Putative histamine-gated chloride channel subunits of the insect visual system and thoracic ganglion

Regulation of proteostasis by sleep through autophagy in Drosophila models of Alzheimer's disease

Sleep and circadian rhythm dysfunctions are common clinical features of Alzheimer's disease (AD). Increasing evidence suggests that in addition to being a symptom, sleep disturbances can also drive the progression of neurodegeneration. Protein aggregation is a pathological hallmark of AD; however, the molecular pathways behind how sleep affects protein homeostasis remain elusive. Here we demonstrate that sleep modulation influences proteostasis and the progression of neurodegeneration in Drosophila models of tauopathy. We show that sleep deprivation enhanced Tau aggregational toxicity resulting in exacerbated synaptic degeneration. In contrast, sleep induction using gaboxadol led to reduced toxic Tau accumulation in neurons as a result of modulated autophagic flux and enhanced clearance of ubiquitinated Tau, suggesting altered protein processing and clearance that resulted in improved synaptic integrity and function. These findings highlight the complex relationship between sleep and regulation of protein homeostasis and the neuroprotective potential of sleep-enhancing therapeutics to slow the progression or delay the onset of neurodegeneration.

A recurrent neural circuit in Drosophila deblurs visual inputs

A critical goal of vision is to detect changes in light intensity, even when these changes are blurred by the spatial resolution of the eye and the motion of the animal. Here we describe a recurrent neural circuit in Drosophila that compensates for blur and thereby selectively enhances the perceived contrast of moving edges. Using in vivo , two-photon voltage imaging, we measured the temporal response properties of L1 and L2, two cell types that receive direct synaptic input from photoreceptors. These neurons have biphasic responses to brief flashes of light, a hallmark of cells that encode changes in stimulus intensity. However, the second phase was often much larger than the first, creating an unusual temporal filter. Genetic dissection revealed that recurrent neural circuitry strongly shapes the second phase of the response, informing the structure of a dynamical model. By applying this model to moving natural images, we demonstrate that rather than veridically representing stimulus changes, this temporal processing strategy systematically enhances them, amplifying and sharpening responses. Comparing the measured responses of L2 to model predictions across both artificial and natural stimuli revealed that L2 tunes its properties as the model predicts in order to deblur images. Since this strategy is tunable to behavioral context, generalizable to any time-varying sensory input, and implementable with a common circuit motif, we propose that it could be broadly used to selectively enhance sharp and salient changes.

Visual processing in the fly, from photoreceptors to behavior

Originally a genetic model organism, the experimental use of Drosophila melanogaster has grown to include quantitative behavioral analyses, sophisticated perturbations of neuronal function, and detailed sensory physiology. A highlight of these developments can be seen in the context of vision, where pioneering studies have uncovered fundamental and generalizable principles of sensory processing. Here we begin with an overview of vision-guided behaviors and common methods for probing visual circuits. We then outline the anatomy and physiology of brain regions involved in visual processing, beginning at the sensory periphery and ending with descending motor control. Areas of focus include contrast and motion detection in the optic lobe, circuits for visual feature selectivity, computations in support of spatial navigation, and contextual associative learning. Finally, we look to the future of fly visual neuroscience and discuss promising topics for further study.

The modulatory effects of biogenic amines on male mating performance in Bactrocera dorsalis

In insects, the emergence of mating behavior requires the interplay among sex-determination hierarchy mechanisms that regulate sex-specific differentiation, perception and integration of different sensory cues, and precisely patterned behavioral outputs. Biogenic amines, including octopamine (OA), dopamine (DA), tyramine (TA), serotonin and histamine, have been identified and proposed as putative neurotransmitters, neurohormones and/or neuromodulators in the central nervous system of insects to influence multiple physiologies and behaviors. The current study provides the physiological roles and pharmacology of these biogenic amines in the mating performance of Bactrocera dorsalis. Silencing gene expressions coding for biosynthetic enzymes of DA and serotonin in male flies could decrease mating rates, while OA, TA and histamine had no such effects on mating. Furthermore, injection of DA or the DA receptor antagonist chlorpromazine could affect mating rate, as well as injection of serotonin. Pharmacological treatments with other biogenic amines or their receptor antagonists in male flies have no roles in regulating mating performance. We conclude that DA and its receptors are involved in regulating male mating behaviors in B. dorsalis, while changes in serotonin levels in male flies could also affect mating rates. In the current study, the modulatory effects of these biogenic amines on mating performance were investigated, and these results will be helpful in providing a new strategy for controlling B. dorsalis.

Identification and pharmacological characterization of histamine-gated chloride channels in the fall armyworm, Spodoptera frugiperda

Histamine-gated chloride channels (HACls) mediate fast inhibitory neurotransmission in invertebrate nervous systems and have important roles in light reception, color processing, temperature preference and light-dark cycle. The fall armyworm, Spodoptera frugiperda is a main destructive pest of grain and row crops. However, the pharmacological characterization of HACls in S. frugiperda remain unknown. In this study, we identified two cDNAs encoding SfHACl1 and SfHACl2 in S. frugiperda. They had similar expression patterns and were most abundantly expressed in the head of larvae and at the egg stage. Electrophysiological analysis with the two-electrode voltage clamp method showed that histamine (HA) and γ-aminobutyric acid (GABA) activated inward currents when SfHACls were singly or collectively expressed with different ratios in Xenopus laevis oocytes. These channels were ≥2000-fold more sensitive to HA than to GABA. They were anion-selective channels, which were highly dependent on changes in external chloride concentrations, but insensitive to changes in external sodium concentrations. The insecticides abamectin (ABM) and emamectin benzoate (EB) also activated these channels with the EC₅₀ to SfHACl1 lower than that to SfHACl2. And the EC_50s of ABM and EB to the co-expressed channels gradually increased with increase in the injection ratio of SfHACl2 cRNA. Homology models and docking simulations revealed that HA bound to the large amino-terminal extracellular domain of SfHACl1 and SfHACl2 by forming 4 and 2 hydrogen bonds, respectively. The docking simulations of ABM and EB had similar binding sites in the transmembrane regions. Overall, these findings indicated that HACls act as targets for macrolide, and this study provides theoretical guidance for further derivatization of abamectin insecticides.

