Regulation of sleep by neuropeptide Y-like system in Drosophila melanogaster

Mechanism of foraging selections regulated by gustatory receptor 43a in Ostrinia furnacalis larvae

BACKGROUND

Omnivores, including humans, have an inborn tendency to avoid risky or non-nutritious foods. However, relatively little is known about how animals perceive and discriminate nutritious foods from risky substances. In this study, we explored the mechanism of feeding selection in Ostrinia furnacalis larvae, one of the most destructive pests to the maize crop.

RESULTS

We identified a gustatory receptor, Gr43a, for feeding regulation in larvae of Ostrinia furnacalis, which highly expresses in the mouthparts of the first- (the period of just hatching out from eggs) and fifth-instar larvae (the period of gluttony). The Gr43a regulates foraging plasticity by discriminating sorbitol, a nonsweet nutritious substance, and sucralose, a sweet non-nutritious substance through the labra of mouthparts, while it differentiates fructose/sucrose and sucralose via the sensilla styloconica of mouthparts. Specially, Gr43a responds to fructose and sucrose via the medial and lateral sensilla styloconica in O. furnacalis, respectively. Furthermore, Gr43a is negatively regulated by the neuropeptide F system, a homologous mammalian neuropeptide Y system.

CONCLUSION

This study reveals a smart feeding strategy for animals to meet both nutritional needs and sweet gratification, and offers an insight into complex feeding selections dependent on food resources in the surrounding environment. © 2023 Society of Chemical Industry.

Genome-wide association in Drosophila identifies a role for Piezo and Proc-R in sleep latency

Sleep latency, the amount of time that it takes an individual to fall asleep, is a key indicator of sleep need. Sleep latency varies considerably both among and within species and is heritable, but lacks a comprehensive description of its underlying genetic network. Here we conduct a genome-wide association study of sleep latency. Using previously collected sleep and activity data on a wild-derived population of flies, we calculate sleep latency, confirming significant, heritable genetic variation for this complex trait. We identify 520 polymorphisms in 248 genes contributing to variability in sleep latency. Tests of mutations in 23 candidate genes and additional putative pan-neuronal knockdown of 9 of them implicated CG44153, Piezo, Proc-R and Rbp6 in sleep latency. Two large-effect mutations in the genes Proc-R and Piezo were further confirmed via genetic rescue. This work greatly enhances our understanding of the genetic factors that influence variation in sleep latency.

Adipose Tissue Exosome circ_sxc Mediates the Modulatory of Adiposomes on Brain Aging by Inhibiting Brain dme-miR-87-3p

Aging of the brain usually leads to the decline of neurological processes and is a major risk factor for various neurodegenerative diseases, including sleep disturbances and cognitive decline. Adipose tissue exosomes, as adipocyte-derived vesicles, may mediate the regulatory processes of adipose tissue on other organs, including the brain; however, the regulatory mechanisms remain unclear. We analyzed the sleep-wake behavior of young (10 days) and old (40 days) Drosophila and found that older Drosophila showed increased sleep fragmentation, which is similar to mammalian aging characteristics. To investigate the cross-tissue regulatory mechanisms of adiposity on brain aging, we extracted 10-day and 40-day Drosophila adipose tissue exosomes and identified circRNAs with age-dependent expression differences by RNA-seq and differential analysis. Furthermore, by combining data from 3 datasets of the GEO database (GSE130158, GSE24992, and GSE184559), circ_sxc that was significantly downregulated with age was finally screened out. Moreover, dme-miR-87-3p, a conserved target of circ_sxc, accumulates in the brain with age and exhibits inhibitory effects in predicted binding relationships with neuroreceptor ligand genes. In summary, the current study showed that the Drosophila brain could obtain circ_sxc by uptake of adipose tissue exosomes which crossed the blood-brain barrier. And circ_sxc suppressed brain miR-87-3p expression through sponge adsorption, which in turn regulated the expression of neurological receptor ligand proteins (5-HT1B, GABA-B-R1, Rdl, Rh7, qvr, NaCP60E) and ensured brain neuronal synaptic signaling normal function of synaptic signaling. However, with aging, this regulatory mechanism is dysregulated by the downregulation of the adipose exosome circ_sxc, which contributes to the brain exhibiting sleep disturbances and other "aging" features.

Biogenic action of Lactobacillus plantarum SBT2227 promotes sleep in Drosophila melanogaster

Lactic acid bacteria (LAB) influence multiple aspects of host brain function via the production of active metabolites in the gut, which is known as the pre/probiotic action. However, little is known about the biogenic effects of LAB on host brain function. Here, we reported that the Lactobacillus plantarum SBT2227 promoted sleep in Drosophila melanogaster. Administration of SBT2227 primarily increased the amount of sleep and decreased sleep latency at the beginning of night-time. The sleep-promoting effects of SBT2227 were independent of the existing gut flora. Furthermore, heat treatment or mechanical crushing of SBT2227 did not suppress the sleep-promoting effects, indicative of biogenic action. Transcriptome analysis and RNAi mini-screening for gut-derived peptide hormones revealed the requirement of neuropeptide F, a homolog of the mammalian neuropeptide Y, for the action of SBT2227. These biogenic effects of SBT2227 on the host sleep provide new insights into the interaction between the brain and gut bacteria.

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Lactobacillus plantarum SBT2227 promotes sleep at the onset of nighttime

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Existing intestinal microbes do not affect the SBT2227 sleep effect

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Heat-stable intracellular/intramembrane components are candidates for active substances

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Neuropeptide F is required for the sleep-promoting effect of SBT2227

Neural Control of Action Selection Among Innate Behaviors

Nervous systems must not only generate specific adaptive behaviors, such as reproduction, aggression, feeding, and sleep, but also select a single behavior for execution at any given time, depending on both internal states and external environmental conditions. Despite their tremendous biological importance, the neural mechanisms of action selection remain poorly understood. In the past decade, studies in the model animal Drosophila melanogaster have demonstrated valuable neural mechanisms underlying action selection of innate behaviors. In this review, we summarize circuit mechanisms with a particular focus on a small number of sexually dimorphic neurons in controlling action selection among sex, fight, feeding, and sleep behaviors in both sexes of flies. We also discuss potentially conserved circuit configurations and neuromodulation of action selection in both the fly and mouse models, aiming to provide insights into action selection and the sexually dimorphic prioritization of innate behaviors.

