Glutamate-induced calcium loads: effects on energy metabolism and neuronal viability

Neurotransmitters in the mediation of cerebral ischemic injury

Under physiological conditions, neurotransmitters shape neuronal networks and control several cellular and synaptic functions. In the mammalian central nervous system (CNS), excitatory and inhibitory neurotransmission are mediated in large part by glutamate and gamma-aminobutyric acid (GABA), which are excitatory and inhibitory neurotransmitters, respectively. Glutamate and GABA also play crucial roles in neurological disorders such as cerebral ischemia. Glutamate in particular causes excitotoxicity, known as one of the hallmark mechanisms in the pathophysiology of cerebral ischemic injury for more than thirty years. Excitotoxicity occurs due to excessive glutamate release leading to overactivation of postsynaptic glutamate receptors, which evokes a downstream cascade that eventually leads to neuronal dysfunction and degeneration. Also, a reduction in GABA receptor response after ischemia impedes these inhibitory effectors from attenuating excitotoxicity and thereby further enabling the excitotoxic insult. This review focuses on the mechanisms by which glutamate and GABA mediate excitotoxicity and ischemic injury. This article is part of the Special Issue entitled 'Cerebral Ischemia'.

Mitochondrial calcium homeostasis: Implications for neurovascular and neurometabolic coupling

Mitochondrial function is critical to maintain high rates of oxidative metabolism supporting energy demands of both spontaneous and evoked neuronal activity in the brain. Mitochondria not only regulate energy metabolism, but also influence neuronal signaling. Regulation of "energy metabolism" and "neuronal signaling" (i.e. neurometabolic coupling), which are coupled rather than independent can be understood through mitochondria's integrative functions of calcium ion (Ca²⁺) uptake and cycling. While mitochondrial Ca²⁺ do not affect hemodynamics directly, neuronal activity changes are mechanistically linked to functional hyperemic responses (i.e. neurovascular coupling). Early in vitro studies lay the foundation of mitochondrial Ca²⁺ homeostasis and its functional roles within cells. However, recent in vivo approaches indicate mitochondrial Ca²⁺ homeostasis as maintained by the role of mitochondrial Ca²⁺ uniporter (mCU) influences system-level brain activity as measured by a variety of techniques. Based on earlier evidence of subcellular cytoplasmic Ca²⁺ microdomains and cellular bioenergetic states, a mechanistic model of Ca²⁺ mobilization is presented to understand systems-level neurovascular and neurometabolic coupling. This integrated view from molecular and cellular to the systems level, where mCU plays a major role in mitochondrial and cellular Ca²⁺ homeostasis, may explain the wide range of activation-induced coupling across neuronal activity, hemodynamic, and metabolic responses.

Cytosolic calcium regulation in rat afferent vagal neurons during anoxia

Sensory neurons are able to detect tissue ischaemia and both transmit information to the brainstem as well as release local vasoactive mediators. Their ability to sense tissue ischaemia is assumed to be primarily mediated through proton sensing ion channels, lack of oxygen however may also affect sensory neuron function. In this study we investigated the effects of anoxia on isolated capsaicin sensitive neurons from rat nodose ganglion. Acute anoxia triggered a reversible increase in [Ca2+]i that was mainly due to Ca2+-efflux from FCCP sensitive stores and from caffeine and CPA sensitive ER stores. Prolonged anoxia resulted in complete depletion of ER Ca2+-stores. Mitochondria were partially depolarised by acute anoxia but mitochondrial Ca2+-uptake/buffering during voltage-gated Ca2+-influx was unaffected. The process of Ca2+-release from mitochondria and cytosolic Ca2+-clearance following Ca2+ influx was however significantly slowed. Anoxia was also found to inhibit SERCA activity and, to a lesser extent, PMCA activity. Hence, anoxia has multiple influences on [Ca2+]i homeostasis in vagal afferent neurons, including depression of ATP-driven Ca2+-pumps, modulation of the kinetics of mitochondrial Ca2+ buffering/release and Ca2+-release from, and depletion of, internal Ca2+-stores. These effects are likely to influence sensory neuronal function during ischaemia.

