Time course for recovery of water maze performance and central cholinergic innervation after fluid percussion injury

Evaluating the Efficacy of Chronic Galantamine on Sustained Attention and Cholinergic Neurotransmission in A Pre-Clinical Model of Traumatic Brain Injury

Cholinergic disruptions underlie attentional deficits following traumatic brain injury (TBI). Yet, drugs specifically targeting acetylcholinesterase (AChE) inhibition have yielded mixed outcomes. Therefore, we hypothesized that galantamine (GAL), a dual-action competitive AChE inhibitor and α7 nicotinic acetylcholine receptor (nAChR) positive allosteric modulator, provided chronically after injury, will attenuate TBI-induced deficits of sustained attention and enhance ACh efflux in the medial prefrontal cortex (mPFC), as assessed by in vivo microdialysis. In Experiment 1, adult male rats (n = 10-15/group) trained in the 3-choice serial reaction time (3-CSRT) test were randomly assigned to controlled cortical impact (CCI) or sham surgery and administered GAL (0.5, 2.0, or 5.0 mg/kg; i.p.) or saline vehicle (VEH; 1 mL/kg; i.p) beginning 24-h post-surgery and once daily thereafter for 27 days. Measures of sustained attention and distractibility were assessed on post-operative days 21-25 in the 3-CSRT, following which cortical lesion volume and basal forebrain cholinergic cells were quantified on day 27. In Experiment 2, adult male rats (n = 3-4/group) received a CCI and 24 h later administered (i.p.) one of the three doses of GAL or VEH for 21 days to quantify the dose-dependent effect of GAL on in vivo ACh efflux in the mPFC. Two weeks after the CCI, a guide cannula was implanted in the right mPFC. On post-surgery day 21, baseline and post-injection dialysate samples were collected in a temporally matched manner with the cohort undergoing behavior. ACh levels were analyzed using reverse phase high-performance liquid chromatography (HPLC) coupled to an electrochemical detector. Cortical lesion volume was quantified on day 22. The data were subjected to ANOVA, with repeated measures where appropriate, followed by Newman-Keuls post hoc analyses. All TBI groups displayed impaired sustained attention versus the pooled SHAM controls (p's < 0.05). Moreover, the highest dose of GAL (5.0 mg/kg) exacerbated attentional deficits relative to VEH and the two lower doses of GAL (p's < 0.05). TBI significantly reduced cholinergic cells in the right basal forebrain, regardless of treatment condition, versus SHAM (p < 0.05). In vivo microdialysis revealed no differences in basal ACh in the mPFC; however, GAL (5.0 mg/kg) significantly increased ACh efflux 30 min following injection compared to the VEH and the other GAL (0.5 and 2.0 mg/kg) treated groups (p's < 0.05). In both experiments, there were no differences in cortical lesion volume across treatment groups (p's > 0.05). In summary, albeit the higher dose of GAL increased ACh release, it did not improve measures of sustained attention or histopathological markers, thereby partially supporting the hypothesis and providing the impetus for further investigations into alternative cholinergic pharmacotherapies such as nAChR positive allosteric modulators.

Transcriptional Pathology Evolves over Time in Rat Hippocampus after Lateral Fluid Percussion Traumatic Brain Injury

Traumatic brain injury (TBI) causes acute and lasting impacts on the brain, driving pathology along anatomical, cellular, and behavioral dimensions. Rodent models offer an opportunity to study the temporal progression of disease from injury to recovery. Transcriptomic and epigenomic analysis were applied to evaluate gene expression in ipsilateral hippocampus at 1 and 14 days after sham (n = 2 and 4, respectively per time point) and moderate lateral fluid percussion injury (n = 4 per time point). This enabled the identification of dynamic changes and differential gene expression (differentially expressed genes; DEGs) modules linked to underlying epigenetic response. We observed acute signatures associated with cell death, astrocytosis, and neurotransmission that largely recovered by 2 weeks. Inflammation and immune signatures segregated into upregulated modules with distinct expression trajectories and functions. Whereas most down-regulated genes recovered by 14 days, two modules with delayed and persistent changes were associated with cholesterol metabolism, amyloid beta clearance, and neurodegeneration. Differential expression was paralleled by changes in histone H3 lysine residue 4 trimethylation at the promoters of DEGs at 1 day post-TBI, with the strongest changes observed for inflammation and immune response genes. These results demonstrate how integrated genomics analysis in the pre-clinical setting has the potential to identify stage-specific biomarkers for injury and/or recovery. Though limited in scope here, our general strategy has the potential to capture pathological signatures over time and evaluate treatment efficacy at the systems level.

Potential of mesenchymal stem cells alone, or in combination, to treat traumatic brain injury

Traumatic brain injury (TBI) causes death and disability in the United States and around the world. The traumatic insult causes the mechanical injury of the brain and primary cellular death. While a comprehensive pathological mechanism of TBI is still lacking, the focus of the TBI research is concentrated on understanding the pathophysiology and developing suitable therapeutic approaches. Given the complexities in pathophysiology involving interconnected immunologic, inflammatory, and neurological cascades occurring after TBI, the therapies directed to a single mechanism fail in the clinical trials. This has led to the development of the paradigm of a combination therapeutic approach against TBI. While there are no drugs available for the treatment of TBI, stem cell therapy has shown promising results in preclinical studies. But, the success of the therapy depends on the survival of the stem cells, which are limited by several factors including route of administration, health of the administered cells, and inflammatory microenvironment of the injured brain. Reducing the inflammation prior to cell administration may provide a better outcome of cell therapy following TBI. This review is focused on different therapeutic approaches of TBI and the present status of the clinical trials.

