Patchy intrinsic projections in visual cortex, area 18, of the cat: morphological and immunocytochemical evidence for an excitatory function.

Small injections of peroxidase-labeled wheat germ agglutinin into cat area 18 gave rise to patches of labeled cells and axon terminals around the injection site. In EM sections it was found that the labeled cells had a pattern of synaptic inputs characteristic of spiny-dendrite neurons (pyramidal or spiny stellate cells). The labeled axon terminals formed type 1 (asymmetric) synapses, most of which were made onto dendritic spines. In other experiments injections of fluorescent beads were made into area 18, giving rise to a similar patchy distribution of labeled cells. The sections were then processed for immunocytochemical demonstration of gamma-aminobutyric acid (GABA). The bead-labeled cells in the patches were GABA negative. The findings suggest that the patchy projections mediate mutual excitation between groups of spiny-dendrite neurons.