Salivation pattern of Rhodnius prolixus (Reduviidae; Triatominae) in mouse skin

https://doi.org/10.1016/j.jinsphys.2006.01.003Get rights and content

Abstract

The objective of this work was to study the pattern of salivation of triatomines during feeding in mouse skin. Rhodnius prolixus was fed with a solution of the dye acridine orange or fluorescein. The saliva was efficiently labelled with acridine orange, probably due to the difference in pH between the salivary gland (⩽6.0) and the hemolymph (6.5–7.0). This procedure was not effective at labelling the saliva of Triatoma infestans, however, fluorescent labelling of R. prolixus saliva allowed us to demonstrate that salivation occurs during entire feeding process. The saliva is released soon after the bite. In the probing phase, saliva is pumped continuously in the host skin, including around the blood vessels. During the engorgement phase, saliva is observed in a bolus within the blood vessel and some of it is sucked up by the insect, together with blood. The frequency of saliva emission inside the vessels was low (0.51±0.18 Hz). The saliva deposition in the microcirculation is continuous and modulated by the frequency of the cibarial pump because, when functioning at high frequency, cibarial pump sucks almost all saliva to the insect gut. This mechanism would determine the quantity of saliva deposited in the microcirculation as necessary, and consequently minimizing the host's immune response to salivary antigens.

Introduction

Triatomines are paurometabolous insects that are obligatory blood feeders in all stages of the life cycle. Their normal hosts are birds or mammals, although they may also feed on other vertebrates. In addition to producing significant blood loss in their hosts (Schofield, 1981), triatomines are medically important as vectors of the flagellate protozoan Trypanosoma cruzi, causative agent of Chagas’ disease in the Americas (Chagas, 1909).

On introducing the mouthparts in the skin of the host in search of blood, hematophagous arthropods unleash a series of physiological responses related to hemostasis, inflammation and immune response (Ribeiro and Francischetti, 2003).

To allow rapid and efficient feeding, triatomines possess a wide variety of bioactive molecules in the saliva, including anticoagulants (Hellmann and Hawkins, 1964, Hellmann and Hawkins, 1965; Ribeiro et al., 1995; Pereira et al., 1996), vasodilators (Ribeiro et al., 1990, Ribeiro et al., 1993; Ribeiro and Nussenzveig, 1993), antihistamine (Ribeiro and Walker, 1994), sialidase (Amino et al., 1998), sodium channel blocker (Dan et al., 1999), immunosuppressor (Kalvachova et al., 1999), pore former (Amino et al., 2002), complement inhibitor system (Cavalcante et al., 2003) and inhibitors of platelet aggregation induced by collagen (Ribeiro and Garcia, 1981; Noeske-Jungblut et al., 1994), ADP (Ribeiro and Garcia, 1980; Sarkis et al., 1986), arachidonic acid (Ribeiro and Sarkis, 1982), thrombin (Noeske-Jungblut et al., 1995; Francischetti et al., 2000), serotonin, epinephrine and norepinephrine (Andersen et al., 2003).

Although triatomine saliva is important to blood feeding (Ribeiro and Garcia, 1981), the process of salivation is still not well known. Thus, the objective of this work was to study the pattern of salivation of triatomines during feeding on the skin of a vertebrate host.

Section snippets

Mice

Hairless mice (HRS/J) aged 30–40 days, reared in the animal facility care from Department of Parasitology (ICB/UFMG) and with a mean weight of 20 g were used in the study. They were provided with standard rodent chow and water ad libitum. Mice used in the experiments had no prior contact with triatomines.

Insects

Third instar specimens of Rhodnius prolixus (Honduras) and second instars of Triatoma infestans (Bolívia) were used, these having been maintained in colonies in the Department of Parasitology,

Labelling of saliva with vital fluorochromes

Although the exoskeletons of R. prolixus and T. infestans fluoresced 24 h after ingesting small quantities of both acridine orange and fluorescein solution, it was only possible to label the saliva of the R. prolixus effectively with the former (Fig. 1A). Third instar nymphs of R. prolixus that ingested this solution were able to feed on the host and complete their life cycle.

Measurement of pH in saliva and hemolymph

The use of indicator stains in capillaries allowed us to measure the pH of saliva and hemolymph for both triatomine

Discussion

In the present study, the fluorochrome acridine orange was used for the first time to label triatomine saliva. The fact that only the saliva of R. prolixus showed fluorescence can probably be explained by the difference in pH between the saliva (⩽6.0) and the hemolymph (6.5–7.0). Due to the higher pH of the hemolymph, many of the molecules of acridine orange dissolved in it are uncharged and thus cross the cell membranes, diffusing to all regions of the insect body. However, on reaching an acid

Acknowledgements

This study was supported by Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), and benefited from international collaboration through the ECLAT network.

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