Conjugated Linoleic Acid Administration Induces Amnesia in Male Sprague Dawley Rats and Exacerbates Recovery from Functional Deficits Induced by a Controlled Cortical Impact Injury

Long-chain polyunsaturated fatty acids like conjugated linoleic acids (CLA) are required for normal neural development and cognitive function and have been ascribed various beneficial functions. Recently, oral CLA also has been shown to increase testosterone (T) biosynthesis, which is known to diminish traumatic brain injury (TBI)-induced neuropathology and reduce deficits induced by stroke in adult rats. To test the impact of CLA on cognitive recovery following a TBI, 5-6 month old male Sprague Dawley rats received a focal injury (craniectomy + controlled cortical impact (CCI; n = 17)) or Sham injury (craniectomy alone; n = 12) and were injected with 25 mg/kg body weight of Clarinol® G-80 (80% CLA in safflower oil; n = 16) or saline (n = 13) every 48 h for 4 weeks. Sham surgery decreased baseline plasma progesterone (P4) by 64.2% (from 9.5 ± 3.4 ng/mL to 3.4 ± 0.5 ng/mL; p = 0.068), T by 74.6% (from 5.9 ± 1.2 ng/mL to 1.5 ± 0.3 ng/mL; p < 0.05), 11-deoxycorticosterone (11-DOC) by 37.5% (from 289.3 ± 42.0 ng/mL to 180.7 ± 3.3 ng/mL), and corticosterone by 50.8% (from 195.1 ± 22.4 ng/mL to 95.9 ± 2.2 ng/mL), by post-surgery day 1. CCI injury induced similar declines in P4, T, 11-DOC and corticosterone (58.9%, 74.6%, 39.4% and 24.6%, respectively) by post-surgery day 1. These results suggest that both Sham surgery and CCI injury induce hypogonadism and hypoadrenalism in adult male rats. CLA treatment did not reverse hypogonadism in Sham (P4: 2.5 ± 1.0 ng/mL; T: 0.9 ± 0.2 ng/mL) or CCI-injured (P4: 2.2 ± 0.9 ng/mL; T: 1.0 ± 0.2 ng/mL, p > 0.05) animals by post-injury day 29, but rapidly reversed by post-injury day 1 the hypoadrenalism in Sham (11-DOC: 372.6 ± 36.6 ng/mL; corticosterone: 202.6 ± 15.6 ng/mL) and CCI-injured (11-DOC: 384.2 ± 101.3 ng/mL; corticosterone: 234.6 ± 43.8 ng/mL) animals. In Sham surgery animals, CLA did not alter body weight, but did markedly increase latency to find the hidden Morris Water Maze platform (40.3 ± 13.0 s) compared to saline treated Sham animals (8.8 ± 1.7 s). In CCI injured animals, CLA did not alter CCI-induced body weight loss, CCI-induced cystic infarct size, or deficits in rotarod performance. However, like Sham animals, CLA injections exacerbated the latency of CCI-injured rats to find the hidden MWM platform (66.8 ± 10.6 s) compared to CCI-injured rats treated with saline (30.7 ± 5.5 s, p < 0.05). These results indicate that chronic treatment of CLA at a dose of 25 mg/kg body weight in adult male rats over 1-month 1) does not reverse craniectomy- and craniectomy + CCI-induced hypogonadism, but does reverse craniectomy- and craniectomy + CCI-induced hypoadrenalism, 2) is detrimental to medium- and long-term spatial learning and memory in craniectomized uninjured rats, 3) limits cognitive recovery following a moderate-severe CCI injury, and 4) does not alter body weight.

Traumatic Brain Injury Stimulates Neural Stem Cell Proliferation via Mammalian Target of Rapamycin Signaling Pathway Activation

Neural stem cells in the adult brain possess the ability to remain quiescent until needed in tissue homeostasis or repair. It was previously shown that traumatic brain injury (TBI) stimulated neural stem cell (NSC) proliferation in the adult hippocampus, indicating an innate repair mechanism, but it is unknown how TBI promotes NSC proliferation. In the present study, we observed dramatic activation of mammalian target of rapamycin complex 1 (mTORC1) in the hippocampus of mice with TBI from controlled cortical impact (CCI). The peak of mTORC1 activation in the hippocampal subgranular zone, where NSCs reside, is 24-48 h after trauma, correlating with the peak of TBI-enhanced NSC proliferation. By use of a Nestin-GFP transgenic mouse, in which GFP is ectopically expressed in the NSCs, we found that TBI activated mTORC1 in NSCs. With 5-bromo-2'-deoxyuridine labeling, we observed that TBI increased mTORC1 activation in proliferating NSCs. Furthermore, administration of rapamycin abolished TBI-promoted NSC proliferation. Taken together, these data indicate that mTORC1 activation is required for NSC proliferation postinjury, and thus might serve as a therapeutic target for interventions to augment neurogenesis for brain repair after TBI.

