Heptanol but not fluoroacetate prevents the propagation of spreading depression in rat hippocampal slices

Anesthetics and Cell-Cell Communication: Potential Ca2+-Calmodulin Role in Gap Junction Channel Gating by Heptanol, Halothane and Isoflurane

Cell–cell communication via gap junction channels is known to be inhibited by the anesthetics heptanol, halothane and isoflurane; however, despite numerous studies, the mechanism of gap junction channel gating by anesthetics is still poorly understood. In the early nineties, we reported that gating by anesthetics is strongly potentiated by caffeine and theophylline and inhibited by 4-Aminopyridine. Neither Ca²⁺ channel blockers nor 3-isobutyl-1-methylxanthine (IBMX), forskolin, CPT-cAMP, 8Br-cGMP, adenosine, phorbol ester or H7 had significant effects on gating by anesthetics. In our publication, we concluded that neither cytosolic Ca²⁺_i nor pH_i were involved, and suggested a direct effect of anesthetics on gap junction channel proteins. However, while a direct effect cannot be excluded, based on the potentiating effect of caffeine and theophylline added to anesthetics and data published over the past three decades, we are now reconsidering our earlier interpretation and propose an alternative hypothesis that uncoupling by heptanol, halothane and isoflurane may actually result from a rise in cytosolic Ca²⁺ concentration ([Ca²⁺]_i) and consequential activation of calmodulin linked to gap junction proteins.

The Critical Role of Spreading Depolarizations in Early Brain Injury: Consensus and Contention

BACKGROUND

When a patient arrives in the emergency department following a stroke, a traumatic brain injury, or sudden cardiac arrest, there is no therapeutic drug available to help protect their jeopardized neurons. One crucial reason is that we have not identified the molecular mechanisms leading to electrical failure, neuronal swelling, and blood vessel constriction in newly injured gray matter. All three result from a process termed spreading depolarization (SD). Because we only partially understand SD, we lack molecular targets and biomarkers to help neurons survive after losing their blood flow and then undergoing recurrent SD.

METHODS

In this review, we introduce SD as a single or recurring event, generated in gray matter following lost blood flow, which compromises the Na⁺/K⁺ pump. Electrical recovery from each SD event requires so much energy that neurons often die over minutes and hours following initial injury, independent of extracellular glutamate.

RESULTS

We discuss how SD has been investigated with various pitfalls in numerous experimental preparations, how overtaxing the Na⁺/K⁺ ATPase elicits SD. Elevated K⁺ or glutamate are unlikely natural activators of SD. We then turn to the properties of SD itself, focusing on its initiation and propagation as well as on computer modeling.

CONCLUSIONS

Finally, we summarize points of consensus and contention among the authors as well as where SD research may be heading. In an accompanying review, we critique the role of the glutamate excitotoxicity theory, how it has shaped SD research, and its questionable importance to the study of early brain injury as compared with SD theory.

Mechanisms of the negative potential associated with Leão's spreading depolarization: A history of brain electrogenesis

Spreading depolarization (SD) is a self-propagated wave that provokes transient disorder of numerous cell and tissue functions, and that may kill neurons in metabolically compromised tissue. We examined the mechanisms underlying the main hallmark of SD, a giant extracellular potential (ΔVo) for which multiple electromotive forces have been proposed. The end-point is that neurons and not glia, dendritic channels and not spatial currents, and increased sodium conductance rather than potassium gradients, appear to be the main actors in the generation of the negative ΔVo. Neuronal currents are established by two mechanisms, a voltage independent dendritic current, and the differential polarization along the neuron membranes. Notably, despite of a marked drop of ion gradients, these evolve significantly during SD, and yet the membrane potential remains clamped at zero no matter how much inward current is present. There may be substantial inward current or none in function of the evolving portion of the neuron dendrites with SD-activated channels. We propose that the ΔVo promotes swelling-induced dendritic damage. Understanding SD electrogenesis requires all elements relevant for membrane potential, action currents, field potentials and volume conduction to be jointly considered, and it has already encouraged the search for new targets to limit SD-related pathology.

Glial Connexins and Pannexins in the Healthy and Diseased Brain

Over the past several decades a large amount of data have established that glial cells, the main cell population in the brain, dynamically interact with neurons and thus impact their activity and survival. One typical feature of glia is their marked expression of several connexins, the membrane proteins forming intercellular gap junction channels and hemichannels. Pannexins, which have a tetraspan membrane topology as connexins, are also detected in glial cells. Here, we review the evidence that connexin and pannexin channels are actively involved in dynamic and metabolic neuroglial interactions in physiological as well as in pathological situations. These features of neuroglial interactions open the way to identify novel non-neuronal aspects that allow for a better understanding of behavior and information processing performed by neurons. This will also complement the "neurocentric" view by facilitating the development of glia-targeted therapeutic strategies in brain disease.

Involvement of neuronal and glial activities in control of the extracellular d-serine concentrations by the AMPA glutamate receptor in the mouse medial prefrontal cortex

It has been well accepted that d-serine may be an exclusive endogenous coagonist for the N-methyl-d-aspartate (NMDA)-type glutamate receptor in mammalian forebrain regions. We have recently found by using an in vivo dialysis method that an intra-medial prefrontal cortex infusion of S-α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (S-AMPA), a selective AMPA-type glutamate receptor agonist, causes a reduction in the extracellular levels of d-serine in a calcium-permeable AMPA receptor antagonist-sensitive manner. The inhibitory influence by the AMPA receptor on the extracellular d-serine, however, contradicts the data obtained from in vitro experiments that the AMPA receptor stimulation leads to facilitation of the d-serine liberation. This discrepancy appears to be due to the different cell setups between the in vivo and in vitro preparations. From the viewpoints of the previous reports indicating (1) the neuronal presence of d-serine synthesizing enzyme, serine racemase, and d-serine-like immunoreactivity and (2) the same high tissue concentrations of d-serine in the glia-enriched white matter and in the neuron-enriched gray matter of the mammalian neocortex, we have now investigated in the mouse medial prefrontal cortex, the effects of attenuation of neuronal and glial activities, by tetrodotoxin or fluorocitrate, respectively, on the S-AMPA-induced downregulation of the extracellular d-serine contents. In vivo dialysis studies revealed that a local infusion of tetrodotoxin or fluorocitrate eliminated the ability of S-AMPA given intra-cortically to cause a significant decrease in the dialysate concentrations of d-serine without affecting the elevating effects of S-AMPA on those of glycine, another intrinsic coagonist for the NMDA receptor. These findings suggest that the control by the AMPA receptor of the extracellular d-serine levels could be modulated by the neuronal and glial activities in the prefrontal cortex. It cannot be excluded that fluorocitrate would indirectly alter the modulation by changing synaptic neurotransmission via glial activity attenuation as previously reported.

