Circadian changes in Drosophila motor terminals

Acute sleep deprivation induces synaptic remodeling at the soleus muscle neuromuscular junction in rats

Sleep is important for cognitive and physical performance. Sleep deprivation not only affects neural functions but also results in muscular fatigue. A good night's sleep reverses these functional derangements caused by sleep deprivation. The role of sleep in brain function has been extensively studied. However, its role in neuromuscular junction (NMJ) or skeletal muscle morphology is sparsely addressed although skeletal muscle atonia and suspended thermoregulation during rapid eye movement sleep possibly provide a conducive environment for the muscle to rest and repair; somewhat similar to slow-wave sleep for synaptic downscaling. In the present study, we have investigated the effect of 24 h sleep deprivation on the NMJ morphology and neurochemistry using electron microscopy and immunohistochemistry in the rat soleus muscle. Acute sleep deprivation altered synaptic ultra-structure viz. mitochondria, synaptic vesicle, synaptic proteins, basal lamina, and junctional folds needed for neuromuscular transmission. Further acute sleep deprivation showed the depletion of the neurotransmitter acetylcholine and the overactivity of its degrading enzyme acetylcholine esterase at the NMJ. The impact of sleep deprivation on synaptic homeostasis in the brain has been extensively reported recently. The present evidence from our studies shows new information on the role of sleep on the NMJ homeostasis and its functioning.

Sleep and memory: The impact of sleep deprivation on transcription, translational control, and protein synthesis in the brain

In countries around the world, sleep deprivation represents a widespread problem affecting school-age children, teenagers, and adults. Acute sleep deprivation and more chronic sleep restriction adversely affect individual health, impairing memory and cognitive performance as well as increasing the risk and progression of numerous diseases. In mammals, the hippocampus and hippocampus-dependent memory are vulnerable to the effects of acute sleep deprivation. Sleep deprivation induces changes in molecular signaling, gene expression and may cause changes in dendritic structure in neurons. Genome wide studies have shown that acute sleep deprivation alters gene transcription, although the pool of genes affected varies between brain regions. More recently, advances in research have drawn attention to differences in gene regulation between the level of the transcriptome compared with the pool of mRNA associated with ribosomes for protein translation following sleep deprivation. Thus, in addition to transcriptional changes, sleep deprivation also affects downstream processes to alter protein translation. In this review, we focus on the multiple levels through which acute sleep deprivation impacts gene regulation, highlighting potential post-transcriptional and translational processes that may be affected by sleep deprivation. Understanding the multiple levels of gene regulation impacted by sleep deprivation is essential for future development of therapeutics that may mitigate the effects of sleep loss.

Circadian Changes of Dendritic Spine Geometry in Mouse Barrel Cortex

The circadian rhythmicity changes the density and shape of dendritic spines in mouse somatosensory barrel cortex, influencing their stability and maturation. In this study, we analyzed the main geometric parameters of dendritic spines reflecting the strength of synapses located on these spines under light/dark (12:12) and constant darkness conditions, in order to distinguish between endogenously regulated and light-driven parameters. Using morphological analysis of serial electron micrographs, as well as three-dimensional reconstructions, we found that the light induces elongation of single-synapse spine necks and increases in the diameter of double-synapse spine necks, increasing and decreasing the isolation of synapses from the parent dendrite, respectively. During the subjective night of constant darkness, we observed an enlargement of postsynaptic density area in inhibitory synapses and an increase in the number of polyribosomes inside double-synapse spines. The results show that both endogenous effect (circadian clock/locomotor activity) and light affect the morphological parameters of single- and double-synapse spines in the somatosensory cortex: light reduces the efficiency of excitatory synapses on single-synapse spines, increases the effect of synaptic transmission in double-synapse spines, and additionally masks the endogenous clock-driven enlargement of inhibitory synapses located on double-synapse spines. This indicates a special role of double-synapse spines and their inhibitory synapses in the regulation of synaptic transmission during both circadian and diurnal cycles in the mouse somatosensory cortex.

