Dynamic inhibition of excitatory synaptic transmission by astrocyte-derived ATP in hippocampal cultures

Regulation of GABAergic neurotransmission by purinergic receptors in brain physiology and disease

Purinergic receptors regulate the processing of neural information in the hippocampus and cerebral cortex, structures related to cognitive functions. These receptors are activated when astrocytic and neuronal populations release adenosine triphosphate (ATP) in an autocrine and paracrine manner, following sustained patterns of neuronal activity. The modulation by these receptors of GABAergic transmission has only recently been studied. Through their ramifications, astrocytes and GABAergic interneurons reach large groups of excitatory pyramidal neurons. Their inhibitory effect establishes different synchronization patterns that determine gamma frequency rhythms, which characterize neural activities related to cognitive processes. During early life, GABAergic-mediated synchronization of excitatory signals directs the experience-driven maturation of cognitive development, and dysfunctions concerning this process have been associated with neurological and neuropsychiatric diseases. Purinergic receptors timely modulate GABAergic control over ongoing neural activity and deeply affect neural processing in the hippocampal and neocortical circuitry. Stimulation of A2 receptors increases GABA release from presynaptic terminals, leading to a considerable reduction in neuronal firing of pyramidal neurons. A1 receptors inhibit GABAergic activity but only act in the early postnatal period when GABA produces excitatory signals. P2X and P2Y receptors expressed in pyramidal neurons reduce the inhibitory tone by blocking GABAA receptors. Finally, P2Y receptor activation elicits depolarization of GABAergic neurons and increases GABA release, thus favoring the emergence of gamma oscillations. The present review provides an overall picture of purinergic influence on GABAergic transmission and its consequences on neural processing, extending the discussion to receptor subtypes and their involvement in the onset of brain disorders, including epilepsy and Alzheimer's disease.

Spiking Neural Membrane Systems with Adaptive Synaptic Time Delay

Spiking neural membrane systems (or spiking neural P systems, SNP systems) are a new type of computation model which have attracted the attention of plentiful scholars for parallelism, time encoding, interpretability and extensibility. The original SNP systems only consider the time delay caused by the execution of rules within neurons, but not caused by the transmission of spikes via synapses between neurons and its adaptive adjustment. In view of the importance of time delay for SNP systems, which are a time encoding computation model, this study proposes SNP systems with adaptive synaptic time delay (ADSNP systems) based on the dynamic regulation mechanism of synaptic transmission delay in neural systems. In ADSNP systems, besides neurons, astrocytes that can generate adenosine triphosphate (ATP) are introduced. After receiving spikes, astrocytes convert spikes into ATP and send ATP to the synapses controlled by them to change the synaptic time delays. The Turing universality of ADSNP systems in number generating and accepting modes is proved. In addition, a small universal ADSNP system using 93 neurons and astrocytes is given. The superiority of the ADSNP system is demonstrated by comparison with the six variants. Finally, an ADSNP system is constructed for credit card fraud detection, which verifies the feasibility of the ADSNP system for solving real-world problems. By considering the adaptive synaptic delay, ADSNP systems better restore the process of information transmission in biological neural networks, and enhance the adaptability of SNP systems, making the control of time more accurate.

PFOS Elicits Cytotoxicity in Neuron Through Astrocyte-Derived CaMKII-DLG1 Signaling In Vitro Rat Hippocampal Model

Both epidemiological investigation and animal experiments demonstrated that pre-/postnatal exposure to perfluorooctane sulfonic acid (PFOS) could induce neurodevelopmental disorders. Previous studies showed that astrocyte was involved in PFOS-induced neurotoxicity, while little information is available. In the present study, the role of astrocyte-derived calmodulin-dependent protein kinase II (CaMKII)-phosphorylated discs large homolog 1 (DLG1) signaling in PFOS eliciting cytotoxicity in neuron was explored with primary cultured hippocampal astrocyte and neuron. The application of PFOS showed a decreased cell viability, synapse length and glutamate transporter 1 (GLT-1) expression, but an increased CaMKII, DLG1 and cyclic AMP response element binding protein (CREB) expression in primary cultured astrocyte. With 2-(2-hydroxyethylamino)-6-aminohexylcarbamic acid tert-butyl ester-9-isopropylpurine (CK59), the CaMKII inhibitor, the disturbed cell viability and molecules induced by PFOS could be alleviated (CREB expression was excluded) in astrocytes. The cytotoxic effect of neuron exposed to astrocyte conditional medium collected from PFOS (PFOS-ACM) pretreated with CK59 was also decreased. These results indicated that PFOS mediated GLT-1 expression through astrocyte-derived CaMKII-DLG signaling, which might be associated with injuries on neurons. The present study gave an insight in further exploration of mechanism in PFOS-induced neurotoxicity.

Extracellular ATP/adenosine dynamics in the brain and its role in health and disease

Extracellular ATP and adenosine are neuromodulators that regulate numerous neuronal functions in the brain. Neuronal activity and brain insults such as ischemic and traumatic injury upregulate these neuromodulators, which exert their effects by activating purinergic receptors. In addition, extracellular ATP/adenosine signaling plays a pivotal role in the pathogenesis of neurological diseases. Virtually every cell type in the brain contributes to the elevation of ATP/adenosine, and various mechanisms underlying this increase have been proposed. Extracellular adenosine is thought to be mainly produced via the degradation of extracellular ATP. However, adenosine is also released from neurons and glia in the brain. Therefore, the regulation of extracellular ATP/adenosine in physiological and pathophysiological conditions is likely far more complex than previously thought. To elucidate the complex mechanisms that regulate extracellular ATP/adenosine levels, accurate methods of assessing their spatiotemporal dynamics are needed. Several novel techniques for acquiring spatiotemporal information on extracellular ATP/adenosine, including fluorescent sensors, have been developed and have started to reveal the mechanisms underlying the release, uptake and degradation of ATP/adenosine. Here, we review methods for analyzing extracellular ATP/adenosine dynamics as well as the current state of knowledge on the spatiotemporal dynamics of ATP/adenosine in the brain. We focus on the mechanisms used by neurons and glia to cooperatively produce the activity-dependent increase in ATP/adenosine and its physiological and pathophysiological significance in the brain.

The important role of glial transmitters released by astrocytes in Alzheimer's disease: A perspective from dynamical modeling

This paper aims to establish a coupling model of neuronal populations and astrocytes and, on this basis, explore the possible mechanism of electroencephalography (EEG) slowing in Alzheimer's disease (AD) from the viewpoint of dynamical modeling. First and foremost, excitatory and inhibitory time constants are shown to induce the early symptoms of AD. The corresponding dynamic nature is mainly due to changes in the amplitude and frequency of the oscillatory behavior. However, there are also a few cases that can be attributed to the change of the oscillation mode caused by the limit cycle bifurcation and birhythmicity. Then, an improved neural mass model influenced by astrocytes is proposed, considering the important effects of glutamate and adenosine triphosphate (ATP) released by astrocytes on the synaptic transmission process reported in experiments. The results show that a dysfunctional astrocyte disrupts the physiological state, causing three typical EEG slowing phenomena reported clinically: the decreased dominant frequency, the decreased rhythmic activity in the α band, and the increased rhythmic activity in the δ+θ band. In addition, astrocytes may control AD when the effect of ATP on synaptic connections is greater than that of glutamate. The control rate depends on the ratio of the effect of glutamate on excitatory and inhibitory synaptic connections. These modeling results can not only reproduce some experimental and clinical results, but, more importantly, may offer a prediction of some underlying phenomena, helping to inspire the disease mechanisms and therapeutic methods of targeting astrocytes.

