Anoxia-induced neuronal injury: role of Na+ entry and Na+-dependent transport

Prophylactic Riluzole Attenuates Oxidative Stress Damage in Spinal Cord Distraction

Spinal cord injury (SCI) without radiographical abnormalities (SCIWORA) presents a significant challenge because of the loss of function despite an apparent normal anatomy. The cause of dysfunction is not understood, and specific treatment options are lacking. Some scoliosis corrective surgeries result in SCIWORA, where stretching of the spinal cord can lead to vascular compromise and hypoxia. The iatrogenic nature of this injury allows for the implantation of neuroprotective strategies that are designed to prevent damage. We utilized a model of atraumatic SCI to evaluate the efficacy of the sodium-channel blocker, riluzole, as a prophylactic neuroprotectant. As expected, the stretch injury caused a significant reduction in intraparenchymal oxygen in distraction (-53.09 ± 22.23%) and riluzole pre-treated distraction animals (-43.04 ± 22.86%). However, in contrast to the oxidative stress and metabolic impairments observed in vehicle-treated distraction animals, in which protein carbonylation increased significantly (5.88 ± 1.3 nmol/mL), riluzole kept these levels within the normal range (1.8 ± 1.0 nmol/mL). This neurprotection also prevented ventral motor neuron hypoplasia and pyknosis, characteristic features of this atraumatic SCI model, and maintained normal gait function (e.g., stride length and stance time). This study provides evidence for the use of prophylactic neuroprotective strategies in which thoracic or spine surgeries present the risk of causing atraumatic SCI.

Elevated intracellular Na+ concentrations in developing spinal neurons

Over 25 years ago it was first reported that intracellular chloride levels (Cl^-_in ) were higher in developing neurons than in maturity. This finding has had significant implications for understanding the excitability of developing networks and recognizing the underlying causes of hyperexcitability associated with disease and neural injury. While there is some evidence that intracellular sodium levels (Na⁺_in ) change during the development of non-neural cells, it has largely been assumed that Na⁺_in is the same in developing and mature neurons. Here, using the sodium indicator SBFI, we test this idea and find that Na⁺_in is significantly higher in embryonic spinal motoneurons and interneurons than in maturity. We find that Na⁺_in reaches ~ 60 mM in mid-embryonic development and is then reduced to ~ 30 mM in late embryonic development. By retrogradely labeling motoneurons with SBFI we can reliably follow Na⁺_in levels in vitro for hours. Bursts of spiking activity, and blocking voltage-gated sodium channels did not influence observed motoneuron sodium levels. On the other hand, Na⁺_in was reduced by blocking the Na⁺ -K⁺ -2Cl^- cotransporter NKCC1, and was highly sensitive to changes in external Na⁺ and a blocker of the Na⁺ /K⁺ ATPase. Our findings suggest that the Na⁺ gradient is weaker in embryonic neuronal development and strengthens in maturity in a manner similar to that of Cl^- .

Identification of a Novel Gnao-Mediated Alternate Olfactory Signaling Pathway in Murine OSNs

It is generally agreed that in olfactory sensory neurons (OSNs), the binding of odorant molecules to their specific olfactory receptor (OR) triggers a cAMP-dependent signaling cascade, activating cyclic-nucleotide gated (CNG) channels. However, considerable controversy dating back more than 20 years has surrounded the question of whether alternate signaling plays a role in mammalian olfactory transduction. In this study, we demonstrate a specific alternate signaling pathway in Olfr73-expressing OSNs. Methylisoeugenol (MIEG) and at least one other known weak Olfr73 agonist (Raspberry Ketone) trigger a signaling cascade independent from the canonical pathway, leading to the depolarization of the cell. Interestingly, this pathway is mediated by Gnao activation, leading to Cl(-) efflux; however, the activation of adenylyl cyclase III (ACIII), the recruitment of Ca(2+) from extra-or intracellular stores, and phosphatidylinositol 3-kinase-dependent signaling (PI signaling) are not involved. Furthermore, we demonstrated that our newly identified pathway coexists with the canonical olfactory cAMP pathway in the same OSN and can be triggered by the same OR in a ligand-selective manner. We suggest that this pathway might reflect a mechanism for odor recognition predominantly used in early developmental stages before olfactory cAMP signaling is fully developed. Taken together, our findings support the existence of at least one odor-induced alternate signal transduction pathway in native OSNs mediated by Olfr73 in a ligand-selective manner.

