Glial glutamate transporter GLT-1 determines susceptibility to spreading depression in the mouse cerebral cortex

The Role of Astrocytes in Migraine with Cortical Spreading Depression: Protagonists or Bystanders? A Narrative Review

Cortical spreading depression (CSD) is a slow wave of cortical depolarization closely associated with migraines with an aura. Previously, it was thought that CSD depolarization was mainly driven by neurons, with characteristic changes in neuronal swelling and increased extracellular potassium (K+) and glutamate. However, the role of astrocytes, a member of the neurovascular unit, in migraine with CSD has recently received increasing attention. In the early stages of CSD, astrocytes provide neurons with energy support and clear K+ and glutamate from synaptic gaps. However, in the late stages of CSD, astrocytes release large amounts of lactic acid to exacerbate hypoxia when the energy demand exceeds the astrocytes' compensatory capacity. Astrocyte endfoot swelling is a characteristic of CSD, and neurons are not similarly altered. It is primarily due to K+ influx and abnormally active calcium (Ca2+) signaling. Aquaporin 4 (AQP-4) only mediates K+ influx and has little role as an aquaporin. Astrocytes endfoot swelling causes perivascular space closure, slowing the glymphatic system flow and exacerbating neuroinflammation, leading to persistent CSD. Astrocytes are double-edged swords in migraine with CSD and may be potential targets for CSD interventions.

Excitotoxic Storms of Ischemic Stroke: A Non-neuronal Perspective

Numerous neurological disorders share a fatal pathologic process known as glutamate excitotoxicity. Among which, ischemic stroke is the major cause of mortality and disability worldwide. For a long time, the main idea of developing anti-excitotoxic neuroprotective agents was to block glutamate receptors. Despite this, there has been little successful clinical translation to date. After decades of "neuron-centered" views, a growing number of studies have recently revealed the importance of non-neuronal cells. Glial cells, cerebral microvascular endothelial cells, blood cells, and so forth are extensively engaged in glutamate synthesis, release, reuptake, and metabolism. They also express functional glutamate receptors and can listen and respond for fast synaptic transmission. This broadens the thoughts of developing excitotoxicity antagonists. In this review, the critical contribution of non-neuronal cells in glutamate excitotoxicity during ischemic stroke will be emphasized in detail, and the latest research progress as well as corresponding therapeutic strategies will be updated at length, aiming to reconceptualize glutamate excitotoxicity in a non-neuronal perspective.

The function of astrocytes and their role in neurological diseases

Astrocytes have countless links with neurons. Previously, astrocytes were only considered a scaffold of neurons; in fact, astrocytes perform a variety of functions, including providing support for neuronal structures and energy metabolism, offering isolation and protection and influencing the formation, function and elimination of synapses. Because of these functions, astrocytes play an critical role in central nervous system (CNS) diseases. The regulation of the secretiory factors, receptors, channels and pathways of astrocytes can effectively inhibit the occurrence and development of CNS diseases, such as neuromyelitis optica (NMO), multiple sclerosis, Alzheimer's disease (AD), Parkinson's disease (PD) and Huntington's disease. The expression of aquaporin 4 in AS is directly related to NMO and indirectly involved in the clearance of Aβ and tau proteins in AD. Connexin 43 has a bidirectional effect on glutamate diffusion at different stages of stroke. Interestingly, astrocytes reduce the occurrence of PD through multiple effects such as secretion of related factors, mitochondrial autophagy and aquaporin 4. Therefore, this review is focused on the structure and function of astrocytes and the correlation between astrocytes and CNS diseases and drug treatment to explore the new functions of astrocytes with the astrocytes as the target. This, in turn, would provide a reference for the development of new drugs to protect neurons and promote the recovery of nerve function.

