Glutamate induced cell death: apoptosis or necrosis?

Antioxidant and protective mechanisms against hypoxia and hypoglycaemia in cortical neurons in vitro

In the present work, we have studied whether cell death could be induced in cortical neurons from rats subjected to different period of O₂ deprivation and low glucose (ODLG). This “in vitro” model is designed to emulate the penumbra area under ischemia. In these conditions, cortical neurons displayed loss of mitochondrial respiratory ability however, nor necrosis neither apoptosis occurred despite ROS production. The absence of cellular death could be a consequence of increased antioxidant responses such as superoxide dismutase-1 (SOD1) and GPX3. In addition, the levels of reduced glutathione were augmented and HIF-1/3α overexpressed. After long periods of ODLG (12–24 h) cortical neurons showed cellular and mitochondrial membrane alterations and did not recuperate cellular viability during reperfusion. This could mean that therapies directed toward prevention of cellular and mitochondrial membrane imbalance or cell death through mechanisms other than necrosis or apoptosis, like authophagy, may be a way to prevent ODLG damage.

Regulation of apoptotic and inflammatory cell signaling in cerebral ischemia: the complex roles of heat shock protein 70

Although heat shock proteins have been studied for decades, new intracellular and extracellular functions in a variety of diseases continue to be discovered. Heat shock proteins function within networks of interacting proteins; they can alter cellular physiology rapidly in response to stress without requiring new protein synthesis. This review focuses on the heat shock protein 70 family and considers especially the functions of the inducible member, heat shock protein 72, in the setting of cerebral ischemia. In general, inhibiting apoptotic signaling at multiple points and up-regulating survival signaling, heat shock protein 70 has a net prosurvival effect. Heat shock protein 70 has both antiinflammatory and proinflammatory effects depending on the cell type, context, and intracellular or extracellular location. Intracellular effects are often antiinflammatory with inhibition of nuclear factor-kappaB signaling. Extracellular effects can lead to inflammatory cytokine production or induction of regulatory immune cells and reduced inflammation.

Role of c-Fos protein on glutamate toxicity in primary neural hippocampal cells

The hippocampus is extremely sensitive to microenvironmental signals and toxic events, including massive glutamate release. Despite the extensive literature related to the cascade of molecular events triggered in postsynaptic neurons, the distinction between proapoptotic and survival pathways is still being discussed. In this study, we have investigated the role of c-Fos in glutamate-induced toxicity in primary cultures of hippocampal neurons by using antisense oligonucleotide (ASO) technology. Exposure of cells (5 days in vitro; DIV) to glutamate 0.5 mM for 24 hr caused massive nuclear alteration. An increase in the number of caspase-3-positive cells was also observed 24 hr after glutamate treatment. The expression of c-fos and c-jun immediate-early genes was increased 30 min after glutamate exposure. The study of c-Fos and c-Jun protein expression revealed an increase in the number of cells positive for both antibodies. To investigate whether the expression of c-Fos protein after glutamate treatment was related to cell death activation or cell survival pathways, cells were exposed to 5 microM of c-fos ASO at 4 DIV, 24 hr before glutamate treatment. The presence of the ASO in the medium significantly decreased the number of altered nuclei, and this was associated with a significant reduction in the number of c-Fos-positive cells after glutamate treatment. Exposure of cells to the c-fos ASO under the conditions described above decreased caspase-3 immunostaining induced by glutamate. These results suggest that the synthesis of c-Fos protein after glutamate exposure favors cell death pathway activation in which caspase-3 is also involved.

