Changes in the biochemical and biophysical parameters of cholinergic synaptic vesicles on transmitter release and during a subsequent period of rest

Synaptic vesicles were isolated on sucrose density gradients from perfused blocks of Torpedo electric organ under varying experimental conditions. Newly synthesized acetylcholine was labelled with ³H]acetate in order to distinguish synaptic vesicles which had been recycled during stimulation-induced transmitter release and, in consequence, had become smaller and denser from the larger, lighter vesicles characteristic of unstimulated tissue. After 1800 pulses at 0.1 Hz, the density of these smaller vesicles increased from a pre-stimulation value of 1.056 g.ml^?1 to 1.067 g.ml^?1, whereas their water space (measured as the space occupied by the permeant solute glycerol), decreased by 34% from 65% to 43% of vesicle volume. During a subsequent 12 h rest period, these changes were partially reversed; water space returned to 52% of control and this change was highly correlated with a decrease in vesicle density and an increase in vesicular acetylcholine. The diameter of vesicles in whole tissue sections showed corresponding changes. An additional 12 h rest period did not lead to further significant recovery, suggesting that the preparation had a limited suitability for following long term processes depending on normal energy metabolism.The results can be explained on the assumption that when vesicles reform after releasing transmitter their core has a lower osmotic pressure than that of fully loaded vesicles. Reloading is accompanied by osmotically induced rehydration.