Insect Neurohormone Research Articles

Octopamine is involved in TRP-induced thermopreference responses in American cockroach

Insects’ thermoregulatory processes depend on thermosensation and further processing of thermal information in the nervous system. It is commonly known that thermosensation involves thermoreceptors, including members of the TRP receptor family, but the involvement of neurotransmitters in thermoregulatory pathways remains unstudied. We conducted test to determine whether octopamine, a biogenic amine that acts as a neurotransmitter and neurohormone in insects, is involved in TRP-induced thermoregulatory responses in Periplaneta americana. We used capsaicin, an activator of the heat-sensitive TRP channel, Painless, to induce thermoregulatory response in cockroaches. Then, we evaluated the behavioural (thermal preferences and grooming), physiological (heart rate) and biochemical responses of insects to capsaicin, octopamine and phentolamine – octopaminergic receptor blocker. Capsaicin, similar to octopamine, increased cockroaches’ grooming activity and heart rate. Moreover, octopamine level and protein kinase A (PKA) activity significantly increased after capsaicin treatment. Blocking octopaminergic receptors with phentolamine diminished cockroaches’ response to capsaicin – thermoregulatory behaviour, grooming and heart rate were abolished. The results indicate that octopamine is a neurotransmitter secreted in insects after the activation of heat receptors.

Structure-dependent receptor subtype selectivity and G protein subtype preference of heterocyclic agonists in heterologously expressed silkworm octopamine receptors

(R)-Octopamine (OA), a major invertebrate biogenic amine, plays an important role in a wide variety of physiological processes as a neurohormone, neuromodulator, and neurotransmitter in insects. OA receptors (OARs) are class A G protein-coupled receptors that specifically bind OA to activate downstream signaling cascades by coupling to G proteins and presumably other regulatory proteins. These receptors are broadly classified as α- and β-adrenergic-like OARs (α- and β-ALOARs). OARs are considered important targets of insecticides and acaricides. In the present study, we examined the actions of an array of 13 heterocyclic OAR agonists with the moieties that correspond to the phenyl group and the basic nitrogen atom of OA on α- and β-ALOARs from the silkworm (Bombyx mori) and the signaling pathways activated through these actions. The results indicated that these compounds display structure-dependent receptor subtype selectivity and G protein subtype preference, underscoring the need to determine which subtype and signaling pathway mediates toxicologically relevant effects for the efficient discovery of novel pest control chemicals. The results of insecticidal assays using B. mori larvae suggested that the activation of signal transduction pathways via α-ALOARs might be mainly responsible for the toxicological effects of the heterocycles.

The Insect Type 1 Tyramine Receptors: From Structure to Behavior.

Tyramine is a neuroactive compound that acts as neurotransmitter, neuromodulator, and neurohormone in insects. Three G protein-coupled receptors, TAR1-3, are responsible for mediating the intracellular pathway in the complex tyraminergic network. TAR1, the prominent player in this system, was initially classified as an octopamine receptor which can also be activated by tyramine, while it later appeared to be a true tyramine receptor. Even though TAR1 is currently considered as a well-defined tyramine receptor and several insect TAR1s have been characterized, a defined nomenclature is still inconsistent. In the last years, our knowledge on the structural, biochemical, and functional properties of TAR1 has substantially increased. This review summarizes the available information on TAR1 from different insect species in terms of basic structure, its regulation and signal transduction mechanisms, and its distribution and functions in the brain and the periphery. A special focus is given to the TAR1-mediated intracellular signaling pathways as well as to their physiological role in regulating behavioral traits. Therefore, this work aims to correlate, for the first time, the physiological relevance of TAR1 functions with the tyraminergic system in insects. In addition, pharmacological studies have shed light on compounds with insecticidal properties having TAR1 as a target and on the emerging trend in the development of novel strategies for pest control.

Adipokinetic hormone enhances CarE-mediated chlorpyrifos resistance in the brown planthopper, Nilaparvata lugens.

