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Reactive microglia or macrophages expressing the histocompatibility glycoprotein HLA-DR were detected in many neurological diseases including Alzheimer's, Parkinson's, Pick's and Huntington's diseases, parkinsonism-dementia of Guam, amyotrophic lateral sclerosis, Shy-Drager syndrome, multiple sclerosis and AIDS encephalopathy. Reactive astrocytes, also present in these conditions, were established as a population distinct from the HLA-DR positive microglia by double immunostaining for glial fibrillary acidic protein and HLA-DR. A distinctive pattern of HLA-DR positive cells was seen in each disease entity. Areas known to contain pathology always stained positively, and, in several cases, reactive microglia appeared in areas that would otherwise not have been suspected of being involved in the pathological process. HLA-DR staining, which outlines the surface membranes of positive cells, was so strong that lesioned areas could frequently be identified in sections with the naked eye. In adjacent sections stained with H&E or sections destained of HLA-DR and then restained with H&E, gliosis was often hard to identify except on close microscopic inspection. The results suggest that HLA-DR staining may be a valuable addition to standard neuropathological methods and might be useful in investigating diseases where pathology has not yet been identified.

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The activity of tyrosine hydroxylase (EC 1.10.3.1) when assayed under ideal conditions in young human brains, was comparable to that in brains of other species in level of activity and distribution. The highest levels of activity were in the putamen, caudate nucleus and substantia nigra, in keeping with data on other species. The caudate activity in human brain appeared to decrease substantially with increasing age. In both humans and baboons, the enzyme in the neostriatum was particle‐bound and inhibited by the 2‐amino‐4‐hydroxy‐6,7‐dimethyltetrahydropteridine cofactor system. In the substantia nigra it was soluble and stimulated by the 2‐amino‐4‐hydroxy‐6,7‐dimethyltetrahydropteridine cofactor system. The data suggest that tyrosine hydroxylase may be produced in a soluble form in the cell bodies of the substantia nigra but become bound as it moves toward the nerve endings in the putamen and caudate nucleus. The bound form of the enzyme was unstable but the soluble form exhibited considerable stability.