Maturation of sleep homeostasis in developing rats: a role for preoptic area neurons

Ontogenesis of the molecular response to sleep loss

Sleep deprivation (SD) results in profound cellular and molecular changes in the adult mammalian brain. Some of these changes may result in, or aggravate, brain disease. However, little is known about how SD impacts gene expression in developing animals. We examined the transcriptional response in the prefrontal cortex (PFC) to SD across postnatal development in male mice. We used RNA sequencing to identify functional gene categories that were specifically impacted by SD. We find that SD has dramatically different effects on PFC genes depending on developmental age. Gene expression differences after SD fall into 3 categories: present at all ages (conserved), present when mature sleep homeostasis is first emerging, and those unique to certain ages. Developmentally conserved gene expression was limited to a few functional categories, including Wnt-signaling which suggests that this pathway is a core mechanism regulated by sleep. In younger ages, genes primarily related to growth and development are affected while changes in genes related to metabolism are specific to the effect of SD in adults.

Ontogenesis of the molecular response to sleep loss

Sleep deprivation (SD) results in profound cellular and molecular changes in the adult mammalian brain. Some of these changes may result in, or aggravate, brain disease. However, little is known about how SD impacts gene expression in developing animals. We examined the transcriptional response in the prefrontal cortex (PFC) to SD across postnatal development in male mice. We used RNA sequencing to identify functional gene categories that were specifically impacted by SD. We find that SD has dramatically different effects on PFC genes depending on developmental age. Gene expression differences after SD fall into 3 categories: present at all ages (conserved), present when mature sleep homeostasis is first emerging, and those unique to certain ages in adults. Developmentally conserved gene expression was limited to a few functional categories, including Wnt-signaling which suggests that this pathway is a core mechanism regulated by sleep. In younger ages, genes primarily related to growth and development are affected while changes in genes related to metabolism are specific to the effect of SD in adults.

Development of network oscillations through adolescence in male and female rats

The primary aim of this research was to study the developmental trajectory of oscillatory synchronization in neural networks of normal healthy rats during adolescence, corresponding to the vulnerable age of schizophrenia prodrome in human. To monitor the development of oscillatory networks through adolescence we used a "pseudo-longitudinal" design. Recordings were performed in terminal experiments under urethane anesthesia, every day from PN32 to PN52 using rats-siblings from the same mother, to reduce individual innate differences between subjects. We found that hippocampal theta power decreased and delta power in prefrontal cortex increased through adolescence, indicating that the oscillations in the two different frequency bands follow distinct developmental trajectories to reach the characteristic oscillatory activity found in adults. Perhaps even more importantly, theta rhythm showed age-dependent stabilization toward late adolescence. Furthermore, sex differences was found in both networks, more prominent in the prefrontal cortex compared with hippocampus. Delta increase was stronger in females and theta stabilization was completed earlier in females, in postnatal days PN41-47, while in males it was only completed in late adolescence. Our finding of a protracted maturation of theta-generating networks in late adolescence is overall consistent with the findings of longitudinal studies in human adolescents, in which oscillatory networks demonstrated a similar pattern of maturation.

The Ontogenesis of Mammalian Sleep: Form and Function

PURPOSE OF REVIEW

To present an up-to-date review and synthesis of findings about perinatal sleep development and function. I discuss landmark events in sleep ontogenesis, evidence that sleep promotes brain development and plasticity, and experimental considerations in this topic.

RECENT FINDINGS

Mammalian sleep undergoes dramatic changes in expression and regulation during perinatal development. This includes a progressive decrease in rapid-eye-movement (REM) sleep time, corresponding increases in nonREM sleep and wake time, and the appearance of mature sleep regulatory processes (homeostatic and circadian). These developmental events coincide with periods of rapid brain maturation and heightened synaptic plasticity. The latter involve an initial experience-independent phase, when circuit development is guided by spontaneous activity, and later occurring critical periods, when these circuits are shaped by experience.

