Altered Glutamate Metabolism Research Articles

Sulforaphane Prevents Methylmercury-Induced Oxidative Damage and Excitotoxicity Through Activation of the Nrf2-ARE Pathway.

Methylmercury (MeHg) is a prominent environmental neurotoxicant, which induces oxidative damage and an indirect excitotoxicity caused by altered glutamate (Glu) metabolism. However, the interaction between oxidative damage and excitotoxicity in MeHg-exposed rats has not been fully recognized. Here, we explored the interaction between oxidative damage and excitotoxicity and evaluated the preventive effects of sulforaphane (SFN) on MeHg-induced neurotoxicity in rat cerebral cortex. Seventy-two rats were randomly assigned to four groups: control group, MeHg-treated groups (4 and 12μmol/kg), and SFN pretreatment group. After treatment (28days), the rats were killed and the cerebral cortex was analyzed. Then, Hg, glutathione (GSH), malondialdehyde (MDA), protein sulfhydryl, protein carbonyl, 8-hydroxy-2-deoxyguanosine (8-OHdG), and the levels of reactive oxygen species (ROS) and apoptosis were examined. Glu and glutamine (Gln) levels, glutamine synthetase (GS), phosphate-activated glutaminase (PAG), superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), Na+-K+-ATPase and Ca2+-ATPase activities, intracellular Ca2+ levels, and the mRNA and protein expressions of Nrf2, Nrf2-regulated gene products, and N-methyl-D-aspartate receptors (NMDARs) were investigated in rat cerebral cortex. In our study, MeHg exposure not only induced Hg accumulation, apoptosis, ROS formation, GSH depletion, inhibition of antioxidant enzyme activities, and activation of Nrf2-ARE pathway signaling but also caused lipid, protein, and DNA peroxidative damage in a dose-dependent manner in rat cerebral cortex. Moreover, MeHg treatment significantly altered Gln/Glu cycling and NMDAR expression and resulted in calcium overloading. Furthermore, the present study also indicated that SFN pretreatment significantly reinforced the activation of the Nrf2-ARE pathway, which could prevent the toxic effects of MeHg exposure. Collectively, MeHg initiates multiple additive or synergistic disruptive mechanisms that lead to oxidative damage and excitotoxicity in rat cerebral cortex; pretreatment with SFN might prevent the MeHg-induced neurotoxicity by reinforcing the activation of the Nrf2-ARE pathway and then downregulating the interaction between oxidative damage and excitotoxicity pathways.

Re-Visiting Glutamate Toxicity: Implications of Monosodium Glutamate Consumption on Glutamate Metabolism and Metabolic Syndrome

Around a quarter of the world’s population is estimated to have metabolic syndrome (metS) which is a major risk factor for type II diabetes, cardiovascular disease and death. While the neurotoxicity of monosodium glutamate (MSG) has long been ruled out, recent trends suggest that excessive MSG intake may be associated with metS. Considering that some enzymes involved in glutamate metabolism predict early onset of metS, and that glutamate is involved in gluconeogenesis, energy production, insulin secretion and fatty acid synthesis and that it creates a perfect environment for the activation of mTOR, which subsequently leads to cell growth and autophagy inhibition, we propose that altered glutamate metabolism plays a central role in the cellular deregulation that leads to the development of metS. While recent studies have highlighted the role of aminotransferase enzymes and glutamine-recycling in metabolic syndrome, here we extend upon these concepts and provide fundamental mechanisms through which altered glutamate metabolism as a result of increased MSG consumption may play a role in the pathogenesis of metS. We propose that excessive MSG consumption leads to metS development via mechanisms involving deregulated glutamate metabolism and this is manifested through changes in glutamate metabolic enzymes and glutamate derived metabolites. Understanding whether MSG plays a role in the pathogenesis of metS is important for policy makers in food additive regulations as this may allow the prevention of debilitating diseases associated with metS. Furthermore, understanding the mechanisms involved in altered glutamate metabolism may provide important targets for diagnosing, preventing and treating metS.

