Calcium elevation in astrocytes causes an NMDA receptor-dependent increase in the frequency of miniature synaptic currents in cultured hippocampal neurons

The role of periaqueductal gray astrocytes in anxiety-like behavior induced by acute stress

Astrocytes in the central nervous system play a vital role in modulating synaptic transmission and neuronal activation by releasing gliotransmitters. The 5-HTergic neurons in the ventrolateral periaqueductal gray (vlPAG) are important in anxiety processing. However, it remains uncertain whether the regulation of astrocytic activity on vlPAG 5-HTergic neurons is involved in anxiety processing. Here, through chemogenetic manipulation, we explored the impact of astrocytic activity in the PAG on the regulation of anxiety. To determine the role of astrocytes in the control of anxiety, we induced anxiety-like behaviors in mice through foot shock and investigated their effects on synaptic transmission and neuronal excitability in vlPAG 5-HTergic neurons. Foot shock caused anxiety-like behaviors, which were accompanied with the increase of the amplitude and frequency of miniature excitatory postsynaptic currents (mEPSCs), the area of slow inward currents (SICs), and the spike frequency of action potentials (AP) in vlPAG 5-HTergic neurons. The chemogenetic inhibition of vlPAG astrocytes was found to attenuate stress-induced anxiety-like behaviors and decrease the heightened synaptic transmission and neuronal excitability of vlPAG 5-HTergic neurons. Conversely, chemogenetic activation of vlPAG astrocytes triggered anxiety-like behaviors, enhanced synaptic transmission, and increased the excitability of vlPAG 5-HTergic neurons in unstressed mice. In summary, this study has provided initial insights into the pathway by which astrocytes influence behavior through the rapid regulation of associated neurons. This offers a new perspective for the investigation of the biological mechanisms underlying anxiety.

The role of glial cells in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) has traditionally been considered a neuron-centric disease. This view is now outdated, with increasing recognition of cell autonomous and non-cell autonomous contributions of central and peripheral nervous system glia to ALS pathomechanisms. With glial research rapidly accelerating, we comprehensively interrogate the roles of astrocytes, microglia, oligodendrocytes, ependymal cells, Schwann cells and satellite glia in nervous system physiology and ALS-associated pathology. Moreover, we highlight the inter-glial, glial-neuronal and inter-system polylogue which constitutes the healthy nervous system and destabilises in disease. We also propose classification based on function for complex glial reactive phenotypes and discuss the pre-requisite for integrative modelling to advance translation. Given the paucity of life-enhancing therapies currently available for ALS patients, we discuss the promising potential of harnessing glia in driving ALS therapeutic discovery.

Astroglial activation: Current concepts and future directions

Astrocytes are abundantly and ubiquitously expressed cell types with diverse functions throughout the central nervous system. Astrocytes show remarkable plasticity and exhibit morphological, molecular, and functional remodeling in response to injury, disease, or infection of the central nervous system, as evident in neurodegenerative diseases. Astroglial mediated inflammation plays a prominent role in the pathogenesis of neurodegenerative diseases. This review focus on the role of astrocytes as essential players in neuroinflammation and discuss their morphological and functional heterogeneity in the normal central nervous system and explore the spatial and temporal variations in astroglial phenotypes observed under different disease conditions. This review discusses the intimate relationship of astrocytes to pathological hallmarks of neurodegenerative diseases. Finally, this review considers the putative therapeutic strategies that can be deployed to modulate the astroglial functions in neurodegenerative diseases. HIGHLIGHTS: Astroglia mediated neuroinflammation plays a key role in the pathogenesis of neurodegenerative diseases. Activated astrocytes exhibit diverse phenotypes in a region-specific manner in brain and interact with β-amyloid, tau, and α-synuclein species as well as with microglia and neuronal circuits. Activated astrocytes are likely to influence the trajectory of disease progression of neurodegenerative diseases, as determined by the stage of disease, individual susceptibility, and state of astroglial priming. Modulation of astroglial activation may be a therapeutic strategy at various stages in the trajectory of neurodegenerative diseases to modify the disease course.

Nedd4-2-dependent regulation of astrocytic Kir4.1 and Connexin43 controls neuronal network activity

Nedd4-2 is an E3 ubiquitin ligase in which missense mutation is related to familial epilepsy, indicating its critical role in regulating neuronal network activity. However, Nedd4-2 substrates involved in neuronal network function have yet to be identified. Using mouse lines lacking Nedd4-1 and Nedd4-2, we identified astrocytic channel proteins inwardly rectifying K+ channel 4.1 (Kir4.1) and Connexin43 as Nedd4-2 substrates. We found that the expression of Kir4.1 and Connexin43 is increased upon conditional deletion of Nedd4-2 in astrocytes, leading to an elevation of astrocytic membrane ion permeability and gap junction activity, with a consequent reduction of γ-oscillatory neuronal network activity. Interestingly, our biochemical data demonstrate that missense mutations found in familial epileptic patients produce gain-of-function of the Nedd4-2 gene product. Our data reveal a process of coordinated astrocytic ion channel proteostasis that controls astrocyte function and astrocyte-dependent neuronal network activity and elucidate a potential mechanism by which aberrant Nedd4-2 function leads to epilepsy.

Role of purines in brain development, from neuronal proliferation to synaptic refinement

The purinergic system includes P1 and P2 receptors, which are activated by ATP and its metabolites. They are expressed in adult neuronal and glial cells and are crucial in brain function, including neuromodulation and neuronal signaling. As P1 and P2 receptors are expressed throughout embryogenesis and development, purinergic signaling also has an important role in the development of the peripheral and central nervous system. In this review, we present the expression pattern and activity of purinergic receptors and of their signaling pathways during embryonic and postnatal development of the nervous system. In particular, we review the involvement of the purinergic signaling in all the crucial steps of brain development i.e. in neural stem cell proliferation, neuronal differentiation and migration as well as in astrogliogenesis and oligodendrogenesis. Then, we review data showing a crucial role of the ATP and adenosine signaling pathways in the formation of the peripheral neuromuscular junction and of central GABAergic and glutamatergic synapses. Finally, we examine the consequences of deregulation of the purinergic system during development and discuss the therapeutic potential of targeting it at adult stage in diseases with reactivation of the ATP and adenosine pathway.

Dynamic amplitude modulation of microstimulation evokes biomimetic onset and offset transients and reduces depression of evoked calcium responses in sensory cortices

BACKGROUND

Intracortical microstimulation (ICMS) is an emerging approach to restore sensation to people with neurological injury or disease. Biomimetic microstimulation, or stimulus trains that mimic neural activity in the brain through encoding of onset and offset transients, could improve the utility of ICMS for brain-computer interface (BCI) applications, but how biomimetic microstimulation affects neural activation is not understood. Current "biomimetic" ICMS trains aim to reproduce the strong onset and offset transients evoked in the brain by sensory input through dynamic modulation of stimulus parameters. Stimulus induced depression of neural activity (decreases in evoked intensity over time) is also a potential barrier to clinical implementation of sensory feedback, and dynamic microstimulation may reduce this effect.