A compound CP-31398 suppresses excitotoxicity-induced neurodegeneration

Neurodegeneration causes dysfunction and degeneration of neurons and is triggered by various factors including genetic defects, free radicals, injury, and glutamate excitotoxicity. Among those, glutamate excitotoxicity is implicated in chronic disorders including AD and ALS, and in acute insults in the CNS including traumatic brain injury. Neurological disorders show hallmark morphological abnormalities such as axon degeneration and cell body death. The molecular mechanisms underlying excitotoxicity-induced neurodegeneration are complex and deciphering a molecular mechanism from one angle is beneficial to understand the process, however, still difficult to develop strategies to suppress excitotoxicity-induced degeneration due to existence of other mechanisms. Thus, directly identifying compounds that can modulate excitotoxicity-induced neurodegeneration and subsequently clarifiying the molecular mechanism is a valid approach to develop effective strategies to suppress neurodegeneration. We searched for compounds that can suppress excitotoxicity-induced neurodegeneration and found that CP-31398, a known compound that can rescue the structure and function of the tumor suppressor protein p53 mutant form and stabilize the active conformation of the p53 wild-type form, suppresses excitotoxicity-induced axon degeneration and cell body death. Moreover, CP-31398 suppresses mitochondrial dysfunction which has a strong correlation with excitotoxicity. Thus, our findings identify a compound that can serve as a novel modulator of neurodegeneration induced by glutamate excitotoxicity.

Glutamate receptors, neurotoxicity and neurodegeneration

Glutamate excitotoxicity is a hypothesis that states excessive glutamate causes neuronal dysfunction and degeneration. As glutamate is a major excitatory neurotransmitter in the central nervous system (CNS), the implications of glutamate excitotoxicity are many and far-reaching. Acute CNS insults such as ischaemia and traumatic brain injury have traditionally been the focus of excitotoxicity research. However, glutamate excitotoxicity has also been linked to chronic neurodegenerative disorders such as amyotrophic lateral sclerosis, multiple sclerosis, Parkinson's disease and others. Despite the continued research into the mechanisms of excitotoxicity, there are currently no pharmacological interventions capable of providing significant neuroprotection in the clinical setting of brain ischaemia or injury. This review addresses the current state of excitotoxic research, focusing on the structure and physiology of glutamate receptors; molecular mechanisms underlying excitotoxic cell death pathways and their interactions with each other; the evidence for glutamate excitotoxicity in acute neurologic diseases; laboratory and clinical attempts at modulating excitotoxicity; and emerging targets for excitotoxicity research.

Mitochondrial calcium function and dysfunction in the central nervous system

The ability of isolated brain mitochondria to accumulate, store and release calcium has been extensively characterized. Extrapolation to the intact neuron led to predictions that the in situ mitochondria would reversibly accumulate Ca(2+) when the concentration of the cation in the vicinity of the mitochondria rose above the 'set-point' at which uptake and efflux were in balance, storing Ca(2+) as a complex with phosphate, and slowly releasing the cation when plasma membrane ion pumps lowered the cytoplasmic free Ca(2+). Excessive accumulation of the cation was predicted to lead to activation of the permeability transition, with catastrophic consequences for the neuron. Each of these predictions has been confirmed with intact neurons, and there is convincing evidence for the permeability transition in cellular Ca(2+) overload associated with glutamate excitotoxicity and stroke, while the neurodegenerative disease in which possible defects in mitochondrial Ca(2+) handling have been most intensively investigated is Huntington's Disease. In this brief review evidence that mitochondrial Ca(2+) transport is relevant to neuronal survival in these conditions will be discussed.

Mitochondria and calcium signaling

The kinetic properties for the uptake, storage and release of Ca2+ from isolated mitochondria accurately predict the behaviour of the organelles within the intact cell. While the steady-state cycling of Ca2+ across the inner membrane between independent uptake and efflux pathways seems at first sight to be symmetrical, the distinctive kinetics of the uniporter, which is highly dependent on external free Ca2+ concentration and the efflux pathway, whose activity is clamped over a wide range of total matrix Ca2+ by the solubility of the calcium phosphate complex provide a mechanism whereby mitochondria reversibly sequester transient elevations in cytoplasmic Ca2+. Under non-stimulated conditions, the same transport processes can regulate matrix Ca2+ concentrations and hence citric acid cycle activity.

