Cellular localization of a metabotropic glutamate receptor in rat brain

Selective RNA editing and subunit assembly of native glutamate receptors

RNA editing and subunit assembly of ionotropic glutamate receptors (GluRs) were examined in an oligodendrocyte progenitor cell line, CG4, which expresses GluR2-GluR4, GluR6, GluR7, KA1, and KA2. AMPA-evoked currents rapidly desensitize, whereas kainate-evoked currents contain a steady-state component with a nearly linear current-voltage relation and a fast desensitizing component that is inwardly rectifying. The Q/R site is edited > 95% to the arginine codon in GluR2(Q607) mRNA, and < 5% in GluR6(Q621) mRNA. Immunoprecipitation experiments demonstrate that GluR6 and/or GluR7 subunits assemble with KA2, but not with GluR2-GluR4. These results indicate that oligodendrocyte progenitor cells selectively edit and assemble glutamate receptors into at least two functionally and structurally distinct heteromeric channels.

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.

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.

Metabotropic glutamate receptor agonist ACPD inhibits some, but not all, muscarinic-sensitive K+ conductances in basolateral amygdaloid neurons

Muscarinic agonists produce membrane depolarization and losses of spike frequency accommodation and the slow afterhyperpolarization (AHP) when applied to neurons of the basolateral amygdala (BLA). Underlying these changes are the muscarinic-induced inhibitions of several K+ conductances, including the voltage-activated M-current (IM), a slowly decaying Ca(2+)-activated current (IAHP), a voltage-insensitive leak current (ILeak), and the hyperpolarization-activated inward rectifier current (IIR). Similar depolarizations and losses of the slow AHP have been observed in other neuronal cell types following stimulation of metabotropic glutamate receptors. Therefore, we tested the effects of the metabotropic glutamate receptor agonist, 1-aminocyclopentane-1s,3R-dicarboxylic acid (ACPD), on pyramidal neurons impaled with a single microelectrode for current- and voltage-clamp recordings in a brain slice preparation of the rat BLA. Application of ACPD (20 or 100 microM) to BLA neurons inhibited IM and IAHP, resulting in membrane depolarization and reductions in the amplitude and duration of the slow AHP. However, ACPD did not inhibit the muscarinic-sensitive current IIR, nor was ILeak blocked in the majority of neurons examined. These findings suggest the possibility that muscarinic cholinergic and metabotropic glutamatergic receptor agonists may activate separate intracellular transduction pathways which have convergent inhibitory effects onto IM and IAHP in BLA pyramidal neurons.

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.

Ionotropic glutamate-receptor gene expression in hypothalamus: localization of AMPA, kainate, and NMDA receptor RNA with in situ hybridization

In situ hybridization and Northern blots were used to study the ionotropic subtypes of the glutamate receptor in the rat hypothalamus. Widespread expression of AMPA, kainate, and NMDA receptor RNA was found in the hypothalamus with the transcripts the same size and number as found in other regions of the brain. Most of the glutamate-receptor subunits studied were expressed in greater amounts in hippocampus than in hypothalamus; GluR5, on the other hand, showed a greater expression in hypothalamus than in hippocampus. On the basis of Northern blot analysis, all regions of the brain examined, including hypothalamus, cerebral cortex, cerebellum, olfactory bulb, and hippocampus, expressed all eight of the subunits studied. Each subunit showed different relative expressions in the different regions. In the hypothalamus, GluR1 and GluR2 were among the most widely expressed of the non-NMDA ionotropic receptors. Other AMPA-preferring receptors, GluR3 and -R4, were also found, but to a lesser extent. Scattered cells expressed the kainate-preferring receptors GluR5, -R6, and -R7. The NMDA receptor NMDAR1 was detected throughout the hypothalamus. In many regions of the hypothalamus, only scattered cells showed detectable expression of the glutamate-receptor mRNA as detected by autoradiographic silver grains over neurons; unlabeled cells were mixed among labeled cells. Every region of the hypothalamus had several different glutamate receptors. The expression of many different types of ionotropic glutamate receptors throughout the hypothalamus suggests that multiple modes of ion channel regulation by glutamate probably operate here and provides further support for the importance of the excitatory transmitter glutamate in hypothalamic regulation.

