Projections from primary somatosensory cortex to the neostriatum: the role of somatotopic continuity in corticostriatal convergence.

We characterized the organization of corticostriatal projections from rodent primary somatosensory cortex (SI), testing the hypothesis that projections from SI areas representing subcomponents of the forelimb exhibit greater neostriatal overlap than projections from areas representing separate body parts. The anterograde tracers Fluoro-Ruby (FR), Alexa Fluor (AF), and biotinylated dextran amine (BDA) were injected into physiologically identified regions of rat SI. Injection locations were confirmed by examining the SI barrel fields and limb representations in tangential sections processed for cytochrome oxidase (CO). Experimental animals were divided into two groups: one group received multiple tracer injections in neighboring SI regions that represent separate body parts (whiskers, forepaw, and hindpaw); the other group received injections in SI areas that represent different components of the forelimb (forepaw, antebrachium, and brachium). The distribution of labeled terminals and their varicosities in the neostriatum and in the thalamus were plotted and quantitatively analyzed. For most animals, tracer overlap in the thalamus was either minimal or completely absent. In the neostriatum, projections from the whisker, forelimb, and hindlimb representations terminated in regions that rarely overlap with each other, while those originating from different parts of the forelimb representation were more likely to terminate in overlapping parts of the neostriatum. To the extent that neostriatal activation depends on corticostriatal convergence, the corticostriatal projections in the sensorimotor channel appeared to be organized so that neostriatal neurons may signal when multiple components of the same body part are activated simultaneously.