Mild acidosis protects hippocampal neurons from injury induced by oxygen and glucose deprivation

How the 'Aerobic/Anaerobic Glycolysis' Meme Formed a 'Habit of Mind' Which Impedes Progress in the Field of Brain Energy Metabolism

The division of glycolysis into two separate pathways, aerobic and anaerobic, depending on the presence or absence of oxygen, respectively, was formulated over eight decades ago. The former ends with pyruvate, while the latter ends with lactate. Today, this division is confusing and misleading as research over the past 35 years clearly has demonstrated that glycolysis ends with lactate not only in cancerous cells but also in healthy tissues and cells. The present essay offers a review of the history of said division and the more recent knowledge that has been gained about glycolysis and its end-product, lactate. Then, it presents arguments in an attempt to explain why separating glycolysis into aerobic and anaerobic pathways persists among scientists, clinicians and teachers alike, despite convincing evidence that such division is not only wrong scientifically but also hinders progress in the field of energy metabolism.

The Brain at High Altitude: From Molecular Signaling to Cognitive Performance

The brain requires over one-fifth of the total body oxygen demand for normal functioning. At high altitude (HA), the lower atmospheric oxygen pressure inevitably challenges the brain, affecting voluntary spatial attention, cognitive processing, and attention speed after short-term, long-term, or lifespan exposure. Molecular responses to HA are controlled mainly by hypoxia-inducible factors. This review aims to summarize the cellular, metabolic, and functional alterations in the brain at HA with a focus on the role of hypoxia-inducible factors in controlling the hypoxic ventilatory response, neuronal survival, metabolism, neurogenesis, synaptogenesis, and plasticity.

Acidosis-related pain and its receptors as targets for chronic pain

Sensing acidosis is an important somatosensory function in responses to ischemia, inflammation, and metabolic alteration. Accumulating evidence has shown that acidosis is an effective factor for pain induction and that many intractable chronic pain diseases are associated with acidosis signaling. Various receptors have been known to detect extracellular acidosis and all express in the somatosensory neurons, such as acid sensing ion channels (ASIC), transient receptor potential (TRP) channels and proton-sensing G-protein coupled receptors. In addition to sense noxious acidic stimulation, these proton-sensing receptors also play a vital role in pain processing. For example, ASICs and TRPs are involved in not only nociceptive activation but also anti-nociceptive effects as well as some other non-nociceptive pathways. Herein, we review recent progress in probing the roles of proton-sensing receptors in preclinical pain research and their clinical relevance. We also propose a new concept of sngception to address the specific somatosensory function of acid sensation. This review aims to connect these acid-sensing receptors with basic pain research and clinical pain diseases, thus helping with better understanding the acid-related pain pathogenesis and their potential therapeutic roles via the mechanism of acid-mediated antinociception.

Neuroprotection against supra-lethal 'stroke in a dish' insults by an anti-excitotoxic receptor antagonist cocktail

The goal of this study was to identify cocktails of drugs able to protect cultured rodent cortical neurons against increasing durations of oxygen-glucose deprivation (OGD). As expected, a cocktail composed of an NMDA and AMPA receptor antagonists and a voltage gated Ca²⁺ channel blocker (MK-801, CNQX and nifedipine, respectively) provided complete neuroprotection against mild OGD. Increasingly longer durations of OGD necessitated increasing the doses of MK-801 and CNQX, until these cocktails ultimately failed to provide neuroprotection against supra-lethal OGD, even at maximal drug concentrations. Surprisingly, supplementation of any of these cocktails with blockers of TRPM7 channels for increasing OGD durations was not neuroprotective, unless these blockers possessed the ability to inhibit NMDA receptors. Supplementation of the maximally effective cocktail with other NMDA receptor antagonists augmented neuroprotection, suggesting insufficient NMDAR blockade by MK-801. Substitution of MK-801 in cocktails with high concentrations of a glycine site NMDA receptor antagonist caused the greatest improvements in neuroprotection, with the more potent SM-31900 superior to L689,560. Substitution of CQNX in cocktails with AMPA receptor antagonists at high concentrations also improved neuroprotection, particularly with the combination of SYM 2206 and NBQX. The most neuroprotective cocktail was thus composed of SM-31900, SYM2206, NBQX, nifedipine and the antioxidant trolox. Thus, the cumulative properties of antagonist potency and concentration in a cocktail dictate neuroprotective efficacy. The central target of supra-lethal OGD is excitotoxicity, which must be blocked to the greatest extent possible to minimize ion influx.

Neuroprotective Role of Acidosis in Ischemia: Review of the Preclinical Evidence

Efforts to develop effective neuroprotective therapies for ischemic stroke have had little success to date. One promising approach to neuroprotection is ischemic postconditioning, which utilizes brief bouts of ischemia after acute ischemic stroke to elicit neuroprotection, although the mechanism is largely unknown. As the primary components of transient ischemia are local hypoxia and acidosis, and hypoxic postconditioning has had little success, it is possible that the acidosis component may be the primary driver. To address the evidence behind this, we performed a systematic review of preclinical studies focused on the neuroprotective role of transient acidosis after ischemia. Animal studies demonstrated that mild-to-moderate acidosis after ischemic events led to better functional neurologic outcomes with reduced infarct volumes, while severe acidosis often led to cerebral edema and worse functional outcomes. In vitro studies demonstrated that mild-to-moderate acidosis improves neuronal survival largely through two means: (1) inhibition of harmful superoxide formation in the excitotoxic pathway and (2) remodeling neuronal mitochondria to allow for efficient ATP production (i.e., oxidative phosphorylation), even in the absence of oxygen. Similar to the animal studies, acidotic postconditioning in humans would entail short cycles of carbon dioxide inhalation, which has already been demonstrated to be safe as part of a hypercapnic challenge when measuring cerebrovascular reactivity. Due to the preclinical efficacy of acidotic postconditioning, its relatively straightforward translation into humans, and the growing need for neuroprotective therapies, future preclinical studies should focus on filling the current knowledge gaps that are currently restricting the development of phase I/II clinical trials.

Acidification can directly affect olfaction in marine organisms

In the past decade, many studies have investigated the effects of low pH/high CO2 as a proxy for ocean acidification on olfactory-mediated behaviours of marine organisms. The effects of ocean acidification on the behaviour of fish vary from very large to none at all, and most of the maladaptive behaviours observed have been attributed to changes in acid-base regulation, leading to changes in ion distribution over neural membranes, and consequently affecting the functioning of gamma-aminobutyric acid-mediated (GABAergic) neurotransmission. Here, we highlight a possible additional mechanism by which ocean acidification might directly affect olfaction in marine fish and invertebrates. We propose that a decrease in pH can directly affect the protonation, and thereby, 3D conformation and charge distribution of odorants and/or their receptors in the olfactory organs of aquatic animals. This can sometimes enhance signalling, but most of the time the affinity of odorants for their receptors is reduced in high CO2/low pH; therefore, the activity of olfactory receptor neurons decreases as measured using electrophysiology. The reduced signal reception would translate into reduced activation of the olfactory bulb neurons, which are responsible for processing olfactory information in the brain. Over longer exposures of days to weeks, changes in gene expression in the olfactory receptors and olfactory bulb neurons cause these neurons to become less active, exacerbating the problem. A change in olfactory system functioning leads to inappropriate behavioural responses to odorants. We discuss gaps in the literature and suggest some changes to experimental design in order to improve our understanding of the underlying mechanisms and their effects on the associated behaviours to resolve some current controversy in the field regarding the extent of the effects of ocean acidification on marine fish.

Excitotoxicity: Still Hammering the Ischemic Brain in 2020

Interest in excitotoxicity expanded following its implication in the pathogenesis of ischemic brain injury in the 1980s, but waned subsequent to the failure of N-methyl-D-aspartate (NMDA) antagonists in high profile clinical stroke trials. Nonetheless there has been steady progress in elucidating underlying mechanisms. This review will outline the historical path to current understandings of excitotoxicity in the ischemic brain, and suggest that this knowledge should be leveraged now to develop neuroprotective treatments for stroke.

