Immunocytochemical localisation and biological activity of diuretic peptides in the housefly, Musca domestica

Leucokinin and Associated Neuropeptides Regulate Multiple Aspects of Physiology and Behavior in Drosophila

Leucokinins (LKs) constitute a family of neuropeptides identified in numerous insects and many other invertebrates. LKs act on G-protein-coupled receptors that display only distant relations to other known receptors. In adult Drosophila, 26 neurons/neurosecretory cells of three main types express LK. The four brain interneurons are of two types, and these are implicated in several important functions in the fly's behavior and physiology, including feeding, sleep-metabolism interactions, state-dependent memory formation, as well as modulation of gustatory sensitivity and nociception. The 22 neurosecretory cells (abdominal LK neurons, ABLKs) of the abdominal neuromeres co-express LK and a diuretic hormone (DH44), and together, these regulate water and ion homeostasis and associated stress as well as food intake. In Drosophila larvae, LK neurons modulate locomotion, escape responses and aspects of ecdysis behavior. A set of lateral neurosecretory cells, ALKs (anterior LK neurons), in the brain express LK in larvae, but inconsistently so in adults. These ALKs co-express three other neuropeptides and regulate water and ion homeostasis, feeding, and drinking, but the specific role of LK is not yet known. This review summarizes Drosophila data on embryonic lineages of LK neurons, functional roles of individual LK neuron types, interactions with other peptidergic systems, and orchestrating functions of LK.

Leucokinins: Multifunctional Neuropeptides and Hormones in Insects and Other Invertebrates

Leucokinins (LKs) constitute a neuropeptide family first discovered in a cockroach and later identified in numerous insects and several other invertebrates. The LK receptors are only distantly related to other known receptors. Among insects, there are many examples of species where genes encoding LKs and their receptors are absent. Furthermore, genomics has revealed that LK signaling is lacking in several of the invertebrate phyla and in vertebrates. In insects, the number and complexity of LK-expressing neurons vary, from the simple pattern in the Drosophila larva where the entire CNS has 20 neurons of 3 main types, to cockroaches with about 250 neurons of many different types. Common to all studied insects is the presence or 1-3 pairs of LK-expressing neurosecretory cells in each abdominal neuromere of the ventral nerve cord, that, at least in some insects, regulate secretion in Malpighian tubules. This review summarizes the diverse functional roles of LK signaling in insects, as well as other arthropods and mollusks. These functions include regulation of ion and water homeostasis, feeding, sleep-metabolism interactions, state-dependent memory formation, as well as modulation of gustatory sensitivity and nociception. Other functions are implied by the neuronal distribution of LK, but remain to be investigated.

Nutrient Sensor in the Brain Directs the Action of the Brain-Gut Axis in Drosophila

Animals can detect and consume nutritive sugars without the influence of taste. However, the identity of the taste-independent nutrient sensor and the mechanism by which animals respond to the nutritional value of sugar are unclear. Here, we report that six neurosecretory cells in the Drosophila brain that produce Diuretic hormone 44 (Dh44), a homolog of the mammalian corticotropin-releasing hormone (CRH), were specifically activated by nutritive sugars. Flies in which the activity of these neurons or the expression of Dh44 was disrupted failed to select nutritive sugars. Manipulation of the function of Dh44 receptors had a similar effect. Notably, artificial activation of Dh44 receptor-1 neurons resulted in proboscis extensions and frequent episodes of excretion. Conversely, reduced Dh44 activity led to decreased excretion. Together, these actions facilitate ingestion and digestion of nutritive foods. We propose that the Dh44 system directs the detection and consumption of nutritive sugars through a positive feedback loop.

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.

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.

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.

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.

The single kinin receptor signals to separate and independent physiological pathways in Malpighian tubules of the yellow fever mosquito

In the past, we have used the kinins of the cockroach Leucophaea (the leucokinins) to evaluate the mechanism of diuretic action of kinin peptides in Malpighian tubules of the yellow fever mosquito Aedes aegypti. Now using the kinins of Aedes (the aedeskinins), we have found that in isolated Aedes Malpighian tubules all three aedeskinins (1 microM) significantly 1) increased the rate of fluid secretion (V(S)), 2) hyperpolarized the basolateral membrane voltage (V(bl)), and 3) decreased the input resistance (R(in)) of principal cells, consistent with the known increase in the Cl(-) conductance of the paracellular pathway in Aedes Malpighian tubules. Aedeskinin-III, studied in further detail, significantly increased V(S) with an EC(50) of 1.5 x 10(-8) M. In parallel, the Na(+) concentration in secreted fluid significantly decreased, and the K(+) concentration significantly increased. The concentration of Cl(-) remained unchanged. While the three aedeskinins triggered effects on V(bl), R(in), and V(S), synthetic kinin analogs, which contain modifications of the COOH-terminal amide pentapeptide core sequence critical for biological activity, displayed variable effects. For example, kinin analog 1578 significantly stimulated V(S) but had no effect on V(bl) and R(in), whereas kinin analog 1708 had no effect on V(S) but significantly affected V(bl) and R(in). These observations suggest separate signaling pathways activated by kinins. One triggers the electrophysiological response, and the other triggers fluid secretion. It remains to be determined whether the two signaling pathways emanate from a single kinin receptor via agonist-directed signaling or from a differentially glycosylated receptor. Occasionally, Malpighian tubules did not exhibit a detectable response to natural and synthetic kinins. Hypothetically, the expression of the kinin receptor may depend on developmental, nutritional, and/or reproductive signals.

