The Neuroprotective Effects of Oxaloacetate in Closed Head Injury in Rats is Mediated by its Blood Glutamate Scavenging Activity: Evidence From the Use of Maleate

Glutamate Efflux across the Blood–Brain Barrier: New Perspectives on the Relationship between Depression and the Glutamatergic System

Depression is a significant cause of disability and affects millions worldwide; however, antidepressant therapies often fail or are inadequate. Current medications for treating major depressive disorder can take weeks or months to reach efficacy, have troubling side effects, and are limited in their long-term capabilities. Recent studies have identified a new set of glutamate-based approaches, such as blood glutamate scavengers, which have the potential to provide alternatives to traditional antidepressants. In this review, we hypothesize as to the involvement of the glutamate system in the development of depression. We identify the mechanisms underlying glutamate dysregulation, offering new perspectives on the therapeutic modalities of depression with a focus on its relationship to blood–brain barrier (BBB) permeability. Ultimately, we conclude that in diseases with impaired BBB permeability, such as depression following stroke or traumatic brain injury, or in neurogenerative diseases, the glutamate system should be considered as a pathway to treatment. We propose that drugs such as blood glutamate scavengers should be further studied for treatment of these conditions.

Glutamate Scavenging as a Neuroreparative Strategy in Ischemic Stroke

Stroke is the second highest reason of death in the world and the leading cause of disability. The ischemic stroke makes up the majority of stroke cases that occur due to the blockage of blood vessels. Therapeutic applications for ischemic stroke include thrombolytic treatments that are in limited usage and only applicable to less than 10% of the total stroke patients, but there are promising new approaches. The main cause of ischemic neuronal death is glutamate excitotoxicity. There have been multiple studies focusing on neuroprotection via reduction of glutamate both in ischemic stroke and other neurodegenerative diseases that ultimately failed due to the obstacles in delivery. At that point, systemic glutamate grabbing, or scavenging is an ingenious way of decreasing glutamate levels upon ischemic stroke. The main advantage of this new therapeutic method is the scavengers working in the circulating blood so that there is no interference with the natural brain neurophysiology. In this review, we explain the molecular mechanisms of ischemic stroke, provide brief information about existing drugs and approaches, and present novel systemic glutamate scavenging methods. This review hopefully will elucidate the potential usage of the introduced therapeutic approaches in stroke patients.

Blood Glutamate Scavenging With Pyruvate as a Novel Preventative and Therapeutic Approach for Depressive-Like Behavior Following Traumatic Brain Injury in a Rat Model

Depression is a common and serious complication following traumatic brain injury (TBI). Both depression and TBI have independently been associated with pathologically elevated extracellular brain glutamate levels. In the setting of TBI, blood glutamate scavenging with pyruvate has been widely shown as an effective method to provide neuroprotection by reducing blood glutamate and subsequent brain glutamate levels. Here we evaluate pyruvate as a novel approach in the treatment and prevention of post-TBI depression-like behavior in a rat model. Rats were divided into five groups: (1) sham-operated control with pyruvate, (2) sham-operated control with placebo, (3) post-TBI with placebo, (4) post-TBI given preventative pyruvate, and (5) post-TBI treated with pyruvate. These groups had an equal number of females and males. Rats were assessed for depressive-like behavior, neurological status, and glutamate levels in the blood and brain. Post-TBI neurological deficits with concurrent elevations in glutamate levels were demonstrated, with peak glutamate levels 24 h after TBI. Following TBI, the administration of either prophylactic or therapeutic pyruvate led to reduced glutamate levels, improved neurologic recovery, and improved depressive-like behavior. Glutamate scavenging with pyruvate may be an effective prophylactic and therapeutic option for post-TBI depression by reducing associated elevations in brain glutamate levels.

Brain endothelial cells metabolize glutamate via glutamate dehydrogenase to replenish TCA-intermediates and produce ATP under hypoglycemic conditions

The endothelial cells of the blood-brain barrier participate in the regulation of glutamate concentrations in the brain interstitial fluid by taking up brain glutamate. However, endothelial glutamate metabolism has not been characterized, nor is its role in brain glutamate homeostasis and endothelial energy production known. The aim of this study was to investigate endothelial glutamate dehydrogenase (GDH) expression and glutamate metabolism and probe its functional significance. The primary brain endothelial cells were isolated from bovine and mouse brains, and human brain endothelial cells were derived from induced pluripotent stem cells. GDH expression on the protein level and GDH function were investigated in the model systems using western blotting, confocal microscopy, ¹³ C-glutamate metabolism, and Seahorse assay. In this study, it was shown that GDH was expressed in murine and bovine brain capillaries and in cultured primary mouse and bovine brain endothelial cells as well as in human-induced pluripotent stem cell-derived endothelial cells. The endothelial GDH expression was confirmed in brain capillaries from mice carrying a central nervous system-specific GDH knockout. Endothelial cells from all tested species metabolized ¹³ C-glutamate to α-ketoglutarate, which subsequently entered the tricarboxylic acid (TCA)-cycle. Brain endothelial cells maintained mitochondrial oxygen consumption rates, when supplied with glutamate alone, whereas glutamate supplied in addition to glucose did not lead to additional oxygen consumption. In conclusion, brain endothelial cells directly take up and metabolize glutamate and utilize the resulting α-ketoglutarate in the tricarboxylic acid cycle to ultimately yield ATP if glucose is unavailable.

Revisiting Traumatic Brain Injury: From Molecular Mechanisms to Therapeutic Interventions

Studying the complex molecular mechanisms involved in traumatic brain injury (TBI) is crucial for developing new therapies for TBI. Current treatments for TBI are primarily focused on patient stabilization and symptom mitigation. However, the field lacks defined therapies to prevent cell death, oxidative stress, and inflammatory cascades which lead to chronic pathology. Little can be done to treat the mechanical damage that occurs during the primary insult of a TBI; however, secondary injury mechanisms, such as inflammation, blood-brain barrier (BBB) breakdown, edema formation, excitotoxicity, oxidative stress, and cell death, can be targeted by therapeutic interventions. Elucidating the many mechanisms underlying secondary injury and studying targets of neuroprotective therapeutic agents is critical for developing new treatments. Therefore, we present a review on the molecular events following TBI from inflammation to programmed cell death and discuss current research and the latest therapeutic strategies to help understand TBI-mediated secondary injury.

Oxaloacetate reduces emotional symptoms in premenstrual syndrome (PMS): results of a placebo-controlled, cross-over clinical trial

Objective

Premenstrual syndrome (PMS) affects millions of women. While over-the-counter products have helped with the physical symptoms of PMS, emotional symptoms have been less well supported. The objective of this trial was to measure the effect of an oxaloacetate/vitamin C combination on the major emotional symptoms of PMS, including depression, anxiety, perceived stress, aggression, and suicidal ideation.

Methods

Forty-eight women experiencing PMS completed a baseline survey comprising the Beck's Depression Inventory, Cohen Perceived Stress Scale, Generalized Anxiety Disorder Test, and Buss-Perry Aggression Questionnaire. After baseline measurements, participants were randomly assigned to take either 2 capsules of 100 mg oxaloacetate/150 mg ascorbic acid, or 2 capsules of rice flour (placebo) for their entire menstrual cycle. At menstruation, the women completed the 4 surveys again. The women then switched capsules in a cross-over design and continued the study for an additional menstrual cycle. The final assessment was repeated at menstruation. Statistical analysis of the 4 surveys was performed to examine efficacy.

Results

Oxaloacetate/vitamin C supplementation during PMS significantly improved depression, perceived stress, anxiety, aggression, and suicidal ideation. The mean improvement in depression was 54.1%, 35.8% for perceived stress, 51.43% for generalized anxiety, and 17.8% for aggression. Suicidal ideation was reduced by 47.9%. All results were highly significant.

Conclusion

A combination of oxaloacetate and vitamin C supplementation helped to alleviate depression, anxiety, perceived stress, aggression, and suicidal ideation symptoms associated with PMS.

Trial Registration

ClinicalTrials.gov Identifier: NCT03509714.

