State-dependent serotonergic long-range modulation of local interneurons in the Drosophila larval antennal lobe

Foraging for food is an essential task for all animals. Olfactory cues are especially important for the detection and evaluation of potential food sources. Modulation of olfactory processing allows flexibility in foraging behaviour and adaptation to changing needs, such as hunger. We investigate the modulatory mechanisms of early olfactory processing in the antennal lobe (AL) of Drosophila larvae in detail. What kind of information is fed back to early olfactory centres via long-range recurrent connections from higher brain regions? How is this information integrated into the complex neural network within the larval AL? We focus on a recurrent serotonergic neuron (CSDn), which modulates the larval AL in a state dependent manner. After food deprivation, its response towards a non-food odour is increased. In this proposal, we first investigate if this is a general mechanism across odours or if this is an odour-specific mechanism. Moreover, in the larval AL, serotonergic modulation partly takes place via activation of an inhibitory 5-HT1A receptor in local AL neurons. In fed larvae, these neurons simultaneously receive glutamatergic inhibition from other local neurons. We will further investigate how this double inhibition affects the excitability of local AL neurons and why a single inhibitory mechanism seems not to be sufficient to suppress neuronal activity. Within the Research Unit, we will compare recurrent modulatory processing mechanisms across invertebrate and vertebrate animal models, which will allow us to detect conserved network principles.