Evidence disputing the importance of excitotoxicity in hippocampal neuron death after experimental traumatic brain injury

mNSCs overexpressing Rimkla transplantation facilitates cognitive recovery in a mouse model of traumatic brain injury

N-acetyl aspartyl-glutamate (NAAG) is easily inactivated for the hydrolysis of NAAG peptidase on the surface of glial cells, thereby losing its endogenous neuroprotective effect after traumatic brain injury. In this study, lentiviral vectors were used to over express/knock out NAAG synthetase II (Rimkla) in mouse embryonic neural stem cells (mNSCs) in vitro and these mNSCs were transplanted at the lesion site in a mouse model of controlled cortical impact (CCI). In vivo experiments showed that transplantation of mNSCs overexpressing Rimkla regulated glutamate-glutamine cycling between adjacent astrocytes and neurons in the subacute phase of CCI, thereby enhancing support for neuronal metabolism and promoting neuronal synaptic repair in the hippocampal CA3 region. Taken together, these findings demonstrate that transplantation of neural stem cells overexpressing Rimkla can effectively increase the NAAG concentration in local brain regions, which opens up new ideas for the maintenance of NAAG neuroprotective effects after TBI.

Peering into the Brain through the Retrosplenial Cortex to Assess Cognitive Function of the Injured Brain

The retrosplenial cortex (RSC) is a posterior cortical area that has been drawing increasing interest in recent years, with a growing number of studies studying its contribution to cognitive and sensory functions. From an anatomical perspective, it has been established that the RSC is extensively and often reciprocally connected with the hippocampus, neocortex, and many midbrain regions. Functionally, the RSC is an important hub of the default-mode network. This endowment, with vast anatomical and functional connections, positions the RSC to play an important role in episodic memory, spatial and contextual learning, sensory-cognitive activities, and multi-modal sensory information processing and integration. Additionally, RSC dysfunction has been reported in cases of cognitive decline, particularly in Alzheimer's disease and stroke. We review the literature to examine whether the RSC can act as a cortical marker of persistent cognitive dysfunction after traumatic brain injury (TBI). Because the RSC is easily accessible at the brain's surface using in vivo techniques, we argue that studying RSC network activity post-TBI can shed light into the mechanisms of less-accessible brain regions, such as the hippocampus. There is a fundamental gap in the TBI field about the microscale alterations occurring post-trauma, and by studying the RSC's neuronal activity at the cellular level we will be able to design better therapeutic tools. Understanding how neuronal activity and interactions produce normal and abnormal activity in the injured brain is crucial to understanding cognitive dysfunction. By using this approach, we expect to gain valuable insights to better understand brain disorders like TBI.

Cerebral Concussion Primes the Lungs for Subsequent Neutrophil-Mediated Injury

OBJECTIVES

Mild traumatic brain injury in the form of concussion is extremely common, and the potential effects on pulmonary priming have been underestimated. The aim of this study was to characterize the pulmonary response following mild traumatic brain injury and assess the pulmonary susceptibility to lung injury after a subsequent innocuous pulmonary insult.

DESIGN

Experimental in vivo study.

SETTING

University research laboratory.

SUBJECTS

Male CD1 mice.

INTERVENTIONS

We developed a model of concussive traumatic brain injury in mice followed by pulmonary acid microaspiration. To assess the dependent role of neutrophils in mediating pulmonary injury, we specifically depleted neutrophils.

MEASUREMENTS AND MAIN RESULTS

Lateral fluid percussion to the brain resulted in neuronal damage and neutrophil infiltration as well as extensive pulmonary interstitial neutrophil accumulation but no alveolar injury. Following subsequent innocuous acid microaspiration, augmented alveolar neutrophil influx led to the development of pulmonary hemorrhage that was reduced following neutrophil depletion.

CONCLUSIONS

This model shows for the first time that innocuous acid microaspiration is sufficient to induce neutrophil-mediated lung injury following mild concussion and that the extracranial effects of mild traumatic brain injury have been underestimated.

Influence of mild traumatic brain injury during pediatric stage on short-term memory and hippocampal apoptosis in adult rats

Traumatic brain injury (TBI) is a leading cause of neurological deficit in the brain, which induces short- and long-term brain damage, cognitive impairment with/without structural alteration, motor deficits, emotional problems, and death both in children and adults. In the present study, we evaluated whether mild TBI in childhood causes persisting memory impairment until adulthood. Moreover, we investigated the influence of mild TBI on memory impairment in relation with hippocampal apoptosis. For this, step-down avoidance task, terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) assay, and immunohistochemistry for caspase-3 were performed. Male Sprague-Dawley rats were used in the experiments. The animals were randomly divided into two groups: sham-operation group and TBI-induction group. The mild TBI model was created with an electromagnetic contusion device activated at a velocity of 3.0 m/sec. The results showed that mild TBI during the pediatric stage significantly decreased memory retention. The numbers of TUNEL-positive and caspase-3-positive cells were increased in the TBI-induction group compared to those in the sham-operation group. Defective memory retention and apoptosis sustained up to the adult stage. The present results shows that mild TBI induces long-lasting cognitive impairment from pediatric to adult stages in rats through the high level of apoptosis. The finding of this study suggests that children with mild TBI may need intensive treatments for the reduction of long-lasting cognitive impairment by secondary neuronal damage.