Primary cerebral and cerebellar astrocytes display differential sensitivity to extracellular sodium with significant effects on apoptosis

Central pontine myelinolysis is one of the idiopathic or iatrogenic brain dysfunction, and the most common cause is excessively rapid correction of chronic hyponatraemia. While myelin disruption is the main pathology, as the diagnostic name indicates, a previous study has reported that astrocyte death precedes the destruction of the myelin sheath after the rapid correction of chronic low Na(+) levels, and interestingly, certain brain regions (cerebral cortex, hippocampus, etc.) are specifically damaged but not cerebellum. Here, using primary astrocyte cultures derived from rat cerebral cortex and cerebellum, we examined how extracellular Na(+) alterations affect astrocyte death and whether the response is different between the two populations of astrocytes. Twice the amount of extracellular [Na(+) ] and voltage-gated Na(+) channel opening induced substantial apoptosis in both populations of astrocytes, while, in contrast, one half [Na(+) ] prevented apoptosis in cerebellar astrocytes, in which the Na(+) -Ca(2+) exchanger, NCX2, was highly expressed but not in cerebral astrocytes. Strikingly, the rapid correction of chronic one half [Na(+) ] exposure significantly increased apoptosis in cerebellar astrocytes but not in cerebral astrocytes. These results indicate that extracellular [Na(+) ] affects astrocyte apoptosis, and the response to alterations in [Na(+) ] is dependent on the brain region from which the astrocyte is derived.

Anoxia-induced NMDA receptor activation opens pannexin channels via Src family kinases

Anoxic depolarization of pyramidal neurons results from a large inward current that is activated, in part, by excessive glutamate release during exposure to anoxia/ischemia. Pannexin-1 (Panx1) channels can be activated both by ischemia and NMDA receptors (NMDARs), but the mechanisms of Panx1 activation are unknown. We used whole-cell recordings to show that pharmacological inhibition or conditional genetic deletion of Panx1 strongly attenuates the anoxic depolarization of CA1 pyramidal neurons in acute brain slices from rats and mice. Anoxia or exogenous NMDA activated Src family kinases (SFKs), as measured by increased phosphorylation of SFKs at Y416. The SFK inhibitor PP2 prevented Src activation and Panx1 opening during anoxia. A newly developed interfering peptide that targets the SFK consensus-like sequence of Panx1 (Y308) attenuated the anoxic depolarization (AD) without affecting SFK activation. Importantly, the NMDAR antagonists, D-APV and R-CPP, attenuated AD currents carried by Panx1, and the combined application of D-APV and (10)panx (a Panx1 blocker) inhibited AD currents to the same extent as either blocker alone. We conclude that activation of NMDARs during anoxia/ischemia recruits SFKs to open Panx1, leading to sustained neuronal depolarizations.

Understanding secondary injury

Secondary injury is a term applied to the destructive and self-propagating biological changes in cells and tissues that lead to their dysfunction or death over hours to weeks after the initial insult (the "primary injury"). In most contexts, the initial injury is usually mechanical. The more destructive phase of secondary injury is, however, more responsible for cell death and functional deficits. This subject is described and reviewed differently in the literature. To biomedical researchers, systemic and tissue-level changes such as hemorrhage, edema, and ischemia usually define this subject. To cell and molecular biologists, "secondary injury" refers to a series of predominately molecular events and an increasingly restricted set of aberrant biochemical pathways and products. These biochemical and ionic changes are seen to lead to death of the initially compromised cells and "healthy" cells nearby through necrosis or apoptosis. This latter process is called "bystander damage." These viewpoints have largely dominated the recent literature, especially in studies of the central nervous system (CNS), often without attempts to place the molecular events in the context of progressive systemic and tissue-level changes. Here we provide a more comprehensive and inclusive discussion of this topic.

