Adenosine A(2A) receptors regulate the activity of sleep regulatory GABAergic neurons in the preoptic hypothalamus

Functions and mechanisms of adenosine and its receptors in sleep regulation

Sleep is a natural and recurring state of life. Long-term insomnia can lead to physical and mental fatigue, inattention, memory loss, anxiety, depression and other symptoms, imposing immense public health and economic burden worldwide. The sleep and awakening regulation system is composed of many nerve nuclei and neurotransmitters in the brain, and it forms a neural network that interacts and restricts each other to regulate the occurrence and maintenance of sleep-wake. Adenosine (AD) is a neurotransmitter in the central nervous system and a driver of sleep. Meanwhile, the functions and mechanisms underlying sleep-promoting effects of adenosine and its receptors are still not entirely clear. However, in recent years, the increasing evidence indicated that adenosine can promote sleep through inhibiting arousal system and activating sleep-promoting system. At the same time, astrocyte-derived adenosine in modulating sleep homeostasis and sleep loss-induced related cognitive and memory deficits plays an important role. This review, therefore, summarizes the current research on the functions and possible mechanisms of adenosine and its receptors in the regulation of sleep and homeostatic control of sleep. Understanding these aspects will provide us better ideas on clinical problems such as insomnia, hypersomnia and other sleep disorders.

"Eritadenine as a regulator of anxiety Disorders: An experimental and docking Approach"

Uncertainty persists regarding the specific chemical causal factors and their corresponding behavioral effects in anxiety disorders. Commonly employed first-line treatments for anxiety target G protein-coupled receptors (GPCRs), including inhibitors of monoaminergic systems. Alternatively, emerging natural bioactive strategies offer potential for mitigating adverse effects. Recent investigations have implicated adenosine in anxiety-triggering mechanisms, while eritadenine, an adenosine analog derived from Shiitake mushroom, has displayed promising attributes. This study explores eritadenine's potential as a bioactive substance for anxiety disorders in mice, employing behavioral tests, pentobarbital-sleep induction, and molecular docking. Behavioral test results reveal a pronounced anxiolytic and sedative-hypnotic pharmacological effect of eritadenine. Our findings suggest that eritadenine may modulate locomotor functions mediated by adenosine receptors, with a stronger affinity for binding to A2AAR over A1AR, thus eliciting these effects.

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.

Role of astrocytes in sleep deprivation: accomplices, resisters, or bystanders?

Sleep plays an essential role in all studied animals with a nervous system. However, sleep deprivation leads to various pathological changes and neurobehavioral problems. Astrocytes are the most abundant cells in the brain and are involved in various important functions, including neurotransmitter and ion homeostasis, synaptic and neuronal modulation, and blood-brain barrier maintenance; furthermore, they are associated with numerous neurodegenerative diseases, pain, and mood disorders. Moreover, astrocytes are increasingly being recognized as vital contributors to the regulation of sleep-wake cycles, both locally and in specific neural circuits. In this review, we begin by describing the role of astrocytes in regulating sleep and circadian rhythms, focusing on: (i) neuronal activity; (ii) metabolism; (iii) the glymphatic system; (iv) neuroinflammation; and (v) astrocyte-microglia cross-talk. Moreover, we review the role of astrocytes in sleep deprivation comorbidities and sleep deprivation-related brain disorders. Finally, we discuss potential interventions targeting astrocytes to prevent or treat sleep deprivation-related brain disorders. Pursuing these questions would pave the way for a deeper understanding of the cellular and neural mechanisms underlying sleep deprivation-comorbid brain disorders.

The stress of losing sleep: Sex-specific neurobiological outcomes

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

Prostaglandin D2 Controls Local Blood Flow and Sleep-Promoting Neurons in the VLPO via Astrocyte-Derived Adenosine

Prostaglandin D₂ (PGD₂) is one of the most potent endogenous sleep-promoting molecules. However, the cellular and molecular mechanisms of the PGD₂-induced activation of sleep-promoting neurons in the ventrolateral preoptic nucleus (VLPO), the major nonrapid eye movement (NREM)-sleep center, still remains unclear. We here show that PGD₂ receptors (DP₁) are not only expressed in the leptomeninges but also in astrocytes from the VLPO. We further demonstrate, by performing real-time measurements of extracellular adenosine using purine enzymatic biosensors in the VLPO, that PGD₂ application causes a 40% increase in adenosine level, via an astroglial release. Measurements of vasodilatory responses and electrophysiological recordings finally reveal that, in response to PGD₂ application, adenosine release induces an A_2AR-mediated dilatation of blood vessels and activation of VLPO sleep-promoting neurons. Altogether, our results unravel the PGD₂ signaling pathway in the VLPO, controlling local blood flow and sleep-promoting neurons, via astrocyte-derived adenosine.

