Electron microscopic study of mossy fiber endings in the hippocampal formation of rats after picrotoxin administration.

The hippocampal mossy fiber endings of rats were examined with electron microscopy, following the administration of picrotoxin. In the control group, many small round clear vesicles (40-60 nm in diameter) filled the nerve endings. Further, it was noted that large dense-core vesicles (80-100 nm in diameter) in small numbers were scattered among these small clear vesicles. After administration of picrotoxin, the large dense-core vesicles increased in number and accumulated next to the presynaptic membrane. These vesicles were often fused to the presynaptic membrane, in the form of omega-shaped profiles.