In vivo potentiation of glutamate-mediated neuronal damage after chronic administration of the glycolysis inhibitor iodoacetate

Can a metabolism-targeted therapeutic intervention successfully subjugate SARS-COV-2? A scientific rational

As a process entailing a high turnover of the host cell molecules, viral replication is required for a successful viral infection and requests virus capacity to acquire the macromolecules required for its propagation. To this end, viruses have adopted several strategies to harness cellular metabolism in accordance with their specific demands. Most viruses upregulate specific cellular anabolic pathways and are largely dependent on such alterations. RNA viruses, for example, upregulate both glycolysisand glycogenolysis providing TCA cycle intermediates essential for anabolic lipogenesis. Also, these infections usually induce the PPP, leading to increased nucleotide levels supporting viral replication. SARS-CoV-2 (the cause of COVID-19)that has so far spread from China throughout the world is also an RNA virus. Owing to the more metabolic plasticity of uninfected cells, a promising approach for specific antiviral therapy, which has drawn a lot of attention in the recent years, would be the targeting of metabolic changes induced by viruses. In the current review, we first summarize some of virus-induced metabolic adaptations and then based on these information as well as SARS-CoV-2 pathogenesis, propose a potential therapeutic modality for this calamitous world-spreading virus with the hope of employing this strategy for near-future clinical application.

Opposing effects of 2-deoxy-d-glucose on interictal- and ictal-like activity when K+ currents and GABAA receptors are blocked in rat hippocampus in vitro

The ketogenic diet (KD), a high-fat, carbohydrate-restricted diet, is used as an alternative treatment for drug-resistant epileptic patients. Evidence suggests that compromised glucose metabolism has a significant role in the anticonvulsant action of the KD; however, it is unclear what part of the glucose metabolism that is important. The present study investigates how selective alterations in glycolysis and oxidative phosphorylation influence epileptiform activity induced by blocking K+ currents and GABAA and NMDA receptors in the hippocampal slice preparation. Blocking glycolysis with the glucose derivative 2-deoxy-d-glucose (2-DG; 10 mM) gave a fast reduction of the frequency of interictal discharge (IED) consistent with findings in other in vitro models. However, this was followed by the induction of seizure-like discharges in area CA1 and CA3. Substituting glucose with sucrose (glucopenia) had effects similar to those of 2-DG, whereas substitution with l-lactate or pyruvate reduced the IED but had a less proconvulsant effect. Blockade of ATP-sensitive K+ channels, glycine or adenosine 1 receptors, or depletion of the endogenous anticonvulsant compound glutathione did not prevent the actions of 2-DG. Baclofen (2 μM) reproduced the effect of 2-DG on IED activity. The proconvulsant effect of 2-DG could be reproduced by blocking the oxidative phosphorylation with the complex I toxin rotenone (4 μM). The data suggest that inhibition of IED, induced by 2-DG and glucopenia, is a direct consequence of impairment of glycolysis, likely exerted via a decreased recurrent excitatory synaptic transmission in area CA3. The accompanying proconvulsant effect is caused by an excitatory mechanism, depending on impairment of oxidative phosphorylation.

NEW & NOTEWORTHY This study reveals two opposing effects of 2-deoxy-d-glucose (2-DG) and glucopenia on in vitro epileptiform discharge observed during combined blockade of K+ currents and GABAA receptors. Interictal-like activity is inhibited by a mechanism that selectively depends on impairment of glycolysis and that results from a decrease in the strength of excitatory recurrent synaptic transmission in area CA3. In contrast, 2-DG and glucopenia facilitate ictal-like activity by an excitatory mechanism, depending on impairment of mitochondrial oxidative phosphorylation.

Disulfide Stress Targets Modulators of Excitotoxicity in Otherwise Healthy Brains

Oxidative stress is a long-hypothesized cause of diverse neurological and psychiatric disorders but the pathways by which physiological redox perturbations may detour healthy brain development and aging are unknown. We reported recently (Foley et al., Neurochem Res 39:2030-2039, 2014) that two-electron oxidations, to disulfides, of protein vicinal thiols can vary markedly in association with more modest oxidations of the glutathione redox couple in brains from healthy adolescent rats whereas levels of protein S-glutathionylation were low and unchanged. Here, we demonstrate that the selective oxidations of protein vicinal thiols, occurring only in the more oxidized brains under study, were linked specifically to a peroxide stress as evidenced by increased oxidations, to disulfides, of the presumed catalytic vicinal thiols of peroxiredoxins 1 and 2. Moreover, we identify the catalytic subunit(s) of Na⁺, K⁺-ATPase, tubulins, glyceraldehyde-3-phosphate dehydrogenase, and protein phosphatase 1, all of which can modulate glutamate neurotransmission and the vulnerability of neurons to excitotoxicity, as non-peroxidase proteins exhibiting prominent oxidations of vicinal thiols. The two-electron pathway, demonstrated here, linking physiological redox perturbations in otherwise healthy brains to protein determinants of excitotoxicity, suggests an alternative to free radical pathways by which oxidative stress may impact brain development and aging.

