Whole Brain Radiation-Induced Impairments in Learning and Memory Are Time-Sensitive and Reversible by Systemic Hypoxia

Whole brain radiation therapy (WBRT) is commonly used for treatment of primary and metastatic brain tumors; however, cognitive impairment occurs in 40–50 % of brain tumor survivors. The etiology of the cognitive impairment following WBRT remains elusive. We recently reported that radiation-induced cerebrovascular rarefaction within hippocampal subregions could be completely reversed by systemic hypoxia. However, the effects of this intervention on learning and memory have not been reported. In this study, we assessed the time-course for WBRT-induced impairments in contextual and spatial learning and the capacity of systemic hypoxia to reverse WBRT-induced deficits in spatial memory. A clinical fractionated series of 4.5Gy WBRT was administered to mice twice weekly for 4 weeks, and after various periods of recovery, behavioral analyses were performed. To study the effects of systemic hypoxia, mice were subjected to 11 % (hypoxia) or 21 % oxygen (normoxia) for 28 days, initiated 1 month after the completion of WBRT. Our results indicate that WBRT induces a transient deficit in contextual learning, disruption of working memory, and progressive impairment of spatial learning. Additionally, systemic hypoxia completely reversed WBRT-induced impairments in learning and these behavioral effects as well as increased vessel density persisted for at least 2 months following hypoxia treatment. Our results provide critical support for the hypothesis that cerebrovascular rarefaction is a key component of cognitive impairment post-WBRT and indicate tha

WBRT-induced learning deficits in spatial learning are evident at 2 months.

<p>(A) Mice were subjected to fractionated WBRT and allowed to recover for 2 months, after which animals were tested in the Barnes maze. Radiated mice showed deficits in learning acquisition as measured by (B) primary errors and (C) primary latency. Performance on the Barnes maze represented as primary errors (D) and primary latency (E) over trials. No differences in memory retention (Day 10 Probe trial) (F). Data Represent Mean ± SEM. N = 9 per group. **p<0.0031; ***p<0.0001 compared to controls.</p

Changes in hippocampal vascular density after re-exposure to ambient oxygen levels.

<p>(A) Representative images of vascular density in the mouse hippocampus. Images were captured using fluorescence microscope at 10× magnification. Scale bar represents 100 µm. (B) Quantification of vascular density (length of vessels per area of tissue). Data represent Mean ± SEM. *p<0.05 vs. Control Normoxic and ^##p<0.01 vs. Radiated Normoxic. N = 5–6 animals per group/treatment.</p

Working memory deficits are evident in radiated mice 4 months post-WBRT.

<p>(A) Schematic of the experimental design used. Impairments in learning occur in the radiated group as measured by: (B) primary errors and (C) primary latency over blocks of trials. Performance during individual trials is shown in (D) as primary errors and (E) as primary latency. Data represent Mean ± SEM. **p<0.00625 vs. Control. N = 9 animals per group.</p

Radiated animals are impaired in both learning and memory retention at 5 months post-treatment.

<p>A schematic of the experimental design is shown in (A). Impairments in acquisition of the task, indicated by increased primary errors (B) and primary latencies (C) are observed in the radiated group. Representation of primary errors (D) and primary latency (E) over individual trials. (F) Long-term learning retention is impaired in the radiated group. Data represent Mean ± SEM. *p<0.0031; ^#p<0.006 vs. controls. N = 8–10 animals per group.</p

Transient cognitive deficits in contextual learning.

<p>(A) Schematic of the experimental design used. (B) At 1 month post treatment, performance of radiated animals was significantly impaired compared to controls at 45 min and 24 h following training. (C) Recovery of learning and memory function was evident at 3 months post-WBRT. Data represent Mean± SEM. *p<0.0167 compared to controls.</p

Benefits of systemic hypoxia are maintained after re-exposure to normal air.

<p>A schematic of the experimental design is shown in (A). Impairments in learning, indicated by increased primary errors (B) and primary latencies (C) are observed in the radiated normoxic group. Animals that were radiated and exposed to systemic hypoxia perform similar to the controls. Data represent Mean ± SEM. ***p<0.0083 vs. Control Normoxic. N = 8–9 animals per group/treatment.</p

Summary of the animals and behavioral tasks used for this study.

<p>Mice were behaviorally tested using the active avoidance task and Barnes maze at 1, 2, 3, 4, and 5 months post-WBRT.</p