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Physiologie des récepteurs gustatifs chez la mouche de vinaigre (Drosophila melanogaster)

Abstract : In most animals including insects, ingestion is preceded by a close examination of the food, for example in order to detect the presence of potentially noxious chemicals. This detection involves specialized gustatory cells, which are generally described as sensitive to “bitter” tastes. Using electrophysiology and behavioral observations, we studied how a model insect, Drosophila melanogaster, can detect potentially toxic substances (described here as “bitter”) when mixed with sugar molecules, with their gustatory neurons. In a first part, we studied how L-canavanine is detected. Lcanavanine is a pseudo amino acid, which is confounded with L-arginine by the metabolism. Proteins which include Lcanavanine are non-functional and this compound is toxic for animals including insects. Using genetic constructions based on the UAS-Gal4 expression system, we showed that Lcanavanine is detected by gustatory cells expressing a receptor protein, GR66a, which is specific to most cells capable of detecting bitter substances. We also showed that, contrary to caffeine, the detection of L-canavanine requires functional Gαo proteins. Then, we studied some aspects of the detection of mixtures of sweet and bitter molecules. In a first approach, we contributed to establish that L-canavanine does not impact sugar detection, while other chemicals like strychnine completely inhibit sugar detection. By using the UAS-Gal4 system to ablate bitter-sensitive cells, we could demonstrate that such inhibition is a specific property of sugar- sensitive cells. These cells should have thus receptors for bitter substances which have not been identified yet. We also examined the reverse interaction, which is a possible role of sweet molecules to inhibit the detection of bitter substances. We examined the detection of denatonium, berberine, caffeine and umbelliferone in the presence of 12 different sugars, using behavioral and electrophysiology observations. By using genetic construction to ablate sugar-sensitive cells, we found that the sugar inhibitory action is not due to the presence of sugar-sensitive cells. It should be noted, however that in our experimental conditions, this inhibitory action is less efficient than the inhibition of bitter upon sugar detection. In a last part, we examined the modulation of gustatory perception by analogs of leucokinine, which is a neuropeptide involved in the diuresis of insects. We show that these analogs, when mixed with sugars in solution, can inhibit sugar detection by gustatory sensilla, both in Aedes aegypti mosquitoes and in Drosophila. The detection of bitter molecules by gustatory neurons in Drosophila thus involves two main coding channels: one is specific, and involves gustatory cells dedicated to the detection of bitter molecules; the second one, less specific, is affecting cells which are dedicated to the detection of sugar molecules. Gustatory coding is thus a more complex phenomenon than previously thought on the basis of examining responses to single molecules, thus urging to study the responses of gustatory receptors to more complex and natural mixtures.
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Moutaz Ali Agha. Physiologie des récepteurs gustatifs chez la mouche de vinaigre (Drosophila melanogaster). Biologie animale. Université Paris Saclay (COmUE), 2016. Français. ⟨NNT : 2016SACLA037⟩. ⟨tel-01761486⟩

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