Differential regulation of circadian pacemaker output by separate clock genes in Drosophila

Clock-gated photic stimulation of timeless expression at cold temperatures and seasonal adaptation in Drosophila

Numerous lines of evidence indicate that the initial photoresponse of the circadian clock in Drosophila melanogaster is the light-induced degradation of TIMELESS (TIM). This posttranslational mechanism is in sharp contrast to the well-characterized pacemakers in mammals and Neurospora, where light evokes rapid changes in the transcriptional profiles of 1 or more clock genes. The authors show that light has novel effects on D. melanogaster circadian pacemakers, acutely stimulating the expression of tim at cold but not warm temperatures. This photoinduction occurs in flies defective for the classic visual phototransduction pathway or the circadian-relevant photoreceptor CRYPTOCHROME (CRY). Cold-specific stimulation of tim RNA abundance is regulated at the transcriptional level, and although numerous lines of evidence indicate that period (per) and tim expression are activated by the same mechanism, light has no measurable acute effect on per mRNA abundance. Moreover, light-induced increases in the levels of tim RNA are abolished or greatly reduced in the absence of functional CLOCK (CLK) or CYCLE (CYC) but not PER or TIM. These findings add to a growing number of examples where molecular and behavioral photoresponses in Drosophila are differentially influenced by "positive" (e.g., CLK and CYC) and "negative" (e.g., PER and TIM) core clock elements. The acute effects of light on tim expression are temporally gated, essentially restricted to the daily rising phase in tim mRNA levels. Because the start of the daily upswing in tim expression begins several hours after dawn in long photoperiods (day length), this gating mechanism likely ensures that sunrise does not prematurely stimulate tim expression during unseasonally cold spring/summer days. The results suggest that the photic stimulation of tim expression at low temperatures is part of a seasonal adaptive response that helps advance the phase of the clock on cold days, enabling flies to exhibit preferential daytime activity despite the (usually) earlier onset of dusk. Taken together with prior findings, the ability of temperature and photoperiod to adjust trajectories in the rising phases of 1 or more clock RNAs constitutes a major mechanism contributing to seasonal adaptation of clock function.

Mechanisms of clock output in the Drosophila circadian pacemaker system

Molecular oscillations that underlie the circadian clock are coupled to different output signals by which daily rhythms in downstream events are evoked and/or synchronized. Here the authors review the literature that describes circadian output mechanisms in Drosophila. They begin at the most proximal level, within oscillator cells themselves, by surveying studies of rhythmic gene expression within Drosophila heads. Next the authors describe the several neuron groups that compose the circadian pacemaker network underlying rhythmic locomotor activity, and they detail current models of how that network is organized and coordinated. The authors outline the body of evidence that describes a role for the neuropeptide pigment dispersing factor (PDF) as a circadian transmitter in the fly brain. Finally, in the context of PDF, they consider studies that address mechanisms of signaling from the circadian pacemaker network to downstream neurons and nonneuronal cells that directly control rhythmic outputs.

Structural isoforms of the circadian neuropeptide PDF expressed in the optic lobes of the cricket Gryllus bimaculatus: immunocytochemical evidence from specific monoclonal antibodies

Pigment-dispersing factor (PDF) is an 18-mer peptide that acts as a principal neurotransmitter of the insect circadian clock. Our previous study, utilizing anti-Uca beta-PDH polyclonal antibody (pAb) to immunolabel the optic lobe of the cricket Gryllus bimaculatus, suggested the existence of an alternative PDF-like peptide in the outer cells of the first neuropile, or lamina (La), which were much less immunoreactive than the inner cells of the second neuropile, the medulla (Me). To obtain structural information about such a PDF-like peptide, we prepared 10 anti-Gryllus PDF monoclonal (mAb) and pAb antibodies and analyzed their detailed epitope specificities. The PDFMe and PDFLa inner cells and their axonal projections were clearly immunoreactive to all these antibodies, revealing the widespread immunocytochemical organization of the PDF system in the optic lobe, as seen previously with anti-Uca beta-PDH pAb and anti-Gryllus PDF mAb, the epitope structures of which were also clarified in this study. The lamina outer cells, which we found lacked a target pdf mRNA, displayed specific immunoreactivities, indicating that the cells contain a distinct PDF-like peptide possessing both N- and C-terminal structures. These cells were not immunolabeled by some other monoclonal antibodies, however, implying that the PDFLa outer cells have a PDF isoform peptide devoid of Asn at positions 6 and 16. This isoform was also identified in a varicose arborization in the lamina. These results suggest not only the structure of the peptide, but also the possibility of additional functions of this novel PDF isoform.

Neuropeptides in interneurons of the insect brain

A large number of neuropeptides has been identified in the brain of insects. At least 35 neuropeptide precursor genes have been characterized in Drosophila melanogaster, some of which encode multiple peptides. Additional neuropeptides have been found in other insect species. With a few notable exceptions, most of the neuropeptides have been demonstrated in brain interneurons of various types. The products of each neuropeptide precursor seem to be co-expressed, and each precursor displays a unique neuronal distribution pattern. Commonly, each type of neuropeptide is localized to a relatively small number of neurons. We describe the distribution of neuropeptides in brain interneurons of a few well-studied insect species. Emphasis has been placed upon interneurons innervating specific brain areas, such as the optic lobes, accessory medulla, antennal lobes, central body, and mushroom bodies. The functional roles of some neuropeptides and their receptors have been investigated in D. melanogaster by molecular genetics techniques. In addition, behavioral and electrophysiological assays have addressed neuropeptide functions in the cockroach Leucophaea maderae. Thus, the involvement of brain neuropeptides in circadian clock function, olfactory processing, various aspects of feeding behavior, and learning and memory are highlighted in this review. Studies so far indicate that neuropeptides can play a multitude of functional roles in the brain and that even single neuropeptides are likely to be multifunctional.

Reevaluation of Drosophila melanogaster's neuronal circadian pacemakers reveals new neuronal classes

In the brain of the fly Drosophila melanogaster, approximately 150 clock-neurons are organized to synchronize and maintain behavioral rhythms, but the physiological and neurochemical bases of their interactions are largely unknown. Here we reevaluate the cellular properties of these pacemakers by application of a novel genetic reporter and several phenotypic markers. First, we describe an enhancer trap marker called R32 that specifically reveals several previously undescribed aspects of the fly's central neuronal pacemakers. We find evidence for a previously unappreciated class of neuronal pacemakers, the lateral posterior neurons (LPNs), and establish anatomical, molecular, and developmental criteria to establish a subclass within the dorsal neuron 1 (DN1) group of pacemakers. Furthermore, we show that the neuropeptide IPNamide is specifically expressed by this DN1 subclass. These observations implicate IPNamide as a second candidate circadian transmitter in the Drosophila brain. Finally, we present molecular and anatomical evidence for unrecognized phenotypic diversity within each of four established classes of clock neurons.

