The role of calcium ions in maintaining the circadian rhythm of suprachiasmatic nucleus (SCN) neuron activity was investigated using rat hypothalamic slice preparations. In normal Krebs solution, the firing rate of SCN neurons was higher in the light period than in the dark period. In Ca2+ -free Krebs solution, SCN neuron activity was low during all periods and did not show diurnal rhythm. These results suggest that the disappearance of circadian rhythmic change of SCN neuron activity in Ca2+ -free Krebs solution may be due to the disappearance of synaptic transmission in the SCN.
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