Age-related synaptic changes in sensorimotor cortex of the Brown Norway X Fischer 344 rat

Investigations of age-related changes in synapse density have yielded contradictory conclusions. The goal of the present study was to determine whether there is a significant decline in the number of cortical synapses in old age. Therefore, brains from 10-, 15-, and 32-month-old Brown Norway X Fischer 344 rats were prepared for electron microscopy and synapses were counted in a stereotaxically-identified region of sensorimotor cortex. Within this cortical area, synapses were counted in layers 2 and 4 and the data have been presented both as number of synapses per volume of neuropil and as the ratio of synapses per neuron. Results indicated that there was a decline in synapse density between 15 and 32 months in layer 2, but not in layer 4. This decline was significant not only for total synapses, but also for subcategories of synapses when classified by presynaptic features or postsynaptic element. Specifically, there was a significant decline in presumptive inhibitory synaptic terminals, i.e., those containing nonround synaptic vesicles, as well as a significant decline in synapses that contact dendritic spines.