Reduced GABAergic inhibition in the basolateral amygdala and the development of anxiety-like behaviors after mild traumatic brain injury

Short-term behavioral and histological findings following a single concussive and repeated subconcussive brain injury in a rodent model

PRIMARY OBJECTIVE

It is unclear of the correlation between a mild traumatic brain injury (mTBI) and repeated subconcussive (RSC) impacts with respect to injury biomechanics. Thus, the present study was designed to determine the behavioral and histological differences between a single mTBI impact and RSC impacts with subdivided cumulative kinetic energies of the single mTBI impact.

RESEARCH DESIGN

Adult male Sprague-Dawley rats were randomly assigned to a single mTBI impact, RSC impact, sham, or repeated sham groups.

METHODS AND PROCEDURES

Following a weight drop injury, anxiety-like behavior and general locomotive activity and were assessed using the open field test, while motor coordination was evaluated using a rotarod unit. Neuronal loss, astrogliosis, and microgliosis were assessed using NeuN, GFAP and Iba-1 immunohistochemistry. All assessments were undertaken at 3- and 7-days post impact.

MAIN OUTCOMES AND RESULTS

No behavioral disturbances were observed in injury groups, however, both injury groups did lead to microgliosis following 3-days post-impact.

CONCLUSIONS

No pathophysiological differences were observed between a single mTBI impact and RSC impacts of the same energy input. Even though a cumulative injury threshold for RSC impacts was not determined, a threshold still may exist where no pathodynamic shift occurs.

IDO-1 inhibition improves outcome after fluid percussion injury in adult male rats

The enzyme indoleamine 2,3 dioxygenase 1 (IDO1) catalyzes the rate-limiting step in the kynurenine pathway (KP) which produces both neuroprotective and neurotoxic metabolites. Neuroinflammatory signals produced as a result of pathological conditions can increase production of IDO1 and boost its enzymatic capacity. IDO1 and the KP have been implicated in behavioral recovery after human traumatic brain injury (TBI), but their roles in experimental models of TBI are for the most part unknown. We hypothesized there is an increase in KP activity in the fluid percussion injury (FPI) model of TBI, and that administration of an IDO1 inhibitor will improve neurological recovery. In this study, adult male Sprague Dawley rats were subjected to FPI or sham injury and received twice-daily oral administration of the IDO1 inhibitor PF-06840003 (100 mg/kg) or vehicle control. FPI resulted in a significant increase in KP activity, as demonstrated by an increased ratio of kynurenine: tryptophan, in the perilesional neocortex and ipsilateral hippocampus 3 days postinjury (DPI), which normalized by 7 DPI. The increase in KP activity was prevented by PF-06840003. IDO1 inhibition also improved memory performance as assessed in the Barnes maze and anxiety behaviors as assessed in open field testing in the first 28 DPI. These results suggest increased KP activity after FPI may mediate neurological dysfunction, and IDO1 inhibition should be further investigated as a potential therapeutic target to improve recovery.

Enhancing cognitive function in chronic TBI: The Role of α7 nicotinic acetylcholine receptor modulation

Traumatic brain injury (TBI) results in several pathological changes within the hippocampus that result in adverse effects on learning and memory. Therapeutic strategies to enhance learning and memory after TBI are still in the early stages of clinical development. One strategy is to target the α7 nicotinic acetylcholine receptor (nAChR), which is highly expressed in the hippocampus and contributes to the formation of long-term memory. In our previous study, we found that AVL-3288, a positive allosteric modulator of the α7 nAChR, improved cognitive recovery in rats after moderate fluid-percussion injury (FPI). However, whether AVL-3288 improved cognitive recovery specifically through the α7 nAChR was not definitively determined. In this study we utilized Chrna7 knockout mice and compared their recovery to wild-type mice treated with AVL-3288 after TBI. We hypothesized that AVL-3288 treatment would improve learning and memory in wild-type mice, but not Chrna7-/- mice after TBI. Adult male C57BL/6 wild-type and Chrna7-/- mice received sham surgery or moderate controlled cortical impact (CCI) and recovered for 3 months. Mice were then treated with vehicle or AVL-3288 at 30 min prior to contextual fear conditioning. At 3 months after CCI, expression of α7 nAChR, choline acetyltransferase (ChAT), high-affinity choline transporter (ChT), and vesicular acetylcholine transporter (VAChT) were found to be significantly decreased in the hippocampus. Treatment of wild-type mice at 3 months after CCI with AVL-3288 significantly improved cue and contextual fear conditioning, whereas no beneficial effects were observed in Chrna7-/- mice. Parietal cortex and hippocampal atrophy were not improved with AVL-3288 treatment in either wild-type or Chrna7-/- mice. Our results indicate that AVL-3288 improves cognition during the chronic recovery phase of TBI through modulation of the α7 nAChR.

Neurobehavioral and inflammatory responses following traumatic brain injury in male and female mice

Traumatic brain injury (TBI) is a leading cause of mortality and is associated with a high rate of functional comorbidities, including motor, cognitive, anxiety, depression, and emotional disorders. TBI pathophysiology and recovery are complicated and involve several mechanistic pathways that control neurobehavioral outcomes. In this study, male and female C57Bl/6 J mice were subjected to a controlled cortical impact model of TBI or sham injury and evaluated for different neurobehavioral and inflammatory outcomes over a month. We demonstrate that TBI mice have increased motor dysfunction at early and late time points following the injury as compared to the sham group. Anxiety-like symptoms were time- and task-dependent, with both sexes having increased anxiety-like behavior 2 weeks post-injury. Cognitive functions measured by T-maze presented greater deficits in TBI mice, while there was no sex or injury-related difference in depressive-like behaviors. Notably, a significant effect of sex was found in empathy-like behavior, with females showing more allogrooming and freezing behavior in the consoling and fear observational tests, respectively. Evaluating the impact of the injury-induced brain damage demonstrated a greater injury volume and neuronal degeneration in males compared to females one month after TBI. Moreover, male mice showed higher peripheral inflammatory responses, as represented by elevated serum levels of peripheral leukocytes and inflammatory markers. These results will have significant implications for understanding TBI's long-term consequences on neurobehavioral and inflammatory responses, which are sex-specific and can be considered for individualized therapeutic strategies in treating TBI.

Emotional Regulation and Adolescent Concussion: Overview and Role of Neuroimaging

Emotional dysregulation symptoms following a concussion are associated with an increased risk for emotional dysregulation disorders (e.g., depression and anxiety), especially in adolescents. However, predicting the emergence or worsening of emotional dysregulation symptoms after concussion and the extent to which this predates the onset of subsequent psychiatric morbidity after injury remains challenging. Although advanced neuroimaging techniques, such as functional magnetic resonance imaging and diffusion magnetic resonance imaging, have been used to detect and monitor concussion-related brain abnormalities in research settings, their clinical utility remains limited. In this narrative review, we have performed a comprehensive search of the available literature regarding emotional regulation, adolescent concussion, and advanced neuroimaging techniques in electronic databases (PubMed, Scopus, and Google Scholar). We highlight clinical evidence showing the heightened susceptibility of adolescents to experiencing emotional dysregulation symptoms following a concussion. Furthermore, we describe and provide empirical support for widely used magnetic resonance imaging modalities (i.e., functional and diffusion imaging), which are utilized to detect abnormalities in circuits responsible for emotional regulation. Additionally, we assess how these abnormalities relate to the emotional dysregulation symptoms often reported by adolescents post-injury. Yet, it remains to be determined if a progression of concussion-related abnormalities exists, especially in brain regions that undergo significant developmental changes during adolescence. We conclude that neuroimaging techniques hold potential as clinically useful tools for predicting and, ultimately, monitoring the treatment response to emotional dysregulation in adolescents following a concussion.

Examining the long-term effects of traumatic brain injury on fear extinction in male rats

There is a strong association between traumatic brain injuries (TBIs) and the development of psychiatric disorders, including post-traumatic stress disorder (PTSD). Exposure-based therapy is a first-line intervention for individuals who suffer from PTSD and other anxiety-related disorders; however, up to 50% of individuals with PTSD do not respond well to this approach. Fear extinction, a core mechanism underlying exposure-based therapy, is a procedure in which a repeated presentation of a conditioned stimulus in the absence of an unconditioned stimulus leads to a decrease in fear expression, and is a useful tool to better understand exposure-based therapy. Identifying predictors of extinction would be useful in developing alternative treatments for the non-responders. We recently found that CO₂ reactivity predicts extinction phenotypes in rats, likely through the activation of orexin receptors in the lateral hypothalamus. While studies have reported mixed results in extinction of fear after TBI, none have examined the long-term durability of this phenotype in the more chronically injured brain. Here we tested the hypothesis that TBI results in a long-term deficit in fear extinction, and that CO₂ reactivity would be predictive of this extinction phenotype. Isoflurane-anesthetized adult male rats received TBI (n = 59) (produced by a controlled cortical impactor) or sham surgery (n = 29). One month post-injury or sham surgery, rats underwent a CO₂ or air challenge, followed by fear conditioning, extinction, and fear expression testing. TBI rats exposed to CO₂ (TBI-CO₂) showed no difference during extinction or fear expression relative to shams exposed to CO₂ (sham-CO₂). However, TBI-CO₂ rats, showed significantly better fear expression than TBI rats exposed to air (TBI-air). In contrast to previous findings, we observed no relationship between CO₂ reactivity and post-extinction fear expression in either the sham or TBI rats. However, compared to the previously observed naïve sample, we observed more variability in post-extinction fear expression but a very similar distribution of CO₂ reactivity in the current sample. Isoflurane anesthesia may lead to interoceptive threat habituation, possibly via action on orexin receptors in the lateral hypothalamus, and may interact with CO₂ exposure, resulting in enhanced extinction. Future work will directly test this possibility.

