The effects of general anaesthetics on ligand ... - Semantic Scholar

British Journal of Anaesthesia 89 (1): 41±51 (2002)

The effects of general anaesthetics on ligand-gated ion channels J. P. Dilger Department of Anesthesiology, State University of New York, Stony Brook, NY 11794-8480, USA Br J Anaesth 2002; 89: 41±51 Keywords: anaesthesia, general; ions, ion channels, ligand-gated; theories of anaesthetic action, molecular

The idea that general anaesthetics produce unconsciousness, analgesia and amnesia by interfering with communication between neurones is conceptually appealing to both scientists and non-scientists. Indeed, there is a long history of considering postsynaptic ligand-gated ion channels (LGICs) as molecular targets for general anaesthetics.59 Advances in experimental techniques, especially in electrophysiology and molecular biology, have fostered a reductionist approach and allowed exploration of the interactions between general anaesthetics and LGICs in increasingly greater molecular detail. Potential binding sites for anaesthetics on some LGICs have now been identi®ed. However, concrete evidence for these sites awaits the determination of the high-resolution structure of LGICs in the absence and presence of anaesthetic. This review is organized in the following way. First, we give a summary of the structure and function of LGICs, and this is followed by an examination of the ways in which anaesthetics (or any drugs) might affect LGIC function, a critique of the methods used to measure LGIC function, and a compilation of the effects of general anaesthetics (and similar agents such as alcohols) on individual families of LGICs.

Structure of ligand-gated ion channels Although it was once thought that all LGICs belonged to a single superfamily of channels, it is now clear that there are at least three distinct superfamilies.60 Currently, ion channels are classi®ed according to their topology with respect to the membrane, i.e. the number of membrane-spanning segments and the number of pore loops (Fig. 1). Members of the cys-loop superfamily contain four membrane-spanning segments without any pore loops (Fig. 1A) and are expressed as pentamers.54 The name `cys-loop' refers to the presence of a pair of disulphidebonded cysteines near the N-terminal of the protein. The muscle-type nicotinic acetylcholine (ACh) receptor

(mnAChR) is a heteropentamer with the stoichiometry a2bgd (embryonic or extrajunctional and Torpedo subtype) or a2bed (adult or junctional subtype). As viewed from the synaptic cleft, the subunits are arranged clockwise in the sequence agabd. The two ligand binding sites are in the synaptic region at the a±g (a±e) and a±d subunit interfaces.84 These two sites have nearly the same af®nity for agonists but antagonists have a higher af®nity for the a±g (a±e) site.93 Most, if not all, of the lining of the pore of the channel is provided by second membrane-spanning segment (M2).42 48 The channel is primarily permeable to monovalent cations;1 the permeability of Ca2+ and Mg2+ relative to Na+ is 0.2. Single channels exhibit a linear current± voltage curve with a conductance of 40 pS (embryonic) or 60 pS (adult). The current±voltage curve produced by a large number of mnAChR channels activated by saturating concentrations of ACh has a small degree of recti®cation because of the weak dependence of channel open-time on voltage. The basic structural features of mnAChRs (four membrane-spanning segments, ligand-binding sites at subunit interfaces and a pore formed by M2) are thought to be preserved in the other members of the cys-loop superfamily. Neuronal nicotinic ACh receptors (nnAChR) are formed from either heteropentamers of two a (a2±6, a10) and three b (b2±4) subunits or homopentamers of ®ve a (a7±9) subunits. The most common subtype combinations are a4b2 (brain), a3b4 (sympathetic ganglia) and a7 (presynaptic terminals).34 nnAChRs are considerably more permeable to divalent cations than are mnAChRs. The permeability of nnAChRs to Ca2+ ranges from 1.5 to 20 times the Na+ permeability.82 This implies that a signi®cant ¯ux of Ca2+ enters the postsynaptic cholinergic neurone during synaptic transmission. nnAChRs exhibit a strongly inwardly rectifying single-channel current±voltage curve with a reversal potential near 0 mV. Recti®cation results from a voltagedependent block of the channel47 by intracellular Mg2+. At

Ó The Board of Management and Trustees of the British Journal of Anaesthesia 2002

Dilger

Fig 1 The structure of ligand-gated ion channels. (A) Structure of the cys-loop superfamily. (B) Structure of the ionotropic glutamate receptor family. (C) Structure of the P2X family of ATP-gated channels.

