Irradiation induces regionally specific alterations in pro-inflammatory environments in rat brain

Purpose: Pro-inflammatory environments in the brain have been implicated in the onset and progression of neurological disorders. In the present study, we investigate the hypothesis that brain irradiation induces regionally specific alterations in cytokine gene and protein expression. Materials and methods: Four month old F344×BN rats received either whole brain irradiation with a single dose of 10 Gy γ-rays or sham-irradiation, and were maintained for 4, 8, and 24h following irradiation. The mRNA and protein expression levels of pro-inflammatory mediators were analysed by real-time reverse transcriptase-polymerase chain reaction (RT-PCR), enzyme-linked immunosorbent assay (ELISA), and immunofluorescence staining. To elucidate the molecular mechanisms of irradiation-induced brain inflammation, effects of irradiation on the DNA-binding activity of pro-inflammatory transcription factors were also examined. Results: A significant and marked up-regulation of mRNA and protein expression of pro-inflammatory mediators, including tumour necrosis factor-α (TNF-α), interleukin-1β (IL-1β), and monocyte chemoattractant protein-1 (MCP-1), was observed in hippocampal and cortical regions isolated from irradiated brain. Cytokine expression was regionally specific since TNF-α levels were significantly elevated in cortex compared to hippocampus (57 greater) and IL-1β levels were elevated in hippocampus compared to cortical samples (126 greater). Increases in cytokine levels also were observed after irradiation of mouse BV-2 microglial cells. A series of electrophoretic mobility shift assays (EMSA) demonstrated that irradiation significantly increased activation of activator protein-1 (AP-1), nuclear factor-κB (NF-κB), and cAMP response element-binding protein (CREB). Conclusion: The present study demonstrated that whole brain irradiation induces regionally specific pro-inflammatory environments through activation of AP-1, NF-κB, and CREB and overexpression of TNF-α, IL-1β, and MCP-1 in rat brain and may contribute to unique pathways for the radiation-induced impairments in tissue function.