Neuronal damage and cognitive impairment associated with hypoglycemia: An integrated view

The aim of the present review is to offer a current perspective about the consequences of hypoglycemia and its impact on the diabetic disorder due to the increasing incidence of diabetes around the world. The main consequence of insulin treatment in type 1 diabetic patients is the occurrence of repetitive periods of hypoglycemia and even episodes of severe hypoglycemia leading to coma. In the latter, selective neuronal death is observed in brain vulnerable regions both in humans and animal models, such as the cortex and the hippocampus. Cognitive damage subsequent to hypoglycemic coma has been associated with neuronal death in the hippocampus. The mechanisms implicated in selective damage are not completely understood but many factors have been identified including excitotoxicity, oxidative stress, zinc release, PARP-1 activation and mitochondrial dysfunction. Importantly, the diabetic condition aggravates neuronal damage and cognitive failure induced by hypoglycemia. In the absence of coma prolonged and severe hypoglycemia leads to increased oxidative stress and discrete neuronal death mainly in the cerebral cortex. The mechanisms responsible for cell damage in this condition are still unknown. Recurrent moderate hypoglycemia is far more common in diabetic patients than severe hypoglycemia and currently important efforts are being done in order to elucidate the relationship between cognitive deficits and recurrent hypoglycemia in diabetics. Human studies suggest impaired performance mainly in memory and attention tasks in healthy and diabetic individuals under the hypoglycemic condition. Only scarce neuronal death has been observed under moderate repetitive hypoglycemia but studies suggest that impaired hippocampal synaptic function might be one of the causes of cognitive failure. Recent studies have also implicated altered mitochondrial function and mitochondrial oxidative stress.

Effect of glycolysis inhibition on mitochondrial function in rat brain

Inhibition of the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase enhances the neural vulnerability to excitotoxicity both in vivo and in vitro through an unknown mechanism possibly related to mitochondrial failure. However, as the effect of glycolysis inhibition on mitochondrial function in brain has not been studied, the aim of the present work was to evaluate the effect of glycolysis inhibition induced by iodoacetate on mitochondrial function and oxidative stress in brain. Mitochondria were isolated from brain cortex, striatum and cerebellum of rats treated systemically with iodoacetate (25 mg/kg/day for 3 days). Oxygen consumption, ATP synthesis, transmembrane potential, reactive oxygen species production, lipoperoxidation, glutathione levels, and aconitase activity were assessed. Oxygen consumption and aconitase activity decreased in the brain cortex and striatum, showing that glycolysis inhibition did not trigger severe mitochondrial impairment, but a slight mitochondrial malfunction and oxidative stress were present.

Glycolysis inhibition decreases the levels of glutamate transporters and enhances glutamate neurotoxicity in the R6/2 Huntington's disease mice

Excitotoxicity has been associated with the loss of medium spiny neurons (MSN) in Huntington's disease (HD). We have previously observed that the content of the glial glutamate transporters, glutamate transporter 1 (GLT-1) and glutamate-aspartate transporter (GLAST), diminishes in R6/2 mice at 14 weeks of age but not at 10 weeks, and that this change correlates with an increased vulnerability of striatal neurons to glutamate toxicity. We have also reported that inhibition of the glycolytic pathway decreases glutamate uptake and enhances glutamate neurotoxicity in the rat brain. We now show that at 10-weeks of age, glutamate excitotoxicity is precipitated in R6/2 mice, after the treatment with iodoacetate (IOA), an inhibitor of the glycolytic enzyme, glyceraldehyde-3-phosphate dehydrogenase (GAPDH). IOA induces a larger inhibition of GAPDH in R6/2 mice, while it similarly reduces the levels of GLT-1 and GLAST in wild-type and transgenic animals. Results suggest that metabolic failure and altered glutamate uptake are involved in the vulnerability of striatal neurons to glutamate excitotoxicity in HD.

Excitotoxic neuronal death and the pathogenesis of Huntington's disease

Huntington's disease (HD) is a neurodegenerative hereditary illness originated by the mutation of the gene encoding the huntingtin-protein (htt). Mutated htt (mhtt) is characterized by an increased number of glutamine repeats in the N-terminal end; when 40 or more glutamine residues are present, the disease is manifested. Expression of mhtt leads to the selective death of the medium spiny neurons (MSN) in the neostriatum, resulting in the appearance of generalized involuntary movements, the main phenotypic alteration of HD. The relationship between the expression of mhtt and the death of the MSN is not fully understood. Nonetheless, according to experimental evidence indicating that MSN are selectively vulnerable to the toxicity of glutamate (excitotoxicity) or its analogues, excitotoxic neuronal death is suggested to be involved in neurodegeneration associated with HD. Support for this hypothesis comes from studies in HD postmortem tissue and transgenic mice models, suggesting a correlation between mhtt expression and altered glutamatergic neurotransmission, mainly altered conductance of the N-methyl-D-aspartate (NMDA) glutamate receptor subtype and decreased levels of glutamate transporters. On the other hand, alterations in energy metabolism are well documented in HD patients, which might facilitate excitotoxicity. Throughout this review we will discuss relevant evidence suggesting that altered glutamatergic neurotransmission plays a role in neurodegeneration associated with HD, as well as the possible contribution of deficient energy metabolism to the development of an excitotoxic cell death cascade in MSN. We show data supporting protection by energy substrates against neuronal damage in a rat model combining energy deficit and glutamate toxicity.

