High oxygen tension leads to acute cell death in organotypic hippocampal slice cultures

Tissue oxygen partial pressure as a viability metric for ex vivo brain tissue slices

BACKGROUND

Despite the prevalent use of the ex vivo brain slice preparation in neurophysiology research, a reliable method for judging tissue viability - and thus suitability of a slice for inclusion in an experiment - is lacking. The utility of indirect electrophysiological measures of tissue health is model-specific and needs to be used cautiously. In this study, we verify a more direct test of slice viability, based on tissue oxygen consumption rate.

NEW METHOD

We hypothesised that the minimum intra-slice partial pressure of oxygen (pO2min) would correlate with tissue oxygen consumption rate, providing an accessible method for reliably assessing tissue viability status. Using mouse brain cortex slices, we measured tissue oxygen consumption rate using a Fick's law diffusion-consumption model applied to full intra-tissue pO2 profiles and compared this to pO2min and 2,3,5-triphenol tetrazolium chloride (TTC) viability staining.

RESULTS

Tissue pO2min correlated strongly with oxygen consumption rate in both neurophysiological active and quiescent tissue (in "no-magnesium" and "normal" artificial cerebrospinal fluid, respectively) (R2 =49.7% and 42.1%, respectively). Both correlated with TTC viability stain. Oxygen consumption rate was positively related to the frequency of seizure-like event activity in no-magnesium artificial cerebrospinal fluid (R2 = 44.8%).

COMPARISON WITH EXISTING METHODS

While measurement of tissue oxygen levels and oxygen consumption is not new, intra-tissue pO2min is a novel approach to assess brain slice viability.

CONCLUSION

The results confirm that tissue oxygen minimum pO2min is a robust metric for estimating tissue viability status - the lower the pO2min, the healthier the tissue.

Methodological standards for in vitro models of epilepsy and epileptic seizures. A TASK1-WG4 report of the AES/ILAE Translational Task Force of the ILAE

In vitro preparations are a powerful tool to explore the mechanisms and processes underlying epileptogenesis and ictogenesis. In this review, we critically review the numerous in vitro methodologies utilized in epilepsy research. We provide support for the inclusion of detailed descriptions of techniques, including often ignored parameters with unpredictable yet significant effects on study reproducibility and outcomes. In addition, we explore how recent developments in brain slice preparation relate to their use as models of epileptic activity.

A Microperfusion and In-Bore Oxygenator System Designed for Magnetic Resonance Microscopy Studies on Living Tissue Explants

Spectrometers now offer the field strengths necessary to visualize mammalian cells but were not designed to accommodate imaging of live tissues. As such, spectrometers pose significant challenges—the most evident of which are spatial limitations—to conducting experiments in living tissue. This limitation becomes problematic upon trying to employ commercial perfusion equipment which is bulky and—being designed almost exclusively for light microscopy or electrophysiology studies—seldom includes MR-compatibility as a design criterion. To overcome problems exclusive to ultra-high magnetic field environments with limited spatial access, we have designed microperfusion and in-bore oxygenation systems capable of interfacing with Bruker’s series of micro surface-coils. These devices are designed for supporting cellular resolution imaging in MR studies of excised, living tissue. The combined system allows for precise control of both dissolved gas and pH levels in the perfusate thus demonstrating applicability for a wide range of tissue types. Its compactness, linear architecture, and MR-compatible material content are key design features intended to provide a versatile hardware interface compatible with any NMR spectrometer. Such attributes will ensure the microperfusion rig’s continued utility as it may be used with a multitude of contemporary NMR systems in addition to those which are currently in development.

Generation of Local CA1 γ Oscillations by Tetanic Stimulation

Neuronal network oscillations are important features of brain activity in health and disease and can be modulated by a range of clinically used drugs. A protocol is provided to generate a model for studying CA1 γ oscillations (20-80 Hz). These γ oscillations are stable for at least 30 min and depend upon excitatory and inhibitory synaptic activity in addition to activation of pacemaker currents. Tetanically stimulated oscillations have a number of reproducible and easily quantifiable characteristics including spike count, oscillation duration, latency and frequency that report upon the network state. The advantages of the electrically stimulated oscillations include stability, reproducibility and episodic acquisition enabling robust characterization of network function. This model of CA1 γ oscillations can be used to study cellular mechanisms and to systematically investigate how neuronal network activity is altered in disease and by drugs. Disease state pharmacology can be readily incorporated by the use of brain slices from genetically modified or interventional animal models to enable selection of drugs that specifically target disease mechanisms.

