Pathogen infection of vertebrate hosts is achieved due to the ability of invading organisms manipulate immunosuppressive host defenses. Such mechanisms are widely investigated in mammalian diseases. However in tropical diseases transmitted by insects the parasite must first be able to evade from vector immune responses in order to achieve colonization and transmission to their ultimate hosts.  Rhodnius prolixus is a blood-sucking bug whose saliva contains several antihaemostatic active compounds. Among them all such salivary glands synthesize and store nitric oxide (NO). NO is a unique molecule used by the bug to feed on blood since of its powerful vasodilatory activity. In the present study we have infected fifth instar Rhodnius prolixus nymphs with T. rangeli in order to determine the mechanism by which such parasite interferes with the enzyme nitric oxide synthase (NOS).  Rhodnius salivary glands were dissected in phosphate buffer in 1, 7 and 21 days after infection. Nitric oxide synthase activity was assayed and its levels in diferent days post-infection were determined by western-blotting. Futhermore, NOS mRNA was reverse transcribed and semi-quantitative RT-PCR was performed. There was a 50% decrease in NOS activity and in infected animals when compared to uninfected ones. The reduced NOS levels and activity observed are in agreement with the down-regulation of NOS mRNA synthesis as shown by RT-PCR assays. Therefore, the parasite modulates NOS at the transcriptional level. We have then tested if a parasite-derived glycoinositolphospholipids (GIPL) may have a role in the down-regulation of nitric oxide synthase in our model. Fifth instar Rhodnius nymphs were injected with 0,1 mM GIPLs. One day later, total mRNA was extracted and semi- quantitative RT-PCR was performed. In this experiment the injection of GIPLs strongly reduced the transcription of NOS mRNA. Such result implies that signaling cascades triggered by the parasite and that down regulate NOS is mimicked only by the presence of GIPL. In the future protein phosphorylation profiles will be constructed with the aid of phosphoproteomic tools in either GIPL treated or non-treated salivary glands in order to obtain a complete map of the vector signaling cascade manipulated by parasite infection on salivary glands. 
 Supported by: CNPq (PIBIC-UFRJ), IFS, FAPERJ, PADCT, OMS