Proinflammatory and proapoptotic markers in relation to mono and di-cations in plasma of autistic patients from Saudi Arabia

Objectives

Autism is a developmental disorder characterized by social and emotional deficits, language impairments and stereotyped behaviors that manifest in early postnatal life. This study aims to clarify the relationship amongst absolute and relative concentrations of K⁺, Na⁺, Ca²⁺, Mg²⁺ and/or proinflammatory and proapoptotic biomarkers.

Materials and methods

Na⁺, K⁺, Ca²⁺, Mg²⁺, Na⁺/K⁺, Ca²⁺/Mg²⁺ together with IL6, TNFα as proinflammatory cytokines and caspase3 as proapoptotic biomarker were determined in plasma of 25 Saudi autistic male patients and compared to 16 age and gender matching control samples.

Results

The obtained data recorded that Saudi autistic patients have a remarkable lower plasma caspase3, IL6, TNFα, Ca²⁺ and a significantly higher K⁺ compared to age and gender matching controls. On the other hand both Mg²⁺ and Na⁺ were non-significantly altered in autistic patients. Pearson correlations revealed that plasma concentrations of the measured cytokines and caspase-3 were positively correlated with Ca²⁺ and Ca²⁺/K⁺ ratio. Reciever Operating Characteristics (ROC) analysis proved that the measured parameters recorded satisfactory levels of specificity and sensitivity.

Conclusion

Alteration of the selected measured ions confirms that oxidative stress and defective mitochondrial energy production could be contributed in the pathogenesis of autism. Moreover, it highlights the relationship between the measured ions, IL6, TNFα and caspase3 as a set of signalling pathways that might have a role in generating this increasingly prevalent disorder. The role of ions in the possible proinflammation and proapoptic mechanisms of autistics' brains were hypothesized and explained.

Sequential degradation of alphaII and betaII spectrin by calpain in glutamate or maitotoxin-stimulated cells

Calpain-catalyzed proteolysis of II-spectrin is a regulated event associated with neuronal long-term potentiation, platelet and leukocyte activation, and other processes. Calpain proteolysis is also linked to apoptotic and nonapoptotic cell death following excessive glutamate exposure, hypoxia, HIV-gp120/160 exposure, or toxic injury. The molecular basis for these divergent consequences of calpain action, and their relationship to spectrin proteolysis, is unclear. Calpain preferentially cleaves II spectrin in vitro in repeat 11 between residues Y1176 and G1177. Unless stimulated by Ca++ and calmodulin (CaM), betaII spectrin proteolysis in vitro is much slower. We identify additional unrecognized sites in spectrin targeted by calpain in vitro and in vivo. Bound CaM induces a second II spectrin cleavage at G1230*S1231. BetaII spectrin is cleaved at four sites. One cleavage only occurs in the absence of CaM at high enzyme-to-substrate ratios near the betaII spectrin COOH-terminus. CaM promotes II spectrin cleavages at Q1440*S1441, S1447*Q1448, and L1482*A1483. These sites are also cleaved in the absence of CaM in recombinant II spectrin fusion peptides, indicating that they are probably shielded in the spectrin heterotetramer and become exposed only after CaM binds alphaII spectrin. Using epitope-specific antibodies prepared to the calpain cleavage sites in both alphaII and betaII spectrin, we find in cultured rat cortical neurons that brief glutamate exposure (a physiologic ligand) rapidly stimulates alphaII spectrin cleavage only at Y1176*G1177, while II spectrin remains intact. In cultured SH-SY5Y cells that lack an NMDA receptor, glutamate is without effect. Conversely, when stimulated by calcium influx (via maitotoxin), there is rapid and sequential cleavage of alphaII and then betaII spectrin, coinciding with the onset of nonapoptotic cell death. These results identify (i) novel calpain target sites in both alphaII and betaII spectrin; (ii) trans-regulation of proteolytic susceptibility between the spectrin subunits in vivo; and (iii) the preferential cleavage of alphaII spectrin vs betaII spectrin when responsive cells are stimulated by engagement of the NMDA receptor. We postulate that calpain proteolysis of spectrin can activate two physiologically distinct responses: one that enhances skeletal plasticity without destroying the spectrin-actin skeleton, characterized by preservation of betaII spectrin; or an alternative response closely correlated with nonapoptotic cell death and characterized by proteolysis of betaII spectrin and complete dissolution of the spectrin skeleton.

