Effects of the sodium channel blocker tetrodotoxin (TTX) on cellular ion homeostasis in rat brain subjected to complete ischemia

Brain vulnerability and viability after ischaemia

The susceptibility of the brain to ischaemic injury dramatically limits its viability following interruptions in blood flow. However, data from studies of dissociated cells, tissue specimens, isolated organs and whole bodies have brought into question the temporal limits within which the brain is capable of tolerating prolonged circulatory arrest. This Review assesses cell type-specific mechanisms of global cerebral ischaemia, and examines the circumstances in which the brain exhibits heightened resilience to injury. We suggest strategies for expanding such discoveries to fuel translational research into novel cytoprotective therapies, and describe emerging technologies and experimental concepts. By doing so, we propose a new multimodal framework to investigate brain resuscitation following extended periods of circulatory arrest.

From Poison to Promise: The Evolution of Tetrodotoxin and Its Potential as a Therapeutic

Tetrodotoxin (TTX) is a potent neurotoxin that was first identified in pufferfish but has since been isolated from an array of taxa that host TTX-producing bacteria. However, determining its origin, ecosystem roles, and biomedical applications has challenged researchers for decades. Recognized as a poison and for its lethal effects on humans when ingested, TTX is primarily a powerful sodium channel inhibitor that targets voltage-gated sodium channels, including six of the nine mammalian isoforms. Although lethal doses for humans range from 1.5-2.0 mg TTX (blood level 9 ng/mL), when it is administered at levels far below LD₅₀, TTX exhibits therapeutic properties, especially to treat cancer-related pain, neuropathic pain, and visceral pain. Furthermore, TTX can potentially treat a variety of medical ailments, including heroin and cocaine withdrawal symptoms, spinal cord injuries, brain trauma, and some kinds of tumors. Here, we (i) describe the perplexing evolution and ecology of tetrodotoxin, (ii) review its mechanisms and modes of action, and (iii) offer an overview of the numerous ways it may be applied as a therapeutic. There is much to be explored in these three areas, and we offer ideas for future research that combine evolutionary biology with therapeutics. The TTX system holds great promise as a therapeutic and understanding the origin and chemical ecology of TTX as a poison will only improve its general benefit to humanity.

An organic neurophysiological tool for neuronal metabolic activity monitoring

Monitoring cell metabolism in vitro is considered a relevant methodology in several scientific fields ranging from fundamental biology research to neuro-toxicology. In the last 20 years, several in vitro neuro-pharmacological and neuro-toxicological approaches have been developed, with the intent of addressing the increasing demand for real-time, non-invasive in vitro systems capable of continuously and reliably monitoring cellular activity. In this paper, an Organic Charge Modulated Field Effect Transistor-based device is proposed as a promising tool for neuro-pharmacological applications, thanks to its ultra-high pH sensitivity and a simple fabrication technology. The preliminary characterization of this versatile organic device with primary neuronal cultures shows how these remarkable properties can be exploited for the realization of ultra-sensitive metabolic probes, which are both reference-less and low cost. These features, together with the already assessed capability of this sensor to also monitor the electrical activity of electrogenic cells, could provide important advances in the fabrication of multi-sensing lab-on-chip devices, thus opening up interesting perspectives in the neuro-pharmacological field.

Neuroprotective effects of levetiracetam, both alone and combined with propylparaben, in the long-term consequences induced by lithium-pilocarpine status epilepticus

Status epilepticus (SE) is a neurological condition that frequently induces severe neuronal injury in the hippocampus, subsequent epileptogenesis and pharmacoresistant spontaneous recurrent seizures (SRS). The repeated administration of LEV (a broad-spectrum antiepileptic drug) during the post-SE period does not prevent the subsequent development of SRS. However, this treatment reduces SE-induced neurodegeneration in the hippocampus. Conversely, propylparaben (PPB) is a widely used antimicrobial that blocks voltage-dependent Na⁺ channels, induces neuroprotection and reduces epileptiform activity in vitro. The present study attempted to determine if the neuroprotective effects induced by LEV are augmented when combined with a sub-effective dose of PPB. Long-term SE-induced consequences (hyperexcitability, high glutamate release, neuronal injury and volume loss) were evaluated in the hippocampus of rats. LEV alone, as well as combined with PPB, did not prevent the occurrence of SRS. However, animals treated with LEV plus PPB showed high prevalence of low frequency oscillations (0.1-4 Hz and 8-90 bands, p < 0.001) and low prevalence of high frequency activity (90-250 bands, p < 0.001) during the interictal period. In addition, these animals presented lower extracellular levels of glutamate, decreased rate of neurodegeneration and a similar hippocampal volume compared to the control conditions. This study's results suggest that LEV associated with PPB could represent a new therapeutic strategy to reduce long-term consequences induced by SE that facilitate pharmacoresistant SRS.

SUMOylation of NaV1.2 channels mediates the early response to acute hypoxia in central neurons

The mechanism for the earliest response of central neurons to hypoxia-an increase in voltage-gated sodium current (INa)-has been unknown. Here, we show that hypoxia activates the Small Ubiquitin-like Modifier (SUMO) pathway in rat cerebellar granule neurons (CGN) and that SUMOylation of NaV1.2 channels increases INa. The time-course for SUMOylation of single NaV1.2 channels at the cell surface and changes in INa coincide, and both are prevented by mutation of NaV1.2-Lys38 or application of a deSUMOylating enzyme. Within 40 s, hypoxia-induced linkage of SUMO1 to the channels is complete, shifting the voltage-dependence of channel activation so that depolarizing steps evoke larger sodium currents. Given the recognized role of INa in hypoxic brain damage, the SUMO pathway and NaV1.2 are identified as potential targets for neuroprotective interventions.

A potent and selective small molecule inhibitor of sirtuin 1 promotes differentiation of pluripotent P19 cells into functional neurons

Sirtuin 1 (SIRT1) is known to suppress differentiation of pluripotent/multipotent cells and neural progenitor cells into neurons by blocking activation of transcription factors critical for neurogenesis. EX-527 is a highly selective and potent inhibitor against SIRT1 and has been used as a chemical probe that modulates SIRT1-associated biological processes. However, the effect of EX-527 on neuronal differentiation in pluripotent cells has not been well elucidated. Here, we report an examination of EX-527 effects on neurogenesis of pluripotent P19 cells. The results showed that EX-527 greatly accelerated differentiation of P19 cells into neurons without generation of cardiac cells and astrocytes. Importantly, neurons derived from P19 cells treated with EX-527 generated voltage-dependent sodium currents and depolarization-induced action potentials. The findings indicate that the differentiated cells have electrophysiological properties. The present study suggests that the selective SIRT1 inhibitor could have the potential of being employed as a chemical inducer to generate functionally active neurons.

Combining Suppression of Stemness with Lineage-Specific Induction Leads to Conversion of Pluripotent Cells into Functional Neurons

Sox2 is a key player in the maintenance of pluripotency and stemness, and thus inhibition of its function would abrogate the stemness of pluripotent cells and induce differentiation into several types of cells. Herein we describe a strategy that relies on a combination of Sox2 inhibition with lineage-specific induction to promote efficient and selective differentiation of pluripotent P19 cells into neurons. When P19 cells transduced with Skp protein, an inhibitor of Sox2, are incubated with a neurogenesis inducer, the cells are selectively converted into neurons that generate depolarization-induced sodium currents and action potentials. This finding indicates that the differentiated neurons are electrophysiologically active. Signaling pathway studies lead us to conclude that a combination of Skp with the neurogenesis inducer enhances neurogenesis in P19 cells by activating Wnt and Notch pathways. The present differentiation protocol could be valuable to selectively generate functionally active neurons from pluripotent cells.

Bumetanide-induced NKCC1 inhibition attenuates oxygen-glucose deprivation-induced decrease in proliferative activity and cell cycle progression arrest in cultured OPCs via p-38 MAPKs

The Na-K-Cl co-transporter 1 (NKCC1; a member of the cation-chloride co-transporter family) mediates the coupled movement of Na(+) and/or K(+) with Cl(-) across the plasma membrane of cells (Haas and Forbush, 2000, Annu. Rev. Physiol., 62, 515-534; Russell, 2000, Physiol. Rev., 80, 211-276). Although it acts as an important regulator of cell volume, secretion, and modulator of cell apoptosis and proliferation (Chen et al., 2005, J. Cereb. Blood Flow Metab., 25, 54-66; Kahle et al., 2008, Nat. Clin. Pract. Neurol., 4, 490-503; Kidokoro et al., 2014, Am. J. Physiol. Ren. Physiol., 306, F1155-F1160; Wang et al., 2011, Cell. Physiol. Biochem., 28, 703-714), NKCC1׳s effects on oligodendrocyte precursor cells (OPCs) have not been characterized. The aim of this study was to investigate whether and to what extent inhibition of NKCC1 alters oxygen glucose deprivation (OGD)-induced cell cycle progression. In the present study, we demonstrated that inhibition of NKCC1 with bumetanide attenuates the decrease in OGD-induced DNA synthesis in cultured OPCs. Western blots showed that NKCC1 inhibition led to an increased expression of cyclin D1, CDK 4, and cyclin E in OGD-treated cells. Furthermore, our results showed bumetanide attenuated the decrease in OGD-induced proliferation and arrest of cell cycle progression via the P-38 MAPK signaling cascade. Thus, NKCC1 plays important roles in the proliferation of OPCs under OGD-induced stress.

