Age-related differences in acute physiologic response to focal traumatic brain injury in piglets

A perfect storm: The distribution of tissue damage depends on seizure duration, hemorrhage, and developmental stage in a gyrencephalic, multi-factorial, severe traumatic brain injury model

The pathophysiology of extensive cortical tissue destruction observed in hemispheric hypodensity, a severe type of brain injury observed in young children, is unknown. Here, we utilize our unique, large animal model of hemispheric hypodensity with multifactorial injuries and insults to understand the pathophysiology of this severe type of traumatic brain injury, testing the effect of different stages of development. Piglets developmentally similar to human infants (1 week old, “infants”) and toddlers (1 month old, “toddlers”) underwent injuries and insults scaled to brain volume: cortical impact, creation of mass effect, placement of a subdural hematoma, seizure induction, apnea, and hypoventilation or a sham injury while anesthetized with a seizure-permissive regimen. Piglets receiving model injuries required overnight intensive care. Hemispheres were evaluated for damage via histopathology. The pattern of damage was related to seizure duration and hemorrhage pattern in “toddlers” resulting in a unilateral hemispheric pattern of damage ipsilateral to the injuries with sparing of the deep brain regions and the contralateral hemisphere. While “infants” had the equivalent duration of seizures as “toddlers”, damage was less than “toddlers”, not correlated to seizure duration, and was bilateral and patchy as is often observed in human infants. Subdural hemorrhage was associate with adjacent focal subarachnoid hemorrhage. The percentage of the hemisphere covered with subarachnoid hemorrhage was positively correlated with damage in both developmental stages. In “infants”, hemorrhage over the cortex was associated with damage to the cortex with sparing of the deep gray matter regions; without hemorrhage, damage was directed to the hippocampus and the cortex was spared. “Infants” had lower neurologic scores than “toddlers”. This multifactorial model of severe brain injury caused unilateral, wide-spread destruction of the cortex in piglets developmentally similar to toddlers where both seizure duration and hemorrhage covering the brain were positively correlated to tissue destruction. Inherent developmental differences may affect how the brain responds to seizure, and thus, affects the extent and pattern of damage. Study into specifically how the “infant” brain is resistant to the effects of seizure is currently underway and may identify potential therapeutic targets that may reduce evolution of tissue damage after severe traumatic brain injury.

The pathologic outcomes and efficacy of epothilone treatment following traumatic brain injury is determined by age

Traumatic brain injury (TBI) can affect individuals at any age, with the potential of causing lasting neurologic consequences. The lack of effective therapeutic solutions and recommendations for patients that acquire a TBI can be attributed, at least in part, to an inability to confidently predict long-term outcomes following TBI, and how the response of the brain differs across the life span. The purpose of this study was to determine how age specifically affects TBI outcomes in a preclinical model. Male Thy1-YFPH mice, that express yellow fluorescent protein in the cytosol of a subset of Layer V pyramidal neurons in the neocortex, were subjected to a lateral fluid percussion injury over the right parietal cortex at distinct time points throughout the life span (1.5, 3, and 12 months of age). We found that the degree of neuronal injury, astrogliosis, and microglial activation differed depending on the age of the animal when the injury occurred. Furthermore, age affected the initial injury response and how it resolved over time. Using the microtubule stabilizing agent Epothilone D, to potentially protect against these pathologic outcomes, we found that the neuronal response was different depending on age. This study clearly shows that age must be taken into account in neurologic studies and preclinical trials involving TBI, and that future therapeutic interventions must be tailored to age.

Increased Intracranial Pressure Damages Optic Nerve Structural Support

Optic nerve sheath diameter (ONSD) is used clinically as a noninvasive measure for elevated intracranial pressure (ICP). This study had two purposes: to investigate the immediate effects optic nerve sheath (ONS) dilation post-ICP increase on trabecular fibers connecting the optic nerve to the ONS and to document any changes in these fibers 30 days post-increased ICP. In a swine model, ICP was increased by inflating a Foley catheter balloon in the epidural space. Three control pigs received the catheter insertion without inflation (no increase in ICP) and four experimental pigs received the catheter with inflation (increased ICP). The control and two randomly selected pigs with increased ICP were euthanized immediately after the procedure. The two other pigs were euthanized 30 days post-catheter inflation. For all pigs, the ONS was removed and imaged using a scanning electron microscope, calculating percent porosity values. Porosity values for the experimental groups (Immediately measured [IM] μ = 0.5749; Delayed measured [DM] μ = 0.5714) were larger than the control group (μ = 0.4336) and statistically significant (IM vs. Control, p = 0.0018; DM vs. Control, p = 0.0092). There was no significant difference (p = 0.9485) in porosity of the DM group when compared with the IM group. This study demonstrated that the trabecular fibers immediately post-increased ICP (ONS dilation) were more porous than the control and remained statistically different (more porous) after 30 days. These results suggest a structural change that occurs in the ONS with elevations in ICP.

