Yu-Chieh David Chen of University of California, Riverside will present a lecture entitled: Control of sugar and amino acid feeding via integration of distinct pharyngeal taste inputs.
Abstract: Gustatory input plays instrumental roles in feeding behaviors, including food choice and intake. In adult Drosophila, the gustatory system comprises multiple taste organs throughout the body, including the labellum, legs, wings and pharynx. Gustatory receptor neurons (GRNs) in each taste sensillum of taste organs express distinct repertoires of chemosensory receptors for detecting various chemicals. To understand the neuronal underpinnings of taste-mediated feeding control, it is critical to determine how distinct classes of GRNs contribute to feeding behaviors. Recent studies have reported distinct taste-evoked responses and chemoreceptor expression in GRNs of different taste sensilla, indicating functional heterogeneity of molecularly distinct GRNs. Nevertheless, poorly defined tools for precise genetic control of individual GRNs have precluded genetic dissection of their functional roles in feeding behaviors. Here, we employed a Pox-neuron (Poxn) mutant as a minimal taste system model in which all the external taste sensilla are transformed into mechanosensory sensilla, while all pharyngeal GRNs remain intact. Combined with the genetic toolkit derived from our recent mapping of pharyngeal taste neurons, we investigated the extent to which taste information is integrated at the cellular level to regulate consumption of sugars and amino acids.
We first genetically silenced all pharyngeal GRNs in Poxn mutants with Ir25a>Kir2.1 and found that feeding attraction to appetitive tastants (sugar, amino acids) and avoidance of aversive tastants (bitter, acid, high salt) were both abolished in binary choice assays. These results indicate that taste input is essential for food choice in short-term feeding assays. We then genetically protected single classes of pharyngeal GRNs via selective expression of the GAL80 suppressor in molecularly defined classes of pharyngeal GRNs in Poxn, Ir25a>Kir2.1 taste-blind flies, which allowed a unique opportunity to test principles of taste coding and behavior in animals that possess only one type of functional taste neuron. We found that pharyngeal Gr43a GRNs drive behavioral responses to both sugars and amino acids, and Ca2+ imaging results confirm their multimodal taste sensing property. Moreover, genetic dissection experiments uncovered functionally redundant pharyngeal Ir20a neurons for detecting amino acids. To further investigate the functions of distinct pharyngeal GRNs we have undertaken a suite of single-fly quantitative FLIC assays, which suggest coordination of pharyngeal GRNs in regulating micro-feeding responses to sugars and amino acids. We are also performing complementary optogenetic activation analyses, which has revealed functional specialization between external and internal taste neurons. Taken together, our work has identified previously unexplored appetitive taste coding principles in the pharynx and found evidence for functional overlap as well as functional subdivision among taste neurons.