Local anesthetics and histrionicotoxin are allosteric inhibitors of the acetylcholine receptor. Studies of clonal muscle cells

Pathways for nicotinic receptor desensitization

Nicotinic acetylcholine receptors (AChRs) are ligand-gated ion channels that generate transient currents by binding agonists and switching rapidly between closed- and open-channel conformations. Upon sustained exposure to ACh, the cell response diminishes slowly because of desensitization, a process that shuts the channel even with agonists still bound. In liganded receptors, the main desensitization pathway is from the open-channel conformation, but after agonists dissociate the main recovery pathway is to the closed-channel conformation. In this Viewpoint, I discuss two mechanisms that can explain the selection of different pathways, a question that has puzzled the community for 60 yr. The first is based on a discrete-state model (the “prism”), in which closed, open, and desensitized conformational states interconnect directly. This model predicts that 5% of unliganded AChRs are desensitized. Different pathways are taken with versus without agonists because ligands have different energy properties (φ values) at the transition states of the desensitization and recovery reactions. The second is a potential energy surface model (the “monkey saddle”), in which the states connect indirectly at a shared transition state region. Different pathways are taken because agonists shift the position of the gating transition state relative to the point where gating and desensitization conformational trajectories intersect. Understanding desensitization pathways appears to be a problem of kinetics rather than of thermodynamics. Other aspects of the two mechanisms are considered, as are experiments that may someday distinguish them.

Molecular Determinants of α3β4 Nicotinic Acetylcholine Receptors Inhibition by Triterpenoids

In a previous work, we reported the regulatory role of the triterpenoids on 5-hydroxytryptamine (5-HT)₃A receptors activity in Xenopus laevis oocytes (Eur. J. Pharmacol., 615, 2009, Lee et al.). In the present report, we studied the modulation of triterpenoids on the activity of the human nicotinic acetylcholine receptor type α3β4. Two-electrode voltage clamp experiments were used to test acetylcholine mediated inward current (I_ACh). Treatment with triterpenoids (dehydroeburicoic acid, 6α-hydroxypolyporenic acid C and pachymic acid) inhibited I_ACh in a concentration dependent and reversible manner. The IC₅₀ values for pachymic acid, dehydroeburicoic acid, and 6α-hydroxypolyporenic acid C were 14.9, 37.7, and 20.9 µM, respectively. The inhibitory regulation of I_ACh by each triterpenoid showed in a non-competitive manner on the activity of α3β4 nicotinic acetylcholine receptors. These results show that triterpenoids (pachymic acid, dehydroeburicoic acid, 6α-hydroxypolyporenic acid C) can be used as agents to modulate the activity of nicotinic acetylcholine receptor type α3β4. Furthermore, molecular docking studies of 6α-hydroxypolyporenic acid C on α3β4 nicotinic acetylcholine receptors in silico showed that this molecule interacted predominantly with residues at cavities in the α3 subunit and β4 subunit. This docking assays indicated four potential binding sites for this ligand in the extracellular region at sensor domain of α3β4 nicotinic acetylcholine receptors. In point mutagenesis of those whose alanine substitution, 6α-hydroxypolyporenic acid C potency decreased on W25A of α3 subunit or N109A of β4 subunit in both mutants. The double mutation of W25A of α3 subunit and N109A of β4 subunit was significantly attenuated inhibitory effects by 6α-hydroxypolyporenic acid C. All taken together, this study revealed that molecular basis of α3β4 nicotinic acetylcholine receptors by triterpenoids and provides a novel potent interaction ligand.

Effect of 50% enantiomeric excess bupivacaine mixture combined with pancuronium on neuromuscular transmission in rat phrenic nerve-diaphragm preparation; a pilot study

BACKGROUND AND AIMS

Local anaesthetics are drugs that are widely used in clinical practice. However, the effects of these drugs on the neuromuscular junction and their influence on the blockade produced by non-depolarising neuromuscular blocking drugs are still under investigation. The aim of this study was to evaluate, in vitro, the influence of a 50% enantiomeric excess bupivacaine mixture on neuromuscular transmission and neuromuscular block produced by pancuronium.

METHODS

Rats were distributed into three groups (n = 5) according to the drug studied namely, 50% enantiomeric excess bupivacaine mixture (5 μg/mL); pancuronium (2 μg/mL); 50% enantiomeric excess bupivacaine mixture + pancuronium. The following parameters were evaluated: (1) Effects of a 50% enantiomeric excess bupivacaine mixture on membrane potential (MP) and miniature endplate potentials (MEPPs); (2) amplitude of diaphragmatic response before and 60 min after the addition of a 50% enantiomeric excess bupivacaine mixture; the degree of neuromuscular block with pancuronium and pancuronium combined with a 50% enantiomeric excess bupivacaine mixture.

RESULTS

A 50% enantiomeric excess bupivacaine mixture did not alter the amplitude of muscle response (MP) but decreased the frequency and amplitude of MEPP. The block produced by pancuronium was potentiated by a 50% enantiomeric excess bupivacaine mixture.

CONCLUSION

A 50% enantiomeric excess bupivacaine mixture used alone did not affect neuromuscular transmission, but potentiated the neuromuscular block produced by pancuronium. No action was shown on the muscle fibre, and alterations on MEPPs demonstrated a presynaptic action.

Inhibitory Effects of Ginsenoside Metabolites, Compound K and Protopanaxatriol, on GABAC Receptor-Mediated Ion Currents

Ginsenosides, one of the active ingredients of Panax ginseng, show various pharmacological and physiological effects, and they are converted into compound K (CK) or protopanaxatriol (M4) by intestinal microorganisms. CK is a metabolite derived from protopanaxadiol (PD) ginsenosides, whereas M4 is a metabolite derived from protopanaxatriol (PT) ginsenosides. The γ-aminobutyric acid receptor_C (GABA_C) is primarily expressed in retinal bipolar cells and several regions of the brain. However, little is known of the effects of ginsenoside metabolites on GABA_C receptor channel activity. In the present study, we examined the effects of CK and M4 on the activity of human recombinant GABA_C receptor (ρ1) channels expressed in Xenopus oocytes by using a 2-electrode voltage clamp technique. In oocytes expressing GABA_C receptor cRNA, we found that CK or M4 alone had no effect in oocytes. However, co-application of either CK or M4 with GABA inhibited the GABA-induced inward peak current (I_GABA). Interestingly, pre-application of M4 inhibited I_GABA more potently than CK in a dose-dependent and reversible manner. The half-inhibitory concentration (IC₅₀) values of CK and M4 were 52.1±2.3 and 45.7±3.9 µM, respectively. Inhibition of I_GABA by CK and M4 was voltage-independent and non-competitive. This study implies that ginsenoside metabolites may regulate GABA_C receptor channel activity in the brain, including in the eyes.

Resveratrol Inhibits GABAC ρ Receptor-Mediated Ion Currents Expressed in Xenopus Oocytes

Resveratrol is a phytoalexin found in grapes, red wine, and berries. Resveratrol has been known to have many beneficial health effects, such as anti-cancer, neuroprotective, anti-inflammatory, and life-prolonging effects. However, relatively little is known about the effects of resveratrol on the regulation of ligand-gated ion channels. We have previously reported that resveratrol regulates subsets of homomeric ligand-gated ion channels such as those of 5-HT_3A receptors. The γ-aminobutyric acid_C (GABA_C) receptor is mainly expressed in retinal bipolar cells and plays an important role in visual processing. In the present study, we examined the effects of resveratrol on the channel activity of homomeric GABA_C receptor expressed in Xenopus oocytes injected with cRNA encoding human GABA_C ρ subunits. Our data show that the application of GABA elicits an inward peak current (I_GABA) in oocytes that express the GABA_C receptor. Resveratrol treatment had no effect on oocytes injected with H₂O or with GABA_C receptor cRNA. Co-treatment with resveratrol and GABA inhibited I_GABA in oocytes with GABA_C receptors. The inhibition of I_GABA by resveratrol was in a reversible and concentration-dependent manner. The IC₅₀ of resveratrol was 28.9±2.8 µM in oocytes expressing GABA_C receptor. The inhibition of I_GABA by resveratrol was in voltage-independent and non-competitive manner. These results indicate that resveratrol might regulate GABA_C receptor expression and that this regulation might be one of the pharmacological actions of resveratrol on the nervous system.

