Swallowing motor pattern triggered and modified by sucrose stimulation in the larvae of the silkworm, Bombyx mori

Enteroendocrine peptides regulate feeding behavior via controlling intestinal contraction of the silkworm Bombyx mori

Our previous study demonstrated that predominant feeding inhibitory effects were found in the crude extracts of foregut and midgut of the silkworm Bombyx mori larvae. To address the entero-intestinal control crucial for the regulation of insect feeding behavior, the present study identified and functionally characterized feeding inhibitory peptides from the midgut of B. mori larvae. Purification and structural analyses revealed that the predominant inhibitory factors in the crude extracts were allatotropin (AT) and GSRYamide after its C-terminal sequence. In situ hybridization revealed that AT and GSRYamide were expressed in enteroendocrine cells in the posterior and anterior midgut, respectively. Receptor screening using Ca²⁺-imaging technique showed that the B. mori neuropeptide G protein-coupled receptor (BNGR)-A19 and -A22 acted as GSRYamide receptors and BNGR-A5 acted as an additional AT receptor. Expression analyses of these receptors and the results of the peristaltic motion assay indicated that these peptides participated in the regulation of intestinal contraction. Exposure of pharynx and ileum to AT and GSRYamide inhibited spontaneous contraction in ad libitum-fed larvae, while exposure of pharynx to GSRYamide did not inhibit contraction in non-fed larvae, indicating that the feeding state changed their sensitivity to inhibitory peptides. These different responses corresponded to different expression levels of their receptors in the pharynx. In addition, injection of AT and GSRYamide decreased esophageal contraction frequencies in the melamine-treated transparent larvae. These findings strongly suggest that these peptides exert feeding inhibitory effects by modulating intestinal contraction in response to their feeding state transition, eventually causing feeding termination.

Gustatory receptors in Lepidoptera: chemosensation and beyond

Lepidoptera is one of the most widespread insect orders and includes several agriculturally important insect species. Ecological success of the lepidopteran insects partly depends on their adaptive chemoreception tactics, which play an important role in the selection of hosts, egg-laying sites and mates. Members of the G-protein coupled receptor family, gustatory receptors (GRs), are an integral part of the Lepidoptera chemosensory machinery. They are expressed in chemosensory neurones and are known to detect different environmental stimuli. Here, we discuss various aspects of the lepidopteran GRs with an emphasis on their roles in different processes such as chemosensation, host selection and adaptation. Phylogenetic analyses have shown that the large diversity of GR genes may have been generated through gene duplication and positive selection events, which also show lineage- and tissue-specific expression. Moreover, lepidopteran GR proteins are diverse and demonstrate broad ligand selectivity for several molecules including sugars, deterrents, salts and CO2 . Binding of ligands to GRs generates multiple downstream changes at the cellular level, which are followed by changes in behaviour. GRs play a critical role in chemosensation and influence the insect's behaviour. Overall, insect GRs are potential targets in the design of effective insect control strategies.

Integrative Proteomics and Metabolomics Analysis of Insect Larva Brain: Novel Insights into the Molecular Mechanism of Insect Wandering Behavior

Before metamorphosis, most holometabolous insects, such as the silkworm studied here, undergo a special phase called the wandering stage. Insects in this stage often display enhanced locomotor activity (ELA). ELA is vital because it ensures that the insect finds a safe and suitable place to live through the pupal stage. The physiological mechanisms of wandering behavior are still unclear. Here, we integrated proteomics and metabolomics approaches to analyze the brain of the lepidopteran insect, silkworm, at the feeding and wandering stages. Using LC-MS/MS and GC-MS, in all we identified 3004 proteins and 37 metabolites at these two stages. Among them, 465 proteins and 22 metabolites were changed. Neural signal transduction proteins and metabolites, such as neurofilament, dopaminergic synapse related proteins, and glutamic acid, were significantly altered, which suggested that active neural conduction occurred in the brain at the wandering stage. We also found decreased dopamine degradation at the wandering stage. The proposed changes in active neural conduction and increased dopamine concentration might induce ELA. In addition, proteins involved in the ubiquitin proteasome system and lysosome pathway were upregulated, revealing that the brain experiences morphological remodeling during metamorphosis. These findings yielded novel insights into the molecular mechanism underlying insect wandering behavior.

