Irradiation of the head reduces adult hippocampal neurogenesis and impairs spatial memory, but leaves overall health intact in rats

Treatment of brain cancer, glioma, can cause cognitive impairment as a side‐effect, possibly because it disrupts the integrity of the hippocampus, a structure vital for normal memory. Radiotherapy is commonly used to treat glioma, but the effects of irradiation on the brain are still poorly understood, and other biological effects have not been extensively studied. Here we exposed healthy adult male rats to small and moderate‐dose irradiation of the head. We found no effect of irradiation on systemic inflammation, weight gain or gut microbiota diversity, although it increased the abundance of Bacteroidaceae family, namely Bacteroides genus in the gut microbiota. Irradiation had no effect on long‐term potentiation in the CA3‐CA1 synapse or endogenous hippocampal electrophysiology, but it did reduce adult hippocampal neurogenesis and impaired short‐term spatial recognition memory. However, no overall cognitive impairment was observed. To summarize, our results suggest that in adult male rats head irradiation does not compromize health or cognition overall even though the number of new, adult‐born hippocampal neurons is decreased. Thus, the sole effects of head irradiation on the body, brain and cognition might be less harmful than previously thought, and the cognitive decline experienced by cancer patients might originate from physiological and mental effects of the disease itself. Therefore, and to increase the translational value of animal studies, the effects of irradiation should be studied together with cancer, in older animals, using varying irradiation protocols and doses.