Minor Functional Deficits in Basic Response Patterns for Reinforcement after Frontal Traumatic Brain Injury in Rats

Traumatic brain injury (TBI) is a major contributor to numerous psychiatric conditions and chronic behavioral dysfunction. Recent studies in experimental brain injury have begun to adopt operant methodologies to assess these deficits, all of which rely on the process of reinforcement. No studies have directly examined how reinforced behaviors are affected by TBI, however. The current study assessed performance under the four most common schedules of reinforcement (fixed ratio, variable ratio, fixed interval, variable interval) and one higher order schedule assessing motivation (progressive ratio) after bilateral, pre-frontal controlled cortical impact injury. TBI-induced differences on the basic schedules were minor, with the exception of the variable ratio, where increased efficacy (more reinforcers, higher response rates, lower interresponse times) at higher requirements was observed as a result of brain injury. Performance on the progressive ratio schedule showed some gross differences between the groups, in that sham rats became more efficient under this schedule while injured rats perseverated in lever pressing. Further, injured rats were specifically impaired at lower response requirements on the progressive ratio. Taken together, these findings indicate that simple reinforced behaviors are mostly unaffected after TBI, except in the case of variable ratio schedules, but the altered performance on the higher-order progressive ratio schedule suggests changes involving motivation or potentially perseveration. These findings validate operant measures of more complex behaviors for brain injury, all of which rely on reinforcement and can be taken into consideration when adapting and developing novel functional assessments.

Chronic Histopathological and Behavioral Outcomes of Experimental Traumatic Brain Injury in Adult Male Animals

The purpose of this review is to survey the use of experimental animal models for studying the chronic histopathological and behavioral consequences of traumatic brain injury (TBI). The strategies employed to study the long-term consequences of TBI are described, along with a summary of the evidence available to date from common experimental TBI models: fluid percussion injury; controlled cortical impact; blast TBI; and closed-head injury. For each model, evidence is organized according to outcome. Histopathological outcomes included are gross changes in morphology/histology, ventricular enlargement, gray/white matter shrinkage, axonal injury, cerebrovascular histopathology, inflammation, and neurogenesis. Behavioral outcomes included are overall neurological function, motor function, cognitive function, frontal lobe function, and stress-related outcomes. A brief discussion is provided comparing the most common experimental models of TBI and highlighting the utility of each model in understanding specific aspects of TBI pathology. The majority of experimental TBI studies collect data in the acute postinjury period, but few continue into the chronic period. Available evidence from long-term studies suggests that many of the experimental TBI models can lead to progressive changes in histopathology and behavior. The studies described in this review contribute to our understanding of chronic TBI pathology.

The potential of endogenous neurogenesis for brain repair and regeneration following traumatic brain injury

Traumatic brain injury (TBI) is the leading cause of death and disability of persons under 45 years old in the United States, affecting over 1.5 million individuals each year. It had been thought that recovery from such injuries is severely limited due to the inability of the adult brain to replace damaged neurons. However, recent studies indicate that the mature mammalian central nervous system (CNS) has the potential to replenish damaged neurons by proliferation and neuronal differentiation of adult neural stem/progenitor cells residing in the neurogenic regions in the brain. Furthermore, increasing evidence indicates that these endogenous stem/progenitor cells may play regenerative and reparative roles in response to CNS injuries or diseases. In support of this notion, heightened levels of cell proliferation and neurogenesis have been observed in response to brain trauma or insults suggesting that the brain has the inherent potential to restore populations of damaged or destroyed neurons. This review will discuss the potential functions of adult neurogenesis and recent development of strategies aiming at harnessing this neurogenic capacity in order to repopulate and repair the injured brain.

Injury timing alters metabolic, inflammatory and functional outcomes following repeated mild traumatic brain injury

Repeated head injuries are a major public health concern both for athletes, and members of the police and armed forces. There is ample experimental and clinical evidence that there is a period of enhanced vulnerability to subsequent injury following head trauma. Injuries that occur close together in time produce greater cognitive, histological, and behavioral impairments than do injuries separated by a longer period. Traumatic brain injuries alter cerebral glucose metabolism and the resolution of altered glucose metabolism may signal the end of the period of greater vulnerability. Here, we injured mice either once or twice separated by three or 20days. Repeated injuries that were separated by three days were associated with greater axonal degeneration, enhanced inflammatory responses, and poorer performance in a spatial learning and memory task. A single injury induced a transient but marked increase in local cerebral glucose utilization in the injured hippocampus and sensorimotor cortex, whereas a second injury, three days after the first, failed to induce an increase in glucose utilization at the same time point. In contrast, when the second injury occurred substantially later (20days after the first injury), an increase in glucose utilization occurred that paralleled the increase observed following a single injury. The increased glucose utilization observed after a single injury appears to be an adaptive component of recovery, while mice with 2 injuries separated by three days were not able to mount this response, thus this second injury may have produced a significant energetic crisis such that energetic demands outstripped the ability of the damaged cells to utilize energy. These data strongly reinforce the idea that too rapid return to activity after a traumatic brain injury can induce permanent damage and disability, and that monitoring cerebral energy utilization may be a tool to determine when it is safe to return to the activity that caused the initial injury.

Elucidating the severity of preclinical traumatic brain injury models: a role for functional assessment?

BACKGROUND

Concussion remains a symptom-based diagnosis clinically, yet preclinical studies investigating traumatic brain injury, of which concussion is believed to represent a "mild" form, emphasize histological end points with functional assessments often minimized or ignored all together. Recently, clinical studies have identified the importance of cognitive and neuropsychiatric symptoms, in addition to somatic concerns, following concussion. How these findings may translate to preclinical studies is unclear at present.

