The ion transport peptide is a new functional clock neuropeptide in the fruit fly Drosophila melanogaster

Ion transport peptide and ion transport peptide-like regulate ecdysis behavior and water transport during ecdysis in Gryllus bimaculatus

Ion transport peptide (ITP) and ITP-like (ITPLs) are pleiotropic bioactive peptides in insects. Although the contribution of these peptides to ecdysis has been studied, the precise regulatory mechanisms remain poorly understood. Here, we characterized the functions of itp and itpl variants in the two-spotted cricket, Gryllus bimaculatus. Reverse transcription-quantitative PCR and whole-mount in situ hybridization revealed that itp was expressed in the brain and terminal abdominal ganglion, whereas itpl variants were expressed in all ganglia of the central nervous system. Simultaneous knockdown of itp and itpls disrupted ecdysis behavior and water transport from the gut into the hemolymph during molting. Nevertheless, knockdown of itpls without influencing itp expression did not significantly affect ecdysis behavior but caused a reduction in hemolymph mass. Although water transport into the hemolymph is considered necessary for the swelling required to split the old cuticle layers during molting, a rescue experiment by injection of water or cricket Ringer's solution into the hemolymph of knockdown crickets did not recover the normal phenotype. Therefore, we propose that ITP/ITPL control ecdysis behavior probably not by regulating water transport from the gut into the hemolymph in crickets.

Light and dopamine impact two circadian neurons to promote morning wakefulness

In both mammals and flies, circadian brain neurons orchestrate physiological oscillations and behaviors like wake and sleep-these neurons can be subdivided by morphology and by gene expression patterns. Recent single-cell sequencing studies identified 17 Drosophila circadian neuron groups. One of these includes only two lateral neurons (LNs), which are marked by the expression of the neuropeptide ion transport peptide (ITP). Although these two ITP+ LNs have long been grouped with five other circadian evening activity cells, inhibiting the two neurons alone strongly reduces morning activity, indicating that they also have a prominent morning function. As dopamine signaling promotes activity in Drosophila, like in mammals, we considered that dopamine might influence this morning activity function. Moreover, the ITP+ LNs express higher mRNA levels than other LNs of the type 1-like dopamine receptor Dop1R1. Consistent with the importance of Dop1R1, cell-specific CRISPR-Cas9 mutagenesis of this receptor in the two ITP+ LNs renders flies significantly less active in the morning, and ex vivo live imaging shows Dop1R1-dependent cyclic AMP (cAMP) responses to dopamine in these two neurons. Notably, the response is more robust in the morning, reflecting higher morning Dop1R1 mRNA levels in the two neurons. As mRNA levels are not elevated in constant darkness, this suggests light-dependent upregulation of morning Dop1R1 transcript levels. Taken together with the enhanced morning cAMP response to dopamine, the data indicate how light and dopamine promote morning wakefulness in flies, mimicking the important effect of light on morning wakefulness in humans.

The Trissin/TrissinR signaling pathway in the circadian network regulates evening activity in Drosophila melanogaster under constant dark conditions

The circadian clock in Drosophila is governed by a neural network comprising approximately 150 neurons, known as clock neurons, which are intricately interconnected by various neurotransmitters. The neuropeptides that play functional roles in these clock neurons have been identified; however, the roles of some neuropeptides, such as Trissin, remain unclear. Trissin is expressed in lateral dorsal clock neurons (LNds), while its receptor, TrissinR, is expressed in dorsal neuron 1 (DN1) and LNds. In this study, we investigated the role of the Trissin/TrissinR signaling pathway within the circadian network in Drosophila melanogaster. Analysis involving our newly generated antibody against the Trissin precursor revealed that Trissin expression in the LNds cycles in a circadian manner. Behavioral analysis further demonstrated that flies with Trissin or TrissinR knockout or knockdown showed delayed evening activity offset under constant darkness conditions. Notably, this observed delay in evening activity offset in TrissinRNAi flies was restored via the additional knockdown of Ion transport peptide (ITP), indicating that the Trissin/TrissinR signaling pathway transmits information via ITP. Therefore, this pathway may be a key regulator of the timing of evening activity offset termination, orchestrating its effects in collaboration with the neuropeptide, ITP.

Wing expansion functional analysis of ion transport peptide gene in Sogatella furcifera (Horváth) (Hemiptera: Delphacidae)

Ion transport peptide (ITP), a superfamily of arthropod neuropeptides, serves a crucial role in regulating various physiological processes such as diuresis, ecdysis behavior, and wing expansion. However, the molecular characteristics, expression profile, and role of ITP in Sogatella furcifera are poorly understood. To elucidate the characteristics and biological function of ITP in S. furcifera, we employed reverse transcription-polymerase chain reaction (RT-PCR) and RNA interference (RNAi) methods. The identified SfITP gene encodes 117 amino acids. The expression of SfITP gradually increased followed the formation of 3-day-old of 5th instar nymph, peaking initially at 40 min after eclosion, and reaching another peak 24 h after eclosion, with particularly high expression levels in thorax and wing tissues. Notably, SfITP RNAi in 3rd instar nymphs of S. furcifera significantly inhibited the transcript levels of SfITP, resulting in 55% mortality and 78% wing deformity. These findings suggests that SfITP is involved in the regulation of wing expansion in S. furcifera, providing insights into the regulation of insect wing expansion and contributing to the molecular understanding of this process.

Light and dopamine impact two circadian neurons to promote morning wakefulness

In both mammals and flies, circadian brain neurons orchestrate physiological oscillations and behaviors like wake and sleep; these neurons can be subdivided by morphology and by gene expression patterns. Recent single-cell sequencing studies identified 17 Drosophila circadian neuron groups. One of these include only two lateral neurons (LNs), which are marked by the expression of the neuropeptide ion transport peptide (ITP). Although these two ITP+ LNs have long been grouped with five other circadian evening activity cells, inhibiting the two neurons alone strongly reduces morning activity; this indicates that they are prominent morning neurons. As dopamine signaling promotes activity in Drosophila like in mammals, we considered that dopamine might influence this morning activity function. Moreover, the ITP+ LNs express higher mRNA levels than other LNs of the type 1-like dopamine receptor Dop1R1. Consistent with the importance of Dop1R1, CRISPR/Cas9 mutagenesis of this receptor only in the two ITP+ LNs renders flies significantly less active in the morning, and ex vivo live imaging shows that dopamine increases cAMP levels in these two neurons; cell-specific mutagenesis of Dop1R1 eliminates this cAMP response to dopamine. Notably, the response is more robust in the morning, reflecting higher morning Dop1R1 mRNA levels in the two neurons. As morning levels are not elevated in constant darkness, this suggests light-dependent upregulation of morning Dop1R1 transcript levels. Taken together with enhanced morning cAMP response to dopamine, the data indicate how light stimulates morning wakefulness in flies, which mimics the important effect of light on morning wakefulness in humans.

