Neuronal and molecular mechanisms of sleep homeostasis

The histone acylation reader ENL/AF9 regulates aging in Drosophila melanogaster

Histone acylation plays a pivotal role in modulating gene expression, ensuring proper neurogenesis and responsiveness to various signals. Recently, the evolutionary conserved YAF9, ENL, AF9, TAF41, SAS5 (YEATS) domain found in four human paralogs, has emerged as a new class of histone acylation reader with a preference for the bulkier crotonyl group lysine over acetylation. Despite advancements, the role of either histone crotonylation or its readers in neurons remains unclear. In this study, we employed Drosophila melanogaster to investigate the role of ENL/AF9 (dENL/AF9) in the nervous system. Pan-neuronal dENL/AF9 knockdown not only extended the lifespan of flies but also enhanced their overall fitness during aging, including improved sleep quality and locomotion. Moreover, a decreased activity of dENL/AF9 in neurons led to an up-regulation of catalase gene expression which combined with reduced levels of malondialdehyde (MDA) and an enhanced tolerance to oxidative stress in aging flies. This study unveiled a novel function of histone crotonylation readers in aging with potential implications for understanding age-related conditions in humans.

Activity Monitoring for Analysis of Sleep in Drosophila melanogaster

Sleep is important for survival, and the need for sleep is conserved across species. In the past two decades, the fruit fly Drosophila melanogaster has emerged as a promising system in which to study the genetic, neural, and physiological bases of sleep. Through significant advances in our understanding of the regulation of sleep in flies, the field is poised to address several open questions about sleep, such as how the need for sleep is encoded, how molecular regulators of sleep are situated within brain networks, and what the functions of sleep are. Here, we describe key findings, open questions, and commonly used methods that have been used to inform existing theories and develop new ways of thinking about the function, regulation, and adaptability of sleep behavior.

Drosophila noktochor regulates night sleep via a local mushroom body circuit

We show that a sleep-regulating, Ig-domain protein (NKT) is secreted from Drosophila mushroom body (MB) α'/β' neurons to act locally on other MB cell types. Pan-neuronal or broad MB expression of membrane-tethered NKT (tNkt) protein reduced sleep, like that of an NKT null mutant, suggesting blockade of a receptor mediating endogenous NKT action. In contrast, expression in neurons requiring NKT (the MB α'/β' cells), or non-MB sleep-regulating centers, did not reduce night sleep, indicating the presence of a local MB sleep-regulating circuit consisting of communicating neural subtypes. We suggest that the leucocyte-antigen-related like (Lar) transmembrane receptor may mediate NKT action. Knockdown or overexpression of Lar in the MB increased or decreased sleep, respectively, indicating the receptor promotes wakefulness. Surprisingly, selective expression of tNkt or knockdown of Lar in MB wake-promoting cells increased rather than decreased sleep, suggesting that NKT acts on wake- as well as sleep-promoting cell types to regulate sleep.

Continuous home cage monitoring of activity and sleep in mice during repeated paroxetine treatment and discontinuation

RATIONALE

Non-invasive home cage monitoring is emerging as a valuable tool to assess the effects of experimental interventions on mouse behaviour. A field in which these techniques may prove useful is the study of repeated selective serotonin reuptake inhibitor (SSRI) treatment and discontinuation. SSRI discontinuation syndrome is an under-researched condition that includes the emergence of sleep disturbances following treatment cessation.

OBJECTIVES

We used passive infrared (PIR) monitoring to investigate changes in activity, sleep, and circadian rhythms during repeated treatment with the SSRI paroxetine and its discontinuation in mice.

METHODS

Male mice received paroxetine (10 mg/kg/day, s.c.) for 12 days, then were swapped to saline injections for a 13 day discontinuation period and compared to mice that received saline injections throughout. Mice were continuously tracked using the Continuous Open Mouse Phenotyping of Activity and Sleep Status (COMPASS) system.

RESULTS

Repeated paroxetine treatment reduced activity and increased behaviourally-defined sleep in the dark phase. These effects recovered to saline-control levels within 24 h of paroxetine cessation, yet there was also evidence of a lengthening of sleep bouts in the dark phase for up to a week following discontinuation.

CONCLUSIONS

This study provides the first example of how continuous non-invasive home cage monitoring can be used to detect objective behavioural changes in activity and sleep during and after drug treatment in mice. These data suggest that effects of paroxetine administration reversed soon after its discontinuation but identified an emergent change in sleep bout duration, which could be used as a biomarker in future preclinical studies to prevent or minimise SSRI discontinuation symptoms.

