Characteristics of an explosive blast-induced brain injury in an experimental model

EEG hyperexcitability and hyperconnectivity linked to GABAergic inhibitory interneuron loss following traumatic brain injury

Traumatic brain injury represents a significant global health burden and has the highest prevalence among neurological disorders. Even mild traumatic brain injury can induce subtle, long-lasting changes that increase the risk of future neurodegeneration. Importantly, this can be challenging to detect through conventional neurological assessment. This underscores the need for more sensitive diagnostic tools, such as electroencephalography, to uncover opportunities for therapeutic intervention. Progress in the field has been hindered by a lack of studies linking mechanistic insights at the microscopic level from animal models to the macroscale phenotypes observed in clinical imaging. Our study addresses this gap by investigating a rat model of mild blast traumatic brain injury using both immunohistochemical staining of inhibitory interneurons and translationally relevant electroencephalography recordings. Although we observed no pronounced effects immediately post-injury, chronic time points revealed broadband hyperexcitability and increased connectivity, accompanied by decreased density of inhibitory interneurons. This pattern suggests a disruption in the balance between excitation and inhibition, providing a crucial link between cellular mechanisms and clinical hallmarks of injury. Our findings have significant implications for the diagnosis, monitoring, and treatment of traumatic brain injury. The emergence of electroencephalography abnormalities at chronic time points, despite the absence of immediate effects, highlights the importance of long-term monitoring in traumatic brain injury patients. The observed decrease in inhibitory interneuron density offers a potential cellular mechanism underlying the electroencephalography changes and may represent a target for therapeutic intervention. This study demonstrates the value of combining cellular-level analysis with macroscale neurophysiological recordings in animal models to elucidate the pathophysiology of traumatic brain injury. Future research should focus on translating these findings to human studies and exploring potential therapeutic strategies targeting the excitation-inhibition imbalance in traumatic brain injury.

A Systematic Review of Traumatic Brain Injury in Modern Rodent Models: Current Status and Future Prospects

According to the Centers for Disease Control and Prevention (CDC), the national public health agency of the United States, traumatic brain injury is among the leading causes of mortality and disability worldwide. The consequences of TBI include diffuse brain atrophy, local post-traumatic atrophy, arachnoiditis, pachymeningitis, meningocerebral cicatrices, cranial nerve lesions, and cranial defects. In 2019, the economic cost of injuries in the USA alone was USD 4.2 trillion, which included USD 327 billion for medical care, USD 69 billion for work loss, and USD 3.8 trillion for the value of statistical life and quality of life losses. More than half of this cost (USD 2.4 trillion) was among working-age adults (25-64 years old). Currently, the development of new diagnostic approaches and the improvement of treatment techniques require further experimental studies focused on modeling TBI of varying severity.

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.

Twenty Years of Blast-Induced Neurotrauma: Current State of Knowledge

Blast-induced neurotrauma (BINT) is an important injury paradigm of neurotrauma research. This short communication summarizes the current knowledge of BINT. We divide the BINT research into several broad categories-blast wave generation in laboratory, biomechanics, pathology, behavioral outcomes, repetitive blast in animal models, and clinical and neuroimaging investigations in humans. Publications from 2000 to 2023 in each subdomain were considered. The analysis of the literature has brought out salient aspects. Primary blast waves can be simulated reasonably in a laboratory using carefully designed shock tubes. Various biomechanics-based theories of BINT have been proposed; each of these theories may contribute to BINT by generating a unique biomechanical signature. The injury thresholds for BINT are in the nascent stages. Thresholds for rodents are reasonably established, but such thresholds (guided by primary blast data) are unavailable in humans. Single blast exposure animal studies suggest dose-dependent neuronal pathologies predominantly initiated by blood-brain barrier permeability and oxidative stress. The pathologies were typically reversible, with dose-dependent recovery times. Behavioral changes in animals include anxiety, auditory and recognition memory deficits, and fear conditioning. The repetitive blast exposure manifests similar pathologies in animals, however, at lower blast overpressures. White matter irregularities and cortical volume and thickness alterations have been observed in neuroimaging investigations of military personnel exposed to blast. Behavioral changes in human cohorts include sleep disorders, poor motor skills, cognitive dysfunction, depression, and anxiety. Overall, this article provides a concise synopsis of current understanding, consensus, controversies, and potential future directions.

Role of Deployment History on the Association Between Epilepsy and Traumatic Brain Injury in Post-9/11 Era US Veterans

BACKGROUND AND OBJECTIVES

Traumatic brain injury (TBI) is a well-established epilepsy risk factor and is common among service members. Deployment-related TBI, where combat/blast may be more common, may have different outcomes than nondeployment-related TBI. This work examined associations of all TBI exposures (not just combat), and epilepsy, while adjusting for comorbidities associated with epilepsy, among veterans by deployment status.

METHODS

The cohort included post-9/11 veterans with ≥2 years of care in both Veterans Health Administration and Defense Health Agency systems. We identified epilepsy using ICD-9/10-CM codes, antiseizure medication, and service-connected disability for epilepsy. We conducted a logistic regression model with interaction terms for conditions by deployment history that adjusted for demographics and military characteristics.

RESULTS

The cohort (n = 938,890) included post-9/11 veterans of whom 27,436 (2.92%) had epilepsy. Most veterans had a history of deployment (70.64%), referred to as "deployed." Epilepsy was more common among veterans who were never deployed ("nondeployed") (3.85% vs 2.54%). Deployed veterans were more likely to have had TBI, compared with the nondeployed veterans (33.94% vs 14.24%), but nondeployed veterans with moderate/severe TBI had higher odds of epilepsy compared with deployed veterans (adjusted odds ratio [aOR] 2.92, 95% CI 2.68-3.17 vs aOR 2.01, 95% CI 1.91-2.11). Penetrating TBI had higher odds of epilepsy among the deployed veterans (aOR 5.33, 95% CI 4.89-5.81), whereas the odds of epilepsy for mild TBI did not significantly differ by deployment status. Although most neurologic conditions were more prevalent among the nondeployed veterans, they were often associated with higher odds of epilepsy in the deployed veterans.

DISCUSSION

Deployment history had a significant differential impact on epilepsy predictors. As expected, penetrating TBI had a greater epilepsy impact among deployed veterans perhaps due to combat/blast. Some epilepsy predictors (moderate/severe TBI, multiple sclerosis, and Parkinson disease) had a stronger association in the nondeployed veterans suggesting a potential healthy warrior effect in which such conditions preclude deployment. Other neurologic conditions (e.g., brain tumor, Alzheimer disease/frontotemporal dementia) had a greater epilepsy impact in the deployed veterans. This may be attributable to deployment-related exposures (combat injury, occupational exposures). A better understanding of deployment effects is critical to provide targeted epilepsy prevention in veterans and military service members.

Porcine Astrocytes and Their Relevance for Translational Neurotrauma Research

Astrocytes are essential to virtually all brain processes, from ion homeostasis to neurovascular coupling to metabolism, and even play an active role in signaling and plasticity. Astrocytic dysfunction can be devastating to neighboring neurons made inherently vulnerable by their polarized, excitable membranes. Therefore, correcting astrocyte dysfunction is an attractive therapeutic target to enhance neuroprotection and recovery following acquired brain injury. However, the translation of such therapeutic strategies is hindered by a knowledge base dependent almost entirely on rodent data. To facilitate additional astrocytic research in the translatable pig model, we present a review of astrocyte findings from pig studies of health and disease. We hope that this review can serve as a road map for intrepid pig researchers interested in studying astrocyte biology.