Mechanisms of spreading depolarization in vertebrate and insect central nervous systems

Spreading depolarization (SD) is generated in the central nervous systems of both vertebrates and invertebrates. SD manifests as a propagating wave of electrical depression caused by a massive redistribution of ions. Mammalian SD underlies a continuum of human pathologies from migraine to stroke damage, whereas insect SD is associated with environmental stress-induced neural shutdown. The general cellular mechanisms underlying SD seem to be evolutionarily conserved throughout the animal kingdom. In particular, SD in the central nervous system of Locusta migratoria and Drosophila melanogaster has all the hallmarks of mammalian SD. Locust SD is easily induced and monitored within the metathoracic ganglion (MTG) and can be modulated both pharmacologically and by preconditioning treatments. The finding that the fly brain supports repetitive waves of SD is relatively recent but noteworthy, since it provides a genetically tractable model system. Due to the human suffering caused by SD manifestations, elucidating control mechanisms that could ultimately attenuate brain susceptibility is essential. Here we review mechanisms of SD focusing on the similarities between mammalian and insect systems. Additionally we discuss advantages of using invertebrate model systems and propose insect SD as a valuable model for providing new insights to mammalian SD.

How spreading depolarization can be the pathophysiological correlate of both migraine aura and stroke

The term spreading depolarization describes a mechanism of abrupt, massive ion translocation between neurons and the interstitial space, which leads to a cytotoxic edema in the gray matter of the brain. In energy-compromised tissue, spreading depolarization is preceded by a nonspreading silencing (depression of spontaneous activity) because of a neuronal hyperpolarization. By contrast, in tissue that is not energy compromised, spreading depolarization is accompanied by a spreading silencing (spreading depression) of spontaneous activity caused by a depolarization block. It is assumed that the nonspreading silencing translates into the initial clinical symptoms of ischemic stroke and the spreading silencing (spreading depression) into the symptoms of migraine aura. In energy-compromised tissue, spreading depolarization facilitates neuronal death, whereas, in healthy tissue, it is relatively innocuous. Therapies targeting spreading depolarization in metabolically compromised tissue may potentially treat conditions of acute cerebral injury such as aneurysmal subarachnoid hemorrhage.

Maintaining the presynaptic glutamate supply for excitatory neurotransmission

Glutamate released from synapses during excitatory neurotransmission must be rapidly recycled to maintain neuronal communication. This review evaluates data from physiological experiments at hippocampal CA3 to CA1 synapses and the calyx of Held synapse in the brainstem to analyze quantitatively the rates of release and resupply of glutamate required to sustain neurotransmission. We calculate that, without efficient recycling, the presynaptic glutamate supply will be exhausted within about a minute of normal synaptic activity. We also discuss replenishment of the presynaptic pool by diffusion from the soma, direct uptake of glutamate back into the presynaptic terminal, and uptake of glutamate precursor molecules. Diffusion of glutamate from the soma is calculated to be fast enough to resupply presynaptic glutamate in the hippocampus but not at the calyx of Held. However, because the somatic cytoplasm will also quickly run out of glutamate and synapses can function continually even if the presynaptic axon is severed, mechanisms other than diffusion must be present to resupply glutamate for release. Direct presynaptic uptake of glutamate is not present at the calyx of Held but may play a role in glutamate recycling in the hippocampus. Alternatively, glutamine or tricarboxylic acid cycle intermediates released from glia can serve as a precursor for glutamate in synaptic terminals, and we calculate that the magnitude of presynaptic glutamine uptake is sufficient to supply enough glutamate to sustain neurotransmission. The nature of these mechanisms, their relative abundance, and the co-ordination between them remain areas of intensive investigation.

Disruption of the blood-brain barrier exacerbates spreading depression in the locust CNS

In response to cellular stress in the nervous system of the locust (Locusta migratoria) neural function is interrupted in association with ionic disturbances propagating throughout nervous tissue (Spreading depression; SD). The insect blood-brain barrier (BBB) plays a critical role in the regulation of ion levels within the CNS. We investigated how a disruption in barrier function by transient exposure to 3M urea affects locusts' vulnerability to disturbances in ion levels. Repetitive SD was induced by bath application of ouabain and the extracellular potassium concentration ([K(+)]o) within the metathoracic ganglion (MTG) was monitored. Urea treatment increased the susceptibility to ouabain and caused a progressive impairment in the ability to maintain baseline [K(+)]o levels during episodes of repetitive SD. Additionally, using a within animal protocol we demonstrate that waves of SD, induced by high K(+), propagate throughout the MTG faster following disruption of the BBB. Lastly, we show that targeting the BBB of intact animals reduces their ability to sustain neural function during anoxic conditions. Our findings indicate that locust's ability to withstand stress is diminished following a reduction in barrier function likely due to an impairment of the ability of neural tissue to maintain ionic gradients.

Pharmacological blockade of gap junctions induces repetitive surging of extracellular potassium within the locust CNS

The maintenance of cellular ion homeostasis is crucial for optimal neural function and thus it is of great importance to understand its regulation. Glial cells are extensively coupled by gap junctions forming a network that is suggested to serve as a spatial buffer for potassium (K(+)) ions. We have investigated the role of glial spatial buffering in the regulation of extracellular K(+) concentration ([K(+)]o) within the locust metathoracic ganglion by pharmacologically inhibiting gap junctions. Using K(+)-sensitive microelectrodes, we measured [K(+)]o near the ventilatory neuropile while simultaneously recording the ventilatory rhythm as a model of neural circuit function. We found that blockade of gap junctions with either carbenoxolone (CBX), 18β-glycyrrhetinic acid (18β-GA) or meclofenamic acid (MFA) reliably induced repetitive [K(+)]o surges and caused a progressive impairment in the ability to maintain baseline [K(+)]o levels throughout the treatment period. We also show that a low dose of CBX that did not induce surging activity increased the vulnerability of locust neural tissue to spreading depression (SD) induced by Na(+)/K(+)-ATPase inhibition with ouabain. CBX pre-treatment increased the number of SD events induced by ouabain and hindered the recovery of [K(+)]o back to baseline levels between events. Our results suggest that glial spatial buffering through gap junctions plays an essential role in the regulation of [K(+)]o under normal conditions and also contributes to a component of [K(+)]o clearance following physiologically elevated levels of [K(+)]o.

Cortical spreading depression and migraine

Cortical spreading depression (CSD), a slowly propagated wave of depolarization followed by suppression of brain activity, is a remarkably complex event that involves dramatic changes in neural and vascular function. Since its original description in the 1940s, CSD has been hypothesized to be the underlying mechanism of the migraine aura. Substantial evidence from animal models provides indirect support for this hypothesis, and studies showing that CSD is common in humans with brain injury clearly demonstrate that the phenomenon can occur in the human brain. Considerable uncertainty about the role of CSD in migraine remains, however, and key questions about how this event is initiated, how it spreads, and how it might cause migraine symptoms remain unanswered. This Review summarizes current concepts of CSD and its potential roles in migraine, and addresses ongoing studies aimed at a clearer understanding of this fundamental brain phenomenon.

Diffusing substances during spreading depolarization: analytical expressions for propagation speed, triggering, and concentration time courses

Spreading depolarization (SD) is an important phenomenon in stroke and migraine. However, the processes underlying the propagation of SD are still poorly understood, and an elementary model that is both physiological and quantitative is lacking. We show that, during the onset and propagation of SD, the concentration time courses of excitatory substances such as potassium and glutamate can be described with a reaction-diffusion equation. This equation contains four physiological parameters: (1) a concentration threshold for excitation; (2) a release rate; (3) a removal rate; and (4) an effective diffusion constant. Solving this equation yields expressions for the propagation velocity, concentration time courses, and the minimum stimulus that can trigger SD. This framework allows for analyzing experimental results in terms of these four parameters. The derived time courses are validated with measurements of potassium in rat brain tissue.