Beetle diuretic peptides: the response of mealworm (Tenebrio molitor) Malpighian tubules to synthetic peptides, and cross-reactivity studies with a dung beetle (Onthophagus gazella)

This paper reports the effects of different diuretic factors on the Malpighian tubules of beetles. Calcitonin (CT)-like peptides from silkmoth and mosquito increase fluid secretion in a dose-dependent manner in the tubules of Tenebrio molitor, but the cockroach CT-like peptide, Dippu-DH(31), has no effect. Thapsigargin induces a small but significant increase in tubule secretion rates. The interactions between different factors in mealworm tubules were explored by testing CT-like peptides, thapsigargin and the mealworm CRF-related diuretic factor Tenmo-DH(37) in various combinations, but no synergistic effects were observed. C-terminal fragments of the CRF-related diuretic peptides Locmi-DH(46) and Dippu-DH(46) fail to increase fluid secretion in mealworm tubules, unlike their corresponding whole peptides. Cross-reactivity of factors between beetle species was investigated using the scarabaeid Onthophagus gazella. Tenmo-DH(37) increases fluid secretion in isolated tubules of O. gazella in a dose-dependent manner, revealing a high degree of cross-reactivity in this distantly related beetle species. However, homogenates of O. gazella brains inhibited fluid secretion in mealworm tubules.

Bioinformatic analysis of neuropeptide and receptor expression profiles during midgut metamorphosis in Drosophila melanogaster

Neuropeptides are important messenger molecules in invertebrates, serving as neuromodulators in the nervous system and as regulatory hormones released into the circulation. Understanding the function of neuropeptides will require the integration of genetic, biochemical, physiological and behavioral information. The advent of DNA microarrays and bioinformatic databases provides a wealth of data describing the expression profiles of thousands of genes during biological processes. One such array catalogs the developmental patterns of gene expression during the metamorphic transformation of the Drosophila midgut. We have mined the data from this experiment to explore changes of expression in genes coding for known neuropeptides, peptide hormones, and their receptors during the metamorphosis of the midgut. We found small but significant changes in the expression of the peptides diuretic hormone, FGLa-type allatostatins, myoinhibiting peptide, ecdysis-triggering hormone, drosokinin and the burs subunit of bursicon, as well as the receptors DAR-2, NPFR1, ALCR-2, Lkr and DH-R. Just as advances have been made in understanding the molecular basis of invertebrate neuropeptide action by analysis of genome projects, data mining of gene expression databases can help to integrate molecular, biochemical and physiological knowledge of biological processes.

Transport mechanisms of diuresis in Malpighian tubules of insects

We have studied Malpighian tubules of Aedes aegypti using a variety of methods: Ramsay fluid secretion assay, electron probe analysis of secreted fluid, in vitro microperfusion and two-electrode voltage clamp. Collectively, these methods have allowed us to elucidate transepithelial transport mechanisms under control conditions and in the presence of diuretic peptides. Mosquito natriuretic peptide (MNP), a corticotropin-releasing factor (CRF)-like diuretic peptide, selectively increases transepithelial secretion of NaCl and water, meeting the NaCl loads of the blood meal. The intracellular messenger of MNP is cAMP, which increases the Na+ conductance and activates the Na+/K+/2Cl- -cotransporter in the basolateral membrane of principal cells. Leucokinin non-selectively increases transepithelial NaCl and KCl secretion, which may deal with hemolymph volume expansions or reduce the flight pay load upon eclosion from the aquatic habitat. The non-selective NaCl and KCl diuresis stems from the increase in septate junctional Cl- conductance activated by leucokinin using Ca2+ as second messenger. Fundamental to diuretic mechanisms are powerful epithelial transport mechanisms in the distal segment of the Malpighian tubules, where transepithelial secretion rates can exceed the capacity of mammalian glomerular kidneys in the renal turnover of the extracellular fluid compartment. In conjunction with powerful epithelial transport mechanisms driven by the V-type H+-ATPase, diuretic hormones enable hematophagous and probably also phytophagous insects to deal with enormous dietary loads, thereby contributing to the evolutionary success of insects.

