Chlorpromazine protects brain tissue in hypoxia by delaying spreading depression-mediated calcium influx

Ca2+-dependent rapid uncoupling of astrocytes upon brief metabolic stress

Astrocytic gap junctional coupling is a major element in neuron-glia interaction. There is strong evidence that impaired coupling is involved in neurological disorders. Reduced coupling was, e.g., demonstrated for core regions of ischemic stroke that suffer from massive cell death. In the surrounding penumbra, cells may recover, but recovery is hampered by spreading depolarizations, which impose additional metabolic stress onto the tissue. Spreading depolarizations are characterized by transient breakdown of cellular ion homeostasis, including pH and Ca2+, which might directly affect gap junctional coupling. Here, we exposed acute mouse neocortical tissue slices to brief metabolic stress and examined its effects on the coupling strength between astrocytes. Changes in gap junctional coupling were assessed by recordings of the syncytial isopotentiality. Moreover, quantitative ion imaging was performed in astrocytes to analyze the mechanisms triggering the observed changes. Our experiments show that a 2-minute perfusion of tissue slices with blockers of glycolysis and oxidative phosphorylation causes a rapid uncoupling in half of the recorded cells. They further indicate that uncoupling is not mediated by the accompanying (moderate) intracellular acidification. Dampening large astrocytic Ca2+ loads by removal of extracellular Ca2+ or blocking Ca2+ influx pathways as well as a pharmacological inhibition of calmodulin, however, prevent the uncoupling. Taken together, we conclude that astrocytes exposed to brief episodes of metabolic stress can undergo a rapid, Ca2+/calmodulin-dependent uncoupling. Such uncoupling may help to confine and reduce cellular damage in the ischemic penumbra in vivo.

Comparative analysis of spreading depolarizations in brain slices exposed to osmotic or metabolic stress

BACKGROUND

Recurrent spreading depolarizations (SDs) occur in stroke and traumatic brain injury and are considered as a hallmark of injury progression. The complexity of conditions associated with SD in the living brain encouraged researchers to study SD in live brain slice preparations, yet methodological differences among laboratories complicate integrative data interpretation. Here we provide a comparative evaluation of SD evolution in live brain slices, in response to selected SD triggers and in various media, under otherwise standardized experimental conditions.

METHODS

Rat live coronal brain slices (350 μm) were prepared (n = 51). Hypo-osmotic medium (Na+ content reduced from 130 to 60 mM, HM) or oxygen-glucose deprivation (OGD) were applied to cause osmotic or ischemic challenge. Brain slices superfused with artificial cerebrospinal fluid (aCSF) served as control. SDs were evoked in the control condition with pressure injection of KCl or electric stimulation. Local field potential (LFP) was recorded via an intracortical glass capillary electrode, or intrinsic optical signal imaging was conducted at white light illumination to characterize SDs. TTC and hematoxylin-eosin staining were used to assess tissue damage.

RESULTS

Severe osmotic stress or OGD provoked a spontaneous SD. In contrast with SDs triggered in aCSF, these spontaneous depolarizations were characterized by incomplete repolarization and prolonged duration. Further, cortical SDs under HM or OGD propagated over the entire cortex and occassionally invaded the striatum, while SDs in aCSF covered a significantly smaller cortical area before coming to a halt, and never spread to the striatum. SDs in HM displayed the greatest amplitude and the most rapid propagation velocity. Finally, spontaneous SD in HM and especially under OGD was followed by tissue injury.

CONCLUSIONS

While the failure of Na+/K+ ATP-ase is thought to impair tissue recovery from OGD-related SD, the tissue swelling-related hyper excitability and the exhaustion of astrocyte buffering capacity are suggested to promote SD evolution under osmotic stress. In contrast with OGD, SD propagating under hypo-osmotic condition is not terminal, yet it is associated with irreversible tissue injury. Further investigation is required to understand the mechanistic similarities or differences between the evolution of SDs spontaneously occurring in HM and under OGD.

Synaptic and Network Contributions to Anoxic Depolarization in Mouse Hippocampal Slices

Ischemic stroke remains the third leading cause of death and leading cause of adult disability worldwide. A key event in the pathophysiology of stroke is the anoxic depolarization (AD) of neurons in the ischemic core. Previous studies have established that both the latency to AD and the time spent in AD prior to re-oxygenation are predictors of neuronal death. The present studies used hippocampal slices from male and female mice to investigate the electrophysiological events that affect latency to AD after oxygen deprivation. The results confirm that the epoch between AD and re-oxygenation largely determines the magnitude of synaptic recovery after anoxic challenge. Using a selective antagonist of adenosine A₁ receptors, we also confirmed that adenosine released during anoxia (ANOX) suppresses synaptic glutamate release; however, this action has no effect on AD latency or the potential for post-anoxic recovery of synaptic transmission. In contrast, antagonism of AMPA- and NMDA-type glutamate receptors significantly prolongs the latency to AD and alters the speed and synchrony of associated depolarizing waves. Experiments using slices with fields Cornu ammonis 3 (CA3) and Cornu ammonis 1 (CA1) disconnected showed that AD latency is longer in CA1 than in CA3; however, the early AD in CA3 is propagated to CA1 in intact slices. Finally, AD latency in CA1 was found to be longer in slices from female mice than in those from age-matched male mice. The results have implications for stroke prevention and for understanding brain adaptations in hypoxia-tolerant animals.

Reappraisal of anoxic spreading depolarization as a terminal event during oxygen-glucose deprivation in brain slices in vitro

Anoxic spreading depolarization (aSD) has been hypothesized as a terminal event during oxygen–glucose deprivation (OGD) in submerged cortical slices in vitro. However, mechanical artifacts caused by aSD-triggered edema may introduce error in the assessment of neuronal viability. Here, using continuous patch-clamp recordings from submerged rat cortical slices, we first confirmed that vast majority of L4 neurons permanently lost their membrane potential during OGD-induced aSD. In some recordings, spontaneous transition from whole-cell to out-side out configuration occurred during or after aSD, and only a small fraction of neurons survived aSD with reperfusion started shortly after aSD. Secondly, to minimize artifacts caused by OGD-induced edema, cells were short-term patched following OGD episodes of various duration. Nearly half of L4 cells maintained membrane potential and showed the ability to spike-fire if reperfusion started less than 10 min after aSD. The probability of finding live neurons progressively decreased at longer reperfusion delays at a rate of about 2% per minute. We also found that neurons in L2/3 show nearly threefold higher resistance to OGD than neurons in L4. Our results suggest that in the OGD ischemia model, aSD is not a terminal event, and that the “commitment point” of irreversible damage occurs at variable delays, in the range of tens of minutes, after OGD-induced aSD in submerged cortical slices.