OBJECTIVE

We evaluated how bio-inspired ICMS trains with dynamic modulation of amplitude and/or frequency change the calcium response, spatial distribution, and depression of neurons in the somatosensory and visual cortices.

METHODS

Calcium responses of neurons were measured in Layer 2/3 of visual and somatosensory cortices of anesthetized GCaMP6s mice in response to ICMS trains with fixed amplitude and frequency (Fixed) and three dynamic ICMS trains that increased the stimulation intensity during the onset and offset of stimulation by modulating the amplitude (DynAmp), frequency (DynFreq), or amplitude and frequency (DynBoth). ICMS was provided for either 1-s with 4-s breaks (Short) or for 30-s with 15-s breaks (Long).

RESULTS

DynAmp and DynBoth trains evoked distinct onset and offset transients in recruited neural populations, while DynFreq trains evoked population activity similar to Fixed trains. Individual neurons had heterogeneous responses primarily based on how quickly they depressed to ICMS, where neurons farther from the electrode depressed faster and a small subpopulation (1-5%) were modulated by DynFreq trains. Neurons that depressed to Short trains were also more likely to depress to Long trains, but Long trains induced more depression overall due to the increased stimulation length. Increasing the amplitude during the hold phase resulted in an increase in recruitment and intensity which resulted in more depression and reduced offset responses. Dynamic amplitude modulation reduced stimulation induced depression by 14.6 ± 0.3% for Short and 36.1 ± 0.6% for Long trains. Ideal observers were 0.031 ± 0.009 s faster for onset detection and 1.33 ± 0.21 s faster for offset detection with dynamic amplitude encoding.

CONCLUSIONS

Dynamic amplitude modulation evokes distinct onset and offset transients, reduces depression of neural calcium activity, and decreases total charge injection for sensory feedback in BCIs by lowering recruitment of neurons during long maintained periods of ICMS. In contrast, dynamic frequency modulation evokes distinct onset and offset transients in a small subpopulation of neurons but also reduces depression in recruited neurons by reducing the rate of activation.

Receptor-receptor interactions and microvesicle exchange as mechanisms modulating signaling between neurons and astrocytes

It is well known that astrocytes play a significant metabolic role in the nervous tissue, maintaining the homeostasis of the extracellular space and of the blood-brain barrier, and providing trophic support to neurons. In addition, however, evidence exists indicating astrocytes as important elements for brain activity through signaling exchange with neurons. Astrocytes, indeed, can sense synaptic activity and their molecular machinery responds to neurotransmitters released by neurons with cytoplasmic Ca²⁺ elevations that, in turn, stimulate the release of neuroactive substances (gliotransmitters) influencing nearby neurons. In both cell types the recognition and transduction of this complex pattern of signals is mediated by specific receptors that are also involved in mechanisms tuning the intercellular cross-talk between astrocytes and neurons. Two of these mechanisms are the focus of the present discussion. The first concerns direct receptor-receptor interactions leading to the formation at the cell membrane of multimeric receptor complexes. The cooperativity that emerges in the actions of orthosteric and allosteric ligands of the monomers forming the assembly provides the cell decoding apparatus with sophisticated and flexible dynamics in terms of recognition and signal transduction pathways. A further mechanism of plasticity involving receptors is based on the transfer of elements of the cellular signaling apparatus via extracellular microvesicles acting as protective containers, which can lead to transient changes in the transmitting/decoding capabilities of the target cell.

The Role of Glutamate Receptors in Epilepsy

Glutamate is an essential excitatory neurotransmitter in the central nervous system, playing an indispensable role in neuronal development and memory formation. The dysregulation of glutamate receptors and the glutamatergic system is involved in numerous neurological and psychiatric disorders, especially epilepsy. There are two main classes of glutamate receptor, namely ionotropic and metabotropic (mGluRs) receptors. The former stimulate fast excitatory neurotransmission, are N-methyl-d-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), and kainate; while the latter are G-protein-coupled receptors that mediate glutamatergic activity via intracellular messenger systems. Glutamate, glutamate receptors, and regulation of astrocytes are significantly involved in the pathogenesis of acute seizure and chronic epilepsy. Some glutamate receptor antagonists have been shown to be effective for the treatment of epilepsy, and research and clinical trials are ongoing.

Revealing the contribution of astrocytes to glutamatergic neuronal transmission

Research on glutamatergic neurotransmission has focused mainly on the function of presynaptic and postsynaptic neurons, leaving astrocytes with a secondary role only to ensure successful neurotransmission. However, recent evidence indicates that astrocytes contribute actively and even regulate neuronal transmission at different levels. This review establishes a framework by comparing glutamatergic components between neurons and astrocytes to examine how astrocytes modulate or otherwise influence neuronal transmission. We have included the most recent findings about the role of astrocytes in neurotransmission, allowing us to understand the complex network of neuron-astrocyte interactions. However, despite the knowledge of synaptic modulation by astrocytes, their contribution to specific physiological and pathological conditions remains to be elucidated. A full understanding of the astrocyte's role in neuronal processing could open fruitful new frontiers in the development of therapeutic applications.

The Implication of Glial Metabotropic Glutamate Receptors in Alzheimer's Disease

Alzheimer's disease (AD) was first identified more than 100 years ago, yet aspects pertaining to its origin and the mechanisms underlying disease progression are not well known. To this date, there is no therapeutic approach or disease-modifying drug that could halt or at least delay disease progression. Until recently, glial cells were seen as secondary actors in brain homeostasis. Although this view was gradually refuted and the relevance of glial cells for the most diverse brain functions such as synaptic plasticity and neurotransmission was vastly proved, many aspects of its functioning, as well as its role in pathological conditions, remain poorly understood. Metabotropic glutamate receptors (mGluRs) in glial cells were shown to be involved in neuroinflammation and neurotoxicity. Besides its relevance for glial function, glutamatergic receptors are also central in the pathology of AD, and recent studies have shown that glial mGluRs play a role in the establishment and progression of AD. AD-related alterations in Ca2+ signalling, APP processing, and Aβ load, as well as AD-related neurodegeneration, are influenced by glial mGluRs. However, different types of mGluRs play different roles, depending on the cell type and brain region that is being analysed. Therefore, in this review, we focus on the current understanding of glial mGluRs and their implication in AD, providing an insight for future therapeutics and identifying existing research gaps worth investigating.

Calcium signaling in astrocytes and gliotransmitter release

Glia are as numerous in the brain as neurons and widely known to serve supportive roles such as structural scaffolding, extracellular ionic and neurotransmitter homeostasis, and metabolic support. However, over the past two decades, several lines of evidence indicate that astrocytes, which are a type of glia, play active roles in neural information processing. Astrocytes, although not electrically active, can exhibit a form of excitability by dynamic changes in intracellular calcium levels. They sense synaptic activity and release neuroactive substances, named gliotransmitters, that modulate neuronal activity and synaptic transmission in several brain areas, thus impacting animal behavior. This "dialogue" between astrocytes and neurons is embodied in the concept of the tripartite synapse that includes astrocytes as integral elements of synaptic function. Here, we review the recent work and discuss how astrocytes via calcium-mediated excitability modulate synaptic information processing at various spatial and time scales.

