During development, growth cones navigate to their targets via numerous interactions with molecular guidance cues, yet the mechanisms of how growth cones translate guidance information into navigational decisions are poorly understood. We have examined the role of intracellular Ca 2ϩ in laminin (LN)-mediated growth cone navigation in vitro, using chick dorsal root ganglion neurons. Subsequent to contacting LN-coated beads with filopodia, growth cones displayed a series of stereotypic changes in

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... behavior, including turning toward LN-coated beads and a phase of increased rates of outgrowth after a pause at LN-coated beads. A pharmacological approach indicated that LN-mediated growth cone turning required an influx of extracellular Ca 2ϩ , likely in filopodia with LN contact, and activation of calmodulin (CaM). Surprisingly, fluorescent Ca 2ϩ imaging revealed no LN-induced rise in intracellular Ca 2ϩ in filopodia attached to their parent growth cone. However, isolation of filopodia by laser-assisted transection unmasked a rapid, LNspecific rise in intracellular Ca 2ϩ (ϩ73 Ϯ 11 nM). Additionally, a second, sustained rise in intracellular Ca 2ϩ (ϩ62 Ϯ 8 nM) occurred in growth cones, with a distinct delay 28 Ϯ 3 min after growth cone filopodia contacted LN-coated beads. This delayed, sustained Ca 2ϩ signal paralleled the phase of increased rates of outgrowth, and both events were sensitive to the inhibition of Ca 2ϩ /CaM-dependent protein kinase II (CaMkinase II) with 2 M KN-62. We propose that LN-mediated growth cone guidance can be attributed, in part, to two temporally and functionally distinct Ca 2ϩ signals linked by a signaling cascade composed of CaM and CaM-kinase II.