Topographical organization of projections from the subiculum to the hypothalamus in the rat

The projections from the subiculum to the hypothalamus were comprehensively examined in the rat by using the anterograde Phaseolus vulgaris leucoagglu tinin (PHA-L) and retrograde cholera toxin B subunit (CTb) methods. Tracing of efferents with PHA-L indicated that the medial preoptic region received projection fibers from the temporal two-thirds of the subiculum, whereas t he anterior, tuberal, and mammillary regions received those from the full l ongitudinal extent of the subiculum. The subicular projections to the anter ior and tuberal hypothalamic regions were also found to be organized in a t opographical manner such that the temporal-to-septal axis of origin in the subiculum determined a ventromedial-to-dorsolateral axis of termination in the medial zone of the hypothalamus: Massive labeled fibers from the tempor almost part of the subiculum terminated in the subparaventricular zone and its caudal continuum around the dorsal and medial aspects of the ventromedi al nucleus, and those from progressively more septal parts terminated in pr ogressively more dorsolateral parts of the medial zone. In addition, the te mporal-to-septal axis of origin in the subiculum tended to determine a medi al-to-lateral axis of termination in the preoptic region as well as a ventr al-to-dorsal axis of termination in the mammillary region. Furthermore, the temporal-to-septal axis of origin in the septal two-thirds of the subiculu m corresponded to a ventrolateral-to-dorsomedial axis of termination in the medial mammillary nucleus. The topographical projections from the subiculu m to the medial zone of the hypothalamus were confirmed by CTb experiments, representatively in the subicular projections to the anterior hypothalamic region. These results suggest that different populations of neurons existi ng along the longitudinal axis of the subiculum may exert their influences on the execution of different hypothalamic functions. (C) 2000 Wiley-Liss, Inc.