GPR68 Is a Neuroprotective Proton Receptor in Brain Ischemia

Supplemental Digital Content is available in the text.

Brain acidosis is prevalent in stroke and other neurological diseases. Acidosis can have paradoxical injurious and protective effects. The purpose of this study is to determine whether a proton receptor exists in neurons to counteract acidosis-induced injury.

Extracellular pH modulation of excitatory synaptic transmission in hippocampal CA3 neurons

In this study, the effect of extracellular pH on glutamatergic synaptic transmission was examined in mechanically dissociated rat hippocampal CA3 pyramidal neurons using a whole-cell patch-clamp technique under voltage-clamp conditions. Native synaptic boutons were isolated without using any enzymes, using a so-called "synapse bouton preparation," and preserved for the electrical stimulation of single boutons. Both the frequency and amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) were found to decrease and increase in response to modest acidic (~pH 6.5) and basic (~pH 8.5) solutions, respectively. These changes in sEPSC frequency were not affected by the addition of TTX but completely disappeared by successive addition of Cd²⁺. However, changes in sEPSC amplitude induced by acidic and basic extracellular solutions were not affected by the addition of neither TTX nor Cd²⁺. The glutamate-induced whole-cell currents were decreased and increased by acidic and basic solutions, respectively. Acidic pH also decreased the amplitude and increased the failure rate (R_f) and paired-pulse rate (PPR) of glutamatergic electrically evoked excitatory postsynaptic currents (eEPSCs), while a basic pH increased the amplitude and decreased both the R_f and PPR of eEPSCs. The kinetics of the currents were not affected by changes in pH. Acidic and basic solutions decreased and increased voltage-gated Ca²⁺ but not Na⁺ channel currents in the dentate gyrus granule cell bodies. Our results indicate that extracellular pH modulates excitatory transmission via both pre- and postsynaptic sites, with the presynaptic modulation correlated to changes in voltage-gated Ca²⁺ channel currents.NEW & NOTEWORTHY The effects of external pH changes on spontaneous, miniature, and evoked excitatory synaptic transmission in CA3 hippocampal synapses were examined using the isolated nerve bouton preparation, which allowed for the accurate regulation of extracellular pH at the synapses. Acidification generally reduced transmission, partly via effects on presynaptic Ca²⁺ channel currents, while alkalization generally enhanced transmission. Both pre- and postsynaptic sites contributed to these effects.

Going the Extra (Synaptic) Mile: Excitotoxicity as the Road Toward Neurodegenerative Diseases

Excitotoxicity is a phenomenon that describes the toxic actions of excitatory neurotransmitters, primarily glutamate, where the exacerbated or prolonged activation of glutamate receptors starts a cascade of neurotoxicity that ultimately leads to the loss of neuronal function and cell death. In this process, the shift between normal physiological function and excitotoxicity is largely controlled by astrocytes since they can control the levels of glutamate on the synaptic cleft. This control is achieved through glutamate clearance from the synaptic cleft and its underlying recycling through the glutamate-glutamine cycle. The molecular mechanism that triggers excitotoxicity involves alterations in glutamate and calcium metabolism, dysfunction of glutamate transporters, and malfunction of glutamate receptors, particularly N-methyl-D-aspartic acid receptors (NMDAR). On the other hand, excitotoxicity can be regarded as a consequence of other cellular phenomena, such as mitochondrial dysfunction, physical neuronal damage, and oxidative stress. Regardless, it is known that the excessive activation of NMDAR results in the sustained influx of calcium into neurons and leads to several deleterious consequences, including mitochondrial dysfunction, reactive oxygen species (ROS) overproduction, impairment of calcium buffering, the release of pro-apoptotic factors, among others, that inevitably contribute to neuronal loss. A large body of evidence implicates NMDAR-mediated excitotoxicity as a central mechanism in the pathogenesis of many neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), and epilepsy. In this review article, we explore different causes and consequences of excitotoxicity, discuss the involvement of NMDAR-mediated excitotoxicity and its downstream effects on several neurodegenerative disorders, and identify possible strategies to study new aspects of these diseases that may lead to the discovery of new therapeutic approaches. With the understanding that excitotoxicity is a common denominator in neurodegenerative diseases and other disorders, a new perspective on therapy can be considered, where the targets are not specific symptoms, but the underlying cellular phenomena of the disease.

Mild Acidosis Protects Neurons during Oxygen-Glucose Deprivation by Reducing Loss of Mitochondrial Respiration

Brain ischemia is often accompanied by brain acidosis and this acidosis can affect ischemic neuronal injury. Ischemic neuronal injury is initiated by a decrease in ATP production which mainly relies on mitochondrial oxidative phosphorylation. Ischemia often causes mitochondrial dysfunction, and acidosis has been found to affect mitochondrial function, suggesting that acidosis accompanying ischemia may influence neurons by targeting mitochondrial metabolism. However, the effects of acidosis on mitochondrial energy metabolism during ischemia lacks thorough investigation. Here, we found that mild acidosis significantly reduced neuronal death possibly by slowing the process of ATP deprivation during oxygen-glucose deprivation (OGD), an in vitro ischemic model. The maintaining of neuronal ATP depended on protecting mitochondrial ATP production. Further investigation of mitochondrial function revealed that mild acidosis alleviated OGD-induced collapse of mitochondrial membrane potentials as well as damage to respiratory function, at least in part by reducing impacts on complex I and II activities. Inhibition of complex I activity aggravated neuronal death, which suggests that the contribution of mild acidosis to maintaining complex I activity promoted neuronal survival during OGD. Our findings reveal maintaining mitochondrial respiration as a new possible protective mechanism of mild acidosis during ischemia, on neurons.

Extracellular mild acidosis decreases the Ca2+ permeability of the human NMDA receptors

NMDA receptors (NMDARs) are glutamate-gated ion channels involved in excitatory synaptic transmission and in others physiological processes such as synaptic plasticity and development. The overload of Ca²⁺ ions through NMDARs, caused by an excessive activation of receptors, leads to excitotoxic neuronal cell death. For this reason, the reduction of Ca²⁺ flux through NMDARs has been a central focus in finding therapeutic strategies to prevent neuronal cell damage. Extracellular H⁺ are allosteric modulators of NMDARs. Starting from previous studies showing that extracellular mild acidosis reduces NMDA-evoked whole cell currents, we analyzed the effects of this condition on the NMDARs Ca²⁺ permeability, measured as "fractional calcium current" (P_f, i.e. the percentage of the total current carried by Ca²⁺ ions), of human NMDARs NR1/NR2A and NR1/NR2B transiently transfected in HeLa cells. Extracellular mild acidosis significantly reduces P_f of both human NR1/NR2A and NR1/NR2B NMDARs, also decreasing single channel conductance in outside out patches for NR1/NR2A receptor. Reduction of Ca²⁺ flux through NMDARs was also confirmed in cortical neurons in culture. A comparative analysis of both NMDA evoked Ca²⁺ transients and whole cell currents showed that extracellular H⁺ differentially modulate the permeation of Na⁺ and Ca²⁺ through NMDARs. Our data highlight the synergy of two distinct neuroprotective mechanisms during acidosis: Ca²⁺ entry through NMDARs is lowered due to the modulation of both open probability and Ca²⁺ permeability. Furthermore, this study provides the proof of concept that it is possible to reduce Ca²⁺ overload in neurons modulating the NMDAR Ca²⁺ permeability.

