Astroglia and glutamate in physiology and pathology: aspects on glutamate transport, glutamate-induced cell swelling and gap-junction communication

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.

Mechanisms of Microglia Proliferation in a Rat Model of Facial Nerve Anatomy

Although microglia exist as a minor glial cell type in the normal state of the brain, they increase in number in response to various disorders and insults. However, it remains unclear whether microglia proliferate in the affected area, and the mechanism of the proliferation has long attracted the attention of researchers. We analyzed microglial mitosis using a facial nerve transection model in which the blood-brain barrier is left unimpaired when the nerves are axotomized. Our results showed that the levels of macrophage colony-stimulating factor (M-CSF), cFms (the receptor for M-CSF), cyclin A/D, and proliferating cell nuclear antigen (PCNA) were increased in microglia in the axotomized facial nucleus (axotFN). In vitro experiments revealed that M-CSF induced cFms, cyclin A/D, and PCNA in microglia, suggesting that microglia proliferate in response to M-CSF in vivo. In addition, M-CSF caused the activation of c-Jun N-terminal kinase (JNK) and p38, and the specific inhibitors of JNK and p38 arrested the microglial mitosis. JNK and p38 were shown to play roles in the induction of cyclins/PCNA and cFms, respectively. cFms was suggested to be induced through a signaling cascade of p38-mitogen- and stress-activated kinase-1 (MSK1)-cAMP-responsive element binding protein (CREB) and/or p38-activating transcription factor 2 (ATF2). Microglia proliferating in the axotFN are anticipated to serve as neuroprotective cells by supplying neurotrophic factors and/or scavenging excite toxins and reactive oxygen radicals.

Ferroptosis and PPAR-gamma in the limelight of brain tumors and edema

Human malignant brain tumors such as gliomas are devastating due to the induction of cerebral edema and neurodegeneration. A major contributor to glioma-induced neurodegeneration has been identified as glutamate. Glutamate promotes cell growth and proliferation in variety of tumor types. Intriguently, glutamate is also an excitatory neurotransmitter and evokes neuronal cell death at high concentrations. Even though glutamate signaling at the receptor and its downstream effectors has been extensively investigated at the molecular level, there has been little insight into how glutamate enters the tumor microenvironment and impacts on metabolic equilibration until recently. Surprisingly, the 12 transmembrane spanning tranporter xCT (SLC7A11) appeared to be a major player in this process, mediating glutamate secretion and ferroptosis. Also, PPARγ is associated with ferroptosis in neurodegeneration, thereby destroying neurons and causing brain swelling. Although these data are intriguing, tumor-associated edema has so far been quoted as of vasogenic origin. Hence, glutamate and PPARγ biology in the process of glioma-induced brain swelling is conceptually challenging. By inhibiting xCT transporter or AMPA receptors in vivo, brain swelling and peritumoral alterations can be mitigated. This review sheds light on the role of glutamate in brain tumors presenting the conceptual challenge that xCT disruption causes ferroptosis activation in malignant brain tumors. Thus, interfering with glutamate takes center stage in forming the basis of a metabolic equilibration approach.

Connexin 43 Phosphorylation: Implications in Multiple Diseases

Connexin 43 (Cx43) is most widely distributed in mammals, especially in the cardiovascular and nervous systems. Its phosphorylation state has been found to be regulated by the action of more than ten kinases and phosphatases, including mitogen-activated protein kinase/extracellular signaling and regulating kinase signaling. In addition, the phosphorylation status of different phosphorylation sites affects its own synthesis and assembly and the function of the gap junctions (GJs) to varying degrees. The phosphorylation of Cx43 can affect the permeability, electrical conductivity, and gating properties of GJs, thereby having various effects on intercellular communication and affecting physiological or pathological processes in vitro and in vivo. Therefore, clarifying the relationship between Cx43 phosphorylation and specific disease processes will help us better understand the disease. Based on the above clinical and preclinical findings, we present in this review the functional significance of Cx43 phosphorylation in multiple diseases and discuss the potential of Cx43 as a drug target in Cx43-related disease pathophysiology, with an emphasis on the importance of connexin 43 as an emerging therapeutic target in cardiac and neuroprotection.

Transient astrocytic accumulation of fluorescein during spreading depolarizations

Spreading depolarizations (SDs) occur frequently in acute cerebral injuries. They are characterized by a breakdown of transmembrane ion gradients resulting in a reduced extracellular sodium ([Na⁺]_o) and increased extracellular potassium concentration ([K⁺]_o). Elevated [K⁺]_o induces astrocytic swelling, another feature of SD; however, the solutes that drive astrocytic swelling remain incompletely understood. We incidentally found astrocytic accumulation of fluorescein (Fluo) - a low molecular weight anionic dye - during SDs induced by elevated [K⁺]_o. Herein, we aimed to explore the properties of astrocytic Fluo accumulation during SDs, electrical stimulation, [K⁺]_o and glutamate elevation and elucidate underlying mechanisms and its relation to swelling. Experiments were performed in acute neocortical slices from adult male C57Bl6 mice and transgenic mice expressing tdTomato in parvalbumin (PV)-positive neurons. We labeled astrocytes with sulforhodamine-101 (SR-101), measured Fluo kinetics using 2-photon laser scanning microscopy and recorded local field potentials (LFP) to detect SDs. Elevations of [K⁺]_o lead to an increase of the astrocytic Fluo intensity in parallel with astrocytic swelling. Pharmacological inhibitors of sodium‑potassium ATPase (Na/K-ATPase), secondary-active transporters and channels were used to address the underlying mechanisms. Fluo accumulation as well as swelling were only prevented by inhibition of the sodium‑potassium ATPase. Application of glutamate or hypoosmolar solution induced astrocytic swelling independent of Fluo accumulation and glutamate opposed Fluo accumulation when co-administered with high [K⁺]_o. Astrocytes accumulated Fluo and swelled during electrical stimulation and even more during SDs. Taken together, Fluo imaging can be used as a tool to visualize yet unidentified anion fluxes during [K⁺]_o- but not glutamate- or hypoosmolarity induced astrocytic swelling. Fluo imaging may thereby help to elucidate mechanisms of astrocytic swelling and associated fluid movements between brain compartments during physiological and pathological conditions, e.g. SDs.

Astrocyte reactivity in the glia limitans superficialis of the rat medial prefrontal cortex following sciatic nerve injury

The glia limitans superficialis (GLS) on the rodent cortical surface consists of astrocyte bodies intermingled with their cytoplasmic processes. Many studies have observed astrocyte reactivity in the medial prefrontal cortex (mPFC) parenchyma induced by a peripheral nerve injury, while the response of GLS astrocytes is still not fully understood. The aim of our study was to identify the reactivity of rat GLS astrocytes in response to sciatic nerve compression (SNC) over different time periods. The alteration of GLS astrocyte reactivity was monitored using immunofluorescence (IF) intensities of glial fibrillary acidic protein (GFAP), glutamine synthetase (GS), and NFκBp65. Our results demonstrated that SNC induced GLS astrocyte reactivity seen as increased intensities of GFAP-IF, and longer extensions of cytoplasmic processes into lamina I. First significant increase of GFAP-IF was observed on post-operation day 7 (POD7) after SNC with further increases on POD14 and POD21. In contrast, dynamic alteration of the extension of cytoplasmic processes into lamina I was detected as early as POD1 and continued throughout the monitored survival periods of both sham and SNC operations. The reactivity of GLS astrocytes was not associated with their proliferation. In addition, GLS astrocytes also displayed a significant decrease in GS immunofluorescence (GS-IF) and NFκB immunofluorescence (NFκB-IF) in response to sham and SNC operation compared with naïve control rats. These results suggest that damaged peripheral tissues (following sham operation as well as peripheral nerve lesions) may induce significant changes in GLS astrocyte reactivity. The signaling mechanism from injured peripheral tissue and nerve remains to be elucidated.

