Altered Glutamate Neurotransmission Research Articles

Integrative transcriptome- and DNA methylation analysis of brain tissue from the temporal pole in suicide decedents and their controls.

Suicide rates have increased steadily world-wide over the past two decades, constituting a serious public health crisis that creates a significant burden to affected families and the society as a whole. Suicidal behavior involves a multi-factorial etiology, including psychological, social and biological factors. Since the molecular neural mechanisms of suicide remain vastly uncharacterized, we examined transcriptional- and methylation profiles of postmortem brain tissue from subjects who died from suicide as well as their neurotypical healthy controls. We analyzed temporal pole tissue from 61 subjects, largely free from antidepressant and antipsychotic medication, using RNA-sequencing and DNA-methylation profiling using an array that targets over 850,000 CpG sites. Expression of NPAS4, a key regulator of inflammation and neuroprotection, was significantly downregulated in the suicide decedent group. Moreover, we identified a total of 40 differentially methylated regions in the suicide decedent group, mapping to seven genes with inflammatory function. There was a significant association between NPAS4 DNA methylation and NPAS4 expression in the control group that was absent in the suicide decedent group, confirming its dysregulation. NPAS4 expression was significantly associated with the expression of multiple inflammatory factors in the brain tissue. Overall, gene sets and pathways closely linked to inflammation were significantly upregulated, while specific pathways linked to neuronal development were suppressed in the suicide decedent group. Excitotoxicity as well as suppressed oligodendrocyte function were also implicated in the suicide decedents. In summary, we have identified central nervous system inflammatory mechanisms that may be active during suicidal behavior, along with oligodendrocyte dysfunction and altered glutamate neurotransmission. In these processes, NPAS4 might be a master regulator, warranting further studies to validate its role as a potential biomarker or therapeutic target in suicidality.

The mGluR2 positive allosteric modulator, SAR218645, improves memory and attention deficits in translational models of cognitive symptoms associated with schizophrenia.

Normalization of altered glutamate neurotransmission through activation of the mGluR2 has emerged as a new approach to treat schizophrenia. These studies describe a potent brain penetrant mGluR2 positive allosteric modulator (PAM), SAR218645. The compound behaves as a selective PAM of mGluR2 in recombinant and native receptor expression systems, increasing the affinity of glutamate at mGluR2 as inferred by competition and GTPγ35S binding assays. SAR218645 augmented the mGluR2-mediated response to glutamate in a rat recombinant mGluR2 forced-coupled Ca2+ mobilization assay. SAR218645 potentiated mGluR2 agonist-induced contralateral turning. When SAR218645 was tested in models of the positive symptoms of schizophrenia, it reduced head twitch behavior induced by DOI, but it failed to inhibit conditioned avoidance and hyperactivity using pharmacological and transgenic models. Results from experiments in models of the cognitive symptoms associated with schizophrenia showed that SAR218645 improved MK-801-induced episodic memory deficits in rats and attenuated working memory impairment in NMDA Nr1neo−/− mice. The drug reversed disrupted latent inhibition and auditory-evoked potential in mice and rats, respectively, two endophenotypes of schizophrenia. This profile positions SAR218645 as a promising candidate for the treatment of cognitive symptoms of patients with schizophrenia, in particular those with abnormal attention and sensory gating abilities.

Pharmacogenetic Analysis of Functional Glutamate System Gene Variants and Clinical Response to Clozapine

