The relationship between spike response and calcium fluorescent signal in insect olfactory receptor neurons

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PROJECT SUMMARY/ABSTRACT The goal of this project is to determine the relationships between odor-induced calcium fluorescence signals and spike responses across morphologically distinctive types of insect olfactory receptor neurons (ORNs). Millions of lives are threatened annually by devastating insect-borne diseases—such as malaria, dengue, and West Nile fever—which are transmitted by vectors that heavily rely on their sense of smell for host-seeking. Correspondingly, growing efforts are dedicated to better understanding insect olfaction to develop tools and strategies to interfere with host-seeking behavior. With the advent of CRISPR genome editing technology in a wide range of insects, in vivo calcium imaging has become a method of choice for identifying ORNs that respond to host odors. However, calcium-induced fluorescence signals may be difficult to interpret, because response amplitudes and dynamic ranges are determined not only by the degree of receptor activation, but also by the intrinsic electrotonic property of neurons which can vary markedly across neuronal types. For example, a 10% increase in fluorescence signal may correspond to a high-frequency, near-saturating spike response for one ORN type, but may instead represent a low spike response at the rising phase of the dosage curve for another. In order to accurately interpret calcium imaging data, it is imperative to understand the spike-calcium relationship associated with individual neuronal types. To achieve this goal, this proposal leverages the powerful genetic toolkit and tractable olfactory system of Drosophila. Similar to other insect species, Drosophila ORNs are categorized under morphological classes based on the sensillum types that encapsulate their sensory dendrites. Each sensillum typically contains two to four ORNs, which exhibit distinctive extracellular spike amplitudes that reflect the size differences between compartmentalized neurons. Given that the electrotonic properties of sensory neurons are influenced by their morphological and morphometric features, any such differences across ORN morphological types will likely impact their spike-calcium relationship. The general hypothesis—that the odor-induced spike and calcium response relationship vary across distinctive ORN morphological types—will be tested via simultaneous in vivo trans-cuticle fluorescence imaging and single-sensillum electrophysiological recording. In this proposal, Aim 1 will determine the calcium response dynamic range and spike-fluorescence relationship in select ORNs representing all morphological classes. Aim 2 will focus on the comparison between compartmentalized ORNs which typically exhibit distinctive morphometric features. Successful execution of the proposal will yield a rich dataset to facilitate meaningful interpretation of calcium-induced fluorescence signals recorded from Drosophila ORN types. Importantly, this information is expected to be generalizable to other insects, including disease vectors and agricultural pests, as these ORN morphological classes are found across insect species. Moreover, this systematic survey will lay the foundation for future studies to determine the mechanistic underpinnings of sensory neurons’ calcium responses.