Effect of electric foot shocks, immobilization, and corticosterone administration on the sleep-wake pattern in the rat

Regulation of stress-induced sleep perturbations by dorsal raphe VGLUT3 neurons in male mice

Exposure to stressors has profound effects on sleep that have been linked to serotonin (5-HT) neurons of the dorsal raphe nucleus (DR). However, the DR also comprises glutamatergic neurons expressing vesicular glutamate transporter type 3 (DRVGLUT3), leading us to examine their role. Cell-type-specific tracing revealed that DRVGLUT3 neurons project to brain areas regulating arousal and stress. We found that chemogenetic activation of DRVGLUT3 neurons mimics stress-induced sleep perturbations. Furthermore, deleting VGLUT3 in the DR attenuated stress-induced sleep perturbations, especially after social defeat stress. In the DR, VGLUT3 is found in subsets of 5-HT and non-5-HT neurons. We observed that both populations are activated by acute stress, including those projecting to the ventral tegmental area. However, deleting VGLUT3 in 5-HT neurons minimally affected sleep regulation. These findings suggest that VGLUT3 expression in the DR drives stress-induced sleep perturbations, possibly involving non-5-HT DRVGLUT3 neurons.

Impact of single-trial avoidance learning on subsequent sleep

Both non-rapid eye movement (NonREM) sleep and rapid eye movement (REM) sleep, as well as sleep spindle and ripple oscillations, are important for memory formation. Through cortical EEG recordings of prefrontal cortex and hippocampus during and after an inhibitory avoidance task, we analysed the dynamic changes in the amounts of sleep, spindle and ripple oscillations related to memory formation. The total amount of NonREM sleep was reduced during the first hour after learning. Moreover, significant decrease of the total spindle and ripple counts was observed at the first hour after learning as well. In addition, foot shock alone, with no associated learning, produced little effect on the dynamics of sleep oscillations, indicating that the learning experience is necessary for these changes to occur.

Sex differences in body temperature and neural power spectra in response to repeated restraint stress

Repeated stress is associated with an increased risk of developing psychiatric illnesses such as post-traumatic stress disorder (PTSD), which is more common in women, yet the neurobiology behind this sex difference is unknown. Habituation to repeated stress is impaired in PTSD, and recent preclinical studies have shown that female rats do not habituate as fully as male rats to repeated stress, which leads to impairments in cognition and sleep. Further research should examine sex differences after repeated stress in other relevant measures, such as body temperature and neural activity. In this study, we analyzed core body temperature and EEG power spectra in adult male and female rats during restraint, as well as during sleep transitions following stress. We found that core body temperature of male rats habituated to repeated restraint more fully than female rats. Additionally, we found that females had a higher average beta band power than males on both days of restraint, indicating higher levels of arousal. Lastly, we observed that females had lower delta band power than males during sleep transitions on Day 1 of restraint, however, females demonstrated higher delta band power than males by Day 5 of restraint. This suggests that it may take females longer to initiate sleep recovery compared with males. These findings indicate that there are differences in the physiological and neural processes of males and females after repeated stress. Understanding the way that the stress response is regulated in both sexes can provide insight into individualized treatment for stress-related disorders.

Modeling integrated stress, sleep, fear and neuroimmune responses: Relevance for understanding trauma and stress-related disorders

Sleep and stress have complex interactions that are implicated in both physical diseases and psychiatric disorders. These interactions can be modulated by learning and memory, and involve additional interactions with the neuroimmune system. In this paper, we propose that stressful challenges induce integrated responses across multiple systems that can vary depending on situational variables in which the initial stress was experienced, and with the ability of the individual to cope with stress- and fear-inducing challenges. Differences in coping may involve differences in resilience and vulnerability and/or whether the stressful context allows adaptive learning and responses. We provide data demonstrating both common (corticosterone, SIH and fear behaviors) and distinguishing (sleep and neuroimmune) responses that are associated with an individual's ability to respond and relative resilience and vulnerability. We discuss neurocircuitry regulating integrated stress, sleep, neuroimmune and fear responses, and show that responses can be modulated at the neural level. Finally, we discuss factors that need to be considered in models of integrated stress responses and their relevance for understanding stress-related disorders in humans.

Alterations in the activation of corticotropin-releasing factor neurons in the paraventricular nucleus following a single or multiple days of sleep restriction

Sleep disturbances are common among disorders associated with hypothalamic pituitary-adrenal (HPA) axis dysfunction, such as depression and anxiety. This comorbidity may partly be the result of the intersection between the role of the HPA axis in mediating the stress response and its involvement in sleep-wake cyclicity. Our previous work has shown that following 20 h of sleep restriction, mice show a blunting of the HPA axis in response to an acute stressor. Furthermore, these responses differ in a sex-dependent manner. This study sought to examine the effect of sleep restriction on corticotropin-releasing factor (CRF)-containing neurons in the paraventricular nucleus (PVN) of the hypothalamus. Male and female Crf-IRES-Cre: Ai14 (Tdtomato) reporter mice were sleep restricted for 20 h daily for either a single or three consecutive days using the modified multiple platform method. These mice allowed the visualization of CRF+ neurons throughout the brain. Animals were subjected to acute restraint stress, and their brains were collected to assess PVN neuronal activation via c-Fos immunohistochemistry. Analyses of cell counts revealed an ablation of the restraint-induced increase in both CRF/c-Fos colocalization and overall c-Fos expression in female mice following both a single day and three days of sleep restriction. Males showed an overall decrease in restraint-induced c-Fos levels following a single day of sleep restriction. However, male mice examined after three days of sleep restriction showed a recovery in PVN-CRF and overall PVN neuronal activation. These data suggest the sex dependent dysregulation in CRF function following sleep restriction.

Effect of Hibiscus syriacus Linnaeus extract and its active constituent, saponarin, in animal models of stress-induced sleep disturbances and pentobarbital-induced sleep

Treatment of sleep disorders promotes the long-term use of commercially available sleep inducers that have several adverse effects, including addiction, systemic fatigue, weakness, loss of concentration, headache, and digestive problems. Therefore, we aimed to limit these adverse effects by investigating a natural product, the extract of the Hibiscus syriacus Linnaeus flower (HSF), as an alternative treatment. In the electric footshock model, we measured anxiety and assessed the degree of sleep improvement after administering HSF extract. In the restraint model, we studied the sleep rate using PiezoSleep, a noninvasive assessment system. In the pentobarbital model, we measured sleep improvement and changes in sleep-related factors. Our first model confirmed the desirable effects of HSF extract and its active constituent, saponarin, on anxiolysis and Wake times. HSF extract also increased REM sleep time. Furthermore, HSF extract and saponarin increased the expression of cortical GABAA receptor α1 (GABAAR α1) and c-Fos in the ventrolateral preoptic nucleus (VLPO). In the second model, HSF extract and saponarin restored the sleep rate and the sleep bout duration. In the third model, HSF extract and saponarin increased sleep maintenance time. Moreover, HSF extract and saponarin increased cortical cholecystokinin (CCK) mRNA levels and the expression of VLPO c-Fos. HSF extract also increased GABAAR α1 mRNA level. Our results suggest that HSF extract and saponarin are effective in maintaining sleep and may be used as a novel treatment for sleep disorder. Eventually, we hope to introduce HSF and saponarin as a clinical treatment for sleep disorders in humans.

