gamma-Aminobutyric acid and glycine in the baboon cochlear nuclei: an immunocytochemical colocalization study with reference to interspecies differences in inhibitory systems.

Previous studies of the cochlear nuclei in cat, rat, and guinea pig have demonstrated neural structures that are enriched in the inhibitory neurotransmitter amino acids gamma-aminobutyric acid (GABA) and glycine. In these mammals, inhibitory terminals are widely distributed throughout the nuclear complex, but somata of inhibitory neurons are concentrated in the dorsal cochlear nucleus, in granule cell regions, and in the cap area. Because these are the subdivisions that undergo the most pronounced phylogenetic changes in primates, we wanted to see whether the inhibitory systems are influenced by changes in cytoarchitecture. Therefore, we applied light microscopic postembedding immunostaining and optical densitometry to the cochlear nuclei of an anthropoid primate, the Senegalese baboon (Papio anubis). Our results demonstrate that, in baboon 1) glycinergic neurons and axons in the ventral cochlear nucleus seem to form a commissural system similar to that of other mammals; 2) the tuberculoventral system appears to be unchanged in morphology but exhibits a higher level of colocalization of GABA with glycine; 3) there is a reduction of the granule/cartwheel cell system, which is reflected in lesser numbers of inhibitory cartwheel, Golgi, and molecular layer stellate cells; 4) the cap area is larger than in rodents and carnivores and contains many neurons that colocalize GABA and glycine; and 5) throughout the nuclear complex, a higher proportion of the inhibitory terminals colocalize GABA and glycine. We conclude that modulation of the ascending auditory pathway in baboon is likely to differ from that in rodents and cat.