The biological activity of diuretic factors in Rhodnius prolixus

The glycoprotein hormone receptor (LGR1) influences Malpighian tubule secretion rate in Rhodnius prolixus

In the hemipteran Rhodnius prolixus, successful post-prandial diuresis is accomplished through the synergistic actions of the peptidergic diuretic hormone RhoprCRF/DH and the biogenic amine 5-hydroxytryptamine (5-HT), and by an antidiuretic hormone RhoprCAPA-2 that terminates diuresis by inhibiting this synergy. Lateral neurosecretory cells (NSCs) in the mesothoracic ganglionic mass release RhoprCRF/DH, while midline NSCs release RhoprCAPA-2 during blood feeding. These NSCs co-express GPA2/GPB5, a conserved glycoprotein hormone involved in various physiological processes across bilaterians. This study investigated the influence of GPA2/GPB5 signaling on Malpighian tubule (MT) fluid secretion in R. prolixus. GPB5-like immunoreactivity in lateral and midline NSCs decreased following a blood meal, suggesting release and a role in diuresis. Downregulating the GPA2/GPB5 receptor LGR1 via RNA interference resulted in an increased basal fluid secretion rate in MTs, which was inhibited by the antidiuretic hormone RhoprCAPA-2. dsLGR1 treatment reduced the effects of RhoprCRF/DH and 5-HT on MT secretion and eliminated their synergism. RT-qPCR revealed that the expression of the diuretic and antidiuretic hormone receptors decreased in MTs of dsLGR1-injected insects, indicating that GPA2/GPB5 influences the expression of these other receptors. Downregulating LGR1 resulted in a smaller blood meal size and disrupted the normal time course of diuresis. As LGR1 is the most abundantly expressed G protein-coupled receptor gene in R. prolixus MTs, our results suggest that GPA2/GPB5 signaling has a critical role in regulating the timing and success of water retention in the unfed state, and in the complex processes associated with feeding and diuresis in R. prolixus.

Serotonergic Modulation of Olfactory Processing in Locust Antennae

Insects have sophisticated olfactory systems that enable them to detect and respond to complex exogenous chemical cues. The encoding mechanisms of these chemical signals have been studied both in their peripheral and central nervous systems (CNS). While many neuromodulators have been shown to play significant roles in olfactory processing within the antennal lobes of the brain, their roles in peripheral olfactory sensory systems, such as the antennae, are less understood. This review focuses on the role of serotonin (5-HT) receptor in the locust antenna, specifically the modulatory function of the serotonin receptor2 on odour inputs. We also review recent studies on the modulation of olfaction in the peripheral nervous systems of other insects and discuss potential directions for future research on the role of neuromodulators in insect peripheral olfactory systems.

Exploring the role of glycoprotein hormone GPA2/GPB5 in the medically important insect, Rhodnius prolixus

Glycoprotein hormones are formed by the heterodimerization of alpha and beta subunits. In vertebrates, there are five glycoprotein hormones, four of which have a common alpha subunit (GPA1) bound to a specific beta subunit (GPB1, GPB2, GPB3, or GPB4), and the fifth, thyrostimulin, is formed by the dimerization of GPA2 and GPB5 subunits. These hormones mediate physiological events such as development, metabolism, and reproduction, although the functional role of thyrostimulin in vertebrates has not been fully elucidated. Recent reports in invertebrates, specifically in holometabolous insects, suggest that GPA2/GPB5 plays a critical role in development, diuresis, and reproduction. In this study, we clone and characterize the transcripts for the glycoprotein hormone GPA2/GPB5 and its receptor (LGR1) in fifth instar Rhodnius prolixus, a hemimetabolous insect vector of Chagas disease. Sequence analyses reveals considerable identity and similarity between GPA2/GPB5 and LGR1 and those reported in other arthropod species. Quantitative PCR (qPCR) shows that both subunit transcripts, GPA2 and GPB5, and LGR1 transcripts are present in a variety of tissues, with greatest expression of the subunits in the central nervous system (CNS) and highest LGR1 expression in the Malpighian tubules (MT). Results from temporal qPCR analyses reveal a decrease in transcript expression 24 h after feeding, followed by an increase as the days post-feeding advance. Using immunohistochemistry, we show that GPB5 is expressed throughout the CNS, and importantly is present in neurosecretory cells in the brain and abdominal neuromeres and their neurohemal organs, indicating a neurohormonal role for this signaling pathway. A reduction in LGR1 transcript expression (via RNA interference) led to a greater weight loss and mortality rate in unfed insects. In addition, when a blood meal is offered, the insects with reduced LGR1 consume a significantly smaller blood meal and have higher mortality rates as the days post-feeding advance. Overall, the results suggest that the GPA2/GPB5 signaling pathway may play roles during a prolonged unfed state and in feeding-related events.

Salivary and Intestinal Transcriptomes Reveal Differential Gene Expression in Starving, Fed and Trypanosoma cruzi-Infected Rhodnius neglectus

Rhodnius neglectus is a potential vector of Trypanosoma cruzi (Tc), the causative agent of Chagas disease. The salivary glands (SGs) and intestine (INT) are actively required during blood feeding. The saliva from SGs is injected into the vertebrate host, modulating immune responses and favoring feeding for INT digestion. Tc infection significantly alters the physiology of these tissues; however, studies that assess this are still scarce. This study aimed to gain a better understanding of the global transcriptional expression of genes in SGs and INT during fasting (FA), fed (FE), and fed in the presence of Tc (FE + Tc) conditions. In FA, the expression of transcripts related to homeostasis maintenance proteins during periods of stress was predominant. Therefore, the transcript levels of Tret1-like and Hsp70Ba proteins were increased. Blood appeared to be responsible for alterations found in the FE group, as most of the expressed transcripts, such as proteases and cathepsin D, were related to digestion. In FE + Tc group, there was a decreased expression of blood processing genes for insect metabolism (e.g., Antigen-5 precursor, Pr13a, and Obp), detoxification (Sult1) in INT and acid phosphatases in SG. We also found decreased transcriptional expression of lipocalins and nitrophorins in SG and two new proteins, pacifastin and diptericin, in INT. Several transcripts of unknown proteins with investigative potential were found in both tissues. Our results also show that the presence of Tc can change the expression in both tissues for a long or short period of time. While SG homeostasis seems to be re-established on day 9, changes in INT are still evident. The findings of this study may be used for future research on parasite-vector interactions and contribute to the understanding of food physiology and post-meal/infection in triatomines.

Fluid Secretion by Malpighian Tubules of Rhodnius prolixus: Neuroendocrine Control With New Insights From a Transcriptome Analysis

Rhodnius prolixus (the kissing bug and a major vector of Chagas disease) is an obligate blood feeder that in the case of the fifth instar consumes up to 10 times its unfed body weight in a single 20-minute feed. A post-prandial diuresis is initiated, within minutes of the start of gorging, in order to lower the mass and concentrate the nutrients of the meal. Thus, R. prolixus rapidly excretes a fluid that is high in NaCl content and hypo-osmotic to the hemolymph, thereby eliminating 50% of the volume of the blood meal within 3 hours of gorging. In R. prolixus, as with other insects, the Malpighian tubules play a critical role in diuresis. Malpighian tubules are not innervated, and their fine control comes under the influence of the neuroendocrine system that releases amines and neuropeptides as diuretic or antidiuretic hormones. These hormones act upon the Malpighian tubules via a variety of G protein-coupled receptors linked to second messenger systems that influence ion transporters and aquaporins; thereby regulating fluid secretion. Much has been discovered about the control of diuresis in R. prolixus, and other model insects, using classical endocrinological studies. The post-genomic era, however, has brought new insights, identifying novel diuretic and antidiuretic hormone-signaling pathways whilst also validating many of the classical discoveries. This paper will focus on recent discoveries into the neuroendocrine control of the rapid post-prandial diuresis in R. prolixus, in order to emphasize new insights from a transcriptome analysis of Malpighian tubules taken from unfed and fed bugs.

