Membrane currents in CA1 pyramidal cells during spreading depression (SD) and SD-like hypoxic depolarization

Spreading depolarizations pose critical energy challenges in acute brain injury

Spreading depolarization (SD) is an electrochemical wave of neuronal depolarization mediated by extracellular K+ and glutamate, interacting with voltage-gated and ligand-gated ion channels. SD is increasingly recognized as a major cause of injury progression in stroke and brain trauma, where the mechanisms of SD-induced neuronal injury are intimately linked to energetic status and metabolic impairment. Here, I review the established working model of SD initiation and propagation. Then, I summarize the historical and recent evidence for the metabolic impact of SD, transitioning from a descriptive to a mechanistic working model of metabolic signaling and its potential to promote neuronal survival and resilience. I quantify the energetic cost of restoring ionic gradients eroded during SD, and the extent to which ion pumping impacts high-energy phosphate pools and the energy charge of affected tissue. I link energy deficits to adaptive increases in the utilization of glucose and O2, and the resulting accumulation of lactic acid and CO2 downstream of catabolic metabolic activity. Finally, I discuss the neuromodulatory and vasoactive paracrine signaling mediated by adenosine and acidosis, highlighting these metabolites' potential to protect vulnerable tissue in the context of high-frequency SD clusters.

Between life and death: the brain twilight zones

Clinically, and legally, death is considered a well-defined state of the organism characterized, at least, by a complete and irreversible cessation of brain activities and functions. According to this pragmatic approach, the moment of death is implicitly represented by a discrete event from which all cerebral processes abruptly cease. However, a growing body of experimental and clinical evidence has demonstrated that cardiorespiratory failure, the leading cause of death, causes complex time-dependent changes in neuronal activity that can lead to death but also be reversed with successful resuscitation. This review synthesizes our current knowledge of the succeeding alterations in brain activities that accompany the dying and resuscitation processes. The anoxia-dependent brain defects that usher in a process of potential death successively include: (1) a set of changes in electroencephalographic (EEG) and neuronal activities, (2) a cessation of brain spontaneous electrical activity (isoelectric state), (3) a loss of consciousness whose timing in relation to EEG changes remains unclear, (4) an increase in brain resistivity, caused by neuronal swelling, concomitant with the occurrence of an EEG deviation reflecting the neuronal anoxic insult (the so-called "wave of death," or "terminal spreading depolarization"), followed by, (5) a terminal isoelectric brain state leading to death. However, a timely restoration of brain oxygen supply-or cerebral blood flow-can initiate a mirrored sequence of events: a repolarization of neurons followed by a re-emergence of neuronal, synaptic, and EEG activities from the electrocerebral silence. Accordingly, a recent study has revealed a new death-related brain wave: the "wave of resuscitation," which is a marker of the collective recovery of electrical properties of neurons at the beginning of the brain's reoxygenation phase. The slow process of dying still represents a terra incognita, during which neurons and neural networks evolve in uncertain states that remain to be fully understood. As current event-based models of death have become neurophysiologically inadequate, I propose a new mixed (event-process) model of death and resuscitation. It is based on a detailed description of the different phases that succeed each other in a dying brain, which are generally described separately and without mechanistic linkage, in order to integrate them into a continuum of declining brain activity. The model incorporates cerebral twilight zones (with still unknown neuronal and synaptic processes) punctuated by two characteristic cortical waves providing real-time biomarkers of death- and resuscitation.

Sudden unexpected death in epilepsy is prevented by blocking postictal hypoxia

Epilepsy is at times a fatal disease. Sudden unexpected death in epilepsy (SUDEP) is the leading cause of epilepsy-related mortality in people with intractable epilepsy and is defined by exclusion; non-accidental, non-toxicologic, and non-anatomic causes of death. While SUDEP often follows a bilateral tonic-clonic seizure, the mechanisms that ultimately lead to terminal apnea and then asystole remain elusive and there is a lack of preventative treatments. Based on the observation that discrete seizures lead to local and postictal vasoconstriction, resulting in hypoperfusion, hypoxia and behavioural disturbances in the forebrain we reasoned those similar mechanisms may play a role in SUDEP when seizures invade the brainstem. Here we tested this neurovascular-based hypothesis of SUDEP in awake non-anesthetized mice by pharmacologically preventing seizure-induced vasoconstriction, with cyclooxygenase-2 or L-type calcium channel antagonists. In both acute and chronic mouse models of seizure-induced premature mortality, ibuprofen and nicardipine extended life while systemic drug levels remained high enough to be effective. We also examined the potential role of spreading depolarization in the acute model of seizure-induced premature mortality. These data provide a proof-of-principle for the neurovascular hypothesis of SUDEP rather than spreading depolarization and the use of currently available drugs to prevent it.

Oxyhemoglobin and Cerebral Blood Flow Transients Detect Infarction in Rat Focal Brain Ischemia

Spreading depolarizations (SD) refer to the near-complete depolarization of neurons that is associated with brain injuries such as ischemic stroke. The present gold standard for SD monitoring in humans is invasive electrocorticography (ECoG). A promising non-invasive alternative to ECoG is diffuse optical monitoring of SD-related flow and hemoglobin transients. To investigate the clinical utility of flow and hemoglobin transients, we analyzed their association with infarction in rat focal brain ischemia. Optical images of flow, oxy-hemoglobin, and deoxy-hemoglobin were continuously acquired with Laser Speckle and Optical Intrinsic Signal imaging for 2 h after photochemically induced distal middle cerebral artery occlusion in Sprague-Dawley rats (n = 10). Imaging was performed through a 6 × 6 mm window centered 3 mm posterior and 4 mm lateral to Bregma. Rats were sacrificed after 24 h, and the brain slices were stained for assessment of infarction. We mapped the infarcted area onto the imaging data and used nine circular regions of interest (ROI) to distinguish infarcted from non-infarcted tissue. Transients propagating through each ROI were characterized with six parameters (negative, positive, and total amplitude; negative and positive slope; duration). Transients were also classified into three morphology types (positive monophasic, biphasic, negative monophasic). Flow transient morphology, positive amplitude, positive slope, and total amplitude were all strongly associated with infarction (p < 0.001). Associations with infarction were also observed for oxy-hemoglobin morphology, oxy-hemoglobin positive amplitude and slope, and deoxy-hemoglobin positive slope and duration (all p < 0.01). These results suggest that flow and hemoglobin transients accompanying SD have value for detecting infarction.

