Determinants of the anesthetic sensitivity of neuronal nicotinic acetylcholine receptors

Mechanisms of the Immunological Effects of Volatile Anesthetics: A Review

Volatile anesthetics (VAs) have been in clinical use for a very long time. Their mechanism of action is yet to be fully delineated, but multiple ion channels have been reported as targets for VAs (canonical VA targets). It is increasingly recognized that VAs also manifest effects outside the central nervous system, including on immune cells. However, the literature related to how VAs affect the behavior of immune cells is very limited, but it is of interest that some canonical VA targets are reportedly expressed in immune cells. Here, we review the current literature and describe canonical VA targets expressed in leukocytes and their known roles. In addition, we introduce adhesion molecules called β2 integrins as noncanonical VA targets in leukocytes. Finally, we propose a model for how VAs affect the function of neutrophils, macrophages, and natural killer cells via concerted effects on multiple targets as examples.

Insights into distinct modulation of α7 and α7β2 nicotinic acetylcholine receptors by the volatile anesthetic isoflurane

Nicotinic acetylcholine receptors (nAChRs) are targets of general anesthetics, but functional sensitivity to anesthetic inhibition varies dramatically among different subtypes of nAChRs. Potential causes underlying different functional responses to anesthetics remain elusive. Here we show that in contrast to the α7 nAChR, the α7β2 nAChR is highly susceptible to inhibition by the volatile anesthetic isoflurane in electrophysiology measurements. Isoflurane-binding sites in β2 and α7 were found at the extracellular and intracellular end of their respective transmembrane domains using NMR. Functional relevance of the identified β2 site was validated via point mutations and subsequent functional measurements. Consistent with their functional responses to isoflurane, β2 but not α7 showed pronounced dynamics changes, particularly for the channel gate residue Leu-249(9'). These results suggest that anesthetic binding alone is not sufficient to generate functional impact; only those sites that can modulate channel dynamics upon anesthetic binding will produce functional effects.

Shaker-related potassium channels in the central medial nucleus of the thalamus are important molecular targets for arousal suppression by volatile general anesthetics

The molecular targets and neural circuits that underlie general anesthesia are not fully elucidated. Here, we directly demonstrate that Kv1-family (Shaker-related) delayed rectifier K(+) channels in the central medial thalamic nucleus (CMT) are important targets for volatile anesthetics. The modulation of Kv1 channels by volatiles is network specific as microinfusion of ShK, a potent inhibitor of Kv1.1, Kv1.3, and Kv1.6 channels, into the CMT awakened sevoflurane-anesthetized rodents. In heterologous expression systems, sevoflurane, isoflurane, and desflurane at subsurgical concentrations potentiated delayed rectifier Kv1 channels at low depolarizing potentials. In mouse thalamic brain slices, sevoflurane inhibited firing frequency and delayed the onset of action potentials in CMT neurons, and ShK-186, a Kv1.3-selective inhibitor, prevented these effects. Our findings demonstrate the exquisite sensitivity of delayed rectifier Kv1 channels to modulation by volatile anesthetics and highlight an arousal suppressing role of Kv1 channels in CMT neurons during the process of anesthesia.

The role of structured water in mediating general anesthetic action on alpha4beta2 nAChR

Water is an essential component for many biological processes. Pauling proposed that water might play a critical role in general anesthesia by forming water clathrates around anesthetic molecules. To examine potential involvement of water in general anesthesia, we analyzed water within alpha4beta2 nAChR, a putative protein target hypersensitive to volatile anesthetics. Experimental structure-derived closed- and open-channel nAChR systems in a fully hydrated lipid bilayer were examined using all-atom molecular dynamics simulations. At the majority of binding sites in alpha4beta2 nAChR, halothane replaced the slow-exchanging water molecules and caused a regional water population decrease. Only two binding sites had an increased quantity of water in the presence of halothane, where water arrangements resemble clathrate-like structures. The small number of such clathrate-like water clusters suggests that the formation of water clathrates is unlikely to be a primary cause for anesthesia. Despite the decrease in water population at most of the halothane binding sites, the number of sites that can be occupied transiently by water is actually increased in the presence of halothane. Many of these water sites were located between two subunits or in regions containing agonist binding sites or critical structural elements for transducing agonist binding to channel gating. Changes in water sites in the presence of halothane affected water-mediated protein-protein interactions and the protein dynamics, which can have direct impact on protein function. Collectively, water contributes to the action of anesthetics in proteins by mediating interactions between protein subunits and altering protein dynamics, instead of forming water clathrates around anesthetics.