Anti-diuretic action of a CAPA neuropeptide against a subset of diuretic hormones in the disease vector Aedes aegypti

The mosquito Aedes aegypti is a vector responsible for transmitting various pathogens to humans, and their prominence as chief vectors of human disease is largely due to their anthropophilic blood feeding behaviour. Larval stage mosquitoes must deal with the potential dilution of their haemolymph in freshwater, whereas the haematophagus A. aegypti female faces the challenge of excess ion and water intake after a blood meal. The excretory system, composed of the Malpighian tubules (MTs) and hindgut, is strictly controlled by neuroendocrine factors, responsible for the regulation of diuresis across all developmental stages. The highly studied insect MTs are influenced by a variety of diuretic hormones and, in some insects, anti-diuretic factors. In the present study, we investigated the effects of AedaeCAPA-1 neuropeptide on larval and adult female A. aegypti MTs stimulated with various diuretic factors including serotonin (5-HT), a corticotropin-related factor (CRF) diuretic peptide, a calcitonin-related diuretic hormone (DH₃₁) and a kinin-related diuretic peptide. Overall, our findings establish that AedaeCAPA-1 specifically inhibits secretion of larval and adult MTs stimulated by 5-HT and DH₃₁, whilst having no activity on MTs stimulated by other diuretic factors. Furthermore, although AedaeCAPA-1 acts as an anti-diuretic, it does not influence the relative proportions of cations transported by adult MTs, thus maintaining the kaliuretic activity of 5-HT and natriuretic activity of DH₃₁ In addition, we tested the effects of the second messenger cGMP in adult MTs. We established that cGMP has similar effects to AedaeCAPA-1, strongly inhibiting 5-HT- and DH₃₁-stimulated fluid secretion, but with only minor effects on CRF-stimulated diuresis. Interestingly, although AedaeCAPA-1 has no inhibitory activity on kinin-stimulated fluid secretion, cGMP strongly inhibited fluid secretion by this diuretic hormone, which targets stellate cells specifically. Collectively, these results support that AedaeCAPA-1 inhibits select diuretic factors acting on the principal cells and this probably involves cGMP as a second messenger. Kinin-stimulated diuresis, which targets stellate cells, is also inhibited by cGMP, suggesting that another anti-diuretic factor in addition to AedaeCAPA-1 exists and may utilize cGMP as a second messenger.

The involvement of Rhopr-CRF/DH in feeding and reproduction in the blood-gorging insect Rhodnius prolixus

Rhodnius prolixus is a blood-gorging insect and a vector for human Chagas disease. The insect transmits the disease following feeding, when it excretes urine and feces contaminated with the Trypanosoma cruzi parasite. A corticotropin-releasing factor-like peptide acts as a diuretic hormone in R. prolixus (Rhopr-CRF/DH); however, its distribution throughout the insect's central nervous system (CNS) and the expression of its receptor in feeding-related tissue as well as the female reproductive system suggests a multifaceted role for the hormone beyond that of diuresis. Here we investigate the involvement of Rhopr-CRF/DH in feeding and reproduction in R. prolixus. Immunohistochemistry of the CNS showed diminished CRF-like staining in neurosecretory cells (NSCs) of the mesothoracic ganglionic mass (MTGM) immediately following feeding, and partial restocking of those same cells two hours later, indicating Rhopr-CRF/DH stores in this regions are involved in feeding. The results of the temporal qPCR analysis were consistent with the immunohistochemical findings, showing an increase in Rhopr-CRF/DH transcript expression in the MTGM immediately after feeding, presumably capturing the restocking of Rhopr-CRF/DH in the lateral NSCs following release of the peptide during feeding. Elevating haemolymph Rhopr-CRF/DH titres by injection of Rhopr-CRF/DH prior to feeding resulted in the intake of a significantly smaller blood meal in 5th instars and adults without an apparent effect on the rate of short-term diuresis. When adult females were injected with Rhopr-CRF/DH, they also produced and laid significantly fewer eggs. Finally, in vitro oviduct contraction assays illustrate that Rhopr-CRF/DH inhibits the amplitude of contractions of the lateral oviducts, highlighting a potential mechanism via which the hormone diminishes reproductive capacity. To conclude, the study of the Rhopr-CRF/DH pathway, its components and mechanisms of action, has implications for vector control by highlighting targets to alter feeding, diuresis, and reproduction of this disease vector.

DINeR: Database for Insect Neuropeptide Research

Neuropeptides are responsible for regulating a variety of functions, including development, metabolism, water and ion homeostasis, and as neuromodulators in circuits of the central nervous system. Numerous neuropeptides have been identified and characterized. However, both discovery and functional characterization of neuropeptides across the massive Class Insecta has been sporadic. To leverage advances in post-genomic technologies for this rapidly growing field, insect neuroendocrinology requires a consolidated, comprehensive and standardised resource for managing neuropeptide information. The Database for Insect Neuropeptide Research (DINeR) is a web-based database-application used for search and retrieval of neuropeptide information of various insect species detailing their isoform sequences, physiological functionality and images of their receptor-binding sites, in an intuitive, accessible and user-friendly format. The curated data includes representatives of 50 well described neuropeptide families from over 400 different insect species. Approximately 4700 FASTA formatted, neuropeptide isoform amino acid sequences and over 200 records of physiological functionality have been recorded based on published literature. Also available are images of neuropeptide receptor locations. In addition, the data include comprehensive summaries for each neuropeptide family, including their function, location, known functionality, as well as cladograms, sequence alignments and logos covering most insect orders. Moreover, we have adopted a standardised nomenclature to address inconsistent classification of neuropeptides. As part of the H2020 nEUROSTRESSPEP project, the data will be actively maintained and curated, ensuring a comprehensive and standardised resource for the scientific community. DINeR is publicly available at the project website: http://www.neurostresspep.eu/diner/.

FLUID AND ION SECRETION BY MALPIGHIAN TUBULES OF LARVAL CHIRONOMIDS, Chironomus riparius: EFFECTS OF REARING SALINITY, TRANSPORT INHIBITORS, AND SEROTONIN

Larvae of Chironomus riparius respond to ion-poor and brackish water (IPW, BW) conditions by activating ion uptake mechanisms in the anal papillae and reducing ion absorption at the rectum, respectively. The role that the Malpighian tubules play in ion and osmoregulation under these conditions is not known in this species. This study examines rates of fluid secretion and major cation composition of secreted fluid from tubules of C. riparius reared in IPW, freshwater (FW) and BW. Fluid secretion of tubules from FW and BW larvae was similar but tubules from IPW larvae secrete fluid at higher rates, are more sensitive to serotonin stimulation, and the secreted fluid contains less Na(+) . Therefore in IPW, tubules work in concert with anal papillae to eliminate excess water while conserving Na(+) in the hemolymph. Tubules do not appear to play a significant role in ion/osmoregulation under BW. Serotonin immunoreactivity in the nervous system and gastrointestinal tract of larval C. riparius was similar to that seen in mosquito larvae with the exception that the hindgut was devoid of staining. Hemolymph serotonin titer was similar in FW and IPW; hence, serotonin is not responsible for the observed high rates of fluid secretion in IPW. Instead, it is suggested that serotonin may work in a synergistic manner with an unidentified hormonal factor in IPW. Ion transport mechanisms in the tubules of C. riparius are pharmacologically similar to those of other insects.

