An antiserum raised against rat glutamate decarboxylase was used to map GABAergic boutons in the inferior olive of rabbit, cat, rhesus monkey, and human. A description of the human periolivary region is also included. The inferior olive of each species contained a dense GABAergic innervation, but immunostaining intensities varied among regions. These intensities were evaluated visually and photomertrically, and the sizes and frequencies of occurence of boutons in various olivary subnuclei were measured. The beta nucleus in all species was intensely immunostained and contained the largest boutons. The caudal subdivision of the dorsal accessory olive stained with a lower intensity than the beta nucleus, but contained similarly large GABAergic boutons. By visual analysis, the rostral subdivision and the subnucleus a of the medial accessory olive and the principal olive were stained with an intermediate intensity, and these regions contained small GABAergic boutons. Photometric analysis of focal regions of the neuropil, however, revealed species differences in the staining intensity of the principal olive, which was lowest in rabbits and highest in primates. In all species, the lowest immunostaining intensity was observed in the subnucleus b of the medial accessory olive. Species variations in bouton sizes and regional staining intensities were observed in the dorsal cap and the dorsomedial cell column. The heterogeneous staining pattern and regional variation of bouton size argue for the existence of separate GABAergic projections to discrete regions of the inferior olive. Since glutamate decarboxylase immunostaining patterns in the olive are largely similar across species, the afferent projections producing these patterns may also be similar.
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