Cellular localization of mGluR3 and mGluR5 mRNAs in normal and injured rat brain

The Physio-Pathological Role of Group I Metabotropic Glutamate Receptors Expressed by Microglia in Health and Disease with a Focus on Amyotrophic Lateral Sclerosis

Microglia cells are the resident immune cells of the central nervous system. They act as the first-line immune guardians of nervous tissue and central drivers of neuroinflammation. Any homeostatic alteration that can compromise neuron and tissue integrity could activate microglia. Once activated, microglia exhibit highly diverse phenotypes and functions related to either beneficial or harmful consequences. Microglia activation is associated with the release of protective or deleterious cytokines, chemokines, and growth factors that can in turn determine defensive or pathological outcomes. This scenario is complicated by the pathology-related specific phenotypes that microglia can assume, thus leading to the so-called disease-associated microglia phenotypes. Microglia express several receptors that regulate the balance between pro- and anti-inflammatory features, sometimes exerting opposite actions on microglial functions according to specific conditions. In this context, group I metabotropic glutamate receptors (mGluRs) are molecular structures that may contribute to the modulation of the reactive phenotype of microglia cells, and this is worthy of exploration. Here, we summarize the role of group I mGluRs in shaping microglia cells' phenotype in specific physio-pathological conditions, including some neurodegenerative disorders. A significant section of the review is specifically focused on amyotrophic lateral sclerosis (ALS) since it represents an entirely unexplored topic of research in the field.

Metabotropic glutamate receptors and cognition: From underlying plasticity and neuroprotection to cognitive disorders and therapeutic targets

Metabotropic glutamate (mGlu) receptors are G protein-coupled receptors that play pivotal roles in mediating the activity of neurons and other cell types within the brain, communication between cell types, synaptic plasticity, and gene expression. As such, these receptors play an important role in a number of cognitive processes. In this chapter, we discuss the role of mGlu receptors in various forms of cognition and their underlying physiology, with an emphasis on cognitive dysfunction. Specifically, we highlight evidence that links mGlu physiology to cognitive dysfunction across brain disorders including Parkinson's disease, Alzheimer's disease, Fragile X syndrome, post-traumatic stress disorder, and schizophrenia. We also provide recent evidence demonstrating that mGlu receptors may elicit neuroprotective effects in particular disease states. Lastly, we discuss how mGlu receptors can be targeted utilizing positive and negative allosteric modulators as well as subtype specific agonists and antagonist to restore cognitive function across these disorders.

The Implication of Glial Metabotropic Glutamate Receptors in Alzheimer's Disease

Alzheimer's disease (AD) was first identified more than 100 years ago, yet aspects pertaining to its origin and the mechanisms underlying disease progression are not well known. To this date, there is no therapeutic approach or disease-modifying drug that could halt or at least delay disease progression. Until recently, glial cells were seen as secondary actors in brain homeostasis. Although this view was gradually refuted and the relevance of glial cells for the most diverse brain functions such as synaptic plasticity and neurotransmission was vastly proved, many aspects of its functioning, as well as its role in pathological conditions, remain poorly understood. Metabotropic glutamate receptors (mGluRs) in glial cells were shown to be involved in neuroinflammation and neurotoxicity. Besides its relevance for glial function, glutamatergic receptors are also central in the pathology of AD, and recent studies have shown that glial mGluRs play a role in the establishment and progression of AD. AD-related alterations in Ca2+ signalling, APP processing, and Aβ load, as well as AD-related neurodegeneration, are influenced by glial mGluRs. However, different types of mGluRs play different roles, depending on the cell type and brain region that is being analysed. Therefore, in this review, we focus on the current understanding of glial mGluRs and their implication in AD, providing an insight for future therapeutics and identifying existing research gaps worth investigating.

Glutamate Metabotropic Receptor Type 3 (mGlu3) Localization in the Rat Prelimbic Medial Prefrontal Cortex

Metabotropic glutamate receptors type 3 (mGlu3, encoded by GRM3) are increasingly related to cognitive functioning, including the working memory operations of the prefrontal cortex (PFC). In rhesus monkeys, mGlu3 are most commonly expressed on glia (36%), but are also very prominent on layer III dendritic spines (23%) in the dorsolateral PFC (dlPFC) where they enhance working memory-related neuronal firing. In contrast, mGlu2 are predominately presynaptic in layer III of macaque dlPFC, indicating a pre- vs. post-synaptic dissociation by receptor subtype. The current study examined the cellular and subcellular localizations of mGlu3 in the rat prelimbic medial PFC (PL mPFC), a region needed for spatial working memory performance in rodents. Multiple label immunofluorescence demonstrated mGlu3 expression in neurons and astrocytes, with rare labeling in microglia. Immunoelectron microscopy of layers III and V found that the predominant location for mGlu3 was on axons (layer III: 35.9%; layer V: 44.1%), with labeling especially prominent within the intervaricose segments distant from axon terminals. mGlu3 were also found on glia (likely astrocytes), throughout the glial membrane (layer III: 28.2%; layer V: 29.5%). Importantly, mGlu3 could be seen on dendritic spines, especially in layer III (layer III: 15.6%; layer V: 8.2%), with minor labeling on dendrites. These data show that there are some similarities between mGlu3 expression in rat PL mPFC and macaque dlPFC, but the spine expression enriches and differentiates in the more recently evolved primate dlPFC.

N-Acetyl-Aspartyl-Glutamate in Brain Health and Disease

N-acetyl-aspartyl-glutamate (NAAG) is the most abundant dipeptide in the brain, where it acts as a neuromodulator of glutamatergic synapses by activating presynaptic metabotropic glutamate receptor 3 (mGluR3). Recent data suggest that NAAG is selectively localized to postsynaptic dendrites in glutamatergic synapses and that it works as a retrograde neurotransmitter. NAAG is released in response to glutamate and provides the postsynaptic neuron with a feedback mechanisms to inhibit excessive glutamate signaling. A key regulator of synaptically available NAAG is rapid degradation by the extracellular enzyme glutamate carboxypeptidase II (GCPII). Increasing endogenous NAAG—for instance by inhibiting GCPII—is a promising treatment option for many brain disorders where glutamatergic excitotoxicity plays a role. The main effect of NAAG occurs through increased mGluR3 activation and thereby reduced glutamate release. In the present review, we summarize the transmitter role of NAAG and discuss the involvement of NAAG in normal brain physiology. We further present the suggested roles of NAAG in various neurological and psychiatric diseases and discuss the therapeutic potential of strategies aiming to enhance NAAG levels.

