The influence of cholesterol on synaptic fluidity and dopamine uptake.

The present study examined how the synaptic cholesterol/phospholipid ratio influences membrane fluidity and a transmembrane process of the dopaminergic system, dopamine uptake. Fluorescence polarization of DPH was used as a measure of membrane fluidity. The cholesterol content of synaptosomal and synaptic plasma membranes was altered using a lipid transfer protein. The results of the present study demonstrate that the transmembrane process of dopamine uptake may be inhibited by an elevated synaptic cholesterol/phospholipid ratio, which correlates with increased fluorescence polarization. Specifically, a 10-30% increase in the cholesterol/phospholipid ratio resulted in an approximately 20% increase in fluorescence polarization, a 3- to 6-fold increase in Km, a 4-fold increase in Vmax, and a marked decrease in the Vmax/Km (the first order rate constant for dopamine uptake at low substrate concentrations).