α1-Adrenergic receptors mediate coordinated Ca2+ signaling of cortical astrocytes in awake, behaving mice

Autoimmune inflammation triggers aberrant astrocytic calcium signaling to impair synaptic plasticity

Cortical pathology involving inflammatory and neurodegenerative mechanisms is a hallmark of multiple sclerosis and a correlate of disease progression and cognitive decline. Astrocytes play a pivotal role in multiple sclerosis initiation and progression but astrocyte-neuronal network alterations contributing to gray matter pathology remain undefined. Here we unveil deregulation of astrocytic calcium signaling and astrocyte-to-neuron communication as key pathophysiological mechanisms of cortical dysfunction in the experimental autoimmune encephalomyelitis (EAE) model of multiple sclerosis. Using two-photon imaging ex vivo and fiber photometry in freely behaving mice, we found that acute EAE was associated with the emergence of spontaneously hyperactive cortical astrocytes exhibiting dysfunctional responses to cannabinoid, glutamate and purinoreceptor agonists. Abnormal astrocyte signaling by Gi and Gq protein coupled receptors was observed in the inflamed cortex. This was mirrored by treatments with pro-inflammatory factors both in vitro and ex vivo, suggesting cell-autonomous effects of the cortical neuroinflammatory environment. Finally, deregulated astrocyte calcium activity was associated with an enhancement of glutamatergic gliotransmission and a shift of astrocyte-mediated short-term and long-term plasticity mechanisms towards synaptic potentiation. Overall, our data identify astrocyte-neuronal network dysfunctions as key pathological features of gray matter inflammation in multiple sclerosis and potentially additional neuroimmunological disorders.

Norepinephrine changes behavioral state via astroglial purinergic signaling

Both neurons and glia communicate via diffusible neuromodulatory substances, but the substrates of computation in such neuromodulatory networks are unclear. During behavioral transitions in the larval zebrafish, the neuromodulator norepinephrine drives fast excitation and delayed inhibition of behavior and circuit activity. We find that the inhibitory arm of this feedforward motif is implemented by astroglial purinergic signaling. Neuromodulator imaging, behavioral pharmacology, and perturbations of neurons and astroglia reveal that norepinephrine triggers astroglial release of adenosine triphosphate, extracellular conversion into adenosine, and behavioral suppression through activation of hindbrain neuronal adenosine receptors. This work, along with a companion piece by Lefton and colleagues demonstrating an analogous pathway mediating the effect of norepinephrine on synaptic connectivity in mice, identifies a computational and behavioral role for an evolutionarily conserved astroglial purinergic signaling axis in norepinephrine-mediated behavioral and brain state transitions.

Norepinephrine Signals Through Astrocytes To Modulate Synapses

Locus coeruleus (LC)-derived norepinephrine (NE) drives network and behavioral adaptations to environmental saliencies by reconfiguring circuit connectivity, but the underlying synapse-level mechanisms are elusive. Here, we show that NE remodeling of synaptic function is independent from its binding on neuronal receptors. Instead, astrocytic adrenergic receptors and Ca2+ dynamics fully gate the effect of NE on synapses as the astrocyte-specific deletion of adrenergic receptors and three independent astrocyte-silencing approaches all render synapses insensitive to NE. Additionally, we find that NE suppression of synaptic strength results from an ATP-derived and adenosine A1 receptor-mediated control of presynaptic efficacy. An accompanying study from Chen et al. reveals the existence of an analogous pathway in the larval zebrafish and highlights its importance to behavioral state transitions. Together, these findings fuel a new model wherein astrocytes are a core component of neuromodulatory systems and the circuit effector through which norepinephrine produces network and behavioral adaptations, challenging an 80-year-old status quo.

Network-level encoding of local neurotransmitters in cortical astrocytes

Astrocytes, the most abundant non-neuronal cell type in the mammalian brain, are crucial circuit components that respond to and modulate neuronal activity through calcium (Ca2+) signalling1-7. Astrocyte Ca2+ activity is highly heterogeneous and occurs across multiple spatiotemporal scales-from fast, subcellular activity3,4 to slow, synchronized activity across connected astrocyte networks8-10-to influence many processes5,7,11. However, the inputs that drive astrocyte network dynamics remain unclear. Here we used ex vivo and in vivo two-photon astrocyte imaging while mimicking neuronal neurotransmitter inputs at multiple spatiotemporal scales. We find that brief, subcellular inputs of GABA and glutamate lead to widespread, long-lasting astrocyte Ca2+ responses beyond an individual stimulated cell. Further, we find that a key subset of Ca2+ activity-propagative activity-differentiates astrocyte network responses to these two main neurotransmitters, and may influence responses to future inputs. Together, our results demonstrate that local, transient neurotransmitter inputs are encoded by broad cortical astrocyte networks over a minutes-long time course, contributing to accumulating evidence that substantial astrocyte-neuron communication occurs across slow, network-level spatiotemporal scales12-14. These findings will enable future studies to investigate the link between specific astrocyte Ca2+ activity and specific functional outputs, which could build a consistent framework for astrocytic modulation of neuronal activity.

Centripetal integration of past events in hippocampal astrocytes regulated by locus coeruleus

An essential feature of neurons is their ability to centrally integrate information from their dendrites. The activity of astrocytes, in contrast, has been described as mostly uncoordinated across cellular compartments without clear central integration. Here we report conditional integration of calcium signals in astrocytic distal processes at their soma. In the hippocampus of adult mice of both sexes, we found that global astrocytic activity, as recorded with population calcium imaging, reflected past neuronal and behavioral events on a timescale of seconds. Salient past events, indicated by pupil dilations, facilitated the propagation of calcium signals from distal processes to the soma. Centripetal propagation to the soma was reproduced by optogenetic activation of the locus coeruleus, a key regulator of arousal, and reduced by pharmacological inhibition of α1-adrenergic receptors. Together, our results suggest that astrocytes are computational units of the brain that slowly and conditionally integrate calcium signals upon behaviorally relevant events.

Modulatory effects of noradrenergic and serotonergic signaling pathway on neurovascular coupling

Dynamic changes in astrocyte Ca2+ are recognized as contributors to functional hyperemia, a critical response to increased neuronal activity mediated by a process known as neurovascular coupling (NVC). Although the critical role of glutamatergic signaling in this process has been extensively investigated, the impact of behavioral state, and the release of behavior-associated neurotransmitters, such as norepinephrine and serotonin, on astrocyte Ca2+ dynamics and functional hyperemia have received less attention. We used two-photon imaging of the barrel cortex in awake mice to examine the role of noradrenergic and serotonergic projections in NVC. We found that both neurotransmitters facilitated sensory stimulation-induced increases in astrocyte Ca2+. Interestingly, while ablation of serotonergic neurons reduced sensory stimulation-induced functional hyperemia, ablation of noradrenergic neurons caused both attenuation and potentiation of functional hyperemia. Our study demonstrates that norepinephrine and serotonin are involved in modulating sensory stimulation-induced astrocyte Ca2+ elevations and identifies their differential effects in regulating functional hyperemia.

