Jana Dunbar

The Permissive Nature of Blood Brain Barrier (BBB) Opening in Edema Formation Following Traumatic Brain Injury

The contribution of blood brain barrier opening to traumatic brain edema is not known. This study compares the course of traumatic BBB disruption and edema formation, with the hypothesis that they are not obligately related. Sprague-Dawley rats were divided into three groups: Group A (n = 47)--Impact Acceleration (IAM); Group B (n = 104)--lateral cortical impact (CCI); Group C (n = 26)--IAM + hypoxia & hypotension (THH). BBB integrity was assessed using i.v. markers (Evans Blue, or gadolinium-DTPA). Edema formation was evaluated with gravimetry, and T1-weighted MRI. In IAM, BBB opened immediately but closed rapidly, and remained closed for at least the next 36 hours whilst 24-hour hemispheric water content (HWC) rose by 0.9% (p < 0.01). In CCI, BBB opened in both hemispheres for up to 4 hours; four hour HWC in the uninjured hemisphere was indistinguishable from Sham, where HWC in the injured hemisphere rose by approximately 1.5% (p < 0.005). We distinguished two THH animals based on Apparent Diffusion Coefficient (ADC) recovery: in ADC-recovery animals 4 hour cortical water content (CWC) was 80.4 +/- 0.6%, cf 81.4 +/- 1.3% in ADC-non-recovery (p < 0.05). In all animals the BBB was open, however two populations of permeability were seen which likely related to flow-limited extravasation of gadolinium. In IAM edema forms despite only brief BBB opening. Although there is diffuse BBB opening with lateral contusion, edema only forms in the injured hemisphere. In THH, edema formation in the face of a widely permeable barrier is driven by ADC changes or cell swelling. Edema formation clearly does not correspond with BBB opening and an open BBB is clearly not required for edema formation. However we hypothesize that a permeable BBB permissively worsens the process, by acting as a low resistance pathway for ion and water movement. These findings are consistent with our general hypothesis that edema formation after TBI is mainly cytotoxic.

The impact-acceleration model of head injury: injury severity predicts motor and cognitive performance after trauma.

This study examines neuropsychological dysfunction after varying severities of the Impact Acceleration Model of diffuse traumatic brain injury. Adult rats (340 g-400 g) were divided into five groups, and exposed to varying degrees of Impact Acceleration Injury (1 m, 2 m, 2.1 m/500 g and second insult). After injury, animals were allowed to recover; acute neurological reflexes, beam walk score, beam balance score, inclined plane score, and Morris Water Maze score were then assessed at multiple time points. Injury of all severities caused significant motor and cognitive deficits. With milder injuries these effects were transient; however, with more severe injuries no recovery in function was seen. The addition of hypoxia and hypotension made a moderate injury worse than a severe injury. The acute neurological reflexes, the beam balance test and the inclined plane test distinguished between the more severely injured groups, but were affected less by mild injury. The beam walk test was sensitive to mild injury, but appeared unable to distinguish between the severe groups. The Morris Water Maze was sensitive for all injury groups, but appeared to adopt a different response profile with secondary insult. This study has for the first time characterized the degree of motor and cognitive deficits in rodents exposed to differing severities of Impact Acceleration Injury. These data confirm that the tests considered, and the Injury Model used, provide a useful system for the consideration of potential therapies which might ameliorate neuropsychological deficits in diffuse brain injury.

The Protein Kinase C Activator Phorbol Myristate Acetate Decreases Brain Edema by Aquaporin 4 Downregulation after Middle Cerebral Artery Occlusion in the Rat

