A Series of Suppressive Signals within the Drosophila Circadian Neural Circuit Generates Sequential Daily Outputs

Overlapping Central Clock Network Circuitry Regulates Circadian Feeding and Activity Rhythms in Drosophila

The circadian system coordinates multiple behavioral outputs to ensure proper temporal organization. Timing information underlying circadian regulation of behavior depends on a molecular circadian clock that operates within clock neurons in the brain. In Drosophila and other organisms, clock neurons can be divided into several molecularly and functionally discrete subpopulations that form an interconnected central clock network. It is unknown how circadian signals are coherently generated by the clock network and transmitted across output circuits that connect clock cells to downstream neurons that regulate behavior. Here, we have exhaustively investigated the contribution of clock neuron subsets to the control of two prominent behavioral outputs in Drosophila: locomotor activity and feeding. We have used cell-specific manipulations to eliminate molecular clock function or induce electrical silencing either broadly throughout the clock network or in specific subpopulations. We find that clock cell manipulations produce similar changes in locomotor activity and feeding, suggesting that overlapping central clock circuitry regulates these distinct behavioral outputs. Interestingly, the magnitude and nature of the effects depend on the clock subset targeted. Lateral clock neuron manipulations profoundly degrade the rhythmicity of feeding and activity. In contrast, dorsal clock neuron manipulations only subtly affect rhythmicity but produce pronounced changes in the distribution of activity and feeding across the day. These experiments expand our knowledge of clock regulation of activity rhythms and offer the first extensive characterization of central clock control of feeding rhythms. Despite similar effects of central clock cell disruptions on activity and feeding, we find that manipulations that prevent functional signaling in an identified output circuit preferentially degrade locomotor activity rhythms, leaving feeding rhythms relatively intact. This demonstrates that activity and feeding are indeed dissociable behaviors, and furthermore suggests that differential circadian control of these behaviors diverges in output circuits downstream of the clock network.

Scientific experimental articles are modernist stories

This paper attempts to revive the epistemological discussion of scientific articles. What are their epistemic aims, and how are they achieved? We argue that scientific experimental articles are best understood as a particular kind of narrative: i.e., modernist narratives (think: Woolf, Joyce), at least in the sense that they employ many of the same techniques, including colligation and the juxtaposition of multiple perspectives. We suggest that this way of writing is necessary given the nature of modern science, but it also has specific epistemic benefits: it provides readers with an effective way to grasp the content of scientific articles which increases their understanding. On the other hand, modernist writing is vulnerable to certain kinds of epistemic abuses, which can be found instantiated in modern scientific writing as well.

On the origin and evolution of the dual oscillator model underlying the photoperiodic clockwork in the suprachiasmatic nucleus

Decades have now passed since Colin Pittendrigh first proposed a model of a circadian clock composed of two coupled oscillators, individually responsive to the rising and setting sun, as a flexible solution to the challenge of behavioral and physiological adaptation to the changing seasons. The elegance and predictive power of this postulation has stimulated laboratories around the world in searches to identify and localize such hypothesized evening and morning oscillators, or sets of oscillators, in insects, rodents, and humans, with experimental designs and approaches keeping pace over the years with technological advances in biology and neuroscience. Here, we recount the conceptual origin and highlight the subsequent evolution of this dual oscillator model for the circadian clock in the mammalian suprachiasmatic nucleus; and how, despite our increasingly sophisticated view of this multicellular pacemaker, Pittendrigh's binary conception has remained influential in our clock models and metaphors.

A four-oscillator model of seasonally adapted morning and evening activities in Drosophila melanogaster

The fruit fly Drosophila melanogaster exhibits two activity peaks, one in the morning and another in the evening. Because the two peaks change phase depending on the photoperiod they are exposed to, they are convenient for studying responses of the circadian clock to seasonal changes. To explain the phase determination of the two peaks, Drosophila researchers have employed the two-oscillator model, in which two oscillators control the two peaks. The two oscillators reside in different subsets of neurons in the brain, which express clock genes, the so-called clock neurons. However, the mechanism underlying the activity of the two peaks is complex and requires a new model for mechanistic exploration. Here, we hypothesize a four-oscillator model that controls the bimodal rhythms. The four oscillators that reside in different clock neurons regulate activity in the morning and evening and sleep during the midday and at night. In this way, bimodal rhythms are formed by interactions among the four oscillators (two activity and two sleep oscillators), which may judiciously explain the flexible waveform of activity rhythms under different photoperiod conditions. Although still hypothetical, this model would provide a new perspective on the seasonal adaptation of the two activity peaks.

Circadian Control of Lipid Metabolism

To ensure optimum health and performance, lipid metabolism needs to be temporally aligned to other body processes and to daily changes in the environment. Central and peripheral circadian clocks and environmental signals such as light provide internal and external time cues to the body. Importantly, each of the key organs involved in insect lipid metabolism contains a molecular clockwork which ticks with a varying degree of autonomy from the central clock in the brain. In this chapter, we review our current knowledge about peripheral clocks in the insect fat body, gut and oenocytes, and light- and circadian-driven diel patterns in lipid metabolites and lipid-related transcripts. In addition, we highlight selected neuroendocrine signaling pathways that are or may be involved in the temporal coordination and control of lipid metabolism.

A new GRAB sensor reveals differences in the dynamics and molecular regulation between neuropeptide and neurotransmitter release

The co-existence and co-transmission of neuropeptides and small molecule neurotransmitters in the same neuron is a fundamental aspect of almost all neurons across various species. However, the differences regarding their in vivo spatiotemporal dynamics and underlying molecular regulation remain poorly understood. Here, we developed a GPCR-activation-based (GRAB) sensor for detecting short neuropeptide F (sNPF) with high sensitivity and spatiotemporal resolution. Furthermore, we explore the differences of in vivo dynamics and molecular regulation between sNPF and acetylcholine (ACh) from the same neurons. Interestingly, the release of sNPF and ACh shows different spatiotemporal dynamics. Notably, we found that distinct synaptotagmins (Syt) are involved in these two processes, as Syt7 and Sytα for sNPF release, while Syt1 for ACh release. Thus, this new GRAB sensor provides a powerful tool for studying neuropeptide release and providing new insights into the distinct release dynamics and molecular regulation between neuropeptides and small molecule neurotransmitters.

A subclass of evening cells promotes the switch from arousal to sleep at dusk

Animals exhibit rhythmic patterns of behavior that are shaped by an internal circadian clock and the external environment. Although light intensity varies across the day, there are particularly robust differences at twilight (dawn/dusk). These periods are also associated with major changes in behavioral states, such as the transition from arousal to sleep. However, the neural mechanisms by which time and environmental conditions promote these behavioral transitions are poorly defined. Here, we show that the E1 subclass of Drosophila evening clock neurons promotes the transition from arousal to sleep at dusk. We first demonstrate that the cell-autonomous clocks of E2 neurons primarily drive and adjust the phase of evening anticipation, the canonical behavior associated with "evening" clock neurons. We next show that conditionally silencing E1 neurons causes a significant delay in sleep onset after dusk. However, rather than simply promoting sleep, activating E1 neurons produces time- and light-dependent effects on behavior. Activation of E1 neurons has no effect early in the day but then triggers arousal before dusk and induces sleep after dusk. Strikingly, these activation-induced phenotypes depend on the presence of light during the day. Despite their influence on behavior around dusk, in vivo voltage imaging of E1 neurons reveals that their spiking rate and pattern do not significantly change throughout the day. Moreover, E1-specific clock ablation has no effect on arousal or sleep. Thus, we suggest that, rather than specifying "evening" time, E1 neurons act, in concert with other rhythmic neurons, to promote behavioral transitions at dusk.

