Experiential effects of appetitive and nonappetitive odors on feeding behavior in the blowfly, Phormia regina: a putative role for tyramine in appetite regulation

Tyramine-Mediated Hyperactivity Modulates the Dietary Habits in Helicoverpa armigera

Helicoverpa armigera exhibits extensive variability in feeding habits and food selection. Neuronal regulation of H. armigera feeding behavior is primarily influenced by biogenic amines such as Tyramine (TA) and Octopamine (OA). The molecular responses of H. armigera to dietary challenges in the presence of TA or OA have yet to be studied. This investigation dissects the impact of OA and TA on H. armigera feeding choices and behaviors under non-host nutritional stress. It has been observed that feeding behavior remains unaltered during the exogenous administration of OA and TA through an artificial diet (AD). Ingestion of higher OA or TA concentrations leads to increased mortality. OA and TA treatment in combination with host and non-host diets results in the induction of feeding and higher locomotion toward food, particularly in the case of TA treatment. Increased expression of markers, prominin-like, and tachykinin-related peptide receptor-like transcripts further assessed increased locomotion activity. Insects subjected to a non-host diet with TA treatment exhibited increased feeding and overexpression of the feeding indicator, the Neuropeptide F receptor, and the feeding regulator, Sulfakinin, compared with other conditions. Expression of sensation and biogenic amine synthesis genesis elevated in insects fed a non-host diet in combination with OA or TA. Metabolomics analysis revealed a decreased concentration of the feeding behavior elicitor, dopamine, in insects fed a non-host diet containing TA. This work highlights the complex interplay between biogenic amine functions during dietary stress and suggests the role of tyramine in feeding promotion under stressed conditions.

Regulation of forager honey bee appetite independent of the glucose-insulin signaling pathway

Introduction

To maintain energetic homeostasis the energetic state of the individual needs to communicate with appetite regulatory mechanisms on a regular basis. Although hunger levels indicated by the energetic state and appetite levels, the desire for food intake, tend to be correlated, and on their own are well studied, how the two cross-talk and regulate one another is less known. Insects, in contrast to vertebrates, tend to have trehalose as the primary sugar found in the hemolymph, which could possibly serve as an alternative monitor of the energetic state in comparison to the glucose-insulin signaling pathway, found in vertebrates.

Methods

We investigate how manipulating hemolymph sugar levels alter the biogenic amines in the honey bee brain, appetite levels, and insulin like peptide gene expression, across three age classes, to determine how the energetic state of the honey bee might be connected to appetite regulation.

Results

We found that only in the forager bees, with a lowering of hemolymph trehalose levels, there was an increase in octopamine and a decrease in tyramine levels in the honey bee brain that corresponded with increased appetite levels, while there was no significant changes in Insulin Like Peptide-1 or 2 gene expression.

Discussion

Our findings suggest that hemolymph trehalose levels aid in regulating appetite levels, in forager bees, via octopamine and tyramine, and this regulation appears to be functioning independent of the glucose insulin signaling pathway. Whether this potentially more direct and rapid appetite regulatory pathway can be generalized to other insects, which also undergo energy demanding activities, remains to be investigated.

Olfactory dysfunction and potential mechanisms caused by volatile organophosphate dichlorvos in the silkworm as a model animal

Volatile pesticides impair olfactory function in workers/farmers and insects, but data on molecular responses and mechanisms are poorly understood. This study aims to reveal the mechanisms of olfactory dysfunction in the silkworm after exposure to volatile dichlorvos. Our results demonstrated that acute exposure for 12 h significantly reduced electroantennogram responses, and over 62.50% of the treated male moths cannot locate the pheromone source. Transcriptional and proteomic responses of the antennae and heads were investigated. A total of 101 differentially expressed genes (DEGs) in the antennae, 138 DEGs in the heads, and 43 differentially expressed proteins (DEPs) in the heads including antennae were revealed. We discovered that upregulations of Arrestin1 and nitric oxide synthase1 (NOS1) may inhibit cyclic nucleotide-gated channels and hinder calcium influx in the antennae. In the central nervous systems (CNS), downregulations of tyrosine hydroxylase (TH) and tyrosine decarboxylase (TDC) may inhibit olfactory signal transduction by reducing the second messenger biosynthesis. Meanwhile, an abnormal increase of brain cell apoptosis was revealed by Annexin V-mCherry staining, often leading to persistent neurologic impairment. Taken together, this study highlighted olfactory dysfunction caused by dichlorvos, which may provide a novel perspective for understanding the toxicity mechanism of volatile pesticides in other organisms.

