The identification of relay neurons in the dorsal lateral geniculate nucleus of monkeys using horseradish peroxidase.

Intraaxonal retrograde transport of the protein horseradish peroxidase (HRP) was used to identify relay neurons in the dorsal lateral geniculate nucleus (LGN) of owl (Aotus trivirgatus) and rhesus (Macaca mulatta) monkeys. In both species, from 94.1-98.6% of the neurons within columns extending through both parvocellular and magnocellular layers were labeled following injection of HRP into striate cortex. Labeled neurons were also identified in the thin ventral-most S(0) Layers. Although most of the cells within the thin interlaminar regions in the LGN of both species were labeled following injections of HRP, many unlabeled neurons were identified within the large cell-rich interlaminar region (IL) between the internal parvocellular and internal magnocellular layers in the LGN of the owl monkey, suggesting that IL may be a specialized region containing a large number of intrinsic neurons. Finally, measurement of the cell diameters of neurons within the densely labeled areas in relay layers revealed that labeled and unlabeled neurons could not be distinguished on the basis of cell body size alone and that some of the smallest cells of the LGN project to striate cortex. These findings indicate that nearly all of the neurons of the main relay layers of the LGN in these two primates are relay cells and that the organization of the LGN in primates may differ significantly from that of other mammals with respect to the percentage of interneurons.