The neuropeptide pigment-dispersing factor signals independently of Bruchpilot-labelled active zones in daily remodelled terminals of Drosophila clock neurons

The small ventrolateral neurons (sLNvs) are key components of the central clock in the Drosophila brain. They signal via the neuropeptide pigment-dispersing factor (PDF) to align the molecular clockwork of different central clock neurons and to modulate downstream circuits. The dorsal terminals of the sLNvs undergo daily morphological changes that affect presynaptic sites organised by the active zone protein Bruchpilot (BRP), a homolog of mammalian ELKS proteins. However, the role of these presynaptic sites for PDF release is ill-defined. Here, we combined expansion microscopy with labelling of active zones by endogenously tagged BRP to examine the spatial correlation between PDF-containing dense-core vesicles and BRP-labelled active zones. We found that the number of BRP-labelled puncta in the sLNv terminals was similar while their density differed between Zeitgeber time (ZT) 2 and 14. The relative distance between BRP- and PDF-labelled puncta was increased in the morning, around the reported time of PDF release. Spontaneous dense-core vesicle release profiles of sLNvs in a publicly available ssTEM dataset (FAFB) consistently lacked spatial correlation to BRP-organised active zones. RNAi-mediated downregulation of brp and other active zone proteins expressed by the sLNvs did not affect PDF-dependent locomotor rhythmicity. In contrast, down-regulation of genes encoding proteins of the canonical vesicle release machinery, the dense-core vesicle-related protein CADPS, as well as PDF impaired locomotor rhythmicity. Taken together, our study suggests that PDF release from the sLNvs is independent of BRP-organised active zones, while BRP may be redistributed to active zones in a time-dependent manner.

Timed receptor tyrosine kinase signaling couples the central and a peripheral circadian clock in Drosophila

Circadian clocks impose daily periodicities to behavior, physiology, and metabolism. This control is mediated by a central clock and by peripheral clocks, which are synchronized to provide the organism with a unified time through mechanisms that are not fully understood. Here, we characterized in Drosophila the cellular and molecular mechanisms involved in coupling the central clock and the peripheral clock located in the prothoracic gland (PG), which together control the circadian rhythm of emergence of adult flies. The time signal from central clock neurons is transmitted via small neuropeptide F (sNPF) to neurons that produce the neuropeptide Prothoracicotropic Hormone (PTTH), which is then translated into daily oscillations of Ca2+ concentration and PTTH levels. PTTH signaling is required at the end of metamorphosis and transmits time information to the PG through changes in the expression of the PTTH receptor tyrosine kinase (RTK), TORSO, and of ERK phosphorylation, a key component of PTTH transduction. In addition to PTTH, we demonstrate that signaling mediated by other RTKs contributes to the rhythmicity of emergence. Interestingly, the ligand to one of these receptors (Pvf2) plays an autocrine role in the PG, which may explain why both central brain and PG clocks are required for the circadian gating of emergence. Our findings show that the coupling between the central and the PG clock is unexpectedly complex and involves several RTKs that act in concert and could serve as a paradigm to understand how circadian clocks are coordinated.

The clock gene Cryptochrome 1 is involved in the photoresponse of embryonic hatching behavior in Bombyx mori

The hatching of insect eggs is a classic circadian behavior rhythm controlled by the biological clock. Its function is considered to impose a daily rhythm on the embryo, allowing it to hatch within a permissible time window. However, the molecular pathways through which the clock affects embryonic hatching behavior remain unclear. Here, we utilized a clock gene Cryptochrome1 (Cry1) knockout mutant to dissect the pathways by which the circadian clock affects embryonic hatching rhythm in the silkworm. In the Cry1 mutant, the embryo hatching rhythm was disrupted. Under the constant light or constant dark incubation conditions, mutant embryos lost their hatching rhythm, while wild-type embryos hatch exhibiting free-running rhythm. In the light-dark cycle (LD), the hatching rhythm of CRY1-deficient silkworms could not be entrained by the LD photoperiod during the incubation period. The messenger RNA levels and enzymatic activities of Cht and Hel in the mutant embryos were significantly reduced at circadian time 24 (CT24). Transcriptome analysis revealed significant differences in gene expression at CT24 between the Cry1 knockout mutant and the wild-type, with 2616 differentially expressed genes identified. The enriched Gene Ontology pathway includes enzyme activity, energy availability, and protein translation. Short neuropeptide F signaling was reduced in the CT24 embryonic brain of the mutant, the expression of the neuropeptide PTTH was also reduced and the rhythm was lost, which further affects ecdysteroid signaling. Our results suggested that the silkworm circadian clock affects neuropeptide-hormone signaling as well as physiological functions related to hatching, which may regulate the hatching rhythm.

Circadian clock disruption promotes the degeneration of dopaminergic neurons in male Drosophila

Sleep and circadian rhythm disruptions are frequent comorbidities of Parkinson's disease (PD), a disorder characterized by the progressive loss of dopaminergic (DA) neurons in the substantia nigra. However, the causal role of circadian clocks in the degenerative process remains uncertain. We demonstrated here that circadian clocks regulate the rhythmicity and magnitude of the vulnerability of DA neurons to oxidative stress in male Drosophila. Circadian pacemaker neurons are presynaptic to a subset of DA neurons and rhythmically modulate their susceptibility to degeneration. The arrhythmic period (per) gene null mutation exacerbates the age-dependent loss of DA neurons and, in combination with brief oxidative stress, causes premature animal death. These findings suggest that circadian clock disruption promotes dopaminergic neurodegeneration.

A subclass of evening cells promotes the switch from arousal to sleep at dusk

Animals exhibit rhythmic patterns of behavior that are shaped by an internal circadian clock and the external environment. While light intensity varies across the day, there are particularly robust differences at twilight (dawn/dusk). These periods are also associated with major changes in behavioral states, such as the transition from arousal to sleep. However, the neural mechanisms by which time and environmental conditions promote these behavioral transitions are poorly defined. Here, we show that the E1 subclass of Drosophila evening clock neurons promotes the transition from arousal to sleep at dusk. We first demonstrate that the cell-autonomous clocks of E2 neurons alone are required to drive and adjust the phase of evening anticipation, the canonical behavior associated with "evening" clock neurons. We next show that conditionally silencing E1 neurons causes a significant delay in sleep onset after dusk. However, rather than simply promoting sleep, activating E1 neurons produces time- and light- dependent effects on behavior. Activation of E1 neurons has no effect early in the day, but then triggers arousal before dusk and induces sleep after dusk. Strikingly, these phenotypes critically depend on the presence of light during the day. Despite their influence on behavior around dusk, in vivo voltage imaging of E1 neurons reveals that their spiking rate does not vary between dawn and dusk. Moreover, E1-specific clock ablation has no effect on arousal or sleep. Thus, we suggest that, rather than specifying "evening" time, E1 neurons act, in concert with other rhythmic neurons, to promote behavioral transitions at dusk.