Potentiation of a metabotropic glutamatergic response following NMDA receptor activation in rat hippocampus

Interactions between metabotropic glutamate and N-methyl-D-aspartate (NMDA) receptor-mediated responses were investigated in hippocampal CA3 cells using the single electrode voltage-clamp method. Bath application (2.5-10 microM, 30 s) or iontophoresis of 1-amino-cyclopentyl-trans-1S,3R-dicarboxylate (ACPD), a selective agonist for metabotropic glutamate receptors, resulted in an inward current associated with a decrease in membrane conductance. Following transient bath application of NMDA (5-10 microM, 30-60 s), the ACPD-induced inward current was potentiated for a period of up to 25 min (by 61 +/- 8% with bath application, by 32 +/- 15% with iontophoresis). Transient application of NMDA did not result in a potentiation of ionotropic RS-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or metabotropic muscarinic responses. ACPD responses were not potentiated following transient AMPA application. Intracellular buffering of calcium with tetrapotassium bis(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA) prevented potentiation by NMDA in all cells. Bath application of arachidonic acid did not mimic the NMDA-induced potentiation. These results demonstrate that activation of NMDA receptors can specifically induce a long-lasting potentiation of a metabotropic glutamatergic response in hippocampal CA3 pyramidal cells. The characterization of this interaction may contribute to the elucidation of the physiological significance of metabotropic glutamate receptors.

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.

Characterization of two alternatively spliced forms of a metabotropic glutamate receptor in the central nervous system of the rat

Amplification of complementary DNA by the polymerase chain reaction and anti-peptide antibodies were used to characterize the expression of two alternatively spliced forms of a metabotropic glutamate receptor (mGluR1 alpha and mGluR1 beta) in the central nervous system of the rat. Polymerase chain reaction analysis showed that mGluR1 alpha was the predominate of the two forms in the cerebellum, diencephalon, mesencephalon, olfactory bulb and brainstem, while mGluR1 beta was the major form present in the hippocampus. Approximately equal amounts of the two receptors were expressed in the cerebral cortex, septum and striatum. Immunochemical analyses of the two receptors were conducted in the rat cerebellum and hippocampus. An mGluR1 alpha-specific antibody labelled a protein with a relative molecular weight of 146,000 on immunoblots of the hippocampus and cerebellum. Immunoblot analysis of the developmental expression of mGluR1 alpha in the hippocampus and cerebellum demonstrated that in both structures, the levels of mGluR1 alpha were at or near their maximum levels in the adult brain. In contrast, two mGluR1 beta-specific antibodies failed to detect mGluR1 beta on immunoblots of brain tissue, thus precluding an immunocytochemical analysis of this receptor. Although low levels of a higher-molecular weight protein, possibly a dimeric form of mGluR1 beta were seen with one of the mGluR1 beta-specific antibodies, we hypothesize that some of the mGluR1 beta present in brain tissue may undergo proteolytic cleavage of the carboxy terminus. Immunocytochemical analysis of mGluR1 alpha showed that very high levels of this receptor were expressed in Purkinje cell bodies and dendrites. In the granule cell layer, some Golgi neurons were immunostained. The granule cells were not labelled. In the hippocampus, mGluR1 alpha immunoreactivity was present in interneurons of the stratum oriens and the dentate hilar region. Double-labelling studies demonstrated that these interneurons were also immunopositive for the neuropeptide somatostatin. The presence of mGluR1 alpha in cells of the hippocampus that are associated with the release of somatostatin, suggest that this receptor could play a role in regulating hippocampal excitability in both normal and epileptic tissues.

Processing of information from different sources: spatial synaptic integration in the dendrites of vertebrate CNS neurons

Most synapses on a neuron are distributed along the dendrites. Inputs from different types of presynaptic neurons often distribute to different dendritic compartments. This provides an anatomical framework for spatial synaptic integration. At the same time, a plethora of time- and voltage-dependent responses are present, usually with a distinct distribution over the somato-dendritic membrane. These intrinsic conductances shape the local dendritic response to ligand-gated conductances, and provide the dendrites with a dynamic way of regulating the interaction between synapses. Recent results from neurons in the vertebrate CNS exemplify these mechanisms of dendritic integration.

Differential expression of metabotropic glutamate receptors in the hippocampus and entorhinal cortex of the rat

Metabotropic glutamate receptors (mGluRs) have been implicated in a number of hippocampal functions including learning and memory. Five subtypes have been molecularly and pharmacologically characterized. Using in situ hybridization with oligonucleotide probes selective for these five mGluRs, we have found that each has a unique pattern of expression in the hippocampus and entorhinal cortex. mGluR1 is expressed predominantly in the dentate gyrus and CA3. mGluR2 is enriched in the dentate gyrus and inner layer of the entorhinal cortex. mGluR3 is also expressed in these two structures, but unlike all the other mGluRs, is found in white matter areas as well. mGluR4 is present predominantly in CA2 while mGluR5 is concentrated in most regions of the hippocampus and entorhinal cortex. Comparative analysis of the distributions of these receptors with that of the components of their putative downstream signal transduction mechanisms suggests that mGluR5 may be the main subtype of mGluR which mediates the excitatory actions of glutamate in CA1 and could contribute to the elevation of calcium levels found in CA1 pyramidal neurons in long term potentiation and in ischemic/hypoxic injury. mGluR2 and mGluR3, the main subtypes contributing to the inhibitory actions of glutamate, are absent in CA1. Thus, the mGluR-mediated excitatory actions of glutamate can occur in all regions of the hippocampus whereas the mGluR-mediated inhibitory actions of glutamate may be restricted to the dentate gyrus and CA3.