Altered glutamate neurotransmission is implicated in the etiology of schizophrenia (SCZ) and the pharmacogenetics of response to clozapine (CLZ), which is the drug of choice for treatment-resistant SCZ. Response to antipsychotic therapy is highly variable, although twin studies suggest a genetic component. We investigated the association of 10 glutamate system gene variants with CLZ response using standard genotyping procedures. GRM2 (rs4067 and rs2518461), SLC1A2 (rs4354668, rs4534557, and rs2901534), SLC6A9 (rs12037805, rs1978195, and rs16831558), GRIA1 (rs2195450), and GAD1 (rs3749034) were typed in 163 European SCZ/schizoaffective disorder patients deemed resistant or intolerant to previous pharmacotherapy. Response was assessed following 6 months of CLZ monotherapy using change in Brief Psychiatric Rating Scale (BPRS) scores. Categorical and continuous response variables were analyzed using χ² tests and analysis of covariance, respectively. We report no significant associations following correction for multiple testing. Prior to correction, nominally significant associations were observed for SLC6A9, SLC1A2, GRM2, and GRIA1. Most notably, CC homozygotes of rs16831558 located in the glycine transporter 1 gene (SLC6A9) exhibited an allele dose-dependent improvement in positive symptoms compared to T allele carriers (p_uncorrected = 0.008, p_corrected = 0.08). To clarify the role of SLC6A9 in clinical response to antipsychotic medication, and CLZ in particular, this finding warrants further investigation in larger well-characterized samples.

P11 modulates L-DOPA therapeutic effects and dyskinesia via distinct cell types in experimental Parkinsonism

The reduced movement repertoire of Parkinson's disease (PD) is mainly due to degeneration of nigrostriatal dopamine neurons. Restoration of dopamine transmission by levodopa (L-DOPA) relieves motor symptoms of PD but often causes disabling dyskinesias. Subchronic L-DOPA increases levels of adaptor protein p11 (S100A10) in dopaminoceptive neurons of the striatum. Using experimental mouse models of Parkinsonism, we report here that global p11 knockout (KO) mice develop fewer jaw tremors in response to tacrine. Following L-DOPA, global p11KO mice show reduced therapeutic responses on rotational motor sensitization, but also develop less dyskinetic side effects. Studies using conditional p11KO mice reveal that distinct cell populations mediate these therapeutic and side effects. Selective deletion of p11 in cholinergic acetyltransferase (ChAT) neurons reduces tacrine-induced tremor. Mice lacking p11 in dopamine D2R-containing neurons have a reduced response to L-DOPA on the therapeutic parameters, but develop dyskinetic side effects. In contrast, mice lacking p11 in dopamine D1R-containing neurons exhibit tremor and rotational responses toward L-DOPA, but develop less dyskinesia. Moreover, coadministration of rapamycin with L-DOPA counteracts L-DOPA-induced dyskinesias in wild-type mice, but not in mice lacking p11 in D1R-containing neurons. 6-OHDA lesioning causes an increase of evoked striatal glutamate release in wild type, but not in global p11KO mice, indicating that altered glutamate neurotransmission could contribute to the reduced L-DOPA responsivity. These data demonstrate that p11 located in ChAT or D2R-containing neurons is involved in regulating therapeutic actions in experimental PD, whereas p11 in D1R-containing neurons underlies the development of L-DOPA-induced dyskinesias.

The Consequences of Early‐Life Adversity: Neurobiological, Behavioural and Epigenetic Adaptations

During the perinatal period, the brain is particularly sensitive to remodelling by environmental factors. Adverse early-life experiences, such as stress exposure or suboptimal maternal care, can have long-lasting detrimental consequences for an individual. This phenomenon is often referred to as 'early-life programming' and is associated with an increased risk of disease. Typically, rodents exposed to prenatal stress or postnatal maternal deprivation display enhanced neuroendocrine responses to stress, increased levels of anxiety and depressive-like behaviours, and cognitive impairments. Some of the phenotypes observed in these models of early-life adversity are likely to share common neurobiological mechanisms. For example, there is evidence for impaired glucocorticoid negative-feedback control of the hypothalamic-pituitary-adrenal axis, altered glutamate neurotransmission and reduced hippocampal neurogenesis in both prenatally stressed rats and rats that experienced deficient maternal care. The possible mechanisms through which maternal stress during pregnancy may be transmitted to the offspring are reviewed, with special consideration given to altered maternal behaviour postpartum. We also discuss what is known about the neurobiological and epigenetic mechanisms that underpin early-life programming of the neonatal brain in the first generation and subsequent generations, with a view to abrogating programming effects and potentially identifying new therapeutic targets for the treatment of stress-related disorders and cognitive impairment.

