Mechanism of spreading depression: a review of recent findings and a hypothesis

Experimental and Clinical Investigation of Cytokines in Migraine: A Narrative Review

The role of neuroinflammation in the pathophysiology of migraines is increasingly being recognized, and cytokines, which are important endogenous substances involved in immune and inflammatory responses, have also received attention. This review examines the current literature on neuroinflammation in the pathogenesis of migraine. Elevated TNF-α, IL-1β, and IL-6 levels have been identified in non-invasive mouse models with cortical spreading depolarization (CSD). Various mouse models to induce migraine attack-like symptoms also demonstrated elevated inflammatory cytokines and findings suggesting differences between episodic and chronic migraines and between males and females. While studies on human blood during migraine attacks have reported no change in TNF-α levels and often inconsistent results for IL-1β and IL-6 levels, serial analysis of cytokines in jugular venous blood during migraine attacks revealed consistently increased IL-1β, IL-6, and TNF-α. In a study on the interictal period, researchers reported higher levels of TNF-α and IL-6 compared to controls and no change regarding IL-1β levels. Saliva-based tests suggest that IL-1β might be useful in discriminating against migraine. Patients with migraine may benefit from a cytokine perspective on the pathogenesis of migraine, as there have been several encouraging reports suggesting new therapeutic avenues.

Immunologic aspects of migraine: A review of literature

Migraine headaches are highly prevalent, affecting 15% of the population. However, despite many studies to determine this disease's mechanism and efficient management, its pathophysiology has not been fully elucidated. There are suggested hypotheses about the possible mediating role of mast cells, immunoglobulin E, histamine, and cytokines in this disease. A higher incidence of this disease in allergic and asthma patients, reported by several studies, indicates the possible role of brain mast cells located around the brain vessels in this disease. The mast cells are more specifically within the dura and can affect the trigeminal nerve and cervical or sphenopalatine ganglion, triggering the secretion of substances that cause migraine. Neuropeptides such as calcitonin gene-related peptide (CGRP), neurokinin-A, neurotensin (NT), pituitary adenylate-cyclase-activating peptide (PACAP), and substance P (SP) trigger mast cells, and in response, they secrete pro-inflammatory and vasodilatory molecules such as interleukin-6 (IL-6) and vascular endothelial growth factor (VEGF) as a selective result of corticotropin-releasing hormone (CRH) secretion. This stress hormone contributes to migraine or intensifies it. Blocking these pathways using immunologic agents such as CGRP antibody, anti-CGRP receptor antibody, and interleukin-1 beta (IL-1β)/interleukin 1 receptor type 1 (IL-1R1) axis-related agents may be promising as potential prophylactic migraine treatments. This review is going to summarize the immunological aspects of migraine.

Hypoxia-related mechanisms inducing acute mountain sickness and migraine

Experimental models of human diseases are vital for pathophysiological and therapeutic research. To investigate the initiation, maintenance, pathophysiology and even termination of a migraine/headache attack these models are urgently needed. Results from different studies promote the profound involvement of hypoxia in migraine and other primary/secondary headaches. The possible mechanisms that drive the induction of headaches through hypoxia are still unknown, but several modes of action, such as increased blood flow, dilation of cerebral arteries, the release of nitroglycerin, calcitonin gene-related peptide and adenosine or increased oxygen extraction are discussed intensively. In studies exposing healthy volunteers and people with a history of migraine to controlled normobaric hypoxia, our research group could demonstrate normobaric hypoxia to be an effective trigger of migraine headaches. Furthermore, a longitudinal measurement of calcitonin gene-related peptide (CGRP), during a hypoxic challenge in migraine patients, revealed increasing CGRP levels with prolonged hypoxic challenge. Since GRP has been linked to migraine and other headache disorders, hypoxia could be regarded as initiator for headaches on a neurotransmitter basis. Furthermore, it has been known for more than 2 decades from studies in vitro and in vivo that hypoxia can induce cortical spreading depression, a phenomenon believed to represent aura. Considering the increased prevalence of migraine in altitude populations and the solid pathophysiological changes on cellular and neurotransmitter level–the role of hypoxia should be investigated in greater detail by the headache community.

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.

Role of Neuroinflammation and Blood-Brain Barrier Permutability on Migraine

Currently, migraine is treated mainly by targeting calcitonin gene-related peptides, although the efficacy of this method is limited and new treatment strategies are desired. Neuroinflammation has been implicated in the pathogenesis of migraine. In patients with migraine, peripheral levels of pro-inflammatory cytokines, such as interleukin-1β (IL-1β) and tumor necrosis factor-α, are known to be increased. Additionally, animal models of headache have demonstrated that immunological responses associated with cytokines are involved in the pathogenesis of migraine. Furthermore, these inflammatory mediators might alter the function of tight junctions in brain vascular endothelial cells in animal models, but not in human patients. Based on clinical findings showing elevated IL-1β, and experimental findings involving IL-1β and both the peripheral trigeminal ganglion and central trigeminal vascular pathways, regulation of the Il-1β/IL-1 receptor type 1 axis might lead to new treatments for migraine. However, the integrity of the blood-brain barrier is not expected to be affected during attacks in patients with migraine.

Experimental and Clinical Evidence of the Effectiveness of Riboflavin on Migraines

Riboflavin, a water-soluble member of the B-vitamin family, plays a vital role in producing energy in mitochondria and reducing inflammation and oxidative stress. Migraine pathogenesis includes neuroinflammation, oxidative stress, and mitochondrial dysfunction. Therefore, riboflavin is increasingly being recognized for its preventive effects on migraines. However, there is no concrete evidence supporting its use because the link between riboflavin and migraines and the underlying mechanisms remains obscure. This review explored the current experimental and clinical evidence of conditions involved in migraine pathogenesis and discussed the role of riboflavin in inhibiting these conditions. Experimental research has demonstrated elevated levels of various oxidative stress markers and pro-inflammatory cytokines in migraines, and riboflavin’s role in reducing these marker levels. Furthermore, clinical research in migraineurs showed increased marker levels and observed riboflavin’s effectiveness in reducing migraines. These findings suggest that inflammation and oxidative stress are associated with migraine pathogenesis, and riboflavin may have neuroprotective effects through its clinically useful anti-inflammatory and anti-oxidative stress properties. Riboflavin’s safety and efficacy suggests its usefulness in migraine prophylaxis; however, insufficient evidence necessitates further study.

Association of cortical spreading depression and seizures in patients with medically intractable epilepsy

OBJECTIVE

Monitoring of the ultra-low frequency potentials, particularly cortical spreading depression (CSD), is excluded in epilepsy monitoring due to technical barriers imposed by the scalp ultra-low frequency electroencephalogram (EEG). As a result, clinical studies of CSD have been limited to invasive EEG. Therefore, the occurrence of CSD and its interaction with epileptiform field potentials (EFP) require investigation in epilepsy monitoring.

METHODS

Using a novel AC/DC-EEG approach, the occurrence of DC potentials in patients with intractable epilepsy presenting different symptoms of aura was investigated during long-term video-EEG monitoring.

