Dynamics of Ionic Shifts in Cortical Spreading Depression

From spreading depolarization to blood-brain barrier dysfunction: navigating traumatic brain injury for novel diagnosis and therapy

Considerable strides in medical interventions during the acute phase of traumatic brain injury (TBI) have brought improved overall survival rates. However, following TBI, people often face ongoing, persistent and debilitating long-term complications. Here, we review the recent literature to propose possible mechanisms that lead from TBI to long-term complications, focusing particularly on the involvement of a compromised blood-brain barrier (BBB). We discuss evidence for the role of spreading depolarization as a key pathological mechanism associated with microvascular dysfunction and the transformation of astrocytes to an inflammatory phenotype. Finally, we summarize new predictive and diagnostic biomarkers and explore potential therapeutic targets for treating long-term complications of TBI.

Probing intracellular potassium dynamics in neurons with the genetically encoded sensor lc-LysM GEPII 1.0 in vitro and in vivo

Neuronal activity is accompanied by a net outflow of potassium ions (K+) from the intra- to the extracellular space. While extracellular [K+] changes during neuronal activity are well characterized, intracellular dynamics have been less well investigated due to lack of respective probes. In the current study we characterized the FRET-based K+ biosensor lc-LysM GEPII 1.0 for its capacity to measure intracellular [K+] changes in primary cultured neurons and in mouse cortical neurons in vivo. We found that lc-LysM GEPII 1.0 can resolve neuronal [K+] decreases in vitro during seizure-like and intense optogenetically evoked activity. [K+] changes during single action potentials could not be recorded. We confirmed these findings in vivo by expressing lc-LysM GEPII 1.0 in mouse cortical neurons and performing 2-photon fluorescence lifetime imaging. We observed an increase in the fluorescence lifetime of lc-LysM GEPII 1.0 during periinfarct depolarizations, which indicates a decrease in intracellular neuronal [K+]. Our findings suggest that lc-LysM GEPII 1.0 can be used to measure large changes in [K+] in neurons in vitro and in vivo but requires optimization to resolve smaller changes as observed during single action potentials.

A comprehensive exploration of astrocytes in migraine: a bibliometric and visual analysis

BACKGROUND

Migraine, as a prevalent neurologic disorder, involves intricate and yet incompletely elucidated pathophysiological mechanisms. A plethora of research findings underscores the pivotal role played by astrocytes in the progression of migraines. In order to elucidate the current advances and directions in research pertaining to astrocytes in migraines, we conducted bibliometric analysis of relevant literature and visualized the results. Subsequently, we expound upon these findings to contribute to the evolving understanding of the role of astrocytes in migraine pathophysiology.

METHODS

On November 21, 2023, we conducted a search on Web of Science (WOS), restricting the document type to articles or reviews and language to English. Following a meticulous selection process involving three researchers, we identified the literature to be included in our analysis. Subsequently, we employed Microsoft Office Excel programs, R, VOSviewer, Scimago Graphica, and CiteSpace software to conduct visualization analysis of basic information and trends regarding journals, countries/regions, and influential authors, institutions, keywords, and papers.

RESULTS

As of November 21, 2023, relevant literature has been published in 71 journals across 27 countries/regions. This corpus comprises contributions from 576 authors affiliated with 220 institutions, encompassing 865 keywords and referencing 6065 scholarly articles. CEPHALALGIA stands out as the most influential journal in this field, while authors PIETROBON D and DALKARA T have significant impact. The United States is highly influential, with CNR and UNIV PADUA emerging as highly influential institutions. The predominant category is Neurosciences.

CONCLUSIONS

Future investigators may continue to focus on migraines with aura, familial hemiplegic migraine (FHM), and the crucial calcitonin gene-related peptide (CGRP) system. Employing advanced observational techniques, such as imaging, researchers should pay attention to cellular and tissue structures, such as microglia and the trigeminal ganglion, as well as mechanisms involving inflammation and central sensitization. Moreover, animal models are paramount in obtaining high-quality evidence.

Imaging the large-scale and cellular response to focal traumatic brain injury in mouse neocortex

Traumatic brain injury (TBI) affects neural function at the local injury site and also at distant, connected brain areas. However, the real-time neural dynamics in response to injury and subsequent effects on sensory processing and behavior are not fully resolved, especially across a range of spatial scales. We used in vivo calcium imaging in awake, head-restrained male and female mice to measure large-scale and cellular resolution neuronal activation, respectively, in response to a mild TBI induced by focal controlled cortical impact (CCI) injury of the motor cortex (M1). Widefield imaging revealed an immediate CCI-induced activation at the injury site, followed by a massive slow wave of calcium signal activation that traveled across the majority of the dorsal cortex within approximately 30 s. Correspondingly, two-photon calcium imaging in primary somatosensory cortex (S1) found strong activation of neuropil and neuronal populations during the CCI-induced traveling wave. A depression of calcium signals followed the wave, during which we observed atypical activity of a sparse population of S1 neurons. Longitudinal imaging in the hours and days after CCI revealed increases in the area of whisker-evoked sensory maps at early time points, in parallel to decreases in cortical functional connectivity and behavioral measures. Neural and behavioral changes mostly recovered over hours to days in our mild-TBI model, with a more lasting decrease in the number of active S1 neurons. Our results provide novel spatial and temporal views of neural adaptations that occur at cortical sites remote to a focal brain injury.

Miniaturized Electrochemical Sensing Platforms for Quantitative Monitoring of Glutamate Dynamics in the Central Nervous System

Glutamate is one of the most important excitatory neurotransmitters within the mammalian central nervous system. The role of glutamate in regulating neural network signaling transmission through both synaptic and extra-synaptic paths highlights the importance of the real-time and continuous monitoring of its concentration and dynamics in living organisms. Progresses in multidisciplinary research have promoted the development of electrochemical glutamate sensors through the co-design of materials, interfaces, electronic devices, and integrated systems. This review summarizes recent works reporting various electrochemical sensor designs and their applicability as miniaturized neural probes to in vivo sensing within biological environments. We start with an overview of the role and physiological significance of glutamate, the metabolic routes, and its presence in various bodily fluids. Next, we discuss the design principles, commonly employed validation models/protocols, and successful demonstrations of multifunctional, compact, and bio-integrated devices in animal models. The final section provides an outlook on the development of the next generation glutamate sensors for neuroscience and neuroengineering, with the aim of offering practical guidance for future research.

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.

Seizure event detection using intravital two-photon calcium imaging data

Significance

Intravital cellular calcium imaging has emerged as a powerful tool to investigate how different types of neurons interact at the microcircuit level to produce seizure activity, with newfound potential to understand epilepsy. Although many methods exist to measure seizure-related activity in traditional electrophysiology, few yet exist for calcium imaging.

Aim

To demonstrate an automated algorithmic framework to detect seizure-related events using calcium imaging-including the detection of pre-ictal spike events, propagation of the seizure wavefront, and terminal spreading waves for both population-level activity and that of individual cells.

Approach

We developed an algorithm for precise recruitment detection of population and individual cells during seizure-associated events, which broadly leverages averaged population activity and high-magnitude slope features to detect single-cell pre-ictal spike and seizure recruitment. We applied this method to data recorded using awake in vivo two-photon calcium imaging during pentylenetetrazol-induced seizures in mice.

