Distribution of metabotropic glutamate receptor DmGlu-A in Drosophila melanogaster central nervous system

CORL Expression and Function in Insulin Producing Neurons Reversibly Influences Adult Longevity in Drosophila

CORL proteins (known as SKOR in mice, Fussel in humans and fussel in Flybase) are a family of CNS specific proteins related to Sno/Ski oncogenes. Their developmental and adult roles are largely unknown. A Drosophila CORL (dCORL) reporter gene is expressed in all Drosophila insulin-like peptide 2 (dILP2) neurons of the pars intercerebralis (PI) of the larval and adult brain. The transcription factor Drifter is also expressed in the PI in a subset of dCORL and dILP2 expressing neurons and in several non-dILP2 neurons. dCORL mutant virgin adult brains are missing all dILP2 neurons that do not also express Drifter. This phenotype is also seen when expressing dCORL-RNAi in neurosecretory cells of the PI. dCORL mutant virgin adults of both sexes have a significantly shorter lifespan than their parental strain. This longevity defect is completely reversed by mating (lifespan increases over 50% for males and females). Analyses of dCORL mutant mated adult brains revealed a complete rescue of dILP2 neurons without Drifter. Taken together, the data suggest that dCORL participates in a neural network connecting the insulin signaling pathway, longevity and mating. The conserved sequence and CNS specificity of all CORL proteins imply that this network may be operating in mammals.

Drep-2 is a novel synaptic protein important for learning and memory

CIDE-N domains mediate interactions between the DNase Dff40/CAD and its inhibitor Dff45/ICAD. In this study, we report that the CIDE-N protein Drep-2 is a novel synaptic protein important for learning and behavioral adaptation. Drep-2 was found at synapses throughout the Drosophila brain and was strongly enriched at mushroom body input synapses. It was required within Kenyon cells for normal olfactory short- and intermediate-term memory. Drep-2 colocalized with metabotropic glutamate receptors (mGluRs). Chronic pharmacological stimulation of mGluRs compensated for drep-2 learning deficits, and drep-2 and mGluR learning phenotypes behaved non-additively, suggesting that Drep 2 might be involved in effective mGluR signaling. In fact, Drosophila fragile X protein mutants, shown to benefit from attenuation of mGluR signaling, profited from the elimination of drep-2. Thus, Drep-2 is a novel regulatory synaptic factor, probably intersecting with metabotropic signaling and translational regulation.

DOI:http://dx.doi.org/10.7554/eLife.03895.001

Synapses are specialized structures that connect nerve cells to one another and allow information to be transmitted between the cells. Synapses are essential for learning and storing memories. Many proteins that regulate how signals are transmitted at synapses have already been studied. In this manner, much has been learned about their function in learning and memory.

Cells can commit suicide by a process called apoptosis, also known as programmed cell death. Apoptosis is not only triggered in damaged cells but is also necessary for an organism to develop correctly. In fruit flies, the protein Drep-2 is a member of a family of proteins that degrade the DNA of cells that undergo apoptosis.

Andlauer et al. found no evidence that Drep-2 plays a role in apoptosis, but have now found Drep-2 at the synapses of the brain of the fruit fly Drosophila. Drep-2 could be observed in close proximity to another type of protein called metabotropic glutamate receptors. Metabotropic glutamate receptors and their signaling pathways are important for regulating certain changes to the synapses that mediate learning processes. Indeed, Andlauer et al. found that flies that have lost the gene that produces Drep-2 were unable to remember smells when these were paired with a punishment. Stimulating the regulatory glutamate receptors with drugs helped to overcome learning deficits that result from the lack of Drep-2.

Alterations in the production of a protein called FMRP cause fragile X syndrome in humans, the most common form of hereditary mental disability originating from a single gene defect. Flies lacking the FMRP protein show learning deficits that are very similar to the ones seen in flies that cannot produce Drep-2. However, Andlauer et al. observed that flies lacking both Drep-2 and FMRP can learn normally.