Migraine Aura, Transient Ischemic Attacks, Stroke, and Dying of the Brain Share the Same Key Pathophysiological Process in Neurons Driven by Gibbs–Donnan Forces, Namely Spreading Depolarization

Neuronal cytotoxic edema is the morphological correlate of the near-complete neuronal battery breakdown called spreading depolarization, or conversely, spreading depolarization is the electrophysiological correlate of the initial, still reversible phase of neuronal cytotoxic edema. Cytotoxic edema and spreading depolarization are thus different modalities of the same process, which represents a metastable universal reference state in the gray matter of the brain close to Gibbs–Donnan equilibrium. Different but merging sections of the spreading-depolarization continuum from short duration waves to intermediate duration waves to terminal waves occur in a plethora of clinical conditions, including migraine aura, ischemic stroke, traumatic brain injury, aneurysmal subarachnoid hemorrhage (aSAH) and delayed cerebral ischemia (DCI), spontaneous intracerebral hemorrhage, subdural hematoma, development of brain death, and the dying process during cardio circulatory arrest. Thus, spreading depolarization represents a prime and simultaneously the most neglected pathophysiological process in acute neurology. Aristides Leão postulated as early as the 1940s that the pathophysiological process in neurons underlying migraine aura is of the same nature as the pathophysiological process in neurons that occurs in response to cerebral circulatory arrest, because he assumed that spreading depolarization occurs in both conditions. With this in mind, it is not surprising that patients with migraine with aura have about a twofold increased risk of stroke, as some spreading depolarizations leading to the patient percept of migraine aura could be caused by cerebral ischemia. However, it is in the nature of spreading depolarization that it can have different etiologies and not all spreading depolarizations arise because of ischemia. Spreading depolarization is observed as a negative direct current (DC) shift and associated with different changes in spontaneous brain activity in the alternating current (AC) band of the electrocorticogram. These are non-spreading depression and spreading activity depression and epileptiform activity. The same spreading depolarization wave may be associated with different activity changes in adjacent brain regions. Here, we review the basal mechanism underlying spreading depolarization and the associated activity changes. Using original recordings in animals and patients, we illustrate that the associated changes in spontaneous activity are by no means trivial, but pose unsolved mechanistic puzzles and require proper scientific analysis.

Questioning Glutamate Excitotoxicity in Acute Brain Damage: The Importance of Spreading Depolarization

BACKGROUND

Within 2 min of severe ischemia, spreading depolarization (SD) propagates like a wave through compromised gray matter of the higher brain. More SDs arise over hours in adjacent tissue, expanding the neuronal damage. This period represents a therapeutic window to inhibit SD and so reduce impending tissue injury. Yet most neuroscientists assume that the course of early brain injury can be explained by glutamate excitotoxicity, the concept that immediate glutamate release promotes early and downstream brain injury. There are many problems with glutamate release being the unseen culprit, the most practical being that the concept has yielded zero therapeutics over the past 30 years. But the basic science is also flawed, arising from dubious foundational observations beginning in the 1950s METHODS: Literature pertaining to excitotoxicity and to SD over the past 60 years is critiqued.

RESULTS

Excitotoxicity theory centers on the immediate and excessive release of glutamate with resulting neuronal hyperexcitation. This instigates poststroke cascades with subsequent secondary neuronal injury. By contrast, SD theory argues that although SD evokes some brief glutamate release, acute neuronal damage and the subsequent cascade of injury to neurons are elicited by the metabolic stress of SD, not by excessive glutamate release. The challenge we present here is to find new clinical targets based on more informed basic science. This is motivated by the continuing failure by neuroscientists and by industry to develop drugs that can reduce brain injury following ischemic stroke, traumatic brain injury, or sudden cardiac arrest. One important step is to recognize that SD plays a central role in promoting early neuronal damage. We argue that uncovering the molecular biology of SD initiation and propagation is essential because ischemic neurons are usually not acutely injured unless SD propagates through them. The role of glutamate excitotoxicity theory and how it has shaped SD research is then addressed, followed by a critique of its fading relevance to the study of brain injury.

CONCLUSIONS

Spreading depolarizations better account for the acute neuronal injury arising from brain ischemia than does the early and excessive release of glutamate.

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.

New Insights and Methods for Recording and Imaging Spontaneous Spreading Depolarizations and Seizure-Like Events in Mouse Hippocampal Slices

Spreading depolarization (SD) is a sudden, large, and synchronous depolarization of principal cells which also involves interneurons and astrocytes. It is followed by depression of neuronal activity, and it slowly propagates across brain regions like cortex or hippocampus. SD is considered to be mechanistically relevant to migraine, epilepsy, and traumatic brain injury (TBI), but there are many questions about its basic neurophysiology and spread. Research into SD in hippocampus using slices is often used to gain insight and SD is usually triggered by a focal stimulus with or without an altered extracellular buffer. Here, we optimize an in vitro experimental model allowing us to record SD without focal stimulation, which we call spontaneous. This method uses only an altered extracellular buffer containing 0 mM Mg2+ and 5 mM K+ and makes it possible for simultaneous patch and extracellular recording in a submerged chamber plus intrinsic optical imaging in slices of either sex. We also add methods for quantification and show the quantified optical signal is much more complex than imaging alone would suggest. In brief, acute hippocampal slices were prepared with a chamber holding a submerged slice but with flow of artificial cerebrospinal fluid (aCSF) above and below, which we call interface-like. As soon as slices were placed in the chamber, aCSF with 0 Mg2+/5 K+ was used. Most mouse slices developed SD and did so in the first hour of 0 Mg2+/5 K+ aCSF exposure. In addition, prolonged bursts we call seizure-like events (SLEs) occurred, and the interactions between SD and SLEs suggest potentially important relationships. Differences between rats and mice in different chambers are described. Regarding optical imaging, SD originated in CA3 and the pattern of spread to CA1 and the dentate gyrus was similar in some ways to prior studies but also showed interesting differences. In summary, the methods are easy to use, provide new opportunities to study SD, new insights, and are inexpensive. They support previous suggestions that SD is diverse, and also suggest that participation by the dentate gyrus merits greater attention.

Neuronal Swelling: A Non-osmotic Consequence of Spreading Depolarization

An acute reduction in plasma osmolality causes rapid uptake of water by astrocytes but not by neurons, whereas both cell types swell as a consequence of lost blood flow (ischemia). Either hypoosmolality or ischemia can displace the brain downwards, potentially causing death. However, these disorders are fundamentally different at the cellular level. Astrocytes osmotically swell or shrink because they express functional water channels (aquaporins), whereas neurons lack functional aquaporins and thus maintain their volume. Yet both neurons and astrocytes immediately swell when blood flow to the brain is compromised (cytotoxic edema) as following stroke onset, sudden cardiac arrest, or traumatic brain injury. In each situation, neuronal swelling is the direct result of spreading depolarization (SD) generated when the ATP-dependent sodium/potassium ATPase (the Na+/K+ pump) is compromised. The simple, and incorrect, textbook explanation for neuronal swelling is that increased Na+ influx passively draws Cl- into the cell, with water following by osmosis via some unknown conduit. We first review the strong evidence that mammalian neurons resist volume change during acute osmotic stress. We then contrast this with their dramatic swelling during ischemia. Counter-intuitively, recent research argues that ischemic swelling of neurons is non-osmotic, involving ion/water cotransporters as well as at least one known amino acid water pump. While incompletely understood, these mechanisms argue against the dogma that neuronal swelling involves water uptake driven by an osmotic gradient with aquaporins as the conduit. Promoting clinical recovery from neuronal cytotoxic edema evoked by spreading depolarizations requires a far better understanding of molecular water pumps and ion/water cotransporters that act to rebalance water shifts during brain ischemia.

