Traumatic brain injury reduces myogenic responses in pressurized rodent middle cerebral arteries

Nervous System Response to Neurotrauma: A Narrative Review of Cerebrovascular and Cellular Changes After Neurotrauma

Neurotrauma is a significant cause of morbidity and mortality worldwide. For instance, traumatic brain injury (TBI) causes more than 30% of all injury-related deaths in the USA annually. The underlying cause and clinical sequela vary among cases. Patients are liable to both acute and chronic changes in the nervous system after such a type of injury. Cerebrovascular disruption has the most common and serious effect in such cases because cerebrovascular autoregulation, which is one of the main determinants of cerebral perfusion pressure, can be effaced in brain injuries even in the absence of evident vascular injury. Disruption of the blood-brain barrier regulatory function may also ensue whether due to direct injury to its structure or metabolic changes. Furthermore, the autonomic nervous system (ANS) can be affected leading to sympathetic hyperactivity in many patients. On a cellular scale, the neuroinflammatory cascade medicated by the glial cells gets triggered in response to TBI. Nevertheless, cellular and molecular reactions involved in cerebrovascular repair are not fully understood yet. Most studies were done on animals with many drawbacks in interpreting results. Therefore, future studies including human subjects are necessarily needed. This review will be of relevance to clinicians and researchers interested in understanding the underlying mechanisms in neurotrauma cases and the development of proper therapies as well as those with a general interest in the neurotrauma field.

Impaired capillary-to-arteriolar electrical signaling after traumatic brain injury

Traumatic brain injury (TBI) acutely impairs dynamic regulation of local cerebral blood flow, but long-term (>72 h) effects on functional hyperemia are unknown. Functional hyperemia depends on capillary endothelial cell inward rectifier potassium channels (Kir2.1) responding to potassium (K+) released during neuronal activity to produce a regenerative, hyperpolarizing electrical signal that propagates from capillaries to dilate upstream penetrating arterioles. We hypothesized that TBI causes widespread disruption of electrical signaling from capillaries-to-arterioles through impairment of Kir2.1 channel function. We randomized mice to TBI or control groups and allowed them to recover for 4 to 7 days post-injury. We measured in vivo cerebral hemodynamics and arteriolar responses to local stimulation of capillaries with 10 mM K+ using multiphoton laser scanning microscopy through a cranial window under urethane and α-chloralose anesthesia. Capillary angio-architecture was not significantly affected following injury. However, K+-induced hyperemia was significantly impaired. Electrophysiology recordings in freshly isolated capillary endothelial cells revealed diminished Ba2+-sensitive Kir2.1 currents, consistent with a reduction in channel function. In pressurized cerebral arteries isolated from TBI mice, K+ failed to elicit the vasodilation seen in controls. We conclude that disruption of endothelial Kir2.1 channel function impairs capillary-to-arteriole electrical signaling, contributing to altered cerebral hemodynamics after TBI.

Cerebral blood vessel damage in traumatic brain injury

Traumatic brain injury is a devastating cause of death and disability. Although injury of brain tissue is of primary interest in head trauma, nearly all significant cases include damage of the cerebral blood vessels. Because vessels are critical to the maintenance of the healthy brain, any injury or dysfunction of the vasculature puts neural tissue at risk. It is well known that these vessels commonly tear and bleed as an immediate consequence of traumatic brain injury. It follows that other vessels experience deformations that are significant though not severe enough to produce bleeding. Recent data show that such subfailure deformations damage the microstructure of the cerebral vessels, altering both their structure and function. Little is known about the prognosis of these injured vessels and their potential contribution to disease development. The objective of this review is to describe the current state of knowledge on the mechanics of cerebral vessels during head trauma and how they respond to the applied loads. Further research on these topics will clarify the role of blood vessels in the progression of traumatic brain injury and is expected to provide insight into improved strategies for treatment of the disease.

