The effect of type D botulinum toxin on frog neuromuscular junctions.

1. Botulinum toxin type D blocks neuromuscular transmission in frogs. Motor nerve impulses continue to invade the nerve terminals but cease to evoke the phasic release of transmitter normally associated with them.2. Sensory receptors in the muscle continue to generate impulses even after 4 days of continuous exposure to botulinum toxin.3. Contrary to expectations, spontaneous miniature end-plate potentials did not disappear completely after botulinum intoxication; they still occurred, although with reduced frequency, in most end-plates. These miniature potentials had a skew amplitude distribution instead of the bell-shaped distribution of normal end-plates.4. The electron-microscopic appearance of botulinum-poisoned end-plates was not obviously altered.5. Even though after botulinum nerve impulses fail to release transmitter, tetanic nerve stimulation causes an increase in the frequency of miniature end-plate potentials. Many of the unit potentials which appear during the tetanic period are of an amplitude which is larger than that of spontaneous potentials, indicating that a different class of unit is being released preferentially. Some implications of this finding are briefly discussed.