Tyramine 1 Receptor Distribution in the Brain of Corbiculate Bees Points to a Conserved Function

Sucrose represents an important carbohydrate source for most bee species. In the Western honeybee (Apis mellifera) it was shown that individual sucrose responsiveness correlates with the task performed in the colony, supporting the response threshold theory which states that individuals with the lowest threshold for a task-associated stimuli will perform the associated task. Tyramine was shown to modulate sucrose responsiveness, most likely via the tyramine 1 receptor. This receptor is located in brain areas important for the processing of gustatory stimuli. We asked whether the spatial expression pattern of the tyramine 1 receptor is a unique adaptation of honeybees or if its expression represents a conserved trait. Using a specific tyramine receptor 1 antibody, we compared the spatial expression of this receptor in the brain of different corbiculate bee species, including eusocial honeybees, bumblebees, stingless bees, and the solitary bee Osmia bicornis as an outgroup. We found a similar labeling pattern in the mushroom bodies, the central complex, the dorsal lobe, and the gnathal ganglia, indicating a conserved receptor expression. With respect to sucrose responsiveness this result is of special importance. We assume that the tyramine 1 receptor expression in these neuropiles provides the basis for modulation of sucrose responsiveness. Furthermore, the tyramine 1 receptor expression seems to be independent of size, as labeling is similar in bee species that differ greatly in their body size. However, the situation in the optic lobes appears to be different. Here, the lobula of stingless bees is clearly labeled by the tyramine receptor 1 antibody, whereas this labeling is absent in other species. This indicates that the regulation of this receptor is different in the optic lobes, while its function in this neuropile remains unclear.

Characterization of Halyomorpha halys TAR1 reveals its involvement in (E)-2-decenal pheromone perception

In insects, tyramine receptor 1 (TAR1) has been shown to control several physiological functions, including olfaction. We investigated the molecular and functional profile of the Halyomorpha halys type 1 tyramine receptor gene (HhTAR1) and its role in olfactory functions of this pest. Molecular and pharmacological analyses confirmed that the HhTAR1 gene codes for a true TAR1. RT-qPCR analysis revealed that HhTAR1 is expressed mostly in adult brain and antennae as well as in early development stages (eggs, 1st and 2nd instar nymphs). In particular, among the antennomeres that compose a typical H. halys antenna, HhTAR1 was more expressed in flagellomeres. Scanning electron microscopy investigation revealed the type and distribution of sensilla on adult H. halys antennae: both flagellomeres appear rich in trichoid and grooved sensilla, known to be associated with olfactory functions. Through an RNAi approach, topically delivered HhTAR1 dsRNA induced a 50% downregulation in gene expression after 24 h in H. halys 2nd instar nymphs. An innovative behavioural assay revealed that HhTAR1 RNAi-silenced 2nd instar nymphs were less susceptible to the alarm pheromone component (E)-2 decenal as compared with controls. These results provide critical information concerning the role of TAR1 in olfaction regulation, especially alarm pheromone reception, in H. halys. Furthermore, considering the emerging role of TAR1 as target of biopesticides, this work opens the way for further investigation on innovative methods for controlling H. halys.

Monoterpenes alter TAR1-driven physiology in Drosophila species

Monoterpenes are molecules with insecticide properties whose mechanism of action is, however, not completely elucidated. Furthermore, they seem to be able to modulate the monoaminergic system and several behavioural aspects in insects. In particular, tyramine (TA) and octopamine (OA) and their associated receptors orchestrate physiological processes such as feeding, locomotion and metabolism. Here, we show that monoterpenes not only act as biopesticides in Drosophila species but also can cause complex behavioural alterations that require functional type 1 tyramine receptors (TAR1s). Variations in metabolic traits as well as locomotory activity were evaluated in both Drosophila suzukii and Drosophila melanogaster after treatment with three monoterpenes. A TAR1-defective D. melanogaster strain (TAR1PL00408) was used to better understand the relationships between the receptor and monoterpene-related behavioural changes. Immunohistochemistry analysis revealed that, in the D. melanogaster brain, TAR1 appeared to be mainly expressed in the pars intercerebralis, lateral horn, olfactory and optic lobes and suboesophageal ganglion lobes. In comparison to wild-type D. melanogaster, the TAR1PL00408 flies showed a phenotype characterized by higher triglyceride levels and food intake as well as lower locomotory activity. The monoterpenes, tested at sublethal concentrations, were able to induce a downregulation of the TAR1 coding gene in both Drosophila species. Furthermore, monoterpenes also altered the behaviour in wild-type D. suzukii and D. melanogaster 24 h after continuous monoterpene exposure. Interestingly, they were ineffective in modifying the physiological performance of TAR1-defective flies. In conclusion, it appears that monoterpenes not only act as biopesticides for Drosophila but also can interfere with Drosophila behaviour and metabolism in a TAR1-dependent fashion.

Visualization of antennal lobe glomeruli activated by nonappetitive D-limonene and appetitive 1-octen-3-ol odors via two types of olfactory organs in the blowfly Phormia regina

Appetite or feeding motivation relies significantly on food odors. In the blowfly Phormia regina, feeding motivation for sucrose is decreased by the odor of D-limonene but increased by the odor of 1-octen-3-ol odor. These flies have antennal lobes (ALs) consisting of several tens of glomerular pairs as a primary olfactory center in the brain. Odor information from different olfactory organs-specifically, the antennae and maxillary palps-goes to the corresponding glomeruli. To investigate how odors differently affect feeding motivation, we identified the olfactory organs and glomeruli that are activated by nonappetitive and appetitive odors. We first constructed a glomerular map of the antennal lobe in P. regina. Anterograde fluorescence labeling of antennal and maxillary afferent nerves, both of which project into the contralateral and ipsilateral ALs, revealed differential staining in glomerular regions. Some of the axonal fiber bundles from the antennae and maxillary palps projected to the subesophageal ganglion (SOG). We visualized the activation of the glomeruli in response to odor stimuli by immunostaining phosphorylated extracellular signal-regulated kinase (pERK). We observed different glomerulus activation under different odor stimulations. Referring to our glomerular map, we determined that antennal exposure to D-limonene odor activated the DA13 glomeruli, while exposure of the maxillary palps to 1-octen-3-ol activated the MxB1 glomeruli. Our results indicated that a nonappetitive odor input from the antennae and an appetitive odor input from the maxillary palps activate different glomeruli in the different regions of ALs in the blowfly P. regina. Collectively, our findings suggest that compartmentalization of glomeruli in AL is essential for proper transmission of odor information.