Traumatic Brain Injury Impairs Myogenic Constriction of Cerebral Arteries: Role of Mitochondria-Derived H2O2 and TRPV4-Dependent Activation of BKca Channels

Traumatic brain injury (TBI) impairs autoregulation of cerebral blood flow, which contributes to the development of secondary brain injury, increasing mortality of patients. Impairment of pressure-induced myogenic constriction of cerebral arteries plays a critical role in autoregulatory dysfunction; however, the underlying cellular and molecular mechanisms are not well understood. To determine the role of mitochondria-derived H₂O₂ and large-conductance calcium-activated potassium channels (BK_Ca) in myogenic autoregulatory dysfunction, middle cerebral arteries (MCAs) were isolated from rats with severe weight drop-impact acceleration brain injury. We found that 24 h post-TBI MCAs exhibited impaired myogenic constriction, which was restored by treatment with a mitochondria-targeted antioxidant (mitoTEMPO), by scavenging of H₂O₂ (polyethylene glycol [PEG]-catalase) and by blocking both BK_Ca channels (paxilline) and transient receptor potential cation channel subfamily V member 4 (TRPV4) channels (HC 067047). Further, exogenous administration of H₂O₂ elicited significant dilation of MCAs, which was inhibited by blocking either BK_Ca or TRPV4 channels. Vasodilation induced by the TRPV4 agonist GSK1016790A was inhibited by paxilline. In cultured vascular smooth muscle cells H₂O₂ activated BK_Ca currents, which were inhibited by blockade of TRPV4 channels. Collectively, our results suggest that after TBI, excessive mitochondria-derived H₂O₂ activates BK_Ca channels via a TRPV4-dependent pathway in the vascular smooth muscle cells, which impairs pressure-induced constriction of cerebral arteries. Future studies should elucidate the therapeutic potential of pharmacological targeting of this pathway in TBI, to restore autoregulatory function in order to prevent secondary brain damage and decrease mortality.

Hypertension-Induced Enhanced Myogenic Constriction of Cerebral Arteries Is Preserved after Traumatic Brain Injury

Traumatic brain injury (TBI) was shown to impair pressure-induced myogenic response of cerebral arteries, which is associated with vascular and neural dysfunction and increased mortality of TBI patients. Hypertension was shown to enhance myogenic tone of cerebral arteries via increased vascular production of 20-hydroxyeicosatrienoic acid (HETE). This adaptive mechanism protects brain tissue from pressure/volume overload; however, it can also lead to increased susceptibility to cerebral ischemia. Although both effects may potentiate the detrimental vascular consequences of TBI, it is not known how hypertension modulates the effect of TBI on myogenic responses of cerebral vessels. We hypothesized that in hypertensive rats, the enhanced myogenic cerebrovascular response is preserved after TBI. Therefore, we investigated the myogenic responses of isolated middle cerebral arteries (MCA) of normotensive and spontaneously hypertensive rats (SHR) after severe impact acceleration diffuse brain injury. TBI diminished myogenic constriction of MCAs isolated from normotensive rats, whereas the 20-HETE-mediated enhanced myogenic response of MCAs isolated from SHRs was not affected by TBI. These results suggest that the optimal cerebral perfusion pressure values and vascular signaling pathways can be different and, therefore, should be targeted differently in normotensive and hypertensive patients following TBI.

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.

Cerebrovascular dysfunction following subfailure axial stretch

Cerebral blood vessels are vital to maintaining the health of the brain. Traumatic brain injury (TBI) commonly results in autoregulatory dysfunction and associated failure of cerebral vessels to maintain homeostasis in the brain. While post-injury changes to brain biochemistry are known to contribute to this dysfunction, tissue deformation may also directly alter vascular smooth muscle cell (SMC) function. As a first step toward understanding stretch-induced dysfunction, this study investigates the effect of overstretch on the contractile behavior of SMCs in middle cerebral arteries (MCAs). We hypothesized that vessel function is altered above a threshold of stretch and strain rate. Twenty-four MCAs from Sprague Dawley rats were tested. Following development of basal SMC tone, vessels were subjected to increasing levels of isosmotic extracellular potassium (K⁺). Samples were then subjected to an axial overstretch of either 1.2*λ_IV or 1.3*λ_IV at strain rates of 0.2 or 20s^-1. Following overstretch, SMC contractile behavior was measured again, both immediately and 60min after overstretch. Control vessels were subjected to the same protocol but without overstretch. SMC contractile behavior was characterized using both percent contraction (%C) relative to the fully dilated inner diameter and the K⁺ dose required to evoke the half maximal contractile response (EC50). Control vessels exhibited increased sensitivity to K⁺ in successive characterization tests, so all effects were quantified relative to the time-matched control response. Samples exhibited the typical biphasic response to extracellular K⁺, dilating and contracting in response to small and large K⁺ concentrations, respectively. As hypothesized, axial overstretch altered SMC contractile behavior, as seen in a decrease in %C for sub-maximal contractile K⁺ doses (p<0.05) and an increase in EC50 (p<0.01), but only for the test group stretched rapidly to 1.3*λ_IV. While the change in %C was only significantly different immediately after overstretch, the change to EC50 persisted for 60min. These results indicate that deformation can alter SMC contractile behavior and thus potentially play a role in cerebrovascular autoregulatory dysfunction independent of the pathological chemical environment in the brain post-TBI.

