Na(+) and K(+) concentrations, extra- and intracellular voltages, and the effect of TTX in hypoxic rat hippocampal slices

Reactive oxygen species mediate the neuroprotection conferred by a mitochondrial ATP-sensitive potassium channel opener during ischemia in the rat hippocampal slice

Reactive oxygen species (ROS) are known to mediate the protection conferred by the opening of mitochondrial ATP-sensitive potassium channels (mitoK(ATP)) during ischemia in heart, but this has not been demonstrated in brain. The present study examined whether ROS mediate the neuroprotection conferred by a mitoK(ATP) opener during ischemia in rat hippocampal slices. Ischemia was simulated by oxygen and glucose deprivation. The direct current potential and population spike were recorded in the stratum pyramidale of the CA1 region, and lactate dehydrogenase (LDH) efflux into the medium was assayed. ROS generation was measured spectrophotofluorometrically. Pretreatment of slices with diazoxide (DIA, 300 microM), a mitoK(ATP) opener, (i) prolonged the latency to ischemic depolarization and decreased its amplitude, (ii) delayed the onset of population spike disappearance and enhanced its recovery after reperfusion, (iii) decreased LDH efflux and (iv) increased ROS levels. The effects induced by DIA were attenuated by 5-hydroxydecanoic acid (200 microM), a mitoK(ATP) blocker. Pretreatment with N-2-mercaptopropionyl glycine (MPG, 500 microM), a ROS scavenger, also abrogated the effects induced by DIA, while treatment with MPG alone had no effect during normoxia and ischemia. These results indicate that ROS participate in the neuroprotection conferred by a mitoK(ATP) opener during ischemia.

Sodium influx pathways during and after anoxia in rat hippocampal neurons

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

Sulfhydryl oxidation reduces hippocampal susceptibility to hypoxia-induced spreading depression by activating BK channels

The cytosolic redox status modulates ion channels and receptors by oxidizing/reducing their sulfhydryl (SH) groups. We therefore analyzed to what degree SH modulation affects hippocampal susceptibility to hypoxia. In rat hippocampal slices, severe hypoxia caused a massive depolarization of CA1 neurons and a negative shift of the extracellular DC potential, the characteristic sign of hypoxia-induced spreading depression (HSD). Oxidizing SH groups by 5,5'-dithiobis 2-nitrobenzoic acid (DTNB, 2 mM) postponed HSD by 30%, whereas their reduction by 1,4-dithio-dl-threitol (DTT, 2 mM) or alkylation by N-ethylmaleimide (500 microM) hastened HSD onset. The DTNB-induced postponement of HSD was not affected by tolbutamide (200 microM), dl-2-amino-5-phosphonovaleric acid (150 microM), or 6-cyano-7-nitroquinoxaline-2,3-dione (25 microM). It was abolished, however, by Ni2+ (2 mM), withdrawal of extracellular Ca2+, charybdotoxin (25 nM), and iberiotoxin (50 nM). In CA1 neurons DTNB induced a moderate hyperpolarization, blocked spontaneous spike discharges and postponed the massive hypoxic depolarization. DTT induced burst firing, depolarized glial cells, and hastened the onset of the massive hypoxic depolarization. Schaffer-collateral/CA1 synapses were blocked by DTT but not by DTNB; axonal conduction remained intact. Mitochondria did not markedly respond to DTNB or DTT. While the targets of DTT are less clear, the postponement of HSD by DTNB indicates that sulfhydryl oxidation increases the tolerance of hippocampal tissue slices against hypoxia. We identified as the underlying mechanism the activation of BK channels in a Ca(2+)-sensitive manner. Accordingly, ionic disregulation and the loss of membrane potential occur later or might even be prevented during short-term insults. Therefore well-directed oxidation of SH groups could mediate neuroprotection.

Heat Stress–Mediated Plasticity in a Locust Looming-Sensitive Visual Interneuron

Neural circuits are strongly affected by temperature and failure ensues at extremes. However, detrimental effects of high temperature on neural pathways can be mitigated by prior exposure to high, but sublethal temperatures (heat shock). Using the migratory locust, Locusta migratoria, we investigated the effects of heat shock on the thermosensitivity of a visual interneuron [the descending contralateral movement detector (DCMD)]. Activity in the DCMD was elicited using a looming stimulus and the response was recorded from the axon using intracellular and extracellular methods. The thoracic region was perfused with temperature-controlled saline and measurements were taken at 5° intervals starting at 25°C. Activity in DCMD was decreased in control animals with increased temperature, whereas heat-shocked animals had a potentiated response such that the peak firing frequency was increased. Significant differences were also found in the thermosensitivity of the action potential properties between control and heat-shocked animals. Heat shock also had a potentiating effect on the amplitude of the afterdepolarization. The concurrent increase in peak firing frequency and maintenance of action potential properties after heat shock could enhance the reliability with which DCMD initiates visually guided behaviors at high temperature.