Stress-induced changes in nociceptive responding post-surgery in preclinical rodent models

Chronic post-surgical pain affects up to 85% of individuals depending on the type of surgery, the extent of inflammation, tissue and/or nerve damage. Pre-surgical stress is associated with greater pain intensity, prolonged recovery and is one of the main risk factors for the development of chronic post-surgical pain. Clinically valid animal models provide an important means of examining the mechanisms underlying the effects of stress on post-surgical pain and identifying potential novel therapeutic targets. This review discusses the current data from preclinical animal studies examining the effect of stress on post-surgical pain, the potential underlying mechanisms and gaps in the knowledge that require further investigation.

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.

Adenosine Downregulates the Activities of Glutamatergic Neurons in the Paraventricular Hypothalamic Nucleus Required for Sleep

Adenosine is an endogenous substance that regulates sleep homeostasis. It plays an important role in sleep induction under physiological condition. So far, the neural mechanisms underlying sleep-promoting effects of adenosine are not completely clear. Recent studies have shown that glutamatergic neurons in the paraventricular hypothalamic nucleus (PVH) play an important role in wakefulness. Using whole-cell patch-clamp, we found that adenosine can inhibit glutamatergic neurons in PVH. This inhibition is mainly achieved by activating adenosine type 1 receptors, thereby reducing hyperpolarization-activated cyclic nucleotide-gated cation channels. By recording electroencephalogram (EEG) and electromyography (EMG), it was found that local administration of adenosine type 1 receptor blocker in PVH could significantly reduce the NREM sleep. On the contrary, if adenosine was given, it could increase the NREM sleep. These results suggest that adenosine can promote sleep by reducing the excitability of PVH neurons. This findings reveal a novel mechanism of adenosine regulating sleep homeostasis.

Neuroinflammation, Sleep, and Circadian Rhythms

Molecules involved in innate immunity affect sleep and circadian oscillators and vice versa. Sleep-inducing inflammatory molecules are activated by increased waking activity and pathogens. Pathologies that alter inflammatory molecules, such as traumatic brain injury, cancer, cardiovascular disease, and stroke often are associated with disturbed sleep and electroencephalogram power spectra. Moreover, sleep disorders, such as insomnia and sleep disordered breathing, are associated with increased dysregulation of inflammatory processes. Inflammatory molecules in both the central nervous system and periphery can alter sleep. Inflammation can also modulate cerebral vascular hemodynamics which is associated with alterations in electroencephalogram power spectra. However, further research is needed to determine the interactions of sleep regulatory inflammatory molecules and circadian clocks. The purpose of this review is to: 1) describe the role of the inflammatory cytokines interleukin-1 beta and tumor necrosis factor-alpha and nucleotide-binding domain and leucine-rich repeat protein-3 inflammasomes in sleep regulation, 2) to discuss the relationship between the vagus nerve in translating inflammatory signals between the periphery and central nervous system to alter sleep, and 3) to present information about the relationship between cerebral vascular hemodynamics and the electroencephalogram during sleep.

Estradiol Influences Adenosinergic Signaling and NREM Sleep Need in Adult Female Rats