RESULTS

Various forms of slow potentials, including simultaneous negative direct current (DC) potentials and prolonged EFP, propagated negative DC potentials, and non-propagated single negative DC potentials were recorded from the scalp of the epileptic patients. The propagated and single negative DC potentials preceded the prolonged EFP with a time lag and seizure appeared at the final shoulder of some instances of the propagated negative DC potentials. The slow potential deflections had a high amplitude and prolonged duration and propagated slowly through the brain. The high-frequency EEG was suppressed in the vicinity of the negative DC potential propagations.

CONCLUSIONS

The study is the first to report the recording of the propagated and single negative DC potentials with EFP at the scalp of patients with intractable epilepsy. The negative DC potentials preceded the prolonged EFP and may trigger seizures. The propagated and single negative DC potentials may be considered as CSD.

SIGNIFICANCE

Recordings of CSD may serve as diagnostic and prognostic monitoring tools in epilepsy.

Effect of Nortriptyline on Spreading Depolarization

Background: Spreading depolarization is associated with the extension of lesion size and complications in some important neurological diseases such as stroke, epilepsy, migraine, and traumatic brain injury. Objectives: This study aimed to reveal some molecular aspects of spreading depolarization and suggesting new therapeutic targets for its control by changing the function of different astrocytic and neuronal ion channels. Methods: The effects of nortriptyline on spreading depolarization in cortical and hippocampal tissues and on the electrophysiological properties of CA1 hippocampal pyramidal neurons were assessed by extra- and intracellular recording, following washing rat brain slices by the drug. Results: Nortriptyline made a significant increase in the amplitude of spreading depolarization in cortical and hippocampal tissues relative to control but did not change the duration significantly in each of the tissues. No significant difference was found in the effects of spreading depolarization on the electrophysiological properties of the CA1 pyramidal neurons between nortriptyline and control groups. Conclusions: The stimulating effect of nortriptyline on spreading depolarization is probably related to the augmentation of extracellular potassium collection in the cortex and hippocampus due to inhibition of astrocytic potassium scavenging function. This change can make more neurons prone to depolarization and increase the overall amplitude of spreading depolarization waves. Further studies should assess the effect of enhancing the clearance function of astrocyte-specific inwardly rectifying potassium channels, Kir4.1, or preventing other factors contributing to spreading depolarization on control of the process.

Preconditioning hippocampal slices with hypothermia promotes rapid tolerance to hypoxic depolarization and swelling: Mediation by erythropoietin

We suggested previously that hippocampal slices were protected from hypoxic depolarization and swelling by preincubating them at room temperature (Kreisman et al., 2000). We postulated that hypothermic preconditioning induced tolerance in our slices, which protected against hypoxic depolarization and swelling. Control hippocampal slices were incubated at 34-35 °C for two hours and the response to 10 min of severe hypoxia was compared to slices which were preconditioned for two hours at room temperature (22-23 °C) prior to warming to 34-35 °C. Recordings of the extracellular DC potential provided an index of tissue depolarization and changes in tissue light transmittance provided an index of swelling. Hypothermic preconditioning significantly reduced hypoxia-induced swelling, particularly in CA3 and the dentate inner blade. Since erythropoietin (EPO) had been shown to mediate hypoxic preconditioning, we tested whether EPO also mediated hypothermic preconditioning in our slices. Recombinant rat EPO (1-10 micromolar) mitigated hypoxia-induced swelling and depolarization in dentate inner blade of unconditioned slices in a dose-dependent manner. We also blocked the protective effects of hypothermic preconditioning on hypoxic depolarization and swelling in the inner blade of the dentate gyrus by administering soluble EPO receptor in the bath and treating slices with wortmannin to block phosphorylation of PI3 kinase, a critical step in the activation of the downstream neuroprotectant, Akt. These results suggest that EPO mediates tolerance to hypoxic depolarization and swelling induced by hypothermic preconditioning. They also emphasize that various preincubation protocols used in experiments with hippocampal slices may differentially affect basal electrophysiological and metabolic properties of those slices.

Cortical Spreading Depolarization (CSD) Recorded from Intact Skin, from Surface of Dura Mater or Cortex: Comparison with Intracortical Recordings in the Neocortex of Adult Rats

In cerebral cortex of anesthetized rats single waves of spreading depolarization (CSD) were elicited by needle prick. CSD-related changes of DC (direct current) potentials were either recorded from the intact skin or together with concomitant changes of potassium concentration with K⁺-selective microelectrodes simultaneously at the surface of the dura mater or of the cortex ([K⁺]_s) and in the extracellular space at a cortical depth of 1200 µm. At the intact skin CSD-related DC-shifts had amplitudes of less than 1 mV and had only in a minority of cases the typical CSD-like shape. In the majority these DC-shifts rose and recovered very slowly and were difficult to identify without further indicators. At dura surface CSD-related DC shifts were significantly smaller and rose and recovered slower than intracortically recorded CSD. Concomitant increases in [K⁺]_s were delayed and reached maximal values of about 5 mM from a baseline of 3 mM. They rose and recovered slower than simultaneously recorded intracortical changes in extracellular potassium concentration ([K⁺]_e) that were up to 65 mM. The results suggest that extracellular potassium during CSD is diffusing through the subarachnoid space and across the dura mater. In a few cases CSD was either absent at the dura or at a depth of 1200 µm. Even full blown CSDs in this cortical depth could remain without concomitant deflections at the dura. Our data confirmed in principle the possibility of non-invasive recordings of CSD-related DC-shifts. For a use in clinical routine sensitivity and specificity will have to be improved.

Down-regulation of Inwardly Rectifying K+ Currents in Astrocytes Derived from Patients with Monge's Disease

Chronic mountain sickness (CMS) or Monge's disease is a disease in highlanders. These patients have a variety of neurologic symptoms such as migraine, mental fatigue, confusion, dizziness, loss of appetite, memory loss and neuronal degeneration. The cellular and molecular mechanisms underlying CMS neuropathology is not understood. In the previous study, we demonstrated that neurons derived from CMS patients' fibroblasts have a decreased expression and altered gating properties of voltage-gated sodium channel. In this study, we further characterize the electrophysiological properties of iPSC-derived astrocytes from CMS patients. We found that the current densities of the inwardly rectifying potassium (Kir) channels in CMS astrocytes (-5.7 ± 2.2 pA/pF at -140 mV) were significantly decreased as compared to non-CMS (-28.4 ± 3.4 pA/pF at -140 mV) and sea level subjects (-28.3 ± 5.3 pA/pF at -140 mV). We further demonstrated that the reduced Kir current densities in CMS astrocytes were caused by their decreased protein expression of Kir4.1 and Kir2.3 channels, while single channel properties (i.e., P_o, conductance) of Kir channel in CMS astrocytes were not altered. In addition, we found no significant differences of outward potassium currents between CMS and non-CMS astrocytes. As compared to non-CMS and sea level subjects, the K⁺ uptake ability in CMS astrocytes was significantly decreased. Taken together, our results suggest that down-regulation of Kir channels and the resulting decreased K⁺ uptake ability in astrocytes could be one of the major molecular mechanisms underlying the neurologic manifestations in CMS patients.

