Local Efficacy of Glutamate Uptake Decreases with Synapse Size

Atypical plume-like events contribute to glutamate accumulation in metabolic stress conditions

Neural glutamate homeostasis is important for health and disease. Ischemic conditions, like stroke, cause imbalances in glutamate release and uptake due to energy depletion and depolarization. We here used the glutamate sensor SF-iGluSnFR(A184V) to probe how chemical ischemia affects the extracellular glutamate dynamics in slice cultures from mouse cortex. SF-iGluSnFR imaging showed spontaneous glutamate release indicating synchronous network activity, similar to calcium imaging with GCaMP6f. Glutamate imaging further revealed local, atypically large, and long-lasting plume-like release events. Plumes occurred with low frequency, independent of network activity, and persisted in tetrodotoxin (TTX). Blocking glutamate uptake with TFB-TBOA favored plumes, whereas blocking ionotropic glutamate receptors (iGluRs) suppressed plumes. During chemical ischemia plumes became more pronounced, overly abundant and contributed to large-scale glutamate accumulation. Similar plumes were previously observed in cortical spreading depression and migraine models, and they may thus be a more general consequence of glutamate uptake dysfunctions in neurological and neurodegenerative diseases.

Astrocyte-synapse structural plasticity in neurodegenerative and neuropsychiatric diseases

Synaptic dysfunction is a common feature across a broad spectrum of brain diseases, spanning from psychopathologies such as post-traumatic stress disorder (PTSD) and substance use disorders (SUD) to neurodegenerative diseases like Alzheimer's and Parkinson's disease (AD and PD). While neuroscience research aiming to understand the mechanisms underlying synaptic dysfunction has traditionally focused on the neuronal elements of the synapse, recent research increasingly acknowledges the contribution of astrocytes as a third element controlling synaptic transmission. This also sparked interest to investigate the tripartite synapse and its role in the etiology of neurological diseases. According to recent evidence, changes in the structural interaction between astrocytes and synapses not only play a pivotal role in modulating synaptic function and behavioral states, but are also implicated in the initiation and progression of various brain diseases. This review aims to integrate recent findings that provide insight into the molecular mechanisms underpinning astrocytic structural changes at the synapse. We offer a comprehensive discussion of the potential implications of compromised astrocyte-synapse interactions, and put forward that astrocytic synaptic coverage is generally reduced in numerous neurological disorders, with the extent of it being disease- and stage- specific. Finally, we propose outstanding questions on astrocyte-synapse structural plasticity that are relevant for future therapeutic strategies to tackle neurodegenerative and neuropsychiatric diseases.

The IP3R2 Knockout Mice in Behavior: A Blessing or a Curse?

The inositol 1,4,5-triphosphate receptor type 2 (IP3R2) plays a critical role in intracellular calcium (Ca2+) signaling, particularly in astrocytes, where it mediates Ca2+ release from the endoplasmic reticulum. This mechanism is vital for astrocytic modulation of neuronal networks, impacting synaptic transmission and broader neural circuit functions. The IP3R2 knockout (IP3R2KO) mouse model has been instrumental in unraveling the nuances of astrocytic somatic Ca2+ dynamics and their implications for brain function. Despite early findings suggesting no significant behavioral or synaptic transmission changes in IP3R2KO mice, further research highlights the model's benefit in exploring cognitive, emotional, and neurodevelopmental processes. IP3R2KO mice revealed key insights into astrocytic Ca2+ signaling diversity, encompassing bulk somatic events and localized microdomain responses, which exhibit temporal and spatial variability. These animals retain alternative Ca2+ mechanisms, likely explaining the absence of severe phenotypes in some contexts. Nevertheless, IP3R2KO mice exhibit impairments in long-term memory retention, working memory, and fear memory, alongside age-related preservation of spatial memory, linking astrocytic IP3R2 signaling to higher-order cognitive functions. Additionally, studies suggest a connection between IP3R2 pathways and depression-like behaviors, with alterations in Brain-Derived Neurotrophic Factor (BDNF) levels and GABAergic signaling, highlighting its relevance to psychiatric conditions. Despite its limitations, such as residual astrocytic Ca2+ activity and inconsistent findings, the IP3R2KO model remains a valuable tool for studying astrocytic contributions to synaptic plasticity and brain function. This underscores the importance of integrating, rather than dismissing, the IP3R2KO model in the development of new methodologies for studying astrocytic Ca2+ dynamics. The use of this model will continue to elucidate the complex interplay between astrocytes and neuronal circuits, fostering advances in understanding astrocytic Ca2+ signaling's role in health and disease.

Loss of Bmal1 impairs the glutamatergic light input to the SCN in mice

Introduction

Glutamate represents the dominant neurotransmitter that conveys the light information to the brain, including the suprachiasmatic nucleus (SCN), the central pacemaker for the circadian system. The neuronal and astrocytic glutamate transporters are crucial for maintaining efficient glutamatergic signaling. In the SCN, glutamatergic nerve terminals from the retina terminate on vasoactive intestinal polypeptide (VIP) neurons, which are essential for circadian functions. To date, little is known about the role of the core circadian clock gene, Bmal1, in glutamatergic neurotransmission of light signal to various brain regions.

Methods

The aim of this study was to further elucidate the role of Bmal1 in glutamatergic neurotransmission from the retina to the SCN. We therefore examined the spontaneous rhythmic locomotor activity, neuronal and glial glutamate transporters, as well as the ultrastructure of the synapse between the retinal ganglion cells (RGCs) and the SCN in adult male Bmal1-/- mice.

Results

We found that the deletion of Bmal1 affects the light-mediated behavior in mice, decreases the retinal thickness and affects the vesicular glutamate transporters (vGLUT1, 2) in the retina. Within the SCN, the immunoreaction of vGLUT1, 2, glial glutamate transporters (GLAST) and VIP was decreased while the glutamate concentration was elevated. At the ultrastructure level, the presynaptic terminals were enlarged and the distance between the synaptic vesicles and the synaptic cleft was increased, indicative of a decrease in the readily releasable pool at the excitatory synapses in Bmal1-/-.

Conclusion

Our data suggests that Bmal1 deletion affects the glutamate transmission in the retina and the SCN and affects the behavioral responses to light.

Super-resolution Analysis of Astrocyte Morphology Using Expansion Microscopy

Analyzing the structure of astrocytes, their specific morphological features, and their remodeling is important for understanding how this cell type fulfils its many functions. This is because cell shape determines the propagation of intracellular signals and their subcellular compartmentalization. At the same time, it determines which other cells in the neuropil an astrocyte is closely in contact with and can most effectively exchange signals with. One experimental challenge has been that the most abundant small astrocytic processes cannot be resolved with diffraction-limited microscopy. Typically, this obstacle was overcome by using electron microscopy, but the continuous development of super-resolution microscopy has produced many alternative techniques. One is expansion microscopy (ExM) [1]. ExM, in principle, expands the tissue while preserving the relative positioning of labels that mark structures of interest (e.g., fluorescent labels), which increases the effective spatial resolution of light microscopy without improving the spatial resolution of the microscope itself. The advantage of ExM is that it requires only a little more laboratory infrastructure than immunohistochemistry combined with confocal fluorescence microscopy. We have previously applied this universal technique to the analysis of the structure of astrocytes and of their fine processes and their perisynaptic arrangement. Here, we present a comprehensive protocol for visualizing and localizing astrocytes, synaptic structures, and synaptic and astrocytic proteins in fixed brain tissue.

