Aaron A. Hall

Blast Exposure Induces Post-Traumatic Stress Disorder-Related Traits in a Rat Model of Mild Traumatic Brain Injury

Blast related traumatic brain injury (TBI) has been a major cause of injury in the wars in Iraq and Afghanistan. A striking feature of the mild TBI (mTBI) cases has been the prominent association with post-traumatic stress disorder (PTSD). However, because of the overlapping symptoms, distinction between the two disorders has been difficult. We studied a rat model of mTBI in which adult male rats were exposed to repetitive blast injury while under anesthesia. Blast exposure induced a variety of PTSD-related behavioral traits that were present many months after the blast exposure, including increased anxiety, enhanced contextual fear conditioning, and an altered response in a predator scent assay. We also found elevation in the amygdala of the protein stathmin 1, which is known to influence the generation of fear responses. Because the blast overpressure injuries occurred while animals were under general anesthesia, our results suggest that a blast-related mTBI exposure can, in the absence of any psychological stressor, induce PTSD-related traits that are chronic and persistent. These studies have implications for understanding the relationship of PTSD to mTBI in the population of veterans returning from the wars in Iraq and Afghanistan.

Assessment of the effects of acute and repeated exposure to blast overpressure in rodents: toward a greater understanding of blast and the potential ramifications for injury in humans exposed to blast

Mild traumatic brain injury (mTBI) resulting from exposure to improvised explosive devices (IEDs) has fueled a requirement to develop animals models that mirror this condition using exposure to blast overpressure (BOP). En route to developing a model of repeated exposure to BOP we sought to initially characterize the effects of acute BOP exposure in rodents, focusing specifically on the levels of BOP exposure that produced clinical mTBI symptoms. We first measured BOP effects on gross motor function on a balance beam. Separate groups of unanesthetized rats were exposed (in different orientations) to 36.6, 74.5, and 116.7 kPa BOP exposure inside a pneumatically driven shock tube. Results demonstrated that rats exposed to 116.7 kPa demonstrated transient alterations or loss of consciousness indicated by a transient loss of righting and by increased latencies on the balance beam. The 116.7 kPa exposure was the threshold for overt pathology for acute BOP exposure with approximately 30% of rats presenting with evidence of subdural hemorrhage and cortical contusions. All animals exposed to 116.7 kPa BOP manifested evidence of significant pulmonary hemorrhage. Anterograde memory deficits were observed in rats exposed to 74.5 kPa facing the BOP wave and rats exposed to 116.7 kPa in the lateral (side) orientation. We next assessed repeated exposure to either lateral or frontal 36.6 kPa BOP in anesthetized rats, once per day for 12 days. Results showed that repeated exposure in the frontal, but not side, orientation to the BOP wave produced a transitory learning deficit on a Morris water maze task as shown by significantly longer latencies to reach the submerged platform in the second and third blocks of a four block session. Implications of these data are discussed in relation to the manifestation of mTBI in military personnel exposed to IEDs. Finally, we suggest that there are multiple types of long-term brain injury from blast exposure.

Blast overpressure induces shear-related injuries in the brain of rats exposed to a mild traumatic brain injury

BackgroundBlast-related traumatic brain injury (TBI) has been a significant cause of injury in the military operations of Iraq and Afghanistan, affecting as many as 10-20% of returning veterans. However, how blast waves affect the brain is poorly understood. To understand their effects, we analyzed the brains of rats exposed to single or multiple (three) 74.5 kPa blast exposures, conditions that mimic a mild TBI.ResultsRats were sacrificed 24 hours or between 4 and 10 months after exposure. Intraventricular hemorrhages were commonly observed after 24 hrs. A screen for neuropathology did not reveal any generalized histopathology. However, focal lesions resembling rips or tears in the tissue were found in many brains. These lesions disrupted cortical organization resulting in some cases in unusual tissue realignments. The lesions frequently appeared to follow the lines of penetrating cortical vessels and microhemorrhages were found within some but not most acute lesions.ConclusionsThese lesions likely represent a type of shear injury that is unique to blast trauma. The observation that lesions often appeared to follow penetrating cortical vessels suggests a vascular mechanism of injury and that blood vessels may represent the fault lines along which the most damaging effect of the blast pressure is transmitted.

Selective vulnerability of the cerebral vasculature to blast injury in a rat model of mild traumatic brain injury

BackgroundBlast-related traumatic brain injury (TBI) is a common cause of injury in the military operations in Iraq and Afghanistan. How the primary blast wave affects the brain is not well understood. The aim of the present study was to examine whether blast exposure affects the cerebral vasculature in a rodent model. We analyzed the brains of rats exposed to single or multiple (three) 74.5 kPa blast exposures, conditions that mimic a mild TBI. Rats were sacrificed 24 hours or between 6 and 10 months after exposure. Blast-induced cerebral vascular pathology was examined by a combination of light microscopy, immunohistochemistry, and electron microscopy.ResultsWe describe a selective vascular pathology that is present acutely at 24 hours after injury. The vascular pathology is found at the margins of focal shear-related injuries that, as we previously showed, typically follow the patterns of penetrating cortical vessels. However, changes in the microvasculature extend beyond the margins of such lesions. Electron microscopy revealed that microvascular pathology is found in regions of the brain with an otherwise normal neuropil. This initial injury leads to chronic changes in the microvasculature that are still evident many months after the initial blast exposure.ConclusionsThese studies suggest that vascular pathology may be a central mechanism in the induction of chronic blast-related injury.

