A family of glutamate receptor genes: evidence for the formation of heteromultimeric receptors with distinct channel properties

AMPA receptors and bacterial periplasmic amino acid-binding proteins share the ionic mechanism of ligand recognition

In order to identify key structural determinants for ligand recognition, we subjected the ligand-binding domain of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-selective glutamate receptor GluR-D subunit to site-directed mutagenesis. Based on the analysis of the [3H]AMPA-binding properties of the mutated binding sites, we constructed a revised three-dimensional model of the ligand-binding site, different in many respects from previously published models. In particular, our results indicate that the residues Arg507 and Glu727 represent the structural and functional correlates of Arg77 and Asp161 in the homologous bacterial lysine/ornithine/arginine-binding protein and histidine-binding protein, and directly interact with the alpha-carboxyl and alpha-amino group of the bound ligand, respectively. In contrast, Glu424, implicated previously in ionic interactions with the alpha-amino group of the agonist, is unlikely to have such a role in ligand binding. Our results indicate that glutamate receptors share with the bacterial polar amino acid-binding proteins the fundamental mechanism of amino acid recognition.

Changes in AMPA receptor-spliced variant expression and shift in AMPA receptor spontaneous desensitization pharmacology during cerebellar granule cell maturation in vitro

Using appropriate internal standards, quantitative reverse transcripase-polymerase chain reaction (RT-PCR), and cerebellar granule cell (CG) in primary cultures we have quantified the expression of mRNAs encoding for GluR1-4 DL-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subunits during neuronal maturation in vitro. GluR1 is the mRNA that increases during CG maturation; the expression changes of the other GluR mRNAs are minimal and the translation products of these mRNAs change with a similar pattern. During CG maturation, there is an 8- to 10-fold increase in the GluR1 FLOP mRNA and a twofold increase in the expression of FLOP mRNA for GluR4 and GluR4C. The GluR1 FLIP mRNA increases, but by a smaller extent. We found that the GluR2 mRNA is completely edited at its Q/R site during CG maturation. The increase on the expression of GluR1 FLIP and FLOP and of GluR4 FLOP mRNA variants during development is associated with a 10-fold increase in AMPA-mediated Na+ currents and in the increased amplification of this current by 7-chloro-3-methyl-3,4 dihydro-2H-1,2,4 benzothiadiazine S-S-dioxide (IDRA21) or by 6-chloro-3,4 dihydro-3-(2-norbornen-5-yl)-7-sulfamoyl-1,2,4-benzothiadiazine 1,1 dioxide (cyclothiazide [CT]).

Progression of change in NMDA, non-NMDA, and metabotropic glutamate receptor function at the developing corticothalamic synapse

The development of receptor function at corticothalamic synapses during the first 20 days of postnatal development is described. Whole cell excitatory postsynaptic currents (EPSCs) were evoked in relay neurons of the ventral posterior nucleus (VP) by stimulation of corticothalamic fibers in in vitro slices of mouse brain from postnatal day 1 (P1). During P1-P12, excitatory postsynaptic conductances showed strong voltage dependence at peak current and at 100 ms after the stimulus and were almost completely antagonized by -2-amino-5-phosphonopentoic acid (APV), indicating that N-methyl--aspartate (NMDA) receptor-mediated currents dominate corticothalamic EPSCs at this time. After P12, in 42% of cells, excitatory postsynaptic conductances showed no voltage-dependence at peak current but still showed voltage-dependence 100-ms poststimulus. This voltage-dependent conductance was antagonized by APV. The nonvoltage-dependent component was APV resistant, showed fast decay, and was antagonized by the nonNMDA antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). In the remaining 58% of cells after P12, excitatory postsynaptic conductances showed moderate voltage dependence at peak conductance and strong voltage dependence 100 ms after the stimulus. Analysis of EPSCs before and after APV showed a significant increase in the relative contribution of the non-NMDA conductance after the second postnatal week. From P1 to P16, there was a significant decrease in the time constant of decay of the NMDA EPSC but no change in the voltage dependence of the NMDA response. After P8, slow EPSPs, 1.5-30 s in duration and mediated by metabotropic glutamate receptors (mGluRs), could be evoked by high-frequency stimulation of corticothalamic fibers in the presence of APV and CNQX. Similar slow depolarizations could be evoked by local application of the mGluR agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD) but from P0. Both conductances were blocked by the mGluR antagonist, (RS)-alpha-methyl-4-carboxyphenylglycine. Hence functional mGluR receptors are present on VP cells from birth, but their synaptic activation at corticothalamic synapses can only be detected after P8. In voltage clamp, the extrapolated reversal potential of the t-ACPD current, with potassium gluconate-based internal solution, was +12 +/- 10 (SE) mV, and the measured reversal potential with cesium gluconate-based internal solution was 1.5 +/- 9.9 mV, suggesting that the mGluR-mediated depolarization was mediated by a nonselective cation current. Replacement of NaCl in the external solution caused the reversal potential of the current to shift to -18 +/- 2 mV, indicating that Na+ is a charge carrier in the current. The current amplitude was not reduced by application of Cs+, Ba2+, and Cd2+, indicating that the t-ACPD current was distinct from the hyperpolarization-activated cation current (IH) and distinct from certain other previously characterized mGluR-activated, nonselective cation conductances.

Molecular and electrophysiological characterizations of fGluR3 alpha, an ionotropic glutamate receptor subunit of a teleost fish

Here we report the cloning and functional analysis of a cDNA encoding a functional glutamate receptor subunit of Oreochromis sp., a freshwater teleost fish. The deduced amino acid sequence of this cDNA clone, fGluR3 alpha, displays the highest sequence identity to that of the mammalian GluR3 subunit. Results of quantitative reverse-transcriptase polymerase chain reaction (RT-PCR) analysis indicated that the expression level of fGluR3 alpha in the cerebellum was much less than that in the telencephalon and optical lobe. Similar to its mammalian counterpart, variants of fGluR3 alpha were created by alternative splicing and RNA editing at the R/G site. The channel properties of homomeric fGluR3 alpha expressed in Xenopus oocytes were similar to those of the mammalian alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-preferring receptors. The rank order of agonist potency of the expressed fGluR3 alpha is AMPA > or = glutamate > or = quisqualate > domoate > or = kainate. This is the first functional glutamate receptor of teleost fish being demonstrated to be sensitive to AMPA. Furthermore, this study suggested a strong functional conservation of AMPA-preferring receptors in vertebrates.

Diversity of glutamate receptors in the mammalian retina

The main neurotransmitters in the vertebrate retina are glutamate, GABA and glycine. Their localization in the different cell types in the retina is well known. In addition, there exists a number of neuropeptides and other neuroactive substances that are only expressed by sparse populations of neurons. In recent years, molecular biology has led to the discovery of a rapidly increasing number of neurotransmitter receptors and the apparent simplicity of neurotransmitters in the mammalian retina is contrasted by the expression of a plethora of neurotransmitter receptors and receptor subunits (not mentioning receptor isoforms). This article will concentrate on glutamate receptors with the intention of reviewing some of the recent data on glutamate receptor expression in the mammalian retina and their possible involvement in retinal function.

Differential distribution of NMDA receptor subunit mRNA in the rat cochlear nucleus

The distribution and expression of mRNAs for different subunits of the N-methyl-D-aspartate receptor (NMDAR) were examined in the cochlear nucleus (CN) of the rat using radioactive in situ hybridization methods. Heavy labeling for NMDAR1 subunit mRNA was observed in all major CN neuronal types with lower labeling for NMDAR2A, 2B, 2C, and 2D mRNA. Silver grain counting was used to compare expression of different NMDAR2 subunits between six of the major CN cell types. Small cells of the small cell cap/shell area had the highest expression of NMDAR2A-C subunit mRNAs of the cell types assessed. These small cells as well as fusiform and corn cells of the dorsal cochlear nucleus had higher NMDAR2C than other NMDAR2 subunits, providing these neuron types with a distinct expression pattern or profile. The other three cell types assessed, spherical bushy cells, granule cells, and octopus cells had relatively equivalent levels of NMDAR2A-C subunit expressions, providing a second distinct profile. NMDAR2D mRNA had low expression in all six cell types assessed.