Potential Biomarkers in Experimental Animal Models for Traumatic Brain Injury

Traumatic brain injury (TBI) is a complex and multifaceted disorder that has become a significant public health concern worldwide due to its contribution to mortality and morbidity. This condition encompasses a spectrum of injuries, including axonal damage, contusions, edema, and hemorrhage. Unfortunately, specific effective therapeutic interventions to improve patient outcomes following TBI are currently lacking. Various experimental animal models have been developed to mimic TBI and evaluate potential therapeutic agents to address this issue. These models are designed to recapitulate different biomarkers and mechanisms involved in TBI. However, due to the heterogeneous nature of clinical TBI, no single experimental animal model can effectively mimic all aspects of human TBI. Accurate emulation of clinical TBI mechanisms is also tricky due to ethical considerations. Therefore, the continued study of TBI mechanisms and biomarkers, of the duration and severity of brain injury, treatment strategies, and animal model optimization is necessary. This review focuses on the pathophysiology of TBI, available experimental TBI animal models, and the range of biomarkers and detection methods for TBI. Overall, this review highlights the need for further research to improve patient outcomes and reduce the global burden of TBI.

Double Blast Wave Primary Effect on Synaptic, Glymphatic, Myelin, Neuronal and Neurovascular Markers

Explosive blasts are associated with neurological consequences as a result of blast waves impact on the brain. Yet, the neuropathologic and molecular consequences due to blast waves vs. blunt-TBI are not fully understood. An explosive-driven blast-generating system was used to reproduce blast wave exposure and examine pathological and molecular changes generated by primary wave effects of blast exposure. We assessed if pre- and post-synaptic (synaptophysin, PSD-95, spinophilin, GAP-43), neuronal (NF-L), glymphatic (LYVE1, podoplanin), myelin (MBP), neurovascular (AQP4, S100β, PDGF) and genomic (DNA polymerase-β, RNA polymerase II) markers could be altered across different brain regions of double blast vs. sham animals. Twelve male rats exposed to two consecutive blasts were compared to 12 control/sham rats. Western blot, ELISA, and immunofluorescence analyses were performed across the frontal cortex, hippocampus, cerebellum, and brainstem. The results showed altered levels of AQP4, S100β, DNA-polymerase-β, PDGF, synaptophysin and PSD-95 in double blast vs. sham animals in most of the examined regions. These data indicate that blast-generated changes are preferentially associated with neurovascular, glymphatic, and DNA repair markers, especially in the brainstem. Moreover, these changes were not accompanied by behavioral changes and corroborate the hypothesis for which an asymptomatic altered status is caused by repeated blast exposures.

Nanoparticle-based drug delivery for the treatment of traumatic brain injury

INTRODUCTION

Traumatic brain injuries (TBIs) impact the breadth of society and remain without any approved pharmacological treatments. Despite successful Phase II clinical trials, the failure of many Phase III clinical trials may be explained by insufficient drug targeting and retention, preventing the proper attainment of an observable dosage threshold. To address this challenge, nanoparticles can be functionalized to protect pharmacological payloads, improve targeted drug delivery to sites of injury, and can be combined with supportive scaffolding to improve secondary outcomes.

AREAS COVERED

This review briefly covers the pathophysiology of TBIs and their subtypes, the current pre-clinical and clinical management strategies, explores the common models of focal, diffuse, and mixed traumatic brain injury employed in experimental animals, and surveys the existing literature on nanoparticles developed to treat TBIs.

EXPERT OPINION

Nanoparticles are well suited to improve secondary outcomes as their multifunctionality and customizability enhance their potential for efficient targeted delivery, payload protection, increased brain penetration, low off-target toxicity, and biocompatibility in both acute and chronic timescales.

Traumatic brain injury and the development of parkinsonism: Understanding pathophysiology, animal models, and therapeutic targets

The clinical translation of therapeutic approaches to combat debilitating neurodegenerative conditions, such as Parkinson's disease (PD), remains as an urgent unmet challenge. The strong molecular association between the pathogenesis of traumatic brain injury (TBI) and the development of parkinsonism in humans has been well established. Therefore, a lot of ongoing research aims to investigate this pathology overlap in-depth, to exploit the common targets of TBI and PD for development of more effective and long-term treatment strategies. This review article intends to provide a detailed background on TBI pathophysiology and its established overlap with PD with an additional emphasis on the recent findings about their effect on perivascular clearance. Although, the traditional animal models of TBI and PD are still being considered, there is a huge focus on the development of combinatory hybrid animal models coupling concussion with the pre-established PD models for a better recapitulation of the human context of PD pathogenesis. Lastly, the therapeutic targets for TBI and PD, and the contemporary research involving exosomes, DNA vaccines, miRNA, gene therapy and gene editing for the development of potential candidates are discussed, along with the recent development of lesser invasive and promising central nervous system (CNS) drug delivery strategies.

Interface astrogliosis in contact sport head impacts and military blast exposure

Exposure to military blast and repetitive head impacts (RHI) in contact sports is associated with increased risk of long-term neurobehavioral sequelae and cognitive deficits, and the neurodegenerative disease chronic traumatic encephalopathy (CTE). At present, the exact pathogenic mechanisms of RHI and CTE are unknown, and no targeted therapies are available. Astrocytes have recently emerged as key mediators of the multicellular response to head trauma. Here, we investigated interface astrogliosis in blast and impact neurotrauma, specifically in the context of RHI and early stage CTE. We compared postmortem brain tissue from former military veterans with a history of blast exposure with and without a neuropathological diagnosis of CTE, former American football players with a history of RHI with and without a neuropathological diagnosis of CTE, and control donors without a history of blast, RHI exposure or CTE diagnosis. Using quantitative immunofluorescence, we found that astrogliosis was higher at the grey-white matter interface in the dorsolateral frontal cortex, with mixed effects at the subpial surface and underlying cortex, in both blast and RHI donors with and without CTE, compared to controls. These results indicate that certain astrocytic alterations are associated with both impact and blast neurotrauma, and that different astroglial responses take place in distinct brain regions.

Air-Evacuation-Relevant Hypobaria Following Traumatic Brain Injury Plus Hemorrhagic Shock in Rats Increases Mortality and Injury to the Gut, Lungs, and Kidneys

ABSTRACT

Rats exposed to hypobaria equivalent to what occurs during aeromedical evacuation within a few days after isolated traumatic brain injury exhibit greater neurologic injury than those remaining at sea level. Moreover, administration of excessive supplemental O2 during hypobaria further exacerbates brain injury. This study tested the hypothesis that exposure of rats to hypobaria following controlled cortical impact (CCI)-induced brain injury plus mild hemorrhagic shock worsens multiple organ inflammation and associated mortality. In this study, at 24 h after CCI plus hemorrhagic shock, rats were exposed to either normobaria (sea level) or hypobaria (=8,000 ft altitude) for 6 h under normoxic or hyperoxic conditions. Injured rats exhibited mortality ranging from 30% for those maintained under normobaria and normoxia to 60% for those exposed to 6 h under hypobaric and hyperoxia. Lung histopathology and neutrophil infiltration at 2 days postinjury were exacerbated by hypobaria and hyperoxia. Gut and kidney inflammation at 30 days postinjury were also worsened by hypobaric hyperoxia. In conclusion, exposure of rats after brain injury and hemorrhagic shock to hypobaria or hyperoxia results in increased mortality. Based on gut, lung, and kidney histopathology at 2 to 30 days postinjury, increased mortality is consistent with multi-organ inflammation. These findings support epidemiological studies indicating that increasing aircraft cabin pressures to 4,000 ft altitude (compared with standard 8,000 ft) and limiting excessive oxygen administration will decrease critical complications during and following aeromedical transport.

