Metabotropic glutamate receptors: beyond the regulation of synaptic transmission

Neurotransmitters: promising immune modulators in the tumor microenvironment

The tumor microenvironment (TME) is modified by its cellular or acellular components throughout the whole period of tumor development. The dynamic modulation can reprogram tumor initiation, growth, invasion, metastasis, and response to therapies. Hence, the focus of cancer research and intervention has gradually shifted to TME components and their interactions. Accumulated evidence indicates neural and immune factors play a distinct role in modulating TME synergistically. Among the complicated interactions, neurotransmitters, the traditional neural regulators, mediate some crucial regulatory functions. Nevertheless, knowledge of the exact mechanisms is still scarce. Meanwhile, therapies targeting the TME remain unsatisfactory. It holds a great prospect to reveal the molecular mechanism by which the interplay between the nervous and immune systems regulate cancer progression for laying a vivid landscape of tumor development and improving clinical treatment.

Group I Metabotropic Glutamate Receptors and Interacting Partners: An Update

Group I metabotropic glutamate (mGlu) receptors (mGlu1/5 subtypes) are G protein-coupled receptors and are broadly expressed in the mammalian brain. These receptors play key roles in the modulation of normal glutamatergic transmission and synaptic plasticity, and abnormal mGlu1/5 signaling is linked to the pathogenesis and symptomatology of various mental and neurological disorders. Group I mGlu receptors are noticeably regulated via a mechanism involving dynamic protein-protein interactions. Several synaptic protein kinases were recently found to directly bind to the intracellular domains of mGlu1/5 receptors and phosphorylate the receptors at distinct amino acid residues. A variety of scaffolding and adaptor proteins also interact with mGlu1/5. Constitutive or activity-dependent interactions between mGlu1/5 and their interacting partners modulate trafficking, anchoring, and expression of the receptors. The mGlu1/5-associated proteins also finetune the efficacy of mGlu1/5 postreceptor signaling and mGlu1/5-mediated synaptic plasticity. This review analyzes the data from recent studies and provides an update on the biochemical and physiological properties of a set of proteins or molecules that interact with and thus regulate mGlu1/5 receptors.

Presynaptic Release-regulating Metabotropic Glutamate Receptors: An Update

Metabotropic glutamate (mGlu) receptors represent the largest family of glutamate receptors in mammals and act as fine tuners of the chemical transmission in central nervous system (CNS). In the last decade, results concerning the expression and the subcellular localization of mGlu receptors further clarified their role in physio-pathological conditions. Concomitantly, their pharmacological characterization largely improved thanks to the identification of new compounds (chemical ligands and antibodies recognizing epitopic sequences of the receptor proteins) that allowed to decipher the protein compositions of the naive receptors. mGlu receptors are expressed at the presynaptic site of chemical synapses. Here, they modulate intraterminal enzymatic pathways controlling the migration and the fusion of vesicles to synaptic membranes as well as the phosphorylation of colocalized receptors. Both the control of transmitter exocytosis and the phosphorylation of colocalized receptors elicited by mGlu receptors are relevant events that dictate the plasticity of nerve terminals, and account for the main role of presynaptic mGlu receptors as modulators of neuronal signalling. The role of the presynaptic mGlu receptors in the CNS has been the matter of several studies and this review aims at briefly summarizing the recent observations obtained with isolated nerve endings (we refer to as synaptosomes). We focus on the pharmacological characterization of these receptors and on their receptor-receptor interaction / oligo-dimerization in nerve endings that could be relevant to the development of new therapeutic approaches for the cure of central pathologies.

5-Hydroxytryptamine, Glutamate, and ATP: Much More Than Neurotransmitters

5-hydroxytryptamine (5-HT) is derived from the essential amino acid L-tryptophan. Although the compound has been studied extensively for its neuronal handling and synaptic actions, serotonin 5-HT receptors can be found extra-synaptically and not only in neurons but in many types of mammalian cells, inside and outside the central nervous system (CNS). In sharp contrast, glutamate (Glu) and ATP are better known as metabolism-related molecules, but they also are neurotransmitters, and their receptors are expressed on almost any type of cell inside and outside the nervous system. Whereas 5-hydroxytryptamine and Glu are key regulators of the immune system, ATP actions are more general. 5-hydroxytryptamine, ATP and Glu act through both G protein-coupled receptors (GPCRs), and ionotropic receptors, i.e., ligand gated ion channels. These are the three examples of neurotransmitters whose actions as holistic regulatory molecules are briefly put into perspective here.