Dysregulation of Ambient Glutamate and Glutamate Receptors in Epilepsy: An Astrocytic Perspective

Given the important functions that glutamate serves in excitatory neurotransmission, understanding the regulation of glutamate in physiological and pathological states is critical to devising novel therapies to treat epilepsy. Exclusive expression of pyruvate carboxylase and glutamine synthetase in astrocytes positions astrocytes as essential regulators of glutamate in the central nervous system (CNS). Additionally, astrocytes can significantly alter the volume of the extracellular space (ECS) in the CNS due to their expression of the bi-directional water channel, aquaporin-4, which are enriched at perivascular endfeet. Rapid ECS shrinkage has been observed following epileptiform activity and can inherently concentrate ions and neurotransmitters including glutamate. This review highlights our emerging knowledge on the various potential contributions of astrocytes to epilepsy, particularly supporting the notion that astrocytes may be involved in seizure initiation via failure of homeostatic responses that lead to increased ambient glutamate. We also review the mechanisms whereby ambient glutamate can influence neuronal excitability, including via generation of the glutamate receptor subunit GluN2B-mediated slow inward currents, as well as indirectly affect neuronal excitability via actions on metabotropic glutamate receptors that can potentiate GluN2B currents and influence neuronal glutamate release probabilities. Additionally, we discuss evidence for upregulation of System x c - , a cystine/glutamate antiporter expressed on astrocytes, in epileptic tissue and changes in expression patterns of glutamate receptors.

Targeting mGlu5 for Methamphetamine Use Disorder

Methamphetamine abuse leads to devastating consequences, including addiction, crime, and death. Despite decades of research, no medication has been approved by the U.S. Food and Drug Administration for the treatment of Methamphetamine Use Disorder. Thus, there is a need for new therapeutic approaches. Animal studies demonstrate that methamphetamine exposure dysregulates forebrain function involving the Group-I metabotropic glutamate receptor subtype 5 (mGlu₅), which is predominantly localized to postsynaptic sites. Allosteric modulators of mGlu₅ offer a unique opportunity to modulate glutamatergic neurotransmission selectively, thereby potentially ameliorating methamphetamine-induced disruptions. Negative allosteric modulators of mGlu₅ attenuate the effects of methamphetamine, including rewarding/reinforcing properties of the drug across animal models, and have shown promising effects in clinical trials for Anxiety Disorder and Major Depressive Disorder. Preclinical studies have also sparked great interest in mGlu₅ positive allosteric modulators, which exhibit antipsychotic and anxiolytic properties, and facilitate extinction learning when access to methamphetamine is removed, possibly via the amelioration of methamphetamine-induced cognitive deficits. Clinical research is now needed to elucidate the mechanisms underlying the mGlu₅ receptor-related effects of methamphetamine and the contributions of these effects to addictive behaviors. The growing array of mGlu₅ allosteric modulators provides excellent tools for this purpose and may offer the prospect of developing tailored and effective medications for Methamphetamine Use Disorder.

Glutamatergic System is Affected in Brain from an Hyperthermia-Induced Seizures Rat Model

One of the most frequent neurological disorders in children is febrile seizures (FS), a risk for epilepsy in adults. Glutamate is the main excitatory neurotransmitter in CNS acting through ionotropic and metabotropic receptors. Excess of glutamate in the extracellular space elicits excitotoxicity and has been associated with neurological disorders, such as epilepsy. The removal of extracellular glutamate by excitatory amino acid transporters (EATT) plays an important neuroprotective role. GLT-1 is the main EAAT present in the cortex brain. On the other hand, an increase in metabotropic glutamate receptors 5 (mGlu₅R) levels or their overstimulation have been related to the appearance of seizure events in different animal models and in temporal lobe epilepsy in humans. In this work, the status of several components of the glutamatergic system has been analysed in the cortex brain from an FS rat model at short (48 h) and long (20 days) term after hyperthermia-induced seizures. At the short term, we detected increased GLT-1 levels, reduced glutamate concentration, and unchanged mGlu₅R levels, without neuronal loss. However, at the long term, an increase in mGlu₅R levels together with a decrease in both GLT-1 and glutamate levels were observed. These changes were associated with the appearance of an anxious phenotype. These results suggest a neuroprotective role of the glutamatergic components mGlu₅R and GLT-1 at the short term. However, this neuroprotective effect seems to be lost at the long term, leading to an anxious phenotype and suggesting an increased vulnerability and propensity to epileptic events in adults.

Attenuation of Acute Intracerebral Hemorrhage-Induced Microglial Activation and Neuronal Death Mediated by the Blockade of Metabotropic Glutamate Receptor 5 In Vivo

The activation of microglia in response to intracerebral hemorrhagic stroke is one of the principal components of the progression of this disease. It results in the formation of pro-inflammatory cytokines that lead to neuronal death, a structural deterioration that, in turn interferes with functional recovery. Metabotropic glutamate receptor 5 (mGluR5) is highly expressed in reactive microglia and is involved in the pathological processes of brain disorders, but its role in intracerebral hemorrhage (ICH) remains unknown. We hypothesized that mGluR5 regulates microglial activation and ICH maintenance. In this study, collagenase-induced ICH mice received a single intraperitoneal injection of the mGluR5 antagonist-, MTEP, or vehicle 2 h after injury. We found that acute ICH upregulated mGluR5 and microglial activation. mGluR5 was highly localized in reactive microglia in the peri-hematomal cortex and striatum on days 3 and 7 post-ICH. The MTEP-mediated pharmacological inhibition of mGluR5 in vivo resulted in the substantial attenuation of acute microglial activation and IL-6, and TNF-α release. We also showed that the blockade of mGluR5 markedly reduced cell apoptosis, and neurodegeneration and markedly elevated neuroprotection. Furthermore, the MTEP-mediated inhibition of mGluR5 significantly reduced the lesion volume and improved functional recovery. Taken together, our results demonstrate that ICH injury enhances mGluR5 expression in the acute and subacute stages and that mGluR5 is highly localized in reactive microglia. The blockade of mGluR5 reduces ICH-induced acute microglial activation, provides neuroprotection and promotes neurofunctional recovery after ICH. The inhibition of mGluR5 may be a relevant therapeutic target for intracerebral hemorrhagic stroke.

