Adenosinergic inhibition of basal forebrain wakefulness-active neurons: a simultaneous unit recording and microdialysis study in freely behaving cats

Calcium Dynamics of the Ventrolateral Preoptic GABAergic Neurons during Spontaneous Sleep-Waking and in Response to Homeostatic Sleep Demands

The ventrolateral preoptic area (VLPO) contains GABAergic sleep-active neurons. However, the extent to which these neurons are involved in expressing spontaneous sleep and homeostatic sleep regulatory demands is not fully understood. We used calcium (Ca2+) imaging to characterize the activity dynamics of VLPO neurons, especially those expressing the vesicular GABA transporter (VGAT) across spontaneous sleep-waking and in response to homeostatic sleep demands. The VLPOs of wild-type and VGAT-Cre mice were transfected with GCaMP6, and the Ca2+ fluorescence of unidentified (UNID) and VGAT cells was recorded during spontaneous sleep-waking and 3 h of sleep deprivation (SD) followed by 1 h of recovery sleep. Although both VGAT and UNID neurons exhibited heterogeneous Ca2+ fluorescence across sleep-waking, the majority of VLPO neurons displayed increased activity during nonREM/REM (VGAT, 120/303; UNID, 39/106) and REM sleep (VGAT, 32/303; UNID, 19/106). Compared to the baseline waking, VLPO sleep-active neurons (n = 91) exhibited higher activity with increasing SD that remained elevated during the recovery period. These neurons also exhibited increased Ca2+ fluorescence during nonREM sleep, marked by increased slow-wave activity and REM sleep during recovery after SD. These findings support the notion that VLPO sleep-active neurons, including GABAergic neurons, are components of neuronal circuitry that mediate spontaneous sleep and homeostatic responses to sustained wakefulness.

Sleep, sleep homeostasis and arousal disturbances in alcoholism

The effects of alcohol on human sleep were first described almost 70 years ago. Since then, accumulating evidences suggest that alcohol intake at bed time immediately induces sleep [reduces the time to fall asleep (sleep onset latency), and consolidates and enhances the quality (delta power) and the quantity of sleep]. Such potent sleep promoting activity makes alcohol as one of the most commonly used "over the counter" sleep aid. However, the somnogenic effects, after alcohol intake, slowly wane off and often followed by sleep disruptions during the rest of the night. Repeated use of alcohol leads to the development of rapid tolerance resulting into an alcohol abuse. Moreover, chronic and excessive alcohol intake leads to the development of alcohol use disorder (AUD). Alcoholics, both during drinking periods and during abstinences, suffer from a multitude of sleep disruptions manifested by profound insomnia, excessive daytime sleepiness, and altered sleep architecture. Furthermore, subjective and objective indicators of sleep disturbances are predictors of relapse. Finally, within the USA, it is estimated that societal costs of alcohol-related sleep disorders exceed $18 billion. Thus, although alcohol associated sleep problems have significant economic and clinical consequences, very little is known about how and where alcohol acts to affect sleep. In this review, a conceptual framework and clinical research focused on understanding the relationship between alcohol and sleep is first described. In the next section, our new and exciting preclinical studies, to understand the cellular and molecular mechanism of how acute and chronic alcohol affects sleep, are described. In the end, based on observations from our recent findings and related literature, opportunities for the development of innovative strategies to prevent and treat AUD are proposed.

Integrative Proteome and Ubiquitinome Analyses Reveal the Substrates of BTBD9 and Its Underlying Mechanism in Sleep Regulation

Ubiquitination is a major posttranslational modification of proteins that affects their stability, and E3 ligases play a key role in ubiquitination by specifically recognizing their substrates. BTBD9, an adaptor of the Cullin-RING ligase complex, is responsible for substrate recognition and is associated with sleep homeostasis. However, the substrates of BTBD9-mediated ubiquitination remain unknown. Here, we generated an SH-SY5Y cell line stably expressing BTBD9 and performed proteomic analysis combined with ubiquitinome analysis to identify the downstream targets of BTBD9. Through this approach, we identified four potential BTBD9-mediated ubiquitination substrates that are targeted for degradation. Among these candidate substrates, inosine monophosphate dehydrogenase (IMPDH2), a novel target of BTBD9-mediated degradation, is a potential risk gene for sleep dysregulation. In conclusion, these findings not only demonstrate that proteomic analysis can be a useful general approach for the systematic identification of E3 ligase substrates but also identify novel substrates of BTBD9, providing a resource for future studies of sleep regulation mechanisms.

Interaction between cocaine use and sleep behavior: A comprehensive review of cocaine's disrupting influence on sleep behavior and sleep disruptions influence on reward seeking

Dopamine, orexin (hypocretin), and adenosine systems have dual roles in reward and sleep/arousal suggesting possible mechanisms whereby drugs of abuse may influence both reward and sleep/arousal. While considerable variability exists across studies, drugs of abuse such as cocaine induce an acute sleep loss followed by an immediate recovery pattern that is consistent with a normal response to loss of sleep. Under more chronic cocaine exposure conditions, an abnormal recovery pattern is expressed that includes a retention of sleep disturbance under withdrawal and into abstinence conditions. Conversely, experimentally induced sleep disturbance can increase cocaine seeking. Thus, complementary, sleep-related therapeutic approaches may deserve further consideration along with development of non-human models to better characterize sleep disturbance-reward seeking interactions across drug experience.

The role of PHOX2B-derived astrocytes in chemosensory control of breathing and sleep homeostasis

KEY POINTS

The embryonic PHOX2B-progenitor domain generates neuronal and glial cells which together are involved in chemosensory control of breathing and sleep homeostasis. Ablating PHOX2B-derived astrocytes significantly contributes to secondary hypoxic respiratory depression as well as abnormalities in sleep homeostasis. PHOX2B-derived astrocyte ablation results in axonal pathologies in the retrotrapezoid nucleus.

ABSTRACT

We identify in mice a population of ∼800 retrotrapezoid nucleus (RTN) astrocytes derived from PHOX2B-positive, OLIG3-negative progenitor cells, that interact with PHOX2B-expressing RTN chemosensory neurons. PHOX2B-derived astrocyte ablation during early life results in adult-onset O2 chemoreflex deficiency. These animals also display changes in sleep homeostasis, including fragmented sleep and disturbances in delta power after sleep deprivation, all without observable changes in anxiety or social behaviours. Ultrastructural evaluation of the RTN demonstrates that PHOX2B-derived astrocyte ablation results in features characteristic of degenerative neuro-axonal dystrophy, including abnormally dilated axon terminals and increased amounts of synapses containing autophagic vacuoles/phagosomes. We conclude that PHOX2B-derived astrocytes are necessary for maintaining a functional O2 chemosensory reflex in the adult, modulate sleep homeostasis, and are key regulators of synaptic integrity in the RTN region, which is necessary for the chemosensory control of breathing. These data also highlight how defects in embryonic development may manifest as neurodegenerative pathology in an adult.