Astrocytes in Parkinson's Disease: From Role to Possible Intervention

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the loss of dopaminergic neurons. While neuronal dysfunction is central to PD, astrocytes also play important roles, both positive and negative, and such roles have not yet been fully explored. This literature review serves to highlight these roles and how the properties of astrocytes can be used to increase neuron survivability. Astrocytes normally have protective functions, such as releasing neurotrophic factors, metabolizing glutamate, transferring healthy mitochondria to neurons, or maintaining the blood-brain barrier. However, in PD, astrocytes can become dysfunctional and contribute to neurotoxicity, e.g., via impaired glutamate metabolism or the release of inflammatory cytokines. Therefore, astrocytes represent a double-edged sword. Restoring healthy astrocyte function and increasing the beneficial effects of astrocytes represents a promising therapeutic approach. Strategies such as promoting neurotrophin release, preventing harmful astrocyte reactivity, or utilizing regional astrocyte diversity may help restore neuroprotection.

Intracellular accumulation of tau oligomers in astrocytes and their synaptotoxic action rely on Amyloid Precursor Protein Intracellular Domain-dependent expression of Glypican-4

Several studies including ours reported the detrimental effects of extracellular tau oligomers (ex-oTau) on glutamatergic synaptic transmission and plasticity. Astrocytes greatly internalize ex-oTau whose intracellular accumulation alters neuro/gliotransmitter handling thereby negatively affecting synaptic function. Both amyloid precursor protein (APP) and heparan sulfate proteoglycans (HSPGs) are required for oTau internalization in astrocytes but the molecular mechanisms underlying this phenomenon have not been clearly identified yet. Here we found that a specific antibody anti-glypican 4 (GPC4), a receptor belonging to the HSPG family, significantly reduced oTau uploading from astrocytes and prevented oTau-induced alterations of Ca2+-dependent gliotransmitter release. As such, anti-GPC4 spared neurons co-cultured with astrocytes from the astrocyte-mediated synaptotoxic action of ex-oTau, thus preserving synaptic vesicular release, synaptic protein expression and hippocampal LTP at CA3-CA1 synapses. Of note, the expression of GPC4 depended on APP and, in particular, on its C-terminal domain, AICD, that we found to bind Gpc4 promoter. Accordingly, GPC4 expression was significantly reduced in mice in which either APP was knocked-out or it contained the non-phosphorylatable amino acid alanine replacing threonine 688, thus becoming unable to produce AICD. Collectively, our data indicate that GPC4 expression is APP/AICD-dependent, it mediates oTau accumulation in astrocytes and the resulting synaptotoxic effects.

ATP-mediated signalling in the central synapses

ATP released from the synaptic terminals and astrocytes can activate neuronal P2 receptors at a variety of locations across the CNS. Although the postsynaptic ATP-mediated signalling does not bring a major contribution into the excitatory transmission, it is instrumental for slow and diffuse modulation of synaptic dynamics and neuronal firing in many CNS areas. Neuronal P2X and P2Y receptors can be activated by ATP released from the synaptic terminals, astrocytes and microglia and thereby can participate in the regulation of synaptic homeostasis and plasticity. There is growing evidence of importance of purinergic regulation of synaptic transmission in different physiological and pathological contexts. Here, we review the main mechanisms underlying the complexity and diversity of purinergic signalling and purinergic modulation in central neurons.

Ocular P2 receptors and glaucoma

Adenosine triphosphate (ATP), an energy source currency in cells, is released or leaked to the extracellular space under both physiological and pathological conditions. Extracellular ATP functions as an intercellular signaling molecule through activation of purinergic P2 receptors. Ocular tissue and cells release ATP in response to physiological stimuli such as intraocular pressure (IOP), and P2 receptor activation regulates IOP elevation or reduction. Dysregulated purinergic signaling may cause abnormally elevated IOP, which is one of the major risk factors for glaucoma. Glaucoma, a leading cause of blindness worldwide, is characterized by progressive degeneration of optic nerves and retinal ganglion cells (RGCs), which are essential retinal neurons that transduce visual information to the brain. An elevation in IOP may stress RGCs and increase the risk for glaucoma pathogenesis. In the aqueous humor of human patients with glaucoma, the ATP level is significantly elevated. Such excess amount of ATP may directly cause RGC death via a specific subtype of P2 receptors. Dysregulated purinergic signaling may also trigger inflammation, oxidative stress, and excitotoxicity via activating non-neuronal cell types such as glial cells. In this review, we discussed the physiological roles of extracellular nucleotides in the ocular tissue and their potential role in the pathogenesis of glaucoma. This article is part of the Special Issue on 'Purinergic Signaling: 50 years'.

P2Y1 Receptor as a Catalyst of Brain Neurodegeneration

Different brain disorders display distinctive etiologies and pathogenic mechanisms. However, they also share pathogenic events. One event systematically occurring in different brain disorders, both acute and chronic, is the increase of the extracellular ATP levels. Accordingly, several P2 (ATP/ADP) and P1 (adenosine) receptors, as well as the ectoenzymes involved in the extracellular catabolism of ATP, have been associated to different brain pathologies, either with a neuroprotective or neurodegenerative action. The P2Y1 receptor (P2Y1R) is one of the purinergic receptors associated to different brain diseases. It has a widespread regional, cellular, and subcellular distribution in the brain, it is capable of modulating synaptic function and neuronal activity, and it is particularly important in the control of astrocytic activity and in astrocyte–neuron communication. In diverse brain pathologies, there is growing evidence of a noxious gain-of-function of P2Y1R favoring neurodegeneration by promoting astrocyte hyperactivity, entraining Ca2+-waves, and inducing the release of glutamate by directly or indirectly recruiting microglia and/or by increasing the susceptibility of neurons to damage. Here, we review the current evidence on the involvement of P2Y1R in different acute and chronic neurodegenerative brain disorders and the underlying mechanisms.

Sensational astrocytes: Mechanotransduction in adult brain function

Sensing the mechanical microenvironment is an essential aspect of all life, yet its mechanism remains poorly understood. In this issue of Neuron, Chi et al. reveal the role of astrocyte mechanosensitive Piezo1 channel in adult neurogenesis and cognitive function.

A requirement for astrocyte IP3R2 signaling for whisker experience-dependent depression and homeostatic upregulation in the mouse barrel cortex

Changes to sensory experience result in plasticity of synapses in the cortex. This experience-dependent plasticity (EDP) is a fundamental property of the brain. Yet, while much is known about neuronal roles in EDP, very little is known about the role of astrocytes. To address this issue, we used the well-described mouse whiskers-to-barrel cortex system, which expresses a number of forms of EDP. We found that all-whisker deprivation induced characteristic experience-dependent Hebbian depression (EDHD) followed by homeostatic upregulation in L2/3 barrel cortex of wild type mice. However, these changes were not seen in mutant animals (IP₃R2^–/–) that lack the astrocyte-expressed IP₃ receptor subtype. A separate paradigm, the single-whisker experience, induced potentiation of whisker-induced response in both wild-type (WT) mice and IP₃R2^–/– mice. Recordings in ex vivo barrel cortex slices reflected the in vivo results so that long-term depression (LTD) could not be elicited in slices from IP₃R2^–/– mice, but long-term potentiation (LTP) could. Interestingly, 1 Hz stimulation inducing LTD in WT paradoxically resulted in NMDAR-dependent LTP in slices from IP₃R2^–/– animals. The LTD to LTP switch was mimicked by acute buffering astrocytic [Ca²⁺]_i in WT slices. Both WT LTD and IP₃R2^–/– 1 Hz LTP were mediated by non-ionotropic NMDAR signaling, but only WT LTD was P38 MAPK dependent, indicating an underlying mechanistic switch. These results demonstrate a critical role for astrocytic [Ca²⁺]_i in several EDP mechanisms in neocortex.

