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

Nicotinic acetylcholine receptors at the single-channel level

Over the past four decades, the patch clamp technique and nicotinic ACh (nACh) receptors have established an enduring partnership. Like all good partnerships, each partner has proven significant in its own right, while their union has spurred innumerable advances in life science research. A member and prototype of the superfamily of pentameric ligand-gated ion channels, the nACh receptor is a chemo-electric transducer, binding ACh released from nerves and rapidly opening its channel to cation flow to elicit cellular excitation. A subject of a Nobel Prize in Physiology or Medicine, the patch clamp technique provides unprecedented resolution of currents through single ion channels in their native cellular environments. Here, focusing on muscle and α7 nACh receptors, we describe the extraordinary contribution of the patch clamp technique towards understanding how they activate in response to neurotransmitter, how subtle structural and mechanistic differences among nACh receptor subtypes translate into significant physiological differences, and how nACh receptors are being exploited as therapeutic drug targets.

LINKED ARTICLES

This article is part of a themed section on Nicotinic Acetylcholine Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.11/issuetoc/.

Quinacrine-induced occlusive fibrosis in the human fallopian tube is due to a unique inflammatory response and modification of repair mechanisms

Quinacrine has been widely used in treatment of parasitic diseases such as malaria and giardiasis, and in the treatment of autoimmune diseases. Quinacrine has also been used as an effective substitute for surgical contraception by causing occlusion of the fallopian tube. This minimally invasive treatment protocol involves intrauterine insertion of the drug in the form of pellets and has been studied in humans in a number of countries, including the United States. Despite its development in the 1970s, the cellular and molecular events induced by quinacrine in the human fallopian tube have not been described. Here we describe a plausible mechanism for quinacrine action in the fallopian tube. This is manifested as an acute pro-inflammatory response in the uterus and fallopian tube, characterized by loss of epithelial cell adhesion. This response relies on properties of gated channels found on the surface of epithelial cells in the reproductive tract. While the uterus returns to normal, the inflammatory response affects the uterotubal junction and transmural segment of the human fallopian tube, and initiates formation of mature collagen in the lumen of the fallopian tube, resulting in its permanent occlusion. The response within the fallopian tube appears similar to the protective mechanisms that have evolved in women to minimize the likelihood of systemic infection from Neisseria gonorrhoeae, and to some extent from Chlamydia trachomatis. This review could assist in development of experimental models used in investigating the mechanisms of fibrotic responses in humans as well as development of techniques for permanent non-surgical female contraception.

Positive and negative modulation of nicotinic receptors

Nicotinic acetylcholine receptors (AChRs) are one of the best characterized ion channels from the Cys-loop receptor superfamily. The study of acetylcholine binding proteins and prokaryotic ion channels from different species has been paramount for the understanding of the structure-function relationship of the Cys-loop receptor superfamily. AChR function can be modulated by different ligand types. The neurotransmitter ACh and other agonists trigger conformational changes in the receptor, finally opening the intrinsic cation channel. The so-called gating process couples ligand binding, located at the extracellular portion, to the opening of the ion channel, located at the transmembrane region. After agonist activation, in the prolonged presence of agonists, the AChR becomes desensitized. Competitive antagonists overlap the agonist-binding sites inhibiting the pharmacological action of agonists. Positive allosteric modulators (PAMs) do not bind to the orthostetic binding sites but allosterically enhance the activity elicited by agonists by increasing the gating process (type I) and/or by decreasing desensitization (type II). Instead, negative allosteric modulators (NAMs) produce the opposite effects. Interestingly, this negative effect is similar to that found for another class of allosteric drugs, that is, noncompetitive antagonists (NCAs). However, the main difference between both categories of drugs is based on their distinct binding site locations. Although both NAMs and NCAs do not bind to the agonist sites, NACs bind to sites located in the ion channel, whereas NAMs bind to nonluminal sites. However, this classification is less clear for NAMs interacting at the extracellular-transmembrane interface where the ion channel mouth might be involved. Interestingly, PAMs and NAMs might be developed as potential medications for the treatment of several diseases involving AChRs, including dementia-, skin-, and immunological-related diseases, drug addiction, and cancer. More exciting is the potential combination of specific agonists with specific PAMs. However, we are still in the beginning of understanding how these compounds act and how these drugs can be used therapeutically.

