Astroglial cell death induced by excessive influx of sodium ions

Neuroprotective Role of Astroglia in Parkinson Disease by Reducing Oxidative Stress Through Dopamine-Induced Activation of Pentose-Phosphate Pathway

Oxidative stress plays an important role in the onset and progression of Parkinson disease. Although released dopamine at the synaptic terminal is mostly reabsorbed by dopaminergic neurons, some dopamine is presumably taken up by astroglia. This study examined the dopamine-induced astroglial protective function through the activation of the pentose-phosphate pathway (PPP) to reduce reactive oxygen species (ROS). In vitro experiments were performed using striatal neurons and cortical or striatal astroglia prepared from Sprague-Dawley rats or C57BL/6 mice. The rates of glucose phosphorylation in astroglia were evaluated using the [¹⁴C]deoxyglucose method. PPP activity was measured using [1-¹⁴C]glucose and [6-¹⁴C]glucose after acute (60 min) or chronic (15 hr) exposure to dopamine. ROS production was measured using 2',7'-dichlorodihydrofluorescein diacetate. The involvement of the Kelch-like ECH-associated protein 1 (Keap1) or nuclear factor-erythroid-2-related factor 2 (Nrf2) system was evaluated using Nrf2 gene knockout mice, immunohistochemistry, and quantitative reverse transcription polymerase chain reaction analysis for heme oxygenase-1. Acute exposure to dopamine elicited increases in astroglial glucose consumption with lactate release. PPP activity in astroglia was robustly enhanced independently of Na⁺-dependent monoamine transporters. In contrast, chronic exposure to dopamine induced moderate increases in PPP activity via the Keap1/Nrf2 system. ROS production from dopamine increased gradually over 12 hr. Dopamine induced neuronal cell damage that was prevented by coculturing with astroglia but not with Nrf2-deficient astroglia. Dopamine-enhanced astroglial PPP activity in both acute and chronic manners may possibly reduce neuronal oxidative stress.

Rapid Na+ accumulation by a sustained action potential impairs mitochondria function and induces apoptosis in HEK293 cells expressing non-inactivating Na+ channels

The mechanisms underlying neuronal cell death induced by the rise of intracellular Na⁺ concentration ([Na⁺]_i) following abnormal hyperexcitation are not fully understood. Previously, we have established a recombinant cell line derived from HEK293 cells, in which the occurrence of a sustained action potential (AP) induces cell death. Mutated voltage-gated Nav1.5 channel (IFM/QQQ) lacking inactivation, and inward rectifying K⁺ channel (Kir2.1) were co-expressed in HEK293 cells (IFM/QQQ + Kir2.1 cells). In this cell line, the rise of [Na⁺]_i due to a sustained AP reached maximum within 15 min without concomitant [Ca²⁺]_i rise, and then elicited significant externalization of phosphatidylserine and enhancement of caspase activity. Marked decreases in mitochondrial transmembrane potential and ATP concentration were also detected. The significant cell death occurred at 3 h from the AP onset and reached a steady state at around 12 h. The significant release of lactate dehydrogenase was not detected even after 12 h. These results provide novel findings that Na⁺ accumulation or/and possibly concomitant K⁺ loss elicits apoptosis presumably due to the mitochondrial dysfunction, which is attributable to neither the membrane depolarization nor [Ca²⁺]_i change. This apoptotic mechanism may be involved, at least in part, in neuronal cell death under pathophysiological settings with abnormal hyperexcitability.