Unresponsive correlated motion in alpha7 nAChR to halothane binding explains its functional insensitivity to volatile anesthetics

Neuronal nicotinic acetylcholine receptors (nAChRs) have been implicated as targets for general anesthetics, but the functional responses to anesthetic modulation vary considerably among different subtypes of nAChRs. Inhaled general anesthetics, such as halothane, could effectively inhibit the channel activity of the alpha4beta2 nAChR but not the homologous alpha7 nAChR. To understand why alpha7 is insensitive to inhaled general anesthetics, we performed multiple sets of 20 ns molecular dynamics (MD) simulations on the closed- and open-channel alpha7 in the absence and presence of halothane and critically compared the results with those from our studies on the alpha4beta2 nAChR (Liu et al. J. Phys. Chem. B 2009, 113, 12581 and Liu et al. J. Phys. Chem. B 2010, 114, 626). Several halothane binding sites with fairly high binding affinities were identified in both closed- and open-channel alpha7, suggesting that a lack of sensitive functional responses of the alpha7 nAChR to halothane in the previous experiments was unlikely due to a lack of halothane interaction with alpha7. The binding affinities of halothane in alpha7 seemed to be protein conformation-dependent. Overall, halothane affinity was higher in the closed-channel alpha7. Halothane binding to alpha7 did not induce profound changes in alpha7 structure and dynamics that could be related to the channel function. In contrast, correlated motion of the open-channel alpha4beta2 was reduced substantially in the presence of halothane, primarily due to the more susceptible nature of beta2 to anesthetic modulation. The amphiphilic extracellular and transmembrane domain interface of the beta2 subunit is attractive to halothane and is susceptible to halothane perturbation, which may be why alpha4beta2 is functionally more sensitive to halothane than alpha7.

Computational studies on the interactions of inhalational anesthetics with proteins

Despite the widespread clinical use of anesthetics since the 19th century, a clear understanding of the mechanism of anesthetic action has yet to emerge. On the basis of early experiments by Meyer, Overton, and subsequent researchers, the cell's lipid membrane was generally concluded to be the primary site of action of anesthetics. However, later experiments with lipid-free globular proteins, such as luciferase and apoferritin, shifted the focus of anesthetic action to proteins. Recent experimental studies, such as photoaffinity labeling and mutagenesis on membrane proteins, have suggested specific binding sites for anesthetic molecules, further strengthening the proteocentric view of anesthetic mechanism. With the increased availability of high-resolution crystal structures of ion channels and other integral membrane proteins, as well as the availability of powerful computers, the structure-function relationship of anesthetic-protein interactions can now be investigated in atomic detail. In this Account, we review recent experiments and related computer simulation studies involving interactions of inhalational anesthetics and proteins, with a particular focus on membrane proteins. Globular proteins have long been used as models for understanding the role of protein-anesthetic interactions and are accordingly examined in this Account. Using selected examples of membrane proteins, such as nicotinic acetyl choline receptor (nAChR) and potassium channels, we address the issues of anesthetic binding pockets in proteins, the role of conformation in anesthetic effects, and the modulation of local as well as global dynamics of proteins by inhaled anesthetics. In the case of nicotinic receptors, inhalational anesthetic halothane binds to the hydrophobic cavity close to the M2-M3 loop. This binding modulates the dynamics of the M2-M3 loop, which is implicated in allosterically transmitting the effects to the channel gate, thus altering the function of the protein. In potassium channels, anesthetic molecules preferentially potentiate the open conformation by quenching the motion of the aromatic residues implicated in the gating of the channel. These simulations suggest that low-affinity drugs (such as inhalational anesthetics) modulate the protein function by influencing local as well as global dynamics of proteins. Because of intrinsic experimental limitations, computational approaches represent an important avenue for exploring the mode of action of anesthetics. Molecular dynamics simulations-a computational technique frequently used in the general study of proteins-offer particular insight in the study of the interaction of inhalational anesthetics with membrane proteins.

