Interaction of local anesthetics with Torpedo californica membrane-bound acetylcholine receptor

Mechanisms Underlying the Strong Inhibition of Muscle-Type Nicotinic Receptors by Tetracaine

Nicotinic acetylcholine (ACh) receptors (nAChRs) are included among the targets of a variety of local anesthetics, although the molecular mechanisms of blockade are still poorly understood. Some local anesthetics, such as lidocaine, act on nAChRs by different means through their ability to present as both charged and uncharged molecules. Thus, we explored the mechanisms of nAChR blockade by tetracaine, which at physiological pH is almost exclusively present as a positively charged local anesthetic. The nAChRs from Torpedo electroplaques were transplanted to Xenopus oocytes and the currents elicited by ACh (I_ACh s), either alone or co-applied with tetracaine, were recorded. Tetracaine reversibly blocked I_ACh , with an IC₅₀ (i.e., the concentration required to inhibit half the maximum I_ACh ) in the submicromolar range. Notably, at very low concentrations (0.1 μM), tetracaine reduced I_ACh in a voltage-dependent manner, the more negative potentials produced greater inhibition, indicating open-channel blockade. When the tetracaine concentration was increased to 0.7 μM or above, voltage-independent inhibition was also observed, indicating closed-channel blockade. The I_ACh inhibition by pre-application of just 0.7 μM tetracaine before superfusion of ACh also corroborated the notion of tetracaine blockade of resting nAChRs. Furthermore, tetracaine markedly increased nAChR desensitization, mainly at concentrations equal or higher than 0.5 μM. Interestingly, tetracaine did not modify desensitization when its binding within the channel pore was prevented by holding the membrane at positive potentials. Tetracaine-nAChR interactions were assessed by virtual docking assays, using nAChR models in the closed and open states. These assays revealed that tetracaine binds at different sites of the nAChR located at the extracellular and transmembrane domains, in both open and closed conformations. Extracellular binding sites seem to be associated with closed-channel blockade; whereas two sites within the pore, with different affinities for tetracaine, contribute to open-channel blockade and the enhancement of desensitization, respectively. These results demonstrate a concentration-dependent heterogeneity of tetracaine actions on nAChRs, and contribute to a better understanding of the complex modulation of muscle-type nAChRs by local anesthetics. Furthermore, the combination of functional and virtual assays to decipher nAChR-tetracaine interactions has allowed us to tentatively assign the main nAChR residues involved in these modulating actions.

Probing pore constriction in a ligand-gated ion channel by trapping a metal ion in the pore upon agonist dissociation

Eukaryotic pentameric ligand-gated ion channels (pLGICs) are receptors activated by neurotransmitters to rapidly transport ions across cell membranes, down their electrochemical gradients. Recent crystal structures of two prokaryotic pLGICs were interpreted to imply that the extracellular side of the transmembrane pore constricts to close the channel (Hilf, R. J., and Dutzler, R. (2009) Nature 457, 115-118; Bocquet, N., Nury, H., Baaden, M., Le Poupon, C., Changeux, J. P., Delarue, M., and Corringer, P. J. (2009) Nature 457, 111-114). Here, we utilized a eukaryotic acetylcholine (ACh)-serotonin chimeric pLGIC that was engineered with histidines to coordinate a metal ion within the channel pore, at its cytoplasmic side. In a previous study, the access of Zn(2+) ions to the engineered histidines had been explored when the channel was either at rest (closed) or active (open) (Paas, Y., Gibor, G., Grailhe, R., Savatier-Duclert, N., Dufresne, V., Sunesen, M., de Carvalho, L. P., Changeux, J. P., and Attali, B. (2005) Proc. Natl. Acad. Sci. U.S.A. 102, 15877-15882). In this study, the interactions of Zn(2+) with the pore were probed upon agonist (ACh) dissociation that triggers the transition of the receptor from the active conformation to the resting conformation (i.e. during deactivation). Application of Zn(2+) onto ACh-bound open receptors obstructed their pore and prevented ionic flow. Removing ACh from its extracellular binding sites to trigger deactivation while Zn(2+) is still bound led to tight trapping of Zn(2+) within the pore. Together with single-channel recordings, made to explore single pore-blocking events, we show that dissociation of ACh causes the gate to shut on a Zn(2+) ion that effectively acts as a "foot in the door." We infer that, upon deactivation, the cytoplasmic side of the pore of the ACh-serotonin receptor chimera constricts to close the channel.

Dynamic Structural Investigations on the Torpedo Nicotinic Acetylcholine Receptor by Time-Resolved Photoaffinity Labeling

An increasing number of high-resolution structures of membrane-embedded ion channels (or soluble homologues) have emerged during the last couple of years. The most pressing need now is to understand the complex mechanism underlying ion-channel function. Time-resolved photoaffinity labeling is a suitable tool for investigating the molecular function of membrane proteins, especially when high-resolution structures of related proteins are available. However until now this methodology has only been used on the Torpedo nicotinic acetylcholine receptor (nAChR). nAChRs are allosteric cation-selective receptor channels that are activated by the neurotransmitter acetylcholine (ACh) and implicated in numerous physiological and pathological processes. Time-resolved photoaffinity labeling has already enabled local motions of nAChR subdomains (i.e. agonist binding sites, ion channel, subunit interface) to be understood at the molecular level, and has helped to explain how small molecules can exert their physiological effect, an important step toward the development of drug design. Recent analytical and technical improvements should allow the application of this powerful methodology to other membrane proteins in the near future.