Dopamine and noradrenaline activate spinal astrocyte endfeet via D1-like receptors

Astrocytes, the most abundant glial cells in the central nervous system, respond to a wide variety of neurotransmitters binding to metabotropic receptors. Here, we investigated the intracellular calcium responses of spinal cord astrocytes to dopamine and noradrenaline, two catecholamines released by specific descending pathways. In a slice preparation from the spinal cord of neonatal mice, puff application of dopamine resulted in intracellular calcium responses that remained in the endfeet. Noradrenaline induced stronger responses that also started in the endfeet but spread to neighbouring compartments. The intracellular calcium responses were unaffected by blocking neuronal activity or inhibiting various neurotransmitter receptors, suggesting a direct effect of dopamine and noradrenaline on astrocytes. The intracellular calcium responses induced by noradrenaline and dopamine were inhibited by the D1 receptor antagonist SCH 23390. We assessed the functional consequences of these astrocytic responses by examining changes in arteriole diameter. Puff application of dopamine or noradrenaline resulted in vasoconstriction of spinal arterioles. However, blocking D1 receptors or manipulating astrocytic intracellular calcium levels did not abolish the vasoconstrictions, indicating that the observed intracellular calcium responses in astrocyte endfeet were not responsible for the vascular changes. Our findings demonstrate a compartmentalized response of spinal cord astrocytes to catecholamines and expand our understanding of astrocyte-neurotransmitter interactions and their potential roles in the physiology of the central nervous system.

Astrocytes modulate cerebral blood flow and neuronal response to cocaine in prefrontal cortex

Cocaine affects both cerebral blood vessels and neuronal activity in brain. Cocaine can also disrupt astrocytes, which modulate neurovascular coupling-a process that regulates cerebral hemodynamics in response to neuronal activation. However, separating neuronal and astrocytic effects from cocaine's direct vasoactive effects has been challenging, partially due to limitations of neuroimaging techniques able to differentiate vascular from neuronal and glial effects at high temporal and spatial resolutions. Here, we used a newly-developed multi-channel fluorescence and optical coherence Doppler microscope (fl-ODM) that allows for simultaneous measurements of neuronal and astrocytic activities (reflected by the intracellular calcium changes in neurons Ca2+N and astrocytes Ca2+A, respectively) alongside their vascular interactions in vivo to address this challenge. Using green and red genetically-encoded Ca2+ indicators differentially expressed in astrocytes and neurons, fl-ODM enabled concomitant imaging of large-scale astrocytic and neuronal Ca2+ fluorescence and 3D cerebral blood flow velocity (CBFv) in vascular networks in the mouse cortex. We assessed cocaine's effects in the prefrontal cortex (PFC) and found that the CBFv changes triggered by cocaine were temporally correlated with astrocytic Ca2+A activity. Chemogenetic inhibition of astrocytes during the baseline state resulted in blood vessel dilation and CBFv increases but did not affect neuronal activity, suggesting modulation of spontaneous blood vessel's vascular tone by astrocytes. Chemogenetic inhibition of astrocytes during a cocaine challenge prevented its vasoconstricting effects alongside the CBFv decreases, but it also attenuated the neuronal Ca2+N increases triggered by cocaine. These results document a role of astrocytes both in regulating vascular tone and consequently blood flow, at baseline and for modulating the vasoconstricting and neuronal activation responses to cocaine in the PFC. Strategies to inhibit astrocytic activity could offer promise for ameliorating vascular and neuronal toxicity from cocaine misuse.

Regulation of nociception threshold by norepinephrine through adrenergic α2 receptor in rat models of Parkinson's disease

BACKGROUND

The mechanism of pain symptoms in Parkinson's disease (PD) is unclear. Norepinephrine (NE) regulates neuropathic pain through ascending and descending pathways. However, the loss of NE neurons in the brain of patients with PD is obvious, it is speculated that NE is involved in the occurrence of PD pain symptoms.

AIMS

To investigate the effect of NE on the activation of brain cells through adrenergic α2 receptor, so as to regulate the nociception threshold in a 6-OHDA-induced animal model of PD.

METHODS

PD rat model was established by 6-OHDA injection (6-OHDA group). DSP-4 (or anti-DBH-saporin) was used to reduce the NE level of the PD rat brain. The heat sensitivity threshold (HST) and pressure withdrawal threshold (PWT) were measured. Tyrosine hydroxylase and NE in rat brains were detected by Elisa. The percentage of GFAP-positive cells in the prefrontal cortex, cingulate gyrus and striatum of rats was detected by immunohistochemistry and immunofluorescence. GFAP protein was semiquantified by method of western blot. Then yohimbine and guanfacine were used to increase the NE level in PD rats, and the above experimental changes were observed after drug application.

RESULTS

The contents of NE in the brain of 6-OHDA-induced PD rats were lower than that of control group. After DSP-4 (or anti-DBH-saporin) injection, PD rats showed the lowest NE level (compared with 6-OHDA group, p ≤ 0.05), and after yohimbine and guanfacine were applied to 6-OHDA group, the contents of NE increased in the prefrontal cortex of rats. The HST and PWT of 6-OHDA group were significantly lower than those of control group, and after DSP-4 (or anti-DBH-saporin) injection, the HST and PWT of rats were lower than those of 6-OHDA group, and after the administration of yohimbine and guanfacine, both HST and PWT were significantly increased. GFAP-positive cells increased in prefrontal cortex and anterior cingulate gyrus of 6-OHDA group rats, and more significantly increased after DSP-4 (or anti-DBH-saporin) injection, and significantly reduced after yohimbine and guanfacine were used.

CONCLUSIONS

The change of norepinephrine content can affect the activation of prefrontal and cingulate gyrus glial cells and participate in the regulation of nociception threshold in PD rats. Adrenergic α2 receptor agonist and central presynaptic membrane α2 receptor blocker both affect cell activation and improve hyperalgesia.

Microglia regulate sleep through calcium-dependent modulation of norepinephrine transmission

Sleep interacts reciprocally with immune system activity, but its specific relationship with microglia-the resident immune cells in the brain-remains poorly understood. Here, we show in mice that microglia can regulate sleep through a mechanism involving Gi-coupled GPCRs, intracellular Ca2+ signaling and suppression of norepinephrine transmission. Chemogenetic activation of microglia Gi signaling strongly promoted sleep, whereas pharmacological blockade of Gi-coupled P2Y12 receptors decreased sleep. Two-photon imaging in the cortex showed that P2Y12-Gi activation elevated microglia intracellular Ca2+, and blockade of this Ca2+ elevation largely abolished the Gi-induced sleep increase. Microglia Ca2+ level also increased at natural wake-to-sleep transitions, caused partly by reduced norepinephrine levels. Furthermore, imaging of norepinephrine with its biosensor in the cortex showed that microglia P2Y12-Gi activation significantly reduced norepinephrine levels, partly by increasing the adenosine concentration. These findings indicate that microglia can regulate sleep through reciprocal interactions with norepinephrine transmission.