General anesthetic binding to neuronal alpha4beta2 nicotinic acetylcholine receptor and its effects on global dynamics

The neuronal alpha4beta2 nicotinic acetylcholine receptor (nAChR) is a target for general anesthetics. Currently available experimental structural information is inadequate to understand where anesthetics bind and how they modulate the receptor motions essential to function. Using our published open-channel structure model of alpha4beta2 nAChR, we identified and evaluated six amphiphilic interaction sites for the volatile anesthetic halothane via flexible ligand docking and subsequent 20-ns molecular dynamics simulations. Halothane binding energies ranged from -6.8 to -2.4 kcal/mol. The primary binding sites were located at the interface of extracellular and transmembrane domains, where halothane perturbed conformations of, and widened the gap among, the Cys loop, the beta1-beta2 loop, and the TM2-TM3 linker. The halothane with the highest binding affinity at the interface between the alpha4 and beta2 subunits altered interactions between the protein and nearby lipids by competing for hydrogen bonds. Gaussian network model analyses of the alpha4beta2 nAChR structures at the end of 20-ns simulations in the absence or presence of halothane revealed profound changes in protein residue mobility. The concerted motions critical to protein function were also perturbed considerably. Halothane's effect on protein dynamics was not confined to the residues adjacent to the binding sites, indicating an action on a more global scale.

Volatile anesthetics and endogenous cannabinoid anandamide have additive and independent inhibitory effects on alpha(7)-nicotinic acetylcholine receptor-mediated responses in Xenopus oocytes

In earlier studies, the volatile anesthetics and the endogenous cannabinoid anandamide have been shown to inhibit the function of alpha(7)-nicotinic acetylcholine receptors. In the present study, interactions between the effects of volatile anesthetics and anandamide on the function of alpha(7)-nicotinic acetylcholine receptors expressed in Xenopus oocytes were investigated using the two-electrode voltage-clamp technique. Anandamide and volatile anesthetics isoflurane and halothane inhibited currents evoked with acetylcholine (100 microM) in a reversible and concentration-dependent manner. Coapplication of anandamide and volatile anesthetics caused a significantly greater inhibition of alpha(7)-nicotinic acetylcholine receptor function than anandamide or volatile anesthetics alone. Analyses of oocytes by matrix-assisted laser desorption/ionization mass spectroscopy indicated that volatile anesthetics did not alter the lipid profile of oocytes. Results of studies with chimeric alpha(7)-nicotinic acetylcholine-5-HT(3) receptors comprised of the N-terminal domain of the alpha(7)-nicotinic acetylcholine receptor and the transmembrane and carboxyl-terminal domains of 5-HT(3) receptors suggest that while isoflurane inhibition of the alpha(7)-nicotinic acetylcholine receptor is likely to involve the N-terminal region of the receptor, the site of action for anandamide involves transmembrane and carboxyl-terminal domains of the receptors. These data indicate that endocannabinoids and isoflurane have additive inhibitory effects on alpha(7)-nicotinic acetylcholine receptor function through allosteric binding sites located on the distinct regions of the receptor.

Partitioning of anesthetics into a lipid bilayer and their interaction with membrane-bound peptide bundles

Molecular dynamics simulations have been performed to investigate the partitioning of the volatile anesthetic halothane from an aqueous phase into a coexisting hydrated bilayer, composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) lipids, with embedded alpha-helical peptide bundles based on the membrane-bound portions of the alpha- and delta-subunits, respectively, of nicotinic acetylcholine receptor. In the molecular dynamics simulations halothane molecules spontaneously partitioned into the DOPC bilayer and then preferentially occupied regions close to lipid headgroups. A single halothane molecule was observed to bind to tyrosine (Tyr-277) residue in the alpha-subunit, an experimentally identified specific binding site. The binding of halothane attenuated the local loop dynamics of alpha-subunit and significantly influenced global concerted motions suggesting anesthetic action in modulating protein function. Steered molecular dynamics calculations on a single halothane molecule partitioned into a DOPC lipid bilayer were performed to probe the free energy profile of halothane across the lipid-water interface and rationalize the observed spontaneous partitioning. Partitioned halothane molecules affect the hydrocarbon chains of the DOPC lipid, by lowering of the hydrocarbon tilt angles. The anesthetic molecules also caused a decrease in the number of peptide-lipid contacts. The observed local and global effects of anesthetic binding on protein motions demonstrated in this study may underlie the mechanism of action of anesthetics at a molecular level.