After-effects of acute footshock stress on sleep states and rhythmic masticatory muscle activity during sleep in guinea pigs

This study investigated the effects of acute footshock stress (FS) on the occurrence of rhythmic masticatory muscle activity (RMMA) during sleep in guinea pigs. Animals were prepared for chronic recordings from electroencephalogram, electrooculogram and electromyograms of neck and masseter muscles. The signals were recorded for six hours on the two successive days: the first day with stress-free condition (non-FS condition) and the second day with acute FS (FS condition). Sleep/wake states and RMMA were scored visually. Sleep variables and the frequency of RMMA occurring during non-rapid eye movement (NREM) sleep were compared during 6-h periods between the two conditions. Compared to non-FS condition, the amount of total sleep and NREM sleep significantly reduced during 2 h following the acute FS in the FS condition. Similarly, the frequency of RMMA significantly increased during 2 h following the acute FS for the FS condition compared to non-FS condition. During 2-6 h after FS in the FS condition, sleep variables and the frequency of RMMA did not differ from those without FS in the non-FS condition. These results suggest that acute experimental stress can induce transient changes in sleep-wake states and the occurrence of RMMA in experimental animals.

Behavioral methods to study anxiety in rodents

Stress is a precipitating factor for anxiety-related disorders, which are among the leading forms of psychiatric illness and impairment in the modern world. Rodent-based behavioral tests and models are widely used to understand the mechanisms by which stress triggers anxiety-related behaviors and to identify new treatments for anxiety-related disorders. Although substantial progress has been made and many of the key neural circuits and molecular pathways mediating stress responsiveness have been characterized, these advances have thus far failed to translate into fundamentally new treatments that are safer and more efficacious in humans. The purpose of this article is to describe methods that have been historically used for this type of research and to highlight new approaches that align with recent conceptualizations of disease symptomatology and that may ultimately prove to be more fruitful in facilitating the development of improved therapeutics.

Nitrergic neurons of the dorsal raphe nucleus encode information about stress duration

Nitrergic neurons of the dorsal raphe nucleus (DRN) may play a role in physiological stress responses. The caudal lateral wings (CLW) are unique compared to other rostral-caudal DRN sub-regions because they contain distinct nitric oxide (NO) synthase (NOS) populations that are independent of tryptophan hydroxylase (TPH). NOS neurons in the CLW are also highly activated during acute restraint stress. However, the effects of acute stress duration on NOS activation in the CLW are unclear. Here NADPH-d, an index of NOS activity, is used to show that sub-regions of the DRN have differential NOS activation in response to 6 hours of restraint stress in rats. We report increased NOS activity through 6 hours of restraint in the caudal lateral wings and ventromedial sub-regions. These data suggest that, NOS neurons may play a dynamic role in the response to stress duration.

Homeostatic response to sleep/rest deprivation by constant water flow in larval zebrafish in both dark and light conditions

Sleep-or sleep-like states-have been reported in adult and larval zebrafish using behavioural criteria. These reversible quiescent periods, displaying circadian rhythmicity, have been used in pharmacological, genetic and neuroanatomical studies of sleep-wake regulation. However, one of the important criteria for sleep, namely sleep homeostasis, has not been demonstrated unequivocally. To study rest homeostasis in zebrafish larvae, we rest-deprived 1-week-old larvae with a novel, ecologically relevant method: flow of water. Stereotyped startle responses to sensory stimuli were recorded after the rest deprivation to study arousal threshold using a high-speed camera, providing an appropriate time resolution to detect species-specific behavioural responses occurring in a millisecond time-scale. Rest-deprived larvae exhibited fewer startle responses than control larvae during the remaining dark phase and the beginning of the light phase, which can be interpreted as a sign of rest homeostasis-often used as equivalent of sleep homeostasis. To address sleep homeostasis further, we probed the adenosinergic system, which in mammals regulates sleep homeostasis. The adenosine A1 receptor agonist, cyclohexyladenosine, administered during the light period, decreased startle responses and increased immobility bouts, while the adenosine antagonist, caffeine, administered during the dark period, decreased immobility bouts. These results suggest that the regulation of sleep homeostasis in zebrafish larvae consists of the same elements as that of other species.

Effects of the IGF-1/PTEN/Akt/FoxO signaling pathway on male reproduction in rats subjected to water immersion and restraint stress

The aim of the present study was to determine the effects of the insulin-like growth factor 1 (IGF-1)/phosphatase and tensin homologue deleted on chromosome 10 (PTEN)/Akt/forkhead box (FoxO) signaling pathway on male reproduction in rats subjected to water immersion and restraint stress (WRS). Sperm morphology, sperm malformation rate, and serum testosterone concentration were analyzed following WRS. In addition, the expression levels and immunolocalization of IGF-1, PTEN, Akt and FoxO proteins, as well as the rate of cell apoptosis in rat testes, were investigated. The results indicated that sperm malformation rate, serum testosterone concentration, and the number of terminal deoxynucleotidyl transferase dUTP nick end labeling-positive cells were increased in the testes after WRS. Furthermore, IGF-1 and FoxO1 proteins were predominantly localized in the sperm cytoplasm during the late stages of spermatogenesis. FoxO1 protein was also localized in Leydig cell cytoplasm. PTEN and total Akt proteins were predominantly expressed in the cytoplasm of Leydig cells and spermatogonia. PTEN protein was also detected in vascular endothelial cells. In addition, IGF-1, PTEN, Akt1, Akt2, FoxO3 and FoxO4 gene expression levels were upregulated following WRS, and peaked after 7 h of WRS. During the recovery period, the expression levels of these genes gradually returned to normal levels. The present study demonstrated that WRS induced sperm damage in the testes. In addition, the results indicated that the IGF-1/PTEN/Akt/FoxO signaling pathway may serve an anti-stress role in the testes of rats subjected to WRS.

Voluntary Sleep Loss in Rats

STUDY OBJECTIVES

Animal sleep deprivation (SDEP), in contrast to human SDEP, is involuntary and involves repeated exposure to aversive stimuli including the inability of the animal to control the waking stimulus. Therefore, we explored intracranial self-stimulation (ICSS), an operant behavior, as a method for voluntary SDEP in rodents.

