Nitric oxide via macrophage iNOS induces apoptosis following traumatic spinal cord injury.

To investigate the pathophysiological mechanisms involved in post-traumatic impairment of the spinal cord, we analyzed expression patterns of the inducible nitric oxide synthase (iNOS) gene following acute injury of rat spinal cord using a weight drop technique. PCR analysis revealed that iNOS mRNA appeared at 3-12 h after injury and declined thereafter. Immunohistochemical analysis showed that iNOS-positive cells invaded the lesioned area through the perivascular space at 6 h after injury. The population of these cells peaked at 24 h and then declined to disappear 3 days after injury. The iNOS-positive cells were also stained with ED-2 but not with ED-1 or OX-42, indicating that these cells were macrophages and/or perivascular cells. In parallel with the appearance of iNOS-positive cells, other cells emerged that were positively stained by the terminal deoxynucleotidyl-transferase-mediated dUDP-biotin nick end-labeling (TUNEL) assay. TUNEL-positive cells were scattered in the lesioned area 1 day after injury, but some in the surrounding area close to iNOS-positive cells. Administration of L-Ng-nitro-arginine methylester, a competitive inhibitor of NOS, resulted in a reduction of TUNEL-positive cells in the lesioned area. These results suggest that nitric oxide generated by iNOS of macrophages and/or perivascular cells plays a significant role in eliminating damaged cells from the lesioned area by apoptosis.