Molecular diversity of glutamate receptors and implications for brain function

Rat hippocampal kindling induces changes in the glutamate receptor mRNA expression patterns in dentate granule neurons

The expression level of the mRNAs encoding the Flip and Flop versions of the AMPA-selective glutamate receptor subunits A, B, C and D was studied using in situ hybridization in the hippocampus of rats kindled by Schaffer collateral/commissural fibre stimulation. The expression levels of the Flip variant of GluR-A, B and C mRNAs were bilaterally enhanced in the dentate granule neurons of fully kindled animals 24 h after the last seizure. These changes were already observed after the sixth kindling stimulation (preconvulsive-stage), but not after a single afterdischarge. Four weeks after the last seizure, when the animals were still hypersensitive to kindling stimulations, only GluR-A Flip expression was enhanced. These results suggest that kindling epileptogenesis is accompanied by an increased number and enhanced sensitivity of the expressed AMPA type glutamate receptors in the fascia dentata, leading to an enhanced excitatory synaptic transmission which may contribute to the process of kindling epileptogenesis.

Immunohistochemical localization of metabotropic glutamate receptors, mGluR2 and mGluR3, in rat cerebellar cortex

The distribution of the metabotropic glutamate receptors mGluR2 and mGluR3 was immunohistochemically examined in the rat cerebellar cortex at both light and electron microscope levels. An antibody was raised against a fusion protein containing a C-terminal portion of mGluR2. On immunoblot, the antibody reacted with both mGluR2 and mGluR3 in rat brain. mGluR2/3 immunoreactivity was expressed in cell bodies, dendrites, and axon terminals of Golgi cells, as well as in presumed glial processes. Golgi axon terminals with mGluR2/3 immunoreactivity were often encountered in the vicinity of glutamatergic mossy fiber terminals. The results suggest that transmitter glutamate may exert control influences upon Golgi cells not only through dendritic mGluR2/3, but also through axonal mGluR2/3.

Regulation of the second-messenger systems in the rat spinal cord during prolonged peripheral inflammation

Unilateral intraplantar injection of Freund's complete adjuvant (FCA) into 1 hind paw of rats was used as a model of peripheral inflammation and persistent pain in order to examine time course effects of a continuous barrage of nociceptive input on the second-messenger transducing systems in the spinal cord. cAMP, cGMP and inositol 1,4,5-trisphosphate (insP3) were extracted from the lumbosacral cord at days 1, 7, 14, 21 and 42 following FCA injection and quantified by either radioreceptor-assay (RRA) or radioimmunoassay (RIA). The lumbosacral contents of cAMP and cGMP when quantified in whole lumbosacral cord segment were not significantly changed by FCA treatment at all time points. InsP3 accumulation was significantly increased on days 14, 21 and 42 following FCA injection relative to sham-treated time-matched controls. However, cGMP and insP3 contents were significantly increased in the left longitudinal half of the lumbar enlargement ipsilateral to the injected paw on day 21 following FCA treatment, but not in the sham-treated time-matched controls. With [3H]insP3 as a ligand, Scatchard (Rosenthal) analyses of the concentration-dependent saturation curves showed that the densities (Bmax) of insP3 receptors (insP3R) were significantly increased throughout the time course of adjuvant-induced peripheral inflammation. The binding affinities (KD) for insP3R were significantly decreased on days 7, 14 and 21 following FCA injection corresponding to the times of most stable and peak inflammation. InsP3R from the cerebelli of the same rats as used in the lumbosacral insP3R characterization was used as a positive control in this study and did not show any change in both Bmax and KD as a result of FCA treatment, thus demonstrating that the changes in lumbosacral insP3R characteristics might be specific to the nociceptive sensory pathway such as the spinal cord. Thus it appears that sustained afferent nociceptive input induced by FCA injection increased the accumulation of cGMP, insP3 and insP3R density in the spinal cord through increased neuronal activities of functional receptors coupled to major classes of chemical mediators of nociception including neuropeptides and excitatory aminoacids. Changes in insP3 accumulation in the lumbosacral cord following FCA injection were significantly correlated with changes in insP3R density. Changes in the ratios of lumbosacral insP3 contents and insP3R density were also significantly correlated with changes in body weight and hind paw size induced by FCA injection.(ABSTRACT TRUNCATED AT 400 WORDS)

Utilization of the resolved L-isomer of 2-amino-6-phosphonohexanoic acid (L-AP6) as a selective agonist for a quisqualate-sensitized site in hippocampal CA1 pyramidal neurons

Brief exposure of rat hippocampal slices to quisqualic acid (QUIS) sensitizes neurons to depolarization by the alpha-amino-omega-phosphonate excitatory amino acid (EAA) analogues AP4, AP5 and AP6. These phosphonates interact with a novel QUIS-sensitized site. Whereas L-AP4 and D-AP5 cross-react with other EAA receptors, DL-AP6 has been shown to be relatively selective for the QUIS-sensitized site. This specificity of DL-AP6, in conjunction with the apparent preference of this site for L-isomers, suggested that the hitherto unavailable L-isomer of AP6 would be a potent and specific agonist. We report the resolution of the D- and L-enantiomers of AP6 by fractional crystallization of the L-lysine salt of DL-AP6. We also report the pharmacological responses of kainate/AMPA, NMDA, lateral perforant path L-AP4 receptors and the CA1 QUIS-sensitized site to D- and L-AP6, and compare these responses to the D- and L-isomers of AP3, AP4, AP5 and AP7. The D-isomers of AP4, AP5 and AP6 were 5-, 3- and 14-fold less potent for the QUIS-sensitized site than their respective L-isomers. While L-AP4 and L-AP5 cross-reacted with NMDA and L-AP4 receptors, L-AP6 was found to be highly potent and specific for the QUIS-sensitized site (IC50 = 40 microM). Its IC50 values for kainate/AMPA, NMDA and L-AP4 receptors were > 10, 3 and 0.8 mM, respectively. As with AP4 and AP5, sensitization to L-AP6 was reversed by L-alpha-aminoadipate.

Light and electron microscopic immunocytochemical localization of AMPA-selective glutamate receptors in the rat spinal cord

alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-type glutamate receptors are probably the most widespread excitatory neurotransmitter receptors of the central nervous system, and they play a role in most normal and pathological neural activities. However, previous detailed studies of AMPA subunit distribution have been limited mainly to the brain. Thus, a comprehensive study of AMPA receptor subunit distribution was carried out on sections of rat spinal cord and dorsal root ganglia, which were immunolabeled with antibodies made against peptides corresponding to C-terminal portions of the AMPA receptor subunits: GluR1, GluR2/3, and GluR4. In the spinal cord, labeling was most prominent in the superficial dorsal horn, motoneurons, and nuclei containing preganglionic autonomic neurons. Immunostaining also was observed in neurons in other regions including those known to contain Renshaw cells and Ia inhibitory cells. Although overall immunostaining was lighter with antibody to GluR1 than with GluR2/3 and 4, there were neurons that preferentially stained with antibody to GluR1. These "GluR1 intense" neurons were usually fusiform and most concentrated in lamina X. In dorsal root ganglia, immunostaining of ganglion cell bodies was moderate to dense with antibody to GluR2/3 and light to moderate with antibody to GluR4. Possible neuroglia in the spinal cord (mainly GluR2/3 and 4) and satellite cells in dorsal root ganglia (GluR4) were immunostained. Electron microscopic studies of the superficial dorsal horn and lateral motor column showed staining that was restricted mainly to postsynaptic densities and associated dendritic and cell body cytoplasm. In dorsal horn, colocalization of dense-cored vesicles with clear, round synaptic vesicles was observed in unstained presynaptic terminals apposed to stained postsynaptic densities. Subsynaptic dense bodies (Taxi-bodies) were associated with some stained postsynaptic densities in both the superficial dorsal horn and lateral motor column. Based on several morphological features including vesicle structure and presence of Taxi-bodies, it is likely that at least some of the postsynaptic staining seen in this study is apposed to glutamatergic input from primary sensory afferent terminals.

