Perivascular nitric oxide and superoxide in neonatal cerebral hypoxia-ischemia

Decreased cerebral blood flow (CBF) has been observed following the resuscitation from neonatal hypoxic-ischemic injury, but its mechanism is not known. We address the hypothesis that reduced CBF is due to a change in nitric oxide (NO) and superoxide anion O-2(-) balance secondary to endothelial NO synthase (eNOS) uncoupling with vascular injury. Wistar rats (7 day old) were subjected to cerebral hypoxia-ischemia by unilateral carotid occlusion under isoflurane anesthesia followed by hypoxia with hyperoxic or normoxic resuscitation. Expired CO2 was determined during the period of hyperoxic or normoxic resuscitation. Laser-Doppler flowmetry was used with isoflurane anesthesia to monitor CBF, and cerebral perivascular NO and O-2(-) were determined using fluorescent dyes with fluorescence microscopy. The effect of tetrahydrobiopterin supplementation on each of these measurements and the effect of apocynin and N-omega-nitro-L-arginine methyl ester (L-NAME) administration on NO and O-2(-) were determined. As a result, CBF in the ischemic cortex declined following the onset of resuscitation with 100% O-2 (hyperoxic resuscitation) but not room air (normoxic resuscitation). Expired CO2 was decreased at the onset of resuscitation, but recovery was the same in normoxic and hyperoxic resuscitated groups. Perivascular NO-induced fluorescence intensity declined, and O-2(-)-induced fluorescence increased in the ischemic cortex after hyperoxic resuscitation up to 24 h postischemia. L-NAME treatment reduced O-2(-) relative to the nonischemic cortex. Apocynin treatment increased NO and reduced O-2(-) relative to the nonischemic cortex. The administration of tetrahydrobiopterin following the injury increased perivascular NO, reduced perivascular O2-, and increased CBF during hyperoxic resuscitation. These results demonstrate that reduced CBF follows hyperoxic resuscitation but not normoxic resuscitation after neonatal hypoxic-ischemic injury, accompanied by a reduction in perivascular production of NO and an increase in O2-. The finding that tetrahydrobiopterin, apocynin, and L-NAME normalized radical production suggests that the uncoupling of perivascular NOS, probably eNOS, due to acquired relative tetrahydrobiopterin deficiency occurs after neonatal hypoxic-ischemic brain injury. It appears that both NOS uncoupling and the activation of NADPH oxidase participate in the changes of reactive oxygen concentrations seen in cerebral hypoxic-ischemic injury.