Effect of lidocaine (40 mg) mixed to prevent injection pain of propofol on the intubating conditions and onset time of rocuronium

Background

To analyze how lidocaine 40 mg mixed prevents injection pain of propofol affects the onset time of rocuronium, tracheal intubating conditions and intubation related hemodynamic changes.

Methods

This study consisted of 70 patients with an American Society of Anesthesiologists (ASA) physical status class 1 or 2 for general anesthesia. All the patients were randomly allocated into two groups: propofol 2 mg/kg plus normal saline 2 ml (Group C) and propofol 2 mg/kg plus 2% lidocaine 40 mg (Group L). Each group was administrated intravenously during induction and the patient was intubated 1 minute after an injection of 0.6 mg/kg of rocuronium. The time at disappearance of the first twitch and intubation scores were recorded. Also, blood pressure and heart rate were measured at the baseline, after intravenous injection of propofol, before intubation, and at 0, 1, 2, 3 and 5 minutes after intubation.

Results

There were no significant differences between group C and L (P > 0.05).

Conclusions

40 mg of lidocaine mixed with propofol to prevent injection pain did not affect the onset time of rocuronium, intubating conditions and intubation related hemodynamic changes.

Inhibitory Effects of Quercetin on Muscle-type of Nicotinic Acetylcholine Receptor-Mediated Ion Currents Expressed in Xenopus Oocytes

The flavonoid quercetin is a low molecular weight compound generally found in apple, gingko, tomato, onion and other red-colored fruits and vegetables. Like other flavonoids, quercetin has diverse pharmacological actions. However, relatively little is known about the influence of quercetin effects in the regulation of ligand-gated ion channels. Previously, we reported that quercetin regulates subsets of nicotinic acetylcholine receptors such as α3β4, α7 and α9α10. Presently, we investigated the effects of quercetin on muscle-type of nicotinic acetylcholine receptor channel activity expressed in Xenopus oocytes after injection of cRNA encoding human fetal or adult muscle-type of nicotinic acetylcholine receptor subunits. Acetylcholine treatment elicited an inward peak current (I(ACh)) in oocytes expressing both muscle-type of nicotinic acetylcholine receptors and co-treatment of quercetin with acetylcholine inhibited I(ACh). Pre-treatment of quercetin further inhibited I(ACh) in oocytes expressing adult and fetal muscle-type nicotinic acetylcholine receptors. The inhibition of I(ACh) by quercetin was reversible and concentration-dependent. The IC(50) of quercetin was 18.9±1.2 µM in oocytes expressing adult muscle-type nicotinic acetylcholine receptor. The inhibition of I(ACh) by quercetin was voltage-independent and non-competitive. These results indicate that quercetin might regulate human muscle-type nicotinic acetylcholine receptor channel activity and that quercetin-mediated regulation of muscle-type nicotinic acetylcholine receptor might be coupled to regulation of neuromuscular junction activity.

Quercetin Inhibits α3β4 Nicotinic Acetylcholine Receptor-Mediated Ion Currents Expressed in Xenopus Oocytes

Quercetin mainly exists in the skin of colored fruits and vegetables as one of flavonoids. Recent studies show that quercetin, like other flavonoids, has diverse pharmacological actions. However, relatively little is known about quercetin effects in the regulations of ligand-gated ion channels. In the previous reports, we have shown that quercetin regulates subsets of homomeric ligand-gated ion channels such as glycine, 5-HT(3A) and α7 nicotinic acetylcholine receptors. In the present study, we examined quercetin effects on heteromeric neuronal α3β4 nicotinic acetylcholine receptor channel activity expressed in Xenopus oocytes after injection of cRNA encoding bovine neuronal α3 and β4 subunits. Treatment with acetylcholine elicited an inward peak current (I(ACh)) in oocytes expressing α3β4 nicotinic acetylcholine receptor. Co-treatment with quercetin and acetylcholine inhibited I(ACh) in oocytes expressing α3β4 nicotinic acetylcholine receptors. The inhibition of I(ACh) by quercetin was reversible and concentration-dependent. The half-inhibitory concentration (IC(50)) of quercetin was 14.9±0.8 µM in oocytes expressing α3β4 nicotinic acetylcholine receptor. The inhibition of I(ACh) by quercetin was voltage-independent and non-competitive. These results indicate that quercetin might regulate α3β4 nicotinic acetylcholine receptor and this regulation might be one of the pharmacological actions of quercetin in nervous systems.

The influence of lidocaine and racemic bupivacaine on neuromuscular blockade produced by rocuronium: a study in rat phrenic nerve-diaphragm preparation

PURPOSE: To evaluate in vitro lidocaine and racemic bupivacaine effects in neuromuscular transmission and in neuromuscular blockade produced by rocuronium. METHODS: Rats were distributed in 5 groups (n = 5) in agreement with the studied drugs: lidocaine, racemic bupivacaine, rocuronium, separately (Groups I, II, III); rocuronium in preparations exposed to local anesthetics (Groups IV, V). The concentrations used were: 20 µg/mL, 5 µg/mL and 4 µg/mL, for lidocaine, bupivacaine and rocuronium, respectively. It was evaluated: 1) amplitude of diaphragm muscle response to indirect stimulation, before and 60 minutes after separately addition of lidocaine, racemic bupivacaine and rocuronium and the association of local anesthetics - rocuronium; 2) membrane potentials (MP) and miniature end-plate potentials (MEPP). RESULTS: Lidocaine and bupivacaine separately didn't alter the amplitude of muscle response and MP. In preparations previously exposed to lidocaine and racemic bupivacaine, the rocuronium blockade was significantly larger (90.10 ± 9.15% and 100%, respectively), in relation to the produced by rocuronium separately (73.12 ± 9.89%). Lidocaine caused an increase in the frequency of MEPP, being followed by blockade; racemic bupivacaine produced decrease being followed by blockade. CONCLUSIONS: Local anesthetics potentiated the blockade caused by rocuronium. The alterations of MEPP identify presynaptic action.

A role for Leu247 residue within transmembrane domain 2 in ginsenoside-mediated alpha7 nicotinic acetylcholine receptor regulation

Nicotinic acetylcholine receptors (nAChRs) play important roles in nervous system functions and are involved in a variety of diseases. We previously demonstrated that ginsenosides, the active ingredients of Panax ginseng, inhibit subsets of nAChR channel currents, but not alpha7, expressed in Xenopus laevis oocytes. Mutation of the highly conserved Leu247 to Thr247 in the transmembrane domain 2 (TM2) channel pore region of alpha7 nAChR induces alterations in channel gating properties and converts alpha7 nAChR antagonists into agonists. In the present study, we assessed how point mutations in the Leu247 residue leading to various amino acids affect 20(S)-ginsenoside Rg(3) (Rg(3)) activity against the alpha7 nAChR. Mutation of L247 to L247A, L247D, L247E, L247I, L247S, and L247T, but not L247K, rendered mutant receptors sensitive to Rg(3). We further characterized Rg(3) regulation of L247T receptors. We found that Rg(3) inhibition of mutant alpha7 nAChR channel currents was reversible and concentration-dependent. Rg(3) inhibition was strongly voltage-dependent and noncompetitive manner. These results indicate that the interaction between Rg(3) and mutant receptors might differ from its interaction with the wild-type receptor. To identify differences in Rg(3) interactions between wild-type and L247T receptors, we utilized docked modeling. This modeling revealed that Rg(3) forms hydrogen bonds with amino acids, such as Ser240 of subunit I and Thr244 of subunit II and V at the channel pore, whereas Rg(3) localizes at the interface of the two wild-type receptor subunits. These results indicate that mutation of Leu247 to Thr247 induces conformational changes in the wild-type receptor and provides a binding pocket for Rg(3) at the channel pore.

