Astrocytic control of synaptic NMDA receptors

Astrocytic PAR1 and mGluR2/3 control synaptic glutamate time course at hippocampal CA1 synapses

Astrocytes play an essential role in regulating synaptic transmission. This study describes a novel form of modulation of excitatory synaptic transmission in the mouse hippocampus by astrocytic G-protein-coupled receptors (GPCRs). We have previously described astrocytic glutamate release via protease-activated receptor-1 (PAR1) activation, although the regulatory mechanisms for this are complex. Through electrophysiological analysis and modeling, we discovered that PAR1 activation consistently increases the concentration and duration of glutamate in the synaptic cleft. This effect was not due to changes in the presynaptic glutamate release or alteration in glutamate transporter expression. However, blocking group II metabotropic glutamate receptors (mGluR2/3) abolished PAR1-mediated regulation of synaptic glutamate concentration, suggesting a role for this GPCR in mediating the effects of PAR1 activation on glutamate release. Furthermore, activation of mGluR2/3 causes glutamate release through the TREK-1 channel in hippocampal astrocytes. These data show that astrocytic GPCRs engage in a novel regulatory mechanism to shape the time course of synaptically-released glutamate in excitatory synapses of the hippocampus.

The SWELL1 Channel Promotes Ischemic Brain Damage by Mediating Neuronal Swelling and Glutamate Toxicity

Cytotoxic neuronal swelling and glutamate excitotoxicity are two hallmarks of ischemic stroke. However, the underlying molecular mechanisms are not well understood. Here, it is reported that SWELL1, the essential subunit of the volume-regulated anion channel (VRAC), plays a dual role in ischemic injury by promoting neuronal swelling and glutamate excitotoxicity. SWELL1 expression is upregulated in neurons and astrocytes after experimental stroke in mice. The neuronal SWELL1 channel is activated by intracellular hypertonicity, leading to Cl- influx-dependent cytotoxic neuronal swelling and subsequent cell death. Additionally, the SWELL1 channel in astrocytes mediates pathological glutamate release, indicated by increases in neuronal slow inward current frequency and tonic NMDAR current. Pharmacologically, targeting VRAC with a new inhibitor, an FDA-approved drug Dicumarol, attenuated cytotoxic neuronal swelling and cell death, reduced astrocytic glutamate release, and provided significant neuroprotection in mice when administered either before or after ischemia. Therefore, these findings uncover the pleiotropic effects of the SWELL1 channel in neurons and astrocytes in the pathogenesis of ischemic stroke and provide proof of concept for therapeutically targeting it in this disease.

Gene Expression at the Tripartite Synapse: Bridging the Gap Between Neurons and Astrocytes

Astrocytes, a major class of glial cells, are an important element at the synapse where they engage in bidirectional crosstalk with neurons to regulate numerous aspects of neurotransmission, circuit function, and behavior. Mutations in synapse-related genes expressed in both neurons and astrocytes are central factors in a vast number of neurological disorders, making the proteins that they encode prominent targets for therapeutic intervention. Yet, while the roles of many of these synaptic proteins in neurons are well established, the functions of the same proteins in astrocytes are largely unknown. This gap in knowledge must be addressed to refine therapeutic approaches. In this chapter, we integrate multiomic meta-analysis and a comprehensive overview of current literature to show that astrocytes express an astounding number of genes that overlap with the neuronal and synaptic transcriptomes. Further, we highlight recent reports that characterize the expression patterns and potential novel roles of these genes in astrocytes in both physiological and pathological conditions, underscoring the importance of considering both cell types when investigating the function and regulation of synaptic proteins.

Eliciting calcium transients with UV nanosecond laser stimulation in adult patient-derived glioblastoma brain cancer cellsin vitro

Objective.Glioblastoma (GBM) is the most common and lethal type of high-grade adult brain cancer. The World Health Organization have classed GBM as an incurable disease because standard treatments have yielded little improvement with life-expectancy being 6-15 months after diagnosis. Different approaches are now crucial to discover new knowledge about GBM communication/function in order to establish alternative therapies for such an aggressive adult brain cancer. Calcium (Ca2+) is a fundamental cell molecular messenger employed in GBM being involved in a wide dynamic range of cellular processes. Understanding how the movement of Ca2+behaves and modulates activity in GBM at the single-cell level is relatively unexplored but holds the potential to yield opportunities for new therapeutic strategies and approaches for cancer treatment.Approach.In this article we establish a spatially and temporally precise method for stimulating Ca2+transients in three patient-derived GBM cell-lines (FPW1, RN1, and RKI1) such that Ca2+communication can be studied from single-cell to larger network scales. We demonstrate that this is possible by administering a single optimized ultra-violet (UV) nanosecond laser pulse to trigger GBM Ca2+transients.Main results.We determine that 1.58µJµm-2is the optimal UV nanosecond laser pulse energy density necessary to elicit a single Ca2+transient in the GBM cell-lines whilst maintaining viability, functionality, the ability to be stimulated many times in an experiment, and to trigger further Ca2+communication in a larger network of GBM cells.Significance.Using adult patient-derived mesenchymal GBM brain cancer cell-lines, the most aggressive form of GBM cancer, this work is the first of its kind as it provides a new effective modality of which to stimulate GBM cells at the single-cell level in an accurate, repeatable, and reliable manner; and is a first step toward Ca2+communication in GBM brain cancer cells and their networks being more effectively studied.

Hemokinin-1 induces transcriptomic alterations in pain-related signaling processes in rat primary sensory neurons independent of NK1 tachykinin receptor activation

The tachykinin hemokinin-1 (HK-1) is involved in immunological processes, inflammation, and pain. Although the neurokinin 1 receptor (NK1R) is described as its main target, several effects are mediated by currently unidentified receptor(s). The role of HK-1 in pain is controversial, depending on the involvement of peripheral and central sensitization mechanisms in different models. We earlier showed the ability of HK-1 to activate the trigeminovascular system, but the mechanisms need to be clarified. Therefore, in this study, we investigated HK-1-induced transcriptomic alterations in cultured rat trigeminal ganglion (TRG) primary sensory neurons. HK-1 was applied for 6 or 24 h in 1 μM causing calcium-influx in these neurons, 500 nM not inducing calcium-entry was used for comparison. Next-generation sequencing was performed on the isolated RNA, and transcriptomic changes were analyzed to identify differentially expressed (DE) genes. Functional analysis was performed for gene annotation using the Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG), and Reactome databases. NK1R and Neurokinin receptor 2 (NK2R) were not detected. Neurokinin receptor 3 (NK3R) was around the detection limit, which suggests the involvement of other NKR isoforms or other receptors in HK-1-induced sensory neuronal activation. We found protease-activated receptor 1 (PAR1) and epidermal growth factor receptor (EGFR) as DE genes in calcium signaling. The transmembrane protein anthrax toxin receptor 2 (ANTXR2), a potential novel pain-related target, was upregulated. Acid-sensing ion channel 1; 3 (Asic1,3), N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptors decreased, myelin production and maintenance related genes (Mbp, Pmp2, Myef2, Mpz) and GNDF changed by HK-1 treatment. Our data showed time and dose-dependent effects of HK-1 in TRG cell culture. Result showed calcium signaling as altered event, however, we did not detect any of NK receptors. Presumably, the activation of TRG neurons is independent of NK receptors. ANTXR2 is a potential new target, PAR-1 has also important role in pain, however their connection to HK-1 is unknown. These findings might highlight new targets or key mediators to solve how HK-1 acts on TRG.

