Inhibitory neuronal changes following a mixed diffuse‐focal model of traumatic brain injury

Cortical calretinin-positive neurons: Functional and ontogenetic characteristics and their relationship to brain pathologies

Cortical GABAergic interneurons can be classified according to electrophysiological, biochemical, and/or morphological criteria. In humans, the use of calcium-binding proteins allows us to differentiate three subpopulations of GABAergic interneurons with minimal overlap. Cortical calretinin-positive neurons mainly include bipolar and double-bouquet morphologies, with a largely non-rapid and adaptive firing pattern, originating from the ganglionic eminence and the ventricular and subventricular regions of the developing brain. These cells are distributed from layer I to VI of the neocortex, with predominance in layers II and III. Given their morphology, distribution of processes, and elucidated synaptic contacts, these neurons are considered important in the control of intraminicolumnar processing through vertical inhibition. They have been extensively studied in the context of pathologies characterized by excitation/inhibition imbalance, such as Alzheimer's disease, epilepsy, traumatic brain injury, and autism. In light of the current evidence, this review considers these aspects in depth and discusses the pathophysiological role and selective vulnerability (pathoclisis) vs. the resistance that these interneurons can present against different types of injury.

Hippocampal interneuronal dysfunction and hyperexcitability in a porcine model of concussion

Cognitive impairment is a common symptom following mild traumatic brain injury (mTBI or concussion) and can persist for years in some individuals. Hippocampal slice preparations following closed-head, rotational acceleration injury in swine have previously demonstrated reduced axonal function and hippocampal circuitry disruption. However, electrophysiological changes in hippocampal neurons and their subtypes in a large animal mTBI model have not been examined. Using in vivo electrophysiology techniques, we examined laminar oscillatory field potentials and single unit activity in the hippocampal network 7 days post-injury in anesthetized minipigs. Concussion altered the electrophysiological properties of pyramidal cells and interneurons differently in area CA1. While the firing rate, spike width and amplitude of CA1 interneurons were significantly decreased post-mTBI, these parameters were unchanged in CA1 pyramidal neurons. In addition, CA1 pyramidal neurons in TBI animals were less entrained to hippocampal gamma (40-80 Hz) oscillations. Stimulation of the Schaffer collaterals also revealed hyperexcitability across the CA1 lamina post-mTBI. Computational simulations suggest that reported changes in interneuronal physiology may be due to alterations in voltage-gated sodium channels. These data demonstrate that a single concussion can lead to significant neuronal and circuit level changes in the hippocampus, which may contribute to cognitive dysfunction following mTBI.

Chronic motor performance following different traumatic brain injury severity-A systematic review

Introduction

Traumatic brain injury (TBI) is now known to be a chronic disease, causing ongoing neurodegeneration and linked to increased risk of neurodegenerative motor diseases, such as Parkinson's disease and amyotrophic lateral sclerosis. While the presentation of motor deficits acutely following traumatic brain injury is well-documented, however, less is known about how these evolve in the long-term post-injury, or how the initial severity of injury affects these outcomes. The purpose of this review, therefore, was to examine objective assessment of chronic motor impairment across the spectrum of TBI in both preclinical and clinical models.

Methods

PubMed, Embase, Scopus, and PsycINFO databases were searched with a search strategy containing key search terms for TBI and motor function. Original research articles reporting chronic motor outcomes with a clearly defined TBI severity (mild, repeated mild, moderate, moderate-severe, and severe) in an adult population were included.

Results

A total of 97 studies met the inclusion criteria, incorporating 62 preclinical and 35 clinical studies. Motor domains examined included neuroscore, gait, fine-motor, balance, and locomotion for preclinical studies and neuroscore, fine-motor, posture, and gait for clinical studies. There was little consensus among the articles presented, with extensive differences both in assessment methodology of the tests and parameters reported. In general, an effect of severity was seen, with more severe injury leading to persistent motor deficits, although subtle fine motor deficits were also seen clinically following repeated injury. Only six clinical studies investigated motor outcomes beyond 10 years post-injury and two preclinical studies to 18-24 months post-injury, and, as such, the interaction between a previous TBI and aging on motor performance is yet to be comprehensively examined.

Conclusion

Further research is required to establish standardized motor assessment procedures to fully characterize chronic motor impairment across the spectrum of TBI with comprehensive outcomes and consistent protocols. Longitudinal studies investigating the same cohort over time are also a key for understanding the interaction between TBI and aging. This is particularly critical, given the risk of neurodegenerative motor disease development following TBI.

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.

