Avoidance task training potentiates phasic pontine-wave density in the rat: A mechanism for sleep-dependent plasticity

The importance of REM sleep fragmentation in the effects of stress on sleep: Perspectives from preclinical studies

Psychological stress poses a risk for sleep disturbances. Importantly, trauma-exposed individuals who develop posttraumatic stress disorder (PTSD) frequently report insomnia and recurrent nightmares. Clinical studies have provided insight into the mechanisms of these sleep disturbances. We review polysomnographic findings in PTSD and identify analogous measures that have been made in animal models of PTSD. There is a rich empirical and theoretical literature on rapid eye movement sleep (REMS) substrates of insomnia and nightmares, with an emphasis on REMS fragmentation. For future investigations of stress-induced sleep changes, we recommend a focus on tonic, phasic and other microarchitectural REMS measures. Power spectral density analysis of the sleep EEG should also be utilized. Animal models with high construct validity can provide insight into gender and time following stressor exposure as moderating variables. Ultimately, preclinical studies with translational potential will lead to improved treatment for stress-related sleep disturbances.

Individual differences in information processing during sleep and wake predict sleep-based memory consolidation of complex rules

Memory is critical for many cognitive functions, from remembering facts, to learning complex environmental rules. While memory encoding occurs during wake, memory consolidation is associated with sleep-related neural activity. Further, research suggests that individual differences in alpha frequency during wake (∼7 - 13 Hz) modulate memory processes, with higher individual alpha frequency (IAF) associated with greater memory performance. However, the relationship between wake-related EEG individual differences, such as IAF, and sleep-related neural correlates of memory consolidation has been largely unexplored, particularly in a complex rule-based memory context. Here, we aimed to investigate whether wake-derived IAF and sleep neurophysiology interact to influence rule learning in a sample of 35 healthy adults (16 males; mean age = 25.4, range: 18 - 40). Participants learned rules of a modified miniature language prior to either 8hrs of sleep or wake, after which they were tested on their knowledge of the rules in a grammaticality judgement task. Results indicate that sleep neurophysiology and wake-derived IAF do not interact but modulate memory for complex linguistic rules separately. Phase-amplitude coupling between slow oscillations and spindles during non-rapid eye-movement (NREM) sleep also promoted memory for rules that were analogous to the canonical English word order. As an exploratory analysis, we found that rapid eye-movement (REM) sleep theta power at posterior regions interacts with IAF to predict rule learning and proportion of time in REM sleep predicts rule learning differentially depending on grammatical rule type. Taken together, the current study provides behavioural and electrophysiological evidence for a complex role of NREM and REM sleep neurophysiology and wake-derived IAF in the consolidation of rule-based information.

The Ponto-Geniculo-Occipital (PGO) Waves in Dreaming: An Overview

Rapid eye movement (REM) sleep is the main sleep correlate of dreaming. Ponto-geniculo-occipital (PGO) waves are a signature of REM sleep. They represent the physiological mechanism of REM sleep that specifically limits the processing of external information. PGO waves look just like a message sent from the pons to the lateral geniculate nucleus of the visual thalamus, the occipital cortex, and other areas of the brain. The dedicated visual pathway of PGO waves can be interpreted by the brain as visual information, leading to the visual hallucinosis of dreams. PGO waves are considered to be both a reflection of REM sleep brain activity and causal to dreams due to their stimulation of the cortex. In this review, we summarize the role of PGO waves in potential neural circuits of two major theories, i.e., (1) dreams are generated by the activation of neural activity in the brainstem; (2) PGO waves signaling to the cortex. In addition, the potential physiological functions during REM sleep dreams, such as memory consolidation, unlearning, and brain development and plasticity and mood regulation, are discussed. It is hoped that our review will support and encourage research into the phenomenon of human PGO waves and their possible functions in dreaming.

A medullary hub for controlling REM sleep and pontine waves

Rapid-eye-movement (REM) sleep is a distinct behavioral state associated with vivid dreaming and memory processing. Phasic bursts of electrical activity, measurable as spike-like pontine (P)-waves, are a hallmark of REM sleep implicated in memory consolidation. However, the brainstem circuits regulating P-waves, and their interactions with circuits generating REM sleep, remain largely unknown. Here, we show that an excitatory population of dorsomedial medulla (dmM) neurons expressing corticotropin-releasing-hormone (CRH) regulates both REM sleep and P-waves in mice. Calcium imaging showed that dmM CRH neurons are selectively activated during REM sleep and recruited during P-waves, and opto- and chemogenetic experiments revealed that this population promotes REM sleep. Chemogenetic manipulation also induced prolonged changes in P-wave frequency, while brief optogenetic activation reliably triggered P-waves along with transiently accelerated theta oscillations in the electroencephalogram (EEG). Together, these findings anatomically and functionally delineate a common medullary hub for the regulation of both REM sleep and P-waves.

What are the neural mechanisms and physiological functions of dreams?

Dreams are mental experiences, including perceptions, thoughts, and emotions, that occur during sleep. In dreams, hallucinatory perceptions, particularly visual and motoric, are often accompanied by negative emotions. When people dream, they perceive them as real even though they are bizarre and distorted in time and space. People often cannot recall their dreams, even though people dream every night. Dreaming is a strange physiological phenomenon. Research has demonstrated that dreaming is closely associated with rapid eye movement (REM) sleep. It is known that dreaming also occurs during non-REM (NREM) sleep, but the content appears to be different. Dreams during REM sleep tend to be longer, more vivid, more story-like, and more bizarre than those during NREM sleep. In this review, the neural circuits underlying dreaming and the physiological functions associated with it are summarized. Two major theories have been proposed regarding the neural circuits involved in dreaming. One is that dreams are generated by the activation of neural activity in the brainstem and its signal transmission to the cortex. The other is that dreams are caused by forebrain activation by dopamine. Whereas the physiological function of dreams remains unclear, several hypotheses have been proposed that are associated with memory and emotions.

