Connectivity of the Claustrum–Endopiriform Complex with the Presubiculum and Hippocampal Regions in the Common Marmoset (Callithrix jacchus)

ABSTRACT We have investigated the hippocampal connectivity of the marmoset presubiculum (PreS) and reported that major connections of PreS in the rat were conserved in the marmoset. Moreover, our results indicated the presence of several additional projections that were almost absent in the rat brain, but abundant in the marmoset, such as direct projections from CA1 to PreS. However, little is known about the connectivity between the frontal brain regions and PreS or hippocampal formation. Therefore, we investigated the distribution of cells of the origins and terminals of the presubicular and hippocampal projections in the marmoset frontal brain regions using the retrograde and anterograde tracer cholera toxin B subunit. In cases of tracer injections into all layers of PreS, many neurons and terminals were labeled in the claustrum–endopiriform (Cl–En) complex almost entirely along the rostrocaudal axis. Even in cases where the injection site involved the superficial (not deep) layers of PreS, labeled neurons and terminals were distributed over a wide rostrocaudal range of the Cl–En complex, but their number and density were significantly lower than the whole‐layer injection cases. In cases where the injection site was confined to the hippocampal formation, labeled cells and terminals were localized at a restricted portion of the Cl–En complex. Here, we demonstrate for what we believe to be the first time the strong, reciprocal connections of the Cl–En complex with PreS and projections from the Cl–En complex to the hippocampal regions (CA1 and the subiculum) in the marmoset. Our findings indicate that the Cl–En complex may exert a strong influence on the cortical and subcortical outputs from PreS and, in turn, the entire memory circuitry in the marmoset brain.