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

Functional implications of axon initial segment cytoskeletal disruption in stroke

Axon initial segment (AIS) is the proximal part of the axon, which is not covered with a myelin sheath and possesses a distinctive, specialized assembly of voltage-gated ion channels and associated proteins. AIS plays critical roles in synaptic integration and action potential generation in central neurons. Recent evidence shows that stroke causes rapid, irreversible calpain-mediated proteolysis of the AIS cytoskeleton of neurons surrounding the ischemic necrotic core. A better understanding of the molecular mechanisms underlying this "non-lethal" neuronal damage might provide new therapeutic strategies for improving stroke outcome. Here, we present a brief overview of the structure and function of the AIS. We then discuss possible mechanisms underlying stroke-induced AIS damage, including the roles of calpains and possible sources of Ca(2+) ions, which are necessary for the activation of calpains. Finally, we discuss the potential functional implications of the loss of the AIS cytoskeleton and ion channel clusters for neuronal excitability.

Identification of an Intracranial Pressure (ICP) Response Function from Continuously Acquired Electroencephalographic and ICP Signals in Burst-Suppressed Patients

Continuous intracranial pressure (ICP) and electroencephalographic (EEG) monitoring are used in the management of patients with brain injury. It is possible that these two signals could be related through neurovascular coupling. To explore this mechanism, we modeled the ICP response to brain activity by treating spontaneous burst activity in burst-suppressed patients as an impulse, and identified the ICP response function (ICPRF) as the subsequent change in ICP.Segments of ICP were filtered, classified as elevating or stable, and suitable ICPRFs were identified. After calibration, each ICPRF was convolved with the EEG to produce the estimated ICP. The mean error (ME) versus distance from the selected ICPRF was calculated and the elevating and stable ICP segments compared.Eighty-four ICPRFs were identified from 15 data segments. The ME of the elevating segments increased at an average rate of 57 mmHg/min, whereas the average ME of the stable segments increased at a rate of 0.05 mmHg/min.These findings demonstrate that deriving an ICPRF from a burst-suppressed patient is a suitable approach for stable segments. To completely model the ICP response to EEG activity, a more robust model should be developed.

Human Albumin Improves Long-Term Behavioral Sequelae After Subarachnoid Hemorrhage Through Neurovascular Remodeling

OBJECTIVE

Subarachnoid hemorrhage results in significant long-lasting neurologic sequelae. Here, we investigated whether human albumin improves long-term outcomes in experimental subarachnoid hemorrhage and whether neurovascular remodeling is involved in the protection of albumin.

DESIGN

Laboratory investigation.

SETTING

Hospital research laboratory.

SUBJECTS

Male Sprague-Dawley rats.

INTERVENTIONS

Rats underwent subarachnoid hemorrhage by endovascular perforation. Albumin of either 0.63 or 1.25 g/kg was injected IV immediately after the surgery. Modified Garcia test, beam-walking test, novel object recognition, and Morris water maze were employed to determine the behavioral deficits. The effects of albumin on early neurovascular dysfunction and chronic synaptic plasticity were also studied.

MEASUREMENTS AND MAIN RESULTS

Both doses of albumin significantly improved the sensorimotor scores (F = 31.277; p = 0.001) and cognitive performance (F = 7.982; p = 0.001 in novel object recognition test; and F = 3.431; p = 0.026 in the latency analysis of Morris water maze test) for at least 40 days after subarachnoid hemorrhage. There were remarkable microvasculature hypoperfusion, intracranial pressure rise, early vasoconstriction, neural apoptosis, and degeneration in subarachnoid hemorrhage rats, with albumin significantly attenuating such neurovascular dysfunction. Furthermore, albumin markedly prevented blood-brain barrier disruption, as indicated by less blood-brain barrier leakage, preserved blood-brain barrier-related proteins, and dampened gelatinase activities. The expressions of key synaptic elements were up-regulated with albumin supplementation in both acute and chronic phases. Accordingly, a higher dendritic spine density was observed in the prefrontal and hippocampal areas of albumin-treated subarachnoid hemorrhage animals.

CONCLUSIONS

Albumin at low-to-moderate doses markedly improves long-term neurobehavioral sequelae after subarachnoid hemorrhage, which may involve an integrated process of neurovascular remodeling.

Focal cerebral ischemia and neurovascular protection: a bench-to-bedside update

PURPOSE OF REVIEW

To date, many pharmacological approaches, or combination of approaches, have been applied to experimental models of focal cerebral ischemia (FCI), but their translation to clinically effective agents has proved unsuccessful. To date, only thrombolysis with recombinant tissue-type plasminogen activator, or other 'clot-breaking' or 'clot-removal' approaches, have proved effective for acute stroke. This review, therefore, focuses on the 'vascular' phenomena involved in the development of FCI.

RECENT FINDINGS

Recent advances in the experimental literature on FCI describe the microvascular characteristics of the ischemic penumbra, the consequences of cortical spreading depression on impairing cerebral perfusion, and the potential neuroprotective mechanisms of ischemic preconditioning via antithrombotic effects on the neurovascular unit.

SUMMARY

This review provides a perspective about the neurovascular components contributing to the pathophysiology of FCI, and some relevant clinical strategies available on the horizon that hold promise for improved cerebral perfusion in FCI.

Cell swelling increases the severity of spreading depression in Locusta migratoria

Progressive accumulation of extracellular potassium ions can trigger propagating waves of spreading depression (SD), which are associated with dramatic increases in extracellular potassium levels ([K(+)]o) and arrest in neural activity. In the central nervous system the restricted nature of the extracellular compartment creates an environment that is vulnerable to disturbances in ionic homeostasis. Here we investigate how changes in the size of the extracellular space induced by alterations in extracellular osmolarity affect locust SD. We found that hypotonic exposure increased susceptibility to experimentally induced SD evidenced by a decrease in the latency to onset and period between individual events. Hypertonic exposure was observed to delay the onset of SD or prevent the occurrence altogether. Additionally, the magnitude of extracellular K(+) concentration ([K(+)]o) disturbance during individual SD events was significantly greater and they were observed to propagate more quickly under hypotonic conditions compared with hypertonic conditions. Our results are consistent with a conclusion that hypotonic exposure reduced the size of the extracellular compartment by causing cell swelling and thus facilitated the accumulation of K(+) ions. Lastly, we found that pharmacologically reducing the accumulation of extracellular K(+) using the K(+) channel blocker tetraethylammonium slowed the rate of SD propagation while increasing [K(+)]o through inhibition of the Na-K-2Cl cotransporter increased propagation rates. Overall our findings indicate that treatments or conditions that act to reduce the accumulation of extracellular K(+) help to protect against the development of SD and attenuate the spread of ionic disturbance adding to the evidence that diffusion of K(+) is a leading event during locust SD.

