Do astrocytes really exocytose neurotransmitters?

Emerging Role of Neuron-Glia in Neurological Disorders: At a Glance

Based on the diverse physiological influence, the impact of glial cells has become much more evident on neurological illnesses, resulting in the origins of many diseases appearing to be more convoluted than previously happened. Since neurological disorders are often random and unknown, hence the construction of animal models is difficult to build, representing a small fraction of people with a gene mutation. As a result, an immediate necessity is grown to work within in vitro techniques for examining these illnesses. As the scientific community recognizes cell-autonomous contributions to a variety of central nervous system illnesses, therapeutic techniques involving stem cells for treating neurological diseases are gaining traction. The use of stem cells derived from a variety of sources is increasingly being used to replace both neuronal and glial tissue. The brain's energy demands necessitate the reliance of neurons on glial cells in order for it to function properly. Furthermore, glial cells have diverse functions in terms of regulating their own metabolic activities, as well as collaborating with neurons via secreted signaling or guidance molecules, forming a complex network of neuron-glial connections in health and sickness. Emerging data reveals that metabolic changes in glial cells can cause morphological and functional changes in conjunction with neuronal dysfunction under disease situations, highlighting the importance of neuron-glia interactions in the pathophysiology of neurological illnesses. In this context, it is required to improve our understanding of disease mechanisms and create potential novel therapeutics. According to research, synaptic malfunction is one of the features of various mental diseases, and glial cells are acting as key ingredients not only in synapse formation, growth, and plasticity but also in neuroinflammation and synaptic homeostasis which creates critical physiological capacity in the focused sensory system. The goal of this review article is to elaborate state-of-the-art information on a few glial cell types situated in the central nervous system (CNS) and highlight their role in the onset and progression of neurological disorders.

Excitatory Synaptic Transmission in Ischemic Stroke: A New Outlet for Classical Neuroprotective Strategies

Stroke is one of the leading causes of death and disability in the world, of which ischemia accounts for the majority. There is growing evidence of changes in synaptic connections and neural network functions in the brain of stroke patients. Currently, the studies on these neurobiological alterations mainly focus on the principle of glutamate excitotoxicity, and the corresponding neuroprotective strategies are limited to blocking the overactivation of ionic glutamate receptors. Nevertheless, it is disappointing that these treatments often fail because of the unspecificity and serious side effects of the tested drugs in clinical trials. Thus, in the prevention and treatment of stroke, finding and developing new targets of neuroprotective intervention is still the focus and goal of research in this field. In this review, we focus on the whole processes of glutamatergic synaptic transmission and highlight the pathological changes underlying each link to help develop potential therapeutic strategies for ischemic brain damage. These strategies include: (1) controlling the synaptic or extra-synaptic release of glutamate, (2) selectively blocking the action of the glutamate receptor NMDAR subunit, (3) increasing glutamate metabolism, and reuptake in the brain and blood, and (4) regulating the glutamate system by GABA receptors and the microbiota–gut–brain axis. Based on these latest findings, it is expected to promote a substantial understanding of the complex glutamate signal transduction mechanism, thereby providing excellent neuroprotection research direction for human ischemic stroke (IS).

Glucocorticoid Receptor-Dependent Astrocytes Mediate Stress Vulnerability

BACKGROUND

Major depressive disorder is a devastating psychiatric illness that affects approximately 17% of the population worldwide. Astrocyte dysfunction has been implicated in its pathophysiology. Traumatic experiences and stress contribute to the onset of major depressive disorder, but how astrocytes respond to stress is poorly understood.

METHODS

Using Western blotting analysis, we identified that stress vulnerability was associated with reduced astrocytic glucocorticoid receptor (GR) expression in mouse models of depression. We further investigated the functions of astrocytic GRs in regulating depression and the underlying mechanisms by using a combination of behavioral studies, fiber photometry, biochemical experiments, and RNA sequencing methods.

RESULTS

GRs in astrocytes were more sensitive to stress than those in neurons. GR absence in astrocytes induced depressive-like behaviors, whereas restoring astrocytic GR expression in the medial prefrontal cortex prevented the depressive-like phenotype. Furthermore, we found that GRs in the medial prefrontal cortex affected astrocytic Ca²⁺ activity and dynamic ATP (adenosine 5'-triphosphate) release in response to stress. RNA sequencing of astrocytes isolated from GR deletion mice identified the PI3K-Akt (phosphoinositide 3-kinase-Akt) signaling pathway, which was required for astrocytic GR-mediated ATP release.

CONCLUSIONS

These findings reveal that astrocytic GRs play an important role in stress response and that reduced astrocytic GR expression in the stressed subject decreases ATP release to mediate stress vulnerability.

The Circadian Molecular Machinery in CNS Cells: A Fine Tuner of Neuronal and Glial Activity With Space/Time Resolution

The circadian molecular machinery is a fine timekeeper with the capacity to harmonize physiological and behavioral processes with the external environment. This tight-knit regulation is coordinated by multiple cellular clocks across the body. In this review, we focus our attention on the molecular mechanisms regulated by the clock in different brain areas and within different cells of the central nervous system. Further, we discuss evidence regarding the role of circadian rhythms in the regulation of neuronal activity and neurotransmitter systems. Not only neurons, but also astrocytes and microglia actively participate in the maintenance of timekeeping within the brain, and the diffusion of circadian information among these cells is fine-tuned by neurotransmitters (e.g., dopamine, serotonin, and γ-aminobutyric acid), thus impacting on the core clock machinery. The bidirectional interplay between neurotransmitters and the circadian clockwork is fundamental in maintaining accuracy and precision in daily timekeeping throughout different brain areas. Deepening the knowledge of these correlations allows us to define the basis of drug interventions to restore circadian rhythms, as well as to predict the onset of drug treatment/side effects that might promote daily desynchronization. Furthermore, it may lead to a deeper understanding of the potential impacts of modulations in rhythmic activities on the pace of aging and provide an insight in to the pathogenesis of psychiatric diseases and neurodegenerative disorders.

CXCR4/CX43 Regulate Diabetic Neuropathic Pain via Intercellular Interactions between Activated Neurons and Dysfunctional Astrocytes during Late Phase of Diabetes in Rats and the Effects of Antioxidant N-Acetyl-L-Cysteine

