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A Functional Dissection of the Larval Drosophila Olfactory Neural Circuit
AuthorClark, David Anthony
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The survival of larval insects depends on their ability to navigate toward appetitive odor sources or avoid predation. Both of these tasks involve sophisticated spatiotemporal neural computations performed by the olfactory neural circuit to resolve information that may help locate or avoid odor sources, such as odor identity, concentration, and continuity of often turbulent odor plumes. Drosophila melanogaster larvae can resolve the characteristics of an odor using a relatively simple neural circuit, which consists of Olfactory Receptor Neurons (ORNs), Projection Neurons (PNs), and Local Neurons (LNs). While considerable research has been conducted to determine receptive capabilities of individual ORNs, the behavior motor programs underlying activities of individual or groups of olfactory neurons such as PNs and LNs remains poorly understood, moreover, understanding the spatiotemporal mechanisms by which these distinct groups of neurons work together to create an odor code is the ultimate goal of olfaction science. The volatile nature of odor plumes presents a particular difficulty in replicating naturally occurring temporal variations in odor plume density. Additionally, nonspecific ORN activation by chemical stimuli has thus far been a limiting factor for functional analyses of olfactory behaviors. Here, I describe a novel method that effectively overcomes these challenges by imposing optogenetic or chemical control of individual or groups of olfactory neurons for subsequent behavioral analysis. Behavioral analysis in response to chemical or optogenetic stimulation of ORNs, PNs, or LNs revealed several intriguing findings including distinct motor programs associated with individual ORNs and temporal sensitivity among ORNs where different temporal patterns of stimulation applied to the same ORN can induce different behaviors. Additionally, we found direct behavioral consequences of inducing activity among LNs such as initiation of head sweeps, a behavior analogous to sniffing. While the focus of this research is on olfaction, the same method could be used to study individual or groups of neurons of any type, not just olfactory. The novel optogenetics apparatus described here, has already and will continue to yield fundamental insights into the most basic question in neuroscience: how is behavior controlled by neural activity?