Development of a Novel Cell-Based Assay System for High-Throughput Screening of Compounds Acting on Background Two-Pore Domain K+ Channels

Two-pore domain K⁺ (K_2P) channels are thought to be druggable targets. However, only a few agents specific for K_2P channels have been identified, presumably due to the lack of an efficient screening system. To develop a new high-throughput screening (HTS) system targeting these channels, we have established a HEK293-based "test cell" expressing a mutated Na⁺ channel (Nav1.5) with markedly slowed inactivation, as well as a K⁺ channel (Kir2.1) that sets the membrane potential quite negative, close to K⁺ equilibrium potential. We found in this system that Kir2.1 block by 100 μM Ba²⁺ application consistently elicited a large depolarization like a long-lasting action potential. This maneuver resulted in cell death, presumably due to the sustained Na⁺ influx. When either the TWIK-related acid-sensitive K⁺ (TASK)-1 or TASK-3 channel was expressed in the test cells, Ba²⁺-induced cell death was markedly weakened. Stronger activation of TASK-1 by extracellular acidification further decreased the cell death. In contrast, the presence of K_2P channel blockers enhanced cell death. IC₅₀ values for TASK-1 and/or TASK-3 blockers acquired by measurements of relative cell viability were comparable to those obtained using patch-clamp recordings. Both blockers and openers of K_2P channels can be accurately assessed with high efficiency and throughput by this novel HTS system.

Na+, K+-ATPase inhibition induces neuronal cell death in rat hippocampal slice cultures: Association with GLAST and glial cell abnormalities

Na⁺, K⁺-ATPase is a highly expressed membrane protein. Dysfunction of Na⁺, K⁺-ATPase has been implicated in the pathophysiology of several neurodegenerative and psychiatric disorders, however, the underlying mechanism of neuronal cell death resulting from Na⁺, K⁺-ATPase dysfunction is poorly understood. Here, we investigated the mechanism of neurotoxicity due to Na⁺, K⁺-ATPase inhibition using rat organotypic hippocampal slice cultures. Treatment with ouabain, a Na⁺, K⁺-ATPase inhibitor, increased the ratio of propidium iodide-positive cells among NeuN-positive cells in the hippocampal CA1 region, which was prevented by MK-801 and d-AP5, specific blockers of the N-methyl-d-aspartate (NMDA) receptor. EGTA, a Ca²⁺-chelating agent, also protected neurons from ouabain-induced injury. We observed that astrocytes expressed the glutamate aspartate transporter (GLAST), and ouabain changed the immunoreactive area of GFAP-positive astrocytes as well as GLAST. We also observed that ouabain increased the number of Iba1-positive microglial cells in a time-dependent manner. Furthermore, lithium carbonate, a mood-stabilizing drug, protected hippocampal neurons and reduced disturbances of astrocytes and microglia after ouabain treatment. Notably, lithium carbonate improved ouabain-induced decreases in GLAST intensity in astrocytes. These results suggest that glial cell abnormalities resulting in excessive extracellular concentrations of glutamate contribute to neurotoxicity due to Na⁺, K⁺-ATPase dysfunction in the hippocampal CA1 region.

Actin filament reorganization in astrocyte networks is a key functional step in neuroinflammation resulting in persistent pain: novel findings on network restoration

In recent years, the importance of glial cell activation in the generation and maintenance of long-term pain has been investigated. One novel mechanism underlying long-lasting pain is injury-induced inflammation in the periphery, followed by microglial activation in the dorsal horn of the spinal cord, which results in local neuroinflammation. An increase in neuronal excitability may follow, with intense signaling along the pain tracts to the thalamus and the parietal cortex along with other cortical regions for the identification and recognition of the injury. If the local neuroinflammation develops into a pathological state, then the astrocytes become activated. Previous studies in which lipopolysaccharide (LPS) was used to induce inflammation have shown that in a dysfunctional astrocyte network, the actin cytoskeleton is reorganized from the normally occurring F-actin stress fibers into the more diffusible, disorganized, ring-form globular G-actin. In addition, Ca(2+) signaling systems are altered, Na(+)- and glutamate transporters are downregulated, and pro-inflammatory cytokines, particularly IL-1β, are released in dysfunctional astrocyte networks. In a series of experiments, we have demonstrated that these LPS-induced changes in astrocyte function can be restored by stimulation of Gi/o and inhibition of Gs with a combination of a μ-receptor agonist and ultralow concentrations of a μ-receptor antagonist and by inhibition of cytokine release, particularly IL-1β, by the antiepileptic drug levetiracetam. These findings could be of clinical significance and indicate a novel treatment for long-term pain.