The selective blockade of metabotropic glutamate receptor-5 attenuates fat accumulation in an <em>in vitro</em> model of benign steatosis

It has been previously found that the blockade of metabotropic glutamate receptor type 5 (mGluR5) protects against hepatic ischemia/reperfusion injury and acetaminophen toxicity. The role of mGluR5 in NAFLD has not yet been elucidated. Here, we evaluated the effects of mGluR5 blockade in an in vitro model of steatosis. HepG2 cells were pre-incubated for 12 h with an mGluR5 agonist, a negative allosteric modulator (DHPG and MPEP, respectively) or vehicle, then treated with 1.5 mM oleate/palmitate (O/P) for another 12 h. Cell viability was evaluated with the MTT assay; fat accumulation was measured using the fluorescent dye nile red; SREBP-1, PPAR-α, iNOS and Caspase-3 protein expression were evaluated by Western blot; NFkB activity was evaluated as pNFkB/NFkB ratio. mGluR5 modulation did not alter cell viability in O/P-incubated cells; MPEP prevented intracellular lipid accumulation in O/P treated cells; MPEP administration was also associated with a reversion of O/P-induced changes in SREBP-1 and PPAR-α expression, involved in free fatty acid (FFA) metabolism and uptake. No changes were observed in iNOS and Caspase-3 expression, or in NFkB activity. In conclusion, mGluR5 pharmacological blockade reduced fat accumulation in HepG2 cells incubated with O/P, probably by modulating the expression of SREBP-1 and PPAR-α.

Selective Blockade of the Metabotropic Glutamate Receptor mGluR5 Protects Mouse Livers in In Vitro and Ex Vivo Models of Ischemia Reperfusion Injury

2-Methyl-6-(phenylethynyl)pyridine (MPEP), a negative allosteric modulator of the metabotropic glutamate receptor (mGluR) 5, protects hepatocytes from ischemic injury. In astrocytes and microglia, MPEP depletes ATP. These findings seem to be self-contradictory, since ATP depletion is a fundamental stressor in ischemia. This study attempted to reconstruct the mechanism of MPEP-mediated ATP depletion and the consequences of ATP depletion on protection against ischemic injury. We compared the effects of MPEP and other mGluR5 negative modulators on ATP concentration when measured in rat hepatocytes and acellular solutions. We also evaluated the effects of mGluR5 blockade on viability in rat hepatocytes exposed to hypoxia. Furthermore, we studied the effects of MPEP treatment on mouse livers subjected to cold ischemia and warm ischemia reperfusion. We found that MPEP and 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) deplete ATP in hepatocytes and acellular solutions, unlike fenobam. This finding suggests that mGluR5s may not be involved, contrary to previous reports. MPEP, as well as MTEP and fenobam, improved hypoxic hepatocyte viability, suggesting that protection against ischemic injury is independent of ATP depletion. Significantly, MPEP protected mouse livers in two different ex vivo models of ischemia reperfusion injury, suggesting its possible protective deployment in the treatment of hepatic inflammatory conditions.

Localization and role of metabotropic glutamate receptors subtype 5 in the gastrointestinal tract

