Energy failure in astrocytes increases the vulnerability of neurons to spreading depression

Astrocytic Regulation of Endocannabinoid-Dependent Synaptic Plasticity in the Dorsolateral Striatum

Astrocytes are pivotal for synaptic transmission and may also play a role in the induction and expression of synaptic plasticity, including endocannabinoid-mediated long-term depression (eCB-LTD). In the dorsolateral striatum (DLS), eCB signaling plays a major role in balancing excitation and inhibition and promoting habitual learning. The aim of this study was to outline the role of astrocytes in regulating eCB signaling in the DLS. To this end, we employed electrophysiological slice recordings combined with metabolic, chemogenetic and pharmacological approaches in an attempt to selectively suppress astrocyte function. High-frequency stimulation induced eCB-mediated LTD (HFS-LTD) in brain slices from both male and female rats. The metabolic uncoupler fluorocitrate (FC) reduced the probability of transmitter release and depressed synaptic output in a manner that was independent on cannabinoid 1 receptor (CB1R) activation. Fluorocitrate did not affect the LTD induced by the CB1R agonist WIN55,212-2, but enhanced CB1R-dependent HFS-LTD. Reduced neurotransmission and facilitated HFS-LTD were also observed during chemogenetic manipulation using Gi-coupled DREADDs targeting glial fibrillary acidic protein (GFAP)-expressing cells, during the pharmacological inhibition of connexins using carbenoxolone disodium, or during astrocytic glutamate uptake using TFB-TBOA. While pretreatment with the N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonopentanoic acid (APV) failed to prevent synaptic depression induced by FC, it blocked the facilitation of HFS-LTD. While the lack of tools to disentangle astrocytes from neurons is a major limitation of this study, our data collectively support a role for astrocytes in modulating basal neurotransmission and eCB-mediated synaptic plasticity.

The evolutionary meaning of migraine

INTRODUCTION

Migraine's astonishing prevalence and preserved genetic background contrast with the definition of a disease and the biological meaning of experiencing recurrent, severe headache attacks is still puzzling.

METHODS

To provide a comprehensive explanation of the migraine evolutionary meaning, we review (i) the putative role of the autonomic nervous system in migraine attacks, (ii) the inter-ictal autonomic, functional, and metabolic signature of migraine patients, (iii) the bio-behavioral perspective of pain, and (iv) the allostatic perception of migraine chronification.

RESULTS

Migraineurs have inter-ictal cortical hyperexcitability and metabolic dysfunction that predisposes to brain energetic imbalance. Multiple precipitating factors may lead to brain energy consumption over the migraine attack generation threshold. In response, the brain engenders adaptive, evolutionary conserved, autonomic-behavior responses through the antidromic activation of the trigeminovascular system. The sickness behavior and severe pain experienced during migraine attacks result in avoiding mental and physical activity, allowing brain energy restoration. Chronic exposure to stressors may result in an allostatic overload, leading to maladaptive chronic activation of these responses. In this bio-behavioral perspective, the chronification of migraine should be envisioned as a pathological process, whereas the migraine itself should not.

CONCLUSION

Migraine has an evolutionary (Darwinian) meaning.

Nonpharmacological modulation of cortical spreading depolarization

AIMS

Cortical spreading depolarization (CSD) is a wave of pathologic neuronal dysfunction that spreads through cerebral gray matter, causing neurologic disturbance in migraine and promoting lesion development in acute brain injury. Pharmacologic interventions have been found to be effective in migraine with aura, but their efficacy in acutely injured brains may be limited. This necessitates the assessment of possible adjunctive treatments, such as nonpharmacologic methods. This review aims to summarize currently available nonpharmacological techniques for modulating CSDs, present their mechanisms of action, and provide insight and future directions for CSD treatment.

MAIN METHODS

A systematic literature review was performed, generating 22 articles across 3 decades. Relevant data is broken down according to method of treatment.

KEY FINDINGS

Both pharmacologic and nonpharmacologic interventions can mitigate the pathological impact of CSDs via shared molecular mechanisms, including modulating K⁺/Ca²⁺/Na⁺/Cl^- ion channels and NMDA, GABA_A, serotonin, and CGRP ligand-based receptors and decreasing microglial activation. Preclinical evidence suggests that nonpharmacologic interventions, including neuromodulation, physical exercise, therapeutic hypothermia, and lifestyle changes can also target unique mechanisms, such as increasing adrenergic tone and myelination and modulating membrane fluidity, which may lend broader modulatory effects. Collectively, these mechanisms increase the electrical initiation threshold, increase CSD latency, slow CSD velocity, and decrease CSD amplitude and duration.

SIGNIFICANCE

Given the harmful consequences of CSDs, limitations of current pharmacological interventions to inhibit CSDs in acutely injured brains, and translational potentials of nonpharmacologic interventions to modulate CSDs, further assessment of nonpharmacologic modalities and their mechanisms to mitigate CSD-related neurologic dysfunction is warranted.

Plasticity of perisynaptic astroglia during ischemia-induced spreading depolarization

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

Adenosine Improves Mitochondrial Function and Biogenesis in Friedreich's Ataxia Fibroblasts Following L-Buthionine Sulfoximine-Induced Oxidative Stress

Adenosine is a nucleoside that is widely distributed in the central nervous system and acts as a central excitatory and inhibitory neurotransmitter in the brain. The protective role of adenosine in different pathological conditions and neurodegenerative diseases is mainly mediated by adenosine receptors. However, its potential role in mitigating the deleterious effects of oxidative stress in Friedreich's ataxia (FRDA) remains poorly understood. We aimed to investigate the protective effects of adenosine against mitochondrial dysfunction and impaired mitochondrial biogenesis in L-buthionine sulfoximine (BSO)-induced oxidative stress in dermal fibroblasts derived from an FRDA patient. The FRDA fibroblasts were pre-treated with adenosine for 2 h, followed by 12.50 mM BSO to induce oxidative stress. Cells in medium without any treatments or pre-treated with 5 µM idebenone served as the negative and positive controls, respectively. Cell viability, mitochondrial membrane potential (MMP), aconitase activity, adenosine triphosphate (ATP) level, mitochondrial biogenesis, and associated gene expressions were assessed. We observed disruption of mitochondrial function and biogenesis and alteration in gene expression patterns in BSO-treated FRDA fibroblasts. Pre-treatment with adenosine ranging from 0-600 µM restored MMP, promoted ATP production and mitochondrial biogenesis, and modulated the expression of key metabolic genes, namely nuclear respiratory factor 1 (NRF1), transcription factor A, mitochondrial (TFAM), and NFE2-like bZIP transcription factor 2 (NFE2L2). Our study demonstrated that adenosine targeted mitochondrial defects in FRDA, contributing to improved mitochondrial function and biogenesis, leading to cellular iron homeostasis. Therefore, we suggest a possible therapeutic role for adenosine in FRDA.

Energy metabolism disturbance in migraine: From a mitochondrial point of view

Migraine is a serious central nervous system disease with a high incidence rate. Its pathogenesis is very complex, which brings great difficulties for clinical treatment. Recently, many studies have revealed that mitochondrial dysfunction may play a key role in migraine, which affects the hyperosmotic of Ca²⁺, the excessive production of free radicals, the decrease of mitochondrial membrane potential, the imbalance of mPTP opening and closing, and the decrease of oxidative phosphorylation level, which leads to neuronal energy exhaustion and apoptosis, and finally lessens the pain threshold and migraine attack. This article mainly introduces cortical spreading depression, a pathogenesis of migraine, and then damages the related function of mitochondria, which leads to migraine. Oxidative phosphorylation and the tricarboxylic acid cycle are the main ways to provide energy for the body. 95 percent of the energy needed for cell survival is provided by the mitochondrial respiratory chain. At the same time, hypoxia can lead to cell death and migraine. The pathological opening of the mitochondrial permeability transition pore can promote the interaction between pro-apoptotic protein and mitochondrial, destroy the structure of mPTP, and further lead to cell death. The increase of mPTP permeability can promote the accumulation of reactive oxygen species, which leads to a series of changes in the expression of proteins related to energy metabolism. Both Nitric oxide and Calcitonin gene-related peptide are closely related to the attack of migraine. Recent studies have shown that changes in their contents can also affect the energy metabolism of the body, so this paper reviews the above mechanisms and discusses the mechanism of brain energy metabolism of migraine, to provide new strategies for the prevention and treatment of migraine and promote the development of individualized and accurate treatment of migraine.