Astrocytic Piezo1-mediated mechanotransduction determines adult neurogenesis and cognitive functions

Adult brain activities are generally believed to be dominated by chemical and electrical transduction mechanisms. However, the importance of mechanotransduction mediated by mechano-gated ion channels in brain functions is less appreciated. Here, we show that the mechano-gated Piezo1 channel is expressed in the exploratory processes of astrocytes and utilizes its mechanosensitivity to mediate mechanically evoked Ca²⁺ responses and ATP release, establishing Piezo1-mediated mechano-chemo transduction in astrocytes. Piezo1 deletion in astrocytes causes a striking reduction of hippocampal volume and brain weight and severely impaired (but ATP-rescuable) adult neurogenesis in vivo, and it abolishes ATP-dependent potentiation of neural stem cell (NSC) proliferation in vitro. Piezo1-deficient mice show impaired hippocampal long-term potentiation (LTP) and learning and memory behaviors. By contrast, overexpression of Piezo1 in astrocytes sufficiently enhances mechanotransduction, LTP, and learning and memory performance. Thus, astrocytes utilize Piezo1-mediated mechanotransduction mechanisms to robustly regulate adult neurogenesis and cognitive functions, conceptually highlighting the importance of mechanotransduction in brain structure and function.

Ischemic Tolerance Induced by Glial Cells

Ischemic tolerance is a phenomenon in which resistance to subsequent invasive ischemia is acquired by a preceding noninvasive ischemic application, and is observed in many organs, including the brain, the organ most vulnerable to ischemic insult. To date, much research has been conducted on cerebral ischemic tolerance as a cell-autonomous action of neurons. In this article, we review the essential roles of microglia and astrocytes in the acquisition of ischemic tolerance through neuron-non-autonomous mechanisms, where the two types of glial cells function in a concerted manner to induce ischemic tolerance.

Control of complex behavior by astrocytes and microglia

Evidence that glial cells influence behavior has been gaining a steady foothold in scientific literature. Out of the five main subtypes of glial cells in the brain, astrocytes and microglia have received an outsized share of attention with regard to shaping a wide spectrum of behavioral phenomena and there is growing appreciation that the signals intrinsic to these cells as well as their interactions with surrounding neurons reflect behavioral history in a brain region-specific manner. Considerable regional diversity of glial cell phenotypes is beginning to be recognized and may contribute to behavioral outcomes arising from circuit-specific computations within and across discrete brain nuclei. Here, we summarize current knowledge on the impact of astrocyte and microglia activity on behavioral outcomes, with a specific focus on brain areas relevant to higher cognitive control, reward-seeking, and circadian regulation.

Glial Purinergic Signals and Psychiatric Disorders

Emotion-related neural networks are regulated in part by the activity of glial cells, and glial dysfunction can be directly related to emotional diseases such as depression. Here, we discuss three different therapeutic strategies involving astrocytes that are effective for treating depression. First, the antidepressant, fluoxetine, acts on astrocytes and increases exocytosis of ATP. This has therapeutic effects via brain-derived neurotrophic factor-dependent mechanisms. Second, electroconvulsive therapy is a well-known treatment for drug-resistant depression. Electroconvulsive therapy releases ATP from astrocytes to induce leukemia inhibitory factors and fibroblast growth factor 2, which leads to antidepressive actions. Finally, sleep deprivation therapy is well-known to cause antidepressive effects. Sleep deprivation also increases release of ATP, whose metabolite, adenosine, has antidepressive effects. These independent treatments share the same mechanism, i.e., ATP release from astrocytes, indicating an essential role of glial purinergic signals in the pathogenesis of depression.

Astrocytes in cocaine addiction and beyond

Drug addiction remains a key biomedical challenge facing current neuroscience research. In addition to neural mechanisms, the focus of the vast majority of studies to date, astrocytes have been increasingly recognized as an "accomplice." According to the tripartite synapse model, astrocytes critically regulate nearby pre- and postsynaptic neuronal substrates to craft experience-dependent synaptic plasticity, including synapse formation and elimination. Astrocytes within brain regions that are implicated in drug addiction exhibit dynamic changes in activity upon exposure to cocaine and subsequently undergo adaptive changes themselves during chronic drug exposure. Recent results have identified several key astrocytic signaling pathways that are involved in cocaine-induced synaptic and circuit adaptations. In this review, we provide a brief overview of the role of astrocytes in regulating synaptic transmission and neuronal function, and discuss how cocaine influences these astrocyte-mediated mechanisms to induce persistent synaptic and circuit alterations that promote cocaine seeking and relapse. We also consider the therapeutic potential of targeting astrocytic substrates to ameliorate drug-induced neuroplasticity for behavioral benefits. While primarily focusing on cocaine-induced astrocytic responses, we also include brief discussion of other drugs of abuse where data are available.

CMOS-Based Redox-Type Label-Free ATP Image Sensor for In Vitro Sensitive Imaging of Extracellular ATP

Adenosine 5'-triphosphate (ATP) plays a crucial role as an extracellular signaling molecule in the central nervous system and is closely related to various nerve diseases. Therefore, label-free imaging of extracellular ATP dynamics and spatiotemporal analysis is crucial for understanding brain function. To decrease the limit of detection (LOD) of imaging extracellular ATP, we fabricated a redox-type label-free ATP image sensor by immobilizing glycerol-kinase (GK), L-α-glycerophosphate oxidase (LGOx), and horseradish peroxidase (HRP) enzymes in a polymer film on a gold electrode-modified potentiometric sensor array with a 37.3 µm-pitch. Hydrogen peroxide (H₂O₂) is generated through the enzymatic reactions from GK to LGOx in the presence of ATP and glycerol, and ATP can be detected as changes in its concentration using an electron mediator. Using this approach, the LOD for ATP was 2.8 µM with a sensitivity of 77 ± 3.8 mV/dec., under 10 mM working buffers at physiological pH, such as in in vitro experiments, and the LOD was great superior 100 times than that of the hydrogen ion detection-based image sensor. This redox-type ATP image sensor may be successfully applied for in vitro sensitive imaging of extracellular ATP dynamics in brain nerve tissue or cells.

Abnormal Ca2+ Signals in Reactive Astrocytes as a Common Cause of Brain Diseases

In pathological brain conditions, glial cells become reactive and show a variety of responses. We examined Ca2+ signals in pathological brains and found that reactive astrocytes share abnormal Ca2+ signals, even in different types of diseases. In a neuropathic pain model, astrocytes in the primary sensory cortex became reactive and showed frequent Ca2+ signals, resulting in the production of synaptogenic molecules, which led to misconnections of tactile and pain networks in the sensory cortex, thus causing neuropathic pain. In an epileptogenic model, hippocampal astrocytes also became reactive and showed frequent Ca2+ signals. In an Alexander disease (AxD) model, hGFAP-R239H knock-in mice showed accumulation of Rosenthal fibers, a typical pathological marker of AxD, and excessively large Ca2+ signals. Because the abnormal astrocytic Ca2+ signals observed in the above three disease models are dependent on type II inositol 1,4,5-trisphosphate receptors (IP3RII), we reanalyzed these pathological events using IP3RII-deficient mice and found that all abnormal Ca2+ signals and pathologies were markedly reduced. These findings indicate that abnormal Ca2+ signaling is not only a consequence but may also be greatly involved in the cause of these diseases. Abnormal Ca2+ signals in reactive astrocytes may represent an underlying pathology common to multiple diseases.

A sensitive GRAB sensor for detecting extracellular ATP in vitro and in vivo

The purinergic transmitter ATP (adenosine 5'-triphosphate) plays an essential role in both the central and peripheral nervous systems, and the ability to directly measure extracellular ATP in real time will increase our understanding of its physiological functions. Here, we developed a sensitive GPCR activation-based ATP sensor called GRAB_ATP1.0, with a robust fluorescence response to extracellular ATP when expressed in several cell types. This sensor has sub-second kinetics, has ATP affinity in the range of tens of nanomolar, and can be used to localize ATP release with subcellular resolution. Using this sensor, we monitored ATP release under a variety of in vitro and in vivo conditions, including stimuli-induced and spontaneous ATP release in primary hippocampal cultures, injury-induced ATP release in a zebrafish model, and lipopolysaccharides-induced ATP-release events in individual astrocytes in the mouse cortex. Thus, the GRAB_ATP1.0 sensor is a sensitive, versatile tool for monitoring ATP release and dynamics under both physiological and pathophysiological conditions.