Upregulation of IP3 receptor mediates APP-induced defects in synaptic downscaling and sleep homeostasis

Evidence suggests that impaired synaptic and firing homeostasis represents a driving force of early Alzheimer's disease (AD) progression. Here, we examine synaptic and sleep homeostasis in a Drosophila model by overexpressing human amyloid precursor protein (APP), whose duplication and mutations cause familial early-onset AD. We find that APP overexpression induces synaptic hyperexcitability. RNA-seq data indicate exaggerated expression of Ca²⁺-related signaling genes in APP mutants, including genes encoding Dmca1D, calcineurin (CaN) complex, and IP₃R. We further demonstrate that increased CaN activity triggers transcriptional activation of Itpr (IP₃R) through activating nuclear factor of activated T cells (NFAT). Strikingly, APP overexpression causes defects in synaptic downscaling and sleep deprivation-induced sleep rebound, and both defects could be restored by inhibiting IP₃R. Our findings uncover IP₃R as a shared signaling molecule in synaptic downscaling and sleep homeostasis, and its dysregulation may lead to synaptic hyperexcitability and AD progression at early stage.

Regulation of Mating-Induced Increase in Female Germline Stem Cells in the Fruit Fly Drosophila melanogaster

In many insect species, mating stimuli can lead to changes in various behavioral and physiological responses, including feeding, mating refusal, egg-laying behavior, energy demand, and organ remodeling, which are collectively known as the post-mating response. Recently, an increase in germline stem cells (GSCs) has been identified as a new post-mating response in both males and females of the fruit fly, Drosophila melanogaster. We have extensively studied mating-induced increase in female GSCs of D. melanogaster at the molecular, cellular, and systemic levels. After mating, the male seminal fluid peptide [e.g. sex peptide (SP)] is transferred to the female uterus. This is followed by binding to the sex peptide receptor (SPR), which evokes post-mating responses, including increase in number of female GSCs. Downstream of SP-SPR signaling, the following three hormones and neurotransmitters have been found to act on female GSC niche cells to regulate mating-induced increase in female GSCs: (1) neuropeptide F, a peptide hormone produced in enteroendocrine cells; (2) octopamine, a monoaminergic neurotransmitter synthesized in ovary-projecting neurons; and (3) ecdysone, a steroid hormone produced in ovarian follicular cells. These humoral factors are secreted from each organ and are received by ovarian somatic cells and regulate the strength of niche signaling in female GSCs. This review provides an overview of the latest findings on the inter-organ relationship to regulate mating-induced female GSC increase in D. melanogaster as a model. We also discuss the remaining issues that should be addressed in the future.

Metabolic control of daily locomotor activity mediated by tachykinin in Drosophila

Metabolism influences locomotor behaviors, but the understanding of neural curcuit control for that is limited. Under standard light-dark cycles, Drosophila exhibits bimodal morning (M) and evening (E) locomotor activities that are controlled by clock neurons. Here, we showed that a high-nutrient diet progressively extended M activity but not E activity. Drosophila tachykinin (DTk) and Tachykinin-like receptor at 86C (TkR86C)-mediated signaling was required for the extension of M activity. DTk neurons were anatomically and functionally connected to the posterior dorsal neuron 1s (DN1_ps) in the clock neuronal network. The activation of DTk neurons reduced intracellular Ca²⁺ levels in DN1_ps suggesting an inhibitory connection. The contacts between DN1_ps and DTk neurons increased gradually over time in flies fed a high-sucrose diet, consistent with the locomotor behavior. DN1_ps have been implicated in integrating environmental sensory inputs (e.g., light and temperature) to control daily locomotor behavior. This study revealed that DN1_ps also coordinated nutrient information through DTk signaling to shape daily locomotor behavior.

Lee and colleagues report the effect of a high-sucrose diet on Drosophila locomotor activity via DTk-TkR86C neuropeptide signalling. This signalling pattern appears to involve a circadian element, with pacemaker neuron involvement having a possible time-of-day effect on locomotor behaviour.

Integration of Circadian Clock Information in the Drosophila Circadian Neuronal Network

Circadian clocks are biochemical time-keeping machines that synchronize animal behavior and physiology with planetary rhythms. In Drosophila, the core components of the clock comprise a transcription/translation feedback loop and are expressed in seven neuronal clusters in the brain. Although it is increasingly evident that the clocks in each of the neuronal clusters are regulated differently, how these clocks communicate with each other across the circadian neuronal network is less clear. Here, we review the latest evidence that describes the physical connectivity of the circadian neuronal network . Using small ventral lateral neurons as a starting point, we summarize how one clock may communicate with another, highlighting the signaling pathways that are both upstream and downstream of these clocks. We propose that additional efforts are required to understand how temporal information generated in each circadian neuron is integrated across a neuronal circuit to regulate rhythmic behavior.

Transcriptome Analysis of NPFR Neurons Reveals a Connection Between Proteome Diversity and Social Behavior

Social behaviors are mediated by the activity of highly complex neuronal networks, the function of which is shaped by their transcriptomic and proteomic content. Contemporary advances in neurogenetics, genomics, and tools for automated behavior analysis make it possible to functionally connect the transcriptome profile of candidate neurons to their role in regulating behavior. In this study we used Drosophila melanogaster to explore the molecular signature of neurons expressing receptor for neuropeptide F (NPF), the fly homolog of neuropeptide Y (NPY). By comparing the transcription profile of NPFR neurons to those of nine other populations of neurons, we discovered that NPFR neurons exhibit a unique transcriptome, enriched with receptors for various neuropeptides and neuromodulators, as well as with genes known to regulate behavioral processes, such as learning and memory. By manipulating RNA editing and protein ubiquitination programs specifically in NPFR neurons, we demonstrate that the proper expression of their unique transcriptome and proteome is required to suppress male courtship and certain features of social group interaction. Our results highlight the importance of transcriptome and proteome diversity in the regulation of complex behaviors and pave the path for future dissection of the spatiotemporal regulation of genes within highly complex tissues, such as the brain.