Long-term depression of synaptic transmission in the cerebellum: cellular and molecular mechanisms revisited

Long-term depression (LTD) of synaptic transmission at parallel fiber (PF)-Purkinje cell (PC) synapses in the cerebellum has been the first established example of enduring decrease of synaptic efficacy in the central nervous system. This review focuses on the underlying cellular and molecular mechanisms. Thus, at the level of the postsynaptic membranes of PCs, induction of LTD requires concommitent activation of voltage-gated calcium channels (VGCCs) and of ionotropic and metabotopic glutamate receptors, of the alpha-amino-3 hydroxy-5-methyl-isoxalone-4-propionate (AMPA) and mGluR1 alpha types respectively. Subsequent intracellular cascades involve production of nitric oxide from arginine and of cGMP, activation of phospholipase A2 and of several protein kinases including protein kinase C and tyrosine kinases. Activation of protein kinase G and of phosphatases are also likely to be involved in LTD induction. In contrast, there are still uncertainties concerning a major role of release of calcium from internal stores in LTD induction. Finally protein synthesis is required for a late phase of LTD to occur. All available experimental evidence points towards a postsynaptic site for LTD expression. In particular, electrophysiological data demonstrate a genuine modification of the functional properties of AMPA receptors of PCs during LTD, and immunocytochemical evidence suggests that this might result from a phosphorylation of these receptors.

Characterization of the kainate-binding domain of the glutamate receptor GluR-6 subunit

Recombinant fragments of the kainate-selective glutamate recepto subunit GluR-6 were expressed in insect cells and analysed for [3H]kainate binding activity in order to characterize the structural determinants responsible for ligand recognition. Deletion of the N-terminal approximately 400 amino-acid-residue segment and the C-terminal approximately 90 residues resulted in a membrane-bound core fragment which displayed pharmacologically native-like [3H]kainate binding properties. Further replacement of the membrane-embedded segments M1-M3 by a hydrophilic linker peptide gave rise to a soluble polypeptide which was accumulated in the culture medium. When bound to chelating Sepharose beads via a C-terminal histidine tag, the soluble fragment showed low-affinity binding of [3H]kainate, which was displaced in a concentration-dependent manner by unlabelled domoic acid, L-glutamate and 6-cyano-7-nitroquinoxaline-2,3-dione. Our results indicate that the kainate-binding site is formed exclusively by the two discontinuous extracellular segments (S1 and S2) which are homologous to bacterial amino-acid-binding proteins. Ligand binding characteristics of soluble S1-S2 chimaeras between the GluR-6 and GluR-D subunits showed that, whereas both S1 and S2 segments contribute to agonist-selectivity, the N-terminal one-third of the GluR-D S2 segment is sufficient to confer alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-binding capacity to the chimaeric ligand-binding domain.

Molecular biology of glutamate receptors in the central nervous system and their role in excitotoxicity, oxidative stress and aging

Forty years of research into the function of L-glutamic acid as a neurotransmitter in the vertebrate central nervous system (CNS) have uncovered a tremendous complexity in the actions of this excitatory neurotransmitter and an equally great complexity in the molecular structures of the receptors activated by L-glutamate. L-Glutamate is the most widespread excitatory transmitter system in the vertebrate CNS and in addition to its actions as a synaptic transmitter it produces long-lasting changes in neuronal excitability, synaptic structure and function, neuronal migration during development, and neuronal viability. These effects are produced through the activation of two general classes of receptors, those that form ion channels or "ionotropic" and those that are linked to G-proteins or "metabotropic". The pharmacological and physiological characterization of these various forms over the past two decades has led to the definition of three forms of ionotropic receptors, the kainate (KA), AMPA, and NMDA receptors, and three groups of metabotropic receptors. Twenty-seven genes are now identified for specific subunits of these receptors and another five proteins are likely to function as receptor subunits or receptor associated proteins. The regulation of expression of these protein subunits, their localization in neuronal and glial membranes, and their role in determining the physiological properties of glutamate receptors is a fertile field of current investigations into the cell and molecular biology of these receptors. Both ionotropic and metabotropic receptors are linked to multiple intracellular messengers, such as Ca2+, cyclic AMP, reactive oxygen species, and initiate multiple signaling cascades that determine neuronal growth, differentiation and survival. These cascades of complex molecular events are presented in this review.

The macro- and microarchitectures of the ligand-binding domain of glutamate receptors

Over the last decade, a large body of information regarding the amino acid sequences and tertiary structures of many proteins has accumulated. Subtle similarities in sequence patterns identified between glutamate receptors and bacterial periplasmic substrate-binding proteins have suggested that structural kinship exists between these protein families. Many of the bacterial periplasmic binding proteins but none of the glutamate receptors have been crystallized so far. The following article reviews how the resemblance between these two protein families led to computer-assisted structural models of crucial elements involved in ligand binding by various glutamate receptors. A plausible dynamic model of the molecular mechanism of activation and desensitization of glutamate-receptor channels is also discussed.

Synaptic distribution of GluR2 in hippocampal GABAergic interneurons and pyramidal cells: a double-label immunogold analysis

GluR2 is the regulatory subunit in the AMPA family of glutamate receptors (GluRs) in that its presence inhibits calcium flux and dominates the current/ voltage characteristics of AMPA receptors. Studies from other laboratories have shown that GABAergic interneurons have a lower ratio of GluR2/GluR1 mRNA than pyramidal cells as well as possessing AMPA receptors that have a higher relative permeability to calcium. We hypothesized that such differences might be related to differences in the subunit stoichiometry at the AMPA synapses in each cell class, and used a GluR2-specific monoclonal antibody in a double-label immunogold protocol with anti-GABA and anti-CaM kinase II to compare the GluR2 representation at asymmetric synapses in GABA neurons to that of pyramidal cells in rat CA1. Virtually all CA1 pyramidal cells as well as the majority of GABAergic interneurons were GluR2 positive. EM immunogold labeling also showed that GABAergic interneurons had distinctive ultrastructural features and contained GluR2 in both their soma and their dendrites, as did the spines and shafts of pyramidal cells. GluR2 immunoreactivity was frequently preferentially located at asymmetric synapses on both pyramidal cell spines and shafts as well as the dendritic processes and soma of GABAergic interneurons. However, the labeled synapses on GABAergic neurons had a significantly lower number of immunogold particles than those on pyramidal cells. In fact, 90% of the labeled asymmetric synapses on GABAergic cells had one to three gold particles, whereas greater than 70% of the labeled asymmetric synapses on pyramidal cells had four or more gold particles associated with the synapse. These data suggest that while both cell classes contain GluR2, they differ in the relative representation of GluR2 at their AMPA synapses, such that GABAergic neurons might possess AMPA receptors with higher calcium permeability on average than pyramidal cells. Such differences in subunit representation at AMPA-receptor-mediated synapses would not only lead to differences in calcium permeability and functional characteristics across these two cell classes, but might also be relevant to the hippocampal patterns of selective vulnerability with respect to excitotoxicity and neurodegeneration.

Synaptic modulation of oscillatory activity of hypothalamic neuronal networks in vitro

1. Rhythmic bursts of action potentials in neurosecretory cells are a key factor in hypothalamic neurosecretion. Rhythmicity and synchronization may be accomplished by pacemaker cells synaptically driving follower cells or by a network oscillator. 2. In this review we describe a hypothalamic cell culture which may serve as a model for a hypothalamic network oscillator. An overview is given of neurochemical phenotypes, synaptic mechanisms and their development, properties of receptors for fast synaptic transmission, and membrane properties of cells in dissociated rat embryonic hypothalamic culture. 3. Rhythmic activity spreads in the cultured network through synapses that release glutamate, activating a heteromultimeric AMPA-type receptor containing a GluR2 subunit which is associated with a high-conductance channel for Na+ and K+. Rhythmic activity is controlled by synapses that release GABA to activate GABAA receptors. The presumed function of the two receptor types is facilitated by their respective location, GABAA receptors predominating near the soma and AMPA receptors being abundant in dendrites. 4. Network oscillators may be more reliable for the presumed function than single-cell oscillators. They are controlled through synaptic modulation, which may prove to represent a process important for the release of hormones.

The regulation of AMPA receptor-binding sites

A wide variety of mechanisms have been identified that can regulate the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-receptor complex. Modulation has been shown to occur at the nucleic acid level via RNA editing and alternative splicing. At the posttranslational level, processes such as phosphorylation, glycosylation, chemical modification of reactive groups on the receptor proteins, interaction with a putative receptor-associated modulatory protein, and changes in the lipid environment have been reported to regulate receptor binding and function. In this review, we discuss general aspects of the cell biology, pharmacology, and function of AMPA receptors. In particular, we focus on some factors shown to modulate agonist binding and discuss possible molecular mechanisms underlying the regulation observed.

