The Wuerzburg hybridoma library against Drosophila brain

Transgenic line for characterizing GABA-receptor expression to study the neural basis of olfaction in the yellow-fever mosquito

The mosquito Aedes aegypti is an important vector of diseases including dengue, Zika, chikungunya, and yellow fever. Olfaction is a critical modality for mosquitoes enabling them to locate hosts, sources of nectar, and sites for oviposition. GABA is an essential neurotransmitter in olfactory processing in the insect brain, including the primary olfactory center, the antennal lobe. Previous work with Ae. aegypti has suggested that antennal lobe inhibition via GABA may be involved in the processing of odors. However, little is known about GABA receptor expression in the mosquito brain, or how they may be involved in odor attraction. In this context, generating mutants that target the mosquito's olfactory responses, and particularly the GABAergic system, is essential to achieve a better understanding of these diverse processes and olfactory coding in these disease vectors. Here we demonstrate the potential of a transgenic line using the QF2 transcription factor, GABA-B1QF2-ECFP, as a new neurogenetic tool to investigate the neural basis of olfaction in Ae. aegypti. Our results show that the gene insertion has a moderate impact on mosquito fitness. Moreover, the line presented here was crossed with a QUAS reporter line expressing the green fluorescent protein and used to determine the location of the metabotropic GABA-B1 receptor expression. We find high receptor expression in the antennal lobes, especially the cell bodies surrounding the antennal lobes. In the mushroom bodies, receptor expression was high in the Kenyon cells, but had low expression in the mushroom body lobes. Behavioral experiments testing the fruit odor attractants showed that the mutants lost their behavioral attraction. Together, these results show that the GABA-B1QF2-ECFP line provides a new tool to characterize GABAergic systems in the mosquito nervous system.

An atlas of the developing Tribolium castaneum brain reveals conservation in anatomy and divergence in timing to Drosophila melanogaster

Insect brains are formed by conserved sets of neural lineages whose fibers form cohesive bundles with characteristic projection patterns. Within the brain neuropil, these bundles establish a system of fascicles constituting the macrocircuitry of the brain. The overall architecture of the neuropils and the macrocircuitry appear to be conserved. However, variation is observed, for example, in size, shape, and timing of development. Unfortunately, the developmental and genetic basis of this variation is poorly understood, although the rise of new genetically tractable model organisms such as the red flour beetle Tribolium castaneum allows the possibility to gain mechanistic insights. To facilitate such work, we present an atlas of the developing brain of T. castaneum, covering the first larval instar, the prepupal stage, and the adult, by combining wholemount immunohistochemical labeling of fiber bundles (acetylated tubulin) and neuropils (synapsin) with digital 3D reconstruction using the TrakEM2 software package. Upon comparing this anatomical dataset with the published work in Drosophila melanogaster, we confirm an overall high degree of conservation. Fiber tracts and neuropil fascicles, which can be visualized by global neuronal antibodies like antiacetylated tubulin in all invertebrate brains, create a rich anatomical framework to which individual neurons or other regions of interest can be referred to. The framework of a largely conserved pattern allowed us to describe differences between the two species with respect to parameters such as timing of neuron proliferation and maturation. These features likely reflect adaptive changes in developmental timing that govern the change from larval to adult brain.

Sexually dimorphic peripheral sensory neurons regulate copulation duration and persistence in male Drosophila

Peripheral sensory neurons are the gateway to the environment across species. In Drosophila, olfactory and gustatory senses are required to initiate courtship, as well as for the escalation of courtship patterns that lead to copulation. To be successful, copulation must last long enough to ensure the transfer of sperm and seminal fluid that ultimately leads to fertilization. The peripheral sensory information required to regulate copulation duration is unclear. Here, we employed genetic manipulations that allow driving gene expression in the male genitalia as a tool to uncover the role of these genitalia specific neurons in copulation. The fly genitalia contain sex-specific bristle hairs innervated by mechanosensory neurons. To date, the role of the sensory information collected by these peripheral neurons in male copulatory behavior is unknown. We confirmed that these MSNs are cholinergic and co-express both fru and dsx. We found that the sensory information received by the peripheral sensory neurons from the front legs (GRNs) and mechanosensory neurons (MSNs) at the male genitalia contribute to the regulation of copulation duration. Moreover, our results show that their function is required for copulation persistence, which ensures copulation is undisrupted in the presence of environmental stress before sperm transfer is complete.

Endocrine signals fine-tune daily activity patterns in Drosophila

Animals need to balance competitive behaviors to maintain internal homeostasis. The underlying mechanisms are complex but typically involve neuroendocrine signaling. Using Drosophila, we systematically manipulated signaling between energy-mobilizing endocrine cells producing adipokinetic hormone (AKH), octopaminergic neurons, and the energy-storing fat body to assess whether this neuroendocrine axis involved in starvation-induced hyperactivity also balances activity levels under ad libitum access to food. Our results suggest that AKH signals via two divergent pathways that are mutually competitive in terms of activity and rest. AKH increases activity via the octopaminergic system during the day, while it prevents high activity levels during the night by signaling to the fat body. This regulation involves feedback signaling from octopaminergic neurons to AKH-producing cells (APCs). APCs are known to integrate a multitude of metabolic and endocrine signals. Our results add a new facet to the versatile regulatory functions of APCs by showing that their output contributes to shape the daily activity pattern under ad libitum access to food.

Neuropeptides as potential modulators of behavioral transitions in the ant Cataglyphis nodus

Age-related behavioral plasticity is a major prerequisite for the ecological success of insect societies. Although ecological aspects of behavioral flexibility have been targeted in many studies, the underlying intrinsic mechanisms controlling the diverse changes in behavior along the individual life history of social insects are not completely understood. Recently, the neuropeptides allatostatin-A, corazonin, and tachykinin have been associated with the regulation of behavioral transitions in social insects. Here, we investigated changes in brain localization and expression of these neuropeptides following major behavioral transitions in Cataglyphis nodus ants. Our immunohistochemical analyses in the brain revealed that the overall branching pattern of neurons immunoreactive (ir) for the three neuropeptides is largely independent of the behavioral stages. Numerous allatostatin-A- and tachykinin-ir neurons innervate primary sensory neuropils and high-order integration centers of the brain. In contrast, the number of corazonergic neurons is restricted to only four neurons per brain hemisphere with cell bodies located in the pars lateralis and axons extending to the medial protocerebrum and the retrocerebral complex. Most interestingly, the cell-body volumes of these neurons are significantly increased in foragers compared to freshly eclosed ants and interior workers. Quantification of mRNA expression levels revealed a stage-related change in the expression of allatostatin-A and corazonin mRNA in the brain. Given the presence of the neuropeptides in major control centers of the brain and the neurohemal organs, these mRNA-changes strongly suggest an important modulatory role of both neuropeptides in the behavioral maturation of Cataglyphis ants.

Compartment and cell-type specific hypoxia responses in the developing Drosophila brain

Environmental factors such as the availability of oxygen are instructive cues that regulate stem cell maintenance and differentiation. We used a genetically encoded biosensor to monitor the hypoxic state of neural cells in the larval brain of Drosophila. The biosensor reveals brain compartment and cell-type specific levels of hypoxia. The values correlate with differential tracheolation that is observed throughout development between the central brain and the optic lobe. Neural stem cells in both compartments show the strongest hypoxia response while intermediate progenitors, neurons and glial cells reveal weaker responses. We demonstrate that the distance between a cell and the next closest tracheole is a good predictor of the hypoxic state of that cell. Our study indicates that oxygen availability appears to be the major factor controlling the hypoxia response in the developing Drosophila brain and that cell intrinsic and cell-type specific factors contribute to modulate the response in an unexpected manner.