Results

We demonstrate that our detected recruitment times are concordant with visually identified labels provided by an expert reviewer and are sufficiently accurate to model the spatiotemporal progression of seizure-associated traveling waves.

Conclusions

Our algorithm enables accurate cell recruitment detection and will serve as a useful tool for researchers investigating seizure dynamics using calcium imaging.

Astrocytes as critical players of the fine balance between inhibition and excitation in the brain: spreading depolarization as a mechanism to curb epileptic activity

Spreading depolarizations (SD) are slow waves of complete depolarization of brain tissue followed by neuronal silencing that may play a role in seizure termination. Even though SD was first discovered in the context of epilepsy research, the link between SD and epileptic activity remains understudied. Both seizures and SD share fundamental pathophysiological features, and recent evidence highlights the frequent occurrence of SD in experimental seizure models. Human data on co-occurring seizures and SD are limited but suggestive. This mini-review addresses possible roles of SD during epileptiform activity, shedding light on SD as a potential mechanism for terminating epileptiform activity. A common denominator for many forms of epilepsy is reactive astrogliosis, a process characterized by morphological and functional changes to astrocytes. Data suggest that SD mechanisms are potentially perturbed in reactive astrogliosis and we propose that this may affect seizure pathophysiology.

Regulation of specific abnormal calcium signals in the hippocampal CA1 and primary cortex M1 alleviates the progression of temporal lobe epilepsy

Temporal lobe epilepsy is a multifactorial neurological dysfunction syndrome that is refractory, resistant to antiepileptic drugs, and has a high recurrence rate. The pathogenesis of temporal lobe epilepsy is complex and is not fully understood. Intracellular calcium dynamics have been implicated in temporal lobe epilepsy. However, the effect of fluctuating calcium activity in CA1 pyramidal neurons on temporal lobe epilepsy is unknown, and no longitudinal studies have investigated calcium activity in pyramidal neurons in the hippocampal CA1 and primary motor cortex M1 of freely moving mice. In this study, we used a multi-channel fiber photometry system to continuously record calcium signals in CA1 and M1 during the temporal lobe epilepsy process. We found that calcium signals varied according to the grade of temporal lobe epilepsy episodes. In particular, cortical spreading depression, which has recently been frequently used to represent the continuously and substantially increased calcium signals, was found to correspond to complex and severe behavioral characteristics of temporal lobe epilepsy ranging from grade II to grade V. However, vigorous calcium oscillations and highly synchronized calcium signals in CA1 and M1 were strongly related to convulsive motor seizures. Chemogenetic inhibition of pyramidal neurons in CA1 significantly attenuated the amplitudes of the calcium signals corresponding to grade I episodes. In addition, the latency of cortical spreading depression was prolonged, and the above-mentioned abnormal calcium signals in CA1 and M1 were also significantly reduced. Intriguingly, it was possible to rescue the altered intracellular calcium dynamics. Via simultaneous analysis of calcium signals and epileptic behaviors, we found that the progression of temporal lobe epilepsy was alleviated when specific calcium signals were reduced, and that the end-point behaviors of temporal lobe epilepsy were improved. Our results indicate that the calcium dynamic between CA1 and M1 may reflect specific epileptic behaviors corresponding to different grades. Furthermore, the selective regulation of abnormal calcium signals in CA1 pyramidal neurons appears to effectively alleviate temporal lobe epilepsy, thereby providing a potential molecular mechanism for a new temporal lobe epilepsy diagnosis and treatment strategy.

Synaptic Zn2+ contributes to deleterious consequences of spreading depolarizations

Spreading depolarizations (SDs) are profound waves of neuroglial depolarization that can propagate repetitively through injured brain. Recent clinical work has established SD as an important contributor to expansion of acute brain injuries and have begun to extend SD studies into other neurological disorders. A critical challenge is to determine how to selectively prevent deleterious consequences of SD. In the present study, we determined whether a wave of profound Zn2+ release is a key contributor to deleterious consequences of SD, and whether this can be targeted pharmacologically. Focal KCl microinjection was used to initiate SD in the CA1 region of the hippocampus in murine brain slices. An extracellular Zn2+ chelator with rapid kinetics (ZX1) increased SD propagation rates and improved recovery of extracellular DC potential shifts. Under conditions of metabolic compromise, tissues showed sustained impairment of functional and structural recovery following a single SD. ZX1 effectively improved recovery of synaptic potentials and intrinsic optical signals in these vulnerable conditions. Fluorescence imaging and genetic deletion of a presynaptic Zn2+ transporter confirmed synaptic release as the primary contributor to extracellular accumulation and deleterious consequences of Zn2+ during SD. These results demonstrate a role for synaptic Zn2+ release in deleterious consequences of SD and show that targeted extracellular chelation could be useful for disorders where repetitive SD enlarges infarcts in injured tissues.

Analysis of Functional NMDA Receptors in Astrocytes

Neuronal N-methyl-D-aspartate (NMDA) receptors are well known for their pivotal role in memory formation. Originally, they were thought to be exclusive to neurons. However, numerous studies revealed their functional expression also on various types of glial cells in the nervous system. Here, the methodology on how to study the physiology of NMDA receptors selectively on astrocytes will be described in detail. Astrocytes are the main class of neuroglia that control transmitter and ion homeostasis, which link cerebral blood flow and neuronal energy demands, but also affect synaptic transmission directly.

Increased membrane Ca2+ permeability drives astrocytic Ca2+ dynamics during neuronal stimulation at excitatory synapses

Astrocytes are intricately involved in the activity of neural circuits; however, their basic physiology of interacting with nearby neurons is not well established. Using two-photon imaging of neurons and astrocytes during higher frequency stimulation of hippocampal CA3-CA1 Schaffer collateral (Scc) excitatory synapses, we could show that increasing levels of released glutamate accelerated local astrocytic Ca2+ elevation. However, blockage of glutamate transporters did not abolish this astrocytic Ca2+ response, suggesting that astrocytic Ca2+ elevation is indirectly associated with an uptake of extracellular glutamate. However, during the astrocytic glutamate uptake, the Na+ /Ca2+ exchanger (NCX) reverse mode was activated, and mediated extracellular Ca2+ entry, thereby triggering the internal release of Ca2+ . In addition, extracellular Ca2+ entry via membrane P2X receptors further facilitated astrocytic Ca2+ elevation via ATP binding. These findings suggest a novel mechanism of activity induced Ca2+ permeability increases of astrocytic membranes, which drives astrocytic responses during neuronal stimulation of CA3-CA1 Scc excitatory synapses.

Cerebral Ischemic Reperfusion Injury: Preventative and Therapeutic Strategies

Acute ischemic stroke is a leading cause of morbidity and mortality in the United States. Treatment goals remain focused on restoring blood flow to compromised areas. However, a major concern arises after reperfusion occurs. Cerebral ischemic reperfusion injury is defined as damage to otherwise salvageable brain tissue occurring with the reestablishment of the vascular supply to that region. The pool of eligible patients for revascularization continues to grow, especially with the recently expanded endovascular therapeutic window. Neurointensivists should understand and manage complications of successful recanalization. In this review, we examine the pathophysiology, diagnosis, and potential management strategies in cerebral ischemic reperfusion injury.

