Blast-induced "PTSD": Evidence from an animal model

Metabotropic Glutamate Receptor 2 Expression Is Chronically Elevated in Male Rats With Post-Traumatic Stress Disorder Related Behavioral Traits Following Repetitive Low-Level Blast Exposure

Many military veterans who experienced blast-related traumatic brain injuries in the conflicts in Iraq and Afghanistan currently suffer from chronic cognitive and mental health problems that include depression and post-traumatic stress disorder (PTSD). Male rats exposed to repetitive low-level blast develop cognitive and PTSD-related behavioral traits that are present for more than 1 year after exposure. We previously reported that a group II metabotropic receptor (mGluR2/3) antagonist reversed blast-induced behavioral traits. In this report, we explored mGluR2/3 expression following blast exposure in male rats. Western blotting revealed that mGluR2 protein (but not mGluR3) was increased in all brain regions studied (anterior cortex, hippocampus, and amygdala) at 43 or 52 weeks after blast exposure but not at 2 weeks or 6 weeks. mGluR2 RNA was elevated at 52 weeks while mGluR3 was not. Immunohistochemical staining revealed no changes in the principally presynaptic localization of mGluR2 by blast exposure. Administering the mGluR2/3 antagonist LY341495 after behavioral traits had emerged rapidly reversed blast-induced effects on novel object recognition and cued fear responses 10 months following blast exposure. These studies support alterations in mGluR2 receptors as a key pathophysiological event following blast exposure and provide further support for group II metabotropic receptors as therapeutic targets in the neurobehavioral effects that follow blast injury.

Bedside to bench: the outlook for psychedelic research

There has recently been a resurgence of interest in psychedelic compounds based on studies demonstrating their potential therapeutic applications in treating post-traumatic stress disorder, substance abuse disorders, and treatment-resistant depression. Despite promising efficacy observed in some clinical trials, the full range of biological effects and mechanism(s) of action of these compounds have yet to be fully established. Indeed, most studies to date have focused on assessing the psychological mechanisms of psychedelics, often neglecting the non-psychological modes of action. However, it is important to understand that psychedelics may mediate their therapeutic effects through multi-faceted mechanisms, such as the modulation of brain network activity, neuronal plasticity, neuroendocrine function, glial cell regulation, epigenetic processes, and the gut-brain axis. This review provides a framework supporting the implementation of a multi-faceted approach, incorporating in silico, in vitro and in vivo modeling, to aid in the comprehensive understanding of the physiological effects of psychedelics and their potential for clinical application beyond the treatment of psychiatric disorders. We also provide an overview of the literature supporting the potential utility of psychedelics for the treatment of brain injury (e.g., stroke and traumatic brain injury), neurodegenerative diseases (e.g., Parkinson's and Alzheimer's diseases), and gut-brain axis dysfunction associated with psychiatric disorders (e.g., generalized anxiety disorder and major depressive disorder). To move the field forward, we outline advantageous experimental frameworks to explore these and other novel applications for psychedelics.

Neuro-psychiatric symptoms in directly and indirectly blast exposed civilian survivors of urban missile attacks

BACKGROUND

Blast-explosion may cause traumatic brain injury (TBI), leading to post-concussion syndrome (PCS). In studies on military personnel, PCS symptoms are highly similar to those occurring in post-traumatic stress disorder (PTSD), questioning the overlap between these syndromes. In the current study we assessed PCS and PTSD in civilians following exposure to rocket attacks. We hypothesized that PCS symptomatology and brain connectivity will be associated with the objective physical exposure, while PTSD symptomatology will be associated with the subjective mental experience.

METHODS

Two hundred eighty nine residents of explosion sites have participated in the current study. Participants completed self-report of PCS and PTSD. The association between objective and subjective factors of blast and clinical outcomes was assessed using multivariate analysis. White-matter (WM) alterations and cognitive abilities were assessed in a sub-group of participants (n = 46) and non-exposed controls (n = 16). Non-parametric analysis was used to compare connectivity and cognition between the groups.

RESULTS

Blast-exposed individuals reported higher PTSD and PCS symptomatology. Among exposed individuals, those who were directly exposed to blast, reported higher levels of subjective feeling of danger and presented WM hypoconnectivity. Cognitive abilities did not differ between groups. Several risk factors for the development of PCS and PTSD were identified.

CONCLUSIONS

Civilians exposed to blast present higher PCS/PTSD symptomatology as well as WM hypoconnectivity. Although symptoms are sub-clinical, they might lead to the future development of a full-blown syndrome and should be considered carefully. The similarities between PCS and PTSD suggest that despite the different etiology, namely, the physical trauma in PCS and the emotional trauma in PTSD, these are not distinct syndromes, but rather represent a combined biopsychological disorder with a wide spectrum of behavioral, emotional, cognitive and neurological symptoms.

(2R,6R)-Hydroxynorketamine Treatment of Rats Exposed to Repetitive Low-Level Blast Injury

Many military veterans who experienced blast-related traumatic brain injuries (TBIs) in the conflicts in Iraq and Afghanistan suffer from chronic cognitive and mental health problems, including post-traumatic stress disorder (PTSD). Male rats subjected to repetitive low-level blast exposure develop chronic cognitive and PTSD-related traits that develop in a delayed manner. Ketamine has received attention as a treatment for refractory depression and PTSD. (2R,6R)-hydroxynorketamine [(2R,6R)-HNK] is a ketamine metabolite that exerts rapid antidepressant actions. (2R,6R)-HNK has become of clinical interest because of its favorable side-effect profile, low abuse potential, and oral route of administration. We treated three cohorts of blast-exposed rats with (2R,6R)-HNK, beginning 7-11 months after blast exposure, a time when the behavioral phenotype is established. Each cohort consisted of groups (n = 10-13/group) as follows: 1) Sham-exposed treated with saline, 2) blast-exposed treated with saline, and 3) blast-exposed treated with a single dose of 20 mg/kg of (2R,6R)-HNK. (2R,6R)-HNK rescued blast-induced deficits in novel object recognition (NOR) and anxiety-related features in the elevated zero maze (EZM) in all three cohorts. Exaggerated acoustic startle was reversed in cohort 1, but not in cohort 3. (2R,6R)-HNK effects were still present in the EZM 12 days after administration in cohort 1 and 27 days after administration in NOR testing of cohorts 2 and 3. (2R,6R)-HNK may be beneficial for the neurobehavioral syndromes that follow blast exposure in military veterans. Additional studies will be needed to determine whether higher doses or more extended treatment regimens may be more effective.

