Astrocyte responses to neuronal activity

Novel insight into astrocyte-mediated gliotransmission modulates the synaptic plasticity in major depressive disorder

Major depressive disorder (MDD) is a form of glial cell-based synaptic dysfunction disease in which glial cells interact closely with neuronal synapses and perform synaptic information processing. Glial cells, particularly astrocytes, are active components of the brain and are responsible for synaptic activity through the release gliotransmitters. A reduced density of astrocytes and astrocyte dysfunction have both been identified the brains of patients with MDD. Furthermore, gliotransmission, i.e., active information transfer mediated by gliotransmitters between astrocytes and neurons, is thought to be involved in the pathogenesis of MDD. However, the mechanism by which astrocyte-mediated gliotransmission contributes to depression remains unknown. This review therefore summarizes the alterations in astrocytes in MDD, including astrocyte marker, connexin 43 (Cx43) expression, Cx43 gap junctions, and Cx43 hemichannels, and describes the regulatory mechanisms of astrocytes involved in synaptic plasticity. Additionally, we investigate the mechanisms acting of the glutamatergic, gamma-aminobutyric acidergic, and purinergic systems that modulate synaptic function and the antidepressant mechanisms of the related receptor antagonists. Further, we summarize the roles of glutamate, gamma-aminobutyric acid, d-serine, and adenosine triphosphate in depression, providing a basis for the identification of diagnostic and therapeutic targets for MDD.

Lateral parabrachial nucleus astrocytes control food intake

Food intake behavior is under the tight control of the central nervous system. Most studies to date focus on the contribution of neurons to this behavior. However, although previously overlooked, astrocytes have recently been implicated to play a key role in feeding control. Most of the recent literature has focused on astrocytic contribution in the hypothalamus or the dorsal vagal complex. The contribution of astrocytes located in the lateral parabrachial nucleus (lPBN) to feeding behavior control remains poorly understood. Thus, here, we first investigated whether activation of lPBN astrocytes affects feeding behavior in male and female rats using chemogenetic activation. Astrocytic activation in the lPBN led to profound anorexia in both sexes, under both ad-libitum feeding schedule and after a fasting challenge. Astrocytes have a key contribution to glutamate homeostasis and can themselves release glutamate. Moreover, lPBN glutamate signaling is a key contributor to potent anorexia, which can be induced by lPBN activation. Thus, here, we determined whether glutamate signaling is necessary for lPBN astrocyte activation-induced anorexia, and found that pharmacological N-methyl D-aspartate (NMDA) receptor blockade attenuated the food intake reduction resulting from lPBN astrocyte activation. Since astrocytes have been shown to contribute to feeding control by modulating the feeding effect of peripheral feeding signals, we further investigated whether lPBN astrocyte activation is capable of modulating the anorexic effect of the gut/brain hormone, glucagon like peptide -1, as well as the orexigenic effect of the stomach hormone - ghrelin, and found that the feeding effect of both signals is modulated by lPBN astrocytic activation. Lastly, we found that lPBN astrocyte activation-induced anorexia is affected by a diet-induced obesity challenge, in a sex-divergent manner. Collectively, current findings uncover a novel role for lPBN astrocytes in feeding behavior control.

Control of astrocytic Ca2+ signaling by nitric oxide-dependent S-nitrosylation of Ca2+ homeostasis modulator 1 channels

BACKGROUND

Astrocytes Ca2+ signaling play a central role in the modulation of neuronal function. Activation of metabotropic glutamate receptors (mGluR) by glutamate released during an increase in synaptic activity triggers coordinated Ca2+ signals in astrocytes. Importantly, astrocytes express the Ca2+-dependent nitric oxide (NO)-synthetizing enzymes eNOS and nNOS, which might contribute to the Ca2+ signals by triggering Ca2+ influx or ATP release through the activation of connexin 43 (Cx43) hemichannels, pannexin-1 (Panx-1) channels or Ca2+ homeostasis modulator 1 (CALHM1) channels. Hence, we aim to evaluate the participation of NO in the astrocytic Ca2+ signaling initiated by stimulation of mGluR in primary cultures of astrocytes from rat brain cortex.

RESULTS

Astrocytes were stimulated with glutamate or t-ACPD and NO-dependent changes in [Ca2+]i and ATP release were evaluated. In addition, the activity of Cx43 hemichannels, Panx-1 channels and CALHM1 channels was also analyzed. The expression of Cx43, Panx-1 and CALHM1 in astrocytes was confirmed by immunofluorescence analysis and both glutamate and t-ACPD induced NO-mediated activation of CALHM1 channels via direct S-nitrosylation, which was further confirmed by assessing CALHM1-mediated current using the two-electrode voltage clamp technique in Xenopus oocytes. Pharmacological blockade or siRNA-mediated inhibition of CALHM1 expression revealed that the opening of these channels provides a pathway for ATP release and the subsequent purinergic receptor-dependent activation of Cx43 hemichannels and Panx-1 channels, which further contributes to the astrocytic Ca2+ signaling.

CONCLUSIONS

Our findings demonstrate that activation of CALHM1 channels through NO-mediated S-nitrosylation in astrocytes in vitro is critical for the generation of glutamate-initiated astrocytic Ca2+ signaling.

The neurovascular unit of capillary blood vessels in the rat nervous system. A rapid-Golgi electron microscopy study

We describe a pericapillary organ in the rat forebrain and cerebellar cortex. It consists of a series of tripartite synapses with synaptic extensions enveloped by astrocytic endfeet that are linked to the capillary wall by synaptic extensions. Reciprocal specializations of the pericyte-capillary blood vessel (CBV) with such specialized synapses suggest a mechanoreceptor role. In Golgi-impregnated and 3D reconstructions of the cerebral cortex and thalamus, a series of TSs appear to be sequentially ordered in a common dendrite, paralleled by synaptic outgrowths termed golf club synaptic extensions (GCE) opposed to a longitudinal crest (LC) from the capillary basal lamina (BL). Our results show that, in the cerebellar cortex, afferent fibers and interneurons display microanatomical structures that strongly suggest an interaction with the capillary wall. Afferent mossy fiber (MF) rosettes and ascending granule cell axons and their dendrites define the pericapillary passage interactions that are entangled by endfeet. The presence of mRNA of the mechanosensitive channel Piezo1 in the MF rosettes, together with the surrounding end-feet and the capillary wall form mechanosensory units. The ubiquity of such units to modulate synaptic transmission is also supported by Piezo1 mRNA expressing pyramidal isocortical and thalamic neurons. This scenario suggests that ascending impulses to the cerebellar and cortical targets are presynaptically modulated by the reciprocal interaction with the mechanosensory pericapillary organ that ultimately modulates the vasomotor response.