Isolation and characterization of the corticotropin-releasing factor-related diuretic hormone receptor in Rhodnius prolixus

Rhodnius prolixus, the vector of human Chagas disease, is a hemipteran insect that undergoes rapid post-feeding diuresis following ingestion of a blood meal that can be up to 10 times its initial body weight. Corticotropin-releasing factor-related diuretic hormone (Rhopr-CRF/DH) and serotonin are neurohormones that are synergistic in increasing rates of fluid secretion by Malpighian tubules during this rapid post-feeding diuresis. A Rhopr-CRF/DH receptor transcript has now been isolated and characterized from fifth instar R. prolixus. The receptor is a family B1 (secretin) G protein-coupled receptor (GPCR) and was deorphaned in a heterologous cellular system using Chinese hamster ovary (CHO) cells stably expressing a promiscuous G-protein (Gα16). This assay was also used to demonstrate the presence of Rhopr-CRF/DH in the haemolymph of R. prolixus in response to blood-gorging. Two additional cell lines were used in this heterologous assay to verify that the cyclic adenosine monophosphate (cAMP) pathway and not the inositol triphosphate (IP3) pathway was stimulated upon activation of the receptor. Lastly, quantitative PCR demonstrated strong receptor expression in digestive tissues, upper Malpighian tubules and reproductive tissues. Identification of the Rhopr-CRF/DH receptor now provides tools for a more detailed understanding into the precise coordination of diuresis and other physiological processes in R. prolixus.

Serotonin triggers cAMP and PKA-mediated intracellular calcium waves in Malpighian tubules of Rhodnius prolixus

Rhodnius prolixus is a hematophagous insect vector of Chagas disease capable of ingesting up to 10 times its unfed body weight in blood in a single meal. The excess water and ions ingested with the meal are expelled through a rapid postprandial diuresis driven by the Malpighian tubules. Diuresis is triggered by at least two diuretic hormones, a CRF-related peptide and serotonin, which were traditionally believed to trigger cAMP as an intracellular second messenger. Recently, calcium has been suggested to act as a second messenger in serotonin-stimulated Malpighian tubules. Thus, we tested the role of calcium in serotonin-stimulated Malpighian tubules from R. prolixus. Our results show that serotonin triggers cAMP-mediated intracellular Ca(2+) waves that were blocked by incubation in Ca(2+)-free saline containing the cell membrane-permeant Ca(2+) chelator BAPTA-AM, or the PKA blocker H-89. Treatment with 8-Br-cAMP triggered Ca(2+) waves that were blocked by H-89 and BAPTA-AM. Analysis of the secreted fluid in BAPTA-AM-treated tubules showed a 75% reduction in fluid secretion rate with increased K(+) concentration, reduced Na(+) concentration. Taken together, the results indicate that serotonin triggers cAMP and PKA-mediated Ca(2+) waves that are required for maximal ion transport rate.

The molecular characterization of the kinin transcript and the physiological effects of kinins in the blood-gorging insect, Rhodnius prolixus

The dramatic feeding-related activities of the Chagas' disease vector, Rhodnius prolixus are under the neurohormonal regulation of serotonin and various neuropeptides. One such family of neuropeptides, the insect kinins, possess diuretic, digestive and myotropic activities in many insects. In this study, we have cloned and examined the spatial expression of the R. prolixus kinin (Rhopr-kinin) transcript. In addition, in situ hybridization has been used to map the distribution of neurons expressing the kinin transcript. Physiological bioassays demonstrate the myostimulatory effects of selected Rhopr-kinin peptides and also illustrate the augmented responses of hindgut contractions to co-application of Rhopr-kinin and a R. prolixus diuretic hormone. Two synthetic kinin analogs have also been examined on the hindgut. These reveal interesting properties including a relatively irreversible effect on hindgut contractions and activity at very low concentrations.

An insight into the transcriptome of the digestive tract of the bloodsucking bug, Rhodnius prolixus

The bloodsucking hemipteran Rhodnius prolixus is a vector of Chagas' disease, which affects 7-8 million people today in Latin America. In contrast to other hematophagous insects, the triatomine gut is compartmentalized into three segments that perform different functions during blood digestion. Here we report analysis of transcriptomes for each of the segments using pyrosequencing technology. Comparison of transcript frequency in digestive libraries with a whole-body library was used to evaluate expression levels. All classes of digestive enzymes were highly expressed, with a predominance of cysteine and aspartic proteinases, the latter showing a significant expansion through gene duplication. Although no protein digestion is known to occur in the anterior midgut (AM), protease transcripts were found, suggesting secretion as pro-enzymes, being possibly activated in the posterior midgut (PM). As expected, genes related to cytoskeleton, protein synthesis apparatus, protein traffic, and secretion were abundantly transcribed. Despite the absence of a chitinous peritrophic membrane in hemipterans - which have instead a lipidic perimicrovillar membrane lining over midgut epithelia - several gut-specific peritrophin transcripts were found, suggesting that these proteins perform functions other than being a structural component of the peritrophic membrane. Among immunity-related transcripts, while lysozymes and lectins were the most highly expressed, several genes belonging to the Toll pathway - found at low levels in the gut of most insects - were identified, contrasting with a low abundance of transcripts from IMD and STAT pathways. Analysis of transcripts related to lipid metabolism indicates that lipids play multiple roles, being a major energy source, a substrate for perimicrovillar membrane formation, and a source for hydrocarbons possibly to produce the wax layer of the hindgut. Transcripts related to amino acid metabolism showed an unanticipated priority for degradation of tyrosine, phenylalanine, and tryptophan. Analysis of transcripts related to signaling pathways suggested a role for MAP kinases, GTPases, and LKBP1/AMP kinases related to control of cell shape and polarity, possibly in connection with regulation of cell survival, response of pathogens and nutrients. Together, our findings present a new view of the triatomine digestive apparatus and will help us understand trypanosome interaction and allow insights into hemipteran metabolic adaptations to a blood-based diet.

Post-feeding physiology in Rhodnius prolixus: the possible role of FGLamide-related allatostatins

Allatostatins (ASTs) are neuropeptides that were first identified as inhibitors of juvenile hormone biosynthesis by the corpora allata of some insect species. The FGLamide-related ASTs (FGLa/ASTs) belong to one of three families of insect ASTs. Previously, we showed that Rhodnius prolixus FGLa/ASTs (Rhopr-FGLa/ASTs) are present throughout the R. prolixus central nervous system and are associated with 5 dorsal unpaired median (DUM) neurons in the mesothoracic ganglionic mass. A similar set of neurons contain serotonin which is a diuretic hormone in R. prolixus. Rhopr-FGLa/ASTs inhibit both spontaneous contractions of the anterior midgut and leucokinin-1-induced hindgut contractions. Since these tissues are involved with post-feeding diuresis, these data suggest a possible role for FGLa/ASTs in events associated with feeding, and a possible interaction with serotonin. To investigate this possibility, we have examined the DUM neurons in more detail with regard to their peptide content, examined the potential release of Rhopr-FGLa/ASTs into the haemolymph following feeding, and further investigated the effects of Rhopr-FGLa/ASTs on feeding-related tissues. There are 10 DUM neurons in the abdominal neuromeres, 5 of which express serotonin-like immunoreactivity and the other 5 express FGLa/AST-like immunoreactivity. FGLa/AST-like immunoreactivity is reduced in the 5 DUM neuron cell bodies and their neurohaemal sites on abdominal nerves at 3-5 h post feeding. Rhopr-FGLa/ASTs do not inhibit serotonin-stimulated anterior midgut absorption or Malpighian tubule secretion but do inhibit hindgut contractions induced by an endogenous kinin, suggesting that they may only indirectly affect post-feeding diuresis in R. prolixus.

