The dose–response of different concentrations of 6-hydroxydopamine (6-OHDA) (0–200 μM) used to induce neurodegeneration in 1 × 105cells/well. Pheochromocytoma (PC 12) cells for the duration of 24 h. The viability of PC12 cells was determined using the MTT reduction assay. Values are the percentages of viable cells, with the viability of untreated control cells taken as 100%. Data are mean and S.E. values from three independent experiments (n = 4). **p < 0.01, relative to untreated cells.