Overexpression of a glutamate receptor (GluR2) ligand binding domain in Escherichia coli: application of a novel protein folding screen

Expression of the S1S2 ligand binding domain [Kuusinen, A., Arvola, M. & Keinänen, K. (1995) EMBO J. 14, 6327-6332] of the rat alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid-selective glutamate receptor GluR2 in Escherichia coli under control of a T7 promoter leads to production of >100 mg/liter of histidine-tagged S1S2 protein (HS1S2) in the form of inclusion bodies. Using a novel fractional factorial folding screen and a rational, step-by-step approach, multiple conditions were determined for the folding of the HS1S2 alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid binding domain. Characterization of the HS1S2 ligand binding domain showed that it is water-soluble, monomeric, has significant secondary structure, and is sensitive to trypsinolysis at sites close to the beginning of the putative transmembrane regions. Application of a fractional factorial folding screen to other proteins may provide a useful means to evaluate E. coli as an economical and convenient expression host.

Changes in glutamate receptor subunit composition in hippocampus and cortex in patients with refractory epilepsy

An assessment of glutamate receptor subunit profiles was made in hippocampus and temporal lobe cortex of patients with refractory epilepsy. Molecular biological analyses using reverse transcription reaction (RT) followed by polymerase chain reaction (PCR) revealed changes in the distribution profile of the transcripts of AMPA/KA glutamate receptor subunits in hippocampal and cortical tissue from patients with refractory epilepsy when compared to similar tissue from six human and four non-human primate samples with no history of seizures or seizure medication. A severe mean decrease (38% of control) in mRNA for the GluR1 subunit was found in 400 mm cross-sections of hippocampus from patients with epilepsy. Less severe but significant reductions in that GluR1 subunit expression (54% of control) were exhibited in samples of excised temporal pole cortex from the same subjects. Message for the GluR4 subunit was also significantly decreased in hippocampus (68% of control), but in contrast to GluR1, GluR4 mRNA level was not decreased in temporal cortex. Levels of GluR2 mRNA were not significantly changed in epileptic hippocampal and cortical tissue relative to control samples. Protein levels of the GluR1 and GluR4 subunits quantified by Western blot analysis were also reduced in hippocampal and cortical tissue from epilepsy patients. Two other kainate subunit transcripts, GluR6 and KA1 also showed significant changes compared to non-epileptic tissue (136% and 71% of control, respectively). Results are discussed in terms of possible mechanisms by which protracted seizures could produce selective loss of certain AMPA/KA subunits.

Expression of glutamate receptor subunits in alpha-motoneurons

Whole-cell recordings from 6.5 day embryonic chick alpha-motoneurons indicated the presence of AMPA, kainate, and NMDA glutamate receptor subtypes in each motoneuron tested. AMPA consistently evoked a desensitizing response, while kainate could evoke either a desensitizing or non-desensitizing whole-cell response. In excised membrane patches, desensitizing AMPA responses appeared to be colocalized with non-desensitizing kainate responses. Desensitizing kainate responses were seen in some patches which were not responsive to AMPA, suggesting that kainate selective subunits and AMPA selective subunits localize separately on the motoneuron membrane. To determine which of the known glutamate receptor subunits might underlie these responses, we used RT-PCR amplification to detect subunits present in mRNA isolated from adult rat spinal cord and from a highly enriched motoneuron population from embryonic chick. Sequencing of the amplified cDNA was used to verify the identity of the products and of the alternative splice variants of GluR1-4. In rat spinal cord, all subunits that we attempted to detect, including AMPA selective subunits GluR1-4, kainate selective subunits GluR5-7 and KA1-2, and NMDA subunit NR1 were present. The isolated motoneurons also contained AMPA subunits GluR1, 2, and 4, and kainate subunits GluR6 and 7. The GluR2 and 4 subunits were specifically processed by splicing, present primarily as the flip splice form.

Analysis of AMPA receptor properties during postnatal development of mouse hippocampal astrocytes

Glial cells in the mammalian brain express various types of voltage- and ligand-gated ion channels, including glutamate receptors (GluRs) of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-subtype. In the present study we followed developmental changes in the functional properties of AMPA receptor (AMPA-R) channels expressed by astrocytes of the mouse hippocampus between postnatal days (P) 5-35 to learn more about the physiological significance of these glial receptors. A fast concentration clamp technique was applied to cells acutely isolated from the CA1 stratum radiatum subregion to quantitatively analyze rapidly activating and desensitizing receptor responses. The equilibrium responses of glutamate and kainate differed between P5 and P12. Although the maximum current induced by kainate was almost the same at all developmental stages, a steep rise in the maximum glutamate response was observed within the same time range. Between P5 and P12 there was an increase in the potentiation of AMPA-R currents with cyclothiazide (CTZ); at the same time, the dissociation kinetics of CTZ became significantly slower. These changes in the pharmacological properties suggested a variation in splice variant expression. With proceeding maturation, we observed an increase in the degree of desensitization of the glutamate- and AMPA-induced receptor currents. In addition to the shift in flip/flop splicing, these findings could hint at a developmental regulation of RNA editing in the arginine/glycine site. Altogether, the present results demonstrate changes in astrocytic AMPA-R functioning early in postnatal development, although after P12 the receptor properties remained almost constant. Although the overall Ca2+ permeability did not vary during development, the prolonged receptor opening in the early postnatal period causes an enhanced Na+/Ca2+ influx into the immature astrocytes. This could influence glial proliferation and differentiation during CNS ontogenesis.

Cellular, subcellular, and subsynaptic distribution of AMPA-type glutamate receptor subunits in the neostriatum of the rat

Glutamate released in the basal ganglia is involved in the expression of clinical symptoms of neurodegenerative diseases like Parkinson's or Huntington's. Neostriatal neurons are the targets of glutamatergic inputs derived from the cortex and the thalamus acting via AMPA-type as well as other glutamate receptors. To determine the location of subunits of the AMPA subclass of glutamate receptors (GluR) in the rat neostriatum, we applied multiple immunocytochemical techniques using anti-peptide antibodies against the GluR1, GluR2/3, and GluR4 subunits at both the light and electron microscopic levels. All medium spiny efferent neurons, some of which were identified as striatonigral neurons, displayed immunoreactivity for GluR1 and GluR2/3 subunits. Double immunofluorescence revealed that at least 70-90% of parvalbumin-immunopositive GABAergic interneurons were immunoreactive for each of GluR1, GluR2/3, or GluR4 subunits and that at least 40% of choline acetyltransferase-immunopositive cholinergic interneurons were immunopositive for GluR1 or GluR4 subunits. The majority of nitric oxide synthase-immunopositive neurons had no detectable immunoreactivity for any of the AMPA receptor subunits. Electron microscopic analysis confirmed the presence of immunoreactivity for GluR1 and GluR2/3 in the perikarya of spiny neurons and interneurons and GluR4 in perikarya of interneurons only. GluR1 and GluR2/3 subunits were detected in dendrites and spines. A significant population of extrasynaptic receptors was revealed by pre-embedding immunogold labeling along the plasma membranes of perikarya, dendrites, and spines. Receptors were concentrated in the postsynaptic membrane specialization of asymmetrical synapses, as revealed by the postembedding immunogold method. Quantitative analysis demonstrated that immunoreactivity for the GluR1 and GluR2/3 subunits is higher at the periphery than at the middle of the postsynaptic membrane specialization. Our results demonstrate that AMPA receptor subunits are distributed widely and heterogeneously among striatal neurons and are concentrated on the postsynaptic membrane of asymmetrical synaptic specializations, although extrasynaptic receptors are also present.