LY354740 Reduces Extracellular Glutamate Concentration, Inhibits Phosphorylation of Fyn/NMDARs, and Expression of PLK2/pS129 α-Synuclein in Mice Treated With Acute or Sub-Acute MPTP

Glutamate overactivity in basal ganglia critically contributes to the exacerbation of dopaminergic neuron degeneration in Parkinson's disease (PD). Activation of group II metabotropic glutamate receptors (mGlu_2/3 receptors), which can decrease excitatory glutamate neurotransmission, provides an opportunity to slow down the degeneration of the dopaminergic system. However, the roles of mGlu_2/3 receptors in relation to PD pathology were partially recognized. By using mGlu_2/3 receptors agonist (LY354740) and mGlu_2/3 receptors antagonist (LY341495) in mice challenged with different cumulative doses of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), we demonstrated that systemic injection of LY354740 reduced the level of extracellular glutamate and the extent of nigro-striatal degeneration in both acute and sub-acute MPTP mice, while LY341495 amplified the lesions in sub-acute MPTP mice only. LY354740 treatment improved behavioral dysfunctions mainly in acute MPTP mice and LY341495 treatment seemed to aggravate motor deficits in sub-acute MPTP mice. In addition, ligands of mGlu_2/3 receptors also influenced the total amount of glutamate and dopamine in brain tissue. Interestingly, compared with normal mice, MPTP-treated mice abnormally up-regulated the expression of polo-like kinase 2 (PLK2)/pS129 α-synuclein and phosphorylation of Fyn/N-methyl-D-aspartate receptor subunit 2A/2B (GluN2A/2B). Both acute and sub-acute MPTP mice treated with LY354740 dose-dependently reduced all the above abnormal expression. Compared with MPTP mice treated with vehicle, mice pretreated with LY341495 exhibited much higher expression of p-Fyn Tyr416/p-GluN2B Tyr1472 and PLK2/pS129 α-synuclein in sub-acute MPTP mice models. Thus, our current data indicated that mGlu_2/3 receptors ligands could influence MPTP-induced toxicity, which supported a role for mGlu_2/3 receptors in PD pathogenesis.

Prolonged febrile seizure history exacerbates seizure severity in a pentylenetetrazole rat model of epilepsy

Epilepsy is a debilitating neurological illness that affects all aspect of an individual life. Despite advancement in research there is little reduction in the incidence of this disease. Prolonged febrile seizure (PFS) has been linked to epilepsy however, the pathophysiology of this is still not clear. We therefore looked at the effect of PFS on the development of epilepsy in a pentylenetetrazole (PTZ) rat model of epilepsy. A total of 42 male Sprague-Dawley rats were used for the experiment. On post-natal day (PND) 14, PFS was induced in 14 rats. This was followed by the induction of epilepsy in the 14 PFS animal and 14 animals from the remaining 28 rats by an initial injection of PTZ at a dose of 60 mg/kg on day one followed by 35 mg/kg on alternate day until kindle. We looked at the effect of PFS on the onset and the stage of convulsion at kindle. We also observed it effect on the hippocampal glial fibrillary acidic protein (GFAP), synaptophysin and metabotropic glutamate receptor 3 (mGluR3) expression measured with immunofluorescence, LI Cor Tissue florescence and immunohistochemistry respectively. Our study showed that PFS reduced seizure threshold by decreasing the time it took animals to kindle and also increased the stage of convulsion. The hippocampal GFAP, synaptophysin and mGluR3 expressions where upregulated in PTZ rats with PFS history when compared to PTZ rats alone.These findings indicated that PFS may increase the severity of epilepsy and alter brain expression of GFAP, synaptophysin and mGluR3 proteins.

Expression of mGlu Receptor Genes in the Hippocampus After Intoxication with Trimethyltin

A variety of localization and signaling properties of eight subtypes of metabotropic glutamate receptors (mGluRs) in the brain provide glutamate an important regulatory role in many processes, including neurodegeneration and repair of neuronal damage. To identify specific subtypes of mGluRs, which are involved in neurodegeneration process, we assessed expression levels of their genes under pathophysiological conditions. Using quantitative real-time RT-PCR analysis, we studied transcription levels of mGlu2-5 and mGlu7 genes in the hippocampus after its damage by neurotoxicant trimethyltin chloride (TMT) in Wistar rats. This organotin compound is known to cause neurodegeneration in the brain, especially in the hippocampus. Morphological studies confirmed neuronal damage in CA3-CA4 subfields of the hippocampus 6 weeks after the treatment with TMT. Step-through passive avoidance test revealed memory deterioration in rat-treated TMT. Interestingly, 3 and 6 weeks after the treatment with TMT, expression levels of the mGlu2 and mGlu7 genes were not changed in comparison to the control values while expression level of mGlu4 genes was upregulated throughout the whole studied period of TMT action. The dynamics of mGlu3 gene expression revealed the existence of neuroinflammation 3 weeks after the treatment with TMT, which was further confirmed by the upregulation of cyclooxygenase-2 gene expression. The expression level of mGlu5 receptors was downregulated 6 weeks after the treatment with TMT. Our results revealed a significant role of mGlu4, mGlu5, and mGlu3 receptors in the neurodegenerative/reparative processes in the hippocampus after the treatment with TMT. Ligands of these receptor subtypes can be, therefore, considered potential therapeutic targets for prevention or reduction of neurodegeneration.

Chloride ions stabilize the glutamate-induced active state of the metabotropic glutamate receptor 3

Due to the essential roles of glutamate, detection and response to a large range of extracellular concentrations of this excitatory amino acid are necessary for the fine-tuning of brain functions. Metabotropic glutamate receptors (mGluRs) are implicated in shaping the activity of many synapses in the central nervous system. Among the eight mGluR subtypes, there is increasing interest in studying the mGlu₃ receptor which has recently been linked to various diseases, including psychiatric disorders. This receptor displays striking functional properties, with a high and, often, full basal activity, making its study elusive in heterologous systems. Here, we demonstrate that Cl^- ions exert strong positive allosteric modulation of glutamate on the mGlu₃ receptor. We have also identified the molecular and structural determinants lying behind this allostery: a unique interactive "chloride-lock" network. Indeed, Cl^- ions dramatically stabilize the glutamate-induced active state of the extracellular domain of the mGlu₃ receptor. Thus, the mGlu₃ receptors' large basal activity does not correspond to a constitutive activity in absence of agonist. Instead, it results mostly from a Cl^-mediated amplified response to low ambient glutamate concentrations, such as those measured in cell media. This strong interaction between glutamate and Cl^- ions allows the mGlu₃ receptor to sense and efficiently react to sub-micromolar concentrations of glutamate, making it the most sensitive member of mGluR family.