Run down of GABAergic depolarization during metabolic inhibition of rat hippocampal CA1 neurons

We investigated the effects of metabolic inhibition on both the shift in the equilibrium potential for Cl(-) (E(Cl)) and the run down of GABA(A) receptor responses, using nystatin- and gramicidin-perforated patch-clamp recordings from rat hippocampal CA1 neurons. Metabolic inhibition with NaCN decreased outward GABAergic currents while increasing inward GABAergic currents. E(Cl) showed a positive shift almost immediately after metabolic poisoning. This shift always occurred prior to GABA receptor run down, which was observed as decreases in whole cell conductance during application of a GABA(A) receptor agonist. The results indicate that GABAergic responses tend to become depolarizing during metabolic inhibition and the run down of the GABAergic response may therefore be neuroprotective against excitotoxicity. Furthermore the results illustrate the importance of considering both changes in receptor function and current driving force, and their temporal relationship, in order to understand the physiological response of the GABAergic system during metabolic stress.

Sodium influx pathways during and after anoxia in rat hippocampal neurons

Mechanisms that contribute to Na+ influx during and immediately after 5 min anoxia were investigated in cultured rat hippocampal neurons loaded with the Na+-sensitive fluorophore sodium-binding benzofuran isophthalate. During anoxia, an influx of Na+ in the face of reduced Na+,K+-ATPase activity caused a rise in [Na+]i. After the return to normoxia, Na+,K+-ATPase activity mediated the recovery of [Na+]i despite continued Na+ entry. Sodium influx during and after anoxia occurred through multiple pathways and increased the longer neurons were maintained in culture. Under the experimental conditions used, Na+ entry during anoxia did not reflect the activation of ionotropic glutamate receptors, TTX- or lidocaine-sensitive Na+ channels, plasmalemmal Na+/Ca2+ exchange, Na+/H+ exchange, or HCO3--dependent mechanisms; rather, contributions were received from a Gd3+-sensitive pathway activated by reactive oxygen species and Na+/K+/2Cl- cotransport in neurons maintained for 6-10 and 11-14 d in vitro (DIV), respectively. Sodium entry immediately after anoxia was not attributable to the activation of ionotropic glutamate receptors, voltage-activated Na+ channels, or Na+/K+/2Cl- cotransport; rather, it occurred via Na+/Ca2+ exchange, Na+/H+ exchange, and a Gd3+-sensitive pathway similar to that observed during anoxia; 11-14 DIV neurons received an additional contribution from an -dependent mechanism(s). The results provide insight into the intrinsic mechanisms that contribute to disturbed internal Na+ homeostasis during and immediately after anoxia in rat hippocampal neurons and, in this way, may play a role in the pathogenesis of anoxic or ischemic cell injury.

Calcium and pH homeostasis in neurons during hypoxia and ischemia

One of the important events during hypoxia or ischemia in the brain (or other organs for that matter, including the myocardium) is the accumulation of Ca2+ ions intracellularly. Although various studies have shown various sources of and routes for Ca2+ entry and accumulation, it is clear now that it is likely that there is a multitude rather than a single mechanism for this accumulation. In this review, we highlight this Ca2+ accumulation during low O2 states and discuss some of the mechanisms leading to accumulation for two main reasons: (a) an accumulation of Ca2+ in the cytosol has been proven to be deleterious for cell function although this accumulation of Ca2+ and consequences represent only a limited view of events that can lead to cell injury during such stress and (b) developing therapeutic strategies involving the reduction or elimination of this accumulation depends, by and large, on the mechanism of entry. In addition to reviewing some of these Ca2+ events, we will also review the relation between pH (H+) and Ca2+ since these two ions and their regulation are tied to each other in a major way. For example, extracellular acidosis, which can occur during ischemia, has a remarkable effect on the function of some of the Ca2+ entry routes.