Appetite is correlated with octopamine and hemolymph sugar levels in forager honeybees

Insects have rapidly changing energy demands, so they primarily rely on hemolymph and other carbohydrates to carry out life activities. However, how gustatory responsiveness and hemolymph sugar levels coordinate with one another to maintain energetic homeostasis in insects remains largely unknown for the highly social honeybee that goes through large physiological and behavioral changes. The potential role of biogenic amines and neuropeptides in the connection between the regulation of appetite and fluctuating sugar levels in the hemolymph, due to starvation, as the bee ages, was investigated. The largest appetite increase due to the starvation treatment was within the forager age class and this corresponded with an increase in octopamine levels in the brain along with a decline in hemolymph sugar levels. Adipokinetic hormone (AKH) was found in very small quantities in the brain and there were no significant changes in response to starvation treatment. Our findings suggest that the particularly dynamic levels of hemolymph sugar levels may serve as a monitor of the forager honeybee energetic state. Therefore, there may be a pathway in forager bees via octopamine responsible for their precise precipitous regulation of appetite, but to determine cause and effect relationships further investigation is needed.

Food-derived volatiles enhance consumption in Drosophila melanogaster

Insects use multiple sensory modalities when searching for and accepting a food source, in particular odor and taste cues. Food-derived odorants are generally involved in mediating long- and short-range attraction. Taste cues, in contrast, act directly by contact with the food source, promoting the ingestion of nutritious food and the avoidance of toxic substances. It is possible, however, that insects integrate information from these sensory modalities during the process of feeding itself. Here, using a simple feeding assay, we investigated whether odors modulate food consumption in the fruit fly Drosophila melanogaster We found that the presence of both single food-derived odorants and complex odor mixtures enhanced consumption of an appetitive food. Feeding enhancement depended on the concentration and the chemical identity of the odorant. Volatile cues alone were sufficient to mediate this effect, as feeding was also increased when animals were prevented from contacting the odor source. Both males and females, including virgin females, increased ingestion in the presence of food-derived volatiles. Moreover, the presence of food-derived odorants significantly increased the consumption of food mixtures containing aversive bitter compounds, suggesting that flies integrate diverse olfactory and gustatory cues to guide feeding decisions, including situations in which animals are confronted with stimuli of opposite valence. Overall, these results show that food-derived olfactory cues directly modulate feeding in D. melanogaster, enhancing ingestion.

Levels and activity of cyclic guanosine monophosphate-dependent protein kinase in nurse and forager honeybees

Age-dependent division of labour in honeybees was shown to be connected to sensory response thresholds. Foragers show a higher gustatory responsiveness than nurse bees. It is generally assumed that nutrition-related signalling pathways underlie this behavioural plasticity. Here, one important candidate gene is the foraging gene, which encodes a cyclic guanosine monophosphate-dependent protein kinase (PKG). Several roles of members of this enzyme family were analysed in vertebrates. They own functions in important processes such as growth, secretion and neuronal adaptation. Honeybee foraging messenger RNA expression is upregulated in the brain of foragers. In vivo activation of PKG can modulate gustatory responsiveness. We present for the first time PKG protein level and activity data in the context of social behaviour and feeding. Protein level was significantly higher in brains of foragers than in those of nurse bees, substantiating the role of PKG in behavioural plasticity. However, enzyme activity did not differ between behavioural roles. The mediation of feeding status appears independent of PKG signalling. Neither PKG content nor enzyme activity differed between starved and satiated individuals. We suggest that even though nutrition-related pathways are surely involved in controlling behavioural plasticity, which involves changes in PKG signalling, mediation of satiety itself is independent of PKG.

Comparison of RNAi knockdown effect of tyramine receptor 1 induced by dsRNA and siRNA in brains of the honey bee, Apis mellifera

RNA interference (RNAi) is a powerful tool for artificially manipulating gene expression in diverse organisms. In the honey bee, Apis mellifera, both long double stranded RNA (dsRNA) and small interference RNA (siRNA) have been successfully used to reduce targeted gene expression and induce specific phenotypes. However, whether dsRNA and siRNA have different effects and efficiencies in gene silencing has never been investigated in honey bees. Thus, we tested the effect of dsRNA and siRNA on the tyramine receptor 1 (tyr1), which encodes a receptor of neurotransmitter tyramine, in honey bee brains at mRNA and protein levels over time. We found that both dsRNA and siRNA achieved successful gene knockdown. The siRNA mixes affected tyr1 gene expression faster than dsRNA, and the duration of the knockdown between dsRNA and siRNA varied. We also found that the turnover rate of TYR1 protein was relatively fast, which is consistent with its role as a neurotransmitter receptor. Our study reveals the different efficiencies of dsRNA and siRNA in honey bee brains. We show that consideration of the gene regions targeted by RNAi, prior screening for RNAi molecules and combing siRNAs are important strategies to enhance RNAi efficiency.