Translational approach towards determining the role of cerebral autoregulation in outcome after traumatic brain injury

Cerebral autoregulation is impaired after traumatic brain injury (TBI), contributing to poor outcome. In the context of the neurovascular unit, cerebral autoregulation contributes to neuronal cell integrity and clinically Glasgow Coma Scale is correlated to intactness of autoregulation after TBI. Cerebral Perfusion Pressure (CPP) is often normalized by use of vasoactive agents to increase mean arterial pressure (MAP) and thereby limit impairment of cerebral autoregulation and neurological deficits. However, current vasoactive agent choice used to elevate MAP to increase CPP after TBI is variable. Vasoactive agents, such as phenylephrine, dopamine, norepinephrine, and epinephrine, clinically have not sufficiently been compared regarding effect on CPP, autoregulation, and survival after TBI. The cerebral effects of these clinically commonly used vasoactive agents are incompletely understood. This review will describe translational studies using a more human like animal model (the pig) of TBI to identify better therapeutic strategies to improve outcome post injury. These studies also investigated the role of age and sex in outcome and mechanism(s) involved in improvement of outcome in the setting of TBI. Additionally, this review considers use of inhaled nitric oxide as a novel neuroprotective strategy in treatment of TBI.

The pig as a preclinical traumatic brain injury model: current models, functional outcome measures, and translational detection strategies

Traumatic brain injury (TBI) is a major contributor of long-term disability and a leading cause of death worldwide. A series of secondary injury cascades can contribute to cell death, tissue loss, and ultimately to the development of functional impairments. However, there are currently no effective therapeutic interventions that improve brain outcomes following TBI. As a result, a number of experimental TBI models have been developed to recapitulate TBI injury mechanisms and to test the efficacy of potential therapeutics. The pig model has recently come to the forefront as the pig brain is closer in size, structure, and composition to the human brain compared to traditional rodent models, making it an ideal large animal model to study TBI pathophysiology and functional outcomes. This review will focus on the shared characteristics between humans and pigs that make them ideal for modeling TBI and will review the three most common pig TBI models-the diffuse axonal injury, the controlled cortical impact, and the fluid percussion models. It will also review current advances in functional outcome assessment measures and other non-invasive, translational TBI detection and measurement tools like biomarker analysis and magnetic resonance imaging. The use of pigs as TBI models and the continued development and improvement of translational assessment modalities have made significant contributions to unraveling the complex cascade of TBI sequela and provide an important means to study potential clinically relevant therapeutic interventions.

Angiopoietin/Tie2 Axis Regulates the Age-at-Injury Cerebrovascular Response to Traumatic Brain Injury

Although age-at-injury influences chronic recovery from traumatic brain injury (TBI), the differential effects of age on early outcome remain understudied. Using a male murine model of moderate contusion injury, we investigated the underlying mechanism(s) regulating the distinct response between juvenile and adult TBI. We demonstrate similar biomechanical and physical properties of naive juvenile and adult brains. However, following controlled cortical impact (CCI), juvenile mice displayed reduced cortical lesion formation, cell death, and behavioral deficits at 4 and 14 d. Analysis of high-resolution laser Doppler imaging showed a similar loss of cerebral blood flow (CBF) in the ipsilateral cortex at 3 and 24 h post-CCI, whereas juvenile mice showed enhanced subsequent restoration at 2-4 d compared with adults. These findings correlated with reduced blood-brain barrier (BBB) disruption and increased perilesional vessel density. To address whether an age-dependent endothelial cell (EC) response affects vessel stability and tissue outcome, we magnetically isolated CD31⁺ ECs from sham and injured cortices and evaluated mRNA expression. Interestingly, we found increased transcripts for BBB stability-related genes and reduced expression of BBB-disrupting genes in juveniles compared with adults. These differences were concomitant with significant changes in miRNA-21-5p and miR-148a levels. Accompanying these findings was robust GFAP immunoreactivity, which was not resolved by day 35. Importantly, pharmacological inhibition of EC-specific Tie2 signaling abolished the juvenile protective effects. These findings shed new mechanistic light on the divergent effects that age plays on acute TBI outcome that are both spatial and temporal dependent.SIGNIFICANCE STATEMENT Although a clear "window of susceptibility" exists in the developing brain that could deter typical developmental trajectories if exposed to trauma, a number of preclinical models have demonstrated evidence of early recovery in younger patients. Our findings further demonstrate acute neuroprotection and improved restoration of cerebral blood flow in juvenile mice subjected to cortical contusion injury compared with adults. We also demonstrate a novel role for endothelial cell-specific Tie2 signaling in this age-related response, which is known to promote barrier stability, is heightened in the injured juvenile vasculature, and may be exploited for therapeutic interventions across the age spectrum following traumatic brain injury.