A role for excitatory amino acids in diabetic eye disease

Diabetic retinopathy is a leading cause of vision loss. The primary clinical hallmarks are vascular changes that appear to contribute to the loss of sight. In a number of neurodegenerative disorders there is an appreciation that increased levels of excitatory amino acids are excitotoxic. The primary amino acid responsible appears to be the neurotransmitter glutamate. This review examines the nature of glutamatergic signaling at the retina and the growing evidence from clinical and animal model studies that glutamate may be playing similar excitotoxic roles at the diabetic retina.

Dopamine receptor activation reveals a novel, kynurenate-sensitive component of striatal N-methyl-D-aspartate neurotoxicity

The N-methyl-d-aspartate (NMDA) subtype of glutamate receptors plays an important role in brain physiology, but excessive receptor stimulation results in seizures and excitotoxic nerve cell death. NMDA receptor-mediated neuronal excitation and injury can be prevented by high, non-physiological concentrations of the neuroinhibitory tryptophan metabolite kynurenic acid (KYNA). Here we report that endogenous KYNA, which is formed in and released from astrocytes, controls NMDA receptors in vivo. This was revealed with the aid of the dopaminergic drugs d-amphetamine and apomorphine, which cause rapid, transient decreases in striatal KYNA levels in rats. Intrastriatal injections of the excitotoxins NMDA or quinolinate (but not the non-NMDA receptor agonist kainate) at the time of maximal KYNA reduction resulted in two- to threefold increases in excitotoxic lesion size. Pre-treatment with a kynurenine 3-hydroxylase inhibitor or with dopamine receptor antagonists, i.e., two classes of pharmacological agents that prevented the reduction in brain KYNA caused by dopaminergic stimulation, abolished the potentiation of neurotoxicity. Thus, the present study identifies a previously unappreciated role of KYNA as a functional link between dopamine receptor stimulation and NMDA neurotoxicity in the striatum.

Sustained metabolic inhibition induces an increase in the content and phosphorylation of the NR2B subunit of N-methyl-d-aspartate receptors and a decrease in glutamate transport in the rat hippocampus in vivo

The concentration of glutamate is regulated to ensure neurotransmission with a high temporal and local resolution. It is removed from the extracellular medium by high-affinity transporters, dependent on the maintenance of the Na(+) gradient through the activity of Na(+),K(+)-ATPases. Failure of glutamate clearance can lead to neuronal damage, named excitotoxic damage, due to the prolonged activation of glutamate receptors. Severe impairment of glycolytic metabolism during ischemia and hypoglycemia, leads to glutamate transport dysfunction inducing the elevation of extracellular glutamate and aspartate, and neuronal damage. Altered glucose metabolism has also been associated with some neurodegenerative diseases such as Alzheimer's and Huntington's, and a role of excitotoxicity in the neuropathology of these disorders has been raised. Alterations in glutamate transporters and N-methyl-D-aspartate (NMDA) receptors have been observed in these patients, suggesting altered glutamatergic neurotransmission. We hypothesize that inhibition of glucose metabolism might induce changes in glutamatergic neurotransmission rendering neurons more vulnerable to excitotoxicity. We have previously reported that sustained glycolysis impairment in vivo induced by inhibition of glyceraldehyde 3-phosphate dehydrogenase (GAPDH), facilitates glutamate-mediated neuronal damage. We have now investigated whether this facilitating effect involves altered glutamate uptake, and/or NMDA receptors in the rat hippocampus in vivo. Results indicate that metabolic inhibition leads to the progressive elevation of extracellular glutamate and aspartate levels in the hippocampus, which correlates with decreased content of the GLT-1 glutamate transporter and diminished glutamate uptake. In addition, we observed increased Tyr(1472) phosphorylation and protein content of the NR2B subunit of the NMDA receptor. Results suggest that moderate sustained glycolysis inhibition alters glutamatergic neurotransmission.

Hypoxia, drug therapy and toxicity

Hypoxia is defined as a decrease in available oxygen reaching the tissues of the body. It is linked to the pathology of cancer, cardiovascular disease, and stroke, the leading causes of death in the United States. Cells under hypoxic stress either induce an adaptive response that includes increasing the rates of glycolysis and angiogenesis or undergo cell death by promoting apoptosis or necrosis. The ability of cells to maintain a balance between adaptation and cell death is regulated by a family of transcription factors called the hypoxia inducible factors (HIF). HIF1, the most widely studied HIF, is essential for regulating the expression of a battery of hypoxia-responsive genes involved in the adaptive and cell death responses. The ability of HIF1 to balance these 2 responses likely lies in the regulation of HIF1alpha stability and transcriptional activity by post-translational hydroxylation and its ability to respond to other cellular factors including key metabolites and growth factors. Targeting HIF1 signaling for therapeutics, therefore, requires an understanding of how these various signals converge upon HIF1 and regulate its role in maintaining the balance between adaptation and cell death. In addition, one must understand how this balance can be perturbed during toxicant-induced tissue damage. This review will summarize our current understanding of hypoxia signaling as it applies to drug therapy and toxicity and describe how these processes can influence the HIF-mediated balance between adaptation and cell death.

