Species-specific action and distribution of tachykinin-related peptides in the foregut of the cockroaches Leucophaea maderae and Periplaneta americana

Comparative and Evolutionary Aspects of the Digestive System and Its Enteric Nervous System Control

All life forms must gain nutrients from the environment and from single cell organisms to mammals a digestive system is present. Components of the digestive system that are recognized in mammals can be seen in the sea squirt that has had its current form for around 500my. Nevertheless, in mammals, the organ system that is most varied is the digestive system, its architecture being related to the dietary niche of each species. Forms include those of foregut or hindgut fermenters, single or multicompartment stomachs and short or capacious large intestines. Dietary niches include nectarivores, folivores, carnivores, etc. The human is exceptional in that, through food preparation (>80% of human consumption is prepared food in modern societies), humans can utilize a wider range of foods than other species. They are cucinivores, food preparers. In direct descendants of simple organisms, such as sponges, there is no ENS, but as the digestive tract becomes more complex, it requires integrated control of the movement and assimilation of its content. This is achieved by the nervous system, notably the enteric nervous system (ENS) and an array of gut hormones. An ENS is first observed in the phylum cnidaria, exemplified by hydra. But hydra has no collections of neurons that could in any way be regarded as a central nervous system. All animals more complex than hydra have an ENS, but not all have a CNS. In mammals, the ENS is extensive and is necessary for control of movement, enteric secretions and local blood flow, and regulation of the gut immune system. In animals with a CNS, the ENS and CNS have reciprocal connections. From hydra to human, an ENS is essential to life.

Recent advances in neuropeptide signaling in Drosophila, from genes to physiology and behavior

This review focuses on neuropeptides and peptide hormones, the largest and most diverse class of neuroactive substances, known in Drosophila and other animals to play roles in almost all aspects of daily life, as w;1;ell as in developmental processes. We provide an update on novel neuropeptides and receptors identified in the last decade, and highlight progress in analysis of neuropeptide signaling in Drosophila. Especially exciting is the huge amount of work published on novel functions of neuropeptides and peptide hormones in Drosophila, largely due to the rapid developments of powerful genetic methods, imaging techniques and innovative assays. We critically discuss the roles of peptides in olfaction, taste, foraging, feeding, clock function/sleep, aggression, mating/reproduction, learning and other behaviors, as well as in regulation of development, growth, metabolic and water homeostasis, stress responses, fecundity, and lifespan. We furthermore provide novel information on neuropeptide distribution and organization of peptidergic systems, as well as the phylogenetic relations between Drosophila neuropeptides and those of other phyla, including mammals. As will be shown, neuropeptide signaling is phylogenetically ancient, and not only are the structures of the peptides, precursors and receptors conserved over evolution, but also many functions of neuropeptide signaling in physiology and behavior.

The first brain: Species comparisons and evolutionary implications for the enteric and central nervous systems

BACKGROUND

The enteric nervous system (ENS) and the central nervous system (CNS) of mammals both contain integrative neural circuitry and similarities between them have led to the ENS being described as the brain in the gut.

PURPOSE

To explore relationships between the ENS and CNS across the animal kingdom. We found that an ENS occurs in all animals investigated, including hydra, echinoderms and hemichordates that do not have a CNS. The general form of the ENS, which consists of plexuses of neurons intrinsic to the gut wall and an innervation that controls muscle movements, is similar in species as varied and as far apart as hydra, sea cucumbers, annelid worms, octopus and humans. Moreover, neurochemical similarities across phyla imply a common origin of the ENS. Investigation of extant species suggests that the ENS developed in animals that preceded the division that led to cnidaria (exemplified by hydra) and bilateria, which includes the vertebrates. The CNS is deduced to be a bilaterian development, later than the divergence from cnidaria. Consistent with the ENS having developed independent of the CNS, reciprocal connections between ENS and CNS occur in mammals, and separate neurons of ENS and CNS origin converge on visceral organs and prevertebral ganglia. We conclude that an ENS arose before and independently of the CNS. Thus the ENS can be regarded as the first brain.