Managing fuels and fluids: Network integration of osmoregulatory and metabolic hormonal circuits in the polymodal control of homeostasis in insects

Osmoregulation in insects is an essential process whereby changes in hemolymph osmotic pressure induce the release of diuretic or antidiuretic hormones to recruit individual osmoregulatory responses in a manner that optimizes overall homeostasis. However, the mechanisms by which different osmoregulatory circuits interact with other homeostatic networks to implement the correct homeostatic program remain largely unexplored. Surprisingly, recent advances in insect genetics have revealed several important metabolic functions are regulated by classic osmoregulatory pathways, suggesting that internal cues related to osmotic and metabolic perturbations are integrated by the same hormonal networks. Here, we review our current knowledge on the network mechanisms that underpin systemic osmoregulation and discuss the remarkable parallels between the hormonal networks that regulate body fluid balance and those involved in energy homeostasis to provide a framework for understanding the polymodal optimization of homeostasis in insects.

The head mesodermal cell couples FMRFamide neuropeptide signaling with rhythmic muscle contraction in C. elegans

FMRFamides are evolutionarily conserved neuropeptides that play critical roles in behavior, energy balance, and reproduction. Here, we show that FMRFamide signaling from the nervous system is critical for the rhythmic activation of a single cell of previously unknown function, the head mesodermal cell (hmc) in C. elegans. Behavioral, calcium imaging, and genetic studies reveal that release of the FLP-22 neuropeptide from the AVL neuron in response to pacemaker signaling activates hmc every 50 s through an frpr-17 G protein-coupled receptor (GPCR) and a protein kinase A signaling cascade in hmc. hmc activation results in muscle contraction through coupling by gap junctions composed of UNC-9/Innexin. hmc activation is inhibited by the neuronal release of a second FMRFamide-like neuropeptide, FLP-9, which functions through its GPCR, frpr-21, in hmc. This study reveals a function for two opposing FMRFamide signaling pathways in controlling the rhythmic activation of a target cell through volume transmission.

Pigment-dispersing factor is present in circadian clock neurons of pea aphids and may mediate photoperiodic signalling to insulin-producing cells

The neuropeptide pigment-dispersing factor (PDF) plays a pivotal role in the circadian clock of most Ecdysozoa and is additionally involved in the timing of seasonal responses of several photoperiodic species. The pea aphid, Acyrthosiphon pisum, is a paradigmatic photoperiodic species with an annual life cycle tightly coupled to the seasonal changes in day length. Nevertheless, PDF could not be identified in A. pisum so far. In the present study, we identified a PDF-coding gene that has undergone significant changes in the otherwise highly conserved insect C-terminal amino acid sequence. A newly generated aphid-specific PDF antibody stained four neurons in each hemisphere of the aphid brain that co-express the clock protein Period and have projections to the pars lateralis that are highly plastic and change their appearance in a daily and seasonal manner, resembling those of the fruit fly PDF neurons. Most intriguingly, the PDF terminals overlap with dendrites of the insulin-like peptide (ILP) positive neurosecretory cells in the pars intercerebralis and with putative terminals of Cryptochrome (CRY) positive clock neurons. Since ILP has been previously shown to be crucial for seasonal adaptations and CRY might serve as a circadian photoreceptor vital for measuring day length, our results suggest that PDF plays a critical role in aphid seasonal timing.

Differential expression of ITP and ITPL indicate multiple functions in the silkworm Bombyx mori

Ion transport peptide (ITP) and a longer ITP-like (ITPL) are alternatively spliced insect neuropeptides involved in the regulation of development and water homeostasis. Using in situ hybridisation and immunohistochemistry, we determined site- and stage-specific expression of each peptide in Bombyx mori. Each peptide was differentially expressed, except for the prominent overlapping expression of both peptides in six pairs of the brain neurosecretory cells Ia₂. After metamorphosis, ITP appeared in the male-specific neurons of the abdominal neuromere 9 (MAN₉) that innervate the reproductive organs. ITPL was detected in a pair of dorsolateral interneurons (IN-DL) in each thoracic and abdominal ganglion, and in the thoracic neurosecretory cells (NS-VTL₂) which terminate in the vicinity of the prothoracic gland. Feeding larvae showed ITPL expression in the abdominal neurosecretory cells M5. ITPL was also expressed in the peripheral L1 neurons that project axons into the thoracic and abdominal transverse nerves. Our results suggest that ITP and ITPL exhibit different sex- and stage-specific functions that may include regulation of reproduction and steroid production. For future functional studies, we identified an upstream regulatory region controlling ITP/ITPL expression in the brain and L1 neurons, and prepared stable transgenic line pITP-Gal4.2 using the piggyBac system.

Alcohol tolerance encoding in sleep regulatory circadian neurons in Drosophila

Alcohol tolerance is a simple form of behavioral and neural plasticity that occurs with the first drink. Neural plasticity in tolerance is likely a substrate for longer term adaptations that can lead to alcohol use disorder. Drosophila develop tolerance with characteristics similar to vertebrates, and it is useful model for determining the molecular and circuit encoding mechanisms in detail. Rapid tolerance, measured after the first alcohol exposure is completely metabolized, is localized to specific brain regions that are not interconnected in an obvious way. We used a forward neuroanatomical screen to identify three new neural sites for rapid tolerance encoding. One of these was comprised of two groups of neurons, the DN1a and DN1p glutamatergic neurons, that are part of the Drosophila circadian clock. We localized rapid tolerance to the two DN1a neurons that regulate arousal by light at night, temperature-dependent sleep timing, and night-time sleep. Two clock neurons that regulate evening activity, LNd6 and the 5th LNv, are postsynaptic to the DN1as and they promote rapid tolerance via the metabotropic glutamate receptor. Thus, rapid tolerance to alcohol overlaps with sleep regulatory neural circuitry, suggesting a mechanistic link.

Ion transport peptide regulates energy intake, expenditure, and metabolic homeostasis in Drosophila

In mammals, energy homeostasis is regulated by the antagonistic action of hormones insulin and glucagon. However, in contrast to the highly conserved insulin, glucagon is absent in most invertebrates. Although there are several endocrine regulators of energy expenditure and catabolism (such as the adipokinetic hormone), no single invertebrate hormone with all of the functions of glucagon has been described so far. Here, we used genetic gain- and loss-of-function experiments to show that the Drosophila gene Ion transport peptide (ITP) codes for a novel catabolic regulator that increases energy expenditure, lowers fat and glycogen reserves, and increases glucose and trehalose. Intriguingly, Ion transport peptide has additional functions reminiscent of glucagon, such as inhibition of feeding and transit of the meal throughout the digestive tract. Furthermore, Ion transport peptide interacts with the well-known signaling via the Adipokinetic hormone; Ion transport peptide promotes the pathway by stimulating Adipokinetic hormone secretion and transcription of the receptor AkhR. The genetic manipulations of Ion transport peptide on standard and Adipokinetic hormone-deficient backgrounds showed that the Adipokinetic hormone peptide mediates the hyperglycemic and hypertrehalosemic effects of Ion transport peptide, while the other metabolic functions of Ion transport peptide seem to be Adipokinetic hormone independent. In addition, Ion transport peptide is necessary for critical processes such as development, starvation-induced foraging, reproduction, and average lifespan. Altogether, our work describes a novel master regulator of fly physiology with functions closely resembling mammalian glucagon.