Phenylglycine derivatives antagonize the excitatory response to Purkinje cells to 1S,3R-ACPD: an in vivo and in vitro study

The interactions of the phenylglycine derivatives (S)-4-carboxyphenylglycine (S-4CPG) and (R,S)-alpha-methyl-4-carboxyphenylglycine (MCPG) with responses of rat cerebellar Purkinje cells to (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) were examined by intracellular recordings in acute cerebellar slices and extracellular recordings in vivo, using multibarrel electrodes. In vitro, both S-4CPG (100 microM to 1 mM) and MCPG (250 microM to 1 mM) reversibly and dose-dependently reduced an inward current induced by bath-applied 1S,3R-ACPD, an agonist at metabotropic glutamate receptors (mGluRs), in Purkinje cells voltage-clamped at -60 to -65 mV. S-4CPG applied at a concentration of 1 mM reduced the 1S,3R-ACPD induced current to 17% of control values but when applied alone also produced an inward current amounting to 26.8% of that induced by 1S,3R-ACPD. MCPG bath-applied at 250 microM, 500 microM, or 1 mM reduced the 1S,3R-ACPD-induced current to 85%, 56% or 3% of control values, respectively, and did not cause any current when applied alone even at a concentration of 1 mM. In vivo, iontophoretic application of 1S,3R-ACPD induced a transient increase followed by a decrease in the firing rate of Purkinje cells. The excitatory response of Purkinje cells to 1S,3R-ACPD was suppressed during ejection of either one of the phenylglycine derivatives, while the mechanism resulting in the decreased firing rate was not affected. Our observations demonstrate that both S-4CPG and MCPG antagonized the excitatory response of cerebellar Purkinje cells to 1S,3R-ACPD.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabotropic glutamate receptor modulation of cAMP accumulation in the neonatal rat hippocampus

The pharmacology and cellular mechanism by which metabotropic glutamate receptor (mGluR) activation modulates cAMP formation was studied in cross-chopped hippocampal slices from neonatal (7 day old) rats. The selective mGluR agonist 1S,3R-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), and other non-selective mGluR agonists produced concentration-related stimulation of basal cAMP formation in this tissue. The relative agonist potency order was 1S,3R-ACPD = quisqualate > ibotenate >> 1R,3S-ACPD. 1S,3R-ACPD stimulated cAMP accumulation was antagonized in a stereoselective manner by L-2-amino-3-phosphonopropionate (L-AP3), but not by higher chain homologues such as L-2-amino-4-phosphonobutyrate (L-AP4) and 2-amino-5-phosphonopentanoate (AP5). 1S,3R-ACPD-enhanced cAMP formation was greatly inhibited by incubation with adenosine deaminase. In the adult rat hippocampus, 1S,3R-ACPD did not appreciably increase basal cAMP, but inhibited forskolin-stimulated cAMP formation, and this effect was observed with or without adenosine deaminase. In the presence of the adenosine receptor antagonist and cAMP phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine (IBMX), 1S,3R-ACPD did not enhance cAMP formation in the neonatal hippocampus, but inhibited forskolin-stimulated cAMP (like in the adult tissue). These results demonstrate that mGluRs that increase cAMP in the neonatal hippocampus have a unique pharmacology when compared to mGluRs that decrease cAMP accumulation and increase phosphoinositide hydrolysis. 1S,3R-ACPD stimulation of cAMP in the neonatal rat hippocampal slice involves potentiation of responses to endogenous adenosine. Negatively coupled cAMP linked mGluRs are also present in the neonatal tissue, but are masked by the predominance of the positively coupled mGluR cAMP response.

Metabotropic glutamate receptors: an original family of G protein-coupled receptors

In 1985, we discovered a new glutamate receptor which was coupled to phospholipase C via a G protein and which was later termed metabotropic glutamate receptor (mGluR). In this review, both the diversity of mGluRs and the cellular events they control are discussed, as well as their roles in physiological regulation and brain function.