Motor patterning, ion regulation and spreading depolarization during CNS shutdown induced by experimental anoxia in Locusta migratoria

Anoxia induces a reversible coma in insects. Coma onset is triggered by the arrest of mechanisms responsible for maintaining membrane ion homeostasis in the CNS, resulting in a wave of neuronal and glial depolarization known as spreading depolarization (SD). Different methods of anoxia influence the behavioural response but their effects on SD are unknown. We investigated the effects of CO₂, N₂, and H₂O on the characteristics of coma induction and recovery in Locusta migratoria. Water immersion delayed coma onset and recovery, likely due to involvement of the tracheal system and the nature of asphyxiation but otherwise resembled N₂ delivery. The main difference between N₂ and CO₂ was that CO₂ hastened onset of neural failure and SD and delayed recovery. In the CNS, this was associated with CO₂ inducing an abrupt and immediate decrease of interstitial pH and increase of extracellular [K⁺]. Recording of the transperineurial potential showed that SD propagation and a postanoxic negativity (PAN) were similar with both gases. The PAN increased with ouabain treatment, likely due to removal of the counteracting electrogenic effect of Na⁺/K⁺-ATPase, and was inhibited by bafilomycin, a proton pump inhibitor, suggesting that it was generated by the electrogenic effect of a Vacuolar-type ATPase (VA). Muscle fibres depolarized by ~20 mV, which happened more rapidly with CO₂ compared with N₂. Wing muscle motoneurons depolarized nearly completely in two stages, with CO₂ causing more rapid onset and slower recovery than N₂. Other parameters of SD onset and recovery were similar with the two gases. Electrical resistance across the ganglion sheath increased during anoxia and at SD onset. We provisionally attribute this to cell swelling reducing the dimensions of the interstitial pathway from neuropil to the bathing saline. Neuronal membrane resistance decreased abruptly at SD onset indicating opening of an unidentified membrane conductance. Consideration of the intracellular recording relative to the saline suggests that the apical membrane of perineurial glia depolarizes prior to neuron depolarization. We propose that SD is triggered by events at the perineurial sheath and then propagates laterally and more deeply into the neuropil. We conclude that the fundamental nature of SD is not dependent on the method of anoxia however the timing of onset and recovery are influenced; water immersion is complicated by the tracheal system and CO₂ delivery has more rapid and longer lasting effects, associated with severe interstitial acidosis.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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.

NMDA receptors sustain but do not initiate neuronal depolarization in spreading depolarization

Spreading depolarization (SD) represents a neurological process characterized by a massive, self-sustaining wave of brain cell depolarization. Understanding its mechanism is important for treating ischemic or hemorrhagic stroke and migraine with aura. Many believed that ion fluxes through NMDA receptors (NMDARs) are responsible for neuronal transmembrane currents of SD. However, the explicit role of NMDARs remains ambiguous. This is in part due to the limitation of traditional pharmacological approaches in resolving the contribution of NMDARs in different intercellular and intracellular processes of SD. Here, we applied single-cell blockade and genetic deletion methods to remove functional NMDARs from individual hippocampal CA1 neurons in order to examine the role of NMDARs in the depolarization mechanism without affecting the propagation of SD. We analyzed neuronal membrane potential changes to demonstrate that NMDARs are not required for initiating the depolarization. Consistently, neuronal input resistance (R_N) revealed a sharp decline at the start of SD, which was unaffected by blocking NMDARs. Instead, the recovery of both membrane potential and R_N during the late phase of SD was facilitated by inhibition of NMDARs, indicating that NMDARs are responsible for sustaining the depolarization. Our results strongly indicate that NMDAR activation is not a determinant of the initiation of depolarization but is important for sustaining transmembrane ion fluxes during SD.

Neural shutdown under stress: an evolutionary perspective on spreading depolarization

Neural function depends on maintaining cellular membrane potentials as the basis for electrical signaling. Yet, in mammals and insects, neuronal and glial membrane potentials can reversibly depolarize to zero, shutting down neural function by the process of spreading depolarization (SD) that collapses the ion gradients across membranes. SD is not evident in all metazoan taxa with centralized nervous systems. We consider the occurrence and similarities of SD in different animals and suggest that it is an emergent property of nervous systems that have evolved to control complex behaviors requiring energetically expensive, rapid information processing in a tightly regulated extracellular environment. Whether SD is beneficial or not in mammals remains an open question. However, in insects, it is associated with the response to harsh environments and may provide an energetic advantage that improves the chances of survival. The remarkable similarity of SD in diverse taxa supports a model systems approach to understanding the mechanistic underpinning of human neuropathology associated with migraine, stroke, and traumatic brain injury.

Mitochondrial dysfunction and role in spreading depolarization and seizure

The effect of pathological phenomena such as epileptic seizures and spreading depolarization (SD) on mitochondria and the potential feedback of mitochondrial dysfunction into the dynamics of those phenomena are complex and difficult to study experimentally due to the simultaneous changes in many variables governing neuronal behavior. By combining a model that accounts for a wide range of neuronal behaviors including seizures, normoxic SD, and hypoxic SD (HSD), together with a detailed model of mitochondrial function and intracellular Ca²⁺ dynamics, we investigate mitochondrial dysfunction and its potential role in recovery of the neuron from seizures, HSD, and SD. Our results demonstrate that HSD leads to the collapse of mitochondrial membrane potential and cellular ATP levels that recover only when normal oxygen supply is restored. Mitochondrial organic phosphate and pH gradients determine the strength of the depolarization block during HSD and SD, how quickly the cell enters the depolarization block when the oxygen supply is disrupted or potassium in the bath solution is raised beyond the physiological value, and how fast the cell recovers from SD and HSD when normal potassium concentration and oxygen supply are restored. Although not as dramatic as phosphate and pH gradients, mitochondrial Ca²⁺ uptake has a similar effect on neuronal behavior during these conditions.

Susceptibility of the cerebral cortex to spreading depolarization in neurological disease states: The impact of aging

Secondary injury following acute brain insults significantly contributes to poorer neurological outcome. The spontaneous, recurrent occurrence of spreading depolarization events (SD) has been recognized as a potent secondary injury mechanism in subarachnoid hemorrhage, malignant ischemic stroke and traumatic brain injury. In addition, SD is the underlying mechanism of the aura symptoms of migraineurs. The susceptibility of the nervous tissue to SD is subject to the metabolic status of the tissue, the ionic composition of the extracellular space, and the functional status of ion pumps, voltage-gated and other cation channels, glutamate receptors and excitatory amino acid transporters. All these mechanisms tune the excitability of the nervous tissue. Aging has also been found to alter SD susceptibility, which appears to be highest at young adulthood, and decline over the aging process. The lower susceptibility of the cerebral gray matter to SD in the old brain may be caused by the age-related impairment of mechanisms implicated in ion translocations between the intra- and extracellular compartments, glutamate signaling and surplus potassium and glutamate clearance. Even though the aging nervous tissue is thus less able to sustain SD, the consequences of SD recurrence in the old brain have proven to be graver, possibly leading to accelerated lesion maturation. Taken that recurrent SDs may pose an increased burden in the aging injured brain, the benefit of therapeutic approaches to restrict SD generation and propagation may be particularly relevant for elderly patients.

