Daily torpor in laboratory mice: physiological phenotyping and role of orexins.

Spontaneous torpor is a physiological phenomenon used in extreme circumstances to save energy. The physiological mechanisms ruling the torpor bouts are still unknown. Orexins (ORXs) are neuropeptides involved in the control of food behavior, in thermoregulation and sleep-wake cycle regulation this suggests that ORXs could have a role in torpor regulation. The aims of this study were to elucidate: 1) the role of ORXs in the entrance and/or exiting from torpor; 2) the level of glycemia at the onset of torpor differs from both glucose during arousal from torpor and from baseline conditions. In order to evaluate the role of orexins, 8 KO-ORX and 8 WT mice, were implanted with a telemetric blood pressure transducer (Data Sciences International, DSI), two cranial electrodes for the discrimination of the wake-sleep state and a thermistor in the brain cortex. For the study of changes in glycemia related to torpor, 6 WT mice were implanted intraperitoneally with glucose telemeter (DSI). To induce torpor, mice were calorically restricted and exposed to an ambient temperature of 20°C. The lack of ORXs does not cause significative differences in the physiological parameters during the different torpor phases. During deep torpor, in both the experimental groups, electroencephalogram (EEG) trace is similar to that described during active wakefulness while electromyogram (EMG) is almost flat, similarly to NREM sleep. This unusual sleep tracings can be observed when the minimum Tb is recorded. We found a strong positive and linear correlation between circulating glucose and Tb during ad libitum feeding at thermoneutrality. Low blood glucose itself was not predictive of a bout of torpor, the onset of torpor was associated with the combination of low blood glucose and hyperactivity Torpor can be considered a multifactorial and complex mechanism involving both metabolism and central nervous system control.