Astrocytes and the modulation of sleep

The Interplay Between the Sleep Slow Oscillation and Cerebrospinal Fluid: New Vistas for Insomnia Research

Insomnia disorder affects about 10% of the global population, representing a major health concern. Despite the availability of evidence-based treatments, the neurobiological mechanisms underpinning this disorder remain poorly understood. Recently, the investigation of the less than 1 Hz oscillations (commonly termed slow oscillations), a hallmark of slow wave sleep, has gained increased interest in research on insomnia. In this context, an intriguing perspective arises from the association between slow oscillations and metabolic waste clearance, an impaired process in individuals suffering from insomnia disorder. Indeed, the exploration of the relationships between cerebrospinal fluid dynamics and glymphatic system functions, which relate to brain metabolic clearance, and sleep slow oscillations may represent a promising avenue for future research in this field. This narrative review examines current knowledge about the intricate interplay among these mechanisms and their implications for insomnia disorder. Particular attention is given to the role of sleep slow oscillations in the clearance of metabolic waste during sleep, their coupling with cerebrospinal fluid oscillations, and the regulatory mechanisms underlying glymphatic function. The review emphasises the relevance of investigating sleep slow oscillations-related mechanisms in insomnia, intending to provide novel insights into the neurophysiological underpinnings of the disorder and contribute to more accurate diagnostic approaches. Furthermore, a deeper understanding of these mechanisms could pave the way for the development of innovative or adjunctive therapeutic strategies targeting sleep slow oscillations-related alterations in insomnia disorder.

Adenosine and Cortical Plasticity

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

Neuron-Astrocyte Interactions and Circadian Timekeeping in Mammals

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

Microglial morphology aligns with vigilance stage-specific neuronal oscillations in a brain region-dependent manner

Microglia, the resident immune cells in the brain, dynamically adapt their morphology based on their functional state. This study explored the relationship between microglial morphology and sleep-wake cycles in mice. Using Iba1 immunostaining to identify microglia, we quantified morphological changes in microglia at different timepoints in multiple brain regions (cortex, hippocampus, basal forebrain, hindbrain, and cerebellum) in B6 male mice using semi-automated 3D structural analysis. Simultaneously, in a separate group, we monitored wake and sleep stage-specific brain activity using EEG/EMG recordings. During natural sleep-wake cycles, we observed increased microglial complexity (enlarged volume, territorial coverage, and ramification) during wakefulness, characterized by high-frequency theta (8-12 Hz) and gamma activity (30-80 Hz). Conversely, during NREM sleep, which is dominated by delta activity (0.5-4 Hz), microglia displayed reduced complexity. Notably, this pattern was absent in brain regions lacking direct functional connections to areas generating vigilance stage-dependent thalamocortical oscillations. We then extended wakefulness to decouple circadian influence from sleep-wake-specific neuronal activity. This procedure attenuated the decrease in microglial complexity observed during natural sleep, suggesting a crucial role for neuronal activity. Subsequent recovery sleep restored microglial features, independent of the time of day (zeitgeber time). These findings reveal a dynamic interplay between vigilance stage-specific thalamocortical activity and microglial morphology across various brain regions. This suggests a potential role for microglia in sleep regulation and warrants further investigation to understand the underlying mechanisms.

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.

Optogenetic targeting of cortical astrocytes selectively improves NREM sleep in an Alzheimer's disease mouse model

Alzheimer's disease (AD) is a progressive neurodegenerative condition marked by memory impairments and distinct histopathological features such as amyloid-beta (Aβ) accumulations. Alzheimer's patients experience sleep disturbances at early stages of the disease. APPswe/PS1dE9 (APP) mice exhibit sleep disruptions, including reductions in non-rapid eye movement (NREM) sleep, that contribute to their disease progression. In addition, astrocytic calcium transients associated with a sleep-dependent brain rhythm, slow oscillations prevalent during NREM sleep, are disrupted in APP mice. However, at present it is unclear whether restoration of circuit function by targeting astrocytic activity could improve sleep in APP mice. To that end, APP mice expressing channelrhodopsin-2 (ChR2) targeted to astrocytes underwent optogenetic stimulation at the slow oscillation frequency. Optogenetic stimulation of astrocytes significantly increased NREM sleep duration but not duration of rapid eye movement (REM) sleep. Optogenetic treatment increased delta power and reduced sleep fragmentation in APP mice. Thus, optogenetic activation of astrocytes increased sleep quantity and improved sleep quality in an AD mouse model. Astrocytic activity provides a novel therapeutic avenue to pursue for enhancing sleep and slowing AD progression.

An integrative analysis of cell-specific transcriptomics and nuclear proteomics of sleep-deprived mouse cerebral cortex

Sleep regulation follows a homeostatic pattern. The mammalian cerebral cortex is the repository of homeostatic sleep drive and neurons and astrocytes of the cortex are principal responders of sleep need. The molecular mechanisms by which these two cell types respond to sleep loss are not yet clearly understood. By combining cell-type specific transcriptomics and nuclear proteomics we investigated how sleep loss affects the cellular composition and molecular profiles of these two cell types in a focused approach. The results indicate that sleep deprivation regulates gene expression and nuclear protein abundance in a cell-type-specific manner. Our integrated multi-omics analysis suggests that this distinction arises because neurons and astrocytes employ different gene regulatory strategies under accumulated sleep pressure. These findings provide a comprehensive view of the effects of sleep deprivation on gene regulation in neurons and astrocytes.

The role of ATP in sleep-wake regulation: In adenosine-dependent and -independent manner

ATP plays a crucial role as an energy currency in the body's various physiological functions, including the regulation of the sleep-wake cycle. Evidence from genetics and pharmacology demonstrates a strong association between ATP metabolism and sleep. With the advent of new technologies such as optogenetics, genetically encoded biosensors, and novel ATP detection methods, the dynamic changes in ATP levels between different sleep states have been further uncovered. The classic mechanism for regulating sleep by ATP involves its conversion to adenosine, which increases sleep pressure when accumulated extracellularly. However, emerging evidence suggests that ATP can directly bind to P2 receptors and influence sleep-wake regulation through both adenosine-dependent and independent pathways. The outcome depends on the brain region where ATP acts and the expression type of P2 receptors. This review summarizes the experimental evidence on the relationship between ATP levels and changes in sleep states and outlines the mechanisms by which ATP is involved in regulating the sleep-wake cycle through both adenosine-dependent and independent pathways. Hopefully, this review will provide a comprehensive understanding of the current research basis and progress in this field and promote further investigations into the specific mechanisms of ATP in regulating sleep.

