Effect of Mouse (Mus musculus) Sex and C57BL/6 Substrain on Sensitivity to Isoflurane and Ketamine-Xylazine-Acepromazine Anesthesia

Anesthesia is commonly performed with mice in the research setting. Standard doses of anesthetic drugs are typically recommended, without customization to strain, substrain, or sex. The purpose of this study was to evaluate C57BL/6 substrain and sex differences in response to isoflurane and ketamine/xylazine/acepromazine (KXA) injectable anesthesia. Female and male C57BL/6NTac, C57BL/6J, C57BL6/6NHsd, and C57BL/6NCrl mice were sourced from 4 different vendors. Isoflurane anesthesia trials were performed with a subset of the mice (n = 24) to determine the minimum alveolar concentration (MAC). Loss of righting reflex, total loss of righting time, time to loss of pedal withdrawal reflex, and total time at surgical plane were evaluated for mice (n = 64) administered 100 mg/kg ketamine, 10 mg/kg xylazine, and 1 mg/kg acepromazine by intraperitoneal injection. Heart rate, respiratory rate, and hemoglobin oxygen saturation (SpO2) were monitored throughout each anesthetic event. Isoflurane MAC was not affected by sex or substrain. In the KXA trials, male mice exhibited a longer duration of loss of righting reflex and remained at a surgical plane of anesthesia significantly longer than the female mice. No significant differences in substrain were detected in the depth or duration of anesthesia. Evaluation of physiologic parameters revealed differences in heart rate between substrains, with C57BL/6NHsd mice exhibiting significantly lower heart rates than the other 3 substrains during both isoflurane and KXA anesthesia. C57BL/6J mice had the highest heart rates during KXA anesthesia. These heart rate differences can impact clinical monitoring practices and are important to consider when selecting strains for study models, especially for cardiovascular studies. In conclusion, the male C57BL/6 mice exhibited a longer duration of anesthesia in response to KXA, while no substrain differences were detected for anesthetic depth or duration of either isoflurane or KXA anesthesia.