Neurotoxic protein expression reveals connections between the circadian clock and mating behavior in Drosophila

To investigate the functions of circadian neurons, we added two strategies to the standard Drosophila behavioral genetics repertoire. The first was to express a polyglutamine-expanded neurotoxic protein (MJDtr78Q; MJD, Machado-Joseph disease) in the major timeless (tim)-expressing cells of the adult brain. These Tim-MJD flies were viable, in contrast to the use of cell-death gene expression for tim neuron inactivation. Moreover, they were more arrhythmic than flies expressing other neurotoxins and had low but detectable tim mRNA levels. The second extended standard microarray technology from fly heads to dissected fly brains. By combining the two approaches, we identified a population of Tim-MJD-affected mRNAs. Some had been previously identified as sex-specific and relevant to courtship, including mRNAs localized to brain-proximal fat-body tissue and brain courtship centers. Finally, we found a decrease in the number of neurons that expressed male-specific forms of the fruitless protein in the laterodorsal region of the brain. The decrease was not a consequence of toxic protein expression within these specialized cells but a likely effect of communication with neighboring TIM-expressing neurons. The data suggest a functional interaction between adjacent circadian and mating circuits within the fly brain, as well as an interaction between circadian circuits and brain-proximal fat body.

Sex- and clock-controlled expression of the neuropeptide F gene in Drosophila

Drosophila neuropeptide F (NPF), a homolog of vertebrate neuropeptide Y, functions in feeding and coordination of behavioral changes in larvae and in modulation of alcohol sensitivity in adults, suggesting diverse roles for this peptide. To gain more insight into adult-specific NPF neuronal functions, we studied how npf expression is regulated in the adult brain. Here, we report that npf expression is regulated in both sex-nonspecific and male-specific manners. Our data show that male-specific npf (ms-npf) expression is controlled by the transformer (tra)-dependent sex-determination pathway. Furthermore, fruitless, one of the major genes functioning downstream of tra, is apparently an upstream regulator of ms-npf transcription. Males lacking ms-npf expression (through tra(F)-mediated feminization) or npf-ablated male flies display significantly reduced male courtship activity, suggesting that one function of ms-npf neurons is to modulate fruitless-regulated sexual behavior. Interestingly, one of the ms-npf neuronal groups belongs to the previously defined clock-controlling dorsolateral neurons. Such ms-npf expression in the dorsolateral neurons is absent in arrhythmic Clock(Jrk) and cycle(02) mutants, suggesting that npf is under dual regulation by circadian and sex-determining factors. Based on these data, we propose that NPF also plays a role in clock-controlled sexual dimorphism in adult Drosophila.

Defective transfer of seminal-fluid materials during matings of semi-fertile fruitless mutants in Drosophila

In context of the semi-sterility exhibited by Drosophila males expressing certain mating-enabling fruitless (fru) mutant genotypes, we examined the transfer of seminal fluid using a transgene that encodes the Sex Peptide (SP) oligopeptide fused to Green Fluorescent Protein (GFP). We found that this fusion construct expresses SP-GFP in a valid manner within accessory glands of the male reproductive system in normal and fru-mutant males. Transfer of SP-GFP to live females was readily detectable during and after copulation. With respect to the pertinent combinations of fru mutations, we demonstrated that these abnormal genotypes cause males to transmit mating-related materials in two aberrant ways: one involving whether any seminal-fluid entities are transferred at all during a given mating; the other revealing an intriguing aspect of these fruitless effects, such that the mutations in question cause males to transfer female-affecting materials in a manner that varies among copulations. In this regard, certain mutant males that do not transfer SP nevertheless are able to transfer sperm: a fru-mated female possessing no GFP who was not fecund initially could produce progeny when seminal-fluid proteins were subsequently supplied by mating with a male that was spermless owing to the effects of a tudor mutation.

Molecular genetics of the fruit-fly circadian clock

The circadian clock percolates through every aspect of behaviour and physiology, and has wide implications for human and animal health. The molecular basis of the Drosophila circadian clock provides a model system that has remarkable similarities to that of mammals. The various cardinal clock molecules in the fly are outlined, and compared to those of their actual and 'functional' homologues in the mammal. We also focus on the evolutionary tinkering of these clock genes and compare and contrast the neuronal basis for behavioural rhythms between the two phyla.

Drosophila CLOCK is constitutively expressed in circadian oscillator and non-oscillator cells

CLOCK (CLK) is a core component of the transcriptional feedback loops that comprise the circadian timekeeping mechanism in Drosophila. As a heterodimer with CYCLE (CYC), CLK binds E-boxes to activate the transcription of rhythmically expressed genes within and downstream of the circadian clock, but this activation unexpectedly occurs at times when CLK is at its lowest levels on Western blots. Recent studies demonstrate that CLK also regulates nonrhythmic gene expression and behaviors. Despite the critical roles CLK plays within and outside the circadian clock, its spatial expression pattern has not been characterized. Using a newly developed CLK antibody, the authors show that CLK is coexpressed with PERIOD (PER) in canonical oscillator cells throughout the head and body. In contrast to PER, however, the levels of CLK immunoreactivity do not cycle in intensity, CLK is detected primarily in the nucleus throughout the circadian cycle, and CLK is expressed in non-oscillator cells within the lateral and dorsal brain, including Kenyon cells, which mediate various forms of learning and memory. These results indicate that constitutive levels of nuclear CLK regulate rhythmic transcription in circadian oscillator cells and suggest that CLK contributes to other behavioral processes by regulating gene expression in non-oscillator cells.

PDF cycling in the dorsal protocerebrum of the Drosophila brain is not necessary for circadian clock function

In Drosophila, the neuropeptide pigment-dispersing factor (PDF) is a likely circadian molecule, secreted by central pacemaker neurons (LNvs). PDF is expressed in both small and large LNvs (sLNvs and lLNvs), and there are striking circadian oscillations of PDF staining intensity in the small cell termini, which require a functional molecular clock. This cycling may be relevant to the proposed role of PDF as a synchronizer of the clock system or as an output signal connecting pacemaker cells to locomotor activity centers. In this study, the authors use a generic neuropeptide fusion protein (atrial natriuretic factor-green fluorescent protein [ANF-GFP]) and show that it can be expressed in the same neurons as PDF itself. Yet, ANF-GFP as well as PDF itself does not manifest any cyclical accumulation in sLNv termini in adult transgenic flies. Surprisingly, the absence of detectable PDF cycling is not accompanied by any detectable behavioral pheno-type, since these transgenic flies have normal morning and evening anticipation in a light-dark cycle (LD) and are fully rhythmic in constant darkness (DD). The molecular clock is also not compromised. The results suggest that robust PDF cycling in sLNv termini plays no more than a minor role in the Drosophila circadian system and is apparently not even necessary for clock output function.

Programmed cell death mechanisms of identifiable peptidergic neurons in Drosophila melanogaster

The molecular basis of programmed cell death (PCD) of neurons during early metamorphic development of the central nervous system (CNS) in Drosophila melanogaster are largely unknown, in part owing to the lack of appropriate model systems. Here, we provide evidence showing that a group of neurons (vCrz) that express neuropeptide Corazonin (Crz) gene in the ventral nerve cord of the larval CNS undergo programmed death within 6 hours of the onset of metamorphosis. The death was prevented by targeted expression of caspase inhibitor p35, suggesting that these larval neurons are eliminated via a caspase-dependent pathway. Genetic and transgenic disruptions of ecdysone signal transduction involving ecdysone receptor-B (EcR-B) isoforms suppressed vCrz death, whereas transgenic re-introduction of either EcR-B1 or EcR-B2 isoform into the EcR-B-null mutant resumed normal death. Expression of reaper in vCrz neurons and suppression of vCrz-cell death in a reaper-null mutant suggest that reaper functions are required for the death, while no apparent role was found for hid or grim as a death promoter. Our data further suggest that diap1 does not play a role as a central regulator of the PCD of vCrz neurons. Significant delay of vCrz-cell death was observed in mutants that lack dronc or dark functions, indicating that formation of an apoptosome is necessary, but not sufficient, for timely execution of the death. These results suggest that activated ecdysone signaling determines precise developmental timing of the neuronal degeneration during early metamorphosis, and that subsequent reaper-mediated caspase activation occurs through a novel DIAP1-independent pathway.

