The Journal of Neurophysiology Vol. 81 No. 6 June 1999, pp. 2696-2700
Copyright ©1999 by the American Physiological Society

Empty Synaptic Vesicles Recycle and Undergo Exocytosis at Vesamicol-Treated Motor Nerve Terminals

 ¹Department of Anatomy and Neurobiology, University of Vermont, College of Medicine, Burlington, Vermont 05405-0160; and  ²Department of Physiology and Pharmacology, University of Strathclyde, Strathclyde Institute for Biomedical Sciences, Glasgow G4 0NR, Scotland

Parsons, Rodney L., Michelle A. Calupca, Laura A. Merriam, and Chris Prior. Empty Synaptic Vesicles Recycle and Undergo Exocytosis at Vesamicol-Treated Motor Nerve Terminals. J. Neurophysiol. 81: 2696-2700, 1999.Empty synaptic vesicles recycle and undergo exocytosis at vesamicol-treated motor nerve terminals. We investigated whether recycled cholinergic synaptic vesicles, which were not refilled with ACh, would join other synaptic vesicles in the readily releasable store near active zones, dock, and continue to undergo exocytosis during prolonged stimulation. Snake nerve-muscle preparations were treated with 5 µM vesamicol to inhibit the vesicular ACh transporter and then were exposed to an elevated potassium solution, 35 mM potassium propionate (35 KP), to release all preformed quanta of ACh. At vesamicol-treated endplates, miniature endplate current (MEPC) frequency increased initially from 0.4 to >300 s¹ in 35 KP but then declined to <1 s¹ by 90 min. The decrease in frequency was not accompanied by a decrease in MEPC average amplitude. Nerve terminals accumulated the activity-dependent dye FM1-43 when exposed to the dye for the final 6 min of a 120-min exposure to 35 KP. Thus synaptic membrane endocytosis continued at a high rate, although MEPCs occurred infrequently. After a 120-min exposure in 35 KP, nerve terminals accumulated FM1-43 and then destained, confirming that exocytosis also still occurred at a high rate. These results demonstrate that recycled cholinergic synaptic vesicles that were not refilled with ACh continued to dock and undergo exocytosis after membrane retrieval. Thus transport of ACh into recycled cholinergic vesicles is not a requirement for repeated cycles of exocytosis and retrieval of synaptic vesicle membrane during prolonged stimulation of motor nerve terminals.