Low-level blast exposure induces chronic vascular remodeling, perivascular astrocytic degeneration and vascular-associated neuroinflammation

Cerebral vascular injury as a consequence of blast-induced traumatic brain injury is primarily the result of blast wave-induced mechanical disruptions within the neurovascular unit. In rodent models of blast-induced traumatic brain injury, chronic vascular degenerative processes are associated with the development of an age-dependent post-traumatic stress disorder-like phenotype. To investigate the evolution of blast-induced chronic vascular degenerative changes, Long-Evans rats were blast-exposed (3 × 74.5 kPa) and their brains analyzed at different times post-exposure by X-ray microcomputed tomography, immunohistochemistry and electron microscopy. On microcomputed tomography scans, regional cerebral vascular attenuation or occlusion was observed as early as 48 h post-blast, and cerebral vascular disorganization was visible at 6 weeks and more accentuated at 13 months post-blast. Progression of the late-onset pathology was characterized by detachment of the endothelial and smooth muscle cellular elements from the neuropil due to degeneration and loss of arteriolar perivascular astrocytes. Development of this pathology was associated with vascular remodeling and neuroinflammation as increased levels of matrix metalloproteinases (MMP-2 and MMP-9), collagen type IV loss, and microglial activation were observed in the affected vasculature. Blast-induced chronic alterations within the neurovascular unit should affect cerebral blood circulation, glymphatic flow and intramural periarterial drainage, all of which may contribute to development of the blast-induced behavioral phenotype. Our results also identify astrocytic degeneration as a potential target for the development of therapies to treat blast-induced brain injury.

Beirut Ammonium Nitrate Blast: Analysis, Review, and Recommendations

A massive chemical detonation occurred on August 4, 2020 in the Port of Beirut, Lebanon. An uncontrolled fire in an adjacent warehouse ignited ~2,750 tons of Ammonium Nitrate (AN), producing one of the most devastating blasts in recent history. The blast supersonic pressure and heat wave claimed the lives of 220 people and injured more than 6,500 instantaneously, with severe damage to the nearby dense residential and commercial areas. This review represents one of the in-depth reports to provide a detailed analysis of the Beirut blast and its health and environmental implications. It further reviews prior AN incidents and suggests actionable recommendations and strategies to optimize chemical safety measures, improve emergency preparedness, and mitigate the delayed clinical effects of blast and toxic gas exposures. These recommended actionable steps offer a starting point for government officials and policymakers to build frameworks, adopt regulations, and implement chemical safety protocols to ensure safe storage of hazardous materials as well as reorganizing healthcare system disaster preparedness to improve emergency preparedness in response to similar large-scale disasters and promote population safety. Future clinical efforts should involve detailed assessment of physical injuries sustained by blast victims, with systemic mitigation and possible treatment of late blast effects involving individuals, communities and the region at large.

Localizing Clinical Patterns of Blast Traumatic Brain Injury Through Computational Modeling and Simulation

Blast traumatic brain injury is ubiquitous in modern military conflict with significant morbidity and mortality. Yet the mechanism by which blast overpressure waves cause specific intracranial injury in humans remains unclear. Reviewing of both the clinical experience of neurointensivists and neurosurgeons who treated service members exposed to blast have revealed a pattern of injury to cerebral blood vessels, manifested as subarachnoid hemorrhage, pseudoaneurysm, and early diffuse cerebral edema. Additionally, a seminal neuropathologic case series of victims of blast traumatic brain injury (TBI) showed unique astroglial scarring patterns at the following tissue interfaces: subpial glial plate, perivascular, periventricular, and cerebral gray-white interface. The uniting feature of both the clinical and neuropathologic findings in blast TBI is the co-location of injury to material interfaces, be it solid-fluid or solid-solid interface. This motivates the hypothesis that blast TBI is an injury at the intracranial mechanical interfaces. In order to investigate the intracranial interface dynamics, we performed a novel set of computational simulations using a model human head simplified but containing models of gyri, sulci, cerebrospinal fluid (CSF), ventricles, and vasculature with high spatial resolution of the mechanical interfaces. Simulations were performed within a hybrid Eulerian—Lagrangian simulation suite (CTH coupled via Zapotec to Sierra Mechanics). Because of the large computational meshes, simulations required high performance computing resources. Twenty simulations were performed across multiple exposure scenarios—overpressures of 150, 250, and 500 kPa with 1 ms overpressure durations—for multiple blast exposures (front blast, side blast, and wall blast) across large variations in material model parameters (brain shear properties, skull elastic moduli). All simulations predict fluid cavitation within CSF (where intracerebral vasculature reside) with cavitation occurring deep and diffusely into cerebral sulci. These cavitation events are adjacent to high interface strain rates at the subpial glial plate. Larger overpressure simulations (250 and 500kPa) demonstrated intraventricular cavitation—also associated with adjacent high periventricular strain rates. Additionally, models of embedded intraparenchymal vascular structures—with diameters as small as 0.6 mm—predicted intravascular cavitation with adjacent high perivascular strain rates. The co-location of local maxima of strain rates near several of the regions that appear to be preferentially damaged in blast TBI (vascular structures, subpial glial plate, perivascular regions, and periventricular regions) suggest that intracranial interface dynamics may be important in understanding how blast overpressures leads to intracranial injury.

Periventricular White Matter Alterations From Explosive Blast in a Large Animal Model: Mild Traumatic Brain Injury or "Subconcussive" Injury?

The neuropathology of mild traumatic brain injury in humans resulting from exposure to explosive blast is poorly understood as this condition is rarely fatal. A large animal model may better reflect the injury patterns in humans. We investigated the effect of explosive blasts on the constrained head minimizing the effects of whole head motion. Anesthetized Yucatan minipigs, with body and head restrained, were placed in a 3-walled test structure and exposed to 1, 2, or 3 explosive blast shock waves of the same intensity. Axonal injury was studied 3 weeks to 8 months postblast using β-amyloid precursor protein immunohistochemistry. Injury was confined to the periventricular white matter as early as 3-5 weeks after exposure to a single blast. The pattern was also present at 8 months postblast. Animals exposed to 2 and 3 blasts had more axonal injury than those exposed to a single blast. Although such increases in axonal injury may relate to the longer postblast survival time, it may also be due to the increased number of blast exposures. It is possible that the injury observed is due to a condition akin to mild traumatic brain injury or subconcussive injury in humans, and that periventricular injury may have neuropsychiatric implications.

Explosive-driven double-blast exposure: molecular, histopathological, and behavioral consequences

Traumatic brain injury generated by blast may induce long-term neurological and psychiatric sequelae. We aimed to identify molecular, histopathological, and behavioral changes in rats 2 weeks after explosive-driven double-blast exposure. Rats received two 30-psi (~ 207-kPa) blasts 24 h apart or were handled identically without blast. All rats were behaviorally assessed over 2 weeks. At Day 15, rats were euthanized, and brains removed. Brains were dissected into frontal cortex, hippocampus, cerebellum, and brainstem. Western blotting was performed to measure levels of total-Tau, phosphorylated-Tau (pTau), amyloid precursor protein (APP), GFAP, Iba1, αII-spectrin, and spectrin breakdown products (SBDP). Kinases and phosphatases, correlated with tau phosphorylation were also measured. Immunohistochemistry for pTau, APP, GFAP, and Iba1 was performed. pTau protein level was greater in the hippocampus, cerebellum, and brainstem and APP protein level was greater in cerebellum of blast vs control rats (p < 0.05). GFAP, Iba1, αII-spectrin, and SBDP remained unchanged. No immunohistochemical or neurobehavioral changes were observed. The dissociation between increased pTau and APP in different regions in the absence of neurobehavioral changes 2 weeks after double blast exposure is a relevant finding, consistent with human data showing that battlefield blasts might be associated with molecular changes before signs of neurological and psychiatric disorders manifest.