Adenosine Differentially Modulates Synaptic Transmission of Excitatory and Inhibitory Microcircuits in Layer 4 of Rat Barrel Cortex

Adenosine is considered to be a key regulator of sleep homeostasis by promoting slow-wave sleep through inhibition of the brain's arousal centers. However, little is known about the effect of adenosine on neuronal network activity at the cellular level in the neocortex. Here, we show that adenosine differentially modulates synaptic transmission between different types of neurons in cortical layer 4 (L4) through activation of pre- and/or postsynaptically located adenosine A1 receptors. In recurrent excitatory connections between L4 spiny neurons, adenosine suppresses synaptic transmission through activation of both pre- and postsynaptic A1 receptors. In reciprocal excitatory and inhibitory connections between L4 spiny neurons and interneurons, adenosine strongly suppresses excitatory transmission via activating presynaptic A1 receptors but only slightly suppresses inhibitory transmission via activating postsynaptic A1 receptors. Adenosine has no effect on inhibitory transmission between L4 interneurons. The effect of adenosine is concentration dependent and first visible at a concentration of 1 μM. The effect of adenosine is blocked by the specific A1 receptor antagonist, 8-cyclopentyltheophylline or the nonspecific adenosine receptor antagonist, caffeine. By differentially affecting excitatory and inhibitory synaptic transmission, adenosine changes the excitation-inhibition balance and causes an overall shift to lower excitability in L4 primary somatosensory (barrel) cortical microcircuits.

Striatal adenosine A2A receptor neurons control active-period sleep via parvalbumin neurons in external globus pallidus

Dysfunction of the striatum is frequently associated with sleep disturbances. However, its role in sleep-wake regulation has been paid little attention even though the striatum densely expresses adenosine A_2A receptors (A_2ARs), which are essential for adenosine-induced sleep. Here we showed that chemogenetic activation of A_2AR neurons in specific subregions of the striatum induced a remarkable increase in non-rapid eye movement (NREM) sleep. Anatomical mapping and immunoelectron microscopy revealed that striatal A_2AR neurons innervated the external globus pallidus (GPe) in a topographically organized manner and preferentially formed inhibitory synapses with GPe parvalbumin (PV) neurons. Moreover, lesions of GPe PV neurons abolished the sleep-promoting effect of striatal A_2AR neurons. In addition, chemogenetic inhibition of striatal A_2AR neurons led to a significant decrease of NREM sleep at active period, but not inactive period of mice. These findings reveal a prominent contribution of striatal A_2AR neuron/GPe PV neuron circuit in sleep control.

Lesion of the basal forebrain cholinergic neurons attenuates sleepiness and adenosine after alcohol consumption

Alcohol has a profound effect on sleep. However, neuronal substrates mediating sleep-promoting effects of alcohol are unknown. Since the basal forebrain (BF) cholinergic neurons are implicated in the homeostatic regulation of sleep, we hypothesized that the BF cholinergic neurons may have an important role in sleepiness observed after alcohol consumption. 192-IgG-saporin (bilateral BF infusions) was used to selectively lesion BF cholinergic neurons in adult male Sprague-Dawley rats. Standard surgical procedures were used to implant sleep recording electrodes or microdialysis guide cannulas. The first experiment used between-group design [lesion and sham (controls)] and examined effects of BF cholinergic neuronal lesions on alcohol (3 g/Kg; ig) induced sleep promotion. The second experiment used within-group design [lesion (ipsilateral BF) and sham (controls; contralateral BF) in same animal] and local reverse microdialysis infusion of alcohol (300 mM) to examine the effects of cholinergic neuronal lesions on extracellular adenosine in the BF. Alcohol had a robust sleep promoting effect in controls as evidenced by a significant reduction in sleep onset latency and wakefulness; non-rapid eye movement sleep was significantly increased. No such alcohol-induced sleep promotion was observed in lesioned rats with significantly fewer BF cholinergic neurons. Rapid eye movement sleep was minimally affected. Adenosine release was significantly reduced following local infusion of alcohol on the lesion side, with significantly fewer cholinergic neurons as compared with the control side. Based on these results, we suggest that alcohol promotes sleep by increasing extracellular adenosine via its action on cholinergic neurons of the BF. Read the Editorial Highlight for this article on page 620.

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.

Sleep-Wake Regulation and Its Impact on Working Memory Performance: The Role of Adenosine

The sleep-wake cycle is regulated by a fine-tuned interplay between sleep-homeostatic and circadian mechanisms. Compelling evidence suggests that adenosine plays an important role in mediating the increase of homeostatic sleep pressure during time spent awake and its decrease during sleep. Here, we summarize evidence that adenosinergic mechanisms regulate not only the dynamic of sleep pressure, but are also implicated in the interaction of homeostatic and circadian processes. We review how this interaction becomes evident at several levels, including electrophysiological data, neuroimaging studies and behavioral observations. Regarding complex human behavior, we particularly focus on sleep-wake regulatory influences on working memory performance and underlying brain activity, with a specific emphasis on the role of adenosine in this interplay. We conclude that a change in adenosinergic mechanisms, whether exogenous or endogenous, does not only impact on sleep-homeostatic processes, but also interferes with the circadian timing system.

Synaptic plasticity model of therapeutic sleep deprivation in major depression

Therapeutic sleep deprivation (SD) is a rapid acting treatment for major depressive disorder (MDD). Within hours, SD leads to a dramatic decrease in depressive symptoms in 50-60% of patients with MDD. Scientifically, therapeutic SD presents a unique paradigm to study the neurobiology of MDD. Yet, up to now, the neurobiological basis of the antidepressant effect, which is most likely different from today's first-line treatments, is not sufficiently understood. This article puts the idea forward that sleep/wake-dependent shifts in synaptic plasticity, i.e., the neural basis of adaptive network function and behavior, represent a critical mechanism of therapeutic SD in MDD. Particularly, this article centers on two major hypotheses of MDD and sleep, the synaptic plasticity hypothesis of MDD and the synaptic homeostasis hypothesis of sleep-wake regulation, and on how they can be integrated into a novel synaptic plasticity model of therapeutic SD in MDD. As a major component, the model proposes that therapeutic SD, by homeostatically enhancing cortical synaptic strength, shifts the initially deficient inducibility of associative synaptic long-term potentiation (LTP) in patients with MDD in a more favorable window of associative plasticity. Research on the molecular effects of SD in animals and humans, including observations in the neurotrophic, adenosinergic, monoaminergic, and glutamatergic system, provides some support for the hypothesis of associative synaptic plasticity facilitation after therapeutic SD in MDD. The model proposes a novel framework for a mechanism of action of therapeutic SD that can be further tested in humans based on non-invasive indices and in animals based on direct studies of synaptic plasticity. Further determining the mechanisms of action of SD might contribute to the development of novel fast acting treatments for MDD, one of the major health problems worldwide.