Hypothalamic astrocytes control systemic glucose metabolism and energy balance

The hypothalamus is key in the control of energy balance. However, strategies targeting hypothalamic neurons have failed to provide viable options to treat most metabolic diseases. Conversely, the role of astrocytes in systemic metabolic control has remained largely unexplored. Here, we show that obesity promotes anatomically restricted remodeling of hypothalamic astrocyte activity. In the paraventricular nucleus (PVN) of the hypothalamus, chemogenetic manipulation of astrocytes results in bidirectional control of neighboring neuron activity, autonomic outflow, glucose metabolism, and energy balance. This process recruits a mechanism involving the astrocytic control of ambient glutamate levels, which becomes defective in obesity. Positive or negative chemogenetic manipulation of PVN astrocyte Ca2+ signals, respectively, worsens or improves metabolic status of diet-induced obese mice. Collectively, these findings highlight a yet unappreciated role for astrocytes in the direct control of systemic metabolism and suggest potential targets for anti-obesity strategy.

In vivo spatiotemporal dynamics of astrocyte reactivity following neural electrode implantation

Brain computer interfaces (BCIs), including penetrating microelectrode arrays, enable both recording and stimulation of neural cells. However, device implantation inevitably causes injury to brain tissue and induces a foreign body response, leading to reduced recording performance and stimulation efficacy. Astrocytes in the healthy brain play multiple roles including regulating energy metabolism, homeostatic balance, transmission of neural signals, and neurovascular coupling. Following an insult to the brain, they are activated and gather around the site of injury. These reactive astrocytes have been regarded as one of the main contributors to the formation of a glial scar which affects the performance of microelectrode arrays. This study investigates the dynamics of astrocytes within the first 2 weeks after implantation of an intracortical microelectrode into the mouse brain using two-photon microscopy. From our observation astrocytes are highly dynamic during this period, exhibiting patterns of process extension, soma migration, morphological activation, and device encapsulation that are spatiotemporally distinct from other glial cells, such as microglia or oligodendrocyte precursor cells. This detailed characterization of astrocyte reactivity will help to better understand the tissue response to intracortical devices and lead to the development of more effective intervention strategies to improve the functional performance of neural interfacing technology.

Insulin-like growth factor 1 regulates excitatory synaptic transmission in pyramidal neurons from adult prefrontal cortex

Insulin-like growth factor 1 (IGF1) influences synaptic function in addition to its role in brain development and aging. Although the expression levels of IGF1 and IGF1 receptor (IGF1R) peak during development and decline with age, the adult brain has abundant IGF1 or IGF1R expression. Studies reveal that IGF1 regulates the synaptic transmission in neurons from young animals. However, the action of IGF1 on neurons in the adult brain is still unclear. Here, we used prefrontal cortical (PFC) slices from adult mice (∼8 weeks old) to characterize the role of IGF1 on excitatory synaptic transmission in pyramidal neurons and the underlying molecular mechanisms. We first validated IGF1R expression in pyramidal neurons using translating ribosomal affinity purification assay. Then, using whole-cell patch-clamp recording, we found that IGF1 attenuated the amplitude of evoked excitatory postsynaptic current (EPSC) without affecting the frequency and amplitude of miniature EPSC. Furthermore, this decrease in excitatory neurotransmission was blocked by pharmacological inhibition of IGF1R or conditionally knockdown of IGF1R in PFC pyramidal neurons. In addition, we determined that IGF1-induced decrease of EPSC amplitude was due to postsynaptic effect (internalization of a-amino-3-hydroxy-5-methyl-4- isoxazolepropionic acid receptors [AMPAR]) rather than presynaptic glutamate release. Finally, we found that inhibition of metabotropic glutamate receptor subtype-1 (mGluR1) abolished IGF1-induced attenuation of evoked EPSC amplitude and decrease of AMPAR expression at synaptic membrane, suggesting mGluR1-mediated endocytosis of AMPAR was involved. Taken together, these data provide the first evidence that IGF1 regulates excitatory synaptic transmission in adult PFC via the interaction between IGF1R-dependent signaling pathway and mGluR1-mediated AMPAR endocytosis.

A multilayer network model of neuron-astrocyte populations in vitro reveals mGluR5 inhibition is protective following traumatic injury

Astrocytes communicate bidirectionally with neurons, enhancing synaptic plasticity and promoting the synchronization of neuronal microcircuits. Despite recent advances in understanding neuron-astrocyte signaling, little is known about astrocytic modulation of neuronal activity at the population level, particularly in disease or following injury. We used high-speed calcium imaging of mixed cortical cultures in vitro to determine how population activity changes after disruption of glutamatergic signaling and mechanical injury. We constructed a multilayer network model of neuron-astrocyte connectivity, which captured distinct topology and response behavior from single-cell-type networks. mGluR₅ inhibition decreased neuronal activity, but did not on its own disrupt functional connectivity or network topology. In contrast, injury increased the strength, clustering, and efficiency of neuronal but not astrocytic networks, an effect that was not observed in networks pretreated with mGluR₅ inhibition. Comparison of spatial and functional connectivity revealed that functional connectivity is largely independent of spatial proximity at the microscale, but mechanical injury increased the spatial-functional correlation. Finally, we found that astrocyte segments of the same cell often belong to separate functional communities based on neuronal connectivity, suggesting that astrocyte segments function as independent entities. Our findings demonstrate the utility of multilayer network models for characterizing the multiscale connectivity of two distinct but functionally dependent cell populations.

Astrocytes communicate bidirectionally with neurons, enhancing synaptic plasticity and promoting the synchronization of neuronal microcircuits. We constructed a multilayer network model of neuron-astrocyte connectivity based on calcium activity in mixed cortical cultures, and used this model to evaluate the effect of glutamatergic inhibition and mechanical injury on network topology. We found that injury increased the strength, clustering, and efficiency of neuronal but not astrocytic networks, an effect that was not observed in injured networks pretreated with a glutamate receptor antagonist. Our findings demonstrate the utility of multilayer network models for characterizing the multiscale connectivity of two distinct but functionally dependent cell populations.

Challenges and opportunities of advanced gliomodulation technologies for excitation-inhibition balance of brain networks

Recent neuroscience studies have highlighted the critical role of glial cells in information processing. This has increased the demand for technologies that selectively modulate glial cells that regulate the excitation-inhibition balance of neural network function. Engineered technologies that modulate glial activity may be necessary for precise tuning of neural network activity in higher-order brain function. This perspective summarizes how glial cells regulate excitation and inhibition of neural circuits, highlights available technologies for glial modulation, and discusses current challenges and potential opportunities for glial engineering technologies.