Adipokinetic hormone (AKH), the principal stress-responsive neurohormone in insects, has been implicated in insect responses to insecticides. However, the functionality of AKH and its mode of signalling in insecticide resistance are unknown. Herein, we demonstrated that the enhanced activity of carboxylesterases (CarEs) is involved in the chlorpyrifos resistance in Nilaparvata lugens [brown planthopper (BPH)]. Chlorpyrifos exposure significantly induced the expression of AKH and its receptor AKHR in the susceptible BPH (Sus), and these two AKH signalling genes were over-expressed in the chlorpyrifos-resistant strain (Res) compared to Sus. RNA interference (RNAi) against AKH or AKHR decreased the CarE activity and suppressed the BPH's resistance to chlorpyrifos in Res. Conversely, AKH peptide injection elevated the CarE activity and enhanced the BPH's survival against chlorpyrifos in Sus. Furthermore, five CarE genes were identified to be positively affected by the AKH pathway using RNAi and AKH injection. Among these CarE genes, CarE and Esterase E4-1 were found to be over-expressed in Res compared to Sus, and knockdown of either gene decreased the BPH's resistance to chlorpyrifos. In conclusion, AKH plays a role in enhancing chlorpyrifos resistance in the BPH through positive influence on the expression of CarE genes and CarE enzyme activity.

Corazonin Signaling Is Required in the Male for Sperm Transfer in the Oriental Fruit Fly Bactrocera dorsalis.

Corazonin (Crz) is a widely distributed neuropeptide (or neurohormone) in insects with diverse physiological functions. The present study aimed to reveal the functions of Crz and its receptor (CrzR) in the regulation of sexual behavior and fertility in male Bactrocera dorsalis. Tissue-specific expression analyses showed that the BdCrz transcript was most abundant in the central nervous system (CNS), and the BdCrzR transcript was most abundant in both the fat body and CNS. Immunochemical localization confirmed that three pairs of Crz-immunoreactive neurons are located in the dorsolateral protocerebrum region of male adult brain. Importantly, RNAi-mediated Crz knockdown lengthened mating duration in males, and knockdown of Crz or CrzR strongly decreased male fertility in the following 3 days, while the courtship behavior and mating efficiency were not affected. The reduced number of sperm in the reproductive organs of mated females indicated that Crz knockdown in males reduced sperm transfer. The findings of this study indicate that Crz contributes to the reproductive physiology of the oriental fruit fly B. dorsalis by regulating sperm transfer in male adults.

Differential effects of predator cues versus activation of fight-or-flight behaviour on reproduction in the cricket Gryllus texensis

How prey animals determine predation risk remains uncertain. We propose that one signal of high predation risk is repeated activation of fight-or-flight behaviour. We activated escape runs in the cricket Gryllus texensis by blowing air on the cerci. Escape runs were induced for 5 min, three times per day, three times per week for 4 weeks. Repeated fight-or-flight behaviour led to a loss in mass and decreased life span, suggesting a decline in somatic maintenance. However, there was an increase in egg laying, which we interpret as terminal reproductive investment. Stress responses remained robust. Octopamine (OA), a stress neurohormone in insects, increased in concentration in the haemolymph after running, and the magnitude of the increase was the same even after repeated activation (i.e. there was no habituation of the response). There was also no increase in basal OA haemolymph levels. In a second experiment, crickets were exposed to a mantid (predator, Tenodera sinensis), a walking stick (nonpredator, Carausius morosus), or an empty container. None of the crickets exhibited fight-or-flight behaviour. However, mantid-exposed crickets decreased egg laying. There was no decrease in life span or mass. There was no change in basal levels of OA, or in the magnitude of the OA increase after running. These results are consistent with the hypothesis that repeated fight-or-flight behaviour induces reproductive responses that would be adaptive for a shortened life span. These responses differ from those produced by predator cues alone. Even short-lived animals, such as crickets, appear to alter reproduction depending on the relative predation risk and their residual reproductive potential.

Neuronal distribution of tyramine and the tyramine receptor AmTAR1 in the honeybee brain

AbstractTyramine is an important neurotransmitter, neuromodulator, and neurohormone in insects. In honeybees, it is assumed to have functions in modulating sensory responsiveness and controlling motor behavior. Tyramine can bind to two characterized receptors in honeybees, both of which are coupled to intracellular cAMP pathways. How tyramine acts on neuronal, cellular and circuit levels is unclear. We investigated the spatial brain expression of the tyramine receptor AmTAR1 using a specific antibody. This antibody detects a membrane protein of the expected molecular weight in western blot analysis. In honeybee brains, it labels different structures which process sensory information. Labeling along the antennal nerve, in projections of the dorsal lobe and in the gnathal ganglion suggest that tyramine receptors are involved in modulating gustatory and tactile perception. Furthermore, the ellipsoid body of the central complex and giant synapses in the lateral complex show AmTAR1‐like immunoreactivity (AmTAR1‐IR), suggesting a role of this receptor in modulating sky‐compass information and/or higher sensor‐motor control. Additionally, intense signals derive from the mushroom bodies, higher‐order integration centers for olfactory, visual, gustatory and tactile information. To investigate whether AmTAR1‐expressing brain structures are in vicinity to tyramine releasing sites, a specific tyramine antibody was applied. Tyramine‐like labeling was observed in AmTAR1‐IR positive structures, although it was sometimes weak and we did not always find a direct match of ligand and receptor. Moreover, tyramine‐like immunoreactivity was also found in brain regions without AmTAR1‐IR (optic lobes, antennal lobes), indicating that other tyramine‐specific receptors may be expressed there.