D-beta-hydroxybutyrate prevents glutamate-mediated lipoperoxidation and neuronal damage elicited during glycolysis inhibition in vivo

Excitotoxic neuronal death mediated by over-activation of glutamate receptors has been implicated in ischemia, hypoglycemia and some neurodegenerative diseases. It involves oxidative stress and is highly facilitated during impairment of energy metabolism. We have shown previously that in vivo systemic glycolysis inhibition with iodoacetate (IOA), exacerbates glutamate excitotoxicity. We have now investigated whether this effect involves oxidative damage to membrane lipids, as evaluated by the presence of thiobarbituric acid-reactive substances. We have also tested whether the ketone body, D-beta-hydroxybutyrate (D-BHB), prevents lipoperoxidation and tissue damage. Results show that glutamate intrastriatal injection in control rats transiently enhances lipoperoxidation, while in IOA-treated animals increased lipoperoxidation is sustained. Treatment with D-BHB significantly reduces striatal lesions and lipoperoxidation. Vitamin E also reduced neuronal damage and lipoperoxidation. Results suggest that glycolysis impairment favors a pro-oxidant condition and situates oxidative damage as an important mediator of in vivo induced excitotoxicity. Results provide evidence for the neuroprotective effect of D-BHB against glutamate toxicity.

The anion channel blocker, 4,4′-dinitrostilbene-2,2′-disulfonic acid prevents neuronal death and excitatory amino acid release during glycolysis inhibition in the hippocampus in vivo

Neuronal death associated with cerebral ischemia and hypoglycemia is related to increased release of excitatory amino acids (EAA) and energy failure. The intrahippocampal administration of the glycolysis inhibitor, iodoacetate (IOA), induces the accumulation of EAA and neuronal death. We have investigated by microdialysis the role of exocytosis, glutamate transporters and volume-sensitive organic anion channel (VSOAC) on IOA-induced EAA release. Results show that the early component of EAA release is inhibited by riluzole, a voltage-dependent sodium channel blocker, and by the VSOAC blocker, tamoxifen, while the early and late components are blocked by the glutamate transport inhibitors, L-trans-pyrrolidine 2,4-dicarboxylate (PDC) and DL-threo-beta-benzyloxyaspartate (DL-TBOA); and by the VSOAC blocker 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS). Riluzole, DL-TBOA and tamoxifen did not prevent IOA-induced neuronal death, while PDC and DNDS did. The VSOAC blockers 5-nitro-2-(3-phenylpropyl-amino) benzoic acid (NPPB) and phloretin had no effect either on EAA efflux or neuronal damage. Results suggest that acute inhibition of glycolytic metabolism promotes the accumulation of EAA by exocytosis, impairment or reverse action of glutamate transporters and activation of a DNDS-sensitive mechanism. The latest is substantially involved in the triggering of neuronal death. To our knowledge, this is the first study to show protection of neuronal death by DNDS in an in vivo model of neuronal damage, associated with deficient energy metabolism and EAA release, two conditions involved in some pathological states such as ischemia and hypoglycemia.

Role of glutamate transporters in the clearance and release of glutamate during ischemia and its relation to neuronal death

Glutamate neurotransmitter action on postsynaptic receptors is terminated by its clearance from the synaptic cleft by transporter proteins located in neurons and glial cells. Failure of glutamate removal can lead to neuronal death due to its well-known neurotoxic properties. Glutamate transporters are dependent on external Na+, and thus on the activity of Na+/K+ ATPases, which maintain the Na+ concentration gradient. When the energy brain requirements are not fulfilled by the appropriate blood supply of glucose and oxygen, the Na+ gradient collapses leading to impaired glutamate and aspartate removal, or even to the release of these amino acids through the reverse operation of their transporters. Such a scenario would be associated with brain ischemia and hypoglycemia due to the prompt decline in ATP levels. In addition, some evidence suggests that downregulation of glutamate transporters after the ischemic period, or the dysfunction induced by oxidation, contributes to the accumulation of extracellular glutamate and neuronal death. Neuronal damage is associated with excitotoxicity, a type of cell death triggered by the overactivation of glutamate receptors and the loss of calcium homeostasis. Throughout this review we will discuss recent evidence suggesting that failure of glutamate transport during ischemia contributes to the elevation of extracellular glutamate and to the induction of excitotoxicity. We will also discuss the contribution of glial vs. neuronal glutamate transporters in ischemic damage, and the involvement of the different glutamate transporter subtypes. We will focus on experimental data from rodent models, because many of the studies on glutamate transport and ischemic damage have been performed in these animal species.