The tertiary amine local anesthetic dibucaine binds to the membrane skeletal protein spectrin

The quinoline-based tertiary amine dibucaine has been shown to bind the membrane skeletal protein spectrin with a dissociation constant of 3.5x10(-5) M at 25 degrees C. Such binding is detected by monitoring the quenching of the tryptophan fluorescence intensity with increasing concentrations of dibucaine only and not with the benzene-based local anesthetics procaine, tetracaine and lidocaine. Binding of dibucaine also indicated changes in the tertiary structure of spectrin indicated by a circular dichroism spectrum in the near-UV region due to absorption of the aromatic side chains. The thermodynamic parameters associated with the binding indicated the interaction of dibucaine and spectrin to be enthalpy-driven and insensitive to an increase in the ionic strength of the buffer.

Effect of cholesterol on interaction of dibucaine with phospholipid vesicles: a fluorescence study

Interaction of the local anesthetic dibucaine with small unilamellar vesicles of dimyristoylphosphatidylcholine (DMPC) and dioleoyl phosphatidylcholine (DOPC) containing different mol percents of cholesterol has been studied by fluorescence spectroscopy. Fluorescence measurements on dibucaine in presence of phospholipid vesicles containing various amounts of cholesterol yielded a pattern of variation of wavelength at emission maximum and steady-state anisotropy which indicated that the microenvironment of dibucaine is more polar and flexible in membranes that contain cholesterol than in membranes without cholesterol. Experiments on quenching of fluorescence from membrane-associated dibucaine by potassium iodide showed a marked increase in quenching efficiency as the cholesterol content of the vesicles was increased, demonstrating increased accessibility of the iodide quenchers to dibucaine in the presence of cholesterol, when compared to that in its absence. Total emission intensity decay profiles of dibucaine yielded two lifetime components of approximately 1 ns and approximately 2.8--3.1 ns with mean relative contributions of approximately 25 and approximately 75%, respectively. The mean lifetime in vesicles was 20--30% smaller than in the aqueous medium and showed a moderate variation with cholesterol content. Fluorescence measurements at two different temperatures in DMPC SUVs, one at 33 degrees C, above the phase transition temperature and another at 25 degrees C, around the main phase transition, indicated two different mode of dibucaine localization. At 25 degrees C dibucaine partitioned differentially in presence and absence of cholesterol. However, at 33 degrees C the apparent partition coefficients remained unaltered indicating differences in the microenvironment of dibucaine in presence and absence of cholesterol in the phospholipid membranes.

A conformational intermediate between the resting and desensitized states of the nicotinic acetylcholine receptor

The structural changes induced in the nicotinic acetylcholine receptor by two noncompetitive channel blockers, proadifen and phencyclidine, have been studied by infrared difference spectroscopy and using the conformationally sensitive photoreactive noncompetitive antagonist 3-(trifluoromethyl)-3-m-([(125)I]iodophenyl)diazirine. Simultaneous binding of proadifen to both the ion channel pore and neurotransmitter sites leads to the loss of positive markers near 1663, 1655, 1547, 1430, and 1059 cm(-)(1) in carbamylcholine difference spectra, suggesting the stabilization of a desensitized conformation. In contrast, only the positive markers near 1663 and 1059 cm(-)(1) are maximally affected by the binding of either blocker to the ion channel pore suggesting that the conformationally sensitive residues vibrating at these two frequencies are stabilized in a desensitized-like conformation, whereas those vibrating near 1655 and 1430 cm(-)(1) remain in a resting-like state. The vibrations at 1547 cm(-)(1) are coupled to those at both 1663 and 1655 cm(-)(1) and thus exhibit an intermediate pattern of band intensity change. The formation of a structural intermediate between the resting and desensitized states in the presence of phencyclidine is further supported by the pattern of 3-(trifluoromethyl)-3-m-([(125)I]iodophenyl)diazirine photoincorporation. In the presence of phencyclidine, the subunit labeling pattern is distinct from that observed in either the resting or desensitized conformations; specifically, there is a concentration-dependent increase in the extent of photoincorporation into the delta-subunit. Our data show that domains of the nicotinic acetylcholine receptor interconvert between the resting and desensitized states independently of each other and suggest a revised model of channel blocker action that involves both low and high affinity agonist binding conformational intermediates.

Thermal destabilization of transmembrane proteins by local anaesthetics

Local anaesthetics, in addition to anaesthesia, induce the synthesis of heat shock proteins (HSPs), sensitize cells to hyperthermia, and increase the aggregation of nuclear proteins during heat shock. Anaesthetics are membrane active agents, and anaesthesia appears to be due to altered ion channel activity; however, the direct effect of heat shock is protein denaturation. These observations suggest that local anaesthetics may sensitize cells to hyperthermia by interacting with and destabilizing membrane proteins such that protein denaturation is increased. It is shown, using differential scanning calorimetry (DSC), that the local anaesthetics procaine, lidocaine, tetracaine and dibucaine destabilize the transmembrane domains of the Ca2+ -ATPase of sarcoplasmic reticulum and the band III anion transporter of red blood cells. The transmembrane domain of the Ca2+ -ATPase has a transition temperature (Tm) of denaturation of 61 degrees C which is decreased, for example, to 53 degrees C by 15 mM lidocaine. The degree of destabilization (deltaTm) by each anaesthetic is proportional to the lipid to water partition coefficient, and the increased sensitization by anaesthetics with larger partition coefficients and at higher pH suggests that the uncharged forms of the anaesthetics are responsible for destabilization. A Hill analysis of deltaTm for the Ca2+ -ATPase as a function of the concentration of anaesthetic in water gives dissociation constants (Kd) on the order of 10(-4) M, if binding occurs directly from the aqueous phase. This demonstrates moderate affinity binding. However, dissociation constants of 1-3 M are obtained, if binding occurs through the lipid phase, which demonstrates low affinity binding. Thus, the interaction of local anaesthetics with the Ca2+ -ATPase may be moderately specific or non-specific depending on the mechanism of interaction. The observation that local anaesthetics also destabilize the transmembrane domain of the band III protein of erythrocytes suggests that destabilization of transmembrane proteins is a general property of anaesthetics, which is at least in part a mechanism of sensitization to hyperthermia.

