Mild traumatic lesion of the right parietal cortex of the rat: selective behavioural deficits in the absence of neurological impairment

Lateral fluid percussion injury: A rat model of experimental traumatic brain injury

Traumatic brain injury (TBI) represents one of the leading causes of disability and death worldwide. The annual economic impact of TBI-including direct and indirect costs-is high, particularly impacting low- and middle-income countries. Despite extensive research, a comprehensive understanding of the primary and secondary TBI pathophysiology, followed by the development of promising therapeutic approaches, remains limited. These fundamental caveats in knowledge have motivated the development of various experimental models to explore the molecular mechanisms underpinning the pathogenesis of TBI. In this context, the Lateral Fluid Percussion Injury (LFPI) model produces a brain injury that mimics most of the neurological and systemic aspects observed in human TBI. Moreover, its high reproducibility makes the LFPI model one of the most widely used rodent-based TBI models. In this chapter, we provide a detailed surgical protocol of the LFPI model used to induce TBI in adult Wistar rats. We further highlight the neuroscore test as a valuable tool for the evaluation of TBI-induced sensorimotor consequences and their severity in rats. Lastly, we briefly summarize the current knowledge on the pathological aspects and functional outcomes observed in the LFPI-induced TBI model in rodents.

Weight-drop model as a valuable tool to study potential neurobiological processes underlying behavioral and cognitive changes secondary to mild traumatic brain injury

The pathophysiology of post-traumatic brain injury (TBI) behavioral and cognitive changes is not fully understood, especially in its mild presentation. We designed a weight drop TBI model in mice to investigate the role of neuroinflammation in behavioral and cognitive sequelae following mild TBI. C57BL/6 mice displayed depressive-like behavior at 72 h after mild TBI compared with controls, as indicated by a decrease in the latency to first immobility and climbing time in the forced swim test. Additionally, anxiety-like behavior and hippocampal-associated spatial learning and memory impairment were found in the elevated plus maze and in the Barnes maze, respectively. Levels of a set of inflammatory mediators and neurotrophic factors were analyzed at 6 h, 24 h, 72 h, and 30 days after injury in ipsilateral and contralateral hemispheres of the prefrontal cortex and hippocampus. Principal components analysis revealed two principal components (PC), which represented 59.1% of data variability. PC1 (cytokines and chemokines) expression varied between both hemispheres, while PC2 (neurotrophic factors) expression varied only across the investigated brain areas. Our model reproduces mild TBI-associated clinical signs and pathological features and might be a valuable tool to broaden the knowledge regarding mild TBI pathophysiology as well as to test potential therapeutic targets.

Focal Cortical Dysplasia in Childhood Epilepsy

Focal cortical dysplasia is a common cause of medication resistant epilepsy. A better understanding of its presentation, pathophysiology and consequences have helped us improved its treatment and outcome. This paper reviews the most recent classification, pathophysiology and imaging findings in clinical research as well as the knowledge gained from studying genetic and lesional animal models of focal cortical dysplasia. This review of this recently gained knowledge will most likely help develop new research models and new therapeutic targets for patients with epilepsy associated with focal cortical dysplasia.

Use of Anisotropy, 3D Segmented Atlas, and Computational Analysis to Identify Gray Matter Subcortical Lesions Common to Concussive Injury from Different Sites on the Cortex

Traumatic brain injury (TBI) can occur anywhere along the cortical mantel. While the cortical contusions may be random and disparate in their locations, the clinical outcomes are often similar and difficult to explain. Thus a question that arises is, do concussions at different sites on the cortex affect similar subcortical brain regions? To address this question we used a fluid percussion model to concuss the right caudal or rostral cortices in rats. Five days later, diffusion tensor MRI data were acquired for indices of anisotropy (IA) for use in a novel method of analysis to detect changes in gray matter microarchitecture. IA values from over 20,000 voxels were registered into a 3D segmented, annotated rat atlas covering 150 brain areas. Comparisons between left and right hemispheres revealed a small population of subcortical sites with altered IA values. Rostral and caudal concussions were of striking similarity in the impacted subcortical locations, particularly the central nucleus of the amygdala, laterodorsal thalamus, and hippocampal complex. Subsequent immunohistochemical analysis of these sites showed significant neuroinflammation. This study presents three significant findings that advance our understanding and evaluation of TBI: 1) the introduction of a new method to identify highly localized disturbances in discrete gray matter, subcortical brain nuclei without postmortem histology, 2) the use of this method to demonstrate that separate injuries to the rostral and caudal cortex produce the same subcortical, disturbances, and 3) the central nucleus of the amygdala, critical in the regulation of emotion, is vulnerable to concussion.

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.