The barrier and interface mechanisms of the brain barrier, and brain drug delivery

Three different barriers are formed between the cerebrovascular and the brain parenchyma: the blood-brain barrier (BBB), the blood-cerebrospinal fluid barrier (BCSFB), and the cerebrospinal fluid-brain barrier (CBB). The BBB is the main regulator of blood and central nervous system (CNS) material exchange. The semipermeable nature of the BBB limits the passage of larger molecules and hydrophilic small molecules, Food and Drug Administration (FDA)-approved drugs for the CNS have been generally limited to lipid-soluble small molecules. Although the complexity of the BBB affects CNS drug delivery, understanding the composition and function of the BBB can provide a platform for the development of new methods for CNS drug delivery. This review summarizes the classification of the brain barrier, the composition and role of the basic structures of the BBB, and the transport, barrier, and destruction mechanisms of the BBB; discusses the advantages and disadvantages of different drug delivery methods and prospects for future drug delivery strategies.

Events Occurring in the Axotomized Facial Nucleus

Transection of the rat facial nerve leads to a variety of alterations not only in motoneurons, but also in glial cells and inhibitory neurons in the ipsilateral facial nucleus. In injured motoneurons, the levels of energy metabolism-related molecules are elevated, while those of neurofunction-related molecules are decreased. In tandem with these motoneuron changes, microglia are activated and start to proliferate around injured motoneurons, and astrocytes become activated for a long period without mitosis. Inhibitory GABAergic neurons reduce the levels of neurofunction-related molecules. These facts indicate that injured motoneurons somehow closely interact with glial cells and inhibitory neurons. At the same time, these events allow us to predict the occurrence of tissue remodeling in the axotomized facial nucleus. This review summarizes the events occurring in the axotomized facial nucleus and the cellular and molecular mechanisms associated with each event.

Impedance-Based Phenotypic Readout of Transporter Function: A Case for Glutamate Transporters

Excitatory amino acid transporters (EAAT/SLC1) mediate Na⁺-dependent uptake of extracellular glutamate and are potential drug targets for neurological disorders. Conventional methods to assess glutamate transport in vitro are based on radiolabels, fluorescent dyes or electrophysiology, which potentially compromise the cell’s physiology and are generally less suited for primary drug screens. Here, we describe a novel label-free method to assess human EAAT function in living cells, i.e., without the use of chemical modifications to the substrate or cellular environment. In adherent HEK293 cells overexpressing EAAT1, stimulation with glutamate or aspartate induced cell spreading, which was detected in real-time using an impedance-based biosensor. This change in cell morphology was prevented in the presence of the Na⁺/K⁺-ATPase inhibitor ouabain and EAAT inhibitors, which suggests the substrate-induced response was ion-dependent and transporter-specific. A mechanistic explanation for the phenotypic response was substantiated by actin cytoskeleton remodeling and changes in the intracellular levels of the osmolyte taurine, which suggests that the response involves cell swelling. In addition, substrate-induced cellular responses were observed for cells expressing other EAAT subtypes, as well as in a breast cancer cell line (MDA-MB-468) with endogenous EAAT1 expression. These findings allowed the development of a label-free high-throughput screening assay, which could be beneficial in early drug discovery for EAATs and holds potential for the study of other transport proteins that modulate cell shape.

Emerging therapeutic targets for cerebral edema

INTRODUCTION

Cerebral edema is a key contributor to death and disability in several forms of brain injury. Current treatment options are limited, reactive, and associated with significant morbidity. Targeted therapies are emerging based on a growing understanding of the molecular underpinnings of cerebral edema.

AREAS COVERED

We review the pathophysiology and relationships between different cerebral edema subtypes to provide a foundation for emerging therapies. Mechanisms for promising molecular targets are discussed, with an emphasis on those advancing in clinical trials, including ion and water channels (AQP4, SUR1-TRPM4) and other proteins/lipids involved in edema signaling pathways (AVP, COX2, VEGF, and S1P). Research on novel treatment modalities for cerebral edema [including recombinant proteins and gene therapies] is presented and finally, insights on reducing secondary injury and improving clinical outcome are offered.

EXPERT OPINION

Targeted molecular strategies to minimize or prevent cerebral edema are promising. Inhibition of SUR1-TRPM4 (glyburide/glibenclamide) and VEGF (bevacizumab) are currently closest to translation based on advances in clinical trials. However, the latter, tested in glioblastoma multiforme, has not demonstrated survival benefit. Research on recombinant proteins and gene therapies for cerebral edema is in its infancy, but early results are encouraging. These newer modalities may facilitate our understanding of the pathobiology underlying cerebral edema.

SGK1 inhibition in glia ameliorates pathologies and symptoms in Parkinson disease animal models

Astrocytes and microglia are brain-resident glia that can establish harmful inflammatory environments in disease contexts and thereby contribute to the progression of neuronal loss in neurodegenerative disorders. Correcting the diseased properties of glia is therefore an appealing strategy for treating brain diseases. Previous studies have shown that serum/ glucocorticoid related kinase 1 (SGK1) is upregulated in the brains of patients with various neurodegenerative disorders, suggesting its involvement in the pathogenesis of those diseases. In this study, we show that inhibiting glial SGK1 corrects the pro-inflammatory properties of glia by suppressing the intracellular NFκB-, NLRP3-inflammasome-, and CGAS-STING-mediated inflammatory pathways. Furthermore, SGK1 inhibition potentiated glial activity to scavenge glutamate toxicity and prevented glial cell senescence and mitochondrial damage, which have recently been reported as critical pathologic features of and therapeutic targets in Parkinson disease (PD) and Alzheimer disease (AD). Along with those anti-inflammatory/neurotrophic functions, silencing and pharmacological inhibition of SGK1 protected midbrain dopamine neurons from degeneration and cured pathologic synuclein alpha (SNCA) aggregation and PD-associated behavioral deficits in multiple in vitro and in vivo PD models. Collectively, these findings suggest that SGK1 inhibition could be a useful strategy for treating PD and other neurodegenerative disorders that share the common pathology of glia-mediated neuroinflammation.

Astrocytic Connexin43 Channels as Candidate Targets in Epilepsy Treatment

In epilepsy research, emphasis is put on exploring non-neuronal targets such as astrocytic proteins, since many patients remain pharmacoresistant to current treatments, which almost all target neuronal mechanisms. This paper reviews available data on astrocytic connexin43 (Cx43) signaling in seizures and epilepsy. Cx43 is a widely expressed transmembrane protein and the constituent of gap junctions (GJs) and hemichannels (HCs), allowing intercellular and extracellular communication, respectively. A plethora of research papers show altered Cx43 mRNA levels, protein expression, phosphorylation state, distribution and/or functional coupling in human epileptic tissue and experimental models. Human Cx43 mutations are linked to seizures as well, as 30% of patients with oculodentodigital dysplasia (ODDD), a rare genetic condition caused by mutations in the GJA1 gene coding for Cx43 protein, exhibit neurological symptoms including seizures. Cx30/Cx43 double knock-out mice show increased susceptibility to evoked epileptiform events in brain slices due to impaired GJ-mediated redistribution of K⁺ and glutamate and display a higher frequency of spontaneous generalized chronic seizures in an epilepsy model. Contradictory, Cx30/Cx43 GJs can traffic nutrients to high-energy demanding neurons and initiate astrocytic Ca²⁺ waves and hyper synchronization, thereby supporting proconvulsant effects. The general connexin channel blocker carbenoxolone and blockers from the fenamate family diminish epileptiform activity in vitro and improve seizure outcome in vivo. In addition, interventions with more selective peptide inhibitors of HCs display anticonvulsant actions. To conclude, further studies aiming to disentangle distinct roles of HCs and GJs are necessary and tools specifically targeting Cx43 HCs may facilitate the search for novel epilepsy treatments.