OBJECTIVE

To address the contrasting end points used clinically compared with those in preclinical studies and the potential role of functional assessments in a commonly used model of diffuse axonal injury (DAI).

METHODS

Animals were subjected to DAI by the use of the impact-acceleration model. Functional and behavioral assessments were conducted during 1 week following DAI before the completion of the histological assessment at 1 week post-DAI.

RESULTS

We show, despite the suggestion that this model represents concussive injury, no functional impairments as determined by using the common measures of motor, sensorimotor, cognitive, and neuropsychiatric function following injury over the course of 1 week. The lack of functional deficits is in sharp contrast to neuropathological findings indicating neural degeneration, astrocyte reactivity, and microglial activation.

CONCLUSION

Future studies are needed to identify functional assessments, neurophysiologic techniques, and imaging assessments more apt to distinguish differences following so-called "mild" traumatic brain injury in preclinical models and determine whether these models are truly studying concussive or subconcussive injury. These studies are needed not only to understand the mechanism of injury and production of subsequent deficits, but also to rigorously evaluate potential therapeutic agents.

Functional assessment of long-term deficits in rodent models of traumatic brain injury

Traumatic brain injury (TBI) ranks as the leading cause of mortality and disability in the young population worldwide. The annual US incidence of TBI in the general population is estimated at 1.7 million per year, with an estimated financial burden in excess of US$75 billion a year in the USA alone. Despite the prevalence and cost of TBI to individuals and society, no treatments have passed clinical trial to clinical implementation. The rapid expansion of stem cell research and technology offers an alternative to traditional pharmacological approaches targeting acute neuroprotection. However, preclinical testing of these approaches depends on the selection and characterization of appropriate animal models. In this article we consider the underlying pathophysiology for the focal and diffuse TBI subtypes, discuss the existing preclinical TBI models and functional outcome tasks used for assessment of injury and recovery, identify criteria particular to preclinical animal models of TBI in which stem cell therapies can be tested for safety and efficacy, and review these criteria in the context of the existing TBI literature. We suggest that 2 months post-TBI is the minimum period needed to evaluate human cell transplant efficacy and safety. Comprehensive review of the published TBI literature revealed that only 32% of rodent TBI papers evaluated functional outcome ≥1 month post-TBI, and only 10% evaluated functional outcomes ≥2 months post-TBI. Not all published papers that evaluated functional deficits at a minimum of 2 months post-TBI reported deficits; hence, only 8.6% of overall TBI papers captured in this review demonstrated functional deficits at 2 months or more postinjury. A 2-month survival and assessment period would allow sufficient time for differentiation and integration of human neural stem cells with the host. Critically, while trophic effects might be observed at earlier time points, it will also be important to demonstrate the sustainability of such an effect, supporting the importance of an extended period of in vivo observation. Furthermore, regulatory bodies will likely require at least 6 months survival post-transplantation for assessment of toxicology/safety, particularly in the context of assessing cell abnormalities.

Exploring temporospatial changes in glucose metabolic disorder, learning, and memory dysfunction in a rat model of diffuse axonal injury

Diffuse axonal injury (DAI) is the predominant effect of severe traumatic brain injury and contributes significantly to cognitive deficits. The mechanisms underlying these cognitive deficits are often associated with complex metabolic alterations. However, the relationships between temporospatial alterations in cerebral glucose metabolism and the pathophysiology of DAI-related learning and memory dysfunction are not yet completely understood. We used a small animal positron emission tomography (PET) scanner with 2-[F-18]-fluoro-2-deoxy-D-glucose (¹⁸F-FDG) as a molecular probe to evaluate temporospatial glucose metabolism in vulnerable areas of rats with DAI. The Morris water maze (MWM) was used to evaluate the development and progression of learning and memory dysfunction. Compared to the sham-treated group, PET-MRI fusion images showed that glucose metabolism was reduced in animals with DAI. In addition, the standardized uptake value (SUV) of ¹⁸F-FDG was significantly decreased in the sensorimotor cortex, hippocampus, corpus callosum, caudate putamen, brain stem, and cerebellum at days 1, 3, and 7 after injury. SUV returned to baseline levels by 30 days after injury. The escape latency of the injured group was significantly increased, and the percentages of distance travelled and time spent in the target quadrant were significantly decreased 1 month after injury. These effects persisted for 3 months. SUVs in the hippocampus at the acute stage were significantly correlated with MWM performance during the recovery stage of DAI. These results demonstrate that microstructural injury-induced hypometabolism in the hippocampus at the acute stage are all significantly correlated with learning and memory dysfunctions during the recovery stage of DAI.

