Short-term homeostasis of active sleep and the architecture of sleep in the rat

A predictive propensity measure to enter REM sleep

Introduction

During sleep periods, most mammals alternate multiple times between rapid-eye-movement (REM) sleep and non-REM (NREM) sleep. A common theory proposes that these transitions are governed by an "hourglass-like" homeostatic need to enter REM sleep that accumulates during the inter-REM interval and partially discharges during REM sleep. However, markers or mechanisms for REM homeostatic pressure remain undetermined. Recently, an analysis of sleep in mice demonstrated that the cumulative distribution function (CDF) of the amount of NREM sleep between REM bouts correlates with REM bout duration, suggesting that time in NREM sleep influences REM sleep need. Here, we build on those results and construct a predictive measure for the propensity to enter REM sleep as a function of time in NREM sleep since the previous REM episode.

Methods

The REM propensity measure is precisely defined as the probability to enter REM sleep before the accumulation of an additional pre-specified amount of NREM sleep.

Results

Analyzing spontaneous sleep in mice, we find that, as NREM sleep accumulates between REM bouts, the REM propensity exhibits a peak value and then decays to zero with further NREM accumulation. We show that the REM propensity at REM onset predicts features of the subsequent REM bout under certain conditions. Specifically, during the light phase and for REM propensities occurring before the peak in propensity, the REM propensity at REM onset is correlated with REM bout duration, and with the probability of the occurrence of a short REM cycle called a sequential REM cycle. Further, we also find that proportionally more REM sleep occurs during sequential REM cycles, supporting a correlation between high values of our REM propensity measure and high REM sleep need.

Discussion

These results support the theory that a homeostatic need to enter REM sleep accrues during NREM sleep, but only for a limited range of NREM sleep accumulation.

Orexin Neurons to Sublaterodorsal Tegmental Nucleus Pathway Prevents Sleep Onset REM Sleep-Like Behavior by Relieving the REM Sleep Pressure

Proper timing of vigilance states serves fundamental brain functions. Although disturbance of sleep onset rapid eye movement (SOREM) sleep is frequently reported after orexin deficiency, their causal relationship still remains elusive. Here, we further study a specific subgroup of orexin neurons with convergent projection to the REM sleep promoting sublaterodorsal tegmental nucleus (OXSLD neurons). Intriguingly, although OXSLD and other projection-labeled orexin neurons exhibit similar activity dynamics during REM sleep, only the activation level of OXSLD neurons exhibits a significant positive correlation with the post-inter-REM sleep interval duration, revealing an essential role for the orexin-sublaterodorsal tegmental nucleus (SLD) neural pathway in relieving REM sleep pressure. Monosynaptic tracing reveals that multiple inputs may help shape this REM sleep-related dynamics of OXSLD neurons. Genetic ablation further shows that the homeostatic architecture of sleep/wakefulness cycles, especially avoidance of SOREM sleep-like transition, is dependent on this activity. A positive correlation between the SOREM sleep occurrence probability and depression states of narcoleptic patients further demonstrates the possible significance of the orexin-SLD pathway on REM sleep homeostasis.

SleepInvestigatoR: A flexible R function for analyzing scored sleep in rodents

Analyzing scored sleep is a fundamental prerequisite to understanding how sleep changes between health and disease. Classically, this is accomplished by manually calculating various measures (e.g., percent of non-rapid eye movement sleep) from a collection of scored sleep files. This process can be tedious and error prone especially when studies include a large number of animals or involve long recording sessions. To address this issue, we present SleepInvestigatoR, a versatile tool that can quickly organize and analyze multiple scored sleep files into a single output. The function is written in the open-source statistical language R and has a total of 25 parameters that can be set to match a wide variety of experimenter needs. SleepInvestigatoR delivers a total of 22 unique measures of sleep, including all measures commonly reported in the rodent literature. A simple plotting function is also provided to quickly graph and visualize the scored data. All code is designed to be implemented with little formal coding knowledge and step-by-step instructions are provided on the corresponding GitHub page. Overall, SleepInvestigatoR provides the sleep researcher a critical tool to increase efficiency, interpretation, and reproducibility in analyzing scored rodent sleep.

Ultradian sleep cycles: Frequency, duration, and associations with individual and environmental factors-A retrospective study

OBJECTIVE

Sleep varies between individuals in response to sleep-wake history and various environmental factors, including light and noise. Here we report on the intranight variation of the ultradian nonrapid eye movement-rapid eye movement (NREM-REM) sleep cycle in 369 participants who have contributed to different laboratory studies from 1994 to 2020 at the Centre for Chronobiology, Basel, Switzerland.

RESULTS

We observed a large interindividual variability in sleep cycle duration, including NREM and REM sleep episodes in healthy participants who were given an 8-hour sleep opportunity at habitual bedtime in controlled laboratory settings. The median sleep cycle duration was 96 minutes out of 6064 polysomnographically-recorded cycles. The number and duration of cycles were not normally distributed, and the distribution became narrower for NREM sleep and wider for REM sleep later in the night. The first cycle was consistently shorter than subsequent cycles, and moderate presleep light or nocturnal noise exposure had no significant effects on ultradian sleep cycle duration. Age and sex significantly affected NREM and REM sleep duration, with older individuals having longer NREM and shorter REM sleep particularly in the end of the night, and females having longer NREM sleep episodes. High sleep pressure (ie, sleep deprivation) and low sleep pressure (ie, multiple naps) altered ultradian sleep cycles, with high sleep pressure leading to longer NREM sleep in the first cycle, and low sleep pressure leading to longer REM sleep episodes. Positive correlations were observed between N2 and NREM duration, and between N1 and REM duration. Weak intrasleep REM sleep homeostasis was also evident in our data set.

CONCLUSIONS

We conclude that ultradian sleep cycles are endogenous biological rhythms modulated by age, sex, and sleep homeostasis, but not directly responsive to (moderate levels of) environmental cues in healthy good sleepers.

Is paradoxical sleep setting up innate and acquired complex sensorimotor and adaptive behaviours?: A proposed function based on literature review

We summarize here the progress in identifying the neuronal network as well as the function of paradoxical sleep and the gaps of knowledge that should be filled in priority. The core system generating paradoxical sleep localized in the brainstem is now well identified, and the next step is to clarify the role of the forebrain in particular that of the hypothalamus including the melanin-concentrating hormone neurons and of the basolateral amygdala. We discuss these two options, and also the discovery that cortical activation during paradoxical sleep is restricted to a few limbic cortices activated by the lateral supramammillary nucleus and the claustrum. Such activation nicely supports the findings recently obtained showing that neuronal reactivation occurs during paradoxical sleep in these structures, and induces both memory consolidation of important memory and forgetting of less relevant ones. The question that still remains to be answered is whether paradoxical sleep is playing more crucial roles in processing emotional and procedural than other types of memories. One attractive hypothesis is that paradoxical sleep is responsible for erasing negative emotional memories, and that this function is not properly functioning in depressed patients. On the other hand, the presence of a muscle atonia during paradoxical sleep is in favour of a role in procedural memory as new types of motor behaviours can be tried without harm during the state. In a way, it also fits with the proposed role of paradoxical sleep in setting up the sensorimotor system during development.

