Monoamine oxidase A (MAO-A) is a chemical in the brain that breaks down other chemicals including serotonin. Serotonin is associated with good mood and high levels of MAO-A have been associated with depression. Researchers at the Centre for Addiction and Mental Health in Toronto used PET (positron emisssion tomography) scanning to look at the levels of MAO-A and serotonin in people's brains as they were being treated with antidepressants. The study found that the drugs raised serotonin levels but did nothing to decrease levels of MAO-A. Some early antidepressants did target MAO-A but they fell out of favour in the 1970s due to adverse reactions with certain foods. These problems have been overcome now so it could be time to look again at the effectiveness of MAO-A inhibitors.