Tissue vulnerability is increased following repetitive mild traumatic brain injury in the rat

Repetitive mild traumatic brain injury (rmTBI) is an important medical concern for active sports and military personnel. Multiple mild injuries may exacerbate tissue damage resulting in cumulative brain injury and poor functional recovery. In the present study, we investigated the time course of brain vulnerability to rmTBI in a rat model of mild cortical controlled impact. An initial mild injury was followed by a second injury unilaterally at an interval of 1, 3, or 7 days. RmTBI animals were compared to single mTBI and sham treated animals. Neuropathology was assessed using multi-modal magnetic resonance imaging (MRI), followed by ex vivo tissue immunohistochemistry. Neurological and behavioral outcomes were evaluated in a subset of animals receiving rmTBI 3 days apart and shams. RmTBI 1 or 3 days apart but not 7 days apart revealed significantly exacerbated MRI-definable lesion volumes compared to single mTBI and shams. Increases in cortical tissue damage, extravascular iron and glial activation assessed by histology/immunohistochemistry correlated with in vivo MRI findings where shorter intervals (1 or 3 days apart) resulted in greater tissue pathology. There were no neurological deficits associated with rmTBI 3 day animals. At 1 mo post-injury, animals with rmTBI 3 days apart showed reduced exploratory behaviors and subtle spatial learning memory impairments were observed. Collectively, our findings suggest that the mildly-impacted brain is more vulnerable to repetitive injury when delivered within 3 days following initial mTBI.

The effect of injury severity on behavior: a phenotypic study of cognitive and emotional deficits after mild, moderate, and severe controlled cortical impact injury in mice

Traumatic brain injury (TBI) can cause a broad array of behavioral problems including cognitive and emotional deficits. Human studies comparing neurobehavioral outcomes after TBI suggest that cognitive impairments increase with injury severity, but emotional problems such as anxiety and depression do not. To determine whether cognitive and emotional impairments increase as a function of injury severity we exposed mice to sham, mild, moderate, or severe controlled cortical impact (CCI) and evaluated performance on a variety of neurobehavioral tests in the same animals before assessing lesion volume as a histological measure of injury severity. Increasing cortical impact depth successfully produced lesions of increasing severity in our model. We found that cognitive impairments in the Morris water maze increased with injury severity, as did the degree of contralateral torso flexion, a measure of unilateral striatal damage. TBI also caused deficits in emotional behavior as quantified in the forced swim test, elevated-plus maze, and prepulse inhibition of acoustic startle, but these deficits were not dependent on injury severity. Stepwise regression analyses revealed that Morris water maze performance and torso flexion predicted the majority of the variability in lesion volume. In summary, we find that cognitive deficits increase in relation to injury severity, but emotional deficits do not. Our data suggest that the threshold for emotional changes after experimental TBI is low, with no variation in behavioral deficits seen between mild and severe brain injury.

Shift of circadian feeding pattern by high-fat diets is coincident with reward deficits in obese mice

Recent studies provide evidence that high-fat diets (HF) trigger both i) a deficit of reward responses linked to a decrease of mesolimbic dopaminergic activity, and ii) a disorganization of circadian feeding behavior that switch from a structured meal-based schedule to a continuous snacking, even during periods normally devoted to rest. This feeding pattern has been shown to be a cause of HF-induced overweight and obesity. Our hypothesis deals with the eventual link between the rewarding properties of food and the circadian distribution of meals. We have investigated the effect of circadian feeding pattern on reward circuits by means of the conditioned-place preference (CPP) paradigm and we have characterized the rewarding properties of natural (food) and artificial (cocaine) reinforcers both in free-feeding ad libitum HF mice and in HF animals submitted to a re-organized feeding schedule based on the standard feeding behavior displayed by mice feeding normal chow (“forced synchronization”). We demonstrate that i) ad libitum HF diet attenuates cocaine and food reward in the CPP protocol, and ii) forced synchronization of feeding prevents this reward deficit. Our study provides further evidence that the rewarding impact of food with low palatability is diminished in mice exposed to a high-fat diet and strongly suggest that the decreased sensitivity to chow as a positive reinforcer triggers a disorganized feeding pattern which might account for metabolic disorders leading to obesity.

Concussive brain injury enhances fear learning and excitatory processes in the amygdala

BACKGROUND

Mild traumatic brain injury (cerebral concussion) results in cognitive and emotional dysfunction. These injuries are a significant risk factor for the development of anxiety disorders, including posttraumatic stress disorder. However, because physically traumatic events typically occur in a highly emotional context, it is unknown whether traumatic brain injury itself is a cause of augmented fear and anxiety.

