[Exocytotic steps in cell-free system after cholesterol deprivation in synaptosomal plasma membranes and synaptic vesicles].

Using a cell-free system we investigated a specific role of cholesterol in exocytotic processes. To modulate the cholesterol content in membrane methyl-beta-cyclodextrin was used as a cholesterol binding agent. The experimental conditions for cholesterol depletion from synaptosomal membrane structures were determined and depended on methyl-beta-cyclodextrin concentration, time and mediums temperature. The role of cholesterol was studied on the stages of synaptic vesicles docking and Ca(2+)-stimulated fusion which are the components of multivesicular compound exocytosis. Using dynamic light scattering technique we have found that after cholesterol depletion from synaptic vesicles the process of their aggregation (docking) remains unchanged. It was found that the rate of calcium-triggered fusion of synaptic vesicles depends on the membrane level of cholesterol. The decreasing level of synaptosomal plasma membrane cholesterol by 8% leads to suppression of the Ca(2+)-dependent membrane fusion with synaptic vesicles. But, under 25% reduction of plasma membrane cholesterol the level of membrane merging with synaptic vesicles did not differ from control; probably this is due to changes in physical properties of lipid bilayer and/ or disturbances in function of membrane proteins driving this process. In cholesterol depleted synaptosomes the exocytotic release of glutamate stimulated by calcium was decreased by 32%. Obtained data suggest that the cholesterol concenration in synaptosomal plasma membranes or synaptic vesicles is the crucial determinant for synaptic transmission efficiency in nerve terminals.