This article has an associated First Person interview with the first author of the paper.

Summary: A fluorescent biosensor reveals cell type specific hypoxia levels in the Drosophila brain in unprecedented detail. It paves the way for further functional studies addressing the role of oxygen in neural stem cell maintenance and differentiation.

The Organization of the Second Optic Chiasm of the Drosophila Optic Lobe

Visual pathways from the compound eye of an insect relay to four neuropils, successively the lamina, medulla, lobula, and lobula plate in the underlying optic lobe. Among these neuropils, the medulla, lobula, and lobula plate are interconnected by the complex second optic chiasm, through which the anteroposterior axis undergoes an inversion between the medulla and lobula. Given their complex structure, the projection patterns through the second optic chiasm have so far lacked critical analysis. By densely reconstructing axon trajectories using a volumetric scanning electron microscopy (SEM) technique, we reveal the three-dimensional structure of the second optic chiasm of Drosophila melanogaster, which comprises interleaving bundles and sheets of axons insulated from each other by glial sheaths. These axon bundles invert their horizontal sequence in passing between the medulla and lobula. Axons connecting the medulla and lobula plate are also bundled together with them but do not decussate the sequence of their horizontal positions. They interleave with sheets of projection neuron axons between the lobula and lobula plate, which also lack decussations. We estimate that approximately 19,500 cells per hemisphere, about two thirds of the optic lobe neurons, contribute to the second chiasm, most being Tm cells, with an estimated additional 2,780 T4 and T5 cells each. The chiasm mostly comprises axons and cell body fibers, but also a few synaptic elements. Based on our anatomical findings, we propose that a chiasmal structure between the neuropils is potentially advantageous for processing complex visual information in parallel. The EM reconstruction shows not only the structure of the chiasm in the adult brain, the previously unreported main topic of our study, but also suggest that the projection patterns of the neurons comprising the chiasm may be determined by the proliferation centers from which the neurons develop. Such a complex wiring pattern could, we suggest, only have arisen in several evolutionary steps.

Implications of the Sap47 null mutation for synapsin phosphorylation, longevity, climbing proficiency and behavioural plasticity in adult Drosophila

The Sap47 gene of Drosophila melanogaster encodes a highly abundant 47 kDa synaptic vesicle-associated protein. Sap47 null mutants show defects in synaptic plasticity and larval olfactory associative learning but the molecular function of Sap47 at the synapse is unknown. We demonstrate that Sap47 modulates the phosphorylation of another highly abundant conserved presynaptic protein, synapsin. Site-specific phosphorylation of Drosophila synapsin has repeatedly been shown to be important for behavioural plasticity but it was not known where these phospho-synapsin isoforms are localized in the brain. Here, we report the distribution of serine-6-phosphorylated synapsin in the adult brain and show that it is highly enriched in rings of synapses in the ellipsoid body and in large synapses near the lateral triangle. The effects of knockout of Sap47 or synapsin on olfactory associative learning/memory support the hypothesis that both proteins operate in the same molecular pathway. We therefore asked if this might also be true for other aspects of their function. We show that knockout of Sap47 but not synapsin reduces lifespan, whereas knockout of Sap47 and synapsin, either individually or together, affects climbing proficiency, as well as plasticity in circadian rhythms and sleep. Furthermore, electrophysiological assessment of synaptic properties at the larval neuromuscular junction (NMJ) reveals increased spontaneous synaptic vesicle fusion and reduced paired pulse facilitation in Sap47 and synapsin single and double mutants. Our results imply that Sap47 and synapsin cooperate non-uniformly in the control of synaptic properties in different behaviourally relevant neuronal networks of the fruitfly.

The Jewel Wasp Standard Brain: Average shape atlas and morphology of the female Nasonia vitripennis brain

Nasonia, a genus of parasitoid wasps, is a promising model system in the study of developmental and evolutionary genetics, as well as complex traits such as learning. Of these "jewel wasps", the species Nasonia vitripennis is widely spread and widely studied. To accelerate neuroscientific research in this model species, fundamental knowledge of its nervous system is needed. To this end, we present an average standard brain of recently eclosed naïve female N. vitripennis wasps obtained by the iterative shape averaging method. This "Jewel Wasp Standard Brain" includes the optic lobe (excluding the lamina), the anterior optic tubercle, the antennal lobe, the lateral horn, the mushroom body, the central complex, and the remaining unclassified neuropils in the central brain. Furthermore, we briefly describe these well-defined neuropils and their subregions in the N. vitripennis brain. A volumetric analysis of these neuropils is discussed in the context of brains of other insect species. The Jewel Wasp Standard Brain will provide a framework to integrate and consolidate the results of future neurobiological studies in N. vitripennis. In addition, the volumetric analysis provides a baseline for future work on age- and experience-dependent brain plasticity.

An essential role of the mouse synapse-associated protein Syap1 in circuits for spontaneous motor activity and rotarod balance

Synapse-associated protein 1 (Syap1) is the mammalian homologue of synapse-associated protein of 47 kDa (Sap47) in Drosophila. Genetic deletion of Sap47 leads to deficiencies in short-term plasticity and associative memory processing in flies. In mice, Syap1 is prominently expressed in the nervous system, but its function is still unclear. We have generated Syap1 knockout mice and tested motor behaviour and memory. These mice are viable and fertile but display distinct deficiencies in motor behaviour. Locomotor activity specifically appears to be reduced in early phases when voluntary movement is initiated. On the rotarod, a more demanding motor test involving control by sensory feedback, Syap1-deficient mice dramatically fail to adapt to accelerated speed or to a change in rotation direction. Syap1 is highly expressed in cerebellar Purkinje cells and cerebellar nuclei. Thus, this distinct motor phenotype could be due to a so-far unknown function of Syap1 in cerebellar sensorimotor control. The observed motor defects are highly specific since other tests in the modified SHIRPA exam, as well as cognitive tasks like novel object recognition, Pavlovian fear conditioning, anxiety-like behaviour in open field dark-light transition and elevated plus maze do not appear to be affected in Syap1 knockout mice.

Summary: Knockout of the Syap1 gene in mice causes a distinct motor behaviour phenotype characterised by reduced initial locomotor activity and impaired rotarod performance.

Dystrobrevin is required postsynaptically for homeostatic potentiation at the Drosophila NMJ

Evolutionarily conserved homeostatic systems have been shown to modulate synaptic efficiency at the neuromuscular junctions of organisms. While advances have been made in identifying molecules that function presynaptically during homeostasis, limited information is currently available on how postsynaptic alterations affect presynaptic function. We previously identified a role for postsynaptic Dystrophin in the maintenance of evoked neurotransmitter release. We herein demonstrated that Dystrobrevin, a member of the Dystrophin Glycoprotein Complex, was delocalized from the postsynaptic region in the absence of Dystrophin. A newly-generated Dystrobrevin mutant showed elevated evoked neurotransmitter release, increased bouton numbers, and a readily releasable pool of synaptic vesicles without changes in the function or numbers of postsynaptic glutamate receptors. In addition, we provide evidence to show that the highly conserved Cdc42 Rho GTPase plays a key role in the postsynaptic Dystrophin/Dystrobrevin pathway for synaptic homeostasis. The present results give novel insights into the synaptic deficits underlying Duchenne Muscular Dystrophy affected by a dysfunctional Dystrophin Glycoprotein complex.

