The anterior stomach of larval mosquitoes (Aedes aegypti): effects of neuropeptides on transepithelial ion transport and muscular motility

Differential expression of neuropeptide F in the digestive organs of female freshwater prawn, Macrobrachium rosenbergii, during the ovarian cycle

Neuropeptide F is a key hormone that controls feeding in invertebrates, including decapod crustaceans. We investigated the differential expression of Macrobrachium rosenbergii neuropeptide F (MrNPF) in the digestive organs of female prawns, M. rosenbergii, during the ovarian cycle. By using RT-qPCR, the expression of MrNPF mRNA in the esophagus (ESO), cardia (CD), and pylorus (PY) of the foregut (FG) gradually increased from stage II and peaked at stage III. In the midgut (MG), hindgut (HG), and hepatopancreas (HP), MrNPF mRNA increased from stage I, reaching a maximal level at stage II, and declined by about half at stages III and IV (P < 0.05). In the ESO, CD, and PY, strong MrNPF-immunoreactivities were seen in the epithelium, muscle, and lamina propria. Intense MrNPF-ir was found in the MG cells and the muscular layer. In the HG, MrNPF-ir was detected in the epithelium of the villi and gland regions, while MrNPF-ir was also more intense in the F-, R-, and B-cells in the HP. However, we found little colocalization between the MrNPF and PGP9.5/ChAT in digestive tissues, implying that most of the positive cells might not be neurons but could be digestive tract-associated endocrine cells that produce and secrete MrNPF to control digestive organ functions in feeding and utilizing feed. Taken together, our first findings indicated that MrNPF was differentially expressed in digestive organs in correlation with the ovarian cycle, suggesting an important link between MrNPF, the physiology of various digestive organs in feeding, and possibly ovarian maturation in female M. rosenbergii.

Identification and expression of short neuropeptide F and its receptors in the tick Ixodes ricinus

In Europe, the tick Ixodes ricinus is the most important vector of numerous pathogens that are transmitted during blood feeding on their vertebrate hosts. To elucidate mechanisms controlling blood intake and associated transmission of pathogens we identified and described expression of short neuropeptide F (sNPF) and its receptors which are known to regulate feeding in insects. Using in situ hybridization (ISH) and immunohistochemistry (IHC) we stained numerous neurons producing sNPF in the central nervous system (CNS; synganglion), while a few peripheral neurons were detected anteriorly to the synganglion, and on the surface of the hindgut and leg muscles. Apparent sNPF expression was also found in enteroendocrine cells individually scattered in anterior lobes of the midgut. In silico analyses and BLAST search for sNPF receptors revealed two putative G protein-coupled receptors (sNPFR1 and sNPFR2) in the I. ricinus genome. Aequorin-based functional assay in CHO cells showed that both receptors were specific and sensitive to sNPF in nanomolar concentrations. Increased expression levels of these receptors in the gut during blood intake suggest that sNPF signaling may be involved in regulation of feeding and digestion processes of I. ricinus.

Allatostatin A Signalling: Progress and New Challenges From a Paradigmatic Pleiotropic Invertebrate Neuropeptide Family

Neuropeptides have gained broad attraction in insect neuroscience and physiology, as new genetic tools are increasingly uncovering their wide-ranging pleiotropic functions with high cellular resolution. Allatostatin A (AstA) peptides constitute one of the best studied insect neuropeptide families. In insects and other panarthropods, AstA peptides qualify as brain-gut peptides and have regained attention with the discovery of their role in regulating feeding, growth, activity/sleep and learning. AstA receptor homologs are found throughout the protostomia and group with vertebrate somatostatin/galanin/kisspeptin receptors. In this review, we summarise the current knowledge on the evolution and the pleiotropic and cell-specific non-allatostatic functions of AstA. We speculate about the core functions of AstA signalling, and derive open questions and challengesfor future research on AstA and invertebrate neuropeptides in general.

FMRF-related peptides in Aedes aegypti midgut: neuromuscular connections and enteric nervous system

FMRFamide-related peptides (FaRPs) are a class of neuropeptides that participate in a variety of physiological processes in invertebrates. They occur in nerves of stomatogastric ganglia and enteroendocrine cells of the insect digestive tract, where they may control muscle functions. However, their direct involvement in muscle function has never been shown in situ. We studied the relationship between FaRPs and midgut muscle during larval-pupal transition of the mosquito Aedes aegypti. In late L4, FaRP-positive neuronal extensions attach to the bundles of the external circular muscle layer, and muscle stem cells start to undergo mitosis in the internal circular layer. Thereafter, the external muscle layer degenerates, disappearing during early pupal development, and is completely absent in the adult mosquito. Our results indicate that FaRP-based neural signals are involved in the reorganization of the muscle fibers of the mosquito midgut during the larval-pupal transition. In addition to confirming FaRP involvement in muscle function, we show that the mosquito midgut muscles are largely innervated, and that circular and longitudinal muscle have specific neuron bodies associated with them.

The neuropeptide RhoprCCHamide2 inhibits serotonin-stimulated transcellular Na+ transport across the anterior midgut of the vector of Chagas disease, Rhodnius prolixus

Rhodnius prolixus is a blood-feeding insect vector of Trypanosoma cruzi, a protozoan parasite that causes Chagas disease. During each blood meal, the animals ingest large volumes of blood, that may be up to 12 times the unfed body mass. These blood meals impose a significant osmotic stress for the animals due to the hyposmotic condition of the ingested blood compared with the insect's hemolymph. Thus the insect undergoes a massive postprandial diuresis that allows for the excretion of the plasma fraction of the blood in less than two hours. Diuresis is performed by the excretory system, consisting of the Malpighian tubules and gut, under the control of diuretic and anti-diuretic factors. We investigated the ion transport machinery triggered by stimulation with the diuretic factor serotonin in the anterior midgut (i.e. crop) and the effect of the diuretic modulator RhoprCCHamide2. Ussing chamber assays revealed that serotonin-stimulated increase in transepithelial short-circuit current (Isc) was more sensitive to the blockage with amiloride than 5-N-ethyl-N-isopropyl amiloride (EIPA), suggesting the involvement of Na+ channels. Incubation in Na+-free, but not Cl--free saline, blocked the effect of serotonin on Isc. Moreover, treatment with Na+-K+-2Cl- cotransporter (NKCC) and Na+-Cl- cotransporter (NCC) blockers had no effect on fluid secretion but was blocked by amiloride. Blockage of Na+/K+-ATPase with ouabain inhibited Isc but the H+-ATPase inhibitor bafilomycin had no effect. The neuropeptide RhoprCCHamide2 diminished serotonin-stimulated Isc across the crop. The results suggest that Na+ undergoes active transport via an apical amiloride-sensitive Na+ channel and a basolateral ouabain-sensitive Na+/K+-ATPase, while Cl- is transported through a passive paracellular pathway.

"Urate and NOX5 Control Blood Digestion in the Hematophagous Insect Rhodnius prolixus"

Low levels of reactive oxygen species (ROS) are now recognized as essential players in cell signaling. Here, we studied the role of two conserved enzymes involved in redox regulation that play a critical role in the control of ROS in the digestive physiology of a blood-sucking insect, the kissing bug Rhodnius prolixus. RNAi-mediated silencing of RpNOX5 and RpXDH induced early mortality in adult females after a blood meal. Recently, a role for RpNOX5 in gut motility was reported, and here, we show that midgut peristalsis is also under the control of RpXDH. Together with impaired peristalsis, silencing either genes impaired egg production and hemoglobin digestion, and decreased hemolymph urate titers. Ultrastructurally, the silencing of RpNOX5 or RpXDH affected midgut cells, changing the cells of blood-fed insects to a phenotype resembling the cells of unfed insects, suggesting that these genes work together in the control of blood digestion. Injection of either allopurinol (an XDH inhibitor) or uricase recapitulated the gene silencing effects, suggesting that urate itself is involved in the control of blood digestion. The silencing of each of these genes influenced the expression of the other gene in a complex way both in the unfed state and after a blood meal, revealing signaling crosstalk between them that influences redox metabolism and nitrogen excretion and plays a central role in the control of digestive physiology.

