The monoamines include the catecholamine neurotransmitters dopamine, norepinephrine, and 5-hydroxytryptamine. Monoamine oxidases (MAOs) are intracellular enzymes found in the outer mitochondrial membrane that catabolize these amines (1). The first MAO identified was tyramine oxidase in 1928 (2). Monoamine oxidase inhibitors (MAOIs) inhibit the action of MAOs.
In the 1950s and 1960s, patients were reported to "dance in the hall" following treatment with antituberculosis drugs (3). One of these medications, iproniazid, was noted to have potent MAO inhibitory properties (4). An open-label trial of iproniazid found significant mood improvement in institutionalized depressed patients (5), and it was later introduced as the first antidepressant medication (6).
In 1965, Knoll and Ecseri (7,8) synthesized phenylisopropyl-N-methyl-propinylamine or E-250. E-250 was a strong, irreversible MAO inhibitor that metabolized tyramine (9), phenylethylamine (9), and benzylamine (8). The compound was separated into two isomers, and the L-form was named deprenyl. In 1968, Johnston synthesized a compound structurally similar to deprenyl, 2,3-dichlorophenoxy-propyl-N-methylpropinylamine, or clorgyline (10). He arbitrarily called the MAO with greater affinity for clorgyline MAO-A and for deprenyl MAO-B.
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