Memory and the circadian system: Identifying candidate mechanisms by which local clocks in the brain may regulate synaptic plasticity

The circadian system is an endogenous biological network responsible for coordinating near-24-h cycles in behavior and physiology with daily timing cues from the external environment. In this review, we explore how the circadian system regulates memory formation, retention, and recall. Circadian rhythms in these memory processes may arise through several endogenous pathways, and recent work highlights the importance of genetic timekeepers found locally within tissues, called local clocks. We evaluate the circadian memory literature for evidence of local clock involvement in memory, identifying potential nodes for direct interactions between local clock components and mechanisms of synaptic plasticity. Our discussion illustrates how local clocks may pervasively modulate neuronal plastic capacity, a phenomenon that we designate here as circadian metaplasticity. We suggest that this function of local clocks supports the temporal optimization of memory processes, illuminating the potential for circadian therapeutic strategies in the prevention and treatment of memory impairment.

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.

Neuron-specific overexpression of core clock genes improves stress-resistance and extends lifespan of Drosophila melanogaster

Gene expression is much altered in aging. We observed age-dependent decline of core clock genes' expression in the whole body of the fruit fly. We hypothesized that inducible overexpression of clock genes (cry, per, tim, cyc and Clk) in the nervous system can improve healthspan of D. melanogaster. We studied the lifespan of transgenic Drosophila and showed life extension for cry, per, cyc and tim genes. It was also the significant positive changes in the stress-resistance of flies overexpressing core clock genes in conditions of hyperthermia, hyperoxia, starvation and persistent lighting. The overexpression of per and cry restore circadian rhythms of locomotor activity. The results presented support the hypotheses that the compensation of circadian oscillator genes expression can improve the healthspan in Drosophila melanogaster.

Circadian Plasticity in the Brain of Insects and Rodents

In both vertebrate and invertebrate brains, neurons, glial cells and synapses are plastic, which means that the physiology and structure of these components are modified in response to internal and external stimuli during development and in mature brains. The term plasticity has been introduced in the last century to describe experience-dependent changes in synapse strength and number. These changes result from local functional and morphological synapse modifications; however, these modifications also occur more commonly in pre- and postsynaptic neurons. As a result, neuron morphology and neuronal networks are constantly modified during the life of animals and humans in response to different stimuli. Nevertheless, it has been discovered in flies and mammals that the number of synapses and size and shape of neurons also oscillate during the day. In most cases, these rhythms are circadian since they are generated by endogenous circadian clocks; however, some rhythmic changes in neuron morphology and synapse number and structure are controlled directly by environmental cues or by both external cues and circadian clocks. When the circadian clock is involved in generating cyclic changes in the nervous system, this type of plasticity is called circadian plasticity. It seems to be important in processing sensory information, in learning and in memory. Disruption of the clock may affect major brain functions.

Neuronal and Glial Clocks Underlying Structural Remodeling of Pacemaker Neurons in Drosophila

A number of years ago we reported that ventral Lateral Neurons (LNvs), which are essential in the control of rest-activity cycles in Drosophila, undergo circadian remodeling of their axonal projections. This structural plasticity gives rise to changes in the degree of connectivity, which could provide a means of transmitting time of day information. Thus far, work from different laboratories has shown that circadian remodeling of adult projections relies on activity-dependent and -independent mechanisms. In terms of clock- dependent mechanisms, several neuronal types undergoing circadian remodeling hinted to a differential effect of clock genes; while per mutants exhibited poorly developed axonal terminals giving rise to low complexity arbors, tim mutants displayed a characteristic hyper branching phenotype, suggesting these genes could be playing additional roles to those ascribed to core clock function. To shed light onto this possibility we altered clock gene levels through RNAi- mediated downregulation and expression of a dominant negative form exclusively in the adult LNvs. These experiments confirmed that the LNv clock is necessary to drive the remodeling process. We next explored the contribution of glia to the structural plasticity of the small LNvs through acute disruption of their internal clock. Interestingly, impaired glial clocks also abolished circadian structural remodeling, without affecting other clock-controlled outputs. Taken together our data shows that both neuronal and glial clocks are recruited to define the architecture of the LNv projections along the day, thus enabling a precise reconfiguration of the circadian network.

