The contribution of GABA-ergic neurons to horizontal intrinsic connections in upper layers of the cat's striate cortex.

The contribution of neurons containing gamma-aminobutyric acid (GABA) to horizontal intrinsic projections in layers I-III of cat's striate cortex was investigated by combining GABA-immunohistochemistry with axonal tracing. After intracortical injections of Rhodamine-labelled latex microspheres Rhodamine-labelled neurons form patch- or bandlike aggregations (clusters) separated from each other by regions containing fewer, evenly distributed or no labelled neurons. Of the Rhodamine-labelled neurons about 5% display GABA-immunoreactive material (double labelled = DL-neurons). Approximately 70% of the DL-neurons occur at distances of less than 1 mm, and the remaining 30% at distances between 1 mm and 2.5 mm from the injection. About 60% of the DL-neurons reside within clusters and 40% are located in regions between clusters; the respective percentages of the Rhodamine labelled GABA-negative neurons are about 85 and 15. Considering their small number and their spatial distribution inhibitory interneurons seem to make only minor contributions to the clustered pattern of intrinsic connections. Our results demonstrate that the topographical organization of neurons giving origin to lateral inhibitory interactions in upper layers of cat's striate cortex is different from that of neurons mediating excitatory functions.