The Dynamic Relationship between the Glymphatic System, Aging, Memory, and Sleep

The process of memory entails the activation of numerous neural networks and biochemical pathways throughout the brain. The phenomenon of memory decline in relation to aging has been the subject of extensive research for several decades. The correlation between the process of aging and memory is intricate and has various aspects to consider. Throughout the aging process, there are various alterations that take place within the brain and, as expected, affect other functions that have already been linked to memory and its function such as involving microcirculation and sleep. Recent studies provide an understanding of how these mechanisms may be interconnected through the relatively new concept of the glymphatic system. The glymphatic system is strongly correlated to sleep processes. Sleep helps the glymphatic system remove brain waste solutes. Astrocytes expand and contract to form channels for cerebrospinal fluid (CSF) to wash through the brain and eliminate waste. However, the details have not been totally elusive, but the discovery of what we call the glymphatic system enables us to connect many pieces of physiology to understand how such factors are interconnected and the interplay between them. Thus, the purpose of this review is to discuss how the glymphatic system, sleep, memory, and aging are interconnected through a network of complex mechanisms and dynamic interactions.

The Effects of Acute and Chronic Sleep Deprivation on the Immune Profile in the Rat

BACKGROUND

Acute and chronic sleep deprivation present many health-related problems in modern societies, mainly concerning the immune system. Immune factors, particularly the interleukins, regulate sleep and, therefore, may be altered by sleep deprivation (SD).

OBJECTIVES

We aimed to investigate the possible effects of acute and chronic sleep deprivation on selected cytokines, including interleukins (IL-1β, IL-9, IL-17, and IL-23) and tumor necrosis factor- alpha (TNF-α).

METHODS

The animals were grouped into acute sleep-deprived (SD; for 24 hours) and chronic sleep-deprived (8 hours a day for 10, 20, and 30-days). The SD was induced using the multipleplatforms model. The serum levels of cytokines were measured using commercially available ELISA.

RESULTS

The serum levels of IL-1β were significantly reduced after acute SD, whereas they were increased after 20-days of chronic SD. The IL-9 levels were reduced after acute SD, increased after 10-days of SD, and reduced again after 30-days of SD. Conversely, the levels of IL-23 were not changed after acute SD, reduced after 10 days of SD, and increased after 30-days of SD. Levels of TNF-α were not changed after acute SD, whereas they were increased after 20 and 30- days of SD.

CONCLUSION

In conclusion, both acute and chronic SD distinctly disturb the immune profile, which might result in the emergence of various pathologies presented during sleep deprivation.

Acetylcholine deficit causes dysfunctional inhibitory control in an aging-dependent manner

Inhibitory control is a key executive function that limits unnecessary thoughts and actions, enabling an organism to appropriately execute goal-driven behaviors. The efficiency of this inhibitory capacity declines with normal aging or in neurodegenerative dementias similar to memory or other cognitive functions. Acetylcholine signaling is crucial for executive function and also diminishes with aging. Acetylcholine's contribution to the aging- or dementia-related decline in inhibitory control, however, remains elusive. We addressed this in Drosophila using a Go/No-Go task that measures inhibition capacity. Here, we report that inhibition capacity declines with aging in wild-type flies, which is mitigated by lessening acetylcholine breakdown and augmented by reducing acetylcholine biosynthesis. We identified the mushroom body (MB) γ neurons as a chief neural site for acetylcholine's contribution to the aging-associated inhibitory control deficit. In addition, we found that the MB output neurons MBON-γ2α'1 having dendrites at the MB γ2 and α'1 lobes and axons projecting to the superior medial protocerebrum and the crepine is critical for sustained movement suppression per se. This study reveals, for the first time, the central role of acetylcholine in the aging-associated loss of inhibitory control and provides a framework for further mechanistic studies.

Sleep Modulates Alcohol Toxicity in Drosophila

Alcohol abuse is a significant public health problem. While considerable research has shown that alcohol use affects sleep, little is known about the role of sleep deprivation in alcohol toxicity. We investigated sleep as a factor modulating alcohol toxicity using Drosophila melanogaster, a model for studies of sleep, alcohol, and aging. Following 24 h of sleep deprivation using a paradigm that similarly affects males and females and induces rebound sleep, flies were given binge-like alcohol exposures. Sleep deprivation increased mortality, with no sex-dependent differences. Sleep deprivation also abolished functional tolerance measured at 24 h after the initial alcohol exposure, although there was no effect on alcohol absorbance or clearance. We investigated the effect of chronic sleep deprivation using mutants with decreased sleep, insomniac and insulin-like peptide 2, finding increased alcohol mortality. Furthermore, we investigated whether pharmacologically inducing sleep prior to alcohol exposure using the GABA_A-receptor agonist 4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol (THIP) mitigated the effects of alcohol toxicity on middle-aged flies, flies with environmentally disrupted circadian clocks, and flies with short sleep. Pharmacologically increasing sleep prior to alcohol exposure decreased alcohol-induced mortality. Thus, sleep prior to binge-like alcohol exposure affects alcohol-induced mortality, even in vulnerable groups such as aging flies and those with circadian dysfunction.

