in chimeric NR2 subunits. In cells expressing the NR1-NR2B, -NR2C, and -NR2D channels DTT elicited only a slowly developing, persistent potentiation and increased the deactivation time course. In these, but not in NR1-NR2A, the DTT effect was rendered insensitive to reoxidation by alkylation. Reduced glutathione mimicked the DTT effects only in the NR1-NR2A receptor. Hence, molecularly distinct NMDA receptors differ profoundly in their responses to sulfhydryl redox agents.

Alanine↗
PubMed1994

Differences in Ca2+ permeability of AMPA-type glutamate receptor channels in neocortical neurons caused by differential GluR-B subunit expression.

Fast excitatory synaptic transmission in the CNS is mediated by AMPA-type glutamate receptor (GluR) channels. Heterologous expression suggested that the Ca2+ permeability of these receptors critically depends on the subunit composition. Using patch-clamp techniques in brain slices, we found that the Ca2+ permeability of native AMPA-type GluRs was markedly higher in nonpyramidal (PCa/PK approximately 0.63) than in pyramidal (PCa/PK approximately 0.05) neurons of rat neocortex. Analysis of mRNA in single cells indicated that the relative abundance of GluR-B-specific mRNA was significantly lower in nonpyramidal (GluR-B/GluR-non-B approximately 0.3) than in pyramidal (GluR-B/GluR-non-B approximately 3) cells. This suggests that differences in relative abundance of GluR-B-specific mRNA generate functional diversity of AMPA-type GluRs in neurons with respect to Ca2+ permeability.

Animals↗
PubMed1993

Argiotoxin detects molecular differences in AMPA receptor channels.

Argiotoxin, a component of the spider venom from Argiope lobata, blocks AMPA receptor channels expressed in homomeric and heteromeric configuration in Xenopus oocytes. Argiotoxin acts as an open channel blocker in a voltage-dependent manner and discriminates between the functionally diverse AMPA receptors. Importantly, a transmembrane region 2 determinant for divalent cation permeability also determines argiotoxin sensitivity. Subunit-specific differences in the time courses of block and recovery demonstrate that heteromeric AMPA receptors can assemble in variable ratios. Thus, argiotoxin can be used as a tool in analyzing the subunit composition of AMPA receptors in native membranes.

Animals↗
PubMed1992

Hippocampal kindling increases the expression of glutamate receptor-A Flip and -B Flip mRNA in dentate granule cells.

The level of the mRNAs encoding the AMPA-selective glutamate receptors-A and -B, alternatively splice variants, Flip and Flop, was studied by in situ hybridization in the brains of rats kindled by Schaffer collateral/commissural-fiber stimulation. In comparison to control animals, the expression level of the Flip variant of both GluR-A and GluR-B mRNAs was bilaterally enhanced in the dentate granule neurons of kindled animals 24 h after last-generalized seizure, whereas no obvious alterations were observed in the GluR-A Flop and GluR-B Flop mRNA variants. In kindled animals, studied 1 month after the last seizure, GluR-A Flip and GluR-B Flip mRNA had returned to control levels. We suggest that these changes may result in an enhanced glutamate receptor sensitivity in the fascia dentata during kindling.

Alternative Splicing↗
PubMed1992

Control by asparagine residues of calcium permeability and magnesium blockade in the NMDA receptor.

The N-methyl-D-aspartate (NMDA) receptor forms a cation-selective channel with a high calcium permeability and sensitivity to channel block by extracellular magnesium. These properties, which are believed to be important for the induction of long-term changes in synaptic strength, are imparted by asparagine residues in a putative channel-forming segment of the protein, transmembrane 2 (TM2). In the NR1 subunit, replacement of this asparagine by a glutamine residue decreases calcium permeability of the channel and slightly reduces magnesium block. The same substitution in NR2 subunits strongly reduces magnesium block and increases the magnesium permeability but barely affects calcium permeability. These asparagines are in a position homologous to the site in the TM2 region (Q/R site) of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors that is occupied by either glutamine (Q) or arginine (R) and that controls divalent cation permeability of the AMPA receptor channel. Hence AMPA and NMDA receptor channels contain common structural motifs in their TM2 segments that are responsible for some of their ion selectivity and conductance properties.

Amino Acid Sequence↗
PubMed1992

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.

Animals↗