Astrocytic adenosine: from synapses to psychiatric disorders

Although it is considered to be the most complex organ in the body, the brain can be broadly classified into two major types of cells, neuronal cells and glial cells. Glia is a general term that encompasses multiple types of non-neuronal cells that function to maintain homeostasis, form myelin, and provide support and protection for neurons. Astrocytes, a major class of glial cell, have historically been viewed as passive support cells, but recently it has been discovered that astrocytes participate in signalling activities both with the vasculature and with neurons at the synapse. These cells have been shown to release D-serine, TNF-α, glutamate, atrial natriuretic peptide (ANP) and ATP among other signalling molecules. ATP and its metabolites are well established as important signalling molecules, and astrocytes represent a major source of ATP release in the nervous system. Novel molecular and genetic tools have recently shown that astrocytic release of ATP and other signalling molecules has a major impact on synaptic transmission. Via actions at the synapse, astrocytes have now been shown to regulate complex network signalling in the whole organism with impacts on respiration and the sleep-wake cycle. In addition, new roles for astrocytes are being uncovered in psychiatric disorders, and astrocyte signalling mechanisms represents an attractive target for novel therapeutic agents.

Brain pathways to recovery from alcohol dependence

This article highlights the research presentations at the satellite symposium on "Brain Pathways to Recovery from Alcohol Dependence" held at the 2013 Society for Neuroscience Annual Meeting. The purpose of this symposium was to provide an up to date overview of research efforts focusing on understanding brain mechanisms that contribute to recovery from alcohol dependence. A panel of scientists from the alcohol and addiction research field presented their insights and perspectives on brain mechanisms that may underlie both recovery and lack of recovery from alcohol dependence. The four sessions of the symposium encompassed multilevel studies exploring mechanisms underlying relapse and craving associated with sustained alcohol abstinence, cognitive function deficit and recovery, and translational studies on preventing relapse and promoting recovery. Gaps in our knowledge and research opportunities were also discussed.

Alcohol disrupts sleep homeostasis

Alcohol is a potent somnogen and one of the most commonly used "over the counter" sleep aids. In healthy non-alcoholics, acute alcohol decreases sleep latency, consolidates and increases the quality (delta power) and quantity of NREM sleep during the first half of the night. However, sleep is disrupted during the second half. Alcoholics, both during drinking periods and during abstinences, suffer from a multitude of sleep disruptions manifested by profound insomnia, excessive daytime sleepiness, and altered sleep architecture. Furthermore, subjective and objective indicators of sleep disturbances are predictors of relapse. Finally, within the USA, it is estimated that societal costs of alcohol-related sleep disorders exceeds $18 billion. Thus, although alcohol-associated sleep problems have significant economic and clinical consequences, very little is known about how and where alcohol acts to affect sleep. In this review, we have described our attempts to unravel the mechanism of alcohol-induced sleep disruptions. We have conducted a series of experiments using two different species, rats and mice, as animal models. We performed microdialysis, immunohistochemical, pharmacological, sleep deprivation and lesion studies which suggest that the sleep-promoting effects of alcohol may be mediated via alcohol's action on the mediators of sleep homeostasis: adenosine (AD) and the wake-promoting cholinergic neurons of the basal forebrain (BF). Alcohol, via its action on AD uptake, increases extracellular AD resulting in the inhibition of BF wake-promoting neurons. Since binge alcohol consumption is a highly prevalent pattern of alcohol consumption and disrupts sleep, we examined the effects of binge drinking on sleep-wakefulness. Our results suggest that disrupted sleep homeostasis may be the primary cause of sleep disruption observed following binge drinking. Finally, we have also shown that sleep disruptions observed during acute withdrawal, are caused due to impaired sleep homeostasis. In conclusion, we suggest that alcohol may disrupt sleep homeostasis to cause sleep disruptions.

Dexmedetomidine-induced sedation does not mimic the neurobehavioral phenotypes of sleep in Sprague Dawley rat

STUDY OBJECTIVES

Dexmedetomidine is used clinically to induce states of sedation that have been described as homologous to nonrapid eye movement (NREM) sleep. A better understanding of the similarities and differences between NREM sleep and dexmedetomidine-induced sedation is essential for efforts to clarify the relationship between these two states. This study tested the hypothesis that dexmedetomidine-induced sedation is homologous to sleep.

DESIGN

This study used between-groups and within-groups designs.

SETTING

University of Michigan.

PARTICIPANTS

Adult male Sprague Dawley rats (n = 40).

INTERVENTIONS

Independent variables were administration of dexmedetomidine and saline or Ringer's solution (control). Dependent variables included time spent in states of wakefulness, sleep, and sedation, electroencephalographic (EEG) power, adenosine levels in the substantia innominata (SI), and activation of pCREB and c-Fos in sleep related forebrain regions.

MEASUREMENTS AND RESULTS

Dexmedetomidine significantly decreased time spent in wakefulness (-49%), increased duration of sedation (1995%), increased EEG delta power (546%), and eliminated the rapid eye movement (REM) phase of sleep for 16 h. Sedation was followed by a rebound increase in NREM and REM sleep. Systemically administered dexmedetomidine significantly decreased (-39%) SI adenosine levels. Dialysis delivery of dexmedetomidine into SI did not decrease adenosine level. Systemic delivery of dexmedetomidine did not alter c-Fos or pCREB expression in the horizontal diagonal band, or ventrolateral, median, and medial preoptic areas of the hypothalamus.

CONCLUSIONS

Dexmedetomidine significantly altered normal sleep phenotypes, and the dexmedetomidine-induced state did not compensate for sleep need. Thus, in the Sprague Dawley rat, dexmedetomidine-induced sedation is characterized by behavioral, electrographic, and immunohistochemical phenotypes that are distinctly different from similar measures obtained during sleep.

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.

Nicotine infusion in the wake-promoting basal forebrain enhances alcohol-induced activation of nucleus accumbens

BACKGROUND

Nicotine and alcohol co-abuse is highly prevalent. Recently, we have shown that nicotine infusion in the basal forebrain (BF) increases alcohol consumption. As nucleus accumbens (NAc) is the terminal brain region associated with drug addiction, we hypothesize that nicotine infusion in the BF may enhance alcohol-induced activation of NAc.

METHODS

Adult male Sprague-Dawley rats were surgically implanted with bilateral guide cannulas in the BF. Following postoperative recovery, rats were divided into 4 groups: (i) ACSF + W group received artificial cerebrospinal fluid (ACSF; 500 nl/side) in the BF and systemic water (intragastric [ig]; 10 ml/kg; N = 5), (ii) ethanol (EtOH) group received ACSF in the BF (500 nl/side) and systemic alcohol (ig; 3 g/kg; N = 5), (iii) NiC group received nicotine in the BF (75 pmole/500 nl/side) and systemic water (ig; 10 ml/kg; N = 5), and (iv) NiC + EtOH group received nicotine in the BF (75 pmole/500 nl/side) and systemic alcohol (ig; 3 g/kg; N = 5). Rats were euthanized 2 hours after treatment to examine c-Fos expression in the NAc by immunohistochemistry.