Network-level encoding of local neurotransmitters in cortical astrocytes

Astrocytes-the most abundant non-neuronal cell type in the mammalian brain-are crucial circuit components that respond to and modulate neuronal activity via calcium (Ca 2+ ) signaling 1-8 . Astrocyte Ca 2+ activity is highly heterogeneous and occurs across multiple spatiotemporal scales: from fast, subcellular activity 3,4 to slow, synchronized activity that travels across connected astrocyte networks 9-11 . Furthermore, astrocyte network activity has been shown to influence a wide range of processes 5,8,12 . While astrocyte network activity has important implications for neuronal circuit function, the inputs that drive astrocyte network dynamics remain unclear. Here we used ex vivo and in vivo two-photon Ca 2+ imaging of astrocytes while mimicking neuronal neurotransmitter inputs at multiple spatiotemporal scales. We find that brief, subcellular inputs of GABA and glutamate lead to widespread, long-lasting astrocyte Ca 2+ responses beyond an individual stimulated cell. Further, we find that a key subset of Ca 2+ activity-propagative events-differentiates astrocyte network responses to these two major neurotransmitters, and gates responses to future inputs. Together, our results demonstrate that local, transient neurotransmitter inputs are encoded by broad cortical astrocyte networks over the course of minutes, contributing to accumulating evidence across multiple model organisms that significant astrocyte-neuron communication occurs across slow, network-level spatiotemporal scales 13-15 . We anticipate that this study will be a starting point for future studies investigating the link between specific astrocyte Ca 2+ activity and specific astrocyte functional outputs, which could build a consistent framework for astrocytic modulation of neuronal activity.

Localization and neurochemical identity of alpha1-adrenergic receptor-containing elements in the mouse locus coeruleus

The locus coeruleus (LC) is the major source for norepinephrine (NE) in the brain and projects to areas involved in learning and memory, reward, arousal, attention, and autonomic functions related to stress. There are three types of adrenergic receptors that respond to NE: alpha1-, alpha2-, and beta-adrenergic receptors. Previous behavioral studies have shown the alpha1-adrenergic receptor (α1AR) to be present in the LC, however, with conflicting results. For example, it was shown that α1ARs in the LC are involved in some of the motivational effects of stimulation of the medial forebrain bundle, which was reduced by α1AR antagonist terazosin. Another study showed that during novelty-induced behavioral activation, the α1AR antagonist prazosin reduced c-fos expression in brain regions known to contain motoric α1ARs, except for the LC, where c-fos expression was enhanced. Despite new research delineating more specific connectivity of the neurons in the LC, and some roles of the adrenergic receptors, the α1ARs have not been localized at the subcellular level. Therefore, in order to gain a greater understanding of the aforementioned studies, we used immunohistochemistry at the electron microscopic (EM) level to determine which neuronal or glial elements in the LC express the α1AR. We hypothesized, based on previous work in the ventral periaqueductal gray area, that the α1AR would be found mainly presynaptically in axon terminals, and possibly in glial elements. Single labeling immunohistochemistry at the EM revealed that about 40% of labeled elements that contained the α1AR were glial elements, while approximately 50% of the labeled neuronal elements were axon terminals or small unmyelinated axons in the LC. Double labeling immunohistochemistry found the α1AR expressed in GFAP-labeled astrocytes, in both GABAergic and glutamatergic axon terminals, and in a portion of the α1AR dendrites, colocalized with tyrosine hydroxylase (TH, a marker for noradrenergic neurons). This study sheds light on the neuroanatomical framework underlying the effects of NE and pharmaceuticals acting directly or indirectly on α1ARs in the LC.

Astrocytes in human central nervous system diseases: a frontier for new therapies

Astroglia are a broad class of neural parenchymal cells primarily dedicated to homoeostasis and defence of the central nervous system (CNS). Astroglia contribute to the pathophysiology of all neurological and neuropsychiatric disorders in ways that can be either beneficial or detrimental to disorder outcome. Pathophysiological changes in astroglia can be primary or secondary and can result in gain or loss of functions. Astroglia respond to external, non-cell autonomous signals associated with any form of CNS pathology by undergoing complex and variable changes in their structure, molecular expression, and function. In addition, internally driven, cell autonomous changes of astroglial innate properties can lead to CNS pathologies. Astroglial pathophysiology is complex, with different pathophysiological cell states and cell phenotypes that are context-specific and vary with disorder, disorder-stage, comorbidities, age, and sex. Here, we classify astroglial pathophysiology into (i) reactive astrogliosis, (ii) astroglial atrophy with loss of function, (iii) astroglial degeneration and death, and (iv) astrocytopathies characterised by aberrant forms that drive disease. We review astroglial pathophysiology across the spectrum of human CNS diseases and disorders, including neurotrauma, stroke, neuroinfection, autoimmune attack and epilepsy, as well as neurodevelopmental, neurodegenerative, metabolic and neuropsychiatric disorders. Characterising cellular and molecular mechanisms of astroglial pathophysiology represents a new frontier to identify novel therapeutic strategies.

Exploring Astrocyte-Mediated Mechanisms in Sleep Disorders and Comorbidity

Astrocytes, the most abundant cells in the brain, are integral to sleep regulation. In the context of a healthy neural environment, these glial cells exert a profound influence on the sleep-wake cycle, modulating both rapid eye movement (REM) and non-REM sleep phases. However, emerging literature underscores perturbations in astrocytic function as potential etiological factors in sleep disorders, either as protopathy or comorbidity. As known, sleep disorders significantly increase the risk of neurodegenerative, cardiovascular, metabolic, or psychiatric diseases. Meanwhile, sleep disorders are commonly screened as comorbidities in various neurodegenerative diseases, epilepsy, and others. Building on existing research that examines the role of astrocytes in sleep disorders, this review aims to elucidate the potential mechanisms by which astrocytes influence sleep regulation and contribute to sleep disorders in the varied settings of brain diseases. The review emphasizes the significance of astrocyte-mediated mechanisms in sleep disorders and their associated comorbidities, highlighting the need for further research.

Activation of Basal Forebrain Astrocytes Induces Wakefulness without Compensatory Changes in Sleep Drive

Mammalian sleep is regulated by a homeostatic process that increases sleep drive and intensity as a function of prior wake time. Sleep homeostasis has traditionally been thought to be a product of neurons, but recent findings demonstrate that this process is also modulated by glial astrocytes. The precise role of astrocytes in the accumulation and discharge of sleep drive is unknown. We investigated this question by selectively activating basal forebrain (BF) astrocytes using designer receptors exclusively activated by designer drugs (DREADDs) in male and female mice. DREADD activation of the Gq-protein-coupled pathway in BF astrocytes produced long and continuous periods of wakefulness that paradoxically did not cause the expected homeostatic response to sleep loss (e.g., increases in sleep time or intensity). Further investigations showed that this was not because of indirect effects of the ligand that activated DREADDs. These findings suggest that the need for sleep is not only driven by wakefulness per se, but also by specific neuronal-glial circuits that are differentially activated in wakefulness.SIGNIFICANCE STATEMENT Sleep drive is controlled by a homeostatic process that increases sleep duration and intensity based on prior time spent awake. Non-neuronal brain cells (e.g., glial astrocytes) influence this homeostatic process, but their precise role is unclear. We used a genetic technique to activate astrocytes in the basal forebrain (BF) of mice, a brain region important for sleep and wake expression and sleep homeostasis. Astroglial activation induced prolonged wakefulness without the expected homeostatic increase in sleep drive (i.e., sleep duration and intensity). These findings indicate that our need to sleep is also driven by non-neuronal cells, and not only by time spent awake.