Chaos and commotion in the wake of cortical spreading depression and spreading depolarizations

Punctuated episodes of spreading depolarizations erupt in the brain, encumbering tissue structure and function, and raising fascinating unanswered questions concerning their initiation and propagation. Linked to migraine aura and headache, cortical spreading depression contributes to the morbidity in the world's migraine with aura population. Even more ominously, erupting spreading depolarizations accelerate tissue damage during brain injury. The once-held view that spreading depolarizations may not exist in the human brain has changed, largely because of the discovery of migraine genes that confer cortical spreading depression susceptibility, the application of sophisticated imaging tools and efforts to interrogate their impact in the acutely injured human brain.

Progesterone inhibition of neuronal calcium signaling underlies aspects of progesterone-mediated neuroprotection

Progesterone is being utilized as a therapeutic means to ameliorate neuron loss and cognitive dysfunction following traumatic brain injury. Although there have been numerous attempts to determine the means by which progesterone exerts neuroprotective effects, studies describing the underlying molecular mechanisms are lacking. What has become clear, however, is the notion that progesterone can thwart several physiological processes that are detrimental to neuron function and survival, including inflammation, edema, demyelination and excitotoxicity. One clue regarding the means by which progesterone has restorative value comes from the notion that these aforementioned biological processes all share the common theme of eliciting pronounced increases in intracellular calcium. Thus, we propose the hypothesis that progesterone regulation of calcium signaling underlies its ability to mitigate these cellular insults, ultimately leading to neuroprotection. Further, we describe recent findings that indicate neuroprotection is achieved via progesterone block of voltage-gated calcium channels, although additional outcomes may arise from blockade of various other ion channels and neurotransmitter receptors. This article is part of a Special Issue entitled 'Neurosteroids'.

Examining protection from anoxic depolarization by the drugs dibucaine and carbetapentane using whole cell recording from CA1 neurons

As an immediate consequence of stroke onset, failure of the Na(+)-K(+)-ATPase pump evokes a propagating anoxic depolarization (AD) across gray matter. Acute neuronal swelling and dendritic beading arise within seconds in the future ischemic core, imaged as changes in light transmittance (ΔLT). AD is itself not a target for drug-based reduction of stroke injury because it is generated in the 1st min of stroke onset. Peri-infarct depolarizations (PIDs) are milder AD-like events that recur during the hours following AD and contribute to infarct expansion. Inhibiting PIDs with drugs could limit expansion. Two types of drugs, "caines" and σ(1)-receptor ligands, have been found to inhibit AD onset (and may also oppose PID initiation), yet their underlying actions have not been examined. Imaging ΔLT in the CA1 region simultaneously with whole cell current-clamp recording from CA1 pyramidal neurons reveal that the elevated LT front and onset of the AD are coincident. Either dibucaine or carbetapentane pretreatment significantly delays AD onset without affecting resting membrane potential or neuronal input resistance. Dibucaine decreases excitability by raising spike threshold and decreasing action potential (AP) frequency, whereas carbetapentane eliminates the fast afterhyperpolarization while accentuating the slow afterhyperpolarization to reduce AP frequency. Orthodromic and antidromic APs are eliminated by dibucaine within 15 min but not by carbetapentane. Thus both drugs reduce cortical excitability at the level of the single pyramidal neuron but through strikingly different mechanisms. In vivo, both drugs would likely inhibit recurring PIDs in the expanding penumbra and so potentially could reduce developing neuronal damage over many hours poststroke when PIDs occur.

Calcium dependence of damage to mouse motor nerve terminals following oxygen/glucose deprivation

Motor nerve terminals are especially sensitive to an ischemia/reperfusion stress. We applied an in vitro model of this stress, oxygen/glucose deprivation (OGD), to mouse neuromuscular preparations to investigate how Ca(2+) contributes to stress-induced motor terminal damage. Measurements using an ionophoretically-injected fluorescent [Ca(2+)] indicator demonstrated an increase in intra-terminal [Ca(2+)] following OGD onset. When OGD was terminated within 20-30min of the increase in resting [Ca(2+)], these changes were sometimes reversible; in other cases [Ca(2+)] remained high and the terminal degenerated. Endplate innervation was assessed morphometrically following 22min OGD and 120min reoxygenation (32.5°C). Stress-induced motor terminal degeneration was Ca(2+)-dependent. Median post-stress endplate occupancy was only 26% when the bath contained the normal 1.8mM Ca(2+), but increased to 81% when Ca(2+) was absent. Removal of Ca(2+) only during OGD was more protective than removal of Ca(2+) only during reoxygenation. Post-stress endplate occupancy was partially preserved by pharmacological inhibition of various routes of Ca(2+) entry into motor terminals, including voltage-dependent Ca(2+) channels (ω-agatoxin-IVA, nimodipine) and the plasma membrane Na(+)/Ca(2+) exchanger (KB-R7943). Inhibition of a Ca(2+)-dependent protease with calpain inhibitor VI was also protective. These results suggest that most of the OGD-induced motor terminal damage is Ca(2+)-dependent, and that inhibition of Ca(2+) entry or Ca(2+)-dependent proteolysis can reduce this damage. There was no significant difference between the response of wild-type and presymptomatic superoxide dismutase 1 G93A mutant terminals to OGD, or in their response to the protective effect of the tested drugs.

The Na-K-Cl Co-transporter in astrocyte swelling

Ion channels, exchangers and transporters are known to be involved in cell volume regulation. A disturbance in one or more of these systems may result in loss of ion homeostasis and cell swelling. In particular, activation of the Na(+)-K(+)-Cl(-) cotransporters has been shown to regulate cell volume in many conditions. The Na(+)-K(+)-Cl- cotransporters (NKCC) are a class of membrane proteins that transport Na, K, and Cl ions into and out of a wide variety of epithelial and nonepithelial cells. Studies have established the role of NKCC1 in astrocyte swelling/brain edema in ischemia and trauma. Our recent studies suggest that NKCC1 activation is also involved in astrocyte swelling induced by ammonia and in the brain edema in the thioacetamide model of acute liver failure. This review will focus on mechanisms of NKCC1 activation and its contribution to astrocyte swelling/brain edema in neurological disorders, with particular emphasis on ammonia neurotoxicity and acute liver failure.

Ionic storm in hypoxic/ischemic stress: can opioid receptors subside it?

Neurons in the mammalian central nervous system are extremely vulnerable to oxygen deprivation and blood supply insufficiency. Indeed, hypoxic/ischemic stress triggers multiple pathophysiological changes in the brain, forming the basis of hypoxic/ischemic encephalopathy. One of the initial and crucial events induced by hypoxia/ischemia is the disruption of ionic homeostasis characterized by enhanced K(+) efflux and Na(+)-, Ca(2+)- and Cl(-)-influx, which causes neuronal injury or even death. Recent data from our laboratory and those of others have shown that activation of opioid receptors, particularly delta-opioid receptors (DOR), is neuroprotective against hypoxic/ischemic insult. This protective mechanism may be one of the key factors that determine neuronal survival under hypoxic/ischemic condition. An important aspect of the DOR-mediated neuroprotection is its action against hypoxic/ischemic disruption of ionic homeostasis. Specially, DOR signal inhibits Na(+) influx through the membrane and reduces the increase in intracellular Ca(2+), thus decreasing the excessive leakage of intracellular K(+). Such protection is dependent on a PKC-dependent and PKA-independent signaling pathway. Furthermore, our novel exploration shows that DOR attenuates hypoxic/ischemic disruption of ionic homeostasis through the inhibitory regulation of Na(+) channels. In this review, we will first update current information regarding the process and features of hypoxic/ischemic disruption of ionic homeostasis and then discuss the opioid-mediated regulation of ionic homeostasis, especially in hypoxic/ischemic condition, and the underlying mechanisms.

