Cold-induced Suspension and Resetting of Ca 2+ and Transcriptional Rhythms in the Suprachiasmatic Nucleus Neurons

Abstract Mammalian circadian rhythms are coordinated by the master clock located in the hypothalamic suprachiasmatic nucleus (SCN). Under severe environmental conditions, such as during the harsh winter season for food, certain mammalian species reduce their basal metabolism and thermogenesis, thereby undergoing torpor, a controlled state of hypothermia, which naturally returns to the normothermic state. A long-lasting debate focused on whether the SCN with a temperature-compensated clock remains functional during hypothermia. However, so far, no direct and quantitative evidence has been reported of temperature sensitivity in living SCN neurons. In this study, we performed dual-color fluorescence imaging of clock gene transcriptions and intracellular Ca 2+ in mouse SCN neurons, using slices at various temperatures. We demonstrated that the Bmal1 transcription and Ca 2+ circadian rhythms persisted at 22°C–28°C, although the two rhythms underwent temporal dissociation at 22 °C. Notably, Ca 2+ , Bmal1 , and Per2 rhythms were suspended at 15°C, coupled with a significant Ca 2+ increase, and all rhythms were reset by rewarming to 35°C. Upon rewarming, the Ca 2+ rhythm showed stable oscillations immediately, whereas the Bmal1 and Per2 rhythms took several days to reach stable oscillations and recover their phase relationship with the Ca 2+ rhythm. Taken together, we concluded that SCN neurons remain functional under moderate hypothermic conditions at approximately 22°C–28°C but stop ticking time in deep hypothermia at 15°C and that the rhythms reset after deep hypothermia. These data also indicate that the stable Ca 2+ oscillation precedes clock gene transcriptional rhythms in the SCN neurons.