Effects of In Vivo Intracellular ATP Modulation on Neocortical Extracellular Potassium Concentration

Neuronal and glial activity are dependent on the efflux of potassium ions into the extracellular space. Efflux of K is partly energy-dependent as the activity of pumps and channels which are involved in K transportation is ATP-dependent. In this study, we investigated the effect of decreased intracellular ATP concentration ([ATP]i) on the extracellular potassium ion concentration ([K]o). Using in vivo electrophysiological techniques, we measured neocortical [K]o and the local field potential (LFP) while [ATP]i was reduced through various pharmacological interventions. We observed that reducing [ATP]i led to raised [K]o and DC-shifts resembling spreading depolarization-like events. We proposed that most likely, the increased [K]o is mainly due to the impairment of the Na/K ATPase pump and the ATP-sensitive potassium channel in the absence of sufficient ATP, because Na/K ATPase inhibition led to increased [K]o and ATP-sensitive potassium channel impairment resulted in decreased [K]o. Therefore, an important consequence of decreased [ATP]i is an increased [K]o. The results of this study acknowledge one of the mechanisms involved in [K]o dynamics.

Astrocytes modulate extracellular neurotransmitter levels and excitatory neurotransmission in dorsolateral striatum via dopamine D2 receptor signaling

Astrocytes provide structural and metabolic support of neuronal tissue, but may also be involved in shaping synaptic output. To further define the role of striatal astrocytes in modulating neurotransmission we performed in vivo microdialysis and ex vivo slice electrophysiology combined with metabolic, chemogenetic, and pharmacological approaches. Microdialysis recordings revealed that intrastriatal perfusion of the metabolic uncoupler fluorocitrate (FC) produced a robust increase in extracellular glutamate levels, with a parallel and progressive decline in glutamine. In addition, FC significantly increased the microdialysate concentrations of dopamine and taurine, but did not modulate the extracellular levels of glycine or serine. Despite the increase in glutamate levels, ex vivo electrophysiology demonstrated a reduced excitability of striatal neurons in response to FC. The decrease in evoked potentials was accompanied by an increased paired pulse ratio, and a reduced frequency of spontaneous excitatory postsynaptic currents, suggesting that FC depresses striatal output by reducing the probability of transmitter release. The effect by FC was mimicked by chemogenetic inhibition of astrocytes using G_i-coupled designer receptors exclusively activated by designer drugs (DREADDs) targeting GFAP, and by the glial glutamate transporter inhibitor TFB-TBOA. Both FC- and TFB-TBOA-mediated synaptic depression were inhibited in brain slices pre-treated with the dopamine D2 receptor antagonist sulpiride, but insensitive to agents acting on presynaptic glutamatergic autoreceptors, NMDA receptors, gap junction coupling, cannabinoid 1 receptors, µ-opioid receptors, P2 receptors or GABA_A receptors. In conclusion, our data collectively support a role for astrocytes in modulating striatal neurotransmission and suggest that reduced transmission after astrocytic inhibition involves dopamine.

Contribution of astrocytes to neurovascular coupling in the spinal cord of the rat

Functional magnetic resonance imaging (fMRI) of the spinal cord relies on the integrity of neurovascular coupling (NVC) to infer neuronal activity from hemodynamic changes. Astrocytes are a key component of cerebral NVC, but their role in spinal NVC is unclear. The objective of this study was to examine whether inhibition of astrocyte metabolism by fluorocitrate alters spinal NVC. In 14 rats, local field potential (LFP) and spinal cord blood flow (SCBF) were recorded simultaneously in the lumbosacral enlargement during noxious stimulation of the sciatic nerve before and after a local administration of fluorocitrate (N  = 7) or saline (N  = 7). Fluorocitrate significantly reduced SCBF responses (p  < 0.001) but not LFP amplitude (p  = 0.22) compared with saline. Accordingly, NVC was altered by fluorocitrate compared with saline (p  < 0.01). These results support the role of astrocytes in spinal NVC and have implications for spinal cord imaging with fMRI for conditions in which astrocyte metabolism may be altered.

Neuroprotective effects of Hericium erinaceus (Bull.: Fr.) Pers. against high-dose corticosterone-induced oxidative stress in PC-12 cells

BACKGROUND

Hericium erinaceus is a culinary and medicinal mushroom in Traditional Chinese Medicines. It has numerous pharmacological effects including immunomodulatory, anti-tumour, anti-microbial, anti-aging and stimulation of nerve growth factor (NGF) synthesis, but little is known about its potential role in negating the detrimental effects of oxidative stress in depression. The present study investigated the neuroprotective effects of H. erinaceus standardised aqueous extract (HESAE) against high-dose corticosterone-induced oxidative stress in rat pheochromocytoma (PC-12) cells, a cellular model mimicking depression.

METHODS

PC-12 cells was pre-treated with HESAE for 48 h followed by 400 μM corticosterone for 24 h to induce oxidative stress. Cells in complete medium without any treatment or pre-treated with 3.125 μg/mL desipramine served as the negative and positive controls, respectively. The cell viability, lactate dehydrogenase (LDH) release, endogenous antioxidant enzyme activities, aconitase activity, mitochondrial membrane potentials (MMPs), intracellular reactive oxygen species (ROS) levels and number of apoptotic nuclei were quantified. In addition, HESAE ethanol extract was separated into fractions by chromatographic methods prior to spectroscopic analysis.

RESULTS

We observed that PC-12 cells treated with high-dose corticosterone at 400 μM had decreased cell viability, reduced endogenous antioxidant enzyme activities, disrupted mitochondrial function, and increased oxidative stress and apoptosis. However, pre-treatment with HESAE ranging from 0.25 to 1 mg/mL had increased cell viability, decreased LDH release, enhanced endogenous antioxidant enzyme activities, restored MMP, attenuated intracellular ROS and protected from ROS-mediated apoptosis. The neuroprotective effects could be attributed to significant amounts of adenosine and herierin III isolated from HESAE.

CONCLUSIONS

HESAE demonstrated neuroprotective effects against high-dose corticosterone-induced oxidative stress in an in vitro model mimicking depression. HESAE could be a potential dietary supplement to treat depression.

Valeriana officinalis Counteracts Rotenone Effects on Spreading Depression in the Rat Brain in vivo and Protects Against Rotenone Cytotoxicity Toward Rat Glioma C6 Cells in vitro

Astrocytes can protect neurons against oxidative stress and excitability-dependent disorders, such as epilepsy. Valeriana officinalis has been used as anticonvulsant and can exert an antioxidant effect, which may underlie its opposing action against the toxic effects of the pesticide rotenone. We investigated the V. officinalis/rotenone interaction in the cortical spreading depression (CSD), a phenomenon that depends upon brain excitability (in vivo model). In addition, we analyzed the protective action of V. officinalis against the cytotoxic effects of rotenone in cultures of rat C6 glioma cells (in vitro model). For the CSD study, Wistar rats received either V. officinalis (250 mg/kg/day via gavage for 15 days; n = 8) or 10 mg/kg/day rotenone via subcutaneous injections for 7 days (n = 7), or they received both substances (n = 5). Two control groups received either saline (vehicle for V. officinalis; n = 8) or 1% Tween-80 aqueous solution (vehicle for rotenone; n = 9). After treatment, CSD was recorded for 4 h. The rotenone- and V. officinalis-treated groups presented, respectively, with lower (2.96 ± 0.14 mm/min), and higher CSD propagation velocity (3.81 ± 0.10 mm/min) when compared with the controls (Tween-80, 3.37 ± 0.06 mm/min and saline, 3.35 ± 0.08 mm/min; p < 0.05). The rotenone plus V. officinalis-treated group displayed a CSD velocity (3.38 ± 0.07 mm/min) that was similar to controls. In line with these results, in vitro experiments on rat glioma C6 cells revealed a protective effect (MTT assay) of V. officinalis against rotenone-induced cytotoxicity. These results suggest the therapeutic potential of V. officinalis for treating neurological diseases involving redox imbalance and astrocyte dysfunction.

