Organization of CA1 projections to the subiculum: A PHA‐L analysis in the rat

The organization of CA1 projections to the rat subiculum was investigated with the anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHA-L). Discrete iontophoretic injections of PHA-L were placed into various transverse positions of the CA1 field at different septotemporal levels of the hippocampus. The distribution of CA1 projections was observed in dissected and extended hippocampal preparations. CA1 cells located proximally in the field, i.e., close to the CA2 field, gave rise to projections that terminated in the distal third of the subiculum, i.e., close to the presubiculum. CA1 cells located distally in the field, i.e., close to the subiculum, gave rise to projections that terminated proximally in the subiculum, i.e., just across the CA1/subiculum border. CA1 cells in the middle of the field projected to a midtransverse portion of the subiculum. The same general pattern of projections was observed at all septotemporal levels of the hippocampus. Varicose fibers from the CA1 neurons terminated among the basal dendrites of the subicular pyramidal cells, within the pyramidal cell layer, and in the deep portion of the molecular layer. In addition to the CA1 to subiculum projections, the discrete PHA-L injections provided the opportunity of examining the extent of local and associational connections within CA1. In general, associational connections in CA1 are far less extensive than in CA3. CA1 is not entirely without local connections, however. CA1 cells located close to the subicular border, for example, originated axons that first innervated the proximal subiculum and then reentered the CA1 field at the interface between stratum radiatum and stratum lacunosum-moleculare. In most of the experimental cases, there were collaterals located in stratum oriens of CA1 that branched from the fibers directed toward the subiculum. Thus, the basal dendrites of CA1 cells may receive associational inputs. The organization of the CA1 projections to the subiculum is discussed in relation to the organization of CA3 projections to CA1 and the differential output of transverse regions of the subiculum. The possibility is raised that information may be "channeled" through the hippocampal formation via the transverse organization of these connections and ultimately distributed to different recipients of hippocampal efferent projections.