Stop the crop: Insights into the insecticidal mode of action of cinnamodial against mosquitoes

Cinnamodial (CDIAL) is a drimane sesquiterpene dialdehyde found in the bark of Malagasy medicinal plants (Cinnamosma species; family Canellaceae). We previously demonstrated that CDIAL was insecticidal, antifeedant, and repellent against Aedes aegypti mosquitoes. The goal of the present study was to generate insights into the insecticidal mode of action for CDIAL, which is presently unknown. We evaluated the effects of CDIAL on the contractility of the ventral diverticulum (crop) isolated from adult female Ae. aegypti. The crop is a food storage organ surrounded by visceral muscle that spontaneously contracts in vitro. We found that CDIAL completely inhibited spontaneous contractions of the crop as well as those stimulated by the agonist 5-hydroxytryptamine. Several derivatives of CDIAL with known insecticidal activity also inhibited crop contractions. Morphometric analyses of crops suggested that CDIAL induced a tetanic paralysis that was dependent on extracellular Ca²⁺ and inhibited by Gd³⁺, a non-specific blocker of plasma membrane Ca²⁺ channels. Screening of numerous pharmacological agents revealed that a Ca²⁺ ionophore (A23187) was the only compound other than CDIAL to completely inhibit crop contractions via a tetanic paralysis. Taken together, our results suggest that CDIAL induces a tetanic paralysis of the crop by elevating intracellular Ca²⁺ through the activation of plasma membrane Ca²⁺ channels, which may explain the insecticidal effects of CDIAL against mosquitoes. Our pharmacological screening experiments also revealed the presence of two regulatory pathways in mosquito crop contractility not previously described: an inhibitory glutamatergic pathway and a stimulatory octopaminergic pathway. The latter pathway was also completely inhibited by CDIAL.

Aspirin Administration Affects Neurochemical Characterization of Substance P-Like Immunoreactive (SP-LI) Nodose Ganglia Neurons Supplying the Porcine Stomach

Background

Acetylsalicylic acid (ASA) is a commonly used anti-inflammatory, antipyretic, and analgesic drug, which has many side effects on the gastric mucosal layer. Despite this, knowledge concerning the influence of ASA on neuronal cells supplying the stomach is very scanty.

Methods

This investigation was performed on ten immature gilts of the Large White Polish race divided into two groups (five animals in each): a control group and animals which were treated with ASA. The retrograde neuronal tracer Fast Blue (FB) was injected into the prepyloric region of the stomach in all animals. ASA was then given orally to the experimental (ASA) group of gilts from the seventh day after FB injection to the 27th day of the experiment. After this period, all animals were euthanized. Immediately after euthanasia, nodose ganglia (NG) were collected and subjected to a standard double-labelling immunofluorescence technique using antibodies directed toward substance P (SP) and other selected neuronal factors, such as galanin (GAL), neuronal isoform of nitric oxide synthase (nNOS), vasoactive intestinal polypeptide (VIP), and calcitonin gene-related peptide (CGRP). Key Results. The obtained results show that SP-LI neurons located in NG supplying the porcine stomach were also immunoreactive to all the above-mentioned neuronal factors. Moreover, ASA administration caused an increase in the degree of colocalization of SP with other neuronal active substances, and the most visible changes concerned the number of neurons simultaneously immunoreactive to SP and CGRP. Conclusions and Inferences. These observations indicate that the population of SP-LI neurons supplying the stomach is not homogeneous and may undergo changes after ASA administration. These changes are probably connected with inflammatory processes and/or neuroprotective reactions although their exact mechanisms remain unknown.

A journey into the world of insect lipid metabolism

Lipid metabolism is fundamental to life. In insects, it is critical, during reproduction, flight, starvation, and diapause. The coordination center for insect lipid metabolism is the fat body, which is analogous to the vertebrate adipose tissue and liver. Fat body contains various different cell types; however, adipocytes and oenocytes are the primary cells related to lipid metabolism. Lipid metabolism starts with the hydrolysis of dietary lipids, absorption of lipid monomers, followed by lipid transport from midgut to the fat body, lipogenesis or lipolysis in the fat body, and lipid transport from fat body to other sites demanding energy. Lipid metabolism is under the control of hormones, transcription factors, secondary messengers and posttranscriptional modifications. Primarily, lipogenesis is under the control of insulin-like peptides that activate lipogenic transcription factors, such as sterol regulatory element-binding proteins, whereas lipolysis is coordinated by the adipokinetic hormone that activates lipolytic transcription factors, such as forkhead box class O and cAMP-response element-binding protein. Calcium is the primary-secondary messenger affecting lipid metabolism and has different outcomes depending on the site of lipogenesis or lipolysis. Phosphorylation is central to lipid metabolism and multiple phosphorylases are involved in lipid accumulation or hydrolysis. Although most of the knowledge of insect lipid metabolism comes from the studies on the model Drosophila; other insects, in particular those with obligatory or facultative diapause, also have great potential to study lipid metabolism. The use of these models would significantly improve our knowledge of insect lipid metabolism.

The Role of Peptide Hormones in Insect Lipid Metabolism

Lipids are the primary storage molecules and an essential source of energy in insects during reproduction, prolonged periods of flight, starvation, and diapause. The coordination center for insect lipid metabolism is the fat body, which is analogous to the vertebrate adipose tissue and liver. The fat body is primarily composed of adipocytes, which accumulate triacylglycerols in intracellular lipid droplets. Genomics and proteomics, together with functional analyses, such as RNA interference and CRISPR/Cas9-targeted genome editing, identified various genes involved in lipid metabolism and elucidated their functions. However, the endocrine control of insect lipid metabolism, in particular the roles of peptide hormones in lipogenesis and lipolysis are relatively less-known topics. In the current review, the neuropeptides that directly or indirectly affect insect lipid metabolism are introduced. The primary lipolytic and lipogenic peptide hormones are adipokinetic hormone and the brain insulin-like peptides (ILP2, ILP3, ILP5). Other neuropeptides, such as insulin-growth factor ILP6, neuropeptide F, allatostatin-A, corazonin, leucokinin, tachykinins and limostatin, might stimulate lipolysis, while diapause hormone-pheromone biosynthesis activating neuropeptide, short neuropeptide F, CCHamide-2, and the cytokines Unpaired 1 and Unpaired 2 might induce lipogenesis. Most of these peptides interact with one another, but mostly with insulin signaling, and therefore affect lipid metabolism indirectly. Peptide hormones are also involved in lipid metabolism during reproduction, flight, diapause, starvation, infections and immunity; these are also highlighted. The review concludes with a discussion of the potential of lipid metabolism-related peptide hormones in pest management.

