Post-Surgical Analgesia in Rainbow Trout: Is Reduced Cardioventilatory Activity a Sign of Improved Animal Welfare or the Adverse Effects of an Opioid Drug?

The use of fish models in biomedical research is increasing. Since behavioural and physiological consequences of surgical procedures may affect experimental results, these effects should be defined and, if possible, ameliorated. Thus, the use of post-surgical analgesia should be considered after invasive procedures also in fish, but presently, little information exists on the effects of analgesics in fish. This study assessed the effects of an opioid drug, buprenorphine (0.05 mg/kg IM), on resting ventilation and heart rates during 7 days of postsurgical recovery in rainbow trout (Oncorhynchus mykiss) at 10°C by non-invasively recording bioelectric potentials from the fish via electrodes in the water. Baseline ventilation and heart rates were considerably lower compared to previously reported values for rainbow trout at 10°C, possibly due to the non-invasive recording technique. Buprenorphine significantly decreased both ventilation and heart rates further, and the effects were most pronounced at 4–7 days after anaesthesia, surgical procedures and administration of the drug. Somewhat surprisingly, the same effects of buprenorphine were seen in the two control groups that had not been subject to surgery. These results indicate that the reductions in ventilation and heart rates are not caused by an analgesic effect of the drug, but may instead reflect a general sedative effect acting on both behaviour as well as e.g. central control of ventilation in fishes. This resembles what has previously been demonstrated in mammals, although the duration of the drug effect is considerably longer in this ectothermic animal. Thus, before using buprenorphine for postoperative analgesic treatment in fish, these potentially adverse effects need further characterisation.