Exactly how Drep-2 works in synapses to help with memory formation remains to be discovered, although there are indications that it boosts the effects of signaling from the glutamate receptors and counteracts FMRP. Further research will be needed to establish whether the mammalian proteins related to Drep-2 perform similar roles in the brains of mammals.

DOI:http://dx.doi.org/10.7554/eLife.03895.002

Glutamate is an inhibitory neurotransmitter in the Drosophila olfactory system

Glutamatergic neurons are abundant in the Drosophila central nervous system, but their physiological effects are largely unknown. In this study, we investigated the effects of glutamate in the Drosophila antennal lobe, the first relay in the olfactory system and a model circuit for understanding olfactory processing. In the antennal lobe, one-third of local neurons are glutamatergic. Using in vivo whole-cell patch clamp recordings, we found that many glutamatergic local neurons are broadly tuned to odors. Iontophoresed glutamate hyperpolarizes all major cell types in the antennal lobe, and this effect is blocked by picrotoxin or by transgenic RNAi-mediated knockdown of the GluClα gene, which encodes a glutamate-gated chloride channel. Moreover, antennal lobe neurons are inhibited by selective activation of glutamatergic local neurons using a nonnative genetically encoded cation channel. Finally, transgenic knockdown of GluClα in principal neurons disinhibits the odor responses of these neurons. Thus, glutamate acts as an inhibitory neurotransmitter in the antennal lobe, broadly similar to the role of GABA in this circuit. However, because glutamate release is concentrated between glomeruli, whereas GABA release is concentrated within glomeruli, these neurotransmitters may act on different spatial and temporal scales. Thus, the existence of two parallel inhibitory transmitter systems may increase the range and flexibility of synaptic inhibition.

The Drosophila DmGluRA is required for social interaction and memory

Metabotropic glutamate receptors (mGluRs) have well-established roles in cognition and social behavior in mammals. Whether or not these roles have been conserved throughout evolution from invertebrate species is less clear. Mammals have eight mGluRs whereas Drosophila has a single DmGluRA, which has both Gi and Gq coupled signaling activity. We have utilized Drosophila to examine the role of DmGluRA in social behavior and various phases of memory. We have found that flies that are homozygous or heterozygous for loss of function mutations of DmGluRA have impaired social behavior in male Drosophila. Futhermore, flies that are heterozygous for loss of function mutations of DmGluRA have impaired learning during training, immediate-recall memory, short-term memory, and long-term memory as young adults. This work demonstrates a role for mGluR activity in both social behavior and memory in Drosophila.

Dramatic expansion and developmental expression diversification of the methuselah gene family during recent Drosophila evolution

Functional studies of the methuselah/methuselah-like (mth/mthl) gene family have focused on the founding member mth, but little is known regarding the developmental functions of this receptor or any of its paralogs. We undertook a comprehensive analysis of developmental expression and sequence divergence in the mth/mthl gene family. Using in situ hybridization techniques, we detect expression of six genes (mthl1, 5, 9, 11, 13, and 14) in the embryo during gastrulation and development of the gut, heart, and lymph glands. Four receptors (mthl3, 4, 6, and 8) are expressed in the larval central nervous system, imaginal discs, or both, and two receptors (mthl10 and mth) are expressed in both embryos and larvae. Phylogenetic analysis of all mth/mthl genes in five Drosophila species, mosquito and flour beetle structured the mth/mthl family into several subclades. mthl1, 5, and 14 are present in most species, each forming a separate clade. A newly identified Drosophila mthl gene (CG31720; herein mthl15) formed another ancient clade. The remaining Drosophila receptors, including mth, are members of a large "superclade" that diversified relatively recently during dipteran evolution, in many cases within the melanogaster subgroup. Comparing the expression patterns of the mth/mthl "superclade" paralogs to the embryonic expression of the singleton ortholog in Tribolium suggests both subfunctionalization and acquisition of novel functionalities. Taken together, our findings shed novel light on mth as a young member of an adaptively evolving developmental gene family.

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.

