Cellular action of MAO inhibitors.

Sites of action of monoamine oxidase (MAO) inhibitors in the human brain were analyzed by studying the distribution of the respective MAO subtype, MAO-A or -B. Blood brain barrier (BBB), glial cells and neurons of several brain areas were examined with biochemical and immunocytochemical methods. Whereas MAO-A was localized in microvessels and neurons of locus ceruleus, MAO-B was present predominantly in neuronal cells of dorsal raphe nucleus. The neurons of substantia nigra revealed no immunohistochemically detectable MAO. Astrocytes expressed both subtypes and were observed in all cortical areas examined and all over the brain stem including medulla reticular formation, pontine reticular formation, reticular formation of mesencephalon, midbrain reticular formation, red nucleus, substantia nigra, caudate nucleus and putamen (sections according to DeArmond et al., 1976). MAO activity in several brain areas seems mostly to be due to astrocytes rather than to neuronal cells. As MAO inhibitors have beneficial effects in the treatment of malfunctions of aminergic transmitter systems (Johnstone and Marsh, 1973; Robinson et al., 1973; Birkmayer et al., 1977; Knoll, 1981; Zarifian, 1984; Riederer and Youdim, 1986) and a large amount of MAO activity seems to be localized in brain astrocytes, the function of glial cells especially with regard to their neuromodulating capacity should be focused in more detailed research in future.