Alpha-linolenic acid enhances the facilitation of GABAergic neurotransmission in the BLA and CA1

Hyperexcitability is a major mechanism implicated in several neuropsychiatric disorders, such as organophosphate-induced status epilepticus (SE), primary epilepsy, stroke, spinal cord injury, traumatic brain injury, schizophrenia, and autism spectrum disorders. Underlying mechanisms are diverse, but a functional impairment and loss of GABAergic inhibitory neurons are common features in many of these disorders. While novel therapies abound to correct for the loss of GABAergic inhibitory neurons, it has been difficult at best to improve the activities of daily living for the majority of patients. Alpha-linolenic acid (ALA) is an essential omega-3 polyunsaturated fatty acid found in plants. ALA exerts pleiotropic effects in the brain that attenuate injury in chronic and acute brain disease models. However, the effect of ALA on GABAergic neurotransmission in hyperexcitable brain regions involved in neuropsychiatric disorders, such as the basolateral amygdala (BLA) and CA1 subfield of the hippocampus, is unknown. Administration of a single dose of ALA (1500 nmol/kg) subcutaneously increased the charge transfer of inhibitory postsynaptic potential currents mediated by GABAA receptors in pyramidal neurons by 52% in the BLA and by 92% in the CA1 compared to vehicle animals a day later. Similar results were obtained in pyramidal neurons from the BLA and CA1 when ALA was bath-applied in slices from naïve animals. Importantly, pretreatment with the high-affinity, selective TrkB inhibitor, k252, completely abolished the ALA-induced increase in GABAergic neurotransmission in the BLA and CA1, suggesting a brain-derived neurotrophic factor (BDNF)-mediated mechanism. Addition of mature BDNF (20 ng/mL) significantly increased GABAA receptor inhibitory activity in the BLA and CA1 pyramidal neurons similar to the results obtained with ALA. ALA may be an effective treatment for neuropsychiatric disorders where hyperexcitability is a major feature.

GABAA Receptor and Serotonin Transporter Expression Changes Dissociate Following Mild Traumatic Brain Injury: Influence of Sex and Estrus Cycle Phase in Rats

Mild traumatic brain injuries (mild TBIs) can affect both males and females, but females are more likely to report long-term psychological complications, including changes in mood and generalized anxiety. Additionally, reproductive cycle phase has been shown to affect mild TBI symptom expression within females. These variances may result from sex differences in mild TBI-induced alterations to neurotransmission in brain regions that influence mood and emotion, possibly mediated by sex steroids. The hippocampus and amygdala are implicated in stress responses and anxiety, and within these regions, gamma-aminobutyric acid (GABA) and serotonin modulate output and behavioral expression. Metabolites of progesterone can allosterically enhance GABAergic signaling, and sex steroids are suggested to regulate the expression of the serotonin transporter (SERT). To determine how mild TBI might alter GABA receptor and SERT expression in males and females, immunocytochemistry was used to quantify expression of the alpha-1 subunit of the GABA_A receptor (α1-GABA_A), SERT, and a neuronal marker (NeuN) in the brains of adult male and naturally-cycling female rats, both with and without mild TBI, 17 days after injury. Mild TBI altered the expression of α1-GABA_A in the amygdala and hippocampus in both sexes, but the direction of change observed depended on sex and reproductive cycle phase. In contrast, mild TBI had little effect on SERT expression. However, SERT expression differed between sexes and varied with the cycle phase. These findings demonstrate that regulation of neurotransmission following mild TBI differs between males and females, with implications for behavioral outcomes and the efficacy of therapeutic strategies.

An Updated Guide to Robust Statistical Methods in Neuroscience

There is a vast array of new and improved methods for comparing groups and studying associations that offer the potential for substantially increasing power, providing improved control over the probability of false positives, and yielding a deeper and more nuanced understanding of data. These new techniques effectively deal with four insights into when and why conventional methods can be unsatisfactory. But for the non-statistician, this vast array of techniques for comparing groups and studying associations can seem daunting. This article briefly reviews when and why conventional methods can have relatively low power and yield misleading results. The main goal is to suggest guidelines regarding the use of modern techniques that improve upon classic approaches such as Pearson's correlation, ordinary linear regression, ANOVA, and ANCOVA. This updated version includes recent advances dealing with effect sizes, including situations where there is a covariate. The R code, figures, and accompanying notebooks have been updated as well. © 2023 The Authors. Current Protocols published by Wiley Periodicals LLC.

Dopaminergic and cholinergic modulation of the amygdala is altered in female mice with oestrogen receptor β deprivation

The amygdala is modulated by dopaminergic and cholinergic neurotransmission, and this modulation is altered in mood disorders. Therefore, this study was designed to evaluate the presence/absence of quantitative alterations in the expression of main dopaminergic and cholinergic markers in the amygdala of mice with oestrogen receptor β (ERβ) knock-out which exhibit increased anxiety, using immunohistochemistry and quantitative methods. Such alterations could either contribute to increased anxiety or be a compensatory mechanism for reducing anxiety. The results show that among dopaminergic markers, the expression of tyrosine hydroxylase (TH), dopamine transporter (DAT) and dopamine D₂-like receptor (DA₂) is significantly elevated in the amygdala of mice with ERβ deprivation when compared to matched controls, whereas the content of dopamine D₁-like receptor (DA₁) is not altered by ERβ knock-out. In the case of cholinergic markers, muscarinic acetylcholine type 1 receptor (AChR_M1) and alpha-7 nicotinic acetylcholine receptor (AChR_α7) display overexpression while the content of acetylcholinesterase (AChE) and vesicular acetylcholine transporter (VAChT) remains unchanged. In conclusion, in the amygdala of ERβ knock-out female the dopaminergic and cholinergic signalling is altered, however, to determine the exact role of ERβ in the anxiety-related behaviour further studies are required.

GABAergic circuits of the basolateral amygdala and generation of anxiety after traumatic brain injury

Traumatic brain injury (TBI) has reached epidemic proportions around the world and is a major public health concern in the United States. Approximately 2.8 million individuals sustain a traumatic brain injury and are treated in an Emergency Department yearly in the U.S., and about 50,000 of them die. Persistent symptoms develop in 10-15% of the cases including neuropsychiatric disorders. Anxiety is the second most common neuropsychiatric disorder that develops in those with persistent neuropsychiatric symptoms after TBI. Abnormalities or atrophy in the temporal lobe has been shown in the overwhelming number of TBI cases. The basolateral amygdala (BLA), a temporal lobe structure that consolidates, stores and generates fear and anxiety-based behavioral outputs, is a critical brain region in the anxiety circuitry. In this review, we sought to capture studies that characterized the relationship between human post-traumatic anxiety and structural/functional alterations in the amygdala. We compared the human findings with results obtained with a reproducible mild TBI animal model that demonstrated a direct relationship between the alterations in the BLA and an anxiety-like phenotype. From this analysis, both preliminary insights, and gaps in knowledge, have emerged which may open new directions for the development of rational and more efficacious treatments.

