Synaptic atrophy in the senescent hippocampus.

Quantitative analyses of electron micrographs have shown a decrease in the number of synapses in the dentate gyrus of the senescent Fischer-344 rat. The loss of synapses, involving both dendritic spines and shafts and axon terminals of more than one population of presynaptic neurons, did not depend upon the antecedent loss of postsynaptic neurons or their dendrites. These findings suggest that the age-related loss of synapses in the dentate gyrus may depend upon an inability of presynaptic elements to maintain the structural integrity of synapses in senescence. It is proposed that a change in the glycoprotein component of presynaptic plasma membranes resulting from a deficiency in axonal transport mechanisms in the septo-hippocampal pathway may underly this presynaptic malfunction. The resulting partial deafferentation of neurons in the dentate gyrus in senescence appears to be associated with a secondary atrophy of dendrites, which results in a loss of postsynaptic membranes before a loss of postsynaptic neurons can be documented.