Decreased prefrontal glutamatergic function is associated with a reduced astrocyte-related gene expression in treatment-resistant depression

Glutamatergic dysfunction is involved in the pathophysiology of treatment-resistant depression (TRD). However, few physiological studies have evaluated its pathophysiology in vivo in individuals with TRD. Transcranial magnetic stimulation-electroencephalography (TMS-EEG) techniques can assess intracortical facilitation (ICF), which reflects glutamatergic neurophysiological function in specific cortical regions. The objectives of this study were (1) to compare glutamatergic receptor-mediated function as indexed with ICF TMS-EEG in the dorsolateral prefrontal cortex (DLPFC) between participants with TRD and healthy controls (HCs) and (2) to explore the relationships between cell-specific gene expression levels and the group difference in glutamatergic neural propagation using virtual histology approach. Sixty participants with TRD and thirty HCs were examined with ICF TMS-EEG measure (80 single-pulse TMS and paired-pulse ICF) in the left DLPFC. Both sensor and source-level ICF measures were computed to compare them between the TRD and HC groups. Furthermore, we conducted spatial correlation analyses interregionally between ICF glutamatergic activity and cell-specific gene expression levels employing the Allen Human Brain Atlas dataset. DLPFC-ICF at the sensor level was not significantly different between the two groups, whereas DLPFC-ICF at the source level was reduced in the TRD group compared with the HC group (p = 0.026). Moreover, the reduced ICF signal propagation of TRD correlated with astrocyte-specific gene expression level (p < 0.0001). The glutamatergic neural activities indexed by ICF in the left DLPFC were decreased in participants with TRD. Additionally, a relative reduction in glutamatergic signal propagation originating from the DLPFC in TRD may be associated with astrocytic abnormality.

Computational modeling of the relationship between morphological heterogeneity and functional responses in mouse hippocampal astrocytes

Recent studies indicate that astrocytes show heterogeneity in morphology and physiological function. They integrate synaptic signals and release calcium in reaction to active neurons. These calcium signals are not yet fully understood as they are highly dependent on the cell's morphology, which can vary across and within brain regions. We found structural heterogeneity among mouse hippocampal CA1 astrocytes based on geometric features, clustering 741 cells into six classes. Of those, we selected 84 cells and reconstructed their morphology based on confocal microscope images and converted them into multi-compartment models with a high detailedness. We applied a computational biophysical model simulating the intracellular ion and IP3 signaling and diffusion in those 3D cell geometries. The cells were stimulated with three different glutamate stimuli. Calcium mainly oscillated in the stimulated and the neighboring compartment but not in the soma. Significant differences were found in the peak width, mean prominence, and mean peak amplitude of the calcium signal when comparing the signals in the stimulated and neighboring compartments. Overall, this study highlights the influence of the complex morphology of astrocytes on intracellular ionic signaling.

Astrocyte coverage of excitatory synapses correlates to measures of synapse structure and function in ferret primary visual cortex

Most excitatory synapses in the mammalian brain are contacted or ensheathed by astrocyte processes, forming tripartite synapses. Astrocytes are thought to be critical regulators of the structural and functional dynamics of synapses. While the degree of synaptic coverage by astrocytes is known to vary across brain regions and animal species, the reason for and implications of this variability remains unknown. Further, how astrocyte coverage of synapses relates to in vivo functional properties of individual synapses has not been investigated. Here, we characterized astrocyte coverage of synapses of pyramidal neurons in the ferret visual cortex and, using correlative light and electron microscopy, examined their relationship to synaptic strength and sensory-evoked Ca2+ activity. Nearly, all synapses were contacted by astrocytes, and most were contacted along the axon-spine interface. Structurally, we found that the degree of synaptic astrocyte coverage directly scaled with synapse size and postsynaptic density complexity. Functionally, we found that the amount of astrocyte coverage scaled with how selectively a synapse responds to a particular visual stimulus and, at least for the largest synapses, scaled with the reliability of visual stimuli to evoke postsynaptic Ca2+ events. Our study shows astrocyte coverage is highly correlated with structural metrics of synaptic strength of excitatory synapses in the visual cortex and demonstrates a previously unknown relationship between astrocyte coverage and reliable sensory activation.

Bidirectional dysregulation of synaptic glutamate signaling after transient metabolic failure

Ischemia leads to a severe dysregulation of glutamate homeostasis and excitotoxic cell damage in the brain. Shorter episodes of energy depletion, for instance during peri-infarct depolarizations, can also acutely perturb glutamate signaling. It is less clear if such episodes of metabolic failure also have persistent effects on glutamate signaling and how the relevant mechanisms such as glutamate release and uptake are differentially affected. We modeled acute and transient metabolic failure by using a chemical ischemia protocol and analyzed its effect on glutamatergic synaptic transmission and extracellular glutamate signals by electrophysiology and multiphoton imaging, respectively, in the mouse hippocampus. Our experiments uncover a duration-dependent bidirectional dysregulation of glutamate signaling. Whereas short chemical ischemia induces a lasting potentiation of presynaptic glutamate release and synaptic transmission, longer episodes result in a persistent postsynaptic failure of synaptic transmission. We also observed unexpected differences in the vulnerability of the investigated cellular mechanisms. Axonal action potential firing and glutamate uptake were surprisingly resilient compared to postsynaptic cells, which overall were most vulnerable to acute and transient metabolic stress. We conclude that short perturbations of energy supply lead to a lasting potentiation of synaptic glutamate release, which may increase glutamate excitotoxicity well beyond the metabolic incident.

Mesenchymal Stem Cells Promote an Increase in Neuronal Oscillation via Glutamate Tonic Release

Mesenchymal stromal cells (MSCs) hold therapeutic potential for neurological disorders, but their impact on neuronal activity remains unclear. We investigated the effects of SB623 cells (Notch-1 intracellular domain-transfected MSCs) and parental MSCs on human induced pluripotent stem cell (iPSC)-derived neurons using multi-electrode arrays. SB623 cells significantly increased neuronal activity and oscillation in a dose-dependent manner, surpassing astrocytes in promoting network bursts. Strikingly, glutamatergic neurons showed a rapid increase in activity and bursts compared to GABAergic neurons, suggesting glutamate release from SB623 cells. We confirmed this by finding high glutamate levels in SB623 cell conditioned medium, which were reduced by glutaminase inhibition. Glutamate release was further implicated by the reduced excitability in co-cultures with astrocytes, known glutamate scavengers. Our findings reveal a novel mechanism for MSCs: promoting neuronal activity and network formation through tonic glutamate release, with potential implications for MSC-based therapies.

