Immunohistological localization of serotonin in the CNS and feeding system of the stable fly Stomoxys calcitrans L. (Diptera: Muscidae)

Morphological analysis and lipid composition of the stable fly Stomoxys calcitrans salivary glands reinforce important features of feeding behaviour

Stomoxys calcitrans L. (Diptera: Muscidae), the stable fly, is a hematophagous insect of great veterinary importance, because it is a mechanical vector of diverse pathogens in livestock. The saliva of blood-feeding insects presents important pharmacologically active molecules that impair blood clotting, promote vasodilation and modulate the host immune system response, crucial processes for successful feeding. These properties also enable pathogens' transmission. In the present work, we describe an efficient protocol to dissect S. calcitrans salivary glands, their morphological characteristics and lipid profile. The mean length of the tubular gland is 3.23 mm with a bulbous posterior end and a narrow anterior end. Histological analysis revealed a monolayer of large polygonal epithelial cells with voluminous nuclei and high lipid content in their cytoplasm. Ultrastructural analysis showed that the epithelium is rich in mitochondria, free ribosomes, Golgi complex cisternae, presenting a great extension of rough endoplasmic reticulum that contains an electron-dense material. Lipid analysis by thin-layer chromatography showed that neutral fatty acids and phosphatidylcholine are predominant in the fly salivary glands. Lysophosphatidylcholine, an important signalling biomolecule involved in different metabolic processes, including host's immunomodulation and pathogens proliferation and differentiation, is also present.

Serotonergic Neurons in the Brain and Gnathal Ganglion of Larval Spodoptera frugiperda

The fall armyworm Spodoptera frugiperda (S. frugiperda) (Lepidoptera: Noctuidae) is a worldwide, disruptive, agricultural pest species. The larvae of S. frugiperda feed on seedling, leave, and kernel of crops with chewing mouthparts, resulting in reduced crop yields. Serotonin is an important biogenic amine acting as a neural circuit modulator known to mediate lots of behaviors including feeding in insects. In order to explore the serotonergic neural network in the nervous system of larval S. frugiperda, we performed immunohistochemical experiments to examine the neuropil structure of the brain and the gnathal ganglion with antisynapsin and to examine their serotonergic neurons with antiserotonin serum. Our data show that the brain of larval S. frugiperda contains three neuromeres: the tritocerebrum, the deutocerebrum, and the protocerebrum. The gnathal ganglion also contains three neuromeres: the mandibular neuromere, the maxillary neuromere, and the labial neuromere. There are about 40 serotonergic neurons in the brain and about 24 serotonergic neurons in the gnathal ganglion. Most of these neurons are wide-field neurons giving off processes in several neuropils of the brain and the gnathal ganglion. Serotonergic neuron processes are mainly present in the protocerebrum. A pair of serotonergic neurons associated with the deutocerebrum has arborizations in the contralateral antennal lobe and bilateral superior lateral protocerebra. In the gnathal ganglion, the serotonergic neuron processes are also widespread throughout the neuropil and some process projections extend to the tritocerebrum. These findings on the serotonergic neuron network in larval S. frugiperda allow us to explore the important roles of serotonin in feeding and find a potential approach to modulate the feeding behavior of the gluttonous pest and reduce its damage.

Small-Molecule Inhibitors of Inward Rectifier Potassium (Kir) Channels Reduce Bloodmeal Feeding and Have Insecticidal Activity Against the Horn Fly (Diptera: Muscidae)

Bloodmeal feeding by the horn fly, Haematobia irritans (L.), is associated with reduced milk production and blood loss that ultimately prevents weight gain of calves and yearlings. Thus, blood feeding by H. irritans causes significant economic losses in several continents. As with other arthropods, resistance to the majority of commercialized insecticides reduces the efficacy of current control programs. Thus, innovative technologies and novel biochemical targets for horn fly control are needed. Salivary gland and Malpighian tubule function are critical for H. irritans survivorship as they drive bloodmeal acquisition and maintain ion- and fluid homeostasis during bloodmeal processing, respectively. Experiments were conducted to test the hypothesis that pharmacological modulation of H. irritans inward rectifier potassium (Kir) channels would preclude blood feeding and induce mortality by reducing the secretory activity of the salivary gland while simultaneously inducing Malpighian tubule failure. Experimental results clearly indicate structurally diverse Kir channel modulators reduce the secretory activity of the salivary gland by up to fivefold when compared to control and the reduced saliva secretion was highly correlated to a reduction in bloodmeal acquisition in adult flies. Furthermore, adult feeding on blood treated with Kir channel modulators resulted in significant mortality. In addition to validating the Kir channels of H. irritans as putative insecticide targets, the knowledge gained from this study could be applied to develop novel therapeutic technologies targeting salivary gland or Malpighian tubule function to reduce the economic burden of horn fly ectoparasitism on cattle health and production.

