Adenosine and behavioral state control: adenosine increases c-Fos protein and AP1 binding in basal forebrain of rats

Inhibition of Piezo1/Ca2+/calpain signaling in the rat basal forebrain reverses sleep deprivation-induced fear memory impairments

In this study, we tested the hypothesis that the Piezo1/Ca²⁺/calpain pathway of the basal forebrain (BF) modulates impaired fear conditioning caused by sleep deprivation. Adult male Wistar rats were subjected to 6 h of total sleep deprivation using the gentle handling protocol. Step-down inhibitory avoidance tests revealed that sleep deprivation induced substantial short- and long-term fear memory impairment in rats, which was accompanied by increased Piezo1 protein expression (P < 0.01) and increased cleavage of full-length tropomyocin receptor kinase B (TrkB-FL) (P < 0.01) in the BF area. Microinjection of the Piezo1 activator Yoda1 into the BF mimicked these sleep deprivation-induced phenomena; TrkB-FL cleavage was increased (P < 0.01) and short- and long-term fear memory was impaired (both P < 0.01) by Yoda1. Inhibition of Piezo1 by GsMTx4 in the BF area reduced TrkB-FL degradation (P < 0.01) and partially reversed short- and long-term fear memory impairments in sleep-deprived rats (both P < 0.01). Inhibition of calpain activation, downstream of Piezo1 signaling, also improved short- and long-term fear memory impairments (P = 0.038, P = 0.011) and reduced TrkB degradation (P < 0.01) in sleep-deprived rats. Moreover, sleep deprivation induced a lower pain threshold than the rest control, which was partly reversed by microinjection of GsMTx4 or PD151746. Neither sleep deprivation nor the abovementioned drugs affected locomotion and sedation. Taken together, these results indicate that BF Piezo1/Ca²⁺/calpain signaling plays a role in sleep deprivation-induced TrkB signaling disruption and fear memory impairments in rats.

Exogenous adenosine activates A2A adenosine receptor to inhibit RANKL-induced osteoclastogenesis via AP-1 pathway to facilitate bone repair

BACKGROUND

Adenosine is a purine nucleoside involved in regulating bone homeostasis through binding to A1, A2A, A2B, and A3 adenosine receptors (A1R, A2AR, A2BR, and A3R, respectively). However, the underlying mechanisms by which adenosine and receptor subtypes regulate osteoclast differentiation remain uncertain. This study aims to assess the role of exogenous adenosine and receptor subtypes in receptor activator of NF-κB ligand (RANKL)-induced osteoclast formation and explore the underlying molecular mechanisms.

METHODS AND RESULTS

The nanofibrous mats incorporated with adenosine exhibited robust ability to facilitate rat critical-size calvarial defect healing with decreased number of osteoclasts. Moreover, exogenous adenosine substantially enhanced the expression of A2AR and suppressed tartrate-resistant acid phosphatase-positive osteoclast formation and expression of osteoclast-related genes Ctsk, NFATc1, MMP9, and ACP5. This enhancement and suppression could be reversed by adding an A2AR antagonist, ZM241385, in RAW264.7 cells. Finally, RNA sequencing showed that the expression of Fos-related antigen 2 (Fra2) was distinctly downregulated through stimulation of adenosine in RAW264.7 cells treated with RANKL. This downregulation was reversed by ZM241385 according to real-time PCR, Western blot, and immunofluorescence analyses.

CONCLUSIONS

These findings demonstrated that exogenous adenosine binding to A2AR attenuated osteoclast differentiation via the inhibition of activating protein-1 (AP-1, including Fra2 subunit) pathway both in vitro and in vivo.

Regular Caffeine Intake Delays REM Sleep Promotion and Attenuates Sleep Quality in Healthy Men

Acute caffeine intake can attenuate homeostatic sleep pressure and worsen sleep quality. Caffeine intake—particularly in high doses and close to bedtime—may also affect circadian-regulated rapid eye movement (REM) sleep promotion, an important determinant of subjective sleep quality. However, it is not known whether such changes persist under chronic caffeine consumption during daytime. Twenty male caffeine consumers (26.4 ± 4 years old, habitual caffeine intake 478.1 ± 102.8 mg/day) participated in a double-blind crossover study. Each volunteer completed a caffeine (3 × 150 mg caffeine daily for 10 days), a withdrawal (3 × 150 mg caffeine for 8 days then placebo), and a placebo condition. After 10 days of controlled intake and a fixed sleep-wake cycle, we recorded electroencephalography for 8 h starting 5 h after habitual bedtime (i.e., start on average at 04:22 h which is around the peak of circadian REM sleep promotion). A 60-min evening nap preceded each sleep episode and reduced high sleep pressure levels. While total sleep time and sleep architecture did not significantly differ between the three conditions, REM sleep latency was longer after daily caffeine intake compared with both placebo and withdrawal. Moreover, the accumulation of REM sleep proportion was delayed, and volunteers reported more difficulties with awakening after sleep and feeling more tired upon wake-up in the caffeine condition compared with placebo. Our data indicate that besides acute intake, also regular daytime caffeine intake affects REM sleep regulation in men, such that it delays circadian REM sleep promotion when compared with placebo. Moreover, the observed caffeine-induced deterioration in the quality of awakening may suggest a potential motive to reinstate caffeine intake after sleep.

Effects of N6 -(4-hydroxybenzyl) adenine riboside in stress-induced insomnia in rodents

Adenosine exhibits a somnogenic effect; however, there is no adenosinergic hypnotic because of cardiovascular effects. This study investigated whether N6-(4-hydroxybenzyl) adenine riboside (T1-11), extracted from Gastrodia elata, produces somnogenic effects in rodents. We determined the involvement of adenosine 2A receptors (A2ARs) in GABAergic neurons of the ventrolateral preoptic area (VLPO) and the cardiovascular effects. Change of cage bedding is employed as a stressor to induce insomnia in rodents, and electroencephalograms and electromyograms were used to acquire and analyse sleep-wake activity. We found that intracerebroventricular administration of T1-11 before a dark period increased non-rapid eye movement (NREM) and rapid eye movement (REM) sleep during a dark period, and T1-11-induced sleep increases were blocked by the A2AR antagonist, SCH58261, in naïve rats. Oral administration of T1-11 increased NREM sleep during both dark and light periods. Microinjection of the A2AR antagonist, SCH58261, into the VLPO blocked sleep effects of T1-11. In addition to the somnogenic effect in naïve mice, T1-11 suppressed the stress-induced insomnia and this suppressive effect was blocked by SCH58261. C-fos expression in GABAergic neurons of VLPO was increased after administration of T1-11 in Gad2-Cre::Ai14 mice, suggesting the activation of GABAergic neurons in the VLPO. T1-11 exhibited no effects on heart rate and the low frequency/high frequency ratio of heart rate variability. We concluded that T1-11 elicited somnogenic effects and effectively ameliorated acute stress-induced insomnia. The somnogenic effect is mediated by A2ARs to activate GABAergic neurons in the VLPO. This adenosine analogue could be a potential hypnotic because of no sympathetic and parasympathetic effects on the cardiovascular system.

