Gliotransmitters travel in time and space

Inhibitory gamma-aminobutyric acidergic neurons in the anterior cingulate cortex participate in the comorbidity of pain and emotion

Pain is often comorbid with emotional disorders such as anxiety and depression. Hyperexcitability of the anterior cingulate cortex has been implicated in pain and pain-related negative emotions that arise from impairments in inhibitory gamma-aminobutyric acid neurotransmission. This review primarily aims to outline the main circuitry (including the input and output connectivity) of the anterior cingulate cortex and classification and functions of different gamma-aminobutyric acidergic neurons; it also describes the neurotransmitters/neuromodulators affecting these neurons, their intercommunication with other neurons, and their importance in mental comorbidities associated with chronic pain disorders. Improving understanding on their role in pain-related mental comorbidities may facilitate the development of more effective treatments for these conditions. However, the mechanisms that regulate gamma-aminobutyric acidergic systems remain elusive. It is also unclear as to whether the mechanisms are presynaptic or postsynaptic. Further exploration of the complexities of this system may reveal new pathways for research and drug development.

Interpreting the rich dialogue between astrocytes and neurons: An overview in Rett syndrome

Rett syndrome (RTT) is a severe neurodevelopmental disorder primarily affecting females, with an incidence of 1 in 10,000 live births. It is caused mainly by de novo mutations in the X-linked MECP2 gene, which encodes methyl-CpG binding protein 2 (Mecp2), a key epigenetic regulator. MECP2 mutations have profound impacts on neurons, which exhibit morphological, synaptic and functional impairments. However, more recent evidence highlights a crucial role of astrocytes in RTT pathogenesis. Indeed, RTT astrocytes exhibit structural and functional impairments, failing to support neuronal growth and function through non-cell autonomous mechanisms. Studies reveal that MECP2 deficient astrocytes secrete abnormal factors that impair neuronal growth and synaptic function. Furthermore, they show dysregulated calcium signalling, disrupted glutamate and potassium homeostasis, and increased inflammatory responses, all of which contribute to neuronal dysfunction. Understanding these neuron-astrocyte interactions may offer novel therapeutic targets for RTT. In the review we aim at presenting the current knowledge of astrocyte-neuron crosstalk in RTT, describing the different mechanisms highlighted so far through which MECP2 mutant astrocytes impair neurons. Finally, we discuss existing and prospective methodological approaches for investigating cell-to-cell communication in RTT.

GFOD1 regulates oxidative stress-induced damage in ADHD via NF-κB signaling pathway

Attention-deficit/hyperactivity disorder (ADHD) is a mental behavioral disorder that poses a serious health risk. Oxidative stress, which damages the function of neurons and astrocytes, has been discovered as a key factor contributing to ADHD pathology. A newly identified gene, Glucose-fructose oxidoreductase domain 1 (GFOD1), may be linked to the development of ADHD. It plays a role in regulating oxidative stress in ADHD; however, its exact role is unclear. This manuscript investigates the changes of GFOD1 expression and aim to correlate this with oxidative stress induced by NF-κB signaling pathway in the rat brains with ADHD and in vitro astrocytes. Our results revealed an increase in GFOD1 expression in the prefrontal cortex and cerebellar cortex of rats with ADHD, accompanied by neuronal injury and increased glial fibrillary acidic protein (GFAP) expression in astrocytes, concomitant with activation of the NF-κB p65/NOX2 signaling pathway. Along with this, GFOD1 overexpression in astrocytes resulted in an up-regulation of this signaling pathway similarly. Both ADHD rats and astrocytes in overexpressing GFOD1 showed elevated levels of reactive oxygen species (ROS) and Malondialdehyde (MDA), reduced activity of superoxide dismutase (SOD). Furthermore, treatment with the methylphenidate (MPH) did not affect GFOD1 expression. But it impacted the levels of oxidative stress mediated by the NF-κB p65/NOX2 signaling pathway. Overall, it is suggested that GFOD1 may contribute to increased levels of oxidative stress specifically in the prefrontal cortex and cerebellar cortex regions and astrocytes affected by ADHD via up-regulation of the NF-κB p65/NOX2/oxidative stress axis.

Neuron-astrocyte coupling in lateral habenula mediates depressive-like behaviors

The lateral habenula (LHb) neurons and astrocytes have been strongly implicated in depression etiology, but it was not clear how the two dynamically interact during depression onset. Here, using multi-brain-region calcium photometry recording in freely moving mice, we discover that stress induces a most rapid astrocytic calcium rise and a bimodal neuronal response in the LHb. LHb astrocytic calcium requires the α1A-adrenergic receptor and depends on a recurrent neural network between the LHb and locus coeruleus (LC). Through the gliotransmitter glutamate and ATP/adenosine, LHb astrocytes mediate the second-wave LHb neuronal activation and norepinephrine (NE) release. Activation or inhibition of LHb astrocytic calcium signaling facilitates or prevents stress-induced depressive-like behaviors, respectively. These results identify a stress-induced positive feedback loop in the LHb-LC axis, with astrocytes being a critical signaling relay. The identification of this prominent neuron-glia interaction may shed light on stress management and depression prevention.

From quiescence to self-sustained activity: How astrocytes reshape neural dynamics

Astrocytes are currently gaining attention from the neuroscience community due to their contribution to a multitude of functions, which includes but are not limited to their ability to change the neuron's spiking frequency, their capacity to alter neuron's firing activity, and their involvement in memory formation and synaptic plasticity regulation. To date, many computational models describing the dynamics of astrocytes, together with their complex interaction with neurons, were proposed, however, these could still be improved since the exploration of their functions and mechanisms was way later than neurons. Hence, in this paper, investigation of the steady state behavior of the neuron-astrocyte interaction through a presentation of phase plane analysis and one parameter bifurcation were primarily performed. Adaptive Exponential Integrate-and-Fire model was utilized to describe the firing dynamics of the neuron while the model from the work of Postnov and collaborators was used to describe the calcium dynamics of the astrocyte. The findings demonstrate that astrocytic modulation can significantly shape neuronal activity, including initiating spikes, inducing self-sustained oscillations, and exerting both inhibitory and excitatory effects depending on synaptic strength. These highlight the crucial role that the contribution of astrocytes to the synapse plays in regulating neuronal activity and producing a range of neuronal firing behaviors within the neuron-astrocyte ensemble. They may impact neuronal synchronization, an attribute of several neurological illnesses, including epilepsy, and, on the other hand, may enhance brain information processing.

Chemogenetic Control of Striatal Astrocytes Improves Parkinsonian Motor Deficits in Mice

Parkinson's disease (PD) is characterized by the degeneration of dopaminergic nigrostriatal inputs, which causes striatal network dysfunction and leads to pronounced motor deficits. Recent evidence highlights astrocytes as a potential local source for striatal neuromodulation. There is substantial evidence for norepinephrine-mediated recruitment of cortical astrocyte activity during movement and locomotion. However, it is unclear how astrocytes in the striatum, a region devoid of norepinephrine neuromodulatory inputs, respond during locomotion. Moreover, it remains unknown how dopamine loss affects striatal astrocyte activity and whether astrocyte activity regulates behavioral deficits in PD. We addressed these questions by performing astrocyte-specific calcium recordings and manipulations using in vivo fiber photometry and chemogenetics. We find that locomotion elicits astrocyte calcium activity over a slower timescale than neurons. Acute pharmacological blockade of dopamine receptors only moderately reduced locomotion-related astrocyte activity. Yet, unilateral dopamine depletion significantly attenuated astrocyte calcium responses. Chemogenetic stimulation of Gi-coupled receptors partially improved this functional astrocyte deficit in dopamine-lesioned mice. In parallel, chemogenetic manipulation restored asymmetrical motor deficits and moderately improved open-field exploratory behavior. Together, our results establish a novel role for functional striatal astrocyte signaling in modulating motor function in PD and highlight non-neuronal targets for potential PD therapeutics.

