Cholesterol Exchange and Lateral Cholesterol Pools in Synaptosomal Membranes of Pair‐Fed Control and Chronic Ethanol‐Treated Mice

Most studies on effects of ethanol on membrane cholesterol have reported on changes in the total or bulk amount of cholesterol. Membrane cholesterol, however, can be described in terms of its kinetics and domains. The kinetics and size of lateral cholesterol exchangeable and nonexchangeable pools were examined in synaptosomes of pair‐fed controls and chronic ethanol‐treated mice. Effects of sphingomyelin, an exofacial leaflet phospholipid, that has been shown to affect cholesterol pools, were also examined. Radiolabeled small unilamellar vesicles were used to exchange cholesterol with synaptosomes. The total amounts of membrane cholesterol, phospholipid phosphorus, and the ratio of cholesterol to phospholipid did not differ between the pair‐fed control and ethanol groups. In control mice, the rate constant (hr^‐1) and the t_1/2 (hr) of cholesterol exchange were 0.065 ± 0.001 and 10.7 ± 0.25 (hr), respectively. The rate constant was significantly slower (0.053 ± 0.001, p < 0.05) and the t_1/2 significantly longer (13.33 ± 0.58, p < 0.05) in synaptosomes of the ethanol group compared with the control group. The size of the exchangeable pool of cholesterol did not differ significantly between the two groups. Sphingomyelinase‐induced hydrolysis of sphingomyelin significantly slowed cholesterol exchange with the largest effect in synaptosomes of the control group as compared with the ethanol group (p < 0.05). Hydrolysis of sphingomyelin had significantly (p < 0.05) less of an effect on cholesterol exchange in synaptosomes of the ethanol group. Membrane cholesterol can be described in terms of total content, transbilayer distribution, kinetics, and size of lateral pools. This study supports the general hypothesis, that not all properties or domains of membrane cholesterol are equally affected by ethanol.