Transcriptional Feedback Links Lipid Synthesis to Synaptic Vesicle Pools in Drosophila Photoreceptors

Neurons can maintain stable synaptic connections across adult life. However, the signals that regulate expression of synaptic proteins in the mature brain are incompletely understood. Here, we describe a transcriptional feedback loop between the biosynthesis and repertoire of specific phospholipids and the synaptic vesicle pool in adult Drosophila photoreceptors. Mutations that disrupt biosynthesis of a subset of phospholipids cause degeneration of the axon terminal and loss of synaptic vesicles. Although degeneration of the axon terminal is dependent on neural activity, activation of sterol regulatory element binding protein (SREBP) is both necessary and sufficient to cause synaptic vesicle loss. Our studies demonstrate that SREBP regulates synaptic vesicle levels by interacting with tetraspanins, critical organizers of membranous organelles. SREBP is an evolutionarily conserved regulator of lipid biosynthesis in non-neuronal cells; our studies reveal a surprising role for this feedback loop in maintaining synaptic vesicle pools in the adult brain.

Dystrophin is required for normal synaptic gain in the Drosophila olfactory circuit

The Drosophila olfactory system provides an excellent model to elucidate the neural circuits that control behaviors elicited by environmental stimuli. Despite significant progress in defining olfactory circuit components and their connectivity, little is known about the mechanisms that transfer the information from the primary antennal olfactory receptor neurons to the higher order brain centers. Here, we show that the Dystrophin Dp186 isoform is required in the olfactory system circuit for olfactory functions. Using two-photon calcium imaging, we found the reduction of calcium influx in olfactory receptor neurons (ORNs) and also the defect of GABA_A mediated inhibitory input in the projection neurons (PNs) in Dp186 mutation. Moreover, the Dp186 mutant flies which display a decreased odor avoidance behavior were rescued by Dp186 restoration in the Drosophila olfactory neurons in either the presynaptic ORNs or the postsynaptic PNs. Therefore, these results revealed a role for Dystrophin, Dp 186 isoform in gain control of the olfactory synapse via the modulation of excitatory and inhibitory synaptic inputs to olfactory projection neurons.

Neuroarchitecture of the Drosophila central complex: A catalog of nodulus and asymmetrical body neurons and a revision of the protocerebral bridge catalog

The central complex, a set of neuropils in the center of the insect brain, plays a crucial role in spatial aspects of sensory integration and motor control. Stereotyped neurons interconnect these neuropils with one another and with accessory structures. We screened over 5,000 Drosophila melanogaster GAL4 lines for expression in two neuropils, the noduli (NO) of the central complex and the asymmetrical body (AB), and used multicolor stochastic labeling to analyze the morphology, polarity, and organization of individual cells in a subset of the GAL4 lines that showed expression in these neuropils. We identified nine NO and three AB cell types and describe them here. The morphology of the NO neurons suggests that they receive input primarily in the lateral accessory lobe and send output to each of the six paired noduli. We demonstrate that the AB is a bilateral structure which exhibits asymmetry in size between the left and right bodies. We show that the AB neurons directly connect the AB to the central complex and accessory neuropils, that they target both the left and right ABs, and that one cell type preferentially innervates the right AB. We propose that the AB be considered a central complex neuropil in Drosophila. Finally, we present highly restricted GAL4 lines for most identified protocerebral bridge, NO, and AB cell types. These lines, generated using the split-GAL4 method, will facilitate anatomical studies, behavioral assays, and physiological experiments.

Characterization of the Sexually Dimorphic fruitless Neurons That Regulate Copulation Duration

Male courtship in Drosophila melanogaster is a sexually dimorphic innate behavior that is hardwired in the nervous system. Understanding the neural mechanism of courtship behavior requires the anatomical and functional characterization of all the neurons involved. Courtship involves a series of distinctive behavioral patterns, culminating in the final copulation step, where sperms from the male are transferred to the female. The duration of this process is tightly controlled by multiple genes. The fruitless (fru) gene is one of the factors that regulate the duration of copulation. Using several intersectional genetic combinations to restrict the labeling of GAL4 lines, we found that a subset of a serotonergic cluster of fru neurons co-express the dopamine-synthesizing enzyme, tyrosine hydroxylase, and provide behavioral and immunological evidence that these neurons are involved in the regulation of copulation duration.

t-GRASP, a targeted GRASP for assessing neuronal connectivity

BACKGROUND

Understanding how behaviors are generated by neural circuits requires knowledge of the synaptic connections between the composite neurons. Methods for mapping synaptic connections, such as electron microscopy and paired recordings, are labor intensive and alternative methods are thus desirable.

NEW METHOD

Development of a targeted GFP Reconstitution Across Synaptic Partners(GRASP) method, t-GRASP, for assessing neural connectivity is described.

RESULTS

Numerous different pre-synaptic and post-synaptic/dendritic proteins were tested for enhancing the specificity of GRASP signal to synaptic regions. Pairing of both targeted pre- and post-t-GRASP constructs resulted in strong preferential GRASP signal in synaptic regions in Drosophila larval sensory neurons, larval neuromuscular junctions, and adult photoreceptor neurons with minimal false-positive signal.

COMPARISON WITH EXISTING METHODS

Activity-independent t-GRASP exhibits an enhancement of GRASP signal specificity for synaptic contact sites as compared to existing Drosophila GRASP methods. Fly strains were developed for expression of both pre- and post-t-GRASP with each of the three Drosophila binary transcription systems, thus enabling GRASP assays to be performed between any two driver pairs of any transcription system in either direction, an option not available for existing Drosophila GRASP methods.

CONCLUSIONS

t-GRASP is a novel targeted GRASP method for assessing synaptic connectivity between Drosophila neurons. Its flexibility of use with all three Drosophila binary transcription systems significantly expands the potential use of GRASP in Drosophila.

The Circadian Clock of the Ant Camponotus floridanus Is Localized in Dorsal and Lateral Neurons of the Brain

The circadian clock of social insects has become a focal point of interest for research, as social insects show complex forms of timed behavior and organization within their colonies. These behaviors include brood care, nest maintenance, foraging, swarming, defense, and many other tasks, of which several require social synchronization and accurate timing. Ants of the genus Camponotus have been shown to display a variety of daily timed behaviors such as the emergence of males from the nest, foraging, and relocation of brood. Nevertheless, circadian rhythms of isolated individuals have been studied in few ant species, and the circadian clock network in the brain that governs such behaviors remains completely uncharacterized. Here we show that isolated minor workers of Camponotus floridanus exhibit temperature overcompensated free-running locomotor activity rhythms under constant darkness. Under light-dark cycles, most animals are active during day and night, with a slight preference for the night. On the neurobiological level, we show that distinct cell groups in the lateral and dorsal brain of minor workers of C. floridanus are immunostained with an antibody against the clock protein Period (PER) and a lateral group additionally with an antibody against the neuropeptide pigment-dispersing factor (PDF). PER abundance oscillates in a daily manner, and PDF-positive neurites invade most parts of the brain, suggesting that the PER/PDF-positive neurons are bona fide clock neurons that transfer rhythmic signals into the relevant brain areas controlling rhythmic behavior.