Dynamics of the Hypoxia-Induced Tissue Edema in the Rat Barrel Cortex in vitro

Cerebral edema is a major, life threatening complication of ischemic brain damage. Previous studies using brain slices have revealed that cellular swelling and a concomitant increase in tissue transparency starts within minutes of the onset of metabolic insult in association with collective anoxic spreading depolarization (aSD). However, the dynamics of tissue swelling in brain slices under ischemia-like conditions remain elusive. Here, we explored the dynamics of brain tissue swelling induced by oxygen-glucose deprivation (OGD) in submerged rat barrel cortex slices. Video monitoring of the vertical and horizontal position of fluorescent dye-filled neurons and contrast slice surface imaging revealed elevation of the slice surface and a horizontal displacement of the cortical tissue during OGD. The OGD-induced tissue movement was also associated with an expansion of the slice borders. Tissue swelling started several minutes after aSD and continued during reperfusion with normal solution. Thirty minutes after aSD, slice borders had expanded by ~130 μm and the slice surface had moved up to attain a height of ~70 μm above control levels, which corresponded to a volume increase of ~30%. Hyperosmotic sucrose solution partially reduced the OGD-induced slice swelling. Thus, OGD-induced cortical slice tissue swelling in brain slices in vitro recapitulates many features of ischemic cerebral edema in vivo, its onset is tightly linked to aSD and it develops at a relatively slow pace after aSD. We propose that this model of cerebral edema in vitro could be useful for the exploration of the pathophysiological mechanisms underlying ischemic cerebral edema and in the search for an efficient treatment to this devastating condition.

Changes in hippocampal neuronal activity during and after unilateral selective hippocampal ischemia in vivo

The hippocampal formation is one of the brain regions most sensitive to ischemic damage. However, there are no studies about changes in hippocampal neuronal activity during and after a selective unilateral hippocampal ischemia. We developed a novel unilateral cerebrovascular ischemia model in mice that selectively shuts down blood supply to the ipsilateral hippocampal formation. Using a modified version of the photothrombotic method, we stereotaxically targeted the initial ascending part of the longitudinal hippocampal artery in urethane anesthetized and rose bengal-injected mice. To block blood flow in the targeted artery, we photoactivated the rose bengal by illuminating the longitudinal hippocampal artery through an optical fiber inserted into the brain. In vivo field potential recordings in the CA1 region of the hippocampus before, during and after the induction of ischemia demonstrated a high-frequency discharge (HFD) reaching frequencies of >300 Hz and lasting 7-24 s during the illumination consistent with a massive synchronous neuronal activity. The HFD was invariably followed by a DC voltage shift and a decreased activity at both low (30-57 Hz)- and high (63-119 Hz)-gamma frequencies. This decrease in gamma activity lasted for the entire duration of the recordings (∼160 min) following ischemia. The contralateral hippocampus displayed HFDs but with different frequency spectra and without DC voltage shifts or long-lasting decreases in gamma oscillations. Our findings reveal for the first time the acute effects of unilateral hippocampal ischemia on ensemble hippocampal neuronal activities.

Contributions of Ca2+ and Zn2+ to spreading depression-like events and neuronal injury

The phenomenon of spreading depression (SD) involves waves of profound neuronal and glial depolarization that spread throughout brain tissue. Under many conditions, tissue recovers full function after SD has occurred, but SD-like events are also associated with spread of injury following ischemia or trauma. Initial large cytosolic Ca2+ increases accompany all forms of SD, but persistently elevated Ca2+ loading is likely responsible for neuronal injury following SD in tissues where metabolic capacity is insufficient to restore ionic gradients. Ca2+ channels are also involved in the propagation of SD, but the channel subtypes and cation fluxes differ significantly when SD is triggered by different types of stimuli. Ca2+ influx via P/Q type channels is important for SD generated by localized application of high K+ solutions. In contrast, SD-like events recorded in in vitro ischemia models are not usually prevented by Ca2+ removal, but under some conditions, Zn2+ influx via L-type channels contributes to SD initiation. This review addresses different roles of Ca2+ in the initiation and consequences of SD, and discusses recent evidence that selective chelation of Zn2+ can be sufficient to prevent SD under circumstances that may have relevance for ischemic injury.

Zn2+ influx is critical for some forms of spreading depression in brain slices

Spreading depression (SD) is wave of profound depolarization that propagates throughout brain tissue and can contribute to the spread of injury after stroke or traumatic insults. The contribution of Ca(2+) influx to SD differs depending on the stimulus, and we show here that Zn(2+) can play a critical complementary role in murine hippocampal slices. In initial studies, we used the Na(+)/K(+) ATPase inhibitor ouabain and found conditions in which SD was always prevented by L-type Ca(2+) channel blockers; however, Ca(2+) influx was not responsible for L-type effects. Cytosolic Ca(2+) increases were not detectable in CA1 neurons before SD, and removal of extracellular Ca(2+) did not prevent ouabain-SD. In contrast, cytosolic Zn(2+) increases were observed in CA1 neurons before ouabain-SD, and L-type channel block prevented the intracellular Zn(2+) rises. A slow mitochondrial depolarization observed before ouabain-SD was abolished by L-type channel block, and Zn(2+) accumulation contributed substantially to initial mitochondrial depolarizations. Selective chelation of Zn(2+) with N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) abolished SD, implying that Zn(2+) entry can play a critical role in the generation of ouabain-SD. TPEN was most effective when synaptic activity was reduced by adenosine A(1) receptor activation, and a combination of Ca(2+) and Zn(2+) removal was required to prevent ouabain-SD when A(1) receptors were blocked. Similarly, Zn(2+) chelation could prevent SD triggered by oxygen/glucose deprivation but Zn(2+) accumulation did not contribute to SD triggered by localized high K(+) exposures. These results identify Zn(2+) as a new target for the block of spreading depolarizations after brain injury.

Effects of high-frequency stimulation on epileptiform activity in vitro: ON/OFF control paradigm

PURPOSE

To determine the effects of high-frequency electrical stimulation on electrographic seizure activity during and after stimulation (ON-effect and OFF-effect).

METHODS

The modulation and suppression of epileptiform activity during (ON-effect) and after (OFF-effect) high-frequency electrical stimulation was investigated using the high-K(+) and picrotoxin hippocampal slice epilepsy models. Uniform sinusoidal fields (50 Hz) were applied with various intensity levels for 1 min across brain slices. Extracellular and intracellular activity were monitored during and after stimulation.

RESULTS

The ON-effects of high-frequency stimulation were highly variable across individual slices and models; ON-effects included modulation of activity, pacing, partial suppression, or activity resembling spreading-depression. On average, epileptic activity, measured as power in the extracellular fields, increased significantly during stimulation. Following the termination of electrical stimulation, a robust poststimulation suppression period was observed. This OFF suppression was observed even at relatively moderate stimulation intensities. The duration of OFF suppression increased with stimulation intensity, independent of ON-effects. Antagonism of GABA(A)function did not directly effect OFF suppression duration.

CONCLUSIONS

The present results suggest that "rational" seizure control protocols using intermittent high-frequency electrical stimulation should control for both ON and OFF effects.

Sulfhydryl oxidation: a potential strategy to achieve neuroprotection during severe hypoxia?

Previously we reported that sulfhydryl (SH) modulation affects the susceptibility of rat hippocampal slices to severe hypoxia. SH-oxidation by DTNB (5,5'-dithiobis 2-nitrobenzoic acid) or H2O2 postponed the onset of hypoxia-induced spreading depression (HSD), thereby delaying the loss of neuronal function, whereas SH-reduction by DTT (1,4-dithio-dl-threitol) hastened HSD onset. To judge the neuroprotective merit that might arise from a postponement of HSD by oxidants, we have extended our earlier observations by multiparametric recordings and screened for changes in the extracellular K+ accumulation, HSD propagation velocity, and its maximum spread. As parameters for neuronal network function, the failure of synapses during hypoxia and their posthypoxic recovery were analyzed. DTNB (2 mM) or H2O2 (5 mM) postponed HSD but did not attenuate the rise in extracellular K+ concentration ([K+](o)), HSD propagation velocity or its maximum spread. H2O2 slightly postponed the synaptic failure during hypoxia; the posthypoxic recovery of synapses was, however, incomplete. DTNB slowed the synaptic recovery upon reoxygenation. DTT (2 mM) hastened HSD onset, but HSD propagation velocity and tissue invasion were not affected. Upon reoxygenation, however, normalization of [K+](o) was disturbed and synaptic recovery failed. Therefore, SH-reducing conditions at the onset of HSD proved to be devastating for the hippocampal network. In conclusion, the only merit of DTNB or H2O2 treatment is a delayed HSD onset, i.e. some extra time before neuronal function is lost during severe hypoxia. Attenuation of the severe changes during HSD or an improved outcome was not observed. Nevertheless, combination of SH-oxidants with established neuroprotectants might be a potential therapeutic approach.