Ultrastructural comparison of the Drosophila larval and adult ventral abdominal neuromuscular junction

Drosophila melanogaster has recently emerged as model system for studying synaptic transmission and plasticity during adulthood, aging and neurodegeneration. However, still little is known about the basic neuronal mechanisms of synaptic function in the adult fly. Per se, adult Drosophila neuromuscular junctions should be highly suited for studying these aspects as they allow for genetic manipulations in combination with ultrastructural and electrophysiological analyses. Although different neuromuscular junctions of the adult fly have been described during the last years, a direct ultrastructural comparison with their larval counterpart is lacking. The present study was designed to close this gap by providing a detailed ultrastructural comparison of the larval and the adult neuromuscular junction of the ventrolongitudinal muscle. Assessment of several parameters revealed similarities but also major differences in the ultrastructural organisation of the two model neuromuscular junctions. While basic morphological parameters are retained from the larval into the adult stage, the analysis discovered major differences of potential functional relevance in the adult: The electron-dense membrane apposition of the presynaptic and postsynaptic membrane is shorter, the subsynaptic reticulum is less elaborated and the number of synaptic vesicles at a certain distance of the presynaptic membrane is higher.

Sleep-Dependent Structural Synaptic Plasticity of Inhibitory Synapses in the Dendrites of Hypocretin/Orexin Neurons

Sleep is tightly regulated by the circadian clock and homeostatic mechanisms. Although the sleep/wake cycle is known to be associated with structural and physiological synaptic changes that benefit the brain, the function of sleep is still debated. The hypothalamic hypocretin/orexin (Hcrt) neurons regulate various functions including feeding, reward, sleep, and wake. Continuous imaging of single neuronal circuits in live animals is vital to understanding the role of sleep in regulating synaptic dynamics, and the transparency of the zebrafish model enables time-lapse imaging of single synapses during both day and night. Here, we use the gephyrin (Gphnb) protein, a central inhibitory synapse organizer, as a fluorescent post-synaptic marker of inhibitory synapses. Double labeling showed that Gphnb-tagRFP and collybistin-EGFP clusters co-localized in dendritic inhibitory synapses. Using a transgenic hcrt:Gphnb-EGFP zebrafish, we showed that the number of inhibitory synapses in the dendrites of Hcrt neurons was increased during development. To determine the effect of sleep on the inhibitory synapses, we performed two-photon live imaging of Gphnb-EGFP in Hcrt neurons during day and night, under light/dark and constant light and dark conditions, and following sleep deprivation (SD). We found that synapse number increased during the night under light/dark conditions but that these changes were eliminated under constant light or dark conditions. SD reduced synapse number during the night, and the number increased during post-deprivation daytime sleep rebound. These results suggest that rhythmic structural plasticity of inhibitory synapses in Hcrt dendrites is independent of the circadian clock and is modulated by consolidated wake and sleep.

CRTC Potentiates Light-independent timeless Transcription to Sustain Circadian Rhythms in Drosophila

Light is one of the strongest environmental time cues for entraining endogenous circadian rhythms. Emerging evidence indicates that CREB-regulated transcription co-activator 1 (CRTC1) is a key player in this pathway, stimulating light-induced Period1 (Per1) transcription in mammalian clocks. Here, we demonstrate a light-independent role of Drosophila CRTC in sustaining circadian behaviors. Genomic deletion of the crtc locus causes long but poor locomotor rhythms in constant darkness. Overexpression or RNA interference-mediated depletion of CRTC in circadian pacemaker neurons similarly impairs the free-running behavioral rhythms, implying that Drosophila clocks are sensitive to the dosage of CRTC. The crtc null mutation delays the overall phase of circadian gene expression yet it remarkably dampens light-independent oscillations of TIMELESS (TIM) proteins in the clock neurons. In fact, CRTC overexpression enhances CLOCK/CYCLE (CLK/CYC)-activated transcription from tim but not per promoter in clock-less S2 cells whereas CRTC depletion suppresses it. Consistently, TIM overexpression partially but significantly rescues the behavioral rhythms in crtc mutants. Taken together, our data suggest that CRTC is a novel co-activator for the CLK/CYC-activated tim transcription to coordinate molecular rhythms with circadian behaviors over a 24-hour time-scale. We thus propose that CRTC-dependent clock mechanisms have co-evolved with selective clock genes among different species.

