Calcium influx accompanies but does not cause excitotoxin-induced neuronal necrosis in retina

Mechanistic Insights into the Anti-Proliferative Action of Gut Microbial Metabolites against Breast Adenocarcinoma Cells

The gut microbiota undergoes metabolic processes to produce by-products (gut metabolites), which play a vital role in the overall maintenance of health and prevention of disease within the body. However, the use of gut metabolites as anticancer agents and their molecular mechanisms of action are largely unknown. Therefore, this study evaluated the anti-proliferative effects of three key gut microbial metabolites-sodium butyrate, inosine, and nisin, against MCF7 and MDA-MB-231 breast adenocarcinoma cell lines. To determine the potential mechanistic action of these gut metabolites, flow cytometric assessments of apoptotic potential, reactive oxygen species (ROS) production measurements and proteomics analyses were performed. Sodium butyrate exhibited promising cytotoxicity, with IC50 values of 5.23 mM and 5.06 mM against MCF7 and MDA-MB-231 cells, respectively. All three metabolites were found to induce apoptotic cell death and inhibit the production of ROS in both cell lines. Nisin and inosine indicated a potential activation of cell cycle processes. Sodium butyrate indicated the possible initiation of signal transduction processes and cellular responses to stimuli. Further investigations are necessary to ascertain the effective therapeutic dose of these metabolites, and future research on patient-derived tumour spheroids will provide insights into the potential use of these gut metabolites in cancer therapy.

Excitotoxicity and stroke: identifying novel targets for neuroprotection

Excitotoxicity, the specific type of neurotoxicity mediated by glutamate, may be the missing link between ischemia and neuronal death, and intervening the mechanistic steps that lead to excitotoxicity can prevent stroke damage. Interest in excitotoxicity began fifty years ago when monosodium glutamate was found to be neurotoxic. Evidence soon demonstrated that glutamate is not only the primary excitatory neurotransmitter in the adult brain, but also a critical transmitter for signaling neurons to degenerate following stroke. The finding led to a number of clinical trials that tested inhibitors of excitotoxicity in stroke patients. Glutamate exerts its function in large by activating the calcium-permeable ionotropic NMDA receptor (NMDAR), and different subpopulations of the NMDAR may generate different functional outputs, depending on the signaling proteins directly bound or indirectly coupled to its large cytoplasmic tail. Synaptic activity activates the GluN2A subunit-containing NMDAR, leading to activation of the pro-survival signaling proteins Akt, ERK, and CREB. During a brief episode of ischemia, the extracellular glutamate concentration rises abruptly, and stimulation of the GluN2B-containing NMDAR in the extrasynaptic sites triggers excitotoxic neuronal death via PTEN, cdk5, and DAPK1, which are directly bound to the NMDAR, nNOS, which is indirectly coupled to the NMDAR via PSD95, and calpain, p25, STEP, p38, JNK, and SREBP1, which are further downstream. This review aims to provide a comprehensive summary of the literature on excitotoxicity and our perspectives on how the new generation of excitotoxicity inhibitors may succeed despite the failure of the previous generation of drugs.

Glutamate-induced oxidative stress, but not cell death, is largely dependent upon extracellular calcium in mouse neuronal HT22 cells

Elucidating the relationship of glutamate-induced Ca2+ flux and oxidative death of neuronal cells may be of great relevance for neurodegenerative diseases in human beings. Mouse hippocampal HT22 cells provide a model system to study this relationship at the molecular level. Here we show that stimulation of HT22 cells with 5 mM glutamate is cytotoxic. Glutamate-induced cytotoxicity was associated with the generation of reactive oxygen species (ROS) and activation of the death executioner caspases 1 and 3. Treatment of HT22 cells with the calcium chelator, EGTA, and the calcium channel blocker, CoCl2, revealed that glutamate-induced cell death was dependent, in part, on glutamate-induced Ca2+ influx from extracellular stores. However, activation of caspases 1 and 3 and death of HT22 cells were also observed when Ca2+ was lacking in the extracellular milieu and ROS production abrogated. These findings led us to conclude that glutamate-induced death of mouse HT22 cells utilizes a complex mechanism that relies only in part on Ca2+ influx and ROS production. Additional studies are warranted to evaluate glutamate-induced death mechanisms that operate independently of Ca2+ influx and generation of ROS.

Retinal ischemia: mechanisms of damage and potential therapeutic strategies

Retinal ischemia is a common cause of visual impairment and blindness. At the cellular level, ischemic retinal injury consists of a self-reinforcing destructive cascade involving neuronal depolarisation, calcium influx and oxidative stress initiated by energy failure and increased glutamatergic stimulation. There is a cell-specific sensitivity to ischemic injury which may reflect variability in the balance of excitatory and inhibitory neurotransmitter receptors on a given cell. A number of animal models and analytical techniques have been used to study retinal ischemia, and an increasing number of treatments have been shown to interrupt the "ischemic cascade" and attenuate the detrimental effects of retinal ischemia. Thus far, however, success in the laboratory has not been translated to the clinic. Difficulties with the route of administration, dosage, and adverse effects may render certain experimental treatments clinically unusable. Furthermore, neuroprotection-based treatment strategies for stroke have so far been disappointing. However, compared to the brain, the retina exhibits a remarkable natural resistance to ischemic injury, which may reflect its peculiar metabolism and unique environment. Given the increasing understanding of the events involved in ischemic neuronal injury it is hoped that clinically effective treatments for retinal ischemia will soon be available.

EGCG attenuates AMPA-induced intracellular calcium increase in hippocampal neurons

We have investigated the protective effect of (-)-epigallocatechin gallate (EGCG) on alpha-amino-3-hydroxy-5-methyl-4-isoxazolo propionate (AMPA)-induced toxicity in cultured rat hippocampal neurons. Treatment of 24 h AMPA (10 microM) reduced the neuronal viability in both survival neuron counting and MTT reduction assay compared with control, with increase in cellular concentrations of hydrogen peroxide and malondialdehyde. These responses to AMPA were significantly blocked by co-treatments with EGCG (10 microM), which effect was very similar to the protective ability of a known antioxidant catalase (2000 U/ml). AMPA (50 microM) elicited the increase in intracellular calcium concentration ([Ca(2+)]i) on which EGCG significantly attenuated both peak amplitude and sustained nature of that [Ca(2+)]i increase in a dose-dependent manner. These data suggest that EGCG has a neuroprotective effect against AMPA through inhibition of AMPA-induced [Ca(2+)]i increase and consequent attenuation of reactive oxygen species production and lipid peroxidation as an antioxidant and a radical scavenger.

Citrate, beneficial or deleterious in the CNS?

Cerebellar granule neurons were incubated with or without glucose (3 mM) in the presence or absence of citrate (20 mM) using normoxic and/or hypoxic incubation conditions. During 4 h of hypoglycemia and also during hypoxia plus hypoglycemia, citrate increased lactate dehydrogenase (LDH) leakage from the cells and decreased mitochondrial activity, the latter was also the case in the presence of glucose. After 24 h of hypoglycemia, however, citrate decreased LDH leakage slightly, possibly due to its metabolism in the tricarboxylic acid cycle under these conditions. It should be noted that during mild hypoxia plus hypoglycemia a reduced LDH leakage was observed when compared to hypoglycemia alone. The 4 h low oxygen period did protect the neurons also during the 20 h re-oxygenation period. The present study might indicate that incubation of brain cell cultures in an atmosphere of air (30% oxygen) and 5% CO2, which is used in most laboratories, can be toxic and that oxygen concentration should be lowered considerably to mimic conditions in the brain.

