Spatial and temporal characteristics of neurodegeneration after controlled cortical impact in mice: more than a focal brain injury

Progesterone: Therapeutic opportunities for neuroprotection and myelin repair

Progesterone and its metabolites promote the viability of neurons in the brain and spinal cord. Their neuroprotective effects have been documented in different lesion models, including traumatic brain injury (TBI), experimentally induced ischemia, spinal cord lesions and a genetic model of motoneuron disease. Progesterone plays an important role in developmental myelination and in myelin repair, and the aging nervous system appears to remain sensitive to some of progesterone's beneficial effects. Thus, the hormone may promote neuroregeneration by several different actions by reducing inflammation, swelling and apoptosis, thereby increasing the survival of neurons, and by promoting the formation of new myelin sheaths. Recognition of the important pleiotropic effects of progesterone opens novel perspectives for the treatment of brain lesions and diseases of the nervous system. Over the last decade, there have been a growing number of studies showing that exogenous administration of progesterone or some of its metabolites can be successfully used to treat traumatic brain and spinal cord injury, as well as ischemic stroke. Progesterone can also be synthesized by neurons and by glial cells within the nervous system. This finding opens the way for a promising therapeutic strategy, the use of pharmacological agents, such as ligands of the translocator protein (18 kDa) (TSPO; the former peripheral benzodiazepine receptor or PBR), to locally increase the synthesis of steroids with neuroprotective and neuroregenerative properties. A concept is emerging that progesterone may exert different actions and use different signaling mechanisms in normal and injured neural tissue.

Proteomic identification of oxidized mitochondrial proteins following experimental traumatic brain injury

Experimental traumatic brain injury (TBI) results in a significant loss of cortical tissue at the site of injury, and in the ensuing hours and days a secondary injury exacerbates this primary injury, resulting in significant neurological dysfunction. The mechanism of the secondary injury is not well understood, but evidence implicates a critical role for mitochondria in this cascade. This mitochondrial dysfunction is believed to involve excitotoxicity, disruption of Ca(2+) homeostasis, production of reactive oxygen species (ROS), ATP depletion, oxidative damage of mitochondrial proteins, and an overall breakdown of mitochondrial bioenergetics. Although oxidative damage occurs following TBI, the identities of proteins undergoing oxidative modification after TBI have not been investigated. In the present study, we utilized the 3-h post-injury controlled cortical impact model of experimental TBI in 20 young adult male Sprague-Dawley rats, coupled with proteomics to identify specific mitochondrial fraction proteins from the cortex and hippocampus that were oxidatively modified after TBI. We identified, from the cortex, pyruvate dehydrogenase, voltage-dependent anion channel, fumarate hydratase 1, ATP synthase, and prohibitin. From the hippocampus, we identified cytochrome C oxidase Va, isovaleryl coenzyme A dehydrogenase, enolase-1, and glyceraldehyde-3-phosphate dehydrogenase as proteins that had undergone oxidative modification following TBI. In addition, we have also shown that, following TBI, there is a reduction in the activities of pyruvate dehydrogenase (PDH), complex I, and complex IV. These findings demonstrate that, following TBI, several proteins involved in mitochondrial bioenergetics are highly oxidatively modified, which may possibly underlie the massive breakdown of mitochondrial energetics and eventual cell death known to occur in this model. The identification of these proteins provides new insights into the mechanisms that take place following TBI and may provide avenues for possible therapeutic interventions after TBI.

Clinical significance of alphaII-spectrin breakdown products in cerebrospinal fluid after severe traumatic brain injury

Following traumatic brain injury (TBI), the cytoskeletal protein alpha-II-spectrin is proteolyzed by calpain and caspase-3 to signature breakdown products. To determine whether alpha -II-spectrin proteolysis is a potentially reliable biomarker for TBI in humans, the present study (1) examined levels of spectrin breakdown products (SBDPs) in cerebrospinal fluid (CSF) from adults with severe TBI and (2) examined the relationship between these levels, severity of injury, and clinical outcome. This prospective case control study enrolled 41 patients with severe TBI, defined by a Glasgow Coma Scale (GCS) score of < or =8, who underwent intraventricular intracranial pressure monitoring. Patients without TBI requiring CSF drainage for other medical reasons served as controls. Ventricular CSF was sampled from each patient at 6, 12, 24, 48, 72, 96, and 120 h following TBI and analyzed for SBDPs. Outcome was assessed using the Glasgow Outcome Score (GOS) 6 months after injury. Calpain and caspase-3 mediated SBDP levels in CSF were significantly increased in TBI patients at several time points after injury, compared to control subjects. The time course of calpain mediated SBDP150 and SBDP145 differed from that of caspase-3 mediated SBDP120 during the post-injury period examined. Mean SBDP densitometry values measured early after injury correlated with severity of injury, computed tomography (CT) scan findings, and outcome at 6 months post-injury. Taken together, these results support that alpha -II-spectrin breakdown products are potentially useful biomarker of severe TBI in humans. Our data further suggests that both necrotic/oncotic and apoptotic cell death mechanisms are activated in humans following severe TBI, but with a different time course after injury.

