Repetitive closed-head impact model of engineered rotational acceleration (CHIMERA) injury in rats increases impulsivity, decreases dopaminergic innervation in the olfactory tubercle and generates white matter inflammation, tau phosphorylation and degeneration

An overview of preclinical models of traumatic brain injury (TBI): relevance to pathophysiological mechanisms

Background

Traumatic brain injury (TBI) is a major cause of morbidity and mortality, affecting millions annually worldwide. Although the majority of TBI patients return to premorbid baseline, a subset of patient can develop persistent and often debilitating neurocognitive and behavioral changes. The etiology of TBI within the clinical setting is inherently heterogenous, ranging from sport related injuries, fall related injuries and motor vehicle accidents in the civilian setting, to blast injuries in the military setting.

Objective

Animal models of TBI, offer the distinct advantage of controlling for injury modality, duration and severity. Furthermore, preclinical models of TBI have provided the necessary temporal opportunity to study the chronic neuropathological sequelae of TBI, including neurodegenerative sequelae such as tauopathy and neuroinflammation within the finite experimental timeline. Despite the high prevalence of TBI, there are currently no disease modifying regimen for TBI, and the current clinical treatments remain largely symptom based. The preclinical models have provided the necessary biological substrate to examine the disease modifying effect of various pharmacological agents and have imperative translational value.

Methods

The current review will include a comprehensive survey of well-established preclinical models, including classic preclinical models including weight drop, blast injury, fluid percussion injury, controlled cortical impact injury, as well as more novel injury models including closed-head impact model of engineered rotational acceleration (CHIMERA) models and closed-head projectile concussive impact model (PCI). In addition to rodent preclinical models, the review will include an overview of other species including large animal models and Drosophila.

Results

There are major neuropathological perturbations post TBI captured in various preclinical models, which include neuroinflammation, calcium dysregulation, tauopathy, mitochondrial dysfunction and oxidative stress, axonopathy, as well as glymphatic system disruption.

Conclusion

The preclinical models of TBI continue to offer valuable translational insight, as well as essential neurobiological basis to examine specific disease modifying therapeutic regimen.

Mapping the neuroanatomical abnormalities in a phenotype of male compulsive rats

Compulsivity is considered a transdiagnostic dimension in obsessive-compulsive and related disorders, characterized by heterogeneous cognitive and behavioral phenotypes associated with abnormalities in cortico-striatal-thalamic-cortical circuitry. The present study investigated the structural morphology of white and gray matter in rats selected for low- (LD) and high- (HD) compulsive drinking behavior on a schedule-induced polydipsia (SIP) task. Regional brain morphology was assessed using ex-vivo high-resolution magnetic resonance imaging (MRI). Voxel-based morphometry of segmented MRI images revealed larger white matter volumes in anterior commissure and corpus callosum of HD rats compared with LD rats. HD rats also showed significantly larger regional volumes of dorsolateral orbitofrontal cortex, striatum, amygdala, hippocampus, midbrain, sub-thalamic nucleus, and cerebellum. By contrast, the medial prefrontal cortex was significantly smaller in HD rats compared with LD rats with no significant group differences in whole brain, ventricular, or cerebrospinal fluid volumes. These findings show that limbic cortico-basal ganglia structures implicated in impulse control disorders are distinct in rats that are vulnerable to develop compulsive behavior. Such abnormalities may be relevant to the etiology of compulsive disorders in humans.

Traumatic brain injury and the pathways to cerebral tau accumulation

Tau is a protein that has received national mainstream recognition for its potential negative impact to the brain. This review succinctly provides information on the structure of tau and its normal physiological functions, including in hibernation and changes throughout the estrus cycle. There are many pathways involved in phosphorylating tau including diabetes, stroke, Alzheimer's disease (AD), brain injury, aging, and drug use. The common mechanisms for these processes are put into context with changes observed in mild and repetitive mild traumatic brain injury (TBI). The phosphorylation of tau is a part of the progression to pathology, but the ability for tau to aggregate and propagate is also addressed. Summarizing both the functional and dysfunctional roles of tau can help advance our understanding of this complex protein, improve our care for individuals with a history of TBI, and lead to development of therapeutic interventions to prevent or reverse tau-mediated neurodegeneration.

