To the Editor:
Preservation of basic animal physiology is important for in vivo models of disease. The mouse with acetaminophen (APAP) intoxication is widely used for studying mechanisms of liver injury, despite its poor reproducibility and high inter-animal variability.1 These mice develop hypothermia, probably as a result of the pharmacological action of APAP2 and the development of liver congestion, hypovolemia and inflammation.3, 4 As shown in Fig. 1, APAP induces hypothermia in a dose-dependent manner in C57BL6 mice. Moreover, body temperature differs by several °C within mice receiving the same dose. Our experience and that of others4 show that these mice may remain hypothermic for prolonged periods. Because mouse strain and nutrition state affect APAP-induced liver damage, they may also influence its effect on body temperature. It is well established that temperature is a major determinant of the rate of all biochemical reactions and, therefore, it could influence processes of liver injury studied in the model.
To assess whether body temperature has been routinely measured in studies of mice with APAP intoxication, we reviewed the full-text manuscripts published from 1994 that were retrieved in a PubMed search using the key words “acetaminophen AND mice”. Of the 481 resulting hits, 251 were original articles written in English on APAP hepatotoxicity in mice. In 52% of these studies, mice were fasted prior to APAP administration, which was usually given intraperitoneally (189 of 251). Measurement of body temperature was only performed in 2 of the 251 articles (0.8%)4, 5; no method or data regarding the measurement of body temperature were mentioned in the remaining 249 articles. Importantly, APAP doses higher than 300 mg/kg, which frequently cause severe hypothermia, were used in 76% of the studies.
Lessons from experimental studies of the cerebral complications of liver failure6, 7 suggest that body temperature is a physiological variable that should be assessed in all animal models of liver disease. Hypothermia protects from ischemic liver injury,8 and preliminary experience suggests that it also influences survival and the degree of liver injury in mice with APAP intoxication.9 Conversely, the degree of hypothermia is influenced by the severity of the damage. Variability on the degree of hypothermia could explain the low reproducibility and discordant results reported using this model. Given the multitude of temperature-sensitive hepatic and cerebral processes, studies using mice with APAP intoxication in which body temperature was not rigorously controlled should be interpreted with caution. Ideally, the body temperature of every APAP-treated mouse should be monitored and maintained within the same values as in control animals in future experiments. This approach is logistically and technically difficult to accomplish, but it is the only way to avoid the introduction of this confounding factor and to interpret findings in an appropriate context. While telemetry is well established as a monitoring tool, new methods to measure temperature are being explored.10 The relevance of body temperature in the clinical setting of patients with acute liver failure requires further study.