Regulation of REM sleep by inhibitory neurons in the dorsomedial medulla

The two major stages of mammalian sleep-rapid eye movement sleep (REMs) and non-REM sleep (NREMs)-are characterized by distinct brain rhythms ranging from millisecond to minute-long (infraslow) oscillations. The mechanisms controlling transitions between sleep stages and how they are synchronized with infraslow rhythms remain poorly understood. Using opto- and chemogenetic manipulation in mice, we show that GABAergic neurons in the dorsomedial medulla (dmM) promote the initiation and maintenance of REMs, in part through their projections to the dorsal and median raphe nuclei. Fiber photometry revealed that their activity is strongly increased during REMs and fluctuates during NREMs in close synchrony with infraslow oscillations in the sleep spindle band of the electroencephalogram. The phase of this rhythm influenced the latency and probability with which dmM activation induced REMs. Thus, dmM inhibitory neurons strongly promote REMs, and their slow activity fluctuations may coordinate the timing of REMs episodes with infraslow brain rhythms.

A probabilistic model for the ultradian timing of REM sleep in mice

A salient feature of mammalian sleep is the alternation between rapid eye movement (REM) and non-REM (NREM) sleep. However, how these two sleep stages influence each other and thereby regulate the timing of REM sleep episodes is still largely unresolved. Here, we developed a statistical model that specifies the relationship between REM and subsequent NREM sleep to quantify how REM sleep affects the following NREM sleep duration and its electrophysiological features in mice. We show that a lognormal mixture model well describes how the preceding REM sleep duration influences the amount of NREM sleep till the next REM sleep episode. The model supports the existence of two different types of sleep cycles: Short cycles form closely interspaced sequences of REM sleep episodes, whereas during long cycles, REM sleep is first followed by an interval of NREM sleep during which transitions to REM sleep are extremely unlikely. This refractory period is characterized by low power in the theta and sigma range of the electroencephalogram (EEG), low spindle rate and frequent microarousals, and its duration proportionally increases with the preceding REM sleep duration. Using our model, we estimated the propensity for REM sleep at the transition from NREM to REM sleep and found that entering REM sleep with higher propensity resulted in longer REM sleep episodes with reduced EEG power. Compared with the light phase, the buildup of REM sleep propensity was slower during the dark phase. Our data-driven modeling approach uncovered basic principles underlying the timing and duration of REM sleep episodes in mice and provides a flexible framework to describe the ultradian regulation of REM sleep in health and disease.

REM sleep-dependent short-term and long-term hourglass processes in the ultradian organization and recovery of REM sleep in the rat

STUDY OBJECTIVES

To evaluate the contribution of long-term and short-term REM sleep homeostatic processes to REM sleep recovery and the ultradian organization of the sleep wake cycle.

METHODS

Fifteen rats were sleep recorded under a 12:12 LD cycle. Animals were subjected during the rest phase to two protocols (2T2I or 2R2I) performed separately in non-consecutive experimental days. 2T2I consisted of 2 h of total sleep deprivation (TSD) followed immediately by 2 h of intermittent REM sleep deprivation (IRD). 2R2I consisted of 2 h of selective REM sleep deprivation (RSD) followed by 2 h of IRD. IRD was composed of four cycles of 20-min RSD intervals alternating with 10 min of sleep permission windows.

RESULTS

REM sleep debt that accumulated during deprivation (9.0 and 10.8 min for RSD and TSD, respectively) was fully compensated regardless of cumulated NREM sleep or wakefulness during deprivation. Protocol 2T2I exhibited a delayed REM sleep rebound with respect to 2R2I due to a reduction of REM sleep transitions related to enhanced NREM sleep delta-EEG activity, without affecting REM sleep consolidation. Within IRD permission windows there was a transient and duration-dependent diminution of REM sleep transitions.

CONCLUSIONS

REM sleep recovery in the rat seems to depend on a long-term hourglass process activated by REM sleep absence. Both REM sleep transition probability and REM sleep episode consolidation depend on the long-term REM sleep hourglass. REM sleep activates a short-term REM sleep refractory period that modulates the ultradian organization of sleep states.

An Asymmetrical Hypothesis for the NREM-REM Sleep Alternation-What Is the NREM-REM Cycle?

Since the discovery of rapid eye movement (REM) sleep (Aserinsky and Kleitman, 1953), sleep has been described as a succession of cycles of non-REM (NREM) and REM sleep episodes. The hypothesis of short-term REM sleep homeostasis, which is currently the basis of most credible theories on sleep regulation, is built upon a positive correlation between the duration of a REM sleep episode and the duration of the interval until the next REM sleep episode (inter-REM interval): the duration of REM sleep would therefore predict the duration of this interval. However, the high variability of inter-REM intervals, especially in polyphasic sleep, argues against a simple oscillator model. A new “asymmetrical” hypothesis is presented here, where REM sleep episodes only determine the duration of a proportional post-REM refractory period (PRRP), during which REM sleep is forbidden and the only remaining options are isolated NREM episodes or waking. After the PRRP, all three options are available again (NREM, REM, and Wake). I will explain why I think this hypothesis also calls into question the notion of NREM-REM sleep cycles.

Neural and Homeostatic Regulation of REM Sleep

Rapid eye movement (REM) sleep is a distinct, homeostatically controlled brain state characterized by an activated electroencephalogram (EEG) in combination with paralysis of skeletal muscles and is associated with vivid dreaming. Understanding how REM sleep is controlled requires identification of the neural circuits underlying its initiation and maintenance, and delineation of the homeostatic processes regulating its expression on multiple timescales. Soon after its discovery in humans in 1953, the pons was demonstrated to be necessary and sufficient for the generation of REM sleep. But, especially within the last decade, researchers have identified further neural populations in the hypothalamus, midbrain, and medulla that regulate REM sleep by either promoting or suppressing this brain state. The discovery of these populations was greatly facilitated by the availability of novel technologies for the dissection of neural circuits. Recent quantitative models integrate findings about the activity and connectivity of key neurons and knowledge about homeostatic mechanisms to explain the dynamics underlying the recurrence of REM sleep. For the future, combining quantitative with experimental approaches to directly test model predictions and to refine existing models will greatly advance our understanding of the neural and homeostatic processes governing the regulation of REM sleep.

