Different polysialic acid-neural cell adhesion molecule expression patterns in distinct types of mossy fiber boutons in the adult hippocampus.

Dentate granule cells continue to be generated in the adult hippocampus, and the newly generated granule cells express the highly polysialylated neural cell adhesion molecule (PSA-NCAM), which has been shown to be important in neural development and plasticity. In the present study, the PSA-NCAM expression pattern in morphologically distinct types of mossy fiber boutons in adult rats was examined by immunoelectron and confocal laser scanning microscopy. Although many unmyelinated axons within the mossy fiber bundles expressed PSA-NCAM, most of the mature type of mossy fiber boutons were negative for polysialic acid (PSA) but positive for NCAM peptides, suggesting that NCAM is less polysialylated in the mature mossy fiber boutons. On the other hand, PSA was expressed by small round varicosities, irregularly shaped boutons, and the presumptive immature type of mossy fiber boutons. The PSA-positive small boutons were found to make synaptic contacts with CA3 pyramidal cells and nonpyramidal cells. The PSA-expressing presumptive immature boutons contained fewer clear synaptic vesicles and mitochondria, and, in some instances, they were invaginated by the PSA-positive, finger-like dendritic outgrowths arising from the dendritic shafts of the pyramidal cells, which are known to develop into a mossy fiber bouton-thorny excrescence complex. These findings indicate that distinct types of the mossy fiber boutons possess different PSA expression patterns in the adult hippocampus, and they also imply that PSA expression allows the mossy fibers to have the ability to regulate the bouton formation and remodeling that accompany synapse formation at the contact sites with pyramidal cells and nonpyramidal cells.