Establishing a Clinically Relevant Large Animal Model Platform for TBI Therapy Development: Using Cyclosporin A as a Case Study

We have developed the first immature large animal translational treatment trial of a pharmacologic intervention for traumatic brain injury (TBI) in children. The preclinical trial design includes multiple doses of the intervention in two different injury types (focal and diffuse) to bracket the range seen in clinical injury and uses two post-TBI delays to drug administration. Cyclosporin A (CsA) was used as a case study in our first implementation of the platform because of its success in multiple preclinical adult rodent TBI models and its current use in children for other indications. Tier 1 of the therapy development platform assessed the short-term treatment efficacy after 24 h of agent administration. Positive responses to treatment were compared with injured controls using an objective effect threshold established prior to the study. Effective CsA doses were identified to study in Tier 2. In the Tier 2 paradigm, agent is administered in a porcine intensive care unit utilizing neurological monitoring and clinically relevant management strategies, and intervention efficacy is defined as improvement in longer term behavioral endpoints above untreated injured animals. In summary, this innovative large animal preclinical study design can be applied to future evaluations of other agents that promote recovery or repair after TBI.

Pre-Clinical Traumatic Brain Injury Common Data Elements: Toward a Common Language Across Laboratories

Traumatic brain injury (TBI) is a major public health issue exacting a substantial personal and economic burden globally. With the advent of "big data" approaches to understanding complex systems, there is the potential to greatly accelerate knowledge about mechanisms of injury and how to detect and modify them to improve patient outcomes. High quality, well-defined data are critical to the success of bioinformatics platforms, and a data dictionary of "common data elements" (CDEs), as well as "unique data elements" has been created for clinical TBI research. There is no data dictionary, however, for preclinical TBI research despite similar opportunities to accelerate knowledge. To address this gap, a committee of experts was tasked with creating a defined set of data elements to further collaboration across laboratories and enable the merging of data for meta-analysis. The CDEs were subdivided into a Core module for data elements relevant to most, if not all, studies, and Injury-Model-Specific modules for non-generalizable data elements. The purpose of this article is to provide both an overview of TBI models and the CDEs pertinent to these models to facilitate a common language for preclinical TBI research.

Animal models of traumatic brain injury

Traumatic brain injury (TBI) is a major health issue comprising a heterogeneous and complex array of pathologies. Over the last several decades, numerous animal models have been developed to address the diverse nature of human TBI. The clinical relevance of these models has been a major point of reflection given the poor translation of pharmacologic TBI interventions to the clinic. While previously characterized broadly as either focal or diffuse, this classification is falling out of favor with increased awareness of the overlap in pathologic outcomes between models and an emerging consensus that no one model is sufficient. Moreover, an appreciation of injury biomechanics is essential in recapitulating and interpreting the spectrum of TBI neuropathology observed in various established models of dynamic closed-head TBI. While these models have replicated many specific features of human TBI, an enhanced context with clinical relevancy will facilitate the further elucidation of the mechanisms and treatment of injury.

Combined hemorrhage/trauma models in pigs-current state and future perspectives

The majority of injury combinations in multiply injured patients entail the chest, abdomen, and extremities. Numerous pig models focus on the investigation of posttraumatic pathophysiology, organ performance monitoring and on potential treatment options. Depending on the experimental question, previous authors have included isolated insults (controlled or uncontrolled hemorrhage, chest trauma) or a combination of these injuries (hemorrhage with abdominal trauma, chest trauma, traumatic brain injury, and/or long-bone fractures). Combined trauma models in pigs can provide a high level of clinical relevance, when they are properly designed and mimicking the clinical situation. Most of these models focus on the first hours after trauma, to assess the acute sequel of traumatic hemorrhage. However, hemorrhagic shock and the associated mass transfusion are also major causes for organ failure and mortality in the later clinical course. Thus, most models lack information on the pathomechanisms during the late posttraumatic phase. Studying new therapies only during the early phase is also not reflective of the clinical situation. Therefore, a longer observation period is required to study the effects of therapeutic approaches during intensive care treatment when using animal models. These long-term studies of combined trauma models will allow the development of valuable therapeutic approaches relevant for the later posttraumatic course. This review summarizes the existing porcine models and outlines the need for long-term models to provide real effective novel therapeutics for multiply injured patients to improve organ function and clinical outcome.

Neurocritical care monitoring correlates with neuropathology in a swine model of pediatric traumatic brain injury

BACKGROUND

Small-animal models have been used in traumatic brain injury (TBI) research to investigate the basic mechanisms and pathology of TBI. Unfortunately, successful TBI investigations in small-animal models have not resulted in marked improvements in clinical outcomes of TBI patients.

OBJECTIVE

To develop a clinically relevant immature large-animal model of pediatric neurocritical care following TBI.

METHODS

Eleven 4-week-old piglets were randomly assigned to either rapid axial head rotation without impact (n = 6) or instrumented sham (n = 5). All animals had an intracranial pressure monitor, brain tissue oxygen tension (Pbto(2)) probe, and cerebral microdialysis probe placed in the frontal lobe and data collected for 6 hours following injury.

