Olfactory Transduction and Taste Processing in Drosophila

Abstract

We completed two separate studies examining chemosensation in Drosophila. The first study investigated taste processing. It was our aim in this study to identify and characterize higher-order gustatory neurons. Our strategy for tackling this problem involved complementary functional and anatomical approaches. First, we used calcium imaging to screen for cells responding to stimulation of gustatory receptor neurons. Second, we used photo-activatable GFP to localize the cell bodies of neurons innervating the gustatory neuropil. Third, based on the information we gained from these imaging experiments, we were able to identify some promising Gal4 lines that labeled candidate gustatory neurons. Fourth and finally, we made whole-cell patch clamp recordings from these candidate gustatory neurons while stimulating the proboscis with tastants. Unfortunately, none of these candidates turned out to be gustatory neurons. However, this study illustrates a flexible and powerful general approach to identifying and characterizing sensory neurons in the Drosophila brain. The second study investigated olfactory transduction. Specifically, we examined the effect of air speed on olfactory receptor neuron responses (ORNs) in Drosophila. We constructed an odor delivery device that allowed us to independently vary concentration and air speed, and we used a fast photoionization detector to precisely measure the actual odor concentration at the antenna while simultaneously recording spikes from ORNs in vivo. Our results demonstrate that Drosophila ORN odor responses are invariant to air speed, as long as odor concentration is kept constant. This finding was true across a >100-fold range of air speeds. Because odor hydrophobicity has been proposed to affect the air speed dependence of olfactory transduction, we tested a >1,000-fold range of hydrophobicity values, and found that ORN responses are invariant to air speed across this full range. These results have implications for the mechanisms of odor delivery to Drosophila ORNs. Our findings are also significant because flies have a limited ability to control air flow across their antennae, unlike terrestrial vertebrates which can control air flow within their nasal cavity. Thus, for the fly, invariance to air speed may be adaptive because it confers robustness to changing wind conditions.