Large-conductance Ca2+-activated potassium channels are potently involved in the inverse neurovascular response to spreading depolarization

Recurrent spreading depolarizations occur in the cerebral cortex from minutes up to weeks following acute brain injury. Clinical evidence suggests that the immediate reduction of cerebral blood flow in response to spreading depolarization importantly contributes to lesion progression as the wave propagates over vulnerable tissue zones, characterized by potassium concentration already elevated prior to the passage of spreading depolarization. Here we demonstrate with two-photon microscopy in anesthetized mice that initial vasoconstriction in response to SD triggered experimentally with 1 M KCl is coincident in space and time with the large extracellular accumulation of potassium, as shown with a potassium indicator fluorescent dye. Moreover, pharmacological manipulations in combination with the use of potassium-sensitive microelectrodes suggest that large-conductance Ca²⁺-activated potassium (BK) channels and L-type voltage-gated calcium channels play significant roles in the marked initial vasoconstriction under elevated baseline potassium. We propose that potassium efflux through BK channels is a central component in the devastating neurovascular effects of spreading depolarizations in tissue at risk.

Protein kinase C epsilon delays latency until anoxic depolarization through arc expression and GluR2 internalization

Global cerebral ischemia is a debilitating injury that damages the CA1 region of the hippocampus, an area important for learning and memory. Protein kinase C epsilon (PKCɛ) activation is a critical component of many neuroprotective treatments. The ability of PKCɛ activation to regulate AMPA receptors (AMPARs) remains unexplored despite the role of AMPARs in excitotoxicity after brain ischemia. We determined that PKCɛ activation increased expression of a protein linked to learning and memory, activity-regulated cytoskeleton-associated protein (arc). Also, arc is necessary for neuroprotection and confers protection through decreasing AMPAR currents via GluR2 internalization. In vivo, activation of PKCɛ increased arc expression through a BDNF/TrkB pathway, and decreased GluR2 mRNA levels. In hippocampal cultured slices, PKCɛ activation decreased AMPAR current amplitudes in an arc- and GluR2-dependent manner. Additionally, PKCɛ activation triggered an arc- and GluR2 internalization-dependent delay in latency until anoxic depolarization. Inhibiting arc also blocked PKCɛ-mediated neuroprotection against lethal oxygen and glucose deprivation. These data characterize a novel PKCɛ-dependent mechanism that for the first time defines a role for arc and AMPAR internalization in conferring neuroprotection.

The role of glutamate in neuronal ion homeostasis: A case study of spreading depolarization

Simultaneous changes in ion concentrations, glutamate, and cell volume together with exchange of matter between cell network and vasculature are ubiquitous in numerous brain pathologies. A complete understanding of pathological conditions as well as normal brain function, therefore, hinges on elucidating the molecular and cellular pathways involved in these mostly interdependent variations. In this paper, we develop the first computational framework that combines the Hodgkin-Huxley type spiking dynamics, dynamic ion concentrations and glutamate homeostasis, neuronal and astroglial volume changes, and ion exchange with vasculature into a comprehensive model to elucidate the role of glutamate uptake in the dynamics of spreading depolarization (SD)-the electrophysiological event underlying numerous pathologies including migraine, ischemic stroke, aneurysmal subarachnoid hemorrhage, intracerebral hematoma, and trauma. We are particularly interested in investigating the role of glutamate in the duration and termination of SD caused by K+ perfusion and oxygen-glucose deprivation. Our results demonstrate that glutamate signaling plays a key role in the dynamics of SD, and that impaired glutamate uptake leads to recovery failure of neurons from SD. We confirm predictions from our model experimentally by showing that inhibiting astrocytic glutamate uptake using TFB-TBOA nearly quadruples the duration of SD in layers 2-3 of visual cortical slices from juvenile rats. The model equations are either derived purely from first physical principles of electroneutrality, osmosis, and conservation of particles or a combination of these principles and known physiological facts. Accordingly, we claim that our approach can be used as a future guide to investigate the role of glutamate, ion concentrations, and dynamics cell volume in other brain pathologies and normal brain function.

Mechanisms of Neuronal Silencing After Cortical Spreading Depression

Cortical spreading depression (CSD) is associated with migraine, stroke, and traumatic brain injury, but its mechanisms remain poorly understood. One of the major features of CSD is an hour-long silencing of neuronal activity. Though this silencing has clear ramifications for CSD-associated disease, it has not been fully explained. We used in vivo whole-cell recordings to examine the effects of CSD on layer 2/3 pyramidal neurons in mouse somatosensory cortex and used in vitro recordings to examine their mechanism. We found that CSD caused a reduction in spontaneous synaptic activity and action potential (AP) firing that lasted over an hour. Both pre- and postsynaptic mechanisms contributed to this silencing. Reductions in frequency of postsynaptic potentials were due to a reduction in presynaptic transmitter release probability as well as reduced AP activity. Decreases in postsynaptic potential amplitude were due to an inhibitory shift in the ratio of excitatory and inhibitory postsynaptic currents. This inhibitory shift in turn contributed to the reduced frequency of APs. Thus, distinct but complementary mechanisms generate the long neuronal silence that follows CSD. These cellular changes could contribute to wider network dysfunction in CSD-associated disease, while the pre- and postsynaptic mechanisms offer separate targets for therapy.

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.

Anions Govern Cell Volume: A Case Study of Relative Astrocytic and Neuronal Swelling in Spreading Depolarization

Cell volume changes are ubiquitous in normal and pathological activity of the brain. Nevertheless, we know little about the dynamics of cell and tissue swelling, and the differential changes in the volumes of neurons and glia during pathological states such as spreading depolarizations (SD) under ischemic and non–ischemic conditions, and epileptic seizures. By combining the Hodgkin–Huxley type spiking dynamics, dynamic ion concentrations, and simultaneous neuronal and astroglial volume changes into a comprehensive model, we elucidate why glial cells swell more than neurons in SD and the special case of anoxic depolarization (AD), and explore the relative contributions of the two cell types to tissue swelling. Our results demonstrate that anion channels, particularly Cl⁻, are intrinsically connected to cell swelling and blocking these currents prevents changes in cell volume. The model is based on a simple and physiologically realistic description. We introduce model extensions that are either derived purely from first physical principles of electroneutrality, osmosis, and conservation of particles, or by a phenomenological combination of these principles and known physiological facts. This work provides insights into numerous studies related to neuronal and glial volume changes in SD that otherwise seem contradictory, and is broadly applicable to swelling in other cell types and conditions.