SUMMARY

These ontogenetic changes suggest important interactions between sleep and brain development. More specifically, sleep may promote developmental programs of synaptogenesis and synaptic pruning and influence the opening and closing of critical periods of brain plasticity.

Role of Ovarian Hormones in the Modulation of Sleep in Females Across the Adult Lifespan

Ovarian hormones, including 17β-estradiol, are implicated in numerous physiological processes, including sleep. Beginning at puberty, girls report more sleep complaints than boys, which is maintained throughout the reproductive life stage. Sleep problems are exacerbated during the menopausal transition, evidenced by greater risk for sleep disorders. There is emerging evidence that menopause-associated hormone loss contributes to this elevated risk, but age is also an important factor. The extent to which menopause-associated sleep disturbance persists into postmenopause above and beyond the effects of age remains unknown. Untreated sleep disturbances have important implications for cognitive health, as they are emerging as risk factors for dementia. Given that sleep loss impairs memory, an important knowledge gap concerns the role played by menopause-associated hormone loss in exacerbating sleep disturbance and, ultimately, cognitive function in aging women. In this review, we take a translational approach to illustrate the contribution of ovarian hormones in maintaining the sleep-wake cycle in younger and middle-aged females, with evidence implicating 17β-estradiol in supporting the memory-promoting effects of sleep. Sleep physiology is briefly reviewed before turning to behavioral and neural evidence from young females linking 17β-estradiol to sleep-wake cycle maintenance. Implications of menopause-associated 17β-estradiol loss is also reviewed before discussing how ovarian hormones may support the memory-promoting effects of sleep, and why menopause may exacerbate pathological aging via effects on sleep. While still in its infancy, this research area offers a new sex-based perspective on aging research, with a focus on a modifiable risk factor for pathological aging.

Melanin-concentrating hormone (MCH) neurons in the developing chick brain

The present study was undertaken because no previous developmental studies exist on MCH neurons in any avian species. After validating a commercially-available antibody for use in chickens, immunohistochemical examinations first detected MCH neurons around embryonic day (E) 8 in the posterior hypothalamus. This population increased thereafter, reaching a numerical maximum by E20. MCH-positive cell bodies were found only in the posterior hypothalamus at all ages examined, restricted to a region showing very little overlap with the locations of hypocretin/orexin (H/O) neurons. Chickens had fewer MCH than H/O neurons, and MCH neurons also first appeared later in development than H/O neurons (the opposite of what has been found in rodents). MCH neurons appeared to originate from territories within the hypothalamic periventricular organ that partially overlap with the source of diencephalic serotonergic neurons. Chicken MCH fibers developed exuberantly during the second half of embryonic development, and they became abundant in the same brain areas as in rodents, including the hypothalamus (by E12), locus coeruleus (by E12), dorsal raphe nucleus (by E20) and septum (by E20). These observations suggest that MCH cells may play different roles during development in chickens and rodents; but once they have developed, MCH neurons exhibit similar phenotypes in birds and rodents.

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

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

Development of Circadian Sleep Regulation in the Rat: A Longitudinal Study Under Constant Conditions

Study Objectives

To better understand the development of sleep, we characterized the development of circadian rhythms in sleep and wakefulness in the artificially-reared, isolated rat pup using an experimental design that minimized the effects of maternal separation.

Methods

Neonatal rats were reared in constant conditions (dim red light) while electroencephalographic and electromyographic signals were continuously recorded for up to 3 weeks. This time period spanned the preweaned and weaned ages. The distribution of sleep-wake states was analyzed to estimate the emergence of circadian rhythms.

Results

Overt ~24-hour rhythms in time spent awake and asleep appear by postnatal day (P)17. A marked bi-modal sleep-wake pattern was also observed, evidenced by the appearance of a pronounced ~12-hour component in the periodogram over the subsequent 3 days (P17-P21). This suggested the presence of two ~24-hour components consistent with the dual-oscillator concept. During this 3-day time window, waking bouts became longer resulting in a repartition of the duration of intervals without non-rapid-eye movement (NREM) sleep into short (<30 minutes) and longer inter-NREM sleep episodes. These longer waking bouts did not immediately result in an increase in NREM sleep delta (0.5-4.0 Hz) power, which is an index of sleep homeostasis in adult mammals. The sleep homeostatic response did not fully mature until P25.