Hippocampal theta power pressure builds over non-REM sleep and dissipates within REM sleep episodes

The theta rhythm during waking has been associated with voluntary motor activity and learning processes involving the hippocampus. Theta also occurs continuously during rapid eye movement (REM) sleep where it likely serves memory consolidation. Theta amplitude builds across wakefulness and is the best indicator of the homeostatic need for non-REM (NREM) sleep. Although REM sleep is homeostatically regulated independently of NREM sleep, the drivers of REM sleep regulation are under debate. The dynamics of theta within REM sleep bouts have not been thoroughly explored. We equipped 20 male rats with sleep instrumentation and hippocampal electrodes to measure theta across normal sleep/waking periods over the first 4 h of the sleep phase on two consecutive days. We found that theta power decreased by a third, on average, within individual REM sleep bouts, but recovered between bouts. Thus, there was no general decline in theta power across the duration of the recording period or between days. The time constant of theta power decline within a REM sleep bout was the same whether the bout was short, midlength, or long, and did not predict the behavioral state immediately following the REM sleep bout. Interestingly, the more time spent in NREM sleep prior to REM sleep, the larger the decline in theta power during REM sleep, indicating that REM sleep theta may be homeostatically driven by NREM sleep just as NREM delta power is driven by the length of prior waking and by waking theta. Potential causes and implications for this phenomenon are discussed.

Changes in Brain-Derived Neurotrophic Factor Expression Influence Sleep-Wake Activity and Homeostatic Regulation of Rapid Eye Movement Sleep

Study Objectives

Brain-derived neurotrophic factor (BDNF) expression and homeostatic regulation of rapid eye movement (REM) sleep are critical for neurogenesis and behavioral plasticity. Accumulating clinical and experimental evidence suggests that decreased BDNF expression is causally linked with the development of REM sleep-associated neuropsychiatric disorders. Therefore, we hypothesize that BDNF plays a role in sleep–wake (S–W) activity and homeostatic regulation of REM sleep.

Methods

Male and female wild-type (WT; BDNF +/+) and heterozygous BDNF (KD; BDNF +/−) rats were chronically implanted with S–W recording electrodes to quantify baseline S–W activity and REM sleep homeostatic regulatory processes during the light phase.

Results

Molecular analyses revealed that KD BDNF rats had a 50% decrease in BDNF protein levels. During baseline S–W activity, KD rats exhibited fewer REM sleep episodes that were shorter in duration and took longer to initiate. Also, the baseline S–W activity did not reveal any sex difference. During the 3-hour selective REM sleep deprivation, KD rats failed to exhibit a homeostatic drive for REM sleep and did not exhibit rebound REM sleep during the recovery S–W period.

Conclusion

Interestingly, both genotypes did not reveal any sex difference in the quality and/or quantity of REM sleep. Collectively, these results, for the first time, unequivocally demonstrate that an intact BDNF system in both sexes is a critical modulator for baseline and homeostatic regulation of REM sleep. This study further suggests that heterozygous BDNF knockdown rats are a useful animal model for the study of the cellular and molecular mechanisms of sleep regulation and cognitive functions of sleep.

Social partnering alters sleep in fear-conditioned Wistar rats

Social support, when provided following a traumatic experience, is associated with a lower incidence of stress-related psychiatric disorders. Our hypothesis was that providing a social interaction period with a naive conspecific would improve sleep architecture in response to cued fear conditioning in Wistar rats. Rats were randomly assigned to either the socially isolated or socially partnered groups. Rats assigned to the socially isolated group were individually housed following electrode implantation and fear conditioning. Rats assigned to the socially partnered group were initially paired-housed, and then one rat from each pair was randomly chosen for sleep electrode implantation and fear conditioning. Rats from both groups were habituated to a recording chamber, and baseline sleep was recorded over 22 hours. One day later (Training Day), they were fear-conditioned to 10 presentations of a tone (800 Hz, 90 dB, 5 sec) co-terminating with a mild electric foot shock (1.0 mA, 0.5 sec), at 30-sec intervals. While rats in the socially isolated group were left undisturbed in their home cage for 30-min, socially partnered rats interacted for 30 minutes with their non-stressed rat partner immediately after fear conditioning and while the auditory tones were presented on Days 1 and 14. The results indicated that social interaction increased sleep efficiency in partnered rats compared to isolated rats following the fear conditioning procedure. This was due to an increase in the amount of rapid eye movement sleep (REMS) during the light phase. Evaluation of REMS microarchitecture revealed that the increase in REMS was due to an increase in the number of single REMS episodes (siREMS), which represented a more consolidated REMS pattern. A surprising finding was that partnered rats had a greater number of sequential REMS episodes (seqREMS) at Baseline, on the Training Day and on Day 1 when compared to isolated rats. The greater number of seqREMS episodes in partnered rats may be due to the partnering procedure and not fear conditioning, as the effect was also seen at Baseline. Thus it appears that while the partnering procedure may have given rise to a fragmented REMS pattern, social partnering promoted a greater consolidation of REMS in response to the fear conditioning procedure.

Cellular and Molecular Mechanisms of REM Sleep Homeostatic Drive: A Plausible Component for Behavioral Plasticity

Homeostatic regulation of REM sleep drive, as measured by an increase in the number of REM sleep transitions, plays a key role in neuronal and behavioral plasticity (i.e., learning and memory). Deficits in REM sleep homeostatic drive (RSHD) are implicated in the development of many neuropsychiatric disorders. Yet, the cellular and molecular mechanisms underlying this RSHD remain to be incomplete. To further our understanding of this mechanism, the current study was performed on freely moving rats to test a hypothesis that a positive interaction between extracellular-signal-regulated kinase 1 and 2 (ERK1/2) activity and brain-derived neurotrophic factor (BDNF) signaling in the pedunculopontine tegmentum (PPT) is a causal factor for the development of RSHD. Behavioral results of this study demonstrated that a short period (<90 min) of selective REM sleep restriction (RSR) exhibited a strong RSHD. Molecular analyses revealed that this increased RSHD increased phosphorylation and activation of ERK1/2 and BDNF expression in the PPT. Additionally, pharmacological results demonstrated that the application of the ERK1/2 activation inhibitor U0126 into the PPT prevented RSHD and suppressed BDNF expression in the PPT. These results, for the first time, suggest that the positive interaction between ERK1/2 and BDNF in the PPT is a casual factor for the development of RSHD. These findings provide a novel direction in understanding how RSHD-associated specific molecular changes can facilitate neuronal plasticity and memory processing.