Blood glutamate scavenging as a novel glutamate-based therapeutic approach for post-stroke depression

Post-stroke depression (PSD) is a major complication of stroke that significantly impacts functional recovery and quality of life. While the exact mechanism of PSD is unknown, recent attention has focused on the association of the glutamatergic system in its etiology and treatment. Minimizing secondary brain damage and neuropsychiatric consequences associated with excess glutamate concentrations is a vital part of stroke management. The blood glutamate scavengers, oxaloacetate and pyruvate, degrade glutamate in the blood to its inactive metabolite, 2-ketoglutarate, by the coenzymes glutamate-oxaloacetate transaminase (GOT) and glutamate-pyruvate transaminase (GPT), respectively. This reduction in blood glutamate concentrations leads to a subsequent shift of glutamate down its concentration gradient from the blood to the brain, thereby decreasing brain glutamate levels. Although there are not yet any human trials that support blood glutamate scavengers for clinical use, there is increasing evidence from animal research of their efficacy as a promising new therapeutic approach for PSD. In this review, we present recent evidence in the literature of the potential therapeutic benefits of blood glutamate scavengers for reducing PSD and other related neuropsychiatric conditions. The evidence reviewed here should be useful in guiding future clinical trials.

Elimination of glutamate using CRRT for 72 h in patients with post-cardiac arrest syndrome: A randomized clinical pilot trial

AIM

Glutamine and glutamate are major mediators of secondary brain cell death during post-cardiac arrest syndrome. As there is an equilibrium between brain tissue and plasma concentrations of glutamine and glutamate, their elimination from systemic circulation by extracorporeal blood purification may ultimately lead to reduced secondary cell death in the brain. We hypothesized that systemic glutamine and glutamate can be significantly reduced by continuous venovenous hemodiafiltration (CVVHDF).

METHODS

This was a prospective, randomized clinical trial in post cardiac-arrest survivors evaluating standard of care or additional CVVHDF over 72 h immediately after admission. Glutamine and glutamate plasma concentrations were analyzed at eight time points in both groups. Primary endpoint was reduction of glutamine and glutamate plasma concentrations. The trial has been registered at clinical trial.gov (NCT02963298).

RESULTS

In total, 41 patients were randomized over a period of 12 months (control n = 21, CVVHDF n = 20). The primary aim reduction of glutamine and glutamate plasma concentrations by CVVHDF, was not achieved; both groups-maintained concentrations within a normal range over the study period (glutamate: 4.7-11.1 mg/dL; glutamine: 0.2-3.7 mg/dL). However, post-filter concentrations of glutamine and glutamate in CRRT patients were significantly decreased as compared to pre-filter concentrations (glutamate: pre-filter median 8.85 mg/dL IQR 7.1-9.6; post-filter 0.95 mg/dL IQR 0.5-2; p < 0.001; glutamine: pre-filter 0.7 mg/dL IQR 0.6-1; post-filter 0.2 mg/dL IQR 0-0.2; p < 0.001).

CONCLUSION

In this trial, CVVHDF was not able to statistically significantly lower systemic plasma glutamine and glutamate levels. Post-cardiac arrest patients had plasma glutamine and glutamate levels within the normal range.

Repurposing of the β-Lactam Antibiotic, Ceftriaxone for Neurological Disorders: A Review

To date, there is no cure or disease-modifying agents available for most well-known neurological disorders. Current therapy is typically focused on relieving symptoms and supportive care in improving the quality of life of affected patients. Furthermore, the traditional de novo drug discovery technique is more challenging, particularly for neurological disorders. Therefore, the repurposing of existing drugs for these conditions is believed to be an efficient and dynamic approach that can substantially reduce the investments spent on drug development. Currently, there is emerging evidence that suggests the potential effect of a beta-lactam antibiotic, ceftriaxone (CEF), to alleviate the symptoms of different experimentally-induced neurological disorders: Parkinson's disease, Alzheimer's disease, amyotrophic lateral sclerosis, epileptic-seizure, brain ischemia, traumatic brain injuries, and neuropathic pain. CEF also affects the markers of oxidative status and neuroinflammation, glutamatergic systems as well as various aggregated toxic proteins involved in the pathogenesis of different neurological disorders. Moreover, it was found that CEF administration to drug dependent animal models improved the withdrawal symptoms upon drug discontinuation. Thus, this review aimed to describe the effects of CEF against multiple models of neurological illnesses, drug dependency, and withdrawal. It also emphasizes the possible mechanisms of neuroprotective actions of CEF with respective neurological maladies.

Homeostasis of the Intraparenchymal-Blood Glutamate Concentration Gradient: Maintenance, Imbalance, and Regulation

It is widely accepted that glutamate is the most important excitatory neurotransmitter in the central nervous system (CNS). However, there is also a large amount of glutamate in the blood. Generally, the concentration gradient of glutamate between intraparenchymal and blood environments is stable. However, this gradient is dramatically disrupted under a variety of pathological conditions, resulting in an amplifying cascade that causes a series of pathological reactions in the CNS and peripheral organs. This eventually seriously worsens a patient’s prognosis. These two “isolated” systems are rarely considered as a whole even though they mutually influence each other. In this review, we summarize what is currently known regarding the maintenance, imbalance and regulatory mechanisms that control the intraparenchymal-blood glutamate concentration gradient, discuss the interrelationships between these systems and further explore their significance in clinical practice.

Characterization of the L-glutamate clearance pathways across the blood-brain barrier and the effect of astrocytes in an in vitro blood-brain barrier model

The aim was to characterize the clearance pathways for L-glutamate from the brain interstitial fluid across the blood-brain barrier using a primary in vitro bovine endothelial/rat astrocyte co-culture. Transporter profiling was performed using uptake studies of radiolabeled L-glutamate with co-application of transporter inhibitors and competing amino acids. Endothelial abluminal L-glutamate uptake was almost abolished by co-application of an EAAT-1 specific inhibitor, whereas luminal uptake was inhibited by L-glutamate and L-aspartate (1 mM). L-glutamate uptake followed Michaelis-Menten-like kinetics with high and low affinity at the abluminal and luminal membrane, respectively. This indicated that L-glutamate is taken up via EAAT-1 at the abluminal membrane and exits at the luminal membrane via a low affinity glutamate/aspartate transporter. Metabolism of L-glutamate and transport of metabolites was examined using [U-¹³C] L-glutamate. Intact L-glutamate and metabolites derived from oxidative metabolism were transported through the endothelial cells. High amounts of L-glutamate-derived lactate in the luminal medium indicated cataplerosis via malic enzyme. Thus, L-glutamate can be transported intact from brain to blood via the concerted action of abluminal and luminal transport proteins, but the total brain clearance is highly dependent on metabolism in astrocytes and endothelial cells followed by transport of metabolites.

Blood Glutamate Reducing Effect of Hemofiltration in Critically Ill Patients

Glutamate toxicity plays a well-established role in secondary brain damage following acute and chronic brain insults. Previous studies have demonstrated the efficacy of hemodialysis and peritoneal dialysis in reducing blood glutamate levels. However, these methods are not viable options for hemodynamically unstable patients. Given more favorable hemodynamics, longer treatment, and less needed anticoagulation, we investigated whether hemofiltration could be effective in lowering blood glutamate levels. Blood samples were taken from 10 critically ill patients immediately before initiation of hemofiltration and after 1, 2, 4, 6, and 12 h, for a total of 6 blood samples. Samples were sent for determination of glutamate, glutamate oxaloacetate transaminase (GOT), glutamate pyruvate transaminase (GPT), hemoglobin, hematocrit, urea, creatinine, glucose, sodium, potassium, platelet, and white blood cell (WBC) levels. There was a statistically significant reduction in blood glutamate levels at all time points compared to baseline levels. There was no difference in levels of GOT or GPT. Hemofiltration can be a promising method of reducing blood glutamate levels, especially in critically ill patients where hemodialysis and peritoneal dialysis may be contraindicated.