Suckling in litters with different sizes, and early and late swimming exercise differentially modulates anxiety-like behavior, memory and electrocorticogram potentiation after spreading depression in rats

OBJECTIVE

Analyze the hypothesis that swimming exercise, in rats suckled under distinct litter sizes, alters behavioral parameters suggestive of anxiety and recognition memory, and the electrocorticogram potentiation that occurs after the excitability-related phenomenon that is known as cortical spreading depression (CSD).

METHODS

Male Wistar rats were suckled in litters with six or 12 pups (L₆ and L₁₂ groups). Animals swam at postnatal days (P) 8-23, or P60-P75 (early-exercised or late-exercised groups, respectively), or remained no-exercised. Behavioral tests (open field - OF and object recognition - OR) were conducted between P77 and P80. Between P90 and P120, ECoG was recorded for 2 hours. After this 'baseline' recording, CSD was elicited every 30 minutes over the course of 2 hours.

RESULTS

Early swimming enhanced the number of entries and the percentage of time in the OF-center (P < 0.05). In animals that swam later, this effect occurred in the L6 group only. Compared to the corresponding sedentary groups, OR-test showed a better memory in the L6 early exercised rats, and a worse memory in all other groups (P < 0.05). In comparison to baseline values, ECoG amplitudes after CSD increased 14-43% for all groups (P < 0.05). In the L₆ condition, early swimming and late swimming, respectively, reduced and enhanced the magnitude of the post-CSD ECoG potentiation in comparison with the corresponding L₆ no-exercised groups (P < 0.05).

DISCUSSION

Our data suggest a differential effect of early- and late-exercise on the behavioral and electrophysiological parameters, suggesting an interaction between the age of exercise and the nutritional status during lactation.

Migraine with brainstem aura: Why not a cortical origin?

Background Migraine with brainstem aura is defined as a migraine with aura including at least two of the following symptoms: dysarthria, vertigo, tinnitus, hypacusis, diplopia, ataxia and/or decreased level of consciousness. Aim The aim of this study is to review data coming from clinical observations and functional mapping that support the role of the cerebral cortex in the initiation of brainstem aura symptoms. Results Vertigo can result from a vestibular cortex dysfunction, while tinnitus and hypacusis can originate within the auditory cortex. Diplopia can reflect a parieto-occipital involvement. Dysarthria can be caused by dysfunctions located in precentral gyri. Ataxia can reflect abnormal processing of vestibular, sensory, or visual inputs by the parietal lobe. Alteration of consciousness can be caused by abnormal neural activation within specific consciousness networks that include prefrontal and posterior parietal cortices. Conclusion Any symptom of so-called brainstem aura can originate within the cortex. Based on these data, we suggest that brainstem aura could have a cortical origin. This hypothesis would explain the co-occurrence of typical and brainstem aura during attacks and would fit with the theory of cortical spreading depression. We propose that migraine with brainstem aura should be classified as a typical migraine aura.

Spreading depolarization triggered by elevated potassium is weak or absent in the rodent lower brain

We examined in live coronal slices from rat and mouse which brain regions generate potassium-triggered spreading depolarization (SD_Kt). This technique simulates cortical spreading depression, which underlies migraine aura in the intact brain. An SD_Kt episode was evoked by increasing bath [K⁺]_o and recorded as a propagating front of elevated light transmittance representing transient neuronal swelling in gray matter of neocortex, hippocampus, striatum, and thalamus. In contrast, SD_Kt was not imaged in hypothalamic nuclei or brainstem with exception of those nuclei near the dorsal brainstem surface. In rat slices, single neurons were whole-cell current clamped during SD_Kt. "Higher" neurons depolarized to near zero millivolts indicating SD_Kt generation. In contrast, seven types of neurons in hypothalamus and brainstem only slowly depolarized without generating SD_Kt, supporting our imaging findings. Therefore, SD_Kt is not a default of CNS neurons but rather displays a region-specific susceptibility, similar to anoxic depolarization, which we have proposed is correlated with a region's vulnerability to traumatic brain injury. In the higher brain, SD_Kt may be a vestigial spreading depolarization that originally evolved to shut down and vasoconstrict gray matter regions more exposed to impact and contusion.

Laryngospasm, central and obstructive apnea during seizures: Defining pathophysiology for sudden death in a rat model

Seizure spread into the autonomic nervous system can result in life-threatening cardiovascular and respiratory dysfunction. Here we report on a less-studied consequence of such autonomic derangements-the possibility of laryngospasm and upper-airway occlusion. We used parenteral kainic acid to induce recurring seizures in urethane-anesthetized Sprague Dawley rats. EEG recordings and combinations of cardiopulmonary monitoring, including video laryngoscopy, were performed during multi-unit recordings of recurrent laryngeal nerve (RLN) activity or head-out plethysmography with or without endotracheal intubation. Controlled occlusions of a tracheal tube were used to study the kinetics of cardiac and respiratory changes after sudden obstruction. Seizure activity caused significant firing increases in the RLN that were associated with abnormal, high-frequency movements of the vocal folds. Partial airway obstruction from laryngospasm was evident in plethysmograms and was prevented by intubation. Complete glottic closure (confirmed by laryngoscopy) occurred in a subset of non-intubated animals in association with the largest increases in RLN activity, and cessation of airflow was followed in all obstructed animals within tens of seconds by ST-segment elevation, bradycardia, and death. Periods of central apnea occurred in both intubated and non-intubated rats during seizures for periods up to 33s and were associated with modestly increased RLN activity, minimal cardiac derangements, and an open airway on laryngoscopy. In controlled complete airway occlusions, respiratory effort to inspire progressively increased, then ceased, usually in less than 1min. Respiratory arrest was associated with left ventricular dilatation and eventual asystole, an elevation of systemic blood pressure, and complete glottic closure. Severe laryngospasm contributed to the seizure- and hypoxemia-induced conditions that resulted in sudden death in our rat model, and we suggest that this mechanism could contribute to sudden death in epilepsy.

Hypoglycemia-induced alterations in hippocampal intrinsic rhythms: Decreased inhibition, increased excitation, seizures and spreading depression

Seizures are the most common clinical presentation of severe hypoglycemia, usually as a side effect of insulin treatment for juvenile onset type 1 diabetes mellitus and advanced type 2 diabetes. We used the mouse thick hippocampal slice preparation to study the pathophysiology of hypoglycemia-induced seizures and the effects of severe glucose depletion on the isolated hippocampal rhythms from the CA3 circuitry.

METHODS AND RESULTS

Dropping the glucose perfusate concentration from the standard 10 mM to 1 mM produced epileptiform activity in 14/16 of the slices. Seizure-like events (SLEs) originated in the CA3 region and then spread into the CA1 region. Following the SLE, a spreading-depression (SD)-like event occurred (12/16 slices) with irreversible synaptic failure in the CA1 region (8/12 slices). CA3 SD-like events followed ~30 s after the SD-like event in the CA1 region. Less commonly, SD-like events originated in the CA3 region (4/12). Additionally, prior to the onset of the SLE in the CA3 area, there was decreased GABA correlated baseline SPW activity (bSPW), while there was increased large-amplitude sharp wave (LASW) activity, thought to originate from synchronous pyramidal cell firing. CA3 pyramidal cells displayed progressive tonic depolarization prior to the seizure which was resistant to synaptic transmission blockade. The initiation of hypoglycemic seizures and SD was prevented by AMPA/kainate or NMDA receptor blockade.