Histamine gustatory aversion in Drosophila melanogaster

Many foods and drinks contain histamine; however, the mechanisms that drive histamine taste perception have not yet been investigated. Here, we use a simple model organism, Drosophila melanogaster, to dissect the molecular sensors required to taste histamine. We first investigated histidine and histamine taste perception by performing a binary food choice assay and electrophysiology to identify essential sensilla for histamine sensing in the labellum. Histamine was found to activate S-type sensilla, which harbor bitter-sensing gustatory receptor neurons. Moreover, unbiased genetic screening for chemoreceptors revealed that a gustatory receptor, GR22e and an ionotropic receptor, IR76b are required for histamine sensing. Ectopic expression of GR22e was sufficient to induce a response in I-type sensilla, which normally do not respond to histamine. Taken together, our findings provide new insights into the mechanisms by which insects discriminate between the toxic histamine and beneficial histidine via their taste receptors.

Reduction of histamine and enhanced spinning behavior of Daphnia magna caused by scarlet mutant

The ABC transporter, Scarlet, and its binding partner, White are involved in pigment synthesis in the insect eye and mutations in these genes are used as genetic markers. Recent studies have suggested that these transporters also have additional functions in the neuronal system. In our previous study, we generated scarlet mutant in the small crustacean, Daphnia magna and showed that the mutant lacked the eye pigment in the mutant. Here, we show that the scarlet mutant exhibits spinning behavior. This phenotype is partly associated with the presence of light. Metabolomic analysis of a juvenile mutant revealed that the scarlet mutant has approximately one-tenth of the histamine content of the wild type. Application of histamine to the scarlet mutant rescued the spinning behavior in juveniles, suggesting that the spinning behavior of the mutant is caused by the reduction of histamine. However, the altered behavior was not rescued in the adult mutant by the addition of histamine, suggesting that Scarlet plays an irreversible role in the development of histaminergic neurons. These results suggest that Scarlet plays an important role in histaminergic signaling, which might be related to control the spinning behavior, in addition to its role in eye pigmentation.

Better Sleep at Night: How Light Influences Sleep in Drosophila

Sleep-like states have been described in Drosophila and the mechanisms and factors that generate and define sleep-wake profiles in this model organism are being thoroughly investigated. Sleep is controlled by both circadian and homeostatic mechanisms, and environmental factors such as light, temperature, and social stimuli are fundamental in shaping and confining sleep episodes into the correct time of the day. Among environmental cues, light seems to have a prominent function in modulating the timing of sleep during the 24 h and, in this review, we will discuss the role of light inputs in modulating the distribution of the fly sleep-wake cycles. This phenomenon is of growing interest in the modern society, where artificial light exposure during the night is a common trait, opening the possibility to study Drosophila as a model organism for investigating shift-work disorders.

Heterogeneous expression of GABA receptor-like subunits LCCH3 and GRD reveals functional diversity of GABA receptors in the honeybee Apis mellifera

BACKGROUND AND PURPOSE

Despite a growing awareness, annual losses of honeybee colonies worldwide continue to reach threatening levels for food safety and global biodiversity. Among the biotic and abiotic stresses probably responsible for these losses, pesticides, including those targeting ionotropic GABA receptors, are one of the major drivers. Most insect genomes include the ionotropic GABA receptor subunit gene, Rdl, and two GABA-like receptor subunit genes, Lcch3 and Grd. Most studies have focused on Rdl which forms homomeric GABA-gated chloride channels, and a complete analysis of all possible molecular combinations of GABA receptors is still lacking.

EXPERIMENTAL APPROACH

We cloned the Rdl, Grd, and Lcch3 genes of Apis mellifera and systematically characterized the resulting GABA receptors expressed in Xenopus oocytes, using electrophysiological assays, fluorescence microscopy and co-immunoprecipitation techniques.

KEY RESULTS

The cloned subunits interacted with each other, forming GABA-gated heteromeric channels with particular properties. Strikingly, these heteromers were always more sensitive than AmRDL homomer to all the pharmacological agents tested. In particular, when expressed together, Grd and Lcch3 form a non-selective cationic channel that opens at low concentrations of GABA and with sensitivity to insecticides similar to that of homomeric Rdl channels.

CONCLUSION AND IMPLICATIONS

For off-target species like the honeybee, chronic sublethal exposure to insecticides constitutes a major threat. At these concentration ranges, homomeric RDL receptors may not be the most pertinent target to study and other ionotropic GABA receptor subtypes should be considered in order to understand more fully the molecular mechanisms of sublethal toxicity to insecticides.

Color vision in insects: insights from Drosophila

Color vision is an important sensory capability that enhances the detection of contrast in retinal images. Monochromatic animals exclusively detect temporal and spatial changes in luminance, whereas two or more types of photoreceptors and neuronal circuitries for the comparison of their responses enable animals to differentiate spectral information independent of intensity. Much of what we know about the cellular and physiological mechanisms underlying color vision comes from research on vertebrates including primates. In insects, many important discoveries have been made, but direct insights into the physiology and circuit implementation of color vision are still limited. Recent advances in Drosophila systems neuroscience suggest that a complete insect color vision circuitry, from photoreceptors to behavior, including all elements and computations, can be revealed in future. Here, we review fundamental concepts in color vision alongside our current understanding of the neuronal basis of color vision in Drosophila, including side views to selected other insects.

The HisCl1 histamine receptor acts in photoreceptors to synchronize Drosophila behavioral rhythms with light-dark cycles

In Drosophila, the clock that controls rest-activity rhythms synchronizes with light-dark cycles through either the blue-light sensitive cryptochrome (Cry) located in most clock neurons, or rhodopsin-expressing histaminergic photoreceptors. Here we show that, in the absence of Cry, each of the two histamine receptors Ort and HisCl1 contribute to entrain the clock whereas no entrainment occurs in the absence of the two receptors. In contrast to Ort, HisCl1 does not restore entrainment when expressed in the optic lobe interneurons. Indeed, HisCl1 is expressed in wild-type photoreceptors and entrainment is strongly impaired in flies with photoreceptors mutant for HisCl1. Rescuing HisCl1 expression in the Rh6-expressing photoreceptors restores entrainment but it does not in other photoreceptors, which send histaminergic inputs to Rh6-expressing photoreceptors. Our results thus show that Rh6-expressing neurons contribute to circadian entrainment as both photoreceptors and interneurons, recalling the dual function of melanopsin-expressing ganglion cells in the mammalian retina.