Somatostatin interneurons exhibit enhanced functional output and resilience to axotomy after mild traumatic brain injury

Mild traumatic brain injury (mTBI) gives rise to a remarkable breadth of pathobiological consequences, principal among which are traumatic axonal injury and perturbation of the functional integrity of neuronal networks that may arise secondary to the elimination of the presynaptic contribution of axotomized neurons. Because there exists a vast diversity of neocortical neuron subtypes, it is imperative to elucidate the relative vulnerability to axotomy among different subtypes. Toward this end, we exploited SOM-IRES-Cre mice to investigate the consequences of the central fluid percussion model of mTBI on the microanatomical integrity and the functional efficacy of the somatostatin (SOM) interneuron population, one of the principal subtypes of neocortical interneuron. We found that the SOM population is resilient to axotomy, representing only 10% of the global burden of inhibitory interneuron axotomy, a result congruous with past work demonstrating that parvalbumin (PV) interneurons bear most of the burden of interneuron axotomy. However, the intact structure of SOM interneurons after injury did not translate to normal cellular function. One day after mTBI, the SOM population is more intrinsically excitable and demonstrates enhanced synaptic efficacy upon post-synaptic layer 5 pyramidal neurons as measured by optogenetics, yet the global evoked inhibitory tone within layer 5 is stable. Simultaneously, there exists a significant increase in the frequency of miniature inhibitory post-synaptic currents within layer 5 pyramidal neurons. These results are consistent with a scheme in which 1 day after mTBI, SOM interneurons are stimulated to compensate for the release from inhibition of layer 5 pyramidal neurons secondary to the disproportionate axotomy of PV interneurons. The enhancement of SOM interneuron intrinsic excitability and synaptic efficacy may represent the initial phase of a dynamic process of attempted autoregulation of neocortical network homeostasis secondary to mTBI.

Post-Traumatic Epilepsy and Comorbidities: Advanced Models, Molecular Mechanisms, Biomarkers, and Novel Therapeutic Interventions

Post-traumatic epilepsy (PTE) is one of the most devastating long-term, network consequences of traumatic brain injury (TBI). There is currently no approved treatment that can prevent onset of spontaneous seizures associated with brain injury, and many cases of PTE are refractory to antiseizure medications. Post-traumatic epileptogenesis is an enduring process by which a normal brain exhibits hypersynchronous excitability after a head injury incident. Understanding the neural networks and molecular pathologies involved in epileptogenesis are key to preventing its development or modifying disease progression. In this article, we describe a critical appraisal of the current state of PTE research with an emphasis on experimental models, molecular mechanisms of post-traumatic epileptogenesis, potential biomarkers, and the burden of PTE-associated comorbidities. The goal of epilepsy research is to identify new therapeutic strategies that can prevent PTE development or interrupt the epileptogenic process and relieve associated neuropsychiatric comorbidities. Therefore, we also describe current preclinical and clinical data on the treatment of PTE sequelae. Differences in injury patterns, latency period, and biomarkers are outlined in the context of animal model validation, pathophysiology, seizure frequency, and behavior. Improving TBI recovery and preventing seizure onset are complex and challenging tasks; however, much progress has been made within this decade demonstrating disease modifying, anti-inflammatory, and neuroprotective strategies, suggesting this goal is pragmatic. Our understanding of PTE is continuously evolving, and improved preclinical models allow for accelerated testing of critically needed novel therapeutic interventions in military and civilian persons at high risk for PTE and its devastating comorbidities.

Regulation of Parvalbumin Interactome in the Perilesional Cortex after Experimental Traumatic Brain Injury

Traumatic brain injury (TBI) causes 10-20% of structural epilepsy, with seizures typically originating in the cortex. Alterations in the neuronal microcircuits in the cortical epileptogenic zone, however, are poorly understood. Here, we assessed TBI-induced changes in perisomatic gamma aminobutyric acid (GABA)-ergic innervation in the perilesional cortex. We hypothesized that TBI will damage parvalbumin (PV)-immunoreactive inhibitory neurons and induce regulation of the associated GABAergic molecular interactome. TBI was induced in adult male Sprague-Dawley rats by lateral fluid-percussion injury. At 1-month post-TBI, the number of PV-positive somata was plotted on unfolded cortical maps and the distribution and density of immunopositive terminals analyzed. Qualitative analysis revealed either patchy microlesions of several hundred micrometers in diameter or diffuse neuronal loss. Quantitative analysis demonstrated a reduction in the number of PV-positive interneurons in patches down to 0% of that in sham-operated controls in the perilesional cortex. In the majority of patches, the cell numbers ranged from 71% to 90% that of the controls. The loss of PV-positive somata was accompanied by decreased axonal labeling. In situ hybridization revealed downregulated PV mRNA expression in the perilesional cortex. Gene Set Enrichment Analysis indicated a robustly downregulated expression profile of PV-related genes, which was confirmed by quantitative reverse transcriptase polymerase chain reaction. Specifically, we found that genes encoding postsynaptic GABA-A receptor genes, Gabrg2 and Gabrd, were downregulated in TBI animals compared with controls. Our data suggests that patchy reduction in PV-positive perisomatic inhibitory innervation contributes to the development of focal cortical inhibitory deficit after TBI.