Astrocyte regulation of extracellular space parameters across the sleep-wake cycle

Multiple subfields of neuroscience research are beginning to incorporate astrocytes into current frameworks of understanding overall brain physiology, neuronal circuitry, and disease etiology that underlie sleep and sleep-related disorders. Astrocytes have emerged as a dynamic regulator of neuronal activity through control of extracellular space (ECS) volume and composition, both of which can vary dramatically during different levels of sleep and arousal. Astrocytes are also an attractive target of sleep research due to their prominent role in the glymphatic system, a method by which toxic metabolites generated during wakefulness are cleared away. In this review we assess the literature surrounding glial influences on fluctuations in ECS volume and composition across the sleep-wake cycle. We also examine mechanisms of astrocyte volume regulation in glymphatic solute clearance and their role in sleep and wake states. Overall, findings highlight the importance of astrocytes in sleep and sleep research.

Astrocytic calcium waves: unveiling their roles in sleep and arousal modulation

Neuron-astrocyte interactions are vital for the brain's connectome. Understanding astrocyte activities is crucial for comprehending the complex neural network, particularly the population-level functions of neurons in different cortical states and associated behaviors in mammals. Studies on animal sleep and wakefulness have revealed distinct cortical synchrony patterns between neurons. Astrocytes, outnumbering neurons by nearly fivefold, support and regulate neuronal and synaptic function. Recent research on astrocyte activation during cortical state transitions has emphasized the influence of norepinephrine as a neurotransmitter and calcium waves as key components of ion channel signaling. This summary focuses on a few recent studies investigating astrocyte-neuron interactions in mouse models during sleep, wakefulness, and arousal levels, exploring the involvement of noradrenaline signaling, ion channels, and glutamatergic signaling in different cortical states. These findings highlight the significant impact of astrocytes on large-scale neuronal networks, influencing brain activity and responsiveness. Targeting astrocytic signaling pathways shows promise for treating sleep disorders and arousal dysregulation. More research is needed to understand astrocytic calcium signaling in different brain regions and its implications for dysregulated brain states, requiring future human studies to comprehensively investigate neuron-astrocyte interactions and pave the way for therapeutic interventions in sleep- and arousal-related disorders.

Resilience Building Practices for Women Physicians

Women now make up more than half of the physician workforce, but they are disproportionately plagued by burnout. Medicine is a fast-paced stressful field, the practice of which is associated with significant chronic stress due to systems issues, crowding, electronic medical records, and patient case mix. Hospitals and health care systems are responsible for mitigating system-based burnout-prone conditions, but often their best efforts fail. Physicians, particularly women, must confront their stressors and the daily burden of significant system strain when this occurs. Those who routinely exceed their cumulative stress threshold may experience burnout, career dissatisfaction, and second victim syndrome and, ultimately, may prematurely leave medicine. These conditions affect women in medicine more often than men and may also produce a higher incidence of health issues, including depression, substance use disorder, and suicide. The individual self-care required to maintain health and raise stress thresholds is not widely ingrained in provider practice patterns or behavior. However, the successful long-term practice of high-stress occupations, such as medicine, requires that physicians, especially women physicians, attend to their wellness. In this article, we address one aspect of health, resilience, and review six practices that can create additional stores of personal resilience when proactively integrated into a daily routine.

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.

Sleep, mood disorders, and the ketogenic diet: potential therapeutic targets for bipolar disorder and schizophrenia

Bipolar disorder and schizophrenia are serious psychiatric conditions that cause a significant reduction in quality of life and shortened life expectancy. Treatments including medications and psychosocial support exist, but many people with these disorders still struggle to participate in society and some are resistant to current therapies. Although the exact pathophysiology of bipolar disorder and schizophrenia remains unclear, increasing evidence supports the role of oxidative stress and redox dysregulation as underlying mechanisms. Oxidative stress is an imbalance between the production of reactive oxygen species generated by metabolic processes and antioxidant systems that can cause damage to lipids, proteins, and DNA. Sleep is a critical regulator of metabolic homeostasis and oxidative stress. Disruption of sleep and circadian rhythms contribute to the onset and progression of bipolar disorder and schizophrenia and these disorders often coexist with sleep disorders. Furthermore, sleep deprivation has been associated with increased oxidative stress and worsening mood symptoms. Dysfunctional brain metabolism can be improved by fatty acid derived ketones as the brain readily uses both ketones and glucose as fuel. Ketones have been helpful in many neurological disorders including epilepsy and Alzheimer's disease. Recent clinical trials using the ketogenic diet suggest positive improvement in symptoms for bipolar disorder and schizophrenia as well. The improvement in psychiatric symptoms from the ketogenic diet is thought to be linked, in part, to restoration of mitochondrial function. These findings encourage further randomized controlled clinical trials, as well as biochemical and mechanistic investigation into the role of metabolism and sleep in psychiatric disorders. This narrative review seeks to clarify the intricate relationship between brain metabolism, sleep, and psychiatric disorders. The review will delve into the initial promising effects of the ketogenic diet on mood stability, examining evidence from both human and animal models of bipolar disorder and schizophrenia. The article concludes with a summary of the current state of affairs and encouragement for future research focused on the role of metabolism and sleep in mood disorders.

Linking brain structure, cognition, and sleep: insights from clinical data

STUDY OBJECTIVES

To use relatively noisy routinely collected clinical data (brain magnetic resonance imaging (MRI) data, clinical polysomnography (PSG) recordings, and neuropsychological testing), to investigate hypothesis-driven and data-driven relationships between brain physiology, structure, and cognition.

METHODS

We analyzed data from patients with clinical PSG, brain MRI, and neuropsychological evaluations. SynthSeg, a neural network-based tool, provided high-quality segmentations despite noise. A priori hypotheses explored associations between brain function (measured by PSG) and brain structure (measured by MRI). Associations with cognitive scores and dementia status were studied. An exploratory data-driven approach investigated age-structure-physiology-cognition links.

RESULTS

Six hundred and twenty-three patients with sleep PSG and brain MRI data were included in this study; 160 with cognitive evaluations. Three hundred and forty-two participants (55%) were female, and age interquartile range was 52 to 69 years. Thirty-six individuals were diagnosed with dementia, 71 with mild cognitive impairment, and 326 with major depression. One hundred and fifteen individuals were evaluated for insomnia and 138 participants had an apnea-hypopnea index equal to or greater than 15. Total PSG delta power correlated positively with frontal lobe/thalamic volumes, and sleep spindle density with thalamic volume. rapid eye movement (REM) duration and amygdala volume were positively associated with cognition. Patients with dementia showed significant differences in five brain structure volumes. REM duration, spindle, and slow-oscillation features had strong associations with cognition and brain structure volumes. PSG and MRI features in combination predicted chronological age (R2 = 0.67) and cognition (R2 = 0.40).

CONCLUSIONS

Routine clinical data holds extended value in understanding and even clinically using brain-sleep-cognition relationships.