Neuronal distribution of tyramine and the tyramine receptor AmTAR1 in the honeybee brain

Tyramine is an important neurotransmitter, neuromodulator, and neurohormone in insects. In honeybees, it is assumed to have functions in modulating sensory responsiveness and controlling motor behavior. Tyramine can bind to two characterized receptors in honeybees, both of which are coupled to intracellular cAMP pathways. How tyramine acts on neuronal, cellular and circuit levels is unclear. We investigated the spatial brain expression of the tyramine receptor AmTAR1 using a specific antibody. This antibody detects a membrane protein of the expected molecular weight in western blot analysis. In honeybee brains, it labels different structures which process sensory information. Labeling along the antennal nerve, in projections of the dorsal lobe and in the gnathal ganglion suggest that tyramine receptors are involved in modulating gustatory and tactile perception. Furthermore, the ellipsoid body of the central complex and giant synapses in the lateral complex show AmTAR1-like immunoreactivity (AmTAR1-IR), suggesting a role of this receptor in modulating sky-compass information and/or higher sensor-motor control. Additionally, intense signals derive from the mushroom bodies, higher-order integration centers for olfactory, visual, gustatory and tactile information. To investigate whether AmTAR1-expressing brain structures are in vicinity to tyramine releasing sites, a specific tyramine antibody was applied. Tyramine-like labeling was observed in AmTAR1-IR positive structures, although it was sometimes weak and we did not always find a direct match of ligand and receptor. Moreover, tyramine-like immunoreactivity was also found in brain regions without AmTAR1-IR (optic lobes, antennal lobes), indicating that other tyramine-specific receptors may be expressed there.

Cardioregulatory Functions of Neuropeptides and Peptide Hormones in Insects.

Neuropeptides and peptide hormones from non-neuronal tissues play important roles in the regulation of insect life. In recent years, the rapid development of analytical techniques has contributed to the discovery of more than 30 families of peptide neurohormones that differ structurally and functionally. Although the discovery of the first neuropeptide occurred almost forty years ago, our knowledge about their full mode of activities, primary structures, synthesis, interactions with receptors or places of action increases gradually and there is still much to unravel. However, one thing is certain. Neuropeptides perform an extremely diverse range of activities. One neuropeptide can affect physiology in different ways. The neuropeptides can act as neurotransmitters, co-transmitters as well as neuromodulators. Most of these molecules have diverse pleiotropic activities on different tissues and organs. Their mode of action includes allatotropic, myotropic, cardiotropic or gonadotropic effects. Activity of some of them is conserved among most of insect species, indicating crucial roles in insect physiology and age of these systems. On the other hand, activity of other neuropeptides and peptide hormones is highly diverse, depending on species or even stages of development. This may indicate that some compounds have taken over the function of others. Insect heart work is regulated in a very complex manner. Myocardium activity undergoes regulation both, by nervous and hormonal way. What is important is that these same compounds can influent on heart as both nervous and hormonal factors. For that reason, the regulation of myocardium is still unclear. In this paper, we summarize the existing knowledge regarding cardioactivity and the involvement of insect neurohormones and some peptide hormones from non-neural tissues to regulation of insect myocardium.

Differences in the phototaxis of pollen and nectar foraging honey bees are related to their octopamine brain titers

The biogenic amine octopamine is an important neuromodulator, neurohormone and neurotransmitter in insects. We here investigate the role of octopamine signaling in honey bee phototaxis. Our results show that groups of bees differ naturally in their phototaxis. Pollen forgers display a lower light responsiveness than nectar foragers. The lower phototaxis of pollen foragers coincides with higher octopamine titers in the optic lobes but is independent of octopamine receptor gene expression. Increasing octopamine brain titers reduces responsiveness to light, while tyramine application enhances phototaxis. These findings suggest an involvement of octopamine signaling in honey bee phototaxis and possibly division of labor, which is hypothesized to be based on individual differences in sensory responsiveness.