Methyl-CpG binding domain proteins inhibit interspecies courtship and promote aggression in Drosophila

Reproductive isolation and speciation are driven by the convergence of environmental and genetic variation. The integration of these variation sources is thought to occur through epigenetic marks including DNA methylation. Proteins containing a methyl-CpG-binding domain (MBD) bind methylated DNA and interpret epigenetic marks, providing a dynamic yet evolutionarily adapted cellular output. Here, we report the Drosophila MBD-containing proteins, dMBD-R2 and dMBD2/3, contribute to reproductive isolation and survival behavioral strategies. Drosophila melanogaster males with a reduction in dMBD-R2 specifically in octopamine (OA) neurons exhibit courtship toward divergent interspecies D. virilis and D. yakuba females and a decrease in conspecific mating success. Conspecific male-male courtship is increased between dMBD-R2-deficient males while aggression is reduced. These changes in adaptive behavior are separable as males with a hypermethylated OA neuronal genome exhibited a decrease in aggression without altering male-male courtship. These results suggest Drosophila MBD-containing proteins are required within the OA neural circuitry to inhibit interspecies and conspecific male-male courtship and indicate that the genetically hard-wired neural mechanisms enforcing behavioral reproductive isolation include the interpretation of the epigenome.

Age-associated increase of the active zone protein Bruchpilot within the honeybee mushroom body

In honeybees, age-associated structural modifications can be observed in the mushroom bodies. Prominent examples are the synaptic complexes (microglomeruli, MG) in the mushroom body calyces, which were shown to alter their size and density with age. It is not known whether the amount of intracellular synaptic proteins in the MG is altered as well. The presynaptic protein Bruchpilot (BRP) is localized at active zones and is involved in regulating the probability of neurotransmitter release in the fruit fly, Drosophila melanogaster. Here, we explored the localization of the honeybee BRP (Apis mellifera BRP, AmBRP) in the bee brain and examined age-related changes in the AmBRP abundance in the central bee brain and in microglomeruli of the mushroom body calyces. We report predominant AmBRP localization near the membrane of presynaptic boutons within the mushroom body MG. The relative amount of AmBRP was increased in the central brain of two-week old bees whereas the amount of Synapsin, another presynaptic protein involved in the regulation of neurotransmitter release, shows an increase during the first two weeks followed by a decrease. In addition, we demonstrate an age-associated modulation of AmBRP located near the membrane of presynaptic boutons within MG located in mushroom body calyces where sensory input is conveyed to mushroom body intrinsic neurons.

We discuss that the observed age-associated AmBRP modulation might be related to maturation processes or to homeostatic mechanisms that might help to maintain synaptic functionality in old animals.

Initial characterization of a Syap1 knock-out mouse and distribution of Syap1 in mouse brain and cultured motoneurons

Synapse-associated protein 1 (Syap1/BSTA) is the mammalian homologue of Sap47 (synapse-associated protein of 47 kDa) in Drosophila. Sap47 null mutant larvae show reduced short-term synaptic plasticity and a defect in associative behavioral plasticity. In cultured adipocytes, Syap1 functions as part of a complex that phosphorylates protein kinase Bα/Akt1 (Akt1) at Ser(473) and promotes differentiation. The role of Syap1 in the vertebrate nervous system is unknown. Here, we generated a Syap1 knock-out mouse and show that lack of Syap1 is compatible with viability and fertility. Adult knock-out mice show no overt defects in brain morphology. In wild-type brain, Syap1 is found widely distributed in synaptic neuropil, notably in regions rich in glutamatergic synapses, but also in perinuclear structures associated with the Golgi apparatus of specific groups of neuronal cell bodies. In cultured motoneurons, Syap1 is located in axons and growth cones and is enriched in a perinuclear region partially overlapping with Golgi markers. We studied in detail the influence of Syap1 knockdown and knockout on structure and development of these cells. Importantly, Syap1 knockout does not affect motoneuron survival or axon growth. Unexpectedly, neither knockdown nor knockout of Syap1 in cultured motoneurons is associated with reduced Ser(473) or Thr(308) phosphorylation of Akt. Our findings demonstrate a widespread expression of Syap1 in the mouse central nervous system with regionally specific distribution patterns as illustrated in particular for olfactory bulb, hippocampus, and cerebellum.

Synaptic circuitry of identified neurons in the antennal lobe of Drosophila melanogaster

In Drosophila melanogaster olfactory sensory neurons (OSNs) establish synapses with projection neurons (PNs) and local interneurons within antennal lobe (AL) glomeruli. Substantial knowledge regarding this circuitry has been obtained by functional studies, whereas ultrastructural evidence of synaptic contacts is scarce. To fill this gap, we studied serial sections of three glomeruli using electron microscopy. Ectopic expression of a membrane-bound peroxidase allowed us to map synaptic sites along PN dendrites. Our data prove for the first time that each of the three major types of AL neurons is both pre- and postsynaptic to the other two types, as previously indicated by functional studies. PN dendrites carry a large proportion of output synapses, with approximately one output per every three input synapses. Detailed reconstructions of PN dendrites showed that these synapses are distributed unevenly, with input and output sites partially segregated along a proximal-distal gradient and the thinnest branches carrying solely input synapses. Moreover, our data indicate synapse clustering, as we found evidence of dendritic tiling of PN dendrites. PN output synapses exhibited T-shaped presynaptic densities, mostly arranged as tetrads. In contrast, output synapses from putative OSNs showed elongated presynaptic densities in which the T-bar platform was supported by several pedestals and contacted as many as 20 postsynaptic profiles. We also discovered synaptic contacts between the putative OSNs. The average synaptic density in the glomerular neuropil was about two synapses/µm(3) . These results are discussed with regard to current models of olfactory glomerular microcircuits across species.

Synapsin is required to "boost" memory strength for highly salient events

Synapsin is an evolutionarily conserved presynaptic phosphoprotein. It is encoded by only one gene in the Drosophila genome and is expressed throughout the nervous system. It regulates the balance between reserve and releasable vesicles, is required to maintain transmission upon heavy demand, and is essential for proper memory function at the behavioral level. Task-relevant sensorimotor functions, however, remain intact in the absence of Synapsin. Using an odor–sugar reward associative learning paradigm in larval Drosophila, we show that memory scores in mutants lacking Synapsin (syn⁹⁷) are lower than in wild-type animals only when more salient, higher concentrations of odor or of the sugar reward are used. Furthermore, we show that Synapsin is selectively required for larval short-term memory. Thus, without Synapsin Drosophila larvae can learn and remember, but Synapsin is required to form memories that match in strength to event salience—in particular to a high saliency of odors, of rewards, or the salient recency of an event. We further show that the residual memory scores upon a lack of Synapsin are not further decreased by an additional lack of the Sap47 protein. In combination with mass spectrometry data showing an up-regulated phosphorylation of Synapsin in the larval nervous system upon a lack of Sap47, this is suggestive of a functional interdependence of Synapsin and Sap47.

Reward signal in a recurrent circuit drives appetitive long-term memory formation

Dopamine signals reward in animal brains. A single presentation of a sugar reward to Drosophila activates distinct subsets of dopamine neurons that independently induce short- and long-term olfactory memories (STM and LTM, respectively). In this study, we show that a recurrent reward circuit underlies the formation and consolidation of LTM. This feedback circuit is composed of a single class of reward-signaling dopamine neurons (PAM-α1) projecting to a restricted region of the mushroom body (MB), and a specific MB output cell type, MBON-α1, whose dendrites arborize that same MB compartment. Both MBON-α1 and PAM-α1 neurons are required during the acquisition and consolidation of appetitive LTM. MBON-α1 additionally mediates the retrieval of LTM, which is dependent on the dopamine receptor signaling in the MB α/β neurons. Our results suggest that a reward signal transforms a nascent memory trace into a stable LTM using a feedback circuit at the cost of memory specificity.

