Final Activity Report Summary - IMAGING SYNAPTIC ACT (Synaptic modulation during olfactory perception in Drosophila melanogaster) In this project, we have studied the olfactory behaviour of adult Drosophila melanogaster flies whose genetic constitution allows to increase or to reduce the number of synapses established by two types of central neurons in the olfactory pathway, local interneurons and projection neurons. To that end, we have previously identified two genes -previously known as elements of the insulin receptor signalling pathway- : the phosphastidylinositol-3-kinase (PI3K) and the glycogen synthase kinase-3 (GSK3) having a key role in synaptogenic activity. Thus, the PI3K overexpression in specific neurons of the Drosophila brain produces an increase of almost 200% in the number of synapses that they establish with their partners, whereas Shaggy/GSK3 overexpression in the same cells induces a loss of 30% in synapse number. Our results have revealed striking changes in olfactory behaviour due to changes in the synapse number in both types of central olfactory neurons.First, we have targeted specifically the overexpression of the Shaggy/GSK3 gene in a subgroup (30%) of local interneurons of the antennal lobe. This targeted expression produces in these neurons a drastic reduction in their synaptic signal but also in their axonal as well as dendritic branching. It is necessary to note that local inhibitory interneurons ramify profusely and send processes to almost all glomeruli of the antennal lobe where they establish synaptic contacts with sensory as well as projection neurons. The behavioural consequence of this genetic manipulation is an increase in the repulsion response to an extensive array of concentrations (10-1 to 10-6) when compared to control flies. This phenotype is independent of the chemical odour identity of the tested odorants: benzaldehyde, propionic acid as well as ethyl hexanoate, one odour normally considered as very attractive in natural fly environment. We think that the loss of inhibitory synapses due to Shaggy/GSK3 overexpression (quantified as -30% in other neuronal areas) conduces to a large glomerular activation in the antennal lobe. For a huge range of concentrations, this activation can produce a change in the odour perception code shifting to a 'higher concentrated odor' in these modified flies and yielding finally to their abnormal repulsive behavioural response. Second, in targeted projection neurons (almost 50%) the overexpression of PI3K protein produces a great increase in synaptic signal, in neuronal branching and also in the size of the synaptic terminals and boutons. These neurons send the olfactory information to higher olfactory brain areas in Drosophila: the mushroom bodies and the lateral protocerebrum.We have performed behavioural tests in these flies with respect to control flies and we have found remarkable differences between both classes of individuals related crucially to odour identity and concentration. This data implies that the processing of olfactory information in higher areas is strongly dependent on the odour identity and intensity and, more interestingly, that increasing the synapse number in projection neurons can change in a subtle manner the odour perception code in higher areas. Taken together, these current results have allowed also to open a new avenue of research in which we plan to examine the changes of synaptic activity in vivo in selected neurons by employing synaptopHlourin-coupled reporter constructs in the glomeruli of living flies imaged during the application of the odour to the antenna.
