Overlapping corticostriatal projections from the rodent vibrissal representations in primary and secondary somatosensory cortex

To determine whether the neostriatum receives overlapping projections from two somatosensory cortical areas, the anterograde tracers biotinylated dext ran amine (BDA) and fluoro-ruby (FR) were injected into the whisker represe ntations of primary (SI) and secondary (SII) somatosensory cortex. Reconstr uctions of labeled terminals and their beaded varicosities in the neostriat um and thalamus were analyzed quantitatively to compare the extent of overl apping projections to both subcortical structures. Corticostriatal projecti ons from focal sites in both somatosensory areas exhibited substantial amou nts of divergence within the dorsolateral neostriatum. Most of the labeled terminals were concentrated in densely packed arborizations that occupied l amellar-shaped regions along the dorsolateral edge of the neostriatum. Trac er injections in both cortical areas also produced dense anterograde and re trograde labeling in the thalamus, especially in the ventrobasal complex (V B) and in the medial part of the posterior (POm) nucleus. Because these tha lamic regions are topographically organized and have reciprocal connections with corresponding representations in both SI and SII, the amount of label ed overlap in the thalamus was used to indicate the degree of somatotopic c orrespondence at the SI and SII injection sites. We found that the proporti on of overlapping projections to the neostriatum was moderately correlated with the amount of overlap observed in the thalamus. This result strongly i ndicates that specific sites in the dorsolateral neostriatum receive conver gent projections from corresponding somatotopic representations in SI and S II, but also suggests that some of the corticostriatal divergence may refle ct neostriatal integration of somatosensory information from noncorrespondi ng representations in SI and SII. (C) 2000 Wiley-Liss, Inc.