Ambient but not local lactate underlies neuronal tolerance to prolonged glucose deprivation

Neurons require a nearly constant supply of ATP. Glucose is the predominant source of brain ATP, but the direct effects of prolonged glucose deprivation on neuronal viability and function remain unclear. In sparse rat hippocampal microcultures, neurons were surprisingly resilient to 16 h glucose removal in the absence of secondary excitotoxicity. Neuronal survival and synaptic transmission were unaffected by prolonged removal of exogenous glucose. Inhibition of lactate transport decreased microculture neuronal survival during concurrent glucose deprivation, suggesting that endogenously released lactate is important for tolerance to glucose deprivation. Tandem depolarization and glucose deprivation also reduced neuronal survival, and trace glucose concentrations afforded neuroprotection. Mass cultures, in contrast to microcultures, were insensitive to depolarizing glucose deprivation, a difference attributable to increased extracellular lactate levels. Removal of local astrocyte support did not reduce survival in response to glucose deprivation or alter evoked excitatory transmission, suggesting that on-demand, local lactate shuttling is not necessary for neuronal tolerance to prolonged glucose removal. Taken together, these data suggest that endogenously produced lactate available globally in the extracellular milieu sustains neurons in the absence of glucose. A better understanding of resilience mechanisms in reduced preparations could lead to therapeutic strategies aimed to bolster these mechanisms in vulnerable neuronal populations.

Acidotoxicity via ASIC1a Mediates Cell Death during Oxygen Glucose Deprivation and Abolishes Excitotoxicity

Ischemic reperfusion (I/R) injury is associated with a complex and multifactorial cascade of events involving excitotoxicity, acidotoxicity, and ionic imbalance. While it is known that acidosis occurs concomitantly with glutamate-mediated excitotoxicity during brain ischemia, it remains elusive how acidosis-mediated acidotoxicity interacts with glutamate-mediated excitotoxicity. Here, we investigated the effect of acidosis on glutamate-mediated excitotoxicity in acute hippocampal slices. We tested the hypothesis that mild acidosis protects against I/R injury via modulation of NMDAR, but produces injury via activation of acid sensing ion channels (ASIC1a). Using a novel microperfusion approach, we monitored time course of injury in acutely prepared, adult hippocampal slices. We varied the duration of insult to delay the return to preinsult conditions to determine if injury was caused by the primary insult or by the modeled reperfusion phase. We also manipulated pH in presence and absence of oxygen glucose deprivation (OGD). The role of ASIC1a and NMDAR was deciphered by treating the slices with and without an ASIC or NMDAR antagonist. Our results show that injury due to OGD or low pH occurs during the insult rather than the modeled reperfusion phase. Injury mediated by low pH or low pH OGD requires ASIC1a and is independent of NMDAR activation. These findings point to ASIC1a as a mediator of ischemic cell death caused by stroke and cardiac arrest.

The Protective Effect of Omeprazole Against Traumatic Brain Injury: An Experimental Study

BACKGROUND

The development of secondary brain injury via oxidative stress after traumatic brain injury (TBI) is a well-known entity. Consequently, the aim of the present study was to evaluate the role of omeprazole (OM) on rat model of TBI.

METHODS

A total of 24 male rats were used and divided into 4 groups as follows; control, trauma, OM, and methylprednisolone (MP). The trauma, OM, and MP groups were subjected to closed-head contusive weight-drop injuries. Rats received treatment with saline, OM, or MP, respectively. All the animals were sacrificed at 24 hours after trauma and brain tissues were extracted. The oxidant/antioxidant parameters (malondialdehyde, glutathione peroxidase, superoxide dismutase, nitric oxide) and caspase-3 in the cerebral tissue were analyzed, and histomorphologic evaluation of the cerebral tissue was performed.

RESULTS

Levels of MDA and activity of caspase-3 were significantly reduced in the OM and MP groups compared with the trauma group. Glutathione peroxidase and superoxide dismutase levels were increased both in the OM and MP groups compared with the trauma group. The pathology scores were statistically lower in the OM and MP groups than the trauma group.

CONCLUSIONS

The results of the present study showed that OM was as effective as MP in protecting brain from oxidative stress, and apoptosis in the early phase of TBI.

Reduction in N-methyl-D-aspartate Receptor-mediated Cell Death in Hippocampal Neurons by Glucose Reduction Preconditioning

BACKGROUND

Repeated episodes of reduced glucose availability can precondition the brain against damage caused by severe hypoglycemia. Because N-methyl-D-aspartate (NMDA) receptor activation may contribute to neuronal loss in the hippocampus following glucose deprivation, we tested the hypothesis that preconditioning with reduced glucose decreased NMDA receptor-mediated cell death in hippocampal neurons.

METHODS

Hippocampal slice cultures from 7-day old rats were used to study glucose reduction preconditioning and N-methyl-D-aspartate receptor (NMDAR)-mediated cell death. Preconditioning involved reductions in glucose to the following levels: 0.1 mM, 0.5, or 1.0 mM for 30 minutes, 60 minutes, or 90 minutes on 3 consecutive days. Cell death following 1-hour total glucose deprivation was measured with a vital dye technique (SYTOX fluorescence). As an index of NMDAR activity, cell death following application of 1 mM NMDA, was also measured.

RESULTS

A preconditioning protocol of 30 minutes of 0.1 mM glucose per day for 3 days reduced cell death following 1-hour total glucose by 65% to 70%, depending on cellular region. No reduction in NMDAR-mediated cell death was seen following any of the preconditioning treatments. However, when NMDAR-mediated cell death was assessed following preconditioning combined with subsequent total glucose deprivation, cell death was reduced in the cultures that had been preconditioned with 0.1 mM glucose for 30 minutes×3 days.

CONCLUSIONS

We found that that glucose reduction preconditioning protects hippocampal neurons against severe glucose deprivation-induced neuronal damage. This preconditioning was not associated with reductions in NMDAR-mediated cell death except when the preconditioning was combined with an additional exposure to a period of total glucose deprivation.

Lactate modulates the intracellular pH sensitivity of human TREK1 channels

Tissue acidosis and high lactate concentrations are associated with cerebral ischaemia. The degree of acidosis is dependent on circulating glucose concentration, hyperglycaemia being associated with increased acidosis. Among other agents, lactate and protons have been shown to activate the leak potassium channel; TREK1 (TWIK related potassium channel 1) from the intracellular side and its increased activity is implicated in tolerance towards ischaemic cell damage. In the present study, we show that ischaemic concentrations of lactate (30 mM) at pH 7.0 and 6.5, commonly observed during ischemia, cause robust potentiation of human TREK1 (hTREK1) activity at single-channel level in cell-free inside-out membrane patches, while 30 mM lactate at pH 6.0 to 5.5, commonly observed during hyperglycaemic ischemia, reduces hTREK1 channel activity significantly. The biphasic effect of 30 mM lactate (ischaemic concentrations) on modulation of hTREK1 by varying pH conditions is specific since basal concentrations of lactate (3 mM) and 30 mM pyruvate at pH 7.0 and 5.5 failed to show similar effect as lactate. Experiments with deletion and point mutants of hTREK1 channel suggest that lactate changes the pH modulation of hTREK1 by interacting differently with the histidine residue at 328th position (H328) above and below its pKa (∼6.0) in the intracellular carboxyl-terminal domain of TREK1. This lactate-induced pH modulation of hTREK1 is absent in C-terminal deletion mutant, CTDΔ100, and is similar in E321A-hTREK1 mutant as in wild-type hTREK1 suggesting that it is independent of pH-sensitive glutamate residue at 321st position. Such a differential pH-dependent effect of lactate on an ion channel function has not been reported earlier and has important implications in different stages of ischaemia.