NMDA-sensitive neurons profoundly influence delayed staurosporine-induced apoptosis in rat mixed cortical neuronal cultures

We have investigated cell killing in cultured rat embryonic cortical neurons exposed to the protein kinase inhibitor staurosporine, the excitatory amino acid N-methyl-D-aspartate (NMDA), or a combination thereof. Our data indicate that there are several populations of neurons that differ in their response to these agents. Cultures exposed to NMDA undergo cell death typified by lactate dehydrogenase (LDH) leakage which is likely primarily necrotic in that little caspase-3 activation or oligonucleosome formation is observed even when followed for 48 h. Cells exposed to staurosporine (STS) exhibit rapid, extensive activation of caspase-3 with coincident LDH leakage, oligonucleosome formation and TUNEL staining. Both LDH leakage and oligonucleosome content were significantly more elevated at 48 h than at 20 h with STS treatment while caspase-3 activity peaked early (8-20 h) and declined markedly by 48 h. Deletion of NMDA-responsive neurons by pre-treatment of the cultures with NMDA for 4 days prevented the late phase (20-48 h) increases in LDH leakage and oligonucleosomes in the remaining neuronal population. Caspase-3 activity was also completely abolished by NMDA pre-treatment. These results indicate that cells susceptible to acute NMDA-induced toxicity can be killed by non-apoptotic means when exposed to NMDA; however, they undergo a delayed, apoptotic death when exposed to STS. Interestingly, removal of NMDA-responsive cells prevents the processing of procaspase-3; thus, STS-induced apoptosis in cells resistant to NMDA-mediated killing proceeds independent of caspase-3 activation. The data indicate that nearly all neurons in these mixed cultures can undergo apoptosis in response to appropriate stimuli such as STS but that the temporal nature, and the pathways activated in response to STS, vary amongst the subpopulations of neurons. These findings may help to explain the simultaneous appearance of features of both apoptosis and necrosis observed in vivo following cerebral ischemia.

The intact isolated (ex vivo) retina as a model system for the study of excitotoxicity

Excitotoxicity is defined as a mode of neural cell death triggered by overactivation of receptors for the amino acid transmitter glutamate. There is considerable evidence that excitotoxicity is responsible for cell death in several neuropathological states, including some retinal diseases. The isolated retina, particularly from chick embryos, has been used extensively as an experimental system to characterize this process. This paper summarizes the use of isolated retina as a model system for studies of excitotoxicity from a theoretical and methodological point of view, and reviews results obtained from studies utilizing this system.

Ca2+-independent excitotoxic neurodegeneration in isolated retina, an intact neural net: a role for Cl- and inhibitory transmitters

Rapidly triggered excitotoxic cell death is widely thought to be due to excessive influx of extracellular Ca2+, primarily through the N-methyl-D-aspartate subtype of glutamate receptor. By devising conditions that permit the maintenance of isolated retina in the absence of Ca2+, it has become technically feasible to test the dependence of excitotoxic neurodegeneration in this intact neural system on extracellular Ca2+. Using biochemical, Ca2+ imaging, and electrophysiological techniques, we found that (1) rapidly triggered excitotoxic cell death in this system occurs independently of both extracellular Ca2+ and increases in intracellular Ca2+; (2) this cell death is highly dependent on extracellular Cl-; and (3) lethal Cl- entry occurs by multiple paths, but a significant fraction occurs through pathologically activated gamma-aminobutyric acid and glycine receptors. These results emphasize the importance of Ca2+-independent mechanisms and the role that local transmitter circuitry plays in excitotoxic cell death.

Oxidative stress affects the selective ion permeability of voltage-sensitive Ca2+ channels in cultured retinal cells

The effect of ascorbate/Fe2+-induced oxidative stress on the intracellular Ca2+ concentration ([Ca2+]i) and on the voltage-sensitive Ca2+ channels (VSCC) of chick retinal cells was evaluated in this study. We also analyzed the effect of oxidation on the intracellular Na+ concentration ([Na+]i) and on the Ca2+-dependent release of [3H])gamma-aminobutric acid (GABA) evoked by 50 mM KCI. The resting [Ca2+]i was not affected by oxidation, but the [Ca2+]i response (delta[Ca2+]i) to K+-depolarization was significantly inhibited under oxidative stress conditions. The Ca2+ influx stimulated by membrane depolarization was mediated by L- and N-type VSCC, and by N-metyl-D-aspartate (NMDA) receptor channel, activated by endogenous glutamate released by glutamatergic cells. In cultured retinal cells L-type channels are the major route of Ca2+ influx during depolarization and the most affected by oxidative stress. The N-type VSCC seem not to be affected by oxidant conditions; they were found to be involved in glutamatergic transmission and only indirectly in the release of [3H]GABA evoked by K+-depolarization. Although the Ca2+-dependent release of [3H]GABA evoked by 50 mM KCl is mediated by Ca2+ entry through L-type Ca2+ channels, it is not affected by pre-incubation with the oxidant pair. The oxidative stress conditions increased the [Na+]i in Ca2+-free medium, by a process dependent of Na+ entry through L-type VSCC. The increased permeability of L-type VSCC to Na+ may increase the Ca2+-independent release of endogenous glutamate which, by activating the NMDA receptors, induces the release of [3H]GABA by reversal of its transporter. The equilibrium between the release of GABA and glutamate may play an in important role in neuroprotection against excitotoxic insults.

Determination of intracellular Ca2+ concentration can be a useful tool to predict neuronal damage and neuroprotective properties of drugs

The purpose of the present study was to examine the relationship between elevation in intracellular Ca2+ concentration ([Ca2+]i) and development of neuronal damage after cytotoxic hypoxia in vitro. Chick telencephalic neurons were exposed to NaCN 1 mM for up to 2 h. [Ca2+]i was assessed by means of fura-2 based microfluorometry and viability was measured by means of trypan blue exclusion on the same relocated cells for a period 24 h after initiation of hypoxia. Exposure to sodium cyanide resulted in an up to 10 fold increase in [Ca2+]i and led to subsequent neuronal damage. According to [Ca2+]i and viability neurons in four different stages could be revealed. The percentage of neurons showing elevated [Ca2+]i paralleled exactly the percentage of neuronal damage. The elevation in [Ca2+]i clearly preceded neuronal damage suggesting a time window for pharmacological intervention. The NMDA antagonists dizocilpine, memantine and amantadine were capable of reducing the percentage of neurons showing elevated [Ca2+]i and attenuated neuronal damage. Dizocilpine proved to be the most potent and amantadine to be the weakest antagonist whereas the alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo-(F)-quinoxaline (NBQX) was ineffective. Under our experimental conditions, measurement of [Ca2+]i was able to predict the extent of neuronal damage as well as the neuroprotective potency of drugs.

Glutamate receptor-induced toxicity in neostriatal cells

Infrared differential interference contrast (IR DIC) videomicroscopy was used to measure and characterize cell swelling induced by activation of glutamate receptors (GluR) in a neostriatal brain slice preparation. This swelling is, in many cases, a prelude to necrotic cell death. Activation of N-methyl-D-aspartate (NMDA) and non-NMDA ionotropic GluRs caused cell swelling. The concentration-response relationships and the time courses of the onset of agonist-induced swelling were very similar for NMDA and kainate (KA). However, cells were able to recover from KA but not NMDA-induced swelling. Results from ion substitution experiments suggest that sodium, chloride and to a lesser extent calcium ions play critical roles in this swelling. Heterogeneity in the response to NMDA occurred within cells of the neostriatum. Approximately 15% of the cells did not swell when exposed to NMDA. The magnitude of the NMDA-induced swelling also varied depending on the region of the nervous system. Swelling was greater in the neostriatum and neocortex than in the hippocampus and it did not occur in the suprachiasmatic nucleus. In conclusion, IR DIC videomicroscopy can be used to follow quantitatively the dynamics of GluR-evoked responses in single cells and should be instrumental in determining the factors capable of modifying excitotoxicity.

