Sublethal Effect Concentrations for Non-Polar Narcosis in the Zebrafish Embryo

Non-polar narcosis, also known as baseline toxicity, has been described as the minimal toxicity that an organic chemical may elicit based on its lipophilicity. While lethal effects of narcosis-inducing chemicals (NICs) have been thoroughly investigated, knowledge of sublethal effects is still very limited.

We investigated the effects of three well-known NICs (phenanthrene, 1,3,5-trichlorobenzene and pentachlorobenzene) on a variety of organismal endpoints (malformations, swim bladder inflation, respiration, heart rate, swimming activity and turning angles), which can be plausibly linked to narcosis in zebrafish embryos.

Baseline toxicity recorded as mortality is typically observed in similar exposure ranges in a wide variety of species including fish, corresponding to a chemical activity range between 0.01 and 0.1. In the present study we found that sublethal effects occurred at concentrations around 5 times below lethal concentrations.

Altered swimming activity and impaired swim bladder inflation were the most sensitive endpoints occurring at exposure levels below the generally accepted threshold for baseline toxicity for two out of three compounds. Overall, most effective exposure levels across the sublethal endpoints and compounds did fall within the range typically associated with baseline toxicity, and deviations were generally limited to a factor 10.

While there could be benefit in adding sublethal endpoints to toxicity tests, such as the Fish Embryo Acute Toxicity (FET) test, based on the present sublethal endpoints and available evidence from our and other studies, the underestimation of toxicity due to the sole assessment of mortality as an endpoint in a FET test may be limited for narcosis.

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