Effects of a Subanesthetic Ketamine Infusion on Inflammatory and Behavioral Outcomes after Closed Head Injury in Rats

Traumatic brain injury (TBI) affects millions of people annually, and most cases are classified as mild TBI (mTBI). Ketamine is a potent trauma analgesic and anesthetic with anti-inflammatory properties. However, ketamine's effects on post-mTBI outcomes are not well characterized. For the current study, we used the Closed-Head Impact Model of Engineered Rotational Acceleration (CHIMERA), which replicates the biomechanics of a closed-head impact with resulting free head movement. Adult male Sprague-Dawley rats sustained a single-session, repeated-impacts CHIMERA injury. An hour after the injury, rats received an intravenous ketamine infusion (0, 10, or 20 mg/kg, 2 h period), during which locomotor activity was monitored. Catheter blood samples were collected at 1, 3, 5, and 24 h after the CHIMERA injury for plasma cytokine assays. Behavioral assays were conducted on post-injury days (PID) 1 to 4 and included rotarod, locomotor activity, acoustic startle reflex (ASR), and pre-pulse inhibition (PPI). Brain tissue samples were collected at PID 4 and processed for GFAP (astrocytes), Iba-1 (microglia), and silver staining (axonal injury). Ketamine dose-dependently altered locomotor activity during the infusion and reduced KC/GRO, TNF-α, and IL-1β levels after the infusion. CHIMERA produced a delayed deficit in rotarod performance (PID 3) and significant axonal damage in the optic tract (PID 4), without significant changes in other behavioral or histological measures. Notably, subanesthetic doses of intravenous ketamine infusion after mTBI did not produce adverse effects on behavioral outcomes in PID 1-4 or neuroinflammation on PID 4. A further study is warranted to thoroughly investigate beneficial effects of IV ketamine on mTBI given multi-modal properties of ketamine in traumatic injury and stress.

Paired associates learning is disrupted after unilateral parietal lobe controlled cortical impact in rats: A trial-by-trial behavioral analysis

Approximately 60-70 million people suffer from traumatic brain injury (TBI) each year. Animal models continue to be paramount in understanding mechanisms of cellular dysfunction and testing new treatments for TBI. Enhancing the translational potential of novel interventions therefore necessitates testing pre-clinical intervention strategies with clinically relevant cognitive assays. This study used a unilateral parietal lobe controlled cortical impact (CCI) model of TBI and tested rats on a touchscreen-based Paired Associates Learning (PAL) task, which is part of the Cambridge Neuropsychological Test Automated Battery. In humans, the PAL task has been used to assess cognitive deficits in the ability to form stimulus-location associations in a multitude of disease states, including TBI. Although the use of PAL in animal models could be important for understanding the clinical severity of cognitive impairment post-injury and throughout intervention, to date, the extent to which a rat model of TBI produces deficits in PAL task performance has not yet been reported. This study details the behavioral consequences of the CCI injury model with a Trial-by-Trial analysis of PAL performance that enables behavioral strategy use to be inferred. Following behavior, the extent of the injury was quantified with histology and staining for the presence of glial fibrillary acid protein and ionized calcium-binding adapter molecule 1. Rats that received unilateral CCI were impaired on the PAL task and showed more aberrant response-driven behavior. The magnitude of PAL impairment was also correlated with Iba1 staining in the thalamus. These observations suggest that PAL could be useful for pre-clinical assessments of novel interventions for treating TBI.

Repeat Closed-Head Injury in Male Rats Impairs Attention but Causes Heterogeneous Outcomes in Multiple Measures of Impulsivity and Glial Pathology

