Abbreviation for a breakdown enzyme monoamine oxidase. A MAO inhibitor blocks the action of monoamine oxidase, thus raising the levels of the monoamine neurotransmitters such as norepinephrine, epinephrine, dopamine, and serotonin – which have significant effects on mood and behavior. Epinephrine, norepinephrine and serotonin are normally deactivated by MAO-A while dopamine and phenylethylamine are normally metabolized by MAO-B.