Involvement of pGluR1, EAAT2 and EAAT3 in offspring depression induced by prenatal stress

It is widely known that prenatal stress (PS) exposure causes depression-like behaviour to offspring, as well as maladaptive responses including neurobiological and physiological changes. However, the underlying mechanism of PS induced juvenile-onset depression remains largely unravelled. The inadequacies of monoamine deficiency hypothesis, the emerging evidence of altered glutamate neurotransmission in mood disorders, as well as our previous studies inspired us to assess the potential role of glutamatergic system in the pathogenesis of juvenile depression. In this research, we examined the expression of phosphorylated GluR1 subunit of ionotropic receptor alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR), the Na+-dependent glutamate transporters excitatory amino acid transporter 2 (EAAT2) and EAAT3 in the hippocampus, striatum and frontal cortex of 1-month-old rat offspring after mid and late PS exposure. Prenatally stressed offspring rats showed significantly prolonged duration of immobility and shortened immobility latency in tail suspension test. We also detected that PS significantly altered the expression of glutamate receptor and glutamate transporters of these depressed rats. In brief, the changes of phosphorylated GluR1 subunit of AMPAR protein level in the hippocampus and frontal cortex, as well as markedly decreased EAAT2 mRNA expression in the hippocampus, striatum and frontal cortex and EAAT3 mRNA expression in the hippocampus of stressed rats were both observed. These results underpinned that glutamate receptors and glutamate transporters might be involved in the progress of depression-like behaviour in juvenile rat offspring induced by PS.

Influence of polymorphisms in genes SLC1A1, GRIN2B, and GRIK2 on clozapine-induced obsessive–compulsive symptoms

Clinical observations indicate that atypical antipsychotics, especially clozapine, induce obsessive-compulsive (OC) symptoms in schizophrenia patients. Recent data from neuroimaging and clinical trials suggest a role for altered glutamate neurotransmission in the etiology of OC disorder (OCD), and SLC1A1, GRIN2B, and GRIK2 have all been reported to regulate glutamate transmission and affect OCD pathophysiology. This study aimed to determine whether SLC1A1, GRIN2B, and GRIK2 are associated with clozapine-induced OC symptoms. A total of 250 clinically stable schizophrenia patients receiving clozapine treatment were recruited. The Yale-Brown Obsessive Compulsive Scale (Y-BOCS) was used to evaluate the severity of OC symptoms. Based on their Y-BOCS scores, 250 patients were divided into the OC and non-OC groups (patients with or without OC symptoms, respectively). Additionally, three reported OCD susceptibility polymorphisms, SLC1A1 (rs2228622), GRIN2B (rs890), and GRIK2 (rs1556995), were genotyped. Trends of association with OC symptoms were observed in rs2228622A and rs890T alleles. SLC1A1 and GRIN2B interaction was found in the significant two-locus gene-gene interaction model (p = 0.0021), using the multifactor dimensionality reduction method. Further analysis showed a significant interaction between SLC1A1 and GRIN2B on the Y-BOCS score (F 6, 137 = 7.650, p < 0.001), and individuals with AA/TT genotypes had a significantly higher mean Y-BOCS score than those with other genotypes, except AG/TT. These results suggest that SLC1A1, GRIN2B, and interactions between the two may potentially confer a susceptibility to OC symptoms in schizophrenia patients receiving clozapine.