Mechanisms of transport of H+, Na+ and K+, across the distal gastric caecum of larval Aedes aegypti

Measured changes in ion fluxes, transepithelial potential (TEP) and basolateral membrane potential (V_b) in response to ion transporter inhibitors were used to assess the mechanisms of transport of H⁺, Na⁺ and K⁺, across the distal gastric caecum of larval Aedes aegypti, a vector of yellow fever. Preparations were stimulated with 5-hydroxytryptamine (5-HT, 10^-6 M) in order to maintain stable rates of H⁺, Na⁺, and K⁺ transport across the distal caecum. Transepithelial potential (TEP), basolateral membrane potential (V_b), and H⁺, Na⁺ and K⁺ fluxes all declined after the addition of a vacuolar-type H⁺-ATPase (VA) inhibitor, n-ethlymaleimide (NEM), consistent with a primary role for VA in energizing ion transport across the distal gastric caecum. Amiloride also inhibited H⁺, Na⁺, and K⁺ fluxes, consistent with an apically expressed VA that is coupled to a cation:H⁺ antiporter (AeNHE8), analogous to the coupling of apical VA and cation:nH⁺ antiporter in Malpighian tubules. A working model of transport of H⁺, Na⁺ and K⁺ across the distal gastric caecum proposes that coupling of VA and AeNHE8 in the apical membrane leads to the removal of intracellular Na⁺ or K⁺, thus creating favourable ion gradients to promote the activity of two transporters in the basal membrane, cation:H⁺ antiporter (AeNHE3) and a bumetanide-sensitive cation chloride cotransporter (CCC).

Regulation of Feeding and Metabolism by Neuropeptide F and Short Neuropeptide F in Invertebrates

Numerous neuropeptide systems have been implicated to coordinately control energy homeostasis, both centrally and peripherally. However, the vertebrate neuropeptide Y (NPY) system has emerged as the best described one regarding this biological process. The protostomian ortholog of NPY is neuropeptide F, characterized by an RXRF(Y)amide carboxyterminal motif. A second neuropeptide system is short NPF, characterized by an M/T/L/FRF(W)amide carboxyterminal motif. Although both short and long NPF neuropeptide systems display carboxyterminal sequence similarities, they are evolutionary distant and likely already arose as separate signaling systems in the common ancestor of deuterostomes and protostomes, indicating the functional importance of both. Both NPF and short-NPF systems seem to have roles in the coordination of feeding across bilaterian species, but during chordate evolution, the short NPF system appears to have been lost or evolved into the prolactin releasing peptide signaling system, which regulates feeding and has been suggested to be orthologous to sNPF. Here we review the roles of both NPF and sNPF systems in the regulation of feeding and metabolism in invertebrates.

The gastric caecum of larval Aedes aegypti: stimulation of epithelial ion transport by 5-hydroxytryptamine and cAMP

We report measurements of ion transport across the gastric caecum of larvae of Aedes aegypti, a vector of yellow fever that inhabits a variety of aquatic habitats ranging from freshwater to brackish water. We provide the first measurements of the effect of 5-hydroxytryptamine (5-HT) on transepithelial potential (TEP), luminal ion concentrations and electrochemical potentials, as well as basolateral membrane potential and H⁺, Na⁺ and K⁺ fluxes. TEP, basolateral membrane potential, and H⁺, K⁺ and Na⁺ fluxes across the gastric caeca declined within 3-6 min after isolation of the entire midgut from the larva. 5-HT restored both the TEP and active accumulation of H⁺, K⁺ and Na⁺ in the lumen. Additionally, 5-HT restored H⁺, K⁺ and Na⁺ fluxes across the distal caecum of freshwater larvae, and restored H⁺ fluxes across the distal caecum of brackish water larvae. There was no effect of 5-HT on ion fluxes across the proximal caecum. We have also shown that 5-HT restores the basolateral membrane potential in cells of the distal, but not proximal, caecum. Effects of 5-HT on TEP and basolateral membrane potential were mimicked by application of cAMP but not by a phorbol ester. We provide a working model which proposes that 5-HT and cAMP stimulate the vacuolar H⁺-ATPase of the distal caecum. Our results provide evidence that the gastric caecum is functionally distinct from the adjacent anterior midgut and we discuss possible roles of the gastric caecum in osmoregulation. We also describe similarities in the arrangement of ion transporters in the caecum with those of the Malpighian tubules.

Functional characterization of the dual allatostatin-A receptors in mosquitoes

The neuropeptide allatostatin-A (AstA) and its cognate receptors (AstARs) are involved in the modulation of feeding behavior, which in hematophagous insects includes the regulation of the disease vector-related behaviors, host seeking and blood feeding. In mosquitoes and other dipterans, there are two copies of AstAR, contrasting with the single copy found in other insects. In this study, we identified and cloned the dual AstAR system of two important disease vectors Aedes aegypti and Culex quinquefasciatus, and compared them with those previously described, including those in Anopheles coluzzii and Drosophila melanogaster. Phylogenetic analysis of the AstARs revealed that the mosquito AstAR1s has retained a similar amino acid sequence as the AstARs from non-dipteran insect species. Intron analysis revealed that the number of introns accumulated in the AstAR2s is similar to that in other insects, and that introns are conserved within the receptor types, but that only the final two introns are conserved across AstAR1s and 2s. We functionally characterized the dual AstARs in An. coluzzii, Ae. aegypti and Cx. quinquefasciatus by stably expressing the receptors in a Chinese hamster oocyte cell line (CHO) also stably expressing a promiscuous G-protein (G16), and challenged them with the endogenous isoforms of AstA from the three mosquito species. In the culicine mosquitoes, Ae. aegypti and Cx. quinquefasciatus, the AstARs demonstrated differential sensitivity to AstA, with the AstAR2s displaying a higher sensitivity than the AstAR1s, suggesting a divergence of functional roles for these AstARs. In contrast, both An. coluzzii AstARs demonstrated a similar sensitivity to the AstA ligands. We discuss our findings in the light of AstA acting as a regulator of blood feeding in mosquitoes. A better understanding of the regulation of host seeking and blood feeding in vector mosquitoes will lead to the rational development of novel approaches for vector control.

Physiological characterization and regulation of the contractile properties of the mosquito ventral diverticulum (crop)

In adult dipteran insects (flies), the crop is a diverticulum of the esophagus that serves as a food storage organ. The crop pumps stored contents into the alimentary canal for digestion and absorption. The pumping is mediated by peristaltic contractions of the crop musculature. In adult female mosquitoes, the crop (ventral diverticulum) selectively stores sugar solutions (e.g., nectar); proteinaceous blood meals by-pass the crop and are transferred directly to the midgut for digestion. The mechanisms that regulate crop contractions have never been investigated in mosquitoes. Here we provide the first physiological characterization of the contractile properties of the mosquito crop and explore the mechanisms that regulate crop contractions. Using an in vitro bioassay we found that the isolated crop spontaneously contracts in Ringer solution for at least 1 h and its contractions are dependent on extracellular Ca2+. Adding serotonin (5-hydroxytryptamine, 5-HT) or a membrane-permeable analog of cyclic adenosine monophosphate (cAMP) to the extracellular bath increased the frequency of crop contractions. On the other hand, adding benzethonium chloride (BzCl; a chemical that mimics the effects of myosuppressins), H-89 or Rp-cAMPS (inhibitors of protein kinase A, PKA), or carbenoxolone (an inhibitor of gap junctions) reduced the frequency of the unstimulated, spontaneous and/or 5-HT-stimulated crop contractions. Adding aedeskinin III did not detectably alter crop contraction rates. In addition to pharmacological evidence of gap junctions, we demonstrated that the crop expressed several mRNAs encoding gap junctional proteins (i.e. innexins). Furthermore, we localized immunoreactivity for innexin 2 and innexin 3 to muscle and epithelial cells of the crop, respectively. Our results 1) suggest that 5-HT and myosupressins oppositely regulate contractile activity of the mosquito crop, and 2) provide the first evidence for putative roles of cAMP, PKA, and gap junctions in modulating contractile activity of the dipteran crop.

