Reversal of glutamate excitotoxicity by activation of PKC-associated metabotropic glutamate receptors in cerebellar granule cells relies on NR2C subunit expression

Knockdown of the neuronal nitric oxide synthase gene retard the development of the cerebellar granule neurons in vitro

The role of endogenous neuronal nitric oxide synthase (nNOS) gene in the development of cerebellar granule neurons (CGNs) is conflicting. Here, we tested the effect of antisense oligos (AS-ODN) on the endogenous nNOS gene and the development of the CGNs in vitro. The expression of nNOS increased in a development-dependent pattern both in terms of mRNA and protein. AS-ODN down-regulated nNOS gene, but in a posttranscriptional manner. Knockdown of nNOS protein decreased the viability of the CGNs from 7 to 13 days in culture (DIC). This activity of AS-ODN was mimicked by nNOS inhibitor I. The antagonist (nNOSi, MK-801, or ODQ) -induced decrease of cell viability was normalized by the provision of the sodium nitroprusside, an NO donor. This study provides direct evidence that endogenous nNOS, mainly by means of its principal product NO, plays an active role in sustaining the survival of developing CGNs at transition from differentiation to maturation.

Prefrontal cortex lesions cause only minor effects on the hyperlocomotion induced by MK-801 and its reversal by clozapine

The non-competitive NMDA receptor antagonist MK-801 elicits a behavioural syndrome in rodents characterized by hyperlocomotion and stereotypies, which is antagonized by antipsychotic drugs. NMDA receptor antagonists increase prefrontal cortex (PFC) activity in rodents, as assessed by electrophysiological and neurochemical measures. The increase in glutamate outflow induced by systemic MK-801 administration in the medial PFC (mPFC) is prevented by the local administration of clozapine (Clz). In the present study, we examine whether a PFC lesion alters the behavioural syndrome induced by MK-801 in rats and the Clz-induced antagonism of MK-801 actions. We evaluated the hyperlocomotion, stereotypies and other behavioural changes induced by MK-801 in the open field and the effect of electrolytic lesions of the mPFC, and of cortical transection on the behavioural syndrome induced by MK-801 and its reversal by Clz. MK-801 (0.1-0.2 mg/kg i.p.) reduced rearings but only the higher dose induced hyperlocomotion. At this dose, MK-801 also increased disorganized movements, head weavings, and induced ataxia signs. An electrolytic lesion of the mPFC markedly reduced the number of rearings pre-treatment but caused a very slight attenuation of MK-801-induced hyperlocomotion. Cortical transection did not significantly alter MK-801 effects. Clz administration (1 mg/kg s.c.) significantly attenuated hyperlocomotion, head weavings and ataxia signs induced by MK-801 but did not prevent the decrease in rearings. The effect of Clz was essentially unaffected by electrolytic lesions of the mPFC. These results show that MK-801-induced motor syndrome and its reversal by Clz are mostly independent on PFC integrity.

A rapid inhibition of NMDA receptor current by corticosterone in cultured hippocampal neurons

The stress level of corticosterone (CORT) may enhance the vulnerability of neurons to insult by increasing N-methyl-D-aspartate (NMDA) receptor activity. In this study, we present data showing that CORT could exert an inhibitory effect on NMDA currents in cultured neonatal hippocampal neurons. Extracellular application of 0.1,1,10 and 100 microM CORT significantly reduced the inward current evoked by co-application of NMDA (100 microM). Extracellular application of a membrane-impermeable CORT-BSA (10 microM) maintained the CORT effect. RU38486 (10 microM) failed to block the CORT (1 microM) inhibitory effect. Additionally, intracellular application of CORT (10 microM) showing the lack of effectiveness indicated that a non-genomic mechanism mediated the CORT suppression on NMDA receptors in hippocampal neurons. Furthermore, to elevate the activity of protein kinase A by intracellular 8-Br-cAMP maintained the suppressive effect of CORT on NMDA current. Intracellular blockade of protein kinase A by Rp-cAMP (10 microM) or staurosporine (50 nM) reduced NMDA currents and abolished CORT depression of NMDA currents. These data indicated that NMDA current itself is dependent on protein kinase A (PKA) activity and CORT depression of the current could be PKA-dependent at the same time. The rapid inhibitory effect of the stress level CORT on NMDA current might suggest a protective mechanism for neurons exposed to a transient increase in glucocorticoids.