Cerebral Immunohistochemical Characterization of the H2S and the Oxytocin Systems in a Porcine Model of Acute Subdural Hematoma

The hydrogen sulfide (H₂S) and the oxytocin/oxytocin receptor (OT/OTR) systems interact in trauma and are implicated in vascular protection and regulation of fluid homeostasis. Acute brain injury is associated with pressure-induced edema formation, blood brain barrier disruption, and neuro-inflammation. The similarities in brain anatomy: size, gyrencephalic organization, skull structure, may render the pig a highly relevant model for translational medicine. Cerebral biomarkers for pigs for pathophysiological changes and neuro-inflammation are limited. The current study aims to characterize the localization of OT/OTR and the endogenous H₂S producing enzymes together with relevant neuro-inflammatory markers on available porcine brain tissue from an acute subdural hematoma (ASDH) model. In a recent pilot study, anesthetized pigs underwent ASDH by injection of 20 mL of autologous blood above the left parietal cortex and were resuscitated with neuro-intensive care measures. After 54 h of intensive care, the animals were sacrificed, the brain was removed and analyzed via immunohistochemistry. The endogenous H₂S producing enzymes cystathionine-ɤ-lyase (CSE) and cystathionine-β-synthase (CBS), the OTR, and OT were localized in neurons, vasculature and parenchyma at the base of sulci, where pressure-induced injury leads to maximal stress in the gyrencephalic brain. The pathophysiological changes in response to brain injury in humans and pigs, we show here, are comparable. We additionally identified modulators of brain injury to further characterize the pathophysiology of ASDH and which may indicate future therapeutic approaches.

Cavitation threshold evaluation of porcine cerebrospinal fluid using a Polymeric Split Hopkinson Pressure Bar-Confinement chamber apparatus

Studies investigating mild Traumatic Brain Injury (mTBI) in the military population using experimental head surrogates and Finite Element (FE) head models have demonstrated the existence of transient negative pressures occurring within the head at the contrecoup location to the blast wave impingement. It has been hypothesized that this negative pressure may cause cavitation of cerebrospinal fluid (CSF) and possibly lead to brain tissue damage from cavitation bubble collapse. The cavitation pressure threshold of human CSF is presently unknown, although existing FE studies in the literature have assumed a value of -100 kPa. In the present study, the cavitation threshold of degassed porcine CSF at body temperature (37 °C) was measured using a unique modified Polymeric Split Hopkinson Pressure Bar apparatus, and compared to thresholds of distilled water at various conditions. The loading pulse generated in the apparatus was comparable to experimentally measured pressures resulting from blast exposure, and those predicted by an FE model. The occurrence of cavitation was identified using high-speed imaging and the corresponding pressures were determined using a computational model of the apparatus that was previously developed and validated. The probability of cavitation was calculated (ISO/TS, 18506) from forty-one experimental tests on porcine CSF, representing an upper bound for in vivo CSF. The 50% probability of cavitation for CSF (-0.467 MPa ± 7%) was lower than that of distilled water (-1.37 MPa ± 16%) under the same conditions. The lesser threshold of CSF could be related to the constituents such as blood cells and proteins. The results of this study can be used to inform FE head models subjected to blast exposure and improve prediction of the potential for CSF cavitation and response of brain tissue.

Mammalian Models of Traumatic Brain Injury and a Place for Drosophila in TBI Research

Traumatic brain injury (TBI), caused by a sudden blow or jolt to the brain that disrupts normal function, is an emerging health epidemic with ∼2.5 million cases occurring annually in the United States that are severe enough to cause hospitalization or death. Most common causes of TBI include contact sports, vehicle crashes and domestic violence or war injuries. Injury to the central nervous system is one of the most consistent candidates for initiating the molecular and cellular cascades that result in Alzheimer's disease (AD), Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS). Not every TBI event is alike with effects varying from person to person. The majority of people recover from mild TBI within a short period of time, but repeated incidents can have deleterious long-lasting effects which depend on factors such as the number of TBIs sustained, time till medical attention, age, gender and genetics of the individual. Despite extensive research, many questions still remain regarding diagnosis, treatment, and prevention of long-term effects from TBI as well as recovery of brain function. In this review, we present an overview of TBI pathology, discuss mammalian models for TBI and focus on current methods using Drosophila melanogaster as a model for TBI study. The relatively small brain size (∼100,000 neurons and glia), conserved neurotransmitter signaling mechanisms and sophisticated genetics of Drosophila allows for cell biological, molecular and genetic analyses that are impractical in mammalian models of TBI.

Exposure to mild blast forces induces neuropathological effects, neurophysiological deficits and biochemical changes

Direct or indirect exposure to an explosion can induce traumatic brain injury (TBI) of various severity levels. Primary TBI from blast exposure is commonly characterized by internal injuries, such as vascular damage, neuronal injury, and contusion, without external injuries. Current animal models of blast-induced TBI (bTBI) have helped to understand the deleterious effects of moderate to severe blast forces. However, the neurological effects of mild blast forces remain poorly characterized. Here, we investigated the effects caused by mild blast forces combining neuropathological, histological, biochemical and neurophysiological analysis. For this purpose, we employed a rodent blast TBI model with blast forces below the level that causes macroscopic neuropathological changes. We found that mild blast forces induced neuroinflammation in cerebral cortex, striatum and hippocampus. Moreover, mild blast triggered microvascular damage and axonal injury. Furthermore, mild blast caused deficits in hippocampal short-term plasticity and synaptic excitability, but no impairments in long-term potentiation. Finally, mild blast exposure induced proteolytic cleavage of spectrin and the cyclin-dependent kinase 5 activator, p35 in hippocampus. Together, these findings show that mild blast forces can cause aberrant neurological changes that critically impact neuronal functions. These results are consistent with the idea that mild blast forces may induce subclinical pathophysiological changes that may contribute to neurological and psychiatric disorders.

Selective Vulnerability of the Foramen Magnum in a Rat Blast Traumatic Brain Injury Model

Primary blast traumatic brain injury (bTBI) accounts for a significant proportion of wartime trauma. Previous studies have demonstrated direct brain injury by blast waves, but the effect of the location of the blast epicenter on the skull with regard to brain injury remains poorly characterized. In order to investigate the role of the blast epicenter location, we modified a previously established rodent model of cranium-only bTBI to evaluate two specific blast foci: a rostrally focused blast centered on bregma (B-bTBI), which excluded the foramen magnum region, and a caudally focused blast centered on the occipital crest, which included the foramen magnum region (FM-bTBI). At all blast overpressures studied (668-1880 kPa), rats subjected to FM-bTBI demonstrated strikingly higher mortality, increased durations of both apnea and hypoxia, and increased severity of convexity subdural hematomas, than rats subjected to B-bTBI. Together, these data suggest a unique role for the foramen magnum region in mortality and brain injury following blast exposure, and emphasize the importance of the choice of blast focus location in experimental models of bTBI.