Homeostasis theory: What can we learn from dormancy and symbiotic associations?

In this letter, I discuss the notion of dormancy that De Luca Jr. relies on to criticize the theory of homeostasis. In particular, I try to qualify the issues related to the fact that dormancy is not always a free behavior but is in most situations under the influence of environmental factors. To this end, I discuss diapause in arthropods, which can be obligatory (under the influence of endogenous commands) but which is in most cases facultative (under external command). I emphasize that the notion of stability of a dormant organism must be taken with caution. I briefly mention what the study of sleep in animals can contribute to the notion of homeostasis. Finally, I focus on the role of microbial symbionts and the notion of holobiont. Through this, I question the future of the notions of internal environment and homeostasis and I propose to revisit them in the context of the effects of species interactions on the physiology of organisms.

Hugin + neurons provide a link between sleep homeostat and circadian clock neurons

Sleep is controlled by homeostatic mechanisms, which drive sleep after wakefulness, and a circadian clock, which confers the 24-h rhythm of sleep. These processes interact with each other to control the timing of sleep in a daily cycle as well as following sleep deprivation. However, the mechanisms by which they interact are poorly understood. We show here that hugin ⁺ neurons, previously identified as neurons that function downstream of the clock to regulate rhythms of locomotor activity, are also targets of the sleep homeostat. Sleep deprivation decreases activity of hugin ⁺ neurons, likely to suppress circadian-driven activity during recovery sleep, and ablation of hugin ⁺ neurons promotes sleep increases generated by activation of the homeostatic sleep locus, the dorsal fan-shaped body (dFB). Also, mutations in peptides produced by the hugin ⁺ locus increase recovery sleep following deprivation. Transsynaptic mapping reveals that hugin ⁺ neurons feed back onto central clock neurons, which also show decreased activity upon sleep loss, in a Hugin peptide-dependent fashion. We propose that hugin ⁺ neurons integrate circadian and sleep signals to modulate circadian circuitry and regulate the timing of sleep.

A connectome of the Drosophila central complex reveals network motifs suitable for flexible navigation and context-dependent action selection

Flexible behaviors over long timescales are thought to engage recurrent neural networks in deep brain regions, which are experimentally challenging to study. In insects, recurrent circuit dynamics in a brain region called the central complex (CX) enable directed locomotion, sleep, and context- and experience-dependent spatial navigation. We describe the first complete electron microscopy-based connectome of the Drosophila CX, including all its neurons and circuits at synaptic resolution. We identified new CX neuron types, novel sensory and motor pathways, and network motifs that likely enable the CX to extract the fly's head direction, maintain it with attractor dynamics, and combine it with other sensorimotor information to perform vector-based navigational computations. We also identified numerous pathways that may facilitate the selection of CX-driven behavioral patterns by context and internal state. The CX connectome provides a comprehensive blueprint necessary for a detailed understanding of network dynamics underlying sleep, flexible navigation, and state-dependent action selection.

Optogenetic activation of SIFamide (SIFa) neurons induces a complex sleep-promoting effect in the fruit fly Drosophila melanogaster

Sleep is a universal and extremely complicated function. Sleep is regulated by two systems-sleep homeostasis and circadian rhythms. In a wide range of species, neuropeptides have been found to play a crucial role in the communication and synchronization between different components of both systems. In the fruit fly Drosophila melanogaster, SIFamide (SIFa) is a neuropeptide that has been reported to be expressed in 4 neurons in the pars intercerebralis (PI) area of the brain. Previous work has shown that transgenic ablation of SIFa neurons, mutation of SIFa itself, or knockdown of SIFa receptors reduces sleep, suggesting that SIFa is sleep-promoting. However, those were all constitutive manipulations that could have affected development or resulted in compensation, so the role of SIFa signaling in sleep regulation during adulthood remains unclear. In the current study, we examined the sleep-promoting effect of SIFa through an optogenetic approach, which allowed for neuronal activation with high temporal resolution, while leaving development unaffected. We found that activation of the red-light sensor Chrimson in SIFa neurons promoted sleep in flies in a sexually dimorphic manner, where the magnitude of the sleep effect was greater in females than in males. Because neuropeptidergic neurons often also release other transmitters, we used RNA interference to knock down SIFa while also optogenetically activating SIFa neurons. SIFa knockdown only partially reduced the magnitude of the sleep effect, suggesting that release of other transmitters may contribute to the sleep induction when SIFa neurons are activated. Video-based analysis showed that activation of SIFa neurons for as brief a period as 1 second was able to decrease walking behavior for minutes after the stimulus. Future studies should aim to identify the transmitters that are utilized by SIFa neurons and characterize their upstream activators and downstream targets. It would also be of interest to determine how acute optogenetic activation of SIFa neurons alters other behaviors that have been linked to SIFa, such as mating and feeding.

