Glutamatergic stimulation of the basal forebrain elevates extracellular adenosine and increases the subsequent sleep

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

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

Phylogenetic conservation of the interdependent homeostatic relationship of sleep regulation and redox metabolism

Sleep is an essential and evolutionarily conserved process that affects many biological functions that are also strongly regulated by cellular metabolism. The interdependence between sleep homeostasis and redox metabolism, in particular, is such that sleep deprivation causes redox metabolic imbalances in the form of over-production of ROS. Likewise (and vice versa), accumulation of ROS leads to greater sleep pressure. Thus, it is theorized that one of the functions of sleep is to act as the brain's "antioxidant" at night by clearing oxidation built up from daily stress of the active day phase. In this review, we will highlight evidence linking sleep homeostasis and regulation to redox metabolism by discussing (1) the bipartite role that sleep-wake neuropeptides and hormones have in redox metabolism through comparing cross-species cellular and molecular mechanisms, (2) the evolutionarily metabolic changes that accompanied the development of sleep loss in cavefish, and finally, (3) some of the challenges of uncovering the cellular mechanism underpinning how ROS accumulation builds sleep pressure and cellularly, how this pressure is cleared.

The neurophysiologic basis of the human sleep–wake cycle and the physiopathology of the circadian clock: a narrative review

The objectives of this review were to explain the neurologic processes that control the human sleep–wake cycle as well as the pathophysiology of the human circadian clock. Non-rapid eye movement and rapid eye movement sleep are the two main phases of sleep. When triggered by circadian input from the anterior hypothalamus and sleep–wake homeostatic information from endogenous chemical signals (example, adenosine), the ventrolateral preoptic nucleus initiates the onset of sleep. Arousal in which there is a conscious monitoring of the surroundings and the ability to respond to external stimuli is known as wakefulness. It contrasts the state of sleep, in which receptivity to external stimuli is reduced. The higher the synchronous firing rates of cerebral cortex neurons, the longer the brain has been awake. Sleep–wake disturbances induced by endogenous circadian system disruptions or desynchronization between internal and external sleep–wake cycles are known as circadian rhythm sleep–wake disorder (CRSWD). Patients with CRSWD usually report chronic daytime drowsiness and/or insomnia, which interferes with their activities. CRSWD is diagnosed based on the results of some functional evaluations, which include measuring the circadian phase using core body temperature, melatonin secretion timing, sleep diaries, actigraphy, and subjective experiences (example, using the Morningness–Eveningness Questionnaire). CRSWD is classified as a dyssomnia in the second edition of the International Classification of Sleep Disorders, with six subtypes: advanced sleep phase, delayed sleep phase, irregular sleep–wake, free running, jet lag, and shift work types. CRSWD can be temporary (due to jet lag, shift work, or illness) or chronic (due to delayed sleep–wake phase disorder, advanced sleep–wake phase disorder, non-24-h sleep–wake disorder, or irregular sleep–wake rhythm disorder). The inability to fall asleep and wake up at the desired time is a common symptom of all CRSWDs.

Characterization of basal forebrain glutamate neurons suggests a role in control of arousal and avoidance behavior

The basal forebrain (BF) is involved in arousal, attention, and reward processing but the role of individual BF neuronal subtypes is still being uncovered. Glutamatergic neurons are the least well-understood of the three main BF neurotransmitter phenotypes. Here we analyzed the distribution, size, calcium-binding protein content and projections of the major group of BF glutamatergic neurons expressing the vesicular glutamate transporter subtype 2 (vGluT2) and tested the functional effect of activating them. Mice expressing Cre recombinase under the control of the vGluT2 promoter were crossed with a reporter strain expressing the red fluorescent protein, tdTomato, to generate vGluT2-cre-tdTomato mice. Immunohistochemical staining for choline acetyltransferase and a cross with mice expressing green fluorescent protein selectively in GABAergic neurons confirmed that cholinergic, GABAergic and vGluT2+ neurons represent distinct BF subpopulations. Subsets of BF vGluT2+ neurons expressed the calcium-binding proteins calbindin or calretinin, suggesting that multiple subtypes of BF vGluT2+ neurons exist. Anterograde tracing using adeno-associated viral vectors expressing channelrhodopsin2-enhanced yellow fluorescent fusion proteins revealed major projections of BF vGluT2+ neurons to neighboring BF cholinergic and parvalbumin neurons, as well as to extra-BF areas involved in the control of arousal or aversive/rewarding behavior such as the lateral habenula and ventral tegmental area. Optogenetic activation of BF vGluT2+ neurons elicited a striking avoidance of the area where stimulation was given, whereas stimulation of BF parvalbumin or cholinergic neurons did not. Together with previous optogenetic findings suggesting an arousal-promoting role, our findings suggest that BF vGluT2 neurons play a dual role in promoting wakefulness and avoidance behavior.

