The role of neutrophils in the production of hypoxic-ischemic brain injury in the neonatal rat

Neutrophils contribute to ischemic brain injury in adult animals. The role of neutrophils in perinatal hypoxic-ischemic (HI) brain injury is unknown. Allopurinol reduces neutrophil accumulation after tissue ischemia and is protective against HI brain injury. This study was designed to investigate how neutrophils contribute to perinatal hypoxic ischemic brain injury and how neutropenia compared with allopurinol in its neuroprotective effects. A HI insult was produced in the right cerebral hemisphere of 7-d-old rats by right common carotid artery ligation and systemic hypoxia. Half the rats were rendered neutropenic with an anti-neutrophil serum (ANS). At 15 min of recovery from hypoxia, half the neutropenic and nonneutropenic rats received allopurinol (135 mg/kg, s.c.). The protective effect of the four treatment combinations was determined on brain swelling at 42 h of recovery. Neutropenia reduced brain swelling by about 70%, p < 0.01. Allopurinol alone produced similar protection so that the relatively small number of animals studied did not permit assessment of an additive effect. Neutrophil accumulation in cerebral hemispheres was measured by myeloperoxidase (MPO) activity assay and by neutrophil counts in 6-mu m sections stained by MPO and ANS immunostaining. MPO activity peaked between 4 and 8 h of recovery in both hemispheres. Hemispheric neutrophil counts peaked at the end of the HI insult and again at 18 h of recovery. Neutrophils were stained within blood vessels and did not infiltrate the injured brain before infarction had occurred. We conclude that neutrophils contribute to HI brain injury in the neonate and that neutrophil depletion before the insult is neuroprotective.