Rapid Sprouting of Filopodia in Nerve Terminals of Chromaffin Cells, PC1 2 Cells, and Dorsal Root Neurons Induced by Electrical Stimulation

Rapid morphological changes induced by direct electrical stimulation of nerve terminals were studied by using video- enhanced differential interference contrast microscopy at a very high magnification (12,000 x ). We used mainly cultured bovine chromaffin cells, which developed neurite-like pro- cesses, and PC 12 cells, which showed neuronal differenti- ation upon NGF treatment. In a few cases, primary neurons of the rat dorsal root ganglion were also examined. Brief pulse stimulation of the terminals and varicosities induced exocytosis accompanied by rapid formation of filopodia. These filopodia, 0.1-0.2 Am in diameter and up to 10 pm in length, formed within a few hundreds of milliseconds and then retracted within tens of seconds. They could also be induced by K depolarization. This rapid filopodial sprouting strongly depended on the presence of extracellular Ca2+ and could be abolished in a medium containing a Ca chelator (EGTA) or La3+. Anti-cytoskeletal agents colchicine and cy- tochalasin B failed to block this response completely but lidocaine fully suppressed it. Quantitative analysis of exo- cytosis and filopodial sprouting showed that they were in- dependent events, not directly linked to each other, having different thresholds usually higher for filopodial formation. In PC12 cells, the extent of filopodial sprouting varied with the