METHODS

Male Sprague-Dawley rats were implanted with electroencephalography/electromyography (EEG/EMG) recording electrodes and a unilateral bipolar electrode into the lateral hypothalamus. Rats were allowed to self-stimulate, or underwent gentle handling-induced SDEP (GH-SDEP), during the first 6 h of the light phase, after which they were allowed to sleep. Other rats performed the 6 h ICSS and 1 w later were subjected to 6 h of noncontingent stimulation (NCS). During NCS the individual stimulation patterns recorded during ICSS were replayed.

RESULTS

After GH-SDEP, ICSS, or NCS, time in nonrapid eye movement (NREM) sleep and rapid eye movement (REM) sleep increased. Further, in the 24 h after SDEP, rats recovered all of the REM sleep lost during SDEP, but only 75% to 80% of the NREM sleep lost, regardless of the SDEP method. The magnitude of EEG slow wave responses occurring during NREM sleep also increased after SDEP treatments. However, NREM sleep EEG slow wave activity (SWA) responses were attenuated following ICSS, compared to GH-SDEP and NCS.

CONCLUSIONS

We conclude that ICSS and NCS can be used to sleep deprive rats. Changes in rebound NREM sleep EEG SWA occurring after ICSS, NCS, and GH-SDEP suggest that nonspecific effects of the SDEP procedure differentially affect recovery sleep phenotypes.

Glucocorticoid receptors in the locus coeruleus mediate sleep disorders caused by repeated corticosterone treatment

Stress induced constant increase of cortisol level may lead to sleep disorder, but the mechanism remains unclear. Here we described a novel model to investigate stress mimicked sleep disorders induced by repetitive administration of corticosterone (CORT). After 7 days treatment of CORT, rats showed significant sleep disturbance, meanwhile, the glucocorticoid receptor (GR) level was notably lowered in locus coeruleus (LC). We further discovered the activation of noradrenergic neuron in LC, the suppression of GABAergic neuron in ventrolateral preoptic area (VLPO), the remarkable elevation of norepinephrine in LC, VLPO and hypothalamus, as well as increase of tyrosine hydroxylase in LC and decrease of glutamic acid decarboxylase in VLPO after CORT treatment. Microinjection of GR antagonist RU486 into LC reversed the CORT-induced sleep changes. These results suggest that GR in LC may play a key role in stress-related sleep disorders and support the hypothesis that repeated CORT treatment may decrease GR levels and induce the activation of noradrenergic neurons in LC, consequently inhibit GABAergic neurons in VLPO and result in sleep disorders. Our findings provide novel insights into the effect of stress-inducing agent CORT on sleep and GRs' role in sleep regulation.

Preclinical experimental stress studies: protocols, assessment and comparison

Stress is a state of threatened homeostasis during which a variety of adaptive processes are activated to produce physiological and behavioral changes. Preclinical models are pivotal for understanding these physiological or pathophysiological changes in the body in response to stress. Furthermore, these models are also important for the development of novel pharmacological agents for stress management. The well described preclinical stress models include immobilization, restraint, electric foot shock and social isolation stress. Stress assessment in animals is done at the behavioral level using open field, social interaction, hole board test; at the biochemical level by measuring plasma corticosterone and ACTH; at the physiological level by measuring food intake, body weight, adrenal gland weight and gastric ulceration. Furthermore the comparison between different stressors including electric foot shock, immobilization and cold stressor is described in terms of intensity, hormonal release, protein changes in brain, adaptation and sleep pattern. This present review describes these preclinical stress protocols, and stress assessment at different levels.

Sleep and vigilance linked to melanism in wild barn owls

Understanding the function of variation in sleep requires studies in the natural ecological conditions in which sleep evolved. Sleep has an impact on individual performance and hence may integrate the costs and benefits of investing in processes that are sensitive to sleep, such as immunity or coping with stress. Because dark and pale melanic animals differentially regulate energy homeostasis, immunity and stress hormone levels, the amount and/or organization of sleep may covary with melanin-based colour. We show here that wild, cross-fostered nestling barn owls (Tyto alba) born from mothers displaying more black spots had shorter non-REM (rapid eye movement) sleep bouts, a shorter latency until the occurrence of REM sleep after a bout of wakefulness and more wakefulness bouts. In male nestlings, the same sleep traits also correlated with their own level of spotting. Because heavily spotted male nestlings and the offspring of heavily spotted biological mothers switched sleep-wakefulness states more frequently, we propose the hypothesis that they could be also behaviourally more vigilant. Accordingly, nestlings from mothers displaying many black spots looked more often towards the nest entrance where their parents bring food and towards their sibling against whom they compete. Owlets from heavily spotted mothers might invest more in vigilance, thereby possibly increasing associated costs due to sleep fragmentation. We conclude that different strategies of the regulation of brain activity have evolved and are correlated with melanin-based coloration.

Sleep pattern and learning in knockdown mice with reduced cholinergic neurotransmission

Impaired cholinergic neurotransmission can affect memory formation and influence sleep-wake cycles (SWC). In the present study, we describe the SWC in mice with a deficient vesicular acetylcholine transporter (VAChT) system, previously characterized as presenting reduced acetylcholine release and cognitive and behavioral dysfunctions. Continuous, chronic ECoG and EMG recordings were used to evaluate the SWC pattern during light and dark phases in VAChT knockdown heterozygous (VAChT-KDHET, n=7) and wild-type (WT, n=7) mice. SWC were evaluated for sleep efficiency, total amount and mean duration of slow-wave, intermediate and paradoxical sleep, as well as the number of awakenings from sleep. After recording SWC, contextual fear-conditioning tests were used as an acetylcholine-dependent learning paradigm. The results showed that sleep efficiency in VAChT-KDHET animals was similar to that of WT mice, but that the SWC was more fragmented. Fragmentation was characterized by an increase in the number of awakenings, mainly during intermediate sleep. VAChT-KDHET animals performed poorly in the contextual fear-conditioning paradigm (mean freezing time: 34.4±3.1 and 44.5±3.3 s for WT and VAChT-KDHET animals, respectively), which was followed by a 45% reduction in the number of paradoxical sleep episodes after the training session. Taken together, the results show that reduced cholinergic transmission led to sleep fragmentation and learning impairment. We discuss the results on the basis of cholinergic plasticity and its relevance to sleep homeostasis. We suggest that VAChT-KDHET mice could be a useful model to test cholinergic drugs used to treat sleep dysfunction in neurodegenerative disorders.