Functional roles of REM sleep

Rapid eye movement (REM) sleep is an enigmatic and intriguing sleep state. REM sleep differs from non-REM sleep by its characteristic brain activity and from wakefulness by a reduced anti-gravity muscle tone. In addition to these key traits, diverse physiological phenomena appear across the whole body during REM sleep. However, it remains unclear whether these phenomena are the causes or the consequences of REM sleep. Experimental approaches using humans and animal models have gradually revealed the functional roles of REM sleep. Extensive efforts have been made to interpret the characteristic brain activity in the context of memory functions. Numerous physical and psychological functions of REM sleep have also been proposed. Moreover, REM sleep has been implicated in aspects of brain development. Here, we review the variety of functional roles of REM sleep, mainly as revealed by animal models. In addition, we discuss controversies regarding the functional roles of REM sleep.

Neural Oscillations in Aversively Motivated Behavior

Fear and anxiety-based disorders are highly debilitating and among the most prevalent psychiatric disorders. These disorders are associated with abnormal network oscillations in the brain, yet a comprehensive understanding of the role of network oscillations in the regulation of aversively motivated behavior is lacking. In this review, we examine the oscillatory correlates of fear and anxiety with a particular focus on rhythms in the theta and gamma-range. First, we describe neural oscillations and their link to neural function by detailing the role of well-studied theta and gamma rhythms to spatial and memory functions of the hippocampus. We then describe how theta and gamma oscillations act to synchronize brain structures to guide adaptive fear and anxiety-like behavior. In short, that hippocampal network oscillations act to integrate spatial information with motivationally salient information from the amygdala during states of anxiety before routing this information via theta oscillations to appropriate target regions, such as the prefrontal cortex. Moreover, theta and gamma oscillations develop in the amygdala and neocortical areas during the encoding of fear memories, and interregional synchronization reflects the retrieval of both recent and remotely encoded fear memories. Finally, we argue that the thalamic nucleus reuniens represents a key node synchronizing prefrontal-hippocampal theta dynamics for the retrieval of episodic extinction memories in the hippocampus.

Regulation of Dark Period Sleep by the Amygdala: A microinjection and optogenetics study

The central nucleus of the amygdala (CNA) projects to brainstem regions that generate and regulate rapid eye movement sleep (REM). We used optogenetics to assess the influence of CNA inputs into reticularis pontis oralis (RPO), pedunculopontine tegmentum (PPT) and nucleus subcoeruleus (SubC) on dark period sleep. We compared these results to effects of microinjections into CNA of the GABA_A agonist, muscimol (MUS, inhibition of cell bodies) and tetrodotoxin (TTX, inhibition of cell bodies and fibers of passage). For optogenetics, male Wistar rats received excitatory (AAV5-EF1a-DIO -hChR2(H134R)-EYFP) or inhibitory (AAV-EF1a-DIO-eNpHR3.0-EYFP; DIO-eNpHR3.0) opsins into CNA and AAV5-EF1a-mCherry-IRES-WGA-Cre into RPO, PPT, or SubC. This enabled only CNA neurons synaptically connected to each region to express opsin. Optic cannulae for light delivery into CNA and electrodes for determining sleep were implanted. Sleep was recorded with and without blue or amber light stimulation of CNA. Separate rats received MUS or TTX into CNA prior to recording sleep. Optogenetic activation of CNA neurons projecting to RPO enhanced REM and did not alter non-REM (NREM) whereas activation of CNA neurons projecting to PPT or SubC did not significantly affect sleep. Inhibition of CNA neurons projecting to any region did not significantly alter sleep. TTX inactivation of CNA decreased REM and increased NREM whereas muscimol inactivation did not significantly alter sleep. Thus, the amygdala can regulate decreases and increases in REM, and RPO is important for CNA promotion of REM. Fibers passing through CNA, likely from the basolateral nucleus of the amygdala, also play a role in regulating sleep.

The beneficial effects of green tea on sleep deprivation-induced cognitive deficits in rats: the involvement of hippocampal antioxidant defense

BACKGROUND

The weight of evidence suggests that sleep is essential for the processes of memory consolidation and sleep deprivation (SD) impairs the retention of long-term memory in both humans and experimental animals, which is associated with oxidative stress damage within the brain. Green tea polyphenols have revealed carcinogenic, antioxidant, anti-, and anti-mutagenic properties. We aimed to investigate the possible protective effect of green tea extract (GTE) and its main active catechin, epigallocatechin-3-gallate (EGCG), on post-training total sleep deprivation (TSD) -induced spatial memory deficits and oxidative stress profile in the hippocampus of the rat.

METHODS

Male rats were treated with saline, GTE (100 and 200 mg/kg/day), and EGCG (50 mg/kg/day) intraperitoneally for 21 days and then trained in Morris water maze (MWM) in a single day protocol. Immediately after the end of MWM training, animals were sleep deprived for 6 h by the gentle handling method, and then evaluated for spatial memory. Hippocampal levels of malondialdehyde, (MDA), and thiol was assessed as oxidant and antioxidant markers.

RESULTS

Spatial memory was impaired in the TSD group and GTE at the dose of 200 mg/kg/day as well as EGCG at the dose of 50 mg/kg/day could reverse the impairment to the saline-treated levels. Despite the unchanged MDA levels, hippocampal total thiol was significantly decreased after TSD and EGCG increased it to the basal levels.

CONCLUSION

In conclusion, green tea and its main catechin, EGCG, could prevent memory impairments during 6 h of TSD; probably through normalizing the antioxidant thiol defense system which was impaired during TSD.

