Starved State Dependent Modulation of Olfactory Receptor Neuron Function in Drosophila melanogaster Larva

The ability of olfactory neurons to locate food sources underlies survival in most species of the animal kingdom. This ability of olfactory neurons to process environmental information is often modulated by the animal's internal state such as hunger. The peripheral end of the olfactory circuit consists of first order olfactory receptor neurons (ORNs), that synapse onto the second order projection neurons (PNs), and regulatory local neurons (LNs) that innervate ORNs and PNs. While a considerable amount of information has been generated, in various animal systems, regarding sensory neuron responses to food odorants and modulation of these responses by hunger, much less is known about the extent of modulation that exists among individual sensory neurons and its impact on driving behavioral output. We hypothesized that starvation differentially alters the sensitivity of individual first-order Olfactory Receptor Neurons (ORNs). To test this hypothesis, we exposed starved or non-starved third instar Drosophila larvae to specific odorants to analyze the effect of individual ORN activity on chemotaxis. We used two different behavioral paradigms to analyze the chemotaxis response of larvae to odorants. When tested with odorants that elicit strong physiological responses from individual ORNs, starved and non-starved larvae showed different behavioral responses in these behavioral paradigms. However, the extent of behavioral differences among starved and non-starved larvae varied when different odorants were tested in the assays. Further, we provide evidence that this modulation of ORN function by starvation is mediated by GABA signaling pathway. To investigate the molecular basis for this differential modulation, we used immunohistochemistry and gene expression analysis. We developed an antibody against the GABA (B) receptor to look at the localization of GABA receptors in the olfactory neurons. We found that GABA (B) receptors are localized at the ORN synapses. We used qRT-PCR analysis to identify other molecular players that are involved in starvation control. We conclude that an animal's internal state such as hunger differentially modulates the functions of individual ORNs to impact olfactory information processing. Our results support recent studies from our lab and other groups that suggest that ORNs are functionally diverse. Overall, this research thesis has implications for understanding peripheral odor coding.