Naloxone and ouabain in ultralow concentrations restore Na+/K+-ATPase and cytoskeleton in lipopolysaccharide-treated astrocytes

Astrocytes respond to inflammatory stimuli and may be important modulators of the inflammatory response in the nervous system. This study aimed first to assess how astrocytes in primary culture behave in response to inflammatory stimuli concerning intracellular Ca(2+) responses, expression of Toll-like receptor 4 (TLR4), Na(+)/K(+)-ATPase, actin filament organization, and expression of cytokines. In a cell culture model with lipopolysaccharide (LPS), astrocyte response was assessed first in the acute phase and then after incubation with LPS for 1-48 h. The concentration curve for LPS-stimulated Ca(2+) responses was bell-shaped, and the astrocytes expressed TLR4, which detects LPS and evokes intracellular Ca(2+) transients. After a long incubation with LPS, TLR4 was up-regulated, LPS-evoked Ca(2+) transients were expressed as oscillations, Na(+)/K(+)-ATPase was down-regulated, and the actin filaments were disorganized. Interleukin-1β (IL-1β) release was increased after 24 h in LPS. A second aim was to try to restore the LPS-induced changes in astrocytes with substances that may have dose-dependent anti-inflammatory properties. Naloxone and ouabain were tested separately in ultralow or high concentrations. Both substances evoked intracellular Ca(2+) transients for all of the concentrations from 10(-15) up to 10(-4) M. Neither substance blocked the TLR4-evoked Ca(2+) responses. Naloxone and ouabain prevented the LPS-induced down-regulation of Na(+)/K(+)-ATPase and restored the actin filaments. Ouabain, in addition, reduced the IL-1β release from reactive astrocytes. Notably, ultralow concentrations (10(-12) M) of naloxone and ouabain showed these qualities. Ouabain seems to be more potent in these effects of the two tested substances.

Action potential changes associated with impairment of functional properties of sodium channels in hippocampal neurons induced by melamine

Since the melamine-contamination event happened in September 2008, there have been lots of studies about melamine toxicity, but very limited studies focused on central nervous system (CNS). In the present study, we investigated the effects of melamine (5x10(-4), 5x10(-5) and 5x10(-6)g/ml) on voltage-gated sodium channels (VGSCs) in hippocampal CA1 neurons using whole-cell patch-clamp recordings technique. The results showed that only 5x10(-4)g/ml melamine reduced the amplitude of voltage-gated sodium current (I(Na)). At the concentrations of 5x10(-5) and 5x10(-4)g/ml, melamine produced a hyperpolarizing shift in the steady-state activation curve of I(Na) and also enhanced the steady-state inactivate processing of I(Na). Action potential properties and the pattern of repetitive firing were examined using current-clamp recording, which indicated that peak amplitude and overshoot of the evoked single action potential were decreased. The half-width and the firing rate of repetitive firing were increased in a concentration-dependent manner. The data suggest that melamine alters the action potential of hippocampal CA1 neurons by impairing the functional properties of VGSCs, which may be the underlie mechanisms of neurotoxicity induced by melamine.