Octopamine and Tyramine Contribute Separately to the Counter-Regulatory Response to Sugar Deficit in Drosophila

All animals constantly negotiate external with internal demands before and during action selection. Energy homeostasis is a major internal factor biasing action selection. For instance, in addition to physiologically regulating carbohydrate mobilization, starvation-induced sugar shortage also biases action selection toward food-seeking and food consumption behaviors (the counter-regulatory response). Biogenic amines are often involved when such widespread behavioral biases need to be orchestrated. In mammals, norepinephrine (noradrenalin) is involved in the counterregulatory response to starvation-induced drops in glucose levels. The invertebrate homolog of noradrenalin, octopamine (OA) and its precursor tyramine (TA) are neuromodulators operating in many different neuronal and physiological processes. Tyrosine-ß-hydroxylase (tßh) mutants are unable to convert TA into OA. We hypothesized that tßh mutant flies may be aberrant in some or all of the counter-regulatory responses to starvation and that techniques restoring gene function or amine signaling may elucidate potential mechanisms and sites of action. Corroborating our hypothesis, starved mutants show a reduced sugar response and their hemolymph sugar concentration is elevated compared to control flies. When starved, they survive longer. Temporally controlled rescue experiments revealed an action of the OA/TA-system during the sugar response, while spatially controlled rescue experiments suggest actions also outside of the nervous system. Additionally, the analysis of two OA- and four TA-receptor mutants suggests an involvement of both receptor types in the animals' physiological and neuronal response to starvation. These results complement the investigations in Apis mellifera described in our companion paper (Buckemüller et al., 2017).

Identification of multiple functional receptors for tyramine on an insect secretory epithelium

The biogenic amine tyramine (TA) regulates many aspects of invertebrate physiology and development. Although three TA receptor subtypes have been identified (TAR1-3), specific receptors have not been linked to physiological responses in native tissue. In the Malpighian (renal) tubule of Drosophila melanogaster, TA activates a transepithelial chloride conductance, resulting in diuresis and depolarization of the transepithelial potential. In the current work, mutation or RNAi-mediated knockdown in the stellate cells of the tubule of TAR2 (tyrR, CG7431) resulted in a dramatic reduction, but not elimination, of the TA-mediated depolarization. Mutation or knockdown of TAR3 (tyrRII, CG16766) had no effect. However, deletion of both genes, or knockdown of TAR3 on a TAR2 mutant background, eliminated the TA responses. Thus while TAR2 is responsible for the majority of the TA sensitivity of the tubule, TAR3 also contributes to the response. Knockdown or mutation of TAR2 also eliminated the response of tubules to the related amine octopamine (OA), indicating that OA can activate TAR2. This finding contrasts to reports that heterologously expressed TAR2 is highly selective for TA over OA. This is the first report of TA receptor function in a native tissue and indicates unexpected complexity in the physiology of the Malpighian tubule.

Neuromodulation of Courtship Drive through Tyramine-Responsive Neurons in the Drosophila Brain

Neuromodulators influence the activities of collections of neurons and have profound impacts on animal behavior. Male courtship drive is complex and subject to neuromodulatory control. Using the fruit fly Drosophila melanogaster, we identified neurons in the brain (inferior posterior slope; IPS) that impact courtship drive and were controlled by tyramine-a biogenic amine related to dopamine, whose roles in most animals are enigmatic. We knocked out a tyramine-specific receptor, TyrR, which was expressed in IPS neurons. Loss of TyrR led to a striking elevation in courtship activity between males. This effect occurred only in the absence of females, as TyrR(Gal4) mutant males exhibited a wild-type preference for females. Artificial hyperactivation of IPS neurons caused a large increase in male-male courtship, whereas suppression of IPS activity decreased male-female courtship. We conclude that TyrR is a receptor for tyramine, and suggest that it serves to curb high levels of courtship activity through functioning as an inhibitory neuromodulator.

Suppressive effects of dRYamides on feeding behavior of the blowfly, Phormia regina

Recently, dRYamides-1 and -2 have been identified as ligands of the neuropeptide Y-like receptor CG5811 in Drosophila melanogaster. It has also been reported in brief that injection of dRYamide-1suppresses the early feeding behavior called proboscis extension reflex (PER) in the blowfly Phormia regina. Immunohistochemical analyses by our group using anti-dRYamide-1 antiserum indicated symmetrical localization of 32 immunoreactive cells in the brain of P. regina. In order to analyze the mechanism of feeding regulation, we further investigated the effects of dRYamide-1 and -2 on intake volume, PER exhibition, and activity of the sugar receptor neuron. After injection of dRYamide-1 or -2, flies showed little change in the intake volume of sucrose solution, but a significant depression of PER to sucrose. Injection of dRYamide-1 revealed a significant decrease in the responsiveness of the sugar receptor neuron, although the injection of dRYamide-2 did not. These results suggest that the dRYamide peptides decrease feeding motivation in flies, as evaluated by PER threshold, through a mechanism that partially involves desensitization of the sugar receptor neuron.