An ex vivo rat retinal preparation for excitotoxicity studies

Although the isolated chicken embryo retina has been a very useful in vitro preparation for studying mechanisms of excitotoxicity, it is an avian rather than mammalian tissue and its embryonic age makes it unsuitable for a full range of developmental and aging studies. Therefore, we have explored the feasibility of using the rat retina at various ages for in vitro excitotoxicity studies. In this model, retinal segments were isolated in artificial cerebrospinal fluid (CSF) at 5 degrees C then incubated under various conditions at 30 degrees C and assessed histologically for signs of neurodegenerative changes. Retinal segments from 7-, 30-, 120- and 660-day-old rats incubated in CSF for 3 h and from 30-day-old rats incubated for 24 h retained a normal histological appearance. Thus, this preparation is suitable for in vitro studies pertaining to either acute or delayed excitotoxic phenomena in the mammalian CNS at any age from infancy to old age. Excitotoxin agonist experiments in the 30-day-old rat retina revealed the surprising result that the non-NMDA agonists, kainate and AMPA, at a low concentration (100 microM) damaged a much larger number of retinal neurons than NMDA did at a very high concentration (10 mM).

Delayed cell death in the contralateral hippocampus following kainate injection into the CA3 subfield

A model of epileptic cell death has been developed employing unilateral injections of kainic acid, a glutamate agonist, into the CA3 subfield of the hippocampus. The contralateral hippocampus, where neuronal damage is induced by hyperactivity in afferent pathways, served as the model structure. The pattern of cell death in this model was shown earlier to correspond to the vulnerable regions in human temporal lobe epilepsy. In the present time-course study we demonstrated that the different subpopulations of vulnerable cells in the contralateral hippocampus of the rat degenerate at different times following kainate injection. Spiny calretinin-containing cells in the hilus and CA3 stratum lucidum disappear at 12-24 h, other types of hilar neurons and CA3c pyramidal cells show shrinkage and argyrophilia at two days, whereas CA1 pyramidal cells degenerate at three days postinjection. The majority of cells destined to die showed a transient expression of the heatshock protein 72, approximately one day (for hilar-CA3c) or two days (for CA1) before degeneration. Parvalbumin-immunoreactivity transiently disappeared from the soma and dendrites of interneurons between the first and the fourth day. The results suggest that seizure-induced cell death is delayed, therefore acute oedema, even if it occurs, is insufficient to kill neurons. The only exception is the population of calretinin-containing interneurons degenerating at 12-24 h. The further one day delay between hilar-CA3c and CA1 cell death is likely to be due to differences in the relative density of glutamate receptor types (kainate versus NMDA) and the source of afferent input of these subfields. Thus, simple pharmacotherapy targeting only one of the excitotoxic mechanisms (i.e. acute oedema of calretinin cells versus delayed death of hilar-CA3c and CA1 cells at different time points) is likely to fail.

Calcium dynamics in the central nervous system

Calcium ions are critically important in many functions of the nervous system from neurotransmitter release to intracellular signal transduction. The large difference between intracellular and extracellular calcium ion concentration ([Ca2+]) highlights the importance of the mechanisms controlling influx and efflux of this ion. Loss of the regulatory ability of these mechanisms and the subsequent increased intracellular calcium levels may be involved in pathological events of brain trauma, stroke, epilepsy and other diseases. Ca2+ dynamics in the CNS ranging from 'waves' to 'spirals' are being studied because of the availability of fluorescent indicators of Ca2+ combined with confocal microscopy. Cellular mechanisms of Ca2+ signal transduction have been extensively reviewed (Tsien and Tsein, 1990; Carafoli, 1992; Berridge, 1993; Berridge and Dupont, 1994; Pozzan et al., 1994; Clapham, 1995; Ghosh and Greenberg, 1995). The aim of this review is to present the types of Ca2+ dynamics observed in the CNS thus far, both in normal brain function as well as in response after injury.

Glutamate-induced intracellular calcium changes and neurotoxicity in cortical neurons in vitro: effect of chemical ischemia

To study the role of calcium in neuronal death during ischemia, we examined the characteristics of intracellular Ca2+ ([Ca2+]i) changes in single rat forebrain neurons exposed for 5 min to glutamate (3 microM + 1 microM glycine), NMDA (30 microM + 1 microM glycine), kainate (100 microM) or high K+ (50 mM), under both normal and ischemic conditions. The parameters of [Ca2+]i change measured included peak [Ca2+]i level, plateau [Ca2+]i level, area under the [Ca2+]i response curve and time taken by [Ca2+]i to recover to 10% of the peak response. Under normal conditions, all the agonists studied produced similar [Ca2+]i changes. Chemical ischemia simulated by application of 5 mM KCN in glucose-free buffer had no effect on the basal level of [Ca2+]i, but significantly enhanced and prolonged the [Ca2+]i changes produced by all the agonists. However, in toxicity studies, chemical ischemia significantly potentiated the toxicity of only glutamate and N-methyl-D-aspartate. In correlation studies, all the neurons which died displayed an irreversible secondary [Ca2+]i load prior to loss of viability. These studies suggest that while Ca2+ entry may play a critical role in neuronal death, the magnitude of initial [Ca2+]i change does not predict the toxicity of an agonist in cortical neurons.

Transient cerebral ischemia disrupts performance on a one-trial passive avoidance task in the domestic chick and is associated with neuronal degeneration in the central nervous system

We have examined the effects of transient cerebral ischemia on performance of a one-trial passive avoidance task by chicks. Transient forebrain ischemia was induced by bilateral carotid artery occlusion for a period of 10 min. In one experimental group, ischemia was produced prior to training on the avoidance task whereas in the other group ischemic intervention was not made until 3 h after initial training. Sham-operated groups were matched to each of the experimental groups. All four groups were tested for retention of the avoidance response 24 h post-surgery. The sham-operated birds and those receiving post-training ischemia showed good retention of the avoidance response, whereas in birds which received ischemia prior to training there was significant amnesia. Neuronal damage, determined qualitatively using a silver impregnation method, was observed in several forebrain regions including the hippocampus, hyperstriatal regions, paleostriatum primitivum, ventral archistriatum, and lateral corticoid area. Damage was also observed in the Purkinje cells of the cerebellum. The behavioural and anatomical effects of transient forebrain ischemia have not been previously investigated in an avian species and the finding of significant amnesia for a learning task following ischemia is in good agreement with several behavioural studies in mammals.

Rapid increase in mitochondrial volume in nucleus magnocellularis neurons following cochlea removal

Second-order auditory neurons in nucleus magnocellularis (NM) of the chick brainstem undergo a series of rapid metabolic changes following unilateral cochlea removal, culminating in the death of 25% of NM neurons. Within hours of cochlea removal, ipsilateral NM neurons show marked increases in histochemical staining for the mitochondrial enzymes succinate dehydrogenase and cytochrome oxidase. We investigated corresponding ultrastructural changes in NM neurons by preparing animals undergoing unilateral cochlea removal for transmission electron microscopy. We quantified changes in NM mitochondrial volume by stereological methods and qualitatively compared mitochondrial morphology between NM neurons destined to survive and those destined to die after cochlea removal. Within hours of cochlea removal, ipsilateral NM neurons show striking increases in mitochondrial volume (84% at 6 hours and 236% at 12 hours after cochlea removal compared to unoperated, control animals). At 2 week survival times, ipsilateral NM neurons contain fewer mitochondria than contralateral neurons. Surprisingly, anesthesia alone causes short-term increases in NM mitochondrial volume. Animals anesthetized with pentobarbital and ketamine and sacrificed 6 or 12 hours later showed a 45% increase in mitochondrial volume compared to previously unanesthetized animals. NM neurons destined to die within days of cochlea removal can be identified within several hours after deafferentation by the appearance of their ribosomes. We observed qualitative differences in mitochondrial morphology in dying neurons. Mitochondria in neurons destined to die consistently showed mitochondrial swelling and vacuolization indicative of metabolic dysfunction. Similar mitochondrial changes have been reported when mitochondria take up excess calcium. Ultrastructural changes in NM after cochlea removal display features of both programmed and pathological cell death, in which increased intracellular calcium is thought to play a role.