Decoding Natural Astrocyte Rhythms: Dynamic Actin Waves Result from Environmental Sensing by Primary Rodent Astrocytes

Astrocytes are key regulators of brain homeostasis, equilibrating ion, water, and neurotransmitter concentrations and maintaining essential conditions for proper cognitive function. Recently, it has been shown that the excitability of the actin cytoskeleton manifests in second-scale dynamic fluctuations and acts as a sensor of chemophysical environmental cues. However, it is not known whether the cytoskeleton is excitable in astrocytes and how the homeostatic function of astrocytes is linked to the dynamics of the cytoskeleton. Here it is shown that homeostatic regulation involves the excitable dynamics of actin in certain subcellular regions of astrocytes, especially near the cell boundary. The results further indicate that actin dynamics concentrate into "hotspot" regions that selectively respond to certain chemophysical stimuli, specifically the homeostatic challenges of ion or water concentration increases. Substrate topography makes the actin dynamics of astrocytes weaker. Super-resolution images demonstrate that surface topography is also associated with the predominant perpendicular alignment of actin filaments near the cell boundary, whereas flat substrates result in an actin cortex mainly parallel to the cell boundary. Additionally, coculture with neurons increases both the probability of actin dynamics and the strength of hotspots. The excitable systems character of actin thus makes astrocytes direct participants in neural cell network dynamics.

Activity-dependent translation dynamically alters the proteome of the perisynaptic astrocyte process

Within eukaryotic cells, translation is regulated independent of transcription, enabling nuanced, localized, and rapid responses to stimuli. Neurons respond transcriptionally and translationally to synaptic activity. Although transcriptional responses are documented in astrocytes, here we test whether astrocytes have programmed translational responses. We show that seizure activity rapidly changes the transcripts on astrocyte ribosomes, some predicted to be downstream of BDNF signaling. In acute slices, we quantify the extent to which cues of neuronal activity activate translation in astrocytes and show that this translational response requires the presence of neurons, indicating that the response is non-cell autonomous. We also show that this induction of new translation extends into the periphery of astrocytes. Finally, synaptic proteomics show that new translation is required for changes that occur in perisynaptic astrocyte protein composition after fear conditioning. Regulation of translation in astrocytes by neuronal activity suggests an additional mechanism by which astrocytes may dynamically modulate nervous system functioning.

Emerging Roles of Microfluidics in Brain Research: From Cerebral Fluids Manipulation to Brain-on-a-Chip and Neuroelectronic Devices Engineering

Remarkable progress made in the past few decades in brain research enables the manipulation of neuronal activity in single neurons and neural circuits and thus allows the decipherment of relations between nervous systems and behavior. The discovery of glymphatic and lymphatic systems in the brain and the recently unveiled tight relations between the gastrointestinal (GI) tract and the central nervous system (CNS) further revolutionize our understanding of brain structures and functions. Fundamental questions about how neurons conduct two-way communications with the gut to establish the gut-brain axis (GBA) and interact with essential brain components such as glial cells and blood vessels to regulate cerebral blood flow (CBF) and cerebrospinal fluid (CSF) in health and disease, however, remain. Microfluidics with unparalleled advantages in the control of fluids at microscale has emerged recently as an effective approach to address these critical questions in brain research. The dynamics of cerebral fluids (i.e., blood and CSF) and novel in vitro brain-on-a-chip models and microfluidic-integrated multifunctional neuroelectronic devices, for example, have been investigated. This review starts with a critical discussion of the current understanding of several key topics in brain research such as neurovascular coupling (NVC), glymphatic pathway, and GBA and then interrogates a wide range of microfluidic-based approaches that have been developed or can be improved to advance our fundamental understanding of brain functions. Last, emerging technologies for structuring microfluidic devices and their implications and future directions in brain research are discussed.

Methamphetamine Dysregulation of the Central Nervous System and Peripheral Immunity

Methamphetamine (METH) is a potent psychostimulant that increases extracellular monoamines, such as dopamine and norepinephrine, and affects multiple tissue and cell types in the central nervous system (CNS) and peripheral immune cells. The reinforcing properties of METH underlie its significant abuse potential and dysregulation of peripheral immunity and central nervous system functions. Together, the constellation of METH's effects on cellular targets and regulatory processes has led to immune suppression and neurodegeneration in METH addicts and animal models of METH exposure. Here we extensively review many of the cell types and mechanisms of METH-induced dysregulation of the central nervous and peripheral immune systems. SIGNIFICANCE STATEMENT: Emerging research has begun to show that methamphetamine regulates dopaminergic neuronal activity. In addition, METH affects non-neuronal brain cells, such as microglia and astrocytes, and immunological cells of the periphery. Concurrent disruption of bidirectional communication between dopaminergic neurons and glia in the CNS and peripheral immune cell dysregulation gives rise to a constellation of dysfunctional neuronal, cell, and tissue types. Therefore, understanding the pathophysiology of METH requires consideration of the multiple targets at the interface between basic and clinical neuroscience.

A Functional Signature in the Developing Cerebellum: Evidence From a Preclinical Model of Autism

Autism spectrum disorders (ASD) are pervasive neurodevelopmental conditions detected during childhood when delayed language onset and social deficits are observed. Children diagnosed with ASD frequently display sensorimotor deficits associated with the cerebellum, suggesting a dysfunction of synaptic circuits. Astroglia are part of the tripartite synapses and postmortem studies reported an increased expression of the glial fibrillary acidic protein (GFAP) in the cerebellum of ASD patients. Astroglia respond to neuronal activity with calcium transients that propagate to neighboring cells, resulting in a functional response known as a calcium wave. This form of intercellular signaling is implicated in proliferation, migration, and differentiation of neural precursors. Prenatal exposure to valproate (VPA) is a preclinical model of ASD in which premature migration and excess of apoptosis occur in the internal granular layer (IGL) of the cerebellum during the early postnatal period. In this study we tested calcium wave propagation in the IGL of mice prenatally exposed to VPA. Sensorimotor deficits were observed and IGL depolarization evoked a calcium wave with astrocyte recruitment. The calcium wave propagation, initial cell recruitment, and mean amplitude of the calcium transients increased significantly in VPA-exposed mice compared to the control group. Astrocyte recruitment was significantly increased in the VPA model, but the mean amplitude of the calcium transients was unchanged. Western blot and histological studies revealed an increased expression of GFAP, higher astroglial density and augmented morphological complexity. We conclude that the functional signature of the IGL is remarkably augmented in the preclinical model of autism.

