The Journal of Cell Biology, Vol 78, 839-855, Copyright © 1978 by The Rockefeller University Press
Aspects of turnover and biogenesis of synaptic vesicles at locust neuromuscular junctions as revealed by zinc iodide-osmium tetroxide (ZIO) reacting with intravesicular SH-groups
M Reinecke and C Walther
Retractor unguis nerve muscle preparations from the locust were subjected
to the zinc iodide-osmium tetroxide reaction (ZIO) after pre- fixation in
glutaraldehyde. Applied for 18 h at 4 degrees C in the dark, ZIO reacts at
pH 4.2--4.0 fairly selectively with the matrix of synaptic vesicles.
Approximately 53% of the vesicles are completely and 4% partially stained.
The percentage of ZIO-positive vesicles is increased to nearly 90% and
reduced to 4% or less by pretreatment with SH-protecting (dithiothreitol)
or SH-blocking (N-ethylmaleimide, p- chloromercuriphenyl sulfonic acid) and
SH-oxidizing (azodicarboxylic acid-bis-dimethylamide) reagents,
respectively. Stimulation of the motor nerve at 20 Hz for 7 min, partially
fatiguing synaptic transmission, reduces the number of vesicles per square
micrometer of terminal area by approximately 52%; 2 min of rest restores
this number of its pre-stimulation level. These changes are chiefly
accounted for by changes in the number of completely ZIO-positive vesicles.
2 min after the end of stimulation, partially ZIO-positive vesicles are
three times more frequent than before. With all experimental conditions,
the average volume of vesicles was as follows: ZIO-negative less than
partially ZIO-positive less than completely ZIO-positive. The average
volume of ZIO-positive vesicles is almost unaffected by stimulation; that
of ZIO-negative vesicles is decreased by 25% immediately after stimulation,
increasing with subsequent rest to the initial level after 1 h. It is
suggested (a) that ZIO demonstrates intravesicular protein(s) containing
SH-groups and (b) that the completely ZIO- positive vesicles represent the
mature ones ready to be used for transmitter release. How the ZIO reaction
differentiates between different developmental stages of vesicles which
could arise from the smooth endoplasmic reticulum is discussed.