Thalamic projections of the hippocampal formation: evidence for an alternate pathway involving the internal capsule.

Experiments were performed using [3H]leucine radioautography and horseradish peroxidase histochemistry to determine the origin and distribution of the projection from the hippocampal formation to the thalamus in the rat. The results confirm recent findings that postcommissural fornix fibers which project to the anterior thalamus arise exclusively from the subicular cortex and not from hippocampal pyramidal cells. A second significant feature of this pathway is that it arises from cells situated in the deepest layer of subicular cortex at dorsal and posterior levels of the hippocampal formation. Thirdly, one component of this projection involves the internal capsule instead of the fornix. Fibers which pass through this fiber bundle arise from the lateral portion of the subicular complex adjacent to field CA1 of the hippocampus (i.e., prosubiculum) and terminate bilaterally throughout the anteromedial thalamic nucleus. In contrast, fibers from the region of subicular cortex adjacent to the retrosplenial cortex (i.e., presubiculum) pass through the postcommissural fornix and terminate in the ventral portion of the anteroventral thalamic nucleus. Fibers from central portions of the subicular complex (i.e., subiculum) project through both the internal capsule and postcommisural fornix and terminate in the anteromedial and adjacent ventral portion of the anteroventral thalamic nuclei. Fibers which project to the dorsal portion of the anteroventral thalamic nucleus arise from adjacent portions of retrosplenial cortex and project through the internal capsule.