Olfactory information in Drosophila is conveyed by projection neurons from olfactory sensory neurons to Kenyon cells (KCs) in the mushroom body (MB). A subset of KCs responds to a given odor molecule, and the combination of these KCs represents a part of the neuronal olfactory code. KCs are also thought to function as coincidence detectors for memory formation, associating odor information with a coincident punishment or reward stimulus. Associative conditioning has been shown to modify KC output. This plasticity occurs in the vertical lobes of MBs containing α/α' branches of KCs, which is revealed by measuring the average Ca²⁺ levels in the branch of each lobe. We devised a method to quantitatively describe the population activity patterns recorded from axons of >1000 KCs at the α/α' branches using two-photon Ca²⁺ imaging. Principal component analysis of the population activity patterns clearly differentiated the responses to distinct odors. In order to asses neuronal dynamics of those neurons, we established an in vivo Ca²⁺ imaging preparation to monitor plasticity of molecular and physiological properties in the brains, induced by pairing odour and shock. Employing two-photon microscopy, we could monitor specific patterns of MB activities to different odours at cellular-resolution. After pairing an odour with electric shock, a part of MB cells showed increased Ca²⁺ signal to the conditioned odour. Currently we are analysing neural plasticity in one of the MB output neurones to reveal the functional connectivity with MB neurones upon learning.
The functional imaging allows us to analyse temporal and spatial dynamics of many neurons simultaneously. This is an important feature because populations of MB cells, which consist of ~2000 cells, will be almost heterogeneous in function and fate after training. In addition, available fluorescent probes for several molecules (Ca²⁺, cAMP, PKA etc.) and recent tremendous progress of developing new probes will cover more comprehensive trace of cells physiology.