Color Processing in the Early Visual System of Drosophila

Color vision extracts spectral information by comparing signals from photoreceptors with different visual pigments. Such comparisons are encoded by color-opponent neurons that are excited at one wavelength and inhibited at another. Here, we examine the circuit implementation of color-opponent processing in the Drosophila visual system by combining two-photon calcium imaging with genetic dissection of visual circuits. We report that color-opponent processing of UV_short/blue and UV_long/green is already implemented in R7/R8 inner photoreceptor terminals of "pale" and "yellow" ommatidia, respectively. R7 and R8 photoreceptors of the same type of ommatidia mutually inhibit each other directly via HisCl1 histamine receptors and receive additional feedback inhibition that requires the second histamine receptor Ort. Color-opponent processing at the first visual synapse represents an unexpected commonality between Drosophila and vertebrates; however, the differences in the molecular and cellular implementation suggest that the same principles evolved independently.

Expression of Cys-loop receptor subunits and acetylcholine binding protein in the mechanosensory neurons, glial cells, and muscle tissue of the spider Cupiennius salei

The central and peripheral nervous system transcriptomes of the spider Cupiennius salei have 15 Cys-loop receptor subunits and an acetylcholine-binding protein (AChBP). Twelve subunits are predicted to form anion channels gated by γ-aminobutyric acid (GABA), glutamate, histamine, or changes in pH, and three are putative ACh-gated cation channels. Spiders have a variety of mechanosensilla and proprioceptive organs that are innervated by efferents in their peripherally located parts, and efferents also innervate muscle fibers. We investigated Cys-loop gene expression in muscle tissue by qPCR and localized this expression in mechanosensilla via in situ hybridization. The cuticular mechanosensory neurons had only CsGABArdl and CspHCl2 subunits, whereas the muscle tissue expressed a wider variety of subunits, especially CsGABAgrd, CsGABA_A β, CsGluCl1 and CspHCl, but very low levels of the CsGABArdl or CsnACh subunits. An nACh non-α subunit was expressed in a group of unidentified cells in the hypodermis and at low level in the muscle tissue, but the physiological function of this subunit is unknown. The CsnAChα subunit was not expressed in sensory neurons and was expressed at extremely low level in the muscle tissue. None of the probes gave signals in proprioceptive joint receptors, suggesting that efferent innervation to this sense organ employs other receptor types. CsAChBP and a glia-specific homeodomain CsREPO were both expressed in glial cells that surround sensory neurons and also in muscle tissue, probably around the nerve endings of the neuromuscular junction. These locations have large numbers of synapses, suggesting that AChBP may have a function in modulating synaptic transmission. J. Comp. Neurol. 525:1139-1154, 2017. © 2016 Wiley Periodicals, Inc.

Evolution, Expression, and Function of Nonneuronal Ligand-Gated Chloride Channels in Drosophila melanogaster

Ligand-gated chloride channels have established roles in inhibitory neurotransmission in the nervous systems of vertebrates and invertebrates. Paradoxically, expression databases in Drosophila melanogaster have revealed that three uncharacterized ligand-gated chloride channel subunits, CG7589, CG6927, and CG11340, are highly expressed in nonneuronal tissues. Furthermore, subunit copy number varies between insects, with some orders containing one ortholog, whereas other lineages exhibit copy number increases. Here, we show that the Dipteran lineage has undergone two gene duplications followed by expression-based functional differentiation. We used promoter-GFP expression analysis, RNA-sequencing, and in situ hybridization to examine cell type and tissue-specific localization of the three D. melanogaster subunits. CG6927 is expressed in the nurse cells of the ovaries. CG7589 is expressed in multiple tissues including the salivary gland, ejaculatory duct, malpighian tubules, and early midgut. CG11340 is found in malpighian tubules and the copper cell region of the midgut. Overexpression of CG11340 increased sensitivity to dietary copper, and RNAi and ends-out knockout of CG11340 resulted in copper tolerance, providing evidence for a specific nonneuronal role for this subunit in D. melanogaster Ligand-gated chloride channels are important insecticide targets and here we highlight copy number and functional divergence in insect lineages, raising the potential that order-specific receptors could be isolated within an effective class of insecticide targets.

Lack of Dietary Polyunsaturated Fatty Acids Causes Synapse Dysfunction in the Drosophila Visual System

Polyunsaturated fatty acids (PUFAs) are essential nutrients for animals and necessary for the normal functioning of the nervous system. A lack of PUFAs can result from the consumption of a deficient diet or genetic factors, which impact PUFA uptake and metabolism. Both can cause synaptic dysfunction, which is associated with numerous disorders. However, there is a knowledge gap linking these neuronal dysfunctions and their underlying molecular mechanisms. Because of its genetic manipulability and its easy, fast, and cheap breeding, Drosophila melanogaster has emerged as an excellent model organism for genetic screens, helping to identify the genetic bases of such events. As a first step towards the understanding of PUFA implications in Drosophila synaptic physiology we designed a breeding medium containing only very low amounts of PUFAs. We then used the fly’s visual system, a well-established model for studying signal transmission and neurological disorders, to measure the effects of a PUFA deficiency on synaptic function. Using both visual performance and eye electrophysiology, we found that PUFA deficiency strongly affected synaptic transmission in the fly’s visual system. These defects were rescued by diets containing omega-3 or omega-6 PUFAs alone or in combination. In summary, manipulating PUFA contents in the fly’s diet was powerful to investigate the role of these nutrients on the fly´s visual synaptic function. This study aims at showing how the first visual synapse of Drosophila can serve as a simple model to study the effects of PUFAs on synapse function. A similar approach could be further used to screen for genetic factors underlying the molecular mechanisms of synaptic dysfunctions associated with altered PUFA levels.