Interaction of bupropion with muscle-type nicotinic acetylcholine receptors in different conformational states

To characterize the binding sites and the mechanisms of inhibition of bupropion on muscle-type nicotinic acetylcholine receptors (AChRs), structural and functional approaches were used. The results established that bupropion (a) inhibits epibatidine-induced Ca(2+) influx in embryonic muscle AChRs, (b) inhibits adult muscle AChR macroscopic currents in the resting/activatable state with approximately 100-fold higher potency compared to that in the open state, (c) increases the desensitization rate of adult muscle AChRs from the open state and impairs channel opening from the resting state, (d) inhibits binding of [(3)H]TCP and [(3)H]imipramine to the desensitized/carbamylcholine-bound Torpedo AChR with higher affinity compared to the resting/alpha-bungarotoxin-bound AChR, (e) binds to the Torpedo AChR in either state mainly by an entropy-driven process, and (f) interacts with a binding domain located between the serine (position 6') and valine (position 13') rings, by a network of van der Waals, hydrogen bond, and polar interactions. Collectively, our data indicate that bupropion first binds to the resting AChR, decreasing the probability of ion channel opening. The remnant fraction of open ion channels is subsequently decreased by accelerating the desensitization process. Bupropion interacts with a luminal binding domain shared with PCP that is located between the serine and valine rings, and this interaction is mediated mainly by an entropy-driven process.

Glutamine 57 at the complementary binding site face is a key determinant of morantel selectivity for {alpha}7 nicotinic receptors

Nicotinic receptors (AChRs) play key roles in synaptic transmission. We explored activation of neuronal α7 and mammalian muscle AChRs by morantel and oxantel. Our results revealed a novel action of morantel as a high efficacy and more potent agonist than ACh of α7 receptors. The EC₅₀ for activation by morantel of both α7 and α7-5HT_3A receptors is 7-fold lower than that determined for ACh. The minimum morantel concentration required to activate α7-5HT_3A channels is 6-fold lower than that of ACh, and activation episodes are more prolonged than in the presence of ACh. By contrast, oxantel is a weak agonist of α7 and α7-5HT_3A, and both drugs are very low efficacy agonists of muscle AChRs. The replacement of Gln⁵⁷ in α7 by glycine, which is found in the equivalent position of the muscle AChR, decreases the efficacy for activation and turns morantel into a partial agonist. The reverse mutation in the muscle AChR (ϵG57Q) increases 7-fold the efficacy of morantel. The mutations do not affect activation by ACh or oxantel, indicating that this position is selective for morantel. In silico studies show that the tetrahydropyrimidinyl group, common to both drugs, is close to Trp¹⁴⁹ of the principal face of the binding site, whereas the other cyclic group is proximal to Gln⁵⁷ of the complementary face in morantel but not in oxantel. Thus, position 57 at the complementary face is a key determinant of the high selectivity of morantel for α7. These results provide new information for further progress in drug design.

Kinetics of desensitization and recovery from desensitization for human alpha4beta2-nicotinic acetylcholine receptors stably expressed in SH-EP1 cells

AIM

Studies were conducted to define the kinetics of the onset of and recovery from desensitization for human alpha4beta2-nicotinic acetylcholine receptors (nAChR) heterologously expressed in the SH-EP1 human epithelial cell line.

METHODS

Whole-cell patch clamp recordings were performed to evaluate alpha4beta2-nAChR currents.