DOI:http://dx.doi.org/10.7554/eLife.10719.001

An animal that finds particularly nutritious and palatable food will often develop a long-lasting memory—even if they experience that event only once.

One example of this is the ability of the fruit fly Drosophila to form a long-term association between a sugar reward and a specific odor that was present when they received the reward. The consumption of sugar triggers the release of a chemical called dopamine on specific compartments of a brain structure called the mushroom body. Dopamine then acts to modify the connection between cells called “Kenyon cells”, which encode specific odors, and the neurons that send signals out from the mushroom body (called MBONs). The result is the formation of a memory that links the odor with the reward.

However, little is known about how this process differs for long-term vs. short-term memories, and how it can occur when the fly has experienced the odor and reward together on only a single occasion. To find out, Ichinose et al. combined behavioral testing of fruit flies with genetics. The results confirmed that the dopamine neurons and the MBONs that project to a single compartment of the mushroom body, called α1, are both required for the formation of long-term odor-reward memories, but not their short-term equivalents. These neurons are called PAM-α1 and MBON-α1, respectively.

Unexpectedly, anatomical data revealed that PAM-α1 dopamine neurons receive input from MBON-α1; that is, long-term memory formation involves a feedback circuit: from PAM-α1 to Kenyon cells, then to MBON-α1 and back to PAM-α1. Blocking feedback from the MBON-α1 onto the PAM-α1 neurons shortly after odor-reward training disrupted long-term memory formation. Conversely, blocking feedback at a later stage did not. This suggests that prolonged activation of PAM-α1 by MBON-α1 helps to strengthen newly established memories, converting them into memories that will last for a long time.

The discovery of a specific circuit that supports long-term, but not short-term, memory formation in fruit flies is consistent with evidence of distinct mechanisms underlying these processes in mammals. Further work is now required to determine whether feedback circuits similar to those in fruit flies also contribute to reward-based learning in other animals.

DOI:http://dx.doi.org/10.7554/eLife.10719.002

Transcellular spreading of huntingtin aggregates in the Drosophila brain

A key feature of many neurodegenerative diseases is the accumulation and subsequent aggregation of misfolded proteins. Recent studies have highlighted the transcellular propagation of protein aggregates in several major neurodegenerative diseases, although the precise mechanisms underlying this spreading and how it relates to disease pathology remain unclear. Here we use a polyglutamine-expanded form of human huntingtin (Htt) with a fluorescent tag to monitor the spreading of aggregates in the Drosophila brain in a model of Huntington's disease. Upon expression of this construct in a defined subset of neurons, we demonstrate that protein aggregates accumulate at synaptic terminals and progressively spread throughout the brain. These aggregates are internalized and accumulate within other neurons. We show that Htt aggregates cause non-cell-autonomous pathology, including loss of vulnerable neurons that can be prevented by inhibiting endocytosis in these neurons. Finally we show that the release of aggregates requires N-ethylmalemide-sensitive fusion protein 1, demonstrating that active release and uptake of Htt aggregates are important elements of spreading and disease progression.

Drosophila Cbp53E Regulates Axon Growth at the Neuromuscular Junction

Calcium is a primary second messenger in all cells that functions in processes ranging from cellular proliferation to synaptic transmission. Proper regulation of calcium is achieved through numerous mechanisms involving channels, sensors, and buffers notably containing one or more EF-hand calcium binding domains. The Drosophila genome encodes only a single 6 EF-hand domain containing protein, Cbp53E, which is likely the prototypic member of a small family of related mammalian proteins that act as calcium buffers and calcium sensors. Like the mammalian homologs, Cbp53E is broadly though discretely expressed throughout the nervous system. Despite the importance of calcium in neuronal function and growth, nothing is known about Cbp53E's function in neuronal development. To address this deficiency, we generated novel null alleles of Drosophila Cbp53E and examined neuronal development at the well-characterized larval neuromuscular junction. Loss of Cbp53E resulted in increases in axonal branching at both peptidergic and glutamatergic neuronal terminals. This overgrowth could be completely rescued by expression of exogenous Cbp53E. Overexpression of Cbp53E, however, only affected the growth of peptidergic neuronal processes. These findings indicate that Cbp53E plays a significant role in neuronal growth and suggest that it may function in both local synaptic and global cellular mechanisms.

Aging Drosophila melanogaster display altered pre- and postsynaptic ultrastructure at adult neuromuscular junctions

Although age-related changes in synaptic plasticity are an important focus within neuroscience, little is known about ultrastructural changes of synaptic morphology during aging. Here we report how aging affects synaptic ultrastructure by using fluorescence and electron microscopy at the adult Drosophila neuromuscular junction (NMJ) of ventral abdominal muscles. Mainly four striking morphological changes of aging NMJs were revealed. 1) Bouton size increases with proportionally rising number of active zones (AZs). 2) Synaptic vesicle density at AZs is increased in old flies. 3) Late endosomes, cisternae, and multivesicular bodies accumulate in the presynaptic terminal, and vesicles accumulate between membranes of the terminal bouton and the subsynaptic reticulum. 4) The electron-dense pre- and postsynaptic apposition is expanded in aging NMJs, which is accompanied by an expansion of the postsynaptic glutamate receptor fields. These findings suggest that aging is possibly accompanied by impaired synaptic vesicle release and recycling and a potentially compensatory expansion of AZs and postsynaptic densities.

Brain composition in Godyris zavaleta, a diurnal butterfly, Reflects an increased reliance on olfactory information

Interspecific comparisons of brain structure can inform our functional understanding of brain regions, identify adaptations to species-specific ecologies, and explore what constrains adaptive changes in brain structure, and coevolution between functionally related structures. The value of such comparisons is enhanced when the species considered have known ecological differences. The Lepidoptera have long been a favored model in evolutionary biology, but to date descriptions of brain anatomy have largely focused on a few commonly used neurobiological model species. We describe the brain of Godyris zavaleta (Ithomiinae), a member of a subfamily of Neotropical butterflies with enhanced reliance on olfactory information. We demonstrate for the first time the presence of sexually dimorphic glomeruli within a distinct macroglomerular complex (MGC) in the antennal lobe of a diurnal butterfly. This presents a striking convergence with the well-known moth MGC, prompting a discussion of the potential mechanisms behind the independent evolution of specialized glomeruli. Interspecific analyses across four Lepidoptera further show that the relative size of sensory neuropils closely mirror interspecific variation in sensory ecology, with G. zavaleta displaying levels of sensory investment intermediate between the diurnal monarch butterfly (Danaus plexippus), which invests heavily in visual neuropil, and night-flying moths, which invest more in olfactory neuropil. We identify several traits that distinguish butterflies from moths, and several that distinguish D. plexippus and G. zavaleta. Our results illustrate that ecological selection pressures mold the structure of invertebrate brains, and exemplify how comparative analyses across ecologically divergent species can illuminate the functional significance of variation in brain structure.