The computational power of the human brain

At the end of the 20th century, analog systems in computer science have been widely replaced by digital systems due to their higher computing power. Nevertheless, the question keeps being intriguing until now: is the brain analog or digital? Initially, the latter has been favored, considering it as a Turing machine that works like a digital computer. However, more recently, digital and analog processes have been combined to implant human behavior in robots, endowing them with artificial intelligence (AI). Therefore, we think it is timely to compare mathematical models with the biology of computation in the brain. To this end, digital and analog processes clearly identified in cellular and molecular interactions in the Central Nervous System are highlighted. But above that, we try to pinpoint reasons distinguishing in silico computation from salient features of biological computation. First, genuinely analog information processing has been observed in electrical synapses and through gap junctions, the latter both in neurons and astrocytes. Apparently opposed to that, neuronal action potentials (APs) or spikes represent clearly digital events, like the yes/no or 1/0 of a Turing machine. However, spikes are rarely uniform, but can vary in amplitude and widths, which has significant, differential effects on transmitter release at the presynaptic terminal, where notwithstanding the quantal (vesicular) release itself is digital. Conversely, at the dendritic site of the postsynaptic neuron, there are numerous analog events of computation. Moreover, synaptic transmission of information is not only neuronal, but heavily influenced by astrocytes tightly ensheathing the majority of synapses in brain (tripartite synapse). At least at this point, LTP and LTD modifying synaptic plasticity and believed to induce short and long-term memory processes including consolidation (equivalent to RAM and ROM in electronic devices) have to be discussed. The present knowledge of how the brain stores and retrieves memories includes a variety of options (e.g., neuronal network oscillations, engram cells, astrocytic syncytium). Also epigenetic features play crucial roles in memory formation and its consolidation, which necessarily guides to molecular events like gene transcription and translation. In conclusion, brain computation is not only digital or analog, or a combination of both, but encompasses features in parallel, and of higher orders of complexity.

Astrocytes as master modulators of neural networks: Synaptic functions and disease-associated dysfunction of astrocytes

Astrocytes are the most abundant glial cell type in the central nervous system and are essential to the development, plasticity, and maintenance of neural circuits. Astrocytes are heterogeneous, with their diversity rooted in developmental programs modulated by the local brain environment. Astrocytes play integral roles in regulating and coordinating neural activity extending far beyond their metabolic support of neurons and other brain cell phenotypes. Both gray and white matter astrocytes occupy critical functional niches capable of modulating brain physiology on time scales slower than synaptic activity but faster than those adaptive responses requiring a structural change or adaptive myelination. Given their many associations and functional roles, it is not surprising that astrocytic dysfunction has been causally implicated in a broad set of neurodegenerative and neuropsychiatric disorders. In this review, we focus on recent discoveries concerning the contributions of astrocytes to the function of neural networks, with a dual focus on the contribution of astrocytes to synaptic development and maturation, and on their role in supporting myelin integrity, and hence conduction and its regulation. We then address the emerging roles of astrocytic dysfunction in disease pathogenesis and on potential strategies for targeting these cells for therapeutic purposes.

Constraints of vigilance-dependent noradrenergic signaling to mouse cerebellar Bergmann glia

Behavioral state plays an important role in determining astroglia Ca2+ signaling. In particular, locomotion-mediated elevated vigilance has been found to trigger norepinephrine-dependent whole cell Ca2+ elevations in astroglia throughout the brain. For cerebellar Bergmann glia it has recently been found that locomotion-induced transient Ca2+ elevations depend on their α1A -adrenergic receptors. With increasing availability and implementation of locomotion as behavioral parameter it becomes important to understand the constraints of noradrenergic signaling to astroglia. Here we evaluated the effect of speed, duration and interval of locomotion on Ca2+ signals in Bergmann glia as well as cerebellar noradrenergic axon terminals. We found almost no dependence on locomotion speed, but following the initial Ca2+ transient prolonged locomotion events revealed a steady-state Ca2+ elevation. Comparison of time course and recovery of transient Bergmann glia and noradrenergic terminal Ca2+ dynamics suggested that noradrenergic terminal Ca2+ activity determines Bergmann glia Ca2+ activation and does not require noradrenergic receptor desensitization to account for attenuation during prolonged locomotion. Further, analyzing the correlation among Ca2+ dynamics within regions within the field of observation we found that coordinated activity among noradrenergic terminals accounts for fluctuations of steady-state Bergmann glia Ca2+ activity. Together, our findings will help to better understand astroglia Ca2+ dynamics during less controlled awake behavior and may guide the identification of behavioral contexts preferably dependent on astroglia Ca2+ signaling.

Glymphatic influx and clearance are accelerated by neurovascular coupling

Functional hyperemia, also known as neurovascular coupling, is a phenomenon that occurs when neural activity increases local cerebral blood flow. Because all biological activity produces metabolic waste, we here sought to investigate the relationship between functional hyperemia and waste clearance via the glymphatic system. The analysis showed that whisker stimulation increased both glymphatic influx and clearance in the mouse somatosensory cortex with a 1.6-fold increase in periarterial cerebrospinal fluid (CSF) influx velocity in the activated hemisphere. Particle tracking velocimetry revealed a direct coupling between arterial dilation/constriction and periarterial CSF flow velocity. Optogenetic manipulation of vascular smooth muscle cells enhanced glymphatic influx in the absence of neural activation. We propose that impedance pumping allows arterial pulsatility to drive CSF in the same direction as blood flow, and we present a simulation that supports this idea. Thus, functional hyperemia boosts not only the supply of metabolites but also the removal of metabolic waste.

Astrocytes: integrators of arousal state and sensory context

The integration of external information with the internal state of the body is central to the survival of virtually every multicellular organism. However, a complete picture of the mechanisms that govern this process is lacking. In this opinion article, we synthesize evidence demonstrating that astrocytes sense the momentary arousal state - through neuromodulator release - as well as the sensory inputs - through local synaptic activity - and respond to them with changes in calcium (Ca2+) signaling. We hypothesize that astrocytes integrate sensory signals with the internal state and that this process is necessary to secure optimal behavior. Finally, we argue that dysfunctional astrocytic Ca2+ signaling could be an underlying factor in disorders characterized by disrupted sensory processing.