Fear conditioning potentiates the hippocampal CA1 commissural pathway in vivo and increases awake phase sleep

The hippocampus is essential for spatial learning and memory. To assess learning we used contextual fear conditioning (cFC), where animals learn to associate a place with aversive events like foot‐shocks. Candidate memory mechanisms for cFC are long‐term potentiation (LTP) and long‐term depression (LTD), but there is little direct evidence of them operating in the hippocampus in vivo following cFC. Also, little is known about the behavioral state changes induced by cFC. To address these issues, we recorded local field potentials in freely behaving mice by stimulating in the left dorsal CA1 region and recording in the right dorsal CA1 region. Synaptic strength in the commissural pathway was monitored by measuring field excitatory postsynaptic potentials (fEPSPs) before and after cFC. After cFC, the commissural pathway's synaptic strength was potentiated. Although recordings occurred during the wake phase of the light/dark cycle, the mice slept more in the post‐conditioning period than in the pre‐conditioning period. Relative to awake periods, in non‐rapid eye movement (NREM) sleep the fEPSPs were larger in both pre‐ and post‐conditioning periods. We also found a significant negative correlation between the animal's speed and fEPSP size. Therefore, to avoid confounds in the fEFSP potentiation estimates, we controlled for speed‐related and sleep‐related fEPSP changes and still found that cFC induced long‐term potentiation, but no significant long‐term depression. Synaptic strength changes were not found in the control group that simply explored the fear‐conditioning chamber, indicating that exploration of the novel place did not produce the measurable effects caused by cFC. These results show that following cFC, the CA1 commissural pathway is potentiated, likely contributing to the functional integration of the left and right hippocampi in fear memory consolidation. In addition, the cFC paradigm produces significant changes in an animal's behavioral state, which are observable as proximal changes in sleep patterns.

Contributions of post-learning REM and NREM sleep to memory retrieval

It has become clear that sleep after learning has beneficial effects on the later retrieval of newly acquired memories. The neural mechanisms underlying these effects are becoming increasingly clear as well, particularly those of non-REM sleep. However, much is still unknown about the sleep and memory relationship: the sleep state or features of sleep physiology that associate with memory performance often vary by task or experimental design, and the nature of this variability is not entirely clear. This paper describes pertinent features of sleep physiology and provides a detailed review of the scientific literature indicating beneficial effects of post-learning sleep on memory retrieval. This paper additionally introduces a hypothesis which attributes these beneficial effects of post-learning sleep to separable processes of memory reinforcement and memory refinement whereby reinforcement supports one's ability to retrieve a given memory and refinement supports the precision of that memory retrieval in the context of competitive alternatives. It is observed that features of non-REM sleep are involved in a post-learning substantiation of memory representations that benefit memory performance; thus, memory reinforcement is primarily attributed to non-REM sleep. Memory refinement is primarily attributed to REM sleep given evidence of bidirectional synaptic plasticity in REM sleep and findings from studies of selective REM sleep deprivation.

The formation of compensatory contextual fear memory in the absence of dorsal hippocampus does not change sleep architecture

Although the dorsal hippocampus (DH) plays an essential role in the consolidation of contextual fear-conditioned (CxFC) memory, this consolidation may also occur in the absence of DH. It is, however, not known if the development of a compensatory circuit for CxFC memory is time-dependent. The DH-dependent contextual fear memory influences sleep architecture, but whether the compensatory fear memory can influence sleep, is not known. Here, we have studied (a) the temporal progression of compensatory contextual fear memory in the absence of DH and (b) the influence of compensatory contextual fear memory on sleep architecture. Rats were surgically prepared for chronic polysomnographic recordings and drug injections in the DH. They were divided into four groups: DH-non-lesioned and fear-conditioned, DH-non-lesioned and non-fear-conditioned, DH-lesioned and fear-conditioned and DH-lesioned and non-fear-conditioned groups. The DH was lesioned with ibotenic acid. The animals were conditioned to contextual fear twice: 1^st training on Day 5 and testing on Day 6; 2^nd training on Day 10 and testing on Day 11. The DH-lesioned and fear-conditioned animals did not exhibit freezing response during the first testing but showed a robust freezing response when re-trained after a gap of three days. In addition, wakefulness and NREM sleep amount did not change, but REM sleep significantly decreased in the DH-dependent CxFC memory group. Interestingly, REM sleep did not decrease in the DH-independent CxFC memory group. Our findings suggest that the development of compensatory CxFC memory is a time-dependent process and the compensatory CxFC memory may not influence sleep architecture.

Sleep changes following intensive cognitive activity

Studies over the last 40 years have mainly investigated sleep structure changes as a result of wake duration, in the frame of the classical sleep regulation theories. However, wake intervals of the same duration can profoundly differ in their intensity, which actually reflects the degree of cognitive and physical activity. Data on how sleep can be modified by wake intensity changes (initially sparse and of little consistence) have become much more substantial, especially in the frame of the intense research debate on sleep-memory relationships. Our aim is to examine the vast repertoire of sleep modifications that depend on waking cognitive manipulations, highlighting the sleep features that appear most affected. By systematically addressing this issue, we want to set the basis for future research exploring both the specific nature of the mechanisms involved and the applicative psychosocial and clinical fall-outs, in terms of possible behavioural interventions for sleep quality improvement.

The Effect of Cognitive Activity on Sleep Maintenance in a Subsequent Daytime Nap

Background/Objective: The aim of this study is to assess the effects of a learning task on the characteristics of a subsequent daytime nap. Participants and Methods: Thirty-eight subjects were administered a control nap (C) and one preceded by a cognitive training session (TR). Results: Relative to C, TR naps showed significantly increased sleep duration with decreased sleep latency, as well as significantly increased sleep efficiency due to reduced awakening frequency. Meaningful trends were also found toward an increase of Stage 2 sleep proportion and a reduction of Stage 1 sleep, percentage of wake after sleep onset (WASO), and frequency of state transitions. Conclusions: Our results indicate that presleep learning favors sleep propensity and maintenance, offering the possibility to explore planned cognitive training as a low-cost treatment for sleep impairments.