Candidate glutamatergic neurons in the visual system of Drosophila

The visual system of Drosophila contains approximately 60,000 neurons that are organized in parallel, retinotopically arranged columns. A large number of these neurons have been characterized in great anatomical detail. However, studies providing direct evidence for synaptic signaling and the neurotransmitter used by individual neurons are relatively sparse. Here we present a first layout of neurons in the Drosophila visual system that likely release glutamate as their major neurotransmitter. We identified 33 different types of neurons of the lamina, medulla, lobula and lobula plate. Based on the previous Golgi-staining analysis, the identified neurons are further classified into 16 major subgroups representing lamina monopolar (L), transmedullary (Tm), transmedullary Y (TmY), Y, medulla intrinsic (Mi, Mt, Pm, Dm, Mi Am), bushy T (T), translobula plate (Tlp), lobula intrinsic (Lcn, Lt, Li), lobula plate tangential (LPTCs) and lobula plate intrinsic (LPi) cell types. In addition, we found 11 cell types that were not described by the previous Golgi analysis. This classification of candidate glutamatergic neurons fosters the future neurogenetic dissection of information processing in circuits of the fly visual system.

Drosophila glial glutamate transporter Eaat1 is regulated by fringe-mediated notch signaling and is essential for larval locomotion

In the mammalian CNS, glial cells expressing excitatory amino acid transporters (EAATs) tightly regulate extracellular glutamate levels to control neurotransmission and protect neurons from excitotoxic damage. Dysregulated EAAT expression is associated with several CNS pathologies in humans, yet mechanisms of EAAT regulation and the importance of glutamate transport for CNS development and function in vivo remain incompletely understood. Drosophila is an advanced genetic model with only a single high-affinity glutamate transporter termed Eaat1. We found that Eaat1 expression in CNS glia is regulated by the glycosyltransferase Fringe, which promotes neuron-to-glia signaling through the Delta-Notch ligand-receptor pair during embryogenesis. We made Eaat1 loss-of-function mutations and found that homozygous larvae could not perform the rhythmic peristaltic contractions required for crawling. We found no evidence for excitotoxic cell death or overt defects in the development of neurons and glia, and the crawling defect could be induced by postembryonic inactivation of Eaat1. Eaat1 fully rescued locomotor activity when expressed in only a limited subpopulation of glial cells situated near potential glutamatergic synapses within the CNS neuropil. Eaat1 mutants had deficits in the frequency, amplitude, and kinetics of synaptic currents in motor neurons whose rhythmic patterns of activity may be regulated by glutamatergic neurotransmission among premotor interneurons; similar results were seen with pharmacological manipulations of glutamate transport. Our findings indicate that Eaat1 expression is promoted by Fringe-mediated neuron-glial communication during development and suggest that Eaat1 plays an essential role in regulating CNS neural circuits that control locomotion in Drosophila.

Cloning and distribution of a putative octopamine/tyramine receptor in the central nervous system of the freshwater prawn Macrobrachium rosenbergii

There is ample evidence linking octopamine (OA) and tyramine (TA) to several neurophysiological functions in arthropods. In our laboratory we use the freshwater prawn Macrobrachium rosenbergii to study the neural basis of aggressive behavior. As a first step towards understanding the possible role of these amines and their receptors in the modulation of interactive behaviors, we have cloned a putative octopamine/tyramine receptor. The predicted sequence of the cloned OA/TA(Mac) receptor consists of 1,579 base pairs (bp), with an open reading frame of 1,350bp that encodes a 450 amino acid protein. This putative protein displays sequence identities of 70% to an Aedes aegypti mosquito TA receptor, followed by 60% to a Stegomyia aegypti mosquito OA receptor, 59% and 58% to the migratory locust TA-1 and -2 receptors respectively, and 57% with the silkworm OA receptor. We also mapped the OA/TA(Mac) receptor distribution by in-situ hybridization to the receptor's mRNA, and by immunohistochemistry to its protein. We observed stained cell bodies for the receptor's mRNA, mainly in the midline region of the thoracic and in the abdominal ganglia, as well as diffuse staining in the brain ganglia. For the receptor's protein, we observed extensive punctate staining within the neuropil and on the membrane of specific groups of neurons in all ganglia throughout the CNS, including the brain, the midline region and neuropiles of the thoracic ganglia, and ventral part and neuropiles of the abdominal ganglia. The same pattern of stained cells was observed on the thoracic and abdominal ganglia in both in-situ hybridization and immunohistochemistry experiments. Diffuse staining observed with in-situ hybridization also coincides with punctate staining observed in brain, SEG, thoracic, and abdominal ganglia in immunohistochemical preparations. This work provides the first step towards characterizing the neural networks that mediate octopaminergic signaling in prawn.

