Single channel activity differs in growing and nongrowing growth cones of isolated identified neurons of Helisoma.

When plated in cell culture, Helisoma neurons extend new neurites with large growth cones. These growth cones progress from actively growing organelles to stable structures over a period of several days. The ability to distinguish morphologically between these growth states has provided the opportunity to test whether changes in growth status are correlated with changes in the ionic properties of the growth cone membrane. This investigation has begun to study these changes by examining single channel ionic currents using patch clamp recording techniques and has demonstrated the presence of at least one ion channel having a conductance of 70 pS in the membrane of Helisoma growth cones. Recordings from cell-attached patches show that this channel is normally active in growing growth cones, whereas channel activity is absent in recordings from stable growth cones. The absence of channel activity in growth cones that have reached the stable state is not due to the loss of channels from these growth cones. Rather, the channel is present in an inactivated state. The activity of this channel is not dependent on the presence of Ca, as channel activity persists when Ca is removed from the solution that bathes the internal surface of the membrane. These data demonstrate a correlation between channel activity and the growth state of neuronal growth cones.