2653 – Influence of polymorphisms in genes SLC1A1, GRIN2B and GRIK2 on clozapine-induced obsessive-compulsive symptoms

Clinical observations indicate that atypical antipsychotics, especially clozapine, induce obsessive-compulsive (OC) symptoms in schizophrenic patients. Recent data from neuroimaging and clinical trials suggest a role for altered glutamate neurotransmission in the etiology of OCD. It has been reported that SLC1A1, GRIN2B and GRIK2 regulate glutamate transmission and affect OCD pathophysiology. This study aimed to determine whether SLC1A1, GRIN2B and GRIK2 are associated with clozapine-induced OC symptoms. A total of 250 clinically stable schizophrenic patients receiving clozapine treatment and 261 sex- and age-matched controls were recruited. The Yale-Brown Obsessive Compulsive Scale (Y-BOCS) was used to evaluate the severity of OC symptoms. Based on the Y-BOCS score, 250 patients were divided into OC group and non-OC group (patients with and without OC symptoms, respectively). Three reported OCD susceptibility polymorphisms SLC1A1 (rs2228622), GRIN2B (rs890) and GRIK2 (rs1556995) were genotyped. Trends of association with OC symptoms were observed in rs2228622A and rs890T alleles. SLC1A1 and GRIN2B interaction was found in the significant 2-locus, gene-gene interaction model (P=0.0021), using a MDR method. Further analysis showed a significant interaction between SLC1A1 and GRIN2B on the Y-BOCS score (F 6, 137 =7.650, P 0.001), and individuals with AA/TT genotypes had a significantly higher mean Y-BOCS score than those with other genotypes, except AG/TT. These results suggest that SLC1A1, GRIN2B and interactions between the two might confer a susceptibility to OC symptoms in schizophrenic patients receiving clozapine.

Extinction-Dependent Alterations in Corticostriatal mGluR2/3 and mGluR7 Receptors following Chronic Methamphetamine Self-Administration in Rats

Methamphetamine (meth) is a highly addictive and widely abused psychostimulant. Repeated use of meth can quickly lead to dependence, and may be accompanied by a variety of persistent psychiatric symptoms and cognitive impairments. The neuroadaptations underlying motivational and cognitive deficits produced by chronic meth intake remain poorly understood. Altered glutamate neurotransmission within the prefrontal cortex (PFC) and striatum has been linked to both persistent drug-seeking and cognitive dysfunction. Therefore, the current study investigated changes in presynaptic mGluR receptors within corticostriatal circuitry after extended meth self-administration. Rats self-administered meth (or received yoked-saline) in 1 hr/day sessions for 7 days (short-access) followed by 14 days of 6 hrs/day (long-access). Rats displayed a progressive escalation of daily meth intake up to 6 mg/kg per day. After cessation of meth self-administration, rats underwent daily extinction or abstinence without extinction training for 14 days before being euthanized. Synaptosomes from the medial PFC, nucleus accumbens (NAc), and the dorsal striatum (dSTR) were isolated and labeled with membrane-impermeable biotin in order to measure surface mGluR2/3 and mGluR7 receptors. Extended access to meth self-administration followed by abstinence decreased surface and total levels of mGluR2/3 receptors in the NAc and dSTR, while in the PFC, only a loss of surface mGluR2/3 and mGluR7 receptors was detected. Daily extinction trials reversed the downregulation of mGluR2/3 receptors in the NAc and dSTR and mGluR7 in the PFC, but downregulation of surface mGluR2/3 receptors in the PFC was present regardless of post-meth experience. Thus, extinction learning can selectively restore some populations of downregulated mGluRs after prolonged exposure to meth. The present findings could have implications for our understanding of the persistence (or recovery) of meth-induced motivational and cognitive deficits.

Glutamate-based depression GBD

We describe a new term: glutamate-based depression (GBD). GBD is defined as a chronic depressive illness associated with environmental stress and diseases associated with altered glutamate neurotransmission. We hypothesize that glutamate-induced over-activation of extrasynaptic NMDA receptors in the subgenual cingulate area called Brodmann’s 25 plays an important role in the etiology of depression and may be responsible for the high incidence of co-morbid depression associated in diseases with glutamate etiology. While depression is a syndrome with multiple possible etiologies, we propose that a disruption in glutamatergic neurotransmission may underline a substantial proportion of clinically observed depression. The high rates of depressive symptoms associated with various disorders in which altered glutamatergic functions have been identified, may suggest a common pathophysiological mechanism is underlying the diverse clinical presentations.

