Behavioral and histological features of zebrafish following sedation with eugenol or propofol

Abstract

The use of anesthesia during short or lengthy procedures is scientifically and ethically required when dealing with fish. In this setting, the present study investigated whether a 3 h exposure to eugenol (4 and 8 mg/L) or propofol (0.2 and 0.4 mg/L) at sedative levels, followed by a 1 h wash-out time, would interfere with behavioral and histological characteristics of zebrafish. Anxiety-like phenotypes were assessed in the novel tank test using an automated tracking software, and histological features of the gills were determined through light microscopy. In the eugenol trial, zebrafish showed behavioral signs of complete recovery from sedation, but the novel environment diving response was amplified at both concentrations tested, thus pointing to the production of an anxiogenic phenotype. Furthermore, transitory alterations were implemented to increase the functional surface area of the gill and decrease lamellar epithelium thickness in order to improve gas exchange in the individuals bathed in 8 mg/L eugenol. Differently, the zebrafish submitted to 0.2 and 0.4 mg/L propofol presented an anti-anxiety response, since they dwelled much longer in the upper half of the tank; together with other behavioral endpoints which indicated a certain degree of inactivity, it seems that these fish were still under the effect of the drug, despite the 1 h wash-out, and thus neurological function had not been fully restored. Moreover, both concentrations of propofol induced shrinkage of the gill structure, which may have contributed towards an incomplete recovery; hence the behavioral pattern displayed in the novelty-based tank paradigm. This work reports contrasting behavioral and histological effects produced by eugenol and propofol upon zebrafish, thus demonstrating that standardization is a challenge with psychoactive drugs. Therefore, it is paramount to refine anesthetic protocols for this highly qualified test organism in order to maximize welfare and reduce experimental variability.