Drosophila CRYPTOCHROME is a circadian transcriptional repressor

BACKGROUND

Although most circadian clock components are conserved between Drosophila and mammals, the roles assigned to the CRYPTOCHROME (CRY) proteins are very different: Drosophila CRY functions as a circadian photoreceptor, whereas mammalian CRY proteins (mCRY1 and 2) are transcriptional repressors essential for molecular clock oscillations.

RESULTS

Here we demonstrate that Drosophila CRY also functions as a transcriptional repressor. We found that RNA levels of genes directly activated by the transcription factors CLOCK (CLK) and CYCLE (CYC) are derepressed in cry(b) mutant eyes. Conversely, while overexpression of CRY and PERIOD (PER) in the eye repressed CLK/CYC activity, neither PER nor CRY repressed individually. Drosophila CRY also repressed CLK/CYC activity in cell culture. Repression by CRY appears confined to peripheral clocks, since neither cry(b) mutants nor overexpression of PER and CRY together in pacemaker neurons significantly affected molecular or behavioral rhythms. Increasing CLK/CYC activity by removing two repressors, PER and CRY, led to ectopic expression of the timeless clock gene, similar to overexpression of Clk itself.

CONCLUSIONS

Drosophila CRY functions as a transcriptional repressor required for the oscillation of peripheral circadian clocks and for the correct specification of clock cells.

The circadian timekeeping system of Drosophila

Daily rhythms in behavior, physiology and metabolism are controlled by endogenous circadian clocks. At the heart of these clocks is a circadian oscillator that keeps circadian time, is entrained by environmental cues such as light and activates rhythmic outputs at the appropriate time of day. Genetic and molecular analyses in Drosophila have revealed important insights into the molecules and mechanisms underlying circadian oscillator function in all organisms. In this review I will describe the intracellular feedback loops that form the core of the Drosophila circadian oscillator and consider how they are entrained by environmental light cycles, where they operate within the fly and how they are thought to control overt rhythms in physiology and behavior. I will also discuss where work remains to be done to give a comprehensive picture of the circadian clock in Drosophila and likely many other organisms.

GABA modulates Drosophila circadian clock neurons via GABAB receptors and decreases in calcium

Circadian clocks play vital roles in the control of daily rhythms in physiology and behavior of animals. In Drosophila, analysis of the molecular and behavioral rhythm has shown that the master clock neurons are entrained by sensory inputs and are synchronized with other clock neurons. However, little is known about the neuronal circuits of the Drosophila circadian system and the neurotransmitters that act on the clock neurons. Here, we provide evidence for a new neuronal input pathway to the master clock neurons, s-LN(v)s, in Drosophila that utilizes GABA as a slow inhibitory neurotransmitter. We monitored intracellular calcium levels in dissociated larval s-LN(v)s with the calcium-sensitive dye Fura-2. GABA decreased intracellular calcium in the s-LN(v)s and blocked spontaneous oscillations in calcium levels. The duration of this response was dose-dependent between 1 nM and 100 microM. The response to GABA was blocked by a metabotropic GABA(B) receptor (GABA(B)-R) antagonist, CGP54626, but not by an ionotropic receptor antagonist, picrotoxin. The GABA(B)-R agonist, 3-APMPA, produced a response similar to GABA. An antiserum against one of the Drosophila GABA(B)-Rs (GABA(B)-R2) labeled the dendritic regions of the s-LN(v)s in both adults and larvae, as well as the dissociated s-LN(v)s. We found that some GABAergic processes terminate at the dendrites of the LN(v)s, as revealed by GABA immunostaining and a GABA-specific GAL4 line (GAD1-gal4). Our results suggest that the s-LN(v)s receive slow inhibitory GABAergic inputs that decrease intracellular calcium of these clock neurons and block their calcium cycling. This response is mediated by postsynaptic GABA(B) receptors.

Mapping of serotonin, dopamine, and histamine in relation to different clock neurons in the brain of Drosophila

Several sets of clock neurons cooperate to generate circadian activity rhythms in Drosophila melanogaster. To extend the knowledge on neurotransmitters in the clock circuitry, we analyzed the distribution of some biogenic amines in relation to identified clock neurons. This was accomplished by employing clock neuron-specific GAL4 lines driving green fluorescent protein (GFP) expression, combined with immunocytochemistry with antisera against serotonin, histamine, and tyrosine hydroxylase (for dopamine). In the larval and adult brain, serotonin-immunoreactive (-IR) neuron processes are in close proximity of both the dendrites and the dorsal terminals of the major clock neurons, the s-LN(v)s. Additionally, the terminals of the l-LN(v) clock neurons and serotonergic processes converge in the distal medulla. No histamine (HA)-IR processes contact the s-LN(v)s in the larval brain, but possibly impinge on the dorsal clock neurons, DN2. In the adult brain, HA-IR axons of the extraocular eyelet photoreceptors terminate on the dendritic branches of the LN(v)s. A few tyrosine hydroxylase (TH)-IR processes were seen close to the dorsal terminals of the s-LN(v)s, but not their dendrites, in the larval and adult brain. TH-IR processes also converge with the distal medulla branches of the l-LN(v)s in adults. None of the monoamines was detectable in the different clock neurons. By using an imaging system to monitor intracellular Ca(2+) levels in dissociated GFP-labeled larval s-LN(v)s, loaded with Fura-2, we demonstrated that application of serotonin induced dose-dependent decreases in Ca(2+). Thus, serotonergic neurons form functional inputs on the s-LN(v)s in the larval brain and possibly also in adults.

A G protein-coupled receptor, groom-of-PDF, is required for PDF neuron action in circadian behavior

The neuropeptide Pigment-Dispersing Factor (PDF) plays a critical role in mediating circadian control of behavior in Drosophila. Here we identify mutants (groom-of-PDF; gop) that display phase-advanced evening activity and poor free-running rhythmicity, phenocopying pdf mutants. In gop mutants, a spontaneous retrotransposon disrupts a coding exon of a G protein-coupled receptor, CG13758. Disruption of the receptor is accompanied by phase-advanced oscillations of the core clock protein PERIOD. Moreover, effects on circadian timing induced by perturbation of PDF neurons require gop. Yet PDF oscillations themselves remain robust in gop mutants, suggesting that GOP acts downstream of PDF. gop is expressed most strongly in the dorsal brain in regions that lie in proximity to PDF-containing nerve terminals. Taken together, these studies implicate GOP as a PDF receptor in Drosophila.

Drosophila GPCR Han is a receptor for the circadian clock neuropeptide PDF

The pigment-dispersing factor (PDF) is a neuropeptide controlling circadian behavioral rhythms in Drosophila, but its receptor is not yet known. From a large-scale temperature preference behavior screen in Drosophila, we isolated a P insertion mutant that preferred different temperatures during the day and night. This mutation, which we named han, reduced the transcript level of CG13758. We found that Han was expressed specifically in 13 pairs of circadian clock neurons in the adult brain. han null flies showed arrhythmic circadian behavior in constant darkness. The behavioral characteristics of han null mutants were similar to those of pdf null mutants. We also found that PDF binds specifically to S2 cells expressing Han, which results in the elevation of cAMP synthesis. Therefore, we herein propose that Han is a PDF receptor regulating circadian behavioral rhythm through coordination of activities of clock neurons.

