Quantitative electron microscopic analysis of synaptic input from cortical areas 17 and 18 to the dorsal lateral geniculate nucleus in cats.

Cortical feedback is the largest extraretinal projection to the lateral geniculate nucleus. This input is thought to modulate the transfer of visual information in a state-dependent manner. The quantitative distribution and synaptology of axon terminals arising from different cortical areas is still an unsolved question. To address this problem, the synaptic termination pattern of corticogeniculate axons from cortical areas 17 and 18 entering the lateral geniculate nucleus of the cat was examined. The Phaseolus vulgaris leucoagglutinin anterograde tract tracing method was used for the labeling of corticogeniculate terminals. Postsynaptic targets were characterized by postembedding gamma-aminobutyric acid (GABA) immunocytochemistry. In both laminae A and A1, labeled corticogeniculate axons from area 17 established synaptic contacts with GABA-immunopositive, interneuronal dendritic profiles more frequently (17.5% of all axons) than did labeled axon terminals from area 18 (7% of axons). Conversely, 76% of labeled corticogeniculate axons from area 17, as opposed to 87% of labeled axons from area 18, terminated on GABA-immunonegative relay cell dendrites. Furthermore, the mean diameter of GABA-negative relay cell dendrites postsynaptic to labeled axons from area 17 was significantly smaller than the diameter of relay cell dendrites synapsing with labeled terminals from area 18. These results indicate that the corticogeniculate axons from cortical areas 17 and 18 exhibit different synaptic termination patterns in the dorsal lateral geniculate nucleus of the cat, suggesting that these two projections may subserve different functions in visual information processing.