The Complex Formed by Group I Metabotropic Glutamate Receptor (mGluR) and Homer1a Plays a Central Role in Metaplasticity and Homeostatic Synaptic Scaling

G-protein-coupled receptors can be constitutively activated following physical interaction with intracellular proteins. The first example described was the constitutive activation of Group I metabotropic glutamate receptors (mGluR: mGluR1,5) following their interaction with Homer1a, an activity-inducible early-termination variant of the scaffolding protein Homer that lacks dimerization capacity (Ango et al., 2001). Homer1a disrupts the links, maintained by the long form of Homer (cross-linking Homers), between mGluR1,5 and the Shank-GKAP-PSD-95-ionotropic glutamate receptor network. Two characteristics of the constitutive activation of the Group I mGluR-Homer1a complex are particularly interesting: (1) it affects a large number of synapses in which Homer1a is upregulated following enhanced, long-lasting neuronal activity; and (2) it mainly depends on Homer1a protein turnover. The constitutively active Group I mGluR-Homer1a complex is involved in the two main forms of non-Hebbian neuronal plasticity: "metaplasticity" and "homeostatic synaptic scaling," which are implicated in a large series of physiological and pathologic processes. Those include non-Hebbian plasticity observed in visual system, synapses modulated by addictive drugs (rewarded synapses), chronically overactivated synaptic networks, normal sleep, and sleep deprivation.

Chronotherapies for Parkinson's disease

Parkinson's disease (PD) is the second-most common progressive neurodegenerative disorder. Although the clinical diagnosis of PD is still based on its cardinal motor dysfunctions, several non-motor symptoms (NMS) have been established as integral part of the disease. Unlike motor disorders, development of therapies against NMS are still challenging and remain a critical unmet clinical need. During the last decade, several studies have characterised the molecular, physiological and behavioural alterations of the circadian system in PD patients. As a consequence, and given the ubiquitous nature of circadian rhythms in the entire organism, the biological clock has emerged as a potential therapeutic target to ease suffering from both motor and NMS in PD patients. Here we discuss the emerging field of using bright light, physical exercise and melatonin as chronotherapeutic tools to alleviate motor disorders, sleep/wake alterations, anxiety and depression in PD patients. We also highlight the potential of these readily available therapies to improve the general quality of life and wellbeing of PD patients. Finally, we provide specific data- and mechanisms-driven recommendations that might help improve the therapeutic benefit of light and physical exercise in PD patients.

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.

Ca2+ -Dependent Hyperpolarization Pathways in Sleep Homeostasis and Mental Disorders

Although we are beginning to understand the neuronal and biochemical nature of sleep regulation, questions remain about how sleep is homeostatically regulated. Beyond its importance in basic physiology, understanding sleep may also shed light on psychiatric and neurodevelopmental disorders. Recent genetic studies in mammals revealed several non-secretory proteins that determine sleep duration. Interestingly, genes identified in these studies are closely related to psychiatric and neurodevelopmental disorders, suggesting that the sleep-wake cycle shares some common mechanisms with these disorders. Here we review recent sleep studies, including reverse and forward genetic studies, from the perspectives of sleep duration and homeostasis. We then introduce a recent hypothesis for mammalian sleep in which the fast and slow Ca²⁺ -dependent hyperpolarization pathways are pivotal in generating the SWS firing pattern and regulating sleep homeostasis, respectively. Finally, we propose that these intracellular pathways are potential therapeutic targets for achieving depolarization/hyperpolarization (D/H) balance in psychiatric and neurodevelopmental disorders.

