Response of the contralateral hippocampus to lateral fluid percussion brain injury

Delayed TBI-Induced Neuronal Death in the Ipsilateral Hippocampus and Behavioral Deficits in Rats: Influence of Corticosterone-Dependent Survivorship Bias?

Acute and chronic corticosterone (CS) elevations after traumatic brain injury (TBI) may be involved in distant hippocampal damage and the development of late posttraumatic behavioral pathology. CS-dependent behavioral and morphological changes were studied 3 months after TBI induced by lateral fluid percussion in 51 male Sprague-Dawley rats. CS was measured in the background 3 and 7 days and 1, 2 and 3 months after TBI. Tests including open field, elevated plus maze, object location, new object recognition tests (NORT) and Barnes maze with reversal learning were used to assess behavioral changes in acute and late TBI periods. The elevation of CS on day 3 after TBI was accompanied by early CS-dependent objective memory impairments detected in NORT. Blood CS levels > 860 nmol/L predicted delayed mortality with an accuracy of 0.947. Ipsilateral neuronal loss in the hippocampal dentate gyrus, microgliosis in the contralateral dentate gyrus and bilateral thinning of hippocampal cell layers as well as delayed spatial memory deficits in the Barnes maze were revealed 3 months after TBI. Because only animals with moderate but not severe posttraumatic CS elevation survived, we suggest that moderate late posttraumatic morphological and behavioral deficits may be at least partially masked by CS-dependent survivorship bias.

Brain Trauma, Glucocorticoids and Neuroinflammation: Dangerous Liaisons for the Hippocampus

Glucocorticoid-dependent mechanisms of inflammation-mediated distant hippocampal damage are discussed with a focus on the consequences of traumatic brain injury. The effects of glucocorticoids on specific neuronal populations in the hippocampus depend on their concentration, duration of exposure and cell type. Previous stress and elevated level of glucocorticoids prior to pro-inflammatory impact, as well as long-term though moderate elevation of glucocorticoids, may inflate pro-inflammatory effects. Glucocorticoid-mediated long-lasting neuronal circuit changes in the hippocampus after brain trauma are involved in late post-traumatic pathology development, such as epilepsy, depression and cognitive impairment. Complex and diverse actions of the hypothalamic–pituitary–adrenal axis on neuroinflammation may be essential for late post-traumatic pathology. These mechanisms are applicable to remote hippocampal damage occurring after other types of focal brain damage (stroke, epilepsy) or central nervous system diseases without obvious focal injury. Thus, the liaisons of excessive glucocorticoids/dysfunctional hypothalamic–pituitary–adrenal axis with neuroinflammation, dangerous to the hippocampus, may be crucial to distant hippocampal damage in many brain diseases. Taking into account that the hippocampus controls both the cognitive functions and the emotional state, further research on potential links between glucocorticoid signaling and inflammatory processes in the brain and respective mechanisms is vital.

Time Course of Remote Neuropathology Following Diffuse Traumatic Brain Injury in the Male Rat

Traumatic brain injury (TBI) can affect different regions throughout the brain. Regions near the site of impact are the most vulnerable to injury. However, damage to distal regions occurs. We investigated progressive neuropathology in the dorsal hippocampus (near the impact) and cerebellum (distal to the impact) after diffuse TBI. Adult male rats were subjected to midline fluid percussion injury or sham injury. Brain tissue was stained by the amino cupric silver stain. Neuropathology was quantified in sub-regions of the dorsal hippocampus at 1, 7, and 28 days post-injury (DPI) and coronal cerebellar sections at 1, 2, and 7 DPI. The highest observed neuropathology in the dentate gyrus occurred at 7 DPI which attenuated by 28 DPI, whereas the highest observed neuropathology was at 1 DPI in the CA3 region. There was no significant neuropathology in the CA1 region at any time point. Neuropathology was increased at 7 DPI in the cerebellum compared to shams and stripes of pathology were observed in the molecular layer perpendicular to the cerebellar cortical surface. Together these data show that diffuse TBI can result in neuropathology across the brain. By describing the time course of pathology in response to TBI, it is possible to build the temporal profile of disease progression.

A role for insulin-like growth factor-1 in hippocampal plasticity following traumatic brain injury

Traumatic brain injury (TBI) initiates a constellation of secondary injury cascades, leading to neuronal damage and dysfunction that is often beyond the scope of endogenous repair mechanisms. Cognitive deficits are among the most persistent morbidities resulting from TBI, necessitating a greater understanding of mechanisms of posttraumatic hippocampal damage and neuroplasticity and identification of therapies that improve recovery by enhancing repair pathways. Focusing here on hippocampal neuropathology associated with contusion-type TBIs, the impact of brain trauma on synaptic structure and function and the process of adult neurogenesis is discussed, reviewing initial patterns of damage as well as evidence for spontaneous recovery. A case is made that insulin-like growth factor-1 (IGF-1), a growth-promoting peptide synthesized in both the brain and the periphery, is well suited to augment neuroplasticity in the injured brain. Essential during brain development, multiple lines of evidence delineate roles in the adult brain for IGF-1 in the maintenance of synapses, regulation of neurotransmission, and modulation of forms of synaptic plasticity such as long-term potentiation. Further, IGF-1 enhances adult hippocampal neurogenesis though effects on proliferation and neuronal differentiation of neural progenitor cells and on dendritic growth of newly born neurons. Post-injury administration of IGF-1 has been effective in rodent models of TBI in improving learning and memory, attenuating death of mature hippocampal neurons and promoting neurogenesis, providing critical proof-of-concept data. More studies are needed to explore the effects of IGF-1-based therapies on synaptogenesis and synaptic plasticity following TBI and to optimize strategies in order to stimulate only appropriate, functional neuroplasticity.

Cannabidiol reduces lesion volume and restores vestibulomotor and cognitive function following moderately severe traumatic brain injury

Despite the high incidence of traumatic brain injury (TBI), there is no universal treatment to safely treat patients. Blunt brain injuries destroy primary neural tissue that results in impaired perfusion, excessive release of glutamate, inflammation, excitotoxicity, and progressive secondary neuronal cell death. We hypothesized that administration of cannabidiol (CBD) directly to a brain contusion site, will optimize delivery to the injured tissue which will reduce local neural excitation and inflammation to spare neural tissue and improve neurological outcome following TBI. CBD was infused into a gelfoam matrix forming an implant (CBDi), then applied over the dura at the contusion site as well as delivered systemically by injection (CBD.IP). Post-injury administration of CBDi+IP greatly reduced defecation scores, lesion volume, the loss of neurons in the ipsilateral hippocampus, the number of injured neurons of the contralateral hippocampus, and reversed TBI-induced glial fibrillary acidic protein (GFAP) upregulation which was superior to either CBD.IP or CBDi treatment alone. Vestibulomotor performance on the beam-balance test was restored by 12 days post-TBI and sustained through 28 days. CBDi+IP treated rats exhibited preinjury levels of spontaneous alternation on the spontaneous alternation T-maze. In the object recognition test, they had greater mobility and exploration of novel objects compared to contusion or implant alone consistent with reduced anxiety and restored cognitive function. These results suggest that dual therapy by targeting the site of injury internally with a CBD-infused medical carrier followed by systemic supplementation may offer a more effective countermeasure than systemic or implant treatment alone for the deleterious effects of penetrating head wounds.

