QUANTITATIVE ELECTRON-MICROSCOPIC ANALYSIS OF SYNAPTIC INPUT FROM CORTICAL AREA-17 AND AREA-18 TO THE DORSAL LATERAL GENICULATE-NUCLEUS IN CATS

Cortical feedback is the largest extraretinal projection to the latera l geniculate nucleus. This input is thought to modulate the transfer o f visual information in a state-dependent manner. The quantitative dis tribution and synaptology of axon terminals arising from different cor tical areas is still an unsolved question. To address this problem, th e synaptic termination pattern of corticogeniculate axons from cortica l areas 17 and 18 entering the lateral geniculate nucleus of the cat w as examined. The Phaseolus vulgaris leucoagglutinin anterograde tract tracing method was used for the labeling of corticogeniculate terminal s. Postsynaptic targets were characterized by postembedding gamma-amin obutyric acid (GABA) immunocytochemistry. In both laminae A and Al, la beled corticogeniculate axons from area 17 established synaptic contac ts with GABA-immunopositive, interneuronal dendritic profiles more fre quently (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, termina ted on GABA-immunonegative relay cell dendrites. Furthermore, the mean diameter of GABA-negative relay cell dendrites postsynaptic to labele d axons from area 17 was significantly smaller than the diameter of re lay cell dendrites synapsing with labeled terminals from area 18. Thes e results indicate that the corticogeniculate axons from cortical area s 17 and 18 exhibit different synaptic termination patterns in the dor sal lateral geniculate nucleus of the cat, suggesting that these two p rojections may subserve different functions in visual information proc essing. (C) 1994 Wiley-Liss, Inc.