De novo assembly and characterization of central nervous system transcriptome reveals neurotransmitter signaling systems in the rice striped stem borer, Chilo suppressalis

Background

Neurotransmitter signaling systems play crucial roles in multiple physiological and behavioral processes in insects. Genome wide analyses of de novo transcriptome sequencing and gene specific expression profiling provide rich resources for studying neurotransmitter signaling pathways. The rice striped stem borer, Chilo suppressalis is a destructive rice pest in China and other Asian countries. The characterization of genes involved in neurotransmitter biosynthesis and transport could identify potential targets for disruption of the neurochemical communication and for crop protection.

Results

Here we report de novo sequencing of the C. suppressalis central nervous system transcriptome, identification and expression profiles of genes putatively involved in neurotransmitter biosynthesis, packaging, and recycling/degradation. A total of 54,411 unigenes were obtained from the transcriptome analysis. Among these unigenes, we have identified 32 unigenes (31 are full length genes), which encode 21 enzymes and 11 transporters putatively associated with biogenic aminergic signaling, acetylcholinergic signaling, glutamatergic signaling and GABAergic signaling. RT-PCR and qRT-PCR results indicated that 12 enzymes were highly expressed in the central nervous system and all the transporters were expressed at significantly high levels in the central nervous system. In addition, the transcript abundances of enzymes and transporters in the central nervous system were validated by qRT-PCR. The high expression levels of these genes suggest their important roles in the central nervous system.

Conclusions

Our study identified genes potentially involved in neurotransmitter biosynthesis and transport in C. suppressalis and these genes could serve as targets to interfere with neurotransmitter production. This study presents an opportunity for the development of specific and environmentally safe insecticides for pest control.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1742-7) contains supplementary material, which is available to authorized users.

Identifying functional connections of the inner photoreceptors in Drosophila using Tango-Trace

In Drosophila, the four inner photoreceptor neurons exhibit overlapping but distinct spectral sensitivities and mediate behaviors that reflect spectral preference. We developed a genetic strategy, Tango-Trace, that has permitted the identification of the connections of the four chromatic photoreceptors. Each of the four stochastically distributed chromatic photoreceptor subtypes make distinct connections in the medulla with four different TmY cells. Moreover, each class of TmY cells forms a retinotopic map in both the medulla and the lobula complex, generating four overlapping topographic maps that could carry different color information. Thus, the four inner photoreceptors transmit spectral information through distinct channels that may converge in both the medulla and lobula complex. These projections could provide an anatomic basis for color vision and may relay information about color to motion sensitive areas. Moreover, the Tango-Trace strategy we used may be applied more generally to identify neural circuits in the fly brain.

Transcriptional regionalization of the fruit fly's airway epithelium

Although airway epithelia are primarily devoted to gas exchange, they have to fulfil a number of different tasks including organ maintenance and the epithelial immune response to fight airborne pathogens. These different tasks are at least partially accomplished by specialized cell types in the epithelium. In addition, a proximal to distal gradient mirroring the transition from airflow conduction to real gas exchange, is also operative. We analysed the airway system of larval Drosophila melanogaster with respect to region-specific expression in the proximal to distal axis. The larval airway system is made of epithelial cells only. We found differential expression between major trunks of the airways and more distal ones comprising primary, secondary and terminal ones. A more detailed analysis was performed using DNA-microarray analysis to identify cohorts of genes that are either predominantly expressed in the dorsal trunks or in the primary/secondary/terminal branches of the airways. Among these differentially expressed genes are especially those involved in signal transduction. Wnt-signalling associated genes for example are predominantly found in secondary/terminal airways. In addition, some G-protein coupled receptors are differentially expressed between both regions of the airways, exemplified by those activated by octopamine or tyramine, the invertebrate counterparts of epinephrine and norepinephrine. Whereas the OAMB is predominantly found in terminal airway regions, the oct3βR has higher expression levels in dorsal trunks. In addition, we observed a significant association of both, genes predominantly expressed in dorsal trunks or in primary to terminal branches branches with those regulated by hypoxia. Taken together, this observed differential expression is indicative for a proximal to distal transcriptional regionalization presumably reflecting functional differences in these parts of the fly’s airway system.

Novel insights into the echinoderm nervous system from histaminergic and FMRFaminergic-like cells in the sea cucumber Leptosynapta clarki

Understanding of the echinoderm nervous system is limited due to its distinct organization in comparison to other animal phyla and by the difficulty in accessing it. The transparent and accessible, apodid sea cucumber Leptosynapta clarki provides novel opportunities for detailed characterization of echinoderm neural systems. The present study used immunohistochemistry against FMRFamide and histamine to describe the neural organization in juvenile and adult sea cucumbers. Histaminergic- and FMRFaminergic-like immunoreactivity is reported in several distinct cell types throughout the body of L. clarki. FMRFamide-like immunoreactive cell bodies were found in the buccal tentacles, esophageal region and in proximity to the radial nerve cords. Sensory-like cells in the tentacles send processes toward the circumoral nerve ring, while unipolar and bipolar cells close to the radial nerve cords display extensive processes in close association with muscle and other cells of the body wall. Histamine-like immunoreactivity was identified in neuronal somatas located in the buccal tentacles, circumoral nerve ring and in papillae distributed across the body. The tentacular cells send processes into the nerve ring, while the processes of cells in the body wall papillae extend to the surface epithelium and radial nerve cords. Pharmacological application of histamine produced a strong coordinated, peristaltic response of the body wall suggesting the role of histamine in the feeding behavior. Our immunohistochemical data provide evidence for extensive connections between the hyponeural and ectoneural nervous system in the sea cucumber, challenging previously held views on a clear functional separation of the sub-components of the nervous system. Furthermore, our data indicate a potential function of histamine in coordinated, peristaltic movements; consistent with feeding patterns in this species. This study on L. clarki illustrates how using a broader range of neurotransmitter systems can provide better insight into the anatomy, function and evolution of echinoderm nervous sytems.

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.

An Update on GABAρ Receptors

The present review discusses the functional and molecular diversity of GABAρ receptors. These receptors were originally described in the mammalian retina, and their functional role in the visual pathway has been recently elucidated; however new studies on their distribution in the brain and spinal cord have revealed that they are more spread than originally thought, and thus it will be important to determine their physiological contribution to the GABAergic transmission in other areas of the central nervous system. In addition, molecular modeling has revealed peculiar traits of these receptors that have impacted on the interpretations of the latest pharmacolgical and biophysical findings. Finally, sequencing of several vertebrate genomes has permitted a comparative analysis of the organization of the GABAρ genes.