Glutamate, T cells and multiple sclerosis

Glutamate is the major excitatory neurotransmitter in the nervous system, where it induces multiple beneficial and essential effects. Yet, excess glutamate, evident in a kaleidoscope of acute and chronic pathologies, is absolutely catastrophic, since it induces excitotoxicity and massive loss of brain function. Both the beneficial and the detrimental effects of glutamate are mediated by a large family of glutamate receptors (GluRs): the ionotropic glutamate receptors (iGluRs) and the metabotropic glutamate receptors (mGluRs), expressed by most/all cells of the nervous system, and also by many non-neural cells in various peripheral organs and tissues. T cells express on their cell surface several types of functional GluRs, and so do few other immune cells. Furthermore, glutamate by itself activates resting normal human T cells, and induces/elevates key T cell functions, among them: T cell adhesion, chemotactic migration, cytokine secretion, gene expression and more. Glutamate has also potent effects on antigen/mitogen/cytokine-activated T cells. Furthermore, T cells can even produce and release glutamate, and affect other cells and themselves via their own glutamate. Multiple sclerosis (MS) and its animal model Experimental Autoimmune Encephalomyelitis (EAE) are mediated by autoimmune T cells. In MS and EAE, there are excess glutamate levels, and multiple abnormalities in glutamate degrading enzymes, glutamate transporters, glutamate receptors and glutamate signaling. Some GluR antagonists block EAE. Enhancer of mGluR4 protects from EAE via regulatory T cells (Tregs), while mGluR4 deficiency exacerbates EAE. The protective effect of mGluR4 on EAE calls for testing GluR4 enhancers in MS patients. Oral MS therapeutics, namely Fingolimod, dimethyl fumarate and their respective metabolites Fingolimod-phosphate and monomethyl fumarate, can protect neurons against acute glutamatergic excitotoxic damage. Furthermore, Fingolimod reduce glutamate-mediated intracortical excitability in relapsing-remitting MS. Glatiramer acetate -COPAXONE®, an immunomodulator drug for MS, reverses TNF-α-induced alterations of striatal glutamate-mediated excitatory postsynaptic currents in EAE-afflicted mice. With regard to T cells of MS patients: (1) The cell surface expression of a specific GluR: the AMPA GluR3 is elevated in T cells of MS patients during relapse and with active disease, (2) Glutamate and AMPA (a selective agonist for glutamate/AMPA iGluRs) augment chemotactic migration of T cells of MS patients, (3) Glutamate augments proliferation of T cells of MS patients in response to myelin-derived proteins: MBP and MOG, (4) T cells of MS patients respond abnormally to glutamate, (5) Significantly higher proliferation values in response to glutamate were found in MS patients assessed during relapse, and in those with gadolinium (Gd)+ enhancing lesions on MRI. Furthermore, glutamate released from autoreactive T cells induces excitotoxic cell death of neurons. Taken together, the evidences accumulated thus far indicate that abnormal glutamate levels and signaling in the nervous system, direct activation of T cells by glutamate, and glutamate release by T cells, can all contribute to MS. This may be true also to other neurological diseases. It is postulated herein that the detrimental activation of autoimmune T cells by glutamate in MS could lead to: (1) Cytotoxicity in the CNS: T cell-mediated killing of neurons and glia cells, which would subsequently increase the extracellular glutamate levels, and by doing so increase the excitotoxicity mediated by excess glutamate, (2) Release of proinflammatory cytokines, e.g., TNFα and IFNγ that increase neuroinflammation. Finally, if excess glutamate, abnormal neuronal signaling, glutamate-induced activation of T cells, and glutamate release by T cells are indeed all playing a key detrimental role in MS, then optional therapeutic tolls include GluR antagonists, although these may have various side effects. In addition, an especially attractive therapeutic strategy is the novel and entirely different therapeutic approach to minimize excess glutamate and excitotoxicity, titled: 'brain to blood glutamate scavenging', designed to lower excess glutamate levels in the CNS by 'pumping it out' from the brain to the blood. The glutamate scavanging is achieved by lowering glutamate levels in the blood by intravenous injection of the blood enzyme glutamate oxaloacetate transaminase (GOT). The glutamate-scavenging technology, which is still experimental, validated so far for other brain pathologies, but not tested on MS or EAE yet, may be beneficial for MS too, since it could decrease both the deleterious effects of excess glutamate on neural cells, and the activation of autoimmune T cells by glutamate in the brain. The topic of glutamate scavenging, and also its potential benefit for MS, are discussed towards the end of the review, and call for research in this direction.

Oxaloacetate and adipose stromal cells-conditional medium synergistically protected potassium/serum deprivation-induced neuronal apoptosis

Adipose stromal cells conditioned media (ASC-CM) protect neurons in a variety of neuronal death models including potassium/serum deprivation-induced neuronal apoptosis. In this study, we found that ASC-CM contained glutamate oxaloacetate transaminase and its substrate, oxaloacetate (OAA) directly protected cerebellar granule neurons (CGN) from apoptosis induced by serum and potassium deprivation. Additionally, OAA inhibited serum and potassium deprivation-induced caspase 3 activation. ASC-CM and OAA in combination had a synergistic neuroprotective effect. Clearly, different from ASC-CM-induced neuroprotection, OAA-induced neuroprotection was Akt- independent but JNK-dependent. These data establish a mechanistic basis supporting that the application of ASC-CM for neuroprotective treatments could be significantly enhanced by addition of OAA.

The Glutamate-Glutamine Cycle in Epilepsy

Epilepsy is a complex, multifactorial disease characterized by spontaneous recurrent seizures and an increased incidence of comorbid conditions such as anxiety, depression, cognitive dysfunction, and sudden unexpected death. About 70 million people worldwide are estimated to suffer from epilepsy, and up to one-third of all people with epilepsy are expected to be refractory to current medications. Development of more effective and specific antiepileptic interventions is therefore requisite. Perturbations in the brain's glutamate-glutamine cycle, such as increased extracellular levels of glutamate, loss of astroglial glutamine synthetase, and changes in glutaminase and glutamate dehydrogenase, are frequently encountered in patients with epilepsy. Hence, manipulations of discrete glutamate-glutamine cycle components may represent novel approaches to treat the disease. The goal of his review is to discuss some of the glutamate-glutamine cycle components that are altered in epilepsy, particularly neurotransmitters and metabolites, enzymes, amino acid transporters, and glutamate receptors. We will also review approaches that potentially could be used in humans to target the glutamate-glutamine cycle. Examples of such approaches are treatment with glutamate receptor blockers, glutamate scavenging, dietary intervention, and hypothermia.

Extracorporeal methods of blood glutamate scavenging: a novel therapeutic modality

Pathologically elevated glutamate concentrations in the brain's extracellular fluid are associated with several acute and chronic brain insults. Studies have demonstrated that by decreasing the concentration of glutamate in the blood, thereby increasing the concentration gradient between the brain and the blood, the rate of brain-to-blood glutamate efflux can be increased. Blood glutamate scavengers, pyruvate and oxaloacetate have shown great promise in providing neuroprotection in many animal models of acute brain insults. However, glutamate scavengers' potential systemic toxicity, side effects and pharmacokinetic properties may limit their use in clinical practice. In contrast, extracorporeal methods of blood glutamate reduction, in which glutamate is filtered from the blood and eliminated, may be an advantageous adjunct in treating acute brain insults. Here, we review the current evidence for the glutamate-lowering effects of hemodialysis, peritoneal dialysis and hemofiltration. The evidence reviewed here highlights the need for clinical trials.

Calorie restriction mimetics: can you have your cake and eat it, too?

Strong consensus exists regarding the most robust environmental intervention for attenuating aging processes and increasing healthspan and lifespan: calorie restriction (CR). Over several decades, this paradigm has been replicated in numerous nonhuman models, and has been expanded over the last decade to formal, controlled human studies of CR. Given that long-term CR can create heavy challenges to compliance in human diets, the concept of a calorie restriction mimetic (CRM) has emerged as an active research area within gerontology. In past presentations on this subject, we have proposed that a CRM is a compound that mimics metabolic, hormonal, and physiological effects of CR, activates stress response pathways observed in CR and enhances stress protection, produces CR-like effects on longevity, reduces age-related disease, and maintains more youthful function, all without significantly reducing food intake, at least initially. Over 16 years ago, we proposed that glycolytic inhibition could be an effective strategy for developing CRM. The main argument here is that inhibiting energy utilization as far upstream as possible provides the highest chance of generating a broad spectrum of CR-like effects when compared to targeting a singular molecular target downstream. As an initial candidate CRM, 2-deoxyglucose, a known anti-glycolytic, was shown to produce a remarkable phenotype of CR, but further investigation found that this compound produced cardiotoxicity in rats at the doses we had been using. There remains interest in 2DG as a CRM but at lower doses. Beyond the proposal of 2DG as a candidate CRM, the field has grown steadily with many investigators proposing other strategies, including novel anti-glycolytics. Within the realm of upstream targeting at the level of the digestive system, research has included bariatric surgery, inhibitors of fat digestion/absorption, and inhibitors of carbohydrate digestion. Research focused on downstream sites has included insulin receptors, IGF-1 receptors, sirtuin activators, inhibitors of mTOR, and polyamines. In the current review we discuss progress made involving these various strategies and comment on the status and future for each within this exciting research field.