Brain Injury Effects on Neuronal Activation and Synaptic Transmission in the Basolateral Amygdala of Adult Male and Female Wistar Rats

Traumatic brain injury (TBI) is defined as brain damage produced by an external mechanical force that leads to behavioral, cognitive, and psychiatric sequelae. The basolateral amygdala (BLA) is involved in emotional regulation, and its function and morphology are altered following TBI. Little is known about potential sex-specific effects of TBI on BLA neuronal function, but it is critical for the field to identify potential sex differences in TBI effects on brain and behavior. Here, we hypothesized that TBI would produce sex-specific acute (1 h) effects on BLA neuronal activation, excitability, and synaptic transmission in adult male and female rats. Forty-nine Wistar rats (n = 23 males and 26 females) were randomized to TBI (using lateral fluid percussion) or Sham groups in two separate studies. Study 1 used in situ hybridization (i.e., RNAscope) to measure BLA expression of c-fos (a marker of cell activation), vGlut, and vGat (markers of glutamatergic and GABAergic neurons, respectively) messenger RNA (mRNA). Study 2 used slice electrophysiology to measure intrinsic excitability and excitatory/inhibitory synaptic transmission in putative pyramidal neurons in the BLA. Physiological measures of injury severity were collected from all animals. Our results show that females exhibit increased apnea duration and reduced respiratory rate post-TBI relative to males. In male and female rats, TBI increased c-fos expression in BLA glutamatergic cells but not in BLA GABAergic cells, and TBI increased firing rate in BLA pyramidal neurons. Further, TBI increased spontaneous excitatory and inhibitory postsynaptic current (sEPSC and sIPSC) amplitude in BLA neurons of females relative to all other groups. TBI increased sEPSC frequency in BLA neurons of females relative to males but did not alter sIPSC frequency. In summary, lateral fluid percussion produced different physiological responses in male and female rats, as well as sex-specific alterations in BLA neuronal activation, excitability, and synaptic transmission 1 h after injury.

Astrocytes in Post-traumatic Stress Disorder

Although posttraumatic stress disorder (PTSD) is on the rise, traumatic events and their consequences are often hidden or minimized by patients for reasons linked to PTSD itself. Traumatic experiences can be broadly classified into mental stress (MS) and traumatic brain injury (TBI), but the cellular mechanisms of MS- or TBI-induced PTSD remain unknown. Recent evidence has shown that the morphological remodeling of astrocytes accompanies and arguably contributes to fearful memories and stress-related disorders. In this review, we summarize the roles of astrocytes in the pathogenesis of MS-PTSD and TBI-PTSD. Astrocytes synthesize and secrete neurotrophic, pro- and anti-inflammatory factors and regulate the microenvironment of the nervous tissue through metabolic pathways, ionostatic control, and homeostatic clearance of neurotransmitters. Stress or trauma-associated impairment of these vital astrocytic functions contribute to the pathophysiological evolution of PTSD and may present therapeutic targets.

The Novel Monoacylglycerol Lipase Inhibitor MJN110 Suppresses Neuroinflammation, Normalizes Synaptic Composition and Improves Behavioral Performance in the Repetitive Traumatic Brain Injury Mouse Model

Modulation of the endocannabinoid system has emerged as an effective approach for the treatment of many neurodegenerative and neuropsychological diseases. However, the underlying mechanisms are still uncertain. Using a repetitive mild traumatic brain injury (mTBI) mouse model, we found that there was an impairment in locomotor function and working memory within two weeks post-injury, and that treatment with MJN110, a novel inhibitor of the principal 2-arachidononyl glycerol (2-AG) hydrolytic enzyme monoacylglycerol lipase dose-dependently ameliorated those behavioral changes. Spatial learning and memory deficits examined by Morris water maze between three and four weeks post-TBI were also reversed in the drug treated animals. Administration of MJN110 selectively elevated the levels of 2-AG and reduced the production of arachidonic acid (AA) and prostaglandin E₂ (PGE₂) in the TBI mouse brain. The increased production of proinflammatory cytokines, accumulation of astrocytes and microglia in the TBI mouse ipsilateral cerebral cortex and hippocampus were significantly reduced by MJN110 treatment. Neuronal cell death was also attenuated in the drug treated animals. MJN110 treatment normalized the expression of the NMDA receptor subunits NR2A and NR2B, the AMPA receptor subunits GluR1 and GluR2, and the GABA_A receptor subunits α1, β2,3 and γ2, which were all reduced at 1, 2 and 4 weeks post-injury. The reduced inflammatory response and restored glutamate and GABA receptor expression likely contribute to the improved motor function, learning and memory in the MJN110 treated animals. The therapeutic effects of MJN110 were partially mediated by activation of CB1 and CB2 cannabinoid receptors and were eliminated when it was co-administered with DO34, a novel inhibitor of the 2-AG biosynthetic enzymes. Our results suggest that augmentation of the endogenous levels of 2-AG can be therapeutically useful in the treatment of TBI by suppressing neuroinflammation and maintaining the balance between excitatory and inhibitory neurotransmission.

Sex and estrous-phase dependent alterations in depression-like behavior following mild traumatic brain injury in adolescent rats

Following mild traumatic brain injury (TBI), high school and collegiate-aged females tend to report more emotional symptoms than males. Adolescent male and female rats (35 days old) were subjected to mild TBI and evaluated for anxiety- and depression-like behaviors using the elevated plus maze and forced swim test (FST), respectively, and cellular alterations. Injured brains did not exhibit an overt lesion, atrophy of tissue or astrocytic reactivity underneath the impact site at 6-week post-injury, suggestive of the mild nature of trauma. Neither male nor female brain-injured rats exhibited anxiety-like behavior at 2 or 6 weeks, regardless of estrous phase at the time of behavior testing. Brain-injured male rats did not exhibit any alterations in immobility, swimming and climbing times in the FST compared to sham-injured rats at either 2- or 6-week post-injury. Brain-injured female rats did, however, exhibit an increase in immobility (in the absence of changes in swimming and climbing times) in the FST at 6 weeks post-injury only during the estrus phase of the estrous cycle, suggestive of a depression-like phenotype. Combined administration of the estrogen receptor antagonist, tamoxifen, and the progesterone receptor antagonist, mifepristone, during proestrus was able to prevent the depression-like phenotype observed during estrus. Taken together, these data suggest that female rats may be more vulnerable to exhibiting behavioral deficits following mild TBI and that estrous phase may play a role in depression-like behavior.

Traumatic brain injury to primary visual cortex produces long-lasting circuit dysfunction

Primary sensory areas of the mammalian neocortex have a remarkable degree of plasticity, allowing neural circuits to adapt to dynamic environments. However, little is known about the effects of traumatic brain injury on visual circuit function. Here we used anatomy and in vivo electrophysiological recordings in adult mice to quantify neuron responses to visual stimuli two weeks and three months after mild controlled cortical impact injury to primary visual cortex (V1). We found that, although V1 remained largely intact in brain-injured mice, there was ~35% reduction in the number of neurons that affected inhibitory cells more broadly than excitatory neurons. V1 neurons showed dramatically reduced activity, impaired responses to visual stimuli and weaker size selectivity and orientation tuning in vivo. Our results show a single, mild contusion injury produces profound and long-lasting impairments in the way V1 neurons encode visual input. These findings provide initial insight into cortical circuit dysfunction following central visual system neurotrauma.

Measuring Anxiety-Like Behaviors in Rodent Models of Traumatic Brain Injury

Anxiety is a common complaint following acquired traumatic brain injury (TBI). However, the measurement of dysfunctional anxiety behavioral states following experimental TBI in rodents is complex. Some studies report increased anxiety after TBI, whereas others find a decreased anxiety-like state, often described as increased risk-taking behavior or impulsivity. These inconsistencies may reflect a lack of standardization of experimental injury models or of behavioral testing techniques. Here, we review the most commonly employed unconditioned tests of anxiety and discuss them in a context of experimental TBI. Special attention is given to the effects of repeated testing, and consideration of potential sensory and motor confounds in injured rodents. The use of multiple tests and alternative data analysis methods are discussed, as well as the potential for the application of common data elements (CDEs) as a means of providing a format for documentation of experimental details and procedures of each published research report. CDEs may improve the rigor, reproducibility, as well as endpoint for better relating findings with clinical TBI phenotypes and the final goal of translation. While this may not resolve all incongruities in findings across laboratories, it is seen as a way forward for standardized and universal data collection for improvement of data quality and sharing, and advance therapies for neuropsychiatric symptoms that often present for decades following TBI.

Increased inhibitory activity in the basolateral amygdala and decreased anxiety during estrus: A potential role for ASIC1a channels

The amygdala is central to emotional behavior, and the excitability level of the basolateral nucleus of the amygdala (BLA) is associated with the level of anxiety. The excitability of neuronal networks is significantly controlled by GABAergic inhibition. Here, we investigated whether GABAergic inhibition in the BLA is altered during the rat estrous cycle. In rat amygdala slices, most principal BLA neurons display spontaneous IPSCs (sIPSCs) in the form of "bursts" of inhibitory currents, occurring rhythmically at a frequency of about 0.5 Hz. The percentage of BLA neurons displaying sIPSC bursts, along with the inhibitory charge transferred by sIPSCs and the frequency of sIPSC bursts, were significantly increased during the estrus phase; increased inhibition was accompanied by reduced anxiety in the open field, the light-dark box, and the acoustic startle response tests. sIPSC bursts were blocked by ibuprofen, an antagonist of acid-sensing-1a channels (ASIC1a), whose activity is known to increase by decreasing temperature. A transient reduction in the temperature of the slice medium, strengthened the sIPSCs bursts; this effect was blocked in the presence of ibuprofen. Further analysis of the sIPSC bursts during estrus showed significantly stronger rhythmic inhibitory activity in early estrus, when body temperature drops, compared with late estrus. To the extent that these results may relate to humans, it is suggested that "a calmer amygdala" due to increased inhibitory activity may underlie the positive affect in women around ovulation time. ASIC1a may contribute to increased inhibition, with their activity facilitated by the body-temperature drop preceding ovulation.

