Exploring the efficacy of immersion analgesics in zebrafish using an integrative approach

Home Office guidelines recommend the use of analgesics for all protected species, including fish during scientific procedures likely to result in pain, suffering, distress and lasting harm. In larger fish species some drugs have shown efficacy in terms of perioperative analgesia when administered intramuscularly or at the site of tissue damage. However, this is impractical in very small species (<1 g) and when large numbers need to be treated. Immersion is a commonly used route of administering anaesthesia in fish and may be a less invasive alternative to administering analgesia. Therefore, this study examined the efficacy of three water soluble analgesic drugs (buffered acetylsalicylic acid, butorphanol tartrate and lidocaine) provided pre- and post-surgery (tail fin clip) through addition to the tank water. The effectiveness of these drugs was measured as the reduction of changes in pain-related post-surgical behaviour and since pain is inherently stressful, HPI axis activity, quantified as whole body cortisol levels. To determine uptake of each drug, whole body concentrations were determined by HPLC-mass spectroscopy. Administration of both lidocaine and acetylsalicylic acid resulted in a significant reduction of post-nociceptive behavioural patterns, compared with animals exposed to saline solution. There was no effect of treatment on whole body cortisol. Finally, uptake was verified for lidocaine with whole-body concentrations showing clear dose dependency, whereas the other two compounds were not detectable. As the only drug with both a behavioural sparing effect and pharmacokinetic validation in this study, low-dose lidocaine immersions can be recommended for the first time to aid perioperative analgesia and provide a valuable refinement for reducing the impact of invasive procedures in zebrafish thereby improving welfare.