Comparison of structurally analogous allatotoxins on the molting and morphogenesis of Rhodnius prolixus,and the reversal of ecdysial stasis by ecdysone

The allatotoxic effect of 3-ethoxy-4-methoxy-6-iso-pentenylphenol on nymphal molting and metamorphosis of Rhodnius prolixus was examined. Continuous contact treatment with IPP induced the formation of precocious adults and retarded molting or initiated a permanent ecdysial stasis. Insects treated with 7-ethoxy-6-methoxy-2,2-dimethylchrornene were similarly affected. Ecdysone given orally counteracted the ecdysial stasis and also reduced the duration of the molting delay caused by IPP.     

Characterization of an ecto-phosphatase activity in malpighian tubules of hematophagous bug Rhodnius prolixus

We have characterized a phosphatase activity present on the external surface of intact Malpighian tubules in Rhodnius prolixus. This phosphatase hydrolyses the substrate p-nitrophenyl phosphate at a rate of 3.38 +/- 0.07 nmol Pi x mg(-1) x min(-1). Phosphatase activity decreased with the increase of the pH from 6.4 to 7.6 pH, a range in which tubules cellular integrity was maintained for at least 1 h. Classical inhibitors of acid phosphatase, such as ammonium molybdate, fluoride, vanadate, mpV-PIC, and bpV-PHEN, caused different patters of inhibition. The ecto-phosphatase present an apparent Km of 1.67 +/- 0.34 mM and Vmax of 5.71 +/- 0.37 nmol Pi x mg(-1) x min(-1) for p-NPP. Zinc chloride inhibited 78.2% of ecto-phosphatase activity, with Ki of 0.35 mM. Such inhibition was reversed by incubation with cysteine and GSH, but not DTT, serine, and GSSG, showing that cysteine residues are important for enzymatic activity. Phosphatase activity increased 141% three days after blood meal, and returned to basal levels 2 days later. These results suggest that ecto-phosphatase activity could be involved in a diuretic mechanism, essential in the initial days after a blood meal for the control of Rhodnius homeostasis.     

Hydrogen peroxide detoxification in the midgut of the blood-sucking insect, Rhodnius prolixus

Here we investigated H2O2 production and detoxification in the hematophagous hemiptera, Rhodnius prolixus. Superoxide dismutase (SOD) catalyzes the dismutation of superoxide radical (O2-). This reaction produces hydrogen peroxide, which is scavenged by antioxidant enzymes such as catalase (CAT). SOD and CAT activities were found in all tissues studied, being highest in the midgut. CAT was dose-dependently inhibited in vivo by injections of 3-amino-1,2,4-triazole (AT). Insects treated with AT showed a twofold increase in H2O2 levels. Injection of DL-buthionine-[S, R]-sulfoximine (BSO), an inhibitor of glutathione synthesis, also resulted in a fourfold increase in H2O2, together with stimulation of CAT activity. Simultaneous administration of both AT and BSO had a synergistic effect on midgut H2O2 content. Taken all together, our results suggest that CAT and glutathione-dependent mechanisms cooperate to control H2O2 concentration in the midgut cell and prevent hydroxyl radical generation by Fenton reaction in this tissue.     

Partial localization of a brain factor inhibiting egg production in the blood-feeding insect,Rhodnius prolixus

A previous study by Davey [Can J Zool 4:243–249 (1987)] showed that egg production in the blood-feeding insect Rhodnius prolixus is greatly enhanced when the corpus allatum (CA) is denervated. This result supported findings of others that the brain of Rhodnius imposes an inhibition on the CA via its connections to the CA. The present study identifies the nervus corporis cardiacum II (NCCII) as the nerve responsible for this inhibitory influence. Transecting the NCCII before feeding causes a dramatic increase in the number of eggs made. Since the NCCII is a relatively small nerve that may contain only axons of lateral and posterior neurosecretory cells in the protocerebrum, this result suggests that one or both of these cell types produce the brain factor inhibiting egg production in this insect. Arch. Insect Biochem. Physiol. 39:126–131, 1998. © 1998 Wiley-Liss, Inc.     

Ultrastructural changes of Drosophila larval and prepupal salivary glands cultured in vitro with ecdysone

Summary Alterations in the ultrastructure of in vitro cultured larval salivary glands of Drosophila melanogaster in response to the steroid hormone ecdysone were studied in relation to complex changes in puffing patterns. We found that the changes in the fine structure of cultured glands reflected progression of the puffing pattern, and they paralleled those seen in vivo. We observed that glue secretion by exocytosis, the main function of salivary glands, took place between puff stage 5 (PS5) and PS7. Glue could not be expectorated under culture conditions but was slowly released from the lumen through a duct into the medium. After the cultured glands reached PS13/PS14, further progress of puffing and fine structural alterations required that the ecdysteroid titer be transiently extremely low or absent. Under in vitro conditions we did not observe the putative new secretory program(s) described for glands in vivo after PS12. However, ultrastructural changes which unambiguously indicated that an autohistolytic process had begun in vitro started to appear after PS17. Many salivary gland cells developed numerous features of progressive self-degradation between PS18 and PS21. Actual degradation of salivary glands in vivo seemed to be rapid, but in vitro degradation was never completed, probably due to a lack of exogenous factors from the hemolymph. Manipulations of ecdysone titer in vitro in the culture medium, known during the larval puffing cycle to cause premature induction of developmentally specific puffing patterns, did not affect the normal development of ultrastructural features of the cytoplasm and nucleus.     