Astrocytic Ca2+ Signaling in Epilepsy

Epilepsy is one of the most common neurological disorders – estimated to affect at least 65 million worldwide. Most of the epilepsy research has so far focused on how to dampen neuronal discharges and to explain how changes in intrinsic neuronal activity or network function cause seizures. As a result, pharmacological therapy has largely been limited to symptomatic treatment targeted at neurons. Given the expanding spectrum of functions ascribed to the non-neuronal constituents of the brain, in both physiological brain function and in brain disorders, it is natural to closely consider the roles of astrocytes in epilepsy. It is now widely accepted that astrocytes are key controllers of the composition of the extracellular fluids, and may directly interact with neurons by releasing gliotransmitters. A central tenet is that astrocytic intracellular Ca²⁺ signals promote release of such signaling substances, either through synaptic or non-synaptic mechanisms. Accruing evidence suggests that astrocytic Ca²⁺ signals play important roles in both seizures and epilepsy, and this review aims to highlight the current knowledge of the roles of this central astrocytic signaling mechanism in ictogenesis and epileptogenesis.

Restoring glutamate homeostasis in the nucleus accumbens via endocannabinoid-mimetic drug prevents relapse to cocaine seeking behavior in rats

Impaired glutamate homeostasis is a key characteristic of the neurobiology of drug addiction in rodent models and contributes to the vulnerability to relapse to drug seeking. Although disrupted astrocytic and presynaptic regulation of glutamate release has been considered to constitute with impaired glutamate homeostasis in rodent model of drug relapse, the involvement of endocannabinoids (eCBs) in this neurobiological process has remained largely unknown. Here, using cocaine self-administration in rats, we investigated the role of endocannabinoids in impaired glutamate homeostasis in the core of nucleus accumbens (NAcore), which was indicated by augmentation of spontaneous synaptic glutamate release, downregulation of metabotropic glutamate receptor 2/3 (mGluR2/3), and mGluR5-mediated astrocytic glutamate release. We found that the endocannabinoid, anandamide (AEA), rather than 2-arachidonoylglycerol elicited glutamate release through presynaptic transient receptor potential vanilloid 1 (TRPV1) and astrocytic cannabinoid type-1 receptors (CB1Rs) in the NAcore of saline-yoked rats. In rats with a history of cocaine self-administration and extinction training, AEA failed to alter synaptic glutamate release in the NAcore, whereas CB1R-mediated astrocytic glutamate release by AEA remained functional. In order to induce increased astrocytic glutamate release via exogenous AEA, (R)-methanandamide (methAEA, a metabolically stable form of AEA) was chronically infused in the NAcore via osmotic pumps during extinction training. Restoration of mGluR2/3 function and mGluR5-mediated astrocytic glutamate release was observed after chronic methAEA infusion. Additionally, priming or cue-induced reinstatement of cocaine seeking was inhibited in methAEA-infused rats. These results demonstrate that enhancing endocannabinoid signaling is a potential pathway to restore glutamate homeostasis and may represent a promising therapeutic strategy for preventing cocaine relapse.

Graphene glial-interfaces: challenges and perspectives

Graphene nanosheets are mechanically strong but flexible, electrically conductive and bio-compatible. Thus, due to these unique properties, they are being intensively studied as materials for the next generation of neural interfaces. Most of the literature focused on optimizing the interface between these materials and neurons. However, one of the most common causes of implant failure is the adverse inflammatory reaction of glial cells. These cells are not, as previously considered, just passive and supportive cells, but play a crucial role in the physiology and pathology of the nervous system, and in the interaction with implanted electrodes. Besides providing structural support to neurons, glia are responsible for the modulation of synaptic transmission and control of central and peripheral homeostasis. Accordingly, knowledge on the interaction between glia and biomaterials is essential to develop new implant-based therapies for the treatment of neurological disorders, such as epilepsy, brain tumours, and Alzheimer's and Parkinson's disease. This work provides an overview of the emerging literature on the interaction of graphene-based materials with glial cells, together with a complete description of the different types of glial cells and problems associated with them. We believe that this description will be important for researchers working in materials science and nanotechnology to develop new active materials to interface, measure and stimulate these cells.

Non-canonical glutamate signaling in a genetic model of migraine with aura

Migraine with aura is a common but poorly understood sensory circuit disorder. Monogenic models allow an opportunity to investigate its mechanisms, including spreading depolarization (SD), the phenomenon underlying migraine aura. Using fluorescent glutamate imaging, we show that awake mice carrying a familial hemiplegic migraine type 2 (FHM2) mutation have slower clearance during sensory processing, as well as previously undescribed spontaneous "plumes" of glutamate. Glutamatergic plumes overlapped anatomically with a reduced density of GLT-1a-positive astrocyte processes and were mimicked in wild-type animals by inhibiting glutamate clearance. Plume pharmacology and plume-like neural Ca2+ events were consistent with action-potential-independent spontaneous glutamate release, suggesting plumes are a consequence of inefficient clearance following synaptic release. Importantly, a rise in basal glutamate and plume frequency predicted the onset of SD in both FHM2 and wild-type mice, providing a novel mechanism in migraine with aura and, by extension, the other neurological disorders where SD occurs.

Modeling Neurodevelopmental and Neuropsychiatric Diseases with Astrocytes Derived from Human-Induced Pluripotent Stem Cells

Accumulating studies demonstrate the morphological and functional diversity of astrocytes, a subtype of glial cells in the central nervous system. Animal models are instrumental in advancing our understanding of the role of astrocytes in brain development and their contribution to neurological disease; however, substantial interspecies differences exist between rodent and human astrocytes, underscoring the importance of studying human astrocytes. Human pluripotent stem cell differentiation approaches allow the study of patient-specific astrocytes in the etiology of neurological disorders. In this review, we summarize the structural and functional properties of astrocytes, including the unique features of human astrocytes; demonstrate the necessity of the stem cell platform; and discuss how this platform has been applied to the research of neurodevelopmental and neuropsychiatric diseases.

Astroglial adrenoreceptors modulate synaptic transmission and contextual fear memory formation in dentate gyrus

Astrocytes perform various supporting functions, including ion buffering, metabolic supplying and neurotransmitter clearance. They can also sense neuronal activity owing to the presence of specific receptors for neurotransmitters. In turn, astrocytes can regulate synaptic activity through the release of gliotransmitters. Evidence has shown that astrocytes are very sensitive to the locus coeruleus (LC) afferents. However, little is known about how LC neuromodulatory norepinephrine (NE) modulates synaptic transmission through astrocytic activity. In mouse dentate gyrus (DG), we demonstrated an increase in the frequency of miniature excitatory postsynaptic currents (mEPSC) in response to NE, which required the release of glutamate from astrocytes. The rise in glutamate release probability is likely due to the activation of presynaptic GluN2B-containing NMDA receptors. Moreover, we showed that the activation of NE signaling in DG is necessary for the formation of contextual learning memory. Thus, NE signaling activation during fear conditioning training contributed to enduring changes in the frequency of mEPSC in DG. Our results strongly support the physiological neuromodulatory role of NE signaling, which is derived from activation of astrocytes.

Novel Therapeutic Approach for Excitatory/Inhibitory Imbalance in Neurodevelopmental and Neurodegenerative Diseases

Excitatory/inhibitory (E/I) balance, defined as the balance between excitation and inhibition of synaptic activity in a neuronal network, accounts in part for the normal functioning of the brain, controlling, for example, normal spike rate. In many pathological conditions, this fine balance is perturbed, leading to excessive or diminished excitation relative to inhibition, termed E/I imbalance, reflected in network dysfunction. E/I imbalance has emerged as a contributor to neurological disorders that occur particularly at the extremes of life, including autism spectrum disorder and Alzheimer's disease, pointing to the vulnerability of neuronal networks at these critical life stages. Hence, it is important to develop approaches to rebalance neural networks. In this review, we describe emerging therapies that can normalize the E/I ratio or the underlying abnormality that contributes to the imbalance in electrical activity, thus improving neurological function in these maladies.

