An Association between OXPHOS-Related Gene Expression and Malignant Hyperthermia Susceptibility in Human Skeletal Muscle Biopsies.

Malignant hyperthermia (MH) is a pharmacogenetic condition of skeletal muscle that manifests in hypermetabolic responses upon exposure to volatile anaesthetics. This condition is caused primarily by pathogenic variants in the calcium-release channel RYR1, which disrupts calcium signalling in skeletal muscle. However, our understanding of MH genetics is incomplete, with no variant identified in a significant number of cases and considerable phenotype diversity. In this study, we applied a transcriptomic approach to investigate the genome-wide gene expression in MH-susceptible cases using muscle biopsies taken for diagnostic testing. Baseline comparisons between muscle from MH-susceptible individuals (MHS, n = 8) and non-susceptible controls (MHN, n = 4) identified 822 differentially expressed genes (203 upregulated and 619 downregulated) with significant enrichment in genes associated with oxidative phosphorylation (OXPHOS) and fatty acid metabolism. Investigations of 10 OXPHOS target genes in a larger cohort (MHN: n = 36; MHS: n = 36) validated the reduced expression of ATP5MD and COQ6 in MHS samples, but the remaining 8 selected were not statistically significant. Further analysis also identified evidence of a sex-linked effect in SDHB and UQCC3 expression, and a difference in ATP5MD expression across individuals with MH sub-phenotypes (trigger from in vitro halothane exposure only, MHSh (n = 4); trigger to both in vitro halothane and caffeine exposure, MHShc (n = 4)). Our data support a link between MH-susceptibility and dysregulated gene expression associated with mitochondrial bioenergetics, which we speculate plays a role in the phenotypic variability observed within MH.

Ractopamine does not rescue Halothane and Rendement Napole metabolism postmortem.

The objective of this study was to determine if ractopamine (RAC) impacts postmortem muscle metabolism and subsequent pork quality in Halothane (HAL) and Rendement Napole (RN) mutant pigs. All RAC fed pigs had increased (P < 0.04) L* values. HAL and RN mutants muscle had lower (P < 0.01) pH values but RAC feeding had no effect. RN mutants had higher and lower (P < 0.05) muscle pH and temperatures, respectfully at 15 min and RN mutant pigs had greater (P < 0.0001) glycogen initially but lactate levels similar to wild type (WT) pigs at 24 h. RAC lowered (P < 0.05) glycogen in RN mutants but not in HAL mutated or WT pig muscle. These data show RAC feeding changes postmortem energy metabolism but does not change pH and pork quality hallmark of two major pig gene mutations and supports our contention that ultimate meat quality traits and their biochemical drivers may be more complex than originally reasoned.

Anesthetics and Cell-Cell Communication: Potential Ca2+-Calmodulin Role in Gap Junction Channel Gating by Heptanol, Halothane and Isoflurane.

Cell-cell communication via gap junction channels is known to be inhibited by the anesthetics heptanol, halothane and isoflurane; however, despite numerous studies, the mechanism of gap junction channel gating by anesthetics is still poorly understood. In the early nineties, we reported that gating by anesthetics is strongly potentiated by caffeine and theophylline and inhibited by 4-Aminopyridine. Neither Ca2+ channel blockers nor 3-isobutyl-1-methylxanthine (IBMX), forskolin, CPT-cAMP, 8Br-cGMP, adenosine, phorbol ester or H7 had significant effects on gating by anesthetics. In our publication, we concluded that neither cytosolic Ca2+i nor pHi were involved, and suggested a direct effect of anesthetics on gap junction channel proteins. However, while a direct effect cannot be excluded, based on the potentiating effect of caffeine and theophylline added to anesthetics and data published over the past three decades, we are now reconsidering our earlier interpretation and propose an alternative hypothesis that uncoupling by heptanol, halothane and isoflurane may actually result from a rise in cytosolic Ca2+ concentration ([Ca2+]i) and consequential activation of calmodulin linked to gap junction proteins.

Scientific Accuracy Matters.