METHODS

Rats were trained with one of five fear-conditioning procedures (n = 105) 2 days after concussive brain trauma. Fear learning was assessed over subsequent days and chronic changes in fear learning and memory circuitry were assessed by measuring N-methyl-D-aspartate receptor subunits and glutamic acid decarboxylase, 67 kDa isoform protein levels in the hippocampus and basolateral amygdala complex (BLA).

RESULTS

Injured rats exhibited an overall increase in fear conditioning, regardless of whether fear was retrieved via discrete or contextual-spatial stimuli. Moreover, injured rats appeared to overgeneralize learned fear to both conditioned and novel stimuli. Although no gross histopathology was evident, injury resulted in a significant upregulation of excitatory N-methyl-D-aspartate receptors in the BLA. There was a trend toward decreased γ-aminobutyric acid-related inhibition (glutamic acid decarboxylase, 67 kDa isoform) in the BLA and hippocampus.

CONCLUSIONS

These results suggest that mild traumatic brain injury predisposes the brain toward heightened fear learning during stressful postinjury events and provides a potential molecular mechanism by which this occurs. Furthermore, these data represent a novel rodent model that can help advance the neurobiological and therapeutic understanding of the comorbidity of posttraumatic stress disorder and traumatic brain injury.

The whisker nuisance task identifies a late-onset, persistent sensory sensitivity in diffuse brain-injured rats

Post-traumatic morbidity reduces the quality of life for traumatic brain injury (TBI) survivors by altering neuropsychological function. After midline fluid percussion injury (FPI), diffuse pathology in the ventral posterior thalamus suggests that somatosensory whisker function may be impaired post-injury. The goals of the present study were to design and validate a task to detect injury-induced somatosensory morbidity (Experiment 1), and to evaluate preliminary applications of the task (Experiment 2). In Experiment 1, male Sprague-Dawley rats were subjected to moderate FPI (approximately 1.9 atm) or sham injury. Over an 8-week time course, the whiskers on both mystacial pads were stimulated manually with an applicator stick in an open field for three 5-min periods. Behavioral responses in this whisker nuisance task were recorded using objective criteria (max score = 16). Sham animals were ambivalent or soothed by whisker stimulation (4.0 +/- 0.8), whereas brain-injured rats showed aggravated responses at 1 week (6.7 +/- 0.9), which became significant at 4 weeks (9.5 +/- 0.5) and 8 weeks (8.4 +/- 1.1) compared to sham injury, indicating chronic injury-induced sensory sensitivity. Total free serum corticosterone levels indicated a significant stress response in brain-injured (125.0 +/- 17.7 ng/mL), but not uninjured animals (74.2 +/- 12.2 ng/mL) in response to whisker stimulation. In Experiment 2, to evaluate applications of the whisker nuisance task, four additional uninjured and brain-injured groups were subjected to mild brain injury only, shaved whiskers after moderate brain injury, repeated whisker nuisance task stimulation after moderate brain injury, or regular opportunities for tactile exploration of an enriched environment after moderate brain injury over 4 weeks post-injury. The whisker nuisance task has the sensitivity to detect mild brain injury (7.7 +/- 1.0), but morbidity was not mitigated by any of the neurorehabilitative interventions. Following diffuse brain injury, the whisker nuisance task is a promising tool to detect post-traumatic morbidity and the efficacy of therapeutic interventions that may restore discrete circuit function in brain-injured patients.

Nothing to fear? Neural systems supporting avoidance behavior in healthy youths

Active avoidance involving controlling and modifying threatening situations characterizes many forms of clinical pathology, particularly childhood anxiety. Presently our understanding of the neural systems supporting human avoidance is largely based on nonhuman research. Establishing the generality of nonhuman findings to healthy children is a needed first step towards advancing developmental affective neuroscience research on avoidance in childhood anxiety. Accordingly, this investigation examined brain activation patterns to threatening cues that prompted avoidance in healthy youths. During functional magnetic resonance imaging, fifteen youths (ages 9-13) completed a task that alternately required approach or avoidance behaviors. On each trial either a threatening 'Snake' cue or a 'Reward' cue advanced towards a bank containing earned points. Directional buttons enabled subjects to move cues away from (Avoidance) or towards the bank (Approach). Avoidance cues elicited activation in regions hypothesized to support avoidance in nonhumans (amygdala, insula, striatum and thalamus). Results also highlighted that avoidance response rates were positively correlated with amygdala activation and negatively correlated with insula and anterior cingulate activation. Moreover, increased amygdala activity was associated with decreased insula and anterior cingulate activity. Our results suggest that nonhuman neurophysiological research findings on avoidance may generalize to neural systems associated with avoidance in childhood. Perhaps most importantly, the amygdala/insula activation observed suggests that threat-related responses can be maintained even when aversive events are consistently avoided, which may account for the persistence of avoidance-coping in childhood anxiety. The present approach may offer developmental affective neuroscience a conceptual and methodological framework for investigating avoidance in childhood anxiety.