Chronic cerebrovascular dysfunction after traumatic brain injury

Traumatic brain injuries (TBI) often involve vascular dysfunction that leads to long-term alterations in physiological and cognitive functions of the brain. Indeed, all the cells that form blood vessels and that are involved in maintaining their proper function can be altered by TBI. This Review focuses on the different types of cerebrovascular dysfunction that occur after TBI, including cerebral blood flow alterations, autoregulation impairments, subarachnoid hemorrhage, vasospasms, blood-brain barrier disruption, and edema formation. We also discuss the mechanisms that mediate these dysfunctions, focusing on the cellular components of cerebral blood vessels (endothelial cells, smooth muscle cells, astrocytes, pericytes, perivascular nerves) and their known and potential roles in the secondary injury cascade. © 2016 Wiley Periodicals, Inc.

Traumatic brain injury disrupts cerebrovascular tone through endothelial inducible nitric oxide synthase expression and nitric oxide gain of function

BACKGROUND

Traumatic brain injury (TBI) has been reported to increase the concentration of nitric oxide (NO) in the brain and can lead to loss of cerebrovascular tone; however, the sources, amounts, and consequences of excess NO on the cerebral vasculature are unknown. Our objective was to elucidate the mechanism of decreased cerebral artery tone after TBI.

METHODS AND RESULTS

Cerebral arteries were isolated from rats 24 hours after moderate fluid‐percussion TBI. Pressure‐induced increases in vasoconstriction (myogenic tone) and smooth muscle Ca2+ were severely blunted in cerebral arteries after TBI. However, myogenic tone and smooth muscle Ca2+ were restored by inhibition of NO synthesis or endothelium removal, suggesting that TBI increased endothelial NO levels. Live native cell NO, indexed by 4,5‐diaminofluorescein (DAF‐2 DA) fluorescence, was increased in endothelium and smooth muscle of cerebral arteries after TBI. Clamped concentrations of 20 to 30 nmol/L NO were required to simulate the loss of myogenic tone and increased (DAF‐2T) fluorescence observed following TBI. In comparison, basal NO in control arteries was estimated as 0.4 nmol/L. Consistent with TBI causing enhanced NO‐mediated vasodilation, inhibitors of guanylyl cyclase, protein kinase G, and large‐conductance Ca2+‐activated potassium (BK) channel restored function of arteries from animals with TBI. Expression of the inducible isoform of NO synthase was upregulated in cerebral arteries isolated from animals with TBI, and the inducible isoform of NO synthase inhibitor 1400W restored myogenic responses following TBI.

CONCLUSIONS

The mechanism of profound cerebral artery vasodilation after TBI is a gain of function in vascular NO production by 60‐fold over controls, resulting from upregulation of the inducible isoform of NO synthase in the endothelium.

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.

Histopathological and behavioral effects of immediate and delayed hemorrhagic shock after mild traumatic brain injury in rats