Continuous recording of excretory water loss from Musca domestica using a flow-through humidity meter: hormonal control of diuresis

Water loss from adult male houseflies was continuously recorded using a flow-through humidity meter, which enabled losses to be apportioned between the sum of cuticular and respiratory transpiration, salivation and excretion. Transpiration accounted for >95% of water lost from sham-injected flies, compared with excretion (3.0%) and salivation (2.4%). In contrast, excretion accounted for 40% of water lost from flies injected with > or =3 microl of saline, whereas salivary losses were unchanged. Saline injections (1-5 microl) expanded the abdomen in the dorsal-ventral plane, and this expansion was positively correlated with the magnitude of the ensuing diuresis, suggesting the signal for diuretic hormone release originates from stretch receptors in abdominal tergal-sternal muscles. The effects of decapitation, severing the ventral nerve cord within the neck or ligaturing the neck, showed the head was needed to initiate and maintain diuresis, but was neither the source of diuretic hormone nor did it control the discharge of urine from the anus. These findings indicate the head is part of the neural-endocrine pathway between abdominal stretch receptors and sites for diuretic hormone release from the thoracic-abdominal ganglion mass. Evidence is presented for Musdo-K having a hormonal role in the control of diuresis, although other neuropeptides may also be implicated.

Drosophila CG8422 encodes a functional diuretic hormone receptor

Diuretic hormone 44 (DH) is a bioactive neuropeptide that mediates osmotic balance in a wide variety of insects through increases in cAMP. It is structurally similar to mammalian corticotrophin releasing factor (CRF) peptides. In the moth Manduca and the cricket Acheta, functional studies have shown that its cognate receptor (DH-R) is related to the mammalian CRF receptor. The Drosophila genome contains two genes (CG8422 and CG12370) orthologous to Manduca and Acheta DH-Rs. Here, we present multiple lines of evidence to support the hypothesis that the orphan CG8422 G-protein-coupled receptor is a functional DH-R. When expressed in mammalian cells, CG8422 conferred selective sensitivity to DH, as indicated by translocation of a beta-arrestin-2-GFP reporter from the cytoplasm to the cell membrane. Consistent with its in vivo activities in other insects, DH activation of CG8422 elicited increases in a cAMP reporter system (CRE-luciferase), with an EC(50) of 1.7 nmol l(-1). CG8422 activation by DH also led to increases in intracellular calcium but at substantially higher doses (EC(50) approximately 300 nmol l(-1)). By microarray analysis, the CG8422 transcript was detectable in Drosophila head mRNA of different genotypes and under different environmental conditions. The identification of a Drosophila receptor for the DH neuropeptide provides a basis for genetic analysis of this critical factor's roles in maintaining physiological homeostasis.

Effects of leucokinin-VIII onAedesMalpighian tubule segments lacking stellate cells

The diuretic peptide leucokinin is known to increase fluid secretion in Malpighian tubules of the yellow fever mosquito Aedes aegypti by increasing a transepithelial Cl- conductance. The present study sought to examine whether stellate cells provided this transepithelial conductance in Aedes Malpighian tubules as they do in Drosophila Malpighian tubules. Aedes Malpighian tubule segments with and without stellate cells were perfused in vitro for measurements of the transepithelial voltage (Vt),resistance (Rt) and Cl- diffusion potentials(DPCl). In 11 tubule segments containing both principal cells and stellate cells, 1 μmol l-1 leucokinin-VIII added to the peritubular bath immediately and significantly decreased Vt from 39.3±14.3 mV to 2.3±0.7 mV,decreased Rt from 12.4±2.6 kΩcm to 2.4±0.3 kΩcm, and increased DPCl from 8.2±1.2 mV to 42.1±5.4 mV. These effects of leucokinin-VIII were qualitatively and quantitatively similar in six tubule segments containing no stellate cells; Vt decreased from 37.8±7.0 mV to 3.4±0.6 mV, Rt decreased from 8.8±2.1 kΩcm to 1.7±0.2 kΩcm, and DPClincreased from 5.8±2.6 mV to 50.0±2.1 mV. Thus, stellate cells are not required for signaling or mediating the effects of leucokinin in Malpighian tubules of Aedes aegypti. The results further support previous observations that principal cells signal the effects of leucokinin to increase the Cl- conductance of the paracellular pathway through septate (or tight) junctions.

Anatomy and functions of brain neurosecretory cells in diptera

In the larval brain of dipteran insects, there are two medial and three lateral groups of neurons innervating the ring gland. One lateral group extends fibers to the corpus allatum. After metamorphosis, a large cluster of the medial group in the pars intercerebralis and two lateral groups in the pars lateralis innervate the retrocerebral complex and some neurons from the lateral group and a few from the medial group extend fibers to the corpus allatum in the adults. Neuropeptides such as insulin-like peptides, FMRFamide related peptides, Locusta-diuretic hormone, beta-pigment dispersing hormone, Manduca sexta-allatostatin, ovary ecdysteroidogenic hormone, and proctolin have been immunocytochemically revealed in medial groups in the pars intercerebralis, and FMRFamide related peptides, beta-pigment dispersing hormone, corazonin, and M. sexta-allatostatin in lateral groups in the pars lateralis of dipteran brains. In mosquitoes after the blood meal, ovary ecdysteroidogenic hormone from 2-3 pairs of medial neurosecretory cells is released at the corpus cardiacum to stimulate the ovaries to secrete ecdysteroid to cause ovarian development. In addition to ovarian development, removal and implantation experiments have shown that neurosecretory cells in the pars intercerebralis and pars lateralis are involved in control of reproductive diapause, cuticular tanning, sugar metabolism, and diures.