Non-neuronal brain biology and its relevance to animal welfare

Non-neuronal cells constitute a significant portion of brain tissue and are seen as having key roles in brain homeostasis and responses to challenges. This review illustrates how non-neuronal biology can bring new perspectives to animal welfare through understanding mechanisms that determine welfare outcomes and highlighting interventions to improve welfare. Most obvious in this respect is the largely unrecognised relevance of neuroinflammation to animal welfare which is increasingly found to have roles in determining how animals respond to challenges. We start by introducing non-neuronal cells and review their involvement in affective states and cognition often seen as core psychological elements of animal welfare. We find that the evidence for a causal involvement of glia in cognition is currently more advanced than the corresponding evidence for affective states. We propose that translational research on affective disorders could usefully apply welfare science derived approaches for assessing affective states. Using evidence from translational research, we illustrate the involvement of non-neuronal cells and neuroinflammatory processes as mechanisms modulating resilience to welfare challenges including disease, pain, and social stress. We review research on impoverished environments and environmental enrichment which suggests that environmental conditions which improve animal welfare also improve resilience to challenges through balancing pro- and anti-inflammatory non-neuronal processes. We speculate that non-neuronal biology has relevance to animal welfare beyond neuro-inflammation including facilitating positive affective states. We acknowledge the relevance of neuronal biology to animal welfare whilst proposing that non-neuronal biology provides additional and relevant insights to improve animals' lives.

Neuron-astrocyte associative memory

Astrocytes, the most abundant type of glial cell, play a fundamental role in memory. Despite most hippocampal synapses being contacted by an astrocyte, there are no current theories that explain how neurons, synapses, and astrocytes might collectively contribute to memory function. We demonstrate that fundamental aspects of astrocyte morphology and physiology naturally lead to a dynamic, high-capacity associative memory system. The neuron-astrocyte networks generated by our framework are closely related to popular machine learning architectures known as Dense Associative Memories. Adjusting the connectivity pattern, the model developed here leads to a family of associative memory networks that includes a Dense Associative Memory and a Transformer as two limiting cases. In the known biological implementations of Dense Associative Memories, the ratio of stored memories to the number of neurons remains constant, despite the growth of the network size. Our work demonstrates that neuron-astrocyte networks follow a superior memory scaling law, outperforming known biological implementations of Dense Associative Memory. Our model suggests an exciting and previously unnoticed possibility that memories could be stored, at least in part, within the network of astrocyte processes rather than solely in the synaptic weights between neurons.

Cell biology of astrocytic adhesive interactions and signaling pathways in regulating neuronal circuits

Astrocytes have attracted attention for their crucial roles in various brain functions. Yet a gap remains in our understanding. The cellular and molecular basis by which astrocytes interact with neuronal circuits are not clear, and how astrocytes leverage their hallmark morphology dominated by intricate processes in implementing their functions require consideration. This review highlights insights into these outstanding questions gained from recent studies featuring mediators and regulators of cell-cell interactions between astrocytes and neurons, focusing on cell adhesion proteins such as cadherins and neuroligins, among others, as well as cell-extracellular matrix interactions, including astrocytic interactions with the perineuronal network.

Neural Stem Cell-Derived Astrogliogenesis: The Hidden Player of the Adult Hippocampal Cytogenic Niche

The adult mammalian brain exhibits remarkable forms of neural plasticity, enabling it to adapt and reorganize in response to internal and external stimuli. These plastic mechanisms include cytogenesis, the capacity of producing new neuronal and glial cells in restricted brain regions through processes known as neuro- and gliogenesis, respectively. Although many advances have been made in understanding adult brain plastic processes associated with cell genesis, as well as its functional and behavioral implications, most of the evidence is focused on neuronal cells. Even though astrocytes play a critical role in maintaining a neurochemical and electrophysiological homeostasis in the brain and provide a pivotal support to neuronal activity, the molecular mechanisms underlying the formation and functional integration of newly formed astroglial cells are poorly understood. However, some studies have provided key insights into the molecular mechanisms driving the generation of adult neural stem cell (NSC)-derived astrocytes, focusing on the dentate gyrus of the hippocampal cytogenic niche. Recent work has demonstrated that intrinsic and extrinsic factors can modulate astrogliogenesis. In the context of neuropathogenesis, this mechanism may be compromised in the hippocampus, contributing to functional and behavioral impairments. Here, we review the mechanisms underlying NSC-derived hippocampal astrogliogenesis, examining current perspectives on how adult-born astrocytes develop in the adult brain, their functional relevance, and the intricate regulation of the astrogliogenic process.

Adenosine transmission from hypothalamic tanycytes to AGRP/NPY neurons regulates energy homeostasis

Tanycytes are a pivotal component of the hypothalamic network that controls energy homeostasis. Despite their importance, the regulatory mechanisms governing tanycyte-neuron interactions in response to metabolic signals remain unexplored. Here we report that adenosine signaling between tanycytes and AGRP/NPY neurons is crucial for tanycytic metabolic regulation mediated by translocator protein 18 kDa (TSPO). Tanycyte-specific Tspo-knockout mice displayed reduced food consumption and weight loss associated with the downregulation of Agrp and Npy expression under high-fat diet feeding. Tspo-deficient tanycytes had elevated levels of intracellular ATP, which was released via connexin 43 hemichannels and extracellularly converted into adenosine by tanycytic ectonucleotidases. The adenosine signal was perceived by adenosine A1 receptors on adjacent AGRP/NPY neurons, reducing ERK phosphorylation, which in turn downregulated Agrp and Npy expression. Our findings underscore the anorexic role of adenosine as a gliotransmitter in the intricate communication between tanycytes and neurons for regulating appetite and body weight.

Dysregulation of Astrocytic ATP/Adenosine Release in the Hippocampus Cause Cognitive and Affective Disorders: Molecular Mechanisms, Diagnosis, and Therapy

The gliotransmitter adenosine 5'-triphosphate (ATP) and its enzymatic degradation product adenosine play a major role in orchestrating in the hippocampus cognitive and affective functions via P2 purinoceptors (P2X, P2Y) and P1 adenosine receptors (A1, A2A). Although numerous reviews exist on purinoceptors that modulate these functions, there is an apparent gap relating to the involvement of astrocyte-derived extracellular ATP. Our review focuses on the following issues: An impeded release of ATP from hippocampal astrocytes through vesicular mechanisms or connexin hemichannels and pannexin channels interferes with spatial working memory in rodents. The pharmacological blockade of P2Y1 receptors (P2Y1Rs) reverses the deficits in learning/memory performance in mouse models of familial Alzheimer's disease (AD). Similarly, in mouse models of major depressive disorder (MDD), based on acute or chronic stress-induced development of depressive-like behavior, a reduced exocytotic/channel-mediated ATP release from hippocampal astrocytes results in the deterioration of these behavioral responses. However, on the opposite, the increased stimulation of the microglial/astrocytic P2X7R-channel by ATP causes neuroinflammation and in consequence depressive-like behavior. In conclusion, there is strong evidence for the assumption that gliotransmitter ATP is intimately involved in the pathophysiology of cognitive and affective neuron/astrocyte-based human illnesses opening new diagnostic and therapeutic vistas for AD and MDD.

Astrocyte Dysfunctions in Obsessive Compulsive Disorder: Rethinking Neurobiology and Therapeutic Targets

Obsessive-compulsive disorder (OCD) has long been conceptualized as a neuron-centric disorder of cortico-striato-thalamo-cortical (CSTC) circuit dysregulation. However, a growing body of evidence is now reframing this narrative, placing astrocytes-once relegated to passive support roles-at the center of OCD pathophysiology. Astrocytes are critical regulators of glutamate and GABA homeostasis, calcium signaling, and synaptic plasticity, all of which are disrupted in OCD. Recent high-resolution molecular and proteomic studies reveal that specific astrocyte subpopulations, including Crym-positive astrocytes, directly shape excitatory/inhibitory balance and control perseverative behaviors by modulating presynaptic inputs from the orbitofrontal cortex. Disruptions in astrocytic neurotransmitter clearance and dopamine metabolism amplify CSTC circuit hyperactivity and reinforce compulsions. This review reframes OCD as a disorder of neuro-glial dysfunctions, proposing that targeting astrocytic signaling, metabolism, and structural plasticity may unlock transformative therapeutic strategies. By integrating human and animal data, we advocate for a glial-centric model of OCD that not only enhances mechanistic understanding but also opens new frontiers for precision treatment.

