Hippocampal commissural connections in the neonatal rat

The presence and specific connections of the hippocampal commissure were investigated in neonatal Sprague-Dawley rats. Unilateral hippocampal injections into specific subfields with rhodamine-labelled microspheres (RLM) were performed between 3 and 5 days of postnatal life. Retrograde transport of the RLM was allowed to proceed for 48 h following the injection, at which time brains were removed and prepared for histology. Frozen sections were cut at 30-microns intervals in the coronal plane and examined with fluorescence microscopy. All 5 brains which had an injection restricted to CA1 demonstrated contralateral retrograde transport to the homotopic CA1 subfield only, and all 5 brains with focal CA3 injections had contralateral homotopic CA3 labelling. Eight brains had injections involving more than one subfield; in 5 of these brains, the results were consistent with the CA3 to CA3 commissural projection demonstrated by the more restricted injections. However, 3 brains in which the injection involved CA1 as well as CA3, contralateral CA1 labelling was not found, most likely due to 'missing' the projection site in CA1. We conclude: (1) the hippocampal commissural projection exists in the early postnatal rat, (2) a CA3 to CA3 homotopic commissural projection exists, as it does in the adult, and (3) a CA1 to CA1 projection is present in the neonatal rat.