Disruption of endoplasmic reticulum calcium stores is involved in neuronal death induced by glycolysis inhibition in cultured hippocampal neurons

Disturbances in neuronal calcium homeostasis have been implicated in a variety of neuropathological conditions, including cerebral ischemia, hypoglycemia, and epilepsy, and possibly constitute part of the cell death process associated with chronic neurodegenerative disorders. We investigated if endoplasmic reticulum (ER) calcium stores participate in neuronal death triggered by moderate glycolysis inhibition induced by iodoacetate, an inhibitor of glyceraldehyde-3-phosphate dehydrogenase, in cultured hippocampal neurons. Results show that exposure to iodoacetate leads to a slow partial decrease in cell survival, which is significantly prevented in the absence of Ca(2+) or in the presence of the calcium chelator BAPTA-AM. Treatment with caffeine and a low (1 microM) concentration of ryanodine, which activates the ryanodine receptor (RyR), exacerbates neuronal death, whereas dantrolene and 25 microM ryanodine, which antagonizes RyR, prevents damage. Xestospongin C (XeC), an antagonist of the inositol-3-phosphate (IP(3)) receptor (IP(3)R) also prevents neuronal damage. Inhibitors of the ER calcium ATPase (sarcoendoplasmic reticulum Ca(2+) ATPase; SERCA) have no effect. The decrease in ATP levels induced by iodoacetate is potentiated by caffeine and prevented by dantrolene. Although only a slight increase in glutamate extracellular levels is observed 3.5 hr after iodoacetate exposure, the N-methyl-D-aspartate (NMDA) glutamate receptor antagonist, MK-801, efficiently prevents neuronal damage. Taken together, the data suggest that neuronal death induced during moderate glycolysis inhibition involves calcium influx through NMDA receptors and calcium release from intracellular ER stores. These results might be relevant to the understanding the mechanisms involved in neuronal damage related to aging and chronic neurodegenerative diseases, which have been associated with decreased glucose metabolism.

Hippocampal infusions of pyruvate reverse the memory-impairing effects of septal muscimol infusions

Hippocampal infusions of glucose reverse memory deficits in spontaneous alternation and in a continuous multiple trial inhibitory avoidance task. The current experiments tested whether glucose metabolism may participate in these effects of glucose. Specifically, these experiments determined whether the glycolytic metabolite pyruvate would mimic these effects of glucose. Male Sprague-Dawley rats were given septal infusions of vehicle or the gamma-aminobutyric acid (GABA) receptor agonist muscimol (0.15 nmol for spontaneous alternation or 5 nmol for continuous multiple trial inhibitory avoidance) combined with hippocampal infusions of vehicle or pyruvate (200 nmol) 15 min prior to assessing spontaneous alternation or training in a continuous multiple trial inhibitory avoidance task. The infusions of muscimol decreased percent alternation scores and continuous multiple trial inhibitory avoidance retention latencies tested 48 h after training. More importantly, hippocampal infusions of pyruvate reversed the deficits produced by septal infusions of muscimol on both tasks. These findings show for the first time that hippocampal infusions of pyruvate influence memory and suggest that glucose may affect memory via glycolytic metabolism.

Tf-lipoplex-mediated NGF gene transfer to the CNS: neuronal protection and recovery in an excitotoxic model of brain injury

The development of efficient systems for in vivo gene transfer to the central nervous system (CNS) may provide a useful therapeutic strategy for the alleviation of several neurological disorders. In this study, we evaluated the feasibility of nonviral gene therapy to the CNS mediated by cationic liposomes. We present evidence of the successful delivery and expression of both a reporter and a therapeutic gene in the rodent brain, as evaluated by immunohistochemical assays. Our results indicate that transferrin-associated cationic liposome/DNA complexes (Tf-lipoplexes) allow a significant enhancement of transfection activity as compared to plain complexes, and that 8/1 (+/-) Tf-lipoplexes constitute the best formulation to mediate in vivo gene transfer. We demonstrated that Tf-lipoplex-mediated nerve growth factor transgene expression attenuates the morphological damages of the kainic acid-induced lesion as assessed by 2,3,5-triphenyltetrazolium chloride (TTC) vital staining. These findings suggest the usefulness of these lipid-based vectors in mediating the delivery of therapeutic genes to the CNS.

Differential effects of the substrate inhibitor l-trans-pyrrolidine-2,4-dicarboxylate (PDC) and the non-substrate inhibitor DL-threo-beta-benzyloxyaspartate (DL-TBOA) of glutamate transporters on neuronal damage and extracellular amino acid levels in rat brain in vivo

