Attacking sleep from a new angle: contributions from zebrafish

A deleterious variant of INTS1 leads to disrupted sleep-wake cycles

Sleep disturbances are common among children with neurodevelopmental disorders. Here, we report a syndrome characterized by prenatal microcephaly, intellectual disability and severe disruption of sleep-wake cycles in a consanguineous family. Exome sequencing revealed homozygous variants (c.5224G>A and c.6506G>T) leading to the missense mutations E1742K and G2169V in integrator complex subunit 1 (INTS1), the core subunit of the Integrator complex. Conservation and structural analyses suggest that G2169V has a minor impact on the structure and function of the complex, while E1742K significantly alters a negatively charged conserved patch on the surface of the protein. The severe sleep-wake cycles disruption in human carriers highlights a new aspect of Integrator complex impairment. To further study INTS1 pathogenicity, we generated Ints1-deficient zebrafish lines. Mutant zebrafish larvae displayed abnormal circadian rhythms of locomotor activity and sleep, as is the case with the affected humans. Furthermore, Ints1-deficent larvae exhibited elevated levels of dopamine β-hydroxylase (dbh) mRNA in the locus coeruleus, a wakefulness-inducing brainstem center. Altogether, these findings suggest a significant, likely indirect, effect of INTS1 and the Integrator complex on maintaining circadian rhythms of locomotor activity and sleep homeostasis across vertebrates.

FlyVISTA, an Integrated Machine Learning Platform for Deep Phenotyping of Sleep in Drosophila

Animal behavior depends on internal state. While subtle movements can signify significant changes in internal state, computational methods for analyzing these "microbehaviors" are lacking. Here, we present FlyVISTA, a machine-learning platform to characterize microbehaviors in freely-moving flies, which we use to perform deep phenotyping of sleep. This platform comprises a high-resolution closed-loop video imaging system, coupled with a deep-learning network to annotate 35 body parts, and a computational pipeline to extract behaviors from high-dimensional data. FlyVISTA reveals the distinct spatiotemporal dynamics of sleep-associated microbehaviors in flies. We further show that stimulation of dorsal fan-shaped body neurons induces micromovements, not sleep, whereas activating R5 ring neurons triggers rhythmic proboscis extension followed by persistent sleep. Importantly, we identify a novel microbehavior ("haltere switch") exclusively seen during quiescence that indicates a deeper sleep stage. These findings enable the rigorous analysis of sleep in Drosophila and set the stage for computational analyses of microbehaviors.

The interplay between sleep and ecophysiology, behaviour and responses to environmental change in fish

Evidence of behavioural sleep has been observed in every animal species studied to date, but current knowledge of the behaviour, neurophysiology and ecophysiology associated with sleep is concentrated on mammals and birds. Fish are a hugely diverse group that can offer novel insights into a variety of sleep-related behaviours across environments, but the ecophysiological relevance of sleep in fish has been largely overlooked. Here, we systematically reviewed the literature to assess the current breadth of knowledge on fish sleep, and surveyed the diverse physiological effects and behaviours associated with sleep. We also discuss possible ways in which unstudied external factors may alter sleep behaviours. For example, predation risk may alter sleep patterns, as has been shown in mammalian, avian and reptilian species. Other environmental factors - such as water temperature and oxygen availability - have the potential to alter sleep patterns in fish differently than for terrestrial endotherms. Understanding the ecological influences on sleep in fish is vital, as sleep deprivation has the potential to affect waking behaviour and fitness owing to cognitive and physiological impairments, possibly affecting ecological phenomena and sensitivity to environmental stressors in ways that have not been considered.

