Pathodynamics of Nitric Oxide Production Within Implanted Glioma Studied With an In Vivo Microdialysis Technique and Immunohistochemistry

Nitric oxide (NO) is thought to be a mediator in many of the processes of malignant brain tumor progression. We examined NO production in the brain of normal conscious, freely moving rats with or without implanted C6 glioma. Both nitrite (NO2−) and nitrate (NO3−) in the dialysates of the two groups were measured using an in vivo microdialysis technique. The mean concentration of NO2− in the glioma group was two-times higher than that in the control group (P<0.01). Concentrations of both NO2− and NO3− in the glioma and control groups decreased following intraperitoneal injection of NG-nitro-L-arginine methyl ester (L-NAME), a non-selective inhibitor of NO synthase (NOS). NO production was also significantly suppressed in the glioma group, but not the control group, by intraperitoneal injection of 2-amino-5,6-dihydro-6-methyl-4H-1,3-thiazine (AMT), a selective inhibitor of inducible NOS (iNOS). On immunohistochemical examination, diffuse iNOS-positive cells were located within glioma tissue. ED1-positive cells (microglia/macrophages) were intermingled between glioma cells on double immunostaining. These results indicate that the basal level of NO production in the glioma group is higher than that in the control group and that the increased NO production was continuously induced by iNOS-expressing cells in glioma.