The extracellular concentration of glutamate is highly regulated by transporter proteins, due to its neurotoxic properties. Dysfunction or reverse activation of these transporters is related to the extracellular accumulation of excitatory amino acids and neuronal damage associated with ischemia and hypoglycemia. We have investigated by microdialysis the effects of the substrate and the non-substrate inhibitors of glutamate transporters, l-trans-2,4-pyrrolidine dicarboxylate (PDC) and DL-threo-beta-benzyloxyaspartate (DL-TBOA), respectively, on the extracellular levels of amino acids in the rat hippocampus in vivo. In addition, we have studied the effect of both inhibitors on neuronal damage after direct administration into the hippocampus and striatum. Electroencephalographic activity was recorded after the intrahippocampal infusion of DL-TBOA or PDC. Microdialysis administration of 500 microM DL-TBOA into the hippocampus increased 3.4- and nine-fold the extracellular levels of aspartate and glutamate, respectively. Upon stereotaxic administration it induced neuronal damage dose-dependently in CA1 and dentate gyrus, and convulsive behavior. Electroencephalographic recording showed the appearance of limbic seizures in the hippocampus after DL-TBOA infusion. In the striatum it also induced dose-dependent neuronal damage. These effects were prevented by the i.p. administration of the glutamate receptor antagonists (+)-5-methyl-10,11-dihydroxy-5H-dibenzo(a,d)cyclohepten-5,10-iminemaleate and 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)-quinoxaline. In contrast to dl-TBOA, PDC (500 microM) induced a more discrete elevation of excitatory amino acids levels (2.6- and three-fold in aspartate and glutamate, respectively), no neuronal damage or behavioral changes, and no alterations in electroencephalographic activity. The differential results obtained with DL-TBOA and PDC might be attributed to their distinct effects on the extracellular concentration of amino acids. Results are relevant to the understanding of the role of glutamate transporters in amino acid removal or release and the induction of excitotoxic cell death.

N-PEP-12 – a novel peptide compound that protects cortical neurons in culture against different age and disease associated lesions

The neuroprotective potency of N-PEP-12, a novel, proprietary compound consisting of biopeptides and amino acids was investigated. Lesion models have been applied in neuronal cultures of embryonic chicken cortex, pre-treated with N-PEP-12 from the first day onwards. On day 8 in vitro neurons were lesioned and cell viability was measured 24 and 48 hours later. To simulate acute brain ischemia, cytotoxic hypoxia was induced by sodium cyanide or by iodoacetate and excitotoxicity by L-glutamate. Ionomycin for up to 48 hours induced calcium overload. The cytoskeleton was disrupted by addition of colchicine. N-PEP-12 shows dose-dependent neuroprotection in all different models. The effect size depends on the recovery time but also on the extent of the lesion. In cases of mild to moderate lesion pronounced dose-dependent effects could be demonstrated. This indicates that chronic exposure to N-PEP-12 is able to prevent neuronal cell death associated to conditions occurring during normal aging and neurological disorders like ischemic stroke, hypoxia, brain trauma, or AD.

Increased mitochondrial respiration maintains the mitochondrial membrane potential and promotes survival of cerebellar neurons in an endogenous model of glutamate receptor activation

It is thought that the combination of extracellular glutamate accumulation and mitochondrial damage is involved in neuronal death associated with brain ischemia and hypoglycemia, and some neurodegenerative diseases such as Huntington's disease. However, the mechanism whereby those two factors interact together to trigger neurodegeneration in this and other neurodegenerative disorders is still elusive. Here, we have addressed this issue using a model of mild and sustained accumulation of extracellular glutamate in cerebellar cultured neurons, which are mostly glutamatergic and commonly used to study glutamate neurotoxicity. The resulting stimulation of glutamate receptors triggered a approximately 50% persistent increase in mitochondrial respiration that was associated with free radicals formation, and which was found to be necessary to prevent the collapse of the mitochondrial membrane potential (Deltapsim) and apoptotic cell death. In fact, hampering the glutamate-mediated increase in mitochondrial respiration with an inhibitor of the mitochondrial respiratory chain stopped neurons from producing free radicals, but led them to undergo rapid and profound Deltapsim collapse and apoptotic cell death. Thus, we suggest that the formation of reactive oxygen species by glutamate receptor activation is the unavoidable consequence of an increase in the mitochondrial respiration aimed to prevent Deltapsim collapse and neurodegeneration. These results may be relevant to understand the pathophysiology of those neurodegenerative diseases associated with both mitochondrial respiratory chain and glutamate transporter defects.

Provocation of kainic acid receptor mRNA changes in the rat paraventricular nucleus by insulin-induced hypoglycaemia

Hypoglycaemia induced by insulin injection is a powerful stimulus to the hypothalamic-pituitary-adrenal (HPA) axis and drives the secretion of corticotropin-releasing hormone and vasopressin from the neurones in the paraventricular nucleus (PVN), as well as the downstream hormones, adrenocorticotropic hormone and corticosterone. In some brain regions, hypoglycaemia also provokes increases in extracellular fluid concentrations of glutamate. Regulation of glutamatergic mechanisms could be involved in the control of the HPA axis during hypoglycaemic stress and one potential site of regulation might be at the receptors for glutamate, which are expressed in the PVN. Insulin (2.0 IU/kg, i.p.) or saline was administered to adult male Sprague-Dawley rats and the animals were sacrificed 30 min, 180 min and 24 h after injection. The amount of several kainic acid-preferring glutamate receptor mRNAs (i.e. KA2, GluR5 and GluR6) were assessed in the PVN by in situ hybridisation histochemistry. Injection of insulin induced a rapid fall in plasma glucose concentrations, which was mirrored by an increase in plasma corticosterone concentrations. KA2 and GluR5 mRNAs are highly expressed within the rat PVN, and responded to hypoglycaemia with robust increases in expression that endured beyond the period of hypoglycaemia itself. However, GluR6 mRNA is expressed in the areas adjacent to the PVN and hypoglycaemic stress failed to alter expression of this mRNA. These experiments suggest that kainic acid-preferring glutamate receptors are responsive to changes in plasma glucose concentrations and may participate in the activation of the PVN neurones during hypoglycaemic stress.

