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.