Augmentation and facilitation of transmitter release. A quantitative description at the frog neuromuscular junction

Endplate potentials were recorded from frog and toad sartorius neuromuscular junctions under conditions of greatly reduced quantal contents. The magnitudes of augmentation increased with the duration and frequency of stimulation, often increasing at an accelerating rate during 10-20-s conditioning trains. The magnitudes of the first and second components of facilitation also increased, but reached apparent steady state values within the first few seconds of stimulation. These observations could be accounted for by assuming (a) that augmentation and the first and second components of facilitation arise from underlying factors in the nerve terminal that act to increase transmitter release; (b) that each nerve impulse adds an increment to each of the underlying factors; (c) that the magnitude of the increment typically increases during the train for augmentation but remains constant for the components of facilitation; and (d) that the underlying factors decay with first-order kinetics with time constants of approximately 7 s for augmentation and 60 and 500 ms for the first and second components of facilitation, respectively. The increments of facilitation added by each impulse were about twice as large in the toad as in the frog. Facilitation was described better by assuming a power relationship between the underlying factor and the observed facilitation than by assuming a linear relationship. Augmentation was described by assuming either a linear or power relationship.