Emerging Pharmacological Treatments for Cerebral Edema: Evidence from Clinical Studies

Cerebral edema, a common and often fatal companion to most forms of acute central nervous system disease, has been recognized since the time of ancient Egypt. Unfortunately, our therapeutic armamentarium remains limited, in part due to historic limitations in our understanding of cerebral edema pathophysiology. Recent advancements have led to a number of clinical trials for novel therapeutics that could fundamentally alter the treatment of cerebral edema. In this review, we discuss these agents, their targets, and the data supporting their use, with a focus on agents that have progressed to clinical trials.

The Multifaceted Role of Astrocyte Connexin 43 in Ischemic Stroke Through Forming Hemichannels and Gap Junctions

Ischemic stroke is a multi-factorial cerebrovascular disease with high worldwide morbidity and mortality. In the past few years, multiple studies have revealed the underlying mechanism of ischemia/reperfusion injury, including calcium overload, amino acid toxicity, oxidative stress, and inflammation. Connexin 43 (Cx43), the predominant connexin protein in astrocytes, has been recently proven to display non-substitutable roles in the pathology of ischemic stroke development and progression through forming gap junctions and hemichannels. Under normal conditions, astrocytic Cx43 could be found in hemichannels or in the coupling with other hemichannels on astrocytes, neurons, or oligodendrocytes to form the neuro-glial syncytium, which is involved in metabolites exchange between communicated cells, thus maintaining the homeostasis of the CNS environment. In ischemic stroke, the phosphorylation of Cx43 might cause the degradation of gap junctions and the opening of hemichannels, contributing to the release of inflammatory mediators. However, the remaining gap junctions could facilitate the exchange of protective and harmful metabolites between healthy and injured cells, protecting the injured cells to some extent or damaging the healthy cells depending on the balance of the exchange of protective and harmful metabolites. In this study, we review the changes in astrocytic Cx43 expression and distribution as well as the influence of these changes on the function of astrocytes and other cells in the CNS, providing new insight into the pathology of ischemic stroke injury; we also discuss the potential of astrocytic Cx43 as a target for the treatment of ischemic stroke.

Regulatory Roles of Metabotropic Glutamate Receptors on Synaptic Communication Mediated by Gap Junctions

Variations of synaptic strength are thought to underlie forms of learning and can functionally reshape neural circuits. Metabotropic glutamate receptors play key roles in regulating the strength of chemical synapses. However, information within neural circuits is also conveyed via a second modality of transmission: gap junction-mediated synapses. We review here evidence indicating that metabotropic glutamate receptors also play important roles in the regulation of synaptic communication mediated by neuronal gap junctions, also known as 'electrical synapses'. Activity-driven interactions between metabotropic glutamate receptors and neuronal gap junctions can lead to long-term changes in the strength of electrical synapses. Further, the regulatory action of metabotropic glutamate receptors on neuronal gap junctions is not restricted to adulthood but is also of critical relevance during brain development and contributes to the pathological mechanisms that follow brain injury.

Danegaptide Enhances Astrocyte Gap Junctional Coupling and Reduces Ischemic Reperfusion Brain Injury in Mice

Ischemic stroke is a complex and devastating event characterized by cell death resulting from a transient or permanent arterial occlusion. Astrocytic connexin43 (Cx43) gap junction (GJ) proteins have been reported to impact neuronal survival in ischemic conditions. Consequently, Cx43 could be a potential target for therapeutic approaches to stroke. We examined the effect of danegaptide (ZP1609), an antiarrhythmic dipeptide that specifically enhances GJ conductance, in two different rodent stroke models. In this study, danegaptide increased astrocytic Cx43 coupling with no significant effects on Cx43 hemichannel activity, in vitro. Using matrix-assisted laser desorption ionization imaging mass spectrometry (MALDI IMS) the presence of danegaptide within brain tissue sections were detected one hour after reperfusion indicating successful transport of the dipeptide across the blood brain barrier. Furthermore, administration of danegaptide in a novel mouse brain ischemia/reperfusion model showed significant decrease in infarct volume. Taken together, this study provides evidence for the therapeutic potential of danegaptide in ischemia/reperfusion stroke.

Acute sensitivity of astrocytes in the Substantia Nigra to oxygen and glucose deprivation (OGD) compared with hippocampal astrocytes in brain slices

The Substantia Nigra is a brainstem nucleus critical for movement control. Although its dopamine-producing neurons degenerate in Parkinsons disease, little is known of the acute effects of ischemia in this region. We recently reported that oxygen and glucose deprivation (OGD) in brain slices, an in vitro ischemia model, evokes a profound depolarization and swelling of GABAergic neurons in the Substantia Nigra pars reticulata (SNr), but not dopaminergic neurons in the Substantia Nigra pars compacta (SNc). The current study characterised the effects of OGD on nigral astrocytes, and compared these with the established responses of astrocytes in the CA1 hippocampal region. Intracellular recordings were made from astrocytes at the border between SNc and SNr subregions, in midbrain slices from postnatal day 21-23 rats. Immunoreactivity for astrocyte-specific proteins was also assessed. OGD evoked a slow, then fast depolarization of nigral astrocytes. The fast phase developed during the anoxic depolarization (indicated by a fast negative shift of extracellular DC potential and increase in light transmittance) and rapid increase in extracellular K⁺ concentration in the SNr. This biphasic response resembled the OGD-evoked depolarization of hippocampal astrocytes. However, unlike the partial repolarization seen in hippocampal cells after reperfusion with O₂ and glucose, nigral astrocytes remained depolarized near 0 mV. In addition, immunoreactivity for glial fibrillary acidic protein-positive astrocytes markedly decreased in the Substantia Nigra after OGD, while in the hippocampus remained unchanged. These data indicate an acute post-ischemic withdrawal of astrocytic support in the Substantia Nigra, but not in the hippocampus.

Cografting astrocytes improves cell therapeutic outcomes in a Parkinson's disease model

Transplantation of neural progenitor cells (NPCs) is a potential therapy for treating neurodegenerative disorders, but this approach has faced many challenges and limited success, primarily because of inhospitable host brain environments that interfere with enriched neuron engraftment and function. Astrocytes play neurotrophic roles in the developing and adult brain, making them potential candidates for helping with modification of hostile brain environments. In this study, we examined whether astrocytic function could be utilized to overcome the current limitations of cell-based therapies in a murine model of Parkinson's disease (PD) that is characterized by dopamine (DA) neuron degeneration in the midbrain. We show here that cografting astrocytes, especially those derived from the midbrain, remarkably enhanced NPC-based cell therapeutic outcomes along with robust DA neuron engraftment in PD rats for at least 6 months after transplantation. We further show that engineering of donor astrocytes with Nurr1 and Foxa2, transcription factors that were recently reported to polarize harmful immunogenic glia into the neuroprotective form, further promoted the neurotrophic actions of grafted astrocytes in the cell therapeutic approach. Collectively, these findings suggest that cografting astrocytes could be a potential strategy for successful cell therapeutic outcomes in neurodegenerative disorders.

