MAO is widely distributed in mammals. Two isoenzymes, MAO-A and MAO-B, are of special interest in psychiatry (Cesura and Pletscher 1992). Both are present in the central nervous system (CNS) and in some peripheral organs. Both MAO-A and MAO-B are present in discrete cell populations within the CNS. MAO-A is present in both dopamine (DA) and norepinephrine (NE) neurons, whereas MAO-B is present to a greater extent in serotonin (5-HT)–containing neurons. They are also present in nonaminergic neurons in various subcortical regions of the brain. Glial cells also express MAO-A and MAO-B (Cesura and Pletscher 1992).