Projection of the entorhinal layer II neurons in the rat as revealed by intracellular pressure-injection of neurobiotin.

A component of the perforant path, projection of the entorhinal layer II neurons, was investigated by recovering intracellularly labeled layer II neurons from the medial or intermediate entorhinal cortex in rats. The labeled layer II spiny stellate neurons had axon collaterals in layers I, II, and III of the entorhinal cortex as well as some axon collaterals in the subiculum. The stem axons gave rise to terminal axon branches that covered the entire extent (suprapyramidal blade, crest, and infrapyramidal blade) of the dentate gyrus and the CA2-3 fields in the transverse plane, forming a sheet-like formation. The axon arbor in the hippocampal formation spread up to 2 mm wide in a septotemporal direction. The sheet-like formation of the axon arbors was a narrow layer in the suprapyramidal blade and in the stratum lacunosum-moleculare of the CA2-3 fields. The layer became wider in the crest and infrapyramidal blade of the dentate gyrus. This study shows that the entorhinohippocampal circuit is not a simple circle from single cells level.