Activation of brain-derived neurotrophic factor signaling in the basal forebrain reverses acute sleep deprivation-induced fear memory impairments

INTRODUCTION

The mechanisms underlying sleep deprivation-induced memory impairments and relevant compensatory signaling pathways remain elusive. We tested the hypothesis that increased brain-derived neurotrophic factor (BDNF) expression in the basal forebrain following acute sleep deprivation was a compensatory mechanism to maintain fear memory performance.

METHODS

Adult male Wistar rats were deprived of 6-hr total sleep from the beginning of the light cycle. The effects of sleep deprivation on BDNF protein expression and activation of downstream tropomyosin receptor kinase B (TrkB)/phospholipase C-γ1 (PLCγ1) signaling in the basal forebrain and fear memory consolidation were examined. BDNF or selective downstream TrkB receptor antagonist ANA-12 was further injected into the basal forebrain bilaterally to observe the changes in fear memory consolidation in response to modulation of the BDNF/TrkB signaling.

RESULTS

Six hours of sleep deprivation-induced both short- and long-term fear memory impairments. Increased BDNF protein expression and TrkB and PLCγ1 phosphorylation in the basal forebrain were observed after sleep deprivation. Microinjection of BDNF into the basal forebrain partly reversed fear memory deficits caused by sleep deprivation, which were accompanied by increased BDNF protein levels and TrkB/PLCγ1 activation. After ANA-12 microinjection, sleep deprivation-induced activation of the BDNF/TrkB pathway was inhibited and impairments of fear memory consolidation were further aggravated.

CONCLUSIONS

Acute sleep deprivation induces compensatory increase of BDNF expression in the basal forebrain. Microinjection of BDNF into the basal forebrain mitigates the fear memory impairments caused by sleep deprivation by activating TrkB/PLCγ1 signaling.

Intracerebral Adenosine During Sleep Deprivation: A Meta-Analysis and New Experimental Data

The neuroregulator adenosine is involved in sleep-wake control. Basal forebrain (BF) adenosine levels increase during sleep deprivation. Only a few studies have addressed the effect of sleep deprivation on extracellular adenosine concentrations in other brain regions. In this paper, we describe a microdialysis experiment as well as a meta-analysis of published data. The 64 h microdialysis experiment determined the extracellular adenosine and adenosine monophosphate (AMP) concentrations in the medial prefrontal cortex of rats before, during and after 12 h of sleep deprivation by forced locomotion. The meta-analysis comprised published sleep deprivation animal experiments measuring adenosine by means of microdialysis. In the animal experiment, the overall median adenosine concentration was 0.36 nM and ranged from 0.004 nM to 27 nM. No significant differences were observed between the five conditions: 12 h of wash-out, baseline light phase, baseline dark phase, 12 h of sleep deprivation and 12 h of subsequent recovery. The overall median AMP concentration was 0.10 nM and ranged from 0.001 nM to 7.56 nM. Median AMP concentration increased during sleep deprivation (T = 47; p = 0.047) but normalised during subsequent recovery. The meta-analysis indicates that BF dialysate adenosine concentrations increase with 74.7% (95% CI: 54.1-95.3%) over baseline during sleep deprivation. Cortex dialysate adenosine concentrations during sleep deprivation were so far only reported by 2 publications. The increase in adenosine during sleep deprivation might be specific to the BF. At this stage, the evidence for adenosine levels in other brain regions is based on single experiments and insufficient for generalised conclusions. Further experiments are currently still warranted.

Intracerebral microdialysis of adenosine and adenosine monophosphate - a systematic review and meta-regression analysis of baseline concentrations

Microdialysis is a method to study the extracellular space in vivo, based on the principle of diffusion. It can be used to measure various small molecules including the neuroregulator adenosine. Baseline levels of the compounds measured with microdialysis vary over studies. We systematically reviewed the literature to investigate the full range of reported baseline concentrations of adenosine and adenosine monophosphate in microdialysates. We performed a meta‐regression analysis to study the influence of flow rate, probe membrane surface area, species, brain area and anaesthesia versus freely behaving, on the adenosine concentration. Baseline adenosine concentrations in microdialysates ranged from 0.8 to 2100 nM. There was limited evidence on baseline adenosine monophosphate concentrations in microdialysates. Across studies, we found effects of flow rate and anaesthesia versus freely behaving on dialysate adenosine concentrations (p ≤ 0.001), but not of probe membrane surface, species, or brain area (p ≥ 0.14). With increasing flow rate, adenosine concentrations decreased. With anaesthesia, adenosine concentrations increased. The effect of other predictor variables on baseline adenosine concentrations, for example, post‐surgical recovery time, could not be analysed because of a lack of reported data. This study shows that meta‐regression can be used as an alternative to new animal experiments to answer research questions in the field of neurochemistry. However, current levels of reporting of primary studies are insufficient to reach the full potential of this approach; 63 out of 133 studies could not be included in the analysis because of insufficient reporting, and several potentially relevant factors had to be excluded from the analyses. The level of reporting of experimental detail needs to improve.

Activation of basal forebrain purinergic P2 receptors promotes wakefulness in mice

The functions of purinergic P2 receptors (P2Rs) for extracellular adenosine triphosphate (ATP) are poorly understood. Here, for the first time, we show that activation of P2Rs in an important arousal region, the basal forebrain (BF), promotes wakefulness, whereas inhibition of P2Rs promotes sleep. Infusion of a non-hydrolysable P2R agonist, ATP-γ-S, into mouse BF increased wakefulness following sleep deprivation. ATP-γ-S depolarized BF cholinergic and cortically-projecting GABAergic neurons in vitro, an effect blocked by antagonists of ionotropic P2Rs (P2XRs) or glutamate receptors. In vivo, ATP-γ-S infusion increased BF glutamate release. Thus, activation of BF P2XRs promotes glutamate release and excitation of wake-active neurons. Conversely, pharmacological antagonism of BF P2XRs decreased spontaneous wakefulness during the dark (active) period. Together with previous findings, our results suggest sleep-wake regulation by BF extracellular ATP involves a balance between excitatory, wakefulness-promoting effects mediated by direct activation of P2XRs and inhibitory, sleep-promoting effects mediated by degradation to adenosine.

