Projections from the nucleus reuniens thalami to the entorhinal cortex, hippocampal field CA1, and the subiculum in the rat arise from different populations of neurons.

The entorhinal cortex, CA1, and the subiculum receive a major input from the thalamic midline nucleus reuniens. At present, it is not known whether reuniens projections to these intimately interconnected regions are collateralized or arise from different cell populations. We employed the multiple fluorescent retrograde tracing technique with Fast Blue, Diamidino Yellow, and Fluoro-Gold to examine the possible collateralization of reuniens projections to the entorhinal cortex, CA1, and the subiculum. In addition, we studied the extent of collateralization within each target area. The results indicate that different, yet morphologically indistinguishable, populations of reuniens cells selectively innervate the entorhinal cortex, CA1, or subiculum. Within each of these areas, reuniens fibers display a locally restricted collateralization instead of distributing collaterals throughout the entire target structure. The rostal two-thirds of the nucleus reuniens is the major source of ipsilateral projections to CA1, subiculum, and entorhinal cortex. The perireuniens nucleus selectively projects to the perirhinal cortex. Reuniens projections to CA1 and medial entorhinal cortex originate in the dorsolateral part and throughout the medial one-half of the nucleus, respectively. For these two projections, no topography could be established. However, subicular afferents are topographically organized such that a dorsal-to-ventral gradient in the nucleus reuniens corresponds to a dorsal-to-ventral gradient along the subicular axis. Lateral entorhinal afferents display a subtle topography such that a lateral-to-medial shift of terminal fields in the lateral entorhinal cortex corresponds to a lateral-to-medial shift of projection neurons in the ventral nucleus reuniens.