Differential changes in monoamine oxidase A and B activity in the aging rat brain.

The activity of monoamine oxidase (MAO) was assayed in the brains of young adult (7-8 weeks old) and aging (95-104 weeks old) male Sprague-Dawley rats. When expressed per milligramme of tissue, total protein content was increased to a similar extent in whole brain and in all seven brain regions of the aging rat, whereas MAO A activity (assayed by using 5-hydroxytryptamine as substrate) was unchanged in whole brain but increased in the cerebellum, and fell in the brainstem, midbrain, hippocampus, and cortex; and MAO B activity (assayed by using beta-phenylethylamine as substrate) increased in whole brain and all regions, except the brainstem. Age-related changes in total protein, MAO A and MAO B were thus independent of each other. Kinetic studies indicated that the Vmax of MAO B was increased in aging rat brain, whereas the Km was unaltered. The increase in MAO B activity was restricted to the extrasynaptosomal mitochondrial fraction of the aging brain, whereas a reduction in MAO A activity was found in the intrasynaptosomal, but not the extrasynaptosomal mitochondrial fraction.