In the grey zone between epilepsy and schizophrenia: alterations in group II metabotropic glutamate receptors

Glutamate is the major excitatory neurotransmitter in the brain. The glutamate system plays an important role in the formation of synapses during brain development and synaptic plasticity. Dysfunctions in glutamate regulation may lead to hyperexcitatory neuronal networks and neurotoxicity. Glutamate excess is possibly of great importance in the pathophysiology of several neurological and psychiatric disorders such as epilepsy and schizophrenia. Interestingly, cross talk between these disorders has been well documented: psychiatric comorbidities are frequent in epilepsy and temporal lobe epilepsy is one of the highest risk factors for developing psychosis. Therefore, dysfunctions in glutamatergic neurotransmission might constitute a common pathological mechanism. A major negative feedback system is regulated by the presynaptic group II metabotropic glutamate (mGlu) receptors including mGlu2/3 receptors. These receptors are predominantly localised extrasynaptically in basal ganglia and limbic structures. Hence, mGlu2/3 receptors are an interesting target for the treatment of disorders like epilepsy and schizophrenia. A dysfunction in the glutamate system may be associated with alterations in mGlu2/3 receptor expression. In this review, we describe the localization of mGlu2/3 receptors in the healthy brain of mice, rats and humans. Secondly, changes in mGlu2/3 receptor density of the brain regions affected in epilepsy and schizophrenia are summarised. Increased mGlu2/3 receptor density might represent a compensatory mechanism of the brain to regulate elevated glutamate levels, while reduced mGlu2/3 receptor density in some brain regions may further contribute to the aberrant hyperexcitability. Further research considering the mGlu2/3 receptor can contribute significantly to the understanding of the etiological and therapeutic role of group II mGlu receptor in epilepsy, epilepsy with psychosis and schizophrenia.

Metabotropic glutamate receptor 3 is downregulated and its expression is shifted from neurons to astrocytes in the mouse lateral septum during the postpartum period

The inhibitory metabotropic glutamate receptor 3 (mGluR3) plays diverse and complex roles in brain function, including synaptic plasticity and neurotransmission. We recently found that mGluR3 is downregulated in the lateral septum (LS) of postpartum females using microarray and qPCR analysis. In this study, we used double fluorescence immunohistochemical approaches to characterize mGluR3 changes in LS of the postpartum brain. The number of mGluR3-immunoractive cells was significantly reduced in the dorsal (LSD) and intermediate (LSI) but not ventral (LSV) parts of the LS in postpartum versus virgin females. mGluR3 immunoreactivity in the LS was found predominantly in neurons (~70%), with a smaller portion (~20%-30%) in astrocytes. Colocalization analysis revealed a reduced mGluR3 expression in neurons but an increased astrocytic localization in postpartum LSI. This change in the pattern of expression suggests that mGluR3 expression is shifted from neurons to astrocytes in postpartum LS, and the decrease in mGluR3 is neuron-specific. Because mGluR3 is inhibitory and negatively regulates glutamate and GABA release, decreases in neuronal expression would increase glutamate and GABA signaling. Given our recent finding that ~90% of LS neurons are GABAergic, the present data suggest that decreases in mGluR3 are a mechanism for elevated GABA in LS in the postpartum state.

Developmental distribution pattern of metabotropic glutamate receptor 5 in prenatal human hippocampus

OBJECTIVE

Metabotropic glutamate receptor 5 (mGluR5) is concentrated in zones of active neurogenesis in the prenatal and postnatal rodent brain and plays an important role in the regulation of neurogenesis. However, little is known about mGluR5 in the prenatal human brain. Here, we aimed to explore the expression pattern and cellular distribution of mGluR5 in human fetal hippocampus.

METHODS

Thirty-four human fetuses were divided into four groups according to gestational age: 9-11, 14-16, 22-24 and 32-36 weeks. The hippocampus was dissected out and prepared. The protein and mRNA expression of mGluR5 were evaluated by Western blot and immunohistochemistry or real-time PCR. The cellular distribution of mGluR5 was observed with double-labeling immunofluorescence.

RESULTS

Both mGluR5 mRNA and protein were detected in the prenatal human hippocampus by real-time PCR and immunoblotting, and the expression levels increased gradually over time. The immunohistochemistry results were consistent with immunoblotting and showed that mGluR5 immunoreactivity was mainly present in the inner marginal zone (IMZ), hippocampal plate (HP) and ventricular zone (VZ). The double-labeling immunofluorescence showed that mGluR5 was present in neural stem cells (nestin-positive), neuroblasts (DCX-positive) and mature neurons (NeuN-positive), but not in typical astrocytes (GFAP-positive). The cells co-expressing mGluR5 and nestin were mainly located in the IMZ, HP and subplate at 11 weeks, all layers at 16 weeks, and CA1 at 24 weeks. As development proceeded, the number of mGluR5/nestin double-positive cells decreased gradually so that there were only a handful of double-labeled cells at 32 weeks. However, mGluR5/DCX double-positive cells were only found in the HP, IZ and IMZ at 11 weeks.

CONCLUSION

The pattern of mGluR5 expression by neural stem/progenitor cells, neuroblasts and neurons provides important anatomical evidence for the role of mGluR5 in the regulation of human hippocampal development.

Bidirectional scaling of astrocytic metabotropic glutamate receptor signaling following long-term changes in neuronal firing rates

Very little is known about the ability of astrocytic receptors to exhibit plasticity as a result of changes in neuronal activity. Here we provide evidence for bidirectional scaling of astrocytic group I metabotropic glutamate receptor signaling in acute mouse hippocampal slices following long-term changes in neuronal firing rates. Plasticity of astrocytic mGluRs was measured by recording spontaneous and evoked Ca²⁺ elevations in both astrocytic somata and processes. An exogenous astrocytic Gq G protein-coupled receptor was resistant to scaling, suggesting that the alterations in astrocyte Ca²⁺ signaling result from changes in activity of the surface mGluRs rather than a change in intracellular G protein signaling molecules. These findings suggest that astrocytes actively detect shifts in neuronal firing rates and adjust their receptor signaling accordingly. This type of long-term plasticity in astrocytes resembles neuronal homeostatic plasticity and might be important to ensure an optimal or expected level of input from neurons.

mGlu3 receptor and astrocytes: partners in neuroprotection

Astrocytes are currently studied intensively because of their now highlighted relevance as key players with neurons that modulate a wide range of central functions, from synaptic plasticity and synaptogenesis to regulation of metabolic and neuroinflammatory processes. Since the discovery of mGlu3 receptors on astrocytes, accumulating evidence supports a role of these receptors not only in maintaining synaptic homeostasis and treating psychiatric disorders but also in promoting astrocyte survival in several pathologic conditions. This review focuses on providing up-to-date knowledge regarding effects of activating astroglial mGlu3 receptors on psychiatric disorders, astrocyte and neuronal survival, and neurodegenerative diseases. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Metabotropic glutamate receptor-mediated signaling in neuroglia

Metabotropic glutamate (mGlu) receptors are G-protein-coupled receptors, which include eight subtypes that have been classified into three groups (I-III) based upon sequence homology, signal transduction mechanism and pharmacological profile. Although most studied with regard to neuronal function and modulation, mGlu receptors are also expressed by neuroglia-including astrocytes, microglia and oligodendrocytes. Activation of mGlu receptors on neuroglia under both physiologic and pathophysiologic conditions mediates numerous actions that are essential for intrinsic glial cell function, as well as for glial-neuronal interactions. Astrocyte mGlu receptors play important physiological roles in regulating neurotransmission and maintaining neuronal homeostasis. However, mGlu receptors on astrocytes and microglia also serve to modulate cell death and neurological function in a variety of pathophysiological conditions such as acute and chronic neurodegenerative disorders. The latter effects are complex and bi-directional, depending on which mGlu receptor sub-types are activated.