Glutamate receptor expression in rat striatum: effect of deafferentation

The cerebral cortex is the primary source of glutamatergic afferents to the neostriatum. We used in situ hybridization to examine the effect of removal of the glutamatergic input to the striatum by unilateral frontal cortical ablation on the expression of genes encoding subunits from three families of glutamate receptors: N-methyl-D-aspartate receptors (NMDAR1, NMDAR2A, and NMDAR2B); alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors (GluR1-4, flip and flop splice variants); and metabotropic receptors (mGluR1-5). Significant changes were restricted to the dorsolateral quadrant of the ipsilateral striatum, the main projection area of the sensorimotor cortex. The expression of those messages which are normally abundant, NMDAR1, NMDAR2A, GluR1-4 flop and mGluR1, 3 and 5, was decreased in the deafferented dorsolateral striatum by 10-39% at 3 days after cortical ablation and subsequently increased to 120-165% of control at 15 and 60 days. mRNAs encoding the flip isoforms of GluR1-4, mGluR2 and 4, and an alternatively spliced region of NMDAR1 (Insertion I) which are undetectable or present at low levels in the striatum were not induced by cortical ablation. In contrast, both glial fibrillary acid protein and beta-actin mRNA expression were markedly enhanced at 3 and 15 days, returning to near normal at 60 days. Striatal NMDA, AMPA and metabotropic type 1 ligand binding sites were increased as early as 3 days after cortical ablation, reached a peak at 15 days and remained increased for up to 60 days, while metabotropic type 2 binding was slightly but significantly reduced at 3 and 15 days and [3H]kainate binding did not change significantly. These results demonstrate that cortical ablation, and subsequent loss of glutamatergic afferents to the striatum, results in alterations in the expression of genes encoding glutamate receptor subunits in striatal neurons. The regulation of these genes appears to be coordinate, so that the relative abundance of the different messages is preserved.

Metabotropic glutamate receptor activation decreases epileptiform activity in rat neocortex

Intracellular and extracellular recordings were obtained from layers II-III of slices of adult rat neocortex maintained in vitro. Excitatory postsynaptic potentials (EPSPs) and epileptiform discharges (paroxysmal depolarizing shifts, PDSs) were evoked in the presence of bicuculline methiodide. Responses were monitored before, during, and after bath application of the putative metabotropic glutamate receptor agonists, 1S, 3R-ACPD and L-AP4. Peak EPSP amplitude and both PDS amplitude and duration were reduced during 1S,3R-ACPD application. Area measurements indicate that PDSs were reduced to 51.7 +/- 19.9% of control. Stimulus threshold for evoking a PDS was increased in the presence of 1S,3R-ACPD or L-AP4. Pretreatment of slices with 1S.3R-ACPD did not prevent the generation of epileptiform events when bicuculline subsequently was applied. These results indicate that mGluR activation by 1S,3R-ACPD has significant suppressive effects on evoked epileptiform activity in the adult rat neocortex in vitro.

Actions of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) in retinal ON bipolar cells indicate that it is an agonist at L-AP4 receptors

Metabotropic glutamate receptors (mGluRs) include receptors sensitive to L-2-amino-4-phosphonobutyrate (L-AP4) and 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD). To determine whether 1S,3R-ACPD is an agonist at retinal L-AP4 receptors, whole cell voltage clamp recordings were obtained from mudpuppy ON bipolar cells in a superfused retinal slice and L-AP4 and 1S,3R-ACPD were bath applied. Both compounds evoked similar outward currents which reversed near 0 mV and were accompanied by an increased input resistance. Responses to both agonists washed out in parallel suggesting they act through the same second messenger pathway(s). Inhibitors of cGMP-PDE activity suppressed responses to both L-AP4 and 1SR,3RS-ACPD, suggesting that both compounds activate cGMP-PDE. Responses to 1S,3R-ACPD were occluded by prior activation of L-AP4 receptors, but not blocked by the non-AP4, mGluR antagonists, L-aminophosphonopropionic acid (L-AP3) or 4-carboxy-3-hydroxyphenylglycine (4C3H-PG). These results indicate that 1S,3R-ACPD is an agonist at L-AP4 receptors. 1S,3S-ACPD and 4C3H-PG evoked outward currents similar to L-AP4 suggesting they may also be L-AP4 receptor agonists. Using the b-wave of the ERG as an assay for ON bipolar cell responses, concentration/response curves were obtained for ACPD enantiomers. The rank-order potency of ACPD enantiomers at L-AP4 receptors in ON bipolar cells is similar to their rank-order potency at non-AP4, mGluRs in brain which suggests that the receptors possess similar binding sites and may be members of a common receptor family.

Antibodies inactivating mGluR1 metabotropic glutamate receptor block long-term depression in cultured Purkinje cells

Antibodies were raised against two distinct extracellular sequences of the rat mGluR1 metabotropic glutamate receptor expressed as bacterial fusion proteins. Both antibodies specifically reacted with mGluR1 in the rat cerebellum and inhibited the mGluR1 activity as assessed by the analysis of glutamate-stimulated inositol phosphate formation in CHO cells expressing mGluR1. Using these antibodies, we examined the role of mGluR1 in the induction of long-term depression in cultured Purkinje cells. In voltage-clamped Purkinje cells, current induced by iontophoretically applied glutamate was persistently depressed by depolarization of the Purkinje cells in conjunction with the glutamate application. The mGluR1 antibodies completely blocked the depression of glutamate-induced current. The results indicate that activation of mGluR1 is necessary for the induction of cerebellar long-term depression and that these mGluR1 antibodies can be used as selective antagonists.

Stimulation of postsynaptic and inhibition of presynaptic adenylyl cyclase activity by metabotropic glutamate receptor activation

To determine the subcellular distribution of cyclic AMP-coupled metabotropic glutamate receptors (mGluRs), the effects of glutamate agonists on adenylyl cyclase activity were examined using two hippocampal membrane preparations. These were synaptosomes (SY), which are composed of presynaptic terminals, and synaptoneurosomes (SN), which are composed of both pre- and postsynaptic elements. In SY, a water-soluble analogue of forskolin (7 beta-forskolin) increased enzyme activity approximately 10-fold at the highest concentration tested. The selective metabotropic receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) inhibited enzyme activity as did glutamate and quisqualate. L-Amino-4-phosphobutanoate (L-AP4) had no effect on enzyme activity at any concentration tested. The metabotropic receptor antagonist L-2-amino-3-phosphopropionic acid (L-AP3) was not effective in the SY in antagonizing the agonist-induced decreases in adenylyl cyclase activity by glutamate or 1S,3R-ACPD. It was, however, effective at antagonizing quisqualate-induced decreases in enzyme activity. In SN, at the highest concentration tested, 7 beta-forskolin produced a 60-fold increase in adenylyl cyclase activity. As was observed in SY, glutamate decreased adenylyl cyclase activity in SN. In contrast, 1S,3R-ACPD, quisqualate, and L-AP4 increased adenylyl cyclase activity. In the SN, L-AP3 was ineffective in antagonizing any agonist-induced increases (1S,3R-ACPD, L-AP4, and quisqualate) or decreases (glutamate) in adenylyl cyclase activity. The data suggest that postsynaptic metabotropic glutamate receptor activation results in stimulation of adenylyl cyclase activity, whereas inhibition of this enzyme appears to be mediated at least partly through presynaptic mechanisms.