AMPA- and kainate-receptors differentially mediate excitatory amino acid-induced dopamine and acetylcholine release from rat striatal slices

Rat striatal slices, preincubated with [3H]dopamine (DA) and [14C]choline, were superfused continuously. Detection of radioactivity was used to monitor the release of the neurotransmitters DA and acetylcholine (ACh). 6-Cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(f)quinoxaline (NBQX) caused a concentration-dependent decrease in 100 microM alpha-amino-3-hydroxy-5-methylisoxazol-4-propionate (AMPA)-, 100 microM kainate- or 100 mM glutamate-induced release of DA and ACh. The IC50 of NBQX is 3-fold higher (for ACh release) and is 2-fold lower (for DA release) than that of CNQX. This is in agreement with the IC50 ratio of NBQX and CNQX on kainate- and AMPA-receptor binding. These two antagonists, at doses that produce an equivalent blockade of kainate-receptor binding (5 microM for NBQX and 1.56 microM for CNQX), caused an approximately equal decrease in ACh- but not DA-release induced by 100 microM kainate or AMPA. At doses that produce an equivalent blockade of AMPA-receptor binding (5 microM for NBQX and 10 microM for CNQX), they caused an approximately equal decrease in DA but not ACh release induced by 100 microM AMPA or kainate. Moreover, concanavalin A (0.3 and 0.5 mg/ml), which selectively potentiates kainate-receptor responses, markedly enhanced 100 microM kainate-induced release of ACh but not DA. Cyclothiazide (10 microM), which selectively potentiates AMPA-receptor responses, significantly increased 100 microM AMPA- or kainate-induced release of DA but not ACh. In summary, these results indicate that AMPA-and kainate-receptor activation, respectively, are predominantly involved in excitatory amino acid (EAA)-induced DA and ACh release in the striatum.

Identification of amino acid residues that control functional behavior in GluR5 and GluR6 kainate receptors

GluR5 and GluR6 kainate receptors differ in their responses to a variety of agonists, despite their relatively high primary sequence homology. We carried out a structure-function study to identify amino acids underlying these divergent responses. Patch clamp analysis of chimeric GluR5-GluR6 receptors indicated that several functionally dominant sites were localized to the C-terminal side of M1. All nonconserved amino acids in the region between M3 and M4 of GluR6 were then individually mutated to their GluR5 counterparts. We found that a single amino acid (N721 in GluR6) controls both AMPA sensitivity and domoate deactivation rates. Additionally, mutation of A689 in GluR6 slowed kainate desensitization. These functional effects were accompanied by alterations in binding affinities. These results support a critical role for these residues in receptor binding and gating activity.

Distribution of glutamate receptor subunits in the primate temporal cortex and hippocampus

The distribution of subunits for the N-methyl-D-aspartate (NR1, NR2A/B), alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (GluR1, GluR2/3, GluR4) and low affinity kainate (GluR5/6/7) ionotropic glutamate receptors was examined by immunocytochemistry in the temporal cortex and hippocampus of the rhesus macaque (Macaca mulatta). Neurons expressing NR1, NR2A/B, GluR2/3, and GluR4 subunits were widely distributed in all of the cortical layers but the overall density of the GluR4-immunopositive neurons was very low. Neurons expressing the GluR1 subunit were found predominantly in cortical layers V and VI while those expressing the GluR5/6/7 subunits were concentrated in layer V and were readily distinguishable by the thick elongate shape of their primary apical dendrites. Subcellular differences in the immunostaining pattern were also noted between the different glutamate receptor subunits. NR1 and NR2A/B immunoreactivity was most pronounced in somatic and primary dendritic compartments and to a lesser extent in cortical and hippocampal molecular layers. GluR1 immunoreactivity was more intense than GluR2/3 in the hippocampal molecular layers whereas GluR4 was undetectable. GluR5/6/7 immunoreactivity was very intense in the dentate molecular layer, and the CA1 pyramidal cells had a subcellular distribution of GluR5/6/7 that was similar to the cortical neurons. Overall, the distribution patterns of the different glutamate receptor subunits was identical in animals that had been ovariectomized and in ovariectomized animals that had subsequently undergone estradiol or estradiol/progesterone hormone replacement. Taken together, these findings demonstrate a differential spatial arrangement of glutamate receptor subunits in the primate temporal cortex and hippocampus, which may have functional significance for the integration of excitatory inputs to these areas. Furthermore, they show that in adult macaques, sex steroids do not play a major role in determining the distribution patterns of these receptor subunits.

Patch clamp detection in capillary electrophoresis

We describe a capillary electrophoresis-patch clamp (CE-PC) analysis of biomolecules that activate ligand-gated ion channels. CE-PC offers a powerful means for identifying receptor ligands based on the combination of the characteristic receptor responses they evoke and their differential electrophoretic migration rates. Corner frequencies, membrane reversal potentials, and mean and unitary single-channel receptor responses were calculated from currents recorded with patch clamp detection. This information was then combined with the electrophoretic mobility of the receptor ligand, which is proportional to the charge-to-frictional-drag ratio of that species. We applied CE-PC to separate and detect the endogenous receptor agonists gamma-aminobutyrate and L-glutamate and the synthetic glutamate receptor agonists N-methyl-D-aspartate and kainic acid. We present dose-response data for electrophoretically separated kainic acid and discuss its implications for making the CE-PC detection system quantitative.

Coexpression of striatal dopamine receptor subtypes and excitatory amino acid subunits

The striatal cellular coexpression patterns for the D(1A) and D2 dopamine (DA) receptor subtypes and the ionotropic excitatory amino acid (EAA) subunits of the N-methyl-D-aspartate (NMDA-R1) and the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) (GluR1 and GluR2/3) receptor subunits were examined morphologically. Their coincidence was assessed by visualization of mRNA transcripts, localization of encoded receptor proteins, and binding analysis using concurrently paired methods of fluorescence detection. The findings indicated that 1) mRNA transcripts for both receptor systems were detected in the medium-sized neuron population, and the distribution of receptor message closely reflected protein and binding patterns, with the exception of the GluR1 subunit; 2) both DA receptor mRNA transcripts were coexpressed with each ionotropic EAA receptor subunit examined and with each other, and NMDA and AMPA receptor subunits also showed coincident expression; 3) D(1A) DA receptor protein was detected in neurons which coexpressed EAA subunit proteins; and 4) GluR2/3 and NMDA-R1 subunit proteins were coexpressed in medium-sized neurons which also demonstrated D2 DA receptor binding sites. These findings suggest morphological receptor "promiscuity" since the coexpression patterns between DA and EAA receptors were found in all permutations. The results provide a spatial framework for physiological findings describing functional interactions between the two DA receptor types and between specific DA and EAA receptors in the striatum.

Comparison of the agonist binding site of homomeric, heteromeric, and chimeric GluR1(o) and GluR3(o) AMPA receptors

A series of AMPA [(R,S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid] analogues were evaluated for activity at homomeric, heteromeric, and chimeric rat GluR1(o) and GluR3(o) receptors expressed in Xenopus oocytes, using the two-electrode voltage clamp technique. The formation of heteromeric receptor complexes was demonstrated by cross-immunoprecipitation of both subunits from solubilized oocyte membranes. The AMPA analogue ACPA [(R,S)-2-amino-3(3-carboxy-5-methyl-4-isoxazolyl)propionic acid] was the most potent and selective agonist tested at GluR1(o) and GluR3(o), with a 10-fold selectivity for GluR3(o). ACPA showed an intermediate potency at both the GluR1(o) + 3(o) heteromeric complex as well as at the homomeric chimeric receptors. These experiments suggest that for receptor activation, agonist binding occurs between the interface of the GluR1 and GluR3 subunits in the heteromeric channel complex, perhaps between the S1 region of one subunit and the S2 region of another. Also, it seems that 1) electronegative group substitutions on the isoxazole ring of AMPA and 2) decreasing the pKa of the sub stituent at position 3 play a major role in determining the degree of receptor activation under steady-state conditions. Future studies will examine the effects of single amino acid mutations in these receptors, giving a more precise localization of the agonist binding site.