Volume-regulated anion channel--a frenemy within the brain

The volume-regulated anion channel (VRAC) is a ubiquitously expressed yet highly enigmatic member of the superfamily of chloride/anion channels. It is activated by cellular swelling and mediates regulatory cell volume decrease in a majority of vertebrate cells, including those in the central nervous system (CNS). In the brain, besides its crucial role in cellular volume regulation, VRAC is thought to play a part in cell proliferation, apoptosis, migration, and release of physiologically active molecules. Although these roles are not exclusive to the CNS, the relative significance of VRAC in the brain is amplified by several unique aspects of its physiology. One important example is the contribution of VRAC to the release of the excitatory amino acid neurotransmitters glutamate and aspartate. This latter process is thought to have impact on both normal brain functioning (such as astrocyte-neuron signaling) and neuropathology (via promoting the excitotoxic death of neuronal cells in stroke and traumatic brain injury). In spite of much work in the field, the molecular nature of VRAC remained unknown until less than 2 years ago. Two pioneer publications identified VRAC as the heterohexamer formed by the leucine-rich repeat-containing 8 (LRRC8) proteins. These findings galvanized the field and are likely to result in dramatic revisions to our understanding of the place and role of VRAC in the brain, as well as other organs and tissues. The present review briefly recapitulates critical findings in the CNS and focuses on anticipated impact on the LRRC8 discovery on further progress in neuroscience research.

Chloride Cotransporters as a Molecular Mechanism underlying Spreading Depolarization-Induced Dendritic Beading

Spreading depolarizations (SDs) are waves of sustained neuronal and glial depolarization that propagate massive disruptions of ion gradients through the brain. SD is associated with migraine aura and recently recognized as a novel mechanism of injury in stroke and brain trauma patients. SD leads to neuronal swelling as assessed in real time with two-photon laser scanning microscopy (2PLSM). Pyramidal neurons do not express aquaporins and thus display low inherent water permeability, yet SD rapidly induces focal swelling (beading) along the dendritic shaft by unidentified molecular mechanisms. To address this issue, we induced SD in murine hippocampal slices by focal KCl microinjection and visualized the ensuing beading of dendrites expressing EGFP by 2PLSM. We confirmed that dendritic beading failed to arise during large (100 mOsm) hyposmotic challenges, underscoring that neuronal swelling does not occur as a simple osmotic event. SD-induced dendritic beading was not prevented by pharmacological interference with the cytoskeleton, supporting the notion that dendritic beading may result entirely from excessive water influx. Dendritic beading was strictly dependent on the presence of Cl(-), and, accordingly, combined blockade of Cl(-)-coupled transporters led to a significant reduction in dendritic beading without interfering with SD. Furthermore, our in vivo data showed a strong inhibition of dendritic beading during pharmacological blockage of these cotransporters. We propose that SD-induced dendritic beading takes place as a consequence of the altered driving forces and thus activity for these cotransporters, which by transport of water during their translocation mechanism may generate dendritic beading independently of osmotic forces.

SIGNIFICANCE STATEMENT

Spreading depolarization occurs during pathological conditions such as stroke, brain injury, and migraine and is characterized as a wave of massive ion translocation between intracellular and extracellular space in association with recurrent transient focal swelling (beading) of dendrites. Numerous ion channels have been demonstrated to be involved in generation and propagation of spreading depolarization, but the molecular machinery responsible for the dendritic beading has remained elusive. Using real-time in vitro and in vivo two-photon laser scanning microscopy, we have identified the transport mechanisms involved in the detrimental focal swelling of dendrites. These findings have clear clinical significance because they may point to a new class of pharmacological targets for prevention of neuronal swelling that consequently will serve as neuroprotective agents.

Minocycline Effectively Protects the Rabbit's Spinal Cord From Aortic Occlusion-Related Ischemia

OBJECTIVES

To identify the minocycline anti-inflammatory and antiapoptotic mechanisms through which it is believed to exert spinal cord protection during aortic occlusion in the rabbit model.

DESIGN

An animal model of aortic occlusion-related spinal cord ischemia. Randomized study with a control group and pre-ischemia and post-ischemia escalating doses of minocycline to high-dose minocycline in the presence of either hyperglycemia, a pro-apoptotic maneuver, or wortmannin, a specific phosphatidylinositol 3-kinase antagonist.

SETTING

Tertiary medical center and school of medicine laboratory.

PARTICIPANTS

Laboratory animals-rabbits.

INTERVENTIONS

Balloon obstruction of infrarenal aorta introduced via femoral artery incision.

RESULTS

Severe hindlimb paralysis (mean Tarlov score 0.36±0.81 out of 3) was observed in all the control group animals (9 of 11 with paraplegia and 2 of 11 with paraparesis) compared with 11 of 12 neurologically intact animals (mean Tarlov score 2.58±0.90 [p = 0.001 compared with control]) in the high-dose minocycline group. This protective effect was observed partially during a state of hyperglycemia and was completely abrogated by wortmannin. Minocycline administration resulted in higher neurologic scores (p = 0.003) and a shift to viable neurons and more apoptotic-stained nuclei resulting from reduced necrosis (p = 0.001).

CONCLUSIONS

In a rabbit model of infrarenal aortic occlusion, minocycline effectively reduced paraplegia by increasing the number of viable neurons in a dose-dependent manner. Its action was completely abrogated by inhibiting the phosphatidylinositol 3-kinase pathway and was inhibited partially by the pro-apoptotic hyperglycemia maneuver, indicating that the activation of cell salvage pathways and mitochondrial sites are possible targets of minocycline action in an ischemic spinal cord.

Electrophysiologic monitoring in acute brain injury

To determine the optimal use and indications of electroencephalography (EEG) in critical care management of acute brain injury (ABI). An electronic literature search was conducted for articles in English describing electrophysiological monitoring in ABI from January 1990 to August 2013. A total of 165 studies were included. EEG is a useful monitor for seizure and ischemia detection. There is a well-described role for EEG in convulsive status epilepticus and cardiac arrest (CA). Data suggest EEG should be considered in all patients with ABI and unexplained and persistent altered consciousness and in comatose intensive care unit (ICU) patients without an acute primary brain condition who have an unexplained impairment of mental status. There remain uncertainties about certain technical details, e.g., the minimum duration of EEG studies, the montage, and electrodes. Data obtained from both EEG and EP studies may help estimate prognosis in ABI patients, particularly following CA and traumatic brain injury. Data supporting these recommendations is sparse, and high quality studies are needed. EEG is used to monitor and detect seizures and ischemia in ICU patients and indications for EEG are clear for certain disease states, however, uncertainty remains on other applications.