Sleep deprivation, sleep fragmentation, and social jet lag increase temperature preference in Drosophila

Despite the fact that sleep deprivation substantially affects the way animals regulate their body temperature, the specific mechanisms behind this phenomenon are not well understood. In both mammals and flies, neural circuits regulating sleep and thermoregulation overlap, suggesting an interdependence that may be relevant for sleep function. To investigate this relationship further, we exposed flies to 12 h of sleep deprivation, or 48 h of sleep fragmentation and evaluated temperature preference in a thermal gradient. Flies exposed to 12 h of sleep deprivation chose warmer temperatures after sleep deprivation. Importantly, sleep fragmentation, which prevents flies from entering deeper stages of sleep, but does not activate sleep homeostatic mechanisms nor induce impairments in short-term memory also resulted in flies choosing warmer temperatures. To identify the underlying neuronal circuits, we used RNAi to knock down the receptor for Pigment dispersing factor, a peptide that influences circadian rhythms, temperature preference and sleep. Expressing UAS-Pdfr^RNAi in subsets of clock neurons prevented sleep fragmentation from increasing temperature preference. Finally, we evaluated temperature preference after flies had undergone a social jet lag protocol which is known to disrupt clock neurons. In this protocol, flies experience a 3 h light phase delay on Friday followed by a 3 h light advance on Sunday evening. Flies exposed to social jet lag exhibited an increase in temperature preference which persisted for several days. Our findings identify specific clock neurons that are modulated by sleep disruption to increase temperature preference. Moreover, our data indicate that temperature preference may be a more sensitive indicator of sleep disruption than learning and memory.

Cholecystokinin neurons in mouse suprachiasmatic nucleus regulate the robustness of circadian clock

The suprachiasmatic nucleus (SCN) can generate robust circadian behaviors in mammals under different environments, but the underlying neural mechanisms remained unclear. Here, we showed that the activities of cholecystokinin (CCK) neurons in the mouse SCN preceded the onset of behavioral activities under different photoperiods. CCK-neuron-deficient mice displayed shortened free-running periods, failed to compress their activities under a long photoperiod, and developed rapid splitting or became arrhythmic under constant light. Furthermore, unlike vasoactive intestinal polypeptide (VIP) neurons, CCK neurons are not directly light sensitive, but their activation can elicit phase advance and counter light-induced phase delay mediated by VIP neurons. Under long photoperiods, the impact of CCK neurons on SCN dominates over that of VIP neurons. Finally, we found that the slow-responding CCK neurons control the rate of recovery during jet lag. Together, our results demonstrated that SCN CCK neurons are crucial for the robustness and plasticity of the mammalian circadian clock.

Polyphasic circadian neural circuits drive differential activities in multiple downstream rhythmic centers

Circadian clocks align various behaviors such as locomotor activity, sleep/wake, feeding, and mating to times of day that are most adaptive. How rhythmic information in pacemaker circuits is translated to neuronal outputs is not well understood. Here, we used brain-wide, 24-h in vivo calcium imaging in the Drosophila brain and searched for circadian rhythmic activity among identified clusters of dopaminergic (DA) and peptidergic neurosecretory (NS) neurons. Such rhythms were widespread and imposed by the PERIOD-dependent clock activity within the ∼150-cell circadian pacemaker network. The rhythms displayed either a morning (M), evening (E), or mid-day (MD) phase. Different subgroups of circadian pacemakers imposed neural activity rhythms onto different downstream non-clock neurons. Outputs from the canonical M and E pacemakers converged to regulate DA-PPM3 and DA-PAL neurons. E pacemakers regulate the evening-active DA-PPL1 neurons. In addition to these canonical M and E oscillators, we present evidence for a third dedicated phase occurring at mid-day: the l-LNv pacemakers present the MD activity peak, and they regulate the MD-active DA-PPM1/2 neurons and three distinct NS cell types. Thus, the Drosophila circadian pacemaker network is a polyphasic rhythm generator. It presents dedicated M, E, and MD phases that are functionally transduced as neuronal outputs to organize diverse daily activity patterns in downstream circuits.

Long-range connections are crucial for synchronization transition in a computational model of Drosophila brain dynamics

The synchronization transition type has been the focus of attention in recent years because it is associated with many functional characteristics of the brain. In this paper, the synchronization transition in neural networks with sleep-related biological drives in Drosophila is investigated. An electrical synaptic neural network is established to research the difference between the synchronization transition of the network during sleep and wake, in which neurons regularly spike during sleep and chaotically spike during wake. The synchronization transition curves are calculated mainly using the global instantaneous order parameters S. The underlying mechanisms and types of synchronization transition during sleep are different from those during wake. During sleep, regardless of the network structure, a frustrated (discontinuous) transition can be observed. Moreover, the phenomenon of quasi periodic partial synchronization is observed in ring-shaped regular network with and without random long-range connections. As the network becomes dense, the synchronization of the network only needs to slightly increase the coupling strength g. While during wake, the synchronization transition of the neural network is very dependent on the network structure, and three mechanisms of synchronization transition have emerged: discontinuous synchronization (explosive synchronization and frustrated synchronization), and continuous synchronization. The random long-range connections is the main topological factor that plays an important role in the resulting synchronization transition. Furthermore, similarities and differences are found by comparing synchronization transition research for the Hodgkin-Huxley neural network in the beta-band and gammma-band, which can further improve the synchronization phase transition research of biologically motivated neural networks. A complete research framework can also be used to study coupled nervous systems, which can be extended to general coupled dynamic systems.

Light triggers a network switch between circadian morning and evening oscillators controlling behaviour during daily temperature cycles

Proper timing of rhythmic locomotor behavior is the consequence of integrating environmental conditions and internal time dictated by the circadian clock. Rhythmic environmental input like daily light and temperature changes (called Zeitgeber) reset the molecular clock and entrain it to the environmental time zone the organism lives in. Furthermore, depending on the absolute temperature or light intensity, flies exhibit their main locomotor activity at different times of day, i.e., environmental input not only entrains the circadian clock but also determines the phase of a certain behavior. To understand how the brain clock can distinguish between (or integrate) an entraining Zeitgeber and environmental effects on activity phase, we attempted to entrain the clock with a Zeitgeber different from the environmental input used for phasing the behavior. 150 clock neurons in the Drosophila melanogaster brain control different aspects of the daily activity rhythms and are organized in various clusters. During regular 12 h light: 12 h dark cycles at constant mild temperature (LD 25°C, LD being the Zeitgeber), so called morning oscillator (MO) neurons control the increase of locomotor activity just before lights-on, while evening oscillator (EO) neurons regulate the activity increase at the end of the day, a few hours before lights-off. Here, using 12 h: 12 h 25°C:16°C temperature cycles as Zeitgeber, we attempted to look at the impact of light on phasing locomotor behavior. While in constant light and 25°C:16°C temperature cycles (LLTC), flies show an unimodal locomotor activity peak in the evening, during the same temperature cycle, but in the absence of light (DDTC), the phase of the activity peak is shifted to the morning. Here, we show that the EO is necessary for synchronized behavior in LLTC but not for entraining the molecular clock of the other clock neuronal groups, while the MO controls synchronized morning activity in DDTC. Interestingly, our data suggest that the influence of the EO on the synchronization increases depending on the length of the photoperiod (constant light vs 12 h of light). Hence, our results show that effects of different environmental cues on clock entrainment and activity phase can be separated, allowing to decipher their integration by the circadian clock.

“If a clock is to provide information involved in controlling important functions, then clearly it must be reasonably reliable” said Colin Pittendrigh, one of the chronobiology pioneers in 1954. The circadian clock allows organisms to synchronize with their ecological niche. For this, the circadian clock uses rhythmic environmental parameters (Zeitgeber), the main ones being light and temperature. Hence, Colin Pittendrigh posted a still unresolved enigma in chronobiology. How can a clock be reliable when its resetting depends on environmental fluctuations that are not so reliable? Both, light and temperature vary a lot on a day-to-day basis, and animals respond to these variations depending on the time of day.