In vivo calcium extrusion from accumbal astrocytes reduces anxiety-like behaviors but increases compulsive-like responses and compulsive ethanol drinking in mice

The ventral striatum is crucially involved in reward processing. The present study investigates the behavioral effects of astrocyte-specific calcium extrusion virus "CalEx" on perseverative responses in the operant five-choice serial reaction time task and ethanol-conditioned place preference. Mice were injected with CalEx via the GfaABC1D promoter to extrude cytosolic calcium from astrocytes within the ventral striatum. We found that CalEx transfection in the ventral striatum reduced evoked response duration, the maximum amplitude, and the response frequency to 500 μM ATP as measured by ΔF/F fluorescence intensity of the genetically encoded calcium indicator targeting astrocytes GCaMP6f. During the five-choice serial reaction time task, CalEx mice persisted in perseverative responses compared to their counterparts. Additionally, during ethanol-conditioned place preference, CalEx mice showed increased place preference for a low ethanol concentration compared to control group. Furthermore, we found that accumbal astrocytic calcium extrusion increased quinine adulterated ethanol drinking. Our findings suggest that diminishing ventral striatum astrocyte calcium activity contributes to compulsive behaviors, ethanol drinking, and enhanced ethanol drug reward.

Astrocytes Mediate Psychostimulant-Induced Alterations of Spike-Timing Dependent Synaptic Plasticity

At cellular and circuit levels, drug addiction is considered a dysregulation of synaptic plasticity. In addition, dysfunction of the glutamate transporter 1 (GLT-1) in the nucleus accumbens (NAc) has also been proposed as a mechanism underlying drug addiction. However, the cellular and synaptic impact of GLT-1 alterations in the NAc remain unclear. Here we show in the NAc that 10 days withdraw after 5 days treatment with cocaine or amphetamine decreases GLT-1 expression in astrocytes, which results in the prolongation of the excitatory postsynaptic potential (EPSP) decay kinetics in D1 receptor-containing medium spiny neurons (D1R-MSNs). Using the spike timing dependent plasticity (STDP) paradigm, we found that enlargement of EPSP duration results in switching the LTP elicited in control animals to LTD in psychostimulant-treated mice. In contrast to D1-MSNs, D2-MSNs did not display changes in EPSP kinetics and synaptic plasticity. Notably, the psychostimulant-induced synaptic transmission and synaptic plasticity effects were absent in IP3R2-/- mice, which lack astrocyte calcium signal, but were mimicked by the selective astrocytes stimulation with DREADDs. Finally, ceftriaxone, which upregulates GLT-1, restored normal GLT-1 function, EPSP kinetics, and synaptic plasticity in psychostimulant-treated mice. Therefore, we propose that cocaine and amphetamine increase dopaminergic levels in the NAc, which stimulates astrocytes and downregulates the GLT-1. The decreased GLT-1 function prolonged the EPSP kinetics, leading to the modulation of the STDP, transforming the LTP observed in control animals into LTD in psychostimulant-treated mice. Present work reveals a novel mechanism underlying the synaptic plasticity changes induced by these drugs of abuse.

Astroglial CB1 Reveal Sex-Specific Synaptic Effects of Amphetamine

The Nucleus Accumbens (NAc) is a critical brain region for the effects of psychostimulant drugs. Type-1 cannabinoid receptors (CB1), the main elements of the endocannabinoid system (ECS) in the brain, participate in these effects and modulate synaptic functions in the NAc. Besides their neuronal expression, CB1 receptors are also present in astrocytes, where they contribute to the regulation of synaptic plasticity and behavior. However, the impact of astroglial CB1 receptors on synaptic plasticity in the NAc and on psychostimulant-induced synaptic and behavioral effects is currently unknown. This study shows that the psychostimulant amphetamine impairs a form of astroglial CB1 receptor-dependent synaptic plasticity in the NAc of male, but not female mice. Consistently, locomotor effects of amphetamine require astroglial CB1 receptors in male, but not female mice. These results, by revealing unforeseen mechanisms underlying sex-dependent effects of amphetamine, pave the way to a better understanding of the diverse impact of psychostimulants in women and men.

The Role of Astrocytes in the Molecular Pathophysiology of Schizophrenia: Between Neurodevelopment and Neurodegeneration

Schizophrenia is a chronic and severe psychiatric disorder affecting approximately 1% of the global population, characterized by disrupted synaptic plasticity and brain connectivity. While substantial evidence supports its classification as a neurodevelopmental disorder, non-canonical neurodegenerative features have also been reported, with increasing attention given to astrocytic dysfunction. Overall, in this study, we explore the role of astrocytes as a structural and functional link between neurodevelopment and neurodegeneration in schizophrenia. Specifically, we examine how astrocytes contribute to forming an aberrant substrate during early neurodevelopment, potentially predisposing individuals to later neurodegeneration. Astrocytes regulate neurotransmitter homeostasis and synaptic plasticity, influencing early vulnerability and disease progression through their involvement in Ca2⁺ signaling and dopamine-glutamate interaction-key pathways implicated in schizophrenia pathophysiology. Astrocytes differentiate via nuclear factor I-A, Sox9, and Notch pathways, occurring within a neuronal environment that may already be compromised in the early stages due to the genetic factors associated with the 'two-hits' model of schizophrenia. As a result, astrocytes may contribute to the development of an altered neural matrix, disrupting neuronal signaling, exacerbating the dopamine-glutamate imbalance, and causing excessive synaptic pruning and demyelination. These processes may underlie both the core symptoms of schizophrenia and the increased susceptibility to cognitive decline-clinically resembling neurodegeneration but driven by a distinct, poorly understood molecular substrate. Finally, astrocytes are emerging as potential pharmacological targets for antipsychotics such as clozapine, which may modulate their function by regulating glutamate clearance, redox balance, and synaptic remodeling.

High urea promotes mitochondrial fission and functional impairments in astrocytes inducing anxiety-like behavior in chronic kidney disease mice

High urea can induce depression and anxiety. Activation of astrocytes is closely associated with psychiatric disorders. However, the pathological mechanism of whether high urea affects astrocyte structure and function to induce anxiety-like behaviors remain unclear. We established a high-urea chronic kidney disease (CKD) mouse model and found that these mice exhibited elevated levels of anxiety through behavioral experiments. Immunofluorescence and transmission electron microscopy studies of astrocytes revealed a decrease in density and branching of mPFC astrocytes. Additionally, we observed a significant reduction in ATP and BDNF levels in the mPFC and primary astrocytes of CKD mice induced by high urea. Analysis of gene expression differences in astrocytes between WT and high-urea mice indicated alterations in mitochondrial dynamics-related signaling pathways in astrocytes. We established a high-urea primary astrocyte model to assess mitochondrial function and levels of fusion and fission proteins. Treatment of primary astrocytes with high urea led to mitochondrial fragmentation and downregulation of Mfn2 expression. These results suggested that high urea downregulates Mfn2 expression in mPFC astrocytes, induced mitochondrial fusion-fission abnormalities, disrupted astrocyte energy metabolism, and promoted high-urea-related anxiety. Mfn2 may represent a potential therapeutic target for high-urea-related anxiety.

Estradiol Mediates Astrocyte-Neuron Communication in the Hippocampus

Accumulating evidence has revealed the existence of functional astrocyte-neuron communication based on the ability of astrocytes to respond to neurotransmitters and release gliotransmitters. However, little is known about how other signaling molecules, such as hormones, impact astrocyte function. Estradiol (E2) is an important hormone known to regulate neuronal activity, synaptic transmission, plasticity, and animal behavior. However, whether E2 specifically signals to astrocytes in situ and the functional consequences on astrocyte-neuron communication remain unknown. Therefore, we investigated the impact of estradiol on astrocyte activity and astrocyte-neuron communication in the mouse hippocampus. Using an RNAscope approach, we determined that estrogen receptors (ERα and ERβ) are expressed in astrocytes in both female and male mice. In both sexes, confocal imaging of hippocampal slices determined that astrocytes respond to locally applied E2 with calcium elevations. In pyramidal neurons, slow inward currents (SICs) are mediated by the activation of extrasynaptic NMDA receptors and indicate gliotransmission. Electrophysiological recordings of hippocampal neurons determined that E2 increases the frequency, but not the amplitude, of SICs. We also recorded excitatory synaptic transmission evoked by Schaffer collateral stimulation. Here, only in females, did E2 produce a reduction in excitatory synaptic transmission. The E2-induced effects on the astrocyte calcium signal and gliotransmission were prevented by the broad estrogen receptor antagonist ICI 182,780. Taken together, these results demonstrate the existence of estradiol-mediated astrocyte-neuron communication in both female and male mice. They reveal that E2 can signal to astrocytes and, through this signaling, E2 may regulate neuronal activity and synaptic transmission.