Bioenergetics of cerebral ischemia: a cellular perspective

In cerebral ischemia survival of neurons, astrocytes, oligodendrocytes and endothelial cells is threatened during energy deprivation and/or following re-supply of oxygen and glucose. After a brief summary of characteristics of different cells types, emphasizing the dependence of all on oxidative metabolism, the bioenergetics of focal and global ischemia is discussed, distinguishing between events during energy deprivation and subsequent recovery attempt after re-circulation. Gray and white matter ischemia are described separately, and distinctions are made between mature and immature brains. Next comes a description of bioenergetics in individual cell types in culture during oxygen/glucose deprivation or exposure to metabolic inhibitors and following re-establishment of normal aerated conditions. Due to their expression of NMDA and non-NMDA receptors neurons and oligodendrocytes are exquisitely sensitive to excitotoxicity by glutamate, which reaches high extracellular concentrations in ischemic brain for several reasons, including failing astrocytic uptake. Excitotoxicity kills brain cells by energetic exhaustion (due to Na(+) extrusion after channel-mediated entry) combined with mitochondrial Ca(2+)-mediated injury and formation of reactive oxygen species. Many (but not all) astrocytes survive energy deprivation for extended periods, but after return to aerated conditions they are vulnerable to mitochondrial damage by cytoplasmic/mitochondrial Ca(2+) overload and to NAD(+) deficiency. Ca(2+) overload is established by reversal of Na(+)/Ca(2+) exchangers following Na(+) accumulation during Na(+)-K(+)-Cl(-) cotransporter stimulation or pH regulation, compensating for excessive acid production. NAD(+) deficiency inhibits glycolysis and eventually oxidative metabolism, secondary to poly(ADP-ribose)polymerase (PARP) activity following DNA damage. Hyperglycemia can be beneficial for neurons but increases astrocytic death due to enhanced acidosis.

Reversed Actrocytic GLT-1 during Ischemia is Crucial to Excitotoxic Death of Neurons, but Contributes to the Survival of Astrocytes themselves

During ischemia, the operation of astrocytic/neuronal glutamate transporters is reversed and glutamate and Na(+) are co-transported to the extracellular space. This study aims to investigate whether this reversed operation of glutamate transporters has any functional meanings for astrocytes themselves. Oxygen/glucose deprivation (OGD) of neuron/astrocyte co-cultures resulted in the massive death of neurons, and the cell death was significantly reduced by treatment with either AP5 or DHK. In cultured astrocytes with little GLT-1 expression, OGD produced Na(+) overload, resulting in the reversal of astrocytic Na(+)/Ca(2+)-exchanger (NCX). The reversed NCX then caused Ca(2+) overload leading to the damage of astrocytes. In contrast, the OGD-induced Na(+) overload and astrocytic damage were significantly attenuated in PACAP-treated astrocytes with increased GLT-1 expression, and the attenuation was antagonized by treatment with DHK. These results suggested that the OGD-induced reversal of GLT-1 contributed to the survival of astrocytes themselves by releasing Na(+) with glutamate via reversed GLT-1.

Metabolic properties of astrocytes differentiated from rat neurospheres

The metabolic properties of astroglia differentiated from neurospheres have not been fully assessed. In this study, the glycolytic and oxidative metabolism of glucose in astroglia differentiated from rat tertiary neurospheres (astroglia(NS)) was compared with that in astroglia prepared from the striata of embryonic day 16 rats (astroglia(ST)). In addition to the basal condition, we also investigated energy metabolism under Na+,K+-ATPase activation. Furthermore, the effects of glucose concentration in the culture medium were assessed. No significant differences in 2-deoxy-D-[1-(14)C]glucose phosphorylation were observed between astroglia(NS) and astrogliaST. The rates of L-[U-14C]lactate ([14C]lactate) and D-[U-14C]glucose ([14C]glucose) oxidation were 5.74+/-0.82 and 2.83+/-0.4 pmol/60 min/microg protein, respectively, in astrogliaNS grown in low glucose (2 mM) and 3.01+/-1.03 and 1.77+/-0.23 pmol/60 min/microg protein, respectively, in astrogliaNS grown in high glucose (22 mM). Neither the [14C]lactate nor the [14C]glucose oxidation rates in astrogliaNS were significantly different from those in astrogliaST. D-aspartate (500 microM) significantly increased the [14C]lactate and [14C]glucose oxidation rates by 127% and 62%, respectively, in astrogliaNS grown in low glucose and by 217% and 115%, respectively, in astroglia(NS) grown in high glucose. D-aspartate also increased the oxidation of [14C]lactate and [14C]glucose to 236% and 147% of the control values, respectively, in astrogliaST grown in low glucose and to 174% and 144%, respectively, in astrogliaST grown in high glucose. Rat astroglia differentiated from neurospheres might possess an equivalent capacity for utilizing energy substrates under both basal and activated conditions to that of astroglia prepared from striatum.