Cholinergic basal forebrain structures are not essential for mediation of the arousing action of glutamate

The cholinergic basal forebrain contributes to cortical activation and receives rich innervations from the ascending activating system. It is involved in the mediation of the arousing actions of noradrenaline and histamine. Glutamatergic stimulation in the basal forebrain results in cortical acetylcholine release and suppression of sleep. However, it is not known to what extent the cholinergic versus non-cholinergic basal forebrain projection neurones contribute to the arousing action of glutamate. To clarify this question, we administered N-methyl-D-aspartate (NMDA), a glutamate agonist, into the basal forebrain in intact rats and after destruction of the cholinergic cells in the basal forebrain with 192 immunoglobulin (Ig)G-saporin. In eight Han-Wistar rats with implanted electroencephalogram/electromyogram (EEG/EMG) electrodes and guide cannulas for microdialysis probes, 0.23 μg 192 IgG-saporin was administered into the basal forebrain, while the eight control animals received artificial cerebrospinal fluid. Two weeks later, a microdialysis probe targeted into the basal forebrain was perfused with cerebrospinal fluid on the baseline day and for 3 h with 0.3 mmNMDA on the subsequent day. Sleep-wake activity was recorded for 24 h on both days. NMDA exhibited a robust arousing effect in both the intact and the lesioned rats. Wakefulness was increased and both non-REM and REM sleep were decreased significantly during the 3-h NMDA perfusion. Destruction of the basal forebrain cholinergic neurones did not abolish the wake-enhancing action of NMDA. Thus, the cholinergic basal forebrain structures are not essential for the mediation of the arousing action of glutamate.

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.

Hypocretin/orexin antagonism enhances sleep-related adenosine and GABA neurotransmission in rat basal forebrain

Hypocretin/orexin (HCRT) neurons provide excitatory input to wake-promoting brain regions including the basal forebrain (BF). The dual HCRT receptor antagonist almorexant (ALM) decreases waking and increases sleep. We hypothesized that HCRT antagonists induce sleep, in part, through disfacilitation of BF neurons; consequently, ALM should have reduced efficacy in BF-lesioned (BFx) animals. To test this hypothesis, rats were given bilateral IgG-192-saporin injections, which predominantly targets cholinergic BF neurons. BFx and intact rats were then given oral ALM, the benzodiazepine agonist zolpidem (ZOL) or vehicle (VEH) at lights-out. ALM was less effective than ZOL at inducing sleep in BFx rats compared to controls. BF adenosine (ADO), γ-amino-butyric acid (GABA), and glutamate levels were then determined via microdialysis from intact, freely behaving rats following oral ALM, ZOL or VEH. ALM increased BF ADO and GABA levels during waking and mixed vigilance states, and preserved sleep-associated increases in GABA under low and high sleep pressure conditions. ALM infusion into the BF also enhanced cortical ADO release, demonstrating that HCRT input is critical for ADO signaling in the BF. In contrast, oral ZOL and BF-infused ZOL had no effect on ADO levels in either BF or cortex. ALM increased BF ADO (an endogenous sleep-promoting substance) and GABA (which is increased during normal sleep), and required an intact BF for maximal efficacy, whereas ZOL blocked sleep-associated BF GABA release, and required no functional contribution from the BF to induce sleep. ALM thus induces sleep by facilitating the neural mechanisms underlying the normal transition to sleep.

Sleep, its regulation and possible mechanisms of sleep disturbances

The state of sleep consists of different phases that proceed in successive, tightly regulated order through the night forming a physiological program, which for each individual is different but stabile from one night to another. Failure to accomplish this program results in feeling of unrefreshing sleep and tiredness in the morning. The program core is constructed by genetic factors but regulated by circadian rhythm and duration and intensity of day time brain activity. Many environmental factors modulate sleep, including stress, health status and ingestion of vigilance-affecting nutrients or medicines (e.g. caffeine). Acute sleep loss results in compromised cognitive performance, memory deficits, depressive mood and involuntary sleep episodes during the day. Moreover, prolonged sleep curtailment has many adverse health effects, as evidenced by both epidemiological and experimental studies. These effects include increased risk for depression, type II diabetes, obesity and cardiovascular diseases. In addition to voluntary restriction of sleep, shift work, irregular working hours, jet lag and stress are important factors that induce curtailed or bad quality sleep and/or insomnia. This review covers the current theories on the function of normal sleep and describes current knowledge on the physiologic effects of sleep loss. It provides insights into the basic mechanisms of the regulation of wakefulness and sleep creating a theoretical background for understanding different disturbances of sleep.