Conclusions

These results demonstrate that the maturation of circadian organization of sleep-wake behavior precedes the expression of mature sleep homeostasis.

Sleep/wake movement velocities, trajectories and micro-arousals during maturation in rats

Background

Sleep is regulated by two main processes. The circadian process provides a 24-h rhythm and the homeostatic process reflects sleep pressure, which increases in the course of wakefulness and decreases during sleep. Both of these processes undergo major changes during development. For example, sleep homeostasis, measured by means of electroencephalogram (EEG) slow-wave activity (SWA, EEG power between 0.5 and 4.5 Hz), peaks around puberty and decreases during adolescence. In humans and rats these changes have been related to cortical maturation. We aimed to explore whether additional parameters as state dynamic (dynamic of sleep/wake behavior) parameters of movement velocity, trajectories and micro-arousals provide markers of rat maturation. The state dynamics reflect the stability of sleep within a specific sleep stage. We applied a state space technique (SST), a quantitative and unbiased method, based on frequency band ratios of the EEG to analyze the development of different sleep/wake states and state dynamics between vigilance states. EEG of recording electrodes at the frontal and parietal lobe were analyzed using conventional scoring criteria and SST.

Results

We found that movement velocity, trajectories between sleep states and micro-arousals changed as an inverse U-shaped curve across maturation. At all ages, movement velocity over the frontal lobe is higher compared to the parietal lobe, while the number of trajectories and micro-arousals are reduced. Furthermore, we showed that SWA correlates negatively with movement velocity and the number of micro-arousals. The velocity in the parietal lobe correlates positively with the number of micro-arousals. As for SWA, trajectories seem primarily to depend on sleep homeostasis regulatory mechanisms while the movement velocity seems to be modulated by other sleep regulators like the circadian rhythms.

Conclusions

New insights in sleep/wake state dynamics are established with the SST, because trajectories, micro-arousals and velocities are not evident by traditional scoring methods. These dynamic measures may represent new indicators for changes in sleep regulatory processes across maturation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12868-017-0343-6) contains supplementary material, which is available to authorized users.

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.

An Adenosine-Mediated Glial-Neuronal Circuit for Homeostatic Sleep

Sleep homeostasis reflects a centrally mediated drive for sleep, which increases during waking and resolves during subsequent sleep. Here we demonstrate that mice deficient for glial adenosine kinase (AdK), the primary metabolizing enzyme for adenosine (Ado), exhibit enhanced expression of this homeostatic drive by three independent measures: (1) increased rebound of slow-wave activity; (2) increased consolidation of slow-wave sleep; and (3) increased time constant of slow-wave activity decay during an average slow-wave sleep episode, proposed and validated here as a new index for homeostatic sleep drive. Conversely, mice deficient for the neuronal adenosine A1 receptor exhibit significantly decreased sleep drive as judged by these same indices. Neuronal knock-out of AdK did not influence homeostatic sleep need. Together, these findings implicate a glial-neuronal circuit mediated by intercellular Ado, controlling expression of homeostatic sleep drive. Because AdK is tightly regulated by glial metabolic state, our findings suggest a functional link between cellular metabolism and sleep homeostasis.

SIGNIFICANCE STATEMENT

The work presented here provides evidence for an adenosine-mediated regulation of sleep in response to waking (i.e., homeostatic sleep need), requiring activation of neuronal adenosine A1 receptors and controlled by glial adenosine kinase. Adenosine kinase acts as a highly sensitive and important metabolic sensor of the glial ATP/ADP and AMP ratio directly controlling intracellular adenosine concentration. Glial equilibrative adenosine transporters reflect the intracellular concentration to the extracellular milieu to activate neuronal adenosine receptors. Thus, adenosine mediates a glial-neuronal circuit linking glial metabolic state to neural-expressed sleep homeostasis. This indicates a metabolically related function(s) for this glial-neuronal circuit in the buildup and resolution of our need to sleep and suggests potential therapeutic targets more directly related to sleep function.