Neuropeptides Modulate Local Astrocytes to Regulate Adult Hippocampal Neural Stem Cells

Neural stem cells (NSCs) in the dentate gyrus (DG) reside in a specialized local niche that supports their neurogenic proliferation to produce adult-born neurons throughout life. How local niche cells interact at the circuit level to ensure continuous neurogenesis from NSCs remains unknown. Here we report the role of endogenous neuropeptide cholecystokinin (CCK), released from dentate CCK interneurons, in regulating neurogenic niche cells and NSCs. Specifically, stimulating CCK release supports neurogenic proliferation of NSCs through a dominant astrocyte-mediated glutamatergic signaling cascade. In contrast, reducing dentate CCK induces reactive astrocytes, which correlates with decreased neurogenic proliferation of NSCs and upregulation of genes involved in immune processes. Our findings provide novel circuit-based information on how CCK acts on local astrocytes to regulate the key behavior of adult NSCs.

Astrocytic BDNF and TrkB regulate severity and neuronal activity in mouse models of temporal lobe epilepsy

Astrocytes have emerged as crucial regulators of neuronal network activity, synapse formation, and underlying behavioral and cognitive processes. Despite some pathways have been identified, the communication between astrocytes and neurons remains to be completely elucidated. Unraveling this communication is crucial to design potential treatments for neurological disorders like temporal lobe epilepsy (TLE). The BDNF and TrkB molecules have emerged as very promising therapeutic targets. However, their modulation can be accompanied by several off-target effects such as excitotoxicity in case of uncontrolled upregulation or dementia, amnesia, and other memory disorders in case of downregulation. Here, we show that BDNF and TrkB from astrocytes modulate neuronal dysfunction in TLE models. First, conditional overexpression of BDNF from astrocytes worsened the phenotype in the lithium-pilocarpine mouse model. Our evidences pointed out to the astrocytic pro-BDNF isoform as a major player of this altered phenotype. Conversely, specific genetic deletion of BDNF in astrocytes prevented the increase in the number of firing neurons and the global firing rate in an in vitro model of TLE. Regarding to the TrkB, we generated mice with a genetic deletion of TrkB specifically in hippocampal neurons or astrocytes. Interestingly, both lines displayed neuroprotection in the lithium-pilocarpine model but only the mice with genetic deletion of TrkB in astrocytes showed significantly preserved spatial learning skills. These data identify the astrocytic BDNF and TrkB molecules as promising therapeutic targets for the treatment of TLE.

G-Protein-Coupled Receptors in Astrocyte-Neuron Communication

Astrocytes, a major type of glial cell, are known to play key supportive roles in brain function, contributing to ion and neurotransmitter homeostasis, maintaining the blood-brain barrier and providing trophic and metabolic support for neurons. Besides these support functions, astrocytes are emerging as important elements in brain physiology through signaling exchange with neurons at tripartite synapses. Astrocytes express a wide variety of neurotransmitter transporters and receptors that allow them to sense and respond to synaptic activity. Principal among them are the G-protein-coupled receptors (GPCRs) in astrocytes because their activation by synaptically released neurotransmitters leads to mobilization of intracellular calcium. In turn, activated astrocytes release neuroactive substances called gliotransmitters, such as glutamate, GABA, and ATP/adenosine that lead to synaptic regulation through activation of neuronal GPCRs. In this review we will present and discuss recent evidence demonstrating the critical roles played by GPCRs in the bidirectional astrocyte-neuron signaling, and their crucial involvement in the astrocyte-mediated regulation of synaptic transmission and plasticity.

Antioxidants and Neuron-Astrocyte Interplay in Brain Physiology: Melatonin, a Neighbor to Rely on

This manuscript is a review focused onto the role of astrocytes in the protection of neurons against oxidative stress and how melatonin can contribute to the maintenance of brain homeostasis. The first part of the review is dedicated to the dependence of neurons on astrocytes by terms of survival under oxidative stress conditions. Additionally, the effects of melatonin against oxidative stress in the brain and its putative role in the protection against diseases affecting the brain are highlighted. The effects of melatonin on the physiology of neurons and astrocytes also are reviewed.

Versatile control of synaptic circuits by astrocytes: where, when and how?

Close structural and functional interactions of astrocytes with synapses play an important role in brain function. The repertoire of ways in which astrocytes can regulate synaptic transmission is complex so that they can both promote and dampen synaptic efficacy. Such contrasting effects raise questions regarding the determinants of these divergent astroglial functions. Recent findings provide insights into where, when and how astroglial regulation of synapses takes place by revealing major molecular and functional intrinsic heterogeneity as well as switches in astrocytes occurring during development or specific patterns of neuronal activity. Astrocytes may therefore be seen as boosters or gatekeepers of synaptic circuits depending on their intrinsic and transformative properties throughout life.

Astrocytic p38α MAPK drives NMDA receptor-dependent long-term depression and modulates long-term memory

NMDA receptor-dependent long-term depression (LTD) in the hippocampus is a well-known form of synaptic plasticity that has been linked to different cognitive functions. The core mechanism for this form of plasticity is thought to be entirely neuronal. However, we now demonstrate that astrocytic activity drives LTD at CA3-CA1 synapses. We have found that LTD induction enhances astrocyte-to-neuron communication mediated by glutamate, and that Ca²⁺ signaling and SNARE-dependent vesicular release from the astrocyte are required for LTD expression. In addition, using optogenetic techniques, we show that low-frequency astrocytic activation, in the absence of presynaptic activity, is sufficient to induce postsynaptic AMPA receptor removal and LTD expression. Using cell-type-specific gene deletion, we show that astrocytic p38α MAPK is required for the increased astrocytic glutamate release and astrocyte-to-neuron communication during low-frequency stimulation. Accordingly, removal of astrocytic (but not neuronal) p38α abolishes LTD expression. Finally, this mechanism modulates long-term memory in vivo.

How astrocytes influence neuronal plasticity remains unclear, as they are typically considered as modulators of core mechanisms driven by neuronal components. Here, authors show that Long-term depression (LTD) induction in the hippocampus triggers calcium signaling in the astrocyte and enhances SNARE-dependent astrocytic glutamate release, which is then responsible for the activation of postsynaptic NMDA receptors and synaptic depression.

Low-grade inflammation causes gap junction-coupled cell dysfunction throughout the body, which can lead to the spread of systemic inflammation

BACKGROUND AND AIMS

Gap junction-coupled cells form networks in different organs in the body. These networks can be affected by inflammatory stimuli and become dysregulated. Cell signaling is also changed through connexin-linked gap junctions. This alteration affects the surrounding cells and extracellular matrix in organs. These changes can cause the spread of inflammatory substances, thus affecting other network-linked cells in other organs in the body, which can give rise to systemic inflammation, which in turn can lead to pain that can turn into chronic.