Extracellular vesicles as precision therapeutics for psychiatric conditions: targeting interactions among neuronal, glial, and immune networks

The critical role of the immune system in brain function and dysfunction is well recognized, yet development of immune therapies for psychiatric diseases has been slow due to concerns about iatrogenic immune deficiencies. These concerns are emphasized by the lack of objective diagnostic tools in psychiatry. A promise to resolve this conundrum lies in the exploitation of extracellular vesicles (EVs) that are physiologically produced or can be synthetized. EVs regulate recipient cell functions and offer potential for EVs-based therapies. Intranasal EVs administration enables the targeting of specific brain regions and functions, thereby facilitating the design of precise treatments for psychiatric diseases. The development of such therapies requires navigating four dynamically interacting networks: neuronal, glial, immune, and EVs. These networks are profoundly influenced by brain fluid distribution. They are crucial for homeostasis, cellular functions, and intercellular communication. Fluid abnormalities, like edema or altered cerebrospinal fluid (CSF) dynamics, disrupt these networks, thereby negatively impacting brain health. A deeper understanding of the above-mentioned four dynamically interacting networks is vital for creating diagnostic biomarker panels to identify distinct patient subsets with similar neuro-behavioral symptoms. Testing the functional pathways of these biomarkers could lead to new therapeutic tools. Regulatory approval will depend on robust preclinical data reflecting progress in these interdisciplinary areas, which could pave the way for the design of innovative and precise treatments. Highly collaborative interdisciplinary teams will be needed to achieve these ambitious goals.

The Duality of Astrocyte Neuromodulation: Astrocytes Sense Neuromodulators and Are Neuromodulators

Neuromodulation encompasses different processes that regulate neuronal and network function. Classical neuromodulators originating from long-range nuclei, such as acetylcholine, norepinephrine, or dopamine, act with a slower time course and wider spatial range than fast synaptic transmission and action potential firing. Accumulating evidence in vivo indicates that astrocytes, which are known to actively participate in synaptic function at tripartite synapses, are also involved in neuromodulatory processes. The present article reviews recent findings obtained in vivo indicating that astrocytes express receptors for neuromodulators that elevate their internal calcium and stimulate the release of gliotransmitters, which regulate synaptic and network function, and hence mediate, at least partially, the effects of neuromodulators. In addition, we propose that astrocytes act in local support of neuromodulators by spatially and temporally integrating neuronal and neuromodulatory signals to regulate neural network function. The presence of astrocyte-neuron hysteresis loops suggests astrocyte-neuron interaction at tripartite synapses scales up to astrocyte-neuronal networks that modulate neural network function. We finally propose that astrocytes sense the environmental conditions, including neuromodulators and network function states, and provide homeostatic control that maximizes the dynamic range of neural network activity. In summary, we propose that astrocytes are critical in mediating the effects of neuromodulators, and they also act as neuromodulators to provide neural network homeostasis thus optimizing information processing in the brain. Hence, astrocytes sense ongoing neuronal activity along with neuromodulators and, acting as neuromodulators, inform the neurons about the state of the internal system and the external world.

Astrocytic GAT-3 Regulates Synaptic Transmission and Memory Formation in the Dentate Gyrus

GABAergic network activity plays a crucial role in a wide array of physiological processes and is implicated in various pathological conditions. While extensive research has been conducted on how GABAergic network activity modulates both excitatory and inhibitory synaptic transmission in the CA1 region, the mechanisms by which it influences synaptic transmission in the entorhinal cortex-dentate gyrus (EC-DG) circuits are still largely unexplored. Using a combination of whole-cell patch-clamp recordings, optogenetics, immunohistochemistry, and behavioral assays, we demonstrate that activation of GABA transporter 3 (GAT-3) in astrocytes triggers an increase in intracellular Ca2+ via the reverse Na+/Ca2+ exchanger. Intriguingly, inhibiting GAT-3 impedes the GABA-induced elevation of astrocytic Ca2+ levels, thereby curtailing the subsequent enhancement of synaptic transmission. Additionally, we show that endogenously released GABA from interneurons also modulates synaptic transmission through GAT-3 in the DG. Crucially, by selectively diminishing astrocytic calcium signals, we observed a concomitant decrease in the GABA-induced enhancement of synaptic transmission, underscoring the crucial role of astrocytes in this regulatory pathway. Moreover, we found that the activation of GAT-3 enhances excitatory transmission via presynaptic GluN2B-containing N-methyl-D-aspartate receptors (GluN2B-NMDARs) in the DG. Finally, our in vivo experiments demonstrate that inhibiting GAT-3 adversely affects the formation of contextual fear memory, highlighting its pivotal role in cognitive processing. These findings underscore the significance of astrocytic GAT-3 in cognitive functions and offer valuable insights into potential therapeutic targets for cognitive impairments, opening new avenues for the treatment of related disorders.

The Astrocytic Zinc Transporter ZIP12 Is a Synaptic Protein That Contributes to Synaptic Zinc Levels in the Mouse Auditory Cortex

Synaptically released zinc is a neuronal signaling system that arises from the actions of the presynaptic vesicular zinc transporter protein zinc transporter 3 (ZnT3). Mechanisms that regulate the actions of zinc at synapses are of great importance for many aspects of synaptic signaling in the brain. Here, we identify the astrocytic zinc transporter protein ZIP12 as a candidate mechanism that contributes to zinc clearance at cortical synapses. We identify small-molecule compounds that antagonize the function of ZIP12 in heterologous expression systems, and we use one of these compounds, ZIP12 modulator 8, to increase the concentration of ZnT3-dependent zinc at synapses in the brain of male and female mice to inhibit the activity of neuronal AMPA and NMDA glutamate receptors. These results identify a cellular mechanism and provide a pharmacological toolbox to target the molecular machinery that supports the actions of synaptic zinc in the brain.

Impact of Microglia-Derived Extracellular Vesicles on Resident Central Nervous System Cell Populations After Acute Brain Injury Under Various External Stimuli Conditions

Acute brain injuries (ABI) caused by various emergencies can lead to structural and functional damage to brain tissue. Common causes include traumatic brain injury, cerebral hemorrhage, ischemic stroke, and heat stroke. Globally, ABI represent a significant portion of neurosurgical cases. Previous studies have emphasized the significant therapeutic potential of stem cell-derived extracellular vesicles (EVs). Recent research indicates that EVs extracted from resident cells in the central nervous system (CNS) also show therapeutic potential following brain injury. Microglia, as innate immune cells of the CNS, respond to changes in the internal environment by altering their phenotype and secreting EVs that impact various CNS cells, including neurons, astrocytes, oligodendrocytes, endothelial cells, neural stem cells (NSCs), and microglia themselves. Notably, under different external stimuli, microglia can either promote neuronal survival, angiogenesis, and myelin regeneration while reducing glial scarring and inflammation, or they can exert opposite effects. This review summarizes and evaluates the current research findings on how microglia-derived EVs influence various CNS cells after ABI under different external stimuli. It analyzes the interaction mechanisms between EVs and resident CNS cells and discusses potential future research directions and clinical applications.

The night's watch: Exploring how sleep protects against neurodegeneration

Sleep loss is often regarded as an early manifestation of neurodegenerative diseases given its common occurrence and link to cognitive dysfunction. However, the precise mechanisms by which sleep disturbances contribute to neurodegeneration are not fully understood, nor is it clear why some individuals are more susceptible to these effects than others. This review addresses critical unanswered questions in the field, including whether sleep disturbances precede or result from neurodegenerative diseases, the functional significance of sleep changes during the preclinical disease phase, and the potential role of sleep homeostasis as an adaptive mechanism enhancing resilience against cognitive decline and neurodegeneration.

