Kinetics of acetylcholine recovery in Torpedo electromotor synapses depleted of synaptic vesicles

Isolated fragments of Torpedo electric organ perfused with choline-free Ringer and stimulated by application of up to 1500 pulses at 1 Hz lost 58% of their synaptic vesicles and 65% of their acetylcholine. The stimulated terminals showed ultrastructural changes of varying intensity which were reversed during rest. The numbers of synaptic vesicles returned to control values by 5 h and total acetylcholine reached the control levels only after 20 h incubation in medium containing excess of choline. Net synthesis of acetylcholine as well as acquisition of acetylcholine by the reformed vesicles followed first order kinetics with respect to the degree of unsaturation of the total acetylcholine pool and the vesicular compartment, respectively.A possible interpretation of these results is that the degree of depletion of the vesicular compartment may regulate the rate of net acetylcholine synthesis, and that the vesicular compartment may account for at least 80% of total acetylcholine in the terminals.