The Physiological Roles of Amyloid-ß Peptide Hint at New Ways to Treat Alzheimer's Disease.

Amyloid-ß (Aß) is best known as the misfolded peptide that is involved in the pathogenesis of Alzheimer's disease (AD), and it is currently the primary therapeutic target in attempts to arrest the course of this disease. This notoriety has overshadowed evidence that Aß serves several important physiological functions. Aß is present throughout the lifespan, it has been found in all vertebrates examined thus far, and its molecular sequence shows a high degree of conservation. These features are typical of a factor that contributes significantly to biological fitness, and this suggestion has been supported by evidence of functions that are beneficial for the brain. The putative roles of Aß include protecting the body from infections, repairing leaks in the blood-brain barrier, promoting recovery from injury, and regulating synaptic function. Evidence for these beneficial roles comes from in vitro and in vivo studies, which have shown that the cellular production of Aß rapidly increases in response to a physiological challenge and often diminishes upon recovery. These roles are further supported by the adverse outcomes of clinical trials that have attempted to deplete Aß in order to treat AD. We suggest that anti-Aß therapies will produce fewer adverse effects if the known triggers of Aß deposition (e.g., pathogens, hypertension, and diabetes) are addressed first.

Pulmonary neuroendocrine carcinoma mimicking neurocysticercosis: a case report.

BACKGROUND: Neurocysticercosis occurs when the eggs of the pork tapeworm (Taenia solium) migrate and hatch into larvae within the central nervous system. Neurocysticercosis is the most common cause of seizures in the developing world and is characterized on brain imaging by cysts in different stages of evolution. In Canada, cases of neurocysticercosis are rare and most of these patients acquire the disease outside of Canada. We report the case of a patient with multiple intracranial lesions whose history and diagnostic imaging were consistent with neurocysticercosis. Pathological investigations ultimately demonstrated that her brain lesions were secondary to malignancy. Brain metastases are considered to be the most common cause of intracranial cystic lesions. CASE PRESENTATION: We present the case of a 60-year-old Canadian-born Caucasian woman with a subacute history of ataxia, lower extremity hyper-reflexia, and otalgia who resided near a pig farm for most of her childhood. Computed tomography and magnetic resonance imaging showed that she had multiple heterogeneous intracranial cysts, suggestive of neurocysticercosis. Despite a heavy burden of disease, serological tests for cysticercosis were negative. This result and a lack of the central scolices on neuroimaging that are pathognomonic of neurocysticercosis prompted whole-body computed tomography imaging to identify another etiology. The whole-body computed tomography revealed right hilar lymphadenopathy associated with soft tissue nodules in her chest wall and abdomen. A biopsy of an anterior chest wall nodule demonstrated high-grade poorly differentiated carcinoma with necrosis, which stained strongly positive for thyroid transcription factor-1 and synaptophysin on immunohistochemistry. A diagnosis of stage 4 metastatic small cell neuroendocrine carcinoma was made and our patient was referred for oncological palliative treatment. CONCLUSIONS: This case illustrates the importance of the diagnostic approach to intracranial lesions. Our patient's diagnosis of neuroendocrine carcinoma was delayed because of her nontraditional presentation. Despite extensive metastatic burden, the lack of perilesional edema and the identification of lesions appearing to be in various stages of development led to a pursuit of neurocysticercosis as the diagnosis. The absence of constitutional symptoms should not discount the possibility of malignancy from the differential diagnosis.

Dietary cholesterol induces hepatic inflammation and blunts mitochondrial function in the liver of high-fat-fed mice.

The present study investigated the role of dietary cholesterol and fat in the development of nonalcoholic fatty liver disease, a common liver disease in metabolic disorders. Mice were fed a diet of regular chow (CH), chow supplemented with 0.2% w/w cholesterol (CHC), high fat (HF, 45kcal%) or HF with cholesterol (HFC) for 17weeks. While both HF and HFC groups displayed hepatic steatosis and metabolic syndrome, only HFC group developed the phenotype of liver injury, as indicated by an increase in plasma level of alanine transaminase (ALT, by 50-80%). There were ~2-fold increases in mRNA expression of tumor necrosis factor α, interleukin 1ß and monocyte chemotactic protein 1 in the liver of HFC-fed mice (vs. HF) but no endoplasmic reticulum stress or oxidative stress was observed. Furthermore, cholesterol suppressed HF-induced increase of peroxisome proliferator-activated receptor γ coactivator 1α and mitochondrial transcription factor A expression and blunted fatty acid oxidation. Interestingly, after switching HFC to HF diet for 5weeks, the increases in plasma ALT and liver inflammatory markers were abolished but the blunted of mitochondrial function remained. These findings suggest that cholesterol plays a critical role in the conversion of a simple fatty liver toward nonalcoholic steatohepatitis possibly by activation of inflammatory pathways together with retarded mitochondrial function.