Two variants of the rat brain sodium-driven chloride bicarbonate exchanger (NCBE): developmental expression and addition of a PDZ motif

Regulation of pH in the central nervous system is critical to normal brain function and response to pathophysiological conditions. Here we identify two novel variants of the sodium-driven chloride bicarbonate exchanger (NCBE) from brain. The developmental expression pattern seen by in situ hybridization for the 90-bp containing insert (insert A) reveals strong expression in spinal cord and brain beginning in embryonic development. High-level expression is seen in cerebellar Purkinje cells and principal cells in hippocampus. The variant missing a 39-bp insert at the 3' end (insert B) encodes a protein in which the deduced carboxyterminal three amino acids are replaced with a unique 21 amino acid stretch terminating in a PDZ motif. rb2NCBE, the PDZ motif-encoding variant, is more highly expressed in astrocytes than is rb1NCBE. Both variants are expressed at similar levels in neurons. Expression varies with age and cell type. The FLAG epitope was fused in-frame at the amino terminus and each variant was expressed using a retroviral vector to study subcellular localization. Both variants were associated with the plasma membrane, but rb2NCBE colocalized with actin filaments to a greater extent, suggesting the PDZ form may interact with the cytoskeleton, whereas rb1NCBE was more often seen in intracellular vesicles. The PDZ motif-containing variant was much more active in pH regulation, with the expected ionic dependence on Na+, HCO3- and Cl- when expressed in 3T3 cells. These results are a first step towards understanding the regulation of expression and activity of this transporter in the brain.

Activation of voltage-sensitive sodium channels during oxygen deprivation leads to apoptotic neuronal death

Sodium (Na(+)) entry into neurons during hypoxia is known to be associated with cell death. However, it is not clear whether Na(+) entry causes cell death and by what mechanisms this increased Na(+) entry induces death. In this study we used cultures of rat neocortical neurons to show that an increase in intracellular sodium (Na(i)(+)) through voltage-sensitive sodium channels (VSSCs), during hypoxia contributes to apoptosis. Hypoxia increased Na(i)(+) and induced neuronal apoptosis, as assessed by electron microscopy, annexin V staining, and terminal UDP nick end labeling staining. Reducing Na(+) entry with the VSSC blocker, tetrodotoxin (TTX), attenuated apoptotic neuronal death via a reduction in caspase-3 activation. Since the attenuation of apoptosis by TTX during hypoxia suggested that the activation of VSSCs and Na(+) entry are crucial events in hypoxia-induced cell death, we also determined whether the activation of VSSCs per se could lead to apoptosis under resting conditions. Increasing Na(+) entry with the VSSC activator veratridine also induced neuronal apoptosis and caspase-3 activation. These data indicate that a) Na(+) entry via VSSCs during hypoxia leads to apoptotic cell death which is mediated, in part, by caspase-3 and b) activation of VSSCs during oxygen deprivation is a major event by which hypoxia induces cell death.

Early ionic and membrane potential changes caused by the pesticide rotenone in striatal cholinergic interneurons

Mitochondrial metabolism impairment has been implicated in the pathogenesis of several neurodegenerative disorders. In the present work, we combined electrophysiological recordings and microfluorometric measurements from cholinergic interneurons obtained from a rat neostriatal slice preparation. Acute application of the mitochondrial complex I inhibitor rotenone produced an early membrane hyperpolarization coupled to a fall in input resistance, followed by a late depolarizing response. Current-voltage relationship showed a reversal potential of -80 +/- 3 mV, suggesting the involvement of a potassium (K+) current. Simultaneous measurement of intracellular sodium [Na+]i or calcium [Ca2+]i concentrations revealed a striking correlation between [Na+]i elevation and the early membrane hyperpolarization, whereas a significant [Ca2+]i rise matched the depolarizing phase. Interestingly, ion and membrane potential changes were mimicked by ouabain, inhibitor of the Na+-K+ATPase, and were insensitive to tetrodotoxin (TTX) or to a combination of glutamate receptor antagonists. The rotenone effects were partially reduced by blockers of ATP-sensitive K+ channels, glibenclamide and tolbutamide, and largely attenuated by a low Na+-containing solution. Morphological analysis of the rotenone effects on striatal slices showed a significant decrease in the number of choline acetyltransferase (ChAT) immunoreactive cells. These results suggest that rotenone rapidly disrupts the ATP content, leading to a decreased Na+-K+ATPase function and, therefore, to [Na+]i overload. In turn, the hyperpolarizing response might be generated both by the opening of ATP-sensitive K+ channels and by Na+-activated K+ conductances. The increase in [Ca2+]i occurs lately and does not seem to influence the early events.