MRS of brain metabolite levels demonstrates the ability of scavenging of excess brain glutamate to protect against nerve agent induced seizures

This study describes the use of in vivo magnetic resonance spectrocopy (MRS) to monitor brain glutamate and lactate levels in a paraoxon (PO) intoxication model. Our results show that the administration of recombinant glutamate-oxaloacetate transaminase (rGOT) in combination with oxaloacetate (OxAc) significantly reduces the brain-accumulated levels of glutamate. Previously we have shown that the treatment causes a rapid decrease of blood glutamate levels and creates a gradient between the brain and blood glutamate levels which leads to the efflux of excess brain glutamate into the blood stream thereby reducing its potential to cause neurological damage. The fact that this treatment significantly decreased the brain glutamate and lactate levels following PO intoxication suggests that it could become a new effective neuroprotective agent.

Taming glutamate excitotoxicity: strategic pathway modulation for neuroprotection

Much work has been carried out in recent years showing that elevated glutamate levels in the extracellular environment of the central nervous system play a pivotal role in neurodegeneration in acute CNS injuries. With the elucidation of the mechanism governing glutamate excitotoxicity, researchers are devising therapeutic strategies to target different parts of the pathway which begins with glutamate accumulation and ultimately results in neuronal cell death. In this article, we review some of the major classes of agents that are currently being investigated and highlight some of the key studies for each. Glutamate scavenging is a relatively new approach that directly decreases glutamate levels in the brain, thus preventing excitotoxicity. Nitric oxide inhibitors and free radical scavengers are more well-studied strategies that continue to yield promising results.

Glutamate efflux at the blood-brain barrier: cellular mechanisms and potential clinical relevance

L-Glutamate is considered the most important excitatory amino acid in the mammalian brain. Strict control of its concentration in the brain interstitial fluid is important to maintain neurotransmission and avoid excitotoxicity. The role of astrocytes in handling L-glutamate transport and metabolism is well known, however endothelial cells may also play an important role through mediating brain-to-blood L-glutamate efflux. Expression of excitatory amino acid transporters has been demonstrated in brain endothelial cells of bovine, human, murine, rat and porcine origin. These can account for high affinity concentrative uptake of L-glutamate from the brain interstitial fluid into the capillary endothelial cells. The mechanisms in between L-glutamate uptake in the endothelial cells and L-glutamate appearing in the blood are still unclear and may involve a luminal transporter for L-glutamate, metabolism of L-glutamate and transport of metabolites or a combination of the two. However, both in vitro and in vivo studies demonstrated blood-to-brain transport of L-glutamate, at least during pathological events. This review summarizes the current knowledge on the brain-to-blood L-glutamate efflux hypothesis including possible mechanisms to account for the transport, in vivo studies on blood glutamate scavenging and potential clinical relevance of the phenomenon.

Brain to blood glutamate scavenging as a novel therapeutic modality: a review

It is well known that abnormally elevated glutamate levels in the brain are associated with secondary brain injury following acute and chronic brain insults. As such, a tight regulation of brain glutamate concentrations is of utmost importance in preventing the neurodegenerative effects of excess glutamate. There has been much effort in recent years to better understand the mechanisms by which glutamate is reduced in the brain to non-toxic concentrations, and in how to safely accelerate these mechanisms. Blood glutamate scavengers such as oxaloacetate, pyruvate, glutamate-oxaloacetate transaminase, and glutamate-pyruvate transaminase have been shown to reduce blood glutamate concentrations, thereby increasing the driving force of the brain to blood glutamate efflux and subsequently reducing brain glutamate levels. In the past decade, blood glutamate scavengers have gained increasing international interest, and its uses have been applied to a wide range of experimental contexts in animal models of traumatic brain injury, ischemic stroke, subarachnoid hemorrhage, epilepsy, migraine, and malignant gliomas. Although glutamate scavengers have not yet been used in humans, there is increasing evidence that their use may provide effective and exciting new therapeutic modalities. Here, we review the laboratory evidence for the use of blood glutamate scavengers. Other experimental neuroprotective treatments thought to scavenge blood glutamate, including estrogen and progesterone, beta-adrenergic activation, hypothermia, insulin and glucagon, and hemodialysis and peritoneal dialysis are also discussed. The evidence reviewed here will hopefully pave the way for future clinical trials.

Blood glutamate scavenging as a novel neuroprotective treatment for paraoxon intoxication

Organophosphate-induced brain damage is an irreversible neuronal injury, likely because there is no pharmacological treatment to prevent or block secondary damage processes. The presence of free glutamate (Glu) in the brain has a substantial role in the propagation and maintenance of organophosphate-induced seizures, thus contributing to the secondary brain damage. This report describes for the first time the ability of blood glutamate scavengers (BGS) oxaloacetic acid in combination with glutamate oxaloacetate transaminase to reduce the neuronal damage in an animal model of paraoxon (PO) intoxication. Our method causes a rapid decrease of blood Glu levels and creates a gradient that leads to the efflux of the excess brain Glu into the blood, thus reducing neurotoxicity. We demonstrated that BGS treatment significantly prevented the peripheral benzodiazepine receptor (PBR) density elevation, after PO exposure. Furthermore, we showed that BGS was able to rescue neurons in the piriform cortex of the treated rats. In conclusion, these results suggest that treatment with BGS has a neuroprotective effect in the PO intoxication. This is the first time that this approach is used in PO intoxication and it may be of high clinical significance for the future treatment of the secondary neurologic damage post organophosphates exposure.

Human recombinant glutamate oxaloacetate transaminase 1 (GOT1) supplemented with oxaloacetate induces a protective effect after cerebral ischemia

Blood glutamate scavenging is a novel and attractive protecting strategy to reduce the excitotoxic effect of extracellular glutamate released during ischemic brain injury. Glutamate oxaloacetate transaminase 1 (GOT1) activation by means of oxaloacetate administration has been used to reduce the glutamate concentration in the blood. However, the protective effect of the administration of the recombinant GOT1 (rGOT1) enzyme has not been yet addressed in cerebral ischemia. The aim of this study was to analyze the protective effect of an effective dose of oxaloacetate and the human rGOT1 alone and in combination with a non-effective dose of oxaloacetate in an animal model of ischemic stroke. Sixty rats were subjected to a transient middle cerebral artery occlusion (MCAO). Infarct volumes were assessed by magnetic resonance imaging (MRI) before treatment administration, and 24 h and 7 days after MCAO. Brain glutamate levels were determined by in vivo MR spectroscopy (MRS) during artery occlusion (80 min) and reperfusion (180 min). GOT activity and serum glutamate concentration were analyzed during the occlusion and reperfusion period. Somatosensory test was performed at baseline and 7 days after MCAO. The three treatments tested induced a reduction in serum and brain glutamate levels, resulting in a reduction in infarct volume and sensorimotor deficit. Protective effect of rGOT1 supplemented with oxaloacetate at 7 days persists even when treatment was delayed until at least 2 h after onset of ischemia. In conclusion, our findings indicate that the combination of human rGOT1 with low doses of oxaloacetate seems to be a successful approach for stroke treatment

The effects of estrogen and progesterone on blood glutamate levels during normal pregnancy in women

The purpose of this study was to examine whether changes in estrogen and progesterone levels observed during normal pregnancy influence blood glutamate levels. One-hundred and sixteen pregnant women were divided into three groups based on gestational age: group 1 included women in their first trimester, group 2 included women in their second trimester, and group 3 included women in their third trimester. A single venous blood sample was collected and analyzed for concentrations of estrogen, progesterone, glutamate-pyruvate transaminase (GPT), glutamate-oxaloacetate transaminase (GOT), and glutamate. Concentrations of blood glutamate were significantly lower during the second trimester (p < 0.001) and third trimester (p < 0.001). Blood glutamate levels were inversely correlated with levels of estrogen and progesterone throughout pregnancy (p < 0.001). Levels of GOT and GPT remained stable during the course of pregnancy, apart from a moderate reduction in GPT during the third trimester. Increases in estrogen and progesterone levels during advanced stages of pregnancy were inversely correlated with maternal blood glutamate concentrations. Once a maximal blood glutamate-reducing effect was achieved, any additional estrogen and progesterone had a negligible effect on blood glutamate. This study demonstrates the glutamate-reducing effects of estrogen and progesterone, which is most likely not mediated by a GOT/GPT conversion mechanism.

The role of hypothermia in the regulation of blood glutamate levels in naive rats

BACKGROUND

The exact mechanism of hypothermia-induced neuroprotection has not been determined yet; however, we hypothesized that it may be mediated by a blood glutamate-scavenging effect. Here, we examine the effect of hypothermic conditions (mild, moderate, and deep) on blood glutamate levels in naive rats. To identify the mechanism of hypothermia-induced glutamate reduction, we also measured concentrations of glutamate oxaloacetate transaminase (GOT) and glutamate pyruvate transaminase (GPT), the primary regulators of glutamate concentration in blood.

