Anna L. Smith

Dose and dose rate effects of whole-body proton-irradiation on lymphocyte blastogenesis and hematological variables: Part II

The goal of part II of this study was to evaluate functional characteristics of leukocytes and circulating blood cell parameters after whole-body proton irradiation at varying doses and at low- and high-dose-rates (LDR and HDR, respectively). C57BL/6 mice (n=51) were irradiated and euthanized at 4 days post-exposure for assay. Significant radiation dose- (but not dose-rate-) dependent decreases were observed in splenocyte responses to T and B cell mitogens when compared to sham-irradiated controls (P<0.001). Spontaneous blastogenesis, also significantly dose-dependent, was increased in both blood and spleen (P<0.001). Red blood cell counts, hemoglobin concentration, and hematocrit were decreased in a dose-dependent manner (P<0.05), whereas thrombocyte numbers were only slightly affected. Comparison of proton- and gamma-irradiated groups (both receiving 3 Gy at HDR) showed a higher level of spontaneous blastogenesis in blood leukocytes and a lower splenocyte response to concanavalin A following proton irradiation (P<0.05). There were no dose rate effects. Collectively, the data demonstrate that the measurements in blood and spleen were largely dependent upon the total dose of proton radiation and that an 80-fold difference in the dose rate was not a significant factor. A difference, however, was found between protons and gamma-rays in the degree of change induced in some of the measurements.

Effects of lipopolysaccharide on 56Fe-particle radiation-induced impairment of synaptic plasticity in the mouse hippocampus.

Abstract Vlkolinský, R., Krucker, T., Smith, A. L., Lamp, T. C., Nelson, G. A. and Obenaus, A. Effects of Lipopolysaccharide on 56Fe-Particle Radiation-Induced Impairment of Synaptic Plasticity in the Mouse Hippocampus. Radiat. Res. 168, 462–470 (2007). Space radiation, including high-mass, high-Z, high-energy particles (HZE; e.g. 56Fe), represents a significant health risk for astronauts, and the central nervous system (CNS) may be a vulnerable target. HZE-particle radiation may directly affect neuronal function, or during immunological challenge, it may alter immune system-to-CNS communication. To test these hypotheses, we exposed mice to accelerated iron particles (56Fe; 600 MeV/nucleon; 1, 2, 4 Gy; brain only) and 1 month later prepared hippocampal slices to measure the effects of radiation on neurotransmission and synaptic plasticity in CA1 neurons. In a model of immune system-to-CNS communication, these electrophysiological parameters were measured in irradiated mice additionally challenged with the peripheral immunological stressor lipopolysaccharide (LPS) injected intraperitoneally 4 h before the slice preparation. Exposure to 56Fe particles alone increased dendritic excitability and inhibited plasticity. In control mice (0 Gy), LPS treatment also inhibited synaptic plasticity. Paradoxically, in mice exposed to 2 Gy, the LPS treatment restored synaptic plasticity to levels similar to those found in controls (0 Gy, no LPS). Our results indicate that HZE-particle radiation alters normal electrophysiological properties of the CNS and the hippocampal response to LPS.

High-LET Radiation-Induced Response of Microvessels in the Hippocampus

Abstract The hippocampus is critical for learning and memory, and injury to this structure is associated with cognitive deficits. The response of the hippocampal microvessels after a relatively low dose of high-LET radiation remains unclear. In this study, endothelial population changes in hippocampal microvessels exposed to 56Fe ions at doses of 0, 0.5, 2 and 4 Gy were quantified using unbiased stereological techniques. Twelve months after exposure, mice that received 0.5 Gy or 2 Gy of iron ions showed a 34% or 29% loss of endothelial cells, respectively, in the hippocampal cornu ammonis region 1 (CA1) compared to age-matched controls or mice that received 4 Gy (P < 0.05). We suggest that this “U-shaped” dose response indicates a repopulation from a sensitive subset of endothelial cells that occurred after 4 Gy that was stimulated by an initial rapid loss of endothelial cells. In contrast to the CA1, in the dentate gyrus (DG), there was no significant difference in microvessel cell and length density between irradiated groups and age-matched controls. Vascular topology differences between CA1 and DG may account for the variation in dose response. The correlation between radiation-induced alterations in the hippocampal microvessels and their functional consequences must be investigated in further studies.

