Segmental vascular resistance after mild controlled cortical impact injury in the rat

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.

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.

Role of fibrinogen in cerebrovascular dysfunction after traumatic brain injury

Traumatic brain injury (TBI) has been associated with various neurological disorders. However, the role of cerebrovascular dysfunction and its mechanisms associated with TBI are still not well understood. Inflammation is the main cause of vascular dysfunction. It affects properties of blood components and the vascular wall leading to changes in blood flow and in interaction of blood components and vascular endothelium exacerbating microcirculatory complications during inflammatory diseases. One of the markers of inflammation is a plasma adhesion protein, fibrinogen (Fg). At elevated levels, Fg can also cause inflammatory responses. One of the manifestations of inflammatory responses is an increase in microvascular permeability leading to accumulation of plasma proteins in the subendothelial matrix and causing vascular remodelling. This has a most devastating effect on cerebral circulation after TBI that is accompanied with an elevation of plasma level of Fg and with an increased cerebrovascular permeability in injury penumbra impairing the normal healing process. This study reviews cerebrovascular alterations after TBI, considers the consequences of increased blood-brain barrier permeability, defines the role of elevated level of Fg and discusses the potential mechanisms of its action leading to vascular dysfunction, which subsequently can cause impairment in neuronal function. Thus, possible mechanisms of vasculo-neuronal dysfunction after TBI are considered.

Analysis of functional pathways altered after mild traumatic brain injury

Concussive injury (or mild traumatic brain injury; mTBI) can exhibit features of focal or diffuse injury patterns. We compared and contrasted the cellular and molecular responses after mild controlled cortical impact (mCCI; a focal injury) or fluid percussion injury (FPI; a diffuse injury) in rats. The rationale for this comparative analysis was to investigate the brain's response to mild diffuse versus mild focal injury to identify common molecular changes triggered by these injury modalities and to determine the functional pathways altered after injury that may provide novel targets for therapeutic intervention. Microarrays containing probes against 21,792 unique messenger RNAs (mRNAs) were used to investigate the changes in cortical mRNA expression levels at 3 and 24 h postinjury. Of the 354 mRNAs with significantly altered expression levels after mCCI, over 89% (316 mRNAs) were also contained within the mild FPI (mFPI) data set. However, mFPI initiated a more widespread molecular response, with over 2300 mRNAs differentially expressed. Bioinformatic analysis of annotated gene ontology molecular function and biological pathway terms showed a significant overrepresentation of genes belonging to inflammation, stress, and signaling categories in both data sets. We therefore examined changes in the protein levels of a panel of 23 cytokines and chemokines in cortical extracts using a Luminex-based bead immunoassay and detected significant increases in macrophage inflammatory protein (MIP)-1α (CCL3), GRO-KC (CXCL1), interleukin (IL)-1α, IL-1β, and IL-6. Immunohistochemical localization of MIP-1α and IL-1β showed marked increases at 3 h postinjury in the cortical vasculature and microglia, respectively, that were largely resolved by 24 h postinjury. Our findings demonstrate that both focal and diffuse mTBI trigger many shared pathobiological processes (e.g., inflammatory responses) that could be targeted for mechanism-based therapeutic interventions.

Blast-induced phenotypic switching in cerebral vasospasm

Vasospasm of the cerebrovasculature is a common manifestation of blast-induced traumatic brain injury (bTBI) reported among combat casualties in the conflicts in Afghanistan and Iraq. Cerebral vasospasm occurs more frequently, and with earlier onset, in bTBI patients than in patients with other TBI injury modes, such as blunt force trauma. Though vasospasm is usually associated with the presence of subarachnoid hemorrhage (SAH), SAH is not required for vasospasm in bTBI, which suggests that the unique mechanics of blast injury could potentiate vasospasm onset, accounting for the increased incidence. Here, using theoretical and in vitro models, we show that a single rapid mechanical insult can induce vascular hypercontractility and remodeling, indicative of vasospasm initiation. We employed high-velocity stretching of engineered arterial lamellae to simulate the mechanical forces of a blast pulse on the vasculature. An hour after a simulated blast, injured tissues displayed altered intracellular calcium dynamics leading to hypersensitivity to contractile stimulus with endothelin-1. One day after simulated blast, tissues exhibited blast force dependent prolonged hypercontraction and vascular smooth muscle phenotype switching, indicative of remodeling. These results suggest that an acute, blast-like injury is sufficient to induce a hypercontraction-induced genetic switch that potentiates vascular remodeling, and cerebral vasospasm, in bTBI patients.

Global cerebral blood flow and CPP after severe head injury: a xenon-CT study

OBJECTIVE

To evaluate the association between global cerebral blood flow and different cerebral perfusion pressure ranges in severe head injury.

DESIGN

A retrospective study

SETTING

Neurosurgical and trauma patients in an intensive care unit in a regional hospital.

PATIENTS AND PARTICIPANTS

Out of a series of 237 consecutive patients with severe head injuries (GCS<or=8), 162 were submitted to ICP monitoring and 89 of them underwent 180 xenon-CT studies and cerebral perfusion pressure (CPP) measurement. The xenon-CT studies did not include any unsalvageable patients nor any mistakenly diagnosed as severe on entry.

