Microglia and macrophages are the major source of tumor necrosis factor in permanent middle cerebral artery occlusion in mice.

The proinflammatory cytokine tumor necrosis factor (TNF) is known to be expressed in brain ischemia; however, its cellular and temporal appearance is not fully settled. In this study, nonradioactive in situ hybridization for murine TNF mRNA was performed on brain sections from adult C57x129 mice at 6 hours, 12 hours, 24 hours, 2 days, 5 days, or 10 days (six to eight mice per group) after induction of permanent focal cerebral ischemia. Cortical infarct volumes were estimated, and TNF mRNA-expressing cells were counted within the infarct and infarct border using Cast-Grid analysis. At 12 hours, a peak of 19.2 +/- 5.1 TNF mRNA-expressing cells/mm2 was counted, contrasting two to three times lower values at 6 and 24 hours (6.4 +/- 4.6 and 9.2 +/- 3.4 cells/mm2, respectively) and <2 cells/mm2 at 48 hours and later stages. The TNF mRNA-expressing cells were distributed along the entire rostrocaudal axis of the cortical infarcts and occasionally within the caudate putamen. At all time points, TNF mRNA colocalized with Mac-1-positive microglia/macrophages but not with Ly-6G (Gr-1)-positive polymorphonuclear leukocytes. Similarly, combined in situ hybridization for TNF mRNA and immunohistochemistry for glial fibrillary acidic protein at 12 and 24 hours revealed no TNF mRNA-expressing astrocytes at these time points. Translation of TNF mRNA into bioactive protein was demonstrated in the neocortex of C57B1/6 mice subjected to permanent middle cerebral artery occlusion. In summary, this study points to a time-restricted microglial/macrophage production of TNF in focal cerebral ischemia in mice.