Isolation and identification of Lom-SG-SASP, a salivation stimulating peptide from the salivary glands of Locusta migratoria

The salivary gland salivation stimulating peptide from Locusta migratoria (Lom-SG-SASP) is not a typical neuropeptide

The salivary gland salivation stimulating peptide was identified from the salivary glands of the migratory locust by its ability to stimulate cAMP production in the same tissue. The gene coding for this peptide has recently been identified and been shown to code for a precursor consisting of a signal peptide, several copies of the peptide separated by Lys–Arg doublets and a few other peptides. These data are consistent with it being a neuropeptide. However, antiserum raised to this peptide labels the acini of the salivary glands while RT-PCR only gives positive results in the salivary gland, but not in any ganglion of the central nervous system. Thus, this peptide is not a typical neuropeptide as previously assumed.

The contribution of the genomes of a termite and a locust to our understanding of insect neuropeptides and neurohormones

The genomes of the migratory locust Locusta migratoria and the termite Zootermopsis nevadensis were mined for the presence of genes encoding neuropeptides, neurohormones, and their G-protein coupled receptors (GPCRs). Both species have retained a larger number of neuropeptide and neuropeptide GPCRs than the better known holometabolous insect species, while other genes that in holometabolous species appear to have a single transcript produce two different precursors in the locust, the termite or both. Thus, the recently discovered CNMa neuropeptide gene has two transcripts predicted to produce two structurally different CNMa peptides in the termite, while the locust produces two different myosuppressin peptides in the same fashion. Both these species also have a calcitonin gene, which is different from the gene encoding the calcitonin-like insect diuretic hormone. This gene produces two types of calcitonins, calcitonins A and B. It is also present in Lepidoptera and Coleoptera and some Diptera, but absent from mosquitoes and Drosophila. However, in holometabolous insect species, only the B transcript is produced. Their putative receptors were also identified. In contrast, Locusta has a highly unusual gene that codes for a salivation stimulatory peptide. The Locusta genes for neuroparsin and vasopressin are particularly interesting. The neuroparsin gene produces five different transcripts, of which only one codes for the neurohormone identified from the corpora cardiaca. The other four transcripts code for neuroparsin-like proteins, which lack four amino acid residues, and that for that reason we called neoneuroparsins. The number of transcripts for the neoneuroparsins is about 200 times larger than the number of neuroparsin transcripts. The first exon and the putative promoter of the vasopressin genes, of which there are about seven copies in the genome, is very well-conserved, but the remainder of these genes is not. The relevance of these findings is discussed.

Feeding and the salivary gland response in free-ranging yellow-winged grasshoppers (Gastrimargus musicus)

Yellow-winged grasshoppers (Gastrimargus musicus) were captured in the field to examine the morphology and amine immunohistochemistry of their salivary glands. Fifty-eight grasshoppers were collected, with only five being males. Eight of 53 female grasshoppers had food in their crop, and the salivary glands of those insects were significantly heavier than those of grasshoppers without food in their crops. The salivary gland of the yellow-winged grasshopper was an acinar-type gland, similar to gland descriptions for other Orthoptera. The primary secretory part of acini of each gland is composed of zymogen and parietal cells. Staining patterns indicated that serotonin and dopamine could act as neurotransmitters and/or neurohormones to stimulate the glands. The pattern of staining of serotonin in the salivary gland suggested that serotonin stimulates both zymogen and parietal cells. Only the parietal cells were positively stained with dopamine. Comparing staining of glands of grasshoppers with food in their crop with the glands of grasshoppers with empty crops suggested a reduction in staining for serotonin in the latter. The differential staining pattern suggests that these amines have different roles in the salivary gland of G. musicus. The lack of difference in structure but increased mass with feeding suggests that all glands were active, but that secretion was actively occurring only in animals with the heavier glands.

