Inhaled NO prevents hyperoxia-induced white matter damage in neonatal rats
Introduction
Bronchopulmonary dysplasia (BPD) and white matter damage (WMD) are the two most common complications of preterm birth. Their incidence reaches 20 to 40% of infants born below 28 weeks of gestation (Jobe, 2011, Volpe, 2009). Whereas advances in neonatal intensive care have resulted in a marked decrease in neonatal mortality, the incidence of both morbidities in these infants remains high. Several common risk factors, such as low gestational age, perinatal inflammation and excessive oxidative stress make more likely the devastating co-occurrence of BPD and WMD (Dammann et al., 2004). In addition, BPD increases the risk for cerebral palsy (CP), which is a common consequence of WMD in infants born prematurely. Among these common risk factors, an excessive release of free radicals induced by oxygen therapy in preterm neonates with respiratory distress was involved in the pathogenesis of WMD as well as BPD. Furthermore, recent experimental studies have supported that hyperoxia causes oxidative stress and triggers maturation-dependent cell death, maturation arrest of developing oligodendrocytes, and disruption of axon–oligodendrocyte integrity, all key features of WMD (Back et al., 2007, Gerstner et al., 2008, Ritter et al., 2013, Schmitz et al., 2011, Vottier et al., 2011). For these reasons, the design of therapeutic strategies that take into account both BPD and WMD is a highly relevant, to date poorly explored challenge.
In the present study, we focused on the therapeutic use of nitric oxide (NO) in neonates. NO is widely recognized as an important messenger and effector molecule in a variety of acute and chronic inflammation systems, and also as a mediator of vascular tone and tolerance to damage (Moncada et al., 1991, Toda and Okamura, 2003, Vaucher et al., 2000). Interestingly, NO is both a physiological mediator of the central nervous system and a key factor for lung angiogenesis and alveolarization, two developmental phenomena involved in BPD pathophysiology (Jobe, 2011, Yun et al., 1996). On the other hand, we have recently reported that inhaled NO (iNO) has several critical properties in the developing brain by promoting myelination and inducing neuroprotection against excitotoxic-induced brain injury and neonatal stroke (Charriaut-Marlangue et al., 2012, Charriaut-Marlangue et al., 2013, Olivier et al., 2010, Pansiot et al., 2010). Interestingly, similar results were reported in preclinical model of adult stroke (Terpolilli et al., 2012, Terpolilli et al., 2013). Thus, iNO appears to be a promising candidate for the prevention and/or the clinical management of WMD and BPD, but remains to be evaluated in an animal model mimicking simultaneously these two developmental diseases.
Here, we used a rat model of prolonged postnatal hyperoxia that induced BPD and WMD. We made the hypothesis that iNO, in addition to its potential effects on lung injury, may be neuroprotective in rat pups exposed to hyperoxia. The present results showed that iNO induces neuroprotection with a significant effect at both histopathological and behavioral levels, whereas it had only transient and mild effects on BPD.
Section snippets
Experimental protocol and gas exposure
This study was approved by the National Institute of Health and Medical Research and complied with the instructions of the Institutional Animal Care and Use Committees INSERM 676—Paris. The day before delivery, pregnant rats (Sprague–Dawley, Janvier S.A.S., Le Genest-St-Isle, France) were placed in a transparent Plexiglas chamber supplied with a gas mixture that either induced hyperoxia (FiO2 = 80 +/− 0.5%) until postnatal day (P)7 or maintained normoxia (FiO2 = 21 +/− 0.5%). Hyperoxia exposure of
Body, lung and brain phenotypes induced by postnatal hyperoxia
Postnatal hyperoxia resulted in a pronounced decrease in body weight at both P3 and P10 (Supplemental Table S3), as well as increased postnatal mortality rates overall (15.5% vs. 1% in controls, p < 0.001). Blood gas analyses performed showed that blood concentrations of pO2 and fraction of oxyhemoglobin (fO2Hb) were significantly elevated no longer than 1 h after the onset of hyperoxia exposure (Supplemental Fig. S1). Hyperoxic condition also induced a significant but transient increased in pCO2
Discussion
In this study, we developed an animal model that combines the two main complications associated with prematurity, BPD and WMD, which share several risk factors including low gestational age, inflammation and oxidative stress. Using this model, we demonstrated that iNO exposure during the first postnatal week significantly attenuated the main features of hyperoxia-induced WMD in neonatal rats and the devastating effects of hyperoxia on learning abilities.
Our animal model simultaneously displayed
Role of funding source
This study was financially supported by the Institut National Sur la Recherche Médicale (INSERM), the INSERM AVENIR Program and the Mairie de Paris. Ikaria Inc. partially supports the experimental research program of Prof. Olivier Baud and co-workers with respect to inhaled nitric oxide. However, Ikaria Inc. has taken part neither in the study design nor in the data analysis.
Acknowledgments
We are grateful to Mathias Schroijen for his help in cognitive assessment. This study has been supported by the Prime Brain project and de Spoelberch Foundation.
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