Gonadal steroids and gender are risk factors for sleep disruptions and insomnia in women. However, the relationship between ovarian steroids and sleep is poorly understood. In rodent models, estradiol (E2) suppresses sleep in females suggesting that E2 may reduce homeostatic sleep need. The current study investigates whether E2 decreases sleep need and the potential mechanisms that govern E2 suppression of sleep. Our previous findings suggest that the median preoptic nucleus (MnPO) is a key nexus for E2 action on sleep. Using behavioral, neurochemical and pharmacological approaches, we tested whether (1) E2 influenced the sleep homeostat and (2) E2 influenced adenosine signaling in the MnPO of adult female rats. In both unrestricted baseline sleep and recovery sleep from 6-hour sleep deprivation, E2 significantly reduced non-rapid eye movement sleep (NREM)-delta power, NREM-Slow Wave Activity (NREM-SWA, 0.5-4.0Hz), and NREM-delta energy suggesting that E2 decreases homeostatic sleep need. However, coordinate with E2-induced changes in physiological markers of homeostatic sleep was a marked increase in MnPO extracellular adenosine (a molecular marker of homeostatic sleep need) during unrestricted and recovery sleep in E2-treated but not oil control animals. While these results seemed contradictory, systemically administered E2 blocked the ability of CGS-21680 (adenosine A2A receptor agonist) microinjected into the MnPO to increase NREM sleep suggesting that E2 may block adenosine signaling. Together, these findings provide evidence that E2 may attenuate the local effects of the A2A receptors in the MnPO which in turn may underlie estrogenic suppression of sleep behavior as well as changes in homeostatic sleep need.

Microglia modulate stable wakefulness via the thalamic reticular nucleus in mice

Microglia are important for brain homeostasis and immunity, but their role in regulating vigilance remains unclear. We employed genetic, physiological, and metabolomic methods to examine microglial involvement in the regulation of wakefulness and sleep. Microglial depletion decreased stable nighttime wakefulness in mice by increasing transitions between wakefulness and non-rapid eye movement (NREM) sleep. Metabolomic analysis revealed that the sleep-wake behavior closely correlated with diurnal variation of the brain ceramide, which disappeared in microglia-depleted mice. Ceramide preferentially influenced microglia in the thalamic reticular nucleus (TRN), and local depletion of TRN microglia produced similar impaired wakefulness. Chemogenetic manipulations of anterior TRN neurons showed that they regulated transitions between wakefulness and NREM sleep. Their firing capacity was suppressed by both microglial depletion and added ceramide. In microglia-depleted mice, activating anterior TRN neurons or inhibiting ceramide production both restored stable wakefulness. These findings demonstrate that microglia can modulate stable wakefulness through anterior TRN neurons via ceramide signaling.

Role of the Preoptic Area in Sleep and Thermoregulation

Sleep and body temperature are tightly interconnected in mammals: warming up our body helps to fall asleep and the body temperature in turn drops while falling asleep. The preoptic area of the hypothalamus (POA) serves as an essential brain region to coordinate sleep and body temperature. Understanding how these two behaviors are controlled within the POA requires the molecular identification of the involved circuits and mapping their local and brain-wide connectivity. Here, we review our current understanding of how sleep and body temperature are regulated with a focus on recently discovered sleep- and thermo-regulatory POA neurons. We further discuss unresolved key questions including the anatomical and functional overlap of sleep- and thermo-regulatory neurons, their pathways and the role of various signaling molecules. We suggest that analysis of genetically defined circuits will provide novel insights into the mechanisms underlying the coordinated regulation of sleep and body temperature in health and disease.

Preoptic Area Modulation of Arousal in Natural and Drug Induced Unconscious States

The role of the hypothalamic preoptic area (POA) in arousal state regulation has been studied since Constantin von Economo first recognized its importance in the early twentieth century. Over the intervening decades, the POA has been shown to modulate arousal in both natural (sleep and wake) as well as drug-induced (anesthetic-induced unconsciousness) states. While the POA is well known for its role in sleep promotion, populations of wake-promoting neurons within the region have also been identified. However, the complexity and molecular heterogeneity of the POA has made distinguishing these two populations difficult. Though multiple lines of evidence demonstrate that general anesthetics modulate the activity of the POA, the region's heterogeneity has also made it challenging to determine whether the same neurons involved in sleep/wake regulation also modulate arousal in response to general anesthetics. While a number of studies show that sleep-promoting POA neurons are activated by various anesthetics, recent work suggests this is not universal to all arousal-regulating POA neurons. Technical innovations are making it increasingly possible to classify and distinguish the molecular identities of neurons involved in sleep/wake regulation as well as anesthetic-induced unconsciousness. Here, we review the current understanding of the POA's role in arousal state regulation of both natural and drug-induced forms of unconsciousness, including its molecular organization and connectivity to other known sleep and wake promoting regions. Further insights into the molecular identities and connectivity of arousal-regulating POA neurons will be critical in fully understanding how this complex region regulates arousal states.