Microglia and astrocyte activation in the spinal cord of lame horses

OBJECTIVE

To determine the microglial and astrocyte response to painful lameness in horses.

STUDY DESIGN

Ionized calcium binding adaptor molecule 1 (Iba-1) and glial fibrillary acidic protein (GFAP) expression, cell density and morphology were determined through immunofluorescence within the dorsal horn of equine spinal cord.

ANIMALS

A total of five adult horses with acute or chronic unilateral lameness, previously scheduled for euthanasia.

METHODS

Musculoskeletal lameness was evaluated in five horses through visual evaluation according to clinical guidelines. Spinal cord samples were obtained immediately after euthanasia, and distal limb lesions were confirmed through dissection and radiography. Iba-1 immunostaining was used for detection and characterization of dorsal horn microglia. GFAP was used for immunostaining of dorsal horn astrocytes. Iba-1 and GFAP labeled cells were quantified in the dorsal horn, and intensity of fluorescence was compared between the ipsi- and contralateral dorsal horn to the affected limb, and between dorsal horn segments of all horses.

RESULTS

Iba-1 expression was higher in the ipsilateral dorsal horn of the affected limb in contrast to the contralateral side dorsal horn. GFAP markers did not demonstrate increased astrocytic activity on the dorsal horn ipsilateral side to the distal limb lesion of affected horses. Horses with acute lameness predominantly had a spherical shape microglial phenotype, while cells from chronic lameness cases had variable morphology. Astrocytes evidenced small somas and large processes in both acute and chronic lameness, with higher GFAP localization in the main branches. As in the case of rodents, the localization of microglia and astrocytes in horses was mainly situated within laminae I, II and III.

CONCLUSIONS AND CLINICAL RELEVANCE

Iba-1 and GFAP are functional and morphological markers of spinal microglial cells and astrocytes in horses with lameness.

Decreased Motor Neuron Support by SMA Astrocytes due to Diminished MCP1 Secretion

Spinal muscular atrophy (SMA) is an autosomal-recessive disorder characterized by severe, often fatal muscle weakness due to loss of motor neurons. SMA patients have deletions and other mutations of the survival of motor neuron 1 (SMN1) gene, resulting in decreased SMN protein. Astrocytes are the primary support cells of the CNS and are responsible for glutamate clearance, metabolic support, response to injury, and regulation of signal transmission. Astrocytes have been implicated in SMA as in in other neurodegenerative disorders. Astrocyte-specific rescue of SMN protein levels has been shown to mitigate disease manifestations in mice. However, the mechanism by which SMN deficiency in astrocytes may contribute to SMA is unclear and what aspect of astrocyte activity is lacking is unknown. Therefore, it is worthwhile to identify defects in SMN-deficient astrocytes that compromise normal function. We show here that SMA astrocyte cultures derived from mouse spinal cord of both sexes are deficient in supporting both WT and SMN-deficient motor neurons derived from male, female, and mixed-sex sources and that this deficiency may be mitigated with secreted factors. In particular, SMN-deficient astrocytes have decreased levels of monocyte chemoactive protein 1 (MCP1) secretion compared with controls and MCP1 restoration stimulates outgrowth of neurites from cultured motor neurons. Correction of MCP1 deficiency may thus be a new therapeutic approach to SMA.SIGNIFICANCE STATEMENT Spinal muscular atrophy (SMA) is caused by the loss of motor neurons, but astrocyte dysfunction also contributes to the disease in mouse models. Monocyte chemoactive protein 1 (MCP1) has been shown to be neuroprotective and is released by astrocytes. Here, we report that MCP1 levels are decreased in SMA mice and that replacement of deficient MCP1 increases differentiation and neurite length of WT and SMN-deficient motor-neuron-like cells in cell culture. This study reveals a novel aspect of astrocyte dysfunction in SMA and indicates a possible approach for improving motor neuron growth and survival in this disease.

A Model of Glial Scarring Analogous to the Environment of a Traumatically Injured Spinal Cord Using Kainate

OBJECTIVE

To develop an in vitro model analogous to the environment of traumatic spinal cord injury (SCI), the authors evaluated change of astrogliosis following treatments with kainate and/or scratch, and degree of neurite outgrowth after treatment with a kainate inhibitor.

METHODS

Astrocytes were obtained from the rat spinal cord. Then, 99% of the cells were confirmed to be GFAP-positive astrocytes. For chemical injury, the cells were treated with kainate at different concentrations (10, 50 or 100 µM). For mechanical injury, two kinds of uniform scratches were made using a plastic pipette tip by removing strips of cells. For combined injury (S/K), scratch and kainate were provided. Cord neurons from rat embryos were plated onto culture plates immediately after the three kinds of injuries and some cultures were treated with a kainate inhibitor.

RESULTS

Astro-gliosis (glial fibrillary acidic protein [GFAP], vimentin, chondroitin sulfate proteoglycan [CSPG], rho-associated protein kinase [ROCK], and ephrin type-A receptor 4 [EphA4]) was most prominent after treatment with 50 µM kainate and extensive scratch injury in terms of single arm (p<0.001) and in the S/K-induced injury model in view of single or combination (p<0.001). Neurite outgrowth in the seeded spinal cord (β-III tubulin) was the least in the S/K-induced injury model (p<0.001) and this inhibition was reversed by the kainate inhibitor (p<0.001).

CONCLUSION

The current in vitro model combining scratch and kainate induced glial scarring and inhibitory molecules and restricted neurite outgrowth very strongly than either the mechanically or chemically-induced injury model; hence, it may be a useful tool for research on SCI.

Disentangling the Role of Astrocytes in Alcohol Use Disorder

Several laboratories recently identified that astrocytes are critical regulators of addiction machinery. It is now known that astrocyte pathology is a common feature of ethanol (EtOH) exposure in both humans and animal models, as even brief EtOH exposure is sufficient to elicit long-lasting perturbations in astrocyte gene expression, activity, and proliferation. Astrocytes were also recently shown to modulate the motivational properties of EtOH and other strongly reinforcing stimuli. Given the role of astrocytes in regulating glutamate homeostasis, a crucial component of alcohol use disorder (AUD), astrocytes might be an important target for the development of next-generation alcoholism treatments. This review will outline some of the more prominent features displayed by astrocytes, how these properties are influenced by acute and long-term EtOH exposure, and future directions that may help to disentangle astrocytic from neuronal functions in the etiology of AUD.

Central Role of Maladapted Astrocytic Plasticity in Ischemic Brain Edema Formation

Brain edema formation and the ensuing brain damages are the major cause of high mortality and long term disability following the occurrence of ischemic stroke. In this process, oxygen and glucose deprivation and the resulting reperfusion injury play primary roles. In response to the ischemic insult, the neurovascular unit experiences both intracellular and extracellular edemas, associated with maladapted astrocytic plasticity. The astrocytic plasticity includes both morphological and functional plasticity. The former involves a reactive gliosis and the subsequent glial retraction. It relates to the capacity of astrocytes to buffer changes in extracellular chemical levels, particularly K⁺ and glutamate, as well as the integrity of the blood-brain barrier (BBB). The latter involves the expression and activity of a series of ion and water transport proteins. These molecules are grouped together around glial fibrillary acidic protein (GFAP) and water channel protein aquaporin 4 (AQP4) to form functional networks, regulate hydromineral balance across cell membranes and maintain the integrity of the BBB. Intense ischemic challenges can disrupt these capacities of astrocytes and result in their maladaptation. The maladapted astrocytic plasticity in ischemic stroke cannot only disrupt the hydromineral homeostasis across astrocyte membrane and the BBB, but also leads to disorders of the whole neurovascular unit. This review focuses on how the maladapted astrocytic plasticity in ischemic stroke plays the central role in the brain edema formation.