Cypermethrin alters Glial Fibrillary Acidic Protein levels in the rat brain

Pyrethroids, widely used insecticides, are biologically active in neurons. Whether they act on the non-neuronal brain cells remains an open question. Thus, the aim of this study was to examine whether Cypermethrin intoxication affects astroglial cells in the rat brain. The levels of Glial Fibrillary Acidic Protein (GFAP) in different brain regions were measured by ELISA following oral treatment with 5 or 10% of LD(50) of Cypermethrin per day for 6 days. A significant decrease of GFAP was observed in different brain regions of treated animals. The cerebral cortex showed the most pronounced effect with GFAP levels reduced to 81% of the controls 2 days after treatment and 77% 21 days after treatment. Although we did not find profound changes in the morphology of astrocytes in Cypermethrin treated animals, the decrease in GFAP suggests that astrocytes were affected by low doses of pyrethroids. The possible consequences were discussed.

Mechanical strain injury increases intracellular sodium and reverses Na+/Ca2+ exchange in cortical astrocytes

Traditionally, astrocytes have been considered less susceptible to injury than neurons. Yet, we have recently shown that astrocyte death precedes neuronal death in a rat model of traumatic brain injury (TBI) (Zhao et al.: Glia 44:140-152, 2003). A main mechanism hypothesized to contribute to cellular injury and death after TBI is elevated intracellular calcium ([Ca2+]i). Since calcium regulation is also influenced by regulation of intracellular sodium ([Na+]i), we used an in vitro model of strain-induced traumatic injury and live-cell fluorescent digital imaging to investigate alterations in [Na+]i in cortical astrocytes after injury. Changes in [Na+]i, or [Ca2+]i were monitored after mechanical injury or L-glutamate exposure by ratiometric imaging of sodium-binding benzofuran isophthalate (SBFI-AM), or Fura-2-AM, respectively. Mechanical strain injury or exogenous glutamate application produced increases in [Na+]i that were dependent on the severity of injury or concentration. Injury-induced increases in [Na+]i were significantly reduced, but not completely eliminated, by inhibition of glutamate uptake by DL-threo-beta-benzyloxyaspartate (TBOA). Blockade of sodium-dependent calcium influx through the sodium-calcium exchanger with 2-[2-[4-(4-Nitrobenzyloxy)phenyl]ethyl]isothiourea mesylate (KB-R7943) reduced [Ca2+]i after injury. KB-R7943 also reduced astrocyte death after injury. These findings suggest that in astrocytes subjected to mechanical injury or glutamate excitotoxicity, increases in intracellular Na+ may be a critical component in the injury cascade and a therapeutic target for reduction of lasting deficits after traumatic brain injury.

Urginea sanguinea: medicinal wonder or death in disguise?

The Southern African plant, Urginea sanguinea Shinz (Hyacinthaceae) is well-known for its poisonous affects on livestock, but it is widely used as herbal medicines by traditional healers. It is most frequently used as a blood purifier, abortifacient, treatment for venereal diseases, abdominal pain, backache and hypertension. In this article we discuss the plant's morphology, distribution, medicinal values but also poisonous characteristics. Furthermore, we discuss the most important chemical components, with special reference to the significance and physiological action of the cardiac glycoside (CG), Transvaalin. Emphasis is placed on the physiological mechanism of CG toxicity involving the Na(+)/K(+)-ATPase pump. This pump is of extreme importance, especially in conducting tissues such as nerve fibers, cardiac-, skeletal- and smooth muscles. Furthermore, we summarize the medicinal uses, the systems it influence as well the adverse effects in case of over dosage.