Purinergic signaling mediates neuroglial interactions to modulate sighs

Sighs prevent the collapse of alveoli in the lungs, initiate arousal under hypoxic conditions, and are an expression of sadness and relief. Sighs are periodically superimposed on normal breaths, known as eupnea. Implicated in the generation of these rhythmic behaviors is the preBötzinger complex (preBötC). Our experimental evidence suggests that purinergic signaling is necessary to generate spontaneous and hypoxia-induced sighs in a mouse model. Our results demonstrate that driving calcium increases in astrocytes through pharmacological methods robustly increases sigh, but not eupnea, frequency. Calcium imaging of preBötC slices corroborates this finding with an increase in astrocytic calcium upon application of sigh modulators, increasing intracellular calcium through g-protein signaling. Moreover, photo-activation of preBötC astrocytes is sufficient to elicit sigh activity, and this response is blocked with purinergic antagonists. We conclude that sighs are modulated through neuron-glia coupling in the preBötC network, where the distinct modulatory responses of neurons and glia allow for both rhythms to be independently regulated.

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.

Neuronal activity-induced, equilibrative nucleoside transporter-dependent, somatodendritic adenosine release revealed by a GRAB sensor

The purinergic signaling molecule adenosine (Ado) modulates many physiological and pathological functions in the brain. However, the exact source of extracellular Ado remains controversial. Here, utilizing a newly optimized genetically encoded GPCR-Activation-Based Ado fluorescent sensor (GRABAdo), we discovered that the neuronal activity-induced extracellular Ado elevation is due to direct Ado release from somatodendritic compartments of neurons, rather than from the axonal terminals, in the hippocampus. Pharmacological and genetic manipulations reveal that the Ado release depends on equilibrative nucleoside transporters but not the conventional vesicular release mechanisms. Compared with the fast-vesicular glutamate release, the Ado release is slow (~40 s) and requires calcium influx through L-type calcium channels. Thus, this study reveals an activity-dependent second-to-minute local Ado release from the somatodendritic compartments of neurons, potentially serving modulatory functions as a retrograde signal.

Molecular Mechanisms of Astrocyte Involvement in Synaptogenesis and Brain Synaptic Plasticity

Astrocytes perform a wide range of important functions in the brain. As structural and functional components of synapses, astrocytes secrete various factors (proteins, lipids, small molecules, etc.) that bind to neuronal receptor and contribute to synaptogenesis and regulation of synaptic contacts. Astrocytic factors play a key role in the formation of neural networks undergoing short- and long-term synaptic morphological and functional rearrangements essential in the memory formation and behavior. The review summarizes the data on the molecular mechanisms mediating the involvement of astrocyte-secreted factors in synaptogenesis in the brain and provides up-to-date information on the role of astrocytes and astrocytic synaptogenic factors in the long-term plastic rearrangements of synaptic contacts.

Brainstem astrocytes control homeostatic regulation of caloric intake

Prolonged high-fat diet (HFD) exposure is associated with hyperphagia, excess caloric intake and weight gain. After initial exposure to a HFD, a brief (24-48 h) period of hyperphagia is followed by the regulation of caloric intake and restoration of energy balance within an acute (3-5 day) period. Previous studies have demonstrated this occurs via a vagally mediated signalling cascade that increases glutamatergic transmission via activation of NMDA receptors located on gastric-projecting neurons of the dorsal motor nucleus of the vagus (DMV). The present study used electrophysiological recordings from thin brainstem slice preparations, in vivo recordings of gastric motility and tone, measurement of gastric emptying rates, and food intake studies to investigate the hypothesis that activation of brainstem astrocytes in response to acute HFD exposure is responsible for the increased glutamatergic drive to DMV neurons and the restoration of caloric balance. Pharmacological and chemogenetic inhibition of brainstem astrocytes reduced glutamatergic signalling and DMV excitability, dysregulated gastric tone and motility, attenuated the homeostatic delay in gastric emptying, and prevented the decrease in food intake that is observed during the period of energy regulation following initial exposure to HFD. Understanding the mechanisms involved in caloric regulation may provide critical insights into energy balance as well as into the hyperphagia that develops as these mechanisms are overcome. KEY POINTS: Initial exposure to a high fat diet is associated with a brief period of hyperphagia before caloric intake and energy balance is restored. This period of homeostatic regulation is associated with a vagally mediated signalling cascade that increases glutamatergic transmission to dorsal motor nucleus of the vagus (DMV) neurons via activation of synaptic NMDA receptors. The present study demonstrates that pharmacological and chemogenetic inhibition of brainstem astrocytes reduced glutamatergic signalling and DMV neuronal excitability, dysregulated gastric motility and tone and emptying, and prevented the regulation of food intake following high-fat diet exposure. Astrocyte regulation of glutamatergic transmission to DMV neurons appears to involve release of the gliotransmitters glutamate and ATP. Understanding the mechanisms involved in caloric regulation may provide critical insights into energy balance as well as into the hyperphagia that develops as these mechanisms are overcome.

Diversification of PAR signaling through receptor crosstalk

Protease activated receptors (PARs) are among the first receptors shown to transactivate other receptors: noticeably, these interactions are not limited to members of the same family, but involve receptors as diverse as receptor kinases, prostanoid receptors, purinergic receptors and ionic channels among others. In this review, we will focus on the evidence for PAR interactions with members of their own family, as well as with other types of receptors. We will discuss recent evidence as well as what we consider as emerging areas to explore; from the signalling pathways triggered, to the physiological and pathological relevance of these interactions, since this additional level of molecular cross-talk between receptors and signaling pathways is only beginning to be explored and represents a novel mechanism providing diversity to receptor function and play important roles in physiology and disease.

Astrocytes Render Memory Flexible by Releasing D-Serine and Regulating NMDA Receptor Tone in the Hippocampus

BACKGROUND

NMDA receptor (NMDAR) hypofunction has been implicated in several psychiatric disorders with impairment of cognitive flexibility. However, the molecular mechanism of how NMDAR hypofunction with decreased NMDAR tone causes the impairment of cognitive flexibility has been minimally understood. Furthermore, it has been unclear whether hippocampal astrocytes regulate NMDAR tone and cognitive flexibility.

METHODS

We employed cell type-specific genetic manipulations, ex vivo electrophysiological recordings, sniffer patch recordings, cutting-edge biosensor for norepinephrine, and behavioral assays to investigate whether astrocytes can regulate NMDAR tone by releasing D-serine and glutamate. Subsequently, we further investigated the role of NMDAR tone in heterosynaptic long-term depression, metaplasticity, and cognitive flexibility.

RESULTS

We found that hippocampal astrocytes regulate NMDAR tone via BEST1-mediated corelease of D-serine and glutamate. Best1 knockout mice exhibited reduced NMDAR tone and impairments of homosynaptic and α₁ adrenergic receptor-dependent heterosynaptic long-term depression, which leads to defects in metaplasticity and cognitive flexibility. These impairments in Best1 knockout mice can be rescued by hippocampal astrocyte-specific BEST1 expression or enhanced NMDAR tone through D-serine supplement. D-serine injection in Best1 knockout mice during initial learning rescues subsequent reversal learning.

CONCLUSIONS

These findings indicate that NMDAR tone during initial learning is important for subsequent learning, and hippocampal NMDAR tone regulated by astrocytic BEST1 is critical for heterosynaptic long-term depression, metaplasticity, and cognitive flexibility.

Chimeric Structures in Mental Illnesses-"Magic" Molecules Specified for Complex Disorders

Mental health problems cover a wide spectrum of diseases, including mild to moderate anxiety, depression, alcohol/drug use disorders, as well as bipolar disorder and schizophrenia. Pharmacological treatment seems to be one of the most effective opportunities to recover function efficiently and satisfactorily. However, such disorders are complex as several target points are involved. This results in a necessity to combine different types of drugs to obtain the necessary therapeutic goals. There is a need to develop safer and more effective drugs. Considering that mental illnesses share multifactorial processes, the paradigm of one treatment with multiple modes of action rather than single-target strategies would be more effective for successful therapies. Therefore, hybrid molecules that combine two pharmacophores in one entity show promise, as they possess the desired therapeutic index with a small off-target risk. This review aims to provide information on chimeric structures designed for mental disorder therapy (i.e., schizophrenia and depression), and new types of drug candidates currently being tested. In addition, a discussion on some benefits and limitations of multifunctional, bivalent drug candidates is also given.