Protective effects of some creatine derivatives in brain tissue anoxia

Some derivatives more lipophylic than creatine, thus theoretically being capable to better cross the blood-brain barrier, were studied for their protective effect in mouse hippocampal slices. We found that N-amidino-piperidine is harmful to brain tissue, and that phosphocreatine is ineffective. Creatine, creatine-Mg-complex (acetate) and phosphocreatine-Mg-complex (acetate) increased the latency to population spike disappearance during anoxia. Creatine and creatine-Mg-complex (acetate) also increased the latency of anoxic depolarization, while the delay induced by phosphocreatine-Mg-complex (acetate) was of borderline significance (P = 0.056). Phosphocreatine-Mg-complex (acetate) significantly reduced neuronal hyperexcitability during anoxia, an effect that no other compound (including creatine itself) showed. For all parameters except reduced hyperexcitability the effects statistically correlated with tissue levels of creatine or phosphocreatine. Summing up, exogenous phosphocreatine and N-amidino piperidine are not useful for brain protection, while chelates of both creatine and phosphocreatine do replicate some of the known protective effects of creatine. In addition, phosphocreatine-Mg-complex (acetate) also reduced neuronal hyperexcitability during anoxia.

NADH hyperoxidation correlates with enhanced susceptibility of aged rats to hypoxia

Aging increases mitochondrial dysfunction and susceptibility to hypoxia. Previous reports have indicated an association between post-hypoxic hyperoxidation of intra-mitochondrial enzymes and delayed neuronal injury. Therefore we investigated the relationship between NADH fluorescence and neuronal function during and after hypoxia across the lifespan. Hippocampal slices were prepared from adult (1 to >22 months) F344 rats. NADH fluorescence, extracellular voltage and tissue PO(2) were recorded from the CA1 region during hypoxia (95% N(2)) of various lengths following onset of hypoxic spreading depression (hsd). Slices from younger rats recovered evoked neuronal responses to a greater degree and exhibited less hyperoxidation after a hypoxic episode, than slices from older rats. However, the use of Ca(2+) free-media in slices from >22 month old rats improved recovery and delayed NADH hyperoxidation (2.5 min hypoxia after hsd). Post-hypoxic decrease of NADH fluorescence (hyperoxidation) was age dependent and correlated with decreased neuronal recovery. Slices exposed to repeated hypoxic episodes yielded data suggesting depletion of the NAD(+) pool, which may have contributed to the deterioration of neuronal function.

Seasonal differences and protection by creatine or arginine pretreatment in ischemia of mammalian and molluscan neurons in vitro

We investigated the dose-response relationship of protection by creatine against ischemic damage, and we asked whether or not such protection may be observed in invertebrate neurons that might provide a simpler experimental model. Rat isolated pyramidal neurons from the CA3 region of hippocampus subjected to ischemia ("in vitro ischemia") showed anoxic depolarization (AD) after 3-7 min of incubation in anoxic medium. Membrane potential (MP) was reduced 25-78% from preanoxic value. Inward current was decreased by an average 49%. Supplementation with creatine protected against these changes, with 1 mM being the minimal effective concentration, 2 mM providing a near-maximal protection, a maximal effect being obtained with 5 mM creatine. No additional protection was provided by up to 20 mM creatine. Isolated giant neurons of Lymnaea stagnalis showed a similar response to in vitro ischemia. However, a clear seasonal dependence of sensitivity of these cells was detected. In cells obtained during summertime (May-August), AD latency ranged from 3 to 10 min; during wintertime (December-March), this response did not occur even after 25-50 min. The addition of creatine to the medium did not cause changes in AD latency, probably because these neurons rely on a phosphoarginine/arginine energy system. However, treatment of the cells, harvested during summertime, with 2 mM arginine did provide clear protection against anoxic-aglycaemic changes. Summing up, besides confirming previous findings on creatine protection in mammalian neurons, we (1) better characterized their dose-response relationship and extended the findings to the CA3 region and to isolated neurons, (2) found that invertebrate neurons are not protected by creatine but by arginine supplementation and (3) reported a novel mechanism of seasonal dependence in sensitivity of in vitro ischemia by invertebrate neurons.

Delirium

BACKGROUND

Delirium is a serious and often undetected neuropsychiatric syndrome. Failure to recognize and manage delirium can lead to longer hospital stays and increased morbidity and mortality, especially among the elderly.

REVIEW SUMMARY

This article reviews definitions and diagnosis. The Diagnostic and Statistical Manual of Mental Disorders, 4th edition, and the International Statistical Classification of Diseases and Related Health Problems, 10th edition, criteria are quite similar in their diagnostic criteria. Risk factors include advanced age, preexisting brain disease or cognitive impairment, multiple medications, and severe medical problems. Delirium in the elderly can be more subtle and recovery more prolonged. Diagnosis is more complex if there is already an underlying dementia. An organized approach should be used to discover etiology and in ordering appropriate laboratory studies. At the cellular level, delirium is considered to be a reversible disregulation of neuronal membrane function. This involves a selective vulnerability of certain populations of neurons and neurotransmitter dysfunction. Practical treatment issues are reviewed.

CONCLUSIONS

Despite advances, delirium is usually still diagnosed at the bedside. Having an organized approach to diagnosis and understanding the underlying pathophysiology should help with overall evaluation and treatment.

Neuronal mechanisms of the anoxia-induced network oscillations in the rat hippocampus in vitro

1. A spindle of fast network oscillations precedes the ischaemia-induced rapid depolarisation in the rat hippocampus in vivo. However, this oscillatory pattern could not be reproduced in slices and the underlying mechanisms remain poorly understood. We have found that anoxia-induced network oscillations (ANOs, 20-40 Hz, lasting for 1-2 min) can be reproduced in the intact hippocampi of postnatal day P7-10 rats in vitro, and we have examined the underlying mechanisms using whole-cell and extracellular field potential recordings in a CA3 pyramidal layer. 2. ANOs were generated at the beginning of the anoxic depolarisation, when pyramidal cells depolarised to subthreshold values. Maximal power of the ANOs was attained when pyramidal cells depolarised to -56 mV; depolarisation above -47 mV resulted in a depolarisation block of pyramidal cells and a waning of ANOs. 3. A multiple unit activity in extracellular field recordings was phase locked to the negative and ascending phases of ANOs. Pyramidal cells recorded in current-clamp mode generated action potentials with an average probability of about 0.05 per cycle. The AMPA receptor-mediated EPSCs and the GABA receptor-mediated IPSCs in CA3 pyramidal cells were also phase locked with ANOs. 4. ANOs were prevented by tetrodotoxin and glutamate receptor antagonists CNQX and APV, and were slowed down by the allosteric GABA(A) receptor modulator diazepam. In the presence of the GABA(A) receptor antagonist bicuculline, ANOs were transformed to epileptiform discharges. 5. In the presence of the A1 adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), the anoxia induced an epileptiform activity and no ANOs were observed. 6. In normoxic conditions, a rise of extracellular potassium to 10 mM induced an epileptiform activity. Increasing extracellular potassium in conjunction with a bath application of the adenosine A1 receptor agonist cyclopentyladenosine induced oscillations similar to ANOs. 7. Multisite recordings along the septo-temporal hippocampal axis revealed that ANOs and anoxic depolarisation originate in the temporal part, and propagate towards the septal pole at a speed of 1.9 mm x min(-1). 8. ANOs were observed starting from P7, i.e. at a developmental stage when the effects of GABA change from depolarisation to hyperpolarisation. 9. These results suggest that the synchronisation of anoxia-induced oscillations relies on synaptic mechanisms; that the inhibition by GABA and adenosine sets the tune for a generation of oscillations and prevents an epileptiform activity; and that a synchronous GABAergic inhibition is instrumental in a phase locking neuronal activity similarly to other types of oscillatory activities in the gamma frequency range.