RESULTS

All injections sites were localized in the BF. Two-way analysis of variance (ig vs. infusion) revealed significant main effects of both treatments (ig and infusion, p < 0.001) on c-Fos expression in the NAc shell, but not in the core. Subsequent post hoc test (Bonferroni's) revealed that as compared to ACSF + W group, c-Fos expression was significantly increased in the shell of NAc of rats in all 3 (EtOH, NiC, and NiC + EtOH) groups with maximal increase observed in NiC + EtOH group.

CONCLUSIONS

The results suggest the following: (i) BF nicotine infusion induced c-Fos in both core and the shell region of NAc at levels comparable to those observed after systemic alcohol administration; (ii) BF nicotine infusion with systemic alcohol induced a significant additive increase in c-Fos expression only in the NAc shell region. These findings implicate the BF in alcohol and nicotine co-use.

Microinjection of adenosine into the hypothalamic ventrolateral preoptic area enhances wakefulness via the A1 receptor in rats

Adenosine (AD) is a nucleic acid component that is critical for energy metabolism in the body. AD modulates numerous neural functions in the central nervous system, including the sleep-wake cycle. Previous studies have indicated that the A1 receptor (A1R) or A2A receptor (A2AR) may mediate the effects of AD on the sleep-wake cycle. The hypothalamic ventrolateral preoptic area (VLPO) initiates and maintains normal sleep. Histological studies have shown A1R are widely expressed in brain tissue, whereas A2AR expression is limited in the brain and undetectable in the VLPO. We hypothesize therefore, that AD modulates the sleep-wake cycle through A1R in the VLPO. In the present study, bilateral microinjection of AD or an AD transporter inhibitor (s-(4-nitrobenzyl)-6-thioinosine) into the VLPO of rats decreased non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. An A1R agonist (N6-cyclohexyladenosine) produced similar effects in the VLPO. Microinjection of an A1R antagonist (8-cyclopentyl-1,3-dimethylxanthine) into the VLPO enhanced NREM sleep and diminished AD-induced wakefulness. These data indicate that AD enhances wakefulness in the VLPO via A1R in rats.

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.

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.

Adenosine inhibits glutamatergic input to basal forebrain cholinergic neurons

Adenosine has been proposed as an endogenous homeostatic sleep factor that accumulates during waking and inhibits wake-active neurons to promote sleep. It has been specifically hypothesized that adenosine decreases wakefulness and promotes sleep recovery by directly inhibiting wake-active neurons of the basal forebrain (BF), particularly BF cholinergic neurons. We previously showed that adenosine directly inhibits BF cholinergic neurons. Here, we investigated 1) how adenosine modulates glutamatergic input to BF cholinergic neurons and 2) how adenosine uptake and adenosine metabolism are involved in regulating extracellular levels of adenosine. Our experiments were conducted using whole cell patch-clamp recordings in mouse brain slices. We found that in BF cholinergic neurons, adenosine reduced the amplitude of AMPA-mediated evoked glutamatergic excitatory postsynaptic currents (EPSCs) and decreased the frequency of spontaneous and miniature EPSCs through presynaptic A(1) receptors. Thus we have demonstrated that in addition to directly inhibiting BF cholinergic neurons, adenosine depresses excitatory inputs to these neurons. It is therefore possible that both direct and indirect inhibition may synergistically contribute to the sleep-promoting effects of adenosine in the BF. We also found that blocking the influx of adenosine through the equilibrative nucleoside transporters or inhibiting adenosine kinase and adenosine deaminase increased endogenous adenosine inhibitory tone, suggesting a possible mechanism through which adenosine extracellular levels in the basal forebrain are regulated.

Adenosine modulates the excitability of layer II stellate neurons in entorhinal cortex through A1 receptors

Stellate neurons in layer II entorhinal cortex (EC) provide the main output from the EC to the hippocampus. It is believed that adenosine plays a crucial role in neuronal excitability and synaptic transmission in the CNS, however, the function of adenosine in the EC is still elusive. Here, the data reported showed that adenosine hyperpolarized stellate neurons in a concentration-dependent manner, accompanied by a decrease in firing frequency. This effect corresponded to the inhibition of the hyperpolarization-activated, cation nonselective (HCN) channels. Surprisingly, the adenosine-induced inhibition was blocked by 3 μM 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective A(1) receptor antagonists, but not by 10 μM 3,7-dimethyl-1-propargylxanthine (DMPX), a selective A(2) receptor antagonists, indicating that activation of adenosine A(1) receptors were responsible for the direct inhibition. In addition, adenosine reduced the frequency but not the amplitude of miniature EPSCs and IPSCs, suggesting that the global depression of glutamatergic and GABAergic transmission is mediated by a decrease in glutamate and GABA release, respectively. Again the presynaptic site of action was mediated by adenosine A(1) receptors. Furthermore, inhibition of spontaneous glutamate and GABA release by adenosine A(1) receptor activation was mediated by voltage-dependent Ca(2+) channels and extracellular Ca(2+) . Therefore, these findings revealed direct and indirect mechanisms by which activation of adenosine A(1) receptors on the cell bodies of stellate neurons and on the presynaptic terminals could regulate the excitability of these neurons.

Plasticity in neurons synthesizing wake/arousal promoting hormone hypocretin/orexin

The hypothalamus is a critical brain structure regulating physiological functions essential to the survival of individuals and species. One of the striking characteristics of this brain region is the abundance of nerve cells (neurons) expressing a great numbers of neurotransmitters and neuromodulators, among which are hormones released into the blood stream through brain neuroendocrinological routes. The neurons in the lateral hypothalamus take part in intra- and extrahypothalamic circuits controlling basic physiological functions essential for the well being of animal bodies (such as cardiovascular function, respiratory function, immune responses, etc.), animal behaviors required for the maintenance of the survival of individuals (food foraging, flight, fight, etc.) and species (reproductive function), and higher brain functions (learning and memory, mental state, etc.). Hypocretin (also called orexin) comprises of two neuropeptides exclusively synthesized by neurons in the perifornical/lateral hypothalamus. Although hypocretin/orexin was initially found to enhance food intake, it is now clear that the functions mediated by hypocretin/orexin are well beyond what were originally proposed. Specifically, hypocretin/orexin is a crucial promoter of wakefulness; deficiency in the hypocretin/orexin system leads to diseases and disorders such as narcolepsy. It is clear that neurons synthesizing hypocretin/orexin are consistently under regulation originating from various parts of the brain and that the status of activity in hypocretin/orexin neurons is closely related with the nutritional and behavioral state of animals. Therefore, the demand to make adaptive changes in hypocretin/orexin neurons to accommodate the changes in the external environment and behavioral state of animals is expected. The latest developments in the studies of plasticity in hypocretin/orexin neurons under the challenges from environmental and behavioral factors have dramatically shaped the understanding of the roles of hypocretin/orexin neurons in the maintenance of the survival of animals. More importantly, the studies of plasticity in hypocretin/orexin neurons as the consequence of physiological, behavioral, and environmental challenges may shed new insight on the understanding and treatment of sleep disorders (such as insomnia).

