Tachykinin-related peptides modulate odor perception and locomotor activity in Drosophila

Neuropeptides in insect mushroom bodies

Owing to their experimental amenability, insect nervous systems continue to be in the foreground of investigations into information processing in - ostensibly - simple neuronal networks. Among the cerebral neuropil regions that hold a particular fascination for neurobiologists are the paired mushroom bodies, which, despite their function in other behavioral contexts, are most renowned for their role in learning and memory. The quest to understand the processes that underlie these capacities has been furthered by research focusing on unraveling neuroanatomical connections of the mushroom bodies and identifying key players that characterize the molecular machinery of mushroom body neurons. However, on a cellular level, communication between intrinsic and extrinsic mushroom body neurons still remains elusive. The present account aims to provide an overview on the repertoire of neuropeptides expressed in and utilized by mushroom body neurons. Existing data for a number of insect representatives is compiled and some open gaps in the record are filled by presenting additional original data.

Presynaptic modulation of early olfactory processing in Drosophila

Most animals are endowed with an olfactory system that is essential for finding foods, avoiding predators, and locating mating partners. The olfactory system must encode the identity and intensity of behaviorally relevant stimuli in a dynamic environmental landscape. How is olfactory information represented? How is a large dynamic range of odor concentrations represented in the olfactory system? How is this representation modulated to meet the demands of different internal physiological states? Recent studies have found that sensory terminals are important targets for neuromodulation. The emerging evidence suggests that presynaptic inhibition scales with sensory input and thus provides a mechanism to increase dynamic range of odor representation. In addition, presynaptic facilitation could be a mechanism to alter behavioral responses in hungry animals. This review will focus on the GABA(B) (gamma-aminobutyric acid) receptor-mediated presynaptic inhibition, and neuropeptide-mediated presynaptic modulation in Drosophila.

CRF-like diuretic hormone negatively affects both feeding and reproduction in the desert locust, Schistocerca gregaria

Diuretic hormones (DH) related to the vertebrate Corticotropin Releasing Factor (CRF) have been identified in diverse insect species. In the migratory locust, Locusta migratoria, the CRF-like DH (CRF/DH) is localized in the same neurosecretory cells as the Ovary Maturating Parsin (OMP), a neurohormone that stimulates oocyte growth, vitellogenesis and hemolymph ecdysteroid levels in adult female locusts. In this study, we investigated whether CRF-like DH can influence feeding and reproduction in the desert locust, Schistocerca gregaria. We identified two highly similar S. gregaria CRF-like DH precursor cDNAs, each of which also encodes an OMP isoform. Alignment with other insect CRF-like DH precursors shows relatively high conservation of the CRF/DH sequence while the precursor region corresponding to OMP is not well conserved. Quantitative real-time RT-PCR revealed that the precursor transcripts mainly occur in the central nervous system and their highest expression level was observed in the brain. Injection of locust CRF/DH caused a significantly reduced food intake, while RNAi knockdown stimulated food intake. Therefore, our data indicate that CRF-like DH induces satiety. Furthermore, injection of CRF/DH in adult females retarded oocyte growth and caused lower ecdysteroid titers in hemolymph and ovaries, while RNAi knockdown resulted in opposite effects. The observed effects of CRF/DH may be part of a wider repertoire of neurohormonal activities, constituting an integrating control system that affects food intake and excretion, as well as anabolic processes like oocyte growth and ecdysteroidogenesis, following a meal. Our discussion about the functional relationship between CRF/DH and OMP led to the hypothesis that OMP may possibly act as a monitoring peptide that can elicit negative feedback effects.

Regulation of insulin-producing cells in the adult Drosophila brain via the tachykinin peptide receptor DTKR

Drosophila insulin-like peptides (DILPs) play important hormonal roles in the regulation of metabolic carbohydrates and lipids, but also in reproduction, growth, stress resistance and aging. In spite of intense studies of insulin signaling in Drosophilag the regulation of DILP production and release in adult fruit flies is poorly understood. Here we investigated the role of Drosophila tachykinin-related peptides (DTKs) and their receptors, DTKR and NKD, in the regulation of brain insulin-producing cells (IPCs) and aspects of DILP signaling. First, we show DTK-immunoreactive axon terminations close to the presumed dendrites of the IPCs, and DTKR immunolabeling in these cells. Second, we utilized targeted RNA interference to knock down expression of the DTK receptor, DTKR, in IPCs and monitored the effects on Dilp transcript levels in the brains of fed and starved flies. Dilp2 and Dilp3, but not Dilp5, transcripts were significantly affected by DTKR knockdown in IPCs, both in fed and starved flies. Both Dilp2 and Dilp3 transcripts increased in fed flies with DTKR diminished in IPCs whereas at starvation the Dilp3 transcript plummeted and Dilp2 increased. We also measured trehalose and lipid levels as well as survival in transgene flies at starvation. Knockdown of DTKR in IPCs leads to increased lifespan and a faster decrease of trehalose at starvation but has no significant effect on lipid levels. Finally, we targeted the IPCs with RNAi or ectopic expression of the other DTK receptor, NKD, but found no effect on survival at starvation. Our results suggest that DTK signaling, via DTKR, regulates the brain IPCs.