Here, we propose a new model where the molecular clock resets to environmental cycles in a robust and independent manner, while the underlying neuronal oscillatory network switches its balance towards specific oscillators depending on the environmental condition thereby leading to distinct behavioral adaptation. To proof this proposed dogma in fruit flies, using temperature cycles as Zeitgeber, we demonstrate a light-induced switch of the network balance. Hence, we supply a foundation that in the future will help to understand how animals use their circadian clock to adapt their behavior to environmental changes.

An extra-clock ultradian brain oscillator sustains circadian timekeeping

The master circadian clock generates 24-hour rhythms to orchestrate daily behavior, even running freely under constant conditions. Traditionally, the master clock is considered self-sufficient in sustaining free-running timekeeping via its cell-autonomous molecular clocks and interneuronal communications within the circadian neural network. Here, we find a set of bona fide ultradian oscillators in the Drosophila brain that support free-running timekeeping, despite being located outside the master clock circuit and lacking clock gene expression. These extra-clock electrical oscillators (xCEOs) generate cell-autonomous ultradian bursts, pacing widespread burst firing and promoting rhythmic resting membrane potentials in clock neurons via parallel monosynaptic connections. Silencing xCEOs disrupts daily electrical rhythms in clock neurons and impairs cycling of neuropeptide pigment dispersing factor, leading to the loss of free-running locomotor rhythms. Together, we conclude that the master clock is not self-sufficient to sustain free-running behavior rhythms but requires additional endogenous inputs to the clock from the extra-clock ultradian brain oscillators.

The Drosophila circadian clock circuit is a nonhierarchical network of peptidergic oscillators

The relatively simple Drosophila circadian clock circuit consists of 150 clock neurons that coordinate rhythmic behavior and physiology, which are generally classified based on neuroanatomical location. Transcriptional and connectomic studies have identified novel subdivisions of these clock neuron populations, and identified neuropeptides not previously known to be expressed in the fly clock circuit. An additional feature of fly clock neurons is daily axonal remodeling, first noted in small ventrolateral neurons, but more recently also found in additional clock neuron groups. These findings raise new questions about the functional roles of clock neuron subpopulations and daily remodeling of network architecture in regulating circadian behavior and physiology.

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.

---Connectomic analysis of the Drosophila lateral neuron clock cells reveals the synaptic basis of functional pacemaker classes

The circadian clock orchestrates daily changes in physiology and behavior to ensure internal temporal order and optimal timing across the day. In animals, a central brain clock coordinates circadian rhythms throughout the body and is characterized by a remarkable robustness that depends on synaptic connections between constituent neurons. The clock neuron network of Drosophila, which shares network motifs with clock networks in the mammalian brain yet is built of many fewer neurons, offers a powerful model for understanding the network properties of circadian timekeeping. Here, we report an assessment of synaptic connectivity within a clock network, focusing on the critical lateral neuron (LN) clock neuron classes within the Janelia hemibrain dataset. Our results reveal that previously identified anatomical and functional subclasses of LNs represent distinct connectomic types. Moreover, we identify a small number of non-clock cell subtypes representing highly synaptically coupled nodes within the clock neuron network. This suggests that neurons lacking molecular timekeeping likely play integral roles within the circadian timekeeping network. To our knowledge, this represents the first comprehensive connectomic analysis of a circadian neuronal network.

Most organisms on Earth possess an internal timekeeping system which ensures that bodily processes such as sleep, wakefulness or digestion take place at the right time. These precise daily rhythms are kept in check by a master clock in the brain. There, thousands of neurons – some of which carrying an internal ‘molecular clock’ – connect to each other through structures known as synapses. Exactly how the resulting network is organised to support circadian timekeeping remains unclear.

To explore this question, Shafer, Gutierrez et al. focused on fruit flies, as recent efforts have systematically mapped every neuron and synaptic connection in the brain of this model organism. Analysing available data from the hemibrain connectome project at Janelia revealed that that the neurons with the most important timekeeping roles were in fact forming the fewest synapses within the network. In addition, neurons without internal molecular clocks mediated strong synaptic connections between those that did, suggesting that ‘clockless’ cells still play an integral role in circadian timekeeping.

With this research, Shafer, Gutierrez et al. provide unexpected insights into the organisation of the master body clock. Better understanding the networks that underpin circadian rhythms will help to grasp how and why these are disrupted in obesity, depression and Alzheimer’s disease.

Circadian programming of the ellipsoid body sleep homeostat in Drosophila

Homeostatic and circadian processes collaborate to appropriately time and consolidate sleep and wake. To understand how these processes are integrated, we scheduled brief sleep deprivation at different times of day in Drosophila and find elevated morning rebound compared to evening. These effects depend on discrete morning and evening clock neurons, independent of their roles in circadian locomotor activity. In the R5 ellipsoid body sleep homeostat, we identified elevated morning expression of activity dependent and presynaptic gene expression as well as the presynaptic protein BRUCHPILOT consistent with regulation by clock circuits. These neurons also display elevated calcium levels in response to sleep loss in the morning, but not the evening consistent with the observed time-dependent sleep rebound. These studies reveal the circuit and molecular mechanisms by which discrete circadian clock neurons program a homeostatic sleep center.

Regulation of PDF receptor signaling controlling daily locomotor rhythms in Drosophila

Each day and in conjunction with ambient daylight conditions, neuropeptide PDF regulates the phase and amplitude of locomotor activity rhythms in Drosophila through its receptor, PDFR, a Family B G protein-coupled receptor (GPCR). We studied the in vivo process by which PDFR signaling turns off, by converting as many as half of the 28 potential sites of phosphorylation in its C terminal tail to a non-phosphorylatable residue (alanine). We report that many such sites are conserved evolutionarily, and their conversion creates a specific behavioral syndrome opposite to loss-of-function phenotypes previously described for pdfr. That syndrome includes increases in the amplitudes of both Morning and Evening behavioral peaks, as well as multi-hour delays of the Evening phase. The precise behavioral effects were dependent on day-length, and most effects mapped to conversion of only a few, specific serine residues near the very end of the protein and specific to its A isoform. Behavioral phase delays of the Evening activity under entraining conditions predicted the phase of activity cycles under constant darkness. The behavioral phenotypes produced by the most severe PDFR variant were ligand-dependent in vivo, and not a consequence of changes to their pharmacological properties, nor of changes in their surface expression, as measured in vitro. The mechanisms underlying termination of PDFR signaling are complex, subject to regulation that is modified by season, and central to a better understanding of the peptidergic modulation of behavior.

In multi-cellular organisms, circadian pacemakers create output as a series of phase markers across the 24 hour day to allow other cells to pattern diverse aspects of daily rhythmic physiology and behavior. Within circadian pacemaker circuits, neuropeptide signaling is essential to help promote coherent circadian outputs. In the fruit fly Drosophila 150 neurons are dedicated circadian clocks and they all tell the same time. In spite of such strong synchronization, they provide diverse phasic outputs in the form of their discrete, asynchronous neuronal activity patterns. Neuropeptide signaling breaks the clock-generated symmetry and drives many pacemakers away from their preferred activity period in the morning. Each day, neuropeptide PDF is released by Morning pacemakers and delays the phase of activity of specific other pacemakers to later parts of the day or night. When and how the PDF that is released in the morning stops acting is unknown. Furthermore, timing of signal termination is not fixed because day length changes each day, hence the modulatory delay exerted by PDF must itself be regulated. Here we test a canonical model of G protein-coupled receptor physiology to ask how PDF receptor signaling is normally de-activated. We use behavioral measures to define sequence elements of the receptor whose post-translational modifications (e.g., phosphorylation) may define the duration of receptor signaling.