IFN-Alpha-Induced Cortical and Subcortical Glutamate Changes Assessed by Magnetic Resonance Spectroscopy

Cytokine effects on behavior may be related to alterations in glutamate metabolism. We therefore measured glutamate concentrations in brain regions shown to be affected by inflammatory stimuli including the cytokine interferon (IFN)-alpha. IFN-alpha is known to alter neural activity in the dorsal anterior cingulate cortex (dACC) and basal ganglia in association with symptoms of depression and increases in peripheral cytokines including the tumor necrosis factor (TNF) and its soluble receptor. Single-voxel magnetic resonance spectroscopy (MRS) was employed to measure glutamate concentrations normalized to creatine (Glu/Cr) in dACC and basal ganglia of 31 patients with hepatitis C before and after ∼ 1 month of either no treatment (n = 14) or treatment with IFN-alpha (n = 17). Depressive symptoms were measured at each visit using the Inventory of Depressive Symptoms-Clinician Rating (IDS-C) and the Multidimensional Fatigue Inventory. IFN-alpha was associated with a significant increase in Glu/Cr in dACC and left basal ganglia. Increases in dACC Glu/Cr were positively correlated with scores on the IDS-C in the group as a whole, but not in either group alone. Glu/Cr increases in left basal ganglia were correlated with decreased motivation in the group as a whole and in IFN-alpha-treated subjects alone. No Glu/Cr changes were found in the right basal ganglia, and no significant correlations were found between Glu/Cr and the inflammatory markers. IFN-alpha-induced increases in glutamate in dACC and basal ganglia are consistent with MRS findings in bipolar depression and suggest that inflammatory cytokines may contribute to glutamate alterations in patients with mood disorders and increased inflammation.

Anti-Inflammatory Therapy in Diabetic Retinopathy

Anti-Inflammatory Therapy in Diabetic Retinopathy

Enzyme activity of the PIP4K2A gene product polymorphism that is implicated in schizophrenia

Enzyme activity of the PIP4K2A gene product polymorphism that is implicated in schizophrenia

114. Inflammation induced changes in Anterior Cingulate Cortex Glutamate is associated with Depression and Fatigue

Behavioral symptoms including depression and fatigue are well-known consequences of activated peripheral immunity. Increased glutamate turnover in mood-regulating brain regions targeted by inflammatory mediators such as cytokines might mediate this relationship. We used single voxel proton magnetic resonance spectroscopy (MRS) to quantify glutamate concentrations in the anterior cingulate cortex (ACC) before and after four weeks of exposure to interferon (IFN)-alpha. IFN-alpha is an inflammatory cytokine used in the treatment of hepatitis C virus (HCV) infection that is known to cause depression and fatigue up to 50% of patients depending on the dose. Sixteen HCV patients underwent MRS scans, behavior ratings and immune marker assessments before and after 4 weeks of IFN-alpha treatment. Fourteen HCV control patients awaiting IFN-alpha therapy were studied in parallel. IFN-alpha administration was associated with significant increases in glutamate concentrations measured as glutamate/creatine (Glu/Cr) ratios in the ACC. Increases in the Glu/Cr ratio in the ACC was in turn correlated with increases in plasma soluble tumor necrosis factor 2 receptor (r = 0.54, p < 0.002), depression (r = 0.53, p < 0.003) and fatigue (r = 0.41, p < 0.03). The above findings are consistent with previously reported changes in ACC function among HCV patients following IFN-alpha treatment and among healthy volunteers administered immune stimuli including typhoid vaccination. These data suggest that altered glutamate metabolism as a result of impaired glial function may contribute to ACC changes following immune activation.

Glutamate Metabolism in Plant Disease and Defense: Friend or Foe?

Plant glutamate metabolism (GM) plays a pivotal role in amino acid metabolism and orchestrates crucial metabolic functions, with key roles in plant defense against pathogens. These functions concern three major areas: nitrogen transportation via the glutamine synthetase and glutamine-oxoglutarate aminotransferase cycle, cellular redox regulation, and tricarboxylic acid cycle-dependent energy reprogramming. During interactions with pathogens, the host GM is markedly altered, leading to either a metabolic state, termed "endurance", in which cell viability is maintained, or to an opposite metabolic state, termed "evasion", in which the process of cell death is facilitated. It seems that endurance-natured modulations result in resistance to necrotrophic pathogens and susceptibility to biotrophs, whereas evasion-related reconfigurations lead to resistance to biotrophic pathogens but stimulate the infection by necrotrophs. Pathogens, however, have evolved strategies such as toxin secretion, hemibiotrophy, and selective amino acid utilization to exploit the plant GM to their own benefit. Collectively, alterations in the host GM in response to different pathogenic scenarios appear to function in two opposing ways, either backing the ongoing defense strategy to ultimately shape an efficient resistance response or being exploited by the pathogen to promote and facilitate infection.

