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Abstract

Taste receptors are essential for recognizing ligands that can signal the presence of nutrients or warn against harmful substances. Although various taste receptor families have been identified, the function of individual receptors is poorly understood. Using Drosophila melanogaster, we investigate the roles that three ionotropic receptors (IR7c, IR56b and IR94e) play in taste processing. These roles allow for optimal food intake and are crucial for the fitness of the fly. Salt must be consumed with control to maintain fluid and ionic balance. In Drosophila melanogaster, low sodium attraction is primarily driven by “sweet” taste neurons that express the sodium-specific receptor IR56b. High salt avoidance is mediated by both “bitter” taste neurons and glutamatergic “high salt” taste neurons. I show that IR7c is expressed in these high salt taste neurons and is necessary for their neuronal responses to monovalent salts. IR25a and IR76b complex with IR7c to detect cation non-selective salts. IR7c mediates avoidance, and this IR7c pathway is essential for modulating KCl (but not NaCl) aversion based on internal salt state. Modulating the palatability of sodium is one way to maintain salt homeostasis. My IR7c findings suggest that a sodium-specific pathway is also required for salt modulation. Indeed, I find that the IR56b pathway is necessary for reducing sodium attraction when flies have been salt-fed. Furthermore, a high-salt diet strongly suppresses the response of sweet taste neurons to sodium but not to sucrose. Thus, the receptor rather than the neuron is modulated by salt intake. This effect is mediated by interoceptive mechanisms rather than sensory adaptation. I investigate which mechanism this might be and how IR56b is regulated. IR94e taste neurons also respond to sodium and may contribute to sodium attraction. We investigated whether the receptor IR94e was involved and found it was not. Instead, we discovered that IR94e was necessary for neuronal responses to glutamate salts. Interestingly, IR94e was not essential for glutamate salt feeding preference but may inhibit feeding. Altogether, I have expanded our understanding of salt taste detection and feeding regulation, both of which are pivotal for health.