The Explosion of Structural Information on Insect Neuropeptides

Comparative analysis of adipokinetic hormones and their receptors in Blattodea reveals novel patterns of gene evolution

Adipokinetic hormone (AKH) is a neuropeptide produced in the insect corpora cardiaca that plays an essential role in mobilising carbohydrates and lipids from the fat body to the haemolymph. AKH acts by binding to a rhodopsin-like G protein-coupled receptor (GPCR), the adipokinetic hormone receptor (AKHR). In this study, we tackle AKH ligand and receptor gene evolution as well as the evolutionary origins of AKH gene paralogues from the order Blattodea (termites and cockroaches). Phylogenetic analyses of AKH precursor sequences point to an ancient AKH gene duplication event in the common ancestor of Blaberoidea, yielding a new group of putative decapeptides. In total, 16 different AKH peptides from 90 species were obtained. Two octapeptides and seven putatively novel decapeptides are predicted for the first time. AKH receptor sequences from 18 species, spanning solitary cockroaches and subsocial wood roaches as well as lower and higher termites, were subsequently acquired using classical molecular methods and in silico approaches employing transcriptomic data. Aligned AKHR open reading frames revealed 7 highly conserved transmembrane regions, a typical arrangement for GPCRs. Phylogenetic analyses based on AKHR sequences support accepted relationships among termite, subsocial (Cryptocercus spp.) and solitary cockroach lineages to a large extent, while putative post-translational modification sites do not greatly differ between solitary and subsocial roaches and social termites. Our study provides important information not only for AKH and AKHR functional research but also for further analyses interested in their development as potential candidates for biorational pest control agents against invasive termites and cockroaches.

Biochemically identified neuropeptides in a caddisfly (Trichoptera) and a pygmy mole cricket (Orthoptera: Caelifera: Tridactyloidea)

One representative of the order Trichoptera, namely the caddisfly Chaetopteryx villosa, was investigated along with the pygmy mole cricket Xya capensis which is a representative of the most basal superfamily of the caeliferan Orthoptera, that is, the Tridactyloidea. From both clades neuropeptides have not been biochemically characterized before this study. Here, members of the adipokinetic hormone family (AKHs) are sequenced via liquid chromatography (LC)-ion trap mass spectrometry from methanolic extracts from the corpora cardiaca of respective species. The corpora cardiaca were dissected, methanolic extracts prepared, peptides separated by liquid chromatography (LC), and AKHs detected and sequenced by ion trap mass spectrometry. Both species investigated contain an octapeptide AKH: the trichopteran species has the peptide with the sequence pGlu-Leu-Thr-Phe-Thr-Pro-Ser-Trp amide; the ambiguity of the isobaric amino acids Leu and Ile at position two was solved by comparing retention times on LC and by co-elution with the synthetic Leu² -form. This peptide is known as Aedae-AKH and found in certain dipteran species and in an alderfly (Megaloptera). The tridactyloid species contains the peptide with the sequence pGlu-Val-Asn-Phe-Ser-Pro-Gly-Trp amide which had first been identified in a member of the order Mantophasmatodea and is called Manto-CC. Comparisons are made between the AKH complements of the sister groups Trichoptera and Lepidoptera and their possible relatedness and, on the other hand, between the AKH of X. capensis with those of closely related caeliferan superfamilies. The biology of the two studied species is used to speculate about a possible function of the elucidated hormones. Lastly, the use of a larval stage as starting material for structural neuropeptide information is discussed.

Unique Members of the Adipokinetic Hormone Family in Butterflies and Moths (Insecta, Lepidoptera)

Lepidoptera is amongst one of the four most speciose insect orders and ecologically very successful because of their ability to fly. Insect flight is always aerobic and exacts a high metabolic demand on the animal. A family of structurally related neuropeptides, generically referred to as adipokinetic hormones (AKHs), play a key role in triggering the release of readily utilizable fuel metabolites into the hemolymph from the storage forms in the fat body. We used mass spectrometry to elucidate AKH sequences from 34 species of Lepidoptera and searched the literature and publicly available databases to compile (in a phylogenetic context) a comprehensive list of all Lepidoptera sequences published/predicted from a total of 76 species. We then used the resulting set of 15 biochemically characterized AKHs in a physiological assay that measures lipid or carbohydrate mobilization in three different lepidopteran species to learn about the functional cross-activity (receptor-ligand interactions) amongst the different butterfly/moth families. Our results include novel peptide structures, demonstrate structural diversity, phylogenetic trends in peptide distribution and order-specificity of Lepidoptera AKHs. There is almost an equal occurrence of octa-, nona-, and decapeptides, with an unparalleled emphasis on nonapeptides than in any insect order. Primitive species make Peram-CAH-II, an octapeptide found also in other orders; the lepidopteran signature peptide is Manse-AKH. Not all of the 15 tested AKHs are active in Pieris brassicae; this provides insight into structure-activity specificity and could be useful for further investigations into possible biorational insecticide development.