Implication of the purinergic system in alcohol use disorders

In the central nervous system, adenosine and adenosine 5'-triphosphate (ATP) play an important role in regulating neuronal activity as well as controlling other neurotransmitter systems, such as, GABA, glutamate, and dopamine. Ethanol increases extracellular adenosine levels that regulate the ataxic and hypnotic/sedative effects of ethanol. Interestingly, ethanol is known to increase adenosine levels by inhibiting an ethanol-sensitive adenosine transporter, equilibrative nucleoside transporter type 1 (ENT1). Ethanol is also known to inhibit ATP-specific P2X receptors, which might result in such similar effects as those caused by an increase in adenosine. Adenosine and ATP exert their functions through P1 (metabotropic) and P2 (P2X-ionotropic and P2Y-metabotropic) receptors, respectively. Purinergic signaling in cortex-striatum-ventral tegmental area (VTA) has been implicated in regulating cortical glutamate signaling as well as VTA dopaminergic signaling, which regulates the motivational effect of ethanol. Moreover, several nucleoside transporters and receptors have been identified in astrocytes, which regulate not only adenosine-ATP neurotransmission, but also homeostasis of major inhibitory-excitatory neurotransmission (i.e., GABA or glutamate) through neuron-glial interactions. This review will present novel findings on the implications of adenosine and ATP neurotransmission in alcohol use disorders.

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.

Sustained antagonism of acute ethanol-induced ataxia following microinfusion of cyclic AMP and cpt-cAMP in the mouse cerebellum

Ataxia is a conspicuous physical manifestation of alcohol consumption in humans and laboratory animals. Previously we reported possible involvement of cAMP in ethanol-induced ataxia. We now report a sustained antagonism of ataxia due to multiple ethanol injections following intracerebellar (ICB) cAMP or cpt-cAMP microinfusion. Adenylyl cyclase drugs cAMP, cpt-cAMP, Sp-cAMP, Rp-cAMP, adenosine A₁ agonist, N⁶-cyclohexyladenosine (CHA) and GABA(A) agonist muscimol were directly microinfused into the cerebellum of CD-1 male mice to evaluate their effect on ethanol (2 g/kg; i.p.) ataxia. Drug microinfusions were made via stereotaxically implanted stainless steel guide cannulas. Rotorod was used to evaluate the ethanol's ataxic response. Intracerebellar cAMP (0.1, 1, 10 fmol) or cpt-cAMP (0.5, 1, 2 fmol) 60 min before ethanol treatment, dose-dependently attenuated ethanol-induced ataxia in general agreement with previous observations. Intracerebellar microinfusion of cAMP (100 fmol) or cpt-cAMP (2 fmol) produced a sustained attenuation of ataxia following ethanol administration at 1, 4, 7 and 25 h or 31 h post-cAMP/cpt-cAMP microinfusion. At 31 h post-cAMP, the ataxic response of ethanol reappeared. Additionally, marked antagonism to the accentuation of ethanol-induced ataxia by adenosine A₁ and GABA(A) agonists, CHA (34 pmol) and muscimol (88 pmol), respectively, was noted 24h after cAMP and cpt-cAMP treatment. This indicated possible participation of AC/cAMP/PKA signaling in the co-modulation of ethanol-induced ataxia by A₁ adenosinergic and GABAergic systems. No change in normal motor coordination was noted when cAMP or cpt-cAMP microinfusion was followed by saline. Finally, Rp-cAMP (PKA inhibitor, 22 pmol) accentuated ethanol-induced ataxia and antagonized its attenuation by cAMP whereas Sp-cAMP (PKA activator, 22 pmol) produced just the opposite effects, further indicating participation of cAMP-dependent PKA downstream. Overall, the results support a role of AC/cAMP/PKA signaling in the expression of ethanol-induced ataxia and its co-modulation by adenosine A₁ and GABA(A) receptors.

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.

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.

Histamine in the regulation of wakefulness

The histaminergic system is exclusively localized within the posterior hypothalamus with projection to almost all the major regions of the central nervous system. Strong and consistent evidence exist to suggest that histamine, acting via H₁ and/or H₃ receptor has a pivotal role in the regulation of sleep-wakefulness. Administration of histamine or H₁ receptor agonists induces wakefulness, whereas administration of H₁ receptor antagonists promotes sleep. The H₃ receptor functions as an auto-receptor and regulates the synthesis and release of histamine. Activation of H₃ receptor reduces histamine release and promotes sleep. Conversely, blockade of H₃ receptor promotes wakefulness. Histamine release in the hypothalamus and other target regions is highest during wakefulness. The histaminergic neurons display maximal activity during the state of high vigilance, and cease their activity during non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. The cerebrospinal levels of histamine are reduced in diseased states where hypersomnolence is a major symptom. The histamine deficient L-histidine decarboxylase knockout (HDC KO) mice display sleep fragmentation and increased REM sleep during the light period along with profound wakefulness deficit at dark onset, and in novel environment. Similar results have been obtained when histamine neurons are lesioned. These studies strongly implicate the histaminergic neurons of the TMN to play a critical role in the maintenance of high vigilance state during wakefulness.

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.

Role of wake-promoting basal forebrain and adenosinergic mechanisms in sleep-promoting effects of ethanol

BACKGROUND

Ethanol intake has significant impact on sleep. However, the cellular substrates responsible for sleep promotion following ethanol intake are unknown. The purine nucleoside, adenosine, is responsible for mediating many neuronal and behavioral responses to ethanol. Studies performed in cell cultures suggest that ethanol inhibits equilibrative nucleoside transporter 1 to block the reuptake of adenosine resulting in increased extracellular adenosine. Adenosine also has a pivotal role in sleep regulation. Adenosine acts via A1 receptor to inhibit the wake-promoting neurons of the basal forebrain (BF) resulting in the promotion of sleep. Is ethanol-induced sleep associated with the inhibition of the BF wake-promoting neurons? Do adenosinergic mechanisms in the BF have a role in sleep-promoting effects of ethanol?

METHODS

To address these questions, we performed 3 experiments in Sprague-Dawley rats. First, we verified the effect of ethanol on sleep promotion. Second, we evaluated the effect of ethanol on c-Fos expression (a marker of neuronal activation) in the BF wake-promoting neurons and third we monitored the effects of A1 receptor blockade in the BF on ethanol-induced sleep.

RESULTS

Significant increase in non-rapid eye movement (NREM) sleep with a concomitant decrease in wakefulness was observed during the first 12 hours postethanol. REM sleep remained unaffected. Ethanol administration caused a significant decrease in the number of BF wake-promoting neurons with c-Fos immunoreactivity. Bilateral microinjections of a selective A1R receptor antagonist 8-cyclopentyl-1, 3-dipropylxanthine into the BF significantly attenuated sleep-promoting effects of ethanol.

