Reciprocal connections between the claustrum and visual thalamus in the tree shrew (Tupaia glis).

We previously described the existence of reciprocal connections between the dorsal claustrum and striate cortex in the tree shrew, Tupaia glis. These projections were found to originate and terminate in a distinct topographic manner within the mid region of the upper portion of the dorsal claustrum. In this investigation, we examined the afferent and efferent projections between the claustrum and the lateral intermediate nucleus (Li) of the visual thalamus using small electrophoretic injections of anterograde and/or retrograde tracers (horseradish peroxidase, wheat germ agglutinin/horseradish peroxidase, or tritiated amino acids) into the claustrum, as well as the Li. Following tracer injections in the dorsal claustrum, labeled cells and/or terminal grains were found throughout the Li, except at the more caudal levels where the activity was confined to the lateral and medial borders. Tracer injections within the same nuclear region of the dorsal thalamus confirmed the existence of reciprocal projections between the Li and the claustrum. Following anterograde tracer injections, labeled terminals were found only within the most ventral zone of the dorsal claustrum--the 'hilum'; while retrograde tracer injections, produced labeled cells principally along the outer margins of the claustrum, including the hilum and tended to encapsulate the nucleus at all levels. Both sets of labeled activity were found to extend in this specific fashion over the majority of the dorsal claustrum, but appeared not to overlap with regions interconnected with striate cortex. These results thus suggest that the claustrum is capable of exerting a neural influence on cortex directly as well as indirectly via the visual thalamus. However, since the thalamic projection terminates in a claustral region not known to project to visual cortex, it is uncertain what function such projections have within the claustrum. It is possible that since Li receives ascending projections from the pretectum and superior colliculus, that it provides a multisensory input to the claustrum for relay onto areas outside of primary visual cortex.