Monoamine oxidase is an enzyme that is found in many parts of the body.
In the brain, monoamine oxidase destroys neurotransmitters, such as norepinephrine and serotonin. So MAO inhibitors, by limiting the activity of monoamine oxidase, block the breakdown of those neurotransmitters. They work more quickly than the tricyclics, but they have more severe side effects and require a change in diet. For people who don't respond to the tricyclics and SSRIs. For cases of atypical depression.