AJVR • Vol 78 • No. 9 • September 2017 1019 U nderstanding of pain in reptiles has expanded greatly within the past 10 years, but some weak- nesses remain, particularly with respect to species differences in response to analgesics, identification of optimal analgesics, and determination of the most appropriate analgesic dosages. This is primarily due to the wide diversity of anatomic, physiologic, and metabolic characteristics in reptile species. Whereas a few studies 1–3 have revealed specific neuroanatom- ic structures associated with pain in reptiles, more recent studies 4–9 have focused on the behavioral re- sponses to various noxious stimuli across several rep- tile species. Studies of nociception and antinociception have been performed in a variety of reptile species, with responses measured to thermal, chemical, or me- chanical noxious stimuli. For example, behavioral re- sponses to chemical noxious stimuli (ie, formalin or capsaicin) have been evaluated in Speke hinge-back tortoises (Kinixys spekii ), 6,7 ball pythons (Python regius), 8 and crocodiles, 9 before and after analgesics Antinociceptive efficacy of intramuscular administration of morphine sulfate and butorphanol tartrate in tegus ( Salvator merianae) William P. Leal MS Adriano B. Carregaro DVM, PhD Thais F. Bressan MS Shayne P. Bisetto Cristiano F. Melo Kurt K. Sladky MS, DVM Received August 27, 2016. Accepted December 12, 2016. From the Department of Veterinary Medicine, School of Animal Science and Food Engineering, University of São Paulo, Pirassununga, São Paulo 13635-900, Brazil (Leal, Carregaro, Bressan, Bisetto, Melo); and the Department of Surgical Sciences, School of Veteri- nary Medicine, University of Wisconsin, Madison, WI 53705 (Sladky). Address correspondence to Dr. Carregaro (carregaro@usp.br). OBJECTIVE To evaluate the antinociceptive ef fcacy of IM morphine sulfate or butorpha- nol tartrate administration in tegus (Salvator merianae). ANIMALS 6 healthy juvenile (12- to 24-month-old) tegus (mean ± SD body weight, 1,484 ± 473 g). PROCEDURES In a crossover study design, tegus were randomly assigned to treatment or- der, with a minimum washout period of 15 days between treatments. Each of 5 treatments was administered IM in a forelimb: saline (0.9% NaCl) solu- tion (0.5 mL), morphine sulfate (5 or 10 mg/kg), or butorphanol tartrate (5 or 10 mg/kg). A withdrawal latency test was used to evaluate antinocicep- tion, with a noxious thermal stimulus applied to the plantar surface of the hind limb before (0 hours; baseline) and 0.5, 1, 2, 3, 4, 6, 12, and 24 hours after each treatment. Observers were unaware of treatment received. RESULTS With saline solution, mean hind limb withdrawal latencies (interval to limb withdrawal from the thermal stimulus) remained constant, except at 12 hours. Tegus had higher than baseline mean withdrawal latencies between 0.5 and 1 hour and at 12 hours with morphine at 5 mg/kg and between 1 and 12 hours with morphine at 10 mg/kg. With butorphanol at 5 and 10 mg/ kg, tegus maintained withdrawal responses similar to baseline at all assess- ment points. CONCLUSIONS AND CLINICAL RELEVANCE Results indicated that morphine, but not butorphanol, provided antinoci- ception at 5 and 10 mg/kg in tegus as measured by thermal noxious stimu- lus testing. These data supported the hypothesis that μ-opioid (but not κ-opioid) receptor agonists provide antinociception in reptiles. (Am J Vet Res 2017;78:1019–1024) were administered. Although evidence from some of these studies suggests that μ-opioid receptor agonists attenuate the nociceptive response, these noxious chemicals can cause severe inflammation and tissue damage, hindering accurate measurement of a noci- ceptive response. Studies 4,5,10,11 in which responses were mea- sured after application of a noxious thermal stimu- lus (hind limb withdrawal apparatus) have yielded consistent results for various reptile species, with responses similar to those in mammals. 10 This type of test has been successfully used for red-eared slid- ers, 4,11 bearded dragons, 5 corn snakes, 5 and green iguanas. 12 When the hind limb withdrawal latency test is used, responses can be measured before and after administration of saline (0.9% NaCl) solution or analgesics. 13,14 Opioids are commonly used for pain management in veterinary medicine. Morphine, a pure μ-opioid re- ceptor agonist from this class, is a commonly used an- algesic; however, it can cause several adverse effects, Unauthenticated | Downloaded 10/01/22 05:48 PM UTC