Essential oil of Ocimum gratissimum L.: Anesthetic effects, mechanism of action and tolerance in silver catfish, Rhamdia quelen

The aim of this study was to determine the time until anesthetic induction and recovery in silver catfish (Rhamdia quelen) exposed to the essential oil of Ocimum gratissimum L. (EO), its potential mechanism of action via GABAergic transmission and the development of tolerance. The EO was obtained from the aerial parts of O. gratissimum L. by hydrodistillation and analyzed by gas chromatograph coupled to mass spectrometer (GC–MS). Eugenol (73.6%) and β-bisabolene (18.3%) are the major compounds of the EO. Juvenile silver catfish were placed in aquaria containing different concentrations of the EO (10, 20, 30, 40, 50, 60, 70, 150 and 300 mg L− 1) to determine the point at which anesthesia was induced and the length of the recovery period. In the following experiments, the anesthetic effect of the EO (10, 20 and 40 mg L− 1) in association with benzodiazepine (BDZ) was determined. The GABAergic-like action and development of tolerance were assessed in fish exposed to BDZ (diazepam 150 μM), EO (40 mg L− 1) or both EO and BDZ (EO + BDZ) at the same concentrations. After the induction of anesthesia, juveniles were transferred to an anesthetic-free aquarium containing either the classic BDZ antagonist flumazenil (5 μM) or water to assess their recovery. The development of tolerance was evaluated in two experiments. In the first, juveniles were exposed to the EO twice in a 30-day interval, and in the second five exposures, weekly intervals were performed. Fish exposed to concentrations above 30 mg L− 1 of the EO were effectively anesthetized without experiencing side effects or mortality. Fast anesthesia (< 4 min) could be obtained using 150 and 300 mg L− 1 of the EO. Synergism was detected for EO + BDZ at the lower concentrations tested. Juveniles exposed to all tested drugs showed a faster recovery in flumazenil than those that recovered in water. The development of tolerance was observed in fish exposed to the EO or BDZ separately after the fifth exposure. Two exposures were sufficient to sensitize the fish to the action of these drugs. However, repeated exposure to EO + BDZ did not induce tolerance and/or sensitization. In conclusion, the EO of O. gratissimum is an effective and safe anesthetic for silver catfish, and its mechanism seems to be related to an interaction with the GABAA-benzodiazepine receptor.

► Anesthetic effect of the essential oil of O. gratissimum in R. quelen was verified.
► Eugenol and β-bisabolene were the major compounds of the essential oil (EO).
► Deep anesthesia was obtained at concentrations of 30–300 mg L–1.
► The EO seems to act on the GABAA–benzodiazepine receptor.
► Tolerance occurred in fish exposed to EO and BDZ after the fifth exposure.