PDF has found its receptor

The neuropeptide pigment-dispersing factor (PDF) is a crucial component of the insect circadian clock, but the identity of its receptor has long remained a mystery. In this issue of Neuron, three papers (Hyun et al., Lear et al., and Mertens et al.) identify the PDF receptor.

Expression and function of variants of slob, slowpoke channel binding protein, in Drosophila

Slob binds to and modulates the Drosophila Slowpoke (dSlo) calcium-activated potassium channel and also recruits the ubiquitous signaling protein 14-3-3 to the channel regulatory complex. RT-PCR reveals the presence of multiple slob transcripts in Drosophila heads. The transcripts are predicted to encode proteins that we call Slob51 (kDa), Slob57, Slob65, and Slob71. Slob51 and Slob65 are splice variants that lack a motif important for the binding of 14-3-3. Previous microarray analyses demonstrated the circadian cycling of slob mRNA, and we show by quantitative PCR that more than one transcript cycles in fly heads. Using in situ hybridization, we observe differences in the expression patterns of the different transcripts. Immunohistochemistry on Drosophila heads reveals Slob71/65 protein to be enriched in the lateral neurons, in contrast to Slob57/51 protein, which is expressed most prominently in the pars intercerebralis neurons and dorsal giant interneurons. Using a heterologous expression system, we show that different Slobs bind to different extents to dSlo and 14-3-3. These data reveal an unexpected diversity of the dSlo/Slob/14-3-3 dynamic regulatory complex.

The Ion Channel Narrow Abdomen Is Critical for Neural Output of the Drosophila Circadian Pacemaker

Circadian clocks consist of transcriptional feedback loops housed in interdependent pacemaker neurons. Yet little is known about the neuronal output components essential for rhythmic behavior. Drosophila mutants of a putative ion channel, narrow abdomen (na), exhibit poor circadian rhythms and suppressed daylight activity. We find that NA is expressed in pacemaker neurons and induced expression within circadian neurons is sufficient to rescue these mutant phenotypes. Selective na rescue in distinct pacemaker neurons influences rhythmicity and timing of behavior. Oscillations of the clock protein PERIOD are intact in na mutants, indicating an output role. Pore residues are required for robust rescue consistent with NA action as an ion channel. In na mutants, expression of potassium currents and the key neuropeptide PDF are elevated, the latter consistent with reduced release. These data implicate NA and the pacemaker neural network in controlling phase and rhythmicity.

Lithium- and valproate-induced alterations in circadian locomotor behavior in Drosophila

Lithium and valproate are commonly used mood stabilizers, but their action pathways are not clearly understood. They also suffer from multiple toxic effects that limit their utility. Elucidating their action mechanisms could lead to newer agents and better understanding of the etiopathogenesis of bipolar disorder. We have expanded the study of signaling mechanisms of lithium and valproate by using Drosophila circadian locomotor activity as a robust behavioral assay that is amenable to genetic manipulations. We demonstrate that lithium affects the circadian system of Drosophila similarly to what has been reported in the mammalian studies. We show that lithium and valproate share effects on the circadian locomotor activity of Drosophila: they lengthen the period of circadian rhythms and increase arrhythmicity. Valproate exerts these effects in a weaker fashion than does lithium. We also tested the circadian alterations in multiple mutant lines of Drosophila bearing defects in the GSK-3beta gene and other clock genes in response to lithium administration. We show that lithium partially rescues the shortening of circadian period when the GSK-3beta gene is overexpressed only in specific circadian pacemaker neurons, thus implicating GSK-3beta as a component in lithium's effect on the circadian oscillator. Moreover, lithium also lengthens the period in GSK-3beta heterozygous mutants and doubletime long mutants. These results establish a basis for using Drosophila genetics to investigate more fully lithium and valproate action mechanisms.

Organization of endogenous clocks in insects

Insect and mammalian circadian clocks show striking similarities. They utilize homologous clock genes, generating self-sustained circadian oscillations in distinct master clocks of the brain, which then control rhythmic behaviour. The molecular mechanisms of rhythm generation were first uncovered in the fruit fly Drosophila melanogaster, whereas cockroaches were among the first animals where the brain master clock was localized. Despite many similarities, there exist obvious differences in the organization and functioning of insect master clocks. These similarities and differences are reviewed on a molecular and anatomical level.

FMRFamide-like immunocytochemistry in the brain and subesophageal ganglion of Triatoma infestans (Insecta: Heteroptera). Coexpression with β-pigment-dispersing hormone and small cardioactive peptide B

The distribution of FMRFamide (FMRFa)-like immunoreactivity (LI) was studied in the brain and subesophageal ganglion of Triatoma infestans, the insect vector of Chagas' disease. The neuropeptide displayed a widespread distribution with immunostained somata in the optic lobe, in the anterior, lateral, and posterior soma rinds of the protocerebrum, and around the antennal sensory and mechanosensory and motor neuropils of the deutocerebrum. FMRFa-immunoreactive profiles of the subesophageal ganglion were seen in the mandibular, maxillary, and labial neuromeres. Immunostained neurites were detected in the medulla and lobula of the optic lobe, the lateral protocerebral neuropil, the median bundle, the calyces and the stalk of the mushroom bodies, and the central body. In the deutocerebrum, the sensory glomeruli showed a higher density of immunoreactive processes than the mechanosensory and motor neuropil, whereas the neuropils of each neuromere of the subesophageal ganglion displayed a moderate density of immunoreactive neurites. Colocalization of FMRFa-LI and crustacean pigment-dispersing hormone-LI was found in perikarya of the proximal optic lobe, the lobula, the sensory deutocerebrum, and the labial neuromere of the subesophageal ganglion. The distribution pattern of small cardioactive peptide B (SCP(B))-LI was also widespread, with immunolabeled somata surrounding every neuropil region of the brain and subesophageal ganglion, except for the optic lobe. FMRFa- and SCP(B)-LIs showed extensive colocalization in the brain of this triatomine species. The presence of immunolabeled perikarya displaying either FMRFa- or SCP(B)-LI confirmed that each antisera identified different peptide molecules. The distribution of FMRFa immunostaining in T. infestans raises the possibility that FMRFa plays a role in the regulation of circadian rhythmicity. The finding of immunolabeling in neurosecretory somata of the protocerebrum suggests that this neuropeptide may also act as a neurohormone.

The Drosophila Fragile X Mental Retardation Protein Controls Actin Dynamics by Directly Regulating Profilin in the Brain

Loss of Fragile X mental retardation protein (FMRP) function causes the highly prevalent Fragile X syndrome [1 and 2]. Identifying targets for the RNA binding FMRP is a major challenge and an important goal of research into the pathology of the disease. Perturbations in neuronal development and circadian behavior are seen in Drosophila dfmr1 mutants. Here we show that regulation of the actin cytoskeleton is under dFMRP control. dFMRP binds the mRNA of the Drosophila profilin homolog and negatively regulates Profilin protein expression. An increase in Profilin mimics the phenotype of dfmr1 mutants. Conversely, decreasing Profilin levels suppresses dfmr1 phenotypes. These data place a new emphasis on actin misregulation as a major problem in fmr1 mutant neurons.