Electromagnetic controlled cortical impact device for precise, graded experimental traumatic brain injury

Genetically modified mice represent useful tools for traumatic brain injury (TBI) research and attractive preclinical models for the development of novel therapeutics. Experimental methods that minimize the number of mice needed may increase the pace of discovery. With this in mind, we developed and characterized a prototype electromagnetic (EM) controlled cortical impact device along with refined surgical and behavioral testing techniques. By varying the depth of impact between 1.0 and 3.0 mm, we found that the EM device was capable of producing a broad range of injury severities. Histologically, 2.0-mm impact depth injuries produced by the EM device were similar to 1.0-mm impact depth injuries produced by a commercially available pneumatic device. Behaviorally, 2.0-, 2.5-, and 3.0-mm impacts impaired hidden platform and probe trial water maze performance, whereas 1.5-mm impacts did not. Rotorod and visible platform water maze deficits were also found following 2.5- and 3.0-mm impacts. No impairment of conditioned fear performance was detected. No differences were found between sexes of mice. Inter-operator reliability was very good. Behaviorally, we found that we could statistically distinguish between injury depths differing by 0.5 mm using 12 mice per group and between injury depths differing by 1.0 mm with 7-8 mice per group. Thus, the EM impactor and refined surgical and behavioral testing techniques may offer a reliable and convenient framework for preclinical TBI research involving mice.

Novel perspectives for progesterone in hormone replacement therapy, with special reference to the nervous system

The utility and safety of postmenopausal hormone replacement therapy has recently been put into question by large clinical trials. Their outcome has been extensively commented upon, but discussions have mainly been limited to the effects of estrogens. In fact, progestagens are generally only considered with respect to their usefulness in preventing estrogen stimulation of uterine hyperplasia and malignancy. In addition, various risks have been attributed to progestagens and their omission from hormone replacement therapy has been considered, but this may underestimate their potential benefits and therapeutic promises. A major reason for the controversial reputation of progestagens is that they are generally considered as a single class. Moreover, the term progesterone is often used as a generic one for the different types of both natural and synthetic progestagens. This is not appropriate because natural progesterone has properties very distinct from the synthetic progestins. Within the nervous system, the neuroprotective and promyelinating effects of progesterone are promising, not only for preventing but also for reversing age-dependent changes and dysfunctions. There is indeed strong evidence that the aging nervous system remains at least to some extent sensitive to these beneficial effects of progesterone. The actions of progesterone in peripheral target tissues including breast, blood vessels, and bones are less well understood, but there is evidence for the beneficial effects of progesterone. The variety of signaling mechanisms of progesterone offers exciting possibilities for the development of more selective, efficient, and safe progestagens. The recognition that progesterone is synthesized by neurons and glial cells requires a reevaluation of hormonal aging.

Temporal relationship of peroxynitrite-induced oxidative damage, calpain-mediated cytoskeletal degradation and neurodegeneration after traumatic brain injury

We assessed the temporal and spatial characteristics of PN-induced oxidative damage and its relationship to calpain-mediated cytoskeletal degradation and neurodegeneration in a severe unilateral controlled cortical impact (CCI) traumatic brain injury (TBI) model. Quantitative temporal time course studies were performed to measure two oxidative damage markers: 3-nitrotyrosine (3NT) and 4-hydroxynonenal (4HNE) at 30 min, 1, 3, 6, 12, 24, 48, 72 h and 7 days after injury in ipsilateral cortex of young adult male CF-1 mice. Secondly, the time course of Ca(++)-activated, calpain-mediated proteolysis was also analyzed using quantitative western-blot measurement of breakdown products of the cytoskeletal protein alpha-spectrin. Finally, the time course of neurodegeneration was examined using de Olmos silver staining. Both oxidative damage markers increased in cortical tissue immediately after injury (30 min) and elevated for the first 3-6 h before returning to baseline. In the immunostaining study, the PN-selective marker, 3NT, and the lipid peroxidation marker, 4HNE, were intense and overlapping in the injured cortical tissue. alpha-Spectrin breakdown products, which were used as biomarker for calpain-mediated cytoskeletal degradation, were also increased after injury, but the time course lagged behind the peak of oxidative damage and did not reach its maximum until 24 h post-injury. In turn, cytoskeletal degradation preceded the peak of neurodegeneration which occurred at 48 h post-injury. These studies have led us to the hypothesis that PN-mediated oxidative damage is an early event that contributes to a compromise of Ca(++) homeostatic mechanisms which causes a massive Ca(++) overload and calpain activation which is a final common pathway that results in post-traumatic neurodegeneration.

