Ipsilateral and contralateral hippocampal CA3-CA1 and CA2-CA1 projections were investigated
in adult male Long-Evans rats by retrograde tracing. Injection of the retrograde tracer
cholera toxin subunit B in the strata oriens and radiatum of dorsal CA1 resulted in
labeling of predominantly pyramidal cells in ipsilateral and contralateral CA3 and
CA2. The contralateral and ipsilateral anterior-posterior extents of CA3 innervation
to CA1 were similar. Fifteen to twenty per cent of the hippocampus proper cells that
give rise to CA1 stratum oriens innervation were CA2 pyramidal cells, whereas CA2
cells were a mere 3% for CA1 stratum radiatum innervation. The preferred projection
of CA2 pyramidal cells to the CA1 stratum oriens was also manifested in transgenic
mice that express GFP under the control of the CACNG5 promoter, in which CA2 cells
express high amounts of GFP. The ratios of ipsilateral to contralateral projections
were compared. For the CA3-CA1 connection, we found that dorsal CA1 stratum radiatum
received more ipsilateral projections whereas CA1 stratum oriens received more contralateral
innervation. Interestingly, ipsilateral connections dominated for both CA2-CA1 stratum
oriens and CA2-CA1 stratum radiatum. These results demonstrate that the primary intrahippocampal
target of CA2 pyramidal cells is the ipsilateral CA1 stratum oriens, in contrast to
CA3 cells which project more diversely to bilateral CA1 regions. Such innervation
patterns may suggest differential dendritic information processing in apical and basal
dendrites of CA1 pyramidal cells.