Fly foregut and transmission of microbes

Two areas of research that have greatly increased in attention are: dipterans as vectors and the microbes they are capable of vectoring. Because it is the front-end of the fly that first encounters these microbes, this review focuses on the legs, mouthparts, and foregut, which includes the crop as major structures involved in dipteran vectoring ability. The legs and mouthparts are generally involved in mechanical transmission of microbes. However, the crop is involved in more than just mechanical transmission, for it is within the lumen of the crop that microbes are taken up with the meal of the fly, stored, and it is within the lumen that horizontal transmission of bacterial resistance has been demonstrated. In addition to storage of microbes, the crop is also involved in depositing the microbes via a process known as regurgitation. Various aspects of crop regulation are discussed and specific examples of crop involvement with microorganisms are discussed. The importance of biofilm and biofilm formation are presented, as well as, some physical parameters of the crop that might either facilitate or inhibit biofilm formation. Finally, there is a brief discussion of dipteran model systems for studying crop microbe interactions.

Distribution of Serotonin-Immunoreactive Neurons in the Brain and Gnathal Ganglion of Caterpillar Helicoverpa armigera

Serotonin (5-hydroxytryptamine, 5-HT) is an important biogenic amine that acts as a neural circuit modulator. It is widespread in the central nervous system of insects. However, little is known about the distribution of serotonin in the nervous system of the cotton bollworm Helicoverpa armigera. In the present study, we performed immunohistochemical experiments with anti-serotonin serum to examine the distribution of serotonin in the central nervous system of H. armigera larvae. We found about 40 serotonin-immunoreactive neurons in the brain and about 20 in the gnathal ganglion. Most of these neurons are wide-field neurons giving rise to processes throughout the neuropils of the brain and the gnathal ganglion. In the central brain, serotonin-immunoreactive processes are present bilaterally in the tritocerebrum, the deutocerebrum, and major regions of the protocerebrum, including the central body (CB), lateral accessory lobes (LALs), clamps, crepine, superior protocerebrum, and lateral protocerebrum. The CB, anterior ventrolateral protocerebrum (AVLP), and posterior optic tubercle (POTU) contain extensive serotonin-immunoreactive process terminals. However, the regions of mushroom bodies, the lateral horn, and protocerebral bridges (PBs) are devoid of serotonin-immunoreactivity. In the gnathal ganglion, the serotonin-immunoreactive processes are also widespread throughout the neuropil, and some process projections extend to the tritocerebrum. Our results provide the first comprehensive description of the serotonergic neuronal network in H. armigera larvae, and they reveal the neural architecture and the distribution of neural substances, allowing us to explore the neural mechanisms of behaviors by using electrophysiological and pharmacological approaches on the target regions.

Physiological characterization and regulation of the contractile properties of the mosquito ventral diverticulum (crop)

In adult dipteran insects (flies), the crop is a diverticulum of the esophagus that serves as a food storage organ. The crop pumps stored contents into the alimentary canal for digestion and absorption. The pumping is mediated by peristaltic contractions of the crop musculature. In adult female mosquitoes, the crop (ventral diverticulum) selectively stores sugar solutions (e.g., nectar); proteinaceous blood meals by-pass the crop and are transferred directly to the midgut for digestion. The mechanisms that regulate crop contractions have never been investigated in mosquitoes. Here we provide the first physiological characterization of the contractile properties of the mosquito crop and explore the mechanisms that regulate crop contractions. Using an in vitro bioassay we found that the isolated crop spontaneously contracts in Ringer solution for at least 1 h and its contractions are dependent on extracellular Ca2+. Adding serotonin (5-hydroxytryptamine, 5-HT) or a membrane-permeable analog of cyclic adenosine monophosphate (cAMP) to the extracellular bath increased the frequency of crop contractions. On the other hand, adding benzethonium chloride (BzCl; a chemical that mimics the effects of myosuppressins), H-89 or Rp-cAMPS (inhibitors of protein kinase A, PKA), or carbenoxolone (an inhibitor of gap junctions) reduced the frequency of the unstimulated, spontaneous and/or 5-HT-stimulated crop contractions. Adding aedeskinin III did not detectably alter crop contraction rates. In addition to pharmacological evidence of gap junctions, we demonstrated that the crop expressed several mRNAs encoding gap junctional proteins (i.e. innexins). Furthermore, we localized immunoreactivity for innexin 2 and innexin 3 to muscle and epithelial cells of the crop, respectively. Our results 1) suggest that 5-HT and myosupressins oppositely regulate contractile activity of the mosquito crop, and 2) provide the first evidence for putative roles of cAMP, PKA, and gap junctions in modulating contractile activity of the dipteran crop.

