FIG. 1. MTT reduction by SHSY5Y cells. A: MTT reduction over 24 h in the presence of increasing concentrations of mercury. B: Effect of mercury (50 µg/L) over 24 h in the presence or absence of a 12-h melatonin (1 µM) preincubation. Included is a group in which melatonin and mercury (Mel + Mer) were premixed for 30 min before administration. Data are mean ± SEM (bars) values of nine observations (from triplicate independent assays). All concentrations with the exception of 1 µg/L showed a significant difference from the control group: p < 0.005 for A. * p < 0.001, significantly different from the control and melatonin groups; [UNK] p < 0.004, significantly different from the melatonin plus mercury group for B. Statistics were calculated by Mann-Whitney U test with Bonferroni's multiple comparison.