Loss of Acid sensing ion channel-1a and bicarbonate administration attenuate the severity of traumatic brain injury

Traumatic brain injury (TBI) is a common cause of morbidity and mortality in people of all ages. Following the acute mechanical insult, TBI evolves over the ensuing minutes and days. Understanding the secondary factors that contribute to TBI might suggest therapeutic strategies to reduce the long-term consequences of brain trauma. To assess secondary factors that contribute to TBI, we studied a lateral fluid percussion injury (FPI) model in mice. Following FPI, the brain cortex became acidic, consistent with data from humans following brain trauma. Administering HCO₃⁻ after FPI prevented the acidosis and reduced the extent of neurodegeneration. Because acidosis can activate acid sensing ion channels (ASICs), we also studied ASIC1a^−/− mice and found reduced neurodegeneration after FPI. Both HCO₃⁻ administration and loss of ASIC1a also reduced functional deficits caused by FPI. These results suggest that FPI induces cerebral acidosis that activates ASIC channels and contributes to secondary injury in TBI. They also suggest a therapeutic strategy to attenuate the adverse consequences of TBI.

Prevention of traumatic brain injury-induced neuronal death by inhibition of NADPH oxidase activation

The present study aimed to evaluate the therapeutic potential of apocynin, an NADPH oxidase assembly inhibitor, on traumatic brain injury. Rat traumatic brain injury (TBI) was performed using a weight drop model. Apocynin (100mg/kg) was injected into the intraperitoneal space 15 min before TBI. Reactive oxygen species (ROS) in the hippocampal CA3 pyramidal neurons were detected by dihydroethidium (dHEt) at 3h after TBI. Oxidative injury was detected by 4-hydroxy-2-nonenal (4HNE) at 6h after TBI. Blood-brain barrier disruption was detected by IgG extravasation and neuronal death was evaluated with Fluoro Jade-B staining 24h after TBI. Microglia activation was detected by CD11b immunohistochemistry in the hippocampus at 1 week after TBI. ROS production was inhibited by apocynin administration in the hippocampal CA3 pyramidal neurons. This pre-treatment with apocynin decreased the blood-brain barrier disruption, the number of degenerating neurons in the hippocampal CA3 region and microglial activation after TBI. The present study indicates that apocynin pre-treatment prevents TBI-induced ROS production, thus decreasing BBB disruption, neuronal death and microglial activation. Therefore, the present study suggests that inhibition of NADPH oxidase by apocynin may have a high therapeutic potential to reduce traumatic brain injury-induced neuronal death.

Prevention of traumatic brain injury-induced neuron death by intranasal delivery of nicotinamide adenine dinucleotide

Traumatic brain injury (TBI) is one of the most devastating injuries experienced by military personnel, as well as the general population, and can result in acute and chronic complications such as cognitive impairments. Since there are currently no effective tools for the treatment of TBI, it is of great importance to determine the mechanisms of neuronal death that characterize this insult. Several studies have indicated that TBI-induced neuronal death arises in part due to excessive activation of poly(ADP-ribose) polymerase-1 (PARP-1), which results in nicotinamide adenine dinucleotide (NAD⁺) depletion and subsequent energy failure. In this study, we investigated whether intranasal administration of NAD⁺ could reduce neuronal death after TBI. Rats were subjected to a weight-drop TBI model that induces cortical and hippocampal neuronal death. The intranasal administration of NAD⁺ (20 mg/kg) immediately after TBI protected neurons in CA1, CA3, and dentate gyrus of the hippocampus, but not in the cortex. In addition, delayed microglial activation normally seen after TBI was reduced by NAD⁺ treatment at 7 days after insult. Neuronal superoxide production and PARP-1 accumulation after TBI were not inhibited by NAD⁺ treatment, indicating that reactive oxygen species (ROS) production and PARP-1 activation are events that occur upstream of NAD⁺ depletion. This study suggests that intranasal delivery of NAD⁺ represents a novel, inexpensive, and non-toxic intervention for preventing TBI-induced neuronal death.