Cerebral Blood Flow Predicts Recovery in Children with Persistent Post-Concussion Symptoms after Mild Traumatic Brain Injury

Persistent post-concussion symptoms (PPCS) following pediatric mild traumatic brain injury (mTBI) are associated with differential changes in cerebral blood flow (CBF). Given its potential as a therapeutic target, we examined CBF changes during recovery in children with PPCS. We hypothesized that CBF would decrease and that such decreases would mirror clinical recovery. In a prospective cohort study, 61 children and adolescents (mean age 14 [standard deviation = 2.6] years; 41% male) with PPCS were imaged with three-dimensional (3D) pseudo-continuous arterial spin-labelled (pCASL) magnetic resonance imaging (MRI) at 4-6 and 8-10 weeks post-injury. Exclusion criteria included any significant past medical history and/or previous concussion within the past 3 months. Twenty-three participants had clinically recovered at the time of the second scan. We found that relative and mean absolute CBF were higher in participants with poor recovery, 44.0 (95% confidence interval [CI]: 43.32, 44.67) than in those with good recovery, 42.19 (95% CI: 41.77, 42.60) mL/min/100 g gray tissue and decreased over time (β = -1.75; p < 0.001). The decrease was greater in those with good recovery (β = 2.29; p < 0.001) and predicted outcome in 77% of children with PPCS (odds ratio [OR] 0.54, 95% CI: 0.36, 0.80; p = 0.002). Future studies are warranted to validate the utility of CBF as a useful predictive biomarker of outcome in PPCS.

Teasing apart trauma: neural oscillations differentiate individual cases of mild traumatic brain injury from post-traumatic stress disorder even when symptoms overlap

Post-traumatic stress disorder (PTSD) and mild traumatic brain injury (mTBI) are highly prevalent and closely related disorders. Affected individuals often exhibit substantially overlapping symptomatology - a major challenge for differential diagnosis in both military and civilian contexts. According to our symptom assessment, the PTSD group exhibited comparable levels of concussion symptoms and severity to the mTBI group. An objective and reliable system to uncover the key neural signatures differentiating these disorders would be an important step towards translational and applied clinical use. Here we explore use of MEG (magnetoencephalography)-multivariate statistical learning analysis in identifying the neural features for differential PTSD/mTBI characterisation. Resting state MEG-derived regional neural activity and coherence (or functional connectivity) across seven canonical neural oscillation frequencies (delta to high gamma) were used. The selected features were consistent and largely confirmatory with previously established neurophysiological markers for the two disorders. For regional power from theta, alpha and high gamma bands, the amygdala, hippocampus and temporal areas were identified. In line with regional activity, additional connections within the occipital, parietal and temporal regions were selected across a number of frequency bands. This study is the first to employ MEG-derived neural features to reliably and differentially stratify the two disorders in a multi-group context. The features from alpha and beta bands exhibited the best classification performance, even in cases where distinction by concussion symptom profiles alone were extremely difficult. We demonstrate the potential of using 'invisible' neural indices of brain functioning to understand and differentiate these debilitating conditions.

Traumatic Brain Injury Broadly Affects GABAergic Signaling in Dentate Gyrus Granule Cells

Traumatic brain injury (TBI) causes cellular and molecular alterations that contribute to neuropsychiatric disease and epilepsy. GABAergic dysfunction figures prominently in the pathophysiology of TBI, yet the effects of TBI on tonic inhibition in hippocampus remain uncertain. We used a mouse model of severe TBI [controlled cortical impact (CCI)] to investigate GABAergic signaling in dentate gyrus granule cells (DGGCs). Basal tonic GABA currents were not affected by CCI. However, tonic currents induced by the δ subunit-selective GABA_A receptor agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP; 10 μm) were reduced by 44% in DGGCs ipsilateral to CCI (CCI-ipsi), but not in contralateral DGGCs. Reduced THIP currents were apparent one week after injury and persisted up to 15 weeks. The frequency of spontaneous IPSCs (sIPSCs) was reduced in CCI-ipsi cells, but the amplitude and kinetics of sIPSCs were unaffected. Immunohistochemical analysis showed reduced expression of GABA_A receptor δ subunits and GABA_B receptor B2 subunits after CCI, by 43% and 40%, respectively. Activation of postsynaptic GABA_B receptors caused a twofold increase in tonic currents, and this effect was markedly attenuated in CCI-ipsi cells (92% reduction). GABA_B receptor-activated K⁺ currents in DGGCs were also significantly reduced in CCI-ipsi cells, confirming a functional deficit of GABA_B receptors after CCI. Results indicate broad disruption of GABAergic signaling in DGGCs after CCI, with deficits in both phasic and tonic inhibition and GABA_B receptor function. These changes are predicted to disrupt operation of hippocampal networks and contribute to sequelae of severe TBI, including epilepsy.

Region-Dependent Modulation of Neural Plasticity in Limbic Structures Early after Traumatic Brain Injury

Traumatic brain injury (TBI)-induced disruptions in synaptic function within brain regions and across networks in the limbic system may underlie a vulnerability for maladaptive plasticity and contribute to behavioral comorbidities. In this study we measured how synaptic proteins respond to lateral fluid percussion injury (FPI) brain regions known to regulate emotion and memory, including the basolateral amygdala (BLA), dorsal and ventral hippocampus (DH, VH), and medial prefrontal cortex (PFC). We investigated proteins involved in regulating plasticity, including synaptic glutamatergic a-amino-3-hydroxy5-methyl-4-isoxazolepropionic acid (AMPA; GluA1, GluA2) and N-methyl-D-aspartate (NMDA; NR1, NR2A, NR2B) receptor subunits as well as inhibitory gamma-aminobutyric acid (GABA) synthetic enzymes (GAD67, GAD65) via western blot. Adult male rats received a mild-moderate lateral FPI or sham surgery and ipsi- and contralateral BLA, DH, VH, and PFC were collected 6 h, 24 h, 48 h, and 7 days post-injury. In the ipsilateral BLA, there was a significant decrease in NR1 and GluA2 24 h after injury, whereas NR2A and NR2B were increased in the contralateral BLA at 48 h compared with sham. GAD67 was increased ipsilaterally at 24 h, but decreased contralaterally at 48 h in the BLA. In the DH, both NMDA (NR2A, NR2B) and GABA-synthetic (GAD65, GAD67) proteins were increased acutely at 6 h compared with sham. GAD67 was also robustly increased in the ipsilateral VH at 6 h. In the contralateral VH, NR2A significantly increased between 6 h and 24 h after FPI, whereas GAD65 was decreased across the same time-points in the contralateral VH. In the medial PFC at 24 h we saw bilateral increases in GAD67 and a contralateral decrease in GluA1. Later, there was a significant decrease in GAD67 in contralateral PFC from 48 h to 7 days post-injury. Collectively, these data suggest that lateral FPI causes a dynamic homeostatic response across limbic networks, leading to an imbalance of the proteins involved in plasticity in neural systems underlying cognitive and emotional regulation.

A Role for the Amygdala in Impairments of Affective Behaviors Following Mild Traumatic Brain Injury

Mild traumatic brain injury (TBI) results in chronic affective disorders such as depression, anxiety, and fear that persist up to years following injury and significantly impair the quality of life for patients. Although a great deal of research has contributed to defining symptoms of mild TBI, there are no adequate drug therapies for brain-injured individuals. Preclinical studies have modeled these deficits in affective behaviors post-injury to understand the underlying mechanisms with a view to developing appropriate treatment strategies. These studies have also unveiled sex differences that contribute to the varying phenotypes associated with each behavior. Although clinical and preclinical studies have viewed these behavioral deficits as separate entities with unique neurobiological mechanisms, mechanistic similarities suggest that a novel approach is needed to advance research on drug therapy. This review will discuss the circuitry involved in the expression of deficits in affective behaviors following mild TBI in humans and animals and provide evidence that the manifestation of impairment in these behaviors stems from an amygdala-dependent emotional processing deficit. It will highlight mechanistic similarities between these different types of affective behaviors that can potentially advance mild TBI drug therapy by investigating treatments for the deficits in affective behaviors as one entity, requiring the same treatment.