Inducing sleep by remote control facilitates memory consolidation in Drosophila

Sleep is believed to play an important role in memory consolidation. We induced sleep on demand by expressing the temperature-gated nonspecific cation channel Transient receptor potential cation channel (UAS-TrpA1) in neurons, including those with projections to the dorsal fan-shaped body (FB). When the temperature was raised to 31°C, flies entered a quiescent state that meets the criteria for identifying sleep. When sleep was induced for 4 hours after a massed-training protocol for courtship conditioning that is not capable of inducing long-term memory (LTM) by itself, flies develop an LTM. Activating the dorsal FB in the absence of sleep did not result in the formation of LTM after massed training.

Synapse loss in olfactory local interneurons modifies perception

Synapse loss correlates with cognitive decline in aging and most neurological pathologies. Sensory perception changes often represent subtle dysfunctions that precede the onset of a neurodegenerative disease. However, a cause-effect relationship between synapse loss and sensory perception deficits is difficult to prove and quantify due to functional and structural adaptation of neural systems. Here we modified a PI3K/AKT/GSK3 signaling pathway to reduce the number of synapses--without affecting the number of cells--in five subsets of local interneurons of the Drosophila olfactory glomeruli and measured the behavioral effects on olfactory perception. The neuron subsets were chosen under the criteria of GABA or ChAT expression. The reduction of one subset of synapses, mostly inhibitory, converted the responses to all odorants and concentrations tested as repulsive, while the reduction of another subset, mostly excitatory, led to a shift toward attraction. However, the simultaneous reduction of both synapse subsets restored normal perception. One group of local interneurons proved unaffected by the induced synapse loss in the perception of some odorants, indicating a functional specialization of these cells. Using genetic tools for space and temporal control of synapse number decrease, we show that the perception effects are specific to the local interneurons, rather than the mushroom bodies, and are not based on major structural changes elicited during development. These findings demonstrate that synapse loss cause sensory perception changes and suggest that normal perception is based on a balance between excitation and inhibition.

Insulin production and signaling in renal tubules of Drosophila is under control of tachykinin-related peptide and regulates stress resistance

The insulin-signaling pathway is evolutionarily conserved in animals and regulates growth, reproduction, metabolic homeostasis, stress resistance and life span. In Drosophila seven insulin-like peptides (DILP1-7) are known, some of which are produced in the brain, others in fat body or intestine. Here we show that DILP5 is expressed in principal cells of the renal tubules of Drosophila and affects survival at stress. Renal (Malpighian) tubules regulate water and ion homeostasis, but also play roles in immune responses and oxidative stress. We investigated the control of DILP5 signaling in the renal tubules by Drosophila tachykinin peptide (DTK) and its receptor DTKR during desiccative, nutritional and oxidative stress. The DILP5 levels in principal cells of the tubules are affected by stress and manipulations of DTKR expression in the same cells. Targeted knockdown of DTKR, DILP5 and the insulin receptor dInR in principal cells or mutation of Dilp5 resulted in increased survival at either stress, whereas over-expression of these components produced the opposite phenotype. Thus, stress seems to induce hormonal release of DTK that acts on the renal tubules to regulate DILP5 signaling. Manipulations of S6 kinase and superoxide dismutase (SOD2) in principal cells also affect survival at stress, suggesting that DILP5 acts locally on tubules, possibly in oxidative stress regulation. Our findings are the first to demonstrate DILP signaling originating in the renal tubules and that this signaling is under control of stress-induced release of peptide hormone.

Blockade of the release of the neuropeptide leucokinin to determine its possible functions in fly behavior: chemoreception assays

Previous studies have revealed leucokinin (LK) expression in the brain and ventral ganglion of Drosophila CNS. One pair of protocerebrum neurons located in the lateral horn area (LHLK) surrounds the peduncles of the mushroom bodies while two pairs of subesophageal neurons (SELKs) project extended processes to the tritocerebrum and through a cervical connection to the ventral ganglion. There, axons of eight or nine pairs of abdominal (ABLK) neurons leave the CNS through the abdominal nerves and processes connecting each other ipsilaterally and contralaterally. The neural functions of LK remain largely unknown, especially those related to Drosophila behavior. Here, we have studied the role of LK in olfactory and gustatory perception by keeping the LK neurons electrically silent through targeted expression of inward rectifier K(+) channels. In order to examine the effects of LK failure, we first analyzed the dehydration response, comparing the leucokinin-silent individuals with their parents as a control. Our results showed significant differences that demonstrate the effectiveness of the method. We then tested the olfactory behavioral response to a set of odorants over a range of concentrations in a T-maze paradigm in which flies were allowed to choose between the odorant and solvent compartments. The feeding preference assays were carried out on microplates in which flies were allowed to choose between two colored tastes. Our results show that the blockade of LK release alters both olfactory and gustatory responses, and are therefore evidence that this neuropeptide also modulates chemosensory responses through LHLK and SELK neurons.