Prognostic and symptomatic aspects of rapid eye movement sleep in a mouse model of posttraumatic stress disorder

Not every individual develops Posttraumatic Stress Disorder (PTSD) after the exposure to a potentially traumatic event. Therefore, the identification of pre-existing risk factors and early diagnostic biomarkers is of high medical relevance. However, no objective biomarker has yet progressed into clinical practice. Sleep disturbances represent commonly reported complaints in PTSD patients. In particular, changes in rapid eye movement sleep (REMS) properties are frequently observed in PTSD patients. Here, we examined in a mouse model of PTSD whether (1) mice developed REMS alterations after trauma and (2) whether REMS architecture before and/or shortly after trauma predicted the development of PTSD-like symptoms. We monitored sleep-wake behavior via combined electroencephalogram/electromyogram recordings immediately before (24 h pre), immediately after (0-48 h post) and 2 months after exposure to an electric foot shock in male C57BL/6N mice (n = 15). PTSD-like symptoms, including hyperarousal, contextual, and generalized fear, were assessed 1 month post-trauma. Shocked mice showed early onset and sustained elevation of REMS compared to non-shocked controls. In addition, REMS architecture before trauma was correlated with the intensity of acoustic startle responses, but not contextual fear, 1 month after trauma. Our data suggest REMS as prognostic (pre-trauma) and symptomatic (post-trauma) marker of PTSD-like symptoms in mice. Translated to the situation in humans, REMS may constitute a viable, objective, and non-invasive biomarker in PTSD and other trauma-related psychiatric disorders, which could guide pharmacological interventions in humans at high risk.

Role of corticosterone on sleep homeostasis induced by REM sleep deprivation in rats

Sleep is regulated by humoral and homeostatic processes. If on one hand chronic elevation of stress hormones impair sleep, on the other hand, rapid eye movement (REM) sleep deprivation induces elevation of glucocorticoids and time of REM sleep during the recovery period. In the present study we sought to examine whether manipulations of corticosterone levels during REM sleep deprivation would alter the subsequent sleep rebound. Adult male Wistar rats were fit with electrodes for sleep monitoring and submitted to four days of REM sleep deprivation under repeated corticosterone or metyrapone (an inhibitor of corticosterone synthesis) administration. Sleep parameters were continuously recorded throughout the sleep deprivation period and during 3 days of sleep recovery. Plasma levels of adrenocorticotropic hormone and corticosterone were also evaluated. Metyrapone treatment prevented the elevation of corticosterone plasma levels induced by REM sleep deprivation, whereas corticosterone administration to REM sleep-deprived rats resulted in lower corticosterone levels than in non-sleep deprived rats. Nonetheless, both corticosterone and metyrapone administration led to several alterations on sleep homeostasis, including reductions in the amount of non-REM and REM sleep during the recovery period, although corticosterone increased delta activity (1.0-4.0 Hz) during REM sleep deprivation. Metyrapone treatment of REM sleep-deprived rats reduced the number of REM sleep episodes. In conclusion, reduction of corticosterone levels during REM sleep deprivation resulted in impairment of sleep rebound, suggesting that physiological elevation of corticosterone levels resulting from REM sleep deprivation is necessary for plentiful recovery of sleep after this stressful event.

Monoaminergic and neuropeptidergic neurons have distinct expression profiles of histone deacetylases

Monoaminergic and neuropeptidergic neurons regulate a wide variety of behaviors, such as feeding, sleep/wakefulness behavior, stress response, addiction, and social behavior. These neurons form neural circuits to integrate different modalities of behavioral and environmental factors, such as stress, maternal care, and feeding conditions. One possible mechanism for integrating environmental factors through the monoaminergic and neuropeptidergic neurons is through the epigenetic regulation of gene expression via altered acetylation of histones. Histone deacetylases (HDACs) play an important role in altering behavior in response to environmental factors. Despite increasing attention and the versatile roles of HDACs in a variety of brain functions and disorders, no reports have detailed the localization of the HDACs in the monoaminergic and neuropeptidergic neurons. Here, we examined the expression profile of the HDAC protein family from HDAC1 to HDAC11 in corticotropin-releasing hormone, oxytocin, vasopressin, agouti-related peptide (AgRP), pro-opiomelanocortin (POMC), orexin, histamine, dopamine, serotonin, and noradrenaline neurons. Immunoreactivities for HDAC1,-2,-3,-5,-6,-7,-9, and -11 were very similar among the monoaminergic and neuropeptidergic neurons, while the HDAC4, -8, and -10 immunoreactivities were clearly different among neuronal groups. HDAC10 expression was found in AgRP neurons, POMC neurons, dopamine neurons and noradrenaline neurons but not in other neuronal groups. HDAC8 immunoreactivity was detected in the cytoplasm of almost all histamine neurons with a pericellular pattern but not in other neuropeptidergic and monoaminergic neurons. Thus, the differential expression of HDACs in monoaminergic and neuropeptidergic neurons may be crucial for the maintenance of biological characteristics and may be altered in response to environmental factors.

Role of sleep in the regulation of the immune system and the pituitary hormones

Sleep is characterized by a reduced response to external stimuli and a particular form of electroencephalographic (EEG) activity. Sleep is divided into two stages: REM sleep, characterized by muscle atonia, rapid eye movements, and EEG activity similar to wakefulness, and non-REM sleep, characterized by slow EEG activity. Around 80% of total sleep time is non-REM. Although it has been intensely studied for decades, the function (or functions) of sleep remains elusive. Sleep is a highly regulated state; some brain regions and several hormones and cytokines participate in sleep regulation. This mini-review focuses on how pituitary hormones and cytokines regulate or affect sleep and how sleep modifies the plasma concentration of hormones as well as cytokines. Also, we review the effects of hypophysectomy and some autoimmune diseases on sleep pattern. Finally, we propose that one of the functions of sleep is to maintain the integrity of the neuro-immune-endocrine system.

REM Sleep Rebound as an Adaptive Response to Stressful Situations

Stress and sleep are related to each other in a bidirectional way. If on one hand poor or inadequate sleep exacerbates emotional, behavioral, and stress-related responses, on the other hand acute stress induces sleep rebound, most likely as a way to cope with the adverse stimuli. Chronic, as opposed to acute, stress impairs sleep and has been claimed to be one of the triggering factors of emotional-related sleep disorders, such as insomnia, depressive- and anxiety-disorders. These outcomes are dependent on individual psychobiological characteristics, conferring even more complexity to the stress-sleep relationship. Its neurobiology has only recently begun to be explored, through animal models, which are also valuable for the development of potential therapeutic agents and preventive actions. This review seeks to present data on the effects of stress on sleep and the different approaches used to study this relationship as well as possible neurobiological underpinnings and mechanisms involved. The results of numerous studies in humans and animals indicate that increased sleep, especially the rapid eye movement phase, following a stressful situation is an important adaptive behavior for recovery. However, this endogenous advantage appears to be impaired in human beings and rodent strains that exhibit high levels of anxiety and anxiety-like behavior.