The purpose of this study was to investigate the increased susceptibility of the brain, after a controlled mild cortical impact injury, to a secondary ischemic insult. The effects of the duration and the timing of the secondary insult after the initial cortical injury were studied. Rats anesthetized with isoflurane underwent a 3 m/sec, 2.5-mm deformation cortical impact injury followed by hypotension to 40 mm Hg induced by withdrawing blood from a femoral vein. The duration of hypotension was varied from 40 to 60 min. The timing of 60 min of hypotension was varied from immediately post-injury to 7 days after the injury. Outcome was assessed by behavioral tasks and histological examination at 2 weeks post-injury. A separate group of animals underwent measurement of the acute physiology including mean blood pressure (MAP), intracranial pressure (ICP), and cerebral blood flow (CBF) using a laser Doppler technique. Increasing durations of hypotension resulted in marked expansion of the contusion, from 6.5±1.8 mm³ with sham hypotension to 27.1±3.9 mm³ with 60 min of hypotension. This worsening of the contusion was found only when then hypotension occurred immediately after injury or at 1 h after injury. CA3 neuron loss followed a similar pattern, but the injury group differences were not significant. Motor tasks, including beam balance and beam walking, were significantly worse following 50 and 60 min of hypotension. Performance on the Morris water maze task was also significantly related to the injury group. Studies of the acute cerebral hemodynamics demonstrated that CBF was significantly more impaired during hypotension in the animals that underwent the mild TBI compared to those that underwent sham TBI. The perfusion deficit was worst at the impact site, but also significant in the pericontusional brain. With 50 and 60 min of hypotension, CBF did not recover following resuscitation at the impact site, and recovered only transiently in the pericontusional brain. These results demonstrate that mild TBI, like more severe levels of TBI, can impair the brain's ability to maintain CBF during a period of hypotension, and result in a worse outcome.

The relationship between transient zinc ion fluctuations and redox signaling in the pathways of secondary cellular injury: relevance to traumatic brain injury

A major obstacle that hampers the design of drug therapy for traumatic brain injury is the incomplete understanding of the biochemical pathways that lead to secondary cellular injury and contribute to cell death. One such pathway involves reactive species that generate potentially cytotoxic zinc ion fluctuations as a major executor of neuronal, and possibly glial, cell death. Whether zinc ions released during traumatic brain injury are toxic or protective is controversial but can be approached by investigating the exact concentrations of free zinc ions, the thresholds of compromised zinc buffering capacity, and the mechanism of cellular homeostatic control of zinc. Rapidly stretch-injured rat pheochromocytoma (PC12) cells express cellular zinc ion fluctuations that depend on the production of nitric oxide. Chelation of cellular zinc ions after rapid stretch injury, however, increases cellular reactive oxygen species. In a rat model of traumatic brain injury, parasagittal fluid percussion, analysis of the metal load of metallothionein was used as an indicator of changes in cellular zinc ion concentrations. The combined results from the cellular and in vivo investigations caution against interpreting zinc ion fluctuations in the early phase (24h) after injury as a primarily cytotoxic event.

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.

Chelation of neurotoxic zinc levels does not improve neurobehavioral outcome after traumatic brain injury

Increases of synaptically released zinc and intracellular accumulation of zinc in hippocampal neurons after traumatic or ischemic brain injury is neurotoxic and chelation of zinc has been shown to reduce neurodegeneration. Although our previous studies showed that zinc chelation in traumatically brain-injured rats correlated with an increase in whole-brain expression of several neuroprotective genes and reduced numbers of apoptotic neurons, the effect on functional outcome has not been determined, and the question of whether this treatment may actually be clinically relevant has not been answered. In the present study, we show that treatment of TBI rats with the zinc chelator calcium EDTA reduces the numbers of injured, Fluoro-Jade-positive neurons in the rat hippocampus 24 h after injury but does not improve neurobehavioral outcome (spatial memory deficits) 2 weeks post-injury. Our data suggest that zinc chelation, despite providing short-term histological neuroprotection, fails to improve long-term functional outcome, perhaps because long-term disruptions in homeostatic levels of zinc adversely influence hippocampus-dependent spatial memory.

Injured Fluoro-Jade-positive hippocampal neurons contain high levels of zinc after traumatic brain injury

Hippocampal damage contributes to cognitive dysfunction after traumatic brain injury (TBI). We previously showed that Fluoro-Jade, a fluorescent stain that labels injured, degenerating brain neurons, quantifies the extent of hippocampal injury after experimental fluid percussion TBI in rats. Coincidentally, we observed that injured neurons in the rat hippocampus also stained with Newport Green, a fluorescent dye specific for free ionic zinc. Here, we show that, regardless of injury severity or therapeutic intervention, the post-TBI population of injured neurons in rat hippocampal subfields CA1, CA3 and dentate gyrus is indistinguishable, both in numbers and anatomical distribution, from the population of neurons containing high levels of zinc. Treatment with lamotrigine, which inhibits presynaptic release of glutamate and presumably zinc that is co-localized with glutamate, reduced numbers of Fluoro-Jade-positive and Newport Green-positive neurons equally as did treatment with nicardipine, which blocks voltage-gated calcium channels through which zinc enters neurons. To confirm using molecular techniques that Fluoro-Jade and Newport Green-positive neurons are equivalent populations, we isolated total RNA from 25 Fluoro-Jade-positive and 25 Newport Green-positive pyramidal neurons obtained by laser capture microdissection (LCM) from the CA3 subfield, linearly amplified the mRNA and used quantitative ribonuclease protection analysis to demonstrate similar expression of mRNA for selected TBI-induced genes. Our data suggest that therapeutic interventions aimed at reducing neurotoxic zinc levels after TBI may reduce hippocampal neuronal injury.