Glutamate uptake inhibitor L-trans-pyrrolidine 2,4-dicarboxylate becomes neurotoxic in the presence of subthreshold concentrations of mitochondrial toxin 3-nitropropionate: involvement of mitochondrial reducing activity and ATP production

An increased concentration of extracellular glutamate is associated with neuronal damage induced by cerebral ischemia. We have demonstrated previously that exposure of cultured cerebellar granule neurons to L-trans-pyrrolidine-2,4-dicarboxylate (PDC), a glutamate uptake inhibitor, increases extracellular glutamate levels but does not induce neuronal damage. Coincubation of PDC, however, with a subthreshold concentration of the mitochondrial toxin, 3-nitropropionic acid (3-NP), results in severe damage to these neurons. We have investigated the time course of changes in mitochondrial reducing capacity and ATP levels in cerebellar granule cells after simultaneous exposure to 3-NP and PDC, and its relation to cell viability and nuclear condensation. Although individually, 3-NP and PDC treatments are not harmful to neurons, the simultaneous exposure to both compounds results in a progressive decline in mitochondrial reducing capacity during the first 4 hr, and a rapid decrease in ATP levels. At 4 hr, cells lose plasma membrane integrity and show condensed nuclei. In the presence of the energy substrates pyruvate and acetoacetate, the N-methyl-D-apartate (NMDA) receptor antagonist, MK-801, and the spin trapper alpha-phenyl-N-tert-butylnitrone (PBN), the decline in mitochondrial activity and ATP levels is prevented, the number of condensed nuclei is reduced, and plasma membrane integrity is preserved. In contrast, the broad-spectrum caspase inhibitor Z-Asp-DCB (Z-Asp-CH2-DCB) prevents nuclear condensation but has no effect on mitochondrial reducing capacity or cell survival. Our results show that glutamate uptake impairment rapidly induces neuronal death during inhibition of succinate dehydrogenase by a mechanism involving mitochondrial dysfunction that, if not prevented, leads to cell death.

Implication of ionotropic glutamate receptors in the release of noradrenaline in hippocampal CA1 and CA3 subregions under oxygen and glucose deprivation

A strong linkage between adrenergic and glutamatergic systems exists in the CNS but it is still unclear whether the excessive release of noradrenaline under ischemic conditions is modulated by excitatory amino acids. We studied the effect of selective glutamate receptor antagonists on the release of [3H]noradrenaline evoked by glucose and oxygen deprivation in hippocampal CA1, CA3 and dentate gyrus subregions. The release of glutamate, aspartate and GABA was measured by HPLC. Omission of oxygen and glucose increased the release of [3H]noradrenaline as well as the release of amino acids. Maximum effect on noradrenaline release was observed in CA1 region. The relative increase of the release after 30 min energy deprivation (R(2)) versus the basal release under normal conditions (R(1)), i.e. the R(2)/R(1) ratio was 7.1+/-1.0, 3.87+/-0.4 and 3.26+/-0.27 for CA1, CA3 and dentate gyrus, respectively. The [3H]noradrenaline outflow in response to glucose and oxygen deprivation was abolished at low temperature, but not by Ca(2+) removal, suggesting a cytoplasmic release process. In CA1 and CA3 [3H]noradrenaline release was significantly attenuated by MK-801, an NMDA receptor antagonist. The AMPA receptor antagonist GYKI-53784 had no effect in CA3, but partly reduced noradrenaline release in CA1. Our results suggest that ionotropic glutamate receptors seem to be implicated in the massive cytoplasmic release of noradrenaline in CA1 what may contribute to its selective vulnerability.

Acetoacetate protects hippocampal neurons against glutamate-mediated neuronal damage during glycolysis inhibition

Glucose is the main substrate that fulfills energy brain demands. However, in some circumstances, such as diabetes, starvation, during the suckling period and the ketogenic diet, brain uses the ketone bodies, acetoacetate and beta-hydroxybutyrate, as energy sources. Ketone body utilization in brain depends directly on its blood concentration, which is normally very low, but increases substantially during the conditions mentioned above. Glutamate neurotoxicity has been implicated in neurodegeneration associated with brain ischemia, hypoglycemia and cerebral trauma, conditions related to energy failure, and to elevation of glutamate extracellular levels in brain. In recent years substantial evidence favoring a close relation between glutamate neurotoxic potentiality and cellular energy levels, has been compiled. We have previously demonstrated that accumulation of extracellular glutamate after inhibition of its transporters, induces neuronal death in vivo during energy impairment induced by glycolysis inhibition. In the present study we have assessed the protective potentiality of the ketone body, acetoacetate, against glutamate-mediated neuronal damage in the hippocampus of rats chronically treated with the glycolysis inhibitor, iodoacetate, and in hippocampal cultured neurons exposed to a toxic concentration of iodoacetate. Results show that acetoacetate efficiently protects against glutamate neurotoxicity both in vivo and in vitro probably by a mechanism involving its role as an energy substrate.