Structural diversity of adipokinetic hormones in the hyperdiverse coleopteran Cucujiformia

Seventeen species of the coleopteran series Cucujiformia are investigated for the presence and sequence of putative adipokinetic hormones (AKHs). Cucujiformia includes species from the major superfamilies, that is, Chrysomeloidea, Curculionoidea, Cucujoidea, and Tenebrionoidea. The clade Phytophaga in which the Chrysomeloidea and Curculionoidea reside, harbor very detrimental species for agriculture and forestry. Thus, this study aims not only to demonstrate the structural biodiversity of AKHs in these beetle species and possible evolutionary trends but also to determine whether the AKHs from harmful pest species can be used as lead substances for a future putative insecticide that is harmless to beneficial insects. Sequence analysis of AKHs is achieved by liquid chromatography coupled to mass spectrometry. Most of the investigated species contain AKH octapeptides in their corpora cardiaca, although previously published work also found a few decapeptides, which we comment on. The signature and sole AKH in cerambycidae Chrysomeloidea and Curculionoidea is Peram-CAH-I (pEVNFSPNW amide), which is also found in the majority of chrysomelidae Chrysomeloidea and in the one investigated species of Cucujoidea albeit in a few cases associated with a second AKH which can be either Peram-CAH-II (pELTFTPNW amide), Emppe-AKH (pEVNFTPNW amide), or Micvi-CC (pEINFTPNW amide). The most often encountered AKH in Tenebrionoidea, family Meloidae as well as family Tenebrionidae, is Tenmo-HrTH (pELNFSPNW amide) followed by Pyrap-AKH (pELNFTPNW amide) and a Tenmo-HrTH extended decapeptide (in Meloidae). Finally, we examine AKH sequences from 43 species of cucujiform beetles, including the superfamily Coccinelloidea for a possible lead compound for producing a cucujiform-specific pesticide.

Evolutionary trends of neuropeptide signaling in beetles - A comparative analysis of Coleopteran transcriptomic and genomic data

Insects employ neuropeptides to regulate their growth & development, behaviour, metabolism and their internal milieu. At least 50 neuropeptides are known to date, with some ancestral to the insects and others more specific to particular taxa. In order to understand the evolution and essentiality of neuropeptides, we data mined publicly available high quality genomic or transcriptomic data for 31 species of the largest insect Order, the Coleoptera, chosen to represent the superfamilies' of the Adephaga and Polyphaga. The resulting neuropeptide distributions were compared against the habitats, lifestyle and other parameters. Around half of the neuropeptide families were represented across the Coleoptera, suggesting essentiality or at least continuing utility. However, the remaining families showed patterns of loss that did not correlate with any obvious life history parameter, suggesting that these neuropeptides are no longer required for the Coleopteran lifestyle. This may perhaps indicate a decreasing reliance on neuropeptide signaling in insects.

Locust flight activity as a model for hormonal regulation of lipid mobilization and transport

Flight activity of insects provides a fascinating yet relatively simple model system for studying the regulation of processes involved in energy metabolism. This is particularly highlighted during long-distance flight, for which the locust constitutes a long-standing favored model insect, which as one of the most infamous agricultural pests additionally has considerable economical importance. Remarkably many aspects and processes pivotal to our understanding of (neuro)hormonal regulation of lipid mobilization and transport during insect flight activity have been discovered in the locust; among which are the peptide adipokinetic hormones (AKHs), synthesized and stored by the neurosecretory cells of the corpus cardiacum, that regulate and integrate lipid (diacylglycerol) mobilization and transport, the functioning of the reversible conversions of lipoproteins (lipophorins) in the hemolymph during flight activity, revealing novel concepts for the transport of lipids in the circulatory system, and the structure and functioning of the exchangeable apolipopotein, apolipophorin III, which exhibits a dual capacity to exist in both lipid-bound and lipid-free states that is essential to these lipophorin conversions. Besides, the lipophorin receptor (LpR) was identified and characterized in the locust. In an integrative approach, this short review aims at highlighting the locust as an unrivalled model for studying (neuro)hormonal regulation of lipid mobilization and transport during insect flight activity, that additionally has offered a broad and profound research model for integrative physiology and biochemistry, and particularly focuses on recent developments in the concept of AKH-induced changes in the lipophorin system during locust flight, that deviates fundamentally from the lipoprotein-based transport of lipids in the circulation of mammals. Current studies in this field employing the locust as a model continue to attribute to its role as a favored model organism, but also reveal some disadvantages compared to model insects with a completely sequenced genome.