How rhythms of the sleeping brain tune memory and synaptic plasticity

Decades of neurobehavioral research has linked sleep-associated rhythms in various brain areas to improvements in cognitive performance. However, it remains unclear what synaptic changes might underlie sleep-dependent declarative memory consolidation and procedural task improvement, and why these same changes appear not to occur across a similar interval of wake. Here we describe recent research on how one specific feature of sleep-network rhythms characteristic of rapid eye movement and non-rapid eye movement-could drive synaptic strengthening or weakening in specific brain circuits. We provide an overview of how these rhythms could affect synaptic plasticity individually and in concert. We also present an overarching hypothesis for how all network rhythms occurring across the sleeping brain could aid in encoding new information in neural circuits.

How Sleep Shapes Thalamocortical Circuit Function in the Visual System

Recent data have shown that sleep plays a beneficial role for cognitive functions such as declarative memory consolidation and perceptual learning. In this article, we review recent findings on the role of sleep in promoting adaptive visual response changes in the lateral geniculate nucleus and primary visual cortex following novel visual experiences. We discuss these findings in the context of what is currently known about how sleep affects the activity and function of thalamocortical circuits and current hypotheses regarding how sleep facilitates synaptic plasticity.

Total sleep deprivation impairs fear memory retrieval by decreasing the basolateral amygdala activity

It is well known that sleep deprivation impairs fear memory processes, but little is known about the underlying mechanisms or circuits. The aim of this study was to evaluate the effects of total sleep deprivation (24 h) on contextual fear memory acquisition, consolidation, and retrieval, as well as c-Fos activity in the hippocampus and amygdala. Fear memory recall was associated with an increase in the number of c-Fos-positive cells in the hippocampal CA1 and CA3 regions and the basolateral amygdala (BLA). Total sleep deprivation before to the acquisition and during consolidation of memory impaired retrieval and blocked the associated c-Fos activity in the hippocampus and amygdala. In contrast, total sleep deprivation before memory recall also impaired retrieval, but selectively prevented the increase of c-Fos activity in the amygdala (but not in the hippocampus). Our data indicate that sleep is essential not only for acquisition and consolidation but also for the retrieval of fear memories. They also suggest a differential susceptibility of specific memory-related neural circuits (hippocampus and BLA) to the absence of sleep.

Sleep influences cognitive performance in lemurs

Primates spend almost half their lives asleep, yet little is known about how sleep influences their waking cognition. We hypothesized that diurnal and cathemeral lemurs differ in their need for consistent, non-segmented sleep for next-day cognitive function-including long-term memory consolidation, self-control, foraging efficiency, and sociality. Specifically, we expected that strictly diurnal Propithecus is more reliant on uninterrupted sleep for cognitive performance, as compared to four other lemur species that are more flexibly active (i.e., cathemeral). We experimentally inhibited sleep and tested next-day performance in 30 individuals of 5 lemur species over 960 total nights at the Duke Lemur Center in Durham, North Carolina. Each set of pair-housed lemurs experienced a sleep restriction and/or deprivation protocol and was subsequently tested in a variety of fitness-relevant cognitive tasks. Within-subject comparisons of performance on these tasks were made by switching the pair from the experimental sleep inhibited condition to a normal sleep environment, thus ensuring cognitive equivalency among individuals. We validated effectiveness of the protocol via actigraphy and infrared videography. Our results suggest that 'normal' non-disrupted sleep improved memory consolidation for all lemurs. Additionally, on nights of normal sleep, diurnal lemurs performed better in foraging efficiency tasks than cathemeral lemurs. Social behaviors changed in species-specific ways after exposure to experimental conditions, and self-control was not significantly linked with sleep condition. Based on these findings, the links between sleep, learning, and memory consolidation appear to be evolutionarily conserved in primates.

How Memory Replay in Sleep Boosts Creative Problem-Solving

Creative thought relies on the reorganisation of existing knowledge. Sleep is known to be important for creative thinking, but there is a debate about which sleep stage is most relevant, and why. We address this issue by proposing that rapid eye movement sleep, or 'REM', and non-REM sleep facilitate creativity in different ways. Memory replay mechanisms in non-REM can abstract rules from corpuses of learned information, while replay in REM may promote novel associations. We propose that the iterative interleaving of REM and non-REM across a night boosts the formation of complex knowledge frameworks, and allows these frameworks to be restructured, thus facilitating creative thought. We outline a hypothetical computational model which will allow explicit testing of these hypotheses.

Sleep-Active Neurons: Conserved Motors of Sleep

Sleep is crucial for survival and well-being. This behavioral and physiological state has been studied in all major genetically accessible model animals, including rodents, fish, flies, and worms. Genetic and optogenetic studies have identified several neurons that control sleep, making it now possible to compare circuit mechanisms across species. The “motor” of sleep across animal species is formed by neurons that depolarize at the onset of sleep to actively induce this state by directly inhibiting wakefulness. These sleep-inducing neurons are themselves controlled by inhibitory or activating upstream pathways, which act as the “drivers” of the sleep motor: arousal inhibits “sleep-active” neurons whereas various sleep-promoting “tiredness” pathways converge onto sleep-active neurons to depolarize them. This review provides the first overview of sleep-active neurons across the major model animals. The occurrence of sleep-active neurons and their regulation by upstream pathways in both vertebrate and invertebrate species suggests that these neurons are general and ancient components that evolved early in the history of nervous systems.

