Differential responses to increasing numbers of mild traumatic brain injury in a rodent closed-head injury model.

Following mild traumatic brain injury (mTBI), further mild impacts can exacerbate negative outcomes. To compare chronic damage and deficits following increasing numbers of repeated mTBIs, a closed-head weight-drop model of repeated mTBI was used to deliver 1, 2 or 3 mTBIs to adult female rats at 24 h intervals. Outcomes were assessed at 3 months following the first mTBI. No gross motor, sensory or reflex deficits were identified (p > 0.05), consistent with current literature. Cognitive function assessed using a Morris water maze revealed chronic memory deficits following 1 and 2, but not 3 mTBI compared to shams (p ≤ 0.05). Oxidative damage to DNA was assessed immunohistochemically in the dentate hilus of the hippocampus and splenium of the corpus callosum; no changes were observed. IBA1-positive microglia were increased in size in the cortex following 1 mTBI and in the corpus callosum following 2 mTBI compared to shams (p ≤ 0.05); no changes were observed in the dentate hilus. Glial fibrillary acidic protein (GFAP)-positive astrocyte immunoreactivity was assessed in all three brain regions and no chronic changes were observed. Integrity of myelin ultrastructure in the corpus callosum was assessed using transmission electron microscopy. G ratio was decreased following 2 mTBIs compared to shams (p ≤ 0.05) at post hoc level only. The changing patterns of damage and deficits following increasing numbers of mTBI may reflect dynamic responses to small numbers of mTBIs or a conditioning effect such that increasing numbers of mTBIs do not necessarily result in worsening pathology. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/. Cover Image for this issue: doi: 10.1111/jnc.14508.

Repeated mild traumatic brain injury in female rats increases lipid peroxidation in neurons.

Negative outcomes of mild traumatic brain injury (mTBI) can be exacerbated by repeated insult. Animal models of repeated closed-head mTBI provide the opportunity to define acute pathological mechanisms as the number of mTBI increases. Furthermore, little is known about the effects of mTBI impact site, and how this may affect brain function. We use a closed head, weight drop model of mTBI that allows head movement following impact, in adult female rats to determine the role of the number and location of mTBI on brain pathology and behaviour. Biomechanical assessment of two anatomically well-defined mTBI impact sites were used, anterior (bregma) and posterior (lambda). Location of the impact had no significant effect on impact forces (450 N), and the weight impact locations were on average 5.4 mm from the desired impact site. No between location vertical linear head kinematic differences were observed immediately following impact, however, in the 300 ms post-impact, significantly higher mean vertical head displacement and velocity were observed in the mTBI lambda trials. Breaches of the blood brain barrier were observed with three mTBI over bregma, associated with immunohistochemical indicators of damage. However, an increased incidence of hairline fractures of the skull and macroscopic haemorrhaging made bregma an unsuitable impact location to model repeated mTBI. Repeated mTBI over lambda did not cause skull fractures and were examined more comprehensively, with outcomes following one, two or three mTBI or sham, delivered at 1 day intervals, assessed on days 1-4. We observe a mild behavioural phenotype, with subtle deficits in cognitive function, associated with no identifiable neuroanatomical or inflammatory changes. However, an increase in lipid peroxidation in a subset of cortical neurons following two mTBI indicates increasing oxidative damage with repeated injury in female rats, supported by increased amyloid precursor protein immunoreactivity with three mTBI. This study of acute events following closed head mTBI identifies lipid peroxidation in neurons at the same time as cognitive deficits. Our study adds to existing literature, providing biomechanics data and demonstrating mild cognitive disturbances associated with diffuse injury, predominantly to grey matter, acutely following repeated mTBI.

Towards the Development of an Integrative, Evidence-Based Suite of Indicators for the Prediction of Outcome Following Mild Traumatic Brain Injury: Results from a Pilot Study.

BACKGROUND: Persisting post-concussion symptoms (PPCS) is a complex, multifaceted condition in which individuals continue to experience the symptoms of mild traumatic brain injury (mTBI; concussion) beyond the timeframe that it typically takes to recover. Currently, there is no way of knowing which individuals may develop this condition. METHOD: Patients presenting to a hospital emergency department (ED) within 48 h of sustaining a mTBI underwent neuropsychological assessment and demographic, injury-related information and blood samples were collected. Concentrations of blood-based biomarkers neuron specific enolase, neurofilament protein-light, and glial fibrillary acidic protein were assessed, and a subset of patients also underwent diffusion tensor-magnetic resonance imaging; both relative to healthy controls. Individuals were classified as having PPCS if they reported a score of 25 or higher on the Rivermead Postconcussion Symptoms Questionnaire at ~28 days post-injury. Univariate exact logistic regression was performed to identify measures that may be predictive of PPCS. Neuroimaging data were examined for differences in fractional anisotropy (FA) and mean diffusivity in regions of interest. RESULTS: Of n = 36 individuals, three (8.33%) were classified as having PPCS. Increased performance on the Repeatable Battery for the Assessment of Neuropsychological Status Update Total Score (OR = 0.81, 95% CI: 0.61-0.95, p = 0.004), Immediate Memory (OR = 0.79, 95% CI: 0.56-0.94, p = 0.001), and Attention (OR = 0.86, 95% CI: 0.71-0.97, p = 0.007) indices, as well as faster completion of the Trails Making Test B (OR = 1.06, 95% CI: 1.00-1.12, p = 0.032) at ED presentation were associated with a statistically significant decreased odds of an individual being classified as having PPCS. There was no significant association between blood-based biomarkers and PPCS in this small sample, although glial fibrillary acidic protein (GFAP) was significantly increased in individuals with mTBI relative to healthy controls. Furthermore, relative to healthy age and sex-matched controls (n = 8), individuals with mTBI (n = 14) had higher levels of FA within the left inferior frontal occipital fasciculus (t (18.06) = -3.01, p = 0.008). CONCLUSION: Performance on neuropsychological measures may be useful for predicting PPCS, but further investigation is required to elucidate the utility of this and other potential predictors.