Feeding state sculpts a circuit for sensory valence

The valence of sensory stimuli (i.e. a measure of attractiveness or aversiveness that an animal attaches to a stimulus) is flexible and determined by a combination of factors including environment, behavioral state, and experience. Together, these factors prime the nervous system in order to drive appropriate behaviors. In this dissertation I investigate how environmental context and behavioral state regulate the valence of the chemosensory cue carbon dioxide (CO2) in the free-living nematode Caenorhabditis elegans. CO2 can signify the presence of food, conspecifics, and predators. When raised in standard laboratory conditions C. elegans avoids CO2, but this response is flexible. Whereas oxygen, salt, and food all promote CO2 avoidance, food-deprivation shifts CO2-response valence from avoidance to attraction. We have identified two sets of neurons, RIG and AIY, that contribute to the starvation-dependent shift. We have demonstrated that dopamine signaling enhances the CO2-evoked responses in both of these neurons in order to drive avoidance in fed worms. All animals must make ecologically relevant decisions to survive, and our results provide fundamental insights into how neural circuits are dynamically sculpted by internal state and context to drive behavior