The protein kinase C activator phorbol 12-myristate 13-acetate (PMA) is known to interact with aquaporin 4 (AQP 4), a water-selective transporting protein that is abundant in astrocytes, and has experimentally been found to decrease osmotically-induced cell swelling. The purpose of this study was to examine whether PMA reduces brain edema following focal ischemia induced by middle cerebral artery (MCA) occlusion by modulation of AQP4 expression. Male Sprague-Dawley rats were randomly assigned to either sham surgery (n = 6), or a continuous intravenous infusion of vehicle (1% dimethylsulfoxide), followed by MCA occlusion (n = 18), and administration of PMA at 50 microg/kg (n = 6) or at 200 microg/kg (n = 6) starting 60 min before or 30 min (200 microg/kg; n = 6) or 60 min (200 microg/kg; n = 6) after MCA occlusion. Cerebral blood flow was monitored with laser Doppler over the MCA territory, and confirmed a 70% reduction during occlusion. After a 2-h period of ischemia and 2 h of reperfusion, the animals were sacrificed for assessment of brain water content and sodium and potassium concentration. AQP4 expression was assessed by immunoblotting and quantified by densitometry (n = 24). Statistical analysis was performed by ANOVA followed by Tukeys post-hoc test. PMA treatment at 200 microg/kg significantly reduced brain water concentration in the infarcted area when started 60 min before or 30 min after occlusion (p < 0.001 and p = 0.022, respectively), and prevented the subsequent sodium shift (p < 0.05). PMA normalized the AQP4 upregulation in ischemia (p = 0.021). A downregulation of AQP4 in the ischemic area paralleling the reduction in brain edema formation following PMA treatment suggests that the effect was mediated by AQP4 modulation.

Acute Blood-Brain Barrier Changes in Experimental Closed Head Injury as Measured by MRI and Gd-DTPA

The objective of this study was to determine the early time course of blood-brain barrier (BBB) changes in diffuse closed head injury (CHI) and to what extent BBB is affected by secondary insult. The BBB disruption was quantified using T1-weighted MRI following administration of Gd-DTPA. The maximal signal intensity (SI) enhancement was used to calculate BBB disruption. A new CHI model was used to induce injury. Adult SD rats were separated into four groups: Group I: Sham (n = 4), II: Hypoxia and Hypotension (HH, n = 4), III: Trauma alone (n = 23), and IV: Trauma coupled with HH (THH, n = 14). Following trauma, a 30 minute insult of hypoxia (PaO2 = 40 mmHg) and hypotension (MABP = 30 mmHg) were imposed. In trauma animals, SI increased dramatically immediately following impact. By 15 minutes, permeability decreased exponentially and by 30 minutes was equal to that of control. In THH animals, SI enhancement was lower after the trauma, consistent with reduced blood pressure and blood flow. However, the SI increased dramatically upon reperfusion and was equal to that of control after 60 minutes. In conclusion we may consider, that CHI is associated with a rapid and transient BBB opening which begins at the time of the trauma and lasts not more than 30 minutes. It has been also shown that addition of hypoxia and hypotension prolongs the time of BBB breakdown.

Coma associated with flaccidity produced by fluid-percussion concussion in the cat. I: Is it due to depression of activity within the brainstem reticular formation?

This study is the first attempt to characterize neurological and behavioural consequences of fluid-percussion concussive head injury in the cat. Both animals initially anaesthetized by N2O as well as unanaesthetized, chronically prepared animals were subjected to injury. Injury with a fluid-pressure wave of 1.9-2.5 atm (duration 21-24 ms) produced a brief generalized areflexia. Following this initial response, injury greater than 2.1 atm frequently produced a period associated with hypotonia of postural muscles and suppression of postural motor responses (flaccidity). A close association between flaccidity and other indices of coma such as absence of eye-opening responses was noted. These consequences of injury can occur without fatal apnoae, circulatory collapse or overt intraparenchymal haemorrhages. This result suggests that mechanical stress predominantly restricted to the brain stem in fluid percussion may be sufficient, at least in the cat, to produce coma associated with flaccidity which has been previously documented for acceleration concussion. There was no evidence that fluid percussion produced EEG depression similar to the effects of lesions in the mesencephalic reticular activating system (RAS). Thus, depression of general levels of brain activity including those within the RAS seems not be necessary for production of this form of reversible coma.

Differential effects of atrial natriuretic peptide on the brain water and sodium after experimental cortical contusion in the rat.