Moderate traumatic brain injury causes acute dendritic and synaptic degeneration in the hippocampal dentate gyrus

Hippocampal injury-associated learning and memory deficits are frequent hallmarks of brain trauma and are the most enduring and devastating consequences following traumatic brain injury (TBI). Several reports, including our recent paper, showed that TBI brought on by a moderate level of controlled cortical impact (CCI) induces immature newborn neuron death in the hippocampal dentate gyrus. In contrast, the majority of mature neurons are spared. Less research has been focused on these spared neurons, which may also be injured or compromised by TBI. Here we examined the dendrite morphologies, dendritic spines, and synaptic structures using a genetic approach in combination with immunohistochemistry and Golgi staining. We found that although most of the mature granular neurons were spared following TBI at a moderate level of impact, they exhibited dramatic dendritic beading and fragmentation, decreased number of dendritic branches, and a lower density of dendritic spines, particularly the mushroom-shaped mature spines. Further studies showed that the density of synapses in the molecular layer of the hippocampal dentate gyrus was significantly reduced. The electrophysiological activity of neurons was impaired as well. These results indicate that TBI not only induces cell death in immature granular neurons, it also causes significant dendritic and synaptic degeneration in pathohistology. TBI also impairs the function of the spared mature granular neurons in the hippocampal dentate gyrus. These observations point to a potential anatomic substrate to explain, in part, the development of posttraumatic memory deficits. They also indicate that dendritic damage in the hippocampal dentate gyrus may serve as a therapeutic target following TBI.

Vascular endothelial growth factor is involved in mediating increased de novo hippocampal neurogenesis in response to traumatic brain injury

Stimulating the endogenous repair process after traumatic brain injury (TBI) can be an important approach in neuroregenerative medicine. Vascular endothelial growth factor (VEGF) is one of the molecules that can increase de novo hippocampal neurogenesis. Here, we tested whether VEGF signaling through Flk1 (VEGF receptor 2) is involved in the neurogenic process after experimental TBI. We found that Flk1 is expressed both by neuroblasts in the subgranular layer (SGL) and by maturing granule neurons in the adult dentate gyrus (DG) of the hippocampus. After lateral fluid percussion TBI (LFP-TBI) in the rat, we detected elevated VEGF levels and also increased numbers of de novo neurons in the ipsilateral DG. To test the involvement of VEGF and Flk1 in the neurogenic process directly, we delivered recombinant VEGF or SU5416, an inhibitor to Flk1, into the ipsilateral cerebral ventricle of injured animals. We found that VEGF infusion significantly increased the number of BrdU+/Prox1+ new neurons, decreased the number of TUNEL+ cells, but did not change the number of BrdU+ newborn cells per se. Infusion with SU5416 caused no significant changes. Our results suggest that (a) VEGF is a part of the molecular signaling network that mediates de novo hippocampal neurogenesis after TBI; (b) VEGF predominantly mediates survival of de novo granule neurons rather than proliferation of neuroblasts in the injured brain; and (c) additional VEGF receptor(s) and/or other molecular mechanism(s) are also involved in mediating increased neurogenesis following injury.

Attitudes toward the elderly with CNS trauma: a cross-sectional study of neuroscientists, clinicians, and allied-health professionals

Despite the potential impact of ageist attitudes on outcomes of central nervous system (CNS)-injured patients, little has been reported on this issue. Given this, we sought to conduct a questionnaire-based survey to assess the attitudes toward the elderly among basic and clinical neuroscientists, clinicians, and allied-health professionals whose research or medical practice is focused on neurotrauma. We also reviewed all abstracts presented in the National Neurotrauma Symposia from 1984 to 2007 and identified previous studies on the potential effects of age/aging on outcomes. The Kogan's Old People (KOP) scale was used to assess attitudes toward elderly individuals among all members of the National Neurotrauma Society (NNS). Of the 504 registered members, 137 subjects completed the survey that was re-mailed for non-respondents 4 weeks apart. There were no significant differences between respondents of the first and second mailings regarding their demographic and professional profiles or regarding their responses. These results support the validity of our findings in spite of the relatively low mail survey response rate (27.2%). Female gender was significantly associated with more positive attitudes toward old people compared to males. Clinicians showed significantly fewer negative attitudes toward old people in comparison with basic and clinical neuroscientists. Of the 4,194 abstracts reviewed, we identified only 44 studies (1.05%) that were explicitly focused on the effects of aging/old age on neurotrauma. In conclusion, our questionnaire-based survey, which appears to be representative of the population of interest, recognized significant differences in the attitudes toward old people among various professional groups and between male and female professionals. These findings may reflect a lack of knowledge and misconceptions regarding the impact of aging and old age on outcomes after CNS trauma. Further research on the impact of aging on outcomes after neurotrauma is required. Moreover, knowledge translation and mobilization appears required to positively influence attitudes among neuroscience research and clinical professionals regarding the issues of neurotrauma and the elderly.

Anatomical integration of newly generated dentate granule neurons following traumatic brain injury in adult rats and its association to cognitive recovery

The hippocampus is particularly vulnerable to traumatic brain injury (TBI), the consequences of which are manifested as learning and memory deficits. Following injury, substantive spontaneous cognitive recovery occurs, suggesting that innate repair mechanisms exist in the brain. However, the underlying mechanism contributing to this is largely unknown. The existence of neural stem cells in the adult hippocampal dentate gyrus (DG) and their proliferative response following injury led us to speculate that neurogenesis may contribute to cognitive recovery following TBI. To test this, we first examined the time course of cognitive recovery following lateral fluid percussion injury in rats. Cognitive deficits were tested at 11-15, 26-30 or 56-60 days post-injury using Morris Water Maze. At 11-15 and 26-30 days post-injury, animals displayed significant cognitive deficits, which were no longer apparent at 56-60 days post-TBI, suggesting an innate cognitive recovery at 56-60 days. We next examined the proliferative response, maturational fate and integration of newly generated cells in the DG following injury. Specifically, rats received BrdU at 2-5 days post-injury followed by Fluorogold (FG) injection into the CA3 region at 56 days post-TBI. We found the majority of BrdU+ cells which survived for 10 weeks became dentate granule neurons, as assessed by NeuN and calbindin labeling, approximately 30% being labeled with FG, demonstrating their integration into the hippocampus. Additionally, some BrdU+ cells were synaptophysin-positive, suggesting they received synaptic input. Collectively, our data demonstrate the extensive anatomical integration of new born dentate granule neurons at the time when innate cognitive recovery is observed.