Adenosine A2B receptor down-regulates metabotropic glutamate receptor 5 in astrocytes during postnatal development

Metabotropic glutamate receptor 5 (mGluR5) in astrocytes is a key molecule for controlling synapse remodeling. Although mGluR5 is abundant in neonatal astrocytes, its level is gradually down-regulated during development and is almost absent in the adult. However, in several pathological conditions, mGluR5 re-emerges in adult astrocytes and contributes to disease pathogenesis by forming uncontrolled synapses. Thus, controlling mGluR5 expression in astrocyte is critical for several diseases, but the mechanism that regulates mGluR5 expression remains unknown. Here, we show that adenosine triphosphate (ATP)/adenosine-mediated signals down-regulate mGluR5 in astrocytes. First, in situ Ca²⁺ imaging of astrocytes in acute cerebral slices from post-natal day (P)7-P28 mice showed that Ca²⁺ responses evoked by (S)-3,5-dihydroxyphenylglycine (DHPG), a mGluR5 agonist, decreased during development, whereas those evoked by ATP or its metabolite, adenosine, increased. Second, ATP and adenosine suppressed expression of the mGluR5 gene, Grm5, in cultured astrocytes. Third, the decrease in the DHPG-evoked Ca²⁺ responses was associated with down-regulation of Grm5. Interestingly, among several adenosine (P1) receptor and ATP (P2) receptor genes, only the adenosine A_2B receptor gene, Adora2b, was up-regulated in the course of development. Indeed, we observed that down-regulation of Grm5 was suppressed in Adora2b knockout astrocytes at P14 and in situ Ca²⁺ imaging from Adora2b knockout mice indicated that the A_2B receptor inhibits mGluR5 expression in astrocytes. Furthermore, deletion of A_2B receptor increased the number of excitatory synapse in developmental stage. Taken together, the A_2B receptor is critical for down-regulation of mGluR5 in astrocytes, which would contribute to terminate excess synaptogenesis during development.

Loss of P2Y1 receptors triggers glaucoma-like pathology in mice

BACKGROUND AND PURPOSE

Glaucoma, the leading cause of blindness, damages the retinal ganglion cells. Elevated intraocular pressure (IOP) is a high-risk factor for glaucoma, so topical hypotensive drugs are usually used for treatment. Because not all patients do not respond adequately to current treatments, there is a need to identify a new molecular target to reduce IOP. Here, we have assessed the role of P2Y1 receptors in mediating elevated IOP.

EXPERIMENTAL APPROACH

P2Y₁ receptor agonist was instilled into the eyes of mice, and the IOP changes were measured by a rebound-type tonometer. Expression of P2Y₁ receptors was estimated by immunohistochemistry. Ocular function was measured by a multifocal electroretinogram.

KEY RESULTS

A single dose of the P2Y₁ receptor agonist transiently reduced IOP and such effects were absent in P2Y₁ receptor-deficient (P2Y₁ KO) mice. P2Y₁ receptors were functionally expressed in the ciliary body, trabecular meshwork and Schlemm's canal. Activation of P2Y₁ receptors negatively regulated aquaporin 4 (AQP4) function but up-regulated endothelial NOS (eNOS). P2Y₁ KO mice showed chronic ocular hypertension regardless of age. P2Y₁ KO mice at 3 months old showed no damage to retinal ganglion cells, whereas 12-month-old mice showed a significant loss of these cells and impairment of ocular functions. Damage to retinal ganglion cells was attenuated by chronic administration of an IOP-reducing agent.

CONCLUSION AND IMPLICATIONS

Activation of P2Y₁ receptors reduced IOP via dual pathways including AQP4 and eNOS. Loss of P2Y₁ receptors resulted in glaucomatous optic neuropathy, suggesting that P2Y₁ receptors might provide an effective target in the treatment of glaucoma.

Extracellular ATP-Induced Alterations in Extracellular H+ Fluxes From Cultured Cortical and Hippocampal Astrocytes

Small alterations in the level of extracellular H⁺ can profoundly alter neuronal activity throughout the nervous system. In this study, self-referencing H⁺-selective microelectrodes were used to examine extracellular H⁺ fluxes from individual astrocytes. Activation of astrocytes cultured from mouse hippocampus and rat cortex with extracellular ATP produced a pronounced increase in extracellular H⁺ flux. The ATP-elicited increase in H⁺ flux appeared to be independent of bicarbonate transport, as ATP increased H⁺ flux regardless of whether the primary extracellular pH buffer was 26 mM bicarbonate or 1 mM HEPES, and persisted when atmospheric levels of CO₂ were replaced by oxygen. Adenosine failed to elicit any change in extracellular H⁺ fluxes, and ATP-mediated increases in H⁺ flux were inhibited by the P2 inhibitors suramin and PPADS suggesting direct activation of ATP receptors. Extracellular ATP also induced an intracellular rise in calcium in cultured astrocytes, and ATP-induced rises in both calcium and H⁺ efflux were significantly attenuated when calcium re-loading into the endoplasmic reticulum was inhibited by thapsigargin. Replacement of extracellular sodium with choline did not significantly reduce the size of the ATP-induced increases in H⁺ flux, and the increases in H⁺ flux were not significantly affected by addition of EIPA, suggesting little involvement of Na⁺/H⁺ exchangers in ATP-elicited increases in H⁺ flux. Given the high sensitivity of voltage-sensitive calcium channels on neurons to small changes in levels of free H⁺, we hypothesize that the ATP-mediated extrusion of H⁺ from astrocytes may play a key role in regulating signaling at synapses within the nervous system.

Massive efflux of adenosine triphosphate into the extracellular space immediately after experimental traumatic brain injury

The aim of the current study was to determine effects of mild traumatic brain injury (TBI), with or without blockade of purinergic ATP Y1 (P2Y1) receptors or store-operated calcium channels, on extracellular levels of ATP, glutamate, glucose and lactate. Concentrations of ATP, glutamate, glucose and lactate were measured in cerebral microdialysis samples obtained from the ipsilateral cortex and underlying hippocampus of rats with mild unilateral controlled cortical impact (CCI) or sham injury. Immediately after CCI, a large release of ATP was observed in the cortex (3.53-fold increase of pre-injury value) and hippocampus (2.97-fold increase of pre-injury value), with ATP returning to the baseline levels within 20 min post-injury and remaining stable for during the 3-h sampling period. In agreement with the results of previous studies, there was a significant increase in glutamate 20 min after CCI, which was concomitant with a decrease in extracellular glucose (20 min) and an increase in lactate (40-60 min) in both brain regions after CCI. Addition of a selective P2Y1 receptor blocker (MRS2179 ammonium salt hydrate) to the microdialysis perfusate significantly lowered pre-injury ATP and glutamate levels, and eliminated the post-CCI peaks. Addition of a blocker of store-operated calcium channels [2-aminoethoxy diphenylborinate (2-APB)] to the microdialysis perfusate significantly lowered pre-injury ATP in the hippocampus, and attenuated the post-CCI peak in both the cortex and hippocampus. 2-APB treatment significantly increased baseline glutamate levels, but the values post-injury did not differ from those in the sham group. Pre-injury glucose levels, but not lactate levels, were increased by MRS2179 and decreased by 2-APB. However, none of these treatments substantially altered the CCI-induced reduction in glucose and increase in lactate in the cortex. In conclusion, the results of the present study demonstrated that a short although extensive release of ATP immediately after experimental TBI can be significantly attenuated by blockade of P2Y1 receptors or store-operated calcium channels.

