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Introduction

Monoamine oxidase inhibitors (MAOIs) increase concentrations of dopamine, norepinephrine, and serotonin (ie, monoamine neurotransmitters).  They are effective for depressive disorders though are reserved for treatment-resistant cases due to the risk of severe adverse effects, including serotonin syndrome and hypertensive crisis.

Mechanism of action

Monoamine oxidase (MAO) is a mitochondrial enzyme that degrades serotonin, norepinephrine, and dopamine and can be found in multiple organs (eg, brain, intestines, heart).  There are 2 types of MAO enzymes:

  • MAO-A:  Primarily breaks down serotonin, norepinephrine, and dopamine
  • MAO-B:  Primarily breaks down dopamine

MAOIs bind and inhibit both MAO-A and MAO-B (ie, are nonselective) ( Figure 1).  The resulting increase in presynaptic monoamine neurotransmitter levels and subsequent release into the synapse are thought to play a role in alleviating depressive symptoms.

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Figures

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