Detection of traumatic axonal injury with diffusion tensor imaging in a mouse model of traumatic brain injury

Traumatic axonal injury (TAI) is thought to be a major contributor to cognitive dysfunction following traumatic brain injury (TBI), however TAI is difficult to diagnose or characterize non-invasively. Diffusion tensor imaging (DTI) has shown promise in detecting TAI, but direct comparison to histologically-confirmed axonal injury has not been performed. In the current study, mice were imaged with DTI, subjected to a moderate cortical controlled impact injury, and re-imaged 4-6 h and 24 h post-injury. Axonal injury was detected by amyloid beta precursor protein (APP) and neurofilament immunohistochemistry in pericontusional white matter tracts. The severity of axonal injury was quantified using stereological methods from APP stained histological sections. Two DTI parameters--axial diffusivity and relative anisotropy--were significantly reduced in the injured, pericontusional corpus callosum and external capsule, while no significant changes were seen with conventional MRI in these regions. The contusion was easily detectable on all MRI sequences. Significant correlations were found between changes in relative anisotropy and the density of APP stained axons across mice and across subregions spanning the spatial gradient of injury. The predictive value of DTI was tested using a region with DTI changes (hippocampal commissure) and a region without DTI changes (anterior commissure). Consistent with DTI predictions, there was histological detection of axonal injury in the hippocampal commissure and none in the anterior commissure. These results demonstrate that DTI is able to detect axonal injury, and support the hypothesis that DTI may be more sensitive than conventional imaging methods for this purpose.

Role of peroxynitrite in secondary oxidative damage after spinal cord injury

Peroxynitrite (PON, ONOO(-)), formed by nitric oxide synthase-generated nitric oxide radical ( NO) and superoxide radical (O(2) (-)), is a crucial player in post-traumatic oxidative damage. In the present study, we determined the spatial and temporal characteristics of PON-derived oxidative damage after a moderate contusion injury in rats. Our results showed that 3-nitrotyrosine (3-NT), a specific marker for PON, rapidly accumulated at early time points (1 and 3 h) and a significant increase compared with sham rats was sustained to 1 week after injury. Additionally, there was a coincident and maintained increase in the levels of protein oxidation-related protein carbonyl and lipid peroxidation-derived 4-hydroxynonenal (4-HNE). The peak increases of 3-NT and 4-HNE were observed at 24 h post-injury. In our immunohistochemical results, the co-localization of 3-NT and 4-HNE results indicates that PON is involved in lipid peroxidative as well as protein nitrative damage. One of the consequences of oxidative damage is an exacerbation of intracellular calcium overload, which activates the cysteine protease calpain leading to the degradation of several cellular targets including cytoskeletal protein (alpha-spectrin). Western blot analysis of alpha-spectrin breakdown products showed that the 145-kDa fragments of alpha-spectrin, which are specifically generated by calpain, were significantly increased as soon as 1 h following injury although the peak increase did not occur until 72 h post-injury. The later activation of calpain is most likely linked to PON-mediated secondary oxidative impairment of calcium homeostasis. Scavengers of PON, or its derived free radical species, may provide an improved antioxidant neuroprotective approach for the treatment of post-traumatic oxidative damage in the injured spinal cord.

Lasting Neuroendocrine-Immune Effects of Traumatic Brain Injury in Rats

Traumatic brain injury (TBI) is a principal cause of long-term physical, cognitive, behavioral, and social deficits in young adults, which frequently coexist with a high incidence of substance abuse disorders. However, few studies have examined the long-term effects of TBI on the neuroendocrine-immune system. TBI was induced in adult male rats under isoflurane anesthesia by cortical contusion injury with a pneumatic piston positioned stereotaxically over the left parietal cortex. Controls underwent sham surgery without injury. At 4 weeks post-injury, the plasma corticosterone response to 30-min restraint stress was significantly blunted in TBI rats compared to the sham controls. One week later, transmitters were implanted for continuous biotelemetric recording of body temperature and spontaneous locomotor activity. At 6 weeks post-injury, the febrile response to i.p. injection of the bacterial endotoxin, lipopolysaccharide (LPS; 50 microg/kg), was significantly lower in TBI than in sham rats. At 8 weeks, swimming in the forced swim test was significantly less in TBI than sham rats. At 9 weeks, rats were rendered ethanol (EtOH) dependent by feeding an EtOH-containing liquid diet for 14 days. Cosine rhythmometry analysis of circadian body temperature Midline Estimating Statistic of Rhythm (MESOR), amplitudes, and acrophases indicated differential effects of EtOH and withdrawal in the two groups. Light- and dark-phase activity analysis indicated that TBI rats were significantly more active than the sham group, and that EtOH and withdrawal differentially affected their activity. Given the extensive interactions of the neuroendocrine-immune systems, these results demonstrate that TBI produces lasting dysregulation amidst the central substrates for allostasis and circadian rhythmicity.

Identification and characterization of PEBP as a calpain substrate

Calpains are calcium- and thiol-dependent proteases whose dysregulation has been implicated in a number of diseases and conditions such as cardiovascular dysfunction, ischemic stroke, and Alzheimer's disease (AD). While the effects of calpain activity are evident, the precise mechanism(s) by which dysregulated calpain activity results in cellular degeneration are less clear. In order to determine the impact of calpain activity, there is a need to identify the range of specific calpain substrates. Using an in vitro proteomics approach we confirmed that phosphatidylethanolamine-binding protein (PEBP) as a novel in vitro and in situ calpain substrate. We also observed PEBP proteolysis in a model of brain injury in which calpain is clearly activated. In addition, with evidence of calpain dysregulation in AD, we quantitated protein levels of PEBP in postmortem brain samples from the hippocampus of AD and age-matched controls and found that PEBP levels were approximately 20% greater in AD. Finally, with previous evidence that PEBP may act as a serine protease inhibitor, we tested PEBP as an inhibitor of the proteasome and found that PEBP inhibited the chymostrypsin-like activity of the proteasome by approximately 30%. Together these data identify PEBP as a potential in vivo calpain substrate and indicate that increased PEBP levels may contribute to impaired proteasome function.

