Chronic postnatal hypoxia increases the numbers of cortical neurons

Infants with neonatal Chronic Lung Disease are associated with delayed auditory conduction in the rostral brainstem after term

AIMS

Very Low Birthweight (VLBW) infants with neonatal Chronic Lung Disease (CLD) have been found to have functional impairment of the brainstem auditory pathway at term. This study investigated the functional status of the brainstem auditory pathway in VLBW infants with CLD after term for any abnormality.

METHODS

Fifty-two VLBW infants were recruited at 50 weeks of Postconceptional Age: 25 with neonatal CLD and 27 without CLD. None had any other major complications to minimize confounding effects. Brainstem Auditory Evoked Responses were studied at 21‒91/s click rates.

RESULTS

Compared with those without CLD, VLBW infants with CLD had relatively shorter latencies of BAER waves I and III, associated with a slightly lower BAER threshold. Wave V latency and I‒V interpeak interval did not differ significantly between the two groups of infants. The I‒III interval in infants with CLD was shorter than in those without CLD at 91/s clicks. However, the III‒V interval was significantly longer than in those without CLD at all click rates (all p < 0.05). There were no significant differences in the amplitudes of BAER wave components between the two groups of infants.

CONCLUSIONS

The main BAER abnormality in VLBW infants with CLD was a prolonged III‒V interval. Auditory conduction is delayed or impaired at more central regions of the brainstem in CLD infants. After term central auditory function is adversely affected by neonatal CLD. Monitoring post-term change is required to provide valuable information for post-term care of CLD infants.

Perinatal Hypoxia and Ischemia in Animal Models of Schizophrenia

Intrauterine or perinatal complications constitute a major risk for psychiatric diseases. Infants who suffered from hypoxia-ischemia (HI) are at twofold risk to develop schizophrenia in later life. Several animal models attempt to reproduce these complications to study the yet unknown steps between an insult in early life and outbreak of the disease decades later. However, it is very challenging to find the right type and severity of insult leading to a disease-like phenotype in the animal, but not causing necrosis and focal neurological deficits. By contrast, too mild, repetitive insults may even be protective via conditioning effects. Thus, it is not surprising that animal models of hypoxia lead to mixed results. To achieve clinically translatable findings, better protocols are urgently needed. Therefore, we compare widely used models of hypoxia and HI and propose future directions for the field.

Pediatric brain repair from endogenous neural stem cells of the subventricular zone

There is great interest in the regenerative potential of the neural stem cells and progenitors that populate the germinal zones of the immature brain. Studies using animal models of pediatric brain injuries have provided a clearer understanding of the responses of these progenitors to injury. In this review, we have compared and contrasted the responses of the endogenous neural stem cells and progenitors of the subventricular zone in animal models of neonatal cerebral hypoxia-ischemia, neonatal stroke, congenital cardiac disease, and pediatric traumatic brain injury. We have reviewed the dynamic shifts that occur within this germinal zone with injury as well as changes in known signaling molecules that affect these progenitors. Importantly, we have summarized data on the extent to which cell replacement occurs in response to each of these injuries, opportunities available, and obstacles that will need to be overcome to improve neurological outcomes in survivors.

Focal Brain Injury Associated with a Model of Severe Hypoxic-Ischemic Encephalopathy in Nonhuman Primates

