Quantitative freeze-fracture analysis of the frog neuromuscular junction synapse--II. Proximal-distal measurements.

Neurotransmitter release from different parts of frog motor nerve terminals is often non-uniform. There is a decrease in release efficacy from the distal regions of frog motor nerve terminal branches. Since release is thought to occur near the double arrays of large intramembranous particles that constitute the pre-synaptic active zones (AZs), we have examined quantitatively the proximal-distal distribution of AZ structure, using a novel freeze-fracture technique that produces replicas of large fractions of terminals, including the region of nerve entry. This enables us to know the proximal distal orientation of each branch. From 23 end-plates we have obtained fractures of 72 branches. For 27 of these branches we have obtained continuous fractures both greater than 25 microm in length and with sufficient information to determine their proximal distal polarity. Only a few of these branches showed a marked distal decrease in AZ length/unit length of terminal, while several junctions had short regions (5-10 microm), either proximally or distally, that exhibited amounts of AZ that were substantially greater or smaller than the mean value for that terminal branch. The terminal area, post-synaptic gutter width and nerve terminal width all exhibit some distal decline concomitant with the distal tapering of nerve terminal branches. AZ length tends to have the least decline compared to the other parameters. Thus, the vast majority of frog motor nerve terminal branches do not display a significant proximal-distal gradient in the amount of AZ structure / microm terminal length. The present data do not provide an obvious ultrastructural correlate for the distal decline in transmitter release that some authors have observed.