Role of local anesthetics on both cholinergic and serotonergic ionotropic receptors

A great body of experimental evidence indicates that the main target for the pharmacological action of local anesthetics (LAs) is the voltage-gated Na+ channel. However, the epidural and spinal anesthesia as well as the behavioral effects of LAs cannot be explained exclusively by its inhibitory effect on the voltage-gated Na+ channel. Thus, the involvement of other ion channel receptors has been suggested. Particularly, two members of the neurotransmitter-gated ion channel receptor superfamily, the nicotinic acetylcholine receptor (AChR) and the 5-hydroxytryptamine receptor (5-HT3R type). In this regard, the aim of this review is to explain and delineate the mechanism by which LAs inhibit both ionotropic receptors from peripheral and central nervous systems. Local anesthetics inhibit the ion channel activity of both muscle- and neuronal-type AChRs in a noncompetitive fashion. Additionally, LAs inhibit the 5-HT3R by competing with the serotonergic agonist binding sites. The noncompetitive inhibitory action of LAs on the AChR is ascribed to two possible blocking mechanisms. An open-channel-blocking mechanism where the drug binds to the open channel and/or an allosteric mechanism where LAs bind to closed channels. The open-channel-blocking mechanism is in accord with the existence of high-affinity LA binding sites located in the ion channel. The allosteric mechanism seems to be physiologically more relevant than the open-channel-blocking mechanism. The inhibitory property of LAs is also elicited by binding to several low-affinity sites positioned at the lipid-AChR interface. However, there is no clearcut evidence indicating whether these sites are located at either the annular or the nonannular lipid domain. Both tertiary (protonated) and quaternary LAs gain the interior of the channel through the hydrophilic pathway formed by the extracellular ion channel's mouth with the concomitant ion flux blockade. Nevertheless, an alternative mode of action is proposed for both deprotonated tertiary and permanently-uncharged LAs: they may pass from the lipid membrane core to the lumen of the ion channel through a hydrophobic pathway. Perhaps this hydrophobic pathway is structurally related to the nonannular lipid domain. Regarding the LA binding site location on the 5-HT3R, at least two amino acids have been involved. Glutamic acid at position 106 which is located in a residue sequence homologous to loop A from the principal component of the binding site for cholinergic agonists and competitive antagonists, and Trp67 which is positioned in a stretch of amino acids homologous to loop F from the complementary component of the cholinergic ligand binding site.

Photoaffinity labeling the torpedo nicotinic acetylcholine receptor with [(3)H]tetracaine, a nondesensitizing noncompetitive antagonist

Tetracaine (N,N-dimethylaminoethyl-4-butylaminobenzoate) and related N,N-dialkylaminoethyl substituted benzoic acid esters have been used to characterize the high-affinity binding site for aromatic amine noncompetitive antagonists in the Torpedo nicotinic acetylcholine receptor (nAChR). [(3)H]Tetracaine binds at equilibrium to a single site with a K(eq) value of 0.5 microM in the absence of agonist or presence of alpha-bungarotoxin and with a K(eq) value of 30 microM in the presence of agonist (i.e., for nAChR in the desensitized state). Preferential binding to nAChR in the absence of agonist is also seen for N,N-DEAE and N,N-diethylaminopropyl esters, both binding with 10-fold higher affinity in the absence of agonist than in the presence, and for the 4-ethoxybenzoic acid ester of N, N-diethylaminoethanol, but not for the 4-amino benzoate ester (procaine). Irradiation at 302 nm of nAChR-rich membranes equilibrated with [(3)H]tetracaine resulted in covalent incorporation with similar efficiency into nAChR alpha, beta, gamma, and delta subunits. The pharmacological specificity of nAChR subunit photolabeling as well as its dependence on [(3)H]tetracaine concentration establish that the observed photolabeling is at the high-affinity [(3)H]tetracaine-binding site. Within alpha subunit, >/=95% of specific photolabeling was contained within a 20-kilodalton proteolytic fragment beginning at Ser(173) that contains the M1 to M3 hydrophobic segments. With all four subunits contributing to [(3)H]tetracaine site, the site in the closed channel state of the nAChR is most likely within the central ion channel domain.

Identification of amino acids of the torpedo nicotinic acetylcholine receptor contributing to the binding site for the noncompetitive antagonist [(3)H]tetracaine

[(3)H]Tetracaine is a noncompetitive antagonist of the Torpedo nicotinic acetylcholine receptor (nAChR) that binds with high affinity in the absence of cholinergic agonist (K(eq) = 0.5 microM) and weakly (K(eq) = 30 microM) in the presence of agonist (i.e., to nAChR in the desensitized state). In the absence of agonist, irradiation at 302 nm of nAChR-rich membranes equilibrated with [(3)H]tetracaine results in specific photoincorporation of [(3)H]tetracaine into each nAChR subunit. In this report, we identify the amino acids of each nAChR subunit specifically photolabeled by [(3)H]tetracaine that contribute to the high-affinity binding site. Subunits isolated from nAChR-rich membranes photolabeled with [(3)H]tetracaine were subjected to enzymatic digestion, and peptides containing (3)H were purified by SDS-polyacrylamide gel electrophoresis followed by reversed phase HPLC. N-terminal sequence analysis of the isolated peptides demonstrated that [(3)H]tetracaine specifically labeled two sets of homologous hydrophobic residues (alphaLeu(251), betaLeu(257), gammaLeu(260), and deltaLeu(265); alphaVal(255) and deltaVal(269)) as well as alphaIle(247) and deltaAla(268) within the M2 hydrophobic segments of each subunit. The labeling of these residues establishes that the high-affinity [(3)H]tetracaine-binding site is located within the lumen of the closed ion channel and provides a definition of the surface of the M2 helices facing the channel lumen.

