Dual effects of TNFα on nerve fiber formation from ventral mesencephalic organotypic tissue cultures

Tumor necrosis factor alpha (TNFα) is toxic to dopamine neurons and increased levels of TNFα are observed in Parkinson's disease. Dopamine nerve fiber outgrowth in organotypic cultures of fetal ventral mesencephalon occurs in two waves. The early appearing nerve fibers are formed in the absence of astroglia, while migrating astrocytes guide the late appearing dopamine nerve fibers. TNFα (40 ng/ml) was added to the medium of organotypic ventral mesencephalic tissue cultures between days 4-7 and 11-14. The cultures were evaluated at days 7 or 19 to study the effects of TNFα on both types of nerve fiber formation. Tyrosine hydroxylase (TH)-immunohistochemistry demonstrated that the number of cultures showing non-glial-guided TH-positive outgrowth was reduced compared to controls, when TNFα was added at day 4. By contrast, the glial-guided TH-positive nerve fiber outgrowth and the astrocytic migration reached significantly longer distances by early TNFα treatment. Ki67-immunohistochemistry revealed that TNFα did not affect proliferation of astrocytes. Treatment with TNFα and antibodies against TNFα receptor 1 between days 4 and 7 revealed that the non-glial-guided TH-positive outgrowth reappeared. TNFα treatment between days 11 and 14 triggered neither the TH-positive glial-guided outgrowth, nor promoted the astrocytic migration to reach longer distances. The number of microglia was significantly increased after the late but not early TNFα treatment. In conclusion, TNFα is toxic for the non-glial dopaminergic nerve fiber outgrowth but stimulates the glial-guided outgrowth and the migration of astrocytes at an early time point. TNFα increased the number of microglia in VM tissue cultures after late but not after early treatment.