Effects of Floral Scents and Their Dietary Experiences on the Feeding Preference in the Blowfly, Phormia regina

The flowers of different plant species have diverse scents with varied chemical compositions. Hence, every floral scent does not uniformly affect insect feeding preferences. The blowfly, Phormia regina, is a nectar feeder, and when a fly feeds on flower nectar, its olfactory organs, antennae, and maxillary palps are exposed to the scent. Generally, feeding preference is influenced by food flavor, which relies on both taste and odor. Therefore, the flies perceive the sweet taste of nectar and the particular scent of the flower simultaneously, and this olfactory information affects their feeding preference. Here, we show that the floral scents of 50 plant species have various effects on their sucrose feeding motivation, which was evaluated using the proboscis extension reflex (PER). Those floral scents were first categorized into three groups, based on their effects on the PER threshold sucrose concentration, which indicates whether a fly innately dislikes, ignores, or likes the target scent. Moreover, memory of olfactory experience with those floral scents during sugar feeding influenced the PER threshold. After feeding on sucrose solutions flavored with floral scents for 5 days, the scents did not consistently show the previously observed effects. Considering such empirical effects of scents on the PER threshold, we categorized the effects of the 50 tested floral scents on feeding preference into 16 of all possible 27 theoretical types. We then conducted the same experiments with flies whose antennae or maxillary palps were ablated prior to PER test in a fly group naïve to floral scents and prior to the olfactory experience during sugar feeding in the other fly group in order to test how these organs were involved in the effect of the floral scent. The results suggested that olfactory inputs through these organs play different roles in forming or modifying feeding preferences. Thus, our study contributes to an understanding of underlying mechanisms associated with the convergent processing of olfactory inputs with taste information, which affects feeding preference or appetite.

Neuronal projections and putative interaction of multimodal inputs in the subesophageal ganglion in the blowfly, Phormia regina

In flies, the maxillary palp possesses olfactory sensilla housing olfactory receptor neurons (ORNs), which project to the primary olfactory center, the antennal lobes (ALs). The labellum possesses gustatory sensilla housing gustatory receptor neurons (GRNs), which project to the primary gustatory center, the subesophageal ganglion (SOG). Using an anterograde staining method, we investigated the axonal projections of sensory receptor neurons from the maxillary palp and labellum to the SOG or other parts of brain in the blowfly, Phormia regina. We show that maxillary mechanoreceptor neurons and some maxillary ORNs project to the SOG where they establish synapses, whereas other maxillary ORNs terminate in the ipsi- and contralateral ALs. The labellar GRNs project to the SOG, and some of these neural projections partially overlap with ORN terminals from the maxillary palp. Based on these anterograde staining data and 3D models of the observed axonal projections, we suggest that interactions occur between GRNs from the labellum and ORNs from the maxillary palp. These observations strongly suggest that olfactory information from the maxillary palp directly interacts with the processing of gustatory information within the SOG of flies.

Fatty acid solubilizer from the oral disk of the blowfly

Background

Blowflies are economic pests of the wool industry and potential vectors for epidemics. The establishment of a pesticide-free, environmentally friendly blowfly control strategy is necessary. Blowflies must feed on meat in order to initiate the cascade of events that are involved in reproduction including juvenile hormone synthesis, vitellogenesis, and mating. During feeding blowflies regurgitate salivary lipase, which may play a role in releasing fatty acids from triglycerides that are found in food. However, long-chain fatty acids show low solubility in aqueous solutions. In order to solubilize and ingest the released hydrophobic fatty acids, the blowflies must use a solubilizer.

Methodology

We applied native PAGE, Edman degradation, cDNA cloning, and RT-PCR to characterize a protein that accumulated in the oral disk of the black blowfly, Phormia regina. In situ hybridization was carried out to localize the expression at the cellular level. A fluorescence competitive binding assay was used to identify potential ligands of this protein.

Conclusion

A protein newly identified from P. regina (PregOBP56a) belonged to the classic odorant-binding protein (OBP) family. This gene was expressed in a cluster of cells that was localized between pseudotracheae on the oral disk, which are not accessory cells of the taste peg chemosensory sensilla that normally synthesize OBPs. At pH 7 and pH 6, PregOBP56a bound palmitic, stearic, oleic, and linoleic acids, that are mainly found in chicken meat. The binding affinity of PregOBP56a decreased at pH 5. We propose that PregOBP56a is a protein that solubilizes fatty acids during feeding and subsequently helps to deliver the fatty acids to the midgut where it may help in the process of reproduction. As such, PregOBP56a is a potential molecular target for controlling the blowfly.

Isolation of the bioactive peptides CCHamide-1 and CCHamide-2 from Drosophila and their putative role in appetite regulation as ligands for G protein-coupled receptors

There are many orphan G protein-coupled receptors (GPCRs) for which ligands have not yet been identified. One such GPCR is the bombesin receptor subtype 3 (BRS-3). BRS-3 plays a role in the onset of diabetes and obesity. GPCRs in invertebrates are similar to those in vertebrates. Two Drosophila GPCRs (CG30106 and CG14593) belong to the BRS-3 phylogenetic subgroup. Here, we succeeded to biochemically purify the endogenous ligands of Drosophila CG30106 and CG14593 from whole Drosophila homogenates using functional assays with the reverse pharmacological technique, and identified their primary amino acid sequences. The purified ligands had been termed CCHamide-1 and CCHamide-2, although structurally identical to the peptides recently predicted from the genomic sequence searching. In addition, our biochemical characterization demonstrated two N-terminal extended forms of CCHamide-2. When administered to blowflies, CCHamide-2 increased their feeding motivation. Our results demonstrated these peptides actually present as the major components to activate these receptors in living Drosophila. Studies on the effects of CCHamides will facilitate the search for BRS-3 ligands.