Experimental traumatic brain injury induces a pervasive hyperanxious phenotype in rats

Mood disturbances, including depression and anxiety disorders, are common and disabling long-term sequelae of traumatic brain injury (TBI). These psychiatric conditions have generally been considered psychosocial consequences of the trauma, but neurobiological alterations and causes have also been implicated. Using a rat model of TBI (lateral fluid-percussion injury), this longitudinal study seeks to assess anxiety and depression-like behaviors following experimental TBI. Male Wistar rats (n = 20) received a severe (approximately 3.5 atmosphere) pressure pulse directed to the right sensorimotor cortex, or sham surgery (n = 15). At 1, 3, and 6 months following injury, all rats underwent four assessments of anxiety and depression-like behaviors: exposure to an open field, elevated plus maze test, the forced swim test, and the sucrose preference test. Injured animals displayed increased anxiety-like behaviors throughout the study, as evidenced by reduced time spent (p = 0.014) and reduced entries (p < 0.001) into the center area of the open field, and reduced proportion of time in the open arms of the plus maze (p = 0.015), compared to sham-injured controls. These striking changes were particularly evident 1 and 3 months after injury. No differences were observed in depression-like behaviors in the forced swim test (a measure of behavioral despair) and the sucrose preference test (a measure of anhedonia). This report provides the first evidence of persistent anxiety-like disturbances in an experimental model of TBI. This finding indicates that the common occurrence of these symptoms in human sufferers is likely to have, at least in part, a neurobiological basis. Studies in this model could provide insight into the mechanisms underlying affective disturbance in brain-injured patients.

DHEAS repeated treatment improves cognitive and behavioral deficits after mild traumatic brain injury

Mild traumatic brain injury (mTBI) is characterized by diffused symptoms, which when combined are called "post-concussion syndrome". Dehydroepiandrosterone sulfate (DHEAS) is a neuroactive neurosteroid. Previously, we have reported that closed head mTBI causes long lasting cognitive deficits and depressive-like behavior. In the present study we describe the effects of DHEAS on the behavior of mice that suffered closed head mTBI. Following the induction of mTBI, mice were treated once a week with DHEAS (s.c. 20 mg/kg) and their performance in the passive avoidance test and the forced swimming test (FST) were evaluated 7, 30, 60 and 90 days post-injury. The most important interactions were between injury and injection (passive avoidance; p<0.001 and FST; p=0.001), meaning that DHEAS has beneficial effects only when given to injured animals. Our results demonstrate that the long-term cognitive and behavioral effects induced by mTBI may be improved by a repeated weekly treatment with DHEAS.

Apoptotic and behavioral sequelae of mild brain trauma in mice

Mild traumatic brain injury (mTBI) is a not uncommon event in adolescents and young adults. Although it does not result in clear morphological brain defects, it is associated with long-term cognitive, emotional, and behavioral problems. Herein, we characterized the biochemical and behavioral changes associated with experimental mTBI in mice that may act as either targets or surrogate markers for interventional therapy. Specifically, mTBI was induced by 30-g and 50-g weight drop, and at 8 and 72 hr thereafter markers of cellular apoptosis-caspase-3, Bax, apoptosis-inducing factor (AIF), and cytochrome-c (Cyt-c)-were quantified by Western blot analysis in hippocampus ipsilateral to the impact. Levels of amyloid-beta precursor protein (APP) were also measured, and specific behavioral tests-passive avoidance, open field, and forced swimming (Porsolt) paradigms-were undertaken to assess learning, emotionality, and emotional memory. In the absence of hemorrhage or infarcts, as assessed by triphenyltetrazolium chloride staining, procaspase-3 and Bax levels were markedly altered following mTBI at both times. No cleaved caspase-3 was detected, and levels of AIF and Cyt-c, but not APP, were significantly changed at 72 hr. Mice subjected to mTBI were indistinguishable from controls by neurological examination at 1 and 24 hr, and by passive avoidance/open field at 72 hr, but could be differentiated in the forced swimming paradigm. In general, this model mimics the diffuse effects of mTBI on brain function associated with the human condition and highlights specific apoptotic proteins and a behavioral paradigm as potential markers for prospective interventional strategies.