Modeling the Long-Term Consequences of Repeated Blast-Induced Mild Traumatic Brain Injuries

Repeated mild traumatic brain injury (rmTBI) caused by playing collision sports or by exposure to blasts during military operations can lead to late onset, chronic diseases such as chronic traumatic encephalopathy (CTE), a progressive neurodegenerative condition that manifests in increasingly severe neuropsychiatric abnormalities years after the last injury. Currently, because of the heterogeneity of the clinical presentation, confirmation of a CTE diagnosis requires post-mortem examination of the brain. The hallmarks of CTE are abnormal accumulation of phosphorylated tau protein, TDP-43 immunoreactive neuronal cytoplasmic inclusions, and astroglial abnormalities, but the pathomechanism leading to these terminal findings remains unknown. Animal modeling can play an important role in the identification of CTE pathomechanisms, the development of early stage diagnostic and prognostic biomarkers, and pharmacological interventions. Modeling the long-term consequences of blast rmTBI in animals is especially challenging because of the complexities of blast physics and animal-to-human scaling issues. This review summarizes current knowledge about the pathobiologies of CTE and rmbTBI and discusses problems as well as potential solutions related to high-fidelity modeling of rmbTBI and determining its long-term consequences.

Region-specific alterations in astrocyte and microglia morphology following exposure to blasts in the mouse hippocampus

Traumatic brain injury (TBI) is a serious public health concern, especially injuries from repetitive insults. The main objective of this study was to immunocytochemically examine morphological alterations in astrocytes and microglia in the hippocampus 48h following a single blast versus multiple blasts in adult C57BL/6 mice. The effects of ketamine and xylazine (KX), two common anesthetic agents used in TBI research, were also evaluated due to the confounding effect of anesthetics on injury outcome. Results showed a significant increase in hypertrophic microglia that was limited to the outer molecular layer of the dentate gyrus, but only in the absence of KX. Although the presence or absence of KX had no effect on astrocytes following a single blast, a significant decrease in astrocytic immunoreactivity was observed in the stratum lacunosum moleculare following multiple blasts in the absence of KX. The morphological changes in astrocytes and microglia reported in this study reveal region-specific differences in the absence of KX that could have significant implications for our interpretation of glial alterations in animal models of injury.

Large animal models of traumatic brain injury

Purpose/Aim: Animal models of traumatic brain injury (TBI) provide powerful tools to study TBI in a controlled, rigorous and cost-efficient manner. The mostly used animals in TBI studies so far are rodents. However, compared with rodents, large animals (e.g. swine, rabbit, sheep, ferret, etc.) show great advantages in modeling TBI due to the similarity of their brains to human brain. The aim of our review was to summarize the development and progress of common large animal TBI models in past 30 years.

MATERIALS AND METHODS

Mixed published articles and books associated with large animal models of TBI were researched and summarized.

RESULTS

We majorly sumed up current common large animal models of TBI, including discussion on the available research methodologies in previous studies, several potential therapies in large animal trials of TBI as well as advantages and disadvantages of these models.

CONCLUSIONS

Large animal models of TBI play crucial role in determining the underlying mechanisms and screening putative therapeutic targets of TBI.

The Effects of Blast Exposure on Protein Deimination in the Brain

Oxidative stress and calcium excitotoxicity are hallmarks of traumatic brain injury (TBI). While these early disruptions may be corrected over a relatively short period of time, long-lasting consequences of TBI including impaired cognition and mood imbalances can persist for years, even in the absence of any evidence of overt injury based on neuroimaging. This investigation examined the possibility that disordered protein deimination occurs as a result of TBI and may thus contribute to the long-term pathologies of TBI. Protein deimination is a calcium-activated, posttranslational modification implicated in the autoimmune diseases rheumatoid arthritis and multiple sclerosis, where aberrant deimination creates antigenic epitopes that elicit an autoimmune attack. The present study utilized proteomic analyses to show that blast TBI alters the deimination status of proteins in the porcine cerebral cortex. The affected proteins represent a small subset of the entire brain proteome and include glial fibrillary acidic protein and vimentin, proteins reported to be involved in autoimmune-based pathologies. The data also indicate that blast injury is associated with an increase in immunoglobulins in the brain, possibly representing autoantibodies directed against novel protein epitopes. These findings indicate that aberrant protein deimination is a biomarker for blast TBI and may therefore underlie chronic neuropathologies of head injury.

Acute death of astrocytes in blast-exposed rat organotypic hippocampal slice cultures

Blast traumatic brain injury (bTBI) affects civilians, soldiers, and veterans worldwide and presents significant health concerns. The mechanisms of neurodegeneration following bTBI remain elusive and current therapies are largely ineffective. It is important to better characterize blast-evoked cellular changes and underlying mechanisms in order to develop more effective therapies. In the present study, our group utilized rat organotypic hippocampal slice cultures (OHCs) as an in vitro system to model bTBI. OHCs were exposed to either 138 ± 22 kPa (low) or 273 ± 23 kPa (high) overpressures using an open-ended helium-driven shock tube, or were assigned to sham control group. At 2 hours (h) following injury, we have characterized the astrocytic response to a blast overpressure. Immunostaining against the astrocytic marker glial fibrillary acidic protein (GFAP) revealed acute shearing and morphological changes in astrocytes, including clasmatodendrosis. Moreover, overlap of GFAP immunostaining and propidium iodide (PI) indicated astrocytic death. Quantification of the number of dead astrocytes per counting area in the hippocampal cornu Ammonis 1 region (CA1), demonstrated a significant increase in dead astrocytes in the low- and high-blast, compared to sham control OHCs. However only a small number of GFAP-expressing astrocytes were co-labeled with the apoptotic marker Annexin V, suggesting necrosis as the primary type of cell death in the acute phase following blast exposure. Moreover, western blot analyses revealed calpain mediated breakdown of GFAP. The dextran exclusion additionally indicated membrane disruption as a potential mechanism of acute astrocytic death. Furthermore, although blast exposure did not evoke significant changes in glutamate transporter 1 (GLT-1) expression, loss of GLT-1-expressing astrocytes suggests dysregulation of glutamate uptake following injury. Our data illustrate the profound effect of blast overpressure on astrocytes in OHCs at 2 h following injury and suggest increased calpain activity and membrane disruption as potential underlying mechanisms.

Neuronal Injury and Glial Changes Are Hallmarks of Open Field Blast Exposure in Swine Frontal Lobe

With the rapid increase in the number of blast induced traumatic brain injuries and associated neuropsychological consequences in veterans returning from the operations in Iraq and Afghanistan, the need to better understand the neuropathological sequelae following exposure to an open field blast exposure is still critical. Although a large body of experimental studies have attempted to address these pathological changes using shock tube models of blast injury, studies directed at understanding changes in a gyrencephalic brain exposed to a true open field blast are limited and thus forms the focus of this study. Anesthetized, male Yucatan swine were subjected to forward facing medium blast overpressure (peak side on overpressure 224-332 kPa; n = 7) or high blast overpressure (peak side on overpressure 350-403 kPa; n = 5) by detonating 3.6 kg of composition-4 charge. Sham animals (n = 5) were subjected to all the conditions without blast exposure. After a 3-day survival period, the brain was harvested and sections from the frontal lobes were processed for histological assessment of neuronal injury and glial reactivity changes. Significant neuronal injury in the form of beta amyloid precursor protein immunoreactive zones in the gray and white matter was observed in the frontal lobe sections from both the blast exposure groups. A significant increase in the number of astrocytes and microglia was also observed in the blast exposed sections compared to sham sections. We postulate that the observed acute injury changes may progress to chronic periods after blast and may contribute to short and long-term neuronal degeneration and glial mediated inflammation.