A C-terminal aldehyde insect kinin analog enhances inhibition of weight gain and induces significant mortality in Helicoverpa zea larvae

The first reported examples of C-terminal aldehyde analogs of an insect neuropeptide are described. They are hexapeptide insect kinin analogs Boc-VFFPWG-H and Fmoc-RFFPWG-H. Activity observed for these modified analogs in an in vitro insect diuretic assay confirms that the C-terminal aldehyde group is tolerated by an insect kinin receptor. The two analogs demonstrate greatly enhanced activity over standard C-terminal amide insect kinins in a larval weight gain inhibition assay in the corn earworm Helicoverpa zea. Treatment with Boc-VFFPWG-H led to significant increases in larval mortality at doses of 500pm (45%) and 5nm (67%). Boc-VFFPWG-H represents a lead analog in the development of novel, environmentally friendly pest insect management agents based on the insect kinin class of neuropeptides.

Comparative distribution of urocortin- and CRF-like immunoreactivities in the nervous system of the earthworm Lumbricus terrestris

Corticotropin-releasing factor (CRF) and urocortin (Ucn) are both members of the CRF neuropeptide family. The distribution of Ucn- and CRF-like immunoreactive (ir) structures in the central nervous system of several vertebrate species has been studied, but little is known about that in non-vertebrates. We used a highly specific polyclonal antibody against rat Ucn and CRF to determine and compare the distribution of Ucn- and CRF-like immunoreactivity in the earthworm nervous system. Several Ucn- and CRF-like ir perikarya were described in the cerebral ganglion, subesophageal and ventral cord ganglia. The majority of Ucn-like ir cells were found in the ventral ganglia, whereas CRF-like ir cells were most abundant in the cerebral ganglion. Scattered Ucn- and CRF-like ir varicose fiber terminals were seen in all areas of the earthworm central nervous system. Ucn-like ir cell bodies and fiber terminals were also demonstrated in the pharyngeal wall. No co-localization of Ucn- and CRF-like ir nervous structures were observed. This study provided morphological evidence that Ucn- and CRF-like neurosecretory products exist in the earthworm central nervous system. Furthermore, both the distribution and morphology of Ucn- and CRF-like ir structures were distinct, therefore, it can be hypothesized that these neuropeptides exert different neurendocrine functions in the earthworm nervous system.

Isolation, identification and localization of a second beetle antidiuretic peptide

We isolated from head extracts of Tenebrio molitor a peptide that inhibits fluid secretion by the Malpighian tubules of this insect. This second antidiuretic factor, ADFb, like the previously published ADFa, works through cyclic GMP as a second messenger. It has primary structure Tyr-Asp-Asp-Gly-Ser-Tyr-Lys-Pro-His-Ile-Tyr-Gly-Phe-OH with an EC(50) of approximately 240 pM in a fluid secretion assay. This peptide is now the second sequenced endogenous insect ADF which inhibits Malpighian tubule fluid secretion. Immunohistochemical techniques show that the peptide is localized in the brain; it appears to be produced mainly in two pairs of bilaterally symmetrical cells in the protocerebrum.

TheDhgene ofDrosophila melanogasterencodes a diuretic peptide that acts through cyclic AMP

Dh, the gene that encodes a CRF-like peptide in Drosophila melanogaster, is described. The product of this gene is a 44-amino-acid peptide (Drome-DH44) with a sequence almost identical to the Musca domestica and Stomoxys calcitrans diuretic hormones. There are no other similar peptides encoded within the known Drosophila genomic sequence. Functional studies showed that the deduced peptide stimulated fluid production, and that this effect was mediated by cyclic AMP in principal cells only: there was no effect on the levels of either cyclic GMP or intracellular calcium. Stimulation also elevated levels of cyclic AMP (but not cyclic GMP) phosphodiesterase, a new mode of action for this class of hormone. The transcript was localised by in situhybridisation, and the peptide by immunocytochemistry, to two groups of three neurones in the pars intercerebralis within the brain. These cells also express receptors for leucokinin, another major diuretic peptide, implying that the cells may be important in homeostatic regulation.

Occurrence of insect kinins in the flesh fly, stable fly and horn fly-mass spectrometric identification from single nerves and diuretic activity

MALDI-TOF mass spectrometric analysis of single lateral abdominal nerves (LANs) demonstrate the presence of the insect kinin Musdo-K in the housefly Musca domestica, and identify heretofore unknown insect kinins in two other Dipteran species as Musdo-K in the stable fly Stomoxys calcitrans and horn fly Haematobia irritans. The insect kinin native to the flesh fly Neobellieria bullata is identified as Drome-K. Musdo-K and Drome-K are identical save for the conservative substitution of Ser for Thr in position 2. The sequences of the insect kinins are, therefore, remarkably conserved throughout Dipterans. The in vitro Malpighian tubule fluid secretion activity of Musdo-K in the stable fly is similar to that in the housefly, whereas that of Drome-K is 30-fold more potent in the flesh fly than in the fruit fly. Given the structural identities of the kinins and CRF-like diuretic hormones of these Dipteran species, the housefly can serve as a model insect for the study of diuretic peptides and their functions in the stable fly and horn fly, both livestock pests.