Multi-ethnic GWAS and meta-analysis of sleep quality identify MPP6 as a novel gene that functions in sleep center neurons

Poor sleep quality can have harmful health consequences. Although many aspects of sleep are heritable, the understandings of genetic factors involved in its physiology remain limited. Here, we performed a genome-wide association study (GWAS) using the Pittsburgh Sleep Quality Index (PSQI) in a multi-ethnic discovery cohort (n = 2868) and found two novel genome-wide loci on chromosomes 2 and 7 associated with global sleep quality. A meta-analysis in 12 independent cohorts (100 000 individuals) replicated the association on chromosome 7 between NPY and MPP6. While NPY is an important sleep gene, we tested for an independent functional role of MPP6. Expression data showed an association of this locus with both NPY and MPP6 mRNA levels in brain tissues. Moreover, knockdown of an orthologue of MPP6 in Drosophila melanogaster sleep center neurons resulted in decreased sleep duration. With convergent evidence, we describe a new locus impacting human variability in sleep quality through known NPY and novel MPP6 sleep genes.

Regulation of circadian behavioural output via clock-responsive miR-276b

Growing evidence indicates that microRNAs play numerous important roles. However, the roles of some microRNAs involved in regulation of circadian rhythm and sleep are still not well understood. In this study, we show that the miR-276b is essential for maintaining both sleep and circadian rhythm by targeting tim, npfr1 and DopR1 genes, with miR-276b deleted mutant flies sleeping more, and vice versa in miR-276b overexpressing flies. Through analysing its promoter, we found that mir-276b is responsive to CLOCK and regulates circadian rhythm through the negative feedback loop of the CLK/CYC-TIM/PER. Furthermore, miR-276b is broadly expressed in the clock neurons and the central complexes such as the mushroom body and the fan-shape body of Drosophila brain, in which up-regulation of miR-276b in tim, npfr1 and DopR1 expressing tissues significantly causes sleep decreases. This study clarifies that the mir-276b is very important for participating in regulation of circadian rhythm and sleep.

Regulatory mechanism of daily sleep by miR-276a

MiRNAs have attracted more attention in recent years as regulators of sleep and circadian rhythms after their roles in circadian rhythm and sleep were discovered. In this study, we explored the roles of the miR-276a on daily sleep in Drosophila melanogaster, and found a regulatory cycle for the miR-276a pathway, in which miR-276a, regulated by the core CLOCK/CYCLE (CLK/CYC) transcription factor upstream, regulates sleep via suppressing targets TIM and NPFR1. (a) Loss of miR-276a function makes the flies sleep more during both daytime and nighttime, while flies with gain of miR-276a function sleep less; (b) MiR-276a is widely expressed in the mushroom body (MB), the pars intercerebralis (PI) and some clock neurons lateral dorsal neurons (LNds), in which tim neurons is important for sleep regulation; (c) MiR-276a promoter is identified to locate in the 8th fragment (aFrag8) of the pre-miR-276a, and this fragment is directly activated and regulated by CLK/CYC; (4) MiR-276a is rhythmically oscillating in heads of the wild-type w¹¹¹⁸ , but this oscillation disappears in the loss of function mutant clk^jrk ; (5) The neuropeptide F receptor 1 (npfr1) was found to be a downstream target of miR-276a. These results clarify that the miR-276a is a very important factor for sleep regulation.

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.

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.

A Symphony of Signals: Intercellular and Intracellular Signaling Mechanisms Underlying Circadian Timekeeping in Mice and Flies

The central pacemakers of circadian timekeeping systems are highly robust yet adaptable, providing the temporal coordination of rhythms in behavior and physiological processes in accordance with the demands imposed by environmental cycles. These features of the central pacemaker are achieved by a multi-oscillator network in which individual cellular oscillators are tightly coupled to the environmental day-night cycle, and to one another via intercellular coupling. In this review, we will summarize the roles of various neurotransmitters and neuropeptides in the regulation of circadian entrainment and synchrony within the mammalian and Drosophila central pacemakers. We will also describe the diverse functions of protein kinases in the relay of input signals to the core oscillator or the direct regulation of the molecular clock machinery.

Wakefulness Is Promoted during Day Time by PDFR Signalling to Dopaminergic Neurons in Drosophila melanogaster

Circadian clocks modulate timing of sleep/wake cycles in animals; however, the underlying mechanisms remain poorly understood. In Drosophila melanogaster, large ventral lateral neurons (l-LN_v) are known to promote wakefulness through the action of the neuropeptide pigment dispersing factor (PDF), but the downstream targets of PDF signalling remain elusive. In a screen using downregulation or overexpression (OEX) of the gene encoding PDF receptor (pdfr), we found that a subset of dopaminergic neurons responds to PDF to promote wakefulness during the day. Moreover, we found that small LN_v (s-LN_v) and dopaminergic neurons form synaptic contacts, and PDFR signalling inhibited dopaminergic neurons specifically during day time. We propose that these dopaminergic neurons that respond to PDFR signalling are sleep-promoting and that during the day when PDF levels are high, they are inhibited, thereby promoting wakefulness. Thus, we identify a novel circadian clock pathway that mediates wake promotion specifically during day time.

Effect of Valerian/Hop Mixture on Sleep-Related Behaviors in Drosophila melanogaster

The aim of this study was to investigate the sleep-promoting effect of a Valerian/Hops mixture in fruit flies. The HPLC analysis showed that Valerenic acid (1260.53 µg/g of extract) and Xanthohumol (Cascade: 827.49 µg/g, Hallertau: 763.60 µg/g, Saaz: 186.93 µg/g) were contained in Valerian and Hop, respectively. The sleep patterns of fruit flies on the Valerian/Hops were examined in both baseline and caffeine-treated conditions. Total activities of flies significantly decreased in 20 mg/mL Valerian (74%), 10 mg/mL Cascade (25%), and 5 mg/mL Hallertau (11%) during nighttime or daytime compared with the control. Valerian/Cascade mixture showed longer sleeping time (ca. 20%) than control group. This mixture-mediated effect was partly observed in caffeine-treated flies. Valerian/Cascade mixture upregulated mRNA expressions of gamma-aminobutyric acid (GABA) receptors and serotonin receptor, and GABA receptors were more strongly regulated than serotonin receptor. In competitive GABA receptor binding assay, Valerian/Cascade mixture extract showed a higher binding ability on GABA receptor than Valerenic acid or/and Xanthohumol which are estimated to be active compounds in the extract. This study demonstrates that a Valerian/Cascade mixture extract improves sleep-related behaviors, including sleeping time, by modulating GABAergic/serotonergic signaling.