Na-K-Cl Cotransporter-1 in the mechanism of ammonia-induced astrocyte swelling

Brain edema and the consequent increase in intracranial pressure and brain herniation are major complications of acute liver failure (fulminant hepatic failure) and a major cause of death in this condition. Ammonia has been strongly implicated as an important factor, and astrocyte swelling appears to be primarily responsible for the edema. Ammonia is known to cause cell swelling in cultured astrocytes, although the means by which this occurs has not been fully elucidated. A disturbance in one or more of these systems may result in loss of ion homeostasis and cell swelling. In particular, activation of the Na-K-Cl cotransporter (NKCC1) has been shown to be involved in cell swelling in several neurological disorders. We therefore examined the effect of ammonia on NKCC activity and its potential role in the swelling of astrocytes. Cultured astrocytes were exposed to ammonia (NH(4)Cl; 5 mm), and NKCC activity was measured. Ammonia increased NKCC activity at 24 h. Inhibition of this activity by bumetanide diminished ammonia-induced astrocyte swelling. Ammonia also increased total as well as phosphorylated NKCC1. Treatment with cyclohexamide, a potent inhibitor of protein synthesis, diminished NKCC1 protein expression and NKCC activity. Since ammonia is known to induce oxidative/nitrosative stress, and antioxidants and nitric-oxide synthase inhibition diminish astrocyte swelling, we also examined whether ammonia caused oxidation and/or nitration of NKCC1. Cultures exposed to ammonia increased the state of oxidation and nitration of NKCC1, whereas the antioxidants N-nitro-l-arginine methyl ester and uric acid all significantly diminished NKCC activity. These agents also reduced phosphorylated NKCC1 expression. These results suggest that activation of NKCC1 is an important factor in the mediation of astrocyte swelling by ammonia and that such activation appears to be mediated by NKCC1 abundance as well as by its oxidation/nitration and phosphorylation.

A review of progress in understanding the pathophysiology and treatment of brain edema

Object

Brain edema resulting from traumatic brain injury (TBI) or ischemia if uncontrolled exhausts volume reserve and leads to raised intracranial pressure and brain herniation. The basic types of edema—vasogenic and cytotoxic—were classified 50 years ago, and their definitions remain intact.

Methods

In this paper the author provides a review of progress over the past several decades in understanding the pathophysiology of the edematous process and the success and failures of treatment. Recent progress focused on those manuscripts that were published within the past 5 years.

Results

Perhaps the most exciting new findings that speak to both the control of production and resolution of edema in both trauma and ischemia are the recent studies that have focused on the newly described “water channels” or aquaporins. Other important findings relate to the predominance of cellular edema in TBI.

Conclusions

Significant new findings have been made in understanding the pathophysiology of brain edema; however, less progress has been made in treatment. Aquaporin water channels offer hope for modulating and abating the devastating effects of fulminating brain edema in trauma and stroke.

NCX and NCKX Operation in Ischemic Neurons

Within the first 2 min of global brain ischemia, extracellular [K+] ([K+]o) increases above 60 mM and [Na+](o) drops to about 50 mM, indicating a massive K+ efflux and Na+ influx, a phenomenon known as anoxic depolarization (AD). Similar ionic shifts take place during repetitive peri-infarct depolarizations (PID) in the area penumbra in focal brain ischemia. The size of ischemic infarct is determined by the duration of AD and PID. However, the mechanism of cytosolic [Ca2+] ([Ca2+]c) elevation during AD or PID is poorly understood. Our data show that the exposure of cultured rat hippocampal CA1 neurons to AD-like conditions promptly elevates [Ca2+]c to about 30 microM. These high [Ca2+]c elevations depend on external Ca2+ and can be prevented by removing Na+ or by simultaneously inhibiting NMDA and AMPA/kainate receptors. These data indicate that [Ca2+]c elevations during AD result from Na+ influx via either NMDA or AMPA/kainate channels. The mechanism of the Na-dependent [Ca2+]c elevations may involve a reversal of plasmalemmal Na+/Ca2+ (NCX) and/or Na+/Ca2+ + K+ (NCKX) exchangers. KB-R7943, an NCX inhibitor, suppresses a fraction of the Na-dependent Ca2+ influx during AD. Therefore, Ca2+ influx via NCX and a KB-R7943-resistant pathway (possibly NCKX) is involved. Inhibition of the Na-dependent Ca2+ influx is likely to decrease ischemic brain damage. No drugs are known that are able to inhibit the KB-R7943-resistant component of Na-dependent Ca2+ influx during AD. The present data encourage development of such agents as potential therapeutic means to limit ischemic brain damage after stroke or heart attack.

Ion channels involved in stroke

Ion channels are membrane proteins that flicker open and shut to regulate the flow of ions down their electrochemical gradient across the membrane and consequently regulate cellular excitability. Every living cell expresses ion channels, as they are critical life-sustaining proteins. Ion channels are generally either activated by voltage or by ligand interaction. For each group of ion channels the channels' molecular biology and biophysics will be introduced and the pharmacology of that group of channels will be reviewed. The in vitro and in vivo literature will be reviewed and, for ion channel groups in which clinical trials have been conducted, the efficacy and therapeutic potential of the neuroprotective compounds will be reviewed. A large part of this article will deal with glutamate receptors, focusing specifically on N-methyl-D-aspartate (NMDA) receptors. Although the outcome of clinical trials for NMDA receptor antagonists as therapeutics for acute stroke is disappointing, the culmination of these failed trials was preceded by a decade of efforts to develop these agents. Sodium and calcium channel antagonists will be reviewed and the newly emerging efforts to develop therapeutics targeting potassium channels will be discussed. The future development of stroke therapeutics targeting ion channels will be discussed in the context of the failures of the last decade in hopes that this decade will yield successful stroke therapeutics.

The role of Na-K-Cl co-transporter in cerebral ischemia

The electroneutral Na-K-Cl co-transporter (NKCC) protein transports Na(+), K(+) and Cl(-) into cells under physiological conditions with a stoichiometry of 1Na(+) :1K(+) :2Cl(-). NKCC is characteristically inhibited by the sulfamoylbenzoic acid "loop'' diuretics, such as bumetanide and furosemide. To date, only two distinct isoforms, NKCC1 and NKCC2, have been identified. NKCC1 has a broad tissue distribution, while the NKCC2 isoform is only found in vertebrate kidney. NKCC serves multiple functions, including ion and fluid movements in secreting or reabsorbing epithelia and cell volume regulation. However, understanding the role of NKCC1 in the central nervous system has just begun. NKCC1 protein is expressed in neurons throughout the brain. Dendritic localization of NKCC1 is found in both pyramidal and non-pyramidal neurons. NKCC1 is important in the maintenance of intracellular Cl(-) in neurons and contributes to GABA-mediated depolarization in immature neurons. Thus, NKCC1 may affect neuronal excitability through regulation of intracellular Cl(-) concentration. Expression of NKCC1 protein has also been found in astrocytes and oligodendrocytes. In addition to its role in the accumulation of Cl(-), NKCC1 may also contribute to K(+) clearance and maintenance of intracellular Na(+) in glia. Our recent studies suggest that NKCC1 activation leads to high [K(+)](o(-)) induced astrocyte swelling and glutamate release, as well as neuronal Na(+) , and Cl(-) influx during acute excitotoxicity. Inhibition of NKCC1 activity significantly reduces infarct volume and cerebral edema following cerebral focal ischemia.

Voltage-gated cation channel modulators for the treatment of stroke

Neuronal voltage-gated cation channels regulate the transmembrane flux of calcium, sodium and potassium. Neuronal ischaemia occurring during acute ischaemic stroke results in the breakdown in the normal function of these ion channels, contributing to a series of pathological events leading to cell death. A dramatic increase in the intracellular concentration of calcium during neuronal ischaemia plays a particularly important role in the neurotoxic cascade resulting in stroke-related acute neurodegeneration. One approach to provide therapeutic benefit following ischaemic stroke has been to target neuronal voltage-gated cation channels, and particularly blockers of calcium and sodium channels, for post-stroke neuroprotection. A recent development has been the identification of openers of large-conductance calcium- and voltage-dependent potassium channels (maxi-K channels), which hyperpolarize ischaemic neurons, reduce excitatory amino acid release, and reduce ischaemic calcium entry. Thus far, targeting these voltage-gated cation channels has not yet yielded significant clinical benefit. The reasons for this may involve the lack of small-molecule blockers of many neuronal members of these ion channel families and the design of preclinical stroke models, which do not adequately emulate the clinical condition and hence lack sufficient rigor to predict efficacy in human stroke. Furthermore, there may be a need for changes in clinical trial designs to optimise the selection of patients and the course of drug treatment to protect neurons during all periods of potential neuronal sensitivity to neuro-protectants. Clinical trials may also have to be powered to detect small effect sizes or be focused on patients more likely to respond to a particular therapy. The development of future solutions to these problems should result in an improved probability of success for the treatment of stroke.

