Rearrangement of intramembrane particles as a possible mechanism for the release of acetylcholine.

1. A chemiluminescent procedure for measuring acetylcholine (ACh) has recently been described. The procedure is based on the hydrolysis of ACh by acetylcholinesterase and on the oxidation of choline to betaine and H2O2 by choline oxidase. The H2O2 generated reacts with luminol in presence of peroxidase to produce a light emission. This method is sensitive in the pmol/ml range. 2. On isolated synaptosomes from electric organ, it is possible to obtain an estimate of the cytoplasmic ACh compartment by measuring the light emission after a single freezing and thawing cycle. The vesicular pool which resists several freezing and thawing cycles is then estimated by opening the compartment with a detergent. Increasing the intensity of stimulation of synaptosomes with different agents depletes the ACh content down to the vesicular pool. 3. The release of ACh is not associated with any change in the number of synaptic vesicles as seen in cryofractured synaptosomes. The only ultrastructural change detected common to all stimulations was a decreased density of P face intramembrane particles smaller than 11 nm and an increased density of E face 8 to 18 nm particles. The very significant particle changes were more intense for the conditions releasing more ACh. It is suggested that these particles are involved in the release of ACh from the cytoplasm. An attempt to directly correlate the release of ACh with intramembrane particle changes is discussed.