The Role of Cell Volume in the Dynamics of Seizure, Spreading Depression, and Anoxic Depolarization

Cell volume changes are ubiquitous in normal and pathological activity of the brain. Nevertheless, we know little of how cell volume affects neuronal dynamics. We here performed the first detailed study of the effects of cell volume on neuronal dynamics. By incorporating cell swelling together with dynamic ion concentrations and oxygen supply into Hodgkin-Huxley type spiking dynamics, we demonstrate the spontaneous transition between epileptic seizure and spreading depression states as the cell swells and contracts in response to changes in osmotic pressure. Our use of volume as an order parameter further revealed a dynamical definition for the experimentally described physiological ceiling that separates seizure from spreading depression, as well as predicted a second ceiling that demarcates spreading depression from anoxic depolarization. Our model highlights the neuroprotective role of glial K buffering against seizures and spreading depression, and provides novel insights into anoxic depolarization and the relevant cell swelling during ischemia. We argue that the dynamics of seizures, spreading depression, and anoxic depolarization lie along a continuum of the repertoire of the neuron membrane that can be understood only when the dynamic ion concentrations, oxygen homeostasis,and cell swelling in response to osmotic pressure are taken into consideration. Our results demonstrate the feasibility of a unified framework for a wide range of neuronal behaviors that may be of substantial importance in the understanding of and potentially developing universal intervention strategies for these pathological states.

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.

Unification of neuronal spikes, seizures, and spreading depression

The pathological phenomena of seizures and spreading depression have long been considered separate physiological events in the brain. By incorporating conservation of particles and charge, and accounting for the energy required to restore ionic gradients, we extend the classic Hodgkin-Huxley formalism to uncover a unification of neuronal membrane dynamics. By examining the dynamics as a function of potassium and oxygen, we now account for a wide range of neuronal activities, from spikes to seizures, spreading depression (whether high potassium or hypoxia induced), mixed seizure and spreading depression states, and the terminal anoxic "wave of death." Such a unified framework demonstrates that all of these dynamics lie along a continuum of the repertoire of the neuron membrane. Our results demonstrate that unified frameworks for neuronal dynamics are feasible, can be achieved using existing biological structures and universal physical conservation principles, and may be of substantial importance in enabling our understanding of brain activity and in the control of pathological states.

Chaos and commotion in the wake of cortical spreading depression and spreading depolarizations

Punctuated episodes of spreading depolarizations erupt in the brain, encumbering tissue structure and function, and raising fascinating unanswered questions concerning their initiation and propagation. Linked to migraine aura and headache, cortical spreading depression contributes to the morbidity in the world's migraine with aura population. Even more ominously, erupting spreading depolarizations accelerate tissue damage during brain injury. The once-held view that spreading depolarizations may not exist in the human brain has changed, largely because of the discovery of migraine genes that confer cortical spreading depression susceptibility, the application of sophisticated imaging tools and efforts to interrogate their impact in the acutely injured human brain.

Oxygen and seizure dynamics: II. Computational modeling

Electrophysiological recordings show intense neuronal firing during epileptic seizures leading to enhanced energy consumption. However, the relationship between oxygen metabolism and seizure patterns has not been well studied. Recent studies have developed fast and quantitative techniques to measure oxygen microdomain concentration during seizure events. In this article, we develop a biophysical model that accounts for these experimental observations. The model is an extension of the Hodgkin-Huxley formalism and includes the neuronal microenvironment dynamics of sodium, potassium, and oxygen concentrations. Our model accounts for metabolic energy consumption during and following seizure events. We can further account for the experimental observation that hypoxia can induce seizures, with seizures occurring only within a narrow range of tissue oxygen pressure. We also reproduce the interplay between excitatory and inhibitory neurons seen in experiments, accounting for the different oxygen levels observed during seizures in excitatory vs. inhibitory cell layers. Our findings offer a more comprehensive understanding of the complex interrelationship among seizures, ion dynamics, and energy metabolism.

Oxygen and seizure dynamics: I. Experiments

We utilized a novel ratiometric nanoquantum dot fluorescence resonance energy transfer (NQD-FRET) optical sensor to quantitatively measure oxygen dynamics from single cell microdomains during hypoxic episodes as well as during 4-aminopyridine (4-AP)-induced spontaneous seizure-like events in rat hippocampal slices. Coupling oxygen sensing with electrical recordings, we found the greatest reduction in the O2 concentration ([O2]) in the densely packed cell body stratum (st.) pyramidale layer of the CA1 and differential layer-specific O2 dynamics between the st. pyramidale and st. oriens layers. These hypoxic decrements occurred up to several seconds before seizure onset could be electrically measured extracellularly. Without 4-AP, we quantified a narrow range of [O2], similar to the endogenous hypoxia found before epileptiform activity, which permits a quiescent network to enter into a seizure-like state. We demonstrated layer-specific patterns of O2 utilization accompanying layer-specific neuronal interplay in seizure. None of the oxygen overshoot artifacts seen with polarographic measurement techniques were observed. We therefore conclude that endogenously generated hypoxia may be more than just a consequence of increased cellular excitability but an influential and critical factor for orchestrating network dynamics associated with epileptiform activity.

Regenerative glutamate release by presynaptic NMDA receptors contributes to spreading depression

Spreading depression (SD) is a slowly propagating neuronal depolarization that underlies certain neurologic conditions. The wave-like pattern of its propagation suggests that SD arises from an unusual form of neuronal communication. We used enzyme-based glutamate electrodes to show that during SD induced by transiently raising extracellular K(+) concentrations ([K(+)]o) in rat brain slices, there was a rapid increase in the extracellular glutamate concentration that required vesicular exocytosis but unlike fast synaptic transmission, still occurred when voltage-gated sodium and calcium channels (VGSC and VGCC) were blocked. Instead, presynaptic N-methyl-D-aspartate (NMDA) receptors (NMDARs) were activated during SD and could generate substantial glutamate release to support regenerative glutamate release and propagating waves when VGSCs and VGCCs were blocked. In calcium-free solutions, high [K(+)]o still triggered SD-like waves and glutamate efflux. Under such a condition, glutamate release was blocked by mitochondrial Na(+)/Ca(2+) exchanger inhibitors that likely blocked calcium release from mitochondria secondary to NMDA-induced Na(+) influx. Therefore presynaptic NMDA receptor activation is sufficient for triggering vesicular glutamate release during SD via both calcium entry and release from mitochondria by mitochondrial Na(+)/Ca(2+) exchanger. Our observations suggest that presynaptic NMDARs contribute to a cycle of glutamate-induced glutamate release that mediate high [K(+)]o-triggered SD.

