gms | German Medical Science

Objective: to assess whether fractionation affects hippocampal neurogenesis decline elicited by cranial irradiation.

Methods: 2-months’ old male C57BL mice (n=40) received a whole brain irradiation – total doses of 5 or 10 Gy, given either as a single dose or in 5 daily fractions. 48 h later, the brains were collected and the numbers of proliferating cells and immature neurons in the dentate gyrus were assessed using immunohistochemistry for Ki67 or doublecortin (DCx), respectively. Next, to determine the effects of irradiation on the survival and fate of newly born cells 3 months after irradiation, the animals received 7 daily injections of BrdU and 3 weeks later the brains were collected, immunostained and analyzed using confocal microscopy as described before (1) in order to assess the numbers of newly born (BrdU+) cells along with their phenotypes. 1. S. Mizumatsu, et al. Cancer Res 63, 4021-4027 (2003).

Results: 48 hr after irradiation with single doses of 5 or 10 Gy, Ki-67+ proliferating cells were decreased by 94% and 97%, respectively. Similarly, DCx+ cells were reduced by 78.9% and 79.6%. After fractionated irradiation, proliferating cells were reduced by 76% after 5 fractions of 1 Gy and by 86% after 5 fractions of 2 Gy while immature neurons were reduced by 76% and 85%, respectively. There was no significant interaction between dose and fractionation for both endpoints. Three months later, the total number of newly born cells (BrdU+) decreased by about 80% after a single dose of 10 Gy. However, when the same total dose was given in 5 fractions, the total number of newly born cells was not different from unirradiated mice. In terms of newly born neurons (NeuN+/BrdU+), a single dose of 10 Gy, reduced total numbers of cells analyzed by about 86% while a fractionated irradiation had no significant effect. Numbers of newly born astrocytes (GFAP+/BrdU+), after a single dose of 10 Gy were reduced by about 86%. Conversely, fractionated irradiation had virtually no effect when compared to unirradiated controls. The effects of both single dose as well as fractionated irradiation on newly born oligodendrocytes populations were not significant. Total numbers of newly born microglia (BrdU+/CD68+) appeared to increase after a single and fractionated dose of 10 Gy but these differences were not statistically significant.

Conclusions: In comparison to a single dose of radiation, fractionated irradiation spares the long-term effects on neurogenesis.