Drosophila alpha/beta mushroom body neurons form a branch-specific, long-term cellular memory trace after spaced olfactory conditioning.

Functional optical imaging showed that odor or electric shock stimuli presented to the fly causes transient calcium influx into the two major axon branches of alpha/beta mushroom body (MB) neurons. One pairing of odor and electric shock stimuli or multiple, massed pairings did not alter odor-evoked calcium influx. In contrast, animals that received multiple, spaced pairings exhibited a robust increase in calcium influx into the MB axons when tested at 9 or 24 hr after training, but not at 3 hr. This modification occurred only in the alpha branch of the neurons and was blocked by mutation of the amnesiac gene, inhibition of protein synthesis, or the expression of a protein blocker of the transcription factor Creb. Thus, behavioral long-term olfactory memory appears to be encoded as a branch-specific modification of calcium influx into the alpha/beta MB neurons that occurs after spaced training in a protein synthesis-, Creb-, and amnesiac-dependent way.