Neuroinflammation Induced by Transgenic Expression of Lipocalin-2 in Astrocytes

Transgenic mice are a useful tool for exploring various aspects of gene function. A key element of this approach is the targeted overexpression of specific genes in cells or tissues. Herein, we report for the first time, the generation and characterization of conditional transgenic (cTg) mice for lipocalin-2 (LCN2) expression. We generated the R26-LCN2-transgenic (LCN2-cTg) mice that carried a loxP-flanked STOP (neo) cassette, Lcn2 cDNA, and a GFP sequence. When bred with Tg mice expressing Cre recombinase under the control of various tissues or cell-specific promoters, Cre-mediated recombination deletes the STOP cassette and allows the expression of LCN2 and GFP. In this study, we achieved the recombination of loxP-flanked LCN2 in hippocampal astrocytes of cTg mouse brain, using a targeted delivery of adeno-associated virus (AAVs) bearing Cre recombinase under the control of a GFAP promoter (AAVs-GFAP-mCherry-Cre). These mice with localized LCN2 overexpression in astrocytes of the hippocampus developed neuroinflammation with enhanced glial activation and increased mRNA and protein levels of proinflammatory cytokines. Furthermore, mice showed impairment in cognitive functions as a typical symptom of hippocampal inflammation. Taken together, our study demonstrates the usefulness of LCN2-cTg mice in targeting specific cells at various organs for conditional LCN2 expression and for subsequent investigation of the functional role of cell-type-specific LCN2 within these sites. Moreover, the LCN2-cTg mice with targeted expression of LCN2 in hippocampal astrocytes are a new in vivo model of neuroinflammation.

Focused Ultrasound Stimulation as a Neuromodulatory Tool for Parkinson's Disease: A Scoping Review

Non-invasive focused ultrasound stimulation (FUS) is a non-ionising neuromodulatory technique that employs acoustic energy to acutely and reversibly modulate brain activity of deep-brain structures. It is currently being investigated as a potential novel treatment for Parkinson's disease (PD). This scoping review was carried out to map available evidence pertaining to the provision of FUS as a PD neuromodulatory tool. In accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analysis Extension for Scoping Reviews, a search was applied to Ovid MEDLINE, Embase, Web of Science and Cochrane Central Register of Controlled Trials on 13 January 2022, with no limits applied. In total, 11 studies were included: 8 were from China and 1 each from Belgium, South Korea and Taiwan. All 11 studies were preclinical (6 in vivo, 2 in vitro, 2 mix of in vivo and in vitro and 1 in silico). The preclinical evidence indicates that FUS is safe and has beneficial neuromodulatory effects on motor behaviour in PD. FUS appears to have a therapeutic role in influencing the disease processes of PD, and therefore holds great promise as an attractive and powerful neuromodulatory tool for PD. Though these initial studies are encouraging, further study to understand the underlying cellular and molecular mechanisms is required before FUS can be routinely used in PD.

Glycine-induced NMDA receptor internalization provides neuroprotection and preserves vasculature following ischemic stroke

Ischemic stroke is the second leading cause of death worldwide. Following an ischemic event, neuronal death is triggered by uncontrolled glutamate release leading to overactivation of glutamate sensitive N-methyl-d-aspartate receptor (NMDAR). For gating, NMDARs require not only the binding of glutamate, but also of glycine or a glycine-like compound as a co-agonist. Low glycine doses enhance NMDAR function, whereas high doses trigger glycine-induced NMDAR internalization (GINI) in vitro. Here, we report that following an ischemic event, in vivo, GINI also occurs and provides neuroprotection in the presence of a GlyT1 antagonist (GlyT1-A). Mice pretreated with a GlyT1-A, which increases synaptic glycine levels, exhibited smaller stroke volume, reduced cell death, and minimized behavioral deficits following stroke induction by either photothrombosis or endothelin-1. Moreover, we show evidence that in ischemic conditions, GlyT1-As preserve the vasculature in the peri-infarct area. Therefore, GlyT1 could be a new target for the treatment of ischemic stroke.

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GINI is a dynamic phenomenon which dampens NMDAR-mediated excitotoxicity during stroke

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GlyT1-antagonists (GlyT1-As) trigger GINI during stroke in vivo

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GlyT1-As mitigate post-stroke behavioral deficits and preserve peri-infarct vasculature

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GlyT1 could be a novel and viable therapeutic target for ischemic stroke

Neuronal and Astrocytic Morphological Alterations Driven by Prolonged Exposure with Δ9-Tetrahydrocannabinol but Not Cannabidiol

Cannabis derivatives are largely used in the general population for recreational and medical purposes, with the highest prevalence among adolescents, but chronic use and abuse has raised medical concerns. We investigated the prolonged effects of Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD) in organotypic hippocampal slices from P7 rats cultured for 2 weeks. Cell death in the CA1 subregion of slices was quantified by propidium iodide (PI) fluorescence, pre-synaptic and post-synaptic marker proteins were analysed by Western blotting and neurodegeneration and astrocytic alterations by NeuN and GFAP by immunofluorescence and confocal laser microscopy. The statistical significance of differences was analysed using ANOVA with a post hoc Dunnett w-test (PI fluorescence intensities and Western blots) or Newman–Keuls (immunohistochemistry data) for multiple comparisons. A probability value (P) of < 0.05 was considered significant. Prolonged (72 h) THC or CBD incubation did not induce cell death but caused modifications in the expression of synaptic proteins and morphological alterations in neurons and astrocytes. In particular, the expression of PSD95 was reduced following incubation for 72 h with THC and was increased following incubation with CBD. THC for 72 h caused disorganisation of CA1 stratum pyramidalis (SP) and complex morphological modifications in a significant number of pyramidal neurons and in astrocytes. Our results suggest that THC or CBD prolonged exposure induce different effects in the hippocampus. In particular, 72 h of THC exposure induced neuronal and glia alterations that must draw our attention to the effects that relatively prolonged use might cause, especially in adolescents.

Neurocoagulation from a Mechanistic Point of View in the Central Nervous System

Coagulation mechanisms are critical for maintaining homeostasis in the central nervous system (CNS). Thrombin, an important player of the coagulation cascade, activates protease activator receptors (PARs), members of the G-protein coupled receptor family. PAR1 is located on neurons and glia. Following thrombin activation, PAR1 signals through the extracellular signal-regulated kinase pathway, causing alterations in neuronal glutamate release and astrocytic morphological changes. Similarly, the anticoagulation factor activated protein C (aPC) can cleave PAR1, following interaction with the endothelial protein C receptor. Both thrombin and aPC are expressed on endothelial cells and pericytes in the blood-brain barrier (BBB). Thrombin-induced PAR1 activation increases cytosolic Ca²⁺ concentration in brain vessels, resulting in nitric oxide release and increasing F-actin stress fibers, damaging BBB integrity. aPC also induces PAR1 activation and preserves BBB vascular integrity via coupling to sphingosine 1 phosphate receptors. Thrombin-induced PAR1 overactivation and BBB disruption are evident in CNS pathologies. During epileptic seizures, BBB disruption promotes thrombin penetration. Thrombin induces PAR1 activation and potentiates N-methyl-D-aspartate receptors, inducing glutamate-mediated hyperexcitability. Specific PAR1 inhibition decreases status epilepticus severity in vivo. In stroke, the elevation of brain thrombin levels further compromises BBB integrity, with direct parenchymal damage, while systemic factor Xa inhibition improves neurological outcomes. In multiple sclerosis (MS), brain thrombin inhibitory capacity correlates with clinical presentation. Both thrombin inhibition by hirudin and the use of recombinant aPC improve disease severity in an MS animal model. This review presents the mechanisms underlying the effects of coagulation on the physiology and pathophysiology of the CNS.