Longitudinal depolarization gradients along the somatodendritic axis of CA1 pyramidal cells: a novel feature of spreading depression

We studied the subcellular correlates of spreading depression (SD) in the CA1 rat hippocampus by combining intrasomatic and intradendritic recordings of pyramidal cells with extracellular DC and evoked field and unitary activity. The results demonstrate that during SD only specific parts of the dendritic membranes are deeply depolarized and electrically shunted. Somatic impalements yielded near-zero membrane potential (V(m)) and maximum decrease of input resistance (R(in)) whether the accompanying extracellular negative potential (V(o)) moved along the basal, the apical or both dendritic arbors. However, apical intradendritic recordings showed a different course of local V(m) that is hardly detected from the soma. A decreasing depolarization gradient was observed from the edge of SD-affected fully depolarized subcellular regions toward distal dendrites. Within apical dendrites, the depolarizing front moved toward and stopped at proximal dendrites during the time course of SD so that distal dendrites had repolarized in part or in full by the end of the wave. The drop of local R(in) was initially maximal at any somatodendritic loci and also recovered partially before the end of SD. This recovery was stronger and faster in far dendrites and is best explained by a wave-like somatopetal closure of membrane conductances. Cell subregions far from SD-affected membranes remain electrically excitable and show evoked unitary and field activity. We propose that neuronal depolarization during SD is caused by current flow through extended but discrete patches of shunted membranes driven by uneven longitudinal depolarization.

Depolarisation phenomena in traumatic and ischaemic brain injury

1. Cortical spreading depression is a non-physiological global depolarisation of neurones and astrocytes that can be initiated with varying degrees of difficulty in the normally perfused cerebral cortex in the experimental laboratory. Induction is typically with electrical stimulation, needling of the cerebral cortex, or superfusion of isotonic or more concentrated potassium chloride solution. The phenomenon propagates across the cerebral cortex at a rate of 2-5 mm per minute, and is accompanied by marked but transient increases in cerebral blood flow, in local tissue oxygen tension, and most probably in metabolic rate. 2. Peri-infarct depolarisation is also a depolarisation event affecting neurones and glia, with an electrophysiological basis similar or identical to CSD, but occurring spontaneously in the ischaemic penumbra or boundary zone in focal cerebral cortical ischaemia. Most such events arise from the edge of the ischaemic core, and propagate throughout the penumbra, at a rate similar to that of cortical spreading depression. 3. Cortical spreading depression in the normally perfused cortex does not result in histological damage whereas peri-infarct depolarisations augment neuronal damage in the penumbra, and are believed by many authors to constitute an important, or the principal, mechanism by which electrophysiological penumbra progressively deteriorates, ultimately undergoing terminal depolarisation and thus recruitment into an expanded core lesion. 4. There is some experimental evidence to suggest that under some circumstances induction of episodes of cortical spreading depression can confer protection against subsequent ischaemic insults. 5. Although cortical spreading depression and peri-infarct depolarisations have been extensively studied in the experimental in vivo models, there is now clear evidence that depolarisations also occur and propagate in the human brain in areas surrounding a focus of traumatic contusion. 6. Whether such events in the injured human brain represent cortical spreading depression or peri-infarct depolarisation is unclear. However, invasive and probably non-invasive monitoring methods are available which may serve to distinguish which event has occurred. 7. Much further work will be needed to examine the relationship of depolarisation events in the injured brain with outcome from cerebral ischaemia or head injury, to examine the factors which influence the frequency of depolarisation events, and to determine which depolarisation events in the human brain augment the injury and should be prevented.

Background sodium current stabilizes bursting in respiratory pacemaker neurons

Endogenous pacemaker properties have been proposed to generate rhythmic activity underlying many behaviors including respiration. For pacemakers to generate regenerative bursting, background currents maintain their membrane potential (Vm) within a range where bi-stable properties are expressed, thereby stabilizing rhythmogenesis. We previously found that the baseline Vm of respiratory pacemakers is stabilized against hyperpolarizing shifts in their Vm. In response to prolonged hyperpolarizing current injection synaptically isolated respiratory pacemakers steadily depolarize and resume bursting, suggesting a stabilizing background current is involved. What is the ionic basis of this background current in respiratory pacemakers? Here we demonstrate that in low-[Na(+)](o) ACSF, synaptically isolated respiratory pacemakers hyperpolarized and remained outside the bursting window, but could burst upon depolarizing current injection. These data suggest that pacemakers possess a background sodium current that is necessary to bring their Vm into a bursting range. Low-[Na(+)](o) ACSF also abolished the depolarizing shift evoked during prolonged hyperpolarizing current injection, and bursting did not resume. This depolarizing shift persisted in the presence of I(h)-current blockers, but was abolished in tetrodotoxin. Although, under control conditions, the Vm of synaptically isolated respiratory pacemaker neurons was not significantly affected when [K(+)](o) was changed from 3 to 8 mM, the Vm is altered when [K(+)](o) was raised in low-[Na(+)](o) ACSF. Thus, current-clamp studies suggest that respiratory pacemaker neurons possess a background sodium current that maintains their membrane potential within a range where they express bursting, thereby stabilizing rhythmogenesis.