CONCLUSIONS

Severe glucose depletion induces rapid changes initiated in the intrinsic CA3 rhythms of the hippocampus including depressed inhibition and enhanced excitation, which may underlie the mechanisms of seizure generation and delayed spreading depression.

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.

Brain and Psyche in Early Christian Asceticism

This study is an 11-part investigation of the psychology and neuropsychology of early Christian asceticism as represented by Evagrius Ponticus (AD 345–399), the tradition's first ascetical theologian and possibly its most influential. Evagrius's biography is reviewed in the first section. The living circumstances and perceptual consequences of desert asceticism are considered in the second. Penitence, dispassion, and the mysticism of “pure prayer” are discussed in the third. Austerities are addressed in the fourth section, particularly fasting, prostrations, and prolonged standing. Ascetical perspectives on sleep, dreams, and the hypnogogic state are analyzed in the fifth. The depressive syndrome of acedia is discussed in the sixth. Evagrius's reports of auditory, olfactory, and visual hallucinations are analyzed in the seventh. Multiple complementary interpretations of demonic phenomena are developed in the eighth section. Evagrius's psychotherapy for anger is reviewed in the ninth. Interpersonal relations among ascetics are considered in the tenth section. The study concludes with a summary.

Potentiation of spontaneous and evoked cortical electrical activity after spreading depression: in vivo analysis in well-nourished and malnourished rats

Cortical spreading depression (CSD) is influenced by brain excitability and is related to neurological diseases, such as epilepsy. In vitro evidence indicates that neuronal electrical activity is potentiated after CSD. Malnutrition can cause electrophysiological changes in the brain, both in animals and in humans. Here, we investigated in vivo whether CSD potentiates the amplitude of electrocorticogram (ECoG) and of transcallosal evoked responses in adult well-nourished (W), early-malnourished (M), and food-restricted rats. ECoG amplitudes were compared before and after CSD, at two parietal regions (designated the anterior and posterior regions). In the anterior region, post-CSD amplitudes of the ECoG waves were 13-23% higher (P < 0.05) than the pre-CSD values in all groups. In the posterior region, amplitudes increased 22% in the M group only (P < 0.05). In a fourth CSD-free group, ECoG amplitude did not change during the four recording hours. Transcallosal electrically evoked cortical responses also increased 21.5 ± 9.6% and 41.8 ± 28.5%, after CSD, in the W and M conditions, respectively, as compared to pre-CSD values. The data support the hypothesis of an in vivo CSD potentiation on cortical excitability as recorded by spontaneous and evoked electrical activity and modulation by nutritional status.

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

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

Altered responses of MeCP2-deficient mouse brain stem to severe hypoxia

Rett syndrome (RTT) patients suffer from respiratory arrhythmias with frequent apneas causing intermittent hypoxia. In a RTT mouse model (methyl-CpG-binding protein 2-deficient mice; Mecp2−/ y) we recently discovered an enhanced hippocampal susceptibility to hypoxia and hypoxia-induced spreading depression (HSD). In the present study we investigated whether this also applies to infant Mecp2−/ y brain stem, which could become life-threatening due to failure of cardiorespiratory control. HSD most reliably occurred in the nucleus of the solitary tract (NTS) and the spinal trigeminal nucleus (Sp5). HSD susceptibility of the Mecp2−/ y NTS and Sp5 was increased on 8 mM K+-mediated conditioning. 5-HT1A receptor stimulation with 8-hydroxy-2-(di-propylamino)tetralin (8-OH-DPAT) postponed HSD by up to 40%, mediating genotype-independent protection. The deleterious impact of HSD on in vitro respiration became obvious in rhythmically active slices, where HSD propagation into the pre-Bötzinger complex (pre-BötC) immediately arrested the respiratory rhythm. Compared with wild-type, the Mecp2−/ y pre-BötC was invaded less frequently by HSD, but if so, HSD occurred earlier. On reoxygenation, in vitro rhythms reappeared with increased frequency, which was less pronounced in Mecp2−/ y slices. 8-OH-DPAT increased respiratory frequency but failed to postpone HSD in the pre-BötC. Repetitive hypoxia facilitated posthypoxic recovery only if HSD occurred. In 57% of Mecp2−/ y slices, however, HSD spared the pre-BötC. Although this occasionally promoted residual hypoxic respiratory activity (“gasping”), it also prolonged the posthypoxic recovery, and thus the absence of central inspiratory drive, which in vivo would lengthen respiratory arrest. In view of the breathing disorders in RTTs, the increased hypoxia susceptibility of MeCP2-deficient brain stem potentially contributes to life-threatening disturbances of cardiorespiratory control.

Potassium-selective microelectrode revealed difference in threshold potassium concentration for cortical spreading depression in female and male rat brain

It has been found with potassium-selective microelectrodes that the threshold extracellular concentration of potassium [K(+)](e) for eliciting fast spreading depression from the site of administration 1.5mm away is 80% lower in the rat female cortex (8.0 ± 0.6 mM) than in the male cortex (14.4 ± 0.4mM). The rate of the first slow phase of [K(+)](e) rise before reaching the threshold and speed of SD propagation were similar for both sexes as well as the rise of the second active fast phase of the [K(+)](e) increase. Lower [K(+)](e) threshold in females can facilitate the rapid K(+) release from the nerve tissue cells that may cause a cerebrovascular vasodilatation and the attack of migraine pain.

Migraine aura pathophysiology: the role of blood vessels and microembolisation

Migraine attacks with auras are sometimes associated with underlying hereditary or acquired cerebrovascular disorders. A unifying pathophysiological explanation linking migraine to these conditions has been difficult to identify. On the basis of genetic and epidemiological evidence, we suggest that changes in blood vessels, hypoperfusion disorders, and microembolisation can cause neurovascular dysfunction and evoke cortical spreading depression, an event that is widely thought to underlie aura symptoms. In fact, recent experimental data have indicated that focal, mild, and transient ischaemia can trigger cortical spreading depression without an enduring tissue signature. Although migraine with aura has many causes (eg, neuronal network excitability), it seems that migraine and stroke might both be triggered by hypoperfusion and could therefore exist on a continuum of vascular complications in a subset of patients who have these hereditary or acquired comorbid vascular conditions.

The relationship between sudden severe hypoxia and ischemia-associated spreading depolarization in adult rat brainstem in vivo

Severe ischemia can induce spreading depolarization (SD) in the cerebral cortex, which is thought to contribute significantly to cerebral dysfunction. Whether the mature brainstem shows SD upon reduced oxygen supply has not been investigated although SDs may significantly influence brainstem functions. In anesthetized adult rats, we induced severe short-lasting hypoxia (SSH) by stopping artificial respiration for about 1 min or by ventilation with pure nitrogen for 1, 2 or 3 min, and milder hypoxia by ventilation with 6% O(2) in N(2) for 10 min. We measured DC potentials in the brainstem and cerebral cortex, systemic arterial blood pressure, heart rate and local blood flow at the brainstem or cerebral cortex surface. SSH lasting up to 1 min did not induce DC shifts in native brainstem but reduced heart rate, systemic blood pressure and blood flow in cortex and brainstem. Longer lasting SSH protocols both reduced systemic blood pressure and induced SD in the brainstem, but the magnitude of the cardiovascular response was not influenced by the simultaneous occurrence of SD. When neuronal excitability in the brainstem was artificially enhanced, SSH of 1 min evoked SD but again the magnitude of cardiovascular changes during SSH was not increased. SSH lasting 3 min evoked non-reversible sustained depolarization. SSH did not render the brainstem more excitable for classical SD evoked by local KCl application. Thus, sudden severe hypoxia/ischemia evokes SDs in the brainstem, but the occurrence of the so-elicited SD does not influence the immediate cardiovascular response to SSH.