Circadian pacemaker neurons display cophasic rhythms in basal calcium level and in fast calcium fluctuations

Circadian pacemaker neurons in the Drosophila brain display daily rhythms in the levels of intracellular calcium. These calcium rhythms are driven by molecular clocks and are required for normal circadian behavior. To study their biological basis, we employed genetic manipulations in conjunction with improved methods of in vivo light-sheet microscopy to measure calcium dynamics in individual pacemaker neurons over complete 24-h durations at sampling frequencies as high as 5 Hz. This technological advance unexpectedly revealed cophasic daily rhythms in basal calcium levels and in high-frequency calcium fluctuations. Further, we found that the rhythms of basal calcium levels and of fast calcium fluctuations reflect the activities of two proteins that mediate distinct forms of calcium fluxes. One is the inositol trisphosphate receptor (ITPR), a channel that mediates calcium fluxes from internal endoplasmic reticulum calcium stores, and the other is a T-type voltage-gated calcium channel, which mediates extracellular calcium influx. These results suggest that Drosophila molecular clocks regulate ITPR and T-type channels to generate two distinct but coupled rhythms in basal calcium and in fast calcium fluctuations. We propose that both internal and external calcium fluxes are essential for circadian pacemaker neurons to provide rhythmic outputs and thereby, regulate the activities of downstream brain centers.

Adaptation of Drosophila melanogaster to Long Photoperiods of High-Latitude Summers Is Facilitated by the ls-Timeless Allele

Circadian clocks help animals to be active at the optimal time of the day whereby for most species the daily light-dark cycle is the most important zeitgeber for their circadian clock. In this respect, long arctic summer days are particularly challenging as light is present almost 24 h per day, and continuous light makes the circadian clocks of many animals arrhythmic. This is especially true for the fruit fly, Drosophila melanogaster, which possesses a very light-sensitive clock. The blue-light photoreceptor Cryptochrome (CRY) and the clock protein Timeless (TIM) are the light-sensitive components of the circadian clock and are responsible for constant light-induced arrhythmicity even at very low light intensities. Nevertheless, D. melanogaster was able to spread from its tropical origin and invade northern latitudes. Here, we tested whether a natural polymorphism at the timeless (tim) locus, s-tim and ls-tim, helped adaptation to very long photoperiods. The recently evolved natural allele, ls-tim, encodes a longer, less light sensitive form of TIM (L-TIM) in addition to the shorter (S-TIM) form, the only form encoded by the ancient s-tim allele. ls-tim has evolved in southeastern Italy and slowly spreads to higher latitudes. L-TIM is known to interact less efficiently with CRY as compared with S-TIM. Here, we studied the locomotor activity patterns of ~40 wild s-tim and ls-tim isofemale lines caught at different latitudes under simulated high-latitude summer light conditions (continuous light or long photoperiods with 20-h daily light). We found that the ls-tim lines were significantly more rhythmic under continuous light than the s-tim lines. Importantly, the ls-tim lines can delay their evening activity under long photoperiods, a behavioral adaptation that appears to be optimal under high-latitude conditions. Our observations suggest that the functional gain associated with ls-tim may drive the northern spread of this allele by directional selection.

Short Neuropeptide F and Its Receptor Regulate Feeding Behavior in Pea Aphid (Acyrthosiphon pisum)

Simple Summary

We know that neuropeptides and G protein-coupled receptors regulate the physiology and behavior of animals and that the pea aphid (Acyrthosiphon pisum) is a serious agricultural pest and model insect. In this study, we investigated the short neuropeptide F and its receptor in pea aphid. Feeding analysis showed that the probing time and total phloem duration significantly decreased in response to sNPF and predicted sNPFR gene silencing in RNAi assays. The silencing of sNPF significantly reduced the aphid’s reproduction but not survival. Our findings will help in the design of control strategies by using the molecular biological approach.

Abstract

Insect short neuropeptide F (sNPF), an ortholog of prolactin-releasing peptide of invertebrates, regulates diverse biological processes, including feeding, olfaction, locomotion, and sleep homeostasis in insects. However, its function is still unclear in an important model insect and agricultural pest, the pea aphid (Acyrthosiphon pisum). Here, we investigated short neuropeptide F (ApsNPF) and its receptor (ApsNPFR) in A. pisum. The sNPF gene contains three exons and two long introns. In addition, the genome contains a single sNPF receptor with seven transmembrane domains. Stage- and tissue-specific transcript profiling by qRT-PCR revealed that ApsNPF and ApsNPFR were mainly expressed in the central nervous system. The receptor was also detected in antennae, midgut, and integument. The highest expression levels were found in first instar nymphs compared to other developmental stages. Besides, the starvation-induced pattern indicated that the sNPF network depends on the nutritional state of the insect. An electrical penetration graph showed that probing time and phloem duration of A. pisum on broad bean plants decreased in response to dssNPF and dssNPFR in RNAi assays. sNPF silencing reduced the number of nymphs per female but not aphid survival. We believe that our results advance in-depth knowledge of the sNPF/sNPFR signaling cascade and its place in regulating feeding behavior in insects. In turn, it may contribute to the potential design of new strategies to control aphids, with a focus on the sNPF system.

Time-course RNASeq of Camponotus floridanus forager and nurse ant brains indicate links between plasticity in the biological clock and behavioral division of labor

Background

Circadian clocks allow organisms to anticipate daily fluctuations in their environment by driving rhythms in physiology and behavior. Inter-organismal differences in daily rhythms, called chronotypes, exist and can shift with age. In ants, age, caste-related behavior and chronotype appear to be linked. Brood-tending nurse ants are usually younger individuals and show “around-the-clock” activity. With age or in the absence of brood, nurses transition into foraging ants that show daily rhythms in activity. Ants can adaptively shift between these behavioral castes and caste-associated chronotypes depending on social context. We investigated how changes in daily gene expression could be contributing to such behavioral plasticity in Camponotus floridanus carpenter ants by combining time-course behavioral assays and RNA-Sequencing of forager and nurse brains.

Results

We found that nurse brains have three times fewer 24 h oscillating genes than foragers. However, several hundred genes that oscillated every 24 h in forager brains showed robust 8 h oscillations in nurses, including the core clock genes Period and Shaggy. These differentially rhythmic genes consisted of several components of the circadian entrainment and output pathway, including genes said to be involved in regulating insect locomotory behavior. We also found that Vitellogenin, known to regulate division of labor in social insects, showed robust 24 h oscillations in nurse brains but not in foragers. Finally, we found significant overlap between genes differentially expressed between the two ant castes and genes that show ultradian rhythms in daily expression.

Conclusion

This study provides a first look at the chronobiological differences in gene expression between forager and nurse ant brains. This endeavor allowed us to identify a putative molecular mechanism underlying plastic timekeeping: several components of the ant circadian clock and its output can seemingly oscillate at different harmonics of the circadian rhythm. We propose that such chronobiological plasticity has evolved to allow for distinct regulatory networks that underlie behavioral castes, while supporting swift caste transitions in response to colony demands. Behavioral division of labor is common among social insects. The links between chronobiological and behavioral plasticity that we found in C. floridanus, thus, likely represent a more general phenomenon that warrants further investigation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08282-x.