Experimental models of traumatic brain injury: do we really need to build a better mousetrap?

Approximately 4000 human beings experience a traumatic brain injury each day in the United States ranging in severity from mild to fatal. Improvements in initial management, surgical treatment, and neurointensive care have resulted in a better prognosis for traumatic brain injury patients but, to date, there is no available pharmaceutical treatment with proven efficacy, and prevention is the major protective strategy. Many patients are left with disabling changes in cognition, motor function, and personality. Over the past two decades, a number of experimental laboratories have attempted to develop novel and innovative ways to replicate, in animal models, the different aspects of this heterogenous clinical paradigm to better understand and treat patients after traumatic brain injury. Although several clinically-relevant but different experimental models have been developed to reproduce specific characteristics of human traumatic brain injury, its heterogeneity does not allow one single model to reproduce the entire spectrum of events that may occur. The use of these models has resulted in an increased understanding of the pathophysiology of traumatic brain injury, including changes in molecular and cellular pathways and neurobehavioral outcomes. This review provides an up-to-date and critical analysis of the existing models of traumatic brain injury with a view toward guiding and improving future research endeavors.

Lateral fluid percussion brain injury: a 15-year review and evaluation

This article comprehensively reviews the lateral fluid percussion (LFP) model of traumatic brain injury (TBI) in small animal species with particular emphasis on its validity, clinical relevance and reliability. The LFP model, initially described in 1989, has become the most extensively utilized animal model of TBI (to date, 232 PubMed citations), producing both focal and diffuse (mixed) brain injury. Despite subtle variations in injury parameters between laboratories, universal findings are evident across studies, including histological, physiological, metabolic, and behavioral changes that serve to increase the reliability of the model. Moreover, demonstrable histological damage and severity-dependent behavioral deficits, which partially recover over time, validate LFP as a clinically-relevant model of human TBI. The LFP model, also has been used extensively to evaluate potential therapeutic interventions, including resuscitation, pharmacologic therapies, transplantation, and other neuroprotective and neuroregenerative strategies. Although a number of positive studies have identified promising therapies for moderate TBI, the predictive validity of the model may be compromised when findings are translated to severely injured patients. Recently, the clinical relevance of LFP has been enhanced by combining the injury with secondary insults, as well as broadening studies to incorporate issues of gender and age to better approximate the range of human TBI within study design. We conclude that the LFP brain injury model is an appropriate tool to study the cellular and mechanistic aspects of human TBI that cannot be addressed in the clinical setting, as well as for the development and characterization of novel therapeutic interventions. Continued translation of pre-clinical findings to human TBI will enhance the predictive validity of the LFP model, and allow novel neuroprotective and neuroregenerative treatment strategies developed in the laboratory to reach the appropriate TBI patients.

Cell proliferation and neuronal differentiation in the dentate gyrus in juvenile and adult rats following traumatic brain injury

It is well known that the cognitive functions of juveniles recover to a greater extent than adult patients following traumatic brain injury (TBI). The exact mechanisms underlying this age-related disparity are unknown; however, we speculate that this improved recovery in juveniles following TBI may be associated with an endogenous neurogenic response in the hippocampus. We, therefore, examined the effects of TBI on cellular proliferation and differentiation in the dentate gyrus (DG) of the hippocampus in juvenile and adult rats following lateral fluid percussion injury (FPI). The temporal profile of the injury-induced proliferative response was determined using BrdU labeling at varying survival times. The differentiation of these newly generated cells was investigated using cell-type specific markers. We found that, following injury, there was a significant increase in cell proliferation in the DG in both injured juveniles and adults at 2 days post injury when compared to shams. When comparing the extent of cell proliferation between juveniles and adults following TBI, the absolute number of cells generated in the subgranular zone (SGZ) was far greater in the juveniles. Moreover, the percentage of newly generated cells in the SGZ that differentiated into neurons was nearly two times higher in the juveniles as compared to adults. Conversely, more glial differentiation was observed in the DG of adult rats. These findings provide compelling evidence that age-related differences in the neurogenic response to injury may underlie the differences observed in cognitive recovery in juvenile mammals as compared to adults following TBI.

