Hippocampal commissural connections in the neonatal rat

Abstract 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-μm 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.