RESULTS

Injured animals had sustained elevations in intracranial pressure and lactate-pyruvate ratio (LPR), and decreased Pbto(2) compared with sham. Pbto(2) and LPR from separate frontal lobes had strong linear correlation in both sham and injured animals. Neuropathologic examination demonstrated significant axonal injury and infarct volumes in injured animals compared with sham at 6 hours postinjury. Averaged over time, Pbto(2) in both injured and sham animals had a strong inverse correlation with total injury volume. Average LPR had a strong correlation with total injury volume.

CONCLUSION

LPR and Pbto(2) can be utilized as serial nonterminal secondary markers in our injury model for neuropathology, and as evaluation metrics for novel interventions and therapeutics in the acute postinjury period. This translational model bridges a vital gap in knowledge between TBI studies in small-animal models and clinical trials in the pediatric TBI population.

Folic acid enhances early functional recovery in a piglet model of pediatric head injury

For stroke and spinal cord injury, folic acid supplementation has been shown to enhance neurodevelopment and to provide neuroprotection. We hypothesized that folic acid would reduce brain injury and improve neurological outcome in a neonatal piglet model of traumatic brain injury (TBI), using 4 experimental groups of 3- to 5-day-old female piglets. Two groups were intubated, anesthetized and had moderate brain injury induced by rapid axial head rotation without impact. One group of injured (Inj) animals received folic acid (Fol; 80 μg/kg) by intraperitoneal (IP) injection 15 min following injury, and then daily for 6 days (Inj + Fol; n = 7). The second group of injured animals received an IP injection of saline (Sal) at the same time points (Inj + Sal; n = 8). Two uninjured (Uninj) control groups (Uninj + Fol, n = 8; Uninj + Sal, n = 7) were intubated, anesthetized and received folic acid (80 μg/kg) or saline by IP injection at the same time points as the injured animals following a sham procedure. Animals underwent neurobehavioral and cognitive testing on days 1 and 4 following injury to assess behavior, memory, learning and problem solving. Serum folic acid and homocysteine levels were collected prior to injury and again before euthanasia. The piglets were euthanized 6 days following injury, and their brains were perfusion fixed for histological analysis. Folic acid levels were significantly higher in both Fol groups on day 6. Homocysteine levels were not affected by treatment. On day 1 following injury, the Inj + Fol group showed significantly more exploratory interest, and better motor function, learning and problem solving compared to the Inj + Sal group. Inj + Fol animals had a significantly lower cognitive composite dysfunction score compared to all other groups on day 1. These functional improvements were not seen on day 4 following injury. Axonal injury measured by β-amyloid precursor protein staining 6 days after injury was not affected by treatment. These results suggest that folic acid may enhance early functional recovery in this piglet model of pediatric head injury. This is the first study to describe the application of complex functional testing to assess an intervention outcome in a swine model of TBI.

The pig as a model animal for studying cognition and neurobehavioral disorders

In experimental animal research, a short phylogenetic distance, i.e., high resemblance between the model species and the species to be modeled is expected to increase the relevance and generalizability of results obtained in the model species. The (mini)pig shows multiple advantageous characteristics that have led to an increase in the use of this species in studies modeling human medical issues, including neurobehavioral (dys)functions. For example, the cerebral cortex of pigs, unlike that of mice or rats, has cerebral convolutions (gyri and sulci) similar to the human neocortex. We expect that appropriately chosen pig models will yield results of high translational value. However, this claim still needs to be substantiated by research, and the area of pig research is still in its infancy. This chapter provides an overview of the pig as a model species for studying cognitive dysfunctions and neurobehavioral disorders and their treatment, along with a discussion of the pros and cons of various tests, as an aid to researchers considering the use of pigs as model animal species in biomedical research.

Physiological and pathological responses to head rotations in toddler piglets

Closed head injury is the leading cause of death in children less than 4 years of age, and is thought to be caused in part by rotational inertial motion of the brain. Injury patterns associated with inertial rotations are not well understood in the pediatric population. To characterize the physiological and pathological responses of the immature brain to inertial forces and their relationship to neurological development, toddler-age (4-week-old) piglets were subjected to a single non-impact head rotation at either low (31.6 +/- 4.7 rad/sec(2), n = 4) or moderate (61.0 +/- 7.5 rad/sec(2), n = 6) angular acceleration in the axial direction. Graded outcomes were observed for both physiological and histopathological responses such that increasing angular acceleration and velocity produced more severe responses. Unlike low-acceleration rotations, moderate-acceleration rotations produced marked EEG amplitude suppression immediately post-injury, which remained suppressed for the 6-h survival period. In addition, significantly more severe subarachnoid hemorrhage, ischemia, and axonal injury by beta-amyloid precursor protein (beta-APP) were observed in moderate-acceleration animals than low-acceleration animals. When compared to infant-age (5-day-old) animals subjected to similar (54.1 +/- 9.6 rad/sec(2)) acceleration rotations, 4-week-old moderate-acceleration animals sustained similar severities of subarachnoid hemorrhage and axonal injury at 6 h post-injury, despite the larger, softer brain in the older piglets. We conclude that the traditional mechanical engineering approach of scaling by brain mass and stiffness cannot explain the vulnerability of the infant brain to acceleration-deceleration movements, compared with the toddler.