Oncostatin M is a neuroprotective cytokine that inhibits excitotoxic injury in vitro and in vivo

Oncostatin M (OsM) is a member of the interleukin (IL)-6 family of cytokines and is well known for its role in inflammation, cell proliferation, and hematopoiesis. OsM, together with its glycoprotein 130 containing receptor complex, is expressed and regulated in most cells of the central nervous system (CNS), yet the function of OsM within this compartment is poorly understood. Here we have investigated the effect of OsM using in vitro and in vivo models of excitotoxic injury. Using primary cultures of mouse cortical neurons, OsM was shown to reduce N-methyl-D-aspartate (NMDA) -induced neuronal death by 50% when added simultaneously with NMDA while pretreatment of neurons with OsM fully prevented NMDA toxicity indicating a profound protective effect of this cytokine. OsM was also shown to inhibit NMDA-mediated increase in levels of free intracellular calcium and to selectively reduce neuronal expression of the NR2C subunit of the NMDA receptor. Finally, using an in vivo model of excitotoxic injury, OsM significantly reduced the NMDA-induced lesion volume when coinjected with NMDA into the mouse striatum. Taken together, these results identify OsM as a powerful neuroprotective cytokine and provide a rational foundation to explore the therapeutic potential for OsM in diseases of the CNS.

N-methyl-D-aspartate (NMDA) and the regulation of mitogen-activated protein kinase (MAPK) signaling pathways: a revolving neurochemical axis for therapeutic intervention?

Excitatory synaptic transmission in the central nervous system (CNS) is mediated by the release of glutamate from presynaptic terminals onto postsynaptic channels gated by N-methyl-D-aspartate (NMDA) and non-NMDA (AMPA and KA) receptors. Extracellular signals control diverse neuronal functions and are responsible for mediating activity-dependent changes in synaptic strength and neuronal survival. Influx of extracellular calcium ([Ca(2+)](e)) through the NMDA receptor (NMDAR) is required for neuronal activity to change the strength of many synapses. At the molecular level, the NMDAR interacts with signaling modules, which, like the mitogen-activated protein kinase (MAPK) superfamily, transduce excitatory signals across neurons. Recent burgeoning evidence points to the fact that MAPKs play a crucial role in regulating the neurochemistry of NMDARs, their physiologic and biochemical/biophysical properties, and their potential role in pathophysiology. It is the purpose of this review to discuss: (i) the MAPKs and their role in a plethora of cellular functions; (ii) the role of MAPKs in regulating the biochemistry and physiology of NMDA receptors; (iii) the kinetics of MAPK-NMDA interactions and their biologic and neurochemical properties; (iv) how cellular signaling pathways, related cofactors and intracellular conditions affect NMDA-MAPK interactions and (v) the role of NMDA-MAPK pathways in pathophysiology and the evolution of disease conditions. Given the versatility of the NMDA-MAPK interactions, the NMDA-MAPK axis will likely form a neurochemical target for therapeutic interventions.

mGluRI targets microglial activation and selectively prevents neuronal cell engulfment through Akt and caspase dependent pathways

Metabotropic glutamate receptors (mGluRs) are expressed throughout the mammalian central nervous system and integrate a host of signal transduction pathways that determine cellular function, plasticity and injury. Yet, one of the more unique regulatory functions of this family of GTP-binding proteins involves cytoprotection in the nervous system. Here, we demonstrate that activation of group I metabotropic glutamate receptors (mGluRIs) in primary hippocampal neurons not only provides intrinsic cellular protection for the maintenance of genomic DNA integrity, but also prevents inflammatory microglial activation and specific neuronal cell engulfment during free radical oxidative stress. Loss of cellular membrane asymmetry and exposure of membrane phosphatidylserine (PS) residues were necessary and sufficient to result in microglial activation and proliferation, since administration of an antibody to the PS receptor could block microglial activity. Through the continuous assessment of individual neurons in real time, activation of mGluRIs was documented to block neuronal PS exposure and prevented subsequent neuronal cell engulfment by microglia seeking "PS tagged" neurons. Furthermore, regulation of both cellular integrity and microglial activity by mGluRI activation was dependent upon the activation and phosphorylation of protein kinase B (Akt1), prevention of mitochondrial membrane depolarization with associated permeability transition pore complex formation, and the down regulation of caspase 9-like activity. Our work defines a significant role of mGluRIs for the modulation of cellular survival and inflammation in the nervous system during oxidative stress.