Connexin 36 promotes cortical spreading depolarization and ischemic brain damage

Cortical spreading depolarization (CSD) promotes the progression of neuronal injury after cerebral ischemia. However, the mechanisms of propagation of postischemic CSD events are still unclear. In this study we characterized the role of the main neuronal gap junction protein connexin 36 (Cx36) in generating postischemic CSDs. In Cx36-deficient mice and controls we occluded the distal middle cerebral artery. To detect CSD events we recorded the direct current and laser Doppler flow. In addition, locomotor function and the infarct size were determined. Cx36-deficient mice had significantly fewer and shorter CSD events than wild-type controls. Additionally, Cx36 deletion is neuroprotective, leading to a better functional outcome and decreased infarct size after ischemia. These results suggest a detrimental role for Cx36 after ischemia, possibly by promoting CSD.

Pannexins in ischemia-induced neurodegeneration

Pannexin 1 (Px1, Panx1) and pannexin 2 (Px2, Panx2) form large-pore nonselective channels in the plasma membrane of cells and were suggested to play a role in the pathophysiology of cerebral ischemia. To directly test a potential contribution of pannexins in ischemia-related mechanisms, we performed experiments in Px1(-/-), Px2(-/-), and Px1(-/-)Px2(-/-) knockout mice. IL-1β release, channel function in astrocytes, and cortical spreading depolarization were not altered in Px1(-/-)Px2(-/-) mice, indicating that, in contrast to previous concepts, these processes occur normally in the absence of pannexin channels. However, ischemia-induced dye release from cortical neurons was lower, indicating that channel function in Px1(-/-)Px2(-/-) neurons was impaired. Furthermore, Px1(-/-)Px2(-/-) mice had a better functional outcome and smaller infarcts than wild-type mice when subjected to ischemic stroke. In conclusion, our data demonstrate that Px1 and Px2 underlie channel function in neurons and contribute to ischemic brain damage.

The effect of a gap-junction blocker, carbenoxolone, on ischemic brain injury and cortical spreading depression

Cortical spreading depression (CSD) has been shown to cause secondary cell loss in experimental models of brain injury and in patients, and blocking of CSD is a potential neuroprotective strategy. Here we tested the hypothesis that gap junctions affect CSD under physiological conditions as well as infarct development in a rat two-vein occlusion model suited to study pathophysiology of the penumbra (n = 71). We applied the gap junction blocker carbenoxolone (CBX) or saline intra-ventricularly. Interestingly, CBX temporarily increased systemic blood pressure and cortical blood flow (41% and 53%, 15 min after 250 μg CBX). We induced CSD with cortical microinjection of potassium chloride (KCl), counted how many spontaneous CSDs after CSD induction were elicited and measured the propagation velocity. After 250 μg CBX administration, significant 37.5 ± 6.5 additional CSDs were seen. CSD velocity increased significantly after 50 μg and 250 μg CBX. Occlusion of two adjacent cortical veins using Rose Bengal dye and fiberoptic illumination followed by 250 μg CBX or saline showed a significant more than doubling of infarct volumes 7 days after CBX. The current experiments provide evidence that CBX can accelerate the initiation and propagation of CSD suggesting opening of gap junctions is not required for CSD propagation. Blocking gap junctions worsens outcome from focal cerebral ischemia. Hence, measures intended to improve spatial buffering via astroglial gap junctions could have therapeutic potential in disease processes involving CSD.

Coma in response to environmental stress in the locust: a model for cortical spreading depression

Spreading depression (SD) is an interesting and important phenomenon due to its role in mammalian pathologies such as migraine, seizures, and stroke. Until recently investigations of the mechanisms involved in SD have mostly utilized mammalian cortical tissue, however we have discovered that SD-like events occur in the CNS of an invertebrate model, Locusta migratoria. Locusts enter comas in response to stress during which neural and muscular systems shut down until the stress is removed, and this is believed to be an adaptive strategy to survive extreme environmental conditions. During stress-induced comas SD-like events occur in the locust metathoracic ganglion (MTG) that closely resemble cortical SD (CSD) in many respects, including mechanism of induction, extracellular potassium ion changes, and propagation in areas equivalent to mammalian grey matter. In this review we describe the generation of comas and the associated SD-like events in the locust, provide a description of the similarities to CSD, and show how they can be manipulated both by stress preconditioning and pharmacologically. We also suggest that locust SD-like events are adaptive by conserving energy and preventing cellular damage, and we provide a model for the mechanism of SD onset and recovery in the locust nervous system.

Spreading depression: from serendipity to targeted therapy in migraine prophylaxis

Despite the relatively well-characterized headache mechanisms in migraine, upstream events triggering individual attacks are poorly understood. This lack of mechanistic insight has hampered a rational approach to prophylactic drug discovery. Unlike targeted abortive and analgesic interventions, mainstream migraine prophylaxis has been largely based on serendipitous observations (e.g. propranolol) and presumed class effects (e.g. anticonvulsants). Recent studies suggest that spreading depression is the final common pathophysiological target for several established or investigational migraine prophylactic drugs. Building on these observations, spreading depression can now be explored for its predictive utility as a preclinical drug screening paradigm in migraine prophylaxis.

Infant Brain Stem Is Prone to the Generation of Spreading Depression During Severe Hypoxia

Spreading depression (SD) resembles a concerted, massive neuronal/glial depolarization propagating within the gray matter. Being associated with cerebropathology, such as cerebral ischemia or hemorrhage, epileptic seizures, and migraine, it is well studied in cortex and hippocampus. We have now analyzed the susceptibility of rat brain stem to hypoxia-induced spreading depression-like depolarization (HSD), which could critically interfere with cardiorespiratory control. In rat brain stem slices, severe hypoxia (oxygen withdrawal) triggered HSD within minutes. The sudden extracellular DC potential shift of approximately −20 mV showed the typical profile known from other brain regions and was accompanied by an intrinsic optical signal (IOS). Spatiotemporal IOS analysis revealed that in infant brain stem, HSD was preferably ignited within the spinal trigeminal nucleus and then mostly spread out medially, invading the hypoglossal nucleus, the nucleus of the solitary tract (NTS), and the ventral respiratory group (VRG). The neuronal hypoxic depolarizations underlying the generation of HSD were massive, but incomplete. The propagation velocity of HSD and the associated extracellular K+ rise were also less marked than in other brain regions. In adult brain stem, HSD was mostly confined to the NTS and its occurrence was facilitated by hypotonic solutions, but not by glial poisoning or block of GABAergic and glycinergic synapses. In conclusion, brain stem tissue reliably generates propagating HSD episodes, which may be of interest for basilar-type migraine and brain stem infarcts. The preferred occurrence of HSD in the infant brain stem and its propagation into the VRG may be of importance for neonatal brain stem pathology such as sudden infant death syndrome.