The depolarization-induced outflow of D-[3H]aspartate from rat brain slices is modulated by metabotropic glutamate receptors

Rat brain slices were used to study the effects of different metabotropic glutamate receptor ligands on (i) the depolarization (30 mM KCl)-induced outflow of previously taken up D-[3H]aspartate; (ii) the inhibition of forskolin (30 microM)-induced cyclic AMP accumulation; and (iii) the hydrolysis of phosphoinositides. In addition, the localization of mRNAs coding for different metabotropic glutamate receptor subtypes was detected using in situ hybridization. (1S-3R)-1-Aminocyclopentane-1,3-dicarboxylic acid (30-300 microM), a non selective metabotropic glutamate receptor agonist, significantly increased the KCl-induced output of radioactivity from cortical slices, whereas it inhibited the output from striatal slices. Conversely, (1S,3S,4S)-carboxycyclopropylglycine (0.1-1 microM), a relatively selective agonist of the mGluR2 metabotropic glutamate receptor subtype, had an inhibitory effect on the output of D-[3H]aspartate from both cortical and striatal slices and proved to be the most potent metabotropic glutamate receptor agonist in inhibiting cyclic AMP accumulation, but not in stimulating phosphoinositide hydrolysis. Since 2-amino-4-phosphonobutyrate (a mGluR4, mGluR6 and mGluR7 agonist) was not active in any of the assays tested, we hypothesized that the mGluR2 subtype could be involved in these events. Accordingly, mGluR2 mRNA expression was abundant in cortical neurons projecting to the striatum. Our experiments suggest that the stimulation of metabotropic glutamate receptors may either decrease or increase transmitter release depending on the subtype that prevails in the region under study.

Is the heterologous expression of metabotropic glutamate receptors (mGluRs) an appropriate method to study the mGluR function? Experience with human embryonic kidney 293 cells transfected with mGluR1

The cloning of metabotropic glutamate receptors (mgluRs) has initiated a new approach to the study of their function: the introduction of mGluR cDNA into cells that do not normally express mGluRs, thus allowing the heterologous receptor expression. We have transfected human embryonic kidney (HEK) 293 cells with the full length mGluR1a cDNA and with its truncated variant which encodes the receptor termed mGluR1T (a receptor lacking the long intracellular domain and similar to the splice variant mGluR1c). Transient transfection of HEK-293 cells with mGluR1a, but not the mGluR1T cDNA, resulted in a significant increase in inositol phosphate (IP) formation in absence of any mGluR agonists. This effect was completely dependent on the presence of extracellular calcium, and unlike the agonist-stimulated IP formation it was insensitive to pertussis toxin. The prolonged activation of IP formation might affect the cell physiology. In an attempt to obtain stably transfected cells, we transfected about 1.5 x 10(6) HEK-293 cells with the plasmid conveying the full-length mGluR1a cDNA and the neomycin-resistance gene. Only 12 clones survived the antibiotic selection, and only one of these 12 clones continued to divide. The size of mRNA from the clone was smaller than the full-length mGluR1a mRNA. The shortened mRNA, revealed in the clone, apparently encoded a functional mGluR that was sensitive to glutamate, but unlike the mGluR1a, it did not respond to 1S,3R-ACPD (1S,3R-aminocyclopentane-1,3-dicarboxylic acid). A prudent use of the heterologous cell transfection technique is necessary in studying the function and the pharmacology of mGluRs.

Excitatory amino acids, free radicals and the pathogenesis of motor neuron disease

The cause of motor neuron disease (MND) remains unknown, but the pathogenic involvement of excitatory amino acid (EAA) neurotransmitters and related exogenous compounds has been proposed. We discuss current concepts of the mechanisms of action of EAAs and the evidence for links between these neurotransmitters and free radical hypotheses of neuronal damage. These concepts are especially pertinent following reports of mutations in the gene encoding the free radical scavenging enzyme, copper-zinc superoxide dismutase, in familial MND. New approaches to treatment are suggested by advances in understanding of the disease.

Hydrodynamic and pharmacological characterization of putative alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate-sensitive L-glutamate receptors solubilized from pig brain

L-[3H]Glutamate binding sites with characteristics resembling that of membrane-bound alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate-subtype L-glutamate receptors have been solubilized from pig brain synaptic junctions by Triton X-114. Binding of [3H]AMPA to these soluble sites in the presence of KSCN results in a curvilinear Scatchard plot that can be resolved into a high-affinity component and a low-affinity component. These Triton-X-114-solubilized sites can be further separated into two species of binding sites by gel-filtration chromatography or sucrose-density-gradient centrifugation. The pharmacological profiles of these two species of binding site are almost identical, and the rank orders of potency for glutamatergic drugs in displacing L-[3H]glutamate binding to these sites are quisqualate > 6,7-dinitroquinoxaline-2,3-dione > 6-cyano-7-nitroquinoxaline-2,3-dione > AMPA > L-glutamate > kainate >> N-methyl-D-aspartate = L-2-amino-4-phosphonobutyrate. Both sites are found to bind [3H]AMPA, and in the presence of KSCN the binding activities are significantly enhanced. Analysis of the hydrodynamic behaviour of these binding sites by sucrose-density-gradient centrifugation in H2O- and 2H2O-based solvents and gel-filtration chromatography has revealed that one of these sites (Stokes radius 8.3 nm, sedimentation coefficient 18.5 S) consists of 562 kDa protein and 281 kDa detergent, and the other site (Stokes radius 9.6 nm, sedimentation coefficient 13.4 S) consists of 352 kDa protein and 569 kDa detergent. Frictional coefficients of these sites indicate that these receptor-detergent complexes are asymmetrical in structure, consistent with large transmembrane proteins.

Glutamate receptor phosphorylation and synaptic plasticity

The recent findings that glutamate receptors are phosphorylated and functionally modulated by protein kinases has provided evidence that phosphorylation of these receptors may play a critical role in mechanisms of synaptic plasticity.

Role of Ca2+ in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-mediated polyphosphoinositide turnover in primary neuronal cultures

Excitatory amino acid (EAA) receptors and EAA-mediated stimulation of polyphosphoinositide (poly-PI) turnover were studied in cultured neurons at different days in vitro (DIV). Six main observations have emerged from these studies: (a) Neurons increased their sensitivity to EAAs as a function of time in culture, indicated by increasing EAA-mediated poly-PI turnover. (b) Extracellular Ca2+ concentration played an important role in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-stimulated poly-PI turnover in cells at 4 DIV, whereas poly-PI turnover mediated by L-glutamate and trans-1-amino-cyclopentane-1,3-dicarboxylic acid was not Ca(2+)-dependent. (c) A marked stimulation of poly-PI turnover by AMPA was seen in the cultured neurons at 4 DIV, but not at 17 DIV, suggesting that a distinct EAA receptor sensitive to AMPA is transiently expressed. (d) The Ca2+ ionophore A23187 increased poly-PI turnover in cultured neurons, suggesting that Ca2+ entry is involved in stimulating poly-PI turnover. (e) Stimulation of poly-PI turnover by carbachol was greater in neurons at 17 DIV as compared with 4 DIV, and appeared to be Ca(2+)-dependent across DIV. (f) 6-Cyano-7-nitroquinoxaline-2,3-dione, an antagonist for non-N-methyl-D-aspartate ionotropic EAA receptors, inhibited 100% and 35% of AMPA- and quisqualate-induced poly-PI turnover, respectively, suggesting an involvement of ionotropic AMPA/quisqualate receptors in stimulating poly-PI turnover.

