Blast-induced Traumatic Brain Injury Research Articles

Low-intensity Blast Wave Model for Preclinical Assessment of Closed-head Mild Traumatic Brain Injury in Rodents.

Traumatic brain injury (TBI) is a large-scale public health problem. Mild TBI is the most prevalent form of neurotrauma and accounts for a large number of medical visits in the United States. There are currently no FDA-approved treatments available for TBI. The increased incidence of military-related, blast-induced TBI further accentuates the urgent need for effective TBI treatments. Therefore, new preclinical TBI animal models that recapitulate aspects of human blast-related TBI will greatly advance the research efforts into the neurobiological and pathophysiological processes underlying mild to moderate TBI as well as the development of novel therapeutic strategies for TBI. Here we present a reliable, reproducible model for the investigation of the molecular, cellular, and behavioral effects of mild to moderate blast-induced TBI. We describe a step-by-step protocol for closed-head, blast-induced mild TBI in rodents using a bench-top setup consisting of a gas-driven shock tube equipped with piezoelectric pressure sensors to ensure consistent test conditions. The benefits of the setup that we have established are its relative low-cost, ease of installation, ease of use and high-throughput capacity. Further advantages of this non-invasive TBI model include the scalability of the blast peak overpressure and the generation of controlled reproducible outcomes. The reproducibility and relevance of this TBI model has been evaluated in a number of downstream applications, including neurobiological, neuropathological, neurophysiological and behavioral analyses, supporting the use of this model for the characterization of processes underlying the etiology of mild to moderate TBI.

Effect of Early Normobaric Hyperoxia on Blast-Induced Traumatic Brain Injury in Rats.

Blast-induced traumatic brain injury (bTBI) is a leading cause of disability and mortality in soldiers during the conflicts in Iraq and Afghanistan. Although substantial clinical and animal studies have investigated the pathophysiology and treatments of bTBI, few effective therapies have been found, especially for the early rescue in the battlefield. The aim of this study is to evaluate neuroprotective effects of early normobaric hyperoxia (NBO) on bTBI. We established a rat model of bTBI caused by explosion in the cabin. It exhibited typical changes of mild bTBI, like impaired neurological function, brain edema, minor intracranial hemorrhage and neuron necrosis. The rats were divided into 4 groups (n = 12): Sham, Vehicle, hyperbaric oxygen(HBO)and NBO. Neurological function of the rats was assessed by the Neurological Severity Scores (NSS) at 24h and 72h after explosion. Serum interleukin-6 (IL-6), neuron specific enolase (NSE)andtauprotein were measured at 24h and 72h after explosion. Brain water content was measured and Aquaporin-4 (AQP4) immunostaining was performed. Neuronal apoptosis was analyzed by TUNEL staining. NBO demonstrated curative effects on protecting the neurological function. Serum levels of NSEandtauprotein were reduced at 24hand72h after explosion. But the levels of IL-6 were not reduced significantly at both time points. Cerebral edema was alleviated. Simultaneously, AQP4 immunostaining of the hippocampus showed remarkably decreased expression after treatment. The number of apoptotic cells in hippocampus was also decreased. Compared with HBO, NBO is simple and convenient, and can be administered in remote areas. It may be a promising therapy for early rescue of bTBI in the battlefield.

Altered monoaminergic levels, spasticity, and balance disability following repetitive blast-induced traumatic brain injury in rats

Spasticity and balance disability are major complications following traumatic brain injury (TBI). Although monoaminergic inputs provide critical adaptive neuromodulations to the motor system, data are not available regarding the levels of monoamines in the brain regions related to motor functions following repetitive blast TBI (bTBI). The objective of this study was to determine if mild, repetitive bTBI results in spasticity/balance deficits and if these are correlated with altered levels of norepinephrine, dopamine, and serotonin in the brain regions related to the motor system. Repetitive bTBI was induced by a blast overpressure wave in male rats on days 1, 4, and 7. Following bTBI, physiological/behavioral tests were conducted and tissues in the central motor system (i.e., motor cortex, locus coeruleus, vestibular nuclei, and lumbar spinal cord) were collected for electrochemical detection of norepinephrine, dopamine, and serotonin by high-performance liquid chromatography. The results showed that norepinephrine was significantly increased in the locus coeruleus and decreased in the vestibular nuclei, while dopamine was significantly decreased in the vestibular nuclei. On the other hand, serotonin was significantly increased in the motor cortex and the lumbar spinal cord. Because these monoamines play important roles in regulating the excitability of neurons, these results suggest that mild, repetitive bTBI-induced dysregulation of monoaminergic inputs in the central motor system could contribute to spasticity and balance disability. This is the first study to report altered levels of multiple monoamines in the central motor system following acute mild, repetitive bTBI.

