Differential modulation of carbachol and trans-ACPD-stimulated phosphoinositide turnover following traumatic brain injury

An overview of preclinical models of traumatic brain injury (TBI): relevance to pathophysiological mechanisms

Background

Traumatic brain injury (TBI) is a major cause of morbidity and mortality, affecting millions annually worldwide. Although the majority of TBI patients return to premorbid baseline, a subset of patient can develop persistent and often debilitating neurocognitive and behavioral changes. The etiology of TBI within the clinical setting is inherently heterogenous, ranging from sport related injuries, fall related injuries and motor vehicle accidents in the civilian setting, to blast injuries in the military setting.

Objective

Animal models of TBI, offer the distinct advantage of controlling for injury modality, duration and severity. Furthermore, preclinical models of TBI have provided the necessary temporal opportunity to study the chronic neuropathological sequelae of TBI, including neurodegenerative sequelae such as tauopathy and neuroinflammation within the finite experimental timeline. Despite the high prevalence of TBI, there are currently no disease modifying regimen for TBI, and the current clinical treatments remain largely symptom based. The preclinical models have provided the necessary biological substrate to examine the disease modifying effect of various pharmacological agents and have imperative translational value.

Methods

The current review will include a comprehensive survey of well-established preclinical models, including classic preclinical models including weight drop, blast injury, fluid percussion injury, controlled cortical impact injury, as well as more novel injury models including closed-head impact model of engineered rotational acceleration (CHIMERA) models and closed-head projectile concussive impact model (PCI). In addition to rodent preclinical models, the review will include an overview of other species including large animal models and Drosophila.

Results

There are major neuropathological perturbations post TBI captured in various preclinical models, which include neuroinflammation, calcium dysregulation, tauopathy, mitochondrial dysfunction and oxidative stress, axonopathy, as well as glymphatic system disruption.

Conclusion

The preclinical models of TBI continue to offer valuable translational insight, as well as essential neurobiological basis to examine specific disease modifying therapeutic regimen.

Blockade of TRPC Channels Limits Cholinergic-Driven Hyperexcitability and Seizure Susceptibility After Traumatic Brain Injury

We investigated the contribution of excitatory transient receptor potential canonical (TRPC) cation channels to posttraumatic hyperexcitability in the brain 7 days following controlled cortical impact model of traumatic brain injury (TBI) to the parietal cortex in male adult mice. We investigated if TRPC1/TRPC4/TRPC5 channel expression is upregulated in excitatory neurons after TBI in contribution to epileptogenic hyperexcitability in key hippocampal and cortical circuits that have substantial cholinergic innervation. This was tested by measuring TRPC1/TRPC4/TRPC5 protein and messenger RNA (mRNA) expression, assays of cholinergic function, neuronal Ca²⁺ imaging in brain slices, and seizure susceptibility after TBI. We found region-specific increases in expression of TRPC1, TRPC4, and TRPC5 subunits in the hippocampus and cortex following TBI. The dentate gyrus, CA3 region, and cortex all exhibited robust upregulation of TRPC4 mRNA and protein. TBI increased cFos activity in dentate gyrus granule cells (DGGCs) and layer 5 pyramidal neurons both at the time of TBI and 7 days post-TBI. DGGCs displayed greater magnitude and duration of acetylcholine-induced rises in intracellular Ca²⁺ in brain slices from mice subjected to TBI. The TBI mice also exhibited greater seizure susceptibility in response to pentylenetetrazol-induced kindling. Blockade of TRPC4/TRPC5 channels with M084 reduced neuronal hyperexcitation and impeded epileptogenic progression of kindling. We observed that the time-dependent upregulation of TRPC4/TRPC5-containing channels alters cholinergic responses and activity of principal neurons acting to increase proexcitatory sensitivity. The underlying mechanism includes acutely decreased acetylcholinesterase function, resulting in greater G_q/11-coupled muscarinic receptor activation of TRPC channels. Overall, our evidence suggests that TBI-induced plasticity of TRPC channels strongly contributes to overt hyperexcitability and primes the hippocampus and cortex for seizures.