Glutamate receptor update

The past year has seen significant advances in matching the actions of recombinant glutamate receptors with the actions of native receptors, and in mapping their distribution and regulation. The discovery of a novel RNA editing mechanism for AMPA receptors and a revised view of the transmembrane topology of the NMDA receptor subunit, NR1, are particularly noteworthy. Seven metabotropic glutamate receptor subtypes have been identified with several interesting expression patterns and transduction mechanisms; results from work on these subtypes has led to a provocative model of the ligand-binding site. Functional studies of metabotropic receptors have been enhanced by the development of the first subtype-specific antagonist.

Stable transfection of the NR1 subunit in Chinese hamster ovary cells fails to produce a functional N-methyl-D-aspartate receptor

The NR1 subunit of the N-methyl-D-aspartate (NMDA) receptor cDNA was stably transfected into Chinese hamster ovary (CHO) cells. Northern analysis revealed 3 clonal cell lines expressing high levels of NMDAR1 mRNA (BA1, BA2 and BA3). NMDAR1 protein was readily detected by Western analysis in only one of the clonal populations, BA1. Whole-cell patch clamp analysis revealed a lack of functional NMDA receptors in all of the cell lines, including BA1. NMDA-mediated responses were obtained only after a second subunit, NMDAR2A, was transiently expressed in BA1 cells. Therefore, the NR1 subunit alone may not be sufficient to form functional NMDA receptors in CHO cells.

Cellular localization and laminar distribution of NMDAR1 mRNA in the rat cerebral cortex

N-methyl-D-aspartate (NMDA) receptors, which play a critical role in many cortical functions, are composed of a heteromeric assembly of different subunits: of these, the NMDA receptor subunit 1 (NMDAR1) is a constant component of, and thus an excellent marker for, NMDA receptors. In this study, we have investigated the cellular localization and laminar distribution of NMDAR1 mRNA in the cerebral cortex of adult rats by in situ hybridization histochemistry with a 35S-labeled cRNA probe. Specificity and background levels were determined in adjacent sections incubated with a 35S-labeled sense RNA. In sections incubated with the antisense RNA probe, specific hybridization signal was observed in a large number of cells. Some cells, however, did not appear to contain NMDAR1 mRNA. The vast majority of these unlabeled cells were small, suggesting that they are astrocytes or other small nonneuronal cells. Double-labeling studies with in situ hybridization histochemistry and immunocytochemistry with antibodies to glial fibrillary acidic protein (GFAP) showed that about 95.7% of the GFAP-positive cells did not express NMDAR1 mRNA, indicating that virtually all astrocytes do not contain this transcript. A semiquantitative evaluation of cortical neurons, defined as those cells larger than the GFAP-positive astrocytes, revealed that about 80% were associated with silver grains. The number of silver grains associated with every neuron was determined from sections exposed for 15 days, the background level was subtracted, and all labeled neurons were grouped into five groups: A (< or = 10 grains), B (11-20 grains), C (21-30 grains), D (31-40 grains), and E (> 40 grains). The number of neurons belonging to each group was then evaluated according to their occurrence in each cortical layer. In layer I all labeled neurons were in group A, whereas in layers II-III and V-VI positive neurons were in group A-E. In layer IV most neurons were in groups A and B, whereas only a few were in group E. These observations indicate that 1) virtually all cortical cells containing NMDAR1 mRNA in adult rats are neurons; 2) about 80% of all cortical neurons express NMDAR1 mRNA; and 3) labeled neurons can be divided into several groups on the basis of NMDAR1 mRNA levels expressed, which presumably reflect the number of NMDA receptors. The existence of neurons with a different number of receptors may be a critical factor for determining the physiological effect of NMDA receptor activation.(ABSTRACT TRUNCATED AT 400 WORDS)

Glutamate increases tau phosphorylation in primary neuronal cultures from fetal rat cerebral cortex

Tau proteins are microtubule-associated proteins which promote microtubule polymerisation and stabilization. AT8 is a new monoclonal antibody raised against a phosphorylated Tau protein probably at Serine 202. Tau protein, recognized by AT8 antibody is present in fetal human and rat brains, and in Alzheimer's brains. Here we report that glutamate an excitatory neurotransmitter and also a potent excitotoxin produces in primary neuronal cultures a rapid increase in phosphorylated Tau protein immunoreactivity using AT8 antibody. Glutamate augments neuronal Tau immunoreactivity by 225% using laser confocal immunocytochemistry and by 355% on immunoblot analysis. This experimental model of Tau protein modifications could help to decipher the intracellular biochemical pathways at the origin of phosphorylated Tau protein.

Interaction between metabotropic receptors and purinergic transmission in rat hippocampal slices

Inhibition of forskolin-stimulated cAMP formation by (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) in rat hippocampal slices was partially obliterated by the adenosine-depleting enzyme, adenosine deaminase, or by the adenosine receptor agonist, 5'-(N-ethylcarboxamido)-adenosine, suggesting that activation of metabotropic glutamate receptors (mGluRs) modulates the release of endogenous adenosine. Consistent with this hypothesis, forskolin stimulated the release of purines from rat hippocampal slices, and this effect was reduced by 1S,3R-ACPD. To establish which transduction pathway is involved in the modulation of forskolin-stimulated purine release, we have tested the novel mGluR2 agonist, (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV), which reduced forskolin-stimulated cAMP formation but, as opposed to 1S,3R-ACPD, did not stimulate polyphosphoinositide hydrolysis. DCG-IV was highly potent and more efficacious than 1S,3R-ACPD in inhibiting forskolin-stimulated purine release. Neither DCG-IV nor 1S,3R-ACPD reduced the release of purines stimulated by depolarizing concentrations of K+, suggesting that their effect was stimulus-specific. These results indicate that, in rat hippocampal slices, activation of mGluR2 receptors attenuates the release of purines induced by forskolin, a process that amplifies the final effect of forskolin on cAMP formation as a result of A2 purinergic receptor activation. Thus, the final effect of mGluR agonists on forskolin-stimulated cAMP formation in hippocampal slices depends on both a direct inhibition of adenylyl cyclase and the inhibition of adenosine release.

Kainate-induced status epilepticus leads to a delayed increase in various specific glutamate metabotropic receptor responses in the hippocampus

Neuronal loss and gliosis were detected in the rat hippocampus soon after unilateral intra-amygdala injection of kainate (KA) (2.5 nmol) while solid mossy fiber sprouting could be seen only fourteen days after this injection. Using this experimental model, we examined the metabotropic glutamate receptor (mGluR)-induced inositol phosphate (IP) formation in hippocampal synaptoneurosomes and slices. In synaptoneurosomes prepared from ipsilateral hippocampi fourteen days following injection, there were no significant changes in mGluR- and carbachol(CARB)-stimulated IPs syntheses when sham-operated and KA-injected animals were compared. In the corresponding hippocampal slices, significant increases of the mGluR responses mediated by ibotenate (IBO) and aminocyclopentane-trans-1,3-dicarboxylate (t-ACPD) were noted after KA application. The net stimulation values respectively expressed in a pair-wise fashion for buffer-injected control and KA-treated animals were IBO: 1,947 +/- 457 and 10,553 +/- 1,242; t-ACPD: 1,557 +/- 662 and 9,449 +/- 2,251 dpm/mg protein respectively. Significantly augmented mGluR responses in hippocampal slices were also measured at 7, 42 and 92 days after KA injection. There were, however, no significant increases in CARB-stimulated phosphoinositide hydrolysis in the hippocampal slices at all time-intervals after KA administration. These findings show that there are differences between the mGluR responses in hippocampal synaptoneurosome and slice preparations, suggesting the presence of two distinct populations of mGluR in each of these two models. The large specific increases in certain mGluR activities after KA-induced status epilepticus in hippocampal slices could represent one of the molecular mechanisms which underlie the profound morphological changes, in particular gliosis or mossy fiber sprouting, which follow the KA-induced status epilepticus.