Astroglial TNFR2 signaling regulates hippocampal synaptic function and plasticity in a sex dependent manner

Astrocytes participate in synaptic transmission and plasticity through tightly regulated, bidirectional communication with pre- and post-synaptic neurons, as well as microglia and oligodendrocytes. A key component of astrocyte-mediated synaptic regulation is the cytokine tumor necrosis factor (TNF). TNF signals via two cognate receptors, TNFR1 and TNFR2, both expressed in astrocytes. While TNFR1 signaling in astrocytes has been long demonstrated to be necessary for physiological synaptic function, the role of astroglial TNFR2 has never been explored. Here, we demonstrate that astroglial TNFR2 is essential for maintaining hippocampal synaptic function and plasticity in physiological conditions. Indeed, Gfap creERT2 :Tnfrsf1b fl/fl mice with selective ablation of TNFR2 in astrocytes exhibited dysregulated expression of neuronal and glial proteins (e.g., SNARE complex molecules, glutamate receptor subunits, glutamate transporters) essential for hippocampal synaptic transmission and plasticity. Hippocampal astrocytes sorted from Gfap creERT2 :Tnfrsf1b fl/fl mice displayed downregulation of genes and pathways implicated in synaptic plasticity, as well as astrocyte-neuron and astrocyte-oligodendrocyte communication. These alterations were accompanied by increased glial reactivity and impaired astrocyte calcium dynamics, and ultimately translated into functional deficits, specifically impaired long-term potentiation (LTP) and cognitive functions. Notably, male Gfap creERT2 :Tnfrsf1b fl/fl mice exhibited more pronounced hippocampal synaptic and cellular alterations, suggesting sex-dependent differences in astroglial TNFR2 regulation of synaptic function. Together, these findings indicate that TNFR2 signaling in astrocytes is essential for proper astrocyte-neuron communication at the basis of synaptic function, and that this is regulated in a sex-dependent manner.

Mechanisms of developmental neurotoxicity mediated by perturbed thyroid hormone homeostasis in the brain: an adverse outcome pathway network

Thyroid hormone (TH) is crucial for proper neurodevelopment. Insufficient TH concentrations in early life are associated with lower IQ and delayed motor development in children. Intracellular levels of TH are modulated via the transmembrane transport of TH and intracellular deiodination, and can mediate gene transcription via binding to the nuclear TH receptor. Chemical exposure can disrupt TH homeostasis via modes of action targeting intracellular mechanisms, thereby potentially influencing TH transport, deiodination or signaling. Understanding the cause and effect relationships of chemical hazards interfering with TH homeostasis in the developing brain is necessary to identify how chemicals might disturb brain development and result in neurodevelopmental disorders. Adverse Outcome Pathways (AOPs) can provide a template for mapping these relationships, and so far multiple AOPs have been developed for TH homeostasis and adverse effects on cognition. The present review aims to expand current AOP networks by (1) summarizing the most important factors in the regulation of brain development under influence of TH, (2) integrating human-based mechanistic information of biological pathways which can be disturbed by TH disrupting chemicals, and (3) by incorporating brain-specific TH-mediated physiology, including barriers and cell specificity, as well as clinical knowledge. TH-specific pathways in the fetal brain are highlighted and supported by distinguishing cell type specific Molecular Initiating Events (MIEs) and downstream Key Events (KEs) for astrocytes, neurons and oligodendrocytes. Two main pathways leading to adverse outcomes (AOs) in the areas of 'cognition' and 'motor function' are decreased myelination due to oligodendrocyte dysfunction, and decreased synaptogenesis and network formation via the neurons. The proposed AOP framework can form a basis for selecting developmental neurotoxic in vitro and in silico test systems for an innovative human-focused hazard testing strategy and risk assessment of chemical exposure.

Molecular signature of primate astrocytes reveals pathways and regulatory changes contributing to human brain evolution

Astrocytes contribute to the development and regulation of the higher-level functions of the brain, the critical targets of evolution. However, how astrocytes evolve in primates is unsettled. Here, we obtain human, chimpanzee, and macaque induced pluripotent stem-cell-derived astrocytes (iAstrocytes). Human iAstrocytes are bigger and more complex than the non-human primate iAstrocytes. We identify new loci contributing to the increased human astrocyte. We show that genes and pathways implicated in long-range intercellular signaling are activated in the human iAstrocytes and partake in controlling iAstrocyte complexity. Genes downregulated in human iAstrocytes frequently relate to neurological disorders and were decreased in adult brain samples. Through regulome analysis and machine learning, we uncover that functional activation of enhancers coincides with a previously unappreciated, pervasive gain of "stripe" transcription factor binding sites. Altogether, we reveal the transcriptomic signature of primate astrocyte evolution and a mechanism driving the acquisition of the regulatory potential of enhancers.

The Role of Complexity Theory in Understanding Brain's Neuron-Glia Interactions

Brain information processing complexity is conventionally recognized as derived from neuronal activity, with neurons and their dynamic signalling responsible for the transfer and processing of information. However, the brain also contains other non-neuronal cells, glial cells, which exceed the number of neurons and are involved in the processes related with information coding by neural networks and underlying brain functions. Decisive advances in the characterization of the molecular and physiological properties of glial cells shed light on their active roles in neurotransmission and neuronal physiopathology. This expanded relationship between neurons and glia challenges traditional neurobiology by highlighting their reciprocal influence, where it is difficult to determine whether neuronal or glial processes initiate and drive the interactions. This interplay creates a dilemma, where the causal hierarchy between these two cell types remains unresolved. A philosophical tool, the 'Theory of Complexity' of Edgard Morin can help to better explain and study the complexity of neuron-glia interactions. Morin's proposal on complexity is useful to transform brain knowledge, in order to review the brain molecular functions in antireductionist pattern. In this manuscript, we will discuss how to use the 'retroactive loop' principle from Morin's 'Theory of Complexity' at the brain molecular level, proposing a new philosophical-experimental grid that can help neuroscientists for a better understanding of the glia-neuron interactions in the brain.

Adenosine and Cortical Plasticity

Brain plasticity is the ability of the nervous system to change its structure and functioning in response to experiences. These changes occur mainly at synaptic connections, and this plasticity is named synaptic plasticity. During postnatal development, environmental influences trigger changes in synaptic plasticity that will play a crucial role in the formation and refinement of brain circuits and their functions in adulthood. One of the greatest challenges of present neuroscience is to try to explain how synaptic connections change and cortical maps are formed and modified to generate the most suitable adaptive behavior after different external stimuli. Adenosine is emerging as a key player in these plastic changes at different brain areas. Here, we review the current knowledge of the mechanisms responsible for the induction and duration of synaptic plasticity at different postnatal brain development stages in which adenosine, probably released by astrocytes, directly participates in the induction of long-term synaptic plasticity and in the control of the duration of plasticity windows at different cortical synapses. In addition, we comment on the role of the different adenosine receptors in brain diseases and on the potential therapeutic effects of acting via adenosine receptors.

Cerebellar Bergmann glia integrate noxious information and modulate nocifensive behaviors

The cerebellum is activated by noxious stimuli and pathological pain but its role in noxious information processing remains unknown. Here, we show that in mice, cutaneous noxious electrical stimuli induced noradrenaline (NA) release from locus coeruleus (LC) terminals in the cerebellar cortex. Bergmann glia (BG) accumulated these LC-NA signals by increasing intracellular calcium in an integrative manner ('flares'). BG flares were also elicited in response to an intraplantar capsaicin injection. Chemogenetic inactivation of LC terminals or BG in the cerebellar cortex or BG-specific knockdown of α1-adrenergic receptors suppressed BG flares, reduced nocifensive licking and had analgesic effects in nerve injury-induced chronic neuropathic pain. Moreover, chemogenetic activation of BG or an intraplantar capsaicin injection reduced Purkinje cell firing, which may disinhibit the output activity of the deep cerebellar nuclei. These results suggest a role for BG in computing noxious information from the LC and in modulating pain-related behaviors by regulating cerebellar output.

Deletion of murine astrocytic vesicular nucleotide transporter increases anxiety and depressive-like behavior and attenuates motivation for reward

Astrocytes are multi-functional glial cells in the central nervous system that play critical roles in modulation of metabolism, extracellular ion and neurotransmitter levels, and synaptic plasticity. Astrocyte-derived signaling molecules mediate many of these modulatory functions of astrocytes, including vesicular release of ATP. In the present study, we used a unique genetic mouse model to investigate the functional significance of astrocytic exocytosis of ATP. Using primary cultured astrocytes, we show that loss of vesicular nucleotide transporter (Vnut), a primary transporter responsible for loading cytosolic ATP into the secretory vesicles, dramatically reduces ATP loading into secretory lysosomes and ATP release, without any change in the molecular machinery of exocytosis or total intracellular ATP content. Deletion of astrocytic Vnut in adult mice leads to increased anxiety, depressive-like behaviors, and decreased motivation for reward, especially in females, without significant impact on food intake, systemic glucose metabolism, cognition, or sociability. These behavioral alterations are associated with significant decreases in the basal extracellular dopamine levels in the nucleus accumbens. Likewise, ex vivo brain slices from these mice show a strong trend toward a reduction in evoked dopamine release in the nucleus accumbens. Mechanistically, the reduced dopamine signaling we observed is likely due to an increased expression of monoamine oxidases. Together, these data demonstrate a key modulatory role of astrocytic exocytosis of ATP in anxiety, depressive-like behavior, and motivation for reward, by regulating the mesolimbic dopamine circuitry.

