Bi-directional changes in fractional anisotropy after experiment TBI: Disorganization and reorganization?

- TBSS analysis of FA, AD, RD, MO, FTD, and covariate of injury severity at 1 and 4 weeks.
- FA decreases driven by RD acutely, but AD and RD chronically.
- Region of AD and RD change greater than FA acutely, less than FA chronically.
- FA increases subcortical-associated with increased fiber tract density and length.
- Data refute the dogma that CCI model of TBI is focal with limited axonal pathology.

The current dogma to explain the extent of injury-related changes following rodent controlled cortical impact (CCI) injury is a focal injury with limited axonal pathology. However, there is in fact good, published histologic evidence to suggest that axonal injury is far more widespread in this model than generally thought. One possibility that might help to explain this is the often-used region-of-interest data analysis approach taken by experimental traumatic brain injury (TBI) diffusion tensor imaging (DTI) or histologic studies that might miss more widespread damage, when compared to the whole brain, statistically robust method of tract-based analysis used more routinely in clinical research. To determine the extent of DTI changes in this model, we acquired in vivo DTI data before and at 1 and 4 weeks after CCI injury in 17 adult male rats and analyzed parametric maps of fractional anisotropy (FA), axial, radial, and mean diffusivity (AD, RD, MD), tensor mode (MO), and fiber tract density (FTD) using tract-based spatial statistics. Contusion volume was used as a surrogate marker of injury severity and as a covariate for investigating severity dependence of the data. Mean fiber tract length was also computed from seeds in the cortical spinal tract regions. In parallel experiments (n = 3-5/group), we investigated corpus callosum neurofilaments and demyelination using immunohistochemistry (IHC) at 3 days and 6 weeks, callosal tract patency using dual-label retrograde tract tracing at 5 weeks, and the contribution of gliosis to DTI parameter maps using GFAP IHC at 4 weeks post-injury. The data show widespread ipsilateral regions of significantly reduced FA at 1 week post-injury, driven by temporally changing values of AD, RD, and MD that persist to 4 weeks. Demyelination, retrograde label tract loss, and reductions in MO (tract degeneration) and FTD were shown to underpin these data. Significant FA increases occurred in subcortical and corticospinal tract regions that were spatially distinct from regions of FA decrease, grossly affected gliotic areas, and MO changes. However, there was good spatial correspondence between regions of increased FA and areas of increased FTD and mean fiber length. We discuss these widespread changes in DTI parameters in terms of axonal degeneration and potential reorganization, with reference to a resting state fMRI companion paper (Harris et al., 2016, Exp. Neurol. 227:124-138) that demonstrated altered functional connectivity data acquired from the same rats used in this study.

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