Developmentally regulated postsynaptic localization of a metabotropic glutamate receptor in rat rod bipolar cells

The retinal bipolar cell receiving glutamate transmission from photoreceptors mediates a key process in segregating visual signals into ON center and OFF center pathways. This transmission involves a G protein-coupled metabotropic glutamate receptor (mGluR). Immunocytochemical and immunoelectron microscopic studies indicate that restricted localization of a specific mGluR subtype, mGluR6, at the postsynaptic site of the rat rod bipolar cell. This specialization is developmentally regulated: mGluR6 is initially distributed in both the soma and dendrites and is finally concentrated on the postsynaptic site. The mGluR6 localization is reversed when photoreceptors degenerate in the mutant rat with retinal dystrophy. Evidence is thus presented indicating specialized, developmentally regulated receptor distribution in the central nervous system and the crucial role of mGluR6 in photoreceptor-bipolar cell synaptic transmission.

Organization of N-methyl-D-aspartate glutamate receptor gene expression in the basal ganglia of the rat

Glutamate is an important neurotransmitter in the circuitry of the basal ganglia. Of the four pharmacological classes of receptors that may mediate the actions of glutamate, the N-methyl-D-aspartate (NMDA) type is of particular interest insofar as it has been implicated in the neural processes underlying long-term synaptic plasticity as well as excitotoxic injury. NMDA ligand binding sites are abundant in the structures of the basal ganglia, and NMDA receptors have been linked to neuronal excitability, neuropeptide gene expression, and regulation of dopamine release in these regions. NMDA receptors are believed to be heterooligomers of subunits from two families: NMDAR1, encoded by a single gene but alternatively spliced to produce eight distinct isoforms (NMDAR1A-H), and NMDAR2, encoded by four separate genes (NMDAR2A-D). We have used in situ hybridization with a total of 13 oligonucleotide probes to examine the expression of these genes in the rat basal ganglia. NMDAR1 subunits are expressed throughout the basal ganglia as well as in the rest of the brain; however, the alternatively spliced amino-terminal region Insertion I is abundantly expressed only in the subthalamic nucleus and is not detectable in the neostriatum, globus pallidus, or substantia nigra pars compacta. In contrast, expression of the carboxy terminus segment Deletion I is prominent in the striatum but is not observed in other elements of the basal ganglia. NMDAR2 subunits also exhibit differential expression: NMDAR2B is abundant in the striatum, but NMDAR2A is present within the striatum only at low levels. NMDAR2C is present in the substantia nigra pars compacta only, while NMDAR2D exhibits an unusual distribution, with high levels of expression in the substantia nigra pars compacta, the subthalamic nucleus, the globus pallidus, and the ventral pallidum. Since each isoform of the NMDAR1 and NMDAR2 subunits can confer distinct properties on the resultant NMDA receptor, these data imply that there is a high degree of regional specialization in the properties of NMDA receptors within the basal ganglia.

Is schizophrenia due to excessive synaptic pruning in the prefrontal cortex? The Feinberg hypothesis revisited

Several lines of evidence support the notion that a substantial reorganization of cortical connections, involving a programmed synaptic pruning, takes place during adolescence in humans. A review of neurobiological abnormalities in schizophrenia indicates that the neurobiological parameters that undergo peripubertal regressive changes may be abnormal in this disorder. An excessive pruning of the prefrontal corticocortical, and corticosubcortical synapses, perhaps involving the excitatory glutamatergic inputs to pyramidal neurons, may underlie schizophrenia. A reciprocal failure of pruning in certain subcortical structures, such as lenticular nuclei, may also occur. Several developmental trajectories, related to early brain insults as well as genetic factors affecting postnatal neurodevelopment, could lead to the illness. These models would have heuristic value and may be consistent with several known facts of the schizophrenic illness, such as its onset in adolescence and the gender differences in its onset and natural course. The relationship between these models and other etiological models of schizophrenia are summarized and approaches to test relevant hypotheses are discussed.

Molecular cloning and characterization of avian N-methyl-D-aspartate receptor type 1 (NMDA-R1) gene

Birds have several advantages in the study of memory formation, as imprinting and passive avoidance behaviors in chick are often used as model systems. However, the primary structure of the bird N-methyl-D-aspartate (NMDA) responsive glutamate receptor, which is assumed to play a critical role in memory formation, has not been determined. In this report we describe the cDNA cloning of a subunit of NMDA receptors (NMDA-R1) from duck and analysis of its structure and distribution in the brain. The N-terminal 898 amino acids of the NMDA-R1 were well conserved between duck and mammals, but the homology was completely lost in the C-terminus. In situ hybridization showed that the duck NMDA-R1 gene was expressed throughout the brain as it is in mammals.

Modulatory effects of NMDA on phosphoinositide responses evoked by the metabotropic glutamate receptor agonist 1S,3R-ACPD in neonatal rat cerebral cortex

1. The effect of NMDA-receptor stimulation on phosphoinositide signalling in response to the metabotropic glutamate receptor agonist 1-aminocyclopentane-1S,3R-dicarboxylic acid (1S,3R-ACPD) has been examined in neonatal rat cerebral cortex slices. 2. Total [3H]-inositol phosphate ([3H]-InsPx) accumulation, in the presence of 5 mM LiCl, in [3H]-inositol pre-labelled slices was concentration-dependently increased by 1S,3R-ACPD (EC50 16.6 microM) and, at a maximally effective concentration, 1S,3R-ACPD (300 microM) increased [3H]-InsPx accumulation by 12.8 fold over basal values. 3. [3H]-InsPx accumulation stimulated by 1S,1R-ACPD was enhanced by low concentrations of NMDA (3-30 microM), but not by higher concentrations (> 30 microM). [3H]-InsPx accumulations stimulated by 1S,3R-ACPD in the absence or presence of 10 microM NMDA were linear with time, at least over the 15 min period examined; however, in the presence of 100 microM NMDA the initial enhancement of 1S,3R-ACPD-stimulated phosphoinositide hydrolysis progressively decreased with time. 4. In the presence of a maximal enhancing concentration of NMDA (10 microM), the response to 1S,3R-ACPD (300 microM) was increased 1.9 fold and the EC50 for agonist-stimulated [3H]-InsPx accumulation decreased about 4 fold. The enhanced response to the metabotropic agonist was concentration-dependently inhibited by competitive and uncompetitive antagonists of NMDA-receptor activation. 5. 1S,3R-ACPD also stimulated inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) mass accumulation with an initial peak response (5-6 fold over basal) at 15 s decaying to a smaller (2 fold), but persistent elevated accumulation (1-10 min). 6. Co-addition of 10 or 100 MicroM NMDA enhanced the initial peak Ins(1,4,5)P3 response to 1S,3RACPD.However, the enhancing effect was only maintained over 10 min in the presence of 1O Micro MNMDA, whilst in contrast, 100 MicroM NMDA ceased to cause a significant enhancement of the metabotropic response by 5 min and completely suppressed lS,3R-ACPD-stimulated Ins(1,4,5)P3 accumulation at 10 min.7. Both basal and 1S,3R-ACPD-stimulated Ins(1,4,5)P3 accumulations were reduced when slices were incubated in nominally Ca2"-free medium. Under these conditions only a concentration-dependent enhancement of the response was observed (EC50 for NMDA facilitation of lS,3R-ACPD-stimulated Ins(1,4,5)P3 accumulation of 32 MicroM).8. These experiments have revealed that at low concentrations, NMDA can dramatically potentiate1S,3R-ACPD-stimulated phosphoinositide hydrolysis, probably by a Ca2"-dependent facilitation of agonist-stimulated phosphoinositide-specific phospholipase C activity. Higher concentrations of NMDA result in time-dependent inhibition of the metabotropic agonist-stimulated response. We believe the former effect could be fundamental in glutamate receptor 'cross-talk', whereas the latter may reflect a Ca2+-dependent neurotoxic effect of NMDA on the neonatal cerebral cortex slices.