Multiphasic responses of cerebellar Purkinje cells to 1S,3R-ACPD: an in vivo study

The effects of iontophoretically applied (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), an agonist of metabotropic glutamate receptors, were examined in rat cerebellar Purkinje cells in vivo. Multibarrel electrodes were used for extracellular recordings of spontaneous single unit discharges and iontophoretic ejection of 1S,3R-ACPD. The effect of 1S,3R-ACPD depended on both the strength and the duration of the iontophoretic current. Application of the agonist with ejection currents at or slightly above the response threshold for up to 60 s resulted in an increased rate of action potential firing. With larger ejection currents of the same duration the initial increase in activity was followed by a depression and eventually a cessation of activity. In the transition phase between low frequency firing and firing arrest, Purkinje cells generated almost exclusively complex spikes. When the drug application was continued for longer durations (1-10 min) the initial response was followed by a characteristic cyclic firing pattern. These cycles consisted of alternating phases of mainly simple spike activity, predominantly complex spike activity and silent intervals. At the end of drug applications using large ejection currents, a prolonged period (on average 66 s) with almost no spiking activity was observed. This period ended with an abrupt onset of simple spike firing. These findings point to an important function of cerebellar metabotropic glutamate receptors in the regulation of Purkinje cell activity.

Immunohistochemical localization of a metabotropic glutamate receptor, mGluR5, in the rat brain

A trpE-fusion protein containing a C-terminal sequence of a rat metabotropic glutamate receptor, mGluR5, was used to produce an antibody. On immunoblot, the antibody specifically reacted with mGluR5 expressed in mammalian cells and rat brain. Immunohistochemical analysis revealed intense mGluR5-like immunoreactivity (LI) in the olfactory bulb, anterior olfactory nuclei, olfactory tubercle, cerebral cortex, hippocampus, lateral septum, striatum, nucleus accumbens, inferior colliculus, and spinal trigeminal nuclei. The distribution pattern of mGluR5-LI corresponds very well with that of mGluR5 mRNA. Electron microscope analysis of the striatum revealed dense accumulation of immunoreaction products in dendrites which were often provided with asymmetrical synapses. These results suggest that mGluR5 is predominantly located in postsynaptic elements.

Identification of the metabotropic glutamate receptor-1 protein in the rat trigeminal ganglion

Anti-metabotropic glutamate receptor-1 monoclonal antibody was raised and applied for immunohistochemistry in the rat trigeminal ganglion. The antibody detected 145-kDa single band of protein in the immunoblot analysis. In immunohistochemistry, neurons in the trigeminal ganglion showed immunostaining with various intensity, almost irrespective of their cell size. The results indicate that metabotrophic glutamate receptors play an important role in somatic sensation together with ionotropic ones.

The regional distribution of extracellularly regulated kinase-1 and -2 messenger RNA in the adult rat central nervous system

It has previously been shown that an intracellular serine/threonine kinase known as extracellularly signal-regulated kinase, also known as microtubule-associated protein kinase, is phosphorylated and activated in response to a range of hormones, growth factors (e.g. nerve growth factor) and neurotransmitters (e.g. N-methyl-D-aspartate) in a variety of cells including neurons. Extracellularly regulated kinases phosphorylate transcription factors, cytoskeletal proteins and enzyme targets. As such they are believed to function in neuronal signal transduction. In situ hybridization histochemistry using synthetic oligonucleotide probes has been used to identify cells in the adult rat central nervous system containing messenger RNAs coding for two isoforms of extracellularly regulated kinase. Extracellularly regulated kinase-2 messenger RNA was observed in many regions including the cerebral cortex, olfactory bulb, hippocampus, amygdala, basal ganglia (except the globus pallidus and endopeduncular nucleus), basal nucleus, thalamus, hypothalamus, brain stem nuclei, cerebellum and neurons in the spinal cord. Extracellularly regulated kinase-1 messenger RNA was confined to fewer regions than extracellularly regulated kinase-2 messenger RNA. Hybridization signals for extracellularly regulated kinase-1 were seen in the olfactory bulb, cortex, regions of the hippocampus, amygdala, nucleus basalis of Maynert, substantia nigra, some hypothalamic and brainstem nuclei and cerebellum, as well as neurons of the spinal cord. Of particular interest, extracellularly regulated kinase-1 messenger RNA was absent from all regions of the basal ganglia and thalamus. Furthermore, extracellularly regulated kinase-1 was almost absent from the CA1 region, whereas extracellularly regulated kinase-2 was present in all neurons of the hippocampus. There were no CNS regions that expressed extracellularly regulated kinase-1 but not extracellularly regulated kinase-2; however, neurons of the dorsal root ganglia showed extracellularly regulated kinase-1 but not extracellularly regulated kinase-2 messenger RNA. Although extracellularly regulated kinase-1 and extracellularly regulated kinase-2 expression was selectively neuronal in the brain, extracellularly regulated kinase-1 messenger RNA was localized to glia in the spinal cord. The distinct cellular distribution of individual extracellularly regulated kinases in the adult rat CNS suggests that they play unique signalling roles.

