Cold-induced suspension and resetting of Ca²⁺ and transcriptional rhythms in the suprachiasmatic nucleus neurons

Does the circadian clock keep running under such hypothermic states as daily torpor and hibernation? This fundamental question has been a research subject for decades but has remained unsettled. We addressed this subject by monitoring the circadian rhythm of clock gene transcription and intracellular Ca²⁺ in the neurons of the suprachiasmatic nucleus (SCN), master circadian clock, in vitro under a cold environment. We discovered that the transcriptional and Ca²⁺ rhythms are maintained at 22°C–28°C, but suspended at 15°C, accompanied by a large Ca²⁺ increase. Rewarming instantly resets the Ca²⁺ rhythms, while transcriptional rhythms reach a stable phase after the transient state and recover their phase relationship with the Ca²⁺ rhythm. We conclude that SCN neurons remain functional under moderate hypothermia but stop ticking in deep hypothermia and that the rhythms reset after rewarming. These data also indicate that stable Ca²⁺ oscillation precedes clock gene transcriptional rhythms in SCN neurons.