Opposite effects of 5-HT/AKH and octopamine on the crop contractions in adult Drosophila melanogaster: Evidence of a double brain-gut serotonergic circuitry

This study showed that in adult Drosophila melanogaster, the type of sugar-either present within the crop lumen or in the bathing solution of the crop-had no effect on crop muscle contraction. What is important, however, is the volume within the crop lumen. Electrophysiological recordings demonstrated that exogenous applications of serotonin on crop muscles increases both the amplitude and the frequency of crop contraction rate, while adipokinetic hormone mainly enhances the crop contraction frequency. Conversely, octopamine virtually silenced the overall crop activity. The present study reports for the first time an analysis of serotonin effects along the gut-brain axis in adult D. melanogaster. Injection of serotonin into the brain between the interocellar area shows that brain applications of serotonin decrease the frequency of crop activity. Based on our results, we propose that there are two different, opposite pathways for crop motility control governed by serotonin: excitatory when added in the abdomen (i.e., directly bathing the crop) and inhibitory when supplied within the brain (i.e., by injection). Finally, our results point to a double brain-gut serotonergic circuitry suggesting that not only the brain can affect gut functions, but the gut can also affect the central nervous system. On the basis of our results, and data in the literature, a possible mechanism for these two discrete serotonergic functions is suggested.

Serotonergic Innervation of the Salivary Glands and Central Nervous System of Adult Glossina pallidipes Austen (Diptera: Glossinidae), and the Impact of the Salivary Gland Hypertrophy Virus (GpSGHV) on the Host

Using a serotonin antibody and confocal microscopy, this study reports for the first time direct serotonergic innervation of the muscle sheath covering the secretory region of the salivary glands of adult tsetse fly, Glossina pallidipes Austen. Reports to date, however, note that up until this finding, dipteran species previously studied lack a muscle sheath covering of the secretory region of the salivary glands. Direct innervation of the salivary gland muscle sheath of tsetse would facilitate rapid deployment of saliva into the host, thus delaying a host response. Our results also suggest that the neuronal and abnormal pattern seen in viral infected glands by the Glossina pallidipes salivary gland hypertrophy virus (GpSGHV) is due to a compensatory increased branching of the neurons of the salivary glands, which is associated with the increased size of the salivary glands in viral infected flies. This study shows for the first time serotonin in the cell bodies of the brain and thoracico-abdominal ganglion in adult tsetse, G. pallidipes Austen (Diptera: Glossinidae). A hypothesis is proposed as to whether innervation of the muscle sheath covering of the secretory region of the salivary glands is present in brachyceran compared with nematoceran dipterans; and, a plea is made that more research is needed to develop a blood feeding model, similar to that in the blow flies, for elucidating the various mechanisms involved in production and deployment of saliva.

The role of serotonin in feeding and gut contractions in the honeybee

Serotonin (5-hydroxytryptamine, 5-HT) is involved in the regulation of feeding and digestion in many animals from worms to mammals. In insects, 5-HT functions both as a neurotransmitter and as a systemic hormone. Here we tested its role as a neurotransmitter in feeding and crop contractions and its role as a systemic hormone that affected feeding in adult foraging honeybees. We found 5-HT immunoreactive processes throughout the gut, including on the surface of the oesophagus, crop, proventriculus, and the midgut, as well as in the ventral nerve cord. mRNA transcripts for all four of the known bee 5-HT receptors (Am5-ht1A,2α,2β,7) were expressed in the crop and the midgut suggesting a functional role for 5-HT in these locations. Application of a cocktail of antagonists with activity against these known receptors to the entire gut in vivo reduced the rate of spontaneous contraction in the crop and proventriculus. Although feeding with sucrose caused a small elevation of endogenous 5-HT levels in the haemolymph, injection of exogenous 5-HT directly into the abdomen of the bee to elevate 5-HT in the haemolymph did not alter food intake. However, when 5-HT was injected into directly into the brain there was a reduction in intake of carbohydrate, amino acid, or toxin-laced food solutions. Our data demonstrate that 5-HT inhibits feeding in the brain and excites muscle contractions in the gut, but general elevation of 5-HT in the bee's haemolymph does not affect food intake.

Effects of reserpine on reproduction and serotonin immunoreactivity in the stable fly Stomoxys calcitrans (L.)

Biogenic amines are known to play critical roles in key insect behaviors such as feeding and reproduction. This study documents the effects of reserpine on mating and egg-laying behaviors of the stable fly, Stomoxys calcitrans (L.) (Diptera: Muscidae), which is one of the most significant biting fly pests affecting cattle. Two sperm staining techniques were adapted successfully to reveal the morphology of stable fly sperm, for the first time, and determine successful mating in females through the assessment of sperm transfer. This approach was also applied to assess sperm transfer by males treated with different doses of reserpine. Mating or sperm transfer did not occur in flies during the first 3 days after emergence. Thereafter, the percentage of females that mated increased with age. Reserpine treatment of males reduced sperm transfer in a dose-dependent manner. Older males were more sensitive to reserpine treatment than younger flies. Reserpine treatment of 5 days old females reduced the number of eggs laid, but had no effect on egg-hatching rates. Results of immunoreactivity (IR) experiments indicated that serotonin in the neuronal processes innervating male testes was completely depleted by reserpine within 5h after treatment. This effect was transient as the serotonin immunoreactive signal was recovered in 33.3% of the males at 1 day post-treatment and in 94.4% of the flies at 3 days post-treatment. The results of this study concur with previous findings in other insect species and extend our knowledge of the critical roles biogenic amines play in mating and oviposition behaviors of the stable fly. The work could provide a foundation to further characterize the specific roles of individual biogenic amines and their receptors in stable fly reproduction.