A gating mechanism proposed from a simulation of a human alpha7 nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor is a well characterized ligand-gated ion channel, yet a proper description of the mechanisms involved in gating, opening, closing, ligand binding, and desensitization does not exist. Until recently, atomic-resolution structural information on the protein was limited, but with the production of the x-ray crystal structure of the Lymnea stagnalis acetylcholine binding protein and the EM image of the transmembrane domain of the torpedo electric ray nicotinic channel, we were provided with a window to examine the mechanism by which this channel operates. A 15-ns all-atom simulation of a homology model of the homomeric human alpha7 form of the receptor was conducted in a solvated palmitoyl-2-oleoyl-sn-glycerol-phosphatidylcholine bilayer and examined in detail. The receptor was unliganded. The structure undergoes a twist-to-close motion that correlates movements of the C loop in the ligand binding domain, via the beta10-strand that connects the two, with the 10 degrees rotation and inward movement of two nonadjacent subunits. The Cys loop appears to act as a stator around which the alpha-helical transmembrane domain can pivot and rotate relative to the rigid beta-sheet binding domain. The M2-M3 loop may have a role in controlling the extent or kinetics of these relative movements. All of this motion, along with essential dynamics analysis, is suggestive of the direction of larger motions involved in gating of the channel.

Gating-enhanced accessibility of hydrophobic sites within the transmembrane region of the nicotinic acetylcholine receptor's {delta}-subunit. A time-resolved photolabeling study

General anesthetics often interact more strongly with sites on open than on closed states of ligand-gated ion channels. To seek such sites, Torpedo membranes enriched in nicotinic acetylcholine receptors (nAChRs) were preincubated with the hydrophobic probe 3-(trifluoromethyl)-3-(m-iodophenyl) diazirine ([125I]TID) and exposed to agonist for either 0 ms (closed state), 1.5 and 10 ms (activated states), 1 s (fast desensitized state), or > or =1 h (equilibrium or slow desensitized state) and then rapidly frozen (<1 ms) and photolabeled. Within 1.5 ms, the fractional change in photoincorporation relative to the closed state decreased to 0.7 in the beta- and gamma-subunits, whereas in the alpha-subunit, it changed little. The most dramatic change occurred in the delta-subunit, where it increased to 1.6 within 10 ms but fell to 0.7 during fast desensitization. Four residues in the delta-subunit's transmembrane domain accounted for the enhanced photoincorporation induced by a 10-ms agonist exposure both when TID was added simultaneously with agonist and when it was preincubated with membranes. In the published closed state structure, two residues (deltaThr274 and deltaLeu278) are situated toward the extracellular end of helix M2, both contralateral to the ion channel and adjacent to the third residue (deltaPhe232) on M1. The fourth labeled residue (deltaIle288) is toward the end of the M2-M3 loop. Contact with these residues occurs on the time scale of a rapid phase of TID inhibition in Torpedo nAChRs, suggesting the formation of a transient hydrophobic pocket between M1, M2, and M3 in the delta-subunit during gating.