A reliable model for gamma oscillations in hippocampal tissue

Gamma oscillations (30-100 Hz) reflect a fast brain rhythm that provides a fundamental mechanism of complex neuronal information processing in the hippocampus and in the neocortex in vivo. Gamma oscillations have been implicated in higher brain functions, such as sensory perception, motor activity, and memory formation. Experimental studies on synaptic transmission and bioenergetics underlying gamma oscillations have primarily used acute slices of the hippocampus. This study tests whether organotypic hippocampal slice cultures of the rat provide an alternative model for cortical gamma oscillations in vitro. Our findings are that 1) slice cultures feature well-preserved laminated architecture and neuronal morphology; 2) slice cultures of different maturation stages (7-28 days in vitro) reliably express gamma oscillations at about 40 Hz as induced by cholinergic (acetylcholine) or glutamatergic (kainate) receptor agonists; 3) the peak frequency of gamma oscillations depends on the temperature, with an increase of ∼ 3.5 Hz per degree Celsius for the range of 28-36 °C; 4) most slice cultures show persistent gamma oscillations for ∼ 1 hr during electrophysiological local field potential recordings, and later alterations may occur; and 5) in slice cultures, glucose at a concentration of 5 mM in the recording solution is sufficient to power gamma oscillations, and additional energy substrate supply with monocarboxylate metabolite lactate (2 mM) exclusively increases the peak frequency by ∼ 4 Hz. This study shows that organotypic hippocampal slice cultures provide a reliable model to study agonist-induced gamma oscillations at glucose levels near the physiological range.

Effects of oxygen and glucose deprivation on synaptic transmission in rat dentate gyrus: role of A2A adenosine receptors

The hippocampus is comprised of two distinct subfields that show different responses to hypoxic-ischemic brain injury: the CA1 region is particularly susceptible whereas the dentate gyrus (DG) is quite resistant. Our aim was to determine the synaptic and proliferative response of the DG to severe oxygen and glucose deprivation (OGD) in acute rat hippocampal slices and to investigate the contribution of A(2A) adenosine receptor antagonism to recovery of synaptic activity after OGD. Extracellular recordings of field excitatory post-synaptic potentials (fEPSPs) in granule cells of the DG in brain slices prepared from male Wistar rats were used. A 9-min OGD is needed in the DG to always induce the appearance of anoxic depolarization (AD) and the irreversible block of synaptic activity, as recorded up to 24 h from the end of the insult, whereas only 7-min OGD is required in the CA1 region. Selective antagonism of A(2A) adenosine receptors by ZM241385 significantly prevents or delays the appearance of AD and protects from the irreversible block of neurotransmission induced by 9-min OGD in the DG. The effects of 9-min OGD on proliferation and maturation of cells localized in the subgranular zone of DG in slices prepared from 5-bromo-2'-deoxyuridine (BrdU) treated rats was investigated. Slices were further incubated with an immature neuronal marker, doublecortin (DCX). The number of BrdU(+) cells was significantly decreased 6 h after 9-min OGD and this effect was antagonized by ZM241385. After 24 h from the end of 9-min OGD, the number of BrdU(+) cells returned to that found before OGD and increased arborization of tertiary dendrites of DCX(+) cells was observed. The adenosine A(2A) antagonist ZM241385 protects from synaptic failure and from decreased proliferation of immature neuronal cells at a precocious time after OGD.

