Immediate cerebrovascular responses to closed head injury in the rat

A Systematic Review of Closed Head Injury Models of Mild Traumatic Brain Injury in Mice and Rats

Mild TBI (mTBI) is a significant health concern. Animal models of mTBI are essential for understanding mechanisms, and pathological outcomes, as well as to test therapeutic interventions. A variety of closed head models of mTBI that incorporate different aspects (i.e., biomechanics) of the mTBI have been reported. The aim of the current review was to compile a comprehensive list of the closed head mTBI rodent models, along with the common data elements, and outcomes, with the goal to summarize the current state of the field. Publications were identified from a search of PubMed and Web of Science and screened for eligibility following PRISMA guidelines. Articles were included that were closed head injuries in which the authors classified the injury as mild in rats or mice. Injury model and animal-specific common data elements, as well as behavioral and histological outcomes, were collected and compiled from a total of 402 articles. Our results outline the wide variety of methods used to model mTBI. We also discovered that female rodents and both young and aged animals are under-represented in experimental mTBI studies. Our findings will aid in providing context comparing the injury models and provide a starting point for the selection of the most appropriate model of mTBI to address a specific hypothesis. We believe this review will be a useful starting place for determining what has been done and what knowledge is missing in the field to reduce the burden of mTBI.

The Effects of Induction and Treatment of Intracranial Hypertension on Cerebral Autoregulation: An Experimental Study

Background

This study aimed to analyse cerebral autoregulation (CA) during induction and treatment of intracranial hypertension (ICH) in an experimental model.

Materials and Methods

Landrace and Duroc piglets were divided into mild and severe ICH groups. Four or seven millilitres of saline solution was infused into paediatric bladder catheter inserted in the parietal lobe (balloon inflation). After 1.5 h, a 3% saline solution was infused via venous catheter, and 30 min later, the bladder catheter balloon was deflated (surgery). The cerebral static autoregulation (sCA) index was evaluated using cerebral blood flow velocities (CBFV) obtained with Doppler ultrasound.

Results

Balloon inflation increased ICP in both groups. The severe ICH group showed significantly lower sCA index values (p=0.001, ANOVA) after balloon inflation (ICH induction) and a higher sCA index after saline injection (p=0.02) and after surgery (p=0.04). ICP and the sCA index were inversely correlated (r=-0.68 and p<0.05). CPP and the sCA index were directly correlated (r=0.74 and p<0.05).

Conclusion

ICH was associated with local balloon expansion, which triggered CA impairment, particularly in the severe ICH group. Moreover, ICP-reducing treatments were associated with improved CA in subjects with severe ICH.

Traumatic brain injury-induced autoregulatory dysfunction and spreading depression-related neurovascular uncoupling: Pathomechanisms, perspectives, and therapeutic implications

Traumatic brain injury (TBI) is a major health problem worldwide. In addition to its high mortality (35-40%), survivors are left with cognitive, behavioral, and communicative disabilities. While little can be done to reverse initial primary brain damage caused by trauma, the secondary injury of cerebral tissue due to cerebromicrovascular alterations and dysregulation of cerebral blood flow (CBF) is potentially preventable. This review focuses on functional, cellular, and molecular changes of autoregulatory function of CBF (with special focus on cerebrovascular myogenic response) that occur in cerebral circulation after TBI and explores the links between autoregulatory dysfunction, impaired myogenic response, microvascular impairment, and the development of secondary brain damage. We further provide a synthesized translational view of molecular and cellular mechanisms involved in cortical spreading depolarization-related neurovascular dysfunction, which could be targeted for the prevention or amelioration of TBI-induced secondary brain damage.

Effects of trauma, hemorrhage and resuscitation in aged rats

Traumatic brain injury (TBI) is a leading cause of death in the elderly and the incidence of mortality and morbidity increases with age. This study tested the hypothesis that, after TBI followed by hemorrhagic hypotension (HH) and resuscitation, cerebral blood flow (CBF) would decrease more in aged compared with young rats. Young adult (4-6 months) and aged (20-24 months) male Sprague-Dawley rats were anesthetized with isoflurane, prepared for parasagittal fluid percussion injury (FPI) and randomly assigned to receive either moderate FPI (2.0 atm) only, moderate FPI+severe HH (40 mm Hg for 45 min) followed by return of shed blood, or sham FPI. Intracranial pressure (ICP), CBF, and mean arterial pressure (MAP) were measured and, after twenty-four hours survival, the rats were euthanized and their brains were sectioned and stained with Fluoro-Jade (FJ), a dye that stains injured neurons. After moderate FPI, severe HH and reinfusion of shed blood, MAP and CBF were significantly reduced in the aged group, compared to the young group. Both FPI and FPI+HH groups significantly increased the numbers of FJ-positive neurons in hippocampal cell layers CA1, CA2 and CA3 (p<0.05 vs Sham) in young and aged rats. Despite differences in post-resuscitation MAP and CBF, there were no differences in the numbers of FJ-positive neurons in aged compared to young rats after FPI, HH and blood resuscitation. Although cerebral hypoperfusion in the aged rats was not associated with increased hippocampal cell injury, the trauma-induced reductions in CBF and post-resuscitation blood pressure may have resulted in damage to brain regions that were not examined or neurological or behavioral impairments that were not assessed in this study. Therefore, the maintenance of normal blood pressure and cerebral perfusion would be advisable in the treatment of elderly patients after TBI.