Worldwide, hypoxic-ischemic encephalopathy (HIE) is a major cause of neonatal mortality and morbidity. To better understand the mechanisms contributing to brain injury and improve outcomes in neonates with HIE, better preclinical animal models that mimic the clinical situation following birth asphyxia in term newborns are needed. In an effort to achieve this goal, we modified our nonhuman primate model of HIE induced by in utero umbilical cord occlusion (UCO) to include postnatal hypoxic episodes, in order to simulate apneic events in human neonates with HIE. We describe a cohort of 4 near-term fetal Macaca nemestrina that underwent 18 min of in utero UCO, followed by cesarean section delivery, resuscitation, and subsequent postnatal mechanical ventilation, with exposure to intermittent daily hypoxia (3 min, 8% O2 3-8 times daily for 3 days). After delivery, all animals demonstrated severe metabolic acidosis (pH 7 ± 0.12; mean ± SD) and low APGAR scores (<5 at 10 min of age). Three of 4 animals had both electrographic and clinical seizures. Serial blood samples were collected and plasma metabolites were determined by 2-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GC × GC-TOFMS). The 4 UCO animals and a single nonasphyxiated animal (delivered by cesarean section but without exposure to UCO or prolonged sedation) underwent brain magnetic resonance imaging (MRI) on day 8 of life. Thalamic injury was present on MRI in 3 UCO animals, but not in the control animal. Following necropsy on day 8, brain histopathology revealed neuronal injury/loss and gliosis in portions of the ventrolateral thalamus in all 4 UCO, with 2 animals also demonstrating putamen/globus pallidus involvement. In addition, all 4 UCO animals demonstrated brain stem gliosis, with neuronal loss present in the midbrain, pons, and lateral medulla in 3 of 4 animals. Transmission electron microscopy imaging of the brain tissues was performed, which demonstrated ultrastructural white matter abnormalities, characterized by perinuclear vacuolation and axonal dilation, in 3 of 4 animals. Immunolabeling of Nogo-A, a negative regulator of neuronal growth, was not increased in the injured brains compared to 2 control animals. Using GC × GC-TOFMS, we identified metabolites previously recognized as potential biomarkers of perinatal asphyxia. The basal ganglia-thalamus-brain stem injury produced by UCO is consistent with the deep nuclear/brainstem injury pattern seen in human neonates after severe, abrupt hypoxic-ischemic insults. The UCO model permits timely detection of biomarkers associated with specific patterns of neonatal brain injury, and it may ultimately be useful for validating therapeutic strategies to treat neonatal HIE.

The role of inflammation in perinatal brain injury

Inflammation is increasingly recognized as being a critical contributor to both normal development and injury outcome in the immature brain. The focus of this Review is to highlight important differences in innate and adaptive immunity in immature versus adult brain, which support the notion that the consequences of inflammation will be entirely different depending on context and stage of CNS development. Perinatal brain injury can result from neonatal encephalopathy and perinatal arterial ischaemic stroke, usually at term, but also in preterm infants. Inflammation occurs before, during and after brain injury at term, and modulates vulnerability to and development of brain injury. Preterm birth, on the other hand, is often a result of exposure to inflammation at a very early developmental phase, which affects the brain not only during fetal life, but also over a protracted period of postnatal life in a neonatal intensive care setting, influencing critical phases of myelination and cortical plasticity. Neuroinflammation during the perinatal period can increase the risk of neurological and neuropsychiatric disease throughout childhood and adulthood, and is, therefore, of concern to the broader group of physicians who care for these individuals.

Erythropoietin as a neuroprotectant for neonatal brain injury: animal models

Prematurity and perinatal hypoxia-ischemia are common problems that result in significant neurodevelopmental morbidity and high mortality worldwide. The Vannucci model of unilateral brain injury was developed to model perinatal brain injury due to hypoxia-ischemia. Because the rodent brain is altricial, i.e., it develops postnatally, investigators can model either preterm or term brain injury by varying the age at which injury is induced. This model has allowed investigators to better understand developmental changes that occur in susceptibility of the brain to injury, evolution of brain injury over time, and response to potential neuroprotective treatments. The Vannucci model combines unilateral common carotid artery ligation with a hypoxic insult. This produces injury of the cerebral cortex, basal ganglia, hippocampus, and periventricular white matter ipsilateral to the ligated artery. Varying degrees of injury can be obtained by varying the depth and duration of the hypoxic insult. This chapter details one approach to the Vannucci model and also reviews the neuroprotective effects of erythropoietin (Epo), a neuroprotective treatment that has been extensively investigated using this model and others.