Brain slice on a chip: opportunities and challenges of applying microfluidic technology to intact tissues

Isolated brain tissue, especially brain slices, are valuable experimental tools for studying neuronal function at the network, cellular, synaptic, and single channel levels. Neuroscientists have refined the methods for preserving brain slice viability and function and converged on principles that strongly resemble the approach taken by engineers in developing microfluidic devices. With respect to brain slices, microfluidic technology may 1) overcome the traditional limitations of conventional interface and submerged slice chambers and improve oxygen/nutrient penetration into slices, 2) provide better spatiotemporal control over solution flow/drug delivery to specific slice regions, and 3) permit successful integration with modern optical and electrophysiological techniques. In this review, we highlight the unique advantages of microfluidic devices for in vitro brain slice research, describe recent advances in the integration of microfluidic devices with optical and electrophysiological instrumentation, and discuss clinical applications of microfluidic technology as applied to brain slices and other non-neuronal tissues. We hope that this review will serve as an interdisciplinary guide for both neuroscientists studying brain tissue in vitro and engineers as they further develop microfluidic chamber technology for neuroscience research.

An organotypic hippocampal slice culture model of excitotoxic injury induced spontaneous recurrent epileptiform discharges

Stroke is the major cause of acquired epilepsy in the adult population. The mechanisms of ischemia-induced epileptogenesis are not completely understood, but glutamate is associated with both ischemia-induced injury and epileptogenesis. The objective of this study was to develop an in vitro model of epileptogenesis induced by glutamate injury in organotypic hippocampal slice cultures (OHSCs), as observed in stroke-induced acquired epilepsy. OHSCs were prepared from 1-week-old Sprague-Dawley rat pups. They were exposed to 3.5 mM glutamate for 35 minutes at 21 days in vitro. Field potential recordings and whole-cell current clamp electrophysiology were used to monitor the development of in vitro seizure events up to 19 days after injury. Propidium iodide uptake assays were used to examine acute cell death following injury. Glutamate exposure produced a subset of hippocampal neurons that died acutely and a larger population of injured but surviving neurons. These surviving neurons manifested spontaneous, recurrent epileptiform discharges in neural networks, characterized by paroxysmal depolarizing shifts and high frequency spiking in both field potential and intracellular recordings. This model also exhibited anticonvulsant sensitivity similar to in vivo models. Our study is the first demonstration of a chronic model of acquired epilepsy in OHSCs following a glutamate injury. This in vitro model of glutamate injury-induced epileptogenesis may help develop therapeutic strategies to prevent epileptogenesis after stroke and elucidate some of the mechanisms that underlie stroke-induced epilepsy in a more anatomically intact system.

The adenosine A2A receptor antagonist ZM241385 enhances neuronal survival after oxygen-glucose deprivation in rat CA1 hippocampal slices

BACKGROUND AND PURPOSE

Activation of adenosine A(2A) receptors in the CA1 region of rat hippocampal slices during oxygen-glucose deprivation (OGD), a model of cerebral ischaemia, was investigated.

EXPERIMENTAL APPROACH

We made extracellular recordings of CA1 field excitatory postsynaptic potentials (fepsps) followed by histochemical and immunohistochemical techniques coupled to Western blots.

KEY RESULTS

OGD (7 or 30 min duration) elicited an irreversible loss of fepsps invariably followed by the appearance of anoxic depolarization (AD), an unambiguous sign of neuronal damage. The application of the selective adenosine A(2A) receptor antagonist, ZM241385 (4-(2-[7-amino-2-{2-furyl}{1,2,4}triazolo{2,3-a}{1,3,5}triazin-5-ylamino]ethyl)phenol; 100-500 nmolxL(-1)) prevented or delayed AD appearance induced by 7 or 30 min OGD and protected from the irreversible fepsp depression elicited by 7 min OGD. Two different selective adenosine A(2A) receptor antagonists, SCH58261 and SCH442416, were less effective than ZM241385 during 7 min OGD. The extent of CA1 cell injury was assessed 3 h after the end of 7 min OGD by propidium iodide. Substantial CA1 pyramidal neuronal damage occurred in untreated slices, exposed to OGD, whereas injury was significantly prevented by 100 nmolxL(-1) ZM241385. Glial fibrillary acid protein (GFAP) immunostaining showed that 3 h after 7 min OGD, astrogliosis was appreciable. Western blot analysis indicated an increase in GFAP 30 kDa fragment which was significantly reduced by treatment with 100 nmolxL(-1) ZM241385.