Modulation by octopamine of olfactory responses to nonpheromone odorants in the cockroach, Periplaneta americana L

Olfactory receptor cells in insects are modulated by neurohormones. Recordings from cockroach olfactory sensilla showed that a subset of sensory neurons increase their responses to selected nonpheromone odorants after octopamine application. With octopamine application, recordings demonstrated increased firing rates by the short but not the long alcohol-sensitive sensilla to the nonpheromone volatile, hexan-1-ol. Within the same sensillum, individual receptor cells are shown to be modulated independently from each other, indicating that the octopamine receptors reside in the receptor not in the accessory cells. A uniform decrease in the amplitude of electroantennogram, which is odorant independent, is suggested to reflect the rise in octopamine concentration in the antennal hemolymph. Perception of general odorants measured as behavioral responses changed qualitatively under octopamine treatment: namely, repulsive hexan-1-ol became neutral, whereas neutral eucalyptol became attractive. Octopamine induced a change in male behavioral responses to general odors that were essentially the same as in the state of sexual arousal. Our findings suggest that sensitivity to odors having different biological significances is modulated selectively at the peripheral as well as other levels of olfactory processing.

Aversive odorant causing appetite decrease downregulates tyrosine decarboxylase gene expression in the olfactory receptor neuron of the blowfly, Phormia regina

In the blowfly Phormia regina, exposure to d-limonene for 5 days during feeding inhibits proboscis extension reflex behavior due to decreasing tyramine (TA) titer in the brain. TA is synthesized by tyrosine decarboxylase (Tdc) and catalyzed into octopamine (OA) by TA ß-hydroxylase (Tbh). To address the mechanisms of TA titer regulation in the blowfly, we cloned Tdc and Tbh cDNAs from P. regina (PregTdc and PregTbh). The deduced amino acid sequences of both proteins showed high identity to those of the corresponding proteins from Drosophila melanogaster at the amino acid level. PregTdc was expressed in the antenna, labellum, and tarsus whereas PregTbh was expressed in the head, indicating that TA is mainly synthesized in the sensory organs whereas OA is primarily synthesized in the brain. d-Limonene exposure significantly decreased PregTdc expression in the antenna but not in the labellum and the tarsus, indicating that PregTdc expressed in the antenna is responsible for decreasing TA titer. PregTdc-like immunoreactive material was localized in the thin-walled sensillum. In contrast, the OA/TA receptor (PregOAR/TAR) was localized to the thick-walled sensillum. The results indicated that d-limonene inhibits PregTdc expression in the olfactory receptor neurons in the thin-walled sensilla, likely resulting in reduced TA levels in the receptor neurons in the antenna. TA may be transferred from the receptor neuron to the specific synaptic junction in the antennal lobe of the brain through the projection neurons and play a role in conveying the aversive odorant information to the projection and local neurons.

Identification of the novel bioactive peptides dRYamide-1 and dRYamide-2, ligands for a neuropeptide Y-like receptor in Drosophila

A number of bioactive peptides are involved in regulating a wide range of animal behaviors, including food consumption. Vertebrate neuropeptide Y (NPY) is a potent stimulator of appetitive behavior. Recently, Drosophila neuropeptide F (dNPF) and short NPF (sNPF), the Drosophila homologs of the vertebrate NPY, were identified to characterize the functions of NPFs in the feeding behaviors of this insect. Dm-NPFR1 and NPFR76F are the receptors for dNPF and sNPF, respectively; both receptors are G protein-coupled receptors (GPCRs). Another GPCR (CG5811; NepYR) was indentified in Drosophila as a neuropeptide Y-like receptor. Here, we identified 2 ligands of CG5811, dRYamide-1 and dRYamide-2. Both peptides are derived from the same precursor (CG40733) and have no significant structural similarities to known bioactive peptides. The C-terminal sequence RYamide of dRYamides is identical to that of NPY family peptides; on the other hand, dNPF and sNPF have C-terminal RFamide. When administered to blowflies, dRYamide-1 suppressed feeding motivation. We propose that dRYamides are related to the NPY family in vertebrates, similar to dNPF and sNPF.

A putative octopamine/tyramine receptor mediating appetite in a hungry fly

In the blowfly Phormia regina, experience of simultaneous feeding with D: -limonene exposure inhibits proboscis extension reflex (PER) due to decreased tyramine (TA) titer in the brain. To elucidate the molecular mechanism of TA signaling pathway related to the associated feeding behavior, we cloned cDNA encoding the octopamine/TA receptor (PregOAR/TAR). The deduced protein is composed of 607 amino acid residues and has 7 predicted transmembrane domains. Based on homology and phylogenetic analyses, this protein belongs to the OAR/TAR family. The PregOAR/TAR was mainly expressed in head, with low levels of expression in other tissues at adult stages. Gene expression profile is in agreement with a plethora of functions ascribed to TA in various insect tissues. The immunolabeled cell bodies and processes were localized in the medial protocerebrum, outer layer of lobula, antennal lobe, and subesophageal ganglion. These results suggest that decrease of TA level in the brain likely affects neurons expressing PregOAR/TAR, causing mediation of the sensitivity in the sensillum and/or output of motor neurons for PER.