Investigations of the signaling cascade involved in diuretic hormone stimulation of Malpighian tubule fluid secretion in Rhodnius prolixus

In insects, the excretory system is comprised of the Malpighian tubules (MTs) and the hindgut, which collectively function to maintain ionic and osmotic balance of the haemolymph and rid the organism of toxic compounds or elements in excess. Secretion by the Malpighian tubules of insects is regulated by a variety of hormones including peptidergic factors as well as biogenic amines. In Rhodnius prolixus, two endogenous diuretic hormones have been identified; the biogenic amine serotonin (5-hydroxytryptamine, 5-HT) and the corticotropin releasing factor-related peptide, RhoprCRF. Both factors significantly increase secretion by MTs and are known to elevate intracellular levels of cAMP. Interestingly, applying sub-maximal doses of these two diuretic factors in combination on isolated MTs in vitro reveals synergistic effects as rates of fluid secretion are significantly higher than would be expected if rates of secretion from MTs treated with each factor alone were summed. This observed synergism suggests that different downstream targets may be activated by the two diuretic factors, but that some cellular elicitors may be shared since cAMP is elevated in response to either diuretic hormone. This study investigated the signaling cascade involved in the diuretic hormone regulation of Malpighian tubule fluid secretion. Bioassays were performed in physiological as well as modified salines (e.g. calcium-free) alone or in the presence of a variety of pharmacological compounds that interfere with prospective intracellular targets, such as the apical cation/H(+) exchanger. Intriguingly, only amiloride yielded differential effects on the two diuretics with 5HT-stimulated secretion being blocked, whereas in contrast, RhoprCRF-stimulated secretion was unaffected. In addition, experiments examining the role of extracellular and intracellular calcium on fluid secretion rate showed that both diuretics are dependent on intracellular calcium availability. Finally, fluid secretion stimulated by either diuretic hormone was also sensitive to inhibition of cAMP-dependent protein kinase A. Taken together, these results suggest that each diuretic hormone activates pathways dependent upon intracellular calcium and cAMP.

Calcitonin-like diuretic hormones in insects

Insect neuropeptides control various biological processes including growth, development, homeostasis and reproduction. The calcitonin-like diuretic hormone (CT/DH) is one such neuropeptide that has been shown to affect salt and water transport by Malpighian tubules of several insects. With an increase in the number of sequenced insect genomes, CT/DHs have been predicted in several insect species, making it easier to characterize the gene encoding this hormone and determine its function in the species in question. This mini review summarizes the current knowledge on insect CT/DHs, focusing on mRNA and peptide structures, distribution patterns, physiological roles, and receptors in insects.

A dynamic paracellular pathway serves diuresis in mosquito Malpighian tubules

Female mosquitoes gorge on vertebrate blood, a rich nutrient source for developing eggs, but gorging meals increase the risk of predation. Mosquitoes are quick to reduce the flight payload with a potent diuresis. Diuretic peptides of the insect kinin family induce a tenfold reduction in the paracellular resistance of Malpighian tubules and increase the paracellular permeation of Cl(-), the counterion of the transepithelial secretion of Na(+) and K(+). As a result, the transepithelial secretion of NaCl and KCl and water increases. Insect kinins signal the opening of the paracellular pathway via G protein-coupled receptors and the elevation of intracellular [Ca(2+)], which leads to the reorganization of the cytoskeleton associated with the septate junction (SJ). The reorganization may affect the septate junctional proteins that control the barrier and permselectivity properties of the paracellular pathway. The proteins involved in the embryonic formation of the SJ and in epithelial polarization are largely known for ectodermal epithelia, but the proteins that form and mediate the dynamic functions of the SJ in Malpighian tubules remain to be determined.

Anti-diuretic factors in insects: the role of CAPA peptides

Insects have adapted to live in a wide variety of habitats and utilize an array of feeding strategies that present challenges to their ability to maintain osmotic balance. Regardless of the feeding strategy, water and ion levels within the haemolymph (insect blood) are maintained within a narrow range. This homeostasis involves the action of a variety of tissues, but is often chiefly regulated by the excretory system. Until recently, most research on the hormonal control of the excretory tissues has focused on factors known to have diuretic activities. In this mini-review, the current state of knowledge on anti-diuretic factors in insects will be discussed with a particular emphasis on the CAPA peptides in the blood-feeding Chagas' disease vector, Rhodnius prolixus.

The blowfly salivary gland - a model system for analyzing the regulation of plasma membrane V-ATPase

Vacuolar H(+)-ATPases (V-ATPases) are heteromultimeric proteins that use the energy of ATP hydrolysis for the electrogenic transport of protons across membranes. They are common to all eukaryotic cells and are located in the plasma membrane or in membranes of acid organelles. In many insect epithelia, V-ATPase molecules reside in large numbers in the apical plasma membrane and create an electrochemical proton gradient that is used for the acidification or alkalinization of the extracellular space, the secretion or reabsorption of ions and fluids, the import of nutrients, and diverse other cellular activities. Here, we summarize our results on the functions and regulation of V-ATPase in the tubular salivary gland of the blowfly Calliphora vicina. In this gland, V-ATPase activity energizes the secretion of a KCl-rich saliva in response to the neurohormone serotonin (5-HT). Because of particular morphological and physiological features, the blowfly salivary glands are a superior and exemplary system for the analysis of the intracellular signaling pathways and mechanisms that modulate V-ATPase activity and solute transport in an insect epithelium.

Natriuresis and diuretic hormone synergism in R. prolixus upper Malpighian tubules is inhibited by the anti-diuretic hormone, RhoprCAPA-α2

Insects contain an array of hormones that coordinate the actions of the excretory system to achieve osmotic and ionic balance. In the hematophagous insect, Rhodnius prolixus, two diuretic hormones have been identified, serotonin (5HT) and a corticotropin releasing factor-related peptide (RhoprDH), and both lead to an increase in fluid secretion by Malpighian tubules (MTs). However, only 5HT activates reabsorption by the lower MTs to recover K(+) and Cl(-). An anti-diuretic hormone (RhoprCAPA-α2) is believed to coordinate the cessation of the rapid diuresis following blood meal engorgement. However, the role of RhoprCAPA-α2 on fluid secretion by MTs stimulated by RhoprDH was previously unknown. Here we demonstrate that, unlike the inhibitory effect on 5HT-stimulated secretion by MTs, RhoprCAPA-α2 does not inhibit secretion stimulated by RhoprDH although it does abolish the synergism that occurs between the two diuretic hormones. In addition, we show that the natriuresis elicited by either diuretic hormone is reduced by RhoprCAPA-α2. Using electrophysiological tools, we investigate the possible mechanism by which this complex regulatory pathway is achieved. Analysis of the pH of secreted fluid as well as the triphasic response in transepithelial potential in MTs treated with diuretic hormones, suggests that RhoprCAPA-α2 does not inhibit the V-type H(+) ATPase. Taken together, these results indicate that RhoprCAPA-α2 functions to reduce the rapid diuresis following blood feeding, and in addition, it inhibits the natriuresis associated with diuretic hormone stimulated MTs. This may reflect an important regulatory mechanism related to the slow diuresis that occurs as the K(+)-rich blood cells are digested.