Quantification of Biological Responses as Predictors of Cognitive Outcome after Developmental TBI

Successful translational studies within the field of Traumatic Brain Injury (TBI) are concerned with determining reliable markers of injury outcome at chronic time points. Determination of injury severity following Fluid Percussion Injury (FPI) has long been limited to the measured atmospheric pressure associated with the delivered pulse. Duration of unresponsiveness to toe pinch (unconsciousness) was next introduced as an extra marker of injury severity. The current study is an effort to assess the utilization of acute injury-induced biological responses (duration of toe pinch unresponsiveness, percent body weight change, quantification of brain edema, and apnea duration) to predict cognitive performance at a subacute time point following developmental brain injury. Cognitive performance, when measured at a subacute phase, after developmental FPI was negatively correlated with the following variables, duration of toe pinch unresponsiveness, percent weight change, and quantified level of brain edema. These finding suggest the potential utilization of reliable severity assessment of injury-induced biological responses in determining outcome measures at subacute time points.

Altered calcium signaling following traumatic brain injury

Cell death and dysfunction after traumatic brain injury (TBI) is caused by a primary phase, related to direct mechanical disruption of the brain, and a secondary phase which consists of delayed events initiated at the time of the physical insult. Arguably, the calcium ion contributes greatly to the delayed cell damage and death after TBI. A large, sustained influx of calcium into cells can initiate cell death signaling cascades, through activation of several degradative enzymes, such as proteases and endonucleases. However, a sustained level of intracellular free calcium is not necessarily lethal, but the specific route of calcium entry may couple calcium directly to cell death pathways. Other sources of calcium, such as intracellular calcium stores, can also contribute to cell damage. In addition, calcium-mediated signal transduction pathways in neurons may be perturbed following injury. These latter types of alterations may contribute to abnormal physiology in neurons that do not necessarily die after a traumatic episode. This review provides an overview of experimental evidence that has led to our current understanding of the role of calcium signaling in death and dysfunction following TBI.

Strain differences in response to traumatic brain injury in Long-Evans compared to Sprague-Dawley rats

The selected strain of rodent used in experimental models of traumatic brain injury is typically dependent upon the experimental questions asked and the familiarity of the investigator with a specific rodent strain. This archival study compares the injury responsiveness and recovery profiles of two popular outbred strains, the Long-Evans (LE) and the Sprague-Dawley (SD), after brain injury induced by lateral fluid percussion injury (LFPI). General findings include a significantly longer duration of unconsciousness in LE rats, but similar durations of apnea. Both strains displayed the same level of initial FPI-induced behavioral deficits, followed by a more rapid rate of functional recovery in SD rats. Cortical volume loss was not significantly different, but close inspection of the data suggests the possibility that LE rats may be more susceptible to damage in the hemisphere contralateral to the injury site than are SD rats. It is hoped that the information provided here encourages greater attention to the subtle differences and similarities between strains in future pre-clinical efficacy studies of traumatic brain injury.

Dicyclomine, an M1 muscarinic antagonist, reduces biomarker levels, but not neuronal degeneration, in fluid percussion brain injury

Recent studies indicate that alphaII-spectrin breakdown products (SBDPs) have utility as biological markers of traumatic brain injury (TBI). However, the utility of SBDP biomarkers for detecting effects of therapeutic interventions has not been explored. Acetylcholine plays a role in pathological neuronal excitation and TBI-induced muscarinic cholinergic receptor activation may contribute to excitotoxic processes. In experiment I, regional and temporal changes in calpain-mediated alpha-spectrin degradation were evaluated at 3, 12, 24, and 48 h using immunostaining for 145-kDa SBDP. Immunostaining of SBDP-145 was only evident in the hemisphere ipsilateral to TBI and was generally limited to the cortex except at 24 h when immunostaining was also prominent in the dentate gyrus and striatum. In Experiment II, cerebral spinal fluid (CSF) samples were analyzed for various SBDPs 24 h after moderate lateral fluid percussion TBI. Rats were administered either dicyclomine (5 mg/kg i.p.) or saline vehicle (n = 8 per group) 5 min prior to injury. Injury produced significant increases (p < 0.001) of 300%, 230%, and >1000% in SBDP-150, -145, and -120, respectively in vehicle-treated rats compared to sham. Dicyclomine treatment produced decreases of 38% (p = 0.077), 37% (p = 0.028), and 63% (p = 0.051) in SBDP-150, -145, and -120, respectively, compared to vehicle-treated injury. Following CSF extraction, coronal brain sections were processed for detecting degenerating neurons using Fluoro-Jade histofluorescence. Stereological techniques were used to quantify neuronal degeneration in the dorsal hippocampus CA2/3 region and in the parietal cortex. No significant differences were detected in numbers of degenerating neurons in the dorsal CA2/3 hippocampus or the parietal cortex between saline and dicyclomine treatment groups. The percent weight loss following TBI was significantly reduced by dicyclomine treatment. These data provide additional evidence that, as TBI biomarkers, SBDPs are able to detect a therapeutic intervention even in the absence of changes in neuronal cell degeneration measured by Fluoro-jade.

