Homeostatic state of microglia in a rat model of chronic sleep restriction

Sleep disturbance in rodent models and its sex-specific implications

Sleep disturbances, encompassing altered sleep physiology or disorders like insomnia and sleep apnea, profoundly impact physiological functions and elevate disease risk. Despite extensive research, the underlying mechanisms and sex-specific differences in sleep disorders remain elusive. While polysomnography serves as a cornerstone for human sleep studies, animal models provide invaluable insights into sleep mechanisms. However, the availability of animal models of sleep disorders is limited, with each model often representing a specific sleep issue or mechanism. Therefore, selecting appropriate animal models for sleep research is critical. Given the significant sex differences in sleep patterns and disorders, incorporating both male and female subjects in studies is essential for uncovering sex-specific mechanisms with clinical relevance. This review provides a comprehensive overview of various rodent models of sleep disturbance, including sleep deprivation, sleep fragmentation, and circadian rhythm dysfunction. We evaluate the advantages and disadvantages of each model and discuss sex differences in sleep and sleep disorders, along with potential mechanisms. We aim to advance our understanding of sleep disorders and facilitate sex-specific interventions.

Differential effects of sleep deprivation on behavior and microglia in a brain-region-specific manner in young and aged male mice

Microglia, resident immune cells in the central nervous system, constantly monitor the state of the surrounding brain activity. The animal model induced by sleep deprivation (SD) is widely used to study the pathophysiological mechanisms of insomnia and bipolar disorder. However, it remains unclear whether SD affects behaviors in young and aged male mice and microglia in various brain regions. In this study, we confirmed brain region-specific changes in microglial density and morphology in the accumbens nucleus (Acb), amygdala (AMY), cerebellum (Cb), corpus callosum (cc), caudate putamen, hippocampus (HIP), hypothalamus (HYP), medial prefrontal cortex (mPFC), and thalamus (TH) of young mice. In addition, the density of microglia in old mice was higher than that in young mice. Compared with young mice, old mice showed a markedly increased microglial size, decreased total length of microglial processes, and decreased maximum length. Importantly, we found that 48-h SD decreased microglial density and morphology in old mice, whereas SD increased microglial density and morphology in most observed brain regions in young mice. SD-induced hyperactivity was observed only in young mice but not in old mice. Moreover, microglial density (HIP, AMY, mPFC, CPu) was significantly positively correlated with behaviors in SD- and vehicle-treated young mice. Contrarily, negative correlations were shown between the microglial density (cc, Cb, TH, HYP, Acb, AMY) and behaviors in vehicle-treated young and old mice. These results suggest that SD dysregulates the homeostatic state of microglia in a region- and age-dependent manner. Microglia may be involved in regulating age-related behavioral responses to SD.

Imaging microglia surveillance during sleep-wake cycles in freely behaving mice

Microglia surveillance manifests itself as dynamic changes in cell morphology and functional remodeling. Whether and how microglia surveillance is coupled to brain state switches during natural sleep-wake cycles remains unclear. To address this question, we used miniature two-photon microscopy (mTPM) to acquire time-lapse high-resolution microglia images of the somatosensory cortex, along with EEG/EMG recordings and behavioral video, in freely-behaving mice. We uncovered fast and robust brain state-dependent changes in microglia surveillance, occurring in parallel with sleep dynamics and early-onset phagocytic microglial contraction during sleep deprivation stress. We also detected local norepinephrine fluctuation occurring in a sleep state-dependent manner. We showed that the locus coeruleus-norepinephrine system, which is crucial to sleep homeostasis, is required for both sleep state-dependent and stress-induced microglial responses and β2-adrenergic receptor signaling plays a significant role in this process. These results provide direct evidence that microglial surveillance is exquisitely tuned to signals and stressors that regulate sleep dynamics and homeostasis so as to adjust its varied roles to complement those of neurons in the brain. In vivo imaging with mTPM in freely behaving animals, as demonstrated here, opens a new avenue for future investigation of microglia dynamics and sleep biology in freely behaving animals.