Effects of triterpenoids from Poria cocos Wolf on the serotonin type 3A receptor-mediated ion current in Xenopus oocytes

Poria cocos Wolf (P. cocos Wolf) is used to treat chronic gastritis, edema, nephrosis, gastric atony, acute gastroenteric catarrh, dizziness, emesis and vomiting. Triterpenoids are a class of natural compounds produced by P. cocos Wolf that contain acyclic 30-carbon precursors. In this study, we investigated the effect of triterpenoids (PA, Pachymic acid; DA, dehydroeburicoic acid; HA, 3beta-hydroxylanosta-7,9(11),24-trien-21-oic acid) on human 5-hydroxytryptamine 3A (5-HT(3A)) receptor channel activity, which is one of the ligand-gated ion channel families. The two-electrode voltage-clamp technique was used to examine the 5-HT3A mediated current. The inhibitory effect of triterpenoids on 5HT-induced inward current (I(5-HT)) occurred in a concentration dependent and reversible manner. Furthermore, the half-inhibitory concentrations (IC(50)) of PA, DA and HA were 3.2+/-0.2, 5.5+/-0.6 and 1.4+/-0.2 microM, respectively. This corresponded to an order of potency for the inhibition of I(5-HT) in oocytes expressing human 5-HT(3A) receptor of HA>PA>DA. Finally, inhibition of I(5HT) by triterpenoids occurred in a non-competitive manner, while inhibition by HA and PA showed more voltage-dependency. Taken together, these results indicate that triterpenoids may regulate the expressed 5-HT(3A) receptors in Xenopus oocytes. Furthermore, this regulation of the ligand-gated ion channel activity by triterpenoids may be one of the pharmacological actions of P. cocos Wolf.

Effect of novel negative allosteric modulators of neuronal nicotinic receptors on cells expressing native and recombinant nicotinic receptors: implications for drug discovery

Allosteric modulation of nAChRs is considered to be one of the most promising approaches for drug design targeting nicotinic acetylcholine receptors (nAChRs). We have reported previously on the pharmacological activity of several compounds that seem to act noncompetitively to inhibit the activation of alpha3beta4(*) nAChRs. In this study, the effects of 51 structurally similar molecules on native and recombinant alpha3beta4 nAChRs are characterized. These 51 molecules inhibited adrenal neurosecretion activated via stimulation of native alpha3beta4(*) nAChR, with IC(50) values ranging from 0.4 to 13.0 microM. Using cells expressing recombinant alpha3beta4 nAChRs, these molecules inhibited calcium accumulation (a more direct assay to establish nAChR activity), with IC(50) values ranging from 0.7 to 38.2 microM. Radiolabeled nAChR binding studies to orthosteric sites showed no inhibitory activity on either native or recombinant nAChRs. Correlation analyses of the data from both functional assays suggested additional, non-nAChR activity of the molecules. To test this hypothesis, the effects of the drugs on neurosecretion stimulated through non-nAChR mechanisms were investigated; inhibitory effects ranged from no inhibition to 95% inhibition at concentrations of 10 microM. Correlation analyses of the functional data confirmed this hypothesis. Several of the molecules (24/51) increased agonist binding to native nAChRs, supporting allosteric interactions with nAChRs. Computational modeling and blind docking identified a binding site for our negative allosteric modulators near the orthosteric binding site of the receptor. In summary, this study identified several molecules for potential development as negative allosteric modulators and documented the importance of multiple screening assays for nAChR drug discovery.

Differential regulations of quercetin and its glycosides on ligand-gated ion channels

Quercetin, one of the flavonoids, is a compound of low molecular weight found in various plants and shows a wide range of diverse neuropharmacological actions. In fruits and vegetables, quercetin exists as monomer- (quercetin-3-O-rhamnoside) (Rham1), dimer- (Rutin), or trimer-glycosides [quercetin-3-(2G-rhamnosylrutinoside)] (Rham2) at carbon-3. In the previous studies, we demonstrated that quercetin inhibits both glycine and 5-hydroxytryptamine type 3, (5-HT3A) receptor channel activities expressed in Xenopus oocytes. However, the effects of quercetin glycosides on glycine and 5-HT3A receptor channel activities are not well known. In the present study, we investigated the effects of quercetin glycosides on the human glycine alpha1 receptor and mouse 5-HT3A receptor channel activities expressed in Xenopus oocytes using a two-electrode voltage clamp technique. In oocytes expressing glycine or 5-HT3A receptors, quercetin- or its glycosides-induced inhibitions on glycine- (IGly) and 5-HT-induced current (I5-HT) were dose-dependent and reversible. Applications of quercetin and its glycosides inhibited IGly in order of quercetin>Rutin> or =Rham1>Rham2. Applications of quercetin and its glycosides inhibited I5-HT in order of Rham2> or =quercetin>Rutin=Rham1. The inhibitions of IGly by quercetin glycosides were non-competitive and voltage-sensitive, whereas the inhibitions of I5-HT by quercetin glycosides were competitive and voltage-insensitive manners. These results also indicate that quercetin glycosides might regulate the human glycine alpha1 and mouse 5-HT3A receptors with differential manners.

Effects of dextrorotatory morphinans on α3β4 nicotinic acetylcholine receptors expressed in Xenopus oocytes

We previously demonstrated that dextromethorphan (DM; 3-methoxy-17-methylmorphinan) analogs have neuroprotective effects, and a recent report showed that DM reduces the adverse effects of morphine and blocks alpha3beta4 nicotinic acetylcholine receptors, a major target of anti-addictive agents. Here, we investigated the effects of DM, three of its analogs (DF, 3-methyl-17-methylmorphinan; AM, 3-allyloxy-17-methoxymorphian; and CM, 3-cyclopropyl-17-methoxymorphinan) and one of its metabolites (HM; 3-methoxymorphinan), on neuronal alpha3beta4 nicotinic acetylcholine receptor channel activity expressed in Xenopus laevis oocytes, using the two-microelectrode voltage clamp technique. We found that intraoocyte injection of neuronal alpha3 and beta4 nicotinic acetylcholine receptor subunit cRNAs elicited an inward current (IACh) in the presence of acetylcholine. Co-treatment with DM, DF, AM, CM or HM inhibited IACh in a dose-dependent, voltage-independent and reversible manner. The IC50 values for DM, DF, AM, CM and HM were 19.5+/-5.2, 15.8+/-4.5, 16.3+/-1.7, 10.1+/-2.8, and 13.5+/-4.0 microM, respectively. The order of potency for the inhibition of IACh was CM>HM>DF=AM>DM in oocytes expressing alpha3beta4 nicotinic acetylcholine receptors. The inhibitions of (IACh) by DM, DF and HM, AM and CM were non-competitive. These results indicate that AM, CM and HM could be novel non-competitive agents regulating alpha3beta4 nicotinic acetylcholine receptor channel activity.