Metabotropic glutamate receptor subtype 5 (mGluR5) is a Group I mGlu subfamily of receptors coupled to the inositol trisphosphate/diacylglycerol pathway. Like other mGluR subtypes, mGluR5s contain a phylogenetically conserved, extracellular orthosteric binding site and a more variable allosteric binding site, located on the heptahelical transmembrane domain. The mGluR5 receptor has proved to be a key pharmacological target in conditions affecting the central nervous system (CNS) but its presence outside the CNS underscores its potential role in pathologies affecting peripheral organs such as the gastrointestinal (GI) tract and accessory digestive organs such as the tongue, liver and pancreas. Following identification of mGluR5s in the mouth, various studies have subsequently demonstrated its involvement in mechanical allodynia, inflammation, pain and oral cancer. mGluR5 expression has also been identified in gastroesophageal vagal pathways. Indeed, experimental and human studies have demonstrated that mGluR5 blockade reduces transient lower sphincter relaxation and reflux episodes. In the intestine, mGluR5s have been shown to be involved in the control of intestinal inflammation, visceral pain and the epithelial barrier function. In the liver, mGluR5s have a permissive role in the onset of ischemic injury in rat and mice hepatocytes. Conversely, livers from mice treated with selective negative allosteric modulators and mGluR5 knockout mice are protected against ischemic injury. Similar results have been observed in experimental models of free-radical injury and in vivo mouse models of acetaminophen intoxication. Finally, mGluR5s in the pancreas are associated with insulin secretion control. The picture is, however, far from complete as the review attempts to establish in particular as regards identifying specific targets and innovative therapeutic approaches for the treatment of GI disorders.

Phenotypic characterization of Grm1crv4 mice reveals a functional role for the type 1 metabotropic glutamate receptor in bone mineralization

Recent increasing evidence supports a role for neuronal type signaling in bone. Specifically glutamate receptors have been found in cells responsible for bone remodeling, namely the osteoblasts and the osteoclasts. While most studies have focused on ionotropic glutamate receptors, the relevance of the metabotropic glutamate signaling in bone is poorly understood. Specifically type 1 metabotropic glutamate (mGlu1) receptors are expressed in bone, but the effect of its ablation on skeletal development has never been investigated. Here we report that Grm1^crv4/crv4 mice, homozygous for an inactivating mutation of the mGlu1 receptor, and mainly characterized by ataxia and renal dysfunction, exhibit decreased body weight, bone length and bone mineral density compared to wild type (WT) animals. Blood analyses of the affected mice demonstrate the absence of changes in circulating factors, such as vitamin D and PTH, suggesting renal damage is not the main culprit of the skeletal phenotype. Cultures of osteoblasts lacking functional mGlu1 receptors exhibit less homogeneous collagen deposition than WT cells, and present increased expression of osteocalcin, a marker of osteoblast maturation. These data suggest that the skeletal damage is directly linked to the absence of the receptor, which in turn leads to osteoblasts dysfunction and earlier maturation. Accordingly, skeletal histomorphology suggests that Grm1^crv4/crv4 mice exhibit enhanced bone maturation, resulting in premature fusion of the growth plate and shortened long bones, and further slowdown of bone apposition rate compared to the WT animals. In summary, this work reveals novel functions of mGlu1 receptors in the bone and indicates that in osteoblasts mGlu1 receptors are necessary for production of normal bone matrix, longitudinal bone growth, and normal skeletal development.

Expression Profiles of Neuropeptides, Neurotransmitters, and Their Receptors in Human Keratocytes In Vitro and In Situ

Keratocytes, the quiescent cells of the corneal stroma, play a crucial role in corneal wound healing. Neuropeptides and neurotransmitters are usually associated with neuronal signaling, but have recently been shown to be produced also by non-neuronal cells and to be involved in many cellular processes. The aim of this study was to assess the endogenous intracellular and secreted levels of the neuropeptides substance P (SP) and neurokinin A (NKA), and of the neurotransmitters acetylcholine (ACh), catecholamines (adrenaline, noradrenaline and dopamine), and glutamate, as well as the expression profiles of their receptors, in human primary keratocytes in vitro and in keratocytes of human corneal tissue sections in situ. Cultured keratocytes expressed genes encoding for SP and NKA, and for catecholamine and glutamate synthesizing enzymes, as well as genes for neuropeptide, adrenergic and ACh (muscarinic) receptors. Keratocytes in culture produced SP, NKA, catecholamines, ACh, and glutamate, and expressed neurokinin-1 and -2 receptors (NK-1R and NK-2R), dopamine receptor D₂, muscarinic ACh receptors, and NDMAR1 glutamate receptor. Human corneal sections expressed SP, NKA, NK-1R, NK-2R, receptor D₂, choline acetyl transferase (ChAT), M₃, M₄ and M₅ muscarinic ACh receptors, glutamate, and NMDAR1, but not catecholamine synthesizing enzyme or the α₁ and β₂ adrenoreceptors, nor M₁ receptor. In addition, expression profiles assumed significant differences between keratocytes from the peripheral cornea as compared to those from the central cornea, as well as differences between keratocytes cultured under various serum concentrations. In conclusion, human keratocytes express an array of neuropeptides and neurotransmitters. The cells furthermore express receptors for neuropeptides/neurotransmitters, which suggests that they are susceptible to stimulation by these substances in the cornea, whether of neuronal or non-neuronal origin. As it has been shown that neuropeptides/neurotransmitters are involved in cell proliferation, migration, and angiogenesis, it is possible that they play a role in corneal wound healing.

