Region-Specific and State-Dependent Astrocyte Ca2+ Dynamics during the Sleep-Wake Cycle in Mice

Astrocytes at the heart of sleep: from genes to network dynamics

Astrocytes have transcended their role from mere structural scaffolds to pivotal regulators of neural circuitry and sleep-wake dynamics. The strategic proximity of their fine processes to blood vessels and synapses positions them as key players in neurobiology, contributing to the tripartite synapse concept. Gap-junction proteins also enable astrocytes to form an extensive network interacting with neuronal assemblies to influence sleep physiology. Recent advances in genetic engineering, neuroimaging and molecular biology have deepened our understanding of astrocytic functions. This review highlights the different mechanisms by which astrocytes regulate sleep, notably through transcriptomic and morphological changes, as well as gliotransmission, whereby intracellular calcium (Ca2+) dynamics plays a significant role in modulating the sleep-wake cycle. In vivo optogenetic stimulation of astrocytes indeed induces ATP release, which is subsequently degraded into adenosine, modulating neuronal excitability in sleep-wake regulatory brain regions. Astrocytes also participate in synaptic plasticity, potentially modulating sleep-associated downscaling, a process essential for memory consolidation and preventing synaptic saturation. Although astrocytic involvement in synaptic maintenance is well supported, the precise molecular mechanisms linking these processes to sleep regulation remain to be elucidated. By highlighting astrocytes' multiple roles in sleep physiology, these insights deepen our understanding of sleep mechanisms and pave the way for improving sleep quality.

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

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

Glia: the cellular glue that binds circadian rhythms and sleep

Glia are increasingly appreciated as serving an important function in the control of sleep and circadian rhythms. Glial cells in Drosophila and mammals regulate daily rhythms of locomotor activity and sleep as well as homeostatic rebound following sleep deprivation. In addition, they contribute to proposed functions of sleep, with different functions mapping to varied glial subtypes. Here, we discuss recent findings in Drosophila and rodent models establishing a role of glia in circadian or sleep regulation of synaptic plasticity, brain metabolism, removal of cellular debris, and immune challenges. These findings underscore the relevance of glia for benefits attributed to sleep and have implications for understanding the neurobiological mechanisms underlying sleep and associated disorders.

Cognitive disorders: Potential astrocyte-based mechanism

Cognitive disorders are a common clinical manifestation, including a deterioration in the patient's memory ability, attention, executive power, language, and other functions. The contributing factors of cognitive disorders are numerous and diverse in nature, including organic diseases and other mental disorders. Neurodegenerative diseases are a common type of organic disease related to the pathology of neuronal death and disruption of glial cell balance, ultimately accompanied with cognitive impairment. Thus, cognitive disorder frequently serves as an extremely critical indicator of neurodegenerative disorders. Cognitive impairments negatively affect patients' daily lives. However, our understanding of the precise pathogenic pathways of cognitive defects remains incomplete. The most prevalent kind of glial cells in the central nervous system are called astrocytes. They have a unique significance in cerebral function because of their wide range of functions in maintaining homeostasis in the central nervous system, regulating synaptic plasticity, and so on. Dysfunction of astrocytes is intimately linked to cognitive disorders, and we are attempting to understand this phenomenon predominantly from those perspectives: neuroinflammation, astrocytic senescence, connexin, Ca2 + signaling, mitochondrial dysfunction, and the glymphatic system.

