Three-dimensional topography of corticopontine projections from rat barrelcortex: Correlations with corticostriatal organization

Subcortical re-entrant projection systems connecting cerebral cortical area s with the basal ganglia and cerebellum are topographically specific and th erefore considered to be parallel circuits or "closed loops." The precision of projections within these circuits, however, has not been characterized sufficiently to indicate whether cortical signals are integrated within or among presumed compartments. To address this issue, we studied the first li nk of the rat cortico-ponto-cerebellar pathway with anterograde axonal trac ing from physiologically defined, individual whisker "barrels" of the prima ry somatosensory cortex (SI). The labeled axons in the pontine nuclei forme d several, sharply delineated clusters. Dual tracer injections into differe nt SI whisker barrels gave rise to partly overlapping, paired clusters, ind icating somatotopic specificity. Three-dimensional reconstructions revealed that the clusters were components of concentrically organized lamellar sub spaces. Whisker barrels in the same row projected to different pontine lame llae (side by side), the somatotopic representation of which followed an in side-out sequence. By contrast, whisker barrels from separate rows projecte d to clusters located within the same lamellar subspace (end to end). In th e neostriatum, this lamellar topography was the opposite, with barrels in t he same row contacting different parts of the same lamellar subspace (end t o end). The degree of overlap among pontine clusters varied as a function o f the proximity of the cortical injections. Furthermore, corticopontine ove rlap was higher among projections from barrels in the same row than among p rojections from different whisker barrel rows. This anisotropy was the same in the corticostriatal projection. These findings have important implicati ons for understanding convergence and local integration in somatosensory-re lated subcortical circuits.