Adaptive changes in BMAL2 with increased locomotion associated with the evolution of unihemispheric slow-wave sleep in mammals

Marine mammals, especially cetaceans, have evolved a very special form of sleep characterized by unihemispheric slow-wave sleep (USWS) and a negligible amount or complete absence of rapid-eye-movement sleep; however, the underlying genetic mechanisms remain unclear. Here, we detected unique, significant selection signatures in basic helix-loop-helix ARNT like 2 (BMAL2; also called ARNTL2), a key circadian regulator, in marine mammal lineages, and identified two nonsynonymous amino acid substitutions (K204E and K346Q) in the important PER-ARNT-SIM domain of cetacean BMAL2 via sequence comparison with other mammals. In vitro assays revealed that these cetacean-specific mutations specifically enhanced the response to E-box-like enhancer and consequently promoted the transcriptional activation of PER2, which is closely linked to sleep regulation. The increased PER2 expression, which was further confirmed both in vitro and in vivo, is beneficial for allowing cetaceans to maintain continuous movement and alertness during sleep. Concordantly, the locomotor activities of zebrafish overexpressing the cetacean-specific mutant bmal2 were significantly higher than the zebrafish overexpressing the wild-type gene. Subsequently, transcriptome analyses revealed that cetacean-specific mutations caused the upregulation of arousal-related genes and the downregulation of several sleep-promoting genes, which is consistent with the need to maintain hemispheric arousal during USWS. Our findings suggest a potential close relationship between adaptive changes in BMAL2 and the remarkable adaptation of USWS and may provide novel insights into the genetic basis of the evolution of animal sleep.

Label-free, whole-brain in vivo mapping in an adult vertebrate with third harmonic generation microscopy

Comprehensive understanding of interconnected networks within the brain requires access to high resolution information within large field of views and over time. Currently, methods that enable mapping structural changes of the entire brain in vivo are extremely limited. Third harmonic generation (THG) can resolve myelinated structures, blood vessels, and cell bodies throughout the brain without the need for any exogenous labeling. Together with deep penetration of long wavelengths, this enables in vivo brain-mapping of large fractions of the brain in small animals and over time. Here, we demonstrate that THG microscopy allows non-invasive label-free mapping of the entire brain of an adult vertebrate, Danionella dracula, which is a miniature species of cyprinid fish. We show this capability in multiple brain regions and in particular the identification of major commissural fiber bundles in the midbrain and the hindbrain. These features provide readily discernable landmarks for navigation and identification of regional-specific neuronal groups and even single neurons during in vivo experiments. We further show how this label-free technique can easily be coupled with fluorescence microscopy and used as a comparative tool for studies of other species with similar body features to Danionella, such as zebrafish (Danio rerio) and tetras (Trochilocharax ornatus). This new evidence, building on previous studies, demonstrates how small size and relative transparency, combined with the unique capabilities of THG microscopy, can enable label-free access to the entire adult vertebrate brain.

Screening effects of HCN channel blockers on sleep/wake behavior in zebrafish

Hyperpolarization-activated cyclic nucleotide-gated (HCN) ion channels generate electrical rhythmicity in various tissues although primarily heart, retina and brain. The HCN channel blocker compound, Ivabradine (Corlanor), is approved by the US Food and Drug Administration (FDA) as a medication to lower heart rate by blocking hyperpolarization activated inward current in the sinoatrial node. In addition, a growing body of evidence suggests a role for HCN channels in regulation of sleep/wake behavior. Zebrafish larvae are ideal model organisms for high throughput drug screening, drug repurposing and behavioral phenotyping studies. We leveraged this model system to investigate effects of three HCN channel blockers (Ivabradine, Zatebradine Hydrochloride and ZD7288) at multiple doses on sleep/wake behavior in wild type zebrafish. Results of interest included shorter latency to daytime sleep at 0.1 μM dose of Ivabradine (ANOVA, p: 0.02), moderate reduction in average activity at 30 μM dose of Zatebradine Hydrochloride (ANOVA, p: 0.024) in daytime, and increased nighttime sleep at 4.5 μM dose of ZD7288 (ANOVA, p: 0.036). Taken together, shorter latency to daytime sleep, decrease in daytime activity and increased nighttime sleep indicate that different HCN channel antagonists affected different parameters of sleep and activity.