Mosquito Peptide Hormones: Diversity, Production, and Function

Mosquitoes, like other insects, produce a diversity of peptide hormones that are processed from different precursor proteins and have a range of activities. Early studies relied on purification of bioactive peptides for hormone identification, but more recently genomic data have provided the information needed to more comprehensively identify peptide hormone genes and associated receptors. The first part of this chapter summarizes the known or predicted peptide hormones that are produced by mosquitoes. The second part of this chapter discusses the sources of these molecules and their functions.

Comprehensive Immunolocalization Studies of a Putative Serotonin Receptor from the Alimentary Canal of Aedes aegypti Larvae Suggest Its Diverse Roles in Digestion and Homeostasis

Serotonin regulates key processes including digestion and homeostasis in insects. Serotonin effects are mediated by serotonin receptors that transduce information through initiation of second messenger signaling pathways. Lack of information on serotonin receptors associated with the alimentary canal impedes the understanding of the serotonergic role in insect physiology. To address this void, the present study has cloned and identified a putative serotonin receptor (hereafter AaSeR-1) from the alimentary canal of Aedes aegypti (yellow fever mosquito) larvae. In addition to in-silico analyses of AaSeR-1 primary sequence, immunohistochemical investigations were carried out to elucidate receptor expression patterns. Specific AaSeR-1 immunofluorescence was detected in the caeca, the mid- and hindgut, including the Malpighian tubules. These findings point out not only receptor ubiquitous nature but also its involvement in regulation of different stages of nutrient processing and homeostasis. Furthermore, AaSeR-1 may mediate an array of effects through its differential expression at various cell compartments. While AaSeR-1 specific immunofluorescence was depicted in the nucleus and nucleolus of principal cells of the anterior midgut, in the posterior, analyses suggest receptor association with the plasma membrane of both principal and regenerative cells. In addition, AaSeR-1 immunofluorescence was also found in some enteroendocrine cells and in both circular and longitudinal muscles that innervate the alimentary canal. Overall, immunohistochemical analyses of AaSeR-1 expression indicate that this receptor exercises multiple roles in digestion- and homeostasis-related mechanisms.

The pleiotropic allatoregulatory neuropeptides and their receptors: A mini-review

Juvenile hormones (JH) are highly pleiotropic insect hormones essential for post-embryonic development. The circulating JH titer in the hemolymph of insects is influenced by enzymatic degradation, binding to JH carrier proteins, uptake and storage in target organs, but evidently also by rates of production at its site of synthesis, the corpora allata (CA). The multiple processes in which JH is involved alongside the critical significance of JH in insect development emphasize the importance for elucidating the control of JH production. Production of JH in CA cells is regulated by different factors: by neurotransmitters, such as dopamine and glutamate, but also by allatoregulatory neuropeptides originating from the brain and axonally transported to the CA where they bind to their G protein-coupled receptors (GPCRs). Different classes of allatoregulatory peptides exist which have other functions aside from acting as influencers of JH production. These pleiotropic neuropeptides regulate different processes in different insect orders. In this mini-review, we will give an overview of allatotropins and allatostatins, and their recently characterized GPCRs with a view to better understand their modes of action and different action sites.

Fluid absorption in the isolated midgut of adult female yellow fever mosquitoes (Aedes aegypti)

The transepithelial voltage (Vte) and the volume of isolated posterior midguts of adult female yellow fever mosquitoes (Aedes aegypti) were monitored. In all experiments, the initial Vte after filling the midgut was lumen negative, but subsequently became lumen positive at a rate of approximately 1 mV min(-1). Simultaneously, the midgut volume decreased, indicating spontaneous fluid absorption. When the midguts were filled and bathed with mosquito saline, the average rate of fluid absorption was 36.5±3.0 nl min(-1) (N=4, ±s.e.m.). In the presence of theophylline (10 mmol l(-1)), Vte reached significantly higher lumen-positive values, but the rate of fluid absorption was not affected (N=6). In the presence of NaCN (5 mmol l(-1)), Vte remained close to 0 mV (N=4) and fluid absorption was reduced (14.4±1.3 nl min(-1), N=3, ±s.e.m.). When midguts were filled with buffered NaCl (154 mmol l(-1) plus 1 mmol l(-1) HEPES) and bathed in mosquito saline with theophylline, fluid absorption was augmented (50.0±5.8 nl min(-1), N=12, ±s.e.m.). Concanamycin A (10 µmol l(-1)), ouabain (1 mmol l(-1)), and acetazolamide (1 mmol l(-1)) affected Vte in different ways, but all reduced fluid absorption by 60-70% of the value before addition of the drugs.

Identification and functional characterization of FGLamide-related allatostatin receptor in Rhodnius prolixus

FGLamide-related ASTs (FGLa/ASTs) are a family of brain/gut peptides with numerous physiological roles, including inhibition of juvenile hormone (JH) biosynthesis by the corpora allata and inhibition of visceral muscle contraction. FGLa/ASTs mediate their effects by binding to a rhodopsin-like G-protein coupled receptor that is evolutionarily related to the vertebrate galanin receptor. Here we determine the cDNA sequence encoding FGLa/AST receptor (FGLa/AST-R) from the Chagas disease vector, Rhodnius prolixus (Rhopr-FGLa/AST-R), determine its spatial expression pattern using quantitative PCR and functionally characterize the receptor using a heterologous assay. Our expression analysis indicates that Rhopr-FGLa/AST-R is highly expressed in the central nervous system. The receptor is also expressed in various peripheral tissues including the dorsal vessel, midgut, hindgut and reproductive tissues of both males and females, suggesting a role in processes associated with feeding and reproduction. The possible involvement of Rhopr-FGLa/ASTs in the inhibition of JH biosynthesis is also implicated due to presence of the receptor transcript in the R. prolixus corpora cardiaca/corpora allata complex. The functional assay showed that various Rhopr-FGLa/ASTs activate the receptor, with EC50 values for the response in the nanomolar range. Moreover, Rhopr-FGLa/AST-R can couple with Gq alpha subunits and cause an increase in intracellular calcium concentration. Lastly, we tested various FGLa/AST analogs in our heterologous assay. These compounds also activated the receptor and thus have the potential to serve as insect growth regulators and aid in pest control.

K+ absorption by locust gut and inhibition of ileal K+ and water transport by FGLamide allatostatins

The Scanning Ion-Selective Electrode Technique (SIET) was utilized for the first time in Locusta migratoria to characterize K+ transport along the digestive tract and to determine the effect of two locust FGLamide allatostatins (FGLa/ASTs) on K+ transport: a previously sequenced FGLa/AST from Schistocerca gregaria (Scg-AST-6; ARPYSFGL-NH2) and a newly sequenced FGLa/AST from L. migratoria (Locmi-FGLa/AST-2; LPVYNFGL-NH2). Regional differences in K+ fluxes along the gut were evident, where K+ efflux in vitro (or absorption into the hemolymph in vivo) was greatest at the anterior ileum, and lowest at the colon. Ileal K+ efflux was inhibited by both Scg-AST-6 and Locmi-FGLa/AST-2, with maximal inhibition at 10-10 and 10-11 M, respectively. Both FGLa/ASTs also inhibited cAMP-stimulated K+ efflux from the ileum. Locmi-FGLa/AST-2 also inhibited efflux of water across the ileum. Locusts are terrestrial insects living in dry climates, risking desiccation and making water conservation a necessity. The results suggest that FGLa/ASTs may be acting as diuretics by increasing K+ excretion and therefore increasing water excretion. Thus, it is likely that FGLa/ASTs are involved in the control of hemolymph water and ion levels during feeding and digestion, to help the locust deal with the excess K+ load (and subsequently fluid) when the meal is processed.