ATP signalling in epilepsy

This paper focuses on a role for ATP neurotransmission and gliotransmission in the pathophysiology of epileptic seizures. ATP along with gap junctions propagates the glial calcium wave, which is an extraneuronal signalling pathway in the central nervous system. Recently astrocyte intercellular calcium waves have been shown to underlie seizures, and conventional antiepileptic drugs have been shown to attenuate these calcium waves. Blocking ATP-mediated gliotransmission, therefore, represents a potential target for antiepileptic drugs. Furthermore, while knowledge of an antiepileptic role for adenosine is not new, a recent study showed that adenosine accumulates from the hydrolysis of accumulated ATP released by astrocytes and is believed to inhibit distant synapses by acting on adenosine receptors. Such a mechanism is consistent with a surround-inhibitory mechanism whose failure would predispose to seizures. Other potential roles for ATP signalling in the initiation and spread of epileptiform discharges may involve synaptic plasticity and coordination of synaptic networks. We conclude by making speculations about future developments.

Variations in tissue resistivity and in the extension of activated neuron domains shape the voltage signal during spreading depression in the CA1 in vivo

Spreading depression (SD), a wave of neuron activity related to migraine and the ischaemic penumbra, features a moving shell of extracellular negative potential shift (V(o)) whose generators are poorly understood. We investigated its subcellular correlates in the hippocampal CA1 in vivo by localizing the neuron domains that generate transmembrane current (I(m)) using field analysis, and the local changes of tissue resistivity, a major determinant of extracellular current flow. A large increase of tissue resistivity occurred in times and dendritic strata displaying large V(o), albeit with different rates. Typically, SD is composed of basal and apical dendritic components. The apical SD lasts much longer, while it becomes gradually restricted to a narrow dendritic region. Strikingly, pyramidal cells displayed a strong surge of inward current only when SD affected a small dendritic region. However, when the V(o) signal covered most of the main neuron axis, only smaller surges of inward current developed at the outer dendritic rims of a wide null current zone. Computational reconstruction showed that this effect was due to strong spatial cancellation of the inward and outward currents in SD-activated isopotential and shunted regions of individual neurons. Consequently, despite former accounts of large conductance increase, the net I(m) is small and the large DeltaV(o) amplitude mostly due to increased tissue resistivity. The particular subcellular evolution indicates an initial explosive opening of conductance along most of the pyramidal neuron followed by a wave-like centripetal closure towards the apical dendrites. The applicability of these mechanisms to SD in other brain regions is discussed.

Stress preconditioning of spreading depression in the locust CNS

Cortical spreading depression (CSD) is closely associated with important pathologies including stroke, seizures and migraine. The mechanisms underlying SD in its various forms are still incompletely understood. Here we describe SD-like events in an invertebrate model, the ventilatory central pattern generator (CPG) of locusts. Using K(+) -sensitive microelectrodes, we measured extracellular K(+) concentration ([K(+)](o)) in the metathoracic neuropile of the CPG while monitoring CPG output electromyographically from muscle 161 in the second abdominal segment to investigate the role K(+) in failure of neural circuit operation induced by various stressors. Failure of ventilation in response to different stressors (hyperthermia, anoxia, ATP depletion, Na(+)/K(+) ATPase impairment, K(+) injection) was associated with a disturbance of CNS ion homeostasis that shares the characteristics of CSD and SD-like events in vertebrates. Hyperthermic failure was preconditioned by prior heat shock (3 h, 45 degrees C) and induced-thermotolerance was associated with an increase in the rate of clearance of extracellular K(+) that was not linked to changes in ATP levels or total Na(+)/K(+) ATPase activity. Our findings suggest that SD-like events in locusts are adaptive to terminate neural network operation and conserve energy during stress and that they can be preconditioned by experience. We propose that they share mechanisms with CSD in mammals suggesting a common evolutionary origin.

A steady-state model of spreading depression predicts the importance of an unknown conductance in specific dendritic domains

Spreading depression (SD) is a pathological wave of transient neuronal inactivation. We recently reported that the characteristic sustained complete depolarization is restricted to specific cell domains where the input resistance (R(in)) first becomes negligible before achieving partial recovery, whereas in adjacent, more polarized membranes it drops by much less. The experimental study of the participating membrane channels is hindered by their mixed contribution and heterogeneous distribution. Therefore, we derived a biophysical model to analyze the conductances that replicate the subcellular profile of R(in) during SD. Systematic variation of conductance densities far beyond the ranges reported failed to fit the experimental values. Besides standard potassium, sodium, and Glu-mediated conductances, the initial opening and gradual closing of an as yet undetermined large conductance is required to account for the evolution of R(in). Potassium conductances follow in the relative contribution and their closing during the late phase is also predicted. Large intracellular potential gradients from zero to rest are readily sustained between shunted and adjacent SD-spared membranes, which remain electroregenerative. The gradients are achieved by a combination of high-conductance subcellular domains and transmembrane ion redistribution in extended but discrete dendritic domains. We conclude that the heterogeneous subcellular behavior is due to local membrane properties, some of which may be specifically activated under extreme SD conditions.

The connexin 36 blockers quinine, quinidine and mefloquine inhibit cortical spreading depression in a rat neocortical slice model in vitro

A protocol for inducing cortical spreading depression (SD) on rat neocortical slices in vitro, upon local application of calibrated approximately nl drops of KCl, 3M was used to elicit SD events, recorded at two different points on the slice. This in vitro model was validated by the inhibition of SD episodes by the NMDA antagonist MK-801 (20 microM), the reference SD blocker. Quinine, its stereoisomer quinidine, and mefloquine consistently inhibited the SD episodes. Quinine and quinidine, 100 and 200 microM reduced the duration, while mefloquine, 100 and 200 microM reduced the amplitude of SD events, all in a concentration-dependent manner. These compounds have been reported to block gap junctions, specifically the neuronal connexin (Cx) 36, but they also exert other cellular effects. While further investigation is warranted to settle whether SD inhibition in vitro by quinine, quinidine and mefloquine reflects an involvement of neuronal Cx36 channels in SD generation/propagation, these results bear potential drug-discovery relevance for the migraine with aura.

Astrocyte control of synaptic transmission and neurovascular coupling

From a structural perspective, the predominant glial cell of the central nervous system, the astrocyte, is positioned to regulate synaptic transmission and neurovascular coupling: the processes of one astrocyte contact tens of thousands of synapses, while other processes of the same cell form endfeet on capillaries and arterioles. The application of subcellular imaging of Ca2+ signaling to astrocytes now provides functional data to support this structural notion. Astrocytes express receptors for many neurotransmitters, and their activation leads to oscillations in internal Ca2+. These oscillations induce the accumulation of arachidonic acid and the release of the chemical transmitters glutamate, d-serine, and ATP. Ca2+ oscillations in astrocytic endfeet can control cerebral microcirculation through the arachidonic acid metabolites prostaglandin E2 and epoxyeicosatrienoic acids that induce arteriole dilation, and 20-HETE that induces arteriole constriction. In addition to actions on the vasculature, the release of chemical transmitters from astrocytes regulates neuronal function. Astrocyte-derived glutamate, which preferentially acts on extrasynaptic receptors, can promote neuronal synchrony, enhance neuronal excitability, and modulate synaptic transmission. Astrocyte-derived d-serine, by acting on the glycine-binding site of the N-methyl-d-aspartate receptor, can modulate synaptic plasticity. Astrocyte-derived ATP, which is hydrolyzed to adenosine in the extracellular space, has inhibitory actions and mediates synaptic cross-talk underlying heterosynaptic depression. Now that we appreciate this range of actions of astrocytic signaling, some of the immediate challenges are to determine how the astrocyte regulates neuronal integration and how both excitatory (glutamate) and inhibitory signals (adenosine) provided by the same glial cell act in concert to regulate neuronal function.