Neuronal NOS activation during oxygen and glucose deprivation triggers cerebellar granule cell death in the later reoxygenation phase

The present study investigated the temporal relationship between neuronal nitric oxide synthase (nNOS) activity and expression and the development of neuronal damage occurring during anoxia and anoxia followed by reoxygenation. For this purpose, cerebellar granule cells were exposed to 2 hr of oxygen and glucose deprivation (OGD) and 24 hr of reoxygenation. To clarify the consequences of nNOS activity inhibition on neuronal survival, cerebellar granule cells were exposed to OGD, both in the absence of extracellular Na(+) ([Na(+)](e)), a condition that by reducing intracellular Ca(2+) ([Ca(2+)](I)) prevents Ca(2+)-dependent nNOS activation, and in the presence of selective and nonselective nNOS inhibitors, such as N(omega)-L-allyl-L-arginine (L-ALA), N(omega)-propyl-L-arginine (NPLA), and L-nitro-arginine-methyl-ester (L-NAME), respectively. The results demonstrated that the removal of [Na(+)](e) hampered the [Ca(2+)](i) increase and decreased expression and activity of nNOS. Similarly, the increase of free radical production present in cerebellar neurons, exposed previously to OGD and OGD/reoxygenation, was abolished completely in the absence of [Na(+)](e). Furthermore, the absence of [Na(+)](e) in cerebellar neurons exposed to 2 hr of OGD led to the improvement of mitochondrial activity and neuronal survival, both after the OGD phase and after 24 hr of reoxygenation. Finally, the exposure of cerebellar neurons to L-ALA (200 nM), and L-NAME (500 microM) was able to effectively reduce NO(*) production and caused an increase in mitochondrial oxidative activity and an improvement of neuronal survival not only during OGD, but also during reoxygenation. Similar results during OGD were obtained also with NPLA (5 nM), another selective nNOS inhibitor. These data suggest that the activation of nNOS is highly accountable for the neuronal damage occurring during the OGD and reoxygenation phases.

Study of the interaction of lubeluzole with cardiac sodium channels

The effects of lubeluzole on sodium currents were examined in guinea-pig isolated cardiac myocytes by use of the whole-cell patch clamp technique. Lubeluzole (0.01-100 microM) reduced peak Na+ current (INa) obtained at a holding potential of -80 mV with an IC50 value of 9.5 (3.5-21.9) microM and a Hill coefficient of 1.1. These effects were rapid and reversible. Lubeluzole (10 microM) produced a shift in the inactivation curve to hyperpolarized potentials (by -9.7 mV, P < 0.05), but produced no change in the voltage-dependence of activation. Lubeluzole (10 microM) produced significant tonic block of INa obtained at a holding potential of -120 mV (2.7 +/- 1.4% and 27.5 +/- 5.8% for control and lubeluzole, respectively; n = 6; P < 0.05). Use-dependent block of INa was also observed. Recovery from block was delayed by lubeluzole (10 microM; tau1=4.4 +/- 6.2, tau2=22.7 +/- 1.5 milliseconds for control and tau1=311 +/- 144, tau2 = 672 +/- 23 milliseconds for lubeluzole; n = 6; P < 0.001) confirming use-dependency of block. The results indicate that lubeluzole produces both tonic and use-dependent block of cardiac sodium channels at concentrations similar to those that block neuronal sodium channels, due mainly to interaction of the drug with channels in the inactivated state.

Secondary injury mechanisms of spinal cord trauma: a novel therapeutic approach for the management of secondary pathophysiology with the sodium channel blocker riluzole

Traumatic spinal cord injury is a consequence of a primary mechanical insult and a sequence of progressive secondary pathophysiological events that confound efforts to mitigate neurological deficits. Pharmacotherapy aimed at reducing the secondary injury is limited by a narrow therapeutic window. Thus, novel drug strategies must target early pathological mechanisms in order to realize the promise of efficacy for this form of neurotrauma. Research has shown that an accumulation of intracellular sodium as a result of trauma-induced perturbation of voltage-sensitive sodium channel activity is a key early mechanism in the secondary injury cascade. As such, voltage-sensitive sodium channels are an important therapeutic target for the treatment of spinal cord trauma. This review describes the evolution of acute spinal cord injury and provides a rationale for the clinical utility of sodium channel blockers, particularly riluzole, in the management of spinal cord trauma.

Radix paeoniae rubra suppression of sodium current in acutely dissociated rat hippocampal CA1 neurons

The effect of Radix paeoniae rubra (RPR) on voltage-gated sodium channel (VGSC) currents (I(Na)) was examined in freshly isolated rat hippocampal CA1 neurons using whole-cell patch-clamp technique under voltage-clamp conditions. RPR suppressed I(Na) without affecting the current activation, inactivation and deactivation. The amplitude of I(Na) decreased by approximately 18.4% within a few seconds of 0.8 mg/ml RPR exposure. RPR (0.8 mg/ml) shifted the steady-state inactivation curves of I(Na) to negative potentials, with hyperpolarizing direction shift of V(1/2) of 10.0 mV. The time course of I(Na) recovery from inactivation was prolonged significantly by 0.8 mg/ml RPR. RPR (0.8 mg/ml) also enhanced the activity-dependent attenuation of I(Na) and decreased the fraction of activated channels. These results suggested that RPR suppressed hippocampal CA1 I(Na) by shifting the inactivation curve in hyperpolarizing direction, slowing the recovery time course from inactivation, enhancing the activity-dependent attenuation and decreasing the number of activatable channels. RPR suppression on I(Na) might predict the protective effect during brain ischemia in hippocampal CA1 neurons.

Ionic mechanism of ouabain-induced concurrent apoptosis and necrosis in individual cultured cortical neurons

Energy deficiency and dysfunction of the Na+, K+-ATPase are common consequences of many pathological insults. The nature and mechanism of cell injury induced by impaired Na+, K+-ATPase, however, are not well defined. We used cultured cortical neurons to examine the hypothesis that blocking the Na+, K+-ATPase induces apoptosis by depleting cellular K+ and, concurrently, induces necrotic injury in the same cells by increasing intracellular Ca2+ and Na+. The Na+, K+-ATPase inhibitor ouabain induced concentration-dependent neuronal death. Ouabain triggered transient neuronal cell swelling followed by cell shrinkage, accompanied by intracellular Ca2+ and Na+ increase, K+ decrease, cytochrome c release, caspase-3 activation, and DNA laddering. Electron microscopy revealed the coexistence of ultrastructural features of both apoptosis and necrosis in individual cells. The caspase inhibitor Z-Val-Ala-Asp(OMe)-fluoromethyl ketone (Z-VAD-FMK) blocked >50% of ouabain-induced neuronal death. Potassium channel blockers or high K+ medium, but not Ca2+ channel blockade, prevented cytochrome c release, caspase activation, and DNA damage. Blocking of K+, Ca2+, or Na+ channels or high K+ medium each attenuated the ouabain-induced cell death; combined inhibition of K+ channels and Ca2+ or Na+ channels resulted in additional protection. Moreover, coapplication of Z-VAD-FMK and nifedipine produced virtually complete neuroprotection. These results suggest that the neuronal death associated with Na+, K+-pump failure consists of concurrent apoptotic and necrotic components, mediated by intracellular depletion of K+ and accumulation of Ca2+ and Na+, respectively. The ouabain-induced hybrid death may represent a distinct form of cell death related to the brain injury of inadequate energy supply and disrupted ion homeostasis.

Role of K(+) efflux in apoptosis induced by AMPA and kainate in mouse cortical neurons

Activation of ionotropic glutamate receptors can induce neuronal apoptosis in vitro and in vivo. We showed previously that activation of the N-methyl-D-aspartic acid (NMDA) subtype of glutamate receptors in a low Ca(2+) and low Na(+) condition induced apoptotic neuronal death, and that the K(+) efflux via NMDA receptor channels was likely a key event in NMDA-induced apoptosis. Since non-NMDA receptors, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) and kainate receptors, are also permeable to K(+), we tested the hypothesis that stimulating K(+) efflux via non-NMDA receptor channels could induce apoptosis in cultured cortical neurons. Using a Ca(2+)-free and Na(+)-free external solution, application of kainate revealed outward membrane currents carried by K(+) efflux. In a low Ca(2+)/low Na(+) medium, a 5-h exposure to 50-500 microM AMPA in the presence of the NMDA receptor antagonist MK801 induced dose-dependent neuronal death 24 h after the onset of the insult, accompanied by intracellular K(+) reduction and caspase-3 activation. The AMPA-induced cell death was attenuated by the caspase inhibitor Z-Val-Ala-Asp(OMe)-fluoromethyl ketone (Z-VAD-FMK) and by the protein synthesis inhibitor cycloheximide. Reducing K(+) efflux by raising extracellular K(+) concentration from 5 to 25 mM attenuated AMPA-triggered cell death, the Ca(2+) channel antagonist nifedipine showed no effect on the AMPA toxicity. Kainate induced similar neuronal death sensitive to attenuation by Z-VAD-FMK or elevated extracellular K(+).We suggest that the non-NMDA receptor-mediated K(+) efflux may participate in apoptotic process and that blocking excessive K(+) efflux mediated by NMDA and non-NMDA receptors may selectively prevent neuronal apoptosis under certain pathological conditions.