Mechanisms of spreading depression and hypoxic spreading depression-like depolarization

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

Quantification of optical signals with electrophysiological signals in neural activities of Di-4-ANEPPS stained rat hippocampal slices

We have quantified the optical signals of synaptically induced neural activities in an in vitro brain slice preparation in terms of electrophysiological signals. The qualification was done using electrophysiologically well known neural activities in the CA1 area of rat hippocampal slices stained with externally applied fluorescent voltage-sensitive dye (VSD; Di-4-ANEPPS). Together with a newly designed CCD-based digital high-speed camera system and epi-fluorescent optics, our improvements were made on a protocol for staining using a newly designed chamber system. These improvements enabled us to make stable and reliable recordings of optical signals and electrophysiological measurements without affecting the physiological status and to make a quantitative comparison between them. The time course and amplitude of the optical signal showed fair agreement with intracellular and extracellular recordings, and was stable over 2 h. The optical signal followed synaptically induced long-term potentiation (LTP) as monitored by the electrophysiological signals. A regional difference in the amount of LTP was found in optical signals and was confirmed in the electrophysiological signals. These results demonstrate the capabilities of our improved method as an alternative but more potent tool to measure the neuronal activities of brain slice in addition to electrophysiological method.

Rat hippocampal slices need bicarbonate for the recovery of synaptic transmission after anoxia

The purpose of this study was to see how the nominal removal of bicarbonate (HCO(-)(3)) from the extracellular space of brain tissue influenced recovery of brain tissue from anoxia. Removal of HCO(-)(3) in HEPES-buffered artificial cerebrospinal fluid (aCSF) inhibited almost completely recovery of synaptic transmission in hippocampal slices after anoxia. Altered pH did not contribute to this finding because adjusting intracellular (pH(i)) and extracellular (pH(o)) pH to control levels did not reduce the effect of HCO(-)(3) removal. Our results suggest that HCO(-)(3) levels are important in determining the extent of anoxic or ischemic brain injury.

Mitochondrial and intrinsic optical signals imaged during hypoxia and spreading depression in rat hippocampal slices

During hypoxia in the CA1 region of the rat hippocampus, spreading-depression-like depolarization (hypoxic spreading depression or HSD) is accompanied by both a negative shift of the extracellular DC potential (DeltaV(o)), and a sharp decrease in light transmittance (intrinsic optical signal or IOS). To investigate alterations in mitochondrial function during HSD and normoxic spreading depression (SD), we simultaneously imaged mitochondrial depolarization, using rhodamine-123 (R123) fluorescence, and IOS while monitoring extracellular voltage. Three major phases of the R123 signal were observed during hypoxia: a gradual, diffuse fluorescence increase, a sharp increase in fluorescence coincident with the HSD-related DeltaV(o), primarily in the CA1 region, and a plateau-like phase if reoxygenation is delayed after HSD onset, persisting until reoxygenation occurs. Two phases occurred following re-oxygenation: an abrupt and then slow decrease in fluorescence to near baseline and a slow secondary increase to slightly above baseline and a late recovery. Parallel phases of the IOS response during hypoxia were also observed though delayed compared with the R123 responses: an initial increase, a large decrease coincident with the HSD-related DeltaV(o), and a trough following HSD. After reoxygenation, there occurred a delayed increase in transmittance and then a slow decrease, returning to near baseline. When Ca(2+) was removed from the external medium, resulting in complete synaptic blockade, the mitochondrial response to hypoxia did not significantly differ from control (normal Ca(2+)) conditions. In slices maintained in low-chloride (2.4 mM) medium, a dramatic reversal in the direction of the IOS signal associated with HSD occurred, and the R123 signal during HSD was severely attenuated. Normoxic SD induced by micro-injection of KCl was also associated with a decrease in light transmittance and a sharp increase in R123 fluorescence but both responses were less pronounced than during HSD. Our results show two mitochondrial responses to hypoxia: an initial depolarization that appears to be caused by depressed electron transport due to lack of oxygen and a later, sudden, sharp depolarization linked to HSD. The depression of the second, sharp depolarization and the inversion of the IOS in low-chloride media suggest a role of Cl(-)-dependent mitochondrial swelling. Lack of effect of Ca(2+)-free medium on the R123 and IOS responses suggests that the protection against hypoxic damage by low Ca(2+) is not due to the prevention of mitochondrial depolarization.

Effects of clomethiazole on spreading depression in the rat hippocampal slice

Clomethiazole is neuroprotective in a variety of animal models of ischaemic stroke, but the mechanism is unclear. This study examined whether clomethiazole is able to modify spreading depression elicited in rat hippocampal slices. When spreading depression was induced by superfusion with high K(+) medium (50 mM), clomethiazole at 100 microM reduced its duration. Both the amplitude and duration of spreading depression were reduced at 200 microM. Clomethiazole at 200 microM tended to reduce the amplitude of the K(+)-induced shift in direct current (DC) potential but this was not statistically significant. When a pair of K(+) pulses were presented, 30 min apart, the second produced a smaller DC potential than the first. Clomethiazole at 200 microM increased the size of the ratio of these responses. Superfusion with a hypoxic solution induced spreading depression observed as a shift in the DC field potential. The amplitude of this was decreased significantly by clomethiazole at 200 microM. With intracellular recordings, the effects of clomethiazole were quantified by measuring the time from the peak K(+)-induced depolarisation to the recovery of membrane potential following the period of hyperpolarisation. Clomethiazole did not reduce this period significantly. It is concluded that clomethiazole can reduce some forms of spreading depression, but only at the higher concentrations tested. It is unlikely that this effect contributes to its neuroprotective properties.

Ischemic cell death in brain neurons

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

Block of (Na+,K+)ATPase with ouabain induces spreading depression-like depolarization in hippocampal slices

We used ouabain (100 microM) to block Na+,K(+)ATPase of in vitro rat hippocampal slices. This treatment was sufficient to cause the sudden depolarization that is the hallmark of both spreading depression (SD) and of the SD-like anoxic depolarization (AD). This depolarization was accompanied by a large and sudden increase in [K](o), also reminiscent of that observed during both SD and AD. Ouabain-induced SD did not require a complete inactivation of Na+,K(+)ATPase, as it occurred when the enzyme was still capable of providing recovery of both V(o) and [K](o). The data indicate that functional inactivation of Na+,K(+)ATPase per se initiates events that lead to an SD-like AD. This ouabain-induced depolarization was not affected by block of synaptic transmission, instead it was abolished by hyperosmolarity of the extracellular space. The possible relevance of these findings to the pathophysiology of AD is discussed.