The effects of octanol on penicillin induced epileptiform activity in rats: an in vivo study

The common features of all types of epilepsy are the synchronized and uncontrolled discharges of nerve cell assemblies. The reason for the pathologically synchronized discharges of the neuron is not exactly known yet. Recent reports claim that gap junctions have a critical role in neuronal synchronization. The present study was planned to investigate the effects of octanol, a gap junction blocker, on penicillin-induced experimental epilepsy. Permanent screw electrodes allowing EEG monitoring from conscious animals and permanent cannula providing the administration of the substances to the brain ventricle were placed into the cranium of rats under general anesthesia. After the postoperative recovery period, epileptiform activity was generated by injecting 300 IU crystallized penicillin through the ventricular cannula. When epileptiform activity, monitored from a digital recording system, reached at its maximum intensity, octanol was applied in the same way as penicillin administered. Application of octanol caused an inhibition in the epileptiform activity. Vehicle solution alone did not affect the epileptiform activity. Results of this study suggest that the blockade of electrical synapses may contribute to the prevention and amelioration of epileptic activity. Production of gap junction blockers selective for connexin types is needed. Further studies on the differential roles of gap junctions on certain epileptiform activities are required.

A role for glutamate and glia in the fast network oscillations preceding spreading depression

The mechanism of the propagation of spreading depression is unclear. Classical theories proposed a self-maintained cycle fed by elevated potassium and/or glutamate in the extracellular space. Earlier we found in vivo a characteristic oscillatory field activity that is synchronous in a strip of tissue ahead of the oncoming wave of neuron depolarization and that occurs before the extracellular potassium level begins to rise [Herreras O, Largo C, Ibarz JM, Somjen GG, Marrín del Río R (1994) Role of neuronal synchronizing mechanisms in the propagation of spreading depression in the in vivo hippocampus. J Neurosci 14:7087-7098]. We investigated here the possible participation of glutamate and the role of glia in the prodromal field oscillations using extra and intracellular recordings and pharmacological manipulations in rat hippocampal slices. As earlier shown in vivo, field oscillations propagated ahead of the negative potential shift covering distances of up to 1 mm. The oscillatory prodromals were initially subthreshold but then each wave became crowned by a population spike. The frequency of the oscillatory prodromals was variable among slices (80-115 Hz), but constant in individual slices. The blockade of ionotropic glutamate receptors decreased the frequency of prodromal oscillations, retarded spreading depression propagation, and shortened the duration of depolarization. Blocking the glutamate membrane transport increased the oscillatory frequency. The selective metabolic poisoning of astrocytes led to gradual disorganization of prodromal oscillations whose frequency first increased and then decreased. Also, the amplitude of the population spikes within the burst diminished as individual cells fired fewer action potentials, although still phase-locked with population spikes. The effects of glial metabolic impairment were observed within the period when neuron electrical properties were still normal, and were blocked by glutamate receptor antagonists. These data suggest that glutamate released from glial cells and possibly also from neurons has a role in the generation of oscillations and neuron firing synchronization that precede the spreading depression-related depolarization, but additional mechanisms are required to fully explain the onset and propagation of spreading depression.

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.

Astrocytes contribute to regulation of extracellular calcium and potassium in the rat cerebral cortex during spreading depression

This study used spreading depression (SD), which is characterized by redistribution of ions, to examine the role of astrocytes in the regulation of extracellular potassium ([K+]o) and calcium ([Ca2+]o) levels. Recurrent spreading depression episodes were induced by application of 3 M potassium chloride to the cortex of adult anesthetized rats while monitoring the extracellular direct current (DC) potential shifts and changes in [K+]o or [Ca2+]o 6-7 mm away. The reversible glial toxins, fluorocitrate (FC) and fluoroacetate (FA), were injected locally into the cortex at doses that are selective for reducing glial function. The peak changes and area under the curve for [K+]o and [Ca2+]o, recovery rate for [K+]o, and interval between spreading depression episodes were measured before and at various times after administration of the toxins. Both fluorocitrate and fluroacetate slowed the recovery of the [K+]o and altered the recovery of the [Ca2+]o. Local injection of glutamate uptake inhibitors or barium had no effect on the peak changes in [K+]o or the rate of recovery of the [K+]o. The slowing of the recovery rate is consistent with the hypothesis that glial cells play a role in the return of [K+]o to baseline after spreading depression in the cortex in vivo. The change in movement of calcium after administration of FC suggests that astrocytes normally extrude calcium during spreading depression, resulting in rapid recovery of the levels of [Ca2+]o with an overshoot. These findings demonstrate that astrocytes contribute to the regulation of both potassium and calcium during and after a stress to the ionic homeostatic mechanisms.

Effects of glial toxins on extracellular acidification in the hippocampal CA1 region in vivo

In the pyramidal cell layer of the CA1 region of the hippocampus in the urethane-anesthetized adult rat, there is an initial alkalinization followed by an acidification in response to synchronized seizure activity induced by stimulus trains. In this study, the role of astrocytes in these extracellular pH changes during neuronal activity was examined using local injection of two relatively selective glial toxins (fluorocitrate (FC) and fluoroacetate (FA)) into the CA1 cell layer. Both glial toxins reduced the peak level of acidification reached after 20 Hz stimulus trains to the contralateral CA3 region, without changing the lengthening of the afterdischarge, when compared to animals that had received a local injection of vehicle. After administration of either glial toxin, the peak level of acidification still correlated with the total discharge duration, but the levels of acidification were consistently lower than in control animals. Administration of either glial toxin had no effect on the peak alkalinization during the stimulus train, or on the rate of recovery from peak level of acidification. Injection of either vehicle, FA, or FC had no effect on the amplitude or frequency of the neuronal discharge during the afterdischarge. The results suggest that, in normal conditions, astrocytes contribute to the acidification of the extracellular space that occurs in response to intense neuronal activity. This acidification may contribute to feedback regulation of neuronal excitability.