Growing evidence suggests that the interactions between astrocytes and neurons exert important functions in the central sensitization of the spinal cord dorsal horn in rodents with diabetes and neuropathic pain (DNP). However, it still remains unclear how signal transmission occurs in the spinal cord dorsal horn between astrocytes and neurons, especially in subjects with DNP. Chemokine CXC receptor 4 (CXCR4) plays critical roles in DNP, and connexin 43 (CX43), which is also primarily expressed by astrocytes, contributes to the development of neuropathy. We thus postulated that astrocytic and neuronal CXCR4 induces and produces inflammatory factors under persistent peripheral noxious stimulation in DNP, while intercellular CX43 can transmit inflammatory stimulation signals. The results showed that streptozotocin-induced type 1 diabetic rats developed heat hyperalgesia and mechanical allodynia. Diabetes led to persistent neuropathic pain. Diabetic rats developed peripheral sensitization at the early phase (2 weeks) and central sensitization at the late phase (5 weeks) after diabetes induction. Both CXCR4 and CX43, which are localized and coexpressed in neurons and astrocytes, were enhanced significantly in the dorsal horn of spinal cord in rats undergoing DNP during late phase of diabetes, and the CXCR4 antagonist AMD3100 reduced the expression of CX43. The nociceptive behavior was reversed, respectively, by AMD3100 at the early phase and by the antioxidant N-acetyl-L-cysteine (NAC) at the late phase. Furthermore, rats with DNP demonstrated downregulation of glial fibrillary acidic protein (GFAP) as well as upregulation of c-fos in the spinal cord dorsal horn at the late phase compared to the controls, and upregulation of GFAP and downregulation of c-fos were observed upon treatment with NAC. Given that GFAP and c-fos are, respectively, makers of astrocyte and neuronal activation, our findings suggest that CXCR4 as an inflammatory stimulation protein and CX43 as an intercellular signal transmission protein both may induce neurons excitability and astrocytes dysfunction in developing DNP.

Cell-Derived Exosomes as Therapeutic Strategies and Exosome-Derived microRNAs as Biomarkers for Traumatic Brain Injury

Traumatic brain injury (TBI) is a complex, life-threatening condition that causes mortality and disability worldwide. No effective treatment has been clinically verified to date. Achieving effective drug delivery across the blood–brain barrier (BBB) presents a major challenge to therapeutic drug development for TBI. Furthermore, the field of TBI biomarkers is rapidly developing to cope with the many aspects of TBI pathology and enhance clinical management of TBI. Exosomes (Exos) are endogenous extracellular vesicles (EVs) containing various biological materials, including lipids, proteins, microRNAs, and other nucleic acids. Compelling evidence exists that Exos, such as stem cell-derived Exos and even neuron or glial cell-derived Exos, are promising TBI treatment strategies because they pass through the BBB and have the potential to deliver molecules to target lesions. Meanwhile, Exos have decreased safety risks from intravenous injection or orthotopic transplantation of viable cells, such as microvascular occlusion or imbalanced growth of transplanted cells. These unique characteristics also create Exos contents, especially Exos-derived microRNAs, as appealing biomarkers in TBI. In this review, we explore the potential impact of cell-derived Exos and exosome-derived microRNAs on the diagnosis, therapy, and prognosis prediction of TBI. The associated challenges and opportunities are also discussed.

Current Understanding of Long-Term Cognitive Impairment After Sepsis

Sepsis is recognized as a life-threatening multi-organ dysfunction resulting from a dysregulated host response to infection. Although the incidence and mortality of sepsis decrease significantly due to timely implementation of anti-infective and support therapies, accumulating evidence suggests that a great proportion of survivors suffer from long-term cognitive impairment after hospital discharge, leading to decreased life quality and substantial caregiving burdens for family members. Several mechanisms have been proposed for long-term cognitive impairment after sepsis, which are not mutually exclusive, including blood-brain barrier disruption, neuroinflammation, neurotransmitter dysfunction, and neuronal loss. Targeting these critical processes might be effective in preventing and treating long-term cognitive impairment. However, future in-depth studies are required to facilitate preventive and/or treatment strategies for long-term cognitive impairment after sepsis.

Relationship between Neuronal Damage/Death and Astrogliosis in the Cerebral Motor Cortex of Gerbil Models of Mild and Severe Ischemia and Reperfusion Injury

Neuronal loss (death) occurs selectively in vulnerable brain regions after ischemic insults. Astrogliosis is accompanied by neuronal death. It can change the molecular expression and morphology of astrocytes following ischemic insults. However, little is known about cerebral ischemia and reperfusion injury that can variously lead to damage of astrocytes according to the degree of ischemic injury, which is related to neuronal damage/death. Thus, the purpose of this study was to examine the relationship between damage to cortical neurons and astrocytes using gerbil models of mild and severe transient forebrain ischemia induced by blocking the blood supply to the forebrain for five or 15 min. Significant ischemia tFI-induced neuronal death occurred in the deep layers (layers V and VI) of the motor cortex: neuronal death occurred earlier and more severely in gerbils with severe ischemia than in gerbils with mild ischemia. Distinct astrogliosis was detected in layers V and VI. It gradually increased with time after both ischemiae. The astrogliosis was significantly higher in severe ischemia than in mild ischemia. The ischemia-induced increase of glial fibrillary acidic protein (GFAP; a maker of astrocyte) expression in severe ischemia was significantly higher than that in mild ischemia. However, GFAP-immunoreactive astrocytes were apparently damaged two days after both ischemiae. At five days after ischemiae, astrocyte endfeet around capillary endothelial cells were severely ruptured. They were more severely ruptured by severe ischemia than by mild ischemia. However, the number of astrocytes stained with S100 was significantly higher in severe ischemia than in mild ischemia. These results indicate that the degree of astrogliosis, including the disruption (loss) of astrocyte endfeet following ischemia and reperfusion in the forebrain, might depend on the severity of ischemia and that the degree of ischemia-induced neuronal damage may be associated with the degree of astrogliosis.

Astrocytes derived from ASD individuals alter behavior and destabilize neuronal activity through aberrant Ca2+ signaling

The cellular mechanisms of autism spectrum disorder (ASD) are poorly understood. Cumulative evidence suggests that abnormal synapse function underlies many features of this disease. Astrocytes regulate several key neuronal processes, including the formation of synapses and the modulation of synaptic plasticity. Astrocyte abnormalities have also been identified in the postmortem brain tissue of ASD individuals. However, it remains unclear whether astrocyte pathology plays a mechanistic role in ASD, as opposed to a compensatory response. To address this, we combined stem cell culturing with transplantation techniques to determine disease-specific properties inherent to ASD astrocytes. We demonstrate that ASD astrocytes induce repetitive behavior as well as impair memory and long-term potentiation when transplanted into the healthy mouse brain. These in vivo phenotypes were accompanied by reduced neuronal network activity and spine density caused by ASD astrocytes in hippocampal neurons in vitro. Transplanted ASD astrocytes also exhibit exaggerated Ca²⁺ fluctuations in chimeric brains. Genetic modulation of evoked Ca²⁺ responses in ASD astrocytes modulates behavior and neuronal activity deficits. Thus, this study determines that astrocytes derived from ASD iPSCs are sufficient to induce repetitive behavior as well as cognitive deficit, suggesting a previously unrecognized primary role for astrocytes in ASD.

Central nervous system regeneration: the roles of glial cells in the potential molecular mechanism underlying remyelination

Glial cells play crucial roles in brain homeostasis and pathogenesis of central nervous system (CNS) injuries and diseases. However, the roles of these cells and the molecular mechanisms toward regeneration in the CNS have not been fully understood, especially the capacity of them toward demyelinating diseases. Therefore, there are still very limited therapeutic strategies to restore the function of adult CNS in diseases such as multiple sclerosis (MS). Remyelination, a spontaneous regeneration process in the CNS, requires the involvement of multiple cellular and extracellular components. Promoting remyelination by therapeutic interventions is a promising novel approach to restore the CNS function. Herein, we review the role of glial cells in CNS diseases and injuries. Particularly, we discuss the roles of glia and their functional interactions and regulatory mechanisms in remyelination, as well as the current therapeutic strategies for MS.