Age-related testicular toxicity of mGluR5 negative allosteric modulators appears to be unrelated to testis drug transporter maturity

Testicular degeneration was observed in exploratory toxicity studies in Wistar rats treated with several mGluR5 negative allosteric modulators. To determine if these testis effects were influenced by animal age, these compounds were administered to male Wistar rats of different ages (8, 10, and 12 weeks old) for 2 weeks followed by evaluation of male reproductive organ weights, testis histopathology, and inhibin B levels. Overall, seminiferous tubule degeneration was observed in 2/15, 5/15, and 0/15 compound treated rats from the 8, 10, and 12 week old cohorts and inhibin B was decreased in 8 and 10 week old animals, but not in 12 week old rats, suggesting that there is an age-related component to this testis toxicity. The gene expression profiles of drug transporters in the testis of rats aged PND 38 through PND 91 were very similar, indicating that immaturity of these transporters is an unlikely factor contributing to the age-related toxicity.

Risk and information evaluation of prioritized genes for complex traits: application to bipolar disorder

Many susceptibility genes for complex traits were identified without conclusive findings. There is a strong need to integrate rapidly accumulated genomic data from multi-dimensional platforms, and to conduct risk evaluation for potential therapeutic and diagnostic usages. We set up an algorithm to computationally search for optimal weight-vector for various data sources, while minimized potential noises. Through gene-prioritization framework, combined scores for the resulting prioritized gene-set were calculated using a genome-wide association (GWA) dataset, following with evaluation using weighted genetic risk score and risk-attributed information using an independent GWA dataset. The significance of association of GWA data was corrected for gene length. Enriched functional pathways were identified for the prioritized gene-set using the Gene Ontology analysis. We illustrated our framework with bipolar disorder. 233 prioritized genes were identified from 10,830 candidates that curated from six platforms. The prioritized genes were significantly enriched (P(adjusted) < 1 × 10(-5)) in 18 biological functions and molecular mechanisms including membrane, synaptic transmission, transmission of nerve impulse, integral to membrane, and plasma membrane. Our risk evaluation demonstrated higher weighted genetic risk score in bipolar patients than controls (P-values ranged from 0.002 to 3.8 × 10(-6)). Substantial risk-information (71%) was extracted from prioritized genes for bipolar illness than other candidate-gene sets. Our evidence-based prioritized gene-set provides opportunity to explore the complex network and to conduct follow-up basic and clinical studies for complex traits.

mGluR5 stimulating Homer-PIKE formation initiates icariin induced cardiomyogenesis of mouse embryonic stem cells by activating reactive oxygen species

Icariin (ICA) has been reported to facilitate cardiac differentiation of mouse embryonic stem (ES) cells; however, the mechanism by which ICA induced cardiomyogenesis has not been fully elucidated yet. Here, an underlying signaling network including metabotropic glutamate receptor 5 (mGluR5), Homer, phosphatidylinositol 3-Kinase Enhancer (PIKE), phosphatidylinositol 3-Kinase (PI3K), reactive oxygen species (ROS) and nuclear factor-kappaB (NF-κB) was investigated in ICA induced cardiomyogenesis. Our results showed that the co-expression of mGluR5 together with α-actinin or Troponin T in embryoid bodies (EBs) treated with ICA was elevated to 10.86% and 9.62%, compared with the case in the control (4.04% and 3.45%, respectively). Exposure of EBs to ICA for 2 h remarkably increased the dimeric form of mGluR5, which was inhibited by small interfering RNA targeting mGluR5 (si-mGluR5). Moreover, the extracellular glutamate concentration in ICA treatment medium was elevated to 28.9±3.5 μM. Furthermore, the activation of mGluR5 by ICA triggered the formation of Homer-PIKE complex and activated PI3K, stimulating ROS generation and NF-κB nuclear translocation. Knockdown of mGluR5 or inhibition of PI3K by LY294002 blocked ICA induced cardiomyogenesis via repressing mGluR5 pathway, reducing ROS and NF-κB activation. These results revealed that the inducible mechanisms of ICA were related to activate mGluR5 pathway.