Neuron-Astrocyte Interactions and Circadian Timekeeping in Mammals

Almost every facet of our behavior and physiology varies predictably over the course of day and night, anticipating and adapting us to their associated opportunities and challenges. These rhythms are driven by endogenous biological clocks that, when deprived of environmental cues, can continue to oscillate within a period of approximately 1 day, hence circa-dian. Normally, retinal signals synchronize them to the cycle of light and darkness, but disruption of circadian organization, a common feature of modern lifestyles, carries considerable costs to health. Circadian timekeeping pivots around a cell-autonomous molecular clock, widely expressed across tissues. These cellular timers are in turn synchronized by the principal circadian clock of the brain: the hypothalamic suprachiasmatic nucleus (SCN). Intercellular signals make the SCN network a very powerful pacemaker. Previously, neurons were considered the sole SCN timekeepers, with glial cells playing supportive roles. New discoveries have revealed, however, that astrocytes are active partners in SCN network timekeeping, with their cell-autonomous clock regulating extracellular glutamate and GABA concentrations to control circadian cycles of SCN neuronal activity. Here, we introduce circadian timekeeping at the cellular and SCN network levels before focusing on the contributions of astrocytes and their mutual interaction with neurons in circadian control in the brain.

Calcium-Dependent Signaling in Astrocytes: Downstream Mechanisms and Implications for Cognition

Astrocytes are glial cells recognized for their diverse roles in regulating brain circuit structure and function. They can sense and adapt to changes in the microenvironment due to their unique structural and biochemical properties. A key aspect of astrocytic function involves calcium (Ca2+)-dependent signaling, which serves as a fundamental mechanism for their interactions with neurons and other cells in the brain. However, while significant progress has been made in understanding the spatio-temporal properties of astrocytic Ca2+ signals, the downstream molecular pathways and exact mechanisms through which astrocytes decode these signals to regulate homeostatic and physiological processes remain poorly understood. To address this topic, we review here the available literature on the sources of intracellular Ca2+, as well as its downstream mechanisms and signaling pathways. We review the well-studied Ca2+-dependent exocytosis but draw attention to additional intracellular Ca2+-dependent mechanisms that are less understood and are, most likely, highly influential for many other cellular functions. Finally, we review how intracellular Ca2+ is thought to underlie neuron-astrocyte signaling in brain regions involved in cognitive processing.

Ca2+ excitability of glia to neuromodulator octopamine in Drosophila living brain is greater than that of neurons

AIM

Octopamine in the Drosophila brain has a neuromodulatory role similar to that of noradrenaline in mammals. After release from Tdc2 neurons, octopamine/tyramine may trigger intracellular Ca2+ signaling via adrenoceptor-like receptors on neural cells, modulating neurotransmission. Octopamine/tyramine receptors are expressed in neurons and glia, but how each of these cell types responds to octopamine remains elusive. This study aimed to characterize Ca2+ responses of neurons and astrocytes to neuromodulatory octopamine signals.

METHODS

We expressed Ca2+ indicator jGCaMP7b in specific cell type in adult Drosophila brains and performed intracellular Ca2+ imaging in the brain optic lobes upon bath application of octopamine by confocal microscopy.

RESULTS

Octopamine-stimulated Ca2+ responses in neurons were different from those of glial cells. The amplitude of octopamine-mediated Ca2+ signals in neurons was 3.4-fold greater than in astrocytes. However, astrocytes were more sensitive to octopamine; the median effective concentration that triggered Ca2+ responses was nearly 6-fold lower in astrocytes than in neurons. In both cell types, Ca2+ transients are shaped by Gq and Gs protein-coupled octopamine/tyramine receptors. Our snRNA-seq database screening uncovered differential expression patterns of these receptors between brain cell types, which may explain the difference in Ca2+ signaling.

CONCLUSION

In the brain optic lobes, astrocytes, not neurons, appear to be the sole responders to low concentration octopamine signals, and therefore likely drive synaptic plasticity and visual processing. Given the interconnectivity of the optic lobes with other brain regions, octopaminergic signals acting through the optic lobe astrocytes may also influence higher-order brain functions including learning and memory.

In Vivo Assessment of Cortical Astrocyte Network Dysfunction During Autoimmune Demyelination: Correlation With Disease Severity

Cortical damage and dysfunction is a pathological hallmark of multiple sclerosis (MS) that correlates with the severity of physical and cognitive disability. Astrocytes participate in MS pathobiology through a variety of mechanisms, and abnormal astrocytic calcium signaling has been pointed as a pathogenic mechanism of cortical dysfunction in MS. However, in vivo evidence supporting deregulation of astrocyte calcium-dependent mechanisms in cortical MS is still limited. Here, we applied fiber photometry to the longitudinal analysis of spontaneous and sensory-evoked astrocyte network activity in the somatosensory cortex of mice in an experimental autoimmune encephalomyelitis (EAE). We found that freely moving EAE mice exhibit spontaneously occurring astrocyte calcium signals of increased duration and reduced amplitude. Concomitantly, cortical astrocytes in EAE mice responded to sensory stimulation with calcium events of decreased amplitude. The emergence of aberrant astrocyte calcium signals in the somatosensory cortex paralleled the onset of neurological symptomatology, and changes in the amplitude of both spontaneous and evoked responses were selectively correlated to the severity of neurological deficits. These results highlight the imbalance of astrocyte network activity in the brain cortex during autoimmune inflammation and further support the relevance of astrocyte-based pathobiology as an underlying mechanism of cortical dysfunction in MS.

Perisynaptic Schwann Cells: Guardians of Neuromuscular Junction Integrity and Function in Health and Disease

The neuromuscular junction (NMJ) is a highly reliable synapse to carry the control of the motor commands of the nervous system over the muscles. Its development, organization, and synaptic properties are highly structured and regulated to support such reliability and efficacy. Yet, the NMJ is also highly plastic, able to react to injury, and able to adapt to changes. This balance between structural stability and synaptic efficacy on one hand and structural plasticity and repair on another hand is made possible by perisynaptic Schwann cells (PSCs), glial cells at this synapse. They regulate synaptic efficacy and structural plasticity of the NMJ in a dynamic, bidirectional manner owing to their ability to decode synaptic transmission and by their interactions with trophic-related factors. Alteration of these fundamental roles of PSCs is also important in the maladapted response of NMJs in various diseases and in aging.

Astrocytes phenomics as new druggable targets in healthy aging and Alzheimer's disease progression

For over a century after their discovery astrocytes were regarded merely as cells located among other brain cells to hold and give support to neurons. Astrocytes activation, "astrocytosis" or A1 functional state, was considered a detrimental mechanism against neuronal survival. Recently, the scientific view on astrocytes has changed. Accumulating evidence indicate that astrocytes are not homogeneous, but rather encompass heterogeneous subpopulations of cells that differ from each other in terms of transcriptomics, molecular signature, function and response in physiological and pathological conditions. In this review, we report and discuss the recent literature on the phenomic differences of astrocytes in health and their modifications in disease conditions, focusing mainly on the hippocampus, a region involved in learning and memory encoding, in the age-related memory impairments, and in Alzheimer's disease (AD) dementia. The morphological and functional heterogeneity of astrocytes in different brain regions may be related to their different housekeeping functions. Astrocytes that express diverse transcriptomics and phenomics are present in strictly correlated brain regions and they are likely responsible for interactions essential for the formation of the specialized neural circuits that drive complex behaviors. In the contiguous and interconnected hippocampal areas CA1 and CA3, astrocytes show different, finely regulated, and region-specific heterogeneity. Heterogeneous astrocytes have specific activities in the healthy brain, and respond differently to physiological or pathological stimuli, such as inflammaging present in normal brain aging or beta-amyloid-dependent neuroinflammation typical of AD. To become reactive, astrocytes undergo transcriptional, functional, and morphological changes that transform them into cells with different properties and functions. Alterations of astrocytes affect the neurovascular unit, the blood-brain barrier and reverberate to other brain cell populations, favoring or dysregulating their activities. It will be of great interest to understand whether the differential phenomics of astrocytes in health and disease can explain the diverse vulnerability of the hippocampal areas to aging or to different damaging insults, in order to find new astrocyte-targeted therapies that might prevent or treat neurodegenerative disorders.

