Serotonin modulates circadian entrainment in Drosophila

Protein kinase C deficiency-induced alcohol insensitivity and underlying cellular targets in Drosophila

Multiple subtypes of protein kinase C (PKC) isozymes are implicated in various neurological disorders including alcohol insensitivity, a trait strongly associated with alcoholism in humans, but molecular and cellular mechanisms underlying the PKC activities remain poorly understood. Here we show that functional knockdown of conventional, novel or atypical PKC in the fly nervous system each resulted in alcohol insensitivity. Neuroanatomical mapping of conventional Ca(2+)-sensitive PKC53E activity uncovers a previously uncharacterized role of Drosophila serotonin neurons in alcohol sensitivity. The deficiency of PKC53E but not novel Ca(2+)-independent PKC98E appears to reduce synaptic serotonin levels, since acute inhibition of serotonin reuptake by citalopram and Prozac reversed alcohol insensitivity in flies expressing PKC53E double-stranded RNA in serotonin neurons. Together, findings from this and our previous studies indicate that PKC53E and PKC98E differentially regulate fly alcohol sensitivity through independent modulation of conserved serotonin and neuropeptide Y-like systems.

Inositol 1,4,5- trisphosphate receptor function in Drosophila insulin producing cells

The Inositol 1,4,5- trisphosphate receptor (InsP₃R) is an intracellular ligand gated channel that releases calcium from intracellular stores in response to extracellular signals. To identify and understand physiological processes and behavior that depends on the InsP₃ signaling pathway at a systemic level, we are studying Drosophila mutants for the InsP₃R (itpr) gene. Here, we show that growth defects precede larval lethality and both are a consequence of the inability to feed normally. Moreover, restoring InsP₃R function in insulin producing cells (IPCs) in the larval brain rescues the feeding deficit, growth and lethality in the itpr mutants to a significant extent. We have previously demonstrated a critical requirement for InsP₃R activity in neuronal cells, specifically in aminergic interneurons, for larval viability. Processes from the IPCs and aminergic domain are closely apposed in the third instar larval brain with no visible cellular overlap. Ubiquitous depletion of itpr by dsRNA results in feeding deficits leading to larval lethality similar to the itpr mutant phenotype. However, when itpr is depleted specifically in IPCs or aminergic neurons, the larvae are viable. These data support a model where InsP₃R activity in non-overlapping neuronal domains independently rescues larval itpr phenotypes by non-cell autonomous mechanisms.

Role of serotonergic neurons in the Drosophila larval response to light

Background

Drosophila larval locomotion consists of forward peristalsis interrupted by episodes of pausing, turning and exploratory behavior (head swinging). This behavior can be regulated by visual input as seen by light-induced increase in pausing, head swinging and direction change as well as reduction of linear speed that characterizes the larval photophobic response. During 3^rd instar stage, Drosophila larvae gradually cease to be repelled by light and are photoneutral by the time they wander in search for a place to undergo metamorphosis. Thus, Drosophila larval photobehavior can be used to study control of locomotion.

Results

We used targeted neuronal silencing to assess the role of candidate neurons in the regulation of larval photobehavior. Inactivation of DOPA decarboxylase (Ddc) neurons increases the response to light throughout larval development, including during the later stages of the 3^rd instar characterized by photoneutral response. Increased response to light is characterized by increase in light-induced direction change and associated pause, and reduction of linear movement. Amongst Ddc neurons, suppression of the activity of corazonergic and serotonergic but not dopaminergic neurons increases the photophobic response observed during 3^rd instar stage. Silencing of serotonergic neurons does not disrupt larval locomotion or the response to mechanical stimuli. Reduced serotonin (5-hydroxytryptamine, 5-HT) signaling within serotonergic neurons recapitulates the results obtained with targeted neuronal silencing. Ablation of serotonergic cells in the ventral nerve cord (VNC) does not affect the larval response to light. Similarly, disruption of serotonergic projections that contact the photoreceptor termini in the brain hemispheres does not impact the larval response to light. Finally, pan-neural over-expression of 5-HT1A_Dro receptors, but not of any other 5-HT receptor subtype, causes a significant decrease in the response to light of 3^rd instar larvae.

Conclusion

Our data demonstrate that activity of serotonergic and corazonergic neurons contribute to the control of larval locomotion by light. We conclude that this control is carried out by 5-HT neurons located in the brain hemispheres, but does not appear to occur at the photoreceptor level and may be mediated by 5-HT1A_Dro receptors. These findings provide new insights into the function of 5-HT neurons in Drosophila larval behavior as well as into the mechanisms underlying regulation of larval response to light.

