Monoamine Oxidase Inhibitors in Parkinson’s Disease

Monoamine oxidase (MAO) is an enzyme involved in the breakdown of catecholamines including dopamine, norepinephrine, and serotonin. MAO inhibitors were discovered in the late 1950s and were first utilized in the treatment of depression. In 1962 Bernheimer showed that MAO inhibitors could potentiate the antiparkinsonian effect of levodopa but caused severe hypertensive crisis (1). In 1968, Johnston identified two types of monoamine oxidase: A and B (2). Each has a separate affinity for various catecholamines and works in different parts of the body. MAO-B is found predominantly in the human brain (3) and platelets and has an affinity for dopamine and benzylamine. MAO-A is found predominantly in the intestinal tract and has an affinity for serotonin and norepinephrine. Both types can oxidize tyramine, though MAO-B does so only at higher concentrations. In 1972, Knoll and Maygar described selegiline (Deprenyl) as a selective irreversible MAO-B inhibitor (4). They also showed that at low doses selegiline did not potentiate the pressor effect of tyramine also known as the ‘‘cheese effect.’’ That same year Squires (3) reported that 80% of