Albuminuria and glomerular damage in mice lacking the metabotropic glutamate receptor 1

The metabotropic glutamate (mGlu) receptor 1 (GRM1) has been shown to play an important role in neuronal cells by triggering, through calcium release from intracellular stores, various signaling pathways that finally modulate neuron excitability, synaptic plasticity, and mechanisms of feedback regulation of neurotransmitter release. Herein, we show that Grm1 is expressed in glomerular podocytes and that a glomerular phenotype is exhibited by Grm1(crv4) mice carrying a spontaneous recessive inactivating mutation of the gene. Homozygous Grm1(crv4/crv4) and, to a lesser extent, heterozygous mice show albuminuria, podocyte foot process effacement, and reduced levels of nephrin and other proteins known to contribute to the maintenance of podocyte cell structure. Overall, the present data extend the role of mGlu1 receptor to the glomerular filtration barrier. The regulatory action of mGlu1 receptor in dendritic spine morphology and in the control of glutamate release is well acknowledged in neuronal cells. Analogously, we speculate that mGlu1 receptor may regulate foot process morphology and intercellular signaling in the podocyte.

Selective activation of metabotropic glutamate receptor 7 induces inhibition of cellular proliferation and promotes astrocyte differentiation of ventral mesencephalon human neural stem/progenitor cells

Expression of group III metabotropic glutamate receptors (mGluR) was established by RT-PCR and immunocytochemistry on a cultured clonal human neural stem/progenitor cell (hNSPC) line derived from fetal ventral mesencephalon (VM). Selective activation of these receptors by the group III mGluR agonist L-(+)-2-amino-4-phosphonobutyric acid (L-AP4) prevented increases in cAMP levels following forskolin stimulation, suggesting these receptors are coupled to their canonical G-protein coupled signal transduction pathway. Tonic exposure of undifferentiated cultures to L-AP4 resulted in a decrease in cellular metabolism and proliferation in the absence of toxicity, as measured by MTT and LDH assays, in a dose-dependent manner. This was confirmed by a reduction in BrdU incorporation into nuclear DNA, suggestive of an anti-proliferative effect of L-AP4. This effect was rescued by co-addition of the broad-spectrum group III mGluR competitive antagonist (RS)-a-cyclopropyl-4-phosphonophenylglycine (CPPG), demonstrating a receptor-mediated mechanism, but not mimicked by application of the cell permeable cAMP analogue dibutyrl cAMP (db-cAMP). The potency of these effects of L-AP4 indicates that this is an mGlu7 subtype-mediated effect. Tonic exposure of undifferentiated cultures to the mGlu7 selective allosteric agonist N,N'-bis(diphenylmethyl)-1,2-ethanediamine dihydrochloride (AMN082), but not the mGlu4 selective allosteric agonist (±)-cis-2-(3,5-dicholorphenylcarbamoyl)cyclohexanecarboxylic acid (VU0155041), or the mGlu8 selective agonist (S)-3,4-dicarboxyphenylglycine ((S)-3,4-DCPG) resulted in an identical anti-proliferative effect to L-AP4, confirming the involvement of the mGlu7 subtype. In differentiating cultures, tonic exposure to L-AP4 or AMN082 resulted in a significant shift towards an astrocyte cell fate. The mGlu7 receptor therefore provides a new opportunity to influence the proliferation and differentiation of ventral mesencephalon-derived hNSPC.