The homeostatic regulation of REM sleep: A role for localized expression of brain-derived neurotrophic factor in the brainstem

Homeostatic regulation of REM sleep plays a key role in neural plasticity and deficits in this process are implicated in the development of many neuropsychiatric disorders. Little is known, however, about the molecular mechanisms that underlie this homeostatic regulation process. This study examined the hypothesis that, during selective REM sleep deprivation (RSD), increased brain-derived neurotrophic factor (BDNF) expression in REM sleep regulating areas is critical for the development of homeostatic drive for REM sleep, as measured by an increase in the number of REM sleep transitions. Rats were assigned to RSD, non-sleep deprived (BSL), or total sleep deprivation (TSD) groups. Physiological recordings were obtained from cortical, hippocampal, and pontine EEG electrodes over a 6h period, in which sleep deprivation occurred during the first 3h. In the RSD, but not the other conditions, homeostatic drive for REM sleep increased progressively. BDNF protein expression was significantly greater in the pedunculopontine tegmentum (PPT) and subcoeruleus nucleus (SubCD) in the RSD as compared to the TSD and BSL groups, areas that regulate REM sleep, but not in the medial preoptic area, which regulates non-REM sleep. There was a significant positive correlation between RSD-induced increases in number of REM sleep episodes and increased BDNF expression in the PPT and SubCD. These increases positively correlated with levels of homeostatic drive for REM sleep. These results, for the first time, suggest that selective RSD-induced increased expression of BDNF in the PPT and SubCD are determinant factors in the development of the homeostatic drive for REM sleep.

Differential effects of duration of sleep fragmentation on spatial learning and synaptic plasticity in pubertal mice

STUDY OBJECTIVE

To examine the differential effects of acute and chronic sleep fragmentation (SF) on spatial learning and memory, and hippocampal long-term potentiation (LTP) in pubertal mice.

METHODS

Two studies were performed during which adolescent C57/Bl6 mice were subjected to acute-SF 24h a day × 3 days or chronic-SF for 12h a day × 2 weeks using a programmable rotating lever that provides tactile stimulus with controls housed in similar cages. Spatial learning and memory was examined using the Morris water maze, and long-term potentiation (LTP) was evaluated after stimulation of Schaffer collaterals in CA1 hippocampus post SF. Actigraphy was used during the period of SF to monitor rest-activity patterns. Electroencephalographic (EEG) recordings were acquired for analysis of vigilance state patterns and delta-power. Serum corticosterone was measured to assess stress levels.

RESULTS

Acute-SF via tactile stimulation negatively impacted spatial learning, as well as LTP maintenance, compared to controls with no tactile stimulation. While actigraphy showed significantly increased motor activity during SF in both groups, EEG data indicated that overall sleep efficiency did not differ between baseline and SF days, but significant increases in number of wakeful bouts and decreases in average NREM and REM bout lengths were seen during lights-on. Acute sleep fragmentation did not impact corticosterone levels.

CONCLUSIONS

The current results indicate that, during development in pubertal mice, acute-SF for 24h a day × 3 days negatively impacted spatial learning and synaptic plasticity. Further studies are needed to determine if any inherent long-term homeostatic mechanisms in the adolescent brain afford greater resistance to the deleterious effects of chronic-SF.