Oxidative stress and neuronal death/survival signaling in cerebral ischemia

It has been demonstrated by numerous studies that apoptotic cell death pathways are implicated in ischemic cerebral injury in ischemia models in vivo. Experimental ischemia and reperfusion models, such as transient focal/global ischemia in rodents, have been thoroughly studied and the numerous reports suggest the involvement of cell survival/death signaling pathways in the pathogenesis of apoptotic cell death in ischemic lesions. In these models, reoxygenation during reperfusion provides oxygen as a substrate for numerous enzymatic oxidation reactions and for mitochondrial oxidative phosphorylation to produce adenosine triphosphate. Oxygen radicals, the products of these biochemical and physiological reactions, are known to damage cellular lipids, proteins, and nucleic acids and to initiate cell signaling pathways after cerebral ischemia. Genetic manipulation of intrinsic antioxidants and factors in the signaling pathways has provided substantial understanding of the mechanisms involved in cell death/survival signaling pathways and the role of oxygen radicals in ischemic cerebral injury. Future studies of these pathways could provide novel therapeutic strategies in clinical stroke.

Extremely rapid induction of outer hair cell apoptosis in the chinchilla cochlea following exposure to impulse noise

We have reported the presence of OHC apoptosis and necrosis in the organ of Corti following exposure to intense noise. The current study was designed to investigate the rapidity and the initial pattern of outer hair cell (OHC) death induced by exposure to impulse noise. Chinchillas were exposed to 75 pairs of impulse noise at 155 dB peak sound pressure level presented over a time period of 75 s. At 5 or 30 min after the noise exposure, the cochleae were examined for morphological and biological indicators of apoptosis and necrosis. In the cochleae collected within 5 min after the 75-s noise exposure, there were clear signs of nuclear condensation and cell body shrinkage, suggesting the presence of OHC apoptosis. Apoptotic OHCs were further detected by positive staining of TUNEL and caspase-3 assays. In contrast to the rapid development of nuclear condensation, appearance of nuclear swelling, a necrotic phenotype, appeared at 30 min after the noise exposure. The results of the study demonstrate that induction of OHC apoptosis after the noise exposure is an extremely rapid process.

Beyond apoptosis: nonapoptotic cell death in physiology and disease

Apoptosis is a morphologically defined form of programmed cell death (PCD) that is mediated by the activation of members of the caspase family. Analysis of death-receptor signaling in lymphocytes has revealed that caspase-dependent signaling pathways are also linked to cell death by nonapoptotic mechanisms, indicating that apoptosis is not the only form of PCD. Under physiological and pathological conditions, cells demonstrate a high degree of flexibility in cell-death responses, as is reflected in the existence of a variety of mechanisms, including necrosis-like PCD, autophagy (or type II PCD), and accidental necrosis. In this review, we discuss recent data suggesting that canonical apoptotic pathways, including death-receptor signaling, control caspase-dependent and -independent cell-death pathways.Key words: apoptosis, necrosis, nonapoptotic programmed cell death, death receptors, ceramides.

Neurotoxicity of amphetamine derivatives is mediated by caspase pathway activation in rat cerebellar granule cells

The neurotoxic action of the abuse drugs methamphetamine (METH) and 3,4-methylenedioxymethamphetamine (MDMA) on cerebellar granule neurones (CGNs) culture was examined. Treatment for 48 h with METH or MDMA (1-5 mM) induced a higher decrease in viability than 24 h treatment. z.VAD.fmk (100 microM) but not MK-801 nor NBQX recovered control viability values. In both cases, cell death was characterised as apoptotic rather than necrotic by morphology cell observation. Apoptosis measured by flow cytometry indicated an increase in the hypodiploid population after 48 h treatment with METH and MDMA. Apoptosis was reverted by the presence of z.VAD.fmk (100 microM) but not by 10 microM MK-801 or NBQX. Similar results were obtained by analysing nuclear chromatine condensation. These results ruled out excitotoxic participation in amphetamine derivative-induced neurotoxicity in CGNs. Participation of radical oxygen species (ROS) was evaluated using alpha-tocopherol (1-15 microM) and cytometric studies. The co-treatment with 4 mM METH or MDMA for 48 h partially reverted neurotoxic action and apoptotic features, indicating ROS implication in CGNs death by amphetamine derivatives. Alteration of mitochondrial function induced cytochrome C (Cyt C) release after 48-h treatment with METH and MDMA (4 mM). There was also indication of caspase-3-like activation, measured by immunoanalysis and biochemically. Finally, neurodegenerative action caused by amphetamine derivatives may be prevented by using caspase inhibitors.

