Why are there parallel hippocampal - diencephalic pathways for event memory (360G-Wellcome-081075_A_06_Z)

Why are the hippocampal projections to the mammillary bodies and to the anterior thalamic nuclei vital for normal memory? Anatomical analyses now reveal that these two sets of direct hippocampal projections only arise from two adjacent cell groups in the subiculum, yet the mammillary bodies also project very densely upon the anterior thalamic nuclei (forming a major, indirect route from the hippocampus to the anterior thalamic nuclei). The functional significance of having two populations of subicular cells supporting the same memory system remains unknown.Anatomical tracing techniques will determine whether the two different populations of subicular cells (subiculum ? mammillary bodies, subiculum ? anterior thalamus) receive their inputs from separate sources. Immunohistochemical methods will determine if these two populations of subicular cells are differentially activated by selected learning tasks. Electrophysiological stimulation/recording studies will test whether the direct and indirect pathways to the anterior thalamic nuclei converge onto the same thalamic cells. Attention will also focus on the types of anterior thalamic/mammillary body plasticity that can be driven by subicular stimulation. The results will shape and refine hypotheses concerning the ways in which hippocampal - diencephalic pathways support memory.