Levels of neurotransmitters called monoamines, which include serotonin, dopamine and neurepinephrine, have long been believed to be lower in the brain during depressive episodes. However, scientists have only just been able to create a convincing model to explain this loss.
In a paper published in the Archives of General Psychiatry this month, researchers show that depressed people have 34 percent higher levels of a protein called monoamine oxidase A, or MAO-A, in their brains. The elevated presence of this substance in turn leads to the faster metabolization of monoamines during depressive episodes (see diagram here), resulting in low levels of the mood-associated chemicals.