Botulinum toxin blocks quantal but not non-quantal release of ACh at the neuromuscular junction

Botulinum (BOT) toxin is known to block quantal acetylcholine (ACh) release at the neuromuscular junction but little is known about its effect on non-quantal ACh release. We have examined the effect of BOT on non-quantal ACh release directly using a variant of the electrophysiological technique described by Katz and Miledi⁶. This method is based on the observation that non-quantally released ACh results in a small, continual depolarization of the postsynaptic membrane, after inhibition of cholinesterase. This depolarization can be revealed by suddenly blocking ACh receptors with a pulse of curare, resulting in an abrupt hyperpolarization, the amplitude of which is presumed to be proportional to the rate of non-quantal ACh release. BOT treatment resulted in a marked decrease in quantal ACh release as shown by miniature endplate potential (m.e.p.p.) frequencies (control 0.65 ± 0.33 m.e.p.p.s/s; BOT 0.03 ± 0.03 m.e.p.p.s/s). However, non-quantal ACh release measured by the curare induced hyperpolarization, was not significantly different in control and BOT treated diaphragms (control 1.01 ± 0.09 mV; BOT 1.03 ± 0.11 mV). Our results show that BOT does not block non-quantal ACh release at a time when it has a profound effect on spontaneous quantal ACh release. This suggests that quantal and non-quantal ACh release take place through different release mechanisms.