New concepts in treatment of pediatric traumatic brain injury

Emerging evidence suggests unique age-dependent responses following pediatric traumatic brain injury. The anesthesiologist plays a pivotal role in the acute treatment of the head-injured pediatric patient. This review provides important updates on the pathophysiology, diagnosis, and age-appropriate acute management of infants and children with severe traumatic brain injury. Areas of important clinical and basic science investigations germane to the anesthesiologist, such as the role of anesthetics and apoptosis in the developing brain, are discussed.

Diffuse optical monitoring of hemodynamic changes in piglet brain with closed head injury

We used a nonimpact inertial rotational model of a closed head injury in neonatal piglets to simulate the conditions following traumatic brain injury in infants. Diffuse optical techniques, including diffuse reflectance spectroscopy and diffuse correlation spectroscopy (DCS), were used to measure cerebral blood oxygenation and blood flow continuously and noninvasively before injury and up to 6 h after the injury. The DCS measurements of relative cerebral blood flow were validated against the fluorescent microsphere method. A strong linear correlation was observed between the two techniques (R=0.89, p<0.00001). Injury-induced cerebral hemodynamic changes were quantified, and significant changes were found in oxy- and deoxy-hemoglobin concentrations, total hemoglobin concentration, blood oxygen saturation, and cerebral blood flow after the injury. The diffuse optical measurements were robust and also correlated well with recordings of vital physiological parameters over the 6-h monitoring period, such as mean arterial blood pressure, arterial oxygen saturation, and heart rate. Finally, the diffuse optical techniques demonstrated sensitivity to dynamic physiological events, such as apnea, cardiac arrest, and hypertonic saline infusion. In total, the investigation corraborates potential of the optical methods for bedside monitoring of pediatric and adult human patients in the neurointensive care unit.

Diffuse brain injury in the immature rat: evidence for an age-at-injury effect on cognitive function and histopathologic damage

Diffuse axonal injury is a significant component of the pathology of moderate-severe pediatric traumatic brain injury in children less than 4 years of age, and is associated with poor cognitive outcome. However, cognitive deficits or gross histopathologic abnormalities are typically not observed following moderate-severe diffuse brain injury in the immature (17-day-old) rat. In order to test whether the age of the immature animal may influence post-traumatic outcome, non-contusive brain trauma was induced in post-natal day (PND) 11 or 17 rats. Brain injury in the PND11 rat, but not in the PND17 rat, was associated with a significant acquisition deficit at 28 days post-injury (p<0.0005 compared with age-matched sham rats, and with brain-injured PND17 rats). All brain-injured animals exhibited a retention deficit in the probe trial (p<0.001), but also demonstrated a significant visual deficit in the visible platform trial (p<0.05 compared to sham animals). Although significantly longer times of apnea and loss of righting reflex were observed in brain-injured PND17 rats compared to PND11 rats (p<0.05), overt cytoarchitectural alterations and reactive gliosis were not observed in the older age group. No focal pathology was observed in the cortex below the impact site in the PND11 rat but by 28 days, the brain-injured PND11 rat exhibited atrophy in multiple brain regions and an enlarged lateral ventricle in the impact hemisphere. Quantitative analysis revealed a time-dependent increase in tissue loss in the injured hemisphere (7-10%) in the younger animals, and a modest extent of tissue loss in the older animals (3-4%). Traumatic axonal injury was observed to similar extents in the white matter and thalamus below the impact site in both brain-injured PND11 and 17 rats. These data demonstrate that non-contusive (diffuse) brain injury of moderate severity in the immature rat is associated with chronic cognitive deficits and long-term histopathologic alterations and suggest that the age-at-injury is an important parameter of behavioral and pathologic outcome following closed head injury in the immature age group.

Chronic cognitive deficits and long-term histopathological alterations following contusive brain injury in the immature rat

Although diffuse axonal injury is the primary pathology in pediatric brain trauma, the additional presence of focal contusions may contribute to the poor prognosis in brain-injured children younger than 4 years of age. Because existing models of pediatric brain trauma focus on diffuse brain injury, a model of contusive brain trauma was developed using postnatal day (PND) 11 and 17 rats, ages that are neurologically equivalent to a human infant and toddler, respectively. Closed head injury was modeled by subjecting the intact skull over the left parietal cortex of the immature rat to an impact with a metal-tipped indenter. Brain trauma on PND11 or PND17 led to significant spatial learning deficits at 28 days post-injury, compared to age-matched control rats (p < 0.05). Although both groups of rats sustained skull fractures on impact, the histopathologic response of the brain was distinctly age-dependent. At 3 days post-injury in PND11 rats, the cortex below the impact site was contused and hemorrhagic, and contained reactive astrocytes, while the subcortical white matter and thalamus contained injured (swollen) axons. At 14 and 28 days post-injury, the cortex, white matter, and hippocampus were substantially atrophied, and the lateral ventricle was enlarged. In contrast, in PND17 rats, the contused cortex observed at 3 days post-injury matured into a pronounced cavity lined with a glia limitans at 14 days; reactive astrocytes were present in both the hippocampus and thalamus up to 28 days post-injury. No evidence of traumatic axonal injury was observed in any region of the brain-injured PND17 rat. These data suggest that contusive brain trauma in the immature rat is associated with chronic cognitive deficits, but underscore the effect of the age-at-injury on behavioral and histopathologic outcomes.