The neurobiological basis of sleep: Insights from Drosophila

Sleep is a biological enigma that has raised numerous questions about the inner workings of the brain. The fundamental question of why our nervous systems have evolved to require sleep remains a topic of ongoing scientific deliberation. This question is largely being addressed by research using animal models of sleep. Drosophila melanogaster, also known as the common fruit fly, exhibits a sleep state that shares common features with many other species. Drosophila sleep studies have unearthed an immense wealth of knowledge about the neuroscience of sleep. Given the breadth of findings published on Drosophila sleep, it is important to consider how all of this information might come together to generate a more holistic understanding of sleep. This review provides a comprehensive summary of the neurobiology of Drosophila sleep and explores the broader insights and implications of how sleep is regulated across species and why it is necessary for the brain.

Neuropeptide Y Regulates Sleep by Modulating Noradrenergic Signaling

Sleep is an essential and evolutionarily conserved behavioral state whose regulation remains poorly understood. To identify genes that regulate vertebrate sleep, we recently performed a genetic screen in zebrafish, and here we report the identification of neuropeptide Y (NPY) as both necessary for normal daytime sleep duration and sufficient to promote sleep. We show that overexpression of NPY increases sleep, whereas mutation of npy or ablation of npy-expressing neurons decreases sleep. By analyzing sleep architecture, we show that NPY regulates sleep primarily by modulating the length of wake bouts. To determine how NPY regulates sleep, we tested for interactions with several systems known to regulate sleep, and provide anatomical, molecular, genetic, and pharmacological evidence that NPY promotes sleep by inhibiting noradrenergic signaling. These data establish NPY as an important vertebrate sleep/wake regulator and link NPY signaling to an established arousal-promoting system.

Genetic and neuronal regulation of sleep by neuropeptide VF

Sleep is an essential and phylogenetically conserved behavioral state, but it remains unclear to what extent genes identified in invertebrates also regulate vertebrate sleep. RFamide-related neuropeptides have been shown to promote invertebrate sleep, and here we report that the vertebrate hypothalamic RFamide neuropeptide VF (NPVF) regulates sleep in the zebrafish, a diurnal vertebrate. We found that NPVF signaling and npvf-expressing neurons are both necessary and sufficient to promote sleep, that mature peptides derived from the NPVF preproprotein promote sleep in a synergistic manner, and that stimulation of npvf-expressing neurons induces neuronal activity levels consistent with normal sleep. These results identify NPVF signaling and npvf-expressing neurons as a novel vertebrate sleep-promoting system and suggest that RFamide neuropeptides participate in an ancient and central aspect of sleep control.

Dissection of the Drosophila neuropeptide F circuit using a high-throughput two-choice assay

In their classic experiments, Olds and Milner showed that rats learn to lever press to receive an electric stimulus in specific brain regions. This led to the identification of mammalian reward centers. Our interest in defining the neuronal substrates of reward perception in the fruit fly Drosophila melanogaster prompted us to develop a simpler experimental approach wherein flies could implement behavior that induces self-stimulation of specific neurons in their brains. The high-throughput assay employs optogenetic activation of neurons when the fly occupies a specific area of a behavioral chamber, and the flies' preferential occupation of this area reflects their choosing to experience optogenetic stimulation. Flies in which neuropeptide F (NPF) neurons are activated display preference for the illuminated side of the chamber. We show that optogenetic activation of NPF neuron is rewarding in olfactory conditioning experiments and that the preference for NPF neuron activation is dependent on NPF signaling. Finally, we identify a small subset of NPF-expressing neurons located in the dorsomedial posterior brain that are sufficient to elicit preference in our assay. This assay provides the means for carrying out unbiased screens to map reward neurons in flies.

Mechanisms of sleep plasticity due to sexual experience in Drosophila melanogaster

Sleep can be altered by an organism's previous experience. For instance, female Drosophila melanogaster experience a post-mating reduction in daytime sleep that is purportedly mediated by sex peptide (SP), one of many seminal fluid proteins (SFPs) transferred from male to female during mating. In the present study, we first characterized this mating effect on sleep more fully, as it had previously only been tested in young flies under 12h light/12h dark conditions. We found that mating reduced sleep equivalently in 3-day-old or 14-day-old females, and could even occur in females who had been mated previously, suggesting that there is not a developmental critical period for the suppression of sleep by mating. In conditions of constant darkness, circadian rhythms were not affected by prior mating. In either constant darkness or constant light, the sleep reduction due to mating was no longer confined to the subjective day but could be observed throughout the 24-hour period. This suggests that the endogenous clock may dictate the timing of when the mating effect on sleep is expressed. We recently reported that genetic elimination of SP only partially blocked the post-mating female siesta sleep reduction, suggesting that the effect was unlikely to be governed solely by SP. We found here that the daytime sleep reduction was also reduced but not eliminated in females mated to mutant males lacking the vast majority of SFPs. This suggested that SFPs other than SP play a minimal role in the mating effect on sleep, and that additional non-SFP signals from the male might be involved. Males lacking sperm were able to induce a normal initial mating effect on female sleep, although the effect declined more rapidly in these females. This result indicated that neither the presence of sperm within the female reproductive tract nor female impregnation are required for the initial mating effect on sleep to occur, although sperm may serve to prolong the effect. Finally, we tested for contributions from other aspects of the mating experience. NorpA and eya2 mutants with disrupted vision showed normal mating effects on sleep. By separating males from females with a mesh, we found that visual and olfactory stimuli from male exposure, in the absence of physical contact, could not replicate the mating effect. Further, in ken/barbie male flies lacking external genitalia, courtship and physical contact without ejaculation were also unable to replicate the mating effect. These findings confirmed that the influence of mating on sleep does in fact require male/female contact including copulation, but may not be mediated exclusively by SP transfer.

Drosophila Neuropeptide F Signaling Independently Regulates Feeding and Sleep-Wake Behavior

Proper regulation of sleep-wake behavior and feeding is essential for organismal health and survival. While previous studies have isolated discrete neural loci and substrates important for either sleep or feeding, how the brain is organized to coordinate both processes with respect to one another remains poorly understood. Here, we provide evidence that the Drosophila Neuropeptide F (NPF) network forms a critical component of both adult sleep and feeding regulation. Activation of NPF signaling in the brain promotes wakefulness and adult feeding, likely through its cognate receptor NPFR. Flies carrying a loss-of-function NPF allele do not suppress sleep following prolonged starvation conditions, suggesting that NPF acts as a hunger signal to keep the animal awake. NPF-expressing cells, specifically those expressing the circadian photoreceptor cryptochrome, are largely responsible for changes to sleep behavior caused by NPF neuron activation, but not feeding, demonstrating that different NPF neurons separately drive wakefulness and hunger.