RESULTS

Application of 0.1 micromol/L nicotine or 1 mmol/L acetylcholine (ACh) for 1 s or longer induced two phases, with time constants of approximately 70 and approximately 700 ms, for the onset of alpha4beta2-nAChR desensitization. For a given duration of agonist exposure, recovery from desensitization induced by nicotine was slower than recovery from ACh-induced desensitization. Comparisons with published reports indicate that time constants for the recovery of alpha4beta2-nAChRs from desensitization are smaller than those for the recovery of human muscle-type nAChRs(1) from desensitization produced by the same concentrations and durations of exposure to an agonist. Moreover, the extent of human alpha4beta2-nAChR desensitization and rate of recovery are the same, regardless of whether they are measured using whole-cell recording or based on published findings(2) using isotopic ion flux assays; this equality demonstrates the equivalent legitimacy of these techniques in the evaluation of nAChR desensitization. Perhaps most significantly, recovery from desensitization also was best fit to a biphasic process. Regardless of whether it was fit to single or double exponentials, however, half-times for recovery from desensitization grew progressively longer with an increased duration of agonist exposure during the desensitizing pulse.

CONCLUSION

These findings indicate the existence of alpha4beta2-nAChRs in many distinctive states of desensitization, as well as the induction of progressively deeper states of desensitization with the increased duration of agonist exposure.

Quinacrine protects neuronal cells against heat-induced injury

The effects of quinacrine (QA) on heat-induced neuronal injury have been explored, with the intention of understanding the mechanisms of QA protection. Primary cultivated striatum neurons from newborn rats were treated with QA 1h before heat treatment at 43 degrees C which lasted for another 1h, and necrosis and apoptosis were detected by Annexin-V-FITC and propidium iodide (PI) double staining. Neuronal apoptosis was determined using terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling (TUNEL) techniques. Cell membrane fluidity, activity of cytosolic phospholipase A(2) (cPLA(2)) and the level of arachidonic acid (AA) were also examined. Membrane surface ultrastructure of striatum neurons was investigated by atomic force microscopy (AFM). Results showed that heat treatment induced great striatum neurons death, with many dying neurons undergoing necrosis rather than apoptosis. QA alone had little effect on the survival of striatum neurons, while QA pretreatment before heat treatment decreased necrosis. Heat treatment also resulted in decreased membrane fluidity and increased cPLA(2) activity as well as arachidonic acid level; these effects were reversed by QA pretreatment. QA pretreatment also significantly prevented damage to the membrane surface ultrastructure of heat-treated neurons. These results suggest that QA protects striatum neurons against heat-induced neuronal necrosis, and also demonstrate that inhibition of cPLA(2) activity and stabilization of membranes may contribute to protective effect of quinacrine.

The local anaesthetics proadifen and adiphenine inhibit nicotinic receptors by different molecular mechanisms

BACKGROUND AND PURPOSE

Many local anaesthetics are non-competitive inhibitors of nicotinic receptors (acetylcholine receptor, AChR). Proadifen induces a high-affinity state of the receptor, but its mechanism of action and that of an analogue, adiphenine, is unknown.

EXPERIMENTAL APPROACH

We measured the effects of proadifen and adiphenine on single-channel and macroscopic currents of adult mouse muscle AChR (wild-type and mutant). We assessed the results in terms of mechanisms and sites of action.

KEY RESULTS

Both proadifen and adiphenine decreased the frequency of ACh-induced single-channel currents. Proadifen did not change cluster properties, but adiphenine decreased cluster duration (36-fold at 100 micromolxL(-1)). Preincubation with proadifen decreased the amplitude (IC(50)= 19 micromolxL(-1)) without changing the decay rate of macroscopic currents. In contrast, adiphenine did not change amplitude but increased the decay rate (IC(50)= 15 micromolxL(-1)). Kinetic measurements demonstrate that proadifen acts on the resting state to induce a desensitized state whose kinetics of recovery resemble those of ACh-induced desensitization. Adiphenine accelerates desensitization from the open state, but previous application of the drug to resting receptors is required. Both drugs stabilize desensitized states, as evidenced by the decrease in the number of clusters elicited by high ACh concentrations. The inhibition by adiphenine is not affected by proadifen, and the mutation alphaE262K decreases the sensitivity of the AChR only for adiphenine, indicating that these drugs act at different sites.