Dose-dependent neuronal injury after traumatic brain injury

The Fluoro-Jade (FJ) stain reliably identifies degenerating neurons after multiple mechanisms of brain injury. We modified the FJ staining protocol to quickly stain frozen hippocampal rat brain sections and to permit systematic counts of stained, injured neurons at 4 and 24 h after mild, moderate or severe fluid percussion traumatic brain injury (TBI). In adjacent sections, laser capture microdissection was used to collect uninjured (FJ negative) CA3 hippocampal neurons to assess the effect of injury severity on mRNA levels of selected genes. Rats were anesthetized, intubated, mechanically ventilated and randomized to sham, mild (1.2 atm), moderate (2.0 atm) or severe (2.3 atm) TBI. Four or 24 h post-TBI, ten frozen sections (10 microm thick, every 15th section) were collected from the hippocampus of each rat, stained with FJ and counterstained with cresyl violet. Fluoro-Jade-positive neurons were counted in hippocampal subfields CA1, CA3 and the dentate gyrus/dentate hilus. At both 4 and 24 h post-TBI, numbers of FJ-positive neurons in all hippocampal regions increased dose-dependently in mildly and moderately injured rats but were not significantly more numerous after severe injury. Although analysis of variance demonstrated no overall difference in expression of mRNA levels for heat shock protein 70, bcl-2, caspase 3, caspase 9 and interleukin-1beta in uninjured CA3 neurons at all injury levels, post hoc analysis suggested that TBI induces increases in neuroprotective gene expression that offset concomitant increases in deleterious gene expression.

Analysis of long-term gene expression in neurons of the hippocampal subfields following traumatic brain injury in rats

After experimental traumatic brain injury (TBI), widespread neuronal loss is progressive and continues in selectively vulnerable brain regions, such as the hippocampus, for months to years after the initial insult. To clarify the molecular mechanisms underlying secondary or delayed cell death in hippocampal neurons after TBI, we compared long-term changes in gene expression in the CA1, CA3 and dentate gyrus (DG) subfields of the rat hippocampus at 24 h and 3, 6, and 12 months after TBI with changes in gene expression in sham-operated rats. We used laser capture microdissection to collect several hundred hippocampal neurons from the CA1, CA3, and DG subfields and linearly amplified the nanogram samples of neuronal RNA with T7 RNA polymerase. Subsequent quantitative analysis of gene expression using ribonuclease protection assay revealed that mRNA expression of the anti-apoptotic gene, Bcl-2, and the chaperone heat shock protein 70 was significantly downregulated at 3, 6 (Bcl-2 only), and 12 months after TBI. Interestingly, the expression of the pro-apoptotic genes caspase-3 and caspase-9 was also significantly decreased at 3, 6 (caspase-9 only), and 12 months after TBI, suggesting that long-term neuronal loss after TBI is not mediated by increased expression of pro-apoptotic genes. The expression of two aging-related genes, p21 and integrin beta3 (ITbeta3), transiently increased 24 h after TBI, returned to baseline levels at 3 months and significantly decreased below sham levels at 12 months (ITbeta3 only). Expression of the gene for the antioxidant glutathione peroxidase-1 also significantly increased 6 months after TBI. These results suggest that decreased levels of neuroprotective genes may contribute to long-term neurodegeneration in animals and human patients after TBI. Conversely, long-term increases in antioxidant gene expression after TBI may be an endogenous neuroprotective response that compensates for the decrease in expression of other neuroprotective genes.

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.