Sleeping Beauty? Developmental Timing, Sleep, and the Circadian Clock in Caenorhabditis elegans

The genetics toolkit is pretty successful in drilling down into minutiae. The big challenge is to integrate the information from this specialty as well as those of biochemistry, physiology, behavior, and anatomy to explain how fundamental biological processes really work. Sleep, the circadian clock and development all qualify as overarching processes that encompass levels from molecule to behavior as part of their known mechanisms. They overlap each other, such that understanding the mechanisms of one can lead to insights into one of the others. In this essay, we consider how the experimental approaches and findings relating to Caenorhabditis elegans development and lethargus on one hand, and to the circadian clock and sleep in higher organisms on the other, could complement and enhance one another.

The RFamide receptor DMSR-1 regulates stress-induced sleep in C. elegans

In response to environments that cause cellular stress, animals engage in sleep behavior that facilitates recovery from the stress. In Caenorhabditis elegans, stress-induced sleep(SIS) is regulated by cytokine activation of the ALA neuron, which releases FLP-13 neuropeptides characterized by an amidated arginine-phenylalanine (RFamide) C-terminus motif. By performing an unbiased genetic screen for mutants that impair the somnogenic effects of FLP-13 neuropeptides, we identified the gene dmsr-1, which encodes a G-protein coupled receptor similar to an insect RFamide receptor. DMSR-1 is activated by FLP-13 peptides in cell culture, is required for SIS in vivo, is expressed non-synaptically in several wake-promoting neurons, and likely couples to a Gi/o heterotrimeric G-protein. Our data expand our understanding of how a single neuroendocrine cell coordinates an organism-wide behavioral response, and suggest that similar signaling principles may function in other organisms to regulate sleep during sickness.

DOI:http://dx.doi.org/10.7554/eLife.19837.001

People often feel fatigued and sleepy when they are sick. Other animals also show signs of sleepiness when ill – they stop eating, move less, and are less responsive to changes in their environment. Sickness-induced sleep helps both people and other animals to recover, and many scientists believe that this type of sleep is different than nightly sleep.

Studies of sickness-induced sleep have made use of a simple worm with a simple nervous system. In this worm, a single nerve cell releases chemicals that cause the worm to fall asleep in response to illness. Animals exposed to one of these chemicals, called FLP-13, fall asleep even when they are not sick. As such, scientists would like to know which cells in the nervous system FLP-13 interacts with, what receptor the cells use to recognize this chemical, and whether it turns on cells that induce sleep or turns off the cells that cause wakefulness.

Now, Iannacone et al. show that FLP-13 likely causes sleep by turning down activity in the cells in the nervous system that promote wakefulness. The experiments sifted through genetic mutations to determine which ones cause the worms not to fall asleep when FLP-13 is released. This revealed that worms with a mutation that causes them to lack a receptor protein called DMSR-1 do not become sleepy in response to FLP-13. This suggests that DMSR-1 must be essential for FLP-13 to trigger sleep. About 10% of cells in the worm’s nervous system have the DMSR-1 receptor. Some of these neurons tell the worm to move forward or to forage around for food. The experiments also showed that FLP-13 is probably not the only chemical that interacts with the DMSR-1 receptor, but the identities of these other chemicals remain unknown.

Additional experiments are now needed to determine if sickness-induced sleepiness in humans and other mammals is triggered by a similar mechanism. If it is, then drugs might be developed to treat people experiencing fatigue associated with sickness as well as other unexplained cases of fatigue.

DOI:http://dx.doi.org/10.7554/eLife.19837.002

The Sleep in Caenorhabditis elegans: What We Know Until Now

Sleep, as one of the most important requirements of our brain, has a mystical nature. Despite long-standing studies, the molecular mechanisms and physiological properties of sleep have not been defined well as the complexity of the mammals' brain make it difficult to investigate the mechanisms and properties of sleep. Although some features of sleep have changed during evolution, its existence in such a simple animal, Caenorhabditis elegans, not only signifies the importance of sleep in even simple animals, but also allows the scientist to assess the core mechanism and biological events in an uncomplicated organism. This article reviews the information which exists about the characteristics of sleep in C. elegans, its circadian rhythm, the neurons and neurotransmitters responsible for each state, and the signaling molecules involved. Although much still remains to be resolved about the sleep of C. elegans, the available knowledge helps the scientists to recognize the properties better of this mysterious function of the brain.

Modulation of light-driven arousal by LIM-homeodomain transcription factor Apterous in large PDF-positive lateral neurons of the Drosophila brain

Apterous (Ap), the best studied LIM-homeodomain transcription factor in Drosophila, cooperates with the cofactor Chip (Chi) to regulate transcription of specific target genes. Although Ap regulates various developmental processes, its function in the adult brain remains unclear. Here, we report that Ap and Chi in the neurons expressing PDF, a neuropeptide, play important roles in proper sleep/wake regulation in adult flies. PDF-expressing neurons consist of two neuronal clusters: small ventral-lateral neurons (s-LNvs) acting as the circadian pacemaker and large ventral-lateral neurons (l-LNvs) regulating light-driven arousal. We identified that Ap localizes to the nuclei of s-LNvs and l-LNvs. In light-dark (LD) cycles, RNAi knockdown or the targeted expression of dominant-negative forms of Ap or Chi in PDF-expressing neurons or l-LNvs promoted arousal. In contrast, in constant darkness, knockdown of Ap in PDF-expressing neurons did not promote arousal, indicating that a reduced Ap function in PDF-expressing neurons promotes light-driven arousal. Furthermore, Ap expression in l-LNvs showed daily rhythms (peaking at midnight), which are generated by a direct light-dependent mechanism rather than by the endogenous clock. These results raise the possibility that the daily oscillation of Ap expression in l-LNvs may contribute to the buffering of light-driven arousal in wild-type flies.

