Monoamine oxidase inhibitors work by stopping the action of an enzyme called monoamine oxidase. This enzyme breaks down a group of brain signalling chemicals called the monoamine neurotransmitters. There are two types of the enzyme, MAO-A and MAO-B. Each type breaks apart different chemicals in the group. MAO-A breaks down melatonin, serotonin, epinephrine and norepinephrine. MAO-B breaks down phenylethylamine and the trace amines. These chemicals (neurotransmitters) are used in the brain for signalling purposes. Dopamine, for example, is a chemical that affects motivation and pleasure. Effectively, they work on balancing the brain's chemistry.