NMDA receptors with different sensitivities to magnesium and ifenprodil control the release of [14C]acetylcholine and [3H]spermidine from rat striatal slices in vitro

KCl (20-100 mM) and N-methyl-D-aspartate (NMDA, 100-1,000 microM) produce concomitant concentration-dependent increases in the release of previously captured [14C]acetylcholine and [3H]spermidine from rat striatal slices in vitro. The effects of NMDA (300 microM) on striatal [14C]acetylcholine and [3H]spermidine release were blocked with equal potencies by the competitive NMDA antagonist CGP 37849, the glycine site antagonist L-689,560, and the NMDA channel blocker dizocilpine. In contrast, although NMDA-evoked [14C]acetylcholine release was antagonized by ifenprodil (IC50 = 5.3 microM) and MgCl2 (IC50 = 200 microM), neither compound antagonized the NMDA-evoked release of [3H]spermidine at concentrations up to 100 microM (ifenprodil) or 1 mM (MgCl2). Distinct NMDA receptor subtypes with different sensitivities to magnesium and ifenprodil therefore exist in the rat striatum.

An ibotenate-selective metabotropic glutamate receptor mediates protein phosphorylation in cultured hippocampal pyramidal neurons

Previous results showed that within 30 s after glutamate stimulation of cultured rat hippocampal pyramidal neurons there occurred an elevation of Ca2+ and diacylglycerol, and the phosphorylation of three acidic protein kinase C substrates, i.e., an 87-kDa protein known as myristoylated alanine-rich C kinase substrate and a 120- and a 48-kDa protein. In addition, it was suggested that a metabotropic-type glutamate receptor might be responsible for the phosphorylation observed. This work examines the ability of metabotropic and inotropic glutamate receptor agonists to quickly activate phospholipases in 1.26 mM versus 50 nM extracellular Ca2+ by measuring the generation of inositol phosphates. NMDA, quisqualate, and trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid did not stimulate the generation of inositol phosphates in the presence of normal or low extracellular Ca2+ in pyramidal neurons. Kainate stimulated the production of inositol phosphates in the presence of 1.26 mM extracellular Ca2+ but not in 50 nM extracellular Ca2+. Other than glutamate, only ibotenate was able to stimulate the generation of inositol phosphatases in both normal and low extracellular Ca2+. The maximal response to ibotenate was approximately equal to that of glutamate, when pyramidal neurons were stimulated in 50 nM extracellular Ca2+. The generation of inositol phosphates by glutamate and ibotenate could be partially blocked (50-60% reduction) by pretreatment of neurons with pertussis toxin (250 ng/ml), suggesting that a GTP-binding protein might be involved. In addition, ibotenate stimulated the immediate phosphorylation of the same three protein kinase C substrates as glutamate. The NMDA receptor blocker MK-801 had no effect on this phosphorylation.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Involvement of glutamate receptors in allodynia induced by prostaglandins E2 and F2 alpha injected into conscious mice

In order to investigate the involvement of glutamate receptor systems in allodynia induced by prostaglandin (PG) E2 or F2 alpha, we co-administered antagonists for N-methyl-D-aspartate (NMDA), non-NMDA, or metabotropic glutamate receptors intrathecally with PGE2 or PGF2 alpha and examined their effects on the allodynia evoked in conscious mice by non-noxious brushing of the flanks. MK-801, a non-competitive NMDA receptor channel blocker, and D-AP-5, a selective NMDA receptor antagonist, dose-dependently blocked PGE2-induced allodynia with an IC50 of 1.60 and 0.52 microgram/mouse, respectively. A glycine binding-site antagonist for the NMDA receptor, 7-Cl-KYNA, did not influence it. None of these NMDA receptor antagonists inhibited PGF2 alpha-evoked allodynia. Non-NMDA receptor antagonists GAMS and CNQX inhibited both PGE2- and PGF2 alpha-induced allodynia. On the other hand, L-AP-3 and L-AP-4, putative metabotropic glutamate receptor antagonists, dose-dependently antagonized the allodynia induced by PGF2 alpha with an IC50 of 0.92 and 3.26 ng/mouse, respectively, but not that induced by PGE2. Intrathecal administration of L-glutamate produced allodynia over a wide range of low doses from 0.1 pg to 0.1 microgram/mouse, and the maximal effect was observed at 1 ng. Similar to allodynia induced by prostaglandins, the response lasted over a 50-min experimental period. These results demonstrate that both PGE2- and PGF2 alpha-evoked allodynia are mediated through a pathway that includes the glutamate receptor system but that subtypes of glutamate receptors involved and sites of action in the spinal cord may be different between them.

The major tyrosine-phosphorylated protein in the postsynaptic density fraction is N-methyl-D-aspartate receptor subunit 2B

The postsynaptic density (PSD) is a specialization of the submembranous cytoskeleton that is visible in the electron microscope on the cytoplasmic face of the postsynaptic membrane. A subcellular fraction enriched in structures with the morphology of PSDs contains signal-transduction molecules thought to regulate receptor localization and function in the central nervous system. We have purified a prominent tyrosine-phosphorylated glycoprotein of apparent molecular mass 180 kDa, termed PSD-gp180, that is highly enriched in the rat forebrain PSD fraction. The sequences of four tryptic peptides generated from the protein reveal that it is the 2B subunit of the N-methyl-D-aspartate (NMDA) type glutamate receptor. We have confirmed the identity of PSD-gp180 by showing that it reacts with antibodies raised against a unique fragment of the 2B subunit of the NMDA receptor. We also show that the 2B subunit is the most prominently tyrosine-phosphorylated protein in the PSD fraction based upon recognition by an anti-phosphotyrosine antibody. Two types of NMDA receptor subunits have been identified by molecular cloning [Nakanishi, S. (1992) Science 258, 597-603]. The single type 1 subunit is expressed throughout the brain and is necessary for formation of the receptor channel. The four type 2 subunits (2A, 2B, 2C, and 2D) are expressed in discrete brain regions, contain unusually long unique C termini, and confer distinct kinetic properties on NMDA receptors that contain them. Our findings suggest that, in the forebrain, NMDA receptor subunit 2B may serve to anchor NMDA receptors at the postsynaptic membrane through its interaction with the PSD. The prominent presence of tyrosine phosphate further suggests that the NMDA receptor may be regulated by tyrosine phosphorylation or that it may participate in signaling through tyrosine phosphorylation and through its ion channel.

Expression of mRNAs of L-AP4-sensitive metabotropic glutamate receptors (mGluR4, mGluR6, mGluR7) in the rat retina

Expression patterns of mRNAs of L-AP4-sensitive metabotropic glutamate receptors (mGluR4, mGluR6, mGluR7) in the rat retina were examined by northern blot analysis and in situ hybridization histochemistry. Expression patterns of mGluR4 and mGluR7 mRNAs were quite different from that of mGluR6 mRNA which was expressed at the outer part of the inner nuclear layer. The mGluR4 mRNA was expressed on the cell bodies of the ganglion cells, but not in the inner or outer nuclear layer. The expression of mGluR7 mRNA was observed throughout the entire region of the inner nuclear layer and on the cell bodies of the ganglion cells.