Activation of AMP-activated protein kinase regulates hippocampal neuronal pH by recruiting Na(+)/H(+) exchanger NHE5 to the cell surface

Strict regulation of intra- and extracellular pH is an important determinant of nervous system function as many voltage-, ligand-, and H(+)-gated cationic channels are exquisitely sensitive to transient fluctuations in pH elicited by neural activity and pathophysiologic events such as hypoxia-ischemia and seizures. Multiple Na(+)/H(+) exchangers (NHEs) are implicated in maintenance of neural pH homeostasis. However, aside from the ubiquitous NHE1 isoform, their relative contributions are poorly understood. NHE5 is of particular interest as it is preferentially expressed in brain relative to other tissues. In hippocampal neurons, NHE5 regulates steady-state cytoplasmic pH, but intriguingly the bulk of the transporter is stored in intracellular vesicles. Here, we show that NHE5 is a direct target for phosphorylation by the AMP-activated protein kinase (AMPK), a key sensor and regulator of cellular energy homeostasis in response to metabolic stresses. In NHE5-transfected non-neuronal cells, activation of AMPK by the AMP mimetic AICAR or by antimycin A, which blocks aerobic respiration and causes acidification, increased cell surface accumulation and activity of NHE5, and elevated intracellular pH. These effects were effectively blocked by the AMPK antagonist compound C, the NHE inhibitor HOE694, and mutation of a predicted AMPK recognition motif in the NHE5 C terminus. This regulatory pathway was also functional in primary hippocampal neurons, where AMPK activation of NHE5 protected the cells from sustained antimycin A-induced acidification. These data reveal a unique role for AMPK and NHE5 in regulating the pH homeostasis of hippocampal neurons during metabolic stress.

NADPH oxidase-2: linking glucose, acidosis, and excitotoxicity in stroke

SIGNIFICANCE

Neuronal superoxide production contributes to cell death in both glutamate excitotoxicity and brain ischemia (stroke). NADPH oxidase-2 (NOX2) is the major source of neuronal superoxide production in these settings, and regulation of NOX2 activity can thereby influence outcome in stroke.

RECENT ADVANCES

Reduced NOX2 activity can rescue cells from oxidative stress and cell death that otherwise occur in excitotoxicity and ischemia. NOX2 activity is regulated by several factors previously shown to affect outcome in stroke, including glucose availability, intracellular pH, protein kinase ζ/δ, casein kinase 2, phosphoinositide-3-kinase, Rac1/2, and phospholipase A2. The newly identified functions of these factors as regulators of NOX2 activity suggest alternative mechanisms for their effects on ischemic brain injury.

CRITICAL ISSUES

Key aspects of these regulatory influences remain unresolved, including the mechanisms by which rac1 and phospholipase activities are coupled to N-methyl-D-aspartate (NMDA) receptors, and whether superoxide production by NOX2 triggers subsequent superoxide production by mitochondria.

FUTURE DIRECTIONS

It will be important to establish whether interventions targeting the signaling pathways linking NMDA receptors to NOX2 in brain ischemia can provide a greater neuroprotective efficacy or a longer time window to treatment than provided by NMDA receptor blockade alone. It will likewise be important to determine whether dissociating superoxide production from the other signaling events initiated by NMDA receptors can mitigate the deleterious effects of NMDA receptor blockade.

The metabolic response to excitotoxicity - lessons from single-cell imaging

Excitotoxicity is a pathological process implicated in neuronal death during ischaemia, traumatic brain injuries and neurodegenerative diseases. Excitotoxicity is caused by excess levels of glutamate and over-activation of NMDA or calcium-permeable AMPA receptors on neuronal membranes, leading to ionic influx, energetic stress and potential neuronal death. The metabolic response of neurons to excitotoxicity is complex and plays a key role in the ability of the neuron to adapt and recover from such an insult. Single-cell imaging is a powerful experimental technique that can be used to study the neuronal metabolic response to excitotoxicity in vitro and, increasingly, in vivo. Here, we review some of the knowledge of the neuronal metabolic response to excitotoxicity gained from in vitro single-cell imaging, including calcium and ATP dynamics and their effects on mitochondrial function, along with the contribution of glucose metabolism, oxidative stress and additional neuroprotective signalling mechanisms. Future work will combine knowledge gained from single-cell imaging with data from biochemical and computational techniques to garner holistic information about the metabolic response to excitotoxicity at the whole brain level and transfer this knowledge to a clinical setting.

Intracellular pH reduction prevents excitotoxic and ischemic neuronal death by inhibiting NADPH oxidase

Sustained activation of N-methyl-d-aspartate (NMDA) -type glutamate receptors leads to excitotoxic neuronal death in stroke, brain trauma, and neurodegenerative disorders. Superoxide production by NADPH oxidase is a requisite event in the process leading from NMDA receptor activation to excitotoxic death. NADPH oxidase generates intracellular H(+) along with extracellular superoxide, and the intracellular H(+) must be released or neutralized to permit continued NADPH oxidase function. In cultured neurons, NMDA-induced superoxide production and neuronal death were prevented by intracellular acidification by as little as 0.2 pH units, induced by either lowered medium pH or by inhibiting Na(+)/H(+) exchange. In mouse brain, superoxide production induced by NMDA injections or ischemia-reperfusion was likewise prevented by inhibiting Na(+)/H(+) exchange and by reduced expression of the Na(+)/H(+) exchanger-1 (NHE1). Neuronal intracellular pH and neuronal Na(+)/H(+) exchange are thus potent regulators of excitotoxic superoxide production. These findings identify a mechanism by which cell metabolism can influence coupling between NMDA receptor activation and superoxide production.

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.

Lactacidosis modulates glutathione metabolism and oxidative glutamate toxicity

Lactate and acidosis increase infarct size in humans and in animal models of cerebral ischemia but the mechanisms by which they exert their neurotoxic effects are poorly understood. Oxidative glutamate toxicity is a form of nerve cell death, wherein glutamate inhibits cystine uptake via the cystine/glutamate antiporter system leading to glutathione depletion, accumulation of reactive oxygen species and, ultimately, programmed cell death. Using the hippocampal cell line, HT22, we show that lactate and acidosis exacerbate oxidative glutamate toxicity and further decrease glutathione levels. Acidosis but not lactate inhibits system , whereas both acidosis and lactate inhibit the enzymatic steps of glutathione synthesis downstream of cystine uptake. In contrast, when glutathione synthesis is completely inhibited by cystine-free medium, acidosis partially protects against glutathione depletion and cell death. Both effects of acidosis are also present in primary neuronal and astrocyte cultures. Furthermore, we show that some neuroprotective compounds are much less effective in the presence of lactacidosis. Our findings indicate that lactacidosis modulates glutathione metabolism and neuronal cell death. Furthermore, lactacidosis may interfere with the action of some neuroprotective drugs rendering these less likely to be therapeutically effective in cerebral ischemia.

Role of the sodium hydrogen exchanger in maitotoxin-induced cell death in cultured rat cortical neurons

Maitotoxin (MTX) is one of the most potent toxins known to date. It causes massive calcium (Ca(2+)) influx and necrotic cell death in various tissues. However, the exact mechanism(s) underlying its cellular toxicity is not fully understood. In the present study, the role of the sodium hydrogen exchanger (NHE) in MTX-induced increases in intracellular Ca(2+) and subsequent cell death were investigated in cultured rat cortical neurons. Intracellular Ca(2+) concentrations ([Ca(2+)](i)) were measured fluorimetrically using FURA-2 as the fluorescence indicator. Cell death was measured with the alamarBlue cell viability assay and the vital dye ethidium bromide (EB) uptake assay. Results showed that MTX increased, in a concentration dependent manner, both [Ca(2+)](i) and cell death in cortical neurons. Decreasing the pH of the treatment medium from 7.5 to 6.0 diminished MTX-induced cell death. The protection offered by lowering extracellular pH was not due to MTX degradation, because it was still effective even if the cells were treated with MTX in normal pH and then switched to a lower pH. Pretreatment of cells with the specific NHE inhibitor, 5-(N-ethyl-N-isopropyl)-amiloride (EIPA), prevented MTX-induced increases in [Ca(2+)](i), as well as cell death in a concentration dependent manner. Furthermore, knockdown of NHE1 by SiRNA transfection suppressed MTX-induced cell death in human embryonic kidney (HEK) cells. Together, these results suggest that NHE1 plays a major role in MTX-induced neurotoxicity.