Cannabinoids and Sleep/Wake Control

The sleep-wake cycle is a complex process that includes wake (W), non-rapid-eye-movement (NREM) and rapid-eye-movement (REM) sleep. Each phase is regulated by specialized brain structures that, by means of different neurotransmitters, maintain the constant expression of the sleep-wake cycle. Molecules like orexin, serotonin, noradrenaline, histamine, for waking; GABA, adenosine, prostaglandins, for NREM sleep and acetylcholine and glutamate for REM sleep, among other molecules are responsible for the expression and maintenance of each phase. When the endocannabinoid system was being described for the first time, almost three decades ago, oleamide's sleep promoting properties were highlighted. Nowadays, enough evidence has been cumulated to support the endocannabinoid system role in the sleep-wake cycle regulation. The endocannabinoids oleamide anandamide, and 2-arachidonylglycerol promote NREM and/or REM sleep via the CB1R, thereby making this system a target to treat sleep disorders, such as insomnia.

Focusing on Adenosine Receptors as a Potential Targeted Therapy in Human Diseases

Adenosine is involved in a range of physiological and pathological effects through membrane-bound receptors linked to G proteins. There are four subtypes of adenosine receptors, described as A1AR, A2AAR, A2BAR, and A3AR, which are the center of cAMP signal pathway-based drug development. Several types of agonists, partial agonists or antagonists, and allosteric substances have been synthesized from these receptors as new therapeutic drug candidates. Research efforts surrounding A1AR and A2AAR are perhaps the most enticing because of their concentration and affinity; however, as a consequence of distressing conditions, both A2BAR and A3AR levels might accumulate. This review focuses on the biological features of each adenosine receptor as the basis of ligand production and describes clinical studies of adenosine receptor-associated pharmaceuticals in human diseases.

Role of Glia in the Regulation of Sleep in Health and Disease

Sleep is a naturally occurring physiological state that is required to sustain physical and mental health. Traditionally viewed as strictly regulated by top-down control mechanisms, sleep is now known to also originate locally. Glial cells are emerging as important contributors to the regulation of sleep-wake cycles, locally and among dedicated neural circuits. A few pioneering studies revealed that astrocytes and microglia may influence sleep pressure, duration as well as intensity, but the precise involvement of these two glial cells in the regulation of sleep remains to be fully addressed, across contexts of health and disease. In this overview article, we will first summarize the literature pertaining to the role of astrocytes and microglia in the regulation of sleep under normal physiological conditions. Afterward, we will discuss the beneficial and deleterious consequences of glia-mediated neuroinflammation, whether it is acute, or chronic and associated with brain diseases, on the regulation of sleep. Sleep disturbances are a main comorbidity in neurodegenerative diseases, and in several brain diseases that include pain, epilepsy, and cancer. Identifying the relationships between glia-mediated neuroinflammation, sleep-wake rhythm disruption and brain diseases may have important implications for the treatment of several disorders. © 2020 American Physiological Society. Compr Physiol 10:687-712, 2020.

Region-specific adenosinergic modulation of the slow-cortical rhythm in urethane-anesthetized rats

Slow cortical rhythm (SCR) is a rhythmic alternation of UP and DOWN states during sleep and anesthesia. SCR-associated slow waves reflect homeostatic sleep functions. Adenosine accumulating during prolonged wakefulness and sleep deprivation (SD) may play a role in the delta power increment during recovery sleep. NREM sleep is a local, use-dependent process of the brain. In the present study, direct effect of adenosine on UP and DOWN states was tested by topical application to frontal, somatosensory and visual cortices, respectively, in urethane-anesthetized rats. Local field potentials (LFPs) were recorded using an electrode array inserted close to the location of adenosine application. Multiple unit activity (MUA) was measured from layer V-VI in close proximity of the recording array. In the frontal and somatosensory cortex, adenosine modulated SCR with slow kinetics on the LFP level while MUA remained mostly unaffected. In the visual cortex, adenosine modulated SCR with fast kinetics. In each region, delta power increment was based on the increased frequency of state transitions as well as increased height of UP-state associated slow waves. These results show that adenosine may directly modulate SCR in a complex and region-specific manner which may be related to the finding that restorative processes may take place with varying duration and intensity during recovery sleep in different cortical regions. Adenosine may play a direct role in the increment of the slow wave power observed during local sleep, furthermore it may shape the region-specific characteristics of the phenomenon.

