Morphine, But Not Butorphanol, Causes Analgesia and Respiratory Depression in Red-Eared Slider Turtles (Trachemys scripta)
Abstract
Pain management is a crucial component of clinical veterinary medicine. While analgesic drugs have been extensively examined in domestic species,1,6 our clinical understanding of analgesic efficacy, pharmacodynamics, and opioid receptor binding characteristics is negligible in non- domestic species.4,8 Relevant data are particularly lacking for reptiles, which are commonly maintained as companion animals, and heavily represented in zoological and scientific laboratory collections. Butorphanol tartrate, a mixed opioid agonist/antagonist with ĸ-agonist activity, is the most widely used analgesic drug in reptile medicine.7 However, there are no clinical data to substantiate its analgesic effect in reptiles. In contrast, morphine, an opioid with µ-agonist activity, has been shown to attenuate behavioral responses to noxious thermal stimuli in anole lizards and crocodiles.3,5 Opioid drugs can cause profound respiratory depression in many species.6 For reptiles, µ-opioid receptor activation abolishes respiratory motor output in isolated turtle brainstems.2 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) using a thermal hind limb withdrawal latency test; and (2) evaluate effects of morphine and butorphanol on respiration in turtles.
Hind limb thermal withdrawal latencies were measured and compared in turtles (n=11) before and after subcutaneous administration of physiologic saline, butorphanol tartrate (2.8 or 28 mg/kg), or morphine sulfate (1.5 or 6.5 mg/kg). Hind limb thermal withdrawal latencies sampled at 1, 2, 4, 8, and 24 hours post injection were no different in turtles receiving saline or either dose of butorphanol. However, hind limb thermal withdrawal latencies increased significantly in turtles after administration of morphine sulfate, indicating that morphine sulfate provided analgesia in this species. Ventilation was measured in freely swimming turtles (n=7) before and after subcutaneous administration of physiologic saline, butorphanol tartrate (28 mg/kg), or morphine sulfate (1.5 mg/kg). Saline and butorphanol had no significant effect on ventilation, although turtles receiving butorphanol exhibited a trend toward ventilatory depression. Morphine, on the other hand, significantly depressed ventilation. Butorphanol tartrate, the most widely used analgesic in reptiles, may not provide adequate analgesia in turtles. However, morphine sulfate appears to be an effective analgesic in turtles but may contribute to significant respiratory depression.
Acknowledgments
This research was supported by a grant from the Morris Animal Foundation, Englewood, CO 80112. The authors gratefully acknowledge Robert Creighton for his excellent technical assistance, and Claudia Hirsch and the animal care staff at the Charmany Research Facility for animal care and logistical assistance.
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