Disruptions in the regulation of extracellular glutamate by neurons and glia in the rat striatum two days after diffuse brain injury

Disrupted regulation of extracellular glutamate in the central nervous system contributes to and can exacerbate the acute pathophysiology of traumatic brain injury (TBI). Previously, we reported increased extracellular glutamate in the striatum of anesthetized rats 2 days after diffuse brain injury. To determine the mechanism(s) responsible for increased extracellular glutamate, we used enzyme-based microelectrode arrays (MEAs) coupled with specific pharmacological agents targeted at in vivo neuronal and glial regulation of extracellular glutamate. After TBI, extracellular glutamate was significantly increased in the striatum by (∼90%) averaging 4.1±0.6 μM compared with sham 2.2±0.4 μM. Calcium-dependent neuronal glutamate release, investigated by local application of an N-type calcium channel blocker, was no longer a significant source of extracellular glutamate after TBI, compared with sham. In brain-injured animals, inhibition of glutamate uptake with local application of an excitatory amino acid transporter inhibitor produced significantly greater increase in glutamate spillover (∼ 65%) from the synapses compared with sham. Furthermore, glutamate clearance measured by locally applying glutamate into the extracellular space revealed significant reductions in glutamate clearance parameters in brain-injured animals compared with sham. Taken together, these data indicate that disruptions in calcium-mediated glutamate release and glial regulation of extracellular glutamate contribute to increased extracellular glutamate in the striatum 2 days after diffuse brain injury. Overall, these data suggest that therapeutic strategies used to regulate glutamate release and uptake may improve excitatory circuit function and, possibly, outcomes following TBI.

Diffuse brain injury elevates tonic glutamate levels and potassium-evoked glutamate release in discrete brain regions at two days post-injury: an enzyme-based microelectrode array study

Traumatic brain injury (TBI) survivors often suffer from a wide range of post-traumatic deficits, including impairments in behavioral, cognitive, and motor function. Regulation of glutamate signaling is vital for proper neuronal excitation in the central nervous system. Without proper regulation, increases in extracellular glutamate can contribute to the pathophysiology and neurological dysfunction seen in TBI. In the present studies, enzyme-based microelectrode arrays (MEAs) that selectively measure extracellular glutamate at 2 Hz enabled the examination of tonic glutamate levels and potassium chloride (KCl)-evoked glutamate release in the prefrontal cortex, dentate gyrus, and striatum of adult male rats 2 days after mild or moderate midline fluid percussion brain injury. Moderate brain injury significantly increased tonic extracellular glutamate levels by 256% in the dentate gyrus and 178% in the dorsal striatum. In the dorsal striatum, mild brain injury significantly increased tonic glutamate levels by 200%. Tonic glutamate levels were significantly correlated with injury severity in the dentate gyrus and striatum. The amplitudes of KCl-evoked glutamate release were increased significantly only in the striatum after moderate injury, with a 249% increase seen in the dorsal striatum. Thus, with the MEAs, we measured discrete regional changes in both tonic and KCl-evoked glutamate signaling, which were dependent on injury severity. Future studies may reveal the specific mechanisms responsible for glutamate dysregulation in the post-traumatic period, and may provide novel therapeutic means to improve outcomes after TBI.

Acute cognitive impairment after lateral fluid percussion brain injury recovers by 1 month: evaluation by conditioned fear response

Conditioned fear associates a contextual environment and cue stimulus to a foot shock in a single training trial, where fear expressed to the trained context or cue indicates cognitive performance. Lesion, aspiration or inactivation of the hippocampus and amygdala impair conditioned fear to the trained context and cue, respectively. Moreover, only bilateral experimental manipulations, in contrast to unilateral, abolish cognitive performance. In a model of unilateral brain injury, we sought to test whether a single lateral fluid percussion brain injury impairs cognitive performance in conditioned fear. Brain-injured mice were evaluated for anterograde cognitive deficits, with the hypothesis that acute injury-induced impairments improve over time. Male C57BL/6J mice were brain-injured, trained at 5 or 27 days post-injury, and tested 48h later for recall of the association between the conditioned stimuli (trained context or cue) and the unconditioned stimulus (foot shock) by quantifying fear-associated freezing behavior. A significant anterograde hippocampal-dependent cognitive deficit was observed at 7 days in brain-injured compared to sham. Cued fear conditioning could not detect amygdala-dependent cognitive deficits after injury and stereological estimation of amygdala neuron number corroborated this finding. The absence of injury-related freezing in a novel context substantiated injury-induced hippocampal-dependent cognitive dysfunction, rather than generalized fear. Variations in the training and testing paradigms demonstrated a cognitive deficit in consolidation, rather than acquisition or recall. By 1-month post-injury, cognitive function recovered in brain-injured mice. Hence, the acute injury-induced cognitive impairment may persist while transient pathophysiological sequelae are underway, and improve as global dysfunction subsides.

