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Brain irradiation: effects on regional MAO and COMT activity in the brain of the squirrel monkey and on the incorporation of [14C]tyrosine into catecholamines.

The effects of brain irradiation on the enzyme activity of monoamine oxidase (MAO) and catecholamine O-methyltransferase (COMT), as well as on the rate of incorporation of [14C]tyrosine into catecholamines (CAs), were examined in brain regions of the squirrel monkey (Saimiri sciureus). Results indicated that proton irradiation of the striate visual cortex at either 10,000 or 20,000 rad drastically affected the MAO activity, moderately affected the COMT activity, and significantly altered the incorporation of [14C]tyrosine into CAs in several brain regions. After 10,000 rad, MAO activity was increased in caudate nucleus (by 135%) and cerebellum (by 41%); significantly decreased in putamen (by 49%), gray matter (by 19%), and hippocampus (by 18%); and unchanged in brainstem. After 20,000 rad, MAO activity was markedly increased in caudate nucleus (by 114%), but decreased in cerebellum (by 24%), putamen (by 38%), gray matter (by 30%), and hippocampus (by 36%), and unchanged in brainstem. After 10,000 rad, COMT activity was significantly decreased in putamen (by 27%) and brainstem (28%). After 20,000 rad, COMT activity was significantly decreased in putamen (by 22%) and cortical gray matter (by 37%) but unchanged in the other brain regions examined. The 10,000 rad significantly increased the rate of incorporation of [14C]tyrosine into CAs in both the cerebellum (by 68%) and hypothalamus (by 26%), but 20,000 rad increased the rate only in hypothalamus (by 17%) and decreased the rate in hippocampus (by 24%). The whole-brain weights were significantly decreased after both 10,000 or 20,000 rad, by 17% and 11%, respectively. The only significant change in regional tissue weight was observed in putamen after 20,000 rad (a decrease by 17%). The protein concentration was significantly increased in hippocampus at both 10,000 (by 13%) and 20,000 (by 30%) but not affected in other brain regions examined. These data support our previous conclusion that a sustained chemical change occurs at the irradiated site and that the effect was transmitted to nonirradiated brain regions. Results also suggest that catecholaminergic pathways may exist between striate visual cortex and other cortical or subcortical brain areas where chemical changes were observed after visual cortical lesion by proton irradiation at various doses.