Neuropeptidases and the metabolic inactivation of insect neuropeptides

Neuropeptidases play a key role in regulating neuropeptide signalling activity in the central nervous system of animals. They are oligopeptidases that are generally found on the surface of neuronal cells facing the synaptic and peri-synaptic space and therefore are ideally placed for the metabolic inactivation of neuropeptide transmitters/modulators. This review discusses the structure of insect neuropeptides in relation to their susceptibility to hydrolysis by peptidases and the need for specialist enzymes to degrade many neuropeptides. It focuses on five neuropeptidase families (neprilysin, dipeptidyl-peptidase IV, angiotensin-converting enzyme, aminopeptidase and dipeptidyl aminopeptidase III) that have been implicated in the metabolic inactivation of neuropeptides in the central nervous system of insects. Experimental evidence for the involvement of these peptidases in neuropeptide metabolism is reviewed and their properties are compared to similar neuropeptide inactivating peptidases of the mammalian brain. We also discuss how the sequencing of insect genomes has led to the molecular identification of candidate neuropeptidase genes.

Molecular modeling of the binding of pheromone biosynthesis activating neuropeptide to its receptor

Moth sex-pheromone biosynthesis follows a circadian cycle, which is cued by the release of the neurohormone pheromone biosynthesis activating neuropeptide (PBAN) to the hemolymph. PBAN binds to a G protein-coupled receptor (GPCR), in pheromone glands, (PG) initially identified by us in Helicoverpa zea moths (HezPBAN-R). In this study, the sequences of the seven transmembrane helices of HezPBAN-R were identified, built, packed and oriented correctly after multiple sequence alignment of the HezPBAN-R and several other GPCRs using the X-ray structure of rhodopsin as a template. Molecular dynamics simulations were run on three different beta-turn types of the C-terminal hexapeptide of PBAN and the results clustered into 12 structurally distinct groups. The lowest energy conformation from each group was used for computer-simulated docking with the model of the HezPBAN-R. Highest scoring complexes were examined and putative binding sites were identified. Experimental studies, using in vitro PG, revealed lower levels of pheromonotropic activity when challenged with pyrokinin-like peptides than with HezPBAN as ligand. Thus, the Drosophila melanogaster pyrokinin-1 receptor (CG9918) was chosen to create chimera receptors by exchanging between the three extracellular loops of the HezPBAN-R and the CG9918 for in silico mutagenesis experiments. The predicted docking model was validated with experimental data obtained from expressed chimera receptors in Sf9 cells.

Cloning and characterization of the pheromone biosynthesis activating neuropeptide receptor gene in Spodoptera littoralis larvae

In noctuid moths cuticular pigmentation is regulated by the pyrokinin/pheromone biosynthesis activating neuropeptide (PK/PBAN) family, which also mediates a variety of other functions in moths and other insects. Numerous studies have shown that these neuropeptides exert their functions through activation of the PBAN receptor (PBAN-R), with subsequent Ca(2+) influx, followed by either activation of cAMP or direct activation of downstream kinases. Recently, several PBAN-Rs have been identified, all of which are from the pheromone gland of adult female moths, but evidence shows that functional PK/PBAN-Rs can also be expressed in insect larvae, where they mediate melanization and possibly other functions (e.g., diapause). Here, we identified a gene encoding a G-protein-coupled receptor from the 5th instar larval tissue of the moth Spodoptera littoralis. The cDNA of this gene contains an open reading frame with a length of 1050 nucleotides, which translates to a 350-amino acid, 42-kDa protein that shares 92% amino acid identity with Helicoverpa zea and Helicoverpa armigera PBAN-R, 81% with Bombyx mori PBAN-R and 72% with Plutella xylostella PBAN-R. The S. littoralis PBAN-R gene was stably expressed in NIH3T3 cells and transiently in HEK293 cells. We show that it mediates the dose-dependent PBAN-induced intracellular Ca(2+) response and activation of the MAP kinase via a PKC-dependent but Galphai-independent signaling mechanism. Other PK/PBAN family peptides (pheromonotropin and a C-terminally PBAN-derived peptide PBAN(28-33)NH(2)) also triggered MAP kinase activation. This receptor, together with the previously cloned PBAN-R, may facilitate our understanding of the cell-specific responses and functional diversities of this diverse neuropeptide family.