Modulation of NMDA receptors in the cerebellum. II. Signaling pathways and physiological modulators regulating NMDA receptor function

NMDA receptors in cerebellum have specific characteristics that make their function and modulation different from those of NMDA receptors in other brain areas. The properties of the NMDA receptor that modulate its function: Subunit composition, post-translational modifications and synaptic localization are summarized in an accompanying article. In this review we summarize how different signaling molecules modulate the function of NMDA receptors. The function of the receptors is modulated by the co-agonists glycine and serine and this modulation is different in cerebellum than in other areas. The NMDA receptor also has binding sites for polyamines that regulate its function. Other signaling molecules that modulate NMDA receptors function are: cAMP, neurotrophic factors such as BDNF, FGF-2 or neuregulins. These and other molecules allow an interplay between NMDA receptors and other receptors for neurotransmitters that may in this way modulate NMDA receptor function. This has been reported, for example, for metabotropic glutamate receptors. The expression and function of NMDA receptor is also modulated by synaptic activity, allowing an adaptation of the receptors function to the external inputs. NMDA receptors modulate important cerebral processes. NMDA receptors in different brain areas seem to modulate different processes. Cerebellar NMDA receptors play a special role in the modulation of motor learning and coordination. This is also briefly reviewed.

Modulation of NMDA receptors in the cerebellum. 1. Properties of the NMDA receptor that modulate its function

NMDA receptors modulate important cerebral processes such as synaptic plasticity, long-term potentiation, learning and memory, etc. NMDA receptors in cerebellum have specific characteristics that make their function and modulation different from those of NMDA receptors in other brain areas. In this and the accompanying review we summarize the information available on the modulation of NMDA receptors in cerebellum. We review the properties of the NMDA receptor that modulate its function: subunit composition, post-translational modifications and synaptic localization. NMDA receptors are heteromeric ligand-gated ion channels assembled from two families of subunits, NR1 and NR2. There are at least eight splicing variant isoforms of the NR1 subunit and four types of NR2 subunits: NR2A, NR2B, NR2C and NR2D. NMDA receptors with different subunit composition or different splice variants of NR1 subunit have different properties. The expression of the different subunits and splicing variants varies during development. Two special characteristics of NMDA receptors in cerebellum that do not occur in other brain areas are the enrichment in the NR2C subunit and in the splice variant NR1b. As a consequence of these and other factors the pharmacology of NMDA receptors is also different in cerebellum than in other brain areas. The function and localization of NMDA receptors is also modulated by postranslational modifications including phosphorylation, glycosylation and nytrosylation. NMDA receptors are phosphorylated in serines of both NR1 and NR2 subunits and in tyrosines of NR2 subunits. Another factor modulating NMDA receptors function is the synaptic localization. The trafficking and clustering of NMDA receptors is modulated by phosphorylation and by interaction with other proteins. The signaling pathways and physiological modulators regulating NMDA receptor function as well as the role of these receptors in motor learning and coordination are reviewed in an accompanying article.

Effects of glutamate transporter and receptor ligands on neuronal glutamate uptake

The excitatory amino acids (EAAs) transporters regulate the balance between physiological and pathological signaling over stimulation of the glutamatergic system pathway. The effect of transportable substrates and glutamate (Glu) receptor agonists on Glu uptake in neuronal cells was assessed at different conditions. Cells pre-incubated with Glu, L- or D-aspartate (Asp) and washed presented an inhibition on [(3)H]-Glu uptake and this effect was not mimicked by Glu receptors agonists. The effects of L- and D-Asp were not altered by the presence of N-methyl-d-aspartate (NMDA) receptor antagonists. Thus, the reduction on Glu uptake induced by EAAs is probably linked to the transporter activity. In contrast, the presence of NMDA or (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (SR-ACPD) during the pre-incubation and the [(3)H]-Glu uptake assay period increased Glu uptake, whilst kainic acid (KA) had no effect. The NMDA effect was not altered by its antagonists (+/-)-2-amino-5-phosphonopentanoic acid (AP-5) or dizocilpine (MK-801). The SR-ACPD effect was due to the activation of metabotropic Glu receptor, since it was abolished by its antagonist, L(+/-)-2-amino-3-phosphonopropionic acid (L-AP3). Thus, the current studies suggest that the neuronal EAAs transporter is regulated in different manner by transportable substrates and Glu receptor agonists. The possible involvement of this modulation after certain neurotoxicity insults is discussed.