Delayed dosing of minocycline plus N-acetylcysteine reduces neurodegeneration in distal brain regions and restores spatial memory after experimental traumatic brain injury

Multiple drugs to treat traumatic brain injury (TBI) have failed clinical trials. Most drugs lose efficacy as the time interval increases between injury and treatment onset. Insufficient therapeutic time window is a major reason underlying failure in clinical trials. Few drugs have been developed with therapeutic time windows sufficiently long enough to treat TBI because little is known about which brain functions can be targeted if therapy is delayed hours to days after injury. We identified multiple injury parameters that are improved by first initiating treatment with the drug combination minocycline (MINO) plus N-acetylcysteine (NAC) at 72 h after injury (MN72) in a mouse closed head injury (CHI) experimental TBI model. CHI produces spatial memory deficits resulting in impaired performance on Barnes maze, hippocampal neuronal loss, and bilateral damage to hippocampal neurons, dendrites, spines and synapses. MN72 treatment restores Barnes maze acquisition and retention, protects against hippocampal neuronal loss, limits damage to dendrites, spines and synapses, and accelerates recovery of microtubule associated protein 2 (MAP2) expression, a key protein in maintaining proper dendritic architecture and synapse density. These data show that in addition to the structural integrity of the dendritic arbor, spine and synapse density can be successfully targeted with drugs first dosed days after injury. Retention of substantial drug efficacy even when first dosed 72 h after injury makes MINO plus NAC a promising candidate to treat clinical TBI.

Neuroinflammation and Neuronal Loss in the Hippocampus Are Associated with Immediate Posttraumatic Seizures and Corticosterone Elevation in Rats

Hippocampal damage after traumatic brain injury (TBI) is associated with late posttraumatic conditions, such as depression, cognitive decline and epilepsy. Mechanisms of selective hippocampal damage after TBI are not well understood. In this study, using rat TBI model (lateral fluid percussion cortical injury), we assessed potential association of immediate posttraumatic seizures and changes in corticosterone (CS) levels with neuroinflammation and neuronal cell loss in the hippocampus. Indices of distant hippocampal damage (neurodegeneration and neuroinflammation) were assessed using histological analysis (Nissl staining, Iba-1 immunohistochemical staining) and ELISA (IL-1β and CS) 1, 3, 7 and 14 days after TBI or sham operation in male Wistar rats (n = 146). IL-1β was elevated only in the ipsilateral hippocampus on day 1 after trauma. CS peak was detected on day 3 in blood, the ipsilateral and contralateral hippocampus. Neuronal cell loss in the hippocampus was demonstrated bilaterally; in the ipsilateral hippocampus it started earlier than in the contralateral. Microglial activation was evident in the hippocampus bilaterally on day 7 after TBI. The duration of immediate seizures correlated with CS elevation, levels of IL-1β and neuronal loss in the hippocampus. The data suggest potential association of immediate post-traumatic seizures with CS-dependent neuroinflammation-mediated distant hippocampal damage.

Diffusion Tensor Imaging Detects Acute Pathology-Specific Changes in the P301L Tauopathy Mouse Model Following Traumatic Brain Injury

Traumatic brain injury (TBI) has been linked with tauopathy. However, imaging methods that can non-invasively detect tau-protein abnormalities following TBI need further investigation. This study aimed to investigate the potential of diffusion tensor imaging (DTI) to detect tauopathy following TBI in P301L mutant-tau-transgenic-pR5-mice. A total of 24 9-month-old pR5 mice were randomly assigned to sham and TBI groups. Controlled cortical injuries/craniotomies were performed for TBI/sham groups followed by DTI data acquisition on days 1 and 7 post-injury. DTI data were analyzed by using voxelwise analysis and track-based spatial statistics for gray matter and white matter. Further, immunohistochemistry was performed for total-tau and phosphorylated-tau, astrocytes, and microglia. To detect the association of DTI with these pathological markers, a correlation analysis was performed between DTI and histology findings. At day 1 post-TBI, DTI revealed a widespread reduction in fractional anisotropy (FA) and axial diffusivity (AxD) in the TBI group compared to shams. On day 7, further reduction in FA, AxD, and mean diffusivity and increased radial diffusivity were observed. FA was significantly increased in the amygdala and cortex. Correlation results showed that in the ipsilateral hemisphere FA reduction was associated with increased phosphorylated-tau and glial-immunoreactivity, whereas in the contralateral regions, the FA increase was associated with increased immunostaining for astrocytes. This study is the first to exploit DTI to investigate the effect of TBI in tau-transgenic mice. We show that alterations in the DTI signal were associated with glial activity following TBI and would most likely reflect changes that co-occur with/without phosphorylated-tau. In addition, FA may be a promising measure to identify discrete pathological processes such as increased astroglia activation, tau-hyperphosphorylation or both in the brain following TBI.

Mild Traumatic Brain Injury Induces Transient, Sequential Increases in Proliferation, Neuroblasts/Immature Neurons, and Cell Survival: A Time Course Study in the Male Mouse Dentate Gyrus

Mild traumatic brain injuries (mTBIs) are prevalent worldwide. mTBIs can impair hippocampal-based functions such as memory and cause network hyperexcitability of the dentate gyrus (DG), a key entry point to hippocampal circuitry. One candidate for mediating mTBI-induced hippocampal cognitive and physiological dysfunction is injury-induced changes in the process of DG neurogenesis. There are conflicting results on how TBI impacts the process of DG neurogenesis; this is not surprising given that both the neurogenesis process and the post-injury period are dynamic, and that the quantification of neurogenesis varies widely in the literature. Even within the minority of TBI studies focusing specifically on mild injuries, there is disagreement about if and how mTBI changes the process of DG neurogenesis. Here we utilized a clinically relevant rodent model of mTBI (lateral fluid percussion injury, LFPI), gold-standard markers and quantification of the neurogenesis process, and three time points post-injury to generate a comprehensive picture of how mTBI affects adult hippocampal DG neurogenesis. Male C57BL/6J mice (6-8 weeks old) received either sham surgery or mTBI via LFPI. Proliferating cells, neuroblasts/immature neurons, and surviving cells were quantified via stereology in DG subregions (subgranular zone [SGZ], outer granule cell layer [oGCL], molecular layer, and hilus) at short-term (3 days post-injury, dpi), intermediate (7 dpi), and long-term (31 dpi) time points. The data show this model of mTBI induces transient, sequential increases in ipsilateral SGZ/GCL proliferating cells, neuroblasts/immature neurons, and surviving cells which is suggestive of mTBI-induced neurogenesis. In contrast to these ipsilateral hemisphere findings, measures in the contralateral hemisphere were not increased in key neurogenic DG subregions after LFPI. Our work in this mTBI model is in line with most literature on other and more severe models of TBI in showing TBI stimulates the process of DG neurogenesis. However, as our DG data in mTBI provide temporal, subregional, and neurogenesis-stage resolution, these data are important to consider in regard to the functional importance of TBI-induction of the neurogenesis process and future work assessing the potential of replacing and/or repairing DG neurons in the brain after TBI.