Activation of brain-derived neurotrophic factor-tropomyosin receptor kinase B signaling in the pedunculopontine tegmental nucleus: a novel mechanism for the homeostatic regulation of rapid eye movement sleep

Rapid eye movement (REM) sleep dysregulation is a symptom of many neuropsychiatric disorders, yet the mechanisms of REM sleep homeostatic regulation are not fully understood. We have shown that, after REM sleep deprivation, the pedunculopontine tegmental nucleus (PPT) plays a critical role in the generation of recovery REM sleep. In this study, we used multidisciplinary techniques to show a causal relationship between brain-derived neurotrophic factor (BDNF)-tropomyosin receptor kinase B (TrkB) signaling in the PPT and the development of REM sleep homeostatic drive. Rats were randomly assigned to conditions of unrestricted sleep or selective REM sleep deprivation (RSD) with PPT microinjections of vehicle control or a dose of a TrkB receptor inhibitor (2, 3, or 4 nmol K252a or 4 nmol ANA-12). On experimental days, rats received PPT microinjections and their sleep-wake physiological signals were recorded for 3 or 6 h, during which selective RSD was performed in the first 3 h. At the end of all 3 h recordings, rats were killed and the PPT was dissected out for BDNF quantification. Our results show that K252a and ANA-12 dose-dependently reduced the homeostatic responses to selective RSD. Specifically, TrkB receptor inhibition reduced REM sleep homeostatic drive and limited REM sleep rebound. There was also a dose-dependent suppression of PPT BDNF up-regulation, and regression analysis revealed a significant positive relationship between REM sleep homeostatic drive and the level of PPT BDNF expression. These data provide the first direct evidence that activation of BDNF-TrkB signaling in the PPT is a critical step for the development of REM sleep homeostatic drive.

Significance of the zero sum principle for circadian, homeostatic and allostatic regulation of sleep-wake state in the rat

Sleep-wake behavior exhibits diurnal rhythmicity, rebound responses to acute total sleep deprivation (TSD), and attenuated rebounds following chronic sleep restriction (CSR). We investigated how these long-term patterns of behavior emerge from stochastic short-term dynamics of state transition. Male Sprague-Dawley rats were subjected to TSD (1day×24h, N=9), or CSR (10days×18h TSD, N=7) using a rodent walking-wheel apparatus. One baseline day and one recovery day following TSD and CSR were analyzed. The implications of the zero sum principle were evaluated using a Markov model of sleep-wake state transition. Wake bout duration (a combined function of the probability of wake maintenance and proportional representations of brief and long wake) was a key variable mediating the baseline diurnal rhythms and post-TSD responses of all three states, and the attenuation of the post-CSR rebounds. Post-NREM state transition trajectory was an important factor in REM rebounds. The zero sum constraint ensures that a change in any transition probability always affects bout frequency and cumulative time of at least two, and usually all three, of wakefulness, NREM and REM. Neural mechanisms controlling wake maintenance may play a pivotal role in regulation and dysregulation of all three states.

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.

REM sleep phase preference in the crepuscular Octodon degus assessed by selective REM sleep deprivation

STUDY OBJECTIVES

To determine rapid eye movement (REM) sleep phase preference in a crepuscular mammal (Octodon degus) by challenging the specific REM sleep homeostatic response during the diurnal and nocturnal anticrepuscular rest phases.

DESIGN

We have investigated REM sleep rebound, recovery, and documented REM sleep propensity measures during and after diurnal and nocturnal selective REM sleep deprivations.

SUBJECTS

Nine male wild-captured O. degus prepared for polysomnographic recordings.

INTERVENTIONS

Animals were recorded during four consecutive baseline and two separate diurnal or nocturnal deprivation days, under a 12:12 light-dark schedule. Three-h selective REM sleep deprivations were performed, starting at midday (zeitgeber time 6) or midnight (zeitgeber time 18).

MEASUREMENTS AND RESULTS

Diurnal and nocturnal REM sleep deprivations provoked equivalent amounts of REM sleep debt, but a consistent REM sleep rebound was found only after nocturnal deprivation. The nocturnal rebound was characterized by a complete recovery of REM sleep associated with an augment in REM/total sleep time ratio and enhancement in REM sleep episode consolidation.

CONCLUSIONS

Our results support the notion that the circadian system actively promotes REM sleep. We propose that the sleep-wake cycle of O. degus is modulated by a chorus of circadian oscillators with a bimodal crepuscular modulation of arousal and a unimodal promotion of nocturnal REM sleep

Absence of systematic relationships between REMS duration episodes and spectral power Delta and Ultra-Slow bands in contiguous NREMS episodes in healthy humans

Previous studies in animals and humans have reported correlations between the durations of rapid eye movement sleep (REMS) episodes and immediately preceding or subsequent non-REMS (NREMS) episodes. The relationship between these two types of sleep is a crucial component in understanding the regulation and neurophysiology of ultradian alternations that occur during sleep. Although the present study replicated previous studies, we also measured NREMS in terms of spectral power Delta and Ultra-Slow bands in addition to duration in examining correlations. The spectral power Delta band, also known as slow-wave activity, measures sleep quantity and is believed to reflect sleep physiology better than mere episode durations. The Ultra-Slow spectral power band was analyzed in parallel. Healthy human participants of both sexes ( n = 26, age range 15–45 yr, n = 12 female) were carefully selected to participate in two consecutive series of home polysomnograms performed after 2 nights of habituation to the equipment. In the analyses, REMS episode durations (minutes) were compared with immediately preceding and immediately subsequent NREMS episodes (Delta and Ultra-Slow power) in each sleep cycle. REMS episode duration was more strongly correlated with preceding NREMS episodes than with subsequent NREMS episodes. However, in most cases, no correlations were observed in either direction. One ultradian sleep regulation hypothesis, which is based on stronger correlations between REMS and subsequent NREMS episode durations, holds that the main purpose of REMS is to reactivate NREMS during each sleep cycle. The present results do not support that hypothesis.