The influence of moderate hypercapnia on neural activity in the anesthetized nonhuman primate

Hypercapnia is often used as vasodilatory challenge in clinical applications and basic research. In functional magnetic resonance imaging (fMRI), elevated CO(2) is applied to derive stimulus-induced changes in the cerebral rate of oxygen consumption (CMRO(2)) by measuring cerebral blood flow and blood-oxygenation-level-dependent (BOLD) signal. Such methods, however, assume that hypercapnia has no direct effect on CMRO(2). In this study, we used combined intracortical recordings and fMRI in the visual cortex of anesthetized macaque monkeys to show that spontaneous neuronal activity is in fact significantly reduced by moderate hypercapnia. As expected, measurement of cerebral blood volume using an exogenous contrast agent and of BOLD signal showed that both are increased during hypercapnia. In contrast to this, spontaneous fluctuations of local field potentials in the beta and gamma frequency range as well as multiunit activity are reduced by approximately 15% during inhalation of 6% CO(2) (pCO(2) = 56 mmHg). A strong tendency toward a reduction of neuronal activity was also found at CO(2) inhalation of 3% (pCO(2) = 45 mmHg). This suggests that CMRO(2) might be reduced during hypercapnia and caution must be exercised when hypercapnia is applied to calibrate the BOLD signal.

The association between cord pH at birth and intellectual function in childhood

BACKGROUND

Acidemia at birth is very common but little is known about its long-term consequences.

AIM

To determine if pH at birth is related to established tests of intellectual function.

SUBJECTS

School children aged 6-8, for whom obstetric data were available, who had been delivered after labour at term, and had an umbilical cord arterial pH>7.00 (i.e. that was not extremely acidemic). STUDY DESIGN/OUTCOMES: Retrospective cohort study correlating birth and arterial pH data with childhood tests for non-verbal intelligence, grammar comprehension and literacy.

METHODS

Relationships between pH and cognitive measures were analysed with parametric correlations. Partial correlations were used to examine these relationships, controlling for possible confounding factors.

RESULTS

Arterial pH was significantly negatively correlated with literacy (p=0.001) and with non-verbal intelligence (p=0.033).

CONCLUSIONS

Lower arterial pH is associated with higher scores on literacy and non-verbal intelligence tests at ages 6-8. This is unlikely to be a chance finding and is further evidence that acidemia in isolation should not be considered an adverse outcome. Further research on the relationship between labour and long-term cognitive measures is required.

Mild acidosis enhances AMPA receptor-mediated intracellular zinc mobilization in cortical neurons

Overactivation of glutamate receptors and subsequent deregulation of the intraneuronal calcium ([Ca2+]i) levels are critical components of the injurious pathways initiated by cerebral ischemia. Another hallmark of stroke is parenchymal acidosis, and we have previously shown that mild acidosis can act as a switch to decrease NMDAR-dependent neuronal loss while potentiating the neuronal loss mediated by AMPARs. Potentiation of AMPAR-mediated neuronal death in an acidotic environment was originally associated only with [Ca2+]i dyshomeostasis, as assessed by Ca2+ imaging; however, intracellular dyshomeostasis of another divalent cation, Zn2+, has recently emerged as another important co-factor in ischemic neuronal injury. Rises in [Zn2+]i greatly contribute to the fluorescent changes of Ca2+-sensitive fluorescent probes, which also have great affinity for Zn2+. We therefore revisited our original findings (Mcdonald et al., 1998) and investigated if AMPAR-mediated fura-2 signals we observed could also be partially due to [Zn2+]i increases. Fura-2 loaded neuronal cultures were exposed to the AMPAR agonist, kainate, in a physiological buffer at pH 7.4 and then washed either at pH 7.4 or pH 6.2. A delayed recovery of fura-2 signals was observed at both pHs. Interestingly this impaired recovery phase was found to be sensitive to chelation of intracellular Zn2+. Experiments with the Zn2+ sensitive (and Ca2+-insensitive) fluorescent probe FluoZin-3 confirmed the idea that AMPAR activation increases [Zn2+]i, a phenomenon that is potentiated by mild acidosis. Additionally, our results show that selective Ca2+ imaging mandates the use of intracellular heavy metal chelators to avoid confounding effects of endogenous metals such as Zn2+.

Brain metabolism during a decrease in cerebral perfusion pressure caused by an elevated intracranial pressure in the porcine neocortex

BACKGROUND

Cranial hypertension coincides with a reduction in cerebral blood flow as well as in oxygen delivery and influences outcome. In this study, we monitored changes in energy-related metabolites in the porcine cortex during an increase of intracranial pressure (ICP) and to determine the level at which damage occurs.

METHODS

Male domestic pigs (32-40 kg) were anesthetized, mechanically ventilated, and randomly assigned to either the experimental (n = 6) or control groups (n = 5). A microdialysis probe (CMA 70) was inserted into the cortex to measure extracellular dialysate concentrations of lactate, pyruvate, glucose, glutamate, and glycerol. Every hour an increase of 10 mm Hg in ICP was preformed in the experimental group by infusion of artificial cerebrospinal fluid into the ventricular system of the brain until a maximum ICP of 50 mm Hg was reached.

RESULTS

We demonstrated a significant increase of lactate and glycerol compared with control at ICP values > or =30 mm Hg and cerebral perfusion pressure (CPP) below 50 mm Hg. The increase of ICP to > or =40 mm Hg in conjunction with a reduction in CPP below 40 mm Hg led to a significant increase in the lactate/pyruvate-ratio and glutamate, as well as a decrease of glucose in relation to control.

CONCLUSIONS

Our data strongly suggest that, during a defined ICP increase, lower CPP values may be tolerable until severe damage occurs. Borderline ICP and CPP values of 30 and 40 mm Hg, respectively, could be advised.

Mild acidosis enhances AMPA receptor-mediated intracellular zinc mobilization in cortical neurons

Overactivation of glutamate receptors and subsequent deregulation of the intraneuronal calcium ([Ca2+]i) levels are critical components of the injurious pathways initiated by cerebral ischemia. Another hallmark of stroke is parenchymal acidosis, and we have previously shown that mild acidosis can act as a switch to decrease NMDAR-dependent neuronal loss while potentiating the neuronal loss mediated by AMPARs. Potentiation of AMPAR-mediated neuronal death in an acidotic environment was originally associated only with [Ca2+]i dyshomeostasis, as assessed by Ca2+ imaging; however, intracellular dyshomeostasis of another divalent cation, Zn2+, has recently emerged as another important co-factor in ischemic neuronal injury. Rises in [Zn2+]i greatly contribute to the fluorescent changes of Ca2+-sensitive fluorescent probes, which also have great affinity for Zn2+. We therefore revisited our original findings (Mcdonald et al., 1998) and investigated if AMPAR-mediated fura-2 signals we observed could also be partially due to [Zn2+]i increases. Fura-2 loaded neuronal cultures were exposed to the AMPAR agonist, kainate, in a physiological buffer at pH 7.4 and then washed either at pH 7.4 or pH 6.2. A delayed recovery of fura-2 signals was observed at both pHs. Interestingly this impaired recovery phase was found to be sensitive to chelation of intracellular Zn2+. Experiments with the Zn2+ sensitive (and Ca2+-insensitive) fluorescent probe FluoZin-3 confirmed the idea that AMPAR activation increases [Zn2+]i, a phenomenon that is potentiated by mild acidosis. Additionally, our results show that selective Ca2+ imaging mandates the use of intracellular heavy metal chelators to avoid confounding effects of endogenous metals such as Zn2+.