CONCLUSIONS AND IMPLICATIONS

Two analogous local anaesthetics bind to different sites and inhibit AChR activity via different mechanisms and conformational states. These results provide new information on drug modulation of AChR.

Pharmacological and neurotoxicological actions mediated by bupropion and diethylpropion

The antiappetite agent diethylpropion (DEP), and the antidepressant and antismoking aid compound bupropion (BP), not only share the same structural motif but also present similar mechanisms of action in the CNS. For example, both drugs induce the release as well as inhibit the reuptake of neurotransmitters such as a dopamine (DA) and norepinephrine (NE). In general, they produce mild side effects, including reversible psychomotor alterations mostly in geriatric patients (by BP), or moderate changes in neurotransmitter contents linked to oxidative damage (by DEP). Therefore, attention must be paid during any therapeutic use of these agents. Regarding the interaction of BP with the DA transporter, residues S359, located in the middle of TM7, and A279, located close to the extracellular end of TM5, contribute to the binding and blockade of translocation mediated by BP, respectively. Additional mechanisms of action have also been determined for each compound. For example, BP is a noncompetitive antagonist (NCA) of several nicotinic acetylcholine receptors (AChRs). Based on this evidence, the dual antidepressant and antinicotinic activity of BP is currently considered to be mediated by its stimulatory action on DA and NE systems as well as its inhibitory action on AChRs. Considering the results obtained in the archetypical mouse muscle AChR, a sequential mechanism can be hypothesized to explain the inhibitory action of BP on neuronal AChRs: (1) BP first binds to AChRs in the resting state, decreasing the probability of ion channel opening, (2) the remnant fraction of open ion channels is subsequently decreased by accelerating the desensitization process, and finally (3) BP interacts with a binding domain located between the serine (position 9') and valine (position 13') rings that is shared with the NCA phencyclidine and other tricyclic antidepressants. The homologous location in the alpha3beta4 AChR is between the serine and valine/phenylalanine rings. This new evidence opens a window for further investigation using AChRs as targets for the action of safer antidepressants and novel antiaddictive compounds.

The interface between extracellular and transmembrane domains of homomeric Cys-loop receptors governs open-channel lifetime and rate of desensitization

The lifetimes of activated postsynaptic receptor channels contribute to the efficiency of synaptic transmission. Here we show that structural differences within the interface dividing extracellular and transmembrane domains of homomeric alpha7 and 5-HT(3A) receptors account for the large differences in open-channel lifetime and time of desensitization onset between these contrasting members of the Cys-loop receptor superfamily. For alpha7 receptors, agonist-evoked single-channel currents appear mainly as isolated brief openings (tau(o) = 0.35 ms), whereas macroscopic currents after a step pulse of agonist desensitize rapidly (tau(d) = 0.4 ms). In contrast for 5-HT(3A) receptors, agonist-evoked single-channel currents appear as clusters of many long openings in quick succession (tau(cluster) = 1.2 s), whereas macroscopic currents desensitize slowly (tau(d) = 1.1 s). A chimeric alpha7-5HT(3A) receptor exhibits functional properties intermediate between those of the parent receptors, but the functional signatures of each parent are reconstituted after substituting the major loops within the interface of the extracellular and transmembrane domains from the corresponding parent receptor. Furthermore, these structural loops contribute to open-channel lifetime and time of desensitization onset in a nonadditive manner. The results suggest that desensitization is the major determinant of the lifetimes of activated alpha7 and 5-HT(3A) receptors and that functional differences between the two receptors arise primarily through structural differences at the interface between extracellular and transmembrane domains.