Characterization of a neuropeptide F receptor in the tsetse fly, Glossina morsitans morsitans

Neuropeptides related to mammalian neuropeptide Y (NPY) and insect neuropeptide F (NPF) are conserved throughout Metazoa and intimately involved in a wide range of biological processes. In insects NPF is involved in regulating feeding, learning, stress and reproductive behavior. Here we identified and characterized an NPF receptor of the tsetse fly, Glossina morsitans morsitans, the sole transmitter of Trypanosoma parasites causing sleeping sickness. We isolated cDNA sequences encoding tsetse NPF (Glomo-NPF) and its receptor (Glomo-NPFR), and examined their spatial and temporal expression patterns using quantitative PCR. In tsetse flies, npfr transcripts are expressed throughout development and most abundantly in the central nervous system, whereas low expression is found in the flight muscles and posterior midgut. Expression of npf, by contrast, shows low transcript levels during development but is strongly expressed in the posterior midgut and brain of adult flies. Expression of Glomo-npf and its receptor in the brain and digestive system suggests that NPF may have conserved neuromodulatory or hormonal functions in tsetse flies, such as in the regulation of feeding behavior. Cell-based activity studies of the Glomo-NPFR showed that Glomo-NPF activates the receptor up to nanomolar concentrations. The molecular data of Glomo-NPF and Glomo-NPFR paves the way for further investigation of its functions in tsetse flies.

Allatostatin A Signalling in Drosophila Regulates Feeding and Sleep and Is Modulated by PDF

Feeding and sleep are fundamental behaviours with significant interconnections and cross-modulations. The circadian system and peptidergic signals are important components of this modulation, but still little is known about the mechanisms and networks by which they interact to regulate feeding and sleep. We show that specific thermogenetic activation of peptidergic Allatostatin A (AstA)-expressing PLP neurons and enteroendocrine cells reduces feeding and promotes sleep in the fruit fly Drosophila. The effects of AstA cell activation are mediated by AstA peptides with receptors homolog to galanin receptors subserving similar and apparently conserved functions in vertebrates. We further identify the PLP neurons as a downstream target of the neuropeptide pigment-dispersing factor (PDF), an output factor of the circadian clock. PLP neurons are contacted by PDF-expressing clock neurons, and express a functional PDF receptor demonstrated by cAMP imaging. Silencing of AstA signalling and continuous input to AstA cells by tethered PDF changes the sleep/activity ratio in opposite directions but does not affect rhythmicity. Taken together, our results suggest that pleiotropic AstA signalling by a distinct neuronal and enteroendocrine AstA cell subset adapts the fly to a digestive energy-saving state which can be modulated by PDF.

Development of a novel-type transgenic cotton plant for control of cotton bollworm

The transgenic Bt cotton plant has been widely planted throughout the world for the control of cotton budworm Helicoverpa armigera (Hubner). However, a shift towards insect tolerance of Bt cotton is now apparent. In this study, the gene encoding neuropeptide F (NPF) was cloned from cotton budworm H. armigera, an important agricultural pest. The npf gene produces two splicing mRNA variants-npf1 and npf2 (with a 120-bp segment inserted into the npf1 sequence). These are predicted to form the mature NPF1 and NPF2 peptides, and they were found to regulate feeding behaviour. Knock down of larval npf with dsNPF in vitro resulted in decreases of food consumption and body weight, and dsNPF also caused a decrease of glycogen and an increase of trehalose. Moreover, we produced transgenic tobacco plants transiently expressing dsNPF and transgenic cotton plants with stably expressed dsNPF. Results showed that H. armigera larvae fed on these transgenic plants or leaves had lower food consumption, body size and body weight compared to controls. These results indicate that NPF is important in the control of feeding of H. armigera and valuable for production of potential transgenic cotton.

The Blood-Brain Barrier: Regulatory Roles in Wakefulness and Sleep

Sleep and its disorders are known to affect the functions of essential organs and systems in the body. However, very little is known about how the blood-brain barrier (BBB) is regulated. A few years ago, we launched a project to determine the impact of sleep fragmentation and chronic sleep restriction on BBB functions, including permeability to fluorescent tracers, tight junction protein expression and distribution, glucose and other solute transporter activities, and mediation of cellular mechanisms. Recent publications and relevant literature allow us to summarize here the sleep-BBB interactions in five sections: (1) the structural basis enabling the BBB to serve as a huge regulatory interface; (2) BBB transport and permeation of substances participating in sleep-wake regulation; (3) the circadian rhythm of BBB function; (4) the effect of experimental sleep disruption maneuvers on BBB activities, including regional heterogeneity, possible threshold effect, and reversibility; and (5) implications of sleep disruption-induced BBB dysfunction in neurodegeneration and CNS autoimmune diseases. After reading the review, the general audience should be convinced that the BBB is an important mediating interface for sleep-wake regulation and a crucial relay station of mind-body crosstalk. The pharmaceutical industry should take into consideration that sleep disruption alters the pharmacokinetics of BBB permeation and CNS drug delivery, being attentive to the chrono timing and activation of co-transporters in subjects with sleep disorders.

Neural clocks and Neuropeptide F/Y regulate circadian gene expression in a peripheral metabolic tissue

Metabolic homeostasis requires coordination between circadian clocks in different tissues. Also, systemic signals appear to be required for some transcriptional rhythms in the mammalian liver and the Drosophila fat body. Here we show that free-running oscillations of the fat body clock require clock function in the PDF-positive cells of the fly brain. Interestingly, rhythmic expression of the cytochrome P450 transcripts, sex-specific enzyme 1 (sxe1) and Cyp6a21, which cycle in the fat body independently of the local clock, depends upon clocks in neurons expressing neuropeptide F (NPF). NPF signaling itself is required to drive cycling of sxe1 and Cyp6a21 in the fat body, and its mammalian ortholog, Npy, functions similarly to regulate cycling of cytochrome P450 genes in the mouse liver. These data highlight the importance of neuronal clocks for peripheral rhythms, particularly in a specific detoxification pathway, and identify a novel and conserved role for NPF/Npy in circadian rhythms.

DOI:http://dx.doi.org/10.7554/eLife.13552.001

Many processes in the body follow rhythms that repeat over 24 hours and are synchronized to the cycle of day and night. Our sleep pattern is a well-known example, but others include daily fluctuations in body temperature and the production of several hormones. Internal clocks located in the brain and other organs drive these rhythms by altering the activity of certain genes depending on the time of day.

Animals have specific organs that contain enzymes needed to break down toxic molecules in the body, and the levels of several of these enzymes rise and fall over each 24-hour period. In mammals, these enzymes are found in the liver, but in insects they are found in an organ called the fat body. Here, Erion, King et al. set out to determine the extent to which the internal clock in the brain influences the daily rhythms of these enzymes.

The experiments show that a hormone released by the nervous system is required for the levels of the detoxifying enzymes to change in 24-hour cycles. This hormone – termed Neuropeptide F in fruit flies and Neuropeptide Y in mice – is also known to stimulate both mice and fruit flies to eat. Since toxic molecules often enter the body during feeding, Erion, King et al. speculate that it may be beneficial to link the detoxification process to feeding by using the same mechanism to control both processes. The next step following on from this work would be to find out exactly how neuropeptide F drives the 24-hour rhythms in the fat body and other organs.