METHODS

Rats were anesthetized with isoflurane, and their rectal temperature was maintained for 6 hours at 36 to 37°C, 33 to 36°C, 30 to 32°C, 18 to 22°C, or was not maintained artificially. At 6 hours, active cooling was discontinued and rats were allowed to rewarm. There were 12 rats in each group for a total of 60 rats. Blood samples were drawn at 0, 3, 6, 12, 24, and 48 hours for the determination of blood glutamate, GOT, and GPT levels.

RESULTS

A strong correlation between body temperature and blood glutamate levels was observed (P<0.001). Mild (33 to 36°C) and moderate (30 to 32°C) hypothermia led to reduced blood glutamate levels (P<0.001). Deep hypothermia (18 to 22°C) was associated with significant elevations in blood glutamate levels (P<0.001). Hypothermia, irrespective of the degree, led to elevations in GOT in plasma (P<0.001).

CONCLUSIONS

Mild and moderate hypothermia led to a reduction in blood glutamate levels in rats, whereas deep hypothermia was associated with a significant elevation in blood glutamate levels. We further demonstrated an elevation of GOT and GPT levels, supporting their involvement in reducing blood glutamate by the conversion of glutamate to 2-ketoglutarate. We suggest that the neuroprotective properties of hypothermia may be partially because of a blood glutamate-scavenging mechanism.

Blood glutamate scavengers prolong the survival of rats and mice with brain-implanted gliomas

L-Glutamate (Glu) plays a crucial role in the growth of malignant gliomas. We have established the feasibility of accelerating a naturally occurring brain to-blood Glu efflux by decreasing blood Glu levels with intravenous oxaloacetate, the respective Glu co-substrate of the blood resident enzyme humane glutamate–oxaloacetate transaminase(hGOT). We wished to demonstrate that blood Glu scavenging provides neuroprotection in the case of glioma.We now describe the neuroprotective effects of blood Glu scavenging in a fatal condition such as brain-implanted C6 glioma in rats and brain-implanted human U87 MG glioma in nude mice. Rat (C-6) or human (U87) glioma cells were grafted stereotactically in the brain of rats or mice. After development of tumors, the animals were drinking oxaloacetate with or without injections of hGOT. In addition, mice were treated with combination treatment, which included drinking oxaloacetate with intracutaneous injections of hGOT and intraperitoneal injection of Temozolomide. Animals drinking oxaloacetate with or without injections of hGOT displayed a smaller tumor volume, reduced invasiveness and prolonged survival than control animals drinking saline. These effects were significantly enhanced by Temozolomide in mice, which increased survival by 237%. This is the first demonstration of blood Glu scavenging in brain cancer, and because of its safety, is likely to be of clinical significance for the future treatment of human gliomas. As we demonstrated, the blood glutamate scavenging treatment in combination with TMZ could be a good candidate or as an alternative treatment to the patients that do not respond to TMZ.

The effects of hemodialysis on blood glutamate levels in chronic renal failure: implementation for neuroprotection

PURPOSE

The purpose of the present study is to investigate whether hemodialysis (HD) is effective in lowering blood glutamate levels. In addition, we examined the effect of HD on glutamate oxaloacetate transaminase (GOT) and glutamate pyruvate transaminase (GPT) levels in the blood and described the rate and pattern of blood glutamate clearance during HD.

MATERIALS AND METHODS

Blood samples were taken from 45 patients with stage V chronic kidney disease immediately after initiation of HD and hourly, for a total of 5 blood samples. Samples were sent for determination of glutamate, glucose, GOT, GPT, hemoglobin, hematocrit, urea, and creatinine levels. A blood sample from 25 healthy volunteers without chronic renal failure was used as a control for the determination of baseline blood levels of glutamate, GOT, and GPT.

RESULTS

Glutamate and GPT levels in patients on HD were higher at baseline compared with healthy controls (P < .001). In the first 3 hours after HD, there was a decrease in blood glutamate levels compared with baseline levels (P < .00001). At the fourth hour, there was an increase in blood glutamate levels compared with the third hour (P < .05).

CONCLUSIONS

Hemodialysis may be a promising method of reducing blood glutamate levels.

Relationship between glutamate, GOT and GPT levels in maternal and fetal blood: a potential mechanism for fetal neuroprotection

BACKGROUND

Excess glutamate in the brain is thought to be implicated in the pathophysiology of fetal anoxic brain injury, yet little is known about the mechanisms by which glutamate is regulated in the fetal brain. This study examines whether there are differences between maternal and fetal glutamate concentrations, and whether a correlation between them exists.

METHODS

10 ml of venous blood was extracted from 87 full-term (>37 weeks gestation) pregnant women in active labor. Immediately after delivery of the neonate, 10 ml of blood from the umbilical artery and vein was extracted. Samples were analyzed for levels of glutamate, glutamate-oxaloacetate transaminase (GOT), and glutamate pyruvate transaminase (GPT).

RESULTS

Fetal blood glutamate concentrations in both the umbilical artery and vein were found to be significantly higher than maternal blood (p<0.001). Similarly, fetal serum GOT levels in the umbilical artery and vein were found to be significantly higher than maternal GOT levels (p<0.001). The difference in GPT levels between maternal and fetal serum was not statistically significant. There was no difference in fetal glutamate, GOT or GPT between the umbilical artery and vein. There was an association observed between glutamate levels in maternal blood and glutamate levels in both venous (R=0.32, p<0.01) and arterial (R=0.33, p<0.05) fetal blood.

CONCLUSIONS

This study demonstrated that higher baseline concentrations of blood glutamate are present in fetal blood compared with maternal blood, and this was associated with elevated GOT, but not GPT levels. An association was observed between maternal and fetal blood glutamate levels.

Blood glutamate scavenging: insight into neuroprotection

Brain insults are characterized by a multitude of complex processes, of which glutamate release plays a major role. Deleterious excess of glutamate in the brain's extracellular fluids stimulates glutamate receptors, which in turn lead to cell swelling, apoptosis, and neuronal death. These exacerbate neurological outcome. Approaches aimed at antagonizing the astrocytic and glial glutamate receptors have failed to demonstrate clinical benefit. Alternatively, eliminating excess glutamate from brain interstitial fluids by making use of the naturally occurring brain-to-blood glutamate efflux has been shown to be effective in various animal studies. This is facilitated by gradient driven transport across brain capillary endothelial glutamate transporters. Blood glutamate scavengers enhance this naturally occurring mechanism by reducing the blood glutamate concentration, thus increasing the rate at which excess glutamate is cleared. Blood glutamate scavenging is achieved by several mechanisms including: catalyzation of the enzymatic process involved in glutamate metabolism, redistribution of glutamate into tissue, and acute stress response. Regardless of the mechanism involved, decreased blood glutamate concentration is associated with improved neurological outcome. This review focuses on the physiological, mechanistic and clinical roles of blood glutamate scavenging, particularly in the context of acute and chronic CNS injury. We discuss the details of brain-to-blood glutamate efflux, auto-regulation mechanisms of blood glutamate, natural and exogenous blood glutamate scavenging systems, and redistribution of glutamate. We then propose different applied methodologies to reduce blood and brain glutamate concentrations and discuss the neuroprotective role of blood glutamate scavenging.

Pharmacokinetics of glutamate-oxaloacetate transaminase and glutamate-pyruvate transaminase and their blood glutamate-lowering activity in naïve rats

Traumatic brain injury (TBI) and stroke lead to elevated levels of glutamate in the brain that negatively affect the neurological outcomes in both animals and humans. Intravenous administration of glutamate-oxaloacetate transaminase (GOT) and glutamate-pyruvate transaminase (GPT) enzymes can be used to lower the blood glutamate levels and to improve the neurological outcome following TBI and stroke. The objective of this study was to analyze the pharmacokinetics and to determine the glutamate-lowering effects of GOT and GPT enzymes in naïve rats. We determined the time course of serum GOT, GPT, and glutamate levels following a single intravenous administration of two different doses of each one of the studied enzymes. Forty-six male rats were randomly assigned into one of 5 treatment groups: saline (control), human GOT at dose 0.03 and 0.06 mg/kg and porcine GPT at dose 0.6 and 1.2 mg/kg. Blood samples were collected at baseline, 5 min, and 2, 4, 8, 12, and 24 h after the drug injection and GOT, GPT and glutamate levels were determined. The pharmacokinetics of both GOT and GPT followed one-compartment model, and both enzymes exhibited substantial glutamate-lowering effects following intravenous administration. Analysis of the pharmacokinetic data indicated that both enzymes were distributed predominantly in the blood (central circulation) and did not permeate to the peripheral organs and tissues. Several-hour delay was present between the time course of the enzyme levels and the glutamate-lowering effects (leading to clock-wise hysteresis on concentration-effect curves), apparently due to the time that is required to affect the pool of serum glutamate. We conclude that the interaction between the systemically-administered enzymes (GOT and GPT) and the glutamate takes place in the central circulation. Thus, glutamate-lowering effects of GOT and GPT apparently lead to redistribution of the excess glutamate from the brain's extracellular fluid into the blood and can reduce secondary brain injury due to glutamate neurotoxicity. The outcomes of this study regarding the pharmacokinetic and pharmacodynamic properties of the GOT and GPT enzymes will be subsequently verified in clinical studies that can lead to design of effective neuroprotective treatment strategies in patients with traumatic brain diseases and stroke.