Optimizing charge-balanced pulse stimulation for desynchronization

Collective synchronization in a large population of self-sustained units appears both in natural and engineered systems. Sometimes this effect is in demand, while in some cases, it is undesirable, which calls for control techniques. In this paper, we focus on pulsatile control, with the goal to either increase or decrease the level of synchrony. We quantify this level by the entropy of the phase distribution. Motivated by possible applications in neuroscience, we consider pulses of a realistic shape. Exploiting the noisy Kuramoto-Winfree model, we search for the optimal pulse profile and the optimal stimulation phase. For this purpose, we derive an expression for the change of the phase distribution entropy due to the stimulus. We relate this change to the properties of individual units characterized by generally different natural frequencies and phase response curves and the population's state. We verify the general result by analyzing a two-frequency population model and demonstrating a good agreement of the theory and numerical simulations.

Astrocyte control of the entorhinal cortex-dentate gyrus circuit: Relevance to cognitive processing and impairment in pathology

The entorhinal cortex-dentate gyrus circuit is centrally involved in memory processing conveying to the hippocampus spatial and nonspatial context information via, respectively, medial and lateral perforant path (MPP and LPP) excitatory projections onto dentate granule cells (GCs). Here, we review work of several years from our group showing that astrocytes sense local synaptic transmission and exert in turn a presynaptic control at PP-GC synapses. Modulation of neurotransmitter release probability by astrocytes sets basal synaptic strength and dynamic range for long-term potentiation of PP-GC synapses. Intriguingly, this astrocyte control is circuit-specific, being present only at MPP-GC (not LPP-GC) synapses, which selectively express atypical presynaptic N-methyl-D-aspartate receptors (NMDAR) suitable to activation by astrocyte-released glutamate. Moreover, the astrocytic control is peculiarly dependent on the cytokine TNFα, which at constitutive levels acts as a gating factor for the astrocyte signaling. During inflammation/infection processes, increased levels of TNFα lead to uncontrolled astrocyte glutamate release, altered PP-GC circuit processing and, ultimately, impaired contextual memory performance. The TNFα-dependent pathological switch of the synaptic control from astrocytes and its deleterious consequences are observed in animal models of HIV brain infection and multiple sclerosis, conditions both known to cause cognitive disturbances in up to 50% of patients. The review also discusses open issues related to the identified astrocytic pathway: its role in contextual memory processing, potential damaging role in Alzheimer's disease, the existence of vesicular glutamate release from DG astrocytes, and the possible synaptic-like connectivity between astrocytic output sites and PP receptive sites.

Astrocytes contribute to pain gating in the spinal cord

Various pain therapies have been developed on the basis of the gate control theory of pain, which postulates that nonpainful sensory inputs mediated by large-diameter afferent fibers (Aβ-fibers) can attenuate noxious signals relayed to the brain. To date, this theory has focused only on neuronal mechanisms. Here, we identified an unprecedented function of astrocytes in the gating of nociceptive signals transmitted by neurokinin 1 receptor–positive (NK1R+) projection neurons in the spinal cord. Electrical stimulation of peripheral Aβ-fibers in naïve mice activated spinal astrocytes, which in turn induced long-term depression (LTD) in NK1R+ neurons and antinociception through activation of endogenous adenosinergic mechanisms. Suppression of astrocyte activation by pharmacologic, chemogenetic, and optogenetic manipulations blocked the induction of LTD in NK1R+ neurons and pain inhibition by Aβ-fiber stimulation. Collectively, our study introduces astrocytes as an important component of pain gating by activation of Aβ-fibers, which thus exert nonneuronal control of pain.

The tripartite glutamatergic synapse

Astroglial cells were long considered as structural and metabolic supporting cells are which do not directly participate in information processing in the brain. Discoveries of responsiveness of astrocytes to synaptically-released glutamate and their capability to release agonists of glutamate receptors awakened extensive studies of glia-neuron communications and led to the revolutionary changes in our understanding of brain cellular networks. Nowadays, astrocytes are widely acknowledged as inseparable element of glutamatergic synapses and role for glutamatergic astrocyte-neuron interactions in the brain computation is emerging. Astroglial glutamate receptors, in particular of NMDA, mGluR3 and mGluR5 types, can activate a variety of molecular cascades leading astroglial-driven modulation of extracellular levels of glutamate and activity of neuronal glutamate receptors. Their preferential location to the astroglial perisynaptic processes facilitates interaction of astrocytes with individual excitatory synapses. Bi-directional glutamatergic communication between astrocytes and neurons underpins a complex, spatially-distributed modulation of synaptic signalling thus contributing to the enrichment of information processing by the neuronal networks. Still, further research is needed to bridge the substantial gaps in our understanding of mechanisms and physiological relevance of astrocyte-neuron glutamatergic interactions, in particular ability of astrocytes directly activate neuronal glutamate receptors by releasing glutamate and, arguably, d-Serine. An emerging roles for aberrant changes in glutamatergic astroglial signalling, both neuroprotective and pathogenic, in neurological and neurodegenerative diseases also require further investigation. This article is part of the special Issue on 'Glutamate Receptors - The Glutamatergic Synapse'.

Digital Reconstruction of the Neuro-Glia-Vascular Architecture

Astrocytes connect the vasculature to neurons mediating the supply of nutrients and biochemicals. They are involved in a growing number of physiological and pathophysiological processes that result from biophysical, physiological, and molecular interactions in this neuro-glia-vascular ensemble (NGV). The lack of a detailed cytoarchitecture severely restricts the understanding of how they support brain function. To address this problem, we used data from multiple sources to create a data-driven digital reconstruction of the NGV at micrometer anatomical resolution. We reconstructed 0.2 mm3 of the rat somatosensory cortex with 16 000 morphologically detailed neurons, 2500 protoplasmic astrocytes, and its microvasculature. The consistency of the reconstruction with a wide array of experimental measurements allows novel predictions of the NGV organization, allowing the anatomical reconstruction of overlapping astrocytic microdomains and the quantification of endfeet connecting each astrocyte to the vasculature, as well as the extent to which they cover the latter. Structural analysis showed that astrocytes optimize their positions to provide uniform vascular coverage for trophic support and signaling. However, this optimal organization rapidly declines as their density increases. The NGV digital reconstruction is a resource that will enable a better understanding of the anatomical principles and geometric constraints, which govern how astrocytes support brain function.