Metabotropic glutamate receptor mGluR5 is not involved in the early hemodynamic response

Activation of astrocytic metabotropic glutamate receptor 5 (mGluR5) is postulated to elicit calcium transients, triggering a chain of events that ultimately regulates cerebral blood flow by changing the tone of smooth muscle cells of nearby arterioles. Using concurrent in vivo optical imaging and determination of receptor occupancy with (11)C-ABP688, we report here that blocking ∼80% of mGluR5 in vivo does not affect transient hemodynamic responses on brief whisker stimulations while transiently reducing neuronal activity as measured by voltage-sensitive dye imaging. Our results show that mechanisms other than activation of mGluR5 are required to trigger the initial hemodynamic response in normal physiological conditions.

Reduced cAMP, Akt activation and p65-c-Rel dimerization: mechanisms involved in the protective effects of mGluR3 agonists in cultured astrocytes

In recent decades, astrocytes have emerged as key pieces in the maintenance of normal functioning of the central nervous system. Any impairment in astroglial function can ultimately lead to generalized disturbance in the brain, thus pharmacological targets associated with prevention of astrocyte death are actually promising. Subtype 3 of metabotropic glutamate receptors (mGluR3) is present in astrocytes, its activation exerting neuroprotective roles. In fact, we have previously demonstrated that mGluR3 selective agonists prevent nitric oxide (NO)-induced astrocyte death. However, mechanisms responsible for that cytoprotective property are still subject to study. Although inhibition of adenylyl cyclase by mGluR3 activation was extensively reported, the involvement of reduced cAMP levels in the effects of mGluR3 agonists and the association between cAMP decrease and the downstream pathways activated by mGluR3 remain neglected. Thus, we studied intracellular signaling mediating anti-apoptotic actions of mGluR3 in cultured rat astrocytes exposed to NO. In the present work, we showed that the cytoprotective effect of mGluR3 agonists (LY379268 and LY404039) requires both the reduction of intracellular cAMP levels and activation of Akt, as assessed by MTT and TUNEL techniques. Moreover, dibutyryl-cAMP impairs Akt phosphorylation induced by LY404039, indicating a relationship between mGluR3-reduced cAMP levels and PI3K/Akt pathway activation. We also demonstrated, by co-immunoprecipitation followed by western-blot, that the mGluR3 agonists not only induce per se survival-linked interaction between members of the NF-κB family p65 and c-Rel, but also impede reduction of levels of p65-c-Rel dimers caused by NO, suggesting a possible anti-apoptotic role for p65-c-Rel. All together, these data suggest that mGluR3 agonists may regulate cAMP/Akt/p65-c-Rel pathway, which would contribute to the protective effect of mGluR3 against NO challenge in astrocytes. Our results widen the knowledge about mechanisms of action of mGluR3, potential targets for the treatment of neurodegenerative disorders where a pathophysiological role for NO has been established.

Positive allosteric modulators of type 5 metabotropic glutamate receptors (mGluR5) and their therapeutic potential for the treatment of CNS disorders

Studies utilizing selective pharmacological antagonists or targeted gene deletion have demonstrated that type 5 metabotropic glutamate receptors (mGluR5) are critical mediators and potential therapeutic targets for the treatment of numerous disorders of the central nervous system (CNS), including depression, anxiety, drug addiction, chronic pain, Fragile X syndrome, Parkinson’s disease, and gastroesophageal reflux disease. However, in recent years, the development of positive allosteric modulators (PAMs) of the mGluR5 receptor have revealed that allosteric activation of this receptor may also be of potential therapeutic benefit for the treatment of other CNS disorders, including schizophrenia, cognitive deficits associated with chronic drug use, and deficits in extinction learning. Here we summarize the discovery and characterization of various mGluR5 PAMs, with an emphasis on those that are systemically active. We will also review animal studies showing that these molecules have potential efficacy as novel antipsychotic agents. Finally, we will summarize findings that suggest that mGluR5 PAMs have pro-cognitive effects such as the ability to enhance synaptic plasticity, improve performance in various learning and memory tasks, including extinction of drug-seeking behavior, and reverse cognitive deficits produced by chronic drug use.

Allosteric modulation of metabotropic glutamate receptors

The development of receptor subtype-selective ligands by targeting allosteric sites of G protein-coupled receptors (GPCRs) has proven highly successful in recent years. One GPCR family that has greatly benefited from this approach is the metabotropic glutamate receptors (mGlus). These family C GPCRs participate in the neuromodulatory actions of glutamate throughout the CNS, where they play a number of key roles in regulating synaptic transmission and neuronal excitability. A large number of mGlu subtype-selective allosteric modulators have been identified, the majority of which are thought to bind within the transmembrane regions of the receptor. These modulators can either enhance or inhibit mGlu functional responses and, together with mGlu knockout mice, have furthered the establishment of the physiologic roles of many mGlu subtypes. Numerous pharmacological and receptor mutagenesis studies have been aimed at providing a greater mechanistic understanding of the interaction of mGlu allosteric modulators with the receptor, which have revealed evidence for common allosteric binding sites across multiple mGlu subtypes and the presence for multiple allosteric sites within a single mGlu subtype. Recent data have also revealed that mGlu allosteric modulators can display functional selectivity toward particular signal transduction cascades downstream of an individual mGlu subtype. Studies continue to validate the therapeutic utility of mGlu allosteric modulators as a potential therapeutic approach for a number of disorders including anxiety, schizophrenia, Parkinson's disease, and Fragile X syndrome.

