Neonatal whisker clipping alters intracortical, but not thalamocortical projections, in rat barrel cortex

Retrograde axonal transport of cholera toxin B subunit (CTB) was used to co mpare the development of intracortical and thalamocortical connections in n ormal rats with those in rats in which all of the whiskers were trimmed con tinuously from birth. In normal animals, injections of CTB into a single ba rrel column resulted in an asymmetrical labeling of cells that were distrib uted preferentially within columns related to the same row in which the inj ection was placed. This anisotropy in the patterns of intracortical connect ions was not observed in whisker-clipped animals. In these animals, there w as a significant reduction in the mean number of labeled cells in the infra granular layers, and labeled cells were distributed symmetrically around th e injection site. The same injections of CTB also labeled thalamocortical n eurons in the ventrobasal thalamus. Analysis of the distribution of these c ells revealed that, in both control and experimental animals, the vast majo rity of labeled cells were restricted to a homologous (i.e., corresponding to the injected cortical barrel) thalamic barreloid. These findings indicat e that manipulations of sensory experience alter patterns of intracortical, but not thalamocortical, connections. (C) 1999 Wiley-Liss, Inc.