Terahertz spectroscopic diagnosis of early blast-induced traumatic brain injury in rats.

The early diagnosis of blast-induced traumatic brain injury (bTBI) is of great clinical significance for prognostication and treatment. Here, we report a new strategy for early bTBI diagnosis through serum and cerebrospinal fluid (CSF) based on terahertz time-domain spectroscopy (THz-TDS). The spectral differences of serum and CSF for different degrees of experimental bTBI in rats have been demonstrated in the early period. In addition, the THz spectra of total protein in the hypothalamus and hippocampus were investigated at different time points after blast exposure, which both showed clear differences with time increasing compared with that in the normal brain. This might help to explain the neurological symptoms caused by bTBI. Moreover, based on the THz absorption spectra of serum and CSF, the principal component analysis and machine learning algorithms were performed to automatically identify the degree of bTBI. The highest diagnostic accuracy was up to 95.5%. It is suggested that this method has potential as an alternative method for high-sensitive, rapid, label-free, economical and early diagnosis of bTBI.

Blast Exposure Leads to Accelerated Cellular Senescence in the Rat Brain.

Blast-induced traumatic brain injury (bTBI) is one of the major causes of persistent disabilities in Service Members, and a history of bTBI has been identified as a primary risk factor for developing age-associated neurodegenerative diseases. Clinical observations of several military blast casualties have revealed a rapid age-related loss of white matter integrity in the brain. In the present study, we have tested the effect of single and tightly coupled repeated blasts on cellular senescence in the rat brain. Isoflurane-anesthetized rats were exposed to either a single or 2 closely coupled blasts in an advanced blast simulator. Rats were euthanized and brains were collected at 24 h, 1 month and 1 year post-blast to determine senescence-associated-β-galactosidase (SA-β-gal) activity in the cells using senescence marker stain. Single and repeated blast exposures resulted in significantly increased senescence marker staining in several neuroanatomical structures, including cortex, auditory cortex, dorsal lateral thalamic nucleus, geniculate nucleus, superior colliculus, ventral thalamic nucleus and hippocampus. In general, the increases in SA-β-gal activity were more pronounced at 1 month than at 24 h or 1 year post-blast and were also greater after repeated than single blast exposures. Real-time quantitative RT-PCR analysis revealed decreased levels of mRNA for senescence marker protein-30 (SMP-30) and increased mRNA levels for p21 (cyclin dependent kinase inhibitor 1A, CDKN1A), two other related protein markers of cellular senescence. The increased senescence observed in some of these affected brain structures may be implicated in several long-term sequelae after exposure to blast, including memory disruptions and impairments in movement, auditory and ocular functions.

Mechanisms of tensile failure of cerebrospinal fluid in blast traumatic brain injury

Mechanisms of blast-induced Traumatic Brain Injury (BTBI), particularly those linked to the primary pressure wave, are still not fully understood. One possible BTBI mechanism is cavitation in the cerebrospinal fluid (CSF) caused by CSF tensile failure, which is likely to increase strain and strain rate in the brain tissue near the CSF. Blast loading of the head can generate rarefaction (expansion) waves and rapid head motion, which both can produce tensile forces in the CSF. However, it is not clear which of these mechanisms is more likely to cause CSF tensile failure. In this study, we used a high-fidelity 3-dimensional computational model of the human head to test whether the CSF tensile failure increases brain deformation near the brain/CSF boundary and to determine the key failure mechanisms. We exposed the head model to a frontal blast wave and predicted strain and strain rate distribution in the cortex. We found that CSF tensile failure significantly increased strain and strain rate in the cortex. We then studied whether the rapid head motion or the rarefaction wave causes strain and strain rate concentration in cortex. We isolated these two effects by conducting simulations with pure head motion loading (i.e. prescribing the skull velocity but eliminating the pressure wave) and pure blast wave loading (i.e. eliminating head motion by fixing the skull base). Our results showed that the strain increase in the cortex was mainly caused by head motion. In contrast, strain rate increase was caused by both rapid head motion and rarefaction waves, but head motion had a stronger effect on elevating strain rate. Our results show that rapid motion of the head produced by blast wave is the key mechanism for CSF tensile failure and subsequent concentration of strain and strain rate in cortex. This finding suggests that mitigation of rapid head motion caused by blast loading needs to be addressed in the design of protective equipment in order to prevent the tensile failure of CSF.

