Differential Changes in Monoamine Oxidase A and B Activity in the Aging Rat Brain

Abstract

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 β-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 V_max of MAO B was increased in aging rat brain, whereas the K^m 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.