DEVELOPMENT AND PLASTICITY OF LOCAL INTRACORTICAL PROJECTIONS WITHIN THE VIBRISSAE REPRESENTATION OF THE RAT PRIMARY SOMATOSENSORY CORTEX

Labelling with -dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perc hlorate (Di-A) was used to assess the development of projections withi n the primary somatosensory cortex (SI) of rats aged between postnatal day 2 and 8 (P-2 and P-8). 1,1'-Dioctadecyl-3,3,3,'' 3'-tetramethylin docarbocyanine perchlorate (Di-I) was used in these same animals to la bel thalamocortical afferents. Particular attention was paid to the em ergence of lamina IV intracortical projections that form a pattern com plementary to vibrissae-related thalamocortical afferents. A vibrissae -related pattern of Di-A-labelled cells and fibers that was restricted largely to the septa regions was not apparent in rats killed on P-2, but it was visible in animals killed on P-4 and later ages. Tracing wi th biotinylated dextran amine (BDA) was used to assess intra-SI projec tions of adult rats that sustained transection of the infraorbital ner ve (ION) on P-0 or P-7 or implantation of a tetrodotoxin (TTX)-impregn ated polymer chip over the cortex on P-0. Rats that sustained ION tran section on P-7 or that had TTX implants demonstrated normal patterns o f projections within SI. The patterns of labelling in the supra- and i nfragranular layers of the cortices of the rats that sustained ION tra nsection on P-0 were generally similar to those in the other groups ev aluated. However, in lamina IV, there was no organization that could b e related to the distribution of the vibrissae. These results indicate that the vibrissae-related pattern of intracortical projections withi n SI develops shortly after birth and that two manipulations that alte r cortical activity, but not the patterning of thalamocortical afferen ts (application of TTX and transection of the ION after thalamocortica l afferent patterns are established), have no significant effect on it . However, a manipulation that alters thalamocortical development (tra nsection of the ION on P-0) profoundly affects the patterning of intra cortical connections. (C) 1996 Wiley-Liss, Inc.