Insect’s olfactory system and brain network: an all-optical investigation

Insect brains are an important model in neurobiology, thanks to their small sizes, but rich performances. We engineered transgenic lines of Drosophila melanogaster co-expressing calcium sensors and opsins in specific brain areas. We further set up an apparatus allowing for blue diode laser optogenetic activation combined with in vivo two-photon calcium imaging. Two-photon microscopy, thanks to its high 3D resolution and deep penetration depth, is an optimal tool to study morphology and functions of the insect olfactory system in vivo, in order to investigate various aspects of information coding and processing. Optogenetic activation of specific nodes of the primary olfactory centers (antennal lobes) allow modulating neuronal activity with high selectivity and precision both in the temporal and spatial domain, permitting us to study the connectivity and the mechanisms of stimulus processing. The all-optical approach in Drosophila allows for neuronal control down to cellular resolution and simultaneous whole brain read-out. We provide our first results on the parameters optimization of this all optical protocol, calibrating the powers of the two lasers in order to obtain defined excitation while minimizing photodamage. Moreover, we show test for limitations due to spectral overlap of opsin and calcium sensor, permitting the use of the best performing molecules, e.g. the combination of ChR2 and GCaMP. Finally, we present preliminary results on antennal lobe connectivity, stimulating specific glomeruli and olfactory units and reading out the response within the primary olfactory centers and the whole brain, respectively.