Migraine and Ischemic Stroke: Deciphering the Bidirectional Pathway

Migraine and stroke are common, disabling neurological conditions with several theories being proposed to explain this bidirectional relationship. Migraine is considered as a benign neurological disorder, but research has revealed a connection between migraine and stroke, predominantly those having migraine with aura (MA). Among migraineurs, females with MA are more susceptible to ischemic stroke and may have a migrainous infarction. Migrainous infarction mostly occurs in the posterior circulation of young women. Although there are several theories about the potential relationship between MA and stroke, the precise pathological process of migrainous infarction is not clear. It is assumed that cortical spreading depression (CSD) might be one of the essential factors for migrainous infarction. Other factors that may contribute to migrainous infarction may be genetic, hormonal fluctuation, hypercoagulation, and right to left cardiac shunts. Antimigraine drugs, such as ergot alkaloids and triptans, are widely used in migraine care. Still, they have been found to cause severe vasoconstriction, which may result in the development of ischemia. It is reported that patients with stroke develop migraines during the recovery phase. Both experimental and clinical data suggest that cerebral microembolism can act as a potential trigger for MA. Further studies are warranted for the treatment of migraine, which may lead to a decline in migraine-related stroke. In this present article, we have outlined various potential pathways that link migraine and stroke.

Metabolism plays a central role in the cortical spreading depression: Evidence from a mathematical model

The slow propagating waves of strong depolarization of neural cells characterizing cortical spreading depression, or depolarization, (SD) are known to break cerebral homeostasis and induce significant hemodynamic and electro-metabolic alterations. Mathematical models of cortical spreading depression found in the literature tend to focus on the changes occurring at the electrophysiological level rather than on the ensuing metabolic changes. In this paper, we propose a novel mathematical model which is able to simulate the coupled electrophysiology and metabolism dynamics of SD events, including the swelling of neurons and astrocytes and the concomitant shrinkage of extracellular space. The simulations show that the metabolic coupling leads to spontaneous repetitions of the SD events, which the electrophysiological model alone is not capable to produce. The model predictions, which corroborate experimental findings from the literature, show a strong disruption in metabolism accompanying each wave of spreading depression in the form of a sharp decrease of glucose and oxygen concentrations, with a simultaneous increase in lactate concentration which, in turn, delays the clearing of excess potassium in extracellular space. Our model suggests that the depletion of glucose and oxygen concentration is more pronounced in astrocyte than neuron, in line with the partitioning of the energetic cost of potassium clearing. The model suggests that the repeated SD events are electro-metabolic oscillations that cannot be explained by the electrophysiology alone. The model highlights the crucial role of astrocytes in cleaning the excess potassium flooding extracellular space during a spreading depression event: further, if the ratio of glial/neuron density increases, the frequency of cortical SD events decreases, and the peak potassium concentration in extracellular space is lower than with equal volume fractions.

Comparison of Spreading Depolarizations in the Motor Cortex and Nucleus Accumbens: Similar Patterns of Oxygen Responses and the Role of Dopamine

Spreading depolarizations (SD) are pathophysiological phenomena that spontaneously arise in traumatized neural tissue and can promote cellular death. Most investigations of SD are performed in the cortex, a brain region that is susceptible to these depolarizing waves and accessible via a variety of monitoring techniques. Here, we describe SD responses in the cortex and the deep brain region of the nucleus accumbens (NAc) of the anesthetized rat with a minimally invasive, implantable sensor. With high temporal resolution, we characterize the time course of oxygen responses to SD in relation to the electrophysiological depolarization signal. The predominant oxygen pattern consists of four phases: (1) a small initial decrease, (2) a large increase during the SD, (3) a delayed increase, and (4) a persistent decrease from baseline after the SD. Oxygen decreases during SD were also recorded. The latter response occurred more often in the NAc than the cortex (56% vs 20% of locations, respectively), which correlates to denser cortical vascularization. We also find that SDs travel more quickly in the cortex than NAc, likely affected by regional differences in cell type populations. Finally, we investigate the previously uncharacterized effects of dopamine release during SD in the NAc with dopamine receptor blockade. Our results support an inhibitory role of the D2 receptor on SD. As such, the data presented here expands the current understanding of within- and between-region variance in responses to SD.

Role of astrocyte connexin hemichannels in cortical spreading depression

Cortical spreading depression (CSD) is an intriguing phenomenon consisting of massive slow brain depolarizations that affects neurons and glial cells. It has been recognized since 1944, but its pathogenesis has only been uncovered during the last decade. Acute brain injuries can be further complicated by CSD in >50% of severe cases. This phenomenon is repetitive and produces a metabolic overload that increments secondary damage. Propagation of CSD is known to be linked to excitotoxicity, but the mechanisms associated with its initiation remain less understood. It has been shown that CSD can be initiated by increases in extracellular [K⁺] ([K⁺]_e), and animal models use high [K⁺]_e to promote CSD. Connexin hemichannel activity increases due to high [K⁺]_e and low extracellular [Ca²⁺], conditions that occur after brain injury. Moreover, glial cell gap junction channels are fundamental in controlling extracellular medium composition, particularly in maintaining normal extracellular glutamate and K⁺ concentrations through "spatial buffering". However, the role of astrocytic gap junctions under tissue stress can change to damage spread in the acute damage zone whereas the reduced communication in adjacent zone would reduce cell dead propagation. Here, we review the main findings associated with CSD, and discuss the possible involvement of astrocytic connexin-based channels in secondary damage propagation. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Binaural blood flow control by astrocytes: listening to synapses and the vasculature

Astrocytes are the most common glial cells in the brain with fine processes and endfeet that intimately contact both neuronal synapses and the cerebral vasculature. They play an important role in mediating neurovascular coupling (NVC) via several astrocytic Ca²⁺ -dependent signalling pathways such as K⁺ release through B_K channels, and the production and release of arachidonic acid metabolites. They are also involved in maintaining the resting tone of the cerebral vessels by releasing ATP and COX-1 derivatives. Evidence also supports a role for astrocytes in maintaining blood pressure-dependent change in cerebrovascular tone, and perhaps also in blood vessel-to-neuron signalling as posited by the 'hemo-neural hypothesis'. Thus, astrocytes are emerging as new stars in preserving the intricate balance between the high energy demand of active neurons and the supply of oxygen and nutrients from the blood by maintaining both resting blood flow and activity-evoked changes therein. Following neuropathology, astrocytes become reactive and many of their key signalling mechanisms are altered, including those involved in NVC. Furthermore, as they can respond to changes in vascular pressure, cardiovascular diseases might exert previously unknown effects on the central nervous system by altering astrocyte function. This review discusses the role of astrocytes in neurovascular signalling in both physiology and pathology, and the impact of these findings on understanding BOLD-fMRI signals.

Mechanisms of spreading depolarization in vertebrate and insect central nervous systems

Spreading depolarization (SD) is generated in the central nervous systems of both vertebrates and invertebrates. SD manifests as a propagating wave of electrical depression caused by a massive redistribution of ions. Mammalian SD underlies a continuum of human pathologies from migraine to stroke damage, whereas insect SD is associated with environmental stress-induced neural shutdown. The general cellular mechanisms underlying SD seem to be evolutionarily conserved throughout the animal kingdom. In particular, SD in the central nervous system of Locusta migratoria and Drosophila melanogaster has all the hallmarks of mammalian SD. Locust SD is easily induced and monitored within the metathoracic ganglion (MTG) and can be modulated both pharmacologically and by preconditioning treatments. The finding that the fly brain supports repetitive waves of SD is relatively recent but noteworthy, since it provides a genetically tractable model system. Due to the human suffering caused by SD manifestations, elucidating control mechanisms that could ultimately attenuate brain susceptibility is essential. Here we review mechanisms of SD focusing on the similarities between mammalian and insect systems. Additionally we discuss advantages of using invertebrate model systems and propose insect SD as a valuable model for providing new insights to mammalian SD.

