X-irradiation inhibited cell proliferation and increased cell death in the dentate gyrus of the hippocampus, and decreased numbers of newly-formed neurons. (A) Mice were X-irradiated at 4 weeks of age, were injected with BrdU (50 mg/kg body weight) one day later, and were then sacrificed after 2 hours. Cell proliferation and cell death were examined in the dentate gyrus of hippocampus. To estimate levels of neurogenesis, (B) we administered BrdU (50 mg/kg body weight, three times a day) from the day after irradiation for three consecutive days. 4 weeks after the last injection, mice were sacrificed and processed for histological analysis. To analyse the effects of stress upon neurogenesis, (C) mice were injected with BrdU at 8 weeks of age after 5 consecutive days of footshock stress, and were then sacrificed at 12 weeks of age. (D) To examine the synergistic effects of stress and X-irradiation, mice were irradiated at 4 weeks of age and treated using the same procedure as described in (C). (E) Strong inhibition of cell proliferation was observed in the 10 Gy-irradiated group compared to the 0 Gy group. (F) Cell death was estimated using the TUNEL method. 10 Gy irradiation caused significant cell death n comparison to 0 Gy. (G) Using immunohistochemistry with anti-BrdU and anti-NeuN antibodies, a significant decrease in the number of newly-formed neurons was observed following 10 Gy irradiation. (H) No inhibitory effects were observed as a result of the stress induction technique used in this study, and nor was any synergistic effect irradiation and stress found. All data were expressed as mean ± S.D., n = 4–6.