VX-765 Alleviates Circadian Rhythm Disorder in a Rodent Model of Traumatic Brain Injury Plus Hemorrhagic Shock and Resuscitation

Severe traumatic brain injury (TBI) can result in persistent complications, including circadian rhythm disorder, that substantially affect not only the injured people, but also the mood and social interactions with the family and the community. Pyroptosis in GFAP-positive astrocytes plays a vital role in inflammatory changes post-TBI. We determined whether VX-765, a low molecular weight caspase-1 inhibitor, has potential therapeutic value against astrocytic inflammation and pyroptosis in a rodent model of TBI plus hemorrhagic shock and resuscitation (HSR). A weight-drop plus bleeding and refusion model was used to establish traumatic exposure in rats. VX-765 (50 mg/kg) was injected via the femoral vein after resuscitation. Wheel-running activity was assessed, brain magnetic resonance images were evaluated, the expression of pyroptosis-associated molecules including cleaved caspase-1, gasdermin D (GSDMD), and interleukin-18 (IL-18) in astrocytes in the region of anterior hypothalamus, were explored 30 days post-trauma. VX-765-treated rats had significant improvement in circadian rhythm disorder, decreased mean diffusivity (MD) and mean kurtosis (MK), increased fractional anisotropy (FA), an elevated number and branches of astrocytes, and lower cleaved caspase-1, GSDMD, and IL-18 expression in astrocytes than TBI + HSR-treated rats. These results demonstrated that inhibition of pyroptosis-associated astrocytic activations in the anterior hypothalamus using VX-765 may ameliorate circadian rhythm disorder after trauma. In conclusion, we suggest that interventions targeting caspase-1-induced astrocytic pyroptosis by VX-765 are promising strategies to alleviate circadian rhythm disorder post-TBI.

Gene Expression at the Tripartite Synapse: Bridging the Gap Between Neurons and Astrocytes

Astrocytes, a major class of glial cells, are an important element at the synapse where they engage in bidirectional crosstalk with neurons to regulate numerous aspects of neurotransmission, circuit function, and behavior. Mutations in synapse-related genes expressed in both neurons and astrocytes are central factors in a vast number of neurological disorders, making the proteins that they encode prominent targets for therapeutic intervention. Yet, while the roles of many of these synaptic proteins in neurons are well established, the functions of the same proteins in astrocytes are largely unknown. This gap in knowledge must be addressed to refine therapeutic approaches. In this chapter, we integrate multiomic meta-analysis and a comprehensive overview of current literature to show that astrocytes express an astounding number of genes that overlap with the neuronal and synaptic transcriptomes. Further, we highlight recent reports that characterize the expression patterns and potential novel roles of these genes in astrocytes in both physiological and pathological conditions, underscoring the importance of considering both cell types when investigating the function and regulation of synaptic proteins.

Mechanisms of Activation of Brain's Drainage during Sleep: The Nightlife of Astrocytes

The study of functions, mechanisms of generation, and pathways of movement of cerebral fluids has a long history, but the last decade has been especially productive. The proposed glymphatic hypothesis, which suggests a mechanism of the brain waste removal system (BWRS), caused an active discussion on both the criticism of some of the perspectives and our intensive study of new experimental facts. It was especially found that the intensity of the metabolite clearance changes significantly during the transition between sleep and wakefulness. Interestingly, at the cellular level, a number of aspects of this problem have been focused on, such as astrocytes-glial cells, which, over the past two decades, have been recognized as equal partners of neurons and perform many important functions. In particular, an important role was assigned to astrocytes within the framework of the glymphatic hypothesis. In this review, we return to the "astrocytocentric" view of the BWRS function and the explanation of its activation during sleep from the viewpoint of new findings over the last decade. Our main conclusion is that the BWRS's action may be analyzed both at the systemic (whole-brain) and at the local (cellular) level. The local level means here that the neuro-glial-vascular unit can also be regarded as the smallest functional unit of sleep, and therefore, the smallest functional unit of the BWRS.

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.

FABP7: a glial integrator of sleep, circadian rhythms, plasticity, and metabolic function

Sleep and circadian rhythms are observed broadly throughout animal phyla and influence neural plasticity and cognitive function. However, the few phylogenetically conserved cellular and molecular pathways that are implicated in these processes are largely focused on neuronal cells. Research on these topics has traditionally segregated sleep homeostatic behavior from circadian rest-activity rhythms. Here we posit an alternative perspective, whereby mechanisms underlying the integration of sleep and circadian rhythms that affect behavioral state, plasticity, and cognition reside within glial cells. The brain-type fatty acid binding protein, FABP7, is part of a larger family of lipid chaperone proteins that regulate the subcellular trafficking of fatty acids for a wide range of cellular functions, including gene expression, growth, survival, inflammation, and metabolism. FABP7 is enriched in glial cells of the central nervous system and has been shown to be a clock-controlled gene implicated in sleep/wake regulation and cognitive processing. FABP7 is known to affect gene transcription, cellular outgrowth, and its subcellular localization in the fine perisynaptic astrocytic processes (PAPs) varies based on time-of-day. Future studies determining the effects of FABP7 on behavioral state- and circadian-dependent plasticity and cognitive processes, in addition to functional consequences on cellular and molecular mechanisms related to neural-glial interactions, lipid storage, and blood brain barrier integrity will be important for our knowledge of basic sleep function. Given the comorbidity of sleep disturbance with neurological disorders, these studies will also be important for our understanding of the etiology and pathophysiology of how these diseases affect or are affected by sleep.

Neuron-astrocyte omnidirectional signaling in neurological health and disease

Astrocytes are an abundantly distributed population of glial cells in the central nervous system (CNS) that perform myriad functions in the normal and injured/diseased brain. Astrocytes exhibit heterogeneous phenotypes in response to various insults, a process known as astrocyte reactivity. The accuracy and precision of brain signaling are primarily based on interactions involving neurons, astrocytes, oligodendrocytes, microglia, pericytes, and dendritic cells within the CNS. Astrocytes have emerged as a critical entity within the brain because of their unique role in recycling neurotransmitters, actively modulating the ionic environment, regulating cholesterol and sphingolipid metabolism, and influencing cellular crosstalk in diverse neural injury conditions and neurodegenerative disorders. However, little is known about how an astrocyte functions in synapse formation, axon specification, neuroplasticity, neural homeostasis, neural network activity following dynamic surveillance, and CNS structure in neurological diseases. Interestingly, the tripartite synapse hypothesis came to light to fill some knowledge gaps that constitute an interaction of a subpopulation of astrocytes, neurons, and synapses. This review highlights astrocytes' role in health and neurological/neurodegenerative diseases arising from the omnidirectional signaling between astrocytes and neurons at the tripartite synapse. The review also recapitulates the disruption of the tripartite synapse with a focus on perturbations of the homeostatic astrocytic function as a key driver to modulate the molecular and physiological processes toward neurodegenerative diseases.