positive membrane potentials, Mg2+ has high af®nity for binding within the pore and blocking the ef¯ux of K+. Nicotinic ACh receptor (AChR) channels permeable to Cl± have been found in molluscan neurones.14 55 However, they have not yet been cloned, so it is not clear how they should be placed in the LGIC classi®cation scheme. Two types of subunit forming LGICs activated by serotonin (5-hydroxytryptamine or 5-HT) have been identi®ed:20 5-HT3A and 5-HT3B. The 5-HT3A subunit forms homopentameric channels with a very small conductance (500 ms. The ionotropic, purinergic receptor (P2X) ATP-activated superfamily of LGICs exhibit two membrane-spanning regions and no pore-loops (Fig. 1C). Seven subunits have been identi®ed (P2X1±7). They assemble into homomeric and heteromeric trimers80 with at least 11 different combinations.97 All of the receptors are about equally 42

Anaesthetics and ligand-gated channels

permeable to Na+ and K+ and also have signi®cant Ca2+ permeability.56 Single-channel conductances are of the order of 30 pS and subconductance levels have also been seen. Profound inward recti®cation has been seen in current±voltage curves from PX2 receptors, which may be attributed to voltage-dependent gating and voltage-dependent single-channel conductance.108 Thus far, none of the LGICs has been crystallized for imaging to atomic resolution. The most detailed structural information about an LGIC comes from electron diffraction images of tubular crystals of the AChR from Torpedo electroplax. The resting state of the channel has been Ê resolution77 and differences between the imaged at 4.6 A Ê resolution.99 resting and open states have been seen at 9 A Structural information about the ligand-binding domains of LGICs has been obtained from crystallographic studies. The Ê ligand-binding domain of GluR has been imaged at 1.9 A resolution to reveal two lobes surrounding a large binding cleft8 in the form of a clamshell. An ACh binding protein was isolated from snail glial cells, puri®ed, crystallized and Ê .15 Remarkably, its amino acid sequence imaged to 2.7 A resembles the extracellular portion of members of the cysloop superfamily and it forms a pentamer just like the mammalian channels. Unlike the ligand-binding domain of GluR, there is no large binding cleft in the ACh-binding protein; rather, ligand-binding sites are formed at the interface between each pair of subunits.

Fig 2 Kinetic schemes used to describe activation and desensitization in ligand-gated ion channels. See text for details. (A) General scheme for ligand-gated channels with two agonist binding sites. (B) The behaviour of mnAChR channels is well described by this part of the general scheme. (C) GABAAR channels have been successfully modelled by this part of the general scheme.

Mechanism of action of ligand-gated ion channels A general scheme for activation and desensitization of LGICs with two agonist binding sites is shown in Figure 2A. Channels can exist in three distinct conformations: resting (R, non-conducting), open (R*, conducting) and desensitized (D, non-conducting). The desensitized state can also be considered as an inactivated state, analogous to voltagegated ion channels. Sequential binding of two agonist (A) molecules is indicated by the horizontal arrows. Gating refers to transitions between resting and open channels (leftto-right diagonal arrows). Desensitization refers to transitions between resting and desensitized channels (vertical arrows) or open and desensitized channels (right-to-left diagonal arrows). In its completely general form, this ninestate model contains 30 rate constants, of which 23 are independent. The typical scheme used for mnAChR channels is illustrated in Figure 2B. This scheme contains only seven states and nine rate constants (assuming identical ligand binding/dissociation rates in the resting conformation). The simpli®cations arise from the observations that non-ligandbound and single-ligand-bound receptors open infrequently,49 that desensitization occurs primarily from the double-ligand-bound open state9 and that recovery from desensitization after removal of agonist occurs primarily

through the desensitized pathway.28 When the channel opens, the agonist becomes more tightly bound (indicated by the lack of a pathway from A2R* to AR*) and this increased af®nity is preserved in the desensitized state. Many studies ignore (do not measure) the pathway for recovery from desensitization, a simpli®cation that reduces the scheme to ®ve states and six rates. The gating reaction favours the open state by a factor of at least 20, so that, at saturating concentrations of ACh, >95% of the channels open. Gating has weak voltage-dependence; channels close faster at depolarized potentials. Desensitization is fast (time constant ~50 ms) and favours the desensitized state by a factor of >10, so that 200 ms after addition of ACh most channels are desensitized. After a brief pulse of agonist (as in a synaptic event), desensitization has not begun so the decay of current is governed by channel-closing. Figure 2C depicts a scheme that has been proposed for the GABAA receptor channel.53 There are seven states and 12 rates (10 after detailed balancing). This scheme differs from that of Figure 2B in several ways. (i) The channel can open to the same conductance level in two distinct ways (other models include a third open state).44 (ii) There are two 43