Effects of hypothermia on cerebral autoregulatory vascular responses in two rodent models of traumatic brain injury

Traumatic brain injury (TBI) can trigger disturbances of cerebral pressure autoregulation that can translate into the generation of secondary insults and increased morbidity/mortality. Few therapies have been developed to attenuate the damaging consequences of disturbed autoregulatory control, although some suggest that hypothermia may exert such protection. Here we reexamine this issue of traumatically induced autoregulatory disturbances and their modulation by hypothermic intervention, examining these phenomena in two different models of TBI. Adult rats were subjected to either impact acceleration injury (IAI) or lateral fluid percussion injury (LFPI) followed by the insertion of cranial windows to assess the pial arteriolar cerebral autoregulatory vascular response to the post-traumatic induction of sequential reductions of arterial blood pressure. The potential for continued pial vasodilation in response to declining blood pressure was directly measured post-injury and compared with that in injured groups subjected to 33° C of hypothermia of 1-2 h duration initiated 1 h post-injury. We observed that the TBI resulted in either impaired or abolished cerebral vascular dilation in response to the sequential declines in blood pressure. Following IAI there was a 50% reduction in the vasculature's ability to dilate in response to the induced hypotension. In contrast, following LFPI, the vascular response to hypotension was abolished both ipsilateral and contralateral to the LFPI. In animals sustaining IAI, the use of 1 h post-traumatic hypothermia preserved vascular dilation in response to declines in blood pressure in contrast to the LFPI in which the use of the same strategy afforded no improvement. However, with LFPI, the use of 2 h of hypothermia provided partial vascular protection. These results clearly illustrate that TBI can alter the cerebral autoregulatory vascular response to sequentially induced hypotensive insult, whereas the use of post-traumatic hypothermia provides benefit. Collectively, these studies also demonstrate that different animal models of TBI can evoke different biological responses to injury.

Cerebrovascular connexin expression: effects of traumatic brain injury

Traumatic brain injury (TBI) results in dysfunction of the cerebrovasculature. Gap junctions coordinate vasomotor responses and evidence suggests that they are involved in cerebrovascular dysfunction after TBI. Gap junctions are comprised of connexin proteins (Cxs), of which Cx37, Cx40, Cx43, and Cx45 are expressed in vascular tissue. This study tests the hypothesis that TBI alters Cx mRNA and protein expression in cerebral vascular smooth muscle and endothelial cells. Anesthetized (1.5% isoflurane) male Sprague-Dawley rats received sham or fluid-percussion TBI. Two, 6, and 24 h after, cerebral arteries were harvested, fresh-frozen for RNA isolation, or homogenized for Western blot analysis. Cerebral vascular endothelial and smooth muscle cells were selected from frozen sections using laser capture microdissection. RNA was quantified by ribonuclease protection assay. The mRNA for all four Cx genes showed greater expression in the smooth muscle layer compared to the endothelial layer. Smooth muscle Cx43 mRNA expression was reduced 2  h and endothelial Cx45 mRNA expression was reduced 24  h after injury. Western blot analysis revealed that Cx40 protein expression increased, while Cx45 protein expression decreased 24  h after injury. These studies revealed significant changes in the mRNA and protein expression of specific vascular Cxs after TBI. This is the first demonstration of cell type-related differential expression of Cx mRNA in cerebral arteries, and is a first step in evaluating the effects of TBI on gap junction communication in the cerebrovasculature.

Blast-induced brain injury and posttraumatic hypotension and hypoxemia

Explosive munitions account for more than 50% of all wounds sustained in military combat, and the proportion of civilian casualties due to explosives is increasing as well. But there has been only limited research on the pathophysiology of blast-induced brain injury, and the contributions of alterations in cerebral blood flow (CBF) or cerebral vascular reactivity to blast-induced brain injury have not been investigated. Although secondary hypotension and hypoxemia are associated with increased mortality and morbidity after closed head injury, the effects of secondary insults on outcome after blast injury are unknown. Hemorrhage accounted for approximately 50% of combat deaths, and the lungs are one of the primary organs damaged by blast overpressure. Thus, it is likely that blast-induced lung injury and/or hemorrhage leads to hypotensive and hypoxemic secondary injury in a significant number of combatants exposed to blast overpressure injury. Although the effects of blast injury on CBF and cerebral vascular reactivity are unknown, blast injury may be associated with impaired cerebral vascular function. Reactive oxygen species (ROS) such as the superoxide anion radical and other ROS, likely major contributors to traumatic cerebral vascular injury, are produced by traumatic brain injury (TBI). Superoxide radicals combine with nitric oxide (NO), another ROS produced by blast injury as well as other types of TBI, to form peroxynitrite, a powerful oxidant that impairs cerebral vascular responses to reduced intravascular pressure and other cerebral vascular responses. While current research suggests that blast injury impairs cerebral vascular compensatory responses, thereby leaving the brain vulnerable to secondary insults, the effects of blast injury on the cerebral vascular reactivity have not been investigated. It is clear that further research is necessary to address these critical concerns.