Animal model of autism induced by prenatal exposure to valproate: Altered glutamate metabolism in the hippocampus

Autism spectrum disorders (ASD) are characterized by deficits in social interaction, language and communication impairments and repetitive and stereotyped behaviors, with involvement of several areas of the central nervous system (CNS), including hippocampus. Although neurons have been the target of most studies reported in the literature, recently, considerable attention has been centered upon the functionality and plasticity of glial cells, particularly astrocytes. These cells participate in normal brain development and also in neuropathological processes. The present work investigated hippocampi from 15 (P15) and 120 (P120) days old male rats prenatally exposed to valproic acid (VPA) as an animal model of autism. Herein, we analyzed astrocytic parameters such as glutamate transporters and glutamate uptake, glutamine synthetase (GS) activity and glutathione (GSH) content. In the VPA group glutamate uptake was unchanged at P15 and increased 160% at P120; the protein expression of GLAST did not change neither in P15 nor in P120, while GLT1 decreased 40% at P15 and increased 92% at P120; GS activity increased 43% at P15 and decreased 28% at P120; GSH content was unaltered at P15 and had a 27% increase at P120. These data highlight that the astrocytic clearance and destination of glutamate in the synaptic cleft might be altered in autism, pointing out important aspects to be considered from both pathophysiologic and pharmacological approaches in ASD.

178. Interferon-alpha-induced fatigue is associated with alterations in CNS glutamate metabolism as measured by magnetic resonance spectroscopy

Peripheral inflammatory states and their behavioral consequences including fatigue have been well-documented. One mechanism that may contribute to inflammation-induced behavioral changes is alterations in central nervous system (CNS) glutamate metabolism. To examine the role of CNS glutamate in inflammation-induced fatigue, proton magnetic resonance spectroscopy ( MRS ) was conducted in 12 patients with hepatitis C (HCV) before and after 4 weeks of treatment with interferon ( IFN )- alpha compared to 12 HCV patients awaiting IFN-alpha therapy. IFN-alpha is an inflammatory cytokine well-known to induce symptoms of fatigue in a high percentage of patients. Compared to controls, the glutamate /creatine ratio in the left basal ganglia significantly increased following 4 weeks of IFN-alpha administration ( p < 0.05). This increase in the glutamate /creatine ratio was in turn significantly correlated with increases in fatigue severity ( r = 0.435, p = 0.034). Increased glutamate concentrations in the left basal ganglia are consistent with previously reported alterations in basal ganglia function following IFN-alpha treatment in cancer patients and patients with HCV. The data are also consistent with changes in basal ganglia function in healthy volunteers administered immune stimuli including endotoxin or typhoid vaccination. These data suggest that altered glutamate metabolism possibly as a result of altered glial function may contribute to basal ganglia changes following acute and chronic immune activation.

Protective effects of MK-801 on methylmercury-induced neuronal injury in rat cerebral cortex: Involvement of oxidative stress and glutamate metabolism dysfunction

Methylmercury (MeHg) is one of the ubiquitous environmental toxicants, which can induce oxidative stress and an indirect excitotoxicity caused by altered glutamate (Glu) metabolism. However, little is known of the interaction between oxidative stress and Glu metabolism play in MeHg poisoning rats. We have investigated the neuroprotective role of MK-801, a non-competitive N-methyl-d-aspartate receptors (NMDAR) antagonist, against MeHg-induced neurotoxicity. Fifty rats were randomly divided into five groups of 10 animals in each group: control group, MK-801 control group, MeHg-treated group (4 and 12μmol/kg) and MK-801 pre-treated group. Administration of MeHg at a dose of 12μmol/kg for four weeks significantly increased in ROS and total Hg levels and that caused lipid, protein and DNA peroxidative damage in cerebral cortex. In addition, MeHg also reduced nonenzymic (reduced glutathione, GSH) and enzymic (glutathione peroxidase, GPx and superoxide dismutase, SOD) antioxidants and enhanced neurocyte apoptosis rate in cerebral cortex. MeHg-induced ROS production appears to inhibit the activity of the glutamine synthetase (GS), leading to Glu metabolism dysfunction. Pretreatment with MK-801 at a dose of 0.3μmol/kg prevented the alterations of the activities of PAG and GS and oxidative stress. In addition, pretreatment with MK-801 significantly alleviated the neurocyte apoptosis rate and histopathological damage. In conclusion, the results suggested ROS formation resulting from MeHg- and Glu-induced oxidative stress contributed to neuronal injury. MK-801 possesses the ability to attenuate MeHg-induced neurotoxicity in the cerebral cortex through mechanisms involving its NMDA receptor binding properties and antioxidation.