CONCLUSION

These results suggest that the inhibition of BF wake-promoting neurons by adenosinergic mechanism may be responsible for the sleep promoting effects of ethanol. We believe our study is the first to investigate the cellular mechanisms responsible for the somnogenic effects of ethanol.

Effects of ethanol on extracellular levels of adenosine in the basal forebrain: an in vivo microdialysis study in freely behaving rats

BACKGROUND

Adenosine is implicated to play a pivotal role in mediating many neuronal responses to ethanol. While in vitro studies performed in cell culture have demonstrated that acute ethanol exposure increases extracellular adenosine levels, this effect has not been demonstrated, in vivo, in the brain. We performed an in vivo microdialysis study to examine the effects of local ethanol perfusion on extracellular levels of adenosine in the basal forebrain (BF).

METHODS

Under sterile conditions and using a standard surgical protocol, adult male Sprague-Dawley rats were implanted with unilateral microdialysis guide cannula targeted toward the BF. Following postoperative recovery, the microdialysis probe was inserted. After allowing at least 12 to 16 hours for probe insertion recovery, the experiment was begun. Artificial cerebrospinal fluid (aCSF) was perfused (0.7 microl/min) for 80 minutes, and 4 x 20-minute pre-ethanol baseline samples were collected. Subsequently, 30, 100, and 300 mM doses of ethanol were perfused. Each ethanol dose was perfused for 80 minutes, and 4 x 20-minute samples were collected. Finally, aCSF was perfused, and 4 x 20 postethanol samples were collected. Adenosine in the microdialysate was separated and measured with HPLC coupled with an UV detector. On completion, the animals were euthanized, brain removed and processed for histology.

RESULTS

Local ethanol perfusion in the BF produced a significant increase in extracellular adenosine with the highest dose of 300 mM ethanol producing a 4-fold increase. Cresyl violet (Nissl) staining did not indicate any toxic damage in the area surrounding the probe tip. Choline acetyltransferase immunohistochemistry revealed that all microdialysis probe sites were localized in the BF.

CONCLUSION

Our study is the first to demonstrate that ethanol acts directly in the brain to increase extracellular adenosine.

Opioid-induced decreases in rat brain adenosine levels are reversed by inhibiting adenosine deaminase

BACKGROUND

Opioids disrupt sleep and adenosine promotes sleep, but no studies have characterized the effects of opioids on adenosine levels in brain regions known to regulate states of arousal. Delivering opioids to the pontine reticular formation (PRF) and substantia innominata (SI) region of the basal forebrain disrupts sleep. In contrast, administering adenosine agonists to the PRF or SI increases sleep. These findings encouraged the current study testing the hypothesis that microdialysis delivery of opioids to the PRF or SI decreases adenosine levels in the PRF or SI, respectively.

METHODS

A microdialysis probe was placed in the PRF of isoflurane anesthetized rats and perfused with Ringer's solution (control) followed by Ringer's solution containing morphine (0, 10, 30, 100, or 300 microm), fentanyl (100 microm), morphine (100 microm) and the adenosine deaminase inhibitor EHNA (100 microm), or naloxone (10 microm) and morphine (100 microm). Additional experiments measured adenosine levels in the SI before and during microdialysis delivery of morphine, fentanyl, and morphine plus EHNA.

RESULTS

Morphine caused a significant (P < 0.05) concentration-dependent decrease in PRF adenosine levels. The significant decrease (-20%) in adenosine caused by 100 microm morphine was blocked by coadministration of naloxone. Fentanyl also significantly decreased (-13.3%) PRF adenosine. SI adenosine levels were decreased by morphine (-26.8%) and fentanyl (-27.4%). In both PRF and SI, coadministration of morphine and EHNA prevented the significant decrease in adenosine levels caused by morphine alone.

CONCLUSIONS

These data support the interpretation that decreased adenosine levels in sleep-regulating brain regions may be one of the mechanisms by which opioids disrupt sleep.

Role of adenosine A(1) receptor in the perifornical-lateral hypothalamic area in sleep-wake regulation in rats

The perifornical-lateral hypothalamic area (PF-LHA) has been implicated in the regulation of arousal. The PF-LHA contains wake-active neurons that are quiescent during non-REM sleep and in the case of neurons expressing the peptide hypocretin (HCRT), quiescent during both non-REM and REM sleep. Adenosine is an endogenous sleep factor and recent evidence suggests that adenosine via A(1) receptors may act on PF-LHA neurons to promote sleep. We examined the effects of bilateral activation as well as blockade of A(1) receptors in the PF-LHA on sleep-wakefulness in freely behaving rats. The sleep-wake profiles of male Wistar rats were recorded during reverse microdialysis perfusion of artificial cerebrospinal fluid (aCSF) and two doses of adenosine A(1) receptor antagonist, 1,3-dipropyl-8-phenylxanthine (CPDX; 5 microM and 50 microM) or A(1) receptor agonist, N(6)-cyclopentyladenosine (CPA; 5 microM and 50 microM) into the PF-LHA for 2 h followed by 4 h of aCSF perfusion. CPDX perfused into the PF-LHA during lights-on phase produced arousal (F=7.035, p<0.001) and concomitantly decreased both non-REM (F=7.295, p<0.001) and REM sleep (F=3.456, p<0.004). In contrast, CPA perfused into the PF-LHA during lights-off phase significantly suppressed arousal (F=7.891, p<0.001) and increased non-REM (F=8.18, p <0.001) and REM sleep (F=30.036, p<0.001). These results suggest that PF-LHA is one of the sites where adenosine, acting via A(1) receptors, inhibits PF-LHA neurons to promote sleep.

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.

The role of the basal forebrain adenosine receptors in sleep homeostasis

Multiple studies indicate that adenosine released in the basal forebrain during prolonged wakefulness could affect recovery sleep. It is still unclear which of adenosine receptors provide its sleep-modulating effects in the basal forebrain. We infused adenosine A1 and A2A receptors antagonists into the rat basal forebrain during sleep deprivation and compared characteristics of recovery non-rapid eye movement (non-REM) sleep (its amount and non-REM sleep delta power) after sleep deprivation, and after sleep deprivation combined with perfusion of antagonists. A1 receptor antagonist significantly reduced recovery sleep amount and delta power, whereas A2A receptor antagonist had no effect on recovery sleep. We conclude that adenosine can promote recovery non-REM sleep when acting through A1 receptors in the basal forebrain.