DOI:http://dx.doi.org/10.7554/eLife.13552.002

Distinct Mechanisms Underlie Quiescence during Two Caenorhabditis elegans Sleep-Like States

Electrophysiological recordings have enabled identification of physiologically distinct yet behaviorally similar states of mammalian sleep. In contrast, sleep in nonmammals has generally been identified behaviorally and therefore regarded as a physiologically uniform state characterized by quiescence of feeding and locomotion, reduced responsiveness, and rapid reversibility. The nematode Caenorhabditis elegans displays sleep-like quiescent behavior under two conditions: developmentally timed quiescence (DTQ) occurs during larval transitions, and stress-induced quiescence (SIQ) occurs in response to exposure to cellular stressors. Behaviorally, DTQ and SIQ appear identical. Here, we use optogenetic manipulations of neuronal and muscular activity, pharmacology, and genetic perturbations to uncover circuit and molecular mechanisms of DTQ and SIQ. We find that locomotion quiescence induced by DTQ- and SIQ-associated neuropeptides occurs via their action on the nervous system, although their neuronal target(s) and/or molecular mechanisms likely differ. Feeding quiescence during DTQ results from a loss of pharyngeal muscle excitability, whereas feeding quiescence during SIQ results from a loss of excitability in the nervous system. Together these results indicate that, as in mammals, quiescence is subserved by different mechanisms during distinct sleep-like states in C. elegans.

Circadian Modulation of Alcohol-Induced Sedation and Recovery in Male and Female Drosophila

Delineating the factors that affect behavioral and neurological responses to alcohol is critical to facilitate measures for preventing or treating alcohol abuse. The high degree of conserved molecular and physiological processes makes Drosophila melanogaster a valuable model for investigating circadian interactions with alcohol-induced behaviors and examining sex-specific differences in alcohol sensitivity. We found that wild-type Drosophila exhibited rhythms in alcohol-induced sedation under light-dark and constant dark conditions with considerably greater alcohol exposure necessary to induce sedation during the late (subjective) day and peak sensitivity to alcohol occurring during the late (subjective) night. The circadian clock also modulated the recovery from alcohol-induced sedation with flies regaining motor control significantly faster during the late (subjective) day. As predicted, the circadian rhythms in sedation and recovery were absent in flies with a mutation in the circadian gene period or arrhythmic flies housed in constant light conditions. Flies lacking a functional circadian clock were more sensitive to the effects of alcohol with significantly longer recovery times. Similar to other animals and humans, Drosophila exhibit sex-specific differences in alcohol sensitivity. We investigated whether the circadian clock modulated the rhythms in the loss-of-righting reflex, alcohol-induced sedation, and recovery differently in males and females. We found that both sexes demonstrated circadian rhythms in the loss-of-righting reflex and sedation with the differences in alcohol sensitivity between males and females most pronounced during the late subjective day. Recovery of motor reflexes following alcohol sedation also exhibited circadian modulation in male and female flies, although the circadian clock did not modulate the difference in recovery times between the sexes. These studies provide a framework outlining how the circadian clock modulates alcohol-induced behaviors in Drosophila and identifies sexual dimorphisms in the circadian modulation of alcohol behaviors.

Call it Worm Sleep

The nematode Caenorhabditis elegans stops feeding and moving during a larval transition stage called lethargus and following exposure to cellular stressors. These behaviors have been termed 'sleep-like states'. We argue that these behaviors should instead be called sleep. Sleep during lethargus is similar to sleep regulated by circadian timers in insects and mammals, and sleep in response to cellular stress is similar to sleep induced by sickness in other animals. Sleep in mammals and Drosophila shows molecular and functional conservation with C. elegans sleep. The simple neuroanatomy and powerful genetic tools of C. elegans have yielded insights into sleep regulation and hold great promise for future research into sleep regulation and function.

Peptidomics of Neuropeptidergic Tissues of the Tsetse Fly Glossina morsitans morsitans

Neuropeptides and peptide hormones are essential signaling molecules that regulate nearly all physiological processes. The recent release of the tsetse fly genome allowed the construction of a detailed in silico neuropeptide database (International Glossina Genome Consortium, Science 344, 380-386 (2014)), as well as an in-depth mass spectrometric analysis of the most important neuropeptidergic tissues of this medically and economically important insect species. Mass spectrometric confirmation of predicted peptides is a vital step in the functional characterization of neuropeptides, as in vivo peptides can be modified, cleaved, or even mispredicted. Using a nanoscale reversed phase liquid chromatography coupled to a Q Exactive Orbitrap mass spectrometer, we detected 51 putative bioactive neuropeptides encoded by 19 precursors: adipokinetic hormone (AKH) I and II, allatostatin A and B, capability/pyrokinin (capa/PK), corazonin, calcitonin-like diuretic hormone (CT/DH), FMRFamide, hugin, leucokinin, myosuppressin, natalisin, neuropeptide-like precursor (NPLP) 1, orcokinin, pigment dispersing factor (PDF), RYamide, SIFamide, short neuropeptide F (sNPF) and tachykinin. In addition, propeptides, truncated and spacer peptides derived from seven additional precursors were found, and include the precursors of allatostatin C, crustacean cardioactive peptide, corticotropin releasing factor-like diuretic hormone (CRF/DH), ecdysis triggering hormone (ETH), ion transport peptide (ITP), neuropeptide F, and proctolin, respectively. The majority of the identified neuropeptides are present in the central nervous system, with only a limited number of peptides in the corpora cardiaca-corpora allata and midgut. Owing to the large number of identified peptides, this study can be used as a reference for comparative studies in other insects. Graphical Abstract ᅟ.

Regulation of Sleep by Insulin-like Peptide System in Drosophila melanogaster

STUDY OBJECTIVES

Most organisms have behavioral and physiological circadian rhythms, which are controlled by an endogenous clock. Although genetic analysis has revealed the intracellular mechanism of the circadian clock, the manner in which this clock communicates its temporal information to produce systemic regulation is still largely unknown.

DESIGN

Sleep behavior was measured using the Drosophila Activity Monitoring System (DAMS) monitor under a 12 h light:12 h dark cycle and constant darkness (DD), and 5 min without recorded activity were defined as a bout of sleep.