Astrocytes Regulate GLP-1 Receptor-Mediated Effects on Energy Balance

Astrocytes are well established modulators of extracellular glutamate, but their direct influence on energy balance-relevant behaviors is largely understudied. As the anorectic effects of glucagon-like peptide-1 receptor (GLP-1R) agonists are partly mediated by central modulation of glutamatergic signaling, we tested the hypothesis that astrocytic GLP-1R signaling regulates energy balance in rats. Central or peripheral administration of a fluorophore-labeled GLP-1R agonist, exendin-4, localizes within astrocytes and neurons in the nucleus tractus solitarius (NTS), a hindbrain nucleus critical for energy balance control. This effect is mediated by GLP-1R, as the uptake of systemically administered fluorophore-tagged exendin-4 was blocked by central pretreatment with the competitive GLP-1R antagonist exendin-(9-39). Ex vivo analyses show prolonged exendin-4-induced activation (live cell calcium signaling) of NTS astrocytes and neurons; these effects are also attenuated by exendin-(9-39), indicating mediation by the GLP-1R. In vitro analyses show that the application of GLP-1R agonists increases cAMP levels in astrocytes. Immunohistochemical analyses reveal that endogenous GLP-1 axons form close synaptic apposition with NTS astrocytes. Finally, pharmacological inhibition of NTS astrocytes attenuates the anorectic and body weight-suppressive effects of intra-NTS GLP-1R activation. Collectively, data demonstrate a role for NTS astrocytic GLP-1R signaling in energy balance control.

SIGNIFICANCE STATEMENT

Glucagon-like peptide-1 receptor (GLP-1R) agonists reduce food intake and are approved by the Food and Drug Administration for the treatment of obesity, but the cellular mechanisms underlying the anorectic effects of GLP-1 require further investigation. Astrocytes represent a major cellular population in the CNS that regulates neurotransmission, yet the role of astrocytes in mediating energy balance is largely unstudied. The current data provide novel evidence that astrocytes within the NTS are relevant for energy balance control by GLP-1 signaling. Here, we report that GLP-1R agonists activate and internalize within NTS astrocytes, while behavioral data suggest the pharmacological relevance of NTS astrocytic GLP-1R activation for food intake and body weight. These findings support a previously unknown role for CNS astrocytes in energy balance control by GLP-1 signaling.

Effects of garlic extract on spreading depression: In vitro and in vivo investigations

OBJECTIVES

The potential use of garlic for prevention and treatment of different types of headaches has been suggested by several medieval literatures. Cortical spreading depression (CSD), a propagating wave of neuroglial depolarization, was established as a target for anti-migraine drugs. This study was designed to investigate the effect of garlic extract on CSD in adult rats.

METHODS

CSD was induced by KCl microinjection in the somatosensory cortex. The effects of five different concentrations of garlic oil (1-500 μl/l) were tested on different characteristic features of CSD in necocortical slices. In in vivo experiments, the effects of garlic oil on electrophysiological and morphological changes induced by CSD were investigated.

RESULTS

Garlic oil in a dose-dependent manner decreased the amplitude of CSD but not its duration and velocity in neocortical brain slices. Garlic oil at concentration of 500 μl/l reversibly reduced the amplitude of the field excitatory post-synaptic potentials and inhibited induction of long-term potentiation in the third layer of neocortical slices. In in vivo studies, systemic application of garlic oil (1 ml/l) for three consecutive days reduced the amplitude and repetition rate of CSD. Garlic oil also prevented of CSD-induced reactive astrocytosis in the neocortex.

DISCUSSION

Garlic oil suppresses CSD, likely via inhibition of synaptic plasticity, and prevents its harmful effects on astrocyte. Further studies are required to identify the exact active ingredient(s) of garlic oil that inhibit CSD and may have the potential to use in treatment of CSD-related disorders.

The Comorbidity of Migraine and Epilepsy in Children and Adolescents

Migraine and epilepsy share a number of clinical attributes, including pathophysiology and clinical expression. Both are paroxysmal in nature and thus constitute episodic disorders, yet either may be chronic and/or recurrent. Epileptic seizures and migraine headaches may be mistaken one for the other and may even overlap. In particular, occipital lobe seizures may be misdiagnosed as migraine auras. In this article, we review the relationship between migraine and epilepsy, including the known genetic contributions to both conditions, prodromal, ictal, and postictal headache and shared pathophysiology and treatment options. We describe clinical conditions in which both migraine and epilepsy are prominent features. Lastly, we discuss electronecephaographic abnormalities that have been known to occur in individuals with migraine.

Do interictal microembolic signals play a role in higher cortical dysfunction during migraine aura?

Introduction

The aim of this study was to evaluate the prevalence and clinical impact of interictal microembolic signals (MES) in patients suffering from migraine with higher cortical dysfunction (HCD), such as language and memory impairment, during an aura.

Patients and methods

This study was carried out on 34 migraineurs with language and memory impairment during aura (HCD group), 31 migraineurs with only visual or visual and somatosensory symptoms during aura (Control group I), and 34 healthy controls (Control group II). We used a Doppler instrument to detect microemboli. Demographic data, disease features and the detection of MES between these groups, as well as the predictors of HCD during the aura, were analyzed.

Results

The duration of aura was longer and the frequency of aura was higher among patients with language and memory impairment during aura compared to Control group I. MES was detected in 29.4% patients from the HCD group, which was significantly higher compared to 3.2% in Control group I and 5.9% in Control group II. Regarding the absence or presence of MES, demographic and aura features were not different in the HCD subgroups. A longer duration of aura, the presence of somatosensory symptoms during the aura and the presence of interictal MES were independent predictors of HCD during the aura.

Conclusion

The present findings indicate that HCD and MES are related in patients with migraine with aura. Further research is needed to better understand the exact pathophysiological mechanism.

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.

Purinergic control of vascular tone in the retina

Purinergic control of vascular tone in the CNS has been largely unexplored. This study examines the contribution of endogenous extracellular ATP, acting on vascular smooth muscle cells, in controlling vascular tone in the in vivo rat retina. Retinal vessels were labelled by i.v. injection of a fluorescent dye and imaged with scanning laser confocal microscopy. The diameters of primary arterioles were monitored under control conditions and following intravitreal injection of pharmacological agents. Apyrase (500 units ml(-1)), an ATP hydrolysing enzyme, dilated retinal arterioles by 40.4 ± 2.8%, while AOPCP (12.5 mm), an ecto-5'-nucleotidase inhibitor that increases extracellular ATP levels, constricted arterioles by 58.0 ± 3.8% (P < 0.001 for both), demonstrating the importance of ATP in the control of basal vascular tone. Suramin (500 μm), a broad-spectrum P2 receptor antagonist, dilated retinal arterioles by 50.9 ± 3.7% (P < 0.001). IsoPPADS (300 μm) and TNP-ATP (50 μm), more selective P2X antagonists, dilated arterioles by 41.0 ± 5.3% and 55.2 ± 6.1% respectively (P < 0.001 for both). NF023 (50 μm), a potent antagonist of P2X1 receptors, dilated retinal arterioles by 32.1 ± 2.6% (P < 0.001). A438079 (500 μm) and AZ10606120 (50 μm), P2X7 antagonists, had no effect on basal vascular tone (P = 0.99 and P = 1.00 respectively). In the ex vivo retina, the P2X1 receptor agonist α,β-methylene ATP (300 nm) evoked sustained vasoconstrictions of 18.7 ± 3.2% (P < 0.05). In vivo vitreal injection of the gliotoxin fluorocitrate (150 μm) dilated retinal vessels by 52.3 ± 1.1% (P < 0.001) and inhibited the vasodilatory response to NF023 (50 μm, 7.9 ± 2.0%; P < 0.01). These findings suggest that vascular tone in rat retinal arterioles is maintained by tonic release of ATP from the retina. ATP acts on P2X1 receptors, although contributions from other P2X and P2Y receptors cannot be ruled out. Retinal glial cells are a possible source of the vasoconstricting ATP.

