Evidence that potassium channels mediate the effects of serotonin on the ocular circadian pacemaker of Aplysia

FMRFamide modulates potassium currents in circadian pacemaker neurons of Bulla gouldiana

The peptide FMRFamide (Phe-Met-Arg-Phe-NH(2)) is known to modulate the circadian pacemaker found in the eye of the marine snail Bulla gouldiana. In the present study, we investigated the cellular mechanisms underlying this modulation by examining the effects of FMRFamide on the membrane properties of the circadian pacemaker cells, known as basal retinal neurons in this preparation. Bath application of FMRFamide (0.1-1 microM) increased the membrane conductance, and hyperpolarized the membrane potential of these neurons. Next, perforated-patch recordings were used to demonstrate that FMRFamide reversibly increased the outward current amplitude due to an augmentation of a non-inactivating calcium-independent current. Reversal potential of the tail currents and its dependence on extracellular potassium concentration suggested potassium ions as the charge carrier for this current. The peptide-modulated outward current was blocked by 54% after bath application of the potassium channel blocker tetraethylammonium chloride and completely blocked by substituting cesium for intracellular potassium. Voltage dependence, activation kinetics and tail current kinetics of the FMRFamide-modulated current were consistent with values found for the delayed rectifier current.Overall, our data suggest that FMRFamide modulates a delayed rectifier potassium current and at least one other, less voltage-dependent conductance. This provides a mechanistic explanation for FMRFamide's ability to both shift the phase and attenuate light-induced phase shifts of the circadian pacemaker in B. gouldiana.

Ionic currents of isolated retinal pacemaker neurons: projected daily phase differences and selective enhancement by a phase-shifting neurotransmitter

The eye of Aplysia expresses a robust circadian rhythm of neuronal activity. We dissociated the retinal cells in primary culture and studied isolated pacemaker neurons to identify ionic currents that may have roles in the circadian clock mechanism. Individual neurons were studied with perforated-patch whole cell recording techniques in current- and voltage-clamp modes. Pacemaker neurons had resting potentials near -40 mV and, if neurites had grown out, produced spontaneous action potentials in darkness at <1 Hz. Depolarizing current injections increased the rate of action potential firing. Hyperpolarizing current injections were followed by slowly decaying (1-3 s) afterhyperpolarizations. Four ionic currents were characterized under voltage-clamp, including a Ca current (I(Ca)), a voltage-gated potassium current (I(KV)), an A current (I(A)), and a hyperpolarization-activated Cl current (I(Cl)). I(Cl) was only seen using Cl(-)-filled electrodes when high concentrations of Cl- diffused from the electrode and is therefore unlikely to be important under physiological conditions. The magnitude of I(KV) was significantly larger during the projected zeitgeber predawn phase than during the postdawn phase, whereas the magnitude of I(A) was constant at these circadian phases, suggesting that only I(KV) is controlled by the circadian clock. Serotonin increased I(KV) by 29%, consistent with reports that serotonin suppresses optic nerve activity and phase shifts the circadian rhythm recorded from the intact eye. The enhancement of I(KV) likely contributes to membrane hyperpolarization, and it may be required for phase shifting. The phase-dependent changes in I(KV) provide evidence that each retinal pacemaker neuron contains a circadian clock, but confirmation must await further recordings made from individual pacemaker neurons that are isolated completely from all other cells in primary culture. From the present experiments, it appears that I(KV) is controlled by the circadian clock, in part, and it may be a required element in the pathway that is activated during serotonin-induced phase shifts.

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

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.

Serotonergic phase advances of the mammalian circadian clock involve protein kinase A and K+ channel opening

The mammalian circadian clock located in the suprachiasmatic nuclei (SCN) continues to oscillate when isolated in a brain slice preparation, and can be phase shifted in vitro by a variety of serotonergic (5-HTergic) agents. We have previously shown that 5-HT and a 5-HT agonist, quipazine, induce phase advances in the daytime and phase delays at night; the phase advances are mimicked by the 5-HT1A-selective agonist 8-OH-DPAT, by analogs of cyclic AMP, and by treatments that increase endogenous levels of cyclic AMP. Here we investigated the intracellular pathway through which these daytime phase advances occur. We find that quipazine- and 8-OH-DPAT-induced phase advances are blocked by two inhibitors of the cyclic AMP-dependent protein kinase, PK-A (H8 and Rp-cAMPS) as well as by a variety of K+ channel blockers (BaCl2, apamin, and charybdotoxin). Furthermore, we confirm previous work showing that a cyclic AMP analog induces phase advances in the daytime, and show that these phase advances are also blocked by BaCl2 and apamin. Finally, we show that a K+ ionophore induces similar phase advances in the subjective day, and these phase advances are blocked by Rp-cAMPS. These results indicate that both activation of PK-A and opening of K+ channels are necessary for 5-HT-induced phase advances of the SCN circadian clock. We propose a model that can account for our results.