Repeated amphetamine administration alters the expression of mRNA for AMPA receptor subunits in rat nucleus accumbens and prefrontal cortex

Recent evidence suggests that behavioral sensitization to amphetamine is associated with alterations in excitatory amino acid (EAA) transmission in perikarya (ventral tegmental area) and terminal regions (nucleus accumbens [NAc]) of the mesoaccumbens dopamine system. The present study determined whether repeated amphetamine administration alters expression of mRNAs for AMPA receptor subunits. We studied the NAc, because it is the site of expression of amphetamine sensitization, and the prefrontal cortex (PFC), because it is the origin of EAA projections that regulate the mesoaccumbens dopamine system. Rats were treated for 5 days with 5 mg/kg/day amphetamine sulfate or vehicle (controls) and perfused 3 or 14 days after the last injection. We used a novel in situ hybridization method that allows quantification of mRNA levels [Lu et al. (1996) J. Neurosci. Methods, 65:69-76]. Repeated amphetamine administration decreased levels of GluR1 and GluR2 but not GluR3 mRNAs in both core and shell subregions of the NAc at the 14 day withdrawal time; no changes were observed after 3 days of withdrawal. In contrast, levels of GluR1 mRNA in the PFC were increased at 3 but not 14 days of withdrawal, while GluR2 and 3 mRNAs were unchanged. Levels of GluR4 mRNA were very low in both NAc and PFC. Functional properties of heteromeric AMPA receptors are determined by subunit composition. Thus, the observed changes in mRNAs for AMPA receptor subunits may result in altered AMPA transmission in NAc and PFC. This, in turn, may influence the responsiveness of the mesoaccumbens DA system to psychomotor stimulants and potentially contribute to behavioral sensitization.

Trends and future developments in the pharmacological treatment of acute ischaemic stroke

Stroke stands as the third leading cause of death. It makes great demands on patients, who must not only survive the complications of the acute stages, but must cope then with the great physical and economic costs of long-term disabilities. Therefore, there is urgent need to establish generally useful regimens for the acute treatment of ischaemic stroke. Three treatment approaches are based upon pathophysiologic concepts derived from experimental work with focal cerebral ischaemia. These include pharmacologic strategies for arterial recanalisation, inhibition of inflammatory processes and neural protection. Focal cerebral ischaemia secondary to occlusion of a brain-supplying artery initiates neuronal and microvascular events, and the simultaneous processes of inflammation which further injure tissue. The use of plasminogen activators to mediate thrombus and lysis in the acute setting has been shown to be clinically beneficial. Further work with arterial reperfusion strategies is under way. Early clinical studies with polymorphonuclear leukocyte-dependent endothelial adhesion receptor antagonists are being completed, but a strategy has yet to emerge. A large effort examining the potential efficacy of agents which may stabilise or protect neurons from ischaemic injury has shown promise in experimental models, and has been translated into clinical trials. Experimental work, and limited clinical experience, have indicated that: (a) the time window for intervention is important in limiting ischaemic and inflammatory injury, and for reducing the risk of haemorrhagic transformation; (b) putative neuroprotective strategies may potentially elongate the time interval for treatment; and (c) limitations from the adverse effects of plasminogen activators and of agents which beneficially affect neuronal dysfunction during ischaemia must yet be overcome. This review surveys pharmacological approaches currently undergoing evaluation which provide the goal of establishing effective strategies for the treatment of patients with acute cerebral ischaemia.

Diversity of channels generated by different combinations of epithelial sodium channel subunits

The epithelial sodium channel is a multimeric protein formed by three homologous subunits: alpha, beta, and gamma; each subunit contains only two transmembrane domains. The level of expression of each of the subunits is markedly different in various Na+ absorbing epithelia raising the possibility that channels with different subunit composition can function in vivo. We have examined the functional properties of channels formed by the association of alpha with beta and of alpha with gamma in the Xenopus oocyte expression system using two-microelectrode voltage clamp and patch-clamp techniques. We found that alpha beta channels differ from alpha gamma channels in the following functional properties: (a) alpha beta channels expressed larger Na+ than Li+ currents (INa+/ILi+ 1.2) whereas alpha gamma channels expressed smaller Na+ than Li+ currents (INa+/ILi+ 0.55); (b) the Michaelis Menten constants (Km of activation of current by increasing concentrations of external Na+ and Li+ of alpha beta channels were larger (Km > 180 mM) than those of alpha gamma channels (Km of 35 and 50 mM, respectively); (c) single channel conductances of alpha beta channels (5.1 pS for Na+ and 4.2 pS for Li+) were smaller than those of alpha gamma channels (6.5 pS for Na+ and 10.8 pS for Li+); (d) the half-inhibition constant (Ki) of amiloride was 20-fold larger for alpha beta channels than for alpha gamma channels whereas the Ki of guanidinium was equal for both alpha beta and alpha gamma. To identify the domains in the channel subunits involved in amiloride binding, we constructed several chimeras that contained the amino terminus of the gamma subunit and the carboxy terminus of the beta subunit. A stretch of 15 amino acids, immediately before the second transmembrane domain of the beta subunit, was identified as the domain conferring lower amiloride affinity to the alpha beta channels. We provide evidence for the existence of two distinct binding sites for the amiloride molecule: one for the guanidium moiety and another for the pyrazine ring. At least two subunits alpha with beta or gamma contribute to these binding sites. Finally, we show that the most likely stoichiometry of alpha beta and alpha gamma channels is 1 alpha: 1 beta and 1 alpha: 1 gamma, respectively.

Immunocytochemical characterization of AMPA-selective glutamate receptor subunits: laminar and compartmental distribution in macaque striate cortex

Subunit proteins that comprise functional AMPA receptors were localized by immunocytochemical methods in the adult macaque primary visual cortex (V1). GluR1, GluR2/3/4c, and GluR4 immunoreactivity consisted of rich plexuses of punctate profiles scattered throughout the neuropil, in radial arrays, and outlining the membrane of somata and proximal dendrites. Cytoplasmic immunoreactivity was limited. GluR2/3/4c immunostaining was more prominent along the somata surface and exhibited greater levels of cytoplasmic immunoreactivity than GluR1 and GluR4 immunostaining. The density of AMPA subunit immunoreactive elements also varied across layers and compartments of macaque V1. Immunoreactivity for GluR1, GluR2/3/4c, and GluR4 was densest in three bands that corresponded to layers IVA, IVC, and VI. Immunostaining for each subunit was also unevenly distributed within many of the layers. In layers II-III, patches of intense immunostaining coincided with cytochrome oxidase (CO)-rich blobs. In layer IVA, intense subunit staining formed a conspicuous honeycomb pattern. In layer IVC, subunit staining formed a radial lattice. GluR2/3/4c subunit immunostaining was also preferentially distributed within the CO-rich blobs of layers V-VI. These findings demonstrate that AMPA subunit immunoreactivity is densely concentrated in layers and compartments receiving direct geniculocortical innervation. This distribution, which differs from that of excitatory synapses, suggests that the density of AMPA receptors is unevenly distributed at synaptic and possibly extrasynaptic sites within macaque visual circuits.

Glutamate, excitotoxicity and amyotrophic lateral sclerosis

The "glutamate hypothesis" is one of three major pathophysiological mechanisms of motor neurone injury towards which current research effort into amyotrophic lateral sclerosis (ALS) is directed. There is great structural and functional diversity in the glutamate receptor family which results from combinations of 14 known gene products and their splice variants, with or without additional RNA editing. It is possible that motor neurones express a unique molecular profile of glutamate receptors. Abnormal activation of glutamate receptors is one of five main candidates as a final common pathway to neuronal death. In classical acute excitotoxicity, there is influx of Na+ and CI-, and destabilisation of intracellular Ca2+ homeostasis, which activates a cascade of harmful biochemical events. The concept of secondary excitotoxicity, where cellular injury by glutamate is triggered by disturbances in neuronal energy status, may be particularly relevant to a chronic neurodegenerative disease such as ALS. Data are now beginning to emerge on the fine molecular structure of the glutamate receptors present on human motor neurones, which have a distinct profile of AMPA receptors. Two important molecular features of motor neurones have been identified that may contribute to their vulnerability to neurodegeneration. The low expression of calcium binding proteins and the low expression of the GluR2 AMPA receptor subunit by vulnerable motor neurone groups may render them unduly susceptible to calcium-mediated toxic events following glutamate receptor activation. Eight lines of evidence that indicate a disturbance of glutamatergic neurotransmission in ALS patients are reviewed. The links between abnormal activation of glutamate receptors and other potential mechanisms of neuronal injury, including activation of calcium-mediated second messenger systems and free radical mechanisms, are emphasised. Riluzole, which modulates the glutamate neurotransmitter system, has been shown to prolong survival in patients with ALS. Further research may allow the development of subunit-specific therapeutic targeting of glutamate receptors and modulation of "downstream" events within motor neurones, aimed at protecting vulnerable molecular targets in specific populations of ALS patients.