Effect of hypobaric hypoxia on the P2X receptors of pyramidal cells in the immature rat hippocampus CA1 sub-field

PRIMARY OBJECTIVE

This study was designed to evaluate the effect of hypobaric hypoxia (HH) on the function and expression of P2X receptors in rat hippocampus CA1 pyramidal cells.

RESEARCH DESIGN

The functional changes of P2X receptors were investigated through the cell HH model and the expressional alterations of P2X receptors were observed through the animal HH model.

METHODS AND PROCEDURE

P2X receptors mediated currents were recorded from the freshly dissociated CA1 pyramidal cells of 7-day-old SD rats by whole cell patch clamp recording. The expression and distribution of P2X receptors were observed through immunohistochemistry and western blot at HH 3-day and 7-day.

MAIN OUTCOMES AND RESULTS

In acute HH conditions, the amplitudes of ATP evoked peak currents were decreased compared to control. The immunohistochemistry and western blot results reflected there was no change in P2X receptors expression after 3 days HH injury, while P2X receptors expression was up-regulated in response to 7 days HH injury.

CONCLUSIONS

These findings supported the possibility that the function of P2X receptors was sensitive to HH damage and long-term function decrease should result in the expression increase of P2X receptors.

Sex differences in a hypoxia model of preterm brain damage

Male sex is a well-established risk factor for poor neurodevelopmental outcome after premature birth. The mechanisms behind this sex-related difference are unknown. The damage associated with prematurity can be mimicked in rodents by prolonged exposure to sublethal postnatal hypoxia. This chronic hypoxia leads to anatomical changes in mice that strongly resemble the loss of volume, decreased myelination, and ventriculomegaly seen in preterm newborns. However, no sex differences have been previously noted in this rodent model. We hypothesized that sex comparisons in hypoxic mice would show sex-related differences in brain volume and white matter loss in response to the same degree of hypoxic insult. Mice were placed in chronic sublethal hypoxia from postnatal day 3-11. Cortical, hippocampal, and cerebellar volumes and myelination indices were measured. We found that the male hippocampus, normally larger than the female, undergoes a greater volume loss compared with females (p < 0.05). Myelination, generally greater in males, was significantly disrupted by hypoxia in neonatal male forebrain. These results support the use of this rodent model to investigate the basis of sex-related susceptibility to brain damage and develop new sex-based neuroprotective strategies.

Sustained hypoxia modulates mitochondrial DNA content in the neonatal rat brain

The effects of placental insufficiency and preterm birth on neurodevelopment can be modeled in experimental settings of neonatal hypoxia in rodents. Here, rat pups were reared in reduced oxygen (9.5%) for 11 days, starting on postnatal day 3 (P3). This led to a significant reduction in brain and body weight gain in hypoxic pups compared to age-matched normoxia-reared controls, plausibly reflecting an inability to fulfill the energetic needs of normal growth and development. Adaptive processes designed to augment energetic capacity in eukaryotes include stimulation of mitochondrial biogenesis. We show that after 11 days of sustained hypoxia, the levels of nuclear respiratory factor-1 and mitochondrial transcription factor A are elevated and the content of mitochondrial DNA (mtDNA) is greater in the hypoxic P14 pup brain compared to normoxic conditions. Corresponding immunohistochemical analyses reveal increased density of mtDNA in large cortical neurons. In contrast, no changes in mtDNA content are observed in the brain of pups reared for 24 h (P3-P4) under hypoxic conditions. Together, these data suggest that prolonged inadequate oxygenation may trigger a compensatory increase in neuronal mitochondrial DNA content to partially mitigate compromised energy homeostasis and reduced energetic capacity in the developing hypoxic brain.