Repetitive mild traumatic brain injury, or concussion, can lead to the development of long-term psychiatric impairments. However, modeling these deficits is challenging in animal models and necessitates sophisticated behavioral approaches. The current set of studies were designed to evaluate whether a rubberized versus metal impact tip would cause functional deficits, the number of injuries required to generate such deficits, and whether different psychiatric domains would be affected. Across two studies, male rats were trained in either the 5-choice serial reaction time task (5CSRT; Experiment 1) to assess attention and motor impulsivity or concurrently on the 5CSRT and the delay discounting task (Experiment 2) to also assess choice impulsivity. After behavior was stable, brain injuries were delivered with the Closed-head Injury Model of Engineered Rotational Acceleration (CHIMERA) either once per week or twice per week (Experiment 1) or just once per week (Experiment 2). Astrocyte and microglia pathology was also assayed in relevant regions of interest. CHIMERA injury caused attentional deficits across both experiments, but only increased motor impulsivity in Experiment 1. Surprisingly, choice impulsivity was actually reduced on the Delay Discounting Task after repeat injuries. However, subsequent analyses suggested potential visual issues which could alter interpretation of these and attentional data. Subtle changes in glial pathology immediately after the injury (Experiment 1) were attenuated after 4 weeks recovery (Experiment 2). Given the heterogenous findings between experiments, additional research is needed to determine the root causes of psychiatric disturbances which may arise as a results of repeated brain injuries.

Meningeal and visual pathway MRI analysis after single and repetitive Closed-Head Impact Model of Engineered Rotational Acceleration (CHIMERA)-induced disruption in male and female mice

The consequences of forceful rotational acceleration on the central nervous system are not fully understood. While traumatic brain injury (TBI) research primarily has focused on effects related to the brain parenchyma, reports of traumatic meningeal enhancement in TBI patients may possess clinical significance. The objective of this study was to evaluate the meninges and brain for changes in dynamic contrast enhancement (DCE) magnetic resonance imaging (MRI) following closed-head impact model of engineered rotational acceleration (CHIMERA)–induced cerebral insult. Adult male and female mice received one (1 × ; n = 19 CHIMERA, n = 19 Sham) or four (4 × one/day; n = 18 CHIMERA, n = 12 Sham) injuries. Each animal underwent three MRI scans: 1 week before injury, immediately after the final injury, and 1 week post-injury. Compared with baseline readings and measures in sham animals, meningeal DCE in males was increased after single impact and repetitive injury. In female mice, DCE was elevated relative to their baseline level after a single impact. One week after CHIMERA, the meningeal enhancement returned to below baseline for single injured male mice, but compared with uninjured mice remained elevated in both sexes in the multiple impact groups. Pre-DCE meningeal T2-weighted relaxation time was increased only after 1 × CHIMERA in injured mice. Since vision is impaired after CHIMERA, visual pathway regions were analyzed through imaging and glial fibrillary acidic protein (GFAP) histology. Initial DCE in the lateral geniculate nucleus (LGN) and superior colliculus (SC) and T2 increases in the optic tract (OPT) and LGN were observed after injury with decreases in DCE and T2 1 week later. Astrogliosis was apparent in the OPT and SC with increased GFAP staining 7 days post-injury. To our knowledge, this is the first study to examine meningeal integrity after CHIMERA in both male and female rodents. DCE-MRI may serve as a useful approach for pre-clinical models of meningeal injury that will enable further evaluation of the underlying mechanisms.

Fibroblast growth factor 21 associating with serotonin and dopamine in the cerebrospinal fluid predicts impulsivity in healthy subjects

Background

Impulsivity is more commonly reported in subjects with mental disorders compared to healthy subjects, suggesting a potential application of impulsivity in predicting impulsivity-related mental disorders. However, no biomarker of impulsivity available so far. This study explored the association between cerebrospinal fluid (CSF) fibroblast growth factor 21 (FGF21), a key hormonal mediator of the stress response, and impulsivity in healthy subjects.

Methods

A total of 126 healthy persons subjected to surgery of anterior cruciate ligament were recruited in the present study. The impulsiveness of the subjects was evaluated by the Chinese version of the Barratt Impulsiveness Scale (BIS)-11 before surgery. CSF and blood samples of the subjects were collected before spinal anesthesia for surgery. The levels of FGF21, serotonin and dopamine in CSF and the level of FGF21 in blood of the subjects were measured by ELISA using commercial kits.

Results

Negative correlations were found between BIS-11 total score and either FGF21, serotonin or dopamine in CSF. However, BIS-11 total score was not correlated with FGF21 in blood. In addition, FGF21 was positively correlated with serotonin and dopamine in CSF, respectively. Multivariable linear regression models indicated that the decrease of FGF21 level associating with the decrease of serotonin and dopamine level in CSF contributed to the higher impulsivity. Furthermore, receiver operating characteristic curve (ROC) analysis indicated an important role of CSF FGF21 predicting high impulsivity.

