Expression of the inducible nitric oxide synthase in distinct cellular types after traumatic brain injury: an in situ hybridization anal immunocytochemical study

The Marmarou's acceleration traumatic brain injury (TBI) model, in situ hyb ridization and immunocytochemistry were utilized to study the temporal expr ession of the inducible form of nitric oxide synthase (iNOS) mRNA and prote in in different cellular compartments of the rat brain. Four hours followin g TBI, expression of iNOS was observed in the endothelial cells of cerebral blood vessels, macrophages and many cortical and hippocampal neurons. In t he cortex labeled neuronal and nonneuronal cells were primarily found in th e superficial layers. In the hippocampus the strongest neuronal labeling wa s observed in the CA1 and CA3 (lateral part) regions. By 24 h post TBI endo thelial cells no longer expressed iNOS mRNA, and the macrophage and neurona l INOS expression was reduced by 30-50%. The reduction was assessed by auto mated quantitation of the silver grains that occupy individual cellular pro files using an image analysis system. Immunocytochemistry revealed de novo iNOS synthesis in non-neuronal cells at the different time points, thus par alleling the changes in iNOS mRNA expression. In contrast, iNOS immunoreact ivity in neurons was not observed before 24 h post TBI, suggesting failure of iNOS protein translation at 4 h after trauma. The results demonstrate co mplex spatial and temporal patterns of iNOS expression in discrete cellular populations, indicating different times of nitric oxide synthesis (and rel ease) following TBI. Uncoupling of iNOS mRNA and protein synthesis in neuro ns suggests differential synthesis of nitric oxide in these cells as compar ed to non-neuronal cellular populations after trauma.