Expression and functional characterization of tachykinin-related peptides in the blood-feeding bug, Rhodnius prolixus

Tachykinins (tachykinin-related peptides, TRPs) are multifunctional neuropeptides that have widespread distribution in the central nervous system (CNS) and in the gastrointestinal tract of many insects, and most have been shown to stimulate contractions of visceral muscles. Invertebrate TRPs carry a characteristic conserved C-terminal pentapeptide (FXGXR-amide) and most of them share some amino acid sequence similarities (approx. 45%) with the vertebrate and mammalian tachykinin family. We have functionally characterized the tachykinins in R. prolixus (Rhopr-TKs) and partially cloned the transcript that encodes for the peptide precursor. The transcript encodes 8 Rhopr-TKs, 7 of which are unique with Rhopr-TK 5 having 2 copies. The spatial distribution analysis of the Rhopr-TK transcript indicates that the highest expression levels are in the CNS, but transcript expression is also associated with salivary glands, fat body, dorsal vessel, and the various gut compartments. Rhopr-TK 1, 2 and 5 significantly increase the frequency and amplitude of peristaltic contractions of the salivary glands. Hindgut muscle also displayed a dose-dependent increase in basal tonus in response to Rhopr-TK1, 2 and 5. TK-like immunoreactivity was seen in a small group of processes that are situated on the lateral margins of the hindgut. Interestingly, kinin-like immunoreactivity is seen in immunoreactive processes on the lateral margin of the hindgut as well as fine processes covering the entire hindgut. Co-localization studies show that TK-like staining is always co-localized with kinin-like immunoreactivity, whereas kinin-like staining is seen in the fine processes that are devoid of TK-like immunoreactivity indicating that TKs are most likely released together with kinins to act on the hindgut. Rhopr-Kinin 2 is a potent stimulator of hindgut muscle contraction in R. prolixus. Addition of Rhopr-Kinin 2 and Rhopr-TK 2 to the hindgut leads to a contraction that was additive of the effects of Rhopr-Kinin 2 and Rhopr-TK 2 alone.

Calcitonin-like peptide hormone (CT/DH) in the frontal ganglia as a feeding regulatory peptide of the silkworm, Bombyx mori

The frontal ganglion (FrG) in insects contributes to the modulation of feeding behavior via the regulation of foregut contraction and other neural networks. Profiling the peptides of the FrG is important to understand endocrine regulation of feeding behavior in insects. High-resolution spiral matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) identified an ion peak, corresponding to calcitonin-like diuretic hormone 31 (CT/DH) in the FrG of silkworm Bombyx mori larvae. RT-PCR confirmed that CT/DH is expressed in the FrG, as are other peptide hormones, including allatoregulatory peptides. A feeding latency assay using synthetic CT/DH revealed that it increases the time to the initiation of feeding in a dose-dependent manner. These data indicate that CT/DH is a candidate regulatory peptide that modulates the feeding behavior of B. mori.

bHLH proneural genes as cell fate determinants of entero-endocrine cells, an evolutionarily conserved lineage sharing a common root with sensory neurons

Entero-endocrine cells involved in the regulation of digestive function form a large and diverse cell population within the intestinal epithelium of all animals. Together with absorptive enterocytes and secretory gland cells, entero-endocrine cells are generated by the embryonic endoderm and, in the mature animal, from a pool of endoderm derived, self-renewing stem cells. Entero-endocrine cells share many structural/functional and developmental properties with sensory neurons, which hints at the possibility of an ancient evolutionary relationship between these two cell types. We will survey in this article recent findings that emphasize the similarities between entero-endocrine cells and sensory neurons in vertebrates and insects, for which a substantial volume of data pertaining to the entero-endocrine system has been compiled. We will then report new findings that shed light on the specification and morphogenesis of entero-endocrine cells in Drosophila. In this system, presumptive intestinal stem cells (pISCs), generated during early metamorphosis, undergo several rounds of mitosis that produce the endocrine cells and stem cells (ISCs) with which the fly is born. Clonal analysis demonstrated that individual pISCs can give rise to endocrine cells expressing different types of peptides. Immature endocrine cells start out as unpolarized cells located basally of the gut epithelium; they each extend an apical process into the epithelium which establishes a junctional complex and apical membrane specializations contacting the lumen of the gut. Finally, we show that the Drosophila homolog of ngn3, a bHLH gene that defines the entero-endocrine lineage in mammals, is expressed and required for the differentiation of this cell type in the fly gut.