The metabotropic glutamate receptor (mGluR1 alpha) is concentrated at perisynaptic membrane of neuronal subpopulations as detected by immunogold reaction

An antiserum to mGluR1 alpha labeled a 160 kd protein in immunoblots of membranes derived from rat brain or cells transfected with mGluR1 alpha. Immunoreactivity for mGluR1 alpha was present in discrete subpopulations of neurons. The GABAergic neurons of the cerebellar cortex were strongly immunoreactive; only some Golgi cells were immunonegative. Somatostatin/GABA-immunopositive cells in the neocortex and hippocampus were enriched in mGluR1 alpha. The hippocampal cells had spiny dendrites that were precisely codistributed with the local axon collaterals of pyramidal and granule cells. Electron microscopic immunometal detection of mGluR1 alpha showed a preferential localization at the periphery of the extensive postsynaptic densities of type 1 synapses in both the cerebellum and the hippocampus. The receptor was also present at sites in the dendritic and somatic membrane where synapses were not located.

Neurotransmitters and second messengers in aging and Alzheimer's disease

A substantial loss of cortical cholinergic nerve endings, along with a much more circumscribed cortical degeneration of pyramidal neurons, almost certainly causes glutamatergic hypoactivity in live Alzheimer's patients. These selective pathologies are discussed in terms of therapy. An additional effect of some proposed treatments is emerging as there is evidence that processing pathways for beta-amyloid precursor proteins in cortical pyramidal neurons, a target cell for acetylcholine, are affected by neuronal activity.

Expression of the metabotropic glutamate receptor mGluR1 alpha and the ionotropic glutamate receptor GluR1 in the brain during the postnatal development of normal mouse and in the cerebellum from mutant mice

Expression of the metabotropic glutamate receptor type 1 alpha (mGluR1 alpha) and the non-N-methyl-D-aspartate (NMDA) ionotropic glutamate receptor type 1 (GluR1) in mouse brain was investigated using the antibodies raised against the synthetic peptides corresponding to their C-terminal amino acid sequences. Both receptor proteins are glycosylated predominantly in an asparagine-linked manner, and are abundant in post-synaptic membranes. We showed that mGluR1 alpha and GluR1 expression within the first 3 postnatal weeks undergoes dramatic changes in time and space, i.e., in the hippocampus and cerebellum. These spatio-temporal expression patterns appear to be correlated with the postnatal ontogenesis and establishment of the glutamatergic neurotransmission system in the hippocampus and cerebellum, cell migration, dendritic and axonal growth, spine formation, and synaptogenesis. In the adult cerebellum, mGluR1 alpha is intensely expressed in Purkinje neurons and GluR1 in Bergmann glial cells. Both receptors are expressed to a fair degree in weaver mutant cerebellum despite granule cell degeneration. However, the intrinsic expression levels of both mGluR1 alpha and GluR1 are markedly reduced in the cerebellum of the Purkinje cell-deficient and underdeveloped mutant mice, Purkinje-cell-degeneration, Lurcher, and staggerer, suggesting that GluR1 expression in Bergmann glia cells may be correlated with the sustained interaction with adjacent Purkinje neurons.

Activation of metabotropic glutamate receptors induces an inward current in rat dopamine mesencephalic neurons

To investigate the electrophysiological effects of the stimulation of the metabotropic excitatory amino acid receptors, we applied trans-1-amino-cyclopentane-1,3-dicarboxylate, an agonist of this type of receptors, on presumed rat dopamine cells intracellularly recorded in vitro. Trans-1-amino-cyclopentane-1,3-dicarboxylate (3-30 microM, t-ACPD) caused a sustained increase of the spontaneous firing rate and a depolarization. When the membrane potential was held at about the resting level (-50, -60 mV), by the single-electrode voltage-clamp technique, t-ACPD induced an inward current. In 57% of the tested cells the inward current was associated with a decrease of the apparent input conductance. In the remaining cells no obvious changes in membrane conductance were observed. The active form of t-ACPD, (1S,3R)-1-amino-cyclopentane-1,3-dicarboxylate [3-50 microM, (1S,3R)-ACPD] also produced a reversible inward current on the dopaminergic cells and this was antagonized by (S)-4-carboxy-3-hydroxyphenylglycine (300 microM), a selective antagonist of the (1S,3R)-ACPD-induced depolarization on central neurons. The (1S,3R)-ACPD-induced inward current was not antagonized by L-2-amino-3-phosphonopropionic acid (100 microM), an antagonist of the t-ACPD-induced activation of inositide synthesis. 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM), an alfa-amino-3-hydroxy-5- methyl-isoxazole propionic acid/kainate antagonist, DL-amino-5-phosphonopentanoic acid (30 microM), an N-methyl-D-aspartate antagonist, and scopolamine (10 microM), a muscarinic antagonist, did not significantly affect the actions of t-ACPD. A block of synaptic transmission obtained by applying tetrodotoxin failed to prevent the action of t-ACPD.(ABSTRACT TRUNCATED AT 250 WORDS)

