Single Ca-activated cation channels in bursting neurons of Helix.

The depolarizing drive that maintains bursting in Helix neurons is carried by a long-lasting calcium-activated inward current. This current was studied using cell-attached and inside-out patches from the right parietal fast burster neuron of Helix pomatia. One population of unitary currents was inward at -50 mV and showed an increased probability of opening when Ca2+ was injected or when excised patches were bathed in solutions with 10(-7) to 10(-5) M free Ca2+ levels. Cell-attached patches (patch electrodes filled with 10(-7) M Ca2+ Ringer) had single channel conductances near 30 pS with reversal potentials near -20 mV; excised patches had similar conductances in symmetrical Na+ solutions and reversal potentials within a few millivolts of zero. Calculations, assuming a simple spherical cell, yield a channel density of only about 1/6 micron2. The increased channel opening probability characteristically persisted well beyond the duration of transient whole-cell inward current. We conclude from this that the later phase of Ca-activated inward currents is normally masked by outward currents.