CONCLUSIONS AND IMPLICATIONS

In the CA1 hippocampus, antagonism of A(2A) adenosine receptors by ZM241385 was protective during OGD (a model of cerebral ischaemia) by delaying AD appearance, decreasing astrocyte activation and improving neuronal survival.

Maintaining network activity in submerged hippocampal slices: importance of oxygen supply

Studies in brain slices have provided a wealth of data on the basic features of neurons and synapses. In the intact brain, these properties may be strongly influenced by ongoing network activity. Although physiologically realistic patterns of network activity have been successfully induced in brain slices maintained in interface-type recording chambers, they have been harder to obtain in submerged-type chambers, which offer significant experimental advantages, including fast exchange of pharmacological agents, visually guided patch-clamp recordings, and imaging techniques. Here, we investigated conditions for the emergence of network oscillations in submerged slices prepared from the hippocampus of rats and mice. We found that the local oxygen level is critical for generation and propagation of both spontaneously occurring sharp wave–ripple oscillations and cholinergically induced fast oscillations. We suggest three ways to improve the oxygen supply to slices under submerged conditions: (i) optimizing chamber design for laminar flow of superfusion fluid; (ii) increasing the flow rate of superfusion fluid; and (iii) superfusing both surfaces of the slice. These improvements to the recording conditions enable detailed studies of neurons under more realistic conditions of network activity, which are essential for a better understanding of neuronal network operation.

Establishing a physiological environment for visualized in vitro brain slice recordings by increasing oxygen supply and modifying aCSF content

Our insights into the basic characteristics of neuronal function were significantly advanced by combining the in vitro slice technique with the visualization of neurons and their processes. The visualization through water immersion objectives requires keeping slices submerged in recording chambers where delivering artificial cerebro-spinal fluid (aCSF) at flow rates of 2-3 ml/min results in a limited oxygen supply [Hájos N, Ellender TJ, Zemankovics R, Mann EO, Exley R, Cragg SJ, et al. Maintaining network activity in submerged hippocampal slices: importance of oxygen supply. Eur J Neurosci 2009;29:319-27]. Here we review two methods aimed at providing sufficient oxygen levels to neurons in submerged slices to enable high energy consuming processes such as elevated firing rates or network oscillations. The use of these methods may also influence the outcome of other electrophysiological experiments in submerged slices including the study of intercellular signaling pathways. In addition, we also emphasize the importance of various aCSF constituents used in in vitro experiments.

Stimulated efflux of amino acids and glutathione from cultured hippocampal slices by omission of extracellular calcium: likely involvement of connexin hemichannels

Omission of extracellular Ca(2+) for 15 min from the incubation medium of cultured hippocampal slices stimulated the efflux of glutathione, phosphoethanolamine, hypotaurine, and taurine. The efflux was reduced by several blockers of gap junctions, i.e. carbenoxolone, flufenamic acid, and endothelin-1, and by the connexin43 hemichannel blocking peptide Gap26 but was unchanged by the P2X(7) receptor inhibitor oxidized ATP, a pannexin1 hemichannel blocking peptide and an inactive analogue of carbenoxolone. Pretreatment of the slices with the neurotoxin N-methyl-d -aspartate left the efflux by Ca(2+) omission unchanged, indicating that the stimulated efflux primarily originated from glia. Elevated glutamate efflux was detected when Ca(2+) omission was combined with the glutamate uptake blocker l-trans-pyrrolidine-2,4-dicarboxylate and when both Ca(2+) and Mg(2+) were omitted from the medium. Omission of Ca(2+) for 15 min alone did not induce delayed toxicity, but in combination with blocked glutamate uptake, significant cell death was observed 24 h later. Our results indicate that omission of extracellular Ca(2+) stimulates efflux of glutathione and specific amino acids including glutamate via opening of glial hemichannels. This type of efflux may have protective functions via glutathione efflux but can aggravate toxicity in situations when glutamate reuptake is impaired, such as following a stroke.