Sleep-wake modulation and pathogenesis of Alzheimer disease: Suggestions for postponement and treatment

Sleep-wake disorders are recognized as one of the earliest symptoms of Alzheimer disease (AD). Accumulating evidence has highlighted a significant association between sleep-wake disorders and AD pathogenesis, suggesting that sleep-wake modulation could be a promising approach for postponing AD onset. The suprachiasmatic nucleus (SCN) and the pineal hormone melatonin are major central modulating components of the circadian rhythm system. Cerebrospinal fluid (CSF) melatonin levels are dramatically decreased in AD. Interestingly, the number of neurofibrillary tangles in the hippocampus, which is one of the two major neuropathologic AD biomarkers, increases in parallel with the decrease in CSF melatonin levels. Furthermore, a decrease in salivary melatonin levels in middle-aged persons is a significant risk factor for the onset of the early stages of AD. Moreover, the disappearance of rhythmic fluctuations in melatonin may be one of the best biomarkers for AD diagnosis. Light therapy combined with melatonin supplementation is the recommended first-line treatment for sleep-wake disorders in AD patients and may be beneficial for ameliorating cognitive impairment. Sleep-wake cycle modulation based on AD risk gene presence is a promising early intervention for AD onset postponement.

Chemogenetic activation of astrocytes modulates sleep-wakefulness states in a brain region-dependent manner

Study Objectives

Astrocytes change their intracellular calcium (Ca2+) concentration during sleep/wakefulness states in mice. Furthermore, the Ca2+ dynamics in astrocytes vary depending on the brain region. However, it remains unclear whether alterations in astrocyte activity can affect sleep-wake states and cortical oscillations in a brain region-dependent manner.

Methods

Astrocyte activity was artificially manipulated in mice using chemogenetics. Astrocytes in the hippocampus and pons, which are 2 brain regions previously classified into different clusters based on their Ca2+ dynamics during sleep-wakefulness, were focused on to compare whether there are differences in the effects of astrocytes from different brain regions.

Results

The chemogenetic activation of astrocytes in the hippocampus significantly decreased the total time of wakefulness and increased the total time of sleep. This had little effect on cortical oscillations in all sleep-wakefulness states. On the other hand, the activation of astrocytes in the pons substantially suppressed rapid eye movement (REM) sleep in association with a decreased number of REM episodes, indicating strong inhibition of REM onset. Regarding cortical oscillations, the delta wave component during non-REM sleep was significantly enhanced.

Conclusions

These results suggest that astrocytes modulate sleep-wakefulness states and cortical oscillations. Furthermore, the role of astrocytes in sleep-wakefulness states appears to vary among brain regions.

Adenosine-Dependent Arousal Induced by Astrocytes in a Brainstem Circuit

Astrocytes play a crucial role in regulating sleep-wake behavior. However, how astrocytes govern a specific sleep-arousal circuit remains unknown. Here, the authors show that parafacial zone (PZ) astrocytes responded to sleep-wake cycles with state-differential Ca2+ activity, peaking during transitions from sleep to wakefulness. Using chemogenetic and optogenetic approaches, they find that activating PZ astrocytes elicited and sustained wakefulness by prolonging arousal episodes while impeding transitions from wakefulness to non-rapid eye movement (NREM) sleep. Activation of PZ astrocytes specially induced the elevation of extracellular adenosine through the ATP hydrolysis pathway but not equilibrative nucleoside transporter (ENT) mediated transportation. Strikingly, the rise in adenosine levels induced arousal by activating A1 receptors, suggesting a distinct role for adenosine in the PZ beyond its conventional sleep homeostasis modulation observed in the basal forebrain (BF) and cortex. Moreover, at the circuit level, PZ astrocyte activation induced arousal by suppressing the GABA release from the PZGABA neurons, which promote NREM sleep and project to the parabrachial nucleus (PB). Thus, their study unveils a distinctive arousal-promoting effect of astrocytes within the PZ through extracellular adenosine and elucidates the underlying mechanism at the neural circuit level.