Classification of advanced methods for evaluating neurotoxicity

Purpose of review

As fields such as neurotoxicity evaluation and neuro-related drug research are increasing in popularity, there is a demand for the expansion of neurotoxicity research. Currently, neurotoxicity is assessed by measuring changes in weight and behavior. However, measurement of such changes does not allow the detection of subtle and inconspicuous neurotoxicity. In this review, methods for advancing neurotoxicity research are divided into molecule-, cell-, circuit-, and animal model-based methods, and the results of previous studies assessing neurotoxicity are provided and discussed.

Recent findings

In coming decades, cooperation between universities, national research institutes, industrial research institutes, governments, and the private sector will become necessary when identifying alternative methods for neurotoxicity evaluation, which is a current goal related to improving neurotoxicity assessment and an appropriate approach to neurotoxicity prediction. Many methods for measuring neurotoxicity in the field of neuroscience have recently been reported. This paper classifies the supplementary and complementary experimental measures for evaluating neurotoxicity.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

Quinacrine directly dissociates amyloid plaques in the brain of 5XFAD transgenic mouse model of Alzheimer's disease

Alzheimer's disease (AD) is the most common type of dementia characterized by the abnormal accumulation of amyloid-β (Aβ) in the brain. Aβ misfolding is associated with neuroinflammation and synaptic dysfunction, leading to learning and memory deficits. Therefore, Aβ production and aggregation have been one of the most popular drug targets for AD. Failures of drug candidates regulating the aforementioned Aβ cascade stimulated development of immunotherapy agents for clearance of accumulated Aβ in the brain. Here, we report that quinacrine, a blood-brain barrier penetrating antimalarial chemical drug, dissociates Aβ plaques in the brain of AD transgenic mice. When co-incubated with pre-formed Aβ fibrils, quinacrine decreased thioflavin T-positive β-sheets in vitro, on top of its inhibitory function on the fibril formation. We confirmed that quinacrine induced dissociation of high-molecular-weight Aβ aggregates into low-molecular-weight species by dot blots in association with size cut-off filtrations. Quinacrine was then administered to adult 5XFAD transgenic mice via weekly intravenous injections for 6 weeks, and we found a significant reduction of Aβ plaques and astrocytosis in their cortex and hippocampus. In western blots of quinacrine-administered mouse brains, amelioration of AD-related biomarkers, glial fibrillary acidic protein, postsynaptic protein 95, phosphorylated cAMP response element-binding protein, phosphorylated c-Jun N-terminal kinase were observed. Lastly, quinacrine-stimulated dissociation of misfolded aggregates induced recovery of synaptic function associated with Aβ in excitatory post-synaptic current recordings of primary rat cortical neurons treated with Aβ aggregates and quinacrine. Collectively, quinacrine can directly dissociate Aβ fibrils and alleviate decreased synaptic functions.

Glia as sculptors of synaptic plasticity

Glial cells are non-neuronal cells in the nervous system that are crucial for proper brain development and function. Three major classes of glia in the central nervous system (CNS) include astrocytes, microglia and oligodendrocytes. These cells have dynamic morphological and functional properties and constantly surveil neural activity throughout life, sculpting synaptic plasticity. Astrocytes form part of the tripartite synapse with neurons and perform many homeostatic functions essential to proper synaptic function including clearing neurotransmitter and regulating ion balance; they can modify these properties, in addition to additional mechanisms such as gliotransmitter release, to influence short- and long-term plasticity. Microglia, the resident macrophage of the CNS, monitor synaptic activity and can eliminate synapses by phagocytosis or modify synapses by release of cytokines or neurotrophic factors. Oligodendrocytes regulate speed of action potential conduction and efficiency of information exchange through the formation of myelin, having important consequences for the plasticity of neural circuits. A deeper understanding of how glia modulate synaptic and circuit plasticity will further our understanding of the ongoing changes that take place throughout life in the dynamic environment of the CNS.

Inhibition of protease-activated receptor 1 (PAR1) ameliorates cognitive performance and synaptic plasticity impairments in animal model of Alzheimer's diseases

INTRODUCTION

Alzheimer's disease (AD) is a progressive brain disorder accompanied with synaptic failures and decline in cognitive and learning processes. Protease-activated receptor 1 (PAR1) is the major thrombin receptor in the brain that is implicated in synaptic plasticity and memory formation. In the current study, we hypothesized that inhibition of PAR1 would theoretically prevent amyloid beta (Aβ) accumulation in the brain and then contribute to reduce risk of AD. The aim of the present study was to evaluate the effect of PAR1 inhibition by using SCH (as an inhibitor of PAR1) on spatial learning, memory, and synaptic plasticity in the CA1 region of the hippocampus in rat model of Alzheimer's disease.

METHODS

For the induction of Alzheimer's disease, amyloid beta (Aβ) 1-42 was injected in the CA1 region of the hippocampus. The rats were divided into four groups: group I (surgical sham); group II rat mode of Alzheimer's disease (AD); group III (SCH) (25 μg/kg) intraperitoneally (i.p.), and group IV (AD + SCH). After 14 days of protocol, the rats in group III received SCH and 30 min after injection behavioral and electrophysiological tests were performed. Learning and memory ability was assessed by Morris water maze and novel object recognition tests. Extracellular evoked field excitatory postsynaptic potentials (fEPSP) were recorded in the stratum radiatum of the CA1 area.

RESULTS

Our results showed that AD rats showed impairments in learning and memory, and long-term potentiation (LTP) was not induced in these rats. However, injection of SCH overcame the AD-induced impairment in LTP generation in the CA1 area of the hippocampus and improved learning and memory impairment.

DMV extrasynaptic NMDA receptors regulate caloric intake in rats

Acute high-fat diet (aHFD) exposure induces a brief period of hyperphagia before caloric balance is restored. Previous studies have demonstrated that this period of regulation is associated with activation of synaptic N-methyl-D-aspartate (NMDA) receptors on dorsal motor nucleus of the vagus (DMV) neurons, which increases vagal control of gastric functions. Our aim was to test the hypothesis that activation of DMV synaptic NMDA receptors occurs subsequent to activation of extrasynaptic NMDA receptors. Sprague-Dawley rats were fed a control or high-fat diet for 3-5 days prior to experimentation. Whole-cell patch-clamp recordings from gastric-projecting DMV neurons; in vivo recordings of gastric motility, tone, compliance, and emptying; and food intake studies were used to assess the effects of NMDA receptor antagonism on caloric regulation. After aHFD exposure, inhibition of extrasynaptic NMDA receptors prevented the synaptic NMDA receptor-mediated increase in glutamatergic transmission to DMV neurons, as well as the increase in gastric tone and motility, while chronic extrasynaptic NMDA receptor inhibition attenuated the regulation of caloric intake. After aHFD exposure, the regulation of food intake involved synaptic NMDA receptor-mediated currents, which occurred in response to extrasynaptic NMDA receptor activation. Understanding these events may provide a mechanistic basis for hyperphagia and may identify novel therapeutic targets for the treatment of obesity.