Calcitonin receptor 1 (AedaeGPCRCAL1) hindgut expression and direct role in myotropic action in females of the mosquito Aedes aegypti (L.)

In anautogenous mosquitoes such as Aedes aegypti females the calcitonin-like diuretic hormone 31 (DH₃₁) stimulates natriuretic fluid excretion from the Malpighian tubules (MTs) after a blood meal. We previously cloned and functionally characterized AedaeGPCRcal1 from A. aegypti, the ortholog of the Drosophila melanogaster DH₃₁ receptor and immunolocalized it in the MTs. However, localization of the calcitonin receptor-like receptor 1 (GPCRCAL1) in the hindgut of any insect is unknown, and specifically, knowledge on its role in hindgut contraction in response to Aedae-DH₃₁ peptide is lacking. We analyzed the expression of AedaeGPCRCAL1 in hindgut by western blot and immunohistochemistry, and evaluated its role in hindgut contractility by application of Aedae-DH₃₁ before and after receptor RNA interference (RNAi). The receptor was detected as a 73 kDa band in western blots of hindgut and immunofluorescence revealed the receptor was expressed in hindgut circular and longitudinal muscles but not in the hindgut epithelial cells. In vitro, incubation in 1 μM solution of Aedae-DH₃₁ peptide significantly increased the hindgut contraction frequency in normal mosquitoes. Hindguts from females treated with AedaeGPCRcal1 dsRNA and incubated with DH₃₁ showed a reduction of 50% percent in their contraction frequency with respect to controls. These results suggest that DH₃₁ hormone released from the brain post-blood meal has a direct and coordinative action on the excretory system, MTs and hindgut, by which AedaeGPCRCAL1 signaling stimulates MT primary urine secretion and hindgut contraction resulting in rapid postprandial fluid excretion.

Discovery and functional study of a novel crustacean tachykinin neuropeptide

Tachykinin-related peptide (TRP) refers to a large and structurally diverse family of neuropeptides found in vertebrate and invertebrate nervous systems. These peptides have various important physiological functions, from regulating stress in mammals to exciting the pyloric (food filtering) rhythm in the stomatogastric nervous system (STNS) of decapod crustaceans. Here, a novel TRP, which we named CalsTRP (Callinectes sapidus TRP), YPSGFLGMRamide (m/z 1026.52), was identified and de novo sequenced using a multifaceted mass spectrometry-based platform in both the central nervous system (CNS) and STNS of C. sapidus. We also found, using isotopic formaldehyde labeling, that CalsTRP in the C. sapidus brain and commissural ganglion (CoG) was up-regulated after food-intake, suggesting that TRPs in the CNS and STNS are involved in regulating feeding in Callinectes. Using imaging mass spectrometry, we determined that the previously identified CabTRP Ia (APSGFLGMRamide) and CalsTRP were co-localized in the C. sapidus brain. Lastly, our electrophysiological studies show that bath-applied CalsTRP and CabTRP Ia each activates the pyloric and gastric mill rhythms in C. sapidus, as shown previously for pyloric rhythm activation by CabTRP Ia in the crab Cancer borealis. In summary, the newly identified CalsTRP joins CabTRP Ia as a TRP family member in the decapod crustacean nervous system, whose actions include regulating feeding behavior.

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.

