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.

In-phasic cytosolic-nuclear Ca2+ rhythms in suprachiasmatic nucleus neurons.

The suprachiasmatic nucleus (SCN) of the hypothalamus is the master circadian clock in mammals. SCN neurons exhibit circadian Ca2+ rhythms in the cytosol, which is thought to act as a messenger linking the transcriptional/translational feedback loop (TTFL) and physiological activities. Transcriptional regulation occurs in the nucleus in the TTFL model, and Ca2+-dependent kinase regulates the clock gene transcription. However, the Ca2+ regulatory mechanisms between cytosol and nucleus as well as the ionic origin of Ca2+ rhythms remain unclear. In the present study, we monitored circadian-timescale Ca2+ dynamics in the nucleus and cytosol of SCN neurons at the single-cell and network levels. We observed robust nuclear Ca2+ rhythm in the same phase as the cytosolic rhythm in single SCN neurons and entire regions. Neuronal firing inhibition reduced the amplitude of both nuclear and cytosolic Ca2+ rhythms, whereas blocking of Ca2+ release from the endoplasmic reticulum (ER) via ryanodine and inositol 1,4,5-trisphosphate (IP3) receptors had a minor effect on either Ca2+ rhythms. We conclude that the in-phasic circadian Ca2+ rhythms in the cytosol and nucleus are mainly driven by Ca2+ influx from the extracellular space, likely through the nuclear pore. It also raises the possibility that nuclear Ca2+ rhythms directly regulate transcription in situ.