Excessive Astrocytic GABA Causes Cortical Hypometabolism and Impedes Functional Recovery after Subcortical Stroke

Glucose hypometabolism in cortical structures after functional disconnection is frequently reported in patients with white matter diseases such as subcortical stroke. However, the molecular and cellular mechanisms have been poorly elucidated. Here we show, in an animal model of internal capsular infarct, that GABA-synthesizing reactive astrocytes in distant cortical areas cause glucose hypometabolism via tonic inhibition of neighboring neurons. We find that reversal of aberrant astrocytic GABA synthesis, by pharmacological inhibition and astrocyte-specific gene silencing of MAO-B, reverses the reduction in cortical glucose metabolism. Moreover, induction of aberrant astrocytic GABA synthesis by cortical injection of putrescine or adenovirus recapitulates cortical hypometabolism. Furthermore, MAO-B inhibition causes a remarkable recovery from post-stroke motor deficits when combined with a rehabilitation regimen. Collectively, our data indicate that cortical glucose hypometabolism in subcortical stroke is caused by aberrant astrocytic GABA and MAO-B inhibition and that attenuating cortical hypometabolism can be a therapeutic approach in subcortical stroke.

Role of Thrombin in Central Nervous System Injury and Disease

Thrombin is a Na+-activated allosteric serine protease of the chymotrypsin family involved in coagulation, inflammation, cell protection, and apoptosis. Increasingly, the role of thrombin in the brain has been explored. Low concentrations of thrombin are neuroprotective, while high concentrations exert pathological effects. However, greater attention regarding the involvement of thrombin in normal and pathological processes in the central nervous system is warranted. In this review, we explore the mechanisms of thrombin action, localization, and functions in the central nervous system and describe the involvement of thrombin in stroke and intracerebral hemorrhage, neurodegenerative diseases, epilepsy, traumatic brain injury, and primary central nervous system tumors. We aim to comprehensively characterize the role of thrombin in neurological disease and injury.

Emerging Roles of Protease-Activated Receptors (PARs) in the Modulation of Synaptic Transmission and Plasticity

Protease-activated receptors (PARs) are a class of G protein-coupled receptors (GPCRs) with a unique mechanism of activation, prompted by a proteolytic cleavage in their N-terminal domain that uncovers a tethered ligand, which binds and stimulates the same receptor. PARs subtypes (PAR1-4) have well-documented roles in coagulation, hemostasis, and inflammation, and have been deeply investigated for their function in cellular survival/degeneration, while their roles in the brain in physiological conditions remain less appreciated. Here, we describe PARs' effects in the modulation of neurotransmission and synaptic plasticity. Available evidence, mainly concerning PAR1-mediated and PAR2-mediated regulation of glutamatergic and GABAergic transmission, supports that PARs are important modulators of synaptic efficacy and plasticity in normal conditions.

AEG-1 Regulates TWIK-1 Expression as an RNA-Binding Protein in Astrocytes

AEG-1, also called MTDH, has oncogenic potential in numerous cancers and is considered a multifunctional modulator because of its involvement in developmental processes and inflammatory and degenerative brain diseases. However, the role of AEG-1 in astrocytes remains unknown. This study aimed to investigate proteins directly regulated by AEG-1 by analyzing their RNA expression patterns in astrocytes transfected with scramble shRNA and AEG-1 shRNA. AEG-1 knockdown down-regulated TWIK-1 mRNA. Real-time quantitative PCR (qPCR) and immunocytochemistry assays confirmed that AEG-1 modulates TWIK-1 mRNA and protein expression. Electrophysiological experiments further revealed that AEG-1 further regulates TWIK-1-mediated potassium currents in normal astrocytes. An RNA immunoprecipitation assay to determine how AEG-1 regulates the expression of TWIK-1 revealed that AEG-1 binds directly to TWIK-1 mRNA. Furthermore, TWIK-1 mRNA stability was significantly increased upon overexpression of AEG-1 in cultured astrocytes (p < 0.01). Our findings show that AEG-1 serves as an RNA-binding protein to regulate TWIK-1 expression in normal astrocytes.

Glial Chloride Channels in the Function of the Nervous System Across Species

In the nervous system, the concentration of Cl- in neurons that express GABA receptors plays a key role in establishing whether these neurons are excitatory, mostly during early development, or inhibitory. Thus, much attention has been dedicated to understanding how neurons regulate their intracellular Cl- concentration. However, regulation of the extracellular Cl- concentration by other cells of the nervous system, including glia and microglia, is as important because it ultimately affects the Cl- equilibrium potential across the neuronal plasma membrane. Moreover, Cl- ions are transported in and out of the cell, via either passive or active transporter systems, as counter ions for K+ whose concentration in the extracellular environment of the nervous system is tightly regulated because it directly affects neuronal excitability. In this book chapter, we report on the Cl- channel types expressed in the various types of glial cells focusing on the role they play in the function of the nervous system in health and disease. Furthermore, we describe the types of stimuli that these channels are activated by, the other solutes that they may transport, and the involvement of these channels in processes such as pH regulation and Regulatory Volume Decrease (RVD). The picture that emerges is one of the glial cells expressing a variety of Cl- channels, encoded by members of different gene families, involved both in short- and long-term regulation of the nervous system function. Finally, we report data on invertebrate model organisms, such as C. elegans and Drosophila, that are revealing important and previously unsuspected functions of some of these channels in the context of living and behaving animals.

Space-Dependent Glia-Neuron Interplay in the Hippocampus of Transgenic Models of β-Amyloid Deposition

This review is focused on the description and discussion of the alterations of astrocytes and microglia interplay in models of Alzheimer's disease (AD). AD is an age-related neurodegenerative pathology with a slowly progressive and irreversible decline of cognitive functions. One of AD's histopathological hallmarks is the deposition of amyloid beta (Aβ) plaques in the brain. Long regarded as a non-specific, mere consequence of AD pathology, activation of microglia and astrocytes is now considered a key factor in both initiation and progression of the disease, and suppression of astrogliosis exacerbates neuropathology. Reactive astrocytes and microglia overexpress many cytokines, chemokines, and signaling molecules that activate or damage neighboring cells and their mutual interplay can result in virtuous/vicious cycles which differ in different brain regions. Heterogeneity of glia, either between or within a particular brain region, is likely to be relevant in healthy conditions and disease processes. Differential crosstalk between astrocytes and microglia in CA1 and CA3 areas of the hippocampus can be responsible for the differential sensitivity of the two areas to insults. Understanding the spatial differences and roles of glia will allow us to assess how these interactions can influence the state and progression of the disease, and will be critical for identifying therapeutic strategies.

Severe reactive astrocytes precipitate pathological hallmarks of Alzheimer's disease via H2O2- production

Although the pathological contributions of reactive astrocytes have been implicated in Alzheimer's disease (AD), their in vivo functions remain elusive due to the lack of appropriate experimental models and precise molecular mechanisms. Here, we show the importance of astrocytic reactivity on the pathogenesis of AD using GiD, a newly developed animal model of reactive astrocytes, where the reactivity of astrocytes can be manipulated as mild (GiDm) or severe (GiDs). Mechanistically, excessive hydrogen peroxide (H₂O₂) originated from monoamine oxidase B in severe reactive astrocytes causes glial activation, tauopathy, neuronal death, brain atrophy, cognitive impairment and eventual death, which are significantly prevented by AAD-2004, a potent H₂O₂ scavenger. These H₂O₂^--induced pathological features of AD in GiDs are consistently recapitulated in a three-dimensional culture AD model, virus-infected APP/PS1 mice and the brains of patients with AD. Our study identifies H₂O₂ from severe but not mild reactive astrocytes as a key determinant of neurodegeneration in AD.