Military Exposures Predict Mental Health Symptoms in Explosives Personnel but Not Always as Expected

OBJECTIVE

The aim of this study was to determine the unique and combined associations of various military stress exposures with positive and negative mental health symptoms in active duty service members.

MATERIALS AND METHODS

We investigated 87 male U.S. Navy Explosive Ordnance Disposal (EOD) technicians (age M ± SE, range 33.7 ± 0.6, 22-47 years). Those who endorsed a positive traumatic brain injury diagnosis were excluded to eliminate the confounding effects on mental health symptoms. Using a survey platform on a computer tablet, EOD technicians self-reported combat exposure, deployment frequency (total number of deployments), blast exposure (vehicle crash/blast or 50-m blast involvement), depression, anxiety, posttraumatic stress, perceived stress, and life satisfaction during an in-person laboratory session.

RESULTS

When controlling for other military stressors, EOD technicians with previous involvement in a vehicle crash/blast endorsed worse mental health than their nonexposed counterparts. The interactions of vehicle crash/blast with deployment frequency and combat exposure had moderate effect sizes, and combat and deployment exposures demonstrated protective, rather than catalytic, effects on negative mental health scores.

CONCLUSIONS

Military stressors may adversely influence self-reported symptoms of negative mental health, but deployment experience and combat exposure may confer stress inoculation.

Progressive Transcriptional Changes in the Amygdala Implicate Neuroinflammation in the Effects of Repetitive Low-Level Blast Exposure in Male Rats

Chronic mental health problems are common among military veterans who sustained blast-related traumatic brain injuries. The reasons for this association remain unexplained. Male rats exposed to repetitive low-level blast overpressure (BOP) exposures exhibit chronic cognitive and post-traumatic stress disorder (PTSD)-related traits that develop in a delayed fashion. We examined blast-induced alterations on the transcriptome in four brain areas (anterior cortex, hippocampus, amygdala, and cerebellum) across the time frame over which the PTSD-related behavioral phenotype develops. When analyzed at 6 weeks or 12 months after blast exposure, relatively few differentially expressed genes (DEGs) were found. However, longitudinal analysis of amygdala, hippocampus, and anterior cortex between 6 weeks and 12 months revealed blast-specific DEG patterns. Six DEGs (hyaluronan and proteoglycan link protein 1 [Hapln1], glutamate metabotropic receptor 2 [Grm2], purinergic receptor P2y12 [P2ry12], C-C chemokine receptor type 5 [Ccr5], phenazine biosynthesis-like protein domain containing 1 [Pbld1], and cadherin related 23 [Cdh23]) were found altered in all three brain regions in blast-exposed animals. Pathway enrichment analysis using all DEGs or those uniquely changed revealed different transcription patterns in blast versus sham. In particular, the amygdala in blast-exposed animals had a unique set of enriched pathways related to stress responses, oxidative phosphorylation, and mitochondrial dysfunction. Upstream analysis implicated tumor necrosis factor (TNF)α signaling in blast-related effects in amygdala and anterior cortex. Eukaryotic initiating factor eIF4E (EIF4e), an upstream regulator of P2ry12 and Ccr5, was predicted to be activated in the amygdala. Quantitative polymerase chain reaction (qPCR) validated longitudinal changes in two TNFα regulated genes (cathepsin B [Ctsb], Hapln1), P2ry12, and Grm2. These studies have implications for understanding how blast injury damages the brain and implicates inflammation as a potential therapeutic target.

Increased Fear Generalization and Amygdala AMPA Receptor Proteins in Chronic Traumatic Brain Injury

Cognitive impairments and emotional lability are common long-term consequences of traumatic brain injury (TBI). How TBI affects interactions between sensory, cognitive, and emotional systems may reveal mechanisms that underlie chronic mental health comorbidities. Previously, we reported changes in auditory-emotional network activity and enhanced fear learning early after TBI. In the current study, we asked whether TBI has long-term effects on fear learning and responses to novel stimuli. Four weeks following lateral fluid percussion injury (FPI) or sham surgery, adult male rats were fear conditioned to either white noise-shock or tone-shock pairing, or shock-only control and subsequently were tested for freezing to context and to the trained or novel auditory cues in a new context. FPI groups showed greater freezing to their trained auditory cue, indicating long-term TBI enhanced fear. Interestingly, FPI-Noise Shock animals displayed robust fear to the novel, untrained tone compared with Sham-Noise Shock across both experiments. Shock Only groups did not differ in freezing to either auditory stimulus. These findings suggest that TBI precipitates maladaptive associative fear generalization rather than non-associative sensitization. Basolateral amygdala (BLA) α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAr) subunits GluA1 and GluA2 levels were analyzed and the FPI-Noise Shock group had increased GluA1 (but not GluA2) levels that correlated with the level of tone fear generalization. This study illustrates a unique chronic TBI phenotype with both a cognitive impairment and increased fear and possibly altered synaptic transmission in the amygdala long after TBI, where stimulus generalization may underlie maladaptive fear and hyperarousal.