The neuronal architecture of the mushroom body provides a logic for associative learning

We identified the neurons comprising the Drosophila mushroom body (MB), an associative center in invertebrate brains, and provide a comprehensive map describing their potential connections. Each of the 21 MB output neuron (MBON) types elaborates segregated dendritic arbors along the parallel axons of ∼2000 Kenyon cells, forming 15 compartments that collectively tile the MB lobes. MBON axons project to five discrete neuropils outside of the MB and three MBON types form a feedforward network in the lobes. Each of the 20 dopaminergic neuron (DAN) types projects axons to one, or at most two, of the MBON compartments. Convergence of DAN axons on compartmentalized Kenyon cell-MBON synapses creates a highly ordered unit that can support learning to impose valence on sensory representations. The elucidation of the complement of neurons of the MB provides a comprehensive anatomical substrate from which one can infer a functional logic of associative olfactory learning and memory.

Neuroarchitecture and neuroanatomy of the Drosophila central complex: A GAL4-based dissection of protocerebral bridge neurons and circuits

Insects exhibit an elaborate repertoire of behaviors in response to environmental stimuli. The central complex plays a key role in combining various modalities of sensory information with an insect's internal state and past experience to select appropriate responses. Progress has been made in understanding the broad spectrum of outputs from the central complex neuropils and circuits involved in numerous behaviors. Many resident neurons have also been identified. However, the specific roles of these intricate structures and the functional connections between them remain largely obscure. Significant gains rely on obtaining a comprehensive catalog of the neurons and associated GAL4 lines that arborize within these brain regions, and on mapping neuronal pathways connecting these structures. To this end, small populations of neurons in the Drosophila melanogaster central complex were stochastically labeled using the multicolor flip-out technique and a catalog was created of the neurons, their morphologies, trajectories, relative arrangements, and corresponding GAL4 lines. This report focuses on one structure of the central complex, the protocerebral bridge, and identifies just 17 morphologically distinct cell types that arborize in this structure. This work also provides new insights into the anatomical structure of the four components of the central complex and its accessory neuropils. Most strikingly, we found that the protocerebral bridge contains 18 glomeruli, not 16, as previously believed. Revised wiring diagrams that take into account this updated architectural design are presented. This updated map of the Drosophila central complex will facilitate a deeper behavioral and physiological dissection of this sophisticated set of structures. J. Comp. Neurol. 523:997–1037, 2015. © 2014 Wiley Periodicals, Inc.

Astrocyte-specific regulation of hMeCP2 expression in Drosophila

Alterations in the expression of Methyl-CpG-binding protein 2 (MeCP2) either by mutations or gene duplication leads to a wide spectrum of neurodevelopmental disorders including Rett Syndrome and MeCP2 duplication disorder. Common features of Rett Syndrome (RTT), MeCP2 duplication disorder, and neuropsychiatric disorders indicate that even moderate changes in MeCP2 protein levels result in functional and structural cell abnormalities. In this study, we investigated two areas of MeCP2 pathophysiology using Drosophila as a model system: the effects of MeCP2 glial gain-of-function activity on circuits controlling sleep behavior, and the cell-type specific regulation of MeCP2 expression. In this study, we first examined the effects of elevated MeCP2 levels on microcircuits by expressing human MeCP2 (hMeCP2) in astrocytes and distinct subsets of amine neurons including dopamine and octopamine (OA) neurons. Depending on the cell-type, hMeCP2 expression reduced sleep levels, altered daytime/nighttime sleep patterns, and generated sleep maintenance deficits. Second, we identified a 498 base pair region of the MeCP2e2 isoform that is targeted for regulation in distinct subsets of astrocytes. Levels of the full-length hMeCP2e2 and mutant RTT R106W protein decreased in astrocytes in a temporally and spatially regulated manner. In contrast, expression of the deletion Δ166 hMeCP2 protein was not altered in the entire astrocyte population. qPCR experiments revealed a reduction in full-length hMeCP2e2 transcript levels suggesting transgenic hMeCP2 expression is regulated at the transcriptional level. Given the phenotypic complexities that are caused by alterations in MeCP2 levels, our results provide insight into distinct cellular mechanisms that control MeCP2 expression and link microcircuit abnormalities with defined behavioral deficits.

Octopamine neuromodulation regulates Gr32a-linked aggression and courtship pathways in Drosophila males

Chemosensory pheromonal information regulates aggression and reproduction in many species, but how pheromonal signals are transduced to reliably produce behavior is not well understood. Here we demonstrate that the pheromonal signals detected by Gr32a-expressing chemosensory neurons to enhance male aggression are filtered through octopamine (OA, invertebrate equivalent of norepinephrine) neurons. Using behavioral assays, we find males lacking both octopamine and Gr32a gustatory receptors exhibit parallel delays in the onset of aggression and reductions in aggression. Physiological and anatomical experiments identify Gr32a to octopamine neuron synaptic and functional connections in the suboesophageal ganglion. Refining the Gr32a-expressing population indicates that mouth Gr32a neurons promote male aggression and form synaptic contacts with OA neurons. By restricting the monoamine neuron target population, we show that three previously identified OA-Fru^M neurons involved in behavioral choice are among the Gr32a-OA connections. Our findings demonstrate that octopaminergic neuromodulatory neurons function as early as a second-order step in this chemosensory-driven male social behavior pathway.

To mate or fight? When meeting other members of their species, male fruit flies must determine whether a second fly is male or female and proceed with the appropriate behavioral patterns. The taste receptor, Gr32a, has been reported to respond to chemical messages (pheromones) that are important for gender recognition, as eliminating Gr32a function impairs both male courtship and aggressive behavior. Here we demonstrate that different subsets of Gr32a-expressing neuron populations mediate these mutually exclusive behaviors and the male Gr32a-mediated behavioral response is amplified through neurons that contain the neuromodulator octopamine (OA, an invertebrate equivalent of norepinephrine). Gr32a-expressing neurons connect functionally and synaptically with distinct OA neurons indicating these amine neurons may function as early as a second-order step in a chemosensory-driven circuit. Our results contribute to understanding how an organism selects an appropriate behavioral response upon receiving external sensory signals.

External and circadian inputs modulate synaptic protein expression in the visual system of Drosophila melanogaster

In the visual system of Drosophila melanogaster the retina photoreceptors form tetrad synapses with the first order interneurons, amacrine cells and glial cells in the first optic neuropil (lamina), in order to transmit photic and visual information to the brain. Using the specific antibodies against synaptic proteins; Bruchpilot (BRP), Synapsin (SYN), and Disc Large (DLG), the synapses in the distal lamina were specifically labeled. Then their abundance was measured as immunofluorescence intensity in flies held in light/dark (LD 12:12), constant darkness (DD), and after locomotor and light stimulation. Moreover, the levels of proteins (SYN and DLG), and mRNAs of the brp, syn, and dlg genes, were measured in the fly's head and brain, respectively. In the head we did not detect SYN and DLG oscillations. We found, however, that in the lamina, DLG oscillates in LD 12:12 and DD but SYN cycles only in DD. The abundance of all synaptic proteins was also changed in the lamina after locomotor and light stimulation. One hour locomotor stimulations at different time points in LD 12:12 affected the pattern of the daily rhythm of synaptic proteins. In turn, light stimulations in DD increased the level of all proteins studied. In the case of SYN, however, this effect was observed only after a short light pulse (15 min). In contrast to proteins studied in the lamina, the mRNA of brp, syn, and dlg genes in the brain was not cycling in LD 12:12 and DD, except the mRNA of dlg in LD 12:12. Our earlier results and obtained in the present study showed that the abundance of BRP, SYN and DLG in the distal lamina, at the tetrad synapses, is regulated by light and a circadian clock while locomotor stimulation affects their daily pattern of expression. The observed changes in the level of synaptic markers reflect the circadian plasticity of tetrad synapses regulated by the circadian clock and external inputs, both specific and unspecific for the visual system.

