Transient NMDA receptor-mediated hypoperfusion following umbilical cord occlusion in preterm fetal sheep

Preventive, rescue and reparative neuroprotective strategies for the fetus and neonate

Neonatal encephalopathy remains a major contributor to death and disability around the world. Acute hypoxia-ischaemia before, during or after birth creates a series of events that can lead to neonatal brain injury. Understanding the evolution of injury underpinned the development of therapeutic hypothermia. This review discusses the determinants of injury, including maturity, the pattern of exposure to HI, impaired placental function, often associated with fetal growth restriction and in the long-term, socio-economic deprivation. Chorioamnionitis has been associated with the presence of NE, but it is important to note that experimentally, inflammation can either sensitize to greater neural injury after HI or alleviate injury, depending on its precise timing. As fetal surveillance tools improve it is likely that improved detection of specific pathways will offer future opportunities for preventive and reparative interventions in utero and after birth.

The Role of Cardiac N-Methyl-D-Aspartate Receptors in Heart Conditioning-Effects on Heart Function and Oxidative Stress

As well as the most known role of N-methyl-D-aspartate receptors (NMDARs) in the nervous system, there is a plethora of evidence that NMDARs are also present in the cardiovascular system where they participate in various physiological processes, as well as pathological conditions. The aim of this study was to assess the effects of preconditioning and postconditioning of isolated rat heart with NMDAR agonists and antagonists on heart function and release of oxidative stress biomarkers. The hearts of male Wistar albino rats were subjected to global ischemia for 20 min, followed by 30 min of reperfusion, using the Langendorff technique, and cardiodynamic parameters were determined during the subsequent preconditioning with the NMDAR agonists glutamate (100 µmol/L) and (RS)-(Tetrazol-5-yl)glycine (5 μmol/L) and the NMDAR antagonists memantine (100 μmol/L) and MK-801 (30 μmol/L). In the postconditioning group, the hearts were perfused with the same dose of drugs during the first 3 min of reperfusion. The oxidative stress biomarkers were determined spectrophotometrically in samples of coronary venous effluent. The NMDAR antagonists, especially MK-801, applied in postconditioning had a marked antioxidative effect with a most pronounced protective effect. The results from this study suggest that NMDARs could be a potential therapeutic target in the prevention and treatment of ischemic and reperfusion injury of the heart.

TLR7 agonist modulation of postasphyxial neurophysiological and cardiovascular adaptations in preterm fetal sheep

Activation of Toll-like receptors (TLRs) after hypoxic-ischemic brain injury can exacerbate injury but also alleviate cell loss, as recently demonstrated with the TLR7 agonist Gardiquimod (GDQ). However, TLR agonists also modulate vascular function and neuronal excitability. Thus, we examined the effects of TLR7 activation with GDQ on cardiovascular function and seizures after asphyxia in preterm fetal sheep at 0.7 gestation (104 days, term ∼147 days). Fetuses received sham asphyxia or asphyxia induced by umbilical cord occlusion for 25 min or asphyxia followed by a continuous intracerebroventricular infusion of 3.34 mg of GDQ from 1 to 4 h after asphyxia. Fetuses were monitored continuously for 72 h postasphyxia. GDQ treatment was associated with sustained, moderate hypertension for 72 h (P < 0.05), with a transient increase in heart rate. Electroencephalographic (EEG) power was suppressed for the entire postasphyxial period in both groups, whereas EEG spectral edge transiently increased during the GDQ infusion compared with asphyxia alone (P < 0.05), with higher β- and lower δ-EEG frequencies (P < 0.05). This increase in EEG frequency was not related to epileptiform activity. After the GDQ infusion, there was earlier onset of high-amplitude stereotypic evolving seizures, with increased numbers of seizures and seizure burden (P < 0.05). Hemodynamic function and seizure activity are important indices of preterm wellbeing. These data highlight the importance of physiological monitoring during preclinical testing of potential neuroprotective strategies.

Early sinusoidal heart rate patterns and heart rate variability to assess hypoxia-ischaemia in near-term fetal sheep

KEY POINTS

•Therapeutic hypothermia needs to be started as early as possible in the first 6 h after acute injury caused by hypoxia-ischaemia (HI), but the severity and timing of HI are often unclear. In this study we evaluated whether measures of heart rate variability (HRV) might provide early biomarkers of HI. •The duration but not magnitude of suppression of HRV power and conversely increased sample entropy of the heart rate were associated with severity of HI, such that changes in the first 3 h did not discriminate between groups. •Relative changes in HRV power bands showed different patterns between groups and therefore may have the potential to evaluate the severity of HI. •Aberrant fetal heart rate patterns and increased arginine vasopressin levels in the first hour after moderate and severe HI were correlated with loss of EEG power after 3 days' recovery, suggesting potential utility as early biomarkers of outcome.

ABSTRACT

Therapeutic hypothermia is partially neuroprotective after acute injury caused by hypoxia-ischaemia (HI), likely because the timing and severity of HI are often unclear, making timely recruitment for treatment challenging. We evaluated the utility of changes in heart rate variability (HRV) after HI as biomarkers of the timing and severity of acute HI. Chronically instrumented fetal sheep at 0.85 gestational age were exposed to different durations of umbilical cord occlusion to produce mild (n = 6), moderate (n = 8) or severe HI (n = 8) or to sham occlusion (n = 5). Heart rate (HR) and HRV indices were assessed until 72 h after HI. All HI groups showed suppressed very low frequency HRV power and elevated sample entropy for the first 3 h; more prolonged changes were associated with greater severity of HI. Analysis of relative changes in spectral power showed that the moderate and severe groups showed a shift towards higher HRV frequencies, which was most marked after severe HI. This shift was associated with abnormal rhythmic HR patterns including sinusoidal patterns in the first hour after HI, and with elevated plasma levels of arginine vasopressin, which were correlated with subsequent loss of EEG power by day 3. In conclusion, absolute changes in HRV power in the first 3 h after acute HI were not significantly related to the severity of HI. The intriguing relative shift in spectral power towards higher frequencies likely reflects greater autonomic dysfunction after severe HI. However, sinusoidal HR patterns and elevated vasopressin levels may have utility as biomarkers of severe HI.