Synaptic Zn2+ contributes to deleterious consequences of spreading depolarizations

Spreading depolarizations (SDs) are profound waves of neuroglial depolarization that can propagate repetitively through injured brain. Recent clinical work has established SD as an important contributor to expansion of acute brain injuries and have begun to extend SD studies into other neurological disorders. A critical challenge is to determine how to selectively prevent deleterious consequences of SD. In the present study, we determined whether a wave of profound Zn2+ release is a key contributor to deleterious consequences of SD, and whether this can be targeted pharmacologically. Focal KCl microinjection was used to initiate SD in the CA1 region of the hippocampus in murine brain slices. An extracellular Zn2+ chelator with rapid kinetics (ZX-1) increased SD propagation rates and improved recovery of extracellular DC potential shifts. Under conditions of metabolic compromise, tissues showed sustained impairment of functional and structural recovery following a single SD. ZX-1 effectively improved recovery of synaptic potentials and intrinsic optical signals in these vulnerable conditions. Fluorescence imaging and genetic deletion of a presynaptic Zn2+ transporter confirmed synaptic release as the primary contributor to extracellular accumulation and deleterious consequences of Zn2+ during SD. These results demonstrate a role for synaptic Zn2+ release in deleterious consequences of SD and show that targeted extracellular chelation could be useful for disorders where repetitive SD enlarges infarcts in injured tissues.

Ca2+-dependent rapid uncoupling of astrocytes upon brief metabolic stress

Astrocytic gap junctional coupling is a major element in neuron-glia interaction. There is strong evidence that impaired coupling is involved in neurological disorders. Reduced coupling was, e.g., demonstrated for core regions of ischemic stroke that suffer from massive cell death. In the surrounding penumbra, cells may recover, but recovery is hampered by spreading depolarizations, which impose additional metabolic stress onto the tissue. Spreading depolarizations are characterized by transient breakdown of cellular ion homeostasis, including pH and Ca2+, which might directly affect gap junctional coupling. Here, we exposed acute mouse neocortical tissue slices to brief metabolic stress and examined its effects on the coupling strength between astrocytes. Changes in gap junctional coupling were assessed by recordings of the syncytial isopotentiality. Moreover, quantitative ion imaging was performed in astrocytes to analyze the mechanisms triggering the observed changes. Our experiments show that a 2-minute perfusion of tissue slices with blockers of glycolysis and oxidative phosphorylation causes a rapid uncoupling in half of the recorded cells. They further indicate that uncoupling is not mediated by the accompanying (moderate) intracellular acidification. Dampening large astrocytic Ca2+ loads by removal of extracellular Ca2+ or blocking Ca2+ influx pathways as well as a pharmacological inhibition of calmodulin, however, prevent the uncoupling. Taken together, we conclude that astrocytes exposed to brief episodes of metabolic stress can undergo a rapid, Ca2+/calmodulin-dependent uncoupling. Such uncoupling may help to confine and reduce cellular damage in the ischemic penumbra in vivo.

Seizure Event Detection Using Intravital Two-Photon Calcium Imaging Data

Significance

Genetic cellular calcium imaging has emerged as a powerful tool to investigate how different types of neurons interact at the microcircuit level to produce seizure activity, with newfound potential to understand epilepsy. Although many methods exist to measure seizure-related activity in traditional electrophysiology, few yet exist for calcium imaging.

Aim

To demonstrate an automated algorithmic framework to detect seizure-related events using calcium imaging - including the detection of pre-ictal spike events, propagation of the seizure wavefront, and terminal spreading waves for both population-level activity and that of individual cells.

Approach

We developed an algorithm for precise recruitment detection of population and individual cells during seizure-associated events, which broadly leverages averaged population activity and high-magnitude slope features to detect single-cell pre-ictal spike and seizure recruitment. We applied this method to data recorded using awake in vivo two-photon calcium imaging during pentylenetetrazol induced seizures in mice.

Results

We demonstrate that our detected recruitment times are concordant with visually identified labels provided by an expert reviewer and are sufficiently accurate to model the spatiotemporal progression of seizure-associated traveling waves.

Conclusions

Our algorithm enables accurate cell recruitment detection and will serve as a useful tool for researchers investigating seizure dynamics using calcium imaging.

A model of neurovascular coupling and its application to cortical spreading depolarization

Cortical spreading depolarization (CSD) is a neuropathological condition involving propagating waves of neuronal silence, and is related to multiple diseases, such as migraine aura, traumatic brain injury (TBI), stroke, and cardiac arrest, as well as poor outcome of patients. While CSDs of different severity share similar roots on the ion exchange level, they can lead to different vascular responses (namely spreading hyperemia and spreading ischemia). In this paper, we propose a mathematical model relating neuronal activities to predict vascular changes as measured with near-infrared spectroscopy (NIRS) and fMRI recordings, and apply it to the extreme case of CSD, where sustained near-complete neuronal depolarization is seen. We utilize three serially connected models (namely, ion exchange, neurovascular coupling, and hemodynamic model) which are described by differential equations. Propagating waves of ion concentrations, as well as the associated vasodynamics and hemodynamics, are simulated by solving these equations. Our proposed model predicts vasodynamics and hemodynamics that agree both qualitatively and quantitatively with experimental literature. Mathematical modeling and simulation offer a powerful tool to help understand the underlying mechanisms of CSD and help interpret the data. In addition, it helps develop novel monitoring techniques prior to data collection. Our simulated results strongly suggest that fMRI is unable to reliably distinguish between spreading hyperemia and spreading ischemia, while NIRS signals are substantially distinct in the two cases.

Meningeal P2X7 Signaling Mediates Migraine-Related Intracranial Mechanical Hypersensitivity

Cortical spreading depolarization (CSD) is a key pathophysiological event that underlies visual and sensory auras in migraine. CSD is also thought to drive the headache phase in migraine by promoting the activation and mechanical sensitization of trigeminal primary afferent nociceptive neurons that innervate the cranial meninges. The factors underlying meningeal nociception in the wake of CSD remain poorly understood but potentially involve the parenchymal release of algesic mediators and damage-associated molecular patterns, particularly ATP. Here, we explored the role of ATP-P2X purinergic receptor signaling in mediating CSD-evoked meningeal afferent activation and mechanical sensitization. Male rats were subjected to a single CSD episode. In vivo, extracellular single-unit recording was used to measure meningeal afferent ongoing activity changes. Quantitative mechanical stimuli using a servomotor force-controlled stimulator assessed changes in the afferent's mechanosensitivity. Manipulation of meningeal P2X receptors was achieved via local administration of pharmacological agents. Broad-spectrum P2X receptor inhibition, selective blockade of the P2X7 receptor, and its related Pannexin 1 channel suppressed CSD-evoked afferent mechanical sensitization but did not affect the accompanying afferent activation response. Surprisingly, inhibition of the pronociceptive P2X2/3 receptor did not affect the activation or sensitization of meningeal afferents post-CSD. P2X7 signaling underlying afferent mechanosensitization was localized to the meninges and did not affect CSD susceptibility. We propose that meningeal P2X7 and Pannexin 1 signaling, potentially in meningeal macrophages or neutrophils, mediates the mechanical sensitization of meningeal afferents, which contributes to migraine pain by exacerbating the headache during normally innocuous physical activities.SIGNIFICANCE STATEMENT Activation and sensitization of meningeal afferents play a key role in migraine headache, but the underlying mechanisms remain unclear. Here, using a rat model of migraine with aura involving cortical spreading depolarization (CSD), we demonstrate that meningeal purinergic P2X7 signaling and its related Pannexin 1 pore, but not nociceptive P2X2/3 receptors, mediate prolonged meningeal afferent sensitization. Additionally, we show that meningeal P2X signaling does not contribute to the increased afferent ongoing activity in the wake of CSD. Our finding points to meningeal P2X7 signaling as a critical mechanism underlying meningeal nociception in migraine, the presence of distinct mechanisms underlying the activation and sensitization of meningeal afferents in migraine, and highlight the need to target both processes for effective migraine therapy.