Electroacupuncture prevents astrocyte atrophy to alleviate depression

Astrocyte atrophy is the main histopathological hallmark of major depressive disorder (MDD) in humans and in animal models of depression. Here we show that electroacupuncture prevents astrocyte atrophy in the prefrontal cortex and alleviates depressive-like behaviour in mice subjected to chronic unpredictable mild stress (CUMS). Treatment of mice with CUMS induced depressive-like phenotypes as confirmed by sucrose preference test, tail suspension test, and forced swimming test. These behavioural changes were paralleled with morphological atrophy of astrocytes in the prefrontal cortex, revealed by analysis of 3D reconstructions of confocal Z-stack images of mCherry expressing astrocytes. This morphological atrophy was accompanied by a decrease in the expression of cytoskeletal linker Ezrin, associated with formation of astrocytic leaflets, which form astroglial synaptic cradle. Electroacupuncture at the acupoint ST36, as well as treatment with anti-depressant fluoxetine, prevented depressive-like behaviours, astrocytic atrophy, and down-regulation of astrocytic ezrin. In conclusion, our data further strengthen the notion of a primary role of astrocytic atrophy in depression and reveal astrocytes as cellular target for electroacupuncture in treatment of depressive disorders.

Multicore fiber optic imaging reveals that astrocyte calcium activity in the cerebral cortex is modulated by internal motivational state

Astrocytes are a direct target of neuromodulators and can influence neuronal activity on broad spatial and temporal scales through their close proximity to synapses. However, our knowledge about how astrocytes are functionally recruited during different animal behaviors and their diverse effects on the CNS remains limited. To enable measurement of astrocyte activity patterns in vivo during normative behaviors, we developed a high-resolution, long working distance, multi-core fiber optic imaging platform that allows visualization of cortical astrocyte calcium transients through a cranial window in freely moving mice. Using this platform, we defined the spatiotemporal dynamics of astrocytes during diverse behaviors, ranging from circadian fluctuations to novelty exploration, showing that astrocyte activity patterns are more variable and less synchronous than apparent in head-immobilized imaging conditions. Although the activity of astrocytes in visual cortex was highly synchronized during quiescence to arousal transitions, individual astrocytes often exhibited distinct thresholds and activity patterns during explorative behaviors, in accordance with their molecular diversity, allowing temporal sequencing across the astrocyte network. Imaging astrocyte activity during self-initiated behaviors revealed that noradrenergic and cholinergic systems act synergistically to recruit astrocytes during state transitions associated with arousal and attention, which was profoundly modulated by internal state. The distinct activity patterns exhibited by astrocytes in the cerebral cortex may provide a means to vary their neuromodulatory influence in response to different behaviors and internal states.

ATP-mediated signalling in the central synapses

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

Goodnight, astrocyte: waking up to astroglial mechanisms in sleep

Astrocytes mediate many important aspects of neural homeostasis, but until recently, their role in sleep was largely unknown. The situation has dramatically changed in the last decade. The use of transgenic animals, optogenetics, chemogenetics, brain imaging and sophisticated molecular assays has led to exciting discoveries. Astrocytes dynamically change their activity across the sleep-wake cycle and may encode sleep need via changes in intracellular signalling pathways. Astrocytes also exocytose/secrete sleep-inducing molecules which modulate brain activity, sleep architecture and sleep regulation. Many of these observations have been made in mice and Drosophila melanogaster, indicating that astroglial sleep mechanisms are evolutionarily conserved. We review recent findings and discuss future directions.

Dissociation Between Neuronal and Astrocytic Calcium Activity in Response to Locomotion in Mice

Locomotion triggers a coordinated response of both neurons and astrocytes in the brain. Here we performed calcium (Ca²⁺) imaging of these two cell types in the somatosensory cortex in head-fixed mice moving on the airlifted platform. Ca²⁺ activity in astrocytes significantly increased during locomotion from a low quiescence level. Ca²⁺ signals first appeared in the distal processes and then propagated to astrocytic somata, where it became significantly larger and exhibited oscillatory behaviour. Thus, astrocytic soma operates as both integrator and amplifier of Ca²⁺ signal. In neurons, Ca²⁺ activity was pronounced in quiescent periods and further increased during locomotion. Neuronal Ca²⁺ concentration ([Ca²⁺]_i) rose almost immediately following the onset of locomotion, whereas astrocytic Ca²⁺ signals lagged by several seconds. Such a long lag suggests that astrocytic [Ca²⁺]_i elevations are unlikely to be triggered by the activity of synapses among local neurons. Ca²⁺ responses to pairs of consecutive episodes of locomotion did not significantly differ in neurons, while were significantly diminished in response to the second locomotion in astrocytes. Such astrocytic refractoriness may arise from distinct mechanisms underlying Ca²⁺ signal generation. In neurons, the bulk of Ca²⁺ enters through the Ca²⁺ channels in the plasma membrane allowing for steady-level Ca²⁺ elevations in repetitive runs. Astrocytic Ca²⁺ responses originate from the intracellular stores, the depletion of which affects subsequent Ca²⁺ signals. Functionally, neuronal Ca²⁺ response reflects sensory input processed by neurons. Astrocytic Ca²⁺ dynamics is likely to provide metabolic and homeostatic support within the brain active milieu.

Shining the Light on Astrocytic Ensembles

While neurons have traditionally been considered the primary players in information processing, the role of astrocytes in this mechanism has largely been overlooked due to experimental constraints. In this review, we propose that astrocytic ensembles are active working groups that contribute significantly to animal conduct and suggest that studying the maps of these ensembles in conjunction with neurons is crucial for a more comprehensive understanding of behavior. We also discuss available methods for studying astrocytes and argue that these ensembles, complementarily with neurons, code and integrate complex behaviors, potentially specializing in concrete functions.

Distinct astrocytic modulatory roles in sensory transmission during sleep, wakefulness, and arousal states in freely moving mice

Despite extensive research on astrocytic Ca²⁺ in synaptic transmission, its contribution to the modulation of sensory transmission during different brain states remains largely unknown. Here, by using two-photon microscopy and whole-cell recordings, we show two distinct astrocytic Ca²⁺ signals in the murine barrel cortex: a small, long-lasting Ca²⁺ increase during sleep and a large, widespread but short-lasting Ca²⁺ spike when aroused. The large Ca²⁺ wave in aroused mice was inositol trisphosphate (IP3)-dependent, evoked by the locus coeruleus-norepinephrine system, and enhanced sensory input, contributing to reliable sensory transmission. However, the small Ca²⁺ transient was IP3-independent and contributed to decreased extracellular K⁺, hyperpolarization of the neurons, and suppression of sensory transmission. These events respond to different pharmacological inputs and contribute to distinct sleep and arousal functions by modulating the efficacy of sensory transmission. Together, our data demonstrate an important function for astrocytes in sleep and arousal states via astrocytic Ca²⁺ waves.

Norepinephrine links astrocytic activity to regulation of cortical state

Cortical state, defined by population-level neuronal activity patterns, determines sensory perception. While arousal-associated neuromodulators-including norepinephrine (NE)-reduce cortical synchrony, how the cortex resynchronizes remains unknown. Furthermore, general mechanisms regulating cortical synchrony in the wake state are poorly understood. Using in vivo imaging and electrophysiology in mouse visual cortex, we describe a critical role for cortical astrocytes in circuit resynchronization. We characterize astrocytes' calcium responses to changes in behavioral arousal and NE, and show that astrocytes signal when arousal-driven neuronal activity is reduced and bi-hemispheric cortical synchrony is increased. Using in vivo pharmacology, we uncover a paradoxical, synchronizing response to Adra1a receptor stimulation. We reconcile these results by demonstrating that astrocyte-specific deletion of Adra1a enhances arousal-driven neuronal activity, while impairing arousal-related cortical synchrony. Our findings demonstrate that astrocytic NE signaling acts as a distinct neuromodulatory pathway, regulating cortical state and linking arousal-associated desynchrony to cortical circuit resynchronization.