The effect of C18 juvenile hormone and Altosid on the efficiency of egg production in Rhodnius prolixus

The product (R) of the weight of the blood meal and the initial weight of the insect is shown to be a reliable predictor of egg production. The egg production efficiency (E), defined as the number of eggs produced per R, has a value characteristic of virgin females, and another, higher, value characteristic of mated females. Topical applications of C18 JH or Altosid to virgin females increase the value of E to the mated level in a fashion which suggests that these compounds act via a trigger mechanism. These compounds do not affect the rate at which oviposition occurs.     

Correlation between larval weights,endocrine gland activities and competence period during differentiation of workers and soldiers in Macrotermes michaelseni (Isoptera:Termitidae)

Correlations were established between corpora allata and prothoracic gland activities, changes in fresh weights and competence period for soldier formation in a higher termite species, Macrotermes michaelseni. There was a time-course differential increase in fresh weights observed in pairs of female third (final)-instar larvae of the same age previously adopted by a pair of female reproductives from incipient colonies. A bimodal distribution of these larvae according to weights was evident from the first day of the instar. Heavier larvae were associated with higher corpora allata and prothoracic gland volumes (activities), and differentiation into presoldiers, while lighter larvae had considerably less-active corpora allata and prothoracic glands and developed into workers. The peak in the activities of these glands in both cases marked the end of the competence period.The chain of events that may precede caste differentiation is discussed briefly and the significance of the findings outlined.     

The effects of juvenile hormone, 20-hydroxyecdysone and precocene II on activity of corpora allata and the mode of negative-feedback regulation of these glands in the adult Colorado potato beetle

When the titre of juvenile hormone III in female Leptinotarsa decemlineata was elevated by the implantation of supernumerary corpora allata or by the injection of the hormone, the rate of endogenous hormone production by the host glands was significantly restrained, as determined by the short-term in vitro radiochemical assay. From denervation studies, it is suggested that during phases of elevated juvenile hormone titre, the corpus allatum activity is regulated via humoral as well as neural factors requiring intact nerve connections. Restrainment of gland activity appears to be mainly via the neural pathway. Isolated corpora allata were not influenced by 10^?5 M juvenile hormone III added to the incubation medium in vitro.Studies with farnesenic acid revealed that the final two enzymatic steps in the biosynthetic pathway of juvenile hormone are also diminished during prolonged neural inhibition of the corpora allata.20-Hydroxyecdysone and precocene II had no apparent effect on the corpus allatum activity of Leptinotarsa decemlineata.     

Inhibition of pupariation and adult development in Calliphora vomitoria treated with ivermectin

Puparium formation and adult development in Calliphora vomitoria L. (Diptera: Calliphoridae) are seriously disrupted after treating final-stage larvae with ivermectin. Fifty % of larvae treated topically with 1.01 g ivermectin 2 days before anticipated pupariation fail to form puparia, while the same response requires 2.24 g given one day before pupariation. Permanent larvae so produced remain alive for up to 2 weeks. Of the larvae that pupariate, there is disruption of adult development, leading to larval-adult intermediates. Only 0.1 g ivermectin induces such disruption in 50% of the treated larvae.The effects on puparium formation and adult development are independent. Inhibition of pupariation can be explained in terms of a disturbance of neuromuscular events leading up to pupariation. The effect on adult development is more difficult to explain. This work shows that ivermectin has great potential in clarifying the sequence of events leading up to puparation in cyclorrhaphous Diptera.

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Résumé L'ivermectine est un insecticide qui a été développé dans le but d'éradiquer les parasites chez les bestiaux. Mais ce produit élimine également beaucoup d'insectes herbivores aussi bien en expériences de laboratoire qu'à l'extérieur. Cet insecticide est effectif dans les synapses entre les nerfs et les muscles, là où la substance chimique GABA est active. Cependant lorsque l'ivermectine est administrée aux larves de mouches à viande, Calliphora vomitoria L., elle produit des réactions différentes.En conditions normales, les larves, une fois qu'elles se sont nourries, errent sans but, deux ou trois jours avant de se transformer en nymphes. Si on administre une dose de 1,01 g d'ivermectine aux larves deux jours avant leur transformation anticipée, 50% d'entr'elles n'accompliront pas leur métamorphose. Elles resteront en permanence à l'état larvaire. Mais, si on leur administre une dose un jour seulement avant la transformation anticipée, il faut alors qu'elle de 2,24 g pour pouvoir empêcher la métamorphose de s'opérer. Ces doses ne tuent pas les larves. Si le traitement est donné le jour même de la transformation, celle ci s'opère alors normalement.Cependant un certain nombre de larves se métamorphosent en nymphes malgré leur traitement par ivermectine, mais beaucoup d'entr'elles n'atteindront jamais le stade adulte; elles resteront á un stade intermédiaire entre celui de nymphe et celui d'adulte. La dose nécéssaire pour empêcher le développement de l'adulte est de 0,1 g d'ivermectine. Il semble que les deux effets soient indépendants l'un de l'autre.L'effet produit par l'ivermectine sur la transformation de larve en nymphe peut s'expliquer par le fait que le traitement soit donné avant la métamorphose, affectant le comportement normal par une perte de mobilité mais non de paralysie. Par contre, l'effet sur le développement des adultes est plus difficile à expliquer. Néanmoins, l'ivermectine pourrait être employée très utilement dans les travaux de recherches sur la métamorphose des diptères.