Astrocytic G Protein-Coupled Receptors in Drug Addiction

Understanding the cellular mechanisms of drug addiction remains a key task in current brain research. While neuron-based mechanisms have been extensively explored over the past three decades, recent evidence indicates a critical involvement of astrocytes, the main type of non-neuronal cells in the brain. In response to extracellular stimuli, astrocytes modulate the activity of neurons, synaptic transmission, and neural network properties, collectively influencing brain function. G protein-coupled receptors (GPCRs) expressed on astrocyte surfaces respond to neuron- and environment-derived ligands by activating or inhibiting astrocytic signaling, which in turn regulates adjacent neurons and their circuitry. In this review, we focus on the dopamine D1 receptors (D1R) and metabotropic glutamate receptor 5 (mGLUR5 or GRM5)-two GPCRs that have been critically implicated in the acquisition and maintenance of addiction-related behaviors. Positioned as an introductory-level review, this article briefly discusses astrocyte biology, outlines earlier discoveries about the role of astrocytes in substance-use disorders (SUDs), and provides detailed discussion about astrocytic D1Rs and mGLUR5s in regulating synapse and network functions in the nucleus accumbens (NAc)-a brain region that mediates addiction-related emotional and motivational responses. This review serves as a stepping stone for readers of Engineering to explore links between astrocytic GPCRs and drug addiction and other psychiatric disorders.

Multifarious astrocyte-neuron dialog in shaping neural circuit architecture

Astrocytes are multifaceted glial cell types that perform structural, functional, metabolic, and homeostatic roles in the brain. Recent studies have revealed mechanisms underlying the diversity of bidirectional communication modes between astrocytes and neurons - the fundamental organizing principle shaping synaptic properties at tripartite synapses. These astrocyte-neuron interactions are critical for the proper functioning of synapses and neural circuits. This review focuses on molecular mechanisms that direct these interactions, highlighting the versatile roles of multiple adhesion-based paths that likely modulate them, often in a context-dependent manner. It also describes how astrocyte-mediated processes go awry in certain brain disorders and provides a timely insight on the pivotal roles of astrocyte-neuron interactions in synaptic integrity and their relevance to understanding and treating neurological disorders.

Neuroglia in anxiety disorders

Anxiety disorders are some of the most prevalent in the world and are extraordinarily debilitating to many individuals, costing millions in disability. One of the most disabling is posttraumatic stress disorder (Snijders et al., 2020). Understanding the pathophysiology of these illnesses further and the cell types involved will allow better targeting of treatments. Glial cells, encompassing microglia, astrocytes, and oligodendrocytes, play critical roles in the pathophysiology of PTSD and other anxiety illnesses. Each of these cell types interacts with aspects of neuro-epigenetics, neuroimmune, and neuronal signaling and may contribute to the pathophysiology of anxiety illnesses. This chapter covers the literature on the role of glial cells in the neurobiology and pathology of anxiety disorders, more specifically PTSD. PTSD is one of the most debilitating anxiety disorders and one of the most complicated from a neurobiologic perspective. This chapter also features a discussion surrounding the current state of treatment and some of the hypothesized mechanisms for novel treatments including tetrahydrocannabidiol and 3,4-methylenedioxymethamphetamine. Finally, thoughts on the future directions for precision treatment and pharmacologic development with a focus on neuroglia are undertaken.

An α7 nicotinic and GABAB receptor-mediated pathway controls acetylcholine release in the tripartite neuromuscular junction

Terminal Schwann cells (TSCs) are capable of regulating acetylcholine (ACh) release at the neuromuscular junction (NMJ). We have identified GABA as a gliotransmitter at mouse NMJs. When ACh activates α7 nicotinic ACh receptor (nAChRs) on TSCs, GABA is released and activates GABAB receptors on the nerve terminal that subsequently reduce ACh release. Indeed, specific deletion of the α7 nAChR in TSCs or inhibition of the metabotropic GABAB receptor prevents the reduction in the quantal content of the end-plate potential induced by cholinesterase inhibitors. The α7/GABAB receptor-mediated pathway is activated when ACh that escapes from collagen Q (ColQ) anchored AChE in the synaptic cleft and from PRiMA-anchored butyrylcholinesterase on the TSC activates α7 nAChRs on the TSC. Consequently, prolonged tetanic stimulation of isolated muscle activates the α7/GABAB receptor pathway, which reduces post-tetanic ACh release. When AChE levels are low in neonatal mice, the α7/GABAB receptor-mediated pathway decreases ACh release and reduces ex vivo muscle fatigue. For ColQ-deficient mice where AChE is not clustered, the decrease in AСh release following activation of this pathway contributes to mouse fatigue in vivo. KEY POINTS: Acetylcholine (ACh) released from the nerve terminal at the neuromuscular junction (NMJ) can activate α7 nicotinic ACh receptor (nAChR) on terminal Schwann cells, releasing gamma-aminobutyric acid (GABA) that activates metabotropic GABAB receptors on the nerve terminal which then reduces further ACh release from the nerve. At the mature NMJ, before reaching α7 nAChRs on terminal Schwann cells ACh is normally hydrolyzed by AChE clustered in the synaptic cleft and by BChE anchored to the TSC. ACh can activate the α7/GABAB receptor-mediated pathway and depress subsequent ACh release when AChE at the NMJ is low, either during development or in congenital myasthenic syndrome. In the latter case, this pathway contributes to muscle fatigue.

Neuroglia and extracellular matrix molecules

This chapter provides a comprehensive overview of the roles of astrocytes, microglia, and the extracellular matrix (ECM) in regulating neuroplasticity and maintaining brain homeostasis. Astrocytes provide essential metabolic support to neurons, regulate synapse development, support neuroplasticity mechanisms, and modulate neurotransmission. Microglia, the resident immune cells of the brain, play a critical role in neuroinflammatory responses and homeostatic brain regulation by modulating synapse formation and pruning. The extracellular space (ECS) mediates intercellular interactions, provides a highly regulated environment for intercellular communication, and is filled with ECM molecules. Proteoglycans and polysaccharides of the ECM play a vital role not only in brain development but also in brain function throughout life. In the injured brain, neuroplasticity and regeneration can be bidirectionally regulated as a result of the interplay between ECM, astrocytes, and microglia. The modulation of synaptic strength, structural remodeling, and modification of intrinsic neuronal properties are among the central mechanisms that contribute to neuronal plasticity in health and disease. We believe that the understanding of ECM-glia interactions and their role in neuroplasticity regulation is key to the development of novel therapeutic strategies in neurologic disorders.

Extracellular ATP Is a Homeostatic Messenger That Mediates Cell-Cell Communication in Physiological Processes and Psychiatric Diseases

Neuronal activity is the basis of information encoding and processing in the brain. During neuronal activation, intracellular ATP (adenosine triphosphate) is generated to meet the high-energy demands. Simultaneously, ATP is secreted, increasing the extracellular ATP concentration and acting as a homeostatic messenger that mediates cell-cell communication to prevent aberrant hyperexcitability of the nervous system. In addition to the confined release and fast synaptic signaling of classic neurotransmitters within synaptic clefts, ATP can be released by all brain cells, diffuses widely, and targets different types of purinergic receptors on neurons and glial cells, making it possible to orchestrate brain neuronal activity and participate in various physiological processes, such as sleep and wakefulness, learning and memory, and feeding. Dysregulation of extracellular ATP leads to a destabilizing effect on the neural network, as found in the etiopathology of many psychiatric diseases, including depression, anxiety, schizophrenia, and autism spectrum disorder. In this review, we summarize advances in the understanding of the mechanisms by which extracellular ATP serves as an intercellular signaling molecule to regulate neural activity, with a focus on how it maintains the homeostasis of neural networks. In particular, we also focus on neural activity issues that result from dysregulation of extracellular ATP and propose that aberrant levels of extracellular ATP may play a role in the etiopathology of some psychiatric diseases, highlighting the potential therapeutic targets of ATP signaling in the treatment of these psychiatric diseases. Finally, we suggest potential avenues to further elucidate the role of extracellular ATP in intercellular communication and psychiatric diseases.