Vasopressin Hypersecretion-Associated Brain Edema Formation in Ischemic Stroke: Underlying Mechanisms

BACKGROUND

Brain edema formation is a major cause of brain damages and the high mortality of ischemic stroke. The aim of this review is to explore the relationship between ischemic brain edema formation and vasopressin (VP) hypersecretion in addition to the oxygen and glucose deprivation and the ensuing reperfusion injury.

METHODS

Pertinent studies involving ischemic stroke, brain edema formation, astrocytes, and VP were identified by a search of the PubMed and the Web of Science databases in January 2016. Based on clinical findings and reports of animal experiments using ischemic stroke models, this systematic review reanalyzes the implication of individual reports in the edema formation and then establishes the inherent links among them.

RESULTS

This systematic review reveals that cytotoxic edema and vasogenic brain edema in classical view are mainly under the influence of a continuous malfunction of astrocytic plasticity. Adaptive VP secretion can modulate membrane ion transport, water permeability, and blood-brain barrier integrity, which are largely via changing astrocytic plasticity. Maladaptive VP hypersecretion leads to disruptions of ion and water balance across cell membranes as well as the integrity of the blood-brain barrier. This review highlights our current understandings of the cellular mechanisms underlying ischemic brain edema formation and its association with VP hypersecretion.

CONCLUSIONS

VP hypersecretion promotes brain edema formation in ischemic stroke by disrupting hydromineral balance in the neurovascular unit; suppressing VP hypersecretion has the potential to alleviate ischemic brain edema.

Molecular pathophysiology of cerebral edema

Advancements in molecular biology have led to a greater understanding of the individual proteins responsible for generating cerebral edema. In large part, the study of cerebral edema is the study of maladaptive ion transport. Following acute CNS injury, cells of the neurovascular unit, particularly brain endothelial cells and astrocytes, undergo a program of pre- and post-transcriptional changes in the activity of ion channels and transporters. These changes can result in maladaptive ion transport and the generation of abnormal osmotic forces that, ultimately, manifest as cerebral edema. This review discusses past models and current knowledge regarding the molecular and cellular pathophysiology of cerebral edema.

Docosahexaenoic acid (DHA) prevents corticosterone-induced changes in astrocyte morphology and function

The many functions of astrocytes, such as glutamate recycling and morphological plasticity, enable them to stabilize synapses environment and protect neurons. Little is known about how they adapt to glucocorticoid-induced stress, and even less about the influence of dietary factors. We previously showed that omega-3 polyunsaturated fatty acids (ω3PUFA), dietary fats which alleviate stress responses, influence the way astroglia regulate glutamatergic synapses. We have explored the role of docosahexaenoic acid (DHA), the main ω3PUFA, in the astroglial responses to corticosterone, the main stress hormone in rodents to determine whether ω3PUFA help astrocytes resist stress. Cultured rat astrocytes were enriched in DHA or arachidonic acid (AA, the main ω6PUFA) and given 100 nM corticosterone for several days. Corticosterone stimulated astrocyte glutamate recycling by increasing glutamate uptake and glutamine synthetase (GS), and altered the astrocyte cytoskeleton. DHA-enriched astrocytes no longer responded to the action of corticosterone on glutamate uptake, had decreased GS, and the cytoskeletal effect of corticosterone was delayed, while AA-enriched cells were unaffected. The DHA-dependent anti-corticosterone effect was related to fewer glucocorticoid receptors, while corticosterone increased DHA incorporation into astrocyte membranes. Thus, DHA helps astrocytes resist the influence of corticosterone, so perhaps promoting a sustainable response by the stressed brain. We show that corticosterone increases the glutamate recycling capacity of rat cortical astrocytes in culture, and alters their morphology, which may be detrimental in the long term. Increasing the membrane incorporation of docosahexaenoic acid (DHA), the main omega-3 in brain, reduces the amount of glucocorticoid receptors (GR) and prevents the effects of corticosterone. This may help the astrocytes maintain a functional phenotype in chronic stress situations.

Mechanisms of long-term cognitive dysfunction of sepsis: from blood-borne leukocytes to glial cells

Several mechanisms are associated with brain dysfunction during sepsis; one of the most important are activation of microglia and astrocytes. Activation of glial cells induces changes in permeability of the blood-brain barrier, secretion of inflammatory cytokines, and these alterations could induce neuronal dysfunction. Furthermore, blood-borne leukocytes can also reach the brain and participate in inflammatory response. Mechanisms involved in sepsis-associated brain dysfunction were revised here, focusing in neuroinflammation and involvement of blood-borne leukocytes and glial cells in this process.

Activity-Dependent Plasticity of Astroglial Potassium and Glutamate Clearance

Recent evidence has shown that astrocytes play essential roles in synaptic transmission and plasticity. Nevertheless, how neuronal activity alters astroglial functional properties and whether such properties also display specific forms of plasticity still remain elusive. Here, we review research findings supporting this aspect of astrocytes, focusing on their roles in the clearance of extracellular potassium and glutamate, two neuroactive substances promptly released during excitatory synaptic transmission. Their subsequent removal, which is primarily carried out by glial potassium channels and glutamate transporters, is essential for proper functioning of the brain. Similar to neurons, different forms of short- and long-term plasticity in astroglial uptake have been reported. In addition, we also present novel findings showing robust potentiation of astrocytic inward currents in response to repetitive stimulations at mild frequencies, as low as 0.75 Hz, in acute hippocampal slices. Interestingly, neurotransmission was hardly affected at this frequency range, suggesting that astrocytes may be more sensitive to low frequency stimulation and may exhibit stronger plasticity than neurons to prevent hyperexcitability. Taken together, these important findings strongly indicate that astrocytes display both short- and long-term plasticity in their clearance of excess neuroactive substances from the extracellular space, thereby regulating neuronal activity and brain homeostasis.

α7 Nicotinic Receptor Promotes the Neuroprotective Functions of Astrocytes against Oxaliplatin Neurotoxicity

Neuropathies are characterized by a complex response of the central nervous system to injuries. Glial cells are recruited to maintain neuronal homeostasis but dysregulated activation leads to pain signaling amplification and reduces the glial neuroprotective power. Recently, we highlighted the property of α7 nicotinic-acetylcholine-receptor (nAChR) agonists to relieve pain and induce neuroprotection simultaneously with a strong increase in astrocyte density. Aimed to study the role of α7 nAChR in the neuron-glia cross-talk, we treated primary rat neurons and astrocytes with the neurotoxic anticancer drug oxaliplatin evaluating the effect of the α7 nAChR agonist PNU-282987 (PNU). Oxaliplatin (1 μM, 48 h) reduced cell viability and increased caspase-3 activity of neuron monocultures without damaging astrocytes. In cocultures, astrocytes were not able to protect neurons by oxaliplatin even if glial cell metabolism was stimulated (pyruvate increase). On the contrary, the coculture incubation with 10 μM PNU improved neuron viability and inhibited apoptosis. In the absence of astrocytes, the protection disappeared. Furthermore, PNU promoted the release of the anti-inflammatory cytokine TGF-β1 and the expression of the glutamate-detoxifying enzyme glutamine synthetase. The α7 nAChR stimulation protects neurons from oxaliplatin toxicity through an astrocyte-mediated mechanism. α7 nAChR is suggested for recovering the homeostatic role of astrocytes.

