REGULATED PLASMALEMMAL EXPANSION IN NERVE GROWTH CONES

To study the mechanisms underlying plasmalemmal expansion in the nerve growth cone, a cell-free assay was developed to quantify membrane addition, using ligand binding and sealed growth cone particles isolated by subcellular fractionation from fetal rat brain. Exposed versus total binding sites of I-125-wheat germ agglutinin were measured in the absence or presence of saponin, respectively, after incubation with various agents. Ca2+-ionophore A23187 in the presence of Ca2+ increases the number of binding sites (B(max)) but does not change their affinity (K(D)), indicating that new receptors appear on the plasma membrane. Similarly, membrane depolarization by high K+ or veratridine significantly induces, in a Ca2+-dependent manner, the externalization of lectin binding sites from an internal pool. Morphometric analysis of isolated growth cones indicates that A23187 and high K+ treatment cause a significant reduction in a specific cytoplasmic membrane compartment, thus confirming the lectin labeling results and identifying the plasmalemmal precursor. The isolated growth cones take up gamma-amino-butyric acid and serotonin, but show no evidence for Ca2+-dependent transmitter release so that transmitter exocytosis is dissociated from plasmalemmal expansion. The data demonstrate that plasmalemmal expansion in the growth cone is a regulated process and identify an internal pool of precursor membrane.