Potentiation of 45Ca uptake and acute toxicity mediated by the N-methyl-D-aspartate receptor: the effect of metal binding agents and transition metal ions

Zinc offers splenic protection through suppressing PERK/IRE1-driven apoptosis pathway in common carp (Cyprinus carpio) under arsenic stress

Arsenic (As) occurs naturally and concentrations in water bodies can reach high levels, leading to accumulation in vital organs like the spleen. Being an important organ in immune response and blood development processes, toxic effects of As on the spleen could compromise immunity and cause associated disorders in affected individuals. Splenic detoxification is key to improving the chances of survival but relatively little is known about the mechanisms involved. Essential trace elements like zinc have shown immune-modulatory effects humans and livestock. This study aimed to investigate the mechanisms involved in As-induced splenic toxicity in the common carp (Cyprinus carpio), and the protective effects of zinc (Zn). Our findings suggest that environmental exposure to As caused severe histological injuries and Ca²⁺ accumulation in the spleen of common carp. Additionally, transcriptional and translational profiles of endoplasmic reticulum stress, apoptosis and autophagy-related genes of the spleen showed upward trends under As toxicity. Treatment with Zn appears to offer protection against As-induced splenic injury in common carp and the pathologic changes above were alleviated. Our results provide additional insight into the mechanism of As toxicity in common carp while elucidating the role of Zn, a natural immune-modulator, as a potential antidote against As poisoning.

Zinc Modulates Nanosilver-Induced Toxicity in Primary Neuronal Cultures

Silver nanoparticles (NAg) have recently become one of the most commonly used nanomaterials. Since the ability of nanosilver to enter the brain has been confirmed, there has been a need to investigate mechanisms of its neurotoxicity. We previously showed that primary neuronal cultures treated with nanosilver undergo destabilization of calcium homeostasis via a mechanism involving glutamatergic NMDA receptors. Considering the fact that zinc interacts with these receptors, the aim of the present study was to examine the role of zinc in mechanisms of neuronal cell death in primary cultures. In cells treated with nanosilver, we noted an imbalance between extracellular and intracellular zinc levels. Thus, the influence of zinc deficiency and supplementation on nanosilver-evoked cytotoxicity was investigated by treatment with TPEN (a chelator of zinc ions), or ZnCl₂, respectively. Elimination of zinc leads to complete death of nanosilver-treated CGCs. In contrast, supplementation with ZnCl₂ increases viability of CGCs in a dose-dependent manner. Addition of zinc provided protection against the extra/intracellular calcium imbalance in a manner similar to MK-801, an antagonist of NMDA receptors. Zinc chelation by TPEN decreases the mitochondrial potential and dramatically increases the rate of production of reactive oxygen species. Our results indicate that zinc supplementation positively influences nanosilver-evoked changes in CGCs. This is presumed to be due to an inhibitory effect on NMDA-sensitive calcium channels.

Is peptic ulcer disease a risk factor of postherpetic neuralgia in patients with herpes zoster?

Postherpetic neuralgia is the most common complication of herpes zoster which is caused by a reactivation of latent varicella zoster virus. The pathogenesis of postherpetic neuralgia may involve peripheral and central mechanisms. Reported risk factors for postherpetic neuralgia include female gender, old age, diminished cell-mediated immunity and nutritional deficiencies. Based on our clinical observation which revealed that peptic ulcer disease (PUD) is one of the common comorbidities in patients with postherpetic neuralgia, we hypothesize that herpes zoster patients with PUD may be at a greater risk for the development of postherpetic neuralgia due to their impaired cellular immunity and depressed nutritional status. Major causes of PUD include Helicobacter pylori infection and usage of ulcerogenic medications. Patients with H. pylori infection may develop T cell dysfunctions and nutritional deficiencies including vitamin C, iron, cobalamin, carotenes and alpha-tocopherol. Ulcerogenic medications such as nonsteroidal anti-inflammatory drugs and steroids have been found not only to be ulcerogenic but also immunosuppressive to T cells. In addition, usage of steroids and nonsteroidal anti-inflammatory drugs may cause deficiencies of alpha-tocopherol, carotenes, cobalamin, iron, zinc and vitamin C. Vitamin C, carotenes and alpha-tocopherol are anti-inflammatory and the major oxidant scavengers in the aqua phase and biomembranes. Deficiencies of these nutrients may induce dysregulated inflammation and oxidative damage leading to neuropathic pain in patients with herpes zoster. Furthermore, nutrient deficiencies including zinc, iron, cobalamin and vitamin C are associated with dysregulation of Ca(v)3.2 T-channels and N-methyl-D-aspartate receptors, upregulation of nitric oxide synthase, the increase of nitric oxide formation and dysfunction of central norepinephrine inhibitory pain pathway. Prospective cohort studies are suggested to test the hypothesis. We further propose that a follow-up study that contains two groups of herpes zoster patients, i.e., with or without gastroendoscopy-proven PUD, be conducted to determine their incidence of postherpetic neuralgia. In addition, despite of the high proportion of zoster patients having been treated with antiviral therapies, prevention and treatment of postherpetic neuralgia remain challenging in clinical practice. The potential risk of postherpetic neuralgia in zoster patients with PUD could mean that physicians need to pay more attention to the comorbidity--PUD in patients with herpes zoster and treat PUD earlier in order to prevent the development of postherpetic neuralgia.