Blast‐induced Traumatic Brain Injury Elicits Sociability Deficits and Aggressive Behaviors Coinciding with Transient Variations in CNS 5‐HT Levels and 5‐HT2A Hypersensitivity

Traumatic brain injury (TBI) is a leading cause of mortality and morbidity. In the United States alone, TBI accounts for 2.8 million emergency department visits and an economic cost of nearly $80 billion annually. The lack of any FDA‐approved therapies for treatment of acute or chronic symptoms of TBI necessitates the identification and characterization of novel drug targets. TBI often results in the generation of neuropsychiatric disorders including major depressive disorder (MDD), anxiety, aggression, and social withdrawal within the first 12 months following injury, disorders linked to altered serotonin (5‐HT) signaling. Currently, the effects of TBI on physiologic function of the 5‐HT signaling system within the CNS are poorly understood. We hypothesize that TBI drives the formation of aberrant behavioral states linked to neuropsychiatric disorders through alterations of normal, physiologic 5‐HT signaling. To test this hypothesis, adult, wild type C57Bl/6J mice were subjected to a single, blast‐induced TBI or sham treatment, followed by behavioral and/or molecular analyses 0–30 days post‐injury (dpi). High performance liquid chromatography (HPLC) revealed a time‐dependent increase and subsequent decrease in total 5‐HT and 5HIAA levels within the dorsal raphe nucleus (DRN) 10 and 30 dpi, respectively. No effect of TBI on 5‐HT levels was found in prefrontal cortex (PFC) or somatosensory cortex (SSC), areas of high 5‐HT innervation. Using the tube test for social dominance, time‐dependent alterations in DRN 5‐HT levels were found to correlate with altered social dominance behavior. TBI resulted in decreased social dominance/aversion 10 dpi, but an increase in dominance behavior 30 dpi. Using the Crawley Three Chamber Sociability Assay, TBI‐induced decreases in sociability were observed 10 dpi, an effect not present at 30 dpi. To investigate alterations in 5‐HT receptor pharmacology associated with TBI, 5‐HT2A receptor sensitivity was determined using DOI‐induced head twitch response (HTR) assays. TBI rapidly increased 5‐HT2A receptor sensitivity, effects persisting for 10 dpi. TBI‐elicited increases in DOI‐induced HTR were attenuated by administration of the 5‐HT2A antagonist M100907, indicating a dependence on intact 5‐HT2A receptor signaling. [3H]Ketanserin receptor binding studies revealed an increase in cortical 5‐HT2A receptor binding at 3 and 10 dpi. Collectively, these data provide evidence that TBI elicits time‐dependent alterations in the 5‐HT architecture of the CNS, effects that coincide with altered social function. A more thorough understanding of specifically how neurotrauma alters 5‐HT signaling will allow for the informed clinical utilization of available 5‐HT‐oriented pharmacotherapies and may provide drug discovery targets for TBI‐elicited neuropsychiatric sequelae.Support or Funding InformationStudies were supported by the Brain & Behavior Research Foundation, PhRMA Foundation, and the University of Cincinnati

Chemokine signaling mediated monocyte infiltration affects anxiety-like behavior following blast injury