The Neuropeptide Galanin Is Required for Homeostatic Rebound Sleep following Increased Neuronal Activity

Sleep pressure increases during wake and dissipates during sleep, but the molecules and neurons that measure homeostatic sleep pressure remain poorly understood. We present a pharmacological assay in larval zebrafish that generates short-term increases in wakefulness followed by sustained rebound sleep after washout. The intensity of global neuronal activity during drug-induced wakefulness predicted the amount of subsequent rebound sleep. Whole-brain mapping with the neuronal activity marker phosphorylated extracellular signal-regulated kinase (pERK) identified preoptic Galanin (Galn)-expressing neurons as selectively active during rebound sleep, and the relative induction of galn transcripts was predictive of total rebound sleep time. Galn is required for sleep homeostasis, as galn mutants almost completely lacked rebound sleep following both pharmacologically induced neuronal activity and physical sleep deprivation. These results suggest that Galn plays a key role in responding to sleep pressure signals derived from neuronal activity and functions as an output arm of the vertebrate sleep homeostat.

Role of the L-PGDS-PGD2-DP1 receptor axis in sleep regulation and neurologic outcomes

To meet the new challenges of modern lifestyles, we often compromise a good night's sleep. In preclinical models as well as in humans, a chronic lack of sleep is reported to be among the leading causes of various physiologic, psychologic, and neurocognitive deficits. Thus far, various endogenous mediators have been implicated in inter-regulatory networks that collectively influence the sleep-wake cycle. One such mediator is the lipocalin-type prostaglandin D2 synthase (L-PGDS)-Prostaglandin D2 (PGD2)-DP1 receptor (L-PGDS-PGD2-DP1R) axis. Findings in preclinical models confirm that DP1R are predominantly expressed in the sleep-regulating centers. This finding led to the discovery that the L-PGDS-PGD2-DP1R axis is involved in sleep regulation. Furthermore, we showed that the L-PGDS-PGD2-DP1R axis is beneficial in protecting the brain from ischemic stroke. Protein sequence homology was also performed, and it was found that L-PGDS and DP1R share a high degree of homology between humans and rodents. Based on the preclinical and clinical data thus far pertaining to the role of the L-PGDS-PGD2-DP1R axis in sleep regulation and neurologic conditions, there is optimism that this axis may have a high translational potential in human therapeutics. Therefore, here the focus is to review the regulation of the homeostatic component of the sleep process with a special focus on the L-PGDS-PGD2-DP1R axis and the consequences of sleep deprivation on health outcomes. Furthermore, we discuss whether the pharmacological regulation of this axis could represent a tool to prevent sleep disturbances and potentially improve outcomes, especially in patients with acute brain injuries.

Neuroendocrine Control of Sleep

Sleep is a phenomenon in animal behavior as enigmatic as it is ubiquitous, and one deeply tied to endocrine function. Though there are still many unanswered questions about the neurochemical basis of sleep and its functions, extensive interactions have been identified between sleep and the endocrine system, in both the endocrine system's effect on sleep and sleep's effect on the endocrine system. Unfortunately, until recent years, much research on sleep behavior largely disregarded its connections with the endocrine system. Use of both clinical studies and rodent models to investigate interactions between neuroendocrine function, including biological sex, and sleep therefore presents a promising area of further exploration. Further investigation of the neurobiological and neuroendocrine basis of sleep could have wide impact on a number of clinical and basic science fields. In this review, we summarize the state of basic sleep biology and its connections to the field of neuroendocrine biology, as well as suggest key future directions for the neuroendocrine regulation of sleep that may significantly impact new therapies for sleep disorders in women and men.