Adult Gross Motor Learning and Sleep: Is There a Mutual Benefit?

Posttraining consolidation, also known as offline learning, refers to neuroplastic processes and systemic reorganization by which newly acquired skills are converted from an initially transient state into a more permanent state. An extensive amount of research on cognitive and fine motor tasks has shown that sleep is able to enhance these processes, resulting in more stable declarative and procedural memory traces. On the other hand, limited evidence exists concerning the relationship between sleep and learning of gross motor skills. We are particularly interested in this relationship with the learning of gross motor skills in adulthood, such as in the case of sports, performing arts, devised experimental tasks, and rehabilitation practice. Thus, the present review focuses on sleep and gross motor learning (GML) in adults. The literature on the impact of sleep on GML, the consequences of sleep deprivation, and the influence of GML on sleep architecture were evaluated for this review. While sleep has proven to be beneficial for most gross motor tasks, sleep deprivation in turn has not always resulted in performance decay. Furthermore, correlations between motor performance and sleep parameters have been found. These results are of potential importance for integrating sleep in physiotherapeutic interventions, especially for patients with impaired gross motor functions.

Short-Term Total Sleep-Deprivation Impairs Contextual Fear Memory, and Contextual Fear-Conditioning Reduces REM Sleep in Moderately Anxious Swiss Mice

The conditioning tasks have been widely used to model fear and anxiety and to study their association with sleep. Many reports suggest that sleep plays a vital role in the consolidation of fear memory. Studies have also demonstrated that fear-conditioning influences sleep differently in mice strains having a low or high anxiety level. It is, therefore, necessary to know, how sleep influences fear-conditioning and how fear-conditioning induces changes in sleep architecture in moderate anxious strains. We have used Swiss mice, a moderate anxious strain, to study the effects of: (i) sleep deprivation on contextual fear conditioned memory, and also (ii) contextual fear conditioning on sleep architecture. Animals were divided into three groups: (a) non-sleep deprived (NSD); (b) stress control (SC); and (c) sleep-deprived (SD) groups. The SD animals were SD for 5 h soon after training. We found that the NSD and SC animals showed 60.57% and 58.12% freezing on the testing day, while SD animals showed significantly less freezing (17.13% only; p < 0.001) on the testing day. Further, we observed that contextual fear-conditioning did not alter the total amount of wakefulness and non-rapid eye movement (NREM) sleep. REM sleep, however, significantly decreased in NSD and SC animals on the training and testing days. Interestingly, REM sleep did not decrease in the SD animals on the testing day. Our results suggest that short-term sleep deprivation impairs fear memory in moderate anxious mice. It also suggests that NREM sleep, but not REM sleep, may have an obligatory role in memory consolidation.

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.

Linking Network Activity to Synaptic Plasticity during Sleep: Hypotheses and Recent Data

Research findings over the past two decades have supported a link between sleep states and synaptic plasticity. Numerous mechanistic hypotheses have been put forth to explain this relationship. For example, multiple studies have shown structural alterations to synapses (including changes in synaptic volume, spine density, and receptor composition) indicative of synaptic weakening after a period of sleep. Direct measures of neuronal activity and synaptic strength support the idea that a period of sleep can reduce synaptic strength. This has led to the synaptic homeostasis hypothesis (SHY), which asserts that during slow wave sleep, synapses are downscaled throughout the brain to counteract net strengthening of network synapses during waking experience (e.g., during learning). However, neither the cellular mechanisms mediating these synaptic changes, nor the sleep-dependent activity changes driving those cellular events are well-defined. Here we discuss potential cellular and network dynamic mechanisms which could underlie reductions in synaptic strength during sleep. We also discuss recent findings demonstrating circuit-specific synaptic strengthening (rather than weakening) during sleep. Based on these data, we explore the hypothetical role of sleep-associated network activity patterns in driving synaptic strengthening. We propose an alternative to SHY—namely that depending on experience during prior wake, a variety of plasticity mechanisms may operate in the brain during sleep. We conclude that either synaptic strengthening or synaptic weakening can occur across sleep, depending on changes to specific neural circuits (such as gene expression and protein translation) induced by experiences in wake. Clarifying the mechanisms underlying these different forms of sleep-dependent plasticity will significantly advance our understanding of how sleep benefits various cognitive functions.

Sleep-Dependent Oscillatory Synchronization: A Role in Fear Memory Consolidation

Sleep plays an important role in memory consolidation through the facilitation of neuronal plasticity; however, how sleep accomplishes this remains to be completely understood. It has previously been demonstrated that neural oscillations are an intrinsic mechanism by which the brain precisely controls neural ensembles. Inter-regional synchronization of these oscillations is also known to facilitate long-range communication and long-term potentiation (LTP). In the present study, we investigated how the characteristic rhythms found in local field potentials (LFPs) during non-REM and REM sleep play a role in emotional memory consolidation. Chronically implanted bipolar electrodes in the lateral amygdala (LA), dorsal and ventral hippocampus (DH, VH), and the infra-limbic (IL), and pre-limbic (PL) prefrontal cortex were used to record LFPs across sleep-wake activity following each day of a Pavlovian cued fear conditioning paradigm. This resulted in three principle findings: (1) theta rhythms during REM sleep are highly synchronized between regions; (2) the extent of inter-regional synchronization during REM and non-REM sleep is altered by FC and EX; (3) the mean phase difference of synchronization between the LA and VH during REM sleep predicts changes in freezing after cued fear extinction. These results both oppose a currently proposed model of sleep-dependent memory consolidation and provide a novel finding which suggests that the role of REM sleep theta rhythms in memory consolidation may rely more on the relative phase-shift between neural oscillations, rather than the extent of phase synchronization.