Circadian Regulation of Synaptic Plasticity

Circadian rhythms refer to oscillations in biological processes with a period of approximately 24 h. In addition to the sleep/wake cycle, there are circadian rhythms in metabolism, body temperature, hormone output, organ function and gene expression. There is also evidence of circadian rhythms in synaptic plasticity, in some cases driven by a master central clock and in other cases by peripheral clocks. In this article, I review the evidence for circadian influences on synaptic plasticity. I also discuss ways to disentangle the effects of brain state and rhythms on synaptic plasticity.

Structural plasticity of the circadian timing system. An overview from flies to mammals

The circadian timing system orchestrates daily variations in physiology and behavior through coordination of multioscillatory cell networks that are highly plastic in responding to environmental changes. Over the last decade, it has become clear that this plasticity involves structural changes and that the changes may be observed not only in central brain regions where the master clock cells reside but also in clock-controlled structures. This review considers experimental data in invertebrate and vertebrate model systems, mainly flies and mammals, illustrating various forms of structural circadian plasticity from cellular to circuit-based levels. It highlights the importance of these plastic events in the functional adaptation of the clock to the changing environment.

Circadian changes in neuronal networks

The circadian clock generates circadian plasticity in some of the clock and non-clock neurons leading to the daily changes in their structure and in the number of synaptic contacts. This plasticity affects neuronal networks in the brain. The two best known examples of circadian changes in neuronal networks are those observed in the first optic neuropil (lamina) of the fly's visual system and between one group of clock neurons, the small ventral lateral neurons (s-LNvs), and their target cells in the dorsal part of the Drosophila brain. Both of these networks are remodeled in the course of the day by the circadian clock and they are further affected by external stimuli.

Mmp1 processing of the PDF neuropeptide regulates circadian structural plasticity of pacemaker neurons

In the Drosophila brain, the neuropeptide PIGMENT DISPERSING FACTOR (PDF) is expressed in the small and large Lateral ventral neurons (LNvs) and regulates circadian locomotor behavior. Interestingly, PDF immunoreactivity at the dorsal terminals changes across the day as synaptic contacts do as a result of a remarkable remodeling of sLNv projections. Despite the relevance of this phenomenon to circuit plasticity and behavior, the underlying mechanisms remain poorly understood. In this work we provide evidence that PDF along with matrix metalloproteinases (Mmp1 and 2) are key in the control of circadian structural remodeling. Adult-specific downregulation of PDF levels per se hampers circadian axonal remodeling, as it does altering Mmp1 or Mmp2 levels within PDF neurons post-developmentally. However, only Mmp1 affects PDF immunoreactivity at the dorsal terminals and exerts a clear effect on overt behavior. In vitro analysis demonstrated that PDF is hydrolyzed by Mmp1, thereby suggesting that Mmp1 could directly terminate its biological activity. These data demonstrate that Mmp1 modulates PDF processing, which leads to daily structural remodeling and circadian behavior.

Circadian clocks have evolved as mechanisms that allow organisms to adapt to the day/night cyclical changes, a direct consequence of the rotation of the Earth. In the last two decades, and due to its amazing repertoire of genetic tools, Drosophila has been at the leading front in the discovery of genes that account for how the clock operates at a single cell level, which are conserved throughout the animal kingdom. Although the biochemical components underlying these molecular clocks have been characterized in certain detail, the mechanisms used by clock neurons to convey information to downstream pathways controlling behavior remain elusive. In the fruit fly, a subset of circadian neurons called the small ventral lateral neurons (sLNvs) are capable of synchronizing other clock cells relying on a neuropeptide named pigment dispersing factor (PDF). In addition, a number of years ago we described another mechanism as a possible candidate for contributing to the transmission of information downstream of the sLNvs, involving adult-specific remodeling of the axonal terminals of these circadian neurons. In this manuscript we describe some of the molecular events that lead to this striking form of structural plasticity on a daily basis.