Calcium responses to caffeine and muscarinic receptor agonists are altered in traumatically injured neurons

A fundamental mechanism that is believed to contribute to neuronal injury and death following traumatic brain injury (TBI) is a disruption in cellular calcium homeostasis. Of primary importance to these homeostatic mechanisms are intracellular calcium stores located on the endoplasmic reticulum. These intracellular stores play an important role in maintaining normal levels of calcium and calcium-mediated signaling through these stores is critical to several physiological processes in neurons. Using an in vitro model of stretch-induced traumatic injury and fura-2 digital calcium imaging, we investigated alterations in calcium-induced calcium release (CICR) and inositol (1,4,5)-trisphosphate (IP(3))-linked signaling through intracellular calcium stores in populations of cultured rat cortical neurons. Caffeine, which stimulates CICR, produced a rapid elevation of intracellular free calcium ([Ca(2+)](i)) in 70% of uninjured neurons. Fifteen min after injury the population of caffeine-responsive neurons was reduced to 30%. The IP(3)-linked muscarinic acetylcholine receptor agonists, CDD-0097 HCl and McN-A-343, produced elevations in [Ca(2+)](i) in 91% and 70% of uninjured neurons, respectively. Following injury the population of responders was reduced to 19% and 26%, respectively. Differential responses to agonists were also noted after injury, in which the majority of neurons within a given culture well were unresponsive to agonists while others elicited a normal elevation of calcium. These results suggest disruptions in intracellular calcium store-mediated signaling and altered calcium signaling population dynamics following injury. These alterations could affect normal neurotransmission in the brain and may contribute to some of the pathology of TBI.

Conventional and functional proteomics using large format two-dimensional gel electrophoresis 24 hours after controlled cortical impact in postnatal day 17 rats

Conventional and functional proteomics have significant potential to expand our understanding of traumatic brain injury (TBI) but have not yet been used. The purpose of the present study was to examine global hippocampal protein changes in postnatal day (PND) 17 immature rats 24 h after moderate controlled cortical impact (CCI). Silver nitrate stains or protein kinase B (PKB) phosphoprotein substrate antibodies were used to evaluate high abundance or PKB pathway signal transduction proteins representing conventional and functional proteomic approaches, respectively. Isoelectric focusing was performed over a nonlinear pH range of 3-10 with immobilized pH gradients (IPG strips) using supernatant from the most soluble cellular protein fraction of hippocampal tissue protein lysates from six paired sham and injured PND 17 rats. Approximately 1,500 proteins were found in each silver stained gel with 40% matching of proteins. Of these 600 proteins, 52% showed a twofold, 20% a fivefold, and 10% a 10-fold decrease or increase. Spot matching with existing protein databases revealed changes in important cytoskeletal and cell signalling proteins. PKB substrate protein phosphorylation was best seen in large format two-dimensional blots and known substrates of PKB such as glucose transporter proteins 3 and 4 and forkhead transcription factors, identified based upon molecular mass and charge, showed altered phosphorylation 24 h after injury. These results suggest that combined conventional and functional proteomic approaches are powerful, complementary and synergistic tools revealing multiple protein changes and posttranslational protein modifications that allow for more specific and comprehensive functional assessments after pediatric TBI.

Subtle alterations in NMDA-stimulated cyclic GMP levels following lateral fluid percussion brain injury

This study examined whether NMDA-stimulated cyclic GMP levels were altered at two different time points following lateral fluid percussion injury. At 60 min and 15 days postinjury, the left and right hippocampi were dissected and chopped into mini-prisms. Each hippocampus was divided into five equal parts and incubated with either the phosphodiesterase inhibitor IBMX (3-isobutyl-1-methylxanthine, 500 microM) alone, IBMX and N-methyl-D-aspartic acid (NMDA) OR IBMX, NMDA, and glycine (10 MM). Two concentrations of NMDA were used: 500 or 1,000 microM. Tissues were then assayed for levels of cyclic GMP. Results indicated that there were no changes in basal levels of cyclic GMP at either postinjury time point. At 60 min postinjury, there were no significant main effects for injury or drug concentration. There was a significant injury x side interaction effect with increased levels of NMDA-stimulated cyclic GMP in the hippocampus ipsilateral to the injury impact and decreased cyclic GMP levels in the contralateral hippocampus. There were no significant alterations in NMDA-stimulated cyclic GMP levels at 15 days postinjury. The data from this study indicated that NMDA-stimulated cyclic GMP accumulation is differentially altered in the hippocampus ipsilateral and contralateral to the site of the injury at 1 h after injury, but is normalized by 15 days postinjury. These findings implicate NMDA-mediated intracellular signaling processes in the acute excitotoxic response to injury.

