Anoxic terminal negative DC-shift in human neocortical slices in vitro

Detection of electrophysiological indicators of neurotoxicity in human and rat brain slices by a three-dimensional microelectrode array

Electrophysiological techniques for the assessment of in vitro neurotoxicology have several advantages over other currently-used methods (for example, morphological techniques), including the ability to detect damage at a very early stage. Novel recording techniques based on microelectrode arrays are available, and could improve recording power. In this study, we investigated how a three-dimensional microelectrode array detects the electrophysiological endpoints of neurotoxicity. We conclude that electrophysiology sensitively reveals neurotoxic actions, and that three-dimensional microelectrode arrays could be proposed for use in neurotoxicology as recording tools that allow easy and sensitive multisite recording, from both rodent and human brain tissue.

Methodological approaches to exploring epileptic disorders in the human brain in vitro

Brain surgery, and in particular epilepsy surgery, offers the unique opportunity to study viable human central nervous tissue in vitro. This does not only open a window to address the basic mechanisms underlying human disease, such as epilepsy, but it allows to venture into investigating neurophysiological functions per se. In the present paper, we describe the most commonly used methods in the electrophysiological (and, at least to some extent, also histochemical and molecular) analysis of human tissue in vitro. In addition, we consider the pitfalls and limitations of such studies, in particular regarding the issue of tissue sampling procedures and control experiments.

Electrophysiology in ischemic neocortical brain slices: species differences vs. influences of anaesthesia and preparation

Ischemia models are indispensable for the evaluation of measures to be clinically applied to brain trauma or stroke patients. Slice models provide good control over experimental parameters and allow for comparative examinations of human and animal brain tissue. Experimental tissue, however, may be altered by anaesthesia, preparatory technique, and, in the case of human tissue, by underlying diseases. These influences on tissue behaviour under ischemia were examined electrophysiologically. Native rat tissue slices were prepared either immediately after decapitation (n = 13), during short ether/barbiturate narcosis (n = 18), or after two hours of inhalation anaesthesia (n = 12) imitating clinical narcosis. Tissue from rats in which generalized amygdala-kindled seizures had been triggered by electric stimulation (n = 10) was prepared according to the decapitation protocol, while human tissue (n = 10) was obtained during epilepsy or tumour surgery. Electrophysiological data (latency and amplitude of anoxic depolarization, recovery of evoked potentials) were recorded during ischemia simulation. Neither details of preparation or anaesthesia nor a history of epileptic fits were associated with significant changes of electrophysiological reactions under ischemia. Human tissue showed a significantly higher ability to uphold transmembrane ion gradients under ischemia. The ability of brain tissue to withstand ischemia is obviously species dependent. For the transfer of experimental results into clinical use it is important that interspecies differences alone can bring about a significant change of tissue behaviour.

Bioelectrical behaviour of hypoxic human neocortical tissue under the influence of nimodipine and dimethyl sulfoxide

Nimodipine and dimethyl sulfoxide (DMSO) have been shown to affect electrophysiological responses in rodent brain tissue in an vitro model of hypoxia. In the present study, the same agents were now examined for their effects on human neocortical brain slices under repeated hypoxic conditions. DMSO (0.4%), with and without addition of nimodipine (40 micromol/l), did not increase the latency of anoxic depolarization (AD). This finding is not in line with our previous observations of DMSO effects, with and without nimodipine, on brain slices of guinea pigs. AD latency was significantly longer in human neocortical brain slices compared with hippocampal slices of rodents even without any pharmacological influence. A possible acute effect of DMSO-nimodipine may therefore be masked by an interspecies difference of hypoxia resistance.

Spontaneous epileptiform activity mediated by GABA(A) receptors and gap junctions in the rat hippocampal slice following long-term exposure to GABA(B) antagonists

Recent evidence suggests that excessive GABA(A) receptor-mediated transmission can lead to neuronal hyperexcitability and hypersynchrony. We show now that exposure of a rat hippocampal slice to GABA(B) receptor antagonists (CGP 55845A and CGP 35348) in the absence of ionotropic glutamatergic transmission leads to a progressive synchronization of spontaneous interneuronal activity. In about 30% of over 200 slices examined, the GABA(A)-mediated spontaneous activity produced field responses in the CA1 soma region with a positive-going phase of up to 5 mV, followed by a long-lasting negative deflection with a simultaneous extracellular K(+) transient. These bicarbonate-dependent GABAergic ictal-like events (GIEs) were associated with biphasic (hyperpolarizing/depolarizing) intracellular responses and with synchronous bursting of the pyramidal neurons. The GIEs could not be reversed by wash-out of the GABA(B) receptor antagonists or by baclofen, but they were inhibited by agonists acting on presynaptic mu-opioid and cannabinoid (CB1) receptors pointing to a down-regulation of presynaptic GABA(B) receptors. GIEs were dependent on intracellular carbonic anhydrase, and potentiated by maneuvers that increase intracellular pH. They were blocked by the Cx36-specific gap-junction (gj) blocker, quinine/quinidine, as well as by the broad-spectrum gj blocker, octanol. These data suggest that enhanced GABAergic activity with functional interneuronal connectivity via gjs is sufficient to trigger epileptiform activity in the absence of ionotropic glutamatergic transmission.