Response of the contralateral hippocampus to lateral fluid percussion brain injury

Traumatic brain injury is a leading cause of death and disability in the United States. Pathological examinations of humans and animal models after brain injury demonstrate hippocampal neuronal damage, which may contribute to cognitive impairments. Data from our laboratories have shown that, at 1 week after brain injury, mice possess significantly fewer neurons in all ipsilateral hippocampal subregions and a cognitive impairment. Since cognitive function is distributed across both cerebral hemispheres, the present paper explores the morphological and physiological response of the contralateral hippocampus to lateral brain injury. We analyzed the contralateral hippocampus using design-based stereology, Fluoro-Jade (FJ) histochemistry, and extracellular field recordings in mice at 7 and 30 days after lateral fluid percussion injury (FPI). At 7 days, all contralateral hippocampal subregions possess significantly fewer healthy neurons compared to sham-injured animals and demonstrate FJ-positive neuronal damage, but not at 30 days. Both the ipsilateral and contralateral dentate gyri demonstrate significantly increased excitability at 7 days post-injury, but only ipsilateral dentate gyrus hyperexcitability persists at 30 days compared to sham. In the contralateral hippocampus, the transient decrease in the number of healthy neurons, concomitant with FJ damage, and electrophysiological alterations establish a stunned period of cellular and circuit dysfunction. The return of healthy neuron number, absence of FJ damage, and sham level of excitability in the contralateral hippocampus suggest recovery of structure and function by 30 days after injury. The cognitive recovery observed after human traumatic brain injury may stem from a differential injury exposure and time course of recovery between homologous regions of the two hemispheres.

Sensorimotor and cognitive deficits after transient middle cerebral artery occlusion in the mouse

Whereas behavioral impairments after stroke are increasingly studied in the rat, little is known about the long-term functional consequences of focal ischemia in the mouse. To address this issue, Swiss mice underwent transient (60 min) intraluminal occlusion of the middle cerebral artery (MCAo) or sham surgery. Sensorimotor (chimney, accelerating rotarod, pole, corner, adhesive removal and staircase tests) and cognitive (passive avoidance and Morris water maze) performances were regularly assessed during 1 month, after which the final histological lesion was measured. Motor coordination and balance, assessed by the chimney and rotarod tests, were transiently altered by MCAo. Moreover, bradykinesia was evidenced by the pole test. The most striking and long-lasting (1 month) sensorimotor deficits were postural asymmetries on the corner test, bilateral skilled forepaw reaching deficits on the staircase test and a contralateral sensorimotor impairment on the adhesive removal test. MCAo animals showed normal spatial learning abilities on the Morris water maze test, but they displayed learning deficits measured by the passive avoidance test. This latter deficit was significantly correlated with both cortical and striatal damage. Our findings demonstrate the usefulness of three tests that had never been reported in the mouse after ischemia: the adhesive removal, staircase and pole tests, which showed deficits 1 month after ischemia and should therefore constitute meaningful tools in mice for assessing both neuroprotective and regenerative therapies in stroke preclinical studies.

Animal models of post-traumatic epilepsy

Epilepsy is a major unfavorable long-term consequence of traumatic brain injury (TBI). Moreover, TBI is one of the most important predisposing factors for the development of epilepsy, particularly in young adults. Understanding the molecular and cellular cascades that lead to the development of post-traumatic epilepsy (PTE) is key for preventing its development or modifying the disease process in such a way that epilepsy, if it develops, is milder and easier-to-treat. Tissue from TBI patients undergoing epileptogenesis is not available for such studies, which underscores the importance of developing clinically relevant animal models of PTE. The goal of this review is to (1) provide a description of PTE in humans, which is critical for the development of clinically relevant models of PTE, (2) review the characteristics of currently available PTE models, and (3) provide suggestions for the development of future models of PTE based on our current understanding of the mechanisms of TBI and epilepsy. The development of clinically relevant models of PTE is critical to advance our understanding of the mechanisms of post-traumatic epileptogenesis and epilepsy, as well as for producing breakthroughs in the development and testing of novel antiepileptogenic treatments.

Mild traumatic brain injury induces persistent cognitive deficits and behavioral disturbances in mice

Victims of mild traumatic brain injury (mTBI) do not show clear morphological brain defects, but frequently suffer from long-lasting cognitive deficits, emotional difficulties and behavioral disturbances. In the present study, we investigated the effects of experimental mTBI in mice on cognition, spatial and non-spatial tasks, and depressive-like behavior in mice. Experimental brain injury was induced using a concussive head trauma, which creates the TBI by a weight-drop device. Different groups of mice were tested at 7, 30, 60, and 90 days post-injury for cognitive function (the swim T-maze and the passive avoidance test) and for depression-like behavior (the forced swimming test). These tests have been used infrequently in the past in mTBI research. Significant differences were observed between the injured mice compared to the controls in both the swim T-maze (day 30: p < 0.001) and passive avoidance (day 30: p < 0.05) tests. In addition, a significant difference was detected in the forced swimming test between the injured mice and the controls (day 7: p < 0.05; day 90: p < 0.01), which showed a depressive- like state in the injured animals beginning 7 days post-injury. These results demonstrate that persistent deficits in these tests of cognitive learning abilities and emergence of depressive-like behavior in injured mice are similar to those reported in human post-concussion syndrome.