Secondary Ca2+ overload indicates early neuronal injury which precedes staining with viability indicators

Spinal neurons, lethally challenged with excitatory amino acids (EAAs) or with high-K+, underwent a biphasic rise in free intracellular calcium concentration ([Ca2+]i). In contrast to the initial rise in [Ca2+]i which recovered, the secondary, irreversible [Ca2+]i increase was unaffected by antagonists of EAA receptors or Ca2+ channels. Also, it correlated highly with cell death, but preceded vital staining with trypan blue and ethidium homodimer, reflecting damaged cellular Ca2+ regulation rather than plasma membrane leakiness. Our findings suggest that delayed Ca2+ overload is the end-product rather than the cause of Ca(2+)-triggered neurotoxic processes.

Excitatory amino acid-mediated cytotoxicity and calcium homeostasis in cultured neurons

A large body of evidence suggests that disturbances of Ca2+ homeostasis may be a causative factor in the neurotoxicity induced by excitatory amino acids (EAAs). The route or routes by which an increase in intracellular calcium concentration ([Ca2+]i) is mediated in vivo are presently not clarified. This may partly reflect the complexity of intact nervous tissue in combination with the relative unspecific action of the available "calcium antagonists," e.g., blockers of voltage-sensitive calcium channels. By using primary cultures of cortical neurons as a model system, it has been found that all EAAs stimulate increases in [Ca2+]i but via different mechanisms. By using the drug dantrolene, it has been shown that 2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)propionate (AMPA) apparently exclusively stimulates Ca2+ influx through agonist-operated calcium channels and voltage-operated calcium channels. Increased [Ca2+]i due to exposure to kainate (KA) is for the major part caused by influx, as in the case of AMPA, but a small part of the increase in [Ca2+]i may be attributed to a release of Ca2+ from intracellular stores. Quisqualate (QA) stimulates Ca2+ release from an intracellular store that is independent of Ca2+ influx; presumably this store is activated by inositol phosphates. The increase in [Ca2+]i due to exposure to glutamate or N-methyl-D-aspartate (NMDA) may be compartmentalized into three components, one of which is related to influx and the other two to Ca2+ release from internal stores. Only one of the latter stores is dependent on Ca2+ influx with regard to release of Ca2+, whereas the other is activated by some other second messengers or, alternatively, directly coupled to the receptor. In muscles dantrolene is known to inhibit Ca2+ release from the sarcoplasmic reticulum, and also in neurons dantrolene inhibits an equivalent release from one or more hitherto unidentified internal Ca2+ pool(s). By using this drug it has been possible to show to what extent these Ca2+ stores are involved in the toxicity observed subsequent to exposure to the EAAs. It turned out that dantrolene, even under conditions allowing Ca2+ influx, inhibited toxicity induced by QA, NMDA, and glutamate, whereas that induced by AMPA or KA was unaffected. In combination with the findings that dantrolene inhibited release from the intracellular stores activated by QA, NMDA, and glutamate, it may be concluded that Ca2+ influx per se is not the primary event causing toxicity following exposure to these EAAs in these neurons. However, it may certainly be involved in the cases of toxicity induced by AMPA and KA.(ABSTRACT TRUNCATED AT 400 WORDS)

N-methyl-D-aspartate receptor excitotoxicity involves activation of polyamine synthesis: protection by alpha-difluoromethylornithine

We investigated the role of polyamines and their regulatory enzyme ornithine decarboxylase in N-methyl-D-aspartate-induced excitotoxicity in embryonic chick retina. N-Methyl-D-aspartate (200 microM) produced an early increase in ornithine decarboxylase activity, putrescine concentration, and Ca2+ entry, leading to selective neuronal death by 30 min. This response was attenuated by the ornithine decarboxylase inhibitor alpha-difluoromethylornithine and the N-methyl-D-aspartate receptor antagonist 5-aminophosphonovaleric acid. Exogenous putrescine increased intracellular putrescine and spermine levels and reversed neuroprotection by alpha-difluoromethylornithine, but not by 5-aminophosphonovaleric acid. N-Methyl-D-aspartate-receptor stimulation of putrescine/polyamine synthesis mediates abnormal Ca2+ entry and acute excitotoxic neuronal death. Postreceptor inhibition of the ornithine decarboxylase/polyamine cascade by alpha-difluoromethylornithine may provide neuroprotection against N-methyl-D-aspartate-induced excitotoxicity.

Evidence that the loss of the voltage-dependent Mg2+ block at the N-methyl-D-aspartate receptor underlies receptor activation during inhibition of neuronal metabolism

In this study, the importance of the Mg2+ blockade of the N-methyl-D-aspartate (NMDA) receptor during metabolic stress was examined in embryonic day 13 chick retina. Retina exposed to mild conditions of metabolic stress (i.e., blockade of glycolysis with 1 mM iodoacetate for 30 min) underwent acute histological somal and neuritic swelling and an increase in gamma-aminobutyric acid (GABA) release into the medium. These acute signs of metabolic stress were eliminated by NMDA antagonists present during pharmacological blockade of glycolysis, occurred in the absence of a net increase in extracellular glutamate or aspartate, and were not affected by the presence or absence of Ca2+ in the incubation medium. One possible explanation for the activation of NMDA receptors in the absence of an increase in extracellular ligand is that NMDA sensitivity during metabolic stress may be governed at the receptor level. Depolarization of membrane potential during metabolic stress may result in the loss of the Mg2+ blockade from the NMDA receptor channel, resulting in an increased potency for glutamate. To test this, the dose-response characteristics for NMDA, glutamate, and kainate in the presence or absence of extracellular Mg2+ and the effects of Mg2+ on metabolic inhibition were examined. The potency for NMDA- or glutamate-mediated acute toxicity was enhanced two- to fivefold in the absence of Mg2+. Omission of Mg2+ greatly decreased the minimal concentration of agonist needed to produce acute excitotoxicity; 25 versus 5 microM for NMDA and 300 versus 10 microM for glutamate in 1.2 or zero Mg2+, respectively. Elevating external Mg2+ to 20 mM completely protected against NMDA-mediated acute toxic effects. In contrast, varying external Mg2+ had no effect on kainate-induced toxicity. Acute toxicity caused by inhibition of metabolism was not potentiated in the absence of Mg2+ but was attenuated by elevating extracellular Mg2+. The protective effect of Mg2+ during metabolic inhibition was not additive with NMDA antagonists, suggesting that the action of Mg2+ was at the level of the NMDA receptor. These findings are consistent with the hypothesis that the Mg2+ block is lifted during metabolic inhibition and may be the primary event resulting in NMDA receptor activation.