Endogenous electric currents might guide rostral migration of neuroblasts

Mechanisms that guide directional migration of neuroblasts from the subventricular zone (SVZ) are not well understood. We report here that endogenous electric currents serve as a guidance cue for neuroblast migration. We identify the existence of naturally occurring electric currents (1.5±0.6 μA/cm(2), average field strength of ∼3 mV/mm) along the rostral migration path in adult mouse brain. Electric fields of similar strength direct migration of neuroblasts from the SVZ in culture and in brain slices. The purinergic receptor P2Y1 mediates this migration. The results indicate that naturally occurring electric currents serve as a new guidance mechanism for rostral neuronal migration.

Sustained NMDA receptor activation by spreading depolarizations can initiate excitotoxic injury in metabolically compromised neurons

Spreading depolarizations (SDs) are slowly propagating waves of near-complete neuronal and glial depolarization. SDs have been recorded in patients with brain injury, and the incidence of SD significantly correlates with outcome severity. Although it is well accepted that the ionic dyshomeostasis of SD presents a severe metabolic burden, there is currently limited understanding of SD-induced injury processes at a cellular level. In the current study we characterized events accompanying SD in the hippocampal CA1 region of murine brain slices, using whole-cell recordings and single-cell Ca(2+) imaging. We identified an excitatory phase that persisted for approximately 2 min following SD onset, and accompanied with delayed dendritic ionic dyshomeostasis. The excitatory phase coincided with a significant increase in presynaptic glutamate release, evidenced by a transient increase in spontaneous EPSC frequency and paired-pulse depression of evoked EPSCs. Activation of NMDA receptors (NMDARs) during this late excitatory phase contributed to the duration of individual neuronal depolarizations and delayed recovery of extracellular slow potential changes. Selectively targeting the NMDAR activation following SD onset (by delayed pressure application of a competitive NMDAR antagonist) significantly decreased the duration of cellular depolarizations. Recovery of dendritic Ca(2+) elevations following SD were also sensitive to delayed NMDA antagonist application. Partial inhibition of neuronal energy metabolism converted SD into an irrecoverable event with persistent Ca(2+) overload and membrane compromise. Delayed NMDAR block was sufficient to prevent these acute injurious events in metabolically compromised neurons. These results identify a significant contribution of a late component of SD that could underlie neuronal injury in pathological circumstances.

Examining protection from anoxic depolarization by the drugs dibucaine and carbetapentane using whole cell recording from CA1 neurons

As an immediate consequence of stroke onset, failure of the Na(+)-K(+)-ATPase pump evokes a propagating anoxic depolarization (AD) across gray matter. Acute neuronal swelling and dendritic beading arise within seconds in the future ischemic core, imaged as changes in light transmittance (ΔLT). AD is itself not a target for drug-based reduction of stroke injury because it is generated in the 1st min of stroke onset. Peri-infarct depolarizations (PIDs) are milder AD-like events that recur during the hours following AD and contribute to infarct expansion. Inhibiting PIDs with drugs could limit expansion. Two types of drugs, "caines" and σ(1)-receptor ligands, have been found to inhibit AD onset (and may also oppose PID initiation), yet their underlying actions have not been examined. Imaging ΔLT in the CA1 region simultaneously with whole cell current-clamp recording from CA1 pyramidal neurons reveal that the elevated LT front and onset of the AD are coincident. Either dibucaine or carbetapentane pretreatment significantly delays AD onset without affecting resting membrane potential or neuronal input resistance. Dibucaine decreases excitability by raising spike threshold and decreasing action potential (AP) frequency, whereas carbetapentane eliminates the fast afterhyperpolarization while accentuating the slow afterhyperpolarization to reduce AP frequency. Orthodromic and antidromic APs are eliminated by dibucaine within 15 min but not by carbetapentane. Thus both drugs reduce cortical excitability at the level of the single pyramidal neuron but through strikingly different mechanisms. In vivo, both drugs would likely inhibit recurring PIDs in the expanding penumbra and so potentially could reduce developing neuronal damage over many hours poststroke when PIDs occur.

Spreading depolarization, a pathophysiological mechanism of stroke and migraine aura

Spreading depolarization is a mechanism of abrupt, massive ion translocation between intraneuronal and extracellular space that entails cytotoxic edema in the brain’s gray matter. It is observed in patients as a large change of the slow electrical potential. Dependent on the energy status of the tissue, spreading depolarization is either preceded by nonspreading silencing due to neuronal hyperpolarization or accompanied by spreading silencing of electrical brain activity due to a depolarization block. Nonspreading silencing seems to translate into the initial clinical symptoms of ischemic stroke and spreading silencing translates into migraine aura. Direct electrophysiological evidence exists that spreading depolarization occurs in abundance in aneurysmal subarachnoid hemorrhage, delayed ischemic stroke after subarachnoid hemorrhage, malignant hemispheric stroke, spontaneous intracerebral hemorrhage and traumatic brain injury. Indirect evidence suggests its occurrence during migraine aura. In animals, spreading depolarizations facilitate neuronal death when they invade metabolically compromised tissue, whereas they are relatively innocuous in healthy tissue. Therapies targeting spreading depolarization may potentially treat these neurological conditions.

Sequential changes in neuronal activity in single neocortical neurons after spreading depression

BACKGROUND

Cortical spreading depression (CSD) has an important role in migraine with aura. Prolonged neuronal depression is followed by a late excitatory synaptic plasticity after CSD.

METHOD

Intra- and extracellular recordings were performed to investigate the effect of CSD on intracellular properties of mouse neocortical tissues in the late excitatory period.

RESULTS

During CSD, changes in the membrane potentials usually began with a relatively short hyperpolarization followed by an abrupt depolarization. These changes occurred roughly at the same time point after CSD as the beginning of the negative extracellular deflection. Forty-five minutes after CSD, neurons showed significantly smaller amplitude of afterhyperpolarization and a reduced input resistance. Depolarization and hyperpolarization of the cells by constant intracellular current injections in this period significantly changed the frequency of the action potentials.

CONCLUSION

These data indicate higher excitability of the neocortical neurons after CSD, which can be assumed to contribute to hyperexcitability of neocortical tissues in patients suffering from migraine.

The role of spreading depression, spreading depolarization and spreading ischemia in neurological disease

The term spreading depolarization describes a wave in the gray matter of the central nervous system characterized by swelling of neurons, distortion of dendritic spines, a large change of the slow electrical potential and silencing of brain electrical activity (spreading depression). In the clinic, unequivocal electrophysiological evidence now exists that spreading depolarizations occur abundantly in individuals with aneurismal subarachnoid hemorrhage, delayed ischemic stroke after subarachnoid hemorrhage, malignant hemispheric stroke, spontaneous intracerebral hemorrhage or traumatic brain injury. Spreading depolarization is induced experimentally by various noxious conditions including chemicals such as potassium, glutamate, inhibitors of the sodium pump, status epilepticus, hypoxia, hypoglycemia and ischemia, but it can can also invade healthy, naive tissue. Resistance vessels respond to it with tone alterations, causing either transient hyperperfusion (physiological hemodynamic response) in healthy tissue or severe hypoperfusion (inverse hemodynamic response, or spreading ischemia) in tissue at risk for progressive damage, which contributes to lesion progression. Therapies that target spreading depolarization or the inverse hemodynamic response may potentially treat these neurological conditions.