Influence of Monovalent Cation Identity on Parvalbumin Divalent Ion-Binding Properties†

Rat alpha- and beta-parvalbumins have distinct monovalent cation-binding properties [Henzl et al. (2000) Biochemistry 39, 5859-5867]. Beta binds two Na(+) or one K(+), and alpha binds one Na(+) and no K(+). Ca(2+) abolishes these binding events, suggesting that the monovalent ions occupy the EF-hand motifs. This study compares alpha and beta divalent ion affinities in Na(+) and K(+) solutions. Solvent cation identity seriously affects alpha. In Hepes-buffered NaCl, at 5 degrees C, the macroscopic Ca(2+)-binding constants are 2.6 x 10(8) and 6.4 x 10(7) M(-1) and the Mg(2+) constants, 1.8 x 10(4) and 4.3 x 10(3) M(-1). In Hepes-buffered KCl, the Ca(2+) values increase to 2.9 x 10(9) and 6.6 x 10(8) M(-1) and the Mg(2+) values to 2.2 x 10(5) and 3.7 x 10(4) M(-1). Monte Carlo simulation of alpha binding data-employing site-specific constants and explicitly considering Na(+) binding-yields a K(Na) of 630 M(-1) and indicates that divalent ion-binding is positively cooperative. NMR data suggest that the lone Na(+) ion occupies the CD loop. Solvent cation identity has a smaller impact on beta. In Na(+), the Ca(2+) constants for the EF and CD sites are 2.3 x 10(7) and 1.5 x 10(6) M(-1), respectively; the Mg(2+) constants are 9.2 x 10(3) and 1.7 x 10(2) M(-1). In K(+), these values shift to 3.1 x 10(7) and 3.8 x 10(6) M(-1) and the latter to 1.4 x 10(4) and 2.9 x 10(2) M(-1). These data suggest that parvalbumin divalent ion affinity, particularly that of rat alpha, can be significantly attenuated by increased intracellular Na(+) levels.

ET-1 induces cortical spreading depression via activation of the ETA receptor/phospholipase C pathway in vivo

Recently, it has been shown that brain topical superfusion of endothelin (ET)-1 at concentrations around 100 nM induces repetitive cortical spreading depressions (CSDs) in vivo. It has remained unclear whether this effect of ET-1 is related to a primary neuronal/astroglial effect, such as an increase in neuronal excitability or induction of interastroglial calcium waves, or a penumbra-like condition after vasoconstriction. In vitro, ET-1 regulates interastroglial communication via combined activation of ET(A) and ET(B) receptors, whereas it induces vasoconstriction via single activation of ET(A) receptors. We have determined the ET receptor profile and intracellular signaling pathway of ET-1-induced CSDs in vivo. In contrast to the ET(B) receptor antagonist BQ-788 and concentration dependently, the ET(A) receptor antagonist BQ-123 completely blocked the occurrence of ET-1-induced CSDs. The ET(B) receptor antagonist did not increase the efficacy of the ET(A) receptor antagonist. Direct stimulation of ET(B) receptors with the selective ET(B) agonist BQ-3020 did not trigger CSDs. The phospholipase C (PLC) antagonist U-73122 inhibited CSD occurrence in contrast to the protein kinase C inhibitor Gö-6983. Our findings indicate that ET-1 induces CSDs through ET(A) receptor and PLC activation. We conclude that the induction of interastroglial calcium waves is unlikely the primary cause of ET-1-induced CSDs. On the basis of the receptor profile, likely primary targets of ET-1 mediating CSD are either neurons or vascular smooth muscle cells.

Differential sensitivity to induction of spreading depression by partial disinhibition in chronically epileptic human and rat as compared to native rat neocortical tissue

Spreading depression (SD) is characterized by a transient breakdown of neuronal function concomitant with a massive failure of ion homeostasis. It is a phenomenon that can be induced in neocortical tissue by raising excitability, e.g. injection of K(+), application of glutamatergic agonists, or blocking Na(+)/K(+) ATPase. Here we report a novel method of SD induction using minimal disinhibition with application of low concentrations (5 microM) of the GABA(A) receptor blocker bicuculline. This procedure-while subthreshold for epileptiform activity-readily induced spontaneous SDs in native rat neocortical slices, accompanied by typical depolarizations of neurons and glial cells. In contrast, in human neocortical preparations obtained from epilepsy surgery, in approximately 20% of the slices spontaneous epileptiform activity appeared with this bicuculline dosage without SDs. Raising the concentration of bicuculline to an epileptogenic dose (10 microM) in human tissue also resulted in the generation of epileptiform activity only. Likewise, in slices from pilocarpine-treated, chronically epileptic rats, bicuculline also only induced epileptiform activity without eliciting SDs. The experiments indicate that chronic epilepsy causes a differential sensitivity to partial GABA(A) receptor blockade with regard to induction of SD.

