The mechanisms of learning and memory are the most important functions in an animal brain. Investigating neuron
circuits and network maps in a brain is the first step toward understanding memory and learning behavior. Since
Drosophila brain is the major model for understanding brain functions, we measure the florescence lifetimes of different
GFP-based reporters expressed in a fly brain. In this work, two Gal4 drivers, OK 107 and MZ 19 were used.
Intracellular calcium ([Ca2+]) concentration is an importation indicator of neuronal activity. Therefore, several groups
have developed GFP-based calcium sensors, among which G-CaMP is the most popular and reliable. The fluorescence
intensity of G-CaMP will increase when it binds to calcium ion; however, individual variation from different animals
prevents quantitative research. In this work, we found that the florescence lifetime of G-CaMP will shrink from 1.8 ns to
1.0 ns when binding to Ca2+. This finding can potentially help us to understand the neuron circuits by fluorescence
lifetime imaging microscopy (FLIM). Channelrhodopsin-2 (ChR2) is a light-activated ion-channel protein on a neuron
cell membrane. In this work, we express ChR2 and G-CaMP in a fly brain. Using a pulsed 470-nm laser to activate the
neurons, we can also record the fluorescence lifetime changes in the structure. Hence, we can trace and manipulate a
specific circuit in this animal. This method provides more flexibility in brain research.