Role of Na(+)/H(+) exchanger during O(2) deprivation in mouse CA1 neurons

To determine the role of membrane transporters in intracellular pH (pH(i)) regulation under conditions of low microenvironmental O(2), we monitored pH(i) in isolated single CA1 neurons using the fluorescent indicator carboxyseminaphthorhodafluor-1 and confocal microscopy. After total O(2) deprivation or anoxia (PO(2) approximately equal to 0 Torr), a large increase in pH(i) was seen in CA1 neurons in HEPES buffer, but a drop in pH(i), albeit small, was observed in the presence of HCO(3)(-). Ionic substitution and pharmacological experiments showed that the large anoxia-induced pH(i) increase in HEPES buffer was totally Na(+) dependent and was blocked by HOE-694, strongly suggesting the activation of the Na(+)/H(+) exchanger (NHE). Also, this pH(i) increase in HEPES buffer was significantly smaller in Na(+)/H(+) exchanger isoform 1 (NHE1) null mutant CA1 neurons than in wild-type neurons, demonstrating that NHE1 is responsible for part of the pH(i) increase following anoxia. Both chelerythrine and H-89 partly blocked, and H-7 totally eliminated, this anoxia-induced pH(i) increase in the absence of HCO. We conclude that 1) O(2) deprivation activates Na(+)/H(+) exchange by enhancing protein kinase activity and 2) membrane proteins, such as NHE, actively participate in regulating pH(i) during low-O(2) states in neurons.

Mechanisms of cell survival in hypoxia and hypothermia

Most animals experience some degree of hypoxia and hypothermia during the course of their natural life history either as a consequence of ambient ‘exposure’ per se or through metabolic, respiratory and/or circulatory insufficiency. A prevailing experimental approach has been to probe tissues from natural models of hypoxia-tolerant and cold-tolerant vertebrates to look for common mechanisms of defence against O2 lack and hypothermia. The ability to sustain vital cellular functions in severe cases of either condition varies widely amongst the vertebrates. Like humans, the vast majority of mammals are unable to survive prolonged periods of hypothermia or O2 deprivation owing to irreversible membrane damage and loss of cellular ion homeostasis in vital organs such as the brain and heart. However, numerous hibernating endotherms, neonatal and diving mammals as well as many ectotherms can tolerate prolonged periods that would, in clinical terms, be called asphyxia or deep hypothermia. The key to their survival under such conditions lies in an inherent ability to downregulate their cellular metabolic rate to new hypometabolic steady states in a way that balances the ATP demand and ATP supply pathways.

Calcium dependence of rapid astrocyte death induced by transient hypoxia, acidosis, and extracellular ion shifts

Exposure to hypoxic, acidic, ion-shifted Ringer (HAIR) for 15-40 min has been shown to cause rapid astrocyte death upon reperfusion with normal media. The ion shifts of the HAIR solution included a rise in extracellular K(+) (e.g., [K(+)](o)) and a fall in [Na(+)](o), [Cl(-)](o), and [Ca(2+)](o), characteristic of ischemic-traumatic brain insults. We investigated the ionic basis of the HAIR-induced injury. After HAIR exposure, reperfusion in 0 Ca(2+)/EGTA media completely protected astrocytes. Preincubation of cells in BAPTA-AM ester was also protective, indicating that the injury was triggered by Ca(2+) influx during reperfusion. Neither nimodipine, CNQX, APV, nor TTX reduced injury. Astrocyte death could be blocked by 100 microM Ni(2+) or 100 microM benzamil, suggesting involvement of Na(+)-Ca(2+) exchange. KB-R7943, which preferentially inhibits reverse Na(+)-Ca(2+) exchange, also protected astrocytes. Elevation of [K(+)](o) was not necessary for astrocyte death. However, when [Na(+)](o) was maintained at 151 mM throughout the HAIR protocol, cell death was markedly reduced. We postulate that [Na(+)](o) shifts aid reversal of Na(+)-Ca(2+) exchange by favoring cytosolic Na(+) loading. Possible means of astrocytic Na(+) accumulation are discussed.