Insulin treatment restores glutamate (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor function in the hippocampus of diabetic rats

Type 1 diabetes is associated with cognitive dysfunction. Cognitive processing, particularly memory acquisition, depends on the regulated enhancement of expression and function of glutamate receptor subtypes in the hippocampus. Impairment of memory was been detected in rodent models of type 1 diabetes induced by streptozotocin (STZ). This study examines the functional properties of synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and the expression of synaptic molecules that regulate glutamatergic synaptic transmission in the hippocampus of STZ-diabetic rats. The AMPA receptor-mediated miniature excitatory postsynaptic currents (mEPSCs) and single-channel properties of synaptosomal AMPA receptors were examined after 4 weeks of diabetes induction. Results show that amplitude and frequency of mEPSCs recorded from CA1 pyramidal neurons were decreased in diabetic rats. In addition, the single-channel properties of synaptic AMPA receptors from diabetic rat hippocampi were different from those of controls. These impairments in synaptic currents gated by AMPA receptors were accompanied by decreased protein levels of AMPA receptor subunit GluR1, the presynaptic protein synaptophysin, and the postsynaptic anchor protein postsynaptic density protein 95 in the hippocampus of diabetic rats. Neural cell adhesion molecule (NCAM), an extracellular matrix molecule abundantly expressed in the brain, and the polysialic acid (PSA) attached to NCAM were also downregulated in the hippocampus of diabetic rats. Insulin treatment, when initiated at the onset of diabetes induction, reduced these effects. These findings suggest that STZ-induced diabetes may result in functional deteriorations in glutamatergic synapses in the hippocampus of rats and that these effects may be reduced by insulin treatment.

Microglial regulation of immunological and neuroprotective functions of astroglia

Microglia and astroglia play critical roles in the development, function, and survival of neurons in the CNS. However, under inflammatory conditions the role of astrogliosis in the inflammatory process and its effects on neurons remains unclear. Here, we used several types of cell cultures treated with the bacterial inflammogen LPS to address these questions. We found that the presence of astroglia reduced inflammation-driven neurotoxicity, suggesting that astrogliosis is principally neuroprotective. Neutralization of supernatant glial cell line-derived neurotrophic factor (GDNF) released from astroglia significantly reduced this neuroprotective effect during inflammation. To determine the immunological role of astroglia, we optimized a highly-enriched astroglial culture protocol and demonstrated that LPS failed to induce the synthesis and release of TNF-α and iNOS/NO. Instead we found significant enhancement of TNF-α and iNOS expression in highly-enriched astroglial cultures required the presence of 0.5-1% microglia, respectively. Thus suggesting that microglial-astroglial interactions are required for LPS to induce the expression of pro-inflammatory factors and GDNF from astroglia. Specifically, we found that microglia-derived TNF-α plays a pivotal role as a paracrine signal to regulate the neuroprotective functions of astrogliosis. Taken together, these findings suggest that astroglia may not possess the ability to directly recognize the innate immune stimuli LPS, but rather depend on crosstalk with microglia to elicit release of neurotrophic factors as a counterbalance to support neuronal survival from the collateral damage generated by activated microglia during neuroinflammation.

Disruption of ion homeostasis in the neurogliovascular unit underlies the pathogenesis of ischemic cerebral edema

Cerebral edema is a major cause of morbidity and mortality following ischemic stroke, but its underlying molecular pathophysiology is incompletely understood. Recent data have revealed the importance of ion flux via channels and transporters expressed in the neurogliovascular unit in the development of ischemia-triggered cytotoxic edema, vasogenic edema, and hemorrhagic conversion. Disruption of homeostatic mechanisms governing cell volume regulation and epithelial/endothelial ion transport due to ischemia-associated energy failure results in the thermodynamically driven re-equilibration of solutes and water across the CSF-blood and blood-brain barriers that ultimately increases the brain's extravascular volume. Additionally, hypoxia, inflammation, and other stress-triggered increases in the functional expression of ion channels and transporters normally expressed at low levels in the neurogliovascular unit cause disruptions in ion homeostasis that contribute to ischemic cerebral edema. Here, we review the pathophysiological significance of several molecular mediators of ion transport expressed in the neurogliovascular unit, including targets of existing FDA-approved drugs, which might be potential nodes for therapeutic intervention.

The glutamate aspartate transporter (GLAST) mediates L-glutamate-stimulated ascorbate-release via swelling-activated anion channels in cultured neonatal rodent astrocytes

Vitamin C (ascorbate) plays important neuroprotective and neuromodulatory roles in the mammalian brain. Astrocytes are crucially involved in brain ascorbate homeostasis and may assist in regenerating extracellular ascorbate from its oxidised forms. Ascorbate accumulated by astrocytes can be released rapidly by a process that is stimulated by the excitatory amino acid, L-glutamate. This process is thought to be neuroprotective against excitotoxicity. Although of potential clinical interest, the mechanism of this stimulated ascorbate-release remains unknown. Here, we report that primary cultures of mouse and rat astrocytes release ascorbate following initial uptake of dehydroascorbate and accumulation of intracellular ascorbate. Ascorbate-release was not due to cellular lysis, as assessed by cellular release of the cytosolic enzyme lactate dehydrogenase, and was stimulated by L-glutamate and L-aspartate, but not the non-excitatory amino acid L-glutamine. This stimulation was due to glutamate-induced cellular swelling, as it was both attenuated by hypertonic and emulated by hypotonic media. Glutamate-stimulated ascorbate-release was also sensitive to inhibitors of volume-sensitive anion channels, suggesting that the latter may provide the conduit for ascorbate efflux. Glutamate-stimulated ascorbate-release was not recapitulated by selective agonists of either ionotropic or group I metabotropic glutamate receptors, but was completely blocked by either of two compounds, TFB-TBOA and UCPH-101, which non-selectively and selectively inhibit the glial Na(+)-dependent excitatory amino acid transporter, GLAST, respectively. These results suggest that an impairment of astrocytic ascorbate-release may exacerbate neuronal dysfunction in neurodegenerative disorders and acute brain injury in which excitotoxicity and/or GLAST deregulation have been implicated.