Circadian regulation of endoplasmic reticulum calcium response in cultured mouse astrocytes

The circadian clock, an internal time-keeping system orchestrates 24 hr rhythms in physiology and behavior by regulating rhythmic transcription in cells. Astrocytes, the most abundant glial cells, play crucial roles in CNS functions, but the impact of the circadian clock on astrocyte functions remains largely unexplored. In this study, we identified 412 circadian rhythmic transcripts in cultured mouse cortical astrocytes through RNA sequencing. Gene Ontology analysis indicated that genes involved in Ca2+ homeostasis are under circadian control. Notably, Herpud1 (Herp) exhibited robust circadian rhythmicity at both mRNA and protein levels, a rhythm disrupted in astrocytes lacking the circadian transcription factor, BMAL1. HERP regulated endoplasmic reticulum (ER) Ca2+ release by modulating the degradation of inositol 1,4,5-trisphosphate receptors (ITPRs). ATP-stimulated ER Ca2+ release varied with the circadian phase, being more pronounced at subjective night phase, likely due to the rhythmic expression of ITPR2. Correspondingly, ATP-stimulated cytosolic Ca2+ increases were heightened at the subjective night phase. This rhythmic ER Ca2+ response led to circadian phase-dependent variations in the phosphorylation of Connexin 43 (Ser368) and gap junctional communication. Given the role of gap junction channel (GJC) in propagating Ca2+ signals, we suggest that this circadian regulation of ER Ca2+ responses could affect astrocytic modulation of synaptic activity according to the time of day. Overall, our study enhances the understanding of how the circadian clock influences astrocyte function in the CNS, shedding light on their potential role in daily variations of brain activity and health.

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.

Mapping multi-regional functional connectivity of astrocyte-neuronal networks during behaviors

Significance

Diverse behaviors rely on coordinated activity and multi-regional functional connectivity within astrocyte-neuronal networks. However, current techniques for simultaneously measuring astrocytic and neuronal activities across multiple brain regions during behaviors remain limited.

Aim

We propose a multi-fiber solution that can simultaneously record activities of astrocyte-neuronal networks across multiple regions during behaviors.

Approach

We employed cell-specific dual-color genetically encoded calcium indicators (GECIs) and multi-fiber photometry to simultaneously measure astrocytic and neuronal Ca2+ transients across multiple brain regions in freely behaving animals.

Results

Our findings demonstrate that both movements and sensory stimuli induce synchronized and highly correlated Ca2+ transients in astrocytes and neurons of freely behaving mice. In addition, we recorded astrocytic and neuronal Ca2+ transients from multiple brain regions during mouse behaviors. Our observations reveal heightened synchronization of astrocytic and neuronal Ca2+ transients across different brain regions during movements or sensory stimuli, indicating enhanced functional connectivity within brain-wide astrocyte-neuronal networks.

Conclusions

Multi-fiber photometry, combined with cell-specific dual-color GECIs, represents a powerful approach for investigating astrocytic and neuronal activities across different brain regions during behaviors. This technique serves as a versatile tool for analyzing the multi-regional functional connectivity map of astrocyte-neuronal networks associated with specific behaviors.

ARViS: a bleed-free multi-site automated injection robot for accurate, fast, and dense delivery of virus to mouse and marmoset cerebral cortex

Genetically encoded fluorescent sensors continue to be developed and improved. If they could be expressed across multiple cortical areas in non-human primates, it would be possible to measure a variety of spatiotemporal dynamics of primate-specific cortical activity. Here, we develop an Automated Robotic Virus injection System (ARViS) for broad expression of a biosensor. ARViS consists of two technologies: image recognition of vasculature structures on the cortical surface to determine multiple injection sites without hitting them, and robotic control of micropipette insertion perpendicular to the cortical surface with 50 μm precision. In mouse cortex, ARViS sequentially injected virus solution into 100 sites over a duration of 100 min with a bleeding probability of only 0.1% per site. Furthermore, ARViS successfully achieved 266-site injections over the frontoparietal cortex of a female common marmoset. We demonstrate one-photon and two-photon calcium imaging in the marmoset frontoparietal cortex, illustrating the effective expression of biosensors delivered by ARViS.