Acute blast injury reduces brain abeta in two rodent species

Blast-induced traumatic brain injury (TBI) has been a major cause of morbidity and mortality in the conflicts in Iraq and Afghanistan. How the primary blast wave affects the brain is not well understood. In particular, it is unclear whether blast injures the brain through mechanisms similar to those found in non-blast closed impact injuries (nbTBI). The β-amyloid (Aβ) peptide associated with the development of Alzheimer’s disease is elevated acutely following TBI in humans as well as in experimental animal models of nbTBI. We examined levels of brain Aβ following experimental blast injury using enzyme-linked immunosorbent assays for Aβ 40 and 42. In both rat and mouse models of blast injury, rather than being increased, endogenous rodent brain Aβ levels were decreased acutely following injury. Levels of the amyloid precursor protein (APP) were increased following blast exposure although there was no evidence of axonal pathology based on APP immunohistochemical staining. Unlike the findings in nbTBI animal models, levels of the β-secretase, β-site APP cleaving enzyme 1, and the γ-secretase component presenilin-1 were unchanged following blast exposure. These studies have implications for understanding the nature of blast injury to the brain. They also suggest that strategies aimed at lowering Aβ production may not be effective for treating acute blast injury to the brain.

Vigabatrin prevents seizure in swine subjected to hyperbaric hyperoxia

Oxygen is the most widely used therapeutic strategy to prevent and treat decompression sickness (DCS). Oxygen prebreathe (OPB) eliminated DCS in 20-kg swine after rapid decompression from saturation at 60 feet of seawater (fsw). However, hyperbaric oxygen (HBO) has risks. As oxygen partial pressure increases, so do its toxic effects. Central nervous system (CNS) oxygen toxicity is the most severe side effect, manifesting as seizure. An adjunctive therapeutic is needed to extend OPB strategies to deeper depths and prevent/delay seizure onset. The Food and Drug Administration-approved anti-epileptic vigabatrin has prevented HBO-induced seizures in rats up to 132 fsw. This study aimed to confirm the rat findings in a higher animal model and determine whether acute high-dose vigabatrin evokes retinotoxicity symptoms seen with chronic use clinically in humans. Vigabatrin dose escalation studies were conducted 20-kg swine exposed to HBO at 132 or 165 fsw. The saline group had seizure latencies of 7 and 11 min at 165 and 132 fsw, respectively. Vigabatrin at 180 mg/kg significantly increased latency (13 and 27 min at 165 and 132 fsw, respectively); 250 mg/kg abolished seizure activity at all depths. Functional electroretinogram and histology of the retinas showed no signs of retinal toxicity in any of the vigabatrin=treated animals. In the 250 mg/kg group there was no evidence of CNS oxygen toxicity; however, pulmonary oxygen toxicity limited HBO exposure. Together, the findings from this study show that vigabatrin therapy is efficacious at preventing CNS oxygen toxicity in swine, and a single dose is not acutely associated with retinotoxicity.

Repeated Low Intensity Blast Exposure Is Associated with Damaged Endothelial Glycocalyx and Downstream Behavioral Deficits

Current clinical research into mild traumatic brain injury (mTBI) has focused on white matter changes as identified by advanced MRI based imaging techniques. However, perivascular tau accumulation in the brains of individuals diagnosed with mTBI suggests that the vasculature plays a key role in the pathology. This study used a rat model to examine whether the endothelial glycocalyx, a layer of the vasculature responsible for sensing luminal shear forces, is damaged by exposure to repeated low intensity blast, and whether this layer is associated with observed behavioral deficits. The blast exposure used consisted of 12, 40 kPa blast exposures conducted with a minimum of 24 h between blasts. We found that repeated blast exposure reduced glycocalyx length and density in various brain regions indicating damage. This blast exposure paradigm was associated with a mild performance decrement in the Morris water maze (MWM) which assesses learning and memory. Administration of hyaluronidase, an enzyme that binds to and degrades hyaluronan (a major structural component of the glycocalyx) prior to blast exposure reduced the observed behavioral deficits and induced a thickening of the glycocalyx layer. Taken together these findings demonstrate that the endothelial glycocalyx degradation following repeated blast is associated with behavioral decrements which can be prevented by treatment with hyaluronidase.