Dopamine and GPCR-mediated modulation of DN1 clock neurons gates the circadian timing of sleep

Neuromodulation is essential for adaptive animal behaviors among other physiological processes. It is essential to reliably manipulate neuromodulator pathways to understand their functions in animal physiology. In this study, we generated a CRISPR-Cas9-based guide library to target every G-Protein Coupled Receptor (GPCR) in the Drosophila genome and applied it to the well-studied clock neuron network. Notably, these GPCRs are highly enriched and differentially expressed in this small network, making it an ideal candidate to investigate their function. We cell-type specifically mutated GPCRs highly efficiently with no background gene editing detected. Applying this strategy to a specific node of the clock network revealed a role for dopamine in prolonging daytime sleep, suggesting network-specific functions of dopamine receptors in sleep-wake regulation.

The metronome-like circadian regulation of sleep timing must still adapt to an uncertain environment. Recent studies in Drosophila indicate that neuromodulation not only plays a key role in clock neuron synchronization but also affects interactions between the clock network and brain sleep centers. We show here that the targets of neuromodulators, G Protein Coupled Receptors (GPCRs), are highly enriched in the fly brain circadian clock network. Single-cell sequencing indicates that they are not only enriched but also differentially expressed and contribute to clock neuron identity. We generated a comprehensive guide library to mutagenize individual GPCRs in specific neurons and verified the strategy by introducing a targeted sequencing approach. Combined with a behavioral screen, the mutagenesis strategy revealed a role of dopamine in sleep regulation by identifying two dopamine receptors and a clock neuron subpopulation that gate the timing of sleep.

Endocrine cybernetics: neuropeptides as molecular switches in behavioural decisions

Plasticity in animal behaviour relies on the ability to integrate external and internal cues from the changing environment and hence modulate activity in synaptic circuits of the brain. This context-dependent neuromodulation is largely based on non-synaptic signalling with neuropeptides. Here, we describe select peptidergic systems in the Drosophila brain that act at different levels of a hierarchy to modulate behaviour and associated physiology. These systems modulate circuits in brain regions, such as the central complex and the mushroom bodies, which supervise specific behaviours. At the top level of the hierarchy there are small numbers of large peptidergic neurons that arborize widely in multiple areas of the brain to orchestrate or modulate global activity in a state and context-dependent manner. At the bottom level local peptidergic neurons provide executive neuromodulation of sensory gain and intrinsically in restricted parts of specific neuronal circuits. The orchestrating neurons receive interoceptive signals that mediate energy and sleep homeostasis, metabolic state and circadian timing, as well as external cues that affect food search, aggression or mating. Some of these cues can be triggers of conflicting behaviours such as mating versus aggression, or sleep versus feeding, and peptidergic neurons participate in circuits, enabling behaviour choices and switches.

A cytorhabdovirus-based expression vector in Nilaparvata lugens, Laodelphax striatellus, and Sogatella furcifera

The brown planthopper (BPH, Nilaparvata lugens), the small brown planthopper (SBPH, Laodelphax striatellus), and the white-backed planthopper (WBPH, Sogatella furcifera) are problematic insect pests and cause severe yield losses through phloem sap-sucking and virus transmission. Barley yellow striate mosaic virus (BYSMV), a plant cytorhabdovirus, has been developed as versatile expression platforms in SBPHs and cereal plants. However, bio-safe overexpression vectors based on recombinant BYSMV (rBYSMV) remain to be developed and applied to the three kinds of planthoppers. Here, we found that rBYSMV was able to infect SBPHs, BPHs and WBPHs through microinjection with crude extracts from rBYSMV-infected barley leaves. To ensure bio-safety of the rBYSMV vectors, we generated an rBYSMV mutant by deleting the accessory protein P3, a putative viral movement protein. As expected, the resulting mutant abolished viral systemic infection in barley plants but had no effects on BYSMV infectivity in insect vectors. Subsequently, we used the modified rBYSMV vector to overexpress iron transport peptide (ITP) in the three kinds of planthoppers and revealed the potential functions of ITP. Overall, our results provide bio-safe overexpression platforms to facilitate functional genomics studies of planthoppers.

The lateral posterior clock neurons (LPN) of Drosophila melanogaster express three neuropeptides and have multiple connections within the circadian clock network and beyond

Drosophila's lateral posterior neurons (LPNs) belong to a small group of circadian clock neurons that is so far not characterized in detail. Thanks to a new highly specific split-Gal4 line, here we describe LPNs' morphology in fine detail, their synaptic connections, daily bimodal expression of neuropeptides, and propose a putative role of this cluster in controlling daily activity and sleep patterns. We found that the three LPNs are heterogeneous. Two of the neurons with similar morphology arborize in the superior medial and lateral protocerebrum and most likely promote sleep. One unique, possibly wakefulness-promoting, neuron with wider arborizations extends from the superior lateral protocerebrum toward the anterior optic tubercle. Both LPN types exhibit manifold connections with the other circadian clock neurons, especially with those that control the flies' morning and evening activity (M- and E-neurons, respectively). In addition, they form synaptic connections with neurons of the mushroom bodies, the fan-shaped body, and with many additional still unidentified neurons. We found that both LPN types rhythmically express three neuropeptides, Allostatin A, Allostatin C, and Diuretic Hormone 31 with maxima in the morning and the evening. The three LPN neuropeptides may, furthermore, signal to the insect hormonal center in the pars intercerebralis and contribute to rhythmic modulation of metabolism, feeding, and reproduction. We discuss our findings in the light of anatomical details gained by the recently published hemibrain of a single female fly on the electron microscopic level and of previous functional studies concerning the LPN. This article is protected by copyright. All rights reserved.

Circadian clock outputs regulating insect photoperiodism: A potential role for glutamate transporter

Many temperate ectotherms survive winter by entering diapause - a state of developmental (or reproductive) suppression or arrest - in response to short autumnal day lengths. Day lengths are assessed by the circadian clock, the biological time-keeping system that governs biological rhythms with a period of approximately 24 h. However, clock output molecules controlling this photoperiodic response are largely unknown for many insects. To identify these molecules in Hemiptera, we performed RNAi knockdowns of several candidate genes in the bean bug Riptortus pedestris to determine whether their silencing affects photoperiodic regulation of ovarian development (reproductive diapause). Knockdown of diuretic hormone 31, short neuropeptide F, neuropeptide F, ion transport peptide, neuropeptide-like precursor 1, and choline acetyltransferase had no effect on ovarian development and were therefore ruled out as regulators of the photoperiodic response. However, knockdown of vesicular glutamate transporter promoted ovarian development under diapause-inducing short days, and this is the first report of the functional involvement of glutamate signalling in insect photoperiodism. Improved knockdown of this transporter (or receptor) and RNAi of other genes involved in glutamate signal transduction is required to verify its role as an output of the circadian clock.