Tyrosine hydroxylase immunostaining in the central complex of dicondylian insects

Dopamine acts as a neurohormone and neurotransmitter in the insect nervous system and controls a variety of physiological processes. Dopaminergic neurons also innervate the central complex (CX), a multisensory center of the insect brain involved in sky compass navigation, goal-directed locomotion and sleep control. To infer a possible influence of evolutionary history and lifestyle on the neurochemical architecture of the CX, we have studied the distribution of neurons immunoreactive to tyrosine hydroxylase (TH), the rate-limiting enzyme in dopamine biosynthesis. Analysis of representatives from 12 insect orders ranging from firebrats to flies revealed high conservation of immunolabeled neurons. One type of TH-immunoreactive neuron was found in all species studied. The neurons have somata in the pars intercerebralis, arborizations in the lateral accessory lobes, and axonal ramifications in the central body and noduli. In all pterygote species, a second type of tangential neuron of the upper division of the central body was TH-immunoreactive. The neurons have cell bodies near the calyces and arborizations in the superior protocerebrum. Both types of neuron showed species-specific variations in cell number and in the innervated areas outside and inside the CX. Additional neurons were found in only two taxa: one type of columnar neuron showed TH immunostaining in the water strider Gerris lacustris, but not in other Heteroptera, and a tritocerebral neuron innervating the protocerebral bridge was immunolabeled in Diptera. The data show largely taxon-specific variations of a common ground pattern of putatively dopaminergic neurons that may be commonly involved in state-dependent modulation of CX function.

Regulation of rhythmic behaviors by astrocytes

Glial astrocytes of vertebrates and invertebrates are important modulators of nervous system development, physiology, and behavior. In all species examined, astrocytes of the adult brain contain conserved circadian clocks, and multiple studies have shown that these glial cells participate in the regulation of circadian behavior and sleep. This short review summarizes recent work, using fruit fly (Drosophila) and mouse models, that document participation of astrocytes and their endogenous circadian clocks in the control of rhythmic behavior. This article is categorized under: Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms Nervous System Development > Flies.

Sleep in honey bees is affected by the herbicide glyphosate

Sleep plays an essential role in both neural and energetic homeostasis of animals. Honey bees (Apis mellifera) manifest the sleep state as a reduction in muscle tone and antennal movements, which is susceptible to physical or chemical disturbances. This social insect is one of the most important pollinators in agricultural ecosystems, being exposed to a great variety of agrochemicals, which might affect its sleep behaviour. The intake of glyphosate (GLY), the herbicide most widely used worldwide, impairs learning, gustatory responsiveness and navigation in honey bees. In general, these cognitive abilities are linked with the amount and quality of sleep. Furthermore, it has been reported that animals exposed to sleep disturbances show impairments in both metabolism and memory consolidation. Consequently, we assessed the sleep pattern of bees fed with a sugar solution containing GLY (0, 25, 50 and 100 ng) by quantifying their antennal activity during the scotophase. We found that the ingestion of 50 ng of GLY decreased both antennal activity and sleep bout frequency. This sleep deepening after GLY intake could be explained as a consequence of the regenerative function of sleep and the metabolic stress induced by the herbicide.

Pumilio Regulates Sleep Homeostasis in Response to Chronic Sleep Deprivation in Drosophila melanogaster

Recent studies have identified the Drosophila brain circuits involved in the sleep/wake switch and have pointed to the modulation of neuronal excitability as one of the underlying mechanisms triggering sleep need. In this study we aimed to explore the link between the homeostatic regulation of neuronal excitability and sleep behavior in the circadian circuit. For this purpose, we selected Pumilio (Pum), whose main function is to repress protein translation and has been linked to modulation of neuronal excitability during chronic patterns of altered neuronal activity. Here we explore the effects of Pum on sleep homeostasis in Drosophila melanogaster, which shares most of the major features of mammalian sleep homeostasis. Our evidence indicates that Pum is necessary for sleep rebound and that its effect is more pronounced during chronic sleep deprivation (84 h) than acute deprivation (12 h). Knockdown of pum, results in a reduction of sleep rebound during acute sleep deprivation and the complete abolishment of sleep rebound during chronic sleep deprivation. Based on these findings, we propose that Pum is a critical regulator of sleep homeostasis through neural adaptations triggered during sleep deprivation.