Preemptive Caffeine Administration Blocks the Increase in Postoperative Pain Caused by Previous Sleep Loss in the Rat: A Potential Role for Preoptic Adenosine A2A Receptors in Sleep-Pain Interactions

Sleep and pain are reciprocally related, but the precise mechanisms underlying this relationship are poorly understood. This study used a rat model of surgical pain to examine the effect of previous sleep loss on postoperative pain and tested the hypothesis that preoptic adenosinergic mechanisms regulate sleep-pain interactions. Relative to ad libitum sleep, 6 hours of total sleep deprivation prior to a surgical incision significantly enhanced postoperative mechanical hypersensitivity in the affected paw and prolonged the time to recovery from surgery. There were no sex-specific differences in these measures. There were also no changes in adrenocorticotropic hormone and corticosterone levels after sleep deprivation, suggesting that this effect was not mediated by the stress associated with the sleep perturbation. Systemic administration of the nonselective adenosine receptor antagonist caffeine at the onset of sleep deprivation prevented the sleep deprivation-induced increase in postoperative hypersensitivity. Microinjection of the adenosine A2A receptor antagonist ZM 241385 into the median preoptic nucleus (MnPO) blocked the increase in surgical pain levels and duration caused by prior sleep deprivation and eliminated the thermal hyperalgesia induced by sleep deprivation in a group of nonoperated (i.e., without surgical incision) rats. These data show that even a brief sleep disturbance prior to surgery worsens postoperative pain and are consistent with our hypothesis that adenosine A2A receptors in the MnPO contribute to regulate these sleep-pain interactions.

Whole-Brain Mapping of Direct Inputs to and Axonal Projections from GABAergic Neurons in the Parafacial Zone

The GABAergic neurons in the parafacial zone (PZ) play an important role in sleep-wake regulation and have been identified as part of a sleep-promoting center in the brainstem, but the long-range connections mediating this function remain poorly characterized. Here, we performed whole-brain mapping of both the inputs and outputs of the GABAergic neurons in the PZ of the mouse brain. We used the modified rabies virus EnvA-ΔG-DsRed combined with a Cre/loxP gene-expression strategy to map the direct monosynaptic inputs to the GABAergic neurons in the PZ, and found that they receive inputs mainly from the hypothalamic area, zona incerta, and parasubthalamic nucleus in the hypothalamus; the substantia nigra, pars reticulata and deep mesencephalic nucleus in the midbrain; and the intermediate reticular nucleus and medial vestibular nucleus (parvocellular part) in the pons and medulla. We also mapped the axonal projections of the PZ GABAergic neurons with adeno-associated virus, and defined the reciprocal connections of the PZ GABAergic neurons with their input and output nuclei. The newly-found inputs and outputs of the PZ were also listed compared with the literature. This cell-type-specific neuronal whole-brain mapping of the PZ GABAergic neurons may reveal the circuits underlying various functions such as sleep-wake regulation.

Body temperature and sleep

Sleep in mammals is accompanied by a decrease in core body temperature (CBT). The circadian clock in the hypothalamic suprachiasmatic nucleus regulates daily rhythms in both CBT and arousal states, and these rhythms are normally coupled. Reductions in metabolic heat production resulting from behavioral quiescence and reduced muscle tone along with changes in autonomic nervous system activity and thermoeffector activity contribute to the sleep-related fall in CBT. Reductions in sympathetic tone to the peripheral vasculature resulting in heat loss through the skin are reflected in a sleep-related increase in distal skin temperature that is a prominent feature of sleep onset in humans. Within a sleep episode, patterns of autonomic nervous system and thermoeffector activity and the ability to defend against heat and cold exposure differ during nonrapid eye movement (NREM) and rapid eye movement sleep. Anatomic and functional integration of the control of arousal states and thermoregulation occur in the preoptic/anterior hypothalamus. Subsets or warm-sensing neurons in the preoptic/anterior hypothalamus implicated in CBT regulation are spontaneously activated during sleep onset and NREM sleep compared to waking and may underlie sleep-related changes in autonomic nervous system and thermoeffector activity.

Neuronal substrates of sleep homeostasis; lessons from flies, rats and mice

Sleep homeostasis is a fundamental property of vigilance state regulation that is highly conserved across species. Neuronal systems and circuits that underlie sleep homeostasis are not well understood. In Drosophila, a neuronal circuit involving neurons in the ellipsoid body and in the dorsal Fan-shaped body is a candidate for both tracing sleep need during waking and translating it to increased sleep drive and expression. Sleep homeostasis in rats and mice involves multiple neuromodulators acting on multiple wake- and sleep-promoting neuronal systems. A functional central homeostat emerges from A₁ receptor mediated actions of adenosine on wake-promoting neurons in the basal forebrain and hypothalamus, and A_2A adenosine receptor-mediated actions on sleep-promoting neurons in the preoptic hypothalamus and nucleus accumbens.