Sleep, clocks, and synaptic plasticity

Sleep is widely believed to play an essential role in synaptic plasticity. However, the precise mechanisms governing this presumptive function are largely unknown. There is also evidence for independent circadian oscillations in synaptic strength and morphology. Therefore, synaptic changes observed after sleep reflect interactions between state-dependent (e.g., wake versus sleep) and state-independent (circadian) processes. In this review we consider how sleep and biological clocks influence synaptic plasticity. We discuss these findings in the context of current plasticity-based theories of sleep function and propose a new model that integrates circadian and brain-state influences on synaptic plasticity.

Rhythmic changes in synapse numbers in Drosophila melanogaster motor terminals

Previous studies have shown that the morphology of the neuromuscular junction of the flight motor neuron MN5 in Drosophila melanogaster undergoes daily rhythmical changes, with smaller synaptic boutons during the night, when the fly is resting, than during the day, when the fly is active. With electron microscopy and laser confocal microscopy, we searched for a rhythmic change in synapse numbers in this neuron, both under light:darkness (LD) cycles and constant darkness (DD). We expected the number of synapses to increase during the morning, when the fly has an intense phase of locomotion activity under LD and DD. Surprisingly, only our DD data were consistent with this hypothesis. In LD, we found more synapses at midnight than at midday. We propose that under LD conditions, there is a daily rhythm of formation of new synapses in the dark phase, when the fly is resting, and disassembly over the light phase, when the fly is active. Several parameters appeared to be light dependent, since they were affected differently under LD or DD. The great majority of boutons containing synapses had only one and very few had either two or more, with a 70∶25∶5 ratio (one, two and three or more synapses) in LD and 75∶20∶5 in DD. Given the maintenance of this proportion even when both bouton and synapse numbers changed with time, we suggest that there is a homeostatic mechanism regulating synapse distribution among MN5 boutons.

The circadian system: plasticity at many levels

Over the years it has become crystal clear that a variety of processes encode time-of-day information, ranging from gene expression, protein stability, or subcellular localization of key proteins, to the fine tuning of network properties and modulation of input signals, ultimately ensuring that physiology and behavior are properly synchronized to a changing environment. The purpose of this review is to put forward examples (as opposed to generate a comprehensive revision of all the available literature) in which the circadian system displays a remarkable degree of plasticity, from cell autonomous to circuit-based levels. In the literature, the term circadian plasticity has been used to refer to different concepts. The obvious one, more literally, refers to any change that follows a circadian (circa=around, diem=day) pattern, i.e. a daily change of a given parameter. The discovery of daily remodeling of neuronal structures will be referred herein as structural circadian plasticity, and represents an additional and novel phenomenon modified daily. Finally, any plasticity that has to do with a circadian parameter would represent a type of circadian plasticity; as an example, adjustments that allow organisms to adapt their daily behavior to the annual changes in photoperiod is a form of circadian plasticity at a higher organizational level, which is an emergent property of the whole circadian system. Throughout this work we will revisit these types of changes by reviewing recent literature delving around circadian control of clock outputs, from the most immediate ones within pacemaker neurons to the circadian modulation of rest-activity cycles.

Circadian dependence of receptors that mediate wake-related excitatory drive to hypoglossal motoneurons

Serotonin (5-HT), norepinephrine and orexins (ORX) are the three best established mediators of wake-related activation of hypoglossal (XII) motoneurons that innervate the muscles of the tongue. Since the tongue's use is temporarily closely aligned with the rest-activity cycle, we tested whether expression of mRNA for relevant 5-HT, norepinephrine and ORX receptors varies in the XII nucleus with the rest-activity cycle. Adult rats (n=7-9/group) were decapitated at 8-9 am (near rest period onset) or at 6-7 pm (near active period onset). Tissue micropunches were extracted from medullary slices containing the XII motor and sensory external cuneate (ECN) nuclei. 5-HT2A, α1-adrenergic and ORX type 2 receptor mRNAs were quantified using RT-PCR. Only 5-HT2A receptor mRNA levels differed between the two time points and were higher at the active period onset; no differences were detected in the ECN. Consistent with the mRNA results, 5-HT2A protein levels were also higher in the XII nucleus at the active period onset than at rest onset. Thus, the endogenous serotonergic excitatory drive to XII motoneurons may be enhanced through circadian- or activity-dependent mechanisms that increase the availability of 5-HT2A receptors prior to the active period. Conversely, reduced levels of 5-HT2A receptors during the rest-sleep period may exacerbate the propensity for sleep-disordered breathing in subjects with anatomically compromised upper airway.