Towards a Functional Understanding of PGO Waves

Ponto-Geniculo-Occipital (PGO) waves are biphasic field potentials identified in a range of mammalian species that are ubiquitous with sleep, but can also be identified in waking perception and eye movement. Their role in REM sleep and visual perception more broadly may constitute a promising avenue for further research, however what was once an active field of study has recently fallen into stasis. With the reality that invasive recordings performed on animals cannot be replicated in humans; while animals themselves cannot convey experience to the extent required to elucidate how PGO waves factor into awareness and behavior, innovative solutions are required if significant research outcomes are to ever be realized. Advances in non-invasive imaging technologies and sophistication in imaging methods now offer substantial scope to renew the study of the electrophysiological substrates of waking and dreaming perception. Among these, Magnetoencephalogram (MEG) stands out through its capacity to measure deep brain activations with high temporal resolution. With the current trend in sleep and dream research to produce translational findings of psychopathological and medical significance, in addition to the clear links that PGO wave generation sites share, pharmacologically, with receptors involved in expression of mental illness; there is a strong case to support scientific research into PGO waves and develop a functional understanding of their broader role in human perception.

Physiological and pathological high-frequency oscillations have distinct sleep-homeostatic properties

OBJECTIVE

The stage of sleep is a known modulator of high-frequency oscillations (HFOs). For instance, high amplitude slow waves during NREM sleep and the subtypes of REM sleep were shown to contribute to a better separation between physiological and pathological HFOs. This study investigated rates and spatial spread of the different HFO types (physiological and pathological ripples in the 80-250 Hz frequency band, and fast ripples above 250 Hz) depending on time spent in sleep across the different sleep cycles.

METHODS

Fifteen patients with focal pharmaco-resistant epilepsy underwent one night of video-polysomnography during chronic intracranial EEG recording for presurgical epilepsy evaluation. The HFO rate and spread across the different sleep cycles were determined with an automatic HFO detector. We built models to explain the observed rate and spread based on time in sleep and other variables i.e. sleep stage, delta band and sigma band activity, and slow wave amplitude. Statistical significance of the different variables was determined by a model comparison using the Akaike information criterion.

RESULTS

The rate of HFOs depends significantly on the accumulated time of sleep. As the night advanced, the rate of pathological ripples and fast ripples decreased during NREM sleep (up to 15% per hour spent in the respective sleep stages), while the rate of physiological ripples increased during REM sleep (8% per hour spent in REM sleep). Interestingly, the stage of sleep but not the sleep cycle determined the extent of spread of HFOs, showing a larger field during NREM sleep and a more restricted field during REM sleep.

CONCLUSION

The different dependence with sleep time for physiological and pathological ripples is in keeping with their distinct underlying generating mechanisms. From a practical point of view, the first sleep cycle seems to be best suitable for studying HFOs in epilepsy, given that the contrast between physiological and pathological ripple rates is largest during this time.

Activation of Retinotopic Visual Areas Is Central to REM Sleep Associated Dreams: Visual Dreams and Visual Imagery Possibly Co-Emerged In Evolution

The latest experimental results support that multiple retinotopic visual systems play a central role not only in the processing of visual signals but also in the integration and processing of internally represented auditory and tactile information. These retinotopic maps have access to higher levels of cognitive processing, performed by the frontal lobes, for example. The occipital cortex may have a special role in multisensory integration. There is a functional basis for the development and maturation of visual memory in association of rapid eye movement sleep (REMS) which is linked to dreams and visual imagery. Physiological and psychological processes of REMS are similar to waking visual imagery. Furthermore, visual imagery during REMS utilize a common visual neural pathway similar to that used in wakefulness. This pathway subserves visual processes accompanied with auditory experiences and intrinsic feelings. We argue that the activation of the retinotopic visual areas is central to REM sleep associated dreams and that REMS associated dreaming and visual imagery may have co-evolved in homeothermic animals during evolution. We also suggest that protoconscious state during REM sleep, as introduced by Hobson many years ago, may be a basic visual process.

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.

Frontal beta-theta network during REM sleep

We lack detailed knowledge about the spatio-temporal physiological signatures of REM sleep, especially in humans. By analyzing intracranial electrode data from humans, we demonstrate for the first time that there are prominent beta (15–35 Hz) and theta (4–8 Hz) oscillations in both the anterior cingulate cortex (ACC) and the DLPFC during REM sleep. We further show that these theta and beta activities in the ACC and the DLPFC, two relatively distant but reciprocally connected regions, are coherent. These findings suggest that, counter to current prevailing thought, the DLPFC is active during REM sleep and likely interacting with other areas. Since the DLPFC and the ACC are implicated in memory and emotional regulation, and the ACC has motor areas and is thought to be important for error detection, the dialogue between these two areas could play a role in the regulation of emotions and in procedural motor and emotional memory consolidation.

DOI:http://dx.doi.org/10.7554/eLife.18894.001

Over the course of a night we cycle through several different stages of sleep. During one of these stages, our eyes move rapidly from side to side behind our closed eyelids. This movement gives this stage its name: rapid eye movement sleep, or REM sleep for short. Most other muscles are paralyzed during REM sleep, possibly to prevent us from acting out the vivid dreams that also occur during this stage of sleep. But despite the distinctive properties of REM sleep, relatively little is known about about why we need it or how the brain generates it.

Vijayan et al. have now obtained new insights into the brain activity that underlies REM sleep by recording from the brains of human patients with epilepsy. The patients all had electrodes temporarily inserted into their brains to help neurologists identify the area of the brain that was responsible for their seizures. By recording from these electrodes overnight, Vijayan et al. were able to study the activity of individual brain regions while the patients slept.