Immunocytochemical characterization of AMPA-selective glutamate receptor subunits: laminar and compartmental distribution in macaque striate cortex

Subunit proteins that comprise functional AMPA receptors were localized by immunocytochemical methods in the adult macaque primary visual cortex (V1). GluR1, GluR2/3/4c, and GluR4 immunoreactivity consisted of rich plexuses of punctate profiles scattered throughout the neuropil, in radial arrays, and outlining the membrane of somata and proximal dendrites. Cytoplasmic immunoreactivity was limited. GluR2/3/4c immunostaining was more prominent along the somata surface and exhibited greater levels of cytoplasmic immunoreactivity than GluR1 and GluR4 immunostaining. The density of AMPA subunit immunoreactive elements also varied across layers and compartments of macaque V1. Immunoreactivity for GluR1, GluR2/3/4c, and GluR4 was densest in three bands that corresponded to layers IVA, IVC, and VI. Immunostaining for each subunit was also unevenly distributed within many of the layers. In layers II-III, patches of intense immunostaining coincided with cytochrome oxidase (CO)-rich blobs. In layer IVA, intense subunit staining formed a conspicuous honeycomb pattern. In layer IVC, subunit staining formed a radial lattice. GluR2/3/4c subunit immunostaining was also preferentially distributed within the CO-rich blobs of layers V-VI. These findings demonstrate that AMPA subunit immunoreactivity is densely concentrated in layers and compartments receiving direct geniculocortical innervation. This distribution, which differs from that of excitatory synapses, suggests that the density of AMPA receptors is unevenly distributed at synaptic and possibly extrasynaptic sites within macaque visual circuits.

Molecular determinants of agonist discrimination by NMDA receptor subunits: analysis of the glutamate binding site on the NR2B subunit

NMDA receptors require both L-glutamate and the coagonist glycine for efficient channel activation. The glycine binding site of these heteromeric receptor proteins is formed by regions of the NMDAR1 (NR1) subunit that display sequence similarity to bacterial amino acid binding proteins. Here, we demonstrate that the glutamate binding site is located on the homologous regions of the NR2B subunit. Mutation of residues within the N-terminal domain and the loop region between membrane segments M3 and M4 significantly reduced the efficacy of glutamate, but not glycine, in channel gating. Some of the mutations also decreased inhibition by the glutamate antagonists, D-AP5 and R-CPP. Homology-based molecular modeling of the glutamate and glycine binding domains indicates that the NR2 and NR1 subunits use similar residues to ligate the agonists' alpha-aminocarboxylic acid groups, whereas differences in side chain interactions and size of aromatic residues determine ligand selectivity.

Developmental changes of RNA editing of glutamate receptor subunits GluR5 and GluR6: in vivo versus in vitro

In the present series of experiments we compared the up-regulation of GluR5 and GluR6 mRNA editing during the transition from the embryonic to the adult state with changes in the extent of editing during neuronal development in vitro. RNA was isolated from rats, from the cerebral cortex, hippocampus and cerebellum of embryonic brains (E19) and adult brains (2 months old), as well as from neurons prepared from the cortex, hippocampus and cerebellum of embryonic brains (E19) and held in tissue culture for 2, 8 or 16 days. Quantification of mRNA editing was achieved by using standards prepared from plasmids with cDNA inserts derived from the edited and unedited state of both GluR5 and GluR6 mRNA. In addition, GluR5 mRNA levels were determined in brain tissue and neuronal cells in culture by quantitative PCR. Developmental changes in the extent of GluR5 and GluR6 mRNA editing were different in vivo compared to in vitro. For GluR5 mRNA editing these differences were most pronounced in cerebellar neurons compared to cerebellar tissue: the extent of GluR5 mRNA editing found in vivo at E19 was significantly down-regulated in cerebellar neurons during the first 8 days in culture, and after 16 days in vitro the extent of editing was still about 50% of that found in the adult state in vivo. For GluR6 mRNA editing these differences were most pronounced in hippocampal neurons compared to the hippocampus in vivo: the extent of GluR6 mRNA editing found in vivo at E19 was significantly down-regulated in vitro during the whole culturing period, most pronounced after 8 days in vivo (to below 40% of that found at E19 and to below 30% of that found in adult hippocampus). GluR5 mRNA levels increased markedly from E19 to the adult brain. However, we could not find any specific pattern of changes in mRNA levels which might account for the development changes in the profile of GluR5 mRNA editing. Comparing developmental changes in the extent of mRNA editing of glutamate receptor subunits may help to elucidate the molecular and regulatory mechanisms of this important editing reaction. Strict control and clear indication of the age of primary neuronal cell cultures used should be required in accounts of electrophysiological or neurotoxicological studies as this would increase comparative usefulness of such experiments, since calcium fluxes through glutamate receptor ion channels are likely to influence the system significantly.

Localization of AMPA-selective glutamate receptors in the auditory brainstem of the barn owl

AMPA receptor subunit-specific antibodies were used to determine if the distribution of excitatory amino acid receptors in the owl's auditory brainstem and midbrain nuclei reflected specializations for temporal processing. Each auditory nucleus displays characteristic levels of immunostaining for the AMPA receptor subunits GluR1-4, with high levels of the subtypes which exhibit rapid desensitization (GluR4 and 2/3). In the auditory brainstem, levels of GluR2/3 and GluR4 were very high in the cochlear nucleus magnocellularis and the nucleus laminaris. The different cell types of the cochlear nucleus angularis and the superior olive were characterized by heterogeneous GluR2/3 and 4 immunostaining. GluR1 levels were very low or undetectable. In the lemniscal nuclei, most neurons contained low levels of GluR1, and dense GluR2/3 and GluR4 immunoreactivity, with high levels of GluR4 in the dendrites. Levels of GluR4 were higher in the anterior portion of the ventral nucleus of the lateral lemniscus. The divisions of the inferior colliculus could be distinguished on the basis of GluR1-4 immunoreactivity, with high levels of GluR4 and moderate levels of GluR1 in the external nucleus. No major differences were observed between the pathways for encoding time and sound level cues.

AMPA receptors in cultured vestibular ganglion neurons: detection and activation

The presence and the activity of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) glutamate receptors were investigated in mouse cultured vestibular ganglion neurons using immunocytochemistry and measurement of intracellular calcium concentration ([Ca2+]i) by spectrofluorimetry. Cultures of dissociated vestibular ganglia from 18 gestation day mouse embryos were grown in vitro for 3-4 days. Immunocytochemical labelling of AMPA receptor subunits GluR2/R3 and GluR4 was detected in neuron cell bodies and proximal neurites and more lightly in glial cells. There was no clear selective subcellular localization of the different subunits. For the GluR1 subunit a signal was observed only in some neurons and neurites and was weak. Vestibular ganglion neurons responded to fast application of 1 mM glutamate and 10 mM aspartate through unknown receptors by a transient increase in [Ca2+]i. The mean amplitude of this rapid increase was about nine times the resting level and recovery was complete within 30-45 s after the application. If separated by an interval of at least 10 min, consecutive applications produced similar calcium responses. AMPA (1 mM) application induced the same type of responses. Five minutes prior to the AMPA exposure, the application of a specific AMPA antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX, 1.5 mM), in the external medium inhibited the response to AMPA. Chelation of external calcium by EGTA (1.5 mM) abolished the responses to drug applications, indicating that an influx of external calcium is involved in the [Ca2+]i increase. These observations suggest that heteromeric AMPA receptors are expressed in vestibular ganglion neurons in culture and play a functional role in their glutamate-induced depolarization. Experiments are in progress using specific AMPA and NMDA antagonists to characterize the participation of the two types of ionotropic glutamate receptors in the glutamate/aspartate-induced intracellular calcium response.

