Increase in the number of presynaptic large intramembrane particles during synaptic transmission at the Torpedo nerve-electroplaque junction

Abstract

Small pieces of Torpedo electric organ were treated with 4-aminopyridine. a drug which greatly increases the duration of transmitter release in a single nerve impulse, transforming the normally brief electroplaque potential to a giant discharge. Specimens of tissue were cryofixed by rapid freezing using liquid coolants at precise time intervals during transmission of a single giant discharge, and then examined by freeze fracture. In each experiment, we monitored the electrical response of one specimen during the freezing run to check the physiological responsiveness of the tissue and to determine the precise time of contact with the cryogenic liquid. The general appearance of nerve terminals after cryofixation was similar to that of terminals from chemically fixed and cryoprotected tissue. The major morphological change observed during the time course of the giant discharge was a marked increase in the density of intramembrane particles larger than 10 nm on both the protoplasmic and external faces of the presynaptic membrane. This change appeared in specimens frozen within the first few milliseconds after the stimulus, that is, at a time corresponding to the onset of the rising phase of the potential (3 ms). At the end of the giant discharge, the particle density returned to control values with the same time course as the potential trace. Pits of 20 nm or larger, probably due to vesicle-membrane interaction, were found in a small proportion of nerve terminals. Their occurrence increased only at 120–150 ms after the stimulus, that is a long time after the beginning of the giant potential and of the change in intramembrane particles. The size distribution of particles was also determined in the membrane of synaptic vesicles exposed by cross fracture of terminal boutons; it was found to be similar to that of the unstimulated presynaptic membrane and it did not change during the giant discharge. Stimulation experiments were also earned out in a modified solution containing no added calcium. 20 mM magnesium and 4-aminopyridine. The propagation of impulses along the nerves to the electric organ was not inhibited in the modified solution but acetylcholine release was prevented and no increase in particle density was found on the presynaptic membrane. These and previous biochemical experiments on this tissue suggest that the release of the neuro. transmitter acetylcholine is associated with a transient occurrence of large intramembrane particles on the two fracture faces of the presynaptic membrane. At variance from the results obtained on the frog neuromuscular junction [Heuser et al. (1979) J. Cell Biol. 81, 275] the present changes of intramembrane particles apparently did not follow vesicle fusion. The possible explanations for the discrepancy between the two sets of results have been discussed.