Biostable multi-Aib analogs of tachykinin-related peptides demonstrate potent oral aphicidal activity in the pea aphid Acyrthosiphon pisum (Hemiptera: Aphidae)

The tachykinin-related peptides (TRPs) are multifunctional neuropeptides found in a variety of arthropod species, including the pea aphid Acyrthosiphon pisum (Hemiptera: Aphidae). Two new biostable TRP analogs containing multiple, sterically hindered Aib residues were synthesized and found to exhibit significantly enhanced resistance to hydrolysis by angiotensin converting enzyme and neprilysin, membrane-bound enzymes that degrade and inactivate natural TRPs. The two biostable analogs were also found to retain significant myostimulatory activity in an isolated cockroach hindgut preparation, the bioassay used to isolate and identify the first members of the TRP family. Indeed one of the analogs (Leuma-TRP-Aib-1) matched the potency and efficacy of the natural, parent TRP peptide in this myotropic bioassay. The two biostable TRP analogs were further fed in solutions of artificial diet to the pea aphid over a period of 3 days and evaluated for antifeedant and aphicidal activity and compared with the effect of treatment with three natural, unmodified TRPs. The two biostable multi-Aib TRP analogs were observed to elicit aphicidal effects within the first 24 h. In contrast natural, unmodified TRPs, including two that are native to the pea aphid, demonstrated little or no activity. The most active analog, double-Aib analog Leuma-TRP-Aib-1 (pEA[Aib]SGFL[Aib]VR-NH(2)), featured aphicidal activity calculated at an LC(50) of 0.0083 nmol/μl (0.0087 μg/μl) and an LT(50) of 1.4 days, matching or exceeding the potency of commercially available aphicides. The mechanism of this activity has yet to be established. The aphicidal activity of the biostable TRP analogs may result from disruption of digestive processes by interfering with gut motility patterns and/or with fluid cycling in the gut; processes shown to be regulated by the TRPs in other insects. These active TRP analogs and/or second generation analogs offer potential as environmentally friendly pest aphid control agents.

Peptide-induced modulation of synaptic transmission and escape response in Drosophila requires two G-protein-coupled receptors

Neuropeptides are found in both mammals and invertebrates and can modulate neural function through activation of G-protein-coupled receptors (GPCRS). The precise mechanisms by which many of these GPCRs modulate specific signaling cascades to regulate neural function are not well defined. We used Drosophila melanogaster as a model to examine both the cellular and behavioral effects of DPKQDFMRFamide, the most abundant peptide encoded by the dFMRF gene. We show that DPKQDFMRFamide enhanced synaptic transmission through activation of two G-protein-coupled receptors, Fmrf Receptor (FR) and Dromyosupressin Receptor-2 (DmsR-2). The peptide increased both the presynaptic Ca(2+) response and the quantal content of released transmitter. Peptide-induced modulation of synaptic function could be abrogated by depleting intracellular Ca(2+) stores or by interfering with Ca(2+) release from the endoplasmic reticulum through disruption of either the ryanodine receptor or the inositol 1,4,5-trisphosphate receptor. The peptide also altered behavior. Exogenous DPKQDFMRFamide enhanced fictive locomotion; this required both the FR and DmsR-2. Likewise, both receptors were required for an escape response to intense light exposure. Thus, coincident detection of a peptide by two GPCRs modulates synaptic function through effects of Ca(2+)-induced Ca(2+) release, and we hypothesize that these mechanisms are involved in behavioral responses to environmental stress.

Multiple neuropeptides in the Drosophila antennal lobe suggest complex modulatory circuits

The fruitfly, Drosophila, is dependent on its olfactory sense in food search and reproduction. Processing of odorant information takes place in the antennal lobes, the primary olfactory center in the insect brain. Besides classical neurotransmitters, earlier studies have indicated the presence of a few neuropeptides in the olfactory system. In the present study we made an extensive analysis of the expression of neuropeptides in the Drosophila antennal lobes by direct profiling using matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF) mass spectrometry and immunocytochemistry. Neuropeptides from seven different precursor genes were unambiguously identified and their localization in neurons was subsequently revealed by immunocytochemistry. These were short neuropeptide F, tachykinin related peptide, allatostatin A, myoinhibitory peptide, SIFamide, IPNamide, and myosuppressin. The neuropeptides were expressed in subsets of olfactory sensory cells and different populations of local interneurons and extrinsic (centrifugal) neurons. In some neuron types neuropeptides were colocalized with classical neurotransmitters. Our findings suggest a huge complexity in peptidergic signaling in different circuits of the antennal lobe.