Structural effects of quinacrine binding in the open channel of the acetylcholine receptor

Noncompetitive inhibitors of the nicotinic acetylcholine (ACh) receptors suppress cation flux directly by binding in and blocking the open channel or indirectly by stabilizing closed states of the receptor. The lidocaine derivative QX-314 and the acridine derivative quinacrine act directly as open channel blockers, but can act indirectly as well. The binding site for quinacrine in the open channel of mouse-muscle ACh receptor was mapped in cysteine-substituted mutants of the alpha subunit expressed with wild-type beta, gamma, and delta subunits. In the open state, substituted cysteines in the inner half of the second membrane-spanning segment (M2), but not in the outer half, were protected by quinacrine from reaction with 2-aminoethyl methanethiosulfonate. In addition, an alkylating derivative, quinacrine mustard, affinity labeled a subset of the substituted cysteines in M2, but only in the open state. These results, mapped onto a model of the open channel surrounded by five alpha-helical M2s, imply that quinacrine binds midway down M2 in the same site previously mapped for QX-314. A cysteine substituted for a residue in the outer third of alphaM1, which reacted with 2-aminoethyl methanethiosulfonate only in the presence of ACh, reacted faster in the additional presence of quinacrine or QX-314. It is proposed that channel opening involves both the opening of the resting gate at the inner end of M2 and the removal of an obstruction formed by the outer end of M1 that retards diffusion of blockers into the closed channel. Blocker binding in the open channel causes a further change in structure.

Naturally occurring mutations at the acetylcholine receptor binding site independently alter ACh binding and channel gating

By defining functional defects in a congenital myasthenic syndrome (CMS), we show that two mutant residues, located in a binding site region of the acetylcholine receptor (AChR) epsilon subunit, exert opposite effects on ACh binding and suppress channel gating. Single channel kinetic analysis reveals that the first mutation, epsilon N182Y, increases ACh affinity for receptors in the resting closed state, which promotes sequential occupancy of the binding sites and discloses rate constants for ACh occupancy of the nonmutant alphadelta site. Studies of the analogous mutation in the delta subunit, deltaN187Y, disclose rate constants for ACh occupancy of the nonmutant alpha epsilon site. The second CMS mutation, epsilon D175N, reduces ACh affinity for receptors in the resting closed state; occupancy of the mutant site still promotes gating because a large difference in affinity is maintained between closed and open states. epsilon D175N impairs overall gating, however, through an effect independent of ACh occupancy. When mapped on a structural model of the AChR binding site, epsilon N182Y localizes to the interface with the alpha subunit, and epsilon D175 to the entrance of the ACh binding cavity. Both epsilon N182Y and epsilon D175 show state specificity in affecting closed relative to desensitized state affinities, suggesting that the protein chain harboring epsilon N182 and epsilon D175 rearranges in the course of receptor desensitization. The overall results show that key residues at the ACh binding site differentially stabilize the agonist bound to closed, open and desensitized states, and provide a set point for gating of the channel.

Mechanism of tacrine block at adult human muscle nicotinic acetylcholine receptors

We used single-channel kinetic analysis to study the inhibitory effects of tacrine on human adult nicotinic receptors (nAChRs) transiently expressed in HEK 293 cells. Single channel recording from cell-attached patches revealed concentration- and voltage-dependent decreases in mean channel open probability produced by tacrine (IC(50) 4.6 microM at -70 mV, 1.6 microM at -150 mV). Two main effects of tacrine were apparent in the open- and closed-time distributions. First, the mean channel open time decreased with increasing tacrine concentration in a voltage-dependent manner, strongly suggesting that tacrine acts as an open-channel blocker. Second, tacrine produced a new class of closings whose duration increased with increasing tacrine concentration. Concentration dependence of closed-times is not predicted by sequential models of channel block, suggesting that tacrine blocks the nAChR by an unusual mechanism. To probe tacrine's mechanism of action we fitted a series of kinetic models to our data using maximum likelihood techniques. Models incorporating two tacrine binding sites in the open receptor channel gave dramatically improved fits to our data compared with the classic sequential model, which contains one site. Improved fits relative to the sequential model were also obtained with schemes incorporating a binding site in the closed channel, but only if it is assumed that the channel cannot gate with tacrine bound. Overall, the best description of our data was obtained with a model that combined two binding sites in the open channel with a single site in the closed state of the receptor.

Effects of ginsenosides, active components of ginseng, on nicotinic acetylcholine receptors expressed in Xenopus oocytes

We investigated the effects of ginsenosides, the active ingredient of ginseng, on neuronal or muscle-type nicotinic acetylcholine receptor channel activity expressed in Xenopus oocytes after injection of cRNA encoding bovine neuronal alpha3beta4, alpha7 or human muscle alphabetadeltavarepsilon subunits. Treatment with acetylcholine elicited an inward peak current (I(ACh)) in oocytes expressing nicotinic acetylcholine receptor subtypes. Cotreatment with ginsenoside Rg2 and acetylcholine inhibited I(ACh) in oocytes expressing with alpha3beta4 or alphabetadeltavarepsilon but not in oocytes expressing alpha7 nicotinic acetylcholine receptors. The inhibition of I(ACh) by ginsenoside Rg2 was reversible and dose-dependent. The half-inhibitory concentrations (IC50) of ginsenoside Rg2 were 60.2+/-14.1 and 15.7+/-3.5 microM in oocytes expressing alpha3beta4 and alphabetadeltavarepsilon nicotinic acetylcholine receptors, respectively. The inhibition of I(ACh) by ginsenoside Rg2 was voltage-independent and noncompetitive. Other ginsenosides besides ginsenoside Rg2 also inhibited I(ACh) in oocytes expressing alpha3beta4 or alphabetadeltavarepsilon nicotinic acetylcholine receptors. The order of potency for the inhibition of I(ACh) was ginsenoside Rg2>Rf>Re>Rg1>Rc>Rb2>Rb1 in oocytes expressing alpha3beta4 nicotinic acetylcholine receptors and was ginsenoside Rg2>Rf>Rg1>Re>Rb1>Rc>Rb2 in oocytes expressing alphabetadeltavarepsilon nicotinic acetylcholine receptors. These results indicate that ginsenosides might regulate nicotinic acetylcholine receptors in a differential manner and this regulation might be one of the pharmacological actions of Panax ginseng.

The noncompetitive inhibitor quinacrine modifies the desensitization kinetics of muscle acetylcholine receptors

Quinacrine has been shown to act as a noncompetitive inhibitor of the nicotinic acetylcholine receptor (nAChR). However, its mechanism of action is still a matter of controversy. We analyzed in detail the action of quinacrine at both the single-channel and macroscopic current levels. The main effect of quinacrine is a profound concentration-dependent decrease in both the frequency of opening events and the duration of clusters elicited by high acetylcholine concentrations. Quinacrine also significantly increases (40-fold at 30 microM) the decay rate of macroscopic currents elicited by rapid perfusion of acetylcholine to outside-out patches. This decay is still well-described by a single exponential. Quinacrine has very little effect on the peak amplitude of the response, suggesting that it acts mainly on open channels. The recovery from desensitization after removal of acetylcholine is delayed in the presence of quinacrine. Results from both single-channel and macroscopic current recordings indicate that quinacrine increases the rate of nAChR desensitization and stabilizes the desensitized state. Interestingly, in equilibrium agonist-binding assays, quinacrine does not promote the typical high-affinity desensitized state. Thus, quinacrine seems to induce an intermediate state exhibiting the permeability but not the agonist binding properties of desensitization.