Alpha-II-spectrin after controlled cortical impact in the immature rat brain

Proteolytic processing plays an important role in regulating a wide range of important cellular functions, including processing of cytoskeletal proteins. Loss of cytoskeletal proteins such as spectrin is an important characteristic in a variety of acute central nervous system injuries including ischemia, spinal cord injury and traumatic brain injury (TBI). The literature contains extensive information on the proteolytic degradation of alpha-II-spectrin after TBI in the adult brain. By contrast, there is limited knowledge on the characteristics and relevance of these important processes in the immature brain. The present experiments examine TBI-induced proteolytic processing of alpha-II-spectrin after TBI in the immature rat brain. Distinct proteolytic products resulting from the degradation of the cytoskeletal protein alpha-II-spectrin by calpain and caspase 3 were readily detectable in cortical brain parenchyma and cerebrospinal fluid after TBI in immature rats.

Time course of post-traumatic mitochondrial oxidative damage and dysfunction in a mouse model of focal traumatic brain injury: implications for neuroprotective therapy

In the present study, we investigate the hypothesis that mitochondrial oxidative damage and dysfunction precede the onset of neuronal loss after controlled cortical impact traumatic brain injury (TBI) in mice. Accordingly, we evaluated the time course of post-traumatic mitochondrial dysfunction in the injured cortex and hippocampus at 30 mins, 1, 3, 6, 12, 24, 48, and 72 h after severe TBI. A significant decrease in the coupling of the electron transport system with oxidative phosphorylation was observed as early as 30 mins after injury, followed by a recovery to baseline at 1 h after injury. A statistically significant (P<0.0001) decline in the respiratory control ratio was noted at 3 h, which persisted at all subsequent time-points up to 72 h after injury in both cortical and hippocampal mitochondria. Structural damage seen in purified cortical mitochondria included severely swollen mitochondria, a disruption of the cristae and rupture of outer membranes, indicative of mitochondrial permeability transition. Consistent with this finding, cortical mitochondrial calcium-buffering capacity was severely compromised by 3 h after injury, and accompanied by significant increases in mitochondrial protein oxidation and lipid peroxidation. A possible causative role for reactive nitrogen species was suggested by the rapid increase in cortical mitochondrial 3-nitrotyrosine levels shown as early as 30 mins after injury. These findings indicate that post-traumatic oxidative lipid and protein damage, mediated in part by peroxynitrite, occurs in mitochondria with concomitant ultrastructural damage and impairment of mitochondrial bioenergetics. The data also indicate that compounds which specifically scavenge peroxynitrite (ONOO(-)) or ONOO(-)-derived radicals (e.g. ONOO(-)+H(+) --> ONOOH --> (*)NO(2)+(*)OH) may be particularly effective for the treatment of TBI, although the therapeutic window for this neuroprotective approach might only be 3 h.

Heavy neurofilament accumulation and alpha-spectrin degradation accompany cerebellar white matter functional deficits following forebrain fluid percussion injury

Evidence for diffuse traumatic axonal injury (TAI) in clinical cases and animal models of traumatic brain injury (TBI) indicate that pathophysiological mechanisms extend to regions remote from the injury epicenter. The potential for indirect cerebellar trauma contributing to TBI pathophysiology is of significance since impairment of motor function and coordination is a common consequence of TBI but is also a domain associated with cerebellar function. The relationship between cerebellar white matter structure and function following traumatic head injury has not been examined. Using the fluid percussion injury (FPI) device applied unilaterally in the forebrain, evoked compound action potential (CAP) recordings from cerebellar white matter of Sprague-Dawley rats indicated a spatial and temporal pattern of electrophysiological deficits throughout the cerebellar vermis. The posterior and middle lobules of the cerebellum exhibited significant declines in evoked CAP amplitude compared to sham controls (p=0.004, p=0.005, respectively). Duration of the CAP decay also increased, suggesting that functional white matter deficits were a combination of axonal loss and compromised axonal integrity. Functional white matter deficits persisted at 14 days post-injury in the posterior and middle regions of the cerebellum. Evidence of heavy chain neurofilament (NF200) degradation was observed at 1 day post-injury by Western blot. Immunohistochemistry labeling for NF200 indicated the presence of highly immunoreactive NF200 axonal swellings consistent with morphological features of TAI. alpha-Spectrin degradation was also observed between 1 and 14 days post-injury. This study demonstrates the electrophysiological consequences of cerebellar white matter injury and a temporal profile of NF200 and spectrin degradation following forebrain FPI.