Decapentaplegic Acutely Defines the Connectivity of Central Pacemaker Neurons in Drosophila

Rhythmic rest-activity cycles are controlled by an endogenous clock. In Drosophila, this clock resides in ∼150 neurons organized in clusters whose hierarchy changes in response to environmental conditions. The concerted activity of the circadian network is necessary for the adaptive responses to synchronizing environmental stimuli. Thus far, work was devoted to unravel the logic of the coordination of different clusters focusing on neurotransmitters and neuropeptides. We further explored communication in the adult male brain through ligands belonging to the bone morphogenetic protein (BMP) pathway. Herein we show that the lateral ventral neurons (LNvs) express the small morphogen decapentaplegic (DPP). DPP expression in the large LNvs triggered a period lengthening phenotype, the downregulation of which caused reduced rhythmicity and affected anticipation at dawn and dusk, underscoring DPP per se conveys time-of-day relevant information. Surprisingly, DPP expression in the large LNvs impaired circadian remodeling of the small LNv axonal terminals, likely through local modulation of the guanine nucleotide exchange factor Trio. These findings open the provocative possibility that the BMP pathway is recruited to strengthen/reduce the connectivity among specific clusters along the day and thus modulate the contribution of the clusters to the circadian network.SIGNIFICANCE STATEMENT The circadian clock relies on the communication between groups of so-called clock neurons to coordinate physiology and behavior to the optimal times across the day, predicting and adapting to a changing environment. The circadian network relies on neurotransmitters and neuropeptides to fine-tune connectivity among clock neurons and thus give rise to a coherent output. Herein we show that decapentaplegic, a ligand belonging to the BMP retrograde signaling pathway required for coordinated growth during development, is recruited by a group of circadian neurons in the adult brain to trigger structural remodeling of terminals on a daily basis.

Distribution and daily oscillation of GABA in the circadian system of the cockroach Rhyparobia maderae

Gamma-aminobutyric acid (GABA) is the prevalent inhibitory neurotransmitter in nervous systems promoting sleep in both mammals and insects. In the Madeira cockroach, sleep-wake cycles are controlled by a circadian clock network in the brain's optic lobes, centered in the accessory medulla (AME) with its innervating pigment-dispersing factor (PDF) expressing clock neurons at the anterior-ventral rim of the medulla. GABA is present in cell clusters that innervate different circuits of the cockroach's AME clock, without colocalizing in PDF clock neurons. Physiological, immunohistochemical, and behavioral assays provided evidence for a role of GABA in light entrainment, possibly via the distal tract that connects the AME's glomeruli to the medulla. Furthermore, GABA was implemented in clock outputs to multiple effector systems in optic lobe and midbrain. Here, GABAergic brain circuits were analyzed further, focusing on the circadian system in search for sleep/wake controlling brain circuits. All GABA-immunoreactive neurons of the cockroach brain were also stained with an antiserum against the GABA-synthesizing enzyme glutamic acid decarboxylase. We found strong overlap of the distribution of GABA-immunoreactive networks with PDF clock networks in optic lobes and midbrain. Neurons in five of the six soma groups that innervate the clock exhibited GABA immunoreactivity. The intensity of GABA immunoreactivity in the distal tract showed daily fluctuations with maximum staining intensity in the middle of the day and weakest staining at the end of the day. Quantification via enzyme-linked immunosorbent assay and quantitative liquid chromatography coupled to electrospray ionization tandem mass spectrometry, likewise, showed higher GABA levels in the optic lobe during the inactivity phase of the cockroach during the day and lower levels during its activity phase at dusk. Our data further support the hypothesis that light- and PDF-dependently the circadian clock network of the cockroach controls GABA levels and thereby promotes sleep during the day.

The Regulation of Drosophila Sleep

Sleep is critical for diverse aspects of brain function in animals ranging from invertebrates to humans. Powerful genetic tools in the fruit fly Drosophila melanogaster have identified - at an unprecedented level of detail - genes and neural circuits that regulate sleep. This research has revealed that the functions and neural principles of sleep regulation are largely conserved from flies to mammals. Further, genetic approaches to studying sleep have uncovered mechanisms underlying the integration of sleep and many different biological processes, including circadian timekeeping, metabolism, social interactions, and aging. These findings show that in flies, as in mammals, sleep is not a single state, but instead consists of multiple physiological and behavioral states that change in response to the environment, and is shaped by life history. Here, we review advances in the study of sleep in Drosophila, discuss their implications for understanding the fundamental functions of sleep that are likely to be conserved among animal species, and identify important unanswered questions in the field.

Drosophila clock cells use multiple mechanisms to transmit time-of-day signals in the brain

Regulation of circadian behavior and physiology by the Drosophila brain clock requires communication from central clock neurons to downstream output regions, but the mechanism by which clock cells regulate downstream targets is not known. We show here that the pars intercerebralis (PI), previously identified as a target of the morning cells in the clock network, also receives input from evening cells. We determined that morning and evening clock neurons have time-of-day-dependent connectivity to the PI, which is regulated by specific peptides as well as by fast neurotransmitters. Interestingly, PI cells that secrete the peptide DH44, and control rest:activity rhythms, are inhibited by clock inputs while insulin-producing cells (IPCs) are activated, indicating that the same clock cells can use different mechanisms to drive cycling in output neurons. Inputs of morning cells to IPCs are relevant for the circadian rhythm of feeding, reinforcing the role of the PI as a circadian relay that controls multiple behavioral outputs. Our findings provide mechanisms by which clock neurons signal to nonclock cells to drive rhythms of behavior.

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.

Genome-wide identification of neuropeptides and their receptor genes in Bemisia tabaci and their transcript accumulation change in response to temperature stresses

Insect neuropeptides play an important role in regulating physiological functions such as growth, development, behavior and reproduction. We identified temperature-sensitive neuropeptides and receptor genes of the cotton whitefly, Bemisia tabaci. We identified 38 neuropeptide precursor genes and 35 neuropeptide receptors and constructed a phylogenetic tree using additional data from other insects. As temperature adaptability enables B. tabaci to colonize a diversity of habitats, we performed quantitative polymerase chain reaction with two temperature stresses (low = 4 °C and high = 40 °C) to screen for temperature-sensitive neuropeptides. We found many neuropeptides and receptors that may be involved in the temperature adaptability of B. tabaci. This study is the first to identify B. tabaci neuropeptides and their receptors, and it will help to reveal the roles of neuropeptides in temperature adaptation of B. tabaci.

A transcriptomic taxonomy of Drosophila circadian neurons around the clock

Many different functions are regulated by circadian rhythms, including those orchestrated by discrete clock neurons within animal brains. To comprehensively characterize and assign cell identity to the 75 pairs of Drosophila circadian neurons, we optimized a single-cell RNA sequencing method and assayed clock neuron gene expression at different times of day. The data identify at least 17 clock neuron categories with striking spatial regulation of gene expression. Transcription factor regulation is prominent and likely contributes to the robust circadian oscillation of many transcripts, including those that encode cell-surface proteins previously shown to be important for cell recognition and synapse formation during development. The many other clock-regulated genes also constitute an important resource for future mechanistic and functional studies between clock neurons and/or for temporal signaling to circuits elsewhere in the fly brain.