Bidirectional dysregulation of synaptic glutamate signaling after transient metabolic failure

Ischemia leads to a severe dysregulation of glutamate homeostasis and excitotoxic cell damage in the brain. Shorter episodes of energy depletion, for instance during peri-infarct depolarizations, can also acutely perturb glutamate signaling. It is less clear if such episodes of metabolic failure also have persistent effects on glutamate signaling and how the relevant mechanisms such as glutamate release and uptake are differentially affected. We modelled acute and transient metabolic failure by using a chemical ischemia protocol and analyzed its effect on glutamatergic synaptic transmission and extracellular glutamate signals by electrophysiology and multiphoton imaging, respectively, in the hippocampus. Our experiments uncover a duration-dependent bidirectional dysregulation of glutamate signaling. Whereas short chemical ischemia induces a lasting potentiation of presynaptic glutamate release and synaptic transmission, longer episodes result in a persistent postsynaptic failure of synaptic transmission. We also observed unexpected differences in the vulnerability of the investigated cellular mechanisms. Axonal action potential firing and glutamate uptake were unexpectedly resilient compared to postsynaptic cells, which overall were most vulnerable to acute and transient metabolic stress. We conclude that even short perturbations of energy supply lead to a lasting potentiation of synaptic glutamate release, which may increase glutamate excitotoxicity well beyond the metabolic incident.

Epileptic activity triggers rapid ROCK1-dependent astrocyte morphology changes

Long-term modifications of astrocyte function and morphology are well known to occur in epilepsy. They are implicated in the development and manifestation of the disease, but the relevant mechanisms and their pathophysiological role are not firmly established. For instance, it is unclear how quickly the onset of epileptic activity triggers astrocyte morphology changes and what the relevant molecular signals are. We therefore used two-photon excitation fluorescence microscopy to monitor astrocyte morphology in parallel to the induction of epileptiform activity. We uncovered astrocyte morphology changes within 10-20 min under various experimental conditions in acute hippocampal slices. In vivo, induction of status epilepticus resulted in similarly altered astrocyte morphology within 30 min. Further analysis in vitro revealed a persistent volume reduction of peripheral astrocyte processes triggered by induction of epileptiform activity. In addition, an impaired diffusion within astrocytes and within the astrocyte network was observed, which most likely is a direct consequence of the astrocyte remodeling. These astrocyte morphology changes were prevented by inhibition of the Rho GTPase RhoA and of the Rho-associated kinase (ROCK). Selective deletion of ROCK1 but not ROCK2 from astrocytes also prevented the morphology change after induction of epileptiform activity and reduced epileptiform activity. Together these observations reveal that epileptic activity triggers a rapid ROCK1-dependent astrocyte morphology change, which is mechanistically linked to the strength of epileptiform activity. This suggests that astrocytic ROCK1 signaling is a maladaptive response of astrocytes to the onset of epileptic activity.

Increased presynaptic excitability in a migraine with aura mutation

Migraine is a common and disabling neurological disorder. The headache and sensory amplifications of migraine are attributed to hyperexcitable sensory circuits, but a detailed understanding remains elusive. A mutation in casein kinase 1 delta (CK1δ) was identified in non-hemiplegic familial migraine with aura and advanced sleep phase syndrome. Mice carrying the CK1δT44A mutation were more susceptible to spreading depolarization (the phenomenon that underlies migraine aura), but mechanisms underlying this migraine-relevant phenotype were not known. We used a combination of whole-cell electrophysiology and multiphoton imaging, in vivo and in brain slices, to compare CK1δT44A mice (adult males) to their wild-type littermates. We found that despite comparable synaptic activity at rest, CK1δT44A neurons were more excitable upon repetitive stimulation than wild-type, with a reduction in presynaptic adaptation at excitatory but not inhibitory synapses. The mechanism of this adaptation deficit was a calcium-dependent enhancement of the size of the readily releasable pool of synaptic vesicles, and a resultant increase in glutamate release, in CK1δT44A compared to wild-type synapses. Consistent with this mechanism, CK1δT44A neurons showed an increase in the cumulative amplitude of excitatory post-synaptic currents, and a higher excitation-to-inhibition ratio during sustained activity compared to wild-type. At a local circuit level, action potential bursts elicited in CK1δT44A neurons triggered an increase in recurrent excitation compared to wild-type, and at a network level, CK1δT44A mice showed a longer duration of 'up state' activity, which is dependent on recurrent excitation. Finally, we demonstrated that the spreading depolarization susceptibility of CK1δT44A mice could be returned to wild-type levels with the same intervention (reduced extracellular calcium) that normalized presynaptic adaptation. Taken together, these findings show a stimulus-dependent presynaptic gain of function at glutamatergic synapses in a genetic model of migraine, that accounts for the increased spreading depolarization susceptibility and may also explain the sensory amplifications that are associated with the disease.

Astrocyte coverage of excitatory synapses correlates to measures of synapse structure and function in primary visual cortex

Most excitatory synapses in the mammalian brain are contacted by astrocytes, forming the tripartite synapse. This interface is thought to be critical for glutamate turnover and structural or functional dynamics of synapses. While the degree of synaptic contact of astrocytes is known to vary across brain regions and animal species, the implications of this variability remain unknown. Furthermore, precisely how astrocyte coverage of synapses relates to in vivo functional properties of individual dendritic spines has yet to be investigated. Here, we characterized perisynaptic astrocyte processes (PAPs) contacting synapses of pyramidal neurons of the ferret visual cortex and, using correlative light and electron microscopy, examined their relationship to synaptic strength and to sensory-evoked Ca2+ activity. Nearly all synapses were contacted by PAPs, and most were contacted along the axon-spine interface (ASI). Structurally, we found that the degree of PAP coverage scaled with synapse size and complexity. Functionally, we found that PAP coverage scaled with the selectivity of Ca2+ responses of individual synapses to visual stimuli and, at least for the largest synapses, scaled with the reliability of visual stimuli to evoke postsynaptic Ca2+ events. Our study shows astrocyte coverage is highly correlated with structural properties of excitatory synapses in the visual cortex and implicates astrocytes as a contributor to reliable sensory activation.

Astrocytes: new evidence, new models, new roles

Astrocytes have been in the limelight of active research for about 3 decades now. Over this period, ideas about their function and role in the nervous system have evolved from simple assistance in energy supply and homeostasis maintenance to a complex informational and metabolic hub that integrates data on local neuronal activity, sensory and arousal context, and orchestrates many crucial processes in the brain. Rapid progress in experimental techniques and data analysis produces a growing body of data, which can be used as a foundation for formulation of new hypotheses, building new refined mathematical models, and ultimately should lead to a new level of understanding of the contribution of astrocytes to the cognitive tasks performed by the brain. Here, we highlight recent progress in astrocyte research, which we believe expands our understanding of how low-level signaling at a cellular level builds up to processes at the level of the whole brain and animal behavior. We start our review with revisiting data on the role of noradrenaline-mediated astrocytic signaling in locomotion, arousal, sensory integration, memory, and sleep. We then briefly review astrocyte contribution to the regulation of cerebral blood flow regulation, which is followed by a discussion of biophysical mechanisms underlying astrocyte effects on different brain processes. The experimental section is closed by an overview of recent experimental techniques available for modulation and visualization of astrocyte dynamics. We then evaluate how the new data can be potentially incorporated into the new mathematical models or where and how it already has been done. Finally, we discuss an interesting prospect that astrocytes may be key players in important processes such as the switching between sleep and wakefulness and the removal of toxic metabolites from the brain milieu.

