Anaesthetic profile of thymol and menthol in zebrafish larvae model

Aquaculture procedures such as capturing, handling, and transporting fish can lead to stress in animals. For this reason, anaesthesia has been recommended to reduce fish stress during these procedures. Synthetic and natural anaesthetics such as tricaine methanesulfonate (MS-222) and eugenol (monoterpene) are commonly used; however, biochemical changes and aversive effects have been reported. Other monoterpene compounds such as thymol and menthol, have shown potential anaesthetic effects in fish species. Thus, further investigation of its anaesthetic properties seems imperative, and the use of the zebrafish in the early stages of its development is an appropriate model due to its advantages. Therefore, the aimed to evaluate the anaesthetic profile of thymol and menthol in zebrafish at 72 hpf (hours post fertilisation). Following the determination of the effective anaesthetic concentration, three concentrations (100, 200 and 300 mg/L for thymol and 200, 400 and 500 mg/L for menthol) were further tested by focusing on mortality rate, heart rate, behaviour features of induction and oxidative effects in comparison to MS-222 and eugenol. Both monoterpenes caused anaesthetic effects in a concentration-dependent manner, with concentrations of 200 mg/L exhibiting a similar profile to MS-222 and eugenol, as evidenced by the increased recovery times.Furthermore, anaesthesia with thymol and menthol caused post-anaesthesia motor pattern abnormalities, which supported some indicators of stress. Slight modifications in physiological parameters and antioxidant changes were observed, but their association with thymol and menthol anaesthesia is unclear. Although supporting its anaesthetic use, more extensive research will be required to fully understand the impact of these natural anaesthetics on fish behaviour and stress levels, ultimately leading to safer and more successful anaesthetic operations in the future.