Purinergic transmission in depressive disorders

Purinergic signaling involves the actions of purine nucleotides and nucleosides (such as adenosine) at P1 (adenosine), P2X, and P2Y receptors. Here, we present recent data contributing to a comprehensive overview of the association between purinergic signaling and depression. We start with background information on adenosine production and metabolism, followed by a detailed characterization of P1 and P2 receptors, with an emphasis on their expression and function in the brain as well as on their ligands. We provide data suggestive of altered metabolism of adenosine in depressed patients, which might be regarded as a disease biomarker. We then turn to considerable amount of preclinical/behavioral data obtained with the aid of the forced swim test, tail suspension test, learned helplessness model, or unpredictable chronic mild stress model and genetic activation/inactivation of P1 or P2 receptors as well as nonselective or selective ligands of P1 or P2 receptors. We also aimed to discuss the reason underlying discrepancies observed in such studies.

Dynamic Transitions in Neuronal Network Firing Sustained by Abnormal Astrocyte Feedback

Astrocytes play a crucial role in neuronal firing activity. Their abnormal state may lead to the pathological transition of neuronal firing patterns and even induce seizures. However, there is still little evidence explaining how the astrocyte network modulates seizures caused by structural abnormalities, such as gliosis. To explore the role of gliosis of the astrocyte network in epileptic seizures, we first established a direct astrocyte feedback neuronal network model on the basis of the hippocampal CA3 neuron-astrocyte model to simulate the condition of gliosis when astrocyte processes swell and the feedback to neurons increases in an abnormal state. We analyzed the firing pattern transitions of the neuronal network when astrocyte feedback starts to change via increases in both astrocyte feedback intensity and the connection probability of astrocytes to neurons in the network. The results show that as the connection probability and astrocyte feedback intensity increase, neuronal firing transforms from a nonepileptic synchronous firing state to an asynchronous firing state, and when astrocyte feedback starts to become abnormal, seizure-like firing becomes more severe and synchronized; meanwhile, the synchronization area continues to expand and eventually transforms into long-term seizure-like synchronous firing. Therefore, our results prove that astrocyte feedback can regulate the firing of the neuronal network, and when the astrocyte network develops gliosis, there will be an increase in the induction rate of epileptic seizures.

Neonatal Seizures and Purinergic Signalling

Neonatal seizures are one of the most common comorbidities of neonatal encephalopathy, with seizures aggravating acute injury and clinical outcomes. Current treatment can control early life seizures; however, a high level of pharmacoresistance remains among infants, with increasing evidence suggesting current anti-seizure medication potentiating brain damage. This emphasises the need to develop safer therapeutic strategies with a different mechanism of action. The purinergic system, characterised by the use of adenosine triphosphate and its metabolites as signalling molecules, consists of the membrane-bound P1 and P2 purinoreceptors and proteins to modulate extracellular purine nucleotides and nucleoside levels. Targeting this system is proving successful at treating many disorders and diseases of the central nervous system, including epilepsy. Mounting evidence demonstrates that drugs targeting the purinergic system provide both convulsive and anticonvulsive effects. With components of the purinergic signalling system being widely expressed during brain development, emerging evidence suggests that purinergic signalling contributes to neonatal seizures. In this review, we first provide an overview on neonatal seizure pathology and purinergic signalling during brain development. We then describe in detail recent evidence demonstrating a role for purinergic signalling during neonatal seizures and discuss possible purine-based avenues for seizure suppression in neonates.

Purinergic signaling orchestrating neuron-glia communication

This review discusses the evidence supporting a role for ATP signaling (operated by P₂X and P₂Y receptors) and adenosine signaling (mainly operated by A₁ and A_2A receptors) in the crosstalk between neurons, astrocytes, microglia and oligodendrocytes. An initial emphasis will be given to the cooperation between adenosine receptors to sharpen information salience encoding across synapses. The interplay between ATP and adenosine signaling in the communication between astrocytes and neurons will then be presented in context of the integrative properties of the astrocytic syncytium, allowing to implement heterosynaptic depression processes in neuronal networks. The process of microglia 'activation' and its control by astrocytes and neurons will then be analyzed under the perspective of an interplay between different P₂ receptors and adenosine A_2A receptors. In spite of these indications of a prominent role of purinergic signaling in the bidirectional communication between neurons and glia, its therapeutical exploitation still awaits obtaining an integrated view of the spatio-temporal action of ATP signaling and adenosine signaling, clearly distinguishing the involvement of both purinergic signaling systems in the regulation of physiological processes and in the control of pathogenic-like responses upon brain dysfunction or damage.

Astrocytes-The Ultimate Effectors of Long-Range Neuromodulatory Networks?

It was long thought that astrocytes, given their lack of electrical signaling, were not involved in communication with neurons. However, we now know that one astrocyte on average maintains and regulates the extracellular neurotransmitter and potassium levels of more than 140,000 synapses, both excitatory and inhibitory, within their individual domains, and form a syncytium that can propagate calcium waves to affect distant cells via release of “gliotransmitters” such as glutamate, ATP, or adenosine. Neuromodulators can affect signal-to-noise and frequency transmission within cortical circuits by effects on inhibition, allowing for the filtering of relevant vs. irrelevant stimuli. Moreover, synchronized “resting” and desynchronized “activated” brain states are gated by short bursts of high-frequency neuromodulatory activity, highlighting the need for neuromodulation that is robust, rapid, and far-reaching. As many neuromodulators are released in a volume manner where degradation/uptake and the confines of the complex CNS limit diffusion distance, we ask the question—are astrocytes responsible for rapidly extending neuromodulator actions to every synapse? Neuromodulators are known to influence transitions between brain states, leading to control over plasticity, responses to salient stimuli, wakefulness, and sleep. These rapid and wide-spread state transitions demand that neuromodulators can simultaneously influence large and diverse regions in a manner that should be impossible given the limitations of simple diffusion. Intriguingly, astrocytes are ideally situated to amplify/extend neuromodulator effects over large populations of synapses given that each astrocyte can: (1) ensheath a large number of synapses; (2) release gliotransmitters (glutamate/ATP/adenosine) known to affect inhibition; (3) regulate extracellular potassium that can affect excitability and excitation/inhibition balance; and (4) express receptors for all neuromodulators. In this review article, we explore the hypothesis that astrocytes extend and amplify neuromodulatory influences on neuronal networks via alterations in calcium dynamics, the release of gliotransmitters, and potassium homeostasis. Given that neuromodulatory networks are at the core of our sleep-wake cycle and behavioral states, and determine how we interact with our environment, this review article highlights the importance of basic astrocyte function in homeostasis, general cognition, and psychiatric disorders.

Exocytosis of large-diameter lysosomes mediates interferon γ-induced relocation of MHC class II molecules toward the surface of astrocytes

Astrocytes are the key homeostatic cells in the central nervous system; initiation of reactive astrogliosis contributes to neuroinflammation. Pro-inflammatory cytokine interferon γ (IFNγ) induces the expression of the major histocompatibility complex class II (MHCII) molecules, involved in antigen presentation in reactive astrocytes. The pathway for MHCII delivery to the astrocyte plasma membrane, where MHCII present antigens, is unknown. Rat astrocytes in culture and in organotypic slices were exposed to IFNγ to induce reactive astrogliosis. Astrocytes were probed with optophysiologic tools to investigate subcellular localization of immunolabeled MHCII, and with electrophysiology to characterize interactions of single vesicles with the plasmalemma. In culture and in organotypic slices, IFNγ augmented the astrocytic expression of MHCII, which prominently co-localized with lysosomal marker LAMP1-EGFP, modestly co-localized with Rab7, and did not co-localize with endosomal markers Rab4A, EEA1, and TPC1. MHCII lysosomal localization was corroborated by treatment with the lysosomolytic agent glycyl-L-phenylalanine-β-naphthylamide, which reduced the number of MHCII-positive vesicles. The surface presence of MHCII was revealed by immunolabeling of live non-permeabilized cells. In IFNγ-treated astrocytes, an increased fraction of large-diameter exocytotic vesicles (lysosome-like vesicles) with prolonged fusion pore dwell time and larger pore conductance was recorded, whereas the rate of endocytosis was decreased. Stimulation with ATP, which triggers cytosolic calcium signaling, increased the frequency of exocytotic events, whereas the frequency of full endocytosis was further reduced. In IFNγ-treated astrocytes, MHCII-linked antigen surface presentation is mediated by increased lysosomal exocytosis, whereas surface retention of antigens is prolonged by concomitant inhibition of endocytosis.

