Reptile Analgesia: Efficacy and Side Effects of Morphine and Butorphanol in Bearded Dragons (Pogona vitticeps), Corn Snakes (Elaphe guttata), and Red-Eared Slider Turtles (Trachemys scripta)
Abstract
Pain and its amelioration are fundamental to the practice of veterinary medicine. In zoological medicine, however, our standard approaches to dealing with clinical analgesia may be providing a false sense of security for us as veterinarians, and little in the way of functional pain management for our patients. This stems from the fact that little is known about mechanisms underlying nociception, analgesic efficacy and side effects, pharmacodynamics, and opioid receptor binding characteristics in non-domestic species, and data are particularly lacking for reptiles.1,5 Butorphanol tartrate, a mixed opioid agonist/antagonist with κ-agonist activity, is an analgesic drug used widely to attenuate pain in zoological medicine, and is the most widely used analgesic drug in reptile medicine.5 However, there are no clinical data to substantiate its analgesic effect in reptiles. In contrast, morphine, an opioid with µ-agonist activity, attenuates behavioral responses to noxious thermal stimuli in red-eared slider turtles, anole lizards and crocodiles.2,3,6 Untoward side effects can be problematic, as opioid drugs frequently cause respiratory depression in many species.4,6 The objectives of this study were to: 1) determine the effects of morphine sulfate and butorphanol tartrate on nociceptive behaviors in adult, red-eared slider turtles (Trachemys scripta), bearded dragons (Pogona vitticeps) and corn snakes (Elaphe guttata) using a thermal hind limb or tail withdrawal latency test; and 2) evaluate effects of morphine and butorphanol on respiration in these three reptile species.
Infrared heat stimuli were applied to the plantar surface of turtle and bearded dragon hindlimbs, and to the ventral tail surface of corn snakes. Thermal withdrawal latencies were measured before and after subcutaneous administration of physiologic saline, butorphanol tartrate (2 or 20 mg/kg in bearded dragons and snakes; 2.8 or 28 mg/kg in turtles), or morphine sulfate (1, 10, 20, and 40 mg/kg in bearded dragons and snakes; 1.5 or 6.5 mg/kg in turtles). Thermal withdrawal latencies sampled at 2, 4, 8, and 24 h post-injection were similar in reptiles receiving saline or butorphanol at any dosage. However, hind limb thermal withdrawal latencies increased in turtles and, less so, in bearded dragons after administration of morphine sulfate, indicating that morphine provided analgesia in these species. Corn snake tail thermal withdrawal latencies after morphine administration were highly variable at all four dosages, which precluded developing any conclusions regarding efficacy of morphine in this species. Because antinociceptive efficacy of morphine was clearly demonstrated in only turtles, ventilation was measured in freely swimming turtles before and after subcutaneous administration of physiologic saline, butorphanol tartrate, or morphine sulfate. Both butorphanol and morphine significantly depressed ventilation in turtles. Butorphanol tartrate, the most widely used analgesic in reptiles, may not provide adequate analgesia in red-eared slider turtles, bearded dragons and corn snakes, and yet cause significant respiratory depression.
Acknowledgments
Supported by grants from the American College of Laboratory Animal Medicine Foundation and the Morris Animal Foundation, Englewood, CO.
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