Cortical spreading depression in traumatic brain injuries: is there a role for astrocytes?

Cortical spreading depression (CSD) is a presumably pathophysiological phenomenon that interrupts local cortical function for periods of minutes to hours. This phenomenon is important due to its association with different neurological disorders such as migraine, malignant stroke and traumatic brain injury (TBI). Glial cells, especially astrocytes, play an important role in the regulation of CSD and in the protection of neurons under brain trauma. The correlation of TBI with CSD and the astrocytic function under these conditions remain unclear. This review discusses the possible link of TBI and CSD and its implication for neuronal survival. Additionally, we highlight the importance of astrocytic function for brain protection, and suggest possible therapeutic strategies targeting astrocytes to improve the outcome following TBI-associated CSD.

Pharmacological blockade of gap junctions induces repetitive surging of extracellular potassium within the locust CNS

The maintenance of cellular ion homeostasis is crucial for optimal neural function and thus it is of great importance to understand its regulation. Glial cells are extensively coupled by gap junctions forming a network that is suggested to serve as a spatial buffer for potassium (K(+)) ions. We have investigated the role of glial spatial buffering in the regulation of extracellular K(+) concentration ([K(+)]o) within the locust metathoracic ganglion by pharmacologically inhibiting gap junctions. Using K(+)-sensitive microelectrodes, we measured [K(+)]o near the ventilatory neuropile while simultaneously recording the ventilatory rhythm as a model of neural circuit function. We found that blockade of gap junctions with either carbenoxolone (CBX), 18β-glycyrrhetinic acid (18β-GA) or meclofenamic acid (MFA) reliably induced repetitive [K(+)]o surges and caused a progressive impairment in the ability to maintain baseline [K(+)]o levels throughout the treatment period. We also show that a low dose of CBX that did not induce surging activity increased the vulnerability of locust neural tissue to spreading depression (SD) induced by Na(+)/K(+)-ATPase inhibition with ouabain. CBX pre-treatment increased the number of SD events induced by ouabain and hindered the recovery of [K(+)]o back to baseline levels between events. Our results suggest that glial spatial buffering through gap junctions plays an essential role in the regulation of [K(+)]o under normal conditions and also contributes to a component of [K(+)]o clearance following physiologically elevated levels of [K(+)]o.

Mitochondrial dysfunction in migraine

Migraine is the most frequent type of headache in children. In the 1980s, scientists first hypothesized a connection between migraine and mitochondrial (mt) disorders. More recent studies have suggested that at least some subtypes of migraine may be related to a mt defect. Different types of evidence support a relationship between mitochondria (mt) and migraine: (1) Biochemical evidence: Abnormal mt function translates into high intracellular penetration of Ca(2+), excessive production of free radicals, and deficient oxidative phosphorylation, which ultimately causes energy failure in neurons and astrocytes, thus triggering migraine mechanisms, including spreading depression. The mt markers of these events are low activity of superoxide dismutase, activation of cytochrome-c oxidase and nitric oxide, high levels of lactate and pyruvate, and low ratios of phosphocreatine-inorganic phosphate and N-acetylaspartate-choline. (2) Morphologic evidence: mt abnormalities have been shown in migraine sufferers, the most characteristic ones being direct observation in muscle biopsy of ragged red and cytochrome-c oxidase-negative fibers, accumulation of subsarcolemmal mt, and demonstration of giant mt with paracrystalline inclusions. (3) Genetic evidence: Recent studies have identified specific mutations responsible for migraine susceptibility. However, the investigation of the mtDNA mutations found in classic mt disorders (mt encephalomyopathy with lactic acidosis and stroke-like episodes, myoclonus epilepsy with ragged red fibers, Kearns-Sayre syndrome, and Leber hereditary optic neuropathy) has not demonstrated any association. Recently, 2 common mtDNA polymorphisms (16519C→T and 3010G→A) have been associated with pediatric cyclic vomiting syndrome and migraine. Also, POLG mutations (eg, p.T851 A, p.N468D, p.Y831C, p.G517V, and p.P163S) can cause disease through impaired replication of mtDNA, including migraine. Further studies to investigate the relationship between mtDNA and migraine will require very large sample sizes to obtain statistically significant results. (4) Therapeutic evidence: Several agents that have a positive effect on mt metabolism have shown to be effective in the treatment of migraines. The agents include riboflavin (B2), coenzyme Q10, magnesium, niacin, carnitine, topiramate, and lipoic acid. Further study is warranted to learn how mt interact with other factors to cause migraines. This will facilitate the development of new and more specific treatments that will reduce the frequency or severity or both of this disease.

Cortical spreading depression as a target for anti-migraine agents

Spreading depression (SD) is a slowly propagating wave of neuronal and glial depolarization lasting a few minutes, that can develop within the cerebral cortex or other brain areas after electrical, mechanical or chemical depolarizing stimulations. Cortical SD (CSD) is considered the neurophysiological correlate of migraine aura. It is characterized by massive increases in both extracellular K⁺ and glutamate, as well as rises in intracellular Na⁺ and Ca²⁺. These ionic shifts produce slow direct current (DC) potential shifts that can be recorded extracellularly. Moreover, CSD is associated with changes in cortical parenchymal blood flow. CSD has been shown to be a common therapeutic target for currently prescribed migraine prophylactic drugs. Yet, no effects have been observed for the antiepileptic drugs carbamazepine and oxcarbazepine, consistent with their lack of efficacy on migraine. Some molecules of interest for migraine have been tested for their effect on CSD. Specifically, blocking CSD may play an enabling role for novel benzopyran derivative tonabersat in preventing migraine with aura. Additionally, calcitonin gene-related peptide (CGRP) antagonists have been recently reported to inhibit CSD, suggesting the contribution of CGRP receptor activation to the initiation and maintenance of CSD not only at the classic vascular sites, but also at a central neuronal level. Understanding what may be lying behind this contribution, would add further insights into the mechanisms of actions for "gepants", which may be pivotal for the effectiveness of these drugs as anti-migraine agents. CSD models are useful tools for testing current and novel prophylactic drugs, providing knowledge on mechanisms of action relevant for migraine.

Interpreting Stress Responses during Routine Toxicity Studies

Stress often occurs during toxicity studies. The perception of sensory stimuli as stressful primarily results in catecholamine release and activation of the hypothalamic–pituitary–adrenal (HPA) axis to increase serum glucocorticoid concentrations. Downstream effects of these neuroendocrine signals may include decreased total body weights or body weight gain; food consumption and activity; altered organ weights (e.g., thymus, spleen, adrenal); lymphocyte depletion in thymus and spleen; altered circulating leukocyte counts (e.g., increased neutrophils with decreased lymphocytes and eosinophils); and altered reproductive functions. Typically, only some of these findings occur in a given study. Stress responses should be interpreted as secondary (indirect) rather than primary (direct) test article–related findings. Determining whether effects are the result of stress requires a weight-of-evidence approach. The evaluation and interpretation of routinely collected data (standard in-life, clinical pathology, and anatomic pathology endpoints) are appropriate and generally sufficient to assess whether or not changes are secondary to stress. The impact of possible stress-induced effects on data interpretation can partially be mitigated by toxicity study designs that use appropriate control groups (e.g., cohorts treated with vehicle and subjected to the same procedures as those dosed with test article), housing that minimizes isolation and offers environmental enrichment, and experimental procedures that minimize stress and sampling and analytical bias.

This article is a comprehensive overview of the biological aspects of the stress response, beginning with a Summary (Section 1) and an Introduction (Section 2) that describes the historical and conventional methods used to characterize acute and chronic stress responses. These sections are followed by reviews of the primary systems and parameters that regulate and/or are influenced by stress, with an emphasis on parameters evaluated in toxicity studies: In-life Procedures (Section 3), Nervous System (Section 4), Endocrine System (Section 5), Reproductive System (Section 6), Clinical Pathology (Section 7), and Immune System (Section 8). The paper concludes (Section 9) with a brief discussion on Minimizing Stress-Related Effects (9.1.), and a final section explaining why Parameters routinely measured are appropriate for assessing the role of stress in toxicology studies (9.2.).