Mechanisms of initiation of cortical spreading depression

BACKGROUND

There is increasing evidence from human and animal studies that cortical spreading depression (CSD) is the neurophysiological correlate of migraine aura and a trigger of migraine pain mechanisms. The mechanisms of initiation of CSD in the brain of migraineurs remain unknown, and the mechanisms of initiation of experimentally induced CSD in normally metabolizing brain tissue remain incompletely understood and controversial. Here, we investigated the mechanisms of CSD initiation by focal application of KCl in mouse cerebral cortex slices.

METHODS

High KCl puffs of increasing duration up to the threshold duration eliciting a CSD were applied on layer 2/3 whilst the membrane potential of a pyramidal neuron located very close to the site of KCl application and the intrinsic optic signal were simultaneously recorded. This was done before and after the application of a specific blocker of either NMDA or AMPA glutamate receptors (NMDARs, AMPARs) or voltage-gated Ca2+ (CaV) channels. If the drug blocked CSD, stimuli up to 12-15 times the threshold were applied.

RESULTS

Blocking either NMDARs with MK-801 or CaV channels with Ni2+ completely inhibited CSD initiation by both CSD threshold and largely suprathreshold KCl stimuli. Inhibiting AMPARs with NBQX was without effect on the CSD threshold and velocity. Analysis of the CSD subthreshold and threshold neuronal depolarizations in control conditions and in the presence of MK-801 or Ni2+ revealed that the mechanism underlying ignition of CSD by a threshold stimulus (and not by a just subthreshold stimulus) is the CaV-dependent activation of a threshold level of NMDARs (and/or of channels whose opening depends on the latter). The delay of several seconds with which this occurs underlies the delay of CSD initiation relative to the rapid neuronal depolarization produced by KCl.

CONCLUSIONS

Both NMDARs and CaV channels are necessary for CSD initiation, which is not determined by the extracellular K+ or neuronal depolarization levels per se, but requires the CaV-dependent activation of a threshold level of NMDARs. This occurs with a delay of several seconds relative to the rapid depolarization produced by the KCl stimulus. Our data give insights into potential mechanisms of CSD initiation in migraine.

Two-photon microscopy: application advantages and latest progress for in vivo imaging of neurons and blood vessels after ischemic stroke

Two-photon microscopy (TPM) plays an important role in the study of the changes of the two important components of neurovascular units (NVU) - neurons and blood vessels after ischemic stroke (IS). IS refers to sudden neurological dysfunction caused by focal cerebral ischemia, which is one of the leading causes of death and disability worldwide. TPM is a new and rapidly developing high-resolution real-time imaging technique used in vivo that has attracted increasing attention from scientists in the neuroscience field. Neurons and blood vessels are important components of neurovascular units, and they undergo great changes after IS to respond to and compensate for ischemic injury. Here, we introduce the characteristics and pre-imaging preparations of TPM, and review the common methods and latest progress of TPM in the neuronal and vascular research for injury and recovery of IS in recent years. With the review, we clearly recognized that the most important advantage of TPM in the study of ischemic stroke is the ability to perform chronic longitudinal imaging of different tissues at a high resolution in vivo. Finally, we discuss the limitations of TPM and the technological advances in recent years.

Astrocyte endfeet may theoretically act as valves to convert pressure oscillations to glymphatic flow

The glymphatic system of cerebrospinal fluid transport through the perivascular spaces of the brain has been implicated in metabolic waste clearance, neurodegenerative diseases and in acute neurological disorders such as stroke and cardiac arrest. In other biological low-pressure fluid pathways such as in veins and the peripheral lymphatic system, valves play an important role in ensuring the flow direction. Though fluid pressure is low in the glymphatic system and directed bulk flow has been measured in pial and penetrating perivascular spaces, no valves have yet been identified. Valves, which asymmetrically favour forward flow to backward flow, would imply that the considerable oscillations in blood and ventricle volumes seen in magnetic resonance imaging could cause directed bulk flow. Here, we propose that astrocyte endfeet may act as such valves using a simple elastic mechanism. We combine a recent fluid mechanical model of viscous flow between elastic plates with recent measurements of in vivo elasticity of the brain to predict order of magnitude flow-characteristics of the valve. The modelled endfeet are effective at allowing forward while preventing backward flow.

Preclinical models of middle cerebral artery occlusion: new imaging approaches to a classic technique

Stroke remains a major burden on patients, families, and healthcare professionals, despite major advances in prevention, acute treatment, and rehabilitation. Preclinical basic research can help to better define mechanisms contributing to stroke pathology, and identify therapeutic interventions that can decrease ischemic injury and improve outcomes. Animal models play an essential role in this process, and mouse models are particularly well-suited due to their genetic accessibility and relatively low cost. Here, we review the focal cerebral ischemia models with an emphasis on the middle cerebral artery occlusion technique, a "gold standard" in surgical ischemic stroke models. Also, we highlight several histologic, genetic, and in vivo imaging approaches, including mouse stroke MRI techniques, that have the potential to enhance the rigor of preclinical stroke evaluation. Together, these efforts will pave the way for clinical interventions that can mitigate the negative impact of this devastating disease.

Meningeal P2X7 signaling mediates migraine-related intracranial mechanical hypersensitivity

Cortical spreading depolarization (CSD) is a key pathophysiological event that underlies visual and sensory auras in migraine. CSD is also thought to drive the headache phase in migraine by promoting the activation and mechanical sensitization of trigeminal primary afferent nociceptive neurons that innervate the cranial meninges. The factors underlying meningeal nociception in the wake of CSD remain poorly understood but potentially involve the parenchymal release of algesic mediators and damage-associated molecular patterns, particularly ATP. Here, we explored the role of ATP-P2X purinergic receptor signaling in mediating CSD-evoked meningeal afferent activation and mechanical sensitization. Male rats were subjected to a single CSD episode. In vivo, extracellular single-unit recording was used to measure meningeal afferent ongoing activity changes. Quantitative mechanical stimuli using a servomotor force-controlled stimulator assessed changes in the afferent's mechanosensitivity. Manipulation of meningeal P2X receptors was achieved via local administration of pharmacological agents. Broad-spectrum P2X receptor inhibition, selective blockade of the P2X7 receptor, and its related pannexin 1 channel suppressed CSD-evoked afferent mechanical sensitization but did not affect the accompanying afferent activation response. Surprisingly, inhibition of the pronociceptive P2X2/3 receptor did not affect the activation or sensitization of meningeal afferents post-CSD. P2X7 signaling underlying afferent mechanosensitization was localized to the meninges and did not affect CSD susceptibility. We propose that meningeal P2X7 and Pannexin 1 signaling, potentially in meningeal macrophages or neutrophils, mediates the mechanical sensitization of meningeal afferents, which contributes to migraine pain by exacerbating the headache during normally innocuous physical activities.