The proteomic analysis of mouse choroid plexus secretome reveals a high protein secretion capacity of choroidal epithelial cells

Choroid plexuses (CP) are involved in multiple functions related to their unique architecture and localization at the interface between the blood and cerebrospinal fluid compartments. These include the release by choroidal epithelial cells (CEC) of biologically active molecules, such as polypeptides, which are distributed globally to the brain. Here, we have used a proteomic approach to get an unbiased overview of the proteins that are secreted by primary cultures enriched in epithelial cells from mice CP. We identified a total of 43 proteins secreted through the classical vesicular pathway in CEC -conditioned medium. They include transport proteins, collagen subunits and other cell matrix proteins, proteases, protease inhibitors and neurotrophic factors. Treating CEC cultures with lipopolysaccharide, increased the secretion of four protein species and induced the release of two additional proteins. Our study also reveals a higher protein secretion capacity of CECs compared with other CP cells or cultured astrocytes. In conclusion, this study provides for the first time the characterization of the major proteins that are secreted by CECs. These proteins may play a critical role in neuronal growth, differentiation and function as well as in brain pathologies.

Nutritional iron deprivation attenuates kainate-induced neurotoxicity in rats: implications for involvement of iron in neurodegeneration

There is evidence suggesting that oxidative stress contributes to kainate neurotoxicity. Since iron promotes oxidative stress, the present study explores how change in nutritional iron content modulates kainate-induced neurotoxicity. Rats received an iron-deficient diet (ID) from 22 days of age for 4 weeks. One control group received the same diet supplemented with iron and another control group received standard rodent diet. Cellular damage after subcutaneous kainate (10 mg/kg) was assessed by silver impregnation and gliosis by staining microglia. ID reduced cellular damage in piriform and entorhinal cortex, in thalamus, and in hippocampal layers CA1-3. ID also attenuated gliosis, except in the hippocampal CA1 layer. Given involvement of zinc in hippocampal neurotransmission and in oxidative stress, we tested for a possible interaction of nutritional iron with nutritional zinc. Rats were made iron-deficient and then assigned to supplementation with iron, zinc, or iron + zinc. Controls were continued on ID diet. After 2 weeks, rats were treated with kainate. Iron supplementation abolished the protective effect of ID in piriform and entorhinal cortex. In hippocampal CA1 and dorsal thalamus, neither iron nor zinc supplementation alone abolished the protective effect of ID against cellular damage. Iron + zinc supplementation abolished ID protection in dorsal thalamus, but not in reuniens nucleus. Kainate-induced gliosis in CA1 remained unaffected by nutritional treatments. Thus, in piriform and entorhinal cortex, nutritional iron has a major impact on cellular damage and gliosis. In hippocampal CA1, gliosis may associate with synaptic plasticity not modulated by nutritional iron, while cellular damage is sensitive to nutritional iron and zinc.

Neuroprotective activity of antazoline against neuronal damage induced by limbic status epilepticus

Imidazoline drugs exert neuroprotective effects in cerebral ischaemia models. They also have effects against mouse cerebellar and striatal neuronal death induced by N-methyl-D-aspartate (NMDA) through the blockade of NMDA currents. Here, we investigated the effects of antazoline on NMDA toxicity and current in rat hippocampal neuronal cultures, and on an in vivo model of status epilepticus. In hippocampal cultures, antazoline (30 microM) decreased NMDA-mediated neurotoxicity and also blocked the NMDA current with voltage-dependent and fast-reversible action (inhibition by 85+/-3% at -60 mV). Status epilepticus was induced by injecting pilocarpine (200 nmol) directly into the right pyriform cortex of male adult rats. The rats then received immediately three consecutive i.p. injections at 30-min intervals of either PBS (control group) or antazoline at 10 mg/kg (low-dose group) or at 45 mg/kg (high-dose group). During the 6-h recording, status epilepticus lasted more than 200 min in all groups. In the high-dose group only, seizures completely ceased 1 h after the third injection of antazoline, then started again 1 h later. Rats were killed 1 week later, and Cresyl Violet-stained sections of their brain were analysed for damage quantification. On the ipsilateral side to the pilocarpine injection, pyriform cortex and hippocampal CA1 and CA3 areas were significantly protected in both antazoline-treated groups, whilst prepyriform and entorhinal cortices were only in the high-dose group. On the contralateral side to the pilocarpine injection, only the hippocampal CA3 area was significantly protected in the low-dose group, but all investigated structures were in the high-dose group. In conclusion, antazoline is a potent neuroprotective drug in different models of neuronal primary culture, as previously shown in striatal and cerebellar granule neurons [Neuropharmacology 39 (2000) 2244], and here in hippocampal neurons. Antazoline is also neuroprotective in vivo in the intra-pyriform pilocarpine-induced status epilepticus model.

Subunit-dependent effects of nickel on NMDA receptor channels

Nickel (Ni2+) is a transition metal that affects different neuronal ionic channels. We investigated its effects on glutamate channels of the NMDA-type in the presence of saturating concentration of glutamate or NMDA (50 microM), in 0 external Mg and in the continuous presence of saturating glycine (30 microM). In neonatal rat cerebellar granule cells, Ni2+ inhibited the current evoked by NMDA at -60 mV with an IC50 close to 40 microM. The inhibition was weakly voltage-dependent and the current at +40 mV was inhibited with IC50=86 microM. Wash out of the metal unmasked a stimulatory effect which persisted for a few seconds. In HEK293 cells transiently transfected with recombinant NR1a-NR2A receptors, Ni2+ inhibited the current elicited by glutamate with an IC50=52 microM at -60 mV and 90 microM at +40 mV. In HEK293 expressing NR1a-NR2B receptors, 0.1-100 microM Ni2+ caused a potentiation of the current, with EC50=4 microM, while with 300 microM, a voltage-dependent block became apparent (IC50=170 microM). As previously reported, the current through both classes of recombinant receptors was steeply dependent on external pH, and in both cases the protonic block had an IC50 close to pH 7.2. Application of Ni2+ showed that stimulation of NR1a-NR2B receptor channels was dependent on external pH, while voltage-independent inhibition of NR1a-NR2A was less sensitive to pH change. These results indicate that Ni2+ has multiple and complex effects on NMDA channels, which are largely dependent on the NR2 subunit.