Metabolic stress responses in Drosophila are modulated by brain neurosecretory cells that produce multiple neuropeptides

In Drosophila, neurosecretory cells that release peptide hormones play a prominent role in the regulation of development, growth, metabolism, and reproduction. Several types of peptidergic neurosecretory cells have been identified in the brain of Drosophila with release sites in the corpora cardiaca and anterior aorta. We show here that in adult flies the products of three neuropeptide precursors are colocalized in five pairs of large protocerebral neurosecretory cells in two clusters (designated ipc-1 and ipc-2a): Drosophila tachykinin (DTK), short neuropeptide F (sNPF) and ion transport peptide (ITP). These peptides were detected by immunocytochemistry in combination with GFP expression driven by the enhancer trap Gal4 lines c929 and Kurs-6, both of which are expressed in ipc-1 and 2a cells. This mix of colocalized peptides with seemingly unrelated functions is intriguing and prompted us to initiate analysis of the function of the ten neurosecretory cells. We investigated the role of peptide signaling from large ipc-1 and 2a cells in stress responses by monitoring the effect of starvation and desiccation in flies with levels of DTK or sNPF diminished by RNA interference. Using the Gal4-UAS system we targeted the peptide knockdown specifically to ipc-1 and 2a cells with the c929 and Kurs-6 drivers. Flies with reduced DTK or sNPF levels in these cells displayed decreased survival time at desiccation and starvation, as well as increased water loss at desiccation. Our data suggest that homeostasis during metabolic stress requires intact peptide signaling by ipc-1 and 2a neurosecretory cells.

Neuropeptides in the Drosophila central complex in modulation of locomotor behavior

The central complex is one of the most prominent neuropils in the insect brain. It has been implicated in the control of locomotor activity and is considered as a pre-motor center. Several neuropeptides are expressed in circuits of the central complex, and thus may be modulators of locomotor behavior. Here we have investigated the roles of two different neuropeptides, Drosophila tachykinin (DTK) and short neuropeptide F (sNPF), in aspects of locomotor behavior. In the Drosophila brain, DTK and sNPF are expressed in interneurons innervating the central complex. We have directed RNA interference (RNAi) towards DTK and sNPF specifically in different central complex neurons. We also expressed a temperature-sensitive dominant negative allele of the fly ortholog of dynamin called shibirets1, essential in membrane vesicle recycling and endocytosis, to disrupt synaptic transmission in central complex neurons. The spontaneous walking activity of the RNAi- or shibirets1-expressing flies was quantified by video tracking. DTK-deficient flies displayed drastically increased center zone avoidance, suggesting that DTK is involved in the regulation of spatial orientation. In addition, DTK deficiency in other central complex neurons resulted in flies with an increased number of activity–rest bouts. Perturbations in the sNPF circuit indicated that this peptide is involved in the fine regulation of locomotor activity levels. Our findings suggest that the contribution of DTK and sNPF to locomotor behavior is circuit dependent and associated with particular neuronal substrates. Thus, peptidergic pathways in the central complex have specific roles in the fine tuning of locomotor activity of adult Drosophila.

Tachykinin-related peptides and their receptors in invertebrates: a current view

Members of the tachykinin peptide family have been well conserved during evolution and are mainly expressed in the central nervous system and in the intestine of both vertebrates and invertebrates. In these animals, they act as multifunctional messengers that exert their biological effects by specifically interacting with a subfamily of structurally related G protein-coupled receptors. Despite the identification of multiple tachykinin-related peptides (TKRPs) in species belonging to the insects, crustaceans, mollusks and echiuroid worms, only five invertebrate receptors harboring profound sequence similarities to mammalian receptors for tachykinins have been functionally characterized to date. Three of these have been cloned from dipteran insect species, i.e. NKD (neurokinin receptor from Drosophila), DTKR (Drosophila tachykinin receptor) and STKR (tachykinin-related peptide receptor from the stable fly, Stomoxys calcitrans). In addition, two receptors from non-insect species, present in echiuroid worms and mollusks, respectively have been identified as well. In this brief review, we will survey some recent findings and insights into the signaling properties of invertebrate tachykinin-related peptides via their respective receptors. In this context, we will also point out the necessity to take into account differences in signaling mechanisms induced by distinct TKRP isoforms in insects.

Neuropeptide biology in Drosophila

Drosophila melanogaster is since decades the most important invertebrate model. With the publishing of the genome sequence, Drosophila also became a pioneer in (neuro)peptide research. Neuropeptides represent a major group of signaling molecules that outnumber all other types of neurotransmitters/modulators and hormones. By means of bioinformatics 119 (neuro)peptide precursor genes have been predicted from the Drosophila genome. Using the neuropeptidomics technology 46 neuropeptides derived from 19 of these precursors could be biochemically characterized. At the cellular level, neuropeptides usually exert their action by binding to membrane receptors, many of which belong to the family of G-protein coupled receptors or GPCRs. Such receptors are the major target for many contemporary drugs. In this chapter, we will describe the identification, localization and functional characterization of neuropeptide-receptor pairs in Drosophila melanogaster.

Neuropeptide signaling near and far: how localized and timed is the action of neuropeptides in brain circuits?