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

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

The role of extracellular potassium dynamics in the different stages of ictal bursting and spreading depression: a computational study

Experimental evidences point out the participation of nonsynaptic mechanisms (e.g., fluctuations in extracellular ions) in epileptiform bursting and spreading depression (SD). During these abnormal oscillatory patterns, it is observed an increase of extracellular potassium concentration [K(+)](o) and a decrease of extracellular calcium concentration [Ca(2+)](o) which raises the neuronal excitability. However, whether the high [K(+)](o) triggers and propagates these abnormal neuronal activities or plays a secondary role into this process is unclear. To better understand the influence of extracellular potassium dynamics in these oscillatory patterns, the experimental conditions of high [K(+)](o) and zero [Ca(2+)](o) were replicated in an extended Golomb model where we added important regulatory mechanisms of ion concentration as Na(+)-K(+) pump, ion diffusion and glial buffering. Within these conditions, simulations of the cell model exhibit seizure-like discharges (ictal bursting). The SD was elicited by the interruption of the Na(+)-K(+) pump activity, mimicking the effect of cellular hypoxia (an experimental protocol to elicit SD, the hypoxia-induced SD). We used the bifurcation theory and the fast-slow method to analyze the interference of K(+) dynamics in the cellular excitability. This analysis indicates that the system loses its stability at a high [K(+)](o), transiting to an elevated state of neuronal excitability. Effects of high [K(+)](o) are observed in different stages of ictal bursting and SD. In the initial stage, the increase of [K(+)](o) creates favorable conditions to trigger both oscillatory patterns. During the neuronal activity, a continuous growth of [K(+)](o) by outward K(+) flow depresses K(+) currents in a positive feedback way. At the last stage, due to the depression of K(+) currents, the Na(+)-K(+) pump is the main mechanism in the end of neuronal activity. Thus, this work suggests that [K(+)](o) dynamics may play a fundamental role in these abnormal oscillatory patterns.

[Cortical spreading depression (CSD): a neurophysiological correlate of migraine aura]

Cortical spreading depression (CSD) is a transient (60-120 s) and at 3-5 mm/min propagating depolarization wave of cortical neurons and glial cells and is characterized by a DC shift of 20-35 mV. It is accompanied by massive redistribution of ions between extracellular and intracellular compartments and by a water influx into the cells. Extracellular potassium ion concentration increases up to 60 mM/l. Potassium ions and the excitatory neurotransmitter glutamate essentially contribute to the initiation and propagation of CSD. Both depolarization and disturbance of brain ion homeostasis regenerate within a few minutes while enhancing energy metabolism, but do not cause damage to normally perfused brain tissue. The similar propagation velocity of CSD and visual scotoma during migraine aura led to the assumption that CSD could be the underlying mechanism of migraine aura. The observation of CSD waves in migraine aura patients with the magnet encephalogram (MEG) technique confirmed this theory. Although many data support the relationship between CSD and aura phase in migraine, the role of CSD in migraine headache is still disputed.

Spreading depression in the brainstem of the adult rat: electrophysiological parameters and influences on regional brainstem blood flow

Cortical spreading depression is a pathophysiological excitation wave that occurs during pathophysiological brain conditions such as ischemic brain infarction, migraine aura, and others. Judged from experiments in rodents, the brainstem is thought to be comparatively resistant to the generation of spreading depression. However, because spreading depression can be elicited in the brainstem of rat pups after superfusing the brainstem with solutions enhancing excitability, we reinvestigated spreading depression in the brainstem of the adult rat. Based on theoretical predictions indicating a major role of extracellular potassium in susceptibility to spreading depression, we used conditioning solutions in which chloride ions were replaced by acetate and tetraethylammonium chloride and a small amount of KCl were added. Under these conditions, spreading depression was reproducibly elicited in the brainstem either by topical application of KCl crystals to the brainstem surface or by local microinjection of KCl into the brainstem. The direct current shifts so elicited were accompanied by typical elevation of extracellular potassium ions, propagated in the brainstem, and were prevented by MK-801, an N-methyl D-aspartate blocker. During spreading depression, the regional blood flow in the brainstem was transiently increased. In addition, systemic arterial blood pressure, but not the heart rate, was transiently enhanced. In the nonconditioned brainstem, KCl stimulation neither elicited spreading depression nor induced changes in regional blood flow and blood pressure. These data show that proper conditioning renders the brainstem susceptible to spreading depression, and that spreading depression at this site elicits changes in local circulation and systemic blood pressure.

Whole isolated neocortical and hippocampal preparations and their use in imaging studies

This study shows that two whole isolated preparations from the young mouse, the neocortical 'slab' and the hippocampal formation, are useful for imaging studies requiring both global monitoring using light transmittance (LT) imaging and high resolution cellular monitoring using 2-photon laser scanning microscopy (2PLSM). These preparations share advantages with brain slices such as maintaining intrinsic neuronal properties and avoiding cardiac or respiratory movement. Important additional advantages include the maintenance of all local input and output pathways, the absence of surfaces injured by slicing and the preservation of three-dimensional tissue structure. Using evoked extracellular field recording, we demonstrate long-term (hours) viability of both whole preparations. We then show that propagating cortical events such as anoxic depolarization (AD) and spreading depression (SD) can be imaged in both preparations, yielding results comparable to those in brain slices but retaining the tissue's three-dimensional structure. Using transgenic mice expressing green fluorescent protein (GFP) in pyramidal and granule cell neurons, 2PLSM confirms that these preparations are free of the surface damage observed in sliced brain tissue. Moreover the neurons undergo swelling with accompanying dendritic beading following AD induced by simulated ischemia, similar to cortical damage described in vivo.

Repeated experiences of air hunger and ventilatory behavior in response to hypercapnia in the standardized rebreathing test: effects of anxiety

In this study, we assessed air hunger (AH) and ventilatory responses to repeated CO(2) exposures in healthy women (N=31), scoring high or low for trait anxiety. A standardized rebreathing test, implying a gradually increasing CO(2) stimulus, was administered three times with 15-min intervals. Respiratory behavior and the intensity of AH perception were measured continuously. Across repeated exposures, maximal tolerance for AH habituated and the slope of AH (increase in AH per unit increase in CO(2)) diminished. Also the dynamics of the breathing response changed across trials. The thresholds for AH and tidal volume (V(T)) moved closer to each other, whereas the threshold for the respiratory rate (RR) was generally postponed. In addition, the association between AH and V(T) was stronger than between AH and RR, and the latter association became weaker over trials, particularly in high anxious persons. This suggests that AH perception became increasingly influenced by psychological factors, especially in high anxious persons. The results suggest that habituation of perceived air hunger is depending on a complex interplay between both changes in respiratory behavior and in perceptual-cognitive processes related to trait anxiety.

