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BRAIN INJURY IN THE NEONATAL RAT IS REDUCED BY NEUTROPHIL DEPLETION INDUCED BEFORE BUT NOT AFTER A HYPOXIC-ISCHEMIC INSULT. • 2254

Neutrophils contribute to reperfusion brain injury in adult animals. Antineutrophil strategies may be neuroprotective after birth asphyxia. Objectives:1) To determine if neutropenia induced before a cerebral hypoxic-ischemic (HI) insult could reduce brain swelling and long term injury in a neonatal rat. 2) To determine if neutrophil depletion after the HI insult(by 8h of recovery) could reduce brain injury. We injected 7 day old (P7) rats with antineutrophil serum and measured the fall in absolute neutrophil count in blood at 2hrly intervals. After 8h the neutrophil count fell from a normal level of 6,180±1,457per mm3 to 389±778 (Mean ±SD). In another 69 rats we induced a HI insult to the R cerebral hemisphere by R common carotid artery ligation. Four hours later we exposed the pups to 2.25h systemic hypoxia in 8% oxygen, then recovered the rats in room air. The Control Group (n=25, not neutropenic) received normal rabbit serum at the time of carotid ligation. The Pre Group (n=18) were injected with antineutrophil serum at the time of ligation and the (Post group, n=24) received antineutrophil serum immediately upon recovery in room air. At 42h recovery we measured hemispheric water content (wet-dry wt) and calculated R hemisphere(RH) swelling.The RH swelled 13.7±9.1%(Mean ±SD) in the Control group and 10.2±9.5% in the Post group. In the Pre group it swelled only 3.2±4.2%,p=<0.001 ANOVA pre vs other groups. To study long term neuroprotection, 60 additional rats were subjected to the same HI insult and treatment groups. The rats were killed after 14d recovery, the brains were immersion fixed, and a 2mm thick coronal section cut. We measured the hemisphere areas and related the areas: R/L ×100 (the lower the ratio the more damaged the RH). The Control non neutropenic rats (n=21) had a ratio 81.8±16.6% (Mean ±SD) the Post HI Group (n=22) a ratio of 78.3±18.2 and the Pre group (n=17) a ratio of 93.6±11.6, p=0.03 ANOVA Pre vs Control. In conclusion neutropenia induced before HI reduces brain swelling and long term injury in 7 day old rats. However, depletion induced by the 8th hour of recovery is not protective.