Differential efferent projections of GABAergic neurons in the basolateral and central nucleus of amygdala in mice

The Basolateral amygdala (BLA) and central nucleus of the amygdala (CEA) have been proved to play a key role in the control of anxiety, stress and fear-related behaviors. BLA is a cortex-like complex consisting of both γ-aminobutyric acidergic (GABAergic) interneurons and glutamatergic neurons. The CEA is a striatum-like output of the amygdala, consisting almost exclusively of GABAergic medium spiny neurons. In this study, we explored the morphology and axonal projections of the GABAergic neurons in BLA and CEA, using conditional anterograde axonal tracing, immunohistochemistry, and VGAT-Cre transgenic mice to further understand their functional roles. We found that the axonal projections of GABAergic neurons from the BLA mainly distributed to the forebrain, whilst GABAergic neurons from the CEA distributed to the forebrain, midbrain and brainstem. In the forebrain, the axonal projections of GABAergic neurons from the BLA projected to the anterior olfactory nucleus, the cerebral cortex, the septum, the striatum, the thalamus, the amygdala and the hippocampus. The axonal projections of GABAergic neurons from the CEA distributed to the nuclei of the prefrontal cortex, the bed nucleus of the stria terminalis, the hypothalamus and the thalamus. In the midbrain and brainstem, the axonal projections of GABAergic neurons from the CEA were found in the periaqueductal gray, the substantia nigra, and the locus coeruleus. These data reveal the neuroanatomical basis for exploring the function of GABAergic neurons in the BLA and CEA, particularly during the processing of fear-related behavior.

Neural Markers of Vulnerability to Anxiety Outcomes after Traumatic Brain Injury

Anxiety outcomes after traumatic brain injury (TBI) are complex, and the underlying neural mechanisms are poorly understood. Here, we developed a multi-dimensional behavioral profiling approach to investigate anxiety-like outcomes in mice that takes into account individual variability. Departing from the tradition of comparing outcomes in TBI versus sham groups, we identified a subgroup within the TBI group that is vulnerable to anxiety dysfunction, and present increased exploration of the anxiogenic zone compared to sham controls or resilient injured animals, by applying dimensionality reduction, clustering, and post hoc validation to behavioral data obtained from multiple assays for anxiety at several post-injury time points. These vulnerable animals expressed distinct molecular profiles in the corticolimbic network, with downregulation in gamma-aminobutyric acid and glutamate and upregulation in neuropeptide Y markers. Indeed, among vulnerable animals, not resilient or sham controls, severity of anxiety-related outcomes correlated strongly with expression of molecular markers. Our results establish a foundational approach, with predictive power, for reliably identifying maladaptive anxiety outcomes after TBI and uncovering neural signatures of vulnerability to anxiety.

Diagnosing mild traumatic brain injury using saliva RNA compared to cognitive and balance testing

BACKGROUND

Early, accurate diagnosis of mild traumatic brain injury (mTBI) can improve clinical outcomes for patients, but mTBI remains difficult to diagnose because of reliance on subjective symptom reports. An objective biomarker could increase diagnostic accuracy and improve clinical outcomes. The aim of this study was to assess the ability of salivary noncoding RNA (ncRNA) to serve as a diagnostic adjunct to current clinical tools. We hypothesized that saliva ncRNA levels would demonstrate comparable accuracy for identifying mTBI as measures of symptom burden, neurocognition, and balance.

METHODS

This case-control study involved 538 individuals. Participants included 251 individuals with mTBI, enrolled ≤14 days postinjury, from 11 clinical sites. Saliva samples (n = 679) were collected at five time points (≤3, 4-7, 8-14, 15-30, and 31-60 days post-mTBI). Levels of ncRNAs (microRNAs, small nucleolar RNAs, and piwi-interacting RNAs) were quantified within each sample using RNA sequencing. The first sample from each mTBI participant was compared to saliva samples from 287 controls. Samples were divided into testing (n = 430; mTBI = 201 and control = 239) and training sets (n = 108; mTBI = 50 and control = 58). The test set was used to identify ncRNA diagnostic candidates and create a diagnostic model. Model accuracy was assessed in the naïve test set.

RESULTS

A model utilizing seven ncRNA ratios, along with participant age and chronic headache status, differentiated mTBI and control participants with a cross-validated area under the curve (AUC) of .857 in the training set (95% CI, .816-.903) and .823 in the naïve test set. In a subset of participants (n = 321; mTBI = 176 and control = 145) assessed for symptom burden (Post-Concussion Symptom Scale), as well as neurocognition and balance (ClearEdge System), these clinical measures yielded cross-validated AUC of .835 (95% CI, .782-.880) and .853 (95% CI, .803-.899), respectively. A model employing symptom burden and four neurocognitive measures identified mTBI participants with similar AUC (.888; CI, .845-.925) as symptom burden and four ncRNAs (.932; 95% CI, .890-.965).

CONCLUSION

Salivary ncRNA levels represent a noninvasive, biologic measure that can aid objective, accurate diagnosis of mTBI.

Blast-Related Mild TBI Alters Anxiety-Like Behavior and Transcriptional Signatures in the Rat Amygdala

The short and long-term neurological and psychological consequences of traumatic brain injury (TBI), and especially mild TBI (mTBI) are of immense interest to the Veteran community. mTBI is a common and detrimental result of combat exposure and results in various deleterious outcomes, including mood and anxiety disorders, cognitive deficits, and post-traumatic stress disorder (PTSD). In the current study, we aimed to further define the behavioral and molecular effects of blast-related mTBI using a well-established (3 × 75 kPa, one per day on three consecutive days) repeated blast overpressure (rBOP) model in rats. We exposed adult male rats to the rBOP procedure and conducted behavioral tests for anxiety and fear conditioning at 1-1.5 months (sub-acute) or 12-13 months (chronic) following blast exposure. We also used next-generation sequencing to measure transcriptome-wide gene expression in the amygdala of sham and blast-exposed animals at the sub-acute and chronic time points. Results showed that blast-exposed animals exhibited an anxiety-like phenotype at the sub-acute timepoint but this phenotype was diminished by the chronic time point. Conversely, gene expression analysis at both sub-acute and chronic timepoints demonstrated a large treatment by timepoint interaction such that the most differentially expressed genes were present in the blast-exposed animals at the chronic time point, which also corresponded to a Bdnf-centric gene network. Overall, the current study identified changes in the amygdalar transcriptome and anxiety-related phenotypic outcomes dependent on both blast exposure and aging, which may play a role in the long-term pathological consequences of mTBI.

Traumatic Brain Injury and Alcohol Drinking Alter Basolateral Amygdala Endocannabinoids in Female Rats

Traumatic brain injury (TBI) affects approximately 3 million Americans yearly and increases vulnerability to developing psychiatric comorbidities. Alcohol use disorder (AUD) is the most prevalent psychiatric diagnosis preceding injury and TBI may increase subsequent alcohol use. The basolateral amygdala (BLA) is a limbic structure commonly affected by TBI that is implicated in anxiety and AUD. Endocannabinoids (eCBs) regulate synaptic activity in the BLA, and BLA eCB modulation alters anxiety-like behavior and stress reactivity. Previous work from our laboratories showed that systemic eCB degradation inhibition ameliorates TBI-induced increases in anxiety-like behavior and motivation to respond for alcohol in male rats. Here, we used a lateral fluid percussion model to test moderate TBI effects on anxiety-like behavior, alcohol drinking, and eCB levels and cell signaling in BLA, as well as the effect of alcohol drinking on anxiety-like behavior and the BLA eCB system, in female rats. Our results show that TBI does not promote escalation of operant alcohol self-administration or increase anxiety-like behavior in female rats. In the BLA, TBI and alcohol drinking alter tissue amounts of 2-arachidonoylglycerol (2-AG) and N-arachidonoylethanolamine (anandamide; AEA) 1 h post-injury, and 2-AG levels remain low 11 days post-injury. Eleven days after injury, BLA pyramidal neurons were hyperexcitable, but measures of synaptic transmission and eCB signaling were unchanged. These data show that TBI impacts BLA 2-AG tissue levels, that this effect is modified by alcohol drinking, and also that TBI increases BLA cell excitability.

Low brain endocannabinoids associated with persistent non-goal directed nighttime hyperactivity after traumatic brain injury in mice

Traumatic brain injury (TBI) is a frequent cause of chronic headache, fatigue, insomnia, hyperactivity, memory deficits, irritability and posttraumatic stress disorder. Recent evidence suggests beneficial effects of pro-cannabinoid treatments. We assessed in mice levels of endocannabinoids in association with the occurrence and persistence of comparable sequelae after controlled cortical impact in mice using a set of long-term behavioral observations in IntelliCages, motor and nociception tests in two sequential cohorts of TBI/sham mice. TBI mice maintained lower body weights, and they had persistent low levels of brain ethanolamide endocannabinoids (eCBs: AEA, OEA, PEA) in perilesional and subcortical ipsilateral brain tissue (6 months), but rapidly recovered motor functions (within days), and average nociceptive responses were within normal limits, albeit with high variability, ranging from loss of thermal sensation to hypersensitivity. TBI mice showed persistent non-goal directed nighttime hyperactivity, i.e. they visited rewarding and non-rewarding operant corners with high frequency and random success. On successful visits, they made more licks than sham mice resulting in net over-licking. The lower the eCBs the stronger was the hyperactivity. In reward-based learning and reversal learning tasks, TBI mice were not inferior to sham mice, but avoidance memory was less stable. Hence, the major late behavioral TBI phenotype was non-goal directed nighttime hyperactivity and "over-licking" in association with low ipsilateral brain eCBs. The behavioral phenotype would agree with a "post-TBI hyperactivity disorder". The association with persistently low eCBs in perilesional and subcortical regions suggests that eCB deficiency contribute to the post-TBI psychopathology.