A structure-based approach to nicotinic receptor pharmacology

Infrared difference spectroscopy has been used to examine the structural effects of local anesthetic (LA) binding to the nicotinic acetylcholine receptor (nAChR). Several LAs induce subtle changes in the vibrational spectrum of the nAChR over a range of concentrations consistent with their reported nAChR-binding affinities. At concentrations of the desensitizing LAs prilocaine and lidocaine consistent with their binding to the ion channel pore, the vibrational changes suggest the stabilization of an intermediate conformation that shares structural features in common with both the resting and desensitized states. Higher concentrations of prilocaine and lidocaine, as well as the LA dibucaine, lead to additional binding to the neurotransmitter-binding site, the formation of physical interactions (most notably cation-tyrosine interactions) between LAs and neurotransmitter-binding-site residues, and the subsequent formation of a presumed desensitized nAChR. Although concentrations of the LA tetracaine consistent with binding to the ion channel pore elicit a reversed pattern of spectral changes suggestive of a resting state-like nAChR, higher concentrations also lead to neurotransmitter site binding and desensitization. Our results suggest that LAs stabilize multiple conformations of the nAChR by binding to at least two conformationally sensitive LA-binding sites. The spectra also reveal subtle differences in the strengths of the physical interactions that occur between LAs and binding-site residues. These differences correlate with LA potency at the nAChR.

Anesthetic-induced structural changes in the nicotinic acetylcholine receptor

The difference between infrared spectra of the nicotinic acetylcholine receptor (nAChR) recorded in the absence and presence of the agonist carbamylcholine (Carb) reveals a complex pattern of positive and negative bands that provides a spectral map of Carb-induced structural change. This spectral map is affected by the presence of either the local anesthetic, dibucaine, or the short chain alcohol, propanol. Both antagonists alter the intensities of difference bands in a manner consistent with the stabilization of a desensitized state. Spectral variations are also observed that are indicative of both the displacement of the anesthetics from the nAChR upon the addition of Carb and physical interactions that occur between the anesthetics and binding site residues.

Topology of ligand binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of both alpha subunits there exist the binding sites for agonists such as the neurotransmitter acetylcholine (ACh) and for competitive antagonists such as d-tubocurarine. Agonists trigger the channel opening upon binding while competitive antagonists compete for the former ones and inhibit its pharmacological action. Identification of all residues involved in recognition and binding of agonist and competitive antagonists is a primary objective in order to understand which structural components are related to the physiological function of the AChR. The picture for the localisation of the agonist/competitive antagonist binding sites is now clearer in the light of newer and better experimental evidence. These sites are mainly located on both alpha subunits in a pocket approximately 30-35 A above the surface membrane. Since both alpha subunits are sequentially identical, the observed high and low affinity for agonists on the receptor is conditioned by the interaction of the alpha subunit with the delta or the gamma chain, respectively. This relationship is opposite for curare-related drugs. This molecular interaction takes place probably at the interface formed by the different subunits. The principal component for the agonist/competitive antagonist binding sites involves several aromatic residues, in addition to the cysteine pair at 192-193, in three loops-forming binding domains (loops A-C). Other residues such as the negatively changed aspartates and glutamates (loop D), Thr or Tyr (loop E), and Trp (loop F) from non-alpha subunits were also found to form the complementary component of the agonist/competitive antagonist binding sites. Neurotoxins such as alpha-, kappa-bungarotoxin and several alpha-conotoxins seem to partially overlap with the agonist/competitive antagonist binding sites at multiple point of contacts. The alpha subunits also carry the binding site for certain acetylcholinesterase inhibitors such as eserine and for the neurotransmitter 5-hydroxytryptamine which activate the receptor without interacting with the classical agonist binding sites. The link between specific subunits by means of the binding of ACh molecules might play a pivotal role in the relative shift among receptor subunits. This conformational change would allow for the opening of the intrinsic receptor cation channel transducting the external chemical signal elicited by the agonist into membrane depolarisation. The ion flux activity can be inhibited by non-competitive inhibitors (NCIs). For this kind of drugs, a population of low-affinity binding sites has been found at the lipid-protein interface of the AChR. In addition, several high-affinity binding sites have been found to be located at different rings on the M2 transmembrane domain, namely luminal binding sites. In this regard, the serine ring is the locus for exogenous NCIs such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222, phencyclidine, and trifluoromethyliodophenyldiazirine. Trifluoromethyliodophenyldiazirine also binds to the valine ring, which is the postulated site for cembranoids. Additionally, the local anaesthetic meproadifen binding site seems to be located at the outer or extracellular ring. Interestingly, the M2 domain is also the locus for endogenous NCIs such as the neuropeptide substance P and the neurotransmitter 5-hydroxytryptamine. In contrast with this fact, experimental evidence supports the hypothesis for the existence of other NCI high-affinity binding sites located not at the channel lumen but at non-luminal binding domains. (ABSTRACT TRUNCATED)

Structural effects of neutral and anionic lipids on the nicotinic acetylcholine receptor. An infrared difference spectroscopy study

The effects of both neutral and anionic lipids on the structure of the nicotinic acetylcholine receptor (nAChR) have been probed using infrared difference spectroscopy. The difference between infrared spectra of the nAChR recorded using the attenuated total reflectance technique in the presence and absence of the neurotransmitter analog, carbamylcholine, exhibits a complex pattern of positive and negative bands that provides a spectral map of the structural changes that occur in the nAChR upon ligand binding and subsequent desensitization. This spectral map is essentially identical in difference spectra recorded from native, native alkaline-extracted, and affinity-purified nAChR reconstituted into either soybean asolectin or egg phosphatidylcholine membranes containing both neutral and anionic lipids. This result suggests both a similar structure of the nAChR and a similar resting to desensitized conformational change in each membrane environment. In contrast, difference spectra recorded from the nAChR reconstituted into egg phosphatidylcholine membranes lacking neutral and/or anionic lipids all exhibit an essentially identical pattern of band intensity variations, which is similar to the pattern of variations observed in difference spectra recorded in the continuous presence of the desensitizing local anesthetic, dibucaine. The difference spectra suggest that the main effect of both neutral and anionic lipids in a reconstituted egg phosphatidylcholine membrane is to help stabilize the nAChR in a resting conformation. In the absence of neutral and/or anionic lipids, the nAChR is converted into an alternate conformation that appears to be analogous to the local anesthetic-induced desensitized state. Significantly, the proportion of receptors found in the resting versus the putative desensitized state appears to be dependent upon the final lipid composition of the reconstituted membrane. A lipid-dependent modulation of the equilibrium between a channel-active resting and channel-inactive desensitized state may account for the modulations of nAChR activity that are observed in different lipid membranes.