Laser capture microdissection and analysis of amplified antisense RNA from distinct cell populations of the young and aged rat brain: effect of traumatic brain injury on hippocampal gene expression

To explore the molecular mechanisms underlying the increased vulnerability of the aged brain to traumatic brain injury (TBI), we compared the expression of several age-related genes in the CA1, CA3 and dentate gyrus subfields of the young and aged rat hippocampus before and after lateral fluid percussion TBI. Using laser capture microdissection (LCM), we obtained hippocampal neurons and glia from the neuropil adjacent to the pyramidal and granule cell layers. Subsequently, we linearly amplified and analyzed the antisense mRNA using Northern blot and ribonuclease protection assays (RPA). Our procedures, which have not been previously applied to quantitative analysis of LCM mRNA from neural tissue, included a modified reverse transcription step to enhance full-length cDNA synthesis, thus enhancing the yield of larger components of in vitro-transcribed mRNA for downstream analysis. Northern analysis showed greater expression of two aging-associated genes, p21 and brain-derived neurotrophic factor (BDNF) in the aged hippocampus. The age-related differences in p21 and BDNF expression were particularly prominent after TBI. By quantitative RPA analysis, we found that the expression of p21, known to be induced in senescent cells, was significantly greater in the CA3 region of aged rats, an area that is selectively vulnerable to TBI. However, expression of genes associated with regenerative and repair functions was significantly decreased in aged hippocampus. Our RPA results indicate that substantial age-dependent differences in the transcriptional profile of distinct regions of the hippocampal formation may account, in part, for their differential susceptibility to brain injury.

Protective effects of zinc chelation in traumatic brain injury correlate with upregulation of neuroprotective genes in rat brain

Chelation of excessive neuronal zinc ameliorates zinc neurotoxicity and reduces subsequent neuronal injury. To clarify the molecular mechanisms of this neuroprotective effect, we used a focused cDNA array of stress-response genes with zinc chelation (calcium EDTA) in our rat model of fluid percussion brain injury at 2 h, 24 h, and 7 days after injury. In parallel experiments, we compared neuronal cell death in TUNEL-stained brain sections in traumatized rats with and without calcium EDTA treatment. Zinc chelation induced the expression of several neuroprotective genes; neuroprotective gene expression correlated with substantially decreased numbers of TUNEL-positive cells.

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.

Potentiated endothelium-derived hyperpolarizing factor-mediated dilations in cerebral arteries following mild head injury

Evidence in the literature suggests that endothelium-derived hyperpolarizing factor (EDHF) may act in a compensatory manner such that during conditions of compromised nitric oxide (NO), EDHF serves as a back-up mechanism. Given that constitutive NO synthase is chronically downregulated after head trauma, we tested the hypothesis that EDHF is potentiated following injury. Male adult rats were subjected to either sham injury (n = 27) or mild controlled cortical impact (CCI) injury (n = 26). Branches of the middle cerebral artery (MCA) directly within the contusion site were harvested either 1 or 24 h later, pressurized to 60 mm Hg in a vessel chamber and allowed to develop spontaneous tone. Relaxation to luminal application of adenosine triphosphate (ATP) was similar in all groups. Relaxation to ATP in the presence of L-NAME (N(G)-nitro-L-arginine methyl ester) and indomethacin was similar in all groups except for vessels isolated at 24 h following mild CCI injury. In this case, L-NAME and indomethacin had no effect on the ATP-mediated dilation. The ATP-mediated dilation in L-NAME and indomethacin-treated MCA branches was inhibited by charybdotoxin, an inhibitor of large conductance Ca2+-sensitive K+ channels. These findings suggest that there is a significant potentiation of the EDHF-mediated dilation to ATP in cerebral arteries isolated at 24 h following mild CCI injury.

Delayed hemorrhagic hypotension exacerbates the hemodynamic and histopathologic consequences of traumatic brain injury in rats