Adenosine role in brain functions: Pathophysiological influence on Parkinson's disease and other brain disorders

Although adenosine plays a key role in multiple motor, affective, and cognitive processes, it has received less attention in the neuroscience field compared to other neurotransmitters (e.g., dopamine). In this review, we highlight the role of adenosine in behavior as well as its interaction with other neurotransmitters, such as dopamine. We also discuss brain disorders impacted by alterations to adenosine, and how targeting adenosine can ameliorate Parkinson's disease motor symptoms. We also discuss the role of caffeine (as an adenosine antagonist) on cognition as well as a neuroprotective agent against Parkinson's disease (PD).

Creatine supplementation reduces sleep need and homeostatic sleep pressure in rats

Sleep has been postulated to promote brain energy restoration. It is as yet unknown if increasing the energy availability within the brain reduces sleep need. The guanidine amino acid creatine (Cr) is a well-known energy booster in cellular energy homeostasis. Oral Cr-monohydrate supplementation (CS) increases exercise performance and has been shown to have substantial effects on cognitive performance, neuroprotection and circadian rhythms. The effect of CS on cellular high-energy molecules and sleep-wake behaviour is unclear. Here, we examined the sleep-wake behaviour and brain energy metabolism before and after 4-week-long oral administration of CS in the rat. CS decreased total sleep time and non-rapid eye movement (NREM) sleep significantly during the light (inactive) but not during the dark (active) period. NREM sleep and NREM delta activity were decreased significantly in CS rats after 6 h of sleep deprivation. Biochemical analysis of brain energy metabolites showed a tendency to increase in phosphocreatine after CS, while cellular adenosine triphosphate (ATP) level decreased. Microdialysis analysis showed that the sleep deprivation-induced increase in extracellular adenosine was attenuated after CS. These results suggest that CS reduces sleep need and homeostatic sleep pressure in rats, thereby indicating its potential in the treatment of sleep-related disorders.

Neurotransmitter receptor availability in the rat brain is constant in a 24 hour-period

Wakefulness and sleep are fundamental characteristics of the brain. We, therefore, hypothesized that transmitter systems contribute to their regulation and will exhibit circadian alterations. We assessed the concentration of various neurotransmitter receptors and transporters including adenosinergic (A₁AR, A_2AAR, and ENT1), dopaminergic (D₁R, D₂R, and DAT), and serotonergic (5-HT_2AR) target proteins. Adult male Sprague Dawley rats were used and maintained in a 12 h light: 12 h dark cycle (lights on from 07:00 h to 19:00 h). We measured receptor and transporter concentrations in different brain regions, including caudate putamen, basal forebrain, and cortex in 4 hour-intervals over a 24 hour-period using quantitative in vitro autoradiography. Investigated receptors and transporters showed no fluctuations in any of the analyzed regions using one-way ANOVA. Only in the horizontal diagonal band of Broca, the difference of A₁AR concentration between light and dark phases (t-test) as well as the cosinor analysis of the 24 hour-course were significant, suggesting that this region underlies receptor fluctuations. Our findings suggest that the availability of the investigated neurotransmitter receptors and transporters does not undergo changes in a 24 hour-period. While there are reports on changes in adenosine and dopamine receptors during sleep deprivation, we found no changes in the investigated adenosine, dopamine, and serotonin receptors during regular and undisturbed day-night cycles.

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.

Disrupted sleep-wake regulation in type 1 equilibrative nucleoside transporter knockout mice

The type 1 equilibrative nucleoside transporter (ENT1) is implicated in regulating levels of extracellular adenosine ([AD]ex). In the basal forebrain (BF) levels of [AD]ex increase during wakefulness and closely correspond to the increases in the electroencephalogram (EEG) delta (0.75-4.5Hz) activity (NRδ) during subsequent non-rapid eye movement sleep (NREMS). Thus in the BF, [AD]ex serves as a biochemical marker of sleep homeostasis. Waking EEG activity in theta range (5-9Hz, Wθ) is also described as a marker of sleep homeostasis. An hour-by-hour temporal relationship between the Wθ and NRδ is unclear. In this study we examined the relationship between these EEG markers of sleep homeostasis during spontaneous sleep-wakefulness and during sleep deprivation (SD) and recovery sleep in the ENT1 gene knockout (ENT1KO) mouse. We observed that baseline NREMS amount was decreased during the light period in ENT1KO mice, accompanied by a weak correlation between Wθ of each hour and NRδ of its subsequent hour when compared to their wild-type (WT) littermates. Perfusion of low dose of adenosine into BF not only strengthened the Wθ-NRδ relationship, but also increased NREMS to match with the WT littermates suggesting decreased [AD]ex in ENT1KO mice. However, the SD-induced [AD]ex increase in the BF and the linear correlation between the EEG markers of sleep homeostasis were unaffected in ENT1KO mice suggesting that during SD, sources other than ENT1 contribute to increase in [AD]ex. Our data provide evidence for a differential regulation of wakefulness-associated [AD]ex during spontaneous vs prolonged waking.

Adenosine receptor neurobiology: overview

Adenosine is a naturally occurring nucleoside that is distributed ubiquitously throughout the body as a metabolic intermediary. In the brain, adenosine functions as an important upstream neuromodulator of a broad spectrum of neurotransmitters, receptors, and signaling pathways. By acting through four G-protein-coupled receptors, adenosine contributes critically to homeostasis and neuromodulatory control of a variety of normal and abnormal brain functions, ranging from synaptic plasticity, to cognition, to sleep, to motor activity to neuroinflammation, and cell death. This review begun with an overview of the gene and genome structure and the expression pattern of adenosine receptors (ARs). We feature several new developments over the past decade in our understanding of AR functions in the brain, with special focus on the identification and characterization of canonical and noncanonical signaling pathways of ARs. We provide an update on functional insights from complementary genetic-knockout and pharmacological studies on the AR control of various brain functions. We also highlight several novel and recent developments of AR neurobiology, including (i) recent breakthrough in high resolution of three-dimension structure of adenosine A2A receptors (A2ARs) in several functional status, (ii) receptor-receptor heterodimerization, (iii) AR function in glial cells, and (iv) the druggability of AR. We concluded the review with the contention that these new developments extend and strengthen the support for A1 and A2ARs in brain as therapeutic targets for neurologic and psychiatric diseases.

