Calcium Circadian Rhythmicity in the Suprachiasmatic Nucleus: Cell Autonomy and Network Modulation

Abstract

Circadian rhythms of mammalian physiology and behavior are coordinated by the suprachiasmatic nucleus (SCN) in the hypothalamus. Within SCN neurons, various aspects of cell physiology exhibit circadian oscillations, including circadian clock gene expression, levels of intracellular Ca2+ ([Ca2+]i), and neuronal firing rate. [Ca2+]i oscillates in SCN neurons even in the absence of neuronal firing. To determine the causal relationship between circadian clock gene expression and [Ca2+]i rhythms in the SCN, as well as the SCN neuronal network dependence of [Ca2+]i rhythms, we introduced GCaMP3, a genetically encoded fluorescent Ca2+ indicator, into SCN neurons from PER2::LUC knock-in reporter mice. Then, PER2 and [Ca2+]i were imaged in SCN dispersed and organotypic slice cultures. In dispersed cells, PER2 and [Ca2+]i both exhibited cell autonomous circadian rhythms, but [Ca2+]i rhythms were typically weaker than PER2 rhythms. This result matches the predictions of a detailed mathematical model in which clock gene rhythms drive [Ca2+]i rhythms. As predicted by the model, PER2 and [Ca2+]i rhythms were both stronger in SCN slices than in dispersed cells and were weakened by blocking neuronal firing in slices but not in dispersed cells. The phase relationship between [Ca2+]i and PER2 rhythms was more variable in cells within slices than in dispersed cells. Both PER2 and [Ca2+]i rhythms were abolished in SCN cells deficient in the essential clock gene Bmal1. These results suggest that the circadian rhythm of [Ca2+]i in SCN neurons is cell autonomous and dependent on clock gene rhythms, but reinforced and modulated by a synchronized SCN neuronal network.

Long-term in vivo recording of circadian rhythms in brains of freely moving mice

“…Previous studies have used GCaMP, a genetically encoded fluorescent calcium indicator, to record circadian Ca 2+ rhythms in mouse SCN slices ex vivo (26,27). Using our in vivo recording system, we demonstrated use of GCaMP6m for long-term monitoring of Ca 2+ levels in SCN VIP neurons (Fig.…”

Verapamil and Cluster Headache: Still a Mystery. A Narrative Review of Efficacy, Mechanisms and Perspectives

“…The hypothalamus is suspected to play a central role in CH pathophysiology and CH has pronounced chronobiological features . Calcium homeostasis is closely associated with circadian rhythms and has been suggested to be a clock‐controlled messenger, and not a generator of rhythmicity . Current through the voltage‐gated calcium channels and intracellular calcium are reduced during the night, and nimodipine reduces the day‐time current more than the night‐time current .…”

Neuronal oscillations on an ultra‐slow timescale: daily rhythms in electrical activity and gene expression in the mammalian master circadian clockwork

“…Elsewhere in the nervous system, oscillations in intracellular calcium signalling underlie most of the fast rhythms in neuronal excitability (Berridge, 1998(Berridge, , 2014. In SCN neurons, steady-state intracellular calcium [Ca 2+ ] i concentration/level oscillates in a circadian manner, peaking during the day and entering a nadir at night [ (Colwell, 2000;Ikeda et al, 2003a;Irwin & Allen, 2010;Enoki et al, 2012;Hong et al, 2012;Brancaccio et al, 2013;Belle et al, 2014;Ikeda & Ikeda, 2014;Noguchi et al, 2017); but see (Ikeda et al, 2003b)]. This peak in global SCN [Ca 2+ ] i anticipates the peak in electrical activity (Ikeda et al, 2003a;Enoki et al, 2017b), raising the possibility that the initial source of [Ca 2+ ] i in SCN neurons is largely through clock-operated intracellular calcium store release (COi-CaSR), and not through depolarised RMP-and action potentialevoked membrane calcium entry via voltage-gated calcium channels (VGCCs).…”