Neurons in our brain form a vast network that processes information to realize higher brain functions such as cognition and learning. The mechanisms of higher brain functions are complex and challenging to understand. However, they are expected to be elucidated in modern science. To clarify such mechanisms, it is essential to have techniques to measure the activity of neurons in the brain accurately. We are developing highly sensitive fluorescent probes necessary for elucidating the mechanisms of higher brain functions. We are also applying the probes to the living brain to study the circuit mechanism of olfactory information processing.