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 exhibit ed a time-dependent, twenty- to thirty-fold increase in their frequenc y at the 5-7 h post-training time. The hyperchromatic cells formed a r ostral-caudal ribbon, 250 mu m in diameter and 60 mu m in depth, in se ctions obtained from -2.6 to -4.5 mm with respect to bregma. This was not observed in passive animals or yoked controls. Ultrastructural ana lysis revealed their cytoplasm and dendrites to be enriched in ribosom es and microtubules, respectively. Dendrites associated with the hyper chromatic cells exhibited a two-fold increase in spine number as compa red 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 neuroplas tic change within this discrete post-training period of memory consoli dation. Copyright (C) 1996 IBRO.