Disruption of metabotropic glutamate receptor signalling is a major defect at cerebellar parallel fibre-Purkinje cell synapses in staggerer mutant mice

Staggerer mutant mice have functional loss of a transcription factor, retinoid-related orphan receptor α (RORα), which is abundantly expressed in Purkinje cells (PCs) of the cerebellum.Homozygous staggerer (sg/sg)mice show cerebellar hypoplasia and congenital ataxia. Sg/sg mice serve as an important extreme mouse model of the hereditary spinocerebellar ataxia type 1 (SCA1), since it has been shown that RORα dysfunction is strongly correlated with SCA1 pathogenesis. However, synaptic abnormalities, especially at parallel fibre (PF)-PC synapses, in SCA1-related sg/sg mice have not been examined in detail electrophysiologically. In this study, we report that PFs can still establish functional synapses onto PCs in sg/sg mice in spite of reduction in the number of PF-PC synapses. Compared with PF-evoked EPSCs in the wild-type or heterozygotes, the success rate of the EPSC recordings in sg/sg was quite low (∼40%) and the EPSCs showed faster kinetics and slightly decreased paired pulse facilitation at short intervals. The prominent synaptic dysfunction is that sg/sg mice lack metabotropic glutamate receptor (mGluR)-mediated slow EPSCs completely. Neither intense PF stimulation nor an exogenously applied mGluR agonist, DHPG, could elicit mGluR-mediated responses.Western blot analysis in the sg/sg cerebellum revealed low-level expression of mGluR1 and TRPC3, both of which underlie mGluR-mediated slow currents in PCs. Immunohistochemical data demonstrated marked mislocalization of mGluR1 on sg/sg PCs.We found that mGluR-mediated retrograde suppression of PF-PC EPSCs by endocannabinoid is also impaired completely in sg/sg mice. These results suggest that disruption of mGluR signalling at PF-PC synapses is one of the major synaptic defects in sg/sg mice and may manifest itself in SCA1 pathology.

Peripheral glutamate signaling in head and neck areas

The major excitatory neurotransmitter glutamate is also found in the periphery in an increasing number of nonexcitable cells. In line with this it became apparent that glutamate can regulate a broad array of peripheral biological responses, as well. Of particular interest is the discovery that glutamate receptor reactive reagents can influence tumor biology. However, the knowledge of glutamate signaling in peripheral tissues is still incomplete and, in the case of head and neck areas, is almost lacking. The roles of glutamate signaling pathways in these regions are manifold and include orofacial pain, periodontal bone production, skin and airway inflammation, as well as salivation. Furthermore, the interrelations between glutamate and cancers in the oral cavity, thyroid gland, and other regions are discussed. In summary, this review shall strengthen the view that glutamate receptor reagents may also be promising targets for novel therapeutic concepts suitable for a number of diseases in peripheral tissues. The contents of this review cover the following sections: Introduction; The "Glutamate System"; The Taste of Glutamate; Glutamate Signaling in Dental Regions; Glutamate Signaling in Head and Neck Areas; Glutamate Signaling in Head and Neck Cancer; A Brief Overview of Glutamate Signaling in Other Cancers; and Conclusion.

Type 2 metabotropic glutamate receptor (mGluR2) fails to negatively couple to cGMP in stably transfected cells

The group II metabotropic glutamate receptors 2 and 3 (mGluR2 and mGluR3) share sequence homology, common pharmacology and negative coupling to cAMP. We recently discovered that mGluR3 also is negatively coupled through a G-protein to the cGMP transduction pathway in rat cerebellar granule cells and astrocytes. To test the hypothesis that mGluR2 also has access to the cGMP pathway, C6 glioma cells were stably transfected with mGluR2 and mGluR3 cDNA and their coupling to cGMP levels was characterized. In contrast to many other cell lines, C6 has a robust cGMP response that makes it attractive in the study of receptor coupling to this second messenger pathway. Consistent with prior studies, the mGluR3 receptor was negatively coupled to cGMP and this coupling was blocked by PTX. In contrast, mGluR2 agonists failed to reduce sodium nitroprusside stimulated cGMP levels in transfected cell lines where the receptor was negatively coupled to cAMP. These data provide further support for the functional divergence between these two closely related receptors.