Proteomic analysis of astrocytic secretion in the mouse. Comparison with the cerebrospinal fluid proteome

Astrocytes, the most abundant cell type in the central nervous system, are intimately associated with synapses. They play a pivotal role in neuronal survival and the brain inflammatory response. Some astrocytic functions are mediated by the secretion of polypeptides. Using a proteomic approach, we have identified more than 30 proteins released by cultured astrocytes. These include proteases and protease inhibitors, carrier proteins, and antioxidant proteins. Exposing astrocytes to brefeldin A, which selectively blocks secretory vesicle assembly, suppressed the release of some of these proteins. This indicates that astrocytes secrete these proteins by a classic vesicular mechanism and others by an alternative pathway. Astrocytes isolated from different brain regions secreted a similar pattern of proteins. However, the secretion of some of them, including metalloproteinase inhibitors and apolipoprotein E, was region-specific. In addition, pro-inflammatory treatments modified the profile of astrocytic protein secretion. Finally, more than two thirds of the proteins identified in the astrocyte-conditioned medium were detectable in the mouse cerebrospinal fluid, suggesting that astrocytes contribute to the cerebrospinal fluid protein content. In conclusion, this study provides the first unbiased characterization of the major proteins released by astrocytes, which may play a crucial role in the modulation of neuronal survival and function.

Akt mediates the anti-apoptotic effect of NMDA but not that induced by potassium depolarization in cultured cerebellar granule cells

Apoptosis of cultured cerebellar granule neurons (CGNs) deprived of serum is prevented by K+ depolarization or moderate concentrations of N-methyl-d-aspartate (NMDA). Here, we have examined the role of the serine/threonine kinase Akt in these protective effects. The exposure of mouse CGNs to NMDA or K+ depolarization increased the phosphorylation of Akt, compared with that measured in cells incubated in a physiological K+ concentration. Only the NMDA-evoked response was reduced by inhibitors of phosphatidylinositol 3-kinase (wortmannin and LY294002) and mitogen-activated protein kinase (PD98059 and U0126). Similarly, the capacity of NMDA to inhibit apoptosis of CGNs deprived of serum was greatly reduced by these inhibitors as well as by the transfection of neurons with a catalytically inactive mutant of Akt, whereas the protective effect of K+ depolarization remained unaffected. These findings indicate that K+ depolarization and NMDA activate Akt through different signalling pathways in CGNs. Moreover, Akt mediates the anti-apoptotic effect of NMDA, but not that evoked by K+ depolarization.

Spreading depression in human neocortical slices

Cortical spreading depression (CSD) occurrence has been suggested to be associated with seizures, migraine aura, head injury and brain ischemia-infarction. Only few studies identified CSD in human neocortical slices and no comprehensive study so far evaluated this phenomenon in human. Using the neocortical tissue excised for treatment of intractable epilepsy, we aimed to investigate CSD in human. CSD was induced by KCl injection and by modulating T-type Ca(2+) currents in incubated human neocortical tissues in an interphase mode. The DC-fluctuations were recorded by inserting microelectrodes into different cortical layers. Local injection of KCl triggered single CSD that propagated at 3.1+/-0.1 mm/min. Repetitive CSD also occurred spontaneously during long lasting application (5 h) of the T-type Ca(2+) channel blockers amiloride (50 microM) or NiCl(2) (10 microM) which was concomitant with a reversible extracellular potassium increase up to 50 mM. CSD could be blocked by the N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonovaleric acid in all cases. The results demonstrate that modulation of the Ca(2+) dynamics conditioned human neocortical slices and increased their susceptibility to generate CSD. Furthermore, these data indicate that glutamatergic pathway plays a role in CSD phenomenon in human.

Voltage-dependent calcium channels in the rat retina: involvement in NMDA-stimulated influx of calcium