Resilience of females to acute blood–brain barrier damage and anxiety behavior following mild blast traumatic brain injury

Low-level blast exposure can result in neurological impairment for military personnel. Currently, there is a lack of experimental data using sex as a biological variable in neurovascular outcomes following blast exposure. To model mild blast traumatic brain injury (mbTBI), male and female rats were exposed to a single 11 psi static peak overpressure blast wave using the McMillan blast device and cohorts were then euthanized at 6 h, 24 h, 7 d and 14 d post-blast followed by isolation of the amygdala. After mbTBI, animals experience immediate bradycardia, although no changes in oxygen saturation levels or weight loss are observed. Male mbTBI animals displayed significantly higher levels of anxiety-like behavior (open field and elevated plus maze) compared to male sham groups; however, there was no anxiety phenotype in female mbTBI animals. Blast-induced neurovascular damage was explored by measuring expression of tight junction (TJ) proteins (zonula occludens-1 (ZO-1), occludin and claudin-5), glial fibrillary acidic protein (GFAP) and astrocyte end-feet coverage around the blood–brain barrier (BBB). Western blot analysis demonstrates that TJ protein levels were significantly decreased at 6 h and 24 h post-mbTBI in male rats, but not in female rats, compared to sham. Female animals have decreased GFAP at 6 h post-mbTBI while male animals display decreased GFAP expression at 24 h post-mbTBI. By 7 d post-mbTBI, there were no significant differences in TJ or GFAP levels between groups in either sex. At 24 h post-mbTBI, vascular integrity and astrocytic end-feet coverage around the BBB was significantly decreased in males following mbTBI. These results demonstrate that loss of GFAP expression may be due to astrocytic damage at the BBB. Our findings also demonstrate sex differences in acute vascular and behavioral outcomes after single mbTBI. Female animals display a lack of BBB pathology after mbTBI corresponding to improved acute neuropsychological outcomes as compared to male animals.

Limbic Responses Following Shock Wave Exposure in Male and Female Mice

Blast traumatic brain injury (bTBI) presents a serious threat to military personnel and often results in psychiatric conditions related to limbic system dysfunction. In this study, the functional outcomes for anxiety- and depressive-like behaviors and neuronal activation were evaluated in male and female mice after exposure to an Advanced Blast Simulator (ABS) shock wave. Mice were placed in a ventrally exposed orientation inside of the ABS test section and received primary and tertiary shock wave insults of approximately 15 psi peak pressure. Evans blue staining indicated cases of blood-brain barrier breach in the superficial cerebral cortex four, but not 24 h after blast, but the severity was variable. Behavioral testing with the elevated plus maze (EPM) or elevated zero maze (EZM), sucrose preference test (SPT), and tail suspension test (TST) or forced swim test (FST) were conducted 8 days–3.5 weeks after shock wave exposure. There was a sex difference, but no injury effect, for distance travelled in the EZM where female mice travelled significantly farther than males. The SPT and FST did not indicate group differences; however, injured mice were less immobile than sham mice during the TST; possibly indicating more agitated behavior. In a separate cohort of animals, the expression of the immediate early gene, c-Fos, was detected 4 h after undergoing bTBI or sham procedures. No differences in c-Fos expression were found in the cerebral cortex, but female mice in general displayed enhanced c-Fos activation in the paraventricular nucleus of the thalamus (PVT) compared to male mice. In the amygdala, more c-Fos-positive cells were observed in injured animals compared to sham mice. The observed sex differences in the PVT and c-Fos activation in the amygdala may correlate with the reported hyperactivity of females post-injury. This study demonstrates, albeit with mild effects, behavioral and neuronal activation correlates in female rodents after blast injury that could be relevant to the incidence of increased post-traumatic stress disorder in women.

Emerging Evidence for the Widespread Role of Glutamatergic Dysfunction in Neuropsychiatric Diseases

The monoamine model of depression has long formed the basis of drug development but fails to explain treatment resistance or associations with stress or inflammation. Recent animal research, clinical trials of ketamine (a glutamate receptor antagonist), neuroimaging research, and microbiome studies provide increasing evidence of glutamatergic dysfunction in depression and other disorders. Glutamatergic involvement across diverse neuropathologies including psychoses, neurodevelopmental, neurodegenerative conditions, and brain injury forms the rationale for this review. Glutamate is the brain's principal excitatory neurotransmitter (NT), a metabolic and synthesis substrate, and an immune mediator. These overlapping roles and multiple glutamate NT receptor types complicate research into glutamate neurotransmission. The glutamate microcircuit comprises excitatory glutamatergic neurons, astrocytes controlling synaptic space levels, through glutamate reuptake, and inhibitory GABA interneurons. Astroglia generate and respond to inflammatory mediators. Glutamatergic microcircuits also act at the brain/body interface via the microbiome, kynurenine pathway, and hypothalamus-pituitary-adrenal axis. Disruption of excitatory/inhibitory homeostasis causing neuro-excitotoxicity, with neuronal impairment, causes depression and cognition symptoms via limbic and prefrontal regions, respectively. Persistent dysfunction reduces neuronal plasticity and growth causing neuronal death and tissue atrophy in neurodegenerative diseases. A conceptual overview of brain glutamatergic activity and peripheral interfacing is presented, including the common mechanisms that diverse diseases share when glutamate homeostasis is disrupted.

Repetitive Blast Exposure Increases Appetitive Motivation and Behavioral Inflexibility in Male Mice

Blast exposure (via detonation of high explosives) represents a major potential trauma source for Servicemembers and Veterans, often resulting in mild traumatic brain injury (mTBI). Executive dysfunction (e.g., alterations in memory, deficits in mental flexibility, difficulty with adaptability) is commonly reported by Veterans with a history of blast-related mTBI, leading to impaired daily functioning and decreased quality of life, but underlying mechanisms are not fully understood and have not been well studied in animal models of blast. To investigate potential underlying behavioral mechanisms contributing to deficits in executive functioning post-blast mTBI, here we examined how a history of repetitive blast exposure in male mice affects anxiety/compulsivity-like outcomes and appetitive goal-directed behavior using an established mouse model of blast mTBI. We hypothesized that repetitive blast exposure in male mice would result in anxiety/compulsivity-like outcomes and corresponding performance deficits in operant-based reward learning and behavioral flexibility paradigms. Instead, results demonstrate an increase in reward-seeking and goal-directed behavior and a congruent decrease in behavioral flexibility. We also report chronic adverse behavioral changes related to anxiety, compulsivity, and hyperarousal. In combination, these data suggest that potential deficits in executive function following blast mTBI are at least in part related to enhanced compulsivity/hyperreactivity and behavioral inflexibility and not simply due to a lack of motivation or inability to acquire task parameters, with important implications for subsequent diagnosis and treatment management.

