Mouse Thermoregulation: Introducing the Concept of the Thermoneutral Point

Estrogen-sensitive medial preoptic area neurons coordinate torpor in mice

Homeotherms maintain a stable internal body temperature despite changing environments. During energy deficiency, some species can cease to defend their body temperature and enter a hypothermic and hypometabolic state known as torpor. Recent advances have revealed the medial preoptic area (MPA) as a key site for the regulation of torpor in mice. The MPA is estrogen-sensitive and estrogens also have potent effects on both temperature and metabolism. Here, we demonstrate that estrogen-sensitive neurons in the MPA can coordinate hypothermia and hypometabolism in mice. Selectively activating estrogen-sensitive MPA neurons was sufficient to drive a coordinated depression of metabolic rate and body temperature similar to torpor, as measured by body temperature, physical activity, indirect calorimetry, heart rate, and brain activity. Inducing torpor with a prolonged fast revealed larger and more variable calcium transients from estrogen-sensitive MPA neurons during bouts of hypothermia. Finally, whereas selective ablation of estrogen-sensitive MPA neurons demonstrated that these neurons are required for the full expression of fasting-induced torpor in both female and male mice, their effects on thermoregulation and torpor bout initiation exhibit differences across sex. Together, these findings suggest a role for estrogen-sensitive MPA neurons in directing the thermoregulatory and metabolic responses to energy deficiency.

Cutaneous Thermal Injury Modulates Blood and Skin Metabolomes Differently in a Murine Model

As the field of metabolomics develops further, investigations of how the metabolome is affected following thermal injury may be helpful to inform diagnostics and guide treatments. In this study, changes to the metabolome were tested and validated in a murine burn injury model. After a 30% total body surface scald injury or sham procedure sera and skin biopsies were collected at 1, 2, 6, or 24 hr.

Burn-specific changes in the metabolome were detected compared to sham animals. The sera metabolome exhibited a more rapid response to burn injury than that of the skin and it peaked more proximal to injury (6 vs 24 hr). Progression of metabolic response in the skin was less synchronous and showed a higher overlap of the significantly modified metabolites (SMMs) among tested time-points. Top affected pathways identified by SMMs of skin included inositol phosphate metabolism, ascorbate and alderate metabolism, caffeine metabolism, and the pentose phosphate pathway. Future research is warranted in human and larger animal models to further elucidate the role of metabolomic perturbations and the pathophysiology following burn injury.

Coupling of energy intake and energy expenditure across a temperature spectrum: impact of diet-induced obesity in mice

Obesity and its metabolic sequelae are implicated in dysfunction of the somatosensory, sympathetic, and hypothalamic systems. Because these systems contribute to integrative regulation of energy expenditure (EE) and energy intake (EI) in response to ambient temperature (T_a) changes, we hypothesized that diet-induced obesity (DIO) disrupts T_a-associated EE-EI coupling. C57BL/6N male mice were fed a high-fat diet (HFD; 45% kcal) or low-fat diet (LFD; 10% kcal) for ∼9.5 wk; HFD mice were then split into body weight (BWT) quartiles (n = 8 each) to study DIO-low gainers (Q1) versus -high gainers (Q4). EI and indirect calorimetry (IC) were measured over 3 days each at 10°C, 20°C, and 30°C. Responses did not differ between LFD, Q1, and Q4; EI and BWT-adjusted EE increased rapidly when transitioning toward 20°C and 10°C. In all groups, EI at 30°C was not reduced despite lower EE, resulting in positive energy balance and respiratory exchange ratios consistent with increased de novo lipogenesis, energy storage, and relative hyperphagia. We conclude that 1) systems controlling T_a-dependent acute EI/EE coupling remained intact in obese mice and 2) rapid coupling of EI/EE at cooler temperatures is an important adaptation to maintain energy stores and defend body temperature, but less critical at thermoneutrality. A post hoc analysis using digestible EI plus IC-calculated EE suggests that standard IC assumptions for EE calculation require further validation in the setting of DIO. The experimental paradigm provides a platform to query the hypothalamic, somatosensory, and sympathetic mechanisms that drive T_a-associated EI/EE coupling.

The contribution of the mouse tail to thermoregulation is modest

Understanding mouse thermal physiology informs the usefulness of mice as models of human disease. It is widely assumed that the mouse tail contributes greatly to heat loss (as it does in rat), but this has not been quantitated. We studied C57BL/6J mice after tail amputation. Tailless mice housed at 22°C did not differ from littermate controls in body weight, lean or fat content, or energy expenditure. With acute changes in ambient temperature from 19 to 39°C, tailless and control mice demonstrated similar body temperatures (Tb), metabolic rates, and heat conductances and no difference in thermoneutral point. Treatment with prazosin, an α1-adrenergic antagonist and vasodilator, increased tail temperature in control mice by up to 4.8 ± 0.8°C. Comparing prazosin treatment in tailless and control mice suggested that the tail's contribution to total heat loss was a nonsignificant 3.4%. Major heat stress produced by treatment at 30°C with CL316243, a β3-adrenergic agonist, increased metabolic rate and Tb and, at a matched increase in metabolic rate, the tailless mice showed a 0.72 ± 0.14°C greater Tb increase and 7.6% lower whole body heat conductance. Thus, the mouse tail is a useful biomarker of vasodilation and thermoregulation, but in our experiments contributes only 5-8% of whole body heat dissipation, less than the 17% reported for rat. Heat dissipation through the tail is important under extreme scenarios such as pharmacological activation of brown adipose tissue; however, non-tail contributions to heat loss may have been underestimated in the mouse.

Melatonin deficiency decreases brown adipose tissue acute thermogenic capacity of in rats measured by 18F-FDG PET

OBJECTIVE

Melatonin has been shown to increase brown adipose tissue (BAT) mass, which can lead to important metabolic effects, such as bodyweight reduction and glycemic improvement. However, BAT mass can only be measured invasively and. The gold standard for non-invasive measurement of BAT activity is positron emission tomography with 2-deoxy-2-[fluorine-18] fluoro-d-glucose (¹⁸F-FDG PET). There is no study, to our knowledge, that has evaluated if melatonin influences BAT activity, measured by this imaging technique in animals.

METHODS

Three experimental groups of Wistar rats (control, pinealectomy, and pinealectomy replaced with melatonin) had an ¹⁸F-FDG PET performed at room temperature and after acute cold exposure. The ratio of increased BAT activity after cold exposure/room temperature was called "acute thermogenic capacity" (ATC) We also measured UCP-1 mRNA expression to correlate with the ¹⁸F-FDG PET results.

RESULTS

Pinealectomy led to reduced acute thermogenic capacity, compared with the other groups, as well as reduced UCP1 mRNA expression.

CONCLUSION

Melatonin deficiency impairs BAT response when exposed to acute cold exposure. These results can lead to future studies of the influence of melatonin on BAT, in animals and humans, without needing an invasive evaluation of BAT.