Significance Statement

Activation and sensitization of meningeal afferents play a key role in migraine headache, but the underlying mechanisms remain unclear. Here, using a rat model of migraine with aura involving cortical spreading depolarization (CSD), we demonstrate that meningeal purinergic P2X7 signaling and its related Pannexin 1 pore, but not nociceptive P2X2/3 receptors, mediate prolonged meningeal afferent sensitization. Additionally, we show that meningeal P2X signaling does not contribute to the increased afferent ongoing activity in the wake of CSD. Our finding points to meningeal P2X7 signaling as a critical mechanism underlying meningeal nociception in migraine, the presence of distinct mechanisms underlying the activation and sensitization of meningeal afferents in migraine, and highlight the need to target both processes for effective migraine therapy.

Plasticity of perisynaptic astroglia during ischemia-induced spreading depolarization

High astroglial capacity for glutamate and potassium clearance aids in recovering spreading depolarization (SD)-evoked disturbance of ion homeostasis during stroke. Since perisynaptic astroglia cannot be imaged with diffraction-limited light microscopy, nothing is known about the impact of SD on the ultrastructure of a tripartite synapse. We used serial section electron microscopy to assess astroglial synaptic coverage in the sensorimotor cortex of urethane-anesthetized male and female mice during and after SD evoked by transient bilateral common carotid artery occlusion. At the subcellular level, astroglial mitochondria were remarkably resilient to SD compared to dendritic mitochondria that were fragmented by SD. Overall, 482 synapses in `Sham' during `SD' and `Recovery' groups were randomly selected and analyzed in 3D. Perisynaptic astroglia was present at the axon-spine interface (ASI) during SD and after recovery. Astrocytic processes were more likely found at large synapses on mushroom spines after recovery, while the length of the ASI perimeter surrounded by astroglia has also significantly increased at large synapses. These findings suggest that as larger synapses have a bigger capacity for neurotransmitter release during SD, they attract astroglial processes to their perimeter during recovery, limiting extrasynaptic glutamate escape and further enhancing the astrocytic ability to protect synapses in stroke.

Modeling the heterogeneity of sodium and calcium homeostasis between cortical and hippocampal astrocytes and its impact on bioenergetics

Emerging evidence indicates that neuronal activity-evoked changes in sodium concentration in astrocytes Na _a represent a special form of excitability, which is tightly linked to all other major ions in the astrocyte and extracellular space, as well as to bioenergetics, neurotransmitter uptake, and neurovascular coupling. Recently, one of us reported that Na _a transients in the neocortex have a significantly higher amplitude than those in the hippocampus. Based on the extensive data from that study, here we develop a detailed biophysical model to further understand the origin of this heterogeneity and how it affects bioenergetics in the astrocytes. In addition to closely fitting the observed experimental Na _a changes under different conditions, our model shows that the heterogeneity in Na _a signaling leads to substantial differences in the dynamics of astrocytic Ca²⁺ signals in the two brain regions, and leaves cortical astrocytes more susceptible to Na⁺ and Ca²⁺ overload under metabolic stress. The model also predicts that activity-evoked Na _a transients result in significantly larger ATP consumption in cortical astrocytes than in the hippocampus. The difference in ATP consumption is mainly due to the different expression levels of NMDA receptors in the two regions. We confirm predictions from our model experimentally by fluorescence-based measurement of glutamate-induced changes in ATP levels in neocortical and hippocampal astrocytes in the absence and presence of the NMDA receptor's antagonist (2R)-amino-5-phosphonovaleric acid.

Inhibition of HSP90 Preserves Blood-Brain Barrier Integrity after Cortical Spreading Depression

Cortical spreading depression (CSD) is a pathophysiological mechanism underlying headache disorders, including migraine. Blood-brain barrier (BBB) permeability is increased during CSD. Recent papers have suggested that heat shock proteins (HSP) contribute to the integrity of the blood-brain barrier. In this study, the possible role of HSP90 in CSD-associated blood-brain barrier leak at the endothelial cell was investigated using an in vitro model, for the blood-endothelial barrier (BEB), and an in vivo model with an intact BBB. We measured barrier integrity using trans endothelial electric resistance (TEER) across a monolayer of rodent brain endothelial cells (bEnd.3), a sucrose uptake assay, and in situ brain perfusion using female Sprague Dawley rats. CSD was induced by application of 60 mM KCl for 5 min in in vitro experiments or cortical injection of KCl (1 M, 0.5 µL) through a dural cannula in vivo. HSP90 was selectively blocked by 17-AAG. Our data showed that preincubation with 17-AAG (1 µM) prevented the reduction of TEER values caused by the KCl pulse on the monolayer of bEnd.3 cells. The elevated uptake of 14C-sucrose across the same endothelial monolayer induced by the KCl pulse was significantly reduced after preincubation with HSP90 inhibitor. Pre-exposure to 17-AAG significantly mitigated the transient BBB leak after CSD induced by cortical KCl injection as determined by in situ brain perfusion in female rats. Our results demonstrated that inhibition of HSP90 with the selective agent 17-AAG reduced CSD-associated BEB/BBB paracellular leak. Overall, this novel observation supports HSP90 inhibition mitigates KCl-induced BBB permeability and suggests the development of new therapeutic approaches targeting HSP90 in headache disorders.

Malignant astrocyte swelling and impaired glutamate clearance drive the expansion of injurious spreading depolarization foci

Spreading depolarizations (SDs) indicate injury progression and predict worse clinical outcome in acute brain injury. We demonstrate in rodents that acute brain swelling upon cerebral ischemia impairs astroglial glutamate clearance and increases the tissue area invaded by SD. The cytotoxic extracellular glutamate accumulation (>15 µM) predisposes an extensive bulk of tissue (4-5 mm²) for a yet undescribed simultaneous depolarization (SiD). We confirm in rat brain slices exposed to osmotic stress that SiD is the pathological expansion of prior punctual SD foci (0.5-1 mm²), is associated with astrocyte swelling, and triggers oncotic neuron death. The blockade of astrocytic aquaporin-4 channels and Na⁺/K⁺/Cl^- co-transporters, or volume-regulated anion channels mitigated slice edema, extracellular glutamate accumulation (<10 µM) and SiD occurrence. Reversal of slice swelling by hyperosmotic mannitol counteracted glutamate accumulation and prevented SiD. In contrast, inhibition of glial metabolism or inhibition of astrocyte glutamate transporters reproduced the SiD phenotype. Finally, we show in the rodent water intoxication model of cytotoxic edema that astrocyte swelling and altered astrocyte calcium waves are central in the evolution of SiD. We discuss our results in the light of evidence for SiD in the human cortex. Our results emphasize the need of preventive osmotherapy in acute brain injury.