Hypoxia Alters The Expression of Inhibitor of Apoptosis Proteins after Brain Trauma in The Mouse

Hypoxia worsens brain injury following trauma, but the mechanisms remain unclear. The purpose of this study was to determine the effect of traumatic brain injury (TBI) and secondary hypoxia (9% oxygen) on apoptosis-related protein expression, cell death, and behavior. Using a murine weight-drop model, TBI led to an early (6 h) increase followed by a later (24 h) decrease in neuronal apoptosis inhibitor protein (NAIP) expression in the olfactory and motor cortex; in contrast, TBI led to a sustained (6 h to 7 days) increase in NAIP in the striatum. The peak increase in the expression of NAIP (6-12 h) following TBI alone was delayed (1-7 days) when hypoxia was added to TBI. Hypoxia following TBI further depleted other apoptosis inhibitor proteins (IAPs) and activated caspases, as well as increased contusion size and worsened cell death. Hypoxia added to TBI also increased motor and feeding activity on days 2 and 4 compared to TBI alone. Hypoxia without TBI had no effect on the expression of IAPs or cell death. These findings show that IAPs have a potential role in the increased vulnerability of brain cells to hypoxia following TBI, and have implications for configuring future therapies for TBI.

Changes in cerebral energy metabolites induced by impact-acceleration brain trauma and hypoxic-hypotensive injury in rats

The aim of this study was to describe, in rats, brain energy metabolites changes after different levels of head trauma (T) complicated by hypoxia-hypotension (HH). Male Sprague Dawley rats (n = 7 per groups) were subjected to T by impact-acceleration with 450-g weight drop from 1.50 or 1.80 m (T 1.50 or T 1.80), or to a 15-min period of HH (controlled hemorrhage to mean arterial pressure [MAP] of 40 mm Hg, and mechanical ventilation with N(2) 90%/O(2) 10%), or to their association (T followed by HH). Invasive MAP, intraparenchymental intracranial pressure (ICP), and cerebral blood flow (CBF using Laser Doppler flowmetry) were recorded during the 5 post-traumatic hours. Cerebral microdialysis was used to measure each hour interstitial brain glucose, lactate, pyruvate, and glutamate. For the entire period, the levels of cerebral glucose, pyruvate, and glutamate were not statistically different between groups. In addition, there were no differences associated with the lactate-glucose ratio. Lactate was significantly higher overtime only in T 1.80 + HH group (p < 0.001 vs. every other groups). The lactate-pyruvate ratio increased with trauma level, and was significantly different vs. sham for the entire study period in T 1.50 + HH, in T 1.80, and in T 1.80 + HH. There was no correlation between CBF variations and the lactate-pyruvate ratio (r(2) = 0.00001). The cerebral perfusion pressure was greater than 70 mm Hg in all groups. The prolonged post-traumatic impairment in brain energy metabolism may be related to traumatic brain injury (TBI) severity. It became worse when T was complicated by HH, but was not related to changes in CBF.

EFFECTS OF HYPERTONIC ARGININE ON CEREBRAL BLOOD FLOW AND INTRACRANIAL PRESSURE AFTER TRAUMATIC BRAIN INJURY COMBINED WITH HEMORRHAGIC HYPOTENSION

Hypertonic saline solutions improve cerebral blood flow (CBF) when used for acute resuscitation from hemorrhagic hypotension accompanying some models of traumatic brain injury (TBI); however, the duration of increased CBF is brief. Because the nitric oxide synthase substrate l-arginine provides prolonged improvement in CBF after TBI, we investigated whether a hypertonic resuscitation fluid containing l-arginine would improve CBF in comparison to hypertonic saline without l-arginine in a model of moderate, paramedian, fluid-percussion TBI followed immediately by hemorrhagic hypotension (mean arterial pressure [MAP] = 60 mm Hg for 45 min). Sprague-Dawley rats were anesthetized with 4.0% isoflurane, intubated and ventilated with 1.5%-2.0% isoflurane in oxygen/air (50:50). After preparation for TBI and measurement of CBF using laser Doppler flowmetry and measurement of intracranial pressure (ICP) using an implanted transducer, rats were subjected to moderate (2.0 atm) TBI, hemorrhaged for 45 min, and randomly assigned to receive an infusion of hypertonic saline (7.5%, 2,400 mOsm total; 6 mL/kg; n = 6) or hypertonic saline with 50, 100, or 300 mg/kg L-arginine (2,400 mOsm; 6 mL/kg; n = 6 in each of the three dose groups) and then monitored for 120 min after the end of infusion. CBF was measured continuously and calculated as a percent of the pre-TBI baseline during the hemorrhage period, after reinfusion of one of the hypertonic arginine solutions, and 30, 60, and 120 min after reinfusion. All four hypertonic solutions initially improved MAP, which, by 120 min after infusion, had decreased nearly to the levels observed during hemorrhage. ICP remained below baseline levels during resuscitation in all groups, although ICP was slightly greater (P = NS) than baseline in the hypertonic saline group. CBF increased similarly in all groups during infusion and then decreased similarly in all groups. At 120 min after infusion, CBF was highest in the group infused with hypertonic saline, but the difference was not significant. We conclude that the improvement of MAP, ICP, and CBF produced by hypertonic saline alone after TBI and hemorrhagic hypotension is not significantly enhanced by the addition of L-arginine at these doses.

