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Abstract

In the vinegar fly Drosophila melanogaster, the majority of olfactory receptors mediating
the detection of volatile chemicals found in their natural habitat have been functionally
characterized (deorphanized) in vivo. In this process, receptors have been assigned
ligands leading to either excitation or inhibition in the olfactory sensory neuron where
they are expressed. In other, non-drosophilid insect species, scientists have not yet
been able to compile datasets about ligand–receptor interactions anywhere near as
extensive as in the model organism D. melanogaster, as genetic tools necessary for
receptor deorphanization are still missing. Recently, it was discovered that exposure to
artificially high concentrations of odorants leads to reliable alterations in mRNA levels of
interacting odorant receptors in mammals. Analyzing receptor expression after odorant
exposure can, therefore, help to identify ligand–receptor interactions in vivo without
the need for other genetic tools. Transfer of the same methodology from mice to a
small number of receptors in D. melanogaster resulted in a similar trend, indicating
that odorant exposure induced alterations in mRNA levels are generally applicable
for deorphanization of interacting chemosensory receptors. Here, we evaluated the
potential of the DREAM (Deorphanization of receptors based on expression alterations
in mRNA levels) technique for high-throughput deorphanization of chemosensory
receptors in insect species using D. melanogaster as a model. We confirmed that in
some cases the exposure of a chemosensory receptor to high concentration of its best
ligand leads to measureable alterations in mRNA levels. However, unlike in mammals, we
found several cases where either confirmed ligands did not induce alterations in mRNA
levels of the corresponding chemosensory receptors, or where gene transcript-levels
were altered even though there is no evidence for a ligand–receptor interaction. Hence,
there are severe limitations to the suitability of the DREAM technique for deorphanization
as a general tool to characterize olfactory receptors in insects.