Mechanisms of glutamate release in the rat spinal cord slices during metabolic inhibition

Glutamate toxicity is a viable hypothesis to explain the expanding tissue degeneration occurring after traumatic or ischemic spinal cord injury. One important component in this process is the acute, excessive release of glutamate. In the current communication, the glycolytic inhibitor iodoacetate was used to induce metabolic inhibition in spinal cord slices and thereby provide an in vitro model to study the mechanisms of pathological glutamate release in the spinal cord. The evoked glutamate release was not Ca2+-dependent. Exclusion of NaCl reduced the evoked release of endogenous glutamate by 56%, while excluding Na+ increased release. Glutamate release was also reduced by the PLA2 inhibitors indomethacin (40%), arachidonyltrifluoromethyl ketone (45%) and 4-bromophenacyl bromide (36%). Blocking reverse glutamate transport by preincubation with 1 mM dihydrokainic acid reduced evoked release by 41%. However, when the dihydrokainic acid and arachidonyltrifluoromethyl ketone treatments were combined, no additive effect of the two substances was seen. These findings suggest that glutamate is released by three mechanisms from the energy compromised spinal cord: (1) in response to cellular swelling, most likely by the regulatory volume decrease, (2) by PLA2-mediated breakdown of the cell membrane and diffusion of glutamate down its concentration gradient, and (3) through reversal of the glutamate transporter.

Mechanisms of spreading depression and hypoxic spreading depression-like depolarization

Spreading depression (SD) and the related hypoxic SD-like depolarization (HSD) are characterized by rapid and nearly complete depolarization of a sizable population of brain cells with massive redistribution of ions between intracellular and extracellular compartments, that evolves as a regenerative, "all-or-none" type process, and propagates slowly as a wave in brain tissue. This article reviews the characteristics of SD and HSD and the main hypotheses that have been proposed to explain them. Both SD and HSD are composites of concurrent processes. Antagonists of N-methyl-D-aspartate (NMDA) channels or voltage-gated Na(+) or certain types of Ca(2+) channels can postpone or mitigate SD or HSD, but it takes a combination of drugs blocking all known major inward currents to effectively prevent HSD. Recent computer simulation confirmed that SD can be produced by positive feedback achieved by increase of extracellular K(+) concentration that activates persistent inward currents which then activate K(+) channels and release more K(+). Any slowly inactivating voltage and/or K(+)-dependent inward current could generate SD-like depolarization, but ordinarily, it is brought about by the cooperative action of the persistent Na(+) current I(Na,P) plus NMDA receptor-controlled current. SD is ignited when the sum of persistent inward currents exceeds persistent outward currents so that total membrane current turns inward. The degree of depolarization is not determined by the number of channels available, but by the feedback that governs the SD process. Short bouts of SD and HSD are well tolerated, but prolonged depolarization results in lasting loss of neuron function. Irreversible damage can, however, be avoided if Ca(2+) influx into neurons is prevented.

Effects of ion channel blockade on the distribution of Na, K, Ca and other elements in oxygen-glucose deprived CA1 hippocampal neurons

The pathophysiology of brain ischemia and reperfusion injury involves perturbation of intraneuronal ion homeostasis. To identify relevant routes of ion flux, rat hippocampal slices were perfused with selective voltage- or ligand-gated ion channel blockers during experimental oxygen-glucose deprivation and subsequent reperfusion. Electron probe X-ray microanalysis was used to quantitate water content and concentrations of Na, K, Ca and other elements in morphological compartments (cytoplasm, mitochondria and nuclei) of individual CA1 pyramidal cell bodies. Blockade of voltage-gated channel-mediated Na+ entry with tetrodotoxin (1 microM) or lidocaine (200 microM) significantly reduced excess intraneuronal Na and Ca accumulation in all compartments and decreased respective K loss. Voltage-gated Ca2+ channel blockade with the L-type antagonist nitrendipine (10 microM) decreased Ca entry and modestly preserved CA1 cell elemental composition and water content. However, a lower concentration of nitrendipine (1 microM) and the N-, P-subtype Ca2+ channel blocker omega-conotoxin MVIIC (3 microM) were ineffective. Glutamate receptor blockade with the N-methyl-D-aspartate (NMDA) receptor-subtype antagonist 3-(2-carboxypiperazin-4-yl) propyl-1-phosphonic acid (CPP; 100 microM) or the alpha-amino-3-hydroxy-5-methyl-4-isoazole propionic acid (AMPA) receptor subtype blocker 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM/100 microM glycine) completely prevented Na and Ca accumulation and partially preserved intraneuronal K concentrations. Finally, the increase in neuronal water content normally associated with oxygen-glucose deprivation/reperfusion was prevented by Na+ channel or glutamate receptor blockade. Results of the present study demonstrate that antagonism of either postsynaptic NMDA or AMPA glutaminergic receptor subtypes provided nearly complete protection against ion and water deregulation in nerve cells subjected to experimental ischemia followed by reperfusion. This suggests activation of ionophoric glutaminergic receptors is involved in loss of neuronal osmoregulation and ion homeostasis. Na+ channel blockade also effectively diminished neuronal ion and water derangement during oxygen-glucose deprivation and reperfusion. Prevention of elevated Nai+ levels is likely to provide neuroprotection by decreasing presynaptic glutamate release and by improving cellular osmoregulation, adenosine triphosphate utilization and Ca2+ clearance. Thus, we suggest that voltage-gated tetrodotoxin-sensitive Na+ channels and glutamate-gated ionotropic NMDA or AMPA receptors are important routes of ion flux during nerve cell injury induced by oxygen-glucose deprivation/reperfusion.

Evaluation of the neuroprotective effects of sodium channel blockers after spinal cord injury: improved behavioral and neuroanatomical recovery with riluzole

OBJECT

Persistent activation of voltage-sensitive Na+ channels is associated with cellular toxicity and may contribute to the degeneration of neural tissue following traumatic brain and spinal cord injury (SCI). Pharmacological blockade of these channels can attenuate secondary pathophysiology and reduce functional deficits acutely.

METHODS

To determine the therapeutic effects of Na+ channel blockers on long-term tissue sparing and functional neurological recovery after traumatic SCI, the authors injected Wistar rats intraperitoneally with riluzole (5 mg/kg), phenytoin (30 mg/kg), CNS5546A, a novel Na+ channel blocker (15 mg/kg), or vehicle (2-HP3CD; 5 mg/kg) 15 minutes after induction of compressive SCI at C7-T1. Functional neurological recovery of coordinated hindlimb function and strength, assessed 1 week postinjury and weekly thereafter for 6 weeks, was significantly enhanced in animals treated with riluzole compared with the other treatment groups. Seven weeks postinjury the preservation of residual tissue and integrity of descending axons were determined with digital morphometrical and fluorescent histochemical analysis. All three Na+ channel blockers significantly enhanced residual tissue area at the injury epicenter compared with control. Riluzole significantly reduced tissue loss in rostrocaudal regions surrounding the epicenter, with overall sparing of gray matter and selective sparing of white matter. Also, counts of red nuclei neurons retrogradely labeled with fluorogold introduced caudal to the injury site were significantly increased in the riluzole group.

CONCLUSIONS

Systemic Na+ channel blockers, in particular riluzole, can confer significant neuroprotection after in vivo SCI and result in behavioral recovery and sparing of both gray and white matter.

Characterization of endogenous amino acid efflux from hippocampal slices during chemically-induced ischemia

Using sodium (NaN3)-induced anoxia plus aglycaemia as a model of chemically-induced ischemia, we have characterized the endogenous release of excitatory and inhibitory amino acids from superfused hippocampal slices. Chemical ischemia produced an azide (1-30 mM) dose-dependent increase in the efflux of glutamate, aspartate and GABA. These increases were attenuated to varying degrees by removal of Ca2+, or the addition of the voltage dependent Na+-channel blocker tetrodotoxin (TTX), the selective Ca2+ channel blockers conotoxin MVIIA, MVIIC, and nifedipine, the NMDA antagonist MK801, the AMPA antagonist GYKI-52466. Similarly, addition of the GLT-1 glutamate transport inhibitor dihydrokainate (DHK) and the anti-estrogen/anion channel blocker tamoxifen also attenuated the efflux of glutamate and GABA. It would therefore appear that the increases in amino acid efflux induced by chemical ischemia originates from both the neuronal pool, via conventional exocytotic release, and glial sources via reversal of the GLT-1 transporter and anion channel regulated cell swelling.

Characterization of transient focal ischemia-induced increases in extracellular glutamate and aspartate in spontaneously hypertensive rats

Using middle cerebral artery occlusion (MCAO) and in vivo microdialysis, we have evaluated the changes in extracellular concentrations of the excitatory amino acids (EAA) glutamate and aspartate during varying periods of MCAO (0, 30, 60 min) in the striatum of spontaneously hypertensive rats (SHR). A positive correlation between occlusion time-dependent elevations in EAAs and the resulting ischemic injury was observed. This is the first demonstration of the temporal profile of EAA efflux during transient focal ischemia in SHRs. Possible sources and mechanisms of ischemia-induced EAA efflux were examined during 60 min of MCAO. Removal of Ca(2+) from the microdialysis infusion media significantly attenuated ischemia-induced increases in both glutamate (from ischemic peak of 4892 +/- 1298 to 1144 +/- 666% of preischemic values) and aspartate (from 2703 +/- 682 to 2090 +/- 599% of preischemic values). Similarly, infusion of the voltage dependent Na(+) channel blocker tetrodotoxin (TTX; 10 microM) significantly attenuated MCAO-induced increases in glutamate (to 1313 +/- 648%) and aspartate (to 359 +/- 114%). Infusion of the GLT-1 selective nontransportable inhibitor, dihydrokainate (DHK; 1 mM) also significantly attenuated the ischemia-induced increases in both EAAs (1285 +/- 508 and 1366 +/- 741% of the preischemic levels, respectively). These results indicate that during transient focal ischemia the increase in extracellular EAAs originates from both the neuronal pool, via conventional exocytotic release, and glial sources via the reversal of the GLT-1 transporter.

