Alteration of synaptic membrane cholesterol/phospholipid ratio using a lipid transfer protein. Effect on gamma-aminobutyric acid uptake.

A procedure was developed to vary the cholesterol-to-phospholipid (Ch/PL) ratio of synaptic plasma membranes and synaptosomes using a nonspecific lipid transfer protein so that membrane lipid composition could be correlated with presynaptic function. In synaptic plasma membranes, Ch/PL molar ratios from 0.21 to 1.19 were produced from a normal value of 0.52 +/- 0.01 by incubation with the transfer protein and an excess of either phosphatidylcholine or cholesterol/phosphatidylcholine liposomes for 60 min at 32 degrees C. In synaptosomes, Ch/PL ratios from 0.16 to 0.81 were similarly produced from a normal value of 0.38 +/- 0.04. Cholesterol loading or depletion of the membranes was accompanied by a decrease or increase, respectively in the phospholipid-to-protein ratio. The fluidity of the synaptic plasma membrane, as estimated by 1,6-diphenylhexatriene anisotropy measurements, was increased by lowering the Ch/PL ratio and decreased by raising the Ch/PL ratio. Decreasing the Ch/PL ratio of synaptosomes and synaptic plasma membrane vesicles resulted in loss of sodium-dependent gamma-aminobutyric acid (GABA) uptake (70-100% loss at Ch/PL ratios decreased to 40% of normal) and reduction in the number of accessible GABA-binding sites. Choline uptake was not affected in these same preparations. GABA uptake was restored by reinserting cholesterol into the membrane. Synaptosomal membrane potential and synaptic plasma membrane sodium permeability were not affected by changing the Ch/PL ratio. Increase in the Ch/PL ratio above normal had no effect on either choline or GABA uptake. Both the decrease in the Ch/PL ratio and the increase in the lipid-to-protein ratio increase membrane "fluidity," lipid-to-protein ratio increase membrane "fluidity," which may modulate the vertical displacement and motional characteristics of the GABA transporter.