Molecular Pathways: Radiation-Induced Cognitive Impairment

Radiation Oncology, Wake Forest School of Medicine.
Clinical Cancer Research (Impact Factor: 8.72). 02/2013; 19(9). DOI: 10.1158/1078-0432.CCR-11-2903
Source: PubMed

ABSTRACT Approximately 200,000/year in the US will receive partial or whole brain irradiation for the treatment of primary or metastatic brain cancer. Early and delayed radiation effects are transient and reversible with modern therapeutic standards; yet late radiation effects (≥6 months postirradiation) remain a significant risk, resulting in progressive cognitive impairment. These include functional deficits in memory, attention, and executive function that severely affect the patient's quality of life (QOL). The mechanisms underlying radiation-induced cognitive impairment remain ill defined. Classically, radiation-induced alterations in vascular and glial cell clonogenic populations were hypothesized to be responsible for radiation-induced brain injury. Recently, preclinical studies have focused on the hippocampus, one of two sites of adult neurogenesis within the brain, which plays an important role in learning and memory. Radiation ablates hippocampal neurogenesis, alters neuronal function, and induces neuroinflammation. Neuronal stem cells implanted into the hippocampus prevent the decrease in neurogenesis and improve cognition following irradiation. Clinically prescribed drugs, including PPAR α and γ agonists, as well as RAS blockers, prevent radiation-induced neuroinflammation and cognitive impairment independent of improved neurogenesis. Translating these exciting findings to the clinic offers the promise of improving the QOL of brain tumor patients who receive radiotherapy.

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    • "Numerous preclinical investigations have supported the sensitivity of the hippocampus to radiation-induced damage, given the fact that radiation decreases hippocampal neurogenesis and induces hippocampal-dependent cognitive deficits in rodents [2]. While patients report cognitive deficits associated with both hippocampus- and non-hippocampus-dependent cognitive domains, there is a lack of preclinical data examining the effects of radiation on behaviors mediated by brain regions other than the hippocampus. "
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    ABSTRACT: The present report describes an animal model for examining the effects of radiation on a range of neurocognitive functions in rodents that are similar to a number of basic human cognitive functions. Fourteen male Long-Evans rats were trained to perform an automated intra-dimensional set shifting task that consisted of their learning a basic discrimination between two stimulus shapes followed by more complex discrimination stages (e.g., a discrimination reversal, a compound discrimination, a compound reversal, a new shape discrimination, and an intra-dimensional stimulus discrimination reversal). One group of rats was exposed to head-only X-ray radiation (2.3 Gy at a dose rate of 1.9 Gy/min), while a second group received a sham-radiation exposure using the same anesthesia protocol. The irradiated group responded less, had elevated numbers of omitted trials, increased errors, and greater response latencies compared to the sham-irradiated control group. Additionally, social odor recognition memory was tested after radiation exposure by assessing the degree to which rats explored wooden beads impregnated with either their own odors or with the odors of novel, unfamiliar rats; however, no significant effects of radiation on social odor recognition memory were observed. These data suggest that rodent tasks assessing higher-level human cognitive domains are useful in examining the effects of radiation on the CNS, and may be applicable in approximating CNS risks from radiation exposure in clinical populations receiving whole brain irradiation.
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    • "The thiazolidinediones are peroxisome proliferatoractivated receptor-gamma (PPAR-) agonists, frequently used in the treatment of diabetes, a condition associated with insulin resistance and decreased neurogenesis [117]. PPARagonists also increase neurogenesis, affording protecton against LPS [118], irradiation [119] and insulin resistance induced decrements in neurogenesis. As to whether these effects are mediated via changes in astrocyte-NPSC interactions, altering fluxes of S1P, FGF1, FGF2, NAS and melatonin requires investigation. "
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    ABSTRACT: Despite the powerful induction of neurogenesis by sphingosine-1-phosphate (S1P), its study in the role of adult neurogenesis has been relatively neglected. S1P, via its differential effects at different S1P receptor subtypes, is a significant determinant of neuronal precursor/stem cell and astrocyte cellular organization. The variations in neurogenesis, classically modelled via the interactions of phosphatase and tensin homolog and Notch, are intimately associated with the co-ordinated regulation of S1P and fibroblast growth factor-1. Incorporating S1P better explains the plasticity and cellular variations in astrocytes and progenitors as well as their interactions. This has treatment implications for both inducing and inhibiting neurogenesis, in conditions such as depression and macrocephaly associated autistic spectrum disorders respectively. Incorporating S1P and fibroblast growth factor-1 also provides a framework for conceptualizing the impact of peripheral inflammation, central inflammation, redox status and medication effects on neurogenesis, as well as future treatment targets.
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