Organizational connectivity among the CA1, subiculum, presubiculum, and entorhinal cortex in the rabbit.

The laminar and topographical organization of connections between the hippocampal formation and parahippocampal regions was investigated in the rabbit following in vivo injection of cholera toxin B subunit as a retro- and antero-grade tracer and biotinylated dextran amine as an anterograde tracer. We confirmed several connectional features different from those of the rat, that is, the rabbit presubiculum received abundant afferents from CA1 and had many reciprocal connections with the entorhinal cortex. On the other hand, we identified many similarities with the rat: both the CA1 and subicular afferents that originated from the entorhinal cortex were abundant; moreover, the presubiculum received many inputs from the subiculum and sent massive projections to the entorhinal cortex. By plotting retrograde and anterograde labels in two-dimensional unfolded maps of the entire hippocampal and parahippocampal regions, we found that each group of entorhinal cells that project to CA1, subiculum, and presubiculum, and also the termination of the presubiculo-entorhinal projection, was distributed in band-like zones in layers II-III, extending across the medial and lateral entorhinal cortex. Our results suggest that the rabbit has a basic connectivity that is common with that of the rat, and also has additional hippocampal-presubicular and entorhino-presubicular connections that may reflect functional evolution in learning and memory.