Long-Term Effects of Low-Intensity Blast Non-Inertial Brain Injury on Anxiety-Like Behaviors in Mice: Home-Cage Monitoring Assessments

Mild traumatic brain injury induced by low-intensity blast (LIB) exposure poses concerns in military personnel. Using an open-field, non-inertial blast model and assessments by conventional behavioral tests, our previous studies revealed early-phase anxiety-like behaviors in LIB-exposed mice. However, the impact of LIB upon long-term anxiety-like behaviors requires clarification. This study applied a highly sensitive automated home-cage monitoring (HCM) system, which minimized human intervention and environmental changes, to assess anxiety-like responses in mice 3 months after LIB exposure. Initial assessment of 72-h spontaneous activities in a natural cage condition over multiple light and dark phases showed altered sheltering behaviors. LIB-exposed mice exhibited a subtle, but significantly decreased, duration of short shelter visits as compared to sham controls. Other measured responses between LIB-exposed mice and sham controls were insignificant. When behavioral assessments were performed in a challenged condition using an aversive spotlight, LIB-exposed mice demonstrated a significantly higher frequency of movements of shorter distance and duration per movement. Taken together, these findings demonstrated the presence of chronic anxiety-like behaviors assessed by the HCM system under both natural and challenged conditions in mice occurring post-LIB exposure. This model thus provides a platform to test for screening and interventions on anxiety disorders occurring after LIB non-inertial brain injury.

Transcranial Laser Therapy Does Not Improve Cognitive and Post-Traumatic Stress Disorder-Related Behavioral Traits in Rats Exposed to Repetitive Low-Level Blast Injury

Many military veterans who experienced blast-related traumatic brain injuries (TBIs) in the conflicts in Iraq and Afghanistan suffer from chronic cognitive and mental health problems, including post-traumatic stress disorder (PTSD). Transcranial laser therapy (TLT) uses low-power lasers emitting light in the far- to near-infrared ranges. Beneficial effects of TLT have been reported in neurological and mental-health-related disorders in humans and animal models, including TBI. Rats exposed to repetitive low-level blast develop chronic cognitive and PTSD-related behavioral traits. We tested whether TLT treatment could reverse these traits. Rats received a 74.5-kPa blast or sham exposures delivered one per day for 3 consecutive days. Beginning at 34 weeks after blast exposure, the following groups of rats were treated with active or sham TLT: 1) Sham-exposed rats (n = 12) were treated with sham TLT; 2) blast-exposed rats (n = 13) were treated with sham TLT; and 3) blast-exposed rats (n = 14) were treated with active TLT. Rats received 5 min of TLT five times per week for 6 weeks (wavelength, 808 nm; power of irradiance, 240 mW). At the end of treatment, rats were tested in tasks found previously to be most informative (novel object recognition, novel object localization, contextual/cued fear conditioning, elevated zero maze, and light/dark emergence). TLT did not improve blast-related effects in any of these tests, and blast-exposed rats were worse after TLT in some anxiety-related measures. Based on these findings, TLT does not appear to be a promising treatment for the chronic cognitive and mental health problems that follow blast injury.

The Neurobehavioral Effects of Buprenorphine and Meloxicam on a Blast-Induced Traumatic Brain Injury Model in the Rat

Previous findings have indicated that pain relieving medications such as opioids and non-steroidal anti-inflammatory drugs (NSAIDs) may be neuroprotective after traumatic brain injury in rodents, but only limited studies have been performed in a blast-induced traumatic brain injury (bTBI) model. In addition, many pre-clinical TBI studies performed in rodents did not use analgesics due to the possibility of neuroprotection or other changes in cognitive, behavioral, and pathology outcomes. To examine this in a pre-clinical setting, we examined the neurobehavioral changes in rats given a single pre-blast dose of meloxicam, buprenorphine, or no pain relieving medication and exposed to tightly-coupled repeated blasts in an advanced blast simulator and evaluated neurobehavioral functions up to 28 days post-blast. A 16.7% mortality rate was recorded in the rats treated with buprenorphine, which might be attributed to the physiologically depressive side effects of buprenorphine in combination with isoflurane anesthesia and acute brain injury. Rats given buprenorphine, but not meloxicam, took more time to recover from the isoflurane anesthesia given just before blast. We found that treatment with meloxicam protected repeated blast-exposed rats from vestibulomotor dysfunctions up to day 14, but by day 28 the protective effects had receded. Both pain relieving medications seemed to promote short-term memory deficits in blast-exposed animals, whereas vehicle-treated blast-exposed animals showed only a non-significant trend toward worsening short-term memory by day 27. Open field exploratory behavior results showed that blast exposed rats treated with meloxicam engaged in significantly more locomotor activities and possibly a lesser degree of responses thought to reflect anxiety and depressive-like behaviors than any of the other groups. Rats treated with analgesics to alleviate possible pain from the blast ate more than their counterparts that were not treated with analgesics, which supports that both analgesics were effective in alleviating some of the discomfort that these rats potentially experienced post-blast injury. These results suggest that meloxicam and, to a lesser extent buprenorphine alter a variety of neurobehavioral functions in a rat bTBI model and, because of their impact on these neurobehavioral changes, may be less than ideal analgesic agents for pre-clinical studies evaluating these neurobehavioral responses after TBI.