Neuronal death/survival signaling pathways in cerebral ischemia

Cumulative evidence suggests that apoptosis plays a pivotal role in cell death in vitro after hypoxia. Apoptotic cell death pathways have also been implicated in ischemic cerebral injury in in vivo ischemia models. Experimental ischemia and reperfusion models, such as transient focal/global ischemia in rodents, have been thoroughly studied and the numerous reports suggest the involvement of cell survival/death signaling pathways in the pathogenesis of apoptotic cell death in ischemic lesions. In these models, reoxygenation during reperfusion provides a substrate for numerous enzymatic oxidation reactions. Oxygen radicals damage cellular lipids, proteins and nucleic acids, and initiate cell signaling pathways after cerebral ischemia. Genetic manipulation of intrinsic antioxidants and factors in the signaling pathways has provided substantial understanding of the mechanisms involved in cell death/survival signaling pathways and the role of oxygen radicals in ischemic cerebral injury. Future studies of these pathways may provide novel therapeutic strategies in clinical stroke.

Reactive oxygen radicals and pathogenesis of neuronal death after cerebral ischemia

Reactive oxygen species have been implicated in brain injury after cerebral ischemia. These oxidants can damage proteins, lipids, and DNA, and lead to cell injury and necrosis. Oxidants are also initiators in intracellular cell death signaling pathways that may lead to apoptosis. The possible targets of this redox signaling include mitochondria, death membrane receptors, and DNA repair enzymes. Genetic manipulation of intrinsic antioxidants and the factors in the signaling pathways has provided substantial progress in understanding the mechanisms in cell death signaling pathways and involvement of oxygen radicals in ischemic brain injury. Future studies of these pathways may provide novel therapeutic strategies in clinical stroke.

Temporal and spatial distribution of activated caspase-3 after subdural kainic acid infusions in rat spinal cord

The molecular events initiating apoptosis following traumatic spinal cord injury (SCI) remain poorly understood. Soon after injury, the spinal cord is exposed to numerous secondary insults, including elevated levels of glutamate, that contribute to cell dysfunction and death. In the present study, we attempted to mimic the actions of glutamate by subdural infusion of the selective glutamate receptor agonist, kainic acid, into the uninjured rat spinal cord. Immunohistochemical colocalization studies revealed that activated caspase-3 was present in ventral horn motor neurons at 24 hours, but not 4 hours or 96 hours, following kainic acid treatment. However, at no time point examined was there evidence of significant neuronal loss. Kainic acid resulted in caspase-3 activation in several glial cell populations at all time points examined, with the most pronounced effect occurring at 24 hours following infusion. In particular, caspase-3 activation was observed in a significant number of oligodendroglia in the dorsal and ventral funiculi, and there was a pronounced loss of oligodendroglia at 96 hours following treatment. The results of these experiments indicate a role for glutamate as a mediator of oligodendroglial apoptosis in traumatic SCI. In addition, understanding the apoptotic signaling events activated by glutamate will be important for developing therapies targeting this cell death process.

Excitotoxic and excitoprotective mechanisms: abundant targets for the prevention and treatment of neurodegenerative disorders