Impact of Developmental Changes of GABAA Receptors on Interneuron-NG2 Glia Transmission in the Hippocampus

NG2 glia receive synaptic input from neurons, but the functional impact of this glial innervation is not well understood. In the developing cerebellum and somatosensory cortex the GABAergic input might regulate NG2 glia differentiation and myelination, and a switch from synaptic to extrasynaptic neuron-glia signaling was reported in the latter region. Myelination in the hippocampus is sparse, and most NG2 glia retain their phenotype throughout adulthood, raising the question of the properties and function of neuron-NG2 glia synapses in that brain region. Here, we compared spontaneous and evoked GABAA receptor-mediated currents of NG2 glia in juvenile and adult hippocampi of mice of either sex and assessed the mode of interneuron-glial signaling changes during development. With patch-clamp and pharmacological analyses, we found a decrease in innervation of hippocampal NG2 glia between postnatal days 10 and 60. At the adult stage, enhanced activation of extrasynaptic receptors occurred, indicating a spillover of GABA. This switch from synaptic to extrasynaptic receptor activation was accompanied by downregulation of γ2 and upregulation of the α5 subunit. Molecular analyses and high-resolution expansion microscopy revealed mechanisms of glial GABAA receptor trafficking and clustering. We found that gephyrin and radixin are organized in separate clusters along glial processes. Surprisingly, the developmental loss of γ2 and postsynaptic receptors were not accompanied by altered glial expression of scaffolding proteins, auxiliary receptor subunits or postsynaptic interaction proteins. The GABAergic input to NG2 glia might contribute to the release of neurotrophic factors from these cells and influence neuronal synaptic plasticity.

Retraction of Astrocyte Leaflets From the Synapse Enhances Fear Memory

BACKGROUND

The formation and retrieval of fear memories depends on orchestrated synaptic activity of neuronal ensembles within the hippocampus, and it is becoming increasingly evident that astrocytes residing in the environment of these synapses play a central role in shaping cellular memory representations. Astrocyte distal processes, known as leaflets, fine-tune synaptic activity by clearing neurotransmitters and limiting glutamate diffusion. However, how astroglial synaptic coverage contributes to mnemonic processing of fearful experiences remains largely unknown.

METHODS

We used electron microscopy to observe changes in astroglial coverage of hippocampal synapses during consolidation of fear memory in mice. To manipulate astroglial synaptic coverage, we depleted ezrin, an integral leaflet-structural protein, from hippocampal astrocytes using CRISPR (clustered regularly interspaced short palindromic repeats)/Cas9 gene editing. Next, a combination of Föster resonance energy transfer analysis, genetically encoded glutamate sensors, and whole-cell patch-clamp recordings was used to determine whether the proximity of astrocyte leaflets to the synapse is critical for synaptic integrity and function.

RESULTS

We found that consolidation of a recent fear memory is accompanied by a transient retraction of astrocyte leaflets from hippocampal synapses and increased activation of NMDA receptors. Accordingly, astrocyte-specific depletion of ezrin resulted in shorter astrocyte leaflets and reduced astrocyte contact with the synaptic cleft, which consequently boosted extrasynaptic glutamate diffusion and NMDA receptor activation. Importantly, after fear conditioning, these cellular phenotypes translated to increased retrieval-evoked activation of CA1 pyramidal neurons and enhanced fear memory expression.

CONCLUSIONS

Together, our data show that withdrawal of astrocyte leaflets from the synaptic cleft is an experience-induced, temporally regulated process that gates the strength of fear memories.

Investigating the ability of astrocytes to drive neural network synchrony

Recent experimental works have implicated astrocytes as a significant cell type underlying several neuronal processes in the mammalian brain, from encoding sensory information to neurological disorders. Despite this progress, it is still unclear how astrocytes are communicating with and driving their neuronal neighbors. While previous computational modeling works have helped propose mechanisms responsible for driving these interactions, they have primarily focused on interactions at the synaptic level, with microscale models of calcium dynamics and neurotransmitter diffusion. Since it is computationally infeasible to include the intricate microscale details in a network-scale model, little computational work has been done to understand how astrocytes may be influencing spiking patterns and synchronization of large networks. We overcome this issue by first developing an "effective" astrocyte that can be easily implemented to already established network frameworks. We do this by showing that the astrocyte proximity to a synapse makes synaptic transmission faster, weaker, and less reliable. Thus, our "effective" astrocytes can be incorporated by considering heterogeneous synaptic time constants, which are parametrized only by the degree of astrocytic proximity at that synapse. We then apply our framework to large networks of exponential integrate-and-fire neurons with various spatial structures. Depending on key parameters, such as the number of synapses ensheathed and the strength of this ensheathment, we show that astrocytes can push the network to a synchronous state and exhibit spatially correlated patterns.

Overview Article Astrocytes as Initiators of Epilepsy

Astrocytes play a dual role in the brain. On the one hand, they are active signaling partners of neurons and can for instance control synaptic transmission and its plasticity. On the other hand, they fulfill various homeostatic functions such as clearance of glutamate and K⁺ released from neurons. The latter is for instance important for limiting neuronal excitability. Therefore, an impairment or failure of glutamate and K⁺ clearance will lead to increased neuronal excitability, which could trigger or aggravate brain diseases such as epilepsy, in which neuronal hyperexcitability plays a role. Experimental data indicate that astrocytes could have such a causal role in epilepsy, but the role of astrocytes as initiators of epilepsy and the relevant mechanisms are under debate. In this overview, we will discuss the potential mechanisms with focus on K⁺ clearance, glutamate uptake and homoeostasis and related mechanisms, and the evidence for their causative role in epilepsy.

Are cytoskeleton changes observed in astrocytes functionally linked to aging?

Astrocytes are active participants in the performance of the Central Nervous System (CNS) in both health and disease. During aging, astrocytes are susceptible to reactive astrogliosis, a molecular state characterized by functional changes in response to pathological situations, and cellular senescence, characterized by loss of cell division, apoptosis resistance, and gain of proinflammatory functions. This results in two different states of astrocytes, which can produce proinflammatory phenotypes with harmful consequences in chronic conditions. Reactive astrocytes and senescent astrocytes share morpho-functional features that are dependent on the organization of the cytoskeleton. However, such changes in the cytoskeleton have yet to receive the necessary attention to explain their role in the alterations of astrocytes that are associated with aging and pathologies. In this review, we summarize all the available findings that connect changes in the cytoskeleton of the astrocytes with aging. In addition, we discuss future avenues that we believe will guide such a novel topic.

Astrocytes in memory formation and maintenance

Learning and memory are fundamental but highly complex functions of the brain. They rely on multiple mechanisms including the processing of sensory information, memory formation, maintenance of short- and long-term memory, memory retrieval and memory extinction. Recent experiments provide strong evidence that, besides neurons, astrocytes crucially contribute to these higher brain functions. However, the complex interplay of astrocytes and neurons in local neuron-glia assemblies is far from being understood. Although important basic cellular principles that govern and link neuronal and astrocytic cellular functions have been established, additional mechanisms clearly continue to emerge. In this short essay, we first review current technologies allowing the experimenter to explore the role of astrocytes in behaving animals, with focus on spatial memory. We then discuss astrocytic signaling mechanisms and their role in learning and memory. We also reveal gaps in our knowledge that currently prevent a comprehensive understanding of how astrocytes contribute to acquisition, storage and retrieval of memory by modulating neuronal signaling in local circuits.