Dynamics of Ionic Shifts in Cortical Spreading Depression

Cortical spreading depression is a slowly propagating wave of near-complete depolarization of brain cells followed by temporary suppression of neuronal activity. Accumulating evidence indicates that cortical spreading depression underlies the migraine aura and that similar waves promote tissue damage in stroke, trauma, and hemorrhage. Cortical spreading depression is characterized by neuronal swelling, profound elevation of extracellular potassium and glutamate, multiphasic blood flow changes, and drop in tissue oxygen tension. The slow speed of the cortical spreading depression wave implies that it is mediated by diffusion of a chemical substance, yet the identity of this substance and the pathway it follows are unknown. Intercellular spread between gap junction-coupled neurons or glial cells and interstitial diffusion of K(+) or glutamate have been proposed. Here we use extracellular direct current potential recordings, K(+)-sensitive microelectrodes, and 2-photon imaging with ultrasensitive Ca(2+) and glutamate fluorescent probes to elucidate the spatiotemporal dynamics of ionic shifts associated with the propagation of cortical spreading depression in the visual cortex of adult living mice. Our data argue against intercellular spread of Ca(2+) carrying the cortical spreading depression wavefront and are in favor of interstitial K(+) diffusion, rather than glutamate diffusion, as the leading event in cortical spreading depression.

Hypoxia triggers high-altitude headache with migraine features: A prospective trial

Background

Given the high prevalence and clinical impact of high-altitude headache (HAH), a better understanding of risk factors and headache characteristics may give new insights into the understanding of hypoxia being a trigger for HAH or even migraine attacks.

Methods

In this prospective trial, we simulated high altitude (4500 m) by controlled normobaric hypoxia (FiO2 = 12.6%) to investigate acute mountain sickness (AMS) and headache characteristics. Clinical symptoms of AMS according to the Lake Louise Scoring system (LLS) were recorded before and after six and 12 hours in hypoxia. O2 saturation was measured using pulse oximetry at the respective time points. History of primary headache, especially episodic or chronic migraine, was a strict exclusion criterion.

Findings

In total 77 volunteers (43 (55.8%) males, 34 (44.2%) females) were enrolled in this study. Sixty-three (81.18%) and 40 (71.4%) participants developed headache at six or 12 hours, respectively, with height and SpO2 being significantly different between headache groups at six hours ( p < 0.05). Binary logistic regression model revealed a significant association of SpO2 and headache development ( p < 0.05, OR 1.123, 95% CI 1.001–1.259). In a subgroup of participants, with history of migraine being a strict exclusion criterion, hypoxia triggered migraine-like headache according to the International Classification of Headache Disorders (ICHD-3 beta) in n = 5 (8%) or n = 6 (15%), at six and 12 hours, respectively.

Interpretation

Normobaric hypoxia is a trigger for HAH and migraine-like headache attacks even in healthy volunteers without any history of migraine. Our study confirms the pivotal role of hypoxia in the development of AMS and beyond that suggests hypoxia may be involved in migraine pathophysiology.

Cortical spreading depolarization: Pathophysiology, implications, and future directions

Cortical spreading depolarization (CSD) is a spreading loss of ion homeostasis, altered vascular response, change in synaptic architecture, and subsequent depression in electrical activity following an inciting neurological injury. First described by Leão in 1944, this disturbance in neuronal electrophysiology has since been demonstrated in a number of animal studies, and recently a few human studies that examine the occurrence of this depolarizing phenomenon in the setting of a variety of pathological states, including migraines, cerebrovascular accidents, epilepsy, intracranial hemorrhages, and traumatic brain injuries. The onset of CSD has been demonstrated experimentally following a disruption in the neuronal environment leading to glutamate-induced toxicity. This initial event leads to pathological changes in the activity of ion channels that maintain membrane potential. Recovery mechanisms such as sodium-potassium pumps that aim to restore homeostasis fail, leading to osmolar shifts of fluid, swelling of the neuron, and ultimately a measurable depression in cortical activity that spreads in the order of millimeters per minute. Equally important is the resulting change in vascular response. In healthy tissue, increased electrical activity is coupled with release of vasodilatory factors such as nitric oxide and arachidonic acid metabolites that increase local blood flow to meet increased energy expenditure. In damaged tissue, not only is the restorative vascular response lacking but a vasoconstrictive response is promoted and the ischemia that follows adds to the severity of the initial injury. Tissue threatened by this ischemic response is then at elevated risk for CSD propagation and falls into a vicious cycle of electrical and hemodynamic disturbance. Efforts have been made to halt this spreading cortical depression using N-methyl-D-aspartate receptor antagonists and other ion channel blockers to minimize the damaging effects of CSD that can persist long after the triggering insult.

Activation of cerebral sodium-glucose transporter type 1 function mediated by post-ischemic hyperglycemia exacerbates the development of cerebral ischemia

The regulation of post-ischemic hyperglycemia plays an important role in suppressing neuronal damage in therapeutic strategies for cerebral ischemia. We previously reported that the cerebral sodium-glucose transporter (SGLT) was involved in the post-ischemic hyperglycemia-induced exacerbation of cerebral ischemic neuronal damage. Cortical SGLT-1, one of the cerebral SGLT isoforms, is dramatically increased by focal cerebral ischemia. In this study, we focused on the involvement of cerebral SGLT-1 in the development of cerebral ischemic neuronal damage. It was previously reported that activation of 5'-adenosine monophosphate-activated protein kinase (AMPK) increases SGLT-1 expression. Moreover, ischemic stress-induced activation of AMPK exacerbates cerebral ischemic neuronal damage. Therefore, we directly confirmed the relationship between cerebral SGLT-1 and cerebral AMPK activation using in vitro primary culture of mouse cortical neurons. An in vivo mouse model of focal cerebral ischemia was generated using a middle cerebral artery occlusion (MCAO). The development of infarct volume and behavioral abnormalities on day 3 after MCAO were ameliorated in cerebral SGLT-1 knock down mice. Cortical and striatal SGLT-1 expression levels were significantly increased at 12h after MCAO. Immunofluorescence revealed that SGLT-1 and the neuronal nuclear antigen (NeuN) were co-localized in the cortex and striatum of MCAO mice. In the in vitro study, primary cortical neurons were cultured for five days before each treatment with reagents. Concomitant treatment with hydrogen peroxide and glucose induced the elevation of SGLT-1 and phosphorylated AMPK/AMPK ratio, and this elevation was suppressed by compound C, an AMPK inhibitor in primary cortical neurons. Moreover, compound C suppressed neuronal cell death induced by concomitant hydrogen peroxide/glucose treatment in primary cortical neurons. Therefore, we concluded that enhanced cerebral SGLT-1 function mediated by post-ischemic hyperglycemia exacerbates the development of cerebral ischemic neuronal damage. One of the mechanisms of cerebral SGLT-1 up-regulation may be involved in the AMPK activation after cerebral ischemia.