Development of histamine-immunoreactivity in the Central nervous system of the two locust species Schistocerca gregaria and Locusta migratoria

Locusts are attractive model preparations for cellular investigations of neurodevelopment. In this study, we investigate the immunocytochemical localization of histamine in the developing ventral nerve cord of two locust species, Schistocerca gregaria and Locusta migratoria. Histamine is the fast neurotransmitter of photoreceptor neurons in the compound eye of insects, but it is also synthesized in interneurons of the central nervous system. In the locust ventral nerve cord, the pattern of histamine-immunoreactive neurons follows a relatively simple bauplan. The histaminergic system comprises a set of single, ascending projection neurons that are segmentally arranged in almost every neuromere. The neurons send out their axons anteriorly, forming branches and varicosities throughout the adjacent ganglia. In the suboesophageal ganglion, the cell bodies lie in a posteriolateral position. The prothoracic ganglion lacks histaminergic neurons. In the posterior ganglia of the ventral nerve cord, the somata of the histaminergic neurons are ventromedially positioned. Histamine-immunoreactivity starts around 50% of embryonic development in interneurons of the brain. Subsequently, the neurons of the more posterior ganglia of the ventral nerve cord become immunoreactive. From 60% embryonic development, the pattern of soma staining in the nerve cord appears mature. Around 65% of embryonic development, the photoreceptor cells show histamine-immunoreactivity. The histaminergic innervation of the neuropile develops from the central branches toward the periphery of the ganglia and is completed right before hatching.

Cys-Loop Ligand-Gated Chloride Channels in Dorsal Unpaired Median Neurons of Locusta migratoria

In insects, inhibitory neurotransmission is generally associated with members of the cys-loop ligand-gated anion channels, such as the glutamate-gated chloride channel (GluCl), the GABA-gated chloride channels (GABACl), and the histamine-gated chloride channels (HisCl). These ionotropic receptors are considered established target sites for the development of insecticides, and therefore it is necessary to obtain a better insight in their distribution, structure, and functional properties. Here, by combining electrophysiology and molecular biology techniques, we identified and characterized GluCl, GABACl, and HisCl in dorsal unpaired median (DUM) neurons of Locust migratoria. In whole cell patch-clamp recordings, application of glutamate, GABA, or histamine induced rapidly activating ionic currents. GluCls were sensitive to ibotenic acid and blocked by picrotoxin and fipronil. The pharmacological profile of the L. migratoria GABACl fitted neither the vertebrate GABAA nor GABAC receptor and was similar to the properties of the cloned Drosophila melanogaster GABA receptor subunit (Rdl). The expression of Rdl-like subunit-containing GABA receptors was shown at the molecular level using RT-PCR. Sequencing analysis indicated that the orthologous GABACl of D. melanogaster CG10357-A is expressed in DUM neurons of L. migratoria. Histamine-induced currents exhibited a fast onset and desensitized completely on continuous application of histamine. In conclusion, within the DUM neurons of L. migratoria, we identified three different cys-loop ligand-gated anion channels that use GABA, glutamate, or histamine as their neurotransmitter.

The functional organisation of glia in the adult brain of Drosophila and other insects

This review annotates and categorises the glia of adult Drosophila and other model insects and analyses the developmental origins of these in the Drosophila optic lobe. The functions of glia in the adult vary depending upon their sub-type and location in the brain. The task of annotating glia is essentially complete only for the glia of the fly's lamina, which comprise: two types of surface glia-the pseudocartridge and fenestrated glia; two types of cortex glia-the distal and proximal satellite glia; and two types of neuropile glia-the epithelial and marginal glia. We advocate that the term subretinal glia, as used to refer to both pseudocartridge and fenestrated glia, be abandoned. Other neuropiles contain similar glial subtypes, but other than the antennal lobes these have not been described in detail. Surface glia form the blood brain barrier, regulating the flow of substances into and out of the nervous system, both for the brain as a whole and the optic neuropiles in particular. Cortex glia provide a second level of barrier, wrapping axon fascicles and isolating neuronal cell bodies both from neighbouring brain regions and from their underlying neuropiles. Neuropile glia can be generated in the adult and a subtype, ensheathing glia, are responsible for cleaning up cellular debris during Wallerian degeneration. Both the neuropile ensheathing and astrocyte-like glia may be involved in clearing neurotransmitters from the extracellular space, thus modifying the levels of histamine, glutamate and possibly dopamine at the synapse to ultimately affect behaviour.

Functional consequences of mutations in the Drosophila histamine receptor HCLB

The gene hclB encodes a histamine-gated chloride channel subunit in Drosophila melanogaster. Mutations in hclB lead to defects in the visual system and altered sensitivity to the action of ivermectin. To investigate whether this member of the Cys-loop receptors is common across the Insecta, we analysed the genomes of seven other insect species (Diptera, Hymenoptera, Coleoptera) and revealed orthologues of hclB in all of them. Sequence comparisons showed high identity levels between the orthologues, indicating similar constraints and conserved function between the species. Two D. melanogaster mutants, hclB(T1) (P293S) and hclB(T2) (W111*, a null mutation) were tested for the lapse into, and recovery from, paralysis induced by high temperature or the anaesthetic action of halothane. At 41 degrees C, the hclB(T2) flies lapsed into coma faster than wild-type or the hclB(T1) flies, while both mutants recovered more slowly. A substantially impaired recovery rate was also observed in hclB(T1) after anaesthesia with halothane. Enhanced synaptic signalling at low-intensity light stimuli was registered on electroretinograms recorded from the two mutant strains. Our results suggest that HCLB may play an essential and conserved role in insect neurophysiology.