Analysing Intercellular Communication in Astrocytic Networks Using "Astral"

Astrocytic networks are critically involved in regulating the activity of neuronal networks. However, a comprehensive and ready-to-use data analysis tool for investigating functional interactions between the astrocytes is missing. We developed the novel software package named "Astral" to analyse intercellular communication in astrocytic networks based on live-cell calcium imaging. Our method for analysing calcium imaging data does not require the assignment of regions of interest. The package contains two applications: the core processing pipeline for detecting and quantifying Ca⁺⁺ events, and the auxiliary visualization tool for controlling data quality. Our method allows for the network-wide quantification of Ca⁺⁺ events and the analysis of their intercellular propagation. In a set of proof-of-concept experiments, we examined Ca⁺⁺ events in flat monolayers of primary astrocytes and confirmed that inter-astrocytic interactions depend on the permeability of gap junctions and connexin hemichannels. The Astral tool is particularly useful for studying astrocyte-neuronal interactions on the network level. We demonstrate that compared with purely astrocytic cultures, spontaneous generation of Ca⁺⁺ events in astrocytes that were co-cultivated with neurons was significantly increased. Interestingly, the increased astrocytic Ca⁺⁺ activity after long-term co-cultivation with neurons was driven by the enhanced formation of gap junctions and connexin hemichannels but was not affected by silencing neuronal activity. Our data indicate the necessity for systematic investigation of astrocyte-neuronal interactions at the network level. For this purpose, the Astral software offers a powerful tool for processing and quantifying calcium imaging data.

Dopamine controls neuronal spontaneous calcium oscillations via astrocytic signal

Dopamine is a neuromodulator and neurotransmitter responsible for a number of physiological processes. Dysfunctions of the dopamine metabolism and signalling are associated with neurological and psychiatric diseases. Here we report that in primary co-culture of neurons and astrocytes dopamine-induces calcium signal in astrocytes and suppress spontaneous synchronous calcium oscillations (SSCO) in neurons. Effect of dopamine on SSCO in neurons was dependent on calcium signal in astrocytes and could be modified by inhibition of dopamine-induced calcium signal or by stimulation of astrocytic calcium rise with ATP. Ability of dopamine to suppress SSCO in neurons was independent on D1- or D2- like receptors but dependent on GABA and alpha-adrenoreceptors. Inhibitor of monoaminoxidase bifemelane blocked effect of dopamine on astrocytes but also inhibited the effect dopamine on SSCO in neurons. These findings suggest that dopamine-induced calcium signal may stimulate release of neuromodulators such as GABA and adrenaline and thus suppress spontaneous calcium oscillations in neurons.

Kidins220/ARMS controls astrocyte calcium signaling and neuron-astrocyte communication

Through their ability to modulate synaptic transmission, glial cells are key regulators of neuronal circuit formation and activity. Kidins220/ARMS (kinase-D interacting substrate of 220 kDa/ankyrin repeat-rich membrane spanning) is one of the key effectors of the neurotrophin pathways in neurons where it is required for differentiation, survival, and plasticity. However, its role in glial cells remains largely unknown. Here, we show that ablation of Kidins220 in primary cultured astrocytes induced defects in calcium (Ca2+) signaling that were linked to altered store-operated Ca2+ entry and strong overexpression of the transient receptor potential channel TRPV4. Moreover, Kidins220-/- astrocytes were more sensitive to genotoxic stress. We also show that Kidins220 expression in astrocytes is required for the establishment of proper connectivity of cocultured wild-type neurons. Altogether, our data reveal a previously unidentified role for astrocyte-expressed Kidins220 in the control of glial Ca2+ dynamics, survival/death pathways and astrocyte-neuron communication.

Disorders of human consciousness in the Tri-partite synapses

Conscious Action Theory extends quantum theory to macroscopic phenomena and suggests physical correlates of consciousness occur at the intersection of external measurement signals and internally generated signals from memories that model the outside world. This physical theory predicts conscious phenomena happen at all scales and differ only by the size and complexity of material organizations involved. At the scale of the human "Brain" consciousness is predicted to happen where the processing loop of activity in the Glial network interfaces with the real world input-output processing loop of the Nuronal network. This happens at the Tripartite synapses creating an intersection plenum in biological systems that produces the experience of empty space and the objects it contains. Analysis of the transmitter-receptor strengths implementing the control and feedback between the Glial and Neuronal networks indicate imbalances can be directly related to schizophrenia, mania, epilepsy, and depression. This paper addresses three topics supporting the above mechanisms for normal consciousness functioning and its medical deviations. First we preset the architecture of a pan-psychic physical theory, which supports the hypothesis that tri-partite synapses are the location of human conscious experience. Second we discuss the inner workings of the Glial network to support long term memory and control functions corresponding to the inner feeling of the "I" self. Third, we consider the relation between psychiatric conditions and the balance states between the number of neuronal transmitters and astrocytic receptors.

Examining the relationship between astrocyte dysfunction and neurodegeneration in ALS using hiPSCs

Amyotrophic lateral sclerosis (ALS) is a complex and fatal neurodegenerative disease for which the causes of disease onset and progression remain unclear. Recent advances in human induced pluripotent stem cell (hiPSC)-based models permit the study of the genetic factors associated with ALS in patient-derived neural cell types, including motor neurons and glia. While astrocyte dysfunction has traditionally been thought to exacerbate disease progression, astrocytic dysfunction may play a more direct role in disease initiation and progression. Such non-cell autonomous mechanisms expand the potential targets of therapeutic intervention, but only a handful of ALS risk-associated genes have been examined for their impact on astrocyte dysfunction and neurodegeneration. This review summarizes what is currently known about astrocyte function in ALS and suggests ways in which hiPSC-based models can be used to more effectively study the role of astrocytes in neurodegenerative disease.