A mouse model of sensorimotor controlled cortical impact: characterization using longitudinal magnetic resonance imaging, behavioral assessments and histology

The present study establishes a new mouse model for traumatic brain injury (TBI), using an electromechanically driven linear motor impactor device to deliver a lateral controlled cortical impact (CCI) injury to the sensorimotor cortex. Lesion cavity size was measured, and inter-animal consistency demonstrated, at 14 days post injury. Qualitative information regarding damage progression over time was obtained by scanning with high field magnetic resonance imaging (MRI) at five time points following injury. Functional impairment and recovery were measured with the Rotarod, gridwalk and cylinder tests, and lesion cavity volume was measured post mortem with thionin-stained tissue sections. The study establishes the reliability of a linear-motor based device for producing repeatable damage in a CCI model, demonstrates the power of longitudinal MRI in studying damage evolution, and confirms that a simple battery of functional tests record sensorimotor impairment and recovery.

Injury severity determines Purkinje cell loss and microglial activation in the cerebellum after cortical contusion injury

Clinical evidence suggests that the cerebellum is damaged after traumatic brain injury (TBI) and experimental studies have validated these observations. We have previously shown cerebellar vulnerability, as demonstrated by Purkinje cell loss and microglial activation, after fluid percussion brain injury. In this study, we examine the effect of graded controlled cortical impact (CCI) injury on the cerebellum in the context of physiologic and anatomical parameters that have been shown by others to be sensitive to injury severity. Adult male rats received mild, moderate, or severe CCI and were euthanized 7 days later. We first validated the severity of the initial injury using physiologic criteria, including apnea and blood pressure, during the immediate postinjury period. Increasing injury severity was associated with an increased incidence of apnea and higher mortality. Severe injury also induced transient hypertension followed by hypotension, while lower grade injuries produced an immediate and sustained hypotension. We next evaluated the pattern of subcortical neuronal loss in response to graded injuries. There was significant neuronal loss in the ipsilateral cortex, hippocampal CA2/CA3, and laterodorsal thalamus that was injury severity-dependent and that paralleled microglial activation. Similarly, there was a distinctive pattern of Purkinje cell loss and microglial activation in the cerebellar vermis that varied with injury severity. Together, these findings emphasize the vulnerability of the cerebellum to TBI. That a selective pattern of Purkinje cell loss occurs regardless of the type of injury suggests a generalized response that is a likely determinant of recovery and a target for therapeutic intervention.

Hippocampal vulnerability following traumatic brain injury: a potential role for neurotrophin-4/5 in pyramidal cell neuroprotection

Traumatic brain injury (TBI) causes selective hippocampal cell death, which is believed to be associated with cognitive impairment observed both in clinical and experimental settings. Although neurotrophin administration has been tested as a strategy to prevent cell death following TBI, the potential neuroprotective role of neurotrophin-4/5 (NT-4/5) in TBI remains unknown. We hypothesized that NT-4/5 would offer neuroprotection for selectively vulnerable hippocampal neurons following TBI. Measurements of NT-4/5 in rats subjected to lateral fluid percussion (LFP) TBI revealed two-threefold increases in the injured cortex and hippocampus in the acute period (1-3 days) following brain injury. Subsequently, the response of NT-4/5 knockout (NT-4/5(-/-)) mice to controlled-cortical impact TBI was investigated. NT-4/5(-/-) mice were more susceptible to selective pyramidal cell loss in Ahmon's corn (CA) subfields of the hippocampus following TBI, and showed impaired motor recovery when compared with their brain-injured wild-type controls (NT-4/5(wt)). Additionally, we show that acute, prolonged administration of recombinant NT-4/5 (5 microg/kg/day) prevented up to 50% of the hippocampal CA pyramidal cell death following LFP TBI in rats. These results suggest that post-traumatic increases in endogenous NT-4/5 may be part of an adaptive neuroprotective response in the injured brain, and that administration of this neurotrophic factor may be useful as a therapeutic strategy following TBI.

Oxidative stress in head trauma in aging

Oxidative damage is proposed as a key mediator of exacerbated morphological responses and deficits in behavioral recovery in aged subjects with traumatic brain injury (TBI). In the present study, we show exacerbated loss of tissue in middle aged (12 months) and aged (22 months) Fisher-344 rats compared to young animals (3 months) subjected to moderate TBI. Analysis of 4-hydroxynonenal (4-HNE) and acrolein, neurotoxic by-products of lipid peroxidation, shows significant (P < 0.05) age-dependent increases in ipsilateral (IP) hippocampus 1 and 7 days post injury. In IP cortex, 4-HNE was significantly elevated 1 day post injury in all age groups, and both 4-HNE and acrolein were elevated in middle aged and aged animals 7 days post injury. Comparison of antioxidant enzyme activities shows significant (P < 0.05) age-dependent decreases of manganese superoxide dismutase in IP hippocampus and cortex 1 and 7 days post injury. Glutathione reductase activity also showed an age-dependent decrease. Overall, our data show increased levels of oxidative damage, diminished antioxidant capacities, and increased tissue loss in TBI in aging.