The TINS/TiPS Lecture. The molecular biology of mammalian glutamate receptor channels

In native brain membranes the principal excitatory neurotransmitter L-glutamate activates cation-conducting channels with distinct biophysical and pharmacological properties. Molecular cloning has revealed the existence of 16 channel subunits that can assemble in homomeric or heteromeric configurations in vitro to form receptor channels with disparate functional properties. This review describes the different channel types obtained by recombinant means and the genetic mechanisms controlling the expression of functionally important channel structures.

Metabotropic glutamate receptors trigger postsynaptic protein synthesis

K+ depolarization or addition of glutamate to a synaptoneurosome preparation triggers a rapid increase in size of polyribosomal aggregates isolated by centrifugation of lysate through 1 M sucrose. The profile of response to the glutamate analogues quisqualate, ibotenate, and 1-aminocyclopentane-1,3-dicarboxylate corresponds to that of metabotropic receptors. Glutamate stimulation is mimicked by the diacylglycerol analogue 1-oleoyl-2-acetylglycerol and by the protein kinase C activator phorbol dibutyrate. The phospholipase blockers 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate and quinacrine reduce the late phase of the response. The protein kinase C inhibitor calphostin C suppresses the response to 1-aminocyclopentane-1,3-dicarboxylate. These data indicate that glutamatergic synapses upregulate postsynaptic protein synthesis via metabotropic glutamate receptors coupled to the phosphatidylinositol second-messenger system. This mechanism could underlie the reported involvement of metabotropic glutamate receptors in long-term potentiation and other forms of neural plasticity.

Multiple subtypes of excitatory amino acid receptors coupled to the hydrolysis of phosphoinositides in rat brain

The excitatory amino acid (EAA) analogues quisqualate, ibotenate, and trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylate (trans-ACPD) activate the metabotropic EAA receptors that are coupled to the hydrolysis of phosphoinositides (PI). Previous studies of hippocampal cross sections demonstrated that PI hydrolysis stimulated by these agonists can be inhibited by either L-aspartate-beta-hydroxamate (L-A beta HA) or DL-2-amino-3-phosphonopropionate (DL-AP3). The goal of the present studies was to determine if all metabotropic EAA receptors are sensitive to L-A beta HA and DL-AP3. Two approaches were used. In the first, using cerebellar cross sections, the effects of these agonists and inhibitors were examined. The EC50 values (the concentrations required to evoke half-maximal stimulation) of quisqualate, ibotenate, and trans-ACPD in cerebellum were similar to the EC50 values that we observed previously in hippocampus, but neither L-A beta HA nor DL-AP3 blocked PI hydrolysis. The EC50 values were 0.65 +/- 0.17 microM for quisqualate, 12.8 +/- 2.5 microM for ibotenate, and 18.1 +/- 3.1 microM for trans-ACPD. All data were best fit to theoretical curves that had Hill slopes of 1. In the second approach, another EAA analogue, D-aspartate, was identified as an agonist that stimulates PI hydrolysis. The EC50 for PI hydrolysis stimulated by D-aspartate was 470 +/- 90 microM in hippocampus. Neither L-A beta HA nor DL-AP3 blocked PI hydrolysis stimulated by D-aspartate in hippocampus.(ABSTRACT TRUNCATED AT 250 WORDS)

The TiPS/TINS lecture: the molecular biology of mammalian glutamate receptor channels

In native brain membranes the principal excitatory neurotransmitter L-glutamate activates cation-conducting channels with distinct biophysical and pharmacological properties. Molecular cloning has revealed the existence of 16 channel subunits that can assemble in homomeric or heteromeric configurations in vitro to form receptor channels with disparate functional properties. This review describes the different channel types obtained by recombinant means and the genetic mechanisms controlling the expression of functionally important channel structures.

Mammalian ionotropic glutamate receptors

Exciting new milestones in glutamate receptor (GluR) channel research include the following: the cloning of N-methyl-D-aspartate (NMDA) receptors; delineation of molecular determinants for ion flow through glutamate-gated channels; the discovery that Ca2+ permeability of non-NMDA receptor channels is determined by RNA editing; the construction of antibodies and their use in immunocytochemical localizations of alpha-amino-3-hydroxy-5-methyl isoxazole-4-propionic acid (AMPA) receptor subunits in the rat brain; and the return to prominence of the high-affinity kainate site with the publication of cDNA sequences for subunits (GluR-5, -6, -7; KA-1, -2) constituting subtypes of this site. Major unresolved issues comprise the transmembrane topology and subunit stoichiometries of native receptor channels.