Angioneurins - Key regulators of blood-brain barrier integrity during hypoxic and ischemic brain injury

The loss of blood-brain barrier (BBB) integrity leading to vasogenic edema and brain swelling is a common feature of hypoxic/ischemic brain diseases such as stroke, but is also central to the etiology of other CNS disorders. In the past decades, numerous proteins, belonging to the family of angioneurins, have gained increasing attention as potential therapeutic targets for ischemic stroke, but also other CNS diseases attributed to BBB dysfunction. Angioneurins encompass mediators that affect both neuronal and vascular function. Recently, increasing evidence has been accumulated that certain angioneurins critically determine disease progression and outcome in stroke among others through multifaceted effects on the compromised BBB. Here, we will give a concise overview about the family of angioneurins. We further describe the most important cellular and molecular components that contribute to structural integrity and low permeability of the BBB under steady-state conditions. We then discuss BBB alterations in ischemic stroke, and highlight underlying cellular and molecular mechanisms. For the most prominent angioneurin family members including vascular endothelial growth factors, angiopoietins, platelet-derived growth factors and erythropoietin, we will summarize current scientific literature from experimental studies in animal models, and if available from clinical trials, on the following points: (i) spatiotemporal expression of these factors in the healthy and hypoxic/ischemic CNS, (ii) impact of loss- or gain-of-function during cerebral hypoxia/ischemia for BBB integrity and beyond, and (iii) potential underlying molecular mechanisms. Moreover, we will highlight novel therapeutic strategies based on the activation of endogenous angioneurins that might improve BBB dysfuntion during ischemic stroke.

Role of astroglia in diet-induced central neuroplasticity

Obesity, characterized by increased adiposity that develops when energy intake outweighs expenditure, is rapidly becoming a serious health crisis that affects millions of people worldwide and is associated with severe comorbid disorders including hypertension, cardiovascular disease, and type II diabetes. Obesity is also associated with the dysregulation of central neurocircuits involved in the control of autonomic, metabolic, and cognitive functions. Systemic inflammation associated with diet-induced obesity (DIO) has been proposed to be responsible for the development of these comorbidities as well as the dysregulation of central neurocircuits. A growing body of evidence suggests, however, that exposure to a high-fat diet (HFD) may cause neuroinflammation and astroglial activation even before systemic inflammation develops, which may be sufficient to cause dysregulation of central neurocircuits involved in energy homeostasis before the development of obesity. The purpose of this review is to summarize the current literature exploring astroglial-dependent modulation of central circuits following exposure to HFD and DIO, including not only dysregulation of neurocircuits involved in energy homeostasis and feeding behavior, but also the dysregulation of learning, memory, mood, and reward pathways.

Lactate transport facilitates neurite outgrowth

How glia affect neurite outgrowth during neural development has not been well elucidated. In the present study, we found that disruption of lactate production using 1,4-dideoxy-1,4-imino-D-arabinitol (DAB) and isofagomine significantly interfered with neurite outgrowth and that exogenous application of L-lactate rescued neurite growth failure. Monocarboxylate transporter-2-knockout, which blocked the lactate shuttle in neurons, showed a remarkable decrease in the length of axons and dendrites. We further demonstrated that Akt activity was decreased while glycogen synthase kinase 3β (GSK3β) activity was increased after astrocytic glycogen phosphorylase blockade. Additionally, GSK3βSer9 mutation reversed neurite growth failure caused by DAB and isofagomine. Our results suggested that lactate transportation played a critical role in neural development and disruption of the lactate shuttle in quiescent condition also affected neurite outgrowth in the central nervous system.

Astroglia contribute to the pathogenesis of spinocerebellar ataxia Type 1 (SCA1) in a biphasic, stage-of-disease specific manner

Spinocerebellar ataxia type 1 (SCA1) is a fatal, dominantly inherited neurodegenerative disease caused by the expansion of CAG repeats in the Ataxin-1 (ATXN1) gene. SCA1 is characterized by balance and coordination deficits due to the predominant loss of Purkinje neurons in the cerebellum. We previously demonstrated that cerebellar astrogliosis beings during the early stages of SCA1, prior to onset of motor deficits and loss of Purkinje neurons. We communicate here that cerebellar astrogliosis contributes to SCA1 pathogenesis in a biphasic, stage of disease dependent manner. We modulated astrogliosis by selectively reducing pro-inflammatory transcriptional regulator nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) signaling in astroglia via a Cre-lox mouse genetic approach. Our results indicate that inhibition of astroglial NF-κB signaling, prior to motor deficit onset, exacerbates disease severity. This is suggestive of a neuroprotective role mediated by astroglia during early stage SCA1. In contrast, inhibition of astroglial NF-κB signaling during late stage of disease ameliorated motor deficits, indicating a potentially harmful role of astroglia late in SCA1. These results indicate that astrogliosis may have a critical and dual role in disease. If so, our results imply that anti-inflammatory astroglia-based therapeutic approaches may need to consider disease progression to achieve therapeutic efficacy.

At the Origin of the History of Glia

The history of brain science is dominated by the study of neurons. However, there are as many glial cells as neurons in the human brain, their complexity increases during evolution, and glial cells play important roles in brain function, behavior, and neurological disorders. Although neurons and glial cells were first described at the same time in the early 19th century, why did the physiological study of glial cells only begin in the 1950s? What are the scientific breakthroughs and conceptual shifts that determined the history of glial cells in relation to that of neurons? What is the impact of the history of glia on the evolution of neuroscience? In order to answer these questions, we reconstructed the history of glial cells, from their first description until the mid-20th century, by examining the relative role of technical developments and scientific interpretations.

An autocrine purinergic signaling controls astrocyte-induced neuronal excitation

Astrocyte-derived gliotransmitters glutamate and ATP modulate neuronal activity. It remains unclear, however, how astrocytes control the release and coordinate the actions of these gliotransmitters. Using transgenic expression of the light-sensitive channelrhodopsin 2 (ChR2) in astrocytes, we observed that photostimulation reliably increases action potential firing of hippocampal pyramidal neurons. This excitation relies primarily on a calcium-dependent glutamate release by astrocytes that activates neuronal extra-synaptic NMDA receptors. Remarkably, our results show that ChR2-induced Ca²⁺ increase and subsequent glutamate release are amplified by ATP/ADP-mediated autocrine activation of P2Y1 receptors on astrocytes. Thus, neuronal excitation is promoted by a synergistic action of glutamatergic and autocrine purinergic signaling in astrocytes. This new mechanism may be particularly relevant for pathological conditions in which ATP extracellular concentration is increased and acts as a major danger signal.

