Effects of commonly used analgesic drugs and trigeminal nerve fiber proportions in the speke-hinged tortoise (kinixys Spekil) and the marsh terrapin (pelomedusa subrufa)

Abstract

This study was undertaken to explore the nociception and antinociception in the speke-hinged tortoise (Kinixys spekii) and the marsh terrapin (Pelomedusa subrufa). Four nociceptive tests, namely the formalin-, hot plate-, capsaicin instillation- and acetic acid instillation tests were adapted and used to study nociception and antinociceptive effects of commonly used analgesics. A histological survey of the fiber proportions in the sensory branches of the trigeminal nerve was also carried out to obtain more information on the nociceptive system of the animals. Forty-two tortoises and thirty-four terrapins were used in the study. In the formalin test, 100 ul, of 12.5% formalin was injected subcutaneously in the hind paw of the animals and the total time spent in pain scored. Both animal species demonstrated a monophasic pain response, characterized by full limb retraction or partial usage of the limb. Thermal stimuli were induced using a hot plate analgesia meter set at 60 "C. Both animals responded by lifting one of the paws and 'attempting to escape'. Tortoises showed a mean response latency of 53.95 ± 3.53 seconds while that of the terrapin was 41.28 ± 3.41 seconds. In the capsaicin instillation test, two drops of capsaicin were directly instilled into the eye and the duration of eye closure measured in blocks of five minutes for 30 minutes. Terrapins showed sensitivity to capsaicin but there was no capsaicin-desensitization effect after repeated application of capsaicin. In the acetic acid instillation test, two drops of 10% acetic acid were directly instilled into the eye and the duration of eye closure scored in blocks of five minutes for 30 minutes. In the formalin and the hot plate tests, both morphine and pethidine showed dose dependent anti-nociceptive effects, which were naloxone reversible, in both animal species. Morphine at dosages less than 7.5 mg/kg and pethidine at dosages less than 20 mg/kg did not induce any significant antinociceptive effects. Acetylsalicylic acid, flunixin meglumin, dexamethasone and hydrocortisone at the dosages used did not show any antinociceptive activity in neither the formalin nor the hot plate tests in any of the animal species. XXI In the ophthalmic nerve of the tortoise and the terrapin, the proportion of nerve fibers with diameters measuring 0.5-5.5 urn was only 17.8 and 18.6 % respectively. In the maxillary branch of the trigeminal nerve, the proportion of nerve fibers with diameters measuring 0.5- 5.5 urn was 20.7 and 27 % in the tortoise and the terrapin respectively. The results suggest that testudines have relatively few nociceptive fibers, which comprise 18-27 % of the sensory afferents in the trigeminal nerve sensory afferents. In conclusion, testudines have a nociceptive system, which is responsive to opioid analgesia. The proportion of nociceptive afferents is low. It is postulated that in testudines the shell, which protects these animals, might be an alternative to a comprehensive nociceptive system.