The Role of Glial Cells in Different Phases of Migraine: Lessons from Preclinical Studies

Migraine is a complex and debilitating neurological disease that affects 15% of the population worldwide. It is defined by the presence of recurrent severe attacks of disabling headache accompanied by other debilitating neurological symptoms. Important advancements have linked the trigeminovascular system and the neuropeptide calcitonin gene-related peptide to migraine pathophysiology, but the mechanisms underlying its pathogenesis and chronification remain unknown. Glial cells are essential for the correct development and functioning of the nervous system and, due to its implication in neurological diseases, have been hypothesised to have a role in migraine. Here we provide a narrative review of the role of glia in different phases of migraine through the analysis of preclinical studies. Current evidence shows that astrocytes and microglia are involved in the initiation and propagation of cortical spreading depolarization, the neurophysiological correlate of migraine aura. Furthermore, satellite glial cells within the trigeminal ganglia are implicated in the initiation and maintenance of orofacial pain, suggesting a role in the headache phase of migraine. Moreover, microglia in the trigeminocervical complex are involved in central sensitization, suggesting a role in chronic migraine. Taken altogether, glial cells have emerged as key players in migraine pathogenesis and chronification and future therapeutic strategies could be focused on targeting them to reduce the burden of migraine.

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.

Astrocytic Glutamate Transporters and Migraine

Glutamate levels and lifetime in the brain extracellular space are dinamically regulated by a family of Na⁺- and K⁺-dependent glutamate transporters, which thereby control numerous brain functions and play a role in numerous neurological and psychiatric diseases. Migraine is a neurological disorder characterized by recurrent attacks of typically throbbing and unilateral headache and by a global dysfunction in multisensory processing. Familial hemiplegic migraine type 2 (FHM2) is a rare monogenic form of migraine with aura caused by loss-of-function mutations in the α2 Na/K ATPase (α2NKA). In the adult brain, this pump is expressed almost exclusively in astrocytes where it is colocalized with glutamate transporters. Knockin mouse models of FHM2 (FHM2 mice) show a reduced density of glutamate transporters in perisynaptic astrocytic processes (mirroring the reduced expression of α2NKA) and a reduced rate of glutamate clearance at cortical synapses during neuronal activity and sensory stimulation. Here we review the migraine-relevant alterations produced by the astrocytic glutamate transport dysfunction in FHM2 mice and their underlying mechanisms, in particular regarding the enhanced brain susceptibility to cortical spreading depression (the phenomenon that underlies migraine aura and can also initiate the headache mechanisms) and the enhanced algesic response to a migraine trigger.

Upregulation of glutamate transporter 1 by mTOR/Akt pathway in astrocyte culture during oxygen-glucose deprivation and reoxygenation

Astrocyte-specific glutamate transporter subtype 1 (GLT-1) plays an important role in influencing glutamate excitatory toxicity and preventing the death of excitatory toxic neurons. Although the mammalian target of rapamycin (mTOR)/protein kinase B(Akt)/nuclear factor kappa B signaling cascade is involved in the upregulation of astrocytic GLT-1 in oxygen-glucose deprivation (OGD), it is unclear whether the mTOR/Akt pathway is involved in astrocytic GLT-1 upregulation in OGD and reoxygenation (OGD/R). In this study, we found that the treatment of cultured astrocytes with rapamycin and triciribine led to the decreased astrocytes' protrusions, smaller nuclei, and an increased apoptotic rate. The inhibitors of mTOR complex 1 significantly increased the expression levels of phosphorylated Akt-Ser473 (p-Akt), phosphorylated Akt-Thr308(p-Akt), and GLT-1, while Akt-specific inhibitors blocked GLT-1 expression, suggesting that the mTOR/Akt pathway is involved in GLT-1 upregulation. We further demonstrated that astrocytes under OGD/R adapted to environmental changes through the mTOR/Akt pathway, mainly by altering cell morphology and apoptosis and upregulating the expression levels of p-Akt and GLT-1. Our results suggested that astrocytes may adapt to short-term ischemic-reperfusion injury by regulating cell morphology, apoptosis and GLT-1 upregulation.

Cortical pain induced by optogenetic cortical spreading depression: from whole brain activity mapping

BACKGROUND

Cortical spreading depression (CSD) is an electrophysiological event underlying migraine aura. Traditional CSD models are invasive and often cause injuries. The aim of the study was to establish a minimally invasive optogenetic CSD model and identify the active networks after CSD using whole-brain activity mapping.

METHODS

CSD was induced in mice by light illumination, and their periorbital thresholds and behaviours in the open field, elevated plus-maze and light-aversion were recorded. Using c-fos, we mapped the brain activity after CSD. The whole brain was imaged, reconstructed and analyzed using the Volumetric Imaging with Synchronized on-the-fly-scan and Readout technique. To ensure the accuracy of the results, the immunofluorescence staining method was used to verify the imaging results.

