Behavioral and electrophysiological evaluation of the anesthetic therapeutic window of thyme essential oil in tambaqui (Colossoma macropomum).

This study aimed to determine the anesthetic therapeutic window of Thymus vulgaris L. essential oil (TVEO) in juvenile Colossoma macropomum (tambaqui), through the integrated analysis of behavioral parameters, electrocardiographic recordings, opercular activity during the induction, and recovery periods of anesthesia, as well as the measurement of blood glucose levels in the animals after the anesthetic procedure. The fish were divided into the control, vehicle, and TVEO-treated groups at concentrations of 18.6 mg. L⁻1, 27.9 mg. L⁻1, 37.2 mg. L⁻1, and 46.5 mg. L⁻1, administered by immersion baths. Anesthetic efficacy was evaluated by means of induction time, loss of righting response, and recovery time. Electrocardiographic recordings were used to monitor heart rate and rhythm, allowing the analysis of possible pharmacological effects on cardiac electrical conduction. A concentration of 18.6 mg. L⁻1 promoted mild anesthesia without loss of postural response or relevant cardiovascular changes. Concentrations of 27.9 mg. L⁻1 and 37.2 mg. L⁻1 induced effective anesthesia, with rapid induction, adequate recovery, and electrocardiographic stability, characterizing the main therapeutic window. The concentration of 46.5 mg. L⁻1 resulted in transient cardiovascular depression, evidenced by a reduction in heart rate, suggesting autonomic modulation and possible interaction with cardiac ion channels. Thymus vulgaris essential oil exhibits concentration-dependent anesthetic action in C. macropomum, with concentrations of 27.9, 37.2, and 46.5 mg. L⁻1 representing the window for anesthesia in moderate to deep planes. The findings indicate the involvement of central and cardiovascular pharmacological mechanisms, reinforcing the potential of TVEO as a natural alternative to synthetic anesthetics in fish.