Stabilization of bursting in respiratory pacemaker neurons

Synaptic and endogenous pacemaker properties have been hypothesized as principal cellular mechanisms for respiratory rhythm generation. This rhythmic activity is thought to originate in the pre-Bötzinger complex, an area that can generate fictive respiration when isolated in brainstem slice preparations of mice. In slice preparations, external potassium concentration ([K+]o) is typically elevated from 3 to 8 mm to induce rhythmic population activity. However, elevated [K+](o) may not simply depolarize respiratory neurons but also change rhythm-generating mechanisms by inducing or altering pacemaker properties. To test this, we examined the membrane potential (V(m)) of nonpacemaker neurons and endogenous bursting properties of pacemaker neurons before and after blockade of excitatory and inhibitory synaptic input in 3 mm [K+]o artificial CSF (aCSF). Most pacemaker neurons (82%) ceased to burst in 3 mm [K+]o aCSF after blockade of glutamatergic transmission. In all of these, endogenous bursting was restored on additional blockade of glycinergic and GABAergic inhibition. Thus, bursting properties are suppressed by endogenous synaptic inhibition, the level of which may determine whether network rhythmicity is generated in 3 mm (n = 12) or 8 mm (n = 40) [K+]o aCSF. In 3 mm [K+]o aCSF, synaptically isolated pacemaker neurons (n = 22) continued to burst over a wide range of imposed V(m). Furthermore, the V(m) of synaptically isolated pacemaker neurons was not significantly affected (p = 0.1; n = 10) when [K+]o was changed from 8 to 3 mm, whereas isolated nonpacemakers hyperpolarized (p < 0.001; n = 14). We conclude that respiratory pacemaker neurons possess membrane properties that stabilize their bursting against changes in [K+]o and imposed changes in V(m).

Dynamic recording of cell death in the in vitro dorsal vagal nucleus of rats in response to metabolic arrest

Anoxic/ischemic neuronal death is usually assessed in cell cultures or in vivo within a time window of 24 h to several days using the nucleic acid stain propidium iodide or histological techniques. Accordingly, there is limited information on the time course of such neuronal death. We loaded acute rat brain stem slices with propidium iodide for dynamic fluorometric recording of metabolic arrest-related cell death in the dorsal vagal nucleus. This model was chosen because dorsal vagal neurons show a graded response to metabolic inhibition: anoxia and aglycemia cause a sustained hyperpolarization, whereas ischemia induces a glutamate-mediated, irreversible depolarization. We found that the number of propidium iodide-labeled cells increased from 27% to 43% of total cell count within 1-7 h after preparation of slices. Compared with these untreated control slices, cyanide-induced anoxia (30 min) or aglycemia (1 h) did not cause further cell death, whereas 3-h aglycemia destroyed an additional 13% of cells. Ischemia (1 h) due to cyanide plus iodoacetate immediately labeled an additional 20% of cells, and an additional 48% of cells were destroyed within the following 3 h of postischemia. Continuous recording of propidium iodide fluorescence showed that loss of membrane integrity started within 25 min after onset of the ischemic depolarization and the concomitant intracellular Ca(2+) rise. The results show that propidium iodide can be used to monitor cell death in acute brain slices. Our findings suggest that pronounced cell death occurs within a period of 1-4 h after onset of metabolic arrest and is apparently due to necrotic/oncotic mechanisms.

Conditions for the triggering of spreading depression studied with computer simulations

In spite of five decades of study, the biophysics of spreading depression (SD) is incompletely understood. Earlier we have modeled seizures and SD, and we have shown that currents through ion channels normally present in neuron membranes can generate SD-like depolarization. In the present study, we define the conditions for triggering SD and the parameters that influence its course in a model of a hippocampal pyramidal cell with more complete representation of ions and channels than the previous version. "Leak" conductances for Na(+), K(+), and Cl(-) and an ion pump were present in the membrane of the entire cell; fast inactivating voltage dependent conductances for sodium and potassium in the soma; "persistent" conductances in soma and apical dendrite, and K(+)- and voltage-dependent N-methyl-D-aspartate (NMDA)-controlled conductance in the apical dendrite. The neuron was surrounded by restricted interstitial space and by a "glia-endothelium" system of extracellular ion regulation bounded by a membrane having leak conductances and an ion pump. Ion fluxes and concentration changes were continuously computed as well as osmotic cell volume changes. As long as reuptake into the neuron and "buffering" by glia kept pace with K(+) released from the neuron, stimulating current applied to the soma evoked repetitive firing that stopped when stimulation ceased. When glial uptake was reduced, K(+) released from neurons could accumulate in the interstitium and keep the neuron depolarized so that strong depolarizing pulses injected into the soma were followed either by afterdischarge or SD. SD-like depolarization was ignited when depolarization spreading into the apical dendrite, activated persistent Na(+) current and NMDA-controlled current. With membrane parameters constant, varying the injected stimulating current influenced SD onset but neither the depolarization nor the increase in extracellular K(+). Glial "leak" conductance influenced SD duration and SD ignition point. Varying maximal conductances (representing channel density) also influenced SD onset time but not the amplitude of the depolarization. Hypoxia was simulated by turning off the Na-K exchange pump, and this resulted in SD-like depolarization. The results confirm that, once ignited, SD runs an all-or-none trajectory, the level of depolarization is governed by feedback involving ion shifts and glutamate acting on ion channels and not by the number of channels open, and SD is ignited if the net persistent membrane current in the apical dendrites turns inward.

