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 Leao 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

Correlates of Spreading Depolarization, Spreading Depression, and Negative Ultraslow Potential in Epidural Versus Subdural Electrocorticography

Spreading depolarizations (SDs) are characterized by near-complete breakdown of the transmembrane ion gradients, neuronal oedema and activity loss (=depression). The SD extreme in ischemic tissue, termed 'terminal SD,' shows prolonged depolarization, in addition to a slow baseline variation called 'negative ultraslow potential' (NUP). The NUP is the largest bioelectrical signal ever recorded from the human brain and is thought to reflect the progressive recruitment of neurons into death in the wake of SD. However, it is unclear whether the NUP is a field potential or results from contaminating sensitivities of platinum electrodes. In contrast to Ag/AgCl-based electrodes in animals, platinum/iridium electrodes are the gold standard for intracranial direct current (DC) recordings in humans. Here, we investigated the full continuum including short-lasting SDs under normoxia, long-lasting SDs under systemic hypoxia, and terminal SD under severe global ischemia using platinum/iridium electrodes in rats to better understand their recording characteristics. Sensitivities for detecting SDs or NUPs were 100% for both electrode types. Nonetheless, the platinum/iridium-recorded NUP was 10 times smaller in rats than humans. The SD continuum was then further investigated by comparing subdural platinum/iridium and epidural titanium peg electrodes in patients. In seven patients with either aneurysmal subarachnoid hemorrhage or malignant hemispheric stroke, two epidural peg electrodes were placed 10 mm from a subdural strip. We found that 31/67 SDs (46%) on the subdural strip were also detected epidurally. SDs that had longer negative DC shifts and spread more widely across the subdural strip were more likely to be observed in epidural recordings. One patient displayed an SD-initiated NUP while undergoing brain death despite continued circulatory function. The NUP's amplitude was -150 mV subdurally and -67 mV epidurally. This suggests that the human NUP is a bioelectrical field potential rather than an artifact of electrode sensitivity to other factors, since the dura separates the epidural from the subdural compartment and the epidural microenvironment was unlikely changed, given that ventilation, arterial pressure and peripheral oxygen saturation remained constant during the NUP. Our data provide further evidence for the clinical value of invasive electrocorticographic monitoring, highlighting important possibilities as well as limitations of less invasive recording techniques

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.Grants to RDA from the Canadian Heart & Stroke Foundation, National Science Engineering and Research Council and the New Frontiers in Research Fund, to E.F from the National Research, Development and Innovation Office of Hungary, grant no. K134377; and the EU’s Horizon 2020 research and innovation program under grant agreement No. 739593, and to JPD from the DFG (German research Council) (DFG DR323/5-1,DFG DR 323/10-1) BMBF Bundesministerium fuer Bildung und Forschung (Era-Net Neuron EBio2, with funds from BMBF 01EW2004)

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.This work was supported by grants from the Heart and Stroke Foundation of Canada and the National Science and Engineering Research Council of Canada to RDA, an NIH grant (NS106901) to CWS, a National Research, Development and Innovation Office of Hungary grant (K1343777) and EU Horizon 2020 research and innovation program (739953) to EF and from DFG Deutsche Forschungsgemeinschaft (German Research Council) (DFG DR 323/5-1), DFG DR 323/10-1, and BMBF Bundesministerium fuer Bildung und Forschung (EraNet Neuron EBio2, with funds from BMBF 01EW2004) to JPD

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. © 2022, The Author(s)

Correlates of Spreading Depolarization, Spreading Depression, and Negative Ultraslow Potential in Epidural Versus Subdural Electrocorticography