Molecular characterization of a family of metalloendopeptidases from the intestinal brush border of Haemonchus contortus

Substantial protection against the economically important parasitic nematode Haemonchus contortus has been achieved by immunizing sheep with a glycoprotein fraction isolated from the intestinal membranes of the worm (H-gal-GP). Previous studies showed that one of the major components of H-gal-GP is a family of at least 4 zinc metalloendopeptidases, designated MEPs 1–4. This paper describes aspects of the molecular architecture of this protease family, including the proteomic analysis of the MEP fraction of the H-gal-GP complex. These enzymes belong to the M13 zinc metalloendopeptidase family (EC 3.4.24.11), also known as neutral endopeptidases or neprilysins. The sequences of MEPs 1 and 3 suggested a typical Type II integral membrane protein structure, whilst MEPs 2 and 4 had putative cleavable signal peptides, typical of secreted proteins. Proteomic analysis of H-gal-GP indicated that the extracellular domain of all 4 MEPs had been cleaved close to the transmembrane region/signal peptide with additional cleavage sites mid-way along the polypeptide. MEP3 was present as a homo-dimer in H-gal-GP, whereas MEP1 or MEP2 formed hetero-dimers with MEP4. It was found that expression of MEP3 was confined to developing 4th-stage larvae and to adult worms, the stages of Haemonchus which feed on blood. MEP-like activity was detected in the H-gal-GP complex over a broad pH range (5–9). Since all 4 MEPs must share a similar microenvironment in the complex, this suggests that each might have a different substrate specificity.

Differential sorting and packaging of capa-gene related products in an insect

A unique costorage of neuropeptides was recently found in the abdominal perisympathetic organs (PSOs) of the American cockroach, Periplaneta americana. Having specific antisera directed against all peptides belonging to this neurosecretory system, we examined the sorting of PSO-peptides in the soma of the median neurosecretory cells of abdominal ganglia by using immunoelectron microscopic double stainings. The data indicate that all six abundant neuropeptides of this neurohormonal system, which includes three capa-gene related products, are primarily incorporated into separate vesicles. These vesicles fuse with each other in the cytoplasm and become translucent on their way to the axon hillock. By means of light microscopy and MALDI-TOF mass spectrometry, an identical population of neuropeptides was found in interneurons of the brain. As revealed by subsequent immunoelectron microscopic analysis, the peptides of these cells are separately packed into dense core vesicles but do not fuse with each other. Thus, hitherto unknown cell-type-specific sorting mechanisms occur in neurosecretory cells and interneurons, respectively.

Identification of leucomyosuppressin in the German cockroach, Blattella germanica, as an inhibitor of food intake

The feeding pattern of the adult female of Blattella germanica peaks in the middle of the vitellogenic cycle. Following the hypothesis that a factor inhibiting gut peristalsis also inhibits food intake and is involved in the regulation of feeding, we searched for the most powerful myoinhibitory peptide in brain extracts from B. germanica females collected after the peak within the feeding cycle. Through HPLC purification and sequence analysis, we obtained the peptide leucomyosuppressin (LMS): pQDVDHVFLRFamide. LMS elicited a powerful myoinhibitory effect on B. germanica foregut and hindgut, with ED(50) values around 10(-10) M. In addition, it inhibited food intake in vivo in a dose-dependent manner at doses between 5 and 50 microg. The study of the distribution of ingested food in the foregut, midgut and hindgut of B. germanica females treated with LMS showed that food accumulates in the foregut, which may be due to the myoinhibitory effects of the peptide. We propose that this accumulation inhibits food intake because of the persistence of the signals from gut stretch receptors.