Role played by sodium in activity-dependent secretion of neurotrophins - revisited

In previous experiments, a causal relationship between sodium influx and secretion of nerve growth factor (NGF) was deduced from the observation that the sodium substitute N-methyl-D-glucamine (NMDG) abolished any activity-mediated NGF secretion that depends on intact internal calcium stores. However, all available experimental evidence speaks against sodium-mediated calcium mobilization from these stores under physiological conditions. We now report that rapid sodium influx initiated by monensin or ouabain did not induce brain-derived neurotrophic factor (BDNF) secretion from either native hippocampal slices or BDNF-transduced hippocampal neuronal cultures. Additionally, we found marked differences between the replacement of sodium by NMDG and sucrose on the one hand, and choline and lithium on the other. Replacement of 100% (and as little as 10%) sodium by NMDG or sucrose not only blocked the activity-mediated neurotrophin secretion, but itself led to a rapid and substantial increase of neurotrophin secretion. In contrast, the replacement of sodium (10% and 100%) by lithium and choline did not result in a release of neurotrophins, and only 100% replacement blocked the activity-mediated neurotrophin secretion. We conclude that the blocking effects of NMDG and sucrose on neurotrophin secretion do not reflect the sodium replacement, but instead represent an independent blocking effect. These differences were also reflected in part by electrophysiological investigations in individually patched hippocampal neurons. The importance of the present observations lies not only in the reevaluation of the involvement of sodium in activity-dependent neurotrophin secretion, but also in the demonstration that sodium replacement may initiate 'side effects' that are unrelated to sodium replacement.

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.

Effects of chloride channel inhibitors on H(2)O(2)-induced renal epithelial cell injury

Oxidative stress contributes to renal epithelial cell injury in certain settings. Chloride influx has also been proposed as an important component of acute renal epithelial cell injury. The present studies examined the role of Cl(-) in H(2)O(2)-induced injury to LLC-PK(1) renal epithelial cells. Exposure of LLC-PK(1) cells to 1 mM H(2)O(2) resulted in the following: depletion of intracellular ATP content; DNA damage; lipid peroxidation; and a loss of membrane integrity to both small molecules, e.g., trypan blue, and macromolecules, e.g., lactate dehydrogenase (LDH), and cell death. Substitution of Cl(-) by isethionate or the inclusion of certain Cl(-) channel blockers, e.g., diphenylamine-2-carboxylate (DPC), 5-nitro-2-(3-phenylpropylamino). benzoate (NPPB), and niflumic acid, prevented the H(2)O(2)-induced loss of membrane integrity to LDH. In addition, the H(2)O(2)-induced loss of membrane integrity was prevented by raising the osmolality of the extracellular solutions, by depletion of cell ATP, and by inhibitors of volume-sensitive Cl(-) channels. However, these maneuvers did not prevent the H(2)O(2)-induced permeability to small molecules or H(2)O(2)-induced ATP depletion, DNA damage, lipid peroxidation, or cell death. These results support the view that volume-sensitive Cl(-) channels play a role in the progressive loss of cell membrane integrity during injury.

Sodium homeostasis in rat hippocampal slices during oxygen and glucose deprivation: role of voltage-sensitive sodium channels

The breakdown in brain ionic homeostasis during energy deprivation (anoxic depolarization [AD]) is intimately linked to neuronal injury. We studied the role of one particular route of Na+ influx, voltage-sensitive Na+ channels, in the AD induced by O2 and/or glucose deprivation. We recorded extracellular Na+ concentration ([Na+]e) and direct current potential (DCP) in the CA1 stratum pyramidale of hippocampal slices using Na+-selective microelectrodes. Tetrodotoxin (0.1-1 microM) delayed the occurrence of AD and reduced the peak change in both [Na+]e and DCP during AD. However the tetrodotoxin effects were overcome by a concomitant reduction in extracellular glucose during anoxia. We conclude that: (1) the activation of voltage-gated Na+ channels is involved in the triggering of AD; (2) there may be a critical level of energy depletion when AD occurs and different mechanisms may underlie AD during hypoxia, compared to O2 and glucose deprivation.

