Gustatory Processing in Drosophila Higher Brain Centers

The sense of taste, which allows animals to distinguish between nutritious and toxic substances, is essential for survival. However, we know very little about how taste information is processed by the brain, and how this processing allows for complex behaviors, such as those involved in learning and memory tasks. Using the model system Drosophila melanogaster, I investigated taste circuitry and the representation of taste information in the higher brain. In the first part of this dissertation, I examine the neural basis of taste memory in Drosophila. First, I establish the role of the mushroom body in aversive taste conditioning, and show that the γ lobe Kenyon Cells, in particular, are necessary for this type of conditioning. I also show the requirement of dopaminergic PPL1 neurons in taste conditioning and demonstrate that most of these neurons are bitter-responsive using calcium imaging. Through an extensive analysis of taste activation in the calyx of the mushroom body, I show how taste is represented in this structure, revealing sparse, modality-specific activation in the mushroom body dendrites in response to taste stimulation. The second part introduces a new class of neurons, the Taste Projection Neurons, or TPNs, which we identify and classify through anatomical, functional and behavioral studies. Each of these neurons overlaps with primary sensory projections of a single modality, and responds in a modality-specific manner. We show that all three of these neurons are capable of influencing proboscis extension, an innate feeding behavior. Two of these neurons, TPNs 2 and 3 extend projections into the higher brain and are necessary for aversive taste conditioning, and activation of TPN3 is sufficient to act as the unconditioned stimulus during associative learning. Together, these studies examine the multiple roles taste information plays in the higher brain and provides a foundation for future studies of taste circuitry and learning and memory in Drosophila