Intracellular Ca2+ signaling pathway is involved in light-induced phase advance, but may not be in phase delay, of the circadian melatonin rhythm in chick pineal cell.

Chick pineal cells have photoreceptive, circadian clock and melatonin synthetic capacities, and express circadian oscillation of melatonin release in vitro. Light pulses cause phase-dependent phase shift of the melatonin rhythm. The purpose of this study was to address the questions whether intracellular calcium is involved in both light-induced phase advance and delay. Thapsigargin and cyclopiazonic acid, which deplete the intracellular calcium stores, blocked the light-induced phase advance in a dose-dependent manner. The pulses of ryanodine receptor antagonist (dantrolene sodium or ruthenium red) also blocked the light-induced phase advance. Most agents did not cause a significant phase shift by themselves. On the other hand, all the agents used, failed to block the light-induced phase delay, even if the magnitude of phase delay was decreased using low intensity light. An antagonist of nitric oxide synthase blocked neither light-induced phase advance nor phase delay. These results indicate the following possibilities: (1) the mechanism of light-induced phase advance and delay may be different in chick pineal cells, or (2) if intracellular calcium is involved in both light-induced phase advance and delay, the sensitivity to light and/or agents used in this study may differ according to Zeitgeber time.