Cortical spreading depression shifts cell fate determination of progenitor cells in the adult cortex

Cortical spreading depression (SD) is propagating neuronal and glial depolarization and is thought to underly the pathophysiology of migraine. We have reported that cortical SD facilitates the proliferative activity of NG2-containing progenitor cells (NG2 cells) that give rise to oligodendrocytes and immature neurons under the physiological conditions in the adult mammalian cortex. Astrocytes have an important role in the maintenance of neuronal functions and alleviate neuronal damage after intense neuronal excitation, including SD and seizures. We here investigated whether SD promotes astrocyte generation from NG2 cells following SD stimuli. Spreading depression was induced by epidural application of 1 mol/L KCl solution in adult rats. We investigated the cell fate of NG2 cells following SD-induced proliferation using 5'-bromodeoxyuridine labeling and immunohistochemical analysis. Newly generated astrocytes were observed only in the SD-stimulated cortex, but not in the contralateral cortex or in normal cortex. The astrocytes were generated from proliferating NG2 cells. Astrogenesis depended on the number of SD stimuli, and was accompanied by suppression of oligodendrogenesis. These observations indicate that the cell fate of NG2 cells was shifted from oligodendrocytes to astrocytes depending on SD stimuli, suggesting activity-dependent tissue remodeling for maintenance of brain functions.

Serum levels of N-acetyl-aspartate in migraine and tension-type headache

Serum levels of N-acetyl-aspartate (NAA) may be considered a useful marker of neuronal functioning. We aimed to measure serum NAA in cohorts of migraine and tension-type headache patients versus controls, performing correlations with main clinical features. A total of 147 migraine patients (including migraine without aura, with aura and chronic migraine), 65 tension-type headache (including chronic and frequent episodic tension-type headache) and 34 sex- and age-matched controls were selected. Serum was stored at -80 °C. Quantification of NAA was achieved by the standard addition approach and analysis was performed with liquid-chromatography-mass-spectrometry (LC/MS) technique. The NAA levels were significantly decreased in migraine group (0.065 ± 0.019 mol/L), compared with both tension-type headache patients (0.078 ± 0.016 mol/L) and controls (0.085 ± 0.013 mol/L). Control subjects were significantly different from migraine with and without aura and chronic migraine, who differed significantly from episodic and chronic tension-type headache. Migraine with aura patients showed lower NAA levels when compared to all the other headache subtypes, including migraine without aura and chronic migraine. In the migraine group, no significant correlation was found between NAA serum levels, and headache frequency, allodynia and interval from the last and the next attack. The low NAA in the serum may be a sign of neuronal dysfunction predisposing to migraine, probably based on reduced mitochondria function.

PK11195 might selectively suppress the quinolinic acid-induced enhancement of anaerobic glycolysis in glial cells

PK11195 was previously reported to attenuate the quinolinic acid (QUIN)-induced enhancement of glucose metabolism in rat brain. In the present study, the effect of PK11195 or anesthesia on [(14)C]2-deoxyglucose ([(14)C]DG) uptake was investigated in order to determine whether the QUIN-induced enhancement of glucose metabolism occurred in glial cells or neurons. We confirmed that the microinjection of QUIN caused a significant enhancement of [(14)C]DG uptake at 2h after the infusion, while the co-injection of PK11195 and QUIN almost completely suppressed this enhancement of [(14)C]DG uptake. No effect of chloral hydrate anesthesia on the QUIN-induced enhancement of [(14)C]DG uptake was observed. In contrast to rats treated with QUIN, PK11195 did not affect the enhancement of [(14)C]DG uptake induced by fluorocitrate (FC); however, chloral hydrate anesthesia completely suppressed the FC-induced increase in [(14)C]DG uptake. These results indicated that the enhancement of glucose metabolism induced by QUIN mainly occurred in glial cells, and the neuroprotective effect of PK11195 in rats injected with QUIN might be related to the suppression of anaerobic glycolysis in glial cells.

New concepts regarding cerebral vasospasm: glial-centric mechanisms

PURPOSE

Poor outcome in patients with cerebral vasospasm following subarachnoid hemorrhage remains a serious clinical problem. The current management with focus on the cerebrovascular constriction accounts for the use of "triple-H" therapy (hypertension, hypervolemia, and hemodilution) to enhance cerebral blood flow through constricted vessels. Recent work suggests that spreading depression (a stereotypical response of cerebral cortical tissue to noxious stimuli with subsequent oligemic blood flow) occurs in patients with cerebral vasospasm. A narrative review was conducted to examine the relationship between spreading depression and subarachnoid hemorrhage and to identify the anesthetic effects on the propagation of spreading depression.

PRINCIPAL FINDINGS

Following review of the literature, an underlying mechanism is advanced that cerebral vasospasm is not primarily a problem of the cerebral vasculature but a consequence of glial cell dysfunction following spreading depression - a glial-centric cause for vasospasm. Such a mechanism for vasospasm becomes manifest when spreading depression waves transition to peri-infarct depolarization waves - with protracted ischemic blood flow in compromised tissue. The extracellular microenvironment with high potassium and low nitric oxide tension can account for conducting vessel narrowing.

CONCLUSIONS

The implication for clinical management is discussed supposing glial cell dysfunction is an underlying mechanism responsible for the vascular spasm.

Emerging role of astroglia in pain hypersensitivity

Recent studies suggest that astroglia, a major non-neuronal cell type in the central nervous system, actively participate in synaptic activity and potentially contribute to neurological disorders including chronic pain. Astroglia exhibit a hyperactive phenotype, also referred to as reactive astrocytosis, in response to peripheral injury. This process is often referred to as astroglial activation. By immunostaining against glial fibrillary acidic proteins, an intermediate cytoskeleton filament protein selectively localized to matured astrocytes, hypertrophy of astrocytes are clearly visible in the spinal cord dorsal horn and spinal trigeminal nucleus following a variety of injuries. Injury-related astroglial activation correlates with behavioral hyperalgesia and conversely, astroglial inhibition attenuates pain hypersensitivity. Astrocytes have a close anatomical relationship with neurons. Interactions between astrocytes and neurons contribute to the mechanisms of chronic pain. Astroglial activation is accompanied by initiation of cellular signal transduction pathways that lead to transcriptional gene regulation and release of a variety of chemical mediators or gliotransmitters, down-regulation of glutamate transporter activity that directly affects synaptic transmission, changes in gap junction proteins by which calcium waves spread, and alterations of the blood brain barrier. These coordinated changes in astroglial functions in turn modulate neuronal activity and facilitate pain transmission.

The induction of HIF-1 reduces astrocyte activation by amyloid beta peptide

Reduced glucose metabolism and astrocyte activation in selective areas of the brain are pathological features of Alzheimer's disease (AD). The underlying mechanisms of low energy metabolism and a molecular basis for preventing astrocyte activation are not, however, known. Here we show that amyloid beta peptide (Abeta)-dependent astrocyte activation leads to a long-term decrease in hypoxia-inducible factor (HIF)-1alpha expression and a reduction in the rate of glycolysis. Glial activation and the glycolytic changes are reversed by the maintenance of HIF-1alpha levels with conditions that prevent the proteolysis of HIF-1alpha. Abeta increases the long-term production of reactive oxygen species (ROS) through the activation of nicotinamide adenine dinucleotide phosphate oxidase and reduces the amount of HIF-1alpha via the activation of the proteasome. ROS are not required for glial activation, but are required for the reduction in glycolysis. These data suggest a significant role for HIF-1alpha-mediated transcription in maintaining the metabolic integrity of the AD brain and identify the probable cause of the observed lower energy metabolism in afflicted areas. They may also explain the therapeutic success of metal chelators in animal models of AD.