Traumatic brain injury in rats results in increased expression of Gap-43 that correlates with behavioral recovery

Traumatic brain injury is associated with behavioral deficits, often in the absence of histopathological or ultrastructural changes. To determine whether membrane remodeling occurs, immunocytochemical techniques were used and the density and distribution of GAP-43 were measured. GAP-43 is a membrane-bound protein, which, when phosphorylated, is thought to regulate metabolic pathways involved in membrane remodeling and neurite growth. Moderate central fluid percussion injury (FPI, 1.9-2.2 atm.) was performed on anesthetized, spontaneously hypertensive Wistar rats (SHR). Behavioral reflex recovery was consistent with moderate levels of brain injury. One, 3, 5, 7 and 9 days after injury, both sham control (n = 4) and FPI (n = 4) animals were sacrificed, the brains were removed, cryosectioned and processed. Density measurements were taken from histological sections taken at interaural 6.20 mm and bregma -2.80 mm and were found to be statistically greater (P < 0.05) than background grey matter readings in the agranular cortices, the frontal, hindlimb, parietal 1 and 2 cortices, and the hippocampus and dentate gyrus, excluding the pyramidal and granular cell layers. Density measurements taken in forelimb and hindlimb cortical regions correlate with forelimb and hindlimb recovery in foot-fault and beam balance tests (P < 0.05). We interpret these data to indicate neuronal membrane remodeling as a result of the disruption of neuronal membranes due to the impact and shearing forces associated with the FPI. The disruption and remodeling of neuronal membranes are in areas that are consistent with the loss and recovery of locomotor and spatial behavior as a result of FPI.

Novel pharmacologic strategies in the treatment of experimental traumatic brain injury: 1998

The mechanisms underlying secondary or delayed cell death following traumatic brain injury are poorly understood. Recent evidence from experimental models suggests that widespread neuronal loss is progressive and continues in selectively vulnerable brain regions for months to years after the initial insult. The mechanisms underlying delayed cell death are believed to result, in part, from the release or activation of endogenous "autodestructive" pathways induced by the traumatic injury. The development of sophisticated neurochemical, histopathological and molecular techniques to study animal models of TBI have enabled researchers to begin to explore the cellular and genomic pathways that mediate cell damage and death. This new knowledge has stimulated the development of novel therapeutic agents designed to modify gene expression, synthesis, release, receptor or functional activity of these pathological factors with subsequent attenuation of cellular damage and improvement in behavioral function. This article represents a compendium of recent studies suggesting that modification of post-traumatic neurochemical and cellular events with targeted pharmacotherapy can promote functional recovery following traumatic injury to the central nervous system.

Effects of muscarinic receptor antagonism on the phosphatidylinositol bisphosphate signal transduction pathway after experimental brain injury

Hippocampal levels of fatty acids extracted from phosphatidylinositol 4,5-bisphosphate (PIP2), free fatty acids (FFA), and lactate were measured after central fluid percussion traumatic brain injury (TBI) in rats. At 5 min after injury, there was a decrease in fatty acids extracted from PIP2 suggesting a decrease in PIP2. At the same time point, total FFA increased in saline-treated TBI rats. Levels of arachidonic acid were significantly decreased in PIP2, while at the same time arachidonic and stearic acids increased in FFA in saline-treated TBI rats. No significant alterations in PIP2-derived fatty acids or FFA were observed at 20 min after TBI. Hippocampal concentrations of lactate were significantly elevated at 5 and 20 min after injury in saline-treated rats. In general, these alterations were blunted by preinjury administration of the muscarinic antagonist, scopolamine. These results suggest that the PIP2 signal transduction pathway is activated in the hippocampus at the onset of central fluid percussion TBI and that the enhanced phospholipase C-catalyzed phosphodiestric breakdown of PIP2 is a major mechanism of liberation of FFA in these sites immediately after such injury. The blunting of PIP2 and FFA alterations in animals treated with scopolamine suggests that activation of muscarinic receptors significantly contributes to the phospholipase C (PLC) signal transduction pathophysiology in TBI. The attenuation of lactate accumulation in scopolamine-treated rats suggests that TBI-induced muscarinic receptor activation also contributes to increased glycolytic metabolism and/or ionic imbalances.

