The color of LOTUS-V bioluminescence changed from cyan to green when neurons in a brain were activated. LOTUS-V is a chimeric protein composed of a bioluminescent protein (cyan), fluorescent protein (lime green), and a voltage-sensitive domain (gray). Membrane depolarization induces conformational changes in the voltage-sensitive domain and shortens the distance between the bioluminescent and fluorescent proteins. Eventually, the efficiency of energy transfer (i.e. FRET) increased and the intensity of lime green was enhanced (conversely, the intensity of cyan decreased).