The voltage-gated potassium channel Shaker promotes sleep via thermosensitive GABA transmission

Genes and neural circuits coordinately regulate animal sleep. However, it remains elusive how these endogenous factors shape sleep upon environmental changes. Here, we demonstrate that Shaker (Sh)-expressing GABAergic neurons projecting onto dorsal fan-shaped body (dFSB) regulate temperature-adaptive sleep behaviors in Drosophila. Loss of Sh function suppressed sleep at low temperature whereas light and high temperature cooperatively gated Sh effects on sleep. Sh depletion in GABAergic neurons partially phenocopied Sh mutants. Furthermore, the ionotropic GABA receptor, Resistant to dieldrin (Rdl), in dFSB neurons acted downstream of Sh and antagonized its sleep-promoting effects. In fact, Rdl inhibited the intracellular cAMP signaling of constitutively active dopaminergic synapses onto dFSB at low temperature. High temperature silenced GABAergic synapses onto dFSB, thereby potentiating the wake-promoting dopamine transmission. We propose that temperature-dependent switching between these two synaptic transmission modalities may adaptively tune the neural property of dFSB neurons to temperature shifts and reorganize sleep architecture for animal fitness.

Ji-hyung Kim and Yoonhee Ki et al. show that low temperatures suppress sleep in Drosophila by increasing GABA transmission in Shaker-expressing GABAergic neurons projecting onto the dorsal fan-shaped body, while high temperatures potentiate dopamine-induced arousal by reducing GABA transmission. This study highlights a role for Shaker in sleep modulation via a temperature-dependent switch in GABA signaling.

Drosophila insulin-like peptide 2 mediates dietary regulation of sleep intensity

Sleep is a nearly universal behavior that is regulated by diverse environmental stimuli and physiological states. A defining feature of sleep is a homeostatic rebound following deprivation, where animals compensate for lost sleep by increasing sleep duration and/or sleep depth. The fruit fly, Drosophila melanogaster, exhibits robust recovery sleep following deprivation and represents a powerful model to study neural circuits regulating sleep homeostasis. Numerous neuronal populations have been identified in modulating sleep homeostasis as well as depth, raising the possibility that the duration and quality of recovery sleep is dependent on the environmental or physiological processes that induce sleep deprivation. Here, we find that unlike most pharmacological and environmental manipulations commonly used to restrict sleep, starvation potently induces sleep loss without a subsequent rebound in sleep duration or depth. Both starvation and a sucrose-only diet result in increased sleep depth, suggesting that dietary protein is essential for normal sleep depth and homeostasis. Finally, we find that Drosophila insulin like peptide 2 (Dilp2) is acutely required for starvation-induced changes in sleep depth without regulating the duration of sleep. Flies lacking Dilp2 exhibit a compensatory sleep rebound following starvation-induced sleep deprivation, suggesting Dilp2 promotes resiliency to sleep loss. Together, these findings reveal innate resilience to starvation-induced sleep loss and identify distinct mechanisms that underlie starvation-induced changes in sleep duration and depth.

Sleep is nearly universal throughout the animal kingdom and homeostatic regulation represents a defining feature of sleep, where animals compensate for lost sleep by increasing sleep over subsequent time periods. Despite the robustness of this feature, the neural mechanisms regulating recovery from different types of sleep deprivation are not fully understood. Fruit flies provide a powerful model for investigating the genetic regulation of sleep, and like mammals, display robust recovery sleep following deprivation. Here, we find that unlike most stimuli that suppress sleep, sleep deprivation by starvation does not require a homeostatic rebound. These findings are likely due to flies engaging in deeper sleep during the period of partial sleep deprivation, suggesting a natural resilience to starvation-induced sleep loss. This unique resilience to starvation-induced sleep loss is dependent on Drosophila insulin-like peptide 2, revealing a critical role for insulin signaling in regulating interactions between diet and sleep homeostasis.

Dietary fatty acids promote sleep through a taste-independent mechanism

Consumption of foods that are high in fat contribute to obesity and metabolism-related disorders. Dietary lipids are comprised of triglycerides and fatty acids, and the highly palatable taste of dietary fatty acids promotes food consumption, activates reward centers in mammals and underlies hedonic feeding. Despite the central role of dietary fats in the regulation of food intake and the etiology of metabolic diseases, little is known about how fat consumption regulates sleep. The fruit fly, Drosophila melanogaster, provides a powerful model system for the study of sleep and metabolic traits, and flies potently regulate sleep in accordance with food availability. To investigate the effects of dietary fats on sleep regulation, we have supplemented fatty acids into the diet of Drosophila and measured their effects on sleep and activity. We found that flies fed a diet of hexanoic acid, a medium-chain fatty acid that is a by-product of yeast fermentation, slept more than flies starved on an agar diet. To assess whether dietary fatty acids regulate sleep through the taste system, we assessed sleep in flies with a mutation in the hexanoic acid receptor Ionotropic receptor 56D, which is required for fatty acid taste perception. We found that these flies also sleep more than agar-fed flies when fed a hexanoic acid diet, suggesting the sleep promoting effect of hexanoic acid is not dependent on sensory perception. Taken together, these findings provide a platform to investigate the molecular and neural basis for fatty acid-dependent modulation of sleep.