Optical and pharmacological tools to investigate the role of mitochondria during oxidative stress and neurodegeneration

Mitochondria are critical for cellular adenosine triphosphate (ATP) production; however, recent studies suggest that these organelles fulfill a much broader range of tasks. For example, they are involved in the regulation of cytosolic Ca(2+) levels, intracellular pH and apoptosis, and are the major source of reactive oxygen species (ROS). Various reactive molecules that originate from mitochondria, such as ROS, are critical in pathological events, such as ischemia, as well as in physiological events such as long-term potentiation, neuronal-vascular coupling and neuronal-glial interactions. Due to their key roles in the regulation of several cellular functions, the dysfunction of mitochondria may be critical in various brain disorders. There has been increasing interest in the development of tools that modulate mitochondrial function, and the refinement of techniques that allow for real time monitoring of mitochondria, particularly within their intact cellular environment. Innovative imaging techniques are especially powerful since they allow for mitochondrial visualization at high resolution, tracking of mitochondrial structures and optical real time monitoring of parameters of mitochondrial function. The techniques discussed include classic imaging techniques, such as rhodamine-123, the highly advanced semi-conductor nanoparticles (quantum dots), and wide field microscopy as well as high-resolution multiphoton imaging. We have highlighted the use of these techniques to study mitochondrial function in brain tissue and have included studies from our laboratories in which these techniques have been successfully applied.

Suppression of EEG gamma activity — an informative measure of spreading depression waves in the neocortex of the conscious rabbit

Spreading depression can arise spontaneously in convulsions, migraine attacks, vascular lesions, and other pathological brain states. However, the methodological complexity of recording a direct current potential in the neocortex in humans and conscious animals significantly limits studies of the functional consequences of spreading depression. The present report describes a detailed analysis of the EEG dynamics at the moment of development of spreading depression waves and identifies specific signs allowing spreading depression to be recorded without recording changes in the direct current potential. At the moment of arrival of spreading depression, the interhemisphere equilibrium is highly disturbed because of large decreases in high-frequency activity. The time parameters of the dynamics of the gamma-1 and gamma-2 ranges were the most informative features of the development of spreading depression waves. Increases in power in the delta range developed with some delay on the background of a deep inhibition of high-frequency activity and are in essence a consequence of spreading depression. The encephalographic criteria of spreading depression waves identified here can significantly simplify the detection of this phenomenon both in experimental and clinical conditions in a number of pathological brain states.

Unilateral cortical spreading depression is an early marker of audiogenic kindling in awake rats

Spreading depression (SD), a self-propagating wave of reversible cellular depolarization, is thought to play an important role in brain pathophysiology. SD and seizures are closely related events but little is known about involvement of SD in chronic epileptogenesis. Here we show that cortical SD is the first and highly reproducible manifestation of audiogenic kindling induced by repeated sound stimulation of WAG/Rij rats with genetic audiogenic and absence epilepsy. Repetition of sound-induced running seizures in freely moving rats led to an appearance and gradual intensification of post-running facial and forelimb clonic convulsions coupled with afterdischarge in the fronto-parietal cortex. Before the development of these traditional manifestations of audiogenic kindling, an unilateral cortical SD wave began to be triggered by audiogenic seizures. Once cortical SD appeared, it became a permanent component of subsequent seizures. SD was always recorded in the hemisphere ipsilateral to the running direction. Only at the late stages of audiogenic kindling SD developed bilaterally. To estimate the contribution of SD in postictal effects of audiogenic seizures, we compared cortical activity after seizures induced SD or not. It was found that only seizures with cortical SD were followed by postictal suppression of spontaneous spike-wave discharges displayed by WAG/Rij rats. The results show that (1) cortical SD is readily triggered by brief sensory-induced seizures in awake animals; (2) SD may be responsible for postictal changes in cortical activity; (3) unilateral initiation of SD suggests asymmetrical recruitment of the cortex into seizure network during audiogenic kindling.

Transhemispheric depolarizations persist in the intracerebral hemorrhage swine brain following corpus callosal transection

Spontaneous episodes of spreading depression (SD) originating in multiple sources adjacent to a focal intracerebral hemorrhage (ICH) propagate into brain regions away from the lesion site soon after injury onset. Although these transient depolarizations have not been established in the opposite hemisphere of the swine ICH model, we have reported a diminishing of sensory responsiveness in this homotopic brain region following induction of a unilateral hemorrhage lesion. This study examined whether transient depolarizations exist in this distant brain region contralateral to the ICH site. Electrocorticographic (ECoG) recordings of brain activity were collected bilaterally from the primary somatosensory (SI) cortices of the swine brain prior to and immediately after an intracerebral injection of collagenase or saline or the insertion of the infusion pipette into the SI cortex of the right hemisphere. Transient depolarizations were present in both hemispheres of all the experimental groups. The earliest negative DC potential shifts were observed in the injured SI cortex within the first hour after collagenase injection, as compared to T = 3 h in the saline-injected group and T = 4 h in the infusion pipette only group. In contrast, transient depolarizations were first detected in the left SI cortex contralateral to the lesioned hemisphere within 2 h after collagenase infusion, by T = 4 h after saline infusion and by T = 3 h in the pipette only group. Propagating waves of negative DC potential shifts continued in both brain hemispheres, particularly in the ICH group, throughout the 11-h recording period. This novel finding of recurrent depolarizing waves in the hemisphere contralateral to the injury site prompted us to examine whether corpus callosal connections may play a role in this transhemispheric phenomenon. In a separate group of animals, the corpus callosum was transected prior to acquiring DC potential recordings and collagenase injection. The onset pattern of negative DC shifts in the callosotomized + collagenase-injected group was similar to the collagenase group with an intact corpus callosum. Initial generation of SD in the callosotomized + collagenase-injected group occurred by T = 1 h in the ICH injured right hemisphere and T = 2 h in the contralateral hemisphere. These transient depolarizations also persisted throughout 11-h recording period indicating that the corpus callosal transection did not hinder these remote propagating waves of depolarization. The presence of SD in the SI cortices of both hemispheres in all experimental groups of this study suggests that a focal mechanical or hemorrhagic injury increases the susceptibility of distant ipsilateral and contralateral brain regions to depolarizing perturbations. The mechanism for these transient depolarizations in the contralateral hemisphere apparently does not involve transhemispheric propagation along corpus callosal fibers.