Catastrophic consequences: can the feline parasite Toxoplasma gondii prompt the purrfect neuroinflammatory storm following traumatic brain injury?

Traumatic brain injury (TBI) is one of the leading causes of morbidity and mortality worldwide; however, treatment development is hindered by the heterogenous nature of TBI presentation and pathophysiology. In particular, the degree of neuroinflammation after TBI varies between individuals and may be modified by other factors such as infection. Toxoplasma gondii, a parasite that infects approximately one-third of the world’s population, has a tropism for brain tissue and can persist as a life-long infection. Importantly, there is notable overlap in the pathophysiology between TBI and T. gondii infection, including neuroinflammation. This paper will review current understandings of the clinical problems, pathophysiological mechanisms, and functional outcomes of TBI and T. gondii, before considering the potential synergy between the two conditions. In particular, the discussion will focus on neuroinflammatory processes such as microglial activation, inflammatory cytokines, and peripheral immune cell recruitment that occur during T. gondii infection and after TBI. We will present the notion that these overlapping pathologies in TBI individuals with a chronic T. gondii infection have the strong potential to exacerbate neuroinflammation and related brain damage, leading to amplified functional deficits. The impact of chronic T. gondii infection on TBI should therefore be investigated in both preclinical and clinical studies as the possible interplay could influence treatment strategies.

Early parosmia signs and affective states predicts depression and anxiety symptoms six months after a mild Traumatic Brain Injury

This longitudinal study aimed to evaluate qualitative (parosmia) and quantitative (hyposmia/anosmia) olfaction 2-4 weeks (baseline) and six months (follow-up) after a mild traumatic brain injury (mTBI). We further evaluated the predictive value of baseline depression, anxiety and olfaction scores on depression and anxiety at follow-up. At baseline, olfactory function and affective state were assessed in 107 participants (53 patients with mild TBI; 54 healthy controls). At follow-up, data were collected on 71 participants (32 patients, 39 controls). Both at baseline and follow-up, patients with mild TBI showed more signs of parosmia, depression and anxiety, compared to controls. However, patients did not, neither at baseline nor follow-up, show quantitative olfactory impairment. Moreover, while baseline scores of depression and anxiety helped predict the development of symptoms of depression and anxiety at follow up, adding parosmia scores to the prediction model significantly increased the amount of explained variances. Clinicians should implement affective and olfactory evaluation to predict patients' affective outcome.

Chrysin attenuates traumatic brain injury-induced recognition memory decline, and anxiety/depression-like behaviors in rats: Insights into underlying mechanisms

RATIONALE

Cortical and hippocampal neuronal apoptosis and neuroinflammation are associated with behavioral deficits following traumatic brain injury (TBI).

OBJECTIVES

The present study was designed to investigate the potential protective effects of flavonoid chrysin against TBI-induced vestibulomotor impairment, exploratory/locomotor dysfunctions, recognition memory decline, and anxiety/depression-like behaviors, as well as the verified possible involved mechanisms.

METHODS

Chrysin (25, 50, or 100 mg/kg/day; P.O.) was administered to rats immediately after diffuse TBI induction, and it was continued for 3 or 14 days. Behavioral functions were assessed by employing standard behavioral paradigms at scheduled points in time. Three days post-TBI, inflammation status was assayed in both cerebral cortex and hippocampus using ELISA kits. Moreover, apoptosis and expression of Bcl-2 family proteins were examined by TUNEL staining and immunohistochemistry, respectively.

RESULTS

The results indicated that treatment with chrysin improved vestibulomotor dysfunction, ameliorated recognition memory deficit, and attenuated anxiety/depression-like behaviors in the rats with TBI. Chrysin treatment also modulated inflammation status, reduced apoptotic index, and regulated Bcl-2 family proteins expression in the brains of rats with TBI.

CONCLUSIONS

In conclusion, the results suggest that chrysin could be beneficial for protection against TBI-associated behavioral deficits, owing to its anti-apoptotic and anti-inflammatory properties.

Panic results in unique molecular and network changes in the amygdala that facilitate fear responses

Recurrent panic attacks (PAs) are a common feature of panic disorder (PD) and post-traumatic stress disorder (PTSD). Several distinct brain regions are involved in the regulation of panic responses, such as perifornical hypothalamus (PeF), periaqueductal gray, amygdala and frontal cortex. We have previously shown that inhibition of GABA synthesis in the PeF produces panic-vulnerable rats. Here, we investigate the mechanisms by which a panic-vulnerable state could lead to persistent fear. We first show that optogenetic activation of glutamatergic terminals from the PeF to the basolateral amygdala (BLA) enhanced the acquisition, delayed the extinction and induced the persistence of fear responses 3 weeks later, confirming a functional PeF-amygdala pathway involved in fear learning. Similar to optogenetic activation of PeF, panic-prone rats also exhibited delayed extinction. Next, we demonstrate that panic-prone rats had altered inhibitory and enhanced excitatory synaptic transmission of the principal neurons, and reduced protein levels of metabotropic glutamate type 2 receptor (mGluR2) in the BLA. Application of an mGluR2-positive allosteric modulator (PAM) reduced glutamate neurotransmission in the BLA slices from panic-prone rats. Treating panic-prone rats with mGluR2 PAM blocked sodium lactate (NaLac)-induced panic responses and normalized fear extinction deficits. Finally, in a subset of patients with comorbid PD, treatment with mGluR2 PAM resulted in complete remission of panic symptoms. These data demonstrate that a panic-prone state leads to specific reduction in mGluR2 function within the amygdala network and facilitates fear, and mGluR2 PAMs could be a targeted treatment for panic symptoms in PD and PTSD patients.

Experimental Traumatic Brain Injury Induces Chronic Glutamatergic Dysfunction in Amygdala Circuitry Known to Regulate Anxiety-Like Behavior

Up to 50% of traumatic brain injury (TBI) survivors demonstrate persisting and late-onset anxiety disorders indicative of limbic system dysregulation, yet the pathophysiology underlying the symptoms is unclear. We hypothesize that the development of TBI-induced anxiety-like behavior in an experimental model of TBI is mediated by changes in glutamate neurotransmission within the amygdala. Adult, male Sprague-Dawley rats underwent midline fluid percussion injury or sham surgery. Anxiety-like behavior was assessed at 7 and 28 days post-injury (DPI) followed by assessment of real-time glutamate neurotransmission in the basolateral amygdala (BLA) and central nucleus of the amygdala (CeA) using glutamate-selective microelectrode arrays. The expression of anxiety-like behavior at 28 DPI coincided with decreased evoked glutamate release and slower glutamate clearance in the CeA, not BLA. Numerous factors contribute to the changes in glutamate neurotransmission over time. In two additional animal cohorts, protein levels of glutamatergic transporters (Glt-1 and GLAST) and presynaptic modulators of glutamate release (mGluR2, TrkB, BDNF, and glucocorticoid receptors) were quantified using automated capillary western techniques at 28 DPI. Astrocytosis and microglial activation have been shown to drive maladaptive glutamate signaling and were histologically assessed over 28 DPI. Alterations in glutamate neurotransmission could not be explained by changes in protein levels for glutamate transporters, mGluR2 receptors, astrocytosis, and microglial activation. Presynaptic modulators, BDNF and TrkB, were significantly decreased at 28 DPI in the amygdala. Dysfunction in presynaptic regulation of glutamate neurotransmission may contribute to anxiety-related behavior and serve as a therapeutic target to improve circuit function.

Interference of neuronal activity-mediated gene expression through serum response factor deletion enhances mortality and hyperactivity after traumatic brain injury

Traumatic brain injury (TBI) is one of the most frequent causes of brain injury and mortality in young adults with detrimental sequelae such as cognitive impairments, epilepsy, and attention-deficit hyperactivity disorder. TBI modulates the neuronal excitability resulting in propagation of a neuronal activity-driven gene expression program. However, the impact of such neuronal activity mediated gene expression in TBI has been poorly studied. In this study we analyzed mouse mutants of the prototypical neuronal activity-dependent transcription factor SRF (serum response factor) in a weight-drop TBI model. Neuron-restricted SRF deletion elevated TBI inflicted mortality suggesting a neuroprotective SRF function during TBI. Behavioral inspection uncovered elevated locomotor activity in Srf mutant mice after TBI in contrast to hypoactivity observed in wild-type littermates. This indicates an SRF role in modulation of TBI-associated alterations in locomotor activity. Finally, induction of a neuronal activity induced gene expression program composed of immediate early genes (IEGs) such as Egr1, Egr2, Egr3, Npas4, Atf3, Arc, Ptgs2, and neuronal pentraxins (Nptx2) was compromised upon SRF depletion. Overall, our data show a role of neuronal activity-mediated gene transcription during TBI and suggest a molecular link between TBI and such post-TBI neurological comorbidities involving hyperactivity phenotypes.