Neuropeptide signaling is functionally very diverse and one and the same neuropeptide may act as a circulating neurohormone, as a locally released neuromodulator or even as a cotransmitter of classical fast-acting neurotransmitters. Thus, neuropeptides are produced by a huge variety of neuron types in different parts of the nervous system. Within the central nervous system (CNS) there are numerous types of peptidergic interneurons, some with strictly localized and patterned branching morphologies, others with widespread and diffuse arborizations. From morphology alone it is often difficult to predict the sphere of influence of a peptidergic interneuron, especially since it has been shown that neuropeptides can diffuse over tens of micrometers within neuropils, and that peptides probably are released exclusively in perisynaptic (or non-synaptic) regions. This review addresses some questions related to peptidergic signaling in the insect CNS. How diverse are the spatial relations between peptidergic neurons and their target neurons and what determines the sphere of functional influence? At one extreme there is volume transmission and at the other targeted cotransmission at synapses. Also temporal aspects of peptidergic signaling are of interest: how transient are peptidergic messages? Factors important for these spatial and temporal aspects of peptidergic signaling are proximity between release sites and cognate receptors, distribution of peptidase activity that can terminate peptide action and colocalization of other neuroactive compounds in the presynaptic peptidergic neuron (and corresponding receptors in target neurons). Other factors such as expression of different channel types, receptor inactivation mechanisms and second messenger systems probably also contribute to the diversity in temporal properties of peptide signaling.

Peptidomic survey of the locust neuroendocrine system

Neuropeptides are important controlling agents in animal physiology. In order to understand their role and the ways in which neuropeptides behave and interact with one another, information on their time and sites of expression is required. We here used a combination of MALDI-TOF and ESI-Q-TOF mass spectrometry to make an inventory of the peptidome of different parts (ganglia and nerves) of the central nervous system from the desert locust Schistocerca gregaria and the African migratory locust Locusta migratoria. This way, we analysed the brain, suboesophageal ganglion, retrocerebral complex, stomatogastric nervous system, thoracic ganglia, abdominal ganglia and abdominal neurohemal organs. The result is an overview of the distribution of sixteen neuropeptide families, i.e. pyrokinins, pyrokinin-like peptides, periviscerokinins, tachykinins, allatotropin, accessory gland myotropin, FLRFamide, (short) neuropeptide F, allatostatins, insulin-related peptide co-peptide, ion-transport peptide co-peptide, corazonin, sulfakinin, orcokinin, hypertrehalosaemic hormone and adipokinetic hormones (joining peptides) throughout the locust neuroendocrine system.

Presynaptic peptidergic modulation of olfactory receptor neurons in Drosophila

The role of classical neurotransmitters in the transfer and processing of olfactory information is well established in many organisms. Neuropeptide action, however, is largely unexplored in any peripheral olfactory system. A subpopulation of local interneurons (LNs) in the Drosophila antannal lobe is peptidergic, expressing Drosophila tachykinins (DTKs). We show here that olfactory receptor neurons (ORNs) express the DTK receptor (DTKR). Using two-photon microscopy, we found that DTK applied to the antennal lobe suppresses presynaptic calcium and synaptic transmission in the ORNs. Furthermore, reduction of DTKR expression in ORNs by targeted RNA interference eliminates presynaptic suppression and alters olfactory behaviors. We detect opposite behavioral phenotypes after reduction and over expression of DTKR in ORNs. Our findings suggest a presynaptic inhibitory feedback to ORNs from peptidergic LNs in the antennal lobe.

Characterization and distribution of NKD, a receptor for Drosophila tachykinin-related peptide 6

Neuropeptides related to vertebrate tachykinins have been identified in Drosophila and are referred to as drosotachykinins, or DTKs. Two Drosophila G protein-coupled receptors, designated NKD (neurokinin receptor from Drosophila; CG6515) and DTKR (Drosophila tachykinin receptor; CG7887), display sequence similarities to mammalian tachykinin receptors. Whereas DTKR was shown to be activated by DTKs [Birse RT, Johnson EC, Taghert PH, Nässel DR. Widely distributed Drosophila G-protein-coupled receptor (CG7887) is activated by endogenous tachykinin-related peptides. J Neurobiol 2006;66:33-46; Poels J, Verlinden H, Fichna J, Van Loy T, Franssens V, Studzian K, et al. Functional comparison of two evolutionary conserved insect neurokinin-like receptors. Peptides 2007;28:103-8] and was localized by immunocytochemistry in Drosophila central nervous system (CNS), agonist-dependent activation and distribution of NKD have not yet been investigated in depth. In the present study, we have challenged NKD-expressing mammalian and insect cells with a library of Drosophila neuropeptides and discovered DTK-6 as a specific agonist that can induce a calcium response in these cells. In addition, we have produced antisera to sequences from NKD protein to analyze receptor distribution. We found that NKD is less abundantly distributed in the central nervous system than DTKR, and only NKD was found in the intestine. In fact, the two receptors are distributed in mutually exclusive patterns in the CNS. The combined distribution of the receptors in brain neuropils corresponds well with the distribution of DTKs. Most interestingly, NKD appears to be activated only by DTK-6, known to possess an Ala-substitution in an otherwise conserved C-terminal core motif. Our findings suggest that NKD and DTKR provide substrates for two functionally and spatially separated peptide signaling systems.

