Patterns of axonal collateralization of single layer V cortical projection neurons in the rat presubiculum.

The presubiculum is one of the important regions of the parahippocampal area known to be responsible for processing and integrating spatial representation information. To understand better the functional roles played by the presubiculum, it is essential to elucidate how output signals from the presubiculum distribute to its target regions. In the present study, the axonal branching patterns of single pyramidal neurons in layer V of the rat presubiculum were investigated by using in vivo injection of a viral vector expressing membrane-targeted palmitoylation site-attached green fluorescent protein. We found that individual layer V neurons provide axonal branches to one or two cortical areas with one or more recurrent collaterals to the presubiculum. These neurons were classified into six types, based on their axonal collateralization pattern: neurons with axon branches to 1) both the retrosplenial granular cortex and the parasubiculum, 2) both the retrosplenial granular cortex and the subiculum, and 3) both the medial entorhinal area and the subiculum, and neurons with axonal branches terminating only in 4) the retrosplenial granular cortex, 5) the subiculum, and 6) the presubiculum. Types 1-5 also provide recurrent axons to the presubiculum. Our data demonstrate that layer V of the presubiculum consists of at least six types of cortical projection neurons with various patterns of axonal collateralization. These findings suggest that single presubicular layer V neurons may distribute information to one or two cortical areas participating in the neural circuitry of spatial representation and also send such information back to the presubiculum itself.