Abstract

The mediation of nociception with analgesic medications has not been well documented in elasmobranchs. They possess many of the same nociceptive anatomic and biochemical structures as mammals, including similar spinothalamic pathways, opioid receptors, endogenous opioids, and cyclooxygenase (COX) enzymes suggesting that pain perception in these animals is likely similar to that in mammals. Many analgesics, including opioids and non-steroidal antiinflammatory drugs, have been used in fish, but no effective dosages have been published. As most studies assessing analgesia in fish have utilized teleosts, a study determining the efficacy of common analgesics in elasmobranchs and documenting effective dosages was warranted.

Most studies of pain in fish have relied on observation of behavioral avoidance of noxious stimuli in assessing pain and/or analgesic effects. In higher vertebrates, an analgesic's ability to permit a reduction in the minimum alveolar concentration (MAC) of an inhalant anesthetic while eliminating responses to aversive stimuli is an objective measurement of analgesic properties. A current, novel study by two of the authors is evaluating the use of common analgesics in permitting a decrease in the MS-222 concentration required to prevent responses to noxious stimuli in rainbow trout. Preliminary results of this study demonstrate that higher dosages of the analgesics evaluated permit a reduction in MS-222 concentration. We hypothesized that intramuscular administration of the opioid agonist-antagonist, butorphanol and the NSAID, ketoprofen, would permit a decrease in the minimum concentration of the immersion anesthetic, MS-222 required to prevent response to noxious stimuli in chain dogfish (Scyliorhinus retifer).

Seventeen subjects were each anesthetized with increasing concentrations of MS-222 and a 22 gauge needle placed into their epaxial muscles as the noxious stimulus. The minimum concentration of MS-222 required to eliminate response to needle placement was deemed the baseline for each animal. Repetitive trials, separated by a minimum time interval of 24 hours, were conducted with butorphanol and ketoprofen at dosages of 0.25, 0.5, 1, 2.5, and 5mg/kg and 1, 1.5, 2, and 4mg/kg, respectively, and the anesthetic procedure repeated. The concentration of MS-222 required to prevent response to needle placement was recorded at each dose for each animal. Individual animals displayed a reduction in MS-222 concentration with various dosages of the analgesics, but overall results appear inconsistent and lack statistical significance. Studies evaluating other analgesic/anesthetic medications in both teleosts and elasmobranchs have shown differences in response patterns between the two groups with the elasmobranchs often showing a reduced or no response to the medications when compared with teleosts. There are several plausible explanations for these differences in response patterns; however, none of these explanations have been thoroughly investigated. Clearly more research into this area is necessary.

Acknowledgements

We would like to thank the Fish and Invertebrate husbandry department at Mystic Aquarium, Emily Eckert, Edward Davis, and the WAVES high school students for their assistance with this project.