Hypothalamic regulation of the sleep/wake cycle

Sleep is one of the most important physiological functions in mammals. It is regulated by not only homeostatic regulation but also circadian clock. Several neuropeptide-producing neurons located in the hypothalamus are implicated in the regulation of sleep/wakefulness. Among them, orexin/hypocretin-producing neurons (orexin neurons) are a crucial component for maintenance of wakefulness, because lack of orexin function results in narcolepsy, which is a sleep disorder. Recent findings have identified substances that excite or inhibit neural activity of orexin neurons. Furthermore neural projections of the neurons which release these substances have been revealed. In addition to orexin, melanin concentrating hormone (MCH)-producing neurons in the lateral hypothalamic area (LHA) are also implicated in the regulation of sleep/wakefulness. MCH neurons are active during sleep but become silent during wakefulness. Recently developed innovative methods including optogenetics and pharmacogenetics have provided substantial insights into the regulation of sleep/wakefulness. In vivo optical recordings and retrograde and anterograde tracing methods will allow us to understand additional details regarding important interactions between these two types of neurons in the LHA and other neurons in the brain. Finally we discuss the circadian clock and sleep/wake cycle. Understanding of the neural networks and its circadian modulation of sleep/wake cycles remain to be investigated.

Circuit-based interrogation of sleep control

Sleep is a fundamental biological process observed widely in the animal kingdom, but the neural circuits generating sleep remain poorly understood. Understanding the brain mechanisms controlling sleep requires the identification of key neurons in the control circuits and mapping of their synaptic connections. Technical innovations over the past decade have greatly facilitated dissection of the sleep circuits. This has set the stage for understanding how a variety of environmental and physiological factors influence sleep. The ability to initiate and terminate sleep on command will also help us to elucidate its functions within and beyond the brain.

The role of adenosine in the maturation of sleep homeostasis in rats

Sleep homeostasis in rats undergoes significant maturational changes during postweaning development, but the underlying mechanisms of this process are unknown. In the present study we tested the hypothesis that the maturation of sleep is related to the functional emergence of adenosine (AD) signaling in the brain. We assessed postweaning changes in 1) wake-related elevation of extracellular AD in the basal forebrain (BF) and adjacent lateral preoptic area (LPO), and 2) the responsiveness of median preoptic nucleus (MnPO) sleep-active cells to increasing homeostatic sleep drive. We tested the ability of exogenous AD to augment homeostatic responses to sleep deprivation (SD) in newly weaned rats. In groups of postnatal day (P)22 and P30 rats, we collected dialysate from the BF/LPO during baseline (BSL) wake-sleep, SD, and recovery sleep (RS). HPLC analysis of microdialysis samples revealed that SD in P30 rats results in significant increases in AD levels compared with BSL. P22 rats do not exhibit changes in AD levels in response to SD. We recorded neuronal activity in the MnPO during BSL, SD, and RS at P22/P30. MnPO neurons exhibited adult-like increases in waking neuronal discharge across SD on both P22 and P30, but discharge rates during enforced wake were higher on P30 vs. P22. Central administration of AD (1 nmol) during SD on P22 resulted in increased sleep time and EEG slow-wave activity during RS compared with saline control. Collectively, these findings support the hypothesis that functional reorganization of an adenosinergic mechanism of sleep regulation contributes to the maturation of sleep homeostasis.

NEW & NOTEWORTHY

Brain mechanisms that regulate the maturation of sleep are understudied. The present study generated first evidence about a potential mechanistic role for adenosine in the maturation of sleep homeostasis. Specifically, we demonstrate that early postweaning development in rats, when homeostatic response to sleep loss become adult like, is characterized by maturational changes in wake-related production/release of adenosine in the brain. Pharmacologically increased adenosine signaling in developing brain facilitates homeostatic responses to sleep deprivation.

Functions and Mechanisms of Sleep

Sleep is a complex physiological process that is regulated globally, regionally, and locally by both cellular and molecular mechanisms. It occurs to some extent in all animals, although sleep expression in lower animals may be co-extensive with rest. Sleep regulation plays an intrinsic part in many behavioral and physiological functions. Currently, all researchers agree there is no single physiological role sleep serves. Nevertheless, it is quite evident that sleep is essential for many vital functions including development, energy conservation, brain waste clearance, modulation of immune responses, cognition, performance, vigilance, disease, and psychological state. This review details the physiological processes involved in sleep regulation and the possible functions that sleep may serve. This description of the brain circuitry, cell types, and molecules involved in sleep regulation is intended to further the reader's understanding of the functions of sleep.

