Calcium plays a central role in phase shifting the ocular circadian pacemaker of Aplysia

Abstract

The eye of the marine mollusk Aplysia californica contains an oscillator that drives a circadian rhythm of spontaneous compound action potentials in the optic nerve. Both light and serotonin are known to influence the phase of this ocular rhythm. The aim of the present study was to evaluate the role of extracellular calcium in both light and serotonin-mediated phase shifts. Low calcium treatments were found to cause phase shifts which resembled those produced by the transmitter serotonin. However, unlike serotonin, low calcium neither increased ocular cAMP levels nor could these phase shifts be prevented by increasing extracellular potassium concentration. Low calcium-induced phase shifts were prevented by the simultaneous application of the translational inhibitor anisomycin and low calcium treatment resulted in changes in [35S]methionine incorporation into several proteins as measured by a two-dimensional electrophoresis gel analysis. Finally, light treatments failed to produce phase shifts in the presence of low calcium or the calcium channel antagonist nickel chloride. These results are consistent with a model in which serotonin phase shifts the ocular pacemaker by decreasing a transmembrane calcium flux through membrane hyperpolarization while light-induced phase shifts are mediated by an increase in calcium flux.

Day-Night Changes in Downstream Regulatory Element Antagonist Modulator/Potassium Channel Interacting Protein Activity Contribute to Circadian Gene Expression in Pineal Gland

“…(1) In retinal cells from Bulla gouldiana and Aplysia californica, clock-regulated K ϩ currents control basal membrane potentials and impose larger rises in intracellular Ca 2ϩ during the subjective night (Michel et al, 1993;Geusz et al, 1994). (2) Light pulses fail to produce phase shifts in ocular rhythms at low extracellular Ca 2ϩ concentrations or in the presence of the Ca 2ϩ channel antagonist, nickel chloride (Colwell et al, 1994). (3) Serotonin-mediated ocular phase shifts in A. californica involve an increase in ocular cAMP levels (Colwell et al, 1994).…”

“…(2) Light pulses fail to produce phase shifts in ocular rhythms at low extracellular Ca 2ϩ concentrations or in the presence of the Ca 2ϩ channel antagonist, nickel chloride (Colwell et al, 1994). (3) Serotonin-mediated ocular phase shifts in A. californica involve an increase in ocular cAMP levels (Colwell et al, 1994). (4) It has been reported recently that a circadian oscillator in cultured chick cones modulates the affinity of cGMP-gated ion channels for cGMP and the nature of the signal transduction cascade after dopamine D2 receptor activation (Ko et al, 2003).…”

Nitric Oxide as an Efferent Modulator of Circadian Pacemaker Neurones in the Eye of the Marine Mollusc Bulla gouldiana~!2008-04-11~!2008-06-27~!2008-08-26~!

“…The BRNs are intrinsically photosensitive and transduce light signals that shift the phase of their pacemaker [84], possibly via an opsin-like photopigment [54]. Furthermore, light induced depolarization leading to a transmembrane calcium influx is essential for light entrainment in both Bulla [5,6,83,85,86] and Aplysia [7]. In mammals, it is hypothesised that the NO/cGMP signalling pathway mediates the phase advancing effects of light on the circadian clock (cf.…”

“…In Bulla, 8-bromo-cAMP applied during the subjective day decreased CAP frequency and produced phase shifts in the circadian rhythm of magnitude and direction depending on the circadian phase [40]. In Aplysia, cAMP is suspected to mediate serotonergic modulation of photic phase shifting [14,88], by activating a hyperpolarizing potassium current, resulting in decreased calcium flux [7]. Serotonin does not occur in the nervous tissues of the Bulla eye [17], but FMRFamide may fulfil a similar role.…”

“…BRNs are depolarised by illumination and by experimentally elevated extracellular potassium ions [5], whereby both stimuli produce similar phase-shifts in the circadian rhythm [6]. Light or depolarisation induced phase shifts required for extracellular calcium, and are blocked by calcium channel antagonists [6,7]. Furthermore, depolarisation of the BRNs causes an influx of calcium ions and a robust elevation of intracellular calcium [8].…”