Use of Thermal Threshold Test Response to Evaluate the Antinociceptive Effects of Butorphanol in Juvenile Green Iguanas (Iguana iguana)
Butorphanol has been widely used as an analgesic in reptiles despite the lack of data on its antinociceptive effects. Six juvenile green iguanas, weighing 35–70 g, were used in this study. Animals were housed together until the night before the experiment, when they were weighed and moved to individual cages. On the morning of each experiment, a probe containing a temperature sensor and heating element was attached to the tail base with tape and attached to a long flexible ribbon cable connected to a thermal threshold tester.1 Skin temperature was recorded and the heater activated. When the iguana responded by kicking a hind leg, the temperature was recorded (thermal threshold; TT) and heating terminated. Three baseline measurements were recorded at 15 min intervals. Treatments were butorphanol (1 mg/kg IM, quadriceps) or 0.9% saline given in a randomized cross-over design with at least 1 month between tests. TT was tested every 15 min for 2 hours post injection, followed by 30-min intervals until 6 hours post injection, and then at 7 hours and 8 hours post injection. Three animals received both treatments with the human observer in view, and all six iguanas were tested after both treatments with the cage covered and the observer hidden but observing via a remote video camera. Skin temperature varied, therefore the difference between this and TT was recorded (∆T). Data was analyzed by a split-plot repeated measures ANOVA.
There was a significant difference in ∆T when the tester was in view compared to when the tester was hidden; mean ± SD baseline of 14.2±3.2°C and 8.8±1.9°C respectively. There was no significant change in ∆T over time after saline or butorphanol under either test situation. This data suggests that the presence of a human increases ∆T in iguanas.
At this dose, butorphanol demonstrated no thermal antinociceptive effects. In other studies it had no anesthetic sparing effect.2 Butorphanol administered at 1 mg/kg is unlikely to alleviate pain in iguanas.
This study was funded by the Morris Animal Foundation. The authors thank Dr. J. Hauptman for statistical analyses.
1. Dixon M.J., S.A. Robertson, and P.M. Taylor. 2002. A thermal threshold testing device for evaluation of analgesics in cats. Res. Vet. Sci. 72:205–210.
2. Mosley C.A., D. Dyson, and D.A. Smith. 2003. Minimum alveolar concentration of isoflurane in green iguanas and the effect of butorphanol on minimum alveolar concentration. J. Am. Vet. Med. Assoc. 222:1559–1564.