Depolarization-induced 65zinc influx into cultured cortical neurons

Toxic Zn(2+) influx may be a key mechanism underlying selective neuronal death after transient global ischemia in rats. To identify routes responsible for neuronal Zn(2+) influx, we measured the accumulation of (65)Zn(2+) into cultured murine cortical cells under depolarizing conditions (60 mM K(+)) associated with severe hypoxia-ischemia in brain tissue. Addition of 60 mM K(+) or 300 microM kainate substantially increased (65)Zn(2+) accumulation into mixed cultures of neurons and glia, but not glia alone. (65)Zn(2+) accumulation was attenuated by increasing concentrations of extracellular Ca(2+) or trypsin pretreatment, but not by late trypsinization, and corresponded to an increase in atomic Zn(2+). Confirming predominantly neuronal entry, K(+)-induced (65)Zn(2+) accumulation was reduced by prior selective destruction of neurons with NMDA. K(+)-induced (65)Zn(2+) influx was not sensitive to glutamate receptor antagonists, but was attenuated by Gd(3+) and Cd(2+) as well as 1 microM nimodipine; it was partially sensitive to 1 microM omega-conotoxin-GVIA, and insensitive to 1 microM omega-agatoxin-IVA. K(+)-induced, Gd(3+)-sensitive (45)Ca(2+) accumulation but not (65)Zn(2+) accumulation was sharply attenuated by lowering extracellular pH to 6.6.

Delayed anoxic depolarizations in hippocampal neurons of mice lacking the excitatory amino acid carrier 1

Hypoxia leads to a rapid increase in vesicular release of glutamate. In addition, hypoxic glutamate release might be caused by reversed operation of neuronal glutamate transporters. An increase in extracellular glutamate concentration might be an important factor in generating anoxic depolarizations (AD) and subsequent neuronal damage. To study the AD and the vesicular release in hippocampal slices from CD1 wild-type mice and mice in which the neuronal glutamate transporter excitatory amino acid carrier 1 (EAAC1) had been knocked out, the authors performed recordings of field potentials and patch clamp recordings of CA1 pyramidal cells. Latency to anoxic depolarizations was enhanced in EAAC1-/- mice, whereas the hypoxia-induced increase in miniature excitatory postsynaptic current frequency occurred with similarly short latencies and to a similar extent in control and mutated animals. Additional block of glial glutamate uptake with TBOA (dl-threo-beta-benzyloxyaspartate), a nontransportable and potent inhibitor, dramatically reduced the latency to onset of AD and abolished the difference between wild-type mice and EAAC1-/- mice. The authors conclude that the neuronal glutamate transporter greatly influences the latency to generation of AD. Because ADs are not prevented in EAAC1-deficient mice, vesicular release mechanisms also seem to be involved. They become prominent when glial glutamate transport is blocked.

ATP-independent anoxic activation of ATP-sensitive K+ channels in dorsal vagal neurons of juvenile mice in situ

The role of ATP in anoxic activation of ATP-sensitive K+ (KATP) channels was studied in dorsal vagal neurons of mouse brainstem slices. In the whole-cell configuration, cyanide-induced chemical anoxia evoked within 10 s a 300-pA outward current that gave rise to a hyperpolarization of 24 mV. These responses were mimicked by nitrogen-aerated saline, rotenone or diazoxide and abolished by tolbutamide. The cyanide-induced hyperpolarization was due to activation of 70 pS K(ATP) channels that were half-maximally blocked by 5 microM internal ATP. Dialyzing the cells with either 1, 20 or 0 mM ATP did not, however, affect the time to onset, the kinetics or the magnitude of the cyanide-induced hyperpolarization. Impairment of ATP consumption by ouabain, vanadate or reduced temperature had no effect either. Thus, anoxia-induced activation of these KATP channels cannot be explained by a fall of cellular ATP or a concomitant rise of ADP. Anoxia-related changes of the actin cytoskeleton or the composition of the plasma membrane are also not likely to be involved, as cytochalasin D did not affect the cyanide-evoked hyperpolarization and phosphatidylinositol 4,5-bisphosphate failed to decrease the ATP sensitivity of single KATP channels. Finally, because of a lack of effects of reduced/oxidized glutathione and the oxidase blocker diphenyliodonium on the cyanide-induced hyperpolarization, cellular redox state does not appear to be involved. Our results indicate that despite a high sensitivity to ATP in excised patches, anoxic activation of KATP channels is independent of cellular ATP. Rather the ATP block seems to be removed as a consequence of impaired mitochondrial function.