Establishment of insomnia model of chronic unpredictable stress in rats

It is well known that stressful situation is one of the important factors causing insomnia, however, the underlying mechanism is still elusive. Therefore, the establishment of a suitable animal model of stress insomnia will be of great help to solve this problem. In this study, by combining with chronic unpredictable stress (multitude of stressors) and sleep deprivation, we attempted to establish a rat model of stress insomnia. It was observed that rats with stress insomnia showed significant weight loss, and less sleep quality in pentobarbital sodium induced sleep test and electroencephalogram detection. Moreover, rats with stress insomnia showed greater depression and anxiety detected by forced swimming, sucrose preference test and open field. Since oxidative stress has been reported to be involved in insomnia, we further evaluated the production of oxidative stress and found that the levels of lipid peroxidation product malondialdehyde (MDA) in liver, serum total bilirubin and urine biopyrrin were all significantly increased in rats with stress insomnia. In addition, we also found that the memory of these rats with stress insomnia was also obviously reduced in water maze. Taken together, these results demonstrate that the emotional behaviors, memory, oxidative and metabolism of the rats were all significantly changed after modeling, indicating a rat model of stress insomnia was successful establishment, and this animal model will provide basis to further explore the underlying mechanism of chronic stress in insomnia.

Alterations in microglial morphology concentrate in the habitual sleeping period of the mouse

In nocturnal animals, waking appears during the dark period while maximal non-rapid-eye-movement sleep (NREMS) with electroencephalographic slow-wave-activity (SWA) takes place at the beginning of the light period. Vigilance states associate with variable levels of neuronal activity: waking with high-frequency activity patterns while during NREMS, SWA influences neuronal activity in many brain areas. On a glial level, sleep deprivation modifies microglial morphology, but only few studies have investigated microglia through the physiological sleep-wake cycle. To quantify microglial morphology (territory, volume, ramification) throughout the 24 h light-dark cycle, we collected brain samples from inbred C57BL male mice (n = 51) every 3 h and applied a 3D-reconstruction method for microglial cells on the acquired confocal microscopy images. As microglia express regional heterogeneity and are influenced by local neuronal activity, we chose to investigate three interconnected and functionally well-characterized brain areas: the somatosensory cortex (SC), the dorsal hippocampus (HC), and the basal forebrain (BF). To temporally associate microglial morphology with vigilance stages, we performed a 24 h polysomnography in a separate group of animals (n = 6). In line with previous findings, microglia displayed de-ramification in the 12 h light- and hyper-ramification in the 12 h dark period. Notably, we found that the decrease in microglial features was most prominent within the early hours of the light period, co-occurring with maximal sleep SWA. By the end of the light period, all features reached maximum levels and remained steadily elevated throughout the dark period with minor regional differences. We propose that vigilance-stage specific neuronal activity, and SWA, could modify microglial morphology.

Microglial Responses and Pain Behaviors Are Exacerbated by Chronic Sleep Deprivation in Rats with Chronic Pain Via Neuroinflammatory Pathways

The inflammatory response of central nervous system (CNS) and microglial activation is important in the development of pain behaviors induced by sleep deprivation. We found that chronic sleep deprivation (CSD) aggravated pain behaviors in rats with chronic pain by upregulating expression of Toll-like receptor 4 (TLR4), NOD-like receptor pyrin domain containing 3 (NLRP3), and interleukin 1β (IL-1β), which promoted microglial activation in the brain. We also found that CSD increased numbers of Iba1⁺ and TLR4⁺ cells, as well as neuronal apoptosis. Inhibitors of TLR4 and NLRP3 (TAK-242 and MCC950, respectively) reduced expression levels of inflammatory factor proteins and M1-related factor mRNA, decreased microglial activation, and relieved the hyperalgesia caused by CSD. These results suggest that CSD aggravated pain behavior in rats with chronic pain through the TLR4/NLRP3/IL-1β signaling pathway, which mediates microglial activation and promotes CNS inflammation and neuronal apoptosis.