The inhibition of neurons in the central nervous pathways for thermoregulatory cold defense induces a suspended animation state in the rat

The possibility of inducing a suspended animation state similar to natural torpor would be greatly beneficial in medical science, since it would avoid the adverse consequence of the powerful autonomic activation evoked by external cooling. Previous attempts to systemically inhibit metabolism were successful in mice, but practically ineffective in nonhibernators. Here we show that the selective pharmacological inhibition of key neurons in the central pathways for thermoregulatory cold defense is sufficient to induce a suspended animation state, resembling natural torpor, in a nonhibernator. In rats kept at an ambient temperature of 15°C and under continuous darkness, the prolonged inhibition (6 h) of the rostral ventromedial medulla, a key area of the central nervous pathways for thermoregulatory cold defense, by means of repeated microinjections (100 nl) of the GABA(A) agonist muscimol (1 mm), induced the following: (1) a massive cutaneous vasodilation; (2) drastic drops in deep brain temperature (reaching a nadir of 22.44 ± 0.74°C), heart rate (from 440 ± 13 to 207 ± 12 bpm), and electroencephalography (EEG) power; (3) a modest decrease in mean arterial pressure; and (4) a progressive shift of the EEG power spectrum toward slow frequencies. After the hypothermic bout, all animals showed a massive increase in NREM sleep Delta power, similarly to that occurring in natural torpor. No behavioral abnormalities were observed in the days following the treatment. Our results strengthen the potential role of the CNS in the induction of hibernation/torpor, since CNS-driven changes in organ physiology have been shown to be sufficient to induce and maintain a suspended animation state.

Balancing Arc synthesis, mRNA decay, and proteasomal degradation: maximal protein expression triggered by rapid eye movement sleep-like bursts of muscarinic cholinergic receptor stimulation

Cholinergic signaling induces Arc/Arg3.1, an immediate early gene crucial for synaptic plasticity. However, the molecular mechanisms that dictate Arc mRNA and protein dynamics during and after cholinergic epochs are little understood. Using human SH-SY5Y neuroblastoma cells, we show that muscarinic cholinergic receptor (mAchR) stimulation triggers Arc synthesis, whereas translation-dependent RNA decay and proteasomal degradation strictly limit the amount and duration of Arc expression. Chronic application of the mAchR agonist, carbachol (Cch), induces Arc transcription via ERK signaling and release of calcium from IP(3)-sensitive stores. Arc translation requires ERK activation, but not changes in intracellular calcium. Proteasomal degradation of Arc (half-life ∼37 min) was enhanced by thapsigargin, an inhibitor of the endoplasmic calcium-ATPase pump. Similar mechanisms of Arc protein regulation were observed in cultured rat hippocampal slices. Functionally, we studied the impact of cholinergic epoch duration and temporal pattern on Arc protein expression. Acute Cch treatment (as short as 2 min) induces transient, moderate Arc expression, whereas continuous treatment of more than 30 min induces maximal expression, followed by rapid decline. Cholinergic activity associated with rapid eye movement sleep may function to facilitate long term synaptic plasticity and memory. Employing a paradigm designed to mimic intermittent rapid eye movement sleep epochs, we show that application of Cch in a series of short bursts generates persistent and maximal Arc protein expression. The results demonstrate dynamic, multifaceted control of Arc synthesis during mAchR signaling, and implicate cholinergic epoch duration and repetition as critical determinants of Arc expression and function in synaptic plasticity and behavior.

Protein kinase A in the pedunculopontine tegmental nucleus of rat contributes to regulation of rapid eye movement sleep

Intracellular signaling mechanisms within the pedunculopontine tegmental (PPT) nucleus for the regulation of recovery rapid eye movement (REM) sleep following REM sleep deprivation remain unknown. This study was designed to determine the role of PPT intracellular cAMP-dependent protein kinase A (cAMP-PKA) in the regulation of recovery REM sleep in freely moving rats. The results show that a brief period (3 h) of selective REM sleep deprivation caused REM sleep rebound associated with increased PKA activity and expression of the PKA catalytic subunit protein (PKA-CU) in the PPT. Local application of a cAMP-PKA-activation-selective inhibitor, RpCAMPS (0.55, 1.1, and 2.2 nmol/100 nl; n = 8 rats/group), bilaterally into the PPT, reduced PKA activity and PKA-CU expression in the PPT, and suppressed the recovery REM sleep, in a dose-dependent manner. Regression analyses revealed significant positive relationships between: PPT levels of PKA activity and the total percentages of REM sleep recovery (Rsqr = 0.944; n = 40 rats); PPT levels of PKA-CU expression and the total percentages of REM sleep recovery (Rsqr = 0.937; n = 40 rats); PPT levels of PKA-CU expression and PKA activity (Rsqr = 0.945; n = 40 rats). Collectively, these results provide evidence that activation of intracellular PKA in the PPT contributes to REM sleep recovery following REM sleep deprivation.

Discovery of 1-[3-(4-bromo-2-methyl-2h-pyrazol-3-yl)-4-methoxyphenyl]-3-(2,4-difluorophenyl)urea (nelotanserin) and related 5-hydroxytryptamine2A inverse agonists for the treatment of insomnia

Insomnia affects a growing portion of the adult population in the U.S. Most current therapeutic approaches to insomnia primarily address sleep onset latency. Through the 5-hydroxytryptamine(2A) (5-HT(2A)) receptor, serotonin (5-HT) plays a role in the regulation of sleep architecture, and antagonists/inverse-agonists of 5-HT(2A) have been shown to enhance slow wave sleep (SWS). We describe here a series of 5-HT(2A) inverse-agonists that when dosed in rats, both consolidate the stages of NREM sleep, resulting in fewer awakenings, and increase a physiological measure of sleep intensity. These studies resulted in the discovery of 1-[3-(4-bromo-2-methyl-2H-pyrazol-3-yl)-4-methoxyphenyl]-3-(2,4-difluorophenyl)urea (Nelotanserin), a potent inverse-agonist of 5-HT(2A) that was advanced into clinical trials for the treatment of insomnia.

The time course of the probability of transition into and out of REM sleep

STUDY OBJECTIVES

A model of rapid eye movement (REM) sleep expression is proposed that assumes underlying regulatory mechanisms operating as inhomogenous Poisson processes, the overt results of which are the transitions into and out of REM sleep.

DESIGN

Based on spontaneously occurring REM sleep episodes ("Episode") and intervals without REM sleep ("Interval"), 3 variables are defined and evaluated over discrete 15-second epochs using a nonlinear logistic regression method: "Propensity" is the instantaneous rate of into-REM transition occurrence throughout an Interval, "Volatility" is the instantaneous rate of out-of-REM transition occurrence throughout an Episode, and "Opportunity" is the probability of being in non-REM (NREM) sleep at a given time throughout an Interval, a requisite for transition.

SETTING

12:12 light:dark cycle, isolated boxes.

