Reciprocal connections of the insular and piriform claustrum with limbic cortex: an anatomical study in the cat.

The connections of the claustrum with non-isocortical limbic and paralimbic cortex in the cat are described, using the anterograde transport of tritiated amino acids and the retrograde transport of various fluorescent tracers and of horseradish peroxidase conjugated to the lectin wheatgerm agglutinin. It could be demonstrated that the claustrum, in addition to its connections with sensory-related areas, is reciprocally and bilaterally connected with widespread limbic and paralimbic cortical regions. These connections are organized such that the area of origin of claustral efferents to a certain cortical region coincides with the area of termination in the claustrum of afferents from that same cortical region. A rostrocaudal topographical organization of the limbic-related connections of the claustrum is not very apparent. However, the results clearly demonstrate a dorsoventral topographical organization in the connections between the claustrum and the cortex. The ventral part of the claustrum has reciprocal connections predominantly with the entorhinal cortex, and possibly with the anterior olfactory nucleus and the prepiriform cortex. A more dorsally located part of the claustrum is preferentially connected with the orbitofrontal, the insular, the perirhinal, the anterior limbic, and the cingular cortices, and with parts of the subicular complex. The most dorsal portion of the claustrum is more heavily connected with parasensory and sensory cortices. It is concluded that the traditional subdivision of the claustrum into two discrete nuclei, i.e. the insular claustrum connected with the isocortex, and the piriform claustrum or endopiriform nucleus connected with the allocortex, does not reflect the actual organization of the cortical connections of the claustrum. The present data provide a more differentiated view, such that the ventral portion of the claustrum is reciprocally connected mainly with the olfactory-related cortices and the entorhinal cortex, whereas the cortical connections of progressively more dorsal parts of the claustrum gradually shift from limbic and paralimbic towards parasensory and sensory cortical connections. The significance of these findings is discussed in the light of a possible function of the claustrum in relation to corticocortical integration and memory processing.