Diisopropylfluorophosphate administration in the pre-weanling period induces long-term changes in anxiety behavior and passive avoidance in adult mice

The developing brain may be particularly vulnerable to exposure to acetylcholinesterase (AChE) inhibitors because of the role of AChE on neuronal development and the effects of cholinergic pathways in mediating behavioral and hormonal responses to stress. C57BL/65 mice of both sexes were injected with 1 mg/kg s.c. diisopropylfluorophosphate (DFP) or saline in three separate experiments, on postnatal days (PNDs) 4-10, 14-20, or 30-36. Anxiety and conditioned avoidance were assessed on the elevated-plus maze (EPM) and step-down passive avoidance (PA) paradigms, respectively, at age 4-5 months. In addition, locomotion and reactivity to pain on the hot plate were assessed. Mice treated on PNDs 4-10 or PNDs 14-20 spent relatively more time and made more entries to the open arms on the first, but not second, exposure to the EPM. Females, but not males, treated with DFP showed deficits in PA retention after 24 h when treated on PNDs 4-10 and on PNDs 14-20. Mice treated on PNDs 30-36 were not impaired in either behavior. Administration of DFP in the preweanling period did not affect locomotor activity or pain reactivity. The results suggest that preweanling exposure to DFP results in anxiolysis in novel conflict situations but exacerbated context-enhanced anxiety.

Animal models of head trauma

Animal models of traumatic brain injury (TBI) are used to elucidate primary and secondary sequelae underlying human head injury in an effort to identify potential neuroprotective therapies for developing and adult brains. The choice of experimental model depends upon both the research goal and underlying objectives. The intrinsic ability to study injury-induced changes in behavior, physiology, metabolism, the blood/tissue interface, the blood brain barrier, and/or inflammatory- and immune-mediated responses, makes in vivo TBI models essential for neurotrauma research. Whereas human TBI is a highly complex multifactorial disorder, animal trauma models tend to replicate only single factors involved in the pathobiology of head injury using genetically well-defined inbred animals of a single sex. Although such an experimental approach is helpful to delineate key injury mechanisms, the simplicity and hence inability of animal models to reflect the complexity of clinical head injury may underlie the discrepancy between preclinical and clinical trials of neuroprotective therapeutics. Thus, a search continues for new animal models, which would more closely mimic the highly heterogeneous nature of human TBI, and address key factors in treatment optimization.

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.

Hippocalcin-deficient mice display a defect in cAMP response element-binding protein activation associated with impaired spatial and associative memory

Hippocalcin is a member of the neuronal calcium sensor (NCS) protein family that is highly expressed in hippocampal pyramidal cells and moderately expressed in the neurons of cerebral cortex, cerebellum and striatum. Here we examined the physiological roles of hippocalcin using targeted gene disruption. Hippocalcin-deficient (-/-) mice displayed no obvious structural abnormalities in the brain including hippocampal formation at the light microscopic level. Deletion of hippocalcin did not result in up-regulation of the hippocalcin-related proteins; neural visinin-like Ca(2+)-binding proteins (NVP) 1, 2, and 3. The synaptic excitability of hippocampal CA1 neurons appeared to be normal, as estimated by the shape of field excitatory postsynaptic potentials elicited by single- and paired-pulse stimuli, and by tetanic stimulation. However, N-methyl-d-aspartate stimulation- and depolarization-induced phosphorylation of cAMP-response element-binding protein (CREB) was significantly attenuated in -/- hippocampal neurons, suggesting an impairment in an activity-dependent gene expression cascade. In the Morris water maze test, the performance of -/- mice was comparable to that of wild-type littermates except in the probe test, where -/- mice crossed the previous location of the platform significantly less often than +/+ mice. Hippocalcin-deficient mice were also impaired on a discrimination learning task in which they needed to respond to a lamp illuminated on the left or right side to obtain food reinforcement. No abnormalities were observed in motor activity, anxiety behavior, or fear learning. These results suggest that hippocalcin plays a crucial role in the Ca(2+)-signaling pathway that underlies long-lasting neural plasticity and that leads to spatial and associative memory.

Mild head injury increasing the brain's vulnerability to a second concussive impact

OBJECT

Mild, traumatic repetitive head injury (RHI) leads to neurobehavioral impairment and is associated with the early onset of neurodegenerative disease. The authors developed an animal model to investigate the behavioral and pathological changes associated with RHI.