Basic science; maturation-dependent response of the immature brain to experimental subdural hematoma

In children less than 2 years of age, the radiographic finding of a subdural hematoma (SDH) in the absence of trauma is highly suggestive of inflicted head injury. Little is understood about the unique pathophysiologic response of the immature brain to a SDH. The goal of the current study was to develop an experimental SDH model to determine whether there is a maturation-dependent response of the immature brain to SDH. Fifteen domestic Yorkshire piglets of three different age groups (five each of 5-days, 1-month, and 4-months old) were selected for study. A volume of blood equal to 10% of the intracranial volume (4.5 cc in the 5-day old, 5.4 cc in the 1-month old, and 9.4 cc in the 4-month old) was injected through a right frontal burr hole. Histologic analysis, including hematoxylin and eosin staining and TUNEL staining, was performed at 7 days survival. A significant difference in percentage of injured hemisphere was noted between the 5-day old group and the 1- and 4-month old animals (p = 0.0382). The number of TUNEL-positive cells/HPF increased significantly with increasing animal age (p = 0.0450). The current study demonstrates a significant maturation-dependent response of the immature brain to SDH, with the youngest animals being quite resistant to a SDH alone. This model will allow further study of additional cerebral insults, such as the addition of apnea or seizures, which may act synergistically along with a SDH to overwhelm the innate neuroprotective capacity of the immature brain to traumatic injury.

The use of pigs in neuroscience: Modeling brain disorders

The use of pigs in neuroscience research has increased in the past decade, which has seen broader recognition of the potential of pigs as an animal for experimental modeling of human brain disorders. The volume of available background data concerning pig brain anatomy and neurochemistry has increased considerably in recent years. The pig brain, which is gyrencephalic, resembles the human brain more in anatomy, growth and development than do the brains of commonly used small laboratory animals. The size of the pig brain permits the identification of cortical and subcortical structures by imaging techniques. Furthermore, the pig is an increasingly popular laboratory animal for transgenic manipulations of neural genes. The present paper focuses on evaluating the potential for modeling symptoms, phenomena or constructs of human brain diseases in pigs, the neuropsychiatric disorders in particular. Important practical and ethical aspects of the use of pigs as an experimental animal as pertaining to relevant in vivo experimental brain techniques are reviewed. Finally, current knowledge of aspects of behavioral processes including learning and memory are reviewed so as to complete the summary of the status of pigs as a species suitable for experimental models of diverse human brain disorders.

Alpha-II-spectrin after controlled cortical impact in the immature rat brain

Proteolytic processing plays an important role in regulating a wide range of important cellular functions, including processing of cytoskeletal proteins. Loss of cytoskeletal proteins such as spectrin is an important characteristic in a variety of acute central nervous system injuries including ischemia, spinal cord injury and traumatic brain injury (TBI). The literature contains extensive information on the proteolytic degradation of alpha-II-spectrin after TBI in the adult brain. By contrast, there is limited knowledge on the characteristics and relevance of these important processes in the immature brain. The present experiments examine TBI-induced proteolytic processing of alpha-II-spectrin after TBI in the immature rat brain. Distinct proteolytic products resulting from the degradation of the cytoskeletal protein alpha-II-spectrin by calpain and caspase 3 were readily detectable in cortical brain parenchyma and cerebrospinal fluid after TBI in immature rats.

Pediatric traumatic brain injury: quo vadis?

In this review, five questions serve as the framework to discuss the importance of age-related differences in the pathophysiology and therapy of traumatic brain injury (TBI). The following questions are included: (1) Is diffuse cerebral swelling an important feature of pediatric TBI and what is its etiology? (2) Is the developing brain more vulnerable than the adult brain to apoptotic neuronal death after TBI and, if so, what are the clinical implications? (3) If the developing brain has enhanced plasticity versus the adult brain, why are outcomes so poor in infants and young children with severe TBI? (4) What contributes to the poor outcomes in the special case of inflicted childhood neurotrauma and how do we limit it? (5) Should both therapeutic targets and treatments of pediatric TBI be unique? Strong support is presented for the existence of unique biochemical, molecular, cellular and physiological facets of TBI in infants and children versus adults. Unique therapeutic targets and enhanced therapeutic opportunities, both in the acute phase after injury and in rehabilitation and regeneration, are suggested.

Large animal models of traumatic injury to the immature brain

Large animal models have been used much less frequently than rodent models to study traumatic brain injury. However, large animal models offer distinct advantages in replicating specific mechanisms, morphology and maturational stages relevant to age-dependent injury responses. This paper reviews how each of these features is relevant in matching a model to a particular scientific question and discusses various scaling strategies, advantages and disadvantages of large animal models for studying traumatic brain injury in infants and children. Progress to date and future directions are outlined.