Visualizing glutamatergic cell bodies and synapses in Drosophila larval and adult CNS

Glutamate is the major excitatory neurotransmitter in the vertebrate central nervous system (CNS) and at Drosophila neuromuscular junctions (NMJs). Although glutamate is also used as a transmitter in the Drosophila CNS, there has been no systematic description of the central glutamatergic signaling system in the fly. With the recent cloning of the Drosophila vesicular glutamate transporter (DVGLUT), it is now possible to mark many, if not all, central glutamatergic neurons and synapses. Here we present the pattern of glutamatergic synapses and cell bodies in the late larval CNS and in the adult fly brain by using an anti-DVGLUT antibody. We also introduce two new tools for studying the Drosophila glutamatergic system: a dvglut promoter fragment fused to Gal4 whose expression labels glutamatergic neurons and a green fluorescent protein (GFP)-tagged DVGLUT transgene that localizes to synapses. In the larval CNS, we find synaptic DVGLUT immunoreactivity prominent in all brain lobe neuropil compartments except for the mushroom body. Likewise in the adult CNS, glutamatergic synapses are abundant throughout all major brain structures except the mushroom body. We also find that the larval ventral nerve cord neuropil is rich in glutamatergic synapses, which are primarily located near the dorsal surface of the neuropil, segregated from the ventrally positioned cholinergic processes. This description of the glutamatergic system in Drosophila highlights the prevalence of glutamatergic neurons in the CNS and presents tools for future study and manipulation of glutamatergic transmission.

Widespread brain distribution of the Drosophila metabotropic glutamate receptor

Glutamate is the predominant excitatory neurotransmitter in the vertebrate brain, whereas acetylcholine has been considered to play the same role in insects. Recent studies have, however, questioned the latter view by showing a rather general distribution of glutamate transporters. Here, we describe the expression pattern of the receptor DmGlu-A (DmGluRA), the unique homolog of vertebrate metabotropic glutamate receptors. Metabotropic glutamate receptors play important roles in the regulation of glutamatergic neurotransmission. Using a specific antibody, we report DmGluRA expression in most neuropile areas in both larvae and adults, but not in the lobes of the mushroom bodies. These observations suggest a key role for glutamate in the insect brain.

Mechanistic relationships between Drosophila fragile X mental retardation protein and metabotropic glutamate receptor A signaling

Fragile X syndrome is caused by loss of the FMRP translational regulator. A current hypothesis proposes that FMRP functions downstream of mGluR signaling to regulate synaptic connections. Using the Drosophila disease model, we test relationships between dFMRP and the sole Drosophila mGluR (DmGluRA) by assaying protein expression, behavior and neuron structure in brain and NMJ; in single mutants, double mutants and with an mGluR antagonist. At the protein level, dFMRP is upregulated in dmGluRA mutants, and DmGluRA is upregulated in dfmr1 mutants, demonstrating mutual negative feedback. Null dmGluRA mutants display defects in coordinated movement behavior, which are rescued by removing dFMRP expression. Null dfmr1 mutants display increased NMJ presynaptic structural complexity and elevated presynaptic vesicle pools, which are rescued by blocking mGluR signaling. Null dfmr1 brain neurons similarly display increased presynaptic architectural complexity, which is rescued by blocking mGluR signaling. These data show that DmGluRA and dFMRP convergently regulate presynaptic properties.