Pattern of neuronal death in the rat hippocampus after status epilepticus. Relationship to calcium binding protein content and ischemic vulnerability

The pattern of hippocampal cell death has been studied following hippocampal seizure activity and status epilepticus induced by 110-min stimulation of the perforant pathway in awake rats. The order of vulnerability of principal cells in the different hippocampal subfields--as determined by silver impregnation--was found to be very similar to the pattern found in ischemia; i.e., dentate hilus greater than CA1, subiculum greater than CA3c greater than CA3a,b greater than dentate granule cells. The hilar somatostatin-containing cells were the most vulnerable cell type, whereas all other subpopulations of nonprincipal neurons--visualized by immunocytochemistry for the calcium binding proteins parvalbumin and calbindin--were remarkably resistant. Pyramidal cells in the CA3 region containing neither of the examined calcium binding proteins were more resistant to overexcitation than CA1 pyramidal cells, most of which do contain calbindin. This indicates that no simple relationship exists between vulnerability in status epilepticus and neuronal calcium binding protein content, and that local and/or systemic hypoxia during status epilepticus may be responsible for the ischemic pattern of cell death.

Developmental differences in antagonism of NMDA toxicity by the polyamine site antagonist ifenprodil

Antagonists of 4 distinct regulatory sites on the N-methyl-D-aspartate (NMDA) receptor were tested for their ability to attenuate NMDA-mediated acute excitotoxicity in isolated chick retina of various embryonic ages between days 11 and 19 in ovo. Acute excitotoxicity was monitored by histology and by release of endogenous gamma-aminobutyric acid (GABA) into the medium during 30 min of incubation with 50 microM NMDA. The uncompetitive PCP channel site antagonist, MK-801, the competitive antagonist, CGS 19755, and the strychnine-insensitive glycine site antagonist, 7-chlorokynurenate, completely blocked NMDA-induced cell swelling and increased GABA release at all ages tested. Potencies versus NMDA were MK-801 greater than CGS 19755 greater than 7-chlorokynurenate with IC50S of 0.02, 0.62, and 15 microM, respectively. NMDA antagonism by the polyamine site antagonist, ifenprodil, differed from other classes of antagonists in several respects. At the earlier embryonic ages tested (E12-13) ifenprodil provided differential protection; completely blocking somal and neuritic swelling in most but not all inner nuclear layer neurons and inner plexiform processes. In dose-response studies, ifenprodil attenuated the NMDA-induced increase in medium GABA at all ages tested with an Imax of 10 microM. Ifenprodil, however, showed a decreased ability to completely protect some NMDA-sensitive neurons. This was reflected both histologically and by GABA release. Maximal attenuation of NMDA evoked GABA release was 83, 80, 62 and 50% at days E12, 13, 15 and 19, respectively. Histologically, differential protection was seen at E12 and 13, in limited areas at E15, and was no longer present at E19.(ABSTRACT TRUNCATED AT 250 WORDS)

Paradoxical potentiation by low extracellular Ca2+ of acute chemical anoxic neuronal injury in cerebellar granule cell culture

Acute chemical anoxic injury was produced in primary cerebellar granule cell cultures incubated with iodoacetate (IAA) alone or IAA combined with potassium cyanide (KCN). Cytotoxicity was assessed using Trypan blue exclusion or LDH release. Four millimolars of KCN induced approx 30% neuron death at 3 h, whereas greater than 50% cell death was produced by 0.2 mM IAA. No potentiation of cytotoxicity was observed by IAA + KCN. A total of 0.2 mM IAA produced an early major reduction of intracellular ATP prior to the onset of neuron injury or reduction in intracellular glutathione (GSH). Medium Na+ replacement by choline, K+, or methylglucamine protected against IAA-induced neuronal injury, reduced the rate of decline of intracellular ATP but had no effect on intracellular GSH. Some 80% neuronal survival was obtained when Na+ was deleted from the medium even after the intracellular ATP had been reduced to less than 10% of control. Removal of Ca2+ from the medium had no effect on control culture, Trypan blue exclusion, GSH, or ATP, but potentiated the onset and magnitude of IAA-induced cytotoxicity. ATP and GSH decline. Loading of granule cells with the Ca2+ chelator Fura-2 did not influence IAA-induced cytotoxicity in control or low Ca2+ media. Addition of 50 microM glutamate had a minimal cytotoxic effect over 3 h and the combined addition of 0.2 mM IAA plus 50 microM glutamate did not potentiate IAA-induced injury. The glutamate receptor antagonists, D-2-amino-5-phosphonovaleric acid (APV) or kynurenate did not block IAA-induced injury in control medium but inhibited the potentiation of toxicity seen in the low Ca2+ medium. This study suggests the use of IAA as a chemical anoxic agent in cerebellar granule cell culture. The early, dose-dependent decline in ATP may be dissociated from GSH change. Acute IAA-induced injury is Na+/Cl- dependent but paradoxically potentiated in low Ca2+ medium. The low Ca2+ potentiated component was sensitive to glutamate/NMDA receptor antagonists and associated with reduction of intracellular GSH.

Thalamic retrograde degeneration following cortical injury: an excitotoxic process?

Traumatic or stroke-like injuries of the cerebral cortex result in the rapid retrograde degeneration of thalamic relay neurons that project to the damaged area. Although this phenomenon has been well documented, neither the basis for the relay neuron's extreme sensitivity to axotomy nor the mechanisms involved in the degenerative process have been clearly identified. Physiological and biochemical studies of the thalamic response to cortical ablation indicate that pathological overexcitation might contribute to the degenerative process. The responses of thalamic projection neurons, protoplasmic astrocytes, and inhibitory thalamic reticular neurons in adult mice were examined from one to 120 days following ablation of the somatosensory cortex as part of an investigation of the role of excitotoxicity in thalamic retrograde degeneration. The responses of thalamic neurons to cortical ablation were compared with those produced by intracortical injection of the convulsant excitotoxin kainic acid, since the degeneration of neurons in connected brain structures distant to the site of kainic acid injection is also thought to occur via an excitotoxic mechanism. Within two days after either type of cortical injury, protoplasmic astrocytes in affected regions of the thalamic ventrobasal complex and the medial division of the posterior thalamic nuclei became reactive and expressed increased levels of immunohistochemically detectable glial fibrillary acidic protein. Within the affected regions of the ventrobasal complex an increased intensity of puncta positive for glutamate decarboxylase immunoreactivity, presumably due to an increase in its content within the terminals of the reciprocally interconnected thalamic reticular neurons, was also evident. These immunohistochemically detectable alterations in the milieu of the damaged thalamic neurons preceded the disappearance of the affected relay neurons by at least two days following cortical ablation and by seven to 10 days following intracortical kainic acid injection. Regions of the thalamus containing reactive astrocytes corresponded very closely to the regions undergoing retrograde degeneration. Protoplasmic astrocytes in these areas remained intensely reactive up to 60 days after cortical injury. Levels of glutamate decarboxylase were only transiently elevated in the degenerating regions of the ventrobasal complex following cortical ablation and returned to normal by 14 days. Increased glutamate decarboxylase immunoreactivity was transiently seen through the entire ventrobasal complex following intracortical kainic acid injection but was markedly more intense in degenerating regions. These patterns of labeling did not return to normal until 50 days after intracortical kainic acid injection, well after the death of the relay neurons. Cortical ablation and intracortical kainic acid injection produce similar alterations in thalamic neuronal and glial populations.(ABSTRACT TRUNCATED AT 400 WORDS)

Intracellular calcium and neurotoxic events

Calcium is important in many intracellular regulatory processes. However, the maintenance of low levels of this cation within the cytosol is essential for maintenance of cell viability, in view of the large concentration gradient of ionic calcium across the plasma membrane. The expenditure of energy is needed to maintain intracellular calcium concentration [Ca2+]i at normal levels. In addition, the integrity of the limiting membrane is also vital for this function. Thus, any disruption of membrane characteristics or of mitochondrial anabolic processes may lead to deleterious levels of [Ca2+]i. The toxicity of a wide range of unrelated agents may, therefore, be in part due to elevation of cytosolic calcium. This general event may synergize with the more selective harmful properties of a compound, thus adversely affecting cell metabolism. The capacity now exists to measure levels of [Ca2+]i in isolated cells or organelles such as synaptosomes. The use of such in vitro models can be of value in the evaluation of the neurotoxic potential of compounds. This method, in conjunction with the use of pharmacological agents known to act at specific sites, and with the use of radioactive calcium in translocation studies, also has utility in the delineation of the biochemical mode of action of neurotoxic agents.