Quantitative Proteomic Analysis Revealed Lovastatin-induced Perturbation of Cellular Pathways in HL-60 Cells

Lovastatin, a member of the statin family of drugs, is widely prescribed for treating hypercholesterolemia. The statin family of drugs, however, also shows promise for cancer treatment and prevention. Although lovastatin is known to be an inhibitor for HMG-CoA reductase, the precise mechanisms underlying the drug's antiproliferative activity remain unclearly defined. Here we utilized mass spectrometry, in conjunction with stable isotope labeling by amino acids in cell culture (SILAC), to analyze the perturbation of protein expression in HL-60 cells treated with lovastatin. We were able to quantify ∼3200 proteins with both forward and reverse SILAC labeling experiments, among which ∼120 exhibited significant alterations in expression levels upon lovastatin treatment. Apart from confirming the expected inhibition of the cholesterol biosynthesis pathway, our quantitative proteomic results revealed that lovastatin perturbed the estrogen receptor signaling pathway, which was manifested by the diminished expression of estrogen receptor α, steroid receptor RNA activator 1, and other related proteins. Lovastatin also altered glutamate metabolism through down-regulation of glutamine synthetase and γ-glutamylcysteine synthetase. Moreover, lovastatin treatment led to a marked down-regulation of carbonate dehydratase II (a.k.a. carbonic anhydrase II) and perturbed the protein ubiquitination pathway. Together, the results from the present study underscored several new cellular pathways perturbed by lovastatin.

Glutamate and glutathione interplay in a motor neuronal model of amyotrophic lateral sclerosis reveals altered energy metabolism

Impairment of mitochondrial function might contribute to oxidative stress associated with neurodegeneration in amyotrophic lateral sclerosis (ALS). Glutamate levels in tissues of ALS patients are sometimes altered. In neurons, mitochondrial metabolism of exogenous glutamine is mainly responsible for the net synthesis of glutamate, which is a neurotransmitter, but it is also necessary for the synthesis of glutathione, the main endogenous antioxidant. We investigated glutathione synthesis and glutamine/glutamate metabolism in a motor neuronal model of familial ALS. In standard culture conditions (with glutamine) or restricting glutamine or cystine, the level of glutathione was always lower in the cell line expressing the mutant (G93A) human Cu, Zn superoxide dismutase (G93ASOD1) than in the line expressing wild-type SOD1. With glutamine the difference in glutathione was associated with a lower glutamate and impairment of the glutamine/glutamate metabolism as evidenced by lower glutaminase and cytosolic malate dehydrogenase activity. d-β-hydroxybutyrate, as an alternative to glutamine as energy substrate in addition to glucose, reversed the decreases of cytosolic malate dehydrogenase activity and glutamate and glutathione. However, in the G93ASOD1 cell line, in all culture conditions the expression of pyruvate dehydrogenase kinase l protein, which down-regulates pyruvate dehydrogenase activity, was induced, together with an increase in lactate release in the medium. These findings suggest that the glutathione decrease associated with mutant SOD1 expression is due to mitochondrial dysfunction caused by the reduction of the flow of glucose-derived pyruvate through the TCA cycle; it implies altered glutamate metabolism and depends on the different mitochondrial energy substrates.

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.

Manganese exposure alters the expression of N‐methyl‐d‐aspartate receptor subunit mRNAs and proteins in rat striatum