Sharing stressful experiences attenuates anxiety-related cognitive and sleep impairments

Anxiety is a growing public health concern that has been shown to impair both sleep and learning, and these associations have been extensively studied in recent years. In the rodent model, oftentimes various foot-shock paradigms are employed to induce stress, and subsequent sleep recordings and/or learning task results are analyzed. Previous studies have focused primarily on an individual animal's response to stress following individual stressor exposure, thereby emulating only an isolated condition. The goal of this study was to investigate the effects of socialization on stress response, and the resultant effects on sleep architecture and aversive learning. A pair-housing/pair-exposure paradigm was utilized, and the effects of unavoidable foot-shock-induced stress on sleep architecture and aversive learning were examined. The results of the present study indicate a large, positive impact of cohabitation and shared stressful experience, as rats failed to develop sleep disturbances or learning deficits. While these results indicate the benefits and importance of companionship, the underlying mechanism of this phenomenon is yet to be elucidated.

Contribution of melanin-concentrating hormone (MCH1) receptor to thermoregulation and sleep stabilization: evidence from MCH1 (-/-) mice

Recent studies have explored the implication of melanin-concentrating hormone (MCH) in the process of vigilance states. The current experiments were carried out in mice lacking the MCH(1) receptor (-/-) and wild-type (WT) littermates, to assess the role of MCH(1) receptor in the regulation of sleep architecture, body temperature (BT) and locomotor activity (LMA) under normal condition and following a 1h restraint stress at lights onset. Under baseline conditions, MCH(1) (-/-) mice exhibited consistent changes in waking and sleeping time across the 24-h recording period. We found an increase in the amount of wakefulness (MCH(1) (-/-) 680.1 ± 15.3 min vs. WT, 601.9 ± 18.1, p<0.05) at the expense of total duration of non rapid eye movement (NREM) sleep (MCH(1) (-/-) 664.1 ± 13.9 min vs. WT 750.1 ± 18.5, p<0.05). Additionally, MCH(1) (-/-) mice had a higher mean basal body temperature (MCH(1) (-/-), 36.6 ± 0.1°C vs. WT, 36.0 ± 0.1°C, p<0.05), particularly during the light-resting period. Restraint stress resulted in an immediate increase in wakefulness with a concomitant reduction in NREM sleep and REM sleep in both genotypes, followed by a homeostatic rebound sleep. A concomitant long lasting increase in BT, independently of the behavioural state accompanied those changes in both genotypes. The elevated basal body temperature and reduction in NREM sleep time resulting from shorter NREM episode durations observed in MCH(1) (-/-) suggests that central MCH(1) receptor has a role in thermoregulation and presumably stabilization of NREM sleep.

Sleep and metabolism: role of hypothalamic neuronal circuitry

Sleep and metabolism are intertwined physiologically and behaviorally, but the neural systems underlying their coordination are still poorly understood. The hypothalamus is likely to play a major role in the regulation sleep, metabolism, and their interaction. And increasing evidence suggests that hypocretin cells in the lateral hypothalamus may provide particularly important contributions. Here we review: 1) direct interactions between biological arousal and metabolic systems in the hypothalamus, and 2) indirect interactions between these two systems mediated by stress or reward, emphasizing the role of hypocretins. An increased understanding of the mechanisms underlying these interactions may provide novel approaches for the treatment of patients with sleep disorders and obesity, as well as suggest new therapeutic strategies for symptoms of aging, stress, or addiction.

Heightened muscle tension and diurnal hyper-vigilance following exposure to a social defeat-conditioned odor cue in rats

Patients with post-traumatic stress disorder (PTSD) exhibit exaggerated daytime muscle tension as well as nocturnal sleep disturbances. Yet, these physiological and behavioral features of the disorder are little studied in animal models of PTSD. Accordingly, the present studies were designed to assess alterations in muscle tension and diurnal hyper-vigilance resulting from exposure to a social defeat stressor paired with an olfactory stimulus, which was then used as a reminder of stressor exposure. In the first series of experiments, rats presented with an olfactory cue paired previously with a single social defeat exhibited a significant increase in muscle tension 4 weeks following defeat. In the second series of experiments, an olfactory cue paired previously with a single social defeat induced a significant increase in locomotor activity among quiescent rats 4 weeks following stressor exposure. The present results thus support the a priori hypotheses that novel physiological and behavioral hallmarks of PTSD can be documented in an animal model of the disorder and that the present overt signs of reactive hyper-vigilance can be triggered by reintroduction of an olfactory stimulus present at the time of initial trauma exposure.

Effects of eszopiclone and zolpidem on sleep-wake behavior, anxiety-like behavior and contextual memory in rats

At present, eszopiclone and zolpidem are the most commonly prescribed drugs for treating insomnia. Despite the established relationship between sleep disturbance and anxiety, it remains unknown whether targeted treatment for insomnia may affect acute anxiety. Therefore, the objective of this study was to examine the effects of three different doses (1, 3, and 10mg/kg) of eszopiclone and zolpidem on the states of sleep and wakefulness, levels of anxiety-like behavior, and long-term contextual memory in footshock-induced anxious rats. The results of this study demonstrated that the administration of eszopiclone and zolpidem both were equally effective in attenuating footshock stressor-induced suppression of slow-wave sleep (SWS). The administration of eszopiclone at 1mg/kg or zolpidem at 1 and 3mg/kg doses showed a tendency for attenuating stressor-induced suppression of REM sleep. However, the REM sleep attenuating effects of these drugs disappeared when they were administered at higher doses. The administration of eszopiclone at 3 and 10mg/kg doses and zolpidem at all three doses reduced the power of electroencephalographic theta band frequencies during wakefulness. In addition, the administration of eszopiclone at 1 and 3mg/kg doses suppressed stressor-induced anxiety-like behavior. The administration of zolpidem at 1, 3, or 10mg/kg doses was not effective in attenuating stressor-induced anxiety-like behavior. Contextual memory after administration of eszopiclone at 1mg/kg dose had no effects, but was reduced significantly with increased dosage. Contextual memory after administration of zolpidem, at all three doses, was severely disrupted. The results of this study suggest that eszopiclone at a low dose could be used effectively to control anxiety and anxiety-induced insomnia.

