DEVELOPMENT OF INTRINSIC CONNECTIONS IN CAT SOMATOSENSORY CORTEX

We explored the development of clustered connections in cat somatosens ory cortex by using intracortical injections of biocytin and carbocyan ine dye (DiI). Biocytin injections in adults revealed clusters of retr ogradely labeled cells, affirming earlier reports of the patchy nature of corticocortical connectivity in the adult cat somatosensory cortex . On postnatal days (PNDs) 1 and 3, a diffuse distribution of axons an d labeled cells were found after DiI injections. A dramatic rearrangem ent of connections had taken place by PND 6. Well-defined clusters of labeled cells surrounded the injection site. Intercluster zones were r elatively free of labeled cells and contained few labeled axons. At la ter ages (PNDs 12 and 15), a clear patchy distribution of intrinsic co nnections was seen. We analyzed neuronal clustering by using spatial p oint process analysis, which corroborated our qualitative observations . The density of labeled neurons was significantly higher in clusters than in the intercluster zones on PND 6. Autocorrelations run on profi le plots of optical density values, along paths parallel to the edge o f the injection sites (reflecting axonal distributions), revealed sign ificant periodicity (P < .05; center-to-center similar to 750 mu m) by PND 6. These data demonstrate that corticocortical connections in cat somatosensory cortex develop from a diffuse network at birth, which t ransforms into a patchy network by the end of the Ist postnatal week. This is substantially earlier than the development of local circuits i n the cat visual cortex and may reflect fundamental differences in the organization of the two cortices. (C) 1997 Wiley-Liss, Inc.