Rapid and slow swelling during hypoxia in the CA1 region of rat hippocampal slices

The role of swelling in hypoxic/ischemic neuronal injury is incompletely understood. We investigated the extent and time course of cell swelling during hypoxia, and recovery of cell volume during reoxygenation, in the CA1 region of rat hippocampal slices in vitro. Cell swelling was measured optically and compared with simultaneous measurements of the extracellular DC potential, extracellular [K+], and synaptic transmission in the presence and absence of hypoxic depolarization. Hypoxia-induced swelling consisted of rapid and/or slow components. Rapid swelling was observed frequently and always occurred simultaneously with hypoxic depolarization. Additionally, rapid swelling was followed by a prolonged phase of swelling that was approximately 15 times slower. Less frequently, slow swelling occurred independently, without either hypoxic depolarization or a preceding rapid swelling. For slices initially swelling rapidly, recovery of both cell volume and the slope of field excitatory postsynaptic potentials were best correlated with the duration of hypoxia (r = 0.77 and 0.87, respectively). This was also the case for slices initially swelling slowly (r = 0.70 and 0.58, respectively). In contrast, the degree of recovery of cell volume was the same at 30 or 60 min of reoxygenation, indicating that prolonging the duration of reoxygenation within these limits was ineffective in improving recovery. Spectral measurements indicated that the hypoxia-induced changes in light transmittance were related to changes in cell volume and not changes in the oxidation state of mitochondrial cytochromes. The persistent impairment of synaptic transmission in slices swelling slowly (i.e., without hypoxic depolarization) indicates that swelling may play a role in this injury and that hypoxic depolarization is not required. Additionally, the correlation between the degree of recovery of cell volume and the degree of recovery of synaptic transmission during reoxygenation supports a role for swelling in hypoxic neuronal injury.

Using hippocampal slices to study how aging alters ion regulation in brain tissue

Aging alters ion regulation in brain tissue. This article describes methods useful for studying such age-related changes in the rat hippocampal slice preparation. Topics considered include (a) selection of appropriate age groups of rats for aging studies, (b) a description of methods for preparing and maintaining hippocampal slices, (c) measurement of intracellular pH with the H+-sensitive dye carboxy-SNARF-1, and (d) measurement of extracellular pH and K+ with cation-selective microelectrodes.

Rapid preconditioning neuroprotection following anoxia in hippocampal slices: role of the K+ ATP channel and protein kinase C

Sublethal cerebral anoxic/ischemic insults may "precondition" and thereby protect brain from subsequent anoxic/ischemic insults. We tested two hypotheses in hippocampal slices: (i) that short periods of anoxia, each followed by reoxygenation, precondition and thereby improve recovery of synaptic activity following "lethal" anoxic insults; and (ii) that the ATP-sensitive potassium channel [K+ ATP] or protein kinase C mediates anoxic preconditioning neuroprotection in hippocampal slices. Hippocampal slices were subjected to three short periods of anoxia, each separated by 10 min of reoxygenation. These anoxic insults were prolonged only until the onset of anoxic depolarization. Thirty minutes following these insults, slices underwent a "test" anoxic insult, which was characterized by an anoxic insult that lasted 1 min of anoxic depolarization. Recovery of evoked potential amplitudes was followed for 30 min of reoxygenation. The beneficial effects of preconditioning was shown by the significant recovery of evoked potentials after "test" anoxic insults in preconditioned slices, when compared to controls that only underwent a "test" anoxic insult. In control slices, transient superfusion with an ATP-sensitive potassium channel agonist (10 microM pinacidil) 30 min prior to "test" anoxia markedly improved evoked potential recovery. Administration of 5 microM of the sulfonylurea tolbutamide, an ATP-sensitive potassium channel antagonist during preconditioning insults, blocked the protection afforded by preconditioning. Transient superfusion of a protein kinase C activator (500 nM phorbol 12-myristate 13-acetate) did not improve evoked potential recovery. Administration of 50 nM chelerythrine, a protein kinase C inhibitor during preconditioning insults did not block the protection afforded by preconditioning. These data support the hypothesis that the ATP-sensitive potassium channel is involved in the neuroprotection afforded by anoxic preconditioning in hippocampal slices. However, protein kinase C activation does not appear to play a role in this neuroprotection.

Persistent block of CA1 synaptic function by prolonged hypoxia

The effects of prolonged hypoxia were studied by field and intracellular recordings from hippocampal slices of the rat, kept submerged at 34 degrees C. When artificial cerebrospinal fluid contained 10 mM glucose, even very long exposures to hypoxia or 300 microM cyanide (21-25 min) did not block field excitatory postsynaptic potentials and population spikes irreversibly. By contrast, in the presence of 4 mM glucose, hypoxia lasting only 9-13 min-ending 2-3 min after the characteristic transient recovery ("hypoxic injury potential")-resulted in irreversible block of synaptic responses. Voltage-dependent sodium channels and N-methyl-D-aspartate receptors are involved, because irreversible block was prevented by tetrodotoxin (0.5 microM), kynurenate (2 mM) or DL-aminophosphonovalerate (50 microM), whereas 6,7-dinitroquinoxaline-2,3-dione (25 microM) suppressed only the transient recovery. The hypoxic suppression of afferent volleys in slices kept in 4 mM glucose was also prevented by kynurenate or aminophosphonovalerate. Intracellular recordings revealed opposite effects of hypoxia according to glucose concentration: in 10 mM glucose, mainly hyperpolarization; in 4 mM glucose, after a brief hyperpolarization, a major and usually irreversible depolarization. In the presence of kynurenate or tetrodotoxin, major depolarizations also occurred, but they were reversible. Thus, large depolarizations of hippocampal neurons do not necessarily lead to irreversible block of synaptic transmission: there is lasting damage only when hypoxia is combined with low glucose, presumably because a reduced supply of glycolytically generated ATP limits the Na+/K+ pump's ability to maintain or restore membrane potentials and thus prevent excessive activation of N-methyl-D-aspartate receptors.

An electrophysiological analysis of the protective effects of felbamate, lamotrigine, and lidocaine on the functional recovery from in vitro ischemia in rat neocortical slices

We used field potential recording techniques to examine whether felbamate (FBM), lamotrigine (LTG), and lidocaine (LID) protect against the irreversible functional damage induced by transient ischemia. Five minutes of ischemia caused a depression of the field potential in rat cortical slices, which did not recover even after more than 1 h of washout. The N-methyl-D-aspartate (NMDA) antagonist ketamine (50 microM) protected against depression of the field caused by ischemia. On the other hand, the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2.3-dione (CNQX) (10 microM) had protective effects only if co-applied with ketamine. We found that either FBM (30-300 microM), which did not modify the amplitude of the field EPSP, or LTG (10-300 microM), which reversibly depressed the excitatory synaptic transmission, had a marked protective effect when superfused before and during the ischemic insult. After FBM (100 microM) and LTG (100 microM), the field EPSP recovered by 84 +/- 1% and 73 +/- 2.7% of control, respectively. Furthermore, LID (30-300 microM) was less effective than FBM and LTG in inducing a functional recovery from the damage caused by ischemia (58 +/- 1.8%). The rank order of potency, based on the maximal protection caused by the three drugs, was FBM > LTG > LID. Our results suggest that a noticeable neuroprotection can be obtained during glucose and O2 deprivation by preventive therapeutic regimens which use the two recently marketed anticonvulsant drugs, FBM and LTG.