Fiber photometry-based investigation of brain function and dysfunction

Fiber photometry is an optical method to monitor fluorescent signals using a fiber optic cannula. Over the past two decades, together with the development of various genetically encoded biosensors, it has been applied to investigate various types of activity in the central nervous system. This includes not only type-specific neuronal population activity, but also non-neuronal activity and neurotransmitter/neuropeptide signals in awake, freely behaving animals. In this perspective, we summarize the recent development of this technique. After describing common technical pitfalls, we discuss future directions of this powerful approach for investigating brain function and dysfunction.

Astrocytes in selective vulnerability to neurodegenerative disease

Selective vulnerability of specific brain regions and cell populations is a hallmark of neurodegenerative disorders. Mechanisms of selective vulnerability involve neuronal heterogeneity, functional specializations, and differential sensitivities to stressors and pathogenic factors. In this review we discuss the growing body of literature suggesting that, like neurons, astrocytes are heterogeneous and specialized, respond to and integrate diverse inputs, and induce selective effects on brain function. In disease, astrocytes undergo specific, context-dependent changes that promote different pathogenic trajectories and functional outcomes. We propose that astrocytes contribute to selective vulnerability through maladaptive transitions to context-divergent phenotypes that impair specific brain regions and functions. Further studies on the multifaceted roles of astrocytes in disease may provide new therapeutic approaches to enhance resilience against neurodegenerative disorders.

Engram reactivation mimics cellular signatures of fear

Engrams, or the physical substrate of memory, recruit heterogeneous cell types. Targeted reactivation of neurons processing discrete memories drives the behavioral expression of memory, though the underlying landscape of recruited cells and their real-time responses remain elusive. To understand how artificial stimulation of fear affects intra-hippocampal neuron-astrocyte dynamics as well as their behavioral consequences, we express channelrhodopsin-2 in an activity-dependent manner within dentate gyrus neurons while recording both cell types with fiber photometry in hippocampal ventral CA1 across learning and memory. Both cell types exhibit shock responsiveness, with astrocytic calcium events uniquely modulated by fear conditioning. Optogenetic stimulation of a hippocampus-mediated engram recapitulates coordinated calcium signatures time locked to freezing, mirroring those observed during natural fear memory recall. Our findings reveal cell-type-specific dynamics in the hippocampus during freezing behavior, emphasizing neuronal-astrocytic coupling as a shared mechanism enabling both natural and artificially induced memory retrieval and the behavioral expression of fear.

Circadian Mechanisms in Brain Fluid Biology

The brain is a complex organ, fundamentally changing across the day to perform basic functions like sleep, thought, and regulating whole-body physiology. This requires a complex symphony of nutrients, hormones, ions, neurotransmitters and more to be properly distributed across the brain to maintain homeostasis throughout 24 hours. These solutes are distributed both by the blood and by cerebrospinal fluid. Cerebrospinal fluid contents are distinct from the general circulation because of regulation at brain barriers including the choroid plexus, glymphatic system, and blood-brain barrier. In this review, we discuss the overlapping circadian (≈24-hour) rhythms in brain fluid biology and at the brain barriers. Our goal is for the reader to gain both a fundamental understanding of brain barriers alongside an understanding of the interactions between these fluids and the circadian timing system. Ultimately, this review will provide new insight into how alterations in these finely tuned clocks may lead to pathology.

Sleep, Glial Function, and the Endocannabinoid System: Implications for Neuroinflammation and Sleep Disorders