Dopamine Signaling in Wake-Promoting Clock Neurons Is Not Required for the Normal Regulation of Sleep in Drosophila

Dopamine is a wake-promoting neuromodulator in mammals and fruit flies. In Drosophila melanogaster, the network of clock neurons that drives sleep/activity cycles comprises both wake-promoting and sleep-promoting cell types. The large ventrolateral neurons (l-LN_vs) and small ventrolateral neurons (s-LN_vs) have been identified as wake-promoting neurons within the clock neuron network. The l-LN_vs are innervated by dopaminergic neurons, and earlier work proposed that dopamine signaling raises cAMP levels in the l-LN_vs and thus induces excitatory electrical activity (action potential firing), which results in wakefulness and inhibits sleep. Here, we test this hypothesis by combining cAMP imaging and patch-clamp recordings in isolated brains. We find that dopamine application indeed increases cAMP levels and depolarizes the l-LN_vs, but, surprisingly, it does not result in increased firing rates. Downregulation of the excitatory D₁-like dopamine receptor (Dop1R1) in the l-LN_vs and s-LN_vs, but not of Dop1R2, abolished the depolarization of l-LN_vs in response to dopamine. This indicates that dopamine signals via Dop1R1 to the l-LN_vs. Downregulation of Dop1R1 or Dop1R2 in the l-LN_vs and s-LN_vs does not affect sleep in males. Unexpectedly, we find a moderate decrease of daytime sleep with downregulation of Dop1R1 and of nighttime sleep with downregulation of Dop1R2. Since the l-LN_vs do not use Dop1R2 receptors and the s-LN_vs also respond to dopamine, we conclude that the s-LN_vs are responsible for the observed decrease in nighttime sleep. In summary, dopamine signaling in the wake-promoting LN_vs is not required for daytime arousal, but likely promotes nighttime sleep via the s-LN_vs.SIGNIFICANCE STATEMENT In insect and mammalian brains, sleep-promoting networks are intimately linked to the circadian clock, and the mechanisms underlying sleep and circadian timekeeping are evolutionarily ancient and highly conserved. Here we show that dopamine, one important sleep modulator in flies and mammals, plays surprisingly complex roles in the regulation of sleep by clock-containing neurons. Dopamine inhibits neurons in a central brain sleep center to promote sleep and excites wake-promoting circadian clock neurons. It is therefore predicted to promote wakefulness through both of these networks. Nevertheless, our results reveal that dopamine acting on wake-promoting clock neurons promotes sleep, revealing a previously unappreciated complexity in the dopaminergic control of sleep.

Molecular Disruption of Ion Transport Peptide Receptor Results in Impaired Water Homeostasis and Developmental Defects in Bombyx mori

Insect ion transport peptides (ITPs) are important regulators of many physiological processes and they exert their functions by interacting with their receptors (ITPRs). In the current study, we comprehensively investigated the physiological functions of ITPR in the lepidopteran model insect, the silkworm (Bombyx mori), using the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein-9 nuclease (Cas9) genome editing technique. Mutations in silkworm ITPR (BNGR-A2) resulted in a prolongnation of the larval stage by 3.5-day as well as failure in wing expansion of moths. The BNGR-A2 mutation accelerated food transition throughout the digestive tract, which is 1.55-fold that of wild type (WT) insects. Excretion was 1.56-fold of WT insects during the larval stage, resulting in the loss of body water content. Loss of BNGR-A2 function induced significant upregulation of nitric oxide synthase (NOS) enzyme activity and nitric oxide (NO) content, as well as downstream Ca2+/NO/cGMP signaling pathways. Key genes in insulin and ecdysone signaling pathways were also affected by BNGR-A2 disruption. Our data show that ITPR plays key roles in regulating insect water homeostasis and development.

Life at High Latitudes Does Not Require Circadian Behavioral Rhythmicity under Constant Darkness

Nearly all organisms evolved endogenous self-sustained timekeeping mechanisms to track and anticipate cyclic changes in the environment. Circadian clocks, with a periodicity of about 24 h, allow animals to adapt to day-night cycles. Biological clocks are highly adaptive, but strong behavioral rhythms might be a disadvantage for adaptation to weakly rhythmic environments such as polar areas [1, 2]. Several high-latitude species, including Drosophila species, were found to be highly arrhythmic under constant conditions [3-6]. Furthermore, Drosophila species from subarctic regions can extend evening activity until dusk under long days. These traits depend on the clock network neurochemistry, and we previously proposed that high-latitude Drosophila species evolved specific clock adaptations to colonize polar regions [5, 7, 8]. We broadened our analysis to 3 species of the Chymomyza genus, which diverged circa 5 million years before the Drosophila radiation [9] and colonized both low and high latitudes [10, 11]. C. costata, pararufithorax, and procnemis, independently of their latitude of origin, possess the clock neuronal network of low-latitude Drosophila species, and their locomotor activity does not track dusk under long photoperiods. Nevertheless, the high-latitude C. costata becomes arrhythmic under constant darkness (DD), whereas the two low-latitude species remain rhythmic. Different mechanisms are behind the arrhythmicity in DD of C. costata and the high-latitude Drosophila ezoana, suggesting that the ability to maintain behavioral rhythms has been lost more than once during drosophilids' evolution and that it might indeed be an evolutionary adaptation for life at high latitudes.

miR-210 controls the evening phase of circadian locomotor rhythms through repression of Fasciclin 2

Circadian clocks control the timing of animal behavioral and physiological rhythms. Fruit flies anticipate daily environmental changes and exhibit two peaks of locomotor activity around dawn and dusk. microRNAs are small non-coding RNAs that play important roles in post-transcriptional regulation. Here we identify Drosophila miR-210 as a critical regulator of circadian rhythms. Under light-dark conditions, flies lacking miR-210 (miR-210KO) exhibit a dramatic 2 hrs phase advance of evening anticipatory behavior. However, circadian rhythms and molecular pacemaker function are intact in miR-210KO flies under constant darkness. Furthermore, we identify that miR-210 determines the evening phase of activity through repression of the cell adhesion molecule Fasciclin 2 (Fas2). Ablation of the miR-210 binding site within the 3' UTR of Fas2 (Fas2ΔmiR-210) by CRISPR-Cas9 advances the evening phase as in miR-210KO. Indeed, miR-210 genetically interacts with Fas2. Moreover, Fas2 abundance is significantly increased in the optic lobe of miR-210KO. In addition, overexpression of Fas2 in the miR-210 expressing cells recapitulates the phase advance behavior phenotype of miR-210KO. Together, these results reveal a novel mechanism by which miR-210 regulates circadian locomotor behavior.

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.