Tripartite synapses: roles for astrocytic purines in the control of synaptic physiology and behavior

Astrocytes are known to release several transmitters to impact neuronal activity. Cell-specific molecular genetic attenuation of vesicular release has shown that ATP is a primary astrocytic transmitter in situ and in vivo. In this review, we discuss the biology of astrocytic ATP release highlighting the exciting discovery that lysosomes might be primary stores for the release of this gliotransmitter. In addition, we discuss the role of ATP and its metabolite adenosine on synaptic transmission and the coordination of synaptic networks. Finally, we discuss the recent elucidation of the involvement of this form of glial signaling in the modulation of mammalian behavior. By controlling neuronal A1-receptor signaling, astrocytes modulate mammalian sleep homeostasis and are essential for mediating the cognitive consequences of sleep deprivation. These discoveries begin to paint a new picture of brain function in which slow-signaling glia modulate fast synaptic transmission and neuronal firing to impact behavioral output. Because these cells have privileged access to synapses, they may be valuable targets for the development of novel therapies for many neurological and psychiatric conditions.

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.

Nitric oxide modulates the discharge rate of basal forebrain neurons

RATIONALE

During prolonged wakefulness, the concentrations of nitric oxide (NO) and adenosine (AD) increase in the basal forebrain (BF). AD inhibits neuronal activity via adenosine (A1) receptors, thus providing a potential mechanism for sleep facilitation. Although NO in the BF increases adenosine and promotes sleep, it is not clear whether the sleep promotion by NO is mediated through adenosine increase, or NO independently of adenosine could modulate sleep.

OBJECTIVE

The objective of the study was to clarify whether NO modulates the discharge rate of BF neurons and whether this effect is mediated via AD.

MATERIALS AND METHODS

We measured the discharge rates of BF neurons in anesthetized rats during microdialysis infusion of NO donor alone or in combination with A1 receptor antagonist, 8-cyclopentyl-1,3-dimethylxanthine.

RESULTS

NO dose dependently modulated the discharge rate of BF neurons. NO donor (0.5 mM) increased the discharge rates in 48% of neurons and decreased it in 22%. A 1-mM dose decreased it in 55% and increased in 18%. Tactile stimulus affected the discharge rates of most neurons: 60% increased (stimulus-on) it and 14% decreased it (stimulus-off). A 1-mM NO donor predominantly inhibited neurons of both stimulus related types. A small proportion of stimulus-on (23%) neurons but none of the stimulus-off neurons were activated by NO donor. The blockade of A1 receptors partly prevented the inhibitory effect of NO on most of the neurons. This response was more prominent in stimulus-on than in stimulus-off neurons.

CONCLUSION

NO modulates the BF neuronal discharge rates in a dose-dependent manner. The inhibitory effect is partly mediated via adenosine A1 receptors.

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.

Adenosine and the homeostatic control of sleep: effects of A1 receptor blockade in the perifornical lateral hypothalamus on sleep-wakefulness

The orexinergic neurons of the lateral hypothalamus (LH) are critical for wakefulness [McCarley RW (2007) Neurobiology of REM and NREM sleep. Sleep Med 8:302-330]. Recent evidence suggests that adenosine (AD), a homeostatic sleep factor, may act via A1 receptor (A1R) to control orexinergic activity and regulate sleep-wakefulness [Thakkar MM, Winston S, McCarley RW (2002) Orexin neurons of the hypothalamus express adenosine A1 receptors. Brain Res 944:190-194; Liu ZW, Gao XB (2006) Adenosine inhibits activity of hypocretin/orexin neurons via A1 receptor in the lateral hypothalamus: a possible sleep-promoting effect. J Neurophysiol]. To evaluate the role of AD in the orexinergic LH and its influences on sleep-wakefulness, we designed two experiments in freely behaving rats: First, we bilaterally microinjected 1,3-dipropyl-8-phenylxanthine (DPX) (1.5 pmol and 15 pmol), a selective A1R antagonist into the LH during the light cycle and examined its effect on spontaneous sleep-wakefulness. Second, we performed 6 h of sleep deprivation. Thirty minutes before the animals were allowed to enter recovery sleep, 15 pmol of DPX was bilaterally microinjected into the LH and its effects on recovery sleep were monitored. Microinjection of DPX into the orexinergic LH produced a significant increase in wakefulness with a concomitant reduction in sleep, both during spontaneous bouts of sleep-wakefulness and during recovery sleep. Local administration of DPX into the LH produced a significant increase in the latency to non-REM sleep during recovery sleep. However, total slow wave (delta) activity during non-REM sleep phase of recovery sleep remained unaffected after DPX treatment. This is the first study that implicates endogenous adenosine to have a functional role in controlling orexinergic tone and influencing the homeostatic regulation of sleep-wakefulness.

Sleep fragmentation elevates behavioral, electrographic and neurochemical measures of sleepiness

Sleep fragmentation, a feature of sleep apnea as well as other sleep and medical/psychiatric disorders, is thought to lead to excessive daytime sleepiness. A rodent model of sleep fragmentation was developed (termed sleep interruption, SI), where rats were awakened every 2 min by the movement of an automated treadmill for either 6 or 24 h of exposure. The sleep pattern of rats exposed to 24 h of SI resembled sleep of the apneic patient in the following ways: sleep was fragmented (up to 30 awakening/h), total rapid eye movement (REM) sleep time was greatly reduced, non-rapid eye movement (NREM) sleep episode duration was reduced (from 2 min, 5 s baseline to 58 s during SI), whereas the total amount of NREM sleep time per 24 h approached basal levels. Both 6 and 24 h of SI made rats more sleepy, as indicated by a reduced latency to fall asleep upon SI termination. Electrographic measures in the recovery sleep period following either 6 or 24 h of SI also indicated an elevation of homeostatic sleep drive; specifically, the average NREM episode duration increased (e.g. for 24 h SI, from 2 min, 5 s baseline to 3 min, 19 s following SI), as did the NREM delta power during recovery sleep. Basal forebrain (BF) levels of extracellular adenosine (AD) were also measured with microdialysis sample collection and high performance liquid chromatography detection, as previous work suggests that increasing concentrations of BF AD are related to sleepiness. BF AD levels were significantly elevated during SI, peaking at 220% of baseline during 30 h of SI exposure. These combined findings imply an elevation of the homeostatic sleep drive following either 6 or 24 h of SI, and BF AD levels appear to correlate more with sleepiness than with the cumulative amount of prior wakefulness, since total NREM sleep time declined only slightly. SI may be partially responsible for the symptom of daytime sleepiness observed in a number of clinical disorders, and this may be mediated by mechanisms involving BF AD.