In vivo and in vitro validation of reference tissue models for the mGluR(5) ligand [(11)C]ABP688

The primary objective of this study was to verify the suitability of reference tissue-based quantification methods of the metabotropic glutamate receptor type 5 (mGluR(5)) with [(11)C]ABP688. This study presents in vivo (Positron Emission Tomography (PET)) and in vitro (autoradiography) measurements of mGluR(5) densities in the same rats and evaluates both noninvasive and blood-dependent pharmacokinetic models for the quantification of [(11)C]ABP688 binding. Eleven rats underwent [(11)C]ABP688 PET scans. In five animals, baseline scans were compared with blockade experiments with the antagonist 1,2-methyl-6-(phenylethynyl)-pyridine (MPEP), and arterial blood samples were drawn and corrected for metabolites. Afterward, saturation-binding autoradiography was performed. Blocking with MPEP resulted in an average decrease of the total distribution volume (V(T)) between 43% and 58% (thalamus and caudate-putamen, respectively) but had no significant effect on cerebellar V(T) (mean reduction: -0.01%). Comparing binding potential (BP(ND)) based on the V(T) with noninvasively determined BP(ND) revealed an average negative bias of 0.7% in the caudate-putamen and an average positive bias of 3.1% in the low-binding regions. Scan duration of 50 minutes is required. The cerebellum is a suitable reference region for the quantification of mGluR(5) availability as measured with [(11)C]ABP688 PET in rats. Blood-based and reference region-based PET quantification shows a significant linear relationship to autoradiographic determinations.

Metabotropic glutamate receptor 3 activation prevents nitric oxide-induced death in cultured rat astrocytes

Altered glial function may contribute to the initiation or progression of neuronal death in neurodegenerative diseases. Thus, modulation of astrocyte death may be essential for preventing pathological processes in the CNS. In recent years, metabotropic glutamate receptor (mGluR) activation has emerged as a key target for neuroprotection. We investigated the effect of subtype 3 mGluR (mGluR3) activation on nitric oxide (NO)-induced astroglial death. A mGluR3 selective agonist, LY379268, reduced inducible NO synthase expression and NO release induced by bacterial lipopolysaccharide and interferon-gamma in cultured rat astrocytes. In turn, a NO donor (diethylenetriamine/NO) induced apoptotic-like death in cultured astrocytes, which showed apoptotic morphology and DNA fragmentation, but no caspase 3 activation. LY379268 prevented astrocyte death induced by NO exposure, which correlates with a reduction in: phosphatidylserine externalization, p53 and Bax activation and mitochondrial permeability. The reported effects of LY379268 were prevented by the mGluR3 antagonist (s)-alpha-ethylglutamic acid. All together, these findings show the protective effect of mGluR3 activation on astroglial death and provide further evidence of a role of these receptors in preventing CNS injury triggered by several inflammatory processes associated with dysregulated NO production.

Effects of early and delayed treatment with an mGluR5 antagonist on motor impairment, nigrostriatal damage and neuroinflammation in a rodent model of Parkinson's disease

The loss of nigrostriatal dopaminergic neurons that characterizes Parkinson's disease (PD) causes complex functional alterations in the basal ganglia circuit. Increased glutamatergic activity at crucial points of the circuit may be central to these alterations, thereby contributing to the onset of PD motor symptoms. Signs of neuroinflammation accompanying the neuronal loss have also been observed; also in this case, glutamate-mediated mechanisms may be involved. Glutamate may therefore intervene at multiple levels in PD pathophysiology, possibly through the modulation of metabotropic receptors. To address this issue, we evaluated the effects of systemic treatment with MPEP (2-methyl-6-(phenylethynyl)-pyridine), an antagonist of metabotropic receptor mGluR5, in a rodent model of progressive nigrostriatal degeneration based on the intrastriatal injection of 6-hydroxydopamine (6-OHDA). Following 6-OHDA injection, Sprague-Dawley rats underwent a 4-week, daily treatment with MPEP (1.5mg/kg, i.p.). To investigate whether the effects varied with the progression of the lesion, subgroups of lesioned animals started the treatment at different time-points: (1) immediately, (2) 1 week, or (3) 4 weeks after the neurotoxin injection. Akinesia, dopaminergic nigrostriatal damage and neuroinflammatory response (microglial and astroglial activation) were investigated. MPEP prompted immediate amelioration of 6-OHDA-induced akinesia, as measured by the Adjusting step test, in all subgroups, regardless of the degree of nigrostriatal damage. Conversely, MPEP did not modify neuronal survival or neuroinflammatory response in the nigrostriatal pathway. In conclusion, chronic treatment with MPEP exerted a pure symptomatic effect, further supporting that mGluR5 modulation may be a viable strategy to counteract the basal ganglia functional modifications underlying PD motor symptoms.

Group II metabotropic glutamate receptor activation by agonist LY379268 treatment increases the expression of brain derived neurotrophic factor in the mouse brain

A number of in vitro and in vivo studies using selective agonists have indicated a neuroprotective role for group-II metabotropic glutamate (mGlu2/3) receptors in various models of neuronal injury. Although an interplay among neurotrophic factors and mGlu2/3 receptors signalling system has been suggested as possible mechanism involved on neuroprotection, at present poor information are available concerning the in vivo regulation by mGlu2/3 receptors activation of specific neurotrophic factors. By using in situ hybridization and western blotting methods the aim of present study was to analyse the potential regulatory role of selective mGluR2/3 agonist LY379268 treatment on brain derived neurotrophic factor (BDNF) expression in the mouse brain. The treatment with LY379268 evidenced a significant upregulation of BDNF mRNA levels in the cerebral cortex and in the hippocampal formation with a peak at 3 h from treatment and its disappearance already at 6 h from treatment. An analysis of dose-effect curve revealed that LY379268 may significantly enhance BDNF mRNA expression already at dose of 0.250 mg/kg b.w. The upregulation of BDNF mRNA expression was followed by a significant increase of BDNF protein levels at 24 h from LY379268 treatment. These effects of LY379268 treatment on BDNF expression were restricted to neuronal cells and were blocked by the new selective mGlu2/3 receptor antagonist LY341495, suggesting a receptor specificity. Taken together these findings suggest that several previous observed neuroprotective and trophic actions of mGluR2/3 agonists treatment may be mediated, at least in the cerebral cortex and hippocampal formation, by upregulation of BDNF expression.

Metabotropic glutamate receptor ligands as potential therapeutics for addiction

There is now compelling evidence that the excitatory amino acid neurotransmitter glutamate plays a pivotal role in drug addiction and alcoholism. As a result, there has been increasing interest in developing glutamate-based therapies for the treatment of addictive disorders. Receptors for glutamate are primarily divided into two classes: ionotropic glutamate receptors (iGluRs) that mediate fast excitatory glutamate transmission, and metabotropic glutamate receptors (mGluRs), which are G-protein coupled receptors that mediate slower, modulatory glutamate transmission. Most iGluR antagonists, while showing some efficacy in animal models of addiction, exhibit serious side effects when tested in humans. mGluR ligands, on the other hand, which have been advanced to testing in clinical trials for various medical conditions, have demonstrated the ability to reduce drug reward, reinforcement, and relapse-like behaviors in animal studies. mGluR ligands that have been shown to be primarily effective are Group I (mGluR1 and mGluR5) negative allosteric modulators and Group II (mGluR2 and mGluR3) orthosteric presynaptic autoreceptor agonists. In this review, we will summarize findings from animal studies suggesting that these mGluR ligands may be of potential benefit in reducing on-going drug self-administration and may aid in the prevention of relapse. The neuroanatomical distribution of mGluR1, mGluR2/3, and mGluR5 receptors and the pharmacological properties of Group I negative allosteric modulators and Group II agonists will also be overviewed. Finally, we will discuss the current status of mGluR ligands in human clinical trials.