Ca2+-ATPase isoforms are expressed in neuroprotection in rat, but not human, neurons

Glutamate excitotoxicity has been implicated in neuronal death and damage in many neurodegenerative disorders. The potential neuroprotective role of the plasma membrane calcium ATPase (PMCA) and the NMDA receptor were investigated in rat and human brain neurons after a glutamate insult. Investigation of potential mechanisms of neuronal survival revealed that surviving rat cerebellar granule cells expressed the mRNA of new PMCA isoforms 2b and 2c. There was no observable change in expression of PMCA isoforms or NMDA receptor NR2 subtypes in human cortical neurons. This study shows that subsets of rat and human neurons are resistant to glutamate-induced excitotoxicity and the mechanisms employed to enable survival differ between rat and human neurons.

Stereoselective synthesis and preliminary evaluation of (+)- and (–)-3-methyl-5-carboxy-thien-2-yl-glycine (3-MATIDA): identification of (+)-3-MATIDA as a novel mGluR1 competitive antagonist

The synthesis of the (+)- and (-)-isomers of 3-methyl-5-carboxy-thyen-2-yl-glycine (3-MATIDA), heterocyle isosters of carboxyphenylglycines (CPGs), a known class of competitive metabotropic glutamate receptors, was accomplished by a stereoselective Ugi condensation. The two isomers were tested as potential rat mGluR1 ligand and the (+) isomer was found to be a moderately potent antagonist, while the (-) one was inactive.

Modulation of stretch-induced enhancement of neuronal NMDA receptor current by mGluR1 depends upon presence of glia

Stretching of cultured neurons has been used to model diffuse axonal injury associated with brain trauma. N-Methyl-D-aspartate receptor (NMDAR) activation and group I metabotropic glutamate receptors (mGluRs) are implicated in the pathophysiology of such injury. Here we detail the effects of culture condition and mGluR1 modulation on stretch-enhanced NMDA receptor activity, and show the presence of mGluR1 in addition to mGluR5 in glia. In cortical neurons grown in the absence (PN) or presence (NG) of a glial monolayer, stretch injury (5.7 mm) enhances NMDAR activity by increasing maximal NMDAR current, decreasing the voltage-dependent Mg(2+) block, and altering the kinetic behavior of these receptors. In PN cultures, activation of mGluR1 increases stretch-enhanced NMDAR activity, whereas in NG cultures, such activity is reduced. In contrast, inhibition of mGluR1 in PN cultures limits stretch-enhanced NMDAR activity, whereas in NG cultures activity is increased. MGluR1 modulate stretch-enhanced NMDAR activity through multiple mechanisms including: altering peak or steady state current, affecting Mg(2+) blockade of the NMDAR, or by changing NMDAR kinetics. The presence of glia significantly alters the nature of mGluR1-mediated modulation of NMDAR activity and stretch-induced injury. Together these data indicate a significant neuronal/glial interaction between glial mGluR1 and neuronal NMDA receptor activity.

The distinct role of mGlu1 receptors in post-ischemic neuronal death

Metabotropic glutamate receptors of the mGlu(1) and mGlu(5) subtypes exhibit a high degree of sequence homology and are both coupled to phospholipase C and intracellular Ca(2+) mobilization. However, functional differences have been detected for these receptor subtypes when they are coexpressed in the same neuronal populations. Experimental evidence indicates that mGlu(1) and mGlu(5) receptors play a differential role in models of cerebral ischemia and that only mGlu(1) receptors are implicated in the pathways leading to post-ischemic neuronal injury. The localization of mGlu(1) receptors in GABA-containing interneurons rather than in hippocampal CA1 pyramidal cells that are vulnerable to ischemia has prompted studies that have provided a new viewpoint on the neuroprotective mechanism of mGlu(1) receptor antagonists. The hypothesis predicts that these pharmacological agents attenuate post-ischemic injury by enhancing GABA-mediated neurotransmission.

Neuroprotective activity of metabotropic glutamate receptor ligands

Metabotropic glutamate receptors form a family of currently eight subtypes (mGluR1-8), subdivided into three groups (I-III). Activation of group-II (mGluR2 and -3) or group-III metabotropic glutamate receptors (mGluR4, -6, -7 and -8) has been established to be neuroprotective in vitro and in vivo. In contrast, group-I mGluRs (mGluR1 and -5) need to be antagonized in order to evoke protection. Initially, all neuroprotective mGluR ligands were analogues of L-glutamate. Those compounds were valuable to demonstrate protection in vitro, but showed limited applicability in animal models, particularly in chronic tests, due to low blood-brain-barrier penetration. Recently, systemically active and more potent and selective ligands became available, e.g., the group-II mGluR agonists LY354740 and LY379268 or group-I antagonists like MPEP (mGluR5-selective) and BAY36-7620 (mGluR1-selective). This new generation of pharmacological agents allows a more stringent assessment of the role of individual mGluR-subtypes or groups of receptors in various nervous system disorders, including ischaemia-induced brain damage, traumatic brain injury, Huntington's and Parkinson's-like pathology or epilepsy. Moreover, the use of genetically modified animals (e.g., knock-out mice) is starting to shed light on specific functions of mGluR-subtypes in experimental neuropathologies.