Neurobiology of REM and NREM sleep

This paper presents an overview of the current knowledge of the neurophysiology and cellular pharmacology of sleep mechanisms. It is written from the perspective that recent years have seen a remarkable development of knowledge about sleep mechanisms, due to the capability of current cellular neurophysiological, pharmacological and molecular techniques to provide focused, detailed, and replicable studies that have enriched and informed the knowledge of sleep phenomenology and pathology derived from electroencephalographic (EEG) analysis. This chapter has a cellular and neurophysiological/neuropharmacological focus, with an emphasis on rapid eye movement (REM) sleep mechanisms and non-REM (NREM) sleep phenomena attributable to adenosine. The survey of neuronal and neurotransmitter-related brainstem mechanisms of REM includes monoamines, acetylcholine, the reticular formation, a new emphasis on GABAergic mechanisms and a discussion of the role of orexin/hypcretin in diurnal consolidation of REM sleep. The focus of the NREM sleep discussion is on the basal forebrain and adenosine as a mediator of homeostatic control. Control is through basal forebrain extracellular adenosine accumulation during wakefulness and inhibition of wakefulness-active neurons. Over longer periods of sleep loss, there is a second mechanism of homeostatic control through transcriptional modification. Adenosine acting at the A1 receptor produces an up-regulation of A1 receptors, which increases inhibition for a given level of adenosine, effectively increasing the gain of the sleep homeostat. This second mechanism likely occurs in widespread cortical areas as well as in the basal forebrain. Finally, the results of a new series of experimental paradigms in rodents to measure the neurocognitive effects of sleep loss and sleep interruption (modeling sleep apnea) provide animal model data congruent with those in humans.

Adenosine and sleep deprivation promote NF-kappaB nuclear translocation in cholinergic basal forebrain

In our investigations related to the homeostatic sleep factor adenosine (AD), we previously demonstrated that the DNA-binding activity of the transcription factor NF-kappaB in rat cholinergic basal forebrain increased following 3 h of sleep deprivation (SD). However, the neurotransmitter nature of the cells and the SD-induced stimuli responsible for NF-kappaB activation were not defined. In this report, we demonstrate, using double labeling immunohistochemistry, that nuclear translocation of NF-kappaB occurs almost exclusively in the cholinergic neurons of the basal forebrain following 3 h of SD. Furthermore, cholinergic basal forebrain microinjection of AD (25 nmol/L) or the A(1) receptor agonist N(6)-cyclo-hexyladenosine (100 nmol/L) induced nuclear translocation of NF-kappaB, thus suggesting that SD-induced increased extracellular concentrations of AD, acting via the A(1) AD receptor, may be responsible for the nuclear translocation of NF-kappaB in cholinergic neurons. Moreover, blocking the nuclear translocation of NF-kappaB by injection of inhibitor peptide, SN50, immediately prior to 6 h SD significantly reduced delta activity (1-4 Hz) during the first two hours of recovery sleep. Together, these data suggest a role in sleep homeostasis for the SD-induced activation of NF-kappaB in cholinergic basal forebrain, and that transcription factor NF-kappaB may code for factor(s) that play a role in sleep homeostasis.

Adenosine and sleep homeostasis in the Basal forebrain

It is currently hypothesized that the drive to sleep is determined by the activity of the basal forebrain (BF) cholinergic neurons, which release adenosine (AD), perhaps because of increased metabolic activity associated with the neuronal discharge during waking, and the accumulating AD begins to inhibit these neurons so that sleep-active neurons can become active. This hypothesis grew from the observation that AD induces sleep and AD levels increase with wake in the basal forebrain, but surprisingly it still remains untested. Here we directly test whether the basal forebrain cholinergic neurons are central to the AD regulation of sleep drive by administering 192-IgG-saporin to lesion the BF cholinergic neurons and then measuring AD levels in the BF. In rats with 95% lesion of the BF cholinergic neurons, AD levels in the BF did not increase with 6 h of prolonged waking. However, the lesioned rats had intact sleep drive after 6 and 12 h of prolonged waking, indicating that the AD accumulation in the BF is not necessary for sleep drive. Next we determined that, in the absence of the BF cholinergic neurons, the selective adenosine A1 receptor agonist N6-cyclohexyladenosine, administered to the BF, continued to be effective in inducing sleep, indicating that the BF cholinergic neurons are not essential to sleep induction. Thus, neither the activity of the BF cholinergic neurons nor the accumulation of AD in the BF during wake is necessary for sleep drive.

Dialysis delivery of an adenosine A2Aagonist into the pontine reticular formation of C57BL/6J mouse increases pontine acetylcholine release and sleep

In vivo microdialysis in C57BL/6J (B6) mouse was used to test the hypothesis that activating adenosine A(2A) receptors in the pontine reticular formation (PRF) increases acetylcholine (ACh) release and rapid eye movement (REM) sleep. Eight concentrations of the adenosine A(2A) receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride (CGS 21680; CGS) were delivered to the PRF and ACh in the PRF was quantified. ACh release was significantly increased by dialysis with 3 mum CGS and significantly decreased by dialysis with 10 and 100 microm CGS. Co-administration of the adenosine A(2A) receptor antagonist 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385; 30 nM) blocked the CGS-induced increase in ACh release. In a second series of experiments, CGS (3 microm) was delivered by dialysis to the PRF for 2 h while recording sleep and wakefulness. CGS significantly decreased time in wakefulness (-51% in h 1; -54% in h 2), increased time in non-rapid eye movement (NREM) sleep (90% in h 1; 151% in h 2), and increased both time in REM sleep (331% in h 2) and the number of REM sleep episodes (488% in h 2). The enhancement of REM sleep is consistent with the interpretation that adenosine A(2A) receptors in the PRF of the B6 mouse contribute to REM sleep regulation, in part, by increasing ACh release in the PRF. A(2A) receptor activation may promote NREM sleep via GABAergic inhibition of arousal promoting neurons in the PRF.

Differential c-Fos immunoreactivity in arousal-promoting cell groups following systemic administration of caffeine in rats

Despite the widespread use of caffeine, the neuronal mechanisms underlying its stimulatory effects are not completely understood. By using c-Fos immunohistochemistry as a marker of neuronal activation, we recently showed that stimulant doses of caffeine activate arousal-promoting hypothalamic orexin (hypocretin) neurons. In the present study, we investigated whether other key neurons of the arousal system are also activated by caffeine, via dual immunostaining for c-Fos and transmitter markers. Rats were administered three doses of caffeine or saline vehicle during the light phase. Caffeine at 10 and 30 mg/kg, i.p., increased motor activities, including locomotion, compared with after saline or a higher dose, 75 mg/kg. The three doses of caffeine induced distinct dose-related patterns of c-Fos immunoreactivity in several arousal-promoting areas, including orexin neurons and adjacent neurons containing neither orexin nor melanin-concentrating hormone; tuberomammillary histaminergic neurons; locus coeruleus noradrenergic neurons; noncholinergic basal forebrain neurons that do not contain parvalbumin; and nondopaminergic neurons in the ventral tegmental area. At any dose used, caffeine induced little or no c-Fos expression in cholinergic neurons of the basal forebrain and mesopontine tegmentum; dopaminergic neurons of the ventral tegmental area, central gray, and substantia nigra pars compacta; and serotonergic neurons in the dorsal raphe nucleus. Saline controls exhibited only few c-Fos-positive cells in most of the cell groups examined. These results indicate that motor-stimulatory doses of caffeine induce a remarkably restricted pattern of c-Fos expression in the arousal-promoting system and suggest that this specific neuronal activation may be involved in the behavioral arousal by caffeine.