Calcium regulation of neurite elongation and growth cone motility

Neurite outgrowth from isolated, identified molluscan (Helisoma trivolvis) neurons in culture can be suppressed by neurotransmitters and electrical activity, both of which increase intraneuronal Ca2+ levels (Haydon et al., 1984; Cohan et al., 1986, 1987). We explored the possibility of a causal relationship between Ca2+ influx from the cell exterior and neurite outgrowth using a spectrum of pharmacological manipulations known to affect transmembrane Ca2+ flux. Ca2+ ionophore A23187, an agent expected to increase Ca2+ influx, suppressed both elongation and motile growth cone structures (i.e., filopodia and lamellipodia) in a dose-dependent (10(8)-10(6) M) and reversible manner. Furthermore, high concentrations of Ca2+ channel blockers (La3+, Cd2+, Co2+; e.g., 10(-4) M La3+) suppressed both elongation and growth cone movements. These data support previous experiments, which indicated that neurite outgrowth is dependent upon a specific range of intracellular Ca2+ concentrations (Connor, 1986; Cohan et al., 1987). However, tests of the dose-dependency of the effects of Ca2+ channel blockers on outgrowth revealed that specific, low concentrations of Ca2+ channel blockers (e.g., 10(-5) M La3+) caused, simultaneously, a reduction of growth cone filopodia and an acceleration of elongation. Consistent with the results using low levels of Ca2+ channel blockers, reduced extracellular Ca2+-stimulated neurite elongation while suppressing growth cone motility. Finally, neurotransmitter regulation of neurite outgrowth was shown to require influx of extracellular Ca2+; serotonin inhibition of neuron B19 was prevented by La3+ (10(-5) M) or by incubation in a reduced Ca2+ environment. Taken together, these results indicate that there are optimum levels of Ca2+ influx that promote normal neurite elongation and growth cone movements; these 2 components of outgrowth appear to have differential sensitivities to Ca2+.