The Timed Depolarization of Morning and Evening Oscillators Phase Shifts the Circadian Clock of Drosophila

Drosophila photoreceptor systems converge in arousal neurons and confer light responsive robustness

Lateral ventral neurons (LNvs) in the fly circadian neural circuit mediate behaviors other than clock resetting, including light-activated acute arousal. Converging sensory inputs often confer functional redundancy. The LNvs have three distinct light input pathways: (1) cell autonomously expressed cryptochrome (CRY), (2) rhodopsin 7 (Rh7), and (3) synaptic inputs from the eyes and other external photoreceptors that express opsins and CRY. We explored the relative photoelectrical and behavioral input contributions of these three photoreceptor systems to determine their functional impact in flies. Patch-clamp electrophysiology measuring light evoked firing frequency (FF) was performed on large LNvs (l-LNvs) in response to UV (365 nm), violet (405 nm), blue (450 nm), or red (635 nm) LED light stimulation, testing controls versus mutants that lack photoreceptor inputs gl60j, cry-null, rh7-null, and double mutant gl60j-cry-null flies. For UV, violet, and blue short wavelength light inputs, all photoreceptor mutants show significantly attenuated action potential FF responses measured in the l-LNv. In contrast, red light FF responses are only significantly attenuated in double mutant gl60j-cry-null flies. We used a light-pulse arousal assay to compare behavioral responses to UV, violet, blue and red light of control and light input mutants, measuring the awakening arousal response of flies during subjective nighttime at two different intensities to capture potential threshold differences (10 and 400 μW/cm2). The light arousal behavioral results are similar to the electrophysiological results, showing significant attenuation of behavioral light responses for mutants compared to control. These results show that the different LNv convergent photoreceptor systems are integrated and together confer functional redundancy for light evoked behavioral arousal.

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.

Drosophila Cryptochrome: Variations in Blue

CRYPTOCHROMES (CRYs) are structurally related to ultraviolet (UV)/blue-sensitive DNA repair enzymes called photolyases but lack the ability to repair pyrimidine dimers generated by UV exposure. First identified in plants, CRYs have proven to be involved in light detection and various light-dependent processes in a broad range of organisms. In Drosophila, CRY's best understood role is the cell-autonomous synchronization of circadian clocks. However, CRY also contributes to the amplitude of circadian oscillations in a light-independent manner, controls arousal and UV avoidance, influences visual photoreception, and plays a key role in magnetic field detection. Here, we review our current understanding of the mechanisms underlying CRY's various circadian and noncircadian functions in fruit flies.

Beyond spikes: Multiscale computational analysis of in vivo long-term recordings in the cockroach circadian clock

The circadian clock of the nocturnal Madeira cockroach is located in the accessory medulla, a small nonretinotopic neuropil in the brain's visual system. The clock comprises about 240 neurons that control rhythms in physiology and behavior such as sleep-wake cycles. The clock neurons contain an abundant number of partly colocalized neuropeptides, among them pigment-dispersing factor (PDF), the insects' most important circadian coupling signal that controls sleep-wake rhythms. We performed long-term loose-patch clamp recordings under 12:12-hr light-dark cycles in the cockroach clock in vivo. A wide range of timescales, from milliseconds to seconds, were found in spike and field potential patterns. We developed a framework of wavelet transform-based methods to detect these multiscale electrical events. We analyzed frequencies and patterns of events with interesting dynamic features, such as mixed-mode oscillations reminiscent of sharp-wave ripples. Oscillations in the beta/gamma frequency range (20-40 Hz) were observed to rise at dawn, when PDF is released, peaking just before the onset of locomotor activity of the nocturnal cockroach. We expect that in vivo electrophysiological recordings combined with neuropeptide/antagonist applications and behavioral analysis will determine whether specific patterns of electrical activity recorded in the network of the cockroach circadian clock are causally related to neuropeptide-dependent control of behavior.