Analysis of the recordings revealed rhythmic waves of neuronal activity in areas at the front of the brain during REM sleep. Two types of brain waves dominated: theta waves, which are relatively slow waves with a frequency of 4–8 cycles per second (Hertz), and beta waves, which are faster with a frequency of 15–35 Hertz. These theta and beta waves were especially pronounced in two subregions of the frontal lobe of the brain, called the dorsolateral prefrontal cortex (DLPFC) and the anterior cingulate cortex (ACC).

The discovery of prominent rhythmic activity in the DLPFC was unexpected. This is because previous studies had shown that this region, which is involved in decision-making and planning, was relatively inactive during REM sleep. Indeed it had been suggested that the limited activity of the DLPFC subregion might be responsible for the often bizarre and illogical nature of our dreams. Instead, Vijayan et al. showed that the ACC and the DLPFC coordinate their activity during REM sleep. The next challenge is to find out whether this dual activity helps support other roles that the two regions share in common, such as the strengthening of memories and the regulation of emotions.

DOI:http://dx.doi.org/10.7554/eLife.18894.002

Different Simultaneous Sleep States in the Hippocampus and Neocortex

STUDY OBJECTIVES

Investigators assign sleep-waking states using brain activity collected from a single site, with the assumption that states occur at the same time throughout the brain. We sought to determine if sleep-waking states differ between two separate structures: the hippocampus and neocortex.

METHODS

We measured electrical signals (electroencephalograms and electromyograms) during sleep from the hippocampus and neocortex of five freely behaving adult male rats. We assigned sleep-waking states in 10-sec epochs based on standard scoring criteria across a 4-h recording, then analyzed and compared states and signals from simultaneous epochs between sites.

RESULTS

We found that the total amount of each state, assigned independently using the hippocampal and neocortical signals, was similar between the hippocampus and neocortex. However, states at simultaneous epochs were different as often as they were the same (P = 0.82). Furthermore, we found that the progression of states often flowed through asynchronous state-pairs led by the hippocampus. For example, the hippocampus progressed from transition-to-rapid eye movement sleep to rapid eye movement sleep before the neocortex more often than in synchrony with the neocortex (38.7 ± 16.2% versus 15.8 ± 5.6% mean ± standard error of the mean).

CONCLUSIONS

We demonstrate that hippocampal and neocortical sleep-waking states often differ in the same epoch. Consequently, electrode location affects estimates of sleep architecture, state transition timing, and perhaps even percentage of time in sleep states. Therefore, under normal conditions, models assuming brain state homogeneity should not be applied to the sleeping or waking brain.

The Role of Sleep in Memory Consolidation and Brain Plasticity: Dream or Reality?

The notion that a good night of sleep improves memory is widely accepted by the general public. Among sleep scientists, however, the idea has been hotly debated for decades. In this review, the authors consider current evidence for and against the hypothesis that sleep facilitates memory consolidation and promotes plastic changes in the brain. They find that despite a steady accumulation of positive findings over the past decade, the precise role of sleep in memory and brain plasticity remains elusive. This impasse may be resolved by more integrated approaches that combine behavioral and neurophysiological measurements in well-described in vivo models of synaptic plasticity.

EEG desynchronization during phasic REM sleep suppresses interictal epileptic activity in humans

Objective

Rapid eye movement (REM) sleep has a suppressing effect on epileptic activity. This effect might be directly related to neuronal desynchronization mediated by cholinergic neurotransmission. We investigated whether interictal epileptiform discharges (IEDs) and high frequency oscillations—a biomarker of the epileptogenic zone—are evenly distributed across phasic and tonic REM sleep. We hypothesized that IEDs are more suppressed during phasic REM sleep because of additional cholinergic drive.

Methods

Twelve patients underwent polysomnography during long‐term combined scalp‐intracerebral electroencephalography (EEG) recording. After sleep staging in the scalp EEG, we identified segments of REM sleep with rapid eye movements (phasic REM) and segments of REM sleep without rapid eye movements (tonic REM). In the intracerebral EEG, we computed the power in frequencies <30 Hz and from 30 to 500 Hz, and marked IEDs, ripples (>80 Hz) and fast ripples (>250 Hz). We grouped the intracerebral channels into channels in the seizure‐onset zone (SOZ), the exclusively irritative zone (EIZ), and the normal zone (NoZ).

Results

Power in frequencies <30 Hz was lower during phasic than tonic REM sleep (p < 0.001), most likely reflecting increased desynchronization. IEDs, ripples and fast ripples, were less frequent during phasic than tonic REM sleep (phasic REM sleep: 39% of spikes, 35% of ripples, 18% of fast ripples, tonic REM sleep: 61% of spikes, 65% of ripples, 82% of fast ripples; p < 0.001). In contrast to ripples in the epileptogenic zone, physiologic ripples were more abundant during phasic REM sleep (phasic REM sleep: 73% in NoZ, 30% in EIZ, 28% in SOZ, tonic REM sleep: 27% in NoZ, 70% in EIZ, 72% in SOZ; p < 0.001).

Significance

Phasic REM sleep has an enhanced suppressive effect on IEDs, corroborating the role of EEG desynchronization in the suppression of interictal epileptic activity. In contrast, physiologic ripples were increased during phasic REM sleep, possibly reflecting REM‐related memory consolidation and dreaming.

Regulation of Hippocampal Firing by Network Oscillations during Sleep

It has been hypothesized that waking leads to higher-firing neurons, with increased energy expenditure, and that sleep serves to return activity to baseline levels. Oscillatory activity patterns during different stages of sleep may play specific roles in this process, but consensus has been missing. To evaluate these phenomena in the hippocampus, we recorded from region CA1 neurons in rats across the 24-hr cycle, and we found that their firing increased upon waking and decreased 11% per hour across sleep. Waking and sleeping also affected lower- and higher-firing neurons differently. Interestingly, the incidences of sleep spindles and sharp-wave ripples (SWRs), typically associated with cortical plasticity, were predictive of ensuing firing changes and were more robustly predictive than other oscillatory events. Spindles and SWRs were initiated during non-REM sleep, yet the changes were incorporated in the network over the following REM sleep epoch. These findings indicate an important role for spindles and SWRs and provide novel evidence of a symbiotic relationship between non-REM and REM stages of sleep in the homeostatic regulation of neuronal activity.