Repeated cocaine administration differentially affects NMDA receptor subunit (NR1, NR2A-C) mRNAs in rat brain

We investigated the effects of intermittent intraperitoneal (i.p.) injections of cocaine (20 mg/kg) on subunit mRNAs of N-methyl-D-aspartate (NMDA) receptors (NR1/NR2A-2C) in the rat brain by in situ hybridization using phosphor screen analysis. The level of NR1 subunit mRNA significantly increased in hippocampal complexes 1 h after a single i.p. injection of cocaine. After repeated cocaine injection, the mean scores of stereotyped behavior were increased with the number of injections. The level of NR1 subunit mRNA was obviously decreased in the striatum and cortices 24 h (early withdrawal) after a final injection following 14 days of subchronic administration. During the early withdrawal period, the amount of the NR1 subunit decreased in the nucleus accumbens, globus pallidus, and subiculum. In the dentate gyrus, the NR1 mRNA level significantly increased during early withdrawal in rats subchronically treated with cocaine. Levels of NR2B subunit mRNA were reduced in the cortices and striatum. During late withdrawal from cocaine, the level of NR2C subunit mRNA in the cerebellum was also reduced. These findings suggest that the disruption of NR1, NR2B, and NR2C subunits in the discrete brain regions occurs under the cocaine-related behavioral abnormalities and would be closely implicated in the initiation and expression of behavioral sensitization induced by repeated cocaine administration. Further studies on the changes in non-NMDA receptors are required to elucidate the biological significance of glutamate receptors for the mechanisms underlying the development of behavioral sensitization.

The novel and systemically active metabotropic glutamate 1 (mGlu1) receptor antagonist 3-MATIDA reduces post-ischemic neuronal death

We examined the pharmacological properties of 3-methyl-aminothiophene dicarboxylic acid (3-MATIDA) by measuring second messenger responses in baby hamster kidney cells stably transfected with mGlu1a, mGlu2, mGlu4a or mGlu5a receptors and ionotropic glutamate receptor agonist-induced depolarizations in mouse cortical wedges. 3-MATIDA was a potent (IC(50)=6.3 microM, 95% confidence limits 3-15) and relatively selective mGlu1 receptor antagonist. When tested on mGlu2, mGlu4 or mGlu5 receptors its IC(50) was >300 microM. When tested in cortical wedges, however, 3-MATIDA was also able to antagonize AMPA or NMDA responses with an IC(50) of 250 microM. When present in the incubation medium of cultured murine cortical cells, 3-MATIDA (1-100 microM) significantly reduced the death of neurons induced by 60 min of oxygen and glucose deprivation (OGD), even when added up to 60 min after OGD. A similar neuroprotective activity was observed when 3-MATIDA was present at 10-100 microM in the medium of rat organotypic hippocampal slice cultures exposed to 30 min OGD. Systemic administration of 3-MATIDA (3-10 mg/kg, immediately and 1 h after the onset of ischemia) reduced the volume of brain infarcts following permanent middle cerebral artery occlusion in rats. Our results show that 3-MATIDA is a potent and possibly selective mGlu 1 receptor antagonist that may be considered as a novel prototype neuroprotective agent.

Excitotoxicity: perspectives based on N-methyl-D-aspartate receptor subtypes

Since excitotoxicity has been implicated in a variety of neuropathological conditions, understanding the pathways involved in this type of cell death is of critical importance to the future clinical treatment of many diseases. The N-methyl-D-aspartate (NMDA) receptor has become a primary focus of excitotoxic research because early studies demonstrated that antagonism of this receptor subtype was neuroprotective. However, initial pharmacological agents were not clinically useful due to the adverse effects of complete NMDA receptor blockade. Understanding the biochemical properties of the multitude of NMDA receptor subtypes offers the possibility of developing more effective and clinically useful drugs. With the discovery of the basis of heterogeneity of NMDA receptors through molecular biological approaches, many new potential therapeutic targets have been uncovered, and several model systems have been developed for the study of NMDA receptor-mediated cell death. This review discusses these models and the current understanding of the relationship between NMDA receptor subtypes and excitotoxicity.

