Induction of Associative Olfactory Memory by Targeted Activation of the Memory Circuits in Drosophila Larvae

Takato Honda^1,2, Chi-Yu Lee², Maki Yoshida-Kasikawa², Ken Honjo² and Katsuo Furukubo-Tokunaga²

1. Ph.D. Program in Human Biology, School of Integrative and Global Majors, University of Tsukuba, Japan
2. Graduate School of Life and Environmental Sciences, University of Tsukuba, Japan

The fruit fly Drosophila melanogaster has been used as a model animal for the study of the genetic and molecular mechanisms of learning and memory. In particular, with simple and identifiable neural networks, Drosophila larvae provide an ideal system for elucidation of underlying neurocircuitry mechanisms. Previous works showed that synaptic output from octopaminergic (OA) neurons is necessary for appetitive memory formation. Signals of OA neurons are integrated with the olfactory information pathway at two prominent brain structures, antennal lobes and mushroom bodies, to establish appetitive associative memory trace. In this study, we took the advantages of optogenetic and thermogenetic techniques to study the neurocuircuitry mechanism of memory induction in flies. We substituted sugar reward stimuli by thermogenetic activation of OA neurons with the dTrpA1 channel, and odor stimuli by optical activation of a specific class of olfactory receptor neurons (ORNs) with Channelrhodopsin-2 (ChR2). We showed that targeted activation of the converging memory circuitry with blue light and heat produces associative memory in the transgenic larvae. We also showed that memory thus produced is specific to the odorant determined by the type of the activated ORNs. Furthermore, we successfully demonstrated that this artificial olfactory memory persists for medium term, and as stable as natural memory produced by the activation of multiple ORNs using a real odorant. Given its simplicity and robustness, this method could be utilized to further our knowledge on the neurocuircuitry mechanisms of memory in the brain.