The effect of alkaloid piperine against the toxicity of 1-methyl-4-phenylpyridinium (MPP(+)) in differentiated PC12 cells was assessed. Piperine treatment revealed a differential effect on the cytotoxicity of MPP(+) and had its maximum inhibitory effect at 1 microM. The addition of piperine (0.5-10 microM) significantly reduced the MPP(+)-induced nuclear damage, mitochondrial membrane permeability changes, formation of reactive oxygen species and depletion of GSH. In contrast, piperine at 50-100 microM showed cytotoxicity and exhibited an additive effect against the MPP(+) toxicity. The results indicate that piperine had a differential effect on the cytotoxicity of MPP(+) depending on concentration. Piperine at low concentrations may reduce the MPP(+)-induced viability loss in PC12 cells by suppressing the changes in the mitochondrial membrane permeability, leading to the release of cytochrome c and subsequent activation of caspase-3. The effects may be ascribed to its inhibitory action on the formation of reactive oxygen species and depletion of GSH.