Conclusions

FGF21, serotonin and dopamine in CSF associate with impulsivity in opposite directions. The decrease of CSF FGF21 is related to higher impulsivity, and indicate that CSF FGF21 may predict impulsivity in healthy subjects.

Higher n-3 Polyunsaturated Fatty Acid Diet Improves Long-Term Neuropathological and Functional Outcome after Repeated Mild Traumatic Brain Injury

Repeated mild traumatic brain injury (TBI) can cause persistent neuropathological effects and is a major risk factor for chronic traumatic encephalopathy. PUFAs (n-3 polyunsaturated fatty acids) were shown to improve acute TBI outcomes in single-injury models in most cases. In this study, we demonstrate positive effects of dietary n-3 PUFA on long-term neuropathological and functional outcome in a clinically relevant model of repeated mild TBI using the Closed-Head Impact Model of Engineered Rotational Acceleration (CHIMERA). Adult mice, reared on n-3 PUFA adequate (higher n-3 PUFA) or deficient (lower n-3 PUFA) diets, were given a mild CHIMERA daily for 3 consecutive days. At 2 months after injury, visual function and spatial memory were evaluated. Glia cell activation was assessed by immunostaining using antibodies of ionized calcium-binding adaptor molecule 1 and glial fibrillary acidic protein, and axonal damage was examined using silver staining. Repeated CHIMERA (rCHIMERA)-induced gliosis was significantly suppressed in the optic tract, corpus callosum, and hippocampus of mice fed the n-3 PUFA adequate diet compared to the deficient diet group. Considerable axonal damage was detected in the optic tract after rCHIMERA, but the adequate diet group displayed less axonal damage compared to the deficient diet group. rCHIMERA induced a drastic reduction in N1 amplitude of the visual evoked potential in both diet groups and the a-wave amplitude of the electroretinogram in the deficient diet group. However, reduction of N1 and a-wave amplitude were less severe in the adequate diet group. The Morris water maze probe test indicated a significant decrease in the number of platform crossings in the deficient diet group compared to the adequate group. In summary, dietary n-3 PUFA can attenuate persistent glial cell activation and axonal damage and improve deficits in visual function and spatial memory after repeated mild TBI. These data support the neuroprotective potential of a higher n-3 PUFA diet in ameliorating the adverse outcome of repeated mild TBI.

Repetitive Mild Traumatic Brain Injury in Rats Impairs Cognition, Enhances Prefrontal Cortex Neuronal Activity, and Reduces Pre-synaptic Mitochondrial Function

A major hurdle preventing effective interventions for patients with mild traumatic brain injury (mTBI) is the lack of known mechanisms for the long-term cognitive impairment that follows mTBI. The closed head impact model of repeated engineered rotational acceleration (rCHIMERA), a non-surgical animal model of repeated mTBI (rmTBI), mimics key features of rmTBI in humans. Using the rCHIMERA in rats, this study was designed to characterize rmTBI-induced behavioral disruption, underlying electrophysiological changes in the medial prefrontal cortex (mPFC), and associated mitochondrial dysfunction. Rats received 6 closed-head impacts over 2 days at 2 Joules of energy. Behavioral testing included automated analysis of behavior in open field and home-cage environments, rotarod test for motor skills, novel object recognition, and fear conditioning. Following rmTBI, rats spent less time grooming and less time in the center of the open field arena. Rats in their home cage had reduced inactivity time 1 week after mTBI and increased exploration time 1 month after injury. Impaired associative fear learning and memory in fear conditioning test, and reduced short-term memory in novel object recognition test were found 4 weeks after rmTBI. Single-unit in vivo recordings showed increased neuronal activity in the mPFC after rmTBI, partially attributable to neuronal disinhibition from reduced inhibitory synaptic transmission, possibly secondary to impaired mitochondrial function. These findings help validate this rat rmTBI model as replicating clinical features, and point to impaired mitochondrial functions after injury as causing imbalanced synaptic transmission and consequent impaired long-term cognitive dysfunction.