Neuroprotective effects of microglial P2Y1 receptors against ischemic neuronal injury

Extracellular ATP, which is released from damaged cells after ischemia, activates P2 receptors. P2Y₁ receptors (P2Y₁R) have received considerable attention, especially in astrocytes, because their activation plays a central role in the regulation of neuron-to-glia communication. However, the functions or even existence of P2Y₁R in microglia remain unknown, despite the fact that many microglial P2 receptors are involved in several brain diseases. Herein, we demonstrate the presence and functional capability of microglial P2Y₁R to provide neuroprotective effects following ischemic stress. Cerebral ischemia resulted in increased microglial P2Y₁R expression. The number of injured hippocampal neurons was significantly higher in P2Y₁ R knockout (KO) mice than wildtype mice after forebrain ischemia. Propidium iodide (PI) uptake, a marker for dying cells, was significantly higher in P2Y₁R KO hippocampal slices compared with wildtype hippocampal slices at 48 h after 40-min oxygen-glucose deprivation (OGD). Furthermore, increased PI uptake following OGD was rescued by ectopic overexpression of P2Y₁R in microglia. In summary, these data suggest that microglial P2Y₁R mediate neuroprotective effects against ischemic stress and OGD insult.

[Epileptogenesis mediated by glial cells]

Epilepsy is one of the most common diseases of the central nervous system. Many epilepsies are controllable because of the existence of different antiepileptic drugs with multiple mechanisms of action. However, about 30% of epilepsy is so-called refractory epilepsy in which existing drugs do not show enough therapeutic effects. Antiepileptic drugs can be roughly divided into two types, i.e., those that suppress the excitability of neuronal cells and those that promote inhibition. Inhibition of excitatory neurons include a variety of ion channel inhibitors such as Na⁺, drugs that inhibit glutamate release and glutamate AMPA receptor, whereas enhancement of inhibitory neurons includes a drug that enhances GABA_A receptor. Both are targeted to neurons. Recent advances in brain science have revealed the importance of the role of glial cells in regulation of brain function and excitability of neurons. Although glia cells themselves are electrically non-excitable cells, they could greatly affect excitability of neurons by controlling extracellular neurotransmitters, glial transmitters, regulating various ions concentration, regulation of energy metabolism, and formation/elimination of synapses. Therefore, when the function of glial cells changes, these regulatory functions also change, which in turn greatly changes the excitability of neurons and neuronal networks. Epilegenicity is a condition in which the brain is likely to undergo spontaneous epileptic seizures and it is suggested that modulation of the above-mentioned glial cell function is greatly related to the acquisition of epileptogenesis. In this article, I focus on astrocytes among glial cells, and describe the relationship between functional modulation and epileptogenesis when changing to the phenotype of reactive astrocytes by epileptic seizures. We also discuss development of antiepileptic drugs targeting reactive astrocytes.

Anti-Depressant Fluoxetine Reveals its Therapeutic Effect Via Astrocytes

Although psychotropic drugs act on neurons and glial cells, how glia respond, and whether glial responses are involved in therapeutic effects are poorly understood. Here, we show that fluoxetine (FLX), an anti-depressant, mediates its anti-depressive effect by increasing the gliotransmission of ATP. FLX increased ATP exocytosis via vesicular nucleotide transporter (VNUT). FLX-induced anti-depressive behavior was decreased in astrocyte-selective VNUT-knockout mice or when VNUT was deleted in mice, but it was increased when astrocyte-selective VNUT was overexpressed in mice. This suggests that VNUT-dependent astrocytic ATP exocytosis has a critical role in the therapeutic effect of FLX. Released ATP and its metabolite adenosine act on P2Y₁₁ and adenosine A2b receptors expressed by astrocytes, causing an increase in brain-derived neurotrophic factor in astrocytes. These findings suggest that in addition to neurons, FLX acts on astrocytes and mediates its therapeutic effects by increasing ATP gliotransmission.

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Anti-depressant FLX acts on astrocytes and increases VNUT-dependent ATP exocytosis.

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Such astrocytic responses are responsible for the FLX-induced therapeutic effects.

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Astrocytic ATP and its metabolite adenosine increase BDNF in astrocytes, and reveal the therapeutic effects.

Kinoshita et al. demonstrated that astrocytes are a therapeutic target of the antidepressant, fluoxetine (FLX). They found that FLX stimulates VNUT-dependent ATP release from astrocytes leading to a BDNF-mediated anti-depressive effect. This study demonstrated the astrocytic regulation of this anti-depressive effect, which complements the previously described conventional mechanism of FLX. Because the involvement of astrocytes in the pathogenesis of depression is of current interest, this new insight into the role of astrocytes in anti-depressive effects should support the establishment of novel therapeutic strategies for depression.

The Metabotropic Purinergic P2Y Receptor Family as Novel Drug Target in Epilepsy

Epilepsy encompasses a heterogeneous group of neurological syndromes which are characterized by recurrent seizures affecting over 60 million people worldwide. Current anti-epileptic drugs (AEDs) are mainly designed to target ion channels and/or GABA or glutamate receptors. Despite recent advances in drug development, however, pharmacoresistance in epilepsy remains as high as 30%, suggesting the need for the development of new AEDs with a non-classical mechanism of action. Neuroinflammation is increasingly recognized as one of the key players in seizure generation and in the maintenance of the epileptic phenotype. Consequently, targeting signaling molecules involved in inflammatory processes may represent new avenues to improve treatment in epilepsy. Nucleotides such as adenosine-5′-triphosphate (ATP) and uridine-5′-triphosphate (UTP) are released in the brain into the extracellular space during pathological conditions such as increased neuronal firing or cell death. Once released, these nucleotides bind to and activate specific purinergic receptors termed P2 receptors where they mediate the release of gliotransmitters and drive neuronal hyperexcitation and neuroinflammatory processes. This includes the fast acting ionotropic P2X channels and slower-acting G-protein-coupled P2Y receptors. While the expression and function of P2X receptors has been well-established in experimental models of epilepsy, emerging evidence is now also suggesting a prominent role for the P2Y receptor subfamily in seizure generation and the maintenance of epilepsy. In this review we discuss data supporting a role for the P2Y receptor family in epilepsy and the most recent finding demonstrating their involvement during seizure-induced pathology and in epilepsy.

Effects of the ecto-ATPase apyrase on microglial ramification and surveillance reflect cell depolarization, not ATP depletion

Microglia, the brain's innate immune cells, have highly motile processes which constantly survey the brain to detect infection, remove dying cells, and prune synapses during brain development. ATP released by tissue damage is known to attract microglial processes, but it is controversial whether an ambient level of ATP is needed to promote constant microglial surveillance in the normal brain. Applying the ATPase apyrase, an enzyme which hydrolyzes ATP and ADP, reduces microglial process ramification and surveillance, suggesting that ambient ATP/ADP maintains microglial surveillance. However, attempting to raise the level of ATP/ADP by blocking the endogenous ecto-ATPase (termed NTPDase1/CD39), which also hydrolyzes ATP/ADP, does not affect the cells' ramification or surveillance, nor their membrane currents, which respond to even small rises of extracellular [ATP] or [ADP] with the activation of K+ channels. This indicates a lack of detectable ambient ATP/ADP and ecto-ATPase activity, contradicting the results with apyrase. We resolve this contradiction by demonstrating that contamination of commercially available apyrase by a high K+ concentration reduces ramification and surveillance by depolarizing microglia. Exposure to the same K+ concentration (without apyrase added) reduced ramification and surveillance as with apyrase. Dialysis of apyrase to remove K+ retained its ATP-hydrolyzing activity but abolished the microglial depolarization and decrease of ramification produced by the undialyzed enzyme. Thus, applying apyrase affects microglia by an action independent of ATP, and no ambient purinergic signaling is required to maintain microglial ramification and surveillance. These results also have implications for hundreds of prior studies that employed apyrase to hydrolyze ATP/ADP.

