Immunocytochemical localization of GABA-, cholecystokinin-, vasoactive intestinal polypeptide-, and somatostatin-like immunoreactivity in the area dentata and hippocampus of the rat

Hippocampal neurons containing GABA‐, cholecystokinin(CCK)‐, vasoactive intestinal polypeptide(VIP)‐, or somatostatin(SS)‐like immunoreactivity (LI) were localized in sections of rat hippocampus. GABA‐, CCK‐, VIP, and SS‐LI are found exclusively in interneurons of the area dentata and hippocampus. In the area dentata, GABA‐LI occurs in cells of all strata but predominates in type 1 and 2 basket cells. CCK‐LI is present in a subset of these basket cells and some hilar cells. VIP‐LI is present in a distinct subset of dentate interneurons that, unlike the type 1 and 2 basket cells, do not contribute to the fiber plexus in the inner molecular layer. These VIP‐LI interneurons send their axons to nearby granule cells and form a plexus in the hilus. SS‐LI, although rare in cells of the molecular and granular layers, is present in a large population of hilar interneurons that do not exhibit GABA‐, CCK‐, or VIP‐LI. In area CA3 of the hippocampus, a variety of morphologically diverse interneurons containing GABA‐, CCK‐, VIP‐, or SS‐LI are present in all strata. In area CA1, SS‐LI is present mainly in cells of strata oriens and pyramidale. GABA‐CCK‐ and VIP‐LI interneurons are present in all strata of CA1 but, unlike the SS‐LI cells, are most numerous in strata pyramidale and radiatum. These findings in the area dentata, taken together with those of Kosaka et al. (J. Comp. Neurol. 239:967–969, '85), indicate that two main populations of interneurons can be discriminated on the basis of the substances they contain. One is a group of GABA‐LI cells, some of which also contain CCK‐ and/or VIP‐LI. These cells innervate the granule cells and the second group of interneurons, the SS‐LI hilar cells, which apparently form part of the dentate ipsilateral associational/commissural projections.