Metabolic challenge to glia activates an adenosine-mediated safety mechanism that promotes neuronal survival by delaying the onset of spreading depression waves

In a model of glial-specific chemical anoxia, we have examined how astrocytes influence both synaptic transmission and the viability of hippocampal pyramidal neurons. This relationship was assessed using electrophysiological, pharmacological, and biochemical techniques in rat slices and cell cultures, and oxidative metabolism was selectively impaired in glial cells by exposure to the mitochondrial gliotoxin, fluoroacetate. We found that synaptic transmission was blocked shortly after inducing glial metabolic stress and peri-infarct-like spreading depression (SD) waves developed within 1 to 2 h of treatment. Neuronal electrogenesis was not affected until SD waves developed, thereafter decaying irreversibly. The blockage of synaptic transmission was totally reversed by A(1) adenosine receptor antagonists, unlike the development of SD waves, which appeared earlier under these conditions. Such blockage led to a marked reduction in the electrical viability of pyramidal neurons 1 h after gliotoxin treatment. Cell culture experiments confirmed that astrocytes indeed release adenosine. We interpret this early glial response as a novel safety mechanism that allocates metabolic resources to vital processes when the glia itself sense an energy shortage, thereby delaying or preventing entry into massive lethal ischemic-like depolarization. The implication of these results on the functional recovery of the penumbra regions after ischemic insults is discussed.

Imaging the impact of cortical microcirculation on synaptic structure and sensory-evoked hemodynamic responses in vivo

In vivo two-photon microscopy was used to image in real time dendrites and their spines in a mouse photothrombotic stroke model that reduced somatosensory cortex blood flow in discrete regions of cortical functional maps. This approach allowed us to define relationships between blood flow, cortical structure, and function on scales not previously achieved with macroscopic imaging techniques. Acute ischemic damage to dendrites was triggered within 30 min when blood flow over >0.2 mm² of cortical surface was blocked. Rapid damage was not attributed to a subset of clotted or even leaking vessels (extravasation) alone. Assessment of stroke borders revealed a remarkably sharp transition between intact and damaged synaptic circuitry that occurred over tens of μm and was defined by a transition between flowing and blocked vessels. Although dendritic spines were normally ~13 μm from small flowing vessels, we show that intact dendritic structure can be maintained (in areas without flowing vessels) by blood flow from vessels that are on average 80 μm away. Functional imaging of intrinsic optical signals associated with activity-evoked hemodynamic responses in somatosensory cortex indicated that sensory-induced changes in signal were blocked in areas with damaged dendrites, but were present ~400 μm away from the border of dendritic damage. These results define the range of influence that blood flow can have on local cortical fine structure and function, as well as to demonstrate that peri-infarct tissues can be functional within the first few hours after stroke and well positioned to aid in poststroke recovery.

The brain is critically dependent on an adequate supply of energy as it consumes up to 20% of the oxygen we breathe. Here we determine the distance scale over which interruptions in blood flow affect synaptic hard wiring and brain function. High-resolution microscopy of live mice was used to image cerebral cortex synapses (the sites of connections between neurons) in real time during targeted interruptions of cortical blood flow that model small survivable strokes. Under normal conditions, synapses were tightly coupled to small brain blood vessels, on average only 13 μm away. During targeted strokes, we find that normal synaptic structure can be maintained by flowing blood vessels at a much greater distance of 80 μm. In contrast to structure, brain function was more sensitive to interruption in blood flow and was only present 400 μm from the border of synaptic structural damage. The identification of intact brain structure in regions lacking function defines brain tissue in which restoration of normal blood flow restores function. Our results define the range of influence that blood flow has on cortical fine structure and function and are important for understanding both the pathology of stroke and how changes in blood flow alter the normal brain.

High-resolution structural and functional imaging of the effects of targeted strokes on individual synapses in somatosensory cortex reveals that blood flow from surrounding intact tissue can aid in the immediate post-stroke recovery.

Exacerbation of excitotoxic neuronal death induced during mitochondrial inhibition in vivo: relation to energy imbalance or ATP depletion?

During the past two decades a close relationship between the energy state of the cell and glutamate neurotoxicity has been suggested. We have previously shown that increasing the extracellular concentration of glutamate does not cause neuronal death unless a deficit in energy metabolism occurs. The mechanisms of glutamate-induced neuronal death have been extensively studied in vitro and it has been associated with a rapid and severe decrease in ATP levels, accompanied with mitochondrial dysfunction. In this study we aimed to investigate the time course of the changes in energy metabolites during glutamate-induced neuronal death, in the presence of a moderate inhibition of mitochondrial metabolism in the rat striatum in vivo. We also aimed to study whether or not, as reported in vitro, changes in ATP levels are related to the extension of neuronal death. Results show that glutamate-induced lesions are exacerbated when rats are previously treated with a subtoxic dose of the mitochondrial toxin 3-nitropropionic acid (3-NP). However, changes in nucleotide levels were similar in rats injected with glutamate alone and in rats injected with glutamate and previously treated with 3-NP. In spite of the presence of an extensive striatal lesion, nucleotide levels were recovered in 3-NP-treated rats 24 h after glutamate injection. Results show that 3-NP pre-treatment induced an imbalance in nucleotide levels that predisposed cells to glutamate toxicity; however it did not influence the bioenergetic changes induced by glutamate alone. Enhancement of glutamate neurotoxicity in 3-NP pre-treated rats is more related to a sustained nucleotide imbalance than just to a rapid decrease in ATP levels.

Role of enteric glia in intestinal physiology: effects of the gliotoxin fluorocitrate on motor and secretory function

The role of enteric glia in gastrointestinal physiology remains largely unexplored. We examined the actions of the gliotoxin fluorocitrate (FC) on intestinal motility, secretion, and inflammation after assessing its efficacy and specificity in vitro. FC (100 microM) caused a significant decrease in the phosphorylation of the glucose analog 2-[N-(7-nitrobenz-2-oxa-1,3-diaz-4-yl)amino]-2-deoxyglucose in enteric glial cultures and a reduction in glial uptake of the fluorescent dipeptide Ala-Lys-7-amino-4-methylcoumarin-3-acetic acid in both the ileum and colon. Dipeptide uptake by resident murine macrophages or guinea pig myenteric neurons was unaffected by FC. Incubation of isolated guinea pig ileal segments with FC caused a specific and significant increase in glial expression of the phosphorylated form of ERK-1/2. Disruption of enteric glial function with FC in mice reduced small intestinal motility in vitro, including a significant decrease in basal tone and the amplitude of contractility in response to electrical field stimulation. Mice treated with 10 or 20 micromol/kg FC twice daily for 7 days demonstrated a concentration-dependent decrease in small intestinal transit. In contrast, no changes in colonic transit or ion transport in vitro were observed. There were no changes in glial or neuronal morphology, any signs of inflammation in the FC-treated mice, or any change in the number of myenteric nitric oxide synthase-expressing neurons. We conclude that FC treatment causes enteric glial dysfunction, without causing intestinal inflammation. Our data suggest that enteric glia are involved in the modulation of enteric neural circuits underlying the regulation of intestinal motility.

Block of neural Kv1.1 potassium channels for neuroinflammatory disease therapy

OBJECTIVE

We asked whether blockade of voltage-gated K+ channel Kv1.1, whose altered axonal localization during myelin insult and remyelination may disturb nerve conduction, treats experimental autoimmune encephalomyelitis (EAE).

METHODS

Electrophysiological, cell proliferation, cytokine secretion, immunohistochemical, clinical, brain magnetic resonance imaging, and spectroscopy studies assessed the effects of a selective blocker of Kv1.1, BgK-F6A, on neurons and immune cells in vitro and on EAE-induced neurological deficits and brain lesions in Lewis rats.

RESULTS

BgK-F6A increased the frequency of miniature excitatory postsynaptic currents in neurons and did not affect T-cell activation. EAE was characterized by ventriculomegaly, decreased apparent diffusion coefficient, and decreased (phosphocreatine + beta-adenosine triphosphate)/inorganic phosphate ratio. Reduced apparent diffusion coefficient and impaired energy metabolism indicate astrocytic edema. Intracerebroventricularly BgK-F6A-treated rats showed attenuated clinical EAE with unexpectedly reduced ventriculomegaly and preserved apparent diffusion coefficient values and (phosphocreatine + beta-adenosine triphosphate)/inorganic phosphate ratio. Thus, under BgK-F6A treatment, brain damage was dramatically reduced and energy metabolism maintained.