Neuropathological abnormalities of astrocytes, GABAergic neurons, and pyramidal neurons in the dorsolateral prefrontal cortices of patients with major depressive disorder

Human post-mortem brain studies have revealed reduced density and size of neurons and glial cells in the dorsolateral prefrontal cortex (dlPFC) in major depressive disorder (MDD). However, the basis of these cytoarchitectural abnormalities and the relationship between them are not understood. We hypothesized that the reduced density of GABAergic neurons and glial cells was associated with altered glutamate neurotransmission in the dlPFC. In order to test this hypothesis, we examined a specific marker type (i.e., calretinin, CR: as a marker of GABAergic neurons) and also attempted to identify the neuropathological markers that correlate with the density of CR-immunoreactive (IR) GABAergic neurons in the dlPFC, using the Stanley Neuropathology Consortium Integrative Database (SNCID, http://sncid.stanleyresearch.org/), which is a web-based tool used to integrate Stanley Medical Research Institute (SMRI) data sets. We found that the density of CR-IR GABAergic neurons was significantly lower in layer I of the dlPFC of MDD patients (n=15) than in that of unaffected controls (n=15) (p=0.021). CR-IR GABAergic neuronal changes were positively correlated with changes in several markers for glial cells and pyramidal neurons in the dlPFC of all SNC subjects (n=60). We also found that the glutamate changes negatively correlated with glial fibrillary acidic protein (GFAP) expression levels and CR-IR GABAergic neuronal density in the prefrontal cortex of all SNC subjects (P<0.05). These findings yield some insight into the mechanism by which increased glutamatergic neurotransmission leads to excitotoxic damage both in neurons and glial cells in the dlPFC of MDD patients.

Chronic cocaine exposure induces putamen glutamate and glutamine metabolite abnormalities in squirrel monkeys

Chronic cocaine exposure has been associated with progressive brain structural and functional changes. Clarifying mechanisms underlying cocaine's progressive brain effects may help in the development of effective cocaine abuse treatments. We used a controlled squirrel monkey model of chronic cocaine exposure (45mg/kg/week for 9months) combined with ultra-high magnetic field (9.4T) proton magnetic resonance spectroscopy to prospectively measure putamen metabolite changes. Proton metabolites were measured with a STEAM sequence, quantified with LCModel using a simulated basis set, and expressed as metabolite/total creatine (tCr) ratios. We found cocaine-induced time-dependent changes in putamen glutamate/tCr and glutamine/tCr metabolite ratios suggestive of altered glutamate compartmentalization, neurotransmission, and metabolism. By contrast, saline-treated monkeys exhibited no metabolite changes over time. The time course of cocaine-induced metabolite abnormalities we detected is consistent with the apparent time course of glutamate abnormalities identified in a cross-sectional study in human cocaine users, as well as with microdialysis findings in rodent models of repeated cocaine exposure. Together, these findings suggests that this squirrel monkey model may be useful for characterizing glutamatergic changes associated with cocaine exposure and for determining efficacies of treatments designed to mitigate cocaine-induced glutamatergic system dysfunction.

A single intra-PFC infusion of BDNF prevents cocaine-induced alterations in extracellular glutamate within the nucleus accumbens.

The glutamatergic pathway arising in the dorsomedial prefrontal cortex (dmPFC) and projecting to the nucleus accumbens (NAc) core is a critical component of the reward circuitry that underlies reinstatement to cocaine-seeking behavior. Brain-derived neurotrophic factor (BDNF) is expressed by and modulates PFC-NAc neurons. BDNF infusion into the dmPFC attenuates reinstatement to cocaine-seeking behavior, as well as some cocaine-induced molecular adaptations within the NAc. In the present study, it is demonstrated that a single intra-dmPFC infusion of BDNF prevents cocaine self-administration-induced reduction in basal extracellular glutamate, as well as cocaine prime-induced increases in extracellular glutamate levels within the NAc. These data suggest that intra-PFC BDNF attenuates reinstatement to cocaine-seeking behavior by normalizing cocaine-induced neuroadaptations that alter glutamate neurotransmission within the NAc.

