Membrane recycling in the neuronal growth cone revealed by FM1-43 labeling

Membrane dynamics within the chick ciliary neuronal growth cone were invest igated by using the membrane-impermeant dye FM1-43. A depolarization-evoked endocytosis was observed that shared many properties with the synaptic ves icle recycling previously described at the presynaptic terminal. In additio n, in the absence of depolarization a basal level of constitutive endocytot ic activity was observed at similar to 30% of the rate of evoked endocytosi s. This constitutive endocytosis accounted for large amounts of membrane re trieval: the equivalent of the entire growth cone surface area could be int ernalized within a 30 min period. Endosomes generated via constitutive and evoked processes were highly mobile and could move considerable distances b oth within the growth cone and out to the neurite. In addition to their dif ferent requirements for formation, evoked and constitutive endosomes displa yed a significant difference in release properties. After a subsequent depo larization of labeled growth cones, evoked endosomes were released although constitutive endosomes were not released. Furthermore, treatment with latr otoxin released evoked endosomes, but not constitutive endosomes. Although the properties of evoked endosomes are highly reminiscent of synaptic vesic les, constitutive endosomes appear to be a separate pool resulting from a d istinct and highly active process within the neuronal growth cone.