The Journal of Neuroscience, November 15, 2000, 20(22):8474-8484
Three-Dimensional Topography of Corticopontine Projections from
Rat Barrel Cortex: Correlations with Corticostriatal Organization
Trygve B.
Leergaard1,
Kevin D.
Alloway2,
Joshua
J.
Mutic2, and
Jan G.
Bjaalie1
1 Department of Anatomy, Institute of Basic Medical
Sciences, University of Oslo, N-0317 Oslo, Norway, and
2 Pennsylvania State University, Department of Neuroscience
and Anatomy, Hershey, Pennsylvania 17033
Subcortical re-entrant projection systems connecting cerebral
cortical areas with the basal ganglia and cerebellum are
topographically specific and therefore 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 link
of the rat cortico-ponto-cerebellar pathway with anterograde axonal
tracing from physiologically defined, individual whisker "barrels"
of the primary somatosensory cortex (SI). The labeled axons in the
pontine nuclei formed several, sharply delineated clusters. Dual tracer
injections into different SI whisker barrels gave rise to partly
overlapping, paired clusters, indicating somatotopic specificity.
Three-dimensional reconstructions revealed that the clusters were
components of concentrically organized lamellar subspaces. Whisker
barrels in the same row projected to different pontine lamellae (side
by side), the somatotopic representation of which followed an
inside-out sequence. By contrast, whisker barrels from separate rows
projected to clusters located within the same lamellar subspace (end to
end). In the neostriatum, this lamellar topography was the opposite,
with barrels in the same row contacting different parts of the same
lamellar subspace (end to end). The degree of overlap among pontine
clusters varied as a function of the proximity of the cortical
injections. Furthermore, corticopontine overlap was higher among
projections from barrels in the same row than among projections from
different whisker barrel rows. This anisotropy was the same in the
corticostriatal projection. These findings have important implications
for understanding convergence and local integration in
somatosensory-related subcortical circuits.
Key words:
3-D reconstruction; basal ganglia; cerebellum; cerebrocerebellar; double anterograde tracing; parallel circuits; pontine nuclei; somatosensory maps; somatotopy
Copyright © 2000 Society for Neuroscience 0270-6474/00/20228474-11$05.00/0
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