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Mossy cell axonal projections to the dentate gyrus molecular layer in the rat hippocampal slice

The ipsilateral associational pathway connects different septotemporal levels of the dentate gyrus. Neurons of the dentate hilus project hundreds of micrometers from the cells of origin to the inner molecular layer. The authors hypothesized that mossy cells, the major cell type of the hilus, also project locally to the inner molecular layer. Within a 400 μm slice, mossy cells were (1) recorded intracellularly while the inner molecular layer was stimulated to test for antidromic responses, and (2) labeled with biocytin and examined with light and electron microscopy for axonal projections into the molecular layer. The authors found that mossy cells can be antidromically activated by inner molecular layer stimulation and that axonal projections to the molecular layer can be visualized within a 400 μm hippocampal slice. In 13 of 19 intracellularly labeled and electrophysiologically characterized mossy cells, collaterals could be traced into the molecular layer. These results suggest that mossy cells contribute to the ipsilateral associational pathway and also participate in local recurrent circuitry to influence granule cell activity.