Spreading depolarizations (SDs) are characterized by near-complete breakdown of the transmembrane ion gradients, neuronal oedema and activity loss (=depression). The SD extreme in ischemic tissue, termed ‘terminal SD,’ shows prolonged depolarization, in addition to a slow baseline variation called ‘negative ultraslow potential’ (NUP). The NUP is the largest bioelectrical signal ever recorded from the human brain and is thought to reflect the progressive recruitment of neurons into death in the wake of SD. However, it is unclear whether the NUP is a field potential or results from contaminating sensitivities of platinum electrodes. In contrast to Ag/AgCl-based electrodes in animals, platinum/iridium electrodes are the gold standard for intracranial direct current (DC) recordings in humans. Here, we investigated the full continuum including short-lasting SDs under normoxia, long-lasting SDs under systemic hypoxia, and terminal SD under severe global ischemia using platinum/iridium electrodes in rats to better understand their recording characteristics. Sensitivities for detecting SDs or NUPs were 100% for both electrode types. Nonetheless, the platinum/iridium-recorded NUP was 10 times smaller in rats than humans. The SD continuum was then further investigated by comparing subdural platinum/iridium and epidural titanium peg electrodes in patients. In seven patients with either aneurysmal subarachnoid hemorrhage or malignant hemispheric stroke, two epidural peg electrodes were placed 10 mm from a subdural strip. We found that 31/67 SDs (46%) on the subdural strip were also detected epidurally. SDs that had longer negative DC shifts and spread more widely across the subdural strip were more likely to be observed in epidural recordings. One patient displayed an SD-initiated NUP while undergoing brain death despite continued circulatory function. The NUP’s amplitude was -150 mV subdurally and -67 mV epidurally. This suggests that the human NUP is a bioelectrical field potential rather than an artifact of electrode sensitivity to other factors, since the dura separates the epidural from the subdural compartment and the epidural microenvironment was unlikely changed, given that ventilation, arterial pressure and peripheral oxygen saturation remained constant during the NUP. Our data provide further evidence for the clinical value of invasive electrocorticographic monitoring, highlighting important possibilities as well as limitations of less invasive recording techniques