Recent advances in neuropeptide signaling in Drosophila, from genes to physiology and behavior

This review focuses on neuropeptides and peptide hormones, the largest and most diverse class of neuroactive substances, known in Drosophila and other animals to play roles in almost all aspects of daily life, as w;1;ell as in developmental processes. We provide an update on novel neuropeptides and receptors identified in the last decade, and highlight progress in analysis of neuropeptide signaling in Drosophila. Especially exciting is the huge amount of work published on novel functions of neuropeptides and peptide hormones in Drosophila, largely due to the rapid developments of powerful genetic methods, imaging techniques and innovative assays. We critically discuss the roles of peptides in olfaction, taste, foraging, feeding, clock function/sleep, aggression, mating/reproduction, learning and other behaviors, as well as in regulation of development, growth, metabolic and water homeostasis, stress responses, fecundity, and lifespan. We furthermore provide novel information on neuropeptide distribution and organization of peptidergic systems, as well as the phylogenetic relations between Drosophila neuropeptides and those of other phyla, including mammals. As will be shown, neuropeptide signaling is phylogenetically ancient, and not only are the structures of the peptides, precursors and receptors conserved over evolution, but also many functions of neuropeptide signaling in physiology and behavior.

Pigment-Dispersing Factor-expressing neurons convey circadian information in the honey bee brain

Pigment-Dispersing Factor (PDF) is an important neuropeptide in the brain circadian network of Drosophila and other insects, but its role in bees in which the circadian clock influences complex behaviour is not well understood. We combined high-resolution neuroanatomical characterizations, quantification of PDF levels over the day and brain injections of synthetic PDF peptide to study the role of PDF in the honey bee Apis mellifera We show that PDF co-localizes with the clock protein Period (PER) in a cluster of laterally located neurons and that the widespread arborizations of these PER/PDF neurons are in close vicinity to other PER-positive cells (neurons and glia). PDF-immunostaining intensity oscillates in a diurnal and circadian manner with possible influences for age or worker task on synchrony of oscillations in different brain areas. Finally, PDF injection into the area between optic lobes and the central brain at the end of the subjective day produced a consistent trend of phase-delayed circadian rhythms in locomotor activity. Altogether, these results are consistent with the hypothesis that PDF is a neuromodulator that conveys circadian information from pacemaker cells to brain centres involved in diverse functions including locomotion, time memory and sun-compass orientation.

Neuropeptides PDF and DH31 hierarchically regulate free-running rhythmicity in Drosophila circadian locomotor activity

Neuropeptides play pivotal roles in modulating circadian rhythms. Pigment-dispersing factor (PDF) is critical to the circadian rhythms in Drosophila locomotor activity. Here, we demonstrate that diuretic hormone 31 (DH31) complements PDF function in regulating free-running rhythmicity using male flies. We determined that Dh31 loss-of-function mutants (Dh31^#51) showed normal rhythmicity, whereas Dh31^#51;Pdf⁰¹ double mutants exhibited a severe arrhythmic phenotype compared to Pdf-null mutants (Pdf⁰¹). The expression of tethered-PDF or tethered-DH31 in clock cells, posterior dorsal neurons 1 (DN1ps), overcomes the severe arrhythmicity of Dh31^#51;Pdf⁰¹ double mutants, suggesting that DH31 and PDF may act on DN1ps to regulate free-running rhythmicity in a hierarchical manner. Unexpectedly, the molecular oscillations in Dh31^#51;Pdf⁰¹ mutants were similar to those in Pdf⁰¹ mutants in DN1ps, indicating that DH31 does not contribute to molecular oscillations. Furthermore, a reduction in Dh31 receptor (Dh31r) expression resulted in normal locomotor activity and did not enhance the arrhythmic phenotype caused by the Pdf receptor (Pdfr) mutation, suggesting that PDFR, but not DH31R, in DN1ps mainly regulates free-running rhythmicity. Taken together, we identify a novel role of DH31, in which DH31 and PDF hierarchically regulate free-running rhythmicity through DN1ps.

Identification and In Vivo Characterisation of Cardioactive Peptides in Drosophila melanogaster

Neuropeptides and peptide hormones serve as critical regulators of numerous biological processes, including development, growth, reproduction, physiology, and behaviour. In mammals, peptidergic regulatory systems are complex and often involve multiple peptides that act at different levels and relay to different receptors. To improve the mechanistic understanding of such complex systems, invertebrate models in which evolutionarily conserved peptides and receptors regulate similar biological processes but in a less complex manner have emerged as highly valuable. Drosophila melanogaster represents a favoured model for the characterisation of novel peptidergic signalling events and for evaluating the relevance of those events in vivo. In the present study, we analysed a set of neuropeptides and peptide hormones for their ability to modulate cardiac function in semi-intact larval Drosophila melanogaster. We identified numerous peptides that significantly affected heart parameters such as heart rate, systolic and diastolic interval, rhythmicity, and contractility. Thus, peptidergic regulation of the Drosophila heart is not restricted to chronotropic adaptation but also includes inotropic modulation. By specifically interfering with the expression of corresponding peptides in transgenic animals, we assessed the in vivo relevance of the respective peptidergic regulation. Based on the functional conservation of certain peptides throughout the animal kingdom, the identified cardiomodulatory activities may be relevant not only to proper heart function in Drosophila, but also to corresponding processes in vertebrates, including humans.

Allatostatin-C/AstC-R2 Is a Novel Pathway to Modulate the Circadian Activity Pattern in Drosophila

Seven neuropeptides are expressed within the Drosophila brain circadian network. Our previous mRNA profiling suggested that Allatostatin-C (AstC) is an eighth neuropeptide and specifically expressed in dorsal clock neurons (DN1s). Our results here show that AstC is, indeed, expressed in DN1s, where it oscillates. AstC is also expressed in two less well-characterized circadian neuronal clusters, the DN3s and lateral-posterior neurons (LPNs). Behavioral experiments indicate that clock-neuron-derived AstC is required to mediate evening locomotor activity under short (winter-like) and long (summer-like) photoperiods. The AstC-Receptor 2 (AstC-R2) is expressed in LNds, the clock neurons that drive evening locomotor activity, and AstC-R2 is required in these neurons to modulate the same short photoperiod evening phenotype. Ex vivo calcium imaging indicates that AstC directly inhibits a single LNd. The results suggest that a novel AstC/AstC-R2 signaling pathway, from dorsal circadian neurons to an LNd, regulates the evening phase in Drosophila.