The activation of resident microglia and infiltrated monocytes are known potent mediators of chronic neuroinflammation following traumatic brain injury (TBI). In this study, we use a mouse model of blast-induced TBI (bTBI) to investigate whether microglia and monocytes contribute to the neuroinflammatory and behavioral consequences of bTBI. Eight-ten week old mice were subject to moderate TBI (180 kPa) in a shock tube. Using double transgenic CCR2RFP/+: CX3CR1GFP/+ mice, we were able to note that in addition to resident Cx3CR1+ microglia, infiltrating CCR2+ monocytes also contributed to the expanding macrophage population that was observed after bTBI. The microglia activation and monocyte infiltration occurred as early as 4 h and lasted up to 30d after blast exposure, suggesting chronic inflammation. The infiltration of monocytes may be partly mediated by chemokine CCL2-CCR2 signaling axis and compromised blood brain barrier permeability. Hence, bTBI-induced infiltration of monocytes and production of IL-1β were prevented in mice lacking CCR2 (CCR2 KO). Finally, this study showed that interference of monocyte infiltration using CCR2 KO, ameliorated the chronic effects of bTBI such as anxiety-like behavior and short-term memory decline. Taken together, these data suggest that bTBI leads to activation of both resident microglia and infiltrated monocytes. The infiltration of monocytes was partly mediated by CCL2-CCR2 signaling, which in turn contributes to increased production of IL-1β leading to behavioral deficits after bTBI. Furthermore, bTBI induced behavioral outcomes were reduced by targeting CCL2-CCR2 signaling, highlighting the significance of this signaling axis in bTBI pathology.

Oxidative stress contributes to cerebral metabolomic profile changes in animal model of blast-induced traumatic brain injury.

Blast-induced neurotrauma (BINT) has been recognized as the common mode of traumatic brain injury amongst military and civilian personnel due to an increased insurgent activity domestically and abroad. Previous studies from this laboratory have identified three major pathological events following BINT which include blood brain barrier disruption the earliest event, followed by oxidative stress and neuroinflammation as secondary events occurring a few hours following blast. Our recent studies have also identified an increase in oxidative stress mediated by the activation of superoxide producing enzyme NADPH oxidase (NOX) in different brain regions at varying levels with neurons displaying higher oxidative stress (NOX activation) compared to any other neural cell. Since neurons have higher energy demands in brain and are more prone to oxidative damage, this study evaluated the effect of oxidative stress on blast-blast induced changes in metabolomics profiles in different brain regions. Animals were exposed to mild/moderate blast injury (180 kPa) and examined the metabolites of energy metabolism, amino acid metabolism as well as the profiles of plasma membrane metabolites in different brain regions at different time points (24 h, 3 day and 7 day) after blast using 1H NMR spectroscopy. Effect of apocynin, an inhibitor of superoxide producing enzyme NADPH oxidase on cerebral metabalomics profiles was also examined. Several metabolomic profile changes were observed in frontal cortex and hippocampus with concomitant decrease in energy metabolism. In addition, glutamate/glutamine and other amino acid metabolism as well as metabolites involved in plasma membrane integrity were also altered. Hippocampus appears metabolically more vulnerable than the frontal cortex. A post-treatment of animals with apocynin, an inhibitor of NOX activation significantly prevented the changes in metabolite profiles. Together these studies indicate that blast injury reduces both cerebral energy and neurotransmitter amino acid metabolism and that oxidative stress contributes to these processes. Thus, strategies aimed at reducing oxidative stress can have a therapeutic benefit in mitigating metabolic changes following BINT.

Effect of mild blast-induced TBI on dendritic architecture of the cortex and hippocampus in the mouse

Traumatic brain injury (TBI) has been designated as a signature injury of modern military conflicts. Blast trauma, in particular, has come to make up a significant portion of the TBIs which are sustained in warzones. Though most TBIs are mild, even mild TBI can induce long term effects, including cognitive and memory deficits. In our study, we utilized a mouse model of mild blast-related TBI (bTBI) to investigate TBI-induced changes within the cortex and hippocampus. We performed rapid Golgi staining on the layer IV and V pyramidal neurons of the parietal cortex and the CA1 basilar tree of the hippocampus and quantified dendritic branching and distribution. We found decreased dendritic branching within both the cortex and hippocampus in injured mice. Within parietal cortex, this decreased branching was most evident within the middle region, while outer and inner regions resembled that of control mice. This study provides important knowledge in the study of how the shockwave associated with a blast explosion impacts different brain regions.