Uptake and Toxicity of Hemin and Iron in Cultured Mouse Astrocytes.

Hemin is a breakdown product of the blood protein, hemoglobin and is responsible for much of the secondary damage caused following a hemorrhagic stroke. Hemin is toxic to cultured astrocytes and it is thought that this toxicity is due to iron that is liberated when hemin is degraded. However, free iron applied to astrocytes is not toxic and the reason for this discrepancy is unknown. The present study exposed primary astrocyte cultures from neonatal mice to hemin-iron (25 µM hemin) or non-hemin iron (25 µM ferric ammonium citrate; FAC) for 12 or 24 h. Perls' and Turnbull's staining, as well as measures of cell viability and iron accumulation, were used to assess the valency, solubility and distribution of iron within cells. While cells accumulated similar amounts of iron from both sources, hemin was shown to be highly toxic to astrocytes, whereas FAC was not. Iron released by the degradation of hemin was present in both valencies (Fe(2+) and Fe(3+)), was mostly soluble and did not induce ferritin expression in most cells, whereas non-hemin iron (from FAC) was present in astrocytes almost exclusively as insoluble Fe(3+) and it induced widespread ferritin expression. These results show that the cellular mechanisms for processing hemin-iron and non-hemin iron are very different. The data suggest that hemin-iron has a greater potential to damage astrocytes by participating in unregulated redox reactions.

Validity of a screening tool for detecting subtle cognitive impairment in the middle-aged and elderly.

The present study tested 121 middle-aged and elderly community-dwelling individuals on the computer-based Subtle Cognitive Impairment Test (SCIT) and compared their performance with that on several neuropsychological tests. The SCIT had excellent internal consistency, as demonstrated by a high split-half reliability measure (0.88-0.93). Performance on the SCIT was unaffected by the confounding factors of sex, education level, and mood state. Many participants demonstrated impaired performance on one or more of the neuropsychological tests (Controlled Oral Word Association Task, Rey Auditory and Verbal Learning Task, Grooved Pegboard [GP], Complex Figures). Performance on SCIT subtests correlated significantly with performance on many of the neuropsychological subtests, and the best and worst performing quartiles on the SCIT subtest discriminated between good and poor performers on other subtests, collectively indicating concurrent validity of the SCIT. Principal components analysis indicated that SCIT performance does not cluster with performance on most of the other cognitive tests, and instead is associated with decision-making efficacy, and processing speed and efficiency. Thus, the SCIT is responsive to the processes that underpin multiple cognitive domains, rather than being specific for a single domain. Since the SCIT is quick and easy to administer, and is well tolerated by the elderly, it may have utility as a screening tool for detecting cognitive impairment in middle-aged and elderly populations.

Phenanthrolines protect astrocytes from hemin without chelating iron.

Hemin, the degradation product of hemoglobin, contributes to the neurodegeneration that occurs in the weeks following a hemorrhagic stroke. The breakdown of hemin in cells releases redox-active iron that can facilitate the production of toxic hydroxyl radicals. The present study used 3-week old primary cultures of mouse astrocytes to compare the toxicity of 33 µM hemin in the presence of the iron chelator 1,10-phenanthroline or its non-chelating analogue, 4,7-phenanthroline. This concentration of hemin killed approximately 75 % of astrocytes within 24 h. Both isoforms of phenanthroline significantly decreased the toxicity of hemin, with the non-chelating analogue providing complete protection at concentrations of 33 µM and above. The decrease in toxicity was associated with less cellular accumulation of hemin. Approximately 90 % of the hemin accumulated was not degraded, irrespective of treatment condition. These observations indicate that chelatable iron is not the cause of hemin toxicity. Cell-free experiments demonstrated that hemin can inactivate a molar excess of hydrogen peroxide (H2O2), and that the rate of inactivation is halved in the presence of either isoform of phenanthroline. We conclude that phenanthrolines may protect astrocytes by limiting hemin uptake and by impairing the capacity of intact hemin to interact with endogenous H2O2.

Effects on cognition of conventional and robotically assisted cardiac valve operation.