Differential early effects of traumatic brain injury on spike-wave discharges in Sprague-Dawley rats

Unprovoked seizures in the late period of traumatic brain injury (TBI) occur in almost 20% of humans and experimental animals, psychiatric comorbidities being common in both situations. The aim of the study was to evaluate epileptiform activity in the early period of TBI induced by lateral fluid percussion brain injury in adult male Srague-Dawley rats and to reveal potential behavioral and pathomorphological correlates of early electrophysiological alterations. One week after TBI the group of animals was remarkably heterogeneous regarding the incidence of bifrontal 7-Hz spikes and spike-wave discharges (SWDs). It consisted of 3 typical groups: a) rats with low baseline and high post-craniotomy SWD level; b)with constantly low both baseline and post-craniotomy SWD levels; c) constantly high both baseline and post-craniotomy SWD levels. Rats with augmented SWD occurrence after TBI demonstrated freezing episodes accompanying SWDs as well as increased anxiety-like behavior (difficulty of choosing). The discharges were definitely associated with sleep phases. The incidence of SWDs positively correlated with the area of glial activation in the neocortex but not in the hippocampus.The translational potential of the data is revealing new pathophysiological links between epileptiform activity appearance, direct cortical and distant hippocampal damage and anxiety-like behavior, putative early predictors of late posttraumatic pathology.

Post-stroke inflammatory response is linked to volume loss in the contralateral hemisphere

OBJECTIVES

There is a delicate homeostatic balance between the central nervous system and immune system. Stroke triggers an immunodepressive state to suppress a potential immune reaction directed against neuroglial tissue; however, this supposedly protective response inadvertently results in an infection-prone, and thereby a pro-inflammatory setting. In this study, we assessed the magnitude of cerebral volume loss in the unaffected contralateral hemisphere following stroke, and determined its relationship with inflammatory cascades.

METHODS

The volume of the hemisphere contralateral to the ischemic insult was measured on admission and follow-up MRI's in 50 ischemic stroke patients. Information related to clinical features, infectious complications, and markers of inflammation (erythrocyte sedimentation rate, neutrophil/lymphocyte ratio, C-reactive protein) were prospectively collected, and their relationship with hemispheric volume change was evaluated using bivariate and multivariate statistics.

RESULTS

The contralateral hemisphere volume decreased by a median (interquartile range) of 14 (4-32) mL after a follow-up duration of 101 (63-123) days (p < .001); the volume reduction was 0.8 (0.2-1.8) % per month with respect to baseline. Old age, atrial fibrillation, stroke severity, C-reactive protein level, neutrophil/lymphocyte ratio, and development of infections during hospitalization were significantly associated with volume loss (p < .05). Stroke severity (NIHSS score or infarct volume) and inflammation related parameters (neutrophil/lymphocyte ratio or systemic infections) remained independently and positively associated with volume loss in multivariate regression models.

CONCLUSIONS

Cerebral tissue changes following stroke are not limited to the ischemic hemisphere. Apart from stroke severity, a pro-inflammatory state and post-stroke infections contribute to cerebral volume loss in the non-ischemic hemisphere.

Mechanoporation is a potential indicator of tissue strain and subsequent degeneration following experimental traumatic brain injury

BACKGROUND

An increases in plasma membrane permeability is part of the acute pathology of traumatic brain injury and may be a function of excessive membrane force. This membrane damage, or mechanoporation, allows non-specific flux of ions and other molecules across the plasma membrane, and may ultimately lead to cell death. The relationships among tissue stress and strain, membrane permeability, and subsequent cell degeneration, however, are not fully understood.

METHODS

Fluorescent molecules of different sizes were introduced to the cerebrospinal fluid space prior to injury and animals were sacrificed at either 10 min or 24 h after injury. We compared the spatial distribution of plasma membrane damage following controlled cortical impact in the rat to the stress and strain tissue patterns in a 3-D finite element simulation of the injury parameters.

FINDINGS

Permeable cells were located primarily in the ipsilateral cortex and hippocampus of injured rats at 10 min post-injury; however by 24 h there was also a significant increase in the number of permeable cells. Analysis of colocalization of permeability marker uptake and Fluorojade staining revealed a subset of permeable cells with signs of degeneration at 24 h, but plasma membrane damage was evident in the vast majority of degenerating cells. The regional and subregional distribution patterns of the maximum principal strain and shear stress estimated by the finite element model were comparable to the cell membrane damage profiles following a compressive impact.

INTERPRETATION

These results indicate that acute membrane permeability is prominent following traumatic brain injury in areas that experience high shear or tensile stress and strain due to differential mechanical properties of the cell and tissue organization, and that this mechanoporation may play a role in the initiation of secondary injury, contributing to cell death.

[Early electrophysiological consequences of dosed traumatic-brain injury in rats]

AIM

To analyze the pathological electrical activity during the acute period after traumatic brain injury (TBI) and to search for potential morphological correlates of this activity in the neocortex and hippocampus.

MATERIAL AND METHODS

The study was performed on male Sprague Dawley rats. TBI was modeled using a lateral hydrodynamic impact in the sensorimotor cortex area. ECoG was continuously recorded one week before and one week after TBI. A histological analysis was performed one week after TBI. Brain slices were Nissl stained as well as immunohistochemically stained for astrocytes (GFAP) and microglia (Isolectin B4). The damage to the neocortex and hippocampus was evaluated.

RESULTS AND CONCLUSION

The slowdown of the background activity one and six hours after TBI and appearance of epileptiform activity in a half of animals one week after TBI were shown. The number of discharges was correlated with the area of astrocyte gliosis in the neocortex and with the number of dark (ischemic-like) neurons in the hippocampus. Microglial activation did not correlate with the epileptiform activity. These data are important to understanding early mechanisms of post-trauma epileptogenesis.

Impact of Traumatic Brain Injury on Neurogenesis

New neurons are generated in the hippocampal dentate gyrus from early development through adulthood. Progenitor cells and immature granule cells in the subgranular zone are responsive to changes in their environment; and indeed, a large body of research indicates that neuronal interactions and the dentate gyrus milieu regulates granule cell proliferation, maturation, and integration. Following traumatic brain injury (TBI), these interactions are dramatically altered. In addition to cell losses from injury and neurotransmitter dysfunction, patients often show electroencephalographic evidence of cortical spreading depolarizations and seizure activity after TBI. Furthermore, treatment for TBI often involves interventions that alter hippocampal function such as sedative medications, neuromodulating agents, and anti-epileptic drugs. Here, we review hippocampal changes after TBI and how they impact the coordinated process of granule cell adult neurogenesis. We also discuss clinical TBI treatments that have the potential to alter neurogenesis. A thorough understanding of the impact that TBI has on neurogenesis will ultimately be needed to begin to design novel therapeutics to promote recovery.

Diffusion Magnetic Resonance Imaging Unveils the Spatiotemporal Microstructural Gray Matter Changes following Injury in the Rodent Brain

Traumatic brain injury (TBI) is associated with gray and white matter alterations in brain tissue. Gray matter alterations are not yet as well studied as those of the white matter counterpart. This work utilized T2-weighted structural imaging, diffusion tensor imaging (DTI), and diffusion kurtosis imaging to unveil the gray matter changes induced in a controlled cortical impact (CCI) mouse model of TBI at 5 h, 1 day, 3 days, 7 days, 14 days, and 30 days post-CCI. A cross-sectional histopathology approach was used to confer validity of the magnetic resonance imaging (MRI) data by performing cresyl violet staining and glial fibrillary acidic protein (GFAP) immunohistochemistry. The results demonstrated a significant increase in lesion volume up to 3 days post-injury followed by a significant decrease in the cavity volume for the period of 1 month. GFAP signals peaked on Day 7 and persisted until Day 30 in both ipsilateral and contralateral hippocampus, ipsilateral cortex, and thalamic areas. An increase in fractional anisotropy (FA) was seen at Day 7 in the pericontusional area but decreased FA in the contralateral cortex, hippocampus, and thalamus. Mean diffusivity (MD) was significantly lower in the pericontusional cortex. Increased MD and decreased mean kurtosis were limited to the injury site on Days 7 to 30 and to the contralateral hippocampus and thalamus on Days 3 and 7. This work is one of the few cross-sectional studies to demonstrate a link between MRI measures and histopathological readings to track gray matter changes in the progression of TBI.