Motor deficits are triggered by reperfusion-reoxygenation injury as diagnosed by MRI and by a mechanism involving oxidants

The early antecedents of cerebral palsy (CP) are unknown but are suspected to be due to hypoxia-ischemia (H-I). In our rabbit model of CP, the MRI biomarker, apparent diffusion coefficient (ADC) on diffusion-weighted imaging, predicted which fetuses will develop postnatal hypertonia. Surviving H-I fetuses experience reperfusion-reoxygenation but a subpopulation manifested a continued decline of ADC during early reperfusion-reoxygenation, which possibly represented greater brain injury (RepReOx). We hypothesized that oxidative stress in reperfusion-reoxygenation is a critical trigger for postnatal hypertonia. We investigated whether RepReOx predicted postnatal neurobehavior, indicated oxidative stress, and whether targeting antioxidants at RepReOx ameliorated motor deficits, which included testing of a new superoxide dismutase mimic (MnTnHex-2-PyP). Rabbit dams, 79% gestation (E25), were subjected to 40 min uterine ischemia. Fetal brain ADC was followed during H-I, immediate reperfusion-reoxygenation, and 4-72 h after H-I. Endpoints were postnatal neurological outcome at E32, ADC at end of H-I, ADC nadir during H-I and reperfusion-reoxygenation, and area under ADC curve during the first 20 min of reperfusion-reoxygenation. Antioxidants targeting RepReOx were administered before and/or after uterine ischemia. The new MRI-ADC biomarker for RepReOx improved prediction of postnatal hypertonia. Greater superoxide production, mitochondrial injury, and oligodendroglial loss occurred in fetal brains exhibiting RepReOx than in those without. The antioxidants, MnTnHex-2-PyP and Ascorbate and Trolox combination, significantly decreased postnatal motor deficits and extent of RepReOx. The etiological link between early injury and later motor deficits can thus be investigated by MRI, and allows us to distinguish between critical oxidative stress that causes motor deficits and noncritical oxidative stress that does not.

Potential biomarkers for hypoxic-ischemic encephalopathy

Cerebral hypothermia reduces brain injury and improves behavioral recovery after hypoxia-ischemia (HI) at birth. However, using current enrolment criteria many infants are not helped, and conversely, a significant proportion of control infants survive without disability. In order to further improve treatment we need better biomarkers of injury. A 'true' biomarker for the phase of evolving, 'treatable' injury would allow us to identify not only whether infants are at risk of damage, but also whether they are still able to benefit from intervention. Even a less specific measure that allowed either more precise early identification of infants at risk of adverse neurodevelopmental outcome would reduce the variance of outcome of trials, improving trial power while reducing the number of infants unnecessarily treated. Finally, valid short-term surrogates for long term outcome after treatment would allow more rapid completion of preliminary evaluation and thus allow new strategies to be tested more rapidly. Experimental studies have demonstrated that there is a relatively limited 'window of opportunity' for effective treatment (up to about 6-8h after HI, the 'latent phase'), before secondary cell death begins. We critically evaluate the utility of proposed biochemical, electronic monitoring, and imaging biomarkers against this framework. This review highlights the two central limitations of most presently available biomarkers: that they are most precise for infants with severe injury who are already easily identified, and that their correlation is strongest at times well after the latent phase, when injury is no longer 'treatable'. This is an important area for further research.

Endogenous alpha2-adrenergic receptor-mediated neuroprotection after severe hypoxia in preterm fetal sheep

Central alpha-adrenergic receptor activity is important for fetal adaptation to hypoxia before birth. It is unclear whether it is also important during recovery. We therefore tested the hypothesis that an infusion of the specific alpha(2)-adrenergic receptor antagonist idazoxan (1 mg/kg/h i.v.) from 15 min to 4 h after profound hypoxia induced by 25 min umbilical cord occlusion in fetal sheep at 70% of gestation (equivalent to the 28-32 weeks in humans) would increase neural injury. After 3 days' recovery, idazoxan infusion was associated with a significant increase in neuronal loss in the hippocampus (P<0.05), expression of cleaved caspase-3 (P<0.05), and numbers of activated microglia (P<0.05). There was no significant effect on other neuronal regions or on loss of O4-positive premyelinating oligodendrocytes in the subcortical white matter. Idazoxan was associated with an increase in evolving epileptiform electroencephalographic (EEG) transient activity after occlusion (difference at peak 2.5+/-1.0 vs. 11.7+/-4.7 counts/min, P<0.05) and significantly reduced average spectral edge frequency, but not EEG intensity, from 54 until 72 h after occlusion (P<0.05). Hippocampal neuronal loss was correlated with total numbers of epileptiform transients during idazoxan infusion (P<0.01; r(2)=0.7). In conclusion, endogenous inhibitory alpha(2)-adrenergic receptor activation after severe hypoxia appears to significantly limit evolving hippocampal damage in the immature brain.