Facilitation, augmentation and potentiation of transmitter release at frog neuromuscular junctions poisoned with botulinum toxin.

Botulinum toxin type A (Botx) is a potent neurotoxin which inhibits specifically cholinergic synaptic transmission by an unknown mechanism. In order to gain further insight into the mode of action of this toxin, the effect of conditioning nerve stimuli on neuromuscular transmission was studied at endplates of Botx-poisoned and unpoisoned control cutaneous pectoris muscles in the frog. Effects of single conditioning stimuli (facilitation) and multiple high-frequency stimuli (augmentation and potentiation) on epp amplitude and mepp frequency were studied. The main results were that initial facilitation was significantly increased and its decay time constant significantly decreased in Botx-poisoned muscles, while augmentation was unchanged and potentiation was abolished. These changes could be detected before the muscle became completely paralysed, suggesting that they reflect a primary disturbance in the Ca2+-dependent release process.