Changes in membrane potential and the intracellular calcium concentration during CSD and OGD in layer V and layer II/III mouse cortical neurons

Cortical spreading depression (CSD) is an episode of electrical silence following intense neuronal activity that propagates across the cortex at ∼3-6 mm/min and is associated with transient neuronal depolarization. CSD is benign in normally perfused brain tissue, but there is evidence suggesting that repetitive CSD contributes to infarct growth following focal ischemia. Studies to date have assumed that the cellular responses to CSD are uniform across neuronal types because there are no data to the contrary. In this study, we investigated the effect of CSD on membrane potential and the intracellular calcium concentration ([Ca(2+)](i)) of mouse layer V and layer II/III pyramidal neurons in brain slices. To place the data in context, we made similar measurements during anoxic depolarization induced by oxygen and glucose deprivation (OGD). The [Ca(2+)](i) was quantified using the low-affinity ratiometric indicator Fura-4F. During both CSD- and OGD-induced depolarization, the membrane potential approached 0 mV in all neurons. In layer V pyramids OGD resulted in an increase in [Ca(2+)](i) to a maximum of 3.69 ± 0.73 (SD) μM (n = 12), significantly greater than the increase to 1.81 ± 0.70 μM in CSD (n = 34; P < 0.0001). Membrane potential and [Ca(2+)](i) returned to nearly basal levels following CSD but not OGD. Layer II/III neurons responded to CSD with a greater peak increase in [Ca(2+)](i) than layer V neurons (2.88 ± 0.6 μM; n = 9; P < 0.01). We conclude there is a laminar difference in the response of pyramidal neurons to CSD; possible explanations are discussed.

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.

An update on the blood vessel in migraine

PURPOSE OF REVIEW

The cranial blood vessel is considered an integral player in the pathophysiology of migraine, but its perceived role has been subject to much discussion and controversy over the years. We will discuss the evolution in our scientific understanding of cranial blood vessels (primarily arteries) in migraine.

RECENT FINDINGS

Recent developments have clarified the role of cranial blood vessels in the trigemino-vascular system and in cortical spreading depression. An underlying theme is the intimate relation between vascular activity and neural function, and we will emphasize the various roles of the blood vessel that go beyond delivering blood. We conclude that migraine cannot be understood, either from a research or clinical point of view, without an understanding of the vascular derangements that accompany it.

SUMMARY

Migraine is accompanied by significant derangements in vascular function that may represent important targets for investigation and treatment.

Generation of Sustained Field Potentials by Gradients of Polarization Within Single Neurons: A Macroscopic Model of Spreading Depression

Spreading depression (SD) is a pathological wave of depolarization of the neural tissue producing a negative macroscopic field potential ( Vo), used as a marker for diagnostic purposes. The cellular basis of SD and neuronal mechanisms of generation of Vo at the microscopic level are poorly understood. Using a CA1 mathematical model and experimental verification, we examined how transmembrane currents in single cells scale up in the extracellular space shaping Vo. The model includes an array of 17,000 realistically modeled neurons (responsible for generating transmembrane currents) dynamically coupled to a virtual aggregate/extracellular space (responsible for Vo). The SD wave in different tissue bands is simulated by imposing membrane shunts in the corresponding dendritic elements as suggested by experimentally assessed drop in membrane resistance. We show that strong isopotential depolarization of wide domains (as in the main SD phase) produce broad central cancellation of axial and transmembrane currents in single cells. When depolarization is restricted to narrow dendritic domains (as in the late SD phase), the internal cancellation shrinks and the transmembrane current increases. This explains why in the laminated CA1 the Vo is smaller in the main phase of SD, when both dendritic layers are seized, than in the SD tail restricted to an apical band. Moreover, scattering of the neuronal somatas (as in cortical regions) further decreases the aggregate Vo due to the volume averaging. Although mechanistically the Vo associated to SD is similar to customary transient fields, its changes maybe related to spatial factors in single cells rather than cell number or depolarization strength.

Biphasic direct current shift, haemoglobin desaturation and neurovascular uncoupling in cortical spreading depression

Cortical spreading depression is a propagating wave of depolarization that plays important roles in migraine, stroke, subarachnoid haemorrhage and brain injury. Cortical spreading depression is associated with profound vascular changes that may be a significant factor in the clinical response to cortical spreading depression events. We used a combination of optical intrinsic signal imaging, electro-physiology, potassium sensitive electrodes and spectroscopy to investigate neurovascular changes associated with cortical spreading depression in the mouse. We identified two distinct phases of altered neurovascular function, one during the propagating cortical spreading depression wave and a second much longer phase after passage of the wave. The direct current shift associated with the cortical spreading depression wave was accompanied by marked arterial constriction and desaturation of cortical haemoglobin. After recovery from the initial cortical spreading depression wave, we observed a second phase of prolonged, negative direct current shift, arterial constriction and haemoglobin desaturation, lasting at least an hour. Persistent disruption of neurovascular coupling was demonstrated by a loss of coherence between electro-physiological activity and perfusion. Extracellular potassium concentration increased during the cortical spreading depression wave, but recovered and remained at baseline after passage of the wave, consistent with different mechanisms underlying the first and second phases of neurovascular dysfunction. These findings indicate that cortical spreading depression is associated with a multiphasic alteration in neurovascular function, including a novel second direct current shift accompanied by arterial constriction and decrease in tissue oxygen supply, that is temporally and mechanistically distinct from the initial propagated cortical spreading depression wave. Vascular/metabolic uncoupling with cortical spreading depression may have important clinical consequences, and the different phases of dysfunction may represent separate therapeutic targets in the disorders where cortical spreading depression occurs.

Transient swelling, acidification, and mitochondrial depolarization occurs in neurons but not astrocytes during spreading depression

Cortical spreading depression (SD) is a propagating wave of neuronal and glial depolarization that manifests in several brain disorders. However, the relative contribution of neurons and astrocytes to SD genesis has remained controversial. This is in part due to a lack of utilizing sophisticated experimental methodologies simultaneously to quantify multiple cellular parameters. To address this, we used simultaneous two-photon imaging, intrinsic optical imaging, and electrophysiological recordings to ascertain the changes in cellular processes that are fundamental to both cell types including cell volume, pH, and metabolism during SD propagation. We found that SD was correlated in neurons with robust yet transient increased volume, intracellular acidification, and mitochondrial depolarization. Our data indicated that a propagating large conductance during SD generated neuronal depolarization, which led to both calcium influx triggering metabolic changes and H(+) entry. Notably, astrocytes did not exhibit changes in cell volume, pH, or mitochondrial membrane potentials associated with SD, but they did show alterations induced by changing external [K(+)]. This suggests that astrocytes are not the primary contributor to SD propagation but are instead activated passively by extracellular potassium accumulation. These data support the hypothesis that neurons are the crucial cell type contributing to the pathophysiological responses of SD.

