What is a suitable method for functional characterization of insect odorant receptors?

Insect olfactory receptors in the peripheral olfactory system play an important role to detect elementary input information from environment. The identification of the OR repertoire is the first step towards understanding how the insect can integrate and process the huge diversity of volatile compounds present in their environment, including signals of food, enemies and mates. Consequentially, several methods for the functional characterization of insect ORs have been developed in which large numbers of biologically relevant odorants can be tested. Therefore, there is a need to compare different approaches to decide which is the best suitable for each specific research problem. Here, we analyze the characteristics of several methods including two-electrode voltage clamp recordings from Xenopus oocyte expressing ORs, and single sensillum recording and behavioral studies with transgenic flies. In our study, we compare methods for functional analysis of ORs in vitro and in vivo taking HassOR13 as the object of our experiments. We found that natural systems are more sensitive than those utilizing transgenic Drosophila. The two electrode voltage-clamp recording is more suitable to functional screenings of large numbers of ORs, while the in vivo transgenic Drosophila system could be more accurate to further validate the function of a specific OR. We also found that, among the different solvents used to dissolve pheromones and odorants, hexane offers a good reproducibility with a high sensitivity. Finally, the function of ORs was indirectly confirmed in transgenic Drosophila, showing that odor-activation of ORs-expressing ORNs can mediate behavioral choice. In summary, our results summarize the advantages and drawbacks of different approaches, thus helping in the choice of the best suitable method to deorphanize insect ORs for each different research purpose.