Rises in intracellular Ca2+ induced by activation of glutamate receptors are of ultimate importance for neuronal excitability and pathophysiological processes. In the present study, we aimed to elucidate the types of voltage-dependent Ca2+ channels involved in the NMDA-stimulated influx of Ca2+ into the isolated rat retina by using selective blockers. Additionally, the number of binding sites for radioligands labelling L- ([3H]nitrendipine), N- ([125I]omega-conotoxin MVIIA) and P/Q-type ([125I]omega-conotoxin MVIIC) Ca2+ channels was assessed in the rat retina and, for further comparison, in the rat cortex. Incubation of isolated rat retinas with 100 microM NMDA produced a three-fold increase in the influx of 45Ca2+ that was completely blunted by MK-801, a NMDA receptor antagonist, and partially attenuated (approximately 20%) by tetrodotoxin, a Na+ channel blocker. The L-type Ca2+ channel blocker nifedipine reduced NMDA-stimulated Ca2+ influx in a dose-related fashion, with a maximum reduction of approximately 50%. Similar effects were observed with verapamil and diltiazem. Blockers of N- and P/Q-type Ca2+ channels had no significant effect on the influx of Ca2+ evoked by NMDA. Co2+, a non-specific Ca2+ channel blocker, caused an inhibition of NMDA-stimulated Ca2+ influx similar to that of nifedipine. Therefore, of all voltage-dependent Ca2+ channels, L-type channels appear to make the greatest contribution (up to 50%) to the NMDA-stimulated influx of Ca2+ into the isolated rat retina. This finding contrasts with evidence obtained in brain neurones supporting a role for L-, N- and P/Q-type channels in NMDA-evoked Ca2+ signals. A comparison of the number of radioligand binding sites associated with L-, N- or P/Q-type Ca2+ channels in the rat cortex and retina revealed that such a difference cannot be ascribed to a distinct expression pattern of these channels in both tissues, although some variations were found. Interestingly, a different affinity of [3H]nitrendipine for L-type Ca2+ channels in the rat retina and cortex was observed which may reflect the expression of different classes of L-type channels in these tissues. The ability of L-type Ca2+ channel blockers to attenuate NMDA-stimulated Ca2+ influx may underlie their neuroprotective effects in the retina.

Mechanisms of L-cysteine neurotoxicity

We review here the possible mechanisms of neuronal degeneration caused by L-cysteine, an odd excitotoxin. L-Cysteine lacks the omega carboxyl group required for excitotoxic actions via excitatory amino acid receptors, yet it evokes N-methyl-D-aspartate (NMDA) -like excitotoxic neuronal death and potentiates the Ca2+ influx evoked by NMDA. Both actions are prevented by NMDA antagonists. One target for cysteine effects is thus the NMDA receptor. The following mechanisms are discussed now: (1) possible increase in extracellular glutamate via release or inhibition of uptake/degradation, (2) generation of cysteine alpha-carbamate, a toxic analog of NMDA, (3) generation of toxic oxidized cysteine derivatives, (4) chelation of Zn2+ which blocks the NMDA receptor-ionophore, (5) direct interaction with the NMDA receptor redox site(s), (6) generation of free radicals, and (7) formation of S-nitrosocysteine. In addition to these, we describe another new alternative for cytotoxicity: (8) generation of the neurotoxic catecholamine derivative, 5-S-cysteinyl-3,4-dihydroxyphenylacetate (cysdopac).

Imidazoline-induced neuroprotective effects result from blockade of NMDA receptor channels in neuronal cultures

Imidazolines have been shown to be neuroprotective in focal and global ischemia in the rat. However, their mechanism of action is still unclear. We have studied the neuroprotective effects of imidazolines against NMDA-induced neuronal death and hypoxic insult in cerebellar and striatal neuronal cultures. All of the imidazolines tested decreased the NMDA-mediated neurotoxicity in a non-competitive manner. Antazoline was the most effective (IC(50) of 5 microM, maximal neuroprotection reaching 90% at 100 microM). The neuroprotective effects were still present when the imidazolines were applied during the post-insult period. Antazoline, idazoxan and guanabenz also showed neuroprotective effects against hypoxia-induced neuronal death (neuroprotection reaching 95% for antazoline at 100 microM). Antazoline was still active if applied during the reoxygenation period (15% neuroprotection). To determine the mechanism of the neuroprotective effects, the possible interaction of imidazolines with NMDA receptors was studied. Imidazolines dose-dependently and non-competitively inhibited NMDA currents. As found for the neuroprotective effects, antazoline was the most effective imidazoline, with an IC(50) of 4 microM and a maximal inhibition of 90% at 100 microM. This blockade was rapid, reversible and voltage-dependent. We compared these effects to those of the classical non-competitive antagonist of NMDA channels, MK-801. In contrast to imidazolines, blockade of the NMDA current by MK-801 was voltage-independent and reversible only at positive potentials. When co-applied with MK-801, antazoline prevented the long lasting blockade of the NMDA current by MK-801. These results are consistent with the existence of overlapping binding sites for these drugs on the NMDA receptor channel. They indicate that imidazolines exert a strong neuroprotective effect against excitotoxicity and hypoxia in cerebellar and striatal primary neuronal cultures by inhibiting NMDA receptors. Since these effects were non-competitive, imidazolines appear to be interesting new drugs with therapeutic potential.

Oxidative glutamate toxicity involves mitochondrial dysfunction and perturbation of intracellular Ca2+ homeostasis

Glutamate toxicity on PC12 cells is mediated by oxidative stress as a consequence of the inhibition of a cystine uptake system with depletion of GSH. In this study we report that glutamate decreases PC12 cell viability, inhibiting the reduction of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). This decrease was prevented by the antioxidants vitamin E, idebenone and L-deprenyl, which were also shown to be effective in reducing the accumulation of reactive oxygen species (ROS) in cells exposed to glutamate, decreasing the fluorescence of 2',7'-dichlorofluorescein (DCF). Incubation of PC12 cells with high glutamate concentrations induced mitochondrial dysfunction, leading to the loss of mitochondrial transmembrane potential, evaluated as a decrease in rhodamine 123 (Rh123) retention by mitochondria, and to the decrease of intracellular ATP levels. The mitochondrial dysfunction, induced by glutamate, can be involved in the observed increase of [Ca2+]i. The elevation of [Ca2+]i occurred after GSH depletion, suggesting that oxidative stress is involved in the disturbances of intracellular calcium homeostasis. In conclusion, our data indicate that glutamate, at concentrations which block cystine uptake in PC12 cells leading to GSH depletion and inducing oxidative stress, increases ROS accumulation and decreases cell survival by a mechanism involving mitochondrial dysfunction and impairment of Ca2+ homeostasis.