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.

Acute protective effect of nimodipine and dimethyl sulfoxide against hypoxic and ischemic damage in brain slices

Nimodipine and dimethyl sulfoxide (DMSO) were tested (alone and in combination) regarding their ability to increase hypoxic tolerance of brain slices under 'hypoxic' (deprivation of oxygen) or 'ischemic' (hypoxia+withdrawal of glucose) conditions. Direct current (DC) and evoked potentials were recorded in the CA1 region of hippocampal slices of adult guinea pigs. After induction of hypoxia or ischemia, the latency of anoxic terminal negativity (ATN) of the DC potential was determined during superfusion with artificial cerebrospinal fluid alone (aCSF), and during superfusion with aCSF containing DMSO [0.1% (14.1 mmol/l) and 0.4% (56.3 mmol/l)] with the addition of nimodipine (40 micromol/l). Latencies of ATN with first hypoxia were 6.7+/-3.7 min in the control group, 9. 3+/-4.2 min in the 0.4% DMSO group and 12.3+/-5.5 min (P=0.007) in the nimodipine/0.4% DMSO group. Latencies of ATN with first ischemia were 2.9+/-2 min in the control group, 4.1+/-1.6 min in the 0.1% DMSO group, 7.1+/-3.9 min in the 0.4% DMSO group (P=0.006), 5.3+/-1. 5 min in the nimodipine/0.1% DMSO group and 7.6+/-3 min (P<0.001) in the nimodipine/0.4% DMSO group. DMSO (0.4%), either alone or in combination with nimodipine, increase the latency of the ATN after acute onset of hypoxia and ischemia.

Dimethyl sulfoxide increases latency of anoxic terminal negativity in hippocampal slices of guinea pig in vitro

Dimethyl sulfoxide (DMSO), which is widely used as a solvent for a variety of drugs, was used in the present study to investigate its ability to increase the hypoxic tolerance of brain tissue in vitro. DC-potentials and evoked potentials (EP, Schaffer collateral stimulation) were recorded in the CA1 region of hippocampal slices from adult guinea pigs. The latencies of the negative DC-potential shift (anoxic terminal negativity, ATN) after onset of hypoxia (95% N2, 5% CO2) were determined during superfusion with artificial cerebrospinal fluid (aCSF) or DMSO 0.4% dissolved in aCSF, respectively. The latencies of ATN were increased by DMSO application from 7.5+/-0.9 min (mean +/- SEM) under control conditions (n = 38) to 11.1+/-1.3 min with DMSO (n = 22, P < 0.01). These results demonstrate a neuroprotective effect of DMSO.

Optical monitoring of PO2 changes and simultaneous recording of bioelectric activity in human and animal brain slices

For investigations of hypoxic effects in nervous tissue, brain slices are often used as a model system. This provides the advantage that parameters of the micromilieu, e.g. pH and temperature can easily be controlled and measurements of different data, e.g. bioelectric potentials, ion activities etc. can be performed. It is of special importance that the PO2 the slice preparation is exposed to is equally controlled under these conditions. Therefore, a PO2 monitoring system is needed which provides representative values for the tissue environment. This requirement is fulfilled by an optical PO2 sensing method based on phosphorescence quenching as a function of PO2. Here, the application of this method as adapted for use in in vitro models is described and compared to the polarographic oxygen-sensing method. Both the optical and polarographic methods are comparable regarding accuracy and response time of measurements. Furthermore, both the optical method and electrophysiological measurements can be combined. Lastly, under experimental conditions, neither the phosphorescent dye Palladium-meso-tetra-4-carboxyphenyl-porphine nor the illumination necessary for excitation of the dye influence bioelectric activity of neuronal tissue in vitro. In conclusion, the optical PO2 sensing method presented here provides a tool for reliable and continuous monitoring of PO2 in the immediate environment of brain slice preparations.