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.

Functions of Astrocytes under Normal Conditions and after a Brain Disease

In the central nervous system (CNS) there are a greater number of glial cells than neurons (between five and ten times more). Furthermore, they have a greater number of functions (more than eight functions). Glia comprises different types of cells, those of neural origin (astrocytes, radial glia, and oligodendroglia) and differentiated blood monocytes (microglia). During ontogeny, neurons develop earlier (at fetal day 15 in the rat) and astrocytes develop later (at fetal day 21 in the rat), which could indicate their important and crucial role in the CNS. Analysis of the phylogeny reveals that reptiles have a lower number of astrocytes compared to neurons and in humans this is reversed, as there have a greater number of astrocytes compared to neurons. These data perhaps imply that astrocytes are important and special cells, involved in many vital functions, including memory, and learning processes. In addition, astrocytes are involved in different mechanisms that protect the CNS through the production of antioxidant and anti-inflammatory proteins and they clean the extracellular environment and help neurons to communicate correctly with each other. The production of inflammatory mediators is important to prevent changes in brain homeostasis. On the contrary, excessive, or continued production appears as a characteristic element in many diseases, such as Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), multiple sclerosis (MS), and in neurodevelopmental diseases, such as bipolar disorder, schizophrenia, and autism. Furthermore, different drugs and techniques have been developed to reverse oxidative stress and/or excess of inflammation that occurs in many CNS diseases, but much remains to be investigated. This review attempts to highlight the functional relevance of astrocytes in normal and neuropathological conditions by showing the molecular and cellular mechanisms of their role in the CNS.

Fragmentation of rest periods, astrocyte activation, and cognitive decline in older adults with and without Alzheimer's disease

INTRODUCTION

Sleep disruption is associated with astrocyte activation and impaired cognition in model organisms. However, the relationship among sleep, astrocyte activation, and cognition in humans is uncertain.

METHODS

We used RNA-seq to quantify the prefrontal cortex expression of a panel of human activated astrocyte marker genes in 1076 older adults in the Religious Orders Study and Rush Memory and Aging Project, 411 of whom had multi-day actigraphy prior to death. We related this to rest fragmentation, a proxy for sleep fragmentation, and to longitudinal cognitive function.

RESULTS

Fragmentation of rest periods was associated with higher expression of activated astrocyte marker genes, which was associated with a lower level and faster decline of cognitive function.

DISCUSSION

Astrocyte activation and fragmented rest are associated with each other and with accelerated cognitive decline. If experimental studies confirm a causal relationship, targeting sleep fragmentation and astrocyte activation may benefit cognition in older adults.

HIGHLIGHTS

Greater fragmentation of rest periods, a proxy for sleep fragmentation, is associated with higher composite expression of a panel of genes characteristic of activated astrocytes. Increased expression of genes characteristic of activated astrocytes was associated with a lower level and more rapid decline of cognitive function, beyond that accounted for by the burden of amyloid and neurofibrillary tangle pathology. Longitudinal and experimental studies are needed to delineate the causal relationships among sleep, astrocyte activation, and cognition.

Sleep deprivation exacerbates microglial reactivity and Aβ deposition in a TREM2-dependent manner in mice

Sleep loss is associated with cognitive decline in the aging population and is a risk factor for Alzheimer's disease (AD). Considering the crucial role of immunomodulating genes such as that encoding the triggering receptor expressed on myeloid cells type 2 (TREM2) in removing pathogenic amyloid-β (Aβ) plaques and regulating neurodegeneration in the brain, our aim was to investigate whether and how sleep loss influences microglial function in mice. We chronically sleep-deprived wild-type mice and the 5xFAD mouse model of cerebral amyloidosis, expressing either the humanized TREM2 common variant, the loss-of-function R47H AD-associated risk variant, or without TREM2 expression. Sleep deprivation not only enhanced TREM2-dependent Aβ plaque deposition compared with 5xFAD mice with normal sleeping patterns but also induced microglial reactivity that was independent of the presence of parenchymal Aβ plaques. We investigated lysosomal morphology using transmission electron microscopy and found abnormalities particularly in mice without Aβ plaques and also observed lysosomal maturation impairments in a TREM2-dependent manner in both microglia and neurons, suggesting that changes in sleep modified neuro-immune cross-talk. Unbiased transcriptome and proteome profiling provided mechanistic insights into functional pathways triggered by sleep deprivation that were unique to TREM2 and Aβ pathology and that converged on metabolic dyshomeostasis. Our findings highlight that sleep deprivation directly affects microglial reactivity, for which TREM2 is required, by altering the metabolic ability to cope with the energy demands of prolonged wakefulness, leading to further Aβ deposition, and underlines the importance of sleep modulation as a promising future therapeutic approach.

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.

The emerging science of Glioception: Contribution of glia in sensing, transduction, circuit integration of interoception

Interoception is the process by which the nervous system regulates internal functions to achieve homeostasis. The role of neurons in interoception has received considerable recent attention, but glial cells also contribute. Glial cells can sense and transduce signals including osmotic, chemical, and mechanical status of extracellular milieu. Their ability to dynamically communicate "listening" and "talking" to neurons is necessary to monitor and regulate homeostasis and information integration in the nervous system. This review introduces the concept of "Glioception" and focuses on the process by which glial cells sense, interpret and integrate information about the inner state of the organism. Glial cells are ideally positioned to act as sensors and integrators of diverse interoceptive signals and can trigger regulatory responses via modulation of the activity of neuronal networks, both in physiological and pathological conditions. We believe that understanding and manipulating glioceptive processes and underlying molecular mechanisms provide a key path to develop new therapies for the prevention and alleviation of devastating interoceptive dysfunctions, among which pain is emphasized here with more focused details.

Purinergic signaling in cognitive impairment and neuropsychiatric symptoms of Alzheimer's disease

About 10 million new cases of dementia develop worldwide each year, of which up to 70% are attributable to Alzheimer's disease (AD). In addition to the widely known symptoms of memory loss and cognitive impairment, AD patients frequently develop non-cognitive symptoms, referred to as behavioral and psychological symptoms of dementia (BPSDs). Sleep disorders are often associated with AD, but mood alterations, notably depression and apathy, comprise the most frequent class of BPSDs. BPSDs negatively affect the lives of AD patients and their caregivers, and have a significant impact on public health systems and the economy. Because treatments currently available for AD are not disease-modifying and mainly aim to ameliorate some of the cognitive symptoms, elucidating the mechanisms underlying mood alterations and other BPSDs in AD may reveal novel avenues for progress in AD therapy. Purinergic signaling is implicated in the pathophysiology of several central nervous system (CNS) disorders, such as AD, depression and sleep disorders. Here, we review recent findings indicating that purinergic receptors, mainly the A₁, A_2A, and P2X7 subtypes, are associated with the development/progression of AD. Current evidence suggests that targeting purinergic signaling may represent a promising therapeutic approach in AD and related conditions. This article is part of the Special Issue on "Purinergic Signaling: 50 years".