Oral nano-antioxidants improve sleep by restoring intestinal barrier integrity and preventing systemic inflammation

Sleep deprivation (SD) is a severe public health threat that can cause systemic inflammation and nerve damage. Few effective and side-effect-free drugs are available to address SD. However, the bidirectional communications between the brain and gut provide new strategies for anti-SD therapeutics. Here we explored oral delivery of fullerene nano-antioxidants (FNAO) in the SD model to improve sleep by regulating abnormal intestinal barrier and systemic inflammation via the brain-gut axis. SD caused excessive reactive oxygen species (ROS) production and hyperactive inflammatory responses in the intestines of zebrafish and mouse models, leading to disturbed sleep patterns and reduced brain nerve activity. Of note, based on the property of the conjugated π bond of the C60 structure to absorb unpaired electrons, oral FNAO efficiently reduced the excessive ROS in the intestines, maintained redox homeostasis and intestinal barrier integrity, and ameliorated intestinal and systemic inflammation, resulting in superior sleep improvement. Our findings suggest that maintaining intestinal homeostasis may be a promising avenue for SD-related nerve injury therapy.

Does EGFR Signaling Mediate Orexin System Activity in Sleep Initiation?

Sleep-wake cycle disorders are an important symptom of many neurological diseases, including Parkinson's disease, Alzheimer's disease, and multiple sclerosis. Circadian rhythms and sleep-wake cycles play a key role in maintaining the health of organisms. To date, these processes are still poorly understood and, therefore, need more detailed elucidation. The sleep process has been extensively studied in vertebrates, such as mammals and, to a lesser extent, in invertebrates. A complex, multi-step interaction of homeostatic processes and neurotransmitters provides the sleep-wake cycle. Many other regulatory molecules are also involved in the cycle regulation, but their functions remain largely unclear. One of these signaling systems is epidermal growth factor receptor (EGFR), which regulates the activity of neurons in the modulation of the sleep-wake cycle in vertebrates. We have evaluated the possible role of the EGFR signaling pathway in the molecular regulation of sleep. Understanding the molecular mechanisms that underlie sleep-wake regulation will provide critical insight into the fundamental regulatory functions of the brain. New findings of sleep-regulatory pathways may provide new drug targets and approaches for the treatment of sleep-related diseases.

Zebrafish (Danio rerio) shoaling in light and dark conditions involves a complex interplay between vision and lateral line

We know little about how - or even if in some species - fish shoal in darkness. We hypothesized that 'dark shoaling' occurs in zebrafish and therefore must depend upon lateral line sensory input. Shoaling in groups of five adult zebrafish was analyzed with motion tracking software. We measured average inter-individual distance, time near the arena wall (thigmotaxis zone) and total distance traveled under normal room light, and in near-complete darkness (infrared light at 850 nm). These observations were repeated in fish treated with cobalt chloride (CoCl₂), which ablates lateral line function. In untreated controls, dark shoaling was reduced compared to in light, but nonetheless still present. Elimination of lateral line sensory input by CoCl₂ treatment similarly reduced, but did not eliminate, shoaling under both light and dark. Our findings indicate that normal zebrafish shoaling in light or dark requires both visual and lateral line inputs, with neither alone sufficient for normal shoaling.

The emergence and influence of internal states

Animal behavior is shaped by a variety of "internal states"-partially hidden variables that profoundly shape perception, cognition, and action. The neural basis of internal states, such as fear, arousal, hunger, motivation, aggression, and many others, is a prominent focus of research efforts across animal phyla. Internal states can be inferred from changes in behavior, physiology, and neural dynamics and are characterized by properties such as pleiotropy, persistence, scalability, generalizability, and valence. To date, it remains unclear how internal states and their properties are generated by nervous systems. Here, we review recent progress, which has been driven by advances in behavioral quantification, cellular manipulations, and neural population recordings. We synthesize research implicating defined subsets of state-inducing cell types, widespread changes in neural activity, and neuromodulation in the formation and updating of internal states. In addition to highlighting the significance of these findings, our review advocates for new approaches to clarify the underpinnings of internal brain states across the animal kingdom.