Evidence for CRF-like and kinin-like peptides as neurohormones in the blood-feeding bug, Rhodnius prolixus

In Rhodnius prolixus, the rapid post-feeding diuresis is under neurohormonal control. While serotonin has been demonstrated to be a diuretic neurohormone [J Exp Biol 156 (1991) 557], a peptide is also known to be involved. Previously, we have demonstrated the presence of corticotropin releasing factor (CRF)-like and kinin-like peptides in the central nervous system (CNS) of 5th instar Rhodnius [J Exp Biol 202 (1999) 2017; Peptides 22 (2001) 161]. These peptides are present in neurohemal sites of the corpus cardiacum and are co-localized in neurohemal sites on abdominal nerves. While various CRF-like peptides have been demonstrated to increase Rhodnius Malpighian tubule secretion the kinin-like peptides do not [Peptides 23 (2002) 671]. The kinin-like peptides do however, increase hindgut contraction which may contribute to the rapid post feeding diuresis by the mixing of hemolymph and/or hindgut contents and the removal of wastes. The presence of these peptides in neurohemal sites suggests that they could be released into the hemolymph and act as neurohormones. We have used immunohistochemical techniques and radioimmunoassay (RIA) to demonstrate qualitative and quantitative changes of CRF-like and kinin-like peptides in the CNS associated with feeding. As well we have examined Malpighian tubule secretion in response to assays of hemolymph from unfed and fed insects. Hemolymph was also partially purified by Sep-Pak and HPLC and the fractions assayed for kinin-like immunoreactivity and the ability to stimulate Malpighian tubule secretion. The results suggest that both kinin-like and CRF-like peptides are neurohormones in Rhodnius, released in response to feeding.

Leucokinin activates Ca(2+)-dependent signal pathway in principal cells of Aedes aegypti Malpighian tubules

The role of Ca(2+) in mediating the diuretic effects of leucokinin-VIII was studied in isolated perfused Malpighian tubules of the yellow fever mosquito, Aedes aegypti. Peritubular leucokinin-VIII (1 microM) decreased the transepithelial resistance from 11.2 to 2.6 kOmega. cm, lowered the transepithelial voltage from 42.8 to 2.7 mV, and increased transepithelial Cl(-) diffusion potentials 5.1-fold. In principal cells of the tubules, leucokinin-VIII decreased the fractional resistance of the basolateral membrane from 0.733 to 0.518. These effects were reversed by the peritubular Ca(2+)-channel blocker nifedipine, suggesting a role of peritubular Ca(2+) and basolateral Ca(2+) channels in signal transduction. In Ca(2+)-free Ringer bath, the effects of leucokinin-VIII were partial and transient but were fully restored after the bath Ca(2+) concentration was restored. Increasing intracellular Ca(2+) with thapsigargin duplicated the effects of leucokinin-VIII, provided that peritubular Ca(2+) was present. The kinetics of the effects of leucokinin-VIII is faster than that of thapsigargin, suggesting the activation of inositol-1,4,5-trisphosphate-receptor channels of intracellular stores. Store depletion may then bring about Ca(2+) entry into principal cells via nifedipine-sensitive Ca(2+) channels in the basolateral membrane.

Neuropeptides in the nervous system of Drosophila and other insects: multiple roles as neuromodulators and neurohormones

Neuropeptides in insects act as neuromodulators in the central and peripheral nervous system and as regulatory hormones released into the circulation. The functional roles of insect neuropeptides encompass regulation of homeostasis, organization of behaviors, initiation and coordination of developmental processes and modulation of neuronal and muscular activity. With the completion of the sequencing of the Drosophila genome we have obtained a fairly good estimate of the total number of genes encoding neuropeptide precursors and thus the total number of neuropeptides in an insect. At present there are 23 identified genes that encode predicted neuropeptides and an additional seven encoding insulin-like peptides in Drosophila. Since the number of G-protein-coupled neuropeptide receptors in Drosophila is estimated to be around 40, the total number of neuropeptide genes in this insect will probably not exceed three dozen. The neuropeptides can be grouped into families, and it is suggested here that related peptides encoded on a Drosophila gene constitute a family and that peptides from related genes (orthologs) in other species belong to the same family. Some peptides are encoded as multiple related isoforms on a precursor and it is possible that many of these isoforms are functionally redundant. The distribution and possible functions of members of the 23 neuropeptide families and the insulin-like peptides are discussed. It is clear that each of the distinct neuropeptides are present in specific small sets of neurons and/or neurosecretory cells and in some cases in cells of the intestine or certain peripheral sites. The distribution patterns vary extensively between types of neuropeptides. Another feature emerging for many insect neuropeptides is that they appear to be multifunctional. One and the same peptide may act both in the CNS and as a circulating hormone and play different functional roles at different central and peripheral targets. A neuropeptide can, for instance, act as a coreleased signal that modulates the action of a classical transmitter and the peptide action depends on the cotransmitter and the specific circuit where it is released. Some peptides, however, may work as molecular switches and trigger specific global responses at a given time. Drosophila, in spite of its small size, is now emerging as a very favorable organism for the studies of neuropeptide function due to the arsenal of molecular genetics methods available.