New roles of reactive astrocytes in the brain; an organizer of cerebral ischemia

The brain consists of neurons and much higher number of glial cells. They communicate each other, by which they control brain functions. The brain is highly vulnerable to several insults such as ischemia, but has a self-protective and self-repairing mechanisms against these. Ischemic tolerance or preconditioning is an endogenous neuroprotective phenomenon, where a mild non-lethal ischemic episode can induce resistance to a subsequent severe ischemic injury in the brain. Because of its neuroprotective effects against cerebral ischemia or stroke, ischemic tolerance has been widely studied. However, almost all studies have been performed from the viewpoint of neurons. Glial cells are structurally in close association with synapses. Recent studies have uncovered the active roles of astrocytes in modulating synaptic connectivity, such as synapse formation, elimination and maturation, during development or pathology. However, glia-mediated ischemic tolerance and/or neuronal repairing have received only limited attention. We and others have demonstrated that glial cells, especially astrocytes, play a pivotal role in regulation of induction of ischemic tolerance as well as repairing/remodeling of neuronal networks by phagocytosis. Here, we review our current understanding of (1) glial-mediated ischemic tolerance and (2) glia-mediated repairing/remodeling of the penumbra neuronal networks, and highlight their mechanisms as well as their potential benefits, problems, and therapeutic application.

Maxi-anion channels play a key role in glutamate-induced ATP release from mouse astrocytes in primary culture

Astrocytes are an abundant source of ATP, which might be released from the cytosol into extracellular spaces under various conditions and even affect cell fate under some circumstances. In the present study, we aimed to evaluate the pathway(s) contributing toward glutamate-induced ATP release from mouse astrocytes. Firstly, our study of cultured astrocytes showed marked ATP release in response to stimuli of glutamate at different concentrations (0.1-1 mM), with an interesting bimodal distribution in time course. Inhibitors or blockers of potential pathways for ATP release such as exocytotic vesicular release, gap junction hemichannels, P2X7 receptors, and volume-sensitive outwardly rectifying chloride channels had no significant effects on the observed ATP release. In contrast, glutamate-induced ATP release from astrocytes was significantly inhibited by gadolinium (50 µM), an inhibitor of a maxi-anion channel; meanwhile, the application of gadolinium can allay glutamate-induced cell injury significantly. Thus, we propose that the maxi-anion channel might play an important role in glutamate-induced ATP release from mouse astrocytes and inhibition of maxi-anion channel activities to reduce ATP release can produce protective effects in the case of glutamate stimuli.

Evidence for astrocyte purinergic signaling in cortical sensory adaptation and serotonin-mediated neuromodulation

In the somatosensory cortex, inhibitory networks are involved in low frequency sensory input adaptation/habituation that can be observed as a paired-pulse depression when using a dual stimulus electrophysiological paradigm. Given that astrocytes have been shown to regulate inhibitory interneuron activity, we hypothesized that astrocytes are involved in cortical sensory adaptation/habituation and constitute effectors of the 5HT-mediated increase in frequency transmission. Using extracellular recordings of evoked excitatory postsynaptic potentials (eEPSPs) in layer II/III of somatosensory cortex, we used various pharmacological approaches to assess the recruitment of astrocyte signaling in paired-pulse depression and serotonin-mediated increase in the paired-pulse ratio (pulse 2/pulse 1). In the absence of neuromodulators or pharmacological agents, the first eEPSP is much larger in amplitude than the second due to the recruitment of long-lasting evoked GABA_A-dependent inhibitory activity from the first stimulus. Disruption of glycolysis or mGluR5 signaling resulted in a very similar loss of paired-pulse depression in field recordings. Interestingly, paired-pulse depression was similarly sensitive to disruption by ATP P2Y and adenosine A2A receptor antagonists. In addition, we show that pharmacological disruption of paired-pulse depression by mGluR5, P2Y, and glycolysis inhibition precluded serotonin effects on frequency transmission (typically increased the paired-pulse ratio). These data highlight the possibility for astrocyte involvement in cortical inhibitory activity seen in this simple cortical network and that serotonin may act on astrocytes to exert some aspects of its modulatory influence.

Astrocytes and ischemic tolerance

A mild non-lethal ischemic episode can induce resistance to a subsequent severe ischemic injury in the brain. This phenomenon is termed ischemic tolerance or ischemic preconditioning, and is an endogenous mechanism that can provide robust neuroprotection. Because of its neuroprotective effects against cerebral ischemia or stroke, ischemic tolerance has been widely studied. However, almost all studies have been performed from the viewpoint of neurons. Accumulating evidence suggests that glial cells have various roles in regulation of brain function, including modulation of synaptic transmission, neuronal excitation, and neuronal structure. In addition, astrocytes are closely related to homeostasis, stability of brain function, and protection of neurons. However, glial cells have received only limited attention with regard to ischemic tolerance. Cross-ischemic preconditioning is a phenomenon whereby non-ischemic preconditioning such as mechanical, thermal, and chemical treatment can induce ischemic tolerance. Of these, chemical treatments that affect the immune system can strongly induce ischemic tolerance, suggesting that glial cells may have important roles in this process. Indeed, we and others have demonstrated that glial cells, especially astrocytes, play a pivotal role in the induction of ischemic tolerance. This glial-mediated ischemic tolerance provides a robust and long-lasting neuroprotection against ischemic injury. In this review, we discuss the mechanisms underlying glial-mediated ischemic tolerance, as well as its potential benefits, problems, and therapeutic application.

The role of neuron-glia interactions in the emergence of ultra-slow oscillations

Ultra-slow cortical oscillatory activity of 1-100 mHz has been recorded in human by electroencephalography and in dissociated cultures of cortical rat neurons, but the underlying mechanisms remain to be elucidated. This study presents a computational model of ultra-slow oscillatory activity based on the interaction between neurons and astrocytes. We predict that the frequency of these oscillations closely depends on activation of astrocytes in the network, which is reflected by oscillations of their intracellular calcium concentrations with periods between tens of seconds and minutes. An increase of intracellular calcium in astrocytes triggers the release of adenosine triphosphate from these cells which may alter transmission at nearby synapses by increasing or decreasing neurotransmitter release. These results provide theoretical support for the emerging awareness of astrocytes as active players in the regulation of neural activity and identify neuron-astrocyte interactions as a potential primary mechanism for the emergence of ultra-slow cortical oscillations.

Astrocyte calcium waves propagate proximally by gap junction and distally by extracellular diffusion of ATP released from volume-regulated anion channels

Wave-like propagation of [Ca²⁺]_i increases is a remarkable intercellular communication characteristic in astrocyte networks, intercalating neural circuits and vasculature. Mechanically-induced [Ca²⁺]_i increases and their subsequent propagation to neighboring astrocytes in culture is a classical model of astrocyte calcium wave and is known to be mediated by gap junction and extracellular ATP, but the role of each pathway remains unclear. Pharmacologic analysis of time-dependent distribution of [Ca²⁺]_i revealed three distinct [Ca²⁺]_i increases, the largest being in stimulated cells independent of extracellular Ca²⁺ and inositol 1,4,5-trisphosphate-induced Ca²⁺ release. In addition, persistent [Ca²⁺]_i increases were found to propagate rapidly via gap junctions in the proximal region, and transient [Ca²⁺]_i increases were found to propagate slowly via extracellular ATP in the distal region. Simultaneous imaging of astrocyte [Ca²⁺]_i and extracellular ATP, the latter of which was measured by an ATP sniffing cell, revealed that ATP was released within the proximal region by volume-regulated anion channel in a [Ca²⁺]_i independent manner. This detailed analysis of a classical model is the first to address the different contributions of two major pathways of calcium waves, gap junctions and extracellular ATP.

