Dopamine (DA) is a neurotransmitter for regulating many motor and cognitive functions. The homeostasis of dopamine is thus vital for the nervous system. To explore dopamine circuits, we aim to develop a probe that could visualize the dopamine dynamics in the fly brain. We generated DA probes by conjugating GFP fluorescent protein and an enzyme monoamine oxidase B (MAO B), which can catalyze dopamine oxidation via its co-factor FAD. The redox reaction of FAD could switch GFP fluorescence upon detecting dopamine release from dopaminergic neurons in which we coined as the shield effect. We are currently using this probe to study of how brain processes information in dopamine circuits in both healthy or diseased conditions.