Materials Characterization of Cranial Simulants for Blast-Induced Traumatic Brain Injury

The mechanical response of brain tissue to high-speed forces in the blast and blunt traumatic brain injury is poorly understood. Object-to-object variation and interspecies differences are current limitations in animal and cadaver studies conducted to study damage mechanisms. Biofidelic and transparent tissue simulants allow the use of high-speed optical diagnostics during a blast event, making it possible to observe deformations and damage patterns for comparison to observed injuries seen post-mortem in traumatic brain injury victims. Material properties of several tissue simulants were quantified using standard mechanical characterization techniques, that is, shear rheometric, tensile, and compressive testing. Polyacrylamide simulants exhibited the best optical and mechanical property matching with the fewest trade-offs in the design of a cranial test object. Polyacrylamide gels yielded densities of ~1.04g/cc and shear moduli ranging 1.3-14.55kPa, allowing gray and white matter simulant tuning to a 30-35% difference in shear for biofidelity. These materials are intended for use as layered cranial phantoms in a shock tube and open field blasts, with focus on observing phenomena occurring at the interfaces of adjacent tissue simulant types or material-fluid boundaries. Mechanistic findings from these studies may be used to inform the design of protective gear to mitigate blast injuries.

Diffusion Tensor Imaging Detects Acute and Subacute Changes in Corpus Callosum in Blast-Induced Traumatic Brain Injury.

There is a critical need for understanding the progression of neuropathology in blast-induced traumatic brain injury using valid animal models to develop diagnostic approaches. In the present study, we used diffusion imaging and magnetic resonance (MR) morphometry to characterize axonal injury in white matter structures of the rat brain following a blast applied via blast tube to one side of the brain. Diffusion tensor imaging was performed on acute and subacute phases of pathology from which fractional anisotropy, mean diffusivity, axial diffusivity, and radial diffusivity were calculated for corpus callosum (CC), cingulum bundle, and fimbria. Ventricular volume and CC thickness were measured. Blast-injured rats showed temporally varying bilateral changes in diffusion metrics indicating persistent axonal pathology. Diffusion changes in the CC suggested vasogenic edema secondary to axonal injury in the acute phase. Axonal pathology persisted in the subacute phase marked by cytotoxic edema and demyelination which was confirmed by ultrastructural analysis. The evolution of pathology followed a different pattern in the cingulum bundle: axonal injury and demyelination in the acute phase followed by cytotoxic edema in the subacute phase. Spatially, structures close to midline were most affected. Changes in the genu were greater than in the body and splenium; the caudal cingulum bundle was more affected than the rostral cingulum. Thinning of CC and ventriculomegaly were greater only in the acute phase. Our results reveal the persistent nature of blast-induced axonal pathology and suggest that diffusion imaging may have potential for detecting the temporal evolution of blast injury.

Supplemental Material for Examining Posttraumatic Stress Disorder as a Key Postinjury Risk Factor in OIF/OEF Veterans With Blast-Induced Mild Traumatic Brain Injury

Supplemental Material for Examining Posttraumatic Stress Disorder as a Key Postinjury Risk Factor in OIF/OEF Veterans With Blast-Induced Mild Traumatic Brain Injury

Direct Observation of Low Strain, High Rate Deformation of Cultured Brain Tissue During Primary Blast.

The Veterans Health Administration determined that over 250,000 U.S. service members were diagnosed with a traumatic brain injury (TBI) between 2008 and 2018, of which a great proportion were due to blast exposure. Although the penetrating (secondary) and inertia-driven (tertiary) phases of blast-induced TBI (bTBI) have been studied thoroughly and are known to be injurious, primary blast brain injury has been less studied. We investigated the biomechanics of primary bTBI in our previously developed in vitro shock tube model with a fluid-filled sample receiver. Using stereoscopic, high-speed cameras and digital image correlation (DIC), we mapped the deformation of organotypic hippocampal slice cultures (OHSCs) following a range of blast exposures to characterize the induced strains. As blast exposure increased, tissue strain increased, although the levels remained relatively low (maximum < 9%), with strains rates between 25 and 85s-1. Both strain magnitude and rate were highly correlated with the in-air blast impulse and in-fluid peak pressure parameters. Comparing biomechanical parameters to previously reported blast-induced electrophysiological dysfunction, a threshold for deficits in long-term potentiation (LTP) was observed for strains between 3.7 and 6.7% and strain rates between 25 and 33s-1. This is the first study to experimentally determine primary blast-induced strain and strain rates in hippocampal tissue.

