Direct synaptic connections between corticothalamic axon terminals and interneurons in the cat ventrobasal complex.

Lesion-induced degeneration was combined with immunocytochemistry to study, with electron microscopy, the synaptic connectivity between corticothalamic axon terminals from the first and second somatosensory areas and local circuit neurons of the ipsilateral ventrobasal complex (VB), selectively labelled with an antibody raised against gamma-aminobutyric acid (GABA). Four days from the cortical ablation many degenerating axon terminals, forming asymmetric synapses, were found on dendritic trees of both labelled and unlabelled neurons of VB and occasionally on presynaptic dendrites. The main finding of the present paper is that 64.01% of degenerating axon terminals synapsed with GABA-immunopositive dendrites, suggesting that the principal target of the cortical projection to VB are local circuit neurons.