Structural synaptic plasticity associated with the induction of long‐term potentiation is preserved in the dentate gyrus of aged rats

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Abstract
Changes in synaptic numbers were examined in the hippocampal dentate gyrus of aged (28 months old) rats following the induction of long‐term potentiation (LTP) by high‐frequency stimulation of the medial perforant path carried out on each of 4 consecutive days. Potentiated animals were sacrificed 1 hour after the fourth stimulation. Stimulated but not potentiated and implanted but not stimulated rats of the same chronological age served as controls. Synapses were analyzed in the middle (MML) and inner (IML) molecular layer of the dentate gyrus. Using the stereological dissector technique, unbiased estimates of the number per neuron were obtained for the following morphological varieties of synapses: axodendritic synaptic junctions involving dendritic shafts, nonperforated axospinous synapses having a continuous postsynaptic density (PSD), and perforated ones distinguished by a fenestrated, horseshoe‐shaped, or segmented PSD. The induction of LTP resulted in a selective increase in the number of synapses with segmented PSDs. This change was detected only in the potentiated synaptic field (MML), but not in an immediately adjacent one (IML), which was not directly stimulated during the induction of LTP. Comparison of these data with the results of our previous LTP study in young adult rats (Geinisman, Y. et al., 1991, Brain Res. 566:77–88) showed that the only significant difference in the absolute number of synaptic contacts per neuron between potentiated animals of the two chronological ages was an age‐related reduction in segmented synapses of the MML. Relative increases in the number of segmented synapses per neuron were, however, virtually of the same magnitude in potentiated rats of both ages as compared with their respective controls. This finding may explain why senescent rats can be potentiated to the same extent as young ones.