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.

Dorsal clock neurons in Drosophila sculpt locomotor outputs but are dispensable for circadian activity rhythms

The circadian system is comprised three components: a network of core clock cells in the brain that keeps time, input pathways that entrain clock cells to the environment, and output pathways that use this information to ensure appropriate timing of physiological and behavioral processes throughout the day. Core clock cells can be divided into molecularly distinct populations that likely make unique functional contributions. Here we clarify the role of the dorsal neuron 1 (DN1) population of clock neurons in the transmission of circadian information by the Drosophila core clock network. Using an intersectional genetic approach that allowed us to selectively and comprehensively target DN1 cells, we show that suppressing DN1 neuronal activity alters the magnitude of daily activity and sleep without affecting overt rhythmicity. This suggests that DN1 cells are dispensable for both the generation of circadian information and the propagation of this information across output circuits.

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Intersectional genetic approach targets DN1 cells comprehensively and selectively

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DN1p silencing alters distribution and amount of activity and sleep across the day

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DN1p cell firing is neither necessary nor sufficient for circadian activity rhythms

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DN1a silencing subtly alters total activity and sleep but leaves rhythmicity intact

Comparative Ca2+ channel contributions to intracellular Ca2+ levels in the circadian clock

Circadian rhythms in mammals are coordinated by the central clock in the brain, located in the suprachiasmatic nucleus (SCN). Multiple molecular and cellular signals display a circadian variation within SCN neurons, including intracellular Ca2+, but the mechanisms are not definitively established. SCN cytosolic Ca2+ levels exhibit a peak during the day, when both action potential firing and Ca2+ channel activity are increased, and are decreased at night, correlating with a reduction in firing rate. In this study, we employ a single-color fluorescence anisotropy reporter (FLARE), Venus FLARE-Cameleon, and polarization inverted selective-plane illumination microscopy to measure rhythmic changes in cytosolic Ca2+ in SCN neurons. Using this technique, the Ca2+ channel subtypes contributing to intracellular Ca2+ at the peak and trough of the circadian cycle were assessed using a pharmacological approach with Ca2+ channel inhibitors. Peak (218 ± 16 nM) and trough (172 ± 13 nM) Ca2+ levels were quantified, indicating a 1.3-fold circadian variance in Ca2+ concentration. Inhibition of ryanodine-receptor-mediated Ca2+ release produced a larger relative decrease in cytosolic Ca2+ at both time points compared to voltage-gated Ca2+channels. These results support the hypothesis that circadian Ca2+ rhythms in SCN neurons are predominantly driven by intracellular Ca2+ channels, although not exclusively so. The study provides a foundation for future experiments to probe Ca2+ signaling in a dynamic biological context using FLAREs.

The Neuropeptide PDF Is Crucial for Delaying the Phase of Drosophila's Evening Neurons Under Long Zeitgeber Periods

Circadian clocks schedule biological functions at a specific time of the day. Full comprehension of the clock function requires precise understanding of their entrainment to the environment. The phase of entrained clock is plastic, which depends on different factors such as the period of endogenous oscillator, the period of the zeitgeber cycle (T), and the proportion of light and darkness (day length). The circadian clock of fruit fly Drosophila melanogaster is able to entrain to a wide range of T-cycles and day lengths. Here, we investigated the importance of the neuropeptide Pigment-Dispersing Factor (PDF) for entrainment by systematically studying locomotor activity rhythms of Pdf ⁰ mutants and wild-type flies under different T-cycles (T22 to T32) and different day lengths (8, 12, and 16 hour [h]). Furthermore, we analysed PERIOD protein oscillations in selected groups of clock neurons in both genotypes under T24 and T32 at a day length of 16 h. As expected, we found that the phase of Drosophila’s evening activity and evening neurons advanced with increasing T in all the day lengths. This advance was much larger in Pdf ⁰ mutants (~7 h) than in wild-type flies causing (1) pronounced desynchrony between morning and evening neurons and (2) evening activity to move in the morning instead of the evening. Most interestingly, we found that the lights-off transition determines the phase of evening neurons in both genotypes and that PDF appears necessary to delay the evening neurons by ~3 h to their wild-type phase. Thus, in T32, PDF first delays the molecular cycling in the evening neurons, and then, as shown in previous studies, delays their neuronal firing rhythms to produce a total delay of ~7 h necessary for a wild-type evening activity phase. We conclude that PDF is crucial for appropriate phasing of Drosophila activity rhythm.

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.

Sleep drive reconfigures wake-promoting clock circuitry to regulate adaptive behavior

Circadian rhythms help animals synchronize motivated behaviors to match environmental demands. Recent evidence indicates that clock neurons influence the timing of behavior by differentially altering the activity of a distributed network of downstream neurons. Downstream circuits can be remodeled by Hebbian plasticity, synaptic scaling, and, under some circumstances, activity-dependent addition of cell surface receptors; the role of this receptor respecification phenomena is not well studied. We demonstrate that high sleep pressure quickly reprograms the wake-promoting large ventrolateral clock neurons to express the pigment dispersing factor receptor (PDFR). The addition of this signaling input into the circuit is associated with increased waking and early mating success. The respecification of PDFR in both young and adult large ventrolateral neurons requires 2 dopamine (DA) receptors and activation of the transcriptional regulator nejire (cAMP response element-binding protein [CREBBP]). These data identify receptor respecification as an important mechanism to sculpt circuit function to match sleep levels with demand.

Natural Zeitgebers Under Temperate Conditions Cannot Compensate for the Loss of a Functional Circadian Clock in Timing of a Vital Behavior in Drosophila

The adaptive significance of adjusting behavioral activities to the right time of the day seems obvious. Laboratory studies implicated an important role of circadian clocks in behavioral timing and rhythmicity. Yet, recent studies on clock-mutant animals questioned this importance under more naturalistic settings, as various clock mutants showed nearly normal diel activity rhythms under seminatural zeitgeber conditions. We here report evidence that proper timing of eclosion, a vital behavior of the fruit fly Drosophila melanogaster, requires a functional molecular clock under quasi-natural conditions. In contrast to wild-type flies, period01 mutants with a defective molecular clock showed impaired rhythmicity and gating in a temperate environment even in the presence of a full complement of abiotic zeitgebers. Although period01 mutants still eclosed during a certain time window during the day, this time window was much broader and loosely defined, and rhythmicity was lower or lost as classified by various statistical measures. Moreover, peak eclosion time became more susceptible to variable day-to-day changes of light. In contrast, flies with impaired peptidergic interclock signaling (Pdf01 and han5304 PDF receptor mutants) eclosed mostly rhythmically with normal gate sizes, similar to wild-type controls. Our results suggest that the presence of natural zeitgebers is not sufficient, and a functional molecular clock is required to induce stable temporal eclosion patterns in flies under temperate conditions with considerable day-to-day variation in light intensity and temperature. Temperate zeitgebers are, however, sufficient to functionally rescue a loss of PDF-mediated clock-internal and -output signaling.

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.