Blocking Tumor Necrosis Factor-Alpha Expression Prevents Blast-Induced Excitatory/Inhibitory Synaptic Imbalance and Parvalbumin-Positive Interneuron Loss in the Hippocampus

Traumatic brain injury (TBI) is a major cause of neurological disorder and death in civilian and military populations. It comprises two components-direct injury from the traumatic impact and secondary injury from ensuing neural inflammatory responses. Blocking tumor necrosis factor-alpha (TNF-α), a central regulator of neural inflammation, has been shown to improve functional recovery after TBI. However, the mechanisms underlying those therapeutic effects are still poorly understood. Here, we examined effects of 3,6'-dithiothalidomide (dTT), a potentially therapeutic TNF-α inhibitor, in mice with blast-induced TBI. We found that blast exposure resulted in elevated expression of TNF-α, activation of microglial cells, enhanced excitatory synaptic transmission, reduced inhibitory synaptic transmission, and a loss of parvalbumin-positive (PV+) inhibitory interneurons. Administration of dTT for 5 days after the blast exposure completely suppressed blast-induced increases in TNF-α transcription, largely reversed blasted-induced synaptic changes, and prevented PV+ neuron loss. However, blocking TNF-α expression by dTT failed to mitigate blast-induced microglial activation in the hippocampus, as evidenced by their non-ramified morphology. These results indicate that TNF-α plays a major role in modulating neuronal functions in blast-induced TBI and that it is a potential target for treatment of TBI-related brain disorders.

Diminished Dentate Gyrus Filtering of Cortical Input Leads to Enhanced Area Ca3 Excitability after Mild Traumatic Brain Injury

Mild traumatic brain injury (mTBI) disrupts hippocampal function and can lead to long-lasting episodic memory impairments. The encoding of episodic memories relies on spatial information processing within the hippocampus. As the primary entry point for spatial information into the hippocampus, the dentate gyrus is thought to function as a physiological gate, or filter, of afferent excitation before reaching downstream area Cornu Ammonis (CA3). Although injury has previously been shown to alter dentate gyrus network excitability, it is unknown whether mTBI affects dentate gyrus output to area CA3. In this study, we assessed hippocampal function, specifically the interaction between the dentate gyrus and CA3, using behavioral and electrophysiological techniques in ex vivo brain slices 1 week following mild lateral fluid percussion injury (LFPI). Behaviorally, LFPI mice were found to be impaired in an object-place recognition task, indicating that spatial information processing in the hippocampus is disrupted. Extracellular recordings and voltage-sensitive dye imaging demonstrated that perforant path activation leads to the aberrant spread of excitation from the dentate gyrus into area CA3 along the mossy fiber pathway. These results suggest that after mTBI, the dentate gyrus has a diminished capacity to regulate cortical input into the hippocampus, leading to increased CA3 network excitability. The loss of the dentate filtering efficacy reveals a potential mechanism by which hippocampal-dependent spatial information processing is disrupted, and may contribute to memory dysfunction after mTBI.

Physical training decreases susceptibility to pilocarpine-induced seizures in the injured rat brain

There is growing evidence that physical activity ameliorates the course of epilepsy in animal models as well as in clinical conditions. Since traumatic brain injury is one of the strongest determinants of epileptogenesis, the present study focuses on the question whether a moderate long-term physical training can decrease susceptibility to seizures evoked following brain damage. Wistar rats received a mechanical brain injury and were subjected to daily running sessions on a treadmill for 21 days. Thereafter, seizures were induced by pilocarpine injections in trained and non-trained, control groups. During the acute period of status epilepticus, the intensity of seizures was assessed within the six-hour observation period. The trained rats showed considerable amelioration of pilocarpine-induced motor symptoms when compared with their non-trained counterparts. Histological investigations of effects of the brain injury and of physical training detected significant quantitative changes in parvalbumin-, calretinin- and NPY-immunopositive neuronal populations. Some of the injury-induced changes, especially those shoved by parvalbumin-immunopositive neurons, were abolished by the subsequent physical training procedure and could, therefore, be considered as neuronal correlates of the observed functional amelioration of the injured brain.

Diminished amygdala activation and behavioral threat response following traumatic brain injury

Each year, approximately 3.8 million people suffer mild to moderate traumatic brain injuries (mTBI) that result in an array of neuropsychological symptoms and disorders. Despite these alarming statistics, the neurological bases of these persistent, debilitating neuropsychological symptoms are currently poorly understood. In this study we examined the effects of mTBI on the amygdala, a brain structure known to be critically involved in the processing of emotional stimuli. Seven days after lateral fluid percussion injury (LFPI), mice underwent a series of physiological and behavioral experiments to assess amygdala function. Brain-injured mice exhibited a decreased threat response in a cued fear conditioning paradigm, congruent with a decrease in amygdala excitability determined with basolateral amygdala (BLA) field excitatory post-synaptic potentials together with voltage-sensitive dye imaging (VSD). Furthermore, beyond exposing a general decrease in the excitability of the primary input of the amygdala, the lateral amygdala (LA), VSD also revealed a decrease in the relative strength or activation of internuclear amygdala circuit projections after LFPI. Thus, not only does activation of the LA require increased stimulation, but the proportion of this activation that is propagated to the primary output of the amygdala, the central amygdala, is also diminished following LFPI. Intracellular recordings revealed no changes in the intrinsic properties of BLA pyramidal neurons after LFPI. This data suggests that mild to moderate TBI has prominent effects on amygdala function and provides a potential neurological substrate for many of the neuropsychological symptoms suffered by TBI patients.

Isolated Primary Blast Inhibits Long-Term Potentiation in Organotypic Hippocampal Slice Cultures

Over the last 13 years, traumatic brain injury (TBI) has affected over 230,000 U.S. service members through the conflicts in Iraq and Afghanistan, mostly as a result of exposure to blast events. Blast-induced TBI (bTBI) is multi-phasic, with the penetrating and inertia-driven phases having been extensively studied. The effects of primary blast injury, caused by the shockwave interacting with the brain, remain unclear. Earlier in vivo studies in mice and rats have reported mixed results for primary blast effects on behavior and memory. Using a previously developed shock tube and in vitro sample receiver, we investigated the effect of isolated primary blast on the electrophysiological function of rat organotypic hippocampal slice cultures (OHSC). We found that pure primary blast exposure inhibited long-term potentiation (LTP), the electrophysiological correlate of memory, with a threshold between 9 and 39 kPa·ms impulse. This deficit occurred well below a previously identified threshold for cell death (184 kPa·ms), supporting our previously published finding that primary blast can cause changes in brain function in the absence of cell death. Other functional measures such as spontaneous activity, network synchronization, stimulus-response curves, and paired-pulse ratios (PPRs) were less affected by primary blast exposure, as compared with LTP. This is the first study to identify a tissue-level tolerance threshold for electrophysiological changes in neuronal function to isolated primary blast.