Early metabolic inhibition-induced intracellular sodium and calcium increase in rat cerebellar granule cells

1. Possible mechanisms responsible for the increases in intracellular calcium ([Ca2+]i) and sodium ([Na+]i) levels seen during metabolic inhibition were investigated by continuous [Ca2+]i and [Na+]i measurement in cultured rat cerebellar granule cells. An initial small mitochondrial Ca2+ release was seen, followed by a large influx of extracellular Ca2+. A large influx of extracellular Na+ was also seen. 2. The large [Ca2+]i increase was not due to opening of voltage-dependent or voltage-independent calcium channels, activation of NMDA/non-NMDA channels, activation of the Na+i-Ca2+o exchanger, or inability of plasmalemmal Ca2+-ATPase to extrude, or mitochondria to take up, calcium. 3. The large [Na+]i increase was not due to activation of the TTX-sensitive Na+ channel, the Na+i-Ca2+o exchanger, the Na+-H+ exchanger, or the Na+-K+-2Cl- cotransporter, or an inability of Na+-K+-ATPase to extrude the intracellular sodium. 4. Phospholipase A2 (PLA2) activation may be involved in the large influx, since both were completely inhibited by PLA2 inhibitors. Moreover, melittin (a PLA2 activator) or lysophosphatidylcholine or arachidonic acid (both PLA2 activation products) caused similar responses. Inhibition of PLA2 activity may help prevent the influx of these ions that may result in serious brain injury and oedema during hypoxia/ischaemia.

Gain control of NMDA-receptor currents by intracellular sodium

The influx of Na+ is fundamental to electrical signalling in the nervous system and is essential for such basic signals as action potentials and excitatory postsynaptic potentials. During periods of bursting or high levels of discharge activity, large increases in intracellular Na+ concentration ([Na+]i) are produced in neuronal soma and dendrites. However, the intracellular signalling function of raised postsynaptic [Na+]i is unknown. Here we show that [Na+]i regulates the function of NMDA (N-methyl-D-aspartate) receptors, a principal subtype of glutamate receptor. NMDA-receptor-mediated whole-cell currents and NMDA-receptor single-channel activity were increased by raising [Na+]i and channel activity decreased upon lowering [Na+]i; therefore, the activity of NMDA channels tracks changes in [Na+]i. We found that the sensitivity of the channel to Na+ was set by a Src kinase that is associated with the channel. Raising [Na+]i selectively increased synaptic responses mediated by NMDA receptors, but not by non-NMDA receptors. Thus, the change in postsynaptic [Na+]i that occurs during neuronal activity is a signal for controlling the gain of excitatory synaptic transmission. This mechanism may be important for NMDA-receptor-dependent plasticity and toxicity in the central nervous system.

Early sodium elevations induced by combined oxygen and glucose deprivation in pyramidal cortical neurons

We investigated the effects of oxygen (O2)/glucose deprivation on intracellular sodium concentration ([Na+]i) of cortical pyramidal cells in a slice preparation of rat frontal cortex. Intracellular recordings were combined with microfluorometric measurements of [Na+]i using the Na+-sensitive dye sodium-binding benzofuran isophthalate (SBFI). Deprivation of O2/glucose caused an initial membrane hyperpolarization that was followed by a slowly developing large depolarization. Levels of [Na+]i started to increase significantly during the phase of membrane hyperpolarization. Neither tetrodotoxin, a combination of ionotropic and metabotropic glutamate receptor antagonists (D-amino-phosphonovalerate, 6-cyano-7-nitroquinoxaline-2,3-dione plus S-methyl-4-carboxyphenylglycine) nor bepridil, an inhibitor of the Na+/Ca2+-exchanger, affected these responses to O2/ glucose. The present results demonstrate that, in cortical neurons, O2/glucose deprivation induces an early rise in [Na+]i which cannot be ascribed to the activity of voltage gated Na+-channels, glutamate receptors or of the Na+/Ca2+-exchanger.