Abstract.-After injection of leucine-H3, 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.The present study was designed to test the practicability of using autoradiography to identify neural structures showing biochemical changes following training on a specific behavioral task.Recent studies have provided some basis for the hypothesis that quantitative changes in RNA and protein metabolism occur with learning. Hyd6n and Lange,' using microdissection and microchemical analysis, found increased leucine-H3 incorporation into hippocampal pyramidal cells in rats trained to reach for food with their initially unfavored paw. Gaito et al. and Zemp et al.2 found increased incorporation of labeled lysine or uridine in homogenates of brain tissue from rats and mice trained in one-way avoidance tasks.Because the last two studies were based on the analysis of homogenates of relatively large blocks of brain tissue, particular brain structures responsible for the observed chemical changes were not determined. Since available microtechniques are difficult to apply to many species of brain cells, we chose autoradiography as a more feasible method for localizing changes within the brain.Seven structures were selected for study. The hippocampus and septal areas were chosen because numerous behavioral studies have implicated them in the learning or performance of avoidance tasks.3 The entorhinal cortex, closely related to the hippocampus both anatomically and electrophysiologically,4 was also studied. Two other areas, the lateral geniculate nucleus and the superior colliculus of the visual system, were chosen for study on the assumption that they would be less likely to be altered during learning. The liver was included as a nonneural control tissue.Materials and Methods.-The subjects were 33 experimentally naive, male SpragueDawley rats from the University of Oregon rat colony, born of rats purchased from Berkeley 692
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