Abstract 2059 Poster Session II, Sunday, 5/2 (poster 234)

Introduction: The most common cerebrovascular lesion affecting the premature asphyxiated infant is damage to the subcortical white matter (myelin), and is pathologically referred to as periventricular leukomalacia (PVL). Overall, PVL has been reported to occur in up to 75% of premature infants weighing < 1500 grams. Pathologically, PVL occurs in a distribution around the anterior horns and trigone of the lateral ventricles; sites recognized as border zones between penetrating branches of the middle, posterior, and anterior cerebral arteries. On gross inspection, multiple cystic cavities can be visualized, and microscopically, oligodendroglial deficiency is the dominant cellular finding, along with loss of myelin and astrogliosis. Clinically, the long term sequelae resulting from PVL are the spastic cerebral palsies. Our laboratory has developed a model of PVL in the immature rat pup of 7-days of age, whereby brief periods of near complete hypoxia-ischemia result in selective damage to developing oligodendroglia in the myelinogenic regions of the brain (Brain Res-in press). In the former study, 7-day rat pups were utilized because it is at this age that oligodendroglia precursors can be identified, and myelination first begins. Given that PVL as a pathologic entity is age specific in the human to premature infants of 26-34 weeks gestation, the purpose of the present study was to determine the effect of an identical insult at different ages, and its' influence on myelin production. Methods: Rat pups of 7, 14, and 21 days of age were anesthetized with halothane (4% induction; 1% maintenance) during which time both common carotid arteries were identified, and a loose ligature placed around each through a midline neck incision. The animals were allowed to recover from anesthesia for 30 min, afterwhich the ligature was tightened to insure complete obstruction of blood flow, and each animal was exposed to hypoxia in 8% oxygen, for a period of 5 min. Following the latter exposure, the ligatures were removed, blood flow was visually noted to be restored, and the midline neck incision was closed. The animals were subsequently returned to their Dams for a period of either 3 or 6 weeks, afterwhich each rat pup was sacrificed, and sections of brain were taken for semi quantitative western blot analysis of myelin basic protein (MBP). Results: At 3 and 6 weeks of recovery, MBP was significantly (p<0.001) reduced to 70% and 80% of control respectively, in those rat pups who experienced the hypoxic-ischemic insult at 7 days of age. Analysis of MBP at 3 and 6 weeks following the insult in 14 and 21 day old rats indicated no significant reduction of myelin production in either of these groups. Determination of the rate of myelin production as a function of time also revealed a significant difference (decrease) between those animals experiencing hypoxia-ischemia at 7 days of age compared to controls, whereas there was no difference between control and those rat pups exposed to hypoxia-ischemia at 14 and 21 days. Conclusions: The described model of PVL is the first in-vivo model which pathologically replicates the pathological findings seen in the human premature infant. The findings in this model appear specific to the 7-day immature rat pup, consistent with the known age predilection of this insult to the premature human. This age specific injury results in permanent damage to the myelin producing cells, and result in a relative reduction MBP with advancing age. Further investigations are required to delineate the vascular and cellular mechanisms by which this insult mimics the human condition.

(The authors gratefully acknowledge the support of the Toronto Hospital for Sick Children Foundation).