The connections of presubiculum and parasubiculum in the rat.

The present study describes the differences and similarities between the connections of the presubiculum and parasubiculum based on retrograde and anterograde tracing experiments. The results demonstrate that both areas have several similar afferent connections, particularly those from subcortical areas such as the claustrum, diagonal band of Broca, anterior thalamus, nucleus reuniens, locus coeruleus, and raphe nuclei. Both subicular areas also are innervated by axons originating in the ipsilateral and contralateral entorhinal cortex, presubiculum, and parasubiculum. In contrast to these similarities, most axons innervating the presubiculum originate in the lateral dorsal thalamic nucleus, the claustrum, and the contralateral presubiculum. Conversely, the parasubiculum is innervated primarily by axons that originate in area CA1 of the hippocampus, the basolateral nucleus of the amygdala, and the contralateral presubiculum and parasubiculum. The major efferent projection from the presubiculum and parasubiculum courses bilaterally to the medial entorhinal cortex; however, the results of the present study confirm previous suggestions that presubicular axons terminate almost exclusively in layers I and III, whereas parasubicular axons innervate layer II. The presubiculum also projects to the anteroventral and laterodorsal nuclei of the thalamus, and the lateral ventral portion of the medial mammillary nucleus, whereas the parasubiculum projects prominently to the anterodorsal nucleus of the thalamus, the contralateral presubiculum and parasubiculum, and the lateral dorsal segment of the medial mammillary nucleus. Thus despite some similarities, the major connections of presubiculum and parasubiculum are distinct from one another and distinct from the projections of the adjacent subiculum and postsubiculum. These results suggest that the subicular cortex is considerably more complex than previously envisioned and indicate that each segment may subserve a distinct role in the processing of information by the hippocampal formation.