Abstract. To study the mechanisms underlying plasmalemmal expansion in the nerve growth cone, a cellfree 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 t25Iwheat germ agglutinin were measured in the absence or presence of saponin, respectively, after incubation with various agents. Ca2+-ionophore A23187 in the presence of Ca 2+ increases the number of binding sites (Bm~,) but does not change their affinity (Kv), indicating that new receptors appear on the plasma membrane. Similarly, membrane depolarization by high K + or veratridine significantly induces, in a Ca2+-dependent T HE neuronal perikaryon is the source and site of synthesis of macromolecular components necessary for neurite growth. Expansion of the neurite's plasmalemma, however, occurs primarily at the growth cone. Labeling studies with fluorescent lectins or carmine particles (Bray, 1970; Feldman et al., 1981), pulse-chase experiments with ferritin-labeled lectins and with phospholipid precursor
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