The roles of cortical astrocytes in chronic pain and other brain pathologies

Astrocytes are the most abundant cell type in the brain. Several decades ago, they were considered to be only support cells in the central nervous system. Recent studies using advanced technologies have clarified that astrocytes play more active roles in regulating neuronal function and remodeling synaptic structures by releasing molecules called gliotransmitters. In addition to various physiological functions, astrocytes are activated under disease conditions, such as chronic pain, releasing molecules that in turn cause reorganization of the central nervous system microstructure and disrupt behavior in pathological conditions. In the present review, we summarize cortical astrocyte function in chronic pain and other neurological disorders and discuss the role of astrocytes in brain pathologies.

Reduced gliotransmitter release from astrocytes mediates tau-induced synaptic dysfunction in cultured hippocampal neurons

Tau is a microtubule-associated protein exerting several physiological functions in neurons. In Alzheimer's disease (AD) misfolded tau accumulates intraneuronally and leads to axonal degeneration. However, tau has also been found in the extracellular medium. Recent studies indicated that extracellular tau uploaded from neurons causes synaptic dysfunction and contributes to tau pathology propagation. Here we report novel evidence that extracellular tau oligomers are abundantly and rapidly accumulated in astrocytes where they disrupt intracellular Ca2+ signaling and Ca2+ -dependent release of gliotransmitters, especially ATP. Consequently, synaptic vesicle release, the expression of pre- and postsynaptic proteins, and mEPSC frequency and amplitude were reduced in neighboring neurons. Notably, we found that tau uploading from astrocytes required the amyloid precursor protein, APP. Collectively, our findings suggests that astrocytes play a critical role in the synaptotoxic effects of tau via reduced gliotransmitter availability, and that astrocytes are major determinants of tau pathology in AD.

Hypoxia-independent mechanisms of HIF-1α expression in astrocytes after ischemic preconditioning

We recently demonstrated that ischemic tolerance was dependent on astrocytes, for which HIF-1α had an essential role. The mild ischemia (preconditioning; PC) increased HIF-1α in a biphasic pattern, that is, a quick and transient increase in neurons, followed by a slow and sustained increase in astrocytes. However, mechanisms underlying such temporal difference in HIF-1α increase remain totally unknown. Here, we show that unlike a hypoxia-dependent mechanism in neurons, astrocytes increase HIF-1α via a novel hypoxia-independent but P2X7-dependent mechanism. Using a middle cerebral artery occlusion (MCAO) model of mice, we found that the PC (a 15-min MCAO period)-evoked increase in HIF-1α in neurons was quick and transient (from 1 to 3 days after PC), but that in astrocytes was slow-onset and long-lasting (from 3 days to at least 2 weeks after PC). The neuronal HIF-1α increase was dependent on inhibition of PHD2, an oxygen-dependent HIF-1α degrading enzyme, whereas astrocytic one was independent of PHD2. Astrocytes even do not possess this enzyme. Instead, they produced a sustained increase in P2X7 receptors, activation of which resulted in HIF-1α increase. The hypoxia-independent but P2X7-receptor-dependent mechanism could allow astrocytes to cause long-lasting HIF-1α expression, thereby leading to induction of ischemic tolerance efficiently. GLIA 2017;65:523-530.

P2Y1 Receptors - Properties and Functional Activities

In this chapter we try to show a comprehensive image of current knowledge of structure, activity and physiological role of the P2Y₁ purinergic receptor. The structure, distribution and changes in the expression of this receptor are summarized, as well as the mechanism of its signaling activity by the intracellular calcium mobilization. We try to show the connection between the components of its G protein activation and cellular or physiological effects, starting from changes in protein phosphorylation patterns and ending with such remote effects as receptor-mediated apoptosis. The special emphasis is put on the role of the P2Y₁ receptor in cancer cells and neuronal plasticity. We concentrate on the P2Y₁ receptor, it is though impossible to completely abstract from other aspects of nucleotide signaling and cross-talk with other nucleotide receptors is here discussed. Especially, the balance between P2Y₁ and P2Y₁₂ receptors, sharing the same ligand but signaling through different pathways, is presented.

Hippocampal GABAergic transmission: a new target for adenosine control of excitability

Physiological network functioning in the hippocampus is dependent on a balance between glutamatergic cell excitability and the activity of diverse local circuit neurons that release the inhibitory neurotransmitter γ-aminobutyric acid (GABA). Tuners of neuronal communication such as adenosine, an endogenous modulator of synapses, control hippocampal network operations by regulating excitability. Evidence has been recently accumulating on the influence of adenosine on different aspects of GABAergic transmission to shape hippocampal function. This review addresses how adenosine, through its high-affinity A₁ (A₁ R) and A_2A receptors (A_2A R), interferes with different GABA-mediated forms of inhibition in the hippocampus to regulate neuronal excitability. Adenosine-mediated modulation of phasic/tonic inhibitory transmission, of GABA transport mechanisms and its interference with other modulatory systems are discussed together with the putative implications for neuronal function in physiological and pathological conditions. This article is part of a mini review series: 'Synaptic Function and Dysfunction in Brain Diseases'.

Understanding epigenetic architecture of suicide neurobiology: A critical perspective

Current understanding of environmental cross-talk with genetic makeup is found to be mediated through an epigenetic interface which is associated with prominent reversible and heritable changes at gene expression level. Recent emergence of epigenetic modulation in shaping the genetic information has become a key regulatory factor in answering the underlying complexities associated with several mental disorders. A comprehensive understanding of the pertinent changes in the epigenetic makeup of suicide phenotype exhibits a characteristic signature with the possibility of using it as a biomarker to help predict the risk factors associated with suicide. Within the scope of this current review, the most sought after epigenetic changes of DNA methylation and histone modification are thoroughly scrutinized to understand their close functional association with the broad spectrum of suicide phenotype.

Anti-nociceptive roles of the glia-specific metabolic inhibitor fluorocitrate in paclitaxel-evoked neuropathic pain

Paclitaxel (Taxol) is a powerful chemotherapy drug used in breast cancers, but it often causes neuropathic pain, leading to the early cessation of therapy and poor treatment outcomes. Approaches for the management of paclitaxel-induced neuropathic pain are urgently needed. The involvement of spinal astrocytes in the pathogenesis of paclitaxel-induced neuropathy has been reported, but little is known about the role of fluorocitrate (FC), a selective inhibitor of astrocyte activation, during neuropathic pain related to paclitaxel treatment. In this study, we investigated the effects of FC on paclitaxel-induced neuropathic pain. Glial fibrillary acidic protein (GFAP) expression was determined to assess astrocyte activation. To explore the mechanisms involved, the expression of glial glutamate transporter 1 (GLT-1) and the activation of mitogen-activated protein kinases in the spinal dorsal horn were analyzed. The results showed that paclitaxel decreased the mechanical nociceptive thresholds and increased GFAP expression, leading to spinal astrocyte activation. After paclitaxel treatment, GLT-1 was significantly down-regulated, and the phosphorylation of ERK1/2 and JNK were obviously up-regulated. However, paclitaxel treatment did not increase p38 phosphorylation. Additional studies showed that paclitaxel-evoked mechanical hypersensitivity was reduced by FC treatment. Moreover, FC treatment inhibited the activation of astrocytes and reversed the changes in GLT-1 expression and MAPK phosphorylation. Further study indicated that FC did not influence the antitumor effect of paclitaxel, suggesting that FC blocked paclitaxel-induced neuropathic pain without antagonizing its antitumor effect. Together, these results suggested that paclitaxel induced astrocyte-specific activation, which may contribute to mechanical allodynia and hyperalgesia, and that FC could be a potential therapeutic agent for paclitaxel-induced neuropathic pain.