Luminal and non-luminal non-competitive inhibitor binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of the alpha subunit exist the binding sites for agonists such as the neurotransmitter acetylcholine, which upon binding trigger the channel opening, and for competitive antagonists such as d-tubocurarine, which compete for the former inhibiting its pharmacological action. For non-competitive inhibitors, a population of low-affinity binding sites have been found at the lipid-protein interface of the nicotinic acetylcholine receptor. In addition, at the M2 transmembrane domain, several high-affinity binding sites have been found for non-competitive inhibitors such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222 and the hydrophobic probe trifluoromethyl-iodophenyldiazirine. They are known as luminal binding sites. Although the local anaesthetic meproadifen seems to be located between the hydrophobic domains M2-M3, this locus is considered to form part of the channel mouth, thus this site can also be called a luminal binding site. In contraposition, experimental evidences support the hypothesis of the existence of other high-affinity binding sites for non-competitive inhibitors located not at the channel lumen, but at non-luminal binding domains. Among them, we can quote the binding site for quinacrine, which is located at the lipid-protein interface of the alpha M1 domain, and the binding site for ethidium, which is believed to interact with the wall of the vestibule very far away from both the lumen channel and the lipid membrane surface. The aim of this review is to discuss these recent findings relative to both structurally and functionally relevant aspects of non-competitive inhibitors of the nicotinic acetylcholine receptor. We will put special emphasis on the description of the localization of molecules with non-competitive antagonist properties that bind with high-affinity to luminal and non-luminal domains. The information described herein was principally obtained by means of methods such as photolabelling and site-directed mutagenesis in combination with patch-clamp. Our laboratory has contributed with data obtained by using biophysical approaches such as paramagnetic electron spin resonance and quantitative fluorescence spectroscopy.

Interaction of dibucaine with the transmembrane domain of the Ca(2+)-ATPase of sarcoplasmic reticulum

The site of interaction of dibucaine with the Ca(2+)-ATPase of rabbit sarcoplasmic reticulum, an ion-transporting membrane protein, was investigated by determining the effect of dibucaine on the denaturation of the transmembrane domain and the aqueous domain containing, respectively, the high-affinity Ca2+ binding sites and the site of ATP hydrolysis. In the absence of Ca2+, a single irreversible denaturation transition with Tm approximately equal to 49 degrees C is observed for the Ca(2+)-ATPase by differential scanning calorimetry (DSC). In the presence of Ca2+, but not Mg2+, Sr2+, or Ba2+, a new high-temperature transition is observed that has been shown to be due to stabilization of the transmembrane region [Lepock, J. R., Rodahl, A. M., Zhang, C., Heynen, M. L., Waters, B., & Cheng, K. H. (1990) Biochemistry 29, 681-689]. The maximum stabilization corresponds to a shift in Tm of 13.8 degrees C, and Hill analysis indicates that the Ca2+ binding site yielding stabilization has a Kd = 2.5 x 10(-4) M with a cooperativity (n) of 1. Thus, stabilization is due to Ca2+ binding not to the high-affinity sites but to one of the previously observed sites of low or intermediate affinity, which must be located in the transmembrane or stalk subdomains. Dibucaine has little effect on the Tm of the aqueous domain, but it decreases the Tm of the transmembrane domain with Kd approximately equal to 4.1 x 10(-4) M and a cooperativity of approximately 1.6, implying that destabilization is due to the binding of dibucaine to sites of intermediate or moderately high affinity.(ABSTRACT TRUNCATED AT 250 WORDS)

Secondary structure of the nicotinic acetylcholine receptor: implications for structural models of a ligand-gated ion channel

The secondary structure and effects of two ligands, carbamylcholine and tetracaine, on the secondary structure of affinity-purified nicotinic acetylcholine receptor (nAChR) from Torpedo has been studied using Fourier transform infrared spectroscopy (FTIR). FTIR spectra of the nAChR were acquired in both 1H2O and 2H2O buffer and exhibit spectral features indicative of a substantial alpha-helical content with lesser amounts of beta-sheet and random coil structures. The resolution enhancement techniques of Fourier self-deconvolution and Fourier derivation reveal seven component bands contributing to both the amide I band and amide I' band contours in 1H2O and 2H2O, respectively. Curve-fitting estimates of the nAChR secondary structure are consistent with the qualitative analysis of the FTIR spectra as follows: 39% alpha-helix, 35% beta-sheet, 6% turn, and 20% random coil. Of particular interest is the estimated alpha-helical content as this value places restrictions on models of the nAChR transmembrane topology and on the types of secondary structures that may contribute to functional domains, such as the ligand-binding site. The estimated alpha-helical content is sufficient to account for four transmembrane alpha-helices in each nAChR subunit as well as a substantial portion of the extracellular and/or the cytoplasmic domains. FTIR spectra were also acquired in the presence and absence of 1 mM carbamylcholine and 5 mM tetracaine to examine the effects of ligand binding on the secondary structure of the nAChR. The similarity of the spectra, even after spectral deconvolution, indicates that the secondary structure of the nAChR is essentially unaffected by desensitization.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of drugs on the incorporation of a conformationally-sensitive, hydrophobic probe into the ion channel of the nicotinic acetylcholine receptor