RESULTS

Here we show that Drosophila insulin-like peptides (DILPs) and their receptor (DInR) regulate sleep behavior. All mutants of the seven dilps and the mutant of their receptor exhibit decreases of total sleep except dilp4 mutants, whereas upregulation of DILP and DInR in the nervous system led to increased sleep. Histological analysis identified four previously unidentified neurons expressing DILP: D1, P1, L1, and L2, of which L1 and L2 belong to the LNd and LNv clock neurons that separately regulate different times of sleep. In addition, dilp2 levels significantly decrease when flies were fasted, which is consistent with a previous report that starvation inhibits sleep, further indicating that the dilp system is involved in sleep regulation.

CONCLUSION

Taken together, the results indicate that the Drosophila insulin-like peptide system is a crucial regulator of sleep.

FMRFamide signaling promotes stress-induced sleep in Drosophila

Enhanced sleep in response to cellular stress is a conserved adaptive behavior across multiple species, but the mechanism of this process is poorly understood. Drosophila melanogaster increases sleep following exposure to septic or aseptic injury, and Caenorhabditis elegans displays sleep-like quiescence following exposure to high temperatures that stress cells. We show here that, similar to C. elegans, Drosophila responds to heat stress with an increase in sleep. In contrast to Drosophila infection-induced sleep, heat-induced sleep is not sensitive to the time-of-day of the heat pulse. Moreover, the sleep response to heat stress does not require Relish, the NFκB transcription factor that is necessary for infection-induced sleep, indicating that sleep is induced by multiple mechanisms from different stress modalities. We identify a sleep-regulating role for a signaling pathway involving FMRFamide neuropeptides and their receptor FR. Animals mutant for either FMRFamide or for the FMRFamide receptor (FR) have a reduced recovery sleep in response to heat stress. FR mutants, in addition, show reduced sleep responses following infection with Serratia marcescens, and succumb to infection at a faster rate than wild-type controls. Together, these findings support the hypothesis that FMRFamide and its receptor promote an adaptive increase in sleep following stress. Because an FMRFamide-like neuropeptide plays a similar role in C. elegans, we propose that FRMFamide neuropeptide signaling is an ancient regulator of recovery sleep which occurs in response to cellular stress.

Clock network in Drosophila

The circadian clock consists of a network of peptidergic neurons in the brain of all animals. The function of this peptidergic network has been largely revealed in the fruit fly Drosophila melanogaster due to the relatively few well characterized clock neurons and because these neurons can be genetically manipulated. Here we review the neuronal organization of the circadian network and the role of individual clock neurons and neuropeptides in it.

Studying circadian rhythm and sleep using genetic screens in Drosophila

The power of Drosophila melanogaster as a model organism lies in its ability to be used for large-scale genetic screens with the capacity to uncover the genetic basis of biological processes. In particular, genetic screens for circadian behavior, which have been performed since 1971, allowed researchers to make groundbreaking discoveries on multiple levels: they discovered that there is a genetic basis for circadian behavior, they identified the so-called core clock genes that govern this process, and they started to paint a detailed picture of the molecular functions of these clock genes and their encoded proteins. Since the discovery that fruit flies sleep in 2000, researchers have successfully been using genetic screening to elucidate the many questions surrounding this basic animal behavior. In this chapter, we briefly recall the history of circadian rhythm and sleep screens and then move on to describe techniques currently employed for mutagenesis and genetic screening in the field. The emphasis lies on comparing the newer approaches of transgenic RNA interference (RNAi) to classical forms of mutagenesis, in particular in their application to circadian behavior and sleep. We discuss the different screening approaches in light of the literature and published and unpublished sleep and rhythm screens utilizing ethyl methanesulfonate mutagenesis and transgenic RNAi from our lab.

Homeostasis in C. elegans sleep is characterized by two behaviorally and genetically distinct mechanisms

Biological homeostasis invokes modulatory responses aimed at stabilizing internal conditions. Using tunable photo- and mechano-stimulation, we identified two distinct categories of homeostatic responses during the sleep-like state of Caenorhabditis elegans (lethargus). In the presence of weak or no stimuli, extended motion caused a subsequent extension of quiescence. The neuropeptide Y receptor homolog, NPR-1, and an inhibitory neuropeptide known to activate it, FLP-18, were required for this process. In the presence of strong stimuli, the correlations between motion and quiescence were disrupted for several minutes but homeostasis manifested as an overall elevation of the time spent in quiescence. This response to strong stimuli required the function of the DAF-16/FOXO transcription factor in neurons, but not that of NPR-1. Conversely, response to weak stimuli did not require the function of DAF-16/FOXO. These findings suggest that routine homeostatic stabilization of sleep may be distinct from homeostatic compensation following a strong disturbance.

DOI:http://dx.doi.org/10.7554/eLife.04380.001

The regenerative properties of sleep are required by all animals, with even the simplest animal, the nematode Caenorhabditis elegans, displaying a sleep-like state called lethargus. During development, nematodes must pass through four larval stages en route to adulthood, and the end of each stage is preceded by a period of lethargus lasting 2 to 3 hr.

Human sleep is divided into distinct stages that recur in a prescribed order throughout the night. Nematodes, on the other hand, simply experience alternating periods of activity and stillness as they sleep. Nevertheless, in both species, any disruptions to sleep automatically lead to adjustments of the rest of the sleep cycle to compensate for the disturbance and to ensure that the organism gets an adequate amount of sleep overall.

To date, it has been assumed that a single mechanism is responsible for adjusting the sleep cycle after any disturbance, regardless of its severity. However, Nagy, Tramm, Sanders et al. now show that this is not the case in C. elegans. Sleeping nematodes that were lightly disturbed by exposing them to light or to vibrations—causing them to briefly increase their activity levels—compensated for the disturbance by lengthening their next inactive period. By contrast, worms that were vigorously agitated by stronger vibrations showed a different response: the alternating pattern of stillness and activity was disrupted for several minutes, followed by an overall increase in the length of time spent in the stillness phase.

Experiments using genetically modified worms revealed that these two responses involve distinct molecular pathways. A signaling molecule called neuropeptide Y affects the response to minor sleep disruptions, whereas a transcription factor called DAF-16/FOXO is involved in the corresponding role after major disruptions. Given that neuropeptide Y has already been implicated in sleep regulation in humans and flies, it is not implausible that similar mechanisms may occur in response to disturbances of our own sleep.

DOI:http://dx.doi.org/10.7554/eLife.04380.002