A delayed rectifier current is modulated by the circadian pacemaker in bulla

Basal retinal neurons of the marine mollusc Bulla gouldiana continue to exp ress a circadian modulation of their membrane conductance for at least two cycles in cell culture. Voltage-dependent currents of these pacemaker cells were recorded using the whole-cell perforated patch-clamp technique to cha racterize outward currents and investigate their putative circadian modulat ion. Three components of the outward potassium current were identified. A t ransient outward current (I-A) was activated after depolarization from hold ing potentials greater than -30 mV, inactivated with a time constant of 50 ms, and partially blocked by 4-aminopyridine (1-5 mM). A Ca2+-dependent pot assium current (I-K(Ca)) was activated by depolarization to potentials more positive than -10 mV and was blocked by removing Ca2+ from the bath or by applying the Ca2+ channel blockers Cd2+ (0.1-0.2 mM) and Ni2+ (1-5 mM). A s ustained Ca2+-independent current component including the delayed rectifier current (I-K) was recorded at potentials positive to -20 mV in the absence of extracellular Na+ and Ca2+ and was partially blocked by tetraethylammon ium chloride (TEA, 30mM). Whole-cell currents recorded before and after the projected dawn and normalized to the cell capacitance revealed a circadian modulation of the delayed rectifier current (I-K). However, the I-A and I- K(Ca) currents were not affected by the circadian pacemaker.