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desensitization pathways (fast and slow); they are accessed through closed states and are voltage-dependent.73 (iii) Recovery from desensitization occurs via the same pathway as activation. (iv) At saturating concentrations of GABA, only about 80% of the channels are open.94 (v) After a brief pulse of agonist, the decay of current is governed by both channel-closing and recovery from desensitization.

How might general anaesthetics affect ligandgated ion channels? Before reviewing speci®c effects of anaesthetics on LGICs, it is useful to consider how anaesthetics might affect channels. The simple answer is that they can either favour open states of the channel (potentiation) or favour closed states of the channel (inhibition). One approach is to simulate this by tweaking the rate constants in the appropriate kinetic scheme (Fig. 2). For example, potentiation might be achieved by increasing agonist af®nity, increasing the open gating rate or decreasing desensitization. Inhibition could arise from tweaks in the opposite direction. The process would be repeated for each experimental concentration of anaesthetic and one would end up with a database describing the dependence of rates on anaesthetic concentration. This is a reasonable approach (and perhaps the only approach) if it is thought that anaesthetics act by partitioning into the cell membrane and modifying the physicochemical properties of the membrane. The alterations in channel rate constants would then represent the behaviour of the protein in the new physicochemical lipid environment induced by the anaesthetic. Even if the lipid theory of anaesthetic action were to ®nd experimental support, this approach to understanding the effects of anaesthetics on channels would not be very enlightening. The relationship between rate constants and anaesthetic concentration would be purely empirical. To go beyond empiricism to predictability, it would be necessary to understand the dependence of the membrane property (call it `¯uidity', for example) on anaesthetic concentration and then understand how ¯uidity affects protein conformational changes. Fortunately, there is much evidence that anaesthetics interact directly with proteins and that this is their primary mode of action. This makes the job of the kinetic modeller simpler: introduce new, anaesthetic-bound states into the model. The underlying assumption is that anaesthetics interact with the receptors in an allosteric fashion.83 Of course, predictability is not guaranteed by this approach because for every additional state there will be additional rate constants to be determined. But you cannot stop a modeller from trying! Figure 3 shows an example of how an anaesthetic (represented by the letter B) may produce inhibition by binding to the pore of the channel and blocking the ¯ow of current through the pore. The scheme for the mnAChR, omitting desensitization, is used as the starting model. It

Fig 3 A kinetic scheme used to describe inhibition of the mnAChR channel by anaesthetics. See text for details.

might be tempting to simply add one additional state to the model, A2R*B, to represent the blocked state. However, this is a restrictive model that implies that the anaesthetic molecule can enter and leave its blocking site only when the gate of the channel is open. The more general model also allows anaesthetic binding to the closed channel conformations and allows that the anaesthetic may bind to these states with different af®nities. One testable prediction of this model is that, under conditions where most of the receptors are in the open state (A2R*), occupancy of the open state will be reduced in the presence of anaesthetic by a factor of (1+[B]/KB)±1, where [B] is the anaesthetic concentration and KB is its equilibrium binding constant for the transition A2R*¬®A2R*B. It is sometimes possible to use electrophysiological techniques to determine the binding of anaesthetics to the resting state, but direct binding experiments are often more useful. If the anaesthetic binds preferentially to either the resting or the open state, then agonist binding and/or channel gating must be different for anaesthetic-bound receptors. Figure 4 uses the GABAAR scheme (Fig. 2C) to illustrate how potentiation by anaesthetics (denoted P*) might be modelled. For clarity, states in which the anaesthetic is bound to the open and desensitized forms of the receptor have been omitted. The scheme in Figure 4A shows how the two commonly observed effects of anaesthetics, direct activation and potentiation, may be understood as consequences of a single binding site for the anaesthetic. Direct activation, in the absence of agonist, is represented by the transition R¬®RP*. In principle, the binding af®nity of P* to R can be obtained by measuring direct activation as a function of anaesthetic concentration. In practice, it may not be possible to achieve saturating concentrations of the anaesthetic. Potentiation is represented by the binding of P* to the single- and double-ligand-bound resting states. A prediction of this model is that the new conducting states introduced by the anaesthetics should be observed as new components in the open-duration histograms obtained from single-channel recording. In the alternative scheme shown 44