Spreading depression expands traumatic injury in neocortical brain slices

Traumatic brain injury (TBI) is particularly common in young people, generating healthcare costs that can span decades. The cellular processes activated in the first minutes following injury are poorly understood, and the 3-4 h following trauma are crucial for reducing subsequent injury. Spreading depression (SD) is a profound inactivation of neurons and glia lasting 1-2 min that arises focally and migrates outward across gray matter. In the hours following focal stroke, the metabolic stress of energy reduction and recurring SD-like events (peri-infarct depolarizations, PIDs) interact to promote neuronal injury. Similar recurring depolarizations might evolve immediately following TBI and exacerbate neuronal damage peripheral to the impact site. To test this possibility and examine if certain drugs might limit damage by inhibiting what we term traumatic spreading depression (tSD), we developed a technique whereby a small weight was dropped onto a live slice of rat neocortex while imaging changes in light transmittance (LT). Imaging revealed a propagating front of increased LT arising at the border of the impact site. Traumatic SD significantly expanded the region of ensuing damage. Both tSD and subsequent damage were blocked by the NMDA receptor antagonist MK-801 (100 microM) or the sigma-1 receptor (sigma1R) ligands dextromethorphan (30 microM) or BD-1063 (100 microM). Co-application of the sigma1R antagonist (+)3-PPP with DM reversed the block as did lowering temperature from 35 degrees C to 32 degrees C. This study provides evidence that an event similar to peri-infarct depolarization can arise from an injury site in neocortex within seconds following impact and act to expand the region of acute neuronal damage.

Inhomogeneous propagation of cortical spreading depression—detection by electro- and magnetoencephalography in rats

Spreading depression (SD) propagates in cortical regions that are different in their morphological and functional characteristics. We tested whether the propagation pattern of spreading depression was different between parts of the cortex. In six adult rats, we recorded the ECoG by a 4 x 4 electrode array that covered parts of the frontal, parietal cortex and the cingulate cortex. Simultaneously a 16-channel magnetoencephalogram was recorded to characterize the development and direction of intracortical ion movements accompanying this phenomenon. Spreading depression was initiated by occipital application of 0.3 molar KCl solution. Depolarization was observed, at first, at lateral cortical regions and then at medial cortical regions. Thereafter, the propagation velocity increased in medial cortical regions and was faster than in lateral regions. Negative potential shifts were detected by all electrodes, but the depolarization reached a maximum over lateral and caudal cortical regions. The recorded magnetic fields indicated the same orientation of currents underlying these fields, which was perpendicular to the wave front and points away from the depolarization region. Overall, the data indicated that propagation patterns of spreading depression differed between parts of the cortex and, thus, propagation was inhomogeneous. This propagation was accompanied by strong currents parallel to the cortical surface.

Decrease in synaptic transmission can reverse the propagation direction of epileptiform activity in hippocampus in vivo

Most types of epileptiform activity with synaptic transmission have been shown to propagate from the CA3 to CA1 region in hippocampus. However, nonsynaptic epileptiform activity induced in vitro is known to propagate slowly from the caudal end of CA1 toward CA2/CA3. Understanding the propagation modes of epileptiform activity, and their causality is important to revealing the underlying mechanisms of epilepsy and developing new treatments. In this paper, the effect of the synaptic transmission suppression on the propagation of epilepsy in vivo was investigated by using multiple-channel recording probes in CA1. Nonsynaptic epileptiform activity was induced by calcium chelator EGTA with varied concentrations of potassium. For comparison, disinhibition synaptic epileptiform activity was induced by picrotoxin (PTX) with or without partial suppression of excitatory synaptic transmission. The propagation velocity was calculated by measuring the time delay between two electrodes separated by a known distance. The results show that in vivo nonsynaptic epileptiform activity propagates with a direction and velocity comparable to those observed in in vitro preparations. The direction of propagation for nonsynaptic activity is reversed from the PTX-induced synaptic activity. A reversal in propagation direction and change in velocity were also observed dynamically during the process of synaptic transmission suppression. Even a partial suppression of synaptic transmission was sufficient to significantly change the propagation direction and velocity of epileptiform activity. These results suggest the possibility that the measurement of propagation can provide important information about the synaptic mechanism underlying epileptic activity.

Blocking the anoxic depolarization protects without functional compromise following simulated stroke in cortical brain slices

Within 2 min of stroke onset, neurons and glia in brain regions most deprived of blood (the ischemic core) undergo a sudden and profound loss of membrane potential caused by failure of the Na+/K+ ATPase pump. This anoxic depolarization (AD) represents a collapse in membrane ion selectivity that causes acute neuronal injury because neurons simply cannot survive the energy demands of repolarization while deprived of oxygen and glucose. In vivo and in live brain slices, the AD resists blockade by antagonists of neurotransmitter receptors (including glutamate) or by ion channel blockers. Our neuroprotective strategy is to identify AD blockers that minimally affect neuronal function. If the conductance underlying AD is not normally active, its selective blockade should not alter neuronal excitability. Imaging changes in light transmittance in live neocortical and hippocampal slices reveal AD onset, propagation, and subsequent dendritic damage. Here we identify several sigma-1 receptor ligands that block the AD in slices that are pretreated with 10-30 microM of ligand. Blockade prevents subsequent cell swelling, dendritic damage, and loss of evoked field potentials recorded in layers II/III of neocortex and in the CA1 region of hippocampus. Even when AD onset is merely delayed, electrophysiological recovery is markedly improved. With ligand treatment, evoked axonal conduction and synaptic transmission remain intact. The large nonselective conductance that drives AD is still unidentified but represents a prime upstream target for suppressing acute neuronal damage arising during the first critical minutes of stroke. Sigma receptor ligands provide insight to better define the properties of the channel responsible for anoxic depolarization. Video clips of anoxic depolarization and spreading depression can be viewed at http://anatomy.queensu.ca/faculty/andrew.cfm.

Changes in intracellular chloride after oxygen-glucose deprivation of the adult hippocampal slice: effect of diazepam

Ischemic injury to the CNS results in loss of ionic homeostasis and the development of neuronal death. An increase in intracellular Ca2+ is well established, but there are few studies of changes in intracellular Cl- ([Cl-]i) after ischemia. We used an in vitro model of cerebral ischemia (oxygen-glucose deprivation) to examine changes in [Cl-]i and GABA(A) receptor-mediated responses in hippocampal slices from adult rats. Changes in [Cl-]i were measured in area CA1 pyramidal neurons using optical imaging of 6-methoxy-N-ethylquinolinium chloride, a Cl--sensitive fluorescent indicator. Oxygen-glucose deprivation induced an immediate rise in [Cl-]i, which recovered within 20 min. A second and more prolonged rise in [Cl-]i occurred within the next hour, during which postsynaptic field potentials failed to recover. The sustained increase in [Cl-]i was not blocked by GABA(A) receptor antagonists. However, oxygen-glucose deprivation caused a progressive downregulation of the K+-Cl- cotransporter (KCC2), which may have contributed to the Cl- accumulation. The rise in [Cl-]i was accompanied by an inability of the GABA(A) agonist muscimol to cause Cl- influx. In vivo, diazepam is neuroprotective when given early after ischemia, although the mechanism by which this occurs is not well understood. Here, we added diazepam early after oxygen-glucose deprivation and prevented the downregulation of KCC2 and the accumulation of [Cl-]i. Consequently, both GABA(A) responses and synaptic transmission within the hippocampus were restored. Thus, after oxygen-glucose deprivation, diazepam may decrease neuronal excitability, thereby reducing the energy demands of the neuron. This may prevent the activation of downstream cell death mechanisms and restore Cl- homeostasis and neuronal function

Extracellular potassium alters frequency and profile of retinal spreading depression waves

The phenomenon of spreading depression (SD) was observed in chicken retina by means of optical registration via a microscope and a CCD camera applying modern methods of image processing for optimized evaluation of the wave profiles. The propagation dynamics of SD waves was investigated as a function of extracellular potassium. Two main findings were obtained. Firstly, the frequency of spontaneous wave generation increased with the increase of K+ concentration. Secondly, there was an effect of potassium on the wave profile. In particular, the recovery zone of SD waves was shortened at increased K+. This effect was not only due to the dispersion relation of waves in excitable media as shown by the result of the mechanically induced wave trains. Applying the basic principles of chemical excitability for the interpretation of the data led us to the conclusion that these potassium effects are due to perturbations of an autocatalytic reaction to be further explored.