Manganese is one of the ubiquitous environmental pollutants that can induce an indirect excitotoxicity caused by altered glutamate (Glu) metabolism. The present study has been carried out to investigate the effect of Mn on the expression of N-methyl-d-aspartate receptor (NR) subunit mRNAs and proteins in rat striatum when rats were in manganism. The rats were divided randomly into four groups of six males and six females each: control group (group 1) and 8, 40, and 200 micromol/kg Mn-treated groups (groups 2-4). The control group rats were subcutaneously (s.c.) injected with normal saline. Manganese-treated rats were s.c. injected with respectively 8, 40, and 200 micromol/kg of MnCl(2) . 6H(2)O in normal saline. The administration of MnCl(2) . 6H(2)O for 4 weeks significantly increased Mn concentration in the striatum. With the increase in administered MnCl(2) dosage, Glu concentration and cell apoptosis rate increased significantly. The relative intensity of NR2A mRNA decreased significantly in 8 micromol/kg Mn-treated rats. However, relative intensities of NR1 and NR2B mRNAs decreased significantly in 40 micromol/kg Mn-treated rats. Similarly, the relative intensity of NR2A protein showed a significant decrease in 40 micromol/kg Mn-treated rats whereas those of NR1 and NR2B decreased significantly in 200 micromol/kg Mn-treated rats. Therefore, the expression of NR2A mRNA and protein were much more sensitive to Mn than that of NR1 and NR2B. In conclusion, the results suggested that Mn induced nerve cell damage by increasing extracellular Glu level and altered expression of NR subunit mRNAs and proteins in rat striatum.

Protective effects of MK-801 on manganese-induced glutamate metabolism disorder in rat striatum

Manganese (Mn) is one of the ubiquitous environmental pollutants that can induce an indirect excitotoxicity caused by altered glutamate (Glu) metabolism. The present study has been carried out to investigate the mechanisms of Glu metabolism disorder and the neuroprotective role of MK-801, a non-competitive N-methyl- d-aspartate receptor antagonist, against Mn-induced excitotoxicity in rat striatum. Fifty rats were randomly divided into five groups with 10 animals in each group: control group, Mn-treated group (8, 40, and 200 μmol/kg), and MK-801-pre-treated group. Administration of MnCl 2·6H 2O at dose of 200 μmol/kg body weight for 4 weeks significantly increased the concentrations of Glu and Mn in the striatum ( P<0.01). In addition, Mn also increased the activity of phosphate-activated glutaminase (PAG) (54.38%, P<0.01), enhanced striatum cell apoptosis rate (65.04%, P<0.01) and decreased the activity of glutamine synthetase (GS) (28.88%, P<0.01). Pre-treatment with MK-801 at a dose of 0.3 μmol/kg body weight for 4 weeks prior to 200 μmol/kg Mn administration prevents the alterations of the activities of PAG and GS and concentrations of glutamine (Gln). In addition, pre-treatment with MK-801 significantly reduced the striatum cell apoptosis rate. In conclusion, the results suggest that MK-801 possesses the ability to attenuate Mn-induced Glu metabolism disorder in the striatum through mechanisms involving disruption enzyme activities of GS and PAG.

Proteome alteration of U251 human astrocytoma cell after inhibiting retinoic acid synthesis

Retinoic acid (Ra) is crucial for the patterning and neuronal differentiation in the central nervous system (CNS). Ra deficiency in animals disrupts the motor activities and memory abilities. The molecular mechanisms underlying these behavior abnormalities remain largely unknown. In the current study, we treated the astrocytoma cells with citral, an inhibitor of Ra synthesis. We analyzed the differences in the protein concentrations between the treated and untreated astrocytoma cells by two-dimensional gel electrophoresis (2-DE), Imagemaster software, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). In total, 39 of 46 altered protein spots with significant mascot scores were identified representing 36 proteins, that were involved in significantly altered glutamate metabolism, lipid metabolism, mitochondrial function, and oxidative stress response by Ingenuity Pathway Analysis (IPA). Altered 3-phosphoglycerate dehydrogenase (PHGDH) was also observed in western blot. These data provide some clues for explaining the behavioral changes caused by Ra deficiency, and support the hypothesis that Ra signaling is associated with some symptoms of neurodegenerative disorders and schizophrenia.

Novos conceitos em retinopatia diabética: dano neurológico versus dano vascular

Diabetic retinopathy is the leading cause of irreversible legal blindness in working-age adults. The number of people worldwide at risk of developing vision loss from diabetes is predicted to double over the next 30 years. Some elements suggest that neurodegenerative changes occur beyond vascular damage. These changes include increased apoptosis, glial cell reactivity, microglial activation, and altered glutamate metabolism, and could explain some of the functional abnormalities that begin soon after the onset of diabetes, as early changes in electroretinogram. This review article will present some evidences that point out neurodegeneration as a possible initial event in diabetic retinopathy.