Sedative effects of Arachis hypogaea L. stem and leaf extracts on sleep-deprived rats

Arachis hypogaea L. stem and leaf extracts (AHSLE) are reputed to aid sleep. The purpose of this study was to evaluate the sedative effects of AHSLE on sleep-deprived (SD) rats and the effect on energy system pathways. Furthermore, we analyzed the essential oil components of Arachis hypogaea L. stems and leaves (AHSL) to explain the sedative effects. AHSLE were obtained by extracting AHSL twice with water at 98°C for 3 h. Animal experiments were performed in the Laboratory Animal Resource Center, University of Tsukuba, Japan, and the levels of neurotransmitters were analyzed by high performance liquid chromatography (HPLC). The essential oil of the AHSL was obtained by simultaneous distillation and extraction (SDE) and analyzed by gas chromatography-mass spectrometry (GC-MS). Following treatment with AHSLE, the adenosine triphosphate (ATP) levels of the SD rats increased, which is a different effect from that previously observed in freely behaving rats. Adenosine (Ad) were not elevated by AHSLE uniformly throughout the brain, but accumulated in site-specific and time-prolonged manners. Following GC-MS analysis of the AHSL essential oil, a total of 37 compounds were identified; the major components were linalool (16.17%, which has sedative-like activity), n-hexadecanoic acid (16.42%), and 1-octen-3-ol (8.48%; a product of linalool decomposition). AHSLE affect the target neurotransmitters related to the rat circadian rhythms in specific brain regions, suggesting that AHSLE have the potential to increase sleep during the SD phase, and the sedative effects of AHSLE may be due to high levels of linalool and its decomposition products. AHSLE are potentially useful as sedatives or sleep aids in hypnotic therapy.

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.

Sleep homeostasis

Research on sleep homeostasis aims to answer the question: how does the brain measure the duration and intensity of previous wakefulness in order to increase the duration and intensity of subsequent sleep? The search of regulatory factors has identified a number of potential molecules that increase their concentration in waking and decrease it during sleep. These factors regulate many physiological functions, including energy metabolism, neural plasticity and immune functions and one molecule may participate in the regulation of all these functions. The method to study regulation of sleep homeostasis is experimental prolongation of waking, which is used also to address the question of physiological purpose of sleep: prolonging wakefulness provokes symptoms that tell us what goes wrong during lack of sleep. The interpretation of the role of each identified factor in the regulation of sleep/sleep homeostasis reflects the theoretical background concept of the research. Presently three main concepts are being actively studied: the energy (depletion) hypothesis, the neural plasticity hypothesis and the (immune) defense hypothesis.

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.

Histamine release in the basal forebrain mediates cortical activation through cholinergic neurons

The basal forebrain (BF) is a key structure in regulating both cortical activity and sleep homeostasis. It receives input from all ascending arousal systems and is particularly highly innervated by histaminergic neurons. Previous studies clearly point to a role for histamine as a wake-promoting substance in the BF. We used in vivo microdialysis and pharmacological treatments in rats to study which electroencephalogram (EEG) spectral properties are associated with histamine-induced wakefulness and whether this wakefulness is followed by increased sleep and increased EEG delta power during sleep. We also investigated which BF neurons mediate histamine-induced cortical activation. Extracellular BF histamine levels rose immediately and remained constant throughout a 6 h period of sleep deprivation, returning to baseline levels immediately afterward. During the spontaneous sleep-wake cycle, we observed a strong correlation between wakefulness and extracellular histamine concentrations in the BF, which was unaffected by the time of day. The perfusion of histamine into the BF increased wakefulness and cortical activity without inducing recovery sleep. The perfusion of a histamine receptor 1 antagonist into the BF decreased both wakefulness and cortical activity. Lesioning the BF cholinergic neurons abolished these effects. Together, these results show that activation of the cholinergic BF by histamine is important in sustaining a high level of cortical activation, and that a lack of activation of the cholinergic BF by histamine may be important in initiating and maintaining nonrapid eye movement sleep. The level of histamine release is tightly connected to behavioral state, but conveys no information about sleep pressure.