The Solubility of Halothane in Blood and Tissue Homogenates. By Larson CP, Eger EI, Severinghaus JW. Anesthesiology 1962; 23:349-55. Measured samples of human and bovine blood, human hemoglobin, and tissue homogenates from human fat and both human and bovine liver, kidney, muscle, whole brain, and separated gray and white cortex were added to stoppered 2,000-ml Erlenmeyer flasks. To each flask, 0.1 ml of liquid halothane was added under negative pressure using a calibrated micropipette. After the flask was agitated for 2 to 4 h to achieve equilibrium between the gas and blood or tissue contents, a calibrated infrared halothane analyzer was used to measure the concentration of halothane vapor. Calculated partition coefficients ranged from 0.7 for water to 2.3 for blood and from 3.5 for human or bovine kidney to 6 for human whole brain or liver and 8 for human muscle. Human peritoneal fat had a value of 138. The human blood-gas partition coefficient of 2.3 as determined by this equilibration method was well below the previously published value of 3.6.

Competitive Interactions between Halothane and Isoflurane at the Carotid Body and TASK Channels.

BACKGROUND: The degree to which different volatile anesthetics depress carotid body hypoxic response relates to their ability to activate TASK potassium channels. Most commonly, volatile anesthetic pairs act additively at their molecular targets. We examined whether this applied to carotid body TASK channels. METHODS: We studied halothane and isoflurane effects on hypoxia-evoked rise in intracellular calcium (Ca2+i, using the indicator Indo-1) in isolated neonatal rat glomus cells, and TASK single-channel activity (patch clamping) in native glomus cells and HEK293 cell line cells transiently expressing TASK-1. RESULTS: Halothane (5%) depressed glomus cell Ca2+i hypoxic response (mean ± SD, 94 ± 4% depression; P < 0.001 vs. control). Isoflurane (5%) had a less pronounced effect (53 ± 10% depression; P < 0.001 vs. halothane). A mix of 3% isoflurane/1.5% halothane depressed cell Ca2+i response (51 ± 17% depression) to a lesser degree than 1.5% halothane alone (79 ± 15%; P = 0.001), but similar to 3% isoflurane alone (44 ± 22%; P = 0.224), indicating subadditivity. Halothane and isoflurane increased glomus cell TASK-1/TASK-3 activity, but mixes had a lesser effect than that seen with halothane alone: 4% halothane/4% isoflurane yielded channel open probabilities 127 ± 55% above control, versus 226 ± 12% for 4% halothane alone (P = 0.009). Finally, in HEK293 cell line cells, progressively adding isoflurane (1.5 to 5%) to halothane (2.5%) reduced TASK-1 channel activity from 120 ± 38% above control, to 88 ± 48% (P = 0.034). CONCLUSIONS: In all three experimental models, the effects of isoflurane and halothane combinations were quantitatively consistent with the modeling of weak and strong agonists competing at a common receptor on the TASK channel.

Reduction of halocarbons to hydrocarbons by NADH models and NADH.

The reduction of halocarbons by NADH models and NADH under ambient conditions is reported as a new type of reactivity pointing towards a hitherto unknown disruptive pathway for NADH/NADPH-dependent processes. The reaction was studied with the omnipresent pesticide DDT, the inhalation anesthetic halothane, and several simple halocarbons. The halide-hydride exchange represents a biochemical equivalent for the reduction of halocarbons by traditional synthetic reagents like silanes (R3Si-H) and stannanes (R3Sn-H). High precision thermochemical calculations (CBS-QB3) reveal the carbon-hydrogen bond dissociation energy of NADH (70.8 kcal·mol-1) to be lower than that of stannane (SnH4: 78.1 kcal·mol-1), approaching that of the elusive plumbane (PbH4: 68.9 kcal·mol-1). The ready synthetic accessibility of NADH models, their low carbon-hydrogen bond dissociation energy, and their dehalogenation activity in the presence of air and moisture recommend these compounds as substitutes for the air-sensitive or toxic metal hydrides currently employed in synthesis.

NMR structures of the human α7 nAChR transmembrane domain and associated anesthetic binding sites.

