Is monoterpene terpinen-4-ol the compound responsible for the anesthetic and antioxidant activity of Melaleuca alternifolia essential oil (tea tree oil) in silver catfish?

The aim of this study was to evaluate the anesthetic efficacy of tea tree oil (TTO; Melaleuca alternifolia) and its three main compounds: terpinen-4-ol, γ-terpinene, and α-terpinene, in silver catfish (Rhamdia quelen). We also aimed to investigate which of these compounds may be responsible for the anesthetic effect, as well as the effect of prolonged exposure on oxidative parameters. Fish were exposed to 100, 300, 500, 800, and 1000 μL L− 1 of TTO, and the corresponding concentrations of terpinen-4-ol, γ-terpinene, and α-terpinene in TTO; anesthesia induction time was recorded based on fish behavior. Anesthesia induction time with 300-1000 μL L− 1 TTO was 185-1.512 s, and with terpinen-4-ol was 103-630 s; for both, the recovery time was 134-673 s. The monoterpenes α-terpinene and γ-terpinene were incapable of causing sedation and anesthesia in R. quelen, but presented with an antagonistic effect when used in association with terpinen-4-ol. Based on this evidence, it can be concluded that terpinen-4-ol itself caused an anesthetic effect, and therefore, can be considered the compound responsible for the anesthetic effect of TTO. Long-term (6 h) exposure to TTO (25 μL L− 1) and terpinen-4-ol (42 μL L− 1) reduced thiobarbituric acid-reactive substances levels in the liver, and terpinen-4-ol (42 and 20 μL L− 1) reduced lipid hydroperoxide in the liver and kidney. Glutathione-S-transferase activity demonstrated an increase in the liver of silver catfish exposed to terpinen-4-ol (20 and 42 μL L− 1), and increased in the kidney of silver catfish exposed to TTO at concentration of 25 μL L− 1 compared to the control group. In summary, TTO and terpinen-4-ol may be recommended to induce faster anesthesia, and may be suitable as sedatives for transport, because they improved the antioxidant status of silver catfish.