Repetitive Closed-Skull Traumatic Brain Injury in Mice Causes Persistent Multifocal Axonal Injury and Microglial Reactivity

Abstract

Repetitive mild or “concussive” traumatic brain injury (TBI) can cause substantial neurological impairment, but the pathology of TBI is not well understood. We report an experimental model of TBI in which the closed skulls of anesthetized male C57BL/6J mice are struck with an electromagnetically controlled rubber impactor twice with an interval of 24 hours between impacts. The mice had deficits in Morris water maze performance in the first week after injury that only partially resolved 7 weeks later. By routine histology there was no apparent bleeding, neuronal cell loss, or tissue disruption and amyloid precursor protein immunohistochemistry demonstrated very few immunoreactive axonal varicosities. In contrast, silver staining revealed extensive abnormalities in the corpus callosum and bilateral external capsule, the ipsilateral cortex and thalamus, and the contralateral hippocampal CA1 stratum radiatum and stratum oriens. Electron microscopy of white matter regions demonstrated axonal cytoskeletal disruption, intra-axonal organelle compaction and irregularities in axon caliber. Reactive microglia were observed in the same areas as the injured axons by both electron microscopy and Iba1 immunohistochemistry. Quantitative analyses of silver staining and Iba1 immunohistochemistry at multiple time points demonstrated transient cortical and thalamic abnormalities; white matter axonal cytoskeletal abnormalities and microglial reaction persisted to 7 weeks after injury. Thus, prominent and long-lasting abnormalities in this TBI model were underestimated using conventional approaches. The model may be useful for mechanistic investigations and preclinical assessment of candidate therapeutics.

Characterization of persistent concussive syndrome using injury reconstruction and finite element modelling

“…Furthermore, persistence of symptoms has been linked to abnormalities in grey matter following strains from TBI (Shitaka et al, 2011), supporting that excessive strain in grey matter might influence persistence of symptoms. It should be noted that the UCDBTM and other finite element models characterize the modulus for the grey matter lower than that for the white matter.…”

“…As a result, this research would suggest that higher stresses and strains are representative of PCS injury in the UCDBTM. In addition, it is likely that these stresses and strains may be representative of the presence of a more severe injury; ones that may lead to the physiological cascades producing the symptomology of concussion (Giza and Hovda 2001) by many different potential mechanisms including: 1) a complex series of ionic, metabolic, and other physiologic changes take place that contribute to the persistent symptoms (Giza and Hovda, 2001), 2) to changes in gene expression (Morrison et al 2000), 3) to sustained activation of microglia for days to weeks Shitaka et al (2011), or 4) other mechanisms.…”

“…No definitive single gene expression has been linked to persistent symptoms after concussive injury. Shitaka et al (2011) suggested the possibility of sustained activation of microglia for days to weeks after concussive injury to be partially responsible for persistent symptoms. Morrison et al, (2000) reported that the changes which occur following the mechanical impact of the brain could influence the severity of clinical outcome.…”

Abnormalities in Diffusional Kurtosis Metrics Related to Head Impact Exposure in a Season of High School Varsity Football

“…25 The increase in astrocytes is a lingering effect seen in animal models after induced TBI. 40 Axonal injury and the associated release of cytokines by activated microglia can induce astrogliosis. Microglial inhibition is now the target of several potential and promising TBI therapies.…”

Reduced bone mass accrual in mouse model of repetitive mild traumatic brain injury

“…Although we indeed found significantly higher Prussian blue staining scores in the impacted versus control mice, indicating that the impact caused more iron deposition and hemorrhage, the Prussian blue staining was mostly localized in just a few areas beneath the impact area on the dorsal side and was not widespread in the brain. Cresyl violet staining is an effective and reliable method to determine the long-term neuronal survival following TBI [36][37]. No visible neuron damage was revealed with this stain, which was indicated by lack of amyloid accumulation and apoptotic neurons in the brain sections of the impacted and control mice.…”