Potassium homeostasis in the ischemic brain

Extracellular [K+] can range within 2.5-3.5 mM under normal conditions to 50-80 mM under ischemic and spreading depression events. Sustained exposure to elevated [K+]o has been shown to cause significant neuronal death even under conditions of abundant glucose supply. Astrocytes are well equipped to buffer this initial insult of elevated [K] through extensive gap junctional coupling, Na+/K+ pump activity (with associated glycogen and glycolytic potential), and endfoot siphoning capability. Their abundant energy availability and alkalinizing mechanisms help sustain Na+/K+ ATPase activity under ischemic conditions. Furthermore, passive K+ uptake mechanisms and water flux mediated through aquaporin-4 channels in endfoot processes are important energy-independent mechanisms. Unfortunately, as the length of ischemic episode is prolonged, these mechanisms increase to a point where they begin to have repercussions on other important cellular functions. Alkalinizing mechanisms induce an elevation of [Na+]i, increasing the energy demand of Na+/K+ ATPase and leading to eventual detrimental reversal of the Na+/glutamate- cotransporter and excitotoxic damage. Prolonged ischemia also results in cell swelling and activates volume regulatory processes that release excessive excitatory amino acids, further exacerbating excitotoxic injury. In the days following ischemic injury, reactive astrocytes demonstrate increased cell size and process thickness, leading to improved spatial buffering capacity in regions outside the lesion core where there is better neuronal survival. There is a substantial heterogeneity among reactive astrocytes, with some close to the lesion showing decreased buffering capacity. However, it appears that both Na+/K+ ATPase activity (along with energy production processes) as well as passive K+ uptake mechanisms are upregulated in gliotic tissue outside the lesion to enhance the above-mentioned homeostatic mechanisms.

A novel toxicity-based assay for the identification of modulators of voltage-gated Na+ channels

Voltage-gated Na+ channels are promising drug targets. Screening of large numbers of putative modulators, however, can be demanding and expensive. In this study, a simple, cheap, and robust assay to test the pharmacological modulation of Na+ channel function is presented. The assay makes use of the fact that the intracellular accumulation of Na+ ions can be cytotoxic. The toxicity of the Na+ channel activator veratridine in the presence of an inhibitor of the Na+/K+ ATPase (ouabain) in a Nav1.2a (rat brain IIA alpha) expressing cell line is assessed. Na+ channel blockers should reduce toxicity in this model. CHO cells which recombinantly expressed rat Nav1.2a subunits were seeded in 96-well plates, and cell survival was tested after 24 h incubation in medium containing veratridine and ouabain in the presence or absence of Na+ channel blockers. Propidium iodide fluorescence was used as toxicity readout. Veratridine (100 microM) or ouabain alone (500 microM) were not toxic to the cells. In the presence of 500 microM ouabain, however, veratridine induced halfmaximal cell death with an EC50 value of 15.1 +/- 2.3 microM. Ouabain's EC50 was 215.3 +/- 16.7 microM (with 30 microM veratridine). The effects of a number of Na+ channel blockers were tested and compared with their Na+ channel blocking activity measured in voltage-clamp experiments. Blockers from various chemical classes reduced toxicity half maximally with IC50 values ranging from 11.7 +/- 1.4 nM (tetrodotoxin) to 280.5 +/- 48.0 microM (lamotrigine). There was a linear relationship between the log IC50 values obtained by the two methods (slope: 1.1 +/- 0.08; correlation coefficient: 0.93). In summary, these data show that this novel toxicity assay is well suited to test Na+ channel blockers.

Increased tolerance to oxygen and glucose deprivation in astrocytes from Na+/H+ exchanger isoform 1 null mice

