Torpor in mice is induced by both leptin-dependent and -independent mechanisms

Abstract

We tested the effect of chronic leptin treatment on fasting-induced torpor in leptin-deficient A-ZIP/F-1 and ob/ob mice. A-ZIP/F-1 mice have virtually no white adipose tissue and low leptin levels, whereas ob/ob mice have an abundance of fat but no leptin. These two models allowed us to examine the roles of adipose tissue and leptin in the regulation of entry into torpor. Torpor is a short-term hibernation-like state that allows conservation of metabolic fuels. We first characterized the A-ZIP/F-1 animals, which have a 10-fold reduction in total body triglyceride stores. Upon fasting, A-ZIP/F-1 mice develop a lower metabolic rate and decreased plasma glucose, insulin, and triglyceride levels, with no increase in free fatty acids or beta-hydroxybutyrate. Unlike control mice, by 24 hr of fasting, they have nearly exhausted their triglycerides and are catabolizing protein. To conserve energy supplies during fasting, A-ZIP/F-1 (but not control) mice entered deep torpor, with a minimum core body temperature of 24 degrees C, 2 degrees C above ambient. In ob/ob mice, fasting-induced torpor was completely reversed by leptin treatment. In contrast, neither leptin nor thyroid hormone prevented torpor in A-ZIP/F-1 mice. These data suggest that there are at least two signals for entry into torpor in mice, a low leptin level and another signal that is independent of leptin and thyroid hormone levels. Studying rodent torpor provides insight into human torpor-like states such as near drowning in cold water and induced hypothermia for surgery.

Figure 1

Reduced energy stores in A-ZIP/F-1 mice. (A) Triglyceride content in 28-week-old male mice (n = 4 per group). (B and C) Glycogen content in liver and muscle of 30- to 33-week-old male mice either fed or after a 24-hr fast (n = 5–6 per group). (D) Energy stored in triglyceride + glycogen was estimated by using data from A–C, assuming that muscle is 40% of body weight, glycogen is 4.18 kcal/g, and fat is 9.35 kcal/g (42). Wild-type (WT) data are depicted with filled bars and A-ZIP/F-1 with open bars.

Figure 2

Time course of fasting in A-ZIP/F-1 mice. Female A-ZIP/F-1 and littermate controls (wild type; 4–6 weeks old, n = 5 per group) were fasted for 0, 2, 4, 6, 12, and 24 hr (see Methods). Body weight (A) and plasma glucose (B), insulin (C), triglyceride (D), FFA (E), and blood urea nitrogen (BUN) (F) concentrations were measured. At 24 hr of fasting, the A-ZIP/F-1 mice were different from the wild-type mice (P < 0.01) for all parameters shown.

Figure 3

Decreased resting metabolic rate in fasting A-ZIP/F-1 mice. Female mice (23–27 weeks old, n = 5) were fasted at 22°C. Oxygen consumption was measured from 3 to 5 hr after the start of the fast (10 a.m.). Fed data were obtained by using the same animals at the same time of the day a week later. The difference between wild type (WT) and A-ZIP/F-1 was significant in the fed (P = 0.045) and fasted (P < 0.001) states; the drop with fasting was significant for the A-ZIP/F-1 (P = 0.001) but not control mice. Weights of the A-ZIP/F-1 and control mice were not different.

Figure 4

A-ZIP/F-1 mice enter torpor upon fasting. T_b was measured in 19- to 20-week-old male wild-type (A, n = 6) and A-ZIP/F-1 (B, n = 5) mice by telemetry. Mice were fasted for the indicated 24 hr starting at 9:30 a.m. During fasting, T_b fell to 33.88 ± 0.44°C in wild-type mice and 24.12 ± 0.69°C in A-ZIP/F-1 mice. The dash-dot line indicates ambient temperature and the black bar indicates lights out.

Figure 5

Leptin prevents fasting-induced torpor in ob/ob but not A-ZIP/F-1 mice. T_b was measured in 12- to 13-week-old male ob/ob and C57BL/6J control (A) and A-ZIP/F-1 and littermate FVB/N control (B) mice. Leptin pumps were inserted (day 0) and mice regained normal circadian T_b rhythms by day 3. The 24-hr fast was begun on day 5. Data are hourly averages ± SEM (■, ob/ob + saline; □, ob/ob + leptin; ●, wild-type + saline; ○, wild-type + leptin; ▾, A-ZIP/F-1 + saline; ▿, A-ZIP/F-1 + leptin). The dash-dot line indicates ambient temperature and the black bar indicates lights out. One A-ZIP/F-1 + saline mouse died (at 12:30 p.m.), and three A-ZIP/F-1 + leptin mice died (at 9:30 a.m., 11:30 a.m., and 11:30 a.m.).