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Identifying higher-order neurons involved in the taste circuitry in Drosophila melanogaster via an optogenetics screen Lau, Celia Kit Si


The ability to detect various tastes greatly enhances survivorship. The detection of nutritious or toxic foods leads to a promotion or inhibition of feeding. Although the characterization of taste-detection at the periphery is extensive, their connection to second-order taste neurons is only beginning to be elucidated. In Drosophila melanogaster, we can harness powerful genetic tools to help map out the neuronal circuitry that translates the sensory inputs of taste to the motor commands of feeding. Our objective is to identify higher-order neurons that bridge this sensorimotor circuitry. Understanding how taste information is processed in fruit flies may shed light on how its mammalian counterparts do so, as well. To accomplish this, we exploit the Sip-Triggered Optogenetic Behaviour Enclosure (STROBE) for its high-efficiency and biological relevance to screen for potential taste neurons. In total, 123 driver lines were selected and screened through optogenetic neuronal activation. One line, in particular, R70C07-GAL4, was chosen for the further characterization of its role in feeding inhibition. It predominantly labels two clusters of cell bodies that are bilateral to the brain’s primary taste center, the subesophageal zone (SEZ). It is predicted to be responsible for aversive feeding behaviour. To test this, we created split-GAL4 lines to narrow down this population before GRASP and additional optogenetic activation experiments were performed to confirm the identity of neurons that are responsible for altering feeding behaviour. Upon activation, one subset of the lateral SEZ population was revealed to induce significantly aversive feeding behaviour, while another induced appetitive feeding. Surprisingly, both subsets show a positive GRASP signal with both sweet and bitter gustatory receptor neurons (GRNs). The evidence presented here demonstrates that clear feeding preferences can be made even by activating a population of neurons that communicates with GRNs of opposing valence. This raises the question of whether feeding decisions are instigated by the relative activity between pathways of opposing taste valences, and if this ratio of activity is already encoded at the level of second-order taste neurons, possibly challenging the extension of the labelled-lines theory into higher-order neurons.

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