Different reactions to irradiation in the juvenile and adult hippocampus

Abstract

Purpose: Cranial radiotherapy is an important tool in the cure of primary brain tumors. Unfortunately, it is associated with late-appearing toxicity to the normal brain tissue, including cognitive impairment, particularly in children. The underlying mechanisms are not fully understood but involve changes in hippocampal neurogenesis. Recent studies report essentially different responses in the juvenile and the adult brain after irradiation, but this has never been verified in a comparative study.

Materials and methods: We subjected juvenile (9-day-old) and adult (6-month-old) male rats to a single dose of 6 Gray (Gy) whole brain irradiation and euthanized them 6 hours, 7 days or 4 weeks later. Hippocampal lysates were analyzed for caspase-3 activity (apoptosis) and the expression of cytokines, chemokines and growth factors. Four weeks after irradiation, the number of microglia (expressing ionized calcium-binding adapter molecule 1, Iba-1), activated microglia (expressing cluster of differentiation 68 [CD68]), bromodeoxyuridine (BrdU) incorporation and granule cell layer (GCL) volume were assessed.

Results: The major findings were (i) higher baseline BrdU incorporation (cell proliferation) in juvenile than in adult controls, which explains the increased susceptibility to irradiation and higher level of acute cell death (caspase activity) in juvenile rats, leading to impaired growth and subsequently a smaller dentate gyrus volume 4 weeks after irradiation, (ii) more activated (CD68-positive) microglia in adult compared to juvenile rats, regardless of irradiation, and (iii) differently expressed cytokines and chemokines after cranial irradiation in the juvenile compared to the adult rat hippocampus, indicating a more pro-inflammatory response in adult brains.

Conclusion: We found essentially diverse irradiation reactions in the juvenile compared to the adult hippocampus, indicating different mechanisms involved in degeneration and regeneration after injury. Strategies to ameliorate the cognitive deficits after cranial radiotherapy should therefore likely be adapted to the developmental level of the brain.

Keywords::

• Cranial radiation therapy
• irradiation
• BrdU
• active caspase-3
• Iba-1
• microglia
• cytokines
• chemokines
• growth factors
• neurogenesis

Acknowledgements

We are grateful for the skillful assistance of Hayde Bolouri for expertise on cytokine assay and statistician Kjell Pettersson for statistical advice.

Declaration of interest

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.

This work was supported by the Swedish Childhood Cancer Foundation (Barncancerfonden), the Swedish Research Council (Vetenskapsrådet), the Swedish Cancer Foundation (Cancerfonden), governmental grants from Agreement concerning research and education of doctors (ALF), Jubileumskliniken's anti-cancer research fund, the Sahlgrenska Academy at the University of Gothenburg, the Sten A. Olsson's Foundation, the Frimurare Barnhus Foundation, the Wilhelm and Martina Lundgren Foundation, the Gothenburg Medical Society, the Aina Wallström's and Mary-Ann Sjöblom's Foundation, the Ulla and Rune Amlöv Foundations, AFA Insurance and the Swedish Society of Medicine.