Loss and gain of Drosophila TDP-43 impair synaptic efficacy and motor control leading to age-related neurodegeneration by loss-of-function phenotypes

Cytoplasmic accumulation and nuclear clearance of TDP-43 characterize familial and sporadic forms of amyotrophic lateral sclerosis and frontotemporal lobar degeneration, suggesting that either loss or gain of TDP-43 function, or both, cause disease formation. Here we have systematically compared loss- and gain-of-function of Drosophila TDP-43, TAR DNA Binding Protein Homolog (TBPH), in synaptic function and morphology, motor control, and age-related neuronal survival. Both loss and gain of TBPH severely affect development and result in premature lethality. TBPH dysfunction caused impaired synaptic transmission at the larval neuromuscular junction (NMJ) and in the adult. Tissue-specific knockdown together with electrophysiological recordings at the larval NMJ also revealed that alterations of TBPH function predominantly affect pre-synaptic efficacy, suggesting that impaired pre-synaptic transmission is one of the earliest events in TDP-43-related pathogenesis. Prolonged loss and gain of TBPH in adults resulted in synaptic defects and age-related, progressive degeneration of neurons involved in motor control. Toxic gain of TBPH did not downregulate or mislocalize its own expression, indicating that a dominant-negative effect leads to progressive neurodegeneration also seen with mutational inactivation of TBPH. Together these data suggest that dysfunction of Drosophila TDP-43 triggers a cascade of events leading to loss-of-function phenotypes whereby impaired synaptic transmission results in defective motor behavior and progressive deconstruction of neuronal connections, ultimately causing age-related neurodegeneration.

Circadian clocks, rhythmic synaptic plasticity and the sleep-wake cycle in zebrafish

The circadian clock and homeostatic processes are fundamental mechanisms that regulate sleep. Surprisingly, despite decades of research, we still do not know why we sleep. Intriguing hypotheses suggest that sleep regulates synaptic plasticity and consequently has a beneficial role in learning and memory. However, direct evidence is still limited and the molecular regulatory mechanisms remain unclear. The zebrafish provides a powerful vertebrate model system that enables simple genetic manipulation, imaging of neuronal circuits and synapses in living animals, and the monitoring of behavioral performance during day and night. Thus, the zebrafish has become an attractive model to study circadian and homeostatic processes that regulate sleep. Zebrafish clock- and sleep-related genes have been cloned, neuronal circuits that exhibit circadian rhythms of activity and synaptic plasticity have been studied, and rhythmic behavioral outputs have been characterized. Integration of this data could lead to a better understanding of sleep regulation. Here, we review the progress of circadian clock and sleep studies in zebrafish with special emphasis on the genetic and neuroendocrine mechanisms that regulate rhythms of melatonin secretion, structural synaptic plasticity, locomotor activity and sleep.

GABAergic synaptic plasticity during a developmentally regulated sleep-like state in C. elegans

Approximately one-fourth of the neurons in Caenorhabditis elegans adults are born during larval development, indicating tremendous plasticity in larval nervous system structure. Larval development shows cyclical expression of sleep-like quiescent behavior during lethargus periods, which occur at larval stage transitions. We studied plasticity at the neuromuscular junction during lethargus using the acetylcholinesterase inhibitor aldicarb. The rate of animal contraction when exposed to aldicarb is controlled by the balance between excitatory cholinergic and inhibitory GABAergic input on the muscle. During lethargus, there is an accelerated rate of contraction on aldicarb. Mutant analysis and optogenetic studies reveal that GABAergic synaptic transmission is reduced during lethargus. Worms in lethargus show partial resistance to GABA(A) receptor agonists, indicating that postsynaptic mechanisms contribute to lethargus-dependent plasticity. Using genetic manipulations that separate the quiescent state from the developmental stage, we show that the synaptic plasticity is dependent on developmental time and not on the behavioral state of the animal. We propose that the synaptic plasticity regulated by a developmental clock in C. elegans is analogous to synaptic plasticity regulated by the circadian clock in other species.