The α7 nicotinic acetylcholine receptor (nAChR), assembled as homomeric pentameric ligand-gated ion channels, is one of the most abundant nAChR subtypes in the brain. Despite its importance in memory, learning and cognition, no structure has been determined for the α7 nAChR TM domain, a target for allosteric modulators. Using solution state NMR, we determined the structure of the human α7 nAChR TM domain (PDB ID: 2MAW) and demonstrated that the α7 TM domain formed functional channels in Xenopus oocytes. We identified the associated binding sites for the anesthetics halothane and ketamine; the former cannot sensitively inhibit α7 function, but the latter can. The α7 TM domain folds into the expected four-helical bundle motif, but the intra-subunit cavity at the extracellular end of the α7 TM domain is smaller than the equivalent cavity in the α4ß2 nAChRs (PDB IDs: 2LLY; 2LM2). Neither drug binds to the extracellular end of the α7 TM domain, but two halothane molecules or one ketamine molecule binds to the intracellular end of the α7 TM domain. Halothane and ketamine binding sites are partially overlapped. Ketamine, but not halothane, perturbed the α7 channel-gate residue L9'. Furthermore, halothane did not induce profound dynamics changes in the α7 channel as observed in α4ß2. The study offers a novel high-resolution structure for the human α7 nAChR TM domain that is invaluable for developing α7-specific therapeutics. It also provides evidence to support the hypothesis: only when anesthetic binding perturbs the channel pore or alters the channel motion, can binding generate functional consequences.

Genetics of residual feed intake in growing pigs: Relationships with production traits, and nitrogen and phosphorus excretion traits.

Residual feed intake (RFI) is defined as the difference between the observed ADFI and the ADFI predicted from production and maintenance requirements. The objectives of this study were to evaluate RFI as a selection criterion to improve feed efficiency and its potential to reduce N and P excretion in 4 pig breeds. Data were collected between 2000 and 2009 in French central test stations for 2 dam breeds [French Landrace (LR) and Large White (LWD)], and 2 sire breeds [Large White (LWS) and Piétrain (PP)]. Numbers of recorded pigs were 6407, 10,694, 2342, and 2448 for the LR, LWD, LWS, and PP breeds, respectively. All PP animals were genotyped for the halothane mutation. This data set was used to calculate RFI equations for each of the 4 breeds, and to estimate genetic parameters for RFI together with growth, carcass, and meat quality traits, and N and P excretion during the test period (35 to 110 kg BW). The RFI explained 20.1% in PP, 26.5% in LWS, 27.6% in LWD, and 29.5% in LR of the phenotypic variability of ADFI. The PP breed differed from the others in this respect, probably due to a lower impact of the variation of body composition on ADFI. Heritability estimates of RFI ranged from 0.21 ± 0.03 (LWD) to 0.33 ± 0.06 (PP) depending on the breed. Heritabilities of N and P excretion traits ranged from 0.29 ± 0.06 to 0.40 ± 0.06. The RFI showed positive genetic correlations with feed conversion ratio (FCR) and excretion traits, these correlations being greater in the sire breeds (from 0.57 to 0.86) than in the dam breeds (from 0.38 to 0.53). Compared with FCR, RFI had weaker genetic correlations with carcass composition, growth rate, and excretion traits. Estimates of genetic correlations between FCR and excretion traits were very close to 1 for all breeds. Finally, excretion traits were, at the genetic level, correlated positively with ADFI, negatively with growth rate and carcass leanness, whereas the halothane n mutation in PP was shown to reduce N and P excretion levels. To conclude, new selection indexes including RFI can be envisaged to efficiently disentangle the responses to selection on growth rate and body composition from those on feed efficiency, with favorable impacts on N and P excretions, particularly in sire pig breeds. However, the switch from FCR to RFI in selection indexes should not resolve the genetic antagonism between feed efficiency and meat quality.

Design and biophysical characterization of a monomeric four-alpha-helix bundle protein Aα4 with affinity for the volatile anesthetic halothane.

A monomeric four-α-helix bundle protein Aα4 was designed as a step towards investigating the interaction of volatile general anesthetics with their putative membrane protein targets. The alpha helices, connected by glycine loops, have the sequence A, B, B', A'. The DNA sequence was designed to make the helices with the same amino acid sequences (helix A and A', B and B', respectively) as different as possible, while using codons which are favorable for expression in E. coli. The protein was bacterially expressed and purified to homogeneity using reversed-phase HPLC. Protein identity was verified using MALDI-TOF mass spectrometry. Far-UV circular dichroism spectroscopy confirmed the predominantly alpha-helical nature of the protein Aα4. Guanidinium chloride induced denaturation showed that the monomeric four-α-helix bundle protein Aα4 is considerably more stable compared to the dimeric di-α-helical protein (Aα2-L38M)2. The sigmoidal character of the unfolding reaction is conserved while the sharpness of the transition is increased 1.8-fold. The monomeric four-α-helix bundle protein Aα4 bound halothane with a dissociation constant (K(d)) of 0.93 ± 0.02mM, as shown by both tryptophan fluorescence quenching and isothermal titration calorimetry. This monomeric four-α-helix bundle protein can now be used as a scaffold to incorporate natural central nervous system membrane protein sequences in order to examine general anesthetic interactions with putative targets in detail.