Superoxide (·O2−) Production in CA1 Neurons of Rat Hippocampal Slices Exposed to Graded Levels of Oxygen

Neuronal signaling, plasticity, and pathologies in CA1 hippocampal neurons are all intimately related to the redox environment and, thus tissue oxygenation. This study tests the hypothesis that hyperoxic superfusate (95% O2) causes a time-dependent increase in superoxide anion (·O2−) production in CA1 neurons in slices, which will decrease as oxygen concentration is decreased. Hippocampal slices (400 μm) from weaned rats were incubated with the fluorescent probe dihydroethidium (DHE), which detects intracellular ·O2− production. Slices were loaded for 30 min using 10 μM DHE and maintained using one-sided superfusion or continuously loaded using 2.5 μM DHE and maintained using two-sided superfusion (36°C). Continuous loading of DHE and two-sided superfusion gave the highest temporal resolution measurements of ·O2− production, which was estimated by the increase in fluorescence intensity units (FIUs) per minute (FIU/min ± SE) over 4 h. Superoxide production (2.5 μM DHE, 2-sided superfusion) was greatest in 95% O2 (6.6 ± 0.4 FIU/min) and decreased significantly during co-exposure with antioxidants (100 μM melatonin, 25 μM MnTMPyP) and lower levels of O2 (60, 40, and 20% O2 at 5.3 ± 0.3, 3.3 ± 0.1, and 1.6 ± 0.2 FIU/min, respectively). CA1 cell death after 4 h (ethidium homodimer-1 staining) was greatest in 95% O2 and lowest in 40 and 20% O2. CA1 neurons generated evoked action potentials in 20% O2 for >4 h, indicating viability at lower levels of oxygenation. We conclude that ·O2− production and cell death in CA1 neurons increases in response to increasing oxygen concentration product (= PO2 × time). Additionally, lower levels of oxygen (20–40%) and antioxidants should be considered to minimize superoxide-induced oxidative stress in brain slices.

Neuroprotective potential of ceftriaxone in in vitro models of stroke

Astrocytic glutamate transporters are considered an important target for neuroprotective therapies as the function of these transporters is abnormal in stroke and other neurological disorders associated with excitotoxicity. Recently, Rothstein et al., [Rothstein JD, Patel S, Regan MR, Haenggeli C, Huang YH, Bergles DE, Jin L, Dykes Hoberg M, Vidensky S, Chung DS, Toan SV, Bruijn LI, Su ZZ, Gupta P, Fisher PB (2005) Beta-lactam antibiotics offer neuroprotection by increasing glutamate transporter expression. Nature 433:73-77] reported that beta-lactam antibiotics (including ceftriaxone, which easily crosses the blood-brain barrier) increase glutamate transporter 1 (GLT-1) expression and reduce cell death resulting from oxygen-glucose deprivation (OGD) in dissociated embryonic cortical cultures. To determine whether a similar neuroprotective mechanism operates in more mature neurons, which show a different pattern of response to ischemia than primary cultures, we exposed acute hippocampal slices obtained from rats treated with ceftriaxone for 5 days (200 mg/kg; i.p.) to OGD. Whole-cell patch clamp recording of glutamate-induced N-methyl-d-aspartate (NMDA) currents from CA1 pyramidal neurons showed a larger potentiation of these currents after application of 15 microM dl-threo-beta-benzyloxyaspartic acid (TBOA; a potent blocker of glutamate transporters) in ceftriaxone-injected animals than in untreated animals, indicating increased glutamate transporter activity. Western blot analysis did not reveal GLT-1 upregulation in the hippocampus. Delay to OGD-induced hypoxic spreading depression (HSD) recorded in slices obtained from ceftriaxone-treated rats was longer (6.3+/-0.2 vs. 5.2+/-0.2 min; P<0.001) than that in the control group, demonstrating a neuroprotective action of the antibiotic in this model. The effect of ceftriaxone was also tested in organotypic hippocampal slices obtained from P7-9 rats (>14 days in vitro). OGD or glutamate (3.5-5.0 mM) damaged CA1 pyramidal neurons as assessed by propidium iodide (PI) fluorescence. Similar damage was observed after pre-treatment with ceftriaxone (10-200 microM; 5 days) and ceftriaxone exposure did not result in GLT-1 upregulation as assayed by Western blot. Treatment of slice cultures with dibutyryl cAMP (100-250 microM; 5 days) increased GLT-1 expression but did not reduce cell damage induced by OGD or glutamate. Thus we confirm the neuroprotective effect of antibiotic exposure on OGD-induced injury, but suggest that this action is related to independent modulation of transporter activity rather than to the level of GLT-1 protein expression. In addition, our results indicate that the protective effects of beta-lactam antibiotics are highly dependent on the experimental model.