Activation of glutamate receptors can trigger the death of neurons and some types of glial cells, particularly when the cells are coincidentally subjected to adverse conditions such as reduced levels of oxygen or glucose, increased levels of oxidative stress, exposure to toxins or other pathogenic agents, or a disease-causing genetic mutation. Such excitotoxic cell death involves excessive calcium influx and release from internal organelles, oxyradical production, and engagement of programmed cell death (apoptosis) cascades. Apoptotic proteins such as p53, Bax, and Par-4 induce mitochondrial membrane permeability changes resulting in the release of cytochrome c and the activation of proteases, such as caspase-3. Events occurring at several subcellular sites, including the plasma membrane, endoplasmic reticulum, mitochondria and nucleus play important roles in excitotoxicity. Excitotoxic cascades are initiated in postsynaptic dendrites and may either cause local degeneration or plasticity of those synapses, or may propagate the signals to the cell body resulting in cell death. Cells possess an array of antiexcitotoxic mechanisms including neurotrophic signaling pathways, intrinsic stress-response pathways, and survival proteins such as protein chaperones, calcium-binding proteins, and inhibitor of apoptosis proteins. Considerable evidence supports roles for excitotoxicity in acute disorders such as epileptic seizures, stroke and traumatic brain and spinal cord injury, as well as in chronic age-related disorders such as Alzheimer's, Parkinson's, and Huntington's disease and amyotrophic lateral sclerosis. A better understanding of the excitotoxic process is not only leading to the development of novel therapeutic approaches for neurodegenerative disorders, but also to unexpected insight into mechanisms of synaptic plasticity.

Molecular mechanisms of cerebral ischemia-induced neuronal death

The mode of neuronal death caused by cerebral ischemia and reperfusion appears on the continuum between the poles of catastrophic necrosis and apoptosis: ischemic neurons exhibit many biochemical hallmarks of apoptosis but remain cytologically necrotic. The position on this continuum may be modulated by the severity of the ischemic insult. The ischemia-induced neuronal death is an active process (energy dependent) and is the result of activation of cascades of detrimental biochemical events that include perturbion of calcium homeostasis leading to increased excitotoxicity, malfunction of endoplasmic reticulum and mitochondria, elevation of oxidative stress causing DNA damage, alteration in proapoptotic gene expression, and activation of the effector cysteine proteases (caspases) and endonucleases leading to the final degradation of the genome. In spite of strong evidence showing that brain infarction can be reduced by inhibiting any one of the above biochemical events, such as targeting excitotoxicity, up-regulation of an antiapoptotic gene, or inhibition of a down-stream effector caspase, it is becoming clear that targeting a single gene or factor is not sufficient for stroke therapeutics. An effective neuroprotective therapy is likely to be a cocktail aimed at all of the above detrimental events evoked by cerebral ischemia and the success of such therapeutic intervention relies upon the complete elucidation of pathways and mechanisms of the cerebral ischemia-induced active neuronal death.

Increased production of tumor necrosis factor-alpha induces apoptosis after traumatic spinal cord injury in rats

We showed previously that, after spinal cord injury (SCI), tumor necrosis factor-alpha (TNF-alpha) may serve as an external signal, initiating apoptosis in neurons and oligodendrocytes. To further characterize the apoptotic cascade initiated by TNF-alpha after SCI, we examined the expression of TNF-alpha, inducible nitric oxide (NO) synthase (iNOS), and the level of NO after SCI. Western blots and reverse transcription polymerase chain reactions showed an early upregulation of TNF-alpha after injury. A peak TNF-alpha expression was observed within 1 h of injury. By 4 h after injury, the expression of iNOS and the level of NO were markedly increased in the injured spinal cord. Terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end labeling (TUNEL)-positive cells were also first observed in the lesioned area 4 h after SCI. The largest number of TUNEL-positive cells was observed between 24-48 h after SCI. Injecting a neutralizing antibody against TNF-alpha into the lesion site after injury significantly reduced the expression of iNOS, the level of NO and the number of TUNEL-positive cells in the injured spinal cord. Injecting the NOS inhibitors, N(G)-monomethyl-L-arginine monoacetate and S-methylisothiourea sulfate, or an NO scavenger, carboxy-PTIO, into the lesion site also significantly reduced the level of NO and the degree of DNA laddering in the injured spinal cord. These data suggest that after SCI, apoptosis induced by TNF-alpha may be mediated in part by NO via upregulation of iNOS, induced in response to TNF-alpha.