GPR110 ligands reduce chronic optic tract gliosis and visual deficit following repetitive mild traumatic brain injury in mice

BACKGROUND

Repetitive mild traumatic brain injury (mTBI) can result in chronic visual dysfunction. G-protein receptor 110 (GPR110, ADGRF1) is the target receptor of N-docosahexaenoylethanolamine (synaptamide) mediating the anti-neuroinflammatory function of synaptamide. In this study, we evaluated the effect of an endogenous and a synthetic ligand of GPR110, synaptamide and (4Z,7Z,10Z,13Z,16Z,19Z)-N-(2-hydroxy-2-methylpropyl) docosa-4,7,10,13,16,19-hexaenamide (dimethylsynaptamide, A8), on the mTBI-induced long-term optic tract histopathology and visual dysfunction using Closed-Head Impact Model of Engineered Rotational Acceleration (CHIMERA), a clinically relevant model of mTBI.

METHODS

The brain injury in wild-type (WT) and GPR110 knockout (KO) mice was induced by CHIMERA applied daily for 3 days, and GPR110 ligands were intraperitoneally injected immediately following each impact. The expression of GPR110 and proinflammatory mediator tumor necrosis factor (TNF) in the brain was measured by using real-time quantitative reverse transcription polymerase chain reaction (qRT-PCR) in an acute phase. Chronic inflammatory responses in the optic tract and visual dysfunction were assessed by immunostaining for Iba-1 and GFAP and visual evoked potential (VEP), respectively. The effect of GPR110 ligands in vitro was evaluated by the cyclic adenosine monophosphate (cAMP) production in primary microglia isolated from adult WT or KO mouse brains.

RESULTS

CHIMERA injury acutely upregulated the GPR110 and TNF gene level in mouse brain. Repetitive CHIMERA (rCHIMERA) increased the GFAP and Iba-1 immunostaining of glia cells and silver staining of degenerating axons in the optic tract with significant reduction of N1 amplitude of visual evoked potential at up to 3.5 months after injury. Both GPR110 ligands dose- and GPR110-dependently increased cAMP in cultured primary microglia with A8, a ligand with improved stability, being more effective than synaptamide. Intraperitoneal injection of A8 at 1 mg/kg or synaptamide at 5 mg/kg significantly reduced the acute expression of TNF mRNA in the brain and ameliorated chronic optic tract microgliosis, astrogliosis, and axonal degeneration as well as visual deficit caused by injury in WT but not in GPR110 KO mice.

CONCLUSION

Our data demonstrate that ligand-induced activation of the GPR110/cAMP system upregulated after injury ameliorates the long-term optic tract histopathology and visual impairment caused by rCHIMERA. Based on the anti-inflammatory nature of GPR110 activation, we suggest that GPR110 ligands may have therapeutic potential for chronic visual dysfunction associated with mTBI.

Long-Term Effects of Repetitive Mild Traumatic Injury on the Visual System in Wild-Type and TDP-43 Transgenic Mice

Little is known about the impairments and pathological changes in the visual system in mild brain trauma, especially repetitive mild traumatic brain injury (mTBI). The goal of this study was to examine and compare the effects of repeated head impacts on the neurodegeneration, axonal integrity, and glial activity in the optic tract (OT), as well as on neuronal preservation, glial responses, and synaptic organization in the lateral geniculate nucleus (LGN) and superior colliculus (SC), in wild-type mice and transgenic animals with overexpression of human TDP-43 mutant protein (TDP-43^G348C) at 6 months after repeated closed head traumas. Animals were also assessed in the Barnes maze (BM) task. Neurodegeneration, axonal injury, and gliosis were detected in the OT of the injured animals of both genotypes. In the traumatized mice, myelination of surviving axons was mostly preserved, and the expression of neurofilament light chain was unaffected. Repetitive mTBI did not induce changes in the LGN and the SC, nor did it affect the performance of the BM task in the traumatized wild-type and TDP-43 transgenic mice. Differences in neuropathological and behavioral assessments between the injured wild-type and TDP-43^G348C mice were not revealed. Results of the current study suggest that repetitive mTBI was associated with chronic damage and inflammation in the OT in wild-type and TDP-43^G348C mice, which were not accompanied with behavioral problems and were not affected by the TDP-43 genotype, while the LGN and the SC remained preserved in the used experimental conditions.