Real-time measurements of synaptic autoinhibition produced by serotonin release in cultured leech neurons

We studied autoinhibition produced immediately after synaptic serotonin (5-HT) release in identified leech Retzius neurons, cultured singly or forming synapses onto pressure-sensitive neurons. Cultured Retzius neurons are isopotential, thus allowing accurate recordings of synaptic events using intracellular microelectrodes. The effects of autoinhibition on distant neuropilar presynaptic endings were predicted from model simulations. Following action potentials (APs), cultured neurons produced a slow hyperpolarization with a rise time of 85.4 +/- 5.2 ms and a half-decay time of 252 +/- 17.4 ms. These inhibitory postpotentials were reproduced by the iontophoretic application of 5-HT and became depolarizing after inverting the transmembranal chloride gradient by using microelectrodes filled with potassium chloride. The inhibitory postpotentials were reversibly abolished in the absence of extracellular calcium and absent in reserpine-treated neurons, suggesting an autoinhibition due to 5-HT acting on autoreceptors coupled to chloride channels. The autoinhibitory responses increased the membrane conductance and decreased subsequent excitability. Increasing 5-HT release by stimulating with trains of ten pulses at 10 or 30 Hz produced 23 +/- 6 and 47 +/- 2% of AP failures, respectively. These failures were reversibly abolished by the serotonergic antagonist methysergide (140 muM). Moreover, reserpine-treated neurons had only 5 +/- 4% of failures during trains at 10 Hz. This percentage was increased to 35 +/- 4% by iontophoretic application of 5-HT. Increases in AP failures correlated with smaller postsynaptic currents. Model simulations predicted that the autoinhibitory chloride conductance reduces the amplitude of APs arriving at neuropilar presynaptic endings. Altogether, our results suggest that 5-HT autoinhibits its subsequent release by decreasing the excitability of presynaptic endings within the same neuron.

Quantitative evaluation of serotonin release and clearance in Drosophila

Serotonin signaling plays a key role in the regulation of development, mood and behavior. Drosophila is well suited for the study of the basic mechanisms of serotonergic signaling, but the small size of its nervous system has previously precluded the direct measurements of neurotransmitters. This study demonstrates the first real-time measurements of changes in extracellular monoamine concentrations in a single larval Drosophila ventral nerve cord. Channelrhodopsin-2-mediated, neuronal type-specific stimulation is used to elicit endogenous serotonin release, which is detected using fast-scan cyclic voltammetry at an implanted microelectrode. Release is decreased when serotonin synthesis or packaging are pharmacologically inhibited, confirming that the detected substance is serotonin. Similar to tetanus-evoked serotonin release in mammals, evoked serotonin concentrations are 280-640nM in the fly, depending on the stimulation length. Extracellular serotonin signaling is prolonged after administering cocaine or fluoxetine, showing that transport regulates the clearance of serotonin from the extracellular space. When ChR2 is targeted to dopaminergic neurons, dopamine release is measured demonstrating that this method is broadly applicable to other neurotransmitter systems. This study shows that the dynamics of serotonin release and reuptake in Drosophila are analogous to those in mammals, making this simple organism more useful for the study of the basic physiological mechanisms of serotonergic signaling.

Relationship between circadian typology and big five personality domains

We explored the relationship between personality, based on the five-factor model, and circadian preference. To this end, 503 participants (280 females, 223 males) were administered the Morningness-Eveningness Questionnaire (MEQ) and the self-report version of the Big Five Observer (BFO) to determine circadian preference and personality features, respectively. Morning types scored significantly higher than evening and intermediate types on the conscientiousness factor. Evening types were found to be more neurotic than morning types. With reference to the big five personality model, our data, together with those of all the previous studies, indicate that the conscientiousness domain is the one that best discriminates among the three circadian types. Results are discussed with reference to neurobiological models of personality.

Serotonin 5-HT(2) and 5-HT(1A)-like receptors differentially modulate aggressive behaviors in Drosophila melanogaster