Kainic acid-induced neuronal cell death in cerebellar granule cells is not prevented by caspase inhibitors

1. We examined the role of non-NMDA receptors in kainic acid (KA)-induced apoptosis in cultures of rat cerebellar granule cells (CGCs). KA (1 - 500 microM) induced cell death in a concentration-dependent manner, which was prevented by NBQX and GYKI 52466, non-NMDA receptor antagonists. Moreover, AMPA blocked KA-induced excitotoxicity, through desensitization of AMPA receptors. 2. Similarly, KA raised the intracellular calcium concentration of CGCs, which was inhibited by NBQX and GYKI 52466. Again, AMPA (100 microM) abolished the KA (100 microM)-induced increase in intracellular calcium concentration. 3. KA-induced cell death in CGCs had apoptotic features, which were determined morphologically, by DNA fragmentation, and by expression of the prostate apoptosis response-4 protein (Par-4). 5. KA (500 microM) slightly (18%) increased caspase-3 activity, which was strongly enhanced by colchicine (1 microM), an apoptotic stimulus. However, neither Z-VAD.fmk, a pan-caspase inhibitor, nor the more specific caspase-3 inhibitor, Ac-DEVD-CHO, prevented KA-induced cell death or apoptosis. In contrast, both drugs inhibited colchicine-induced apoptosis. 5. The calpain inhibitor ALLN had no effect on KA or colchicine-induced neurotoxicity. 6. Our findings indicate that colchicine-induced apoptosis in CGCs is mediated by caspase-3 activation, unlike KA-induced apoptosis.

Apoptosis has a prolonged role in the neurodegeneration after hypoxic ischemia in the newborn rat

Birth asphyxia can cause moderate to severe brain injury. It is unclear to what degree apoptotic or necrotic mechanisms of cell death account for damage after neonatal hypoxia-ischemia (HI). In a 7-d-old rat HI model, we determined the contributions of apoptosis and necrosis to neuronal injury in adjacent Nissl-stained, hematoxylin and eosin-stained, and terminal deoxynucleotidyl transferase-mediated UTP nick end-labeled sections. We found an apoptotic-necrotic continuum in the morphology of injured neurons in all regions examined. Eosinophilic necrotic neurons, typical in adult models, were rarely observed in neonatal HI. Electron microscopic analysis showed "classic" apoptotic and necrotic neurons and "hybrid" cells with intermediate characteristics. The time course of apoptotic injury varied regionally. In CA3, dentate gyrus, medial habenula, and laterodorsal thalamus, the density of apoptotic cells was highest at 24-72 hr after HI and then declined. In contrast, densities remained elevated from 12 hr to 7 d after HI in most cortical areas and in the basal ganglia. Temporal and regional patterns of neuronal death were compared with expression of caspase-3, a cysteine protease involved in the execution phase of apoptosis. Immunocytochemical and Western blot analyses showed increased caspase-3 expression in damaged hemispheres 24 hr to 7 d after HI. A p17 peptide fragment, which results from the proteolytic activation of the caspase-3 precursor, was detected in hippocampus, thalamus, and striatum but not in cerebral cortex. The continued expression of activated caspase-3 and the persistence of cells with an apoptotic morphology for days after HI suggests a prolonged role for apoptosis in neonatal hypoxic ischemic brain injury.

Transient NMDA receptor inactivation provides long-term protection to cultured cortical neurons from a variety of death signals

NMDA receptor antagonists, such as (+)-5-methyl-10, 11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate (MK-801), potently block glutamate-induced neuronal death in myriad in vitro cell models and effectively attenuate ischemic damage in vivo. In this report, a novel role for MK-801 and other NMDA receptor antagonists in preconditioning neurons to withstand a wide range of subsequent lethal insults is described. A brief 30 min exposure to 0.1 microM MK-801, applied up to 96 hr before a "lethal" insult, protected primary cortical neurons from a diverse group of neurotoxic agents, including NMDA, beta-amyloid, staurosporine, etoposide, and oxygen-glucose deprivation. This neuroprotective preconditioning by MK-801 arose from transient NMDA receptor inactivation, because the noncompetitive NMDA receptor antagonists memantine and nylindin and the competitive antagonist AP-5 gave similar effects. MK-801 protection was dependent on new protein synthesis during the first 2 hr, but not from 2 to 5 hr, after MK-801 exposure. The MK-801 transient did not alter the ability of NMDA to trigger normally lethal [Ca(2+)](i) influx 48 hr later, but it did block early downstream signaling events coupled to NMDA neurotoxicity, including PKC inactivation and the activation of calpain. Moreover, MK-801 protected neurons from staurosporine-induced apoptosis, although caspase activation in these cells was unimpeded. It is likely that the stress associated with transient inactivation of NMDA receptors triggered a rapid compensatory survival response that provided long-term protection from a spectrum of insults, inducing apoptotic and nonapoptotic death. The possibility that MK-801 preconditioning blocks an event common to seemingly diverse death mechanisms suggests it will be an important tool for obtaining a clearer understanding of the salient molecular events at work in neuronal death and survival pathways.