Effects of anoxia and aglycemia on cytosolic calcium regulation in rat sensory neurons

Nociceptive neurons play an important role in ischemia by sensing and transmitting information to the CNS and by secreting peptides and nitric oxide, which can have local effects. While these responses are probably primarily mediated by acid sensing channels, other events occurring in ischemia may also influence neuron function. In this study, we have investigated the effects of anoxia and anoxic aglycemia on Ca²⁺ regulation in sensory neurons from rat dorsal root ganglia. Anoxia increased [Ca²⁺]_i by evoking Ca²⁺ release from two distinct internal stores one sensitive to carbonyl cyanide p-(trifluoromethoxy) phenylhydrazone (FCCP) and one sensitive to caffeine, cyclopiazonic acid (CPA), and ryanodine [assumed to be the endoplasmic reticulum (ER)]. Anoxia also promoted progressive decline in ER Ca²⁺ content. Despite partially depolarizing mitochondria, anoxia had relatively little effect on mitochondrial Ca²⁺ uptake when neurons were depolarized but substantially delayed mitochondrial Ca²⁺ release and subsequent Ca²⁺ clearance from the cytosol on repolarization. Anoxia also reduced both sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) activity and Ca²⁺ extrusion [probably via plasma membrane Ca²⁺-ATPase (PMCA)]. Thus anoxia has multiple effects on [Ca²⁺]_i homeostasis in sensory neurons involving internal stores, mitochondrial buffering, and Ca²⁺ pumps. Under conditions of anoxic aglycemia, there was a biphasic and more profound elevation of [Ca²⁺]_i, which was associated with complete ER Ca²⁺ store emptying and progressive, and eventually complete, inhibition of Ca²⁺ clearance by PMCA and SERCA. These data clearly show that loss of oxygen, and exhaustion of glycolytic substrates, can profoundly affect many aspects of cell Ca²⁺ regulation, and this may play an important role in modulating neuronal responses to ischemia.

The protective role of cellular glutathione peroxidase against trauma-induced mitochondrial dysfunction in the mouse brain

Reactive oxygen species are believed to participate in the pathogenesis of traumatic brain injury (TBI). To evaluate the role of cellular glutathione peroxidase (Gpx1), a selenium-containing enzyme functioning in reduction of hydrogen peroxide and alkyl hyperoxides, in protecting animals against TBI, a line of Gpx1 transgenic mice was generated. Overexpression of Gpx1 was found in many organs including the brain of the transgenic mice. This line of transgenic mice and knockout mice deficient in Gpx1 were used in a model of controlled cortical impact injury and the efficiency of oxidative phosphorylation in brain mitochondria was determined. Although a 2-mm depth of mechanical impact caused a drastic decrease in NAD-linked electron transfer activities and energy-coupling capacities in brain mitochondria of nontransgenic mice, the decrease in mitochondrial function was completely prevented by overexpression of Gpx1 in Gpx1 transgenic mice. In addition, a 1-mm deformation depth hardly affected brain mitochondrial function in wild-type (Gpx1+/+) mice, yet resulted in a significant decrease in mitochondrial bioenergetic capacity in brains of homozygous Gpx1 knockout (Gpx1-/-) mice. Further experiments showed that inclusion of calcium chelator egtazic acid in measurement of mitochondrial respiration could completely restore the efficiency of mitochondrial respiration in injured brains of nontransgenic mice and Gpx1-/- mice, suggesting that the observed mitochondrial dysfunction is a direct result of increase in mitochondrion-associated calcium, which is secondary to the increased oxidative stress. These studies not only establish the role of Gpx1 in preventing mitochondrial dysfunction in mouse brain after TBI, but also suggest the species of reactive oxygen responsible for this event.