Repetitive Spreading Depression-Like Events Result in Cell Damage in Juvenile Hippocampal Slice Cultures Maintained in Normoxia

Prolonged seizures, e.g., induced by fever, experienced early in life are considered a precipitating injury for the subsequent development of temporal lobe epilepsy. During in vitro epileptiform activity, spreading depressions (SDs) have often been observed. However, their contribution to changes in the properties of juvenile neuronal tissue is unknown. We therefore used the juvenile hippocampal slice culture preparation (JHSC) maintained in normoxia (20% O(2)-5% CO(2)-75% N(2)) to assess the effect of repetitive SD-like events (SDLEs) on fast field potentials and cell damage. Repetitive SDLEs in the CA1 region could be induced in about two-thirds of the investigated JHSCs (n = 61) by repetitive electrical stimulation with 2-200 pulses. SDLEs were characterized by a transient large negative field potential shift accompanied by intracellular depolarization, ionic redistribution, slow propagation (assessed by intrinsic optical signals) and glutamate receptor antagonist sensitivity. The term "SDLE" was used because evoked fast field potentials were only incompletely suppressed and superimposed discharges occurred. With 20 +/- 1 repetitive SDLEs (interval of 10-15 min, n = 7 JHSCs), the events got longer, their amplitude of the first peak declined, while threshold for induction became reduced. Evoked fast field potentials deteriorated and cell damage (assessed by propidium iodide fluorescence) occurred, predominantly in regions CA1 and CA3. As revealed by measurements of tissue partial oxygen pressure during SDLEs repetitive transient anoxia accompanying SDLE might be critical for the observed cell damage. These results, limited so far to the slice culture preparation, suggest SDs to be harmful events in juvenile neuronal tissue in contrast to what is known about their effect on adult neuronal tissue.

Changes of neuronal activity in areas CA1 and CA3 during anoxia and normoxic or hyperoxic reoxygenation in juvenile rat organotypic hippocampal slice cultures

In neonates, asphyxia is usually followed by hyperoxic treatment. In order to study whether hyperoxic reoxygenation might cause additional impairment of neuronal function, we subjected organotypic hippocampal slice cultures of juvenile rats (7 DIV, P6-8) to 30 min anoxia followed by 60 min hyperoxic or normoxic reoxygenation (95% or 19% O2, respectively). Spontaneous and evoked field potentials as well as [Ca2+]o were recorded in the pyramidal layer of area CA1 or area CA3. In area CA1, 30 min of anoxia led to decline of evoked field potential amplitudes by on average 67% and to profound changes in field potential characteristics and Ca2+ homeostasis which were not related to outcome after reoxygenation. Hyperoxic reoxygenation resulted first in a fast recovery of the field potential amplitude to 82% of the control value and then, in 75% of slice cultures, in a large negative field potential shift accompanied by a prolonged decrease of [Ca2+]o and loss of excitability outlasting the experiment. Recovery of field potential amplitude under normoxic conditions stayed poor, with a first increase to 51% and a second decrease to 22%. In contrast, field potential amplitude in area CA3 recovered to 80% of the initial amplitude, irrespective of the reoxygenation mode. The selective loss of function during hyperoxic reoxygenation in area CA1 might be a first sign of neuronal injury that we observed 1 h after end of hyperoxic reoxygenation in a previous study. Whether the poor outcome after normoxic reoxygenation would favour long-term recovery remains to be determined.