Inhibitors of synaptic vesicle exocytosis reduce surface expression of postsynaptic glutamate receptors

Bafilomycin A1, a vacuolar H⁺-ATPase inhibitor, and botulinum toxin B and tetanus toxin, both vesicle fusion inhibitors, are widely known exocytosis blockers that have been used to inhibit the presynaptic release of neurotransmitters. However, protein trafficking mechanisms, such as the insertion of postsynaptic receptors and astrocytic glutamate-releasing channels into the plasma membrane, also require exocytosis. In our previous study, exocytosis inhibitors reduced the surface expression of astrocytic glutamate-releasing channels. Here, we further investigated whether exocytosis inhibitors influence the surface expression of postsynaptic receptors. Using pH-sensitive superecliptic pHluorin (SEP)-tagged postsynaptic glutamate receptors, including GluA1, GluA2, GluN1, and GluN2A, we found that bafilomycin A1, botulinum toxin B, and/or tetanus toxin reduce the SEP fluorescence of SEP-GluA1, SEP-GluA2, SEP-GluN1, and SEP-GluN2A. These findings indicate that presynaptic vesicle exocytosis inhibitors also affect the postsynaptic trafficking machinery for surface expression. Finally, this study provides profound insights assembling presynaptic, postsynaptic and astrocytic viewpoints into the interpretation of the data obtained using these synaptic vesicle exocytosis inhibitors.

Protease-activated receptor 1 (PAR1) inhibits synaptic NMDARs in mouse nigral dopaminergic neurons

Protease-activated receptor 1 (PAR1) is a G protein-coupled receptor (GPCR), whose activation requires a proteolytic cleavage in the extracellular domain exposing a tethered ligand, which binds to the same receptor thus stimulating Gα_q/11-, Gα_i/o- and Gα_12-13 proteins. PAR1, activated by serine proteases and matrix metalloproteases, plays multifaceted roles in neuroinflammation and neurodegeneration, in stroke, brain trauma, Alzheimer's diseases, and Parkinson's disease (PD). Substantia nigra pars compacta (SNpc) is among areas with highest PAR1 expression, but current evidence on its roles herein is restricted to mechanisms controlling dopaminergic (DAergic) neurons survival, with controversial data showing PAR1 either fostering or counteracting degeneration in PD models. Since PAR1 functions on SNpc DAergic neurons activity are unknown, we investigated if PAR1 affects glutamatergic transmission in this neuronal population. We analyzed PAR1's effects on NMDARs and AMPARs by patch-clamp recordings from DAergic neurons from mouse midbrain slices. Then, we explored subunit composition of PAR1-sensitive NMDARs, with selective antagonists, and mechanisms underlying PAR1-induced NMDARs modulation, by quantifying NMDARs surface expression. PAR1 activation inhibits synaptic NMDARs in SNpc DAergic neurons, without affecting AMPARs. PAR1-sensitive NMDARs contain GluN2B/GluN2D subunits. Moreover, PAR1-mediated NMDARs hypofunction is reliant on NMDARs internalization, as PAR1 stimulation increases NMDARs intracellular levels and pharmacological limitation of NMDARs endocytosis prevents PAR1-induced NMDARs inhibition. We reveal that PAR1 regulates glutamatergic transmission in midbrain DAergic cells. This might have implications in brain's DA-dependent functions and in neurological/psychiatric diseases linked to DAergic dysfunctions.

The molecular mechanism of synaptic activity-induced astrocytic volume transient

KEY POINTS

Neuronal activity causes astrocytic volume change via K⁺ uptake through TREK-1 containing two-pore domain potassium channels. The volume transient is terminated by Cl^- efflux through the Ca²⁺ -activated anion channel BEST1. The source of the Ca²⁺ required to open BEST1 appears to be the stretch-activated TRPA1 channel. Intense neuronal activity is synaptically coupled with a physical change in astrocytes via volume transients.

ABSTRACT

The brain volume changes dynamically and transiently upon intense neuronal activity through a tight regulation of ion concentrations and water movement across the plasma membrane of astrocytes. We have recently demonstrated that an intense neuronal activity and subsequent astrocytic AQP4-dependent volume transient are critical for synaptic plasticity and memory. We have also pharmacologically demonstrated a functional coupling between synaptic activity and the astrocytic volume transient. However, the precise molecular mechanisms of how intense neuronal activity and the astrocytic volume transient are coupled remain unclear. Here we utilized an intrinsic optical signal imaging technique combined with fluorescence imaging using ion sensitive dyes and molecular probes and electrophysiology to investigate the detailed molecular mechanisms in genetically modified mice. We report that a brief synaptic activity induced by a train stimulation (20 Hz, 1 s) causes a prolonged astrocytic volume transient (80 s) via K⁺ uptake through TREK-1 containing two-pore domain potassium (K2P) channels, but not Kir4.1 or NKCC1. This volume change is terminated by Cl^- efflux through the Ca²⁺ -activated anion channel BEST1, but not the volume-regulated anion channel TTYH. The source of the Ca²⁺ required to open BEST1 appears to be the stretch-activated TRPA1 channel in astrocytes, but not IP₃ R2. In summary, our study identifies several important astrocytic ion channels (AQP4, TREK-1, BEST1, TRPA1) as the key molecules leading to the neuronal activity-dependent volume transient in astrocytes. Our findings reveal new molecular and cellular mechanisms for the synaptic coupling of intense neuronal activity with a physical change in astrocytes via volume transients.

Thrombin action on astrocytes in the hindbrain of the rat disrupts glycemic and respiratory control

Severe trauma can produce a postinjury "metabolic self-destruction" characterized by catabolic metabolism and hyperglycemia. The severity of the hyperglycemia is highly correlated with posttrauma morbidity and mortality. Although no mechanism has been posited to connect severe trauma with a loss of autonomic control over metabolism, traumatic injury causes other failures of autonomic function, notably, gastric stasis and ulceration ("Cushing's ulcer"), which has been connected with the generation of thrombin. Our previous studies established that proteinase-activated receptors (PAR1; "thrombin receptors") located on astrocytes in the autonomically critical nucleus of the solitary tract (NST) can modulate gastric control circuit neurons to cause gastric stasis. Hindbrain astrocytes have also been implicated as important detectors of low glucose or glucose utilization. When activated, these astrocytes communicate with hindbrain catecholamine neurons that, in turn, trigger counterregulatory responses (CRR). There may be a convergence between the effects of thrombin to derange hindbrain gastrointestinal control and the hindbrain circuitry that initiates CRR to increase glycemia in reaction to critical hypoglycemia. Our results suggest that thrombin acts within the NST to increase glycemia through an astrocyte-dependent mechanism. Blockade of purinergic gliotransmission pathways interrupted the effect of thrombin to increase glycemia. Our studies also revealed that thrombin, acting in the NST, produced a rapid, dramatic, and potentially lethal suppression of respiratory rhythm that was also a function of purinergic gliotransmission. These results suggest that the critical connection between traumatic injury and a general collapse of autonomic regulation involves thrombin action on astrocytes.

Synaptic Potentiation at Basal and Apical Dendrites of Hippocampal Pyramidal Neurons Involves Activation of a Distinct Set of Extracellular and Intracellular Molecular Cues

In the central nervous system, several forms of experience-dependent plasticity, learning and memory require the activity-dependent control of synaptic efficacy. Despite substantial progress in describing synaptic plasticity, mechanisms related to heterogeneity of synaptic functions at local circuits remain elusive. Here we studied the functional and molecular aspects of hippocampal circuit plasticity by analyzing excitatory synapses at basal and apical dendrites of mouse hippocampal pyramidal cells (CA1 region) in acute brain slices. In the past decade, activity of metalloproteinases (MMPs) has been implicated as a widespread and critical factor in plasticity mechanisms at various projections in the CNS. However, in the present study we discovered that in striking contrast to apical dendrites, synapses located within basal dendrites undergo MMP-independent synaptic potentiation. We demonstrate that synapse-specific molecular pathway allowing MMPs to rapidly upregulate function of NMDARs in stratum radiatum involved protease activated receptor 1 and intracellular kinases and GTPases activity. In contrast, MMP-independent scaling of synaptic strength in stratum oriens involved dopamine D1/D5 receptors and Src kinases. Results of this study reveal that 2 neighboring synaptic systems differ significantly in extracellular and intracellular cascades that control synaptic gain and provide long-searched transduction pathways relevant for MMP-dependent synaptic plasticity.