Treadmill pre-training suppresses the release of glutamate resulting from cerebral ischemia in rats

This study was designed to investigate the neuroprotective effect of treadmill pre-training against the over-release of glutamate resulting from cerebral ischemia. Sprague-Dawley rats underwent 2 weeks of treadmill run-training before cerebral ischemia was performed by middle cerebral artery occlusion. The level of glutamate in brain extracellular fluid was detected before, during and after ischemia/reperfusion. The expression of metabotropic glutamate receptor-1 (mGluR1) mRNA in striatum was examined after ischemia for 80 min and reperfusion for 240 min. Neurological defect score and brain infarction volumes were measured. The treadmill pre-training significantly suppressed the release of glutamate, and reduced the expression of mGluR1 mRNA at 59% (P < 0.01) and 62% (P < 0.05), respectively, as compared with the ischemia group. The neurological defect score and infarction volume were significantly improved by 75% (P < 0.01) and 74% (P < 0.01), respectively, in the pre-training group, as compared to the ischemia group. Treadmill pre-training has a significant neuroprotective function against ischemia/reperfusion injury, by suppressing glutamate release resulting from cerebral ischemia, and this effect may be mediated by downregulation of mGluR1.

AMN082 promotes the proliferation and differentiation of neural progenitor cells with influence on phosphorylation of MAPK signaling pathways

Metabotropic glutamate receptors (mGluRs) are expressed in neural progenitor cells (NPCs) and may play important roles in the neurogenesis during embryonic development and adult brain repair following injuries. In the present study, we investigated the expression of metabotropic glutamate receptor 7 (mGluR7) and the possible roles of this receptor in the proliferation and differentiation of NPCs isolated from embryonic Sprague-Dawley (SD) rats. The results showed that under the normal culture and the hypoxic condition, both mRNA and protein of mGluR 7 are expressed in NPCs. Administration of AMN082, a selective agonist ofmGluR7, promoted the proliferation and differentiation of NPCs. We also demonstrated that activation of JNK and ERK signaling pathways are involved in the differentiation of NPCs into neurons following AMN082 treatment. AMN082 stimulated p-ERK and p-JNK2 expression in both normal and hypoxic conditions at different time points. But p-p38 decreased in normoxia and increased in hypoxia condition at 6h following treated with AMN082 activation. In conclusion, mGluR7 possesses the potential in promoting rat NPCs proliferation and differentiation in vitro with changes in phosphorylation of mitogen-activated protein kinases (MAPK) signaling pathways, suggesting that mGluR7 may exert an important role in brain development and repair of the central nervous system after injury.

Glutamate differently modulates metabotropic glutamate receptors in neuronal and glial cells

Glutamate is an excitatory neurotransmitter implicated in learning and memory processes, but at high concentrations it acts as an excitotoxin causing degeneration and neuronal death. The aim of this work was to determine the excitotoxic effect of glutamate and the regulation of metabotropic glutamate receptors (mGluR) during excitotoxicity in neurons and C6 glioma cells. Results show that glutamate causes excitotoxic damage only in cortical neurons. Loss of cell viability in neurons was glutamate concentration- and time-dependent. Total mGluR levels were significantly reduced in these cells when exposed to glutamate. However, in C6 cells, which have been used as a model of glial cells, these receptors were regulated in a biphasic manner, decreased after 6 h, and increased after 24/48 h of treatment. Results show a cell dependent mGluR regulation by glutamate exposure which could mediate the vulnerability or not to glutamate mediated excitotoxicity.

Neurotransmission in Parkinson's disease: beyond dopamine

Parkinson's disease (PD) is most frequently associated with characteristic motor symptoms that are known to arise with degeneration of dopaminergic neurons. However, patients with this disease also experience a multitude of non-motor symptoms, such as sleep disturbances, fatigue, apathy, anxiety, depression, cognitive impairment, dementia, olfactory dysfunction, pain, sweating and constipation, some of which can be at least as debilitating as the movement disorders and have a major impact on patients' quality of life. Many of these non-motor symptoms may be evident prior to the onset of motor dysfunction. The neuropathology of PD has shown that complex, interconnected neuronal systems, regulated by a number of different neurotransmitters in addition to dopamine, are involved in the aetiology of motor and non-motor symptoms. This review focuses on the non-dopaminergic neurotransmission systems associated with PD with particular reference to the effect that their modulation and interaction with dopamine has on the non-motor symptoms of the disease. PD treatments that focus on the dopaminergic system alone are unable to alleviate both motor and non-motor symptoms, particularly those that develop at early stages of the disease. The development of agents that interact with several of the affected neurotransmission systems could prove invaluable for the treatment of this disease.

