Profiles of evoked release along the length of frog motor nerve terminals.

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Abstract
To determine the relative probability of evoked transmitter release from different parts of frog motor nerve terminals, a technique was developed in which single quantum end-plate potentials (EPP) are recorded by 2 intracellular electrodes, located at opposite ends of identified junctions. The log of the ratio of the amplitudes recorded simultaneously at the 2 electrodes is a linear function of the distance of the site of origin of the event from each of the 2 electrodes. Using on-line computer data acquisition and analysis, and current pulses at known locations for spatial calibration, it is possible to localize the site of single quantum EPP to within .+-. 10-20 .mu.m. Using the frog cutaneous pectoris neuromuscular preparation and a low calcium, high magnesium Ringer solution to ensure mostly single quantum events and failures, several thousand responses were recorded from each junction, allowing construction of a profile of the numbers of single quantum events arising from each portion of the junction. By comparison of junctional morphology and release profiles, it is possible to construct a probability of release per unit length profile for the entire junction. This technique has several advantages over localization of release events by measurements of extracellular synaptic currents. For most junctions, the central 60-90% of the terminal exhibited relatively uniform probability of release, with highest levels typically near the point where the axon first contacted the muscle fibers or in regions with many short terminal branches. No instances were found in which a small region of terminal (10% or less) showed extraordinarily high release levels (30-50% of the total release from the junction). Characteristically, but not invariably, there is reduced release near the ends of terminal branches, especially the longer branches, where release per unit length could be as little as 5-10% of that in proximal portions. Some junctions had large regions of terminal that released very little transmitter. These also showed multiple myelineated axonal inputs and may have been polyneuronally innervated junctions in which one of the inputs was much weaker than the other. No evidence was found for non-random clustering of responses or failures, or for coupled release of consecutive single quantum events from the same site at stimulus intervals of 40 ms or 1s. Absolute single quantum EPP amplitudes were highly variable, but different branches or portions of branches of the same terminal showed similar means and variability, except near the ends of the junction, where there was sometimes a slight decrease in amplitude. There was no evidence for differing quantal size in different terminal branches, even in cases where the branches were releasing very different amounts of transmitter and may have come from different motoneurons.