Allatostatin A-like immunoreactivity in the nervous system and gut of the larval midge, Chironomus riparius (Meigen): Modulation of hindgut motility, rectal K+ transport and implications for exposure to salinity

Evidence for the presence of allatostatin (AST) A-like neuropeptides in the larval midge, Chironomus riparius is reported. Immunohistochemical studies on the nervous system and gut revealed the presence of AST A-like immunoreactive (AST-IR) cells and processes. The nerve cord contained AST-IR processes that originated from cells in the brain and travelled the length of nerve cord to the terminal ganglion. Within each ganglion, these processes gave rise to varicosities suggesting that they formed synapses with neurons in the ganglia. Endocrine cells containing AST-IR were present in three regions of the midgut: near the attachment of the Malpighian tubules, between the anterior and posterior midgut and in the vicinity of the gastric caecae. The terminal ganglion also contained 4 AST-IR cells which gave rise to axons that projected onto the hindgut and posterior midgut. Application of a cockroach AST to the semi-isolated hindgut of larval C. riparius led to dose-dependent inhibition of muscle contractions with an EC50 of ~ 10 nM and a decrease in rectal K+ reabsorption resulting from reduced rectal Na+/K+-ATPase (NKA) and vacuolar type H+-ATPase (VA) activities. The results suggest the presence of endogenous AST-like neuropeptides in the larval midge C. riparius where these factors play a role in the function of the gut. Furthermore, regulation of ion reabsorption by ASTs at the rectum could serve as an ideal mechanism of ion regulation in the face of abrupt and acute elevated salt levels.

Phylogenetic investigation of Peptide hormone and growth factor receptors in five dipteran genomes

Peptide hormones and growth factors bind to membrane receptors and regulate a myriad of processes in insects and other metazoans. The evolutionary relationships among characterized and uncharacterized ("orphan") receptors can provide insights into receptor-ligand biology and narrow target choices in deorphanization studies. However, the large number and low sequence conservation of these receptors make evolutionary analysis difficult. Here, we characterized the G-protein-coupled receptors (GPCRs), receptor guanylyl cyclases (RGCs), and protein kinase receptors (PKRs) of mosquitoes and select other flies by interrogating the genomes of Aedes aegypti, Anopheles gambiae, Culex quinquefasciatus, Drosophila melanogaster, and D. mojavensis. Sequences were grouped by receptor type, clustered using the program CLANS, aligned using HMMR, and phylogenetic trees built using PhyML. Our results indicated that PKRs had relatively few orphan clades whereas GPCRs and RGCs had several. In addition, more than half of the Class B secretin-like GPCRs and RGCs remained uncharacterized. Additional studies revealed that Class B GPCRs exhibited more gain and loss events than other receptor types. Finally, using the neuropeptide F family of insect receptors and the neuropeptide Y family of vertebrate receptors, we also show that functional sites considered critical for ligand binding are conserved among distinct family members and between distantly related taxa. Overall, our results provide the first comprehensive analysis of peptide hormone and growth factor receptors for a major insect group.

The neural and peptidergic control of gut contraction in Locusta migratoria: the effect of an FGLa/AST

The regulation of insect gut physiology is complex and involves the interactions of a number of mechanisms, including the neural regulation of gut contraction by altering neural input and the modulation of gut contractions by neuropeptides directly affecting the muscle. The FGLa-type allatostatins (FGLa/ASTs) are known brain/gut peptides with numerous physiological roles, including modulation of gut contraction and neural input. To further investigate the pleiotropic roles of FGLa/AST peptides in Locusta migratoria, we have examined the role of a locust FGLa/AST (Scg-AST-6) in the gut. Proctolin and Scg-AST-6 have opposing effects on gut contraction, where proctolin dose-dependently increases gut muscle tension, while Scg-AST-6 inhibits both muscle tension and spontaneous and neurogenic contractions in a dose-dependent manner. Results from neurophysiological recordings indicate that there may be a central pattern generator (CPG) within the ventricular ganglia regulated by descending inhibition, and the addition of Scg-AST-6 dose-dependently modulates this ventricular ganglion CPG. This work provides a comprehensive picture of how FGLa/ASTs may modulate and coordinate each region of the locust gut, and shows that FGLa/ASTs have both central effects, on the ventricular ganglion CPG, and peripheral effects on the gut muscle. Overall, this work shows how FGLa/ASTs contribute to the complex regulation and fine tuning of gut contraction.

A comparative review of short and long neuropeptide F signaling in invertebrates: Any similarities to vertebrate neuropeptide Y signaling?

Neuropeptides referred to as neuropeptide F (NPF) and short neuropeptide F (sNPF) have been identified in numerous invertebrate species. Sequence information has expanded tremendously due to recent genome sequencing and EST projects. Analysis of sequences of the peptides and prepropeptides strongly suggest that NPFs and sNPFs are not closely related. However, the NPFs are likely to be ancestrally related to the vertebrate family of neuropeptide Y (NPY) peptides. Peptide diversification may have been accomplished by different mechanisms in NPFs and sNPFs; in the former by gene duplications followed by diversification and in the sNPFs by internal duplications resulting in paracopies of peptides. We discuss the distribution and functions of NPFs and their receptors in several model invertebrates. Signaling with sNPF, however, has been investigated mainly in insects, especially in Drosophila. Both in invertebrates and in mammals NPF/NPY play roles in feeding, metabolism, reproduction and stress responses. Several other NPF functions have been studied in Drosophila that may be shared with mammals. In Drosophila sNPFs are widely distributed in numerous neurons of the CNS and some gut endocrines and their functions may be truly pleiotropic. Peptide distribution and experiments suggest roles of sNPF in feeding and growth, stress responses, modulation of locomotion and olfactory inputs, hormone release, as well as learning and memory. Available data indicate that NPF and sNPF signaling systems are distinct and not likely to play redundant roles.

Neuropeptide F and the corn earworm, Helicoverpa zea: a midgut peptide revisited

The neuropeptide Y family of peptides is implicated in the regulation of feeding across a broad range of animals, including insects. Among vertebrates, neuropeptide Y exerts its actions mainly centrally, whereas peptide YY and pancreatic polypeptide arise from digestive tissues. Among invertebrates, neuropeptide F (NPF) is the sole counterpart of the NPY family. Shared features of NPF sequences derived for Lepidoptera indicate that the midgut peptide (Hez-MP-I) of the corn earworm, Helicoverpa zea, characterized more than a decade ago, is a carboxyl fragment of a full-length NPF. An antibody to Hez-MP-I was used to characterize the peptide's distribution in tissues of larvae, pupae, and adults. Immunostaining demonstrated NPF-related material both in nervous tissues and in abundant endocrine cells of the midgut. Radioimmunoassay of Hez-MP-I in the head, midgut and hemolymph of fifth instar larvae revealed concentration changes corresponding to development and feeding state. As with the vertebrate homologs, NPF may arise both centrally and peripherally to modulate the physiology of feeding and digestion of Lepidoptera.