Optogenetic Stimulation Increases Level of Antiapoptotic Protein Bcl-xL in Neurons

The antiapoptotic protein Bcl-xL is associated with several neuroplastic processes such as formation of synapses, regulation of spontaneous and evoked synaptic responses, and release of neurotransmitters. Dependence of expression on activity of neurons is characteristic for many proteins participating in regulation of neuroplasticity. Whether such property is exhibited by the Bcl-xL protein was analyzed using in vivo optogenetic stimulation of hippocampal glutamatergic neurons expressing channelrhodopsin ChR2H134 under CAMKIIa promoter in the adeno-associated viral vector, followed by immunohistochemical determination of the level of Bcl-xL protein in these neurons and surrounding cells. Increase in the level of early response c-Fos protein following illumination with blue light was indicative of activation of these hippocampal neurons. The optogenetic activation of hippocampus resulted in a significant increase in the level of antiapoptotic protein Bcl-xL in the photosensitive neurons as well as in the surrounding cells. The dependence of the level of expression of Bcl-xL protein on the activity of neurons indicates that this protein possesses one more important property that is essential for participation in neuroplastic processes in the brain.

Role of astrocytic glutamate transporter in alcohol use disorder

Alcohol use disorder (AUD) is one of the most widespread neuropsychiatric conditions, having a significant health and socioeconomic impact. According to the 2014 World Health Organization global status report on alcohol and health, the harmful use of alcohol is responsible for 5.9% of all deaths worldwide. Additionally, 5.1% of the global burden of disease and injury is ascribed to alcohol (measured in disability adjusted life years, or disability adjusted life years). Although the neurobiological basis of AUD is highly complex, the corticostriatal circuit contributes significantly to the development of addictive behaviors. In-depth investigation into the changes of the neurotransmitters in this circuit, dopamine, gamma-aminobutyricacid, and glutamate, and their corresponding neuronal receptors in AUD and other addictions enable us to understand the molecular basis of AUD. However, these discoveries have also revealed a dearth of knowledge regarding contributions from non-neuronal sources. Astrocytes, though intimately involved in synaptic function, had until recently been noticeably overlooked in their potential role in AUD. One major function of the astrocyte is protecting neurons from excitotoxicity by removing glutamate from the synapse via excitatory amino acid transporter type 2. The importance of this key transporter in addiction, as well as ethanol withdrawal, has recently become evident, though its regulation is still under investigation. Historically, pharmacotherapy for AUD has been focused on altering the activity of neuronal glutamate receptors. However, recent clinical evidence has supported the animal-based findings, showing that regulating glutamate homeostasis contributes to successful management of recovery from AUD.

Neurotrophic Activity of Cultured Cell Line U87 is Up-Regulated by Proline-Rich Polypeptide Complex and Its Constituent Nonapeptide

Neurotrophins such as nerve growth factor (NGF) and brain-derived neurotrophic factor, as well as cytokines, for example, interleukin-6 (IL-6) play an important role in neuroprotection and in the control of the central nervous system (CNS) function. Reduced expression of neurotrophic factors can lead to dysregulation of neuron function and neuronal death. There is also evidence for mutual interactions between neurotrophins and IL-6. Therefore, the up-regulating the level of neuroprotective substances is one of the key manners to control the nervous system development and function. It can be a promising aim in the therapy of neurodegenerative disease in which the decreased level of neurotrophins is observed. In our recent studies, the role of proline-rich polypeptide complex (PRP) and its nonapeptide fragment (NP) in the regulation of neurotrophic activity in cultured astrocytes was shown. PRP and NP stimulate human astrocytoma cell line U87 to release the significant amounts of NGF to the extracellular space both in its precursor and mature form. We also provide the evidence that in NP-treated cells, the level of βNGF mRNA was increased. NP-treated cells used in this study produced also increasing amounts of IL-6. This finding indicates that PRP and its nonapeptide fragment NP up-regulate neurotrophic activity of U87 cell line by increase of NGF synthesis and its release into the extracellular space. It was also shown that NP-dependent increased production of IL-6 can enhance the NGF activity.

Astrocyte regulation of blood flow in the brain

Neuronal activity results in increased blood flow in the brain, a response named functional hyperemia. Astrocytes play an important role in mediating this response. Neurotransmitters released from active neurons evoke Ca(2+) increases in astrocytes, leading to the release of vasoactive metabolites of arachidonic acid from astrocyte endfeet onto blood vessels. Synthesis of prostaglandin E2 (PGE2) and epoxyeicosatrienoic acids (EETs) dilate blood vessels, whereas 20-hydroxyeicosatetraenoic acid (20-HETE) constricts vessels. The release of K(+) from astrocyte endfeet may also contribute to vasodilation. Oxygen modulates astrocyte regulation of blood flow. Under normoxic conditions, astrocytic Ca(2+) signaling results in vasodilation, whereas under hyperoxic conditions, vasoconstriction is favored. Astrocytes also contribute to the generation of vascular tone. Tonic release of both 20-HETE and ATP from astrocytes constricts vascular smooth muscle cells, generating vessel tone. Under pathological conditions, including Alzheimer's disease and diabetic retinopathy, disruption of normal astrocyte physiology can compromise the regulation of blood flow.

GABA as a rising gliotransmitter

Gamma-amino butyric acid (GABA) is the major inhibitory neurotransmitter that is known to be synthesized and released from GABAergic neurons in the brain. However, recent studies have shown that not only neurons but also astrocytes contain a considerable amount of GABA that can be released and activate GABA receptors in neighboring neurons. These exciting new findings for glial GABA raise further interesting questions about the source of GABA, its mechanism of release and regulation and the functional role of glial GABA. In this review, we highlight recent studies that identify the presence and release of GABA in glial cells, we show several proposed potential pathways for accumulation and modulation of glial intracellular and extracellular GABA content, and finally we discuss functional roles for glial GABA in the brain.

A statistical method of identifying interactions in neuron-glia systems based on functional multicell Ca2+ imaging

Crosstalk between neurons and glia may constitute a significant part of information processing in the brain. We present a novel method of statistically identifying interactions in a neuron-glia network. We attempted to identify neuron-glia interactions from neuronal and glial activities via maximum-a-posteriori (MAP)-based parameter estimation by developing a generalized linear model (GLM) of a neuron-glia network. The interactions in our interest included functional connectivity and response functions. We evaluated the cross-validated likelihood of GLMs that resulted from the addition or removal of connections to confirm the existence of specific neuron-to-glia or glia-to-neuron connections. We only accepted addition or removal when the modification improved the cross-validated likelihood. We applied the method to a high-throughput, multicellular in vitro Ca2+ imaging dataset obtained from the CA3 region of a rat hippocampus, and then evaluated the reliability of connectivity estimates using a statistical test based on a surrogate method. Our findings based on the estimated connectivity were in good agreement with currently available physiological knowledge, suggesting our method can elucidate undiscovered functions of neuron-glia systems.

