Role of a novel maintained low-voltage-activated inward current permeable to sodium and calcium in pacemaking of insect neurosecretory neurons.

Among ionic currents underlying neuronal pacemaker activity, low-threshold-activated calcium currents contribute to setting the threshold for spike firing. In the insect central nervous system, dorsal unpaired median (DUM) neurons are capable of generating spontaneous electrical activity. It has previously been shown that two distinct (transient and maintained) low-voltage-activated (LVA) calcium currents are responsible for the generation of the pacemaker potential. Whole-cell recordings in voltage- and current-clamp mode were obtained from short-term cultured DUM neurons. Using 100 mM sodium and 2 mM calcium as charge carrier in the external solution as well as conditions that eliminate calcium currents (0.5 mM CdCl2), voltage-clamp experiments showed that a hitherto unanticipated LVA maintained inward current, activated at around -60 mV, was present. The current amplitude was strongly dependent on internal ATP concentration. Sodium-free solution reduced by 80% the current amplitude. Increasing (5 mM) or decreasing (calcium-free) external calcium concentrations enhanced or reduced, respectively, the maximum conductance without any effect on the voltage dependence. This novel ion channel was permeable to barium but manipulating internal or external magnesium concentrations was without effect on current amplitude or reversal potential. Based on IC50 values, the maintained current was 50-fold less sensitive to TTX than the classical transient voltage-dependent sodium current. Furthermore, it was insensitive to ethosuximide and halothane. Voltage-dependent inactivation analysis revealed an unexpected calcium-sensitive process that involved calcineurin. From these results it appears that, besides the two LVA calcium currents previously described, another LVA maintained inward current permeable to both sodium and calcium was also involved in the generation of the predepolarization. Based on these findings, we propose that a novel calcium-dependent mechanism is involved in the regulation of the pacemaker activity.