Action Potentials Are Critical for the Propagation of Focally Elicited Spreading Depolarizations

Spreading depolarizations (SDs) of gray matter occur in the brain in different pathologic conditions, and cause varying degrees of tissue damage depending on the extent of metabolic burden on the tissue. As might be expected for such large depolarizations, neurons exhibit bursts of action potentials (APs) as the wave propagates. However, the specific role of APs in SD propagation is unclear. This is potentially consequential, since sodium channel modulation has not been considered as a therapeutic target for SD-associated disorders, because of ambiguous experimental evidence. Using whole-cell electrophysiology and single-photon imaging in acute cortical slices from male C57Bl6 mice, we tested the effects of AP blockade on SDs generated by two widely used induction paradigms. We found that AP blockade using tetrodotoxin (TTX) restricted propagation of focally induced SDs, and significantly reduced the amplitude of neuronal depolarization, as well as its Ca2+ load. TTX also abolished the suppression of spontaneous synaptic activity that is a hallmark of focally induced SD. In contrast, TTX did not affect the propagation of SD induced by global superfusion of high [K+]e containing artificial CSF (ACSF). Thus, we show that voltage-gated sodium channel (Nav)-mediated neuronal AP bursts are critical for the propagation and downstream effects of focally induced SD but are less important when the ionic balance of the extracellular space is already compromised. In doing so we corroborate the notion that two different SD induction paradigms, each relevant to different clinical situations, vary significantly in their characteristics and potentially their response to treatment.SIGNIFICANCE STATEMENT Our findings suggest that voltage-gated sodium channel (Nav) channels have a critical role in the propagation and downstream neural effects of focally induced spreading depolarization (SD). As SDs are likely induced focally in many disease conditions, these studies support sodium channel modulation, a previously underappreciated therapeutic option in SD-associated disorders, as a viable approach.

Microglia Modulate Cortical Spreading Depolarizations After Ischemic Stroke: A Narrative Review

Cortical spreading depolarizations (CSDs) are characterized by waves of diminished electroencephalography activity that propagate across the cortex with subsequent loss of ionic homeostasis. CSDs have been found in many pathological conditions, including migraine, traumatic brain injury, and ischemic stroke. Because of CSD-associated ionic and metabolic disturbances at the peri-infarct area after ischemic stroke, it is thought that CSDs exacerbate tissue infarction and worsen clinical outcomes. Microglia, the main innate immune cells in the brain, are among the first responders to brain tissue damage. Recent studies demonstrated that microglia play a critical role in CSD initiation and propagation. In this article, we discuss the significance of CSD in the setting of ischemic stroke and how microglia may modulate peri-infarct CSDs, also known as iso-electric depolarizations. Finally, we discuss the significance of microglial Ca²⁺ and how it might be used as a potential therapeutic target for patients with ischemic stroke.

Numerical Simulation of Concussive-Generated Cortical Spreading Depolarization to Optimize DC-EEG Electrode Spacing for Noninvasive Visual Detection

BACKGROUND

Cortical spreading depolarization (SD) is a propagating depolarization wave of neurons and glial cells in the cerebral gray matter. SD occurs in all forms of severe acute brain injury, as documented by using invasive detection methods. Based on many experimental studies of mechanical brain deformation and concussion, the occurrence of SDs in human concussion has often been hypothesized. However, this hypothesis cannot be confirmed in humans, as SDs can only be detected with invasive detection methods that would require either a craniotomy or a burr hole to be performed on athletes. Typical electroencephalography electrodes, placed on the scalp, can help detect the possible presence of SD but have not been able to accurately and reliably identify SDs.

METHODS

To explore the possibility of a noninvasive method to resolve this hurdle, we developed a finite element numerical model that simulates scalp voltage changes that are induced by a brain surface SD. We then compared our simulation results with retrospectively evaluated data in patients with aneurysmal subarachnoid hemorrhage from Drenckhahn et al. (Brain 135:853, 2012).

RESULTS

The ratio of peak scalp to simulated peak cortical voltage, Vscalp/Vcortex, was 0.0735, whereas the ratio from the retrospectively evaluated data was 0.0316 (0.0221, 0.0527) (median [1st quartile, 3rd quartile], n = 161, p < 0.001, one sample Wilcoxon signed-rank test). These differing values provide validation because their differences can be attributed to differences in shape between concussive SDs and aneurysmal subarachnoid hemorrhage SDs, as well as the inherent limitations in human study voltage measurements. This simulated scalp surface potential was used to design a virtual scalp detection array. Error analysis and visual reconstruction showed that 1 cm is the optimal electrode spacing to visually identify the propagating scalp voltage from a cortical SD. Electrode spacings of 2 cm and above produce distorted images and high errors in the reconstructed image.

CONCLUSIONS

Our analysis suggests that concussive (and other) SDs can be detected from the scalp, which could confirm SD occurrence in human concussion, provide concussion diagnosis on the basis of an underlying physiological mechanism, and lead to noninvasive SD detection in the setting of severe acute brain injury.

Validating a Computational Framework for Ionic Electrodiffusion with Cortical Spreading Depression as a Case Study

Cortical spreading depression (CSD) is a wave of pronounced depolarization of brain tissue accompanied by substantial shifts in ionic concentrations and cellular swelling. Here, we validate a computational framework for modeling electrical potentials, ionic movement, and cellular swelling in brain tissue during CSD. We consider different model variations representing wild-type (WT) or knock-out/knock-down mice and systematically compare the numerical results with reports from a selection of experimental studies. We find that the data for several CSD hallmarks obtained computationally, including wave propagation speed, direct current shift duration, peak in extracellular K⁺ concentration as well as a pronounced shrinkage of extracellular space (ECS) are well in line with what has previously been observed experimentally. Further, we assess how key model parameters including cellular diffusivity, structural ratios, membrane water and/or K⁺ permeabilities affect the set of CSD characteristics.

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.

Cortical spreading depression and meningeal nociception

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CSD evoked persistent activation and mechanical sensitization of dural nociceptors is likely to drive the headache phase in migraine with aura.

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The development of neurogenic-mediated dural vasodilatation and increased plasma protein extravasation in the wake of CSD may not contribute to meningeal nociception.

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Cortical vasoconstriction and reduced oxygen availability following CSD do not contribute to meningeal nociception.

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Cortical neuroinflammation, involving neuronal pannexin1 and calcium-independent astrocytic signaling drive meningeal nociception following CSD.

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CSD-related closing of K(ATP) channels and release of COX-driven prostanoids mediate the activation and sensitization of dural nociceptors respectively.

Migraine results in an enormous burden on individuals and societies due to its high prevalence, significant disability, and considerable economic costs. Current treatment options for migraine remain inadequate, and the development of novel therapies is severely hindered by the incomplete understanding of the mechanisms responsible for the pain. The sensory innervation of the cranial meninges is now considered a key player in migraine headache genesis. Recent studies have significantly advanced our understanding of some of the processes that drive meningeal nociceptive neurons, which may be targeted therapeutically to abort or prevent migraine pain. In this review we will summarize our current understanding of the mechanisms that contribute to the genesis of the headache in one migraine subtype – migraine with aura. We will focus on animal studies that address the notion that cortical spreading depression is a critical process that drives meningeal nociception in migraine with aura, and discuss recent insights into some of the proposed underlying mechanisms.

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.

Src family kinases activity is required for transmitting purinergic P2X7 receptor signaling in cortical spreading depression and neuroinflammation

BACKGROUND

Purinergic P2X7 receptor plays an important role in migraine pathophysiology. Yet precise molecular mechanism underlying P2X7R signaling in migraine remains unclear. This study explores the hypothesis that P2X7 receptor transmits signaling to Src family kinases (SFKs) during cortical spreading depression (CSD) and neuroinflammation after CSD.