Inhibition of major cationic inward currents prevents spreading depression-like hypoxic depolarization in rat hippocampal tissue slices

Hypoxia-induced spreading depression-like depolarization (hypoxic SD, or anoxic depolarization) is accompanied by the near-loss of membrane potential, severe reduction of membrane resistance, and influx of Na+, Ca2+, Cl- and water into neurons. The biophysical nature of these membrane changes is incompletely understood. In the present study we applied a pharmacological mixture (10 microM DNQX, 10 microM CPP, 1 microM TTX, and 2 mM Ni2+) to rat hippocampal tissue slices to inhibit major Na+ and Ca2+ inward currents. This inhibitory cocktail slightly depolarized CA1 pyramidal neurons and completely blocked all evoked potentials. In its presence severe hypoxia of up to 20 min duration failed to induce hypoxic SD and the accompanying intrinsic optical signal. Instead, only moderate, very slow negative shifts of the extracellular DC potential were observed. Following 10 min hypoxia and 1 hour wash-out of the inhibitors antidromic and orthodromic responses were still blocked but hypoxic SD with markedly delayed onset could be induced in most slices. In current-clamped CA1 pyramidal cells hypoxia induced a rapid, near-complete depolarization and decreased the input resistance by 89%. In the presence of the cocktail, however, hypoxia caused a gradual, partial depolarization, to about -40 mV; the membrane resistance decreased by only 37%. We conclude that simultaneous blockade of the known major Na+ and Ca2+ channels consistently prevents hypoxic SD. The hypothesis that SD initiation is the consequence of general loss of selective permeability or general membrane breakdown becomes unlikely. Instead, influx of Na+ and Ca2+ might play a crucial role in the generation of the rapid SD-like depolarization.

Flat and steep terminal negativity in the DC-potential after deprivation of oxygen and glucose in human neocortical slices

The so-called terminal negativity (TN) of the DC-potential is a characteristic reaction of neuronal tissue to hypoxia or ischemia. In a previous study on human neocortical slices, two types of TN with flat and steep slopes of rise (< or >10 mV/min) were found with hypoxia. The aim of the present study was to further investigate causes underlying the occurrence of flat and steep TN. Experiments were performed on 23 human neocortical slices (500 micron) resected from 13 patients (epilepsy and tumour surgery). DC-potential and evoked potentials (white matter stimulation) were recorded in layer III. The extracellular potassium concentration ([K+]o) was measured by K+-sensitive microelectrodes. In an interface type chamber, ischemic episodes were induced by oxygen and glucose deprivation. They were terminated when TN had peaked. Both flat and steep TN also existed with ischemic conditions. There was a linear correlation between the slope of rise of TN and the associated slope of rise in [K+]o, respectively, but none regarding latencies of TN or recovery of evoked potentials. Peak levels in [K+]o were 13.9+/-0.9 mmol/l. Compared to control, the slope of rise and latency of TN were clearly increased by addition of dimethyl sulfoxide (DMSO, 0.4%) to the bath solution, whereas nimodipine (40 micromol/l) in 0.4% DMSO had neither an effect on slope of rise of TN nor on latency of TN. As a whole, our observations suggest, that the actual metabolic state determines the occurrence of flat or steep TN.

Neuroprotection of mild hypothermia: differential effects

To estimate whether mild hypothermia during repetitive hypoxia provides a neuroprotective effect on brain tissue, hippocampal slice preparations were subjected to repetitive hypoxic episodes under different temperature conditions. Slices of guinea pig hippocampus (n=40) were placed at the interface of artificial cerebrospinal fluid (aCSF) and gas (normoxia: 95% O2, 5% CO2; hypoxia: 95% N2, 5% CO2). Evoked potentials (EP) and direct current (DC) potentials were recorded from hippocampal CA1 region. Slices were subjected to two repetitive hypoxic episodes under the following temperature conditions: (A) 34 degrees C/34 degrees C, (B) 30 degrees C/30 degrees C and (C) 34 degrees C/30 degrees C. Hypoxic phases lasted until an anoxic terminal negativity (ATN) occurred. The recovery after first hypoxia lasted 30 min. Tissue function was assessed regarding the latency of ATN and the recovery of evoked potentials. The ATN latencies with protocol A (n = 25) for the first and second hypoxia were 5.9+/-1.3 min (mean+/-S.E.M., 1st hypoxia) and 2.4+/-0.9 min (2nd hypoxia), with protocol B the latencies (n = 7) were significantly longer: 25.2+/-7.1 min and 15.6+/-7.7 min. With protocol C (n=8), the latencies were 5.6+/-1.8 and 3.3+/-0.5 min. No differences were seen in the recovery of the EPs with protocols A-C. Our results suggest that a mild hypothermia is only neuroprotective if applied from an initial hypoxia onwards.