Protective effect of the metabotropic glutamate receptor agonist, DCG-IV, against excitotoxic neuronal death

(2S,1'R,2'R,3'R)-2-(2,3-Dicarboxycyclopropyl)glycine (DCG-IV), a potent agonist of subtypes 2 and 3 of metabotropic glutamate receptors (mGluR2 or 3), protected cultured cortical neurons against excitotoxicity induced either by a brief exposure to N-methyl-D-aspartate (NMDA) or a prolonged exposure to kainate. As a neuroprotective agent, DCG-IV was much more potent than the mixed agonists 1S,3R-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) or (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I), suggesting a neuroprotective role for mGluR2 or 3 against excitotoxic neuronal death.

The effect of experimental ischaemia and excitatory amino acid agonists on the GABA and serotonin immunoreactivities in the rabbit retina

The aim of the described experiments was to use immunohistochemistry to visualize the release of GABA from specific retinal amacrine cells following ischaemia and to establish the involvement of defined glutamatergic receptors. In initial experiments, rabbit retinas were exposed in vitro to excitatory amino acid agonists alone or in combination with a putative antagonist, or in physiological solution lacking oxygen and glucose, or in solution containing potassium cyanide for 45 min at 37 degrees C. The nature of the GABA immunoreactivity was then examined by immunohistochemistry. In other in vitro experiments, retinas were first allowed to accumulate exogenous serotonin before exposing the tissues to the combinations as described. These tissues were then processed immunohistochemically for the localization of serotonin. In yet other experiments, the intraocular pressure of a rabbit's eye was raised to about 110 mmHg for 60 min and a reperfusion time of 45 min allowed before dissecting the retina and processing for the localization of GABA immunoreactivity. The other eye served as a control. Of the excitatory amino acid agonists tested, only N-methyl-D-aspartate, kainate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid caused a change in the GABA immunoreactivity. The N-methyl-D-aspartate effect was specifically antagonized by dizocilpine maleate, dextromethorphan and memantine, and was characterized by a reduction in the number of GABA-immunoreactive perikarya. The GABA "staining" in the inner plexiform layer also appeared as four clear bands. The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid- and kainate-induced effects were both antagonized by 6-cyano-2,3-dihydroxy-7-nitroquinoxaline-2,3-dione and partially by kynurenic acid at the concentrations used. Here, the amount of GABA-positive perikarya was greatly reduced and three immunoreactive bands appeared in the inner plexiform layer. However, for low concentrations of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid four GABA-immunoreactive bands could be identified in the inner plexiform layer. The normal GABA immunoreactivity of the inner plexiform layer also appeared to be in defined bands in retinas which received an ischaemic insult either by reducing the availability of glucose and oxygen, exposing the tissue to potassium cyanide or raising the intraocular pressure of an eye. In these cases the number of GABA-positive perikarya was also reduced. Only alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and kainate of the excitatory amino acid agonists tested caused a release of serotonin and this process was antagonized by 6-cyano-2,3-dihydroxy-7-nitroquinoxaline-2,3-dione and partially by kynurenic acid.(ABSTRACT TRUNCATED AT 400 WORDS)

Glutamate receptor-driven activation of transcription factors in primary neuronal cultures

We have used primary neuronal cultures prepared from fetal cerebral hemispheres to investigate the effects of different glutamate receptor agonists and antagonists on the expression of transcription factor encoding genes, such as c-fos, fosB, c-jun, junB, junD, c-myc, and zif/268. The addition of glutamate (100 microM) to the culture medium rapidly activated c-fos, fosB, c-jun, junB and zif/268 gene expression, reaching the maximal level at 30-60 minutes for zif/268 and at 60 minutes for the other genes. The onset of fosB mRNA accumulation was slightly delayed in comparison to the other genes. No clear induction was found for junD and c-myc. Different glutamate receptor agonists, such as NMDA, kainate, quisqualate, trans-(+/-)-1-aminocyclopentane-1,3-dicarboxylic acid (t-ACPD) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) were able to increase c-fos, c-jun, and zif-268 mRNA levels with rapid and transient kinetics similar to those observed after glutamate treatment. Similar results were obtained for junB and fosB after kainate and quisqualate stimulation. Pretreatment with MK-801, a non competitive NMDA antagonist, produced an almost complete inhibition of glutamate-driven expression of transcription factor genes, thus suggesting that NMDA receptor plays a major role in glutamate induced-gene expression. On the contrary the kainate/AMPA receptor antagonist, DNQX, did not influence glutamate induced-gene expression. Under the conditions used in the present study, NMDA was effective in inducing the simultaneous activation of several IEGs even when added to the culture medium containing millimolar concentration of magnesium. When experiments were performed in Krebs solution, NMDA was effective in stimulating zif/268 and c-fos mRNAs only in the absence of Mg2+, while glutamate activated c-fos and zif/268 both in the presence and absence of magnesium ions. As expected, NMDA effect was fully inhibited by MK-801. The level of AP-1 DNA binding activity, as measured by electrophoretic mobility shift assay, increased after addition of glutamate and NMDA to cultured neurons and such increase was antagonized by the pretreatment with MK-801.

Metabotropic glutamate response in acutely dissociated hippocampal CA1 pyramidal neurones of the rat

1. The metabotropic glutamate (mGlu) response was investigated in dissociated rat hippocampal CA1 pyramidal neurones using conventional and nystatin-perforated whole-cell modes of the patch recording configuration. 2. In the perforated patch recording configuration, the application of glutamate (Glu), quisqualate (QA), aspartate (Asp) and N-methyl-D-aspartate (NMDA) induced a slow outward current superimposed on a fast ionotropic inward current, whereas alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and kainate (KA) induced only an ionotropic inward current at a holding potential (VH) of -20 mV. A specific agonist of the mGlu receptor (mGluR), trans-1-aminocyclopentane-1,3-dicarboxylate (tACPD), induced an outward current in approximately 80% of the neurones tested. Asp- and NMDA-induced outward currents were antagonized by D-2-amino-5-phosphonopentanoate (D-AP5) whereas Glu-, QA- and tACPD-induced outward currents were not antagonized by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), 6,7-dinitroquinoxaline-2,3-dione (DNQX) and D-AP5, indicating that the mGlu response is an outward current component. 3. L-2-Amino-3-phosphonopropionate (L-AP3) and DL-2-amino-4-phosphonobutyrate (AP4) did not block the mGlu response. 4. The relative potencies of mGlu agonists were QA > Glu > tACPD. The threshold and EC50 values of metabotropic outward currents were 10-100 times lower than those of the ionotropic inward current (iGlu response). 5. The reversal potential of the mGlu response (EmGlu) was close to EK (K+ equilibrium potential), and it shifted 59.5 mV for a tenfold change in extracellular K+ concentration. 6. In Ca(2+)-free external solution, the mGlu response was elicited by an initial application of Glu, but subsequent applications failed to induce the response. There was also an increase in the intracellular free Ca2+ concentration ([Ca2+]i) during the application of Glu and QA but not of AMPA, indicating Ca2+ release from an intracellular Ca2+ store. 7. During the activation of a Ca(2+)-dependent K+ current (IK(Ca)) by inositol trisphosphate (IP3) in the internal solution, the mGlu response was suppressed. Addition of GDP-beta-S, neomycin or heparin to the internal solution also suppressed the mGlu response, but staurosporine had no effect. The mGlu response was abolished by pretreatment with either caffeine or ryanodine, but treatment with pertussis toxin (IAP) for 6-8 h had no effect. 8. The mGlu response was suppressed by tetraethylammonium, but not by either apamin or iberiotoxin, suggesting that intermediate-conductance Ca(2+)-dependent K+ (KCa+) channels are involved.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of glutamate-induced intracellular energy failure in neonatal cerebral cortical slices by kynurenic acid, dizocilpine, and NBQX

