Memory processes associated with olfactory learning in Drosophila were investigated by training flies to associate a specific odour with a punishing or rewarding stimulus, such as an electric shock or sugar. The sparsely distributed memory trace (that is, the engram) that encodes the association between cue and stimulus is located in Kenyon cell synapses in the mushroom body of the fly brain. The activity of individual Kenyon cell synapses was monitored using a fluorescent Ca2+ sensor expressed throughout the axonal arbour and showed, unexpectedly, that individual boutons showed specific and individual responses to odours, suggesting that the encoding of memory occurs at the level of individual synapses.
Bilz, F. et al. Visualization of a distributed synaptic memory code in the Drosophila brain. Neuron https://doi.org/10.1016/j.neuron.2020.03.010 (2020)