INTRACELLULAR ACIDIFICATION REVERSIBLY REDUCES ENDOCYTOSIS AT THE NEUROMUSCULAR-JUNCTION

The close spatial and temporal coupling of endocytosis and exocytosis in nerve terminals has made it difficult to elucidate the mechanisms a nd the regulation of endocytosis per se. Despite significant advances in our knowledge of the molecules involved in endocytosis, it has not yet been possible to selectively manipulate endocytosis in nerve termi nals. We report that the substitution of propionate for chloride in th e saline bathing a lizard neuromuscular junction reduces internal pH a nd reversibly blocks activity-dependent endocytosis. When intratermina l pH is reduced by similar to 0.7 pH units, the uptake of FM1-43 in ne rve terminals, but not activity-dependent destaining, is reduced. Norm alization of intracellular pH by removing the propionate, raising extr acellular pH, or adding ammonium chloride immediately restores FM1-43 uptake. Electron microscopy indicates that intracellular acidification reversibly reduces activity-dependent endocytosis in nerve terminals, because depolarization in propionate saline leads to a depletion of v esicles and the appearance of large intramembraneous infoldings.