Association of Nepeta cataria L. essential oil and eugenol: synergism and safe anesthesia in tambaqui Colossoma macropomum (Cuvier, 1818).

Anesthetics are widely employed in aquaculture and in research environments utilizing fish as experimental models, with the aim of mitigating stress and hypermotility during handling procedures. Among the most commonly used anesthetic agents, eugenol stands out for its suitability across multiple species, enabling safer handling and transportation while minimizing physiological impact on the animals. For the experimental procedure, three oil mixtures were prepared in a 1:1 ratio. The first contained 50% eugenol and 50% Nepeta cataria essential oil (ENCO); the second mixture was 50% eugenol and 50% corn oil (Zea mays) (ECO); and the third was 50% Nepeta cataria essential oil and 50% corn oil (NCCO). The present study proposes a behavioral and electrophysiological assessment of Colossoma macropomum subjected to immersion baths containing different concentrations of Nepeta cataria L. 50% and eugenol 50%, in addition to investigating the underlying components of the synergism and the mechanism of action of eugenol. A total of 288 juvenile specimens, of mixed sex, with a mean body mass of 25.34 ± 3.2 g, were used. Experiment 1: assessed the synergistic potentiation between the compounds, based on latency times for anesthetic induction and recovery. Experiment 2: performed electrophysiological analyses of muscle (EMG) and cardiac (ECG) functions. Experiment 3: evaluated the influence of flumazenil on the activation of the anesthetic agents. Evaluations were conducted during 10-minute immersion baths at concentrations of 30 µL·L⁻¹, 35 µL·L⁻¹, 40 µL·L⁻¹, 45 µL·L⁻¹, and 50 µL·L⁻¹. The combination of Nepeta cataria essential oil and eugenol promotes good muscle relaxation and minor alterations in the cardiovascular system, promoting easily reversible bradycardia. The findings indicate that the combination of Nepeta cataria essential oil and eugenol constitutes a promising and safe alternative, due to the synergistic reduction in induction time and the maintenance of cardiac safety, which allowed for a reduction in the concentration required without compromising the maintenance of the anesthetic plane. Furthermore, flumazenil was found to exert a significant influence on both the induction process and the recovery phase of anesthesia.