Quantitative peptidomics reveal brain peptide signatures of behavior

The honey bee genome predicts approximately 100 peptides from 36 prohormones, but the functions of many of these peptides are unknown. We used differential isotope labeling combined with mass spectrometric analysis to quantify approximately 50% of known bee brain peptides in the context of foraging, with 8 showing robust and dynamic regulation. Some showed differences in brain abundance as a function of experience; specifically, nectar and pollen collection led to quick changes in abundance. These changes were related to the act of food collection, not ingestion, because foragers bring food back to the hive for storage rather than eating it themselves. Other peptide differences in brain abundance were seen in bees that either flew to a nectar feeder or a pollen feeder, but did not yet collect any food. These differences likely reflect well-known predispositions of some bees to collect either nectar or pollen, but not both. Tachykinin, PBAN, and sNPF were among the peptides with the strongest changes in association with nectar and pollen foraging. These peptides are known to be involved in regulating food intake in solitary insects, suggesting an evolutionary connection between that behavior and social foraging. These results demonstrate that it is now possible to use quantitative peptidomics to help determine which brain peptides are bioactive and to elucidate their function in the regulation of behavior.

Gamma-aminobutyric acid immunostaining in the antennal lobe of the moth Heliothis virescens and its colocalization with neuropeptides

The antennal lobe is the primary processing center for olfactory information in insects. To understand further the neural circuitry of this brain area, we have investigated the distribution of gamma-aminobutyric acid (GABA) and its colocalization with neuropeptides in the antennal lobe of the noctuid moth Heliothis virescens. Immunocytochemical experiments with an antiserum against GABA showed a large number of labeled somata in the antennal lobe; these somata were located exclusively in the lateral cell cluster. Stained neurites innervating all antennal-lobe glomeruli, including the male-specific macroglomerular complex, suggested a prominent role of GABA in processing olfactory information, including signals from pheromones, interspecifically acting odors, and plant odors. Fibers in two antennocerebral tracts (the middle and dorsal antennocerebral tract) exhibited prominent GABA immunoreactivity. Double-labeling experiments revealed that immunostaining for three neuropeptides, viz., A-type allatostatin, Manduca sexta allatotropin, and FMRFamide-related peptides, was largely colocalized with GABA in cell bodies of the lateral cell cluster. The general absence of peptide immunostaining in the antennocerebral tracts strongly indicated that these peptides were colocalized with GABA in local interneurons of the antennal lobe. In contrast, tachykinin-related peptides occurred in a distinct population of local antennal-lobe neurons that did not exhibit GABA immunostaining. Thus, local interneurons that were not GABAergic were present in the moth antennal lobe.

An ontogenetic analysis of locustatachykinin-like expression in the central complex of the grasshopper Schistocerca gregaria

We have investigated the ontogenetic basis of locustatachykinin-like expression in a group of cells located in the pars intercerebralis of the grasshopper midbrain. These cells project fibers to the protocerebral bridge and the central body via a characteristic set of fiber bundles called the w, x, y, z tracts. Lineage analyses associate the immunoreactive cells with one of four neuroblasts (termed W, X, Y, Z) in each protocerebral hemisphere of the early embryo. Locustatachykinin is a ubiquitous myotropic peptide among the insects and its expression in the pars intercerebralis begins at approximately 60-65% of embryogenesis. This coincides with the appearance of the columnar neuroarchitecture characteristic of the central body. The number of immunoreactive cells in a given lineage is initially small, increases significantly in later embryogenesis, and attains the adult situation (about 7% of a lineage) in the first larval instar after hatching. Although each neuroblast generates progeny displaying a spectrum of cell body sizes, there is a clear morphological gradient, which reflects birth order within the lineage. Locustatachykinin expressing cells are located stereotypically at or near the tip of their lineage, which an age profile reveals places them amongst the first born progeny of their respective neuroblasts. Although these neuroblasts begin to generate progeny at approximately 25-27% of embryogenesis, their daughter cells remain quiescent with respect to locustatachykinin expression for over 30% of embryogenesis.

Global and local modulatory supply to the mushroom bodies of the moth Spodoptera littoralis

The moth Spodoptera littoralis, is a major pest of agriculture whose olfactory system is tuned to odorants emitted by host plants and conspecifics. As in other insects, the paired mushroom bodies are thought to play pivotal roles in behaviors that are elicited by contextual and multisensory signals, amongst which those of specific odors dominate. Compared with species that have elaborate behavioral repertoires, such as the honey bee Apis mellifera or the cockroach Periplaneta americana, the mushroom bodies of S. littoralis were originally viewed as having a simple cellular organization. This has been since challenged by observations of putative transmitters and neuromodulators. As revealed by immunocytology, the spodopteran mushroom bodies, like those of other taxa, are subdivided longitudinally into discrete neuropil domains. Such divisions are further supported by the present study, which also demonstrates discrete affinities to different mushroom body neuropils by antibodies raised against two putative transmitters, glutamate and gamma-aminobutyric acid, and against three putative neuromodulatory substances: serotonin, A-type allatostatin, and tachykinin-related peptides. The results suggest that in addition to longitudinal divisions of the lobes, circuits in the calyces and lobes are likely to be independently modulated.