The relationship between sleep, glial cells, and the endocannabinoid system represents a multifaceted regulatory network with profound implications for neuroinflammation and cognitive function. The molecular underpinnings of sleep modulation by the endocannabinoid system and its influence on glial cell activity are discussed, shedding light on the reciprocal relationships that govern these processes. Emphasis is placed on understanding the role of glial cells in mediating neuroinflammatory responses and their modulation by sleep patterns. Additionally, this review examines how the endocannabinoid system interfaces with glia-immune signaling to regulate inflammatory cascades within the central nervous system. Notably, the cognitive consequences of disrupted sleep, neuroinflammation, and glial dysfunction are addressed, encompassing implications for neurodegenerative disorders, mood disturbances, and cognitive decline. Insights into the bidirectional modulation of cognitive function by the endocannabinoid system in the context of sleep and glial activity are explored, providing a comprehensive perspective on the potential mechanisms underlying cognitive impairments associated with sleep disturbances. Furthermore, this review examines potential therapeutic avenues targeting the endocannabinoid system to mitigate neuroinflammation, restore glial homeostasis, and normalize sleep patterns. The identification of novel therapeutic targets within this intricate regulatory network holds promise for addressing conditions characterized by disrupted sleep, neuroinflammation, and cognitive dysfunction. This work aims to examine the complexities of neural regulation and identify potential avenues for therapeutic intervention.

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: 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.

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.

Basolateral Amygdala Astrocytes Are Engaged by the Acquisition and Expression of a Contextual Fear Memory

Astrocytes are key cellular regulators within the brain. The basolateral amygdala (BLA) is implicated in fear memory processing, yet most research has entirely focused on neuronal mechanisms, despite a significant body of work implicating astrocytes in learning and memory. In the present study, we used in vivo fiber photometry in C57BL/6J male mice to record from amygdalar astrocytes across fear learning, recall, and three separate periods of extinction. We found that BLA astrocytes robustly responded to foot shock during acquisition, their activity remained remarkably elevated across days in comparison to unshocked control animals, and their increased activity persisted throughout extinction. Further, we found that astrocytes responded to the initiation and termination of freezing bouts during contextual fear conditioning and recall, and this behavior-locked pattern of activity did not persist throughout the extinction sessions. Importantly, astrocytes do not display these changes while exploring a novel context, suggesting that these observations are specific to the original fear-associated environment. Chemogenetic inhibition of fear ensembles in the BLA did not affect freezing behavior or astrocytic calcium dynamics. Overall, our work presents a real-time role for amygdalar astrocytes in fear processing and provides new insight into the emerging role of these cells in cognition and behavior.SIGNIFICANCE STATEMENT We show that basolateral amygdala astrocytes are robustly responsive to negative experiences, like shock, and display changed calcium activity patterns through fear learning and memory. Additionally, astrocytic calcium responses become time locked to the initiation and termination of freezing behavior during fear learning and recall. We find that astrocytes display calcium dynamics unique to a fear-conditioned context, and chemogenetic inhibition of BLA fear ensembles does not have an impact on freezing behavior or calcium dynamics. These findings show that astrocytes play a key real-time role in fear learning and memory.

Activation of astrocytes in the basal forebrain in mice facilitates isoflurane-induced loss of consciousness and prolongs recovery

OBJECTIVES

General anesthesia results in a state of unconsciousness that is similar to sleep. In recent years, increasing evidence has reported that astrocytes play a crucial role in regulating sleep. However, whether astrocytes are involved in general anesthesia is unknown.

METHODS

In the present study, the designer receptors exclusively activated by designer drugs (DREADDs) approach was utilized to specifically activate astrocytes in the basal forebrain (BF) and observed its effect on isoflurane anesthesia. One the other side, L-α-aminoadipic acid was used to selectively inhibit astrocytes in the BF and investigated its influence on isoflurane-induced hypnotic effect. During the anesthesia experiment, cortical electroencephalography (EEG) signals were recorded as well.

RESULTS

The chemogenetic activation group had a significantly shorter isoflurane induction time, longer recovery time, and higher delta power of EEG during anesthesia maintenance and recovery periods than the control group. Inhibition of astrocytes in the BF delayed isoflurane-induced loss of consciousness, promoted recovery, decreased delta power and increased beta and gamma power during maintenance and recovery periods.

CONCLUSIONS

The present study suggests that astrocytes in the BF region are involved in isoflurane anesthesia and may be a potential target for regulating the consciousness state of anesthesia.

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.

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.

