DEPLETION OF VESICLES FROM FROG NEUROMUSCULAR JUNCTIONS BY PROLONGED TETANIC STIMULATION

TLDR

Previous studies indicate that vesicles can continuously fuse with and reform from the axolemma, as shown by peroxidase labeling in stimulated preparations. Frog neuromuscular preparations were subjected to prolonged indirect stimulation at 2 Hz, then curare was removed and the tissues fixed or treated with black‑widow venom to assess transmitter store depletion. Prolonged stimulation (6–8 hr) nearly exhausted transmitter and vesicle stores in frog nerve terminals, with the bulk of release occurring in the first 4 hr, yet 4‑hr preparations still displayed many vesicles and normal morphology.

Abstract

Curarized cutaneous pectoris nerve muscle preparations from frogs were subjected to prolonged indirect stimulation at 2/sec while recording from end plate regions. At the ends of the periods of stimulation, the curare was removed and the preparations were fixed for electron microscopy or treated with black widow spider venom to determine the degree to which their stores of transmitter had been depleted. After 6–8 hr of stimulation the nerve terminals were almost completely depleted of their stores of transmitter and of their population of vesicles. Most of the transmitter release occurred during the first 4 hr of stimulation, and after this time most (about 80%) of the fibers were depleted of about 80% of their transmitter. The organization of the nerve terminals in 4-hr preparations appeared normal and the terminals still contained many vesicles. When peroxidase was present in the bathing medium, terminals from stimulated preparations showed many vesicles that contained peroxidase, whereas the rested control preparations showed few such vesicles The fact that after 4 hr the total number of vesicles is not markedly changed while a large fraction (up to 45%) contained peroxidase suggests that in our experiments vesicles were continuously fusing with and reforming from the axolemma.