Cyclooxygenases, lipoxygenases, and epoxygenases in CNS: Their role and involvement in neurological disorders

Three enzyme systems, cyclooxygenases that generate prostaglandins, lipoxygenases that form hydroxy derivatives and leukotrienes, and epoxygenases that give rise to epoxyeicosatrienoic products, metabolize arachidonic acid after its release from neural membrane phospholipids by the action of phospholipase A(2). Lysophospholipids, the other products of phospholipase A(2) reactions, are either reacylated or metabolized to platelet-activating factor. Under normal conditions, these metabolites play important roles in synaptic function, cerebral blood flow regulation, apoptosis, angiogenesis, and gene expression. Increased activities of cyclooxygenases, lipoxygenases, and epoxygenases under pathological situations such as ischemia, epilepsy, Alzheimer's disease, Parkinson disease, amyotrophic lateral sclerosis, and Creutzfeldt-Jakob disease produce neuroinflammation involving vasodilation and vasoconstriction, platelet aggregation, leukocyte chemotaxis and release of cytokines, and oxidative stress. These are closely associated with the neural cell injury which occurs in these neurological conditions. The metabolic products of docosahexaenoic acid, through these enzymes, generate a new class of lipid mediators, namely docosatrienes and resolvins. These metabolites antagonize the effect of metabolites derived from arachidonic acid. Recent studies provide insight into how these arachidonic acid metabolites interact with each other and other bioactive mediators such as platelet-activating factor, endocannabinoids, and docosatrienes under normal and pathological conditions. Here, we review present knowledge of the functions of cyclooxygenases, lipoxygenases, and epoxygenases in brain and their association with neurodegenerative diseases.

Effect of acidosis on IL-8 and MCP-1 during hypoxia and reoxygenation in human NT2-N neurons

Inflammation probably plays a significant role in perinatal brain injury. To study the contribution of locally produced cytokines, the effect on cell death of addition of IL-8 and MCP-1 or antibodies to these, and the impact of acidosis, human postmitotic NT2-N neurons were exposed to 3 h of hypoxia and glucose deprivation and reoxygenated for 21 h. After 3 h of hypoxia with neutral medium, IL-8 was significantly increased compared to controls (150 (100-250)% vs. 100 (85-115)%, p=0.023). After 21 h of neutral reoxygenation, both IL-8 (380 (110-710)% vs. 150 (85-260)%, p=0.041) and monocyte chemoattractant protein-1 (MCP-1) (650 (440-2000)% vs. 310 (230-340)%, p=0.007) were significantly increased compared to controls. After 3 h of hypoxia, both IL-8 (p=0.002) and MCP-1 (p=0.008) were significantly lower in cells with acidotic compared with cells with neutral medium. Acidosis during reoxygenation, however, significantly increased IL-8 release, whereas MCP-1 release was diminished. Similar effects of acidosis were seen in normoxic controls. The cells also secreted RANTES and IP-10, but not 8 other cytokines tested. We found no effect on cell death, measured by MTT assay, of addition of IL-8, MCP-1 or antibodies to these. We conclude that human NT2-N neurons release IL-8 and MCP-1 during 21 h of reoxygenation after 3 h of hypoxia. Acidosis led to a differential effect on IL-8 and MCP-1, with increased IL-8 and decreased MCP-1, both during reoxygenation and in normoxic controls. IL-8 and MCP-1 had no effect on cell death.

DPCPX-resistant hypoxic synaptic depression in the CA1 region of hippocampal slices: Possible role of intracellular accumulation of monocarboxylates

Adenosine plays the principal role in synaptic depression during various energy-depleted conditions. However, additional inhibitory factors not associated with A1 adenosine receptors appear to be involved in hypoxic insults. Monocarboxylate accumulation and consequent acidic changes during hypoxia may be responsible for this remaining depression in synaptic activity. Field evoked potentials were recorded in the CA1 region of rat hippocampal slices. Preincubation with 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) disclosed 43% of DPCPX-resistant synaptic depression (DRSD) during oxygen deprivation (OD). In contrast, no DRSD was detected in various conditions with limited glucose utilization, such as glucose deprivation and oxygen-glucose deprivation. Inhibition of anaerobic glycolysis (iodoacetate, sodium fluoride) abolished DRSD during OD, whereas blockade of monocarboxylate utilization with alpha-cyano-4-hydroxycinnamic acid (4-CIN) provoked DRSD in normoxic medium. These observations suggest that an intracellular accumulation of monocarboxylates is responsible for DRSD during hypoxia.

Selective sparing of hippocampal CA3 cells following in vitro ischemia is due to selective inhibition by acidosis

A brief global ischemic insult to the brain leads to a selective degeneration of the pyramidal neurons in the hippocampal CA1 region while the neurons in the neighbouring CA3 region are spared. The reason for this difference is not known. The selective vulnerability of CA1 neurons to ischemia can be reproduced in vitro in murine organotypic slice cultures, if the ion concentrations in the medium during the anoxic/aglycemic insult are similar to that in the brain extracellular fluid during ischemia in vivo. As acidosis develops during ischemia, we studied the importance of extracellular pH for selective vulnerability. We found that cell death in the CA1 and CA3 regions was equally prevented by removal of calcium from the medium or following blockade of the N-methyl-D-aspartate (NMDA) receptor by D-2 amino-5-phosphonopentanoic-acid (D-APV). On the other hand, damage to the CA3 neurons markedly decreased with decreasing pH following in vitro ischemia, while the degeneration of CA1 neurons was less pH dependent. Patch-clamp recordings from pyramidal neurons in the CA1 and CA3 regions, respectively, revealed a pronounced inhibition of NMDA-receptor mediated excitatory postsynaptic currents (EPSCs) at pH 6.5 that was equally pronounced in the two regions. However, when changing pH from 6.5 to 7.4 the recovery of the EPSCs was significantly slower in the CA3 region. We conclude that acidosis selectively protects CA3 pyramidal neurons during in vitro ischemia, and differentially affects the kinetics of NMDA receptor activation, which may explain the difference in vulnerability between CA1 and CA3 pyramidal neurons to an ischemic insult.

Failure and function of intracellular pH regulation in acute hypoxic-ischemic injury of astrocytes

Astrocytes can die rapidly following ischemic and traumatic injury to the CNS. Brain acid-base status has featured prominently in theories of acute astrocyte injury. Failure of astrocyte pH regulation can lead to cell loss under conditions of severe acidosis. By contrast, the function of astrocyte pH regulatory mechanisms appears to be necessary for acute cell death following the simulation of transient ischemia and reperfusion. Severe lactic acidosis, and the failure of astrocytes to regulate intracellular pH (pH(i)) have been emphasized in brain ischemia under hyperglycemic conditions. Direct measurements of astrocyte pH(i) after cardiac arrest demonstrated a mean pH(i) of 5.3 in hyperglycemic rats. In addition, both in vivo and in vitro studies of astrocytes have shown similar pH levels to be cytotoxic. Whereas astrocytes exposed to hypoxia alone may require 12-24 h to die, acidosis has been found to exacerbate and speed hypoxic loss of these cells. Recently, astrocyte cultures were exposed to hypoxic, acidic media in which the large ionic perturbations characteristic of brain ischemia were simulated. Upon return to normal saline ("reperfusion"), the majority of cells died. This injury was dependent on external Ca2+ and was prevented by inhibition of reversed Na(+)-Ca2+ exchange, blockade of Na(+)-H+ exchange, or by low pH of the reperfusion saline. These data suggested that cytotoxic elevation of [Ca2+]i occurred during reperfusion due to a sequence of activated Na(+)-H+ exchange, cytosolic Na+ loading, and resultant reversal of Na(+)-Ca2+ exchange. The significance of this reperfusion model to ischemic astrocyte injury in vivo is discussed.