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.

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.

NCX and NCKX Operation in Ischemic Neurons

Within the first 2 min of global brain ischemia, extracellular [K+] ([K+]o) increases above 60 mM and [Na+](o) drops to about 50 mM, indicating a massive K+ efflux and Na+ influx, a phenomenon known as anoxic depolarization (AD). Similar ionic shifts take place during repetitive peri-infarct depolarizations (PID) in the area penumbra in focal brain ischemia. The size of ischemic infarct is determined by the duration of AD and PID. However, the mechanism of cytosolic [Ca2+] ([Ca2+]c) elevation during AD or PID is poorly understood. Our data show that the exposure of cultured rat hippocampal CA1 neurons to AD-like conditions promptly elevates [Ca2+]c to about 30 microM. These high [Ca2+]c elevations depend on external Ca2+ and can be prevented by removing Na+ or by simultaneously inhibiting NMDA and AMPA/kainate receptors. These data indicate that [Ca2+]c elevations during AD result from Na+ influx via either NMDA or AMPA/kainate channels. The mechanism of the Na-dependent [Ca2+]c elevations may involve a reversal of plasmalemmal Na+/Ca2+ (NCX) and/or Na+/Ca2+ + K+ (NCKX) exchangers. KB-R7943, an NCX inhibitor, suppresses a fraction of the Na-dependent Ca2+ influx during AD. Therefore, Ca2+ influx via NCX and a KB-R7943-resistant pathway (possibly NCKX) is involved. Inhibition of the Na-dependent Ca2+ influx is likely to decrease ischemic brain damage. No drugs are known that are able to inhibit the KB-R7943-resistant component of Na-dependent Ca2+ influx during AD. The present data encourage development of such agents as potential therapeutic means to limit ischemic brain damage after stroke or heart attack.

Measurement of neuronal activity of individual neurons after stroke in the rat using a microwire electrode array

BACKGROUND AND PURPOSE

Ischemic stroke induces spreading depression of brain waves and ischemic depolarizations, suggesting electrical activity of neurons is sensitive to stroke. The present study was designed to measure the electrophysiological response of an array of individual neurons to ischemic stroke in rats.

METHODS

A custom-made microwire electrode array (16 channels) was implanted in the cortical area supplied by the middle cerebral artery, spanning the core and boundary of the ischemic lesion. The electrophysiological activity of individual neurons was simultaneously recorded before, during and one week after middle cerebral artery occlusion (MCAo).

RESULTS

Neuronal activities were significantly reduced immediately after MCAo. Intermittent silent periods (SP) appeared within minutes or hours after MCAo and lasted variable times. Between intermittent SP, neurons fired irregular bursting spikes (BS) with small magnitudes. Intermittent SP and irregular BS progressed in one day post stroke to persistent SP in channels close to the ischemic core or to regular BS with small amplitudes in the penumbral zone. Both persistent SP and regular BS persisted for at least seven days.

CONCLUSIONS

Electrode array can be used to simultaneously record multiple individual neurons in response to ischemic stroke. This study provides the first evidence that the primary electrophysiological activity of multiple individual neurons to ischemic stroke is reduced in the lesion boundary and/or stopped in and adjacent to the lesion core.

Involvement of TRP-like channels in the acute ischemic response of hippocampal CA1 neurons in brain slices

During a period of acute ischemia in vivo or oxygen-glucose deprivation (OGD) in vitro, CA1 neurons depolarize, swell and become overloaded with calcium. Our aim was to test the hypothesis that the initial responses to OGD are at least partly due to transient receptor potential (TRP) channel activation. As some TRP channels are temperature-sensitive, we also compared the effects of pharmacological blockade of the channels with the effects of reducing temperature. Acute hippocampal slices (350 mum) obtained from Wistar rats were submerged in ACSF at 36 degrees C. CA1 neurons were monitored electrophysiologically using extracellular, intracellular or whole-cell patch-clamp recordings. Cell swelling was assessed by recording changes in relative tissue resistance, and changes in intracellular calcium were measured after loading neurons with fura-2 dextran. Blockers of TRP channels (ruthenium red, La3+, Gd3+, 2-APB) or lowering temperature by 3 degrees C reduced responses to OGD. This included: (a) an increased delay to negative shifts of extracellular DC potential; (b) reduction in rate of the initial slow membrane depolarization, slower development of OGD-induced increase in cell input resistance and slower development of whole-cell inward current; (c) reduced tissue swelling; and (d) a smaller rise in intracellular calcium. Mild hypothermia (33 degrees C) and La3+ or Gd3+ (100 microM) showed an occlusion effect when delay to extracellular DC shifts was measured. Expression of TRPM2/TRPM7 (oxidative stress-sensitive) and TRPV3/TRPV4 (temperature-sensitive) channels was demonstrated in the CA1 subfield with RT-PCR. These results indicate that TRP or TRP-like channels are activated by cellular stress and contribute to ischemia-induced membrane depolarization, intracellular calcium accumulation and cell swelling. We also hypothesize that closing of some TRP channels (TRPV3 and/or TRPV4) by lowering temperature may be partly responsible for the neuroprotective effect of hypothermia.

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.

Apoptosis in the adult striatum after transient forebrain ischemia and the effects of ischemic severity

The mechanisms of neuronal injury after cerebral ischemia have been under active investigation. The medium-size neurons in the dorsal striatum die within 24 h after transient cerebral ischemia. Using electron microscopy, the present study examined the nature of neuronal death in the striatum of adult rats following transient forebrain ischemia and tested the hypothesis that the ischemic severity might influence the nature of cell death. After severe ischemia (approximately 21 min ischemic depolarization), most neurons in the dorsal striatum died with swollen organelles and small irregular chromatin clumps resembling necrosis. The tissue damage in the dorsomedial striatum was less severe than that in the dorsolateral striatum and approximately 5% of the neurons in this region died with large chromatin clumps and relatively intact organelles resembling apoptosis. Some neurons displayed a mixture of necrotic- and apoptotic-like appearance. In contrast, the neurons with large somata only exhibited mild ultrastructural changes. After moderate ischemia (approximately 15 min ischemic depolarization), the tissue damage was less severe and the process of necrosis was temporally prolonged compared with that after severe ischemia. The apoptotic-like neuronal death was observed not only in the dorsomedial (approximately 6%) but also in the dorsolateral striatum (approximately 7%). The neurons in the striatum showed transient reversible changes after mild ischemia (approximately 10 min ischemic depolarization). The present study demonstrates that both apoptosis and necrosis occur in the adult striatum following transient forebrain ischemia and apoptosis occurs in the regions with less severe ischemia. These results suggest that ischemic severity might be one of the contributing factors to necrosis or apoptosis following transient global ischemia.