Synapsin function in GABA-ergic interneurons is required for short-term olfactory habituation

In Drosophila, short-term (STH) and long-term habituation (LTH) of olfactory avoidance behavior are believed to arise from the selective potentiation of GABAergic synapses between multiglomerular local circuit interneurons (LNs) and projection neurons in the antennal lobe. However, the underlying mechanisms remain poorly understood. Here, we show that synapsin (syn) function is necessary for STH and that syn(97)-null mutant defects in STH can be rescued by syn(+) cDNA expression solely in the LN1 subset of GABAergic local interneurons. As synapsin is a synaptic vesicle-clustering phosphoprotein, these observations identify a presynaptic mechanism for STH as well as the inhibitory interneurons in which this mechanism is deployed. Serine residues 6 and/or 533, potential kinase target sites of synapsin, are necessary for synapsin function suggesting that synapsin phosphorylation is essential for STH. Consistently, biochemical analyses using a phospho-synapsin-specific antiserum show that synapsin is a target of Ca(2+) calmodulin-dependent kinase II (CaMKII) phosphorylation in vivo. Additional behavioral and genetic observations demonstrate that CaMKII function is necessary in LNs for STH. Together, these data support a model in which CaMKII-mediated synapsin phosphorylation in LNs induces synaptic vesicle mobilization and thereby presynaptic facilitation of GABA release that underlies olfactory STH. Finally, the striking observation that LTH occurs normally in syn(97) mutants indicates that signaling pathways for STH and LTH diverge upstream of synapsin function in GABAergic interneurons.

Identification and structural characterization of interneurons of the Drosophila brain by monoclonal antibodies of the würzburg hybridoma library

Several novel synaptic proteins have been identified by monoclonal antibodies (mAbs) of the Würzburg hybridoma library generated against homogenized Drosophila brains, e.g. cysteine string protein, synapse-associated protein of 47 kDa, and Bruchpilot. However, at present no routine technique exists to identify the antigens of mAbs of our library that label only a small number of cells in the brain. Yet these antibodies can be used to reproducibly label and thereby identify these cells by immunohistochemical staining. Here we describe the staining patterns in the Drosophila brain for ten mAbs of the Würzburg hybridoma library. Besides revealing the neuroanatomical structure and distribution of ten different sets of cells we compare the staining patterns with those of antibodies against known antigens and GFP expression patterns driven by selected Gal4 lines employing regulatory sequences of neuronal genes. We present examples where our antibodies apparently stain the same cells in different Gal4 lines suggesting that the corresponding regulatory sequences can be exploited by the split-Gal4 technique for transgene expression exclusively in these cells. The detection of Gal4 expression in cells labeled by mAbs may also help in the identification of the antigens recognized by the antibodies which then in addition to their value for neuroanatomy will represent important tools for the characterization of the antigens. Implications and future strategies for the identification of the antigens are discussed.

Maggot learning and Synapsin function

Drosophila larvae are focused on feeding and have few neurons. Within these bounds, however, there still are behavioural degrees of freedom. This review is devoted to what these elements of flexibility are, and how they come about. Regarding odour-food associative learning, the emerging working hypothesis is that when a mushroom body neuron is activated as a part of an odour-specific set of mushroom body neurons, and coincidently receives a reinforcement signal carried by aminergic neurons, the AC-cAMP-PKA cascade is triggered. One substrate of this cascade is Synapsin, and therefore this review features a general and comparative discussion of Synapsin function. Phosphorylation of Synapsin ensures an alteration of synaptic strength between this mushroom body neuron and its target neuron(s). If the trained odour is encountered again, the pattern of mushroom body neurons coding this odour is activated, such that their modified output now allows conditioned behaviour. However, such an activated memory trace does not automatically cause conditioned behaviour. Rather, in a process that remains off-line from behaviour, the larvae compare the value of the testing situation (based on gustatory input) with the value of the odour-activated memory trace (based on mushroom body output). The circuit towards appetitive conditioned behaviour is closed only if the memory trace suggests that tracking down the learned odour will lead to a place better than the current one. It is this expectation of a positive outcome that is the immediate cause of appetitive conditioned behaviour. Such conditioned search for reward corresponds to a view of aversive conditioned behaviour as conditioned escape from punishment, which is enabled only if there is something to escape from - much in the same way as we only search for things that are not there, and run for the emergency exit only when there is an emergency. One may now ask whether beyond 'value' additional information about reinforcement is contained in the memory trace, such as information about the kind and intensity of the reinforcer used. The Drosophila larva may allow us to develop satisfyingly detailed accounts of such mnemonic richness - if it exists.

Ral mediates activity-dependent growth of postsynaptic membranes via recruitment of the exocyst

Remodelling neuronal connections by synaptic activity requires membrane trafficking. We present evidence for a signalling pathway by which synaptic activity and its consequent Ca(2+) influx activate the small GTPase Ral and thereby recruit exocyst proteins to postsynaptic zones. In accord with the ability of the exocyst to direct delivery of post-Golgi vesicles, constitutively active Ral expressed in Drosophila muscle causes the exocyst to be concentrated in the region surrounding synaptic boutons and consequently enlarges the membrane folds of the postsynaptic plasma membrane (the subsynaptic reticulum, SSR). SSR growth requires Ral and the exocyst component Sec5 and Ral-induced enlargement of these membrane folds does not occur in sec5(-/-) muscles. Chronic changes in synaptic activity influence the plastic growth of this membrane in a manner consistent with activity-dependent activation of Ral. Thus, Ral regulation of the exocyst represents a control point for postsynaptic plasticity. This pathway may also function in mammals as expression of activated RalA in hippocampal neurons increases dendritic spine density in an exocyst-dependent manner and increases Sec5 in spines.

Age-related plasticity in the synaptic ultrastructure of neurons in the mushroom body calyx of the adult honeybee Apis mellifera

The mushroom bodies are high-order sensory integration centers in the insect brain. In the honeybee, their main sensory input regions are large, doubled calyces with modality-specific, distinct sensory neuropil regions. We investigated adult structural plasticity of input synapses in the microglomeruli of the olfactory lip and visual collar. Synapsin-immunolabeled whole-mount brains reveal that during the natural transition from nursing to foraging, a significant volume increase in the calycal subdivisions is accompanied by a decreased packing density of boutons from input projection neurons. To investigate the associated ultrastructural changes at pre- and postsynaptic sites of individual microglomeruli, we employed serial-section electron microscopy. In general, the membrane surface area of olfactory and visual projection neuron boutons increased significantly between 1-day-old bees and foragers. Both types of boutons formed ribbon and non-ribbon synapses. The percentage of ribbon synapses per bouton was significantly increased in the forager. At each presynaptic site the numbers of postsynaptic partners-mostly Kenyon cell dendrites-likewise increased. Ribbon as well as non-ribbon synapses formed mainly dyads in the 1-day-old bee, and triads in the forager. In the visual collar, outgrowing Kenyon cell dendrites form about 140 contacts upon a projection neuron bouton in the forager compared with only about 95 in the 1-day-old bee, resulting in an increased divergence ratio between the two stages. This difference suggests that synaptic changes in calycal microcircuits of the mushroom body during periods of altered sensory activity and experience promote behavioral plasticity underlying polyethism and social organization in honeybee colonies.