Aggressive behavior is widespread throughout the animal kingdom, and is a complex social behavior influenced by both genetics and environment. Animals typically fight over resources that include food, territory, and sexual partners. Of all the neurotransmitters, serotonin (5-HT) has been the most implicated in modulating aggressive behaviors in mammalian systems. In the fruit fly, Drosophila melanogaster, the involvement of 5-HT itself in aggressive behaviors has been recently established, however, the underlying mechanisms have largely remained elusive. Here we describe the influence of different 5-HT receptor subtypes on aggressive behaviors in Drosophila. Drosophila express homologs of three mammalian 5-HT receptors: the 5-HT(1A), 5-HT(2), and 5-HT(7) receptors. Significantly, these receptors mediate important behaviors in mammalian systems ranging from feeding, aggression, and sleep, to cognition. To examine the role of the 5-HT(2)Dro receptor, we utilized the selective 5-HT(2) receptor agonist (R)-1-[2,5-dimethoxy-4-iodophenyl]-2-aminopropane (DOI), and the 5-HT(2) receptor antagonist, ketanserin. To examine the role of 5-HT(1A)-like receptors we used the 5-HT(1A) receptor agonist 8-hydroxy-2-dipropylaminotetralin hydrobromide (8-OH-DPAT), and the 5-HT(1A) receptor antagonist WAY100635. We find that activation of 5-HT(2) receptors with (R)-DOI appears to decrease overall aggression, whereas activation of 5-HT(1A)-like receptors with 8-OH-DPAT increases overall aggression. Furthermore, the different 5-HT receptor circuitries appear to mediate different aspects of aggression: 5-HT(2) receptor manipulation primarily alters lunging and boxing, whereas 5-HT(1A)-like receptor manipulation primarily affects wing threats and fencing. Elucidating the effects of serotonergic systems on aggression in the fly is a significant advancement not only in establishing the fly as a system to study aggression, but as a system relevant to elucidating molecular mechanisms underlying aggression in mammals, including humans.

Genetic distortion of the balance between punishment and relief learning in Drosophila

An experience with electric shock can support two opposing kinds of behavioral effects: Stimuli that precede shock during training are subsequently avoided as predictors for punishment, whereas stimuli that follow shock during training are later on approached, as they predict relief. We show here, for the fruit fly Drosophila, that upon the loss of white-function, the balance between these two kinds of learning is distorted in favor of punishment learning: white1118 mutants show stronger punishment learning and weaker relief learning, as compared to wild type flies. Thus, white1118 mutants establish, overall, more "negative" memories for the shock experience. This only concerns the mnemonic effects of the shock; the immediate, reflexive responsiveness to shock remains unaltered. Also, learning about reward is apparently unaffected, both in adult and larval Drosophila. Prompted by the proposed function of the White protein as the transporter for biogenic amine precursors, we probed the brains of white1118 mutants for the amounts of biogenic amines (octopamine, tyramine, dopamine, and serotonin) by using high-pressure liquid chromatography coupled to mass spectrometry. Using this method, we found, however, no difference between white1118 and wild type files for any of the probed amines. In any event, analyses of how the white1118 mutation affects the balance between punishment and relief learning should provide a study case of how heritable distortions of such balance can come about. Finally, the effects of the white1118 mutation should be considered as a source of confound when using white as the "marker gene" in behavior-genetic analyses of any sort.

Immunocytochemical localization of serotonin in the central and peripheral chemosensory system of mosquitoes

Female mosquitoes depend on blood to complete their reproductive cycle and rely mainly on chemosensory systems to obtain blood meals. An immunocytochemical analysis reveals a number of serotonin-immunoreactive neurons that innervate the chemosensory systems, suggesting a potential role of serotonin in modulating chemosensory processes. In the primary olfactory system, we identify a single ipsilateral centrifugal neuron with arborizations in higher brain centers; the varicosities of this neuron display volumetric changes in response to both blood feeding and during a circadian rhythm. Six to eight pairs of serotonin-immunoreactive neurons are identified in the primary gustatory neuropil, including the subesophageal ganglion and tritocerebrum. The peripheral chemosensory organs, i.e. the antenna, the maxillary palp and the labium, are described as having extensive serotonergic neurohemal plexi. In addition, we describe the presence of serotonin-immunoreactive fibers in the mechanosensory Johnston's organ. Taking these results together, we discuss the potential role of serotonin as a neuromodulator in the chemosensory system of disease vector mosquitoes.

Identification of novel genes involved in light-dependent CRY degradation through a genome-wide RNAi screen

Circadian clocks regulate many different physiological processes and synchronize these to environmental light:dark cycles. In Drosophila, light is transmitted to the clock by a circadian blue light photoreceptor CRYPTOCHROME (CRY). In response to light, CRY promotes the degradation of the circadian clock protein TIMELESS (TIM) and then is itself degraded. To identify novel genes involved in circadian entrainment, we performed an unbiased genome-wide screen in Drosophila cells using a sensitive and quantitative assay that measures light-induced degradation of CRY. We systematically knocked down the expression of approximately 21,000 genes and identified those that regulate CRY stability. These genes include ubiquitin ligases, signal transduction molecules, and redox molecules. Many of the genes identified in the screen are specific for CRY degradation and do not affect degradation of the TIM protein in response to light, suggesting that, for the most part, these two pathways are distinct. We further validated the effect of three candidate genes on CRY stability in vivo by assaying flies mutant for each of these genes. This work identifies a novel regulatory network involved in light-dependent CRY degradation and demonstrates the power of a genome-wide RNAi approach for understanding circadian biology.