Dysfunction of NG2 glial cells affects neuronal plasticity and behavior

NG2 glia represents a distinct type of macroglial cells in the CNS and is unique among glia because they receive synaptic input from neurons. They are abundantly present in white and gray matter. While the majority of white matter NG2 glia differentiates into oligodendrocytes, the physiological impact of gray matter NG2 glia and their synaptic input are still ill defined. Here, we asked whether dysfunctional NG2 glia affect neuronal signaling and behavior. We generated mice with inducible deletion of the K⁺ channel Kir4.1 in NG2 glia and performed comparative electrophysiological, immunohistochemical, molecular and behavioral analyses. Kir4.1 was deleted at postnatal day 23-26 (recombination efficiency about 75%) and mice were investigated 3-8 weeks later. Notably, these mice with dysfunctional NG2 glia demonstrated improved spatial memory as revealed by testing new object location recognition while working and social memory remained unaffected. Focussing on the hippocampus, we found that loss of Kir4.1 potentiated synaptic depolarizations of NG2 glia and stimulated the expression of myelin basic protein while proliferation and differentiation of hippocampal NG2 glia remained largely unaffected. Mice with targeted deletion of the K⁺ channel in NG2 glia showed impaired long-term potentiation at CA3-CA1 synapses, which could be fully rescued by extracellular application of a TrkB receptor agonist. Our data demonstrate that proper NG2 glia function is important for normal brain function and behavior.

Gray and white matter astrocytes differ in basal metabolism but respond similarly to neuronal activity

Astrocytes are a heterogeneous population of glial cells in the brain, which adapt their properties to the requirements of the local environment. Two major groups of astrocytes are protoplasmic astrocytes residing in gray matter as well as fibrous astrocytes of white matter. Here, we compared the energy metabolism of astrocytes in the cortex and corpus callosum as representative gray matter and white matter regions, in acute brain slices taking advantage of genetically encoded fluorescent nanosensors for the NADH/NAD⁺ redox ratio and for ATP. Astrocytes of the corpus callosum presented a more reduced basal NADH/NAD⁺ redox ratio, and a lower cytosolic concentration of ATP compared to cortical astrocytes. In cortical astrocytes, the neurotransmitter glutamate and increased extracellular concentrations of K⁺ , typical correlates of neuronal activity, induced a more reduced NADH/NAD⁺ redox ratio. While application of glutamate decreased [ATP], K⁺ as well as the combination of glutamate and K⁺ resulted in an increase of ATP levels. Strikingly, a very similar regulation of metabolism by K⁺ and glutamate was observed in astrocytes in the corpus callosum. Finally, strong intrinsic neuronal activity provoked by application of bicuculline and withdrawal of Mg²⁺ caused a shift of the NADH/NAD⁺ redox ratio to a more reduced state as well as a slight reduction of [ATP] in gray and white matter astrocytes. In summary, the metabolism of astrocytes in cortex and corpus callosum shows distinct basal properties, but qualitatively similar responses to neuronal activity, probably reflecting the different environment and requirements of these brain regions.

Induced Remodelling of Astrocytes In Vitro and In Vivo by Manipulation of Astrocytic RhoA Activity

Structural changes of astrocytes and their perisynaptic processes occur in response to various physiological and pathophysiological stimuli. They are thought to profoundly affect synaptic signalling and neuron-astrocyte communication. Understanding the causal relationship between astrocyte morphology changes and their functional consequences requires experimental tools to selectively manipulate astrocyte morphology. Previous studies indicate that RhoA-related signalling can play a major role in controlling astrocyte morphology, but the direct effect of increased RhoA activity has not been documented in vitro and in vivo. Therefore, we established a viral approach to manipulate astrocytic RhoA activity. We tested if and how overexpression of wild-type RhoA, of a constitutively active RhoA mutant (RhoA-CA), and of a dominant-negative RhoA variant changes the morphology of cultured astrocytes. We found that astrocytic expression of RhoA-CA induced robust cytoskeletal changes and a withdrawal of processes in cultured astrocytes. In contrast, overexpression of other RhoA variants led to more variable changes of astrocyte morphology. These induced morphology changes were reproduced in astrocytes of the hippocampus in vivo. Importantly, astrocytic overexpression of RhoA-CA did not alter the branching pattern of larger GFAP-positive processes of astrocytes. This indicates that a prolonged increase of astrocytic RhoA activity leads to a distinct morphological phenotype in vitro and in vivo, which is characterized by an isolated reduction of fine peripheral astrocyte processes in vivo. At the same time, we identified a promising experimental approach for investigating the functional consequences of astrocyte morphology changes.

Glial Cell Modulation of Dendritic Spine Structure and Synaptic Function

Glia comprise a heterogeneous group of cells involved in the structure and function of the central and peripheral nervous system. Glial cells are found from invertebrates to humans with morphological specializations related to the neural circuits in which they are embedded. Glial cells modulate neuronal functions, brain wiring and myelination, and information processing. For example, astrocytes send processes to the synaptic cleft, actively participate in the metabolism of neurotransmitters, and release gliotransmitters, whose multiple effects depend on the targeting cells. Human astrocytes are larger and more complex than their mice and rats counterparts. Astrocytes and microglia participate in the development and plasticity of neural circuits by modulating dendritic spines. Spines enhance neuronal connectivity, integrate most postsynaptic excitatory potentials, and balance the strength of each input. Not all central synapses are engulfed by astrocytic processes. When that relationship occurs, a different pattern for thin and large spines reflects an activity-dependent remodeling of motile astrocytic processes around presynaptic and postsynaptic elements. Microglia are equally relevant for synaptic processing, and both glial cells modulate the switch of neuroendocrine secretion and behavioral display needed for reproduction. In this chapter, we provide an overview of the structure, function, and plasticity of glial cells and relate them to synaptic maturation and modulation, also involving neurotrophic factors. Together, neurons and glia coordinate synaptic transmission in both normal and abnormal conditions. Neglected over decades, this exciting research field can unravel the complexity of species-specific neural cytoarchitecture as well as the dynamic region-specific functional interactions between diverse neurons and glial subtypes.

An astrocytic signaling loop for frequency-dependent control of dendritic integration and spatial learning

Dendrites of hippocampal CA1 pyramidal cells amplify clustered glutamatergic input by activation of voltage-gated sodium channels and N-methyl-D-aspartate receptors (NMDARs). NMDAR activity depends on the presence of NMDAR co-agonists such as D-serine, but how co-agonists influence dendritic integration is not well understood. Using combinations of whole-cell patch clamp, iontophoretic glutamate application, two-photon excitation fluorescence microscopy and glutamate uncaging in acute rat and mouse brain slices we found that exogenous D-serine reduced the threshold of dendritic spikes and increased their amplitude. Triggering an astrocytic mechanism controlling endogenous D-serine supply via endocannabinoid receptors (CBRs) also increased dendritic spiking. Unexpectedly, this pathway was activated by pyramidal cell activity primarily in the theta range, which required HCN channels and astrocytic CB1Rs. Therefore, astrocytes close a positive and frequency-dependent feedback loop between pyramidal cell activity and their integration of dendritic input. Its disruption in mice led to an impairment of spatial memory, which demonstrated its behavioral relevance.