Cholinergic neurons of the basal forebrain mediate biochemical and electrophysiological mechanisms underlying sleep homeostasis

The tight coordination of biochemical and electrophysiological mechanisms underlies the homeostatic sleep pressure (HSP) produced by sleep deprivation (SD). We have reported that during SD the levels of inducible nitric oxide synthase (iNOS), extracellular nitric oxide (NO), adenosine [AD]ex , lactate [Lac]ex and pyruvate [Pyr]ex increase in the basal forebrain (BF). However, it is not clear whether all of them contribute to HSP leading to increased electroencephalogram (EEG) delta activity during non-rapid eye movement (NREM) recovery sleep (RS) following SD. Previously, we showed that NREM delta increase evident during RS depends on the presence of BF cholinergic (ChBF) neurons. Here, we investigated the role of ChBF cells in coordination of biochemical and EEG changes seen during SD and RS in the rat. Increases in low-theta power (5-7 Hz), but not high-theta (7-9 Hz), during SD correlated with the increase in NREM delta power during RS, and with the changes in nitrate/nitrite [NOx ]ex and [AD]ex . Lesions of ChBF cells using IgG 192-saporin prevented increases in [NOx ]ex , [AD]ex and low-theta activity, during SD, but did not prevent increases in [Lac]ex and [Pyr]ex . Infusion of NO donor DETA NONOate into the saporin-treated BF failed to increase NREM RS and delta power, suggesting ChBF cells are important for mediating NO homeostatic effects. Finally, SD-induced iNOS was mostly expressed in ChBF cells, and the intensity of iNOS induction correlated with the increase in low-theta activity. Together, our data indicate ChBF cells are important in regulating the biochemical and EEG mechanisms that contribute to HSP.

Gray matter-specific changes in brain bioenergetics after acute sleep deprivation: a 31P magnetic resonance spectroscopy study at 4 Tesla

STUDY OBJECTIVES

A principal function of sleep may be restoration of brain energy metabolism caused by the energetic demands of wakefulness. Because energetic demands in the brain are greater in gray than white matter, this study used linear mixed-effects models to examine tissue-type specific changes in high-energy phosphates derived using 31P magnetic resonance spectroscopy (MRS) after sleep deprivation and recovery sleep.

DESIGN

Experimental laboratory study.

SETTING

Outpatient neuroimaging center at a private psychiatric hospital.

PARTICIPANTS

A total of 32 MRS scans performed in eight healthy individuals (mean age 35 y; range 23-51 y).

INTERVENTIONS

Phosphocreatine (PCr) and β-nucleoside triphosphate (NTP) were measured using 31P MRS three dimensional-chemical shift imaging at high field (4 Tesla) after a baseline night of sleep, acute sleep deprivation (SD), and 2 nights of recovery sleep. Novel linear mixed-effects models were constructed using spectral and tissue segmentation data to examine changes in bioenergetics in gray and white matter.

MEASUREMENTS AND RESULTS

PCr increased in gray matter after 2 nights of recovery sleep relative to SD with no significant changes in white matter. Exploratory analyses also demonstrated that increases in PCr were associated with increases in electroencephalographic slow wave activity during recovery sleep. No significant changes in β-NTP were observed.

CONCLUSIONS

These results demonstrate that sleep deprivation and subsequent recovery-induced changes in high-energy phosphates primarily occur in gray matter, and increases in PCr after recovery sleep may be related to sleep homeostasis.

Sleep, plasticity and the pathophysiology of neurodevelopmental disorders: the potential roles of protein synthesis and other cellular processes

Sleep is important for neural plasticity, and plasticity underlies sleep-dependent memory consolidation. It is widely appreciated that protein synthesis plays an essential role in neural plasticity. Studies of sleep-dependent memory and sleep-dependent plasticity have begun to examine alterations in these functions in populations with neurological and psychiatric disorders. Such an approach acknowledges that disordered sleep may have functional consequences during wakefulness. Although neurodevelopmental disorders are not considered to be sleep disorders per se, recent data has revealed that sleep abnormalities are among the most prevalent and common symptoms and may contribute to the progression of these disorders. The main goal of this review is to highlight the role of disordered sleep in the pathology of neurodevelopmental disorders and to examine some potential mechanisms by which sleep-dependent plasticity may be altered. We will also briefly attempt to extend the same logic to the other end of the developmental spectrum and describe a potential role of disordered sleep in the pathology of neurodegenerative diseases. We conclude by discussing ongoing studies that might provide a more integrative approach to the study of sleep, plasticity, and neurodevelopmental disorders.

Physiologically-based modeling of sleep-wake regulatory networks

Mathematical modeling has played a significant role in building our understanding of sleep-wake and circadian behavior. Over the past 40 years, phenomenological models, including the two-process model and oscillator models, helped frame experimental results and guide progress in understanding the interaction of homeostatic and circadian influences on sleep and understanding the generation of rapid eye movement sleep cycling. Recent advances in the clarification of the neural anatomy and physiology involved in the regulation of sleep and circadian rhythms have motivated the development of more detailed and physiologically-based mathematical models that extend the approach introduced by the classical reciprocal-interaction model. Using mathematical formalisms developed in the field of computational neuroscience to model neuronal population activity, these models investigate the dynamics of proposed conceptual models of sleep-wake regulatory networks with a focus on generating appropriate sleep and wake state transition patterns as well as simulating disease states and experimental protocols. In this review, we discuss several recent physiologically-based mathematical models of sleep-wake regulatory networks. We identify common features among these models in their network structures, model dynamics and approaches for model validation. We describe how the model analysis technique of fast-slow decomposition, which exploits the naturally occurring multiple timescales of sleep-wake behavior, can be applied to understand model dynamics in these networks. Our purpose in identifying commonalities among these models is to propel understanding of both the mathematical models and their underlying conceptual models, and focus directions for future experimental and theoretical work.

Adenosine receptors: expression, function and regulation

Adenosine receptors (ARs) comprise a group of G protein-coupled receptors (GPCR) which mediate the physiological actions of adenosine. To date, four AR subtypes have been cloned and identified in different tissues. These receptors have distinct localization, signal transduction pathways and different means of regulation upon exposure to agonists. This review will describe the biochemical characteristics and signaling cascade associated with each receptor and provide insight into how these receptors are regulated in response to agonists. A key property of some of these receptors is their ability to serve as sensors of cellular oxidative stress, which is transmitted by transcription factors, such as nuclear factor (NF)-κB, to regulate the expression of ARs. Recent observations of oligomerization of these receptors into homo- and heterodimers will be discussed. In addition, the importance of these receptors in the regulation of normal and pathological processes such as sleep, the development of cancers and in protection against hearing loss will be examined.

