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Environmental contamination by antibiotics has become a global issue. Colistin, a cationic antimicrobial polypeptide, has been widely used in human/veterinary medicine, and growth promotion in aquaculture. However, no study has been conducted to test the toxic effects of colistin on aquatic animals. In this study, we examined the effects of colistin on zebrafish embryos. Zebrafish embryos were incubated in different concentrations (0, 0.01, 0.1, 1, 2, 3, and 10 μM) of colistin for 96 h. Colistin increased the mortality rate in a dose-dependent manner (LC50 was 3.0 μM or 3.5 mg L−1), but it did not change the hatching rate, heart rate, body length, eye size, or yolk size of embryos. However, colistin impaired keratinocytes and lateral-line hair cells in the skin of embryos. Colistin (at concentrations ≥0.1 μM) decreased the number of FM1-43-labeled hair cells and reduced the mechanotransduction-mediated Ca2+ influx at hair bundles, suggesting that sublethal concentrations of colistin can impair lateral line function. To investigate the lethal injury, morphological changes were sequentially observed in post-hatched embryos subjected to lethal concentrations of colistin. We found that skin keratinocytes were severely damaged and detached after exposure, leading to hypotonic swelling of the yolk sac, loss of ion contents, cell lysis, and eventual death. This study revealed that acute colistin exposure can impair skin cells and pose a threat to fish survival.

Ammonia (including NH3 and NH4+) is a major pollutant of freshwater environments. However, the toxic effects of ammonia on the early stages of fish are not fully understood, and little is known about the effects on the sensory system. In this study, we hypothesized that ammonia exposure can cause adverse effects on embryonic development and impair the lateral line system of fish. Zebrafish embryos were exposed to high-ammonia water (10, 15, 20, 25, and 30 mM NH4Cl; pH 7.0) for 96 h (0–96 h post-fertilization). The body length, heart rate, and otic vesicle size had significantly decreased with ≥15 mM NH4Cl, while the number and function of lateral-line hair cells had decreased with ≥10 mM NH4Cl. The mechanoelectrical transduction (MET) channel-mediated Ca2+ influx was measured with a scanning ion-selective microelectrode technique to reveal the function of hair cells. We found that NH4+ (≥5 mM NH4Cl) entered hair cells and suppressed the Ca2+ influx of hair cells. Neomycin and La3+ (MET channel blockers) suppressed NH4+ influx, suggesting that NH4+ enters hair cells via MET channels in hair bundles. In conclusion, this study showed that ammonia exposure (≥10 mM NH4Cl) can cause adverse effects in zebrafish embryos, and lateral-line hair cells are sensitive to ammonia exposure.

The potential toxicity of copper nanoparticles (CuNPs) to early stages of fishes is not fully understood, and little is known about their effects on ionocytes and associated functions. This study used zebrafish embryos as a model to investigate the toxic effects of CuNPs on two subtypes of ionocytes. Zebrafish embryos were exposed to 0.1, 1, and 3 mg L−1 CuNPs for 96 h. After exposure, whole-body Na+ and Ca2+ contents were significantly reduced at ≥0.1 mg L−1, while the K+ content had decreased at ≥1 mg L−1. H+ and NH4+ excretion by the skin significantly decreased at ≥1 mg L−1. The number of living ionocytes labeled with rhodamine-123 had significantly decreased with ≥0.1 mg L−1 CuNPs. The ionocyte subtypes of H+-ATPase-rich (HR) and Na+/K+-ATPase-rich (NaR) cells were labeled by immunostaining and had decreased with ≥1 mg L−1. Shrinkage of the apical opening of ionocytes was revealed by scanning electronic microscopy. Functional impairment was also reflected by changes in gene expressions, including ion transporters/channels and Ca2+-regulatory hormones. This study shows that CuNP exposure can impair two subtypes of ionocytes and their associated functions, including Na+/Ca2+ uptake and H+/NH4+ excretion in zebrafish embryos.