Analysis of Network Models with Neuron-Astrocyte Interactions

Neural networks, composed of many neurons and governed by complex interactions between them, are a widely accepted formalism for modeling and exploring global dynamics and emergent properties in brain systems. In the past decades, experimental evidence of computationally relevant neuron-astrocyte interactions, as well as the astrocytic modulation of global neural dynamics, have accumulated. These findings motivated advances in computational glioscience and inspired several models integrating mechanisms of neuron-astrocyte interactions into the standard neural network formalism. These models were developed to study, for example, synchronization, information transfer, synaptic plasticity, and hyperexcitability, as well as classification tasks and hardware implementations. We here focus on network models of at least two neurons interacting bidirectionally with at least two astrocytes that include explicitly modeled astrocytic calcium dynamics. In this study, we analyze the evolution of these models and the biophysical, biochemical, cellular, and network mechanisms used to construct them. Based on our analysis, we propose how to systematically describe and categorize interaction schemes between cells in neuron-astrocyte networks. We additionally study the models in view of the existing experimental data and present future perspectives. Our analysis is an important first step towards understanding astrocytic contribution to brain functions. However, more advances are needed to collect comprehensive data about astrocyte morphology and physiology in vivo and to better integrate them in data-driven computational models. Broadening the discussion about theoretical approaches and expanding the computational tools is necessary to better understand astrocytes' roles in brain functions.

Rescue of astrocyte activity by the calcium sensor STIM1 restores long-term synaptic plasticity in female mice modelling Alzheimer's disease

Calcium dynamics in astrocytes represent a fundamental signal that through gliotransmitter release regulates synaptic plasticity and behaviour. Here we present a longitudinal study in the PS2APP mouse model of Alzheimer's disease (AD) linking astrocyte Ca²⁺ hypoactivity to memory loss. At the onset of plaque deposition, somatosensory cortical astrocytes of AD female mice exhibit a drastic reduction of Ca²⁺ signaling, closely associated with decreased endoplasmic reticulum Ca²⁺ concentration and reduced expression of the Ca²⁺ sensor STIM1. In parallel, astrocyte-dependent long-term synaptic plasticity declines in the somatosensory circuitry, anticipating specific tactile memory loss. Notably, we show that both astrocyte Ca²⁺ signaling and long-term synaptic plasticity are fully recovered by selective STIM1 overexpression in astrocytes. Our data unveil astrocyte Ca²⁺ hypoactivity in neocortical astrocytes as a functional hallmark of early AD stages and indicate astrocytic STIM1 as a target to rescue memory deficits.

Receptor-receptor interactions and microvesicle exchange as mechanisms modulating signaling between neurons and astrocytes

It is well known that astrocytes play a significant metabolic role in the nervous tissue, maintaining the homeostasis of the extracellular space and of the blood-brain barrier, and providing trophic support to neurons. In addition, however, evidence exists indicating astrocytes as important elements for brain activity through signaling exchange with neurons. Astrocytes, indeed, can sense synaptic activity and their molecular machinery responds to neurotransmitters released by neurons with cytoplasmic Ca²⁺ elevations that, in turn, stimulate the release of neuroactive substances (gliotransmitters) influencing nearby neurons. In both cell types the recognition and transduction of this complex pattern of signals is mediated by specific receptors that are also involved in mechanisms tuning the intercellular cross-talk between astrocytes and neurons. Two of these mechanisms are the focus of the present discussion. The first concerns direct receptor-receptor interactions leading to the formation at the cell membrane of multimeric receptor complexes. The cooperativity that emerges in the actions of orthosteric and allosteric ligands of the monomers forming the assembly provides the cell decoding apparatus with sophisticated and flexible dynamics in terms of recognition and signal transduction pathways. A further mechanism of plasticity involving receptors is based on the transfer of elements of the cellular signaling apparatus via extracellular microvesicles acting as protective containers, which can lead to transient changes in the transmitting/decoding capabilities of the target cell.

Astrocytes in the pathophysiology of neuroinfection

Key homeostasis providing cells in the central nervous system (CNS) are astrocytes, which belong to the class of cells known as atroglia, a highly heterogeneous type of neuroglia and a prominent element of the brain defence. Diseases evolve due to altered homeostatic state, associated with pathology-induced astroglia remodelling represented by reactive astrocytes, astroglial atrophy and astrodegeneration. These features are hallmarks of most infectious insults, mediated by bacteria, protozoa and viruses; they are also prominent in the systemic infection. The COVID-19 pandemic revived the focus into neurotropic viruses such as SARS-CoV2 (Coronaviridae) but also the Flaviviridae viruses including tick-borne encephalitis (TBEV) and Zika virus (ZIKV) causing the epidemic in South America prior to COVID-19. Astrocytes provide a key response to neurotropic infections in the CNS. Astrocytes form a parenchymal part of the blood-brain barrier, the site of virus entry into the CNS. Astrocytes exhibit aerobic glycolysis, a form of metabolism characteristic of highly morphologically plastic cells, like cancer cells, hence a suitable milieu for multiplication of infectious agent, including viral particles. However, why the protection afforded by astrocytes fails in some circumstances is an open question to be studied in the future.

Adenosine-independent regulation of the sleep-wake cycle by astrocyte activity

Astrocytes play a crucial role in regulating sleep-wake behavior, and adenosine signaling is generally thought to be involved. Here we show multiple lines of evidence supporting that modulation of the sleep-wake behavior by astrocyte Ca²⁺ activity could occur without adenosine signaling. In the basal forebrain and the brainstem, two brain regions that are known to be essential for sleep-wake regulation, chemogenetically-induced astrocyte Ca²⁺ elevation significantly modulated the sleep-wake cycle. Although astrocyte Ca²⁺ level positively correlated with the amount of extracellular adenosine, as revealed by a genetically encoded adenosine sensor, we found no detectable change in adenosine level after suppressing astrocyte Ca²⁺ elevation, and transgenic mice lacking one of the major extracellular ATP-adenosine conversion enzymes showed similar extracellular adenosine level and astrocyte Ca²⁺-induced sleep modulation. Furthermore, astrocyte Ca²⁺ is dependent primarily on local neuronal activity, causing brain region-specific regulation of the sleep-wake cycle. Thus, neural activity-dependent astrocyte activity could regulate the sleep-wake behavior independent of adenosine signaling.