Control of sleep and wakefulness

This review summarizes the brain mechanisms controlling sleep and wakefulness. Wakefulness promoting systems cause low-voltage, fast activity in the electroencephalogram (EEG). Multiple interacting neurotransmitter systems in the brain stem, hypothalamus, and basal forebrain converge onto common effector systems in the thalamus and cortex. Sleep results from the inhibition of wake-promoting systems by homeostatic sleep factors such as adenosine and nitric oxide and GABAergic neurons in the preoptic area of the hypothalamus, resulting in large-amplitude, slow EEG oscillations. Local, activity-dependent factors modulate the amplitude and frequency of cortical slow oscillations. Non-rapid-eye-movement (NREM) sleep results in conservation of brain energy and facilitates memory consolidation through the modulation of synaptic weights. Rapid-eye-movement (REM) sleep results from the interaction of brain stem cholinergic, aminergic, and GABAergic neurons which control the activity of glutamatergic reticular formation neurons leading to REM sleep phenomena such as muscle atonia, REMs, dreaming, and cortical activation. Strong activation of limbic regions during REM sleep suggests a role in regulation of emotion. Genetic studies suggest that brain mechanisms controlling waking and NREM sleep are strongly conserved throughout evolution, underscoring their enormous importance for brain function. Sleep disruption interferes with the normal restorative functions of NREM and REM sleep, resulting in disruptions of breathing and cardiovascular function, changes in emotional reactivity, and cognitive impairments in attention, memory, and decision making.

Shared genetic background for regulation of mood and sleep: association of GRIA3 with sleep duration in healthy Finnish women

STUDY OBJECTIVES

Sleeping 7 to 8 hours per night appears to be optimal, since both shorter and longer sleep times are related to increased morbidity and mortality. Depressive disorder is almost invariably accompanied by disturbed sleep, leading to decreased sleep duration, and disturbed sleep may be a precipitating factor in the initiation of depressive illness. Here, we examined whether, in healthy individuals, sleep duration is associated with genes that we earlier found to be associated with depressive disorder.

DESIGN

Population-based molecular genetic study.

SETTING

Regression analysis of 23 risk variants for depressive disorder from 12 genes to sleep duration in healthy individuals.

PARTICIPANTS

Three thousand, one hundred, forty-seven individuals (25-75 y) from population-based Health 2000 and FINRISK 2007 samples.

MEASUREMENTS AND RESULTS

We found a significant association of rs687577 from GRIA3 on the X-chromosome with sleep duration in women (permutation-based corrected empirical P=0.00001, β=0.27; Bonferroni corrected P=0.0052; f=0.11). The frequency of C/C genotype previously found to increase risk for depression in women was highest among those who slept for 8 hours or less in all age groups younger than 70 years. Its frequency decreased with the lengthening of sleep duration, and those who slept for 9 to 10 hours showed a higher frequency of C/A or A/A genotypes, when compared with the midrange sleepers (7-8 hours) (permutation-based corrected empirical P=0.0003, OR=1.81).

CONCLUSIONS

The GRIA3 polymorphism that was previously found to be associated with depressive disorder in women showed an association with sleep duration in healthy women. Mood disorders and short sleep may share a common genetic background and biologic mechanisms that involve glutamatergic neurotransmission.

Sleep duration varies as a function of glutamate and GABA in rat pontine reticular formation

The oral part of the pontine reticular formation (PnO) is a component of the ascending reticular activating system and plays a role in the regulation of sleep and wakefulness. The PnO receives glutamatergic and GABAergic projections from many brain regions that regulate behavioral state. Indirect, pharmacological evidence has suggested that glutamatergic and GABAergic signaling within the PnO alters traits that characterize wakefulness and sleep. No previous studies have simultaneously measured endogenous glutamate and GABA from rat PnO in relation to sleep and wakefulness. The present study utilized in vivo microdialysis coupled on-line to capillary electrophoresis with laser-induced fluorescence to test the hypothesis that concentrations of glutamate and GABA in the PnO vary across the sleep/wake cycle. Concentrations of glutamate and GABA were significantly higher during wakefulness than during non-rapid eye movement sleep and rapid eye movement sleep. Regression analysis revealed that decreases in glutamate and GABA accounted for a significant portion of the variance in the duration of non-rapid eye movement sleep and rapid eye movement sleep episodes. These data provide novel support for the hypothesis that endogenous glutamate and GABA in the PnO contribute to the regulation of sleep duration.