Fluoxetine modifies circadian rhythm by reducing melatonin content in zebrafish

Fluoxetine (FLX), a selective serotonin reuptake inhibitor (SSRI), increases the serotonin levels in the brain to treat depression. Antidepressants have been demonstrated to modulate circadian rhythm, but the underlying mechanisms by which antidepressants regulate circadian rhythm require more research. This study aimed to investigate the role of FLX on circadian rhythm by analyzing the movement behavior and internal circadian oscillations in zebrafish. The results showed that the expression of clock genes clock1a and bmal1b was significantly down-regulated, and the amplitude reduction and phase shift were observed after FLX treatment. Furthermore, FLX exposure inhibited the expression of aanat2, which led to a decrease in nocturnal melatonin secretion. aanat2^-/- larvae showed disrupted circadian rhythm. These findings may help reveal the effect of FLX exposure on the circadian rhythm and locomotor activity. It may provide theoretical data for the clinical application of FLX.

Validation of Candidate Sleep Disorder Risk Genes Using Zebrafish

Sleep disorders and chronic sleep disturbances are common and are associated with cardio-metabolic diseases and neuropsychiatric disorders. Several genetic pathways and neuronal mechanisms that regulate sleep have been described in animal models, but the genes underlying human sleep variation and sleep disorders are largely unknown. Identifying these genes is essential in order to develop effective therapies for sleep disorders and their associated comorbidities. To address this unmet health problem, genome-wide association studies (GWAS) have identified numerous genetic variants associated with human sleep traits and sleep disorders. However, in most cases, it is unclear which gene is responsible for a sleep phenotype that is associated with a genetic variant. As a result, it is necessary to experimentally validate candidate genes identified by GWAS using an animal model. Rodents are ill-suited for this endeavor due to their poor amenability to high-throughput sleep assays and the high costs associated with generating, maintaining, and testing large numbers of mutant lines. Zebrafish (Danio rerio), an alternative vertebrate model for studying sleep, allows for the rapid and cost-effective generation of mutant lines using the CRISPR/Cas9 system. Numerous zebrafish mutant lines can then be tested in parallel using high-throughput behavioral assays to identify genes whose loss affects sleep. This process identifies a gene associated with each GWAS hit that is likely responsible for the human sleep phenotype. This strategy is a powerful complement to GWAS approaches and holds great promise to identify the genetic basis for common human sleep disorders.

Loss of circadian rhythmicity in bdnf knockout zebrafish larvae

Brain-derived neurotrophic factor (BDNF) plays a pivotal role in neuronal growth and differentiation, neuronal plasticity, learning, and memory. Using CRISPR/Cas9 technology, we generated a vital Bdnf null mutant line in zebrafish and carried out its molecular and behavioral characterization. Although no defects are evident on a morphological inspection, 66% of coding genes and 37% of microRNAs turned out to be differentially expressed in bdnf^−/− compared with wild type sibling embryos. We deeply investigated the circadian clock pathway and confirmed changes in the rhythmic expression of clock (arntl1a, clock1a and clock2) and clock-controlled (aanat2) genes. The modulatory role of Bdnf on the zebrafish circadian clock was then validated by behavioral tests highlighting the absence of circadian activity rhythms in bdnf^−/− larvae. The circadian behavior was partially rescued by pharmacological treatment. The bdnf^−/− zebrafish line presented here is the first valuable and stable vertebrate model for the study of BDNF-related neurodevelopmental diseases

•

Generation of a viable bdnf KO line in zebrafish

•

Bdnf deficiency affects locomotor activity and thigmotaxis in larvae

•

Differential RNA-seq analysis shows changes in expression of circadian clock genes

•

Bdnf mutant fails in the generation of the behavioral circadian rhythmicity

Synaptic dysfunction connects autism spectrum disorder and sleep disturbances: A perspective from studies in model organisms