Excitatory amino acid-induced toxicity in chick retina: amino acid release, histology, and effects of chloride channel blockers

Acute excitotoxicity in embryonic chick retina and the ability of Cl- channel blockers to prevent toxicity were evaluated by measurement of endogenous amino acid release and histology. Treatment of retina with kainate, quisqualate, or N-methyl-D-aspartate resulted in a large dose-dependent release of gamma-aminobutyric acid and taurine, moderate release of glutamine and alanine, and no measurable release of glutamate or aspartate. Concentrations inducing maximal gamma-aminobutyric acid release were 50 microM quisquaalate, 100 microM kainate, and 100 microM N-methyl-D-aspartate. Treatment with 1 mM glutamate resulted in significant gamma-aminobutyric acid release, as well as an elevation in medium aspartate levels. Typical excitotoxic retinal lesions were produced by the agonists and, at the lower concentrations tested, revealed a regional sensitivity. There was a positive correlation between the amount of gamma-aminobutyric acid release and the extent of tissue swelling, suggesting that release may be secondary to toxic cellular events. Omission of Cl- completely blocked cytotoxic effects due to kainate or glutamate. Likewise, addition of the Cl-/bicarbonate anion channel blocker 4,4'-diisothiocyanatostilbene-2,2'-disulfonate at 600 microM protected retina from cytotoxic damage from all excitotoxic analogs and restored amino acid levels to baseline values. Furosemide, which blocks Na+/K+/2Cl- cotransport, was only minimally effective in reducing amino acid release induced by the agonists. Consistent with the latter, histological examination showed the continued presence of the lesion but with general reduction of cellular edema. These results indicate that although influx of Cl- is a central component of the acute excitotoxic phenomenon, mechanisms other than passive Cl- flux may be involved.

Cerebral ischemia revisited: new insights as revealed using in vitro brain slice preparations

The elucidation of the pathophysiological mechanisms of cerebral ischemia/hypoxia dictates the use of experimental models which mimic this disabling brain condition. In vivo experimental models have been available for many decades and are responsible for the bulk of, though incomplete, knowledge we have about these mechanisms. Since study in isolation of each postulated mechanism is impossible in vivo, the need for an in vitro experimental model has intensified in recent years. Consequently, rat and guinea pig hippocampal slice preparations have emerged as the models of choice. This review attempts to highlight some of the results obtained using brain slices in the study of cerebral ischemia/hypoxia and compare them to those obtained in vivo. Both the biochemical and the physiological correlates of energy metabolism, ion homeostasis, neurotransmission and neuromodulation of this brain condition are reviewed. The agreements, and especially the disagreements, between the in vivo and in vitro findings are emphasized. Details are given of the possible roles of both lactic acid, Ca2+ and excitotoxins in the neuronal damage inflicted by cerebral ischemia/hypoxia. Recent attempts to protect brain slices against experimental cerebral ischemic/hypoxic damage are also reviewed here briefly.

Neurotoxicity of excitatory amino acid receptor agonists in young rat hippocampal slices

Hippocampal slices from young (8-day-old) rats were evaluated as a model for investigating the mechanisms underlying the neurotoxic action of excitatory amino acid receptor agonists. The slices were exposed to the agonists for up to 30 min and were then postincubated for 90 min in order to allow irreversibly damaged cells to become visibly necrotic. Under control conditions (greater than or equal to 3 h incubation) all regions of the hippocampus and dentate gyrus displayed good preservation. Exposure of the slices to N-methyl-D-aspartate (NMDA) resulted in widespread, oedematous necrosis of all neuronal types (except undifferentiated granule cells) which was maximal after 20 min exposure to a concentration of 100 microM. With 30 min exposure, the EC50 for NMDA was 30 microM; 10 min exposure to NMDA at a concentration of 100 microM was sufficient to destroy 50% of the neurones. Quisqualate produced a degeneration of most (98%) of the CA3 neurones, a proportion (65%) of CA1 neurons and some (25%) of the dentate granule cells. The occurrence of "dark cell degeneration" was prevalent. Half maximal effects on CA3 neurones were estimated to be produced by a concentration of 15 microM (with 30 min exposure) or by 8 min exposure (at 100 microM concentration). Incubation of the slices with kainate (100 microM for 30 min) did not cause widespread damage but led to the necrosis of a small population of cells scattered in all regions of the hippocampus and dentate gyrus. The patterns of toxicity of the different agonists resemble closely those found after their administration in vivo. It is suggested that the hippocampal slices provide a valuable new model system for studying excitatory amino acid toxicity.

Aspartate neurotoxicity on cultured cortical neurons

L-aspartate neurotoxicity was quantitatively characterized in murine cortical cell cultures. Five-minute exposure to 30 microM-3 mM L-aspartate resulted in concentration-dependent (ED50 about 190 microM) neuronal destruction over the next 10 hr; glia were not injured. D-aspartate and L-aspartate were roughly equipotent neurotoxins. Ion substitution experiments suggested that L-aspartate neurotoxicity is comprised of both acute, sodium-dependent "excitotoxicity" and delayed, calcium-dependent degeneration, with the latter predominant under conditions of brief exposure. Aspartate neurotoxicity could be attenuated by D-2-amino-5-phosphonovalerate (D-APV), dextrorphan, ketamine, and kynurenate, but not by L-glutamate diethyl ester or gamma-D-glutamylaminomethyl sulfonate, consistent with principal involvement of N-methyl-D-aspartate receptors. D-APV and dextrorphan produced different effects on the aspartate concentration-toxicity relation; the former drug was consistent with a competitive and the latter with a noncompetitive mechanism of antagonism.

Calcium-binding protein (calbindin-D28k) and parvalbumin immunocytochemistry: localization in the rat hippocampus with specific reference to the selective vulnerability of hippocampal neurons to seizure activity