Protective effect of cerebrocrast on rat brain ischaemia induced by occlusion of both common carotid arteries

Diabetes mellitus is accompanied by several cardiovascular complications including atherosclerosis, cerebral ischaemia and stroke. We examined the neuroprotective effect of a 1,4-dihydropyridine derivative cerebrocrast (C, a new antidiabetic agent, synthesized in the Latvian Institute of Organic Synthesis) on the level of ATP in the brain, and on changes of the EEG and ECG, as well as blood pressure parameters in anaesthetized Wistar male rats before and during 10-min occlusion of both common carotid arteries. Cerebrocrast was administered i.v. at doses of 1.0 and 10 microg/kg in the v. femoralis 20 min prior to ischaemia. After 10-min ischaemia animals were decapitated and the brain was immediately frozen in liquid nitrogen and subsequently used for analysis of changes of ATP contention. Cerebrocrast, administered at doses of 1.0 and 10 microg/kg 20 min prior to occlusion of both common carotid arteries, completely prevented a fall in the ATP content of brain compared with the control rats. In control rats the content of ATP in brain during ischaemia decreased from 2.77 +/- 0.22 (basal level) to 1.74 +/- 0.20 micromol/g as a result of ischaemia. By administration of cerebrocrast 20 min before occlusion of the arteries, the content of ATP in the brain remained at the level of preischaemia (1.0 microg/kg C + ischaemia 2.82 +/- 0.36; 10 microg/kg C + ischaemia 2.42 +/- 0.22 micromol/g). Analysis of EEG parameters both before and during 10 min of occlusion showed that at a C dose of 1.0 microg/kg before occlusion produced a regular alpha rhythm during ischaemia and prevented cerebral bioelectric activity from significant changes. The depression of basal rhythm was observed at a C dose of 10 microg/kg during ischaemia in two rats out of six as well as an increase in the ECG ST segment above the isoelectric line. Blood pressure was decreased by about 10-20 mm Hg. We propose that pretreatment of rats with cerebrocrast at doses of 1.0 or 10 microg/kg 20 min prior to ischaemia can prevent ischaemic damage of rat brain, maintain necessary energy consumption, promote ATP production in brain cells, and prevent significant changes in EEG and ECG parameters. These properties are important in diabetes mellitus and its evoked cardiovascular complications as stroke, ischaemia, etc.

GCPII (NAALADase) inhibition prevents long-term diabetic neuropathy in type 1 diabetic BB/Wor rats

AIMS/HYPOTHESIS

Hyperglutamatergic activity induced by ischemia is believed to underlie neuronal damage in a variety of neurological disorders, including neuropathic pain. Since ischemia is believed to be a prominent mechanism involved in diabetic polyneuropathy (DPN), we investigated the effect of the glutamate carboxypeptidase II (GCPII, EC #3.4-17.21; previously termed NAALADase), an enzyme responsible for the hydrolysis of the neuropeptide NAAG to NAA and glutamate, on the development of DPN in type 1 diabetic BB/Wor rats.

METHODS

Diabetic animals were treated with 10 mg/kg/day i.p. of the selective GCPII inhibitor GPI-5232 from onset of diabetes for 6 months. Hyperalgesia to thermal stimulation and nerve conduction velocity (NCV) were measured monthly. The effect on structural DPN was assessed by scoring of single, teased myelinated fibers, myelinated fiber morphometry and ultrastructural examination of C-fibers at 6 months.

RESULTS

GCPII inhibition showed significant but partial effects on hyperalgesia (p<0.001), nerve conduction slowing (p<0.01) axonal and nodal structural changes (p<0.001), small myelinated fiber atrophy, and degenerative changes of C-fibers.

CONCLUSIONS

GCPII inhibition has beneficial effects on hyperalgesia, nerve function, and structural degenerative changes in DPN, which are likely mediated by inhibition of ischemia-induced glutamate release.

Dose-response curve and optimal dosing regimen of cyclosporin A after traumatic brain injury in rats