PARTICIPANTS

Sixteen male Sprague-Dawley rats.

INTERVENTIONS

None. Spontaneous sleep cycles.

MEASUREMENTS AND RESULTS

The highest levels of volatility and propensity occur, respectively, at the very beginning of Episodes and Intervals. The new condition stabilizes rapidly, and variables reach nadirs at minute 1.25 and 2.50, respectively. Afterward, volatility increases markedly, reaching values close to the initial level. Propensity increases moderately, the increment being stronger through NREM sleep bouts occurring at the end of long Intervals. Short-term homeostasis is evidenced by longer REM sleep episodes lowering propensity in the following Interval.

CONCLUSIONS

The stabilization after transitions into Episodes or Intervals and the destabilization after remaining for some time in either condition may be described as resulting from continuous processes building up during Episodes and intervals. These processes underlie the overt occurrence of transitions.

Relationships between the number of ultradian cycles and key sleep variables in outpatients with major depressive disorder

The regulation of the alternation between rapid eye movement sleep (REMS) and non-rapid eye movement sleep (NREMS) is still a matter of much debate. It is also an important topic for psychiatric research, since both sleep components show anomalies in Major Depressive Disorders (MDD) and related syndromes. In previous studies on healthy controls, we showed preferential links of the number of ultradian cycles with REMS-related variables rather than with NREMS-related variables. REMS Latency (RL), for example, was shown to be inversely related to the number of cycles. The present study replicates these analyses in a group of 29 patients with MDD (age range: 23-56; 16 females), after two adaptation nights. Results showed significant correlations between the number of cycles and REMS, and between the number of cycles and RL, whereas correlations with NREMS were not significant. This indirectly supports regulation hypotheses considering REMS as the main focus of the oscillation, inhibiting and interrupting NREMS. Also, when the RL is shorter, there are more ultradian cycles than when the RL is long. This adds an interesting element in the elucidation of the physiological meaning of anomalies of RL.

Cold exposure and sleep in the rat: REM sleep homeostasis and body size

STUDY OBJECTIVES

Exposure to low ambient temperature (Ta) depresses REM sleep (REMS) occurrence. In this study, both short and long-term homeostatic aspects of REMS regulation were analyzed during cold exposure and during subsequent recovery at Ta 24 degrees C.

DESIGN

EEG activity, hypothalamic temperature, and motor activity were studied during a 24-h exposure to Tas ranging from 10 degrees C to -10 degrees C and for 4 days during recovery.

SETTING

Laboratory of Physiological Regulation during the Wake-Sleep Cycle, Department of Human and General Physiology, Alma Mater Studiorum-University of Bologna.

SUBJECTS

24 male albino rats.

INTERVENTIONS

Animals were implanted with electrodes for EEG recording and a thermistor to measure hypothalamic temperature.

MEASUREMENTS AND RESULTS

REMS occurrence decreased proportionally with cold exposure, but a fast compensatory REMS rebound occurred during the first day of recovery when the previous loss went beyond a "fast rebound" threshold corresponding to 22% of the daily REMS need. A slow REMS rebound apparently allowed the animals to fully restore the previous REMS loss during the following 3 days of recovery.

CONCLUSION

Comparing the present data on rats with data from earlier studies on cats and humans, it appears that small mammals have less tolerance for REMS loss than large ones. In small mammals, this low tolerance may be responsible on a short-term basis for the shorter wake-sleep cycle, and on long-term basis, for the higher percentage of REMS that is quickly recovered following REMS deprivation.

Rapid eye movement (REM) sleep homeostatic regulatory processes in the rat: changes in the sleep-wake stages and electroencephalographic power spectra

The aim of this study was to elucidate physiological processes that are involved in the homeostatic regulation of REM sleep. Adult rats were chronically instrumented with sleep-wake recording electrodes. Following post-surgical recovery, rats were habituated extensively for freely moving polygraphic recording conditions. On the first experimental recording day (baseline day, BLD), polygraphic signs of undisturbed sleep-wake activities were recorded for 4 h (between 11:00 AM and 3:00 PM). During the second experimental recording day (REM sleep deprivation day, RDD), rats were selectively deprived of REM sleep for the first 2 h and then allowed to have normal sleep-wake for the following 2 h. The results demonstrated that during the first 2 h, compared to BLD, RDD recordings exhibited 87.80% less time in REM sleep and 16% more time in non-REM (NREM) sleep. The total percentages of wakefulness remained comparable between the BLD and RDD. During the RDD, the mean number of REM sleep episodes was much higher than in the BLD, indicating increased REM sleep drive. Electroencephalographic (EEG) power spectral analysis revealed that selective REM sleep deprivation increased delta power but decreased theta power during the residual REM sleep. During the last 2 h, after REM sleep deprivation, rats spent 51% more time in REM sleep compared to the BLD. Also during this period, the number of REM sleep episodes with the shortest (5-30 s) and longest (>120 s) duration increased during the RDD. These findings suggest that the REM sleep homeostatic process involves increased delta- and decreased theta-frequency wave activities in the cortical EEG.

A preliminary study of sleep ontogenesis in the ferret (Mustela putorius furo)

We investigated sleep ontogenesis in the ferret-a placental mammal that is highly altricial compared to other mammalian species. Because altriciality is linked with elevated rapid-eye-movement (REM) sleep amounts during infancy, it was expected that ferret kits would display very high levels of this state. Longitudinal polysomnographic measurements were made from 8 ferret kits from approximately eye-opening (postnatal day [P]30)-P50 using an experimental routine that minimized the effects of maternal separation. These data were compared to values from 8 adult ferrets (>3 months of age) and 6 neonatal cats (mean age: P31.7). We find that the polygraphic features of REM and non-REM (NREM) sleep are present by at least P30. Over the next 2 weeks, REM sleep amounts slightly declined while wakefulness and NREM sleep amounts increased. However, a comparison to published values from developing cats and rats showed that the ferret did not exhibit a disproportionate amount of REM sleep at similar postnatal ages or relative to a common developmental milestone (eye-opening).

Ultradian cycles in mice: definitions and links with REMS and NREMS

Sleep can be organized in two quite different ways across homeothermic species: either in one block (monophasic), or in several bouts across the 24 h (polyphasic). Yet, the main relationships between variables, as well as regulating mechanisms, are likely to be similar. Correlations and theories on sleep regulation should thus be examined on both types of sleepers. In previous studies on monophasic humans, we have shown preferential links between the number of ultradian cycles and the rapid eye movement sleep (REMS) time, rather than with its counterpart non-rapid eye movement sleep (NREMS). Here, the sleep of 26 polyphasic mice was examined, both to better describe the NREMS distribution, which is far more complex than in humans, and to replicate the analyses performed on humans. As in humans, the strongest links with the number of cycles were with REMS. Links were not significant with NREMS taken as a whole, although positive correlations were found with the NREMS immediately preceding REMS episodes and inversely significant with the residue. This convergence between monophasic and polyphasic patterns supports the central role played by REMS in sleep alternation.