Brain Injury Impairs Working Memory and Prefrontal Circuit Function

More than 2.5 million Americans suffer a traumatic brain injury (TBI) each year. Even mild to moderate TBI causes long-lasting neurological effects. Despite its prevalence, no therapy currently exists to treat the underlying cause of cognitive impairment suffered by TBI patients. Following lateral fluid percussion injury (LFPI), the most widely used experimental model of TBI, we investigated alterations in working memory and excitatory/inhibitory synaptic balance in the prefrontal cortex. LFPI impaired working memory as assessed with a T-maze behavioral task. Field excitatory postsynaptic potentials recorded in the prefrontal cortex were reduced in slices derived from brain-injured mice. Spontaneous and miniature excitatory postsynaptic currents onto layer 2/3 neurons were more frequent in slices derived from LFPI mice, while inhibitory currents onto layer 2/3 neurons were smaller after LFPI. Additionally, an increase in action potential threshold and concomitant decrease in firing rate was observed in layer 2/3 neurons in slices from injured animals. Conversely, no differences in excitatory or inhibitory synaptic transmission onto layer 5 neurons were observed; however, layer 5 neurons demonstrated a decrease in input resistance and action potential duration after LFPI. These results demonstrate synaptic and intrinsic alterations in prefrontal circuitry that may underlie working memory impairment caused by TBI.

Efficacy, dosage, and duration of action of branched chain amino Acid therapy for traumatic brain injury

Traumatic brain injury (TBI) results in long-lasting cognitive impairments for which there is currently no accepted treatment. A well-established mouse model of mild to moderate TBI, lateral fluid percussion injury (FPI), shows changes in network excitability in the hippocampus including a decrease in net synaptic efficacy in area CA1 and an increase in net synaptic efficacy in dentate gyrus. Previous studies identified a novel therapy consisting of branched chain amino acids (BCAAs), which restored normal mouse hippocampal responses and ameliorated cognitive impairment following FPI. However, the optimal BCAA dose and length of treatment needed to improve cognitive recovery is unknown. In the current study, mice underwent FPI then consumed 100 mM BCAA supplemented water ad libitum for 2, 3, 4, 5, and 10 days. BCAA therapy ameliorated cognitive impairment at 5 and 10 days duration. Neither BCAA supplementation at 50 mM nor BCAAs when dosed 5 days on then 5 days off was sufficient to ameliorate cognitive impairment. These results suggest that brain injury causes alterations in hippocampal function, which underlie and contribute to hippocampal cognitive impairment, which are reversible with at least 5 days of BCAA treatment, and that sustaining this effect is dependent on continuous treatment. Our findings have profound implications for the clinical investigation of TBI therapy.

Cannabinoid agonist rescues learning and memory after a traumatic brain injury

Traumatic brain injury can cause persistent challenges including problems with learning and memory. Previous studies suggest that the activation of the cannabinoid 1 receptor after a traumatic brain injury could be beneficial. We tested the hypothesis that posttraumatic brain injury administration of a cannabinoid 1 receptor agonist can rescue deficits in learning and memory. Young adult male rats were subjected to a moderately severe controlled cortical impact brain injury, with a subset given postinjury i.p. injections of a cannabinoid receptor agonist. Utilizing novel object recognition and the morris water task, we found that the brain-injured animals treated with the agonist showed a marked recovery.

Electrophysiologic recordings in traumatic brain injury

Following a traumatic brain injury (TBI), the brain undergoes numerous electrophysiologic changes. The most common techniques used to evaluate these changes include electroencepalography (EEG) and evoked potentials. In animals, EEGs immediately following TBI can show either diffuse slowing or voltage attenuation, or high voltage spiking. Following a TBI, many animals display evidence of hippocampal excitability and a reduced seizure threshold. Some mice subjected to severe TBI via a fluid percussion injury will eventually develop seizures, which provides a useful potential model for studying the neurophysiology of epileptogenesis. In humans, the EEG changes associated with mild TBI are relatively subtle and may be challenging to distinguish from EEG changes seen in other conditions. Quantitative EEG (QEEG) may enhance the ability to detect post-traumatic electrophysiologic changes following a mild TBI. Some types of evoked potential (EP) and event related potential (ERP) can also be used to detect post-traumatic changes following a mild TBI. Continuous EEG monitoring (cEEG) following moderate and severe TBI is useful in detecting the presence of seizures and status epilepticus acutely following an injury, although some seizures may only be detectable using intracranial monitoring. CEEG can also be helpful for assessing prognosis after moderate or severe TBI. EPs, particularly somatosensory evoked potentials, can also be useful in assessing prognosis following severe TBI. The role for newer technologies such as magnetoencephalography and bispectral analysis (BIS) in the evaluation of patients with TBI remains unclear.

Decoding hippocampal signaling deficits after traumatic brain injury

There are more than 3.17 million people coping with long-term disabilities due to traumatic brain injury (TBI) in the United States. The majority of TBI research is focused on developing acute neuroprotective treatments to prevent or minimize these long-term disabilities. Therefore, chronic TBI survivors represent a large, underserved population that could significantly benefit from a therapy that capitalizes on the endogenous recovery mechanisms occurring during the weeks to months following brain trauma. Previous studies have found that the hippocampus is highly vulnerable to brain injury, in both experimental models of TBI and during human TBI. Although often not directly mechanically injured by the head injury, in the weeks to months following TBI, the hippocampus undergoes atrophy and exhibits deficits in long-term potentiation (LTP), a persistent increase in synaptic strength that is considered to be a model of learning and memory. Decoding the chronic hippocampal LTP and cell signaling deficits after brain trauma will provide new insights into the molecular mechanisms of hippocampal-dependent learning impairments caused by TBI and facilitate the development of effective therapeutic strategies to improve hippocampal-dependent learning for chronic survivors of TBI.

Effect of normabaric hyperoxia treatment on neuronal damage following fluid percussion injury in the striatum of mice: a morphological approach

Traumatic brain injury (TBI) causes significant mortality in most developing countries worldwide. At present, it is imperative to identify a treatment to address the devastating post-TBI consequences. Therefore, the present study has been performed to assess the specific effect of immediate exposure to normabaric hyperoxia (NBO) after fluid percussion injury (FPI) in the striatum of mice. To execute FPI, mice were anesthetised and sorted into (i) a TBI group, (ii) a sham group without injury and (iii) a TBI group treated with immediate exposure to NBO for 3 h. Afterwards, brains were harvested for morphological assessment. The results revealed no changes in morphological and neuronal damage in the sham group as compared to the TBI group. Conversely, the TBI group showed severe morphological changes as well as neuronal damage as compared to the TBI group exposed to NBO for 3 h. Interestingly, our findings also suggested that NBO treatment could diminish the neuronal damage in the striatum of mice after FPI. Neuronal damage was evaluated at different points of injury and the neighbouring areas using morphology, neuronal apoptotic cell death and pan-neuronal markers to determine the complete neuronal structure. In conclusion, immediate exposure to NBO following FPI could be a potential therapeutic approach to reduce neuronal damage in the TBI model.