METHODS

Adult male C57BL/6 mice were subjected to a single injury (43 mice), repetitive injury (two injuries 24 hours apart; 49 mice), or no impact (36 mice). Cognitive function was assessed using the Morris water maze test, and neurological motor function was evaluated using a battery of neuroscore, rotarod, and rotating pole tests. The animals were also evaluated for cardiovascular changes, blood-brain barrier (BBB) breakdown, traumatic axonal injury, and neurodegenerative and histopathological changes between 1 day and 56 days after brain trauma. No cognitive dysfunction was detected in any group. The single-impact group showed mild impairment according to the neuroscore test at only 3 days postinjury, whereas RHI caused pronounced deficits at 3 days and 7 days following the second injury. Moreover, RHI led to functional impairment during the rotarod and rotating pole tests that was not observed in any animal after a single impact. Small areas of cortical BBB breakdown and axonal injury. observed after a single brain injury, were profoundly exacerbated after RHI. Immunohistochemical staining for microtubule-associated protein-2 revealed marked regional loss of immunoreactivity only in animals subjected to RHI. No deposits of beta-amyloid or tau were observed in any brain-injured animal.

CONCLUSIONS

On the basis of their results, the authors suggest that the brain has an increased vulnerability to a second traumatic insult for at least 24 hours following an initial episode of mild brain trauma.

Behavioral effects of protein deprivation and rehabilitation in adult rats: relevance to morphological alterations in the hippocampal formation

In the present study we have analyzed the behavioral and neuroanatomical effects of protein deprivation in adult rats. Starting at 2 months of age, animals were maintained on 8%-casein diet either for 8 months (malnourished group), or for 6 months followed by a 2-month period of nutritional rehabilitation (17%-protein diet, rehabilitated group). Malnourished rats exhibited reduced emotional reactivity and impaired habituation in the open field. In a water maze, these animals did not differ from controls during training, but showed retention deficits on the probe trial. However, working memory, sensorimotor abilities and passive avoidance behavior were not significantly impaired in malnourished rats. The performance of rehabilitated group was similar to that of the control group throughout behavioral testing. Postmortem morphological analysis revealed that the total number of neurons in the granular layer of the dentate gyrus, and in CA3 and CA1 hippocampal fields was reduced in protein-deprived and rehabilitated rats relative to controls. In addition, it was found that protein deprivation caused a 30% loss of synapses established between mossy fibers and dendrites of CA3 pyramidal cells, whereas nutritional rehabilitation resulted in a reversal of this effect. These results show that prolonged malnutrition in adult rats produces marked loss of hippocampal neurons and synapses accompanied by substantial impairments of hippocampal-dependent behaviors. The fact that nutritional rehabilitation results in restoration of the total number of hippocampal synapses and parallel amelioration of the behavioral impairments suggests that the mature CNS possesses a remarkable potential for structural and functional recovery from the damage induced by this type of dietary insult.

Experimental models of brain trauma

A short review of the most widely used and popular experimental models of traumatic brain injury is presented. This review focuses on current animal models of traumatic brain injury that apply mechanical energy to the skull or, after trephination of the skull, to the intact dura. Recent experimental studies evaluating the pathobiology of traumatic brain injury using these models are also discussed. This article attempts to provide a broad overview of current knowledge and controversies in experimental animal research on brain trauma.

Mild experimental brain injury differentially alters the expression of neurotrophin and neurotrophin receptor mRNAs in the hippocampus

The molecular events responsible for impairments in cognition following mild traumatic brain injury are poorly understood. Neurotrophins, such as brain-derived neurotrophic factor (BDNF), have been identified as having a role in learning and memory. We have previously demonstrated that following experimental brain trauma of moderate severity (2.0-2.1 atm), mRNA levels of BDNF and its high-affinity receptor, trkB, are increased bilaterally in the hippocampus for several hours, whereas NT-3 mRNA expression is decreased. In the present study, we used in situ hybridization to compare BDNF, trkB, NT-3, and trkC mRNA expression in rat hippocampus at 3 or 6 h after a lateral fluid percussion brain injury (FPI) of mild severity (1.0 atm) to sham-injured controls at equivalent time points. Mild FPI induced significant increases in hybridization levels for BDNF and trkB mRNAs, and a decrease in NT-3 mRNA in the hippocampus. However, in contrast to the bilateral effects of moderate experimental brain injury, the present changes with mild injury were restricted to the injured side. These findings demonstrate that even a mild traumatic brain injury differentially alters neurotrophin and neurotrophin receptor levels in the hippocampus. Such alterations may have important implications for neural plasticity and recovery of function in people who sustain a mild head injury.