Acute neuropathology following experimental traumatic brain injury results in the rapid necrosis of cortical tissue at the site of injury. This primary injury is exacerbated in the ensuing hours and days via the progression of secondary injury mechanism(s) leading to significant neurological dysfunction. Recent evidence from our laboratory demonstrates that the immunosuppressant cyclosporin A significantly ameliorates cortical damage following traumatic brain injury. The present study extends the previous findings utilizing a unilateral controlled cortical impact model of traumatic brain injury in order to establish a dose-response curve and optimal dosing regimen of cyclosporin A. Following injury to adult rats, cyclosporin A was administrated at various dosages and the therapy was initiated at different times post-injury. In addition to examining the effect of cyclosporin A on the acute disruption of the blood-brain barrier following controlled cortical impact, we also assessed the efficacy of cyclosporin A to reduce tissue damage utilizing the fluid percussion model of traumatic brain injury. The findings demonstrate that the neuroprotection afforded by cyclosporin A is dose-dependent and that a therapeutic window exists up to 24h post-injury. Furthermore, the optimal cyclosporin dosage and regimen markedly reduces disruption of the blood-brain barrier acutely following a cortical contusion injury, and similarly affords significant neuroprotection following fluid percussion injury. These findings clearly suggest that the mechanisms responsible for tissue necrosis following traumatic brain injury are amenable to pharmacological intervention.

Continuous infusion of cyclosporin A postinjury significantly ameliorates cortical damage following traumatic brain injury

Traumatic brain injury (TBI) results in the rapid necrosis of cortical tissue at the site of injury. In the ensuing hours and days, secondary injury exacerbates the original damage resulting in significant neurological dysfunction. Recent reports from our lab demonstrate that a bolus injection of the immunosuppressant cyclosporin A (CsA) is neuroprotective following TBI. CsA transiently inhibits the opening of the mitochondrial permeability transition pore and maintains calcium homeostasis in isolated mitochondria. The present study utilized a unilateral controlled cortical impact model of TBI to assess whether the neuroprotective effects of CsA could be extended by chronic infusion. Adult rats were subjected to a moderate (2 mm) cortical deformation and the extent of cortical damage was assessed using modern stereological techniques. Animals were administrated a 20 mg/kg intraperitoneal bolus of CsA or vehicle 15 min postinjury and osmotic minipumps were implanted subcutaneously to deliver CsA (4.5 or 10 mg/kg/day) or vehicle. All animals receiving CsA demonstrated a significant reduction in lesion volume, with the highest dose offering the most neuroprotection (74% reduction in lesion volume). These results extend our previous findings and demonstrate that chronic infusion of CsA is neuroprotective following TBI. These findings also suggest that the mechanisms responsible for tissue necrosis following TBI are amenable to manipulation.

Mitochondria and neuronal survival

Mitochondria play a central role in the survival and death of neurons. The detailed bioenergetic mechanisms by which isolated mitochondria generate ATP, sequester Ca(2+), generate reactive oxygen species, and undergo Ca(2+)-dependent permeabilization of their inner membrane are currently being applied to the function of mitochondria in situ within neurons under physiological and pathophysiological conditions. Here we review the functional bioenergetics of isolated mitochondria, with emphasis on the chemiosmotic proton circuit and the application (and occasional misapplication) of these principles to intact neurons. Mitochondria play an integral role in both necrotic and apoptotic neuronal cell death, and the bioenergetic principles underlying current studies are reviewed.

Mitochondria regulate the Ca(2+)-exocytosis relationship of bovine adrenal chromaffin cells

The present study expands the contemporary view of mitochondria as important participants in cellular Ca(2+) dynamics and provides evidence that mitochondria regulate the supply of release-competent secretory granules. Using pharmacological probes to inhibit mitochondrial Ca(2+) import, the ability of mitochondria to modulate secretory activity in single, patch-clamped bovine chromaffin cells was examined by simultaneously monitoring rapid changes in membrane surface area (DeltaC(m)) and cytosolic Ca(2+) levels ([Ca(2+)](c)). Repetitive step depolarizations or action potential waveforms were found to raise the [Ca(2+)](c) of chromaffin cells into the 1 microM to tens of micromolar range. Inhibiting mitochondria by treatment with carbonyl cyanide p-(trifuoro-methoxy)phenylhydrazone, antimycin-oligomycin, or ruthenium red revealed that mitochondria are a prominent component for the clearance of Ca(2+) that entered via voltage-activated Ca(2+) channels. Disruption of cellular Ca(2+) homeostasis by poisoning mitochondria enhanced the secretory responsiveness of chromaffin cells by increasing the amplitude of the transient rise and the time course of recovery to baseline of the evoked Delta[Ca(2+)](c). The enhancement of the secretory response was represented by significant deviation of the Ca(2+)-exocytosis relationship from a standard relationship that equates Ca(2+) influx and DeltaC(m). Thus, mitochondria would play a critical role in the control of secretory activity in chromaffin cells that undergo tonic or repetitive depolarizing activity, likely by limiting the Ca(2+)-dependent activation of specific proteins that recruit or prime secretory granules for exocytosis.

