Light switch: A photochromicagonist ATA‐3 (see scheme) of AMPAreceptors, arguably the most important class of ionotropic glutamate receptors, is shown to be subtype selective, activates the AMPAreceptor GluA2 in the dark, and is quickly turned off when irradiated with blue–green light. It can be used to effectively control neuronal activity in the mammalian brain.