Identification of a tachykinin-related peptide with orexigenic properties in the German cockroach

A number of evidences suggest that tachykinin-related peptides (TRPs) of insects can stimulate food consumption after being released from the midgut to the hemolymph. The idea of the present work has been to test this hypothesis in the anautogenous cockroach Blattella germanica. First, we have identified the peptide LemTRP-1 (APSGFLGVR-NH(2)) from brain extracts, by means of an ELISA developed with a polyclonal antibody against this peptide. ELISA studies have also shown that, whereas brain LemTRP-1 levels were fairly constant, midgut levels increase to a maximum on day 3 after adult emergence, falling thereafter until the end of the gonadotrophic cycle. Interestingly, maximum values of food consumption are concomitant with the decrease of LemTRP-1 immunoreactivity in the midgut. Furthermore, starvation decreases LemTRP-1 immunoreactivity in midgut, whereas in the hemolymph it increases. Finally, injection of synthetic LemTRP-1 to adult females significantly stimulates food consumption. The whole observations suggest that LemTRP-1 is released from the midgut to the hemolymph when sustained food consumption is required to maintain vitellogenesis at the highest levels, and that LemTRP-1 in the hemolymph stimulates food consumption in these days.

The effects of Calliphora vomitoria Tachykinin-I and the FMRFamide-related peptide Perisulfakinin on female Phormia regina crop contractions, in vitro

The dipteran crop is an elaborate diverticulation of the foregut that serves as an important food reservoir and feeding regulator. Peptidergic innervation has been associated with the crop of the blow fly Phormia regina and myotropic neuropeptides have been previously demonstrated to affect crop contraction rates, in vitro. The blow fly peptide, callitachykinin-1 was found to increase the rate of contractions and alter the contractile morphology of the P. regina crop, in vitro. The cockroach peptide perisulfakinin, however, had no measurable affect on crop contractions.

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

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

Myotropic effect of helicokinins, tachykinin-related peptides and Manduca sexta allatotropin on the gut of Heliothis virescens (Lepidoptera: Noctuidae)

Different insect neuropeptides (helicokinins, tachykinin-related and allatoregulating peptides) were investigated with regard to their myostimulatory effects using whole-gut preparations isolated from fifth instar Heliothis virescens larvae. The experiments demonstrated that representatives of all three peptide families are able to induce and amplify gut contractions in this species in a dose-dependent manner. Structure-activity studies (alanine scan, D-amino acid scan and truncated analogues) with the helicokinin Hez-K1 supported the finding, that the core sequence for biological activity of kinins is the amidated C-terminal pentapeptide (FSPWG-amide). Similar investigations with insect tachykinin isolated from Leucophaea madera (Lem-TRP1) revealed that the minimum sequence evoking a physiological gut response in H. virescens is the amidated hexapeptide (GFLGVR-amide), which represents the conserved amino acid sequence for Leucophaea TRPs in general. The peptide concentration causing a half-maximal gut contraction (EC(50)) for Lem-TRP1 was about 26 nM. Although the potency of Lem-TRP1 was 9-fold lower compared with Hez-KI (EC(50): 3 nM), the maximal tension of the gut obtained with Lem-TRP1 was 1.7-fold higher compared with Hez-KI. The EC(50) of Manduca sexta allatotropin (Mas-AT; 79 nM) was of lowest potency among all three peptides tested. In a pharmacological study, co-incubation experiments with Lem-TRP1, Hez-KI or Mas-AT and compounds interfering with signal transduction pathways were employed to investigate the mode of action of the myotropic effects of these peptides. Cadmium and the protein kinase C (PKC) inhibitor tamoxifen attenuated the contractile effects of all three peptides tested. The data suggest that in the gut muscle of H. virescens the myotropic peptides bind to G-protein-coupled receptors that cause contraction by promoting the entry of extracellular calcium mediated by a PKC involved pathway.

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.

Intestinal peptides as circulating hormones: release of tachykinin-related peptide from the locust and cockroach midgut

