Transient, learning-induced ultrastructural change in spatially-clustered dentate granule cells of the adult rat hippocampus.

In semithin, Toluidine Blue-stained plastic coronal sections, we have observed hyperchromatic granule cells in the dorsal crest of the adult rat dentate gyrus following passive avoidance learning. These exhibited a time-dependent, twenty- to thirty-fold increase in their frequency at the 5-7 h post-training time. The hyperchromatic cells formed a rostral-caudal ribbon, 250 microns in diameter and 60 microns in depth, in sections obtained from -2.6 to -4.5 mm with respect to bregma. This was not observed in passive animals or yoked controls. Ultrastructural analysis revealed their cytoplasm and dendrites to be enriched in ribosomes and microtubules, respectively. Dendrites associated with the hyperchromatic cells exhibited a two-fold increase in spine number as compared to those of normochromatic cells in the same region of the dorsal mid-molecular layer. These changes are suggested to be associated with modulation of L1 and neural cell adhesion molecule-mediated neuroplastic change within this discrete post-training period of memory consolidation.