Elucidating the temporal dynamics of optical birefringence changes in crustacean nerves

Intrinsic optical properties, such as optical birefringence, may serve as a tool for minimally invasive neuroimaging methods with high spatiotemporal resolution to aid in the study of neuronal activation patterns. To facilitate imaging neuronal activity by sensing dynamic birefringence, temporal characteristics behind the signal must be better understood. We have developed a novel nerve chamber to investigate changes in birefringence at the stimulation site, and at distances ~4-28 mm from that site. Using crustacean nerves with either heterogeneous or homogeneous size distributions of axon diameters, we found that the gradual (slow) recovery of the crossed-polarized signal is not explained by the arrival times of action potentials in smaller axons. Through studying the effects of stimulating current and voltage pulses, we hypothesize that the recovery may be caused by a capacitive-like coupling between firing axons and adjacent tissue structures, and we report data consistent with this hypothesis. This study will aid in the utilization of action-potential-related changes in birefringence to study fast changes in neuronal network activity.

Spreading Depression, Spreading Depolarizations, and the Cerebral Vasculature

Spreading depression (SD) is a transient wave of near-complete neuronal and glial depolarization associated with massive transmembrane ionic and water shifts. It is evolutionarily conserved in the central nervous systems of a wide variety of species from locust to human. The depolarization spreads slowly at a rate of only millimeters per minute by way of grey matter contiguity, irrespective of functional or vascular divisions, and lasts up to a minute in otherwise normal tissue. As such, SD is a radically different breed of electrophysiological activity compared with everyday neural activity, such as action potentials and synaptic transmission. Seventy years after its discovery by Leão, the mechanisms of SD and its profound metabolic and hemodynamic effects are still debated. What we did learn of consequence, however, is that SD plays a central role in the pathophysiology of a number of diseases including migraine, ischemic stroke, intracranial hemorrhage, and traumatic brain injury. An intriguing overlap among them is that they are all neurovascular disorders. Therefore, the interplay between neurons and vascular elements is critical for our understanding of the impact of this homeostatic breakdown in patients. The challenges of translating experimental data into human pathophysiology notwithstanding, this review provides a detailed account of bidirectional interactions between brain parenchyma and the cerebral vasculature during SD and puts this in the context of neurovascular diseases.

Multifaceted roles for astrocytes in spreading depolarization: A target for limiting spreading depolarization in acute brain injury?

Spreading depolarizations (SDs) are coordinated waves of synchronous depolarization, involving large numbers of neurons and astrocytes as they spread slowly through brain tissue. The recent identification of SDs as likely contributors to pathophysiology in human subjects has led to a significant increase in interest in SD mechanisms, and possible approaches to limit the numbers of SDs or their deleterious consequences in injured brain. Astrocytes regulate many events associated with SD. SD initiation and propagation is dependent on extracellular accumulation of K(+) and glutamate, both of which involve astrocytic clearance. SDs are extremely metabolically demanding events, and signaling through astrocyte networks is likely central to the dramatic increase in regional blood flow that accompanies SD in otherwise healthy tissues. Astrocytes may provide metabolic support to neurons following SD, and may provide a source of adenosine that inhibits neuronal activity following SD. It is also possible that astrocytes contribute to the pathophysiology of SD, as a consequence of excessive glutamate release, facilitation of NMDA receptor activation, brain edema due to astrocyte swelling, or disrupted coupling to appropriate vascular responses after SD. Direct or indirect evidence has accumulated implicating astrocytes in many of these responses, but much remains unknown about their specific contributions, especially in the context of injury. Conversion of astrocytes to a reactive phenotype is a prominent feature of injured brain, and recent work suggests that the different functional properties of reactive astrocytes could be targeted to limit SDs in pathophysiological conditions.

High incidence of adverse cerebral blood flow responses to spreading depolarization in the aged ischemic rat brain

Spreading depolarizations (SDs) occur spontaneously in the brain after stroke, exacerbate ischemic injury, and thus emerge as a potential target of intervention. Aging predicts worse outcome from stroke; yet, the impact of age on SD evolution is not clear. Cerebral ischemia was induced by bilateral common carotid artery occlusion in young (8-9 weeks old, n = 8) and old (2 year olds, n = 6) anesthetized rats. Sham-operated animals of both age groups served as control (n = 12). Electrocorticogram, direct current potential, and cerebral blood flow (CBF) variations were acquired via a small craniotomy above the parietal cortex. SDs were elicited by KCl through a second craniotomy distal to the recording site. Ischemia and age delayed the recovery from SD. CBF decreased progressively during ischemia in the old animals selectively, and inverse neurovascular coupling with SD evolved in the old but not in the young ischemic group. We propose that (mal)adaptation of cerebrovascular function with aging impairs the SD-related CBF response, which is implicated in the intensified expansion of ischemic damage in the old brain.

The stroke-migraine depolarization continuum

The term spreading depolarization (SD) refers to waves of abrupt, sustained mass depolarization in gray matter of the CNS. SD, which spreads from neuron to neuron in affected tissue, is characterized by a rapid near-breakdown of the neuronal transmembrane ion gradients. SD can be induced by hypoxic conditions--such as from ischemia--and facilitates neuronal death in energy-compromised tissue. SD has also been implicated in migraine aura, where SD is assumed to ascend in well-nourished tissue and is typically benign. In addition to these two ends of the "SD continuum," an SD wave can propagate from an energy-depleted tissue into surrounding, well-nourished tissue, as is often the case in stroke and brain trauma. This review presents the neurobiology of SD--its triggers and propagation mechanisms--as well as clinical manifestations of SD, including overlaps and differences between migraine aura and stroke, and recent developments in neuromonitoring aimed at better diagnosis and more targeted treatments.

Suppression of spreading depolarization and stabilization of dendritic spines by GLYX-13, an NMDA receptor glycine-site functional partial agonist

Cortical spreading depolarization (SD) is a slow self-propagating wave of mass cellular depolarization in brain tissue, thought to be the underlying cause of migraine scintillating scotoma and aura, and associated with stroke, traumatic brain injury, and termination of status epilepticus. The N-methyl-d-aspartate subtype of glutamate receptor (NMDAR), which gates influx of calcium and is an important trigger of long-term synaptic plasticity, is also a contributor to the initiation and propagation of SD. The current study tested the potential of pharmacological modulation of NMDAR activity through the obligatory co-agonist binding site, to suppress the initiation of SD, and modulate the effects of SD on dendritic spine morphology, in in vitro hippocampal slices. A novel NMDAR functional glycine site partial agonist, GLYX-13, sometimes completely prevented the induction of SD and consistently slowed its rate of propagation. The passage of SD through the hippocampal CA1 region produced a rapid retraction of dendritic spines which reversed after neuronal depolarization had recovered. GLYX-13 improved the rate and extent of return of dendritic spines to their original sizes and locations following SD, suggesting that NMDAR modulators can protect synaptic connections in the brain from structural alterations elicited by SD. These data indicate that NMDAR modulation to renormalize activity may be an effective new treatment strategy for suppression or amelioration of the contribution of SD to short and long-term symptoms of migraine attacks, as well as the effects of SD on tissue damaged by stroke or traumatic brain injury.