Photoreceptor neurons find new synaptic targets when misdirected by overexpressing runt in Drosophila

As a neuron differentiates, it adopts a suite of features specific to its particular type. Fly photoreceptors are of two types: R1-R6, which innervate the first optic neuropile, the lamina; and R7-R8, which innervate the second, the medulla. Photoreceptors R1-R6 normally have large light-absorbing rhabdomeres, express Rhodopsin1, and have synaptic terminals that innervate the lamina. In Drosophila melanogaster, we used the yeast GAL4/UAS system to drive exogenous expression of the transcription factor Runt in subsets of photoreceptors, resulting in aberrant axonal pathfinding and, ultimately, incorrect targeting of R1-R6 synaptic terminals to the medulla, normally occupied by terminals from R7 and R8. Even when subsets of their normal R1-R6 photoreceptor inputs penetrate the lamina, to terminate in the medulla, normal target cells within the lamina persist and maintain expression of cell-specific markers. Some R1-R6 photoreceptors form reciprocal synaptic inputs with their normal lamina targets, whereas supernumerary terminals targeted to the medulla also form synapses. At both sites, tetrad synapses form, with four postsynaptic elements at each release site, the usual number in the lamina. In addition, the terminals at both sites are invaginated by profiles of glia, at organelles called capitate projections, which in the lamina are photoreceptor sites of vesicle endocytosis. The size and shape of the capitate projection heads are identical at both lamina and medulla sites, although those in the medulla are ectopic and receive invaginations from foreign glia. This uniformity indicates the cell-autonomous determination of the architecture of its synaptic organelles by the presynaptic photoreceptor terminal.

The neural substrate of spectral preference in Drosophila

Drosophila vision is mediated by inputs from three types of photoreceptor neurons; R1-R6 mediate achromatic motion detection, while R7 and R8 constitute two chromatic channels. Neural circuits for processing chromatic information are not known. Here, we identified the first-order interneurons downstream of the chromatic channels. Serial EM revealed that small-field projection neurons Tm5 and Tm9 receive direct synaptic input from R7 and R8, respectively, and indirect input from R1-R6, qualifying them to function as color-opponent neurons. Wide-field Dm8 amacrine neurons receive input from 13-16 UV-sensing R7s and provide output to projection neurons. Using a combinatorial expression system to manipulate activity in different neuron subtypes, we determined that Dm8 neurons are necessary and sufficient for flies to exhibit phototaxis toward ultraviolet instead of green light. We propose that Dm8 sacrifices spatial resolution for sensitivity by relaying signals from multiple R7s to projection neurons, which then provide output to higher visual centers.

Altered ivermectin pharmacology and defective visual system in Drosophila mutants for histamine receptor HCLB

The Drosophila gene hclB encodes a histamine-gated chloride channel, which can be activated by the neurotoxin ivermectin when expressed in vitro. We have identified two novel hclB mutants, carrying either a missense mutation (P293S, allele hclB (T1)) or a putative null mutation (W111*, allele hclB (T2)), as well as a novel splice form of the gene. In survival studies, hclB (T1) mutants were more sensitive to ivermectin than wild-type, whereas hclB (T2) were more resistant. Electroretinogram recordings from the two mutants exhibited enlarged peak amplitudes of the transient components, indicating altered synaptic transmission between retinal photoneurons and their target cells. Ivermectin treatment severely affected or completely suppressed these transient components in an allele-specific manner. This suppression of synaptic signals by ivermectin was dose-dependent. These results identify HCLB as an important in vivo target for ivermectin in Drosophila melanogaster, and demonstrate the involvement of this protein in the visual pathway.

Distinct roles for two histamine receptors (hclA and hclB) at the Drosophila photoreceptor synapse

Histamine (HA) is the photoreceptor neurotransmitter in arthropods, directly gating chloride channels on large monopolar cells (LMCs), postsynaptic to photoreceptors in the lamina. Two histamine-gated channel genes that could contribute to this channel in Drosophila are hclA (also known as ort) and hclB (also known as hisCl1), both encoding novel members of the Cys-loop receptor superfamily. Drosophila S2 cells transfected with these genes expressed both homomeric and heteromeric histamine-gated chloride channels. The electrophysiological properties of these channels were compared with those from isolated Drosophila LMCs. HCLA homomers had nearly identical HA sensitivity to the native receptors (EC(50) = 25 microM). Single-channel analysis revealed further close similarity in terms of single-channel kinetics and subconductance states ( approximately 25, 40, and 60 pS, the latter strongly voltage dependent). In contrast, HCLB homomers and heteromeric receptors were more sensitive to HA (EC(50) = 14 and 1.2 microM, respectively), with much smaller single-channel conductances ( approximately 4 pS). Null mutations of hclA (ort(US6096)) abolished the synaptic transients in the electroretinograms (ERGs). Surprisingly, the ERG "on" transients in hclB mutants transients were approximately twofold enhanced, whereas intracellular recordings from their LMCs revealed altered responses with slower kinetics. However, HCLB expression within the lamina, assessed by both a GFP (green fluorescent protein) reporter gene strategy and mRNA tagging, was exclusively localized to the glia cells, whereas HCLA expression was confirmed in the LMCs. Our results suggest that the native receptor at the LMC synapse is an HCLA homomer, whereas HCLB signaling via the lamina glia plays a previously unrecognized role in shaping the LMC postsynaptic response.

Glutamate, GABA and acetylcholine signaling components in the lamina of the Drosophila visual system

Synaptic connections of neurons in the Drosophila lamina, the most peripheral synaptic region of the visual system, have been comprehensively described. Although the lamina has been used extensively as a model for the development and plasticity of synaptic connections, the neurotransmitters in these circuits are still poorly known. Thus, to unravel possible neurotransmitter circuits in the lamina of Drosophila we combined Gal4 driven green fluorescent protein in specific lamina neurons with antisera to γ-aminobutyric acid (GABA), glutamic acid decarboxylase, a GABA_B type of receptor, L-glutamate, a vesicular glutamate transporter (vGluT), ionotropic and metabotropic glutamate receptors, choline acetyltransferase and a vesicular acetylcholine transporter. We suggest that acetylcholine may be used as a neurotransmitter in both L4 monopolar neurons and a previously unreported type of wide-field tangential neuron (Cha-Tan). GABA is the likely transmitter of centrifugal neurons C2 and C3 and GABA_B receptor immunoreactivity is seen on these neurons as well as the Cha-Tan neurons. Based on an rdl-Gal4 line, the ionotropic GABA_A receptor subunit RDL may be expressed by L4 neurons and a type of tangential neuron (rdl-Tan). Strong vGluT immunoreactivity was detected in α-processes of amacrine neurons and possibly in the large monopolar neurons L1 and L2. These neurons also express glutamate-like immunoreactivity. However, antisera to ionotropic and metabotropic glutamate receptors did not produce distinct immunosignals in the lamina. In summary, this paper describes novel features of two distinct types of tangential neurons in the Drosophila lamina and assigns putative neurotransmitters and some receptors to a few identified neuron types.

