theories of general anaesthetic action theories of general anaesthetic action a general anaesthetic (or anesthetic) is a drug that brings about a reversible loss of consciousness. these drugs are generally administered by an anaesthetist/anesthesiologist to induce or maintain general anaesthesia to facilitate surgery. structures of general anaesthetics widely used in medicine. 1 - ethanol, 2 - chloroform, 3 - diethyl ether, 4 - fluroxene, 5 - halothane, 6 - methoxyflurane, 7 - enflurane, 8 - isoflurane, 9 - desflurane, 10 - sevoflurane general anaesthetics have been widely used in surgery since 1842 when crawford long for the first time administered diethyl ether to a patient and performed a painless operation. it has long been believed that general anaesthetics exert their effects (analgesia, unconsciousness, immobility) through a membrane mediated mechanism or by directly modulating the activity of membrane proteins in the neuronal membrane. in general, different anaesthetics exhibit different mechanisms of action such that there are numerous molecular targets at all levels of integration within the central nervous system. however, for certain intravenous anaesthetics, such as propofol and etomidate, the main molecular target has been identified to be gabaa receptor, with particular β subunits playing a crucial role. the concept of specific interactions between receptors and drugs first introduced by paul ehrlich in 1897 states that drugs act only when they are bound to their targets (receptors). the identification of concrete molecular targets for general anaesthetics was made possible only with the modern development of molecular biology techniques for single amino acid mutations in proteins of genetically engineered mice. lipid solubility-anaesthetic potency correlation (the meyer-overton correlation) the meyer-overton correlation for anaesthetics a nonspecific mechanism of general anaesthetic action was first proposed by emil harless and ernst von bibra in 1847. they suggested that general anaesthetics may act by dissolving in the fatty fraction of brain cells and removing fatty constituents from them, thus changing activity of brain cells and inducing anaesthesia. in 1899 hans horst meyer published the first experimental evidence of the fact that anaesthetic potency is related to lipid solubility. two years later a similar theory was published independently by charles ernest overton. meyer compared the potency of many agents, defined as the reciprocal of the molar concentration required to induce anaesthesia in tadpoles, with their olive oil/water partition coefficient. he found a nearly linear relationship between potency and the partition coefficient for many types of anaesthetic molecules such as alcohols, aldehydes, ketones, ethers, and esters. the anaesthetic concentration required to induce anaesthesia in 50% of a population of animals (the ec50) was independent of the means by which the anaesthetic was delivered, i.e., the gas or aqueous phase. meyer and overton had discovered the striking correlation between the physical properties of general anaesthetic molecules and their potency: the greater the lipid solubility of a compound in olive oil, the greater its anaesthetic potency. this correlation is true for a wide range of anaesthetics with lipid solubilities ranging over 4-5 orders of magnitude if olive oil is used as the oil phase. this correlation can be improved considerably in terms of both the quality of the correlation and the increased range of anaesthetics if bulk octanol or a fully hydrated fluid lipid bilayer is used as the "oil" phase. it was also noted that volatile anaesthetics are additive in their effects (a mixture of a half dose of two different volatile anaesthetics gave the same anaesthetic effect as a full dose of either drug alone). early lipid hypotheses of general anaesthetic action as bulky and hydrophobic anaesthetic molecules accumulate inside the neuronal cell membrane, this causes membrane distortion and expansion (thickening) due to volume displacement. membrane thickening reversibly alters function of membrane ion channels thus providing anaesthetic effect. the actual chemical structure of the anaesthetic agent per se was not important, but its molecular volume plays the major role: the more space within membrane is occupied by anaesthetic - the greater is the anaesthetic effect. from the correlation between lipid solubility and anaesthetic potency, both meyer and overton had surmised a unitary mechanism of general anaesthesia. they assumed that solubilization of lipophilic general anaesthetic in lipid bilayer of the neuron causes its malfunction and anaesthetic effect when critical concentration of anaesthetic is reached. later in 1973 miller and smith suggested the critical volume hypothesis also called lipid bilayer expansion hypothesis. they assumed that bulky and hydrophobic anaesthetic molecules accumulate inside the hydrophobic (or lipophilic) regions of neuronal lipid membrane causing its distortion and expansion (thickening) due to volume displacement. accumulation of critical amounts of anaesthetic causes membrane thickening sufficient to reversibly alter function of membrane ion channels thus providing anaesthetic effect. actual chemical structure of the anaesthetic agent per se is not important, but its molecular volume plays the major role: the more space within membrane is occupied by anaesthetic - the greater is the anaesthetic effect. based on this theory, in 1954 mullins suggested that the meyer-overton correlation with potency can be improved if molecular volumes of anaesthetic molecules are taken into account. this theory existed for over 60 years and was supported by experimental fact that increases in atmospheric pressure reverse anaesthetic effect (pressure reversal effect). then