Acute Effects of Iron-Particle Radiation on Immunity. Part I: Population Distributions

Abstract Pecaut, M. J., Dutta-Roy, R., Smith, A. L., Jones, T. A., Nelson, G. A. and Gridley, D. S. Acute Effects of Iron-Particle Radiation on Immunity. Part I: Population Distributions. Radiat. Res. 165, 68–77 (2006). Health risks due to exposure to high-linear energy transfer (LET) charged particles remain unclear. The major goal of this study was to confirm and further characterize the acute effects of high-LET radiation (56Fe26) on erythrocyte, thrombocyte and leukocyte populations in three body compartments after total-body exposure. Adult female C57BL/6 mice were irradiated with total doses of 0, 0.5, 2 and 3 Gy and killed humanely 4 days later. Body and organ masses were determined and blood, spleen and bone marrow leukocytes were evaluated using a hematology analyzer and flow cytometry. Spleen and thymus (but not body, liver and lung) masses were significantly decreased in a dose-dependent manner. In general, red blood cell (RBC) counts and most other RBC parameters were depressed with increasing dose (P < 0.05); the major exception was an increase in cell size at 0.5 Gy. Platelet numbers and volume, total white blood cell counts, and all three major types of leukocytes also decreased (P < 0.05). Lymphocyte populations in blood and spleen exhibited variable degrees of susceptibility to 56Fe-particle radiation (B > T > NK and T cytotoxic > T helper cells). In the bone marrow, leukocytes with granulocytic, lymphocytic (“dim” and “bright”), and monocytic characteristics exhibited proportional variations at the higher radiation doses in the expression of CD34 and/or Ly-6A/E. The data are discussed in relation to our previous investigations with iron ions, other forms of radiation, and space flight in this same animal model.

Magnetic Resonance Imaging and Spectroscopy of the Rat Hippocampus 1 Month after Exposure to 56Fe-Particle Radiation

Abstract Obenaus, A., Huang, L., Smith, A., Favre, C. J., Nelson, G. and Kendall, E. Magnetic Resonance Imaging and Spectroscopy of the Rat Hippocampus 1 Month after Exposure to 56Fe-Particle Radiation. Radiat. Res. 169, 149–161 (2008). The response of the central nervous system to space radiation is largely unknown. The hippocampus, which is known for its critical role in learning and memory, was evaluated for its response to heavy-ion radiation. At 1 month, animals exposed to brain-only 56Fe-particle irradiation (0–4 Gy) were examined using contrast-enhanced T1 imaging (CET1), T2-weighted imaging (T2WI), diffusion weighted imaging (DWI), and 1H-magnetic resonance spectroscopy (MRS). Correlative histology was performed after imaging. The T2WI, DWI and CET1 images revealed no overt anatomical changes after irradiation. Quantitative analysis demonstrated a significant increase in T2 at 2 Gy compared to 0 Gy. The apparent diffusion coefficient (ADC) revealed an inverse dose-dependent quantitative change in water mobility. Compared to 0 Gy, the ADC increased 122% at 1 Gy and declined to 44% above control levels at 4 Gy. MRS showed a significant increase in the N-acetylaspartate/choline ratio at 4 Gy and a lactate peak. Histology demonstrated no overt pathological changes in neuronal and astrocyte populations. However, a significant inverse dose-dependent morphological change in the microglial population was detected in irradiated animals. Our results suggest that early tissue matrix modifications induced by 56Fe-particle radiation can be detected by MRI in the absence of evident histopathology. These changes may indicate fundamental changes in the structure and function of the hippocampus.