The ability of stress to alter sleep in mice is sensitive to reproductive hormones

Though stress causes complex sleep disruptions that are different in females and males, little is known about how sex influences the ability of stress to alter sleep. To date there have been no comprehensive examinations of whether effects of stress on sleep are sensitive to determinants of sex, such as reproductive hormones. Since restraint stress produces a sexually dimorphic increase in rapid eye movement sleep (REMS) amount in mice that is greater in males than females, in the current study we sought to determine whether estrogens and androgens influence the ability of restraint stress to alter sleep states. We removed the gonads from adult female and male C57BL/6J mice and implanted the mice with recording electrodes to monitor sleep-wake states. Gonadectomized females and males exhibited similar amounts of REMS in response to restraint stress. Mice were then implanted with continuous release hormone pellets. Females received 17beta-estradiol and males received testosterone. Hormone replacement (HR) in females decreased the REMS response to restraint stress while HR in males increased the REMS response to restraint stress. The combined effects of HR in females and males restored the sex difference in the ability of restraint stress to alter REMS. These results demonstrate that sex differences in the effects of stress on REMS are dependent on reproductive hormones and support the view that endogenous or exogenous changes in the reproductive hormone environment influence sleep responses to stress.

Disruptions of the mother-infant relationship and stress-related behaviours: altered corticosterone secretion does not explain everything

The hypothalamic-pituitary-adrenal (HPA) axis is the main neuroendocrine system of response to stress, and an imbalance of this system's activity is believed to be at the core of numerous psychiatric pathologies. During the neonatal period, the glucocorticoid response to stress is maintained at low levels by specific maternal behaviours, which is essential for proper brain development. Effective evaluation of the impact of increased secretion of corticosterone during an essentially anabolic developmental period on adulthood behaviour involved separation of the neonate from its mother for periods ranging from 3 to 24h. It has been shown that disinhibition of the stress response is achieved by such procedures. The pioneering studies by Seymour Levine set the stage for a prolific and promising field of study that may help neuroscientists unveil the neurobiological underpinnings of stress-related disorders. Based on a series of studies, we propose that maternal separation and maternal deprivation change stress-related behaviours, but that corticosterone seem to be only partially involved in these changes in adulthood. It appears that extra-hypothalamic corticotrophin-releasing factor and neurotransmitter systems may be the primary mediators of these behavioural outcomes.

The effect of baclofen on alterations in the sleep patterns induced by different stressors in rats

We have previously reported that sleep patterns are significantly affected by both physical and psychological stress induced by a communication box; however, the mechanism by which stress alters sleep patterns was not established. In the present study, we investigated the role of gamma-aminobutyric acid (GABA), acting through the GABA(B) receptor, on stress-induced changes in sleep patterns. Our results show that physical stress increased the total wakefulness time by increasing sleep latency and inhibiting both rapid eye movement (REM) and non rapid eye movement (NREM) sleep during a 6 h sleep-recording period. The GABA(B) agonist baclofen (20 pmol/2 microl) attenuated the effects of physical stress on sleep latency, total wakefulness, and NREM sleep, but not total REM sleep. In contrast, psychological stress enhanced total REM sleep and shortened REM sleep latency without altering other sleep patterns. The effect of psychological stress on total REM sleep was also reversed by baclofen. These results suggest that GABA via GABA(B) receptors may play a role in the regulation of specific sleep patterns by both physical and psychological stress.

Chronic stress during paradoxical sleep deprivation increases paradoxical sleep rebound: association with prolactin plasma levels and brain serotonin content

Previous studies suggest that stress associated to sleep deprivation methods can affect the expression of sleep rebound. In order to examine this association and possible mechanisms, rats were exposed to footshock stress during or immediately after a 96-h period of paradoxical sleep deprivation (PSD) and their sleep and heart rate were recorded. Control rats (maintained in individual home cages) and paradoxical sleep-deprived (PS-deprived) rats were distributed in three conditions (1) no footshock--NF; (2) single footshock--SFS: one single footshock session at the end of the PSD period (6-8 shocks per minute; 100 ms; 2 mA; for 40 min); and (3) multiple footshock--MFS: footshock sessions with the same characteristics as described above, twice a day throughout PSD (at 7:00 h and 19:00 h) and one extra session before the recovery period. After PSD, animals were allowed to sleep freely for 72 h. Additional groups were sacrificed at the end of the sleep deprivation period for blood sampling (ACTH, corticosterone, prolactin and catecholamine levels) and brain harvesting (monoamines and metabolites). Neither SFS nor MFS produced significant alterations in the sleep patterns of control rats. All PS-deprived groups exhibited increased heart rate which could be explained by increased dopaminergic activity in the medulla. As expected, PS deprivation induced rebound of paradoxical sleep in the first day of recovery; however, PSD+MFS group showed the highest rebound (327.3% above the baseline). This group also showed intermediate levels of corticosterone and the highest levels of prolactin, which were positively correlated with the length of PS episodes. Moreover, paradoxical sleep deprivation resulted in elevation of the serotonergic turnover in the hypothalamus, which partly explained the hormonal results, and in the hippocampus, which appears to be related to adaptive responses to stress. The data are discussed in the realm of a prospective importance of paradoxical sleep for processing of traumatic events.

A double exposure to social defeat induces sub-chronic effects on sleep and open field behaviour in rats

Social defeat, resulting from the fight for a territory is based on the resident-intruder paradigm. A male rat intruder is placed in the territory of an older, bigger and more aggressive male resident and is defeated. In the present study, a double exposure to social defeat increased sleep fragmentation due to an increased amount of waking and slow-wave-sleep-1 (SWS-1) episodes. Also, social defeat increased the amount of slow-wave-sleep-2 (SWS-2). In repeated exposures to an open field, socially defeated rats showed low central activity and persistent defecation indicating high emotionality. The strongest effects of social defeat on sleep and open field behaviour were seen sub-chronically after stress. Social defeat did not induce changes in rapid eye movement (REM) sleep (e.g. total amount, latency), sleep latency, sexual activity, body weight or adrenal weight. A negative correlation between habituation in open field central activity and total sleep fragmentation indicates a commonality of effects of social defeat on both behaviour and sleep.

The relationship between anxiety and sleep-wake behavior after stressor exposure in the rat

Disturbed sleep is a common subjective complaint among individuals diagnosed with anxiety disorders. In rodents, sleep is often recorded after exposure to various foot-shock paradigms designed to induce an anxiety state. Although differences in sleep-wake architecture are noted, the relationship to specific level of anxiety is often assumed or absent. Utilizing the elevated plus-maze (EPM) after exposure to escapable shock (ES), inescapable shock (IS) or fear conditioning (FC), resulting differences in sleep architecture were compared to an objective measure of anxiety. Male Wistar rats were implanted with EEG, EMG and hippocampal theta electrodes to record sleep-wake behavior. After recovery and recording of baseline sleep, rats were exposed to one of five manipulations: ES, IS, FC or control (CES or CIS; utilizing either chamber with no shock exposure). Shortly after experimental manipulation, the EPM was employed to quantify traditional and ethological measures of anxiety and polygraphic signs of sleep-wake behavior were recorded continuously for 6 h. Although no significance was observed in EPM measurements across groups, profound differences in sleep architecture were present. Individual correlation analysis revealed no differences in anxiety level and total percentage of time spent in sleep-wake states. These results indicate that differences in sleep architecture after foot-shock exposure may not be simply due to increased anxiety. Rather, individual anxiety may be exacerbated by disrupted sleep. To fully understand the relationship between anxiety and sleep-wake behavior, a more objective analysis of anxiety after stressor exposure is mandated.

