Quantitative Synaptic Changes With Differential Experience In Rat Brain

Significant differences in size and number of synaptic junctions were found between littcrmate rats assigned at weaning (25 days of age) to enriched or impoverished environments and kept there for 30 days. The synapses measured were asymmetrical axodendritic junctions in the neuropil of layer III of the occipital cortex. Rats given experience in the enriched condition (EC) showed, in comparison to littermates in the impoverished condition (1C). synapses that averaged 52% greater in length but that were only 67% as numerous. The EC rats had more large synapses as well as fewer small synapses than did IC rats, so the EC size distribution could not have been derived simply by loss of small synapses from the IC distribution. The total area of synapses in the EC group, taking both size and number of contacts into account, was 40% greater than in IC. Thickness of cortex was 4.0% greater in EC than in IC, a value that compares closely with the 4.6% found in several previous 25-to-55 day EC-1C experiments. Problems of measurement and sampling in electron microscopy are considered. The results are discussed in relation to concepts of brain mechanisms of learning and memory storage, some aspects of which can now be studied directly.