Astrocyte Ca2+-evoked ATP release regulates myelinated axon excitability and conduction speed

In the brain’s gray matter, astrocytes regulate synapse properties, but their role is unclear for the white matter, where myelinated axons rapidly transmit information between gray matter areas. We found that in rodents, neuronal activity raised the intracellular calcium concentration ([Ca2+]i) in astrocyte processes located near action potential–generating sites in the axon initial segment (AIS) and nodes of Ranvier of myelinated axons. This released adenosine triphosphate, which was converted extracellularly to adenosine and thus, through A2a receptors, activated HCN2-containing cation channels that regulate two aspects of myelinated axon function: excitability of the AIS and speed of action potential propagation. Variations in astrocyte-derived adenosine level between wake and sleep states or during energy deprivation could thus control white matter information flow and neural circuit function.

The glutamatergic synapse: a complex machinery for information processing

Being the most abundant synaptic type, the glutamatergic synapse is responsible for the larger part of the brain's information processing. Despite the conceptual simplicity of the basic mechanism of synaptic transmission, the glutamatergic synapse shows a large variation in the response to the presynaptic release of the neurotransmitter. This variability is observed not only among different synapses but also in the same single synapse. The synaptic response variability is due to several mechanisms of control of the information transferred among the neurons and suggests that the glutamatergic synapse is not a simple bridge for the transfer of information but plays an important role in its elaboration and management. The control of the synaptic information is operated at pre, post, and extrasynaptic sites in a sort of cooperation between the pre and postsynaptic neurons which also involves the activity of other neurons. The interaction between the different mechanisms of control is extremely complicated and its complete functionality is far from being fully understood. The present review, although not exhaustively, is intended to outline the most important of these mechanisms and their complexity, the understanding of which will be among the most intriguing challenges of future neuroscience.

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.

Three's Company: Neuroimmune activation, sex, and memory at the tripartite synapse

The neuroimmune system is required for normal cognitive functions such as learning and memory in addition to its critical role in detecting and responding to invading pathogens and injury. Understanding the functional convergence of neurons, astrocytes, and microglia at the synapse, particularly in the hippocampus, is key to understanding the nuances of such diverse roles. In the healthy brain, communication between all three cells is important for regulating neuronal activation and synaptic plasticity mechanisms, and during neuroinflammation, the activity and functions of all three cells can produce and be modulated by inflammatory cytokines. An important remaining component to this system is the conclusive evidence of sex differences in hippocampal plasticity mechanisms, hormone modulation of synaptic plasticity, functional properties of hippocampal neurons, and in neuroimmune activation. Sex as a biological variable here is necessary to consider given sex differences in the prevalence of memory-related disorders such as Alzheimer's disease and Post-Traumatic Stress disorder, both of which present with neuroimmune dysregulation. To make meaningful progress towards a deeper understanding of sex biases in memory-related disease prevalence, I propose that the next chapter of psychoneuroimmune research must focus on the signal integration and transduction at the synapse between experience-dependent plasticity mechanisms, neuroimmune activation, and the influence of biological sex.

Glial PAMPering and DAMPening of Adult Hippocampal Neurogenesis

Adult neurogenesis represents a mature brain's capacity to integrate newly generated neurons into functional circuits. Impairment of neurogenesis contributes to the pathophysiology of various mood and cognitive disorders such as depression and Alzheimer's Disease. The hippocampal neurogenic niche hosts neural progenitors, glia, and vasculature, which all respond to intrinsic and environmental cues, helping determine their current state and ultimate fate. In this article we focus on the major immune communication pathways and mechanisms through which glial cells sense, interact with, and modulate the neurogenic niche. We pay particular attention to those related to the sensing of and response to innate immune danger signals. Receptors for danger signals were first discovered as a critical component of the innate immune system response to pathogens but are now also recognized to play a crucial role in modulating non-pathogenic sterile inflammation. In the neurogenic niche, viable, stressed, apoptotic, and dying cells can activate danger responses in neuroimmune cells, resulting in neuroprotection or neurotoxicity. Through these mechanisms glial cells can influence hippocampal stem cell fate, survival, neuronal maturation, and integration. Depending on the context, such responses may be appropriate and on-target, as in the case of learning-associated synaptic pruning, or excessive and off-target, as in neurodegenerative disorders.

Redefining varicose projection astrocytes in primates

Varicose projection astrocytes (VP-As) are found in the cerebral cortex and have been described to be specific to humans and chimpanzees. To further examine the phylogenetic distribution of this cell type, we analyzed cortical tissue from several primates ranging from primitive primates to primates evolutionary closer to human such as apes. We specifically analyzed tissue from four strepsirrhine species, one tarsier, six species of platyrrhine monkeys, ten species of cercopithecoid monkeys, two hylobatid ape species, four to six cases each of chimpanzee, bonobo, gorilla, and orangutan, and thirteen human. We found that VP-As were present only in human and other apes (hominoids) and were absent in all other species. We showed that VP-As are localized to layer VI and the superficial white matter of the cortex. The presence of VP-As co-occured with interlaminar astrocytes that also had varicosities in their processes. Due to their location, their long tangential processes, and their irregular presence within species, we propose that VP-As are astrocytes that develop varicosities under specific conditions and that are not a distinct astrocyte type.

Exocytosis in Astrocytes

Until recently, astrocytes were thought to be a part of a simple "brain glue" providing only a supporting role for neurons. However, the discoveries of the last two decades have proven astrocytes to be dynamic partners participating in brain metabolism and actively influencing communication between neurons. The means of astrocyte-neuron communication are diverse, although regulated exocytosis has received the most attention but also caused the most debate. Similar to most of eukaryotic cells, astrocytes have a complex range of vesicular organelles which can undergo exocytosis as well as intricate molecular mechanisms that regulate this process. In this review, we focus on the components needed for regulated exocytosis to occur and summarise the knowledge about experimental evidence showing its presence in astrocytes.

Contribution of Neuronal and Glial Two-Pore-Domain Potassium Channels in Health and Neurological Disorders

Two-pore-domain potassium (K2P) channels are widespread in the nervous system and play a critical role in maintaining membrane potential in neurons and glia. They have been implicated in many stress-relevant neurological disorders, including pain, sleep disorder, epilepsy, ischemia, and depression. K2P channels give rise to leaky K⁺ currents, which stabilize cellular membrane potential and regulate cellular excitability. A range of natural and chemical effectors, including temperature, pressure, pH, phospholipids, and intracellular signaling molecules, substantially modulate the activity of K2P channels. In this review, we summarize the contribution of K2P channels to neuronal excitability and to potassium homeostasis in glia. We describe recently discovered functions of K2P channels in glia, such as astrocytic passive conductance and glutamate release, microglial surveillance, and myelin generation by oligodendrocytes. We also discuss the potential role of glial K2P channels in neurological disorders. In the end, we discuss current limitations in K2P channel researches and suggest directions for future studies.

Glial glucose fuels the neuronal pentose phosphate pathway for long-term memory

Brain function relies almost solely on glucose as an energy substrate. The main model of brain metabolism proposes that glucose is taken up and converted into lactate by astrocytes to fuel the energy-demanding neuronal activity underlying plasticity and memory. Whether direct neuronal glucose uptake is required for memory formation remains elusive. We uncover, in Drosophila, a mechanism of glucose shuttling to neurons from cortex glia, an exclusively perisomatic glial subtype, upon formation of olfactory long-term memory (LTM). In vivo imaging reveals that, downstream of cholinergic activation of cortex glia, autocrine insulin signaling increases glucose concentration in glia. Glucose is then transferred from glia to the neuronal somata in the olfactory memory center to fuel the pentose phosphate pathway and allow LTM formation. In contrast, our results indicate that the increase in neuronal glucose metabolism, although crucial for LTM formation, is not routed to glycolysis.

