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Liria Masuda-Nakagawa (Department of Genetics, University of Cambridge)
The mushroom bodies (MBs) of insects are higher brain centers essential for associative olfactory learning. We previously made a sensory map of all 21 olfactory sensory neurons (OSNs) in the calyx, the sensory input region, of Drosophila larval MB. Stimulation of single OSNs evokes selective activity in calyx glomeruli. In contrast to stereotypic projection neuron (PN) innervation, the pattern of connectivity of Kenyon cells (KCs) is consistent with them processing inputs by a random combinatorial mechanism that can discriminate a large number of odors.
To understand the circuit mechanisms governing selectivity of odor representations in KCs, we have identified GAL4 lines that label non-PN non-KC neurons that innervate widely in the calyx. We found three classes of neuron: one is similar in morphology to the adult APL and is the sole detectable GABAergic input in the larval calyx. The larval APL responds to odors, and blocking synaptic output from KCs decreases larval APL activity. Therefore, the larval APL mediates a negative feedback loop connecting MB output to MB input in the calyx. A second class of neuron is Tdc2-octopaminergic neurons, and a third is odd-like neurons.
We are now addressing how the larval APL, octopaminergic and odd-like neurons fit within the calyx circuitry. We have analyzed the polarity of these neurons and are using GRASP to assess the extent of the connections between them. Our data will help reveal the logic of information processing