Adenosine inhibits the excitatory synaptic inputs to Basal forebrain cholinergic, GABAergic, and parvalbumin neurons in mice

Coffee and tea contain the stimulants caffeine and theophylline. These compounds act as antagonists of adenosine receptors. Adenosine promotes sleep and its extracellular concentration rises in association with prolonged wakefulness, particularly in the basal forebrain (BF) region involved in activating the cerebral cortex. However, the effect of adenosine on identified BF neurons, especially non-cholinergic neurons, is incompletely understood. Here we used whole-cell patch-clamp recordings in mouse brain slices prepared from two validated transgenic mouse lines with fluorescent proteins expressed in GABAergic or parvalbumin (PV) neurons to determine the effect of adenosine. Whole-cell recordings were made from BF cholinergic neurons and from BF GABAergic and PV neurons with the size (>20 μm) and intrinsic membrane properties (prominent H-currents) corresponding to cortically projecting neurons. A brief (2 min) bath application of adenosine (100 μM) decreased the frequency but not the amplitude of spontaneous excitatory postsynaptic currents (EPSCs) in all groups of BF cholinergic, GABAergic, and PV neurons we recorded. In addition, adenosine decreased the frequency of miniature EPSCs in BF cholinergic neurons. Adenosine had no effect on the frequency of spontaneous inhibitory postsynaptic currents in cholinergic neurons or GABAergic neurons with large H-currents but reduced them in a group of GABAergic neurons with smaller H-currents. All effects of adenosine were blocked by a selective, adenosine A1 receptor antagonist, cyclopentyltheophylline (CPT, 1 μM). Adenosine had no postsynaptic effects. Taken together, our work suggests that adenosine promotes sleep by an A1 receptor-mediated inhibition of glutamatergic inputs to cortically projecting cholinergic and GABA/PV neurons. Conversely, caffeine and theophylline promote attentive wakefulness by inhibiting these A1 receptors in BF thereby promoting the high-frequency oscillations in the cortex required for attention and cognition.

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

The median preoptic nucleus (MnPN) and the ventrolateral preoptic area (VLPO) are two hypothalamic regions that have been implicated in sleep regulation, and both nuclei contain sleep-active GABAergic neurons. Adenosine is an endogenous sleep regulatory substance, which promotes sleep via A1 and A2A receptors (A2AR). Infusion of A2AR agonist into the lateral ventricle or into the subarachnoid space underlying the rostral basal forebrain (SS-rBF), has been previously shown to increase sleep. We examined the effects of an A2AR agonist, CGS-21680, administered into the lateral ventricle and the SS-rBF on sleep and c-Fos protein immunoreactivity (Fos-IR) in GABAergic neurons in the MnPN and VLPO. Intracerebroventricular administration of CGS-21680 during the second half of lights-on phase increased sleep and increased the number of MnPN and VLPO GABAergic neurons expressing Fos-IR. Similar effects were found with CGS-21680 microinjection into the SS-rBF. The induction of Fos-IR in preoptic GABAergic neurons was not secondary to drug-induced sleep, since CGS-21680 delivered to the SS-rBF significantly increased Fos-IR in MnPN and VLPO neurons in animals that were not permitted to sleep. Intracerebroventricular infusion of ZM-241385, an A2AR antagonist, during the last 2 h of a 3-h period of sleep deprivation caused suppression of subsequent recovery sleep and reduced Fos-IR in MnPN and VLPO GABAergic neurons. Our findings support a hypothesis that A2AR-mediated activation of MnPN and VLPO GABAergic neurons contributes to adenosinergic regulation of sleep.

Sleep-wake sensitive mechanisms of adenosine release in the basal forebrain of rodents: an in vitro study

Adenosine acting in the basal forebrain is a key mediator of sleep homeostasis. Extracellular adenosine concentrations increase during wakefulness, especially during prolonged wakefulness and lead to increased sleep pressure and subsequent rebound sleep. The release of endogenous adenosine during the sleep-wake cycle has mainly been studied in vivo with microdialysis techniques. The biochemical changes that accompany sleep-wake status may be preserved in vitro. We have therefore used adenosine-sensitive biosensors in slices of the basal forebrain (BFB) to study both depolarization-evoked adenosine release and the steady state adenosine tone in rats, mice and hamsters. Adenosine release was evoked by high K⁺, AMPA, NMDA and mGlu receptor agonists, but not by other transmitters associated with wakefulness such as orexin, histamine or neurotensin. Evoked and basal adenosine release in the BFB in vitro exhibited three key features: the magnitude of each varied systematically with the diurnal time at which the animal was sacrificed; sleep deprivation prior to sacrifice greatly increased both evoked adenosine release and the basal tone; and the enhancement of evoked adenosine release and basal tone resulting from sleep deprivation was reversed by the inducible nitric oxide synthase (iNOS) inhibitor, 1400 W. These data indicate that characteristics of adenosine release recorded in the BFB in vitro reflect those that have been linked in vivo to the homeostatic control of sleep. Our results provide methodologically independent support for a key role for induction of iNOS as a trigger for enhanced adenosine release following sleep deprivation and suggest that this induction may constitute a biochemical memory of this state.

Sleep and Anesthesia Interactions: A Pharmacological Appraisal

Anesthetics have been used in clinical practice for over a hundred years, yet their mechanisms of action remain poorly understood. One tempting hypothesis to explain their hypnotic properties posits that anesthetics exert a component of their effects by "hijacking" the endogenous arousal circuitry of the brain. Modulation of activity within sleep- and wake-related neuroanatomic systems could thus explain some of the varied effects produced by anesthetics. There has been a recent explosion of research into the neuroanatomic substrates affected by various anesthetics. In this review, we will highlight the relevant sleep architecture and systems and focus on studies over the past few years that implicate these sleep-related structures as targets of anesthetics. These studies highlight a promising area of investigation regarding the mechanisms of action of anesthetics and provide an important model for future study.

Roles of endoplasmic reticulum and energetic stress in disturbed sleep

Sleep disturbances are contributing factors to health risk for several diseases including hypertension, diabetes, obesity, depression, and stroke. On a molecular level, sleep disturbances that incur sleep loss and sleep fragmentation result in cellular stress, inflammation, and an impaired immune system. It has been hypothesized that sleep deprivation or prolonged waking leads to increased energy demand and thus energetic stress. Sleep loss and sleep fragmentation are also known to lead to cellular stress specifically endoplasmic reticulum (ER) stress. This review will summarize the current knowledge of the roles of ER and energetic stress during sleep loss and fragmentation that are characteristics of many sleep disturbances. Sleep research pertinent to these specific pathways will be discussed.

Astrocyte-derived adenosine and A1 receptor activity contribute to sleep loss-induced deficits in hippocampal synaptic plasticity and memory in mice

Sleep deprivation (SD) can have a negative impact on cognitive function, but the mechanism(s) by which SD modulates memory remains unclear. We have previously shown that astrocyte-derived adenosine is a candidate molecule involved in the cognitive deficits following a brief period of SD (Halassa et al., 2009). In this study, we examined whether genetic disruption of soluble N-ethylmaleimide-sensitive factor attached protein (SNARE)-dependent exocytosis in astrocytes (dnSNARE mice) or pharmacological blockade of A1 receptor signaling using an adenosine A1 receptor (A1R) antagonist, 8-cyclopentyl-1,3-dimethylxanthine (CPT), could prevent the negative effects of 6 h of SD on hippocampal late-phase long-term potentiation (L-LTP) and hippocampus-dependent spatial object recognition memory. We found that SD impaired L-LTP in wild-type mice but not in dnSNARE mice. Similarly, this deficit in L-LTP resulting from SD was prevented by a chronic infusion of CPT. Consistent with these results, we found that hippocampus-dependent memory deficits produced by SD were rescued in dnSNARE mice and CPT-treated mice. These data provide the first evidence that astrocytic ATP and adenosine A1R activity contribute to the effects of SD on hippocampal synaptic plasticity and hippocampus-dependent memory, and suggest a new therapeutic target to reverse the hippocampus-related cognitive deficits induced by sleep loss.