Spatial-specific action of serotonin within the leech midbody ganglion

Serotonin is a conspicuous neuromodulator in the nervous system of many vertebrates and invertebrates. In previous experiments performed in the leech nervous system, we compared the effect of the amine released from endogenous sources [using selective serotonin reuptake inhibitors (SSRIs), e.g. fluoxetine] with that of bath-applied serotonin. The results suggested that the amine does not reach all its targets in a uniform way, but produces the activation of an interneuronal pathway that generated specific synaptic responses on different neurons. Taking into account that the release of the amine is often regulated at the presynaptic level, we have investigated whether autoreceptor antagonists mimic the SSRIs effect. We found that methiothepin (100 microM) produced similar effects than fluoxetine. To further test the hypothesis that endogenous serotonin produce its effect by acting locally at specific sites, we analyzed the effect of iontophoretic applications of serotonin. We found a site in the neuropil of the leech ganglia where serotonin application mimicked the effect of the SSRIs and the 5-HT antagonist. The results further support the view that the effect of serotonin exhibits a spatial specificity that can be relevant to understand its modulatory actions.

Neuroarchitecture of aminergic systems in the larval ventral ganglion of Drosophila melanogaster

Biogenic amines are important signaling molecules in the central nervous system of both vertebrates and invertebrates. In the fruit fly Drosophila melanogaster, biogenic amines take part in the regulation of various vital physiological processes such as feeding, learning/memory, locomotion, sexual behavior, and sleep/arousal. Consequently, several morphological studies have analyzed the distribution of aminergic neurons in the CNS. Previous descriptions, however, did not determine the exact spatial location of aminergic neurite arborizations within the neuropil. The release sites and pre-/postsynaptic compartments of aminergic neurons also remained largely unidentified. We here used gal4-driven marker gene expression and immunocytochemistry to map presumed serotonergic (5-HT), dopaminergic, and tyraminergic/octopaminergic neurons in the thoracic and abdominal neuromeres of the Drosophila larval ventral ganglion relying on Fasciclin2-immunoreactive tracts as three-dimensional landmarks. With tyrosine hydroxylase- (TH) or tyrosine decarboxylase 2 (TDC2)-specific gal4-drivers, we also analyzed the distribution of ectopically expressed neuronal compartment markers in presumptive dopaminergic TH and tyraminergic/octopaminergic TDC2 neurons, respectively. Our results suggest that thoracic and abdominal 5-HT and TH neurons are exclusively interneurons whereas most TDC2 neurons are efferent. 5-HT and TH neurons are ideally positioned to integrate sensory information and to modulate neuronal transmission within the ventral ganglion, while most TDC2 neurons appear to act peripherally. In contrast to 5-HT neurons, TH and TDC2 neurons each comprise morphologically different neuron subsets with separated in- and output compartments in specific neuropil regions. The three-dimensional mapping of aminergic neurons now facilitates the identification of neuronal network contacts and co-localized signaling molecules, as exemplified for DOPA decarboxylase-synthesizing neurons that co-express crustacean cardioactive peptide and myoinhibiting peptides.

Probing the relative importance of molecular oscillations in the circadian clock

Circadian ( approximately 24 hr) rhythms of behavior and physiology are driven by molecular clocks that are endogenous to most organisms. The mechanisms underlying these clocks are remarkably conserved across evolution and typically consist of auto-regulatory loops in which specific proteins (clock proteins) rhythmically repress expression of their own genes. Such regulation maintains 24-hr cycles of RNA and protein expression. Despite the conservation of these mechanisms, however, questions are now being raised about the relevance of different molecular oscillations. Indeed, several studies have demonstrated that oscillations of some critical clock genes can be eliminated without loss of basic clock function. Here, we describe the multiple levels at which clock gene/protein expression and function can be rhythmically regulated-transcription, protein expression, post-translational modification, and localization-and speculate as to which aspect of this regulation is most critical. While the review is focused on Drosophila, we include some discussion of mammalian clocks to indicate the extent to which the questions concerning clock mechanisms are similar, regardless of the organism under study.

Organization of the Drosophila circadian control circuit

Molecular genetics has revealed the identities of several components of the fundamental circadian molecular oscillator - an evolutionarily conserved molecular mechanism of transcription and translation that can operate in a cell-autonomous manner. Therefore, it was surprising when studies of circadian rhythmic behavior in the fruit fly Drosophila suggested that the normal operations of circadian clock cells, which house the molecular oscillator, in fact depend on non-cell-autonomous effects - interactions between the clock cells themselves. Here we review several genetic analyses that broadly extend that viewpoint. They support a model whereby the approximately 150 circadian clock cells in the brain of the fly are sub-divided into functionally discrete rhythmic centers. These centers alternatively cooperate or compete to control the different episodes of rhythmic behavior that define the fly's daily activity profile.

