We examined the effects of focal X-irradiation of the hippocampus in infancy on the partial reinforcement extinction effect (PREE) in weanling rats in a runway and on running wheel activity at 40 days of age. Our results show a dose-dependent X-irradiation-related reduction in granule cell neurogenesis. Weanling rats showed a corresponding increase in running speed in both acquisition and extinction and a dose-dependent reduction in the PREE--an increase in persistence after CRF and a decrease in persistence after PRF training. The same degree of hippocampal granule cell agenesis had no effect on running wheel activity. These results suggest that the enhanced speeds in the runway are incentive-related and do not reflect simple hyperactivity but rather hyperreactivity. The discussion of these results is in part a speculation regarding their possible relation to some explanations of attention deficit-hyperactivity disorder in children.
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