Endothelin-1 potently induces Leão's cortical spreading depression in vivo in the rat: a model for an endothelial trigger of migrainous aura?

According to the 'neuronal' theory, cortical spreading depression (CSD) is the pathophysiological correlate of migrainous aura. However, the 'vascular' theory has implicated altered vascular function in the induction of aura symptoms. The possibility of a vascular origin of aura symptoms is supported, e.g. by the clinical observation that cerebral angiography frequently provokes migrainous aura. This suggests that endothelial irritation may somehow initiate one of the pathways resulting in migrainous aura. Up to now, an endothelium-derived factor has never been shown to trigger CSD. Here, for the first time, we demonstrate and characterize the ability of the vasoconstrictor and astroglial/neuronal modulator endothelin-1 to trigger Leão's 'spreading depression of activity' in vivo in rats. At a concentration range between 10 nM and 1 microM, endothelin-1 induced changes characteristic of CSD with regard to the rate of propagation, steady (direct current) potential and extracellular K(+)-concentration. A spreading hyperaemia followed by oligaemia was observed similar to those in K(+)-induced CSD. Endothelin-1 did not provoke changes characteristic of a terminal depolarization. The mechanism by which endothelin-1 generated CSD involved the N-methyl-D-asparate receptor. Cerebral blood flow decreased slightly, but significantly, before endothelin-1 generated CSD. A vasodilator (NO*-donor) shifted the threshold for CSD induction to higher concentrations of endothelin-1. Endothelin-1, in contrast to K(+), did not induce CSD in rat brain slices suggesting indirectly that endothelin-1 may require intact perfusion to exert its effects. In conclusion, endothelin-1 was found in the experiment to be the most potent inducer of CSD currently known. We propose endothelin-1 as a possible candidate for the yet enigmatic link between endothelial irritation and migrainous aura.

Tissue oxygen tension and brain sensitivity to hypoxia

Mammalian brain is a highly oxidative organ and although it constitutes only a small fraction of total body weight it accounts for a disproportionately large percentage of bodily oxygen consumption (in humans about 2 and 20%, respectively). Yet, the partial pressure and concentration of oxygen in the brain are low and non-uniform. There is a large number of enzymes that use O(2) as a substrate, the most important of which is cytochrome c oxidase, the key to mitochondrial ATP production. The affinity of cytochrome c oxidase for oxygen is very high, which under normal conditions ensures undiminished activity of oxidative phosphorylation down to very low P(O(2)). By contrast, many other relevant enzymes have K(m) values for oxygen within, or above, the ambient cerebral gas tension, thus making their operations very dependent on oxygen level in the physiological range. Among its multiple, versatile functions, oxygen partial pressure and concentration control production of reactive oxygen species, expression of genes and functions of ion channels. Limitation of oxygen supply to the brain below a 'critical' level reduces, and eventually blocks oxidative phosphorylation, drastically decreases cellular (ATP) and leads to a collapse of ion gradients. Neuronal activity ceases and if oxygen is not re-introduced quickly, cells die. The object of this review is to discuss briefly the central oxygen-dependent processes in mammalian brain and the short-term consequences of O(2) deprivation, but not the mechanisms of long-term adaptation to chronic hypoxia. Particular emphasis is placed on issues which have been the focus of recent attention and/or controversy.

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.

GABA(A) and GABA(B) receptors on neocortical neurons are differentially distributed

The distribution of functional neurotransmitter receptors on the surface of neurons is highly relevant for synaptic transmission and signal processing. To map functional GABA(A) and GABA(B) receptors on the somadendritic membrane of rat neocortical layer V pyramidal neurons we used patch-clamp recording in combination with infrared-guided laser stimulation to release gamma-aminobutyric acid (GABA) photolytically. The data strongly suggest that relatively more GABA(A) receptors are located at the apical dendrite and relatively more GABA(B) receptors near the soma. Such a specific distribution of GABA(A) and GABA(B) receptors may serve to compensate for differences in electrotonic voltage attenuation between GABA(A) and GABA(B) receptor-mediated inhibitory postsynaptic potentials (IPSPs).

Suppression of epileptiform activity by high frequency sinusoidal fields in rat hippocampal slices

1. Sinusoidal high frequency (20-50 Hz) electric fields induced across rat hippocampal slices were found to suppress zero-Ca2+, low-Ca2+, picrotoxin, and high-K+ epileptiform activity for the duration of the stimulus and for up to several minutes following the stimulus. 2. Suppression of spontaneous activity by high frequency stimulation was found to be frequency (< 500 Hz) but not orientation or waveform dependent. 3. Potassium-sensitive microelectrodes showed that block of epileptiform activity was always coincident with a stimulus-induced rise in extracellular potassium concentration during stimulation. Post-stimulus inhibition was always associated with a decrease in extracellular potassium activity below baseline levels. 4. Intracellular recordings and optical imaging with voltage-sensitive dyes showed that during suppression neurons were depolarized yet did not fire action potentials. 5. Direct injection of sinusoidal current into individual pyramidal cells did not result in a tonic depolarization. Injection of large direct current (DC) depolarized neurons and suppressed action potential generation. 6. These findings suggest that high frequency stimulation suppresses epileptiform activity by inducing potassium efflux and depolarization block.