Behavioral consequences of radiation exposure to simulated space radiation in the C57BL/6 mouse: open field, rotorod, and acoustic startle

Two experiments were carried out to investigate the consequences of exposure to proton radiation, such as might occur for astronauts during space flight. C57BL/6 mice were exposed, either with or without 15-g/cm2 aluminum shielding, to 0-, 3-, or 4-Gy proton irradiation mimicking features of a solar particle event. Irradiation produced transient direct deficits in open-field exploratory behavior and acoustic startle habituation. Rotorod performance at 18 rpm was impaired by exposure to proton radiation and was impaired at 26 rpm, but only for mice irradiated with shielding and at the 4-Gy dose. Long-term (>2 weeks) indirect deficits in open-field activity appeared as a result of impaired experiential encoding immediately following exposure. A 2-week recovery prior to testing decreased most of the direct effects of exposure, with only rotorod performance at 26 rpm being impaired. These results suggest that the performance deficits may have been mediated by radiation damage to hippocampal, cerebellar, and possibly, forebrain dopaminergic function.

The Effects of Low-Dose, High-LET Radiation Exposure on Three Models of Behavior in C57BL/6 Mice

Abstract Pecaut, M. J., Haerich, P., Zuccarelli Miller, C. N., Smith, A. L., Zendejas, E. D. and Nelson, G. A. The Effects of Low-Dose, High-LET Radiation Exposure on Three Models of Behavior in C57BL/6 Mice. Radiat. Res. 162, 148–156 (2004). To investigate the behavioral consequences of exposure to whole-body irradiation such as might occur for astronauts during space flight, female C57BL/6 mice were exposed to 0, 0.1, 0.5 or 2 Gy accelerated iron ions (56Fe, Z = 26, β = 0.9, LET = 148.2 keV/μm) of 1 GeV per nucleon using the Alternating Gradient Synchrotron at the Brookhaven National Laboratory. Animal testing began 2 weeks after exposure and continued for 8 weeks. Under these conditions, there were few significant effects of radiation on open-field, rotorod or acoustic startle activities at any of the times examined. The lack of radiation effects in these behavioral models appears to offer reassurance to NASA mission designers. These results suggest that there may be negligible effects of HZE radiation on many behaviors during a 2–8-week period immediately after radiation.

Dynamic characteristics of 56Fe-particle radiation-induced alterations in the rat brain: magnetic resonance imaging and histological assessments.

Abstract 56Fe-particle radiation-induced brain disturbances are a major health concern for astronauts during long-term space travel. The present study investigated temporal modifications within the brain after 56Fe-particle exposure using in vivo magnetic resonance imaging (MRI) correlated to histology. Male Sprague-Dawley rats were exposed to brain-only 56Fe-particle radiation. MRI including T2-weighted, diffusion-weighted, pre/postcontrast enhanced T1-weighted imaging was performed 0.25–18 months after exposure. T2 relaxation times and the apparent diffusion coefficient were quantified within the hippocampus, entorhinal cortex, retrosplenial cortex and thalamus, and correlative histopathology was then performed at each time. In the absence of visible lesions on MR images, the apparent diffusion coefficient and T2 relaxation times revealed 56Fe-particle-induced dynamic changes in all ROIs over the 18-month time course. The patterns of MR changes were spatially similar within the different regions. The temporal alterations in the apparent diffusion coefficient corresponded to the glial cell changes within the brain. Quantitative MRI provides a non-invasive approach to monitor spatio-temporal brain alterations after 56Fe-particle irradiation. The apparent diffusion coefficient appears to be a sensitive metric to reveal ongoing tissue modifications involving multiple cellular components in vivo.

Neuroimaging assessment of memory-related brain structures in a rat model of acute space-like radiation.

To investigate the acute effects on the central nervous system (CNS) of 56Fe radiation, a component of high‐energy charged particles (HZE) in space radiation, using quantitative magnetic resonance imaging (MRI) noninvasively.