A morphological analysis of vasoactive intestinal polypeptide (VIP)‐like immunoreactive neurons in the area dentata of the rat brain

Abstract

The general distribution and morphology of vasoactive intestinal polypeptide‐like immunoreactive (VIP‐LI) neurons in the area dentata (AD) have been studied using immunohistochemical methods on thick (70–120 μm) sections from the rat brain. The long (≥ 72 hours) incubation of free‐floating sections in combination with a sensitive immunoperoxidase method resulted in extensive labeling of VIP‐positive cell bodies, along with their dendritic and axonal processes in the hippocampal formation, including the area dentata. The VIP‐positive cells are found in all parts of the area dentata, and plots of the distribution of individual cells showed that there is a preferential clustering of these neurons within, or in close proximity to, the granule cell layer, while the deep zones in the hilus are almost completely devoid of VIP‐LI immunoreactive neurons. Throughout the long axis of the AD the number and relative position of the VIP‐LI neurons remains rather constant. Analysis of the size and shape of the VIP‐positive neurons in the AD shows a remarkable morphological heterogeneity, with the soma ranging from small (long axis: ˜ 10 μm) ovoid or round, to large (long axis: ˜ 25 μm) stellate or multipolar; of these the small to medium sized cells predominate. Comparisons between the morphology of neurons visualized with the VIP antibody and sections stained by the rapid Golgi method allow a rather conclusive classification of some of the VIP‐stained neurons, while the classification of a majority of the VIP nerons remains tentative, perhaps because of the selective visualization by the rapid Golgi method of only certain cell types in the AD. On the basis of this comparison, several displaced granule cells in the molecular layer, as well as granule cells, small ovoid cells, and pyramidal basket cells in the granule cell layer have been identified as VIP‐positive as determined by their somatic shape, dendritic branching pattern, and axonal projections. In addition, several VIP‐positive neurons have been identified with less certainty. These include the dentate basket cell of Cajal, certain small cells in the molecular layer, and fusiform cells in the polymorph layer. Taken together the present results demonstrate the usefulness of “Golgi‐like” immunoperoxidase staining for detailed classification of chemically defined neurons. Furthermore, these studies provide evidence for a morphological heterogeneity of VIP cells in the area dentata, and suggest that although some of the VIP cells may project out of the AD (the granule cells) a majority of the VIP neurons are interneurons that lack extrinsic projections.