NMDA-receptor mediated efflux of N-acetylaspartate: physiological and/or pathological importance?

N-Acetylaspartate (NAA) is a largely neuron specific dianionic amino acid present in high concentration in vertebrate brain. Many fundamental questions concerning N-acetylaspartate in brain remain unanswered. One such issue is the predominantly neuronal synthesis and largely glial catabolism which implies the existence of a regulated efflux from neurons. Here we show that transient (5 min) NMDA-receptor activation (60 microM) induces a long lasting Ca2+ -dependent efflux of N-acetylaspartate from organotypic slices of rat hippocampus. The NMDA-receptor stimulated efflux was unaffected by hyper-osmotic conditions (120 mM sucrose) and no efflux of N-acetylaspartate was evoked by high K+ -depolarization (50 mM) or kainate (300 microM). These results indicate that the efflux induced by NMDA is not related directly to either cell swelling or depolarization but is coupled to Ca2+ -influx via the NMDA-receptor. The efflux of N-acetylaspartate persisted at least 20 min after the omission of NMDA, similar to the efflux of the organic anions glutathione and phosphoethanolamine. The efflux of taurine and hypotaurine was also stimulated by NMDA but returned more quickly to basal levels. The NMDA-receptor stimulated efflux of N-acetylaspartate, glutathione, phosphoethanolamine, taurine and hypotaurine correlated with delayed nerve cell death measured 24 h after the transient NMDA-receptor stimulation. However, exogenous administration of high concentrations of N-acetylaspartate to the culture medium was non-toxic. The results suggest that Ca2+ -influx via the NMDA-receptor regulates the efflux of N-acetylaspartate from neurons which may have both physiological and pathological importance.

Hyperoxia is not an essential condition for status epilepticus induced cell death in organotypic hippocampal slice cultures

The low Mg2+ model of epilepsy in organotypic hippocampal slice cultures is used to elucidate the mechanism underlying neuronal cell death following sustained epileptiform activity. However, the high oxygen tension of 95% widely used in this model is capable of inducing neuronal cell death by itself. Here we demonstrate that even under normoxic conditions 1h of epileptiform activity induced neuronal cell death as assessed by Propidium Iodide uptake. We conclude that hyperoxia is not essential for status epilepticus induced neuronal cell death in this model.

Coupling of neuronal activity and mitochondrial metabolism as revealed by NAD(P)H fluorescence signals in organotypic hippocampal slice cultures of the rat

During physiological activity neurons may experience localised energy demands which require intracellular signals for stimulation of mitochondrial NADH generation and subsequent delivery of ATP. To elucidate these mechanisms, we applied microfluorimetric monitoring of cytoplasmic (Fluo-3) and mitochondrial (Rhod-2) calcium concentration ([Ca(2+)](c), [Ca(2+)](m)), as well as of mitochondrial oxidative metabolism (NAD(P)H), whilst simultaneously measuring changes in extracellular potassium concentration ([K(+)](o)), as an indicator of neuronal activity in hippocampal slice cultures. Changes in neuronal activity were induced by repetitive stimulation at different frequencies (5, 20, 100 Hz) and intensities. Stimulation parameters were chosen to elicit rises in [K(+)](o) of less than 3 mM which is comparable to physiologically occurring rises in [K(+)](o). The mitochondrial uncoupler carbonyl cyanide m-chlorophenyl hydrazone (CCCP) reduced stimulus-induced changes in Rhod-2 fluorescence by 79%, indicating that Rhod-2 signals were primarily of mitochondrial origin. Repetitive stimulation at 20 Hz applied to areas CA1 or CA3 resulted in moderate rises in [K(+)](o) which were associated with stimulus-dependent elevations in [Ca(2+)](c) and [Ca(2+)](m) of up to 15%. The same stimuli also elicited biphasic changes in NAD(P)H fluorescence characterised by an initial decline and a subsequent prolonged elevation of up to 10%. Variation of stimulus parameters revealed close correlations between rises in [K(+)](o), in [Ca(2+)](m) and changes in NAD(P)H fluorescence. To elucidate the role of intracellular Ca(2+) accumulation in induction of NAD(P)H fluorescence signals, the effect of application of Ca(2+)-free solution on these signals evoked by repetitive antidromic stimulation of the alveus during recordings in area CA1 was studied. Lowering extracellular Ca(2+) led to complete blockade of postsynaptic field potential components as well as of rises in [Ca(2+)](c) and [Ca(2+)](m). Amplitudes of NAD(P)H signals were reduced by 59%, though rises in [K(+)](o) were comparable in presence and absence of extracellular Ca(2+). The results suggest i) that mitochondrial oxidative metabolism is fine-tuned to graded physiological activity in neurons and ii) that rapid mitochondrial Ca(2+) signalling represents one of the main determinants for stimulation of oxidative metabolism under physiological conditions.