The effects of 17beta-estradiol on ischemia-induced neuronal damage in the gerbil hippocampus

The effects of 17beta-estradiol, a potent estrogen, on ischemia-induced neuronal damage, membrane depolarization and changes in intracellular Ca2+ concentration were studied in gerbil hippocampi. The histological outcome evaluated seven days after 3 min of transient forebrain ischemia in hippocampal CA1 pyramidal cells was improved by high doses of 17beta-estradiol (30 microg, i.c.v. and 4 mg/kg, i.p.), whereas low doses of 17beta-estradiol (3 and 10 microg, i.c.v.) showed no protective effect. Administration of 17beta-estradiol did not affect the changes in the direct current potential shift in ischemia in the hippocampal CA1 area at any dosage. A hypoxia-induced intracellular Ca2+ increase was evaluated by in vitro microfluorometry in gerbil hippocampal slices. Pretreatment of 17beta-estradiol (4 mg/kg, injected i.p. 1 h before decapitation) suppressed the increase in the intracellular concentration of Ca2+ due to the in vitro hypoxia, affecting both the onset of the increase and the extent. The in vitro hypoxia in the Ca2+-free condition induced an elevation of the intracellular concentration of Ca2+, although the increase was gradual. Pretreatment of 17beta-estradiol (4 mg/kg, i.p.) also inhibited this elevation. These findings imply that high doses of 17beta-estradiol protect the neurons from ischemia by inhibiting the release of Ca2+ from the intracellular Ca2+ stores, as well as by inhibiting the influx of Ca2+ from the extracellular space.

Comparison of brain slice chamber designs for investigations of oxygen deprivation in vitro

We have systematically compared anoxia-induced effects in the CA1 stratum pyramidale of rat hippocampal slices maintained in Haas-type interface, and fully-submerged recording chambers to determine whether heterogeneous responses occur in response to oxygen deprivation, between these brain slice chambers. Extracellular K+ concentration ([K+]e), extracellular d.c. potential, and synaptically-evoked field potentials were measured using a K+-selective microelectrode. In slices maintained in the interface chamber, anoxia resulted in a dramatic disturbance of K homeostasis; during the first approximately 3 min of anoxia, [K+]e increased to ca. 5 mM and synaptic transmission was abolished. Thereafter, [K+]e increased explosively to ca. 35 mM, and a negative shift in the extracellular d.c. potential occurred. In slices maintained fully-submerged, even prolonged anoxia (60 min) caused only a very modest increase in [K+]e to ca. 5 mM and a negative shift in the extracellular potential never occurred. However, when glucose deprivation was combined with anoxia, both a dramatic increase in [K+]e and a negative shift in the d.c. potential were observed. We conclude that major differences exist in the degree of disruption of ionic homeostasis by anoxia in rat hippocampal slices when they are maintained at a gas-liquid interface, compared to when they are completely immersed. Possible factors underlying this difference include heterogeneous rates of glucose diffusion into the tissue slices, and washout from the extracellular space of accumulating substances such as K+ by the perfusing artificial cerebrospinal fluid (ACSF). The changes observed in the interface chamber may more closely replicate the dramatic disturbance of K+ homeostasis that is commonly observed in rodent brain in response to anoxia in vivo.

Prolonged anoxic depolarization exacerbates NADH hyperoxidation and promotes poor electrical recovery after anoxia in hippocampal slices

Mitochondrial dysfunction appears to occur during brain ischemia and following reperfusion. A characteristic event during reoxygenation after anoxia in hippocampal slices is hyperoxidation of the electron carriers of the mitochondrial respiratory chain. Earlier studies suggested that calcium influx due to loss of ion homeostasis during anoxia was linked to neuronal damage. Since a link between cytosolic calcium overload and post-anoxic hyperoxidation (PAMHo) has been suggested in previous studies, present studies sought to test the hypothesis that the length of anoxic depolarization can influence hyperoxidation and electrical activity recovery following anoxia in hippocampal slices. Rat hippocampal slices were made anoxic and then allowed to recover for 60 min. The time of anoxia was defined by the time of anoxic depolarization (AD), and slices were divided in five groups: 0.5, 1, 2, 5 and 10 min of AD. Reduction/oxidation shifts of NADH were measured by rapid scanning spectrofluorometry. Synaptic activity was indicated by population spike amplitudes in the CA1 pyramidal cell subfield of the hippocampus in response to stimulation of the Schaffer collaterals. We report here that mitochondrial hyperoxidation and synaptic activity in hippocampal slices are highly sensitive to the time in which slices remain depolarized (AD).

The influence of glucose on intracellular and extracellular pH in rat hippocampal slices during and after anoxia

In this study we investigated in rat hippocampal slices (1) how glucose availability affected tissue acidosis during and after anoxia, (2) whether the onset of anoxic depolarization was associated with a specific pH, (3) whether glycolysis was the major source of acidification before and during anoxic depolarization, and (4) whether improved recovery of synaptic function with elevated glucose levels was related to changes in tissue acidosis. Intracellular pH (pHi) and extracellular pH (pHo) were measured simultaneously before, during, and after anoxia in hippocampal slices bathed in 0, 5, 10, and 15 mM glucose. Slices exposed to 0 mM glucose were given 20 mM sodium lactate as a metabolic substrate. We found that the pHi and pHo at which anoxic depolarization occurred depended upon glucose concentration. We also found that elevated glucose availability increased acidification in both the intracellular and extracellular compartments during anoxia and delayed recovery of pH homeostasis after anoxia. Our results suggest that glycolysis is the primary source of acidosis before the onset of anoxic depolarization, but not during anoxic depolarization. Our results also suggest that moderate increases in acidosis resulting from increased glycolysis are potentially beneficial for anoxic survival.

The calmodulin antagonist trifluoperazine in transient focal brain ischemia in rats. Anti-ischemic effect and therapeutic window

BACKGROUND AND PURPOSE

This study was performed to assess the efficacy and the therapeutic window for the calmodulin antagonist trifluoperazine in experiments involving transient middle cerebral artery (MCA) occlusion.

METHODS

Male Wistar rats were subjected to transient (2 hours) MCA occlusion by an intraluminal filament technique. Trifluoperazine (5.0 mg.kg-1) was injected intraperitoneally 5 minutes, 1 hour, or 2 hours after the induction of ischemia. Drug administration was repeated 24 hours after the first injection. Neurological scores and infarct volumes were evaluated at 48 hours of reperfusion. The effect of trifluoperazine on cortical blood flow was studied with continuous laser-Doppler flowmetry.

RESULTS

The median value of neurological scores in the control rats (n = 7) was 3, while those in the treated groups were 1 (5-minute group; n = 7, P < .05) and 2 (1-hour and 2-hour groups; each n = 7). The percentage of infarct volume in the control rats was 34.8 +/- 4.9% (mean +/- SD), while those in the treated groups were 11.3 +/- 12.3% (5-minute group; P < .01), 24.8 +/- 15.1% (1-hour group), and 28.8 +/- 8.3% (2-hour group). Trifluoperazine, given at 5 minutes after ischemia, had no influence on blood flow in the neocortical penumbra during and after ischemia.

