Approximately 200,000 patients in the United States are treated each year with partial- or whole-brain irradiation (WBI) 
, and half or more that survive ≥six months develop neural dysfunction due to radiation-induced injury of normal brain tissue 
. Among adults, older patients appear to be at greater risk for radiation-induced neural dysfunction 
. It is not clear why WBI affects cognitive function or why aging may impact that neural response. Since all neural cell types probably are involved in the radiation response 
, it is critical to consider diverse cellular and molecular mechanisms. Studies to date indicate that changes in cell turnover and activation of neuroinflammatory pathways contribute significantly to neural deficits following WBI. These also may be processes where effects of normal aging and of WBI intersect 
WBI kills dividing cells and changes the proliferative potential of surviving progenitor cells. Reduced neurogenesis has been associated with cognitive deficits following WBI in young adult rodents 
and, based largely on those studies, it has been proposed that conforming clinical brain irradiation to avoid neurogenic regions in patients may diminish or prevent cognitive deficits 
. Significantly, however, there is no sustained effect of radiation on neurogenesis in older animals 
. Moreover, WBI clearly alters functions of brain regions in which there is no adult neurogenesis 
. Thus, changes in neurogenesis likely contribute to neural deficits following WBI in the developing and young adult brain, but other mechanisms must contribute and may play a large role in older individuals.
In addition to reducing neurogenesis, brain irradiation can kill oligodendrocytes and alter glial turnover 
. Changes in white matter are the most common finding in imaging studies of patients suffering from WBI-induced cognitive changes, but cognitive dysfunction also may occur without evidence of white matter changes 
. Most cycling cells in the normal adult brain produce oligodendrocytes or cells with the potential to become oligodendrocytes, and oligodendrocyte precursor cells (OPC) are found in both white- and gray matter 
. Thus, radiation-induced death of oligodendrocytes and changes in OPCs could contribute to widespread deficits in neural signaling as both the maintenance and integrity of myelin and supporting functions of oligodendrocytes in gray matter become compromised 
. How oligodendrocyte turnover changes during normal aging is not clear, however, and it is not known whether and how the vulnerability of oligodendrocytes and OPCs to radiation and their contribution to radiation-induced neural dysfunction may be altered in older individuals.
Among neural cells that contribute to neuroinflammation, microglia appear to be central to the development of radiation-induced injury 
. Changes in microglia have been demonstrated repeatedly in the dentate gyrus (DG) of young adult animals examined at 1 week to 18 months after irradiation 
. Since most previous analysis of the microglial response has focused on the DG in developing and young adult animals, little is known about the neuroinflammatory response in regions outside of the DG, which lack neurogenesis and may respond differently to challenges. Similarly, little is known about the neuroinflammatory response to irradiation in the latter part of the lifespan (corresponding to the primary clinical population), despite evidence that both basal levels of neuroinflammatory markers and vulnerability to other pro-inflammatory stimuli clearly vary with age 
. Overall, it is likely that radiation-induced microglial responses and their sequelae may differ both regionally and between younger and older adults.
We previously examined the effects of radiation on neurogenesis, microglial density, and microglial expression of the rat homolog of CD68 (by immunolabeling with the ED1 antibody) in the subgranular zone (SGZ) of the DG 
. This analysis demonstrated that in middle-aged and old animals, in contrast to young adults, WBI did not reduce neurogenesis below the level seen in age-matched control animals. A WBI-induced increase in proliferation of non-neurogenic cells was evident, particularly in older rats, but it was not clear what cell type(s) were involved.
To better understand the radiation-induced normal tissue response, we now have assessed the responses of non-neurogenic regions of gray matter and of white matter to a single 10 Gy dose of WBI. Previous experimental studies of mice 
and rats 
demonstrated that a single dose of 10 Gy WBI produced cognitive deficits, supporting analysis of the response to single-dose WBI in rodents to assess neurobiological mechanisms of normal tissue injury. The analysis of gray matter in the present study compared responses of the CA1 and CA3 subfields of the hippocampus, since the two regions may be differentially susceptible to some aging-related and experimentally-induced changes 
. The analysis of white matter assessed the corpus callosum (CC) as a representative region. In addition to assessing microglial density and ED1 labeling, we examined microglial morphology and quantified proliferation as additional indicators of microglial reactivity. We also analyzed effects of irradiation on the proliferation of non-neurogenic, non-microglial cells, which were identified as OPCs. The cellular markers were examined in rats irradiated at one of three adult ages and in age-matched, sham irradiated controls, in order to evaluate changes across the adult life span and assess aging-related changes in the response to WBI.
These analyses allowed us to test working hypotheses that i) non-neurogenic mechanisms, including but not limited to effects on other proliferating cells, can contribute to WBI-induced normal tissue damage in adults, and ii) that the nature and/or extent of these mechanisms differ among neural regions and are influenced by normal aging processes. The resultant evidence that gray- and white matter exhibit differential responses to aging and to WBI and that CA3 exhibits greater aging-related, and possibly WBI-induced, changes than CA1 indicate that future investigations of the mechanisms and treatment of radiation-induced normal tissue injury should include explicit assessment of region- and age-specific regulation of affected cell populations.