Traumatic axonal injury is exacerbated following repetitive closed head injury in the neonatal pig

Inflicted brain injury is associated with widespread traumatic axonal injury (TAI) and subdural hematoma and is the leading cause of death in infants and children. Anesthetized 3-5-day-old piglets were subjected to either a single (n = 5) or double (n = 6, 15 min apart) rapid (<15 msec), non-impact, axial rotations of the head. Peak rotational velocities (averaging 172 rad/sec for single and 138 rad/sec for double loads) were lower than those utilized to induce severe injuries (240-260 rad/sec; Raghupathi and Margulies, 2002). At 6 h post-injury, brains were evaluated for the presence TAI using immunohistochemistry for the 200-kDa neurofilament protein (NF200). Accumulation of NF200 was observed in both contiguous (swellings) and in disconnected axons (axon bulbs) predominantly in central deep and peripheral subcortical white matter regions in the frontal, temporal, and parietal lobes of all injured piglets. Although the density of injured axons did not significantly increase after two rotational loads, the distribution of injured axons shifted from a few foci (2.2 +/- 2.3 per animal) with 1-2 swellings/bulbs following a single rotation to significantly more foci (14.7 +/- 11.9), and additional foci (2.5 +/- 1.9) containing 3 or more axon swellings/bulbs following two rotational loads. The density and distribution of injured axons following a single mild rotation were significantly reduced compared with those obtained previously following a single more severe rotational load. Collectively, these data are indicative of the graded response of the immature brain to rotational load magnitude, and importantly, the vulnerability to repeated, mild, non-impact loading conditions.

Moderate controlled cortical contusion in pigs: effects on multi-parametric neuromonitoring and clinical relevance

Over the last decade, routine neuromonitoring of ICP and CPP has been extended with new on-line techniques such as microdialysis, tissue oxygen (ptiO(2)), acid-base balance (ptiCO(2), pH) and CBF measurements, which so far have not lead to clear-cut therapy approaches in the neurointensive care unit. This is partially due to the complex pathophysiology following a wide-range of brain injuries, and the lack of suitable animal models allowing simultaneous, clinically relevant neuromonitoring under controlled conditions. Therefore, a controlled cortical impact (CCI) model in large animals (pig) has been developed. After placement of microdialysis, ptiO(2), temperature and ICP catheters, an unilateral CCI injury (2.6-2.8 m/sec velocity, 9 mm depth, 400 ms dwell time) was applied and neuromonitoring continued for 10 h. CCI caused a rapid drop in CPP, ptiO(2) and glucose, whereas ICP, glutamate and lactate increased significantly. Most parameters returned to baseline values within hours. Lactate stayed elevated significantly throughout the experiment, but the lactate-to-pyruvate ratio (LPR) changed only slightly, indicating no severely ischemic CBF. Contralateral parameters were not affected significantly. Evaluation of brain water content and histology (12 h post-CCI) showed ipsilateral brain swelling by 5% and massive cell damage underneath the injury site which correlated with changes of ICP, CPP, glutamate, lactate, and ptiO(2) within the first hours post-CCI. Moderate controlled cortical contusion in pigs induced a complex pattern of pathophysiological processes which led to 'early' histological damage. Thus, this new large animal model will enable us to investigate the effect of therapeutic interventions on multi-parametric neuromonitoring and histological outcome, and to translate the data into clinical practice.

Age-Dependent Changes in Material Properties of the Brain and Braincase of the Rat

Clinical and biomechanical evidence indicates that mechanisms and pathology of head injury in infants and young children may be different from those in adults. Biomechanical computer-based modeling, which can be used to provide insight into the thresholds for traumatic tissue injury, requires data on material properties of the brain, skull, and sutures that are specific for the pediatric population. In this study, brain material properties were determined for rats at postnatal days (PND) 13, 17, 43, and 90, and skull/suture composite (braincase) properties were determined at PND 13, 17, and 43. Controlled 1 mm indentation of a force probe into the brain was used to measure naive, non-preconditioned (NPC) and preconditioned (PC) instantaneous (G(i)) and long-term (G( infinity )) shear moduli of brain tissue both in situ and in vitro. Brains at 13 and 17 PND exhibited statistically indistinguishable shear moduli, as did brains at 43 and 90 PND. However, the immature (average of 13 and 17 PND) rat brain (G(i) = 3336 Pa NPC, 1754 Pa PC; G( infinity )= 786 Pa NPC, 626 Pa PC) was significantly stiffer (p < 0.05) than the mature (average of 43 and 90 PND) brains (G(i) = 1721 Pa NPC, 1232 Pa PC; G( infinity ) = 508 Pa NPC, 398 Pa PC). A "reverse engineering" finite element model approach, which simulated the indentation of the force probe into the intact braincase, was used to estimate the effective elastic moduli of the braincase. Although the skull of older rats was significantly thicker than that of the younger rats, there was no significant age-dependent change in the effective elastic modulus of the braincase (average value = 6.3 MPa). Thus, the increase in structural rigidity of the braincase with age (up to 43 PND) was due to an increase in skull thickness rather than stiffening of the tissue. These observations of a stiffer brain and more compliant braincase in the immature rat compared with the adult rat will aid in the development of age-specific experimental models and in computational head injury simulations. Specifically, these results will assist in the selection of forces to induce comparable mechanical stresses, strains and consequent injury profiles in brain tissues of immature and adult animals.

