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Examining how pharyngeal taste input and internal physiological context influence feeding decisions in Drosophila

University of British Columbia

To survive animals must find and consume nutritive foods. The chemical composition of food sources is evaluated using gustation. Because of its importance to survival, feeding behaviors are tightly regulated. Changes in feeding occur in response to the external environment and the animal’s internal physiological state. Expendable behaviours can be suppressed during starvation to prioritize feeding. This thesis examines the role of two factors in feeding decisions: the location of taste input and starvation. Pharyngeal sense organs in Drosophila are the last evaluation point before food is ingested. It was previously unclear whether they served a unique function in feeding. To investigate this, we focused on appetitive gustatory pharyngeal neurons and showed they express nine gustatory receptors which respond to sweet compounds. Mutants lacking peripheral taste have functional pharyngeal sense organs. In the absence of peripheral taste cues, the pharyngeal sense organs can drive the choice and ingestion of sweet compounds by sustaining consumption. Knocking down pharyngeal neurons in these mutants allowed us to examine a sweet-blind fly in a short term feeding assay. Putatively sweet-blind flies do not show a preference for sweet compounds in a short-term feeding assay, suggesting that nutrient sensing is not operating in this context. Starved flies have an increased tolerance for bitter foods, which is mediated by sensitization and desensitization of sweet and bitter taste, respectively. Mechanisms that cause the sensitization of sweet taste have been studied, but those that underlie desensitization of bitter taste were unknown. We identify a pair of octopaminergic/tyraminergic modulatory neurons called the ventrolateral cluster of octopaminergic neurons (OA-VLs). Because OA-VLs exist in close physical proximity to bitter neuron axon terminals but are not postsynaptic to them, we examined their function as modulators of bitter neuron output. Tonic firing rate of OA-VLs decreases as a function of starvation. Octopamine and tyramine are sufficient to potentiate bitter neuron response in starved flies, suggesting that reduction in OA-VL activity during starvation depotentiates bitter neuron output. Silencing OA-VLs causes a reduction in bitter neuron output and increased acceptance of bitter compounds. OA-VLs may act directly on bitter sensory neurons through the Oct-Tyr receptor.

Text

2017-01-21

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/60200

Zoology

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/