preparations has the advantage of providing a physiological environment for the cells and for observing effects of drugs on synaptic transmission as a whole rather than on one component of the process. In particular, in a slice the time course of neurotransmitter concentration in the synapse is physiological, as are receptor density and the postsynaptic integration of multiple input signals. However, these preparations are not suited to mechanistic studiesÐthere are too many interacting components. Moreover, unless tissues are obtained from transgenic animals, the effects of receptor mutation cannot be studied. Smaller preparations (cells and patches with expressed receptors) provide the experimenter with the control over voltage, concentration and receptor subtype that is necessary for mechanistic studies. The drawbacks of these preparations include varying degrees of arti®ciality of the conditions and the dif®culty of predicting whether drug-induced changes will have a physiological effect in a real synapse or neural circuit. One experimental factor that is sometimes given inadequate attention in cell and patch studies of LGICs is the speed of the change in agonist concentration used to activate currents. This speed varies with the size of the preparation. Solution exchange times for excised patches can be as fast as 100 ms.63 91 It may be possible to perfuse small cells that are not attached to a substrate within 10 ms.3 However, cells attached to a culture dish may require 200 ms.111 Perfusion of whole oocytes requires several seconds. One consequence of this is illustrated in Figure 5. LGIC currents are assumed to be activated instantaneously and then to decay as a result of desensitization. For the control trace, the desensitization time constant was 0.5 s. A drug is assumed to have two effects on this LGIC: inhibition of the peak current by 50% and a change in the rate of desensitization. For test 1, desensitization is twice as fast as the control and for test 2 it is half as fast as the control. If the system being studied is an excised patch, the true relationships between the test currents and the control can be measured. If the

in Figure 4B, the anaesthetic binds preferentially to the double-ligand-bound open state while keeping the channel open. A prediction of this model is that the anaesthetic prolongs one of the open-state components seen in the absence of anaesthetic by a factor of (1+[P]/KP), where [P] is the anaesthetic concentration and KP is its equilibrium binding constant for the transition A2R*¬®A2R*P*.

Experimental approaches to the study of ligand-gated ion channels Some of the techniques used to study LGICs are listed in Table 1. No one technique is ideal. Using intact slice

Fig 4 A kinetic scheme used to describe potentiation of the GABAAR channel by anaesthetics. See text for details.

Table 1 A comparison of methods used to study LGICs. Each category is rated good (+), average (0) or poor (±). The categories are as follows: Ex-cell physiol=how well the extracellular environment resembles in vivo conditions; Int-cell physiol=how well the intracellular environment resembles in vivo conditions; synaptic comm=whether synaptic communication is intact; V control=whether voltage-clamp experiments are possible; [ag] control=whether agonist concentration can be experimentally controlled; [ag](t) control=whether the time course of agonist concentration can be controlled; Subunit control=whether signals from different subunit types in the preparation can be distinguished; Mutant control=whether mutant receptors can be studied; Mech info=whether mechanistic information can be obtained from the experiments Method

Ex-cell physiol

Int-cell physiol

Synaptic comm

V control

[ag] control

[ag](t) control

Subunit control

Mutant control

Mech info

Brain or tissue slice: extracellular recording Brain or tissue slice: intracellular recording Voltage clamp of native receptors: whole cell Voltage clamp of native receptors: cell-attached patch Voltage clamp of native receptors: outside-out patch Voltage clamp of expressed receptors: oocytes Voltage clamp of expressed receptors: whole cell Voltage clamp of expressed receptors: cell-attached patch Voltage clamp of expressed receptors: outside-out patch Stopped-¯ow ¯uorescence spectroscopy Photolabelling or radiolabelling

+ + ± ± ± ± ± ± ± ± ±

+ ± ± + ± ± ± + ± + +

+ + ± ± ± ± ± ± ± ± ±

± ± + + + + + + + ± ±

± ± + + + + + + + + +

± ± 0 ± + ± 0 ± + + ±

± ± ± ± ± + + + + 0 0

± ± ± ± ± + + + + 0 0

± ± 0 + + 0 + + + + +

45

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the `right' answer, this may not be the relevant answer for a real synapse because this depends on factors such as the integration time in the postsynaptic cell. As will be shown in the following sections, multiple effects of anaesthetics on LGICs are commonly observed, so the situation illustrated in Figure 5 is not completely theoretical. Clearly, caution must be used in the interpretation of experiments on LGICs when agonist perfusion is slow and desensitization (or some other process) is fast.

