Heterogeneity in the Dorsal Subiculum of the Rat. Distinct Neuronal Zones Project to Different Cortical and Subcortical Targets

The aim of the present study was to relate the distribution of efferents of the dorsal subiculum to their origin along the proximo‐distal axis of the subiculum. The distribution of subicular projections was studied in detail by means of the sensitive anterograde tracer Phaseolus vulgaris‐leucoagglutinin (PHA‐L), and the precise origin of these projections analysed with retrogradely transported fluorescent tracers, using double‐ and triple‐labelling protocols. Injections of PHA‐L in the proximal part of the dorsal subiculum, i.e. that part which borders field CA1, result in labelling of the infralimbic, entorhinal and perirhinal cortices, the nucleus accumbens and the lateral septa) region, the interanteromedial nucleus of the thalamus, the core of the nucleus gelatinosus, and the mammillary nuclei, in particular in the rostral parts of the medial nucleus. In contrast, injections in the distal part of the dorsal subiculum, i.e. that part which borders the presubiculum, give rise to labelling in the retrosplenial and postrhinal cortices, the presubiculum, the anterior thalamic complex, the shell of the nucleus gelatinosus, and the mammillary nuclei, preferentially in the caudal part of the medial nucleus. The results of injections of different retrograde tracers, simultaneously placed in two or three targets of the subicular efferents, confirm the results of the anterograde tracing experiments. Moreover, they clearly demonstrate that the population of subicular neurons which, for example, projects to the nucleus accumbens and the interanteromedial nucleus of the thalamus is almost completely segregated from the population that projects to the retrosplenial cortex and the anterior complex of the thalamus. Thus within the dorsal subiculum, populations of neurons can be differentiated so that each population projects to a unique set of target structures. These cell populations are differentially positioned along the proximo‐distal axis. In view of additional evidence indicating that some of the major afferents to the subiculum are organized along the same axis, we suggest that the heterogeneity of the dorsal subiculum along the proximo‐distal axis reflects a general organizational characteristic of this hippocampal field.