Simultaneous real-time measurement of EEG/EMG and L-glutamate in mice: A biosensor study of neuronal activity during sleep

We report on electroencephalograph (EEG) and electromyograph (EMG) measurements concurrently with real-time changes in L-glutamate concentration. These data reveal a link between sleep state and extracellular neurotransmitter changes in a freely-moving (tethered) mouse. This study reveals, for the first time in mice, that the extracellular L-glutamate concentration in the pre-frontal cortex (PFC) increases during periods of extended wakefulness, decreases during extended sleep episodes and spikes during periods of REM sleep. Individual sleep epochs (10 s in duration) were scored as wake, slow-wave (SW) sleep or rapid eye movement (REM) sleep, and then correlated as a function of time with measured changes in L-glutamate concentrations. The observed L-glutamate levels show a statistically significant increase of 0.86 ± 0.26 μM (p < 0.05) over 37 wake episodes recorded from all mice (n = 6). Over the course of 49 measured sleep periods longer than 15 min, L-glutamate concentrations decline by a similar amount (0.88 ± 0.37 μM, p < 0.08). The analysis of 163 individual REM sleep episodes greater than one min in length across all mice (n = 6) demonstrates a significant rise in L-glutamate levels as compared to the 1 min preceding REM sleep onset (RM-ANOVA, DF = 20, F = 6.458, p < 0.001). The observed rapid changes in L-glutamate concentration during REM sleep last only between 1 and 3 min. The approach described can also be extended to other regions of the brain which are hypothesized to play a role in sleep. This study highlights the importance of obtaining simultaneous measurements of neurotransmitter levels in conjunction with sleep markers to help elucidate the underlying physiological and ultimately the genetic components of sleep.

The time course of adenosine, nitric oxide (NO) and inducible NO synthase changes in the brain with sleep loss and their role in the non-rapid eye movement sleep homeostatic cascade

Both adenosine and nitric oxide (NO) are known for their role in sleep homeostasis, with the basal forebrain (BF) wakefulness center as an important site of action. Previously, we reported a cascade of homeostatic events, wherein sleep deprivation (SD) induces the production of inducible nitric oxide synthase (iNOS)-dependent NO in BF, leading to enhanced release of extracellular adenosine. In turn, increased BF adenosine leads to enhanced sleep intensity, as measured by increased non-rapid eye movement sleep EEG delta activity. However, the presence and time course of similar events in cortex has not been studied, although a frontal cortical role for the increase in non-rapid eye movement recovery sleep EEG delta power is known. Accordingly, we performed simultaneous hourly microdialysis sample collection from BF and frontal cortex (FC) during 11 h SD. We observed that both areas showed sequential increases in iNOS and NO, followed by increases in adenosine. BF increases began at 1 h SD, whereas FC increases began at 5 h SD. iNOS and Fos-double labeling indicated that iNOS induction occurred in BF and FC wake-active neurons. These data support the role of BF adenosine and NO in sleep homeostasis and indicate the temporal and spatial sequence of sleep homeostatic cascade for NO and adenosine.

Sleep deprivation triggers inducible nitric oxide-dependent nitric oxide production in wake-active basal forebrain neurons

Sleep loss negatively impacts performance, mood, memory, and immune function, but the homeostatic factors that impel sleep after sleep loss are imperfectly understood. Pharmacological studies had implicated the basal forebrain (BF) inducible nitric oxide (NO) synthase (iNOS)-dependent NO as a key homeostatic factor, but its cellular source was obscure. To obtain direct evidence about the cellular source of iNOS-generated NO during sleep deprivation (SD), we used intracerebroventricular perfusion in rats of the cell membrane-permeable dye diaminofluorescein-2/diacetate (DAF-2/DA) that, once intracellular, bound NO and fluoresced. To circumvent the effects of neuronal NOS (nNOS), DAF-2/DA was perfused in the presence of an nNOS inhibitor. SD led to DAF-positive fluorescence only in the BF neurons, not glia. SD increased expression of iNOS, which colocalized with NO in neurons and, more specifically, in prolonged wakefulness-active neurons labeled by Fos. SD-induced iNOS expression in wakefulness-active neurons positively correlated with sleep pressure, as measured by the number of attempts to enter sleep. Importantly, SD did not induce Fos or iNOS in stress-responsive central amygdala and paraventricular hypothalamic neurons, nor did SD elevate corticosterone, suggesting that the SD protocol did not provoke iNOS expression through stress. We conclude that iNOS-produced neuronal NO is an important homeostatic factor promoting recovery sleep after SD.

Integrated brain circuits: neuron-astrocyte interaction in sleep-related rhythmogenesis

Although astrocytes are increasingly recognized as important modulators of neuronal excitability and information transfer at the synapse, whether these cells regulate neuronal network activity has only recently started to be investigated. In this article, we highlight the role of astrocytes in the modulation of circuit function with particular focus on sleep-related rhythmogenesis. We discuss recent data showing that these glial cells regulate slow oscillations, a specific thalamocortical activity that characterizes non-REM sleep, and sleep-associated behaviors. Based on these findings, we predict that our understanding of the genesis and tuning of thalamocortical rhythms will necessarily go through an integrated view of brain circuits in which non-neuronal cells can play important neuromodulatory roles.

Changes in components of energy regulation in mouse cortex with increases in wakefulness

STUDY OBJECTIVES

Increases in ATP production machinery have been described in brain after 3 h of sleep deprivation. Whether this is sustained with longer durations of extended wakefulness is unknown. We hypothesized that energy depletion could be a mechanism leading to difficulty maintaining wakefulness and assessed changes in components of the electron transport chain.

DESIGN

Protein levels of key subunits of complexes IV and V of the electron transport chain (COXI, COXIV, ATP5B) and uncoupling protein 2 (UCP2) in isolated mitochondria by Westerns in mouse cerebral cortex after 3 and 12 h of sleep deprivation were compared to that in control mice. Activity of complex IV enzyme and relevant transcription factors-Nrf1, Nrf2 (Gabp), and phosphorylation of AMP-dependent kinase (AMPK)-were also assessed.

PARTICIPANTS

8-10 week old C57BL/6J male mice (n = 91).

INTERVENTIONS

3, 6, and 12 h of sleep deprivation.