The ubiquitously expressed Na+/H+ exchanger isoform 1 (NHE1) functions as a major intracellular pH (pHi) regulatory mechanism in many cell types, and in some tissues its activity may contribute to ischemic injury. In the present study, cortical astrocyte cultures from wild-type (NHE1+/+) and NHE1-deficient (NHE1−/−) mice were used to investigate the role of NHE1 in pHi recovery and ischemic injury in astrocytes. In the absence of HCO3−, the mean resting pHi levels were 6.86 ± 0.03 in NHE1+/+ astrocytes and 6.53 ± 0.04 in NHE1−/− astrocytes. Removal of extracellular Na+ or blocking of NHE1 activity by the potent NHE1 inhibitor HOE-642 significantly reduced the resting level of pHi in NHE1+/+ astrocytes. NHE1+/+ astrocytes exhibited a rapid pHi recovery (0.33 ± 0.08 pH unit/min) after NH4Cl prepulse acid load. The pHi recovery in NHE1+/+ astrocytes was reversibly inhibited by HOE-642 or removal of extracellular Na+. In NHE1−/− astrocytes, the pHi recovery after acidification was impaired and not affected by either Na+-free conditions or HOE-642. Furthermore, 2 h of oxygen and glucose deprivation (OGD) led to an ∼80% increase in pHi recovery rate in NHE1+/+ astrocytes. OGD induced a 5-fold rise in intracellular [Na+] and 26% swelling in NHE1+/+ astrocytes. HOE-642 or genetic ablation of NHE1 significantly reduced the Na+ rise and swelling after OGD. These results suggest that NHE1 is the major pHi regulatory mechanism in cortical astrocytes and that ablation of NHE1 in astrocytes attenuates ischemia-induced disruption of ionic regulation and swelling.

Functional expression of constitutive nitric oxide synthases regulated by voltage-gated Na+ and Ca2+ channels in cultured human astrocytes

We report the functional characterization of constitutive nitric oxide synthase(s) (NOS) such as neuronal and endothelial NOS in cultured human astrocytes. Exposure of cultured human astrocytes to 1 microM veratridine or 50 mM KCl produced a pronounced increase in a calmodulin-dependent NOS activity estimated from cGMP formation. The functional expression of voltage-gated Na(+) channel, which is estimated by the response to veratridine, appeared to be earlier (at second day in culture) than that of voltage-gated Ca(2+) channels, which are estimated by the response to the KCl stimulation (at fourth day in culture). The KCl-evoked NO synthesis was totally reversed by L-type Ca(2+) channel blockers such as nifedipine and verapamil, but not by omega-conotoxin GVIA, an N-type Ca(2+) channel blocker, or omega-agatoxin IVA, a P/Q-type Ca(2+) channel blocker. In addition, verapamil abolished the KCl-induced increase in the intracellular free Ca(2+) concentration. RT-PCR analysis revealed that mRNA for neuronal and endothelial NOS was expressed in human astrocytes. In addition, Western blot analysis and double labeling of NOS and glial fibrillary acidic protein (GFAP) showed that cultured human astrocytes expressed neuronal NOS and endothelial NOS as well as the alpha(1) subunit of Ca(2+) channel. These results suggest that human astrocytes express constitutive NOS that are regulated by voltage-gated L-type Ca(2+) channel as well as Na(+) channel.

Induction of mRNAs and proteins for Na/K ATPase alpha1 and beta1 subunits following hypoxia/reoxygenation in astrocytes

Characteristics of the cellular response to oxygen deprivation and subsequent reoxygenation (hypoxia/reoxygenation) include redirection of energy metabolism, increased glucose utilization and expression of oxygen-regulated proteins. Inhibition of protein synthesis during early reoxygenation period prevented effective astrocyte adaptation to hypoxia/reoxygenation, resulting in eventual cell death. To elucidate the role of astrocytes in the central nervous system in response to hypoxia/reoxygenation, we analyzed the cDNA library derived from the cultured rat astrocytes subjected to 24 h of hypoxia followed by reoxygenation by differential display, and isolated a cDNA corresponding to Na/K ATPase alpha1 subunit. The expression of Na/K ATPase alpha1 subunit mRNA as well as beta1subunit mRNA was transiently increased after reoxygenation, whereas hypoxia itself did not induce any gene expression change. Na/K ATPase alpha1 subunit protein was transiently increased, whereas the protein expression for Na/K ATPase beta1 subunit showed sustained induction after reoxygenation. Overexpression of beta1 subunit in HEK 293 cells subjected to hypoxia/reoxygenation promoted survival of the cells. These findings suggest that Na/K ATPases may contribute to maintain the cellular environment of astrocytes subjected to hypoxia/reoxygenation.