Short neuropeptide F acts as a functional neuromodulator for olfactory memory in Kenyon cells of Drosophila mushroom bodies

In insects, many complex behaviors, including olfactory memory, are controlled by a paired brain structure, the so-called mushroom bodies (MB). In Drosophila, the development, neuroanatomy, and function of intrinsic neurons of the MB, the Kenyon cells, have been well characterized. Until now, several potential neurotransmitters or neuromodulators of Kenyon cells have been anatomically identified. However, whether these neuroactive substances of the Kenyon cells are functional has not been clarified yet. Here we show that a neuropeptide precursor gene encoding four types of short neuropeptide F (sNPF) is required in the Kenyon cells for appetitive olfactory memory. We found that activation of Kenyon cells by expressing a thermosensitive cation channel (dTrpA1) leads to a decrease in sNPF immunoreactivity in the MB lobes. Targeted expression of RNA interference against the sNPF precursor in Kenyon cells results in a highly significant knockdown of sNPF levels. This knockdown of sNPF in the Kenyon cells impairs sugar-rewarded olfactory memory. This impairment is not due to a defect in the reflexive sugar preference or odor response. Consistently, knockdown of sNPF receptors outside the MB causes deficits in appetitive memory. Altogether, these results suggest that sNPF is a functional neuromodulator released by Kenyon cells.

A multisensory centrifugal neuron in the olfactory pathway of heliothine moths

We have characterized, by intracellular recording and staining, a unique type of centrifugal neuron in the brain olfactory center of two heliothine moth species; one in Heliothis virescens and one in Helicoverpa armigera. This unilateral neuron, which is not previously described in any moth, has fine processes in the dorsomedial region of the protocerebrum and extensive neuronal branches with blebby terminals in all glomeruli of the antennal lobe. Its soma is located dorsally of the central body close to the brain midline. Mass-fills of antennal-lobe connections with protocerebral regions showed that the centrifugal neuron is, in each brain hemisphere, one within a small group of neurons having their somata clustered. In both species the neuron was excited during application of non-odorant airborne signals, including transient sound pulses of broad bandwidth and air velocity changes. Additional responses to odors were recorded from the neuron in Heliothis virescens. The putative biological significance of the centrifugal antennal-lobe neuron is discussed with regard to its morphological and physiological properties. In particular, a possible role in multisensory processes underlying the moth's ability to adapt its odor-guided behaviors according to the sound of an echo-locating bat is considered.

CaMKII is differentially localized in synaptic regions of Kenyon cells within the mushroom bodies of the honeybee brain

Calcium/calmodulin-dependent protein kinase II (CaMKII) has been linked to neuronal plasticity associated with long-term potentiation as well as structural synaptic plasticity. Previous work in adult honeybees has shown that a single CaMKII gene is strongly expressed in the mushroom bodies (MBs), brain centers associated with sensory integration, and learning and memory formation. To study a potential role of CaMKII in synaptic plasticity, the cellular and subcellular distribution of activated (phosphorylated) pCaMKII protein was investigated at various life stages of the honeybee using immunocytochemistry, confocal microscopy, and western blot analyses. Whereas at pupal stages 3-4 most parts of the brain showed high levels of pCaMKII immunoreactivity, the protein was predominantly concentrated in the MBs in the adult brain. The results show that pCaMKII is present in a specific subpopulation of Kenyon cells, the noncompact cells. Within the olfactory (lip) and visual (collar) subregion of the MB calyx neuropil pCaMKII was colocalized with f-actin in postsynaptic compartments of microglomeruli, indicating that it is enriched in Kenyon cell dendritic spines. This suggests a potential role of CaMKII in Kenyon cell dendritic plasticity. Interestingly, pCaMKII protein was absent in two other types of Kenyon cells, the inner compact cells associated with the multimodal basal ring and the outer compact cells. During adult behavioral maturation from nurse bees to foragers, pCaMKII distribution remained essentially similar at the qualitative level, suggesting a potential role in dendritic plasticity of Kenyon cells throughout the entire life span of a worker bee.

Distribution of metabotropic receptors of serotonin, dopamine, GABA, glutamate, and short neuropeptide F in the central complex of Drosophila

The central complex is a prominent set of midline neuropils in the insect brain, known to be a higher locomotor control center that integrates visual inputs and modulates motor outputs. It is composed of four major neuropil structures, the ellipsoid body (EB), fan-shaped body (FB), noduli (NO), and protocerebral bridge (PB). In Drosophila different types of central complex neurons have been shown to express multiple neuropeptides and neurotransmitters; however, the distribution of corresponding receptors is not known. Here, we have mapped metabotropic, G-protein-coupled receptors (GPCRs) of several neurotransmitters to neurons of the central complex. By combining immunocytochemistry with GAL4 driven green fluorescent protein, we examined the distribution patterns of six different GPCRs: two serotonin receptor subtypes (5-HT(1B) and 5-HT(7)), a dopamine receptor (DopR), the metabotropic GABA(B) receptor (GABA(B)R), the metabotropic glutamate receptor (DmGluR(A)) and a short neuropeptide F receptor (sNPFR1). Five of the six GPCRs were mapped to different neurons in the EB (sNPFR1 was not seen). Different layers of the FB express DopR, GABA(B)R, DmGluR(A,) and sNPFR1, whereas only GABA(B)R and DmGluR(A) were localized to the PB. Finally, strong expression of DopR and DmGluR(A) was detected in the NO. In most cases the distribution patterns of the GPCRs matched the expression of markers for their respective ligands. In some nonmatching regions it is likely that other types of dopamine and serotonin receptors or ionotropic GABA and glutamate receptors are expressed. Our data suggest that chemical signaling and signal modulation are diverse and highly complex in the different compartments and circuits of the Drosophila central complex. The information provided here, on receptor distribution, will be very useful for future analysis of functional circuits in the central complex, based on targeted interference with receptor expression.

The genetic analysis of functional connectomics in Drosophila

Fly and vertebrate nervous systems share many organizational features, such as layers, columns and glomeruli, and utilize similar synaptic components, such as ion channels and receptors. Both also exhibit similar network features. Recent technological advances, especially in electron microscopy, now allow us to determine synaptic circuits and identify pathways cell-by-cell, as part of the fly's connectome. Genetic tools provide the means to identify synaptic components, as well as to record and manipulate neuronal activity, adding function to the connectome. This review discusses technical advances in these emerging areas of functional connectomics, offering prognoses in each and identifying the challenges in bridging structural connectomics to molecular biology and synaptic physiology, thereby determining fundamental mechanisms of neural computation that underlie behavior.

Identification of Eps15 as antigen recognized by the monoclonal antibodies aa2 and ab52 of the Wuerzburg Hybridoma Library against Drosophila brain

The Wuerzburg Hybridoma Library against the Drosophila brain represents a collection of around 200 monoclonal antibodies that bind to specific structures in the Drosophila brain. Here we describe the immunohistochemical staining patterns, the Western blot signals of one- and two-dimensional electrophoretic separation, and the mass spectrometric characterization of the target protein candidates recognized by the monoclonal antibodies aa2 and ab52 from the library. Analysis of a mutant of a candidate gene identified the Drosophila homolog of the Epidermal growth factor receptor Pathway Substrate clone 15 (Eps15) as the antigen for these two antibodies.