The stress of losing sleep: Sex-specific neurobiological outcomes

Sleep is a vital and evolutionarily conserved process, critical to daily functioning and homeostatic balance. Losing sleep is inherently stressful and leads to numerous detrimental physiological outcomes. Despite sleep disturbances affecting everyone, women and female rodents are often excluded or underrepresented in clinical and pre-clinical studies. Advancing our understanding of the role of biological sex in the responses to sleep loss stands to greatly improve our ability to understand and treat health consequences of insufficient sleep. As such, this review discusses sex differences in response to sleep deprivation, with a focus on the sympathetic nervous system stress response and activation of the hypothalamic-pituitary-adrenal (HPA) axis. We review sex differences in several stress-related consequences of sleep loss, including inflammation, learning and memory deficits, and mood related changes. Focusing on women's health, we discuss the effects of sleep deprivation during the peripartum period. In closing, we present neurobiological mechanisms, including the contribution of sex hormones, orexins, circadian timing systems, and astrocytic neuromodulation, that may underlie potential sex differences in sleep deprivation responses.

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.

Understanding neuron-glia crosstalk and biological clocks in insomnia

According to the World Health Organization, about one-third of the population experiences insomnia symptoms, and about 10-15% suffer from chronic insomnia, the most common sleep disorder. Sleeping difficulties associated with insomnia are often linked to chronic sleep deprivation, which has a negative health impact partly due to disruption in the internal synchronisation of biological clocks. These are regulated by clock genes and modulate most biological processes. Most studies addressing circadian rhythm regulation have focused on the role of neurons, yet glial cells also impact circadian rhythms and sleep regulation. Chronic insomnia and sleep loss have been associated with glial cell activation, exacerbated neuroinflammation, oxidative stress, altered neuronal metabolism and synaptic plasticity, accelerated age-related processes and decreased lifespan. It is, therefore, essential to highlight the importance of glia-neuron interplay on sleep/circadian regulation and overall healthy brain function. Hence, in this review, we aim to address the main neurobiological mechanisms involved in neuron-glia crosstalk, with an emphasis on microglia and astrocytes, in both healthy sleep, chronic sleep deprivation and chronic insomnia.

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.

State-dependent and region-specific alterations of cerebellar connectivity across stable human wakefulness and NREM sleep states

Sleep regulation and functioning may rely on systematic coordination throughout the whole brain, including the cerebellum. However, whether and how interactions between the cerebellum and other brain regions vary across sleep stages remain poorly understood. Here, using simultaneous EEG-fMRI recordings captured from 73 participants during wakefulness and non-rapid eye movement (NREM) sleep, we constructed cerebellar connectivity among intrinsic functional networks with intra-cerebellar, neocortical and subcortical regions. We uncovered that cerebellar connectivity exhibited sleep-dependent alterations: slight differences between wakefulness and N1/N2 sleep and greater changes in N3 sleep than other states. Region-specific cerebellar connectivity changes between N2 sleep and N3 sleep were also revealed: general breakdown of intra-cerebellar connectivity, enhancement of limbic-cerebellar connectivity and alterations of cerebellar connectivity with spatially specific neocortices. Further correlation analysis showed that functional connectivity between the cerebellar Control II network and regions (including the insula, hippocampus, and amygdala) correlated with delta power during N3 and beta power during N2 sleep. These findings systematically reveal altered cerebellar connectivity among intrinsic networks from wakefulness to deep sleep and highlight the potential role of the cerebellum in sleep regulation and functioning.