Isoform- and cell-specific function of tyrosine decarboxylase in the Drosophila Malpighian tubule

The biogenic amine tyramine (TA) is a potent diuretic factor when applied to the Malpighian tubule (MT) of Drosophila melanogaster, stimulating both urine production and transepithelial chloride conductance. Isolated MTs can respond not only to TA but also to its precursor, tyrosine; this observation led to the proposal that MTs are able to synthesize TA from applied tyrosine through the action of the enzyme tyrosine decarboxylase (TDC). In the current study it is shown that the non-neuronal isoform of TDC, Tdc1, is expressed in the principal cells of the MT. A mutant allele of Tdc1, Tdc1(f03311), was identified that reduced expression of the mature Tdc1 transcript by greater than 100-fold. MTs isolated from Tdc1(f03311) homozygous flies showed no significant depolarization of their transepithelial potential (TEP) or diuresis in response to tyrosine while retaining normal sensitivity to TA. By contrast, a previously identified null mutant allele of the neuronal TDC isoform Tdc2 had no effect on either tyrosine or TA sensitivity. To determine in which cell type of the MT Tdc1 expression is required, flies were generated carrying a UAS-Tdc1 transgene and cell-type-specific Gal4 drivers on a Tdc1(f03311) homozygous background. Rescue of Tdc1 expression in principal cells fully restored sensitivity to tyrosine whereas expression of Tdc1 in stellate cells had no rescuing effect. It is concluded that synthesis of TA by Tdc1 in the principal cells of the MT is required for physiological responses to tyrosine. TA synthesis in the MT is the first reported physiological role for Drosophila Tdc1.

Molecular cloning and pharmacological characterization of a Bombyx mori tyramine receptor selectively coupled to intracellular calcium mobilization

Tyramine (TA) is a biogenic amine in invertebrates. cDNA encoding the TA receptor (TAR) BmTAR2 was cloned from the nerve tissue of the silkworm Bombyx mori. The receptor's functional and pharmacological properties were examined in BmTAR2-transfected HEK-293 cells. In [(3)H]TA binding assays, BmTAR2 showed considerably higher affinity for TA than for other biogenic amines, with an IC(50) value of 57.5 nM. Moreover, TA induced a dose-dependent increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) in cells, with an EC(50) value of 11.6 nM, whereas octopamine and dopamine increased [Ca(2+)](i) only at concentrations above 100 microM. A few antagonists were found to inhibit the TA-induced increases in [Ca(2+)](i); the rank order of potency was yohimbine > chlorpromazine > mianserin. TA showed no effect on intracellular cAMP concentration. The data indicate that BmTAR2 belongs to the second class of TARs, which are selectively coupled to intracellular Ca(2+) mobilization. RT-PCR analysis revealed that BmTAR2 was expressed predominantly in the nervous tissue of B. mori larvae, suggesting that TA has neurotransmitter and neuromodulatory roles that are mediated by BmTAR2.

Tyramine: from octopamine precursor to neuroactive chemical in insects

It is well acknowledged that tyramine acts as the biosynthetic intermediate precursor for octopamine. This fact has biased the interpretation of biological effects of tyramine towards an artifact of it being a partial agonist on octopamine receptors. Over recent years there has been an accumulation of evidence to show that tyramine is in fact a neuroactive chemical in its own right, with diverse physiological/behavioral roles. In addition, tyramine plays a unique role in a non-neuronal tissue, namely the Malpighian tubules. This review examines this evidence, taking into account the criteria that need to be satisfied in order to claim neuroactive chemical status. Thus, the evidence points to tyramine being synthesized by, and present in, neurons; capable of being released from neurons; removed by high affinity plasma membrane transporters; acting upon specific tyramine receptors; and producing physiological/behavioral effects that can be blocked by antagonists. This composite evidence is strong, although the final proof still awaits analysis on a uniquely identifiable tyraminergic neuron as has been possible with octopamine.

Cloning and expression pattern of a putative octopamine/tyramine receptor in antennae of the noctuid moth Mamestra brassicae

In insects, biogenic amines have been shown to play an important role in olfactory plasticity. In a first attempt to decipher the underlying molecular mechanisms, we report the molecular cloning and precise expression pattern of a newly identified octopamine/tyramine-receptor-encoding gene in the antennae of the noctuid moth Mamestra brassicae (MbraOAR/TAR). A full-length cDNA has been obtained through homology cloning in combination with rapid amplification of cDNA ends/polymerase chain reaction; the deduced protein exhibits high identities with previously identified octopamine/tyramine receptors in other moths. In situ hybridization within the antennae has revealed that MbraOAR/TAR is expressed at the bases of both pheromone-sensitive and non-sensitive olfactory sensilla and in cells with a neurone-like shape. In accordance with previous physiological studies that have revealed a role of biogenic amines in the electrical activity of the receptor neurones, our results suggest that biogenic amines (either octopamine or tyramine) target olfactory receptor neurones to modulate olfactory coding as early as the antennal level.