BACKGROUND: The effect of valve surgical procedures on cognition was investigated in patients undergoing conventional or robotically assisted techniques. The confounding factors of surgical procedure, mood state, preexisting cognitive impairment, and repeated experience with cognitive tests were controlled for. METHODS: Patients undergoing conventional valve procedures (n = 15), robotically assisted valve procedures (n = 15), and thoracic surgical procedures (n = 15), along with a nonsurgical control group (n = 15) were tested preoperatively, 1 week after operation, and 8 weeks after operation by use of a battery of cognitive tests and a mood state assessment. Surgical group data were normalized against data from the nonsurgical control group before statistical analysis. RESULTS: Patients undergoing conventional valve procedures performed worse than those undergoing robotically assisted valve procedures on every subtest before operation, and this disadvantage persisted after operation. Age and premorbid intelligence quotient were significantly associated with performance on several cognitive subtests. Anxiety, depression, and stress were not associated with impaired cognitive performance in the surgical groups after operation. A week after operation, patients undergoing conventional valve procedures performed worse on the cognitive tests that had a motor component, which may reflect discomfort caused by the sternotomy. Patients undergoing robotically assisted valve procedures were significantly less impaired on information processing tasks 1 week after operation when compared with those undergoing conventional valve procedures. The majority of patients who were impaired 1 week after operation recovered to preoperation levels within 8 weeks. CONCLUSIONS: The robotically assisted valve surgical procedure results in more rapid recovery of performance on cognitive tests. However, regardless of the type of surgical intervention, the prospect of a recovery of cognitive performance to preoperative levels is high.

Recovery of cognitive function after coronary artery bypass graft operations.

BACKGROUND: The effect of coronary artery bypass grafting (CABG) operations on cognition was examined after controlling for the operation, emotional state, preexisting cognitive impairment, and repeated experience with cognitive tests. METHODS: On-pump CABG patients (n=16), thoracic surgical patients (n=15), and a nonsurgical control group (n=15) were tested preoperatively, and at 1 and 8 weeks postoperatively, using a battery of cognitive tests and an emotional state assessment. Patient groups were similar in age, sex, level of education, and premorbid intelligence quotient score. Surgical group data were normalized against data from the nonsurgical control group before statistical analysis. RESULTS: CABG patients performed worse on every subtest before the operation, and this disadvantage persisted after the operation. Anxiety, depression, and stress were associated with impaired cognitive performance in the surgical groups 1 week after the operation: 44% of CABG patients and 33% of surgical control patients were significantly impaired; yet, by 8 weeks, nearly all patients had recovered to preoperative levels, with 25% of CABG and 13% of surgical control patients improving beyond their preoperative performance. CONCLUSIONS: Stress, anxiety, and depression impair cognitive performance in association with CABG and thoracic operations. Most patients recover to, or exceed, preoperative levels of cognition within 8 weeks. Thus, after controlling for nonsurgical factors, the prospects of a tangible improvement in cognition after CABG are high.

The metabolism and toxicity of hemin in astrocytes.

Hemin is cytotoxic, and contributes to the brain damage that accompanies hemorrhagic stroke. In order to better understand the basis of hemin toxicity in astrocytes, the present study quantified hemin metabolism and compared it to the pattern of cell death. Heme oxygenase-1 (HO-1) expression was first evident after 2 h incubation with hemin, with maximal expression being observed by 24 h. Despite the induction of HO-1, it was found that the proportion of hemin metabolized by astrocytes remained fairly constant throughout the 24 h period, with 70-80% of intracellular hemin remaining intact. A period of cell loss began after 2 h exposure to hemin, which gradually increased in severity to reach a maximum by 24 h. This cell loss could not be attenuated by the iron chelator, 1,10-phenanthroline, or by several antioxidant compounds (Trolox, N-acetyl-L-cysteine and N-tert-butyl-α-phenylnitrone), indicating that the mechanism of hemin toxicity does not involve iron. While these results make it unlikely that hemin toxicity is due to interactions with endogenous H(2)O(2), hemin toxicity was increased in the presence of supraphysiological levels of H(2)O(2) and this increase was ameliorated by PHEN, indicating that the iron released from hemin can be toxic under some pathological conditions. However, when H(2)O(2) is present at physiological levels, the toxicity of hemin appears to be caused by other mechanisms that may involve bilirubin and carbon monoxide in this model system.

Accumulation of non-transferrin-bound iron by neurons, astrocytes, and microglia.