Development of a new biomechanical indicator for primary blast-induced brain injury

Primary blast-induced traumatic brain injury (bTBI) has been observed at the boundary of brain tissue and cerebrospinal fluid (CSF). Such injury can hardly be explained by using the theory of compressive wave propagation, since both the solid and fuid materials have similar compressibility and thus the intracranial pressure (ICP) has a continuous distribution across the boundary. Since they have completely different shear properties, it is hypothesized the injury at the interface is caused by shear wave. In the present study, a preliminary combined numerical and theoretical analysis was conducted based on the theory of shear wave propagation/reflection. Simulation results show that higher lateral acceleration of brain tissue particles is concentrated in the boundary region. Based on this fnding, a new biomechanical vector, termed as strain gradient, was suggested for primary bTBI. The subsequent simple theoretical analysis reveals that this parameter is proportional to the value of lateral acceleration. At the boundary of lateral ventricles, high spatial strain gradient implies that the brain tissue in this area (where neuron cells may be contained) undergo significantly different strains and large velocity discontinuity, which may result in mechanical damage of the neuron cells.

Neuronal and glial changes in the brain resulting from explosive blast in an experimental model

Mild traumatic brain injury (mTBI) is the signature injury in warfighters exposed to explosive blasts. The pathology underlying mTBI is poorly understood, as this condition is rarely fatal and thus postmortem brains are difficult to obtain for neuropathological studies. Here we report on studies of an experimental model with a gyrencephalic brain that is exposed to single and multiple explosive blast pressure waves. To determine injuries to the brain resulting from the primary blast, experimental conditions were controlled to eliminate any secondary or tertiary injury from blasts. We found small but significant levels of neuronal loss in the hippocampus, a brain area that is important for cognitive functions. Furthermore, neuronal loss increased with multiple blasts and the degree of neuronal injury worsened with time post-blast. This is consistent with our findings in the blast-exposed human brain based on magnetic resonance spectroscopic imaging. The studies on this experimental model thus confirm what has been presumed to be the case with the warfighter, namely that exposure to multiple blasts causes increased brain injury. Additionally, as in other studies of both explosive blast as well as closed head mTBI, we found astrocyte activation. Activated microglia were also prominent in white matter tracts, particularly in animals exposed to multiple blasts and at long post-blast intervals, even though injured axons (i.e. β-APP positive) were not found in these areas. Microglial activation appears to be a delayed response, though whether they may contribute to inflammation related injury mechanism at even longer post-blast times than we tested here, remains to be explored. Petechial hemorrhages or other gross signs of vascular injury were not observed in our study. These findings confirm the development of neuropathological changes due to blast exposure. The activation of astrocytes and microglia, cell types potentially involved in inflammatory processes, suggest an important area for future study.

Primary blast injury causes cognitive impairments and hippocampal circuit alterations

Blast-induced traumatic brain injury (bTBI) and its long term consequences are a major health concern among veterans. Despite recent work enhancing our knowledge about bTBI, very little is known about the contribution of the blast wave alone to the observed sequelae. Herein, we isolated its contribution in a mouse model by constraining the animals' heads during exposure to a shockwave (primary blast). Our results show that exposure to primary blast alone results in changes in hippocampus-dependent behaviors that correspond with electrophysiological changes in area CA1 and are accompanied by reactive gliosis. Specifically, five days after exposure, behavior in an open field and performance in a spatial object recognition (SOR) task were significantly different from sham. Network electrophysiology, also performed five days after injury, demonstrated a significant decrease in excitability and increase in inhibitory tone. Immunohistochemistry for GFAP and Iba1 performed ten days after injury showed a significant increase in staining. Interestingly, a threefold increase in the impulse of the primary blast wave did not exacerbate these measures. However, we observed a significant reduction in the contribution of the NMDA receptors to the field EPSP at the highest blast exposure level. Our results emphasize the need to account for the effects of primary blast loading when studying the sequelae of bTBI.

High-Fidelity Simulation of Primary Blast: Direct Effects on the Head

The role of primary blast in blast-induced traumatic brain injury (bTBI) is controversial in part due to the technical difficulties of generating free-field blast conditions in the laboratory. The use of traditional shock tubes often results in artifacts, particularly of dynamic pressure, whereas the forces affecting the head are dependent on where the animal is placed relative to the tube, whether the exposure is whole-body or head-only, and on how the head is actually exposed to the insult (restrained or not). An advanced blast simulator (ABS) has been developed that enables high-fidelity simulation of free-field blastwaves, including sharply defined static and dynamic overpressure rise times, underpressures, and secondary shockwaves. Rats were exposed in head-only fashion to single-pulse blastwaves of 15 to 30 psi static overpressure. Head restraints were configured so as to eliminate concussive and minimize whiplash forces exerted on the head, as shown by kinematic analysis. No overt signs of trauma were present in the animals post-exposure. However, significant changes in brain 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNPase) and neurofilament heavy chain levels were evident by 7 days. In contrast to most studies of primary blast-induced TBI (PbTBI), no elevation of glial fibrillary acidic protein (GFAP) levels was noted when head movement was minimized. The ABS described in this article enables the generation of shockwaves highly representative of free-field blast. The use of this technology, in concert with head-only exposure, minimized head movement, and the kinematic analysis of the forces exerted on the head provide convincing evidence that primary blast directly causes changes in brain function and that GFAP may not be an appropriate biomarker of PbTBI.

Cellular Mechanisms and Behavioral Outcomes in Blast-Induced Neurotrauma: Comparing Experimental Setups

Blast-induced neurotrauma (BINT) has increased in incidence over the past decades and can result in cognitive issues that have debilitating consequences. The exact primary and secondary mechanisms of injury have not been elucidated and appearance of cellular injury can vary based on many factors, such as blast overpressure magnitude and duration. Many methodologies to study blast neurotrauma have been employed, ranging from open-field explosives to experimental shock tubes for producing free-field blast waves. While there are benefits to the various methods, certain specifications need to be accounted for in order to properly examine BINT. Primary cell injury mechanisms, occurring as a direct result of the blast wave, have been identified in several studies and include cerebral vascular damage, blood-brain barrier disruption, axonal injury, and cytoskeletal damage. Secondary cell injury mechanisms, triggered subsequent to the initial insult, result in the activation of several molecular cascades and can include, but are not limited to, neuroinflammation and oxidative stress. The collective result of these secondary injuries can lead to functional deficits. Behavioral measures examining motor function, anxiety traits, and cognition/memory problems have been utilized to determine the level of injury severity. While cellular injury mechanisms have been identified following blast exposure, the various experimental models present both concurrent and conflicting results. Furthermore, the temporal response and progression of pathology after blast exposure have yet to be detailed and remain unclear due to limited resemblance of methodologies. This chapter summarizes the current state of blast neuropathology and emphasizes the need for a standardized preclinical model of blast neurotrauma.

Clinical Traumatic Brain Injury in the Preclinical Setting

Traumatic brain injury (TBI) is the leading cause of death and disability for people under 45 years of age. Clinical TBI is often the result of disparate forces resulting in heterogeneous injuries. Preclinical modeling of TBI is a vital tool for studying the complex cascade of metabolic, cellular, and molecular post-TBI events collectively termed secondary injury. Preclinical models also provide an important platform for studying therapeutic interventions. However, modeling TBI in the preclinical setting is challenging, and most models replicate only certain aspects of clinical TBI. This chapter details the most widely used models of preclinical TBI, including the controlled cortical impact, fluid percussion, blast, and closed head models. Each of these models replicates particular critical aspects of clinical TBI. Prior to selecting a preclinical TBI model, it is important to address what aspect of human TBI is being sought to evaluate.

