ORGANIZATION OF PROJECTION NEURONS OF THE HIPPOCAMPUS

The origin of the fornix system was investigated by the horseradish peroxidase (HRP) technique. Small injections of HRP into the septal region resulted in labeling of neurons in both the pyramidal and nonpyramidal layers. Labeled neurons in the stratum pyramidale were seen in CA3 (bilaterally), CA2, CA1, and the adjacent subicular complex. Different patterns of uptake were observed in those hippocampal subfields. Numerous neurons in stratum oriens also displayed HRP positivity. These neurons were both polymorphic and spindle shape. The neurons were mainly restricted to the ventral portions of the hippocampus but labeled neurons were also seen in more dorsal levels. Furthermore, scattered neurons in stratum radiatum also displayed HRP positivity. These data demonstrate that some neurons generally considered to be interneurons are indeed projection neurons of the hippocampus. © 1979.