Proton MR spectroscopy detected glutamate/glutamine is increased in children with traumatic brain injury

Adults with traumatic brain injury (TBI) have been shown by invasive methods to have increased levels of the excitatory neurotransmitter glutamate. It is unclear whether glutamate release contributes to primary or secondary injury and whether its protracted elevation is predictive of a poor outcome. Preliminary studies at our institution in adults found that early increases in magnetic resonance spectroscopy (MRS)-detected glutamate/glutamine (Glx) were associated with poor outcomes. We therefore studied 38 children (mean age, 11 years; range, 1.6-17 years) who had TBI with quantitative short-echo time (STEAM, TE = 20 msec) proton MRS, a mean of 7 +/- 4 (range, 1-17) days after injury in order to determine if their occipital or parietal Glx levels correlated with the severity of injury or outcome. Occipital Glx was significantly increased in children with TBI compared to controls (13.5 +/- 2.4 vs. 10.7 +/- 1.8; p = 0.002), but there was no difference between children with good compared to poor outcomes as determined by the Pediatric Cerebral Performance Category Scale score at 6-12 months after injury. We also did not find a correlation between the amount of Glx and the initial Glasgow Coma Scale score, duration of coma, nor with changes in spectral metabolites, including N-acetyl aspartate, choline, and myoinositol. In part, this may have occurred because, in this study, most patients with poor outcomes were studied later than patients with good outcomes, potentially beyond the time frame for peak elevation of Glx after injury. Additional early and late studies of patients with varying degrees of injury are required to assess the importance to the pathophysiology of TBI of this excitatory neurotransmitter.

Increased hippocampal CA3 vulnerability to low-level kainic acid following lateral fluid percussion injury

This study was designed to determine whether a secondary increase in neuronal activity induced by a low dose of kainic acid (KA), a glutamate analogue, exacerbates the anatomical damage in hippocampal regions following a mild lateral fluid percussion (LFP) brain injury. KA (9 mg/kg) was injected intraperitoneally in LFP-injured rats (n = 16) 1 h post-trauma. The neuronal loss in the CA3, CA4, and hilar regions at 7 days was quantified by two-dimensional cell counts. Hippocampal activation 15 min following KA injection was assessed by measuring local glucose metabolic rates (lCMR(glc)). Following LFP + KA, the ipsilateral side exhibited a 62.7%, 75.7%, and 52.1% decrease in the number of CA3, CA4 and hilar neurons, respectively, compared to naive rats (n = 3). These CA3 and CA4 neuronal counts were also significantly decreased compared to LFP + saline (n = 5) and sham + KA (n = 9) groups. The median Racine Score, used to rate the severity of behavioral seizures, was 4 in LFP + KA and 2 in sham + KA groups (p < 0.015), suggesting a reduction in seizure threshold following injury. lCMRglc in CA3 following LFP + KA was 121.8 +/- 2.0 (mean +/- SE) ipsilaterally and 71.5 +/- 5.4 contralaterally (p < 0.0012). No changes were found in the BBB permeability as measured by [(14)C]aminoisobutyric acid in CA3, CA4, and hilar regions. We conclude that the presence of low-level KA 1 h after LFP dramatically increases the extent of hippocampal activation and induces a striking loss of ipsilateral CA3 and CA4 pyramidal neurons. Neuronal excitation during a time of cellular vulnerability may trigger or amplify the cycle of secondary damage in functionally impaired, but potentially viable, tissue.

Neonatal monosodium glutamate treatment modifies glutamic acid decarboxylase activity during rat brain postnatal development

Monosodium glutamate (MSG) produces neurodegeneration in several brain regions when it is administered to neonatal rats. From an early embryonic age to adulthood, GABA neurons appear to have functional glutamatergic receptors, which could convert them in an important target for excitotoxic neurodegeneration. Changes in the activity of the GABA synthesizing enzyme, glutamic acid decarboxylase (GAD), have been shown after different neuronal insults. Therefore, this work evaluates the effect of neonatal MSG treatment on GAD activity and kinetics in the cerebral cortex, striatum, hippocampus and cerebellum of the rat brain during postnatal development. Neonatal MSG treatment decreased GAD activity in the cerebral cortex at 21 and 60 postnatal days (PD), mainly due to a reduction in the enzyme affinity (K(m)). In striatum, the GAD activity and the enzyme maximum velocity (V(max)) were increased at PD 60 after neonatal MSG treatment. Finally, in the hippocampus and cerebellum, the GAD activity and V(max) were increased, but the K(m) was found to be lower in the experimental group. The results could be related to compensatory mechanisms from the surviving GABAergic neurons, and suggest a putative adjustment in the GAD isoform expression throughout the development of the postnatal brain, since this enzyme is regulated by the synaptic activity under physiological and/or pathophysiological conditions.

Traumatic brain injury research: a review of clinical studies

There is a growing volume of research on trauma brain injury (TBI) as evidenced by a recent Medline search that reported over 6000 articles published on TBI in the past 5 years.