Ameliorative Effects of Antioxidants on the Hippocampal Accumulation of Pathologic Tau in a Rat Model of Blast-Induced Traumatic Brain Injury

Traumatic brain injury (TBI) can lead to early onset dementia and other related neurodegenerative diseases. We previously demonstrated that damage to the central auditory pathway resulting from blast-induced TBI (bTBI) could be significantly attenuated by a combinatorial antioxidant treatment regimen. In the current study, we examined the localization patterns of normal Tau and the potential blast-induced accumulation of neurotoxic variants of this microtubule-associated protein that are believed to potentiate the neurodegenerative effects associated with synaptic dysfunction in the hippocampus following three successive blast overpressure exposures in nontransgenic rats. We observed a marked increase in the number of both hyperphosphorylated and oligomeric Tau-positive hilar mossy cells and somatic accumulation of endogenous Tau in oligodendrocytes in the hippocampus. Remarkably, a combinatorial regimen of 2,4-disulfonyl α-phenyl tertiary butyl nitrone (HPN-07) and N-acetylcysteine (NAC) resulted in striking reductions in the numbers of both mossy cells and oligodendrocytes positively labeled for these pathological Tau immunoreactivity patterns in response to bTBI. This treatment strategy represents a promising therapeutic approach for simultaneously reducing or eliminating both primary auditory injury and nonauditory changes associated with bTBI-induced hippocampal neurodegeneration.

Chronic Histopathological and Behavioral Outcomes of Experimental Traumatic Brain Injury in Adult Male Animals

The purpose of this review is to survey the use of experimental animal models for studying the chronic histopathological and behavioral consequences of traumatic brain injury (TBI). The strategies employed to study the long-term consequences of TBI are described, along with a summary of the evidence available to date from common experimental TBI models: fluid percussion injury; controlled cortical impact; blast TBI; and closed-head injury. For each model, evidence is organized according to outcome. Histopathological outcomes included are gross changes in morphology/histology, ventricular enlargement, gray/white matter shrinkage, axonal injury, cerebrovascular histopathology, inflammation, and neurogenesis. Behavioral outcomes included are overall neurological function, motor function, cognitive function, frontal lobe function, and stress-related outcomes. A brief discussion is provided comparing the most common experimental models of TBI and highlighting the utility of each model in understanding specific aspects of TBI pathology. The majority of experimental TBI studies collect data in the acute postinjury period, but few continue into the chronic period. Available evidence from long-term studies suggests that many of the experimental TBI models can lead to progressive changes in histopathology and behavior. The studies described in this review contribute to our understanding of chronic TBI pathology.

Pre-Clinical Traumatic Brain Injury Common Data Elements: Toward a Common Language Across Laboratories

Traumatic brain injury (TBI) is a major public health issue exacting a substantial personal and economic burden globally. With the advent of "big data" approaches to understanding complex systems, there is the potential to greatly accelerate knowledge about mechanisms of injury and how to detect and modify them to improve patient outcomes. High quality, well-defined data are critical to the success of bioinformatics platforms, and a data dictionary of "common data elements" (CDEs), as well as "unique data elements" has been created for clinical TBI research. There is no data dictionary, however, for preclinical TBI research despite similar opportunities to accelerate knowledge. To address this gap, a committee of experts was tasked with creating a defined set of data elements to further collaboration across laboratories and enable the merging of data for meta-analysis. The CDEs were subdivided into a Core module for data elements relevant to most, if not all, studies, and Injury-Model-Specific modules for non-generalizable data elements. The purpose of this article is to provide both an overview of TBI models and the CDEs pertinent to these models to facilitate a common language for preclinical TBI research.

Blast Overpressure Waves Induce Transient Anxiety and Regional Changes in Cerebral Glucose Metabolism and Delayed Hyperarousal in Rats

Physiological alterations, anxiety, and cognitive disorders are strongly associated with blast-induced traumatic brain injury (blast TBI), and are common symptoms in service personnel exposed to blasts. Since 2006, 25,000–30,000 new TBI cases are diagnosed annually in U.S. Service members; increasing evidence confirms that primary blast exposure causes diffuse axonal injury and is often accompanied by altered behavioral outcomes. Behavioral and acute metabolic effects resulting from blast to the head in the absence of thoracic contributions from the periphery were examined, following a single blast wave directed to the head of male Sprague-Dawley rats protected by a lead shield over the torso. An 80 psi head blast produced cognitive deficits that were detected in working memory. Blast TBI rats displayed increased anxiety as determined by elevated plus maze at day 9 post-blast compared to sham rats; blast TBI rats spent significantly more time than the sham controls in the closed arms (p < 0.05; n = 8–11). Interestingly, anxiety symptoms were absent at days 22 and 48 post-blast. Instead, blast TBI rats displayed increased rearing behavior at day 48 post-blast compared to sham rats. Blast TBI rats also exhibited suppressed acoustic startle responses, but similar pre-pulse inhibition at day 15 post-blast compared to sham rats. Acute physiological alterations in cerebral glucose metabolism were determined by positron emission tomography 1 and 9 days post-blast using ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG). Global glucose uptake in blast TBI rat brains increased at day 1 post-blast (p < 0.05; n = 4–6) and returned to sham levels by day 9. Our results indicate a transient increase in cerebral metabolism following a blast injury. Markers for reactive astrogliosis and neuronal damage were noted by immunoblotting motor cortex tissue from day 10 post-blast in blast TBI rats compared to sham controls (p < 0.05; n = 5–6).

A novel rat model of blast-induced traumatic brain injury simulating different damage degree: implications for morphological, neurological, and biomarker changes

In current military conflicts and civilian terrorism, blast-induced traumatic brain injury (bTBI) is the primary cause of neurotrauma. However, the effects and mechanisms of bTBI are poorly understood. Although previous researchers have made significant contributions to establishing animal models for the simulation of bTBI, the precision and controllability of blast-induced injury in animal models must be improved. Therefore, we established a novel rat model to simulate blast-wave injury to the brain. To simulate different extents of bTBI injury, the animals were divided into moderate and severe injury groups. The miniature spherical explosives (pentaerythritol tetranitrate) used in each group were of different sizes (2.5 mm diameter in the moderate injury group and 3.0 mm diameter in the severe injury group). A specially designed apparatus was able to precisely adjust the positions of the miniature explosives and create eight rats with bTBI simultaneously, using a single electric detonator. Neurological functions, gross pathologies, histopathological changes and the expression levels of various biomarkers were examined after the explosion. Compared with the moderate injury group, there were significantly more neurological dysfunctions, cortical contusions, intraparenchymal hemorrhages, cortical expression of S-100β, myelin basic protein, neuron-specific enolase, IL-8, IL-10, inducible nitric oxide synthase, and HIF-1α in the severe injury group. These results demonstrate that we have created a reliable and reproducible bTBI model in rats. This model will be helpful for studying the mechanisms of bTBI and developing strategies for clinical bTBI treatment.

Vascular and inflammatory factors in the pathophysiology of blast-induced brain injury

Blast-related traumatic brain injury (TBI) has received much recent attention because of its frequency in the conflicts in Iraq and Afghanistan. This renewed interest has led to a rapid expansion of clinical and animal studies related to blast. In humans, high-level blast exposure is associated with a prominent hemorrhagic component. In animal models, blast exerts a variety of effects on the nervous system including vascular and inflammatory effects that can be seen with even low-level blast exposures which produce minimal or no neuronal pathology. Acutely, blast exposure in animals causes prominent vasospasm and decreased cerebral blood flow along with blood-brain barrier breakdown and increased vascular permeability. Besides direct effects on the central nervous system, evidence supports a role for a thoracically mediated effect of blast; whereby, pressure waves transmitted through the systemic circulation damage the brain. Chronically, a vascular pathology has been observed that is associated with alterations of the vascular extracellular matrix. Sustained microglial and astroglial reactions occur after blast exposure. Markers of a central and peripheral inflammatory response are found for sustained periods after blast injury and include elevation of inflammatory cytokines and other inflammatory mediators. At low levels of blast exposure, a microvascular pathology has been observed in the presence of an otherwise normal brain parenchyma, suggesting that the vasculature may be selectively vulnerable to blast injury. Chronic immune activation in brain following vascular injury may lead to neurobehavioral changes in the absence of direct neuronal pathology. Strategies aimed at preventing or reversing vascular damage or modulating the immune response may improve the chronic neuropsychiatric symptoms associated with blast-related TBI.