Identification of calcium sensing receptor (CaSR) mRNA-expressing cells in normal and injured rat brain

Calcium sensing receptor (CaSR), isolated for the first time from bovine and human parathyroid, is a G-protein-coupled receptors that has been involved in diverse physiological functions. At present a complete in vivo work on the identification of CaSR mRNA-expressing cells in the adult brain lacks and this investigation was undertaken in order to acquire more information on cell type expressing CaSR mRNA in the rat brain and to analyse for the first time its expression in different experimental models of brain injury. The expression of CaSR mRNAs was found mainly in scattered cells throughout almost all the brain regions. A double labeling analysis showed a colocalization of CaSR mRNA expression in neurons and oligodendrocytes, whereas it was not found expressed both in the microglia and in astrocytes. One week after kainate-induced seizure CaSR was found in the injured CA3 region of the hippocampus and very interestingly it was found up-regulated in the neurons of CA1-CA2 and dentate gyrus. Similarly, 1 week following ibotenic acid injection in the hippocampus, CaSR mRNA expression was increased in oligodendrocytes both in the lesioned area and in the contralateral CA1-CA3 pyramidal cell layers and dentate gyrus. One week after needle-induced mechanical lesion an increase of labeled cells expressing CaSR mRNA was observed along the needle track. In conclusion, the present results contribute to extend available data on cell type-expressing CaSR in normal and injured brain and could spur to understand the role of CaSR in repairing processes of brain injury.

Increased vulnerability of hippocampal pyramidal neurons to the toxicity of kainic acid in OASIS-deficient mice

The endoplasmic reticulum (ER) stress response is a defense system for dealing with the accumulation of unfolded proteins in the ER lumen. Old astrocyte specifically induced substance (OASIS) is known to be expressed in astrocytes and involved in the ER stress response; however the function of OASIS in the injured brain has remained unclear. In this study, we examined the roles of OASIS in neuronal degeneration in the hippocampi of mice intraperitoneally injected with kainic acid (KA). OASIS mRNA was strongly induced in response to KA injection, with a similar time course to the induction of ER molecular chaperone immunoglobulin heavy chain binding protein mRNA. In situ hybridization showed that KA injection causes induction of immunoglobulin heavy chain binding protein mRNA in glial fibrillary acidic protein-positive astrocytes as well as in pyramidal neurons, although up-regulation of OASIS mRNA was only detected in glial fibrillary acidic protein-positive astrocytes. Primary cultured astrocytes, but not the neurons of OASIS-/- mice, revealed reduced vulnerability to ER stress. Furthermore, pyramidal neurons in the hippocampi of OASIS-/- mice were more susceptible to the toxicity induced by KA than those of wild-type mice. Taken together, these data suggest that OASIS expressed in astrocytes plays important roles in protection against the neuronal damage induced by KA.

Metabotropic glutamate receptor II in the brains of Parkinsonian patients

Modulation of basal ganglia group II metabotropic glutamate receptors (mGluR2/3) is a potential therapeutic alternative to levodopa in Parkinson disease (PD). We used receptor-binding autoradiography of the mGluR2/3-selective radioligand [H]LY341495 in postmortem brain specimens from PD patients (n = 14) and controls (n=11) to investigate possible contributions of changes in ligand binding of this receptor to levodopa-associated motor complications experienced premortem in PD patients. The PD patients included those with and without histories of dyskinesias and those with and without "wearing off," which is defined as a reduced period of benefit from levodopa. Specific binding of [H]LY341495 to mGluR2/3 in the basal ganglia was higher in the caudate nucleus than the putamen and lower by approximately half in the external and internal globus pallidus (GPi) in controls. [H]LY341495-specific binding was reduced in the caudate and GPi in patients without wearing-off (-22% caudate, -30% GPi), compared with controls and with patients who had experienced wearing-off; there were no differences among PD patients with or without dyskinesias. These data suggest that an adaptive downregulation of mGluR2/3 in PD patients without wearing-off may compensate for increased glutamate. They indicate a key role for mGluR2/3 in control of movement and the potential for mGluR2/3-targeted drugs in the management of wearing-off fluctuations in PD.

Effects of lead exposure on hippocampal metabotropic glutamate receptor subtype 3 and 7 in developmental rats

Background

A complete explanation of the mechanisms by which Pb²⁺ exerts toxic effects on developmental central nervous system remains unknown. Glutamate is critical to the developing brain through various subtypes of ionotropic or metabotropic glutamate receptors (mGluRs). Ionotropic N-methyl-D-aspartate receptors have been considered as a principal target in lead-induced neurotoxicity. The relationship between mGluR3/mGluR7 and synaptic plasticity had been verified by many recent studies. The present study aimed to examine the role of mGluR3/mGluR7 in lead-induced neurotoxicity.

Methods

Twenty-four adult and female rats were randomly selected and placed on control or 0.2% lead acetate during gestation and lactation. Blood lead and hippocampal lead levels of pups were analyzed at weaning to evaluate the actual lead content at the end of the exposure. Impairments of short -term memory and long-term memory of pups were assessed by tests using Morris water maze and by detection of hippocampal ultrastructural alterations on electron microscopy. The impact of lead exposure on mGluR3 and mGluR7 mRNA expression in hippocampal tissue of pups were investigated by quantitative real-time polymerase chain reaction and its potential role in lead neurotoxicity were discussed.

Results

Lead levels of blood and hippocampi in the lead-exposed rats were significantly higher than those in the controls (P < 0.001). In tests using Morris Water Maze, the overall decrease in goal latency and swimming distance was taken to indicate that controls had shorter latencies and distance than lead-exposed rats (P = 0.001 and P < 0.001 by repeated-measures analysis of variance). On transmission electron microscopy neuronal ultrastructural alterations were observed and the results of real-time polymerase chain reaction showed that exposure to 0.2% lead acetate did not substantially change gene expression of mGluR3 and mGluR7 mRNA compared with controls.