Two neuronal calcium-binding proteins, calbindin-D28k (CaBP) and parvalbumin (PV), were localized in the normal rat hippocampus by using immunocytochemical methods to determine 1) their location and 2) whether a correlation exists between the presence of these two calcium-binding proteins and the selective vulnerability of different hippocampal neuronal populations to experimental seizure activity. CaBP-like immunoreactivity (CaBP-LI) is present in all dentate granule cells and some, but not all, CA1 and CA2 pyramidal cells. Some CA1 pyramidal cells lack CaBP-LI, and those that do are lightly stained compared to the dentate granule cells. CA3 pyramidal cells appear to contain neither CaBP- nor PV-LI, and no granule or pyramidal cells exhibit PV-LI. CaBP-LI is present in distinct populations of dentate and hippocampal interneurons but absent from others. In area dentata, CaBP-LI is present in a small number of interneurons of the molecular and granule cell layers and in a small population of presumed basket cells in or below the granule cell layer. Conversely, more presumed dentate basket cells exhibit PV-LI than CaBP-LI. In the hilus of area dentata, few cells are CaBP- or PV-immunoreactive. The hilar somatostatin/neuropeptide Y (NPY)-immunoreactive cells and hilar mossy cells, two distinct and large populations, lack CaBP- and PV-LI. In the CA3 region, CaBP-LI is present in a relatively small number of interneurons in each stratum. PV-immunoreactive interneurons in area CA3 are more numerous. In area CA1, CaBP-LI is present in many interneurons in strata radiatum and lacunosum-moleculare. Some, but relatively fewer, CaBP-positive interneurons are present in strata pyramidale and oriens. Conversely, PV-immunoreactive interneurons are numerous in strata pyramidale and oriens but rare in strata radiatum and lacunosum-moleculare. Staining with the particulate chromagen benzidine hydrochloride revealed a previously undescribed dense band of CaBP-LI in the inner dentate molecular layer, a lamina enriched with kainate-displaceable glutamate-binding sites and innervated by the apparently excitatory ipsilateral associational/commissural (IAC) pathway that originates in the CaBP-negative hilar mossy cells. Bilateral electrical stimulation of the perforant path was performed in order to destroy the hilar mossy cells and to determine if this band of CaBP-LI is normally present within the mossy cell terminals. Perforant path stimulation that destroyed hilar mossy cells throughout the dorsal portions of both hippocampi did not abolish the dense CaBP-like immunoreactivity in the inner molecular layer.

Calcium dependency of N-methyl-D-aspartate toxicity in slices from the immature rat hippocampus

The literature on ionic requirements for excitotoxicity is largely contradictory. Depending on the experimental paradigms, it has been concluded that either Ca2+ or Na+ and Cl- mediate excitotoxicity. In the present study, the dependence on Ca2+ of N-methyl-D-aspartate-induced damage to neurons in immature rat hippocampal slices was investigated with light microscopy. In addition N-methyl-D-aspartate-induced cell damage was followed by measurement of release of lactate dehydrogenase from slices. When incubated in N-methyl-D-aspartate-containing (100 microM) buffer for 30 min, hippocampal neurons displayed fine chromatin aggregation and swelling of neuronal nuclei and neuropil. Slices incubated in standard medium for 90 min after exposure to N-methyl-D-aspartate contained a large number of neurons that failed to recover from the initial lesion. The acute edema was at least as severe in slices incubated in N-methyl-D-aspartate-containing, Ca2+-free buffer. In contrast, clumping of the chromatin could not be observed. CA1 neurons recovered completely from the acute changes, and granule cells recovered to some extent. While omission of Ca2+ had no obvious morphological effects on the tissue in its own right, the efflux of lactate dehydrogenase was significantly increased after incubation in Ca2+-free medium. Slices exposed to N-methyl-D-aspartate released approximately twice as much lactate dehydrogenase as controls 1-5 h after the exposure, and the same rate of release was seen if Ca2+ was absent during N-methyl-D-aspartate treatment. The morphological results suggest that N-methyl-D-aspartate toxicity is Ca2+-dependent in pyramidal cells whereas the toxicity in granule cells is partly Ca2+-independent.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular calcium as an index of neurotoxic damage

The availability of techniques that allow the quantitation of levels of ionized calcium within intact cells and synaptosomes, allows a new approach to understanding the events underlying neurotoxicity. By use of various pharmacological agents, it is possible to dissect out vulnerable loci within the cell that account for the increases in cytosolic calcium which accompany many neurotoxic events. The relation between calcium and cell death can now be more concisely addressed. This article gives examples of the types of questions that this new methodology can resolve.

L-homocysteate is a potent neurotoxin on cultured cortical neurons

L-Homocysteate (L-HCA) neurotoxicity was quantitatively studied in dissociated cell cultures prepared from the fetal mouse neocortex. Five minute exposure to 3 microM-1 mM L-HCA was associated with neuronal cell loss, but not glial cell loss; the extent of neuronal damage was dependent on the concentration of L-HCA, with an ED50 of approximately 40 microM. The stereoisomer D-HCA was a somewhat weaker neurotoxin than L-HCA. Ion substitution experiments suggested that L-HCA neurotoxicity can be separated into two components on the basis of differences in time course and ionic dependence: an acute, sodium-dependent 'excitotoxic' component, marked by rapid early cell swelling; and a late, calcium-dependent component, marked by delayed cell degeneration. L-HCA neurotoxicity could be attenuated by 2-amino-5-phosphonovalerate (APV), ketamine, and kynurenate, but not by L-glutamate diethyl ester or gamma-D-glutamylaminomethyl sulfonate, consistent with a predominant involvement of N-methyl-D-aspartate receptors. APV and ketamine produced different effects on the L-HCA concentration-toxicity relation, the former drug consistent with a competitive, and the latter drug consistent with a non-competitive, mechanism of antagonism.

Pharmacological protection against the toxicity of N-methyl-D-aspartate in immature rat cerebellar slices

In order to delineate the pharmacological characteristics of the toxicity of N-methyl-D-aspartate (NMDA), slices of cerebellum from 7-day old rats were incubated with NMDA, together with various putative protective agents. These comprised three different groups: (i) a competitive receptor antagonist (kynurenic acid), (ii) direct (cobalt ions, flunarizine) and indirect (taurine) calcium entry blockers, (iii) cyclo-oxygenase inhibitors (indomethacin and acetylsalicylic acid) and a blocker of calcium-activated, neutral proteases (leupeptin). When the slices were incubated for 30 min in medium containing 100 microM NMDA, postmigratory granule cell nuclei were rounded and swollen. After 90 min of recovery in normal medium, the nuclei were pyknotic and the cells were irreversibly injured. As expected, these changes were completely blocked by kynurenate, indicating that NMDA receptors mediate the cell death. Cobalt ions abolished the acute toxicity of NMDA, but after recovery, some granule cell nuclei were swollen. This effect could be attributed to the toxicity of cobalt ions and not to delayed toxicity of NMDA. The other inhibitors of the uptake of calcium, flunarizine and taurine, did neither affect acute nor persistent toxicity of NMDA. These results support the previous finding that the toxicity of NMDA is calcium-dependent and that organic calcium channel blockers are ineffective against NMDA-induced uptake of calcium. Leupeptin had no effect on the toxicity of NMDA, suggesting that calcium-activated proteolysis was not the crucial event in excitotoxic necrosis. Indomethacin, but not acetylsalicylic acid, prevented neuronal degeneration provoked by NMDA, but only in very large concentrations (greater than or equal to 100 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

L-homocysteic acid: an endogenous excitotoxic ligand of the NMDA receptor

L-Homocysteic acid (L-HCA) has been proposed as a natural transmitter at the N-methyl-D-aspartate (NMDA) subtype of excitatory amino acid receptor based on recent evidence that L-HCA occurs L-HCA occurs naturally in the mammalian CNS, is released from K+ stimulated brain slices in a calcium-dependent manner and may be contained in nerve terminals located in certain brain regions that have a high density of NMDA receptors. Here we report that L-HCA potently induces a pattern of cytopathology in the ex vivo chick retina which mimics the pattern of NMDA but not kainic acid (KA) neurotoxicity. We also show that known NMDA antagonists, including Mg++, D-aminophosphonopentanoate and certain anesthetics, analgesics, and sedative hypnotics block the neurotoxic actions of L-HCA in direct proportion to their efficacy in blocking NMDA neurotoxicity. While there is a perfect correspondence between agents that block NMDA and L-HCA neurotoxicity, only a few such agents are active against KA neurotoxicity. We find that 3H-Glu binding is inhibited more potently by L-HCA (Ki = 67 microM). Moreover the patterns with which L-HCA and NMDA displace 3H-Glu binding in autoradiograms appear essentially identical. These findings are consistent with the proposal that L-HCA is an endogenous ligand at NMDA receptors.