Neurodegenerative conditions such as Alzheimer's disease, Parkinson's disease, and hemorrhagic stroke are associated with increased levels of non-transferrin-bound iron (NTBI) in the brain, which can promote Fenton chemistry. While all types of brain cells can take up NTBI, their efficiency of accumulation and capacity to withstand iron-mediated toxicity has not been directly compared. The present study assessed NTBI accumulation in cultures enriched in neurons, astrocytes, or microglia after exposure to ferric ammonium citrate (FAC). Microglia were found to be the most efficient in accumulating iron, followed by astrocytes, and then neurons. Exposure to 100 µM FAC for 24 h increased the specific iron content of cultured neurons, astrocytes, and microglial cells by 30-, 80-, and 100-fold, respectively. All cell types accumulated iron against the concentration gradient, resulting in intracellular iron concentrations that were several orders of magnitude higher than the extracellular iron concentrations. Accumulation of these large amounts of iron did not affect the viability of the cell cultures, indicating a high resistance to iron-mediated toxicity. These findings show that neurons, astrocytes and microglia cultured from neonatal mice all have the capacity to accumulate and safely store large quantities of iron, but that glial cells do this more efficiently than neurons. It is concluded that neurodegenerative conditions involving iron-mediated toxicity may be due to a failure of iron transport or storage mechanisms, rather than to the presence of high levels of NTBI.

Two routes of iron accumulation in astrocytes: ascorbate-dependent ferrous iron uptake via the divalent metal transporter (DMT1) plus an independent route for ferric iron.

Astrocytes are central to iron and ascorbate homoeostasis within the brain. Although NTBI (non-transferrin-bound iron) may be a major form of iron imported by astrocytes in vivo, the mechanisms responsible remain unclear. The present study examines NTBI uptake by cultured astrocytes and the involvement of ascorbate and DMT1 (divalent metal transporter 1). We demonstrate that iron accumulation by ascorbate-deficient astrocytes is insensitive to both membrane-impermeant Fe(II) chelators and to the addition of the ferroxidase caeruloplasmin. However, when astrocytes are ascorbate-replete, as occurs in vivo, their rate of iron accumulation is doubled. The acquisition of this additional iron depends on effluxed ascorbate and can be blocked by the DMT1 inhibitor ferristatin/NSC306711. Furthermore, the calcein-accessible component of intracellular labile iron, which appears during iron uptake, appears to consist of only Fe(III) in ascorbate-deficient astrocytes, whereas that of ascorbate-replete astrocytes comprises both valencies. Our data suggest that an Fe(III)-uptake pathway predominates when astrocytes are ascorbate-deficient, but that in ascorbate-replete astrocytes, at least half of the accumulated iron is initially reduced by effluxed ascorbate and then imported by DMT1. These results suggest that ascorbate is intimately involved in iron accumulation by astrocytes, and is thus an important contributor to iron homoeostasis in the mammalian brain.

Astrocytes retain their antioxidant capacity into advanced old age.

Oxidative stress has been implicated in the progression of ageing and in many age-related neurodegenerative conditions. Astrocytes play a major role in the antioxidant protection of the brain, yet little is known about how the antioxidant defenses of astrocytes change across the lifespan. This study assessed the antioxidant capacity and glutathione metabolism of astrocytes cultured from the brains of neonatal (<24 h old), mature (12-month-old), old (25-month-old), and senescent (31-month-old) C57BL/6J mice. When exposed to 100 microM hydrogen peroxide, mature, old, and senescent astrocytes cleared the peroxide approximately 30% more slowly than neonatal astrocytes. This difference persisted when catalase was inhibited by 3-aminotriazole, but was abolished when glutathione was depleted by application of buthionine sulfoximine, suggesting a deficit in the glutathione system. Correspondingly, the specific glutathione reductase activity of mature, old, and senescent astrocytes was approximately 30% lower than that of neonatal cultures, whereas no age-related change was observed in the specific activities of glutathione peroxidase, catalase, or in total antioxidant capacity. In addition, the specific rate of glutathione export was almost identical in mature, old, and senescent astrocytes, but was more than double that of neonatal astrocytes. These results indicate that the antioxidant capacity and glutathione metabolism of astrocytes are preserved from mature adulthood into senescence. It is concluded that the oxidative stress seen in ageing brains is likely due to factors extrinsic to astrocytes, rather than to an impairment of the antioxidative functions of astrocytes.

Histidine, cystine, glutamine, and threonine collectively protect astrocytes from the toxicity of zinc.