Intravenous polyethylene glycol inhibits the loss of cerebral cells after brain injury

We have tested the effectiveness of polyethylene glycol (PEG) to restore the integrity of neuronal membranes after mechanical damage secondary to severe traumatic brain injury (TBI) produced by a standardized head injury model in rats. We provide additional detail on the standardization of this model, particularly the use and storage of foam bedding that serves to both support the animal during the impact procedure-and as a dampener to the acceleration of the brass weight. Further, we employed a dye exclusion technique using ethidium bromide (EB; quantitative evaluation) and horseradish peroxidase (HRP; qualitative evaluation). Both have been successfully used previously to evaluate neural injury in the spinal cord since they enter cells when their plasma membranes are damaged. We quantified EB labeling (90 microM in 110 microL of sterile saline) after injection into the left lateral ventricle of the rat brain 2 h after injury. At six h after injection and 8 h after injury, the animals were sacrificed and the brains were analyzed. In the injured rat brain, EB entered cells lining and medial to the ventricles, particularly the axons of the corpus callosum. There was minimal EB labeling in uninjured control brains, limited to cells lining the luminal surfaces of the ventricles. Intravenous injections of PEG (1 cc of saline, 30% by volume, 2000 MW) immediately after severe TBI resulted in significantly decreased EB uptake compared with injured control animals. A similar result was achieved using the larger marker, HRP. PEG-treated brains closely resembled those of uninjured animals.

Cerebral pressure autoregulation is intact and is not influenced by hypothermia after traumatic brain injury in rats

In head-injured patients and experimental traumatic brain injury (TBI), important cerebrovascular abnormalities include decreases in cerebral blood flow (CBF) and impairment of cerebral pressure autoregulation. We evaluated CBF and pressure autoregulation after fluid percussion injury (FPI) and hypothermia in rats with the hypothesis that hypothermia would ameliorate changes in posttraumatic CBF. Male Sprague-Dawley rats, intubated and mechanically ventilated, were prepared for parasagittal FPI (1.8 atm) and laser Doppler CBF flow (LDF) measurement. The abdominal aorta was cannulated for rapid removal and reinfusion of blood. Baseline autoregulatory testing in all groups consisted of LDF measurements at normothermia and a mean arterial pressure (MAP) of 100 mm Hg, followed by randomly ordered changes of MAP to 80, 60, and 40 mm Hg. Animals were then randomized to one of five groups: normothermic control without FPI; normothermia with FPI; hypothermic control (32 degrees C) without FPI; hypothermia initiated before FPI; and hypothermia initiated immediately after FPI injury. For each group, a complete, randomly ordered autoregulatory sequence was performed at 30 and 60 min after FPI or sham TBI. In a second study, rats were prepared identically, maintained at normothermic temperatures and autoregulation was tested before and after TBI using a set of randomly ordered levels of hypotension or using progressive reductions in MAP (i.e., 80, 60, 40 mm Hg) with the hypothesis that the technical manner and timing of decreasing of the blood pressure would effect CBF after TBI. Due to high acute mortality, the group in which hypothermia was induced before FPI was excluded from the analysis. At baseline, autoregulation was similar in all groups. There was no change in CBF or autoregulation in the normothermic control group at 30 and 60 min. In the other groups at 30 and 60 min, there was a similar, statistically significant decrease in absolute CBF (i.e., a decrease of 27-57% of baseline values), but pressure autoregulation was intact except at the lowest blood pressure tested at 60 min, where there was a slight improvement in the hypothermic group. Thus, in these experiments, absolute CBF decreased with hypothermia and FPI, while neither hypothermia nor FPI significantly altered autoregulation. In the second study, autoregulatory function was not different before TBI when comparing random and sequential blood pressure changes, but, when comparing the groups after TBI at the 60 mm Hg blood pressure level, CBF was significantly lower in the sequential group than in the random order group. This suggests that the mechanism of creating hypotension, whether random or sequential, significantly affects the measurement of CBF and autoregulation after TBI in rats.

Traumatic Cerebral Vascular Injury: The Effects of Concussive Brain Injury on the Cerebral Vasculature

In terms of human suffering, medical expenses, and lost productivity, head injury is one of the major health care problems in the United States, and inadequate cerebral blood flow is an important contributor to mortality and morbidity after traumatic brain injury. Despite the importance of cerebral vascular dysfunction in the pathophysiology of traumatic brain injury, the effects of trauma on the cerebral circulation have been less well studied than the effects of trauma on the brain. Recent research has led to a better understanding of the physiologic, cellular, and molecular components and causes of traumatic cerebral vascular injury. A more thorough understanding of the direct and indirect effects of trauma on the cerebral vasculature will lead to improvements in current treatments of brain trauma as well as to the development of novel and, hopefully, more effective therapeutic strategies.