Inflammation in Traumatic Brain Injury

There is an increasing evidence that inflammation contributes to clinical and functional outcomes in traumatic brain injury (TBI). Many successful target-engaging, lesion-reducing, symptom-alleviating, and function-improving interventions in animal models of TBI have failed to show efficacy in clinical trials. Timing and immunological context are paramount for the direction, quality, and intensity of immune responses to TBI and the resulting neuroanatomical, clinical, and functional course. We present components of the immune system implicated in TBI, potential immune targets, and target-engaging interventions. The main objective of our article is to point toward modifiable molecular and cellular mechanisms that may modify the outcomes in TBI, and contribute to increasing the translational value of interventions that have been identified in animal models of TBI.

An update on the association between traumatic brain injury and Alzheimer's disease: Focus on Tau pathology and synaptic dysfunction

L.P. Li, J.W. Liang and H.J. Fu. An update on the association between traumatic brain injury and Alzheimer's disease: Focus on Tau pathology and synaptic dysfunction. NEUROSCI BIOBEHAV REVXXX-XXX,2020.-Traumatic brain injury (TBI) and Alzheimer's disease (AD) are devastating conditions that have long-term consequences on individual's cognitive functions. Although TBI has been considered a risk factor for the development of AD, the link between TBI and AD is still in debate. Aggregation of hyperphosphorylated tau and intercorrelated synaptic dysfunction, two key pathological elements in both TBI and AD, play a pivotal role in mediating neurodegeneration and cognitive deficits, providing a mechanistic link between these two diseases. In the first part of this review, we analyze the experimental literatures on tau pathology in various TBI models and review the distribution, biological features and mechanisms of tau pathology following TBI with implications in AD pathogenesis. In the second part, we review evidences of TBI-mediated structural and functional impairments in synapses, with a focus on the overlapped mechanisms underlying synaptic abnormalities in both TBI and AD. Finally, future perspectives are proposed for uncovering the complex relationship between TBI and neurodegeneration, and developing potential therapeutic avenues for alleviating cognitive deficits after TBI.

The closed-head impact model of engineered rotational acceleration (CHIMERA) as an application for traumatic brain injury pre-clinical research: A status report

Closed-head traumatic brain injury (TBI) is a worldwide concern with increasing prevalence and cost to society. Rotational acceleration is a primary mechanism in TBI that results from tissue strains that give rise to diffuse axonal injury. The Closed-Head Impact Model of Engineered Rotational Acceleration (CHIMERA) was recently introduced as a method for the study of impact acceleration effects in pre-clinical TBI research. This review provides a survey of the published literature implementing the CHIMERA device and describes pathological, imaging, neurophysiological, and behavioral findings. Findings show CHIMERA inflicts damage in white matter tracts as a key area of injury. Behaviorally, repeated studies have shown motor deficits and more chronic cognitive effects after CHIMERA injury. Good progress with model application has been accomplished by investigators attending to what is required for model validation. However, the majority of CHIMERA studies only utilize adult male mice. To further establish this model, more work with female animals and various age groups need to be performed, as well as studies to further establish and standardize methodologies for validation of the models for clinical relevance. Common data elements to standardize the reporting methodology for the CHIMERA literature are suggested.

Neurochemical organization of the ventral striatum's olfactory tubercle

The ventral striatum is a collection of brain structures, including the nucleus accumbens, ventral pallidum and the olfactory tubercle (OT). While much attention has been devoted to the nucleus accumbens, a comprehensive understanding of the ventral striatum and its contributions to neurological diseases requires an appreciation for the complex neurochemical makeup of the ventral striatum's other components. This review summarizes the rich neurochemical composition of the OT, including the neurotransmitters, neuromodulators and hormones present. We also address the receptors and transporters involved in each system as well as their putative functional roles. Finally, we end with briefly reviewing select literature regarding neurochemical changes in the OT in the context of neurological disorders, specifically neurodegenerative disorders. By overviewing the vast literature on the neurochemical composition of the OT, this review will serve to aid future research into the neurobiology of the ventral striatum.