system being studied is an oocyte and the agonist does not equilibrate with the system for, say, 2 s, very different results will be observed. At 2 s, test 1 appears to inhibit the current by 99% and this receptor might be labelled `supersensitive' to the drug. In test 2, this inhibitory drug actually appears to be potentiating the control current. If measurements were made at ~800 ms, this drug might be considered totally ineffective because the test 2 and control currents are the same. Although the patch experiment gives

Anaesthetics and cys-loop LGICs Table 2 catalogues the effects of a variety of general anaesthetics on LGICs of the cys-loop superfamily. The purpose of this tabulation is to provide an overview of the effects observed and not to be comprehensive. The many subtypes of heteromeric nnAChRs are not listed separately. A recent review tabulates subtype-speci®c effects.104 When possible, we note when both inhibitory and potentiating effects have been observed. When the effects are small at minimum alveolar (MA) concentrations, we indicate whether inhibition (i) or potentiation (p) is observed at high concentrations, so that the trend can be noted. Among the muscle-type receptors, inhibition is the most commonly observed effect, but these receptors are relatively insensitive to a number of anaesthetics. Volatile anaesthetics and alcohols have both inhibitory and potentiating effects on mnAChRs.72 Inhibition is manifested in several ways. At the single-channel level, there is a ¯ickering channel behaviour that decreases the open time per burst.26 78 On the macroscopic current level, there is a decrease in the peak current response to saturating concentrations of agonist27 64 and an acceleration of desensitization.27 89 Both channel ¯ickering and the decreased current response can be interpreted in terms of a channel blocking mechanism (Fig. 3) in which the

Fig 5 Current simulations illustrating the time-dependence of current modulation by an anaesthetic that has two effects on a channel. The control current desensitizes with a time constant (t) of 0.5 s. For current test 1, the anaesthetic is assumed to decrease the peak amplitude by 50% and increase the rate of desensitization by a factor of 2 (t=0.25 s). For current test 2, the anaesthetic is also assumed to decrease the peak amplitude by 50% but to decrease the rate of desensitization by a factor of 2 (t=1 s). The ratio of test to control is shown in the inset on a logarithmic scale. The degree of inhibition or potentiation observed is critically dependent on the time resolution of the experiment (the ®rst time point that can be measured).

Table 2 Effects of anaesthetics on cationic cys-loop LGICs. i=weak inhibition; I=inhibition; II=strong inhibition; p=weak potentiation; P=potentiation; PP=strong potentiation; 0=insensitive Anaesthetic

Muscle AChR

Iso¯urane (or other halogenated ethers)

I26

27

P27

Halothane Ether Nitrous oxide Xenon Cyclopropane Butane Urethane Short-chain alcohols (pentanol) Barbiturates Ketamine Propofol Etomidate Steroid anaesthetics Non-immobilizers