Investigation of the potential involvement of eicosanoid metabolites in anti-diuretic hormone signaling in Rhodnius prolixus

The use of naturally occurring plant-derived compounds for controlling insect pests remains an attractive alternative to potentially dangerous synthetic chemical compounds. One prospective plant-based compound, isoforms of the so-called jack bean urease (JBU) from the jack bean, Canavalia ensiformis, as well a derived peptide, Jaburetox-2Ec, have insecticidal effects on an array of insect species. In the Chagas' disease vector, Rhodnius prolixus, some of the physiological effects attributed to these urease isoforms include inhibition of serotonin (5-HT)-stimulated fluid secretion by the Malpighian tubules (MTs). Here, we investigated whether the effects of these exogenous urease isoforms were targeting the neuroendocrine network involved in the anti-diuretic hormone (RhoprCAPA-2) signaling cascade. We show that pharmacological agents known to interfere with eicosanoid metabolite biosynthesis do not affect RhoprCAPA-2 inhibition of 5-HT-stimulated fluid secretion by MTs. In addition, we demonstrate that RhoprCAPA-2 inhibition of MTs is independent of extracellular or intracellular calcium. Using a heterologous system for analysis of receptor activation, we show that neither JBU nor Jaburetox-2Ec are agonists of the anti-diuretic hormone receptor, RhoprCAPAr1. Finally, activation of the receptor using sub-maximal doses of the natural ligand, RhoprCAPA-2, was not influenced by the presence of either JBU or Jaburetox-2Ec indicating that the urease isoforms do not compete with RhoprCAPA-2 for binding and activation of RhoprCAPAr1. Taken together, these results suggest that at least two distinct mechanisms leading to inhibition of fluid secretion by MTs exist in R. prolixus and, unlike the urease-related effects, the eicosanoid metabolite pathway is not involved in RhoprCAPA-2 mediated anti-diuresis.

Cloning of the cDNA, localization, and physiological effects of FGLamide-related allatostatins in the blood-gorging bug, Rhodnius prolixus

Allatostatins (ASTs) are insect neuropeptides that were first identified as inhibitors of juvenile hormone biosynthesis by the corpora allata. There are three families of ASTs in insects, defined by their C-terminus conserved regions, one of which is FGLamide. Here we determine, for the first time in a hemipteran, the complete 1013 bp cDNA sequence encoding the Rhodnius prolixus FGLa/ASTs (Rhopr-FGLa/ASTs), and confirm the transcript size using northern blot. Phylogenetic analysis suggests that the Rhopr-FGLa/AST prepropeptide is most similar to the FGLa/AST precursors identified in Hymenoptera. Reverse-transcriptase PCR demonstrates that the Rhopr-FGLa/AST transcript is highly expressed in the central nervous system (CNS) in unfed fifth-instar R. prolixus, and is reduced in expression in CNS dissected from one day old blood-fed insects. Fluorescent in situ hybridization shows transcript expression in neurons in each ganglion of the CNS, but also in cells located on peripheral nerves. Rhopr-FGLa/ASTs dose-dependently inhibit contractions of the anterior midgut and hindgut, suggesting a role in feeding-related physiological events.

Identification of the elusive peptidergic diuretic hormone in the blood-feeding bug Rhodnius prolixus: a CRF-related peptide

Probing of a host and ingestion of a blood-meal in a fifth instar Rhodnius prolixus results in a cascade of tightly integrated events. The huge blood-meal is pumped into the anterior midgut during feeding, then modified by diuresis and stored until it is digested. While serotonin is known to be a diuretic hormone in R. prolixus, a peptidergic factor(s) was also known to play a role in diuresis. In the present study we employed molecular techniques and mass spectrometry to determine the sequence of a native CRF-like peptide from R. prolixus (Rhopr DH). In addition, we confirmed the distribution and localization of Rhopr DH using in situ hybridization and immunohistochemistry, and demonstrated its potent biological activity on both the anterior midgut and Malpighian tubules.

Identification of kinin-related peptides in the disease vector, Rhodnius prolixus

We have used an in silico approach to identify a gene from the blood-gorging vector, Rhodnius prolixus, that is predicted to produce an insect kinin prepropeptide. The prepropeptide is 398 amino acids in length and can potentially produce a large number of kinin-related peptides following post-translational processing. A comparison with other insect kinin precursor sequences demonstrates greatest conservation at the C-terminal region of the kinin peptides. Multiple peptides predicted from the kinin gene are phenotypically expressed in R. prolixus, as revealed by MALDI-TOF MS MS, including 12 kinins and one kinin precursor peptide (KPP). Six of these peptides are characterized by the typical insect kinin C-terminal motif FX(1)X(2)WGamide and five of these are also found as truncated forms. Five peptides were identified with an atypical, though similar, FX(1)X(2)WAamide C-terminus. There is also peptide with a C-terminal DDNGamide motif and a number of non-amidated peptides.

The control of Malpighian tubule secretion in a predacious hemipteran insect, the spined soldier bug Podisus maculiventris (Heteroptera, Pentatomidae)

Spined soldier bugs, Podisus maculiventris, are heteropteran insects that feed voraciously on other insects, particular the soft bodied larval forms of Lepidoptera and Coleoptera. The response of P. maculiventris Malpighian tubules (MTs) to serotonin and known diuretic and antidiuretic peptides has been investigated, and is compared with that of MT from the hematophagous and phytophagous heteropteran bugs Rhodnius prolixus and Acrosternum hilare, respectively. A CRF-related peptide diuretic hormone (DH) from the termite Zootermopsis nevadensis (Zoone-DH) stimulated MT secretion, which was reversed by a member of the CAP(2b) family of peptides from A. hilare (Acrhi-CAP(2b)-2), an antidiuretic effect. Serotonin had no effect on secretion, neither did a representative calcitonin-like DH, kinin, tachykinin-related peptide, and an antidiuretic factor from the mealworm Tenebrio molitor (Tenmo-ADFb) in both P. maculiventris or A. hilare. Serotonin is a DH in R. prolixus, and its lack of effect on MT from P. maculiventris and A. hilare suggests this is an adaptation to hematophagy. On the other hand, the antidiuretic activity of members of the CAP(2b) family in all three bugs is consistent with this being a heteropteran feature rather than a specialism for hematophagy.

Neurohormones implicated in the control of Malpighian tubule secretion in plant sucking heteropterans: The stink bugs Acrosternum hilare and Nezara viridula

Plant sucking heteropteran bugs feed regularly on small amounts of K(+)-rich plant material, in contrast to their hematophagous relatives which imbibe large volumes of Na(+)-rich blood. It was anticipated that this would be reflected in the endocrine control of Malpighian tubule (MT) secretion. To explore this, neuroendocrine factors known to influence MT secretion were tested on MT of the pentatomid plant sucking stink bugs, Acrosternum hilare and Nezara viridula, and the results compared with previously published data from Rhodnius prolixus. Serotonin had no effect on N. viridula MT, although it stimulates secretion by R. prolixus MT >1000-fold, and initiates a rapid diuresis to remove excess salt and water from the blood meal. Kinins had no effect on stink bug MT, but secretion was increased by Zoone-DH, a CRF-like peptide, although the response was a modest 2-3-fold acceleration compared with 1000-fold in R. prolixus. Native CAPA peptides, which have diuretic activity in dipteran flies, had antidiuretic activity in MT of the stink bug (Acrhi/Nezvi-CAPA-1 and -2), as previously shown with Rhopr-CAPA-2 in R. prolixus. The antidiuretic activity of Rhopr-CAPA-2 has been linked with terminating the rapid diuresis, but results with stink bugs suggest it is a general feature of heteropteran MT.