The effects of crustacean cardioactive peptide on locust oviducts are calcium-dependent

The role of calcium as a second messenger in the crustacean cardioactive peptide (CCAP)-induced contractions of the locust oviducts was investigated. Incubation of the oviducts in a calcium-free saline containing, a preferential calcium cation chelator, or an extracellular calcium channel blocker, abolished CCAP-induced contractions, indicating that the effects of CCAP on the oviducts are calcium-dependent. In contrast, sodium free saline did not affect CCAP-induced contractions. Co-application of CCAP to the oviducts with preferential L-type voltage-dependent calcium channel blockers reduced CCAP-induced contractions by 32-54%. Two preferential T-type voltage-dependent calcium channel blockers both inhibited CCAP-induced oviduct contractions although affecting different components of the contractions. Amiloride decreased the tonic component of CCAP-induced contractions by 40-55% and flunarizine dihydrochloride decreased the frequency of CCAP-induced phasic contractions by as much as 65%, without affecting tonus. Flunarizine dihydrochloride did not alter the proctolin-induced contractions of the oviducts. Results suggest that the actions of CCAP are partially mediated by voltage-dependent calcium channels similar to vertebrate L-type and T-type channels. High-potassium saline does not abolish CCAP-induced contractions indicating the presence of receptor-operated calcium channels that mediate the actions of CCAP on the oviducts. The involvement of calcium from intracellular stores in CCAP-induced contractions of the oviducts is likely since, an intracellular calcium antagonist decreased CCAP-induced contractions by 30-35%.

Diuretic and myotropic activities of N-terminal truncated analogs of Musca domestica kinin neuropeptide

Musca kinin (Musdo-K; NTVVLGKKQRFHSWG-NH(2)) and N-terminal truncated analogs of 4-14 residues in length were assayed for diuretic and myotropic activity on housefly Malpighian tubules and hindgut, respectively. The pentapeptide was the minimum sequence required for biological activity, but it was > 5 orders of magnitude less potent than the intact peptide. The pharmacological profiles of the different analogs in the two assays were very similar, suggesting the same receptor is present on both tissues. Potency was little affected by the deletion of Asn(1), but was reduced > 10-fold after the removal of Thr(2). Deletion of the next 5 residues had relatively little effect, but after the second lysyl residue (Lys(8)) was removed potency fell by one to two orders of magnitude. There was a similar drop in potency after the removal of Arg(10), and at 100 microM the pentapeptide had only 20% of the diuretic activity of the intact peptide. The importance of Arg(10) was confirmed by comparing dose-response curves for Musdo-K [6-15] and Acheta kinin-V (AFSHWG-NH(2)) in the diuretic assay; the substitution of arginine by alanine produced a significant reduction in potency and some loss of activity.

Enhanced in vivo activity of peptidase-resistant analogs of the insect kinin neuropeptide family

The diuretic/myotropic insect kinin neuropeptides, which share the common C-terminal pentapeptide core FX(1)X(2)WG-NH(2), reveal primary (X(2)-W) and secondary (N-terminal to F) sites of susceptibility to peptidases bound to corn earworm (H. zea) Malpighian tubule tissue. Analogs designed to enhance resistance to tissue-bound peptidases, and pure insect neprilysin and ACE, demonstrate markedly enhanced in vivo activity in a weight gain inhibition assay in H. zea, and strong in vivo diuretic activity in the housefly (M. domestica). The peptidase-resistant insect kinin analog pQK(pQ)FF[Aib]WG-NH(2) demonstrates a longer internal residence time in the housefly than the native muscakinin (MK), and despite a difference of over 4 orders of magnitude in an in vitro Malpighian tubule fluid secretion assay, is equipotent with MK in an in vivo housefly diuretic assay. Aminohexanoic acid (Ahx) is shown to function as a surrogate for N-terminal Lys, while at the same time providing enhanced resistance to aminopeptidase attack. Peptidaese-resistant insect kinin analogs demonstrate enhanced inhibition of weight gain in larvae of the agriculturally destructive corn earworm moth. Potent peptidase resistant analogs of the insect kinins, coupled with an increased understanding of related regulatory factors, offer promise in the development of new, environmentally friendly pest insect control measures.

