Therapeutic Window of Morphine on Cardiac and Respiratory Parameters of Juvenile Tambaqui, Colossoma macropomum

Morphine is widely used as an analgesic in vertebrates, yet its cardiorespiratory safety and effective therapeutic range remain poorly explored in fish. This study investigated the dose-dependent effects of morphine on cardiac and respiratory parameters of juvenile tambaqui (Colossoma macropomum). Juveniles (25.95 ± 4.08 g) were randomly assigned to control, sham (0.9% saline) or morphine groups (24, 28, 32, 36 and 40 mg kg−1, i.p.). Electrocardiogram (ECG) recordings were used to assess heart rate (HR), PQ, RR and QT intervals, and QRS amplitude, while opercular beat rate (OBR) and opercular beat intensity (OBI) were measured to evaluate respiratory responses. Morphine induced a significant dose-dependent bradycardia and QT prolongation, without affecting QRS amplitude or conduction integrity. Respiratory frequency and intensity also decreased with increasing doses, with responses plateauing above 32 mg kg−1. The EC50 for HR reduction was 27.18 mg kg−1, aligning with a safe therapeutic range of 24–32 mg kg−1. By establishing this dose–response dynamic, the study provides the first characterization of a physiologically safe therapeutic window of morphine in tambaqui and highlights its safety profile for cardiorespiratory parameters. Moreover, the present results demonstrate that the opioid system of juvenile tambaqui is functionally developed, providing a physiological basis for future studies on nociception and analgesic efficacy, with relevance to welfare-oriented practices in aquaculture.