Role of GSK3 beta in behavioral abnormalities induced by serotonin deficiency

Dysregulation of brain serotonin (5-HT) neurotransmission is thought to underlie mental conditions as diverse as depression, anxiety disorders, bipolar disorder, autism, and schizophrenia. Despite treatment of these conditions with serotonergic drugs, the molecular mechanisms by which 5-HT is involved in the regulation of aberrant emotional behaviors are poorly understood. Here, we generated knockin mice expressing a mutant form of the brain 5-HT synthesis enzyme, tryptophan hydroxylase 2 (Tph2). This mutant is equivalent to a rare human variant (R441H) identified in few individuals with unipolar major depression. Expression of mutant Tph2 in mice results in markedly reduced ( approximately 80%) brain 5-HT production and leads to behavioral abnormalities in tests assessing 5-HT-mediated emotional states. This reduction in brain 5-HT levels is accompanied by activation of glycogen synthase kinase 3beta (GSK3beta), a signaling molecule modulated by many psychiatric therapeutic agents. Importantly, inactivation of GSK3beta in Tph2 knockin mice, using pharmacological or genetic approaches, alleviates the aberrant behaviors produced by 5-HT deficiency. These findings establish a critical role of Tph2 in the maintenance of brain serotonin homeostasis and identify GSK3beta signaling as an important pathway through which brain 5-HT deficiency induces abnormal behaviors. Targeting GSK3beta and related signaling events may afford therapeutic advantages for the management of certain 5-HT-related psychiatric conditions.

Serotonin modulation of moth central olfactory neurons

In the tobacco hornworm, Manduca sexta, 5-hydroxytryptamine (5HT) acting at the level of the antennal lobes contributes significantly to changing the moth's responsiveness to olfactory stimuli. 5HT targets K(+) conductances in the cells, increasing the excitability of central olfactory neurons and their responsiveness to olfactory cues. Effects of 5HT modulation are apparent not only at the single cell level, but also in the activity patterns of populations of neurons that convey olfactory information from antennal lobes to higher centers of the brain. Evidence suggests that 5HT-induced changes in activity within neural circuits of the antennal lobes might also drive structural plasticity, providing the basis for longer-term changes in antennal lobe function.

Post-translational regulation of the Drosophila circadian clock requires protein phosphatase 1 (PP1)

Phosphorylation is an important timekeeping mechanism in the circadian clock that has been closely studied at the level of the kinases involved but may also be tightly controlled by phosphatase action. Here we demonstrate a role for protein phosphatase 1 (PP1) in the regulation of the major timekeeping molecules in the Drosophila clock, TIMELESS (TIM) and PERIOD (PER). Flies with reduced PP1 activity exhibit a lengthened circadian period, reduced amplitude of behavioral rhythms, and an altered response to light that suggests a defect in the rising phase of clock protein expression. On a molecular level, PP1 directly dephosphorylates TIM and stabilizes it in both S2R(+) cells and clock neurons. However, PP1 does not act in a simple antagonistic manner to SHAGGY (SGG), the kinase that phosphorylates TIM, because the behavioral phenotypes produced by inhibiting PP1 in flies are different from those achieved by overexpressing SGG. PP1 also acts on PER, and TIM regulates the control of PER by PP1, although it does not affect PP2A action on PER. We propose a modified model for post-translational regulation of the Drosophila clock, in which PP1 is critical for the rhythmic abundance of TIM/PER while PP2A also regulates the nuclear translocation of TIM/PER.

5-HT2 receptors in Drosophila are expressed in the brain and modulate aspects of circadian behaviors

Dysregulation of 5-HT(2) receptor function has been strongly implicated in many neuropsychiatric disorders, including schizophrenia. At present, the molecular mechanisms linking 5-HT(2) receptor activation to behaviors is not well understood. In efforts to elucidate these processes, the fruit fly, Drosophila melanogaster, is proposed to serve as a powerful genetically tractable model organism to study 5-HT(2) receptor function. Data are presented here on the expression of the fly ortholog of the mammalian 5-HT(2) receptor, 5-HT(2)Dro, in the larval and adult brain of the fly, and on the involvement of these circuits in certain circadian behaviors. In the adult brain, 5-HT(2)Dro is expressed in the protocerebrum and ellipsoid body, areas believed to participate in higher order behaviors including learning, locomotion, and sensory perception. In the third instar larva, 5-HT(2)Dro receptor expression is detected in a specific pattern that markedly changes from early to late third instar. To probe the function of this receptor we have examined the effects of the 5-HT(2) receptor-specific agonist DOI in wild type and 5-HT(2)Dro hypomorphic flies on circadian behaviors. DOI was found to increase early day activity, eliminate anticipatory behavior, and reduce viability. The effects of DOI were significantly diminished in a 5-HT(2)Dro hypomorphic strain. Identifying the 5-HT(2)Dro receptor circuitry and behaviors they mediate are significant steps towards developing this model system to study conserved molecular mechanisms underlying behaviors mediated by 5-HT(2) receptors in mammalian systems.

