Response of gene expression in zebrafish exposed to pharmaceutical mixtures: Implications for environmental risk

Complex mixtures of pharmaceutical chemicals in surface waters indicate potential for mixture effects in aquatic organisms. The objective of the present study was to evaluate whether effects on target gene expression and enzymatic activity of individual substances at environmentally relevant concentrations were additive when mixed. Expression of zebrafish cytochrome P4501A (cyp1a) and vitellogenin (vtg) genes as well as activity of ethoxyresorufin-O-deethylase (EROD) were analyzed after exposure (96 h) to caffeine-Caf, ibuprofen-Ibu, and carbamazepine-Cbz (0.05 and 5 µM), tamoxifen-Tmx (0.003 and 0.3 µM), and after exposure to pharmaceutical mixtures (low mix: 0.05 µM of Caf, Ibu, Cbz and 0.003 µM of Tmx, and high mix: 5 µM of Caf, Ibu, Cbz and 0.3 µM of Tmx). Pharmaceuticals tested individually caused significant down regulation of both cyp1a and vtg, but EROD activity was not affected. Exposure to low mix did not cause a significant change in gene expression; however, the high mix caused significant up-regulation of cyp1a but did not affect vtg expression. Up-regulation of cyp1a was consistent with induction of EROD activity in larvae exposed to high mix. The complex mixture induced different responses than those observed by the individual substances. Additive toxicity was not supported, and results indicate the need to evaluate complex mixtures rather than models based on individual effects, since in environment drugs are not found in isolation and the effects of their mixtures is poorly understood.