Physiologic, Behavioral, and Histologic Responses to Various Euthanasia Methods in C57BL/6NTac Male Mice

Rodent euthanasia using exposure to increasing concentrations of CO2 has come under scrutiny due to concerns of potential pain during the euthanasia process. Alternatives to CO2, such as isoflurane and barbiturates, have been proposed as more humane methods of euthanasia. In this study, we examined 3 commonly used euthanasia methods in mice: intraperitoneal injection of pentobarbital-phenytoin solution, CO2 inhalation, and isoflurane anesthesia followed by CO2 inhalation. We hypothesized that pentobarbital-phenytoin euthanasia would cause fewer alterations in cardiovascular response, result in less behavioral evidence of pain or stress, and produce lower elevations in ACTH than would the isoflurane and CO2 methods, which we hypothesized would not differ in regard to these parameters. ACTH data suggested that pentobarbital-phenytoin euthanasia may be less stressful to mice than are isoflurane and CO2 euthanasia. Cardiovascular, behavioral, and activity data did not consistently or significantly support isoflurane or pentobarbital-phenytoin euthanasia as less stressful methods than CO2. Euthanasia with CO2 was the fastest method of the 3 techniques. Therefore, we conclude that using CO2 with or without isoflurane is an acceptable euthanasia method. Pathologic alterations in the lungs were most severe with CO2 euthanasia, and alternative euthanasia techniques likely are better suited for studies that rely on analysis of the lungs.