Identification of a tachykinin-related peptide with orexigenic properties in the German cockroach

A number of evidences suggest that tachykinin-related peptides (TRPs) of insects can stimulate food consumption after being released from the midgut to the hemolymph. The idea of the present work has been to test this hypothesis in the anautogenous cockroach Blattella germanica. First, we have identified the peptide LemTRP-1 (APSGFLGVR-NH(2)) from brain extracts, by means of an ELISA developed with a polyclonal antibody against this peptide. ELISA studies have also shown that, whereas brain LemTRP-1 levels were fairly constant, midgut levels increase to a maximum on day 3 after adult emergence, falling thereafter until the end of the gonadotrophic cycle. Interestingly, maximum values of food consumption are concomitant with the decrease of LemTRP-1 immunoreactivity in the midgut. Furthermore, starvation decreases LemTRP-1 immunoreactivity in midgut, whereas in the hemolymph it increases. Finally, injection of synthetic LemTRP-1 to adult females significantly stimulates food consumption. The whole observations suggest that LemTRP-1 is released from the midgut to the hemolymph when sustained food consumption is required to maintain vitellogenesis at the highest levels, and that LemTRP-1 in the hemolymph stimulates food consumption in these days.

A glial amino-acid transporter controls synapse strength and courtship in Drosophila

Mate choice is an evolutionarily critical decision that requires the detection of multiple sex-specific signals followed by central integration of these signals to direct appropriate behavior. The mechanisms controlling mate choice remain poorly understood. Here, we show that the glial amino-acid transporter genderblind controls whether Drosophila melanogaster males will attempt to mate with other males. Genderblind (gb) mutant males showed no alteration in heterosexual courtship or copulation, but were attracted to normally unappealing male species-specific chemosensory cues. As a result, genderblind mutant males courted and attempted to copulate with other Drosophila males. This homosexual behavior could be induced within hours using inducible RNAi, suggesting that genderblind controls nervous system function rather than its development. Consistent with this, and indicating that glial genderblind regulates ambient extracellular glutamate to suppress glutamatergic synapse strength in vivo, homosexual behavior could be turned on and off by altering glutamatergic transmission pharmacologically and/or genetically.

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

Recent studies on Drosophila melanogaster and other insects have revealed important insights into the functions and evolution of neuropeptide signaling. In contrast, in- and output connections of insect peptidergic circuits are largely unexplored. Existing morphological descriptions typically do not determine the exact spatial location of peptidergic axonal pathways and arborizations within the neuropil, and do not identify peptidergic in- and output compartments. Such information is however fundamental to screen for possible peptidergic network connections, a prerequisite to understand how the CNS controls the activity of peptidergic neurons at the synaptic level. We provide a precise 3D morphological description of peptidergic neurons in the thoracic and abdominal neuromeres of the Drosophila larva based on fasciclin-2 (Fas2) immunopositive tracts as landmarks. Comparing the Fas2 "coordinates" of projections of sensory or other neurons with those of peptidergic neurons, it is possible to identify candidate in- and output connections of specific peptidergic systems. These connections can subsequently be more rigorously tested. By immunolabeling and GAL4-directed expression of marker proteins, we analyzed the projections and compartmentalization of neurons expressing 12 different peptide genes, encoding approximately 75% of the neuropeptides chemically identified within the Drosophila CNS. Results are assembled into standardized plates which provide a guide to identify candidate afferent or target neurons with overlapping projections. In general, we found that putative dendritic compartments of peptidergic neurons are concentrated around the median Fas2 tracts and the terminal plexus. Putative peptide release sites in the ventral nerve cord were also more laterally situated. Our results suggest that i) peptidergic neurons in the Drosophila ventral nerve cord have separated in- and output compartments in specific areas, and ii) volume transmission is a prevailing way of peptidergic communication within the CNS. The data can further be useful to identify colocalized transmitters and receptors, and develop peptidergic neurons as new landmarks.

Octopamine-mediated neuromodulation of insect senses

Octopamine functions as a neuromodulator, neurotransmitter, and neurohormone in insect nervous systems. Octopamine has a prominent role in influencing multiple physiological events: (a) as a neuromodulator, it regulates desensitization of sensory inputs, arousal, initiation, and maintenance of various rhythmic behaviors and complex behaviors such as learning and memory; (b) as a neurotransmitter, it regulates endocrine gland activity; and (c) as a neurohormone, it induces mobilization of lipids and carbohydrates. Octopamine exerts its effects by binding to specific proteins that belong to the superfamily of G protein-coupled receptors and share the structural motif of seven transmembrane domains. The activation of octopamine receptors is coupled with different second messenger pathways depending on species, tissue source, receptor type and cell line used for the expression of cloned receptor. The second messengers include adenosine 3',5'-cyclic monophosphate (cAMP), calcium, diacylglycerol (DAG), and inositol 1,4,5-trisphosphate (IP3). The cAMP activates protein kinase A, calcium and DAG activate protein kinase C, and IP3 mobilizes calcium from intracellular stores. Octopamine-mediated generation of these second messengers is associated with changes in cellular response affecting insect behaviors. The main objective of this review is to discuss significance of octopamine-mediated neuromodulation in insect sensory systems.