Microglia Are Necessary to Regulate Sleep after an Immune Challenge

Microglia play a critical role in the neuroimmune response, but little is known about the role of microglia in sleep following an inflammatory trigger. Nevertheless, decades of research have been predicated on the assumption that an inflammatory trigger increases sleep through microglial activation. We hypothesized that mice (n = 30) with depleted microglia using PLX5622 (PLX) would sleep less following the administration of lipopolysaccharide (LPS) to induce inflammation. Brains were collected and microglial morphology was assessed using quantitative skeletal analyses and physiological parameters were recorded using non-invasive piezoelectric cages. Mice fed PLX diet had a transient increase in sleep that dissipated by week 2. Subsequently, following a first LPS injection (0.4 mg/kg), mice with depleted microglia slept more than mice on the control diet. All mice were returned to normal rodent chow to repopulate microglia in the PLX group (10 days). Nominal differences in sleep existed during the microglia repopulation period. However, following a second LPS injection, mice with repopulated microglia slept similarly to control mice during the dark period but with longer bouts during the light period. Comparing sleep after the first LPS injection to sleep after the second LPS injection, controls exhibited temporal changes in sleep patterns but no change in cumulative minutes slept, whereas cumulative sleep in mice with repopulated microglia decreased during the dark period across all days. Repopulated microglia had a reactive morphology. We conclude that microglia are necessary to regulate sleep after an immune challenge.

Microglia are involved in the protection of memories formed during sleep deprivation

Sleep deprivation can generate inflammatory responses in the central nervous system. In turn, this inflammation increases sleep drive, leading to a rebound in sleep duration. Microglia, the innate immune cells found exclusively in the CNS, have previously been found to release inflammatory signals and exhibit altered characteristics in response to sleep deprivation. Together, this suggests that microglia may be partially responsible for the brain's response to sleep deprivation through their inflammatory activity. In this study, we ablated microglia from the mouse brain and assessed resulting sleep, circadian, and sleep deprivation phenotypes. We find that microglia are dispensable for both homeostatic sleep and circadian function and the sleep rebound response to sleep deprivation. However, we uncover a phenomenon by which microglia appear to be essential for the protection of fear-conditioning memories formed during the recovery sleep period following a period of sleep deprivation. This phenomenon occurs potentially through the upregulation of synaptic-homeostasis related genes to protect nascent dendritic spines that may be otherwise removed or downscaled during recovery sleep. These findings further expand the list of known functions for microglia in synaptic modulation.

Sleep Disruption Worsens Seizures: Neuroinflammation as a Potential Mechanistic Link

Sleep disturbances, such as insomnia, obstructive sleep apnea, and daytime sleepiness, are common in people diagnosed with epilepsy. These disturbances can be attributed to nocturnal seizures, psychosocial factors, and/or the use of anti-epileptic drugs with sleep-modifying side effects. Epilepsy patients with poor sleep quality have intensified seizure frequency and disease progression compared to their well-rested counterparts. A better understanding of the complex relationship between sleep and epilepsy is needed, since approximately 20% of seizures and more than 90% of sudden unexpected deaths in epilepsy occur during sleep. Emerging studies suggest that neuroinflammation, (e.g., the CNS immune response characterized by the change in expression of inflammatory mediators and glial activation) may be a potential link between sleep deprivation and seizures. Here, we review the mechanisms by which sleep deprivation induces neuroinflammation and propose that neuroinflammation synergizes with seizure activity to worsen neurodegeneration in the epileptic brain. Additionally, we highlight the relevance of sleep interventions, often overlooked by physicians, to manage seizures, prevent epilepsy-related mortality, and improve quality of life.

Microglia modulate hippocampal synaptic transmission and sleep duration along the light/dark cycle

Microglia, the brain's resident macrophages, actively contribute to the homeostasis of cerebral parenchyma by sensing neuronal activity and supporting synaptic remodeling and plasticity. While several studies demonstrated different roles for astrocytes in sleep, the contribution of microglia in the regulation of sleep/wake cycle and in the modulation of synaptic activity in the different day phases has not been deeply investigated. Using light as a zeitgeber cue, we studied the effects of microglial depletion with the colony stimulating factor‐1 receptor antagonist PLX5622 on the sleep/wake cycle and on hippocampal synaptic transmission in male mice. Our data demonstrate that almost complete microglial depletion increases the duration of NREM sleep and reduces the hippocampal excitatory neurotransmission. The fractalkine receptor CX3CR1 plays a relevant role in these effects, because cx3cr1^GFP/GFP mice recapitulate what found in PLX5622‐treated mice. Furthermore, during the light phase, microglia express lower levels of cx3cr1 and a reduction of cx3cr1 expression is also observed when cultured microglial cells are stimulated by ATP, a purinergic molecule released during sleep. Our findings suggest that microglia participate in the regulation of sleep, adapting their cx3cr1 expression in response to the light/dark phase, and modulating synaptic activity in a phase‐dependent manner.