Regulation of intermediary metabolism and water balance of insects by neuropeptides

Neuropeptides regulate all important physiological, developmental, and behavioral processes in insects. Here, I review two major physiological events that are hormonally controlled, namely intermediary metabolism and ion and water transport. Peptides belonging to the family of adipokinetic hormones (AKHs) increase hemolymph carbohydrates, lipids, and proline by activating the enzyme glycogen phosphorylase or lipase in the fat body. Moreover, these pleiotropic and multifunctional peptides inhibit protein-, lipid-, and RNA synthesis, and stimulate the frequency of contraction of certain muscles. Diuretic hormones that are related to the vertebrate corticotropin-releasing factor (CRF-related DHs) or belong to the family of kinins (which also have a myotropic action) or the cardioacceleratory peptides (CAPs), which increase the frequency of the heartbeat, all stimulate the secretion of fluid in Malpighian tubules (MTs) in vitro. Only a few true antidiuretic hormones are known: those from mealworms that inhibit the fluid transport in MTs in vitro, probably neuroparsins that stimulate water absorption by everted rectal sacs in vitro, and the desert locust's ion-transport peptide (ITP). Biosynthesis, release, receptors, mode of action, inactivation, structure-activity studies, and biological functions are discussed for the various peptides.

The use of peptidomics in endocrine research

In 2002, the Nobel Prize for chemistry was awarded to the inventors of two novel ionization techniques in mass spectrometry, MALDI and ESI. These techniques, often in combination with data from genomic databases, represent an extremely powerful tool in analytical (bio)chemistry, with many applications, e.g., in the field of proteomics. Peptides, which are small proteins, have, despite their importance as controlling agents in numerous physiological processes, as yet been much less intensively studied by these novel techniques than larger proteins. The term peptidomics, i.e., the study of all peptides expressed by a certain cell, organ or organism was only introduced in 2001. In neuroendocrinology, spectacular progress could already be realized and the future looks bright. In this minireview we discuss the different methodologies that are used in peptidomics and give an overview of the wide range of applications.

G Protein-Coupled Receptors in Invertebrates: A State of the Art

G protein-coupled receptors (GPCRs) constitute one of the largest and most ancient superfamilies of membrane-spanning proteins. We focus on neuropeptide GPCRs, in particular on those of invertebrates. In general, such receptors mediate the responses of signaling molecules that constitute the highest hierarchical position in the regulation of physiological processes. Until recently, only a few of these receptors were identified in invertebrates. However, the availability of a plethora of genomic information has boosted the discovery of novel members in several invertebrate species, such as Drosophila, in which 18 neuropeptide GPCRs have been characterized. The finalization of genomic projects in other invertebrates will lead to a similar expansion of GPCR understanding. Many new insights regarding neuropeptide regulation have followed from the discovery of their cognate receptors. Furthermore, information on GPCR signaling is still fragmentary and the elucidation of these pathways in model insects such as Drosophila will lead to further insights in other species, including mammals. In this review we present the current status of what is known about invertebrate GPCRs, discuss some novel perceptions that follow from the identified members, and, finally, present some future prospects.

Enhanced oral availability/pheromonotropic activity of peptidase-resistant topical amphiphilic analogs of pyrokinin/PBAN insect neuropeptides

The peptide bond between active core residues Pro and Arg is the primary site of susceptibility for the pyrokinin/PBAN neuropeptides to insect tissue-bound peptidases, and incorporation of modified Pro residues can enhance resistance to peptidase hydrolysis. An Hyp-containing amphiphilic analog (Hex-FT[Hyp]RLa) is shown to operate as a topically active tissue-bound peptidase-resistant analog of the pyrokinin/PBAN class of insect neuropeptides in adult Heliothis virescens moths. An Oic amphiphilic analog (Hex-FT[Oic]RLa) is ineffective topically, but proves to be a superior tissue-bound, peptidase-resistant pyrokinin/PBAN analog for oral administration; outperforming both the Hyp analog and the orally inactive natural hormone PBAN in the moths. The Oic analog is effective in penetrating an isolated, ligated foregut preparation, but less successful in transmigrating an isolated midgut preparation; whereas the opposite behavior is observed for the Hyp analog. The success of the Oic analog via oral administration may be related to its ability to effectively penetrate the foregut, thereby bypassing the hostile environment of the midgut region.