Membrane electrical excitability is necessary for the free-running larval Drosophila circadian clock

Drosophila larvae and adult pacemaker neurons both express free-running oscillations of period (PER) and timeless (TIM) proteins that constitute the core of the cell-autonomous circadian molecular clock. Despite similarities between the adult and larval molecular oscillators, adults and larvae differ substantially in the complexity and organization of their pacemaker neural circuits, as well as in behavioral manifestations of circadian rhythmicity. We have shown previously that electrical silencing of adult Drosophila circadian pacemaker neurons through targeted expression of either an open rectifier or inward rectifier K(+) channel stops the free-running oscillations of the circadian molecular clock. This indicates that neuronal electrical activity in the pacemaker neurons is essential to the normal function of the adult intracellular clock. In the current study, we show that in constant darkness the free-running larval pacemaker clock-like that of the adult pacemaker neurons they give rise to-requires membrane electrical activity to oscillate. In contrast to the free-running clock, the molecular clock of electrically silenced larval pacemaker neurons continues to oscillate in diurnal (light-dark) conditions. This specific disruption of the free-running clock caused by targeted K(+) channel expression likely reflects a specific cell-autonomous clock-membrane feedback loop that is common to both larval and adult neurons, and is not due to blocking pacemaker synaptic outputs or disruption of pacemaker neuronal morphology.

Comparative analysis of Corazonin-encoding genes (Crz's) in Drosophila species and functional insights into Crz-expressing neurons

To gain insight into regulatory mechanisms of tissue-specific Corazonin (Crz) gene expression and its functions in Drosophila, we cloned the Crz genes from four Drosophila species (D. melanogaster, D. simulans, D. erecta, and D. virilis) and performed comparative analyses of Crz gene sequences and expression patterns using in situ hybridization and immunohistochemistry. Although Crz gene sequences showed a great deal of diversity, its expression patterns in the CNS were highly conserved in the Drosophila species examined here. In D. melanogaster larva, Crz expression was found in four pairs of neurons per cerebral lobe and in eight pairs of bilateral neurons in the ventral nerve cord; in adult, the number of Crz-producing neurons increased to 6-8 in the pars lateralis of each brain lobe, whereas neurons in the ventral nerve cord were no longer detectable. Crz transcripts were also found in the optic lobes; however, these mRNAs do not seem to be translated. Such adult-like Crz expression patterns were established within 48 hours after pupation. Somata of Crz-neurons in the pars lateralis are located in the vicinity of terminals emanating from PDF-containing pacemaking neurons, indicating a functional connection between the two peptidergic nervous systems. A subset of Crz neurons coexpressed the period clock gene; however, normal Crz transcription was unaffected by central clockworks. Two pairs of ectopic Crz cells were detected in the adult brains of behaviorally arrhythmic Clock(Jrk) or cycle(02) mutants, suggesting that CLOCK and CYCLE proteins negatively regulate Crz transcription in a cell-specific manner.

Circadian pacemaker neurons transmit and modulate visual information to control a rapid behavioral response

Circadian pacemaker neurons contain a molecular clock that oscillates with a period of approximately 24 hr, controlling circadian rhythms of behavior. Pacemaker neurons respond to visual system inputs for clock resetting, but, unlike other neurons, have not been reported to transmit rapid signals to their targets. Here we show that pacemaker neurons are required to mediate a rapid behavior. The Drosophila larval visual system, Bolwig's organ (BO), projects to larval pacemaker neurons to entrain their clock. BO also mediates larval photophobic behavior. We found that ablation or electrical silencing of larval pacemaker neurons abolished light avoidance. Thus, circadian pacemaker neurons receive input from BO not only to reset the clock but also to transmit rapid photophobic signals. Furthermore, as clock gene mutations also affect photophobicity, the pacemaker neurons modulate the sensitivity of larvae to light, generating a circadian rhythm in visual sensitivity.

Hemolymph sugar homeostasis and starvation-induced hyperactivity affected by genetic manipulations of the adipokinetic hormone-encoding gene in Drosophila melanogaster

Adipokinetic hormones (AKHs) are metabolic neuropeptides, mediating mobilization of energy substrates from the fat body in many insects. In delving into the roles of the Drosophila Akh (dAkh) gene, its developmental expression patterns were examined and the physiological functions of the AKH-producing neurons were investigated using animals devoid of AKH neurons and ones with ectopically expressing dAkh. The dAkh gene is expressed exclusively in the corpora cardiaca from late embryos to adult stages. Projections emanating from the AKH neurons indicated that AKH has multiple target tissues as follows: the prothoracic gland and aorta in the larva and the crop and brain in the adult. Studies using transgenic manipulations of the dAkh gene demonstrated that AKH induced both hypertrehalosemia and hyperlipemia. Starved wild-type flies displayed prolonged hyperactivity prior to death; this novel behavioral pattern could be associated with food-searching activities in response to starvation. In contrast, flies devoid of AKH neurons not only lacked this type of hyperactivity, but also displayed strong resistance to starvation-induced death. From these findings, we propose another role for AKH in the regulation of starvation-induced foraging behavior.

Techniques that Revealed the Network of the Circadian Clock of Drosophila

The techniques are reviewed that revealed the neuronal network of the circadian clock in the brain of the fruit fly as well as the function and localization of peripheral oscillators. Three principal techniques helped characterize the circadian clock network of Drosophila consisting of pacemaker centers in the brain and oscillators in peripheral tissues: (1) Immunolabeling with antibodies raised against specific clock proteins detected the tissues and cells that express the clock proteins, revealed the subcellular localization of clock molecules, and illuminated their abundance at different time points during the day; (2) reporter genes unraveled the network of clock neurons and reported the circadian cycling of the clock genes in vivo; and (3) genetic manipulations of clock gene expression elucidated the function of specific clock genes and clock cells. These techniques and the results gained by them are reviewed briefly.

Systems approaches to biological rhythms in Drosophila

The chronobiological system of Drosophila is considered from the perspective of rhythm-regulated genes. These factors are enumerated and discussed not so much in terms of how the gene products are thought to act on behalf of circadian-clock mechanisms, but with special emphasis on where these molecules are manufactured within the organism. Therefore, with respect to several such cell and tissue types in the fly head, what is the "systems meaning" of a given structure's function insofar as regulation of rest-activity cycles is concerned? (Systematic oscillation of daily behavior is the principal overt phenotype analyzed in studies of Drosophila chronobiology). In turn, how do the several separate sets of clock-gene-expressing cells interact--or in some cases act in parallel--such that intricacies of the fly's sleep-wake cycles are mediated? Studying Drosophila chrono-genetics as a system-based endeavor also encompasses the fact that rhythm-related genes generate their products in many tissues beyond neural ones and during all stages of the life cycle. What, then, is the meaning of these widespread gene-expression patterns? This question is addressed with regard to circadian rhythms outside the behavioral arena, by considering other kinds of temporally based behaviors, and by contemplating how broadly systemic expression of rhythm-related genes connects with even more pleiotropic features of Drosophila biology. Thus, chronobiologically connected factors functioning within this insect comprise an increasingly salient example of gene versatility--multi-faceted usages of, and complex interactions among, entities that set up an organism's overall wherewithal to form and function. A corollary is that studying Drosophila development and adult-fly actions, even when limited to analysis of rhythm-systems phenomena, involves many of the animal's tissues and phenotypic capacities. It follows that such chronobiological experiments are technically demanding, including the necessity for investigators to possess wide-ranging expertise. Therefore, this chapter includes several different kinds of Methods set-asides. These techniques primers necessarily lack comprehensiveness, but they include certain discursive passages about why a given method can or should be applied and concerning real-world applicability of the pertinent rhythm-related technologies.

