Effects of methylmercury and mercuric chloride on differentiation and cell viability in PC12 cells

The effects of methylmercury (CH,Hg) or mercuric chloride (HgCl2) on neurite outgrowth and cell viability were quantified using undifferentiated (unprimed) and differentiated (primed) pheochromocytoma (PC12) cells. In unprimed cells, following 24-h exposure, CH3Hg significantly decreased NGF-stimulated neurite outgrowth at concentrations of 0.3-3 muM. However, HgCl2 significantly increased both neurite outgrowth and the number of branch points, a component of neurite outgrowth. In primed PC12 cells, following 24-h exposure, both CH3Hg and HgCl2 inhibited NGF-stimulated neurite outgrowth with an EC,, of approximately 0.03 muM; however, there was a difference between CH,Hg and HgCl2 effects on the subcomponents of total neurite outgrowth. CH3Hg significantly decreased both the number of branch points (0.3 muM) and fragment length (0.01 muM), while HgCl2 only decreased fragment length (0.03 muM). Cell. viability was assessed in the same cultures by trypan-blue exclusion. In unprimed cells, the ECS, for cytotoxicity of CH,Hg in the presence and absence of NGF was 0.21 +/- 0.04 and 0.87 +/- 0.12 muM, respectively, and for HgCl2 in the presence and absence of NGF was 8.18 +/- 1.52 and 5.02 +/- 0.74 muM, respectively. In primed cells, the EC, for cytotoxicity of CH,Hg in the presence or absence of NGF was 1.17 +/- 0.38 and 0.73 +/- 0.14 muM, respectively, and for HgCl2 in the presence or absence of NGF was 3.96 +/- 0.82 and 3.81 +/- 0.91 muM, respectively. In the primed PC12 model, cytotoxicity occurred at concentrations that were at least 30-fold higher than the EC50 for neurite outgrowth, suggesting that the mercurial compounds can act selectively on the process of differentiation.