Inhibition of dopamine biosynthesis by protoberberine alkaloids in PC12 cells

Berberine and palmatine exhibit a mild and competitive inhibition on bovine adrenal tyrosine hydroxylase (EC 1.14.16.2; TH). In this study, the inhibitory effects of protoberberine alkaloids (such as berberine, palmatine and coptisine) on dopamine biosynthesis in PC12 cells were investigated. Treatment with berberine and palmatine showed 53.7% and 61.0% inhibition of dopamine content in PC12 cells at a concentration of 20 mu M for 24 hr, respectively. However, coptisine did not reduce dopamine content. The IC50 values of berberine and palmatine were 18.6 mu M and 7.9 mu M. Dopamine content was lowered at 6 hr and reached the minimal level at 24 hr after exposure to berberine and palmatine at 20 mu M. The decreased dopamine level was maintained up to 48 hr, and then recovered to the control level at about 72 hr. TH activity was inhibited at 6 hr following treatment with berberine and palmatine, and was maintained at a reduced level up to 36 hr in PC12 cells (21-27% inhibition at 20 mu M), whereas TH mRNA level was not found to alter for 24 hr. However, the intracellular Ca2+ concentration decreased by treatment with berberine and palmatine at 20 mu M by 22-26% inhibition relative to the control level in PC12 cells. These results give evidence that berberine and palmatine lead to decreased dopamine content by inhibition of TH activity but not by regulation of TH gene expression in PC12 cells.