Alterations in cerebral autoregulation and cerebrovascular reactivity after traumatic brain injury (TBI) may increase the susceptibility of the brain to secondary insults, including arterial hypotension. The purpose of this study was to evaluate the consequences of mild hemorrhagic hypotension on hemodynamic and histopathologic outcome after TBI. Intubated, anesthetized male rats were subjected to moderate (1.94 to 2.18 atm) parasagittal fluid-percussion (FP) brain injury. After TBI, animals were exposed to either normotension (group 1: TBI alone, n = 6) or hypotension (group 2: TBI + hypotension, n = 6). Moderate hypotension (60 mm Hg/30 min) was induced 5 minutes after TBI or sham procedures by hemorrhage. Sham-operated controls (group 3, n = 7) underwent an induced hypotensive period, whereas normotensive controls (group 4, n = 4) did not. For measuring regional cerebral blood flow (rCBF), radiolabeled microspheres were injected before, 20 minutes after, and 60 minutes after TBI (n = 23). For quantitative histopathologic evaluation, separate groups of animals were perfusion-fixed 3 days after TBI (n = 22). At 20 minutes after TBI, rCBF was bilaterally reduced by 57% +/- 6% and 48% +/- 11% in cortical and subcortical brain regions, respectively, under normotensive conditions. Compared with normotensive TBI rats, hemodynamic depression was significantly greater with induced hypotension in the histopathologically vulnerable (P1) posterior parietal cortex (P < 0.01). Secondary hypotension also increased contusion area at specific bregma levels compared with normotensive TBI rats (P < 0.05), as well as overall contusion volume (0.96 +/- 0.46 mm(3) vs. 2.02 +/- 0.51 mm(3), mean +/- SD, P < 0.05). These findings demonstrate that mild hemorrhagic hypotension after FP injury worsens local histopathologic outcome, possibly through vascular mechanisms.

Effects of luminal shear stress on cerebral arteries and arterioles

The effect of luminal shear stress was studied in cerebral arteries and arterioles. Middle cerebral arteries (MCA) and penetrating arterioles (PA) were isolated from male Long-Evans rats, mounted in a tissue bath, and pressurized. After the development of spontaneous tone, inside diameters were 186 +/- 5 microm (n = 28) for MCA and 65 +/- 3 microm (n = 37) for PA. MCA and PA constricted approximately 20% with increasing flow. Flow-induced constriction persisted in MCA and PA after removal of the endothelium. After removal of the endothelium, the luminal application of a polypeptide containing the Arg-Gly-Asp amino acid sequence (inhibitor of integrin attachment) abolished the flow-induced constriction. Similarly, an antibody specific for the beta(3)-chain of the integrin complex significantly inhibited the flow-induced constriction. The shear stress-induced constriction was accompanied by an increase in vascular smooth muscle Ca(2+). For example, a shear stress of 20 dyn/cm(2) constricted MCA 8% (n = 5) and increased Ca(2+) from 209 +/- 17 to 262 +/- 29 nM (n = 5). We conclude that isolated cerebral arteries and arterioles from the rat constrict to increased shear stress. Because the endothelium is not necessary for the response, the shear forces must be transmitted across the endothelium, presumably by the cytoskeletal matrix, to elicit constriction. Integrins containing the beta(3)-chain are involved with the shear stress-induced constrictions.

Peroxynitrite reduces vasodilatory responses to reduced intravascular pressure, calcitonin gene-related peptide, and cromakalim in isolated middle cerebral arteries

Vasodilatory responses to progressive reductions in intravascular pressure or to calcitonin gene-related peptide (CGRP) or cromakalim were determined in rodent middle cerebral arteries (MCAs) before and after treatment with peroxynitrite (ONOO-). Middle cerebral artery diameters in isolated, pressurized MCAs were measured as intravascular pressure was reduced from 100 to 20 mm Hg in 20-mm Hg increments before and after inactive ONOO-, pH-adjusted ONOO-, or 10, 20, or 40 micromol/L ONOO- was added to the bath. In other MCAs, responses to CGRP (1 x 10-9 - 5 x 10-8) or cromakalim (3 x 10-8 - 8 x 10-7) were measured before and after the addition of 25 micromol/L ONOO-. Inactive ONOO- (n = 6, P = 0.40), pH-adjusted ONOO- (n = 6, P = 0.29), and 10 micromol/L ONOO- (n = 6, P = 0.88) did not reduce vasodilatory responses to reduced intravascular pressure. Middle cerebral arteries treated with 20 (n = 6, P < 0.0001) and 40 (n = 6, P > 0.0001) micromol/L ONOO- constricted significantly when intravascular pressure was reduced. Vasodilatory responses to CGRP or cromakalim were reduced by ONOO- (P > 0.02, n = 6 and P > 0.01, n = 7, respectively). ONOO- had no effect on vasoconstriction in response to serotonin or vasodilation in response to KCl. These studies demonstrate that ONOO- reduces multiple cerebral vasodilatory responses.