Acute Effects of Hypothermia and Inhalant Anesthesia on Ultrasonic Vocalizations and Neuroendocrine Markers in Neonatal Rats.

Neonatal rodents undergo anesthesia for numerous procedures and for euthanasia by anesthetic overdose. However, data regarding whether neonatal anesthesia is humane are limited. Hypothermia (cryoanesthesia) is the most commonly used anesthetic protocol for neonatal rats 10 d of age or younger. However, hypothermia has recently been restricted in several countries due to perceived painful effects, including pain on rewarming. Minimizing the potential pain and distress of neonates in research is imperative, although very challenging. Traditional validated and nonvalidated behavioral and physiologic outcome measures used for adult rats undergoing anesthesia are unsuitable for evaluating neonates. Therefore, we investigated the effects of several anesthetic methods on neonatal rats by using the innovative objective approaches of noninvasive ultrasonic vocalizations and more invasive neuroendocrine responses (i. e., serum corticosterone, norepinephrine, glucose). Our results show that hypothermia leads to heightened acute distress in neonatal rats as indicated by prolonged recovery times, increased duration of vocalizations, and elevated corticosterone levels, as compared with neonates undergoing inhalational anesthesia. We demonstrate that inhalational anesthesia is preferable to cryoanesthesia for neonatal rats, and researchers using hypothermia anesthesia should consider using inhalational anesthesia as an alternative method.