Stress-induced changes in sleep in rodents: models and mechanisms

Psychological stressors have a prominent effect on sleep in general, and rapid eye movement (REM) sleep in particular. Disruptions in sleep are a prominent feature, and potentially even the hallmark, of posttraumatic stress disorder (PTSD) (Ross, R.J., Ball, W.A., Sullivan, K., Caroff, S., 1989. Sleep disturbance as the hallmark of posttraumatic stress disorder. American Journal of Psychiatry 146, 697-707). Animal models are critical in understanding both the causes and potential treatments of psychiatric disorders. The current review describes a number of studies that have focused on the impact of stress on sleep in rodent models. The studies are also in Table 1, summarizing the effects of stress in 4-h blocks in both the light and dark phases. Although mild stress procedures have sometimes produced increases in REM sleep, more intense stressors appear to model the human condition by leading to disruptions in sleep, particularly REM sleep. We also discuss work conducted by our group and others looking at conditioning as a factor in the temporal extension of stress-related sleep disruptions. Finally, we attempt to describe the probable neural mechanisms of the sleep disruptions. A complete understanding of the neural correlates of stress-induced sleep alterations may lead to novel treatments for a variety of debilitating sleep disorders.

Sleep pattern in rats under different stress modalities

The present study was designed to evaluate the sleep pattern of rats submitted to chronic stressors (restraint, electrical footshock, swimming and cold) applied to male rats. After 48 h-baseline recording, rats were submitted to 4 days of chronic stress, and electrocorticogram recordings were carried out continuously. The stressors (footshock, swimming and cold) were applied twice a day for periods of 1 h at 9:00 and 16:00 h. Restrained animals were maintained in plastic cylinders for 22 h/day. The findings indicated that sleep efficiency, slow wave sleep (SWS) and paradoxical sleep (PS) were decreased on the third and fourth days of unpredictable shocks compared to baseline while immobilization and swimming presented reduced sleep efficiency in all 4-day recordings. Swimming led to decreased SWS, whereas augmented PS was observed on the first day compared to baseline. Immobilization produced drastic alterations in sleep patterns since it reduced SWS during the 4 days and PS at days 1 to 4 in relation to baseline. Of all stressors, cold was the only one that did not result in any statistical differences in sleep pattern during the light periods. Regarding the effect of stress compared to baseline on the dark recordings, PS was higher during cold stress periods, whereas footshock increased PS on days 2 to 4 and swimming only on day 2. Immobilization decreased PS throughout the 4 days of the stress sessions. Thus, the data suggest that different stress modalities result in distinct sleep responses, with immobilization producing the most dramatic alterations.

Altered sleep latency and arousal regulation in mice lacking norepinephrine

Latency to sleep and the amount of sensory stimulation required to awaken an animal are measures of arousal threshold, which are ultimately modulated by an arousal regulation system involving many brain areas. Among these brain areas and network connections are wake-promoting nuclei of the brainstem and their corresponding neurotransmitters, including norepinephrine (NE). In this study, we used mice that are unable to produce NE to study its role in regulating sleep latency after a variety of interventions, and to study arousal from sleep after sleep deprivation (SD). Sleep latency was measured after gentle awakening or after injections of saline, caffeine or modafinil. Sleep latency was also measured before and after partial restoration of NE pharmacologically. Arousal threshold was measured by recording the number of decibels of white noise required to wake each mouse from NREM sleep after 0, 3 and 3 + 3 h SD (3 h SD followed by < 2 min sleep, followed by an additional 3 h SD). Results showed that when mice were awakened without being touched, there were no differences in sleep latency between the genotypes. However, after an injection of saline, the control mice increased their sleep latency, whereas the NE-deficient mice did not. There were no group differences in sleep latency after treatment with either stimulant. The sleep latency difference between the genotypes was ameliorated by partial restoration of NE. The arousal threshold experiments revealed that significantly more noise was required to wake the NE-deficient mice after 3 and 3 + 3 h of SD. These findings show that mice lacking NE fall asleep more rapidly only after a mild stressor, such as an intraperitoneal injection. NE-deficient mice are also more difficult to wake up using audio stimulation after SD. The results presented here suggest that NE promotes wakefulness during transitions between sleep and wake under conditions involving mild stress and SD, but not under baseline circumstances.

A rodent model of sleep disturbances in posttraumatic stress disorder: the role of context after fear conditioning

BACKGROUND

A prominent sleep disturbance, likely including a disruption of rapid eye movement sleep (REMS) continuity, characterizes posttraumatic stress disorder (PTSD). We set out to develop a fear conditioning paradigm in rats that displays alterations in sleep architecture analogous to those in PTSD.

METHODS

Baseline polysomnographic recordings of rats were performed in a neutral context to which the rats had been habituated for several days. Rats were then shock- or mock-trained in a distinctly different context, and their sleep was studied the following day in that context. A separate group of rats was shock-trained and studied in the neutral context on the following 2 days.

RESULTS

Rats that slept in the neutral context exhibited a REMS-selective increase in sleep 24 hours after training and increases in REMS and non-REMS 48 hours after training. In contrast, rats that slept in the presence of situational reminders of the training context exhibited a REMS-selective decrease in sleep 24 hours later. Animals that were mock-trained showed no changes in sleep.

CONCLUSIONS

Shock training induced days-long changes in sleep architecture that were disrupted when the animal was exposed to situational reminders of the training context.

Behavioural changes after an acute stress: stressor and test types influences

Behavioural consequences of different acute stressors (30 min of restraint, 20 min of forced swim stress, 15 min of inescapable footshocks) applied at the beginning of the active period were assessed in using two behavioural tests: a 20 min light extinction test 24 h after the stressor exposure in order to explore the psychomotor ability and a 10 min open field session within the dark period 48 h after the stressor exposure to estimate the emotional status and the locomotor activity of the rat. Different behavioural responses were observed depending on the nature of the applied stressor. In the light extinction test, the footshock-stressed rats developed a very low activity independent on light conditions whereas the rats submitted to forced swim and restraint exhibited an activity level depending on the strain. Moreover, restrained rats had a higher transient activity than forced swim rats under light condition. In the open field test, none of the stressed rats did develop differences in behaviour. The efficacy of a 24 h recovery period on the behavioural response to an acute stressor exposure depends on the intensity of the applied stressor and the behavioural demands.