Accelerated hippocampal spreading depression and enhanced locomotory activity in mice with astrocyte-directed inactivation of connexin43

Using a human glial fibrillary acidic protein (hGFAP) promoter-driven cre transgene, we have achieved efficient inactivation of a floxed connexin43 (Cx43) gene in astrocytes of adult mice. The loss of Cx43 expression was monitored in a cell-autonomous manner via conditional replacement of the Cx43-coding region by a lacZ reporter gene. In this way, we bypassed the early postnatal lethality previously reported for Cx43 null mice and characterized the phenotypic consequences of Cx43 deficiency in the CNS. Mice lacking Cx43 in astrocytes were viable and showed no evidence of either neurodegeneration or astrogliosis. Spreading depression (SD) is a pathophysiological phenomenon observed in the CNS that is characterized by a propagating wave of depolarization followed by neuronal inactivation. Inhibitors of gap junctional communication have previously been shown to block initiation and propagation of SD. In contrast, we observed an increase in the velocity of hippocampal SD in the stratum radiatum of mice lacking Cx43 in astrocytes. In the same brain subregion, dye-coupling experiments revealed a reduction in overall astrocytic intercellular communication by approximately 50%. This strongly suggests separate and different neuronal and glial contributions of gap junctional intercellular communication to SD. Concomitant with increased velocity of spreading depression, we observed enhanced locomotory activity in mice lacking Cx43 in astrocytes.

Cortical spreading depression in the feline brain following sustained and transient stimuli studied using diffusion-weighted imaging

Cortical spreading depression (CSD) was induced by transient (10 min) applications of KCl in agar upon the cortical surface of alpha-chloralose anaesthetised cats. Its features were compared with CSD resulting from sustained applications of crystalline KCl through a mapping of the apparent diffusion coefficient (ADC) using diffusion-weighted echo planar imaging (DWI) over a poststimulus period of 60-100 min. Individual CSD events were computationally detected with the aid of Savitzky-Golay smoothing applied to critically sampled data derived from regions of interest (ROIs) made up of 2 x 2 pixel matrices. The latter were consistently placed at three selected sites on the suprasylvian gyrus (SG) and six sites on the marginal gyrus (MG). The CSD events thus detected were then quantitatively characterised for each ROI using the original time series. Both stimuli consistently elicited similar spreading patterns of initial, primary CSD events that propagated over the SG and marginal MG and were restricted to the hemispheres on which the stimuli were applied. There followed secondary events over smaller extents of cortical surface. Sustained stimuli elicited primary and secondary CSD events with similar amplitudes of ADC deflection that were distributed around a single mean. The ADC deflections were also conserved in peak amplitude throughout the course of their propagation. The initial primary event showed a poststimulus latency of 1.1 +/- 0.1 min. Successive secondary events followed at longer, but uniform, time intervals of around 10 min. Primary and secondary CSDs showed significantly different velocities of conduction (3.32 +/- 0.43 mm min(-1) vs. 2.11 +/- 0.21 mm min(-1), respectively; n = 5) across the cerebral hemisphere. In contrast, transient stimuli produced significantly fewer numbers of CSD events (3.8 +/- 0.5 events per animal, n = 5) than did sustained stimuli (7.4 +/- 0.5 events per animal, mean +/- S.E.M., n = 5, P = 0.002). The peak ADC deflection of their primary CSD events declined by approximately 30 % as they propagated from their initiation site to the interhemispheric boundary. The primary CSD event following a transient stimulus showed a latency of 1.4 +/- 0.1 min. It was followed by successive and smaller secondary ADC deflections that were separated by progressively longer time intervals. Conduction velocities of secondary events were similar to those of primary events. Conduction velocities of both primary and secondary events were slower than their counterparts following a sustained stimulus. ADC changes associated with CSD thus persist at times well after stimulus withdrawal and vary markedly with the nature of the initiating stimulus even in brain regions remote from the stimulus site.

Fast network oscillations induced by potassium transients in the rat hippocampus in vitro

Brief pressure ejection of solutions containing potassium, caesium or rubidium ions into stratum radiatum of the CA1 or CA3 regions of the hippocampal slice evoked a fast network oscillation. The activity evoked lasted approximately 3-25 s with the predominant frequency component being in the gamma frequency range (30-80 Hz), although beta frequency (15-30 Hz) and ultrafast (> 80 Hz) components could also be seen. The gamma frequency component of the oscillation remained constant, even when large changes in power occurred, and was synchronous across the CA1 region. Measurements with potassium ion-sensitive electrodes revealed that the network oscillation was accompanied by increases (0.5 to 2.0 mM) in the extracellular potassium concentration [K+]o. Bath application of the N-methyl-D-aspartate (NMDA) receptor antagonists D(-)-2-amino-5-phosphonopentanoic acid (D-AP5; 50 microM) had no significant effect but the alpha-amino-3-hydroxy-5-methyl-4-isooxazolepropionic acid (AMPA)/kainate receptor antagonist 2,3,-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulphonamide disodium (NBQX; 20 microM) caused a significant reduction (86.7 +/- 4.5 %) in the power in the gamma frequency range. Residual rhythmic activity, presumably arising in the interneuronal network, was blocked by the GABA(A) receptor antagonist bicuculline. The putative gap junction blocker octanol caused a decrease in the power of the gamma frequency component of 75.5 +/- 5.6 %, while carbenoxolone produced a reduction of only 14 +/- 42 %. These experiments demonstrate that a modest increase in exogenous [K+]o in the hippocampus in vitro is sufficient to evoke a fast network oscillation, which is an emergent property of the synaptically and electrically interconnected neuronal network.

Role of calcium, glutamate neurotransmission, and nitric oxide in spreading acidification and depression in the cerebellar cortex

This study investigated the mechanisms underlying the recently reported fast spreading acidification and transient depression in the cerebellar cortex in vivo. Spreading acidification was evoked by surface stimulation in the rat and mouse cerebellar cortex stained with the pH-sensitive dye neutral red and monitored using epifluorescent imaging. The probability of evoking spreading acidification was dependent on stimulation parameters; greater frequency and/or greater amplitude were more effective. Although activation of the parallel fibers defined the geometry of the spread, their activation alone was not sufficient, because blocking synaptic transmission with low Ca(2+) prevented spreading acidification. Increased postsynaptic excitability was also a major factor. Application of either AMPA or metabotropic glutamate receptor antagonists reduced the likelihood of evoking spreading acidification, but stronger stimulation intensities were still effective. Conversely, superfusion with GABA receptor antagonists decreased the threshold for evoking spreading acidification. Blocking nitric oxide synthase (NOS) increased the threshold for spreading acidification, and nitric oxide donors lowered the threshold. However, spreading acidification could be evoked in neuronal NOS-deficient mice (B6;129S-Nos1(tm1plh)). The depression in cortical excitability that accompanies spreading acidification occurred in the presence of AMPA and metabotropic glutamate receptor antagonists and NOS inhibitors. These findings suggest that spreading acidification is dependent on extracellular Ca(2+) and glutamate neurotransmission with a contribution from both AMPA and metabotropic glutamate receptors and is modulated by nitric oxide. Therefore, spreading acidification involves both presynaptic and postsynaptic mechanisms. We hypothesize that a regenerative process, i.e., a nonpassive process, is operative that uses the cortical architecture to account for the high speed of propagation.

