Validation of a thermal threshold nociceptive model in bearded dragons (Pogona vitticeps)

To validate a thermal threshold (TT) nociceptive model in bearded dragons (Pogona vitticeps) and to document TT changes after administration of morphine.

A two-part randomized, blinded, controlled, experimental study.

Five adult bearded dragons (242-396 g).

A TT device delivered a ramped nociceptive stimulus (0.6 °C second^-1) to the medial thigh until a response (leg kick/escape behavior) was observed or maximum (cut-off) temperature of 62 °C was reached. In phase I, period 1, six TT readings were determined at 20 minute intervals for evaluation of repeatability. Two of these readings were randomly assigned to be sham to assess specificity of the behavioral response. The same experiment was repeated 2 weeks later (period 2) to test reproducibility. In phase II, animals were administered either intramuscular morphine (10 mg kg^-1) or saline 0.9%. TTs (maximum 68 °C) were determined before and 2, 4, 8, 12 and 24 hours after treatment administration. Data were analyzed using one-way anova (temporal changes and repeatability) and paired t tests (reproducibility and treatment comparisons) using Bonferroni correction (p < 0.05).

Mean TT values were 57.4 ± 3.8 °C and 57.3 ± 4.3 °C for periods 1 and 2, respectively. Data were repeatable within each period (p = 0.83 and p = 0.07, respectively). Reproducibility between periods was remarkable (p = 0.86). False-positive responses during sham testing were 10%. TTs were significantly increased after morphine administration at 2, 4 and 8 hours compared with baseline, and at 2 and 4 hours compared with saline 0.9%. The highest TT was 67.7 ± 0.7 °C at 4 hours after morphine administration.

Testing was repeatable, reproducible and well tolerated in bearded dragons. TT nociceptive testing detected morphine administration and may be suitable for studying opioid regimens in bearded dragons.