Free radical-mediated cell damage after experimental status epilepticus in hippocampal slice cultures

Generation of free radicals may have a key role in the nerve cell damage induced by prolonged or frequently recurring convulsions (status epilepticus). Mitochondrial function may also be altered due to production of free radicals during seizures. We therefore studied changes in field potentials (fp) together with measurements of extracellular, intracellular, and intramitochondrial calcium concentration ([Ca(2+)]e, [Ca(2+)]i, and [Ca(2+)]m, respectively), mitochondrial membrane potential (deltapsi), NAD(P)H auto-fluorescence, and dihydroethidium (HEt) fluorescence in hippocampal slice cultures by means of simultaneous electrophysiological and microfluorimetric measurements. As reported previously, each seizure-like event (SLE) resulted in mitochondrial depolarization associated with a delayed rise in oxidation of HEt to ethidum, presumably indicating ROS production. We show here that repeated SLEs led to a decline in intracellular and intramitochondrial Ca(2+) signals despite unaltered Ca(2+) influx. Also, mitochondrial depolarization and the NAD(P)H signal became smaller during recurring SLEs. By contrast, the ethidium fluorescence rises remained constant or even increased from SLE to SLE. After about 15 SLEs, activity changed to continuous afterdischarges with steady depolarization of mitochondrial membranes. Staining with a cell death marker, propidium iodide, indicated widespread cell damage after 2 h of recurring SLEs. The free radical scavenger, alpha-tocopherol, protected the slice cultures against this damage and also reduced the ongoing impairment of NAD(P)H production. These findings suggest involvement of reactive oxygen species (ROS) of mitochondrial origin in the epileptic cell damage and that free radical scavenging may prevent status epilepticus-induced cell loss.

Acute neuronal injury after hypoxia is influenced by the reoxygenation mode in juvenile hippocampal slice cultures

In neonates asphyxia is usually followed by hyperoxia due to resuscitation procedures. In order to study whether hyperoxic reoxygenation might cause additional cell injury we subjected organotypic hippocampal slice cultures of juvenile rats to normoxic or hyperoxic reoxygenation (19 or 85% oxygen, respectively) following hypoxia (3% oxygen) for 30, 60, and 120 min. Cell injury was quantified by lactate dehydrogenase (LDH) release and propidium iodide (PI) fluorescence 1 h after end of the reoxygenation period. In both experimental groups, LDH activity was significantly enhanced by hypoxia as compared to normoxic controls. However, hyperoxic reoxygenation caused a larger increase in LDH activity than normoxic reoxygenation (e.g., by a factor of 1.60 vs. 1.29 following 120 min hypoxia). PI fluorescence increased after hypoxia in all principal cell layers of the hippocampus but again showed a larger enhancement after hyperoxic reoxygenation as compared to normoxic reoxygenation (e.g., by a factor of 3.9 vs. 1.7 for CA1 and 120 min of hypoxia). After normoxic reoxygenation, PI fluorescence intensity was lower in the dentate gyrus as compared to CA1 and CA3, while it reached similar values like CA1 after high oxygen supply. In conclusion, juvenile hippocampal slice cultures subjected to hyperoxic reoxygenation display a greater amount of acute neuronal injury than slice cultures undergoing normoxic reoxygenation.