Histamine operates Cl–-gated channels in crayfish neurosecretory cells

We describe a histamine-activated Cl– conductance in the X-organ neurons from crayfish Cherax quadricarinatus, which has comparable properties to the homomultimeric histamine-gated ion channels described in Drosophila. Topical application of histamine inhibited spontaneous neuronal firing in the X-organ sinus gland tract, concomitant with an increase in the membrane conductance. In X-organ neurons in culture and under voltage-clamp conditions, histamine evoked outward currents at –40 mV that reversed at the Cl– equilibrium potential. Histamine sensitivity in these neurons had a half-maximal response(EC50)=3.3±1 μmol l–1, with a Hill number of 2.6±0.4. The histamine-evoked current was blocked by tiotidine, cimetidine, ranitidine and 256±11 and 483±11 μmol l–1, respectively) and d-tubocurarine(IC50=21±2 μmol l–1), but was insensitive to picrotoxin, bicuculline and strychnine. Neither GABA nor glutamate was capable of desensitizing the histamine response, indicating that histamine activates a particular Cl– conductance. The presence of immunoreactive neurons to histamine in the medulla terminalis with axonal projections to the neuropile suggests a possible histaminergic modulation of the X-organ sinus gland system.

The dynamics of signaling at the histaminergic photoreceptor synapse of arthropods

Histamine, a ubiquitous aminergic messenger throughout the body, also serves as a neurotransmitter in both vertebrates and invertebrates. In particular, the photoreceptors of adult arthropods use histamine, modulating its release to signal increases and decreases in light intensity. Strong evidence from various arthropod species indicates that histamine is synthesized and stored in photoreceptors, undergoes Ca-dependent release, inhibits postsynaptic interneurons by gating Cl channels, and is then recycled. In Drosophila, the synthetic enzyme, histidine decarboxylase, and the subunits of the histamine-gated chloride channel have been cloned. Possible histamine transporters at synaptic vesicles and for reuptake remain elusive. Indeed, the mechanisms that remove histamine from the synaptic cleft, and that help terminate histamine's action, are unexpectedly complex, their details remaining unresolved. A major pathway in Drosophila, and possibly other arthropod species, is by conjugation of histamine to beta-alanine to form carcinine in adjacent glia. This conjugate then returns to the photoreceptors where it is hydrolysed to liberate histamine, which is then loaded into synaptic vesicles. Evidence from other species suggests that direct reuptake of histamine into the photoreceptors may also occur. Light depolarizes the photoreceptors, causing histamine release and postsynaptic inhibition; dimming hyperpolarizes the photoreceptors, causing a decrease in histamine release and an "off" response in the postsynaptic cell. Further pursuit of histamine's action at these highly specialized synapses should lead to an understanding of how they signal minute changes in presynaptic membrane potential, how they reliably extract signals from noise, and how they adapt to a wide range of presynaptic membrane potentials.

The histamine H1 receptor activates the nitric oxide pathway at fertilization

Sperm fusion with the egg initiates a signaling cascade that releases intracellular calcium (Ca(i) (2+)) from the endoplasmic reticulum (ER). In sea urchins, Ca(2+) is released as a single, large transient via two distinct pathways. The first depends on inositol 1,4,5-triphosphate (IP(3)) production and triggers the initial phase of Ca(2+) release, while the second depends on nitric oxide (NO) production and is thought to maintain the duration of the Ca(2+) wave. We identified a sea urchin homolog of the seven trans-membrane G protein-coupled receptor for histamine (suH(1)R) on the egg cell surface that activates NO production. Treatment with histamine (HA) causes fluctuations in the resting levels of NO in the egg, while antagonists or antibodies of H(1)R inhibit the rise of NO normally observed at fertilization. Inhibition of suH(1)R function decreases the maintenance, but not the amplitude, of the Ca(2+) transient and suggests that it is an integral part of the overall pathway leading to egg activation at fertilization in sea urchins.

Phylogenetic shadowing of a histamine-gated chloride channel involved in insect vision

A recently identified gene, hclA (synonym: ort), codes for an ionotrophic histamine receptor subunit in Drosophila melanogaster, and known hclA mutations lead to defects in the visual system, neurologic disorders and changed responsiveness to neurotoxins. To investigate whether this novel class of receptors is common across the Insecta, we analysed the genomes of 15 other insect species (Diptera, Hymenoptera, Coleoptera, Lepidoptera) and revealed orthologs of hclA in all of them. The predicted receptor domain of HCLA is extensively conserved (86-100% of identity) among the 16 proteins. Minor changes in the amino acid sequence that includes the putative transmembrane domains (TMs) 1-3 were found in non-drosophilid species only. Substantial amino acid variability was observed in the signal polypeptides, the intracellular loop domains and in TM4, in good accordance with known data on sequence variations in ligand-gated ion channels. Pairwise comparisons revealed three consensus sequences for N-glycosylation, conserved in HCLAs of all species studied, as well as a drosophilid-specific putative phosphorylation site. Real-time PCR analysis demonstrated that hclA-mRNA is abundant in heads of adult Drosophila. However, species- and sex-specific variations of the hclA expression levels were also observed.

A Novel Chloride Channel in Drosophila melanogaster Is Inhibited by Protons

A systematic analysis of the Drosophila genome data reveals the existence of pHCl, a novel member of ligand-gated ion channel subunits. pHCl shows nearly identical similarity to glutamate-, glycine-, and histamine-gated ion channels, does however not belong to any of these ion channel types. We identified three different sites, where splicing generates multiple transcripts of the pHCl mRNA. The pHCl is expressed in Drosophila embryo, larvae, pupae, and the adult fly. In embryos, in situ hybridization detected pHCl in the neural cord and the hindgut. Functional expression of the three different splice variants of pHCl in oocytes of Xenopus laevis and Sf9 cells induces a chloride current with a linear current-voltage relationship that is inhibited by extracellular protons and activated by avermectins in a pH-dependent manner. Further, currents through pHCl channels were induced by a raise in temperature. Our data give genetic and electrophysiological evidence that pHCl is a member of a new branch of ligand-gated ion channels in invertebrates with, however, a hitherto unique combination of pharmacological and biophysical properties.