Adrenergic signalling to astrocytes in anterior cingulate cortex contributes to pain-related aversive memory in rats

Pain contains both sensory and affective dimensions. We identify the role of norepinephrine in colorectal distention (sub-threshold for acute pain) induced conditioned place avoidance and plasticity gene expression in the anterior cingulate cortex (ACC). Activating locus coeruleus (LC)-projecting ACC neurons facilitates pain-evoked aversive consolidation and memory, while inhibiting LC-projecting ACC neurons reversibly blocks it. Optogenetic activation of ACC astrocytes facilitates aversive behaviour. ACC astrocytic Gi manipulation suppressed aversive behaviour and early plasticity gene expression induced by opto-activation of LC neurons projecting to ACC. Evidences for the critical role of β2AR in ACC astrocytes were provided using AAV encoding β2AR miRNAi to knockdown β2AR in astrocytes. In contrast, opto-activation of ACC astrocytic β2ARs promotes aversion memory. Our findings suggest that projection-specific adrenergic astrocytic signalling in ACC is integral to system-wide neuromodulation in response to visceral stimuli, and plays a key role in mediating pain-related aversion consolidation and memory formation.

Endocannabinoid signaling in astrocytes

The study of the astrocytic contribution to brain functions has been growing in popularity in the neuroscience field. In the last years, and especially since the demonstration of the involvement of astrocytes in synaptic functions, the astrocyte field has revealed multiple functions of these cells that seemed inconceivable not long ago. In parallel, cannabinoid investigation has also identified different ways by which cannabinoids are able to interact with these cells, modify their functions, alter their communication with neurons and impact behavior. In this review, we will describe the expression of different endocannabinoid system members in astrocytes. Moreover, we will relate the latest findings regarding cannabinoid modulation of some of the most relevant astroglial functions, namely calcium (Ca²⁺ ) dynamics, gliotransmission, metabolism, and inflammation.

Calcium signaling in astrocytes and gliotransmitter release

Glia are as numerous in the brain as neurons and widely known to serve supportive roles such as structural scaffolding, extracellular ionic and neurotransmitter homeostasis, and metabolic support. However, over the past two decades, several lines of evidence indicate that astrocytes, which are a type of glia, play active roles in neural information processing. Astrocytes, although not electrically active, can exhibit a form of excitability by dynamic changes in intracellular calcium levels. They sense synaptic activity and release neuroactive substances, named gliotransmitters, that modulate neuronal activity and synaptic transmission in several brain areas, thus impacting animal behavior. This "dialogue" between astrocytes and neurons is embodied in the concept of the tripartite synapse that includes astrocytes as integral elements of synaptic function. Here, we review the recent work and discuss how astrocytes via calcium-mediated excitability modulate synaptic information processing at various spatial and time scales.

Glymphatic Dysfunction Induced Oxidative Stress and Neuro-Inflammation in Major Depression Disorders

Major Depression disorder (MDD) is a potentially life-threatening mental illness, however, many patients have a poor response to current treatments. Recent studies have suggested that stress- or trauma-induced oxidative stress and inflammation could be important factors involved in the development of MDD, but the mechanisms remain unclear. We showed that the glymphatic system is a recently discovered structure in the brain that may be involved in the clearance of large molecular and cell debris in extracellular space. In addition, the glymphatic system can help with the removal of reactive oxygen species (ROS) and cytokines such as IL-1β and HIF-1α. Glymphatic impairment can lead to ROS accumulation in the microenvironment, inducing cellular injury signaling and activating NLRP3 in microglia to induce inflammation and, thus, many brain diseases, including psychiatric disorders. Therefore, trauma-induced glymphatic impairment could induce oxidative stress and inflammation, and thus MDD. This paper will review recent advances with regard to stress-induced glymphatic system impairment and ROS-mediated inflammation in MDD.

Astrocyte Heterogeneity in Regulation of Synaptic Activity

Our awareness of the number of synapse regulatory functions performed by astroglia is rapidly expanding, raising interesting questions regarding astrocyte heterogeneity and specialization across brain regions. Whether all astrocytes are poised to signal in a multitude of ways, or are instead tuned to surrounding synapses and how astroglial signaling is altered in psychiatric and cognitive disorders are fundamental questions for the field. In recent years, molecular and morphological characterization of astroglial types has broadened our ability to design studies to better analyze and manipulate specific functions of astroglia. Recent data emerging from these studies will be discussed in depth in this review. I also highlight remaining questions emerging from new techniques recently applied toward understanding the roles of astrocytes in synapse regulation in the adult brain.

Monoamine Neurotransmitters Control Basic Emotions and Affect Major Depressive Disorders

Major depressive disorder (MDD) is a common and complex mental disorder, that adversely impacts an individual’s quality of life, but its diagnosis and treatment are not accurately executed and a symptom-based approach is utilized in most cases, due to the lack of precise knowledge regarding the pathophysiology. So far, the first-line treatments are still based on monoamine neurotransmitters. Even though there is a lot of progress in this field, the mechanisms seem to get more and more confusing, and the treatment is also getting more and more controversial. In this study, we try to review the broad advances of monoamine neurotransmitters in the field of MDD, and update its effects in many advanced neuroscience studies. We still propose the monoamine hypothesis but paid special attention to their effects on the new pathways for MDD, such as inflammation, oxidative stress, neurotrophins, and neurogenesis, especially in the glial cells, which have recently been found to play an important role in many neurodegenerative disorders, including MDD. In addition, we will extend the monoamine hypothesis to basic emotions; as suggested in our previous reports, the three monoamine neurotransmitters play different roles in emotions: dopamine—joy, norepinephrine—fear (anger), serotonins—disgust (sadness). Above all, this paper tries to give a full picture of the relationship between the MDD and the monoamine neurotransmitters such as DA, NE, and 5-HT, as well as their contributions to the Three Primary Color Model of Basic Emotions (joy, fear, and disgust). This is done by explaining the contribution of the monoamine from many sides for MDD, such the digestive tract, astrocytes, microglial, and others, and very briefly addressing the potential of monoamine neurotransmitters as a therapeutic approach for MDD patients and also the reasons for its limited clinical efficacy, side effects, and delayed onset of action. We hope this review might offer new pharmacological management of MDD.

Dopamine activates astrocytes in prefrontal cortex via α1-adrenergic receptors

The prefrontal cortex (PFC) is a hub for cognitive control, and dopamine profoundly influences its functions. In other brain regions, astrocytes sense diverse neurotransmitters and neuromodulators and, in turn, orchestrate regulation of neuroactive substances. However, basic physiology of PFC astrocytes, including which neuromodulatory signals they respond to and how they contribute to PFC function, is unclear. Here, we characterize divergent signaling signatures in mouse astrocytes of the PFC and primary sensory cortex, which show differential responsiveness to locomotion. We find that PFC astrocytes express receptors for dopamine but are unresponsive through the Gs/Gi-cAMP pathway. Instead, fast calcium signals in PFC astrocytes are time locked to dopamine release and are mediated by α1-adrenergic receptors both ex vivo and in vivo. Further, we describe dopamine-triggered regulation of extracellular ATP at PFC astrocyte territories. Thus, we identify astrocytes as active players in dopaminergic signaling in the PFC, contributing to PFC function though neuromodulator receptor crosstalk.

Pittolo et al. demonstrate that the neuromodulator dopamine targets astrocytes, a type of brain cell, via receptors specific to another neuromodulator—norepinephrine. This study provides groundwork on how dopamine affects non-neuronal brain cells and suggests that crosstalk between neuromodulatory pathways occurs in vivo, with possible clinical implications.

