Intracellular calcium in the entrainment pathway of molluscan circadian pacemakers.

Circadian clock systems contain three components--an entrainment pathway, a pacemaker mechanism, and an output or expressed rhythm. The entrainment pathway for light stimuli can be studied by separating steps involved in light transduction and subsequent events acting on the pacemaker mechanism from the steps critical for continued motion of the pacemaker. Studies indicate that calcium entry across the plasma membrane is a required step in the light entrainment pathway of the ocular circadian pacemaker of the marine snail Bulla gouldiana. A calcium influx due to phase-shifting stimuli has recently been measured using the calcium-sensitive dye Fura-2 in dissociated pacemaker neurons from Bulla. Studies preceding these calcium imaging experiments are presented together with a simple model of the role of Ca2+ influx in entrainment and a discussion of problems in demonstrating that calcium influx alone is a sufficient step in the entrainment pathway.