MEASUREMENTS AND RESULTS

After both 3 and 12 h of sleep deprivation, complex IV proteins and enzyme activity were significantly increased. The complex V catalytic subunit was significantly increased after 12 h of sleep deprivation only. Increased levels of UCP2 protein after 12 h of sleep deprivation suggests that there might be alterations in the ATP/AMP ratio as wakefulness is extended. That phosphorylation of AMPK is increased after 6 h of sleep deprivation supports this assertion. The increase in Nrf1 and Nrf2 (Gabp) mRNA after 6 h of sleep deprivation provides a mechanism by which there is up-regulation of key proteins.

CONCLUSIONS

There are complex dynamic changes in brain energy regulation with extended wakefulness.

Sleep and brain energy levels: ATP changes during sleep

Sleep is one of the most pervasive biological phenomena, but one whose function remains elusive. Although many theories of function, indirect evidence, and even common sense suggest sleep is needed for an increase in brain energy, brain energy levels have not been directly measured with modern technology. We here report that ATP levels, the energy currency of brain cells, show a surge in the initial hours of spontaneous sleep in wake-active but not in sleep-active brain regions of rat. The surge is dependent on sleep but not time of day, since preventing sleep by gentle handling of rats for 3 or 6 h also prevents the surge in ATP. A significant positive correlation was observed between the surge in ATP and EEG non-rapid eye movement delta activity (0.5-4.5 Hz) during spontaneous sleep. Inducing sleep and delta activity by adenosine infusion into basal forebrain during the normally active dark period also increases ATP. Together, these observations suggest that the surge in ATP occurs when the neuronal activity is reduced, as occurs during sleep. The levels of phosphorylated AMP-activated protein kinase (P-AMPK), well known for its role in cellular energy sensing and regulation, and ATP show reciprocal changes. P-AMPK levels are lower during the sleep-induced ATP surge than during wake or sleep deprivation. Together, these results suggest that sleep-induced surge in ATP and the decrease in P-AMPK levels set the stage for increased anabolic processes during sleep and provide insight into the molecular events leading to the restorative biosynthetic processes occurring during sleep.

Adenosine and sleep

Over the last several decades the idea that adenosine (Ado) plays a role in sleep control was postulated due in large part to pharmacological studies that showed the ability of Ado agonists to induce sleep and Ado antagonists to decrease sleep. A second wave of research involving in vitro cellular analytic approaches and subsequently, the use of neurochemical tools such as microdialysis, identified a population of cells within the brainstem and basal forebrain arousal centers, with activity that is both tightly coupled to thalamocortical activation and under tonic inhibitory control by Ado. Most recently, genetic tools have been used to show that Ado receptors regulate a key aspect of sleep, the slow wave activity expressed during slow wave sleep. This review will briefly introduce some of the phenomenology of sleep and then summarize the effect of Ado levels on sleep, the effect of sleep on Ado levels, and recent experiments using mutant mouse models to characterize the role for Ado in sleep control and end with a discussion of which Ado receptors are involved in such control. When taken together, these various experiments suggest that while Ado does play a role in sleep control, it is a specific role with specific functional implications and it is one of many neurotransmitters and neuromodulators affecting the complex behavior of sleep. Finally, since the majority of adenosine-related experiments in the sleep field have focused on SWS, this review will focus largely on SWS; however, the role of adenosine in REM sleep behavior will be addressed.

Simulating Microinjection Experiments in a Novel Model of the Rat Sleep-Wake Regulatory Network

This study presents a novel mathematical modeling framework that is uniquely suited to investigating the structure and dynamics of the sleep-wake regulatory network in the brain stem and hypothalamus. It is based on a population firing rate model formalism that is modified to explicitly include concentration levels of neurotransmitters released to postsynaptic populations. Using this framework, interactions among primary brain stem and hypothalamic neuronal nuclei involved in rat sleep-wake regulation are modeled. The model network captures realistic rat polyphasic sleep-wake behavior consisting of wake, rapid eye movement (REM) sleep, and non-REM (NREM) sleep states. Network dynamics include a cyclic pattern of NREM sleep, REM sleep, and wake states that is disrupted by simulated variability of neurotransmitter release and external noise to the network. Explicit modeling of neurotransmitter concentrations allows for simulations of microinjections of neurotransmitter agonists and antagonists into a key wake-promoting population, the locus coeruleus (LC). Effects of these simulated microinjections on sleep-wake states are tracked and compared with experimental observations. Agonist/antagonist pairs, which are presumed to have opposing effects on LC activity, do not generally induce opposing effects on sleep-wake patterning because of multiple mechanisms for LC activation in the network. Also, different agents, which are presumed to have parallel effects on LC activity, do not induce parallel effects on sleep-wake patterning because of differences in the state dependence or independence of agonist and antagonist action. These simulation results highlight the utility of formal mathematical modeling for constraining conceptual models of the sleep-wake regulatory network.

Sleep deprivation attenuates experimental stroke severity in rats

Indirect epidemiological and experimental evidence suggest that the severity of injury during stroke is influenced by prior sleep history. The aim of our study was to test the effect of acute sleep deprivation on early outcome following experimental stroke. Young male Sprague-Dawley rats (n=20) were subjected to focal cerebral ischemia by reversible right middle cerebral artery occlusion (MCAO) for 90 min. In 10 rats, MCAO was performed just after 6-h of total sleep deprivation (TSD) by "gentle handling", whereas the other rats served as controls. Neurological function during the first week after stroke was monitored using a battery of behavioral tests investigating the asymmetry of sensorimotor deficit (tape removal test and cylinder test), bilateral sensorimotor coordination (rotor-rod and Inclined plane) and memory (T-maze and radial maze). Following MCAO, control rats had impaired behavioral performance in all tests. The largest impairment was noted in the tape test where the tape removal time from the left forelimb (contralateral to MCAO) was increased by approximately 10 fold (p<0.01). In contrast, rats subjected to TSD had complete recovery of sensorimotor performance consistent with a 2.5 fold smaller infarct volume and reduced morphological signs of neuronal injury at day 7 after MCAO. Our data suggest that brief TSD induces a neuroprotective response that limits the severity of a subsequent stroke, similar to rapid ischemic preconditioning.

