Long‐term Anesthesia in the Mexican Axolotl Salamander

Abstract

Research animals are routinely anesthetized for varying durations of time and using different anesthetics. However, the anesthesia is usually limited to the duration of surgical procedures or other interventions usually lasting no more than a few hours.Establishing a long‐term anesthetic model in regenerative competent species, such as the Mexican axolotl, would allow for better estimations of physiological parameters during regeneration as the behavioral aspect is detached. As such, we aim for an anesthetic duration of 60 days, a period of time compatible with regeneration of limb amputations as well as heart and spinal cord injury. In this study, axolotls were anesthetized using propofol for increasing periods of time starting with 24 hours and gradually increasing to 7, 12 and 26 days.During this study, parameters such as oxygen consumption, echocardiography and body mass was assessed. After anesthesia, activity level, response to tactile and nociceptive stimuli, and recovery time was examined to ensure no lasting effect of long‐term anesthesia. Long‐term anesthesia was associated with a significant, but reversible, reduction in oxygen consumption (approximately 50%) and a significant increase in body mass despite fasting. Anesthetic periods exceeding 7 days in hypoosmotic medium like tap water were associated with complications including severe swelling and even death, but these complications were alleviated using isosmotic medium. Tactile and nociceptive stimulation was scored on a scale from 1 to 3 prior to anesthesia. No significant change in response was detected following anesthesia demonstrating that no permanent nerve damage had occurred. Axolotls showed a reversible decrease in activity levels following an anesthetic period with levels returning to baseline within a week of recovery.In conclusion, this study shows that axolotls can be anesthetized for extended periods of at least 26 days presenting new possibilities for future experiments in which long‐term anesthesia or immobilization is preferable. Additionally, stable long‐term anesthesia paves the way for non‐recovery experiments and more severe interventions as animals can be kept anesthetized for the duration.