Parabrachial nucleus astrocytes regulate wakefulness and isoflurane anesthesia in mice

Background: The parabrachial nucleus (PBN) is an important structure regulating the sleep-wake behavior and general anesthesia. Astrocytes in the central nervous system modulate neuronal activity and consequential behavior. However, the specific role of the parabrachial nucleus astrocytes in regulating the sleep-wake behavior and general anesthesia remains unclear. Methods: We used chemogenetic approach to activate or inhibit the activity of PBN astrocytes by injecting AAV-GFAabc1d-hM3Dq-eGFP or AAV-GFAabc1d-hM4Di-eGFP into the PBN. We investigated the effects of intraperitoneal injection of CNO or vehicle on the amount of wakefulness, NREM sleep and REM sleep in sleep-wake behavior, and on the time of loss of righting reflex, time of recovery of righting reflex, sensitivity to isoflurane, electroencephalogram (EEG) power spectrum and burst suppression ratio (BSR) in isoflurane anesthesia. Results: The activation of PBN astrocytes increased wakefulness amount for 4 h, while the inhibition of PBN astrocytes decreased total amount of wakefulness during the 3-hour post-injection period. Chemogenetic activation of PBN astrocytes decreased isoflurane sensitivity and shortened the emergence time from isoflurane-induced general anesthesia. Cortical EEG recordings revealed that PBN astrocyte activation decreased the EEG delta power and BSR during isoflurane anesthesia. Chemogenetic Inhibition of PBN astrocytes increased the EEG delta power and BSR during isoflurane anesthesia. Conclusion: PBN astrocytes are a key neural substrate regulating wakefulness and emergence from isoflurane anesthesia.

Endocannabinoid signaling in synaptic function

In the last decades, astrocytes have emerged as important regulatory cells actively involved in brain function by exchanging signaling with neurons. The endocannabinoid (eCB) signaling is widely present in many brain areas, being crucially involved in multiple brain functions and animal behaviors. The present review presents and discusses current evidence demonstrating that astrocytes sense eCBs released during neuronal activity and subsequently release gliotransmitters that regulate synaptic transmission and plasticity. The eCB signaling to astrocytes and the synaptic regulation mediated by astrocytes activated by eCBs are complex phenomena that exhibit exquisite spatial and temporal properties, a wide variety of downstream signaling mechanisms, and a large diversity of functional synaptic outcomes. Studies investigating this topic have revealed novel regulatory processes of synaptic function, like the lateral regulation of synaptic transmission and the active involvement of astrocytes in the spike-timing dependent plasticity, originally thought to be exclusively mediated by the coincident activity of pre- and postsynaptic neurons, following Hebbian rules for associative learning. Finally, the critical influence of astrocyte-mediated eCB signaling on animal behavior is also discussed.

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.

Control of complex behavior by astrocytes and microglia

Evidence that glial cells influence behavior has been gaining a steady foothold in scientific literature. Out of the five main subtypes of glial cells in the brain, astrocytes and microglia have received an outsized share of attention with regard to shaping a wide spectrum of behavioral phenomena and there is growing appreciation that the signals intrinsic to these cells as well as their interactions with surrounding neurons reflect behavioral history in a brain region-specific manner. Considerable regional diversity of glial cell phenotypes is beginning to be recognized and may contribute to behavioral outcomes arising from circuit-specific computations within and across discrete brain nuclei. Here, we summarize current knowledge on the impact of astrocyte and microglia activity on behavioral outcomes, with a specific focus on brain areas relevant to higher cognitive control, reward-seeking, and circadian regulation.

Astrocyte regulation of neural circuit activity and network states

Astrocytes are known to influence neuronal activity through different mechanisms, including the homeostatic control of extracellular levels of ions and neurotransmitters and the exchange of signaling molecules that regulate synaptic formation, structure, and function. While a great effort done in the past has defined many molecular mechanisms and cellular processes involved in astrocyte-neuron interactions at the cellular level, the consequences of these interactions at the network level in vivo have only relatively recently been identified. This review describes and discusses recent findings on the regulatory effects of astrocytes on the activity of neuronal networks in vivo. Accumulating but still limited, evidence indicates that astrocytes regulate neuronal network rhythmic activity and synchronization as well as brain states. These studies demonstrate a critical contribution of astrocytes to brain activity and are paving the way for a more thorough understanding of the cellular bases of brain function.