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.

An emerging role of astrocytes in aging/neuroinflammation and gut-brain axis with consequences on sleep and sleep disorders

Understanding the role of astrocytes in the central nervous system has changed dramatically over the last decade. The accumulating findings indicate that glial cells are involved not only in the maintenance of metabolic and ionic homeostasis and in the implementation of trophic functions but also in cognitive functions and information processing in the brain. Currently, there are some controversies regarding the role of astrocytes in complex processes such as aging of the nervous system and the pathogenesis of age-related neurodegenerative diseases. Many findings confirm the important functional role of astrocytes in age-related brain changes, including sleep disturbance and the development of neurodegenerative diseases and particularly Alzheimer's disease. Until recent years, neurobiological research has focused mainly on neuron-glial interactions, in which individual astrocytes locally modulate neuronal activity and communication between neurons. The review considers the role of astrocytes in the physiology of sleep and as an important "player" in the development of neurodegenerative diseases. In addition, the features of the astrocytic network reorganization during aging are discussed.

Microglial homeostasis disruption modulates non-rapid eye movement sleep duration and neuronal activity in adult female mice

Sleep is a natural physiological state, tightly regulated through several neuroanatomical and neurochemical systems, which is essential to maintain physical and mental health. Recent studies revealed that the functions of microglia, the resident immune cells of the brain, differ along the sleep-wake cycle. Inflammatory cytokines, such as interleukin-1β and tumor necrosis factor-α, mainly produced by microglia in the brain, are also well-known to promote sleep. However, the contributing role of microglia on sleep regulation remains largely elusive, even more so in females. Given the higher prevalence of various sleep disorders in women, we aimed to determine the role of microglia in regulating the sleep-wake cycle specifically in female mice. Microglia were depleted in adult female mice with inhibitors of the colony-stimulating factor 1 receptor (CSF1R) (PLX3397 or PLX5622), which is required for microglial population maintenance. This led to a 65-73% reduction of the microglial population, as confirmed by immunofluorescence staining against IBA1 (marker of microglia/macrophages) and TMEM119 (microglia-specific marker) in the reticular nucleus of the thalamus and primary motor cortex. The spontaneous sleep-wake cycle was evaluated at steady-state, during microglial homeostasis disruption and after complete microglial repopulation, upon cessation of treatment with the inhibitors of CSF1R, using electroencephalography (EEG) and electromyography (EMG). We found that microglia-depleted female mice spent more time in non-rapid eye movement (NREM) sleep and had an increased number of NREM sleep episodes, which was partially restored after microglial total repopulation. To determine whether microglia could regulate sleep locally by modulating synaptic transmission, we used patch clamp to record spontaneous activity of pyramidal neurons in the primary motor cortex, which showed an increase of excitatory synaptic transmission during the dark phase. These changes in neuronal activity were modulated by microglial depletion in a phase-dependent manner. Altogether, our results indicate that microglia are involved in the sleep regulation of female mice, further strengthening their potential implication in the development and/or progression of sleep disorders. Furthermore, our findings indicate that microglial repopulation can contribute to normalizing sleep alterations caused by their partial depletion.

The Molecular Relationship between Stress and Insomnia

The bi-directional relationship between sleep and stress has been actively researched as sleep disturbances and stress have become increasingly common in society. Interestingly, the brain and underlying neural circuits important for sleep regulation may respond uniquely to stress that leads to post-traumatic stress disorder (PTSD) and stress that does not. In stress that does not lead to PTSD, the hypothalamic-pituitary-adrenal axis (HPA) pathway is activated normally that results in sympathetic nervous system activation that allows the brain and body to return to baseline functioning. However, exposure to stress that leads to PTSD, causes enhanced negative feedback of this same pathway and results in long-term physiological and psychological changes. In this review, how stress regulates glucocorticoid signaling pathways in brain glial cells called astrocytes, and then mediates stress-induced insomnia are examined. Astrocytes are critical sleep regulatory cells and their connections to sleep and stress due to disturbed glucocorticoid signaling provide a novel mechanism to explain how stress leads to insomnia. This review will examine the interactions of stress neurobiology, astrocytes, sleep, and glucocorticoid signaling pathways and will examine the how stress that leads to PTSD and stress that does not impacts sleep-regulatory processes.

Adolescent sleep and the foundations of prefrontal cortical development and dysfunction

Modern life poses many threats to good-quality sleep, challenging brain health across the lifespan. Curtailed or fragmented sleep may be particularly damaging during adolescence, when sleep disruption by delayed chronotypes and societal pressures coincides with our brains preparing for adult life via intense refinement of neural connectivity. These vulnerabilities converge on the prefrontal cortex, one of the last brain regions to mature and a central hub of the limbic-cortical circuits underpinning decision-making, reward processing, social interactions and emotion. Even subtle disruption of prefrontal cortical development during adolescence may therefore have enduring impact. In this review, we integrate synaptic and circuit mechanisms, glial biology, sleep neurophysiology and epidemiology, to frame a hypothesis highlighting the implications of adolescent sleep disruption for the neural circuitry of the prefrontal cortex. Convergent evidence underscores the importance of acknowledging, quantifying and optimizing adolescent sleep's contributions to normative brain development and to lifelong mental health.

Reactogenic sleepiness after COVID-19 vaccination. A hypothesis involving orexinergic system linked to inflammatory signals

Coronavirus disease 2019 (COVID-19) represents a global healthcare crisis that has led to morbidity and mortality on an unprecedented scale. While studies on COVID-19 vaccines are ongoing, the knowledge about the reactogenic symptoms that can occur after vaccination and its generator mechanisms can be critical for healthcare professionals to improve compliance with the future vaccination campaign. Because sleep and immunity are bidirectionally linked, sleepiness or sleep disturbance side effects reported after some of the COVID-19 vaccines advise an academic research line in the context of physiological or pathological neuroimmune interactions. On the recognized basis of inflammatory regulation of hypothalamic neurons in sickness behavior, we hypothesized that IL-1β, INF-γ and TNF-α pro-inflammatory cytokines inhibit orexinergic neurons promoting sleepiness after peripheral activation of the innate immune system induced by the novel COVID-19 vaccines. In addition, based on knowledge of previous vaccines and disease manifestations of SARS-CoV-2 infection, it also suggests that narcolepsy must be included as potential adverse events of particular interest to consider in pharmacovigilance studies.