Potassium-induced enhancement of persistent inward current in hippocampal neurons in isolation and in tissue slices

Previous work suggested a role for the voltage-dependent persistent sodium current, I(Na,P), in the generation of seizures and spreading depression (SD). Ordinarily, I(Na,P) is small in hippocampal neurons. We investigated the effect of raising external K(+) concentration, [K(+)](o), on whole-cell persistent inward current in freshly isolated hippocampal CA1 pyramidal neurons. I(Na,P) was identified by TTX-sensitivity and dependence on external Na(+) concentration. When none of the ion channels were blocked, I(Na,P) was not usually detectable, probably because competing K(+) current masked it, but after raising [K(+)](o) I(Na,P) appeared, while K(+) currents diminished. With K(+) channels blocked, I(Na,P) could usually be evoked in control solution and raising [K(+)](o) caused its reversible increase in most cells. The increase did not depend on external calcium [Ca(2+)](o). In CA1 pyramidal neurons in hippocampal slices a TTX-sensitive persistent inward current was always recorded and when [K(+)](o) was raised, it was reversibly enhanced. Strong depolarization evoked irregular current fluctuations, which were also augmented in high [K(+)](o). The findings support a role of potassium-mediated positive feedback in the generation of seizures and spreading depression.

Na(+) dependence and the role of glutamate receptors and Na(+) channels in ion fluxes during hypoxia of rat hippocampal slices

Spreading depression (SD) as well as hypoxia-induced SD-like depolarization in forebrain gray matter are characterized by near complete depolarization of neurons. The biophysical mechanism of the depolarization is not known. Earlier we reported that simultaneous pharmacological blockade of all known major Na(+) and Ca(2+) channels prevents hypoxic SD. We now recorded extracellular voltage, Na(+), and K(+) concentrations and the intracellular potential of individual CA1 pyramidal neurons during hypoxia of rat hippocampal tissue slices after substituting Na(+) in the bath by an impermeant cation, or in the presence of channel blocking drugs applied individually and in combination. Reducing extracellular Na(+) concentration [Na(+)](o) to 90 mM postponed the hypoxia-induced extracellular DC-potential deflection (DeltaV(o)) and reduced its amplitude, and it also postponed the SD-like depolarization of neurons. After lowering [Na(+)](o) to 25 mM, SD-like DeltaV(o) became very small, indicating that an influx of Na(+) is required for SD; influx of Ca(2+) ions alone is not sufficient. We then asked whether the SD-related Na(+) current flows through glutamate-controlled and/or through voltage-gated Na(+) channels. Administration of either the non-N-methyl-D-aspartate (NMDA) receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX), or the NMDA receptor antagonist (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) postponed the hypoxic DeltaV(o) and depressed its amplitude but the effect of the combined administration of these two drugs was not greater than that of either alone. During the early phase of hypoxia, before SD onset, [K(+)](o) increased faster and reached a much higher level in the presence of glutamate antagonists than in their absence. The [K(+)](o) level reached at the height of hypoxic SD was, however, not affected. When TTX was added to DNQX and CPP, SD was prevented in half the trials. When SD did occur, it was greatly delayed, yet eventually neurons depolarized to the same extent as in normal solution. The SD-related sudden drop in [Na(+)](o) was depressed by only 19% in the presence of the three drugs, indicating that Na(+) can flow into cells through pathways other than ionotropic glutamate receptors and TTX-sensitive Na(+) channels. We conclude that, when they are functional, glutamate-receptor-mediated and voltage-gated Na(+) currents are the major generators of the self-regenerative rapid depolarization, but in their absence other pathways can sometimes take their place. The final level of SD-like depolarization is determined by positive feedback and not by the number of channels available. A schematic flow chart of the events generating hypoxic SD is discussed.