INTERPRETATION

Kv1.1 blockade may target neurons and astrocytes, and modulate neuronal activity and neural cell volume, which may partly account for the attenuation of the neurological deficits. We propose that Kv1.1 blockade has a broad therapeutic potential in neuroinflammatory diseases (multiple sclerosis, stroke, and trauma).

Immunomodulatory effect of combination therapy with lovastatin and 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside alleviates neurodegeneration in experimental autoimmune encephalomyelitis

Combination therapy with multiple sclerosis (MS) therapeutics is gaining momentum over monotherapy for improving MS. Lovastatin, an HMG-CoA reductase inhibitor (statin), was immunomodulatory in an experimental autoimmune encephalomyelitis (EAE) model of MS. Lovastatin biases the immune response from Th1 to a protective Th2 response in EAE by a different mechanism than 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside, an immunomodulating agent that activates AMP-activated protein kinase. Here we tested these agents in combination in an EAE model of MS. Suboptimal doses of these drugs in combination were additive in efficacy against the induction of EAE; clinical symptoms were delayed and severity and duration of disease was reduced. In the central nervous system, the cellular infiltration and proinflammatory immune response was decreased while the anti-inflammatory immune response was increased. Combination treatment biased the class of elicited myelin basic protein antibodies from IgG2a to IgG1 and IgG2b, suggesting a shift from Th1 to Th2 response. In addition, combination therapy lessened inflammation-associated neurodegeneration in the central nervous system of EAE animals. These effects were absent in EAE animals treated with either drug alone at the same dose. Thus, our data suggest that agents with different mechanisms of action such as lovastatin and 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside, when used in combination, could improve therapy for central nervous system demyelinating diseases and provide a rationale for testing them in MS patients.

Some aspects of astroglial functions and aluminum implications for neurodegeneration

The present decade had witnessed an unprecedented attention focused on glial cells as a result of their unusual physiological roles that are being unraveled. It is now known that, rather than being a mere supporter of neurons, astroglia are actively involved in their modulation. The aluminum hypothesis seems to have been laid to rest, probably due to contradictory epidemiological reports on it as a causative factor of neurodegenerative diseases. Surprisingly, newer scientific evidences continue to appear and recent findings have implicated astrocytes as the principal target of its toxic action. In view of the likely detrimental effects of the interaction between these two infamous partners in neuroscience on neurons and nervous system, we have reviewed some aspects of glia-neuron interaction and discussed the implications of aluminum-impaired astrocytic functions on neurodegeneration. Because sporadic causes still account for the majority of the neurodegenerative diseases of which Alzheimer's disease is the most prominent, it has been suggested that neurotoxicologists should not relent in screening for the environmental agents, such as aluminum, and that considerable attention should be given to glial cells in view of the likely implications of environmental toxicants on their never-imagined newly reported roles in the central nervous system (CNS).

The effect of thalidomide on spinal cord ischemia/reperfusion injury in a rabbit model

STUDY DESIGN

Randomized study.

OBJECTIVES

To evaluate the effects of thalidomide on spinal cord ischemia/reperfusion injury via reduced TNF-alpha production.

SETTING

Animal experimental laboratory, Clinical Research Institute of Seoul National University Hospital, Seoul, Korea.

METHODS

Spinal cord ischemia was induced in rabbits by occluding the infrarenal aorta. Rabbits in group N did not undergo ischemic insult, but rabbits in groups C (the untreated group), THA, and THB underwent ischemic insult for 15 min. The THA and THB groups received thalidomide (20 mg/kg) intraperitoneally (i.p.) before ischemia, but only the THB group received thalidomide (i.p., 20 mg/kg) after 24 and 48 h of reperfusion. After evaluating neurologic functions at 1.5 h, 3, and 5 days of reperfusion, rabbits were killed for histopathologic examination and Western blot analysis of TNF-alpha.

RESULTS

The THA and THB groups showed significantly less neurologic dysfunction than the C group at 1.5 h, 3, and 5 days of reperfusion. The number of normal spinal motor neurons in ventral gray matter was higher in THA and THB than in C, but no difference was observed between THA and THB. Western blot analysis showed a significantly higher level of TNF-alpha in C than in THA and THB at 1.5 h of reperfusion, but no difference was observed between C, THA, or THB at 3 or 5 days of reperfusion.

CONCLUSION

Thalidomide treatment before ischemic insult reduces early phase ischemia/reperfusion injury of the spinal cord in rabbits.

Toxicology of fluoroacetate: a review, with possible directions for therapy research

Fluoroacetate (FA; CH2FCOOR) is highly toxic towards humans and other mammals through inhibition of the enzyme aconitase in the tricarboxylic acid cycle, caused by 'lethal synthesis' of an isomer of fluorocitrate (FC). FA is found in a range of plant species and their ingestion can cause the death of ruminant animals. Some fluorinated compounds -- used as anticancer agents, narcotic analgesics, pesticides or industrial chemicals -- metabolize to FA as intermediate products. The chemical characteristics of FA and the clinical signs of intoxication warrant the re-evaluation of the toxic danger of FA and renewed efforts in the search for effective therapeutic means. Antidotal therapy for FA intoxication has been aimed at preventing fluorocitrate synthesis and aconitase blockade in mitochondria, and at providing citrate outflow from this organelle. Despite a greatly improved understanding of the biochemical mechanism of FA toxicity, ethanol, if taken immediately after the poisoning, has been the most acceptable antidote for the past six decades. This review deals with the clinical signs and physiological and biochemical mechanisms of FA intoxication to provide an explanation of why, even after decades of investigation, has no effective therapy to FA intoxication been elaborated. An apparent lack of integrated toxicological studies is viewed as a limiter of progress in this regard. Two principal ways of developing effective therapies for FA intoxication are considered. Firstly, competitive inhibition of FA interaction with CoA and of FC interaction with aconitase. Secondly, channeling the alternative metabolic pathways by orienting the fate of citrate via cytosolic aconitase, and by maintaining the flux of reducing equivalents into the TCA cycle via glutamate dehydrogenase.

Repetitive Spreading Depression-Like Events Result in Cell Damage in Juvenile Hippocampal Slice Cultures Maintained in Normoxia

Prolonged seizures, e.g., induced by fever, experienced early in life are considered a precipitating injury for the subsequent development of temporal lobe epilepsy. During in vitro epileptiform activity, spreading depressions (SDs) have often been observed. However, their contribution to changes in the properties of juvenile neuronal tissue is unknown. We therefore used the juvenile hippocampal slice culture preparation (JHSC) maintained in normoxia (20% O(2)-5% CO(2)-75% N(2)) to assess the effect of repetitive SD-like events (SDLEs) on fast field potentials and cell damage. Repetitive SDLEs in the CA1 region could be induced in about two-thirds of the investigated JHSCs (n = 61) by repetitive electrical stimulation with 2-200 pulses. SDLEs were characterized by a transient large negative field potential shift accompanied by intracellular depolarization, ionic redistribution, slow propagation (assessed by intrinsic optical signals) and glutamate receptor antagonist sensitivity. The term "SDLE" was used because evoked fast field potentials were only incompletely suppressed and superimposed discharges occurred. With 20 +/- 1 repetitive SDLEs (interval of 10-15 min, n = 7 JHSCs), the events got longer, their amplitude of the first peak declined, while threshold for induction became reduced. Evoked fast field potentials deteriorated and cell damage (assessed by propidium iodide fluorescence) occurred, predominantly in regions CA1 and CA3. As revealed by measurements of tissue partial oxygen pressure during SDLEs repetitive transient anoxia accompanying SDLE might be critical for the observed cell damage. These results, limited so far to the slice culture preparation, suggest SDs to be harmful events in juvenile neuronal tissue in contrast to what is known about their effect on adult neuronal tissue.