Post-training infusion of glutamate into the bed nucleus of the stria terminalis enhanced inhibitory avoidance memory: An effect involving norepinephrine

This study examined an interaction between glutamate and norepinephrine in the bed nucleus of the stria terminalis (BNST) in modulating affective memory formation. Male Wistar rats with indwelling cannulae in the BNST were trained on a one-trial step-through inhibitory avoidance task and received pre- or post-training intra-BNST infusion of glutamate, norepinephrine or their antagonists. Results of the 1-day test indicated that post-training intra-BNST infusion of dl-2-amino-5-phosphonovaleric acid (APV) impaired retention in a dose- and time-dependent manner, while infusion of glutamate had an opposite effect. Co-infusion of 0.2 μg glutamate and 0.02 μg norepinephrine resulted in marked retention enhancement by summating non-apparent effects of the two drugs given at a sub-enhancing dose. The amnesic effect of 5.0 μg APV was ameliorated by 0.02 μg norepinephrine, while the memory enhancing effect of 1.0 μg glutamate was attenuated by 5.0 μg propranolol. These findings suggest that training on an inhibitory avoidance task may alter glutamate neurotransmission, which by activating NMDA receptors releases norepinephrine to modulate memory formation via β adrenoceptors in the BNST.

Altered Glutamate Neurotransmission and Behaviour in Dementia: Evidence from Studies of Memantine

Behavioural symptoms are a significant problem in Alzheimers disease (AD). Symptoms including agitation/aggression and psychosis reduce patient quality of life, significantly increase caregiver burden, and often trigger nursing home placement. Underlying changes in the serotonergic, noradrenergic and cholinergic systems have been linked to some behavioural problems, however, the use of antipsychotics in this population has been associated with significant safety concerns. A role for the glutamate system in schizophrenia, as well as in anxiety and depression, has been suggested, and evidence is emerging for a role for dysfunctional glutamate neurotransmission (via N-methyl-D-aspartate (NMDA) receptors) in certain behavioural changes in dementia. For example, the NMDA receptor antagonist, memantine has been shown to improve cognition, function (activities of daily living, ADLs) and, more recently, agitation/aggression, and delusions in AD patients. To date, little information is available regarding the neurochemical basis of agitation/aggression. However, the frontal and cingulate cortices – specifically, the formation of neurofibrillary tangles in glutamatergic pyramidal neurones of these areas – are proposed as regional substrates of these behaviours. Given that memantine displays a favourable tolerability profile, it is relevant to investigate the underlying mechanism linking memantine with the behavioural elements of AD. One hypothesis proposes that memantine corrects dysfunctional glutamatergic neurotransmission in the frontal and cingulate cortices, thereby normalising pathways responsible for causing agitation. An alternative hypothesis is based on the observation that increased tangle formation is associated with agitation, and on recent studies where memantine has been shown to reduce tau phosphorylation via glycogen synthase kinase (GSK)-3 or activation of protein phosphatase (PP)-2A, which might subsequently lead to reduced agitation. Keywords: Alzheimer's disease, glutamate, neurofibrillary tangles, tau, agitation

Altered Glutamate Neurotransmission and Behaviour in Dementia: Evidence from Studies of Memantine

