A hypothetical model on the mechanism of anesthesia

Medical Hypotheses (1987) 23, l-9 ‘1’ Longman Group UK Ltd 1987

FI HYPOTHETICAL

MODEL ON THE MECHANISM OF ANESTHESIA

Sze-Chuh Cheng and Edward A, Brunner. Department of Anesthesia, Northwestern University Medical School, 303 E. Chicago Avenue, Chicago, IL, 60611, U.S,A. RBSTRACT An hypothesis on the mechanism of action of general anesthetic agents is proposed. It is based on a potentiation of chloride influx due to the action of anesthetic agents on the BABd-receptor complex at the lipidprotein interface, This hypothesis accomodates various observations such as lipid solubility of anesthetic molecules, their lack of stringent reversal of 'anesthetic action, and structural requirement, pressure other neurochemical and neurophormacological data, INTRODUCTION Previous attempts to explain the mechanism of general anesthesia have have centered mostly on the lipid layer (1-3) and elaborate hypotheses in the lipid layers of neural been built on the structural changes membranes caused by a large variety of anesthetic molecules, The relationship between lipid solubility and anesthetic potency was elegantly stated by Meyer and Over-ton nearly a century ago. However, the correlation of lipid solubility and anesthetic potency is merely descriptive and does not provide an explanation of the mechanism of action of anesthetic molecules. Similarly, other hypotheses, such as those based on inhibition of metabolism (3,41 or on their effects on acetylcholine receptor (1,3,51 or on luciferase (3,6), have failed to provide a full explanation of why anesthesia occurs. The present proposed model is based upon recent developments in neurochemical research and deals with events occurring at the synaptic level. It accommodates many of the features of anesthesia and can be tested experimentally, In 1973, we advanced the QCIBFlhypothesis (7,B) as a possible mechanism for anesthesia without specifying the loci of anesthetic action. We proposed that anesthetic agents caused increased inhibition of synaptic of the inhibitory transmission through a potentiation of the action nhurotransmitter, OpBFI (gamma-aminobutyric acid). GFlBA is known to cause

membrane hyperpolarizdtion due to increase in chloride conductance, Since the commonly conceived rction of anerthetic agents involve5 a reduction of Cerebrdl function, we proposed that the dction of rnesthetic drugs involves BABA inhibition of synaptic transmission. We now wish to locdte the site of dction of general dne5thetiC dgents dt a molecular level, For this purpose, we shall first direct our attention to the events tdking pldce dt d QABA syndpse and three of the postulated RleChdnismS by which the anesthetic molecules dct at or near the BABA syndpse, First, a general perturbation of the phospholipid bilayer is, in itSelf, now dCCepted ill inddequate to dccount for the dction of anesthetic agents (l-3). Second, volatile anesthetic dgents dct through the inhibition of mitochondrial respiration at the level of complex I (9, 10). If this is correct, a successive build-up of reduced flavoprotein and NADH '(reduced nicotinamide-adenine dinucleotidel should be observed, incresses in NADH have been seen in mitoAlthough rnesthetic-induced chondrial prepardtione dnd in the kidney, the increase 15 minimdl in the brdin (111. It is unlikely that the critical action of these dnepthetic agents is via the inhibition of mitochondrisl respiration. The third, dnd last, postuldted mechanirm Stdtes thdt synaptic events dt the GABA syndpres (7,8) dre dltered, These events include the synthesis of the transmitter, its release for action, its recognition by the and its removal (extrrcellulrr inactivation or uptake with or receptor agents on d without subsequent metdboli5ml, The effect5 of dnerthetic GABA synapse with respect to these events taken together and sepdrdtely vdlues (or MAC vdlues have been reported 1G,12-22). By comparing ED for some dgentsl with 10% effect on edch pro 28 ss (ID 1, we found thdt, GABA of the general anesthetic agents studied, the IDlO of'~ynaptosoma1 “diSp05dlM . JABA “disposdl are close to theiri4respective ED wds a term ured to measure the CO liberdtio~Of~~~u~~CIGABA and it all synaptic eventa ex i! ept cjynthesis. Syntheeis is reduced, represent5 metabolic slowdown which accomprnies the supposedly from a general Inhibition of GABA CdtabOliSm, as measured by GABA dnI!SthetiC State, transrminrse inhibition, wds 5een only with relevdnt concentrdtion5 of thiopental but not with the inhalation agente. GABA relrr5e is not stimulated to any degree by anecthetic agents. GABA Uptdke is inhibited by thiopental, ketarine dnd middzoldm, but not by the voldtile dnesthetic agentr, Clearly, the dction of the volatile anesthetic dgents can not be explained by any dlteration of these syndptic eventc, membrane hyperpoldrizdtion Since GABA action involves and incredsed it is pertinent to exsmine the DABA receptor dnd chloride conductance, the iissocirted chloride Channel (ionophorel (23,241 dnd see whether this mdy be the rite of action of volatile dnesthetic agents, The chloride is surrounded by three principrl components:- receptors distinchdnnel guished by their affinities for GABA, benrodidzepines or picrotoxinl Their interdctions are reflected in the effects of benzobdrbiturates. diazepines and picrotoxin/bdrbiturates on GABA binding dnd chloride and ie reported to be potentidted by bdrbitUrdte8 flux0 GABA binding this potentiation ie augmented by increasing concentrrtions of chloride ion (2S-271. We (22,2G) found that thiOpentd1, hdlothdne dnd enflurdne dlso cdn increase GABA binding to its receptor dt concentrations much