Investigation of wave propagation through head layers with focus on understanding blast wave transmission.

Blast-induced traumatic brain injury (bTBI) is a critical health concern. This issue is being addressed in terms of identifying a cause-effect relationship between the mechanical insult in the form of a blast and resulting injury to the brain. Understanding wave propagation through the head is an important aspect in this regard. The objective of this work was to study the blast wave propagation through the layered architecture of the head with an emphasis on understanding the wave transmission mechanism. Toward this end, one-dimensional (1D) finite element head model is built for a simplified surrogate, human, and rat. Motivated from experimental investigations, four different head layer configurations have been considered. These configurations are: (A)Skull-Brain, (B)Skin-Skull-Brain, (C)Skin-Skull-Dura-Arachnoid-CSF-Pia-Brain, (D)Skin-Skull-Dura-Arachnoid-AT-Pia-Brain. The validated head model is subjected to flattop and Friedlander loading implied in the blast, and the resulting response is evaluated in terms of brain pressures. Our results suggest that wave propagation through head parenchyma plays an important role in blast wave transmission. The thickness, material properties of head layers, and rise time of an input pulse govern the temporal evolution of pressure in the brain. The key findings of this work are: (a) Skin and meninges amplify the applied input pressure, whereas air sinus has an attenuation effect. (b) Model is able to describe experimentally recorded peak pressures and rise times in the brain, including variations within the aforementioned experimental head models of TBI. This reinforces that the wave transmission is an important loading pathway to the brain. (c) Equivalent layer theory for modeling meningeal layers as a single layer has been proposed, and it gives reasonable agreement with each meningeal layer modeled explicitly. This modeling approach has a great utility in 3D head models. The potential applications of 1D head model in evaluation of new helmet materials, brain sensor calibration, and brain pressure estimation for a given explosive strength have also been demonstrated. Overall, these results provide important insights into the understanding of mechanics of blast wave transmission in the head.

Factors associated with surgical site infection in blast-induced traumatic brain injury.

Factors associated with surgical site infection in blast-induced traumatic brain injury.

An assessment of blast modelling techniques for injury biomechanics research.

Blast-induced traumatic brain injury (TBI) has been affecting combatants and civilians. The blast pressure wave is thought to have a significant contribution to blast-related TBI. Due to the limitations and difficulties of conducting blast tests on surrogates, computational modelling has been used as a key method for exploring this field. However, the blast wave modelling methods reported in current literature have drawbacks. They either cannot generate the desirable blast pressure wave history or they are unable to accurately simulate the blast wave/structure interaction. In addition, boundary conditions, which can have significant effects on model predictions, have not been described adequately. Here, we critically assess the commonly used methods for simulating blast wave propagation in air (open-field blast) and its interaction with the human body. We investigate the predicted blast wave time history, blast wave transmission, and the effects of various boundary conditions in three-dimensional (3D) models of blast prediction. We propose a suitable meshing topology, which enables accurate prediction of blast wave propagation and interaction with the human head and significantly decreases the computational cost in 3D simulations. Finally, we predict strain and strain rate in the human brain during blast wave exposure and show the influence of the blast wave modelling methods on the brain response. The findings presented here can serve as guidelines for accurately modelling blast wave generation and interaction with the human body for injury biomechanics studies and design of prevention systems.