NR2B-containing NMDA autoreceptors at synapses on entorhinal cortical neurons

We have previously shown that presynaptic N-methyl-D-aspartate receptors (NMDARs) can facilitate glutamate release onto principal neurons in the entorhinal cortex (EC). In the present study, we have investigated the subunit composition of these presynaptic NMDARs. We recorded miniature alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated excitatory postsynaptic currents (mEPSCs), from visually identified neurons in layers II and V of the EC in vitro. In both layers, bath application of the NR2A/B subunit-selective agonist, homoquinolinic acid (HQA), resulted in a marked facilitation of mEPSC frequency. Blockade of presynaptic Ca(2+) entry through either NMDARs or voltage-gated Ca(2+) channels with Co(2+) prevented the effects of HQA, confirming that Ca(2+) entry to the terminal was required for facilitation. When the NR2B-selective antagonist, ifenprodil, was applied prior to HQA, the increase in mEPSC frequency was greatly reduced. In addition, we found that an NMDAR antagonist blocked frequency-dependent facilitation of evoked release and reduced mEPSC frequency in layer V. Thus we have demonstrated that NMDA autoreceptors in layer V of the EC bear the NR2B subunit, and that NMDARs are also present at terminals onto superficial neurons.

Metabotropic glutamate receptor subtypes as targets for neuroprotective drugs

Metabotropic glutamate (mGlu) receptors have been considered as potential targets for neuroprotective drugs, but the lack of specific drugs has limited the development of neuroprotective strategies in experimental models of acute or chronic central nervous system (CNS) disorders. The advent of potent and centrally available subtype-selective ligands has overcome this limitation, leading to an extensive investigation of the role of mGlu receptor subtypes in neurodegeneration during the last 2 years. Examples of these drugs are the noncompetitive mGlu1 receptor antagonists, CPCCOEt and BAY-36-7620; the noncompetitive mGlu5 receptor antagonists, 2-methyl-6-(phenylethynyl)pyridine, SIB-1893, and SIB-1757; and the potent mGlu2/3 receptor agonists, LY354740 and LY379268. Pharmacologic blockade of mGlu1 or mGlu5 receptors or pharmacologic activation of mGlu2/3 or mGlu4/7/8 receptors produces neuroprotection in a variety of in vitro or in vivo models. MGlu1 receptor antagonists are promising drugs for the treatment of brain ischemia or for the prophylaxis of neuronal damage induced by synaptic hyperactivity. MGlu5 receptor antagonists may limit neuronal damage induced by a hyperactivity of N-methyl-d-aspartate (NMDA) receptors, because mGlu5 and NMDA receptors are physically and functionally connected in neuronal membranes. A series of observations suggest a potential application of mGlu5 receptor antagonists in chronic neurodegenerative disorders, such as amyotrophic lateral sclerosis and Alzheimer disease. MGlu2/3 receptor agonists inhibit glutamate release, but also promote the synthesis and release of neurotrophic factors in astrocytes. These drugs may therefore have a broad application as neuroprotective agents in a variety of CNS disorders. Finally, mGlu4/7/8 receptor agonists potently inhibit glutamate release and have a potential application in seizure disorders. The advantage of all these drugs with respect to NMDA or AMPA receptor agonists derives from the evidence that mGlu receptors do not "mediate," but rather "modulate" excitatory synaptic transmission. Therefore, it can be expected that mGlu receptor ligands are devoid of the undesirable effects resulting from the inhibition of excitatory synaptic transmission, such as sedation or an impairment of learning and memory.

Glutamate receptor subunit expression in primary neuronal and secondary glial cultures

We report on the expression of ionotropic glutamate receptor subunits in primary neuronal cultures from rat cortex, hippocampus and cerebellum and of metabotropic glutamate (mGlu) receptor subtypes in these neuronal cultures as well as in cortical astroglial cultures. We found that the NMDA receptor (NR) subunits NR1, NR2A and NR2B were expressed in all three cultures. Each of the three cultures showed also expression of the four AMPA receptor subunits. Although RT-PCR detected mRNA of all kainate (KA) subunits in the three cultures, western blot showed only expression of Glu6 and KA2 receptor subunits. The expression analysis of mGlu receptors indicated the presence of all mGlu receptor subtype mRNAs in the three neuronal cultures, except for mGlu2 receptor mRNA, which was not detected in the cortical and cerebellar culture. mGlu1a/alpha, -2/3 and -5 receptor proteins were present in all three cultures, whereas mGlu4a and mGlu8a receptor proteins were not detected. Astroglial cultures were grown in either serum-containing or chemically defined medium. Only mGlu5 receptor protein was found in astroglial cultures grown in serum-containing medium. When astrocytes were cultured in chemically defined medium, mGlu3, -5 and -8 receptor mRNAs were detected, but at the protein level, still only mGlu5 receptor was found.

