Evidence for separate projections of hippocampal pyramidal and non-pyramidal neurons to different parts of the septum in the rat brain.

Large (200 nl) intraseptal injections of horseradish peroxidase (HRP) resulted in retrograde axonal labeling of both pyramidal and non-pyramidal neurons throughout all septo-temporal levels of the hippocampal formation in the rat brain. Small (50 nl) injections of HRP into the medial septum labeled cells of non-pyramidal shape in the stratum oriens and the stratum radiatum of regio inferior, stratum oriens of regio superior and the hilus of the area dentata. Small (50 nl) injections of HRP restricted to the lateral septum resulted in retrograde labeling of pyramidal cells in regio inferior and regio superior without labeling of non-pyramidal cells. These results suggest a new efferent projection system from the hippocampus consisting of non-pyramidal neurons which innervate the medial septum/diagonal band complex in the rat brain.