Atrial natriuretic peptide expression is increased in rat cerebral cortex following spreading depression: possible contribution to sd-induced neuroprotection

Cortical spreading depression (CSD) is characterised by slowly propagating waves of cellular depolarization and depression and involves transient changes in blood flow, ion balance and metabolism. In cerebral ischaemia, peri-infarct CSD-like depolarization potentiates infarct growth, whereas preconditioning with a CSD episode protects against subsequent ischaemic insult. Thus, many of the long-lasting molecular changes that occur in CSD-affected tissue are presumed to be part of a 'neuroprotective cascade.' 3',5'-Cyclic guanosine monophosphate (cGMP) has been shown to be a neuroprotective mediator and the nitric oxide system, which increases cGMP production by soluble guanylate cyclase, is up-regulated by CSD. Atrial and C-type natriuretic peptide (ANP/CNP) are present in cerebral cortex and their actions are mediated via particulate guanylate cyclase receptors and cGMP production. Therefore, in further efforts to characterise the role of cGMP-related systems in CSD and neuroprotection, this study investigated possible changes in cortical natriuretic peptide expression following acute, unilateral CSD in rats. Using in situ hybridisation, significant 20-80% increases in ANP mRNA were detected in layers II and VI of ipsilateral cortex at 6 h and 1-14 days after CSD. Ipsilateral cortical levels were again equivalent to control contralateral values after 28 days. Assessment of cortical concentrations of ANP immunoreactivity by radioimmunoassay revealed a significant 57% increase at 7 days after CSD. Despite using a sensitive signal-amplification protocol, authentic ANP-like immunostaining was readily detected in subcortical nerve fibres, but was not reliably detected in normal or CSD-affected neocortex, suggesting the presence of very low levels, and/or active or differential processing of the peptide. Cortical CNP mRNA levels are not altered by CSD, indicating the specificity of the observed effects.Overall, these novel findings demonstrate a prolonged increase in cortical ANP expression after an acute episode of CSD. The overlap between the described time course of CSD-induced protection against ischaemic insult and demonstrated increases in ANP levels, suggest that ANP (like nitric oxide) may contribute to CSD-induced neuroprotection, via effects on cGMP production and other signal-transduction pathways.

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

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

Spreading acidification and depression in the cerebellar cortex

Optical imaging of activity-dependent pH changes using neutral red has revealed a novel form of propagated activity in the cerebellar cortex: spreading acidification and depression (SAD). Evoked by surface stimulation, SAD is characterized by a propagation geometry that reflects the parasagittal architecture of the cerebellum, high speed of propagation across several folia, and a transient depression of the molecular layer circuitry. The properties of SAD differentiate it from other forms of propagating activity in the nervous system including spreading depression and Ca++ waves. Involving several factors, SAD is hypothesized to be a regenerative process that requires a functioning parallel fibers-Purkinje cell circuit, glutamatergic neurotransmission, and is initiated by increased neuronal excitability. Three possible neuronal and glia substrates in the cerebellar cortex could account for the propagation geometry of SAD. Recently, the authors demonstrated that blocking voltage-gated Kv1.1 potassium channels plays a major role in the generation of SAD. This observation has lead to the hypothesis that the episodic and transient disruption in cerebellar function that characterizes episodic ataxia type 1, a Kv1.1 channelopathy, is due to SAD occurring in the cerebellar cortex.

Cortical spreading depression affects Fos expression in the hypothalamic paraventricular nucleus and the cerebral cortex of both hemispheres

The present experiments were performed to clarify the brain sites whose activity is affected exclusively by cortical spreading depression (CoSD). For this purpose, Fos protein, a product of an immediate early gene, was used as a marker of neuronal activation. Because Fos can be induced by many manipulations such as stress stimuli, we verified CoSD-induced Fos expression by excluding the influence of other factors such as anaesthesia and surgical manipulation. CoSD was induced by applying a KCl solution directly to the dura mater over the cerebral cortex, and Fos expression in the brain was assessed by immunohistochemistry using antibodies against Fos protein. We found that during CoSD, Fos expression was increased specifically in the magnocellular region of the hypothalamic paraventricular nucleus (PVN), as well as in the ipsilateral cortex, whereas reduced Fos expression was observed in both the parvocellular region of the PVN and the whole cortex contralateral to the CoSD site. Consistent with the reduced Fos expression, approximately 40% of neurons in the contralateral cortex revealed a suppression of electrical activity during CoSD. These results suggest that in addition to the ipsilateral cortex, CoSD affects Fos expression exclusively in the PVN and the contralateral cortex.

Spreading depression: imaging and blockade in the rat neocortical brain slice

Spreading depression (SD) is a profound but transient depolarization of neurons and glia that migrates across the cortical and subcortical gray at 2-5 mm/min. Under normoxic conditions, SD occurs during migraine aura where it precedes migraine pain but does not damage tissue. During stroke and head trauma, however, SD can arise repeatedly near the site of injury and may promote neuronal damage. We developed a superfused brain slice preparation that can repeatedly support robust SD during imaging and electrophysiological recording to test drugs that may block SD. Submerged rat neocortical slices were briefly exposed to artificial cerebrospinal fluid (ACSF) with KCl elevated to 26 mM. SD was evoked within 2 min, recorded in layers II/III both as a negative DC shift and as a propagating front of elevated light transmittance (LT) representing transient cell swelling in all cortical layers. An SD episode was initiated focally and could be repeatedly evoked and imaged with no damage to slices. As reported in vivo, pretreatment with one of several N-methyl-D-aspartate (NMDA) receptor antagonists blocked SD, but a non-NMDA glutamate receptor antagonist (CNQX) had no effect. NMDA receptor (NMDAR) activation does not initiate SD nor are NMDAR antagonists tolerated therapeutically so we searched for more efficacious drugs to block SD generation. Pretreatment with the sigma-one receptor (sigma(1)R) agonists dextromethorphan (10-100 microM), carbetapentane (100 microM), or 4-IBP (30 microM) blocked SD, even when KCl exposure was extended beyond 5 min. The block was independent of NMDA receptor antagonism. Two sigma(1)R antagonists [(+)-3PPP and BD-1063] removed this block but had no effect upon SD alone. Remarkably, the sigma(1)R agonists also substantially reduced general cell swelling evoked by bath application of 26 mM KCl. More potent sigma(1)R ligands that are therapeutically tolerated could prove useful in reducing SD associated with migraine and be of potential use in stroke or head trauma.