Physiological measurements of acoustic trauma following blast-induced traumatic brain injury (TBI) in laboratory mice

Exposure to a high intensity blast from the detonation of improvised explosive devices (IEDs) can lead to a traumatic brain injury (TBI) and concurrent trauma to major auditory structures. Research on whether this trauma is permanent or temporary has been inconclusive due to heterogeneity of injury acquisition. A blast wave system that generates consistent and controllable blast waves has been developed, allowing for investigation into the co-morbidity of auditory damage and TBI. In this study, the effects of blast exposure on auditory brainstem responses (ABRs) and distortion product otoacoustic emissions (DPOAEs) in male and female mice were measured. ABRs and DPOAEs were collected prior to exposure to a blast to represent the baseline auditory capacity of each individual subject. Mice were tested at 3, 30, and 90 days after the exposure to capture initial trauma and the degree of recovery. A shift in ABRs and DPOAEs thresholds was observed in post-exposure mice, indicating trauma to central and peripheral auditory systems. Following the 90 day tests, brains and cochleae were collected for further analysis. [Work supported by R01DC016641.]Exposure to a high intensity blast from the detonation of improvised explosive devices (IEDs) can lead to a traumatic brain injury (TBI) and concurrent trauma to major auditory structures. Research on whether this trauma is permanent or temporary has been inconclusive due to heterogeneity of injury acquisition. A blast wave system that generates consistent and controllable blast waves has been developed, allowing for investigation into the co-morbidity of auditory damage and TBI. In this study, the effects of blast exposure on auditory brainstem responses (ABRs) and distortion product otoacoustic emissions (DPOAEs) in male and female mice were measured. ABRs and DPOAEs were collected prior to exposure to a blast to represent the baseline auditory capacity of each individual subject. Mice were tested at 3, 30, and 90 days after the exposure to capture initial trauma and the degree of recovery. A shift in ABRs and DPOAEs thresholds was observed in post-exposure mice, indicating trauma to central and periphe...

Conditioned Contextual Freezing is A Neurobehavioral Biomarker of Axonal Injury Indicated by Reduced Fractional Anisotropy in A Mouse Model of Blast-Induced Mild Traumatic Brain Injury.

Mild traumatic brain injury (TBI) is an important public health problem generated by closed head injury. This study is focused on the impact of blast-induced mild TBI on auditory trace and delay fear conditioning, models of declarative and non-declarative memory, respectively, and the correlation of conditioned freezing and fractional anisotropy, a measure of axonal state. A supersonic helium pressure wave was generated by a shock tube to blast 8-week-old male mice on Day 1 for 1.4 msec with an incident pressure of 16 psi, corresponding to a reflected pressure of 56.9 psi at the mouse head. On Day 3, the mice were subjected to auditory trace- or delay-fear conditioning. On Day 4, contextual freezing in the trained context, and precue and cued freezing in a novel context were determined. After cardiac perfusion on Day 5, ex vivo images were obtained with diffusion tensor imaging at 14.1 Tesla. We observed that delay fear conditioning prevented or reversed the decrease in fractional anisotropy in both the medial and lateral corpus callosum suggesting axonal stabilization of potentially behavioral therapeutic significance. Moderately strong and statistically significant Pearson correlations were found between fractional anisotropy and contextual freezing in the medial and lateral corpus callosum of blasted and sham-blasted delay- or trace-fear conditioned mice. Thus, contextual freezing is a neurobehavioral biomarker for axonal injury in mild TBI and is a reliable and high-throughput behavioral assay for the evaluation of potential therapeutics to treat mild TBI.

Response to Blast-like Shear Stresses Associated with Mild Blast-Induced Brain Injury

Toward the goal of understanding the pathophysiology of mild blast-induced traumatic brain injury and identifying the physical forces associated with the primary injury phase, we developed a system that couples a pneumatic blast to a microfluidic channel to precisely and reproducibly deliver shear transients to dissociated human central nervous system (CNS) cells, on a timescale comparable to an explosive blast but with minimal pressure transients. Using fluorescent beads, we have characterized the shear transients experienced by the cells and demonstrate that the system is capable of accurately and reproducibly delivering uniform shear transients with minimal pressure across the cell culture volume. This system is compatible with high-resolution, time-lapse optical microscopy. Using this system, we demonstrate that blast-like shear transients produced with minimal pressure transients and submillisecond rise times activate calcium responses in dissociated human CNS cultures. Cells respond with increased cytosolic free calcium to a threshold shear stress between 8 and 21 Pa; the propagation of this calcium response is a result of purinergic signaling. We propose that this system models, in vitro, the fundamental injury wave produced by shear forces consequent to blast shock waves passing through density inhomogeneity in human CNS cells.