GABAergic organization of the cat medial geniculate body

Abstract

A study of neurons and processes (puncta) immunolabeled by antibodies to gamma-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 postembbedding immunocytochemistry on semithin plastic sections. A second goal was to draw parallels and differences between the auditory thalamus and other thalamic nuclei. Finally, the types of GABAergic puncta and their concentration in the three major subdivisions of the medial geniculate body were analyzed. The results were that (1) each division had many GABAergic neurons, averaging approximately 26% of the neuronal population; (2) the ventral division had the highest proportion of these cells (33%), the medial division the fewest (18%), and the dorsal division was intermediate (26%); (3) there was a gradient in the proportion of GABAergic neurons, i.e., the ventral 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 microm in diameter; (5) a small population of much larger GABAergic neurons was present mainly in the dorsal division; (6) in addition to the fine, granular puncta 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 qualitative differences in the distribution of GABAergic processing within the medial 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 division had two types of GABAergic neurons, and the giant GABAergic puncta represent a new substrate for inhibitory interactions. The medial division also had more than one type of GABAergic neuron and a slightly lower concentration of puncta. These qualitative and quantitative distinctions suggest a morphologic basis for possible differences in inhibitory processing among medial geniculate body subdivisions.

GABAergic circuitry in the dorsal division of the cat medial geniculate nucleus

“…The present study is the first to combine electron microscopy and GABA immunocytochemistry to examine the synaptic circuitry of the cat MGN. Our results are consistent with light microscopic descriptions of GABA‐positive profiles (Winer et al, 1996, 1999b; Huang et al, 1999) and electron microscopic descriptions of normal ultrastructure in cat MGN (Majorossy and Réthelyi, 1968; Morest, 1971, 1975; Jones and Rockel, 1971; Majorossy and Kiss, 1976a,b). The combined use of GABA immunocytochemistry and electron microscopy allowed us to extend the earlier findings and to characterize further GABA‐positive axon terminals (F1 profiles), presynaptic dendrites (F2 profiles), and GABAergic dendritic profiles that are not presynaptic (D + ).…”

“…In the light microscope, the vast majority of GABA‐positive boutons are located in the neuropil (i.e., not associated with cell bodies; Huang et al, 1999), and indeed, all of the GABAergic contacts observed in the present study were made with dendrites. Our sample of synapses did not include GABA‐positive contacts onto relay cell somas or any contacts onto interneuron somas; presumably both of these types of synapses are rare compared with synapses involving GABA‐positive profiles in the neuropil.…”

“…Our sample of synapses did not include GABA‐positive contacts onto relay cell somas or any contacts onto interneuron somas; presumably both of these types of synapses are rare compared with synapses involving GABA‐positive profiles in the neuropil. We expected to observe giant GABA‐positive boutons (Huang et al, 1999; Winer et al, 1999b). In fact, we did not observe such giant profiles, but we frequently observed D − profiles that were surrounded by multiple, small, GABA‐positive profiles (see Fig.…”

“…The presence of interneurons in the medial geniculate nucleus (MGN) has been known since Ramón y Cajal's time (1909). Immunocytochemical studies have supported the idea that interneurons are GABAergic (Winer and Larue, 1988; Winer, 1991; Huang et al, 1999). Electron microscopic studies have examined presumptive interneurons (Majorossy and Réthelyi, 1968; Jones and Rockel, 1971; Morest, 1971, 1975; Majorossy and Kiss, 1976a,b), but these studies were limited by a lack of GABA labeling.…”

“…γ‐Aminobutyric acid (GABA)‐ergic interneurons are one of the most important influences on thalamic activity. In the cat, interneurons constitute 20–35% of the thalamic cells (LeVay and Ferster, 1979; Weber and Kalil, 1983; Madarasz et al, 1985; Huang et al, 1999). In the lateral geniculate nucleus (LGN) and ventroposterior nucleus (VP), the dendrites of interneurons receive ascending sensory input and form dendrodendritic connections with thalamocortical cells.…”