Long Term Cognitive Impairment and Increased Inflammation following Mild Traumatic Brain Injury using the CHIMERA Model

Abstract

IntroductionTraumatic brain injury (TBI) affects a vast number of people worldwide. Mild TBI (mTBI), including concussions, accounts for most of the TBI cases. mTBI patients experience enduring symptoms, including headaches, mild cognitive dysfunction, emotional instability, psychiatric symptoms, and increased risk of neurodegenerative disease. Repetitive closed head impact model of engineered rotational acceleration (rCHIMERA) is a recently developed nonsurgical animal model of TBI that primarily produces diffuse axonal injury characterized by white matter inflammation and axonal damage, which mimic the mTBI in humans. However, the majority of studies on rCHIMERA were conducted in mice, which are not suitable for assessment of complex behavior.MethodsThis study was designed to validate our rCHIMERA‐induced rat mTBI model using advanced and cutting‐edge behavioral tests and immunofluorescence staining of key biomarkers of TBI. Adult male Sprague‐Dawley rats (~350 g) were divided into sham and injured groups. All rats were anesthetized with dexmedetomidine (IV, 0.2 mg/kg/hr). Following anesthesia, the rats designated for injury received 6 closed‐head impacts of 2 Joules (J) energy over 3 days with 1‐day interval (3 impacts on days 1 & 3, with 30‐seconds between impacts, and rested on day 2). The sham rats were handled the same way as the injured without the impacts. The rats were subsequently subjected to a battery of behavioral tests at specified time points post‐injury: neurological severity score (NSS), automated home‐cage (CleverSys), behavioral spectrometer (Biobserve), rotarod test, and fear conditioning. Histological assessment of the injury was performed in targeted rat brain areas from sham and injured rats. Immunohistochemistry staining for amyloid precursor protein (APP), ionized calcium‐binding adapter molecule 1 (Iba‐1) and glial fibrillary acidic protein (GFAP) were used to determine axonal damage, inflammatory response and astrocytic activation, respectively, in sham and injured rats. The weights of the rats were also monitored for the duration of the studies.Results3 months after mTBI, there was no significant difference in body weight and NSS between sham and injured rats. Injury did not affect motor activity and motor learning with the rotarod test, but did impair associative fear learning and memory in fear conditioning tests 4 weeks and 3 months after mTBI (n=9 shams and 9 mTBI). APP immunohistochemistry in the injured brains revealed induced diffusive axonal injury in the the cortex. The number of GFAP positive cells was increased in the prefrontal cortex and hippocampus after injury and the astrocytes showed distinct hypertrophic morphology. The number of Iba‐1 positive cells was also increased after injury, indicating enhanced inflammation.ConclusionThe results suggest that our mTBI model in rats induces short‐term changes in affect and activity, long‐term cognitive impairments, and histological changes. These observations validate the CHIMERA, using the rat, as a model that replicates clinical mTBI.Support or Funding InformationThis work is supported by the Advancing a Healthier Wisconsin Endowment (#5520444) to AC.