Cyclosporin A attenuates acute mitochondrial dysfunction following traumatic brain injury

Experimental traumatic brain injury (TBI) results in a rapid and significant necrosis of cortical tissue at the site of injury. In the ensuring hours and days, secondary injury exacerbates the primary damage, resulting in significant neurological dysfunction. Recent reports from our lab and others have demonstrated that the immunosuppressant cyclosporin A (CsA) is neuroprotective following TBI. The opening of the mitochondrial permeability transition pore (MPTP) is inhibited by CsA, thereby maintaining the mitochondrial membrane potential and calcium homeostasis in isolated mitochondrial. In the present study we utilized a unilateral controlled cortical impact model of TBI to assess mitochondrial dysfunction in both isolated mitochondria and synaptosomes to elucidate the neuroprotective role of CsA. The results demonstrate that administration of CsA 15 min postinjury significantly attenuates mitochondrial dysfunction as measured using several biochemical assays of mitochondria integrity and energetics. Following TBI, mitochondria isolated from the injured cortex of animals treated with CsA demonstrate a significant increase in mitochondria membrane potential and are resistant to the induction of mitochondrial permeability transition compared to vehicle-treated animals. Similarly, synaptosomes isolated from CsA-treated animals demonstrate a significant increase in mitochondria membrane potential, accompanied by lower levels of intramitochondrial Ca2+ and reactive oxygen species production than seen in vehicle-treated animals. These results suggest that the neuroprotective properties of CsA are mediated through modulation of the MPTP and maintenance of mitochondria homeostasis. Amelioration of cortical damage with CsA indicates that pharmacological therapies can be devised which will significantly alter neurological outcome after injury.

Spreading depression determines acute cellular damage in the hippocampal slice during oxygen/glucose deprivation

During ischaemia neurons depolarize and release the neurotransmitter L-glutamate, which accumulates extracellularly and binds to postsynaptic receptors. This initiates a sequence of events thought to culminate in immediate and delayed neuronal death. However, there is growing evidence that during ischaemia the development of spreading depression (SD) can be an important determinant of the degree and extent of ischaemic damage. In contrast, SD without metabolic compromise (as occurs in migraine aura) causes no discernible damage to brain tissue. SD is a profound depolarization of neurons and glia that propagates like a wave across brain tissue. Brain cell swelling, an early event of both the excitotoxic process and of SD, can be assessed by imaging associated intrinsic optical signals (IOSs). We demonstrate here that IOS imaging clearly demarcates the ignition site and migration of SD across the submerged hippocampal slice of the rat. If SD is induced by elevating [K+]O, the tissue fully recovers, but in slices that are metabolically compromised at 37.5 degrees C by oxygen/glucose deprivation (OGD) or by ouabain exposure, cellular damage develops only where SD has propagated. Specifically, the evoked CA1 field potential is permanently lost, the cell bodies of involved neurons swell and their dendritic regions increase in opacity. In contrast to OGD, bath application of L-glutamate (6-10 mM) at 37.5 degrees C evokes a non-propagating LT increase in CA1 that reverses without obvious cellular damage. Moreover, application of 2-20 mM glutamate or various glutamate agonists fail to evoke SD in the submerged hippocampal slice. We propose that SD and OGD together (but not alone) constitute a 'one-two punch', causing acute neuronal death in the slice that is not replicated by elevated glutamate. These findings support the proposal that SD generation during stroke promotes and extends acute ischaemic damage.