Sleep disturbances (SD) accompany many neurodevelopmental disorders, suggesting SD is a transdiagnostic process that can account for behavioral deficits and influence underlying neuropathogenesis. Autism Spectrum Disorder (ASD) comprises a complex set of neurodevelopmental conditions characterized by challenges in social interaction, communication, and restricted, repetitive behaviors. Diagnosis of ASD is based primarily on behavioral criteria, and there are no drugs that target core symptoms. Among the co-occurring conditions associated with ASD, SD are one of the most prevalent. SD often arises before the onset of other ASD symptoms. Sleep interventions improve not only sleep but also daytime behaviors in children with ASD. Here, we examine sleep phenotypes in multiple model systems relevant to ASD, e.g., mice, zebrafish, fruit flies and worms. Given the functions of sleep in promoting brain connectivity, neural plasticity, emotional regulation and social behavior, all of which are of critical importance in ASD pathogenesis, we propose that synaptic dysfunction is a major mechanism that connects ASD and SD. Common molecular targets in this interplay that are involved in synaptic function might be a novel avenue for therapy of individuals with ASD experiencing SD. Such therapy would be expected to improve not only sleep but also other ASD symptoms.

Neuromodulation and Behavioral Flexibility in Larval Zebrafish: From Neurotransmitters to Circuits

Animals adapt their behaviors to their ever-changing needs. Internal states, such as hunger, fear, stress, and arousal are important behavioral modulators controlling the way an organism perceives sensory stimuli and reacts to them. The translucent zebrafish larva is an ideal model organism for studying neuronal circuits regulating brain states, owning to the possibility of easy imaging and manipulating activity of genetically identified neurons while the animal performs stereotyped and well-characterized behaviors. The main neuromodulatory circuits present in mammals can also be found in the larval zebrafish brain, with the advantage that they contain small numbers of neurons. Importantly, imaging and behavioral techniques can be combined with methods for generating targeted genetic modifications to reveal the molecular underpinnings mediating the functions of such circuits. In this review we discuss how studying the larval zebrafish brain has contributed to advance our understanding of circuits and molecular mechanisms regulating neuromodulation and behavioral flexibility.

Neuropeptide VF neurons promote sleep via the serotonergic raphe

Although several sleep-regulating neuronal populations have been identified, little is known about how they interact with each other to control sleep/wake states. We previously identified neuropeptide VF (NPVF) and the hypothalamic neurons that produce it as a sleep-promoting system (Lee et al., 2017). Here we show using zebrafish that npvf-expressing neurons control sleep via the serotonergic raphe nuclei (RN), a hindbrain structure that is critical for sleep in both diurnal zebrafish and nocturnal mice. Using genetic labeling and calcium imaging, we show that npvf-expressing neurons innervate and can activate serotonergic RN neurons. We also demonstrate that chemogenetic or optogenetic stimulation of npvf-expressing neurons induces sleep in a manner that requires NPVF and serotonin in the RN. Finally, we provide genetic evidence that NPVF acts upstream of serotonin in the RN to maintain normal sleep levels. These findings reveal a novel hypothalamic-hindbrain neuronal circuit for sleep/wake control.

Hierarchical Compression Reveals Sub-Second to Day-Long Structure in Larval Zebrafish Behavior

Animal behavior is dynamic, evolving over multiple timescales from milliseconds to days and even across a lifetime. To understand the mechanisms governing these dynamics, it is necessary to capture multi-timescale structure from behavioral data. Here, we develop computational tools and study the behavior of hundreds of larval zebrafish tracked continuously across multiple 24-h day/night cycles. We extracted millions of movements and pauses, termed bouts, and used unsupervised learning to reduce each larva’s behavior to an alternating sequence of active and inactive bout types, termed modules. Through hierarchical compression, we identified recurrent behavioral patterns, termed motifs. Module and motif usage varied across the day/night cycle, revealing structure at sub-second to day-long timescales. We further demonstrate that module and motif analysis can uncover novel pharmacological and genetic mutant phenotypes. Overall, our work reveals the organization of larval zebrafish behavior at multiple timescales and provides tools to identify structure from large-scale behavioral datasets.