Structural Heterogeneity of the GABAergic Tripartite Synapse

The concept of the tripartite synapse describes the close interaction of pre- and postsynaptic elements and the surrounding astrocyte processes. For glutamatergic synapses, it is established that the presence of astrocytic processes and their structural arrangements varies considerably between and within brain regions and between synapses of the same neuron. In contrast, less is known about the organization of astrocytic processes at GABAergic synapses although bi-directional signaling is known to exist at these synapses too. Therefore, we established super-resolution expansion microscopy of GABAergic synapses and nearby astrocytic processes in the stratum radiatum of the mouse hippocampal CA1 region. By visualizing the presynaptic vesicular GABA transporter and the postsynaptic clustering protein gephyrin, we documented the subsynaptic heterogeneity of GABAergic synaptic contacts. We then compared the volume distribution of astrocytic processes near GABAergic synapses between individual synapses and with glutamatergic synapses. We made two novel observations. First, astrocytic processes were more abundant at the GABAergic synapses with large postsynaptic gephyrin clusters. Second, astrocytic processes were less abundant in the vicinity of GABAergic synapses compared to glutamatergic, suggesting that the latter may be selectively approached by astrocytes. Because of the GABA transporter distribution, we also speculate that this specific arrangement enables more efficient re-uptake of GABA into presynaptic terminals.

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).

A perspective on astrocyte regulation of neural circuit function and animal behavior

Studies over the past two decades have demonstrated that astrocytes are tightly associated with neurons and play pivotal roles in neural circuit development, operation, and adaptation in health and disease. Nevertheless, precisely how astrocytes integrate diverse neuronal signals, modulate neural circuit structure and function at multiple temporal and spatial scales, and influence animal behavior or disease through aberrant excitation and molecular output remains unclear. This Perspective discusses how new and state-of-the-art approaches, including fluorescence indicators, opto- and chemogenetic actuators, genetic targeting tools, quantitative behavioral assays, and computational methods, might help resolve these longstanding questions. It also addresses complicating factors in interpreting astrocytes' role in neural circuit regulation and animal behavior, such as their heterogeneity, metabolism, and inter-glial communication. Research on these questions should provide a deeper mechanistic understanding of astrocyte-neuron assemblies' role in neural circuit function, complex behaviors, and disease.

Dysfunction of ABC Transporters at the Surface of BBB: Potential Implications in Intractable Epilepsy and Applications of Nanotechnology Enabled Drug Delivery

Epilepsy is a chronic neurological disorder affecting 70 million people globally. One of the fascinating attributes of brain microvasculature is the (BBB), which controls a chain of distinct features that securely regulate the molecules, ions, and cells movement between the blood and the parenchyma. The barrier's integrity is of paramount importance and essential for maintaining brain homeostasis, as it offers both physical and chemical barriers to counter pathogens and xenobiotics. Dysfunction of various transporters in the (BBB), mainly ATP binding cassette (ABC), is considered to play a vital role in hampering the availability of antiepileptic drugs into the brain. ABC (ATP-binding cassette) transporters constitute a most diverse protein superfamily, which plays an essential part in various biological processes, including cell homeostasis, cell signaling, uptake of nutrients, and drug metabolism. Moreover, it plays a crucial role in neuroprotection by out-flowing various internal and external toxic substances from the interior of a cell, thus decreasing their buildup inside the cell. In humans, forty-eight ABC transporters have been acknowledged and categorized into subfamilies A to G based on their phylogenetic analysis. ABC subfamilies B, C, and G, impart a vital role at the BBB in guarding the brain against the entrance of various xenobiotic and their buildup. The illnesses of the central nervous system have received a lot of attention lately Owing to the existence of the BBB, the penetration effectiveness of most CNS medicines into the brain parenchyma is very limited (BBB). In the development of neurological therapies, BBB crossing for medication delivery to the CNS continues to be a major barrier. Nanomaterials with BBB cross ability have indeed been extensively developed for the treatment of CNS diseases due to their advantageous properties. This review will focus on multiple possible factors like inflammation, oxidative stress, uncontrolled recurrent seizures, and genetic polymorphisms that result in the deregulation of ABC transporters in epilepsy and nanotechnology-enabled delivery across BBB in epilepsy.

A closer look at astrocyte morphology: Development, heterogeneity, and plasticity at astrocyte leaflets

Astrocytes represent an abundant type of glial cell involved in nearly every aspect of central nervous system (CNS) function, including synapse formation and maturation, ion and neurotransmitter homeostasis, blood-brain barrier maintenance, as well as neuronal metabolic support. These various functions are enabled by the morphological complexity that astrocytes adopt. Recent experimental advances in genetic and viral labeling, lineage tracing, and live- and ultrastructural imaging of miniscule astrocytic sub-compartments reveal a complex morphological heterogeneity that is based on the origin, local function, and environmental context in which astrocytes reside. In this minireview, we highlight recent findings that reveal the plastic nature of astrocytes in the healthy brain, particularly at the synapse, and emerging technologies that have advanced our understanding of these morphologically complex cells.

The role of astrocyte structural plasticity in regulating neural circuit function and behavior

Brain circuits undergo substantial structural changes during development, driven by the formation, stabilization, and elimination of synapses. Synaptic connections continue to undergo experience‐dependent structural rearrangements throughout life, which are postulated to underlie learning and memory. Astrocytes, a major glial cell type in the brain, are physically in contact with synaptic circuits through their structural ensheathment of synapses. Astrocytes strongly contribute to the remodeling of synaptic structures in healthy and diseased central nervous systems by regulating synaptic connectivity and behaviors. However, whether structural plasticity of astrocytes is involved in their critical functions at the synapse is unknown. This review will discuss the emerging evidence linking astrocytic structural plasticity to synaptic circuit remodeling and regulation of behaviors. Moreover, we will survey possible molecular and cellular mechanisms regulating the structural plasticity of astrocytes and their non‐cell‐autonomous effects on neuronal plasticity. Finally, we will discuss how astrocyte morphological changes in different physiological states and disease conditions contribute to neuronal circuit function and dysfunction.

Riluzole prevents stress-induced spine plasticity in the hippocampus but mimics it in the amygdala

Stress elicits divergent patterns of structural plasticity in the amygdala and hippocampus. Despite these contrasting effects, at least one of the immediate consequences of stress - elevated levels of extracellular glutamate - is similar in both brain areas. This raises the possibility that the contrasting effects of stress on neuronal plasticity is shaped by differences in astrocytic glutamate clearance in these two brain areas. Although astrocytes play a key role in glutamate reuptake, past analyses of, and interventions against, stress-induced plasticity have focused largely on neurons. Hence, we tested the impact of riluzole, which potentiates glutamate clearance by astrocytic glutamate transporters, on principal neurons and astrocytes in the basal amygdala (BA) and hippocampal area CA1. Chronic immobilization stress reduced spine-density on CA1 pyramidal neurons of male rats. Riluzole, administered in the drinking water during chronic stress, prevented this decrease; but, the drug by itself had no effect. In contrast, the same chronic stress enhanced spine-density on BA principal neurons, and this effect, unlike area CA1, was not reversed by riluzole. Strikingly, riluzole treatment alone also caused spinogenesis in the BA. Thus, the same riluzole treatment that prevented the effect of stress on spines in the hippocampus, mimicked its effect in the amygdala. Further, chronic stress and riluzole alone decreased the neuropil volume occupied by astrocytes in both the BA and CA1 area. Riluzole treatment in stressed animals, however, did not reverse or further add to this reduction in either region. Thus, while the effects on astrocytes were similar, neuronal changes were distinct between the two areas following stress, riluzole and the two together. Therefore, similar to the impact of repeated stress, pharmacological potentiation of glutamate clearance, with or without stress, also leads to differential effects on dendritic spines in principal neurons of the amygdala and hippocampus. This highlights differences in the astrocytic glutamate reuptake machinery that are likely to have important functional consequences for stress-induced dysfunction, and its reversal, in two brain areas implicated in stress-related psychiatric disorders.

