Monoamine oxidase inhibitor
Monoamine oxidase inhibitors (MAOIs) were the first class of antidepressant medications developed. MAOIs increase levels of monoamine neurotransmitters in the synapses between neurons by deactivating one or more subtypes of the enzyme monoamine oxidase. The use of MAOIs is generally disfavored in the current practice of psychiatry due to their side effects, the most prominent of which is the prospect of inducing a hypertensive crisis.
The initial MAOIs produced, such as tranylcypromine, were irreversible, permanently bonding to and deactivating monoamine oxidase enzymes, and unselective, inhibiting both MAO-A and MAO-B. This pharmacology leaves the patient without sufficient monoamine oxidase activity to adequately break down the levels of tyramine present in normal food intake, thus requiring adherence to substantial dietary restrictions to avoid a hypertensive crisis. Aged and cultured foods such as cheese, tenderized meats, etc, must not be ingested. Medications containing significant quantities of pressor amines are likewise forbidden, as is chocolate, due to its phenethylamine content.
More recent developments include MAO-A selective, reversible medications such as moclobemide, which allow patients to consume a normal diet, but are not approved in the United States. Selegiline is MAO-B selective at lower doses, thus predominantly raising synaptic levels of dopamine, and has found use as a therapy for Parkinson's disease.