Evidence for some collateralization between cortical and diencephalic efferent axons of the rat subicular cortex

The present study has used the fluorescent dye tracing technique in order to determine the exact location of neuronal somata within the subicular cortex which project to the diencephalon, telencephalon (entorhinal cortex), or to both via axonal collaterals. The greatest collateralization to the two sites is found in the neurons of the subiculum proper. In this region approximately one-third of all neurons project to both the entorhinal cortex and the hypothalamus (either the mammillary bodies or the ventral medial hypothalamic nucleus). The hypothalamic and cortical projection cell bodies in this region are intermingled extensively with each other. In the cytoarchitectonically more organized regions of the subicular cortex, i.e. the pre-, para- and postsubiculum, the situation is quite different. In these areas neurons project to the hypothalamus or entorhinal cortex but very seldom does a single neuron project to both areas, and the neuronal somata are spatially segregated according to their projections. The entorhinal cortex projecting somata are located in layer two whereas the hypothalamic neurons are in the deeper layer. The somata projecting to the thalamus are the most deeply located neurons in all regions of the subicular cortex, and extremely few collateralize to the entorhinal cortex.