Synaptic plasticity in sleep: learning, homeostasis and disease

Sleep is a fundamental and evolutionarily conserved aspect of animal life. Recent studies have shed light on the role of sleep in synaptic plasticity. Demonstrations of memory replay and synapse homeostasis suggest that one essential role of sleep is in the consolidation and optimization of synaptic circuits to retain salient memory traces despite the noise of daily experience. Here, we review this recent evidence and suggest that sleep creates a heightened state of plasticity, which may be essential for this optimization. Furthermore, we discuss how sleep deficits seen in diseases such as Alzheimer's disease and autism spectrum disorders might not just reflect underlying circuit malfunction, but could also play a direct role in the progression of those disorders.

Circadian organization of behavior and physiology in Drosophila

Circadian clocks organize behavior and physiology to adapt to daily environmental cycles. Genetic approaches in the fruit fly, Drosophila melanogaster, have revealed widely conserved molecular gears of these 24-h timers. Yet much less is known about how these cell-autonomous clocks confer temporal information to modulate cellular functions. Here we discuss our current knowledge of circadian clock function in Drosophila, providing an overview of the molecular underpinnings of circadian clocks. We then describe the neural network important for circadian rhythms of locomotor activity, including how these molecular clocks might influence neuronal function. Finally, we address a range of behaviors and physiological systems regulated by circadian clocks, including discussion of specific peripheral oscillators and key molecular effectors where they have been described. These studies reveal a remarkable complexity to circadian pathways in this "simple" model organism.

Widespread changes in synaptic markers as a function of sleep and wakefulness in Drosophila

Sleep is universal, strictly regulated, and necessary for cognition. Why this is so remains a mystery, although recent work suggests that sleep, memory, and plasticity are linked. However, little is known about how wakefulness and sleep affect synapses. Using Western blots and confocal microscopy in Drosophila, we found that protein levels of key components of central synapses were high after waking and low after sleep. These changes were related to behavioral state rather than time of day and occurred in all major areas of the Drosophila brain. The decrease of synaptic markers during sleep was progressive, and sleep was necessary for their decline. Thus, sleep may be involved in maintaining synaptic homeostasis altered by waking activities.

Circadian control of dendrite morphology in the visual system of Drosophila melanogaster

Background

In the first optic neuropil (lamina) of the fly's visual system, monopolar cells L1 and L2 and glia show circadian rhythms in morphological plasticity. They change their size and shape during the day and night. The most pronounced changes have been detected in circadian size of the L2 axons. Looking for a functional significance of the circadian plasticity observed in axons, we examined the morphological plasticity of the L2 dendrites. They extend from axons and harbor postsynaptic sites of tetrad synaptic contacts from the photoreceptor terminals.

Methodology/Principal Findings

The plasticity of L2 dendrites was evaluated by measuring an outline of the L2 dendritic trees. These were from confocal images of cross sections of L2 cells labeled with GFP. They were in wild-type and clock mutant flies held under different light conditions and sacrified at different time points. We found that the L2 dendrites are longest at the beginning of the day in both males and females. This rhythm observed under a day/night regime (LD) was maintained in constant darkness (DD) but not in continuous light (LL). This rhythm was not present in the arrhythmic per⁰¹ mutant in LD or in DD. In the clock photoreceptor cry^b mutant the rhythm was maintained but its pattern was different than that observed in wild-type flies.

Conclusions/Significance

The results obtained showed that the L2 dendrites exhibit circadian structural plasticity. Their morphology is controlled by the per gene-dependent circadian clock. The L2 dendrites are longest at the beginning of the day when the daytime tetrad presynaptic sites are most numerous and L2 axons are swollen. The presence of the rhythm, but with a different pattern in cry^b mutants in LD and DD indicates a new role of cry in the visual system. The new role is in maintaining the circadian pattern of changes of the L2 dendrite length and shape.