Stress reactivity after traumatic brain injury: implications for comorbid post-traumatic stress disorder

Most people have or will experience traumatic stress at some time over the lifespan, but only a subset of traumatized individuals develop post-traumatic stress disorder (PTSD). Clinical research supports high rates of traumatic brain injury (TBI)-PTSD comorbidity and demonstrates TBI as a significant predictor of the development of PTSD. Biological factors impacted following brain injury that may contribute to increased PTSD risk are unknown. Heightened stress reactivity and dysregulated hypothalamic-pituitary-adrenal (HPA) axis function are common to both TBI and PTSD, and affect amygdalar structure and function, which is implicated in PTSD. In this review, we summarize a growing body of literature that shows HPA axis dysregulation, as well as enhanced fear and amygdalar function after TBI. We present the hypothesis that altered stress reactivity as a result of brain injury impacts the amygdala and defense systems to be vulnerable to increased fear and PTSD development from traumatic stress. Identifying biological mechanisms that underlie this vulnerability, such as dysregulated HPA axis function, may lead to better targeted treatments and preventive measures to support psychological health after TBI.

Connectivity differences between Gulf War Illness (GWI) phenotypes during a test of attention

One quarter of veterans returning from the 1990–1991 Persian Gulf War have developed Gulf War Illness (GWI) with chronic pain, fatigue, cognitive and gastrointestinal dysfunction. Exertion leads to characteristic, delayed onset exacerbations that are not relieved by sleep. We have modeled exertional exhaustion by comparing magnetic resonance images from before and after submaximal exercise. One third of the 27 GWI participants had brain stem atrophy and developed postural tachycardia after exercise (START: Stress Test Activated Reversible Tachycardia). The remainder activated basal ganglia and anterior insulae during a cognitive task (STOPP: Stress Test Originated Phantom Perception). Here, the role of attention in cognitive dysfunction was assessed by seed region correlations during a simple 0-back stimulus matching task (“see a letter, push a button”) performed before exercise. Analysis was analogous to resting state, but different from psychophysiological interactions (PPI). The patterns of correlations between nodes in task and default networks were significantly different for START (n = 9), STOPP (n = 18) and control (n = 8) subjects. Edges shared by the 3 groups may represent co-activation caused by the 0-back task. Controls had a task network of right dorsolateral and left ventrolateral prefrontal cortex, dorsal anterior cingulate cortex, posterior insulae and frontal eye fields (dorsal attention network). START had a large task module centered on the dorsal anterior cingulate cortex with direct links to basal ganglia, anterior insulae, and right dorsolateral prefrontal cortex nodes, and through dorsal attention network (intraparietal sulci and frontal eye fields) nodes to a default module. STOPP had 2 task submodules of basal ganglia–anterior insulae, and dorsolateral prefrontal executive control regions. Dorsal attention and posterior insulae nodes were embedded in the default module and were distant from the task networks. These three unique connectivity patterns during an attention task support the concept of Gulf War Disease with recognizable, objective patterns of cognitive dysfunction.

Aberrant ER Stress Induced Neuronal-IFNβ Elicits White Matter Injury Due to Microglial Activation and T-Cell Infiltration after TBI

Persistent endoplasmic reticulum (ER) stress in neurons is associated with activation of inflammatory cells and subsequent neuroinflammation following traumatic brain injury (TBI); however, the underlying mechanism remains elusive. We found that induction of neuronal-ER stress, which was mostly characterized by an increase in phosphorylation of a protein kinase R-like ER kinase (PERK) leads to release of excess interferon (IFN)β due to atypical activation of the neuronal-STING signaling pathway. IFNβ enforced activation and polarization of the primary microglial cells to inflammatory M1 phenotype with the secretion of a proinflammatory chemokine CXCL10 due to activation of STAT1 signaling. The secreted CXCL10, in turn, stimulated the T-cell infiltration by serving as the ligand and chemoattractant for CXCR3⁺ T-helper 1 (Th1) cells. The activation of microglial cells and infiltration of Th1 cells resulted in white matter injury, characterized by impaired myelin basic protein and neurofilament NF200, the reduced thickness of corpus callosum and external capsule, and decline of mature oligodendrocytes and oligodendrocyte precursor cells. Intranasal delivery of CXCL10 siRNA blocked Th1 infiltration but did not fully rescue microglial activation and white matter injury after TBI. However, impeding PERK-phosphorylation through the administration of GSK2656157 abrogated neuronal induction of IFNβ, switched microglial polarization to M2 phenotype, prevented Th1 infiltration, and increased Th2 and Treg levels. These events ultimately attenuated the white matter injury and improved anxiety and depressive-like behavior following TBI.SIGNIFICANCE STATEMENT A recent clinical study showed that human brain trauma patients had enhanced expression of type-1 IFN; suggests that type-1 IFN signaling may potentially influence clinical outcome in TBI patients. However, it was not understood how TBI leads to an increase in IFNβ and whether induction of IFNβ has any influence on neuroinflammation, which is the primary reason for morbidity and mortality in TBI. Our study suggests that induction of PERK phosphorylation, a characteristic feature of ER stress is responsible for an increase in neuronal IFNβ, which, in turn, activates microglial cells and subsequently manifests the infiltration of T cells to induce neuroinflammation and subsequently white matter injury. Blocking PERK phosphorylation using GSK2656157 (or PERK knockdown) the whole cascade of neuroinflammation was attenuated and improved cognitive function after TBI.

Positive allosteric modulation of the α7 nicotinic acetylcholine receptor as a treatment for cognitive deficits after traumatic brain injury

Cognitive impairments are a common consequence of traumatic brain injury (TBI). The hippocampus is a subcortical structure that plays a key role in the formation of declarative memories and is highly vulnerable to TBI. The α7 nicotinic acetylcholine receptor (nAChR) is highly expressed in the hippocampus and reduced expression and function of this receptor are linked with cognitive impairments in Alzheimer's disease and schizophrenia. Positive allosteric modulation of α7 nAChRs with AVL-3288 enhances receptor currents and improves cognitive functioning in naïve animals and healthy human subjects. Therefore, we hypothesized that targeting the α7 nAChR with the positive allosteric modulator AVL-3288 would enhance cognitive functioning in the chronic recovery period of TBI. To test this hypothesis, adult male Sprague Dawley rats received moderate parasagittal fluid-percussion brain injury or sham surgery. At 3 months after recovery, animals were treated with vehicle or AVL-3288 at 30 min prior to cue and contextual fear conditioning and the water maze task. Treatment of TBI animals with AVL-3288 rescued learning and memory deficits in water maze retention and working memory. AVL-3288 treatment also improved cue and contextual fear memory when tested at 24 hr and 1 month after training, when TBI animals were treated acutely just during fear conditioning at 3 months post-TBI. Hippocampal atrophy but not cortical atrophy was reduced with AVL-3288 treatment in the chronic recovery phase of TBI. AVL-3288 application to acute hippocampal slices from animals at 3 months after TBI rescued basal synaptic transmission deficits and long-term potentiation (LTP) in area CA1. Our results demonstrate that AVL-3288 improves hippocampal synaptic plasticity, and learning and memory performance after TBI in the chronic recovery period. Enhancing cholinergic transmission through positive allosteric modulation of the α7 nAChR may be a novel therapeutic to improve cognition after TBI.

Lifelong consequences of brain injuries during development: From risk to resilience

Traumatic brain injuries in children represent a major public health issue and even relatively mild injuries can have lifelong consequences. However, the outcomes from these injuries are highly heterogeneous, with most individuals recovering fully, but a substantial subset experiencing prolonged or permanent disabilities across a number of domains. Moreover, brain injuries predispose individuals to other kinds of neuropsychiatric and somatic illnesses. Critically, the severity of the injury only partially predicts subsequent outcomes, thus other factors must be involved. In this review, we discuss the psychological, social, neuroendocrine, and autonomic processes that are disrupted following traumatic brain injury during development, and consider the mechanisms the mediate risk or resilience after traumatic brain injury in this vulnerable population.