CONCLUSIONS

The results demonstrate that trifluoperazine markedly reduces infarct volume after 2 hours of MCA occlusion when given 5 minutes after the induction of ischemia. However, the therapeutic window for trifluoperazine seems narrow since the drug had no significant effect when given after 1 or 2 hours.

The influence of age of pH regulation in hippocampal slices before, during, and after anoxia

Changes in intracellular and extracellular pH may influence the vulnerability of brain tissue to anoxic or ischemic damage. In the present study, we investigated whether the increased vulnerability of aged brain tissue to anoxic damage is associated with age-related alterations in pH regulation. We obtained evidence for altered pH regulation by measuring concurrent changes in intracellular and extracellular pH before, during, and after anoxia in hippocampal slices from young adult (6-8 months old) and aged (24-27 months old) rats. We found indications of impaired pH regulation in aged hippocampal slices (a) before anoxia, as seen in a lower resting intracellular pH, (b) during anoxia, as seen in a slower decline in extracellular pH, and (c) during recovery after anoxia, as seen in a slower rate of recovery of intracellular pH. Age-related changes in pH regulation may contribute to the faster onset of anoxic depolarization in aged brain tissue during anoxia.

Hypothermia reduces the propensity of cortical tissue to propagate direct current depolarizations in the rat

Both spreading depression (SD) and spontaneous cortical ischemic depolarizations are known to be sensitive to brain temperature. What is unknown is whether this temperature effect is caused by altered sensitivity of cortical tissue to the initiating stimulus or is attributable to an altered ability of cortex to propagate the depolarization wave. To address this, halothane anesthetized rats underwent surface heating/cooling to produce pericranial temperatures of 33 degrees C, 38 degrees C, or 40 degrees C. Spreading depression was first initiated by electrocortical stimulation and then by topical application of KCl. The electrical threshold for SD and the time to direct current shift onset after KCl administration were unaffected by temperature. In contrast, the ability of cortical tissue to propagate the SD wave was temperature dependent. Decreasing temperature from 40 degrees C to 33 degrees C was associated with a slowing of the rate of propagation by 25-30% while the duration of the propagated direct current (DC) shifts was increased by 80% regardless of the initiating stimulus. After elicitation of persistent local DC shift with KCl, the interval between initial waves of SD was progressively increased as temperature was decreased. For either method of stimulation, once SD was initiated, the amplitude of the waveform was temperature independent. These results confirm the importance of temperature regulation in procedures examining SD in vivo. Further, temperature effects on SD reflect propensity of the tissue to propagate depolarization waves although ability of cortex to depolarize in direct response to the stimulus does not undergo substantive change.

Hypertonic environment prevents depolarization and improves functional recovery from hypoxia in hippocampal slices

Treatments that postpone hypoxic spreading depression (SD)-like depolarization (also called anoxic depolarization) facilitate recovery of function after transient cerebral hypoxia. Hypertonia reduces cerebral excitability, and we tested whether it also offers protection against SD-like depolarization and hypoxia. Oxygen was withdrawn from hippocampal slices bathed in normal artificial cerebrospinal fluid (ACSF) and, simultaneously, from slices cut from the same hippocampus but bathed in strongly hypertonic ACSF. Extracellular osmolarity (pi(o)) was increased by adding 100 mM mannitol or fructose to ACSF. Slices in normal pi(o) underwent SD-like negative extracellular voltage shift (delta Vo). The hypertonic slices usually showed no SD-like delta Vo but only a small, gradual negative voltage shift. Hypertonia also prevented the precipitate drop of interstitial calcium level ([Ca2+]o). When oxygenation and normal osmolarity were restored, synaptic transmission in the previously hypertonic slices recovered completely, but 3 h after reoxygenation orthodromically transmitted population spikes of the control slices recovered only 25.1% of the initial control amplitude. We conclude that hypertonic treatment during hypoxia improves subsequent recovery of synaptic function. The protection is probably due to the prevention of calcium uptake by blocking the SD-like depolarization, with the prevention of hypoxic cell swelling playing a lesser role.

Hypoxic neuronal damage in the absence of hypoxic depolarization in rat hippocampal slices: the role of glutamate receptors

The propensity of neurons to undergo profound and precipitous depolarization is believed to contribute to their characteristic vulnerability to hypoxic injury. The length of time a neuron spends in a depolarized state following hypoxic depolarization (HD) is a critical determinant of the extent of irreversible cell damage. It is less clear, however, what the effects of moderate hypoxia are when HD does not occur. The present study examined the effects of prolonged, moderate hypoxia which does not elicit HD in rat hippocampal slices. Extracellularly-recorded population excitatory postsynaptic potentials (pEPSPs) in stratum radiatum of CA1 were eliminated 10-15 min after initiating hypoxia. Physiological damage was related to the hypoxic duration: full, intermediate, or poor recovery of pEPSP slope was observed after 30, 60, or 120 min of hypoxia, respectively. The glutamate receptor antagonists, D,L-2-amino-5-phosphonovaleric acid (APV) or 6,7-dinitroquinoxaline-2,3-dione (DNQX), enhanced the post-hypoxic recovery of synaptic responses. These findings demonstrate that profound HD is not necessary to elicit physiological damage during moderate hypoxia; moreover, the neuroprotective actions of excitatory transmitter antagonists are not limited to their capacity to delay HD. The precise characterization of cellular responses under these conditions will be of particular importance for understanding the pathophysiology of an ischemic penumbra.

The extent and mechanism of the loss of function caused by strongly hypotonic solutions in rat hippocampal slices

To investigate whether prolonged severe swelling would cause irreversible injury to neurons, we exposed hippocampal tissue slices to hypotonic solutions (142 mosmol/kg) and followed the recovery of evoked responses for 5 h. Orthodromically evoked responses increased during hypotonia, except during recurrent waves of spreading depression (SD). After restoring normal osmotic pressure (pi o), evoked potentials became profoundly depressed. Following 30 min exposure, nearly maximal orthodromic responses recovered completely but responses to submaximal stimuli remained depressed, indicating elevated threshold. Following 60 min exposure, orthodromic transmission remained depressed. In slices from young animals, antidromic population spikes recovered completely, but in slices from older rats they remained partly depressed. Withdrawing calcium and raising magnesium concentration before and during hypotonic exposure resulted in modest but significant improvement of the recovery of synaptically transmitted responses, but made no difference for antidromic responses. With [Ca2+]o reduced and [Mg2+]o elevated, electrographic seizures replaced the episodes of SD during low pi o treatment. We conclude that even 60 min of severe hypotonic swelling did not kill CA1 pyramidal cells in tissue from young rats, but in its aftermath synaptic transmission was disrupted. Uptake of calcium may have played a minor role in the impairment of synaptic transmission. We propose hypothetically that post-hypotonic shrinkage of dendrites disrupted the integrity of excitatory synapses.

Ion channel involvement in anoxic depolarization induced by cardiac arrest in rat brain

Anoxic depolarization (AD) and failure of ion homeostasis play an important role in ischemia-induced neuronal injury. In the present study, different drugs with known ion-channel-modulating properties were examined for their ability to interfere with cardiac-arrest-elicited AD and with the changes in the extracellular ion activity in rat brain. Our results indicate that only drugs primarily blocking membrane Na+ permeability (NBQX, R56865, and flunarizine) delayed the occurrence of AD, while compounds affecting cellular Ca2+ load (MK-801 and nimodipine) did not influence the latency time. The ischemia-induced [Na+]e reduction was attenuated by R56865. Blockade of the ATP-sensitive K+ channels with glibenclamide reduced the [K+]e increase upon ischemia, indicating an involvement of the KATP channels in ischemia-induced K+ efflux. The KATP channel opener cromakalim did not affect the AD or the [K+]e concentration. The ischemia-induced rapid decline of extracellular calcium was attenuated by receptor-operated Ca2+ channel blockers MK-801 and NBQX, but not by the voltage-operated Ca2+ channel blocker nimodipine, R56865, and flunarizine.