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Neuronal glucose metabolism is increased upon long-term memory formation

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Glial cells shuttle glucose to neurons following insulin signaling activation

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Glucose fuels the neuronal pentose phosphate pathway

Effects on Metabolism in Astrocytes Caused by cGAMP, Which Imitates the Initial Stage of Brain Metastasis

The second messenger 2'3'-cyclic-GMP-AMP (cGAMP) is thought to be transmitted from brain carcinomas to astrocytes via gap junctions, which functions to promote metastasis in the brain parenchyma. In the current study, we established a method to introduce cGAMP into astrocytes, which simulates the state of astrocytes that have been invaded by cGAMP around tumors. Astrocytes incorporating cGAMP were analyzed by metabolomics, which demonstrated that cGAMP increased glutamate production and astrocyte secretion. The same trend was observed for γ-aminobutyric acid (GABA). Conversely, glutamine production and secretion were decreased by cGAMP treatment. Due to the fundamental role of astrocytes in regulation of the glutamine-glutamate cycle, such metabolic changes may represent a potential mechanism and therapeutic target for alteration of the central nervous system (CNS) environment and the malignant transformation of brain carcinomas.

Receptor-Receptor Interactions and Glial Cell Functions with a Special Focus on G Protein-Coupled Receptors

The discovery that receptors from all families can establish allosteric receptor-receptor interactions and variably associate to form receptor complexes operating as integrative input units endowed with a high functional and structural plasticity has expanded our understanding of intercellular communication. Regarding the nervous system, most research in the field has focused on neuronal populations and has led to the identification of many receptor complexes representing an important mechanism to fine-tune synaptic efficiency. Receptor-receptor interactions, however, also modulate glia-neuron and glia-glia intercellular communication, with significant consequences on synaptic activity and brain network plasticity. The research on this topic is probably still at the beginning and, here, available evidence will be reviewed and discussed. It may also be of potential interest from a pharmacological standpoint, opening the possibility to explore, inter alia, glia-based neuroprotective therapeutic strategies.

The Lepidopteran KAAT1 and CAATCH1: Orthologs to Understand Structure-Function Relationships in Mammalian SLC6 Transporters

To the SLC6 family belong 20 human transporters that utilize the sodium electrochemical gradient to move biogenic amines, osmolytes, amino acids and related compounds into cells. They are classified into two functional groups, the Neurotransmitter transporters (NTT) and Nutrient amino acid transporters (NAT). Here we summarize how since their first cloning in 1998, the insect (Lepidopteran) Orthologs of the SLC6 family transporters have represented very important tools for investigating functional–structural relationships, mechanism of transport, ion and pH dependence and substate interaction of the mammalian (and human) counterparts.

Glutamatergic receptor expression changes in the Alzheimer's disease hippocampus and entorhinal cortex

Alzheimer's Disease (AD) is the leading form of dementia worldwide. Currently, the pathological mechanisms underlying AD are not well understood. Although the glutamatergic system is extensively implicated in its pathophysiology, there is a gap in knowledge regarding the expression of glutamate receptors in the AD brain. This study aimed to characterize the expression of specific glutamate receptor subunits in post‐mortem human brain tissue using immunohistochemistry and confocal microscopy. Free‐floating immunohistochemistry and confocal laser scanning microscopy were used to quantify the density of glutamate receptor subunits GluA2, GluN1, and GluN2A in specific cell layers of the hippocampal sub‐regions, subiculum, entorhinal cortex, and superior temporal gyrus. Quantification of GluA2 expression in human post‐mortem hippocampus revealed a significant increase in the stratum (str.) moleculare of the dentate gyrus (DG) in AD compared with control. Increased GluN1 receptor expression was found in the str. moleculare and hilus of the DG, str. oriens of the CA2 and CA3, str. pyramidale of the CA2, and str. radiatum of the CA1, CA2, and CA3 subregions and the entorhinal cortex. GluN2A expression was significantly increased in AD compared with control in the str. oriens, str. pyramidale, and str. radiatum of the CA1 subregion. These findings indicate that the expression of glutamatergic receptor subunits shows brain region‐specific changes in AD, suggesting possible pathological receptor functioning. These results provide evidence of specific glutamatergic receptor subunit changes in the AD hippocampus and entorhinal cortex, indicating the requirement for further research to elucidate the pathophysiological mechanisms it entails, and further highlight the potential of glutamatergic receptor subunits as therapeutic targets.

S1P2-Gα12 Signaling Controls Astrocytic Glutamate Uptake and Mitochondrial Oxygen Consumption

Glutamate is the principal excitatory neurotransmitter in the human brain. Following neurotransmission, astrocytes remove excess extracellular glutamate to prevent neurotoxicity. Glutamate neurotoxicity has been reported in multiple neurologic diseases including multiple sclerosis (MS), representing a shared neurodegenerative mechanism. A potential modulator of glutamate neurotoxicity is the bioactive lysophospholipid sphingosine 1-phosphate (S1P) that signals through five cognate G-protein-coupled receptors, S1P₁-S1P₅; however, a clear link between glutamate homeostasis and S1P signaling has not been established. Here, S1P receptor knock-out mice, primary astrocyte cultures, and receptor-selective chemical tools were used to examine the effects of S1P on glutamate uptake. S1P inhibited astrocytic glutamate uptake in a dose-dependent manner and increased mitochondrial oxygen consumption, primarily through S1P₂ Primary cultures of wild-type mouse astrocytes expressed S1P_1,2,3 transcripts, and selective deletion of S1P₁ and/or S1P₃ in cerebral cortical astrocytes, did not alter S1P-mediated, dose-dependent inhibition of glutamate uptake. Pharmacological antagonists, S1P₂-null astrocytes, and Gα₁₂ hemizygous-null astrocytes indicated that S1P₂-Gα₁₂-Rho/ROCK signaling was primarily responsible for the S1P-dependent inhibition of glutamate uptake. In addition, S1P exposure increased mitochondrial oxygen consumption rates (OCRs) in wild-type astrocytes and reduced OCRs in S1P₂-null astrocytes, implicating receptor selective metabolic consequences of S1P-mediated glutamate uptake inhibition. Astrocytic S1P-S1P₂ signaling increased extracellular glutamate, which could contribute to neurotoxicity. This effect was not observed with the FDA-approved S1P receptor modulators, siponimod and fingolimod. Development and use of S1P₂-selective antagonists may provide a new approach to reduce glutamate neurotoxicity in neurologic diseases.

