Electron microscopic and morphometric studies on synaptic plasticity in the hippocampus of the rat following conditioning.

1. In 40 three months old Wistar rats (16 trained animals, 14 active controls, 10 passive controls) the influence of learning (brightness discrimination) on the number of electronmicroscopically detectable synapses per area neuropil was studied in the stratum radiatum (CA 1) of the hippocampus. 2. 70 minutes after completion of training, the mean number of synapses has significantly increased as compared to active controls (trained animals 14.26 +/- 2.95 synapses/25 micrometer2; active controls 10.23 +/- 2.80 synapses/25 micrometer2; p less than 0.001). 3. 24 hours after training this increase in trained animals still persisted (trained animals 14.15 +/- 2.60, active controls 11.39 +/- 2.87 synapses/25 micrometer2; p less tha 0.001). 4. 14 days after training, the number of synapses has decreased considerably, but was nevertheless significantly higher than in the active controls (trained animals 12.49 +/- 2.76, active controls 11.45 +/- 3.10; p less than 0.001). 5. The results are discussed with regard to their significance to the different stages of memory formation in the CNS.