Fig. (3) MTT reduction and LDH release of primary neuronal cells (5 DIV) after 24 hours treatment with increasing concentrations of fentanyl (F1-F9, 0.8, 1.56, 3.125, 6.25, 12.5, 25, 50, 100 and 200 ng/ml, black) compared to controls (white). Data are means ± SEM; n = 3; one-way ANOVA test with Bonferroni post hoc test.