RESULTS

The optogenetic CSD model showed significantly decreased periorbital thresholds, increased facial grooming and freezing behaviours and prominent light-aversion behaviours. Brain activity mapping revealed that the somatosensory, primary sensory, olfactory, basal ganglia and default mode networks were activated. However, the thalamus and trigeminal nucleus caudalis were not activated.

CONCLUSIONS

Optogenetic CSD model could mimic the behaviours of headache and photophobia. Moreover, the optogenetic CSD could activate multiple sensory cortical regions without the thalamus or trigeminal nucleus caudalis to induce cortical pain.

Migraine Pathophysiology Revisited: Proposal of a New Molecular Theory of Migraine Pathophysiology and Headache Diagnostic Criteria

Various explanations for the pathophysiology of migraines have been proposed; however, none of these provide a complete explanation. The author critically reviews previous theories and proposes a new molecular theory of migraine pathophysiology. The diagnosis of primary headaches is generally based on clinical histories and symptoms only because there is no reliable diagnostic examination. The author proposes a new classification system and set of diagnostic criteria for headaches based on molecular markers.

Positive effects of running exercise on astrocytes in the medial prefrontal cortex in an animal model of depression

Depression is one of the most common mental illnesses and seriously affects all aspects of life. Running exercise has been suggested to prevent or alleviate the occurrence and development of depression; however, the underlying mechanisms of these effects remain unclear. Independent studies have indicated that astrocytes play essential roles and that the medial prefrontal cortex (mPFC) is an important brain region involved in the pathology underlying depression. However, it is unknown whether running exercise achieves antidepressant effects by affecting the number of astrocytes and glutamate transport function in the mPFC. Here, animal models of depression were established using chronic unpredictable stress (CUS), and depression-like behavior was assessed by the sucrose preference test. After successfully establishing the depression model, experimental animals performed running exercise. Glial fibrillary acidic protein-positive (GFAP⁺ ) cell number in the mPFC was precisely quantified using immunohistochemical and stereological methods, and the densities of bromodeoxyuridine-positive (BrdU⁺ ) and BrdU⁺ /GFAP⁺ cells in the mPFC were measured using a semiquantitative immunofluorescence assay. Changes in glutamate transporter gene expression in mPFC astrocytes were detected by mRNA sequencing and qRT-PCR. We found that running exercise reversed CUS-induced decreases in sucrose preference, increased astrocyte number and the density of newborn astrocytes, and reversed decreases in gene expression levels of GFAP, S100b, and the glutamate transporters GLT-1 and GLAST in the mPFC of CUS animals. These results suggested that changes in astrocyte number and glutamate transporter function may be potential meditators of the effects of running exercise in the treatment of depression.

Structural insights into inhibitory mechanism of human excitatory amino acid transporter EAAT2

Glutamate is a pivotal excitatory neurotransmitter in mammalian brains, but excessive glutamate causes numerous neural disorders. Almost all extracellular glutamate is retrieved by the glial transporter, Excitatory Amino Acid Transporter 2 (EAAT2), belonging to the SLC1A family. However, in some cancers, EAAT2 expression is enhanced and causes resistance to therapies by metabolic disturbance. Despite its crucial roles, the detailed structural information about EAAT2 has not been available. Here, we report cryo-EM structures of human EAAT2 in substrate-free and selective inhibitor WAY213613-bound states at 3.2 Å and 2.8 Å, respectively. EAAT2 forms a trimer, with each protomer consisting of transport and scaffold domains. Along with a glutamate-binding site, the transport domain possesses a cavity that could be disrupted during the transport cycle. WAY213613 occupies both the glutamate-binding site and cavity of EAAT2 to interfere with its alternating access, where the sensitivity is defined by the inner environment of the cavity. We provide the characterization of the molecular features of EAAT2 and its selective inhibition mechanism that may facilitate structure-based drug design for EAAT2.

EAAT2 is an amino acid transporter implicated in glutamate homeostasis in brain and therapy resistance of cancer cells. Here, the authors report cryo-EM structures and reveal inhibitory mechanisms via selective inhibitor WAY213613.