I102 I29 P64 I102 088 088 I25 78 P64 I25 78 P64 I7 24 I101 i103 i103 i103 I38

Neuronal AChR

90

5-HT3R

Heteromeric

Homomeric

II36

036

100

II33 I79

033 i79

i105 i105 I87 P45 P111 I111 I6 31 I35 i36 035 I81 I87

46

I107 I18 31 I17 036

P67 I (sevo¯urane)92 P (sevo¯urane)92 P67 P109 i105 I95

P50 67 I50 I50 I12 50 I12 0106 i11 057 I12


inhibitory binding site is within the channel pore.37 110 Drugs such as iso¯urane and butanol have equal af®nity for the open and resting states of the channel,27 64 whereas longchain alcohols, such as octanol, have greater af®nity for the open state.37 The molecular site of action for acceleration of desensitization is unknown. The potentiating effects of volatile anaesthetics and alcohols can easily be overlooked. Experiments must be done at low concentrations of ACh or with a partial agonist, such as decamethonium.64 Alternatively, the non-electrophysiological approach of stopped-¯ow ¯uorescence spectroscopy90 may be used. The frequency of bursts of single-channel activity increases in the presence of iso¯urane26 and many alcohols,64 revealing their potentiating effects. Potentiation is most easily studied with ethanol because it occurs at lower concentrations than inhibition.39 Potentiation by ether64 and iso¯urane27 90 arises from the stabilization of ACh binding, whereas potentiation by alcohols39 64 arises from stabilization of the open state. Inhibition of mnAChRs by pentobarbital has also been studied at the mechanistic level.7 24 This barbiturate binds more tightly to the open state of the receptor than to the resting state and acts as a blocker of open channels. Pentobarbital does not accelerate desensitization nor does it have any potentiating effects on the muscle receptor. The most pronounced effect of anaesthetics on nnAChRs is the inhibition observed with heteromeric receptors in the presence of volatile anaesthetics.36 100 These receptors are sensitive to sub-MA concentrations of volatile anaesthetics and are thus considered to be unimportant for anaesthesia itself. This may be a premature assessment for two reasons. First, most of the reported experiments were done with receptors expressed in oocytes. The poor time resolution of such experiments may provide a distorted picture, especially if the anaesthetics have multiple effects (Fig. 5). Secondly, our knowledge about cholinergic synapses in the CNS is lacking. If there is a large margin of safety at these synapses (as is seen at the neuromuscular junction), it may be necessary to inhibit a large fraction of receptors in order to interfere with synaptic communication. Potentiating effects by volatile anaesthetics on heteromeric nnAChRs have not been reported. The problem may be that experiments have not been done under conditions that would favour potentiating effects. However, potentiation has been observed with urethane45 and short-chain alcohols.111 Homomeric nnAChRs appear to be relatively insensitive to the anaesthetics that have been tested thus far. 5-HT3Rs exhibit a wide variety of responses to anaesthetics. Although iso¯urane, halothane67 and ether109 potentiate these receptors, sevo¯urane has mostly inhibitory effects.92 Short-chain alcohols have potentiating and inhibitory effects.50 67 Inhibition is seen with long-chain alcohols,50 barbiturates12 50 and a steroid anaesthetic.12 Propofol inhibits only at high concentrations.12

Table 3 Effects of anaesthetics on anionic cys-loop LGICs. For explanation see legend of Table 2 Anaesthetic

GABAAR

GlyR

AChR with chloride permeability

Iso¯urane (or other halogenated ethers) Halothane Ether Nitrous oxide Xenon Cyclopropane Butane Urethane Short-chain alcohols (pentanol) Barbiturates Ketamine Propofol Etomidate Steroid anaesthetics Non-immobilizers

P52I10 P52 I10 P62 p105 p105 088 088 P45 P23 P23 P 4 I4 P62 0105 P62 P46 P19 075

P32 P32

I71 I71

p105 p105 P45 P2 p86 0105 P86 057

I71 I71

Effects of anaesthetics on the anionic members of the cysloop superfamily are listed in Table 3. The neuronal AChR that is permeable to chloride is included in this table, although its superfamily relationship is unknown. In general, family members containing different subunit combinations may be affected to different degrees by the drugs, but for clarity (but not completeness) we have not listed these separately. One important exception is the homomeric GABACR formed from the r1 subunit. This receptor is inhibited rather than potentiated by volatile anaesthetics and alcohols and is not affected by pentobarbital, propofol or alphaxalone.74 This observation prompted the (GABAC r1)±(Gly a1) chimera receptor experiments that led to the identi®cation of residues that confer sensitivity to potentiation by anaesthetics.76 GABAARs and GlyRs are potentiated by many, but not all, anaesthetics. The exceptions are nitrous oxide (weak potentiation), xenon (weak potentiation), cyclopropane (no effect at the concentrations studied) and butane (no effect at the concentrations studied). In addition, GlyRs are only weakly potentiated by pentobarbital and are not affected by ketamine or etomidate. Site-directed mutagenesis experiments have localized residues on GABAARs and GlyRs that determine anaesthetic sensitivity.76 The current model envisages a cavity between the membrane-spanning segments of the receptors with Leu232 on M1, Ser270 on M2, and Ala291 on M3 contributing to this cavity.51 This model is discussed in more detail elsewhere in this issue.98 Like the cationic members of the cys-loop superfamily, anaesthetics have multiple actions on GABAA and GlyRs. Direct activation of these channels by some anaesthetics has been observed. Inhibition of the receptors by volatile anaesthetics and barbiturates has been reported to occur at somewhat higher concentrations than those needed to produce potentiation. It is possible that inhibition also 47

