Drugs targeting the ‘happy hormone’ are widely used for depression. But some question whether the condition is caused by a chemical imbalance in the brain

Daniela da Silva is feeling good. Lying cocooned under fleece blankets inside a medical scanner, her eyes are closed and her mind is focused and remarkably unperturbed by negative thoughts. Three hours earlier, the 39-year-old yoga teacher and neuroscience student was given a dose of the stimulant drug dextroamphetamine, which is often used to treat ADHD. “I’m having a serotonin increase. Oh definitely,” she predicts before entering the PET scanner.

Da Silva is a healthy volunteer in a trial using a pioneering brain imaging technique designed to measure serotonin changes in the brains of living people. Last year, scientists used the scan to obtain what they claimed to be the first direct evidence that serotonin release is blunted in the brains of people with depression. The findings added fuel to a fiercely fought debate over the role of the brain chemical – if any – in depression. Just months earlier, a high-profile scientific review caused a stir when it reached the opposite conclusion that “after a vast amount of research, conducted over several decades, there is no convincing evidence” for the idea that depression is caused by a chemical imbalance in the brain.