Inverse association between Slow Wave Activity per cycle and the number of ultradian sleep cycles per night in healthy humans

OBJECTIVE

Comparisons of sleep Slow Wave Activity (SWA) during successive sleep cycles rely on the assumption that SWA in a given cycle is independent of the number of ultradian cycles present in a night. This assumption was evaluated here.

METHODS

Twenty-six healthy controls with no medical, sleep or psychiatric disorders were selected among 84 candidates and their sleep was recorded at home across 2 consecutive nights after two habituation nights.

RESULTS

In comparison with nights with less cycles, nights with more cycles showed significantly more REMS but not more NREMS. No correlation was found between the number of cycles and the integrated SWA per night (epochs visually scored as NREMS). However, inverse correlations were found between the number of cycles and the SWA per cycle. This was significant on both nights in Cycle 1 and strong trends were found for the two subsequent cycles on Night 2. Comparable results were found after removal of nights containing suspected Skipped First REMS episodes.

CONCLUSIONS

The SWA in a cycle was found to be inversely correlated to the number of cycles in the first 3 cycles in at least one of the two analyzed nights.

SIGNIFICANCE

Differences in the number of cycles per night are a potential bias in the comparisons of SWA per cycles.

Sleep homeostasis in rats assessed by a long-term intermittent paradoxical sleep deprivation protocol

Numerous studies have evaluated the sleep homeostasis of rats after short- or long-periods of sleep deprivation, but none has assessed the effects of prolonged sleep restriction on the rat's sleep pattern. The purpose of the present study, therefore, was to evaluate the sleep homeostasis of rats under a protocol of chronic sleep restriction. Male Wistar rats were implanted with electrodes for EEG and EMG recordings. Using the single platform method, the animals were submitted to 18 h of sleep restriction, beginning at 16:00 h (lights on at 07:00 h), followed by a 6 h sleep window (from 10:00 h to 16:00 h) for 21 days. Immediately after this period, rats were allowed to sleep freely for 4 days (recovery period). The sleep-wake cycle was recorded throughout the entire experiment and the results showed that during the 6h sleep window there was an increase on the percentage of sleep time, reflected by augmented time in high amplitude slow wave sleep and in paradoxical sleep, when compared to baseline sleep, whereas bouts of awakening longer than 1.5 min were greatly reduced, with the animals exhibiting a monophasic-type sleep pattern. During the deprivation period, paradoxical sleep was abolished. High amplitude slow wave sleep was also greatly affected by the protocol. Nonetheless, one day of recovery was sufficient to restore the normal sleep pattern. These findings indicate that this protocol was capable to induce many changes in the rat's sleep patterns, suggesting that during the 6h sleep window there is a sleep adaptive homeostatic process.

Beta2-containing nicotinic receptors contribute to the organization of sleep and regulate putative micro-arousals in mice

The cholinergic system is involved in arousal and in rapid eye movement sleep (REMS). To evaluate the contribution of nicotinic acetylcholine receptors (nAChRs) to these functions, we studied with polygraphic recordings the regulation of sleep in mice lacking the beta2 subunit gene of the nAChRs, a major component of high-affinity nicotine binding sites in the brain. Nicotine (1-2 mg/kg, i.p.) increased wakefulness in wild-type but not knock-out animals, indicating that beta2-containing nAChRs mediate the arousing properties of nicotine. Under normal conditions, the beta2-/- mice displayed the same amounts of waking, non-REM sleep (NREMS) and REMS as their wild-type counterparts. However, they exhibited longer REMS episodes and a reduced fragmentation of NREMS by events characterized notably by a transient drop in EEG power and frequently associated with EMG activation, tentatively referred to as micro-arousals. Respiration monitoring showed that these events were accompanied with, but not caused by, breathing irregularities. Sleep deprivation of beta2-/- mice resulted in a normal increase in REMS episode duration and NREMS delta power but yielded a reduction of the number of micro-arousals in NREMS. In contrast, in beta2-/- mice, a 1 hr immobilization stress failed to produce the normal rebound in REMS in the following 12 hr and, instead, was associated with increased NREMS fragmentation and sustained corticosterone levels. Our results show that the beta2-containing nAChRs contribute to the organization of sleep by regulating the transient phasic activity in NREMS, the REMS onset and duration, and the REMS-promoting effect of stress.

The cerebral circulation during sleep: regulation mechanisms and functional implications

Cerebral blood flow measurements during sleep are reviewed and discussed in relation to the different techniques utilized (Positron Emission Tomography, functional Magnetic Resonance Imaging, Flowmeters, Radioactive MicroIspheres, Brain Temperature Recordings, Spectrophotometry) since these methodological approaches aim at diverse features of circulation changes in the spatial or temporal domain. The regulation of cerebral circulation during sleep reveals no specific state-dependent features, flow-activity coupling being the prevailing mechanism, with O(2) as the primary candidate for the metabolic side of the link. On a general level, the latest data on brain circulation are compatible with the classical hypothesis of a "restorative" function of sleep processes.

Correlations using the NREM-REM sleep cycle frequency support distinct regulation mechanisms for REM and NREM sleep

Polysomnograms of most homeothermic species distinguish two states, rapid eye movement (REM) and non-REM (NREM) sleep. These alternate several times during the night for reasons and following rules that remain poorly understood. It is unknown whether each state has its own function and regulation or whether they represent two facets of the same process. The present study compared the mean REM/NREM sleep ratio and the mean number of NREM-REM sleep cycles across 3 consecutive nights. The rationale was that, if REM and NREM sleep are tightly associated, their ratio should be comparable whatever the cycle frequency in the night. Twenty-six healthy subjects of both sexes were recorded at their home for 4 consecutive nights. The correlation between the REM/NREM sleep ratio and the number of cycles was highly significant. Of the two sleep components, REM sleep was associated to the number of cycles, whereas NREM sleep was not. This suggests that the relationship between REM sleep and NREM sleep is rather weak within cycles, does not support the concept of NREM-REM sleep cycles as miniature units of the sleep process, and favors the concept of distinct mechanisms of regulation for the two components.