A Significant Increase in Both Basal and Maximal Calcineurin Activity following Fluid Percussion Injury in the Rat

Calcineurin, a neuronally enriched, calcium-stimulated phosphatase, is an important modulator of many neuronal processes, including several that are physiologically related to the pathology of traumatic brain injury. This study examined the effects of moderate, central fluid percussion injury on the activity of this important neuronal enzyme. Animals were sacrificed at several time-points postinjury and cortical, hippocampal, and cerebellar homogenates were assayed for calcineurin activity by dephosphorylation of p-nitrophenol phosphate. A significant brain injury-dependent increase was observed in both hippocampal and cortical homogenates under both basal and maximally-stimulated reaction conditions. This increase persisted 2-3 weeks post-injury. Brain injury did not alter substrate affinity, but did induce a significant increase in the apparent maximal dephosphorylation rate. Unlike the other brain regions, no change in calcineurin activity was observed in the cerebellum following brain injury. No brain region tested displayed a significant change in calcineurin enzyme levels as determined by Western blot, demonstrating that increased enzyme synthesis was not responsible for the observed increase in activity. The data support the conclusion that fluid percussion injury results in increased calcineurin activity in the rat forebrain. This increased activity has broad physiological implications, possibly resulting in altered cellular excitability or a greater likelihood of neuronal cell death.

Ex VivoGene Therapy Using Targeted Engraftment of NGF-Expressing Human NT2N Neurons Attenuates Cognitive Deficits Following Traumatic Brain Injury in Mice

Infusion of nerve growth factor (NGF) has been shown to be neuroprotective following traumatic brain injury (TBI). In this study, we tested the hypothesis that NGF-expressing human NT2N neurons transplanted into the basal forebrain of brain-injured mice can attenuate long-term cognitive dysfunction associated with TBI. Undifferentiated NT2 cells were transduced in vitro with a lentiviral vector to release NGF, differentiated into NT2N neurons by exposure to retinoic acid and transplanted into the medial septum of mice 24 h following controlled cortical impact (CCI) brain injury or sham injury. Adult mice (n = 78) were randomly assigned to one of four groups: (1) sham-injured and vehicle (serum-free medium)-treated, (2) brain-injured and vehicle-treated, (3) brain-injured engrafted with untransduced NT2N neurons, and (4) brain-injured engrafted with transduced NGF-NT2N neurons. All groups were immunosuppressed daily with cyclosporin A (CsA) for 4 weeks. At 1 month post-transplantation, animals engrafted with NGF-expressing NT2N neurons showed significantly improved learning ability (evaluated with the Morris water maze) compared to brain-injured mice receiving either vehicle (p < 0.05) or untransduced NT2N neurons (p < 0.01). No effect of NGF-secreting NT2N cells on motor function deficits at 1-4 weeks post-transplantation was observed. These data suggest that NGF gene therapy using transduced NT2N neurons (as a source of delivery) may selectively improve cognitive function following TBI.

Motor and cognitive function evaluation following experimental traumatic brain injury

Traumatic brain injury (TBI) in humans may cause extensive sensorimotor and cognitive dysfunction. As a result, many TBI researchers are beginning to assess behavioral correlates of histologically determined damage in animal models. Although this is an important step in TBI research, there is a need for standardization between laboratories. The ability to reliably test treatments across laboratories and multiple injury models will close the gap between treatment success in the lab and success in the clinic. The goal of this review is to describe and evaluate the tests employed to assess functional outcome after TBI and to overview aspects of cognitive, sensory, and motor function that may be suitable targets for therapeutic intervention.

Closed-head minimal traumatic brain injury produces long-term cognitive deficits in mice

Victims of minimal traumatic brain injury (mTBI) do not show clear morphological brain defects, but frequently suffer lasting cognitive deficits, emotional difficulties and behavioral disturbances. In the present study we adopted a non-invasive closed-head weight-drop mouse model to produce mTBI. We examined the effects of 20, 25, or 30 g weight drop 7, 30, 60 and 90 days following injury on mice's ability to perform the Morris water maze. The mice suffered profound long-lasting learning and memory deficits that were force- and time-dependent. Although the injured mice could acquire the task, they could not improve their initial escape latency by more than 50%, while normal mice improved by up to 450% (P<0.001). In order to directly compare the learning ability of individual mice following our mTBI we have devised a new measure which we term learning rate. We define learning rate as the rate the mouse improved its own performance in consecutive trials in a given experimental day. The learning rate of control mice increased linearly throughout the testing period with a slope of approximately 0.9. Injured mice that sustained 20 and 25 g weight drop could also improve their learning rate linearly but with a slope of only 0.2. Mice who sustained 30 g weight drop could not improve their learning rate linearly and reached a plateau after the third experimental learning day. These results indicate that the severity of injury may correlate with the degree of integration of the learning task. These cognitive deficits occurred without any other clear neurological damage, no evident brain edema, no notable damage to the blood-brain barrier and no early anatomical changes to the brain (observed by magnetic resonance imaging imaging). These results demonstrate that persistent deficits of cognitive learning abilities in mice, similar to those observed in human post-concussive syndrome, can follow mTBI without any anatomical damage to the brain and its surrounding tissue.

Nucleus basalis of Meynert pathology in the human brain after fatal head injury

Dysfunction of the basal forebrain cholinergic system has been hypothesized to contribute to deficits of memory and cognition after head injury. We have previously reported reduced levels of choline acetyltransferase activity in the cerebral cortex of patients who died after a head injury, demonstrating that there is a loss of cortical cholinergic innervation. In the present study, we examined the nucleus basalis of Meynert (NBM), which provides cortical cholinergic innervation, in fatally head-injured patients and in controls. The majority of head-injured patients had histological evidence of neuronal damage in the NBM, which was due to mechanical distortion of the tissue and/or ischemic damage. The findings demonstrate that the NBM is vulnerable after severe head injury and that secondary insults play an important role in the damage. Thus, both neuronal perikarya and terminals of the basal forebrain cholinergic system are damaged after human fatal head injury. This damage may contribute to persisting dysfunction of memory and cognition in head-injured patients who survive.