Histamine, Neuroinflammation and Neurodevelopment: A Review

The biogenic amine, histamine, has been shown to critically modulate inflammatory processes as well as the properties of neurons and synapses in the brain, and is also implicated in the emergence of neurodevelopmental disorders. Indeed, a reduction in the synthesis of this neuromodulator has been associated with the disorders Tourette's syndrome and obsessive-compulsive disorder, with evidence that this may be through the disruption of the corticostriatal circuitry during development. Furthermore, neuroinflammation has been associated with alterations in brain development, e.g., impacting synaptic plasticity and synaptogenesis, and there are suggestions that histamine deficiency may leave the developing brain more vulnerable to proinflammatory insults. While most studies have focused on neuronal sources of histamine it remains unclear to what extent other (non-neuronal) sources of histamine, e.g., from mast cells and other sources, can impact brain development. The few studies that have started exploring this in vitro, and more limited in vivo, would indicate that non-neuronal released histamine and other preformed mediators can influence microglial-mediated neuroinflammation which can impact brain development. In this Review we will summarize the state of the field with regard to non-neuronal sources of histamine and its impact on both neuroinflammation and brain development in key neural circuits that underpin neurodevelopmental disorders. We will also discuss whether histamine receptor modulators have been efficacious in the treatment of neurodevelopmental disorders in both preclinical and clinical studies. This could represent an important area of future research as early modulation of histamine from neuronal as well as non-neuronal sources may provide novel therapeutic targets in these disorders.

NMDARs, Coincidence Detectors of Astrocytic and Neuronal Activities

Synaptic plasticity is an extensively studied cellular correlate of learning and memory in which NMDARs play a starring role. One of the most interesting features of NMDARs is their ability to act as a co-incident detector. It is unique amongst neurotransmitter receptors in this respect. Co-incident detection is possible because the opening of NMDARs requires membrane depolarisation and the binding of glutamate. Opening of NMDARs also requires a co-agonist. Although the dynamic regulation of glutamate and membrane depolarization have been well studied in coincident detection, the role of the co-agonist site is unexplored. It turns out that non-neuronal glial cells, astrocytes, regulate co-agonist availability, giving them the ability to influence synaptic plasticity. The unique morphology and spatial arrangement of astrocytes at the synaptic level affords them the capacity to sample and integrate information originating from unrelated synapses, regardless of any pre-synaptic and post-synaptic commonality. As astrocytes are classically considered slow responders, their influence at the synapse is widely recognized as modulatory. The aim herein is to reconsider the potential of astrocytes to participate directly in ongoing synaptic NMDAR activity and co-incident detection.

Role of Purinergic Signalling in Endothelial Dysfunction and Thrombo-Inflammation in Ischaemic Stroke and Cerebral Small Vessel Disease

The cerebral endothelium is an active interface between blood and the central nervous system. In addition to being a physical barrier between the blood and the brain, the endothelium also actively regulates metabolic homeostasis, vascular tone and permeability, coagulation, and movement of immune cells. Being part of the blood–brain barrier, endothelial cells of the brain have specialized morphology, physiology, and phenotypes due to their unique microenvironment. Known cardiovascular risk factors facilitate cerebral endothelial dysfunction, leading to impaired vasodilation, an aggravated inflammatory response, as well as increased oxidative stress and vascular proliferation. This culminates in the thrombo-inflammatory response, an underlying cause of ischemic stroke and cerebral small vessel disease (CSVD). These events are further exacerbated when blood flow is returned to the brain after a period of ischemia, a phenomenon termed ischemia-reperfusion injury. Purinergic signaling is an endogenous molecular pathway in which the enzymes CD39 and CD73 catabolize extracellular adenosine triphosphate (eATP) to adenosine. After ischemia and CSVD, eATP is released from dying neurons as a damage molecule, triggering thrombosis and inflammation. In contrast, adenosine is anti-thrombotic, protects against oxidative stress, and suppresses the immune response. Evidently, therapies that promote adenosine generation or boost CD39 activity at the site of endothelial injury have promising benefits in the context of atherothrombotic stroke and can be extended to current CSVD known pathomechanisms. Here, we have reviewed the rationale and benefits of CD39 and CD39 therapies to treat endothelial dysfunction in the brain.

Astrocyte-Neuron Signaling in Synaptogenesis

Astrocytes are the key component of the central nervous system (CNS), serving as pivotal regulators of neuronal synapse formation and maturation through their ability to dynamically and bidirectionally communicate with synapses throughout life. In the past 20 years, numerous astrocyte-derived molecules promoting synaptogenesis have been discovered. However, our understanding of the cell biological basis underlying intra-neuron processes and astrocyte-mediated synaptogenesis is still in its infancy. Here, we provide a comprehensive overview of the various ways astrocytes talk to neurons, and highlight astrocytes’ heterogeneity that allow them to displays regional-specific capabilities in boosting synaptogenesis. Finally, we conclude with promises and future directions on how organoids generated from human induced pluripotent stem cells (hiPSCs) effectively address the signaling pathways astrocytes employ in synaptic development.

Two-photon calcium imaging of neuronal and astrocytic responses: the influence of electrical stimulus parameters and calcium signaling mechanisms

Objective. Electrical brain stimulation has been used to ameliorate symptoms associated with neurologic and psychiatric disorders. The astrocytic activation and its interaction with neurons may contribute to the therapeutic effects of electrical stimulation. However, how the astrocytic activity is affected by electrical stimulation and its calcium signaling mechanisms remain largely unknown. This study is to explore the influence of electrical stimulus parameters on cellular calcium responses and corresponding calcium signaling mechanisms, with a focus on the heretofore largely overlooked astrocytes.Approach. Usingin vivotwo-photon microscopy in mouse somatosensory cortex, the calcium activity in neurons and astrocytes were recorded.Main results. The cathodal stimulation evoked larger responses in both neurons and astrocytes than anodal stimulation. Both neuronal and astrocytic response profiles exhibited the unimodal frequency dependency, the astrocytes prefer higher frequency stimulation than neurons. Astrocytes need longer pulse width and higher current intensity than neurons to activate. Compared to neurons, the astrocytes were not capable of keeping sustained calcium elevation during prolonged electrical stimulation. The neuronal Ca²⁺influx involves postsynaptic effects and direct depolarization. The Ca²⁺surge of astrocytes has a neuronal origin, the noradrenergic and glutamatergic signaling act synergistically to induce astrocytic activity.Significance. The astrocytic activity can be regulated by manipulating stimulus parameters and its calcium activation should be fully considered when interpreting the mechanisms of action of electrical neuromodulation. This study brings considerable benefits in the application of electrical stimulation and provides useful insights into cortical signal transduction, which contributes to the understanding of mechanisms underlying the therapeutic efficacy of electrical stimulation for neurorehabilitation applications.

