Calcium dynamics and circadian rhythms in suprachiasmatic nucleus neurons

The hypothalamic suprachiasmatic nucleus (SCN) has a pivotal role in the mammalian circadian clock. SCN neurons generate circadian rhythms in action potential firing frequencies and neurotransmitter release, and the core oscillation is thought to be driven by "clock gene" transcription-translation feedback loops. Cytosolic Ca²⁺ mobilization followed by stimulation of various receptors has been shown to reset the gene transcription cycles in SCN neurons, whereas contribution of steady-state cytosolic Ca²⁺ levels to the rhythm generation is unclear. Recently, circadian rhythms in cytosolic Ca²⁺ levels have been demonstrated in cultured SCN neurons. The circadian Ca²⁺ rhythms are driven by the release of Ca²⁺ from ryanodine-sensitive internal stores and resistant to the blockade of action potentials. These results raise the possibility that gene translation/transcription loops may interact with autonomous Ca²⁺ oscillations in the production of circadian rhythms in SCN neurons.