Spontaneous sleep interruptions during extended nights. Relationships with NREM and REM sleep phases and effects on REM sleep regulation

OBJECTIVES

There is no agreement in the literature as to whether sleep interruption causes rapid eye movement (REM) pressure to increase, and if so, whether this increase is expressed as shortened REM latency, increased REM density, or increased duration of REM sleep. The purpose of the present study was to examine the effect of different durations of spontaneous sleep interruptions on the regulation of REM sleep that occurs after return to sleep.

METHODS

The occurrence of spontaneous periods of wakefulness and their effects on subsequent REM sleep periods were analysed in a total sample of 1189 sleep interruptions which occurred across 364 extended nights in 13 normal subjects.

RESULTS

Compared with sleep interruptions that last less than 10 min, sleep interruptions that last longer than 10 min occur preferentially out of REM sleep. In both the short and long types of sleep interruptions, the duration of REM periods that ended in wakefulness were shorter than the duration of those that were not interrupted by wakefulness. REM densities of the REM periods that terminated in periods of wakefulness were higher than those of uninterrupted REM periods. The proportion of episodes of wakefulness following REM sleep that were long-lasting progressively increased over the course of the extended night period. The sleep episodes that followed the periods of wakefulness were characterised by a short REM latency. REM duration was increased in episodes that followed long sleep interruptions compared to those that followed short sleep interruptions. REM density did not appear to change significantly in the episodes that followed sleep interruption.

CONCLUSIONS

REM sleep mechanisms appear to be the main force controlling sleep after a spontaneous sleep interruption, presumably because during the second half of the night, where more sleep interruptions occur, the pressure for non-rapid eye movement sleep is reduced and the circadian rhythm in REM sleep propensity reaches its peak. Processes promoting REM sleep at the end of the night are consistent with the Pittendrigh and Daan dual oscillator model of the circadian pacemaker.

On-line detection of sleep-wake states and application to produce intermittent hypoxia only in sleep in rats

Sleep-disordered breathing is associated with adverse clinical consequences such as daytime sleepiness and hypertension. The mechanisms behind these associations have been studied in animal models, especially rats, but intermittent stimuli such as hypoxia have been applied without reference to sleep-wake states. To determine mechanisms underlying the adverse physiological consequences of stimuli associated with sleep-disordered breathing requires criteria for detection of sleep-wake states on-line to trigger stimuli only in sleep. This study aimed to develop such a system for freely behaving rats. Twelve rats with implanted electroencephalogram and neck electromyogram electrodes were studied in the light and dark phases. Electroencephalogram frequencies in the high (20–30 Hz) and low (2–4 Hz) frequency bands distinguished non-rapid eye movement (REM) sleep, whereas neck electromyogram distinguished REM. Using these parameters in a simple algorithm led to detection accuracies of 94.5 ± 1.0 (SE) % for wakefulness, 96.2 ± 0.8% for non-REM sleep, and 92.3 ± 1.6% for REM compared with blinded human judgment. The algorithm was then used to trigger hypoxic stimuli only in sleep. Because frequency and amplitude analysis is readily performed using a variety of commercial systems, incorporation of these parameters into such an algorithm will facilitate studies investigating mechanisms underlying the physiological consequences of sleep-related respiratory stimuli in a fashion that more effectively models clinical disorders.

The rat as an experimental model for sleep neurophysiology

The aim of this study was to develop a sleep-wake recording system for rats that would yield results more comparable to those obtained from cats than those that are usually reported. For 18 male Sprague-Dawley rats, the authors combined measures of cortical and hippocampal electroencephalogram (EEG) and neck muscle electromyogram with the electrooculogram and pontine EEG, so that the behavioral states could be identified with greater confidence with the use of polygraphic criteria developed in the cat and so that the distinctive phasic events of REM sleep could be more easily studied in the rat. The results suggest that for many neurophysiological studies, the rat is a suitable alternative to the cat.

Homeostasis of REM sleep after total and selective sleep deprivation in the rat

During specific rapid eye movement (REM) sleep deprivation its homeostatic regulation is expressed by progressively more frequent attempts to enter REM and by a compensatory rebound after the deprivation ends. The buildup of pressure to enter REM may be hypothesized to depend just on the time elapsed without REM or to be differentially related to non-REM (NREM) and wakefulness. This problem bears direct implications on the issue of the function of REM and its relation to NREM. We compared three protocols that combined REM-specific and total sleep deprivation so that animals underwent similar 3-h REM deprivations but different concomitant NREM deprivations for the first 2 (2T1R), 1 (1T2R), or 0 (3R) hours. Deprivation periods started at hour 6 after lights on. Twenty-two chronically implanted rats were recorded. The median amount of REM during all three protocols was approximately 1 min. The deficits of median amount of NREM in minutes within the 3-h deprivation periods as compared with their baselines were, respectively for 2T1R, 1T2R, and 3R, 35 (43%), 25 (25%), and 7 (7%). Medians of REM rebound in the three succeeding hours, in minutes above baseline, were, respectively, 8 (44%), 9 (53%), and 9 (50%), showing no significant differences among protocols. Attempted transitions to REM showed a rising trend during REM deprivations reaching a final value that did not differ significantly among the three protocols. These results support the hypothesis that the build up of REM pressure and its subsequent rebound is primarily related to REM absence independent of the presence of NREM.

Observations on delta homeostasis, the one-stimulus model of NREM-REM alternation and the neurobiologic implications of experimental dream studies

We first review the concept of delta homeostasis as formulated in our 1974 model and as developed quantitatively by Borbely and colleagues in several versions of the two-process model. We illustrate difficulties in the application of this quantitative model to the negative delta rebound in the rat and we put forward additional evidence that the negative rebound is a pathological rather than a homeostatic response to sleep deprivation. We next review experiments on rats in which the waking metabolic rate of limbic structures was increased by blockade of the NMDA-gated cation channel with ketamine and MK-801. As predicted by the 1974 homeostatic model, NREM delta increased during subsequent sleep. However, it remains to be shown that this powerful effect is actually caused by the metabolic change and that it is an intensification of physiological sleep rather than a non-specific increase in EEG slow waves caused by neurotoxicity. We then outline our one-stimulus model of NREM/REM alternation. In this model NREM sleep is induced by periodic (neuroendocrine?) pulses. These pulses increase delta EEG amplitude and density, depress arousal level and inhibit neural activity. When the strength of the pulsatile stimulus falls below a critical level, REM emerges as neuronal escape. Last, we discuss the neurobiologic implications of two robust findings in experimental dream studies: the relation of dream reports to arousal and the consistent failure of controlled studies to demonstrate qualitative differences between NREM and REM mentation.