Constraints on hippocampal processing imposed by the connectivity between CA1, subiculum and subicular targets

A long-held fundamental function of the rat hippocampus is to provide a spatial map of the environment. The principal hippocampal output region for spatial information is area CA1 and the major target of CA1 is the subiculum. Thus, one possible role of the subiculum is to receive, process and transmit information regarding location to areas outside the hippocampus. Anatomical experiments in the rat have shown that the projections from CA1 to subiculum exist in a series of 'nested loops'. Thus, each part of CA1 does not connect with each part of the subiculum. In turn, the majority of subicular principal neurons at the end of these loops have only one axonal projection target. The identity of these target areas depends on the location of the projecting cell within the subiculum, that is, neurons in different loops (and different septo-temporal levels of the subiculum) project to different sites. Principal neurons in subiculum and CA1 are not highly interconnected, suggesting that spatial information within any one of the nested loops is not shared amongst the others. This anatomical arrangement suggests that spatial information from any one part of CA1 is sent only to the sub-population of subicular principal cells within the same loop. In turn, this sub-population of subicular cells projects information to only a subset of the total range of subicular target areas. Thus, these subicular targets receive spatial information from only a subset of those in CA1. One conclusion from this is that for subicular targets to receive spatial information from the whole environment, area CA] must have multiple spatial maps, one for each major projection (nested loop) to the subiculum. (c) 2006 Elsevier B.V. All rights reserved.