Identification of nicotinic acetylcholine receptor amino acids photolabeled by the volatile anesthetic halothane

To identify inhalational anesthetic binding domains in a ligand-gated ion channel, we photolabeled nicotinic acetylcholine receptor (nAChR)-rich membranes from Torpedo electric organ with [(14)C]halothane and determined by Edman degradation some of the photolabeled amino acids in nAChR subunit fragments isolated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and high-performance liquid chromatography. Irradiation at 254 nm for 60 s in the presence of 1 mM [(14)C]halothane resulted in incorporation of approximately 0.5 mol of (14)C/mol of subunit, with photolabeling distributed within the nAChR extracellular and transmembrane domains, primarily at tyrosines. GammaTyr-111 in ACh binding site segment E was labeled, while alphaTyr-93 in segment A was not. Within the transmembrane domain, alphaTyr-213 within alphaM1 and deltaTyr-228 within deltaM1 were photolabeled, while no labeled amino acids were identified within the deltaM2 ion channel domain. Although the efficiency of photolabeling at the subunit level was unaffected by agonist, competitive antagonist, or isoflurane, state-dependent photolabeling was seen in a delta subunit fragment beginning at deltaPhe-206. Labeling of deltaTyr-212 in the extracellular domain was inhibited >90% by d-tubocurarine, whereas addition of either carbamylcholine or isoflurane had no effect. Within M1, the level of photolabeling of deltaTyr-228 with [(14)C]halothane was increased by carbamylcholine (90%) or d-tubocurarine (50%), but it was inhibited by isoflurane (40%). Within the structure of the nAChR transmembrane domain, deltaTyr-228 projects into an extracellular, water accessible pocket formed by amino acids from the deltaM1-deltaM3 alpha-helices. Halothane photolabeling of deltaTyr-228 provides initial evidence that halothane and isoflurane bind within this pocket with occupancy or access increased in the nAChR desensitized state compared to the closed channel state. Halothane binding at this site may contribute to the functional inhibition of nAChRs.

Unique general anesthetic binding sites within distinct conformational states of the nicotinic acetylcholine receptor

General anesthesia is a complex behavioral state provoked by the pharmacological action of a broad range of structurally different hydrophobic molecules called general anesthetics (GAs) on receptor members of the genetically linked ligand-gated ion channel (LGIC) superfamily. This superfamily includes nicotinic acetylcholine (AChRs), type A and C gamma-aminobutyric acid (GABAAR and GABACR), glycine (GlyR), and type 3 5-hydroxytryptamine (5-HT3R) receptors. This review focuses on recent advances in the localization of GA binding sites on conformationally and compositionally distinct AChRs. The experimental evidence outlined in this review suggests that: 1. Several neuronal-type AChRs might be targets for the pharmacological action of distinct GAs. 2. The molecular components of a specific GA binding site on a certain receptor subtype are different from the structural determinants of the locus for the same GA on a different receptor subtype. 3. There are unique binding sites for distinct GAs in the same receptor protein. 4. A GA can activate, potentiate, or inhibit an ion channel, indicating the existence of more than one binding site for the same GA. 5. The affinity of a specific GA depends on the conformational state of the receptor. 6. GAs inhibition channels by at least two mechanisms, an open-channel-blocking and/or an allosteric mechanism. 7. Certain GAs may inhibit AChR function by competing for the agonist binding sites or by augmenting the desensitization rate.

Unconventional ligands and modulators of nicotinic receptors

Evidence gathered from epidemiologic and behavioral studies have indicated that neuronal nicotinic receptors (nAChRs) are intimately involved in the pathogenesis of a number of neurologic disorders, including Alzheimer's disease, Parkinson's disease, and schizophrenia. In the mammalian brain, neuronal nAChRs, in addition to mediating fast synaptic transmission, modulate fast synaptic transmission mediated by the major excitatory and inhibitory neurotransmitters glutamate and GABA, respectively. Of major interest, however, is the fact that the activity of the different subtypes of neuronal nAChR is also subject to modulation by substances of endogenous origin such as choline, the tryptophan metabolite kynurenic acid, neurosteroids, and beta-amyloid peptides and by exogenous substances, including the so-called nicotinic allosteric potentiating ligands, of which galantamine is the prototype, and psychotomimetic drugs such as phencyclidine and ketamine. The present article reviews and discusses the effects of unconventional ligands on nAChR activity and briefly describes the potential benefits of using some of these compounds in the treatment of neuropathologic conditions in which nAChR function/expression is known to be altered.

The effects of general anaesthetics on ligand-gated ion channels

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 specifically 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 amino-acid 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 insignificant, 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. 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.