The effect of blood glutamate scavengers oxaloacetate and pyruvate on neurological outcome in a rat model of subarachnoid hemorrhage

Blood glutamate scavengers have been shown to effectively reduce blood glutamate concentrations and improve neurological outcome after traumatic brain injury and stroke in rats. This study investigates the efficacy of blood glutamate scavengers oxaloacetate and pyruvate in the treatment of subarachnoid hemorrhage (SAH) in rats. Isotonic saline, 250 mg/kg oxaloacetate, or 125 mg/kg pyruvate was injected intravenously in 60 rats, 60 minutes after induction of SAH at a rate of 0.1 ml/100 g/min for 30 minutes. There were 20 additional rats that were used as a sham-operated group. Blood samples were collected at baseline and 90 minutes after SAH. Neurological performance was assessed at 24 h after SAH. In half of the rats, glutamate concentrations in the cerebrospinal fluid were measured 24 h after SAH. For the remaining half, the blood brain barrier permeability in the frontal and parieto-occipital lobes was measured 48 h after SAH. Blood glutamate levels were reduced in rats treated with oxaloacetate or pyruvate at 90 minutes after SAH (p < 0.001). Cerebrospinal fluid glutamate was reduced in rats treated with pyruvate (p < 0.05). Neurological performance was significantly improved in rats treated with oxaloacetate (p < 0.05) or pyruvate (p < 0.01). The breakdown of the blood brain barrier was reduced in the frontal lobe in rats treated with pyruvate (p < 0.05) and in the parieto-occipital lobes in rats treated with either pyruvate (p < 0.01) or oxaloacetate (p < 0.01). This study demonstrates the effectiveness of blood glutamate scavengers oxaloacetate and pyruvate as a therapeutic neuroprotective strategy in a rat model of SAH.

β2 adrenergic-mediated reduction of blood glutamate levels and improved neurological outcome after traumatic brain injury in rats

BACKGROUND

Isoflurane-anesthetized rats subjected to traumatic brain injury (TBI) show a transient reduction in blood L-glutamate levels. Having previously observed that isoproterenol produces a sustained decrease in blood glutamate levels in naive rats, we investigated the possible effects of nonselective and selective β1 and β2 adrenergic agonists and antagonists both on blood glutamate levels and on the neurological outcomes of rats subjected to TBI.

METHODS

Rats received either 10 mL/kg of isotonic saline 1 hour after TBI, 50 µg/kg of isoproterenol pretreatment 30 minutes before TBI, 10 mg/kg of propranolol pretreatment 60 minutes before TBI, 10 mg/kg of metoprolol pretreatment 60 minutes before TBI, or 10 mg/kg of butaxamine pretreatment 40 minutes before TBI and 10 minutes before pretreatment with 50 µg/kg isoproterenol or 10 mg/kg of propranolol 60 minutes after TBI. A neurological severity score (NSS) was measured at 1, 24, and 48 hours after TBI. Blood glutamate, blood glucose, mean arterial blood pressure, and heart rate were measured at the time of drug injection, at the time of TBI, 60 minutes after TBI, and 90 minutes after TBI.

RESULTS

Blood glutamate levels decreased spontaneously by 60 minutes after TBI in the control group (P<0.05), reverting to baseline levels by 90 minutes after TBI. A pretreatment with either 10 mg/kg of metoprolol 60 minutes before TBI or with 50 µg/kg of isoproterenol 30 minutes before TBI also reduced blood glutamate levels (P<0.05) both at 90 minutes after TBI and improved the NSS measured 24 and 48 hours after TBI in comparison with the control saline-treated group. However, a 10-mg/kg butoxamine pretreatment 40 minutes before TBI and 10 minutes before pretreatment with 50 µg/kg of isoproterenol or 10 mg/kg of propranolol 60 minutes before TBI neither affected blood glutamate levels across time after TBI nor caused any significant change in the NSS measured 24 and 48 hours after TBI in comparison with the control saline-treated group. A strong correlation (r(2)=0.73) was demonstrated between the percent decrease in blood glutamate levels at 90 minutes after TBI and the percent improvement of NSS measured 24 hours after TBI.

CONCLUSIONS

The results suggest that the transient blood glutamate reduction seen after TBI is the result of a stress response and of the activation of the sympathetic nervous system through the β2 adrenergic receptors, causing an increase of the brain-to-blood efflux of glutamate observed with excess brain glutamate levels after a brain insult. This strongly correlates with the neurological improvement observed 24 hours after TBI.

Blood glutamate scavengers prolong the survival of rats and mice with brain-implanted gliomas

L-Glutamate (Glu) plays a crucial role in the growth of malignant gliomas. We have established the feasibility of accelerating a naturally occurring brain to-blood Glu efflux by decreasing blood Glu levels with intravenous oxaloacetate, the respective Glu co-substrate of the blood resident enzyme humane glutamate-oxaloacetate transaminase (hGOT). We wished to demonstrate that blood Glu scavenging provides neuroprotection in the case of glioma. We now describe the neuroprotective effects of blood Glu scavenging in a fatal condition such as brain-implanted C6 glioma in rats and brain-implanted human U87 MG glioma in nude mice. Rat (C-6) or human (U87) glioma cells were grafted stereotactically in the brain of rats or mice. After development of tumors, the animals were drinking oxaloacetate with or without injections of hGOT. In addition, mice were treated with combination treatment, which included drinking oxaloacetate with intracutaneous injections of hGOT and intraperitoneal injection of Temozolomide. Animals drinking oxaloacetate with or without injections of hGOT displayed a smaller tumor volume, reduced invasiveness and prolonged survival than control animals drinking saline. These effects were significantly enhanced by Temozolomide in mice, which increased survival by 237%. This is the first demonstration of blood Glu scavenging in brain cancer, and because of its safety, is likely to be of clinical significance for the future treatment of human gliomas. As we demonstrated, the blood glutamate scavenging treatment in combination with TMZ could be a good candidate or as an alternative treatment to the patients that do not respond to TMZ.

The effects of insulin, glucagon, glutamate, and glucose infusion on blood glutamate and plasma glucose levels in naive rats

BACKGROUND

Elevated levels of glutamate in brain fluids, in the context of several neurodegenerative conditions, are associated with a worsened neurological outcome. Because there is a clear relationship between brain glutamate levels and glutamate levels in the blood, and an association of the latter with stress, the purpose of this study was to investigate the effects of glucose, insulin, and glucagon on rat blood glutamate levels.

METHODS

Rats received either 1 mL/100 g of rat body weight (BW) intravenous isotonic saline (control), 150 mg/1 mL/100 g BW intravenous glucose, 75 mg/1 mL/100 g BW intravenous glutamate, 50 g/100 g BW intraparitoneal glucagon, or 0.2 UI/100 g BW intraparitoneal insulin. Blood samples were subsequently drawn at 0, 30, 60, 90, and 120 minutes for determination of blood glutamate and glucose levels.

RESULTS

We observed a significant decrease in blood glutamate levels at 30 minutes after injection of glucose (P<0.05), at 30 and 60 minutes after injection of insulin (P<0.05), and at 90 and 120 minutes after injection of glucagon. Plasma glucose levels were elevated after infusion of glutamate and glucose but were decreased after injection of insulin.

CONCLUSIONS

The results of this study demonstrate that glucose, insulin, and glucagon significantly reduce blood glutamate levels. The effect of insulin is immediate and transient, whereas the effect of glucagon is delayed but longer lasting, suggesting that the sensitivity of pancreatic glucagon and insulin-secreting cells to glutamate is dependent on glucose concentration. The results of this study provide insight into blood glutamate homeostasis and may assist in the implementation of new therapies for brain neuroprotection from excess glutamate.