Combination of temperature-sensitive stem cells and mild hypothermia: a new potential therapy for severe traumatic brain injury

Stem cell transplantation holds great potential for the treatment of traumatic brain injury (TBI). However, the micro-environment of reduced oxygen and accumulated toxins leads to low survival rates of grafted cells, which dramatically limits their clinical application. Mild hypothermia has been demonstrated to improve the micro-environment after severe TBI. Thus, we speculate that combinational therapy of mild hypothermia may promote survival of grafted cells, especially temperature-sensitive stem cells, which show the most activity in mild temperatures. In this study, we first isolated mesenchymal stem cells from umbilical cord (UCSMCs) and generated the temperature-sensitive UCSMCs (tsUCSMCs) by infection with a retrovirus carrying the temperature-sensitive tsA58 SV40 LT antigen gene. We demonstrated that tsUCSMCs grew and proliferated with more activity at 33°C than at 37°C by counting cell numbers with a hematocytometer, measuring the cell cycle with flow cytometry, and detecting proliferating cell nuclear antigen (PCNA) with immunofluorescence staining. Thereafter, we established the rat severe TBI model by fluid percussion, and injected PBS, UCSMCs, or tsUCSMCs into the injured region, and subject the animals to normothermia or mild hypothermia (33°C). We found that, compared with UCSMC or tsUCSMC treatment alone, their combination with hypothermia could significantly improve motor and cognitive function with more survival of the grafted cells. Furthermore, we observed that combined therapy with hypothermia and tsUCSMCs exerted the most protective effect on the recovery of neurological function of all the tested treatments, with the highest survival and proliferation rates, and the lowest apoptosis rate. Thus this may represent a new therapeutic strategy for the treatment of severe TBI.

Differential effects of voluntary and forced exercise on stress responses after traumatic brain injury

Voluntary exercise increases levels of brain-derived neurotrophic factor (BDNF) after traumatic brain injury (TBI) when it occurs during a delayed time window. In contrast, acute post-TBI exercise does not increase BDNF. It is well known that increases in glucocorticoids suppress levels of BDNF. Moreover, recent work from our laboratory showed that there is a heightened stress response after fluid percussion injury (FPI). In order to determine if a heightened stress response is also observed with acute exercise, at post-injury days 0-4 and 7-11, corticosterone (CORT) and adrenocorticotropic hormone (ACTH) release were measured in rats running voluntarily or exposed to two daily 20-min periods of forced running wheel exercise. Forced, but not voluntary exercise, continuously elevated CORT. ACTH levels were initially elevated with forced exercise, but decreased by post-injury day 7 in the control, but not the FPI animals. As previously reported, voluntary exercise did not increase BDNF in the FPI group as it did in the control animals. Forced exercise did not increase levels of BDNF in any group. It did, however, decrease hippocampal glucocorticoid receptors in the control group. The results suggest that exercise regimens with strong stress responses may not be beneficial during the early post-injury period.

Development of post-traumatic epilepsy after controlled cortical impact and lateral fluid-percussion-induced brain injury in the mouse

The present study investigated the development of hyperexcitability and epilepsy in mice with traumatic brain injury (TBI) induced by controlled cortical impact (CCI) or lateral fluid-percussion injury (FPI), which are the two most commonly used experimental models of human TBI in rodents. TBI was induced with CCI to 50 (14 controls) and with lateral FPI to 45 (15 controls) C57BL/6S adult male mice. The animals were followed-up for 9 months, including three 2-week periods of continuous video-electroencephalographic (EEG) monitoring, and a seizure susceptibility test with pentylenetetrazol (PTZ). In the end, the animals were perfusion-fixed for histology. The experiment included two independent cohorts of animals. Late post-traumatic spontaneous electrographic seizures were detected in 9% of mice after CCI and 3% after lateral FPI. Eighty-two percent of mice after CCI and 71% after lateral FPI had spontaneous epileptiform spiking on EEG. In addition, 58% of mice with lateral FPI showed spontaneous epileptiform discharges. A PTZ test demonstrated increased seizure susceptibility in the majority of mice in both models, compared to control mice. There was no further progression in the occurrence of epilepsy or epileptiform spiking when follow-up was extended from 6 to 9 months. The severity of cortical or hippocampal damage did not differentiate mice with or without epileptiform activity in either model. Finally, two independent series of experiments in both injury models provided comparable data demonstrating reproducibility of the modeling. These data show that different types of impact can trigger epileptogenesis in mice. Even though the frequency of spontaneous seizures in C57BL/6S mice is low, a large majority of animals develop hyperexcitability.

Neurodegeneration in the somatosensory cortex after experimental diffuse brain injury

Disruption and consequent reorganization of central nervous system circuits following traumatic brain injury may manifest as functional deficits and behavioral morbidities. We previously reported axotomy and neuronal atrophy in the ventral basal (VB) complex of the thalamus, without gross degeneration after experimental diffuse brain injury in adult rats. Pathology in VB coincided with the development of late-onset aberrant behavioral responses to whisker stimulation, which lead to the current hypothesis that neurodegeneration after experimental diffuse brain injury includes the primary somatosensory barrel cortex (S1BF), which receives projection of VB neurons and mediates whisker somatosensation. Over 28 days after midline fluid percussion brain injury, argyrophilic reaction product within superficial layers and layer IV barrels at 1 day progresses into the cortex to subcortical white matter by 7 days, and selective inter-barrel septa and subcortical white matter labeling at 28 days. Cellular consequences were determined by stereological estimates of neuronal nuclear volumes and number. In all cortical layers, neuronal nuclear volumes significantly atrophied by 42-49% at 7 days compared to sham, which marginally attenuated by 28 days. Concomitantly, the number of healthy neurons was reduced by 34-45% at 7 days compared to sham, returning to control levels by 28 days. Progressive neurodegeneration, including argyrophilic reaction product and neuronal nuclear atrophy, indicates injury-induced damage and reorganization of the reciprocal thalamocortical projections that mediate whisker somatosensation. The rodent whisker barrel circuit may serve as a discrete model to evaluate the causes and consequences of circuit reorganization after diffuse brain injury.

The pathophysiology of concussions in youth

Mild traumatic brain injury, especially sport-related concussion, is common among young persons. Consequences of transient pathophysiologic dysfunction must be considered in the context of a developing or immature brain, as must the potential for an accumulation of damage with repeated exposure. This review summarizes the underlying neurometabolic cascade of concussion, with emphasis on the young brain in terms of acute pathophysiology, vulnerability, alterations in plasticity and activation, axonal injury, and cumulative risk from chronic, repetitive damage, and discusses their implications in the context of clinical care for the concussed youth, highlighting areas for future investigation.

Hippocampal θ dysfunction after lateral fluid percussion injury

Chronic memory deficits are a major cause of morbidity following traumatic brain injury (TBI). In the rat, the hippocampal theta rhythm is a well-studied correlate of memory function. This study sought to investigate disturbances in hippocampal theta rhythm following lateral fluid percussion injury in the rat. A total of 13 control rats and 12 TBI rats were used. Electrodes were implanted in bilateral hippocampi and an electroencephalogram (EEG) was recorded while the rats explored a new environment, and also while navigating a modified version of the Barnes maze. Theta power and peak theta frequency were significantly attenuated in the injured animals. Further, injured rats were less likely to develop a spatial strategy for Barnes maze navigation compared to control rats. In conclusion, rats sustaining lateral fluid percussion injury demonstrated deficits in hippocampal theta activity. These deficits may contribute to the underlying memory problems seen in chronic TBI.