Neuronal and astroglial correlates underlying spatiotemporal intrinsic optical signal in the rat hippocampal slice

Widely used for mapping afferent activated brain areas in vivo, the label-free intrinsic optical signal (IOS) is mainly ascribed to blood volume changes subsequent to glial glutamate uptake. By contrast, IOS imaged in vitro is generally attributed to neuronal and glial cell swelling, however the relative contribution of different cell types and molecular players remained largely unknown. We characterized IOS to Schaffer collateral stimulation in the rat hippocampal slice using a 464-element photodiode-array device that enables IOS monitoring at 0.6 ms time-resolution in combination with simultaneous field potential recordings. We used brief half-maximal stimuli by applying a medium intensity 50 Volt-stimulus train within 50 ms (20 Hz). IOS was primarily observed in the str. pyramidale and proximal region of the str. radiatum of the hippocampus. It was eliminated by tetrodotoxin blockade of voltage-gated Na(+) channels and was significantly enhanced by suppressing inhibitory signaling with gamma-aminobutyric acid(A) receptor antagonist picrotoxin. We found that IOS was predominantly initiated by postsynaptic Glu receptor activation and progressed by the activation of astroglial Glu transporters and Mg(2+)-independent astroglial N-methyl-D-aspartate receptors. Under control conditions, role for neuronal K(+)/Cl(-) cotransporter KCC2, but not for glial Na(+)/K(+)/Cl(-) cotransporter NKCC1 was observed. Slight enhancement and inhibition of IOS through non-specific Cl(-) and volume-regulated anion channels, respectively, were also depicted. High-frequency IOS imaging, evoked by brief afferent stimulation in brain slices provide a new paradigm for studying mechanisms underlying IOS genesis. Major players disclosed this way imply that spatiotemporal IOS reflects glutamatergic neuronal activation and astroglial response, as observed within the hippocampus. Our model may help to better interpret in vivo IOS and support diagnosis in the future.

Implications and mechanism of action of gabapentin in neuropathic pain

Gabapentin is an anti-epileptic agent but now it is also recommended as first line agent in neuropathic pain, particularly in diabetic neuropathy and post herpetic neuralgia. α2δ-1, an auxillary subunit of voltage gated calcium channels, has been documented as its main target and its specific binding to this subunit is described to produce different actions responsible for pain attenuation. The binding to α2δ-1 subunits inhibits nerve injury-induced trafficking of α1 pore forming units of calcium channels (particularly N-type) from cytoplasm to plasma membrane (membrane trafficking) of pre-synaptic terminals of dorsal root ganglion (DRG) neurons and dorsal horn neurons. Furthermore, the axoplasmic transport of α2δ-1 subunits from DRG to dorsal horns neurons in the form of anterograde trafficking is also inhibited in response to gabapentin administration. Gabapentin has also been shown to induce modulate other targets including transient receptor potential channels, NMDA receptors, protein kinase C and inflammatory cytokines. It may also act on supra-spinal region to stimulate noradrenaline mediated descending inhibition, which contributes to its anti-hypersensitivity action in neuropathic pain.

Imaging hypothalamic activity using diffusion weighted magnetic resonance imaging in the mouse and human brain

Hypothalamic appetite regulation is a vital homeostatic process underlying global energy balance in animals and humans, its disturbances resulting in feeding disorders with high morbidity and mortality. The objective evaluation of appetite remains difficult, very often restricted to indirect measurements of food intake and body weight. We report here, the direct, non-invasive visualization of hypothalamic activation by fasting using diffusion weighted magnetic resonance imaging, in the mouse brain as well as in a preliminary study in the human brain. The brain of fed or fasted mice or humans were imaged at 7 or 1.5 Tesla, respectively, by diffusion weighted magnetic resonance imaging using a complete range of b values (10<b<2000s.mm(-2)). The diffusion weighted image data sets were registered and analyzed pixel by pixel using a biexponential model of diffusion, or a model-free Linear Discriminant Analysis approach. Biexponential fittings revealed statistically significant increases in the slow diffusion parameters of the model, consistent with a neurocellular swelling response in the fasted hypothalamus. Increased resolution approaches allowed the detection of increases in the diffusion parameters within the Arcuate Nucleus, Ventromedial Nucleus and Dorsomedial Nucleus. Independently, Linear Discriminant Analysis was able to classify successfully the diffusion data sets from mice and humans between fed and fasted states. Present results are consistent with increased glutamatergic neurotransmission during orexigenic firing, a process resulting in increased ionic accumulation and concomitant osmotic neurocellular swelling. This swelling response is spatially extendable through surrounding astrocytic networks until it becomes MRI detectable. Present findings open new avenues for the direct, non-invasive, evaluation of appetite disorders and other hypothalamic pathologies helping potentially in the development of the corresponding therapies.

Cellular abnormalities in depression: evidence from postmortem brain tissue

During the past two decades, in vivo neuroimaging studies have permitted significant insights into the general location of dysfunctional brain regions in depression. In parallel and often intersecting ways, neuroanatomical, pharmacological, and biochemical studies of postmortem brain tissue are permitting new insights into the pathophysiology of depression. In addition to long-recognized neurochemical abnormalities in depression, novel studies at the microscopic level support the contention that mood disorders are associated with abnormalities in cell morphology and distribution. In the past 6 years, cell-counting studies have identified changes in the density and size of both neurons and glia in a number of frontolimbic brain regions, including dorsolateral prefrontal, orbitofrontal, and anterior cingulate cortex, and the amygdala and hippocampus. Convergence of cellular changes at the microscopic level with neuroimaging changes detected in vivo provides a compelling integration of clinical and basic research for disentangling the pathophysiology of depression. The ultimate integration of these two research approaches will occur with premortem longitudinal clinical studies on well-characterized patients linked to postmortem studies of the same subjects.

Gap junction pathology in multiple sclerosis lesions and normal-appearing white matter

Oligodendrocyte gap junctions (GJs) are vital for central nervous system myelination, but their involvement in multiple sclerosis (MS) pathology remains unknown. The aim of this study was to examine alterations of oligodendrocyte and related astrocyte GJs in MS lesions and normal-appearing white matter (NAWM). Post-mortem brain samples from 9 MS and 11 age-matched non-MS control patients were studied. Tissue sections that included both chronic active and inactive lesions were characterized neuropathologically with Luxol Fast Blue staining and immunostaining for myelin oligodendrocyte glycoprotein (MOG) and the microglial marker Iba1. We analyzed the expression of Cx32 and Cx47 in oligodendrocytes and of Cx43, the major astrocytic partner in oligodendrocyte-astrocyte (O/A) GJs by quantitative immunoblot and real-time PCR. Formation of GJ plaques was quantified by immunohistochemistry. Compared to control brains, both Cx32 and Cx47 GJ plaques and protein levels were reduced in and around MS lesions, while Cx43 was increased as part of astrogliosis. In the NAWM, Cx32 was significantly reduced along myelinated fibers whereas Cx47 showed increased expression mainly in oligodendrocyte precursor cells (OPCs). However, OPCs showed only limited connectivity to astrocytes. Cx43 showed modestly increased levels in MS NAWM compared to controls, while GJ plaque counts were unchanged. Our findings indicate that oligodendrocyte GJs are affected not only in chronic MS lesions but also in NAWM, where disruption of Cx32 GJs in myelinated fibers may impair myelin structure and function. Moreover, limited O/A GJ connectivity of recruited OPCs in the setting of persistent inflammation and astrogliosis may prevent differentiation and remyelination.