Situation-Based Neuromorphic Memory in Spiking Neuron-Astrocyte Network

Mammalian brains operate in very special surroundings: to survive they have to react quickly and effectively to the pool of stimuli patterns previously recognized as danger. Many learning tasks often encountered by living organisms involve a specific set-up centered around a relatively small set of patterns presented in a particular environment. For example, at a party, people recognize friends immediately, without deep analysis, just by seeing a fragment of their clothes. This set-up with reduced "ontology" is referred to as a "situation." Situations are usually local in space and time. In this work, we propose that neuron-astrocyte networks provide a network topology that is effectively adapted to accommodate situation-based memory. In order to illustrate this, we numerically simulate and analyze a well-established model of a neuron-astrocyte network, which is subjected to stimuli conforming to the situation-driven environment. Three pools of stimuli patterns are considered: external patterns, patterns from the situation associative pool regularly presented to the network and learned by the network, and patterns already learned and remembered by astrocytes. Patterns from the external world are added to and removed from the associative pool. Then, we show that astrocytes are structurally necessary for an effective function in such a learning and testing set-up. To demonstrate this we present a novel neuromorphic computational model for short-term memory implemented by a two-net spiking neural-astrocytic network. Our results show that such a system tested on synthesized data with selective astrocyte-induced modulation of neuronal activity provides an enhancement of retrieval quality in comparison to standard spiking neural networks trained via Hebbian plasticity only. We argue that the proposed set-up may offer a new way to analyze, model, and understand neuromorphic artificial intelligence systems.

Non-ionotropic NMDAR signalling activates Panx1 to induce P2X4R-dependent long-term depression in the hippocampus

In recent years, evidence supporting non-ionotropic signalling by the NMDA receptor (niNMDAR) has emerged, including roles in long-term depression (LTD). Here, we investigated whether niNMDAR-pannexin-1 (Panx1) contributes to LTD at the CA3-CA1 hippocampal synapse. Using whole-cell, patch clamp electrophysiology in rat hippocampal slices, we show that a low-frequency stimulation (3 Hz) of the Schaffer collaterals produces LTD that is blocked by continuous but not transient application of the NMDAR competitive antagonist, MK-801. After transient MK-801, LTD involved pannexin-1 and sarcoma (Src) kinase. We show that pannexin-1 is not permeable to Ca2+, but probably releases ATP to induce LTD via P2X4 purinergic receptors because LTD after transient MK-801 application was prevented by 5-BDBD. Thus, we conclude that niNMDAR activation of Panx1 can link glutamatergic and purinergic pathways to produce LTD following low frequency synaptic stimulation when NMDARs are transiently inhibited. KEY POINTS: Differential effect of short-term D-APV and MK-801 application on long-term depression (LTD) suggests that the NMDA receptor (niNMDAR) contributes to later phases of synaptic depression. niNMDAR LTD involved sarcoma (Src) kinase and pannexin-1 (Panx1), which is a pathway previously identified to be active during excitotoxicity. Panx1 was not calcium permeable but may contribute to late phase LTD via ATP release. Panx1 blockers prevent LTD, and this was rescued with exogenous ATP application. Inhibition of LTD with 5-BDBD suggests the downstream involvement of postsynaptic P2X4 receptors.

Research progress on astrocyte-derived extracellular vesicles in the pathogenesis and treatment of neurodegenerative diseases

Neurodegenerative disorders, including Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and Huntington's disease (HD), pose significant global health risks and represent a substantial public health concern in the contemporary era. A primary factor in the pathophysiology of these disorders is aberrant accumulation and aggregation of pathogenic proteins within the brain and spinal cord. Recent investigations have identified extracellular vesicles (EVs) in the central nervous system (CNS) as potential carriers for intercellular transport of misfolded proteins associated with neurodegenerative diseases. EVs are involved in pathological processes that contribute to various brain disorders including neurodegenerative disorders. Proteins linked to neurodegenerative disorders are secreted and distributed from cell to cell via EVs, serving as a mechanism for direct intercellular communication through the transfer of biomolecules. Astrocytes, as active participants in CNS intercellular communication, release astrocyte-derived extracellular vesicles (ADEVs) that are capable of interacting with diverse target cells. This review primarily focuses on the involvement of ADEVs in the development of neurological disorders and explores their potential dual roles - both advantageous and disadvantageous in the context of neurological disorders. Furthermore, this review examines the current studies investigating ADEVs as potential biomarkers for the diagnosis and treatment of neurodegenerative diseases. The prospects and challenges associated with the application of ADEVs in clinical settings were also comprehensively reviewed.

A spatial threshold for astrocyte calcium surge

Astrocytes are active cells involved in brain function through the bidirectional communication with neurons, in which astrocyte calcium plays a crucial role. Synaptically evoked calcium increases can be localized to independent subcellular domains or expand to the entire cell, i.e., calcium surge. Because a single astrocyte may contact ~100,000 synapses, the control of the intracellular calcium signal propagation may have relevant consequences on brain function. Yet, the properties governing the spatial dynamics of astrocyte calcium remains poorly defined. Imaging subcellular responses of cortical astrocytes to sensory stimulation in mice, we show that sensory-evoked astrocyte calcium responses originated and remained localized in domains of the astrocytic arborization, but eventually propagated to the entire cell if a spatial threshold of >23% of the arborization being activated was surpassed. Using Itpr2-/- mice, we found that type-2 IP3 receptors were necessary for the generation of astrocyte calcium surge. We finally show using in situ electrophysiological recordings that the spatial threshold of the astrocyte calcium signal consequently determined the gliotransmitter release. Present results reveal a fundamental property of astrocyte physiology, i.e., a spatial threshold for astrocyte calcium propagation, which depends on astrocyte intrinsic properties and governs astrocyte integration of local synaptic activity and subsequent neuromodulation.

Activation of Bradykinin B2 Receptors in Astrocytes Stimulates the Release of Leukemia Inhibitory Factor for Autocrine and Paracrine Signaling

Communications between different cell types within a tissue are often critical for the proper functioning of an organ. In the central nervous system, interactions among neurons and glial cells are known to modulate neurotransmission, energy metabolism, extracellular ion homeostasis, and neuroprotection. Here we showed that bradykinin, a proinflammatory neuropeptide, can be detected by astrocytes, resulting in the secretion of cytokines that act on neurons. In astrocytic cell lines and primary astrocytes, bradykinin and several other ligands acting on Gq-coupled receptors stimulated Ca2+ mobilization, which subsequently led to the release of leukemia inhibitory factor (LIF) and interleukin-6 (IL-6). The bradykinin B2 receptor antagonist, HOE-140, effectively blocked the ability of bradykinin to mobilize Ca2+ and stimulate mitogen-activated protein kinases (MAPKs) in astrocytes. Interestingly, incubation of neuronal cell lines and primary cortical neurons with conditioned media from bradykinin-treated astrocytes resulted in the activation of STAT3, a key component downstream of LIF and IL-6 receptors. LIF was apparently the major active factor in the conditioned media as the STAT3 response was almost completely neutralized by an anti-LIF antiserum. The presence of kininogen and kallikrein transcripts in neuronal cells but not in astrocytic cells indicates that neurons can produce bradykinin. Correspondingly, conditioned media from neuronal cells stimulated MAPKs in astrocytes in a HOE-140-sensitive manner. These studies demonstrate that paracrine signaling between neurons and astrocytes may involve ligands of Gq-coupled receptors and cytokines such as LIF.

Glial Regulation of Circuit Wiring, Firing, and Expiring in the Drosophila Central Nervous System

Molecular genetic approaches in small model organisms like Drosophila have helped to elucidate fundamental principles of neuronal cell biology. Much less is understood about glial cells, although interest in using invertebrate preparations to define their in vivo functions has increased significantly in recent years. This review focuses on our current understanding of the three major neuron-associated glial cell types found in the Drosophila central nervous system (CNS)-astrocytes, cortex glia, and ensheathing glia. Together, these cells act like mammalian astrocytes and microglia; they associate closely with neurons including surrounding neuronal cell bodies and proximal neurites, regulate synapses, and engulf neuronal debris. Exciting recent work has shown critical roles for these CNS glial cells in neural circuit formation, function, plasticity, and pathology. As we gain a more firm molecular and cellular understanding of how Drosophila CNS glial cells interact with neurons, it is clear that they share significant molecular and functional attributes with mammalian glia and will serve as an excellent platform for mechanistic studies of glial function.