Unconventional animal models for traumatic brain injury and chronic traumatic encephalopathy

Traumatic brain injury (TBI) is one of the main causes of death worldwide. It is a complex injury that influences cellular physiology, causes neuronal cell death, and affects molecular pathways in the brain. This in turn can result in sensory, motor, and behavioral alterations that deeply impact the quality of life. Repetitive mild TBI can progress into chronic traumatic encephalopathy (CTE), a neurodegenerative condition linked to severe behavioral changes. While current animal models of TBI and CTE such as rodents, are useful to explore affected pathways, clinical findings therein have rarely translated into clinical applications, possibly because of the many morphofunctional differences between the model animals and humans. It is therefore important to complement these studies with alternative animal models that may better replicate the individuality of human TBI. Comparative studies in animals with naturally evolved brain protection such as bighorn sheep, woodpeckers, and whales, may provide preventive applications in humans. The advantages of an in-depth study of these unconventional animals are threefold. First, to increase knowledge of the often-understudied species in question; second, to improve common animal models based on the study of their extreme counterparts; and finally, to tap into a source of biological inspiration for comparative studies and translational applications in humans.

Drug Repurposing: Promises of Edaravone Target Drug in Traumatic Brain Injury

Edaravone is a potent free-radical scavenger that has been in the market for more than 30 years. It was originally developed in Japan to treat strokes and has been used there since 2001. Aside from its anti-oxidative effects, edaravone demonstrated beneficial effects on proinflammatory responses, nitric oxide production, and apoptotic cell death. Interestingly, edaravone has shown neuroprotective effects in several animal models of diseases other than stroke. In particular, edaravone administration was found to be effective in halting amyotrophic lateral sclerosis (ALS) progression during the early stages. Accordingly, after its success in Phase III clinical studies, edaravone has been approved by the FDA as a treatment for ALS patients. Considering its promises in neurological disorders and its safety in patients, edaravone is a drug of interest that can be repurposed for traumatic brain injury (TBI) treatment. Drug repurposing is a novel approach in drug development that identifies drugs for purposes other than their original indication. This review presents the biochemical properties of edaravone along with its effects on several neurological disorders in the hope that it can be adopted for treating TBI patients.

Behavioral and Histopathological Impairments Caused by Topical Exposure of the Rat Brain to Mild-Impulse Laser-Induced Shock Waves: Impulse Dependency

Although an enormous number of animal studies on blast-induced traumatic brain injury (bTBI) have been conducted, there still remain many uncertain issues in its neuropathology and mechanisms. This is partially due to the complex and hence difficult experimental environment settings, e.g., to minimize the effects of blast winds (tertiary mechanism) and to separate the effects of brain exposure and torso exposure. Since a laser-induced shock wave (LISW) is free from dynamic pressure and its energy is spatially well confined, the effects of pure shock wave exposure (primary mechanism) solely on the brain can be examined by using an LISW. In this study, we applied a set of four LISWs in the impulse range of 15–71 Pa·s to the rat brain through the intact scalp and skull; the interval between each exposure was ~5 s. For the rats, we conducted locomotor activity, elevated plus maze and forced swimming tests. Axonal injury in the brain was also examined by histological analysis using Bodian silver staining. Only the rats with exposure at higher impulses of 54 and 71 Pa·s showed significantly lower spontaneous movements at 1 and 2 days post-exposure by the locomotor activity test, but after 3 days post-exposure, they had recovered. At 7 days post-exposure, however, these rats (54 and 71 Pa·s) showed significantly higher levels of anxiety-related and depression-like behaviors by the elevated plus maze test and forced swimming test, respectively. To the best of the authors' knowledge, there have been few studies in which a rat model showed both anxiety-related and depression-like behaviors caused by blast or shock wave exposure. At that time point (7 days post-exposure), histological analysis showed significant decreases in axonal density in the cingulum bundle and corpus callosum in impulse-dependent manners; axons in the cingulum bundle were found to be more affected by a shock wave. Correlation analysis showed a statistically significant correlation between the depression like-behavior and axonal density reduction in the cingulum bundle. The results demonstrated the dependence of behavior deficits and axonal injury on the shock wave impulse loaded on the brain.

Behavioral and Myelin-Related Abnormalities after Blast-Induced Mild Traumatic Brain Injury in Mice

In civilian and military settings, mild traumatic brain injury (mTBI) is a common consequence of impacts to the head, sudden blows to the body, and exposure to high-energy atmospheric shockwaves from blast. In some cases, mTBI from blast exposure results in long-term emotional and cognitive deficits and an elevated risk for certain neuropsychiatric diseases. Here, we tested the effects of mTBI on various forms of auditory-cued fear learning and other measures of cognition in male C57BL/6J mice after single or repeated blast exposure (blast TBI; bTBI). bTBI produced an abnormality in the temporal organization of cue-induced freezing behavior in a conditioned trace fear test. Spatial working memory, evaluated by the Y-maze task performance, was also deleteriously affected by bTBI. Reverse-transcription quantitative real-time polymerase chain reaction (RT-qPCR) analysis for glial markers indicated an alteration in the expression of myelin-related genes in the hippocampus and corpus callosum 1-8 weeks after bTBI. Immunohistochemical and ultrastructural analyses detected bTBI-related myelin and axonal damage in the hippocampus and corpus callosum. Together, these data suggest a possible link between blast-induced mTBI, myelin/axonal injury, and cognitive dysfunction.