A Biphasic Change of Regional Blood Volume in the Frontal Cortex during Non-Rapid Eye Movement Sleep: A Near-Infrared Spectroscopy Study

STUDY OBJECTIVES

Current knowledge on hemodynamics in sleep is limited because available techniques do not allow continuous recordings and mainly focus on cerebral blood flow while neglecting other important parameters, such as blood volume (BV) and vasomotor activity.

DESIGN

Observational study.

PARTICIPANTS AND SETTINGS

Continuous measures of hemodynamics over the left forehead and biceps were performed using near-infrared spectroscopy (NIRS) during nocturnal polysomnography in 16 healthy participants in sleep laboratory.

MEASUREMENTS AND RESULTS

Temporal dynamics and mean values of cerebral and muscular oxygenated hemoglobin (HbO2), deoxygenated hemoglobin (HHb), and BV during different sleep stages were compared. A biphasic change of cerebral BV was observed which contrasted a monotonic increase of muscular BV during non-rapid eye movement sleep. A significant decrement in cerebral HbO2 and BV accompanied by an increase of HHb was recorded at sleep onset (Phase I). Prior to slow wave sleep (SWS) HbO2 and BV turned to increase whereas HHb began to decrease in subsequent Phase II suggested increased brain perfusion during SWS. The cerebral HbO2 slope correlated to BV slope in Phase I and II, but it only correlated to HHb slope in Phase II. The occurrence time of inflection points correlated to SWS latencies.

CONCLUSION

Initial decrease of brain perfusion with decreased blood volume (BV) and oxygenated hemoglobin (HbO2) together with increasing muscular BV fit thermoregulation process at sleep onset. The uncorrelated and correlated slopes of HbO2 and deoxygenated hemoglobin indicate different mechanisms underlying the biphasic hemodynamic process in light sleep and slow wave sleep (SWS). In SWS, changes in vasomotor activity (i.e., increased vasodilatation) may mediate increasing cerebral and muscular BV.

A validation study of the use of near-infrared spectroscopy imaging in primary and secondary motor areas of the human brain

The electroencephalographically measured Bereitschafts (readiness)-potential in the supplementary motor area (SMA) serves as a signature of the preparation of motor activity. Using a multichannel, noninvasive near-infrared spectroscopy (NIRS) imager, we studied the vascular correlate of the readiness potential. Sixteen healthy subjects performed a self-paced or externally triggered motor task in a single or repetitive pattern, while NIRS simultaneously recorded the task-related responses of deoxygenated hemoglobin (HbR) in the primary motor area (M1) and the SMA. Right-hand movements in the repetitive sequence trial elicited a significantly greater HbR response in both the SMA and the left M1 compared to left-hand movements. During the single sequence condition, the HbR response in the SMA, but not in the M1, was significantly greater for self-paced than for externally cued movements. Nonetheless, an unequivocal temporal delay was not found between the SMA and M1. Near-infrared spectroscopy is a promising, noninvasive bedside tool for the neuromonitoring of epileptic seizures or cortical spreading depolarizations (CSDs) in patients with epilepsy, stroke, or brain trauma because these pathological events are associated with typical spatial and temporal changes in HbR. Propagation is a characteristic feature of these events which importantly supports their identification and characterization in invasive recordings. Unfortunately, the present noninvasive study failed to show a temporal delay during self-paced movements between the SMA and M1 as a vascular correlate of the readiness potential. Although this result does not exclude, in principle, the possibility that scalp-NIRS can detect a temporal delay between different regions during epileptic seizures or CSDs, it strongly suggests that further technological development of NIRS should focus on both improved spatial and temporal resolution. This article is part of a Special Issue entitled Status Epilepticus.

Computational modeling of neurostimulation in brain diseases

Neurostimulation as a therapeutic tool has been developed and used for a range of different diseases such as Parkinson's disease, epilepsy, and migraine. However, it is not known why the efficacy of the stimulation varies dramatically across patients or why some patients suffer from severe side effects. This is largely due to the lack of mechanistic understanding of neurostimulation. Hence, theoretical computational approaches to address this issue are in demand. This chapter provides a review of mechanistic computational modeling of brain stimulation. In particular, we will focus on brain diseases, where mechanistic models (e.g., neural population models or detailed neuronal models) have been used to bridge the gap between cellular-level processes of affected neural circuits and the symptomatic expression of disease dynamics. We show how such models have been, and can be, used to investigate the effects of neurostimulation in the diseased brain. We argue that these models are crucial for the mechanistic understanding of the effect of stimulation, allowing for a rational design of stimulation protocols. Based on mechanistic models, we argue that the development of closed-loop stimulation is essential in order to avoid inference with healthy ongoing brain activity. Furthermore, patient-specific data, such as neuroanatomic information and connectivity profiles obtainable from neuroimaging, can be readily incorporated to address the clinical issue of variability in efficacy between subjects. We conclude that mechanistic computational models can and should play a key role in the rational design of effective, fully integrated, patient-specific therapeutic brain stimulation.

The macrosphere model-an embolic stroke model for studying the pathophysiology of focal cerebral ischemia in a translational approach

The main challenge of stroke research is to translate promising experimental findings from the bench to the bedside. Many suggestions have been made how to achieve this goal, identifying the need for appropriate experimental animal models as one key issue. We here discuss the macrosphere model of focal cerebral ischemia in the rat, which closely resembles the pathophysiology of human stroke both in its acute and chronic phase. Key pathophysiological processes such as brain edema, cortical spreading depolarizations (CSD), neuroinflammation, and stem cell-mediated regeneration are observed in this stroke model, following characteristic temporo-spatial patterns. Non-invasive in vivo imaging allows studying the macrosphere model from the very onset of ischemia up to late remodeling processes in an intraindividual and longitudinal fashion. Such a design of pre-clinical stroke studies provides the basis for a successful translation into the clinic.