Reciprocal Relationship Between Calcium Signaling and Circadian Clocks: Implications for Calcium Homeostasis, Clock Function, and Therapeutics

In animals, circadian clocks impose a daily rhythmicity to many behaviors and physiological processes. At the molecular level, circadian rhythms are driven by intracellular transcriptional/translational feedback loops (TTFL). Interestingly, emerging evidence indicates that they can also be modulated by multiple signaling pathways. Among these, Ca²⁺ signaling plays a key role in regulating the molecular rhythms of clock genes and of the resulting circadian behavior. In addition, the application of in vivo imaging approaches has revealed that Ca²⁺ is fundamental to the synchronization of the neuronal networks that make up circadian pacemakers. Conversely, the activity of circadian clocks may influence Ca²⁺ signaling. For instance, several genes that encode Ca²⁺ channels and Ca²⁺-binding proteins display a rhythmic expression, and a disruption of this cycling affects circadian function, underscoring their reciprocal relationship. Here, we review recent advances in our understanding of how Ca²⁺ signaling both modulates and is modulated by circadian clocks, focusing on the regulatory mechanisms described in Drosophila and mice. In particular, we examine findings related to the oscillations in intracellular Ca²⁺ levels in circadian pacemakers and how they are regulated by canonical clock genes, neuropeptides, and light stimuli. In addition, we discuss how Ca²⁺ rhythms and their associated signaling pathways modulate clock gene expression at the transcriptional and post-translational levels. We also review evidence based on transcriptomic analyzes that suggests that mammalian Ca²⁺ channels and transporters (e.g., ryanodine receptor, ip3r, serca, L- and T-type Ca²⁺ channels) as well as Ca²⁺-binding proteins (e.g., camk, cask, and calcineurin) show rhythmic expression in the central brain clock and in peripheral tissues such as the heart and skeletal muscles. Finally, we discuss how the discovery that Ca²⁺ signaling is regulated by the circadian clock could influence the efficacy of pharmacotherapy and the outcomes of clinical interventions.

Collective Dynamics of Neural Networks With Sleep-Related Biological Drives in Drosophila

The collective electrophysiological dynamics of the brain as a result of sleep-related biological drives in Drosophila are investigated in this paper. Based on the Huber-Braun thermoreceptor model, the conductance-based neurons model is extended to a coupled neural network to analyze the local field potential (LFP). The LFP is calculated by using two different metrics: the mean value and the distance-dependent LFP. The distribution of neurons around the electrodes is assumed to have a circular or grid distribution on a two-dimensional plane. Regardless of which method is used, qualitatively similar results are obtained that are roughly consistent with the experimental data. During wake, the LFP has an irregular or a regular spike. However, the LFP becomes regular bursting during sleep. To further analyze the results, wavelet analysis and raster plots are used to examine how the LFP frequencies changed. The synchronization of neurons under different network structures is also studied. The results demonstrate that there are obvious oscillations at approximately 8 Hz during sleep that are absent during wake. Different time series of the LFP can be obtained under different network structures and the density of the network will also affect the magnitude of the potential. As the number of coupled neurons increases, the neural network becomes easier to synchronize, but the sleep and wake time described by the LFP spectrogram do not change. Moreover, the parameters that affect the durations of sleep and wake are analyzed.

Daily rewiring of a neural circuit generates a predictive model of environmental light

Behavioral responsiveness to external stimulation is shaped by context. We studied how sensory information can be contextualized, by examining light-evoked locomotor responsiveness of Drosophila relative to time of day. We found that light elicits an acute increase in locomotion (startle) that is modulated in a time-of-day-dependent manner: Startle is potentiated during the nighttime, when light is unexpected, but is suppressed during the daytime. The internal daytime-nighttime context is generated by two interconnected and functionally opposing populations of circadian neurons-LNvs generating the daytime state and DN1as generating the nighttime state. Switching between the two states requires daily remodeling of LNv and DN1a axons such that the maximum presynaptic area in one population coincides with the minimum in the other. We propose that a dynamic model of environmental light resides in the shifting connectivities of the LNv-DN1a circuit, which helps animals evaluate ongoing conditions and choose a behavioral response.

GABA from vasopressin neurons regulates the time at which suprachiasmatic nucleus molecular clocks enable circadian behavior

The suprachiasmatic nucleus (SCN), the central circadian pacemaker in mammals, is a network structure composed of multiple types of γ-aminobutyric acid (GABA)-ergic neurons and glial cells. However, the roles of GABA-mediated signaling in the SCN network remain controversial. Here, we report noticeable impairment of the circadian rhythm in mice with a specific deletion of the vesicular GABA transporter in arginine vasopressin (AVP)-producing neurons. These mice showed disturbed diurnal rhythms of GABAA receptor-mediated synaptic transmission in SCN neurons and marked lengthening of the activity time in circadian behavioral rhythms due to the extended interval between morning and evening locomotor activities. Synchrony of molecular circadian oscillations among SCN neurons did not significantly change, whereas the phase relationships between SCN molecular clocks and circadian morning/evening locomotor activities were altered significantly, as revealed by PER2::LUC imaging of SCN explants and in vivo recording of intracellular Ca2+ in SCN AVP neurons. In contrast, daily neuronal activity in SCN neurons in vivo clearly showed a bimodal pattern that correlated with dissociated morning/evening locomotor activities. Therefore, GABAergic transmission from AVP neurons regulates the timing of SCN neuronal firing to temporally restrict circadian behavior to appropriate time windows in SCN molecular clocks.

Demyelination Regulates the Circadian Transcription Factor BMAL1 to Signal Adult Neural Stem Cells to Initiate Oligodendrogenesis

Circadian clocks are endogenous oscillators that generate cell-autonomous rhythms that govern cellular processes and are synchronized by external cues in the local macro- and micro-environments. Demyelination, a common brain pathology with variable degrees of recovery, changes the microenvironment via damaged myelin and activation of glial cells. How these microenvironmental changes affect local circadian clocks and with what consequences is mostly unknown. Here, we show that within demyelinating lesions, astrocyte circadian clocks produce the Wnt inhibitors SFRP1 and SFRP5. Unexpectedly, SFRP1 and SFRP5 signal to the subventricular zone (SVZ) to reduce the circadian transcription factor BMAL1. This sequence of events causes adult neural stem cells in the SVZ to differentiate into oligodendrocyte lineage cells, which are then supplied to demyelinated lesions. Our findings show that circadian clocks in demyelinating lesions respond to microenvironmental changes and communicate with the SVZ to enhance a natural repair system of spontaneous remyelination.

Circadian Structural Plasticity Drives Remodeling of E Cell Output

Behavioral outputs arise as a result of highly regulated yet flexible communication among neurons. The Drosophila circadian network includes 150 neurons that dictate the temporal organization of locomotor activity; under light-dark (LD) conditions, flies display a robust bimodal pattern. The pigment-dispersing factor (PDF)-positive small ventral lateral neurons (sLNv) have been linked to the generation of the morning activity peak (the "M cells"), whereas the Cryptochrome (CRY)-positive dorsal lateral neurons (LNds) and the PDF-negative sLNv are necessary for the evening activity peak (the "E cells") [1, 2]. While each group directly controls locomotor output pathways [3], an interplay between them along with a third dorsal cluster (the DN1ps) is necessary for the correct timing of each peak and for adjusting behavior to changes in the environment [4-7]. M cells set the phase of roughly half of the circadian neurons (including the E cells) through PDF [5, 8-10]. Here, we show the existence of synaptic input provided by the evening oscillator onto the M cells. Both structural and functional approaches revealed that E-to-M cell connectivity changes across the day, with higher excitatory input taking place before the day-to-night transition. We identified two different neurotransmitters, acetylcholine and glutamate, released by E cells that are relevant for robust circadian output. Indeed, we show that acetylcholine is responsible for the excitatory input from E cells to M cells, which show preferential responsiveness to acetylcholine during the evening. Our findings provide evidence of an excitatory feedback between circadian clusters and unveil an important plastic remodeling of the E cells' synaptic connections.