Unilateral parietal brain injury increases risk-taking on a rat gambling task

Traumatic brain injury (TBI) affects millions of individuals every year. Many of these injuries lead to lasting effects, particularly impairments in domains broadly classified as executive functions, such as impulse control and decision-making. While these impairments have been historically associated with frontal brain damage, other injuries such as concussion or parietal injury also contribute to similar dysfunction. However, it is unknown whether animal models of TBI would replicate these broad effects that are observed in human patients. In the current study, we delivered a unilateral parietal controlled cortical impact injury and assessed the performance of rats on a motoric task (rotarod) and a test of decision-making and impulsivity (rodent gambling task). TBI rats demonstrated significant motor impairments on the rotarod task; however, this did not extend to difficulties inhibiting motor actions (impulsivity). In addition, TBI caused chronic alterations to risk-based decision-making, extending out to 12 weeks post-injury. Specifically, rats with TBI preferred the riskiest, and most suboptimal option over all others. The current data suggest that models of unilateral TBI are sufficient for replicating some aspects of executive dysfunction (risky decision-making), while others are limited to frontal damage (impulsivity). These models may be used to develop therapeutics targeted at the chronic post-injury period when these symptoms often manifest in patients, a critically understudied area in preclinical TBI research.

Increased severity of the CHIMERA model induces acute vascular injury, sub-acute deficits in memory recall, and chronic white matter gliosis

Traumatic brain injury (TBI) is a leading cause of death and disability in modern societies. Diffuse axonal and vascular injury are nearly universal consequences of mechanical energy impacting the head and contribute to disability throughout the injury severity spectrum. CHIMERA (Closed Head Impact Model of Engineered Rotational Acceleration) is a non-surgical, impact-acceleration model of rodent TBI that reliably produces diffuse axonal injury characterized by white matter gliosis and axonal damage. At impact energies up to 0.7 joules, which result in mild TBI in mice, CHIMERA does not produce detectable vascular or grey matter injury. This study was designed to expand CHIMERA's capacity to induce more severe injuries, including vascular damage and grey matter gliosis. This was made possible by designing a physical interface positioned between the piston and animal's head to allow higher impact energies to be transmitted to the head without causing skull fracture. Here, we assessed interface-assisted single CHIMERA TBI at 2.5 joules in wild-type mice using a study design that spanned 6 h-60 d time points. Injured animals displayed robust acute neurological deficits, elevated plasma total tau and neurofilament-light levels, transiently increased proinflammatory cytokines in brain tissue, blood-brain barrier (BBB) leakage and microstructural vascular abnormalities, and grey matter microgliosis. Memory deficits were evident at 30 d and resolved by 60 d. Intriguingly, white matter injury was not remarkable at acute time points but evolved over time, with white matter gliosis being most extensive at 60 d. Interface-assisted CHIMERA thus enables experimental modeling of distinct endophenotypes of TBI that include acute vascular and grey matter injury in addition to chronic evolution of white matter damage, similar to the natural history of human TBI.

Multiple Mild Traumatic Brain Injuries Lead to Visual Dysfunction in a Mouse Model

Visual dysfunction is a common occurrence after traumatic brain injury (TBI). We investigated in this study effects of single or multiple mild TBI on visual function in mice using a closed head injury model that permits unconstrained head movement after impact. Adult mice were briefly anesthetized with isoflurane and given one or three mild TBI with the closed head injury by mechanically engineered rotational acceleration (CHIMERA) device with an interinjury interval of 24 h. Mice were then tested in the Morris water maze, visual cliff, and open field tests from day 19 to day 32 and for visual evoked potential at 5 weeks after the last injury and euthanized. Mice with multiple TBI showed impaired performance in the visible platform water maze test and had increased errors in the visual cliff test. Further, there was a graded difference in visual evoked potential, with the single injury mice showing modest reduction in N1 amplitude whereas the multiple injuries produced significant reduction compared to sham and single injury groups. The optic tract of the injured mice showed increases in glial cell immunostaining. The increase in glial fibrillary acid protein immunostaining reached statistical significance for both injured groups whereas the ionized calcium binding adaptor molecule 1 immunostaining was only significantly increased in the optic tract of repeatedly injured mice. These results indicate that multiple injuries using CHIMERA may result in visual deficits, which can affect certain behavioral performances. The change in vision may be a useful marker when monitoring repeated TBI outcome and screening for protective agents from TBI.