GABAergic organization of the cat medial geniculate body

A study of neurons and processes (puncta) immunolabeled by antibodies to ga mma-aminobutyric acid (GABA) or glutamic acid decarboxylase was undertaken in the medial geniculate body of the adult cat. The proportion and types of GABAergic cells were determined with high resolution methods, including po stembbedding immunocytochemistry on semithin plastic sections. A second goa l was to draw parallels and differences between the auditory thalamus and o ther thalamic nuclei. Finally, the types of GABAergic puncta and their conc entration in the three major subdivisions of the medial geniculate body wer e analyzed. The results were that (1) each division had many GABAergic neur ons, averaging approximately 26% of the neuronal population; (2) the ventra l division had the highest proportion of these cells (33%), the medial divi sion the fewest (18%), and the dorsal division was intermediate (26%); (3) there was a gradient in the proportion of GABAergic neurons, i.e., the vent ral and medial division values increased caudorostrally, whereas the value in the dorsal division declined; (4) the predominant GABAergic cell type in each division was a small neuron with a soma approximately 10-12 mu m in d iameter; (5) a small population of much larger GABAergic neurons was presen t mainly in the dorsal division; (6) in addition to the fine, granular punc ta in each division, a type of giant GABAergic puncta was found only in the dorsal division nuclei. The results obtained with the two antibodies were essentially identical. These findings suggest a structural basis for qualit ative differences in the distribution of GABAergic processing within the me dial geniculate complex. The GABAergic arrangement in the ventral division was stereotyped, with only one type of putative GABAergic interneuron, and the puncta were correspondingly homogeneous. In contrast, the dorsal divisi on had two types of GABAergic neurons, and the giant GABAergic puncta repre sent a new substrate for inhibitory interactions. The medial division also had more than one type of GABAergic neuron and a slightly lower concentrati on of puncta. These qualitative and quantitative distinctions suggest a mor phologic basis for possible differences in inhibitory processing among medi al geniculate body subdivisions. (C) 1994 Wiley-Liss, Inc.