Effects of the cyclopeptide mycotoxin destruxin A on the Malpighian tubules of Rhodnius prolixus (Stål)

The production of peptide toxins by entomopathogenic fungi during the infection process plays critical roles in pathogenesis. To gain insight into the mechanism of action of these mycotoxins on insect internal organs, we have evaluated the effects of destruxin A, a cyclic peptide produced by Metarhizium anispliae, on Rhodnius prolixus Malpighian tubules measuring fluid secretion rate, transepithelial electrical potential (TEP), pH and ion composition of secreted fluid, and ATP content. Destruxin A dramatically inhibited fluid secretion rate on tubules stimulated by 5-Hydroxytryptamine (5-HT) or cAMP. The calculated IC(50) for destruxin A on 5-HT-stimulated tubules was 3 x 10(-7) M. Fluid secretion rate by Malpighian tubules exposed for 20 min to 10(-6) M destruxin A recovered completely when tubules were washed with saline; however, when tubules were exposed to 5 x 10(-6) M destruxin A the fluid secretion rate only partially recovered upon wash off. The use of Ca(2+)-free saline or addition of the calcium channel blocker CoCl(2) to the bathing saline did not interfere with the effects of destruxin A, and neither did the modification of intracellular calcium by TMB-8. Measurement of TEP of tubules challenged with 5-HT after preincubation for 10 min in saline containing 10(-6) M destruxin A showed that the second and third phases of the typical triphasic response to 5-HT were disrupted. Likewise, the positive shift in TEP in response to 5-HT in chloride-free bathing saline was significantly reduced when tubules were preincubated for 10 min in 10(-6) M destruxin A. The pH of the secreted fluid, but not the Na(+) or K(+) concentration, increased significantly when 5-HT-stimulated tubules were exposed to 10(-6) M destruxin A. The ATP content was not significantly different when tubules stimulated with 5-HT were exposed to destruxin A. Taken together, these results show that destruxin A, without interfering with the intracellular ATP production, strongly inhibits fluid secretion rate by the Malpighian tubules of R. prolixus. Changes in properties of the TEP suggest that one of the target sites for this peptide toxin might be associated with inhibition of the apical V-type H(+) ATPase of tubule cells.

Biological activity of diuretic factors on the anterior midgut of the blood-feeding bug, Rhodnius prolixus

Probing of a host and ingestion of a blood-meal in a fifth instar Rhodnius prolixus results in a cascade of tightly integrated events, including salivary gland secretion, plasticization of the abdominal cuticle, increased ion and water movement across the anterior midgut (crop) and Malpighian tubules (which rapidly produce urine) and the regular expulsion of urine from the hindgut. In this study we have focussed on the role of the anterior midgut during the rapid postprandial diuresis. The huge blood-meal is pumped into the anterior midgut, during feeding, then modified by diuresis and stored until it is digested. Changes in the anterior midgut activity are rapid. Within minutes of the commencement of feeding there is an increase in the frequency of anterior midgut contractions and diuresis begins with the movement of salt and water across the epithelium of the anterior midgut into the haemolymph. While serotonin, a diuretic hormone in R. prolixus, is known to play a role in the physiological activity of the anterior midgut, we were interested in exploring further the role of serotonin, and other diuretic peptides. We have tested the activity of several peptides, including R. prolixus calcitonin-like diuretic hormone (Rhopr-DH 31), corticotropin-releasing factor (CRF)-like peptide from Zootermopsis nevadensis DH (Zoone-DH) and a kinin from Leucophaea maderae, Leucokinin 1 (LK1). These peptides families are known to be present in the central nervous system of R. prolixus, are putative neurohormones released into the haemolymph after the start of feeding, and have been shown to have activity on a variety of tissues involved in post-feeding diuresis. We show here that both serotonin and Zoone-DH increase the cAMP content of the anterior midgut and that serotonin, Zoone-DH and cAMP analogues increase absorption of water from the anterior midgut, increase the short circuit current and voltage, while decreasing the resistance across the epithelium. While LK1 and Rhopr-DH 31 do not significantly increase absorption, or short circuit current, LK1 does significantly decrease the resistance and transepithelial voltage of the anterior midgut epithelium. All of the factors studied increase the frequency of contractions of the anterior midgut.

Neuroendocrine control of ionic homeostasis in blood-sucking insects

The pioneering work of Simon Maddrell established that the rapid postprandial diuresis of the haematophagous insect Rhodnius prolixus is controlled by a diuretic hormone and demonstrated the role of the Malpighian tubules in meeting the volumic, osmotic and ionic challenges posed by an enormous blood meal. A number of diuretic and antidiuretic hormones that control secretion of primary urine by Malpighian tubules have now been identified, but little is known of the interplay between these hormones and those that regulate transport processes in the hindgut. This review therefore focuses on the control of ionic homeostasis in Rhodnius and mosquitoes, because primary urine is voided virtually unchanged during the rapid diuresis that follows a blood meal. At such times, the hindgut has a negligible impact on the volume and composition of the final urine, and neurohormones acting on the Malpighian tubules have a dominant role in the control of ionic homeostasis.

Allatotropin-like peptide in Malpighian tubules: insect renal tubules as an autonomous endocrine organ

Malpighian tubules (MTs) are recognised as the main excretory organ in insects, ensuring water and mineral balance. Haematophagous insects incorporate with each meal a large quantity of blood, producing a particularly large volume of urine in a few hours. In the present study, we report the presence of an allatotropin-like (AT-like) peptide in MTs of Triatoma infestans (Klug). The AT-like content in MTs decreased during the first hours after blood-intake, correlating with the post-prandial diuresis. In vivo artificial dilution of haemolymph showed a similar effect. Isolated MTs challenged with a diluted saline solution resulted in an autonomous and reversible response of the organ regulating the quantity of peptide released to the medium, and suggesting that MTs synthesise the AT-like peptide. While MTs are recognised as the target for several hormones, our results corroborate that they also have the ability to produce and secrete a hormone in an autonomous way.

Physiological activity of neuropeptide f on the hindgut of the blood-feeding hemipteran, Rhodnius prolixus

Current hypotheses propose that, in the invertebrates, neuropeptide F (NPF), the vertebrate NPY homologue, may be capable of regulating responses to diverse cues related to nutritional status and feeding. An investigation into the effects of Drosophila melanogaster NPF (DrmNPF) and Anopheles gambiae NPF (AngNPF) on hindgut physiology of Rhodnius prolixus Stal (Heimptera: Reduviidae) suggests a myoinhibitory role for these peptides and the R. prolixus native peptide. Extracts of the central nervous system of R. prolixus were processed and several HPLC-fractions revealed NPF-like activity within the nanomolar equivalent range when tested using the hindgut contraction assay. Although NPF has been shown to decrease epithelial membrane potential in Aedes aegypti larval midgut preparations, NPF does not appear to play a role in epithelial transport of potassium in the hindgut. While the function of NPF has yet to be established, NPF-like effects suggest multiple physiological roles for NPF among invertebrates.

Isolation, cloning, and expression mapping of a gene encoding an antidiuretic hormone and other CAPA-related peptides in the disease vector, Rhodnius prolixus

After a blood meal, Rhodnius prolixus undergoes a rapid diuresis to eliminate excess water and salts. During the voiding of this primary urine, R. prolixus acts as a vector of Chagas' disease, with the causative agent, Trypanosoma cruzi, infecting the human host via the urine. Diuresis in R. prolixus is under the neurohormonal control of serotonin and peptidergic diuretic hormones, and thus, diuretic hormones play an important role in the transmission of Chagas' disease. Although diuretic hormones may be degraded or excreted, resulting in the termination of diuresis, it would also seem appropriate, given the high rates of secretion, that a potent antidiuretic factor could be present and act to prevent excessive loss of water and salts after the postgorging diuresis. Despite the medical importance of R. prolixus, no genes for any neuropeptides have been cloned, including obviously, those that control diuresis. Here, using molecular biology in combination with matrix-assisted laser desorption ionization-time of flight-tandem mass spectrometry, we determined the sequence of the CAPA gene and CAPA-related peptides in R. prolixus, which includes a peptide with anti-diuretic activity. We have characterized the expression of mRNA encoding these peptides in various developmental stage and also examined the tissue-specific distribution in fifth-instars. The expression is localized to numerous bilaterally paired cell bodies within the central nervous system. In addition, our results show that RhoprCAPA gene expression is also associated with the testes, suggesting a novel role for this family of peptides in reproduction.