Not all sugars are created equal: some mask aversive tastes better than others in an herbivorous insect

Manduca sexta caterpillars are unusual because they exhibit strong peripheral gustatory responses to sugars, but nevertheless fail to show immediate appetitive responses to them. We hypothesized that the primary function of the peripheral gustatory response to sugars is to mask the taste of noxious compounds, which abound in host plants of M. sexta. We compared 10 s biting responses to water with those to mixtures of a noxious compound [caffeine (Caf) or aristolochic acid (AA)] and various combinations of sugars [i.e. sucrose (Suc), glucose (Glu), inositol (Ino), Suc+Glu, Suc+Ino or Glu+Ino]. The biting assays indicated that the aversive taste of AA was completely masked by Suc+Ino, and partially masked by Suc+Glu, Glu+Ino and Suc, whereas that of Caf was completely masked by Suc+Ino and Suc+Glu, and partially masked by Glu+Ino, Suc and Ino. To examine the contribution of the peripheral taste system to the masking phenomenon, we recorded responses of the maxillary gustatory sensilla to each stimulus mixture. The sugars differed greatly in their capacity to suppress peripheral gustatory responses to AA and Caf: Suc+Ino and Suc+Glu produced the greatest suppression, and Glu and Ino the least. Further, the extent to which each sugar stimulus suppressed the peripheral gustatory responses to AA reliably predicted the extent to which it masked the taste of AA in biting assays; no such predictive relationship was observed for the sugar/Caf mixtures. We conclude that some, but not all, sugars act on both peripheral and central elements of the gustatory system to mask the taste of noxious compounds.

Gustatory feedback affects feeding related motor pattern generation in starved 3rd instar larvae of Calliphora vicina

Gustatory feedback allows animals to distinguish between edible and noxious food and adapts centrally generated feeding motor patterns to environmental demands. In reduced preparations obtained from starved Calliphora larvae, putatively appetitive (ethanol), aversive (sodium acetate) and neutral (glucose) gustatory stimuli were applied to the anterior sense organs. The resulting sensory response was recorded from the maxillary and antennal nerves. All three stimuli increased the neural activity in both nerves. Recordings obtained from the antennal nerve to monitor the activation pattern of the cibarial dilator muscles, demonstrated an effect of gustatory input on the central pattern generator for feeding. Ethanol consistently enhanced the rhythmic activity of the CDM motor neurons either by speeding up the rhythm or by increasing the burst duration. Ethanol also had an enhancing effect on the motor patterns of a protractor muscle which moves the cephalopharyngeal skeleton relative to the body. Sodium acetate showed a state dependent effect: in preparations without spontaneous CDM activity it initiated rhythmic motor patterns, while an ongoing CDM rhythm was inhibited. Surprisingly glucose had an enhancing effect which was less pronounced than that of ethanol. Gustatory feedback therefore can modify and adapt the motor output of the multifunctional central pattern generator for feeding.

Identification of a novel hemolymph peptide that modulates silkworm feeding motivation

Phytophagous insects do not constantly chew their diets; most of their time is spent in a non-feeding quiescent state even though they live on or around their diets. Following starvation, phytophagous insect larvae exhibit enhanced foraging behaviors such as nibbling and walking similar to the sequential behavior that occurs prior to each meal. Although extensive physiological studies have revealed regularly occurring feeding behaviors in phytophagous insects, little has been elucidated regarding the mechanism at the molecular level. Here, we report identification and characterization of a novel 62-amino acid peptide, designated as hemolymph major anionic peptide (HemaP), from the hemolymph of Bombyx mori larvae that induces foraging behaviors. The endogenous HemaP levels are significantly increased by diet deprivation, whereas refeeding after starvation returns them to basal levels. In larvae fed ad libitum, hemolymph HemaP levels fluctuate according to the feeding cycle, indicating that locomotor-associated feeding behaviors of B. mori larvae are initiated when HemaP levels exceed an unidentified threshold. Furthermore, administration of exogenous HemaP mimics the starvation-experienced state by affecting dopamine levels in the suboesophageal ganglion, which coordinates neck and mandible movements. These data strongly suggest that fluctuation of hemolymph HemaP levels modulates the regularly occurring feeding-motivated behavior in B. mori by triggering feeding initiation.