In epilepsy, traumatic brain injury, and ischemic stroke, toxic levels of zinc released from neurons contribute to the brain damage associated with these disorders. Zinc causes oxidative stress by increasing the generation of reactive oxygen species and by inhibiting glutathione reductase (GR). This study investigated whether naturally occurring amino acids can protect cultured mouse astrocytes from zinc. Astrocytes incubated for 24h with 33 microM zinc acetate in a minimal medium displayed a loss of GR activity, a depletion of total glutathione, and a loss of total antioxidant capacity. These changes were accompanied by extensive cell death. Four amino acids (200 microM L-histidine, 201 microM L-cystine, 4mM L-glutamine, 798 microM L-threonine), which are a subset of the 15 present in Dulbecco's modified Eagle medium, were found to collectively prevent zinc toxicity. Histidine was the most protective, followed by cystine, glutamine, and threonine. It is proposed that each of these amino acids protects against different aspects of zinc toxicity by chelating zinc, by serving as precursors for glutathione, or by converting to tricarboxylic acid cycle intermediates. It is possible that these 4 amino acids contribute in vivo to the protection of brain cells from the toxic effects of zinc.

Synergistic accumulation of iron and zinc by cultured astrocytes.

Iron and zinc are essential for normal brain function, yet the mechanisms used by astrocytes to scavenge non-transferrin-bound iron (NTBI) and zinc are not well understood. Ischaemic stroke, traumatic brain injury and Alzheimer's disease are associated with perturbations in the metabolism of NTBI and zinc, suggesting that these two metals may collectively contribute to pathology. The present study has investigated the accumulation of NTBI and zinc by rat primary astrocyte cultures. It was found that astrocytes express mRNA for both divalent metal transporter 1 (DMT1) and Zip14, indicating the potential for these transporters to contribute to the accumulation of NTBI and zinc by these cells. Astrocytes were found to accumulate iron from ferric chloride in a time- and dose-dependent manner, and the rate of accumulation was strongly stimulated by co-incubation with zinc acetate. In addition, cultured astrocytes rapidly accumulated zinc from zinc acetate, and this accumulation was stimulated by co-incubation with ferric chloride. Because a synergistic stimulation of iron and zinc accumulation is inconsistent with the known properties of DMT1 and Zip14, the present results suggest that additional mechanisms assist astrocytes to scavenge iron and zinc when they are present together in the extracellular compartment. These mechanisms may be involved in disorders that involve elevations in the extracellular concentrations of these metal ions.

A role for Na+/H+ exchangers and intracellular pH in regulating vitamin C-driven electron transport across the plasma membrane.

Ascorbate (vitamin C) is the major electron donor to a tPMET (transplasma membrane electron transport) system that was originally identified in human erythrocytes. This plasma membrane redox system appears to transfer electrons from intracellular ascorbate to extracellular oxidants (e.g. non-transferrin-bound iron). Although this phenomenon has been observed in nucleated cells, its mechanism and regulation are not well understood. In the present study we have examined both facets of this phenomenon in K562 cells and primary astrocyte cultures. Using ferricyanide as the analytical oxidant we demonstrate that tPMET is enhanced by dehydroascorbate uptake via facilitative glucose transporters, and subsequent accumulation of intracellular ascorbate. Additionally, we demonstrate that this stimulation is not due to ascorbate that is released from the cells, but is dependent only on a restricted intracellular pool of the vitamin. Substrate-saturation kinetics suggest an enzyme-catalysed reaction across the plasma membrane by an as-yet-unidentified reductase that relies on extensive recycling of intracellular ascorbate. Inhibition of ascorbate-stimulated tPMET by the NHE (Na(+)/H(+)-exchanger) inhibitors amiloride and 5-(N-ethyl-N-isopropyl)amiloride, which is diminished by bicarbonate, suggests that tPMET activity may be regulated by intracellular pH. In support of this hypothesis, tPMET in astrocytes was significantly inhibited by ammonium chloride-pulse-induced intracellular acidification, whereas it was significantly stimulated by bicarbonate-induced intracellular alkalinization. These results suggest that ascorbate-dependent tPMET is enzyme-catalysed and is modulated by NHE activity and intracellular pH.

Effects of carboxylic acids on the uptake of non-transferrin-bound iron by astrocytes.

