Biochemical and morphometric studies of the relationship of acetylcholine synthesis and vesicle numbers after stimulation of frog neuromuscular junctions: the effect of a choline-O-acetyltransferase inhibitor.

The present study compares choline acetylase (ChAc) activity with morphometric determinations of synaptic vesicles at the neuromuscular junctions of frog pectoralis muscle subjected to high and low frequency stimulation in the presence or absence of NVP, a ChAc inhibitor. Muscles stimulated at 10/sec for 20 min with one hour rest, in the presence of NVP, showed an approximately 50% reduction in synthesis of Ach, and a 50--60% reduction in the numerical density of synaptic vesicles relative to controls preparations. Similar nerve muscle preparations stimulated at 2/sec for one hour without rest, in the presence of NVP, also showed approximately 50% less synthesis of Ach and vesicle numbers 43% lower than that seen in control muscles treated in the same way in the absence of drug. The results indicate a correlation between the inhibitory effect of NVP on ChAc and the numbers of vesicles present within terminals. In addition, stimulation of nerve terminals, with or without NVP, produced a significant increase in the numerical density of synaptic vesicles over respective unstimulated controls. These and other results are discussed in relation to current hypotheses concerning recirculation of synaptic vesicles.