Opioids In Fish

Opioids, nonsteroidal anti-inflammatory drugs (NSAIDs) and local anaesthetics are three types of analgesic drugs that have been studied as potential analgesic agents in fish.16 Opioid analgesic drugs exert their effects through the activation of mu-, delta- and kappa-opioid receptors, thus inhibiting and modulating pain signals.88 Morphine is a pure mu-opioid receptor agonist with a great hypoalgesic effect and very used in veterinary medicine.89 Since pure mu-agonist opioids cause addiction in humans, the use of these opioids is strictly controlled, which may limit their use. Butorphanol is a kappa-opioid receptor agonist and a mu-antagonist, which limits its analgesic properties and efficacy.89 Buprenorphine is a partial mu-opioid receptor …show more content…
However, difficulties in assessing pain responses in these species, and the likely wide variations of responses between species, make recommendations of effective dose rates very difficult. Furthermore, these drugs are injected by intramuscular, intraperitoneal and subcutaneous routes, which, in fish, limits their use mainly to the intraoperative period or short time post-operative period without extra animal restraint and anaesthesia. Also, a very important point in analgesia therapy is the action time and time to peak obtained with the different drugs. For example, butorphanol’s analgesic onset is rapid but the mild analgesia is of short duration, while buprenorphine’s time to peak analgesic effect is quite slow but its analgesic duration can be quite long. When administered together, butorphanol’s short-term analgesia wanes as buprenorphine is reaching its peak …show more content…
Regarding welfare, it is important to use an anaesthetic protocol able to produce anaesthesia with analgesia, when needed. The optimal anaesthetic protocol should be stable during the entire experimental procedure, inducing short recovery times with minor or no physiological and behavioural alterations. Studies addressing the effects of anaesthetics in fish used in research are still scarce (table 1), which make it difficult to find an appropriate dose regime suitable for each species of fish. Besides, the size of the fish within a species may largely influence the best anaesthetic dose. The difficulty to choose the anaesthetic agent that suits better a certain species is also in part due to the lack of information regarding the anaesthetic stage achieved, duration of anaesthesia, times of induction and recovery, and side effects. Due to the countless variables that can interfere with the anaesthetic efficacy, it is always advisable to perform a pilot study before starting the experimental protocol to find the optimal anaesthetic conditions for your species, strain, age of the animal, husbandry conditions, and experimental requirements, thus ensuring the fish welfare and reliable data