Cortical superficial siderosis: detection and clinical significance in cerebral amyloid angiopathy and related conditions

Cortical superficial siderosis describes a distinct pattern of blood-breakdown product deposition limited to cortical sulci over the convexities of the cerebral hemispheres, sparing the brainstem, cerebellum and spinal cord. Although cortical superficial siderosis has many possible causes, it is emerging as a key feature of cerebral amyloid angiopathy, a common and important age-related cerebral small vessel disorder leading to intracerebral haemorrhage and dementia. In cerebral amyloid angiopathy cohorts, cortical superficial siderosis is associated with characteristic clinical symptoms, including transient focal neurological episodes; preliminary data also suggest an association with a high risk of future intracerebral haemorrhage, with potential implications for antithrombotic treatment decisions. Thus, cortical superficial siderosis is of relevance to neurologists working in neurovascular, memory and epilepsy clinics, and neurovascular emergency services, emphasizing the need for appropriate blood-sensitive magnetic resonance sequences to be routinely acquired in these clinical settings. In this review we focus on recent developments in neuroimaging and detection, aetiology, prevalence, pathophysiology and clinical significance of cortical superficial siderosis, with a particular emphasis on cerebral amyloid angiopathy. We also highlight important areas for future investigation and propose standards for evaluating cortical superficial siderosis in research studies.

Blood-brain barrier, bulk flow, and interstitial clearance in epilepsy

Understanding the pathophysiology of epilepsy implies elucidating the neurovascular modifications occurring before or at time of seizures. Cerebrovascular dysfunction provokes or sustains seizures and loss of selective blood-brain barrier (BBB) permeability is a modulator of seizure threshold. However, cerebrovascular pathology in epilepsy extends beyond BBB "leakage" to encompass vascular and parenchymal events. Whenever abnormal accumulation of protein is observed surrounding brain blood vessels, BBB disruption (BBBD) was invoked. Recent clinical and laboratory findings challenged an exclusive role of BBBD in perivascular accumulation of serum-derived products. The circulation of interstitial fluid (ISF) and its bulk flow have emerged as candidate mechanisms which play a role in clearance of CNS waste. Although controversy exists, changes of ISF flow may contribute to CNS disorders through a mechanism encompassing incomplete parenchymal clearance and accompanying accumulation of toxic byproducts. We summarize the evidence in favor and against ISF bulk flow and propose a scenario where abnormal ISF in the epileptic brain allows accumulation of brain protein, sustaining pathophysiology and altering the pharmacology of antiepileptic drugs. We also describe the methods routinely used to dissect out the contribution of BBB-dependent, vascular or paracellular mechanisms to altered neuronal excitability.

Supply-demand mismatch transients in susceptible peri-infarct hot zones explain the origins of spreading injury depolarizations

Peri-infarct depolarizations (PIDs) are seemingly spontaneous spreading depression-like waves that negatively impact tissue outcome in both experimental and human stroke. Factors triggering PIDs are unknown. Here, we show that somatosensory activation of peri-infarct cortex triggers PIDs when the activated cortex is within a critical range of ischemia. We show that the mechanism involves increased oxygen utilization within the activated cortex, worsening the supply-demand mismatch. We support the concept by clinical data showing that mismatch predisposes stroke patients to PIDs as well. Conversely, transient worsening of mismatch by episodic hypoxemia or hypotension also reproducibly triggers PIDs. Therefore, PIDs are triggered upon supply-demand mismatch transients in metastable peri-infarct hot zones due to increased demand or reduced supply. Based on the data, we propose that minimizing sensory stimulation and hypoxic or hypotensive transients in stroke and brain injury would reduce PID incidence and their adverse impact on outcome.

VIDEO ABSTRACT

Aristides Leão: a birth centennial homage with comments on his spreading depression

The year of 2014 is the birth centenary of Aristides Azevedo Pacheco Leão (1914-1993), and also marks seventy years of the publication of his discovery of the novel electrophysiological phenomenon, named by him “spreading depression” (SD), soon designated “Leão’s wave” or “Leão’s spreading depression”. This was a remarkable scientific milestone, and the author must be celebrated for this achievement, as the studies he triggered proceeded worldwide, with new concepts, as spreading depolarization, until the present days. Robust experimental and clinical evidence emerged to suggest that these and related electrophysiological phenomena are involved in the mechanisms of migraine aura, acute cerebrovascular diseases, traumatic brain injury, transient global amnesia, epileptic seizures, and their pathophysiological characteristics come to offer new therapeutic perspectives. He was a remarkable and complex personality, and the authors remit the readers to a paper where his personal life is contemplated.

Concordant occipital and supraorbital neurostimulation therapy for hemiplegic migraine; initial experience; a case series

INTRODUCTION

Hemiplegic migraine is a particularly severe form of the disease that often evolves to a debilitating chronic illness that is resistant to commonly available therapies. Peripheral neurostimulation has been found to be a beneficial therapy for some patients among several diagnostic classes of migraine, but its potential has not been specifically evaluated for hemiplegic migraine.

MATERIALS AND METHODS

Four patients with hemiplegic migraine were treated with concordant, combined occipital and supraorbital neurostimulation over periods ranging 6-92 months. The clinical indicators followed included assessments of headache frequency and severity, frequency of hemiplegic episodes, functional impairment, medication usage, and patient satisfaction.

RESULTS

All reported a positive therapeutic response, as their average headache frequency decreased by 92% (30 to 2.5 headache days/month); Visual Analog Score by 44% (9.5 to 5.3); frequency of hemiplegic episodes by 96% (7.5 to 0.25 hemiplegic episodes/month); headache medication usage by 96% (6 to 0.25 daily medications); and Migraine Disability Assessment score by 98% (249 to 6). All were satisfied and would recommend the therapy, and all preferred combined occipital-supraorbital neurostimulation to occipital neurostimulation alone.

CONCLUSIONS

Concordant combined occipital and supraorbital neurostimulation may provide effective therapy for both the pain and motor aura in some patients with hemiplegic migraine.

Measurement of distinctive features of cortical spreading depolarizations with different MRI contrasts

Growing clinical evidence suggests critical involvement of spreading depolarizations (SDs) in the pathophysiology of neurological disorders such as migraine and stroke. MRI provides powerful tools to detect and assess co-occurring cerebral hemodynamic and cellular changes during SDs. This study reports the feasibility and advantages of two MRI scans, based on balanced steady-state free precession (b-SSFP) and diffusion-weighted multi-spin-echo (DT2), heretofore unexplored for monitoring SDs. These were compared with gradient-echo MRI. SDs were induced by KCl application in rat brain. Known for high SNR, the T2- and T1-based b-SSFP contrast was hypothesized to provide higher spatiotemporal specificity than T2*-based gradient-echo scanning. DT2 scanning was designed to provide simultaneous T2 and apparent diffusion coefficient (ADC) measurements, thus enabling combined quantitative assessment of hemodynamic and cellular changes during SDs. Procedures were developed to automate identification of SD-induced responses in all the scans. These responses were analyzed to determine detection sensitivity and temporal characteristics of signals from each scanning method. Cluster analysis was performed to elucidate unique temporal patterns for each contrast. All scans allowed detection of SD-induced responses. b-SSFP scans showed significantly larger relative intensity changes, narrower peak widths and greater spatial specificity compared with gradient-echo MRI. SD-induced effects on ADC, calculated from DT2 scans, showed the most pronounced signal changes, displaying about 20% decrease, as against 10-15% signal increases observed with b-SSFP and gradient-echo scanning. Cluster analysis revealed additional temporal sub-patterns, such as an initial dip on gradient-echo scans and temporally shifted T2 and proton density changes in DT2 data. To summarize, b-SSFP and DT2 scanning provide distinct information on SDs compared with gradient-echo MRI. DT2 scanning, with its potential to simultaneously provide cellular and hemodynamic information, can offer unique information on the inter-relationship between these processes in pathologic brain, which may improve monitoring of spreading depolarizations in (pre)clinical settings.