Extracellular protons differentially potentiate the responses of native AMPA receptor subtypes regulating neurotransmitter release

1. The effects of pH changes on the basal and evoked release of [(3)H]noradrenaline ([(3)H]NA) and [(3)H]5-hydrohytryptamine ([(3)H]5-HT) from hippocampal synaptosomes and of [(3)H]dopamine ([(3)H]DA) and [(3)H]acetylcholine ([(3)H]ACh) from striatal and cortical synaptosomes were investigated in rat brain. 2. Changing pH between 6.4 and 8.0 did not affect the spontaneous release of the four [(3)H]neurotransmitters; alkalinization to pH 8.8 significantly enhanced release. Acidification to pH 6.4 augmented the AMPA-evoked overflows of [(3)H]NA, [(3)H]5-HT and [(3)H]DA, but not that of [(3)H]ACh. In contrast, lowering pH to 6.4 decreased the K(+)-evoked overflows of [(3)H]NA, [(3)H]5-HT, [(3)H]DA and [(3)H]ACh. 3. AMPA released transmitters in a Ca(2+)-dependent, exocytotic manner since its effects, at pH 7.4 or 6.4, were abolished by omitting external Ca(2+) or by depleting vesicular transmitter stores with bafilomycin A1. AMPA did not evoke carrier-mediated release because the uptake blockers nisoxetine, 6-nitroquipazine, GBR12909 and hemicholinium-3 could not inhibit the AMPA-induced release of [(3)H]NA, [(3)H]5-HT, [(3)H]DA and [(3)H]ACh. 4. Extraterminal acidification to pH 6.4 prevented the potentiating effect of cyclothiazide on the AMPA-evoked release of [(3)H]NA, [(3)H]DA and [(3)H]5-HT, whereas the proton-insensitive AMPA-evoked release of [(3)H]ACh, previously found to be cyclothiazide-insensitive at pH 7.4 was cyclothiazide-resistant also at pH 6.4. 5. To conclude, the cyclothiazide-sensitive AMPA receptors mediating release of NA, 5-HT and DA, but not the cyclothiazide-insensitive AMPA receptors mediating the release of ACh, become more responsive when external pH is lowered to pathophysiologically relevant values. The results with cyclothiazide suggest that H(+) ions may prevent desensitization of some AMPA receptor subtypes.

Acidosis during reoxygenation has an early detrimental effect on neuronal metabolic activity

We recently showed that acidosis is protective during hypoxia and detrimental during reoxygenation. We hypothesized that the detrimental effect of acidosis during reoxygenation was due to a negative effect on mitochondrial function. Human postmitotic NT2-N neurons were exposed to 3 h of hypoxia and glucose deprivation and then reoxygenated for 0, 1, 4, 9, or 21 h. The detrimental effect of acidotic reoxygenation on metabolic activity was evident already after 1 h of reoxygenation, when MTT [3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide] reduction (percentage of normoxic controls) was significantly higher in cells reoxygenated with neutral compared with acidotic medium both after acidotic hypoxia (83+/-26% versus 67+/-27%, p=0.006) and after neutral hypoxia (51+/-12% versus 41+/-7%, p=0.005). Hypoxanthine, a marker of cellular energy failure, increased more with acidotic compared with neutral reoxygenation both after acidotic hypoxia (after 21 h: 7.7+/-2.7 versus 3.1+/-1.9 microM, p<0.001) and after neutral hypoxia (10.4+/-2.6 versus 7.9+/-2.8 microM, p=0.001). During hypoxia and reoxygenation, there was an earlier reduction in the activity of complex IV compared with complexes II+III, and the ratio between these complexes fell during the first hour of reoxygenation. The reduction in complex IV activity was alleviated with acidotic hypoxia. Acidosis during reoxygenation, however, had no effect on the activity of either complex IV or complexes II+III. We conclude that acidosis during hypoxia increases neuronal survival and preserves complex IV activity. Acidosis during reoxygenation has an early detrimental effect on metabolic activity, but this is not mediated through an effect on the mitochondrial complexes IV or II+III.

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.

Effects of hyperglycemia and hypercapnia on lipid metabolism during complete brain ischemia

Ischemic damage is greatly enhanced by preischemic hyperglycemia or hypercapnia, which affects many intracellular responses including protein kinase C (PKC) translocation. We explored whether hyperglycemic or hypercapnic ischemia affects lipid metabolism, especially ischemia-induced release of free fatty acids (FFAs) and diacylglycerols (DAGs). A change in intraischemic level of acidosis was induced either by injecting glucose (hyperglycemic, HG) or by adding CO(2) (hypercapnic, HC). Complete cerebral ischemia was induced, and the brain was frozen in situ after 3, 5, and 10 min at 37 degrees C. Frontoparietal neocortex was dissected for FFA and DAG lipid analysis by thin-layer chromatography and gas-liquid chromatography. Significant differences were shown between normoglycemic and either hypercapnic or hyperglycemic values for individual and total FFAs. A significant delay in the release of FFA in ischemia with hyperglycemia or hypercapnia was observed. Significant differences were also shown in individual DAG-acyl groups and total DAGs. Hyperglycemic or hypercapnic ischemia resulted in a significant decrease of DAG at 10 min of ischemia. This was unexpected because a previous study showed that PKC translocation was significantly enhanced under similar condition at this time point. Upon cellular depolarization, massive influx of calcium and FFA accumulation may decrease the PKC dependence of DAG for translocation. In addition, PKC activation may lead to a negative feedback inhibition of phospholipase C.

In, out, shake it all about: elevation of [Ca2+]i during acute cerebral ischaemia

Because of the extensive second messenger role played by calcium, free intracellular calcium levels are strictly regulated. Under normal physiological conditions, this is achieved through a combination of restricted calcium entry, efficient efflux and restricted intracellular mobility. Overall, the process of regulating free calcium is dependent on ATP derived from oxidative metabolism. Under conditions of cerebral ischaemia, ATP levels fall rapidly and calcium homeostasis becomes significantly disturbed resulting in the initiation of calcium-dependent neurodegenerative processes. In this review, the mechanisms underlying physiological calcium homeostasis and the links between calcium disregulation and neurodegeneration will be discussed.

Cerebral lactic acidosis correlates with neurological impairment in MELAS

OBJECTIVE

To evaluate the role of chronic cerebral lactic acidosis in mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS).

METHODS

The authors studied 91 individuals from 34 families with MELAS and the A3243G point mutation and 15 individuals from two families with myoclonus epilepsy and ragged red fibers (MERRF) and the A8344G mutation. Subjects were divided into four groups. Paternal relatives were studied as controls (Group 1). The maternally related subjects were divided clinically into three groups: asymptomatic (no clinical evidence of neurologic disease) (Group 2), oligosymptomatic (neurologic symptoms but without the full clinical picture of MELAS or MERRF) (Group 3), and symptomatic (fulfilling MELAS or MERRF criteria) (Group 4). The authors performed a standardized neurologic examination, neuropsychological testing, MRS, and leukocyte DNA analysis in all subjects.

RESULTS

The symptomatic and oligosymptomatic MELAS subjects had significantly higher ventricular lactate than the other groups. There was a significant correlation between degree of neuropsychological and neurologic impairment and cerebral lactic acidosis as estimated by ventricular MRS lactate levels.

CONCLUSIONS

High levels of ventricular lactate, the brain spectroscopic signature of MELAS, are associated with more severe neurologic impairment.

Association between brain tissue pH and brain injury during asphyxia in piglets

BACKGROUND

Acidosis may contribute to brain injury from asphyxia, but its role is unclear. In order to evaluate the association between brain acidosis and cerebral injury, we subjected piglets to hypoxia and hypotension (HYP-HOTN) or hypoxia alone (HYP) to inflict varying amounts of brain damage. We hypothesized that piglets with a more severe brain injury would have a lower brain pH.

METHODS

Piglets had a pH microprobe inserted into the cerebral cortex. HYP animals breathed 5-8% O(2)/7% CO(2) for 30 min with mean arterial pressure (MAP) maintained at >40 mmHg. HYP-HOTN animals breathed the same gas for 30 min, but during the last 15 min, MAP was reduced to 25-30 mmHg by withdrawing blood. After 4 h of recovery, the animals were perfusion-fixed and pathology assessed. Somatosensory-evoked potentials (SEP) were also monitored.

RESULTS

HYP-HOTN piglets had more neuropathology than HYP animals. During the last 15 min of injury, brain pH in the HYP-HOTN group was significantly higher than that in HYP. However, recovery of brain pH was prolonged in the HYP-HOTN animals. The amount of time for brain pH to recover to > or =7.00 correlated very well with both the degree of neuropathology and SEP recovery. The reduction in brain pH, either absolute or relative to baseline, was not associated with the severity of damage.