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

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

Amino acid sequence and biological activity of a calcitonin-like diuretic hormone (DH31) from Rhodnius prolixus

Diuresis in the blood-gorging hemipteran Rhodnius prolixus is under neurohormonal control and involves a variety of processes and tissues. These include ion and water movement across the epithelium of the crop and the Malpighian tubules, and muscle contractions of the crop, hindgut and dorsal vessel, which facilitate mixing of the blood-meal, mixing of the haemolymph, as well as the expulsion of waste. One of the neurohormones that might play a role in this rapid diuresis belongs to the calcitonin-like diuretic hormone (DH(31)) family of insect peptides. Previously we have demonstrated the presence of DH(31)-like peptides in the central nervous system (CNS) and gut of R. prolixus 5th instars. In the present work, a DH(31) from the CNS of 5th instar R. prolixus was isolated using reversed-phase liquid chromatography (RPLC), monitored with an enzyme-linked immunosorbent assay (ELISA) combined with matrix-assisted laser desorption/ionisation time-of-flight (MALDI-TOF) mass spectrometry, and sequenced using tandem mass spectrometry and Edman degradation. This neuropeptide is the first to be sequenced in R. prolixus and has a sequence identical to that found previously for Dippu-DH(31) from the cockroach Diploptera punctata. In previous studies testing Rhopr/Dippu-DH(31) in Malpighian tubule secretion assays, we demonstrated increases in the rate of secretion that were small, relative to that induced by serotonin, but nevertheless 14-fold over baseline. In the present study, we investigated second messenger pathways in response to Rhopr/Dippu-DH(31) and found no increase or decrease in cyclic adenosine monophosphate (cyclic AMP) content of the Malpighian tubules. DH(31)-like immunoreactivity is present over the dorsal hindgut, anterior dorsal vessel and dorsal diaphragm, and bioassays of the R. prolixus dorsal vessel and hindgut indicate that Rhopr/Dippu-DH(31) increases the frequency of muscle contractions of both tissues. Second messenger pathways were also investigated for the dorsal vessel and hindgut.

Differential actions of diuretic factors on the Malpighian tubules of Rhodnius prolixus

The effects of corticotropin-releasing factor (CRF)-related (ZooneDH), calcitonin (CT)-related (RhoprDH(31)) and kinin-related (leucokinin I) peptides on the ion composition of fluid secreted by upper Rhodnius prolixus Malpighian tubules and on KCl reabsorption by the lower tubules were assessed. ZooneDH stimulated fluid secretion while increasing the [Na(+)] of secreted fluid at the expense of [K(+)]. Upper tubules responded to ZooneDH with a characteristic triphasic change in the transepithelial potential (TEP), reminiscent of the response to 5-hydroxytryptamine (5HT). RhoprDH(31) produced a small (~9 mV) lumen-positive shift in TEP of the upper tubule but had no effect on the rate of fluid secretion or ion composition of the secreted fluid. In contrast to 5HT, both peptides failed to activate KCl reabsorption by the lower tubule. Leucokinin I had no effect on the ion composition of fluid secreted by whole or upper Malpighian tubules. We propose that: (1) 5HT and a native CRF-related peptide similar to ZooneDH activate the same second messenger systems and ion transporters in the upper tubule cells; (2) CRF-related peptide is utilized to maintain high rates of fluid secretion during the post-feeding diuresis and is additionally used at times when KCl reabsorption is unnecessary or detrimental. The differential actions of multiple diuretic factors allows for intricate control of ionic and osmotic balance in R. prolixus.

Distribution and activity of a Dippu DH31-like peptide in the large milkweed bug Oncopeltus fasciatus

The milkweed bug, Oncopeltus fasciatus, is a plant feeding hemipteran. While there has been much research done on the neurohormonal control of the post-feeding diuresis in the blood-feeding hemipteran, Rhodnius prolixus, little is known about the control of the post-feeding diuresis in O. fasciatus. One of the neurohormones that may play a role in this rapid diuresis belongs to the calcitonin-like diuretic hormone (DH31) family of insect peptides. In this study we demonstrate the presence of DH31-like immunoreactivity in the central nervous system (CNS) and gut of O. fasciatus 5th instars. As well, DH31-like material was quantified and partially purified from the CNS of 5th instar O. fasciatus using reversed-phase liquid chromatography (RPLC) and monitored with an enzyme-linked immunosorbent assay (ELISA). When tested on O. fasciatus 5th instar Malpighian tubules, DH31-like peptides significantly increased the rate of secretion over saline controls. The results suggest that there is a DH31-like peptide(s) present in the CNS of O. fasciatus and that this peptide may play a role in the control of Malpighian tubule secretion.

Rhodnius prolixus Malpighian tubules and control of diuresis by neurohormones

Rhodnius prolixus Malpighian tubules (MTs) are a good model for fluid and ion secretion studies in view of the dramatic postprandial diuresis, which follows its massive blood meals. Ingestion of a blood meal equals to 10-12 times their initial body mass, leads to rapid activation of high output by excretory system, which eliminates 40-50% of the fluid mass. Secretion of ions and water is stimulated 1000-fold by serotonin and diuretic hormone. These hormones cooperate synergistically to activate adenylate cyclase activity from MTs cells, which increase the level of intracellular cAMP. The anti-diuretic hormones have also an important role in the fluid maintenance of Rhodnius prolixus. Several hours after insect feeding occurs a reduction in urine flow, that has been thought to result from a decreased diuretic hormone release or from a novel mechanism of anti-diuresis involving insect cardioacceleratory peptide 2b (CAP2b) and cyclic GMP. In this article it is discussed how the hormone regulation of fluid transport is done in Rhodnius prolixus MTs.

The endocrine control of salt balance in insects

An overview is given of the role of Malpighian (renal) tubules and the hindgut (ileum and rectum) in the excretory process of insects. The review focuses on the mechanism of primary urine production by Malpighian tubules and its control by neurohormones, which includes serotonin and neuropeptides resembling mammalian corticotropin-releasing factor (CRF) and calcitonin. Particular emphasis is given to in vitro studies of the effect of neurohormones on Malpighian tubule ion transport and a consideration of their likely role in the regulation of salt balance in vivo.

Actions of kinin peptides in the stomatogastric ganglion of the crab Cancer borealis

To fully understand neuronal network operation, the influence of all inputs onto that network must be characterized. As in most systems, many neuronal and hormonal pathways influence the multifunctional motor circuits of the crustacean stomatogastric ganglion (STG), but the actions of only some of them are known. Therefore, we characterized the influence of the kinin peptide family on the gastric mill (chewing) and pyloric (filtering of chewed food) motor circuits in the STG of the crab Cancer borealis. The kinins are myoactive in arthropods and they occur within the arthropod central nervous system (CNS), but their CNS actions are not well characterized in any species. The pevkinins were first identified in the shrimp Penaeus vannamei, but they have yet to be studied in the STG of any species. We identified kinin-like immunolabeling (KLI) in the pericardial organs (POs) in C. borealis, but there was no KLI within the STG. The POs are a major source of hormonal influence on the STG. Pevkinin peptides activated the pyloric circuit and they caused a modest increase in the speed of ongoing pyloric rhythms. This modest influence on cycle speed resulted in part from pevkinin excitation of the lateral pyloric neuron, whose strengthened inhibitory synapse onto the pyloric pacemaker neurons limited the pevkinin-mediated increase in cycle speed. The pevkinin excitation of the pyloric rhythm was not strong enough to interfere with the previously documented, gastric mill rhythm-mediated weakening of the pyloric rhythm. Pevkinin also had little influence on the gastric mill rhythm. These results indicate that the kinin peptides have distinct and selective modulatory actions on the pyloric rhythm.