Tired and stressed: Examining the need for sleep

A key feature of circadian rhythms is the sleep/wake cycle. Sleep causes reduced responsiveness to the environment, which puts animals in a particularly vulnerable state; yet sleep has been conserved throughout evolution, indicating that it fulfils a vital purpose. A core function of sleep across species has not been identified, but substantial advances in sleep research have been made in recent years using the genetically tractable model organism, Drosophila melanogaster. This review describes the universality of sleep, the regulation of sleep, and current theories on the function of sleep, highlighting a historical and often overlooked theory called the Free Radical Flux Theory of Sleep. Additionally, we summarize our recent work with short-sleeping Drosophila mutants and other genetic and pharmacological tools for manipulating sleep which supports an antioxidant theory of sleep and demonstrates a bi-directional relationship between sleep and oxidative stress.

Aging and the clock: Perspective from flies to humans

Endogenous circadian oscillators regulate molecular, cellular and physiological rhythms, synchronizing tissues and organ function to coordinate activity and metabolism with environmental cycles. The technological nature of modern society with round-the-clock work schedules and heavy reliance on personal electronics has precipitated a striking increase in the incidence of circadian and sleep disorders. Circadian dysfunction contributes to an increased risk for many diseases and appears to have adverse effects on aging and longevity in animal models. From invertebrate organisms to humans, the function and synchronization of the circadian system weakens with age aggravating the age-related disorders and pathologies. In this review, we highlight the impacts of circadian dysfunction on aging and longevity and the reciprocal effects of aging on circadian function with examples from Drosophila to humans underscoring the highly conserved nature of these interactions. Additionally, we review the potential for using reinforcement of the circadian system to promote healthy aging and mitigate age-related pathologies. Advancements in medicine and public health have significantly increased human life span in the past century. With the demographics of countries worldwide shifting to an older population, there is a critical need to understand the factors that shape healthy aging. Drosophila melanogaster, as a model for aging and circadian interactions, has the capacity to facilitate the rapid advancement of research in this area and provide mechanistic insights for targeted investigations in mammals.

The Neuropeptide Galanin Is Required for Homeostatic Rebound Sleep following Increased Neuronal Activity

Sleep pressure increases during wake and dissipates during sleep, but the molecules and neurons that measure homeostatic sleep pressure remain poorly understood. We present a pharmacological assay in larval zebrafish that generates short-term increases in wakefulness followed by sustained rebound sleep after washout. The intensity of global neuronal activity during drug-induced wakefulness predicted the amount of subsequent rebound sleep. Whole-brain mapping with the neuronal activity marker phosphorylated extracellular signal-regulated kinase (pERK) identified preoptic Galanin (Galn)-expressing neurons as selectively active during rebound sleep, and the relative induction of galn transcripts was predictive of total rebound sleep time. Galn is required for sleep homeostasis, as galn mutants almost completely lacked rebound sleep following both pharmacologically induced neuronal activity and physical sleep deprivation. These results suggest that Galn plays a key role in responding to sleep pressure signals derived from neuronal activity and functions as an output arm of the vertebrate sleep homeostat.

Sleep in Drosophila and Its Context

A prominent idea emerging from the study of sleep is that this key behavioural state is regulated in a complex fashion by ecologically and physiologically relevant environmental factors. This concept implies that sleep, as a behaviour, is plastic and can be regulated by external agents and changes in internal state. Drosophila melanogaster constitutes a resourceful model system to study behaviour. In the year 2000, the utility of the fly to study sleep was realised, and has since extensively contributed to this exciting field. At the centre of this review, we will discuss studies showing that temperature, food availability/quality, and interactions with conspecifics can regulate sleep. Indeed the relationship can be reciprocal and sleep perturbation can also affect feeding and social interaction. In particular, different environmental temperatures as well as gradual changes in temperature regulate when, and how much flies sleep. Moreover, the satiation/starvation status of an individual dictates the balance between sleep and foraging. Nutritional composition of diet also has a direct impact on sleep amount and its fragmentation. Likewise, aggression between males, courtship, sexual arousal, mating, and interactions within large groups of animals has an acute and long-lasting effect on sleep amount and quality. Importantly, the genes and neuronal circuits that relay information about the external environment and internal state to sleep centres are starting to be elucidated in the fly and are the focus of this review. In conclusion, sleep, as with most behaviours, needs the full commitment of the individual, preventing participation in other vital activities. A vast array of behaviours that are modulated by external and internal factors compete with the need to sleep and thus have a significant role in regulating it.