Pathophysiology of anoxic depolarization: new findings and a working hypothesis

Anoxic depolarization has been linked to the generation of hypoxic irreversible damage. Treatments that postpone its occurrence during hypoxia protect against irreversible damage. This work investigates possible mechanisms leading to anoxic depolarization and ways to prevent or delay it. Exogenous creatine (a compound that delays ATP depletion during hypoxia by increasing the intracellular store of phosphocreatine) doubles the latency of anoxic depolarization. Ouabain (100 microM) reproduces in normoxic slices the depolarization of anoxic depolarization and the concurrent changes in [K+]0; thus, failure of (Na+, K+)ATPase (which is likely to occur during hypoxia due to ATP depletion) is sufficient to cause anoxic depolarization. Electrophysiological evidence, however, suggests that failure of this ATPase causes anoxic depolarization through some intermediate event, probably Na(+)-induced cell swelling. In accordance with this hypothesis, increasing extracellular osmolarity with mannitol (25 mM) increases anoxic depolarization latency by approximately 25%. Other possible mechanisms of anoxic depolarization are also discussed.

Phenytoin delays ischemic depolarization, but cannot block its long-term consequences, in the rat hippocampal slice

The anticonvulsant phenytoin has been reported to block anoxia-induced losses of synaptic activity in the rat hippocampal slice and experimental ischemia-induced losses of synaptic activity in the guinea pig hippocampal slice. We examined phenytoin in our rat hippocampal slice model of experimental ischemia (anoxia +2 mM D-glucose). In this model, ischemic depolarization (ID) occurs 4-5 min after the introduction of anoxic medium, and oxygen and D-glucose are restored 1 min after the onset of ID. In control slices, synaptic recovery is never observed following ID in 2 mM D-glucose. Phenytoin (30,100 and 300 microM), perfused for 20 min prior to, and for 10 min following anoxia, did not allow for synaptic recovery following ID. At the higher concentrations, however, it did increase the latency to ID. In addition, the presynaptic volley (PV), which normally disappears at the time of ID, was lost substantially earlier in the presence of phenytoin. These findings suggest that the anti-ischemic effects of phenytoin reported by others are due to delay of ID. This may suggest that phenytoin will be effective in preventing global ischemia-induced damage only when the ischemic insult is of short duration.

Age-related alterations in energy metabolism contribute to the increased vulnerability of the aging brain to anoxic damage

Aging increases the vulnerability of brain tissue to anoxia and ischemia. We investigated whether age-related alterations in energy metabolism underlie this increased vulnerability. Energy metabolism was manipulated in hippocampal slices from Fischer 344 rats of ages 6-9 (young adult), 16-19 (middle-aged adult), and 26-29 (aged adult) months by altering glucose concentrations or by using lactate instead of glucose as the metabolic substrate. Extracellular K+ activity (K+o) and synaptic excitability were monitored in stratum pyramidale of hippocampal subfield CA1. Aging diminished how well increasing concentrations of glucose delayed onset of anoxic depolarization and improved postanoxic recovery of K+o homeostasis and synaptic transmission. Hippocampal slices from all age groups responded to anoxia similarly when lactate was present instead of glucose. Also, no age-related differences were seen in normoxic ATP and phosphocreatine levels. These results suggest that an age-related decline in the glycolytic capacity of brain cells contributes to earlier onset of anoxic depolarization and poorer recovery of ion homeostasis and synaptic transmission in aging brain tissue.

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

Anoxic depolarization (AD) and failure of the cellular ion homeostasis are suggested to play a key role in ischemia-induced neuronal death. Recent studies show that the blockade of Na+ influx significantly improved the neuronal outcome. In the present study, we investigated the effects of 10 microM tetrodotoxin (TTX) on ischemia-induced disturbances of ion homeostasis in the isolated perfused rat brain. TTX inhibited the spontaneous EEG activity, delayed the ischemia-induced tissue acidification, and significantly postponed the occurrence of AD by 65%. The [Ca2+]e elevation prior to AD was attenuated from 17.8% to 6% while the increase of the [Na+]e in this period was enhanced (from 2.9% to 7.3%). These findings implied that the ischemia-induced early cellular sodium load and the corresponding shrinkage of the extracellular space was counteracted by TTX. Our results suggest that the Na+ influx via voltage-dependent channels preceding complete breakdown of ion homeostasis is one major factor leading to cell depolarization. The massive Na+ influx coinciding with AD, however, may be mainly via non-selective cation channels or/and receptor-operated channels. Persistent Na+ influx deteriorates neuronal tissue integrity by favouring Ca2+ influx and edema formation. Blockade of ischemia-induced excessive Na+ influx is, therefore, a promising pharmacological approach for stroke treatment.

Anoxia reduces depolarization induced calcium uptake in the rat hippocampal slice

Veratridine-induced depolarization caused a large increase in Ca uptake in the rat hippocampal slice (30.2 vs. 9.0 nM/mg dry weight). This uptake was reduced to 18.4 nM/mg when veratridine was combined with anoxia. When compared with veratridine exposure alone, the combination of anoxia and veratridine increased intracellular Na (460 vs. 380 microM/g), decreased intracellular K (30 vs. 40 microM/g) and decreased ATP levels (0.1 vs. 0.8 nM/mg). The changes in Na, K, and ATP should enhance net Ca uptake, yet Ca uptake was reduced. This suggests an effect of anoxia to block Ca channels. In summary anoxia attenuates depolarization-induced Ca uptake. This may represent a mechanism by which neurons are partially protected against anoxic damage which could be more severe if depolarization-induced Ca uptake was not limited.

Evolving concepts about the role of acidosis in ischemic neuropathology

Cerebral ischemia is one of the most common neurological insults. Many pathological events are undoubtedly triggered by ischemia, but only recently has it become accepted that ischemic cell injury arises from a complex interaction between multiple biochemical cascades. Tissue acidosis is a well established feature of ischemic brain tissue, but its role in ischemic neuropathology is still not fully understood. Within the last few years, new evidence has challenged the historically negative view of acidosis and suggests that it may play more of a beneficial role than previously thought. This review reintroduces the concept of acidosis to ischemic brain injury and presents some new perspectives on its neuroprotective potential.

Glycolysis and recovery of potassium ion homeostasis and synaptic transmission in hippocampal slices after anoxia or stimulated potassium release

The present study was undertaken to determine whether glycolytic energy production was critical to the survival of brain tissue subjected to metabolic stress. Specifically, the contributions of glycolysis (1) to recovery of ion homeostasis after anoxia or high frequency electrical stimulation, and (2) to recovery of synaptic transmission after anoxia, were examined. Energy metabolism in rat hippocampal slices was manipulated by varying glucose concentrations, and by substituting lactate for glucose. Ion transport was slower and recovery of synaptic transmission after anoxia was greatly impeded in the absence of glycolysis. These results support the hypothesis that glycolytic ATP production is tied directly or indirectly to ion transport. The results also suggest that recovery of synaptic transmission following anoxia requires glycolytic ATP.