The biological activity of diuretic factors in Rhodnius prolixus

The rapid post-feeding diuresis of Rhodnius prolixus is under neurohormonal control and involves the integrated activity of the crop, Malpighian tubules and hindgut. One of the factors which is involved in this rapid diuresis is serotonin, however a peptide(s) is also considered to be involved. In other insects, corticotropin releasing factor (CRF)-like and kinin-like, calcitonin-like peptides and CAP(2b) have been demonstrated to be diuretic factors/hormones. In the present study, serotonin and CRF-like peptides increased secretion rate and cAMP content of Rhodnius Malpighian tubules, while the kinin-like peptides tested did not increase secretion rate or cAMP content of the tubules. Extracts of the CNS were processed and several HPLC fractions revealed kinin-like immunoreactivity but these fractions did not increase secretion rate when tested on Malpighian tubules. However, these same fractions did possess activity when tested on the hindgut contraction assay. In addition, material eluting at higher acetonitrile concentrations from the HPLC increased secretion and cAMP content of Rhodnius Malpighian tubules. This material eluted at concentrations of acetonitrile consistent with the elution time of CRF-like peptide standards. Synergism was demonstrated using the pharmacological agent forskolin and serotonin, tested on the rate of secretion of Rhodnius Malpighian tubules, in agreement with data of Maddrell et al. As well, synergism could be demonstrated using mesothoracic ganglionic mass (MTGM) homogenates and serotonin at some concentrations of serotonin. However, combinations of CRF-like material and serotonin increased secretion additively, not synergistically. Kinin-like peptides, tested along with CRF-like material and serotonin, at low concentrations, did not increase secretion above that of those factors tested alone.

Antagonistic control of fluid secretion by the Malpighian tubules ofTenebrio molitor: effects of diuretic and antidiuretic peptides and their second messengers

Fluid secretion by insect Malpighian tubules is controlled by haemolymph-borne factors. The mealworm Tenebrio molitor provides the first known example of antagonistic interactions between endogenous neuropeptides acting on Malpighian tubules. The two corticotropin-releasing-factor (CRF)-related diuretic peptides previously isolated from Tenebrio molitor, Tenmo-DH37 and Tenmo-DH47, were found to stimulate Tenebrio molitor tubules in vitro in a dose-dependent manner with EC50 values of 0.12 nmol l–1 and 26 nmol l–1 respectively. However, no synergistic or additive effect was observed when these two peptides were tested simultaneously. We then investigated antagonism between second messengers: dose–response curves were constructed for stimulation of Tenebrio molitor tubules by cyclic AMP and their inhibition by cyclic GMP. When both cyclic nucleotides were included in the bathing Ringer, the stimulatory effect of cyclic AMP was neutralised by cyclic GMP. Similarly, the stimulatory effect of Tenmo-DH37 was reversed on addition of an antidiuretic peptide (Tenmo-ADF), which was recently isolated from Tenebrio molitor and acts via cyclic GMP. The cardioacceleratory peptide CAP2b, originally isolated from Manduca sexta, also increases intracellular cyclic GMP levels and inhibited fluid secretion by Tenebrio molitor tubules, with an EC50 value of 85 nmol l–1. This inhibitory effect was reversed by Tenmo-DH37. Endogenous diuretic and antidiuretic peptides, effective at low concentrations and acting via antagonistic second messengers, have the potential for fine control of secretion rates in the Malpighian tubules of Tenebrio molitor.

The distribution of a kinin-like peptide and its co-localization with a CRF-like peptide in the blood-feeding bug, Rhodnius prolixus

Rhodnius prolixus, a blood-feeding hemipteran insect, ingests large meals which are followed by rapid diuresis to eliminate excess water and salt. In Rhodnius, serotonin and an unidentified peptide(s) [33,34] have been shown to act as neurohormonal diuretic factors. In other insects, two families of diuretic peptides, the corticotropin releasing factor (CRF)-like, and kinin peptides [9], have been identified and sequenced. Recently, we demonstrated the presence of a CRF-like diuretic peptide in the CNS and digestive system of Rhodnius [47] using immunohistochemistry and bioassay. In this study, combining immunohistochemistry and radioimmunoassay (RIA) techniques, we show the presence of leucokinin-like peptide(s) in the CNS and digestive system of Rhodnius 5th instar. Additionally, double-label immunohistochemistry demonstrates that the leucokinin-like and CRF-like peptides are co-localized in the posterior lateral neurosecretory cells of the mesothoracic ganglionic mass (MTGM) and in neurohaemal areas on abdominal nerves one and two, suggesting the possibility of co-release of the peptides into the hemolymph.Partially purified extracts of the CNS and neurohaemal tissue were tested in vitro on Malpighian tubule secretion and cAMP assays. The factors eluting with increasing acetonitrile percentages from Sep-Pak cartridges were assayed in the presence or absence of ketanserin, a serotonin antagonist which blocks the effects of serotonin on Malpighian tubules. The results indicate activity of serotonin and a CRF-like diuretic peptide on Rhodnius Malpighian tubules, but fail to demonstrate activity of the leucokinin-like peptide(s). The rapid diuresis following feeding is a highly coordinated event, requiring the movement of water and salt across the epithelial cells of the crop into the hemolymph, and from the hemolymph across the cells of the Malpighian tubules. The urine then travels along the Malpighian tubules into the hindgut in order to be expelled. The presence of a leucokinin-like peptide(s) in the CNS and digestive system, which co-localizes with a CRF-like peptide(s), suggests that kinins may play a role in the rapid diuresis, although possibly not directly on the Malpighian tubules.