The CCHamide1 Neuropeptide Expressed in the Anterior Dorsal Neuron 1 Conveys a Circadian Signal to the Ventral Lateral Neurons in Drosophila melanogaster

The fruit fly Drosophila melanogaster possesses approximately 150 brain clock neurons that control circadian behavioral rhythms. Even though individual clock neurons have self-sustaining oscillators, they interact and synchronize with each other through a network. However, little is known regarding the factors responsible for these network interactions. In this study, we investigated the role of CCHamide1 (CCHa1), a neuropeptide expressed in the anterior dorsal neuron 1 (DN1a), in intercellular communication of the clock neurons. We observed that CCHa1 connects the DN1a clock neurons to the ventral lateral clock neurons (LNv) via the CCHa1 receptor, which is a homolog of the gastrin-releasing peptide receptor playing a role in circadian intercellular communications in mammals. CCHa1 knockout or knockdown flies have a generally low activity level with a special reduction of morning activity. In addition, they exhibit advanced morning activity under light-dark cycles and delayed activity under constant dark conditions, which correlates with an advance/delay of PAR domain Protein 1 (PDP1) oscillations in the small-LNv (s-LNv) neurons that control morning activity. The terminals of the s-LNv neurons show rather high levels of Pigment-dispersing factor (PDF) in the evening, when PDF is low in control flies, suggesting that the knockdown of CCHa1 leads to increased PDF release; PDF signals the other clock neurons and evidently increases the amplitude of their PDP1 cycling. A previous study showed that high-amplitude PDP1 cycling increases the siesta of the flies, and indeed, CCHa1 knockout or knockdown flies exhibit a longer siesta than control flies. The DN1a neurons are known to be receptive to PDF signaling from the s-LNv neurons; thus, our results suggest that the DN1a and s-LNv clock neurons are reciprocally coupled via the neuropeptides CCHa1 and PDF, and this interaction fine-tunes the timing of activity and sleep.

The thirsty fly: Ion transport peptide (ITP) is a novel endocrine regulator of water homeostasis in Drosophila

Animals need to continuously adjust their water metabolism to the internal and external conditions. Homeostasis of body fluids thus requires tight regulation of water intake and excretion, and a balance between ingestion of water and solid food. Here, we investigated how these processes are coordinated in Drosophila melanogaster. We identified the first thirst-promoting and anti-diuretic hormone of Drosophila, encoded by the gene Ion transport peptide (ITP). This endocrine regulator belongs to the CHH (crustacean hyperglycemic hormone) family of peptide hormones. Using genetic gain- and loss-of-function experiments, we show that ITP signaling acts analogous to the human vasopressin and renin-angiotensin systems; expression of ITP is elevated by dehydration of the fly, and the peptide increases thirst while repressing excretion, promoting thus conservation of water resources. ITP responds to both osmotic and desiccation stress, and dysregulation of ITP signaling compromises the fly's ability to cope with these stressors. In addition to the regulation of thirst and excretion, ITP also suppresses food intake. Altogether, our work identifies ITP as an important endocrine regulator of thirst and excretion, which integrates water homeostasis with feeding of Drosophila.

NonA and CPX Link the Circadian Clockwork to Locomotor Activity in Drosophila

Drosophila NonA and its mammalian ortholog NONO are members of the Drosophila behavior and human splicing (DBHS) family. NONO also has a strong circadian connection: it associates with the circadian repressor protein PERIOD (PER) and contributes to circadian timekeeping. Here, we investigate NonA, which is required for proper levels of evening locomotor activity as well as a normal free-running period in Drosophila. NonA is associated with the positive transcription factor CLOCK/CYCLE (CLK/CYC), interacts directly with complexin (cpx) pre-mRNA, and upregulates gene expression, including the gene cpx. Downregulation of cpx expression in circadian neurons phenocopies NonA downregulation, whereas cpx overexpression rescues the nonA RNAi phenotypes, indicating that cpx is an important NonA target gene. As the cpx protein contributes to proper neurotransmitter and neuropeptide release in response to calcium, these results and others indicate that this control is important for the normal circadian regulation of locomotor activity.

The Underlying Genetics of Drosophila Circadian Behaviors

Life is shaped by circadian clocks. This review focuses on how behavioral genetics in the fruit fly unveiled what is known today about circadian physiology. We will briefly summarize basic properties of the clock and focus on some clock-controlled behaviors to highlight how communication between central and peripheral oscillators defines their properties.

Reconfiguration of a Multi-oscillator Network by Light in the Drosophila Circadian Clock

The brain clock that drives circadian rhythms of locomotor activity relies on a multi-oscillator neuronal network. In addition to synchronizing the clock with day-night cycles, light also reformats the clock-driven daily activity pattern. How changes in lighting conditions modify the contribution of the different oscillators to remodel the daily activity pattern remains largely unknown. Our data in Drosophila indicate that light readjusts the interactions between oscillators through two different modes. We show that a morning s-LNv > DN1p circuit works in series, whereas two parallel evening circuits are contributed by LNds and other DN1ps. Based on the photic context, the master pacemaker in the s-LNv neurons swaps its enslaved partner-oscillator-LNd in the presence of light or DN1p in the absence of light-to always link up with the most influential phase-determining oscillator. When exposure to light further increases, the light-activated LNd pacemaker becomes independent by decoupling from the s-LNvs. The calibration of coupling by light is layered on a clock-independent network interaction wherein light upregulates the expression of the PDF neuropeptide in the s-LNvs, which inhibits the behavioral output of the DN1p evening oscillator. Thus, light modifies inter-oscillator coupling and clock-independent output-gating to achieve flexibility in the network. It is likely that the light-induced changes in the Drosophila brain circadian network could reveal general principles of adapting to varying environmental cues in any neuronal multi-oscillator system.

Coordination between Differentially Regulated Circadian Clocks Generates Rhythmic Behavior

Specialized groups of neurons in the brain are key mediators of circadian rhythms, receiving daily environmental cues and communicating those signals to other tissues in the organism for entrainment and to organize circadian physiology. In Drosophila, the "circadian clock" is housed in seven neuronal clusters, which are defined by their expression of the main circadian proteins, Period, Timeless, Clock, and Cycle. These clusters are distributed across the fly brain and are thereby subject to the respective environments associated with their anatomical locations. While these core components are universally expressed in all neurons of the circadian network, additional regulatory proteins that act on these components are differentially expressed, giving rise to "local clocks" within the network that nonetheless converge to regulate coherent behavioral rhythms. In this review, we describe the communication between the neurons of the circadian network and the molecular differences within neurons of this network. We focus on differences in protein-expression patterns and discuss how such variation can impart functional differences in each local clock. Finally, we summarize our current understanding of how communication within the circadian network intersects with intracellular biochemical mechanisms to ultimately specify behavioral rhythms. We propose that additional efforts are required to identify regulatory mechanisms within each neuronal cluster to understand the molecular basis of circadian behavior.