Odour of limonene affects feeding behaviour in the blowfly, Phormia regina

Experiences of odours at meals are thought to affect food preference in many animals. To study appetite modulation by odours, we established a new experimental system based on the modification of a previous method, where flies fed sucrose flavoured with D-limonene subsequently showed reduced appetite to plain sucrose. In this new experimental system, a fly population was divided into two groups: (1) the "simultaneous" group of flies was exposed to D-limonene and sucrose simultaneously for 10 min, and (2) the "separate" group of flies was exposed to sucrose alone for 10 min, and 1h later, to limonene for 10 min. The appetite of flies in the "separate" group for sucrose was unaffected by the experiment, but the appetite of flies in the "simultaneous" group was significantly decreased, and this effect lasted for > or = 3 days. To investigate if this appetite modulation by D-limonene was based on long-term memory formation (protein synthesis), we examined the effects of the protein synthesis inhibitor, cycloheximide. Injection of cycloheximide 1h after exposure to limonene and sucrose inhibited the appetite suppression in the "simultaneous" group of flies. In addition, to exclude the possibility that D-limonene exerted its effect through taste, rather than odour, we examined the effect of removing of the olfactory organs, antennae and maxillary palps on appetite modulation by D-limonene. When the olfactory organs were removed, no reduction in appetite was observed in the flies in the "simultaneous" group, indicating olfaction. In our new and effective appetite-modulation system for flies, modulation of appetite by olfactory detection of D-limonene was shown.

Enhanced serotonin transporter function during depression in seasonal affective disorder

Decreased synaptic serotonin during depressive episodes is a central element of the monoamine hypothesis of depression. The serotonin transporter (5-HTT, SERT) is a key molecule for the control of synaptic serotonin levels. Here we aimed to detect state-related alterations in the efficiency of 5-HTT-mediated inward and outward transport in platelets of drug-free depressed patients suffering from seasonal affective disorder (SAD). 5-HTT turnover rate, a measure for the number of inward transport events per minute, and tyramine-induced, 5-HTT-mediated outward transport were assessed at baseline, after 4 weeks of bright light therapy, and in summer using a case-control design in a consecutive sample of 73 drug-free depressed patients with SAD and 70 nonseasonal healthy controls. Patients were drug-naive or medication-free for at least 6 months prior to study inclusion, females patients were studied in the follicular phase of the menstrual cycle. All participants were genotyped for a 5-HTT-promoter polymorphism (5-HTTLPR) to assess the influence of this polymorphism on 5-HTT parameters. Efficiency of 5-HTT-mediated inward (p=0.014) and outward (p=0.003) transport was enhanced in depressed patients. Both measures normalized toward control levels after therapy and in natural summer remission. Changes in outward transport showed a clear correlation with treatment response (rho=0.421, p=0.001). Changes in inward transport were mediated by changes in 5-HTT transport efficiency rather than affinity or density. 5-HTTLPR was not associated with any of the 5-HTT parameters. In sum, we conclude that the 5-HTT is in a hyperfunctional state during depression in SAD and normalizes after light therapy and in natural summer remission.

Multimodal chemosensory integration through the maxillary palp in Drosophila

Drosophila melanogaster has an olfactory organ called the maxillary palp. It is smaller and numerically simpler than the antenna, and its specific role in behavior has long been unclear. Because of its proximity to the mouthparts, I explored the possibility of a role in taste behavior. Maxillary palp was tuned to mediate odor-induced taste enhancement: a sucrose solution was more appealing when simultaneously presented with the odorant 4-methylphenol. The same result was observed with other odors that stimulate other types of olfactory receptor neuron in the maxillary palp. When an antennal olfactory receptor was genetically introduced in the maxillary palp, the fly interpreted a new odor as a sweet-enhancing smell. These results all point to taste enhancement as a function of the maxillary palp. It also opens the door for studying integration of multiple senses in a model organism.

Regulation of feeding behaviour and locomotor activity by takeout in Drosophila

The hormonal regulation of feeding behaviour is well known in vertebrates, whereas it remains poorly understood in insects. Here, we report that the takeout gene is an essential component of nutritional homeostasis in Drosophila. takeout encodes a putative juvenile hormone (JH) binding protein and has been described as a link between circadian rhythm and feeding behaviour. However, the physiological role of takeout and its putative link to JH remain unknown. In this study, we show that takeout (to(1)) flies failed to adapt their food intake according to food availability and that most defects could be genetically rescued. When food is abundant, to(1) are hyperphagic, yielding to hypertrophy of the fat body. When food reappears after a starvation period, to(1) flies do not increase their food intake as much as wild-type flies. This defect in food intake regulation is partly based on the action of Takeout on taste neurons, because the sensitivity of to(1) gustatory neurons to sugars does not increase after starvation, as in wild-type neurons. This lack of regulation is also evident at the locomotor activity, which normally increases during starvation, a behaviour related to food foraging. In addition, to(1) flies lack sexual dimorphism of locomotor activity, which has previously been linked to the JH circulating level. Moreover, application of the JH analog methoprene rescues the phenotype. These results suggest that takeout plays a central role as a feeding regulator and may act by modulating the circulating JH level.