Posttraumatic hypothermia followed by slow rewarming protects the cerebral microcirculation

In the clinical and laboratory setting, multiple reports have suggested the efficacy of hypothermia in blunting the damaging consequences of traumatic brain injury (TBI). With the use of posttraumatic hypothermia, it has been recognized that the time of initiation and duration of hypothermia are important variables in determining the degree of neuroprotection provided. Further, it has been recently recognized that the rate of posttraumatic rewarming is an important variable, with rapid rewarming exacerbating neuronal/axonal damage in contrast to slow rewarming which appears to provide enhanced neuroprotection. Although these findings have been confirmed in the brain parenchyma, no information exists for the cerebral microcirculation on the potential benefits of posttraumatic hypothermia followed by either slow or rapid rewarming. In the current communication we assess these issues in the pial circulation using a well-characterized model of TBI. Rats were prepared for the placement of cranial widows for direct assessment of the pial microcirculation prior to and after the induction of impact acceleration injury followed by moderate hypothermia with either subsequent slow or rapid rewarming strategies. The cranial windows allowed for the measurement of pial vessel diameter to assess ACh-dependent and CO2 reactivity in the chosen paradigms. ACh was applied topically to assess ACh-dependent dilation, while CO2 reactivity was assessed by changing the concentration of the inspired gas. Through this approach, it was found that posttraumatic hypothermia followed by slow rewarming maintained normal arteriolar vascular responses in terms of ACh-dependent dilation and CO2 reactivity. In contrast, arterioles subjected to TBI followed by normothermia or hypothermia and rapid rewarming showed impaired vasoreactivity in terms of their ACh-dependent and CO2 responses. This study provides additional evidence of the benefits of posttraumatic hypothermia followed by slow rewarming, demonstrating for the first time that the previously described neuroprotective effects extend to the cerebral microcirculation.

Effects of elevated ICP on brain function: can the multiparametric monitoring system detect the 'Cushing Response'?

The 'Cushing Response' is a significant phenomenon associated with elevated ICP. The purpose of our study was to examine the effects of the intracranial hypertension level and duration on the cerebral tissue physiology, using a Multiprobe assembly (MPA). The parameters monitored simultaneously included ICP, CBF, mitochondrial NADH redox state, extracellular K+ and H+ levels, DC potential and ECoG, calculated CPP and blood pressure. Two groups of rats were used. In one group, ICP was elevated to 50-60 mmHg for 13-15 min and, in the second group, ICP was elevated to 20 mmHg for 30 min. The results show that ICP of 50-60 mmHg led to CPP reduction below the lower limits of autoregulation. However, ICP of 20 mmHg, even for a prolonged period of time is completely tolerated. Additionally, we found that the 'Cushing Response', developed in the moderate treatment (ICP = 20 mmHg) is beneficial, assuring high CBF levels under intracranial hypertension. Furthermore, CBF and CPP monitoring, apparently, are not sufficient for autoregulation assessment; more parameters are needed.

Delayed changes in regional brain energy metabolism following cerebral concussion in rats

Traumatic brain injury (TBI) results in an acute altered metabolic profile of brain tissue which resolves within hours of initial insult and yet some of the functional deficits and cellular perturbations persist for days. It is hypothesized that a delayed change in energy status does occur and is a factor in the neural tissue's ability to survive and regain function. Regional metabolic profile and glucose consumption were determined at either 1 or 3 days following two different intensities of parasagittal fluid-percussion (F-P). A significant decrease in both 1CMRgluc and levels of ATP and P-creatine was evident in the hemisphere ipsilateral to the trauma at 1 day after the insult. The effect was greater in the cortical than the subcortical regions and was more pronounced at the higher trauma intensity. Normalization of glucose consumption and energy levels was essentially complete by 3 days. It would appear that the delayed metabolic changes at 1 day postinsult cannot be explained by a secondary ischemia since the changes in the metabolite profile do not elicit an increase in the consumption of glucose. These changes in energy metabolites may account for and contribute to the chronic neurological deficits following TBI.

High-intensity focused ultrasound selectively disrupts the blood-brain barrier in vivo

High-intensity focused ultrasound (HIFU) has been shown to generate lesions that destroy brain tissue while disrupting the blood-brain barrier (BBB) in the periphery of the lesion. BBB opening, however, has not been shown without damage, and the mechanisms by which HIFU induces BBB disruption remain unknown. We show that HIFU is capable of reversible, nondestructive, BBB disruption in a targeted region-of-interest (ROI) (29 of 55 applications; 26 of 55 applications showed no effect); this opening reverses after 72 h. Light microscopy demonstrates that HIFU either entirely preserves brain architecture while opening the BBB (18 of 29 applications), or generates tissue damage in a small volume within the region of BBB opening (11 of 29 applications). Electron microscopy supports these observations and suggests that HIFU disrupts the BBB by opening capillary endothelial cell tight junctions, an isolated ultrastructural effect that is different from the mechanisms through which other (untargeted) modalities, such as hyperosmotic solutions, hyperthermia and percussive injury disrupt the BBB.