Relationship of calpain-mediated proteolysis to the expression of axonal and synaptic plasticity markers following traumatic brain injury in mice

The role of neuronal plasticity and repair on the final functional outcome following traumatic brain injury (TBI) remains poorly understood. Moreover, the relationship of the magnitude of post-traumatic secondary injury and neurodegeneration to the potential for neuronal repair has not been explored. To address these questions, we employed Western immunoblotting techniques to examine how injury severity affects the spatial and temporal expression of markers of axonal growth (growth-associated protein GAP-43) and synaptogenesis (pre-synaptic vesicular protein synaptophysin) following either moderate (0.5 mm, 3.5 M/s) or severe (1.0 mm, 3.5 M/s) lateral controlled cortical impact traumatic brain injury (CCI-TBI) in young adult male CF-1 mice. Moderate CCI increased GAP-43 levels at 24 and 48 h post-insult in the ipsilateral hippocampus relative to sham, non-injured animals. This increase in axonal plasticity occurred prior to maximal hippocampal neurodegeneration, as revealed by de Olmos silver staining, at 72 h. However, moderate CCI-TBI did not elevate GAP-43 expression in the ipsilateral cortex where neurodegeneration was extensive by 6 h post-TBI. In contrast to moderate injury, severe CCI-TBI failed to increase hippocampal GAP-43 levels and instead resulted in depressed GAP-43 expression in the ipsilateral hippocampus and cortex at 48 h post-insult. In regards to injury-induced changes in synaptogenesis, we found that moderate CCI-TBI elevated synaptophysin levels in the ipsilateral hippocampus at 24, 48, 72 h and 21 days, but this effect was not present after severe injury. Together, these data highlights the adult brain's ability for axonal and synaptic plasticity following a focal cortical injury, but that severe injuries may diminish these endogenous repair mechanisms. The differential effects of moderate versus severe TBI on the post-traumatic plasticity response may be related to the calpain-mediated proteolytic activity occurring after a severe injury preventing increased expression of proteins required for plasticity. Supporting this hypothesis is the fact that GAP-43 is a substrate for calpain along with our data demonstrating that calpain-mediated degradation of the cytoskeletal protein, alpha-spectrin, is approximately 10 times greater in ipsilateral hippocampal tissue following severe compared to moderate CCI-TBI. Thus, TBI severity has a differential effect on the injury-induced neurorestorative response with calpain activation being one putative factor contributing to neuroregenerative failure following severe CCI-TBI. If true, then calpain inhibition may lead to both neuroprotective effects and an enhancement of neuronal plasticity/repair mechanisms post-TBI.

Purkinje cell vulnerability to mild and severe forebrain head trauma

Pathophysiological changes in the cortex, thalamus, and hippocampus have been implicated as contributors to motor and cognitive deficits in a number of animal models of traumatic brain injury (TBI). Indirect cerebellar injury may contribute to TBI pathophysiology because impairment of motor function and coordination are common consequences of TBI, but are also domains associated with cerebellar function. However, there is a lack of direct evidence to support this claim. Hence, in this study, a dose-response relationship of the cerebellum's susceptibility was determined at four grades of fluid percussion injury (1.5, 2.0, 2.5, and 3.0 atm) applied in the right lateral cerebral cortex of adult male Sprague-Dawley rats. Evidence suggests primary and secondary injury mechanisms resulting in selective cerebellar Purkinje neuron (PN) loss, whereas interneurons of the molecular layer were spared. The posterior region of the cerebellar vermis displayed significant PN loss (p = 0.001) at 1 day postinjury, whereas the gyrus of the horizontal fissure and gyrus of lobules III and IV exhibited delayed PN loss at higher levels of injury severity. Interestingly, neither terminal deoxynucleotidyl transferase biotin-dUTP nick end labeling (TUNEL) or cleaved caspase-3 colocalized with PNs at any time point or injury severity. Expression of calbindin-28k increased in regions of greatest PN loss, suggesting that the surviving PNs possess higher calcium-buffering capacities, which may account for their survival.

Differential regulation of the CXCR2 chemokine network in rat brain trauma: Implications for neuroimmune interactions and neuronal survival

Chemokine receptors represent promising targets to attenuate inflammatory responses and subsequent secondary damage after brain injury. We studied the response of the chemokines CXCL1/CINC-1 and CXCL2/MIP-2 and their receptors CXCR1 and CXCR2 after controlled cortical impact injury in adult rats. Rapid upregulation of CXCL1/CINC-1 and CXCL2/MIP-2, followed by CXCR2 (but not CXCR1), was observed after injury. Constitutive neuronal CXCR2 immunoreactivity was detected in several brain areas, which rapidly but transiently downregulated upon trauma. A second CXCR2-positive compartment, mainly colocalized with the activated microglia/macrophage marker ED1, was detected rapidly after injury in the ipsilateral cortex, progressively emerging into deeper areas of the brain later in time. It is proposed that CXCR2 has a dual role after brain injury: (i) homologous neuronal CXCR2 downregulation would render neurons more vulnerable to injury, whereas (ii) chemotaxis and subsequent differentiation of blood-borne cells into a microglial-like phenotype would be promoted by the same receptor.