The distribution of glutamate receptors in cultured rat hippocampal neurons: postsynaptic clustering of AMPA-selective subunits

The distribution of several glutamate receptor subunits was investigated in cultured rat hippocampal neurons by in situ hybridization and immunocytochemistry. The AMPA/kainate-selective receptors GluR1-6 exhibited two patterns of mRNA expression: most neurons expressed GluR1, R2, and R6, whereas only about 20% expressed significant levels of GluR3, R4, and R5. By immunocytochemistry, the metabotropic glutamate receptor mGluR1 alpha was detectable only in a subpopulation of GABAergic interneurons. GluR1 and GluR2/3 segregated to the somatodendritic domain within the first week in culture, even in the absence of synaptogenesis. Glutamate receptor-enriched spines developed later and were present only on presumptive pyramidal cells, not on GABAergic interneurons. Clusters of GluR1 and GluR2/3 completely colocalized and were restricted to a subset of postsynaptic sites. Thus, glutamate receptor subunits exhibit both a cell type-specific expression and a selective subcellular localization.

Co-activation of metabotropic glutamate and N-methyl-D-aspartate receptors is involved in mechanisms of long-term potentiation maintenance in rat hippocampal CA1 neurons

Slices of hippocampal area CA1 in the rat were employed to test the hypothesis that the activation of metabotropic glutamate receptors during tetanization is necessary for the late maintenance of long-term potentiation. If the metabotropic glutamate receptor antagonist L-2-amino-3-phosphonopropionate was present during tetanization, post-tetanic and early long-term potentiation of the population spike as well as field excitatory postsynaptic potential developed almost normally. However, 100 min after tetanization, long-term potentiation of the field excitatory postsynaptic potential decreased in an irreversible manner. The same concentration of D-2-amino-3-phosphonopropionate was ineffective. If L-2-amino-3-phosphonopropionate was applied 120 min after tetanization, it did not influence long-term potentiation. The presence of the metabotropic glutamate receptor agonist trans-D,L-1-aminocyclopentane-1,3-dicarboxylic acid during tetanization weakly enhanced the slope of field excitatory postsynaptic potential long-term potentiation. The influence of L-2-amino-3-phosphonopropionate and D,L-1-aminocyclopentane-1,3-dicarboxylic acid on ionotropic glutamate receptors was studied using whole-cell voltage-clamp and pressure application techniques. No effect of L-2-amino-3-phosphonopropionate on either early or late components of excitatory postsynaptic currents could be detected at the concentration used to block long-term potentiation. It is therefore unlikely that the effect of L-2-amino-3-phosphonopropionate on long-term potentiation is due to an interaction with N-methyl-D-aspartate receptors or alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors. However, bath-applied 1S,3R-D,L-1-aminocyclopentane-1,3-dicarboxylic acid facilitated the N-methyl-D-aspartate-induced depolarization in response to N-methyl-D-aspartate pressure application in a reversible manner. These data suggest that besides the involvement of N-methyl-D-aspartate receptors the activation of a 2-amino-3-phosphonopropionate-sensitive metabotropic glutamate receptors during or immediately after tetanization is necessary for subsequent mechanisms responsible for the maintenance of long-term potentiation. A link between metabotropic glutamate receptors and protein kinase C activation during long-term potentiation is discussed considering the similar time course of long-term potentiation blockade after application of L-2-amino-3-phosphonopropionate and protein kinase C inhibitors.