Tachykinin-related peptides (TRPs) in the locust Locusta migratoria and the cockroach Leucophaea maderae have stimulatory effects on some muscles that are not innervated by TRP-containing neurons. Thus, these tissues may be affected by circulating TRPs. Here, we have investigated whether the midgut is the source of circulating TRPs. TRP-immunoreactive material in the locust midgut is found only in the endocrine cells of the gut epithelium. In both species of insect, the endocrine cells contain several isoforms of TRPs, as determined by immunocytochemistry and a combination of chromatography (HPLC) and enzyme immunoassay (ELISA). The release of TRPs was investigated by ELISA using isolated midguts of the locust and cockroach. Elevated levels of K(+) in the bathing saline induced the release of TRP from the midgut of both species. To examine the release of TRPs into the circulation in vivo, we measured haemolymph levels of TRPs in fed and starved locusts. The concentration of TRP-immunoreactive material in fed locusts was estimated to be 0.15 nmol l(−1), and this increased approximately fourfold in insects starved for 24 h. In accordance with this observation, the content of TRP-immunoreactive material in the midgut was lower in starved locusts than in fed locusts. Although part of the increased blood concentration of TRPs may be due to reduced blood volume, our data suggest that TRPs are released as hormones from the midgut of the locust and cockroach and that this release may be linked to nutritional status.

Myostimulatory neuropeptides in cockroaches: structures, distribution, pharmacological activities, and mimetic analogs

In this brief overview we give the historical background on the discovery of myostimulatory neuropeptides in cockroaches. Related peptides were later found in other insect groups as well. We summarize the current knowledge on primary structures, localization, physiological and pharmacological effects of the different cockroach neuropeptides, including kinins, sulfakinins, pyrokinins, tachykinin-related peptides, periviscerokinins, corazonin, and proctolin. In addition, we briefly comment on the development of mimetic pseudopeptide analogs in the context of their possible use in insect pest management.

Efficacy of native FXPRLamides (pyrokinins) and synthetic analogs on visceral muscles of the American cockroach

Six pyrokinins, members of a widely distributed neuropeptide family in insects (FXPRLamides), have been identified from the American cockroach. Five of these peptides, Pea-PK-1-5, were tested in different myotropic bioassays, including hyperneural muscle, hindgut, foregut and oviduct. Among these muscles, the hyperneural muscle exhibited the highest sensitivity to pyrokinin applications. The efficacy of the different pyrokinins differed dramatically. No muscle specific effectiveness was obtained; the ranking order in all muscle assays was as follows: PK-1>PK-4>PK-3>PK-2>PK-5. Testing of synthetic analogs revealed the importance of the amino acid at the variable -4 position of the C-terminus. PK-5, the only one of the five tested peptides which is stored in abdominal perisympathetic organs, has probably no myotropic function at all. This is further evidence that these neurohemal release sites are not necessary to compensate the open circulatory system of insects but have rather specific functions which are totally unknown as yet.

Cardioacceleratory action of tachykinin-related neuropeptides and proctolin in two coleopteran insect species

Several cardioactive peptides have been identified in insects and most of them are likely to act on the heart as neurohormones. Here we have investigated the cardioactive properties of members of a family of insect tachykinin-related peptides (TRPs) in heterologous bioassays with two coleopteran insects, Tenebrio molitor and Zophobas atratus. Their effects were compared with the action of the pentapeptide proctolin. We tested the cardiotropic activity of LemTRP-4 isolated from the midgut of the cockroach Leucophaea maderae, CavTK-I and CavTK-II isolated from the blowfly Calliphora vomitoria. The semi-isolated hearts of the two coleopteran species were strongly stimulated by proctolin. We observed a dose dependent increase in heartbeat frequency (a positive chronotropic effect) and a decrease in amplitude of contractions (a negative inotropic effect). In both beetles the TRPs are less potent cardiostimulators and exert lower maximal frequency responses than proctolin. LemTRP-4 applied at 10(-9)-10(-6) M was cardiostimulatory in both species inducing an increase of heart beat frequency. The amplitude of contractions was stimulated only in Z. atratus. CavTK-I and CavTK-II also exerted cardiostimulatory effects in Z. atratus at 10(-9)-10(-6) M. Both peptides stimulated the frequency, but only CavTK-II increased the amplitude of the heart beat. In T. molitor, however, the CavTKs induced no significant effect on the heart. Immunocytochemistry with antisera to the locust TRPs LomTK-I and LomTK-II was employed to identify the source of TRPs acting on the heart. No innervation of the heart by TRP immunoreactive axons could detected, instead it is possible that TRPs reach the heart by route of the circulation. The likely sources of circulating TRPs in these insects are TRP-immunoreactive neurosecretory cells of the median neurosecretory cell group in the brain with terminations in the corpora cardiaca and endocrine cells in the midgut. In conclusion, LemTRP-4, CavTK-I and CavTK-II are less potent cardiostimulators than proctolin and also exert stimulatory rather than inhibitory action on amplitude of contractions. The differences in the responses to proctolin and TRPs suggest that the peptides regulate heart activity by different mechanisms.