Diffusion-weighted MRI used to detect in vivo modulation of cortical spreading depression: comparison of sumatriptan and tonabersat

Spreading cortical depolarization and depression of electroencephalographic activity (SD) may underlie the aura and spreading neurovascular events of migraine. Cortical depolarization may also precipitate the progressive development of cerebral pathology following ischemia. However, data on SD in the human brain are sparse, most likely reflecting the technical difficulties involved in performing such clinical studies. We have previously shown that the transient cerebral water disturbances during SD can be quantitatively investigated in the gyrencephalic brain using repetitive diffusion-weighted magnetic resonance imaging (DWI). To investigate whether DWI could detect modulation of the spatiotemporal properties of SD in vivo, the effects of the antimigraine drug sumatriptan (0.3 mg/kg iv) and the novel anticonvulsant tonabersat (10 mg/kg ip) were evaluated in the cat brain. Supporting previous findings, sumatriptan did not affect the numbers of events (range, 4-8), the duration of SD activity (39.8 +/- 4.4 min, mean +/- SEM), and event velocity (2.2 +/- 0.4 mm min(-1)); tonabersat significantly reduced SD event initiation (range, 0-3) and duration (13.2 +/- 5.0 min) and increased primary event velocity (5.4 +/- 0.7 mm min(-1)). However, both drugs significantly decreased, by >50%, the spatial extent of the first KCl-evoked SD event, and sumatriptan significantly increased event propagation across the suprasylvian sulcus (5.5 +/- 0.6 vs 2.4 +/- 0.4 events in controls). These results demonstrate (1) the feasibility of using DWI to evaluate therapeutic effects on SD, and (2) that sumatriptan may directly modulate the spatial distribution of SD activity in the gyrencephalic brain.

Can gap-junction blockade preferentially inhibit neuronal hypersynchrony vs. excitability?

Epileptic activity involves a hypersynchronous firing of large populations of brain neurons, some of which are hyperexcitable. This study explored to what extent gap-junction blockade inhibits neuronal synchronization vs. neuronal excitability. We investigated the effects of the gap-junction blockers (GJBs) 1-heptanol, 1-octanol and carbenoxolone vs. the loop diuretic furosemide on spontaneous and evoked epileptiform field potentials, induced in CA3 area of rat hippocampal slices by a 'high K(+)-low Ca(2+)' perfusion fluid. This milieu induced frequent (>30 min(-1)) spontaneous bursts, led single fimbrial stimuli to evoke repetitive population spikes (PSs), and increased PS amplitudes. Both furosemide and the three GJBs gradually reduced spontaneous field bursting, or even stopped it within one hour. The anti-bursting activity of carbenoxolone showed dose-response dependence in the concentration range 50-400 microM. 1-heptanol and 1-octanol markedly and similarly depressed all the epileptiform markers of the evoked responses, whereas carbenoxolone did not reduce the number of repetitive PSs evoked by single stimuli. By its minor effect on evoked responses, carbenoxolone resembled furosemide, rather than its congeners GJBs. These results favor the possibility that selective gap-junction blockade might antagonize epileptic synchronization, without depressing neuronal excitability.

The effect of ethanol on metabolic, hemodynamic and electrical responses to cortical spreading depression

Alcohol induces a decrease in cerebral blood flow (CBF) and metabolic rate, mitochondrial damage and other impairments in brain function and structure. Cortical spreading depression (CSD) is a phenomenon causing changes in ion homeostasis and raises energy demand, mitochondrial activity and CBF. It is of great interest to study the effect of ethanol on brain response under a challenge of increasing oxygen demand by inducing CSD. A special multisite assembly (MSA) was constructed to evaluate metabolic (mitochondrial NADH), hemodynamic (reflectance) and electrical (DC potential) activities from four parasagittally adjacently arranged areas of the cerebral cortex, continuously and simultaneously in vivo. Three CSD cycles were initiated every 30 min before and after ethanol or saline infusion over 4.5 h. During CSD amplitude changes of reflectance, NADH and DC potential as well as propagation rates and wave frequency were calculated. After ethanol infusion CSD showed a decrease in the negative shift of the DC potential, and alterations in the biphasic responses in reflectance, which may indicate alteration in blood volume: unclear responses in the initial vasoconstriction phase and a significant increase in the subsequent vasodilatation phase. The reduction in the amplitude of the NADH oxidation cycle may depict a decrease in energy production, which could also be indicated by a decline in wave frequency (prolonging the recovery phase of the CSD). The decrease in propagation rate indicates a decline in tissue excitability and in the CSD initiation mechanism induced by ethanol treatment.

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.

Optical recording of spreading depression in rat neocortical slices

A spreading depression (SD) was elicited in adult rat neocortical slices by microdrop application of high potassium and the SD propagation pattern was analyzed by recording simultaneously the extracellular DC potential and the changes in the intrinsic optical signal. The electrical SD with an average peak amplitude of 13.2+/-3.4 mV showed a good spatial and temporal correlation with the optical signal. In 79% of the slices, the SD was characterized by an initial increase of light reflectance by 2.3+/-1.6%, followed by a reflectance decrease of 0.5+/-2.4% and finally a larger and long-lasting increase by 5+/-2.4%. In the remaining slices, the SD revealed an initial decrease in light reflectance by 5.8+/-1.8% followed by an increase of 1.4+/-1.2%. In all slices, the recovery in the DC recording was faster as in the optical signal. The SD preferentially propagated within layers I-IV and could be blocked in most experiments by a vertical incision through upper layers or by local glutamate receptor blockade following microdrop application of kynurenic acid in layers II-III. The SD could be also blocked by bath application of kynurenic acid, MK-801 and octanol, but not by the more specific gap junction blocker carbenoxolone. Our results indicate that the high density of dendritic processes and glutamate receptors in layers II-IV promote the horizontal spread of the SD in these cortical layers and that gap junctions are not required for the propagation of SD in neocortical slices.

Carbenoxolone depresses spontaneous epileptiform activity in the CA1 region of rat hippocampal slices

An important contributor to the generation of epileptiform activity is the synchronization of burst firing in a group of neurons. The aim of this study was to investigate whether gap junctions are involved in this synchrony using an in vitro model of epileptiform activity. Hippocampal slices (400 microm) were prepared from female Sprague-Dawley rats (120-170 g). The perfusion of slices with a medium containing no added magnesium and 4-aminopyridine (50 microM) resulted in the generation of spontaneous bursts of population spikes of a fast frequency along with less frequent negative-going bursts. The frequency of the bursts produced was consistent over a 3h period. Carbenoxolone (100 microM), a gap junction blocker and mineralocorticoid agonist, perfused for 75 min, reduced the frequency of both types of spontaneous burst activity. Perfusion of spironolactone (1 microM), a mineralocorticosteroid antagonist, for 15 min prior to and during carbenoxolone perfusion did not alter the ability of carbenoxolone to depress the frequency of spontaneous activity. The incubation of hippocampal slices in carbenoxolone prior to recording increased the time taken for the spontaneous activity to start on change to the zero magnesium/4-aminopyridine medium and decreased the total number of spontaneous bursts over the first 60 min period. The ability of carbenoxolone to delay induction of epileptiform activity and reduce established epileptiform activity suggests that gap junctions contribute to the synchronization of neuronal firing in this model.