Glutamate and its metabotropic receptor in Drosophila clock neuron circuits

Identification of the neurotransmitters in clock neurons is critical for understanding the circuitry of the neuronal network that controls the daily behavioral rhythms in Drosophila. Except for the neuropeptide pigment-dispersing factor, no neurotransmitters have been clearly identified in the Drosophila clock neurons. Here we show that glutamate and its metabotropic receptor, DmGluRA, are components of the clock circuitry and modulate the rhythmic behavior pattern of Drosophila. The dorsal clock neurons, DN1s in the larval brain and some DN1s and DN3s in the adult brain, were immunolabeled with antibodies against Drosophila vesicular glutamate transporter (DvGluT), suggesting that they are glutamatergic. Because the DN1s may communicate with the primary pacemaker neurons, s-LN(v)s, we tested glutamate responses of dissociated larval s-LN(v)s by means of calcium imaging. Application of glutamate dose dependently decreased intracellular calcium in the s-LN(v)s. Pharmacology of the response suggests the presence of DmGluRA on the s-LN(v)s. Antibodies against DmGluRA labeled dissociated s-LN(v)s and the LN(v) dendrites in the intact larval and adult brain. The role of metabotropic glutamate signaling was tested in behavior assays in transgenic larvae and flies with altered DmGluRA expression in the LN(v)s and other clock neurons. Larval photophobic behavior was enhanced in DmGluRA mutants. For adults, we could induce altered activity patterns in the dark phase under LD conditions and increase the period during constant darkness by knockdown of DmGluRA expression in LN(v)s. Our results suggest that a glutamate signal from some of the DNs modulates the rhythmic behavior pattern via DmGluRA on the LN(v)s in Drosophila.

Characterization of a metabotropic glutamate receptor in the honeybee (Apis mellifera): implications for memory formation

G-protein-coupled metabotropic glutamate receptors (GPC mGluRs) are important constituents of glutamatergic synapses where they contribute to synaptic plasticity and development. Here we characterised a member of this family in the honeybee. We show that the honeybee genome encodes a genuine mGluR (AmGluRA) that is expressed at low to medium levels in both pupal and adult brains. Analysis of honeybee protein sequence places it within the type 3 GPCR family, which includes mGlu receptors, GABA-B receptors, calcium-sensing receptors, and pheromone receptors. Phylogenetic comparisons combined with pharmacological evaluation in HEK 293 cells transiently expressing AmGluRA show that the honeybee protein belongs to the group II mGluRs. With respect to learning and memory AmGluRA appears to be required for memory formation. Both agonists and antagonists selective against the group II mGluRs impair long-term (24 h) associative olfactory memory formation when applied 1 h before training, but have no effect when injected post-training or pre-testing. Our results strengthen the notion that glutamate is a key neurotransmitter in memory processes in the honeybee.

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

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

Differential regulation of two distinct voltage-dependent sodium currents by group III metabotropic glutamate receptor activation in insect pacemaker neurons

Using whole cell patch-clamp technique and immunocytochemistry on adult dorsal unpaired median (DUM) neurons isolated from the cockroach Periplaneta americana CNS, we reported the characterization of a native mGluR, sharing pharmacological properties with vertebrate metabotropic glutamate receptor III (mGluRIII) that regulated voltage-dependent sodium current (I(Na)). The global I(Na) was dissociated by means of l-glutamate sensitivity, deactivation time constant, voltage dependence of activation and inactivation, recovery from inactivation, and intracellular regulation process. These two currents were respectively designated I(Na1) and I(Na2) for l-glutamate-sensitive and -insensitive sodium currents. l-glutamate selectively reduced I(Na1) by an increase of intracellular cAMP level. Using different activators and/or inhibitors of G proteins and cAMP/PKA cascade, together with St-Ht31 (an inhibitor of PKA binding to AKAP) and AKAP-79 antibodies, we established that mGluRIII was linked to I(Na1) by a Gi/o and a suspected Gs protein. According to the activated signaling pathway, l-glutamate elevated the cAMP level, which thereby activated cytosolic PKA and released PKA bound to AKAP. As expected from both biophysical and pharmacological studies, we showed that, through an inhibition of I(Na1), l-glutamate increased DUM neuron spontaneous electrical activity. These results indicated that such mGluRIII-activated dual processes provided a new physiological control of pacemaker neuronal firing.