To what extent do neurobiological sleep-waking processes support psychoanalysis?

Sigmund Freud's thesis was that there is a censorship during waking that prevents memory of events, drives, wishes, and feelings from entering the consciousness because they would induce anxiety due to their emotional or ethical unacceptability. During dreaming, because the efficiency of censorship is decreased, latent thought contents can, after dream-work involving condensation and displacement, enter the dreamer's consciousness under the figurative form of manifest content. The quasi-closed dogma of psychoanalytic theory as related to unconscious processes is beginning to find neurobiological confirmation during waking. Indeed, there are active processes that suppress (repress) unwanted memories from entering consciousness. In contrast, it is more difficult to find neurobiological evidence supporting an organized dream-work that would induce meaningful symbolic content, since dream mentation most often only shows psychotic-like activities.

Pharmacokinetics and cerebral distribution of glycine administered to rats

High doses of glycine have been reported to improve negative schizophrenic symptoms, suggesting that ingested glycine activates glutamatergic transmission via N-methyl-D-aspartate (NMDA) receptors. However, the pharmacokinetics of administered glycine in the brain has not been evaluated. In the present study, the time- and dose-dependent distributions of administered glycine were investigated from a pharmacokinetic viewpoint. Whole-body autoradiography of radiolabeled glycine was performed, and time-concentration curves for glycine and serine in plasma, cerebrospinal fluid (CSF), and brain tissues were obtained. Furthermore, pharmacokinetic parameters were calculated. For a more detailed analysis, the amount of glycine uptake in the brain was evaluated using the brain uptake index method. Radiolabeled glycine was distributed among periventricular organs in the brain. Oral administration of 2 g/kg of glycine significantly elevated the CSF glycine concentration above the ED50 value for NMDA receptors. The glycine levels in CSF were 100 times lower than those in plasma. Glycine levels were elevated in brain tissue, but with a slower time-course than in CSF. Serine, a major metabolite of glycine, was elevated in plasma, CSF, and brain tissue. Glycine uptake in brain tissue increased in a dose-dependent manner. Time-concentration curves revealed that glycine was most likely transported via the blood-CSF barrier and activated NMDA receptors adjacent to the ventricles. The pharmacokinetic analysis and the brain uptake index for glycine suggested that glycine was transported into brain tissue by passive diffusion. These results provide further insight into the potential therapeutic applications of glycine.

Unilateral cortical spreading depression affects sleep need and induces molecular and electrophysiological signs of synaptic potentiation in vivo

Cortical spreading depression (CSD) is an electrophysiological phenomenon first described by Leao in 1944 as a suppression of spontaneous electroencephalographic activity, traveling across the cerebral cortex. In vitro studies suggest that CSD may induce synaptic potentiation. One recent study also found that CSD is followed by a non-rapid eye movement (NREM) sleep duration increase, suggesting an increased need for sleep. Recent experiments in animals and humans show that the occurrence of synaptic potentiation increases subsequent sleep need as measured by larger slow wave activity (SWA) during NREM sleep, prompting the question whether CSD can affect NREM SWA. Here, we find that, in freely moving rats, local CSD induction increases corticocortical evoked responses and strongly induces brain derived neurotrophic factor (BDNF) in the affected cortical hemisphere but not in the contralateral one, consistent with synaptic potentiation in vivo. Moreover, for several hours after CSD, large slow waves occur in the affected hemisphere during rapid eye movement sleep and quiet waking but disappear during active exploration. Finally, we find that CSD increases NREM sleep duration and SWA, the latter specifically in the affected hemisphere. These effects are consistent with an increase in synaptic strength triggered by CSD, although nonphysiological phenomena associated with CSD may also play a role.

