Electronically and chemically mediated increases in intracellular calcium in neuronal growth cones.

In the present report we used the calcium indicator fura-2 to compare intracellular levels of free calcium in growth cones of isolated Helisoma neurons under a variety of experimental conditions. We tested whether two different signals which inhibit growth cone mobility, action potentials and serotonin, changed calcium levels in growth cones. Electrical stimulation of the cell body caused a rise in calcium levels at the growth cone. After brief stimulation, calcium levels quickly recovered to normal values, whereas longer stimulation periods required longer recovery times. The application of serotonin to growth cones caused an increase in calcium levels in growth cones of neuron B19 but not neuron B4 or B5. We also found that growing growth cones had higher free calcium levels than nongrowing growth cones. Furthermore, the distribution of calcium in neurons contained growing growth cones was heterogeneous; calcium levels were always higher in the growth cone than in the neurite or soma. These data which indicate that calcium levels in growth cones vary in different states of outgrowth and that calcium levels can be modulated by both electrical and chemical signals.