Effects of mild traumatic brain injury on immunoreactivity for the inducible transcription factors c-Fos, c-Jun, JunB, and Krox-24 in cerebral regions associated with conditioned fear responding

We have previously demonstrated that mild traumatic brain injury (TBI) of the right parietal cortex results in a relatively selective deficit in conditioned fear responding. However, this behavioural deficit is very consistent and unrelated to the extent of the cortical necrotic lesion. We were therefore interested in determining if other brain regions might show a consistent response to mild TBI, and therefore, more reliably relate to the behavioural change. Increased expression of inducible transcription factors (ITFs) has been used to study which brain regions respond to a variety of events. In the present study, we examined the expression patterns of immunoreactivity (IR) for four ITFs (c-Fos, c-Jun, JunB, and Krox-24) at 3 h after mild fluid percussion TBI. Changes in ITF expression were only observed ipsilateral to the side of TBI. The clearest changes were observed in brain regions known to be involved in conditioned fear responding, such as the amygdala complex and hippocampal formation and several cortical regions. In contrast, no changes in IR for any of the ITFs were observed in the striatum, nucleus accumbens, nucleus basalis magnocellularis, septum or periacqueductal grey. Unlike the extent of visible damage to the cortex at the site of impact, the overexpression of ITFs showed a notable consistency between animals subjected to TBI. This consistency in regions known to be involved in conditioned fear responding (i.e., amygdala complex and hippocampal formation) lead us to suggest that it is these changes, rather than the more variable cortical necrotic lesion, that is responsible for the behavioural deficits we observe following mild TBI. Importantly, our results demonstrate that like the hippocampus, the amygdala is a sub-cortical structure particularly sensitive to the effects of mild brain trauma and underline the fact that cerebral regions distant from the location of the fluid impact can be affected.

Neuroprotective effect of eliprodil: attenuation of a conditioned freezing deficit induced by traumatic injury of the right parietal cortex in the rat

We have previously demonstrated that a lateral fluid percussion-induced traumatic lesion of the right parietal cortex can lead to a deficit in a conditioned freezing response and that this deficit can be attenuated by both pre- and postlesion administration of the NMDA receptor antagonist dizocilpine. In the present study, we investigated the effects of eliprodil, a noncompetitive NMDA receptor antagonist acting at the polyamine modulatory site, which also acts as a Ca2+ channel blocker, on the trauma-induced conditioned freezing deficit. Eliprodil produced a 50% reduction in this deficit when administered as three 1 mg/kg injections i.v. at 15 min, 6 h, and 24 h following the lesion. Approximately the same degree of protection was afforded when 2 x 1.5 mg/kg were administered 6 and 24 h and equally at 12 and 24 h after surgery (56% and 59%, respectively). A single treatment (3 mg/kg) at 24 h was ineffective against the deficit. The protection afforded with treatment at 6 and 24 h after lesion was dose dependent, with a minimal active dose of 2 x 0.75 mg/kg. These data complement those previously published on the ability of eliprodil to reduce lesion volume following traumatic brain injury and show, in addition, that the neuroprotective effect has functional consequences.

Mild traumatic lesion of the right parietal cortex in the rat: characterisation of a conditioned freezing deficit and its reversal by dizocilpine

We have previously demonstrated that traumatic injury of the lateral aspect of the right parietal cortex results in reduced acquisition of the passive avoidance task but enhanced learning in an active avoidance procedure. In order to try to explain the apparent dichotomy between these findings a series of experiments examined the effect of fluid percussion-induced traumatic brain injury (FP-TBI) on the conditioned freezing response to a context previously paired with an aversive stimulus. Rats subjected to FP-TBI displayed less conditioned freezing than the sham-operated controls. This effect was particularly marked when the delay between context exposure and footshock was short (< or = 30 s) and was no longer significant when this delay was 3 min, indicating that the injured animals did not have an impaired freezing response per se. This phenomenon was enduring such that it could still be observed 2 months following the surgery. There was no significant freezing deficit after FP-TBI of the motor cortex, demonstrating that the site of injury is important and that the freezing deficit is not a general response to CNS trauma. The NMDA receptor antagonist dizocilpine (MK-801, 1 mg/kg i.v.) significantly reduced the trauma-induced freezing deficit when administered as a single bolus 15 min prior to the surgery, or as three repeated treatments (3 x 0.33 mg/kg) 15 min, and 6 and 24 h following lesion. The trauma-induced deficit in conditioned freezing can explain the differences in active and passive avoidance behaviours and appears to be specific to lesion of the lateral parietal cortex. In addition, the behavioural deficit can be attenuated using the neuroprotective agent dizocilpine, suggesting that it may prove useful as a sensitive and specific measure of cortical damage following traumatic injury.