Synaptic connections between corticogeniculate axons and interneurons in the dorsal lateral geniculate nucleus of the cat

Anatomical evidence is provided for direct synaptic connections by axons from visual cortex with interneurons in lamina A of the cat's dorsal lateral geniculate nucleus. Corticogeniculate axon terminals were labeled selectively with 3H-proline and identified by means of electron microscopic autoradiography. Interneurons in the lateral geniculate nucleus were stained with antibodies that had been raised against gamma aminobutyric acid (GABA). We found that corticogeniculate terminals synapsed with dendrites stained positively for GABA about three times as often as with unstained dendrites. Of the corticogeniculate terminals that contacted GABA-positive dendrites, 97% made synaptic connections with dendritic shafts. Only 3% synapsed with F profiles, the vesicle-filled dendritic appendages characteristic of lateral geniculate interneurons. These results suggest that the corticogeniculate pathway in the cat is directed primarily at interneurons and is organized synaptically to influence the integrated output of these cells, rather than the local interactions in which their dendritic specializations participate.