CONCLUSIONS

The time needed for brain pH to recover after asphyxia, but not its severity, was associated with the amount of brain injury. Further study is warranted to determine whether immediate restoration of brain pH will reduce brain damage.

Reoxygenation with 100 or 21% Oxygen after Cerebral Hypoxemia-Ischemia-Hypercapnia in Newborn Piglets

We tested if reoxygenation with 100% O(2) was superior to 21% O(2) after combined cerebral hypoxemia-ischemia-hypercapnia (HIH) in newborn piglets. Twenty-eight piglets were randomized to reoxygenation with 100 or 21% O(2) following asphyxia. Asphyxia was induced by ventilation with 8% O(2), adding CO(2), and temporary occlusion of both common carotid arteries. After 20 min, reoxygenation-reperfusion was started with 21% O(2) (HIH 21% group, n = 13) or 100% O(2) (HIH 100% group, n = 11) for 30 min followed by 21% O(2). All piglets were observed for 2 h. We measured mean arterial blood pressure (MABP), changes in microcirculation in the cerebral cortex (laser Doppler), and extracellular concentrations of hypoxanthine in the cortex and amino acids in the striatum (microdialysis). We found significantly higher MABP and better restoration of microcirculation after reoxygenation with 100% compared with 21% O(2), but no differences in biochemical markers were found between the groups. This indicates that the brain tolerated reoxygenation with 21% as well as with 100% O(2) in the present model of experimental asphyxia in spite of the differences in MABP and cerebral microcirculation.

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.

A potentially critical role of phospholipases in central nervous system ischemic, traumatic, and neurodegenerative disorders

Phospholipases are a diverse group of enzymes whose activation may be responsible for the development of injury following insult to the brain. Amongst the numerous isoforms of phospholipase proteins expressed in mammals are 19 different phospholipase A2's (PLA2s), classified functionally as either secretory, calcium dependent, or calcium independent, 11 isozymes belonging to three structural groups of PLC, and 3 PLD gene products. Many of these phospholipases have been identified in selected brain regions. Under normal conditions, these enzymes regulate the turnover of free fatty acids (FFAs) in membrane phospholipids affecting membrane stability, fluidity, and transport processes. The measurement of free fatty acids thus provides a convenient method to follow phospholipase activity and their regulation. Phospholipase activity is also responsible for the generation of an extensive list of intracellular messengers including arachidonic acid metabolites. Phospholipases are regulated by many factors including selective phosphorylation, intracellular calcium and pH. However, under abnormal conditions, excessive phospholipase activation, along with a decreased ability to resynthesize membrane phospholipids, can lead to the generation of free radicals, excitotoxicity, mitochondrial dysfunction, and apoptosis/necrosis. This review evaluates the critical contribution of the various phospholipases to brain injury following ischemia and trauma and in neurodegenerative diseases.

Lactate and glucose as energy substrates during, and after, oxygen deprivation in rat hippocampal acute and cultured slices

The effects of raised brain lactate levels on neuronal survival following hypoxia or ischemia is still a source of controversy among basic and clinical scientists. We have sought to address this controversy by studying the effects of glucose and lactate on neuronal survival in acute and cultured hippocampal slices. Following a 1-h hypoxic episode, neuronal survival in cultured hippocampal slices was significantly higher if glucose was present in the medium compared with lactate. However, when the energy substrate during the hypoxic period was glucose and then switched to lactate during the normoxic recovery period, the level of cell damage in the CA1 region of organotypic cultures was significantly improved from 64.3 +/- 2.1 to 74.6 +/- 2.1% compared with cultures receiving glucose during and after hypoxia. Extracellular field potentials recorded from the CA1 region of acute slices were abolished during oxygen deprivation for 20 min, but recovered almost fully to baseline levels with either glucose (82.6 +/- 10.0%) or lactate present in the reperfusion medium (108.1 +/- 8.3%). These results suggest that lactate alone cannot support neuronal survival during oxygen deprivation, but a combination of glucose followed by lactate provides for better neuroprotection than either substrate alone.

Treatment of metabolic acidosis

Metabolic acidosis is characterized by a decrease of the blood pH associated with a decrease in the bicarbonate concentration. This may be secondary to a decrease in the strong ion difference or to an increase in the weak acids concentration, mainly the inorganic phosphorus. From a conceptual point of view, two types of nontoxic metabolic acidosis must be differentiated: the mineral metabolic acidosis that reveals the presence of an excess of nonmetabolizable anions, and the organic metabolic acidosis that reveals an excess of metabolizable anions. Significance and consequences of these two types of acidosis are radically different. Mineral acidosis is not caused by a failure in the energy metabolic pathways, and its treatment is mainly symptomatic by correcting the blood pH (alkali therapy) or accelerating the elimination of excessive mineral anions (renal replacement therapy). On the other hand, organic acidosis gives evidence that a severe underlying metabolic distress is in process. No reliable argument exists to prove that this acidosis is harmful under these conditions in humans. Experimental data even show that hypoxic cells are able to survive only if the medium is kept acidic. The management of an acute organic metabolic acidosis is therefore primarily based on the cause of the acidosis, and no scientific argument exists to justify the correction of the acid-base imbalance in this context.

Relation between bicarbonate concentration and voltage dependence of sodium currents in freshly isolated CA1 neurons of the rat

It recently has been shown that whole cell calcium and sodium currents are modulated by CO(2)/HCO(3)(-)-buffered saline. While the bicarbonate ion, but not CO(2), has been proven to modulate calcium currents, this information is lacking for sodium currents. Furthermore, it is not known whether the strength of modulation dependents on the bicarbonate concentration or whether it is an all-or-nothing phenomenon. To answer these questions, we used the whole cell voltage-clamp technique on freshly isolated hippocampal CA1 neurons from the rat. A voltage step from -130 to -20 mV elicited a sodium current with an amplitude of -5.1 +/- 0.5 nA (mean +/- SE, n = 17) when cells were superfused with HEPES-buffered saline. The amplitude of this current increased during a subsequent superfusion with solutions containing increasing amounts of bicarbonate and CO(2) (%CO(2)/mM HCO(3)(-): 2.5/5.6; 5.0/18; 10/37), with a maximal increment in 10% CO(2)/37 mM HCO(3)(-) of -6.9 +/- 0.8 nA. The increase in amplitude was associated with a linear negative shift (slope: -0.7 mV/mM HCO(3)(-)) of the potential of half-maximal activation (DeltaV(h,a): -19.4 +/- 1.8 mV in 10% CO(2)) but not with an alteration in the maximal conductance (g(max): HEPES: 203.1 +/- 21.0 nS and 10% CO(2)/37 mM HCO(3)(-): 207.3 +/- 21.3 nS). In addition, the potential of half-maximal inactivation (V(h,i)) shifted to more negative potentials (slope: -0.6 mV/mM HCO(3)(-)) with increasing amounts of bicarbonate and CO(2) (HEPES: -53.6 +/- 11.8 mV; 10% CO(2)/37 mM HCO(3)(-): -69.8 +/- 2.1 mV), making the amplitude of the current highly sensitive for small potential changes at resting membrane potential. The same negative shift in voltage dependence arose when cells were exposed to solutions with different amounts of bicarbonate (5.6; 18; 26 mM) but constant CO(2) (5%) with slope rates of -0.5 mV/mM HCO(3)(-) for V(h,a) and -0.5 mV/mM HCO(3)(-) for V(h,i). Again, there was no correlation between bicarbonate concentration and the size of g(max). When currents were evoked in solutions containing a constant concentration (18 mM) of bicarbonate but different amounts of CO(2) (2.5; 5.0 10%), no significant changes have been observed. The present data demonstrate that bicarbonate ions, and not CO(2), modulate voltage-gated sodium currents in a concentration-dependent manner. Because the amplitude of the sodium current becomes highly sensitive to membrane potential changes concomitant with increased bicarbonate amounts, this may be critical for the excitability of the neuronal network in situations (like metabolic acidosis, respiratoric alkalosis and hypercapnia) in which the concentration of this ion can alter.