Membranes, Ions, and Clocks: Testing the Njus–Sulzman–Hastings Model of the Circadian Oscillator

Current circadian clock models based on interlocking autoregulatory transcriptional?translational negative feedback loops have arisen out of an explosion of molecular genetic data obtained over the last decade (for review, see Stanewsky, 2003; Young and Kay, 2001). An earlier model of circadian oscillation was based on feedback interactions between membrane ion transport systems and ion concentration gradients (Njus et al., 1974, 1976). This membrane model was posited as a more plausible alternative at the time to the even earlier "chronon" model, which was based on autoregulatory genetic feedback loops (Ehret and Trucco, 1967). The membrane model has been tested in a number of experimental systems by pharmacologically manipulating either ionic gradients across the plasma membrane or ion transport systems, but with inconsistent results. In the meantime, the scope and explanatory power of the genetic models overshadowed inquiries into the role of membrane ion fluxes in clock function. However, several recently developed techniques described in this article have provided a new glimpse into the essential role that membrane ion fluxes play in the mechanism of the core circadian oscillator and indicate that a complete understanding of the clock must include both genetic and membrane-based feedback loops.

Genetic and biochemical strategies for identifying Drosophila genes that function in circadian control

Explicit biochemical models have been elaborated for the circadian oscillators of cyanobacterial, fungal, insect, and mammalian species. In contrast, much remains to be learned about how such circadian oscillators regulate rhythmic physiological processes. This article summarizes contemporary genetic and biochemical strategies that are useful for identifying gene products that have a role in circadian control.

Lmo mutants reveal a novel role for circadian pacemaker neurons in cocaine-induced behaviors

Drosophila has been developed recently as a model system to investigate the molecular and neural mechanisms underlying responses to drugs of abuse. Genetic screens for mutants with altered drug-induced behaviors thus provide an unbiased approach to define novel molecules involved in the process. We identified mutations in the Drosophila LIM-only (LMO) gene, encoding a regulator of LIM-homeodomain proteins, in a genetic screen for mutants with altered cocaine sensitivity. Reduced Lmo function increases behavioral responses to cocaine, while Lmo overexpression causes the opposite effect, reduced cocaine responsiveness. Expression of Lmo in the principal Drosophila circadian pacemaker cells, the PDF-expressing ventral lateral neurons (LN(v)s), is sufficient to confer normal cocaine sensitivity. Consistent with a role for Lmo in LN(v)function,Lmomutants also show defects in circadian rhythms of behavior. However, the role for LN(v)s in modulating cocaine responses is separable from their role as pacemaker neurons: ablation or functional silencing of the LN(v)s reduces cocaine sensitivity, while loss of the principal circadian neurotransmitter PDF has no effect. Together, these results reveal a novel role for Lmo in modulating acute cocaine sensitivity and circadian locomotor rhythmicity, and add to growing evidence that these behaviors are regulated by shared molecular mechanisms. The finding that the degree of cocaine responsiveness is controlled by the Drosophila pacemaker neurons provides a neuroanatomical basis for this overlap. We propose that Lmo controls the responsiveness of LN(v)s to cocaine, which in turn regulate the flies' behavioral sensitivity to the drug.

Regulation of copulation duration by period and timeless in Drosophila melanogaster

The circadian clock involves several clock genes encoding interacting transcriptional regulators. Mutations in clock genes in Drosophila melanogaster, period (per), timeless (tim), Clock (Clk), and cycle (cyc), produce multiple phenotypes associated with physiology, behavior, development, and morphology. It is not clear whether these genes always work as clock components or may also act in some unknown pleiotropic fashion. We report here that per and tim are involved in a novel, male-specific phenotype that affects behavioral timing on the order of minutes. Males lacking per or tim copulate significantly longer than males with normal per or tim function, while females do not show this effect. No correlation between fertility and extended copulation duration was found. Several lines of evidence suggest that the time in copula (TIC) is not regulated by the known clock mechanism. First, the period of free-running clock oscillations does not appear to affect this phenotype. Second, constant light, which abolishes the clock function, does not alter TIC. Finally, mutations in the positively acting clock transcription factors, Clk and cyc, do not affect TIC. Our study extends the repertoire of behavioral functions involving per and tim genes and uncovers another time scale over which these genes may act.

Pigment-dispersing hormone (PDH)-immunoreactive neurons form a direct coupling pathway between the bilaterally symmetric circadian pacemakers of the cockroach Leucophaea maderae

Circadian locomotor activity rhythms of the cockroach Leucophaea maderae are driven by two bilaterally paired and mutually coupled pacemakers that reside in the optic lobes of the brain. Transplantation studies have shown that this circadian pacemaker is located in the accessory medulla (AMe), a small neuropil of the medulla of the optic lobe. The AMe is densely innervated by about 12 anterior pigment-dispersing-hormone-immunoreactive (PDH-ir) medulla (PDHMe) neurons. PDH-ir neurons are circadian pacemaker candidates in the fruitfly and cockroach. A subpopulation of these neurons also appears to connect both optic lobes and may constitute at least one of the circadian coupling pathways. To determine whether PDHMe neurons directly connect both accessory medullae, we injected rhodamine-labeled dextran as neuronal tracer into one AMe and performed PDH immunocytochemistry. Double-labeled fibers in the anterior, shell, and internodular neuropil of the AMe contralaterally to the injection site showed that PDH-ir fibers directly connect both accessory medullae. This connection is formed by three anterior PDHMe neurons of each optic lobe, which, thus, fulfill morphological criteria for a direct circadian coupling pathway. Our double-label studies also showed that all except one of the midbrain projection areas of anterior PDHMe neurons were innervated ipsilaterally and contralaterally. Thus, anterior PDHMe neurons seem to play multiple roles in generating circadian rhythms. They also deliver timing information output and perform mutual pacemaker coupling in L. maderae.

Expression and release patterns of neuropeptides during embryonic development and hatching of the green shore crab, Carcinus maenas

Crustacean ecdysis is controlled by at least three neuropeptides:moult-inhibiting hormone (MIH), which represses ecdysteroid synthesis;crustacean hyperglycaemic hormone (CHH), which not only influences ecdysteroid synthesis but also water uptake during moulting; and crustacean cardioactive peptide (CCAP), which is involved in stereotyped ecdysis behaviour. During embryonic development, moulting takes place in the egg, but there is little information regarding developmental expression of these neuropeptides during this period or during hatching – an event that is analogous to eclosion in insects. To address this problem, we determined expression profiles of MIH and CHH mRNA by quantitative RT-PCR, together with developmental peptide expression studies [confocal immunocytochemistry (ICC) and radioimmunoassay(RIA)]. Likely homologous events relating to neuropeptide surges of both CHH and CCAP were seen during larval hatching, when compared to the adult moult,and cell-specific copy concentration of both MIH and CHH mRNAs was identical to that of the adult during late embryonic development. We measured parallel mRNA and peptide expression of two neuropeptides (red pigment-concentrating hormone RPCH) and pigment-dispersing hormone (PDH) during development, as these have roles as neuromodulators and as classical neurohormonal roles. For MIH and CHH, gene expression was in accordance with peptide expression, but novel sites of CHH expression were found (abdominal peripheral neurones), the expression and release patterns of which may be related to larval eclosion and water uptake necessary for eggshell rupture and hatching. For RPCH and PDH,gene transcription and peptide expression were not in accordance. A significant contribution of maternally derived (non-translated) PDH mRNA to the embryo was seen, and for RPCH, high-level mRNA and peptide expression during late embryogenesis is related to a long ignored, but potentially important release site – the enigmatic post-commissural organs –which are the most prominent structures expressing RPCH during late embryogenesis.