METHODS

This is a review based on literature search and our own research data of inflammatory stimuli that can affect different organs and particularly gap-junction-coupled cells throughout the body.

CONCLUSIONS

A remaining question is which cell type or tissue is first affected by inflammatory stimuli. Can endotoxin exposure through the air, water and body start the process and are mast cells the first target cells that have the capacity to alter the physiological status of gap junction-coupled cells, thereby causing breakdown of different barrier systems?

IMPLICATIONS

Is it possible to address the right cellular and biochemical parameters and restore inflammatory systems to a normal physiological level by therapeutic strategies?

Opioid-Mediated Astrocyte-Neuron Signaling in the Nucleus Accumbens

Major hallmarks of astrocyte physiology are the elevation of intracellular calcium in response to neurotransmitters and the release of neuroactive substances (gliotransmitters) that modulate neuronal activity. While μ-opioid receptor expression has been identified in astrocytes of the nucleus accumbens, the functional consequences on astrocyte–neuron communication remains largely unknown. The present study has investigated the astrocyte responsiveness to μ-opioid signaling and the regulation of gliotransmission in the nucleus accumbens. Through the combination of calcium imaging and whole-cell patch clamp electrophysiology in brain slices, we have found that μ-opioid receptor activation in astrocytes elevates astrocyte cytoplasmic calcium and stimulates the release of the gliotransmitter glutamate, which evokes slow inward currents through the activation of neuronal N-methyl-D-aspartate (NMDA) receptors. These results indicate the existence of molecular mechanisms underlying opioid-mediated astrocyte–neuron signaling in the nucleus accumbens.

Extenuation of in utero toxic effects of MeHg in the developing neurons by Fisetin via modulating the expression of synaptic transmission and plasticity regulators in hippocampus of the rat offspring

The neurotoxic environmental contaminant, methylmercury (MeHg), has shown to have detrimental effects on the developing brain when exposed during gestation. We have shown in our earlier studies that gestational administration of 3,3',4',7-Tetrahydroxyflavone or Fisetin reduces the toxic effects of MeHg in the developing rat brain. The current study has pivoted to study the mechanism behind the mitigating action of Fisetin against prenatal MeHg exposure induced neurotoxicity. Negligible data is available about the toxicity targets of MeHg in the developing brain. Studies have exhibited that MeHg exposure cause toxic effects on synaptic transmission and plasticity in the offspring brain. Hence, we aimed to study the effect of Fisetin on MeHg induced alterations in the expressions of regulatory genes and proteins involved in synaptic plasticity and transmission. Pregnant rats were grouped according to the type of oral administration as, (i) Control, (ii) MeHg (1.5 mg/kg b. w.), (iii) MeHg + Fisetin (30 mg/kg b. w.) and (iv) Fisetin (30 mg/kg b. w). Maternal administration of Fisetin prevented MeHg exposure induced downregulation of neurogranin (Nrgn), dendrin (Ddn), Syntaxin 1 A (Stx1a), Lin-7 homolog A (Lin7a), Complexin-2 (Cplx2) and Exocyst complex component 8 (Exoc8) genes in the offspring rat. Fisetin also prevented MeHg exposure induced downregulation of brain derived neurotrophic factor (BDNF), Glial-cell derived neurotrophic factor (GDNF) protein expressions and hampered reactive astrogliosis in the hippocampus of F₁ generation rats. Hence, through this study, we conclude that Fisetin modulates the expression of regulatory genes and proteins involved in synaptic transmission and plasticity and extenuates MeHg neurotoxicity in the developing rat brain.

Stores, Channels, Glue, and Trees: Active Glial and Active Dendritic Physiology

Glial cells and neuronal dendrites were historically assumed to be passive structures that play only supportive physiological roles, with no active contribution to information processing in the central nervous system. Research spanning the past few decades has clearly established this assumption to be far from physiological realities. Whereas the discovery of active channel conductances and their localized plasticity was the turning point for dendritic structures, the demonstration that glial cells release transmitter molecules and communicate across the neuroglia syncytium through calcium wave propagation constituted path-breaking discoveries for glial cell physiology. An additional commonality between these two structures is the ability of calcium stores within their endoplasmic reticulum (ER) to support active propagation of calcium waves, which play crucial roles in the spatiotemporal integration of information within and across cells. Although there have been several demonstrations of regulatory roles of glial cells and dendritic structures in achieving common physiological goals such as information propagation and adaptability through plasticity, studies assessing physiological interactions between these two active structures have been few and far. This lacuna is especially striking given the strong connectivity that is known to exist between these two structures through several complex and tightly intercoupled mechanisms that also recruit their respective ER structures. In this review, we present brief overviews of the parallel literatures on active dendrites and active glial physiology and make a strong case for future studies to directly assess the strong interactions between these two structures in regulating physiology and pathophysiology of the brain.

Gi/o protein-coupled receptors inhibit neurons but activate astrocytes and stimulate gliotransmission

G protein-coupled receptors (GPCRs) play key roles in intercellular signaling in the brain. Their effects on cellular function have been largely studied in neurons, but their functional consequences on astrocytes are less known. Using both endogenous and chemogenetic approaches with DREADDs, we have investigated the effects of G_q and G_i/o GPCR activation on astroglial Ca²⁺ -based activity, gliotransmitter release, and the functional consequences on neuronal electrical activity. We found that while G_q GPCR activation led to cellular activation in both neurons and astrocytes, G_i/o GPCR activation led to cellular inhibition in neurons and cellular activation in astrocytes. Astroglial activation by either G_q or G_i/o protein-mediated signaling stimulated gliotransmitter release, which increased neuronal excitability. Additionally, activation of G_q and G_i/o DREADDs in vivo increased astrocyte Ca²⁺ activity and modified neuronal network electrical activity. Present results reveal additional complexity of the signaling consequences of excitatory and inhibitory neurotransmitters in astroglia-neuron network operation and brain function.

Astrocytes Maintain Glutamate Homeostasis in the CNS by Controlling the Balance between Glutamate Uptake and Release

Glutamate is one of the most prevalent neurotransmitters released by excitatory neurons in the central nervous system (CNS); however, residual glutamate in the extracellular space is, potentially, neurotoxic. It is now well-established that one of the fundamental functions of astrocytes is to uptake most of the synaptically-released glutamate, which optimizes neuronal functions and prevents glutamate excitotoxicity. In the CNS, glutamate clearance is mediated by glutamate uptake transporters expressed, principally, by astrocytes. Interestingly, recent studies demonstrate that extracellular glutamate stimulates Ca2+ release from the astrocytes' intracellular stores, which triggers glutamate release from astrocytes to the adjacent neurons, mostly by an exocytotic mechanism. This released glutamate is believed to coordinate neuronal firing and mediate their excitatory or inhibitory activity. Therefore, astrocytes contribute to glutamate homeostasis in the CNS, by maintaining the balance between their opposing functions of glutamate uptake and release. This dual function of astrocytes represents a potential therapeutic target for CNS diseases associated with glutamate excitotoxicity. In this regard, we summarize the molecular mechanisms of glutamate uptake and release, their regulation, and the significance of both processes in the CNS. Also, we review the main features of glutamate metabolism and glutamate excitotoxicity and its implication in CNS diseases.

