White matter injury after repeated endotoxin exposure in the preterm ovine fetus

Acute fetal hypoxia: the modulating effect of infection

The fetal brain is protected from the effects of acute hypoxia by a range of haemodynamic and metabolic compensations. Hypoxia alone is therefore an unusual cause of perinatal brain injury in either preterm or term infants. More recently, materno-fetal infection has been implicated as a causative factor in cases of cerebral palsy associated with preterm and term birth. This paper explores the concept that exposure to infection, and in particular pro-inflammatory cytokines, may reduce the threshold at which hypoxia becomes neurotoxic, so making the brain much more vulnerable to even mild hypoxic insults. The hypothesis is supported by an increasing body of evidence from animal studies that also demonstrate the importance of duration between exposure to infection and subsequent hypoxia. There are a number of clinical and research implications that centre around the role of antibiotics, mode and timing of delivery, maternal cooling during labour and the role of immune-modulating drugs.

Preconditioning and the developing brain

Preconditioning occurs when a subinjurious exposure renders the brain less vulnerable to a subsequent damaging exposure. In this essay, various models of preconditioning in the immature brain are discussed. Adenosine, excitatory amino acids, nitric oxide, hypoxia-inducible factor, ATP-sensitive K+ channels, caspases, heat shock proteins, inflammatory mediators and gene expression all seem to be involved in sensing, transducing and executing preconditioning resistance. Also reviewed in this essay is evidence that some subinjurious exposures render the brain more vulnerable to a subsequent damaging exposure. We believe that unraveling the mechanisms of how the developing brain becomes inherently resilient or vulnerable will offer important insights into the pathogenesis of injury. Preconditioning of the brain or induction of tolerance of the immune system might be utilized in the future to decrease CNS vulnerability and the occurrence of perinatal brain injury.

Determining the fetal inflammatory response in an experimental model of intrauterine inflammation in rats

Intrauterine infection is a risk factor for developmental brain injuries in childhood. A variety of cytokines known to be toxic to developing brain cells have been isolated from mothers or children at risk for developmental disabilities, and these cytokines have been proposed as mediators of these injuries. We have developed a model of intrauterine inflammation that damages the developing white matter and we now hypothesize that selected cytokines are increased after our experimental inflammatory stimulus. Timed-pregnant Fischer 344 and Lewis rats were injected with 0.1 mg/kg of lipopolysaccharide (LPS) into the cervix at E15. Tumor necrosis factor-alpha (TNF-alpha), interferon-gamma (IFN-gamma), IL-6, and IL-10 were measured in homogenates of fetal brain and placenta at serial time periods within the first 24 h after the inflammatory stimulus. TNF-alpha was increased 20-fold in the placenta and more than 5-fold in the fetal brain after the stimulus. IFN-gamma was only increased within the fetal brain (20-fold) and IL-6 was only increased in the placenta (10-fold). IL-10 was mildly increased in the placenta and was decreased slightly in the fetal brain. Our observations show that an intrauterine inflammatory stimulus can cause large increases in Th1 cytokines within the fetal brain. The placenta can produce selected cytokines but fails to produce IFN-gamma, suggesting that the fetal immune system produces this cytokine in response to our stimulus. By studying placental and brain cytokine responses in models such as ours, the mechanisms responsible for the damage to developing white matter can be determined.

Brain injury induced by intracerebral injection of interleukin-1beta and tumor necrosis factor-alpha in the neonatal rat

To examine the possible role of inflammatory cytokines in mediating neonatal brain injury, we investigated effects of intracerebral injection of IL-1beta (IL-1beta) or tumor necrosis factor-alpha (TNFalpha) on brain injury in the neonatal rat. A stereotaxic intracerebral injection of IL-1beta or TNFalpha (10 ng per pup) was performed in postnatal day 5 (P5) SD rats. Although no necrosis of neurons was found, increased astrogliosis, as indicated by GFAP positive staining was observed 24 and 72 h following the injection of IL-1beta or TNFalpha. IL-1beta induced apoptotic cell death in the rat brain 24 h after the injection, as indicated by increases in positive TUNEL staining and caspase-3 activity, and apoptotic cell death was partially blocked by systemic administration of NBQX, an antagonist of the AMPA glutamate receptor. IL-1beta also significantly reduced the number of developing oligodendrocytes (OLs) 24 h after the injection and this impairment was not prevented by NBQX. On the contrary, TNFalpha induced a much smaller increase in the number of TUNEL positive cells and did not reduce the number of developing OLs. By P8, myelin basic protein (MBP) was clearly detected in the control rat brain, while MBP positive staining was very weak, if any, in the IL-1beta treated rat brain. MBP expression in the TNFalpha treated rat brain was less affected. The overall results indicate that IL-1beta may directly cause injuries to developing OLs and impair myelination in the neonatal rat brain and TNFalpha may have different roles in mediating brain injury.

Inflammatory brain damage in preterm newborns--dry numbers, wet lab, and causal inferences

Epidemiologic observations support the contention that infection, inflammation, and neonatal white matter damage (WMD) are associated. We also have documentation from multiple experimental models that infection/inflammation can damage developing white matter. Based on these observations in humans and animals, we offer causal inferences using widely accepted causal criteria and the multivariable model of causation. As much as we want to, however, we are reluctant to state unequivocally that inflammation causes WMD in humans born much before term. The main reason is that we lack convincing evidence that inflammation precedes WMD (temporal evidence). We also need more (and more detailed) observational studies clarifying the presumed infection --> inflammation --> WMD sequence before we can initiate intervention trials to reduce the risk of WMD.

