Drug Tolerance in Biomembranes

B tolerance to drugs, particularly alcohol, has been part of folklore for most of recorded history; it is common wisdom that some who drink heartily and frequently are better able to “hold their liquor” than are others who consume a thim­ bleful of sacramental wine during the year. The mechanism(s) of alcoholic intoxication, let alone tolerance, still are not under­ stood, although many researchers have thought that alcohol af­ fects the central nervous system in a manner similar to that of inhalation anesthetics. Anesthetic ef­ fects are governed by the Meyer­ Overton rule, which in its original formulation almost a century ago held that the potency of a general anesthetic is directly correlated with its lipid solubility (i.e., how readily it dissolves in tissues containing fats). Thus, the more soluble an anesthetic is in lipids, the more po­ tent are its effects. With the develop­ ment of cell biology in the 1960’s, it became clear that the lipids of the Meyer­Overton law are the phospho­ lipids (i.e., a type of lipid, or fat) that forms the structural basis of cell membranes. Further progress in this area came in the late 1960’s, when Hubbell and McConnell (1969) demonstrated that general anesthet­ ics, including alcohol, cause mo­ lecular disordering of biological membranes. This article and subse­ quently those of other researchers (e.g., Trudell et al. 1973) suggested that alcohol exerts its intoxicating ef­ fects by dissolving in the membrane and “fluidizing” that structure, there­ by in some way altering the function of the cell. Although the aforementioned studies were relevant to the study of anesthesia and alcohol intoxication, they did not address the problem of behavioral tolerance to alcohol and thus seemed distant from the con­ cerns of alcoholism or chronic alco­ hol abuse. It was in this context that Chin and Goldstein (1977) were able to link, for the first time, the induc­ tion of a “tolerant­dependent” state to changes in a physical parameter. These authors demonstrated that por­ tions of the membranes of nerve ter­ minals (i.e., synaptosomes) from mice treated chronically with alcohol until they had reached a state of al­ cohol tolerance and physical dependence were resistant to the fluidizing effect of alcohol in vitro. In Chin and Goldstein’s study, the molecular order, or “fluidity,” of the isolated mem­ branes in vitro was unchanged by chronic alcohol intake as long as alcohol was not present in the solution. However, when alco­ hol was added to the medium, the expected fluidizing response was blunted. This seminal article actually had wider implications than the link established between a behavioral phenomenon and a biolog­ ical alteration. It also demonstrated in general that mammalian cells can adapt to perturbing conditions by modulating the “flu­ idity” (and by inference the composition) of their membranes. The study by Chin and Goldstein stimulated numerous other studies of “membrane tolerance.” The resistance to fluidization was later extended to include other membranes as well as com­