METHODS

CSD was recorded using electrophysiology in rats and intrinsic optical imaging in mouse brain slices. Cortical IL-1β and TNFα mRNA levels were detected using qPCR. Glutamate release from mouse brain slices was detected using glutamate assay.

RESULTS

The data showed that deactivation of SFKs by systemic injection of PP2 reduced cortical susceptibility to CSD in rats and CSD-induced IL-1β and TNF-α gene expression in rat ipsilateral cortices. Consistently, in mouse brain slices, inhibition of SFKs activity by saracatinib and P2X7 receptor by A740003 similarly reduced cortical susceptibility to CSD. When the interaction of P2X7 receptor and SFKs was disrupted by TAT-P2X7, a marked reduction of cortical susceptibility to CSD, IL-1β gene expression and glutamate release after CSD induction were observed in mouse brain slices. The reduced cortical susceptibility to CSD by TAT-P2X7 was restored by NMDA, and disrupting the Fyn-NMDA interaction using TAT-Fyn (39-57) but not disrupting Src-NMDA receptor interaction using TAT-Src (40-49) reduced cortical susceptibility to CSD. Furthermore, activation of P2X7 receptor by BzATP restored the TAT-Fyn (39-57)-reduced cortical susceptibility to CSD.

CONCLUSION

This study reveals that SFKs activity transmits P2X7 receptor signaling to facilitate CSD propagation via glutamatergic pathway and promote neuroinflammation, which is of particular relevance to migraine.

Cerebellar spreading depolarization mediates paroxysmal movement disorder

Paroxysmal kinesigenic dyskinesia (PKD) is the most common paroxysmal dyskinesia, characterized by recurrent episodes of involuntary movements provoked by sudden changes in movement. Proline-rich transmembrane protein 2 (PRRT2) has been identified as the major causative gene for PKD. Here, we report that PRRT2 deficiency facilitates the induction of cerebellar spreading depolarization (SD) and inhibition of cerebellar SD prevents the occurrence of dyskinetic movements. Using Ca²⁺ imaging, we show that cerebellar SD depolarizes a large population of cerebellar granule cells and Purkinje cells in Prrt2-deficient mice. Electrophysiological recordings further reveal that cerebellar SD blocks Purkinje cell spiking and disturbs neuronal firing of the deep cerebellar nuclei (DCN). The resultant aberrant firing patterns in DCN are tightly, temporally coupled to dyskinetic episodes in Prrt2-deficient mice. Cumulatively, our findings uncover a pivotal role of cerebellar SD in paroxysmal dyskinesia, providing a potent target for treating PRRT2-related paroxysmal disorders.

OnabotulinumtoxinA affects cortical recovery period but not occurrence or propagation of cortical spreading depression in rats with compromised blood-brain barrier

ABSTRACT

OnabotulinumtoxinA (BoNT-A) is an Food and Drug Administration-approved, peripherally acting preventive migraine drug capable of inhibiting meningeal nociceptors. Expanding our view of how else this neurotoxin attenuates the activation of the meningeal nociceptors, we reasoned that if the stimulus that triggers the activation of the nociceptor is lessened, the magnitude and/or duration of the nociceptors' activation could diminish as well. In the current study, we further examine this possibility using electrocorticogram recording techniques, immunohistochemistry, and 2-photon microscopy. We report (1) that scalp (head) but not lumbar (back) injections of BoNT-A shorten the period of profound depression of spontaneous cortical activity that follows a pinprick-induced cortical spreading depression (CSD); (2) that neither scalp nor lumbar injections prevent the induction, occurrence, propagation, or spreading velocity of a single wave of CSD; (3) that cleaved SNAP25-one of the most convincing tools to determine the anatomical targeting of BoNT-A treatment-could easily be detected in pericranial muscles at the injection sites and in nerve fibers of the intracranial dura, but not within any cortical area affected by the CSD; (4) that the absence of cleaved SNAP25 within the cortex and pia is unrelated to whether the blood-brain barrier is intact or compromised; and (5) that BoNT-A does not alter vascular responses to CSD. To the best of our knowledge, this is the first report of peripherally applied BoNT-A's ability to alter a neuronal function along a central nervous system pathway involved in the pathophysiology of migraine.

Headache in people with epilepsy

Epidemiological estimates indicate that individuals with epilepsy are more likely to experience headaches, including migraine, than individuals without epilepsy. Headaches can be temporally unrelated to seizures, or can occur before, during or after an episode; seizures and migraine attacks are mostly not temporally linked. The pathophysiological links between headaches (including migraine) and epilepsy are complex and have not yet been fully elucidated. Correct diagnoses and appropriate treatment of headaches in individuals with epilepsy is essential, as headaches can contribute substantially to disease burden. Here, we review the insights that have been made into the associations between headache and epilepsy over the past 5 years, including information on the pathophysiological mechanisms and genetic variants that link the two disorders. We also discuss the current best practice for the management of headaches co-occurring with epilepsy and highlight future challenges for this area of research.

Astrocytes mediate migraine-related intracranial meningeal mechanical hypersensitivity

ABSTRACT

The genesis of the headache phase in migraine with aura is thought to be mediated by cortical spreading depression (CSD) and the subsequent activation and sensitization of primary afferent neurons that innervate the intracranial meninges and their related large vessels. Yet, the exact mechanisms underlying this peripheral meningeal nociceptive response remain poorly understood. We investigated the relative contribution of cortical astrocytes to CSD-evoked meningeal nociception using extracellular single-unit recording of meningeal afferent activity and 2-photon imaging of cortical astrocyte calcium activity, in combination with 2 pharmacological approaches to inhibit astrocytic function. We found that fluoroacetate and l-α-aminoadipate, which inhibit astrocytes through distinct mechanisms, suppressed CSD-evoked afferent mechanical sensitization, but did not affect afferent activation. Pharmacological inhibition of astrocytic function, which ameliorated meningeal afferents' sensitization, reduced basal astrocyte calcium activity but had a minimal effect on the astrocytic calcium wave during CSD. We propose that calcium-independent signaling in cortical astrocytes plays an important role in driving the sensitization of meningeal afferents and the ensuing intracranial mechanical hypersensitivity in migraine with aura.

Low-Intensity Pulsed Ultrasound Stimulation Inhibits Cortical Spreading Depression

Cortical spreading depression (CSD), which is closely correlated with migraine aura, cerebral ischemia, seizure, and brain injury, is a spreading wave of neuronal and glial depolarization. The purpose of this study is to investigate whether low-intensity pulsed ultrasound stimulation (PUS) inhibits CSD by modulating neural activity and hemodynamics. Behavioral test, intrinsic signal optical imaging and western blot analysis were used for evaluating the inhibition effect of PUS on CSD in rat. We found that: 1) 30 min of PUS can significantly improve motor activity of rat with CSD. 2) Both 30 s and 30 min of PUS can significantly reduce count and propagation speed of CSD in rat and the inhibitory effect was enhanced with increase of ultrasound intensity. 3) 30 min of PUS significantly enhanced levels of brain-derived neurotrophic factor protein in brain tissue with CSD. These results suggest that PUS has the potential to treat brain disorders associated with CSD.