Serotonin and neuropeptide F have opposite modulatory effects on fly aggression

Both serotonin (5-HT) and neuropeptide Y have been shown to affect a variety of mammalian behaviors, including aggression. Here we show in Drosophila melanogaster that both 5-HT and neuropeptide F, the invertebrate homolog of neuropeptide Y, modulate aggression. We show that drug-induced increases of 5-HT in the fly brain increase aggression. Elevating 5-HT genetically in the serotonergic circuits recapitulates these pharmacological effects, whereas genetic silencing of these circuits makes the flies behaviorally unresponsive to the drug-induced increase of 5-HT but leaves them capable of aggression. Genetic silencing of the neuropeptide F (npf) circuit also increases fly aggression, demonstrating an opposite modulation to 5-HT. Moreover, this neuropeptide F effect seems to be independent of 5-HT. The implication of these two modulatory systems in fly and mouse aggression suggest a marked degree of conservation and a deep molecular root for this behavior.

The Drosophila Circadian Network Is a Seasonal Timer

Previous work in Drosophila has defined two populations of circadian brain neurons, morning cells (M-cells) and evening cells (E-cells), both of which keep circadian time and regulate morning and evening activity, respectively. It has long been speculated that a multiple oscillator circadian network in animals underlies the behavioral and physiological pattern variability caused by seasonal fluctuations of photoperiod. We have manipulated separately the circadian photoentrainment pathway within E- and M-cells and show that E-cells process light information and function as master clocks in the presence of light. M-cells in contrast need darkness to cycle autonomously and dominate the network. The results indicate that the network switches control between these two centers as a function of photoperiod. Together with the different entraining properties of the two clock centers, the results suggest that the functional organization of the network underlies the behavioral adjustment to variations in daylength and season.

Clock genes beyond the clock: CLOCK genotype biases neural correlates of moral valence decision in depressed patients

Gene polymorphisms in the mammalian biological clock system influence individual rhythms. A single nucleotide polymorphism (SNP) in the 3' flanking region of CLOCK (3111 T/C; rs1801260) influenced diurnal preference in healthy humans and caused sleep phase delay and insomnia in patients affected by bipolar disorder. Genes of the biological clock are expressed in many brain structures other than in the 'master clock' suprachiasmatic nuclei. These areas, such as cingulate cortex, are involved in the control of many human behaviors. Clock genes could then bias 'nonclock' functions such as information processing and decision making. Thirty inpatients affected by a major depressive episode underwent blood oxygen-level dependent (BOLD) functional magnetic resonance imaging (fMRI). The cognitive activation paradigm was based on a go/no-go task. Morally connoted words were presented. Genotyping of CLOCK was performed for each patients. We measured activity levels through actimetry during the day before the fMRI study. CLOCK 3111 T/C SNP was associated with activity levels in the second part of the day, neuropsychological performance and BOLD fMRI correlates (interaction of genotype and moral valence of the stimuli). Our results support the hypothesis that individual clock genotype may influence several variables linked with human behaviors in normal and psychopathological conditions.

Development and morphology of the clock-gene-expressing lateral neurons of Drosophila melanogaster

The clock-gene-expressing lateral neurons are essential for the locomotor activity rhythm of Drosophila melanogaster. Traditionally, these neurons are divided into three groups: the dorsal lateral neurons (LN(d)), the large ventral lateral neurons (l-LN(v)), and the small ventral lateral neurons (s-LN(v)), whereby the latter group consists of four neurons that express the neuropeptide pigment-dispersing factor (PDF) and a fifth PDF-negative neuron. So far, only the l-LN(v) and the PDF-positive s-LN(v) have been shown to project into the accessory medulla, a small neuropil that contains the circadian pacemaker center in several insects. We show here that the other lateral neurons also arborize in the accessory medulla, predominantly forming postsynaptic sites. Both the l-LN(v) and LN(d) are anatomically well suited to connect the accessory medullae. Whereas the l-LN(v) may receive ipsilateral photic input from the Hofbauer-Buchner eyelet, the LN(d) invade mainly the contralateral accessory medulla and thus may receive photic input from the contralateral side. Both the LN(d) and the l-LN(v) differentiate during midmetamorphosis. They do so in close proximity to one another and the fifth PDF-negative s-LN(v), suggesting that these cell groups may derive from common precursors.