Masculine sexual activity affects slow wave sleep in Golden hamsters

The sleep pattern is modified by events occurring during wakefulness. In rats, it has been shown that male sexual behavior has a direct influence on sleeping patterns, increasing slow wave sleep (SWS) duration. On the other hand, the sexual behavior pattern of the male Golden hamster differs from the copulatory pattern of male rats. Male hamsters copulate faster and they do not display the motor inhibition observed in rats after each ejaculation. Moreover, close to exhaustion, hamsters display a behavioral pattern known as Long Intromission, which has been linked to an sexual inhibitory process. The present study was performed to determine the effects of male sexual activity on the sleep pattern in hamsters. Subjects were allowed to copulate for 30 and 60 min. In addition, the effect of locomotor activity was also assessed. The results show that male sexual behavior induced a significant increase of SWS II, with a reduction of wakefulness. No effect was observed on REM sleep. Locomotor activity produced only a slight effect on sleep. The results are discussed in terms of the similarities between the effects observed after sexual behavior on sleep in rats and hamsters, despite the substantial differences in the behavioral pattern.

Copulatory activity increases slow-wave sleep in the male rat

It is believed that sexual activity increases the need to sleep in many species. However, the relationship between copulatory activity and sleep has been poorly studied. Several studies have observed variations in the sleep of female rats and women as a function of their reproductive state. These effects have been correlated with the effects of female steroid hormones, but not with sexual activity. The aim of the present study was to evaluate the sleep-wake pattern of male rats immediately after different conditions of copulatory activity. Sexually experienced male rats were chronically implanted with a standard set of electrodes for sleep recording. After a control sleep recording of 8 h, the males were randomly assigned to one of the following experimental conditions: 30 min in the presence of an ovariectomized (OVX) rat; 30 min in the presence of an intact non-receptive female (NRF); with a receptive female until reaching one ejaculation (1E); and with a receptive female until reaching three ejaculations (3E). In addition, after 10 days, males were randomly exposed to one of the following copulatory conditions during 4 h: to remain in the presence of an OVX rat; to remain in the presence of an NRF female, and with receptive females until reaching sexual satiety (SS). Male sexual behavior was assessed just after the onset of the dark period, and sleep recordings were obtained during 8 h immediately after experimental testing. Both the three ejaculations group (3E) in the first experiment and the sexual satiety group (SS) in the second experiment showed enhanced percentages of time spent in slow wave sleep (SWS) II and a shorter latency to the first SWS II episode than in the control group or under basal conditions. In addition, neither the presence of a non-receptive female or an OVX female, nor sexual behavior until reaching one ejaculation induced any effect on the sleep stages. These findings suggest that the increase in SWS II induced by both 3E and SS may be governed by some specific mechanism that is essentially independent of physical exercise or stress. Copulatory activity might be the source of neurohormonal processes that induce sleep and may involve the participation of gamma-aminobutyric acid, serotonin or other endogenous regulators of sleep and wakefulness. Nevertheless, the precise mechanism by which the sexual behavior increases SWS is still to be determined.

Involvement of 5-HT1A receptors in homeostatic and stress-induced adaptive regulations of paradoxical sleep: studies in 5-HT1A knock-out mice

For the last two decades, the involvement of 5-HT(1A) receptors in the regulation of vigilance states has been studied extensively thanks to pharmacological tools, but clear-cut conclusion has not been reached yet. By studying mutant mice that do not express this receptor type (5-HT(1A)-/-) and their wild-type 129/Sv counterparts, we herein demonstrate that 5-HT(1A) receptors play key roles in the control of spontaneous sleep-wakefulness cycles, as well as in homeostatic regulation and stress-induced adaptive changes of paradoxical sleep. Both strains of mice exhibited a diurnal sleep-wakefulness rhythm, but 5-HT(1A)-/- animals expressed higher amounts of paradoxical sleep than wild-type mice during both the light and the dark phases. In wild-type mice, pharmacological blockade of 5-HT(1A) receptors by WAY 100635 (0.5 mg/kg, i.p.) promoted paradoxical sleep, whereas the 5-HT(1A) agonist 8-OH-DPAT (0.25-1 mg/kg, s.c.) had an opposite effect. In contrast, none of the 5-HT(1A) receptor ligands affected sleep significantly in 5-HT(1A)-/- mice. However, 5-HT(1B) receptor stimulation by CP 94253 (1-3 mg/kg, i.p.) induced a reduction in paradoxical sleep in both strains, this effect being more pronounced in 5-HT(1A)-/- mutants. Finally, in contrast to wild-type mice, 5-HT(1A)-/- mutants did not exhibit any rebound of paradoxical sleep after either a 9 hr instrumental paradoxical sleep deprivation or a 90 min immobilization stress. Altogether, these data indicate that, in the mouse, 5-HT(1A) receptors participate in the spontaneous and homeostatic regulation, as well as in stress-induced adaptive changes of paradoxical sleep.

The effect of restraint stress on paradoxical sleep is influenced by the circadian cycle

It is well known that the physiological impact imposed by events or behaviors displayed during the waking period determines the way organisms sleep. Among the situations known to affect sleep both in its duration and quality, stress has been widely studied and it is now admitted that its effects on sleep architecture depend on several factors specific to the stressor or the individual itself. Although numerous reports have highlighted the prominent role of the circadian cycle in the physiological, endocrine and behavioral consequences of restraint stress, a possible circadian influence in the effects of stress on the sleep-wake cycle has never been studied. Thus the present study was designed to compare the effects on sleep of a 1 h-lasting restraint stress applied at light onset to those observed after the same stressor was applied at light offset. We report that in both conditions stress induced a marked paradoxical sleep increase, whereas wakefulness displayed a moderate decrease and slow wave sleep a moderate augmentation. Although the effects of stress at lights on were of similar magnitude than those of stress at lights off, important differences in the sleep rebound latencies were observed: whatever the time of day the stress was applied, its effects on sleep always occurred during the dark period. This result thus shows that restraint stress could be efficiently used to study the interaction between the circadian and homeostatic components of sleep regulation.

Sleeping brain, learning brain. The role of sleep for memory systems

The hypothesis that sleep participates in the consolidation of recent memory traces has been investigated using four main paradigms: (1) effects of post-training sleep deprivation on memory consolidation, (2) effects of learning on post-training sleep, (3) effects of within sleep stimulation on the sleep pattern and on overnight memories, and (4) re-expression of behavior-specific neural patterns during post-training sleep. These studies convincingly support the idea that sleep is deeply involved in memory functions in humans and animals. However, the available data still remain too scarce to confirm or reject unequivocally the recently upheld hypothesis that consolidations of non-declarative and declarative memories are respectively dependent upon REM and NREM sleep processes.