Regional generation of leukotriene C4 after experimental brain injury in anesthetized rats

Regional concentrations of leukotriene C4 and extravasation of Evans blue were measured after lateral fluid-percussion brain injury in rats. Tissue levels of LTC4 were elevated in the injured cortex at 10 min, 30 min, and 1 h after injury; these levels returned to normal by 2 h after injury. Increases in the levels of LTC4 were also observed in the ipsilateral hippocampus after brain injury, and these elevations persisted for 2 h after injury. No significant increase in levels of LTC4 was observed in the contralateral cortex at any time after injury. A substantial extravasation of Evans blue was observed only in the ipsilateral cortex and hippocampus at 3 h and 6 h after brain injury. Although a temporal association between LTC4 and blood-brain barrier (BBB) breakdown is suggested by these data, no cause-and-effect relationship has been addressed in this study. However, it is possible that, as is true for cerebral ischemia, LTC4 may play a role as a mediator in the BBB breakdown associated with fluid-percussion brain injury in rats.

Exposure to environmental complexity promotes recovery of cognitive function after traumatic brain injury

This study was designed to determine whether exposure to a complex environment after traumatic brain injury (TBI) would promote the recovery of cognitive function. Rats were injured at a moderate level of fluid percussion injury (2.1 atm) or were prepared for injury but were not injured (sham injury). Immediately after the injury or sham injury, the injured/complex (n = 8) and the sham/complex (n = 7) groups were placed into a complex environment. The complex environment was a 89 x 89-cm enclosure with different types of bedding and objects that provided motor, olfactory, tactile, and visual stimulation. The injured/standard (n = 8) and the sham/standard (n = 8) groups were returned to the animal vivarium where they were housed individually in standard wire mesh cages (24 x 20 x 18 cm). On days 11-15 (postinjury), performance in the Morris water maze was assessed. Analysis of the latency to reach the goal platform indicated that injured animals recuperating in the complex environment performed significantly better than injured animals recovering in the standard environment (p < 0.01). In fact, injured animals in the complex environment performed as well as both sham-injured groups. The improved performance of injured rats recovering in the enriched environment occurred in the absence of environmentally induced alterations in brain weight. These results indicate that exposure to environmental complexity enhances recovery of cognitive function after TBI.

Differential consequences of lateral and central fluid percussion brain injury on receptor coupling in rat hippocampus

We have identified alterations in the responses of muscarinic and metabotropic receptors in rat hippocampus that persist for at least 15 days after central fluid percussion injury. This study compares the effect of lateral fluid percussion and central fluid percussion on these responses. Moderate injury was obtained by displacement and deformation of the brain within the closed cranial cavity using a fluid percussion device positioned either centrally or laterally. Carbachol and (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD)-stimulated polyphosphoinositide (PPI) hydrolysis was assayed in hippocampus from injured and sham-injured controls at 15 days following injury. At 15 days after central fluid percussion traumatic brain injury (TBI), the response to carbachol was enhanced by 30% and the response to trans-ACPD was enhanced by 75% compared to sham-injured animals. At 15 days after lateral fluid percussion TBI the response to trans-ACPD was enhanced by 40% both ipsilateral and contralateral to the side of injury. In contrast, the response to carbachol was enhanced by 29% contralateral to the side of injury but was diminished by 12% ipsilateral to the side of injury. Cresyl violet staining shows no hippocampal cell death after central fluid percussion injury or on the side contralateral to lateral fluid percussion injury but on the ipsilateral side cell death was identified in hippocampal area CA3. Thus, abnormal hippocampal cell signaling through the phosphoinositide pathway occurs in the absence of cell death and may contribute to cognitive impairment.

Metabotropic glutamate antagonist, MCPG, treatment of traumatic brain injury in rats

The metabotropic glutamate receptor (mGluR) antagonist, alpha-methyl-4-carboxyphenylglycine (MCPG) was administered into the left lateral ventricle 5 min prior to fluid percussion traumatic brain injury (TBI) in the rat. A single 5.0 microliters ventricular infusion of the active isomer. (+)-MCPG (0.2 mumol), significantly reduced beam walking motor deficits on days 1-5 after injury and learning/memory deficits measured on days 11-15 after injury. Neither a lower dose of (+)-MCPG (0.2 mumol) affected behavioral outcome. (+)-MCPG (0.2 mumol) did not affect systemic hemodynamic responses to injury. These results suggest that TBI induced activation of mGluRs contributes to behavioral morbidity and that blockade of certain mGluR subtypes (mGluR1, mGluR5 and/or mGluR2) may reduce these pathophysiological responses.