Effects of chloride transport inhibition and chloride substitution on neuron function and on hypoxic spreading-depression-like depolarization in rat hippocampal slices

Chloride fluxes play a crucial role in synaptic inhibition, cell pH regulation, as well as in cell volume control. In many neuropathological processes, cell swelling is a pivotal parameter, since cell volume changes and the dimension of the interstitial space critically modulate synchronized neuronal activity as well as the tissue's susceptibility to seizures or spreading depression. This study therefore focuses on the effects of different Cl(-) transport inhibitors and Cl(-) substitution on neuronal function and hypoxia-induced changes in rat hippocampal tissue slices. Orthodromically evoked focal excitatory postsynaptic potentials were depressed by furosemide (2mM), 4,4'-diisothiocyanatostilbene-2, 2'-disulfonic acid (1mM) and Cl(-) substitution by methylsulfate, but were enhanced by 4,4'-dinitrostilbene-2,2'-disulfonic acid (1mM). All four treatments induced multiple population spike firing in response to single orthodromic volleys, suggesting reduced synaptic inhibition. Antidromic population spikes increased following Cl(-) withdrawal, were unaffected in the presence of furosemide and 4, 4'-dinitrostilbene-2,2'-disulfonic acid, but were abolished by 4, 4'-diisothiocyanatostilbene-2,2'-disulfonic acid. The amplitude of the hypoxic spreading-depression-like extracellular potential shift was reduced by furosemide, 4,4'-diisothiocyanatostilbene-2, 2'-disulfonic acid and Cl(-) withdrawal, i.e. by the same treatments that depressed orthodromically evoked postsynaptic potentials. Furosemide prolonged the time to onset and the duration of the spreading-depression-like extracellular potential shift, while 4, 4'-dinitrostilbene-2,2'-disulfonic acid shortened the time to onset. Spreading-depression-related cell swelling was recorded as the shrinkage of relative interstitial space, which was measured as tetramethylammonium-chloride space. Neither the Cl(-) transport inhibitors nor Cl(-) withdrawal had any detectable effect on spreading-depression-related cell swelling. CA1 pyramidal neurons usually hyperpolarized during drug application and their input resistance decreased. Cl(-) withdrawal increased their input resistance and caused spontaneous burst firing. Hypoxia caused the expected spreading-depression-like rapid, near complete depolarization of single pyramidal neurons and drastically reduced their input resistance. The three Cl(-) transport inhibitors and Cl(-) withdrawal delayed the onset of the hypoxic depolarization. In low Cl(-) solutions, the apparent threshold potential at which spreading depression was triggered shifted to more positive membrane potentials. The final voltage of the hypoxic depolarization was, however, not affected. It appears from these results that the reduction in the hypoxic spreading-depression-like extracellular potential shifts by Cl(-) transport inhibitors is at least partially attributable to desynchronization of depolarization, not to decreased depolarization in individual cells. Other contributing factors could be changes in recording conditions, depression of swelling-induced amino acid release from glial cells and unspecific side-effects of the applied drugs. Desynchronization could also account for the delayed spreading-depression onset. It is concluded that Cl(-) fluxes play a role in the triggering of spreading depression, but the spreading-depression-like depolarization itself or its self-regenerative character is not mediated by Cl(-).

Simulated seizures and spreading depression in a neuron model incorporating interstitial space and ion concentrations

Sustained inward currents in neuronal membranes underlie tonic-clonic seizure discharges and spreading depression (SD). It is not known whether these currents flow through abnormally operating physiological ion channels or through pathological pathways that are not normally present. We have now used the NEURON simulating environment of Hines, Moore, and Carnevale to model seizure discharges and SD. The geometry and electrotonic properties of the model neuron conformed to a hippocampal pyramidal cell. Voltage-controlled transient and persistent sodium currents (I(Na,T) and I(Na,P)), potassium currents (I(K,DR) and I(K,A)), and N-methyl-D-aspartate (NMDA) receptor-controlled currents (I(NMDA)), were inserted in the appropriate regions of the model cell. The neuron was surrounded by an interstitial space where extracellular potassium and sodium concentration ([K(+)](o) and [Na(+)](o)) could rise or fall. Changes in intra- and extracellular ion concentrations and the resulting shifts in the driving force for ionic currents were continuously computed based on the amount of current flowing through the membrane. A Na-K exchange pump operated to restore ion balances. In addition, extracellular potassium concentration, [K(+)](o), was also controlled by a "glial" uptake function. Parameters were chosen to resemble experimental data. As long as [K(+)](o) was kept within limits by the activity of the Na-K pump and the "glial" uptake, a depolarizing current pulse applied to the cell soma evoked repetitive firing that ceased when the stimulating current stopped. If, however, [K(+)](o) was allowed to rise, then a brief pulse provoked firing that outlasted the stimulus. At the termination of such a burst, the cell hyperpolarized and then slowly depolarized and another burst erupted without outside intervention. Such "clonic" bursting could continue indefinitely maintained by an interplay of the rise and fall of potassium and sodium concentrations with membrane currents and threshold levels. SD-like depolarization could be produced in two ways, 1) by a dendritic NMDA-controlled current. Glutamate was assumed to be released in response to rising [K(+)](o). And 2) by the persistent (i.e., slowly inactivating) Na-current, I(Na,P). When both I(NMDA) and I(Na,P) were present, the two acted synergistically. We conclude that epileptiform neuronal behavior and SD-like depolarization can be generated by the feedback of ion currents that change ion concentrations, which, in turn, influence ion currents and membrane potentials. The normal stability of brain function must depend on the efficient control of ion activities, especially that of [K(+)](o).