UCP4 expression changes in larval and pupal fat bodies of the beetle Zophobas atratus under adipokinetic hormone treatment

We investigated the influence of adipokinetic hormone (AKH), an insect neurohormone, on uncoupling protein 4 (ZaUCP4) expression and activity in larval and pupal fat body mitochondria of the beetle Zophobas atratus in relation to intermediary metabolism. Homologous Tenmo-AKH was administered to the beetle larvae and pupae as either a single dose or as two doses of 20pmol during a 24h interval. In the larval and pupal fat bodies, downregulation of ZaUCP4 expression at the mRNA and protein levels was observed 24h and 48h after AKH treatment, respectively. In both developmental stages, ZaUCP4 activity was lowered in fat body mitochondria 48h after AKH treatment. In the AKH-injected larvae, changes in ZaUCP4 expression were accompanied by the mobilization of carbohydrate reserves, no change in the concentration of total lipids and an increase in the free fatty acid level. In contrast, AKH had no effect on carbohydrate metabolism in the pupal fat body but induced lipid mobilization. It seems that AKH influences ZaUCP4 expression by triggering multiple events and that it has different physiological roles in controlling intermediary metabolism in the fat body of the beetle larvae and pupae.

Characterization of tyramine β-hydroxylase, an enzyme upregulated by stress in Periplaneta americana

Octopamine (OA) is an important neuroactive substance that modulates several physiological functions and behaviors of various invertebrate species. This biogenic monoamine, structurally related to noradrenaline, acts as a neurotransmitter, a neuromodulator, or a neurohormone in insects. The tyramine β-hydroxylase (TBH) catalyzes the last step in OA biosynthesis and thus plays a key role in the regulation of synthesis and secretion of OA in neurons. The aim of this study was to characterize TBH in the cockroach Periplaneta americana and to get a better understanding of its regulation under stress conditions in this insect. First of all, five full-length cDNAs encoding TBH isoforms were cloned from the nerve cord of the physiological model P. americana. PaTBH transcripts were found mainly expressed in nervous tissues and in octopaminergic dorsal unpaired median neurons. In addition, a new ELISA assay was developed so as to allow determination of both OA level and TBH activity in stressed cockroaches. Mechanical stressful stimulation led to a significant increase in TBH activity after 1 and 24 h, with a higher induction after 1 h than after 24 h. Thus, TBH could be considered as a promising biomarker of stress in insects rather than OA.

Bursicon, the tanning hormone of insects: recent advances following the discovery of its molecular identity

Bursicon was identified in 1965 as a peptide neurohormone that initiates the tanning of the insect cuticle immediately after the shedding of the old one during the final stages of the molting process. Its molecular identity as an approximately 30 kDa bioactive heterodimer consisting of two cystine knot proteins resisted elucidation for 43 years. The sequence of the two bursicon subunits is highly conserved among arthropods, and this conservation extends even to echinoderms. We review the efforts leading to bursicon's characterization, the identification of its leucine-rich repeat-containing, G protein-coupled receptor (LGR2), and the progress towards revealing its various functions. It is now clear that bursicon regulates different aspects of wing inflation in Drosophila melanogaster besides being involved at various points in the cuticle tanning process in different insects. We also describe the current knowledge of the expression of bursicon in the central nervous system of different insects in large homologous neurosecretory cells, and the changes in its expression during the development of Manduca sexta and D. melanogaster. Although much remains to be learned, the elucidation of its molecular identity and that of its receptor has provided the breakthrough needed for investigating the diverse actions of this critical insect neurohormone.

Molecular characterization and functional distribution of N-ethylmaleimide-sensitive factor in Helicoverpa armigera

N-ethylmaleimide-sensitive fusion protein (NSF) is an essential component for the neurotransmitter or neurohormone release apparatus present in all eukaryotic cells. Here, a new NSF orthologue was characterized from the cotton bollworm, Helicoverpa armigera (Har). Northern blot exhibited a high expression in larval brain. Southern analysis indicated that a single copy of the gene is present in a haploid genome. Using antibodies labeled with fluoresceins, we directly proved that NSF is co-localized with two crucial neurohormones, prothoracicotropic hormone and diapause hormone, both of which regulate insect development. These findings suggest that Har-NSF may be involved in regulating insect neurohormone release.

The parable of the mandarin

In September 2004, a PhD student and a postdoctoral fellow identified an insect neurohormone that had eluded scientists for more than 40 years. They were convinced that they had made a valuable discovery, after having read an editorial on the subject that was published the previous July in a major high‐impact journal, hereafter referred to as MHIJ. The editorial commented on an article published in a high‐impact journal (HIJ‐1), which claimed to have solved the problem, whereas our two researchers had their own evidence that things were more complicated. They consolidated their findings and submitted a paper to the MHIJ on December 8. On December 22, the researchers received notification from the editor who had written the editorial that their study had not been given a high priority rating and would not …

Bombyxin exhibits an insulin-like response to modification in the N-terminal region of the A chain.