Excitatory amino acid receptor potency and subclass specificity of sulfur-containing amino acids

The sulfur-containing amino acids, L- and D-cysteate, L-cysteine, L- and D-cysteine sulfinate, L- and D-cysteine-S-sulfate, L-cystine, L- and D-homocysteate, L- and D-homocysteine sulfinate, L-homocysteine, L-serine-O-sulfate, and taurine were tested in two excitatory amino acid receptor functional assays and in receptor binding assays designed to label specifically the AA1/N-methyl-D-aspartate (NMDA), AA2/quisqualate, and AA3/kainate receptor recognition sites, as well as a CaCl2-dependent L-2-amino-4-phosphonobutanoate site, and a putative glutamate uptake site. Agonist efficacies were determined by chick retinal excitotoxicity and stimulated sodium efflux from rat brain slices. D-Homocysteine sulfinate, L-homocysteate, and L-serine-O-sulfate had affinities most selective for the NMDA binding site, whereas the binding affinities of D-cysteate, D-cysteine sulfinate, D-homocysteate, and L-homocysteine sulfinate were less selective. However, the correlation of agonist activity sensitive to blockade by D-2-amino-7-phosphonoheptanoate or D-2-amino-5-phosphonopentanoate in the functional assays with affinity in the NMDA binding assay (r = 0.87, p less than 0.005 and r = 0.98, p less than 0.005 for excitotoxicity and sodium efflux, respectively) allows characterization of these sulfur-containing amino acids as acting at NMDA subclass receptors. L-Homocysteate, which has been found in the brain, and L-serine-O-sulfate are selective agonists and could serve as endogenous neurotransmitters at the NMDA receptor.

Neurotoxicity of calcium ionophore A23187 in immature rat cerebellar slices

The causal role of Ca2+ in neuronal necrosis is controversial and it has been suggested that neuronal Ca2+ uptake is only a secondary effect to cell death. Here, I address this question directly by studying the morphological effects of calcium ionophore A23187 on immature cerebellar slices. Parasagittal slices were prepared and incubated for 30, 90 or 120 min in physiological saline with or without A23187. In some cases Ca2+ was omitted from the incubation medium. Slices were processed for light microscopy. A23187 produced nuclear changes indicative of cell death that encompassed cells of the external granule cell layer at short incubation times (30 min) and more deeply situated cells at longer times (120 min). This indicates that A23187 diffusion is limited in the slice. The histological changes produced by 30 min exposure to the ionophore could not be reversed by incubation for 90 min in normal medium. Necrosis was never observed when slices were exposed to A23187 in Ca2+-free medium. The results demonstrate that influx of excessive amounts of Ca2+ kills cells of the central nervous system.

Cortical neurons exposed to glutamate rapidly leak preloaded 51chromium

The acute toxic effects of excess glutamate exposure on cortical neurons in culture was followed using a novel adaptation of the 51chromium efflux assay. Although the acute, sodium-dependent phase of glutamate neurotoxicity may contribute to several acute disease settings, including sustained seizures and stroke, functional aspects of the phenomenon have not been previously studied. We report here that the earliest morphologic sign of glutamate neurotoxicity, neuronal swelling, is accompanied by a large efflux of complexed 51chromium from preloaded neurons in the first hour after exposure, and that this efflux is detectable as early as 15 min after the onset of glutamate exposure. We suggest that this pathological burst of 51chromium may result from glutamate-induced "leakiness" of neuronal cell membranes.

Effects of Ca2+ entry blockers on kainate-induced changes in extracellular amino acids and Ca2+ in vivo

The effect of organic Ca2+ channel blockers and Co2+ on kainate-induced changes in 45Ca2+ efflux and amino acid release was studied in the rabbit hippocampus with the dialysis-perfusion technique. Administration of 1 mM kainate caused a transient, 50% drop of extracellular Ca2+. This effect was insensitive to 100 microM flunarizine or verapamil, 10 microM nimodipine, and 6 mM CoCl2. The organic Ca2+ entry blockers did not significantly influence kainate-induced changes in extracellular amino acids, whereas Co2+ affected both basal and kainic acid stimulated release of amino acids. These results indicate that kainate-regulated Ca2+ ionophores differ from Ca2+ channels in peripheral tissues in terms of sensitivity to Ca2+ entry inhibitors.

Amino acid neurotoxicity: intracellular sites of calcium accumulation associated with the onset of irreversible damage to rat cerebellar neurones in vitro

Electron microscopy and the combined oxalate-pyroantimonate technique were used to locate calcium in intracerebellar nucleus neurones of rat cerebellar slices subjected to a neurotoxic concentration of N-methyl-D-aspartate. After a sub-lethal exposure period (5 min) calcium pyroantimonate deposits were found in swollen cisterns of the Golgi apparatus and, in lesser amounts, in the nuclei. Deposits were more prominent in the nuclei after a just-lethal exposure (10 min) when they were additionally observed within a population of swollen mitochondria and also apparently free in the dendritic and somatic cytoplasm. The results support the proposal that amino acid neurotoxicity is a consequence of an intracellular Ca2+ overload brought about by excessive Ca2+ influx.

Blockade of N-methyl-D-aspartate-sensitive acidic amino acid receptors inhibits ischemia-induced accumulation of purine catabolites in the rat striatum

The effect of blocking N-methyl-D-aspartate (NMDA)-sensitive excitatory amino acid (EAA) receptors during brain ischemia was studied in order to test a link between EAAs and neuronal energy metabolism. The receptors were blocked unilaterally in the rat striatum before, during and after an ischemic insult. The receptor blocker, D-2-amino-5-phosphonovalerate (D-APV) was administered by dialysis perfusion, which also allowed continuous sampling for analysis of adenosine triphosphate degradation products, i.e. purine catabolites, in control and D-APV-treated striata. Purine catabolites were analysed with reversed-phase liquid chromatography. Hypoxanthine, xanthine, inosine and adenosine increased dramatically in the striatum during ischemia and reached maximum levels during early reperfusion. D-APV reduced the extracellular accumulation of all measured purine catabolites during ischemia/reflow and improved to some extent the recovery of the striatal electroencephalographic activity in the majority of the animals. The results suggest that NMDA receptor blockade attenuates acute changes in energy metabolism during ischemia.

The anti-excitotoxic effects of certain anesthetics, analgesics and sedative-hypnotics

Various agents were tested for their ability to antagonize the acute excitotoxic action of N-methyl-DL-aspartate (NMA) and kainic acid (KA) on neurons in the in vitro chick embryo retina. The following compounds (in order of descending potencies) were effective in completely blocking the neurotoxic activity of NMA: phencyclidine, ketamine, (+/-)-SKF 10,047, pentazocine, D-aminophosphonovalerate, D-amino-phosphonoheptanoate, D-alpha-aminoadipate, OH-quinoxaline carboxylate, kynurenate, (+/-)-cis-2,3-piperidine dicarboxylate, secobarbital, amobarbital and pentobarbital. The latter 6 agents also protected against KA toxicity but complete protection was observed only from relatively high concentrations. At 20 mM, Mg2+ blocked NMA toxicity but at concentrations up to 30 mM did not block KA toxicity. Compounds that failed to block either NMA or KA toxicity include D- and L-aminophosphonobutyrate, L-glutamic acid diethyl ester, xanthurenate, GABA and taurine. The chick embryo retina is a useful preparation for identifying agents that have either excitotoxic or anti-excitotoxic activity.