G protein-coupled receptor kinase-mediated desensitization of metabotropic glutamate receptor 1A protects against cell death

Metabotropic glutamate receptors (mGluRs) constitute a unique subclass of G protein-coupled receptors (GPCRs) that bear little sequence homology to other members of the GPCR superfamily. The mGluR subtypes that are coupled to the hydrolysis of phosphoinositide contribute to both synaptic plasticity and glutamate-mediated excitotoxicity in neurons. In the present study, the expression of mGluR1a in HEK 293 cells led to agonist-independent cell death. Since G protein-coupled receptor kinases (GRKs) desensitize a diverse variety of GPCRs, we explored whether GRKs contributed to the regulation of both constitutive and agonist-stimulated mGluR1a activity and thereby may prevent mGluR1a-mediated excitotoxicity associated with mGluR1a overactivation. We find that the co-expression of mGluR1a with GRK2 and GRK5, but not GRK4 and GRK6, reduced both constitutive and agonist-stimulated mGluR1a activity. Agonist-stimulated mGluR1a phosphorylation was enhanced by the co-expression of GRK2 and was blocked by two different GRK2 dominant-negative mutants. Furthermore, GRK2-dependent mGluR1a desensitization protected against mGluR1a-mediated cell death, at least in part by blocking mGluR1a-stimulated apoptosis. Our data indicate that as with other members of the GPCR superfamily, a member of the structurally distinct mGluR family (mGluR1a) serves as a substrate for GRK-mediated phosphorylation and that GRK-dependent "feedback" modulation of mGluR1a responsiveness protects against pathophysiological mGluR1a signaling.

Lipid alterations in transient forebrain ischemia: possible new mechanisms of CDP-choline neuroprotection

We have previously demonstrated that cytidine 5'-diphosphocholine (CDP-choline or citicoline) attenuated arachidonic acid (ArAc) release and provided significant protection for the vulnerable hippocampal CA(1) neurons of the cornu ammonis after transient forebrain ischemia of gerbil. ArAc is released by the activation of phospholipases and the alteration of phosphatidylcholine (PtdCho) synthesis. Released ArAc is metabolized by cyclooxygenases/lipoxygenases to form eicosanoids and reactive oxygen species (ROS). ROS contribute to neurotoxicity through generation of lipid peroxides and the cytotoxic byproducts 4-hydroxynonenal and acrolein. ArAc can also stimulate sphingomyelinase to produce ceramide, a potent pro-apoptotic agent. In the present study, we examined the changes and effect of CDP-choline on ceramide and phospholipids including PtdCho, phosphatidylethanolamine (PtdEtn), phosphatidylinositol (PtdIns), phosphatidylserine (PtdSer), sphingomyelin, and cardiolipin (an exclusive inner mitochondrial membrane lipid essential for electron transport) following ischemia/1-day reperfusion. Our studies indicated significant decreases in total PtdCho, PtdIns, PtdSer, sphingomyelin, and cardiolipin and loss of ArAc from PtdEtn in gerbil hippocampus after 10-min forebrain ischemia/1-day reperfusion. CDP-choline (500 mg/kg i.p. immediately after ischemia and at 3-h reperfusion) significantly restored the PtdCho, sphingomyelin, and cardiolipin levels as well as the ArAc content of PtdCho and PtdEtn but did not affect PtdIns and PtdSer. These data suggest multiple beneficial effects of CDP-choline: (1) stabilizing the cell membrane by restoring PtdCho and sphingomyelin (prominent components of outer cell membrane), (2) attenuating the release of ArAc and limiting its oxidative metabolism, and (3) restoring cardiolipin levels.

