Ultrastructural identification of satellite interneurons in the rat dorsal lateral geniculate nucleus

Multiple injections of horseradish peroxidase (HRP) were placed in the occipital cortex of rats, including all striate and extrastriate visual areas, to examine with the electron microscope the population of retrogradely labeled and unlabeled cells in the dorsal lateral geniculate nucleus (LGN), for the purpose of ultrastructurally identifying geniculate interneurons. O-tolidine was used as chromogen in the histochemical procedure. EM analysis of LGN showed dense HRP reaction products diffusely distributed in the cytoplasm of virtually all large and medium size geniculate cells, identifying the geniculo-cortical relay cells. The classes of unlabeled cells distinguished in this study are oligodendrocytes, astrocytes, and other small cells whose glial or neural nature is difficult to assess in light microscopic semithin sections. In serial ultrathin sections, these small cells were seen to be presynaptic in somato-somatic synapses with relay cells, and to give origin to presynaptic dendrites. These neurons are cytologically identifiable by their small size, scant cytoplasm, characteristic chromatin clumps in nuclei that are not indented, and by having extended contacts with the cytologically different relay cells. We conclude that these neurons were not retrogradely labeled because of their intrinsic nature in LGN and, because of their common contacts with relay cells, we term them "satellite interneurons".