Glomerular maps without cellular redundancy at successive levels of the Drosophila larval olfactory circuit

BACKGROUND

Drosophila larvae possess only 21 odorant-receptor neurons (ORNs), whereas adults have 1,300. Does this suggest that the larval olfactory system is built according to a different design than its adult counterpart, or is it just a miniature version thereof?

RESULTS

By genetically labeling single neurons with FLP-out and MARCM techniques, we analyze the connectivity of the larval olfactory circuit. We show that each of the 21 ORNs is unique and projects to one of 21 morphologically identifiable antennal-lobe glomeruli. Each glomerulus seems to be innervated by a single projection neuron. Each projection neuron sends its axon to one or two of about 28 glomeruli in the mushroom-body calyx. We have discovered at least seven types of projection neurons that stereotypically link an identified antennal-lobe glomerulus with an identified calycal glomerulus and thus create an olfactory map in a higher brain center.

CONCLUSIONS

The basic design of the larval olfactory system is similar to the adult one. However, ORNs and projection neurons lack cellular redundancy and do not exhibit any convergent or divergent connectivity; 21 ORNs confront essentially similar numbers of antennal-lobe glomeruli, projection neurons, and calycal glomeruli. Hence, we propose the Drosophila larva as an "elementary" olfactory model system.

Glutamatergic innervation of the heart initiates retrograde contractions in adult Drosophila melanogaster

The adult abdominal heart of Drosophila melanogaster receives extensive innervation from glutamatergic neurons at specific cardiac regions during metamorphosis. Here, we show that the neurons form presynaptic specializations, as indicated by the localization of synaptotagmin and active zone markers, adjacent to postsynaptic sites that have aggregates of glutamate IIA receptors. To determine the role of this innervation in cardiac function, we developed an optical technique, based on the movement of green fluorescent protein-labeled nerve terminals, to monitor heart beat in intact and semi-intact preparations. Simultaneous monitoring of adjacent cardiac chambers revealed the direction of contractions and allowed correlation with volume changes. The cardiac cycle is composed of an anterograde beat in alternation with a retrograde beat, which correlate respectively with systole and diastole of this multichambered heart. The periodic change in hemolymph direction is referred to as cardiac reversal. Intracellular recordings from muscles of the first abdominal cardiac chamber, the conical chamber, revealed pacemaker action potentials and the excitatory effect of local glutamate application, which initiated retrograde contractions in semi-intact preparations. Unilateral electrical stimulation of the transverse nerve containing the glutamatergic neuron that serves the conical chamber caused a chronotropic effect and initiation of retrograde contractions. This effect is distinct from that of peripheral crustacean cardioactive peptide (CCAP) neurons, which potentiate the anterograde beat. Cardiac reversal was evoked pharmacologically by sequentially applying CCAP and glutamate to the heart.

Pharmacological rescue of synaptic plasticity, courtship behavior, and mushroom body defects in a Drosophila model of fragile X syndrome

Fragile X syndrome is a leading heritable cause of mental retardation that results from the loss of FMR1 gene function. A Drosophila model for Fragile X syndrome, based on the loss of dfmr1 activity, exhibits phenotypes that bear similarity to Fragile X-related symptoms. Herein, we demonstrate that treatment with metabotropic glutamate receptor (mGluR) antagonists or lithium can rescue courtship and mushroom body defects observed in these flies. Furthermore, we demonstrate that dfmr1 mutants display cognitive deficits in experience-dependent modification of courtship behavior, and treatment with mGluR antagonists or lithium restores these memory defects. These findings implicate enhanced mGluR signaling as the underlying cause of the cognitive, as well as some of the behavioral and neuronal, phenotypes observed in the Drosophila Fragile X model. They also raise the possibility that compounds having similar effects on metabotropic glutamate receptors may ameliorate cognitive and behavioral defects observed in Fragile X patients.