Intranasal Administration of Oxytocin Attenuates Social Recognition Deficits and Increases Prefrontal Cortex Inhibitory Postsynaptic Currents following Traumatic Brain Injury

Pediatric traumatic brain injury (TBI) results in heightened risk for social deficits that can emerge during adolescence and adulthood. A moderate TBI in male and female rats on postnatal day 11 (equivalent to children below the age of 4) resulted in impairments in social novelty recognition, defined as the preference for interacting with a novel rat compared with a familiar rat, but not sociability, defined as the preference for interacting with a rat compared with an object in the three-chamber test when tested at four weeks (adolescence) and eight weeks (adulthood) postinjury. The deficits in social recognition were not accompanied by deficits in novel object recognition memory and were associated with a decrease in the frequency of spontaneous inhibitory postsynaptic currents (IPSCs) recorded from pyramidal neurons within Layer II/III of the medial prefrontal cortex (mPFC). Whereas TBI did not affect the expression of oxytocin (OXT) or the OXT receptor (OXTR) mRNAs in the hypothalamus and mPFC, respectively, intranasal administration of OXT before behavioral testing was found to reduce impairments in social novelty recognition and increase IPSC frequency in the mPFC in brain-injured animals. These results suggest that TBI-induced deficits in social behavior may be linked to increased excitability of neurons in the mPFC and suggests that the regulation of GABAergic neurotransmission in this region as a potential mechanism underlying these deficits.

Hippocampal-Dependent Cognitive Dysfunction following Repeated Diffuse Rotational Brain Injury in Male and Female Mice

Cognitive dysfunction is a common, often long-term complaint following acquired traumatic brain injury (TBI). Cognitive deficits suggest dysfunction in hippocampal circuits. The goal of the studies described here is to phenotype in both male and female mice the hippocampal-dependent learning and memory deficits resulting from TBI sustained by the Closed-Head Impact Model of Engineered Rotational Acceleration (CHIMERA) device—a model that delivers both a contact–concussion injury as well as unrestrained rotational head movement. Mice sustained either sham procedures or four injuries (0.7 J, 24-h intervals). Spatial learning and memory skills assessed in the Morris water maze (MWM) approximately 3 weeks following injuries were significantly impaired by brain injuries; however, slower swimming speeds and poor performance on visible platform trials suggest that measurement of cognitive impairment with this test is confounded by injury-induced motor and/or visual impairments. A separate experiment confirmed hippocampal-dependent cognitive deficits with trace fear conditioning (TFC), a behavioral test less dependent on motor and visual function. Male mice had greater injury-induced deficits on both the MWM and TFC tests than female mice. Pathologically, the injury was characterized by white matter damage as observed by silver staining and glial fibrillary acidic protein (astrogliosis) in the optic tracts, with milder damage seen in the corpus callosum, and fimbria and brainstem (cerebral peduncles) of some animals. No changes in the density of GABAergic parvalbumin-expressing cells in the hippocampus, amygdala, or parietal cortex were found. This experiment confirmed significant sexually dimorphic cognitive impairments following a repeated, diffuse brain injury.

Effect of fractionated whole-brain irradiation on brain and plasma in a rat model: Metabolic, volumetric and histopathological changes

In the present study, we investigated the correlation between histopathological, metabolic, and volumetric changes in the brain and plasma under experimental conditions. Adult male Wistar rats received fractionated whole-brain irradiation (fWBI) with a total dose of 32 Gy delivered in 4 fractions (dose 8 Gy per fraction) once a week on the same day for 4 consecutive weeks. Proton magnetic resonance spectroscopy (¹H MRS) and imaging were used to detect metabolic and volumetric changes in the brain and plasma. Histopathological changes in the brain were determined by image analysis of immunofluorescent stained sections. Metabolic changes in the brain measured by ¹H MRS before, 48 h, and 9 weeks after the end of fWBI showed a significant decrease in the ratio of total N-acetylaspartate to total creatine (tNAA/tCr) in the corpus striatum. We found a significant decrease in glutamine + glutamate/tCr (Glx/tCr) and, conversely, an increase in gamma-aminobutyric acid to tCr (GABA/tCr) in olfactory bulb (OB). The ratio of astrocyte marker myoinositol/tCr (mIns/tCr) significantly increased in almost all evaluated areas. Magnetic resonance imaging (MRI)-based brain volumetry showed a significant increase in volume, and a concomitant increase in the T₂ relaxation time of the hippocampus. Proton nuclear magnetic resonance (¹H NMR) plasma metabolomics displayed a significant decrease in the level of glucose and glycolytic intermediates and an increase in ketone bodies. The histomorphological analysis showed a decrease to elimination of neuroblasts, increased astrocyte proliferation, and a mild microglia response. The results of the study clearly reflect early subacute changes 9-11 weeks after fWBI with strong manifestations of brain edema, astrogliosis, and ongoing ketosis.