Photonics tools begin to clarify astrocyte calcium transients

Astrocytes integrate information from neurons and the microvasculature to coordinate brain activity and metabolism. Using a variety of calcium-dependent cellular mechanisms, these cells impact numerous aspects of neurophysiology in health and disease. Astrocyte calcium signaling is highly diverse, with complex spatiotemporal features. Here, we review astrocyte calcium dynamics and the optical imaging tools used to measure and analyze these events. We briefly cover historical calcium measurements, followed by our current understanding of how calcium transients relate to the structure of astrocytes. We then explore newer photonics tools including super-resolution techniques and genetically encoded calcium indicators targeted to specific cellular compartments and how these have been applied to astrocyte biology. Finally, we provide a brief overview of analysis software used to accurately quantify the data and ultimately aid in our interpretation of the various functions of astrocyte calcium transients.

Jacob, a Synapto-Nuclear Protein Messenger Linking N-methyl-D-aspartate Receptor Activation to Nuclear Gene Expression

Pyramidal neurons exhibit a complex dendritic tree that is decorated by a huge number of spine synapses receiving excitatory input. Synaptic signals not only act locally but are also conveyed to the nucleus of the postsynaptic neuron to regulate gene expression. This raises the question of how the spatio-temporal integration of synaptic inputs is accomplished at the genomic level and which molecular mechanisms are involved. Protein transport from synapse to nucleus has been shown in several studies and has the potential to encode synaptic signals at the site of origin and decode them in the nucleus. In this review, we summarize the knowledge about the properties of the synapto-nuclear messenger protein Jacob with special emphasis on a putative role in hippocampal neuronal plasticity. We will elaborate on the interactome of Jacob, the signals that control synapto-nuclear trafficking, the mechanisms of transport, and the potential nuclear function. In addition, we will address the organization of the Jacob/NSMF gene, its origin and we will summarize the evidence for the existence of splice isoforms and their expression pattern.

Cytokine Signalling at the Microglial Penta-Partite Synapse

Microglial cell processes form part of a subset of synaptic contacts that have been dubbed microglial tetra-partite or quad-partite synapses. Since tetrapartite may also refer to the presence of extracellular matrix components, we propose the more precise term microglial penta-partite synapse for synapses that show a microglial cell process in close physical proximity to neuronal and astrocytic synaptic constituents. Microglial cells are now recognised as key players in central nervous system (CNS) synaptic changes. When synaptic plasticity involving microglial penta-partite synapses occurs, microglia may utilise their cytokine arsenal to facilitate the generation of new synapses, eliminate those that are not needed anymore, or modify the molecular and structural properties of the remaining synaptic contacts. In addition, microglia–synapse contacts may develop de novo under pathological conditions. Microglial penta-partite synapses have received comparatively little attention as unique sites in the CNS where microglial cells, cytokines and other factors they release have a direct influence on the connections between neurons and their function. It concerns our understanding of the penta-partite synapse where the confusion created by the term “neuroinflammation” is most counterproductive. The mere presence of activated microglia or the release of their cytokines may occur independent of inflammation, and penta-partite synapses are not usually active in a neuroimmunological sense. Clarification of these details is the main purpose of this review, specifically highlighting the relationship between microglia, synapses, and the cytokines that can be released by microglial cells in health and disease.

Super-resolution imaging to reveal the nanostructure of tripartite synapses

Even though neurons are the main drivers of information processing in the brain and spinal cord, other cell types are important to mediate adequate flow of information. These include electrically passive glial cells such as microglia and astrocytes, which recently emerged as active partners facilitating proper signal transduction. In disease, these cells undergo pathophysiological changes that propel disease progression and change synaptic connections and signal transmission. In the healthy brain, astrocytic processes contact pre- and postsynaptic structures. These processes can be nanoscopic, and therefore only electron microscopy has been able to reveal their structure and morphology. However, electron microscopy is not suitable in revealing dynamic changes, and it is labour- and time-intensive. The dawn of super-resolution microscopy, techniques that 'break' the diffraction limit of conventional light microscopy, over the last decades has enabled researchers to reveal the nanoscopic synaptic environment. In this review, we highlight and discuss recent advances in our understanding of the nano-world of the so-called tripartite synapses, the relationship between pre- and postsynapse as well as astrocytic processes. Overall, novel super-resolution microscopy methods are needed to fully illuminate the intimate relationship between glia and neuronal cells that underlies signal transduction in the brain and that might be affected in diseases such as Alzheimer's disease and epilepsy.

Deciphering the functional nano-anatomy of the tripartite synapse using stimulated emission depletion microscopy

A major challenge for studying neuron-astrocyte communication lies in visualizing the tripartite synapse, which is the physical site where astrocytic processes contact and interact with neuronal synapses. While conventional light microscopy cannot resolve the anatomical details of the tripartite synapse, electron microscopy only provides ultrastructural snapshots that tell us little about its living state and dynamics. Stimulated emission depletion (STED) microscopy is a super-resolution fluorescence imaging technique that can provide live images of tripartite synapses with nanoscale spatial resolution. It is compatible with physiology experiments and imaging in the intact brain in vivo, opening up new opportunities to link the nanoscale structure of the tripartite system with functional readouts of neurons and astrocytes or even behavior. In this review, we first summarize the findings and insights from previous studies addressing the structure-function relationship of the tripartite synapse using conventional imaging techniques. We then explain the basic principle of STED microscopy and the main challenges facing its application to live-tissue imaging of fine astrocytic processes. We summarize insights from our recent STED studies, which revealed new aspects of the structure and physiology of the tripartite synapse and the surrounding extracellular space. Finally, we discuss how the STED approach and other advanced optical techniques can illuminate the role of astrocytes for brain physiology and animal behavior.

Super-resolving Microscopy in Neuroscience

Fluorescence imaging techniques play a pivotal role in our understanding of the nervous system. The emergence of various super-resolution microscopy methods and specialized fluorescent probes enables direct insight into neuronal structure and protein arrangements in cellular subcompartments with so far unmatched resolution. Super-resolving visualization techniques in neurons unveil a novel understanding of cytoskeletal composition, distribution, motility, and signaling of membrane proteins, subsynaptic structure and function, and neuron-glia interaction. Well-defined molecular targets in autoimmune and neurodegenerative disease models provide excellent starting points for in-depth investigation of disease pathophysiology using novel and innovative imaging methodology. Application of super-resolution microscopy in human brain samples and for testing clinical biomarkers is still in its infancy but opens new opportunities for translational research in neurology and neuroscience. In this review, we describe how super-resolving microscopy has improved our understanding of neuronal and brain function and dysfunction in the last two decades.

Assessment of astrocytes as a mediator of memory and learning in rodents

The classical view of astrocytes is that they provide supportive functions for neurons, transporting metabolites and maintaining the homeostasis of the extracellular milieu. This view is gradually changing with the advent of molecular genetics and optical methods allowing interrogation of selected cell types in live experimental animals. An emerging view that astrocytes additionally act as a mediator of synaptic plasticity and contribute to learning processes has gained in vitro and in vivo experimental support. Here we focus on the literature published in the past two decades to review the roles of astrocytes in brain plasticity in rodents, whereby the roles of neurotransmitters and neuromodulators are considered to be comparable to those in humans. We outline established inputs and outputs of astrocytes and discuss how manipulations of astrocytes have impacted the behavior in various learning paradigms. Multiple studies suggest that the contribution of astrocytes has a considerably longer time course than neuronal activation, indicating metabolic roles of astrocytes. We advocate that exploring upstream and downstream mechanisms of astrocytic activation will further provide insight into brain plasticity and memory/learning impairment.