Immunocytochemical mapping and quantification of expression of a putative type 1 serotonin receptor in the crayfish nervous system

Serotonin is an important neurotransmitter that is involved in modulation of sensory, motor, and higher functions in many species. In the crayfish, which has been developed as a model for nervous system function for over a century, serotonin modulates several identified circuits. Although the cellular and circuit effects of serotonin have been extensively studied, little is known about the receptors that mediate these signals. Physiological data indicate that identified crustacean cells and circuits are modulated via several different serotonin receptors. We describe the detailed immunocytochemical localization of the crustacean type 1 serotonin receptor, 5-HT1crust, throughout the crayfish nerve cord and on abdominal superficial flexor muscles. 5-HT1crust is widely distributed in somata, including those of several identified neurons, and neuropil, suggesting both synaptic and neurohormonal roles. Individual animals show very different levels of 5-HT1crust immunoreactivity (5-HT(1crust)ir) ranging from preparations with hundreds of labeled cells per ganglion to some containing only a handful of 5-HT(1crust)ir cells in the entire nerve cord. The interanimal variability in 5-HT(1crust)ir is great, but individual nerve cords show a consistent level of labeling between ganglia. Quantitative RT-PCR shows that 5-HT1crust mRNA levels between animals are also variable but do not directly correlate with 5-HT(1crust)ir levels. Although there is no correlation of 5-HT1crust expression with gender, social status, molting or feeding, dominant animals show significantly greater variability than subordinates. Functional analysis of 5-HT1crust in combination with this immunocytochemical map will aid further understanding of this receptor's role in the actions of serotonin on identified circuits and cells.

The Drosophila metabotropic glutamate receptor DmGluRA regulates activity-dependent synaptic facilitation and fine synaptic morphology

In vertebrates, several groups of metabotropic glutamate receptors (mGluRs) are known to modulate synaptic properties. In contrast, the Drosophila genome encodes a single functional mGluR (DmGluRA), an ortholog of vertebrate group II mGluRs, greatly expediting the functional characterization of mGluR-mediated signaling in the nervous system. We show here that DmGluRA is expressed at the glutamatergic neuromuscular junction (NMJ), localized in periactive zones of presynaptic boutons but excluded from active sites. Null DmGluRA mutants are completely viable, and all of the basal NMJ synaptic transmission properties are normal. In contrast, DmGluRA mutants display approximately a threefold increase in synaptic facilitation during short stimulus trains. Prolonged stimulus trains result in very strongly increased ( approximately 10-fold) augmentation, including the appearance of asynchronous, bursting excitatory currents never observed in wild type. Both defects are rescued by expression of DmGluRA only in the neurons, indicating a specific presynaptic requirement. These phenotypes are reminiscent of hyperexcitable mutants, suggesting a role of DmGluRA signaling in the regulation of presynaptic excitability properties. The mutant phenotypes could not be replicated by acute application of mGluR antagonists, suggesting that DmGluRA regulates the development of presynaptic properties rather than directly controlling short-term modulation. DmGluRA mutants also display mild defects in NMJ architecture: a decreased number of synaptic boutons accompanied by an increase in mean bouton size. These morphological changes bidirectionally correlate with DmGluRA levels in the presynaptic terminal. These data reveal the following two roles for DmGluRA in presynaptic mechanisms: (1) modulation of presynaptic excitability properties important for the control of activity-dependent neurotransmitter release and (2) modulation of synaptic architecture.