Interaction of the catecholamine release-inhibitory peptide catestatin (human chromogranin A(352-372)) with the chromaffin cell surface and Torpedo electroplax: implications for nicotinic cholinergic antagonism

The catecholamine release-inhibitory chromogranin A fragment catestatin (chromogranin A(344-364)) exhibits non-competitive antagonism of nicotinic cholinergic signaling in chromaffin cells. A previous homology model of catestatin's likely structure suggested a mode of interaction of the peptide with the nicotinic receptor, but direct evidence has been lacking. Here we found that [125I]-catestatin binds to the surface of intact PC12 and bovine chromaffin cells with high affinity (K(D)=15.2+/-1.53 nM) and specificity (lack of displacement by another [N-terminal] fragment of chromogranin A). Nicotinic agonist (carbamylcholine) did not displace [125I]-catestatin from chromaffin cells, nor did catestatin displace the nicotinic agonist [3H]-epibatidine; these observations indicate a catestatin binding site separate from the agonist binding pocket on the nicotinic receptor, a finding consistent with catestatin's non-competitive nicotinic mechanism. [125I]-catestatin could be displaced from chromaffin cells by substance P (IC(50) approximately 5 microM), though at far lower potency than displacement by catestatin itself (IC(50) approximately 350-380 nM), suggesting that catestatin and substance P occupy an identical or overlapping non-competitive site on the nicotinic receptor, at different affinities (catestatin > substance P). Small, non-peptide non-competitive nicotinic antagonists (hexamethonium or clonidine) did not diminish [125I]-catestatin binding, suggesting distinct non-competitive binding sites on the nicotinic receptor for peptide and non-peptide antagonists. Similar binding and inhibitory profiles for [125I]-catestatin were observed on chromaffin cells as well as nicotinic receptor-enriched Torpedo membranes. Covalent cross-linking of [125I]-catestatin to Torpedo membranes suggested specific contacts of [125I]-catestatin with the delta, gamma, and beta subunits of the nicotinic receptor, a finding consistent with prior homology modeling of the interaction of catestatin with the extracellular face of the nicotinic heteropentamer. We conclude that catestatin occludes the nicotinic cation pore by interacting with multiple nicotinic subunits at the pore vestibule. Such binding provides a physical explanation for non-competitive antagonism of the peptide at the nicotinic receptor.

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.

Acetylcholine and epibatidine binding to muscle acetylcholine receptors distinguish between concerted and uncoupled models

The muscle acetylcholine receptor (AChR) has served as a prototype for understanding allosteric mechanisms of neurotransmitter-gated ion channels. The phenomenon of cooperative agonist binding is described by the model of Monod et al. (Monod, J., Wyman, J., and Changeux, J. P. (1965) J. Mol. Biol. 12, 88-118; MWC model), which requires concerted switching of the two binding sites between low and high affinity states. The present study examines binding of acetylcholine (ACh) and epibatidine, agonists with opposite selectivity for the two binding sites of mouse muscle AChRs. We expressed either fetal or adult AChRs in 293 HEK cells and measured agonist binding by competition against the initial rate of 125I-alpha-bungarotoxin binding. We fit predictions of the MWC model to epibatidine and ACh binding data simultaneously, taking as constants previously determined parameters for agonist binding and channel gating steps, and varying the agonist-independent parameters. We find that the MWC model describes the apparent dissociation constants for both agonists but predicts Hill coefficients that are far too steep. An Uncoupled model, which relaxes the requirement of concerted state transitions, accurately describes binding of both ACh and epibatidine and provides parameters for agonist-independent steps consistent with known aspects of AChR function.

Delimiting the binding site for quaternary ammonium lidocaine derivatives in the acetylcholine receptor channel

The triethylammonium QX-314 and the trimethylammonium QX-222 are lidocaine derivatives that act as open-channel blockers of the acetylcholine (ACh) receptor. When bound, these blockers should occlude some of the residues lining the channel. Eight residues in the second membrane-spanning segment (M2) of the mouse-muscle alpha subunit were mutated one at a time to cysteine and expressed together with wild-type beta, gamma, and delta subunits in Xenopus oocytes. The rate constant for the reaction of each substituted cysteine with 2-aminoethyl methanethiosulfonate (MTSEA) was determined from the time course of the irreversible effect of MTSEA on the ACh-induced current. The reactions were carried out in the presence and absence of ACh and in the presence and absence of QX-314 and QX-222. These blockers had no effect on the reactions in the absence of ACh. In the presence of ACh, both blockers retarded the reaction of extracellularly applied MTSEA with cysteine substituted for residues from alphaVal255, one third of the distance in from the extracellular end of M2, to alphaGlu241, flanking the intracellular end of M2, but not with cysteine substituted for alphaLeu258 or alphaGlu262, at the extracellular end of M2. The reactions of MTSEA with cysteines substituted for alphaLeu258 and alphaGlu262 were considerably faster in the presence of ACh than in its absence. That QX-314 and QX-222 did not protect alphaL258C and alphaE262C against reaction with MTSEA in the presence of ACh implies that protection of the other residues was due to occlusion of the channel and not to the promotion of a less reactive state from a remote site. Given the 12-A overall length of the blockers and the alpha-helical conformation of M2 in the open state, the binding site for both blockers extends from alphaVal255 down to alphaSer248.

Desensitization of mouse nicotinic acetylcholine receptor channels. A two-gate mechanism

The rate constants of acetylcholine receptor channels (AChR) desensitization and recovery were estimated from the durations and frequencies of clusters of single-channel currents. Diliganded-open AChR desensitize much faster than either unliganded- or diliganded-closed AChR, which indicates that the desensitization rate constant depends on the status of the activation gate rather than the occupancy of the transmitter binding sites. The desensitization rate constant does not change with the nature of the agonist, the membrane potential, the species of permeant cation, channel block by ACh, the subunit composition (epsilon or gamma), or several mutations that are near the transmitter binding sites. The results are discussed in terms of cyclic models of AChR activation, desensitization, and recovery. In particular, a mechanism by which activation and desensitization are mediated by two distinct, but interrelated, gates in the ion permeation pathway is proposed.

Distinctions in agonist and antagonist specificity conferred by anionic residues of the nicotinic acetylcholine receptor

Two anionic residues in the nicotinic acetylcholine receptor, Asp-152 in the alpha-subunit and Asp-174 in the gamma-subunit or the corresponding Asp-180 in the delta-subunit, are presumed to reside near the two agonist binding sites at the alphagamma and alphadelta subunit interfaces of the receptor and have been implicated in electrostatic attraction of cationic ligands. Through site-directed mutagenesis and analysis of state changes in the receptor elicited by agonists, we have distinguished the roles of anionic residues in conferring ligand specificity and ligand-induced state changes. alphaAsp-152 affects agonist and antagonist affinity similarly, whereas gammaAsp-174 and deltaAsp-180 primarily affect agonist affinity. Combining charge neutralization on the alpha subunit with that on the gamma and delta subunits shows an additivity in free energy changes for carbamylcholine and d-tubocurarine, suggesting independent contributions of these residues to stabilizing the bound ligands. Since both aromatic and anionic residues stabilize cationic ligands, we substituted tyrosines (Y) for the aspartyl residues. While the substitution, alphaD152Y, reduced the affinities for agonists and antagonists, the gammaD174Y/deltaD180Y mutations reduced the affinity for agonist binding, but surprisingly enhanced the affinity for d-tubocurarine. To ascertain whether selective changes in agonist binding stem from the capacity of agonists to form the desensitized state of the receptor, carbamylcholine binding was measured in the presence of an allosteric inhibitor, proadifen. Mutant nAChRs carrying alphaD152Q or gammaD174N/deltaD180N show similar reductions in dissociation constants for the desensitized compared with activable receptor state and a similar proadifen concentration dependence. Hence, these mutations influence ligand recognition rather than the capacity of the receptor to desensitize. By contrast, the alphaD200Q mutation diminishes the ratio of dissociation constants for two states and requires higher proadifen concentrations to induce desensitization. Thus, the contributions of alphaAsp-152, gamma/deltaAsp-174/180, and alphaAsp-200 in stabilizing ligand binding can be distinguished by the interactions between agonists and allosteric inhibitors.