The thoracic muscular system and its innervation in third instar Calliphora vicina Larvae. I. Muscles of the pro- and mesothorax and the pharyngeal complex

An anatomical description is given by the muscles in the pro- and mesothorax, and those associated with the feeding apparatus (cephalopharyngeal skeleton, CPS) that participate in feeding behavior in third instar Calliphora larvae. The body wall muscles in the pro- and mesothoracic segments are organized in three layers: internal, intermedial, and external. The muscles were labeled with roman numerals according to the nomenclature in use for the abdominal segments. Muscles associated with the CPS are labeled according to their function. The prothorax bears five pairs of lateral symmetrically longitudinal segmental body wall muscles and lacks the transversal muscle group present in the mesothorax and abdominal segments. Additionally, four pairs of intersegmental muscles project from the prothorax to the second, fourth, and fifth segment. The mesothorax bears 15 pairs of segmental longitudinal and 18 pairs of transversal muscles. The accessory pharyngeal muscles span the CPS and the cuticle. Three pairs of protractors and retractors and two pairs of mouth hook accessors (MH(AC)) exist, which move the CPS relative to the body. The pharyngeal muscles are exclusively attached to the structures of the CPS. The mouth hook elevators and depressors, which mediate the hooks rotation are attached to the ventral arm of the CPS and project to a dorsal (elevators) or ventral (depressors) protuberance of the mouth hooks. The cibarial dilator muscles (CDM) span the dorsal arms of the CPS and the dorsal surface of the esophagus and mediate food ingestion. The labial retractors (LRs) lack antagonists and project from the ventral surface of the CPS to the unpaired labium. Contractions of these muscles open the mouth cavity.

The brain can eat: establishing the existence of a central pattern generator for feeding in third instar larvae of Drosophila virilis and Drosophila melanogaster

To establish the existence of a central pattern generator for feeding in the larval central nervous system of two Drosophila species, the gross anatomy of feeding related muscles and their innervation is described, the motor units of the muscles identified and rhythmic motor output recorded from the isolated CNS. The cibarial dilator muscles that mediate food ingestion are innervated by the frontal nerve. Their motor pathway projects from the brain through the antennal nerves, the frontal connectives and the frontal nerve junction. The mouth hook elevator and depressor system is innervated by side branches of the maxillary nerve. The motor units of the two muscle groups differ in amplitude: the elevator is always activated by a small unit, the depressor by a large one. The dorsal protractors span the cephalopharyngeal skeleton and the body wall hence mediating an extension of the CPS. These muscles are innervated by the prothoracic accessory nerve. Rhythmic motor output produced by the isolated central nervous system can simultaneously be recorded from all three nerves. The temporal pattern of the identified motor units resembles the sequence of muscle contractions deduced from natural feeding behavior and is therefore considered as fictive feeding. Phase diagrams show an almost identical fictive feeding pattern is in both species.

A homeotic mutation influences the wing vibration patterns during mating in males of the silkworm moth Bombyx mori

An abnormality in the wing vibration pattern in males of the E(Nc) homeotic mutant of Bombyx mori was investigated. The wild-type (+/+) males show a switching of the rhythmic wing vibrations from a sequential pattern to an intermittent pattern during mating, whereas the E(Nc) mutants show a sequential pattern both before and during mating. Wing motions in +/+ males became small during mating, but those in +/E(Nc) males did not. Ablation of the head ganglia of +/+ and +/E(Nc) males during mating caused no change in the motor patterns of wing vibrations. Ablation or cooling of the posterior abdomen in the +/+ males during mating caused sequential wing vibrations, suggesting that the change in wing vibrations is induced by signals from the posterior abdomen. The pterothoracic ganglion in the +/E(Nc) males is separated into two ganglia, in contrast to the complete ganglionic fusion in the +/+ males. The neurons in the pterothoracic ganglion stained from abdominal nerve cords are homologus in +/+ and +/E(Nc) males, but many of these in +/E(Nc) males are elongated along the anteroposterior axis. These results suggest that the wing vibration pattern is restricted by genetic factors through reconstruction of the thoracic nervous system during metamorphosis.