Repetitive Blast Promotes Chronic Aversion to Neutral Cues Encountered in the Peri-Blast Environment

Repetitive mild traumatic brain injury (mTBI) has been called the "signature injury" of military service members in the Iraq and Afghanistan wars and is highly comorbid with post-traumatic stress disorder (PTSD). Correct attribution of adverse blast-induced mTBI and/or PTSD remains challenging. Pre-clinical research using animal models can provide important insight into the mechanisms by which blast produces injury and dysfunction-but only to the degree by which such models reflect the human experience. Avoidance of trauma reminders is a hallmark of PTSD. Here, we sought to understand whether a mouse model of blast reproduces this phenomenon, in addition to blast-induced physical injuries. Drawing on well-established work from the chronic stress and Pavlovian conditioning literature, we hypothesized that even while one is anesthetized during blast exposure, environmental cues encountered in the peri-blast environment could be conditioned to evoke aversion/dysphoria and re-experiencing of traumatic stress. Using a pneumatic shock tube that recapitulates battlefield-relevant open-field blast forces, we provide direct evidence that stress is inherent to repetitive blast exposure, resulting in chronic aversive/dysphoric-like responses to previous blast-paired cues. The results in this report demonstrate that, although both single and repetitive blast exposures produce acute stress responses (weight loss, corticosterone increase), only repetitive blast exposure also results in co-occurring aversive/dysphoric-like stress responses. These results extend appreciation of the highly complex nature of repetitive blast exposure; and lend further support for the potential translational relevance of animal modeling approaches currently used by multiple laboratories aimed at elucidating the mechanisms (both molecular and behavioral) of repetitive blast exposure.

Repetitive blast mild traumatic brain injury increases ethanol sensitivity in male mice and risky drinking behavior in male combat veterans

BACKGROUND

Mild traumatic brain injury (mTBI) is common in civilians and highly prevalent among military service members. mTBI can increase health risk behaviors (e.g., sensation seeking, impulsivity) and addiction risk (e.g., for alcohol use disorder (AUD)), but how mTBI and substance use might interact to promote addiction risk remains poorly understood. Likewise, potential differences in single vs. repetitive mTBI in relation to alcohol use/abuse have not been previously examined.

METHODS

Here, we examined how a history of single (1×) or repetitive (3×) blast exposure (blast-mTBI) affects ethanol (EtOH)-induced behavioral and physiological outcomes using an established mouse model of blast-mTBI. To investigate potential translational relevance, we also examined self-report responses to the Alcohol Use Disorders Identification Test-Consumption questions (AUDIT-C), a widely used measure to identify potential hazardous drinking and AUD, and used a novel unsupervised machine learning approach to investigate whether a history of blast-mTBI affected drinking behaviors in Iraq/Afghanistan Veterans.

RESULTS

Both single and repetitive blast-mTBI in mice increased the sedative properties of EtOH (with no change in tolerance or metabolism), but only repetitive blast potentiated EtOH-induced locomotor stimulation and shifted EtOH intake patterns. Specifically, mice exposed to repetitive blasts showed increased consumption "front-loading" (e.g., a higher rate of consumption during an initial 2-h acute phase of a 24-h alcohol access period and decreased total daily intake) during an intermittent 2-bottle choice condition. Examination of AUDIT-C scores in Iraq/Afghanistan Veterans revealed an optimal 3-cluster solution: "low" (low intake and low frequency), "frequent" (low intake and high frequency), and "risky" (high intake and high frequency), where Veterans with a history of blast-mTBI displayed a shift in cluster assignment from "frequent" to "risky," as compared to Veterans who were deployed to Iraq/Afghanistan but had no lifetime history of TBI.

CONCLUSIONS

Together, these results offer new insight into how blast-mTBI may give increase AUD risk and highlight the increased potential for adverse health risk behaviors following repetitive blast-mTBI.

Laterality and region-specific tau phosphorylation correlate with PTSD-related behavioral traits in rats exposed to repetitive low-level blast

Military veterans who experience blast-related traumatic brain injuries often suffer from chronic cognitive and neurobehavioral syndromes. Reports of abnormal tau processing following blast injury have raised concerns that some cases may have a neurodegenerative basis. Rats exposed to repetitive low-level blast exhibit chronic neurobehavioral traits and accumulate tau phosphorylated at threonine 181 (Thr181). Using data previously reported in separate studies we tested the hypothesis that region-specific patterns of Thr181 phosphorylation correlate with behavioral measures also previously determined and reported in the same animals. Elevated p-tau Thr181 in anterior neocortical regions and right hippocampus correlated with anxiety as well as fear learning and novel object localization. There were no correlations with levels in amygdala or posterior neocortical regions. Particularly striking were asymmetrical effects on the right and left hippocampus. No systematic variation in head orientation toward the blast wave seems to explain the laterality. Levels did not correlate with behavioral measures of hyperarousal. Results were specific to Thr181 in that no correlations were observed for three other phospho-acceptor sites (threonine 231, serine 396, and serine 404). No consistent correlations were linked with total tau. These correlations are significant in suggesting that p-tau accumulation in anterior neocortical regions and the hippocampus may lead to disinhibited amygdala function without p-tau elevation in the amygdala itself. They also suggest an association linking blast injury with tauopathy, which has implications for understanding the relationship of chronic blast-related neurobehavioral syndromes in humans to neurodegenerative diseases.

Progressive Cognitive and Post-Traumatic Stress Disorder-Related Behavioral Traits in Rats Exposed to Repetitive Low-Level Blast

Many military veterans who experienced blast-related traumatic brain injuries (TBI) in the conflicts in Iraq and Afghanistan currently have chronic cognitive and mental health problems including post-traumatic stress disorder (PTSD). Besides static symptoms, new symptoms may emerge or existing symptoms may worsen. TBI is also a risk factor for later development of neurodegenerative diseases. In rats exposed to repetitive low-level blast overpressure (BOP), robust and enduring cognitive and PTSD-related behavioral traits develop that are present for at least one year after blast exposure. Here we determined the time-course of the appearance of these traits by testing rats in the immediate post-blast period. Three cohorts of rats examined within the first eight weeks exhibited no behavioral phenotype or, in one cohort, features of anxiety. None showed the altered cued fear responses or impaired novel object recognition characteristic of the fully developed phenotype. Two cohorts retested 36 to 42 weeks after blast exposure exhibited the expanded behavioral phenotype including anxiety as well as altered cued fear learning and impaired novel object recognition. Combined with previous work, the chronic behavioral phenotype has been observed in six cohorts of blast-exposed rats studied at 3-4 months or longer after blast injury, and the three cohorts studied here document the progressive nature of the cognitive/behavioral phenotype. These studies suggest the existence of a latent, delayed emerging and progressive blast-induced cognitive and behavioral phenotype. The delayed onset has implications for the evolution of post-blast neurobehavioral syndromes in military veterans and its modeling in experimental animals.