Upregulation of iNOS expression and phosphorylation of eIF-2alpha are paralleled by suppression of protein synthesis in rat hypothalamus in a closed head trauma model

When the inducible form of nitric oxide synthase (iNOS) is expressed after challenge to the nervous system, it results in abnormally high concentrations of nitric oxide (NO). Under such conditions, NO could phosphorylate the eukaryotic translation initiation factor (eIF)-2alpha, thus suppressing protein synthesis in neurons that play a role in endocrine and autonomic functions. Using the Marmarou model of traumatic brain injury (TBI), we observed a rapid increase (at 4 h after TBI) of iNOS mRNA in magno- and parvocellular supraoptic and paraventricular neurons, declining gradually by approximately 30% at 24 h and by approximately 80% at 48 h. Western analysis indicated a trend towards increased iNOS protein synthesis at 4 h, which peaked at 8 h, and tended to decrease at the later time points. At the same time points, we detected immunocytochemically the phosphorylated form of eIF-2alpha (eIF-2alpha[P]) as cytoplasmic and more often as nuclear labeling. The incidence of double-labeled [iNOS and eIF-2alpha(P)] neuronal profiles, particularly at 24 h and 48 h after TBI, was high. De novo protein synthesis assessed quantitatively after infusion of 35S methionine/cysteine was reduced by approximately 20% at 4 h, remained depressed at 24 h, and did not return to control levels up to 48 h following the trauma. The results suggest that iNOS may trigger phosphorylation of eIF-2alpha, which in turn interferes with protein synthesis at the translational (ribosomal complex) and transcriptional (chromatin) levels. The depression in protein synthesis may include downregulation of iNOS itself, which could be an autoregulatory inhibitory feedback mechanism for NO synthesis. Excessive amounts of NO may also participate in dysfunction of hypothalamic circuits that underlie endocrine and autonomic alterations following TBI.

Multiparametric monitoring of brain under elevated intracranial pressure in a rat model

Intracranial hypertension may develop in most patients exposed to traumatic head injury. In many cases, patients enduring elevated intracranial pressure (ICP) will incur morbidity or mortality. Several methods are used in animal models to investigate the influence of ICP elevation on physiological parameters. In this study, we developed a cisterna magna model by adding a mechanism for warming the mock cerebrospinal fluid (CSF) entering the cisterna space to a temperature of 37 degrees C and combined this method for ICP elevation with the multiparametric monitoring system (Multiprobe Assembly [MPA]). Using the MPA, we monitored, for the first time, mitochondrial NADH redox state as well as ionic homeostasis under elevated ICP in a rat model. In addition, we monitored cerebral blood flow (CBF) by laser Doppler flowmetry, ECoG (bipolar electrodes), and surface temperature. Blood pressure was measured in the cannulated femoral artery. The ICP (monitored by Camino probe) was elevated to 50-60 mm Hg for 13-15 min, followed by 2 h of recovery. The results show that CBF was decreased by 90%, while NADH was elevated by 80% as compared to the normoxic levels. Complete depolarization occurred as evidence by the decrease in extracellular Ca2+ and a significant increase in K+. All parameters recovered 10 min after reopening the cannula to the cisterna magna to air pressure. We conclude that ICP elevation through the cisterna magna infusion method, used simultaneously with multiparametric monitoring, supplies reliable information on the brain tissue metabolic state with intracranial hypertension in a rat model.

Impaired autoregulation of cerebral blood flow in an experimental model of traumatic brain injury

In order to study the pathophysiology and the intracranial hemodynamics of traumatic brain injury, we have developed a modified closed-head injury model of impact-acceleration that expresses several features of severe head injury in humans, including acute and long-lasting intracranial hypertension, diffuse axonal injury, neuronal necrosis, bleeding, and edema. In view of the clinical relevance of impaired autoregulation of cerebral blood flow after traumatic brain injury, and aiming at further characterization of the model, we investigated the autoregulation efficiency 24 h after experimental closed-head injury. Cortical blood flow was continuously monitored with a laser-Doppler flowmeter, and the mean arterial blood pressure was progressively decreased by controlled hemorrhage. Relative laser-Doppler flow was plotted against the corresponding mean arterial blood pressure, and a two-line segmented model was applied to determine the break point and slopes of the autoregulation curves. The slope of the curve at the right hand of the break point was significantly increased in the closed head injury group (0.751 +/- 0.966%/mm Hg versus -0.104 +/- 0.425%/mm Hg,p = 0.028). The break point tended towards higher values in the closed head injury group (62.2 +/- 20.8 mm Hg versus 46.9 +/- 12.7 mm Hg; mean +/- SD, p = 0.198). It is concluded that cerebral autoregulation in this modified closed head injury model is impaired 24 h after traumatic brain injury. This finding, in addition to other characteristic features of severe head injury established earlier in this model, significantly contributes to its clinical relevance.

Neuroprotective effect of hypothermia on neuronal injury in diffuse traumatic brain injury coupled with hypoxia and hypotension