Direct peptide profiling of lateral cell groups of the antennal lobes ofManduca sextareveals specific composition and changes in neuropeptide expression during development

The paired antennal lobes are the first integration centers for odor information in the insect brain. In the sphinx moth Manduca sexta, like in other holometabolous insects, they are formed during metamorphosis. To further understand mechanisms involved in the formation of this particularly well investigated brain area, we performed a direct peptide profiling of a well defined cell group (the lateral cell group) of the antennal lobe throughout development by MALDI-TOF mass spectrometry. Although the majority of the about 100 obtained ion signals represent still unknown substances, this first peptidomic characterization of this cell group indicated the occurrence of 12 structurally known neuropeptides. Among these peptides are helicostatin 1, cydiastatins 2, 3, and 4, M. sexta-allatotropin (Mas-AT), M. sexta-FLRFamide (Mas-FLRFamide) I, II, and III, nonblocked Mas-FLRFamide I, and M. sexta-myoinhibitory peptides (Mas-MIPs) III, V, and VI. The identity of two of the allatostatins (cydiastatins 3 and 4) and Mas-AT were confirmed by tandem mass spectrometry (MALDI-TOF/TOF). During development of the antennal lobe, number and frequency of ion signals including those representing known peptides generally increased at the onset of glomeruli formation at pupal Stage P7/8, with cydiastatin 2, helicostatin 1, and Mas-MIP V being the exceptions. Cydiastatin 2 showed transient occurrence mainly during the period of glomerulus formation, helicostatin 1 was restricted to late pupae and adults, while Mas-MIP V occurred exclusively in adult antennal lobes. The power of the applied direct mass spectrometric profiling lies in the possibility of chemically identifying neuropeptides of a given cell population in a fast and reliable manner, at any developmental stage in single specimens. The identification of neuropeptides in the antennal lobes now allows to specifically address the function of these signaling molecules during the formation of the antennal lobe network.

Functional comparison of two evolutionary conserved insect neurokinin-like receptors

Tachykinins are multifunctional neuropeptides that have been identified in vertebrates as well as invertebrates. The C-terminal FXGXRa-motif constitutes the consensus active core region of invertebrate tachykinins. In Drosophila, two putative G protein-coupled tachykinin receptors have been cloned: DTKR and NKD. This study focuses on the functional characterization of DTKR, the Drosophila ortholog of the stable fly's tachykinin receptor (STKR). Tachykinins containing an alanine residue instead of the highly conserved glycine (FXAXRa) display partial agonism on STKR-mediated Ca(2+)-responses, but not on cAMP-responses. STKR therefore seems to differentiate between a number of tachykinins. Gly- and Ala-containing tachykinins are both encoded in the Drosophila tachykinin precursor, thus raising the question of whether DTKR can also distinguish between these two tachykinin types. DTKR was activated by all Drosophila tachykinins and inhibited by tachykinin antagonists. Ala-containing analogs did not produce the remarkable activation behavior previously observed with STKR, suggesting different mechanisms of discerning ligands and/or activating effector pathways for STKR and DTKR.

Neuropeptides in interneurons of the insect brain

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

Distribution of neuropeptides in the primary olfactory center of the heliothine moth Heliothis virescens

Neuropeptides are a diverse widespread class of signaling substances in the nervous system. As a basis for the analysis of peptidergic neurotransmission in the insect olfactory system, we have studied the distribution of neuropeptides in the antennal lobe of the moth Heliothis virescens. Immunocytochemical experiments with antisera recognizing A-type allatostatins (AST-As), Manduca sexta allatotropin (Mas-AT), FMRFamide-related peptides (FaRPs), and tachykinin-related peptides (TKRPs) have shown that members of all four peptide families are present in local interneurons of the antennal lobe. Whereas antisera against AST-As, Mas-AT, and FaRPs give similar staining patterns characterized by dense meshworks of processes confined to the core of all antennal-lobe glomeruli, TKRPs are present only in neurons with blebby processes distributed throughout each glomerulus. In addition to local neurons, a pair of centrifugal neurons with cell bodies in the lateral subesophageal ganglion, arborizations in the antennal lobe, and projections in the inner antenno-cerebral tracts exhibits tachykinin immunostaining. Double-label immunofluorescence has detected the co-localization of AST-As, Mas-AT, and FaRPs in certain local interneurons, whereas TKRPs occurs in a distinct population. MALDI-TOF mass spectrometry has revealed nearly 50 mass peaks in the antennal lobe. Seven of these masses (four AST-As, two N-terminally extended FLRFamides, and Mas-AT) match known moth neuropeptides. The data thus show that local interneurons of the moth antennal lobe are highly differentiated with respect to their neuropeptide content. The antennal lobe therefore represents an ideal preparation for the future analysis of peptide signaling in insect brain.