Astrocytic AEG-1 regulates expression of TREK-1 under acute hypoxia

TREK-1 (TWIK-related K+ channel), a member of the two-pore domain K+ (K2P) channel family, is highly expressed in astrocytes, where it plays a key role in glutamate release and passive conductance. In addition, TREK-1 is induced to protect neurons under pathological conditions such as hypoxia. However, the upstream regulation of TREK-1 remains poorly understood. In this study, we found that AEG-1 (astrocyte elevated gene-1) regulates the expression of astrocytic TREK-1 under hypoxic conditions. Upregulation of AEG-1 increased expression of TREK-1 in astrocytes, and knockdown of AEG-1 dramatically decreased the mRNA and protein levels of TREK-1, which were restored by expression of shRNA-insensitive AEG-1. In addition, expression of TREK-1 was not regulated in the absence of AEG-1, even when HIF1α was present. Together, these results suggest that AEG-1 acts as a major upstream regulator of TREK-1 channels in astrocytes under hypoxia. SIGNIFICANCE OF THE STUDY: Previous studies have reported that hypoxia increases the expression of astrocytic TREK-1 and that increased TREK-1 expression protects neuronal cells from apoptosis. However, its cellular mechanism is not clear. In this study we first showed that AEG-1 is a major mediator of hypoxic-regulated TREK-1 expression in normal astrocytes independently of HIF-1α.

Increased matrix metalloproteinase levels and perineuronal net proteolysis in the HIV-infected brain; relevance to altered neuronal population dynamics

HIV-associated neurocognitive disorders (HAND) continue to persist despite effective control of viral replication. Although the mechanisms underlying HAND are poorly understood, recent attention has focused on altered neuronal population activity as a correlate of impaired cognition. However, while alterations in neuronal population activity in the gamma frequency range are noted in the setting of HAND, the underlying mechanisms for these changes is unclear. Perineuronal nets (PNNs) are a specialized extracellular matrix that surrounds a subset of inhibitory neurons important to the expression of neuronal oscillatory activity. In the present study, we observe that levels of PNN-degrading matrix metalloproteinases (MMPs) are elevated in HIV-infected post-mortem human brain tissue. Furthermore, analysis of two PNN components, aggrecan and brevican, reveals increased proteolysis in HIV-infected brains. In addition, local field potential recordings from ex vivo mouse hippocampal slices demonstrate that the power of carbachol-induced gamma activity is increased following PNN degradation. Together, these results provide a possible mechanism whereby increased MMP proteolysis of PNNs may stimulate altered neuronal oscillatory activity and contribute to HAND symptoms.

Ultrasonic Neuromodulation via Astrocytic TRPA1

Low-intensity, low-frequency ultrasound (LILFU) is the next-generation, non-invasive brain stimulation technology for treating various neurological and psychiatric disorders. However, the underlying cellular and molecular mechanism of LILFU-induced neuromodulation has remained unknown. Here, we report that LILFU-induced neuromodulation is initiated by opening of TRPA1 channels in astrocytes. The Ca²⁺ entry through TRPA1 causes a release of gliotransmitters including glutamate through Best1 channels in astrocytes. The released glutamate activates NMDA receptors in neighboring neurons to elicit action potential firing. Our results reveal an unprecedented mechanism of LILFU-induced neuromodulation, involving TRPA1 as a unique sensor for LILFU and glutamate-releasing Best1 as a mediator of glia-neuron interaction. These discoveries should prove to be useful for optimization of human brain stimulation and ultrasonogenetic manipulations of TRPA1.

CRAC channels regulate astrocyte Ca2+ signaling and gliotransmitter release to modulate hippocampal GABAergic transmission

Astrocytes are the major glial subtype in the brain and mediate numerous functions ranging from metabolic support to gliotransmitter release through signaling mechanisms controlled by Ca²⁺ Despite intense interest, the Ca²⁺ influx pathways in astrocytes remain obscure, hindering mechanistic insights into how Ca²⁺ signaling is coupled to downstream astrocyte-mediated effector functions. Here, we identified store-operated Ca²⁺ release-activated Ca²⁺ (CRAC) channels encoded by Orai1 and STIM1 as a major route of Ca²⁺ entry for driving sustained and oscillatory Ca²⁺ signals in astrocytes after stimulation of metabotropic purinergic and protease-activated receptors. Using synaptopHluorin as an optical reporter, we showed that the opening of astrocyte CRAC channels stimulated vesicular exocytosis to mediate the release of gliotransmitters, including ATP. Furthermore, slice electrophysiological recordings showed that activation of astrocytes by protease-activated receptors stimulated interneurons in the CA1 hippocampus to increase inhibitory postsynaptic currents on CA1 pyramidal cells. These results reveal a central role for CRAC channels as regulators of astrocyte Ca²⁺ signaling, gliotransmitter release, and astrocyte-mediated tonic inhibition of CA1 pyramidal neurons.

Glutamate-Releasing SWELL1 Channel in Astrocytes Modulates Synaptic Transmission and Promotes Brain Damage in Stroke

By releasing glutamate, astrocytes actively regulate synaptic transmission and contribute to excitotoxicity in neurological diseases. However, the mechanisms of astrocytic glutamate release have been debated. Here, we report non-vesicular release of glutamate through the glutamate-permeable volume-regulated anion channel (VRAC). Both cell swelling and receptor stimulation activated astrocytic VRAC, which requires its only obligatory subunit, Swell1. Astrocyte-specific Swell1 knockout mice exhibited impaired glutamatergic transmission due to the decreases in presynaptic release probability and ambient glutamate level. Consistently, the mutant mice displayed hippocampal-dependent learning and memory deficits. During pathological cell swelling, deletion of astrocytic Swell1 attenuated glutamate-dependent neuronal excitability and protected mice from brain damage after ischemic stroke. Our identification of a new molecular mechanism for channel-mediated glutamate release establishes a role for astrocyte-neuron interactions in both synaptic transmission and brain ischemia. It provides a rationale for targeting VRAC for the treatment of stroke and other neurological diseases associated with excitotoxicity.

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.

Activation of Astrocytic μ-opioid Receptor Elicits Fast Glutamate Release Through TREK-1-Containing K2P Channel in Hippocampal Astrocytes

Recently, μ-opioid receptor (MOR), one of the well-known Gi-protein coupled receptors (Gi-GPCR), was reported to be highly expressed in the hippocampal astrocytes. However, the role of astrocytic MOR has not been investigated. Here we report that activation of astrocytic MOR by [D-Ala²,N-MePhe⁴,Gly-ol]-enkephalin (DAMGO), a selective MOR agonist, causes a fast glutamate release using sniffer patch technique. We also found that the DAMGO-induced glutamate release was not observed in the astrocytes from MOR-deficient mice and MOR-short hairpin RNA (shRNA)-expressed astrocytes. In addition, the glutamate release was significantly reduced by gene silencing of the TREK-1-containing two-pore potassium (K2P) channel, which mediates passive conductance in astrocytes. Our findings were consistent with the previous study demonstrating that activation of Gi-GPCR such as cannabinoid receptor CB1 and adenosine receptor A1 causes a glutamate release through TREK-1-containing K2P channel from hippocampal astrocytes. We also demonstrated that MOR and TREK-1 are significantly co-localized in the hippocampal astrocytes. Furthermore, we found that both MOR and TREK-1-containing K2P channels are localized in the same subcellular compartments, soma and processes, of astrocytes. Our study raises a novel possibility that astrocytic MOR may participate in several physiological and pathological actions of opioids, including analgesia and addiction, through astrocytically released glutamate and its signaling pathway.

