INDUCIBLE NITRIC-OXIDE SYNTHASE EXPRESSION IN CEREBROVASCULAR SMOOTH-MUSCLE AND NEUTROPHILS AFTER TRAUMATIC BRAIN INJURY IN IMMATURE RATS

The inflammatory response after traumatic brain injury (TBI) includes cytokine production, leukocyte infiltration, and microglial activation . Production of nitric oxide by inducible nitric oxide synthase (iNOS) occurs during acute inflammation outside of the CNS and in models of cerebral ischemia, and therefore may contribute to the inflammatory re sponse after TBI. The purpose of this study was to localize and define the time course of iNOS expression after TBI in the immature rat. Imm ature Wistar rats (age 3.5-4.5 wk) were anesthetized and subjected to percussive trauma to the right parietal cortex. Nontraumatized rats we re used as controls (n = 7). At 2, 24, 48, or 168 h (n = 3/group) post trauma rats were killed by perfusion fixation. Brains were removed, fr ozen, sectioned, immunostained with antibodies against iNOS and glial fibrillary acidic protein (GFAP, a marker specific for astrocytes), an d imaged using fluorescent detection systems. There was no detectable expression of iNOS in control brains. At 2 h, minimal cerebrovascular iNOS expression was seen in the peritrauma area. At 24 and 48 h, there was marked peritrauma cerebrovascular INOS expression that appeared t o be restricted to vascular smooth muscle cells and infiltrated leukoc ytes. Further dual-immunolabeling showed that the leukocytes expressin g iNOS were predominantly neutrophils. At 168 h, iNOS expression was n o longer detectable. iNOS was not detectable in GFAP-positive cells. T he prominent expression of iNOS protein after TBI in cerebrovascular s mooth muscle cells and infiltrated neutrophils suggests that iNOS may play a role in cerebrovascular disturbances and secondary brain injury after trauma.