The pattern of incorporation of the hydrophobic photolabel 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine([125I]TID) into the nicotinic acetylcholine receptor (AChR) is a sensitive measure of AChR conformation (resting state or desensitized). We determined the ability of tetracaine, dibucaine, procaine, lidocaine, chlorpromazine or phencyclidine to inhibit [125I]TID photolabeling of the AChR as a function of drug concentration, both as a measure of the ability of these drugs to desensitize the AChR, and to characterize the [125I]TID binding site. To localize the site(s) of drug action, experiments were performed in the absence and presence of saturating concentrations of alpha-bungarotoxin (BgTx), to block drug binding to the agonist binding site. On the basis of the concentration dependence of their effects, which was not altered by the presence of BgTx, tetracaine and dibucaine appeared to block [125I]TID incorporation competitively, suggesting that the high-affinity [125I]TID binding site is the non-competitive blocker binding site presumed to exist in the interior of the AChR ion channel. Procaine, chlorpromazine, lidocaine and phencyclidine blocked [125I]TID incorporation at lower concentrations in the absence of BgTx than in its presence, suggesting that these drugs block incorporation by inducing desensitization when bound to their high-affinity non-competitive blocker binding sites and that BgTx countered the drug effect by allosterically stabilizing the resting state.

Photoaffinity labeling of Torpedo acetylcholine receptor at multiple sites

The acetylcholine receptor from Torpedo californica electroplax was labeled with the photoaffinity reagent bis(3-azidopyridinium)decane perchlorate. All four receptor subunits (alpha, beta, gamma, and delta) were specifically labeled. In the presence of cholinergic agonists the gamma-, beta-, and delta-subunit labeling was decreased significantly, whereas labeling of the alpha subunit was minimally affected. Full occupancy of the two high-affinity sites involving the alpha subunits in the vicinity of alpha-Cys-192-Cys-193 by covalent reaction with bromoacetylcholine also caused a large decrease of gamma-subunit labeling by the photoaffinity reagent and lesser but significant decreases in beta- and delta-subunit labeling. No decrease in labeling of the alpha subunit was seen. Labeling of the alpha subunit could, however, be inhibited by high concentrations of the agonist carbamoylcholine. We conclude that the binding sites of high-affinity reside at interfaces of the alpha subunit and other subunits and that the alpha subunit also contributes to formation of a low-affinity site(s) for cholinergic compounds.

Conformations of dibucaine and tetracaine in small unilamellar phosphatidylcholine vesicles as studied by nuclear Overhauser effects in 1H nuclear magnetic resonance spectroscopy

Conformations of dibucaine and tetracaine in small unilamellar phosphatidylcholine vesicles have been investigated by nuclear Overhauser effects (NOEs) in 1H nuclear magnetic resonance spectroscopy. Two-dimensional NOE and chemical exchange correlated spectroscopy (NOESY) and rotating frame NOE spectroscopy (ROESY) methods have been applied for obtaining the NOEs. In the NOESY spectra, NOEs between protons within the drug were overwhelmed by spin diffusion even at a short mixing time. This observation reduced the usefulness of the NOESY method on the one hand, however, on the other hand it facilitated remarkably in revealing signals due to the drug, hidden in the broad resonances of the membranes. In the ROESY spectra, the spin diffusion phenomena were less effective; accordingly the conformations of the drugs interacting with membranes were determined by the ROESY method. The observed NOE data showed that dibucaine takes more than two conformations and that both dibucaine and tetracaine are present as a dimer in the membranes. Molecular dynamics calculations supported these findings.

Interaction of the local anesthetics dibucaine and tetracaine with sarcoplasmic reticulum membranes. Differential scanning calorimetry and fluorescence studies

The local anesthetics dibucaine and tetracaine inhibit the (Ca2+ + Mg2+)-ATPase from skeletal muscle sarcoplasmic reticulum [DeBoland, A. R., Jilka, R. L., & Martonosi, A. N. (1975) J. Biol. Chem. 250, 7501-7510; Suko, J., Winkler, F., Scharinger, B., & Hellmann, G. (1976) Biochim. Biophys. Acta 443, 571-586]. We have carried out differential scanning calorimetry and fluorescence measurements to study the interaction of these drugs with sarcoplasmic reticulum membranes and with purified (Ca2+ + Mg2+)-ATPase. The temperature range of denaturation of the (Ca2+ + Mg2+)-ATPase in the sarcoplasmic reticulum membrane, determined from our scanning calorimetry experiments, is ca. 45-55 degrees C and for the purified enzyme ca. 40-50 degrees C. Millimolar concentrations of dibucaine and tetracaine, and ethanol at concentrations higher than 1% v/v, lower a few degrees (degrees C) the denaturation temperature of the (Ca2+ + Mg2+)-ATPase. Other local anesthetics reported to have no effect on the ATPase activity, such as lidocaine and procaine, did not significantly alter the differential scanning calorimetry pattern of these membranes up to a concentration of 10 mM. The order parameter of the sarcoplasmic reticulum membranes, calculated from measurements of the polarization of the fluorescence of diphenylhexatriene, is not significantly altered at the local anesthetic concentrations that shift the denaturation temperature of the (Ca2+ + Mg2+)-ATPase.(ABSTRACT TRUNCATED AT 250 WORDS)

Procaine rapidly inactivates acetylcholine receptors from Torpedo and competes with agonist for inhibition sites