Interaction of d-tubocurarine analogs with the mouse nicotinic acetylcholine receptor. Ligand orientation at the binding site

The binding of d-tubocurarine and several of its analogs to the mouse nicotinic acetylcholine receptor (AChR) was measured by competition against the initial rate 125I-alpha-bungarotoxin binding to BC3H-1 cells. The changes in affinity due to methylation or halogenation at various functional groups on d-tubocurarine was measured to both the high affinity (alphagamma-site) and the low affinity site (alphadelta-site). We show that quaternization by methylation of the 2'-N ammonium group enhances the affinity for both the acetylcholine binding sites of mouse AChR, whereas this change does not affect affinity for the Torpedo AChR sites. The effect of N-methylation suggests the presence of interactions with the ammonium moiety that cannot be readily attributed to the known conserved residues thought to stabilize this functional group. Methylation of both the 7'- and 12'-phenols produced net affinity changes at both sites. The changes resulted from contributions at both the 7'- and the 12'-positions; however, these effects were dependent on whether the ammoniums were also methylated. Substitution of bromine or iodine at the 13'-position decreased the affinity considerably to the high affinity alphagamma-site of mouse AChR, whereas the affinity for the Torpedo alphagamma-site was slightly increased. Furthermore, binding to the mouse AChR was unaffected by the conformational state, whereas these ligands strongly preferred the desensitized conformation of the Torpedo AChR. Comparison of binding changes upon 13'-halogenation to the changes in amino acid residues at the ACh binding sites of the mouse and Torpedo AChR shows mouse residue Ile-gamma116 as likely to be involved in interacting with the 13'-position of d-tubocurarine. It is predicted that this residue is involved in the conformational equilibrium between the resting and desensitized conformations.

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)

Molecular approaches to receptors as targets for drug discovery

The cloning of a great number of receptors and channels has revealed that many of these targets for drug discovery can be grouped into superfamilies based on sequence and structural similarities. This review presents an overview of how molecular biological approaches have revealed a plethora of receptor subtypes, led to new definitions of subtypes and isoforms, and played a role in the development of high selective drugs. Moreover, the diversity of subtypes has molded current views of the structure and function of receptor families. Practical difficulties and limitations inherent in the characterization of the ligand binding and signaling properties of expressed recombinant receptors are discussed. The importance of evaluating drug-receptor interactions that differ with temporally transient and distinct receptor conformational states is emphasized. Structural motifs and signal transduction features are presented for the following major receptor superfamilies: ligand-gated ion channel, voltage-dependent ion channel, G-protein coupled, receptor tyrosine-kinase, receptor protein tyrosine-phosphatase, cytokine and nuclear hormone. In addition, a prototypic receptor is analyzed to illustrate functional properties of a given family. The review concludes with a discussion of future directions in receptor research that will impact drug discovery, with a specific focus on orphan receptors as targets for drug discovery. Methods for classifying orphan receptors based upon homologies with members of existing superfamilies are presented together with molecular approaches to the greater challenge of defining their physiological roles. Besides revealing new orphan receptors, the human genome sequencing project will result in the identification of an abundance of novel receptors that will be molecular targets for the development of highly selective drugs. These findings will spur the discovery and development of an exciting new generation of receptor-subtype specific drugs with enhanced therapeutic specificity.

Spiropyrrolizidines: a new class of blockers of nicotinic receptors

The spiropyrrolizidine oximes 236 and 222 and a related spiropyrrolizidine alkaloid, nitropolyzonamine, block nicotinic receptor channels in rat pheochromocytoma PC12 cells and in human medulloblastoma TE671 cells. In PC12 cells with an alpha 3 beta 4(5)-nicotinic receptor, both the spiropyrrolizidine oxime 236 and nitropolyzonamine had IC50 values of about 1.5 microM, while spiropyrrolizidine oxime 222 had an IC50 value of 2.6 microM versus carbamylcholine-elicited sodium-22 influx. In TE671 cells with an alpha 1 beta 1 gamma delta nicotinic receptor, the spiropyrrolizidine oximes 236, 222, and nitropolyzonamine had IC50 values of 9.5, 14, and 67 microM, respectively. The inhibitions by the spiropyrrolizidine oxime 236 and nitropolyzonamine appeared to be noncompetitive in nature in both cell lines. In rat cerebral cortical membranes, binding of [3H]nicotine to alpha 4 beta 2 nicotinic receptors was not inhibited significantly by 10 microM concentrations of the spiropyrrolizidine oxime 236, or by nitropolyzonamine, as expected for a noncompetitive blocker. Both compounds at 10 microM had marginal effects on a variety of central receptors, but did inhibit binding of [3H]1,3-di(2-tolyl) guanidine to sigma receptors in mouse brain membranes with IC50 values of about 0.5 microM. The spiropyrrolizidine oxime 236 at 10 microM had no effect on batrachotoxin-elicited sodium influx in guinea pig cerebral cortical synaptoneurosomes or on ATP-elicited calcium influx in PC12 cells. Such spiropyrrolizidines represent a new structural class of blockers of nicotinic receptor channels with selectivity for ganglionic-type receptors.

Congenital myasthenic syndrome caused by decreased agonist binding affinity due to a mutation in the acetylcholine receptor epsilon subunit

We describe the genetic and kinetic defects for a low-affinity fast channel disease of the acetylcholine receptor (AChR) that causes a myasthenic syndrome. In two unrelated patients with very small miniature end plate (EP) potentials, but with normal EP AChR density and normal EP ultrastructure, patch-clamp studies demonstrated infrequent AChR channel events, diminished channel reopenings during ACh occupancy, and resistance to desensitization by ACh. Each patient had two heteroallelic AChR epsilon subunit gene mutations: a common epsilon P121L mutation, a signal peptide mutation (epsilon G-8R) (patient 1), and a glycosylation consensus site mutation (epsilon S143L) (patient 2). AChR expression in HEK fibroblasts was normal with epsilon P121L but was markedly reduced with the other mutations. Therefore, epsilon P121L defines the clinical phenotype. Studies of the engineered epsilon P121L AChR revealed a markedly decreased rate of channel opening, little change in affinity of the resting state for ACh, but reduced affinity of the open channel and desensitized states.

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.

Mutation of the acetylcholine receptor alpha subunit causes a slow-channel myasthenic syndrome by enhancing agonist binding affinity

In five members of a family and another unrelated person affected by a slow-channel congenital myasthenic syndrome (SCCMS), molecular genetic analysis of acetylcholine receptor (AChR) subunit genes revealed a heterozygous G to A mutation at nucleotide 457 of the alpha subunit, converting codon 153 from glycine to serine (alpha G153S). Electrophysiologic analysis of SCCMS end plates revealed prolonged decay of miniature end plate currents and prolonged activation episodes of single AChR channels. Engineered mutant AChR expressed in HEK fibroblasts exhibited prolonged activation episodes strikingly similar to those observed at the SCCMS end plates. Single-channel kinetic analysis of engineered alpha G153S AChR revealed a markedly decreased rate of ACh dissociation, which causes the mutant AChR to open repeatedly during ACh occupancy. In addition, ACh binding measurements combined with the kinetic analysis indicated increased desensitization of the mutant AChR. Thus, ACh binding affinity can dictate the time course of the synaptic response, and alpha G153 contributes to the low binding affinity for ACh needed to speed the decay of the synaptic response.