Effect of estrogens on blood glutamate levels in relation to neurological outcome after TBI in male rats

PURPOSE

Estrogen has been shown to possess neuroprotective properties both in vitro and in vivo. Traumatic brain injury (TBI) in ovulating females results in favorable neurological outcomes when compared to males with similar insults. The brain-to-blood glutamate gradient removes excess glutamate from brain extracellular fluids (ECF). Enhancing this gradient leads to improved neurological outcomes following TBI. In this study we investigate the effect of female gonadal steroids on blood glutamate levels and neurological outcomes.

METHODS

Forty male Sprague-Dawley rats were assigned to one of five groups: (1) sham, (2) Premarin treatment, (3) TBI, (4) TBI + Premarin treatment, and (5) TBI + Premarin pretreatment. TBI was induced, and estrogen and glutamate levels were determined at 0, 60, 120, 135, and 150 min. Neurological recovery was evaluated using the Neurological Severity Score (NSS) at 1 h and reassessed at 24 h post TBI.

RESULTS

Premarin treatment groups demonstrated a decline in blood glutamate levels by 60 min. This decline was found to be more pronounced in the TBI + Premarin group, which maintained the decline throughout the experiment. At 120 min, the difference between groups was most pronounced (TBI + Premarin 99 ± 36 μM/l vs. control 200 ± 46 μM/l, p < 0.01). Neurological recovery was significantly better in the Premarin treatment group (NSS at 24 h 6 ± 1 vs. control 11 ± 1).

CONCLUSIONS

Premarin injected into male rats significantly decreases blood glutamate levels in rats suffering TBI. This decrease is associated with improved neurological outcomes, thus implicating the role of estrogen in neuroprotection.

The preclinical discovery of amyotrophic lateral sclerosis drugs

INTRODUCTION

Amyotrophic lateral sclerosis (ALS), also referred to as Lou Gehrig's disease, is characterized by the progressive loss of cells in the brain and spinal cord that leads to debilitation and death in 3 - 5 years. Only one therapeutic drug, riluzole, has been approved for ALS and this drug improves survival by 2 - 3 months. The need for new therapeutics that can postpone or slow the progression of the motor deficits and prolong survival is still a strong unmet medical need.

AREAS COVERED

Although there are a number of drugs currently in clinical trials for ALS, this review provides an overview of the most promising biological targets and preclinical strategies that are currently being developed and deployed. The list of targets for ALS was compiled from a variety of websites including individual companies that have ALS programs and include those from the author's experience.

EXPERT OPINION

Progress is being made in the identification of possible new therapeutics for ALS with recent efforts in understanding the genetic causes of the disease, susceptibility factors and the development of additional preclinical animal models. However, many challenges remain in the identification of new ALS therapeutics including: the use of relevant biomarkers, the need for an earlier diagnosis of the disease and additional animal models. Multiple strategies need to be tested in the clinic in order to determine what will be effective in patients.

Could fever and neuroinflammation play a role in the neurobiology of autism? A subject worthy of more research

Autism is neuropsychiatric disorder in which a hyperglutamate state may play a role. It is suggested here that fever or hyperthermia may be able to alter glutamate levels in the brain and may therefore be able to impact on the symptoms of autism. More study on this possibility is clearly warranted.

Pyruvate's blood glutamate scavenging activity contributes to the spectrum of its neuroprotective mechanisms in a rat model of stroke

In previous studies, we have shown that by increasing the brain-to-blood glutamate efflux upon scavenging blood glutamate with either oxaloacetate or pyruvate, one achieves highly significant neuroprotection particularly in the context of traumatic brain injury. The current study examines, for the first time, how the blood glutamate scavenging properties of glutamate-pyruvate transaminase (GPT), alone or in combination with pyruvate, may contribute to the spectrum of its neuroprotective mechanisms and improve the outcome of rats exposed to brain ischemia, as they do after head trauma. Rats that were exposed to permanent middle cerebral artery occlusion (MCAO) and treated with intravenous 250 mg/kg pyruvate had a smaller volume of infarction and reduced brain edema, resulting in an improved neurological outcome and reduced mortality compared to control rats treated with saline. Intravenous pyruvate at the low dose of 31.3 mg/kg did not demonstrate any neuroprotection. However, when combined with 0.6 mg/kg of GPT there was a similar neuroprotection observed as seen with pyruvate at 250 mg/kg. Animals treated with 1.69 g/kg glutamate had a worse neurological outcome and a larger extent of brain edema. The decrease in mortality, infarcted brain volume and edema, as well as the improved neurological outcome following MCAO, was correlated with a decrease in blood glutamate levels. We therefore suggest that the blood glutamate scavenging activity of GPT and pyruvate contributes to the spectrum of their neuroprotective mechanisms and may serve as a new neuroprotective strategy for the treatment of ischemic stroke.

OAT2 catalyses efflux of glutamate and uptake of orotic acid

OAT (organic anion transporter) 2 [human gene symbol SLC22A7 (SLC is solute carrier)] is a member of the SLC22 family of transport proteins. In the rat, the principal site of expression of OAT2 is the sinusoidal membrane domain of hepatocytes. The particular physiological function of OAT2 in liver has been unresolved so far. In the present paper, we have used the strategy of LC (liquid chromatography)-MS difference shading to search for specific and cross-species substrates of OAT2. Heterologous expression of human and rat OAT2 in HEK (human embryonic kidney)-293 cells stimulated accumulation of the zwitterion trigonelline; subsequently, orotic acid was identified as an excellent and specific substrate of OAT2 from the rat (clearance=106 μl·min⁻¹·mg of protein⁻¹) and human (46 μl·min⁻¹·mg of protein⁻¹). The force driving uptake of orotic acid was identified as glutamate antiport. Efficient transport of glutamate by OAT2 was directly demonstrated by uptake of [³H]glutamate. However, because of high intracellular glutamate, OAT2 operates as glutamate efflux transporter. Thus expression of OAT2 markedly increased the release of glutamate (measured by LC-MS) from cells, even without extracellular exchange substrate. Orotic acid strongly trans-stimulated efflux of glutamate. We thus propose that OAT2 physiologically functions as glutamate efflux transporter. OAT2 mRNA was detected, after laser capture microdissection of rat liver slices, equally in periportal and pericentral regions; previous reports of hepatic release of glutamate into blood can now be explained by OAT2 activity. A specific OAT2 inhibitor could, by lowering plasma glutamate and thus promoting brain-to-blood efflux of glutamate, alleviate glutamate exotoxicity in acute brain conditions.

The activation of β2-adrenergic receptors in naïve rats causes a reduction of blood glutamate levels: relevance to stress and neuroprotection

This study examines the effects of the activation of β1 and β2-adrenergic receptors on glutamate homeostasis in the blood of naïve rats. Forty five male Sprague-Dawley rats were randomly assigned into one of seven treatment groups that were treated with various β-adrenergic receptor agonist and antagonist drugs. Blood glutamate levels were determined at t = 0, 30, 60, 90, and 120 min. The activation of β1 and β2-adrenergic receptors via isoproterenol hydrochloride administration produced a marked sustained decrease in blood glutamate levels by 60 min after treatment (ANOVA, t = 60, 90 min: P < 0.05, t = 120 min: P < 0.01). Pretreatment with propranolol hydrochloride (a non-selective β-adrenergic receptor blocker) or butaxamine hydrochloride (a selective β2-adrenergic receptor blocker) occluded the isoproterenol-mediated decrease in blood glutamate levels. Propranolol alone had no effect on blood glutamate levels. Selective β1-adrenergic receptor blockade with metoprolol resulted in decreased blood glutamate levels (ANOVA, t = 90 min: P < 0.05, t = 120 min: P < 0.01). Butaxamine hydrochloride alone resulted in a delayed-onset increase in glutamate levels (ANOVA, t = 120 min: P < 0.05). The results suggest that the activation of β2 receptors plays an important role in the homeostasis of glutamate in rat blood.