Cortical neuronal activity does not regulate sleep homeostasis

The neural substrate of sleep homeostasis is unclear, but both cortical and subcortical structures are thought to be involved in sleep regulation. To test whether prior neuronal activity in the cortex or in subcortical regions drives sleep rebound, we systemically administered atropine (100mg/kg) to rats, producing a dissociated state with slow-wave cortical electroencephalogram (EEG) but waking behavior (e.g. locomotion). Atropine injections during the light period produced 6h of slow-wave cortical EEG but also subcortical arousal. Afterward, rats showed a significant increase in non-rapid eye movement (NREM) sleep, compared to the same period on a baseline day. Consistent with the behavioral and cortical EEG state produced by systemic atropine, c-Fos expression was low in the cortex but high in multiple subcortical arousal systems. These data suggest that subcortical arousal and behavior are sufficient to drive sleep homeostasis, while a sleep-like pattern of cortical activity is not sufficient to satisfy sleep homeostasis.

The truncated TrkB receptor influences mammalian sleep

Brain-derived neurotrophic factor (BDNF) is a neurotrophin hypothesized to play an important role in mammalian sleep expression and regulation. In order to investigate the role of the truncated receptor for BDNF, TrkB.T1, in mammalian sleep, we examined sleep architecture and sleep regulation in adult mice constitutively lacking this receptor. We find that TrkB.T1 knockout mice have increased REM sleep time, reduced REM sleep latency, and reduced sleep continuity. These results demonstrate a novel role for the TrkB.T1 receptor in sleep expression and provide new insights into the relationship between BDNF, psychiatric illness, and sleep.

Sleep patterns and homeostatic mechanisms in adolescent mice

Sleep changes were studied in mice (n = 59) from early adolescence to adulthood (postnatal days P19–111). REM sleep declined steeply in early adolescence, while total sleep remained constant and NREM sleep increased slightly. Four hours of sleep deprivation starting at light onset were performed from ages P26 through adulthood (>P60). Following this acute sleep deprivation all mice slept longer and with more consolidated sleep bouts, while NREM slow wave activity (SWA) showed high interindividual variability in the younger groups, and increased consistently only after P42. Three parameters together explained up to 67% of the variance in SWA rebound in frontal cortex, including weight-adjusted age and increase in alpha power during sleep deprivation, both of which positively correlated with the SWA response. The third, and strongest predictor was the SWA decline during the light phase in baseline: mice with high peak SWA at light onset, resulting in a large SWA decline, were more likely to show no SWA rebound after sleep deprivation, a result that was also confirmed in parietal cortex. During baseline, however, SWA showed the same homeostatic changes in adolescents and adults, declining in the course of sleep and increasing across periods of spontaneous wake. Thus, we hypothesize that, in young adolescent mice, a ceiling effect and not the immaturity of the cellular mechanisms underlying sleep homeostasis may prevent the SWA rebound when wake is extended beyond its physiological duration.

Adolescent sleep patterns in humans and laboratory animals

This article is part of a Special Issue "Puberty and Adolescence". One of the defining characteristics of adolescence in humans is a large shift in the timing and structure of sleep. Some of these changes are easily observable at the behavioral level, such as a shift in sleep patterns from a relatively morning to a relatively evening chronotype. However, there are equally large changes in the underlying architecture of sleep, including a >60% decrease in slow brain wave activity, which may reflect cortical pruning. In this review we examine the developmental forces driving adolescent sleep patterns using a cross-species comparison. We find that behavioral and physiological sleep parameters change during adolescence in non-human mammalian species, ranging from primates to rodents, in a manner that is often hormone-dependent. However, the overt appearance of these changes is species-specific, with polyphasic sleepers, such as rodents, showing a phase-advance in sleep timing and consolidation of daily sleep/wake rhythms. Using the classic two-process model of sleep regulation, we demonstrate via a series of simulations that many of the species-specific characteristics of adolescent sleep patterns can be explained by a universal decrease in the build-up and dissipation of sleep pressure. Moreover, and counterintuitively, we find that these changes do not necessitate a large decrease in overall sleep need, fitting the adolescent sleep literature. We compare these results to our previous review detailing evidence for adolescent changes in the regulation of sleep by the circadian timekeeping system (Hagenauer and Lee, 2012), and suggest that both processes may be responsible for adolescent sleep patterns.