Potential neurotoxicity of spinal anesthesia with lidocaine

Spinal (intrathecal) anesthesia has evolved into a safe, widely accepted method of anesthesia with many advantages. However, the past decade has seen a large number of case reports and incidence studies that implicate the local anesthetic (LA) lidocaine as being more neurotoxic than other commonly used LAs such as bupivacaine and tetracaine, based on patterns of clinical use current at the time of those reports. Available studies suggest a risk of persistent lumbosacral neuropathy after spinal lidocaine by single injection in about 1 in 1300 procedures and a risk as high as about 1 in 200 after continuous spinal anesthesia with lidocaine. While uncommon, this risk is probably an order of magnitude higher than the risk reported for other commonly used LAs or for general anesthesia. Spinal lidocaine is also implicated in the syndrome of transient neurologic symptoms (previously referred to as transient radicular irritation), manifest by pain or dysesthesia in the buttocks or legs after recovery from anesthesia. Although the pain typically resolves within 1 week without lasting sequelae, it can be severe in up to one third of patients with the syndrome. In addition to clinical studies, both whole animal and in vitro studies have shown that lidocaine can be neurotoxic at clinically available concentrations and that lidocaine is more neurotoxic than equipotent concentrations of other commonly used LAs. The mechanism of this neurotoxicity may involve changes in cytoplasmic calcium homeostasis and mitochondrial membrane potential.

Role of voltage-gated Na+ channels in hypoxia-induced neuronal injuries

1. Mammalian neurons in the central nervous system are vulnerable to oxygen deprivation. In clinical conditions, such as stroke or apnoea, permanent loss of neuronal functions can occur within minutes of severe hypoxia. 2. Recent studies have focused on the role of Na+ in acute neuronal responses to hypoxia. These studies have shown that the influx of extracellular Na+ is an important factor in hypoxia-induced injury and that blockade of voltage-gated Na+ channels reduces hypoxic responses and injury of neurons. Yet, the mechanism underlying the effect of blockade of Na+ channels on hypoxic injury is unclear. 3. The aim of the present review is to discuss the above topics given the current understanding of the role of Na+ channels in hypoxia and its implications on therapeutic strategy for preventing hypoxia-induced neurological damage. 4. It has been known that the maintenance of ionic homeostasis and membrane properties in neurons are improved by reducing the activity of voltaged-gated Na+ channels during acute hypoxia. 5. Recent studies suggest that persistent Na+ current and Na+-dependent exchangers may play a role in Na+ influx and neuronal injury during hypoxia. 6. The neuroprotective action of blockers of the Na+ channel may also be via the improved maintainance of intracellular energy levels because the action is dependent on cellular energy levels and extracellular glucose during hypoxia. 7. Hence, the blockade of voltage-gated Na+ channels reduces the excitability of neurons, Na+ influx and the accumulation of intracellular Na+. These improve the ionic homeostasis and cellular energy levels and, thus, prevent hypoxia-induced neuronal injury and neuronal damage mediated by Ca2+ overload.

Inhibition of different pathways influencing Na(+) homeostasis protects organotypic hippocampal slice cultures from hypoxic/hypoglycemic injury

A prominent feature of cerebral ischemia is the excessive intracellular accumulation of both Na(+) and Ca(2+), which results in subsequent cell death. A large number of studies have focused on pathways involved in the increase of the intracellular Ca(2+) concentration [Ca(2+)](i), whereas the elevation of intracellular Na(+) has received less attention. In the present study we investigated the effects of inhibitors of different Na(+) channels and of the Na(+)/Ca(2+) exchanger, which couples the Na(+) to the Ca(2+) gradient, on ischemic damage in organotypic hippocampal slice cultures. The synaptically evoked population spike in the CA1 region was taken as a functional measure of neuronal integrity. Neuronal cell death was assessed by propidium iodide staining. The Na(+) channel blocker tetrodotoxin, and the NMDA receptor blocker MK 801, but not the AMPA/kainate receptor blocker NBQX prevented ischemic cell death. The novel Na(+)/Ca(2+) exchange inhibitor 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulfonate (KB-R7943), which preferentially acts on the reverse mode of the exchanger, leading to Ca(2+) accumulation, also reduced neuronal damage. At higher concentrations, KB-R7943 also inhibits Ca(2+) extrusion by the forward mode of the exchanger and exaggerates neuronal cell death. Neuroprotection by KB-R7943 may be due to reducing the [Ca(2+)](i) increase caused by the exchanger.

Threshold of ischemic depolarization for neuronal injury following four-vessel occlusion in the rat cortex

Previous studies have indicated that prolonging the onset of ischemic depolarization reduces neuronal damage. However, the relationship between the duration of ischemic depolarization and its histological outcome has not been quantitatively evaluated. Rats were anesthetized (with 1% halothane), intubated, and placed in a stereotaxic frame. Direct current (DC)-potential in the 5th layer of the parietal cortex was measured. Changes in cerebral blood flow (CBF) were monitored by a laser-Doppler flow probe placed adjacent to the DC-electrode. Animals were subjected to four-vessel occlusion for 5 minutes, 10 minutes, or 20 minutes. The histological outcome was evaluated at the DC-recorded site 7 days after the insult. Initiation of ischemia reduced CBF to 9%-28% of the preischemic value. Initial DC-deflection (14+/-3 mV), indicating ischemic depolarization, was observed at 172+/-112 seconds after the onset of ischemia. Total durations of ischemic depolarization were 6.4+/-1.5 minutes, 10.6+/-3.5 minutes and 19.8+/-4.6 minutes in rats in which ischemia was induced for 5 minutes, 10 minutes, and 20 minutes, respectively. The regression curve showed that there was a close relationship between percentage of damaged neurons and duration of ischemic depolarization (r2 = 0.80, P = .001). The durations of ischemic depolarization for neuronal injury in 30%, 50%, and 70% of pyramidal neurons in the parietal cortex were 15.6 minutes (95% confidence interval, 14.2-17.0 minutes), 20.3 minutes (18.5-22.7 minutes) and 25.0 minutes (22.7-28.5 minutes), respectively. It is thought that shortening the duration of ischemic depolarization is a rational approach for reducing the severity of ischemic injury.

The protective action of tetrodotoxin and (+/-)-kavain on anaerobic glycolysis, ATP content and intracellular Na+ and Ca2+ of anoxic brain vesicles

Because recent reports point to Na+ channel blockers as protective agents directed against anoxia-induced neuronal damage including protection of anaerobic glycolysis, the influences of tetrodotoxin (TTX) and (+/-)-kavain on anoxic rat brain vesicles were investigated with respect to lactate synthesis, vesicular ATP content and cytosolic free Na+ and Ca2+ ([Na+]i, [Ca2+]i), both of the latter determined fluorometrically employing SBFI and FURA-2, respectively. After anoxia, basal lactate production was increased from 2.9 to 9.8 nmol lactate/min/mg protein. Although lactate synthesis seemed to be stable for at least 45 min of anoxia, as deduced from the linearity of lactate production, the ATP content declined continuously with a half life (tau 1/2) of 14.5 min, indicating that anaerobic glycolysis was insufficient to cover the energy demand of anoxic vesicles. Correspondingly, [Na+]i and [Ca2+]i increased persistently after anoxia by 22.1 mmol/l Na+ and 274.9 nmol/l Ca2+, determined 6.3 min after onset. An additional stimulation of vesicles with veratridine accelerated the drop of ATP (tau 1/2 = 5.1 min) and provoked a massive Na+ overload, which levelled off to 119 mmol/l Na+ within a few minutes. Concomitantly, [Ca2+]i increased linearly with a rate of 355 nmol Ca2+/l/min. Despite the massive perturbation of ion homeostasis, lactate production was unaffected during the first 8 min of veratridine stimulation. However, complete inhibition of lactate synthesis took place 30 min after veratridine was added. The Na+ channel blockers TTX and (+/-)-kavain, if applied before anoxia, preserved vesicular ATP content, diminished anoxia-induced increases in [Na+]i and [Ca2+]i and prevented both the veratridine-induced increases of [Na+]i and [Ca2+]i and the inhibition of lactate production. The data indicate a considerable Na+ influx via voltage-dependent Na+ channels during anoxia, which speeds up the decline in ATP and provokes an increase in [Ca2+]i. A massive Na+ and Ca2+ overload induced by veratridine failed to influence lactate synthesis directly, but initiated its inhibition.

