LIGHT-MICROSCOPIC AND ELECTRON-MICROSCOPIC ANALYSIS OF SYNAPTIC INPUTFROM CORTICAL AREA-17 TO THE LATERAL POSTERIOR NUCLEUS IN CATS

The morphology and synaptic organization of the corticothalamic (CT) F ibres from area 17 were studied in the lateral posterior nucleus (LP) of the thalamus in cats. Injection of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHAL) into primary visual cortex labelled a band of CT fibres in the LP with terminal field confined to its later al division ''LP1'', PHAL-labelled CT axons in the LP1 gave rise to bo th en passant and terminal boutons. They usually established several s ynaptic contacts - often in complex glomerulus-like synaptic arrangeme nts - with dendritic shafts of large diameter and presynaptic dendrite s containing pleomorphic vesicles. Postsynaptic targets of the PHAL-la belled CT boutons were characterized by postembedding gamma-aminobutyr ic acid (GABA) immunocytochemistry. It appeared that, in the LP1 of th e cat, almost half (44.5%) of the postsynaptic dendrites to CT boutons from area 17 belonged to the GABA-immunopositive interneurons and the majority (41%) of these GABA-immunopositive dendrites were F2 termina ls. These results indicate that the CT axons from the striate cortex i n the LP of the cat, in addition to a direct excitatory action, exert a powerful feed-forward inhibition on the thalamic principal cells.