No loss of orexin/hypocretin, melanin-concentrating hormone or locus coeruleus noradrenergic neurons in a rat model of chronic sleep restriction

Chronic sleep restriction (CSR) is common in modern society, adversely affecting cognitive performance and health. Yet how it impacts neurons regulating sleep remains unclear. Several studies using mice reported substantial losses of wake-active orexin/hypocretin and locus coeruleus (LC) noradrenergic neurons, but not rapid eye movement sleep-active melanin-concentrating hormone (MCH) neurons, following CSR. Here, we used immunohistochemistry and stereology to examine orexin, MCH and LC noradrenergic neurons in a rat model of CSR that uses programmed wheel rotation (3 h on/1 h off; '3/1' protocol). Adult male Wistar rats underwent one or four cycles of the 4-day 3/1 CSR protocol, with 2-day recovery between cycles in home cages. Time-matched control rats were housed in locked wheels/home cages. We found no significant differences in the numbers of orexin, MCH and LC noradrenergic neurons following either one- or four-cycle CSR protocol compared to respective controls. Similarly, the four-cycle CSR protocol had no effect on the densities of orexin axon terminals in the LC, noradrenergic dendrites in the LC and noradrenergic axon terminals in the frontal cortex. Body weights, however, decreased after one cycle of CSR and then increased with diminishing slope over the next three cycles. Thus, we found no evidence for loss of orexin or LC noradrenergic neurons following one and four cycles of the 4-day 3/1 CSR protocol in rats. Differences in CSR protocols and/or possible species differences in neuronal vulnerability to sleep loss may account for the discrepancy between the current results in rats and previous findings in mice.

Microglia dynamics in sleep/wake states and in response to sleep loss

Sleep has an essential role for optimal brain function, but the cellular substrates for sleep regulation are not fully understood. Microglia, the immune cells of the brain, have gained increasingly more attention over the last two decades for their important roles in various brain functions that extend beyond their well-known immune function, including brain development, neuronal protection, and synaptic plasticity. Here we review recent advances in understanding: i) morphological and phenotypic dynamics of microglia including process motility/growth and gene/protein expression, and ii) microglia-neuron interactions including phagocytosis and contact at synapses which alters neuronal circuit activity, both under physiological state in the adult brain. We discuss how the microglia-neuron interactions particularly at synapses could influence microglia and neuronal activities across circadian cycles and sleep/wake states. We also review recent findings on how microglia respond to sleep loss. We conclude by pointing out key questions and proposing suggestions for future research to better understand the role of microglia in sleep regulation, sleep homeostasis, and the function of sleep.

Synaptic elimination by microglia and disturbed higher brain functions

Microglial cells in normal mature brains have long been considered to be cells that are resting until pathological events take place, activating the microglial cells. However, it is currently well known that the microglia that have resting ramified morphology in normal mature brains move actively in the brain parenchyma and phagocytose synapses, thus forming and maintaining neural circuits. This review summarizes recent findings on the roles of microglia in mature brains, with special reference to phagocytosis of synapses and higher brain functions. Phagocytic elimination of synapses by microglia may affect the balance between excitatory and inhibitory synaptic transmission, termed the E/I balance. When impaired synaptic elimination by microglia leads to disturbed E/I balance, various problems may follow in brain functions: in memory and cognitive functions, sleep, movement, social behaviors, and thinking. In addition to the roles of microglia in normal developing and mature brains, impaired microglial phagocytosis functions also correlate with disturbances to these higher brain functions that are caused by neurological, mental, and developmental disorders; Parkinson's and Alzheimer's diseases, autism spectrum disorder, attention deficit/hyperactivity disorder, and schizophrenia.