Functional neuroanatomy of Drosophila olfactory memory formation.

New approaches, techniques and tools invented over the last decade and a half have revolutionized the functional dissection of neural circuitry underlying Drosophila learning. The new methodologies have been used aggressively by researchers attempting to answer three critical questions about olfactory memories formed with appetitive and aversive reinforcers: (1) Which neurons within the olfactory nervous system mediate the acquisition of memory? (2) What is the complete neural circuitry extending from the site(s) of acquisition to the site(s) controlling memory expression? (3) How is information processed across this circuit to consolidate early-forming, disruptable memories to stable, late memories? Much progress has been made and a few strong conclusions have emerged: (1) Acquisition occurs at multiple sites within the olfactory nervous system but is mediated predominantly by the γ mushroom body neurons. (2) The expression of long-term memory is completely dependent on the synaptic output of α/β mushroom body neurons. (3) Consolidation occurs, in part, through circuit interactions between mushroom body and dorsal paired medial neurons. Despite this progress, a complete and unified model that details the pathway from acquisition to memory expression remains elusive.