The concentrations of non-transferrin-bound iron are elevated in the brain during pathological conditions such as stroke and Alzheimer's disease. Astrocytes are specialised for sequestering this iron, however little is known about the mechanisms involved. Carboxylates, such as citrate, have been reported to facilitate iron uptake by intestinal cells. Citrate binds iron and limits its redox activity. The presence of high citrate concentrations in the interstitial fluid of the brain suggests that citrate may be an important ligand for iron transport by astrocytes. This study investigates whether iron accumulation by cultured rat astrocytes is facilitated by citrate or other carboxylates. Contrary to expectations, citrate, tartrate and malate were found to block iron accumulation in a concentration-dependent manner; alpha-ketoglutarate had limited effects, while fumarate, succinate and glutarate had no effect. This blockade was not due to an inhibition of ferric reductase activity. Instead, it appeared to be related to the capacity of these carboxylates to bind iron, since phosphate, which also binds iron, diminished the capacity of citrate, tartrate and malate to block the cellular accumulation of iron. These findings raise the possibility that citrate may have therapeutic potential in the management of neurodegenerative conditions that involve cellular iron overload.

Uptake of ferrous iron by cultured rat astrocytes.

Astrocytes are considered to play an important role in iron homeostasis of the brain, yet the mechanisms involved in the uptake of iron into astrocytes remain elusive. To investigate the uptake of iron into astrocytes, we have applied ferric ammonium citrate (FAC) to rat astrocyte-rich primary cultures. These cultures express the mRNAs of two membrane-bound ferric reductases, Dcytb and SDR2, and reduce extracellular ferric iron (100 muM) with a rate of 3.2 +/- 0.4 nmol/(hr x mg). This reduction rate is substantially lower than the rate of cellular iron accumulation from 100 muM FAC [24.7 +/- 8.9 nmol/(hr x mg)], which suggests that iron accumulation from FAC does at best partially depend on extracellular ferric reduction. Nonetheless, when the iron in FAC was almost completely reduced by an excess of exogenous ascorbate, astrocytes accumulated iron in a time- and concentration-dependent manner with specific iron accumulation rates that increased linearly for concentrations of up to 100 muM ferrous iron. This accumulation was attenuated by lowering the incubation temperature, by the presence of ferrous iron chelators, or by lowering the pH from 7.4 to 6.8. These data indicate that, in addition to the DMT1-mediated uptake of ferrous iron, astrocytes can accumulate ferric and ferrous iron by mechanisms that are independent of DMT1 or transferrin.

The pivotal role of astrocytes in the metabolism of iron in the brain.

Iron is essential for the normal functioning of cells but since it is also capable of generating toxic reactive oxygen species, the metabolism of iron is tightly regulated. The present article advances the view that astrocytes are largely responsible for distributing iron in the brain. Capillary endothelial cells are separated from the neuropil by the endfeet of astrocytes, so astrocytes are ideally positioned to regulate the transport of iron to other brain cells and to protect them if iron breaches the blood-brain barrier. Astrocytes do not appear to have a high metabolic requirement for iron yet they possess transporters for transferrin, haemin and non-transferrin-bound iron. They store iron efficiently in ferritin and can export iron by a mechanism that involves ferroportin and ceruloplasmin. Since astrocytes are a common site of abnormal iron accumulation in ageing and neurodegenerative disorders, they may represent a new therapeutic target for the treatment of iron-mediated oxidative stress.

Zinc stimulates the production of toxic reactive oxygen species (ROS) and inhibits glutathione reductase in astrocytes.

The release of zinc (Zn) from glutamatergic synapses contributes to the neuropathology of ischemia, traumatic brain injury, and stroke. Astrocytes surround glutamatergic synapses and are vulnerable to the toxicity of Zn, which impairs their antioxidative glutathione (GSH) system and elevates the production of reactive oxygen species (ROS). It is not known whether one or both of these actions are the primary cause of Zn-induced cell death in astrocytes. Using primary rat astrocyte cultures we have examined whether Zn-mediated impairment of GSH redox cycling is the main source of its toxicity. Zn acetate at concentrations of 100 microM or greater were found to inactivate glutathione reductase (GR) via an NADPH-dependent mechanism, while concentrations of 150 microM and above caused substantial cell death. Furthermore, Zn increased the ratio of GSSG:GSH in astrocytes, increased their export of GSSG, slowed their clearance of exogenous H2O2, and promoted the intracellular production of ROS. In contrast, the GR inhibitor, carmustine, did not induce cell death, cause the production of ROS, or alter the GSH thiol redox balance. Taken together these results indicate that Zn toxicity in astrocytes is primarily associated with the generation of intracellular ROS, rather than the inhibition of GR.