Characterization of the two distinct subtypes of metabotropic glutamate receptors from honeybee, Apis mellifera

L-Glutamate is a major neurotransmitter at the excitatory synapses in the vertebrate brain. It is also the excitatory neurotransmitter at neuromuscular junctions in insects, however its functions in their brains remain to be established. We identified and characterized two different subtypes (AmGluRA and AmGluRB) of metabotropic glutamate receptors (mGluRs) from an eusocial insect, honeybee. Both AmGluRA and AmGluRB form homodimers independently on disulfide bonds, and bind [3H]glutamate with K(D) values of 156.7 and 80.7 nM, respectively. AmGluRB is specifically expressed in the brain, while AmGluRA is expressed in the brain and other body parts, suggesting that AmGluRA is also present at the neuromuscular junctions. Both mGluRs are expressed in the mushroom bodies and the brain regions of honeybees, where motor neurons are clustered. Their expression in the brain apparently overlaps, suggesting that they may interact with each other to modulate the glutamatergic neurotransmission.

Chemical neuroanatomy of the fly's movement detection pathway

In Diptera, subsets of small retinotopic neurons provide a discrete channel from achromatic photoreceptors to large motion-sensitive neurons in the lobula complex. This pathway is distinguished by specific affinities of its neurons to antisera raised against glutamate, aspartate, gamma-aminobutyric acid (GABA), choline acetyltransferase (ChAT), and a N-methyl-D-aspartate type 1 receptor protein (NMDAR1). Large type 2 monopolar cells (L2) and type 1 amacrine cells, which in the external plexiform layer are postsynaptic to the achromatic photoreceptors R1-R6, express glutamate immunoreactivity as do directionally selective motion-sensitive tangential neurons of the lobula plate. L2 monopolar cells ending in the medulla are accompanied by terminals of a second efferent neuron T1, the dendrites of which match NMDAR1-immunoreactive profiles in the lamina. L2 and T1 endings visit ChAT and GABA-immunoreactive relays (transmedullary neurons) that terminate from the medulla in a special layer of the lobula containing the dendrites of directionally selective retinotopic T5 cells. T5 cells supply directionally selective wide-field neurons in the lobula plate. The present results suggest a circuit in which initial motion detection relies on interactions among amacrines and T1, and the subsequent convergence of T1 and L2 at transmedullary cell dendrites. Convergence of ChAT-immunoreactive and GABA-immunoreactive transmedullary neurons at T5 dendrites in the lobula, and the presence there of local GABA-immunoreactive interneurons, are suggested to provide excitatory and inhibitory elements for the computation of motion direction. A comparable immunocytological organization of aspartate- and glutamate-immunoreactive neurons in honeybees and cockroaches further suggests that neural arrangements providing directional motion vision in flies may have early evolutionary origins.

Drosophila mutants of the kynurenine pathway as a model for ageing studies

A search for Drosophila mutants with phenotypes similar to human diseases might help to unravel evolutionary conserved genes implicated in polygenic human disorders. Among these are neurodegenerative diseases, characterized by a late onset disturbance of memory, structural brain impairments and altered content of the intermediates of the kynurenine pathway. The ratio between kynurenate (KYNA) and 3-hydroxykynurenine (3-HOK) in the brain is a critical determinant of neuronal viability. Therefore, the Drosophila mutants cinnabar (KYNA excess) and cardinal (3-HOK excess) allow an evaluation of the specific roles of these metabolites which present in physiologic concentrations and mimic systemic administration. Previously we have demonstrated that the mutant cardinal can serve as a model for dementia and can help to unravel the earliest manifestations of brain dysfunction. Here we show that a state of the brain control of locomotor coordination characterized by the parameters of sound production in males results from the neuroprotective and neurotoxic effects of KYNA and 3-HOK accumulated in young and aged Drosophila mutants. The high instability of 1) cycle form and number in pulses; 2) of pulse amplitude and 3) rhythm in the courtship song of aged cardinal males are similar to the alterations in mutants with defective central complex of the brain. The cardinal mutants demonstrate apoptosis in the brain after stress treatment. This might reflect the misbalance in the content of excitatory amino acids' and the glycine site agonists revealed by HPLC-determination. The mutant cinnabar proved to be normal in respect of the parameters studied.