Intraneuronal ion distribution during experimental oxygen/glucose deprivation. Routes of ion flux as targets of neuroprotective strategies

Ischemic neuronal injury appears to be mediated by disruption of subcellular ion distribution and, therefore, prevention of ion relocation might be neuroprotective. X-ray microanalysis was used to measure concentrations of Na, K, Ca and other elements in subcellular compartments (e.g., mitochondria) of CA1 neurons from oxygen/glucose-deprived (OGD) hippocampal slices. Results showed that OGD produced progressive loss of ion regulation in CA1 cells. Post-OGD reperfusion with normal media exacerbated the initial ion deregulation. To study neuroprotective mechanisms, we determined the ability of hypothermia (31 degrees C) or ion channel blockade to retard intraneuronal ion disruption induced by OGD/reperfusion. Whereas Ca2+ channel blockade (omega-conotoxin MVIIC, 3 microM) was ineffective, hypothermia and Na+ channel blockers (tetrodotoxin, TTX, 1 microM; lidocaine, 200 microM) reduced ion deregulation in subneuronal compartments. Blockade of glutamate receptors (AMPA, 10 microM; the non-NMDA receptor antagonist CNQX, 10 microM/100 microM glycine; the NMDA receptor antagonist CCP, 100 microM) during OGD/reperfusion provided nearly complete protection. These findings provide a foundation for identifying potential pharmacotherapeutic approaches and for discerning corresponding mechanisms of neuroprotection.

Sodium overload through voltage-dependent Na(+) channels induces necrosis and apoptosis of rat superior cervical ganglion cells in vitro

Using the failure to exclude trypan blue as a criterion for cell death, we found that veratridine, the voltage-dependent Na(+) channel activator, exerted its toxicity to cultured sympathetic neurons in a dose-dependent manner (half-maximal toxicity occurred at 2 microM). The co-presence of tetrodotoxin completely reversed the toxicity only at concentrations of veratridine < 20 microM. Veratridine neurotoxicity was due to the influx of Na(+); a medium low in Na(+) (36 mM) completely abolished its neurotoxicity, whereas a Ca(2+)-free medium did not attenuate its neurotoxicity. Furthermore, the buffering action of 1, 2-Bis-(2-aminophenoxy)ethane-N,N,N',N',-tetraacetate (BAPTA) on veratridine-induced increase in intracellular Ca(2+) levels neither blocked veratridine neurotoxicity in normal medium, nor attenuated the low Na(+) effect. Elevated K(+) effectively blocked veratridine neurotoxicity in a Ca(2+)-dependent manner. Cytoplasmic pH measurements using a fluorescent pH indicator demonstrated that cellular acidification (from pH 7.0 to pH 6.5) occurred upon treatment with veratridine. Both veratridine-induced acidification and cell death were ameliorated by 5-(N-ethyl-N-isopropyl)amiloride, the specific inhibitor of the Na(+)/H(+) exchanger (IC(50) = 0.5 microM). Finally, necrosis occurred predominantly in veratridine neurotoxicity, but both staining with bis-benzimide and TUNEL analysis showed nuclear features of apoptosis in sympathetic neurons undergoing cell death.

Na(+) and K(+) concentrations, extra- and intracellular voltages, and the effect of TTX in hypoxic rat hippocampal slices

Severe hypoxia causes rapid depolarization of CA1 neurons and glial cells that resembles spreading depression (SD). In brain slices in vitro, the SD-like depolarization and the associated irreversible loss of function can be postponed, but not prevented, by blockade of Na(+) currents by tetrodotoxin (TTX). To investigate the role of Na(+) flux, we made recordings from the CA1 region in hippocampal slices in the presence and absence of TTX. We measured membrane changes in single CA1 pyramidal neurons simultaneously with extracellular DC potential (V(o)) and either extracellular [K(+)] or [Na(+)]; alternatively, we simultaneously recorded [Na(+)](o), [K(+)](o), and V(o). Confirming previous reports, early during hypoxia, before SD onset, [K(+)](o) began to rise, whereas [Na(+)](o) still remained normal and V(o) showed a slight, gradual, negative shift; neurons first hyperpolarized and then began to gradually depolarize. The SD-like abrupt negative DeltaV(o) corresponded to a near complete depolarization of pyramidal neurons and an 89% decrease in input resistance. [K(+)](o) increased by 47 mM and [Na(+)](o) dropped by 91 mM. Changes in intracellular Na(+) and K(+) concentrations, estimated on the basis of the measured extracellular ion levels and the relative volume fractions of the neuronal, glial, and extracellular compartment, were much more moderate. Because [Na(+)](o) dropped more than [K(+)](o) increased, simple exchange of Na(+) for K(+) cannot account for these ionic changes. The apparent imbalance of charge could be made up by Cl(-) influx into neurons paralleling Na(+) flux and release of Mg(2+) from cells. The hypoxia-induced changes in interneurons resembled those observed in pyramidal neurons. Astrocytes responded with an initial slow depolarization as [K(+)](o) rose. It was followed by a rapid but incomplete depolarization as soon as SD occurred, which could be accounted for by the reduced ratio, [K(+)](i)/[K(+)](o). TTX (1 microM) markedly postponed SD, but the SD-related changes in [K(+)](o) and [Na(+)](o) were only reduced by 23 and 12%, respectively. In TTX-treated pyramidal neurons, the delayed SD-like depolarization took off from a more positive level, but the final depolarized intracellular potential and input resistance were not different from control. We conclude that TTX-sensitive channels mediate only a fraction of the Na(+) influx, and that some of the K(+) is released in exchange for Na(+). Even though TTX-sensitive Na(+) currents are not essential for the self-regenerative membrane changes during hypoxic SD, in control solutions their activation may trigger the transition from gradual to rapid depolarization of neurons, thereby synchronizing the SD-like event.

The relationship between the apparent diffusion coefficient measured by magnetic resonance imaging, anoxic depolarization, and glutamate efflux during experimental cerebral ischemia

A reduction in the apparent diffusion coefficient (ADC) of water measured by magnetic resonance imaging (MRI) has been shown to occur early after cerebrovascular occlusion. This change may be a useful indicator of brain tissue adversely affected by inadequate blood supply. The objective of this study was to test the hypothesis that loss of membrane ion homeostasis and depolarization can occur simultaneously with the drop in ADC. Also investigated was whether elevation of extracellular glutamate ([GLU]e) would occur before ADC changes. High-speed MRI of the trace of the diffusion tensor (15-second time resolution) was combined with simultaneous recording of the extracellular direct current (DC) potential and on-line [GLU]e from the striatum of the anesthetized rat. After a control period, data were acquired during remote middle cerebral artery occlusion for 60 minutes, followed by 30 minutes of reperfusion, and cardiac arrest-induced global ischemia. After either focal or global ischemia, the ADC was reduced by 10 to 25% before anoxic depolarization occurred. After either insult, the time for half the maximum change in ADC was significantly shorter than the corresponding DC potential parameter (P < 0.05). The [GLU]e remained at low levels during the entire period of varying ADC and DC potential and did not peak until much later after either ischemic insult. This study demonstrates that ADC changes can occur before membrane depolarization and that high [GLU]e has no involvement in the early rapid ADC decrease.

Biology of ischemic cerebral cell death

With the approval of alteplase (tPA) therapy for stroke, it is likely that combination therapy with tPA to restore blood flow, and agents like glutamate receptor antagonists to halt or reverse the cascade of neuronal damage, will dominate the future of stroke care. The authors describe events and potential targets of therapeutic intervention that contribute to the excitotoxic cascade underlying cerebral ischemic cell death. The focal and global animal models of stroke are the basis for the identification of these events and therapeutic targets. The signalling pathways contributing to ischemic neuronal death are discussed based on their cellular localization. Cell surface signalling events include the activities of both voltage-gated K+, Na+, and Ca2+ channels and ligand-gated glutamate, gamma-aminobutyric acid and adenosine receptors and channels. Intracellular signalling events include alterations in cytosolic and subcellular Ca2+ dynamics, Ca2+ -dependent kinases and immediate early genes whereas intercellular mechanisms include free radical formation and the activation of the immune system. An understanding of the relative importance and temporal sequence of these processes may result in an effective stroke therapy targeting several points in the cascade. The overall goal is to reduce disability and enhance quality of life for stroke survivors.

