Validation of an Optogenetic Approach to the Study of Olfactory Behavior in the T-Maze of Drosophila melanogaster Adults

Abstract

Optogenetics enables the alteration of neural activity using genetically targeted expression of light activated proteins for studying behavioral circuits in several species including Drosophila. The main idea behind this approach is to replace the native behavioral stimulus by the light-induced electrical activation of different points of the circuit. Therefore, its effects on subsequent steps of the circuit or on the final behavior can be analyzed. However, the use of optogenetics to dissect the receptor elements of the adult olfactory behavior presents a challenge due to one additional factor: Most odorants elicit attraction or avoidance depending on their concentration; this complicates the representative replacement of odor activation of olfactory sensory neurons (OSNs) by light. Here, we explore a dual excitation model where the subject is responding to odors while the OSNs are optogenetically activated. Thereby, we can assess if and how the olfactory behavior is modified. We measure the effects of light excitation on the response to several odorant concentrations. The dose-response curve of these flies still depends on odor concentration but with reduced sensitivity compared to olfactory stimulation alone. These results are consistent with behavioral tests performed with a background odor and suggest an additive effect of light and odor excitation on OSNs.

Neuron-glia interaction at the receptor level affects olfactory perception in adult Drosophila

“…In our setup, intense light fully illuminates the T-maze when flies make their choice. 42

Figure 3 Effects of optogenetic Mz317 cell activation or inhibition on olfactory behavior in the T-maze (A) Optogenetic activation is achieved in flies expressing channel rhodopsin in Mz317 cells by light. Only these flies (Mz317-Gal4/UAS-ChR2XXL) in the presence of light (L), N = 25, respond significantly differently from flies that either choose the odorant in the dark (D), N = 24, or cannot express the channel (Canton-S/UAS-ChR2XXL) tested both with light (L), N = 25, or in the dark (D), N = 24, in response to ethyl acetate (10 −1.5 ).

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“…Female flies were used in functional studies as they present stable and robust olfactory responses. 42 …”

“…For the optogenetics experiments the medium was supplemented with all-trans-retinal 300 μM (Sigma-Aldrich, Germany) and the flies were kept in the dark as previously described. 42 …”

“…For optogenetic experiments, a previously described setup was used. 42 In short, an intense white LED light of 200 W, 4000 K and 18,000 lumens (Alverlamp LSPR0200W40, Valencia, Spain), which serves to stimulate both channel rhodopsin and halorhodopsin, was placed 22 cm perpendicular to and illuminated both election tubes. Flies subjected to the light stimulus were subjected to 5 min of light before the test because it yielded more consistent results.…”