mGluR7-like receptor and GABA(B) receptor activation enhance neurotoxic effects of N-methyl-D-aspartate in cultured mouse striatal GABAergic neurones

Presynaptic metabotropic glutamate receptors (mGluRs) of group III constitute possible targets for putative neuroprotective drugs acting against glutamate excitotoxic insults. Indeed, in glutamatergic cerebellar granule neurones in culture, high concentrations of L-2-amino-4-phosphonobutyrate (L-AP4, above 0.3 mM, thus activating mGluR7) inhibit NMDA-induced cell death. In contrast, in striatal cultures which are enriched in GABAergic neurones, we show that high concentrations of L-AP4 increased neuronal death in control as well as in NMDA-stimulated cultures. Moreover, similar results were obtained with the GABA(B)R agonist. baclofen. Both the neuroprotective effects in cerebellar granule cells and the neurotoxic effects in striatal neurones were mediated via Gi-Go-coupled mGluRs, suggesting that these effects were probably mediated by mGluR7a or b and GABA(B)R expressed in these neurones. In striatal neurones, we found that L-AP4 and baclofen inhibited both basal and NMDA-stimulated GABA release. These inhibitions of GABA release may be responsible for the increase in basal and NMDA-stimulated neuronal death. Indeed, blockade of GABA(A) receptors with bicuculline increased neuronal death of control and NMDA-treated striatal cultures. Taken together, these results suggest that L-AP4 and baclofen, via mGluR7 and GABA(B)R, reduced the neuroprotective effect of GABA present in striatal cultures acting via GABA(A) receptors. Although caution must be taken when extrapolating from in vitro to in vivo situations, the present experiments and the recent observations that mGluR7 and GABA(B)R are expressed in heterologous synapses, should be taken into consideration when evaluating the neuroprotective action of future mGluR7 specific agonists or GABA(B)R specific antagonists.

Magnetic resonance spectroscopy studies on changes in cerebral calcium and zinc and the energy state caused by excitotoxic amino acids

Under control conditions, superfused hippocampal slices exhibited a significantly higher phosphocreatine (PCr)/ATP ratio than cortical slices; the evidence suggests that this is due to lower concentrations of ATP, rather than higher concentrations of PCr. Glutamate caused relatively rapid decreases in PCr and ATP levels to approximately 45%, accompanied or immediately followed by an increased free intracellular calcium concentration ([Ca2+]i) and the release of Zn2+ in the cortex. In the hippocampus PCr and ATP decreased further to approximately 20% of control values, but the changes in [Ca2+]i and Zn2+ content were slower. This is in contrast to the effects of depolarisation, which produced the same rapid changes in the energy state and [Ca2+]i, with no detectable Zn2+, in both tissues. NMDA causes effects similar to those of glutamate in the cortex (decreases in the energy state, increased [Ca2+]i, and release of Zn2+). Pretreatment of the cortex for 1 h with the NMDA blocker MK-801 prevented all of the observed effects of NMDA. In contrast, pretreatment with MK-801 had no detectable effect on the increase in [Ca2+]i or the decreases in PCr and ATP caused by glutamate, although it prevented the release of zinc. The results are discussed in relation to the function of the NMDA subtype of glutamate receptor in excitotoxicity.

mGluR7-like metabotropic glutamate receptors inhibit NMDA-mediated excitotoxicity in cultured mouse cerebellar granule neurons

Glutamate-induced glutamate release may be involved in the delayed neuronal death induced by N-methyl-D-aspartate (NMDA). In order to examine a possible modulatory effect of the presynaptic group III mGluRs on glutamate excitotoxicity, the effect of L-2-amino-4-phosphonobutyrate (L-AP4) was examined on NMDA-induced delayed death of mouse cerebellar granule neurons in culture. We found that L-AP4, at high concentration (in the millimolar range), inhibited in a non-competitive manner the NMDA-induced toxicity. This effect was mimicked by high concentration of L-serine-o-phosphate (L-SOP), and was inhibited by pertussis toxin (PTX) indicating the involvement of a Gi/o protein. This suggests the involvement of mGluR7 in the L-AP4 effect, and this was consistent with the detection of both mGluR7 protein and mRNA in these cultured neurons. To examine the mechanism of the L-AP4-induced protection from excitotoxic damage, the effect of L-AP4 on glutamate release was examined. L-AP4 (> or = 1 mM) noncompetitively inhibited by more than 60% the glutamate release induced by NMDA during the insult. We also observed that the 10-min NMDA receptor stimulation resulted in a dramatic increase in the extracellular glutamate concentration reaching 6000% of the control value 24 h after the insult. This large increase was also inhibited when NMDA was applied in the presence of > or = 1 mM L-AP4. Part of the L-AP4-induced protection from excitotoxic damage of granule neurons may therefore result from the inhibition of the vicious cycle: dying cells release glutamate, glutamate induced cell death. The present results add to the hypothesis that presynaptic mGluRs, probably mGluR7, may be the targets of drugs decreasing glutamate release and then neuronal death observed in some pathological situations.