From calcium signaling to cell death: two conformations for the mitochondrial permeability transition pore. Switching from low- to high-conductance state

The permeability transition pore (PTP) is a channel of the inner mitochondrial membrane that appears to operate at the crossroads of two distinct physiological pathways, i.e., the Ca2+ signaling network during the life of the cell, and the effector phase of the apoptotic cascade during Ca2+-dependent cell death. Correspondingly, two open conformations of the PTP can also be observed in isolated organelles. A low-conductance state, that allows the diffusion of small ions like Ca2+, is pH-operated, promoting spontaneous closure of the channel. A high-conductance state, that allows the unselective diffusion of big molecules, stabilizes the channel in the open conformation, disrupting in turn the mitochondrial structure and causing the release of proapoptotic factors. Our current results indicate that switching from low- to high-conductance state is an irreversible process that is strictly dependent on the saturation of the internal Ca2+-binding sites of the PTP. Thus, the high-conductance state of the PTP, which was shown to play a pivotal role in the course of excitotoxic and thapsigargin-induced cell death, might result from a Ca2+-dependent conformational shift of the low-conductance state, normally participating in the regulation of cellular Ca2+ homeostasis as a pH-operated channel. These observations lead us to propose a simple biophysical model of the transition between Ca2+ signaling and Ca2+-dependent apoptosis.

The regulation of reactive oxygen species production during programmed cell death

Reactive oxygen species (ROS) are thought to be involved in many forms of programmed cell death. The role of ROS in cell death caused by oxidative glutamate toxicity was studied in an immortalized mouse hippocampal cell line (HT22). The causal relationship between ROS production and glutathione (GSH) levels, gene expression, caspase activity, and cytosolic Ca2+ concentration was examined. An initial 5-10-fold increase in ROS after glutamate addition is temporally correlated with GSH depletion. This early increase is followed by an explosive burst of ROS production to 200-400-fold above control values. The source of this burst is the mitochondrial electron transport chain, while only 5-10% of the maximum ROS production is caused by GSH depletion. Macromolecular synthesis inhibitors as well as Ac-YVAD-cmk, an interleukin 1beta-converting enzyme protease inhibitor, block the late burst of ROS production and protect HT22 cells from glutamate toxicity when added early in the death program. Inhibition of intracellular Ca2+ cycling and the influx of extracellular Ca2+ also blocks maximum ROS production and protects the cells. The conclusion is that GSH depletion is not sufficient to cause the maximal mitochondrial ROS production, and that there is an early requirement for protease activation, changes in gene expression, and a late requirement for Ca2+ mobilization.

Intrinsic optical signaling denoting neuronal damage in response to acute excitotoxic insult by domoic acid in the hippocampal slice

Using the seafood contaminant domoic acid (an AMPA/kainate receptor agonist), we demonstrate a distinct excitotoxic sequence of events leading to acute neuronal damage in the hippocampal slice as measured by (1) loss of the evoked CA1 field potential, (2) irreversible changes in light transmittance, (3) histopathology, and (4) lucifer yellow injection of single CA1 pyramidal neurons. Change in light transmittance (LT) through the submerged slice indirectly measures altered cell volume, both neuronal and glial. At 37 degrees C, a 1-min superfusion of 10 mu M domoate induced a prolonged reversible increase in LT, primarily in the dendritic regions of CA1 and dentate granule cells (GC), but not in the CA3 region. Spectral analysis (400-800 nm) revealed a wide-band transmittance increase, indicating cell swelling as a major source of the intrinsic signal. The evoked field potential recorded in the CA1 cell body region (PYR) was lost as LT peaked, but completely recovered upon return to the baseline LT level. Increasing domoate exposure to 10 min elicited a different and distinct LT sequence in CA1 and dentate regions. An initial LT increase in dendritic regions evolved in an irreversible decrease in LT. At the same time, LT irreversibly increased in cell body regions (CA1 PYR and GC) and the evoked field potential was irretrievably lost. Also, there was histological damage to cell body and dendritic regions of CA1 and granule cells. Injection of lucifer yellow into single CA1 neurons in slices displaying the irreversible LT sequence revealed extensive dendritic beading, whereas CA1 cells in control slices displayed a smoothly contoured arbor. Consistent with acute neuronal damage, the optical changes generated by domoate did not require extracellular Ca2+, and lowering the temperature protected the slice from irreversible damage to CA1 and GC regions. Although glial changes may also occur, we conclude that imaging light transmittance reveals dynamic and compartmentalized excitotoxic changes in neuronal volume. Beading of the dendritic arbor increases light scatter, thereby decreasing LT and highlighting damaged dendritic regions.

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.