The relationship between the high-affinity procaine channel inhibition site (apparent dissociation constant Kp congruent to 200 microM) and the agonist self-inhibition site on acetylcholine receptors (AChRs) from Torpedo electroplaque was investigated by using rapid 86Rb+ quenched-flux assays at 4 degrees C in native AChR-rich vesicles on which 50-60% of ACh activation sites were blocked with alpha-bungarotoxin (alpha-BTX). In the presence of channel-activating acetylcholine (ACh) concentrations (10 microM-10 mM) alone, AChR undergoes one phase of inactivation (fast desensitization, rate = kd) in under a second. Addition of procaine produces two-phase inactivation similar to that seen with self-inhibiting (greater than 10 mM) ACh concentrations [Forman & Miller (1988) Biophys. J. 54, 149-158]--rapid inactivation (rate = kr) complete in 30-75 ms is followed by fast desensitization at the same kd observed without procaine. The dependence of kr on [procaine] is consistent with a bimolecular association between procaine and its AChR site with kon = 2.5 X 10(5) M-1 s-1, koff = 36 s-1, and Kp = 145 +/- 36 microM). Inhibition of AChR function by mixtures of procaine (up to 12Kp) plus self-inhibiting concentrations of ACh or suberyldicholine ([SubCh] up to 13 X the 50% self-inhibiting agonist concentration, KB) was studied by reducing the level of alpha-BTX block in vesicles. The apparent KB increased in the presence of procaine, and the apparent KP increased linearly with [SubCh], indicating mutually exclusive actions at a common AChR site. Our data support a mechanism where procaine binds preferentially to the open-channel AChR state, since no procaine-induced inactivation is observed without agonist and kr's dependence on [ACh] in the channel-activating range closely parallels that of 86Rb+ flux response to ACh.

Inhibition of human serum and rabbit muscle cholinesterase by local anesthetics

The effects of tertiary amine local anesthetics (procaine, mepivacaine, lidocaine, tetracaine, dibucaine, and bupivacaine) and chlorpromazine were investigated for rabbit muscle acetylcholinesterase and human serum cholinesterase. The muscle enzyme was poorly inhibited by local anesthetics containing an amide linkage. The serum cholinesterase was inhibited by all those compounds, their relative potencies being proportional to their octanol/water partition coefficients. The dissociation constants of tetracaine and procaine, ester anesthetics, were 1000-fold and 100-fold, respectively, that which would be expected from their partition coefficient basis respective to the other amide anesthetics. Procaine showed competitive inhibition of serum cholinesterase, whereas for most anesthetics a mixed type of inhibition was observed. Procaine probably binds at the main anionic site, while the other positively charged anesthetics bind to either the catalytic centre or to the peripheral or modulator anionic site, modifying the kinetic behaviour of cholinesterase as has been demonstrated by the appearance of negative cooperativity for binding to the substrate.

Rotifer neuropharmacology--II. Synergistic effect of acetylcholine on local anesthetic activity in Brachionus calyciflorus (Rotifera, Aschelminthes)

A number of compounds showing general anesthetic action in the rotifer Brachionus calyciflorus were investigated in the presence of acetylcholine. Non-ionizing anesthetics, including tricaine, showed no interaction with acetylcholine. However, highly ionized compounds like the local anesthetics procaine and lidocaine, the muscarinic blocker and local anesthetic atropine, and the beta-adrenergic blocker propranolol showed a synergistic effect with acetylcholine. ACh increased the general anesthetic effect of these compounds in a statistically highly significant dose-dependent fashion. To account for the mechanism of this unusual and novel effect it is proposed that these compounds interact with the anesthetic binding site of the rotifer cholinoceptor ionophore in the open state. It is also proposed that non-ionizing compounds have a general membrane effect only. In addition to anesthesia, atropine and propranolol cause foot paralysis in B. calyciflorus. This other novel effect is also enhanced by acetylcholine as well as decamethonium, a neuromuscular blocker.

Demonstration and affinity labeling of a stereoselective binding site for a benzomorphan opiate on acetylcholine receptor-rich membranes from Torpedo electroplaque

The interaction of an optically pure benzomorphan opiate, (-)-N-allyl-N-normetazocine [(-)-ANMC], with the nicotinic acetylcholine receptor from Torpedo electroplaque was studied by using radioligand binding and affinity labeling. The binding was complex with at least two specific components having equilibrium dissociation constants of 0.3 microM and 2 microM. The affinity of the higher affinity component was decreased by carbamoylcholine but not by alpha-bungarotoxin. The effect of carbamoylcholine was not blocked by alpha-bungarotoxin. In comparison, the affinity of [3H]phencyclidine, a well-characterized ligand for a high-affinity site for noncompetitive blockers on the acetylcholine receptor, is increased by carbamoylcholine and the increase is blocked by alpha-bungarotoxin. The binding of (-)-[3H]ANMC was inhibited by a number of other benzomorphans, with (-) isomers being 4- to 5-fold more potent than (+) isomers. Phencyclidine inhibits the binding of (-)-[3H]ANMC to its high-affinity site by a mechanism that is not competitive. UV-catalyzed affinity labeling indicated that the high-affinity-binding site for (-)-[3H]ANMC is at least partially associated with the delta subunit. Tryptic degradation of the Torpedo marmorata delta chain suggested that (-)-ANMC labeled a 16,000-dalton COOH-terminal portion of the subunit. In contrast, 5-azido-[3H]trimethisoquin, a photoaffinity label of the high-affinity site for noncompetitive blockers, labels a 47,000-dalton NH2-terminal fragment of the delta subunit. These results suggest that (-)-[3H]ANMC binds to sites completely distinct from the binding sites for acetylcholine. The high-affinity-binding site for (-)-ANMC and that for phencyclidine and 5-azidotrimethisoquin are allosterically coupled but are regulated differently and are probably physically distinct.