AMPA glutamate receptor subunits are differentially distributed in rat brain

To demonstrate the regional, cellular and subcellular distributions of non-N-methyl-D-aspartate glutamate receptors in rat brain, we generated antipeptide antibodies that recognize the C-terminal domains of individual subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-preferring glutamate receptors (i.e. GluR1, GluR4, and a region highly conserved in GluR2, GluR3 and GluR4c). On immunoblots, antibodies detect distinct proteins with mol. wts ranging from 102,000 to 108,000 in homogenates of rat brain. Immunocytochemistry shows that glutamate receptor subunits are distributed abundantly and differentially within neuronal cell bodies and processes in cerebral cortex, basal ganglia, limbic system, thalamus, cerebellum and brainstem. The precise patterns and cellular localizations of glutamate receptor subunit immunoreactivities are unique for each antibody. In neocortex and hippocampus, pyramidal neurons express GluR1 and GluR2/3/4c immunoreactivities; many non-pyramidal, calcium-binding, protein-enriched neurons in cerebral cortex are selectively immunoreactive for GluR1. In striatum, the cellular localizations of GluR1, GluR2/3/4c and GluR4 immunoreactivities are different; in this region, GluR1 co-localizes with many cholinergic neurons but is only present in a minor proportion of nicotinamide adenine dinucleotide phosphate diaphorase-positive striatal neurons. GluR1 co-localizes with most dopaminergic neurons within the substantia nigra. In several brain regions, astrocytes show GluR4 immunoreactivity. Within the cerebellar cortex, cell bodies and processes of Bergmann glia express intense GluR4 and GluR1 immunoreactivities; perikarya and dendrites of Purkinje cells show GluR2/3/4c immunoreactivity but no evidence of GluR1 or GluR4. Ultrastructurally, GluR subunit immunoreactivities are localized within cell bodies, dendrites and dendritic spines of specific subsets of neurons and, in the case of GluR1 and GluR4, in some populations of astrocytes. This investigation demonstrates that individual AMPA-preferring glutamate receptor subunits are distributed differentially in the brain and suggests that specific neurons and glial cells selectively express glutamate receptors composed of different subunit combinations. Thus, the co-expression of all AMPA receptor subunits within individual cells may not be obligatory for the functions of this glutamate receptor in vivo.

Biochemical and immunocytochemical characterization of antipeptide antibodies to a cloned GluR1 glutamate receptor subunit: cellular and subcellular distribution in the rat forebrain

Antibodies were made to synthetic peptides corresponding to residues 253-367, 757-771 and 877-889 of the published amino acid sequence of the rat brain glutamate receptor GluR1 subunit [Hollmann et al. (1989) Nature 342, 643-648]. The peptides were synthesized both as multiple copies on a branching lysyl matrix (multiple antigenic peptides) and conventional linear peptides using solid-phase synthesis. Rabbits were immunized with these peptides either without conjugation (multiple antigenic peptides) or following coupling to ovalbumin with glutaraldehyde (monomeric peptides). The antibodies from immune sera were then purified by affinity chromatography using reactigel coupled monomeric peptides. All the rabbits produced good antipeptide responses, and were characterized by immunoprecipitation of solubilized alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate and kainate binding activity and by their staining patterns on immunoblots. Antibody to peptide 253-267 specifically immunoprecipitated 12 +/- 3, 50 +/- 3 and 44 +/- 4% of solubilized alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate binding activity from cortex, hippocampus and cerebellum, respectively. Under identical conditions, antibody against the 877-889 peptide removed 23 +/- 4, 9 +/- 4 and 15 +/- 9% of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate binding sites from these areas. On immunoblots of rat brain membrane samples separated by sodium dodecyl sulphate-polyacrylamide gel electrophoresis, antibodies labelled a 105,000 mol. wt immunoreactive band. GluR1 was immunoaffinity-purified using subunit-specific antibodies against both N-terminal (253-267) and C-terminal (877-889) residues, covalently attached to protein A-agarose. Analysis of the purified product from each column showed a major immunoreactive band, recognized by both sera at 105,000 mol. wt and silver staining identified the same major protein. After exhaustive immunoprecipitation of solubilized membrane samples with antibody against the C-terminal of the subunit, a subpopulation of GluR1 was labelled with antibodies specific for the N-terminal part of the receptor. These observations suggest that the GluR1 subunit consists of at least two isoforms possessing a common N-terminal region but a distinct C-terminus. Immunocytochemistry, using immunoperoxidase staining, was performed for the GluR1 subunit in rat forebrain with antisera raised against the N-terminal (253-267) and the C-terminal parts (877-889) of the molecule. Both antisera gave a similar distribution of immunoreactivity at the light-microscopic level. Immunoreactivity for the GluR1 subunit was selectively distributed throughout the rat forebrain. The hippocampus, septum, amygdala and olfactory bulb exhibited the strongest immunoreactivity.(ABSTRACT TRUNCATED AT 400 WORDS)

Mediation of thalamic sensory responses in vivo by ACPD-activated excitatory amino acid receptors

The existence of the so-called metabotropic excitatory amino acid receptor has been known for some years. Various functions have been suggested for this receptor, but the lack of selective antagonists for (IS, 3R)-aminocyclopentane dicarboxylic acid (ACPD) has precluded the direct demonstration of a functional role for this receptor in synaptic processes. We describe here a specific antagonism of the excitatory responses of thalamic neurons to ACPD by two novel antagonists, and a parallel antagonism by these compounds of sensory synaptic responses to noxious stimuli of the same neurons. This provides the first direct pharmacological evidence for a functional role of ACPD-sensitive receptors in central neurotransmission, and indicates that these receptors may play an important part in central sensory processes.