Computational predictions of volatile anesthetic interactions with the microtubule cytoskeleton: implications for side effects of general anesthesia.

The cytoskeleton is essential to cell morphology, cargo trafficking, and cell division. As the neuronal cytoskeleton is extremely complex, it is no wonder that a startling number of neurodegenerative disorders (including but not limited to Alzheimer's disease, Parkinson's disease and Huntington's disease) share the common feature of a dysfunctional neuronal cytoskeleton. Recently, concern has been raised about a possible link between anesthesia, post-operative cognitive dysfunction, and the exacerbation of neurodegenerative disorders. Experimental investigations suggest that anesthetics bind to and affect cytoskeletal microtubules, and that anesthesia-related cognitive dysfunction involves microtubule instability, hyper-phosphorylation of the microtubule-associated protein tau, and tau separation from microtubules. However, exact mechanisms are yet to be identified. In this paper the interaction of anesthetics with the microtubule subunit protein tubulin is investigated using computer-modeling methods. Homology modeling, molecular dynamics simulations and surface geometry techniques were used to determine putative binding sites for volatile anesthetics on tubulin. This was followed by free energy based docking calculations for halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) on the tubulin body, and C-terminal regions for specific tubulin isotypes. Locations of the putative binding sites, halothane binding energies and the relation to cytoskeleton function are reported in this paper.

Trifluoroacetate is an allosteric modulator with selective actions at the glycine receptor.

Trifluoroacetic acid is a metabolite of the inhaled anesthetics halothane, desflurane and isoflurane as well as a major contaminant in HPLC-purified peptides. Ligand-gated ion channels, including cys-loop receptors such as the glycine receptor, have been the targets of peptide-based drug design and are considered to be likely candidates for mediating the effects of anesthetics in vivo, but the possible secondary contributions of contaminants and metabolites to these effects have not been studied. We used two-electrode voltage-clamp electrophysiology to test glycine, GABA(A) and 5-HT3 receptors expressed in Xenopus oocytes for their sensitivities to sodium trifluoroacetate. Trifluoroacetate (100 µM-3mM) enhanced the currents elicited by low concentrations of glycine applied to α1 homomeric and α1ß heteromeric glycine receptors, but it had no effects when co-applied with a maximally-effective glycine concentration. Trifluoroacetate had no effects on α1ß2γ2S GABA(A) or 5-HT3A receptors at any GABA or serotonin concentration tested. The results demonstrate that trifluoroacetate acts as an allosteric modulator at the glycine receptor with greater specificity than other known modulators. These results have important implications for both the secondary effects of volatile anesthetics and the presence of contaminating trifluoroacetate in HPLC-purified peptides, which is potentially an important source of experimental variability or error that requires control.

Formation/fate of reactive metabolites from general anesthetics and a comparison of toxic and non-toxic analogues: a DFT study.

Chloroform and Halothane are well known hepatotoxic anesthetics for which toxicity is attributed to their reactive metabolites. The molecular level details of reactions leading to the formation of reactive metabolites from chloroform and halothane have not been explored. Potential energy surface (PES) for the formation of phosgene (a toxic intermediate) from Chloroform has been studied using quantum chemical methods. The HOOH mediated reaction of chloroform to give phosgene has been found to be exothermic by 81.24 kcal/mol with a barrier of ~ 3 kcal/mol through the water catalyzed transition sate. The quantum chemical studies on the reactivity profile of phosgene indicated that urea derivatives need to be considered on the mechanism leading to toxicity. Similarly, metabolic pathways of Halothane oxidation have been studied. The C-H bond dissociation energies (BDE) and radical stabilization energies (RSE) for Chloroform and Halothane (< 95 kcal/mol and > 10 kcal/mol) were found to be significantly different for these toxic anesthetics in comparison to their safer analogues (> 100 kcal/mol and < 5 kcal/mol) respectively; thus these parameters can be employed to distinguish toxic and non-toxic general anesthetics. Enthalpy for the Cpd I, a widely used model for CYP450 enzymes, mediated reactions also agreed well with these results.