What is there left to learn about the Drosophila clock?

Circadian rhythms offer probably the best understanding of how genes control behavior, and much of this understanding has come from studies in Drosophila. More recently, genetic manipulation of clock neurons in Drosophila has helped identify how daily patterns of activity are programmed by different clock neuron groups. Here, we review some of the more recent findings on the fly molecular clock and ask what more the fly model can offer to circadian biologists.

Drosophila melanogaster neurobiology, neuropharmacology, and how the fly can inform central nervous system drug discovery

Central nervous system (CNS) drug discovery in the post-genomic era is rapidly evolving. Older empirical methods are giving way to newer technologies that include bioinformatics, structural biology, genetics, and modern computational approaches. In the search for new medical therapies, and in particular treatments for disorders of the central nervous system, there has been increasing recognition that identification of a single biological target is unlikely to be a recipe for success; a broad perspective is required. Systems biology is one such approach, and has been increasingly recognized as a very important area of research, as it places specific molecular targets within a context of overall biochemical action. Understanding the complex interactions between the components within a given biological system that lead to modifications in output, such as changes in behavior or development, may be important avenues of discovery to identify new therapies. One avenue to drug discovery that holds tremendous potential is the use of model genetic organisms such as the fruit fly, Drosophila melanogaster. The similarity between mode of drug action, behavior, and gene response in D. melanogaster and mammalian systems, combined with the power of genetics, have recently made the fly a very attractive system to study fundamental neuropharmacological processes relevant to human diseases. The promise that the use of model organisms such as the fly offers is speed, high throughput, and dramatically reduced overall costs that together should result in an enhanced rate of discovery.

Influences of octopamine and juvenile hormone on locomotor behavior and period gene expression in the honeybee, Apis mellifera

Octopamine (OA) and juvenile hormone (JH) are implicated in the regulation of age-based division of labor in the honeybee, Apis mellifera. We tested the hypothesis that these two neuroendocrine signals influence task-associated plasticity in circadian and diurnal rhythms, and in brain expression of the clock gene period (per). Treatment with OA, OA antagonist (epinastine), or both, did not affect the age at onset of circadian rhythmicity or the free running period in constant darkness (DD). Young bees orally treated with OA in light-dark (LD) illumination regime for 6 days followed by DD showed reduced alpha (the period between the daily onset and offset of activity) during the first 4 days in LD and the first 4 days in DD. Oral treatment with OA, epinastine, or both, but not manipulations of JH levels, caused increased average daily levels and aberrant patterns of brain per mRNA oscillation in young bees. These results suggest that OA and JH do not influence the development or function of the central pacemaker but rather that OA influences the brain expression of a clock gene and characteristics of locomotor behavior that are not thought to be under direct control of the circadian pacemaker.

Regulating a Circadian Clock's Period, Phase and Amplitude by Phosphorylation: Insights from Drosophila

Much progress has been made in understanding the molecular underpinnings governing circadian ( approximately 24 h) rhythms. Despite the increased complexity in metazoans whereby inter-cellular networks form the basis for driving overt rhythms, such as wake-sleep cycles in animals, single isolated cells can exhibit all the formal properties of a circadian pacemaker. How do these cell-autonomous rhythm generators operate? Breakthrough studies in Drosophila melanogaster led to the realization that the molecular logic underlying circadian clocks are highly shared. Most notably, interconnected transcriptional-translational feedback loops produce coordinated rhythms in "clock" RNAs and proteins that are required for the daily progression of clocks, synchronization to local time and transducing temporal signals to downstream effector pathways. More recent findings indicate prominent roles for reversible phosphorylation of clock proteins in the core oscillatory mechanism. In this review we focus on findings in Drosophila to explore the multiple levels that reversible phosphorylation plays in clock function. Specific clock proteins in this system are subjected to different phosphorylation programs, which affect three key properties of a circadian oscillator, its period, amplitude and phase. The role of phosphorylation in clocks is of clear relevance to human health because mutations that affect the PERIOD (PER) phosphorylation program are associated with familial sleep disorders. In addition, the central role of phosphorylation in the assembly of a circadian oscillator was dramatically shown recently by the ability to reconstitute a circadian phosphorylation/dephosphorylation cycle in vitro, suggesting that the dynamics of clock protein phosphorylation are at the "heart" of circadian time-keeping.