Inhibition of synaptosomal enzymes by local anesthetics

The effects of tertiary amine local anesthetics (procaine, lidocaine, tetracaine and dibucaine) and chlorpromazine were investigated for three enzyme activities associated with rat brain synaptosomal membranes, i.e., (Na+ + K+)-ATPase (ouabain-sensitive), Mg2+-ATPase (ouabain-insensitive) and acetylcholinesterase. Approximately the same concentrations of each agent gave 50% inhibition of both ATPases, for example 7.9 and 10 mM tetracaine for Mg2+-ATPase and (Na+ + K+)-ATPase, respectively; these concentrations are 10-fold higher than required for inhibition of mitochondrial F1-ATPase. The relative inhibitory potency of the several agents was proportional to their octanol/water partition coefficients. Acetylcholinesterase was inhibited by all agents tested, but the ester anesthetics (procaine and tetracaine) were considerably more potent than the others after correction for partition coefficient differences. For tetracaine, 0.18 mM gave 50% inhibition and showed competitive inhibition on a Lineweaver-Burk plot, but for dibucaine a mixed type of inhibition was observed, and 0.63 mM was required for 50% inhibition. Tetracaine evidently binds at the active site, and dibucaine at the peripheral or modulator site, on this enzyme.

Effects of amphipathic drugs on L'[3H]glutamate binding to synaptic membranes and the purified binding protein

Four amphipathic molecules with known local anesthetic activity, dibucaine, tetracaine, chlorpomazine, and quinacrine, inhibited the binding of L-[3H]glutamic acid to rat brain synaptic plasma membranes and to the purified glutamate binding protein. Neither haloperidol nor diphenylhydantoin had significant inhibitory effects on the glutamate binding activity of the membranes or of the purified protein. The amphipathic drugs apparently inhibited L-[3H]glutamate binding to synaptic membranes by a mixed type of inhibition. The inhibitory activity of quinacrine on glutamate binding to the synaptic membranes was greater in a low ionic strength, Ca2+-free buffer medium, than in a physiologic medium (Krebs-Henseleit buffer). Removal of Ca2+ from the Krebs solution enhanced quinacrine's inhibition of glutamate binding. Quinacrine up to 1 mM concentration did not inhibit the high affinity Na+-dependent L-glutamate transport in these membrane preparations. The importance of Ca2+ in the expression of quinacrine's effects on the glutamate binding activity of synaptic membranes and the observed tetracaine and chlorpromazine-induced increases in the transition temperature for the glutamate binding process of these membranes, were indicative of an interaction of the local anesthetics with the lipid environment of the glutamate binding sites.

Effects of local anesthetics and histrionicotoxin on the binding of carbamoylcholine to membrane-bound acetylcholine receptor

The effects of local anesthetics and perhydrohistrionicotoxin on the kinetic mechanism of carbamoylcholine binding to the membrane-bound acetylcholine receptor have been studied by stopped-flow methods. Receptor-enriched membrane fragments from Torpedo californica were reduced and then alkylated by 5-(iodoacetamido)salicylic acid, and the agonist binding kinetics were monitored by the fluorescence changes of this bound probe. The alkylation procedures did not alter the ability of the receptor to mediate agonist-induced cation flux. Preincubation of such modified receptor preparations with saturating concentrations of lidocaine, prilocaine, or dimethisoquin did not significantly affect the equilibrium dissociation constant for carbamoylcholine binding. The multiphasic kinetic signal which accompanies the binding of the agonist was, however, much simplified in the presence of local anesthetics, and the observed kinetics could be described by a mechanism in which a single conformational change follows the formation of the initial complex. Perhydrohistrionicotoxin did not act in the same way as the local anesthetics examined since saturating concentrations did not significantly perturb the agonist binding kinetics.

Multiple sites of inhibition of mitochondrial electron transport by local anesthetics

Local anesthetics and alcohols were found to inhibit mitochondrial electron transport at several points along the chain. THe anesthetics employed were the tertiary amines procaine, tetracaine, dibucaine, and chlorpromazine, and the alcohols were n-butamol, n-pentanol, n-hexanol, and benzyl alcohol. Uncoupled sonic submitochondrial particles from beef heart and rat liver were studied. We report the following: (1) All of the anesthetics were found to inhibit each of the segments of the electron transport chain assayed; these included cytochrome c oxidase, durohydroquinone oxidase, succinate oxidase, NADH oxidase, succinate dehydrogenase, succinate-cytochrome c oxidoreductase, and NADH-cytochrome c oxidoreductase. (2) NADH oxidase and NADH-cytochrome c oxidoreductase required the lowest concentration of anesthetic for inhibition, and cytochrome c oxidase required the highest concentrations. (3) We conclude that there are several points along the chain at which inhibition occurs, the most sensitive being in the region of Complex I (NADH dehydrogenase). (4) Beef heart submitochondrial particles are less sensitive to inhibition than are rat liver particles. (5) Low concentrations of several of the anesthetics gave enhancement of electron transport activity, whereas higher concentrations of the same agents caused inhibition. (6) The concentrations of anesthetics (alcohol and tertiary amine) which gave 50% inhibition of NADH oxidase were lower than the reported concentrations required for blockage of frog sciatic nerve.

Topographic studies of Torpedo acetylcholine receptor subunits as a transmembrane complex

The exposure of the four subunits of the acetylcholine receptor from Torpedo californica on both the extracellular and cytoplasmic faces of the postsynaptic membranes of the electroplaque cells has been investigated. Sealed membrane vesicles containing no protein components other than the receptor were isolated and were shown to have 95% of their synaptic surfaces facing the medium. The susceptibility of the four receptor subunits in these preparations to hydrolysis by trypsin both from the external and from the internal medium was used to investigate the exposure of the subunits on the synaptic and cytoplasmic surfaces of the membrane. It was shown by sodium dodecyl sulfate gel electrophoresis of the tryptic products that all four subunits are exposed on the extracellular surface to a similar degree. All four subunits are also exposed on the internal surface of the membrane, but the apparent degree of exposure varies with the subunit size, the larger subunits being more exposed. The results are discussed in terms of a possible topographic model of the receptor as a transmembrane protein complex.