Behavioural symptoms are a significant problem in Alzheimer's disease (AD). Symptoms including agitation/aggression and psychosis reduce patient quality of life, significantly increase caregiver burden, and often trigger nursing home placement. Underlying changes in the serotonergic, noradrenergic and cholinergic systems have been linked to some behavioural problems, however, the use of antipsychotics in this population has been associated with significant safety concerns. A role for the glutamate system in schizophrenia, as well as in anxiety and depression, has been suggested, and evidence is emerging for a role for dysfunctional glutamate neurotransmission (via N-methyl-D-aspartate (NMDA) receptors) in certain behavioural changes in dementia. For example, the NMDA receptor antagonist, memantine has been shown to improve cognition, function (activities of daily living, ADLs) and, more recently, agitation/aggression, and delusions in AD patients. To date, little information is available regarding the neurochemical basis of agitation/aggression. However, the frontal and cingulate cortices--specifically, the formation of neurofibrillary tangles in glutamatergic pyramidal neurones of these areas--are proposed as regional substrates of these behaviours. Given that memantine displays a favourable tolerability profile, it is relevant to investigate the underlying mechanism linking memantine with the behavioural elements of AD. One hypothesis proposes that memantine corrects dysfunctional glutamatergic neurotransmission in the frontal and cingulate cortices, thereby normalising pathways responsible for causing agitation. An alternative hypothesis is based on the observation that increased tangle formation is associated with agitation, and on recent studies where memantine has been shown to reduce tau phosphorylation via glycogen synthase kinase (GSK)-3 or activation of protein phosphatase (PP)-2A, which might subsequently lead to reduced agitation.

Activation of mGlu2/3 receptors as a new approach to treat schizophrenia: a randomized Phase 2 clinical trial

Schizophrenia is a chronic, complex and heterogeneous mental disorder, with pathological features of disrupted neuronal excitability and plasticity within limbic structures of the brain. These pathological features manifest behaviorally as positive symptoms (including hallucinations, delusions and thought disorder), negative symptoms (such as social withdrawal, apathy and emotional blunting) and other psychopathological symptoms (such as psychomotor retardation, lack of insight, poor attention and impulse control). Altered glutamate neurotransmission has for decades been linked to schizophrenia, but all commonly prescribed antipsychotics act on dopamine receptors. LY404039 is a selective agonist for metabotropic glutamate 2/3 (mGlu2/3) receptors and has shown antipsychotic potential in animal studies. With data from rodents, we provide new evidence that mGlu2/3 receptor agonists work by a distinct mechanism different from that of olanzapine. To clinically test this mechanism, an oral prodrug of LY404039 (LY2140023) was evaluated in schizophrenic patients with olanzapine as an active control in a randomized, three-armed, double-blind, placebo-controlled study. Treatment with LY2140023, like treatment with olanzapine, was safe and well-tolerated; treated patients showed statistically significant improvements in both positive and negative symptoms of schizophrenia compared to placebo (P < 0.001 at week 4). Notably, patients treated with LY2140023 did not differ from placebo-treated patients with respect to prolactin elevation, extrapyramidal symptoms or weight gain. These data suggest that mGlu2/3 receptor agonists have antipsychotic properties and may provide a new alternative for the treatment of schizophrenia.

Elevated Glucose Changes the Expression of Ionotropic Glutamate Receptor Subunits and Impairs Calcium Homeostasis in Retinal Neural Cells

Altered glutamatergic neurotransmission and calcium homeostasis may contribute to retinal neural cell dysfunction and apoptosis in diabetic retinopathy (DR). The purpose of this study was to determine the effect of high glucose on the protein content of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and kainate glutamate receptor subunits, particularly the GluR2 subunit, because it controls Ca2+ permeability of AMPA receptor-associated channels. The effect of high glucose on the concentration of cytosolic free calcium ([Ca2+]i) was also investigated. The protein content of GluR1, GluR2, GluR6/7, and KA2 subunits was assessed by Western blot. Cobalt staining was used to identify cells containing calcium/cobalt-permeable AMPA receptors. The [Ca2+]i changes evoked by KCl or kainate were recorded by live-cell confocal microscopy in R28 cells and in primary cultures of rat retina, loaded with fluo-4. In primary cultures, high glucose significantly decreased the protein content of GluR1 and GluR6/7 subunits and increased the protein content of GluR2 and KA2 subunits. High glucose decreased the number of cobalt-positive cells, suggesting a decrease in calcium permeability through AMPA receptor-associated channels. In high-glucose-treated cells, changes in [Ca2+]i were greater than in control cells, and the recovery to basal levels was delayed. However, in the absence of Na+, to prevent the activation of voltage-sensitive calcium channels, the [Ca2+]i changes evoked by kainate in the presence of cyclothiazide, which inhibits AMPA receptor desensitization, were significantly lower in high-glucose-treated cells than in control cultures, further indicating that AMPA receptors were less permeable to calcium. Mannitol, used as an osmotic control, did not cause significant changes compared with the control. The results suggest that elevated glucose may alter glutamate neurotransmission and calcium homeostasis in the retina, which may have implications for the mechanisms of vision loss in DR.