The pathophysiology of traumatic brain injury in α7 nicotinic cholinergic receptor knockout mice

The alpha7 nicotinic cholinergic receptor is a ligand-gated ion channel with calcium permeability similar to that of ionotrophic glutamate receptors. Previous studies from our laboratory have implicated changes in expression alpha7 nicotinic cholinergic receptors in the pathophysiology of traumatic brain injury (TBI). In rats, TBI causes a time-dependent and significant decrease in cortical and hippocampal alpha-[(125)I]-bungarotoxin (BTX) binding. We have postulated that deficits in alpha7 expression may contribute to TBI-induced cognitive impairment and that nicotinic receptor agonists can reverse alpha7 binding deficits and result in significant cognitive improvement compared to saline-treated controls. Thus, alpha7 nAChRs could be involved in a form of cholinergically mediated excitotoxicity following brain injury. In the current study, wild-type, heterozygous and null mutant mice were employed to test the hypothesis that genotypic depletion of the alpha7 receptor would render animals less sensitive to tissue loss and brain inflammation following experimental brain injury. Mice were anesthetized and subjected to a 0.5-mm cortical contusion injury of the somatosensory cortex. Brain inflammation, changes in nicotinic receptor expression and cortical tissue sparing were evaluated in wild-type, heterozygous and homozygous mice 1 week following TBI. In wild-type mice, brain injury caused a significant decrease in BTX binding in several hippocampal regions, consistent with what we have measured in rat brain following TBI. However, there were no genotypic differences in cortical tissue sparing or brain inflammation in this experiment. Although the results of this study were largely negative, it is still plausible that changes in the activity/expression of native alpha7 receptors contribute to pathophysiology following TBI. However, when null mutant mice develop in the absence of central alpha7 expression, it is possible that compensatory changes occur that confound the results obtained.

Quantitative analysis of the relationship between intra- axonal neurofilament compaction and impaired axonal transport following diffuse traumatic brain injury

Traumatic axonal injury (TAI) following traumatic brain injury (TBI) contributes to morbidity and mortality. TAI involves intra-axonal changes assumed to progress to impaired axonal transport (IAT), disconnection, and axonal bulb formation. Immunocytochemical studies employing antibodies to amyloid precursor protein (APP), a marker of IAT and RMO14, a marker of neurofilament compaction (NFC), have shown that TAI involves both NFC and IAT, with the suggestion that NFC leads to IAT. Recently, new data has suggested that NFC may occur independently of IAT. The objective of this study was to determine quantitatively the precise relationship between NFC and IAT. Following TBI, rats were studied at 30 min, 3 h, and 24 h. Using single-label immunocytochemistry employing the antibodies RM014, APP, or a combined labeling strategy targeting APP/RMO14 in aggregate, the immunoreactive (IR) profiles were counted in the corticospinal tract (CSpT) and medial lemniscus (ML). In the CSpT, the number of axons demonstrating RMO14-IR approximated the number of axons showing APP-IR, with the APP-IR population showing a significant increase over 24 h (p < 0.05). The sum of both single-label counts equaled the aggregate APP/RMO14 numbers, demonstrating little relationship between NFC and IAT. In the ML, 75% of fibers demonstrated a separation of APP-IR and NFC-IR; however, 25% of the ML fibers showed co-localization of APP-IR and RMO14. The results of these studies indicate that, in the majority of damaged axons, NFC is not associated with IAT. Our findings argue for the use of multiple markers when evaluating the extent of TAI or the efficacy of therapies targeting the treatment of TAI.

Physiologic progesterone reduces mitochondrial dysfunction and hippocampal cell loss after traumatic brain injury in female rats

Growing literature suggests important sex-based differences in outcome following traumatic brain injury (TBI) in animals and humans. Progesterone has emerged as a key hormone involved in many potential neuroprotective pathways after acute brain injury and may be responsible for some of these differences. Many studies have utilized supraphysiologic levels of post-traumatic progesterone to reverse pathologic processes after TBI, but few studies have focused on the role of endogenous physiologic levels of progesterone in neuroprotection. We hypothesized that progesterone at physiologic serum levels would be neuroprotective in female rats after TBI and that progesterone would reverse early mitochondrial dysfunction seen in this model. Female, Sprague-Dawley rats were ovariectomized and implanted with silastic capsules containing either low or high physiologic range progesterone at 7 days prior to TBI. Control rats received ovariectomy with implants containing no hormone. Rats underwent controlled cortical impact to the left parietotemporal cortex and were evaluated for evidence of early mitochondrial dysfunction (1 h) and delayed hippocampal neuronal injury and cortical tissue loss (7 days) after injury. Progesterone in the low physiologic range reversed the early postinjury alterations seen in mitochondrial respiration and reduced hippocampal neuronal loss in both the CA1 and CA3 subfields. Progesterone in the high physiologic range had a more limited pattern of hippocampal neuronal preservation in the CA3 region only. Neither progesterone dose significantly reduced cortical tissue loss. These findings have implications in understanding the sex-based differences in outcome following acute brain injury.

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.