Prenatal Cord Clamping in Newborn Macaca nemestrina: A Model of Perinatal Asphyxia

Our objective was to establish a nonhuman primate model of perinatal asphyxia appropriate for preclinical evaluation of neuroprotective treatment strategies under conditions that closely resemble human neonatal emergencies, and to begin testing the safety and efficacy of erythropoietin neuroprotective treatment. Prior to delivery by hysterotomy, the umbilical cords of near term Macaca nemestrina (n = 8) were clamped for times ranging between 12 and 15 min. Animals received erythropoietin (5,000 U/kg/dose x 2 i.v., n = 3), or vehicle (n = 5) after resuscitation. We assessed physiologic parameters, continuous electroencephalogram, magnetic resonance imaging/spectroscopy, safety parameters and behavior. Animals were euthanized at 4 months of age. Mean birth weight was 507 +/- 62 g. Initial arterial pH ranged from 6.75 to 7.12, with base deficits of 17-25 mEq. Animals were flaccid at birth, with attenuated electroencephalograms, and seizures occurred in 3 of 8 animals. We demonstrated magnetic resonance imaging/spectroscopy changes consistent with hypoxia (elevated lactate levels were present in some animals), significant motor and behavioral abnormalities (particularly with 15 min of cord clamping), and evidence of gliosis at the time of death. We have established a reproducible model of moderate to severe perinatal hypoxic-ischemic injury in M. nemestrina newborns. This model, which combines structural, biochemical, and behavioral assessments over time can be used to assess the safety and efficacy of neuroprotective strategies.

Cerebral cortex thickness in 15-year-old adolescents with low birth weight measured by an automated MRI-based method

Infants with low birth weight are at increased risk of perinatal brain injury. Disruption of normal cortical development may have consequences for later motor, behavioural and cognitive development. The aim of this study was to measure cerebral cortical thickness, area and volume with an automated MRI technique in 15-year-old adolescents who had low birth weight. Cerebral MRI for morphometric analysis was performed on 50 very low birth weight (VLBW, birth weight </=1500 g), 49 term small for gestational age births (SGA, birth weight <10th percentile at term) and 58 control adolescents. A novel method of cortical surface models yielded measurements of cortical thickness and area for each subject's entire brain and computed cross-subject statistics based on cortical anatomy. The cortical surface models demonstrated regional thinning of the parietal, temporal and occipital lobes in the VLBW group, whereas regional thickening was demonstrated in the frontal and occipital lobes. The areas of change were greatest in those with the shortest gestational age at birth and lowest birth weight. Cortical surface area and cortical volume were lower in the VLBW than in the Control group. Within the VLBW group, there was an association between surface area and estimation of the intelligence quotient IQ (IQ(est)) and between cortical volume and IQ(est). Furthermore, cortical grey matter as a proportion of brain volume was significantly lower in the VLBW, but not in the SGA group compared with Controls. This observed reorganization of the developing brain offers a unique opportunity to investigate any relationship between changes in cortical anatomy and cognitive and social impairments, and the increase in psychiatric disorders that have been found in VLBW children and adolescents.

Long-term effects of acute and of chronic hypoxia on behavior and on hippocampal histology in the developing brain

Ten-day-old rat pups (P10) subjected to acute hypoxia (down to 4% O2) had as adults increased aggression (handling test), memory impairment (water maze test), and decreased CA1 cell counts. Pups subjected to chronic hypoxia (10% O2 from P0 to P21) had increased aggression, hyperactivity (open-field test), and decreased CA1 cell counts. Chronic hypoxia with superimposed acute hypoxia resulted in consequences that were not different from those of chronic hypoxia.

