Monoamine oxidase (MAO) inhibitors refers to drugs used to treat depressions that inhibit the activity of the enzyme Monoamine oxidase, which degrades the neurotransmitters of norepinephrine and serotonin ; a drug that blocks the enzyme Monoamine oxidase (MAO), a presynaptic terminal enzyme that metabolizes catecholamines and serotonin into inactive forms Please see also Monoamine oxidase inhibitors (MAOIs)