Osmotic forces and gap junctions in spreading depression: a computational model

In a computational model of spreading depression (SD), ionic movement through a neuronal syncytium of cells connected by gap junctions is described electrodiffusively. Simulations predict that SD will not occur unless cells are allowed to expand in response to osmotic pressure gradients and K+ is allowed to move through gap junctions. SD waves of [K+]out approximately 25 to approximately 60 mM moving at approximately 2 to approximately 18 mm/min are predicted over the range of parametric values reported in gray matter, with extracellular space decreasing up to approximately 50%. Predicted waveform shape is qualitatively similar to laboratory reports. The delayed-rectifier, NMDA, BK, and Na+ currents are predicted to facilitate SD, while SK and A-type K+ currents and glial activity impede SD. These predictions are consonant with recent findings that gap junction poisons block SD and support the theories that cytosolic diffusion via gap junctions and osmotic forces are important mechanisms underlying SD.

Autonomous activity and autophosphorylation of CAMPK-II in rat hippocampal slices: effects of tissue preparation

Measurement of the proportion of calcium/calmodulin-stimulated protein kinase II (CaMPK-II) that is autonomously active or phosphorylated on Thr(286) is thought to provide an index of the degree to which CaMPK-II in a tissue has been activated. We have examined how various ways of handling hippocampal tissue can alter these properties. Both autonomous activity and phospho-Thr(286) content was high in freshly dissected hippocampus or freshly cut hippocampal slices. After incubation of hippocampal slices in artificial cerebrospinal fluid for 120 min, both properties of CaMPK-II decreased to a steady state level. Freeze-thaw or cutting the equilibrated slices could rapidly increase both autonomous activity and phospho-Thr(286) immunoreactivity of CaMPK-II. These increases were comparable to changes induced by experimental treatment. Therefore, our results suggest that considerable care needs to be taken over the way in which hippocampal slices are handled.

The effects of anesthetics on cortical spreading depression elicitation and c-fos expression in rats

The effects of anesthetics on the generation of cortical spreading depression (CSD) were investigated. Volatile anesthetics halothane, isoflurane, sevoflurane (0.5, 1.0, and 2.0 MAC), and the intravenous anesthetic pentobarbital were studied. Cortical spreading depression was induced by 3M-KCl applied to a surface of brain cortex for 30 minutes. Direct current (DC) potential was recorded, and the number, amplitude, and duration of CSDs were observed. With increasing concentrations of each volatile anesthetic, there was a dose-related reduction in CSD frequency but not in CSD amplitude. At 2.0 MAC of sevoflurane the suppression of CSD was less than with the other volatile anesthetics. In addition, the influence of anesthetics on expression of c-fos mRNA was investigated. Additional animals anesthetized by isoflurane or sevoflurane were studied. Five CSDs were elicited by electric stimulation (0.5 mV, 1 second) in each animal. In situ hybridization with 35S-labeled oligonucleotides was used to evaluate the level of c-fos mRNA. The expression of c-fos was observed in the hemisphere in which CSD was elicited, but there was no difference in expression of c-fos among the groups. We conclude that volatile anesthetics can induce suppression of CSD elicitation in a dose dependent manner, but that at high concentrations sevoflurane is significantly less effective than other volatile agents. Pentobarbital has the least effect on KCl-induced CSD. These data suggest that the choice of anesthetics can impact the results of studies examining membrane depolarization and the ionic changes initiated by CSD.

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

Severe hypoxia causes rapid depolarization of CA1 neurons and glial cells that resembles spreading depression (SD). In brain slices in vitro, the SD-like depolarization and the associated irreversible loss of function can be postponed, but not prevented, by blockade of Na(+) currents by tetrodotoxin (TTX). To investigate the role of Na(+) flux, we made recordings from the CA1 region in hippocampal slices in the presence and absence of TTX. We measured membrane changes in single CA1 pyramidal neurons simultaneously with extracellular DC potential (V(o)) and either extracellular [K(+)] or [Na(+)]; alternatively, we simultaneously recorded [Na(+)](o), [K(+)](o), and V(o). Confirming previous reports, early during hypoxia, before SD onset, [K(+)](o) began to rise, whereas [Na(+)](o) still remained normal and V(o) showed a slight, gradual, negative shift; neurons first hyperpolarized and then began to gradually depolarize. The SD-like abrupt negative DeltaV(o) corresponded to a near complete depolarization of pyramidal neurons and an 89% decrease in input resistance. [K(+)](o) increased by 47 mM and [Na(+)](o) dropped by 91 mM. Changes in intracellular Na(+) and K(+) concentrations, estimated on the basis of the measured extracellular ion levels and the relative volume fractions of the neuronal, glial, and extracellular compartment, were much more moderate. Because [Na(+)](o) dropped more than [K(+)](o) increased, simple exchange of Na(+) for K(+) cannot account for these ionic changes. The apparent imbalance of charge could be made up by Cl(-) influx into neurons paralleling Na(+) flux and release of Mg(2+) from cells. The hypoxia-induced changes in interneurons resembled those observed in pyramidal neurons. Astrocytes responded with an initial slow depolarization as [K(+)](o) rose. It was followed by a rapid but incomplete depolarization as soon as SD occurred, which could be accounted for by the reduced ratio, [K(+)](i)/[K(+)](o). TTX (1 microM) markedly postponed SD, but the SD-related changes in [K(+)](o) and [Na(+)](o) were only reduced by 23 and 12%, respectively. In TTX-treated pyramidal neurons, the delayed SD-like depolarization took off from a more positive level, but the final depolarized intracellular potential and input resistance were not different from control. We conclude that TTX-sensitive channels mediate only a fraction of the Na(+) influx, and that some of the K(+) is released in exchange for Na(+). Even though TTX-sensitive Na(+) currents are not essential for the self-regenerative membrane changes during hypoxic SD, in control solutions their activation may trigger the transition from gradual to rapid depolarization of neurons, thereby synchronizing the SD-like event.

Modulation of burst frequency, duration, and amplitude in the zero-Ca(2+) model of epileptiform activity

Incubation of hippocampal slices in zero-Ca(2+) medium blocks synaptic transmission and results in spontaneous burst discharges. This seizure-like activity is characterized by negative shifts (bursts) in the extracellular field potential and a K(+) wave that propagates across the hippocampus. To isolate factors related to seizure initiation, propagation, and termination, a number of pharmacological agents were tested. K(+) influx and efflux mechanisms where blocked with cesium, barium, tetraethylammonium (TEA), and 4-aminopyridine (4-AP). The effect of the gap junction blockers, heptanol and octanol, on zero-Ca(2+) bursting was evaluated. Neuronal excitability was modulated with tetrodotoxin (TTX), charge screening, and applied electric fields. Glial cell function was examined with a metabolism antagonist (fluroacetate). Neuronal hyperpolarization by cation screening or applied fields decreased burst frequency but did not affect burst amplitude or duration. Heptanol attenuated burst amplitude and duration at low concentration (0.2 mM), and blocked bursting at higher concentration (0.5 mM). CsCl(2) (1 mM) had no effect, whereas high concentrations (1 mM) of BaCl(2) blocked bursting. TEA (25 mM) and low concentration of BaCl(2) (300 microM) resulted in a two- to sixfold increase in burst duration. Fluroacetate also blocked burst activity but only during prolonged application (>3 h). Our results demonstrate that burst frequency, amplitude, and duration can be independently modulated and suggest that neuronal excitability plays a central role in burst initiation, whereas potassium dynamics establish burst amplitude and duration.