Quantitative and Qualitative Behavioral Measurements to Assess Pain in Axolotls (Ambystoma mexicanum)

Effective pain relief in animals relies on the ability to discern pain and assess its severity. However, few objective measures exist to assess the presence and severity of pain in axolotls, and few resources are available regarding drugs and appropriate doses to provide pain relief in this species. This study evaluated behavioral tools for cageside pain assessment and validated a reproducible and reliable quantitative method to evaluate analgesic efficacy in axolotls. Animals were divided into control and treatment groups (n = 6 per group); treatment groups received buprenorphine through injection (50 mg/kg every 24 h for 48 h intracelomically) or butorphanol immersion (0.50 or 0.75 mg/L every 24 h for 48 h). Qualitative behavioral tests, adapted from other amphibian studies, included tapping on the home tank, directing water jets or physically touching specific anatomic points on the animal, and placing a novel object in the home tank. Quantitative methods used to produce noxious stimuli were the acetic acid test and von Frey aesthesiometers. Animals that were treated with analgesics did not demonstrate a significant difference compared with controls during behavioral assessment at 1, 6, 12, 25, 30, and 48 h after analgesia administration. The acetic acid test revealed a reproducible, concentration-dependent pain response. However, a significant difference in the AAT response was not observed between control and treated groups with the tested analgesics and doses.