Effect of sleep deprivation on multi-unit discharge activity of basal forebrain

The basal forebrain (BF) is an important wakefulness/arousal-promoting structure involved in homeostatic responses to sleep deprivation (SD). However, the effects of SD and subsequent sleep recovery on the BF discharge have not been investigated. Multi-unit BF activity was recorded on freely moving rats during 8 h of baseline (BL) and, on the following day, during 4 h of SD by gentle handling followed by 4 h of recovery. The effect of SD on the waking discharge was evaluated during the last 10 min of each hour when attentive waking was induced. The wakefulness level was defined based on the ratio between theta and delta electroencephalogram (EEG) powers, and epochs with ratios >or=1 but <2 (T/D-1) and >or=2 but <4 (T/D-2) were analysed separately. During T/D-1 wakefulness, the BF multi-unit discharge rate increased significantly during the second and third hours of SD and decreased during the third hour of recovery when compared with corresponding hours of BL. Non-rapid eye movement sleep discharge rate during recovery decreased significantly in the second and third versus the first and last hours. The results suggest that maintenance of the level of vigilance necessary for adequate performance during SD requires increased activation of BF neurones when compared with the BL, whereas the same level of vigilance after several hours of recovery can be maintained with lesser activation of BF neurones.

c-Fos protein expression is increased in cholinergic neurons of the rodent basal forebrain during spontaneous and induced wakefulness

It has been proposed that cholinergic neurons of the basal forebrain (BF) may play a role in vigilance state control. Since not all vigilance states have been studied, we evaluated cholinergic neuronal activation levels across spontaneously occurring states of vigilance, as well as during sleep deprivation and recovery sleep following sleep deprivation. Sleep deprivation was performed for 2h at the beginning of the light (inactive) period, by means of gentle sensory stimulation. In the rodent BF, we used immunohistochemical detection of the c-Fos protein as a marker for activation, combined with labeling for choline acetyl-transferase (ChAT) as a marker for cholinergic neurons. We found c-Fos activation in BF cholinergic neurons was highest in the group undergoing sleep deprivation (12.9% of cholinergic neurons), while the spontaneous wakefulness group showed a significant increase (9.2%), compared to labeling in the spontaneous sleep group (1.8%) and a sleep deprivation recovery group (0.8%). A subpopulation of cholinergic neurons expressed c-Fos during spontaneous wakefulness, when possible confounds of the sleep deprivation procedure were minimized (e.g., stress and sensory stimulation). Double-labeling in the sleep deprivation treatment group was significantly elevated in select subnuclei of the BF (medial septum/vertical limb of the diagonal band, horizontal limb of the diagonal band, and the magnocellular preoptic nucleus), when compared to spontaneous wakefulness. These findings support and provide additional confirming data of previous reports that cholinergic neurons of BF play a role in vigilance state regulation by promoting wakefulness.

Sleep deprivation increases A(1) adenosine receptor density in the rat brain

Adenosine, increasing after sleep deprivation and acting via the A(1) adenosine receptor (A(1)AR), is likely a key factor in the homeostatic control of sleep. This study examines the impact of sleep deprivation on A(1)AR density in different parts of the rat brain with [(3)H]CPFPX autoradiography. Binding of [(3)H]CPFPX was significantly increased in parietal cortex (PAR) (7%), thalamus (11%) and caudate-putamen (9%) after 24 h of sleep deprivation compared to a control group with an undisturbed circadian sleep-wake rhythm. Sleep deprivation of 12 h changed receptor density regionally between -5% and +9% (motor cortex (M1), statistically significant) compared to the circadian control group. These results suggest cerebral A(1)ARs are involved in effects of sleep deprivation and the regulation of sleep. The increase of A(1)AR density could serve the purpose of not only maintaining the responsiveness to increased adenosine levels but also amplifying the effect of sleep deprivation and is in line with a sleep-induced homoeostatic reorganization at the synaptic level.

Nitric oxide mediated recovery sleep is attenuated with aging

Nitric oxide (NO) produced by inducible nitric oxide synthase (iNOS) in the cholinergic basal forebrain (BF) during sleep deprivation (SD) is implicated in adenosine (AD) release and induction of recovery sleep. Aging is associated with impairments in sleep homeostasis, such as decrease in non-rapid eye movement sleep (NREM) intensity following SD. We hypothesized that age related changes in sleep homeostasis may be induced by impairments in NO-mediated sleep induction. To test this hypothesis we measured levels of NO and iNOS in the BF during SD as well as recovery sleep after SD and NO-donor (DETA/NO) infusion into the BF in three age groups of rats (young, 4 months; middle-aged, 14 months; old, 24 months). We found that in aged rats as compared to young (1) recovery NREM sleep intensity was significantly decreased, (2) neither iNOS nor NO increased in the BF during SD, and (3) DETA/NO infusion failed to induce sleep. Together, these results support our hypothesis that aging impairs the mechanism through which NO in the BF induces sleep.

The role of cholinergic basal forebrain neurons in adenosine-mediated homeostatic control of sleep: lessons from 192 IgG-saporin lesions

A topic of high current interest and controversy is the basis of the homeostatic sleep response, the increase in non-rapid-eye-movement (NREM) sleep and NREM-delta activity following sleep deprivation (SD). Adenosine, which accumulates in the cholinergic basal forebrain (BF) during SD, has been proposed as one of the important homeostatic sleep factors. It is suggested that sleep-inducing effects of adenosine are mediated by inhibiting the wake-active neurons of the BF, including cholinergic neurons. Here we examined the association between SD-induced adenosine release, the homeostatic sleep response and the survival of cholinergic neurons in the BF after injections of the immunotoxin 192 immunoglobulin G (IgG)-saporin (saporin) in rats. We correlated SD-induced adenosine level in the BF and the homeostatic sleep response with the cholinergic cell loss 2 weeks after local saporin injections into the BF, as well as 2 and 3 weeks after i.c.v. saporin injections. Two weeks after local saporin injection there was an 88% cholinergic cell loss, coupled with nearly complete abolition of the SD-induced adenosine increase in the BF, the homeostatic sleep response, and the sleep-inducing effects of BF adenosine infusion. Two weeks after i.c.v. saporin injection there was a 59% cholinergic cell loss, correlated with significant increase in SD-induced adenosine level in the BF and an intact sleep response. Three weeks after i.c.v. saporin injection there was an 87% cholinergic cell loss, nearly complete abolition of the SD-induced adenosine increase in the BF and the homeostatic response, implying that the time course of i.c.v. saporin lesions is a key variable in interpreting experimental results. Taken together, these results strongly suggest that cholinergic neurons in the BF are important for the SD-induced increase in adenosine as well as for its sleep-inducing effects and play a major, although not exclusive, role in sleep homeostasis.

The energy hypothesis of sleep revisited

One of the proposed functions of sleep is to replenish energy stores in the brain that have been depleted during wakefulness. Benington and Heller formulated a version of the energy hypothesis of sleep in terms of the metabolites adenosine and glycogen. They postulated that during wakefulness, adenosine increases and astrocytic glycogen decreases reflecting the increased energetic demand of wakefulness. We review recent studies on adenosine and glycogen stimulated by this hypothesis. We also discuss other evidence that wakefulness is an energetic challenge to the brain including the unfolded protein response, the electron transport chain, NPAS2, AMP-activated protein kinase, the astrocyte-neuron lactate shuttle, production of reactive oxygen species and uncoupling proteins. We believe the available evidence supports the notion that wakefulness is an energetic challenge to the brain, and that sleep restores energy balance in the brain, although the mechanisms by which this is accomplished are considerably more complex than envisaged by Benington and Heller.