Compensatory loss of axosomatic synapses in the dentate gyrus of the senescent rat.

A portentous reorganization of the dentate gyrus occurs characteristically in senescent rats. This reorganization includes atrophy of dendrites, hypertrophy of astrocytes and a 27% loss of axodendritic synapses in the molecular layer of the dentate gyrus. A coincident loss of axosomatic synapses is now reported. These synapses on granule cell somata were counted in electron micrographs of representative coronal sections through the dentate gyri of five 3-month-old and five 25-month-old Fischer-344 male rats. A 15% decrease in the number of axosomatic synapses per 100 micron length of granule cell plasma membrane and a 22% decrease in the amount of neuronal surface covered by synapses were found in the senescent, as compared with the young adult, animals. These differences were statistically significant. As synapses on granule cell somata are inhibitory terminals of GABAergic interneurons it is suggested that their loss in senescence may be compensatory for the loss of axodendritic synapses, which are excitatory. By means of a compensatory loss of synapses it may be possible for the aging animal to maintain a reasonably adaptive level of function in spite of ongoing changes in the level of granule cell excitation resulting from the reorganization of the dentate gyrus in senescence.