Glutamate receptors in the olfactory bulb synaptic circuitry: heterogeneity and synaptic localization of N-methyl-D-aspartate receptor subunit 1 and AMPA receptor subunit 1

In this study, we analysed the molecular heterogeneity and synaptic localization of the N-methyl-D-aspartate receptor subunit 1 and the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor subunit 1 in the olfactory bulb glomerular synaptic circuitry. Semiquantitative reverse transcriptase polymerase chain reaction showed that approximately 40% of the N-methyl-D-aspartate receptor subunit 1 messenger RNA splice variants contain the N1 exon, which conveys specific functional properties on the channel. In other forebrain and hindbrain regions that we examined, the ratio of the N1-containing (receptor subunit 1(1XX)) to N1-lacking (receptor subunit 1(0XX)) N-methyl-D-aspartate receptor subunit 1 messenger RNAs varied considerably. The cellular and subcellular distribution of N-methyl-D-aspartate receptor subunit 1 and AMPA receptor subunit 1 was investigated with antibodies generated against the C-terminal domain of the individual subunits [Petralia R. S. et al. (1994) J. Neurosci. 14, 667 696; Wenthold R. J. et al. (1992) J. biol Chem. 267, 501 507]. Both N-methyl-D-aspartate receptor subunit 1 and AMPA receptor subunit 1 were localized to the postsynaptic density of asymmetric synapses established by olfactory receptor neuron terminals with the dendrites of mitral and tufted cells. Not all of these synapses, however, were labelled. These results are consistent with the notion that glutamate is the neurotransmitter at the olfactory nerve to mitral and tufted cell synapses, and suggest a high heterogeneity in the expression of the postsynaptic glutamate receptors.

Kainate/AMPA receptors expressed on human fetal astrocytes in long-term culture

Long-term cultivation of primary human fetal brain cells has yielded a homogeneous population of glial progenitors of extended life span. These human astrocyte precursor (HAP-1) cells have been in culture for greater than 1 year, are diploid, and do not form colonies in soft agar. The culture was established in 10% fetal calf serum (FCS), although cells greatly increase their proliferative rate when both basic fibroblast growth factor and FCS are present in the culture media. HAP-1 cells express the cytoskeletal proteins glial fibrillary acidic protein, vimentin, and nestin. HAP-1 cells express the AMPA/kainate receptor subunit genes GluRs 1, 3, and 4 and the kainate receptor subunit genes GluR6, KA1, and KA2. Immunohistochemistry confirms the expression of GluR subunit proteins. HAP-1 cells demonstrate a kainate-responsive current found to be blockable by CNQX. HAP-1 cells will serve in the study of human glial cells and ligand-gated ion channels and in the identification of compounds which might act as agonists or antagonists at these receptor-ion channel complexes.

Expression of the AMPA-selective receptor subunits in the vestibular nuclei of the chinchilla

The distribution of the AMPA type glutamate receptor has been investigated throughout the central nervous system; however, no detailed description of its distribution is available in the vestibular nuclei. In the present study, in situ hybridization histochemistry and immunohistochemistry were used to localize the messenger RNAs and proteins of the AMPA-selective receptor subunits GluR1, GluR2, GluR3 and GluR4 in the vestibular nuclei of the chinchilla. Immunohistochemistry with subunits specific antisera showed differential distribution of the subunits in the vestibular nuclei. GluR2/3 antiserum labeled the most neurons, suggesting that many if not all vestibular neurons receive glutamatergic input. GluR1-positive neurons were fewer than GluR2/3 immunoreactive neurons and GluR4 immunoreactivity was found in the fewest number of neurons. GluR1 and GluR4 immunoreactivity was also found in astrocyte-like structures. In situ hybridization with 35S-labeled complementary RNA probes confirmed the distribution of the AMPA receptor subunits obtained by immunohistochemistry. Quantitative analysis of the levels of hybridization showed a high degree of diversity in the levels of expression of the GluR2 subunit mRNA, with the highest levels of expression in the giant Deiter's cells of the lateral vestibular nuclei and the lowest levels in the small neurons throughout the vestibular nuclei. The subunit compositions of the AMPA receptors determine their physiological properties. Differential distribution and levels of expression of the receptor subunits in the vestibular nuclei may be related to the characteristics of information processing through the vestibular system.

Cellular, subcellular, and subsynaptic distribution of AMPA-type glutamate receptor subunits in the neostriatum of the rat

Glutamate released in the basal ganglia is involved in the expression of clinical symptoms of neurodegenerative diseases like Parkinson's or Huntington's. Neostriatal neurons are the targets of glutamatergic inputs derived from the cortex and the thalamus acting via AMPA-type as well as other glutamate receptors. To determine the location of subunits of the AMPA subclass of glutamate receptors (GluR) in the rat neostriatum, we applied multiple immunocytochemical techniques using anti-peptide antibodies against the GluR1, GluR2/3, and GluR4 subunits at both the light and electron microscopic levels. All medium spiny efferent neurons, some of which were identified as striatonigral neurons, displayed immunoreactivity for GluR1 and GluR2/3 subunits. Double immunofluorescence revealed that at least 70-90% of parvalbumin-immunopositive GABAergic interneurons were immunoreactive for each of GluR1, GluR2/3, or GluR4 subunits and that at least 40% of choline acetyltransferase-immunopositive cholinergic interneurons were immunopositive for GluR1 or GluR4 subunits. The majority of nitric oxide synthase-immunopositive neurons had no detectable immunoreactivity for any of the AMPA receptor subunits. Electron microscopic analysis confirmed the presence of immunoreactivity for GluR1 and GluR2/3 in the perikarya of spiny neurons and interneurons and GluR4 in perikarya of interneurons only. GluR1 and GluR2/3 subunits were detected in dendrites and spines. A significant population of extrasynaptic receptors was revealed by pre-embedding immunogold labeling along the plasma membranes of perikarya, dendrites, and spines. Receptors were concentrated in the postsynaptic membrane specialization of asymmetrical synapses, as revealed by the postembedding immunogold method. Quantitative analysis demonstrated that immunoreactivity for the GluR1 and GluR2/3 subunits is higher at the periphery than at the middle of the postsynaptic membrane specialization. Our results demonstrate that AMPA receptor subunits are distributed widely and heterogeneously among striatal neurons and are concentrated on the postsynaptic membrane of asymmetrical synaptic specializations, although extrasynaptic receptors are also present.

Modulation and block of ion channels: a new biology of polyamines

The endogenous polyamines, spermine, spermidine, and putrescine have effects on several types of cation channels. Intracellular polyamines, in particular spermine, contribute to intrinsic gating and rectification of strong inward rectifier K+ channels. Intracellular spermine is also responsible for inward rectification of some types of Ca(2+)-permeable AMPA and kainate receptors. Spermine has a number of effects on the activity of the NMDA subtype of glutamate receptor, involving two or more extracellular polyamine binding sites on the NMDA receptor. In K+ channels and glutamate receptors, some of the amino acids in the receptor/channel structure that influence to polyamines have been identified, leading to a partial understanding of the effects of polyamines at a molecular level. Block of K+ channels by intracellular polyamines is likely to be an important receptors by intracellular spermine and modulation by extracellular spermine may affect excitability and the influx of Ca2+ in neurons and glial cells of the nervous system.

Regional gene expression of the glutamate receptor subtypes GluR1, GluR2, and GluR3 in human postmortem brain

Although glutamatergic receptors are localized throughout the mammalian central nervous system (CNS), the specific cellular localization of the various glutamatergic receptor subtypes throughout human brain remains largely unknown. PCR fragments to human GluR1, GluR2, and GluR3 receptor subtypes were cloned and used as probes for in situ hybridization in order to examine the anatomical and cellular localization of glutamate receptor subtype gene expression in dissected regions of human postmortem brain tissue. Although hybridization was observed throughout the CNS, results indicated that the highest levels of hybridization were in the hippocampus, with localization primarily to cells in the pyramidal cell layer of the CA1-CA3 region, and the granular cells of the dentate gyrus. Prominent hybridization also was observed in the medium to large neurons of the cingulate cortex, temporal lobe, septum, and amygdala, as well as in scattered neurons in the thalamus, cerebral cortex, and medulla. A striking pattern of differential hybridization was observed within the cerebellum. GluR1 demonstrated light hybridization along the Purkinje/Bergmann glia layer, with GluR2 and GluR3 demonstrating hybridization to Purkinje cells, and GluR3 also to cells within the molecular layer, previously identified as stellate-basket cells. Changes in glutamate receptor function have been shown to be important in the pathogenesis of a number of neurological disorders. Therefore, an examination of glutamatergic receptor expression in human postmortem brain tissue may provide important information on the molecular basis of a variety of neurological and psychiatric disorders of the CNS.

Heteromultimeric CLC chloride channels with novel properties

The skeletal muscle chloride channel CLC-1 and the ubiquitous volume-activated chloride channel CLC-2 belong to a large gene family whose members often show overlapping expression patterns. CLC-1 and CLC-2 are coexpressed in skeletal and smooth muscle and in the heart. By coexpressing CLC-1 and CLC-2 in Xenopus oocytes, we now show the formation of novel CLC-1/CLC-2 heterooligomers that yield time-independent linear chloride currents with a chloride-->bromide-->iodide selectivity sequence. Formation of heterooligomeric CLC channels increases the number and possible functions of chloride channels.