Neuropeptidomics of the mosquito Aedes aegypti

Neuropeptidomic data were collected on the mosquito Ae. aegypti, which is considered the most tractable mosquito species for physiological and endocrine studies. The data were solely obtained by direct mass spectrometric profiling, including tandem fragmentation, of selected tissues from single specimens, which yielded a largely complete accounting of the putative bioactive neuropeptides; truncated neuropeptides with low abundance were not counted as mature peptides. Differential processing within the CNS was detected for the CAPA-precursor, and differential post-translational processing (pyroglutamate formation) was detected for AST-C and CAPA-PVK-2. For the first time in insects, we succeeded in the direct mass spectrometric profiling of midgut tissue which yielded a comprehensive and immediate overview of the peptides involved in the endocrine system of the gut. Head peptides which were earlier identified as the most abundant RFamides of Ae. aegypti, were not detected in any part of the CNS or midgut. This study provides a framework for future investigations on mosquito endocrinology and neurobiology. Given the high sequence similarity of neuropeptide precursors identified in other medically important mosquitoes, conclusions regarding the peptidome of Ae. aegypti likely are applicable to these mosquitoes.

Cellular mechanisms of acid secretion in the posterior midgut of the larval mosquito (Aedes aegypti)

The gut contents of larval mosquitoes are alkalinized by the anterior midgut and reacidified by the posterior midgut. In the present study the cellular mechanisms of reacidification were studied in isolated, perfused posterior midgut by measuring the transepithelial voltage (V(te)) and the rate of acid secretion as indicated by the color change of m-cresol purple during intervals of perfusion stop. The lumen-positive V(te) and reacidification were significantly increased by serotonin (0.2 mumol l(-1)). The V-type H(+)-ATPase inhibitor concanamycin A (10 mumol l(-1)) on the luminal side inhibited acidification and decreased V(te). On the hemolymph side the carbonic anhydrase (CA) inhibitor acetazolamide (1 mmol l(-1)) almost abolished V(te), but had no effect on acidification. Similarly, hemolymph-side DIDS (0.1 mmol l(-1)), DPC (0.5 mmol l(-1)), amiloride (1 mmol l(-1)) and ouabain (2.5 mmol l(-1)) significantly reduced V(te), whereas Ba(2+) (5 mmol l(-1)) was without effect. DPC and amiloride also reduced V(te) when applied to the luminal side of the epithelium. Unilateral substitution of gluconate for Cl(-) affected V(te) in a way consistent with a greater permeability for Cl(-) versus Na(+). Cl(-) replacement in the lumen decreased V(te), whereas replacement on the hemolymph side increased it. Bilateral replacement left the control voltage unaffected. Na(+) replacement on either side of the tissue reduced V(te) to different degrees. Omission of luminal amino acids was followed by a significant decrease in V(te). Except for concanamycin A, none of the above manipulations impaired acidification, indicating that acidification requires only the apical proton pump. However, the chemical source of secreted H(+) is still unknown and needs to be investigated.

Molecular characterization of neuropeptide F from the eastern subterranean termite Reticulitermes flavipes (Kollar) (Isoptera: Rhinotermitidae)

Neuropeptide F (NPF)-like immunoreactivity was previously found to be abundant in the eastern subterranean termite, Reticulitermes flavipes. Purification of the NPF from a whole body extract of worker termites was accomplished in the current study by HPLC and heterologous radioimmunoassay for an NPF-related peptide, Helicoverpa zea Midgut Peptide-I. A partial amino acid sequence allowed determination of the corresponding cDNA that encoded an open reading frame deduced for authentic R. flavipes NPF (Ref NPF): KPSDPEQLADTLKYLEELDRFYSQVARPRFa. Effects of synthetic NPFs on muscle contractions were investigated for isolated foreguts and hindguts of workers, with Drm NPF inhibiting spontaneous contractions of hindguts. Phylogenetic analysis of invertebrate NPF sequences reveals two separate groupings, with Ref NPF occurring within a clade composed exclusively of arthropods.

Effects of serotonergic agents on survival and hemolymph composition of the larval mosquito Aedes aegypti (Diptera: Culicidae, L.) in vivo: does serotonin regulate hemolymph acid-base homeostasis?

The role of serotonin in the regulation of larval Aedes aegypti hemolymph composition was investigated in vivo using two reuptake inhibitors (SSRIs), alaproclate HCl and 6-nitroquipazine maleate, and the receptor antagonist methiothepin mesylate. Larvae were placed in media differing in pH and salinity in the presence and absence of serotonergic agents. The toxicity of each agent was strongly influenced by ambient pH. For each agent, toxicity was negligible in acidic media, intermediate in neutral media and greatest in alkaline media. By contrast, toxicity of all agents was independent of salinity. No effects on mass-specific body water or hemolymph volume were observed whereas hemolymph osmotic pressure, Na(+) concentrations and pH differed significantly among treatments. 6-nitroquipazine caused a decrease in Na(+) from 115+/-1.7 to 103+/-0.9 mmol l(-1), and alaproclate caused alkalosis of the hemolymph from pH 7.55+/-0.026 to pH 7.72+/-0.044. Methiothepin decreased hemolymph osmotic pressure from 329+/-9.9 to 304+/-8.8 and showed the greatest overall toxicity. Control larvae excreted net base in pH 4 media (1.4 micromol g(-1) h(-1)) and net acid in pH 7 (1.2 micromol g(-1) h(-1)) and pH 11 (5.1 micromol g(-1) h(-1)) media. In pH 4 media, alaproclate and methiothepin caused a shift to net H(+) excretion (1.1 and 1.5 micromol g(-1) h(1), respectively) whereas these agents did not influence acid excretion rates in pH 7 or pH 11 media. The hypothesis that serotonin is involved in hemolymph acid-base balance is discussed.

The antidiuretic neurohormone RhoprCAPA-2 downregulates fluid transport across the anterior midgut in the blood-feeding insect Rhodnius prolixus

Osmotic balance in insects is regulated by the excretory system, consisting of Malpighian tubules and the gut under the control of diuretic and antidiuretic factors. Terrestrial insects must conserve water, and antidiuresis is the norm, only interrupted by brief diuretic periods. Surprisingly, little is known about antidiuresis in insects. Two antidiuretic strategies have been described. The first antidiuretic mechanism involves the reabsorption of fluid from the primary urine in the hindgut. More recently, a second antidiuretic strategy was reported, consisting of inhibition of primary urine formation by the Malpighian tubules. Recently, we isolated, characterized, and cloned the gene encoding for the antidiuretic neurohormone (the neuropeptide RhoprCAPA-2) acting on the Malpighian tubules of Rhodnius prolixus. Here we describe a third, novel mechanism central to the antidiuretic strategy of R. prolixus, the inhibition of ion and fluid transport across the anterior midgut by RhoprCAPA-2. Our results show that RhoprCAPA-2 (1 micromol/l) reduces serotonin-stimulated fluid transport from 83 +/- 11 to 12 +/- 12 nl/min and equivalent short-circuit current from 20 +/- 4 to 5 +/- 0.7 microA/cm(2) in diuretic hormone-stimulated anterior midgut. RhoprCAPA-2 appears to function independently of intracellular cGMP or Ca(2+) in the midgut. Thus, the antidiuretic neurohormone RhoprCAPA-2 has multiple target tissues, and we hypothesize that RhoprCAPA-2 functions to coordinate the transport activity of the anterior midgut and Malpighian tubules so that the rate of fluid transport into the haemolymph by the anterior midgut matches the transport rate of Malpighian tubules to maintain the volume and ion composition of haemolymph.

