Distribution and origin of vesicular glutamate transporter 2-immunoreactive fibers in the rat hippocampus.

This study examined the distribution of vesicular glutamate transporter 2 (VGLUT2)-immunoreactive neuronal structures in the ipsilateral and contralateral hippocampi of unilateral fimbria/fornix transected, unilateral entorhinal cortex ablated, and intact female and male rats. In the hippocampi of intact animals, the highest density of VGLUT2-positive boutons was observed in the supragranular layer of the dentate gyrus, followed by the CA2 pyramidal and oriens layers, and the stratum lacunosum-moleculare of the CA1 field. This staining pattern was identical both in males and in females. Electron microscopic examination revealed that the immunolabeling was confined to axon terminals forming exclusively asymmetric synaptic contacts. The quantitative analysis of the synaptic targets of VGLUT2-positive terminals showed that in the dentate gyrus, 59% of the synaptic targets were dendritic spines, followed by dendritic shafts (22%) and granule cell somata (19%). In the pyramidal layer of the CA2 field, VGLUT2-immunoreactive boutons contacted mostly dendritic shafts (85%), only some of which (15%) synapsed with spines. The synaptic targets of VGLUT2-positive varicosities were dendritic spines (71%) and shafts (29%) in the stratum lacunosum-moleculare of the CA1 field. The fimbria/fornix transection caused a significant reduction in the density of VGLUT2-positive boutons only in the CA2 field, while entorhinal cortex ablation elicited no change in fiber density in any of the areas analyzed. Furthermore, our latest experiments on colchicine-treated animals revealed a large population of VGLUT2-positive neurons in the hippocampus that may be a possible intrinsic source of hippocampal VGLUT2 boutons. Our results suggest that the most likely sources of VGLUT2-positive boutons in the dentate supragranular layer, the CA2 area, as well as in the stratum lacunosum-moleculare of the CA1 field, might be the mossy cells, the supramammillary area, and the nucleus reuniens thalami, respectively.