Insect neuropeptide antagonists

The development of a new integrated approach to the generation of a novel type of insect neuropeptide (Np) antagonists and putative insect control agents based on backbone cyclic compounds is described. The approach, termed the backbone cyclic neuropeptide-based antagonist (BBC-NBA), was applied to the insect pyrokinin (PK)/pheromone biosynthesis activating neuropeptide (PBAN) family as a model, and led to the discovery of a potent linear lead antagonist and several highly potent, metabolically stable BBC antagonists, devoid of agonistic activity, which inhibited PBAN-mediated activities in moths in vivo. This review briefly summarizes our knowledge of insect Nps, describes the PK/PBAN Np family, presents the basic concepts behind the BBC-NBA approach, and introduces the advantages of this method for generation of Np agonists, antagonists and insecticide prototype molecules.

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.

Comparative topical pheromonotropic activity of insect pyrokinin/PBAN amphiphilic analogs incorporating different fatty and/or cholic acid components

This study presents a comparison of the topical pheromonotropic activity in the tobacco budworm moth of a series of amphiphilic pseudopeptide analogs of the insect pyrokinin/PBAN peptide class incorporating fatty acids of varying chain lengths. While the C16 analog fails to penetrate the moth cuticle, and the C12 only moderately so, shorter chain analogs transmigrate the moth cuticle readily with decreasing cuticle-retention properties. A cholic acid analog topically induces twice the maximal pheromone titer of injected native hormone. From a pest management perspective, these non-aromatic hydrophobic components are expected to be more environmentally benign than benzenoid components previously used in topical insect peptide analogs.

Newly discovered functions for some myotropic neuropeptides in locusts

The field of neuropeptide research in insects during the past twenty years can be characterized by the enormous number of peptides that have been identified. In the locusts, Locusta migratoria and Schistocerca gregaria only, structural information is now available for more than 60 peptides. Quite a number of these peptides were isolated on the basis of their effect on visceral muscle contraction in vitro. A very limited number of reports describe the 'in vivo' function of a myotropic neuropeptide. Moreover, for most of the brain neuropeptides, we ignore whether they have a hormonal function. In this paper, we describe the recently discovered in vivo effects of some of the myotropic peptides, identified in locusts in the past decade. Schistocerca-neuropeptide F accelerates egg development; locustasulfakinin inhibits food intake and [His(7)]-corazonin induces body color pigmentation.

Sulfakinins reduce food intake in the desert locust, Schistocerca gregaria

In vertebrates, the peptides cholecystokinin (CCK), neuropeptide Y, galanin, and bombesin are known to be involved in the control of food intake. We report here that insect sulfakinins, peptides which display substantial sequence similarities with the vertebrate gastrin/CCK peptide family, significantly inhibit food uptake in fifth instar nymphs of the locust, Schistocerca gregaria. Upon injection of Lom-sulfakinin, a neuropeptide present in the corpus cardiacum of locusts, food intake was significantly reduced in a dose-dependent manner within a fixed 20 min time period. The induced effect ranged from 13% inhibition (10 pmol of injected peptide) to over 50% inhibition at 1 nmol. Other naturally occurring sulfakinins from different insect species also elicited this satiety effect. Analogous to the satiety effect of CCK in vertebrates, the sulfate group is required for activity. No effect on the palptip resistance was found after injection with sulfakinin. Therefore it seems unlikly that sulfakinins reduce food intake by decreasing the sensitivity of the taste receptors.

Cyclic AMP mediates the elevation of proline by AKH peptides in the cetoniid beetle, Pachnoda sinuata

The role of cyclic nucleotides in the transduction of the hyperprolinaemic and hypertrehalosaemic signal of the endogenous neuropeptide Mem-CC was investigated in the cetoniid beetle Pachnoda sinuata. Flight and injection of Mem-CC into the haemocoel of the beetle induce an increase of cAMP levels in the fat body of the beetle. This increase is tissue-specific and does not occur in brain and flight muscles. An elevation of cAMP levels was also found when in vitro preparations of fat body tissue were subjected to Mem-CC. Elevation of the cAMP concentration after injection of Mem-CC is time- and dose-dependent: the maximum response is measured after 1 min, and a dose of 25 pmol Mem-CC is needed. Injection of cpt-cAMP, a cAMP analogue which penetrates the cell membrane, causes a stimulation of proline synthesis but no mobilisation of carbohydrate reserves. The same is measured when IBMX, an inhibitor of phosphodiesterase, is injected. cGMP seems not to be involved in synthesis of proline nor carbohydrate release, because injection of cpt-cGMP has no influence on the levels of proline, alanine and carbohydrates in the haemolymph. Although glycogen phosphorylase of the fat body is activated by Mem-CC in a time- and dose-dependent manner, it cannot be stimulated by cpt-cAMP. The combined data suggest that cAMP is involved in regulation of proline levels by Mem-CC but not in regulation of carbohydrates. Octopamine has no effect on metabolites in the haemolymph and is not capable of activating glycogen phosphorylase, indicating that it is not involved in the regulation of substrates in this beetle. Furthermore, the requirements of the receptor of Mem-CC are different for eliciting a hypertrehalosaemic and a hyperprolinaemic effect, respectively, suggesting that differentiation in signal transduction begins at the receptor level.