Controlled cortical impact injury affects dopaminergic transmission in the rat striatum

The therapeutic benefits of dopamine (DA) agonists after traumatic brain injury (TBI) imply a role for DA systems in mediating functional deficits post-TBI. We investigated how experimental TBI affects striatal dopamine systems using fast scan cyclic voltammetry (FSCV), western blot, and d-amphetamine-induced rotational behavior. Adult male Sprague-Dawley rats were injured by a controlled cortical impact (CCI) delivered unilaterally to the parietal cortex, or were naïve controls. Amphetamine-induced rotational behavior was assessed 10 days post-CCI. Fourteen days post-CCI, animals were anesthetized and underwent FSCV with bilateral striatal carbon fiber microelectrode placement and stimulating electrode placement in the medial forebrain bundle (MFB). Evoked DA overflow was assessed in the striatum as the MFB was electrically stimulated at 60 Hz for 10 s. In 23% of injured animals, but no naïve animals, rotation was observed with amphetamine administration. Compared with naïves, striatal evoked DA overflow was lower for injured animals in the striatum ipsilateral to injury (p < 0.05). Injured animals exhibited a decrease in V(max) (52% of naïve, p < 0.05) for DA clearance in the hemisphere ipsilateral to injury compared with naïves. Dopamine transporter (DAT) expression was proportionally decreased in the striatum ipsilateral to injury compared with naïve animals (60% of naïve, p < 0.05), despite no injury-related changes in vesicular monoamine transporter or D2 receptor expression (DRD2) in this region. Collectively, these data appear to confirm that the clinical efficacy of dopamine agonists in the treatment of TBI may be related to disruptions in the activity of subcortical dopamine systems.

Synaptogenesis in the Hippocampal CA1 Field following Traumatic Brain Injury

Traumatic brain injury (TBI) results in both acute and chronic disruption of cognitive ability that may be mediated through a disruption of hippocampal circuitry. Experimental models of TBI have demonstrated that cortical contusion injuries can result in the loss of specific neurons in the CA3 subfield of the ipsilateral hippocampus, resulting in partial loss of afferents to the CA1 subfield. Numerous studies have documented the ability of the central nervous system to compensate for deafferentation by initiating a plasticity response capable of restoring lost synaptic contacts. The present study was designed to examine the time course of loss and replacement of synaptic contacts in stratum radiatum dendritic field of CA1. Young adult rats were subjected to a lateral cortical contusion injury and assayed for total synaptic numbers using unbiased stereology coupled with transmission electron microscopy. Injured animals demonstrated a 60% loss of synapses in CA1 at 2 days post-injury, followed by a reinnervation process that was apparent as early as 10 days post-injury. By 60 days post-injury, total synaptic numbers had approached pre-injury levels but were still significantly lower. Some animals were behaviorally tested for spatial memory in a Morris Water Maze at 15 and 30 days post-injury. While there was some improvement in spatial memory, injured animals continued to demonstrate a significant deficit in acquisition. These results show that the hippocampus ipsilateral to the cortical contusion is capable of a significant plasticity response but that synapse replacement in this area does not necessarily result in significant improvement in spatial learning.

Lack of a Gender Difference in Post-Traumatic Neurodegeneration in the Mouse Controlled Cortical Impact Injury Model

Recent studies using a mouse model of weight-drop-induced "diffuse" traumatic brain injury (TBI) have demonstrated a substantial gender difference in the time course and magnitude of post-traumatic neurodegeneration following a severe level of injury. The time of maximal damage, as assessed by the de Olmos aminocupric silver staining method, occurred at 72 h in male mice, whereas the peak of neurodegeneration was not observed until 14 days in females and was less in magnitude compared to males. This difference, favoring females, has been postulated to relate to the neuroprotective actions of estrogen and progesterone. In the presently reported experiments, we compared the time course and peak of neurodegeneration in male and female mice after a severe level of "focal" controlled cortical impact (CCI; 1 mm, 3.5 m/sec) TBI using the same strain (CF-1) and weight (29-31 g) as employed in the "diffuse" TBI study. The volume of silver staining was measured using image analysis methods at 24, 48, and 72 h, and 1, 2 and 4 weeks. In male and female mice, a significant increase in neurodegeneration was observed at 24 h, and the volume was not significantly different between the two genders. In both gender groups, the maximal neurodegeneration was seen at 48 h after injury. Although the female mice exhibited a trend toward higher mean volumes of silver staining, this difference was not significantly different compared to males. Furthermore, the rate of resolution of staining between 48 h and 4 weeks was similar. However, injured females still exhibited a significantly higher volume of staining compared to sham, non-injured females at 4 weeks, whereas the difference in staining volume between sham and injured males was no longer significant at that time point. These results show that, following a "focal" CCI, there is no gender difference that favors females, in contrast to that seen with the "diffuse" injury paradigm. The disparity between the effects of gender in the two models may be due to the fact that, in the "focal" CCI model, the timing of post-traumatic neurodegeneration is significantly more rapid than that seen in the "diffuse" model, which may overwhelm the neuroprotective effects of estrogen and progesterone and obscure the appearance of a gender difference.