In Vivo Synaptic Density Imaging With 18f-Ucb-H After Space Irradiation

86 Introduction: Previous findings have shown that doses of charged particles as low as 5 cGy can result in substantial reductions of synapse number in the mouse that are persistent over many months ( Parihar et al. Sci. Adv. 2015;1:e1400256). To non-invasively assess which brain regions are most sensitive to chronic, low-level radiation exposure, we examined adult male Wistar rats irradiated with 252Cf neutrons with PET imaging using a radioligand (18F-UCB-H) which binds specifically to the synaptic vesicle protein SV2A and serves as a surrogate marker of synaptic density. For comparison, region-specific c-fos expression following a behavioral stimulus in a subset of animals was performed to determine the strength of behavior-neuron activation coupling. Methods: Adult male, 6 month old Wistar rats (n=15) were irradiated chronically with 252Cf neutrons at 1 mGy/day for 6 months to a total dose of 180 mGy and compared to controls (n=16). PET images of the brain were acquired in twelve rats (6 irradiated and 6 control; 869±74 g) 65-67 days post-irradiation using the Rodent R4 microPET scanner (Concorde Systems). A 40 minute dynamic list mode scan was performed immediately after injection of 18F-UCB-H (76.6±5.5 MBq). Corrected images were reconstructed by OSEM 2D (4 iterations, 256x256, ramp filter, pixel size of 0.4x0.4x1.2 mm). Summed images (0-40 min) were used for rigid coregistration with MRI T2 weighted brain images (11.7T Bruker) and a standard rat brain template available in Vivoquant™ software (version 4.0; InVicro). Atlas based volumes of interest, representing the brain distribution of SV2A measured as Standard Uptake Value (SUV) were used for comparisons between unirradiated and irradiated animals. A separate cohort of animals (14 irradiated and 12 controls) were tested in four behavioral paradigms (1 x 1 meter square Open Field Task, Elevated Zero Maze test, Novel Object recognition task, and Social Dominance Tube test) two months after irradiation. At 3 months after irradiation animals were allowed to explore a 1 meter diameter open field for 30 min.to drive c-fos expression then euthanized at 90 min after completion of the task for brain extraction and fixation followed by antibody staining of the immediate early gene, c-fos, which is expressed in activated neurons. Results: PET imaging analysis revealed three brain regions with statistically decreased mean SUV values in irradiated rats compared to controls; the entorhinal cortex (p=0.038), the ventral tegmental area (p=0.045), and the substantia nigra (p=.046). C-fos imaging after open field exploration showed significant decrements in c-fos positive cells in hippocampus CA1 (p = 0.0014, t-test) and a trend in primary motor cortex (p = 0.08, t-test) but not in prelimbic, cingulate and entorhinal cortex, dentate gyrus, amygdala or substantia nigra. Open field and elevated zero mazes showed reduced movement and exploration in neutron-irradiated animals (P