Habenula bibliometrics: Thematic development and research fronts of a resurgent field

The habenula (Hb) is a small structure of the posterior diencephalon that is highly conserved across vertebrates but nonetheless has attracted relatively little research attention until the past two decades. The resurgent interest is motivated by neurobehavioral studies demonstrating critical functions in a broad spectrum of motivational and cognitive processes, including functions relevant to psychiatric diseases. The Hb is widely conceived as an “anti-reward” center that acts by regulating brain monoaminergic systems. However, there is still no general conceptual framework for habenula research, and no study has focused on uncovering potentially significant but overlooked topics that may advance our understanding of physiological functions or suggest potential clinical applications of Hb-targeted interventions. Using science mapping tools, we quantitatively and qualitatively analyzed the relevant publications retrieved from the Web of Science Core Collection (WoSCC) database from 2002 to 2021. Herein we present an overview of habenula-related publications, reveal primary research trends, and prioritize some key research fronts by complementary bibliometric analysis. High-priority research fronts include Ventral Pallidum, Nucleus Accumbens, Nicotine and MHb, GLT-1, Zebrafish, and GCaMP, Ketamine, Deep Brain Stimulation, and GPR139. The high intrinsic heterogeneity of the Hb, extensive connectivity with both hindbrain and forebrain structures, and emerging associations with all three dimensions of mental disorders (internalizing, externalizing, and psychosis) suggest that the Hb may be the neuronal substrate for a common psychopathology factor shared by all mental illnesses termed the p factor. A future challenge is to explore the therapeutic potential of habenular modulation at circuit, cellular, and molecular levels.

Converging Mechanisms of Epileptogenesis and Their Insight in Glioblastoma

Glioblastoma (GBM) is the most common and advanced form of primary malignant tumor occurring in the adult central nervous system, and it is frequently associated with epilepsy, a debilitating comorbidity. Seizures are observed both pre- and post-surgical resection, indicating that several pathophysiological mechanisms are shared but also prompting questions about how the process of epileptogenesis evolves throughout GBM progression. Molecular mutations commonly seen in primary GBM, i.e., in PTEN and p53, and their associated downstream effects are known to influence seizure likelihood. Similarly, various intratumoral mechanisms, such as GBM-induced blood-brain barrier breakdown and glioma-immune cell interactions within the tumor microenvironment are also cited as contributing to network hyperexcitability. Substantial alterations to peri-tumoral glutamate and chloride transporter expressions, as well as widespread dysregulation of GABAergic signaling are known to confer increased epileptogenicity and excitotoxicity. The abnormal characteristics of GBM alter neuronal network function to result in metabolically vulnerable and hyperexcitable peri-tumoral tissue, properties the tumor then exploits to favor its own growth even post-resection. It is evident that there is a complex, dynamic interplay between GBM and epilepsy that promotes the progression of both pathologies. This interaction is only more complicated by the concomitant presence of spreading depolarization (SD). The spontaneous, high-frequency nature of GBM-associated epileptiform activity and SD-associated direct current (DC) shifts require technologies capable of recording brain signals over a wide bandwidth, presenting major challenges for comprehensive electrophysiological investigations. This review will initially provide a detailed examination of the underlying mechanisms that promote network hyperexcitability in GBM. We will then discuss how an investigation of these pathologies from a network level, and utilization of novel electrophysiological tools, will yield a more-effective, clinically-relevant understanding of GBM-related epileptogenesis. Further to this, we will evaluate the clinical relevance of current preclinical research and consider how future therapeutic advancements may impact the bidirectional relationship between GBM, SDs, and seizures.

Epigenetic Connection of the Calcitonin Gene-Related Peptide and Its Potential in Migraine

The calcitonin gene-related peptide (CGRP) is implicated in the pathogenesis of several pain-related syndromes, including migraine. Targeting CGRP and its receptor by their antagonists and antibodies was a breakthrough in migraine therapy, but the need to improve efficacy and limit the side effects of these drugs justify further studies on the regulation of CGRP in migraine. The expression of the CGRP encoding gene, CALCA, is modulated by epigenetic modifications, including the DNA methylation, histone modification, and effects of micro RNAs (miRNAs), circular RNAs, and long-coding RNAs (lncRNAs). On the other hand, CGRP can change the epigenetic profile of neuronal and glial cells. The promoter of the CALCA gene has two CpG islands that may be specifically methylated in migraine patients. DNA methylation and lncRNAs were shown to play a role in the cell-specific alternative splicing of the CALCA primary transcript. CGRP may be involved in changes in neural cytoarchitecture that are controlled by histone deacetylase 6 (HDAC6) and can be related to migraine. Inhibition of HDAC6 results in reduced cortical-spreading depression and a blockade of the CGRP receptor. CGRP levels are associated with the expression of several miRNAs in plasma, making them useful peripheral markers of migraine. The fundamental role of CGRP in inflammatory pain transmission may be epigenetically regulated. In conclusion, epigenetic connections of CGRP should be further explored for efficient and safe antimigraine therapy.