Heterogeneity and Development of Fine Astrocyte Morphology Captured by Diffraction-Limited Microscopy

The fine processes of single astrocytes can contact many thousands of synapses whose function they can modulate through bi-directional signaling. The spatial arrangement of astrocytic processes and neuronal structures is relevant for such interactions and for the support of neuronal signaling by astrocytes. At the same time, the geometry of perisynaptic astrocyte processes is variable and dynamically regulated. Studying these fine astrocyte processes represents a technical challenge, because many of them cannot be fully resolved by diffraction-limited microscopy. Therefore, we have established two indirect parameters of astrocyte morphology, which, while not fully resolving local geometry by design, provide statistical measures of astrocyte morphology: the fraction of tissue volume that astrocytes occupy and the density of resolvable astrocytic processes. Both are straightforward to obtain using widely available microscopy techniques. We here present the approach and demonstrate its robustness across various experimental conditions using mainly two-photon excitation fluorescence microscopy in acute slices and in vivo as well as modeling. Using these indirect measures allowed us to analyze the morphology of relatively large populations of astrocytes. Doing so we captured the heterogeneity of astrocytes within and between the layers of the hippocampal CA1 region and the developmental profile of astrocyte morphology. This demonstrates that volume fraction (VF) and segment density are useful parameters for describing the structure of astrocytes. They are also suitable for online monitoring of astrocyte morphology with widely available microscopy techniques.

Regulation of Glutamate, GABA and Dopamine Transporter Uptake, Surface Mobility and Expression

Neurotransmitter transporters limit spillover between synapses and maintain the extracellular neurotransmitter concentration at low yet physiologically meaningful levels. They also exert a key role in providing precursors for neurotransmitter biosynthesis. In many cases, neurons and astrocytes contain a large intracellular pool of transporters that can be redistributed and stabilized in the plasma membrane following activation of different signaling pathways. This means that the uptake capacity of the brain neuropil for different neurotransmitters can be dynamically regulated over the course of minutes, as an indirect consequence of changes in neuronal activity, blood flow, cell-to-cell interactions, etc. Here we discuss recent advances in the mechanisms that control the cell membrane trafficking and biophysical properties of transporters for the excitatory, inhibitory and modulatory neurotransmitters glutamate, GABA, and dopamine.

The Effect of GLT-1 Upregulation on Extracellular Glutamate Dynamics

Pharmacological upregulation of glutamate transporter-1 (GLT-1), commonly achieved using the beta-lactam antibiotic ceftriaxone, represents a promising therapeutic strategy to accelerate glutamate uptake and prevent excitotoxic damage in neurological conditions. While excitotoxicity is indeed implicated in numerous brain diseases, it is typically restricted to select vulnerable brain regions, particularly in early disease stages. In healthy brain tissue, the speed of glutamate uptake is not constant and rather varies in both an activity- and region-dependent manner. Despite the widespread use of ceftriaxone in disease models, very little is known about how such treatments impact functional measures of glutamate uptake in healthy tissue, and whether GLT-1 upregulation can mask the naturally occurring activity-dependent and regional heterogeneities in uptake. Here, we used two different compounds, ceftriaxone and LDN/OSU-0212320 (LDN), to upregulate GLT-1 in healthy wild-type mice. We then used real-time imaging of the glutamate biosensor iGluSnFR to investigate functional consequences of GLT-1 upregulation on activity- and regional-dependent variations in glutamate uptake capacity. We found that while both ceftriaxone and LDN increased GLT-1 expression in multiple brain regions, they did not prevent activity-dependent slowing of glutamate clearance nor did they speed basal clearance rates, even in areas characterized by slow uptake (e.g., striatum). Unexpectedly, ceftriaxone but not LDN decreased glutamate release in the cortex, suggesting that ceftriaxone may alter release properties independent of its effects on GLT-1 expression. In sum, our data demonstrate the complexities of glutamate uptake by showing that GLT-1 expression does not necessarily translate to accelerated uptake. Furthermore, these data suggest that the mechanisms underlying activity- and regional-dependent differences in glutamate dynamics are independent of GLT-1 expression levels.

Astrocyte glutamate uptake coordinates experience-dependent, eye-specific refinement in developing visual cortex

The uptake of glutamate by astrocytes actively shapes synaptic transmission, however its role in the development and plasticity of neuronal circuits remains poorly understood. The astrocytic glutamate transporter, GLT1 is the predominant source of glutamate clearance in the adult mouse cortex. Here, we examined the structural and functional development of the visual cortex in GLT1 heterozygous (HET) mice using two-photon microscopy, immunohistochemistry and slice electrophysiology. We find that though eye-specific thalamic axonal segregation is intact, binocular refinement in the primary visual cortex is disrupted. Eye-specific responses to visual stimuli in GLT1 HET mice show altered binocular matching, with abnormally high responses to ipsilateral compared to contralateral eye stimulation and a greater mismatch between preferred orientation selectivity of ipsilateral and contralateral eye responses. Furthermore, we observe an increase in dendritic spine density in the basal dendrites of layer 2/3 excitatory neurons suggesting aberrant spine pruning. Monocular deprivation induces atypical ocular dominance plasticity in GLT1 HET mice, with an unusual depression of ipsilateral open eye responses; however, this change in ipsilateral responses correlates well with an upregulation of GLT1 protein following monocular deprivation. These results demonstrate that a key function of astrocytic GLT1 function during development is the experience-dependent refinement of ipsilateral eye inputs relative to contralateral eye inputs in visual cortex.

Disruption of Glutamate Transport and Homeostasis by Acute Metabolic Stress

High-affinity, Na⁺-dependent glutamate transporters are the primary means by which synaptically released glutamate is removed from the extracellular space. They restrict the spread of glutamate from the synaptic cleft into the perisynaptic space and reduce its spillover to neighboring synapses. Thereby, glutamate uptake increases the spatial precision of synaptic communication. Its dysfunction and the entailing rise of the extracellular glutamate concentration accompanied by an increased spread of glutamate result in a loss of precision and in enhanced excitation, which can eventually lead to neuronal death via excitotoxicity. Efficient glutamate uptake depends on a negative resting membrane potential as well as on the transmembrane gradients of the co-transported ions (Na⁺, K⁺, and H⁺) and thus on the proper functioning of the Na⁺/K⁺-ATPase. Consequently, numerous studies have documented the impact of an energy shortage, as occurring for instance during an ischemic stroke, on glutamate clearance and homeostasis. The observations range from rapid changes in the transport activity to altered expression of glutamate transporters. Notably, while astrocytes account for the majority of glutamate uptake under physiological conditions, they may also become a source of extracellular glutamate elevation during metabolic stress. However, the mechanisms of the latter phenomenon are still under debate. Here, we review the recent literature addressing changes of glutamate uptake and homeostasis triggered by acute metabolic stress, i.e., on a timescale of seconds to minutes.