other, physicochemical theories of anaesthetic action emerged that took into account the diverse chemical nature of general anaesthetics and suggested that anaesthetic effect is exerted through some perturbation of the lipid bilayer. several types of bilayer perturbations were proposed to cause anaesthetic effect, including (1) changes in phase separation, (2) changes in bilayer thickness, (3) changes in order parameters, or (4) changes in curvature elasticity. according to the lateral phase separation theory anaesthetics exert their action by fluidizing nerve membranes to a point when phase separations in the critical lipid regions disappear. this anaesthetic-induced fluidization makes membranes less able to facilitate the conformational changes in proteins that may be the basis for such membrane events as ion gating, synaptic transmitter release, and transmitter binding to receptors. more recent techniques with super resolution imaging show that the anesthetics do not overcome phase separation—the phase separation persists. rather saturated lipids within the phase separation can undergo a transitions from ordered to disordered which is dramatically affected by anesthetics. nonetheless the concept of proteins moving between phase separated lipids in response to anesthetic has now been shown to be correct. all these early lipid theories were generally thought to suffer from four weaknesses (full description with rebuttals see in sections below): stereoisomers of an anaesthetic drug have very different anaesthetic potency whereas their oil/gas partition coefficients are similar certain drugs that are highly soluble in lipids, therefore expected to act as anaesthetics, exert convulsive effect instead (and therefore were called nonimmobilizers). a small increase in body temperature affects membrane density and fluidity as much as general anaesthetics, yet it does not cause anaesthesia. increasing the chain length in a homologous series of straight-chain alcohols or alkanes increases their lipid solubility, but their anaesthetic potency stops increasing beyond a certain cutoff length. the correlation between lipid solubility and potency of general anaesthetics was thought to be a necessary but insufficient condition for inferring a lipid target site. general anaesthetics could equally well be binding to hydrophobic target sites on proteins in the brain, but given the chemical diversity of anesthetics this would likely need to include more than one site and those sites would not inherently preclude a site in the membrane. for protein's, one reason that more polar general anaesthetics could be less potent is that they have to cross the blood–brain barrier to exert their effect on neurons in the brain. objections to the early lipid hypotheses 1. stereoisomers of an anaesthetic drug stereoisomers that represent mirror images of each other are termed enantiomers or optical isomers (for example, the isomers of r-(+)- and s-(−)-etomidate). physicochemical effects of enantiomers are always identical in an achiral environment (for example in the lipid bilayer). however, in vivo enantiomers of many general anaesthetics (e.g. isoflurane, thiopental, etomidate) can differ greatly in their anaesthetic potency despite the similar oil/gas partition coefficients. for example, the r-(+) isomer of etomidate is 10 times more potent anaesthetic than its s-(-) isomer. this means that optical isomers partition identically into lipid, but have differential effects on ion channels and synaptic transmission. this objection provides a compelling evidence that the primary target for anaesthetics is not the achiral lipid bilayer itself but rather stereoselective binding sites on membrane proteins that provide a chiral environment for specific anaesthetic-protein docking interactions. rebuttal to the objection: 1) stereo selective transport of the anesthetic was never considered. anesthetics are hydrophobic and transported bound to proteins in the blood. any stereo selective binding to the transport protein would change the concentration at the site of action. furthermore a protein sink in the membrane could bind one of the isomers slightly better and reduce the effective concentration the membrane experiences. all stereo isomers are effective anesthetics, they only shifted the sensitivity, suggesting selective transport and selective protein sinks need to be considered. 2) lipids are chiral, the same as proteins. and like proteins, lipids have ordered and disordered regions. the field failed to investigate the chirality of ordered lipids due to a lack of knowledge of their existence. 2. nonimmobilizers all general anaesthetics induce immobilization (absence of movement in response to noxious stimuli) through depression of spinal cord functions, whereas their amnesic actions are exerted within the brain. according to the meyer-overton correlation the anaesthetic potency of the drug is directly proportional to its lipid solubility, however, there are many compounds that do not satisfy this rule. these drugs are strikingly similar to potent general anaesthetics and are predicted to be potent anaesthetics based on their lipid solubility, but they exert only one constituent of the anaesthetic action (amnesia) and do not suppress movement (i.e. do not depress spinal cord functions) as all anaesthetics do. these drugs are referred to as nonimmobilizers. the existence of nonimmobilizers suggests that anaesthetics induce different components of anaesthetic effect (amnesia and immobility) by affecting different molecular targets and not just the one target (neuronal bilayer) as it was believed earlier. good example of non-immobilizers are halogenated alkanes that are very hydrophobic, but fail to suppress movement in response to noxious stimulation at appropriate concentrations.