This was an initial study testing the effects of a single, minor head impact on the development of early edema in a closed head, momentum exchange model of brain injury and the ability of a V1a receptor antagonist to reduce or block observed changes. The ADC from diffusion weighted imaging was used as a proxy for vasogenic edema. In this model of mild head injury, edema was significantly increased in whole brain and a subset of the 11 regions that were tested, including basal ganglia, cerebellum, and thalamus, without evidence of structural damage. The observed edema was transient because at 24 hours post-injury, vehicle-treated rats with mild head injury did not differ from sham controls in ADC. V1a antagonist treatment significantly reduced edema compared to vehicle treatment in whole brain, thalamus, and cerebellum.
Edema is a critical component in the pathophysiology of TBI and can occur within the first few hrs post-injury[29-31]. The type ofedema occurring after TBI can be determined using DWI with ADC, a quantitative measure of water mobility[29, 32-34].Vasogenic edema, characterized by higher ADC values, contributes to parenchymal swelling and an increase in intracranial pressure. In the present study, only higher ADC values were found in the affected regions, indicating that a single, mild head injury can induce vasogenic edema.
An important question is why the cerebellum and thalamus, regions caudal to the site of impact on the forehead, but not the prefrontal cortex immediately beneath the site of impact, show the greatest increases in ADC.The transmission of mechanical force down white matter tracts and along brain areas abutting the ventricular system may offer an explanation . The cerebellum is recognized as beingparticularly vulnerable to mild TBI [36-39] andneuroradiological evidence of cerebellar dysfunction has beenadvanced as a diagnostic biomarker of TBI . Because this is the only studywe knowof that used a closed-head, mild injury protocol, we cannot readily compare our findings to other rodent models that producestructural brain damage.
The increase in ADC, which reflects vasogenic edema, suggests a change in the permeability of the BBB with a single mild head impact. BBB permeability increases within 1-4 hr hrs after brain damage in rodent models of TBI based on extravasation of blood biomarkers, e.g. Evan blue, plasma immunoglobulins, horse radish peroxidase, and albumin[40-43]. Similar results, albeit in moderate TBI models, have been reported in rats usingdynamic contrast enhanced (DCE) MRI to measure BBB permeability [44, 45] and in humans, DCE MRI shows increased BBB permeability around damaged tissue within 24 hrs of injury that persists for days .Athletes playing American football and thought to have mild,sub-concussive head injuries show evidence of increased BBB permeability with DCE MRI. Using DCE-MRI, O’Keeffe and colleagues reported BBB disruption in response to repetitive mild concussions in rugby players after a season and mixed martial arts fighters within days after a competitive fight . There were no significant differences in self-reported concussions between these players and control subjects (track and field athletes).Players with multiple subconcussive collisions in American football show elevated blood levels of astrocytic protein S100B thought to be due to BBB disruption . Interestingly, professional boxers show an increase in ADC values in large white matter tracts and a general increase in brain diffusivity attributed to the cumulative effect of multiple non-severe head impacts [50-52].To our knowledge, these are the first data reporting evidence of global and region-specific increases in brain edema to a single, mild head injury in the absence of structural brain damage.
These data reinforce concerns expressed by others about the long-term effect of mild head injury . For example, mild head injury begins with an impact that by itself may be recorded but not pursued as needing medical attention. Yet here we show there are global changes in vasogenic edema presumably caused by disruption in the BBB. Newsomeet al. reported modest neurocognitiveabnormalities and increased functional connectivity in adolescent athletes with sports related concussions compared to age-matched, orthopedically injured controls. Despite these signs of altered cortical function, post-concussive symptoms were judged to be resolvedand athletes could return to competition. Indeed, more adolescent players are suffering from undetected neurological problems than previously thought . A single season of high school American football producesDWI changes in diffusivity in the absence of clinical concussion. Multiple mild head injuries are a risk factor for cognitive and emotional impairment and neurodegenerative disorders in athletes.
Evidence for the involvement AVP in the development of edema associated with TB is substantial. Following a moderate impact to the brain, the tissue surrounding the damage parenchyma increases the expression of V1a receptors on neurons, astrocytes and endothelium of microvessels, an effect that occurs within hrs and persists for days [57, 58]. AVP appears to have a role in post-traumatic neuroinflammation as the proinflammatory chemokines in endothelial cells and astrocytes of damaged tissue are reduced in AVP-deficient Brattleboro rats compared to wild-type controls [59, 60].With brain damage,the water content from vasogenic edema goes up dramatically in the first 5 to 24 hrs [23, 30, 31]. The increase in brain edema can be reduced by the continuous intravenous infusion of SR49059, a V1a receptor antagonist,beginning immediately after head injury [22, 23, 30]. The intracerebroventricular injection of V1a receptor antagonists e.g., SR-49059, V1880 or the peptidic deamino-Pen(1), O-Me-Tyr(2), Arg(8)]-vasopressin[31, 62], are also effective but only when given within the first few hrs of injury.Moderate head injury is associated with elevated levels of the water channel AQP4, GFAP, and V1a receptor, and disruption in sodium/potassium balance, all of which can be corrected by continuous exposure to SR49059[22, 23, 30]. The edema in the these moderate TBI models is reduced in V1a null mice.In a recent study, we reported that AVN576, an orally active V1a receptor antagonist,givenipbeginning 24 hrs post injury for 5 days, could effectively reduce edema following TBI with brain damage.
Limitations and Data Interpretation
This was aninitial study with a limited number of animals to explore the possibility that we could measure significant changes in ADC following a single mild head impact. As such, there were several limitations: 1) The sensitivity of DWI did not allow us to examine 150 individual brain areas; instead, we were only able to observe change in ADC values in the whole brain and in three brain regions, cerebellum, thalamus, and basal ganglia;2) Would there have been sex differences if female rats also had been tested? We are aware of the many studies reporting sex differences in response to TBI, e.g. [63-68]. Recent reviews report there are male/female differences in morbidity and mortality following TBI [69, 70]. However, the data collected over decades of preclinical and clinical research are inconsistent, certainly as it relates to the role of estrogen and progesterone. Further, these initial studies were not designed to address the issues surrounding hormones and head injury;3) While there was no evidence of structural brain damage, post mortem histology looking at site-specific or regional signs of inflammation or alterations in BBB permeability would have helpedto interpret the findings. 4) Animals were exposed to light isoflurane anesthesia four times over a 24 hr period. This necessary condition over this abbreviated time span for imaging precluded taking any meaningful measures of motor or cognitive behavior.