ABSTRACT:The premature infant is at increased risk of cerebral white matter injury. Melatonin is neuroprotective in adult models of focal cerebral ischemia and attenuates ibotenate-induced white matter cysts in neonatal mice. Clinically, melatonin has been used to treat sleep disorders in children without major side effects. The aim of this study was to investigate the protective and anti-inflammatory effects of melatonin in the immature brain following intrauterine asphyxia. Fetal sheep at 90 d of gestation were subjected to umbilical cord occlusion. Melatonin (20 mg/kg, n ϭ 9) or vehicle (n ϭ 10) was administered IV to the fetus, starting 10 min after the start of reperfusion and continued for 6 h. Melatonin treatment resulted in a slower recovery of fetal blood pressure following umbilical cord occlusion, but without changes in fetal heart rate, acid base status or mortality. The production of 8-isoprostanes following umbilical cord occlusion was attenuated and there was a reduction in the number of activated microglia cells and TUNEL-positive cells in melatonin treated fetuses, suggesting a protective effect of melatonin. In conclusion, this study shows that melatonin attenuates cell death in the fetal brain in association with a reduced inflammatory response in the blood and the brain following intrauterine asphyxia in mid-gestation fetal sheep. T he premature infant is at increased risk of cerebral white matter injury, often referred to as periventricular leukomalacia (PVL), which is associated with subsequent development of cerebral palsy and cognitive impairment (1,2). The precise etiology of white matter damage remains unclear, but ischemia-reperfusion and generation of free radicals seem to play an important role (3). Presently, there are no effective treatments for preterm brain injury, although encouraging studies suggest that hypothermic treatment in term infants following birth asphyxia is successful in reducing mortality/ morbidity (4,5).Melatonin is the major secretory product of the pineal gland and its main physiologic function is to mediate circadian rhythmicity and seasonality (6). Clinically, melatonin has been used to treat sleep disorders in children and jet-lag (7,8) and melatonin administered to neonates reduced oxidative stress in association with septicaemia (9). Furthermore, treatment with melatonin in asphyxiated newborns reduced levels of malondialdehyde and nitrite/nitrate in the blood (10).In adult animals, melatonin has been shown to be neuroprotective in models of focal cerebral ischemia (11) and to reduce microglia activation in the hippocampus after kainateinduced inflammation in rats (12). Melatonin also attenuated ibotenate-induced white matter cysts in neonatal mice (13). In addition, melatonin given to pregnant rats prevented oxidative mitochondria damage after ischemia-reperfusion in premature fetal rat brain (14).We have previously demonstrated white matter injury following systemic asphyxia in the preterm fetal sheep, which is similar to the injury seen in preterm infan...