Veratridine, but not elevated K+, inhibits excitatory amino acid transporter activity in rat hippocampal slices

PURPOSE

Excitatory amino acid transporter (EAAT) activity prevents Glu from reaching toxic levels, but their contribution to epileptogenesis remains controversial. We examined how the convulsant veratridine causes inhibition of EAAT activity and how it differs from the effects of another convulsant, high (50 mM) K+, that also increases Na+ conductance.

METHODS

Transverse rat hippocampal slices were incubated for 1 h with 100 microM veratridine in oxygenated artificial cerebrospinal fluid (aCSF) with or without extracellular Ca2+. The medium was replaced by 50 microM[(3)H]glutamate in aCSF, and the slices incubated for 10 min at 37 degrees C. The slices were washed 3 times with cold aCSF after removal of the extracellular medium, and the radioactivity was quantified after solubilization of the slices.

RESULTS

Veratridine caused a time- and dose-dependent decrease, whereas high K+ had no effect on EAAT activity. The effects of veratridine on EAAT activity were not prevented by tetrodotoxin (TTX; 10 microM). Coincubation of ouabain with veratridine resulted in further decreases of EAAT activity. Removal of extracellular Ca2+ potentiated the inhibitory effects of veratridine (and other convulsants) on EAAT activity. Chelation of intracellular Ca2+ with BAPTA also increased the inhibitory effects of veratridine on EAAT activity.

CONCLUSIONS

Veratridine caused changes Ca2+ dynamics that led to inhibition of EAAT activity. Such changes in EAAT activity can contribute to the sustained epileptiform activity caused by veratridine.

A "minimal" sodium channel construct consisting of ligated S5-P-S6 segments forms a toxin-activatable ionophore

The large size (six membrane-spanning repeats in each of four domains) and asymmetric architecture of the voltage-dependent Na+ channel has hindered determination of its structure. With the goal of determining the minimum structure of the Na+ channel permeation pathway, we created two stable cell lines expressing the voltage-dependent rat skeletal muscle Na+ channel (micro1) with a polyhistidine tag on the C terminus (muHis) and pore-only micro1 (muPore) channels with S1-S4 in all domains removed. Both constructs were recognized by a Na+ channel-specific antibody on a Western blot. muHis channels exhibited the same functional properties as wild-type micro1. In contrast, muPore channels did not conduct Na+ currents nor did they bind [3H]saxitoxin. Veratridine caused 40 and 54% cell death in muHis- and muPore-expressing cells, respectively. However, veratridine-induced cell death could only be blocked by tetrodotoxin in cells expressing muHis, but not muPore. Furthermore, using a fluorescent Na+ indicator, we measured changes in intracellular Na+ induced by veratridine and a brevotoxin analogue, pumiliotoxin. When calibrated to the maximum signal after addition of gramicidin, the maximal percent increases in fluorescence (deltaF) were 35 and 31% in cells expressing muHis and muPore, respectively. Moreover, in the presence of 1 microm tetrodotoxin, deltaF decreased significantly to 10% in muHis- but not in muPore-expressing cells (43%). In conclusion, S5-P-S6 segments of micro1 channels form a toxin-activable ionophore but do not reconstitute the Na+ channel permeation pathway with full fidelity.

Rhabdomyolysis, acute renal failure, and death after monensin ingestion

We report a case of human monensin intoxication; to our knowledge, this is the first reported case in the medical literature. The patient took a dose of monensin three times higher than a dose considered lethal for cattle and developed a clinical picture similar to that reported in veterinary medicine. There was an early and extremely severe rhabdomyolysis followed by acute renal failure, heart failure, and death. The main changes observed at autopsy were extensive skeletal muscle necrosis, complement deposition at the myocardial level, pulmonary edema, and acute tubular damage.