High-affinity zinc inhibition of NMDA NR1-NR2A receptors

Micromolar concentrations of extracellular Zn2+ are known to antagonize native NMDA receptors via a dual mechanism involving both a voltage-independent and a voltage-dependent inhibition. We have tried to evaluate the relative importance of these two effects and their subunit specificity on recombinant NMDA receptors expressed in HEK 293 cells and Xenopus oocytes. The comparison of NR1a-NR2A and NR1a-NR2B receptors shows that the voltage-dependent inhibition is similar in both types of receptors but that the voltage-independent inhibition occurs at much lower Zn2+ concentrations in NR1a-NR2A receptors (IC50 in the nanomolar range) than in NR1a-NR2B receptors (IC50 in the micromolar range). The high affinity of the effect observed with NR1a-NR2A receptors was found to be attributable mostly to the slow dissociation of Zn2+ from its binding site. By analyzing the effects of Zn2+ on varied combinations of NR1 (NR1a or NR1b) and NR2 (NR2A, NR2B, NR2C), we show that both the NR1 and the NR2 subunits contribute to the voltage-independent Zn2+ inhibition. We have observed further that under control conditions, i.e., in zero nominal Zn2+ solutions, the addition of low concentrations of heavy metal chelators markedly potentiates the responses of NR1a-NR2A receptors, but not of NR1a-NR2B receptors. This result suggests that traces of a heavy metal (probably Zn2+) contaminate standard solutions and tonically inhibit NR1a-NR2A receptors. Chelation of a contaminant metal also could account for the rapid NR2A subunit-specific potentiations produced by reducing compounds like DTT or glutathione.

Is high extracellular glutamate the key to excitotoxicity in traumatic brain injury?

Traumatic brain injury (TBI) increases extracellular levels of the excitatory amino acid glutamate and aspartate, and N-methyl-D aspartate (NMDA)-receptor antagonists protect against experimental TBI. These two findings have led to the prevalent hypothesis that excitatory amino acid efflux is a major contributor to the development of neuronal damage subsequent to traumatic injury. However, as with stroke, the hypothesis that high extracellular glutamate is the key to excitotoxicity in TBI conflicts with important data. For example, the initial increase in extracellular glutamate is cleared within 5 min after moderate TBI, whereas antagonists of glutamate receptors and the so- called presynaptic glutamate release inhibitors remain effective when administered 30 min after insult. In this article, we argue that the current concept of excitotoxicity in TBI, centered on high extracellular glutamate, does not withstand scientific scrutiny. As alternatives to explain the beneficial actions of glutamate antagonists in experimental TBI, we propose abnormalities of glutamatergic neurotransmission, such as deficient Mg2+ block of NMDA-receptor ionophore complexes, and phenomena such as spreading depression, which requires activation of glutamate receptors and is detrimental to neurons in damaged/vulnerable brain regions. Finally, we introduce the notion that beneficial effects of glutamate receptor antagonists in experimental models of neurological disorders do not necessarily imply the occurrence of excitotoxic processes. Indeed, glutamate-receptor blockade may be protective by reducing the energy demand required to counterbalance Na+ influx associated with glutamatergic synaptic transmission. In other words, glutamate receptor antagonists (and blockers of voltage-gated Na+-channels) may help nervous tissue to cope with increased permeability of the cellular membrane to ions and reduced efficacy of Na+ extrusion, and thus prevent the decay of transmembrane ionic concentrations gradients.

Aluminum potentiates glutamate-induced calcium accumulation and iron-induced oxygen free radical formation in primary neuronal cultures

Aluminum is a neurotoxic metal that may be involved in the progression of neurodegenerative diseases, including Alzheimer disease and amyotrophic lateral sclerosis (ALS). Although the mechanism of action is not known, aluminum has been shown to alter Ca2+ flux and homeostasis, and facilitate peroxidation of membrane lipids. Since abnormal increases of intracellular Ca2+ and oxygen free radicals have both been implicated in pathways leading to neurodegeneration, we examined the effect of aluminum on these parameters in vitro using primary cultures of cerebellar granule cells. Exposure to glutamate (1-300 microM) caused a concentration-dependent uptake of 45Ca in granule cells to a maximum of 280% of basal. Pretreatment with AlCl3 (1-1000 microM) had no effect on 45Ca accumulation, but increased the uptake induced by glutamate. Similarly, AlCl3 had no effect on intracellular free Ca2+ levels measured using fluorescent probe fura-2, but potentiated the increase induced by glutamate. The production of reactive oxygen species (ROS) was examined using the fluorescent probe dichlorofluorescin. By itself, AlCl3 had little effect on ROS production. However, AlCl3 pretreatment potentiated the ROS production induced by 50 microM Fe2+. These results suggest that aluminum may facilitate increases in intracellular Ca2+ and ROS, and potentially contribute to neurotoxicity induced by other neurotoxicants.

Altered glutamatergic transmission in neurological disorders: from high extracellular glutamate to excessive synaptic efficacy

This review is a critical appraisal of the widespread assumption that high extracellular glutamate, resulting from enhanced pre-synaptic release superimposed on deficient uptake and/or cytosolic efflux, is the key to excessive glutamate-mediated excitation in neurological disorders. Indeed, high extracellular glutamate levels do not consistently correlate with, nor necessarily produce, neuronal dysfunction and death in vivo. Furthermore, we exemplify with spreading depression that the sensitivity of an experimental or pathological event to glutamate receptor antagonists does not imply involvement of high extracellular glutamate levels in the genesis of this event. We propose an extension to the current, oversimplified concept of excitotoxicity associated with neurological disorders, to include alternative abnormalities of glutamatergic transmission which may contribute to the pathology, and lead to excitotoxic injury. These may include the following: (i) increased density of glutamate receptors; (ii) altered ionic selectivity of ionotropic glutamate receptors; (iii) abnormalities in their sensitivity and modulation; (iv) enhancement of glutamate-mediated synaptic efficacy (i.e. a pathological form of long-term potentiation); (v) phenomena such as spreading depression which require activation of glutamate receptors and can be detrimental to the survival of neurons. Such an extension would take into account the diversity of glutamate-receptor-mediated processes, match the complexity of neurological disorders pathogenesis and pathophysiology, and ultimately provide a more elaborate scientific basis for the development of innovative treatments.