Perfluorocarbon in Delayed Recompression with a Mixed Gender Swine Model of Decompression Sickness

INTRODUCTION Perfluorocarbons (PFC) are fluorinated hydrocarbons that dissolve gases to a much greater degree than plasma and hold promise in treating decompression sickness (DCS). The efficacy of PFC in a mixed gender model of DCS and safety in recompression therapy has not been previously explored. METHODS Swine (25 kg; N = 104; 51 male and 53 female) were randomized into normal saline solution (NSS) or PFC emulsion treatment groups and subjected to compression on air in a hyperbaric chamber at 200 fsw for 31 min. Then the animals were decompressed and observed for signs of DCS. Afterwards, they were treated with oxygen and either PFC (4 cc · kg-1) or NSS (4 cc · kg-1). Surviving animals were observed for 4 h, at which time they underwent recompression therapy using a standard Navy Treatment Table 6. After 24 h the animals were assessed and then euthanized. RESULTS Survival rates were not significantly different between NSS (74.04%) and PFC (66.67%) treatment groups. All swine that received recompression treatment survived to the end of the study and no seizures were observed in either PFC or NSS animals. Within the saline treated swine group there were no significant differences in DCS survival between male (75.00%, N = 24) and female (73.08%, N = 26) swine. Within the PFC treated swine, survival of females (51.85%, N = 27) was significantly lower than males (81.48%, N = 27). DISCUSSION In this large animal mixed gender efficacy study in DCS, PFC did not improve mortality or spinal cord injury, but appears safe during recompressive therapy. Gender differences in DCS treatment with PFC will need further study.Cronin WA, Hall AA, Auker CR, Mahon RT. Perfluorocarbon in delayed recompression with a mixed gender swine model of decompression sickness. Aerosp Med Hum Perform. 2018; 89(1):14-18.

Propranolol Effects on Decompression Sickness in a Simulated DISSUB Rescue in Swine

INTRODUCTION Disabled submarine (DISSUB) survivors may face elevated CO2 levels and inert gas saturation, putting them at risk for CO2 toxicity and decompression sickness (DCS). Propranolol was shown to reduce CO2 production in an experimental DISSUB model in humans but its effects on DCS in a DISSUB rescue scenario are unknown. A 100% oxygen prebreathe (OPB) reduces DCS incidence and severity and is incorporated into some DISSUB rescue protocols. We used a swine model of DISSUB rescue to study the effect of propranolol on DCS incidence and mortality with and without an OPB. METHODS In Experiment 1, male Yorkshire Swine (70 kg) were pressurized to 2.8 ATA for 22 h. Propranolol 1.0 mg · kg-1 (IV) was administered at 21.25 h. At 22 h, the animal was rapidly decompressed and observed for DCS type, onset time, and mortality. Experimental animals (N = 21; 69 ± 4.1 kg), PROP1.0, were compared to PROP1.0-OPB45 (N = 8; 69 ± 2.8 kg) with the same dive profile, except for a 45 min OPB prior to decompression. In Experiment 2, the same methodology was used with the following changes: swine pressurized to 2.8 ATA for 28 h; experimental group (N = 25; 67 ± 3.3 kg), PROP0.5 bis, propranolol 0.5 mg · kg-1 bis (twice) (IV) was administered at 22 h and 26 h. Control animals (N = 25; 67 ± 3.9 kg) received normal saline. RESULTS OPB reduced mortality in PROP1.0-OBP45 compared to PROP1.0 (0% vs. 71%). PROP0.5 bis had increased mortality compared to CONTROL (60-% vs. 4%). DISCUSSION Administration of beta blockers prior to saturation decompression appears to increase DCS and worsen mortality in a swine model; however, their effects in bounce diving remain unknown.Forbes AS, Regis DP, HallAA, Mahon RT, Cronin WA. Propranolol effects on decompression sickness in a simulated DISSUB rescue in swine. Aerosp Med Hum Perform. 2017; 88(4):385-391.

Correlation of rheoencephalography and laser Doppler flow: a rat study

Abstract Measuring brain electrical impedance (rheoencephalography) is a potential technique for noninvasive, continuous neuro-monitoring of cerebral blood flow autoregulation in humans. In the present rat study, we compared changes in cerebral blood flow autoregulation during CO2 inhalation measured by rheoencephalography to changes measured by laser Doppler flowmetry, an invasive continuous monitoring modality. Our hypothesis was that both modalities would reflect cerebral blood flow autoregulation. Male Sprague-Dawley rats (n=28; 28 control and 82 CO2 challenges) were measured under anesthesia. The surgical preparation involved implantation of intracerebral REG electrodes and an LDF probe into the brain. Analog waveforms were stored in a computer. CO2 inhalation caused transient, simultaneous increases in the signals of both laser Doppler flow (171.99 ± 46.68 %) and rheoencephalography (329.88 ± 175.50%). These results showed a correlation between the two measured modalities; the area under the receiver operating characteristic curve was 0.8394. The similar results obtained by measurements made with laser Doppler flowmetry and rheoencephalography indicate that rheo-encephalography, like laser Doppler flowmetry, reflects cerebral blood flow autoregulation. Rheoencephalography therefore shows potential for use as a continuous neuro-monitoring technique.