Maternal LPS induces cytokines in the amniotic fluid and corticotropin releasing hormone in the fetal rat brain

Perinatal infections are a risk factor for fetal neurological pathologies, including cerebral palsy and schizophrenia. Cytokines that are produced as part of the inflammatory response are proposed to partially mediate the neurological injury. This study investigated the effects of intraperitoneal injections of lipopolysaccharide (LPS) to pregnant rats on the production of cytokines and stress markers in the fetal environment. Gestation day 18 pregnant rats were treated with LPS (100 μg/kg body wt ip), and maternal serum, amniotic fluid, placenta, chorioamnion, and fetal brain were harvested at 1, 6, 12, and 24 h posttreatment to assay for LPS-induced changes in cytokine protein (ELISA) and mRNA (real-time RT-PCR) levels. We observed induction of proinflammatory cytokines interleukin (IL)-1β, IL-6, and tumor necrosis factor-α (TNF-α) as well as the anti-inflammatory cytokine IL-10 in the maternal serum within 6 h of LPS exposure. Similarly, proinflammatory cytokines were induced in the amniotic fluid in response to LPS; however, no significant induction of IL-10 was observed in the amniotic fluid. LPS-induced mRNA changes included upregulation of the stress-related peptide corticotropin-releasing factor in the fetal whole brain, TNF-α, IL-6, and IL-10 in the chorioamnion, and TNF-α, IL-1β, and IL-6 in the placenta. These findings suggest that maternal infections may lead to an unbalanced inflammatory reaction in the fetal environment that activates the fetal stress axis.

Brain development during fetal life: influences of the intra-uterine environment

The intrauterine environment can significantly affect fetal brain development. Here we review our recent findings using animal models that mimic adverse intrauterine conditions which could exist during human pregnancy. We have focused on effects of both acute and chronic hypoxic and inflammatory insults. Relatively brief periods of hypoxemic compromise can have significant effects on the fetal brain causing neuronal loss and cerebral white matter damage. Subtle brain injury can occur, for example to a particular class of neuron, and this can have a significant effect on the function of a specific system. Chronic mild placental insufficiency can result in long term deficits in neuronal connectivity affecting function postnatally as demonstrated in the auditory and visual systems. Repeated acute exposure to an inflammatory agent results in diffuse subcortical white matter damage and in some cases periventricular necrosis. We have demonstrated that the timing and severity of these prenatal insults are determinants of the outcomes, in terms of the severity of the damage and the regions of the brain affected.

Animal models of hypoxic-ischemic brain damage in the newborn

Controversy continues over which animal model to use as a reflection of human disease states. With respect to perinatal brain disorders, scientists must contend with a disease in evolution. In that regard, the perinatal brain is at risk during a time of extremely rapid development and maturation, involving processes that are required for normal growth. Interfering with these processes, as part of therapeutic intervention must be efficacious and safe. To date, numerous models have provided tremendous information regarding the pathophysiology of brain damage to term and preterm infants. Our challenges will continue to be in identifying those infants at greatest risk for permanent injury, and adapting therapies that provide more benefit than harm. Using animal models to conduct these studies will bring us closer to that goal.

Chorioamnionitis induced by subchorionic endotoxin infusion in sheep

OBJECTIVE

This study was undertaken to determine whether subchorionic endotoxin infusion causes chorioamnionitis and preterm lung maturation, as occurs after intra-amniotic endotoxin.

STUDY DESIGN

From day 118 of pregnancy, sheep received infusions of endotoxin (subchorionic 7.5 mg/d, n=11; intra-amniotic 2.5 mg/d, n=9) until delivery of lambs at 120 or 124 days. Other sheep received a single intra-amniotic injection of endotoxin (10 mg, n=7) at 118 days before delivery at 124 days. Controls (n=9) received equivalent saline solution treatments.

RESULTS

Chorioamnionitis accompanied all endotoxin treatments. Lung inflammation occurred after intra-amniotic endotoxin infusion or injection but not after subchorionic endotoxin. Umbilical arterial pH was lower and Pco(2) was higher than control after subchorionic endotoxin. Lung compliance and surfactant were increased after intra-amniotic endotoxin infusion or injection but not after subchorionic endotoxin.

CONCLUSION

Chorioamnionitis may result from inflammatory stimuli at various intrauterine sites, with different sites causing different fetal effects and not all cases of chorioamnionitis being accompanied by enhanced lung maturation.

The effects of repeated endotoxin exposure on placental structure in sheep

Intrauterine infection has been associated with fetal brain injury and preterm birth. We have recently shown that repeated exposure to bacterial endotoxin leads to hypoxia and brain injury in the preterm ovine fetus and we considered it possible that endotoxin could also damage the placenta. Our aim therefore was to assess placental structure following repeated exposure to endotoxin. Endotoxin was administered on 3-5 occasions (1 microg/kg, i.v.) over 5 days from 95-99 days of gestation (term approximately 147 days) to 6 fetal sheep and placental structure assessed at 105 days. In LPS-exposed animals there was a 17 per cent reduction (P<0.05) in placental weight and the average cross-sectional area of placentomes was reduced (P<0.05) by 20 per cent. In addition, all LPS-exposed placentae showed significant injury as evidenced by calcium deposits associated with areas of infarcted tissue. We conclude that repeated endotoxin exposure results in damage to the placenta which could lead to persistent alterations in placental exchange function.