Diuresis in the housefly (Musca domestica) and its control by neuropeptides

Diuresis was studied in vivo by measuring the loss of tritiated water. The basal rate of water loss (5 nl/min) represents respiratory and cuticular losses, whereas higher rates reflect urine output, which reaches 20 nl/min after injection of 1 microl distilled water. This response to hypervolemia involves release of a diuretic hormone(s) into the hemolymph. However, housefly diuretic peptides increased urine output to a maximum of only 7 nl/min, and higher rates may require fluid reabsorption from the hindgut to be reduced. Diuresis is partially blocked by injected anti-muscakinin antibodies, providing evidence of a hormonal function for this insect myokinin.

Effect of helicokinins and ACE inhibitors on water balance and development of Heliothis virescens larvae

The diuretic activity of the helicokinins I (YFSPWG-amide), II (VRFSPWG-amide) and III (KVKFSAWG-amide) was tested on Heliothis virescens larvae. All three kinins increased fluid secretion in isolated Malpighian tubules in a dose-dependent manner. Injections into the haemolymph caused a significant reduction in weight gain after 24 h and, in the case of helicokinin I, led to an increased mortality of 43% within 6 days. When truncated analogues of helicokinin I were tested in vitro, only the pentapeptide (FSPWG-amide) stimulated fluid secretion. Tested in vivo the pentapeptide did not influence normal development of the larvae. An alanine scan of helicokinin I showed that the substitution of phenylalanine, tryptophan and glycine led to a massive decrease or even loss of diuretic activity. The substitution of the other amino acids had no effect in vitro. The ACE inhibitors captopril, enalapril-maleate and lisinopril were tested for their influence on the development of the larvae. In combination with one of the helicokinins the in vivo injection of the ACE inhibitors led to increased rates of mortality and/or reductions in pupal weight.

The kinin peptide family in invertebrates

Kinins comprise a family of peptides that were first found in the central nervous system of insects and recently also in mollusks and crustaceans. After the isolation of the first members of the kinin family, the leukokinins from Leucophaea maderae, leukokinin-related peptides were found in the cricket Acheta domesticus and the locust Locusta migratoria, all through their ability to induce Leucophaea maderae hindgut contraction. Subsequently, kinins were found in the mosquitoes Culex salinarius and Aedes aegypti and in the earworm Helicoverpa zea. The first noninsect member of this family was isolated from a mollusk, the pond snail Lymnaea stagnalis. Most recently our group has isolated the first kinins from crustaceans. Six kinins were isolated from the white shrimp Penaeus vannamei. To date, 35 members of this family have been isolated. The first relatively small family of insect kinins has grown into an expanding and rather large family with members in insects, crustaceans, and mollusks. In this paper we discuss the kinin family in terms of method of isolation, structure, in vitro and in vivo activity, distribution, receptors, and signal transduction. We will compare the crustacean and insect members of the kinin family, using the data available on crustacea.

Isolation, characterization and biological activity of a diuretic myokinin neuropeptide from the housefly, Musca domestica

A competitive ELISA employing a polyclonal antiserum raised against leucokinin-I was used to isolate and purify a myokinin (muscakinin) from 1.05 kg of adult houseflies (Musca domestica). Following solid-phase purification, seven HPLC column steps were used to purify 4.8 nmol of leucokinin-immunoreactive material. Sequence analysis and mass spectrometry were consistent with the structure Asn-Thr-Val-Val-Leu-Gly Lys-Lys-Gln-Arg-Phe-His-Ser-Trp-Gly NH2. This peptide was synthesized and co-eluted with the natural peptide on three different HPLC columns. The activities of natural and synthetic muscakinin were identical, with both producing a 4-5 fold increase in fluid secretion by housefly Malpighian tubules at nanomolar concentrations. The presence of a pair of basic residues (Lys-Lys) suggested muscakinin might be processed further, with the peptide pGlu-Arg-Phe-His-Ser-Trp-Gly NH2 being produced by conversion of an N-terminal glutamine to pyroglutamic acid. However, this analog was 1000-fold less active than the intact peptide, comparable to the activity of AK-V which shares the same C-terminal pentapeptide sequence. The diuretic activity of muscakinin is more than double that of a previously identified CRF-related diuretic peptide (Musca-DP) from the housefly, and the two peptides act synergistically in stimulating fluid secretion. Muscakinin also increased the frequency and amplitude of contractions by housefly hindgut which might further contribute to the excretory process.