Role of Rhodopsins as Circadian Photoreceptors in the Drosophila melanogaster

Light profoundly affects the circadian clock and the activity levels of animals. Along with the systematic changes in intensity and spectral composition, over the 24-h day, light shows considerable irregular fluctuations (noise). Using light as the Zeitgeber for the circadian clock is, therefore, a complex task and this might explain why animals utilize multiple photoreceptors to entrain their circadian clock. The fruit fly Drosophila melanogaster possesses light-sensitive Cryptochrome and seven Rhodopsins that all contribute to light detection. We review the role of Rhodopsins in circadian entrainment, and of direct light-effects on the activity, with a special emphasis on the newly discovered Rhodopsin 7 (Rh7). We present evidence that Rhodopsin 6 in receptor cells 8 of the compound eyes, as well as in the extra retinal Hofbauer-Buchner eyelets, plays a major role in entraining the fly’s circadian clock with an appropriate phase-to-light–dark cycles. We discuss recent contradictory findings regarding Rhodopsin 7 and report original data that support its role in the compound eyes and in the brain. While Rhodopsin 7 in the brain appears to have a minor role in entrainment, in the compound eyes it seems crucial for fine-tuning light sensitivity to prevent overshooting responses to bright light.

Hub-organized parallel circuits of central circadian pacemaker neurons for visual photoentrainment in Drosophila

Circadian rhythms are orchestrated by a master clock that emerges from a network of circadian pacemaker neurons. The master clock is synchronized to external light/dark cycles through photoentrainment, but the circuit mechanisms underlying visual photoentrainment remain largely unknown. Here, we report that Drosophila has eye-mediated photoentrainment via a parallel pacemaker neuron organization. Patch-clamp recordings of central circadian pacemaker neurons reveal that light excites most of them independently of one another. We also show that light-responding pacemaker neurons send their dendrites to a neuropil called accessary medulla (aMe), where they make monosynaptic connections with Hofbauer–Buchner eyelet photoreceptors and interneurons that transmit compound-eye signals. Laser ablation of aMe and eye removal both abolish light responses of circadian pacemaker neurons, revealing aMe as a hub to channel eye inputs to central circadian clock. Taken together, we demonstrate that the central clock receives eye inputs via hub-organized parallel circuits in Drosophila.

The central circadian clock in Drosophila is made up of ~ 150 anatomically distributed neurons; the circuits underlying photoentrainment is unclear. This study describes ex vivo patch-clamp recording of the eye-mediated light response of all known circadian clock neurons, and shows that they are organized in parallel circuits centered around a hub.

Cryptochrome Interacts With Actin and Enhances Eye-Mediated Light Sensitivity of the Circadian Clock in Drosophila melanogaster

Cryptochromes (CRYs) are a class of flavoproteins that sense blue light. In animals, CRYs are expressed in the eyes and in the clock neurons that control sleep/wake cycles and are implied in the generation and/or entrainment of circadian rhythmicity. Moreover, CRYs are sensing magnetic fields in insects as well as in humans. Here, we show that in the fruit fly Drosophila melanogaster CRY plays a light-independent role as "assembling" protein in the rhabdomeres of the compound eyes. CRY interacts with actin and appears to increase light sensitivity of the eyes by keeping the "signalplex" of the phototransduction cascade close to the membrane. By this way, CRY also enhances light-responses of the circadian clock.

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.

