Membrane effects of ethanol in excitable cells

Ethanol has a wide range of biochemical and behavioral effects. Many of these can be explained by the ability of ethanol to reduce the amount of order, or increase the fluidity, in biological membranes. During chronic ethanol administration, membrane fluidity in the absence of ethanol and the sensitivity of membrane fluidity to added ethanol are decreased. Changes in membrane lipid composition that are consistent with decreased fluidity or with resistance to ethanol including increases in membrane cholesterol, reductions in the double bond index of phospholipid acyl chains, and increases in anionic phospholipids, have been reported during ethanol treatment. These changes are not found uniformly, however, and membrane tolerance and resistance have been reported in their absence. A variety of changes in lipid metabolism have been reported; their possible relevance to these adaptive changes is discussed. Ethanol treatment affects several transport systems in membranes; Na+, K+-ATPase, Ca++-ATPase, and other Ca++ transport systems all appear to undergo adaptive changes during ethanol treatment. Alterations in these systems may account for some of the effects of ethanol on activity-dependent energy metabolism and neurotransmitter release. Paradoxes in the membrane actions of ethanol remain to be resolved, including the weakness of membrane fluidization by ethanol in vitro compared to the evidence that it occurs in vivo, and the consistency with which adaptive changes in membrane fluidity and in Na+, K+-ATPase are observed compared to the inconsistency in changes in membrane composition.