Imipramine treatment improves cognitive outcome associated with enhanced hippocampal neurogenesis after traumatic brain injury in mice

Previous animal and human studies have demonstrated that chronic treatment with several different antidepressants can stimulate neurogenesis, neural remodeling, and synaptic plasticity in the normal hippocampus. Imipramine is a commonly used tricyclic antidepressant (TCA). We employed a controlled cortical impact (CCI) mouse model of traumatic brain injury (TBI) to assess the effect of imipramine on neurogenesis and cognitive and motor function recovery after TBI. Mice were given daily imipramine injections for either 2 or 4 weeks after injury. Bromodeoxyuridine (BrdU) was administered 3-7 days post-brain injury to label the cells that proliferated as a result of the injury. We assessed the effects of imipramine on post-traumatic motor function using a beam-walk test and an assessment of cognitive function: the novel object recognition test (NOR). Histological analyses were performed at 2 and 4 weeks after CCI. Brain-injured mice treated with imipramine showed significantly improved cognitive function compared to a saline-treated group (p<0.001). However, there was no significant difference in motor function recovery between imipramine-treated and saline-treated mice. Histological examination revealed increased preservation of proliferation of Ki-67- and BrdU-positive cells in the hippocampal dentate gyrus (DG) at 2 and 4 weeks after TBI. Immunofluorescence double-labeling with BrdU and neuron-specific markers at 4 weeks after injury showed that most progenitors became neurons in the DG and astrocytes in the hilus. Notably, treatment with imipramine increased preservation of the total number of newly-generated neurons. Our findings provide direct evidence that imipramine treatment contributes to cognitive improvement after TBI, perhaps by enhanced hippocampal neurogenesis.

Strain-related differences after experimental traumatic brain injury in rats

The present study directly compares the effects of experimental brain injury in two commonly used rat strains: Fisher 344 and Sprague-Dawley. We previously found that Fisher rats have a higher mortality rate and more frequent seizure attacks at the same injury level than Sprague-Dawley rats. Although strain differences in rats are commonly accepted as contributing to variability among studies, there is a paucity of literature addressing strain influence in experimental neurotrauma. Therefore this study compares outcome measures in two rat strains following lateral fluid percussion injury. Fisher 344 and Sprague-Dawley rats were monitored for changes in physiological measurements, intracranial pressure, and electroencephalographic activity. We further analyzed neuronal degeneration and cell death in the injured brain using Fluoro-Jade-B (FJB) histochemistry and caspase-3 immunostaining. Behavioral studies using the beam walk and Morris water maze were conducted to characterize strain differences in both motor and cognitive functional recovery following injury. We found that Fisher rats had significantly higher intracranial pressure, prolonged seizure activity, increased FJB-positive staining in the injured cortex and thalamus, and increased caspase-3 expression than Sprague-Dawley rats. On average, Fisher rats displayed a greater amount of total recording time in seizure activity and had longer ictal durations. The Fisher rats also had increased motor deficits, correlating with the above results. In spite of these results, Fisher rats performed better on cognitive tests following injury. The results demonstrate that different rat strains respond to injury differently, and thus in preclinical neurotrauma studies strain influence is an important consideration when evaluating outcomes.

Heightening of the stress response during the first weeks after a mild traumatic brain injury

The effects of a mild traumatic brain injury range from white matter disruption to affective disorders. We set out to determine the response to restraint-induced stress after a mild fluid-percussion injury (FPI), an experimental model for brain injury. Hypothalamic-pituitary-adrenal (HPA) axis regulation of corticosterone (CORT) and adrenocorticotropic hormone (ACTH) was determined during the first post-injury weeks, which corresponds to the same time period when rehabilitative exercise has been shown to be ineffective after a mild FPI. Adult male rats underwent either an FPI or sham injury. Additional rats were only exposed to anesthesia. HPA regulation was evaluated by measuring the effects of dexamethasone (DEX) treatment on CORT and ACTH. Tail vein blood was collected following 30-min restraint stress, at post-injury days (PID) 1, 7 and 14, prior to (0 min) and at 30, 60, 90 and 120 min after stress onset. Results from these studies indicate that the stress response was significantly more pronounced after FPI in that CORT and ACTH restraint-induced increases were more pronounced and longer lasting compared to controls. DEX suppression of CORT and ACTH was observed in all groups, suggesting that stress hyper-responsiveness after mild FPI is not attributable to reduced sensitivity of CORT feedback regulation. The increased sensitivity to stressful events in the first two post-injury weeks after a mild FPI may have a negative impact on early rehabilitative therapies.

Conditional knockout of brain-derived neurotrophic factor in the hippocampus increases death of adult-born immature neurons following traumatic brain injury

It has been reported that the hippocampus is particularly vulnerable to traumatic brain injury (TBI), the consequence of which results in hippocampal-dependent cognitive impairment. In the previous study we found that adult-born immature neurons in the hippocampal dentate gyrus are the most vulnerable cell type to moderate TBI insult. However, the molecular mechanisms that regulate the survival of adult-born immature neurons in the hippocampus following TBI are still not well understood. Here, we conditionally knocked out brain-derived neurotrophic factor (BDNF) in the hippocampal dentate gyrus and examined the death of adult-born immature neurons following moderate TBI. The results showed that the amount of adult-born immature neuron death in the hippocampal dentate gyrus significantly increased in the BDNF conditional knockout mice. This result suggests that BDNF is involved in regulating the survival of adult-born immature neurons in the hippocampus following TBI, and potentially might be a useful target for preventing the adult-born immature neurons from death following TBI.

Mild fluid percussion injury in mice produces evolving selective axonal pathology and cognitive deficits relevant to human brain injury

Mild traumatic brain injury (TBI) accounts for up to 80% of clinical TBI and can result in cognitive impairment and white matter damage that may develop and persist over several years. Clinically relevant models of mild TBI for investigation of neurobiological changes and the development of therapeutic strategies are poorly developed. In this study we investigated the temporal profile of axonal and somal injury that may contribute to cognitive impairments in a mouse model of mild TBI. Neuronal perikaryal damage (hematoxylin and eosin and Fluoro-Jade C), myelin integrity (myelin basic protein and myelin-associated glycoprotein), and axonal damage (amyloid precursor protein), were evaluated by immunohistochemistry at 4 h, 24 h, 72 h, 4 weeks, and 6 weeks after mild lateral fluid percussion brain injury (0.9 atm; righting time 167 +/- 15 sec). At 3 weeks post-injury spatial reference learning and memory were tested in the Morris water maze (MWM). Levels of damage to neuronal cell bodies were comparable in the brain-injured and sham groups. Myelin integrity was minimally altered following injury. Clear alterations in axonal damage were observed at various time points after injury. Axonal damage was localized to the cingulum at 4 h post-injury. At 4 and 6 weeks post-injury, axonal damage was evident in the external capsule, and was seen at 6 weeks in the dorsal thalamic nuclei. At 3 weeks post-injury, injured mice showed an impaired ability to learn the water maze task, suggesting injury-induced alterations in search strategy learning. The evolving localization of axonal damage points to ongoing degeneration after injury that is concomitant with a deficit in learning.