Questioning Glutamate Excitotoxicity in Acute Brain Damage: The Importance of Spreading Depolarization

BACKGROUND

Within 2 min of severe ischemia, spreading depolarization (SD) propagates like a wave through compromised gray matter of the higher brain. More SDs arise over hours in adjacent tissue, expanding the neuronal damage. This period represents a therapeutic window to inhibit SD and so reduce impending tissue injury. Yet most neuroscientists assume that the course of early brain injury can be explained by glutamate excitotoxicity, the concept that immediate glutamate release promotes early and downstream brain injury. There are many problems with glutamate release being the unseen culprit, the most practical being that the concept has yielded zero therapeutics over the past 30 years. But the basic science is also flawed, arising from dubious foundational observations beginning in the 1950s METHODS: Literature pertaining to excitotoxicity and to SD over the past 60 years is critiqued.

RESULTS

Excitotoxicity theory centers on the immediate and excessive release of glutamate with resulting neuronal hyperexcitation. This instigates poststroke cascades with subsequent secondary neuronal injury. By contrast, SD theory argues that although SD evokes some brief glutamate release, acute neuronal damage and the subsequent cascade of injury to neurons are elicited by the metabolic stress of SD, not by excessive glutamate release. The challenge we present here is to find new clinical targets based on more informed basic science. This is motivated by the continuing failure by neuroscientists and by industry to develop drugs that can reduce brain injury following ischemic stroke, traumatic brain injury, or sudden cardiac arrest. One important step is to recognize that SD plays a central role in promoting early neuronal damage. We argue that uncovering the molecular biology of SD initiation and propagation is essential because ischemic neurons are usually not acutely injured unless SD propagates through them. The role of glutamate excitotoxicity theory and how it has shaped SD research is then addressed, followed by a critique of its fading relevance to the study of brain injury.

CONCLUSIONS

Spreading depolarizations better account for the acute neuronal injury arising from brain ischemia than does the early and excessive release of glutamate.

Astroglia and Obsessive Compulsive Disorder

Obsessive compulsive disorder (OCD) has a prevalence rate of 1-3% in the general population and has been ranked as one of the top ten leading causes of illness-related disability (American Psychiatric Association 2013; Kessler et al. 2005). OCD is characterized by persistent intrusive thoughts (obsessions) and repetitive behaviors (compulsions) (Leckman et al. 1997). There are various OCD-related disorders, including Tourette syndrome (TS), grooming disorders (e.g., skin-picking, trichotillomania), and autism spectrum disorders (ASD) that share considerable overlapping features with OCD (Browne et al. 2014). Although the neurobiological basis of OCD still remains obscure, neuroimaging studies in patients with OCD and OCD-related disorders have consistently identified hyperactivity in orbitofrontal cortex and striatum (Cerliani et al. 2015; Hou et al. 2014; Jung et al. 2017; Neuner et al. 2014). However, the cellular and synaptic abnormalities underlying this hyperactivity are unclear. The most prominent theory regarding the underlying mechanisms of OCD and OCD-related disorders is an increased excitation to inhibition (E/I) ratio due to increased glutamatergic excitation or reduced GABAergic inhibition (Albin and Mink 2006; Rubenstein and Merzenich 2003; Wu et al. 2012). A proper E/I ratio is achieved by factors expressed in neuron and glia. In astrocytes, both the glutamate transporter GLT1 and GABA transporter GAT-3 are critical for regulating the E/I balance (Aida et al. 2015; Aizawa et al. 2020; Boddum et al. 2016; Cui et al. 2014; Kersanté et al. 2013; Kiryk et al. 2008; Matos et al. 2018; Scimemi 2014; Sugimoto et al. 2018; Sugiyama et al. 2017; Tanaka et al. 1997; Zhao et al. 2018). Although astrocyte dysfunction has not been directly explored in OCD patients, several animal studies have found that astrocytes are involved in the pathophysiology of OCD. In this chapter, I highlight recent studies in which astrocyte dysfunction contributed to E/I imbalance, leading to pathological repetitive behaviors shared between patients with OCD, TS, and ASD.