Different glutamate sources and endogenous co-agonists activate extrasynaptic NMDA receptors on amacrine cells of the rod pathway microcircuit

The NMDA receptors (NMDARs) expressed by AII and A17 amacrine cells, the two main inhibitory interneurons of the rod pathway microcircuit in the mammalian retina, are exclusively extrasynaptic, activated by ambient levels of glutamate, and molecularly distinct, with AII and A17 amacrines expressing GluN2B- and GluN2A-containing receptors, respectively. This important sensory microcircuit thus provides a unique model to study the activation and function of extrasynaptic NMDARs. Here, we investigated the sources of glutamate and the endogenous co-agonists (d-serine or glycine) that activate these distinct populations of NMDARs. With acute slices from rat retina, we used whole-cell voltage-clamp recording and measurement of current noise to monitor levels of NMDAR activity. Pre-incubation of retina with bafilomycin A1 (an inhibitor of neurotransmitter uptake into synaptic vesicles) abolished NMDAR-mediated noise in AII, but not A17 amacrines, suggesting a vesicular source of glutamate activates AII NMDARs, whereas a non-vesicular source activates A17 NMDARs. Pre-incubation of retina with l-methionine sulfoximine (an inhibitor of glutamine synthetase) also abolished NMDAR-mediated noise in AII, but not A17 amacrines, suggesting a neuronal source of glutamate activates AII NMDARs, whereas a glial source activates A17 NMDARs. Enzymatic breakdown of d-serine reduced NMDAR-mediated noise in AII, but not A17 amacrines, suggesting d-serine is the endogenous co-agonist at AII, but not A17 NMDARs. Our results reveal unique characteristics of these two populations of extrasynaptic NMDARs. The differential and independent activation of these receptors is likely to provide specific contributions to the signal processing and plasticity of the cellular components of the rod pathway microcircuit.

Neuronal and glial cell number is altered in a cortical layer-specific manner in autism

LAY ABSTRACT

The cerebral cortex affected with autism spectrum disorder presents changes in the number of neurons and glia cells, possibly leading to a dysregulation of brain circuits and affecting behavior. However, little is known about cell number alteration in specific layers of the cortex in autism spectrum disorder. We found an increase in the number of neurons and a decrease in the number of astrocytes in specific layers of the prefrontal cortex in postmortem human brains from autism spectrum disorder cases. We hypothesize that this may be due to a failure in neural stem cells to shift differentiation from neurons to glial cells during prenatal brain development. These data provide key anatomical findings that contribute to the bases of autism spectrum disorder pathogenesis.

Interactions of neuroimmune signaling and glutamate plasticity in addiction

Chronic use of drugs of abuse affects neuroimmune signaling; however, there are still many open questions regarding the interactions between neuroimmune mechanisms and substance use disorders (SUDs). Further, chronic use of drugs of abuse can induce glutamatergic changes in the brain, but the relationship between the glutamate system and neuroimmune signaling in addiction is not well understood. Therefore, the purpose of this review is to bring into focus the role of neuroimmune signaling and its interactions with the glutamate system following chronic drug use, and how this may guide pharmacotherapeutic treatment strategies for SUDs. In this review, we first describe neuroimmune mechanisms that may be linked to aberrant glutamate signaling in addiction. We focus specifically on the nuclear factor-kappa B (NF-κB) pathway, a potentially important neuroimmune mechanism that may be a key player in driving drug-seeking behavior. We highlight the importance of astroglial-microglial crosstalk, and how this interacts with known glutamatergic dysregulations in addiction. Then, we describe the importance of studying non-neuronal cells with unprecedented precision because understanding structure-function relationships in these cells is critical in understanding their role in addiction neurobiology. Here we propose a working model of neuroimmune-glutamate interactions that underlie drug use motivation, which we argue may aid strategies for small molecule drug development to treat substance use disorders. Together, the synthesis of this review shows that interactions between glutamate and neuroimmune signaling may play an important and understudied role in addiction processes and may be critical in developing more efficacious pharmacotherapies to treat SUDs.

Chemogenetic manipulation of astrocytic activity: Is it possible to reveal the roles of astrocytes?

Astrocytes are the major glial cells in the central nervous system, but unlike neurons, they do not produce action potentials. For many years, astrocytes were considered supporting cells in the central nervous system (CNS). Technological advances over the last two decades are changing the face of glial research. Accumulating data from recent investigations show that astrocytes display transient calcium spikes and regulate synaptic transmission by releasing transmitters called gliotransmitters. Many new powerful technologies are used to interfere with astrocytic activity, in order to obtain a better understanding of the roles of astrocytes in the brain. Among these technologies, chemogenetics has recently been used frequently. In this review, we will summarize new functions of astrocytes in the brain that have been revealed using this cutting-edge technique. Moreover, we will discuss the possibilities and challenges of manipulating astrocytic activity using this technology.

Glial amplification of synaptic signals

KEY POINTS

Recent studies have repeatedly demonstrated the cross-talk of heterogeneous signals between neuronal and glial circuits. Here, we investigated the mechanism and the influence of physiological interactions between neurons and glia in the cerebellum. We found that the cerebellar astrocytes, Bergmann glial cells, react to exogenously applied glutamate, glutamate transporter substrate (d-aspartate) and synaptically released glutamate. In response, the Bergmann glial cells release glutamate through volume-regulated anion channels. It is generally assumed that all of the postsynaptic current is mediated by presynaptically released glutamate. However, we showed that a part of the postsynaptic current is mediated by glutamate released from Bergmann glial cells. Optogenetic manipulation of Bergmann glial state with archaerhodpsin-T or channelrhodopsin-2 reduced or augmented the amount of glial glutamate release, respectively. Our data indicate that glutamate-induced glutamate release in Bergmann glia serves as an effective amplifier of excitatory information processing in the brain.

ABSTRACT

Transmitter released from presynaptic neurons has been considered to be the sole generator of postsynaptic excitatory signals. However, astrocytes of the glial cell population have also been shown to release transmitter that can react on postsynaptic receptors. Therefore, we investigated whether astrocytes take part in generation of at least a part of the synaptic current. In this study, mice cerebellar acute slices were prepared and whole cell patch clamp recordings were performed. We found that Bergmann glial cells (BGs), a type of astrocyte in the cerebellum, reacts to a glutamate transporter substrate, d-aspartate (d-Asp) and an anion conductance is generated and glutamate is released from the BGs. Glutamate release was attenuated or augmented by modulating the state of BGs with activation of light-sensitive proteins, archaerhodopsin-T (ArchT) or channelrhodopsin-2 (ChR2) expressed on BGs, respectively. Glutamate release appears to be mediated by anion channels that can be blocked by a volume-regulated anion channel-specific blocker. Synaptic response to a train of parallel fibre stimulation was recorded from Purkinje cells. The latter part of the response was also attenuated or augmented by glial modulation with ArchT or ChR2, respectively. Thus, BGs effectively function as an excitatory signal amplifier, and a part of the 'synaptic' current is actually mediated by glutamate released from BGs. These data show that the state of BGs have potential for having direct and fundamental consequences on the functioning of information processing in the brain.

Star power: the emerging role of astrocytes as neuronal partners during cortical plasticity

Plasticity is a fundamental property of neuronal circuits, allowing them to adapt to alterations in activation. Generally speaking, plasticity has been viewed from a 'neuron-centric' perspective, with changes in circuit function attributed to alterations in neuronal excitability, synaptic strength or neuronal connectivity. However, it is now clear that glial cells, in particular astrocytes, are key regulators of neuronal plasticity. This article reviews recent progress made in understanding astrocyte function and attempts to summarize these functions into a coherent framework that positions astrocytes as central players in the plasticity process.