A Systems-Based Computational Model for Dose-Response Comparisons of Two Mode of Action Hypotheses for Ethanol-Induced Neurodevelopmental Toxicity

Investigations into the potential mechanisms for ethanol-induced developmental toxicity have been ongoing for over 30 years since Fetal Alcohol Syndrome (FAS) was first described. Neurodevelopmental endpoints are particularly sensitive to in utero exposure to alcohol as suggested by the more prevalent alcohol-related neurodevelopmental disorder (ARND). The inhibition of proliferation during neurogenesis and the induction of apoptosis during the period of synaptogenesis have been identified as potentially important mechanisms for ARND. However, it is unclear how these two mechanisms quantitatively relate to the dose and timing of exposure. We have extended our model of neocortical neurogenesis to evaluate apoptosis during synaptogenesis. This model construct allows quantitative evaluation of the relative impacts on neuronal proliferation versus apoptosis during neocortical development. Ethanol-induced lengthening of the cell cycle of neural progenitor cells during rat neocortical neurogenesis (G13-G19) is used to compute the number of neurons lost after exposure during neurogenesis. Ethanol-induced dose-dependent increases in cell death rates are applied to our apoptosis model during rat synaptogenesis (P0-P14), when programmed cell death plays a major role in shaping the future neocortex. At a human blood ethanol concentration that occurs after 3-5 drinks ( approximately 150 mg/dl), our model predicts a 20-30% neuronal deficit due to inhibition of proliferation during neurogenesis, while a similar exposure during synaptogenesis suggests a 7-9% neuronal loss through induction of cell death. Experimental in vitro and in vivo dose-response research and stereological research on long-term neuronal loss after developmental exposure to ethanol is compared to our model predictions. Our computational model allows for quantitative, systems-level comparisons of mechanistic hypotheses for perturbations during specific neurodevelopmental periods.

The role of cell death during neocortical neurogenesis and synaptogenesis: implications from a computational model for the rat and mouse

We are quantitatively evaluating the acquisition of neocortical neurons through key stages of development including neurogenesis, migration, and synaptogenesis. Here we expand upon a previous computational model describing neocortical neurogenesis in the rat and mouse [Dev. Neurosci. 24 (2002) 467], to include the period of synaptogenesis (P0-P14) when programmed cell death (PCD) is known to play a major role in shaping the neocortex. We also quantitatively evaluate differing hypotheses on the role of cell death during neurogenesis. This new model construct allows prediction of acquisition of adult neuronal number in the rat and mouse neocortex from the beginning of neurogenesis through synaptogenesis. The mathematical model output is validated by independently derived stereologically determined neuron number estimates in the adult rat and mouse. Simulations suggest cell death during synaptogenesis reduces the neocortical neuronal population by 20-30%, while cell death of progenitor cells and newly formed neurons during neurogenesis may reduce output by as much as 24%. However, higher death rates during neurogenesis as suggested by some research would deplete the progenitor population, not allowing for the vast expansion that is the hallmark of the mammalian neocortex. Furthermore, our simulations suggest the clearance time of dying neurons labeled by TUNEL or pyknosis is relatively short, between 1 and 4 h, corroborating experimental research. This novel mathematical model for adult neocortical neuronal acquisition allows for in silico analysis of normal and perturbed states of neocortical development as well as interspecies and evolutionary analyses of neocortical development.

Brain Imaging Studies of the Anatomical and Functional Consequences of Preterm Birth for Human Brain Development

Premature birth can have devastating effects on brain development and long-term functional outcome. Rates of psychiatric illness and learning difficulties are high, and intelligence on average is lower than population means. Brain imaging studies of infants born prematurely have demonstrated reduced volumes of parietal and sensorimotor cortical gray matter regions. Studies of school-aged children have demonstrated reduced volumes of these same regions, as well as in temporal and premotor regions, in both gray and white matter. The degrees of these anatomical abnormalities have been shown to correlate with cognitive outcome and with the degree of fetal immaturity at birth. Functional imaging studies have shown that these anatomical abnormalities are associated with severe disturbances in the organization and use of neural systems subserving language, particularly for school-aged children who have low verbal IQs. Animal models suggest that hypoxia-ischemia may be responsible at least in part for some of the anatomical and functional abnormalities. Increasing evidence suggests that a host of mediators for hypoxic-ischemic insults likely contribute to the disturbances in brain development in preterm infants, including increased apoptosis, free-radical formation, glutamatergic excitotoxicity, and alterations in the expression of a large number of genes that regulate brain maturation, particularly those involved in the development of postsynaptic neurons and the stabilization of synapses. The collaboration of both basic neuroscientists and clinical researchers is needed to understand how normal brain development is derailed by preterm birth and to develop effective prevention and early interventions for these often devastating conditions.

