The topography of tangential inhibitory connections in the postnatally developing and mature striate cortex of the cat.

Clustered intrinsic connections in the striate cortex of kittens originate from an unclustered, diffusely organized pattern prevailing during the first postnatal week. In order to study the participation of inhibitory neurons in this reorganization of the connections, we determined the topography of the inhibitory tangenital connections in the striate cortex of cats ranging in age between 7 and 330 days by combining retrograde transport of fluorescent microspheres with GABA immunohistochemistry. After small intracortical injections of tracer, neurons containing either microspheres only (non-GABAergic neurons) or GABA-like immunoreactivity in addition to microspheres (GABAergic neurons) are labelled at various horizontal distances from the injection. At the end of the first postnatal week, both GABAergic and non-GABAergic neurons are distributed in the horizontal plane in an unclustered fashion. During the second postnatal week, the tangential connections rearrange rapidly to form clusters. The tendency of the cells to form clusters is much weaker, however, in GABAergic than in non-GABAergic neurons. In regions > 500 microns distant from the centre of injection approximately 90% of the non-GABAergic neurons (range 87.5-92.6%) but only 63% (range 57.1-72.3%) of the GABAergic neurons reside within the clusters formed by the non-GABAergic neurons. These proportions do not change systematically with age. In the regions outside the non-GABAergic clusters, GABAergic neurons appear to be evenly distributed and not to aggregate in clusters. From postnatal day 7 forward GABAergic neurons largely retain their overall distribution and density in the horizontal plane. When considering all cortical layers (including the superficial white matter) the lateral spread of the GABAergic neurons is more restricted than that of the non-GABAergic neurons. Systematic changes in the lateral spread of inhibitory connections according to postnatal age were not observed. We conclude that, like the non-GABAergic neurons, the GABAergic neurons have attained an adult-like topography in the horizontal plane by about the end of the second postnatal week. From that time until adulthood they display much weaker clustering, a higher relative occurrence of short axon collaterals and a more restricted lateral distribution than do the excitatory neurons.