Using a novel optogenetic tool, researchers have successfully controlled, reproduced and visualized serotonin receptor signals in neural cells. To this end, they modified a photosensitive membrane receptor in the eye, namely melanopsin. They were able to switch the receptor on and off using light; it also acted like a sensor indicating via fluorescence if specific signalling pathways in the cell had been activated. The sensor was, moreover, specifically designed to migrate to those domains in the neural cells that are sensitive to the neurotransmitter serotonin.