The morphology of rat soleus neuromuscular junctions treated in vitro with purified beta-bungarotoxin

Pairs of isolated rat soleus nerve-muscle preparations were maintained in vitro in oxygenated Ringer solution containing 3 microgram/ml beta-bungarotoxin prepared by the method of MacDermot et al. End-plate potentials and twitch tensions generated in response to continuous stimulation via the nerve at 0.5 Hz were recorded from one muscle, while miniature end-plate potential amplitude and frequencies were monitored in the corresponding unstimulated member of the pair. The results obtained were compared with the fine structure of end-plates from identical preparations sampled after varying periods of intoxication, and from unpoisoned muscles maintained for comparable times in vitro. The electron micrographs obtained were analyzed quantitatively using simple stereological techniques. Our results show that: (1) nerve terminal synaptic vesicle content declines for the first hour of intoxication but returns to near normal levels following conduction block; (2) there is a rapid increase in the 'coated' vesicle population of the nerve terminals, both attached to the plasmalemma and within the terminal cytoplasm, which persists to the end of the experiment; and that (3) there is no significant increase in the incidence of damaged mitochondria within the poisoned nerve terminals. These findings are discussed in relation to previous observations and the putative mechanisms of action of beta-bungarotoxin.