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.

Neural activity and survival in the developing nervous system

Recent evidence suggests that blockade of normal excitation in the immature nervous system may have profound effects on neuronal survival during the period of natural cell death. Cell loss following depression of electrical activity in the central nervous system (CNS) may explain the neuropsychiatric deficits in humans exposed to alcohol or other CNS depressants during development. Thus, understanding the role of electrical activity in the survival of young neurons is an important goal of modern basic and clinical neuroscience. Here we review the evidence from in vivo and in vitro model systems that electrical activity participates in promoting neuronal survival. We discuss the potential role of moderate elevations of intracellular calcium in promoting survival, and we address the possible ways in which activity and conventional trophic factors may interact.

Slow death of postnatal hippocampal neurons by GABA(A) receptor overactivation

Neurotransmitters can have both toxic and trophic functions in addition to their role in neural signaling. Surprisingly, chronic blockade of GABA(A) receptor activity for 5-8 d in vitro enhanced survival of hippocampal neurons, suggesting that GABA(A) receptor overactivation may be neurotoxic. Potentiating GABA(A) receptor activity by chronic treatment with the endogenous neurosteroid (3alpha,5alpha)-3-hydroxypregnan-20-one caused massive cell loss over 1 week in culture. Other potentiators of GABA(A) receptors, including benzodiazepines, mimicked the cell loss, suggesting that potentiating endogenous GABA activity is sufficient to produce neuronal death. Neurosteroid-treated neurons had lower resting intracellular calcium levels than control cells and produced smaller calcium rises in response to depolarizing challenges. Manipulating intracellular calcium levels with chronic elevated extracellular potassium or with the calcium channel agonist Bay K 8644 protected neurons. The results may have implications for the mechanisms of programmed cell death in the developing CNS as well as implications for the long-term consequences of chronic GABAmimetic drug use during development.

Spermine-induced toxicity in cerebellar granule neurons is independent of its actions at NMDA receptors

The neurotoxic actions of polyamines such as spermine have been linked to their modulation of NMDA receptors, resulting in an excitotoxic cell death. Here, we demonstrate that chronic exposure to the polyamine spermine and acute exposure to the combination of spermine and glutamate result in significant toxicity to primary cultures of cerebellar granule neurons (CGNs). However, in both cases this cell death (a) lacks the characteristic cell swelling associated with the necrotic cell death induced by glutamate and (b) is characterized by the widespread formation of apoptotic nuclei. Whereas dizocilpine (MK-801) blocks the synergistic cell death resulting from acute exposure to spermine plus glutamate, neither MK-801 nor the calcium chelator EGTA appreciably attenuates CGN death resulting from chronic exposure to spermine. Consistent with previous reports, glutamate, both acute and chronic, causes CGN death that is characterized by cell swelling, sensitivity to MK-801 and EGTA, and only small numbers of apoptotic nuclei. Spermine-induced toxicity is not blocked by either the protein synthesis inhibitor cycloheximide or the pancaspase inhibitor tert-butoxycarbonyl-Asp-(O-methyl) fluoromethyl ketone. However, the antioxidant butylated hydroxyanisole is an effective blocker of spermine-induced CGN death, suggesting a free-radical component to this cell death. The intact spermine molecule, rather than a catabolic by-product, is required for cell death because the amine oxidase inhibitors N1,N2-bis(2,3-butadienyl)-1,4-butanediamine and aminoguanidine fail to block this toxicity. Thus, in CGNs, spermine-induced toxicity does not occur by its modulation of NMDA receptors, although, under some circumstances, NMDA receptor activation can modulate spermine-induced toxicity.