Electrophysiological and pharmacological characterization of spreading depolarization in the adult zebrafish tectum

Spreading depolarization (SD) is a slowly propagating wave of neuronal and glial depolarization. A growing number of studies show that SD and SD-like phenomena play a role in neurological disorders such as migraine, stroke, and traumatic brain injury. Despite the clinical importance of SD, its underlying molecular and cellular mechanisms remain elusive, possibly because of insufficient animal model allowing genetic manipulation. Such a model would also allow high-throughput screening for SD-suppressing drug development. To address this, we developed a novel experimental system to study SD using zebrafish. Electrophysiological recordings in the immobilized adult zebrafish revealed that increasing extracellular potassium concentration elicited SD with a large and long-lasting negative shift of direct current (DC) potential in the optic tectum. It also reduced the oscillatory activity in the extracellular field potential and increased the expression of the immediate early gene c-fos. Pharmacological blocking of the N-methyl-d-aspartate (NMDA) glutamate receptor attenuated the propagation of SD, suggesting that glutamatergic neurotransmission mediated tectal SD in zebrafish. Our analyses revealed that the zebrafish tectum and rodent cortex had similar SD kinetics. The current study provides electrophysiological and pharmacological evidence that zebrafish SD and mammal SD are comparable. This zebrafish SD model is suitable for genetic manipulation and cost-effective high-throughput screening. It could pave the way to novel diagnostic and therapeutic methods applicable to SD-associated neurological disorders.NEW & NOTEWORTHY Previous studies have implicated spreading depolarization (SD) in stroke and migraine. Here, we demonstrate SD, for the first time, in the adult zebrafish tectum showing waveform kinetics, c-fos expression, and attenuation by N-methyl-d-aspartate glutamate receptor blocker as observed in the rodent cortex. Since the zebrafish is an animal model amenable to genetic manipulation and chemical screening, this result could pave the way to novel diagnostic and therapeutic methods applicable to SD-associated neurological disorders.

Understanding the Connection Between Common Stroke Comorbidities, Their Associated Inflammation, and the Course of the Cerebral Ischemia/Reperfusion Cascade

Despite the enormous progress in the understanding of the course of the ischemic stroke over the last few decades, a therapy that effectively protects neurovascular units (NVUs) and significantly improves neurological functions in stroke patients has still not been achieved. The reasons for this state are unclear, but it is obvious that the cerebral ischemia and reperfusion cascade is a highly complex phenomenon, which includes the intense neuroinflammatory processes, and comorbid stroke risk factors strongly worsen stroke outcomes and likely make a substantial contribution to the pathophysiology of the ischemia/reperfusion, enhancing difficulties in searching of successful treatment. Common concomitant stroke risk factors (arterial hypertension, diabetes mellitus and hyperlipidemia) strongly drive inflammatory processes during cerebral ischemia/reperfusion; because these factors are often present for a long time before a stroke, causing low-grade background inflammation in the brain, and already initially disrupting the proper functions of NVUs. Broad consideration of this situation in basic research may prove to be crucial for the success of future clinical trials of neuroprotection, vasculoprotection and immunomodulation in stroke. This review focuses on the mechanism by which coexisting common risk factors for stroke intertwine in cerebral ischemic/reperfusion cascade and the dysfunction and disintegration of NVUs through inflammatory processes, principally activation of pattern recognition receptors, alterations in the expression of adhesion molecules and the subsequent pathophysiological consequences.

Mice with reduced glutamate transporter GLT1 expression exhibit behaviors related to attention-deficit/hyperactivity disorder

Attention-deficit/hyperactivity disorder (ADHD) is a common neuropsychiatric disorder in children. Although animal models and human brain imaging studies indicate a significant role for glutamatergic dysfunction in ADHD, there is no direct evidence that glutamatergic dysfunction is sufficient to induce ADHD-like symptoms. The glial glutamate transporter GLT1 plays a critical role in glutamatergic neurotransmission. We report here the generation of mice expressing only 20% of normal levels of the GLT1. Unlike conventional GLT1 knockout mice, these mice survive to adulthood and exhibit ADHD-like phenotypes, including hyperactivity, impulsivity and impaired memory. These findings indicate that glutamatergic dysfunction due to GLT1 deficiency, a mechanism distinct from the dopaminergic deficit hypothesis of ADHD, underlies ADHD-like symptoms.