Bioelectronics for Millimeter-Sized Model Organisms

Advances in microfabrication technologies and biomaterials have enabled a growing class of electronic devices that can stimulate and record bioelectronic signals. Many of these devices have been developed for humans or vertebrate animals, where miniaturization allows for implantation within the body. There are, however, another class of bioelectronic interfaces that exploit microfabrication and nanoelectronics to record signals from tiny, millimeter-sized organisms. In these cases, rather than implanting a device inside an animal, animals themselves are loaded in large numbers into bioelectronic devices for neural circuit and behavioral interrogation. These scalable interfaces provide platforms to develop new therapeutics as well as better understand basic principles of bioelectronic communication, neuroscience, and behavior. Here we review recent progress in these bioelectronic technologies and describe how they can complement on-chip optical, mechanical, and chemical interrogation methods to achieve high-throughput, multimodal studies of millimeter-sized small animals.

Evolutionarily conserved regulation of sleep by epidermal growth factor receptor signaling

The genetic bases for most human sleep disorders and for variation in human sleep quantity and quality are largely unknown. Using the zebrafish, a diurnal vertebrate, to investigate the genetic regulation of sleep, we found that epidermal growth factor receptor (EGFR) signaling is necessary and sufficient for normal sleep levels and is required for the normal homeostatic response to sleep deprivation. We observed that EGFR signaling promotes sleep via mitogen-activated protein kinase/extracellular signal-regulated kinase and RFamide neuropeptide signaling and that it regulates RFamide neuropeptide expression and neuronal activity. Consistent with these findings, analysis of a large cohort of human genetic data from participants of European ancestry revealed that common variants in genes within the EGFR signaling pathway are associated with variation in human sleep quantity and quality. These results indicate that EGFR signaling and its downstream pathways play a central and ancient role in regulating sleep and provide new therapeutic targets for sleep disorders.

The Serotonergic Raphe Promote Sleep in Zebrafish and Mice

The role of serotonin (5-HT) in sleep is controversial: early studies suggested a sleep-promoting role, but eventually the paradigm shifted toward a wake-promoting function for the serotonergic raphe. Here, we provide evidence from zebrafish and mice that the raphe are critical for the initiation and maintenance of sleep. In zebrafish, genetic ablation of 5-HT production by the raphe reduces sleep, sleep depth, and the homeostatic response to sleep deprivation. Pharmacological inhibition or ablation of the raphe reduces sleep, while optogenetic stimulation increases sleep. Similarly, in mice, ablation of the raphe increases wakefulness and impairs the homeostatic response to sleep deprivation, whereas tonic optogenetic stimulation at a rate similar to baseline activity induces sleep. Interestingly, burst optogenetic stimulation induces wakefulness in accordance with previously described burst activity of the raphe during arousing stimuli. These results indicate that the serotonergic system promotes sleep in both diurnal zebrafish and nocturnal rodents. VIDEO ABSTRACT.

On the cause of sleep: Protein fragments, the concept of sentinels, and links to epilepsy

The molecular-level cause of sleep is unknown. In 2012, we suggested that the cause of sleep stems from cumulative effects of numerous intracellular and extracellular protein fragments. According to the fragment generation (FG) hypothesis, protein fragments (which are continually produced through nonprocessive cleavages by intracellular, intramembrane, and extracellular proteases) can be beneficial but toxic as well, and some fragments are eliminated slowly during wakefulness. We consider the FG hypothesis and propose that, during wakefulness, the degradation of accumulating fragments is delayed within natural protein aggregates such as postsynaptic densities (PSDs) in excitatory synapses and in other dense protein meshworks, owing to an impeded diffusion of the ∼3,000-kDa 26S proteasome. We also propose that a major function of sleep involves a partial and reversible expansion of PSDs, allowing an accelerated destruction of PSD-localized fragments by the ubiquitin/proteasome system. Expansion of PSDs would alter electrochemistry of synapses, thereby contributing to a decreased neuronal firing during sleep. If so, the loss of consciousness, a feature of sleep, would be the consequence of molecular processes (expansions of protein meshworks) that are required for degradation of protein fragments. We consider the concept of FG sentinels, which signal to sleep-regulating circuits that the levels of fragments are going up. Also discussed is the possibility that protein fragments, which are known to be overproduced during an epileptic seizure, may contribute to postictal sleep and termination of seizures. These and related suggestions, described in the paper, are compatible with current evidence about sleep and lead to testable predictions.