Brain Glucose Metabolism: Integration of Energetics with Function

Glucose is the long-established, obligatory fuel for brain that fulfills many critical functions, including ATP production, oxidative stress management, and synthesis of neurotransmitters, neuromodulators, and structural components. Neuronal glucose oxidation exceeds that in astrocytes, but both rates increase in direct proportion to excitatory neurotransmission; signaling and metabolism are closely coupled at the local level. Exact details of neuron-astrocyte glutamate-glutamine cycling remain to be established, and the specific roles of glucose and lactate in the cellular energetics of these processes are debated. Glycolysis is preferentially upregulated during brain activation even though oxygen availability is sufficient (aerobic glycolysis). Three major pathways, glycolysis, pentose phosphate shunt, and glycogen turnover, contribute to utilization of glucose in excess of oxygen, and adrenergic regulation of aerobic glycolysis draws attention to astrocytic metabolism, particularly glycogen turnover, which has a high impact on the oxygen-carbohydrate mismatch. Aerobic glycolysis is proposed to be predominant in young children and specific brain regions, but re-evaluation of data is necessary. Shuttling of glucose- and glycogen-derived lactate from astrocytes to neurons during activation, neurotransmission, and memory consolidation are controversial topics for which alternative mechanisms are proposed. Nutritional therapy and vagus nerve stimulation are translational bridges from metabolism to clinical treatment of diverse brain disorders.

Does shuttling of glycogen-derived lactate from astrocytes to neurons take place during neurotransmission and memory consolidation?

Glycogen levels in resting brain and its utilization rates during brain activation are high, but the functions fulfilled by glycogenolysis in living brain are poorly understood. Studies in cultured astrocytes have identified glycogen as the preferred fuel to provide ATP for Na⁺ ,K⁺ -ATPase for the uptake of extracellular K⁺ and for Ca²⁺ -ATPase to pump Ca²⁺ into the endoplasmic reticulum. Studies in astrocyte-neuron co-cultures led to the suggestion that glycogen-derived lactate is shuttled to neurons as oxidative fuel to support glutamatergic neurotransmission. Furthermore, both knockout of brain glycogen synthase and inhibition of glycogenolysis prior to a memory-evoking event impair memory consolidation, and shuttling of glycogen-derived lactate as neuronal fuel was postulated to be required for memory. However, lactate shuttling has not been measured in any of these studies, and procedures to inhibit glycogenolysis and neuronal lactate uptake are not specific. Testable alternative mechanisms to explain the observed findings are proposed: (i) disruption of K⁺ and Ca²⁺ homeostasis, (ii) release of gliotransmitters, (iii) imposition of an energy crisis on astrocytes and neurons by inhibition of mitochondrial pyruvate transport by compounds used to block neuronal monocarboxylic acid transporters, and (iv) inhibition of astrocytic filopodial movements that secondarily interfere with glutamate and K⁺ uptake from the synaptic cleft. Evidence that most pyruvate/lactate derived from glycogen is not oxidized and does not accumulate suggests predominant glycolytic metabolism of glycogen to support astrocytic energy demands. Sparing of blood-borne glucose for use by neurons is a reasonable explanation for the requirement for glycogenolysis in neurotransmission and memory processing.

Exploring the Role of Astroglial Glutamate Release and Association With Synapses in Neuronal Function and Behavior

Astrocytes are stellate cells whose appearance can resemble a pointed star, especially when visualizing glial fibrillary acidic protein, a canonical marker for astrocytes. Accordingly, there is a commonly made connection between the points of light that shine in the night sky and the diffuse and abundant cells that buffer ions and provide support for neurons. An exceptional amount of function has been attributed to, negated for, and potentially reaffirmed for these cells, especially regarding their ability to release neuroactive molecules and influence synaptic plasticity. This makes the precise role of astrocytes in tuning neural communication seem difficult to grasp. However, data from animal models of addiction demonstrate that a variety of drug-induced molecular adaptations responsible for relapse vulnerability take place in astrocyte systems that regulate glutamate uptake and release. These findings highlight astrocytes as a critical component of the neural systems responsible for addiction, serving as a key component of the plasticity responsible for relapse and drug seeking. Here I assemble recent findings that utilize genetic tools to selectively manipulate or measure flux of internal calcium in astrocytes, focusing on G protein-coupled receptor-mediated mobilization of calcium and the induction of glutamate release. Further, I compile evidence regarding astrocyte glutamate release as well as astrocyte association with synapses with respect to the impact of these cellular phenomena in shaping synaptic transmission. I also place these findings in the context of the previous studies of Scofield et al., who explored the role of astrocytes in the nucleus accumbens in the neural mechanisms underlying cocaine seeking.

Emerging Roles of Astrocytes in Neuro-Vascular Unit and the Tripartite Synapse With Emphasis on Reactive Gliosis in the Context of Alzheimer's Disease

Astrocytes, which are five-fold more numerous than neurons in the central nervous system (CNS), are traditionally viewed to provide simple structural and nutritional supports for neurons and to participate in the composition of the blood brain barrier (BBB). In recent years, the active roles of astrocytes in regulating cerebral blood flow (CBF) and in maintaining the homeostasis of the tripartite synapse have attracted increasing attention. More importantly, astrocytes have been associated with the pathogenesis of Alzheimer's disease (AD), a major cause of dementia in the elderly. Although microglia-induced inflammation is considered important in the development and progression of AD, inflammation attributable to astrogliosis may also play crucial roles. A1 reactive astrocytes induced by inflammatory stimuli might be harmful by up-regulating several classical complement cascade genes thereby leading to chronic inflammation, while A2 induced by ischemia might be protective by up-regulating several neurotrophic factors. Here we provide a concise review of the emerging roles of astrocytes in the homeostasis maintenance of the neuro-vascular unit (NVU) and the tripartite synapse with emphasis on reactive astrogliosis in the context of AD, so as to pave the way for further research in this area, and to search for potential therapeutic targets of AD.

Response of catecholaminergic neurons in the mouse hindbrain to glucoprivic stimuli is astrocyte dependent

Hindbrain catecholaminergic (CA) neurons are required for critical autonomic, endocrine, and behavioral counterregulatory responses (CRRs) to hypoglycemia. Recent studies suggest that CRR initiation depends on hindbrain astrocyte glucose sensors (McDougal DH, Hermann GE, Rogers RC. Front Neurosci 7: 249, 2013; Rogers RC, Ritter S, Hermann GE. Am J Physiol Regul Integr Comp Physiol 310: R1102-R1108, 2016). To test the proposition that hindbrain CA responses to glucoprivation are astrocyte dependent, we utilized transgenic mice in which the calcium reporter construct (GCaMP5) was expressed selectively in tyrosine hydroxylase neurons (TH-GCaMP5). We conducted live cell calcium-imaging studies on tissue slices containing the nucleus of the solitary tract (NST) or the ventrolateral medulla, critical CRR initiation sites. Results show that TH-GCaMP5 neurons are robustly activated by a glucoprivic challenge and that this response is dependent on functional astrocytes. Pretreatment of hindbrain slices with fluorocitrate (an astrocytic metabolic suppressor) abolished TH-GCaMP5 neuronal responses to glucoprivation, but not to glutamate. Pharmacologic results suggest that the astrocytic connection with hindbrain CA neurons is purinergic via P2 receptors. Parallel imaging studies on hindbrain slices of NST from wild-type C57BL/6J mice, in which astrocytes and neurons were prelabeled with a calcium reporter dye and an astrocytic vital dye, show that both cell types are activated by glucoprivation but astrocytes responded significantly sooner than neurons. Pretreatment of these hindbrain slices with P2 antagonists abolished neuronal responses to glucoprivation without interruption of astrocyte responses; pretreatment with fluorocitrate eliminated both astrocytic and neuronal responses. These results support earlier work suggesting that the primary detection of glucoprivic signals by the hindbrain is mediated by astrocytes.