Role of adipokinetic hormone in stimulation of salivary gland activities: the fire bug Pyrrhocoris apterus L. (Heteroptera) as a model species

The effect of adipokinetic hormone (Pyrap-AKH) in stimulating the function of insect salivary glands (SGs) in extra-oral digestive processes was studied in the firebug, Pyrrhocoris apterus L. (Heteroptera). The analyses were performed on samples of SGs and extracts of linden seeds, a natural source of the bug's food. The SGs from 3-day old P. apterus females (when the food ingestion culminates), primarily contained polygalacturonase (PG) enzyme activity, whereas the level of lipase, peptidase, amylase and α-glucosidase was negligible. The transcription of PG mRNA and enzymatic activity were significantly increased in SGs after Pyrap-AKH treatment. The piercing and sucking of linden seeds by the bugs stimulated the intrinsic enzymatic cocktail of seeds (lipase, peptidase, amylase, glucosidase), and moreover the activity of these enzymes was significantly enhanced when the seeds were fed on by the Pyrap-AKH treated bugs. Similarly, a significant increase in PG activity was recorded in linden seeds fed on by hormonally-treated bugs or when injected by SG extract from hormonally treated ones as compared to untreated controls. The mechanism of AKH action in SGs is unknown, but likely involves cAMP (and excludes cGMP) as a second messenger, since the content of this compound doubled in SGs after Pyrap-AKH treatment. This new and as yet undescribed function of AKH in SGs is compared with the effect of this hormone on digestive processes in the midgut elucidated earlier.

Peptidergic control of food intake and digestion in insects 1This review is part of a virtual symposium on recent advances in understanding a variety of complex regulatory processes in insect physiology and endocrinology, including development, metabolism, cold hardiness, food intake and digestion, and diuresis, through the use of omics technologies in the postgenomic era

Like all heterotrophic organisms, insects require a strict control of food intake and efficient digestion of food into nutrients to maintain homeostasis and to fulfill physiological tasks. Feeding and digestion are steered by both external and internal signals that are transduced by a multitude of regulatory factors, delivered either by neurons innervating the gut or mouthparts, or by midgut endocrine cells. The present review gives an overview of peptide regulators known to control feeding and digestion in insects. We describe the discovery and functional role in these processes for insect allatoregulatory peptides, diuretic hormones, FMRFamide-related peptides, (short) neuropeptide F, proctolin, saliva production stimulating peptides, kinins, and tachykinins. These peptides control either gut myoactivity, food intake, and (or) release of digestive enzymes. Some peptides exert their action at multiple levels, possibly having a biological function that depends on their site of delivery. Many regulatory peptides have been physically extracted from different insect species. However, multiple peptidomics, proteomics, transcriptomics, and genome sequencing projects have led to increased discovery and prediction of peptide (precursor) and receptor sequences. In combination with physiological experiments, these large-scale projects have already led to important steps forward in unraveling the physiology of feeding and digestion in insects.

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.

Dopaminergic and serotonergic innervation of cockroach salivary glands:distribution and morphology of synapses and release sites

The paired salivary glands in the cockroach are composed of acini with ion-transporting peripheral P-cells and protein-secreting central C-cells, and a duct system for the modification of the primary saliva. Secretory activity is controlled by serotonergic and dopaminergic neurons, whose axons form a dense plexus on the glands. The spatial relationship of release sites for serotonin and dopamine to the various cell types was determined by anti-synapsin immunofluorescence confocal microscopy and electron microscopy. Every C-cell apparently has only serotonergic synapses on its surface. Serotonergic and dopaminergic fibres on the acini have their release zones at a distance of ∼0.5 μm from the P-cells. Nerves between acinar lobules may serve as neurohaemal organs and contain abundant dopaminergic and few serotonergic release sites. Some dopaminergic and serotonergic release sites reside in the duct epithelium, the former throughout the duct system, the latter only in segments next to acini. These findings are consistent with the view that C-cells respond exclusively to serotonin, P-cells to serotonin and dopamine, and most duct cells only to dopamine. Moreover, the data suggest that C-cells are stimulated by serotonin released close to their surface,whereas P-cells and most duct cells are exposed to serotonin/dopamine liberated at some distance.

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.

