Body Temperature Measurements for Metabolic Phenotyping in Mice

Abstract

Key Points Rectal probing is subject to procedural bias. This method is suitable for first-line phenotyping, provided probe depth and measurement duration are standardized. It is also useful for detecting individuals with out-of-range body temperatures (during hypothermia, torpor).The colonic temperature attained by inserting the probe >2 cm deep is a measure of deep (core) body temperature.IR imaging of the skin is useful for detecting heat leaks and autonomous thermoregulatory alterations, but it does not measure body temperature.Temperature of the hairy or shaved skin covering the inter-scapular brown adipose tissue can be used as a measure of BAT thermogenesis. However, obtaining such measurements of sufficient quality is very difficult, and interpreting them can be tricky. Temperature differences between the inter-scapular and lumbar areas can be a better measure of the thermogenic activity of inter-scapular brown adipose tissue.Implanted probes for precise determination of BAT temperature (changes) should be fixed close to the Sulzer's vein. For measurement of BAT thermogenesis, core body temperature and BAT temperature should be recorded simultaneously.Tail temperature is suitable to compare the presence or absence of vasoconstriction or vasodilation.Continuous, longitudinal monitoring of core body temperature is preferred over single probing, as the readings are taken in a non-invasive, physiological context.Combining core body temperature measurements with metabolic rate measurements yields insights into the interplay between heat production and heat loss (thermal conductance), potentially revealing novel thermoregulatory phenotypes. Endothermic organisms rely on tightly balanced energy budgets to maintain a regulated body temperature and body mass. Metabolic phenotyping of mice, therefore, often includes the recording of body temperature. Thermometry in mice is conducted at various sites, using various devices and measurement practices, ranging from single-time probing to continuous temperature imaging. Whilst there is broad agreement that body temperature data is of value, procedural considerations of body temperature measurements in the context of metabolic phenotyping are missing. Here, we provide an overview of the various methods currently available for gathering body temperature data from mice. We explore the scope and limitations of thermometry in mice, with the hope of assisting researchers in the selection of appropriate approaches, and conditions, for comprehensive mouse phenotypic analyses.

Keywords: body temperature; metabolism; mouse; mouse models; phenotyping; telemetric recordings; thermography.

Figure 1

Twenty four-hour brown adipose tissue (BAT) thermometry and abdominal body temperature readings in a freely-moving C57BL/6 mouse maintained at 26°C. Abdominal temperature was measured using an implanted telemetry transmitter (DSI, ETA-F10). BAT temperature was measured using a thermistor (NTH5G10P, muRata, Kyoto, Japan) implanted between the BAT and the underlying muscle layer in the inter-scapular region near Sulzer's vein, and was connected to a swivel. The resolution of readings was set to 1 Hz. Y. Ootsuka, unpublished data.

Figure 2

(A) Adrenergically-stimulated thermogenesis [heat production (HP)] following 1 mg/kg norepinephrine, s.c. (Arterenol, Merck; the arrow indicates the time-point of injection), and transient hyperthermia in a male AKR/J mouse kept at 30°C (N. Rink and C. W. Meyer; unpublished data). Body temperature (T_b) of the individual was recorded with an intraperitoneally-implanted probe (MiniMitter, Sunriver, OR, USA) at a frequency of 2 min. HP was determined in parallel using indirect calorimetry (Meyer et al., 2015). (B) Genotype-dependent variation in lipopolysaccharide (LPS)-induced (15 μg/g, i.p.) transient hypothermia in mice, as determined by an intra-abdominal temperature sensor (DSI TA-F10). KO: knockout mouse, WT: wild-type control (B. Strilic, unpublished data). (C) Simultaneous metabolic rate (MR) and abdominal body temperature (T_b) recordings (E-Mitter Series 3000 XM-FH, 4-min resolution) of a physiological torpor event in a female C57BL/6 mouse, compared to a hypothermic individual. (D) MR was measured using an indirect calorimetry set-up (Heldmaier and Ruf, 1992). In (C,D), shaded areas indicate the duration of “lights off.” Note in (C), the steep decrease in MR and T_b during the middle of the dark phase, and the spontaneous arousal shortly after “lights-on,” in contrast to the slowly-decreasing MR and T_b in (D). The hypothermic mouse in example (D) was removed from the calorimetry cage (arrow) and externally rewarmed without experiencing consequential damage from hypothermia. Activity counts in (C,D) were measured via integrated gross motor detection of the E-Mitter in the receiver field. Data are taken from Oelkrug et al. (2011).

Figure 3

(A) Intra-abdominal body temperature (E-Mitter Series 3000 XM-FH) recorded every 4 min in a wild-type (WT) and an uncoupling protein-1 (UCP1)-knockout (KO) mouse previously acclimated to 18°C and acutely exposed to 5°C. Note the pronounced episodic fluctuations in body temperature of the WT mouse that are absent in the UCP1-KO mouse. Using the same data sets, we simulated hourly probing (B), demonstrating resolution legacy and the information potentially missed from less-frequent sampling. Data are taken from Meyer et al. (2010).

Figure 4

Infrared (IR) thermography in mouse metabolic studies and phenotyping. In each panel, specific color coding of radiant heat is indicated to the right. (A) Dorsal view from an unrestrained, conscious wild-type mouse, captured by IR thermography (T335, FLIR Systems), demonstrating heterogeneity in surface temperatures by color coding. The ambient temperature was set to 22–23°C. Image kindly provided by R. Oelkrug and J. Mittag, unpublished. (B) Radiant temperature from mouse tails reveals enhanced skin vasoconstriction and altered autonomous vasomotor control in transient receptor potential vanilloid-1 (Trpv1)-knockout (KO) mice compared to wild-type controls (Garami et al., 2011). The IR camera (ThermoVision A20M, FLIR Systems) was positioned above a group of confined, conscious mice inside a climatic chamber at 32°C. The mice had been previously habituated to the experimental setup by extensive handling. (C) Whole-body thermography in neonates (p1–p3), highlighting reduced inter-scapular skin-surface temperature in association with genetic knockout of uncoupling protein-1 (UCP-1) and impaired non-shivering thermogenesis. For the measurement, pups were placed in 6-well cell culture plates at 22–23°C ambient temperature (Maurer et al., 2015). (D) Lateral view from an unrestrained, conscious wild-type mouse, captured by IR thermography (T335, FLIR Systems) for specific measurement of external acoustic meatus temperature. Ambient temperature was set to 22–23°C. Image kindly provided by R. Oelkrug and J. Mittag, unpublished.

Figure 5

(A) Assessment of thermal conductance using resting heat production rates collected in wild-type and uncoupling-protein-1 (UCP1-KO)-deficient littermates at different ambient temperatures (“Scholander-Irving plot”; Scholander et al., 1950). Average slopes corresponding to predicted minimal thermal conductance (expressed as positive values) are indicated for each genotype (p = 0.08, t-test). The arrow highlights the x-axis intersection points corresponding to the predicted average body temperature during resting conditions (not different between genotypes; p = 0.44, t-test). (B) Calculated thermal conductance (see Box 6) involving core body temperature readings (E-Mitter Series 3000 XM-FH) in the animals shown in (A). Each point indicates mean ± SD (n = 4–7). Both analyses, (A,B), are supporting lower thermal conductance and thus an altered thermoregulatory strategy involving improved heat conservation in UCP1-KO mice. Data are taken from Meyer et al. (2010).