Anesthetic agents modulate ECoG potentiation after spreading depression, and insulin-induced hypoglycemia does not modify this effect

Cortical spreading depression (CSD) is characterized by reversible reduction of spontaneous and evoked electrical activity of the cerebral cortex. Experimental evidence suggests that CSD may modulate neural excitability and synaptic activity, with possible implications for long-term potentiation. Systemic factors like anesthetics and insulin-induced hypoglycemia can influence CSD propagation. In this study, we examined whether the post-CSD ECoG potentiation can be modulated by anesthetics and insulin-induced hypoglycemia. We found that awake adult rats displayed increased ECoG potentiation after CSD, as compared with rats under urethane+chloralose anesthesia or tribromoethanol anesthesia. In anesthetized rats, insulin-induced hypoglycemia did not modulate ECoG potentiation. Comparison of two cortical recording regions in awake rats revealed a similarly significant (p<0.05) potentiation effect in both regions, whereas in the anesthetized groups the potentiation was significant only in the recording region nearer to the stimulating point. Our data suggest that urethane+chloralose and tribromoethanol anesthesia modulate the post-CSD potentiation of spontaneous electrical activity in the adult rat cortex, and insulin-induced hypoglycemia does not modify this effect. Data may help to gain a better understanding of excitability-dependent mechanisms underlying CSD-related neurological diseases.

Characterization of the relationship between intracranial pressure and electroencephalographic monitoring in burst-suppressed patients

BACKGROUND

The objective of this study is to characterize the relationship between ICP and EEG METHODS: Simultaneous ICP and EEG data were obtained from burst-suppressed patients and segmented by EEG bursts. Segments were categorized as increasing/decreasing and peak/valley to investigate relationship between ICP changes and EEG burst duration. A generalized ICP response was obtained by averaging all segments time-aligned at burst onsets. A vasodilatation index (VDI) was derived from the ICP pulse waveform and calculated on a sliding interval to investigate cerebrovascular changes post-burst.

RESULTS

Data from two patients contained 309 bursts. 246 ICP segments initially increased, of which 154 peaked. 63 ICP segments decreased, and zero reached a valley. The change in ICP (0.54 ± 0.85 mmHg) was significantly correlated with the burst duration (p < 0.001). Characterization of the ICP segments showed a peak at 8.1 s and a return to baseline at 14.7 s. The VDI for increasing segments was significantly elevated (median 0.56, IQR 0.31, p < 0.001) and correlated with burst duration (p < 0.001).

CONCLUSIONS

Changes in the ICP and pulse waveform shape after EEG burst suggest that these signals can be related within the context of neurovascular coupling.

SIGNIFICANCE

Existence of a physiological relationship between ICP and EEG may allow the study of neurovascular coupling in acute brain injury patients.

Cortical spreading depolarization stimulates gliogenesis in the rat entorhinal cortex

Recently, we showed that cortical spreading depolarizations (CSDs) are a potent trigger of hippocampal neurogenesis. Here, we evaluated CSD-induced cytogenesis in the entorhinal cortex (EC), which provides the major afferent input to the dentate gyrus. Cortical spreading depolarizations were induced by epidural application of 3 mol/L KCl, controls received equimolar NaCl. Cytogenesis was analyzed at different time points thereafter by means of intraperitoneal 5-bromodeoxyuridine injections (day 2, 4, or days 1 to 7) and immunohistochemistry. Recurrent CSD significantly increased numbers of newborn cells in the ipsilateral EC. The majority of these cells expressed glial markers. Microglia proliferation was maximal at day 2, whereas NG2 glia and astrocytes responded for a prolonged period of time (days 2 to 4). Newborn glia remained detectable for 6 weeks after CSD. Whereas we furthermore detected newborn cells immunopositive for doublecortin, a marker for immature neuronal cells, we found no evidence for the generation of new neurons in the EC. Our results indicate that CSD is a potent gliogenic stimulus, leading to rapid and enduring changes in the glial cellular composition of the affected brain tissue. Thus, CSD facilitates ongoing structural remodeling of the directly affected cortex that might contribute to the pathophysiology of CSD-related brain pathologies.

Hyperperfusion counteracted by transient rapid vasoconstriction followed by long-lasting oligemia induced by cortical spreading depression in anesthetized mice

Cortical spreading depression (CSD) involves mass depolarization of neurons and glial cells accompanied with changes in regional cerebral blood flow (rCBF) and energy metabolism. To further understand the mechanisms of CBF response, we examined the temporal diametric changes in pial arteries, pial veins, and cortical capillaries. In urethane-anesthetized mice, the diameters of these vessels were measured while simultaneously recording rCBF with a laser Doppler flowmeter. We observed a considerable increase in rCBF during depolarization in CSD induced by application of KCl, accompanied by a transient dip of rCBF with marked vasoconstriction of pial arteries, which resembled the response to pin-prick-induced CSD. Arterial constriction diminished or disappeared during the second and third passages of CSD, whereas the rCBF increase was maintained without a transient dip. Long-lasting oligemia with a decrease in the reciprocal of mean transit time of injected dye and mild constriction of pial arteries was observed after several passages of the CSD wave. These results indicate that CSD-induced rCBF changes consist of initial hyperemia with a transient dip and followed by a long-lasting oligemia, partially corresponding to the diametric changes of pial arteries, and further suggest that vessels other than pial arteries, such as intracortical vessels, are involved.