Differential distribution of AMPA receptors and glutamate during pre- and postnatal development in the visual cortex of ferrets

Immunohistochemical methods were used to study the distribution and time-course of appearance of cells expressing glutamate and alpha-amino-3-hydroxy-5-methyl-4-isoaxazole propionic acid (AMPA)-type glutamate receptors (GluR1 and GluR2/3) during development of the ferret visual cortex. Glutamate is present in many neurons in the ventricular zone, intermediate zone, developing cortical plate, and marginal zone as early as embryonic day (E) 34 (birth is at E41 in ferrets). Glutamate attains its adult distribution coincident with the completion of cellular migration. By contrast, GluR1 immunoreactivity emerges more slowly. By birth, GluR1 immunoreactivity is present only in a few neurons in the marginal zone and ventricular zone but is abundant in the marginal zone and subplate, where synaptogenesis commences. The number and staining intensity of GluR1-positive cells increases dramatically during the first two postnatal weeks and is maximal between the second and third week, before slowly declining to adult levels. Cortical cells immunopositive for GluR2/3 follow a similar pattern, although their distribution differs: GluR2/3-positive cells are mainly pyramidal cells. During the first postnatal week, GluR2/3 is also transiently present in fibers in the intermediate zone, which at this stage contains many thalamocortical and callosal and corticofugal axons. The abundance of glutamate at fetal stages, especially in the ventricular zone, is consistent with the previously proposed role of glutamate in mediating trophic effects in vivo, as previously demonstrated in vitro. The expression of AMPA receptors, as well as their transient overexpression, confirms the results of in situ hybridization studies and may imply a developmental role in neuronal differentiation for these receptors, in addition to their mature role in mediating cortical transmission.

A study of the oligomeric state of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-preferring glutamate receptors in the synaptic junctions of porcine brain

The number of the subunits in an alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-preferring L-glutamate receptor in the synaptic junctions of porcine brain was investigated in this study. Upon incubation of the synaptic junctions with three cross-linking regents, dimethyl adipimidate (DMA), dimethyl suberimidate (DMS) and N-succinimidyl-(4-azidophenyl)-1,3'-dithiopropionate (SADP), AMPA receptor subunits in higher-molecular-mass aggregates were detected by immunoblotting. These aggregates migrated as proteins of approx. 200, 300 and 400 kDa. The number and identity of the subunits in a solubilized AMPA receptor were also investigated here. Two samples, W1 and W2, enriched in AMPA receptors were prepared from synaptic junctions by a combination of detergent-solubilization, anion-exchange chromatography and wheatgerm agglutinin affinity chromatography. Hydrodynamic behaviour analyses revealed that the majority of the AMPA receptors in either one of these samples were asymmetrical detergent-surrounded particles with a protein mass around 350 kDa. SDS/PAGE analysis revealed that the majority of AMPA receptors in the W1 sample were comprised of dimers of 106 kDa subunits which were covalently linked by disulphide bonds. Cross-linking these receptors with SADP yielded a new band of approx. 400 kDa. The results obtained here, either from the studies of AMPA receptors embedding in synaptic junctions or from those of detergent-solubilized and partially purified receptors, suggest that AMPA receptors contain a basic core structure comprising of four 106 kDa subunits.

Identification of the amino acid subsets accounting for the ligand binding specificity of a glutamate receptor

In a situation so far unique among neurotransmitter receptors, glutamate receptors share amino acid sequence similarities with the bacterial periplasmic binding proteins (PBPs). On the basis of the primary structure similarity of two bacterial periplasmic proteins (lysine/arginine/ornithine- and phosphate-binding proteins) with the chick cerebellar kainate-binding protein (KBP), a member of the ionotropic glutamate receptor family, we have generated a three-dimensional model structure of the KBP extracellular domain. By an interplay between homology modeling and site-directed mutagenesis, we have investigated the kainate binding properties of 55 different mutants (corresponding to 43 positions) and studied the interactions of some of these mutants with various glutamatergic ligands. As a result, we present here the subsets of amino acids accounting for the binding free energies and specificities of KBP for kainate, glutamate, and CNQX and propose a three-dimensional model, at the microarchitectural level, of the glutamatergic binding domain.

Distribution of glutamate receptor subunit GluR1 and GABA in human cerebral neocortex: a double immunolabelling and electron microscopic study

Specimens of human cerebral neocortex were obtained during neurosurgical operations and studied by immunocytochemistry and electron microscopy, using antibodies to the glutamate receptor subunit GluR1 and gamma-aminobutyric acid (GABA). Many GluR1-positive pyramidal neurons and fewer GluR1-positive non-pyramidal neurons were present in the cortex. Non-pyramidal neurons were more heavily labelled for GluR1 than pyramidal neurons. Most GABAergic neurons were labelled for GluR1. The white matter was unstained, except for occasional labelled neurons. This pattern of GluR1 immunostaining is similar to that in rat cerebral cortex, but is different from that in the hippocampus and amygdala, where large numbers of pyramidal or projection neurons, but few non-pyramidal or GABAergic neurons, were labelled for GluR1.

Purification and biochemical characterization of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate-sensitive L-glutamate receptors of pig brain

Two preparations of glutamate receptors were purified from the synaptic junctions of pig brain by a combination of detergent solubilization, anion-exchange chromatography, wheat-germ agglutinin affinity chromatography and sedimentation through sucrose gradients. These preparations were enriched in specific L-[3H]glutamate binding activity (> 5000 pmol of glutamate binding sites/mg of protein), and the rank order of ligand affinity for binding to these preparations was: quisqualate > 6-cyano-7- nitroquinoxaline-2,3-dione > alpha-amino-3-hydroxy-5-methyl-4- isoxazolepropionic acid (AMPA) > L-glutamate > kainate > > N-methyl-D-aspartate approximately L-2-amino-4-phosphonobutyrate. SDS/PAGE analysis revealed that more than 80% of the protein in either of these preparations appeared as a single protein band of 106 kDa. Two-dimensional gel electrophoresis further revealed that these 106 kDa proteins consisted of a series of acidic proteins which were recognized by antibodies against rat AMPA receptor subunits. These 106 kDa proteins were also recognized by wheatgerm agglutinin and concanavalin A; in addition, peptide N-glycosidase F treatment of these preparations decreased their size to 99 kDa. Our results suggest that the putative glutamate receptors isolated here are likely to belong to the AMPA subtype of glutamate receptors in pig brain. Using the purification procedure reported here, 5 micrograms of AMPA receptor proteins can be isolated from 250 g of pig brain tissue.

Distinct distributions of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor subunits and a related 53,000 M(R) antigen (GR53) in brain tissue

Polyclonal antibodies against specific carboxy-terminal sequences of known alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor subunits (GluR-4) were used to screen regional homogenates and subcellular fractions from rat brain. Affinity purified anti-GluR1 (against amino acids 877-899), anti-GluR2/3 (850-862), and anti-GluR4a and anti-GluR4b (868-881) labeled distinct subunits with the expected molecular weight of approximately 105,000. These antigens were shown to have distinct distributions in the brain. While GluR2/3 epitopes had a distribution profile similar to that of the presynaptic marker synaptophysin, GluR1 was notable for its abundance in the hippocampus and its relatively low density in neocortical areas, and GluR4 was highly enriched in cerebellar tissue. An additional antigen (glutamate receptor-related, GR53) of lower molecular weight (50,000-59,000) was recognized in rat, human, frog, chick and goldfish brain samples by anti-GluR4a as well as by anti-GluR1 at, an antibody that specifically recognizes the extracellular aminoterminal domain of GluR1 (amino acids 163-188). Both antibodies also labeled antigens of approximately 105,000 mol. wt in brain tissue from all species tested. The approximately 53,000 mol. wt antigen was concentrated 10-20-fold in synaptic membranes vs homogenates across rat brain regions. Both the 105,000 and the 53,000 mol. wt proteins were also concentrated in postsynaptic densities, and neither of the two antigens were evident in seven non-brain tissue samples. These data indicate that AMPA receptors have regionally different subunit combinations and that some AMPA receptor composites include proteins other than the conventional 105,000 mol. wt GluR subunits.