Electron microscopic study of mossy fiber endings of the hippocampal formation in El mice.

The effects of seizure activity on the mossy fiber endings of El mice were studied by electron microscopy. During epileptic seizures of El mice, the number of clear round vesicles (50 nm) in the mossy fiber endings of the hippocampal formation decreased, while the number of large densecore vesicles (100 nm) increased. In these endings, the large dense-core vesicles were scattered during the resting state, but after seizure activity they tended to accumulate together and attach to the presynaptic membrane. Omega-shaped profiles, which seemed to be due to exocytosis of the large dense-core vesicles, were seen in the presynaptic membrane.