Role of adenosine and wake-promoting basal forebrain in insomnia and associated sleep disruptions caused by ethanol dependence

Insomnia is a severe symptom of alcohol withdrawal; however, the underlying neuronal mechanism is yet unknown. We hypothesized that chronic ethanol exposure will impair basal forebrain (BF) adenosinergic mechanism resulting in insomnia-like symptoms. We performed a series of experiments in Sprague-Dawley rats to test our hypothesis. We used Majchrowicz's chronic binge ethanol protocol to induce ethanol dependency. Our first experiment verified the effects of ethanol withdrawal on sleep-wakefulness. Significant increase in wakefulness was observed during ethanol withdrawal. Next, we examined c-Fos expression (marker of neuronal activation) in BF wake-promoting neurons during ethanol withdrawal. There was a significant increase in the number of BF wake-promoting neurons with c-Fos immunoreactivity. Our third experiment examined the effects of ethanol withdrawal on sleep deprivation induced increase in BF adenosine levels. Sleep deprivation did not increase BF adenosine levels in ethanol dependent rats. Our last experiment examined the effects of ethanol withdrawal on equilibrative nucleoside transporter 1 and A1 receptor expression in the BF. There was a significant reduction in A1 receptor and equilibrative nucleoside transporter 1 expression in the BF of ethanol dependent rats. Based on these results, we suggest that insomnia observed during ethanol withdrawal is caused because of impaired adenosinergic mechanism in the BF.

Activation of the basal forebrain by the orexin/hypocretin neurones

The orexin neurones play an essential role in driving arousal and in maintaining normal wakefulness. Lack of orexin neurotransmission produces a chronic state of hypoarousal characterized by excessive sleepiness, frequent transitions between wake and sleep, and episodes of cataplexy. A growing body of research now suggests that the basal forebrain (BF) may be a key site through which the orexin-producing neurones promote arousal. Here we review anatomical, pharmacological and electrophysiological studies on how the orexin neurones may promote arousal by exciting cortically projecting neurones of the BF. Orexin fibres synapse on BF cholinergic neurones and orexin-A is released in the BF during waking. Local application of orexins excites BF cholinergic neurones, induces cortical release of acetylcholine and promotes wakefulness. The orexin neurones also contain and probably co-release the inhibitory neuropeptide dynorphin. We found that orexin-A and dynorphin have specific effects on different classes of BF neurones that project to the cortex. Cholinergic neurones were directly excited by orexin-A, but did not respond to dynorphin. Non-cholinergic BF neurones that project to the cortex seem to comprise at least two populations with some directly excited by orexin-A that may represent wake-active, GABAergic neurones, whereas others did not respond to orexin-A but were inhibited by dynorphin and may be sleep-active, GABAergic neurones. This evidence suggests that the BF is a key site through which orexins activate the cortex and promote behavioural arousal. In addition, orexins and dynorphin may act synergistically in the BF to promote arousal and improve cognitive performance.

Basal forebrain lactate release and promotion of cortical arousal during prolonged waking is attenuated in aging

The wake-promoting basal forebrain (BF) is critically involved in sustaining cortical arousal. In the present study, we investigated how aging affects the capacity of the BF to cope with continuous activation during prolonged waking. Increased neuronal activity induces lactate release in the activated brain area, and BF stimulation increases cortical arousal. We used in vivo microdialysis to measure lactate levels in the BF, and electroencephalography (EEG) to measure cortical arousal, during 3 h sleep deprivation (SD) in three age groups of rats. Lactate increased during SD in young but not in aged (middle-aged and old) rats. The increase in high-frequency (HF) EEG theta power (7-9 Hz), a marker of cortical arousal and active waking, was attenuated in the aged. Furthermore, a positive correlation between BF lactate release and HF EEG theta increase was found in young but not in aged rats. We hypothesized that these age-related attenuations result from reduced capacity of the BF to respond to increased neuronal activation. This was tested by stimulating the BF with glutamate receptor agonist NMDA. Whereas BF stimulation increased waking in young and old rats, lactate increase and the HF EEG theta increase were attenuated in the old. Also, the homeostatic increase in sleep intensity after SD was attenuated in aged rats. Our results suggest that an age-related attenuation in BF function reduces cortical arousal during prolonged waking. As the quality of waking is important in regulating the subsequent sleep, reduced cortical arousal during SD may contribute to the age-related reduction in sleep intensity.