Dilger Table 4 Effects of anaesthetics on glutamate-activated and P2X-ATP activated LGICs. For explanation see legend of Table 2 Anaesthetic

NMDA-R

AMPA-R

Kainate-R

P2X-R

Iso¯urane (or other halogenated ethers) Halothane Ether Nitrous oxide Xenon Cyclopropane Butane Urethane Short-chain alcohols (pentanol) Barbiturates Ketamine Propofol

i22 i22

i22 i22

PP22 PP22

I70

II105 II40

i105

p105

I45 I85

I45 I23 023 0104 I65 I22

I23 023 I22 0104 0104

Etomidate Steroid anaesthetics Non-immobilizers

057 0104 022

I22 I66 0104

057 0104

occurs with other anaesthetics but the ideal conditions for observing inhibition and potentiation are different, so the proper experiments may not have been performed. While much of the recent interest on the interactions of anaesthetics with GABAARs has to do with the identi®cation of sites, research speci®cally on the mechanism of potentiation has been neglected. A notable exception is single-channel studies of the interaction of pentobarbital with expressed GABAARs.5 94 One of these studies addresses the question of whether potentiation results from slowing of agonist dissociation (binding) or from slowing of channel closure (gating). The results are consistent with the idea that pentobarbital stabilizes one of the open states.94

0104 0104 022

I61 06 I41 i41p (P2X4)96 0 (P2X2)96

Summary The experimental effort that has been expended in investigating the effects of general anaesthetics on LGICs has been enormous over the past decade. Members of all three LGIC superfamilies have been examined using electrophysiological techniques. Anaesthetics that have been examined include volatile anaesthetics, gaseous anaesthetics, alcohols, i.v. anaesthetics and non-immobilizers. Obsolete anaesthetics (ether, cyclopropane, butane) have been used in order to increase the variability of the structure and polarity of experimental compounds. The tools of molecular biology have been used to make chimeric receptors and to make single-site mutations. Interestingly, this work has been taking place in parallel with efforts to understand the structure of these proteins. Anaesthetic research often stimulates structural research as well as vice versa. There are some common themes in the interactions between anaesthetics and the three superfamilies of LGICs. In many cases, anaesthetics have both inhibitory and potentiating effects on the channels. It is likely that the number of examples of this will increase when experiments are designed to look speci®cally for one or the other type of effect. So we must conclude that there are multiple binding sites for anaesthetics on LGICs. The degree of inhibition or potentiation is not easily predictable. In retrospect, this is not surprising when we consider that the sensitivity of a channel to anaesthetics can be altered by a single aminoacid mutation. The large structural differences between the cys-loop, glutamate-activated and P2X superfamilies do not lead to large differences in anaesthetic sensitivity. It is the smaller, almost insigni®cant, changes that do this. This observation that small changes may lead to large effects reinforces the idea that at least some of the interactions between anaesthetics and LGICs are direct drug±protein interactions that are not mediated by the lipids.

Anaesthetics, ionotropic glutamate ligandgated ion channels and P2X ATP ligandgated ion channels NMDA receptors (NMDA-Rs) are inhibited by many general anaesthetics (Table 4). Of particular interest is the fact that they are strongly inhibited by both nitrous oxide and xenon,40 105 two of the anaesthetics that do not potentiate GABAARs or GlyRs. These recent observations, added to the long-standing observation that ketamine inhibits NMDA-Rs,66 have caused some to speculate that there are (at least) two different routes to anaesthesia, volatile anaesthetics potentiating GABAergic synapses and xenon inhibiting glutamatergic synapses.21 One test of this hypothesis will be to extend measurements on NMDA-Rs to other anaesthetics that do not potentiate GABAARs. Also, it is not clear how the potentiation by volatile anaesthetics of kainate receptors would ®t into this scheme. The P2X-R is the least comprehensively studied LGIC superfamily when it comes to general anaesthetics (Table 4). Inhibition by volatile anaesthetics, short-chain alcohols and ketamine has been reported. 48


This review has not addressed the question of whether the effects of anaesthetics seen on LGICs are relevant to anaesthesia. This question cannot really be answered at present. Although potent effects can be observed on the channels themselves, we have only begun to try to understand whether these effects are important for a synapse, a neuronal circuit or the function of an animal's nervous system. We have studied the trees; now we must go on to study the forest and the ecosystem.

17

18 19 20

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