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

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

The Function of Astrocyte Mediated Extracellular Vesicles in Central Nervous System Diseases

Astrocyte activation plays an important role during disease-induced inflammatory response in the brain. Exosomes in the brain could be released from bone marrow (BM)-derived stem cells, neuro stem cells (NSC), mesenchymal stem cells (MSC), etc. We summarized that exosomes release and transport signaling to the target cells, and then produce function. Furthermore, we discussed the pathological interactions between astrocytes and other brain cells, which are related to brain diseases such as stroke, Alzheimer’s disease (AD), Parkinson’s disease (PD), amyotrophic lateral sclerosis (ALS) disease, multiple sclerosis (MS), psychiatric, traumatic brain injury (TBI), etc. We provide up-to-date, comprehensive and valuable information on the involvement of exosomes in brain diseases, which is beneficial for basic researchers and clinical physicians.

Neuron-derived factors negatively modulate ryanodine receptor-mediated calcium release in cultured mouse astrocytes

Changes in intracellular Ca²⁺ concentration ([Ca²⁺]_i) produced by ryanodine receptor (RyR) agonist, caffeine (caf), and ionotropic agonists: N-methyl-d-aspartate (NMDA) receptor (NMDAR) agonist, NMDA and P2X7 receptor (P2X7R) agonist, 3'-O-(4-benzoyl)benzoyl adenosine 5'-triphosphate (BzATP) were measured in cultured mouse cortical astrocytes loaded with the fluorescent calcium indicator Fluo3-AM in a confocal laser scanning microscope. In mouse astrocytes cultured in standard medium (SM), treatment with caf increased [Ca²⁺]_i, with a peak response occurring about 10 min after stimulus application. Peak responses to NMDA or BzATP were observed about <1 min and 4.5 min post stimulus, respectively. Co-treatment with NMDA or BzATP did not alter the peak response to caf in astrocytes cultured in SM, the absence of the effects being most likely due to asynchrony between the response to caf, NMDA and BzATP. Incubation of astrocytes with neuron-condition medium (NCM) for 24 h totally abolished the caf-evoked [Ca²⁺]_i increase. In NCM-treated astrocytes, peak of [Ca²⁺]_i rise evoked by NMDA was delayed to about 3.5 min, and that induced by BzATP occurred about three minutes earlier than in SM. The results show that neurons secrete factors that negatively modulate RyR-mediated Ca²⁺-induced Ca²⁺ release (CICR) in astrocytes and alter the time course of Ca²⁺ responses to ionotropic stimuli.

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

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

Cortical Interlaminar Astrocytes Are Generated Prenatally, Mature Postnatally, and Express Unique Markers in Human and Nonhuman Primates

Interlaminar astrocytes (ILAs) are a subset of cortical astrocytes that reside in layer I, express GFAP, have a soma contacting the pia, and contain long interlaminar processes that extend through several cortical layers. We studied the prenatal and postnatal development of ILAs in three species of primates (rhesus macaque, chimpanzee, and human). We found that ILAs are generated prenatally likely from radial glial (RG) cells, that ILAs proliferate locally during gestation, and that ILAs extend interlaminar processes during postnatal stages of development. We showed that the density and morphological complexity of ILAs increase with age, and that ILAs express multiple markers that are expressed by RG cells (Pax6, Sox2, and Nestin), specific to inner and outer RG cells (Cryab and Hopx), and astrocyte markers (S100β, Aqp4, and GLAST) in prenatal stages and in adult. Finally, we demonstrated that rudimentary ILAs in mouse also express the RG markers Pax6, Sox2, and Nestin, but do not express S100β, Cryab, or Hopx, and that the density and morphological complexity of ILAs differ between primate species and mouse. Together these findings contribute new information on astrogenesis of this unique class of cells and suggest a lineal relationship between RG cells and ILAs.

Neuropeptides Modulate Local Astrocytes to Regulate Adult Hippocampal Neural Stem Cells

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

Fine structure of the central brain in the octopod Eledone cirrhosa (Lamarck, 1798) (Mollusca-Octopoda)

This study aims to investigate the fine structure of the different cell types in the central brain of Eledone cirrhosa; the organelles in the neurons and the glial cells; the glial hemolymph-brain barrier; the neuro-secretions and the relationships between glial and nerve cells. The brain is surrounded by a non-cellular neurilemma followed by a single layer of perilemmal cells. Ependymal cells, highly prismatic glial cells, astrocytes, oligodendrocytes and epithelial processes were observed. The perikarya of the neurons are filled with slightly oval nuclei with heterochromatin, a strongly tortuous ER, numerous mitochondria and Golgi apparatus with two types of vesicles. In the cellular cortex, glial cells are much less numerous than the neurons and they are located preferably at the border between perikarya and neuropil. Furthermore, they send many branching shoots between the surrounding neuron perikarya and the axons. The glial cytoplasmic matrix appears more electrodense than that of the neurons. Only few ribosomes are attached to the membranes of the ER; the vast majorities are free. In the perikarya of the glial cells, mitochondria, multi-vesicular bodies, various vacuoles and vesicles are present. The essential elements of the hemolymph-brain barrier are the endothelial cells with their tight junctions. The cytoplasm contains various vesicles and mitochondria. However, two other cell types are present, the pericytes and the astrocytes, which are of great importance for the function of the hemolymph-brain barrier. The cell-cell interactions between endothelial cells, pericytes and astrocytes are as close as no other cells.

Ketamine/xylazine and barbiturates modulate microglial morphology and motility differently in a mouse model

Microglia, the resident immune cells of the brain, are highly ramified and motile and their morphology is strongly linked to their function. Microglia constantly monitor the brain parenchyma and are crucial for maintaining brain homeostasis and fine-tuning neuronal networks. Besides affecting neurons, anesthetics may have wide-ranging effects mediated by non-neuronal cells and in particular microglia. We thus examined the effect of two commonly used anesthetic agents, ketamine/xylazine and barbiturates, on microglial motility and morphology. A combination of two-photon in vivo imaging and electroencephalography (EEG) recordings in unanesthetized and anesthetized mice as well as automated analysis of ex vivo sections were used to assess morphology and dynamics of microglia. We found that administration of ketamine/xylazine and pentobarbital anesthesia resulted in quite distinct EEG profiles. Both anesthetics reduced microglial motility, but only ketamine/xylazine administration led to reduction of microglial complexity in vivo. The change of cellular dynamics in vivo was associated with a region-dependent reduction of several features of microglial cells ex vivo, such as the complexity index and the ramification length, whereas thiopental altered the size of the cytoplasm. Our results show that anesthetics have considerable effects on neuronal activity and microglial morphodynamics and that barbiturates may be a preferred anesthetic agent for the study of microglial morphology. These findings will undoubtedly raise compelling questions about the functional relevance of anesthetics on microglial cells in neuronal physiology and anesthesia-induced neurotoxicity.