Dysregulation of Astrocyte Ion Homeostasis and Its Relevance for Stroke-Induced Brain Damage

Ischemic stroke is a leading cause of mortality and chronic disability. Either recovery or progression towards irreversible failure of neurons and astrocytes occurs within minutes to days, depending on remaining perfusion levels. Initial damage arises from energy depletion resulting in a failure to maintain homeostasis and ion gradients between extra- and intracellular spaces. Astrocytes play a key role in these processes and are thus central players in the dynamics towards recovery or progression of stroke-induced brain damage. Here, we present a synopsis of the pivotal functions of astrocytes at the tripartite synapse, which form the basis of physiological brain functioning. We summarize the evidence of astrocytic failure and its consequences under ischemic conditions. Special emphasis is put on the homeostasis and stroke-induced dysregulation of the major monovalent ions, namely Na+, K+, H+, and Cl-, and their involvement in maintenance of cellular volume and generation of cerebral edema.

Glia and Orofacial Pain: Progress and Future Directions

Orofacial pain is a universal predicament, afflicting millions of individuals worldwide. Research on the molecular mechanisms of orofacial pain has predominately focused on the role of neurons underlying nociception. However, aside from neural mechanisms, non-neuronal cells, such as Schwann cells and satellite ganglion cells in the peripheral nervous system, and microglia and astrocytes in the central nervous system, are important players in both peripheral and central processing of pain in the orofacial region. This review highlights recent molecular and cellular findings of the glia involvement and glia–neuron interactions in four common orofacial pain conditions such as headache, dental pulp injury, temporomandibular joint dysfunction/inflammation, and head and neck cancer. We will discuss the remaining questions and future directions on glial involvement in these four orofacial pain conditions.

Influence of glutamate and GABA transport on brain excitatory/inhibitory balance

An optimally functional brain requires both excitatory and inhibitory inputs that are regulated and balanced. A perturbation in the excitatory/inhibitory balance-as is the case in some neurological disorders/diseases (e.g. traumatic brain injury Alzheimer's disease, stroke, epilepsy and substance abuse) and disorders of development (e.g. schizophrenia, Rhett syndrome and autism spectrum disorder)-leads to dysfunctional signaling, which can result in impaired cognitive and motor function, if not frank neuronal injury. At the cellular level, transmission of glutamate and GABA, the principle excitatory and inhibitory neurotransmitters in the central nervous system control excitatory/inhibitory balance. Herein, we review the synthesis, release, and signaling of GABA and glutamate followed by a focused discussion on the importance of their transport systems to the maintenance of excitatory/inhibitory balance.

Disruption of Glutamate Transport and Homeostasis by Acute Metabolic Stress

High-affinity, Na⁺-dependent glutamate transporters are the primary means by which synaptically released glutamate is removed from the extracellular space. They restrict the spread of glutamate from the synaptic cleft into the perisynaptic space and reduce its spillover to neighboring synapses. Thereby, glutamate uptake increases the spatial precision of synaptic communication. Its dysfunction and the entailing rise of the extracellular glutamate concentration accompanied by an increased spread of glutamate result in a loss of precision and in enhanced excitation, which can eventually lead to neuronal death via excitotoxicity. Efficient glutamate uptake depends on a negative resting membrane potential as well as on the transmembrane gradients of the co-transported ions (Na⁺, K⁺, and H⁺) and thus on the proper functioning of the Na⁺/K⁺-ATPase. Consequently, numerous studies have documented the impact of an energy shortage, as occurring for instance during an ischemic stroke, on glutamate clearance and homeostasis. The observations range from rapid changes in the transport activity to altered expression of glutamate transporters. Notably, while astrocytes account for the majority of glutamate uptake under physiological conditions, they may also become a source of extracellular glutamate elevation during metabolic stress. However, the mechanisms of the latter phenomenon are still under debate. Here, we review the recent literature addressing changes of glutamate uptake and homeostasis triggered by acute metabolic stress, i.e., on a timescale of seconds to minutes.