Astrocyte-derived adenosine is central to the hypnogenic effect of glucose

Sleep has been hypothesised to maintain a close relationship with metabolism. Here we focus on the brain structure that triggers slow-wave sleep, the ventrolateral preoptic nucleus (VLPO), to explore the cellular and molecular signalling pathways recruited by an increase in glucose concentration. We used infrared videomicroscopy on ex vivo brain slices to establish that glucose induces vasodilations specifically in the VLPO via the astrocytic release of adenosine. Real-time detection by in situ purine biosensors further revealed that the adenosine level doubles in response to glucose, and triples during the wakefulness period. Finally, patch-clamp recordings uncovered the depolarizing effect of adenosine and its A2A receptor agonist, CGS-21680, on sleep-promoting VLPO neurons. Altogether, our results provide new insights into the metabolically driven release of adenosine. We hypothesise that adenosine adjusts the local energy supply to local neuronal activity in response to glucose. This pathway could contribute to sleep-wake transition and sleep intensity.

Strategies of Functional Foods Promote Sleep in Human Being

Sleep is a vital segment of life, however, the mechanisms of diet promoting sleep are unclear and are the focus of research. Insomnia is a general sleep disorder and functional foods are known to play a key role in the prevention of insomnia. A number of studies have demonstrated that major insomnia risk factors in human being are less functional foods in dietary. There are higher functional components in functional foods promoting sleep, including tryptophan, GABA, calcium, potassium, melatonin, pyridoxine, L-ornithine and hexadecanoic acid; but wake-promoting neurochemical factors include serotonin, noradrenalin, acetylcholine, histamine, orexin and so on. The factors promoting sleep in human being are the functional foods include barley grass powder, whole grains, maca, panax, Lingzhi, asparagus powder, lettuce, cherry, kiwifruits, walnut, schisandra wine, and milk; Barley grass powder with higher GABA and calcium, as well as potassium is the most ideal functional food promoting sleep, however, the sleep duration for modern humans is associated with food structure of ancient humans. In this review, we put forward possible mechanisms of functional components in foods promoting sleep. Although there is clear relevance between sleep and diet, their molecular mechanisms need to be studied further.

Neuronal activity in the preoptic hypothalamus during sleep deprivation and recovery sleep

The preoptic hypothalamus is implicated in sleep regulation. Neurons in the median preoptic nucleus (MnPO) and the ventrolateral preoptic area (VLPO) have been identified as potential sleep regulatory elements. However, the extent to which MnPO and VLPO neurons are activated in response to changing homeostatic sleep regulatory demands is unresolved. To address this question, we continuously recorded the extracellular activity of neurons in the rat MnPO, VLPO and dorsal lateral preoptic area (LPO) during baseline sleep and waking, during 2 h of sleep deprivation (SD) and during 2 h of recovery sleep (RS). Sleep-active neurons in the MnPO (n = 11) and VLPO (n = 13) were activated in response to SD, such that waking discharge rates increased by 95.8 ± 29.5% and 59.4 ± 17.3%, respectively, above waking baseline values. During RS, non-rapid eye movement (REM) sleep discharge rates of MnPO neurons initially increased to 65.6 ± 15.2% above baseline values, then declined to baseline levels in association with decreases in EEG delta power. Increase in non-REM sleep discharge rates in VLPO neurons during RS averaged 40.5 ± 7.6% above baseline. REM-active neurons (n = 16) in the LPO also exhibited increased waking discharge during SD and an increase in non-REM discharge during RS. Infusion of A2A adenosine receptor antagonist into the VLPO attenuated SD-induced increases in neuronal discharge. Populations of LPO wake/REM-active and state-indifferent neurons and dorsal LPO sleep-active neurons were unresponsive to SD. These findings support the hypothesis that sleep-active neurons in the MnPO and VLPO, and REM-active neurons in the LPO, are components of neuronal circuits that mediate homeostatic responses to sustained wakefulness.