Peptidergic control of the corpus cardiacum-corpora allata complex of locusts

The brain-corpora cardiaca-corpora allata complex of insects is the physiological equivalent of the brain-hypophysis axis of vertebrates. In locusts there is only one corpus cardiacum as a result of fusion, while most other insect species have a pair of such glands. Like the pituitary of vertebrates, the corpus cardiacum consists of a glandular lobe and a neurohemal lobe. The glandular lobe synthesizes and releases adipokinetic hormones. In the neurohemal part many peptide hormones, which are produced in neurosecretory cells in the brain, are released into the hemolymph. The corpora allata, which have no counterpart in vertebrates, synthesize and release juvenile hormones. The control of the locust corpus cardiacum-corpora allata complex appears to be very complex. Numerous brain factors have been reported to have an effect on biosynthesis and release of juvenile hormone or adipokinetic hormone. Many neuropeptides are present in nerves projecting from the brain into the corpora cardiaca-corpora allata complex, the most important ones being neuroparsins, ovary maturating parsin, insulin-related peptide, diuretic peptide, tachykinins, FLRFamides, FXPRLamides, accessory gland myotropin I, crustacean cardioactive peptide, and schistostatins. In this paper, the cellular distribution, posttranslational processing, peptide-receptor interaction, and inactivation of these peptides are reviewed. In addition, the signal transduction pathways in the release of adipokinetic hormone and juvenile hormone from, respectively, the corpora cardiaca and corpora allata are discussed.

Insect neuropeptides and their receptors new leads for medical and agricultural applications

Diversification of messenger and receptor molecules is the result of evolution; however, the principles of intercellular signaling mechanisms are very similar in all metazoans. Recent discoveries of insect peptides provide new leads for applications in medicine and agriculture. (Trends Endocrinol Metab 1997;8:321-326). (c) 1997, Elsevier Science Inc.

Peptides in the locusts, Locusta migratoria and Schistocerca gregaria

The first peptide identified in locusts was adipokinetic hormone I (AKH-I), a neurohormone mobilizing lipids from the fat body. No other locusts peptides were isolated until 1985. From then on peptide identification started to boom at such a tremendously fast rate that even specialists in the field could hardly keep track. At this moment the total number of different insect neuropeptide sequences exceeds 100. Currently, the locusts Locusta migratoria and Schistocerca gregaria are the species from which the largest number of neuropeptides has been isolated and sequenced, namely 56. Myotropic bioassays have played a major role in the isolation and subsequent structural characterization of locust neuropeptides. They have been responsible for the discovery of locustamyotropins, locustapyrokinins, locustatachykinins, locustakinin, locusta accessory gland myotropins, locustasulfakinin, cardioactive peptide, and locustamyoinhibiting peptides. Members of the myotropin peptide families have been associated with a variety of physiological activities such as myotropic activities, pheromonotropic activities, diapause induction, stimulation of cuticular melanization, diuresis, pupariation, and allatostatic activities. Recently, we have identified in Schistocerca 10 peptides belonging to the allatostatin peptide family, which inhibit peristaltic movements of the oviduct. Some of the myotropins appear to be important neurotransmitters or modulators innervating the locust oviduct, the salivary glands, the male accessory glands, and the heart, whereas others are stored in neurohemal organs until release in the hemolymph. Some myotropic peptides have been found to be releasing factors of neurohormones from the corpora cardiaca. Several peptides isolated in locusts appear to be unique to insects or arthropods; others seem to be members of peptides families spanning across phyla: two vasopressin-like peptides, FMRFamide-related peptides, Locusta diuretic hormone (CRF-like), Locusta insulin-related peptide, locustatachykinins, locustasulfakinin (gastrin/CCK-like). In a systematic structural study of neuropeptides in Locusta, several novel peptides have been isolated from the corpora cardiaca and the pars intercerebralis. They include the neuroparsins, two 6-kDa dimeric peptides, and three proteinase inhibitors. Ovary maturating parsin is the first gonadotropin identified in insects. The isolation of a peptide from an ovary extract that inhibits ovary maturation in Schistocerca gregaria is currently underway in our lab. The proteinase inhibitors, recently found to be mainly transcribed in the fat body, are believed to play a role in defense reactions of insects. Finally, a locust ion transport peptide and a peptide stimulating salivation recently can be added to this extensive list of locust peptides.