Unusual functional properties of homo- and heteromultimeric histamine-gated chloride channels of Drosophila melanogaster: spontaneous currents and dual gating by GABA and histamine

Histamine acts as a neurotransmitter of photoreceptors in insects and other arthropods, where it directly activates a chloride channel and mediates rapid inhibitory responses. Homo- and heteromultimeric histamine-gated ion channels formed by HisCl-alpha2 or HisCl-alpha1 + alpha2 subunits from Drosophila melanogaster were characterized by two-electrode voltage-clamp measurements of functionally expressed ion channels in Xenopus laevis oocytes. The sensitivity of heteromultimeric histamine receptors with an EC(50) of 2.3 microM is lower than that of either homomultimeric receptor. They can be further distinguished from the homomultimeric channels by their reduced sensitivity to d-tubocurarine. Heteromultimeric channels generate a spontaneous current in the absence of any agonist. This spontaneous current can be blocked in the absence of an agonist by d-tubocurarine and the histamine antagonists cimetidine, thioperamide and pyrilamine. Homomultimeric HisCl-alpha2 channels are dually gated by histamine (IC(50)=9.4 microM) and GABA (IC(50)=1.0mM), both of which are full agonists. The action of both agonists can be blocked with comparable IC(50) values by the histamine antagonists cimetidine, thioperamide and pyrilamine but not by the GABA antagonist bicuculline. Picrotoxin blocked with an IC(50) of 403 microM. Our data show that histamine and GABA act on the same ion channel, which thus might function as a site of integration of the action of different neurotransmitters.

Differential transcription in defined parts of the insect brain: comparative study utilizing Drosophila melanogaster and Schistocerca gregaria

The brain of all higher organisms has a modular architecture. Processing of various tasks, such as learning, olfaction, or motor control is performed in specialized brain areas, characterized by morphological and molecular peculiarities. To identify those genes that are transcribed in only one region of the insect brain, we chose two different approaches, differential display PCR and DNA array hybridization, with two different insect species, the desert locust Schistocerca gregaria and the fruitfly Drosophila melanogaster. The optic lobes (centers of visual information processing), the midbrain (the region of the brain where almost all "higher" centers are localized), and the thoracic ganglia (regions required to control various peripheral organs) were compared in both types of experiments. Both, the differential display PCR screen of the different parts of the locust brain as well as the DNA array screen of the Drosophila brain revealed almost identical numbers of transcripts exclusively present in either of the three above-mentioned brain areas. Interestingly, the brain areas with the largest number of differential transcripts are the thoracic ganglia and not the midbrain.

CSTX-13, a highly synergistically acting two-chain neurotoxic enhancer in the venom of the spider Cupiennius salei (Ctenidae)

The survival of the spider Cupiennius salei depends on its hunting success, which largely relies on its immediately paralyzing multicomponent venom. Here, we report on the isolation and characterization of CSTX-13, a neurotoxic enhancer in the spider venom. De novo elucidation of the disulfide bridge pattern of CSTX-13 and the neurotoxin CSTX-1 by tandem MS revealed an identical arrangement. However, in contrast to CSTX-1, CSTX-13 is a two-chain peptide with two interchain and two intrachain disulfide bridges. Furthermore, the insecticidal activity of CSTX-13 is synergistically increased in the presence of K+ ions as well as of the cytolytic peptide cupiennin 1a. We demonstrated that the weakly neurotoxic CSTX-13 enhances the paralytic activity of the neurotoxin CSTX-1 by 65% when it is administered with the latter at its entirely nontoxic physiological concentration, which is 440 times below its LD50 concentration.

Drosophila melanogaster GRD and LCCH3 subunits form heteromultimeric GABA-gated cation channels

In addition to its action as a fast inhibitory neurotransmitter, gamma-aminobutyric acid (GABA) is thought to mediate excitatory action by activating cation currents in some cell types in invertebrates. However, to date no GABA receptor capable of mediating such action has been identified at the molecular level in insects. Using a systematic expression screening approach, we found that the Drosophila ligand-gated ion channel subunits GRD and LCCH3 combine to form cation-selective GABA-gated ion channels when coexpressed in Xenopus laevis oocytes. The heteromultimeric receptor is activated by GABA (EC50=4.5 microm), muscimol (EC50=4.8 microm) and trans-4-aminocrotonic acid (EC50=104.5 microm), and partially by cis-4-aminocrotonic acid (EC50=106.3 microm). Picrotoxin effectively blocked the GABA-gated channel (IC50=0.25 microm), but bicuculline, TPMTA, dieldrin and lindane did not. The benzodiazepines flunitrazepam and diazepam did not potentiate the GABA-evoked current. Our data suggest that heteromultimeric channels composed of GRD and LCCH3 subunits form GABA-gated cation channels in insects.

Histamine modulates thalamocortical activity by activating a chloride conductance in ferret perigeniculate neurons

In the mammalian central nervous system only gamma-aminobutyric acid (GABA) and glycine have been firmly linked to inhibition of neuronal activity through increases in membrane Cl(-) conductance, and these responses are mediated by ionotropic receptors. Iontophoretic application of histamine can also cause inhibitory responses in vivo, although the mechanisms of this inhibition are unknown and may involve pre- or postsynaptic factors. Here, we report that application of histamine to the GABAergic neurons of the thalamic perigeniculate nucleus (PGN), which is innervated by histaminergic fibers from the tuberomammillary nucleus of the hypothalamus, causes a slow membrane hyperpolarization toward a reversal potential of -73 mV through a relatively small increase in membrane conductance to Cl(-). This histaminergic action appears to be mediated by the H(2) subclass of histaminergic receptors and inhibits the single-spike activity of these PGN GABAergic neurons. Application of histamine to the PGN could halt the generation of spindle waves, indicating that increased activity in the tuberomammillary histaminergic system may play a functional role in dampening thalamic oscillations in the transition from sleep to arousal.