A Secreted Ig-Domain Protein Required in Both Astrocytes and Neurons for Regulation of Drosophila Night Sleep

Endogenous rhythmic behaviors are evolutionarily conserved and essential for life. In mammalian and invertebrate models, well-characterized neuronal circuits and evolutionarily conserved mechanisms regulate circadian behavior and sleep [1-4]. In Drosophila, neuronal populations located in multiple brain regions mediate arousal, sleep drive, and homeostasis (reviewed in [3, 5-7]). Similar to mammals [8], there is also evidence that fly glial cells modulate the neuronal circuits controlling rhythmic behaviors, including sleep [1]. Here, we describe a novel gene (CG14141; aka Nkt) that is required for normal sleep. NKT is a 162-amino-acid protein with a single IgC2 immunoglobulin (Ig) domain and a high-quality signal peptide [9], and we show evidence that it is secreted, similar to its C. elegans ortholog (OIG-4) [10]. We demonstrate that Nkt-null flies or those with selective knockdown in either neurons or glia have decreased and fragmented night sleep, indicative of a non-redundant requirement in both cell types. We show that Nkt is required in fly astrocytes and in a specific set of wake-promoting neurons-the mushroom body (MB) α'β' cells that link sleep to memory consolidation [11]. Importantly, Nkt gene expression is required in the adult nervous system for normal sleep, consistent with a physiological rather than developmental function for the Ig-domain protein.

Optogenetic activation of short neuropeptide F (sNPF) neurons induces sleep in Drosophila melanogaster

Sleep abnormalities have widespread and costly public health consequences, yet we have only a rudimentary understanding of the events occurring at the cellular level in the brain that regulate sleep. Several key signaling molecules that regulate sleep across taxa come from the family of neuropeptide transmitters. For example, in Drosophila melanogaster, the neuropeptide Y (NPY)-related transmitter short neuropeptide F (sNPF) appears to promote sleep. In this study, we utilized optogenetic activation of neuronal populations expressing sNPF to determine the causal effects of precisely timed activity in these cells on sleep behavior. Combining sNPF-GAL4 and UAS-Chrimson transgenes allowed us to activate sNPF neurons using red light. We found that activating sNPF neurons for as little as 3 s at a time of day when most flies were awake caused a rapid transition to sleep that persisted for another 2+ hours following the stimulation. Changing the timing of red light stimulation to times of day when flies were already asleep caused the control flies to wake up (due to the pulse of light), but the flies in which sNPF neurons were activated stayed asleep through the light pulse, and then showed further increases in sleep at later points when they would have normally been waking up. Video recording of individual fly responses to short-term (0.5-20 s) activation of sNPF neurons demonstrated a clear light duration-dependent decrease in movement during the subsequent 4-min period. These results provide supportive evidence that sNPF-producing neurons promote long-lasting increases in sleep, and show for the first time that even brief periods of activation of these neurons can cause changes in behavior that persist after cessation of activation. We have also presented evidence that sNPF neuron activation produces a homeostatic sleep drive that can be dissipated at times long after the neurons were stimulated. Future studies will determine the specific roles of sub-populations of sNPF-producing neurons, and will also assess how sNPF neurons act in concert with other neuronal circuits to control sleep.

Inverse Control of Turning Behavior by Dopamine D1 Receptor Signaling in Columnar and Ring Neurons of the Central Complex in Drosophila

Action selection is a prerequisite for decision-making and a fundamental aspect to any goal-directed locomotion; it requires integration of sensory signals and internal states to translate them into action sequences. Here, we introduce a novel behavioral analysis to study neural circuits and mechanisms underlying action selection and decision-making in freely moving Drosophila. We discovered preferred patterns of motor activity and turning behavior. These patterns are impaired in FoxP mutant flies, which present an altered temporal organization of motor actions and turning behavior, reminiscent of indecisiveness. Then, focusing on central complex (CX) circuits known to integrate different sensory modalities and controlling premotor regions, we show that action sequences and turning behavior are regulated by dopamine D1-like receptor (Dop1R1) signaling. Dop1R1 inputs onto CX columnar ellipsoid body-protocerebral bridge gall (E-PG) neuron and ellipsoid body (EB) R2/R4m ring neuron circuits both negatively gate motor activity but inversely control turning behavior. Although flies deficient of D1 receptor signaling present normal turning behavior despite decreased activity, restoring Dop1R1 level in R2/R4m-specific circuitry affects the temporal organization of motor actions and turning. We finally show EB R2/R4m neurons are in contact with E-PG neurons that are thought to encode body orientation and heading direction of the fly. These findings suggest that Dop1R1 signaling in E-PG and EB R2/4 m circuits are compared against each other, thereby modulating patterns of activity and turning behavior for goal-directed locomotion.