Recovery of water maze performance in aged versus young rats after brain injury with the impact acceleration model

Clinically, elderly patients have a higher cognitive morbidity from head trauma than young patients. We have modeled injury in aged rats in an effort to elucidate the pathophysiology of this enhanced sensitivity and, in particular, to determine if there are susceptibility differences in forebrain cholinergic innervation in young versus aged rats. Aged (20-23 months) and young (2-3 months) rats were subjected to injury under halothane anesthesia using the Marmarou impact acceleration model. Injury parameters required adjustment downward for the aged rats (323 g at 1.61 m versus 494 g at 2.06 m) to provide equivalent mortality (30% versus 20%) and loss of righting-reflex times (10-12 min average). At 1 week following injury, the aged animals were markedly more impaired in water maze performance than were young rats, and this difference persisted at least up to 5 weeks following injury. The extent of improvement in performance from 1 to 5 weeks was markedly worse for aged animals compared to young animals. Forebrain synaptosomal choline uptake was decreased in aged injured rats by 8-14% at 1, 3, and 5 weeks postinjury, but not decreased in young injured rats. No differences were noted in entorhinal cortex or hippocampal choline uptake. This model effectively demonstrates the markedly increased susceptibility of older animals to head injury and their decreased capacity for recovery. The neurophysiological basis for this difference is presently unknown, but the differences in cognitive dysfunction between young and aged rats appears to be much greater than would seem to be explained by the small differences in forebrain cholinergic innervation.

Cognitive impairment and synaptosomal choline uptake in rats following impact acceleration injury

Traumatic brain injury is well known to cause deficits in learning and memory, which typically improve with time. Animal studies with fluid percussion or controlled cortical impact injury have identified transient disturbances in forebrain cholinergic innervation which may contribute to such cognitive problems. This study examines the extent to which water maze performance and forebrain synaptosomal choline uptake are affected one week after injury using the newly developed impact acceleration injury model. Injury or sham injury was delivered to adult male Sprague-Dawley rats under halothane anesthesia using a 500-g 2.1-m weight drop. Based on righting reflex, injured rats were divided into moderate (< or = 12 min) or severe (>12 min) groups. Water maze testing was performed on days 5-7 postinjury. On day 7, choline uptake was determined in synaptosomes from hippocampus, a parietal cortex, and entorhinal cortex. Maze learning was severely impaired in the severe injury group but not in the moderate injury group. Learning retention was slightly impaired in the moderate injury group and severely affected in the severe injury group. There was a very strong correlation between the severity of injury as determined by prolongation of righting times and disruption of maze learning at 1 week postinjury. There was no change in synaptosomal choline uptake in any of the forebrain regions in the severe injury group, but a slight (14%) decrease in the hippocampus and parietal cortex of the moderate injury group. Correlation analysis showed no relationship between synaptosomal choline uptake in any brain region and performance in either water maze learning or retention. This study shows that the impact acceleration model produces cognitive impairments equivalent to those seen with fluid percussion injury and controlled cortical impact. Compared with those models, the impact acceleration model does not produce a similar disruption of forebrain cholinergic nerve terminals.

Secondary hypoxemia exacerbates the reduction of visual discrimination accuracy and neuronal cell density in the dorsal lateral geniculate nucleus resulting from fluid percussion injury

The purpose of this study was to determine the impact of secondary hypoxemia on visual discrimination accuracy after parasagittal fluid percussion injury (FPI). Rats lived singly in test cages, where they were trained to repeatedly execute a flicker-frequency visual discrimination for food. After learning was complete, all rats were surgically prepared and then retested over the following 4-5 days to ensure recovery to presurgery levels of performance. Rats were then assigned to one of three groups [FPI + Hypoxia (IH), FPI + Normoxia (IN), or Sham Injury + Hypoxia (SH)] and were anesthetized with halothane delivered by compressed air. Immediately after injury or sham injury, rats in groups IH and SH were switched to a 13% O2 source to continue halothane anesthesia for 30 min before being returned to their test cages. Anesthesia for rats in group IN was maintained using compressed air for 30 min after injury. FPI significantly reduced visual discrimination accuracy and food intake, and increased incorrect choices. Thirty minutes of immediate posttraumatic hypoxemia significantly (1) exacerbated the FPI-induced reductions of visual discrimination accuracy and food intake, (2) further increased numbers of incorrect choices, and (3) delayed the progressive recovery of visual discrimination accuracy. Thionine stains of midbrain coronal sections revealed that, in addition to the loss of neurons seen in several thalamic nuclei following FPI, cell loss in the ipsilateral dorsal lateral geniculate nucleus (dLG) was significantly greater after FPI and hypoxemia than after FPI alone. In contrast, neuropathological changes were not evident following hypoxemia alone. These results show that, although hypoxemia alone was without effect, posttraumatic hypoxemia exacerbates FPI-induced reductions in visual discrimination accuracy and secondary hypoxemia interferes with control of the rat's choices by flicker frequency, perhaps in part as a result of neuronal loss and fiber degeneration in the dLG. These results additionally confirm the utility of this visual discrimination procedure as a sensitive, noninvasive means of assessing behavioral function after experimental traumatic brain injury.