Opioid signalling prevents regeneration through control of ROS production in zebrafish. (a) Scheme of the experiment. Caudal fins of adult fish were amputated and then allowed to regenerate for 16 or 72 hours. (b) Quantification of the size of the regenerated tissue at 72 hpa (hours post-amputation) in the control (H₂O) (○) and in fish treated with NAL-M (●) or TRAM (⦻). (c) Representative images 72 hpa of caudal fins challenged to regenerate in the presence of NAL-M or TRAM. (d) ROS detection (representative images) at the level of the amputation plane at 16 hpa. (e) ROS quantification in the control (H₂O) (white) and in fish treated with NAL-M (black) or TRAM (white checkered). Data are represented as mean ± SEM. (**p < 0.01; ***p < 0.0001). NAL-M: naloxone methiodide, TRAM: Tramadol.