Regional and temporal characterization of neuronal, glial, and axonal response after traumatic brain injury in the mouse.

Fluid percussion injury (FPI) is a commonly used and clinically relevant model of traumatic brain injury (TBI) in the rat. Recently, our lab successfully adapted FPI to mice. To account for differences in response to injury between mice and rats and provide a foundation for further use of FPI in gene-targeting studies, we sought to characterize the temporal and regional response to FPI in male C57BL/6 mice. Animals were killed at 10 min, 24 h, and 4, 7, 14, and 35 days (n = 3 for each group) after a very severe parasagittal FPI (> 4.0 atm) or sham injury (n = 3). Extensive numbers of damaged neurons were consistently found in the ipsilateral cortex, thalamus, and hippocampus by 10 min. This damage was nearly identical at 24 h, but quickly declined at subsequent time points. Activated microglia were found only in regions of neuronal injury at the earliest time points. Glial fibrillary acidic protein immunoreactivity reached significantly higher levels compared with controls at 7 days (P < 0. 05) in the cortex, thalamus, and hippocampus and remained elevated for 35 days. White matter degeneration was present in all regions examined. This damage did not appear until at least day 4, but progressed up to day 35. The spatial pattern of damage we observed in mice after FPI is similar to that seen in rats. However, the temporal progression of neuronal injury in mice is comparatively abbreviated in the hippocampus and thalamus. In conclusion, these results suggest that FPI in mice may be a particularly useful tool for studying mechanisms of TBI in gene-targeting studies.

Evidence disputing the importance of excitotoxicity in hippocampal neuron death after experimental traumatic brain injury.

The hippocampus is selectively vulnerable to experimental traumatic brain injury (TBI). Beneficial effects of glutamate receptor antagonists and increased extracellular levels of glutamate have suggested that glutamate-mediated excitotoxicity may be responsible for this selective damage. In order to clarify this important issue, we applied a severe parasagittal fluid percussion injury (FPI) to strains of mice shown to be susceptible and resistant to kainic acid (KA)-induced excitotoxic hippocampal damage. Dystrophic neurons were present by 10 min after FPI in the hippocampi of both strains. Damaged hippocampal neurons were absent at 4 days and 7 days. Additionally, there was no significant difference (p = 1.00) in CA3 neuron survival between KA-susceptible and -resistant mice at 4 days. In conclusion, excitotoxicity does not significantly contribute to hippocampal neuron loss after FPI and, in contrast to classic studies of excitotoxicity in vivo, the pattern of hippocampal cell death after TBI is extremely acute.

Adaptation of the fluid percussion injury model to the mouse.

Fluid percussion injury (FPI) is a well-characterized experimental model of traumatic brain injury (TBI) in the rat. Many pathophysiologic consequences and mechanisms of recovery after TBI rely on neurochemical pathways that can be examined in genetically altered mice. Therefore, FPI applied to mice may be a useful experimental tool to investigate TBI at the molecular level. In the present study, we establish FPI as a viable model of TBI in the mouse by characterizing acute neurological, histopathological, and behavioral changes. Right-sided parasagittal FPI or sham treatment was administered in male C57BL/6 mice. Acute neurological evaluation revealed righting reflexes in the injured animals (p < 0.001). Deficits in spatial learning and memory were observed in the Morris water maze (MWM) 5 and 6 days after injury. A novel MWM data analysis protocol is described. The injured group (n = 18) demonstrated impaired performance in the MWM during acquisition (p < 0.05) and probe trials (p < 0.025) compared to sham animals (n = 16). At 7 days postinjury, glial fibrillary acidic protein immunohistochemistry revealed intense cortical, callosal, and hippocampal gliosis. The modified Gallyas silver degeneration stain consistently labeled cell bodies and terminals throughout the ipsilateral cortex, axons in the gray matter-white matter interface above the corpus callosum and within the corpus callosum bilaterally, and terminals and fibers in the thalamus bilaterally. Additionally, the mouse FPI model described is immediately employable in labs already using the FPI rat model with no modifications to a pre-existing FPI apparatus.