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Freely moving Drosophila present preferred patterns of activity and turning behavior

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FoxP mutations affect temporal distribution of motor actions and turning behavior

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Central complex columnar E-PG and R2/4 m ring neurons inversely regulate turning

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Dopamine D1-like receptor signaling in R2/R4m ring neurons modulates behavior

Neuroendocrine Control of Sleep

Sleep is a phenomenon in animal behavior as enigmatic as it is ubiquitous, and one deeply tied to endocrine function. Though there are still many unanswered questions about the neurochemical basis of sleep and its functions, extensive interactions have been identified between sleep and the endocrine system, in both the endocrine system's effect on sleep and sleep's effect on the endocrine system. Unfortunately, until recent years, much research on sleep behavior largely disregarded its connections with the endocrine system. Use of both clinical studies and rodent models to investigate interactions between neuroendocrine function, including biological sex, and sleep therefore presents a promising area of further exploration. Further investigation of the neurobiological and neuroendocrine basis of sleep could have wide impact on a number of clinical and basic science fields. In this review, we summarize the state of basic sleep biology and its connections to the field of neuroendocrine biology, as well as suggest key future directions for the neuroendocrine regulation of sleep that may significantly impact new therapies for sleep disorders in women and men.

Astrocyte expression of the Drosophila TNF-alpha homologue, Eiger, regulates sleep in flies

Sleep contributes to cognitive functioning and is sufficient to alter brain morphology and function. However, mechanisms underlying sleep regulation remain poorly understood. In mammals, tumor necrosis factor-alpha (TNFα) is known to regulate sleep, and cytokine expression may represent an evolutionarily ancient mechanism in sleep regulation. Here we show that the Drosophila TNFα homologue, Eiger, mediates sleep in flies. We show that knockdown of Eiger in astrocytes, but not in neurons, significantly reduces sleep duration, and total loss-of-function reduces the homeostatic response to sleep loss. In addition, we show that neuronal, but not astrocyte, expression of the TNFα receptor superfamily member, Wengen, is necessary for sleep deprivation-induced homeostatic response and for mediating increases in sleep in response to human TNFα. These data identify a novel astrocyte-to-neuron signaling mechanism in the regulation of sleep homeostasis and show that the Drosophila cytokine, Eiger, represents an evolutionarily conserved mechanism of sleep regulation across phylogeny.

Every animal sleeps, from fruit flies to humans. However, the function of sleep is still currently unknown. Identifying conserved mechanisms of sleep regulation in evolutionarily ancient organisms may help us to understand the function of sleep. Therefore, we have examined whether Eiger, the homologue of the cytokine tumor necrosis factor-alpha (TNFα), regulates sleep in the fruit fly as it does in higher mammals. Cytokines are inflammatory molecules and are typically elevated following infection or fever and may contribute to increased sleepiness when sick. We found that, in the fruit fly, Eiger regulates sleep duration just like TNFα does in mammals: increasing cytokine levels increased sleep duration while decreasing Eiger reduced sleep. In addition, we found that Eiger expression in glial astrocytes, is responsible for the alteration in sleep duration. We also examined the necessity of Eiger receptor activation on neurons and found that astrocyte-to-neuron communication was required for regulating the normal increases in sleep following sleep deprivation. These data show that a novel cytokine mechanism regulates sleep in flies and mammals, and provides insight into conserved roles of astrocytes in sleep behavior.

Roles for sleep in memory: insights from the fly

Sleep has been universally conserved across animal species. The basic functions of sleep remain unclear, but insufficient sleep impairs memory acquisition and retention in both vertebrates and invertebrates. Sleep is also a homeostatic process that is influenced not only by the amount of time awake, but also by neural activity and plasticity. Because of the breadth and precision of available genetic tools, the fruit fly has become a powerful model system to understand sleep regulation and function. Importantly, these tools enable the dissection of memory-encoding circuits at the level of individual neurons, and have allowed the development of genetic tools to induce sleep on-demand. This review describes recent investigations of the role for sleep in memory using Drosophila and current hypotheses of sleep's functions for supporting plasticity, learning, and memory.