Circadian Clocks in Fish-What Have We Learned so far?

Zebrafish represent the one alternative vertebrate, genetic model system to mice that can be easily manipulated in a laboratory setting. With the teleost Medaka (Oryzias latipes), which now has a significant following, and over 30,000 other fish species worldwide, there is great potential to study the biology of environmental adaptation using teleosts. Zebrafish are primarily used for research on developmental biology, for obvious reasons. However, fish in general have also contributed to our understanding of circadian clock biology in the broadest sense. In this review, we will discuss selected areas where this contribution seems most unique. This will include a discussion of the issue of central versus peripheral clocks, in which zebrafish played an early role; the global nature of light sensitivity; and the critical role played by light in regulating cell biology. In addition, we also discuss the importance of the clock in controlling the timing of fundamental aspects of cell biology, such as the temporal control of the cell cycle. Many of these findings are applicable to the majority of vertebrate species. However, some reflect the unique manner in which “fish” can solve biological problems, in an evolutionary context. Genome duplication events simply mean that many fish species have more gene copies to “throw at a problem”, and evolution seems to have taken advantage of this “gene abundance”. How this relates to their poor cousins, the mammals, remains to be seen.

Genetic sleep deprivation: using sleep mutants to study sleep functions

Sleep is a fundamental conserved physiological state in animals and humans. It may serve multiple functions, ranging from energy conservation to higher brain operation. Understanding sleep functions and the underlying mechanisms requires the study of sleeplessness and its consequences. The traditional approach to remove sleep is sleep deprivation (SD) by sensory stimulation. However, stimulation-induced SD can be stressful and can cause non-specific side effects. An emerging alternative method is "genetic SD", which removes sleep using genetics or optogenetics. Sleep requires sleep-active neurons and their regulators. Thus, genetic impairment of sleep circuits might lead to more specific and comprehensive sleep loss. Here, I discuss the advantages and limits of genetic SD in key genetic sleep model animals: rodents, zebrafish, fruit flies and roundworms, and how the study of genetic SD alters our view of sleep functions. Genetic SD typically causes less severe phenotypes compared with stimulation-induced SD, suggesting that sensory stimulation-induced SD may have overestimated the role of sleep, calling for a re-investigation of sleep functions.

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.

Electrophysiological characterization of sleep/wake, activity and the response to caffeine in adult cynomolgus macaques

Most preclinical sleep studies are conducted in nocturnal rodents that have fragmented sleep in comparison to humans who are primarily diurnal, typically with a consolidated sleep period. Consequently, we sought to define basal sleep characteristics, sleep/wake architecture and electroencephalographic (EEG) activity in a diurnal non-human primate (NHP) to evaluate the utility of this species for pharmacological manipulation of the sleep/wake cycle. Adult, 9–11 y.o. male cynomolgus macaques (n = 6) were implanted with telemetry transmitters to record EEG and electromyogram (EMG) activity and Acticals to assess locomotor activity under baseline conditions and following injections either with vehicle or the caffeine (CAF; 10 mg/kg, i.m.) prior to the 12 h dark phase. EEG/EMG recordings (12–36 h in duration) were analyzed for sleep/wake states and EEG spectral composition. Macaques exhibited a sleep state distribution and architecture similar to previous NHP and human sleep studies. Acute administration of CAF prior to light offset enhanced wakefulness nearly 4-fold during the dark phase with consequent reductions in both NREM and REM sleep, decreased slow wave activity during wakefulness, and increased higher EEG frequency activity during NREM sleep. Despite the large increase in wakefulness and profound reduction in sleep during the dark phase, no sleep rebound was observed during the 24 h light and dark phases following caffeine administration. Cynomolgus macaques show sleep characteristics, EEG spectral structure, and respond to CAF in a similar manner to humans. Consequently, monitoring EEG/EMG by telemetry in this species may be useful both for basic sleep/wake studies and for pre-clinical assessments of drug-induced effects on sleep/wake.