Anesthetic binding in a pentameric ligand-gated ion channel: GLIC.

Cys-loop receptors are molecular targets of general anesthetics, but the knowledge of anesthetic binding to these proteins remains limited. Here we investigate anesthetic binding to the bacterial Gloeobacter violaceus pentameric ligand-gated ion channel (GLIC), a structural homolog of cys-loop receptors, using an experimental and computational hybrid approach. Tryptophan fluorescence quenching experiments showed halothane and thiopental binding at three tryptophan-associated sites in the extracellular (EC) domain, transmembrane (TM) domain, and EC-TM interface of GLIC. An additional binding site at the EC-TM interface was predicted by docking analysis and validated by quenching experiments on the N200W GLIC mutant. The binding affinities (K(D)) of 2.3 ± 0.1 mM and 0.10 ± 0.01 mM were derived from the fluorescence quenching data of halothane and thiopental, respectively. Docking these anesthetics to the original GLIC crystal structure and the structures relaxed by molecular dynamics simulations revealed intrasubunit sites for most halothane binding and intersubunit sites for thiopental binding. Tryptophans were within reach of both intra- and intersubunit binding sites. Multiple molecular dynamics simulations on GLIC in the presence of halothane at different sites suggested that anesthetic binding at the EC-TM interface disrupted the critical interactions for channel gating, altered motion of the TM23 linker, and destabilized the open-channel conformation that can lead to inhibition of GLIC channel current. The study has not only provided insights into anesthetic binding in GLIC, but also demonstrated a successful fusion of experiments and computations for understanding anesthetic actions in complex proteins.

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.

Effect of PSD-95/SAP90 and/or PSD-93/chapsyn-110 deficiency on the minimum alveolar anesthetic concentration of halothane in mice.

BACKGROUND: The authors have previously shown that the clinically relevant concentrations of inhalational anesthetics dose-dependently inhibit the postsynaptic density protein (PSD)-95, Dlg, and ZO-1 domain-mediated protein interactions between N-methyl-d-aspartate receptors and PSD-95/synaptic-associated protein (SAP) 90 or PSD-93/Chapsyn-110 and that the knockdown of spinal PSD-95/SAP90 significantly reduces the minimum alveolar concentration (MAC) for isoflurane in rats. METHODS: The authors designed antisense oligodeoxynucleotides based on the mouse PSD-95/SAP90 and PSD-93/Chapsyn-110 messenger RNAs that correspond to their PSD-95, Dlg, and ZO-1 domain nucleotides and can specifically knock down the respective proteins. The authors intrathecally injected antisense oligodeoxynucleotides into wild-type and PSD-93/Chapsyn-110 knockout mice to investigate the effect of PSD-95/SAP90 and/or PSD-93/Chapsyn-110 deficiency on halothane anesthesia. RESULTS: Both PSD-95/SAP90 and PSD-93/Chapsyn-110 antisense oligodeoxynucleotides caused a dose-dependent and significant reduction in the MAC of halothane in wild-type mice. The intrathecal injection of PSD-95/SAP90 antisense oligodeoxynucleotide at different doses (25 and 50 microg) reduced halothane MAC by 40 and 55%, respectively, and intrathecal injection of PSD-93/Chapsyn-110 antisense oligodeoxynucleotide at different doses (12 and 24 microg) reduced halothane MAC by 25 and 53%, respectively. The PSD-95/SAP90 antisense oligodeoxynucleotide showed similar effect on halothane MAC in wild-type and PSD-93/Chapsyn-110 knockout mice, suggesting that the combination of PSD-95/SAP90 knockdown with PSD-93/Chapsyn-110 deletion did not have an additive effect on halothane anesthesia. CONCLUSIONS: The current results indicate that PSD-95/SAP90 and PSD-93/Chapsyn-110 are involved in the molecular mechanisms of halothane anesthesia and that the functional role of PSD-95/SAP90 in halothane anesthesia is not enhanced after PSD-93/Chapsyn-110 deletion.

