| Olfactory-mediated behaviors are critical for fish survival, but studies have shown that these behaviors can be disrupted on exposure to certain metals. Although aquatic Superfund sites often contain elevated levels of various metals, few have been characterized for potential olfactory toxicity. A larval zebrafish behavioral assay was developed to characterize dose-response curves for zinc (Zn), hexavalent chromium (Cr), and arsenate (As) olfaction inhibition. Cadmium (Cd), a known olfactory toxicant, was used as a positive control. As expected, following a 24-hour exposure to Cd, we observed a reduced response to taurocholic acid (TCA), a known aversive odorant, validating the behavioral assay. At higher concentrations, Zn also elicited a decrease in response to TCA (IC50: 34.9 microg/L and 69.0 microg/L respectively). Behavioral analysis using a secondary odorant, L-cysteine, revealed potential preferential toxicity of Zn towards ciliated olfactory sensory neurons. No significant changes in olfactory responses were observed for Cr and As, even at exposures far exceeding environmental concentrations. Binary mixtures of Cd and Zn demonstrated a protective effect of Zn against Cd toxicity at low concentrations and this protective response disappeared at higher Zn concentrations. Quantitative analysis of whole larvae gene expression of 8 genes, important in mitigating metal induced whole larvae oxidative stress, confirmed an antioxidant response by Cd, but not for the other metals, suggesting that oxidative stress was not a primary mechanism of Zn induced olfactory dysfunction. In summary, our study has identified Zn as an olfactory inhibitor in zebrafish at environmental concentrations and a modifier of olfactory function in situations where Zn and Cd may exist as mixtures. Our studies further extend the utility of zebrafish as a model species to assess olfactory toxicity in fish. |