Lophotoxin is a slow binding irreversible inhibitor of nicotinic acetylcholine receptors

Lophotoxin and the bipinnatins are members of the lophotoxin family of marine neurotoxins, which covalently react with Tyr190 in the alpha-subunits of the nicotinic acetylcholine receptor. Bipinnatin-A, -B, and -C are protoxins that have been shown to spontaneously convert from inactive to active toxins during preincubation in buffer. However, in this report, we show that preincubation of lophotoxin did not result in an increase in the subsequent rate of irreversible inhibition of nicotinic receptors. Thus, unlike the bipinnatins, lophotoxin does not appear to be an inactive protoxin. Lophotoxin preferentially inhibited one of the two acetylcholine-binding sites on the receptor, and this preference resulted from both a higher reversible affinity and a faster rate of irreversible inhibition at this site. Association of 125I-alpha-bungarotoxin in the presence of lophotoxin was analyzed to obtain the apparent reversible association and dissociation rate constants for lophotoxin. The apparent association rate constant of lophotoxin was approximately 10(6)-fold slower than expected for a diffusion-limited interaction, indicating that lophotoxin is a slow binding irreversible inhibitor. The kinetic constants that describe the interaction of lophotoxin with the receptor did not change in the presence of dibucaine, suggesting that, unlike agonists, the slow apparent association of lophotoxin does not result from a slow transition of the receptor to a desensitized conformation.

The effect of epidural bupivacaine on vecuronium-induced neuromuscular blockade in children

The effect of epidural bupivacaine on potency and duration of action of vecuronium-induced neuromuscular blockade (NMB) was evaluated in 30 general surgical paediatric patients (ASA I-II) of three to ten years of age. Premedication was midazolam 0.5 mg kg-1 orally (max 15 mg). In addition to general anaesthesia, 15 of the children received a lumbar epidural block with 0.5% bupivacaine 2.5 mg kg-1. Anaesthesia was induced and maintained with N2O:O2 (2:1), propofol and alfentanil. NMB was monitored by adductor pollicis EMG with the train-of-four stimulus every 20 sec. Thirty minutes following the epidural bupivacaine injection (mean plasma concentration 0.86 micrograms ml-1) or induction of anaesthesia a cumulative dose-response curve of vecuronium was established to achieve a 95% depression of the twitch response. Thereafter, NMB was allowed to recover spontaneously. ED doses of vecuronium were 19-22% greater in the control group than in the epidural group. ED50 doses were 33.8 (s.e. mean 1.3) micrograms kg-1 and 28.4 (2.2) micrograms kg-1, respectively (P < 0.05). There were no differences in recovery times from NMB between control and epidural group, the recovery index (time of twitch height to recover from 25 to 75%) being 6.4 (0.4) min and 7.0 (0.9) min, respectively. However, a negative correlation was found between bupivacaine plasma concentration and an ED50 dose of vecuronium (P = 0.01). Our results indicate that vecuronium is slightly more potent in children with bupivacaine epidural block than in children without it.

Serotonin modulates nicotinic responses of adrenal chromaffin cells

5-Hydroxytryptamine (5-HT) specifically and reversibly inhibits nicotine-induced currents and catecholamine release in bovine adrenal chromaffin cells in culture. Pharmacological analysis indicates that the inhibition is not mediated by known 5-HT receptor subtypes. The inhibition is noncompetitive over a range of nicotine concentrations between 1 and 100 microM. Preincubation with either 5-HT or substance P significantly protects the response from nicotine-induced desensitization. It is concluded that 5-HT inhibits nicotinic acetylcholine receptors on bovine adrenal chromaffin cells, probably by binding to a noncompetitive site on the receptor itself. Because both blood and the chromaffin cells contain 5-HT, the inhibition provides an opportunity for negative control of catecholamine secretion from the adrenals.

Effects of substance P on the binding of agonists to the nicotinic acetylcholine receptor of Torpedo electroplaque

The effect of the neuropeptide substance P on the binding of the cholinergic ligands to the nicotinic acetylcholine receptor of Torpedo electroplaque membranes was examined at a physiological concentration of NaCl (150 mM). Substance P had no effect on the initial rate of 125I-alpha-bungarotoxin binding at concentrations of < 100 microM. The peptide did not bind to the high-affinity local anesthetic site but allosterically modulated [3H]phencyclidine binding, positively in the absence of agonist and negatively in the presence of agonist. Substance P increased the apparent affinity of the cholinergic agonists carbamylcholine and acetylcholine at equilibrium. The effect of substance P on the equilibrium binding of [3H]acetylcholine was examined directly, and the peptide appeared to increase the affinity of the binding of the second molecule of agonist, with no effect on the binding of the first. This indicates that substance P can affect the cooperative interactions between agonist binding sites. Substance P appeared to increase the rate of carbamylcholine-induced desensitization; however, the data are also consistent with an allosteric mechanism that does not involve the desensitized state. To attempt to differentiate between these mechanisms, the rates of recovery were determined after exposure to peptide and/or agonist. The kinetics of recovery are consistent with stabilization of the desensitized state by substance P if the peptide remains bound long enough to allow rapid recovery to the low-affinity state. However, an allosteric modulation of agonist binding that does not involve the desensitized state cannot be ruled out.

Desensitization of acetylcholine receptors in BC3H-1 cells

We studied desensitization of nicotinic acetylcholine (ACh) receptor channels in the clonal BC3H-1 cell line. We measured the current response to rapid perfusion of outside-out patches with 1 microM to 5 mM ACh, carbamylcholine and suberyldicholine. After binding to the receptors and opening the ion channels, all agonists induce a rapid, concentration-dependent decay of channel activity. The time constant of the current decay ranged from several seconds at low agonist concentrations to about 50 ms at saturating concentrations. The decay rate at saturating concentrations was independent of voltage. The ratio of steady-state to peak current ranged from 0.5 at low agonist concentrations to 0.02 or less at high concentrations. The rate of recovery from desensitization after removal of agonist was also measured. For ACh, the recovery time constant was 320 ms; recovery from desensitization by carbamylcholine was twice as fast. A linear kinetic results. The data are consistent with a cyclic model, although, it is not possible to uniquely determine all of the rate constants in this scheme. The results are compared with competitive binding and single channel studies of desensitization in BC3H-1 cells.

Decahydroquinoline alkaloids: noncompetitive blockers for nicotinic acetylcholine receptor-channels in pheochromocytoma cells and Torpedo electroplax

In pheochromocytoma PC12 cells, (+)-cis-decahydroquinoline 195A (5-methyl-2-propyl-cis-decahydroquinoline) and (+)-perhydro-cis-decahydroquinoline 219A (2,5-dipropyl-cis-decahydroquinoline) inhibit carbamylcholine-elicited sodium flux with IC50 values of 1.0 and 1.5 microM, respectively. Both of these decahydroquinolines appear to enhance desensitization, although apparent lack of complete removal of (+)-perhydro-cis-219A by washing complicates interpretation of the effects of that agent. A series of cis- and trans-decahydroquinolines with substituents in the 2- and 5-position also exhibit structure-dependent inhibition of carbamylcholine-elicited sodium flux in PC12 cells and all of the decahydroquinolines inhibit binding of the noncompetitive blocking agent [3H]perhydrohistrionicotoxin to muscle-type nicotinic acetylcholine receptor-channels in membranes from Torpedo electroplax. The Ki values in electroplax membranes range from 1.4 to 7.9 microM, making these alkaloids comparable in potencies to the histrionicotoxins. Potencies are increased 2- to 3-fold in the presence of an agonist, carbamylcholine. The profile of activities are similar in PC12 cells and electroplax membranes. The cis- and trans-decahydroquinolines represent another class of noncompetitive blockers for acetylcholine receptor-channels with similar activity for both muscle-type and ganglionic type nicotinic receptors.