It is well established in mechanical head trauma that posttraumatic secondary insults, such as hypoxia and hypotension exacerbate neuronal injury and lead to worse outcome. In this study, the neuroprotective effect of hypothermia on the reduction of supraventricular subcortical neuronal damage was evaluated using an impact-acceleration model of diffuse traumatic brain injury coupled with both moderate and severe periods of hypoxia and hypotension. A total of 135 adult male Sprague-Dawley rats (340-375 g) were divided into three experimental studies: (I) physiological evaluation (n = 36); (II) quantitative analysis of the effect of trauma coupled with moderate and severe hypotension on neuronal damage assessed at 4 (n = 39) and 24 h (n = 24); and (III) the neuroprotective effect of hypothermia following moderate secondary insult (n = 36). Induction of hypothermia occurred at 15 min postinjury, to a level of 30 degrees C for 60 min. At the designated time points (4 and 24 h), the animals were sacrificed via standard transcardial perfusion techniques for histological processing. Quantitative assessment of neuronal damage using routine H&E staining at 4 hours showed neuronal damage which correlated with the severity of secondary insult. Animals exposed to trauma alone had a mean number of damaged neurons of 7.61 +/- 3.08/high powered field (hpf) compared with a mean of 1.21 +/- 0.30/hpf in the sham operated group (p = 0.015). Animals exposed to trauma with 10 min of hypoxia and hypotension (THH-10) showed a statistically significant number of damaged neurons compared to the sham-operated animals (7.50 +/- 2.15 damaged neurons/hpf, p = 0.013), whereas, neuronal damage in animals undergoing trauma with a 30-min secondary insult of hypoxia and hypotension (THH-30) was markedly increased (100 +/- 30.20/hpf, p = 0.002). Statistical analysis showed no significant difference in neuronal damage in animals subjected to secondary insult alone. At 24 h, the evolution of neuronal damage in the trauma alone group (5.08 +/- 1.63/hpf) was relatively static; however, there was a remarkable increase in the neuronal damage of the THH-10 group (29.88 50 +/- 8.20/hpf). However, hypothermia provided nearly complete protection against secondary insults, and neuronal damage was equal to that of the trauma alone group (p = 0.42). The results of this study confirm that hypothermia provides remarkable protection against the adverse effects of neuronal damage exacerbated by secondary injury. This study also presents a new model of secondary insult, which can be used experimentally to further define the mechanism of increased vulnerability of the injured brain.

Effects of L-NAME and 7-NI on NOS catalytic activity and behavioral outcome after traumatic brain injury in the rat

Traumatic brain injury (TBI) produces transient increases in constitutive nitric oxide synthase (cNOS) activity and prolonged behavioral abnormalities. This study investigated the effects of nitro-L-arginine-methyl ester (L-NAME) and 3-bromo-7-nitroindazole (7-NI) treatment on cNOS catalytic activity and sensorimotor behavioral outcome after TBI. Rats underwent moderate (1.8-2.2 atm) parasagittal fluid percussion brain injury (FPI). At 5 min after FPI, cNOS activity was significantly increased within the damaged cerebral cortex of vehicle-treated rats compared to the noninjured contralateral cortex (206.7 +/- 150.5 % of contralateral, p < 0.01). Pretreatment with L-NAME and 7-NI significantly reduced injury-induced cNOS activation (47.7 +/- 42.6 %, p < 0.05, and 96.16 +/- 12.76, p < 0.05, respectively). Pretreatment with L-NAME and 7-NI also inhibited cNOS activity within the contralateral noninjured cerebral cortex compared to vehicle-treated rats (L-NAME 43.7 +/- 12.47%, p < 0.05; 7-NI 36.8 +/- 7.47%, p < 0.05). Furthermore, pretreatment with 7-NI, but not L-NAME, significantly reduced forelimb placing sensorimotor deficits (3.14 +/- 1.07, p < 0.05) at 1 day after TBI compared to vehicle-treated rats (5.38 +/- 0.42). These data indicate that inhibition of injury-induced elevations in neuronal NOS activity has a beneficial effect on neurological outcome after parasagittal FPI brain injury.

Temporal effect of severe controlled cortical impact injury in the rat on the myogenic response of the middle cerebral artery

The present study examined the effect of severe traumatic brain injury (TBI) on the myogenic response in the rat middle cerebral artery (MCA). Rats were subjected to severe controlled cortical impact (CCI; 5 m/sec, 130-msec duration, 3-mm deformation) injury over the right parietal cortex. At 2, 24, and 120 h postinjury, ipsilateral and contralateral segments of MCAs were isolated, mounted in a vessel chamber, and pressurized. After equilibration, the myogenic tone, the difference in vessel diameter in the presence and absence of calcium for a given pressure, and the myogenic response (the active contractile response elicited by a vessel to increasing pressure), were measured. At 24 h postsurgery, there was a significant interaction between myogenic tone and pressure in the ipsilateral and contralateral MCAs when TBI was compared to shams. However, this was not apparent, at the 2- and 120-h time points. At 2- and 24-h postsurgery, there was a significant interaction between myogenic response and pressure in the ipsilateral MCAs when TBI was compared to shams. While the response of the vascular smooth muscle was altered following injury, it was still functioning, suggesting that these vessels compensate, perhaps through alternate mechanisms or by relying on those remaining intact mechanisms.

Failure of cerebral autoregulation in an experimental diffuse brain injury model

The normal cerebral circulation has the ability to maintain a stable cerebral blood flow over a wide range of cerebral perfusion pressures and this is known as cerebral autoregulation. Autoregulation may be impaired in the injured brain. Closed head injury was induced in 28 Sprague-Dawley rats weighing 400-450 g. Four groups were studied: control and groups, head injured by weight drop from one meter height using 350 g, 400 g and 450 g respectively. CBF was monitored using laser-Doppler flowmetry along with monitoring of ICP and arterial blood pressure. If the correlation coefficient between CBF and CPP was > 0.85 and CPP was within normal range, loss of autoregulation was hypothesized. Loss of autoregulation was seen in all groups of injured rats during first four hours. A statistically significant difference (p = 0.041) was seen in the trequency of loss of autoregulation between injured and control animals. No loss of autoregulation was observed in the control group. In conclusion CBF and CPP provide information about loss of autoregulation in diffuse brain injury. Decrease in CBF and increase of ICP is observed as a result of loss of cerebral autoregulation. Knowledge of loss of autoregulation could help in the management of head injured patients.