Coupled oscillators control morning and evening locomotor behaviour of Drosophila

Daily rhythms of physiology and behaviour are precisely timed by an endogenous circadian clock. These include separate bouts of morning and evening activity, characteristic of Drosophila melanogaster and many other taxa, including mammals. Whereas multiple oscillators have long been proposed to orchestrate such complex behavioural programmes, their nature and interplay have remained elusive. By using cell-specific ablation, we show that the timing of morning and evening activity in Drosophila derives from two distinct groups of circadian neurons: morning activity from the ventral lateral neurons that express the neuropeptide PDF, and evening activity from another group of cells, including the dorsal lateral neurons. Although the two oscillators can function autonomously, cell-specific rescue experiments with circadian clock mutants indicate that they are functionally coupled.

cDNA cloning of the housefly pigment-dispersing factor (PDF) precursor protein and its peptide comparison among the insect circadian neuropeptides

Pigment-dispersing factor (PDF), an 18-amino acid neuropeptide, is a principal circadian neurotransmitter for the circadian rhythms of the locomotor activity in flies. Recently, two completely different types of PDF precursor were clarified; that of the cricket Gryllus bimaculatus and that of the last-summer cicada Meimuna opalifera. The G. bimaculatus PDF precursor is extraordinarily short and comprises a nuclear localization signal (NLS), while the M. opalifera PDF precursor is of ordinary length, comparable to that seen for the precursors of crustacean beta-PDH homologues. Although their PDF peptide regions were exactly the same, the regions containing a signal peptide combined with a PDF-associated peptide (PAP) were remarkably different from each other. Such a grouping suggested a fundamental role for the PAP peptide in the circadian clock, perhaps associated with PDF function. In the present study, the cDNA cloning of PDF from the adult brains of the housefly Musca domestica was carried out and it was found that an isolated clone (527 bp) encodes a PDF precursor protein of ordinary length. The PDF peptide shows a high sequence identity (78%-94%) and similarity (89%-100%) to insect PDFs and also to the crustacean beta-PDH peptides. In particular, there is only a single amino acid difference between the PDFs of Musca and Drosophila; at position 14 Ser for Musca PDF and Asn for Drosophila PDF. A characteristic Ser10 in Drosophila was retained in Musca, indicating the presence of a structural profile unique to these PDFs. The results of sequence analyses suggest that Musca and Drosophila PDFs are to be considered members of a single group that has evolved structurally. When the primary structure of the PAP regions was compared, the Musca PDF precursor also belonged to the same group as that to which the Drosophila PDF precursor belongs.

Neurobiology of the fruit fly's circadian clock

Studying the fruit fly Drosophila melanogaster has revealed mechanisms underlying circadian clock function. Rhythmic behavior could be assessed to the function of several clock genes that generate circadian oscillations in certain brain neurons, which finally modulate behavior in a circadian manner. This review outlines how individual circadian pacemaker neurons in the fruit fly's brain control rhythm in locomotor activity and eclosion.

A constitutively active cryptochrome in Drosophila melanogaster

Light-activated cryptochrome (CRY) regulates circadian photoresponses in Drosophila melanogaster. Removing the carboxy (C) terminus to create CRYDelta produces, in yeast, a light-independent, constitutively active form. Here we show that flies overexpressing CRYDelta have a longer free-running period of locomotor activity, as well as altered cycling kinetics of the clock proteins timeless (TIM) and period (PER). Moreover, at the cellular level, they show a reduction in the level of TIM and in the nuclear localization of TIM and PER in two significant clusters of behavioral pacemaker cells: the large and the small ventral lateral neurons (LN(v)s). These effects are similar to those seen in wild-type flies under continuous light and suggest a regulatory role for the C terminus of CRY on the photosensitive, photolyase-like part of the protein.

Excitatory and inhibitory switches for courtship in the brain of Drosophila melanogaster

BACKGROUND

Courtship is the best-studied behavior in Drosophila melanogaster, and work on its anatomical basis has concentrated mainly on the functional identification of sexually dimorphic sites in the brain. Much less is known of the more expansive, nondimorphic, but nonetheless essential, neural elements subserving male courtship behavior.

RESULTS

Sites in the CNS mediating initiation and early steps of male courtship in Drosophila melanogaster were identified by analyzing the behavior of mosaic flies expressing transgenes designed either to suppress neurotransmission or enhance neuronal excitability. Suppression of neurotransmission was accomplished by means of the dominantly acting, temperature-sensitive dynamin mutation shibire(ts1), whereas enhanced neuronal excitability was produced by means of a novel, dominantly acting, truncated eag potassium channel. By using a new, landmark-based procedure for aligning diverse expression patterns among the various mosaic strains, a comparison of courtship performance and affected brain sites in strains expressing the transgenes identified a cluster of cells in the posterior lateral protocerebrum that exerts reciprocal effects on the initiation of courtship, suppressing it when they are inactivated and enhancing it when they are hyperactivated, indicative of cells that normally play an excitatory, triggering role. A separate group of nearby cells, slightly more anterior in the lateral protocerebrum, was found to inhibit courtship when its activity is enhanced, indicative of an inhibitory role in courtship.

CONCLUSIONS

A cluster of cells, some excitatory and some inhibitory, in the lateral protocerebrum regulates courtship initiation in Drosophila. These cells are likely to be an integration center for the multiple sensory inputs that trigger male courtship.

Circadian clocks in antennal neurons are necessary and sufficient for olfaction rhythms in Drosophila

BACKGROUND

The Drosophila circadian clock is controlled by interlocked transcriptional feedback loops that operate in many neuronal and nonneuronal tissues. These clocks are roughly divided into a central clock, which resides in the brain and is known to control rhythms in locomotor activity, and peripheral clocks, which comprise all other clock tissues and are thought to control other rhythmic outputs. We previously showed that peripheral oscillators are required to mediate rhythmic olfactory responses in the antenna, but the identity and relative autonomy of these peripheral oscillators has not been defined.

RESULTS

Targeted ablation of lateral neurons by using apoptosis-promoting factors and targeted clock disruption in antennal neurons with newly developed dominant-negative versions of CLOCK and CYCLE show that antennal neurons, but not central clock cells, are necessary for olfactory rhythms. Targeted rescue of antennal neuron oscillators in cyc(01) flies through wild-type CYCLE shows that these neurons are also sufficient for olfaction rhythms.

CONCLUSIONS

Antennal neurons are both necessary and sufficient for olfaction rhythms, which demonstrates for the first time that a peripheral tissue can function as an autonomous pacemaker in Drosophila. These results reveal fundamental differences in the function and organization of circadian oscillators in Drosophila and mammals and suggest that components of the olfactory signal transduction cascade could be targets of circadian regulation.

