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

To test the postexposure analgesic efficacy of low doses of eugenol in zebrafish. Prospective experimental study. A total of 76 large adult zebrafish (Danio rerio). 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. 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. 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.