Inhaled Nitric Oxide Reduces Brain Damage by Collateral Recruitment in a Neonatal Stroke Model

Abstract

N eonatal hypoxia-ischemia is a common cause of neonatal brain injury and results in cerebral palsy, learning disabilities, and epilepsy.1 In addition to global cerebral ischemia arising from systemic asphyxia, recent data suggest a higher incidence of focal ischemia-reperfusion leading to stroke in near-term neonates.2 Tissue-type plasminogen activator is the only approved agent capable of improving reperfusion after ischemia in the adult brain. 3 However, no safe neuroprotective molecule is currently available to protect the immature brain.Among vasoactive molecules, nitric oxide (NO) is a small, highly diffusible and reactive molecule produced by the NO synthases and released from endothelial cells 4 and perivascular nitrergic neurons. 5,6 Endogenous NO is widely recognized as an important messenger and effector molecule for vascular tone and tolerance to damage as well as a mediator in a variety of acute and chronic inflammatory systems. 7 NO is also involved in several critical processes in the developing brain; indeed, exposure to inhaled NO (iNO) during the first week of postnatal life has recently been shown to play a key role in myelination in the developing brain 8 and to significantly reduce the size of excitotoxic lesions in neonatal rat pups. 9 We recently demonstrated, by using 2-dimensional color-coded pulsed ultrasound imaging, that collateral recruitment or failure during ischemia, as revealed by changes in blood flow velocities in the basilar trunk (BT), determined the extent of the ischemic lesion in P7 rats 10 and wasBackground and Purpose-We recently demonstrated that endogenous nitric oxide (NO) modulates collateral blood flow in a neonatal stroke model in rats. The inhalation of NO (iNO) has been found to be neuroprotective after ischemic brain damage in adults. Our objective was to examine whether iNO could modify cerebral blood flow during ischemiareperfusion and reduce lesions in the developing brain. Methods-In vivo variations in cortical NO concentrations occurring after 20-ppm iNO exposure were analyzed using the voltammetric method in P7 rat pups. Inhaled NO-mediated blood flow velocities were measured by ultrasound imaging with sequential Doppler recordings in both internal carotid arteries and the basilar trunk under basal conditions and in a neonatal model of ischemia-reperfusion. The hemodynamic effects of iNO (5 to 80 ppm) were correlated with brain injury 48 hours after reperfusion. Results-Inhaled NO (20 ppm) significantly increased NO concentrations in the P7 rat cortex and compensated for the blockade of endogenous NO synthesis under normal conditions. Inhaled NO (20 ppm) during ischemia increased blood flow velocities and significantly reduced lesion volumes by 43% and cellular damage. In contrast, both 80 ppm iNO given during ischemia and 5 or 20 ppm iNO given 30 minutes after reperfusion were detrimental. Blood flow increases in the BT mirror the efficiency of collateral support through the circle of Willis and the opening of cortical arterial anastomoses among anter...

Sildenafil Mediates Blood-Flow Redistribution and Neuroprotection After Neonatal Hypoxia-Ischemia

“…8 Sildenafil has been shown to similarly increase CBF at the ischemic boundary in adult rats with embolic stroke. 20 Tadalafil, another specific PDE-5 inhibitor, alleviates microvascular ischemia and fully restores the blood-flow regulation after a single dose (10 mg/kg per os) in Becker muscular dystrophy, 21 an effect that is both marked and immediate.…”

“…34,35 Indeed, exposure to 20 ppm of iNO is beneficial when given during ischemia but not after reperfusion in a neonatal stroke model. 8 Similarly, iNO at 50 ppm reduces neuronal damage when given during HI, whereas lower doses increase lesion size after HI. Together, these data suggest that excess NO during reperfusion exacerbates oxidative stress 8 and is detrimental to the immature brain.…”

“…8 Similarly, iNO at 50 ppm reduces neuronal damage when given during HI, whereas lower doses increase lesion size after HI. Together, these data suggest that excess NO during reperfusion exacerbates oxidative stress 8 and is detrimental to the immature brain. 36 Sildenafil does not act by increasing endogenous NO but prevents the rapid degradation of its main vasoactive mediator, cGMP, and therefore improves CBF in hypoxic microvessels.…”

“…7 In contrast, in neonatal or juvenile rats, iNO seems to be beneficial only when given during ischemia but is deleterious when administered during reperfusion. 8 When the Rice-Vannucci model is applied to P9 mouse pups, iNO (50 ppm) reduces neuronal damage when given during hypoxia. 6 Conversely, iNO at 10 or 40 ppm has been reported to increase lesion size when given after the HI insult.…”

Nitric Oxide Pathway and Proliferation of Neural Progenitors in the Neonatal Rat

“…However, several lines of evidence demonstrate that low doses of iNO not only act locally on the pulmonary vasculature but also have remote effects on the developing brain under basal or pathological conditions by modulating white matter maturation, inflammation and subsequent brain repair [8]. Indeed, we recently reported that the NO pathway is able to promote oligodendroglial maturation, myelination and neuroprotection in preclinical models of neonatal stroke and white matter damage induced by postnatal hyperoxia or excitotoxic insult [9,10,11,12]. Several other reports have demonstrated that iNO is associated with significant neuroprotection in preclinical models of adult brain damage [13,14,15].…”