Modulation of cardiac rhythm by allatostatins in the cockroach Blattella germanica (L.) (Dictyoptera, Blattellidae)

Cardiac rhythm was measured in Blattella germanica females during the reproductive cycle. The rate increased from day 0 to 1, remained constant during the vitellogenic period and fell by about 20% during the period of oothecal transport. The effects of allatostatins, allatostatin analogues and corazonin were tested on semi-isolated heart preparations. Allatostatins showed a rapid, reversible and dose-dependent cardioinhibitory activity. Blattella allatostatin 1 (BLAST-1: LYDFGL-NH(2)), was the most active, eliciting 76% inhibition at 10(-7) M and even 19% inhibition at 10(-9) M. BLAST-2 (DRLYSFGL-NH(2)), BLAST-3 (AGSDGRLYSFGL-NH(2)) and BLAST-4 (APSSAQRLYGFGL-NH(2)) were less active. An analogue of BLAST-2 with C-terminus in acid form and a pseudopeptide analogue of BLAST-2 with a methyleneamino Psi[CH(2)NH] peptide bond surrogate between residues L(3) and Y(4) were inactive. Corazonin elicited rapid, reversible and dose-dependent cardioacceleratory activity. When tested together with BLAST-1, corazonin overrode the cardioinhibitory effect of allatostatin. Our previous results had shown that high levels of allatostatin were maintained during the period of oothecal transport. This and the fact that physiological concentrations of allatostatins produce physiological levels of inhibition, suggest that allatostatins are involved in the modulation of cardiac rhythm in this cockroach.

Differential distribution of pyrokinin-isoforms in cerebral and abdominal neurohemal organs of the American cockroach

Different pyrokinin isoforms were identified from major neurohemal organs of the American cockroach. During their isolation they were recognized by bioassay using a hyperneural muscle preparation that is sensitive to pyrokinins. All structures were elucidated by sequence analysis and mass spectrometry. The primary structures of the novel peptides isolated from the retrocerebral complex are LVPFRPRL-NH2 (designated Pea-PK-3) and DHLPHDVYSPRL-NH2 (designated Pea-PK-4). A pyrokinin, labeled Pea-PK-5, was isolated from abdominal perisympathetic organs. Structural analysis of this peptide yielded the sequence GGGGSGETSGMWFGPRL-NH2. The threshold concentrations of the identified pyrokinins for an eliciting effect on contractions of the hyperneural muscle preparations differed dramatically. This indicates that the different distribution of pyrokinin-isoform observed in neurohemal organs may be associated with different functions. This is the first report of a differential distribution of peptide-isoforms in the neurohemal organs of insects.

Isolation and structural elucidation of two pyrokinins from the retrocerebral complex of the American cockroach

By monitoring the contractile activity of the hyperneural muscle of the American cockroach in vitro two peptides were isolated from the retrocerebral complex of the American cockroach. Three purification steps using reversed-phase high performance liquid chromatography on C-18 columns containing trifluoroacetic acid or heptafluorobutyric acid as organic modifiers were sufficient to achieve homogeneous peptide preparations. The structures of both peptides were elucidated by a combination of Edman degradation and mass spectrometry which yielded the following structures: His-Thr-Ala-Gly Phe-Ile-Pro-Arg-Leu-NH2 (Pea-PK-1) and Ser-Pro-Pro-Phe-Ala-Pro-Arg-Leu-NH2 (Pea-PK-2). The C-terminal sequence Phe-X-Pro-Arg-Leu-NH2 characterized the peptides as members of the insect pyrokinin family. The synthetic peptides were shown to have the same retention times as the natural peptides. The occurrence of both peptides in the retrocerebral complex suggests a physiological role as neurohormones. The effects of the synthetic pyrokinis were clearly distinguishable in their actions on the hyperneural muscle. Regarding the threshold concentrations, Pea-PK-2 was only 0.3% as active as Pea-PK-1.