Clocks and meals keep mice from being cool

Daily torpor is used by small mammals to reduce daily energy expenditure in response to energetic challenges. Optimizing the timing of daily torpor allows mammals to maximize its energetic benefits and, accordingly, torpor typically occurs in the late night and early morning in most species. The regulatory mechanisms underlying such temporal regulation have however not been elucidated. Direct control by the circadian clock and indirect control through the timing of food intake have both been suggested as possible mechanisms. Here, feeding cycles outside of the circadian range and brain-specific mutations of circadian clock genes (Vgat-Cre(+)CK1delta(fl/fl)(fl/+); Vgat-Cre(+)Bmal1(fl/fl) ) were used to separate the roles of the circadian clock and food timing in controlling the timing of daily torpor in mice. These experiments revealed that the timing of daily torpor is transiently inhibited by feeding, while the circadian clock is the major determinant of the timing of torpor. Torpor never occurred during the early part of the circadian active phase, but is preferentially initiated late in the subjective night. Food intake disrupted torpor in the first 4-6 h after feeding by preventing or interrupting torpor bouts. Following interruption, re-initiation of torpor was unlikely until after the next circadian active phase. Overall, these results demonstrate that feeding transiently inhibits torpor while the central circadian clock gates the timing of daily torpor in response to energetic challenges by restricting the initiation of torpor to a specific circadian phase

Norepinephrine Controls Both Torpor Initiation and Emergence via Distinct Mechanisms in the Mouse

Some mammals, including laboratory mice, enter torpor in response to food deprivation, and leptin can attenuate these bouts of torpor. We previously showed that dopamine β-hydroxylase knockout (Dbh −/−) mice, which lack norepinephrine (NE), do not reduce circulating leptin upon fasting nor do they enter torpor. To test whether the onset of torpor in mice during a fast requires a NE-mediated reduction in circulating leptin, double mutant mice deficient in both leptin (ob/ob) and DBH (DBL MUT) were generated. Upon fasting, control and ob/ob mice entered torpor as assessed by telemetric core Tb acquisition. While fasting failed to induce torpor in Dbh −/− mice, leptin deficiency bypassed the requirement for NE, as DBL MUT mice readily entered torpor upon fasting. These data indicate that sympathetic activation of white fat and suppression of leptin is required for the onset of torpor in the mouse. Emergence from torpor was severely retarded in DBL MUT mice, revealing a novel, leptin-independent role for NE in torpor recovery. This phenotype was mimicked by administration of a β3 adrenergic receptor antagonist to control mice during a torpor bout. Hence, NE signaling via β3 adrenergic receptors presumably in brown fat is the first neurotransmitter-receptor system identified that is required for normal recovery from torpor

Heart rate dynamics in a marsupial hibernator

The eastern pygmy possum (Cercartetus nanus) is a small marsupial that can express spontaneous short bouts of torpor, as well as multi-day bouts of deep hibernation. To examine heart rate (fH) control at various stages of torpor in a marsupial hibernator, and to see whether fH variability differs from that of deep placental hibernators, we used radiotelemetry to measure ECG and body temperature (Tb) while measuring the rate of O2 consumption and ventilation. fH and O2 consumption rate during euthermia were at a minimum (321±34 beats min−1, 0.705±0.048 ml O2 g−1 h−1) at an ambient temperature (Ta) of 31°C. fH had an inverse linear relationship with Ta to a maximum of 630±19 beats min−1 at a Ta of 20°C. During entry into torpor at a Ta of 20°C, fH slowed primarily as a result of episodic periods of cardiac activity where electrical activity of the heart occurred in groups of 3 or 4 heart beats. When Tb was stable at 24°C in these torpor bouts, the episodic nature of fH had disappeared (i.e. no asystoles) with a rate of 34±3 beats min−1. For multi-day bouts of deep torpor, Ta was lowered to 6.6±0.8°C. During these deep bouts of torpor, Tb reached a minimum of 8.0±1.0°C, with a minimum fH of 8 beats min−1 and a minimum O2 consumption rate of 0.029±0.07 ml O2 g−1 h−1. Shivering bouts occurred in deep torpor about every 8 min, during which ventilation occurred, and fH was elevated to 40 beats min−1. The duration of the QRS complex increased from 12 ms during euthermia to 69 ms at a Tb of 8°C. These findings demonstrate the dynamic functioning range of fH to be about 600 beats min−1 (∼80-fold), one of the largest known ranges in mammals. Our study shows that despite a separation of ∼160 million years, the control and function of the cardiac system seems indistinguishable in marsupial and placental hibernating mammals

Administration of a β3 antagonist slows rate of emergence from torpor.

<p>16 control mice were fasted at the onset of the dark cycle. Once in torpor, mice were either injected subcutaneously with 2 mg of the β3 antagonist, SR59230A, or vehicle. The maximum rate of temperature elevation was calculated as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0004038#pone-0004038-g002" target="_blank">Figure 2</a>. The rates for individual mice in each group are shown. [t-test: t = 2.844, df = 14, p = 0.013 vs. vehicle injection].</p

Emergence from torpor is retarded in DBL MUT mice.

<p>(A) Typical tracings from control, <i>ob/ob</i>, and DBL MUT mice during emergence from torpor. Both control and <i>ob/ob</i> mice exit torpor within 45 minutes of initiation, whereas emergence from torpor in this DBL MUT mouse was much slower. (B) The maximum rate of temperature gain over a 30 minute window (a measure of emergence from torpor) was calculated during both the fed state and fasted state based off the first derivative of core body temperature curves. Rapid emergence from torpor was seen in control, <i>ob/ob</i>, and <i>Dbh</i> −/− mice treated with DOPS to acutely replace NE. Data for “<i>Dbh</i> −/−+DOPS” groups was reanalyzed from ref. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0004038#pone.0004038-Swoap1" target="_blank">[10]</a>. [F(39,4) = 42.68, P<0.0001]. b : p<0.05 vs. <i>Dbh −/−</i>. c : p<0.05 vs. <i>ob/ob</i>. d: p<0.05 vs. DBL MUT. e : p<0.05 vs. <i>Dbh −/−</i>+DOPS.</p