Sleep and circadian rhythm disturbances: multiple genes and multiple phenotypes

Sleep is regulated by two broad mechanisms: the circadian system, which generates 24-h rhythms of sleep propensity and a wake-dependent homeostatic sleep process whereby sleep pressure increases during wake and dissipates during sleep. These, in turn, regulate multiple brain structures and neurotransmitter systems. In view of the complexity of sleep it is not surprising that there is considerable variation between individuals in both sleep timing and propensity. Furthermore, marked abnormalities in sleep are commonly encountered in psychiatric and neurodegenerative disorders. Teasing apart the genetic versus environmental contributions to normal and abnormal sleep is complex. Here we attempt to summarise what recent progress has been made, and what will be needed in the future to gain a more complete understanding of this fundamental aspect of physiology.

The Drosophila fragile X mental retardation gene regulates sleep need

Sleep need is affected by developmental stage and neuronal plasticity, but the underlying mechanisms remain unclear. The fragile X mental retardation gene Fmr1, whose loss-of-function mutation causes the most common form of inherited mental retardation in humans, is involved in synaptogenesis and synaptic plasticity, and its expression depends on both developmental stage and waking experience. Fmr1 is highly conserved across species and Drosophila mutants carrying dFmr1 loss-of-function or gain-of-function mutations are well characterized: amorphs have overgrown dendritic trees with larger synaptic boutons, developmental defects in pruning, and enhanced neurotransmission, while hypermorphs show opposite defects, including dendritic and axonal underbranching and loss of synapse differentiation. We find here that dFmr1 amorphs are long sleepers and hypermorphs are short sleepers, while both show increased locomotor activity and shortened lifespan. Both amorphs and hypermorphs also show abnormal sleep homeostasis, with impaired waking performance and no sleep rebound after sleep deprivation. An impairment in the circadian regulation of sleep cannot account for the altered sleep phenotype of dFmr1 mutants, nor can an abnormal activation of glutamatergic metabotropic receptors. Moreover, overexpression of dFmr1 throughout the mushroom bodies is sufficient to reduce sleep. Finally, dFmr1 protein levels are modulated by both developmental stage and behavioral state, with increased expression immediately after eclosure and after prolonged wakefulness. Thus, dFmr1 expression dose-dependently affects both sleep and synapses, suggesting that changes in sleep time in dFmr1 mutants may derive from changes in synaptic physiology.

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.

A causal role for brain-derived neurotrophic factor in the homeostatic regulation of sleep

Slow-wave activity (SWA), the EEG power between 0.5 and 4 Hz during non-rapid eye movement (NREM) sleep, is one of the best characterized markers of sleep need, because it increases as a function of preceding waking duration and decreases during sleep, but the underlying mechanisms remain unknown. We hypothesized that SWA is high at sleep onset because it reflects the occurrence, during the previous waking period, of widespread synaptic potentiation in cortical and subcortical areas. Consistent with this hypothesis, we recently showed that the more rats explore, the stronger is the cortical expression of BDNF during wakefulness, and the larger is the increase in SWA during the subsequent sleep period. There is compelling evidence that BDNF plays a causal role in synaptic potentiation, and exogenous application of BDNF in vivo is sufficient to induce long-term increases in synaptic strength. We therefore performed cortical unilateral microinjections of BDNF in awake rats and measured SWA during the subsequent sleep period. SWA during NREM sleep was higher in the injected hemisphere relative to the contralateral one. The effect was reversible within 2 h, and did not occur during wakefulness or rapid eye movement sleep. Asymmetries in NREM SWA did not occur after vehicle injections. Furthermore, microinjections, during wakefulness, of a polyclonal anti-BDNF antibody or K252a, an inhibitor of BDNF TrkB receptors, led to a local SWA decrease during the following sleep period. These effects were also reversible and specific for NREM sleep. These results show a causal link between BDNF expression during wakefulness and subsequent sleep regulation.