A low voltage-activated calcium conductance in embryonic chick sensory neurons.

Isolated Ca currents in cultured dorsal root ganglion (DRG) cells were studied using the patch clamp technique. The currents persisted in the presence of 30 microM tetrodotoxin (TTX) or when external Na was replaced by choline. They were fully blocked by millimolar additions of Cd2+ and Ni2+ to the bath. Two components of an inward-going Ca current were observed. In 5 mM external Ca, a current of small amplitude, turned on already during steps changes to -60 mV membrane potential, leveled off at -30 mV to a value of approximately 0.2 nA. A second, larger current component, which resembled the previously described Ca current in other cells, appeared at more positive voltages (-20 to -10 mV) and had a maximum approximately 0 mV. The current component activated at the more negative membrane potentials showed the stronger dependence on external Ca. The presence of a time- and a voltage-dependent activation was indicated by the current's sigmoidal rise, which became faster with increased depolarization. Its tail currents were generally slower than those associated with the Ca currents of larger amplitude. From -60 mV holding potential, the maximum obtainable amplitude of the low depolarization-activated current was only one-tenth of that achieved from a holding potential of -90 mV. Voltage-dependent inactivation of this current component was fast compared with that of the other component. The properties of this low voltage-activated and fully inactivating Ca current suggest it is the same as the inward current that has been postulated in several central neurons (Llinas, R., and Y. Yarom, 1981, J. Physiol. (Lond.), 315:569-584), which produce depolarizing potential waves and burst-firing only when membrane hyperpolarization precedes.