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

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

Astrocyte calcium dysfunction causes early network hyperactivity in Alzheimer's disease

Dysfunctions of network activity and functional connectivity (FC) represent early events in Alzheimer’s disease (AD), but the underlying mechanisms remain unclear. Astrocytes regulate local neuronal activity in the healthy brain, but their involvement in early network hyperactivity in AD is unknown. We show increased FC in the human cingulate cortex several years before amyloid deposition. We find the same early cingulate FC disruption and neuronal hyperactivity in App^NL-F mice. Crucially, these network disruptions are accompanied by decreased astrocyte calcium signaling. Recovery of astrocytic calcium activity normalizes neuronal hyperactivity and FC, as well as seizure susceptibility and day/night behavioral disruptions. In conclusion, we show that astrocytes mediate initial features of AD and drive clinically relevant phenotypes.

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The cingulate cortex of humans and mice shows early functional deficits in AD

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Astrocyte calcium signaling is decreased before the presence of amyloid plaques

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Recovery of astrocyte calcium signals mitigates neuronal hyperactivity

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Recovery of astrocytes normalizes cingulate connectivity and behavior disruptions

Glucocorticoid Receptor-Dependent Astrocytes Mediate Stress Vulnerability

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Impaired astrocytic Ca2+ signaling in awake-behaving Alzheimer's disease transgenic mice

Increased astrocytic Ca²⁺ signaling has been shown in Alzheimer’s disease mouse models, but to date no reports have characterized behaviorally induced astrocytic Ca²⁺ signaling in such mice. Here, we employ an event-based algorithm to assess astrocytic Ca²⁺ signals in the neocortex of awake-behaving tg-ArcSwe mice and non-transgenic wildtype littermates while monitoring pupil responses and behavior. We demonstrate an attenuated astrocytic Ca²⁺ response to locomotion and an uncoupling of pupil responses and astrocytic Ca²⁺ signaling in 15-month-old plaque-bearing mice. Using the genetically encoded fluorescent norepinephrine sensor GRAB_NE, we demonstrate a reduced norepinephrine signaling during spontaneous running and startle responses in the transgenic mice, providing a possible mechanistic underpinning of the observed reduced astrocytic Ca²⁺ responses. Our data points to a dysfunction in the norepinephrine–astrocyte Ca²⁺ activity axis, which may account for some of the cognitive deficits observed in Alzheimer’s disease.

Neurodegenerative conditions such as Parkinson’s or Alzheimer’s disease are characterized by neurons dying and being damaged. Yet neurons are only one type of brain actors; astrocytes, for example, are star-shaped ‘companion’ cells that have recently emerged as being able to fine-tune neuronal communication. In particular, they can respond to norepinephrine, a signaling molecule that acts to prepare the brain and body for action. This activation results, for instance, in astrocytes releasing chemicals that can act on neurons.

Certain cognitive symptoms associated with Alzheimer’s disease could be due to a lack of norepinephrine. In parallel, studies in anaesthetized mice have shown perturbed astrocyte signaling in a model of the condition. Disrupted norepinephrine-triggered astrocyte signaling could therefore be implicated in the symptoms of the disease. Experiments in awake mice are needed to investigate this link, especially as anesthesia is known to disrupt the activity of astrocytes.

To explore this question, Åbjørsbråten, Skaaraas et al. conducted experiments in naturally behaving mice expressing mutations found in patients with early-onset Alzheimer’s disease. These mice develop hallmarks of the disorder. Compared to their healthy counterparts, these animals had reduced astrocyte signaling when running or being startled. Similarly, a fluorescent molecular marker for norepinephrine demonstrated less signaling in the modified mice compared to healthy ones.

Over 55 million individuals currently live with Alzheimer’s disease. The results by Åbjørsbråten, Skaaraas et al. suggest that astrocyte–norepinephrine communication may be implicated in the condition, an avenue of research that could potentially lead to developing new treatments.

Noradrenergic Signaling in Astrocytes Influences Mammalian Sleep Homeostasis

Astrocytes influence sleep expression and regulation, but the cellular signaling pathways involved in these processes are poorly defined. We proposed that astrocytes detect and integrate a neuronal signal that accumulates during wakefulness, thereby leading to increased sleep drive. Noradrenaline (NA) satisfies several criteria for a waking signal integrated by astrocytes. We therefore investigated the role of NA signaling in astrocytes in mammalian sleep. We conditionally knocked out (cKO) β2-adrenergic receptors (β2-AR) selectively in astrocytes in mice and recorded electroencephalographic and electromyographic activity under baseline conditions and in response to sleep deprivation (SDep). cKO of astroglial β2-ARs increased active phase siesta duration under baseline conditions and reduced homeostatic compensatory changes in sleep consolidation and non-rapid eye movement slow-wave activity (SWA) after SDep. Overall, astroglial NA β2-ARs influence mammalian sleep homeostasis in a manner consistent with our proposed model of neuronal-astroglial interactions.

Critical Spatial-Temporal Dynamics and Prominent Shape Collapse of Calcium Waves Observed in Human hNT Astrocytes in Vitro

Networks of neurons are typically studied in the field of Criticality. However, the study of astrocyte networks in the brain has been recently lauded to be of equal importance to that of the neural networks. To date criticality assessments have only been performed on networks astrocytes from healthy rats, and astrocytes from cultured dissociated resections of intractable epilepsy. This work, for the first time, presents studies of the critical dynamics and shape collapse of calcium waves observed in cultures of healthy human astrocyte networks in vitro, derived from the human hNT cell line. In this article, we demonstrate that avalanches of spontaneous calcium waves display strong critical dynamics, including power-laws in both the size and duration distributions. In addition, the temporal profiles of avalanches displayed self-similarity, leading to shape collapse of the temporal profiles. These findings are significant as they suggest that cultured networks of healthy human hNT astrocytes self-organize to a critical point, implying that healthy astrocytic networks operate at a critical point to process and transmit information. Furthermore, this work can serve as a point of reference to which other astrocyte criticality studies can be compared.

Astrocytic Calcium and cAMP in Neurodegenerative Diseases

It is commonly accepted that the role of astrocytes exceeds far beyond neuronal scaffold and energy supply. Their unique morphological and functional features have recently brough much attention as it became evident that they play a fundamental role in neurotransmission and interact with synapses. Synaptic transmission is a highly orchestrated process, which triggers local and transient elevations in intracellular Ca²⁺, a phenomenon with specific temporal and spatial properties. Presynaptic activation of Ca²⁺-dependent adenylyl cyclases represents an important mechanism of synaptic transmission modulation. This involves activation of the cAMP-PKA pathway to regulate neurotransmitter synthesis, release and storage, and to increase neuroprotection. This aspect is of paramount importance for the preservation of neuronal survival and functionality in several pathological states occurring with progressive neuronal loss. Hence, the aim of this review is to discuss mutual relationships between cAMP and Ca²⁺ signaling and emphasize those alterations at the Ca²⁺/cAMP crosstalk that have been identified in neurodegenerative disorders, such as Alzheimer's and Parkinson's disease.