Moeyaert, B. et al. Nat. Commun. 9, 4440 (2018).

Active neurons have traditionally been identified by their expression of immediate early genes. More recently, an alternative tool called CAMPARI has become available. CAMPARI is a coincidence detector that photoconverts from a green to a red fluorescent protein in the presence of calcium and blue light. Thus, this tool allows specific labeling of neurons that are active at a particular time. Moeyaert et al. have developed CAMPARI2, which exhibits brighter fluorescence in both the green and the red form, as well as faster calcium unbinding kinetics and higher contrast between the two forms. Furthermore, variants with a range of calcium affinities are available, which provides options that can be chosen according to the properties of the cells of interest. The researchers demonstrate the improved performance of CAMPARI2 in vitro, in neuronal cell culture, in brain slices, in larval zebrafish, and in the mouse visual cortex. As a bonus, the researchers have also generated an antibody that specifically binds to the red form of CAMPARI2 and its precursors, facilitating the detection of red CAMPARI in fixed tissue.