TOPOGRAPHICALLY SPECIFIC HIPPOCAMPAL PROJECTIONS TARGET FUNCTIONALLY DISTINCT PREFRONTAL AREAS IN THE RHESUS-MONKEY

The sources of ipsilateral projections from the hippocampal formation, the presubiculum, area 29a-c, and parasubiculum to medial, orbital, a nd lateral prefrontal cortices were studied with retrograde tracers in 27 rhesus monkeys. Labeled neurons within the hippocampal formation ( CA1, CA1', prosubiculum, and subiculum) were found rostrally, although some were noted throughout the entire rostrocaudal extent of the hipp ocampal formation. Most labeled neurons in the hippocampal formation p rojected to medial prefrontal cortices, followed by orbital areas. In addition, there were differences in the topography of afferent neurons projecting to medial when compared with orbital cortices. Labeled neu rons innervating medial cortices were found mainly in the CA1' and CA1 fields rostrally, but originated in the subicular fields caudally. In contrast, labeled neurons which innervated orbital cortices were cons iderably more focal, emanating from the same relative position within a field throughout the rostrocaudal extent of the hippocampal formatio n. In marked contrast to the pattern of projection to medial and orbit al prefrontal cortices, lateral prefrontal areas received projections from only a few labeled neurons found mostly in the subicular fields. Lateral prefrontal cortices received the most robust projections from the presubiculum and the supracallosal area 29a-c. Orbital, and to a l esser extent medial, prefrontal areas received projections from a smal ler but significant number of neurons from the presubiculum and area 2 9a-c. Only a few labeled neurons were found in the parasubiculum, and most projected to medial prefrontal areas. The results suggest that fu nctionally distinct prefrontal cortices receive projections from diffe rent components of the hippocampal region. Medial and orbital prefront al cortices may have a role in long-term mnemonic processes similar to those associated with the hippocampal formation with which they are l inked. Moreover, the preponderance of projection neurons from the hipp ocampal formation innervating medial when compared with orbital prefro ntal areas followed the opposite trend from what we had observed previ ously for the amygdala (Barbas and De Olmos [1990] (I Comp Neurol 301: 1-23). Thus, the hippocampal formation, associated with mnemonic proce sses, targets predominantly medial prefrontal cortices, whereas the am ygdala, associated with emotional aspects of memory, issues robust pro jections to orbital limbic cortices. Lateral prefrontal cortices recei ve robust projections from the presubiculum and area 29a-c and sparse projections from the hippocampal formation. These findings are consist ent with the idea that the role of lateral prefrontal cortices in memo ry is distinct from that of either medial or orbital cortices. The res ults suggest that signals from functionally distinct limbic structures to some extent follow parallel pathways to functionally distinct pref rontal cortices. (C) 1995 Wiley-Liss, Inc.