Reward taste conditioning in Drosophila melanogaster

Animals tend to approach and consume food that is palatable, energetically advantageous or nutritious. Although appetitive taste behavior is hardwired, it can be modulated by experience. Previously, we were able to suppress it by pairing appetitive tastants with aversive stimuli, such as bitter quinine. Here, we present a taste memory assay where a taste stimulus is paired with rewarding sugar intake. We tested an array of neutral or appetitive substances (conditioned stimulus, CS) and rewarding reinforcements (unconditioned stimulus, US), and determined optimal conditions for a robust and reliable reward taste conditioning (RTC) assay. We ultimately used a moderate concentration of fructose as CS paired with a sweeter sucrose as US, allowing us to characterize the properties of the reinforcement and the resulting memory. We found that tight forward pairing of CS and US is necessary for memory formation, and repeated unpaired presentation of the CS extinguishes the memory. Without further interference, the memory is sustained for at least 60 min after training. We determined that genes involved in short-term memory and intact mushroom bodies are required for RTC. This assay is a fast and robust way to investigate the involvement of genes specific to reward taste learning, and to determine neuronal sub-types and the circuits involved in appetitive taste memory. Considering that the reward and aversive taste assays are designed to be very similar, it will allow for comparison between memories with opposite valence and further our understanding of the detailed neuronal architecture underlying acquired taste memories.