The nature of sites of general anaesthetic action.

The molecular nature of the site of general anaesthesia has long been sought through the process of comparing the in vivo potencies of general anaesthetics with their physical properties, particularly their ability to dissolve in solvents of various polarities. This approach has led to the conclusion that the site of general anaesthesia is largely apolar but contains a strong polar component. However, there is growing evidence that several physiological targets underlie general anaesthesia, and that different agents may act selectively on subsets of these targets. Consequently research now focuses on the details of general-anaesthetic-protein interactions. There are large amounts of structural data that identify cavities where anaesthetics bind on soluble proteins that are readily crystallizable. These proteins serve as models, having no role in anaesthesia. Two problems make studies of the more likely targets--excitable membrane proteins--difficult. One is that they rarely crystallize and the other is that the sites have their highest affinity for general anaesthetics when the channels are in the open state. Such states rarely exist for more than tens of milliseconds. Crystallographers are making progress with the first problem, whilst anaesthesia researchers have developed a number of strategies for addressing the second. Some of these (kinetic analysis, site-directed mutagenesis) provide indirect evidence for sites and their nature, whilst others seek direct identification of sites by employing newly developed general anaesthetics that are photoaffinity labels. Such studies on acetylcholine, glycine and GABA receptors point to the existence of sites located within the plane of the membrane either within the ion channel lumen (acetylcholine receptor), or on the outer side of the alpha-helix lining that lumen (GABAA and glycine receptors). Bound anaesthetics generally exert their actions on ion channels by binding to allosteric sites whose topology varies from one conformation to another, but definitive proof for this mechanism remains elusive.