The etiology and outcome of cerebral ventriculomegaly at term in very low birth weight preterm infants

BACKGROUND

Despite improvements in survival data, the incidence of neurodevelopmental handicaps in preterm infants remains high. To prevent these handicaps, one must understand the pathophysiology behind them. For preterm infants, cerebral ventriculomegaly (VM) may be associated with adverse neurodevelopmental outcome. We hypothesized that although the causes of VM are multiple, the incidence of handicap at 4.5 years of age in preterm infants with this ultrasonographic finding at term would be high.

METHODS

To test this hypothesis, we provided neurodevelopmental follow-up for all 440 very low birth weight survivors of the Multicenter Randomized Indomethacin Intraventricular Hemorrhage (IVH) Prevention Trial. A total of 384 children (87%) were evaluated at 54 months' corrected age (CA), and 257 subjects were living in English-speaking, monolingual households and are included in the following data analysis.

RESULTS

Moderate to severe low pressure VM at term was documented in 11 (4%) of the English-speaking, monolingual survivors. High grade IVH and bronchopulmonary dysplasia (BPD) were both risk factors for the development of VM. Of 11 (45%) children with VM, 5 suffered grades 3 to 4 IVH, compared with 2/246 (1%) children without VM who experienced grades 3 to 4 IVH. Similarly, 9/11 (82%) children with VM had BPD, compared with 120/246 (49%) children without VM who had BPD. Logistic regression analysis was performed using birth weight, gestational age, gender, Apgar score at 5 minutes, BPD, sepsis, moderate to severe VM, periventricular leukomalacia, grade of IVH, and maternal education to predict IQ <70. Although maternal education was an important and independent predictor of adverse cognitive outcome, in this series of very low birth weight prematurely born children, VM was the most important predictor of IQ <70 (OR: 19.0; 95% CI: 4.5, 80.6). Of children with VM, 6/11 (55%) had an IQ <70, compared with 31/246 (13%) of children without VM. Children with VM had significantly lower verbal and performance scores compared with children without VM.

CONCLUSIONS

These data suggest that, for preterm neonates, VM at term is a consequence of the vulnerability of the developing brain. Furthermore, its presence is an important and independent predictor of adverse cognitive and motor development at 4.5 years' CA.

Association of chronic sublethal hypoxia with ventriculomegaly in the developing rat brain

Bronchopulmonary dysplasia remains a major cause of neurodevelopmental handicap in preterm infants. Because bronchopulmonary dysplasia may be associated with prolonged hypoxemia without obvious changes in systemic blood pressure, we developed an animal model of chronic sublethal hypoxia to test the hypothesis that this insult results in significant alterations in corticogenesis in the developing brain. Three groups of newborn rats were placed in a chamber with FIO2 9.5% on postnatal day 3 (P3). One group was sacrificed at P13; a second group was sacrificed at P33, and the third group was removed at P33 and reared in normoxia until sacrifice at P63. Control rats were those raised in room air for the corresponding periods of time. Rats were transcardially perfused and the brains were embedded in celloidin and prepared for morphometric analysis using standard stereology methods. Although experimental rat pups in the third group demonstrated 'catch-up' of body weight following return to normoxia, these studies demonstrated both failure of brain growth (p<0.01) and progressive cerebral ventriculomegaly (p<0.01). Decreased subcortical white matter (p<0. 05) and corpus callosum size (p<0.01) were noted at P63 in pups reared under conditions of chronic hypoxia. Decreases in cortical volume (p<0.05) were noted at all three experimental time points for hypoxic-reared pups when compared to control animals. These data suggest that chronic sublethal hypoxia may lead to severe impairments in corticogenesis in an animal model of developing brain.