Calcium and cAMP directly modulate the speed of the Drosophila circadian clock

Circadian clocks impose daily periodicities to animal behavior and physiology. At their core, circadian rhythms are produced by intracellular transcriptional/translational feedback loops (TTFL). TTFLs may be altered by extracellular signals whose actions are mediated intracellularly by calcium and cAMP. In mammals these messengers act directly on TTFLs via the calcium/cAMP-dependent transcription factor, CREB. In the fruit fly, Drosophila melanogaster, calcium and cAMP also regulate the periodicity of circadian locomotor activity rhythmicity, but whether this is due to direct actions on the TTFLs themselves or are a consequence of changes induced to the complex interrelationship between different classes of central pacemaker neurons is unclear. Here we investigated this question focusing on the peripheral clock housed in the non-neuronal prothoracic gland (PG), which, together with the central pacemaker in the brain, controls the timing of adult emergence. We show that genetic manipulations that increased and decreased the levels of calcium and cAMP in the PG caused, respectively, a shortening and a lengthening of the periodicity of emergence. Importantly, knockdown of CREB in the PG caused an arrhythmic pattern of eclosion. Interestingly, the same manipulations directed at central pacemaker neurons caused arrhythmicity of eclosion and of adult locomotor activity, suggesting a common mechanism. Our results reveal that the calcium and cAMP pathways can alter the functioning of the clock itself. In the PG, these messengers, acting as outputs of the clock or as second messengers for stimuli external to the PG, could also contribute to the circadian gating of adult emergence.

Circadian clocks impose daily periodicities to animal behavior and physiology. At their core, circadian rhythms are produced by intracellular transcriptional/translational feedback loops (TTFL). TTFLs may be altered by extracellular signals whose actions are mediated intracellularly by calcium and cAMP. In Drosophila, calcium and cAMP levels affect the periodicity of Drosophila circadian rhythms, but whether this is due to direct actions on the TTFLs themselves or is a consequence of changes induced to the complex interrelationship between different classes of central pacemaker neurons is unclear. Here we used the non-neuronal circadian clock located in the prothoracic gland (PG) to show that these messengers affect the speed of the circadian clock that controls the timing of adult emergence and suggest that these actions are mediated by CREB. Importantly, since calcium and cAMP are also output signals of the clock, they may contribute to the mechanism that imposes a circadian gating to the timing of adult emergence.

Sleep in Insects

Sleep is essential for proper brain function in mammals and insects. During sleep, animals are disconnected from the external world; they show high arousal thresholds and changed brain activity. Sleep deprivation results in a sleep rebound. Research using the fruit fly, Drosophila melanogaster, has helped us understand the genetic and neuronal control of sleep. Genes involved in sleep control code for ion channels, factors influencing neurotransmission and neuromodulation, and proteins involved in the circadian clock. The neurotransmitters/neuromodulators involved in sleep control are GABA, dopamine, acetylcholine, serotonin, and several neuropeptides. Sleep is controlled by the interplay between sleep homeostasis and the circadian clock. Putative sleep-wake centers are located in higher-order brain centers that are indirectly connected to the circadian clock network. The primary function of sleep appears to be the downscaling of synapses that have been built up during wakefulness. Thus, brain homeostasis is maintained and learning and memory are assured.

Mapping Quantitative Trait Loci Underlying Circadian Light Sensitivity in Drosophila

Despite the significant advance in our understanding of the molecular basis of light entrainment of the circadian clock in Drosophila, the underlying genetic architecture is still largely unknown. The aim of this study was to identify loci associated with variation in circadian photosensitivity, which are important for the evolution of this trait. We have used complementary approaches that combined quantitative trait loci (QTL) mapping, complementation testing, and transcriptome profiling to dissect this variation. We identified a major QTL on chromosome 2, which was subsequently fine mapped using deficiency complementation mapping into 2 smaller regions spanning 139 genes, some of which are known to be involved in functions that have been previously implicated in light entrainment. Two genes implicated with the clock and located within that interval, timeless and cycle, failed to complement the QTL, indicating that alleles of these genes contribute to the variation in light response. Specifically, we find that the timeless s/ ls polymorphism that has been previously shown to constitute a latitudinal cline in Europe is also segregating in our recombinant inbred lines and is contributing to the phenotypic variation in light sensitivity. We also profiled gene expression in 2 recombinant inbred strains that differ significantly in their photosensitivity and identified a total of 368 transcripts that showed differential expression (false discovery rate < 0.1). Of 131 transcripts that showed a significant recombinant inbred line by treatment interaction (i.e., putative expression QTL), 4 are located within QTL2.

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.