Effects of blast overpressure on neurons and glial cells in rat organotypic hippocampal slice cultures

Due to recent involvement in military conflicts, and an increase in the use of explosives, there has been an escalation in the incidence of blast-induced traumatic brain injury (bTBI) among US military personnel. Having a better understanding of the cellular and molecular cascade of events in bTBI is prerequisite for the development of an effective therapy that currently is unavailable. The present study utilized organotypic hippocampal slice cultures (OHCs) exposed to blast overpressures of 150 kPa (low) and 280 kPa (high) as an in vitro bTBI model. Using this model, we further characterized the cellular effects of the blast injury. Blast-evoked cell death was visualized by a propidium iodide (PI) uptake assay as early as 2 h post-injury. Quantification of PI staining in the cornu Ammonis 1 and 3 (CA1 and CA3) and the dentate gyrus regions of the hippocampus at 2, 24, 48, and 72 h following blast exposure revealed significant time dependent effects. OHCs exposed to 150 kPa demonstrated a slow increase in cell death plateauing between 24 and 48 h, while OHCs from the high-blast group exhibited a rapid increase in cell death already at 2 h, peaking at ~24 h post-injury. Measurements of lactate dehydrogenase release into the culture medium also revealed a significant increase in cell lysis in both low- and high-blast groups compared to sham controls. OHCs were fixed at 72 h post-injury and immunostained for markers against neurons, astrocytes, and microglia. Labeling OHCs with PI, neuronal, and glial markers revealed that the blast-evoked extensive neuronal death and to a lesser extent loss of glial cells. Furthermore, our data demonstrated activation of astrocytes and microglial cells in low- and high-blasted OHCs, which reached a statistically significant difference in the high-blast group. These data confirmed that our in vitro bTBI model is a useful tool for studying cellular and molecular changes after blast exposure.

Injury biomechanics, neuropathology, and simplified physics of explosive blast and impact mild traumatic brain injury

Explosive blast shock waves and blunt impact to the head are two types of loading shown to result in mild traumatic brain injury (mTBI). While mTBI from these two causes shares some common features behaviorally, there are distinct differences in the pathophysiology of the underlying injury mechanisms. Various elucidations have been offered in the literature to explain the organic damage associated with mTBI resulting from both types of loading. The current state of understanding in this field is somewhat limited by the degree of appreciation of the physics and biomechanics governing the effects of explosive blast shock waves and blunt impact on the head, which has resulted in the various approaches to the investigation of the operative brain injury "wounding mechanisms". In this chapter we provide a simplified description of terminology associated with forces on the head from explosive blast shock waves and blunt impact, to assist readers in the field in evaluating interpretations of brain injury "wounding" processes. Remarkably, mTBI from either loading is shown generally to result in only a small loss of neurons, with hippocampal neurons appearing to be particularly vulnerable to explosive blast shock waves. Explosive blast studies in large animal models show a unique pattern of periventricular injury, which is different from the classic diffuse axonal injury. Both astrocyte and microglial activation are also seen in explosive blast as well as impact trauma, but this may be a general secondary brain injury response, nonspecific to explosive blast or blunt trauma. Additionally, while moderate to severe impact closed head injuries sometimes result in petechial hemorrhages or hematomas, they do not appear to be associated with explosive blast mTBI even with repeated exposure to blasts.

Animal models of traumatic brain injury

Traumatic brain injury (TBI) is a major health issue comprising a heterogeneous and complex array of pathologies. Over the last several decades, numerous animal models have been developed to address the diverse nature of human TBI. The clinical relevance of these models has been a major point of reflection given the poor translation of pharmacologic TBI interventions to the clinic. While previously characterized broadly as either focal or diffuse, this classification is falling out of favor with increased awareness of the overlap in pathologic outcomes between models and an emerging consensus that no one model is sufficient. Moreover, an appreciation of injury biomechanics is essential in recapitulating and interpreting the spectrum of TBI neuropathology observed in various established models of dynamic closed-head TBI. While these models have replicated many specific features of human TBI, an enhanced context with clinical relevancy will facilitate the further elucidation of the mechanisms and treatment of injury.

Brain injury from explosive blast: description and clinical management

Accumulating clinical experience is indicating that explosive blast brain injury is becoming recognized as a disease distinct from the penetrating form of blast injury as well as the classic closed head injury (CHI). In recent US conflicts in Iraq and Afghanistan, over 60% of combat casualties were from explosive blast with the hallmark explosive weapon being the improvised explosive device (IED). Explosive blast TBI is a condition afflicting many combat injured warfighters potentially constituting another category of TBI. Clinically, it shares many features with conventional TBI but possesses some unique aspects. In its mild form, it also shares many clinical features with PTSD but here again has distinct aspects. Although military medical providers depend on civilian standard of care guidelines when managing explosive blast mTBI, they are continually adapting their medical practice in order to optimize the treatment of this disease, particularly in a theater of war. It is clear that further rigorous scientific study of explosive blast mTBI at both the basic science and clinical levels is needed. This research must include improved understanding of the causes and mechanisms of explosive blast TBI as well as comprehensive epidemiologic studies to determine the prevalence of this disease and its risk factors. A widely accepted unambiguous clinical description of explosive blast mTBI with diagnostic criteria would greatly improve diagnosis. It is hoped that through appropriate research meaningful prevention, mitigation, and treatment strategies for explosive blast mTBI can be speedily realized.

The problem of axonal injury in the brains of veterans with histories of blast exposure

Introduction

Blast injury to brain, a hundred-year old problem with poorly characterized neuropathology, has resurfaced as health concern in recent deployments in Iraq and Afghanistan. To characterize the neuropathology of blast injury, we examined the brains of veterans for the presence of amyloid precursor protein (APP)-positive axonal swellings typical of diffuse axonal injury (DAI) and compared them to healthy controls as well as controls with opiate overdose, anoxic-ischemic encephalopathy, and non-blast TBI (falls and motor vehicle crashes).

Results

In cases with blast history, we found APP (+) axonal abnormalities in several brain sites, especially the medial dorsal frontal white matter. In white matter, these abnormalities were featured primarily by clusters of axonal spheroids or varicosities in a honeycomb pattern with perivascular distribution. Axonal abnormalities colocalized with IBA1 (+) reactive microglia and had an appearance that was distinct from classical DAI encountered in TBI due to motor vehicle crashes. Opiate overdose cases also showed APP (+) axonal abnormalities, but the intensity of these lesions was lower compared to cases with blast histories and there was no clear association of such lesions with microglial activation.

Conclusions

Our findings demonstrate that many cases with history of blast exposure are featured by APP (+) axonopathy that may be related to blast exposure, but an important role for opiate overdose, antemortem anoxia, and concurrent blunt TBI events in war theater or elsewhere cannot be discounted.

Electronic supplementary material

The online version of this article (doi:10.1186/s40478-014-0153-3) contains supplementary material, which is available to authorized users.