A role for glia in cellular and systemic metabolism: insights from the fly

Excitability and synaptic transmission make neurons high-energy consumers. However, neurons do not store carbohydrates or lipids. Instead, they need support cells to fuel their metabolic demands. This role is assumed by glia, both in vertebrates and invertebrates. Many questions remain regarding the coupling between neuronal activity and energy demand on the one hand, and nutrient supply by glia on the other hand. Here, we review recent advances showing that fly glia, similar to their role in vertebrates, fuel neurons in times of high energetic demand, such as during memory formation and long-term storage. Vertebrate glia also play a role in the modulation of neurons, their communication, and behavior, including food search and feeding. We discuss recent literature pointing to similar roles of fly glia in behavior and metabolism.

From Brain-Body Function to Conscious Interactions

In this review, we discuss empirical results inspiring the introduction of a formal mathematical multilayer model for the biological neuroscience of conscious experience. First, we motivate the discussion through evidence regarding the dynamic brain. Second, we review different brain-body couplings associated with conscious experience and its potential role in driving brain dynamics. Third, we introduce the machinery of multilayer networks to account for several types of interactions in brain-body systems. Then, a multilayer structure consists of two main generalizations: a formal semantic to study biological systems, and an integrative account for several signatures and models of consciousness. Finally, under this framework, we define composition of layers to account for entangled features of brain-body systems related to conscious experience. As such, a multilayer mathematical framework is highly integrative and thus may be more complete than other models. In this short review, we discuss a variety of empirical results inspiring the introduction of a formal mathematical multilayer model for the biological neuroscience of conscious experience.

Monocarboxylate transporters (MCTs) in skeletal muscle and hypothalamus of less or more physically active mice exposed to aerobic training

AIMS

The synthesis of monocarboxylate transporters (MCTs) can be stimulated by aerobic training, but few is known about this effect associated or not with non-voluntary daily activities. We examined the effect of eight weeks of aerobic training in MCTs on the skeletal muscle and hypothalamus of less or more physically active mice, which can be achieved by keeping them in two different housing models, a small cage (SC) and a large cage (LC).

MAIN METHODS

Forty male C57BL/6J mice were divided into four groups. In each housing condition, mice were divided into untrained (N) and trained (T). For 8 weeks, the trained animals ran on a treadmill with an intensity equivalent to 80 % of the individual critical velocity (CV), considered aerobic capacity, 40 min/day, 5 times/week. Protein expression of MCTs was determined with fluorescence Western Blot.

KEY FINDINGS

T groups had higher hypothalamic MCT2 than N groups (ANOVA, P = 0.032). Significant correlations were detected between hypothalamic MCT2 and CV. There was a difference between the SC and LC groups in relation to MCT4 in the hypothalamus (LC > SC, P = 0.044). Trained mice housed in LC (but not SC-T) exhibited a reduction in MCT4 muscle (P < 0.001).

SIGNIFICANCE

Our findings indicate that aerobically trained mice increased the expression of MCT2 protein in the hypothalamus, which has been related to the uptake of lactate in neurons. Changes in energy metabolism in physically active mice (kept in LC) may be related to upregulation of hypothalamic MCT4, probably participating in the regulation of satiety.

Astrocytic GABA transporter controls sleep by modulating GABAergic signaling in Drosophila circadian neurons

A precise balance between sleep and wakefulness is essential to sustain a good quality of life and optimal brain function. GABA is known to play a key and conserved role in sleep control, and GABAergic tone should, therefore, be tightly controlled in sleep circuits. Here, we examined the role of the astrocytic GABA transporter (GAT) in sleep regulation using Drosophila melanogaster. We found that a hypomorphic gat mutation (gat33-1) increased sleep amount, decreased sleep latency, and increased sleep consolidation at night. Interestingly, sleep defects were suppressed when gat33-1 was combined with a mutation disrupting wide-awake (wake), a gene that regulates the cell-surface levels of the GABAA receptor resistance to dieldrin (RDL) in the wake-promoting large ventral lateral neurons (l-LNvs). Moreover, RNAi knockdown of rdl and its modulators dnlg4 and wake in these circadian neurons also suppressed gat33-1 sleep phenotypes. Brain immunohistochemistry showed that GAT-expressing astrocytes were located near RDL-positive l-LNv cell bodies and dendritic processes. We concluded that astrocytic GAT decreases GABAergic tone and RDL activation in arousal-promoting LNvs, thus determining proper sleep amount and quality in Drosophila.

Astroglial cannabinoid signaling and behavior

In neuroscience, the explosion of innovative and advanced technical accomplishments is fundamental to understanding brain functioning. For example, the possibility to distinguish glial and neuronal activities at the synaptic level and/or the appearance of new genetic tools to specifically monitor and manipulate astroglial functions revealed that astrocytes are involved in several facets of behavioral control. In this sense, the discovery of functional presence of type-1 cannabinoid receptors in astrocytes has led to identify important behavioral responses mediated by this specific pool of cannabinoid receptors. Thus, astroglial type-1 cannabinoid receptors are in the perfect place to play a role in a complex scenario in which astrocytes sense neuronal activity, release gliotransmitters and modulate the activity of other neurons, ultimately controlling behavioral responses. In this review, we will describe the known behavioral implications of astroglial cannabinoid signaling and highlight exciting unexplored research avenues on how astroglial cannabinoid signaling could affect behavior.

Rapid-acting antidepressants and the circadian clock

A growing number of epidemiological and experimental studies has established that circadian disruption is strongly associated with psychiatric disorders, including major depressive disorder (MDD). This association is becoming increasingly relevant considering that modern lifestyles, social zeitgebers (time cues) and genetic variants contribute to disrupting circadian rhythms that may lead to psychiatric disorders. Circadian abnormalities associated with MDD include dysregulated rhythms of sleep, temperature, hormonal secretions, and mood which are modulated by the molecular clock. Rapid-acting antidepressants such as subanesthetic ketamine and sleep deprivation therapy can improve symptoms within 24 h in a subset of depressed patients, in striking contrast to conventional treatments, which generally require weeks for a full clinical response. Importantly, animal data show that sleep deprivation and ketamine have overlapping effects on clock gene expression. Furthermore, emerging data implicate the circadian system as a critical component involved in rapid antidepressant responses via several intracellular signaling pathways such as GSK3β, mTOR, MAPK, and NOTCH to initiate synaptic plasticity. Future research on the relationship between depression and the circadian clock may contribute to the development of novel therapeutic strategies for depression-like symptoms. In this review we summarize recent evidence describing: (1) how the circadian clock is implicated in depression, (2) how clock genes may contribute to fast-acting antidepressants, and (3) the mechanistic links between the clock genes driving circadian rhythms and neuroplasticity.