Glutamatergic mechanisms in schizophrenia: Current concepts

Schizophrenia is a severe mental illness affecting millions of people worldwide, with a substantial impact on patients, family, and society. Recent studies have established support for a hypothesis of abnormal glutamatergic neurotransmission in specific brain regions in schizophrenia involving myriad molecules associated with glutamate signaling. After a brief description of these molecules of the glutamatergic synapse, this review focuses on recent experimental evidence for glutamate abnormalities in schizophrenia, and discusses data from genetic, postmortem brain, in vivo imaging, and pharmacologic studies. These convergent findings implicate altered glutamate neurotransmission in the pathophysiology of schizophrenia, and suggest that novel therapeutic strategies targeted at modulation of glutamate neurotransmission may be useful in this illness.

In vivo evaluation of [ 11C]N-(2-chloro-5-thiomethylphenyl)-N′- (3-methoxy-phenyl)-N′-methylguanidine ([ 11C]GMOM) as a potential PET radiotracer for the PCP/NMDA receptor

The development of imaging methods to measure changes in NMDA ion channel activation would provide a powerful means to probe the mechanisms of drugs and device based treatments (e.g., ECT) thought to alter glutamate neurotransmission. To provide a potential NMDA/PCP receptor PET tracer, we synthesized the radioligand [ 11C]GMOM ( k i = 5.2 ±0.3 nM; log P = 2.34) and evaluated this ligand in vivo in awake male rats and isoflurane anesthetized baboons. In rats, the regional brain uptake of [ 11C]GMOM ranged from 0.75±0.13% ID/g in the medulla and pons to 1.15±0.17% ID/g in the occipital cortex. MK801 (1 mg/kg i.v.) significantly reduced (24-28%) [ 11C]GMOM uptake in all regions. D-serine (10 mg/kg i.v.) increased [ 11C]GMOM %ID/g values in all regions (10-24%) reaching significance in the frontal cortex and cerebellum only. The NR2B ligand RO 25-6981 (10 mg/kg i.v.) reduced [ 11C]GMOM uptake significantly (24-38%) in all regions except for the cerebellum and striatum. Blood activity was 0.11±0.03 %ID/g in the controls group and did not vary significantly across groups. PET imaging in isoflurane-anesthetized baboons with high specific activity [ 11C]GMOM provided fairly uniform regional brain distribution volume ( V T ) values (12.8–17.1 ml g −1). MK801 (0.5 mg/kg, i.v., n = 1, and 1.0 mg/kg, i.v., n = 1) did not significantly alter regional V T values, indicating a lack of saturable binding. However, the potential confounding effects associated with ketamine induction of anesthesia along with isoflurane maintenance must be considered because both agents are known to reduce NMDA ion channel activation. Future and carefully designed studies, presumably utilizing an optimized NMDA/PCP site tracer, will be carried out to further explore these hypotheses. We conclude that, even though [ 11C]GMOM is not an optimized PCP site radiotracer, its binding is altered in vivo in awake rats as expected by modulation of NMDA ion channel activity by MK801, D-serine or RO 25-6981. The development of higher affinity NMDA/PCP site radioligands is in progress.