Nicotinic receptor-elicited sodium flux in rat pheochromocytoma PC12 cells: effects of agonists, antagonists, and noncompetitive blockers

Nicotinic agonists stimulate 22Na flux in rat pheochromocytoma PC12 cells. The stimulatory effect of carbamylcholine is maximal at 1 mM, while the stimulatory effect of nicotine and anatoxin maximize at the same level at 100 microM and 10 microM, respectively. The tertiary amines arecolone and isoarecolone have no effect on flux at 100 microM, while the methiodides at 100 microM stimulate flux to an extent similar to 1 mM carbamylcholine. Dihydro and alcohol analogues of isoarecolone methiodide have markedly smaller effects on flux. A preincubation for 2 to 20 min with carbamylcholine (2 mM), nicotine (300 microM), anatoxin (30 microM) or isoarecolone methiodide (100 microM) causes marked desensitization to a subsequent carbamylcholine-elicited stimulation of flux. d-Tubocurarine, mecamylamine, hexamethonium, and chlorisondamine inhibit carbamylcholine-elicited flux with IC50 values of 1.0, 0.8, 43, and 0.020 microM, respectively. Atropine has no effect at 1 microM, but reduces the response to carbamylcholine by 50% at 8.6 microM, presumably as a noncompetitive blocker. Other noncompetitive blockers of nicotinic acetylcholine-receptors, such as histrionicotoxins, gephyrotoxin, pumiliotoxin C, phencyclidine, bupivacaine and piperocaine, inhibit carbamylcholine-elicited stimulation of 22Na flux with IC50 values from 0.3 to 1.8 microM. In contrast to d-tubocurarine, which inhibits carbamylcholine-elicited desensitization, and mecamylamine, which has no apparent effect on desensitization, chlorisondamine and certain noncompetitive blockers appear to enhance desensitization. The effects of agonists, antagonists and noncompetitive blockers at the neuronal nicotinic acetylcholine receptor-channel of PC12 cells are compared to their effects on binding of [125I]alpha-bungarotoxin to agonist-recognition sites and of [3H]perhydrohistrionicotoxin to noncompetitive blocker sites of the nicotinic acetylcholine receptor-channel of electric ray (Torpedo) electroplax membranes. There are marked differences in relative potencies for the two types of nicotinic acetylcholine receptor-channel.

Modes of hexamethonium action on acetylcholine receptor channels in frog skeletal muscle

1. The antagonism between hexamethonium and cholinoceptor agonists was investigated in frog skeletal muscle fibres with voltage-clamp techniques. Hexamethonium caused a voltage-dependent reduction in the amplitude of endplate currents. For neurally evoked endplate currents, the reduction increased e-fold with a 38 mV membrane hyperpolarization. 2. The effect of hexamethonium on the time course of endplate currents was small, and was most apparent as a slight prolongation of the decay phase at hyperpolarized potentials (more negative than -100 mV). A similar small prolongation of single channel lifetime was detected with fluctuation analysis techniques. Hexamethonium produced a voltage-dependent reduction in apparent single channel conductance as the membrane was hyperpolarized. 3. Log (concentration-response) curves for acetylcholine (ACh)-induced currents, determined either from currents accompanying ramp changes in membrane potential or from steady state currents in voltage-jump experiments, were less steep for responses in the presence of hexamethonium. This reduction in slope became more pronounced at more negative membrane potentials. Observations at +50 mV suggested that the equilibrium constant for competitive antagonism was approximately 200 microM. 4. In voltage-jump experiments with a two-microelectrode voltage clamp, the current evoked by ACh in the presence of hexamethonium differed from that recorded with ACh alone. In the presence of hexamethonium, the expected 'instantaneous' ohmic increase in membrane current in response to a hyperpolarizing step was not detected; instead a decrease in current was observed. This problem was further investigated with a vaseline-gap voltage-clamp technique which provides improved temporal resolution. With this method a rapid decrease in the ACh-induced inward current was observed with step hyperpolarizations in the presence of hexamethonium. 5. When the membrane potential was stepped back to its resting level from a more hyperpolarized potential in the presence of hexamethonium, there was a surge of ACh-induced inward current that decayed with a time constant of less than 100 microseconds. 6. The slow relaxation in the ACh-induced current that followed a voltage step recorded in the presence of hexamethonium was slower than that recorded with ACh alone. In the presence of hexamethonium the time constant of this relaxation increased e-fold for a 67 mV hyperpolarization. 7. The results are consistent with a rapid voltage-dependent block of ACh-activated channels by hexamethonium with hyperpolarization, and voltage-dependent unblock with depolarization. The voltagedependent block is combined with competitive antagonism at the ACh receptors. However, not all observations appear to be compatible with a simple sequential block of open ion channels, but rather suggest that occupation of the channel by hexamethonium may not prevent channel closure.

Effect of epidurally administered bupivacaine on atracurium-induced neuromuscular blockade

The effect of epidurally administered bupivacaine on duration, intensity and reversal characteristics of atracurium-induced neuromuscular blockade was studied in 30 healthy patients anaesthetized with thiopentone, fentanyl, midazolam and nitrous oxide. Fifteen patients received, in addition, epidural anaesthesia with bupivacaine. The remaining patients served as controls. The ulnar nerve was stimulated at the wrist and the evoked twitch response from the adductor pollicis was measured with a force displacement transducer. Neuromuscular blockade was induced with atracurium 0.5 mg i.v. and maintained with repeated doses of atracurium 0.15 mg/kg whenever the twitch height had recovered to 15% of the initial twitch height. After operation, the neuromuscular blockade was reversed with neostigmine when the twitch height had recovered to 15%. In the epidural group the clinical duration of neuromuscular blockade, time until first response to train-of-four (TOF) and reversal time were all significantly prolonged (P less than 0.05). Post-tetanic count (PTC) after 20 min was also significantly lower in the epidural group (P less than 0.05). It is therefore concluded that epidurally administered bupivacaine prolongs atracurium-induced neuromuscular blockade. The clinical implication of the modest prolongation is, however, limited.

Association of spin-labeled local anesthetics at the hydrophobic surface of acetylcholine receptor in native membranes from Torpedo marmorata

The interactions between a series of spin-labeled local anesthetic analogues and the nicotinic acetylcholine receptor (AChR) have been investigated by means of electron spin resonance (ESR) and fluorescence spectroscopy. The paramagnetic local anesthetic analogues quenched the intrinsic tryptophan fluorescence of AChR-rich membranes in an agonist-dependent manner, demonstrating a direct interaction with the AChR. The quenching efficiency was greater for the benzocaine than for the thioprocaine analogue. The protein was found to restrict directly the molecular motion of the spin-labeled analogues, as seen by the appearance of a highly anisotropic component in the ESR spectrum. The relative affinity of the population of local anesthetic probes which interacts directly with the integral protein of the AChR-rich membranes was calculated on the basis of relative association constants, Kr, determined by ESR. By comparison with the relative association constant for spin-labeled phospholipid, Kro, it was possible to differentiate between local anesthetic analogues interacting with high (Kr/Kro greater than 2), intermediate (Kr/Kro = 1.6-1.9), and low (Kr/Kro less than or equal to 1.3) specificity and to calculate the fraction of protein-associated probe in each case. Differences were observed in the presence of agonist (0.1 mM carbamylcholine) with some, but not all, of the spin-labeled derivatives. The role of the protonatable diethylammonium group in the specificity of the interaction of the procaine and thioprocaine analogues was investigated. Only in the uncharged form, or in the charged form at high ionic strength, was there a preferential association of these two local anesthetic analogues.(ABSTRACT TRUNCATED AT 250 WORDS)