Magnetic resonance imaging studies of age-dependent responses to scaled focal brain injury in the piglet

OBJECT

Whether the brain differs in its response to traumatic injury as a function of age remains unclear. To further investigate the age-dependent response of the brain to mechanical trauma, a cortical contusion model scaled for brain growth during maturation was used to study the evolution of injury over time as demonstrated on serial magnetic resonance (MR) imaging studies in piglets of different ages.

METHODS

Sixteen Yorkshire piglets received scaled cortical contusions. Animals were either 5 days (six animals), 1 month (five animals), or 4 months (five animals) of age at injury. These ages correspond developmentally to human infants, toddlers, and early adolescents, respectively. Serial MR imaging examinations, including fluid-attenuated inversion-recovery and T1-, T2-, and diffusion-weighted sequences were performed at 24 hours, 1 week, and 1 month after injury. Lesions were quantified and expressed as a ratio of the lesion volume divided by the volume of the uninjured hemisphere for each animal and each MR sequencing. Differences in relative lesion volume among the varied ages at a single time point and in lesion volume over time at each age were compared. In addition, the relationship between age and evolution of injury were analyzed using a two-compartment mathematical model. Histological features were examined at 1 month postinjury. Despite comparable injury inputs, the youngest animals had lesions whose volumes peaked earlier and resolved more quickly than those in older animals. The intermediate-age piglets (toddler) had the most pronounced swelling of any age group, and the oldest piglets (adolescent) had the latest peak in lesion volume.

CONCLUSIONS

Scaled cortical contusions in piglets demonstrated age-dependent differences in injury response, both in magnitude and time course. These observations may shed light on development-related trauma response in the gyrencephalic brain.

Traumatic brain injury in piglets of different ages: techniques for lesion analysis using histology and magnetic resonance imaging

Quantitation of lesions in large gyrencephalic brains presents a variety of technical challenges. Specific techniques are required when comparing lesions in subjects of different ages in order to assess maturational effects. We have modified existing techniques to attain reliable, consistent and reproducible paraffin-embedded histological sections for volumetric lesion analysis and correlation with magnetic resonance imaging (MRI) in piglets of different ages following focal traumatic brain injury. Twenty-four Yorkshire domestic piglets at three different ages (5 days, 1 month, and 4 months old) underwent scaled cortical impact injury to the fronto-parietal cortex. This contusion model utilizes a rapid volume of indentation scaled proportionally to the growth of the brain, allowing for examination of maturational influences on the brain's response to focal mechanical trauma. To overcome problems with differential processing and embedding of brains ranging from 43 to 107 g, we developed a piglet parallel brain slicing apparatus. Along with specific methods for processing, embedding, mounting, and slide preparation, these techniques enabled excellent quality 10-microm serial coronal sections to be obtained for histology and immunohistochemical analysis. Accurate co-registration of histologic, immunohistochemical and radiologic images at different ages was possible, which may enhance understanding of developmental aspects of brain injury pathophysiology.

Traumatic axonal injury after closed head injury in the neonatal pig

Closed head injury is the leading cause of morbidity and mortality in infants and children, and results in pathologies such as diffuse axonal injury (DAI) and subarachnoid hematoma (SAH). To better understand the mechanical environment associated with closed head injury in the pediatric population, animal models that include salient features of human infant brain must be utilized. Based on detailed information regarding the parallels between brain development in the pig and the human, the 3-5-day-old piglet was used to represent the infant at less than 3 months of age. Anesthetized piglets (n = 7) were subjected to rapid, inertial (nonimpact) rotation of the head about its axial plane and sacrificed at 6 h postinjury. Immediately following injury, five of seven piglets were apneic, with an absence of pupillary and pain reflexes. All piglets exhibited severe coma immediately postinjury, but recovered by sacrifice time. Blood was present on the surface of the frontal lobes, cerebellum, and brainstem, and subarachnoid hemorrhage was evident in the frontal cortex. In six of seven brain-injured piglets, accumulation of the 68-kDa neurofilament protein was evident in contiguous axons (swollen) and occasionally in disconnected axons (axonal bulbs), suggestive of traumatic axonal injury (TAI). Mapping of the regional pattern of TAI revealed injured axons predominantly in central and peripheral white matter tracts in the frontal and temporal lobes and in the midbrain. The number of injured axons was equivalent in both hemispheres, and did not correlate to the load applied to the head. Together, these data demonstrate that rapid rotation of the piglet head without impact results in SAH and TAI, similar to that observed in children following severe brain trauma.