Astrocyte Ca2+ Waves and Subsequent Non-Synchronized Ca2+ Oscillations Coincide with Arteriole Diameter Changes in Response to Spreading Depolarization

Spreading depolarization (SD) is a wave of mass depolarization that causes profound perfusion changes in acute cerebrovascular diseases. Although the astrocyte response is secondary to the neuronal depolarization with SD, it remains to be explored how glial activity is altered after the passage of SD. Here, we describe post-SD high frequency astrocyte Ca²⁺ oscillations in the mouse somatosensory cortex. The intracellular Ca²⁺ changes of SR101 labeled astrocytes and the SD-related arteriole diameter variations were simultaneously visualized by multiphoton microscopy in anesthetized mice. Post-SD astrocyte Ca²⁺ oscillations were identified as Ca²⁺ events non-synchronized among astrocytes in the field of view. Ca²⁺ oscillations occurred minutes after the Ca²⁺ wave of SD. Furthermore, fewer astrocytes were involved in Ca²⁺ oscillations at a given time, compared to Ca²⁺ waves, engaging all astrocytes in the field of view simultaneously. Finally, our data confirm that astrocyte Ca²⁺ waves coincide with arteriolar constriction, while post-SD Ca²⁺ oscillations occur with the peak of the SD-related vasodilation. This is the first in vivo study to present the post-SD astrocyte Ca²⁺ oscillations. Our results provide novel insight into the spatio-temporal correlation between glial reactivity and cerebral arteriole diameter changes behind the SD wavefront.

Non-canonical glutamate signaling in a genetic model of migraine with aura

Migraine with aura is a common but poorly understood sensory circuit disorder. Monogenic models allow an opportunity to investigate its mechanisms, including spreading depolarization (SD), the phenomenon underlying migraine aura. Using fluorescent glutamate imaging, we show that awake mice carrying a familial hemiplegic migraine type 2 (FHM2) mutation have slower clearance during sensory processing, as well as previously undescribed spontaneous "plumes" of glutamate. Glutamatergic plumes overlapped anatomically with a reduced density of GLT-1a-positive astrocyte processes and were mimicked in wild-type animals by inhibiting glutamate clearance. Plume pharmacology and plume-like neural Ca2+ events were consistent with action-potential-independent spontaneous glutamate release, suggesting plumes are a consequence of inefficient clearance following synaptic release. Importantly, a rise in basal glutamate and plume frequency predicted the onset of SD in both FHM2 and wild-type mice, providing a novel mechanism in migraine with aura and, by extension, the other neurological disorders where SD occurs.

"Shooting pain" in lumbar radiculopathy and trigeminal neuralgia, and ideas concerning its neural substrates

Patients with radicular low back pain (radicular LBP, sciatica) frequently describe their pain as "shooting" or "radiating." The dictionary meaning of these words implies rapid movement, and indeed, many sufferers report feeling pain moving rapidly from the lower back or buttock into the leg. But, others do not. Moreover, the sensation of movement is paradoxical; it is neither predicted nor accounted for by current ideas about the pathophysiology of radicular LBP. We have used a structured questionnaire to evaluate the sensory qualities associated with "shooting" and "radiating" in 155 patients, 98 with radicular LBP and 57 with trigeminal neuralgia, a second chronic pain condition in which shooting/radiating are experienced. Results indicated a spectrum of different sensations in different people. Although many sciatica patients reported rapid downward movement of their pain, even more reported downward expansion of the area of pain, some reported upward movement, and for some, there was no spatial dynamic at all. The velocity of movement or expansion was also variable. By cross-referencing sensations experienced in the sciatica and trigeminal neuralgia cohorts with known signal processing modes in the somatosensory system, we propose testable hypotheses concerning the pathophysiology of the various vectorial sensations reported, their direction and velocity, and the structures in which they are generated. Systematic evaluation of qualitative features of "shooting" and "radiating" pain at the time of diagnosis can shed light on the pain mechanism in the individual patient and perhaps contribute to a better therapeutic outcomes.

Microglial Calcium Waves During the Hyperacute Phase of Ischemic Stroke

BACKGROUND AND PURPOSE

Ischemic injury triggers multiple pathological responses in the brain tissue, including spreading depolarizations across the cerebral cortex (cortical spreading depolarizations [CSD]). Microglia have been recently shown to play a significant role in the propagation of CSD. However, the intracellular responses of myeloid cells during ischemic stroke have not been investigated.

METHODS

We have studied intracellular calcium activity in cortical microglia in the stroke model of the middle cerebral artery occlusion, using the murine Polr2a-based and Cre-dependent GCaMP5 and tdTomato reporter (PC::G5-tdT). High-speed 2-photon microscopy through cranial windows was employed to record signals from genetically encoded indicators of calcium. Inflammatory stimuli and pharmacological inhibition were used to modulate microglial calcium responses in the somatosensory cortex.

RESULTS

In vivo imaging revealed periodical calcium activity in microglia during the hyperacute phase of ischemic stroke. This activity was more frequent during the first 6 hours after occlusion, but the amplitudes of calcium transients became larger at later time points. Consistent with CSD nature of these events, we reproducibly triggered comparable calcium transients with microinjections of potassium chloride (KCl) into adjacent cortical areas. Furthermore, lipopolysaccharide-induced peripheral inflammation, mimicking sterile inflammation during ischemic stroke, produced significantly greater microglial calcium transients during CSD. Finally, in vivo pharmacological analysis with CRAC (calcium release-activated channel) inhibitor CM-EX-137 demonstrated that CSD-associated microglial calcium transients after KCl microinjections are mediated at least in part by the CRAC mechanism.

CONCLUSIONS

Our findings demonstrate that microglia participate in ischemic brain injury via previously undetected mechanisms, which may provide new avenues for therapeutic interventions.

Neocortical in vivo focal and spreading potassium responses and the influence of astrocytic gap junctional coupling

Raised extracellular potassium ion (K⁺) concentration is associated with several disorders including migraine, stroke, neurotrauma and epilepsy. K⁺ spatial buffering is a well-known mechanism for extracellular K⁺ regulation/distribution. Astrocytic gap junction-mediated buffering is a controversial candidate for K⁺ spatial buffering. To further investigate the existence of a K⁺ spatial buffering and to assess the involvement of astrocytic gap junctional coupling in K⁺ redistribution, we hypothesized that neocortical K⁺ and concomitant spreading depolarization (SD)-like responses are controlled by powerful local K⁺ buffering mechanisms and that K⁺ buffering/redistribution occurs partially through gap junctional coupling. Herein, we show, in vivo, that a threshold amount of focally applied KCl is required to trigger local and/or distal K⁺ responses, accompanied by a SD-like response. This observation indicates the presence of powerful local K⁺ buffering which mediates a rapid return of extracellular K⁺ to the baseline. Application of gap junctional blockers, carbenoxolone and Gap27, partially modulated the amplitude and shape of the K⁺ response and noticeably decreased the velocity of the spreading K⁺ and SD-like responses. Opening of gap junctions by trimethylamine, slightly decreased the amplitude of the K⁺ response and markedly increased the velocity of redistribution of K⁺ and SD-like events. We conclude that spreading K⁺ responses reflect powerful local K⁺ buffering mechanisms which are partially modulated by gap junctional communication. Gap junctional coupling mainly affected the velocity of the K⁺ and SD-like responses.