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Although the mode of action of lower than their refpective,EDJe values. be established the pentobarbital the latter agents still remains o effect has been shown to be a prol:ngation of the assopotentiation ciation of OABA with its receptor f20). Presumably this leads to a prolonged opening of the chloride channel (301 and causes an exaggerated chloride flux. Since, the outside chloride concentration is normally higher than that inside the nerve cell in the central nervous system, This increased the increased chloride flux would be directed inwards. influx will produce hyperpolarization and synaptic inhibition. If the volatile anesthetic agents act in a manner similar to that of pentobarbital, they all must exert a stabilizing effect on the cQnforcomplex so that the chloride channel can mation of the QABA-receptor the well-known remain Qpen for a longer duration. This effect mimics allosteric regulation of enzyme or receptor action. However, a major difference exists. The classical allosteric effecters are all hydroand their binding to the protein enzyme molecule philic compounds usually involves .steric fit and ionic and hydrogen bonding. Volatile anesthetic molecules are usually hydrophobic molecules with little structural similarity, Therefore , specific binding between the anesthetic molecule and the OABA-receptor complex becomes unlikely and a mechanism other than allosterism must be sought, HYPOTHESIS The proposed hypothetical model described below (Figure 1) attempts to resolve this dilemma. In the absence of attached: GABA (A in Figure 11, the receptor keeps the chloride channel closed while the fatty acids in the phospholipid bilayer next to the receptor remain straight. When a OABA-receptor complex is formed (R in Figure 11, the receptor undergoes cQnformationa1 changes causing the chloride channel to open and creating a cavity on the side of the receptor molecule, This forces the fatty acid chains to bend and fit into the cavity, Such altered and mQre restrictive configuration would certainly decrease the entropy and lead to a higher energy, and therefore less stable, state, In other WQrds, while QABA binding causes the receptor to change from its configuration in A (with closed chloride channel1 to that in E (with open chloride channel), the hydrophobic interaction between the fatty acids of the phospholipids with the receptor would tend to convert the receptor from the configuration in B back to that in A, However, if anesthetic molecules are present, they might diffuse to and fit into the cavity and thus permit the fatty acid chains to resume the lower energy state (C in Figure 11, The new tertiary "complex" is thus at a lower energy level than the BABA-receptor bimolecular complex. The open state of the chloride channel becomes more stable and allows a prolonged influx of chloride ions. No rigid structural requirement of the anesthetic molecule is necessary, Rather, portion of one, one or few molecules alter the microenvironment of the fatty acids abutting a hydrophobic region of the GABA-receptor compl@x. When the anesthetic molecule(s) diffuses out of the cavity, the system reverses back towards A and the chloride channel ClQSCS, Different lipid soluble molecules may be more or less effective as anesthetic agents depending on how they fit into the cavity. In other words, the anesthetic potency of moleculres may depend not only on avail-

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Cl- Channel

A GABA Receptor Benzodiazepine/ Barbiturates Receptor Picrotoxin Receptor

R

+ GABA 11 \I

GABA \

Figure 1, Diagramotic representation of the action of anesthetic molccule. (CI- normally closed chloride channel surrounded by three receptor proteins; E - less stable open channel after GAEL attachment to its receptor, causing a cavity to develop in the protein which in turn produces bending of fatty acid chains next to it; and C - more stable open with anesthetic molecule(s) filling in the cavity thus allowing channel the fatty acid chains to straighten up. The cavity for anesthetic molewas drawn on the ORB&-receptor, it could be on the other recepcule(s) tors of the same complex.)

