Demonstration of axonal projections of neurons in the rat hippocampus and subiculum by intracellular injection of HRP.

Hippocampal formation neurons of rat were injected intracellularly with horseradish peroxidase in order to trace intrinsic and extrinsic axonal projections. CA3 pyramids (n = 9) projected axons rostrally toward the fimbria, one or more Schafer collaterals toward CA1, and in two cases fibers that crossed the hippocampal commissure. Pyramids of CA1 (n = 5) projected axons to the alveus where they proceeded caudally toward the subiculum. A subset (n = 3) also projected an axonal branch rostrally toward the fimbria. These findings confirm not only major target regions of Ammon's horn pyramids, but also emphasize their divergent axonal projections that are not necessarily lamellar in organization. Axons from subicular pyramids (n = 12) projected rostrally, caudally, or in both directions. They could be traced to several other cortical regions, specifically Ammon's horn, entorhinal cortex and cingulate cortex. The results further confirm that subicular neurons are the recipient of input from the hippocampus proper and are a principal source of efferents from the hippocampal formation. A multi-process neuron in CA1 with physiologic properties associated with inhibitory interneurons was filled and traced in detail. It most resembled the poligonal basket cells that Lorente de Nó described, having long radially oriented dendrites extending as far as stratum lacunosum-moleculare. The presence of putative inhibitory interneuron dendrites in stratum lacunosum-moleculare suggests some role other than traditional recurrent inhibition for these dendritic segments, and two possible circuits are described.