Is calcium accumulation post-injury an indicator of cell damage?

It is generally agreed that excessive intracellular calcium accumulation is the main culprit for nerve cell damage following brain injury. Many autoradiographic studies of the post-injury brain have demonstrated an accumulation of 45Ca2+ in regions exhibiting neuronal damage. We have recently observed, after cortical contusion trauma [10], that there was a discrepancy between the extent of cell damage and the extent of 45Ca2+ in autoradiograms; rather the distribution of 45Ca2+ followed that of serum proteins. In addition 45Ca2+ was also observed in white matter, which had no signs of damage. We tested the hypothesis that 45Ca2+ accumulation was coupled to the presence of protein by directly injecting albumin into the brain cortex. There was a highly significant correlation between the content of 45Ca2+ and of albumin as measured by ELISA. A similar pattern was found after a cortical freeze-lesion in the contralateral hemisphere. However, in the ipsilateral hemisphere where cell damage was observed, the relation broke down and calcium accumulated in excess. We conclude that calcium accumulation in the brain is not only the result of cell damage but also the presence of calcium-binding proteins, e.g. albumin.

Balanced interaction of growth factors and taurine regulate energy metabolism, neuronal survival, and function of cultured mouse cerebellar cells under depolarizing conditions

The development of neuronal cells in a given cellular environment requires mechanisms that dynamically regulate the balanced interactions of multiple factors which are known to control maintenance and plasticity in function of neurons throughout constantly changing extracellular conditions. Periodic release of excitatory amino acids from both developing glial and neuronal cells into the extracellular environment and their uptake has been shown to stimulate neuronal function in concert with growth factors that control the degree of depolarization and, therefore, neuronal function. This study attempts to characterize the critical concentrations of these factors either alone or together in relation to energy metabolism, cell survival and function. We demonstrate a close correlation between energy metabolism of neuronal cells, controlled by the combination of growth-factors (beta FGF, BDNF), and glutamate-taurine as well as K+ in depolarizing concentrations (10-25 mM), during the balancing act of neuronal survival or death, and neuronal function. These functions depend on medium conditions (energy sources, ion composition), the ratio of glial cells versus neurons and cell density. Granule cell migration as a measure of developmental neuronal function was analyzed in the presence of various combinations of growth factors and taurine under various depolarizing conditions (glutamate, K+). We found that K+ concentrations > 7 mM in BME and 10% horse serum blocked migration in less than 30 min. Taurine did not prevent this effect. However, in the presence of HEPES as well as in F12-medium with HEPES, taurine restored granule cell migration. On the other hand, glutamate-or NMDA-mediated depolarization stopped migrating granule cells while NMDA antagonists extended the period of migration. Taurine amplified the stop-signal in the presence of glutamate agonists but increased the number of migrating cells in the absence of glutamate. Thus, the mechanisms of glutamate receptor mediated excitotoxicity, possibly by reducing Ca2+ influx under depolarizing conditions, but amplifies the stop-signal, Ca2+ levels may not control granule cell migration.

Citrate modulates the regulation by Zn2+ of N-methyl-D-aspartate receptor-mediated channel current and neurotransmitter release

The effect of the two metal-ion chelators EDTA and citrate on the action of N-methyl-D-aspartate (NMDA) receptors was investigated by use of cultured mouse cerebellar granule neurons and Xenopus oocytes, respectively, to monitor either NMDA-evoked transmitter release or membrane currents. Transmitter release from the glutamatergic neurons was determined by superfusion of the cells after preloading with the glutamate analogue D-[3H]aspartate. The oocytes were injected with mRNA isolated from mouse cerebellum and, after incubation to allow translation to occur, currents mediated by NMDA were recorded electrophysiologically by voltage clamp at a holding potential of -80 mV. It was found that citrate as well as EDTA could attenuate the inhibitory action of Zn2+ on NMDA receptor-mediated transmitter release from the neurons and membrane currents in the oocytes. These effects were specifically related to the NMDA receptor, since the NMDA receptor antagonist MK-801 abolished the action and no effects of Zn2+ and its chelators were observed when kainate was used to selectively activate non-NMDA receptors. Since it was additionally demonstrated that citrate (and EDTA) preferentially chelated Zn2+ rather than Ca2+, the present findings strongly suggest that endogenous citrate released specifically from astrocytes into the extracellular space in the brain may function as a modulator of NMDA receptor activity. This is yet another example of astrocytic influence on neuronal activity.