Arsenic exposure and glutamate-induced gliotransmitter release from astrocytes

The present study used cultures of primary astrocytes, isolated from neonatal rats, to verify the hypothesis that arsenite-induced neurotoxicity can influence neuronal function by altering glutamate-induced gliotransmitter release. Primary astrocytes were exposed to 0, 2.5, 5, 10, 20 or 30 µM arsenite for 24 hours. Cell viability and morphological observations revealed that 5 µM arsenic exposure could induce cytotoxicity. Cells were then cultured in the presence of 0, 2.5, 5, or 10 µM arsenite for 24 hours and stimulated with 25 µM glutamate for 10 minutes. Results showed that [Ca²⁺]_i in astrocytes exposed to 5 and 10 µM arsenite was significantly increased and levels of D-serine, γ-aminobutyric acid and glycine in cultures exposed to 2.5–10 µM arsenite were also increased. However, glutamate levels in the media were significantly increased only after treatment with 10 µM arsenite. In conclusion, our findings suggest that arsenic exposure may affect glutamate-induced gliotransmitter release from astrocytes and further disturb neuronal function.

Thiamine deficiency: an update of pathophysiologic mechanisms and future therapeutic considerations

Thiamine is an essential vitamin that is necessary to maintain the functional integrity of cells in the brain. Its deficiency is the underlying cause of Wernicke's encephalopathy (WE), a disorder primarily associated with, but not limited to, chronic alcoholism. Thiamine deficiency leads to the development of impaired energy metabolism due to mitochondrial dysfunction in focal regions of the brain resulting in cerebral vulnerability. The consequences of this include oxidative stress, excitotoxicity, inflammatory responses, decreased neurogenesis, blood-brain barrier disruption, lactic acidosis and a reduction in astrocyte functional integrity involving a loss of glutamate transporters and other astrocyte-specific proteins which together contribute in a major way to the resulting neurodegeneration. Exactly how these factors acting in concert lead to the demise of neurons is unclear. In this review we reassess their relative importance in the light of more recent findings and discuss therapeutic possibilities that may provide hope for the future for individuals with WE.

Maternal dietary loads of alpha-tocopherol increase synapse density and glial synaptic coverage in the hippocampus of adult offspring

An increased intake of the antioxidant α-Tocopherol (vitamin E) is recommended in complicated pregnancies, to prevent free radical damage to mother and fetus. However, the anti-PKC and antimitotic activity of α-Tocopherol raises concerns about its potential effects on brain development. Recently, we found that maternal dietary loads of α-Tocopherol through pregnancy and lactation cause developmental deficit in hippocampal synaptic plasticity in rat offspring. The defect persisted into adulthood, with behavioral alterations in hippocampus-dependent learning. Here, using the same rat model of maternal supplementation, ultrastructural morphometric studies were carried out to provide mechanistic interpretation to such a functional impairment in adult offspring by the occurrence of long-term changes in density and morphological features of hippocampal synapses. Higher density of axo-spinous synapses was found in CA1 stratum radiatum of α-Tocopherol-exposed rats compared to controls, pointing to a reduced synapse pruning. No morphometric changes were found in synaptic ultrastructural features, i.e., perimeter of axon terminals, length of synaptic specializations, extension of bouton-spine contact. Glia-synapse anatomical relationship was also affected. Heavier astrocytic coverage of synapses was observed in Tocopherol-treated offspring, notably surrounding axon terminals; moreover, the percentage of synapses contacted by astrocytic endfeet at bouton-spine interface (tripartite synapses) was increased. These findings indicate that gestational and neonatal exposure to supranutritional tocopherol intake can result in anatomical changes of offspring hippocampus that last through adulthood. These include a surplus of axo-spinous synapses and an aberrant glia-synapse relationship, which may represent the morphological signature of previously described alterations in synaptic plasticity and hippocampus-dependent learning.

Spontaneous and nicotine-induced Ca2+ oscillations mediated by Ca2+ influx in rat pinealocytes

The pineal gland regulates circadian rhythm through the synthesis and secretion of melatonin. The rise of intracellular Ca(2+) concentration ([Ca(2+)]i) following nicotinic acetylcholine receptor (nAChR) stimulation due to parasympathetic nerve activity downregulates melatonin production. Important characteristics and roles of Ca(2+) mobilization due to nAChR stimulation remain to be clarified. We report here that spontaneous Ca(2+) oscillations can be observed in ∼15% of the pinealocytes in slice preparations from rat pineal glands when this dissociation procedure is done within 6 h from a dark-to-light change. The frequency and half-life of [Ca(2+)]i rise were 0.86 min(-1) and 19 s, respectively. Similar spontaneous Ca(2+) oscillations were recorded in 17% of rat pinealocytes that were primary cultured for several days. Simultaneous measurement of [Ca(2+)]i and membrane potential revealed that spontaneous Ca(2+) oscillations were triggered by periodic membrane depolarizations. Spontaneous Ca(2+) oscillations in cultured pinealocytes were abolished by extracellular Ca(2+) removal or application of nifedipine, a blocker of voltage-dependent Ca(2+) channel (VDCC). In contrast, blockers of intracellular Ca(2+)-release channels, 2-aminoethoxydiphenylborate and ryanodine, have no effect. Our results also reveal that, in 23% quiescent pinealocytes, Ca(2+) oscillations were observed following the withdrawal of nicotine. Norepinephrine-induced melatonin secretion from whole pineal glands was significantly decreased by the coapplication of acetylcholine (ACh). This inhibitory effect of ACh was attenuated by nifedipine. In conclusion, both spontaneous and evoked Ca(2+) oscillations are due to membrane depolarization following activation of VDCCs. This consists of VDCC α1F subunit, and the associated Ca(2+) influx can strongly regulate melatonin secretion in pineal glands.

Controversies and evolving new mechanisms in subarachnoid hemorrhage

Despite decades of study, subarachnoid hemorrhage (SAH) continues to be a serious and significant health problem in the United States and worldwide. The mechanisms contributing to brain injury after SAH remain unclear. Traditionally, most in vivo research has heavily emphasized the basic mechanisms of SAH over the pathophysiological or morphological changes of delayed cerebral vasospasm after SAH. Unfortunately, the results of clinical trials based on this premise have mostly been disappointing, implicating some other pathophysiological factors, independent of vasospasm, as contributors to poor clinical outcomes. Delayed cerebral vasospasm is no longer the only culprit. In this review, we summarize recent data from both experimental and clinical studies of SAH and discuss the vast array of physiological dysfunctions following SAH that ultimately lead to cell death. Based on the progress in neurobiological understanding of SAH, the terms "early brain injury" and "delayed brain injury" are used according to the temporal progression of SAH-induced brain injury. Additionally, a new concept of the vasculo-neuronal-glia triad model for SAH study is highlighted and presents the challenges and opportunities of this model for future SAH applications.