Evidence of elevated glutamate in multiple sclerosis using magnetic resonance spectroscopy at 3 T

Histopathological reports of multiple sclerosis and its animal models have shown evidence of a link between axonal injury in active lesions and impaired glutamate metabolism. Mature oligodendrocytes play a role in glutamate uptake to maintain glutamate homeostasis but in multiple sclerosis white matter the loss of expression of glutamate transporters in the lesion vicinity results in ineffective glutamate removal. Using a magnetic resonance spectroscopy technique that isolates the glutamate resonance at 3 T, we compared glutamate levels between normal subjects and multiple sclerosis patients in different brain areas. Metabolite concentrations (glutamate, glutamine, N-acetyl-aspartate, myo-inositol, choline, creatine) were derived from LCmodel and corrected for T1 relaxation time. Glutamate concentrations were found to be elevated in acute lesions (P = 0.02) and normal-appearing white matter (P = 0.03), with no significant elevation in chronic lesions (P = 0.77). The N-acetyl-aspartate level in chronic lesions was significantly lower (P < 0.001) than in acute lesions and normal-appearing white matter. The choline level in acute lesions was significantly higher (P < 0.001) than in chronic lesions. Evidence was also found for increased glial activity in multiple sclerosis, with significantly higher (P < 0.001) myo-inositol levels in acute lesions compared with control white matter. These in vivo results support the hypothesis that altered glutamate metabolism is present in brains of multiple sclerosis patients.

A new view of diabetic retinopathy: a neurodegenerative disease of the eye

Diabetic retinopathy (DR) is a common complication of diabetes and a leading cause of legal blindness in working-age adults. The clinical hallmarks of DR include increased vascular permeability, leading to edema, and endothelial cell proliferation. Much of the research effort has been focused on vascular changes, but it is becoming apparent that other degenerative changes occur beyond the vascular cells of the retina. These include increased apoptosis, glial cell reactivity, microglial activation, and altered glutamate metabolism. When occurring together, these changes may be considered as neurodegenerative and could explain some of the functional deficits in vision that begin soon after the onset of diabetes. This review will present the current evidence that neurodegeneration of the retina is a critical component of DR. There are two basic hypotheses that account for loss of cells in the neural retina. First, the loss of blood–retinal barrier integrity, which initially manifests as an increase in vascular permeability, causes a failure to control the composition of the extracellular fluid in the retina, which in turn leads to edema and neuronal cell loss. Alternatively, diabetes has a direct effect on metabolism within the neural retina, leading to an increase in apoptosis, which in turn causes breakdown of the blood–retinal barrier. It is not clear which hypothesis will be found to be correct, and, in fact, it is likely that vascular permeability and neuronal apoptosis are closely linked components of DR. However, the gradual loss of neurons suggests that progress of the disease is ultimately irreversible, since these cells cannot usually be replaced. In light of this possibility, new treatments for DR should be preventive in nature, being implemented before overt clinical symptoms develop. While vascular permeability is the target that is primarily considered for new treatments of DR, evidence presented here suggests that apoptosis of neurons is also an essential target for pharmacological studies. The vision of people with diabetes will be protected only when we have discovered a means to prevent the gradual but constant loss of neurons within the inner retina.

Manganese neurotoxicity and glutamate-GABA interaction

Brain extracellular concentrations of amino acids (e.g. aspartate, glutamate, taurine) and divalent metals (e.g. zinc, copper, manganese) are primarily regulated by astrocytes. Adequate glutamate homeostasis is essential for the normal functioning of the central nervous system (CNS). Glutamate is of central importance for nitrogen metabolism and, along with aspartate, is the primary mediator of the excitatory pathways in the brain. Similarly, the maintenance of proper manganese levels is important for normal brain functioning. Several in vivo and in vitro studies have linked increased manganese concentrations with alterations in the content and metabolism of neurotransmitters, namely dopamine, γ-aminobutyric acid, and glutamate. It has been reported by our laboratory and others, that cultured rat primary astrocytes exposed to manganese displayed decreased glutamate uptake, thereby increasing the excitotoxic potential of glutamate. Furthermore, decreased uptake of glutamate has been associated with decreased gene expression of glutamate:aspartate transporter (GLAST) in manganese-exposed astroctyes. Additional studies have suggested that attenuation of astrocytic glutamate uptake by manganese may be a consequence of reactive oxygen species (ROS) generation. Collectively, these data suggest that excitotoxicity may occur due to manganese-induced altered glutamate metabolism, representing a proximate mechanism for manganese-induced neurotoxicity.