From behavior to fictive feeding: anatomy, innervation and activation pattern of pharyngeal muscles of Calliphora vicina 3rd instar larvae

A description of the muscles and nerves involved in feeding of larval Calliphora vicina is given as a prerequisite to establish fictive feeding patterns recorded from the isolated central nervous system. Feeding Diptera larvae show a repetitive sequence of pro- and retraction of the cephalopharyngeal skeleton (CPS), elevation and depression of the mouth hooks and food ingestion. The corresponding pharyngeal muscles are protractors, mouth hook elevators and depressors, the labial retractor and cibarial dilator muscles. These muscles are innervated by the prothoracic accessory nerve (PaN), maxillary nerve (MN) and antennal nerve (AN) as shown electrophysiologically by recording action potentials from the respective nerve that correlate to post-synaptic potentials on the muscles. All three nerves show considerably more complex branching patterns than indicated in the literature. Extracellular recordings from the stumps of PaN, MN and AN connected to an isolated CNS show spontaneous rhythmic motor patterns that reflect the feeding sequence in intact larvae. Variability of the feeding pattern observed in behavioral experiments is also evident from the level of motor output from an isolated CNS. The data obtained from Calliphora will facilitate electrophysiological investigations dealing with the genetic background of feeding behavior in Drosophila larvae.

The hungry caterpillar: an analysis of how carbohydrates stimulate feeding inManduca sexta

In most insects, the taste of carbohydrates stimulates an immediate appetitive response. The caterpillar of Manduca sexta is an exception to this general pattern. Despite eliciting a strong peripheral gustatory response, high concentrations of carbohydrates (e.g. glucose or inositol)stimulate the same intensity of biting as water during 2-min tests. We suspected that the lack of feeding stimulation reflected the fact that prior studies used single carbohydrates (e.g. sucrose), which M. sextawould rarely encounter in its host plants. We hypothesized that the feeding control system of M. sexta responds selectively to carbohydrate mixtures. To test this hypothesis, we ran three experiments. First, we stimulated the two taste sensilla that respond to carbohydrates (the lateral and medial styloconic) with a battery of carbohydrates. These sensilla responded exclusively to sucrose, glucose and inositol. Second, we determined the response properties of the carbohydrate-sensitive taste cells within both sensilla. We found that one class of carbohydrate-sensitive taste cell responded to sucrose, and two other classes each responded to glucose and inositol. Third, we examined the initial biting responses of caterpillars to disks treated with solutions containing single carbohydrates (sucrose, glucose or inositol) or binary mixtures of these carbohydrates. The only solutions that stimulated sustained biting were those that activated all three classes of taste cell (i.e. sucrose+inositol or sucrose+glucose). We propose that the brain of M. sexta monitors input from the different classes of carbohydrate-sensitive taste cell, and generates protracted feeding responses only when all three classes are activated.

Anatomical and functional characterisation of the stomatogastric nervous system of blowfly (Calliphora vicina) larvae

The anatomy and functionality of the stomatogastric nervous system (SNS) of third-instar larvae of Calliphora vicina was characterised. As in other insects, the Calliphora SNS consists of several peripheral ganglia involved in foregut movement regulation. The frontal ganglion gives rise to the frontal nerve and is connected to the brain via the frontal connectives and antennal nerves (ANs). The recurrent nerve connects the frontal- to the hypocerebral ganglion from which the proventricular nerve runs to the proventricular ganglion. Foregut movements include rhythmic contractions of the cibarial dilator muscles (CDM), wavelike movements of crop and oesophagus and contractions of the proventriculus. Transections of SNS nerves indicate mostly myogenic crop and oesophagus movements and suggest modulatory function of the associated nerves. Neural activity in the ANs, correlating with postsynaptic potentials on the CDM, demonstrates a motor pathway from the brain to CDM. Crop volume is monitored by putative stretch receptors. The respective sensory pathway includes the recurrent nerve and the proventricular nerve. The dorsal organs (DOs) are directly connected to the SNS. Mechanical stimulation of the DOs evokes sensory activity in the AN. This suggests the DOs can provide sensory input for temporal coordination of feeding behaviour.