Astrocytes differentially respond to inflammatory autoimmune insults and imbalances of neural activity

BACKGROUND

Neuronal activity intimately communicates with blood flow through the blood-brain barrier (BBB) in the central nervous system (CNS). Astrocyte endfeet cover more than 90% of brain capillaries and interact with synapses and nodes of Ranvier. The roles of astrocytes in neurovascular coupling in the CNS remain poorly understood.

RESULTS

Here we show that astrocytes that are intrinsically different are activated by inflammatory autoimmune insults and alterations of neuronal activity. In the progression of experimental autoimmune encephalomyelitis (EAE), both fibrous and protoplasmic astrocytes were broadly and reversibly activated in the brain and spinal cord, indicated by marked upregulation of glial fibrillary acidic protein (GFAP) and other astrocytic proteins. In early and remitting EAE, upregulated GFAP and astrocytic endfoot water channel aquaporin 4 (AQP4) enclosed white matter lesions in spinal cord, whereas they markedly increased and formed bundles in exacerbated lesions in late EAE. In cerebellar cortex, upregulation of astrocytic proteins correlated with EAE severity. On the other hand, protoplasmic astrocytes were also markedly activated in the brains of ankyrin-G (AnkG) and Kv3.1 KO mice, where neuronal activities are altered. Massive astrocytes replaced degenerated Purkinje neurons in AnkG KO mice. In Kv3.1 KO mice, GFAP staining significantly increased in cerebellar cortex, where Kv3.1 is normally highly expressed, but displayed in a patchy pattern in parts of the hippocampus.

CONCLUSIONS

Thus, astrocytes can detect changes in both blood and neurons, which supports their central role in neurovascular coupling. These studies contribute to the development of new strategies of neuroprotection and repair for various diseases, through activity-dependent regulation of neurovascular coupling.

Neurovascular interactions in the retina: physiological and pathological roles

Increasing evidence suggests that the complex interactions among multiple cell types including neuronal, glial, and vascular cells, are critical for maintaining adequate cerebral blood flow that is necessary for normal brain function and survival. The disturbance of these interactions contributes to the pathogenesis of central nervous system disorders such as stroke and Alzheimer's disease. The retina is part of the central nervous system, and the properties of vasculature in the retina are similar to those in the brain. The interactions among multiple cell types in the retina also play an important role in the maintenance of tissue homeostasis, and the impairment of interactions can contribute to the onset and/or progression of retinal diseases. In this review, we describe the neurovascular interactions in the retina and alternations of interactions in pathological conditions such as diabetic retinopathy and glaucoma.

Towards neuronal organoids: a method for long-term culturing of high-density hippocampal neurons

One of the goals in neuroscience is to obtain tractable laboratory cultures that closely recapitulate in vivo systems while still providing ease of use in the lab. Because neurons can exist in the body over a lifetime, long-term culture systems are necessary so as to closely mimic the physiological conditions under laboratory culture conditions. Ideally, such a neuronal organoid culture would contain multiple cell types, be highly differentiated, and have a high density of interconnected cells. However, before these types of cultures can be created, certain problems associated with long-term neuronal culturing must be addressed. We sought to develop a new protocol which may further prolong the duration and integrity of E18 rat hippocampal cultures. We have developed a protocol that allows for culturing of E18 hippocampal neurons at high densities for more than 120 days. These cultured hippocampal neurons are (i) well differentiated with high numbers of synapses, (ii) anchored securely to their substrate, (iii) have high levels of functional connectivity, and (iv) form dense multi-layered cellular networks. We propose that our culture methodology is likely to be effective for multiple neuronal subtypes-particularly those that can be grown in Neurobasal/B27 media. This methodology presents new avenues for long-term functional studies in neurons.

A dialogue between glia and neurons in the retina: modulation of neuronal excitability

Bidirectional signaling between neurons and glial cells has been demonstrated in brain slices and is believed to mediate glial modulation of synaptic transmission in the CNS. Our laboratory has characterized similar neuron-glia signaling in the mammalian retina. We find that light-evoked neuronal activity elicits Ca(2+) increases in Müller cells, which are specialized retinal glial cells. Neuron to glia signaling is likely mediated by the release of ATP from neurons and is potentiated by adenosine. Glia to neuron signaling has also been observed and is mediated by several mechanisms. Stimulation of glial cells can result in either facilitation or depression of synaptic transmission. Release of D-serine from Müller cells might also potentiate NMDA receptor transmission. Müller cells directly inhibit ganglion cells by releasing ATP, which, following hydrolysis to adenosine, activates neuronal A(1) receptors. The existence of bidirectional signaling mechanisms indicates that glial cells participate in information processing in the retina.

Astroglial networks and implications for therapeutic neuromodulation of epilepsy

Epilepsy is a common chronic neurologic disorder affecting approximately 1% of the world population. More than one-third of all epilepsy patients have incompletely controlled seizures or debilitating medication side effects in spite of optimal medical management. Medically refractory epilepsy is associated with excess injury and mortality, psychosocial dysfunction, and significant cognitive impairment. Effective treatment options for these patients can be limited. The cellular mechanisms underlying seizure activity are incompletely understood, though we here describe multiple lines of evidence supporting the likely contribution of astroglia to epilepsy, with focus on individual astrocytes and their network functions. Of the emerging therapeutic modalities for epilepsy, one of the most intriguing is the field of neuromodulation. Neuromodulatory treatment, which consists of administering electrical pulses to neural tissue to modulate its activity leading to a beneficial effect, may be an option for these patients. Current modalities consist of vagal nerve stimulation, open and closed-loop stimulation, and transcranial magnetic stimulation. Due to their unique properties, we here present astrocytes as likely important targets for the developing field of neuromodulation in the treatment of epilepsy.

Neuroprotective effect of sinapic acid in a mouse model of amyloid β(1-42) protein-induced Alzheimer's disease

Sinapic acid (SA) is a phenylpropanoid compound with anti-inflammatory and neuroprotective activities. The neuroprotective effects of SA in a mouse model of amyloid β (Aβ)(1-42) protein-induced Alzheimer's disease (AD) were investigated. Mice received a bilateral injection of Aβ(1-42) protein into the hippocampus to verify the efficacy of SA. Mice were treated with SA (10mg/kg/day, p.o.) for 7days beginning immediately after Aβ(1-42) protein injection, and an acquisition trial of the passive avoidance task was conducted 1h after the last administration of SA. Retention trial was conducted 24h after the acquisition trial, and mice were sacrificed for immunohistochemistry immediately after the retention trial. SA rescued neuronal cell death in the hippocampal CA1 region and also attenuated the increase of iNOS expression, glial cell activations and nitrotyrosine expressions induced by Aβ(1-42) protein. SA significantly attenuated memory impairment in the passive avoidance task. These results suggest that SA ameliorated Aβ(1-42) protein-related pathology including neuronal cell death and cognitive dysfunction via its anti-oxidative and anti-inflammatory activities, and may be an efficacious treatment for AD.