An activity-dependent switch from facilitation to inhibition in the control of excitotoxicity by group I metabotropic glutamate receptors

Activation of group I metabotropic glutamate receptors (mGlu1 or -5 receptors) is known to either enhance or attenuate excitotoxic neuronal death depending on the experimental conditions. We have examined the possibility that these receptors may switch between two different functional modes in regulating excitotoxicity. In mixed cultures of cortical cells, the selective mGlu1/5 agonist, 3,5-dihydroxyphenylglycine (DHPG), amplified neurodegeneration induced by a toxic pulse of NMDA. This effect was observed when DHPG was either combined with NMDA or transiently applied to the cultures prior to the NMDA pulse. However, two consecutive applications of DHPG consistently produced neuroprotection. Similar effects were observed with DHPG or quisqualate (a potent agonist of mGlu1/5 receptors) in pure cultures of cortical neurons virtually devoid of astrocytes. In cultures of hippocampal pyramidal neurons, however, only protective effects of DHPG were seen suggesting that, in these particular cultures, group I mGlu receptors were endogenously switched into a "neuroprotective mode". The characteristics of the activity-dependent switch from facilitation to inhibition were examined in mixed cultures of cortical cells. The switch in the response to DHPG was observed when the two applications of the drug were separated by an interval ranging from 1-45 min, but was lost when the interval was extended to 90 min. In addition, this phenomenon required the initial activation of mGlu5 receptors (as indicated by the use of subtype-selective antagonists) and was mediated by the activation of protein kinase C. We conclude that group I mGlu receptors are subjected to an activity-dependent switch in regulating excitotoxic neuronal death and, therefore, the recent "history" of these receptors is critical for the response to agonists or antagonists.

Activation of type 5 metabotropic glutamate receptors enhances NMDA responses in mice cortical wedges

1. We measured the effects of agonists and antagonists of metabotropic glutamate (mGlu) receptors (types 1 and 5) on NMDA-induced depolarization of mouse cortical wedges in order to characterize the mGlu receptor type responsible for modulating NMDA responses. We also characterized a number of mGlu receptor agents by measuring [3H]-inositol phosphate (IP) formation in cortical slices and in BHK cells expressing either mGlu 1 or mGlu 5 receptors. 2. (S)-3,5-dihydroxyphenylglycine (DHPG), an agonist of both mGlu 1 and mGlu 5 receptors, at concentrations ranging from 1-10 microM, enhanced up to 105+/-15% the NMDA-induced depolarization. Larger concentrations (100-300 microM) of the compound were inactive in this test. When evaluated on [3H]-IP synthesis in cortical slices or in cells expressing either mGlu 1 or mGlu 5 receptors, DHPG responses (1-300 microM) increased in a concentration-dependent manner. 3. (RS)-2-chloro-5-hydroxyphenylglycine (CHPG) and (S:)-(+)-2-(3'-carboxybicyclo[1.1.1]pentyl)-glycine (CBPG), had partial agonist activity on mGlu 5 receptors, with maximal effects reaching approximately 50% that of the full agonists. These compounds, however, enhanced NMDA-evoked currents with maximal effects not different from those induced by DHPG. Thus the enhancement of [3H]-IP synthesis and the potentiation of NMDA currents were not directly related. 4. 2-methyl-6-(phenylethynyl)-pyridine (MPEP, 1-10 microM), a selective mGlu 5 receptor antagonist, reduced DHPG effects on NMDA currents. 7-(hydroxyimino)cyclopropan[b]-chromen-1a-carboxylic acid ethylester (CPCCOEt, 30 microM), a preferential mGlu 1 receptor antagonist, did not reduce NMDA currents. 5. These results show that mGlu 5 receptor agonists enhance while mGlu 5 receptor antagonists reduce NMDA currents. Thus the use of mGlu 5 receptor agents may be suggested in a number of pathologies related to altered NMDA receptor function.

