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.