Atrial natriuretic peptide (ANP) plays an important role in the regulation of water and sodium in the body via cyclic GMP (cGMP) pathway. Although ANP has been shown to be protective in cerebral ischemia or intracerebral hemorrhage, its role in traumatic brain injury (TBI) has yet to be elucidated. We herein assessed ANP effects on brain water and sodium in TBI. Controlled cortical impact (3 mm depth, 6 m/sec) was used to induce an experimental cortical contusion in rats. Continuous administration of ANP 0.2 (n = 6) or 0.7 μg/kg/24 h (n = 6), cGMP analogue (8-Bromo-cGMP) 0.1 (n = 5) or 0.3 mg/kg/24 h (n = 5), or vehicle (n = 6) was begun 15 minutes after injury, using a mini-osmotic pump implanted into the peritoneal cavity. At 24 hours after injury, ANP significantly exacerbated brain edema in the injured hemisphere in a dose-dependent manner while it reduced brain sodium concentrations in both hemispheres. These ANP effects could be mimicked by a cGMP analogue. In the second series (n = 20), BBB integrity was assessed by evaluating the extravasation of Evans blue dye. ANP or cGMP analogue significantly worsened BBB disruption in the injured hemisphere at 24 hours after injury. These findings suggest that ANP administration exacerbates brain edema after the experimental cortical contusion in rats, possibly because of an increase in the BBB permeability via cGMP pathway, whereas it reduces brain sodium levels.

Coma associated with flaccidity produced by fluid-percussion concussion in the cat. II: Contribution of activity in the pontine inhibitory system

In the preceding paper we reported that concussive levels of fluid-percussion head injury can produce transient flaccidity of postural muscles associated with other indices of coma. This reversible coma associated with flaccidity follows an initial period of generalized areflexia and occurs in the absence of EEG slow waves. The present study investigated the physiological mechanisms underlying the flaccidity following concussive head injury be recording dorsal and ventral root potentials of the spinal cord. Studies indicated that, during the initial period of generalized areflexia, afferent input transmission was depressed although the excitability of motoneuronal pools was increased. In contrast, during periods of flaccidity, spinal cord somatomotor functions were depressed while transmission of afferent inputs was recovering. Systematic transection of the brain stem showed that activity within structures lying between collicular and midpontine levels is necessary to produce this latter condition. Cholinergic activation of pontine inhibitory areas within this same region of the rostral pons can produce profound descending inhibitory influences on postural somatomotor function in conjunction with other features of coma including suppression of eye-opening responses. Such effects occur without EEG slow waves. Moreover, other data indicate that local rates of glucose utilization within this pontine inhibitory area increase following concussive head injury. Thus, it is possible that a predominance of activity within the pontine inhibitory area could provide at least one neural basis for the reversible comatose state following concussive head injury characterized by close association between flaccidity and other indices of coma. Possible relationships of these data to clinically observed features of concussion are discussed.

The effect of Cyclosporin A on brain edema formation following experimental cortical contusion

Cyclosporin A (CsA) has been shown by our laboratory and others to be neuroprotective in the experimental animal model of traumatic brain injury. However, we found that the intrathecal administration resulted in a concomitant increase of brain edema. The aim of this study was to assess whether intravascular administration may also influence brain edema formation. This project includes two independent series in which different doses of CsA were intravenously given to Sprague-Dawley rats of each group. In the first series, the animals were exposed to focal brain injury by a controlled cortical impact (CCI, 6 m/sec, 3 mm depth) and randomized into the following two groups: 20 mg/kg CsA and control vehicle. In the second series, animals were also injured by CCI and randomized into 35 mg/kg CsA and control vehicle. The intravenous continuous (1 h) infusion was begun 30 minutes after the insult. All animals were sacrificed at 24 hours post injury to assess the brain water content using the gravimetric method. Intravenously-administrated CsA of either 20 or 35 mg/kg did not significantly change the brain water content. We therefore suggest that an intravascular route may be better for CsA administration because the intrathecal injection may exacerbate brain edema as found in our previous study.

Diffuse Brain Injury Complicated by Acute Subdural Hematoma and Secondary Insults in the Rodents: The Effect of Surgical Evacuation

Head Trauma associated with acute sudural hematoma (SDH) and complicated by secondary insult is a grave clinical combination with complex pathophysiology. The aim of this study was to develop a clinically relevant injury model, which can be used to study the interaction between injury mechanisms. We present a novel model of SDH combined with diffuse brain injury (DBI) and a hypoxic secondary insult, and investigate the effects of surgical evacuation. Adult Sprague-Dawley rats were given a 300 microliters SDH and 20 minute-hypoxia following Impact Acceleration DBI. Hematoma was evacuated at one hour post-injury. Physiological parameters were measured for 5 hours, together with assessment of brain water content. Secondary insult after traumatic SDH was associated with significant brain swelling and stimulated refractory rise in ICP. In traumatic SDH complicated by secondary insult, brain swelling is exacerbated by surgical evacuation.