Hippocampal CA3 and CA2 have distinct bilateral innervation patterns to CA1 in rodents

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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 thestrata oriensandradiatumof 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 CA1stratum oriensinnervation were CA2 pyramidal cells, whereas CA2 cells were a mere 3% for CA1stratum radiatuminnervation. The preferred projection of CA2 pyramidal cells to the CA1stratum orienswas 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 CA1stratum radiatumreceived more ipsilateral projections whereas CA1stratum oriensreceived more contralateral innervation. Interestingly, ipsilateral connections dominated for both CA2–CA1stratum oriensand CA2–CA1stratum radiatum. These results demonstrate that the primary intrahippocampal target of CA2 pyramidal cells is the ipsilateral CA1stratum 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.

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