Spreading depolarizations mediate excitotoxicity in the development of acute cortical lesions

Spreading depolarizations (SD) are mass depolarizations of neurons and astrocytes that occur spontaneously in acute brain injury and mediate time-dependent lesion growth. Glutamate excitotoxicity has also been extensively studied as a mechanism of neuronal injury, although its relevance to in vivo pathology remains unclear. Here we hypothesized that excitotoxicity in acute lesion development occurs only as a consequence of SD. Using glutamate-sensitive microelectrodes, we found that SD induced by KCl in normal rat cortex elicits increases in extracellular glutamate (11.6±1.3μM) that are synchronous with the onset, sustainment, and resolution of the extracellular direct-current shift of SD. Inhibition of glutamate uptake with d,l-threo-β-benzyloxyaspartate (TBOA, 0.5 and 1mM) significantly prolonged the duration of the direct-current shift (148% and 426%, respectively) and the glutamate increase (167% and 374%, respectively) in a dose-dependent manner (P<0.05). These prolonged events produced significant cortical lesions as indicated by Fluoro-Jade staining (P<0.05), while no lesions were observed after SD in control conditions or after cortical injection of 1mM glutamate (extracellular increase: 243±50.8μM) or 0.5mM TBOA (glutamate increase: 8.5±1.6μM) without SD. We then used an embolic focal ischemia model to determine whether glutamate elevations occur independent of SD in the natural evolution of a cortical lesion. In both the ischemic core and penumbra, glutamate increased only in synchrony with anoxic terminal SD (6.1±1.1μM) and transient SDs (11.8±2.4μM), and not otherwise. Delayed terminal SDs were also observed in two animals at 98 and 150min after ischemic onset and induced similar glutamate elevations. Durations of SDs and glutamate increases were significantly correlated in both normal and ischemic animals (P<0.05). These data suggest that pathologically prolonged SDs are a required mechanism of acute cortical lesion development and that glutamate elevations and the mass electrochemical changes of SD and are merely different facets of the same pathophysiologic process.

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

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

Cortical hot spots and labyrinths: why cortical neuromodulation for episodic migraine with aura should be personalized

Stimulation protocols for medical devices should be rationally designed. For episodic migraine with aura we outline model-based design strategies toward preventive and acute therapies using stereotactic cortical neuromodulation. To this end, we regard a localized spreading depression (SD) wave segment as a central element in migraine pathophysiology. To describe nucleation and propagation features of the SD wave segment, we define the new concepts of cortical hot spots and labyrinths, respectively. In particular, we firstly focus exclusively on curvature-induced dynamical properties by studying a generic reaction-diffusion model of SD on the folded cortical surface. This surface is described with increasing level of details, including finally personalized simulations using patient's magnetic resonance imaging (MRI) scanner readings. At this stage, the only relevant factor that can modulate nucleation and propagation paths is the Gaussian curvature, which has the advantage of being rather readily accessible by MRI. We conclude with discussing further anatomical factors, such as areal, laminar, and cellular heterogeneity, that in addition to and in relation to Gaussian curvature determine the generalized concept of cortical hot spots and labyrinths as target structures for neuromodulation. Our numerical simulations suggest that these target structures are like fingerprints, they are individual features of each migraine sufferer. The goal in the future will be to provide individualized neural tissue simulations. These simulations should predict the clinical data and therefore can also serve as a test bed for exploring stereotactic cortical neuromodulation.

Injury and repair in perinatal brain injury: Insights from non-invasive MR perfusion imaging

Injury to the developing brain remains an important complication in critically ill newborns, placing them at risk for future neurodevelopment impairments. Abnormal brain perfusion is often a key mechanism underlying neonatal brain injury. A better understanding of how alternations in brain perfusion can affect normal brain development will permit the development of therapeutic strategies that prevent and/or minimize brain injury and improve the neurodevelopmental outcome of these high-risk newborns. Recently, non-invasive MR perfusion imaging of the brain has been successfully applied to the neonatal brain, which is known to be smaller and have lower brain perfusion compared to older children and adults. This article will present an overview of the potential role of non-invasive perfusion imaging by MRI to study maturation, injury, and repair in perinatal brain injury and demonstrate why this perfusion sequence is an important addition to current neonatal imaging protocols, which already include different sequences to assess the anatomy and metabolism of the neonatal brain.

Aneurysmal subarachnoid hemorrhage: pathobiology, current treatment and future directions

Aneurysmal subarachnoid hemorrhage is the most devastating form of stroke. Many pathological mechanisms ensue after cerebral aneurysm rupture, including hydrocephalus, apoptosis of endothelial cells and neurons, cerebral edema, loss of blood-brain barrier, abnormal cerebral autoregulation, microthrombosis, cortical spreading depolarization and macrovascular vasospasm. Although studied extensively through experimental and clinical trials, current treatment guidelines to prevent delayed cerebral ischemia is limited to oral nimodipine, maintenance of euvolemia, induction of hypertension if ischemic signs occur and endovascular therapy for patients with continued ischemia after induced hypertension. Future investigations will involve agents targeting vasodilation, anticoagulation, inhibition of apoptosis pathways, free radical neutralization, suppression of cortical spreading depolarization and attenuation of inflammation.

Cerebral vasospasm

Cerebral vasospasm causes delayed ischemic neurologic deficits after aneurysmal subarachnoid hemorrhage. This is a well-established clinical entity with significant associated morbidity and mortality. The underlying patholphysiology is highly complex and poorly understood. Large-vessel vasospasm, autoregulatory dysfunction, inflammation, genetic predispositions, microcirculatory failure, and spreading cortical depolarization are aspects of delayed neurologic deterioration that have been described in the literature. This article presents a perspective on cerebral vasospasm, as guided by the literature to date, specifically examining the mechanism, diagnosis, and treatment of cerebral vasospasm.

Anatomy and physiology of the blood-brain barrier

Essential requisite for the preservation of normal brain activity is to maintain a narrow and stable homeostatic control in the neuronal environment of the CNS. Blood flow alterations and altered vessel permeability are considered key determinants in the pathophysiology of brain injuries. We will review the present-day literature on the anatomy, development and physiological mechanisms of the blood-brain barrier, a distinctive and tightly regulated interface between the CNS and the peripheral circulation, playing a crucial role in the maintenance of the strict environment required for normal brain function.

Analyzing critical propagation in a reaction-diffusion-advection model using unstable slow waves

The effect of advection on the propagation and in particular on the critical minimal speed of traveling waves in a reaction-diffusion model is studied. Previous theoretical studies estimated this effect on the velocity of stable fast waves and predicted the existence of a critical advection strength below which propagating waves are not supported anymore. In this paper, an analytical expression for the advection-velocity relation of the unstable slow wave is derived. In addition, the critical advection strength is calculated taking into account the unstable slow wave solution. We also analyze a two-variable reaction-diffusion-advection model numerically in a wide parameter range. Due to the new control parameter (advection) we can find stable wave propagation in the otherwise non-excitable parameter regime, if the advection strength exceeds a critical value. Comparing theoretical predictions to numerical results, we find that they are in good agreement. Theory provides an explanation for the observed behaviour.