Sensory sensitivity as a link between concussive traumatic brain injury and PTSD

Traumatic brain injury (TBI) is one of the most common injuries to military personnel, a population often exposed to stressful stimuli and emotional trauma. Changes in sensory processing after TBI might contribute to TBI-post traumatic stress disorder (PTSD) comorbidity. Combining an animal model of TBI with an animal model of emotional trauma, we reveal an interaction between auditory sensitivity after TBI and fear conditioning where 75 dB white noise alone evokes a phonophobia-like phenotype and when paired with footshocks, fear is robustly enhanced. TBI reduced neuronal activity in the hippocampus but increased activity in the ipsilateral lateral amygdala (LA) when exposed to white noise. The white noise effect in LA was driven by increased activity in neurons projecting from ipsilateral auditory thalamus (medial geniculate nucleus). These data suggest that altered sensory processing within subcortical sensory-emotional circuitry after TBI results in neutral stimuli adopting aversive properties with a corresponding impact on facilitating trauma memories and may contribute to TBI-PTSD comorbidity.

Sex differences in cued fear responses and parvalbumin cell density in the hippocampus following repetitive concussive brain injuries in C57BL/6J mice

There is strong evidence to suggest a link between repeated head trauma and cognitive and emotional disorders, and Repetitive concussive brain injuries (rCBI) may also be a risk factor for depression and anxiety disorders. Animal models of brain injury afford the opportunity for controlled study of the effects of injury on functional outcomes. In this study, male and cycling female C57BL/6J mice sustained rCBI (3x) at 24-hr intervals and were tested in a context and cued fear conditioning paradigm, open field (OF), elevated zero maze and tail suspension test. All mice with rCBI showed less freezing behavior than sham control mice during the fear conditioning context test. Injured male, but not female mice also froze less in response to the auditory cue (tone). Injured mice were hyperactive in an OF environment and spent more time in the open quadrants of the elevated zero maze, suggesting decreased anxiety, but there were no differences between injured mice and sham-controls in depressive-like activity on the tail suspension test. Pathologically, injured mice showed increased astrogliosis in the injured cortex and white matter tracts (optic tracts and corpus callosum). There were no changes in the number of parvalbumin-positive interneurons in the cortex or amygdala, but injured male mice had fewer parvalbumin-positive neurons in the hippocampus. Parvalbumin-reactive interneurons of the hippocampus have been previously demonstrated to be involved in hippocampal-cortical interactions required for memory consolidation, and it is possible memory changes in the fear-conditioning paradigm following rCBI are the result of more subtle imbalances in excitation and inhibition both within the amygdala and hippocampus, and between more widespread brain regions that are injured following a diffuse brain injury.

Histone Deacetylase 4 Downregulation Elicits Post-Traumatic Psychiatric Disorders through Impairment of Neurogenesis

An enduring deficit in neurogenesis largely contributes to the development of severe post-traumatic psychiatric disorders such as anxiety, depression, and memory impairment following traumatic brain injury (TBI); however, the mechanism remains obscure. Here we have shown that an imbalance in the generation of γ-aminobutyric acid (GABA)ergic and glutamatergic neurons due to aberrant induction of vesicular glutamate transporter 1 (vGlut1)-positive glutamatergic cells is responsible for impaired neuronal differentiation in the hippocampus following TBI. To elucidate the molecular mechanism, we found that TBI activates a transcription factor, Pax3, by increasing its acetylation status, and subsequently induces Ngn2 transcription. This event, in turn, augments the vGlut1-expressing glutamatergic neurons and accumulation of excess glutamate in the hippocampus that can affect neuronal differentiation. In our study the acetylation of Pax3 was increased due to loss of its interaction with a deacetylase, histone deacetylase 4 (HDAC4), which was downregulated after TBI. TBI-induced activation of GSK3β was responsible for the degradation of HDAC4. We also showed that overexpression of HDAC4 before TBI reduces Pax3 acetylation by restoring an interaction between HDAC4 and Pax3 in the hippocampus. This event prevents the aberrant induction of vGlut1-positive glutamatergic neurons by decreasing the Ngn2 level and subsequently reinforces the balance between GABAergic and glutamatergic neurons following TBI. Further, we found that overexpression of HDAC4 in the hippocampus improves anxiety, depressive-like behavior, and memory functions following TBI.

Neurotransmitter changes after traumatic brain injury: an update for new treatment strategies

Traumatic brain injury (TBI) is a pervasive problem in the United States and worldwide, as the number of diagnosed individuals is increasing yearly and there are no efficacious therapeutic interventions. A large number of patients suffer with cognitive disabilities and psychiatric conditions after TBI, especially anxiety and depression. The constellation of post-injury cognitive and behavioral symptoms suggest permanent effects of injury on neurotransmission. Guided in part by preclinical studies, clinical trials have focused on high-yield pathophysiologic mechanisms, including protein aggregation, inflammation, metabolic disruption, cell generation, physiology, and alterations in neurotransmitter signaling. Despite successful treatment of experimental TBI in animal models, clinical studies based on these findings have failed to translate to humans. The current international effort to reshape TBI research is focusing on redefining the taxonomy and characterization of TBI. In addition, as the next round of clinical trials is pending, there is a pressing need to consider what the field has learned over the past two decades of research, and how we can best capitalize on this knowledge to inform the hypotheses for future innovations. Thus, it is critically important to extend our understanding of the pathophysiology of TBI, particularly to mechanisms that are associated with recovery versus development of chronic symptoms. In this review, we focus on the pathology of neurotransmission after TBI, reflecting on what has been learned from both the preclinical and clinical studies, and we discuss new directions and opportunities for future work.

Endocannabinoid and Muscarinic Signaling Crosstalk in the 3xTg-AD Mouse Model of Alzheimer's Disease

The endocannabinoid system, which modulates emotional learning and memory through CB1 receptors, has been found to be deregulated in Alzheimer's disease (AD). AD is characterized by a progressive decline in memory associated with selective impairment of cholinergic neurotransmission. The functional interplay of endocannabinoid and muscarinic signaling was analyzed in seven-month-old 3xTg-AD mice following the evaluation of learning and memory of an aversive stimulus. Neurochemical correlates were simultaneously studied with both receptor and functional autoradiography for CB1 and muscarinic receptors, and regulations at the cellular level were depicted by immunofluorescence. 3xTg-AD mice exhibited increased acquisition latencies and impaired memory retention compared to age-matched non-transgenic mice. Neurochemical analyses showed changes in CB1 receptor density and functional coupling of CB1 and muscarinic receptors to Gi/o proteins in several brain areas, highlighting that observed in the basolateral amygdala. The subchronic (seven days) stimulation of the endocannabinoid system following repeated WIN55,212-2 (1 mg/kg) or JZL184 (8 mg/kg) administration induced a CB1 receptor downregulation and CB1-mediated signaling desensitization, normalizing acquisition latencies to control levels. However, the observed modulation of cholinergic neurotransmission in limbic areas did not modify learning and memory outcomes. A CB1 receptor-mediated decrease of GABAergic tone in the basolateral amygdala may be controlling the limbic component of learning and memory in 3xTg-AD mice. CB1 receptor desensitization may be a plausible strategy to improve behavior alterations associated with genetic risk factors for developing AD.

Alpha-Linolenic Acid Treatment Reduces the Contusion and Prevents the Development of Anxiety-Like Behavior Induced by a Mild Traumatic Brain Injury in Rats

Approximately, 1.7 million Americans suffer a TBI annually and TBI is a major cause of death and disability. The majority of the TBI cases are of the mild type and while most patients recover completely from mild TBI (mTBI) about 10% result in persistent symptoms and some result in lifelong disability. Anxiety disorders are the second most common diagnosis post-TBI. Of note, TBI-induced anxiety disorders are difficult to treat and remain a chronic condition suggesting that new therapies are needed. Previous work from our laboratory demonstrated that a mild TBI induced an anxiety-like phenotype, a key feature of the human condition, associated with loss of GABAergic interneurons and hyperexcitability in the basolateral amygdala (BLA) in rodents 7 and 30 days after a controlled cortical impact (CCI) injury. We now confirm that animals display significantly increased anxiety-like behavior 30 days after CCI. The anxiety-like behavior was associated with a significant loss of GABAergic interneurons and significant reductions in the frequency and amplitude of spontaneous and miniature GABA_A-receptor-mediated inhibitory postsynaptic currents (IPSCs) in the BLA. Significantly, subchronic treatment with alpha-linolenic acid (ALA) after CCI prevents the development of anxiety-like behavior, the loss of GABAergic interneurons, hyperexcitability in the BLA and reduces the impact injury. Taken together, administration of ALA after CCI is a potent therapy against the neuropathology and pathophysiological effects of mTBI in the BLA.

A Robust Nonparametric Measure of Effect Size Based on an Analog of Cohen's d, Plus Inferences About the Median of the Typical Difference

The paper describes a nonparametric analog of Cohen's d, Q. It is established that a confidence interval for Q can be computed via a method for computing a confidence interval for the median of D = X1 − X2, which in turn is related to making inferences about P(X1 < X2).