Bombyxin is an insect neurohormone with an insulin-like structure. The N-terminal A chain helix, a region which is considered part of the active site in insulin, is almost identical between the two hormones. Bombyxin analogues with modifications at the N-terminus of the A-chain were synthesized and investigated for their ability to bind to bombyxin-specific receptors. While N-acetylation reduced the affinity to the bombyxin receptor to 18% the removal of glycine (A1) inactivated the hormone completely. Replacement of glycine (A1) by L-amino acids caused a significant loss in activity (11%) while its replacement by D-amino acid resulted in active bombyxin analogues (55%). Comparative CD spectroscopy indicated a change in structure for desGly(A1)bombyxin. Although the insect hormone does not have an insulin-like function it exhibits mammalian insulin-like structural sensitivity for A chain modifications.

Coordination and Integration of Metabolism in Insect Flight*

Insect flight is the most energy-demanding activity of animals. It requires the coordination and cooperation of many tissues, with the nervous system and neurohormones controlling the performance and energy metabolism of muscles, and of the fat body, ensuring that the muscles and nerves are supplied with essential fuels throughout flight. Muscle metabolism can be based on several different fuels, the proportions of which vary according to the insect species and the stage in flight activity. Octopamine, which acts as neurotransmitter, neuromodulator or neurohormone in insects, has a central role in flight. It is present in brain, ventral ganglia and nerves, supplying peripheral tissues such as the flight muscles, and its concentration in hemolymph increases during flight. Octopamine has multiple effects during flight in coordinating and stimulating muscle contraction and also energy metabolism partly by activating phosphofructokinase via the glycolytic activator, fructose 2,6-bisphosphate. One important muscle fuel is trehalose, synthesized by the fat body from a variety of precursors, a process that is regulated by neuropeptide hormones. Other fuels for flight include proline, glycerol and ketone bodies. The roles of these and possible regulation in some insect species are discussed.

Insect prothoracicotropic hormone: a new member of the vertebrate growth factor superfamily

Prothoracicotropic hormone (PTTH) is a brain neurosecretory protein that controls insect development. PTTH of the silkmoth Bombyx mori is a homodimeric protein, the subunit of which consists of 109 amino acids. Clear-cut sequence similarity to any other proteins has not been observed. By disulfide-bond pattern analysis and modeling of the PTTH structure based on the known three-dimensional (3D) structures of growth factor family with cystine-knot motif, we propose that the PTTH protomer adopts the fold unique to the structural superfamily of the growth factors, β-nerve growth factor (β-NGF), transforming growth factor-β2 (TGF-β2), and platelet-derived growth factor-BB (PDGF-BB). The insect neurohormone PTTH appears to be a member of the growth factor superfamily, sharing a common ancestral gene with the three vertebrate growth factors, β-NGF, TGF-β2 and PDGF-BB.

Chemical synthesis of a 7 kDa insect gonadotropic neurohormone.

An original insect neurohormone of 65 residues was synthesized by the solid-phase methodology using t-Boc strategy and Boc-Val-PAM-resin. The purification, conducted by several steps of liquid chromatography having mass, polarity or charge as separative criteria, yielded the product with the correct molecular weight of 6922 Da determined by mass spectrometry. The synthetic peptide had both the same affinity for the anti-native neurohormone serum and the same biological activity as the native neurohormone.

Octopamine-sensitive adenylyl cyclase and G proteins in Ceratitis capitata brain during aging

The loss of the ability in signalling transduction constitutes an attractive hypothesis to explain the age-related loss of functions in the nervous system. In this paper we have examined adenylyl cyclase and G proteins in Ceratitis capitata brain during aging. The intermediate level of complexity of the Mediterranean fruit fly and its short lifespan make it a particularly interesting system for aging studies. Adenylyl cyclase basal activity decreased in the course of aging. By contrast, neither guanine nucleotide-induced activation of adenylyl cyclase nor Gs protein levels were modified. However, adenylyl cyclase activation by octopamine, which is a major neurotransmitter, neuromodulator and neurohormone in insects, was lost during aging. This observation correlated with a decrease in octopamine binding to brain plasma membranes that was due to a decrease in both receptor affinity and binding sites. On the other hand, we observed an increase in the expression of C. capitata Go protein with age, as revealed by pertussis toxin-catalysed ADP-ribosylation and immunoblotting experiments, that was not correlated with an increase in β subunit levels. This report constitutes the first direct evidence for the participation of a Go protein in aging in the nervous system.