CSF dynamics in a rodent model of closed head injury

Using ICP measurements and the bolus injection technique dynamic parameters of the cerebrospinal fluid system as there are pressure-volume-index (PVI) and resistance to CSF outflow (Rout) were investigated in a new model of diffuse closed head injury (CHI) in the rat. It was found that in the absence of brain oedema and ICP alterations an increase in PVI and Rout was present in the early (4h) period following head injury. This may be indicative for a reduction in cerebral blood flow and cerebral blood volume, both shown previously to occur after CHI. Furthermore an early impairment of CSF absorption mechanisms is evident. To answer the question, whether bolus injection techniques are advisable for clinical routine and whether results might have a predictive value, further investigations covering longer observation intervals and in the presence of secondary insults to the brain are necessary.

A model of diffuse traumatic brain injury in the immature rat

Diffuse cerebral swelling after severe traumatic brain injury (TBI) develops more commonly in children than adults; however, models of diffuse brain injury in immature animals are lacking. The authors developed a new model of diffuse severe TBI in immature rats by modifying a recently described closed head injury model for adult rats. A total of 105 Sprague-Dawley immature rats (17 days old; average weight 38.5 +/- 5.46 g) were subjected to head impact using variable weights (0 g (sham), 75 g, 100 g, or 125 g) delivered from a height of 2 m onto a metal disk cemented to the intact cranium. Mortality, physiological and neurological parameters (from early reflex recovery to escape), and early histopathological changes were assessed. During the acute period after severe injury (SI) (100 g delivered from a height of 2 m; 50 rats), apnea was frequently observed and the mortality rate was 38%. Neurological recovery was complete in the sham-injured animals (11 rats) by 4.1 +/- 0.23 minutes (mean +/- standard error of the mean), but was delayed in both moderately injured (MI) (75 g/2 m; 11 rats) (14.97 +/- 3.99 minutes) and SI (20.57 +/- 1.31 minutes (p < 0.05)) rats. In the first 24 hours, the sham-injured animals were more active than the injured ones as reflected by a greater net weight gain: 2.9 +/- 1.0 g, 1.2 +/- 1.6 g, and -0.6 +/- 2.1 g in sham-injured, MI, and SI animals, respectively. Immediately after injury, transient hypertension (lasting < 15 seconds) was followed by hypotension (lasting < 3 minutes) and loss of temperature regulation. Both injuries also induced apnea (0.75 +/- 0.7 minutes and 1.27 +/- 0.53 minutes in MI and SI groups, respectively), which either resolved or deteriorated to death. Intubation and assisted ventilation in animals with SI for 9.57 +/- 3.27 minutes in the peritrauma period eliminated mortality (p < 0.05, intubated vs. nonintubated). Histologically, after SI, there was diffuse edema throughout the corpus callosum below the region of injury and in the thalami. Other injuries included neuronal death in the deep nuclei, bilateral disruption of CA3, diffuse subarachnoid hemorrhage, and, in some, ventriculomegaly. Following a diffuse TBI in immature rats, SI produced a mortality rate, neurological deficit, and histological changes similar to those previously reported for an injury resulting from a 450-g weight dropped from 2 m in adult rats. A graded insult was achieved by maintaining the height of the weight drop but varying the weights. Weight loss, acute physiological instability, and acute neurological deficits were also indicative of an SI. Mortality was eliminated when ventilatory support was used during the peritrauma period. This model should be useful in studying the response of the immature rat to diffuse severe TBI.

Acute alterations in [Ca2+]i in NG108-15 cells subjected to high strain rate deformation and chemical hypoxia: an in vitro model for neural trauma

The short-term (less than 2 min) alterations in the intracellular free calcium concentration in differentiated NG108-15 (neuroblastoma cross glioma) cells exposed to dynamic mechanical deformation with and without superimposed chemical hypoxia were determined. A previously developed device, modified for these studies, was used to apply deformations at a magnitude and rate representative of those experienced by neural tissue in Traumatic Brain Injury. Chemical hypoxia was imposed using a combination of 2-deoxy-D-glucose and salicylate, anaerobic and aerobic metabolic blockers, respectively. Real time measurement of intracellular free calcium concentration using Fura-2 and a custom epifluorescence microscopy system provided a quantitative index of cell response. At high rates of deformation (approximately 10 sec-1), increases in intracellular free calcium concentration were exponentially related to the magnitude of the applied deformation. Chemical hypoxia had no effect on this acute response. At low rates of deformation, small increases in intracellular free calcium concentration were independent of the magnitude of the deformation. These findings indicate that strategies for reducing severity of TBI should focus on minimizing the rate of deformation of neural cells. Together with data from animal, physical, and finite element models, these data can be employed in the development of physiologic injury tolerance criteria for the whole head.