•

Cynomolgus macaques show diurnal sleep/wake architecture similar to humans.

•

Caffeine enhanced wakefulness with consequent reductions in both NREM and REM sleep.

•

Caffeine decreased slow wave activity during wakefulness.

•

Caffeine increased higher EEG frequency activity during NREM sleep.

•

No sleep rebound was observed during the subsequent 24 h light and dark phases after CAF treatment.

Neuronal Mechanisms for Sleep/Wake Regulation and Modulatory Drive

Humans have been fascinated by sleep for millennia. After almost a century of scientific interrogation, significant progress has been made in understanding the neuronal regulation and functions of sleep. The application of new methods in neuroscience that enable the analysis of genetically defined neuronal circuits with unprecedented specificity and precision has been paramount in this endeavor. In this review, we first discuss electrophysiological and behavioral features of sleep/wake states and the principal neuronal populations involved in their regulation. Next, we describe the main modulatory drives of sleep and wakefulness, including homeostatic, circadian, and motivational processes. Finally, we describe a revised integrative model for sleep/wake regulation.

Impaired Sleep, Circadian Rhythms and Neurogenesis in Diet-Induced Premature Aging

Chronic high caloric intake (HCI) is a risk factor for multiple major human disorders, from diabetes to neurodegeneration. Mounting evidence suggests a significant contribution of circadian misalignment and sleep alterations to this phenomenon. An inverse temporal relationship between sleep, activity, food intake, and clock mechanisms in nocturnal and diurnal animals suggests that a search for effective therapeutic approaches can benefit from the use of diurnal animal models. Here, we show that, similar to normal aging, HCI leads to the reduction in daily amplitude of expression for core clock genes, a decline in sleep duration, an increase in scoliosis, and anxiety-like behavior. A remarkable decline in adult neurogenesis in 1-year old HCI animals, amounting to only 21% of that in age-matched Control, exceeds age-dependent decline observed in normal 3-year old zebrafish. This is associated with misalignment or reduced amplitude of daily patterns for principal cell cycle regulators, cyclins A and B, and p20, in brain tissue. Together, these data establish HCI in zebrafish as a model for metabolically induced premature aging of sleep, circadian functions, and adult neurogenesis, allowing for a high throughput approach to mechanistic studies and drug trials in a diurnal vertebrate.

Light-Dependent Regulation of Sleep and Wake States by Prokineticin 2 in Zebrafish

Light affects sleep and wake behaviors by providing an indirect cue that entrains circadian rhythms and also by inducing a direct and rapid regulation of behavior. While circadian entrainment by light is well characterized at the molecular level, mechanisms that underlie the direct effect of light on behavior are largely unknown. In zebrafish, a diurnal vertebrate, we found that both overexpression and mutation of the neuropeptide prokineticin 2 (Prok2) affect sleep and wake behaviors in a light-dependent but circadian-independent manner. In light, Prok2 overexpression increases sleep and induces expression of galanin (galn), a hypothalamic sleep-inducing peptide. We also found that light-dependent, Prok2-induced sedation requires prokineticin receptor 2 (prokr2) and is strongly suppressed in galn mutants. These results suggest that Prok2 antagonizes the direct wake-promoting effect of light in zebrafish, in part through the induction of galn expression in the hypothalamus.