beta-Amyloid neurotoxicity is exacerbated during glycolysis inhibition and mitochondrial impairment in the rat hippocampus in vivo and in isolated nerve terminals: implications for Alzheimer's disease

Senile plaques composed mainly by beta-amyloid (Abeta) protein are one of the pathological hallmarks of Alzheimer's disease (AD). In vitro, Abeta and its active fragment 25-35 have been shown either to be directly neurotoxic or to exacerbate the damaging effect of other neurotoxic insults. However, the attempts to replicate Abeta neurotoxicity in vivo have yielded conflicting results. One of the most consistent alterations in AD is a reduced resting glucose utilization. Important evidence suggests that impairment of brain energy metabolism can lead to neuronal damage or facilitate the deleterious effects of some neurotoxic agents. In the present study we have investigated the influence of glycolysis inhibition induced by iodoacetate, and mitochondrial impairment induced by 3-nitropropionic acid (3-NP), in the toxicity of Abeta. We have studied Abeta neurotoxicity during energy deficiency both in vivo in the dentate gyrus of the hippocampal formation and in presynaptic terminals isolated from neocortex and hippocampus. Results show that during metabolic inhibition an enhanced vulnerability of hippocampal neurons to Abeta peptide toxicity occurs, probably resulting from decreased glucose metabolism and mitochondrial ATP production. Synaptosomal response to energy impairment and Abeta toxicity was evaluated by the MTT assay. Results suggest that synapses may be particularly sensitive to metabolic perturbation, which in turn exacerbates Abeta toxicity. The present data provide experimental support to the hypothesis that certain risk factors such as metabolic dysfunction and amyloid accumulation may interact to exacerbate AD, and that metabolic substrates such as pyruvate may play a role as a therapeutic tool.

Neurotoxicity of glutamate uptake inhibition in vivo: correlation with succinate dehydrogenase activity and prevention by energy substrates

Impairment of glutamate uptake or the reverse action of its transporters has been suggested as the mechanism responsible for the increased glutamate extracellular levels associated with ischemic neuronal damage. In previous studies we have shown that glutamate uptake inhibition by L-trans-pyrrolidine-2,4-dicarboxylate (PDC) in the rat striatum and hippocampus in vivo does not induce neuronal death despite the notable increase in the extracellular levels of glutamate and aspartate. However, PDC intracerebral administration leads to neuronal death in rats chronically injected with the mitochondrial toxin 3-nitropropionic acid (3-NP), an inhibitor of succinate dehydrogenase (SDH). In the present study we have determined the time course of inhibition of SDH activity in the striatum of rats acutely injected with a single dose of 3-NP (20 mg/kg), and studied its relation to PDC neurotoxicity. PDC induced larger lesions when administered during maximum inhibition of SDH activity while smaller lesions were found when it was injected during recovery of enzyme activity. We also studied the neuroprotective effect of different energy substrates such as creatine, pyruvate, and the ketone bodies beta-hydroxybutyrate and acetoacetate in this experimental model. Our results show partial protection with all compounds except for beta-hydroxybutyrate that showed no protection, while MK-801 completely prevented PDC-induced neuronal damage. We believe that the present results might be of relevance for the understanding of the mechanisms responsible for ischemic neuronal death and its prevention.

Strategies for neuroprotection against L-trans-2,4-pyrrolidine dicarboxylate-induced neuronal damage during energy impairment in vitro

Increased levels of extracellular excitatory amino acids and failure of energy metabolism are two conditions associated with brain ischemia. In the present study we have combined the simultaneous inhibition of glutamate uptake and mitochondrial electron transport chain to simulate neuronal damage associated with brain ischemia. Results show that cerebellar granule neurons are not vulnerable to transient glutamate uptake inhibition by L-trans-pyrrolidine-2,4-dicarboxylate (PDC) despite the increase in the extracellular concentration of glutamate, unless they are simultaneously exposed to the mitochondrial toxins 3-nitropropionic acid (3-NP) or sodium azide. Cell damage was assessed by light microscopy observation, by reduction of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), and by the fluorescent markers for live and dead cells, calcein and ethidium homodimer, respectively. The protective effect of alternative energy substrates, such as pyruvate, acetoacetate, and beta-hydroxybutyrate against PDC-induced neuronal death during 3-NP exposure was studied and compared to the effects of the antioxidant vitamin E, the spin trapper alpha-phenyl-N-tert-butylnitrone (PBN), voltage-dependent calcium channel antagonists, and glutamate receptor antagonists. Results show that neuronal damage can be efficiently prevented in the presence of pyruvate and the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801, whereas the non-NMDA receptor antagonist NBQX, acetoacetate, vitamin E, and PBN showed partial protection. In contrast, beta-hydroxybutyrate and voltage-dependent calcium channels blockers did not show any protective effect at the concentrations tested.