Riluzole and gabapentinoids activate glutamate transporters to facilitate glutamate-induced glutamate release from cultured astrocytes

We have recently demonstrated that the glutamate transporter activator riluzole paradoxically enhanced glutamate-induced glutamate release from cultured astrocytes. We further showed that both riluzole and the α(2)δ subunit ligand gabapentin activated descending inhibition in rats by increasing glutamate receptor signaling in the locus coeruleus and hypothesized that these drugs share common mechanisms to enhance glutamate release from astrocytes. In the present study, we examined the effects of riluzole and gabapentin on glutamate uptake and release and glutamate-induced Ca(2+) responses in primary cultures of astrocytes. Riluzole and gabapentin facilitated glutamate-induced glutamate release from astrocytes and significantly increased glutamate uptake, the latter being completely blocked by the non-selective glutamate transporter blocker DL-threo-β-benzyloxyaspartic acid (DL-TBOA). Riluzole and gabapentin also enhanced the glutamate-induced increase in intracellular Ca(2+) concentrations. Some α(2)δ subunit ligands, pregabalin and L-isoleucine, enhanced the glutamate-induced Ca(2+) response, whereas another, 3-exo-aminobicyclo[2.2.1]heptane-2-exo-carboxylic acid (ABHCA), did not. The enhancement of glutamate-induced intracellular Ca(2+) response by riluzole and gabapentin was blocked by the DL-TBOA and an inhibitor of Na(+)/Ca(2+) exchange, 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiurea (KB-R7943). Gabapentin's enhancement of Ca(2+) increase was specific to glutamate stimulation, as it was not mimicked with stimulation by ATP. These results suggest that riluzole and gabapentin enhance Na(+)-glutamate co-transport through glutamate transporters, induce subsequent Ca(2+) influx via the reverse mode of Na(+)/Ca(2+) exchange, and thereby facilitate Ca(2+)-dependent glutamate release by glutamate in astrocytes. The present study also demonstrates a novel target of gabapentinoid action in astrocytes other than α(2)δ subunits in neurons.

Changes in kynurenine pathway metabolism in the brain, liver and kidney of aged female Wistar rats

The kynurenine pathway of tryptophan catabolism plays an important role in several biological systems affected by aging. We quantified tryptophan and its metabolites kynurenine (KYN), kynurenine acid (KYNA), picolinic acid (PIC) and quinolinic acid (QUIN), and activity of the kynurenine pathway enzymes indoleamine 2,3-dioxygenase (IDO), tryptophan 2,3-dioxygenase (TDO) and quinolinic acid phosphoribosyltransferase (QPRTase), in the brain, liver and kidney of young, middle-aged and old female Wistar rats. Tryptophan levels and TDO activity decreased in all tissues with age. In contrast, brain IDO activity increased with age, while liver and kidney IDO activity decreased with age. The levels of KYN, KYNA, QUIN and PIC in brain all increased with age, while the levels of KYN in the liver and kidney showed a tendency to decrease. The levels of KYNA in the liver did not change, but the levels of KYNA in the kidney increased. The levels of PIC and QUIN increased significantly in the liver but showed a tendency to decrease in the kidney. QPRTase activity in both brain and liver decreased with age but was elevated in the kidney in middle-aged (12-month-old) rats. These age-associated changes in tryptophan metabolism have the potential to impact upon major biological processes, including lymphocyte function, pyridine (NAD(P)(H)) synthesis and N-methyl-d-aspartate (NMDA)-mediated synaptic transmission, and may therefore contribute to several degenerative changes of the elderly.

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.

High frequency stimulation abolishes thalamic network oscillations: an electrophysiological and computational analysis

Deep brain stimulation (DBS) of the thalamus has been demonstrated to be effective for the treatment of epilepsy. To investigate the mechanism of action of thalamic DBS, we examined the effects of high frequency stimulation (HFS) on spindle oscillations in thalamic brain slices from ferrets. We recorded intracellular and extracellular electrophysiological activity in the nucleus reticularis thalami (nRt) and in thalamocortical relay (TC) neurons in the lateral geniculate nucleus, stimulated the slice using a concentric bipolar electrode, and recorded the level of glutamate within the slice. HFS (100 Hz) of TC neurons generated excitatory post-synaptic potentials, increased the number of action potentials in both TC and nRt neurons, reduced the input resistance, increased the extracellular glutamate concentration, and abolished spindle wave oscillations. HFS of the nRt also suppressed spindle oscillations. In both locations, HFS was associated with significant and persistent elevation in extracellular glutamate levels and suppressed spindle oscillations for many seconds after the cessation of stimulation. We simulated HFS within a computational model of the thalamic network, and HFS also disrupted spindle wave activity, but the suppression of spindle activity was short-lived. Simulated HFS disrupted spindle activity for prolonged periods of time only after glutamate release and glutamate-mediated activation of a hyperpolarization-activated current (I(h)) was incorporated into the model. Our results suggest that the mechanism of action of thalamic DBS as used in epilepsy may involve the prolonged release of glutamate, which in turn modulates specific ion channels such as I(h), decreases neuronal input resistance, and abolishes thalamic network oscillatory activity.

Microglial self-defence mediated through GLT-1 and glutathione

Glutamate is stored in synaptic vesicles in presynaptic neurons. It is released into the synaptic cleft to provide signalling to postsynaptic neurons. Normally, the astroglial glutamate transporters GLT-1 and GLAST take up glutamate to mediate a high signal-to-noise ratio in the synaptic signalling, and also to prevent excitotoxic effects by glutamate. In astrocytes, glutamate is transformed into glutamine, which is safely transported back to neurons. However, in pathological conditions, such as an ischemia or virus infection, astroglial transporters are down-regulated which could lead to excitotoxicity. Lately, it was shown that even microglia can express glutamate transporters during pathological events. Microglia have two systems for glutamate transport: GLT-1 for transport into the cells and the x (c) (-) system for transport out of the cells. We here review results from our work and others, which demonstrate that microglia in culture express GLT-1, but not GLAST, and transport glutamate from the extracellular space. We also show that TNF-α can induce increased microglial GLT-1 expression, possibly associating the expression with inflammatory systems. Furthermore, glutamate taken up through GLT-1 may be used for direct incorporation into glutathione and to fuel the intracellular glutamate pool to allow cystine uptake through the x (c) (-) system. This can lead to a defence against oxidative stress and have an antiviral function.

Implications for glycine receptors and astrocytes in ethanol-induced elevation of dopamine levels in the nucleus accumbens

Elevated dopamine levels are believed to contribute to the rewarding sensation of ethanol (EtOH), and previous research has shown that strychnine-sensitive glycine receptors in the nucleus accumbens (nAc) are involved in regulating dopamine release and in mediating the reinforcing effects of EtOH. Furthermore, the osmoregulator taurine, which is released from astrocytes treated with EtOH, can act as an endogenous ligand for the glycine receptor, and increase extracellular dopamine levels. The aim of this study was to address if EtOH-induced swelling of astrocytes could contribute to elevated dopamine levels by increasing the extracellular concentration of taurine. Cell swelling was estimated by optical sectioning of fluorescently labeled astrocytes in primary cultures from rat, and showed that EtOH (25-150 mM) increased astrocyte cell volumes in a concentration- and ion-dependent manner. The EtOH-induced cell swelling was inhibited in cultures treated with the Na(+) /K(+) /2Cl⁻ cotransporter blocker furosemide (1 mM), Na(+) /K(+) -ATPase inhibitor ouabain (0.1 mM), potassium channel inhibitor BaCl₂ (50 µM) and in cultures containing low extracellular sodium concentration (3 mM). In vivo microdialysis performed in the nAc of awake and freely moving rats showed that local treatment with EtOH enhanced the concentrations of dopamine and taurine in the microdialysate, while glycine and β-alanine levels were not significantly modulated. EtOH-induced dopamine release was antagonized by local treatment with the glycine receptor antagonist strychnine (20 µM) or furosemide (100 µM or 1 mM). Furosemide also prevented EtOH-induced taurine release in the nAc. In conclusion, our data suggest that extracellular concentrations of dopamine and taurine are interconnected and that swelling of astrocytes contributes to the acute rewarding sensation of EtOH.