Triheteromeric GluN1/GluN2A/GluN2C NMDARs with Unique Single-Channel Properties Are the Dominant Receptor Population in Cerebellar Granule Cells

NMDA-type glutamate receptors (NMDARs) are ligand-gated ion channels that mediate excitatory neurotransmission in the CNS. Here we describe functional and single-channel properties of triheteromeric GluN1/GluN2A/GluN2C receptors, which contain two GluN1, one GluN2A, and one GluN2C subunits. This NMDAR has three conductance levels and opens in bursts similar to GluN1/GluN2A receptors but with a single-channel open time and open probability reminiscent of GluN1/GluN2C receptors. The deactivation time course of GluN1/GluN2A/GluN2C receptors is intermediate to GluN1/GluN2A and GluN1/GluN2C receptors and is not dominated by GluN2A or GluN2C. We show that triheteromeric GluN1/GluN2A/GluN2C receptors are the predominant NMDARs in cerebellar granule cells and propose that co-expression of GluN2A and GluN2C in cerebellar granule cells occludes cell surface expression of diheteromeric GluN1/GluN2C receptors. This new insight into neuronal GluN1/GluN2A/GluN2C receptors highlights the complexity of NMDAR signaling in the CNS.

Modulation of spinal motor networks by astrocyte-derived adenosine is dependent on D1-like dopamine receptor signaling

Astrocytes modulate many neuronal networks, including spinal networks responsible for the generation of locomotor behavior. Astrocytic modulation of spinal motor circuits involves release of ATP from astrocytes, hydrolysis of ATP to adenosine, and subsequent activation of neuronal A₁ adenosine receptors (A₁Rs). The net effect of this pathway is a reduction in the frequency of locomotor-related activity. Recently, it was proposed that A₁Rs modulate burst frequency by blocking the D₁-like dopamine receptor (D₁LR) signaling pathway; however, adenosine also modulates ventral horn circuits by dopamine-independent pathways. Here, we demonstrate that adenosine produced upon astrocytic stimulation modulates locomotor-related activity by counteracting the excitatory effects of D₁LR signaling and does not act by previously described dopamine-independent pathways. In spinal cord preparations from postnatal mice, a D₁LR agonist, SKF 38393, increased the frequency of locomotor-related bursting induced by 5-hydroxytryptamine and N-methyl-d-aspartate. Bath-applied adenosine reduced burst frequency only in the presence of SKF 38393, as did adenosine produced after activation of protease-activated receptor-1 to stimulate astrocytes. Furthermore, the A₁R antagonist 8-cyclopentyl-1,3-dipropylxanthine enhanced burst frequency only in the presence of SKF 38393, indicating that endogenous adenosine produced by astrocytes during network activity also acts by modulating D₁LR signaling. Finally, modulation of bursting by adenosine released upon stimulation of astrocytes was blocked by protein kinase inhibitor-(14-22) amide, a protein kinase A (PKA) inhibitor, consistent with A₁R-mediated antagonism of the D₁LR/adenylyl cyclase/PKA pathway. Together, these findings support a novel, astrocytic mechanism of metamodulation within the mammalian spinal cord, highlighting the complexity of the molecular interactions that specify motor output. NEW & NOTEWORTHY Astrocytes within the spinal cord produce adenosine during ongoing locomotor-related activity or when experimentally stimulated. Here, we show that adenosine derived from astrocytes acts at A₁ receptors to inhibit a pathway by which D₁-like receptors enhance the frequency of locomotor-related bursting. These data support a novel form of metamodulation within the mammalian spinal cord, enhancing our understanding of neuron-astrocyte interactions and their importance in shaping network activity.

Acute high-fat diet upregulates glutamatergic signaling in the dorsal motor nucleus of the vagus

Obesity is associated with dysregulation of vagal neurocircuits controlling gastric functions, including food intake and energy balance. In the short term, however, caloric intake is regulated homeostatically although the precise mechanisms responsible are unknown. The present study examined the effects of acute high-fat diet (HFD) on glutamatergic neurotransmission within central vagal neurocircuits and its effects on gastric motility. Sprague-Dawley rats were fed a control or HFD diet (14% or 60% kcal from fat, respectively) for 3-5 days. Whole cell patch-clamp recordings and brainstem application of antagonists were used to assess the effects of acute HFD on glutamatergic transmission to dorsal motor nucleus of the vagus (DMV) neurons and subsequent alterations in gastric tone and motility. After becoming hyperphagic initially, caloric balance was restored after 3 days following HFD exposure. In control rats, the non- N-methyl-d-aspartate (NMDA) receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX), but not the NMDA receptor antagonist, amino-5-phosphonopentanoate (AP5), significantly decreased excitatory synaptic currents and action potential firing rate in gastric-projecting DMV neurons. In contrast, both AP5 and DNQX decreased excitatory synaptic transmission and action potential firing in acute HFD neurons. When microinjected into the brainstem, AP5, but not DNQX, decreased gastric motility and tone in acute HFD rats only. These results suggest that acute HFD upregulates NMDA receptor-mediated currents, increasing DMV neuronal excitability and activating the vagal efferent cholinergic pathway, thus increasing gastric tone and motility. Although such neuroplasticity may be a persistent adaptation to the initial exposure to HFD, it may also be an important mechanism in homeostatic regulation of energy balance. NEW & NOTEWORTHY Vagal neurocircuits are critical to the regulation of gastric functions, including satiation and food intake. Acute high-fat diet upregulates glutamatergic signaling within central vagal neurocircuits via activation of N-methyl-d-aspartate receptors, increasing vagal efferent drive to the stomach. Although it is possible that such neuroplasticity is a persistent adaptation to initial exposure to the high-fat diet, it may also play a role in the homeostatic control of feeding.

In the Telencephalon, GluN2C NMDA Receptor Subunit mRNA is Predominately Expressed in Glial Cells and GluN2D mRNA in Interneurons

N-methyl-D-aspartate receptors (NMDARs) are widely distributed in the brain with high concentrations in the telencephalon where they modulate synaptic plasticity, working memory, and other functions. While the actions of the predominate GluN2 NMDAR subunits, GluN2A and GluN2B are relatively well understood, the function of GluN2C and GluN2D subunits in the telencephalon is largely unknown. To better understand the possible role of GluN2C subunits, we used fluorescence in situ hybridization (FISH) together with multiple cell markers to define the distribution and type of cells expressing GluN2C mRNA. Using a GluN2C-KO mouse as a negative control, GluN2C mRNA expression was only found in non-neuronal cells (NeuN-negative cells) in the hippocampus, striatum, amygdala, and cerebral cortex. For these regions, a significant fraction of GFAP-positive cells also expressed GluN2C mRNA. Overall, for the telencephalon, the globus pallidus and olfactory bulb were the only regions where GluN2C was expressed in neurons. In contrast to GluN2C, GluN2D subunit mRNA colocalized with neuronal and not astrocyte markers or GluN2C mRNA in the telencephalon (except for the globus pallidus). GluN2C mRNA did, however, colocalize with GluN2D in the thalamus where neuronal GluN2C expression is found. These findings strongly suggest that GluN2C has a very distinct function in the telencephalon compared to its role in other brain regions and compared to other GluN2-containing NMDARs. NMDARs containing GluN2C may have a specific role in regulating L-glutamate or D-serine release from astrocytes in response to L-glutamate spillover from synaptic activity.