Die Rolle von Alpha-Isoformen der Na+/K+-ATPase in Spreading Depolarization

The Na+/K+-ATPase is the single greatest energy consumer of brain cells and accounts for at least 50% of ATP consumption under resting conditions. When Na+/K+-ATPase function is compromised, a neuropathophysiological phenomenon is triggered, known as spreading depolarization (SD). SD is characterized by massive, unparalleled redistribution of ions across cell membranes and widespread, sustained depolarization that propagates as a wave through the gray matter of the central nervous system. SD has been shown to occur abundantly in humans in acute, life threatening medical conditions and it is widely accepted as the cellular mechanism underlying migraine aura. Mutations in ATP1A2, the gene that encodes the α2 isoform of the Na+/K+-ATPase are associated with the occurrence of a severe subtype of migraine with aura: familial hemiplegic migraine type 2 (FHM2). This association suggests a role of the α2 isoform in SD. Despite research on FHM2 knock-in mouse models, the roles in SD of the other two α isoforms that are expressed in the mammalian brain are largely unknown. In my thesis I investigated the role of all three brain-expressed α isoforms employing three distinct knock-out mouse lines. Combining genetic isoform ablation of Na+/K+-ATPase α1 (ubiquitous), α2 (astrocytic) and α3 (neuronal) with pharmacological inhibition we compared the resulting SD phenotypes under similar conditions in the acute brain slice preparation and in vivo. We found that only α2-deficient mice displayed increased SD susceptibility in acute brain slices. Intriguingly, this susceptibility effect was dependent on high baseline [K+]o in the bathing medium and was abolished under normal [K+]o in brain slices and in vivo. Furthermore, we found that the extracellular K+ clearance upon intense neuronal stimulation was surprisingly well compensated in α2 deficient mice. In vivo, we found indications that the Na+/K+-ATPase α2 isoform is implicated in modulation of the vascular tone which was evidenced by a pronounced post-SD hypoemic response that was not reproduced in α1- or α3-deficient mice. By contrast, deficiency of α3 resulted in increased resistance against electrically-induced SD in vivo whereas α1 deficiency did not affect the SD phenotype. These data support a pivotal role of the α2 isoform in SD that is not replicated by α1 or α3 and that suggests specialized function through functional coupling to secondary active transporters. The observed vascular effect is particularly important in the context of migraine and stroke and warrants further research to unravel its mechanistic basis.Mehr als 50% des ATP-Bedarfs unter Normalbedingungen sind auf die Na+/K+-ATPase zurückzuführen und machen diese damit zum größten Einzel-Energieverbraucher von Zellen im Gehirn. Bei Beeinträchtigung der normalen Na+/K+-ATPase-Funktion kann es im Gehirn zur Ausbildung von sog. Spreading Depolarizations (SD) kommen. Diese Massen-Depolarisationswellen sind gekennzeichnet durch eine außergewöhnlich ausgeprägte Verschiebung von Ionen über die Nervenzellmembran und gehen mit einer starken, anhaltenden Depolarisation einher, welche sich wellenartig in der grauen Substanz ausbreitet. SDs werden regelhaft in vielen akuten Krankheitsbildern des Gehirns angetroffen und gelten allgemein als der zugrunde liegende Mechanismus der Migräne-Aura. Mutationen in ATP1A2, dem humanen Gen, das für die α2-Isoform der Na+/K+-ATPase kodiert, wurden bei Patienten gefunden, die an einer schweren Form der Migräne mit Aura leiden, der familären hemiplegischen Migräne Typ 2 (FHM2). Diese Verbindung weist auf eine mögliche Rolle der α2-Isoform in der Entwicklung von SD hin. Trotz Bemühungen die Funktion der α2-Isoform in Knockin-Mausmodellen zu erforschen, ist nur wenig über die Rolle der anderen beiden α-Isoformen bekannt, die im Gehirn exprimiert werden. In der vorliegenden Arbeit habe ich die Rolle dieser drei Na+/K+-ATPase α-Isoformen mittels dreier verschiedener Knockout-Mauslinien untersucht. Dafür haben wir unter gleichbleibenden Bedingungen die genetische Reduktion von α1 (ubiquitär), α2 (astrozytär) und α3 (neuronal) mit pharmakologischer Hemmung kombiniert und die resultierenden SD-Phänotypen miteinander verglichen. Wir konnten zeigen, dass einzig die Reduktion der α2-Isoform zu einer Erhöhung der Empfänglichkeit für SD führte. Interessanterweise war dieser Effekt abhängig von einer erhöhten extrazellulären K+-Konzentration [K+]o und verschwand unter normalen Bedingungen. Darüber hinaus fanden wir heraus, dass die Fähigkeit zur extrazellulären K+-Pufferung während intensiver neuronaler Stimulation in α2-heterozygoten Mäusen nahezu unberührt blieb. Wir fanden auch Hinweise für eine Modulierung des zerebralen Gefäßtonus durch die Na+/K+-ATPase-α2-Isoform, die in einer verstärkten Ausbildung der Post-SD-Hypoperfusion resultierte und in α1- sowie α3-Heterozygoten nicht nachweisbar war. Im Gegensatz dazu konnten wir nachweisen, dass α3-haploinsuffiziente Tiere eine erhöhte Widerstandsfähigkeit gegenüber elektrisch ausgelöster SD aufwiesen, Tiere mit einem Mangel an α1-Isoform jedoch keinerlei Veränderungen des SD-Phänotyps zeigten. Die vorliegenden Ergebnisse sprechen für eine zentrale Rolle der α2-Isoform in der Modulation von SD-Empfänglichkeit und der assoziierten Blutflussregulation. Diese Spezifität ist am ehesten auf die funktionelle Kopplung mit sekundär aktiven Transportern zurückzuführen. Insbesondere der beobachtete Blutfluss-Effekt ist im Hinblick auf die Krankheitsbilder Migräne und Schlaganfall von Bedeutung und rechtfertigt eine weitere Abklärung zugrundeliegender Mechanismen