REM-Enriched Naps Are Associated with Memory Consolidation for Sad Stories and Enhance Mood-Related Reactivity

Emerging evidence suggests that emotion and affect modulate the relation between sleep and cognition. In the present study, we investigated the role of rapid-eye movement (REM) sleep in mood regulation and memory consolidation for sad stories. In a counterbalanced design, participants (n = 24) listened to either a neutral or a sad story during two sessions, spaced one week apart. After listening to the story, half of the participants had a short (45 min) morning nap. The other half had a long (90 min) morning nap, richer in REM and N2 sleep. Story recall, mood evolution and changes in emotional response to the re-exposure to the story were assessed after the nap. Although recall performance was similar for sad and neutral stories irrespective of nap duration, sleep measures were correlated with recall performance in the sad story condition only. After the long nap, REM sleep density positively correlated with retrieval performance, while re-exposure to the sad story led to diminished mood and increased skin conductance levels. Our results suggest that REM sleep may not only be associated with the consolidation of intrinsically sad material, but also enhances mood reactivity, at least on the short term.

Recent Advances in the Study of Sleep in the Anxiety Disorders, Obsessive-Compulsive Disorder, and Posttraumatic Stress Disorder

Sleep disturbance is frequently associated with generalized anxiety disorder, panic disorder, obsessive-compulsive disorder, and posttraumatic stress disorder. This article reviews recent advances in understanding the mechanisms of the sleep disturbances in these disorders and discusses the implications for developing improved treatments.

The role of REM sleep theta activity in emotional memory

While non-REM (NREM) sleep has been strongly implicated in the reactivation and consolidation of memory traces, the role of rapid-eye movement (REM) sleep remains unclear. A growing body of research on humans and animals provide behavioral evidence for a role of REM sleep in the strengthening and modulation of emotional memories. Theta activity-which describes low frequency oscillations in the local field potential within the hippocampus, amygdala and neocortex-is a prominent feature of both wake and REM sleep in humans and rodents. Theta coherence between the hippocampus and amygdala drives large-scale pontine-geniculo-occipital (PGO) waves, the density of which predicts increases in plasticity-related gene expression. This could potentially facilitate the processing of emotional memory traces within the hippocampus during REM sleep. Further, the timing of hippocampal activity in relation to theta phase is vital in determining subsequent potentiation of neuronal activity. This could allow the emotionally modulated strengthening of novel and gradual weakening of consolidated hippocampal memory traces during REM sleep. Hippocampal theta activity is also correlated with REM sleep levels of achetylcholine - which is thought to reduce hippocampal inputs in the neocortex. The additional low levels of noradrenaline during REM sleep, which facilitate feedback within the neocortex, could allow the integration of novel memory traces previously consolidated during NREM sleep. We therefore propose that REM sleep mediates the prioritized processing of emotional memories within the hippocampus, the integration of previously consolidated memory traces within the neocortex, as well as the disengagement of consolidated neocortical memory traces from the hippocampus.

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.

The role of rapid eye movement sleep for amygdala-related memory processing

Over the years, rapid eye movement (REM) sleep has been associated with general memory consolidation, specific consolidation of perceptual, procedural, emotional and fear memories, brain maturation and preparation of waking consciousness. More recently, some of these associations (e.g., general and procedural memory consolidation) have been shown to be unlikely, while others (e.g., brain maturation and consciousness) remain inconclusive. In this review, we argue that both behavioral and neurophysiological evidence supports a role of REM sleep for amygdala-related memory processing: the amygdala-hippocampus-medial prefrontal cortex network involved in emotional processing, fear memory and valence consolidation shows strongest activity during REM sleep, in contrast to the hippocampus-medial prefrontal cortex only network which is more active during non-REM sleep. However, more research is needed to fully understand the mechanisms.

Stress, arousal, and sleep

Stress is considered to be an important cause of disrupted sleep and insomnia. However, controlled and experimental studies in rodents indicate that effects of stress on sleep-wake regulation are complex and may strongly depend on the nature of the stressor. While most stressors are associated with at least a brief period of arousal and wakefulness, the subsequent amount and architecture of recovery sleep can vary dramatically across conditions even though classical markers of acute stress such as corticosterone are virtually the same. Sleep after stress appears to be highly influenced by situational variables including whether the stressor was controllable and/or predictable, whether the individual had the possibility to learn and adapt, and by the relative resilience and vulnerability of the individual experiencing stress. There are multiple brain regions and neurochemical systems linking stress and sleep, and the specific balance and interactions between these systems may ultimately determine the alterations in sleep-wake architecture. Factors that appear to play an important role in stress-induced wakefulness and sleep changes include various monominergic neurotransmitters, hypocretins, corticotropin releasing factor, and prolactin. In addition to the brain regions directly involved in stress responses such as the hypothalamus, the locus coeruleus, and the amygdala, differential effects of stressor controllability on behavior and sleep may be mediated by the medial prefrontal cortex. These various brain regions interact and influence each other and in turn affect the activity of sleep-wake controlling centers in the brain. Also, these regions likely play significant roles in memory processes and participate in the way stressful memories may affect arousal and sleep. Finally, stress-induced changes in sleep-architecture may affect sleep-related neuronal plasticity processes and thereby contribute to cognitive dysfunction and psychiatric disorders.