The effect of hyperthermia on blood glutamate levels

INTRODUCTION

Glutamate neurotoxicity is determined by the balance between glutamate release within the brain and efflux of excess glutamate from the brain. Brain-to-blood efflux of glutamate is increased by decreasing the concentration of glutamate in blood. Little is known about the effect of hyperthermia on blood glutamate concentrations, and the effectiveness of blood glutamate-decreasing mechanisms in these conditions. Although hyperthermia is hypothesized to decrease blood glutamate concentrations by activation of stress mechanisms, blunting the stress response by blocking β-adrenergic receptors should prevent this decrease. Furthermore, during hyperthermia there should be a concurrent process of leakage of glutamate from muscle tissue into blood, resulting in a contradictory increase of blood glutamate concentrations. In this study we investigated the effects of hyperthermia on blood glutamate levels and studied the effects of the β-adrenergic receptor antagonist propranolol on stress-induced changes in glutamate levels. We then studied the effectiveness of the blood glutamate scavenger oxaloacetate on hyperthermia-induced increases of glutamate levels.

MATERIALS AND METHODS

Twenty-four rats were randomly divided into 3 groups. Rats' body temperatures were increased (by 1°C every 40 minutes) from 37°C to 42°C. The first group received 1 mL per 100 g of isotonic saline (control). The second group received 1 mL per 100 g of 1M oxaloacetate when the temperature reached 39°C. The third group received 10 mg/kg of propranolol before initiation of the warming.

RESULTS

Warming the rats from 37°C to 39°C decreased the blood glutamate levels in the control group (P < 0.01) and oxaloacetate treatment group (P < 0.0001), whereas further increases in temperature from 40°C to 42°C increased the blood glutamate levels (P < 0.01 and P < 0.0001, respectively). Pretreatment with propranolol prevented the decrease in blood glutamate concentrations seen in mild hyperthermia and did not affect the increase in blood glutamate levels seen at temperatures of 41°C and 42°C (P < 0.005).

DISCUSSION

The results of this study demonstrated that hyperthermia leads to decreases in glutamate levels in the blood, presumably by activation of the sympathetic nervous system. Oxaloacetate, previously reported to reduce blood glutamate levels at 37°C, was ineffective at temperatures over 40°C. Propranolol pretreatment blunted the initial decrease in blood glutamate, and thereafter had no effect when compared with control and treatment groups. Understanding the mechanisms underlying glutamate regulation in the blood during states of hyperthermia and stress has important clinical implications in treating neurodegenerative conditions.

The effects of estrogen and progesterone on blood glutamate levels: evidence from changes of blood glutamate levels during the menstrual cycle in women

The gonadal steroids estrogen and progesterone have been shown to have neuroprotective properties against various neurodegenerative conditions. Excessive concentrations of glutamate have been found to exert neurotoxic properties. We hypothesize that estrogen and progesterone provide neuroprotection by the autoregulation of blood and brain glutamate levels. Venous blood samples (10 ml) were taken from 31 men and 45 women to determine blood glutamate, estrogen, progesterone, glucose, glutamate-pyruvate transaminase (GPT), and glutamate-oxaloacetate transaminase (GOT) levels, collected on Days 1, 7, 12, and 21 of the female participants' menstrual cycle. Blood glutamate concentrations were higher in men than in women at the start of menstruation (P < 0.05). Blood glutamate levels in women decreased significantly on Days 7 (P < 0.01), 12 (P < 0.001), and 21 (P < 0.001) in comparison with blood glutamate levels on Day 1. There was a significant decrease in blood glutamate levels on Days 12 (P < 0.001) and 21 (P < 0.001) in comparison with blood glutamate levels on Day 7. Furthermore, there was an increase in blood glutamate levels on Day 21 compared with Day 12 (P < 0.05). In women, there were elevated levels of estrogen on Days 7 (P < 0.05), 12, and 21 (P < 0.001), and elevated levels of progesterone on Days 12 and 21 (P < 0.001). There were no differences between men and women with respect to blood glucose concentrations. Concentrations of GOT (P < 0.05) and GPT (P < 0.001) were significantly higher in men than in women during the entire cycle. The results of this study demonstrate that blood glutamate levels are inversely correlated to levels of plasma estrogen and progesterone.

Nitrogen anabolism underlies the importance of glutaminolysis in proliferating cells

Glutaminolysis and Warburg effect are the two most noticeable metabolic features of tumor cells whereas their biological significance in cell proliferation remains elusive. A widely accepted current hypothesis is that tumor cells use glutamine as a preferred carbon source for energy and reducing power, which has been used to explain both glutaminolysis and the Warburg effect. Here we provide evidence to show that supplying nitrogen, not the carbon skeleton, underlies the major biological importance of glutaminolysis for proliferating cells. We show alternative nitrogen supplying mechanisms rescue cell proliferation in glutamine-free media. Particularly, we show that ammonia is sufficient to maintain a long-term survival and proliferation of Hep3B in glutamine-free media. We also observed that nitrogen source restriction repressed carbon metabolic pathways including glucose utilization. Based on these new observations and metabolic pathways well established in published literature, we propose an alternative model that cellular demand for glutamate as a key molecule in nitrogen anabolism is the driving force of glutaminolysis in proliferating cells. Our model suggests that the Warburg effect may be a metabolic consequence secondary to the nitrogen anabolism.

Neuroprotective effect of CPDT on THA-induced cortical motor neuron death in an organotypic culture model

Brain stroke, trauma, and motor neuron disease each can result in cortical motoneuron (CMN) death or impairment. Glutamate excitotoxicity induces motor neuron damage in both acute motor neuron loss and chronic motor neuron degeneration. It is necessary to find effective strategies to protect CMNs from excitotoxicity in a variety of pathological conditions. 5,6-Dihydrocyclopenta-1,2-dithiole-3-thione (CPDT) is one of the phase II enzyme inducers. In our previous report, CPDT was shown to have neuroprotective effects on the spinal cord, by activating the Nrf2/ARE pathway to increase antioxidative capacity. In this study, in order to figure out whether CPDT can prevent CMN's from THA-induced death, we set up an organotypic brain slice culture system. Threo-hydroxyaspartate (THA), a glutamate transport inhibitor, was added to the culture medium to induce CMN death by glutamate excitotoxicity. Brain slices were pretreated with CPDT for 48h, then treated with CPDT and THA simultaneously for 3 weeks. We found that pretreatment with CPDT significantly increased CMN survival. Glutamate concentration in the culture medium was significantly greater following THA treatment, whereas no significant decrease was found in the CPDT pretreatment group. However, both Nrf2 and HO-1 protein expression was significantly elevated in the CPDT pretreatment group, and Nrf2 protein translocated to the nucleus after CPDT stimulation. These findings suggest that CPDT can protect CMNs from THA-induced motor neuron death by activating the Nrf2 pathway and increasing HO-1 protein expression. Therefore, increasing antioxidative defense capacity should benefit to upper motor neuron survival following a glutamate excitotoxicity insult.

Distribution of radiolabeled l-glutamate and d-aspartate from blood into peripheral tissues in naive rats: significance for brain neuroprotection

Excess l-glutamate (glutamate) levels in brain interstitial and cerebrospinal fluids (ISF and CSF, respectively) are the hallmark of several neurodegenerative conditions such as stroke, traumatic brain injury or amyotrophic lateral sclerosis. Its removal could prevent the glutamate excitotoxicity that causes long-lasting neurological deficits. As in previous studies, we have established the role of blood glutamate levels in brain neuroprotection, we have now investigated the contribution of the peripheral organs to the homeostasis of glutamate in blood. We have administered naive rats with intravenous injections of either l-[1-(14)C] Glutamic acid (l-[1-(14)C] Glu), l-[G-(3)H] Glutamic acid (l-[G-(3)H] Glu) or d-[2,3-(3)H] Aspartic acid (d-[2,3-(3)H] Asp), a non-metabolized analog of glutamate, and have followed their distribution into peripheral organs. We have observed that the decay of the radioactivity associated with l-[1-(14)C] Glu and l-[G-(3)H] Glu was faster than that associated with glutamate non-metabolized analog, d-[2,3-(3)H] Asp. l-[1-(14)C] Glu was subjected in blood to a rapid decarboxylation with the loss of (14)CO(2). The three major sequestrating organs, serving as depots for the eliminated glutamate and/or its metabolites were skeletal muscle, liver and gut, contributing together 92% or 87% of total l-[U-(14)C] Glu or d-[2,3-(3)H] Asp radioactivity capture. l-[U-(14)C] Glu and d-[2,3-(3)H] Asp showed a different organ sequestration pattern. We conclude that glutamate is rapidly eliminated from the blood into peripheral tissues, mainly in non-metabolized form. The liver plays a central role in glutamate metabolism and serves as an origin for glutamate metabolites that redistribute into skeletal muscle and gut. The findings of this study suggest now that pharmacological manipulations that reduce the liver glutamate release rate or cause a boosting of the skeletal muscle glutamate pumping rate are likely to cause brain neuroprotection.