Fyn kinase inhibition reduces protein aggregation, increases synapse density and improves memory in transgenic and traumatic Tauopathy

Accumulation of misfolded phosphorylated Tau (Tauopathy) can be triggered by mutations or by trauma, and is associated with synapse loss, gliosis, neurodegeneration and memory deficits. Fyn kinase physically associates with Tau and regulates subcellular distribution. Here, we assessed whether pharmacological Fyn inhibition alters Tauopathy. In P301S transgenic mice, chronic Fyn inhibition prevented deficits in spatial memory and passive avoidance learning. The behavioral improvement was coupled with reduced accumulation of phospho-Tau in the hippocampus, with reductions in glial activation and with recovery of presynaptic markers. We extended this analysis to a trauma model in which very mild repetitive closed head injury was paired with chronic variable stress over 2 weeks to produce persistent memory deficits and Tau accumulation. In this model, Fyn inhibition beginning 24 h after the trauma ended rescued memory performance and reduced phospho-Tau accumulation. Thus, inhibition of Fyn kinase may have therapeutic benefit in clinical Tauopathies.

Psychological Stress-Induced Immune Response and Risk of Alzheimer's Disease in Veterans from Operation Enduring Freedom and Operation Iraqi Freedom

PURPOSE

Psychological stress is a significant health problem in veterans and their family members. Traumatic brain injury (TBI) and stress lead to the onset, progression, and worsening of several inflammatory and neurodegenerative diseases in veterans and civilians. Alzheimer's disease (AD) is a progressive, irreversible neuroinflammatory disease that causes problems with memory, thinking, and behavior. TBIs and chronic psychological stress cause and accelerate the pathology of neuroinflammatory diseases such as AD. However, the precise molecular and cellular mechanisms governing neuroinflammation and neurodegeneration are currently unknown, especially in veterans. The purpose of this review article was to advance the hypothesis that stress and TBI-mediated immune response substantially contribute and accelerate the pathogenesis of AD in veterans and their close family members and civilians.

METHODS

The information in this article was collected and interpreted from published articles in PubMed between 1985 and 2020 using the key words stress, psychological stress, Afghanistan war, Operation Enduring Freedom (OEF), Iraq War, Operation Iraqi Freedom (OIF), Operation New Dawn (OND), traumatic brain injury, mast cell and stress, stress and neuroimmune response, stress and Alzheimer's disease, traumatic brain injury, and Alzheimer's disease.

FINDINGS

Chronic psychological stress and brain injury induce the generation and accumulation of beta-amyloid peptide, amyloid plaques, neurofibrillary tangles, and phosphorylation of tau in the brain, thereby contributing to AD pathogenesis. Active military personnel and veterans are under enormous psychological stress due to various war-related activities, including TBIs, disabilities, fear, new environmental conditions, lack of normal life activities, insufficient communications, explosions, military-related noise, and health hazards. Brain injury, stress, mast cell, and other immune cell activation can induce headache, migraine, dementia, and upregulate neuroinflammation and neurodegeneration in veterans of Operation Enduring Freedom, Operation Iraqi Freedom, and Operation New Dawn. TBIs, posttraumatic stress disorder, psychological stress, pain, glial activation, and dementia in active military personnel, veterans, or their family members can cause AD several years later in their lives. We suggest that there are increasing numbers of veterans with TBIs and stress and that these veterans may develop AD late in life if no appropriate therapeutic intervention is available.

IMPLICATIONS

Per these published reports, the fact that TBIs and psychological stress can accelerate the pathogenesis of AD should be recognized. Active military personnel, veterans, and their close family members should be evaluated regularly for stress symptoms to prevent the pathogenesis of neurodegenerative diseases, including AD.

Laurent-Arriot K, Tsirka SE, Bergold PJ. Increased Behavioral Deficits and Inflammation in a Mouse Model of Co-Morbid Traumatic Brain Injury and Post-Traumatic Stress Disorder

Comorbid post-traumatic stress disorder with traumatic brain injury (TBI) produce more severe affective and cognitive deficits than PTSD or TBI alone. Both PTSD and TBI produce long-lasting neuroinflammation, which may be a key underlying mechanism of the deficits observed in co-morbid TBI/PTSD. We developed a model of co-morbid TBI/PTSD by combining the closed head (CHI) model of TBI with the chronic variable stress (CVS) model of PTSD and examined multiple behavioral and neuroinflammatory outcomes. Male C57/Bl6 mice received sham treatment, CHI, CVS, CHI then CVS (CHI → CVS) or CVS then CHI (CVS → CHI). The CVS → CHI group had deficits in Barnes maze or active place avoidance not seen in the other groups. The CVS → CHI, CVS and CHI → CVS groups displayed increased basal anxiety level, based on performance on elevated plus maze. The CVS → CHI had impaired performance on Barnes Maze, and Active Place Avoidance. These performance deficits were strongly correlated with increased hippocampal Iba-1 level an indication of activated MP/MG. These data suggest that greater cognitive deficits in the CVS → CHI group were due to increased inflammation. The increased deficits and neuroinflammation in the CVS → CHI group suggest that the order by which a subject experiences TBI and PTSD is a major determinant of the outcome of brain injury in co-morbid TBI/PTSD.

