MONOAMINE OXIDASE-B-POSITIVE GRANULAR STRUCTURES IN THE HIPPOCAMPUS OF AGED SENESCENCE-ACCELERATED MOUSE (SAMP8)

We examined the histochemical localization of monoamine oxidase in the hippocampus of young and old senescence-accelerated mouse (SAM). We f ound a monoamine oxidase-B-positive granular structure (MGS) in the hi ppocampus of old SAMP8, an accelerated senescence-prone line of SAM. T he MGS was a round-shaped granular structure of 0.5 to 5 mu m diameter and usually formed a cluster, the largest diameter of which ranged fr om 50 to 150 mu m. No MGS were found in the hippocampus of young SAMP8 or of young SAMR1, an accelerated senescence resistant line of SAM, a nd only few, if any, were seen in old SAMR1. A monoamine oxidase-posit ive astrocyte was usually observed in the central area of each cluster of MGS. Furthermore, the MGS was in close anatomical relationship wit h monoamine oxidase-positive astrocytic processes. The enzyme inhibiti on experiments showed that monoamine oxidase activities localized in t he MGS and astrocytes were both predominantly of type B. These finding s suggest MGS occurs at least partly in monoamine oxidase-B-positive a strocytes. Furthermore, the MGS was similar to a periodic acid-Schiff- positive granular structure, a polyglucosan body previously documented in the brains of old SAMP8 and some other aged mice strains including C57BL/6 and nude mice, in terms of their size, morphological appearan ces and topographical distribution in the hippocampus. Thus, the prese nt results suggest that monoamine oxidase type B is a proteinaceous co mponent of the periodic acid-Schiff-positive granular structure in age d mice brains, and might provide some clues for clarifying the mechani sms of age-related occurrence of periodic acid-Schiff-positive granula r structures in mice brains.