The severity and rapidity of acute, glutamate-induced energy failure were compared in live cerebral cortical slices. In each experiment 80 live cerebral cortical slices (350 microns thick) were obtained from neonatal Sprague-Dawley rats, suspended and perfused in a nuclear magnetic resonance (NMR) tube, and studied at 4.7 T with interleaved 31P/1H NMR spectroscopy. NMR spectra, obtained continually, were determined as 5-min averages. Slices were perfused for 60 min with artificial cerebrospinal fluid (ACSF) containing either glutamate alone or glutamate mixed with one of three glutamate-receptor antagonists: kynurenate, dizocilpine (MK-801), and 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(F)quinoxaline (NBQX). Dose-dependent decreases in high-energy phosphates were studied during glutamate exposure (0.5 to 10 mM), with and without antagonist protection. Energy recovery after glutamate exposures was measured during a 60-min washout with glutamate-free, antagonist-free ACSF. Reversible and irreversible energy failures were characterized by changes in intracellular pH, and by changes in relative concentrations of ATP, phosphocreatine (PCr), and inorganic phosphate. No changes were observed in intracellular levels of N-acetylaspartate and lactate. Some special studies were also done using R-(-)-2-amino-5-phosphonovaleric acid (100 microM) and tetrodotoxin (1 mM) to examine glutamate receptor specificity in this tissue model. Dizocilpine (150 microM) best ameliorated the energy failure caused by 2.0 mM glutamate. With dizocilpine the maximum ATP decrease was only 6 +/- 5%, instead of 35 +/- 7%.(ABSTRACT TRUNCATED AT 250 WORDS)

NMDA and non-NMDA receptor gene expression following global brain ischemia in rats: effect of NMDA and non-NMDA receptor antagonists

Transient forebrain or global ischemia in rats induces selective and delayed damage of hippocampal CA1 neurons. In a previous study, we have shown that expression of GluR2, the kainate/alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit that governs Ca2+ permeability, is preferentially reduced in CA1 at a time point preceding neuronal degeneration. Postischemic administration of the selective AMPA receptor antagonist, 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)quinoxaline (NBQX), protects CA1 neurons against delayed death. In this study we examined the effects of NBQX (at a neuroprotective dose) and of MK-801 (a selective NMDA receptor antagonist, not protective in this model) on kainate/AMPA receptor gene expression changes after global ischemia. We also examined the effects of transient forebrain ischemia on expression of the NMDA receptor subunit NMDAR1. In ischemic rats treated with saline, GluR2 and GluR3 mRNAs were markedly reduced in CA1 but were unchanged in CA3 or dentate gyrus. GluR1 and NMDAR1 mRNAs were not significantly changed in any region examined. Administration of NBQX or MK-801 did not alter the ischemia-induced changes in kainate/AMPA receptor gene expression. These findings suggest that NBQX affords neuroprotection by a direct blockade of kainate/AMPA receptors, rather than by a modification of GluR2 expression changes.

Expression of N-methyl-D-aspartate receptor subunit NR1 messenger RNA by identified striatal somatostatin cells

At present it is not clear whether N-methyl-D-aspartate and N-methyl-D-aspartate receptor agonists have a direct excitotoxic effect on somatostatin interneurons in rat striatum. The N-methyl-D-aspartate receptor comprises a multivariant complex encoded by a family of subunit complementary DNAs. Evidence suggests that expression of the N-methyl-D-aspartate receptor subunit NR1 (zeta 1) is essential for functional receptors. To investigate the expression of NR1 messenger RNA by striatal somatostatin cells, a dual in situ hybridization technique was applied to fresh frozen tissue sections. Cellular sites of NR1 and somatostatin gene expression were visualized in the same tissue section using [35S]NR1 and alkaline phosphatase-labelled somatostatin oligonucleotides. Only 8-18% of striatal somatostatin cells expressed a strong NR1 hybridization signal; most cells (> 80%) expressed a weak or undetectable signal. In contrast NR1 messenger RNA was enriched in neighbouring medium-sized non-somatostatin cells. These data suggest that while the NR1 gene is expressed in some striatal somatostatin cells most do not express a strong NR1 signal, a finding which may explain, in part, the preferential survival of somatostatin cells in Huntington's disease.

Depolarization and neurotransmitters increase neuronal protein tyrosine phosphorylation

In rat hippocampal slices and in neurons in primary culture, K(+)-induced depolarization increased markedly and rapidly tyrosine phosphorylation of a 110-kDa protein (pp110) and, to a lesser degree, of a 120-kDa protein (pp120), in a calcium-dependent fashion. Glutamate, 1-aminocyclopentane-trans-1,3-dicarboxylic acid (an agonist of metabotropic glutamate receptors), and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (an agonist of ionotropic glutamate receptors) stimulated also tyrosine phosphorylation of pp110 and pp120. These effects were not observed in astrocytes in primary culture. In hippocampal slices tyrosine phosphorylation of pp110 and pp120 was stimulated by Ca(2+)-ionophores and by phorbol esters and antagonized by a chelator of intracellular Ca2+ and by drugs that inhibit protein kinase C. Stimulation of muscarinic and alpha 1-adrenergic receptors increased also tyrosine phosphorylation of pp110 and pp120. These results demonstrate that membrane depolarization and stimulation of neurotransmitter receptors activate a tyrosine phosphorylation pathway in neurons. This pathway involves an increase in intracellular Ca2+ concentrations and the activation of protein kinase C. It may provide a biochemical basis for some neurotrophic effects of electrical activity and neurotransmitters and may contribute to the role of tyrosine phosphorylation in long-term potentiation.

N-methyl-D-aspartate (NMDA) receptors in the spinal cord and motor cortex in motor neuron disease: a quantitative autoradiographic study using [3H]MK-801

The distribution and density of NMDA receptors in spinal cord and motor cortex was compared in motor neuron disease (MND; 10 cases) and controls (8 cases) using [3H]MK-801 autoradiography. In the spinal ventral horn of MND cases, [3H]MK-801 binding was reduced and there were fewer focal hot spots of binding. These changes are likely to reflect loss of motor neurons (MN) bearing NMDA receptors. [3H]MK-801 binding was increased in intermediate spinal grey matter and deeper layers of the motor cortex in MND cases compared to controls. This may represent either an adaptive response to MN loss or a pathophysiological phenomenon contributing to MN degeneration.

Quantitative analysis of the distributions of glutamatergic ligand binding sites in goldfish brain

Goldfish brain is a widely used model system for the study of the mechanisms involved in neuronal regeneration and synaptic plasticity. Because of the proposed role of glutamate receptors in these processes we have investigated the anatomical localisations of [3H]AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate), [3H]kainate, [3H]CNQX (6-cyano-7-nitroquinoxaline-2,3-dione) and [3H]L-glutamate binding sites in horizontal and sagittal sections. Binding sites for [3H]L-glutamate were the most widespread and both NMDA (N-methyl-D-aspartate) and non-NMDA sensitive components were detected. The density of [3H]kainate binding was very high in the cerebellum compared to other regions and in comparison with the other radioligands used. Conversely, relatively low amounts of [3H]AMPA binding were present with the telencephalon being the most densely labelled structure. [3H]CNQX binding was most densely localised in the tectum with the cerebellum also possessing high binding. In addition, there was a small population of [3H]CNQX binding sites located in the telencephalon and lobus vagi that appeared insensitive to AMPA and kainate.