Conclusion

Exposure to lead before and after birth can damage short-term and long-term memory ability of young rats and hippocampal ultrastructure. However, the current study does not provide evidence that the expression of rat hippocampal mGluR3 and mGluR7 can be altered by systemic administration of lead during gestation and lactation, which are informative for the field of lead-induced developmental neurotoxicity noting that it seems not to be worthwhile to include mGluR3 and mGluR7 in future studies.

Glutamate elicits release of BDNF from basal forebrain astrocytes in a process dependent on metabotropic receptors and the PLC pathway

A key neurotrophin responsible for the survival and function of basal forebrain (BF) cholinergic neurons is brain-derived neurotrophic factor (BDNF). A number of studies now indicate that a source of this factor may be BF astrocytes. This study was designed to define the role of BF-astrocyte-derived BDNF on cholinergic neurons. Moreover, it investigated regulatory events that modulate BDNF content and release. In initial work BDNF derived from BF-astrocyte-conditioned medium (ACM) was found to increase both numbers of BF acetylcholinesterase (AChE+) cholinergic neurons and the cholinergic synthetic enzyme choline acetyltransferase (ChAT). Western blots, immunocytochemistry and pharmacological inhibition studies revealed that glutamate, through group I metabotropic glutamate receptors (mGluR), increases the intracellular levels of BDNF in BF astrocytes in culture, as well as its release. Furthermore, the release of BDNF is mediated by the actions of PLC, IP3 and internal stores of Ca2+. These results suggest that BF astrocytes serve as local sources of BDNF for cholinergic neurons, and that they may be regulated as such by the neuronal signal, glutamate, through the mediation of group I metabotropic receptors and the PLC pathway.

Chronic CXCL10 alters neuronal properties in rat hippocampal culture

The chemokine CXCL10 is expressed in the central nervous system (CNS) during neuroinflammatory conditions. Neurons express CXCR3, the receptor for CXCL10, and neuronal function has been shown to be altered by acute exposure to CXCL10. Little is known about the effects of chronic exposure to CXCL10 on neuronal function. Results from our studies show that chronic exposure of cultured rat hippocampal neurons to CXCL10 results in altered levels of protein for GABA and glutamate receptors and altered synaptic network activity. These effects of CXCL10 may contribute to altered CNS function that occurs in some chronic neuroinflammatory conditions.

Metabotropic glutamate receptors as targets for multipotential treatment of neurological disorders

Glutamate is a major excitatory neurotransmitter in the CNS that is involved in numerous cellular functions, including cell death and survival. Metabotropic glutamate receptors (mGluR) are G-protein coupled receptors that have been classified into three groups on the basis of signal transduction pathways and pharmacological profiles. Group I, II, and III mGluRs are found on cell types within and peripheral to the CNS, including neurons, microglia, astrocytes, oligodendrocytes, T- and B-cell lymphocytes, osteoblasts, hepatocytes, and endothelial cells, among others. These receptors have a number of effects on cells that can influence outcome after trauma, including reducing neuronal and oligodendroglial cell death, inflammation, and endothelial permeability. Thus, mGluRs are a promising multipotential therapeutic approach. Because the pathology of CNS trauma and neurodegeneration is multifactorial (including, for example, oxidative stress, mitochondrial breakdown, and inflammation), therapies that serve to modulate multiple pathophysiological pathways may prove more effective than those directed at a single target. This review examines the multipotential therapeutic utility of mGluR modulation in acute and chronic injury and neurodegeneration.

Differential microglial activation between acute stress and lipopolysaccharide treatment

Acute stress was demonstrated to induce morphological microglial activation in several brain regions including the midbrain periaqueductal gray (PAG), an area that plays important roles in behavioral responses to uncontrollable stress, threat, anxiety, and pain. To determine whether neuronal activation may be involved in the stress-induced microglial activation, the present study investigated the correlation between neuronal activity measured as c-Fos expression and morphological microglial activation in the PAG. Acute stress was followed by morphological activation of microglia and increased c-Fos expression in the PAG but not in the surrounding midbrain. Double immunohistochemistry and topological analysis demonstrated that microglial activation occurred adjacent to responsive neurons. By contrast, lipopolysaccharide (LPS) treatment induced microglial activation even in the absence of neuronal responses in the PGA as well as in the rest of the midbrain. These findings suggest that the mechanism of microglial activation during stress may differ from those of infection or inflammation. It also indicates that the neuronal cells expressing c-Fos protein may play some roles to trigger microglial activation.

Influence of Neuronal Nicotinic Receptors over Nicotine Addiction and Withdrawal

Cigarette smoking represents an enormous, global public health threat. Nearly five million premature deaths during a single year are attributable to smoking. Despite the resounding message of risks associated with smoking and numerous public health initiatives, cigarette smoking remains the most common preventable cause of disease in the United States. Fortunately, even in an adult smoker, smoking cessation can reverse many of the potential harmful effects. The symptoms associated with nicotine withdrawal represent the major obstacle to smoking cessation. This minireview examines the roles of various nicotinic receptors in the mechanisms of nicotine dependence, discusses the potential role of the habenula-interpeduncular nucleus axis in nicotine withdrawal, and highlights nicotinic receptors containing the β4 subunit as a potential pharmacological target for smoking cessation strategies.

The group II metabotropic glutamate receptor 3 (mGluR3, mGlu3, GRM3): expression, function and involvement in schizophrenia

Group II metabotropic glutamate receptors (mGluRs) comprise mGluR2 (mGlu2; encoded by GRM2) and mGluR3 (mGlu3; encoded by GRM3) and modulate glutamate neurotransmission and synaptic plasticity. Here we review the expression and function of mGluR3 and its involvement in schizophrenia. mGluR3 is expressed by glia and neurons in many brain regions and has a predominantly presynaptic distribution, consistent with its role as an inhibitory autoreceptor and heteroceptor. mGluR3 splice variants exist in human brain but are of unknown function. Differentiation of mGluR3 from mGluR2 has been problematic because of the lack of selective ligands and antibodies; the available data suggest particular roles for mGluR3 in long-term depression, in glial function and in neuroprotection. Some but not all studies find genetic association of GRM3 polymorphisms with psychosis, with the risk alleles also being associated with schizophrenia-related endophenotypes such as impaired cognition, cortical activation and glutamate markers. The dimeric form of mGluR3 may be reduced in the brain in schizophrenia. Finally, preclinical findings have made mGluR3 a putative therapeutic target, and now direct evidence for antipsychotic efficacy of a group II mGluR agonist has emerged from a randomised clinical trial in schizophrenia. Together these data implicate mGluR3 in aetiological, pathophysiological and pharmacotherapeutic aspects of the disorder.

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.