The effect of epidermal growth factor in the injured brain after trauma in rats

Epidermal growth factor (EGF) is a known mitogen for neural stem and progenitor cells (NS/NPCs) in the central nervous system (CNS). In vitro, EGF maintains NS/NPCs in the proliferative state, whereas in the normal rodent brain it promotes their proliferation and migration in the subventricular zone (SVZ). Additionally, EGF administration can augment neuronal replacement in the ischemic-injured adult striatum. Recently we found that the SVZ and the hippocampus display an injury-induced proliferative response following traumatic brain injury (TBI) that is linked to increased EGF expression. As adult neurogenesis is associated with cognitive function, we hypothesized that post-TBI administration of EGF could affect neurogenesis and cognitive recovery. Adult rats were intraventricularly infused with EGF or vehicle for 7 days following TBI. 5-Bromo-2-deoxyuridine (BrdU) was administered to label proliferating cells and the animals were sacrificed at 1 or 4 weeks post-injury. Using immunohistochemistry and stereology, we found that at 1 week post-injury, compared to vehicle-infused animals EGF-infused animals had significantly more BrdU-positive cells in the SVZ and hippocampus concomitant with enhanced EGF receptor expression. At 4 weeks post-injury, the number of BrdU-positive cells in the hippocampus was similar in both groups, suggesting that EGF does not support long-term survival of newly generated cells. Furthermore, we found that the EGF-induced proliferative population differentiated preferentially toward astroglial phenotype. Nevertheless, animals treated with EGF showed significant improvement in cognitive function, which was accompanied by reduced hippocampal neuronal cell loss. Collectively, the data from this study demonstrate that EGF exerts a neuroprotective rather than neurogenic effect in protecting the brain from injury.

Recovery of afferent function and synaptic strength in hippocampal CA1 following traumatic brain injury

Cortical contusion injury can result in the partial loss of ipsilateral CA3 neurons within 48 h, leading to a proportional reduction in the number of afferent fibers to CA1 stratum radiatum. While the loss of afferent input to CA1 exhibits a remarkable, albeit incomplete, recovery over the next few weeks, little is known about the functional status of presynaptic afferents during the depletion and recovery phases following injury. Here, we prepared hippocampal slices from adult Sprague Dawley rats at 2, 7, and 14 days after lateral cortical contusion injury and measured fiber volley (FV) amplitudes extracellularly in CA1 stratum radiatum. Field excitatory post-synaptic potentials (EPSPs) were also measured and plotted as a function of FV amplitude to assess relative synaptic strength of residual and/or regenerated synaptic contacts. At 2 days post-injury, FV amplitude and synaptic strength were markedly reduced in the ipsilateral, relative to the contralateral, hippocampus. FV amplitude in ipsilateral CA1 showed a complete recovery by 7 days, indicative of a post-injury sprouting response. Synaptic strength in ipsilateral CA1 also showed a dramatic recovery over this time; however, EPSP-to-FV curves remained slightly suppressed at both the 7 and 14 day time points. Despite these deficits, ipsilateral slices retained the capacity to express long-term potentiation, indicating that at least some mechanisms for synaptic plasticity remain intact, or are compensated for. These results are in agreement with anatomical evidence showing a profound deafferentation, followed by a remarkable re-enervation, of ipsilateral CA1 in the first few weeks after traumatic brain injury. Although plasticity mechanisms appear to remain intact, synaptic strength deficits in CA1 could limit information throughput in the hippocampus, leading to persistent memory dysfunction.

Increase of GABAA receptor-mediated tonic inhibition in dentate granule cells after traumatic brain injury

Traumatic brain injury (TBI) can result in altered inhibitory neurotransmission, hippocampal dysfunction, and cognitive impairments. GABAergic spontaneous and miniature inhibitory postsynaptic currents (sIPSCs and mIPSCs) and tonic (extrasynaptic) whole cell currents were recorded in control rat hippocampal dentate granule cells (DGCs) and at 90days after controlled cortical impact (CCI). At 34 degrees C, in CCI DGCs, sIPSC frequency and amplitude were unchanged, whereas mIPSC frequency was decreased (3.10+/-0.84Hz, n=16, and 2.44+/-0.67Hz, n=7, p<0.05). At 23 degrees C, 300nM diazepam increased peak amplitude of mIPSCs in control and CCI DGCs, but the increase was 20% higher in control (26.81+/-2.2pA and 42.60+/-1.22pA, n=9, p=0.031) compared to CCI DGCs (33.46+/-2.98pA and 46.13+/-1.09pA, n=10, p=0.047). At 34 degrees C, diazepam did not prolong decay time constants (6.59+/-0.12ms and 6.62+/-0.98ms, n=9, p=0.12), the latter suggesting that CCI resulted in benzodiazepine-insensitive pharmacology in synaptic GABA(A) receptors (GABA(A)Rs). In CCI DGCs, peak amplitude of mIPSCs was inhibited by 100microM furosemide (51.30+/-0.80pA at baseline and 43.50+/-5.30pA after furosemide, n=5, p<0.001), a noncompetitive antagonist of GABA(A)Rs with an enhanced affinity to alpha4 subunit-containing receptors. Potentiation of tonic current by the GABA(A)R delta subunit-preferring competitive agonist THIP (1 and 3microM) was increased in CCI DGCs (47% and 198%) compared to control DGCs (13% and 162%), suggesting the presence of larger tonic current in CCI DGCs; THIP (1microM) had no effect on mIPSCs. Taken together, these results demonstrate alterations in synaptic and extrasynaptic GABA(A)Rs in DGCs following CCI.

Dietary branched chain amino acids ameliorate injury-induced cognitive impairment

Neurological dysfunction caused by traumatic brain injury results in profound changes in net synaptic efficacy, leading to impaired cognition. Because excitability is directly controlled by the balance of excitatory and inhibitory activity, underlying mechanisms causing these changes were investigated using lateral fluid percussion brain injury in mice. Although injury-induced shifts in net synaptic efficacy were not accompanied by changes in hippocampal glutamate and GABA levels, significant reductions were seen in the concentration of branched chain amino acids (BCAAs), which are key precursors to de novo glutamate synthesis. Dietary consumption of BCAAs restored hippocampal BCAA concentrations to normal, reversed injury-induced shifts in net synaptic efficacy, and led to reinstatement of cognitive performance after concussive brain injury. All brain-injured mice that consumed BCAAs demonstrated cognitive improvement with a simultaneous restoration in net synaptic efficacy. Posttraumatic changes in the expression of cytosolic branched chain aminotransferase, branched chain ketoacid dehydrogenase, glutamate dehydrogenase, and glutamic acid decarboxylase support a perturbation of BCAA and neurotransmitter metabolism. Ex vivo application of BCAAs to hippocampal slices from injured animals restored posttraumatic regional shifts in net synaptic efficacy as measured by field excitatory postsynaptic potentials. These results suggest that dietary BCAA intervention could promote cognitive improvement by restoring hippocampal function after a traumatic brain injury.

Moderate traumatic brain injury promotes proliferation of quiescent neural progenitors in the adult hippocampus

Recent evidence shows that traumatic brain injury (TBI) regulates proliferation of neural stem/progenitor cells in the dentate gyrus (DG) of adult hippocampus. There are distinct classes of neural stem/progenitor cells in the adult DG, including quiescent neural progenitors (QNPs), which carry stem cell properties, and their progeny, amplifying neural progenitors (ANPs). The response of each class of progenitors to TBI is not clear. We here used a transgenic reporter Nestin-GFP mouse line, in which QNP and ANP cells are easily visualized and quantified, to determine the targets of the TBI in the DG. We examined changes in proliferation of QNPs and ANPs in the acute phase following TBI and found that QNPs were induced by TBI insult to enter the cell cycle whereas proliferation of ANPs was not significantly affected. These results indicate that different subtypes of neural stem/progenitor cells respond differently to TBI insult. Stem cell activation by the TBI may reflect the induction of innate repair and plasticity mechanisms by the injured brain.