Substrates for Neuronal Cotransmission With Neuropeptides and Small Molecule Neurotransmitters in Drosophila

It has been known for more than 40 years that individual neurons can produce more than one neurotransmitter and that neuropeptides often are colocalized with small molecule neurotransmitters (SMNs). Over the years much progress has been made in understanding the functional consequences of cotransmission in the nervous system of mammals. There are also some excellent invertebrate models that have revealed roles of coexpressed neuropeptides and SMNs in increasing complexity, flexibility, and dynamics in neuronal signaling. However, for the fly Drosophila there are surprisingly few functional studies on cotransmission, although there is ample evidence for colocalization of neuroactive compounds in neurons of the CNS, based both on traditional techniques and novel single cell transcriptome analysis. With the hope to trigger interest in initiating cotransmission studies, this review summarizes what is known about Drosophila neurons and neuronal circuits where different neuropeptides and SMNs are colocalized. Coexistence of neuroactive substances has been recorded in different neuron types such as neuroendocrine cells, interneurons, sensory cells and motor neurons. Some of the circuits highlighted here are well established in the analysis of learning and memory, circadian clock networks regulating rhythmic activity and sleep, as well as neurons and neuroendocrine cells regulating olfaction, nociception, feeding, metabolic homeostasis, diuretic functions, reproduction, and developmental processes. One emerging trait is the broad role of short neuropeptide F in cotransmission and presynaptic facilitation in a number of different neuronal circuits. This review also discusses the functional relevance of coexisting peptides in the intestine. Based on recent single cell transcriptomics data, it is likely that the neuronal systems discussed in this review are just a fraction of the total set of circuits where cotransmission occurs in Drosophila. Thus, a systematic search for colocalized neuroactive compounds in further neurons in anatomically defined circuits is of interest for the near future.

Neuroanatomical details of the lateral neurons of Drosophila melanogaster support their functional role in the circadian system

Drosophila melanogaster is a long‐standing model organism in the circadian clock research. A major advantage is the relative small number of about 150 neurons, which built the circadian clock in Drosophila. In our recent work, we focused on the neuroanatomical properties of the lateral neurons of the clock network. By applying the multicolor‐labeling technique Flybow we were able to identify the anatomical similarity of the previously described E2 subunit of the evening oscillator of the clock, which is built by the 5th small ventrolateral neuron (5th s‐LN_v) and one ITP positive dorsolateral neuron (LN_d). These two clock neurons share the same spatial and functional properties. We found both neurons innervating the same brain areas with similar pre‐ and postsynaptic sites in the brain. Here the anatomical findings support their shared function as a main evening oscillator in the clock network like also found in previous studies. A second quite surprising finding addresses the large lateral ventral PDF‐neurons (l‐LN_vs). We could show that the four hardly distinguishable l‐LN_vs consist of two subgroups with different innervation patterns. While three of the neurons reflect the well‐known branching pattern reproduced by PDF immunohistochemistry, one neuron per brain hemisphere has a distinguished innervation profile and is restricted only to the proximal part of the medulla‐surface. We named this neuron “extra” l‐LN_v (l‐LN_vx). We suggest the anatomical findings reflect different functional properties of the two l‐LN_v subgroups.

The characterization of the circadian clock in the olive fly Bactrocera oleae (Diptera: Tephritidae) reveals a Drosophila-like organization

The olive fruit fly, Bactrocera oleae, is the single most important pest for the majority of olive plantations. Oxitec's self-limiting olive fly technology (OX3097D-Bol) offers an alternative management approach to this insect pest. Because of previously reported asynchrony in the mating time of wild and laboratory strains, we have characterized the olive fly circadian clock applying molecular, evolutionary, anatomical and behavioural approaches. Here we demonstrate that the olive fly clock relies on a Drosophila melanogaster-like organization and that OX3097D-Bol carries a functional clock similar to wild-type strains, confirming its suitability for operational use.

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.

A Neural Network Underlying Circadian Entrainment and Photoperiodic Adjustment of Sleep and Activity in Drosophila

A sensitivity of the circadian clock to light/dark cycles ensures that biological rhythms maintain optimal phase relationships with the external day. In animals, the circadian clock neuron network (CCNN) driving sleep/activity rhythms receives light input from multiple photoreceptors, but how these photoreceptors modulate CCNN components is not well understood. Here we show that the Hofbauer-Buchner eyelets differentially modulate two classes of ventral lateral neurons (LNvs) within the Drosophila CCNN. The eyelets antagonize Cryptochrome (CRY)- and compound-eye-based photoreception in the large LNvs while synergizing CRY-mediated photoreception in the small LNvs. Furthermore, we show that the large LNvs interact with subsets of "evening cells" to adjust the timing of the evening peak of activity in a day length-dependent manner. Our work identifies a peptidergic connection between the large LNvs and a group of evening cells that is critical for the seasonal adjustment of circadian rhythms.

SIGNIFICANCE STATEMENT

In animals, circadian clocks have evolved to orchestrate the timing of behavior and metabolism. Consistent timing requires the entrainment these clocks to the solar day, a process that is critical for an organism's health. Light cycles are the most important external cue for the entrainment of circadian clocks, and the circadian system uses multiple photoreceptors to link timekeeping to the light/dark cycle. How light information from these photorecptors is integrated into the circadian clock neuron network to support entrainment is not understood. Our results establish that input from the HB eyelets differentially impacts the physiology of neuronal subgroups. This input pathway, together with input from the compound eyes, precisely times the activity of flies under long summer days. Our results provide a mechanistic model of light transduction and integration into the circadian system, identifying new and unexpected network motifs within the circadian clock neuron network.

Circadian Rhythm Neuropeptides in Drosophila: Signals for Normal Circadian Function and Circadian Neurodegenerative Disease

Circadian rhythm is a ubiquitous phenomenon in many organisms ranging from prokaryotes to eukaryotes. During more than four decades, the intrinsic and exogenous regulations of circadian rhythm have been studied. This review summarizes the core endogenous oscillation in Drosophila and then focuses on the neuropeptides, neurotransmitters and hormones that mediate its outputs and integration in Drosophila and the links between several of these (pigment dispersing factor (PDF) and insulin-like peptides) and neurodegenerative disease. These signaling molecules convey important network connectivity and signaling information for normal circadian function, but PDF and insulin-like peptides can also convey signals that lead to apoptosis, enhanced neurodegeneration and cognitive decline in flies carrying circadian mutations or in a senescent state.

Neuropeptides as Regulators of Behavior in Insects

Neuropeptides are by far the largest and most diverse group of signaling molecules in multicellular organisms. They are ancient molecules important in regulating a multitude of processes. Their small proteinaceous character allowed them to evolve and radiate quickly into numerous different molecules. On average, hundreds of distinct neuropeptides are present in animals, sometimes with unique classes that do not occur in distantly related species. Acting as neurotransmitters, neuromodulators, hormones, or growth factors, they are extremely diverse and are involved in controlling growth, development, ecdysis, digestion, diuresis, and many more physiological processes. Neuropeptides are also crucial in regulating myriad behavioral actions associated with feeding, courtship, sleep, learning and memory, stress, addiction, and social interactions. In general, behavior ensures that an organism can survive in its environment and is defined as any action that can change an organism's relationship to its surroundings. Even though the mode of action of neuropeptides in insects has been vigorously studied, relatively little is known about most neuropeptides and only a few model insects have been investigated. Here, we provide an overview of the roles neuropeptides play in insect behavior. We conclude that multiple neuropeptides need to work in concert to coordinate certain behaviors. Additionally, most neuropeptides studied to date have more than a single function.