Circadian modulation of light-induced locomotion responses in Drosophila melanogaster

The relationship between light and the circadian system has long been a matter of discussion. Many studies have focused on entrainment of light with the internal biological clock. Light also functions as an environmental stimulus that affects the physiology and behaviour of animals directly. In this study, we used light as an unexpected stimulus for flies at different circadian times. We found that wildtype flies showed circadian changes in light-induced locomotion responses. Elevation of locomotor activity by light occurred during the subjective night, and performance in response to light pulses declined to trough during the subjective day. Moreover, arrhythmic mutants lost the rhythm of locomotion responses to light, with promotion of activity by light in timeless(01)mutants and inhibition of activity by light in Clock(ar)mutants. However, neither ablation of central oscillators nor disturbance of the functional clock inside compound eyes was sufficient to disrupt the rhythm of light responses. We show that, compound eyes, which have been identified as the control point for normal masking (promotion of activity by light), are sufficient but not necessary for paradoxical masking (suppression of activity by light) under high light intensity. This, taken together with the clear difference of light responses in wildtype flies, suggests that two different masking mechanisms may underlie the circadian modulation of light-induced locomotion responses.

Circadian remodeling of neuronal circuits involved in rhythmic behavior

Clock output pathways are central to convey timing information from the circadian clock to a diversity of physiological systems, ranging from cell-autonomous processes to behavior. While the molecular mechanisms that generate and sustain rhythmicity at the cellular level are well understood, it is unclear how this information is further structured to control specific behavioral outputs. Rhythmic release of pigment dispersing factor (PDF) has been proposed to propagate the time of day information from core pacemaker cells to downstream targets underlying rhythmic locomotor activity. Indeed, such circadian changes in PDF intensity represent the only known mechanism through which the PDF circuit could communicate with its output. Here we describe a novel circadian phenomenon involving extensive remodeling in the axonal terminals of the PDF circuit, which display higher complexity during the day and significantly lower complexity at nighttime, both under daily cycles and constant conditions. In support to its circadian nature, cycling is lost in bona fide clockless mutants. We propose this clock-controlled structural plasticity as a candidate mechanism contributing to the transmission of the information downstream of pacemaker cells.

Circadian systems evolved as a mechanism that allows organisms to adapt to the environmental changes in light and dark which occur as a consequence of the rotation of Earth. Because of its unique repertoire of genetic tools, Drosophila is a well established model for the study of the circadian clock. Although the biochemical components underlying the molecular oscillations have been characterized in detail, the mechanisms used by the clock neurons to convey information to the downstream pathways remain elusive. In the fruit fly, the small ventral lateral neurons (LNv) are capable of synchronizing other clock cells relying on a neuropeptide named pigment dispersing factor. In this work we introduce a novel mechanism as a possible candidate for contributing to the transmission of information downstream of the small LNvs, involving clock-controlled remodeling of their axonal morphology. By labeling the entire neuronal membrane and analyzing the complexity of the axonal arbor at different times we showed that there is a circadian variation in the complexity of the axonal arbor. This phenomenon was not observed in flies carrying null mutations in two canonical clock genes, underscoring the dependence of the circadian clock for the structural plasticity of its pacemaker neurons.

The circadian clock controls a wide array of biological phenomena ranging from basal transcription to overt behavior. Now, new evidence shows that the clock affects a striking remodeling of the circuit controlling rest-activity cycles inDrosophila.

The Drosophila circadian pacemaker circuit: Pas De Deux or Tarantella?

Molecular genetic analysis of the fruit fly Drosophila melanogaster has revolutionized our understanding of the transcription/translation loop mechanisms underlying the circadian molecular oscillator. More recently, Drosophila has been used to understand how different neuronal groups within the circadian pacemaker circuit interact to regulate the overall behavior of the fly in response to daily cyclic environmental cues as well as seasonal changes. Our present understanding of circadian timekeeping at the molecular and circuit level is discussed with a critical evaluation of the strengths and weaknesses of present models. Two models for circadian neural circuits are compared: one that posits that two anatomically distinct oscillators control the synchronization to the two major daily morning and evening transitions, versus a distributed network model that posits that many cell-autonomous oscillators are coordinated in a complex fashion and respond via plastic mechanisms to changes in environmental cues.