NMR investigations of amyloid-ß peptide interactions with propofol at clinically relevant concentrations with and without aqueous halothane solution.

Oligomerization of amyloid-ß peptide (Aß) is an important stage in Alzheimer's disease. Recently, it has been shown that in an experimental model, smaller sized anesthetics (e.g., isoflurane, desflurane, etc.) induce Aß oligomerization. Using state-of-the-art solution nuclear magnetic resonance, spectroscopic studies on Aß interaction with propofol indicated that propofol does not interact with the G29, A30, and I31 residues of Aß peptide at a clinically relevant concentration (0.083 mM), and no Aß oligomerization was observed after 69 days. However, Aß oligomerization was observed when treated with propofol (clinically relevant concentration) coadministered with aqueous halothane solution. Furthermore, dose dependence studies at various propofol concentrations (0.32 mM, 2.07 mM, and 53.4 mM) indicate the effect of propofol concentration on Aß oligomerization revealing the hydrophobic nature of interactions between propofol with these critical residues (G29, A30, and I31). These experimental findings reaffirm that smaller molecular sized anesthetics (e.g., halothane) do play a leading role in Aß oligomerization.

Higher susceptibility to halothane modulation in open- than in closed-channel alpha4beta2 nAChR revealed by molecular dynamics simulations.

The neuronal alpha4beta2 nicotinic acetylcholine receptor (nAChR) is a potential molecular target for general anesthetics. It is unclear, however, whether anesthetic action produces the same effect on the open and closed channels. Computations parallel to our previous open channel study (J. Phys. Chem. B 2009, 113, 12581) were performed on the closed-channel alpha4beta2 nAChR to investigate the conformation-dependent anesthetic effects on channel structures and dynamics. Flexible ligand docking and over 20 ns molecular dynamics simulations revealed similar halothane-binding sites in the closed and open channels. The sites with relatively high binding affinities (approximately -6.0 kcal/mol) were identified at the interface of extracellular (EC) and transmembrane (TM) domains or at the interface between alpha4 and beta2 subunits. Despite similar sites for halothane binding, the closed-channel conformation showed much less sensitivity than the open channel to the structural and dynamical perturbations from halothane. Compared to the systems without anesthetics, the amount of water inside the pore decreased by 22% in the presence of halothane in the open channel but only by 6% in the closed channel. Comparison of the nonbonded interactions at the EC/TM interfaces suggested that the beta2 subunits were more prone than the alpha4 subunits to halothane binding. In addition, our data support the notion that halothane exerts its effect by disturbing the quaternary structure and dynamics of the channel. The study concludes that sensitivity and global dynamics responsiveness of alpha4beta2 nAChR to halothane are conformation dependent. The effect of halothane on the global dynamics of the open-channel conformation might also account for the action of other inhaled general anesthetics.

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.

Mechanism of interaction between the general anesthetic halothane and a model ion channel protein, I: Structural investigations via X-ray reflectivity from Langmuir monolayers.

We previously reported the synthesis and structural characterization of a model membrane protein comprised of an amphiphilic 4-helix bundle peptide with a hydrophobic domain based on a synthetic ion channel and a hydrophilic domain with designed cavities for binding the general anesthetic halothane. In this work, we synthesized an improved version of this halothane-binding amphiphilic peptide with only a single cavity and an otherwise identical control peptide with no such cavity, and applied x-ray reflectivity to monolayers of these peptides to probe the distribution of halothane along the length of the core of the 4-helix bundle as a function of the concentration of halothane. At the moderate concentrations achieved in this study, approximately three molecules of halothane were found to be localized within a broad symmetric unimodal distribution centered about the designed cavity. At the lowest concentration achieved, of approximately one molecule per bundle, the halothane distribution became narrower and more peaked due to a component of approximately 19A width centered about the designed cavity. At higher concentrations, approximately six to seven molecules were found to be uniformly distributed along the length of the bundle, corresponding to approximately one molecule per heptad. Monolayers of the control peptide showed only the latter behavior, namely a uniform distribution along the length of the bundle irrespective of the halothane concentration over this range. The results provide insight into the nature of such weak binding when the dissociation constant is in the mM regime, relevant for clinical applications of anesthesia. They also demonstrate the suitability of both the model system and the experimental technique for additional work on the mechanism of general anesthesia, some of it presented in the companion parts II and III under this title.