Periviscerokinins in cockroaches: release, localization, and taxon-specific action on the hyperneural muscle

The periviscerokinins of Periplaneta americana (Pea-PVKs) were the first neuropeptides directly isolated from perisympathetic organs (PSOs), neurohemal swellings of the transverse and median nerves of insects. It has been demonstrated that Pea-PVK-1 release can be induced from the abdominal PSOs of P. americana by in vitro depolarization. A myotropic effect of Pea-PVK-1 on the hyperneural muscle is restricted to blattid cockroaches, whereas proctolin induces contractions of this muscle in all cockroach species investigated. The location and morphology of Pea-PVK-1-like immunoreactive neurons in species of different cockroach taxa are very similar to those previously described for P. americana. Pea-PVK-1-like immunoreactivity is restricted to cells of the abdominal ganglia which constitute a neurohemal system and project via the median nerve to the abdominal PSOs. Despite interspecific differences in the topography of the transverse nerves of the terminal ganglion, Pea-PVK-1-like immunoreactive fibers always innervate transversal nerves VII and VIII of the terminal ganglion. The results suggest that PVKs act as neurohormones throughout the cockroaches, although they may have different effect(s) depending on the species.

The evolution of neuronal circuits underlying species-specific behavior

The nervous system is evolutionarily conservative compared to the peripheral appendages that it controls. However, species-specific behaviors may have arisen from very small changes in neuronal circuits. In particular, changes in neuromodulatory systems may allow multifunctional circuits to produce different sets of behaviors in closely related species. Recently, it was demonstrated that even species differences in complex social behavior may be attributed to a change in the promoter region of a single gene regulating a neuromodulatory action.

Tachykinin-related peptides in invertebrates: a review

Peptides with sequence similarities to members of the tachykinin family have been identified in a number of invertebrates belonging to the mollusca, echiuridea, insecta and crustacea. These peptides have been designated tachykinin-related peptides (TRPs) and are characterized by the preserved C-terminal pentapeptide FX1GX2Ramide (X1 and X2 are variable residues). All invertebrate TRPs are myostimulatory on insect hindgut muscle, but also have a variety of additional actions: they can induce contractions in cockroach foregut and oviduct and in moth heart muscle, trigger a motor rhythm in the crab stomatogastric ganglion, depolarize or hyperpolarize identified interneurons of locust and the snail Helix and induce release of adipokinetic hormone from the locust corpora cardiaca. Two putative TRP receptors have been cloned from Drosophila; both are G-protein coupled and expressed in the nervous system. The invertebrate TRPs are distributed in interneurons of the CNS of Limulus, crustaceans and insects. In the latter two groups TRPs are also present in the stomatogastric nervous system and in insects endocrine cells of the midgut display TRP-immunoreactivity. In arthropods the distribution of TRPs in neuronal processes of the brain displays similar patterns. Also in coelenterates, flatworms and molluscs TRPs have been demonstrated in neurons. The activity of different TRPs has been explored in several assays and it appears that an amidated C-terminal hexapeptide (or longer) is required for bioactivity. In many invertebrate assays the first generation substance P antagonist spantide I is a potent antagonist of invertebrate TRPs and substance P. Locustatachykinins stimulate adenylate cyclase in locust interneurons and glandular cells of the corpora cardiaca, but in other tissues the putative second messenger systems have not yet been identified. The heterologously expressed Drosophila TRP receptors coupled to the phospholipase C pathway and could induce elevations of inositol triphosphate. The structures, distributions and actions of TRPs in various invertebrates are compared and it is concluded that the TRPs are multifunctional peptides with targets both in the central and peripheral nervous system and other tissues, similar to vertebrate tachykinins. Invertebrate TRPs may also be involved in developmental processes.