Genome-Wide Association Study and Genetic Correlation Scan Provide Insights into Its Genetic Architecture of Sleep Health Score in the UK Biobank Cohort

PURPOSE

Most previous genetic studies of sleep behaviors were conducted individually, without comprehensive consideration of the complexity of various sleep behaviors. Our aim is to identify the genetic architecture and potential biomarker of the sleep health score, which more powerfully represents overall sleep traits.

PATIENTS AND METHODS

We conducted a genome-wide association study (GWAS) of sleep health score (overall assessment of sleep duration, snoring, insomnia, chronotype, and daytime dozing) using 336,463 participants from the UK Biobank. Proteome-wide association study (PWAS) and transcriptome-wide association study (TWAS) were then performed to identify candidate genes at the protein and mRNA level, respectively. We finally used linkage disequilibrium score regression (LDSC) to estimate the genetic correlations between sleep health score and other functionally relevance traits.

RESULTS

GWAS identified multiple variants near known candidate genes associated with sleep health score, such as MEIS1, FBXL13, MED20 and SMAD5. HDHD2 (PPWAS = 0.0146) and GFAP (PPWAS = 0.0236) were identified associated with sleep health score by PWAS. TWAS identified ORC4 (PTWAS = 0.0212) and ZNF732 (PTWAS = 0.0349) considering mRNA expression level. LDSC found significant genetic correlations of sleep health score with 3 sleep behaviors (including insomnia, snoring, dozing), 4 psychiatry disorders (major depressive disorder, attention deficit/hyperactivity disorder, schizophrenia, autism spectrum disorder), and 9 plasma protein (such as Stabilin-1, Stromelysin-2, Cytochrome c) (all LDSC PLDSC < 0.05).

CONCLUSION

Our results advance the comprehensive understanding of the aetiology and genetic architecture of the sleep health score, refine the understanding of the relationship of sleep health score with other traits and diseases, and may serve as potential targets for future mechanistic studies of sleep phenotype.

From Synapses to Circuits, Astrocytes Regulate Behavior

Astrocytes are non-neuronal cells that regulate synapses, neuronal circuits, and behavior. Astrocytes ensheath neuronal synapses to form the tripartite synapse where astrocytes influence synapse formation, function, and plasticity. Beyond the synapse, recent research has revealed that astrocyte influences on the nervous system extend to the modulation of neuronal circuitry and behavior. Here we review recent findings on the active role of astrocytes in behavioral modulation with a focus on in vivo studies, primarily in mice. Using tools to acutely manipulate astrocytes, such as optogenetics or chemogenetics, studies reviewed here have demonstrated a causal role for astrocytes in sleep, memory, sensorimotor behaviors, feeding, fear, anxiety, and cognitive processes like attention and behavioral flexibility. Current tools and future directions for astrocyte-specific manipulation, including methods for probing astrocyte heterogeneity, are discussed. Understanding the contribution of astrocytes to neuronal circuit activity and organismal behavior will be critical toward understanding how nervous system function gives rise to behavior.

Astrocytes: The Housekeepers and Guardians of the CNS

Astroglia are a diverse group of cells in the central nervous system. They are of the ectodermal, neuroepithelial origin and vary in morphology and function, yet, they can be collectively defined as cells having principle function to maintain homeostasis of the central nervous system at all levels of organisation, including homeostasis of ions, pH and neurotransmitters; supplying neurones with metabolic substrates; supporting oligodendrocytes and axons; regulating synaptogenesis, neurogenesis, and formation and maintenance of the blood-brain barrier; contributing to operation of the glymphatic system; and regulation of systemic homeostasis being central chemosensors for oxygen, CO₂ and Na⁺. Their basic physiological features show a lack of electrical excitability (inapt to produce action potentials), but display instead a rather active excitability based on variations in cytosolic concentrations of Ca²⁺ and Na⁺. It is expression of neurotransmitter receptors, pumps and transporters at their plasmalemma, along with transports on the endoplasmic reticulum and mitochondria that exquisitely regulate the cytosolic levels of these ions, the fluctuation of which underlies most, if not all, astroglial homeostatic functions.

Entrainment of Astrocytic and Neuronal Ca2+ Population Dynamics During Information Processing of Working Memory in Mice

Astrocytes are increasingly recognized to play an active role in learning and memory, but whether neural inputs can trigger event-specific astrocytic Ca²⁺ dynamics in real time to participate in working memory remains unclear due to the difficulties in directly monitoring astrocytic Ca²⁺ dynamics in animals performing tasks. Here, using fiber photometry, we showed that population astrocytic Ca²⁺ dynamics in the hippocampus were gated by sensory inputs (centered at the turning point of the T-maze) and modified by the reward delivery during the encoding and retrieval phases. Notably, there was a strong inter-locked and antagonistic relationship between the astrocytic and neuronal Ca²⁺ dynamics with a 3-s phase difference. Furthermore, there was a robust synchronization of astrocytic Ca²⁺ at the population level among the hippocampus, medial prefrontal cortex, and striatum. The inter-locked, bidirectional communication between astrocytes and neurons at the population level may contribute to the modulation of information processing in working memory.

Sleep and perivascular spaces in the middle-aged and elderly population

Sleep has been hypothesised to facilitate waste clearance from the brain. We aimed to determine whether sleep is associated with perivascular spaces on brain magnetic resonance imaging (MRI), a potential marker of impaired brain waste clearance, in a population-based cohort of middle-aged and elderly people. In 559 participants (mean [SD] age 62 [6] years, 52% women) from the population-based Rotterdam Study, we measured total sleep time, sleep onset latency, wake after sleep onset and sleep efficiency with actigraphy and polysomnography. Perivascular space load was determined with brain MRI in four regions (centrum semiovale, basal ganglia, hippocampus, and midbrain) via a validated machine learning algorithm using T2-weighted MR images. Associations between sleep characteristics and perivascular space load were analysed with zero-inflated negative binomial regression models adjusted for various confounders. We found that higher actigraphy-estimated sleep efficiency was associated with a higher perivascular space load in the centrum semiovale (odds ratio 1.10, 95% confidence interval 1.04-1.16, p = 0.0008). No other actigraphic or polysomnographic sleep characteristics were associated with perivascular space load in other brain regions. We conclude that, contrary to our hypothesis, associations of sleep with perivascular space load in this middle-aged and elderly population remained limited to an association of a high actigraphy-estimated sleep efficiency with a higher perivascular space load in the centrum semiovale.