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

We studied the Drosophila circadian neural circuit using whole-brain imaging in vivo. Five major groups of pacemaker neurons display synchronized molecular clocks, yet each exhibits a distinct phase of daily Ca²⁺ activation. Light and neuropeptide pigment dispersing factor (PDF) from morning cells (s-LNv) together delay the phase of the evening (LNd) group by ∼12 hr; PDF alone delays the phase of the DN3 group by ∼17 hr. Neuropeptide sNPF, released from s-LNv and LNd pacemakers, produces Ca²⁺ activation in the DN1 group late in the night. The circuit also features negative feedback by PDF to truncate the s-LNv Ca²⁺ wave and terminate PDF release. Both PDF and sNPF suppress basal Ca²⁺ levels in target pacemakers with long durations by cell-autonomous actions. Thus, light and neuropeptides act dynamically at distinct hubs of the circuit to produce multiple suppressive events that create the proper tempo and sequence of circadian pacemaker neuronal activities.

Circadian Rhythms and Sleep in Drosophila melanogaster

The advantages of the model organism Drosophila melanogaster, including low genetic redundancy, functional simplicity, and the ability to conduct large-scale genetic screens, have been essential for understanding the molecular nature of circadian (∼24 hr) rhythms, and continue to be valuable in discovering novel regulators of circadian rhythms and sleep. In this review, we discuss the current understanding of these interrelated biological processes in Drosophila and the wider implications of this research. Clock genes period and timeless were first discovered in large-scale Drosophila genetic screens developed in the 1970s. Feedback of period and timeless on their own transcription forms the core of the molecular clock, and accurately timed expression, localization, post-transcriptional modification, and function of these genes is thought to be critical for maintaining the circadian cycle. Regulators, including several phosphatases and kinases, act on different steps of this feedback loop to ensure strong and accurately timed rhythms. Approximately 150 neurons in the fly brain that contain the core components of the molecular clock act together to translate this intracellular cycling into rhythmic behavior. We discuss how different groups of clock neurons serve different functions in allowing clocks to entrain to environmental cues, driving behavioral outputs at different times of day, and allowing flexible behavioral responses in different environmental conditions. The neuropeptide PDF provides an important signal thought to synchronize clock neurons, although the details of how PDF accomplishes this function are still being explored. Secreted signals from clock neurons also influence rhythms in other tissues. SLEEP is, in part, regulated by the circadian clock, which ensures appropriate timing of sleep, but the amount and quality of sleep are also determined by other mechanisms that ensure a homeostatic balance between sleep and wake. Flies have been useful for identifying a large set of genes, molecules, and neuroanatomic loci important for regulating sleep amount. Conserved aspects of sleep regulation in flies and mammals include wake-promoting roles for catecholamine neurotransmitters and involvement of hypothalamus-like regions, although other neuroanatomic regions implicated in sleep in flies have less clear parallels. Sleep is also subject to regulation by factors such as food availability, stress, and social environment. We are beginning to understand how the identified molecules and neurons interact with each other, and with the environment, to regulate sleep. Drosophila researchers can also take advantage of increasing mechanistic understanding of other behaviors, such as learning and memory, courtship, and aggression, to understand how sleep loss impacts these behaviors. Flies thus remain a valuable tool for both discovery of novel molecules and deep mechanistic understanding of sleep and circadian rhythms.

Desensitization and recovery of crayfish photoreceptors. Dependency on circadian time, and pigment-dispersing hormone

In this work, we studied the characteristics of recovery from desensitization of the light-elicited current of crayfish. Applying a two-flash protocol, we found that the first flash triggers a current that activates with a noticeable latency, reaches a peak value, and thereafter decays along a single exponential time course. In comparison with the first-elicited current, the current elicited by the second flash not only presents an expected smaller peak current, depending on the time between flashes, but it also displays a different latency and decay time constant. Recovery of the first flash values of these current parameters depends on the circadian time at which the experiments are conducted, and on the presence of pigment-dispersing hormone. Our data also suggest the existence of distinctive desensitized states, whose induction depends on circadian time and the presence of pigment-dispersing hormone.