Traumatic Brain Injury and Posttraumatic Stress Disorder: Comorbid Consequences of War

Scientific literature is reviewed supporting a “consequence of war syndrome (CWS)” in Operation Enduring Freedom/Operation Iraqi Freedom/Operation New Dawn soldiers. CWS constituents include chronic pain and insomnia, other physical complaints, posttraumatic stress disorder (PTSD), anxiety, depression, and neuropsychological deficits. The foundation of CWS lies with the chronic stressors inherent to deployment and the cascade of biological events mediated and maintained by hypothalamic-pituitary-adrenal (HPA) axis dysregulation. Such dysregulation is modified by the individual’s specific experiences at war, difficulty reintegrating to post-deployment life, and the onset or exacerbation of the chronic and comorbid physical, emotional, and cognitive disorders. The circuit network between the prefrontal cortex (PFC), amygdala, and hippocampus is particularly sensitive to the consequences of war. The review’s specific conclusions are as follows: HPA axis dysregulation contributes to the chronic insomnia and hyperarousal seen in soldiers. There is considerable symptom overlap between PTSD and blast-related head injury, and it is difficult to determine the relative contributions of the two disorders to abnormal imaging studies. In some cases, traumatic brain injury (TBI) may directly precipitate PTSD symptoms. While not intuitive, the relationship between TBI and postconcussion syndrome appears indirect and mediated through PTSD. Blast-related or conventional head injury may have little long-term impact on neuropsychological functioning; contrarily, PTSD particularly accounts for current cognitive deficits. The psychological experience of CWS includes a “war-within” where soldiers continue to battle an internalized enemy. Successful treatment of CWS entails transdisciplinary care that addresses each of the constituent disorders.

Long-Term Effects of Blast Exposure: A Functional Study in Rats Using an Advanced Blast Simulator

Anecdotal observations of blast victims indicate that significant neuropathological and neurobehavioral defects may develop at later stages of life. To pre-clinically model this phenomenon, we have examined neurobehavioral changes in rats up to 1 year after exposure to single and tightly coupled repeated blasts using an advanced blast simulator. Neurobehavioral changes were monitored at acute, sub-acute, and chronic time-points using Morris water maze test of spatial learning and memory, novel object recognition test of short-term memory, open field exploratory activity as a test of anxiety/depression, a rotating pole test for vestibulomotor function, and a rotarod balance test for motor coordination. Single and repeated blasts resulted in significant functional deficits at both acute and chronic time-points. In most functional tests, rats exposed to repeated blasts performed more poorly than rats exposed to single blast. Interestingly, several functional deficits post-blast were most pronounced at 6 months and beyond. Significant neuromotor impairments occurred at early stages after blast exposure and the severity increased with repeated exposures. The novel object recognition testing revealed short-term memory deficits at 6 and 12 months post-blast. The water maze test revealed impairments at acute and chronic stages after blast exposure. The most substantial changes in the blast-exposed rats were observed with the center time and margin time legacies in the open field exploration test at 6, 9, and 12 months post-blast. Notably, these two outcome measures were minimally altered acutely, recovered during sub-acute stages, and were markedly affected during the chronic stages after blast exposures and may implicate development of chronic anxiety and depressive-like behaviors.

Relationship of traumatic brain injury to chronic mental health problems and dementia in military veterans

Traumatic brain injury (TBI) is an unfortunately common event in military life. The conflicts in Iraq and Afghanistan have increased public awareness of TBI in the military. Certain injury mechanisms are relatively unique to the military, the most prominent being blast exposure. Blast-related mild TBI (mTBI) has been of particular concern in the most recent veterans although controversy remains concerning separation of the postconcussion syndrome associated with mTBI from post-traumatic stress disorder. TBI is also a risk factor for the development of neurodegenerative diseases including chronic traumatic encephalopathy (CTE) and Alzheimer's disease (AD). AD, TBI, and CTE are all associated with chronic inflammation. Genome wide association studies (GWAS) have identified multiple genetic loci associated with AD that implicate inflammation and - in particular microglia - as key modulators of the AD- and TBI-related degenerative processes. At the molecular level, recent studies have identified TREM2 and TYROBP/DAP12 as components of a key molecular hub linking inflammation and microglia to the pathophysiology of AD and possibly TBI. Evidence concerning the relationship of TBI to chronic mental health problems and dementia is reviewed in the context of its relevance to military veterans.

Low-level blast exposure disrupts gliovascular and neurovascular connections and induces a chronic vascular pathology in rat brain

Much concern exists over the role of blast-induced traumatic brain injury (TBI) in the chronic cognitive and mental health problems that develop in veterans and active duty military personnel. The brain vasculature is particularly sensitive to blast injury. The aim of this study was to characterize the evolving molecular and histologic alterations in the neurovascular unit induced by three repetitive low-energy blast exposures (3 × 74.5 kPa) in a rat model mimicking human mild TBI or subclinical blast exposure. High-resolution two-dimensional differential gel electrophoresis (2D-DIGE) and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry of purified brain vascular fractions from blast-exposed animals 6 weeks post-exposure showed decreased levels of vascular-associated glial fibrillary acidic protein (GFAP) and several neuronal intermediate filament proteins (α-internexin and the low, middle, and high molecular weight neurofilament subunits). Loss of these proteins suggested that blast exposure disrupts gliovascular and neurovascular interactions. Electron microscopy confirmed blast-induced effects on perivascular astrocytes including swelling and degeneration of astrocytic endfeet in the brain cortical vasculature. Because the astrocyte is a major sensor of neuronal activity and regulator of cerebral blood flow, structural disruption of gliovascular integrity within the neurovascular unit should impair cerebral autoregulation. Disrupted neurovascular connections to pial and parenchymal blood vessels might also affect brain circulation. Blast exposures also induced structural and functional alterations in the arterial smooth muscle layer. Interestingly, by 8 months after blast exposure, GFAP and neuronal intermediate filament expression had recovered to control levels in isolated brain vascular fractions. However, despite this recovery, a widespread vascular pathology was still apparent at 10 months after blast exposure histologically and on micro-computed tomography scanning. Thus, low-level blast exposure disrupts gliovascular and neurovascular connections while inducing a chronic vascular pathology.