Circadian Clocks: Mosquitoes Master the Dark Side of the Room

Aedes aegypti and Anopheles coluzzii mosquitoes exhibit diurnal and nocturnal behaviors, respectively. Baik et al. reveal the clock network architecture underlying each species' light preferences.

Coupling Neuropeptide Levels to Structural Plasticity in Drosophila Clock Neurons

We have previously reported that pigment dispersing factor (PDF) neurons, which are essential in the control of rest-activity cycles in Drosophila, undergo circadian remodeling of their axonal projections, a phenomenon called circadian structural plasticity. Axonal arborizations display higher complexity during the day and become simpler at night, and this remodeling involves changes in the degree of connectivity. This phenomenon depends on the clock present within the ventrolateral neurons (LNvs) as well as in glia. In this work, we characterize in detail the contribution of the PDF neuropeptide to structural plasticity at different times across the day. Using diverse genetic strategies to temporally restrict its downregulation, we demonstrate that even subtle alterations to PDF cycling at the dorsal protocerebrum correlate with impaired remodeling, underscoring its relevance for the characteristic morning spread; PDF released from the small LNvs (sLNvs) and the large LNvs (lLNvs) contribute to the process. Moreover, forced depolarization recruits activity-dependent mechanisms to mediate growth only at night, overcoming the restriction imposed by the clock on membrane excitability. Interestingly, the active process of terminal remodeling requires PDF receptor (PDFR) signaling acting locally through the cyclic-nucleotide-gated channel ion channel subunit A (CNGA). Thus, clock-dependent PDF signaling shapes the connectivity of these essential clock neurons on daily basis.

Norpa Signalling and the Seasonal Circadian Locomotor Phenotype in Drosophila

In this paper, we review the role of the norpA-encoded phospholipase C in light and thermal entrainment of the circadian clock in Drosophila melanogaster. We extend our discussion to the role of norpA in the thermo-sensitive splicing of the per 3′ UTR, which has significant implications for seasonal adaptations of circadian behaviour. We use the norpA mutant-generated enhancement of per splicing and the corresponding advance that it produces in the morning (M) and evening (E) locomotor component to dissect out the neurons that are contributing to this norpA phenotype using GAL4/UAS. We initially confirmed, by immunocytochemistry and in situ hybridisation in adult brains, that norpA expression is mostly concentrated in the eyes, but we were unable to unequivocally reveal norpA expression in the canonical clock cells using these methods. In larval brains, we did see some evidence for co-expression of NORPA with PDF in clock neurons. Nevertheless, downregulation of norpA in clock neurons did generate behavioural advances in adults, with the eyes playing a significant role in the norpA seasonal phenotype at high temperatures, whereas the more dorsally located CRYPTOCHROME-positive clock neurons are the likely candidates for generating the norpA behavioural effects in the cold. We further show that knockdown of the related plc21C encoded phospholipase in clock neurons does not alter per splicing nor generate any of the behavioural advances seen with norpA. Our results with downregulating norpA and plc21C implicate the rhodopsins Rh2/Rh3/Rh4 in the eyes as mediating per 3′ UTR splicing at higher temperatures and indicate that the CRY-positive LNds, also known as ‘evening’ cells are likely mediating the low-temperature seasonal effects on behaviour via altering per 3′UTR splicing.

A Functional Clock Within the Main Morning and Evening Neurons of D. melanogaster Is Not Sufficient for Wild-Type Locomotor Activity Under Changing Day Length

A major challenge for all organisms that live in temperate and subpolar regions is to adapt physiology and activity to different photoperiods. A long-standing model assumes that there are morning (M) and evening (E) oscillators with different photoreceptive properties that couple to dawn and dusk, respectively, and by this way adjust activity to the different photoperiods. In the fruit fly Drosophila melanogaster, M and E oscillators have been localized to specific circadian clock neurons in the brain. Here, we investigate under different photoperiods the activity pattern of flies expressing the clock protein PERIOD (PER) only in subsets of M and E oscillators. We found that all fly lines that expressed PER only in subsets of the clock neurons had difficulties to track the morning and evening in a wild-type manner. The lack of the E oscillators advanced M activity under short days, whereas the lack of the M oscillators delayed E activity under the same conditions. In addition, we found that flies expressing PER only in subsets of clock neurons showed higher activity levels at certain times of day or night, suggesting that M and E clock neurons might inhibit activity at specific moments throughout the 24 h. Altogether, we show that the proper interaction between all clock cells is important for adapting the flies’ activity to different photoperiods and discuss our findings in the light of the current literature.

Busted! A Dope Ring with Activity Clocked at Dawn and Dusk

Clock neurons generate circadian rhythms in behavioral activity, but the relevant pathways remain poorly understood. In this issue of Neuron, Liang et al. (2019) show that distinct clock neurons independently drive movement-promoting "ring neurons" in Drosophila through dopaminergic relays to support morning and evening locomotor activity.

Flies as models for circadian clock adaptation to environmental challenges

Life on earth is assumed to have developed in tropical regions that are characterized by regular 24 hr cycles in irradiance and temperature that remain the same throughout the seasons. All organisms developed circadian clocks that predict these environmental cycles and prepare the organisms in advance for them. A central question in chronobiology is how endogenous clocks changed in order to anticipate very different cyclical environmental conditions such as extremely short and long photoperiods existing close to the poles. Flies of the family Drosophilidae can be found all over the world-from the tropics to subarctic regions-making them unprecedented models for studying the evolutionary processes that underlie the adaptation of circadian clocks to different latitudes. This review summarizes our current understanding of these processes. We discuss evolutionary changes in the clock genes and in the clock network in the brain of different Drosophilids that may have caused behavioural adaptations to high latitudes.

Molecular and circuit mechanisms mediating circadian clock output in the Drosophila brain

A central question in the circadian biology field concerns the mechanisms that translate ~24-hr oscillations of the molecular clock into overt rhythms. Drosophila melanogaster is a powerful system that provided the first understanding of how molecular clocks are generated and is now illuminating the neural basis of circadian behavior. The identity of ~150 clock neurons in the Drosophila brain and their roles in shaping circadian rhythms of locomotor activity have been described before. This review summarizes mechanisms that transmit time-of-day signals from the clock, within the clock network as well as downstream of it. We also discuss the identification of functional multisynaptic circuits between clock neurons and output neurons that regulate locomotor activity.

A Catalog of GAL4 Drivers for Labeling and Manipulating Circadian Clock Neurons in Drosophila melanogaster

Daily rhythms of physiology, metabolism, and behavior are orchestrated by a central circadian clock. In mice, this clock is coordinated by the suprachiasmatic nucleus, which consists of 20,000 neurons, making it challenging to characterize individual neurons. In Drosophila, the clock is controlled by only 150 clock neurons that distribute across the fly's brain. Here, we describe a comprehensive set of genetic drivers to facilitate individual characterization of Drosophila clock neurons. We screened GAL4 lines that were obtained from Drosophila stock centers and identified 63 lines that exhibit expression in subsets of central clock neurons. Furthermore, we generated split-GAL4 lines that exhibit specific expression in subsets of clock neurons such as the 2 DN2 neurons and the 6 LPN neurons. Together with existing driver lines, these newly identified ones are versatile tools that will facilitate a better understanding of the Drosophila central circadian clock.