Serotonin: A coordinator of feeding-related physiological events in the blood-gorging bug, Rhodnius prolixus

Rhodnius prolixus is an obligatory blood-feeder that can ingest blood meals of up to 10 times its mass. Rapid production of urine commences within 2-3 min of the start of feeding in order to eliminate the load of water and salts, and so there is an increase of Malpighian tubule secretion greater than 1,000 fold in response to feeding. Feeding and post-prandial diuresis in Rhodnius are highly coordinated events, including for example, host recognition, probing, injection of saliva, cuticle plasticization, passage of blood through the digestive system, diuresis and excretion. This review illustrates that many of the known functions of serotonin in Rhodnius are feeding-related. Serotonin coordinates or 'orchestrates' feeding-related physiological events either as a neurotransmitter/neuromodulator, delivered to target tissues in the nerve supply, or as a neurohormone, delivered by the haemolymph. Thus, serotonin has physiological effects upon the salivary glands, cuticle, digestive tract, cardiac muscle, and Malpighian tubules. By discussing these aspects, the review illustrates that serotonin acts in a coordinated manner to prepare Rhodnius for this energy-demanding process of feeding and diuresis.

Neuropeptide Control of Fluid Balance in Insects

This minireview considers various aspects of the control of hydromineral balance in insects with particular reference to the control of diuresis and natriuresis in mosquitoes, with new information on the diuretic peptides of Anopheles gambiae.

Identification of tick periviscerokinin, the first neurohormone of Ixodidae: single cell analysis by means of MALDI-TOF/TOF mass spectrometry

The first peptidergic neurohormone from the ticks Ixodes ricinus and Boophilus microplus has been identified by using a combination of immunocytochemistry and mass spectrometric analysis of single cells. The novel peptide (Ixori-PVK, PALIPFPRV-NH2) shows a high sequence homology with members of the insect periviscerokinin/CAP2b peptides that in insects are involved in the regulation of water balance. The function of this peptide in ticks is still unknown, but these pests consume large amounts of blood in a single blood meal which is a challenge for the regulation of diuretic processes. Thus, the novel peptide may be involved in one of the key physiological processes in ticks. High energy collision-induced dissociation was successfully used to distinguish between Leu/Ile ambiguities in single cell preparations. This is the first successful de novo sequencing of a peptide from single cell preparations of arthropods.

Conservation of capa peptide-induced nitric oxide signalling in Diptera

In D. melanogaster Malpighian (renal) tubules, the capa peptides stimulate production of nitric oxide (NO) and guanosine 3', 5'-cyclic monophosphate (cGMP), resulting in increased fluid transport. The roles of NO synthase (NOS), NO and cGMP in capa peptide signalling were tested in several other insect species of medical relevance within the Diptera (Aedes aegypti, Anopheles stephensi and Glossina morsitans) and in one orthopteran out-group, Schistocerca gregaria. NOS immunoreactivity was detectable by immunocytochemistry in tubules from all species studied. D. melanogaster, A. aegypti and A. stephensi express NOS in only principal cells, whereas G. morsitans and S. gregaria show more general NOS expression in the tubule. Measurement of associated NOS activity (NADPH diaphorase) shows that both D. melanogaster capa-1 and the two capa peptides encoded in the A. gambiae genome, QGLVPFPRVamide (AngCAPA-QGL) and GPTVGLFAFPRVamide (AngCAPA-GPT), all stimulate NOS activity in D. melanogaster, A. aegypti, A. stephensi and G. morsitans tubules but not in S. gregaria. Furthermore, capa-stimulated NOS activity in all the Diptera was inhibited by the NOS inhibitor l-NAME. All capa peptides stimulate an increase in cGMP content across the dipteran species, but not in the orthopteran S. gregaria. Similarly, all capa peptides tested stimulate fluid secretion in D. melanogaster, A. aegypti, A. stephensi and G. morsitans tubules but are either without effect or are inhibitory on S. gregaria. Consistent with these results, the Drosophila capa receptor was shown to be expressed in Drosophila tubules, and its closest Anopheles homologue was shown to be expressed in Anopheles tubules. Thus, we provide the first demonstration of physiological roles for two putative A. gambiae neuropeptides. We also demonstrate neuropeptide modulation of fluid secretion in tsetse tubule for the first time. Finally, we show the generality of capa peptide action, to stimulate NO/cGMP signalling and increase fluid transport, across the Diptera, but not in the more primitive Orthoptera.

Presence and activity of a Dippu-DH31-like peptide in the blood-feeding bug, Rhodnius prolixus

The blood-feeding bug, Rhodnius prolixus, ingests large blood meals, then undergoes a period of rapid diuresis which is under neurohormonal control. In both cockroach (Diploptera punctata) and fruit fly (Drosophila melanogaster) a calcitonin-like DH31 neuropeptide has been identified [Coast GM, Webster SG, Schegg KM, Tobe SS, Schooley DA. The Drosophila melanogaster homologue of an insect calcitonin-like diuretic peptide stimulates V-ATPase activity in fruit fly Malpighian tubules. J Exp Biol 2001;204:1795-804; Furuya K, Milchak RJ, Schegg KM, Zhang J, Tobe SS, Coast GM, et al. Cockroach diuretic hormones: characterization of a calcitonin-like peptide in insects. Proc Natl Acad Sci USA 2000;97:6469-74] and demonstrated to be active on Malpighian tubule secretion [Coast GM, Webster SG, Schegg KM, Tobe SS, Schooley DA. The Drosophila melanogaster homologue of an insect calcitonin-like diuretic peptide stimulates V-ATPase activity in fruit fly Malpighian tubules. J Exp Biol 2001;204:1795-804; Furuya K, Milchak RJ, Schegg KM, Zhang J, Tobe SS, Coast GM, et al. Cockroach diuretic hormones: characterization of a calcitonin-like peptide in insects. Proc Natl Acad Sci USA 2000;97:6469-74]. Using an antibody raised against D. punctata (Dippu) DH31, we demonstrate the presence of Dippu-DH31-like immunoreactivity in the CNS, salivary glands, hindgut and neurohemal sites of 5th instar Rhodnius. Double-label immunohistochemistry for Dippu-DH31-like and serotonin-like immunoreactivity demonstrates some co-localization of these factors in cells of the mesothoracic ganglionic mass (MTGM) and in neurohemal sites on the abdominal nerves. When tested on Rhodnius 5th instar Malpighian tubules, Dippu-DH31 stimulated minor increases in rate of secretion. Dippu-DH31 tested in combination with serotonin resulted in increases in the rate of secretion which were at least additive.

The distribution and activity of tachykinin-related peptides in the blood-feeding bug, Rhodnius prolixus

The invertebrate tachykinin-related peptides (TRPs) with the conserved C-terminal sequence FX1GX2Ramide shows sequence similarity to the vertebrate tachykinins after which they are named, and are hypothesized to be ancestrally related. In this study a polyclonal antiserum generated against a locust tachykinin (LomTK I), was used to demonstrate the presence and describe the distribution of LomTK-like immnoreactivity in the CNS and gut of Rhodnius prolixus. Reverse phase high performance liquid chromatography (RP-HPLC) was used in combination with a sensitive radioimmunoassay (RIA) to demonstrate picomolar amounts of immunoreactive material in the CNS, and femptomolar amounts associated with the hindgut. Furthermore, the results from CNS extracts separated by RP-HPLC, suggest that at least two tachykinin isoforms exist in R. prolixus. A hindgut contraction assay was developed to quantify the myotropic effects of selected LomTKs on R. prolixus hindgut contraction. Both LomTK I and II caused an increase in the frequency of hindgut contractions with EC50 values of 3.6x10(-8)M and 3.8x10(-8)M for LomTK I and II, respectively.