Serotonin-induced high intracellular pH aids in alkali secretion in the anterior midgut of larval yellow fever mosquito Aedes aegypti L

The anterior midgut of the larval yellow fever mosquito Aedes aegypti generates a luminal pH in excess of 10 in vivo and similar values are attained by isolated and perfused anterior midgut segments after stimulation with submicromolar serotonin. In the present study we investigated the mechanisms of strong luminal alkalinization using the intracellular fluorescent indicator BCECF-AM. Following stimulation with serotonin, we observed that intracellular pH (pH(i)) of the anterior midgut increased from a mean of 6.89 to a mean of 7.62, whereas pH(i) of the posterior midgut did not change in response to serotonin. Moreover, a further increase of pH(i) to 8.58 occurred when the pH of the luminal perfusate was raised to an in vivo-like value of 10.0. Luminal Zn(2+) (10 micromol l(-1)), an inhibitor of conductive proton pathways, did not inhibit the increase in pH(i), the transepithelial voltage, or the capacity of the isolated tissue to alkalinize the lumen. Finally, the transapical voltage did not significantly respond to luminal pH changes induced either by perfusion with pH 10 or by stopping the luminal perfusion with unbuffered solution which results in spontaneous luminal alkalinization. Together, these results seem to rule out the involvement of conductive pathways for proton absorption across the apical membrane and suggest that a serotonin-induced alkaline pH(i) plays an important role in the generation of an alkaline lumen.

Serotonergic modulation of crayfish hindgut

The crayfish hindgut is a morphologically differentiated tube that varies along its length in the distribution of muscles and glands, contractile properties, serotonergic innervation, patterns of 5-HT receptor expression, and sensitivity to serotonin (5-HT). Anatomical differences divide the hindgut into five distinct segments along its length. Spontaneous pulsatile contractions produced by the isolated hindgut decrease in force and increase in frequency along the anterior-posterior axis. Central input to the hindgut comes from a large cluster of 5-HT-immunoreactive neurons in the terminal abdominal ganglion that form a large nerve plexus on the hindgut. 5-HT(1alpha) and 5-HT(2beta) receptors vary in their distribution along the hindgut, and are associated with longitudinal and circular muscles and with axon collaterals of the 5-HT-immunoreactive neurons. Application of 30 nmol l(-1) to 1 mumol l(-1) 5-HT to rostral, middle, or caudal sections of hindgut produced tension changes that varied with the concentration and section. 5-HT also initiated antiperistaltic waves in the posterior hindgut. These results indicate that 5-HT is an important neuromodulator for initiating contractions and coordinating activity in the different functional compartments along the rostral-to-caudal axis of the hindgut.

Strong alkalinization in the anterior midgut of larval yellow fever mosquitoes (Aedes aegypti): involvement of luminal Na+/K+-ATPase

Recently, Na(+)/K(+)-ATPase has been detected in the luminal membrane of the anterior midgut of larval yellow fever mosquitoes (Aedes aegypti) with immunohistochemical techniques. In this study, the possible involvement of this ATPase in strong alkalinization was investigated on the level of whole larvae, isolated and perfused midgut preparations and on the molecular level of the Na(+)/K(+)-ATPase protein. Ouabain (5 mM) did not inhibit the capability of intact larval mosquitoes to alkalinize their anterior midgut. Also in isolated and perfused midgut preparations the perfusion of the lumen with ouabain (5 mM) did not result in a significant change of the transepithelial voltage or the capacity of luminal alkalinization. Na(+)/K(+)-ATPase activity was completely abolished when KCl was substituted with choline chloride, suggesting that the enzyme cannot act as an ATP-driven Na(+)/H(+)-exchanger. Altogether the results of the present investigation indicate that apical Na(+)/K(+)-ATPase is not of direct importance for strong luminal alkalinization in the anterior midgut of larval yellow fever mosquitoes.

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.

Neuropeptides associated with the regulation of feeding in insects

The stomatogastric nervous system plays a pivotal role in feeding behaviour. Central to this system is the frontal ganglion, which is responsible for foregut motor activity, and hence the passage of food through the gut. Many insect peptides, which exhibit myoactivity on the visceral muscles of the gut in vitro, have been detected in the stomatogastric nervous system by immunochemical or mass spectrometric techniques. This localisation of myoactive peptides, particularly in the frontal ganglion, implies roles for these peptides in the neural control and modulation of feeding in insects. Insect sulfakinins, tachykinins, allatotropin and proctolin have all been shown to stimulate the foregut muscles, whereas myosuppressins, myoinhibitory peptides and allatostatins all inhibited spontaneous contractions of the foregut in a variety of insects. Some of these peptides, when injected, inhibited feeding in vivo. Both the A-type and B-type allatostatins suppressed feeding activity when injected into the cockroach, Blattella germanica and the Manduca sexta C-type allatostatin and allatotropin inhibited feeding when injected into the larvae of two noctuid moths, Lacanobia oleracea and Spodoptera frugiperda, respectively. Injection of sulfakinins into the fly Phormia regina, the locust Schistocera gregaria and the cockroach B. germanica also suppressed feeding, whereas silencing the sulfakinin gene through the injection of double stranded RNA resulted in an increase in food consumption in the cricket Gryllus bimaculatus. The regulation of feeding in insects is clearly very complex, and involves the interaction of a number of mechanisms, one of which is the release, either centrally or locally, of neuropeptides. However, the role of neuropeptides, their mechanisms of action, interactions with each other, and their release are still poorly understood. It is also unclear why insects possess such a number of different peptides, some with multiples copies or homologues, which stimulate or inhibit gut motility, and how their release, sometimes from the same neurone, is regulated. These neuropeptides may also act at sites other than visceral muscles, such as centrally through the brain or on gut stretch receptors.

Peptidergic paracrine and endocrine cells in the midgut of the fruit fly maggot

Endocrine cells in the larval midgut of Drosophila melanogaster are recognized by antisera to seven regulatory peptides: the allatostatins A, B, and C, short neuropeptide F, neuropeptide F, diuretic hormone 31, and the tachykinins. These are the same peptides that are produced by the endocrine cells of the adult midgut, except for short neuropeptide F, which is absent in adult midgut endocrine cells. The anterior larval midgut contains two types of endocrine cells. The first produces short neuropeptide F, which is also recognized by an antiserum to the receptor for the diuretic hormone leucokinin. The second type in the anterior midgut is recognized by an antiserum to diuretic hormone 31. The latter cell type is also found in the junction between the anterior and middle midgut; an additional type of endocrine cell in this region produces allatostatin B, a peptide also known as myoinhibitory peptide. Both types of endocrine cells in the junction between the anterior and middle midgut can express the homeodomain transcription factor labial. The copper cell region contains small cells that either produce allatostatin C or a combination of neuropeptide F, allatostatin B, and diuretic hormone 31. The latter cell type is also found in the region of the large flat cells. The posterior midgut possesses strongly immunoreactive allatostatin C endocrine cells immediately behind the iron cells. In the next part of the posterior midgut, two cell types have been found: one produces diuretic hormone 31, and a second is strongly immunoreactive to antiserum against the leucokinin receptor and weakly immunoreactive to antisera against allatostatins B and C and short neuropeptide F. The last part of the posterior midgut again has two types of endocrine cells: those that produce allatostatin A, and those that produce tachykinins. Many of the latter cells are also weakly immunoreactive to the antiserum against diuretic hormone 31. As in the adult, the insulin-like peptide 3 gene appears to be expressed by midgut muscles, but not by midgut endocrine cells.

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.