Astroglial Modulation of Hydromineral Balance and Cerebral Edema

Maintenance of hydromineral balance (HB) is an essential condition for life activity at cellular, tissue, organ and system levels. This activity has been considered as a function of the osmotic regulatory system that focuses on hypothalamic vasopressin (VP) neurons, which can reflexively release VP into the brain and blood to meet the demand of HB. Recently, astrocytes have emerged as an essential component of the osmotic regulatory system in addition to functioning as a regulator of the HB at cellular and tissue levels. Astrocytes express all the components of osmoreceptors, including aquaporins, molecules of the extracellular matrix, integrins and transient receptor potential channels, with an operational dynamic range allowing them to detect and respond to osmotic changes, perhaps more efficiently than neurons. The resultant responses, i.e., astroglial morphological and functional plasticity in the supraoptic and paraventricular nuclei, can be conveyed, physically and chemically, to adjacent VP neurons, thereby influencing HB at the system level. In addition, astrocytes, particularly those in the circumventricular organs, are involved not only in VP-mediated osmotic regulation, but also in regulation of other osmolality-modulating hormones, including natriuretic peptides and angiotensin. Thus, astrocytes play a role in local/brain and systemic HB. The adaptive astrocytic reactions to osmotic challenges are associated with signaling events related to the expression of glial fibrillary acidic protein and aquaporin 4 to promote cell survival and repair. However, prolonged osmotic stress can initiate inflammatory and apoptotic signaling processes, leading to glial dysfunction and a variety of brain diseases. Among many diseases of brain injury and hydromineral disorders, cytotoxic and osmotic cerebral edemas are the most common pathological manifestation. Hyponatremia is the most common cause of osmotic cerebral edema. Overly fast correction of hyponatremia could lead to central pontine myelinolysis. Ischemic stroke exemplifies cytotoxic cerebral edema. In this review, we summarize and analyze the osmosensory functions of astrocytes and their implications in cerebral edema.

Astrocytic Activation Generates De Novo Neuronal Potentiation and Memory Enhancement

Astrocytes respond to neuronal activity and were shown to be necessary for plasticity and memory. To test whether astrocytic activity is also sufficient to generate synaptic potentiation and enhance memory, we expressed the Gq-coupled receptor hM3Dq in CA1 astrocytes, allowing their activation by a designer drug. We discovered that astrocytic activation is not only necessary for synaptic plasticity, but also sufficient to induce NMDA-dependent de novo long-term potentiation in the hippocampus that persisted after astrocytic activation ceased. In vivo, astrocytic activation enhanced memory allocation; i.e., it increased neuronal activity in a task-specific way only when coupled with learning, but not in home-caged mice. Furthermore, astrocytic activation using either a chemogenetic or an optogenetic tool during acquisition resulted in memory recall enhancement on the following day. Conversely, directly increasing neuronal activity resulted in dramatic memory impairment. Our findings that astrocytes induce plasticity and enhance memory may have important clinical implications for cognitive augmentation treatments.

The Neuroglial Dialog Between Cannabinoids and Hemichannels

The formation of gap junctions was initially thought to be the central role of connexins, however, recent evidence had brought to light the high relevance of unopposed hemichannels as an independent mechanism for the selective release of biomolecules during physiological and pathological conditions. In the healthy brain, the physiological opening of astrocyte hemichannels modulates basal excitatory synaptic transmission. At the other end, the release of potentially neurotoxic compounds through astroglial hemichannels and pannexons has been insinuated as one of the functional alterations that negatively affect the progression of multiple brain diseases. Recent insights in this matter have suggested encannabinoids (eCBs) as molecules that could regulate the opening of these channels during diverse conditions. In this review, we discuss and hypothesize the possible interplay between the eCB system and the hemichannel/pannexon-mediated signaling in the inflamed brain and during event of synaptic plasticity. Most findings indicate that eCBs seem to counteract the activation of major neuroinflammatory pathways that lead to glia-mediated production of TNF-α and IL-1β, both well-known triggers of astroglial hemichannel opening. In contrast to the latter, in the normal brain, eCBs apparently elicit the Ca²⁺-activation of astrocyte hemichannels, which could have significant consequences on eCB-dependent synaptic plasticity.

Glial Cells in the Genesis and Regulation of Circadian Rhythms

Circadian rhythms are biological oscillations with a period of ~24 h. These rhythms are orchestrated by a circadian timekeeper in the suprachiasmatic nucleus of the hypothalamus, the circadian "master clock," which exactly adjusts clock outputs to solar time via photic synchronization. At the molecular level, circadian rhythms are generated by the interaction of positive and negative feedback loops of transcriptional and translational processes of the so-called "clock genes." A large number of clock genes encode numerous proteins that regulate their own transcription and that of other genes, collectively known as "clock-controlled genes." In addition to the sleep/wake cycle, many cellular processes are regulated by circadian rhythms, including synaptic plasticity in which an exquisite interplay between neurons and glial cells takes place. In particular, there is compelling evidence suggesting that glial cells participate in and regulate synaptic plasticity in a circadian fashion, possibly representing the missing cellular and physiological link between circadian rhythms with learning and cognition processes. Here we review recent studies in support of this hypothesis, focusing on the interplay between glial cells, synaptic plasticity, and circadian rhythmogenesis.

Astrocytic modulation of excitatory synaptic signaling in a mouse model of Rett syndrome

Studies linking mutations in Methyl CpG Binding Protein 2 (MeCP2) to physiological defects in the neurological disease, Rett syndrome, have focused largely upon neuronal dysfunction despite MeCP2 ubiquitous expression. Here we explore roles for astrocytes in neuronal network function using cortical slice recordings. We find that astrocyte stimulation in wild-type mice increases excitatory synaptic activity that is absent in male mice lacking MeCP2 globally. To determine the cellular basis of the defect, we exploit a female mouse model for Rett syndrome that expresses wild-type MeCP2-GFP in a mosaic distribution throughout the brain, allowing us to test all combinations of wild-type and mutant cells. We find that the defect is dependent upon MeCP2 expression status in the astrocytes and not in the neurons. Our findings highlight a new role for astrocytes in regulation of excitatory synaptic signaling and in the neurological defects associated with Rett syndrome.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.