Serotonin is a neurotransmitter that's partly responsible for feelings of happiness and for mood regulation in humans. This makes it a common target for antidepressants, which block serotonin from being reabsorbed by neurons after it has dispatched its signal, so more of it stays floating around the brain. Now researchers have developed an imaging technique that, for the first time, enables 3D mapping of serotonin as it's reabsorbed into neurons, across multiple regions of the living brain. This technique could be a powerful tool for the research and development of antidepressants.