Autoradiographic demonstration of biochemical changes in the limbic system during avoidance training.

After injection of leucine-H(3), members from eight triplets of rats were assigned to one of three treatment groups: avoidance learning, the same handling and shocks without opportunity to learn, or passive control. Autoradiographic grains over all nuclei were counted "blind." In the hippocampus, the number of grains for the learning rats was reliably greater than that for either control. The differences approached statistical significance in the entorhinal cortex and the septal area but not in other brain areas or in the liver. Although the processes responsible for the increased incorporation are not yet defined, these changes probably are the result of learning rather than of stress.