Characterization of neuropeptide F-like immunoreactivity in the blood-feeding hemipteran, Rhodnius prolixus

The invertebrate neuropeptide Y (NPY) homolog, neuropeptide F (NPF), has been characterized for a wide range of invertebrate phyla, including platyhelminthes, molluscs, and arthropods. Current hypotheses suggest that NPF may be capable of regulating responses to diverse external cues related to nutritional status and feeding. The qualitative and quantitative distribution of an NPF-like peptide in fifth instar Rhodnius prolixus was undertaken using an antiserum raised against Drosophila NPF. Immunohistochemistry reveals NPF-like immunoreactive neurons and processes in the central nervous system, stomatogastric nervous system and peripheral nervous system. The distribution of NPF-like immunoreactivity within the medial neurosecretory cells of the brain and neurohemal areas of the corpus cardiacum and dorsal vessel, suggests NPF may act as a neurohormone. Immunoreactive processes are present over the surface of the hindgut and the immunoreactivity in these processes is greatly reduced in intensity 24h post-feeding. The quantification of partially purified NPF-like material in the CNS of R. prolixus was conducted by HPLC fractionation and radioimmunoassay. The results suggest that NPF-like material is present in fifth instar R. prolixus and likely released into the hemolymph following a blood meal.

Alkalinization in the isolated and perfused anterior midgut of the larval mosquito, Aedes aegypti

In the present study, isolated midguts of larval Aedes aegypti L. (Diptera: Culicidae) were mounted on perfusion pipettes and bathed in high buffer mosquito saline. With low buffer perfusion saline, containing m-cresol purple, transepithelial voltage was monitored and luminal alkalinization became visible through color changes of m-cresol purple after perfusion stop. Lumen negative voltage and alkalinization depended on metabolic energy and were stimulated in the presence of serotonin (0.2 micromol l(-1)). In some experiments a pH microelectrode in the lumen recorded pH values up to 10 within minutes after perfusion stop. The V-ATPase inhibitor concanamycin (50 micromol l(-1)) on the hemolymph side almost abolished V(te) and inhibited luminal alkalinization. The carbonic anhydrase inhibitor, methazolamide (50 micromol l(-1)), on either the luminal or hemolymph-side, or the inhibitor of anion transport, DIDS (1 mmol l(-1)) on the luminal side, had no effect on V(te) or alkalinization. Cl(-) substitution in the lumen or on both sides of the tissue affected V(te), but the color change of m-cresol purple was unchanged from control conditions. Hemolymph-side Na(+) substitution or addition of the Na(+)/H(+) exchange inhibitor, amiloride (200 micromol l(-1)), reduced V(te) and luminal alkalinization. Luminal amiloride (200 micromol l(-1)) was without effects on V(te) or alkalinization. High K(+) (60 mmol l(-1)) in the lumen reduced V(te) without affecting alkalinization. These results indicate that strong luminal alkalinization in isolated and perfused anterior midgut of larval A. aegypti depends on basolateral V-ATPase, but is apparently independent of carbonic anhydrase, apical Cl(-)/HCO(3)(-) exchange or apical K(+)/2H(+) antiport.

Organization, ultrastructure, and development of midgut visceral muscle in larval Aedes aegypti

The midgut muscularis of larvae of the mosquito Aedes aegypti takes the form of a grid of longitudinal and circular muscle bands. The longitudinal and circular bands overlap at near right angles at many areas of intersection. The longitudinal bands run the length of the midgut. However, some bands of circular muscle, located in the anterior midgut, pass only partway around the gut. An unusual feature was observed at some regions where longitudinal and circular bands of muscle intersect: filaments oriented at near right angles to one another were present in the same membrane-bound fiber. These cruciform regions send contractile elements into both circular and longitudinal bands. The muscularis was fixed in a contracted state, so most of the sarcomeres are represented by complete overlap of myosin and lighter staining actin filaments. Features characteristic of supercontracting muscle, including perforated Z-lines, were seen in sarcomeres of circular muscle bands. Small invaginations resembling transverse tubules were present but a sarcoplasmic reticulum was not observed. While occasional cells that may be neurons or neurosecretory cells were observed, a network that might serve to coordinate the segmentation and peristaltic movement of the muscularis was not apparent.

Characterization and expression of the short neuropeptide F receptor in the African malaria mosquito, Anopheles gambiae

A short neuropeptide F (sNPF) precursor and a sNPF receptor (sNPFR) were characterized for the mosquito, Anopheles gambiae. The sNPFR was expressed in CHO-K1 cells, and it exhibited high affinity binding, IC(50) approximately 3-5 nM, for specific sNPFs. sNPF1 potently inhibited forskolin-stimulated cAMP production by transfected cells, suggesting sNPFR acts via G(i/o). Transcripts for sNPF and sNPFR were present in all body regions of larvae, pupae, and adults, and immunoblots for sNPFR confirmed this distribution in females. Membranes from female heads and thoraces exhibited prototypical high affinity binding for radiolabeled sNPF, indicating sNPFR is a bona fide endogenous receptor.

Structural studies of Drosophila short neuropeptide F: Occurrence and receptor binding activity

Among insects, short neuropeptide Fs (sNPF) have been implicated in regulation of reproduction and feeding behavior. For Drosophila melanogaster, the nucleotide sequence for the sNPF precursor protein encodes four distinctive candidate sNPFs. In the present study, all four peptides were identified by mass spectrometry in body extracts of D. melanogaster; some also were identified in hemolymph, suggesting potential neuroendocrine roles. Actions of sNPFs in D. melanogaster are mediated by the G protein-coupled receptor Drm-NPFR76F. Mammalian CHO-K1 cells were stably transfected with the Drm-NPFR76F receptor for membrane-based radioreceptor studies. Binding assays revealed that longer sNPF peptides comprised of nine or more amino acids were clearly more potent than shorter ones of eight or fewer amino acids. These findings extend understanding of the relationship between structure and function of sNPFs.

The isolated anterior stomach of larval mosquitoes (Aedes aegypti): Voltage-clamp measurements with a tubular epithelium

The anterior stomach of larval Aedes aegypti was isolated and perfused via two pipettes. For transepithelial voltage (V(te)) measurement, the inflow pipette and the bath were connected via agar bridges to calomel electrodes. For voltage-clamping, the lumen of the tissue contained an Ag/AgCl wire held by the outflow pipette, and the preparation was placed in a bath within a spiral of Ag/AgCl wire. After equilibrating the tissue in mosquito saline on both sides, a V(te) of -8+/-1 mV was measured (+/-S.E.M., N=32). Current-voltage curves (+/-100 mV) demonstrated ohmic behaviour of the epithelium. Short-circuiting resulted in a current (I(sc)) of 103+/-16 microA cm(-2) and a mean transepithelial conductance (G(te)) of 11.8+/-1.3 mS cm(-2) (+/-S.E.M., N=32). A Yonath-Civan plot of G(te) of individual preparations over the corresponding I(sc) resulted in a straight line (r(2)=0.8422), indicating that the difference in I(sc) of individual preparations is mainly based on different transcellular conductances (G(c)). This analysis allowed to estimate the mean leak conductance (G(l) approximately 3.9 mS cm(-2)) and the mean transcellular electromotive force (E(c) approximately 13 mV). After administering 0.2 micromol L(-1) serotonin, I(sc) and G(te) significantly increased, to 457+/-49 microA cm(-2) and to 21.3+/-2.3 mS cm(-2) (+/-S.E.M., N=31, P<0.05), respectively. The Yonath-Civan plot after serotonin resulted again in a straight line (r(2)=0.8219), indicating a mean G(l) of about 1 mS cm(-2) and a mean E(c) of about 22 mV. Dinitrophenol (2.5 mmol L(-1)) almost abolished I(sc) and significantly reduced G(te) (N=6). Concanamycin A (100 micromol L(-1)) reduced I(sc) by more than 90% without significantly affecting G(te).