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ability, also on

which how

well

can

be measured

these

molecule6

as oil-water can

fill

the

partition cavity.

coefficient, Experiments

Collect data to te6t this hypothesis are just being developed. line6 of observation are, however, in harmony with this model.

but to

Several

Corollary Any pertinent model must accommodate a phenomenon unique to anesthetic (3,31). Since the association of an action, namely pressure reversal must be anesthetic and/or a BABpl molecule to the QABp receptor complex reversible, it murt al60 be dependent upon conformational changes of the #ny factor that can cau6e alteration in protein configuration protein. may also affect binding. In the case of pressure, it is well known that preesure induce6 depolymerization of aicrotubulee, ruggesting conformational change. In the cd68 of luciferase, a protein, preosure i6 suppored to squeeze the anesthetic molecule6 away from the target sites (321, Furthermore, there is indeed evidence rhowing that high pre66ure affects the sodium channel (331, It is, therefore, possible that prescomplex sure may also cause conformational change of the QABCI receptor protein(r) leading to reduced binding of the ligands, in turn causing a In other words, pressure may cause the reduction of anesthetic effects, system to 6hift toward6 the QABA unbound configuration (Figure 1A). readily reversible anesthetic state may be related to the fitting of anesthetic molecule, without etrict chemical bonding, into the GFIBAthe receptor, This lack of 6trict chemical binding created cavity on

The the

facilitate6 the easy formation and dissolution of the prOpO6ed tertiary and the extent of thi6 "complex" formation depenCi6 only on "complex" ma65 action where both anesthetic concentration in the lipid bilayer and the rate of diffusion are the controlling factors. More "complex" forinaTherefore, the more lipid soluble tion leads to deeper anesthetic state, the more pharmacologically effective it the anesthetic molecule is, becomes, correct, 6everal observations can be explained. The If thio model is system proabundance of the chloride ionophore in the central nervous duces the phenomenon of general anelrthesia instead of more localized or selective effects. The prolonged opening of the chloride channel is equivalent to an increare in MBA concentration in the 6ynaptic cleft. In other words, one doe6 not need to have an increaoed MBA concentrrtion to cause an increa6ed chloride conductance, In vivo increa6es in GFlEA concentrations under the influence of various anesthetic agents one recent report in the rat pans region have not been found except (34). other 4160,

Barbiturate6 and benzodiazepiner may exert their effects via the also parts of the chloride ionophore. two receptors which are GABS, agonist6 6hould mimic GABCI action in viva, Thir has been obacid (35) but also with a with gamma-hydroxybutyric served not only bicyclic QCIBCI analog THIP (4,3,6,7-tetradydroiooxazolotS,4-cl-pyridinthis model also explain6 why Eignificantly large S-01) (36). Lastly, changes in membrane charPcteri6tic6 are not induced by anesthetic concentrations of these druge. If

an

increase

in chloride

influx

is

the

ultimate

answer

to the mecha-

nism of anesthesia, then why not include the glycine system as has been recently (37)'? The plycine system is located primarily in the supperted (38). Anesthesia lower brain stem, an‘d especially in the spinal cord involves not only the loss of brain stem and cord functions but also loss of function6 of higher cerebral centers. Therefore, it Seems unlikely that the glycine system alone iS responsible for anesthesia althouph anesthetic-induced alteration of brain stem or spinal function6 may be affected through the glycine system, This model appears similar to the model recently proposed with respect to anesthetic action on luciferase activity (6) but is fundamentally different. That model is based on competitive substrate inhibition of enzyme activity where the primary intermolecular forces are hydrophilic in nature. Since most of the general anerthetic molecule6 are basically hydrophobic in nature, it becomes difficult to conceive the type of bonding that could be established between anesthetic molecules of diverse structure and the protein receptor molecule. Furthermore, if the anetthetic molecules only serve to fill a cavity, then there will be no reason to suspect that there should be a natural endopenous lipand which can function as an anesthetic agent, An alternative (no diagram) to this cavity-creating GABA effect on the receptor complex appear6 simpler: why not a bulge? The fatty acid chains in the lipid bilayer, beinp orderly, will tend to resist this bulge and pu6h it back towards the configuration in Figure lA, thereby closing the chloride channel. When anesthetic molecules are present, they tend to increase the disorientation of these fatty acid chains by increased fluidity (21, This reduces the tendency to push the bulge back and stabilizes the opening of the chloride channel, Only a local effect around the receptor complex is necessary and the model would be valid enrrpywice, One can even rationalize preosure reversal on anesthetic action with this alternative. The problem is, in this alternative, any molecule which is lipid soluble and increases fluidity should be an anesthetic, This is clearly not the case. The fitness of a hydrophobic molecule into the cavity as proposed earlier (see Figure 1) would distinguish an anesthetic molecule from a non-anesthetic one, CONCLUSION An original hypothesis postulating a potentiation of GAGA action by anesthetic apents as one of the possible mechanisms of anesthesia is version postulates an extended to the molecular level, This revi6ed of the anesthetic molecule via the lipid bilayer to the GABA approach ionophore) and an anestheticreceptor complex (i.e., the chloride of GABA binding to this receptor causing an exaginduced prolongation gerated chloride influx. The increase in chloride influx, in turn, leads to membrane hyperpolarization, reduced synaptic activity and anesthesia. Thie hypothesis require6 neither a specific location of action in the brain nor a change in GABA content anywhere in the brain, It can accomodate the lipid rolubility property of the wide variety of anesthetic apents, the lack of stringent structural specificity of them, and the phenomenon of pressure reversal.

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of THIP, a Q#BA

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