INTERVENTIONS

None.

MEASUREMENTS AND RESULTS

Most (95.6%) of xenon-CT studies were obtained with CPP values within the 50-90 mmHg range. Perfusion data were grouped according to CPP values: (1) below 50 mmHg, (2) 50-60 mmHg, (3) 60-70 mmHg, (4) above 70 mmHg. Global cerebral blood flow did not differ among the groups (p=0.49). No differences in physiological variables were found among the CPP groups, except for intracranial pressure, higher in the group with CPP below 50 mmHg, and mean arterial pressure, higher in the CPP above 70 mmHg group (p<0.0001). No differences were found for cerebral metabolic rate of oxygen and lactate.

CONCLUSIONS

There was little correlation between CPP values and global cerebral blood flow levels in our selected patients, probably because pressure autoregulation was preserved. Global metabolism measurements were constant within the groups, suggesting that in patients with controlled physiological variables an interplay between cerebral blood flow and metabolism might be more relevant than the relationship between CPP and cerebral blood flow.

Atorvastatin reduction of intravascular thrombosis, increase in cerebral microvascular patency and integrity, and enhancement of spatial learning in rats subjected to traumatic brain injury

OBJECT

Atorvastatin, a beta-hydroxy-beta-methylglutaryl coenzyme A reductase inhibitor, has pleiotropic effects, such as promoting angiogenesis, increasing fibrinolysis, and reducing inflammatory responses, and has shown promise in enhancing recovery in animals with traumatic brain injury (TBI) and stroke. The authors tested the effect of atorvastatin on vascular changes after TBI.

METHODS

Male Wistar rats subjected to controlled cortical impact injury were perfused at different time points with fluorescein isothiocyanate (FITC)--conjugated dextran 1 minute before being killed. Spatial memory function had been measured using a Morris Water Maze test at various points before and after TBI. The temporal profile of intravascular thrombosis and vascular changes was measured on brain tissue sections by using a microcomputer imaging device and a laser confocal microscopy. The study revealed the following results. 1) Vessels in the lesion boundary zone and hippocampal CA3 region showed a variety of damage, morphological alterations, reduced perfusion, and intraluminal microthrombin formation. 2) Atorvastatin enhanced FITC-dextran perfusion of vessels and reduced intravascular coagulation. 3) Atorvastatin promoted the restoration of spatial memory function.

CONCLUSIONS

These results indicated that atorvastatin warrants investigation as a potential therapeutic drug for TBI.

Atorvastatin reduction of intracranial hematoma volume in rats subjected to controlled cortical impact

Object. Atorvastatin, a β-hydroxy-β-methylglutaryl coenzyme A reductase inhibitor, has pleiotropic effects such as improving thrombogenic profile, promoting angiogenesis, and reducing inflammatory responses and has shown promise in enhancing neurological functional improvement and promoting neuroplasticity in animal models of traumatic brain injury (TBI), stroke, and intracranial hemorrhage. The authors tested the effect of atorvastatin on intracranial hematoma after TBI.

Methods. Male Wistar rats were subjected to controlled cortical impact, and atorvastatin (1 mg/kg) was orally administered 1 day after TBI and daily for 7 days thereafter. Rats were killed at 1, 8, and 15 days post-TBI. The temporal profile of intraparenchymal hematoma was measured on brain tissue sections by using a MicroComputer Imaging Device and light microscopy.

Conclusions. Data in this study showed that intraparenchymal and intraventricular hemorrhages are present 1 day after TBI and are absorbed at 15 days after TBI. Furthermore, atorvastatin reduces the volume of intracranial hematoma 8 days after TBI.

Responses of posttraumatic pericontusional cerebral blood flow and blood volume to an increase in cerebral perfusion pressure

In and around traumatic contusions, cerebral blood flow (CBF) is often near or below the threshold for ischemia. Increasing cerebral perfusion pressure (CPP) in patients with head injuries may improve CBF in these regions. However, the pericontusional response to this intervention has not been studied. Using positron emission tomography (PET), we have quantified the response to an increase in CPP in and around contusions in 18 contusions in 18 patients. Regional CBF and cerebral blood volume (CBV) were measured with PET at CPPs of 70 and 90 mmHg using norepinephrine to control CPP. Based upon computed tomography, regions of interest (ROIs) were placed as two concentric ellipsoids, each of 1-cm width, around the core of the contusions. Measurements were compared with a control ROI in tissue with normal anatomic appearance. Baseline CBF and CBV increased significantly with increasing distance from the core of the lesion. The increase in CPP led to small increases in CBF in all ROIs except the core. The largest absolute CBF increase was found in the control ROI. Relative CBF increases did not differ between ROIs so that ischemic areas remained ischemic. Pericontusional oedema on computed tomography was associated with lower absolute values of CBF and CBV but did not differ from nonoedematous tissue in the relative response to CPP elevation.