Lack of postexposure analgesic efficacy of low concentrations of eugenol in zebrafish.

OBJECTIVE: To test the postexposure analgesic efficacy of low doses of eugenol in zebrafish.

STUDY DESIGN: Prospective experimental study.

ANIMALS: A total of 76 large adult zebrafish (Danio rerio).

METHODS: Fish swimming behavior (median velocity, freeze time, high-speed swimming and distance moved in the vertical direction) was recorded in a 1.6 L video arena before and after exposure to eugenol (0, 1, 2, 5, 10 and 20 mg L-1 ). In a second experiment, fish were anesthetized with 2-phenoxy-ethanol and treated with an injection of 5% acetic acid (noxious stimulus), and then exposed to 0, 1, 2 and 5 mg L-1 eugenol. The fish swimming behavior was also recorded.

RESULTS: The higher doses (10 and 20 mg L-1 ) reduced the median velocity, high-speed swimming and distance moved in the vertical direction, and increased the freeze time. Zebrafish behavior was not altered by eugenol (1, 2 and 5 mg L-1 ) after noxious stimulation.

CONCLUSIONS AND CLINICAL RELEVANCE: The change in the behavior of zebrafish associated with a noxious stimulus can be monitored and is a good model for studying analgesia in fish. Eugenol (10 and 20 mg L-1 ) induced zebrafish sedation. The response after a noxious stimulus was not affected by postexposure to lower doses, and thus we cannot recommend its use as an analgesic.