Expression of functional metabotropic glutamate receptors in primary cultured rat osteoblasts. Cross-talk with N-methyl-D-aspartate receptors

Osteoblasts and osteoclasts express functional N-methyl-d-aspartate (NMDA) receptors, which participate in regulation of bone matrix. In rat femoral osteoblasts held in whole cell clamp there is a robust NMDA current but little if any response to l-glutamate. We have investigated expression of metabotropic glutamate receptors (mGluRs) in these cells. By reverse transcription polymerase chain reaction, we have detected expression of mGluR1b (but not mGluR1a, 2, 3, 4, 5, or 6). Blockade of mGluRs with (+/-)-alpha-methyl-carboxyphenyl-glycine resulted in an enlarged l-glutamate-induced current that resembled the response to NMDA. Conversely, prior stimulation of mGluRs with trans-(+/-)-1-amino-1, 3-cyclopentanedicarboxylic acid (1S,3R-ACPD; mGluR agonist) reduced the NMDA-induced current by 77%. Monitoring of [Ca(2+)](i) showed that NMDA induced a sustained elevation of [Ca(2+)](i), which was dependent upon [Ca(2+)](o). Treatment with 1S,3R-ACPD generated an initial transient that was independent of [Ca(2+)](o), followed by a sustained, [Ca(2+)](o)-dependent phase, a response consistent with phospholipase C-mediated mobilization of stored Ca(2+). Investigations of the interaction between the two receptors confirmed inhibitory modulation of the NMDA receptor-induced rise in [Ca(2+)](i) by mGluRs. Parathyroid hormone, which also activates phospholipase C in osteoblasts, had a similar inhibitory effect on the NMDA receptor-induced [Ca(2+)](i) response. Elevation of [Ca(2+)](i) mediated by mGluR activation was reduced by subsequent stimulation of NMDA receptors. This is the first description of mGluRs in bone and shows that complex glutamatergic signaling can occur in this tissue.

The metabotropic glutamate system promotes neuronal survival through distinct pathways of programmed cell death

Activation of the metabotropic glutamate receptor (mGluR) system can prevent free radical, nitric oxide (NO)-induced programmed cell death (PCD). To investigate the mechanisms utilized by the mGluR system to regulate the induction of PCD, we examined the course of PCD in real time in individual, living, primary hippocampal neurons. We assessed both phosphatidylserine (PS) externalization, an early event in PCD, and DNA fragmentation during NO toxicity and mGluR modulation to determine the individual contributions of PS externalization and genomic DNA fragmentation during neuronal PCD. Exposure to the NO donors (300 microM SNP or 300 microM NOC-9) induced PCD in approximately 75% of neurons over a 24-h period. The externalization of PS in neurons increased to 21 +/- 2% as early as 3 h following NO exposure and then increased to 80 +/- 2% over a 24-h period. The externalization of PS was independent of the loss of membrane integrity. Agonists for individual mGluR subgroups were equally able to prevent NO-induced neuronal death and DNA degradation, yet they possessed differential abilities to regulate PS externalization. The group I agonist DHPG (750 microM) and the group III agonist L-AP4 (750 microM) both prevented and reversed NO-induced PS externalization. In contrast, activation of group II subtypes using L-CCG-I (750 microM) did not prevent PS externalization. Employing an experimental model that independently led to the externalization of PS residues, we demonstrated that PS externalization does not immediately impact on neuronal survival. Yet, subsequent neuronal survival may ultimately depend upon preventing PS externalization to avoid neuronal tagging for phagocytosis. Since group I and III mGluR subtypes possess the unique ability to maintain genomic integrity and membrane PS asymmetry, these agents may provide superior overall protection against NO-induced neuronal injury.

Group-I metabotropic glutamate receptors: hypotheses to explain their dual role in neurotoxicity and neuroprotection

The role of group-I metabotropic glutamate receptors (mGlu1 and 5) in neurodegeneration is still controversial. While antagonists of these receptors are consistently neuroprotective, agonists have been found to either amplify or attenuate excitotoxic neuronal death. At least three variables affect responses to agonists: (i) the presence of the NR2C subunit in the NMDA receptor complex; (ii) the existence of an activity-dependent functional switch of group-I mGlu receptors, similar to that described for the regulation of glutamate release; and (iii) the presence of astrocytes expressing mGlu5 receptors. Thus, a number of factors, including the heteromeric composition of NMDA receptors, the exposure time to drugs or to ambient glutamate, and the function of astrocytes clearing extracellular glutamate and producing neurotoxic or neuroprotective factors, must be taken into account when examining the role of group-I mGlu receptors in neurodegeneration/neuroprotection.