Review and Hypothesis: A Potential Common Link Between Glial Cells, Calcium Changes, Modulation of Synaptic Transmission, Spreading Depression, Migraine, and Epilepsy-H. Front Cell Neurosci 2021;15:693095. [PMID: 34539347 PMCID: PMC8446203 DOI: 10.3389/fncel.2021.693095]

There is significant evidence to support the notion that glial cells can modulate the strength of synaptic connections between nerve cells, and it has further been suggested that alterations in intracellular calcium are likely to play a key role in this process. However, the molecular mechanism(s) by which glial cells modulate neuronal signaling remains contentiously debated. Recent experiments have suggested that alterations in extracellular H+ efflux initiated by extracellular ATP may play a key role in the modulation of synaptic strength by radial glial cells in the retina and astrocytes throughout the brain. ATP-elicited alterations in H+ flux from radial glial cells were first detected from Müller cells enzymatically dissociated from the retina of tiger salamander using self-referencing H+-selective microelectrodes. The ATP-elicited alteration in H+ efflux was further found to be highly evolutionarily conserved, extending to Müller cells isolated from species as diverse as lamprey, skate, rat, mouse, monkey and human. More recently, self-referencing H+-selective electrodes have been used to detect ATP-elicited alterations in H+ efflux around individual mammalian astrocytes from the cortex and hippocampus. Tied to increases in intracellular calcium, these ATP-induced extracellular acidifications are well-positioned to be key mediators of synaptic modulation. In this article, we examine the evidence supporting H+ as a key modulator of neurotransmission, review data showing that extracellular ATP elicits an increase in H+ efflux from glial cells, and describe the potential signal transduction pathways involved in glial cell-mediated H+ efflux. We then examine the potential role that extracellular H+ released by glia might play in regulating synaptic transmission within the vertebrate retina, and then expand the focus to discuss potential roles in spreading depression, migraine, epilepsy, and alterations in brain rhythms, and suggest that alterations in extracellular H+ may be a unifying feature linking these disparate phenomena.

Reactive astrocytes: The nexus of pathological and clinical hallmarks of Alzheimer's disease

Astrocyte reactivity is a hallmark of neuroinflammation that arises with Alzheimer’s disease (AD) and nearly every other neurodegenerative condition. While astrocytes certainly contribute to classic inflammatory processes (e.g. cytokine release, waste clearance, and tissue repair), newly emerging technologies for measuring and targeting cell specific activities in the brain have uncovered essential roles for astrocytes in synapse function, brain metabolism, neurovascular coupling, and sleep/wake patterns. In this review, we use a holistic approach to incorporate, and expand upon, classic neuroinflammatory concepts to consider how astrocyte dysfunction/reactivity modulates multiple pathological and clinical hallmarks of AD. Our ever-evolving understanding of astrocyte signaling in neurodegeneration is not only revealing new drug targets and treatments for dementia but is suggesting we reimagine AD pathophysiological mechanisms.

From Physiology to Pathology of Cortico-Thalamo-Cortical Oscillations: Astroglia as a Target for Further Research

The electrographic hallmark of childhood absence epilepsy (CAE) and other idiopathic forms of epilepsy are 2.5-4 Hz spike and wave discharges (SWDs) originating from abnormal electrical oscillations of the cortico-thalamo-cortical network. SWDs are generally associated with sudden and brief non-convulsive epileptic events mostly generating impairment of consciousness and correlating with attention and learning as well as cognitive deficits. To date, SWDs are known to arise from locally restricted imbalances of excitation and inhibition in the deep layers of the primary somatosensory cortex. SWDs propagate to the mostly GABAergic nucleus reticularis thalami (NRT) and the somatosensory thalamic nuclei that project back to the cortex, leading to the typical generalized spike and wave oscillations. Given their shared anatomical basis, SWDs have been originally considered the pathological transition of 11-16 Hz bursts of neural oscillatory activity (the so-called sleep spindles) occurring during Non-Rapid Eye Movement (NREM) sleep, but more recent research revealed fundamental functional differences between sleep spindles and SWDs, suggesting the latter could be more closely related to the slow (<1 Hz) oscillations alternating active (Up) and silent (Down) cortical activity and concomitantly occurring during NREM. Indeed, several lines of evidence support the fact that SWDs impair sleep architecture as well as sleep/wake cycles and sleep pressure, which, in turn, affect seizure circadian frequency and distribution. Given the accumulating evidence on the role of astroglia in the field of epilepsy in the modulation of excitation and inhibition in the brain as well as on the development of aberrant synchronous network activity, we aim at pointing at putative contributions of astrocytes to the physiology of slow-wave sleep and to the pathology of SWDs. Particularly, we will address the astroglial functions known to be involved in the control of network excitability and synchronicity and so far mainly addressed in the context of convulsive seizures, namely (i) interstitial fluid homeostasis, (ii) K⁺ clearance and neurotransmitter uptake from the extracellular space and the synaptic cleft, (iii) gap junction mechanical and functional coupling as well as hemichannel function, (iv) gliotransmission, (v) astroglial Ca²⁺ signaling and downstream effectors, (vi) reactive astrogliosis and cytokine release.

Electrophysiological features of sleep in children with Kir4.1 channel mutations and Autism-Epilepsy phenotype: a preliminary study

Study Objectives

Recently, a role for gain-of-function (GoF) mutations of the astrocytic potassium channel Kir4.1 (KCNJ10 gene) has been proposed in subjects with Autism–Epilepsy phenotype (AEP). Epilepsy and autism spectrum disorder (ASD) are common and complexly related to sleep disorders. We tested whether well characterized mutations in KCNJ10 could result in specific sleep electrophysiological features, paving the way to the discovery of a potentially relevant biomarker for Kir4.1-related disorders.

Methods

For this case–control study, we recruited seven children with ASD either comorbid or not with epilepsy and/or EEG paroxysmal abnormalities (AEP) carrying GoF mutations of KCNJ10 and seven children with similar phenotypes but wild-type for the same gene, comparing period-amplitude features of slow waves detected by fronto-central bipolar EEG derivations (F3-C3, F4-C4, and Fz-Cz) during daytime naps.

Results

Children with Kir4.1 mutations displayed longer slow waves periods than controls, in Fz-Cz (mean period = 112,617 ms ± SE = 0.465 in mutated versus mean period = 105,249 ms ± SE = 0.375 in controls, p < 0.001). An analog result was found in F3-C3 (mean period = 125,706 ms ± SE = 0.397 in mutated versus mean period = 120,872 ms ± SE = 0.472 in controls, p < 0.001) and F4-C4 (mean period = 127,914 ms ± SE = 0.557 in mutated versus mean period = 118,174 ms ± SE = 0.442 in controls, p < 0.001).

Conclusion

This preliminary finding suggests that period-amplitude slow wave features are modified in subjects carrying Kir4.1 GoF mutations. Potential clinical applications of this finding are discussed.