Type-3 metabotropic glutamate receptors negatively modulate bone morphogenetic protein receptor signaling and support the tumourigenic potential of glioma-initiating cells

Targeted-therapies enhancing differentiation of glioma-initiating cells (GICs) are potential innovative approaches to the treatment of malignant gliomas. These cells support tumour growth and recurrence and are resistant to radiotherapy and chemotherapy. We have found that GICs express mGlu3 metabotropic glutamate receptors. Activation of these receptors sustained the undifferentiated state of GICs in culture by negatively modulating the action of bone morphogenetic proteins, which physiologically signal through the phosphorylation of the transcription factors, Smads. The cross-talk between mGlu3 receptors and BMP receptors was mediated by the activation of the mitogen-activated protein kinase pathway. Remarkably, pharmacological blockade of mGlu3 receptors stimulated the differentiation of cultured GICs into astrocytes, an effect that appeared to be long lasting, independent of the growth conditions, and irreversible. In in vivo experiments, a 3-month treatment with the brain-permeant mGlu receptor antagonist, LY341495 limited the growth of infiltrating brain tumours originating from GICs implanted into the brain parenchyma of nude mice. While clusters of tumour cells were consistently found in the brain of control mice, they were virtually absent in a large proportion of mice treated with LY341495. These findings pave the way to a new non-cytotoxic treatment of malignant gliomas based on the use of mGlu3 receptor antagonists.

Synthesis and characterization of 1,3-dihydro-benzo[b][1,4]diazepin-2-one derivatives: Part 3. New potent non-competitive metabotropic glutamate receptor 2/3 antagonists

A series of 1,3-dihydro-benzo[b][1,4]diazepin-2-one derivatives was evaluated as non-competitive mGluR2/3 antagonists. Replacement of the (2-aryl)-ethynyl-moiety in 8-position with smaller less lipophilic substituents produced compounds inhibiting the binding of [3H]-LY354740 to rat mGluR2 with low nanomolar affinity and consistent functional effect at both mGluR2 and mGluR3. These compounds were able to reverse LY354740-mediated inhibition of field excitatory postsynaptic potentials in the rat dentate gyrus and in vivo activity could be demonstrated by reversal of the LY354740-induced hypoactivity in mice after oral administration.

Protection from fatal viral encephalomyelitis: AMPA receptor antagonists have a direct effect on the inflammatory response to infection

Neuronal cell death during fatal acute viral encephalomyelitis can result from damage caused by virus replication, glutamate excitotoxicity, and the immune response. A neurovirulent strain of the alphavirus Sindbis virus (NSV) causes fatal encephalomyelitis associated with motor neuron death in adult C57BL/6 mice that can be prevented by treatment with the prototypic noncompetitive alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) glutamate receptor antagonist GYKI 52466 [Nargi-Aizenman J, et al. (2004) Ann Neurol 55:541-549]. To determine the mechanism of protection, NSV-infected mice were treated with 7-acetyl-5-(4-aminophenyl)-8(R)-methyl-8,9-dihydro-7H-1,3-dioxolo-(4,5-h)-benzodiazepine (talampanel), a potent, orally available member of the 2,3 benzodiazepine class of noncompetitive AMPA glutamate receptor antagonists. Talampanel-treated mice were protected from NSV-induced paralysis and death. Examination of the brain during infection showed significantly less mononuclear cell infiltration and no increase in astrocyte expression of glial fibrillary acidic protein in treated mice compared with untreated mice. Lack of CNS inflammation was attributable to failure of treated mice to induce activation and proliferation of lymphocytes in secondary lymphoid tissue in response to infection. Antibody responses to NSV were also suppressed by talampanel treatment, and virus clearance was delayed. These studies reveal a previously unrecognized effect of AMPA receptor antagonists on the immune response and suggest that prevention of immune-mediated damage, in addition to inhibition of excitotoxicity, is a mechanism by which these drugs protect from death of motor neurons caused by viral infection.