GABA release by hippocampal astrocytes

Astrocytes can directly influence neuronal activity through the release of various transmitters acting on membrane receptors expressed by neurons. However, in contrast to glutamate and ATP for instance, the release of GABA (γ-amino-butyric acid) by astrocytes is still poorly documented. Here, we used whole-cell recordings in rat acute brain slices and electron microscopy to test whether hippocampal astrocytes release the inhibitory transmitter GABA. We observed that slow transient inhibitory currents due to the activation of GABA_A receptors occur spontaneously in principal neurons of the three main hippocampal fields (CA1, CA3, and dentate gyrus). These currents share characteristics with the slow NMDA receptor-mediated currents previously shown to result from astrocytic glutamate release: they occur in the absence of synaptic transmission and have variable kinetics and amplitudes as well as low frequencies. Osmotic pressure reduction, known to enhance transmitter release from astrocytes, similarly increased the frequency of non-synaptic GABA and glutamate currents. Simultaneous occurrence of slow inhibitory and excitatory currents was extremely rare. Yet, electron microscopy examination of immunostained hippocampal sections shows that about 80% of hippocampal astrocytes [positive for glial fibrillary acidic protein (GFAP)] were immunostained for GABA. Our results provide quantitative characteristics of the astrocyte-to-neuron GABAergic signaling. They also suggest that all principal neurons of the hippocampal network are under a dual, excitatory and inhibitory, influence of astrocytes. The relevance of the astrocytic release of GABA, and glutamate, on the physiopathology of the hippocampus remains to be established.

Resveratrol preconditioning modulates inflammatory response in the rat hippocampus following global cerebral ischemia

Considerable evidence has been accumulated to suggests that blocking the inflammatory reaction promotes neuroprotection and shows therapeutic potential for clinical treatment of ischemic brain injury. Consequently, anti-inflammatory therapies are being explored for prevention and treatment of these diseases. Induction of brain tolerance against ischemia by pretreatment with resveratrol has been found to influence expression of different molecules. It remains unclear, however, whether and how resveratrol preconditioning changes expression of inflammatory mediators after subsequent global cerebral ischemia/reperfusion (I/R). Therefore, we investigated the effect of resveratrol pretreatment on NF-κB inflammatory cascade, COX-2, iNOS and JNK levels in experimental I/R. Adult male rats were subjected to 10 min of four-vessel occlusion and sacrificed at selected post-ischemic time points. Resveratrol (30 mg/kg) pretreatment was injected intraperitoneally 7 days prior to I/R induction. We found that resveratrol treatment before insult remarkably reduced astroglial and microglial activation at 7 days after I/R. It greatly attenuated I/R-induced NF-κB and JNK activation with decreased COX-2 and iNOS production. In conclusion, the neuroprotection of resveratrol preconditioning may be due in part to the suppression of the inflammatory response via regulation of NF-κB, COX-2 and iNOS induced by I/R. JNK was also suggested to play a protective role through in neuroprotection of resveratrol, which may also be contributing to reduction in neuroinflammation. The study adds to a growing literature that resveratrol can have important anti-inflammatory actions in the brain.

Barrel cortical neurons and astrocytes coordinately respond to an increased whisker stimulus frequency

Background

Nerve cells program the brain codes to manage well-organized cognitions and behaviors. It remains unclear how a population of neurons and astrocytes work coordinately to encode their spatial and temporal activity patterns in response to frequency and intensity signals from sensory inputs.

Results

With two-photon imaging and electrophysiology to record cellular functions in the barrel cortex in vivo, we analyzed the activity patterns of neurons and astrocytes in response to whisker stimuli with increasing frequency, an environmental stimulus pattern that rodents experience in the accelerated motion. Compared to the resting state, whisker stimulation caused barrel neurons and astrocytes to be activated more synchronously. An increased stimulus frequency up-regulated the activity strength of neurons and astrocytes as well as coordinated their interaction. The coordination among the barrel neurons and astrocytes was fulfilled by increasing their functional connections.

Conclusions

Our study reveals that the nerve cells in the barrel cortex encode frequency messages in whisker tactile inputs through setting their activity coordination.

Glial cells in (patho)physiology

Neuroglial cells define brain homeostasis and mount defense against pathological insults. Astroglia regulate neurogenesis and development of brain circuits. In the adult brain, astrocytes enter into intimate dynamic relationship with neurons, especially at synaptic sites where they functionally form the tripartite synapse. At these sites, astrocytes regulate ion and neurotransmitter homeostasis, metabolically support neurons and monitor synaptic activity; one of the readouts of the latter manifests in astrocytic intracellular Ca(2+) signals. This form of astrocytic excitability can lead to release of chemical transmitters via Ca(2+) -dependent exocytosis. Once in the extracellular space, gliotransmitters can modulate synaptic plasticity and cause changes in behavior. Besides these physiological tasks, astrocytes are fundamental for progression and outcome of neurological diseases. In Alzheimer's disease, for example, astrocytes may contribute to the etiology of this disorder. Highly lethal glial-derived tumors use signaling trickery to coerce normal brain cells to assist tumor invasiveness. This review not only sheds new light on the brain operation in health and disease, but also points to many unknowns.

Probing astrocyte function in fragile X syndrome

Astrocytes have been recognized as a class of cells that fill the space between neurons for more than a century. From their humble beginnings in the literature as merely space filling cells, an ever expanding list of functions in the CNS now exceeds the list of functions performed by neurons. In virtually all developmental and pathological conditions in the brain, astrocytes are involved in some capacity that directly affects neuronal function. Today we recognize that astrocytes are involved in the development and function of synaptic communication. Increasing evidence suggests that abnormal synaptic function may be a prominent contributing factor to the learning disability phenotype. With the discovery of FMRP in astrocytes, coupled with a role of astrocytes in synaptic function, research directed to glial neurobiology has never been more important. This chapter highlights the current knowledge of astrocyte function with a focus on their involvement in Fragile X syndrome.