Phylogeny of a serotonin-immunoreactive neuron in the primary olfactory center of the insect brain

Serotonin (5-hydroxytryptamine; 5HT) functions in insects as a neurotransmitter, neuromodulator, and neurohormone. In the sphinx moth Manduca sexta, each of the paired antennal lobes (ALs; the primary olfactory centers in the insect brain) has one 5HT-immunoreactive (5HT-ir) neuron that projects into the protocerebrum, crosses the posterior midline, and innervates the contralateral AL; this is referred to as the contralaterally projecting, serotonin-immunoreactive deutocerebral (CSD) neuron. These neurons are thought to function as centrifugal modulators of olfactory sensitivity. To examine the phylogenetic distribution of 5HT-ir neurons apparently homologous to the CSD neuron, we imaged 5HT-like immunoreactivity in the brains of 40 species of insects belonging to 38 families in nine orders. CSD neurons were found in other Lepidoptera, Trichoptera, Diptera, Coleoptera, and Neuroptera but not in the Hymenoptera. In the paraneopteran and polyneopteran species (insects that undergo incomplete metamorphosis) examined, AL 5HT neurons innervate the ispsilateral AL and project to the protocerebrum. Our findings suggest that the characteristic morphology of the CSD neurons originated in the holometabolous insects (those that undergo complete metamorphosis) and were lost in the Hymenoptera. In a subset of the Diptera, the CSD neurons branch within the contralateral AL and project back to the ipsilateral AL via the antennal commissure. The evolution of AL 5HT neurons is discussed in the context of the physiological actions of 5HT observed in the lepidopteran AL.

Fluoxetine modulates the circadian biological clock via phase advances of suprachiasmatic nucleus neuronal firing

BACKGROUND

The documented ability of serotonin (5-HT) to directly modulate circadian rhythms prompted interest in a similar role for therapeutic agents that readily enhance 5-HT neurotransmission, namely the selective serotonin reuptake inhibitors (SSRIs).

METHODS

Extracellular recordings of unit firing of suprachiasmatic nucleus (SCN) neurons maintained in slice culture enabled determinations of circadian rhythmicity. Shifts in the peak of activity were determined during the next circadian cycle following drug exposure.

RESULTS

Fluoxetine (10 microm, 60 minutes incubation) produced robust phase advances only in the presence of L-tryptophan (.5 microm), added to maintain serotonergic tone.

CONCLUSIONS

Actions of SSRIs at the level of the circadian biological clock add to the list of pharmacological effects for this drug class and encourage speculation as to their importance clinically.

A Sleep-Promoting Role for the Drosophila Serotonin Receptor 1A

BACKGROUND

Although sleep is an important process essential for life, its regulation is poorly understood. The recently developed Drosophila model for sleep provides a powerful system to genetically and pharmacologically identify molecules that regulate sleep. Serotonin is an important neurotransmitter known to affect many behaviors, but its role in sleep remains controversial.

RESULTS

We generated or obtained flies with genetically altered expression of each of three Drosophila serotonin receptor subtypes (d5-HT1A, d5-HT1B, and d5-HT2) and assayed them for baseline sleep phenotypes. The data indicated a sleep-regulating role for the d5-HT1A receptor. d5-HT1A mutant flies had short and fragmented sleep, which was rescued by expressing the receptor in adult mushroom bodies, a structure associated with learning and memory in Drosophila. Neither the d5-HT2 receptor nor the d5-HT1B receptor, which was previously implicated in circadian regulation, had any effect on baseline sleep, indicating that serotonin affects sleep and circadian rhythms through distinct receptors. Elevating serotonin levels, either pharmacologically or genetically, enhanced sleep in wild-type flies. In addition, serotonin promoted sleep in some short-sleep mutants, suggesting that it can compensate for some sleep deficits.

CONCLUSIONS

These data show that serotonin promotes baseline sleep in Drosophila. They also link the regulation of sleep behavior by serotonin to a specific receptor in a distinct region of the fly brain.

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.

Neural circuits underlying circadian behavior in Drosophila melanogaster

Circadian clocks include control systems for organizing daily behavior. Such a system consists of a time-keeping mechanism (the clock or pacemaker), input pathways for entraining the clock, and output pathways for producing overt rhythms in behavior and physiology. In Drosophila melanogaster, as in mammals, neural circuits play vital roles in all three functional subdivisions of the circadian system. Regarding the pacemaker, multiple clock neurons, each with cell-autonomous pacemaker capability, are coupled to each other in a network. The outputs of different sets of clock neurons in this network combine to produce the normal bimodal pattern of locomotor activity observed in Drosophila. Regarding input, multiple sensory modalities (including light, temperature, and pheromones) use their own circuitry to entrain the clock. Regarding output, distinct circuits are likely involved for controlling the timing of eclosion and for generating the locomotor activity rhythms. This review summarizes work on all of these circadian circuits, and discusses the broader utility of studying the fly's circadian system.