Ischemic cell death in brain neurons

This review is directed at understanding how neuronal death occurs in two distinct insults, global ischemia and focal ischemia. These are the two principal rodent models for human disease. Cell death occurs by a necrotic pathway characterized by either ischemic/homogenizing cell change or edematous cell change. Death also occurs via an apoptotic-like pathway that is characterized, minimally, by DNA laddering and a dependence on caspase activity and, optimally, by those properties, additional characteristic protein and phospholipid changes, and morphological attributes of apoptosis. Death may also occur by autophagocytosis. The cell death process has four major stages. The first, the induction stage, includes several changes initiated by ischemia and reperfusion that are very likely to play major roles in cell death. These include inhibition (and subsequent reactivation) of electron transport, decreased ATP, decreased pH, increased cell Ca(2+), release of glutamate, increased arachidonic acid, and also gene activation leading to cytokine synthesis, synthesis of enzymes involved in free radical production, and accumulation of leukocytes. These changes lead to the activation of five damaging events, termed perpetrators. These are the damaging actions of free radicals and their product peroxynitrite, the actions of the Ca(2+)-dependent protease calpain, the activity of phospholipases, the activity of poly-ADPribose polymerase (PARP), and the activation of the apoptotic pathway. The second stage of cell death involves the long-term changes in macromolecules or key metabolites that are caused by the perpetrators. The third stage of cell death involves long-term damaging effects of these macromolecular and metabolite changes, and of some of the induction processes, on critical cell functions and structures that lead to the defined end stages of cell damage. These targeted functions and structures include the plasmalemma, the mitochondria, the cytoskeleton, protein synthesis, and kinase activities. The fourth stage is the progression to the morphological and biochemical end stages of cell death. Of these four stages, the last two are the least well understood. Quite little is known of how the perpetrators affect the structures and functions and whether and how each of these changes contribute to cell death. According to this description, the key step in ischemic cell death is adequate activation of the perpetrators, and thus a major unifying thread of the review is a consideration of how the changes occurring during and after ischemia, including gene activation and synthesis of new proteins, conspire to produce damaging levels of free radicals and peroxynitrite, to activate calpain and other Ca(2+)-driven processes that are damaging, and to initiate the apoptotic process. Although it is not fully established for all cases, the major driving force for the necrotic cell death process, and very possibly the other processes, appears to be the generation of free radicals and peroxynitrite. Effects of a large number of damaging changes can be explained on the basis of their ability to generate free radicals in early or late stages of damage. Several important issues are defined for future study. These include determining the triggers for apoptosis and autophagocytosis and establishing greater confidence in most of the cellular changes that are hypothesized to be involved in cell death. A very important outstanding issue is identifying the critical functional and structural changes caused by the perpetrators of cell death. These changes are responsible for cell death, and their identity and mechanisms of action are almost completely unknown.

Rapid monitoring of diffusion, DC potential, and blood oxygenation changes during global ischemia. Effects of hypoglycemia, hyperglycemia, and TTX

BACKGROUND AND PURPOSE

The increasing interest in diffusion-weighted MRI (MRI) for diagnosis and monitoring of acute stroke in humans calls for a sound understanding of the underlying mechanisms of this image contrast in acute cerebral ischemia. The present study aimed to show that a rapid decrease in brain-water apparent diffusion coefficient (ADC) occurs coincident with anoxic depolarization and that this change is delayed by hyperglycemia and sodium channel blockade but accelerated by hypoglycemia.

METHODS

Rats were divided into groups: normoglycemic, hypoglycemic, and hyperglycemic, and those given local tetrodotoxin (TTX) application. Cardiac arrest was effected by intravenous KCl injection during serial high-speed diffusion and blood oxygenation-sensitive gradient-recalled echo MRI. Brain DC potential was recorded simultaneously. Serial ADC maps were calculated from the diffusion-weighted data and fitted to a model function to measure the delay between cardiac arrest and rapid ADC decrease.

RESULTS

The time of anoxic depolarization indicated by DC change agreed well with the rapid drop in ADC in all groups; both were accelerated with hypoglycemia and delayed by hyperglycemia. A more gradual ADC decline occurred before anoxic depolarization, which was more pronounced in hyperglycemic animals and less pronounced in hypoglycemic animals. Rapid drop in ADC was also delayed by local TTX application. Changes in gradient-recalled echo image intensity were not significantly different among groups.

CONCLUSIONS

While much of the ADC decrease in ischemia occurs during anoxic depolarization, significant but gradual ADC changes occur earlier that may not be due to a massive loss in ion homeostasis.

Anoxia-evoked intracellular pH and Ca2+ concentration changes in cultured postnatal rat hippocampal neurons

The ratiometric indicators 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein and Fura-2 were employed to examine, respectively, intracellular pH (pHi) and calcium ([Ca2+]i) changes evoked by anoxia in cultured postnatal rat hippocampal neurons at 37 degrees C. Under both HCO3-/CO2- and HEPES-buffered conditions, 3-, 5- or 10-min anoxia induced a triphasic change in pHi consisting of an initial fall in pHi, a subsequent rise in pHi in the continued absence of O2 and, finally, a further rise in pHi upon the return to normoxia, which recovered towards preanoxic steady-state pHi values if the duration of the anoxic insult was < or = 5 min. In parallel experiments performed on sister cultures, anoxia of 3, 5 or 10 min duration evoked rises in [Ca2+]i which, in all cases, commenced after the start of the fall in pHi, reached a peak at or just following the return to normoxia and then declined towards preanoxic resting levels. Removal of external Ca2+ markedly attenuated increases in [Ca2+]i, but failed to affect the pHi changes evoked by 5 min anoxia. The latency from the start of anoxia to the start of the increase in pHi observed during anoxia was increased by perfusion with media containing either 2 mM Na+, 20 mM glucose or 1 microM tetrodotoxin. Because each of these manoeuvres is known to delay the onset and/or attenuate the magnitude of anoxic depolarization, the results suggest that the rise in pHi observed during anoxia may be consequent upon membrane depolarization. This possibility was also suggested by the findings that Zn2+ and Cd2+, known blockers of voltage-dependent proton conductances, reduced the magnitude of the rise in pHi observed during anoxia. Under HCO3-/CO2-free conditions, reduction of external Na+ by substitution with N-methyl-D-glucamine (but not Li+) attenuated the magnitude of the postanoxic alkalinization, suggesting that increased Na+/H+ exchange activity contributes to the postanoxic rise in pHi. In support, rates of pHi recovery from internal acid loads imposed following anoxia were increased compared to control values established prior to anoxia in the same neurons. In contrast, rates of pHi recovery from acid loads imposed during anoxia were reduced, suggesting the possibility that Na+/H+ exchange is inhibited during anoxia. We conclude that the steady-state pHi response of cultured rat hippocampal neurons to transient anoxia is independent of changes in [Ca2+]i and is characterized by three phases which are determined, at least in part, by alterations in Na+/H- exchange activity and, possibly, by a proton conductance which is activated during membrane depolarization.

Block of rapid depolarization induced by in vitro energy depletion of rat dorsal vagal motoneurones

1. The ionic mechanisms contributing to the rapid depolarization (RD) induced by in vitro ischaemia have been studied in dorsal vagal motoneurones (DVMs) of brainstem slices. Compared with CA1 hippocampal neurones, RD of DVMs was slower, generally occurred from a more depolarized membrane potential and was accompanied by smaller increases in [K+]o. 2. RD was not induced by elevation of [K+]o to values measured around DVMs during in vitro ischaemia or by a combination of raised [K+]o and 2-5 microM ouabain. 3. Neither TTX (5-10 microM) nor TTX combined with bepridil (10-30 microM), a Na+-Ca2+ exchange inhibitor, slowed RD. Block of voltage-dependent Ca2+ channels with Cd2+ (0.2 mM) and Ni2+ (0.3 mM) led to an earlier onset of RD, possibly because [K+]o was higher than that measured during in vitro ischaemia in the absence of divalent ions. 4. When [Na+]o was reduced to 11.25-25 mM, RD did not occur, although a slow depolarization was observed. RD was slowed (i) by 10 mM Mg2+ and 0.5 mM Ca2+, (ii) by a combination of TTX (1.5-5 microM), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) and D-2-amino-5-phosphonovalerate (AP5, 50 microM) and (iii) by TTX (1.5-5 microM) and AP5 (50 microM). 5. Ni2+ at concentrations of 0.6 or 1.33 mM blocked RD whereas 0.6 mM Cd2+ did not. A combination of Cd2+ (0.2 mM), Ni2+ (0.3 mM), AP5 (50 microM) and bepridil (10 microM) was largely able to mimic the effects of high concentrations of Ni2+. 6. It is concluded that RD is due to Na+ entry, predominantly through N-methyl-D-aspartate receptor ionophores, and to Ca2+ entry through voltage-dependent Ca2+ channels. These results are consistent with known changes in the concentrations of extracellular ions when ischaemia-induced rapid depolarization occurs.

NMDA receptor-mediated K+ efflux and neuronal apoptosis

Neuronal death induced by activating N-methyl-D-aspartate (NMDA) receptors has been linked to Ca2+ and Na+ influx through associated channels. Whole-cell recording from cultured mouse cortical neurons revealed a NMDA-evoked outward current, INMDA-K, carried by K+ efflux at membrane potentials positive to -86 millivolts. Cortical neurons exposed to NMDA in medium containing reduced Na+ and Ca2+ (as found in ischemic brain tissue) lost substantial intracellular K+ and underwent apoptosis. Both K+ loss and apoptosis were attenuated by increasing extracellular K+, even when voltage-gated Ca2+ channels were blocked. Thus NMDA receptor-mediated K+ efflux may contribute to neuronal apoptosis after brain ischemia.