Use of targetable gfp-tagged neuropeptide for visualizing neuropeptide release following execution of a behavior

Previous work has shown that a transgene consisting of a fusion between the rat atrial natriuretic peptide and a green fluorescent protein reporter (ANF-gfp) is processed, localized, and released, as would be an endogenous neuropeptide when it is expressed in the nervous system of Drosophila melanogaster using the GAL4/UAS expression system. Here we have tested the utility of this targetable transgene for detecting neuropeptide release following the execution of a peptide-controlled behavior. For the behavior we used ecdysis, the behavior expressed by insects to shed their old cuticle at the end of the molt. We found that larval ecdysis was accompanied by a readily detectable reduction in gfp fluorescence from relevant secretory cells in the periphery and peptidergic neurons in the CNS. We also found that expression of the ANF-gfp products did not have detrimental effects on larval ecdysis or adult circadian rhythmicity, when the transgene was expressed in peptidergic cells that are known to control these behaviors. Finally, we used a broadly expressed GAL4 driver to show that the UAS-ANF-gfp transgene could be used to identify axons that show a reduction in gfp fluorescence following the expression of ecdysis behavior. These findings, coupled with the availability of an increasing number of strains bearing different GAL4 drivers, suggest that this transgene will be a useful tool for identifying peptidergic neurons and secretory cells (and, eventually, their secretory product) that release their peptide content during the occurrence, in the intact animal, of a developmental, physiological or behavioral process of interest.

Ap-let neurons--a peptidergic circuit potentially controlling ecdysial behavior in Drosophila

Here we describe a novel set of peptidergic neurons conserved throughout all developmental stages in the Drosophila central nervous system (CNS). We show that a small complement of 28 apterous-expressing cells (Ap-let neurons) in the ventral nerve cord (VNC) of Drosophila larvae co-express numerous gene products. The products include the neuroendocrine-specific bHLH regulator called Dimmed (Dimm), four neuropeptide biosynthetic enzymes (PC2, Fur1, PAL2, and PHM), and a specific dopamine receptor subtype (dDA1). For the PC2, Fur1, and PAL2 enzymes, and for the dDA1 receptor, this neuronal pattern represents the vast majority of their total expression in the VNC. In addition, while Dimm and PHM are present in the peritracheal Inka cells in larvae, pupae, and adults, Ap, PC2, Fur1, PAL2, and dDA1 are not. PC2, PAL2, and DA1 receptor expression were all controlled by both dimm and ap. Previous genetic analysis of animals deficient in PC2 revealed an abnormal larval ecdysis phenotype. Together, these data support the hypothesis that the small cohort of Ap-let interneurons regulates larval ecdysis behavior by secretion of an unidentified amidated peptide(s). This hypothesis further predicts that the production of the Ap-let neuropeptide(s) is dependent on each of four specific enzymes, and that a certain aspect(s) of its production and/or release is regulated by dopamine input.

The Doubletime and CKII Kinases Collaborate to Potentiate Drosophila PER Transcriptional Repressor Activity

In all genetically studied model organisms, a negative feedback loop of gene expression contributes to the circadian rhythm mechanism. In the Drosophila system, it has been proposed that the delay between the synthesis and function of clock proteins is due to phosphorylation-regulated nuclear entry. To test this hypothesis, we assayed the relationship between PER phosphorylation, nuclear localization, and transcriptional repression activity in cultured S2 cells. The results indicate that the two putative PER kinases DBT and CKII work together to phosphorylate PER and increase repression activity. Experiments combining kinase inhibition with inhibition of PER nuclear export suggest that phosphorylation directly affects PER repression activity and that PER nuclear localization is an indirect consequence of the association of active PER with DNA or chromatin. This interpretation suggests further that the circadian regulation of PER nuclear localization in flies reflects changes in PER transcriptional activity rather than in PER nuclear import or export activity.

Acetylcholine increases intracellular Ca2+ via nicotinic receptors in cultured PDF-containing clock neurons of Drosophila

Light entrains the biological clock both in adult and larval Drosophila melanogaster. The Bolwig organ photoreceptors most likely constitute one substrate for this light entrainment in larvae. Acetylcholine (ACh) has been suggested as the neurotransmitter in these photoreceptors, but there is no evidence that ACh signaling is involved in photic input onto circadian pacemaker neurons. Here we demonstrate that the putative targets of the Bolwig photoreceptors, the PDF-containing clock neurons (LNs), in the larval brain express functional ACh receptors (AChRs). With the use of GAL4-UAS-driven expression of green fluorescent protein (GFP), we were able to identify LNs in dissociated cell culture. After loading with the Ca(2+)-sensitive dye fura-2, we monitored changes in intracellular Ca(2+) levels ([Ca(2+)](i)) in GFP-marked LNs while applying candidate neurotransmitters. ACh induced transient increases in [Ca(2+)](i) at physiological concentrations. These increases were dependent on extracellular Ca(2+) and Na(+) and were likely caused by activation of voltage-dependent Ca(2+) channels. Application of nicotinic and muscarinic agonists and antagonists showed that the AChRs on cultured LNs have a nicotinic pharmacology. Antibodies to several subunits of nicotinic AChRs (nAChRs) labeled the putative contact site of the Bolwig organ axon terminals with the dendrites of LNs, as well as dissociated LNs in culture. Our findings support a role of ACh as input factor onto the LNs and suggest that Ca(2+) is used as a second messenger mediating cholinergic input within the LNs. Experiments using a more general GAL4-UAS-driven expression of GFP showed that functional expression of nAChRs is a widespread phenomenon in peptidergic neurons.

Development of PDF-immunoreactive cells, possible clock neurons, in the housefly Musca domestica

Even though the housefly Musca domestica shows clear circadian rhythms in its behavioural and physiological processes, a circadian pacemaker system controlling these rhythms has not yet been described morphologically in this species. In M. domestica, neurons immunoreactive to pigment-dispersing factor (PDF), a neurotransmitter/neuromodulator of circadian information arising from a circadian clock and transmitted to target cells, are similar in their number and distribution to the PDF neurons of Drosophila melanogaster. In D. melanogaster these neurons co-localize PER protein and have been identified as clock neurons in that species. Here we report PDF-immunoreactive cells in the housefly's brain during postembryonic development in the larval and pupal stages, as well as in the adult fly soon after eclosion. In the housefly's brain, there are three groups of PDF-immunoreactive neurons: two groups with small (sPDFMe) and large (lPDFMe) cell bodies in the proximal medulla of the optic lobe; and one group in the dorsal protocerebrum (PDFD). Three out of four sPDFMe can be detected during the first hour of larval development, but the fourth sPDFMe is observed in the larva only from 48 hours after hatching, along with five lPDFMe neurons, seen first as two subgroups, and three out of four PDFD neurons. During postembryonic development these neurons show changes in their structure and immunoreactivity. New PDF neurons are observed during pupal development but these neurons mostly do not survive into adulthood. In the adult fly's brain, the PDF neurons have also been examined in double-labelled preparations made with a second antibody directed against the product of one of several clock genes: period (per), timeless (tim), or cryptochrome (cry). Among them, only immunoreactivity to CRY-like protein has been detected in the brain of M. domestica and has shown a daily rhythm in its concentration, as examined immunocytochemically. CRY was co-localized with PDF in the sPDFMe of the housefly's brain fixed during the day. The possibility that the sPDFMe neurons are the housefly's clock neurons is discussed.