Complex correlation between excitatory amino acid-induced increase in the intracellular Ca2+ concentration and subsequent loss of neuronal function in individual neocortical neurons in culture

Primary cultures of cerebral cortical neurons and single-cell imaging of intracellular free Ca2+ concentration ([Ca2+]i) with the ratiometric dye fura-2 were used to assess excitatory amino acid (EAA)-induced neurotoxicity; the loss of neuronal function as defined by the ability of the cells to respond to K(+)-induced depolarization by a transient increase in Ca2+ influx was measured. The responsiveness of individual neurons was measured quantitatively as the [Ca2+]i values of the second KCl (2.KCl) stimulation divided by those of the first KCl (1.KCl) stimulation, giving the value of the ratio (2.KCl/1.KCl). Exposure to EAAs led to an increase in [Ca2+]i, but no simple correlation between the increase in [Ca2+]i and neuronal responsiveness could be demonstrated. Rather, below a threshold level of [Ca2+]i (ca. 1 microM), the neuronal responsiveness was largely independent of the glutamate receptor-agonist-induced increase in [Ca2+]i. However, when [Ca2+]i increased above this threshold level, the neurons almost invariably lost the ability to respond to a K(+)-induced depolarization, particularly after exposure to glutamate. Therefore, the cortical neurons were found to be exceptionally vulnerable to the glutamate-induced loss of function when compared with the effect induced by the glutamate receptor subtype-specific agonists, N-methyl-D-aspartate, quisqualate, and 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl) propionate. The findings suggest that the loss of neuronal membrane polarization precedes plasma membrane disruption and is a sensitive marker of EAA-induced neurodegeneration observed at the single-cell level.

Glutamate-induced neuronal death in cerebellar culture is mediated by two distinct components: a sodium-chloride component and a calcium component

The relative contribution of sodium, chloride and calcium ions in the neuronal death induced by glutamate is controversial. We have therefore reassessed the effects of extracellular ion substitution on glutamate-induced neuronal death in cerebellar granule cell culture. Sodium or chloride substitution by impermeant ions prevented the initial swelling observed after glutamate exposure (100 microM, 15 min) in balanced salt solution but did not prevent the progressive degeneration of cerebellar neurons over the next few hours. In low calcium medium, glutamate exposure also led to degeneration of granule neurons. In contrast, sodium or chloride substitution and calcium omission prevented both the initial swelling and the delayed neuronal death after glutamate exposure. These morphological observations were confirmed both by measurement of the intracellular water space with [3H]methylglucose and by quantification of cell viability by 3-(4,5-dimethylthiazol-2-yl-)-2,5-diphenyl tetrazolium bromide (MTT) staining. We conclude that glutamate-induced neuronal death is mediated by two distinct components: a calcium-independent sodium-chloride dependent component and a calcium-dependent component. Each one of these components leads to the death of cerebellar neurons after glutamate exposure.

Excitotoxic amino acids cause appearance of magnetic resonance spectroscopy-observable zinc in superfused cortical slices

(1) The effects of glutamate and NMDA on the free intracellular calcium concentration ([Ca2+]i) have been followed in superfused cortical slices using the 19F-magnetic resonance indicator 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid (5FBAPTA). (2) Glutamate (0.5 or 1 mM) caused a 75-100% increase in [Ca2+]i, and a new resonance was attributed to zinc-5FBAPTA, which was confirmed from its disappearance in the presence of a high-affinity chelator of heavy metals, N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine. The appearance of zinc occurred with or just after the rise in [Ca2+]i and was independent of Mg2+. (3) NMDA, N-methyl-DL-aspartate, or N-methyl-L-aspartate (10-200 microM) caused a slower increase in [Ca2+]i, and zinc was observed in some but not all experiments. When present, zinc appeared later than the increase in [Ca2+]i. These changes were also independent of Mg2+. (4) Decreases in both phosphocreatine and ATP were observed in all of these studies. (5) The results are discussed in terms of the proposed role of zinc as a modulator of excitotoxicity. Observations of zinc after exposure to glutamate or more slowly to NMDA, but not after depolarisation or deprivation of glucose and O2 (where increases also occur in [Ca2+]i), suggest that the cellular damage caused by the latter insults (depolarisation and fuel deprivation as in ischaemia) involves mechanisms not solely attributable to release of excitotoxins.

The quantity of calcium that appears to induce neuronal death

Excessive entry of Ca2+ through the NMDA receptor is thought to be the major cause of glutamate toxicity in brain neurons. However, actual quantitation of the calcium overload has not been achieved. Here we show that the absolute amount of 45Ca2+ taken up via the NMDA receptor correlates quantitatively with the amount of acute cell death in cultured cerebellar granule cells of the rat. Analysis of 9- and 16-day cultures reveals that the NMDA-induced Ca2+ uptake is about the same at these ages, whereas the Ca-dependent lethal process is more developed in the older neurons. The calculated lethal concentration of 45Ca taken up exceeds by approximately 10,000 times the maximal concentration of [Ca2+]i that can be measured by fluorescence imaging. It is suggested that the Ca2+ taken up induces the lethal process in a subcellular structure in which it has been segregated.

Down-regulation of NMDA receptor activity by NMDA

Rat cerebellar granule cells were cultured in a medium containing 25 mM KCl. The presence of NMDA during culture caused strong down-regulation of 45Ca uptake through the NMDA receptor channel. The process affected neither the viability nor the protein content of the cells. The developmental program of NMDA receptor activity was resumed after removal of NMDA from the culture medium, dependent apparently on protein synthesis. The down-regulation also rendered the neurons resistant to NMDA toxicity. It permitted replenishment of the culture with fresh medium, which is extremely toxic for cells cultured in absence of NMDA. Such down-regulation might perhaps play a role in adjusting the activity of post synaptic NMDA receptors, following synaptogenesis.