Discriminating between cellular and misfolded prion protein by using affinity to 9-aminoacridine compounds

Quinacrine and related 9-aminoacridine compounds are effective in eliminating the alternatively folded prion protein, termed PrP(Sc), from scrapie-infected cultured cells. Clinical evaluations of quinacrine for the treatment of human prion diseases are progressing in the absence of a clear understanding of the molecular mechanism by which prion replication is blocked. Here, insight into the mode of action of 9-aminoacridine compounds was sought by using a chemical proteomics approach to target identification. Cellular macromolecules that bind 9-aminoacridine ligands were affinity-purified from tissue lysates by using a 9-aminoacridine-functionalized solid-phase matrix. Although the 9-aminoacridine matrix was conformationally selective for PrP(Sc), it was inefficient: approximately 5 % of PrP(Sc) was bound under conditions that did not support binding of the cellular isoform, PrP(C). Our findings suggest that 9-aminoacridine compounds may reduce the PrP(Sc) burden either by occluding epitopes necessary for templating on the surface of PrP(Sc) or by altering the stability of PrP(Sc) oligomers, where a one-to-one stoichiometry is not necessary.

Molecular determinants of pyrantel selectivity in nicotinic receptors

Nicotinic receptors (acetylcholine receptors, AChRs) play key roles in synaptic transmission throughout the nervous system. AChRs mediate neuromuscular transmission in nematodes, and they are targets for antiparasitic drugs. The anthelmintic agents levamisole and pyrantel, which are potent agonists of nematode muscle AChRs, are partial agonists of mammalian muscle AChRs. To further explore the structural basis of the differential activation of AChR subtypes by anthelmintics, we studied the activation of alpha7 AChRs using the high-conductance form of the alpha7-5-hydroxytryptamine-3A receptor, which is a good model for pharmacological studies involving the extracellular region of alpha7. Macroscopic and single-channel current recordings show that levamisole is a weak agonist of alpha7. It is interesting that pyrantel is a more potent agonist of alpha7 than acetylcholine (ACh). To identify determinants of this differential activation, we replaced residues of the complementary face of the binding site by the homologous residues in the muscle epsilon subunit and evaluated changes in activation. The mutation Q57G does not affect the activation by either ACh or levamisole. However, it increases EC50 values and decreases the maximal response to pyrantel. Kinetic analysis shows that gating of the mutant channel activated by pyrantel is profoundly impaired. The decreased sensitivity of alpha7-Q57G to pyrantel agrees with its weak action at muscle AChRs, indicating that when glycine occupies position 57, as in the mammalian muscle AChR, pyrantel behaves as a partial agonist. Thus, position 57 located at the complementary face of the binding site plays a key role in the selective activation of AChRs by pyrantel.

Single-channel kinetic analysis of chimeric alpha7-5HT3A receptors

The receptor chimera alpha7-5HT3A has served as a prototype for understanding the pharmacology of alpha7 neuronal nicotinic receptors, yet its low single channel conductance has prevented studies of the activation kinetics of single receptor channels. In this study, we show that introducing mutations in the M3-M4 cytoplasmic linker of the chimera alters neither the apparent affinity for the agonist nor the EC50 but increases the amplitude of agonist-evoked single channel currents to enable kinetic analysis. Channel events appear as single brief openings flanked by long closings or as bursts of several openings in quick succession. Both the open and closed time distributions are described as the sum of multiple exponential components, but these do not change over a wide range of acetylcholine (ACh), nicotine, or choline concentrations. Bursts elicited by a saturating concentration of ACh contain brief and long openings and closings, and a cyclic scheme containing two open and two closed states is found to adequately describe the data. The analysis indicates that once fully occupied, the receptor opens rapidly and efficiently, and closes and reopens several times before it desensitizes. Channel closing and desensitization occur at similar rates and account for the invariant open and closed time distributions.

Coupling of agonist binding to channel gating in an ACh-binding protein linked to an ion channel

Neurotransmitter receptors from the Cys-loop superfamily couple the binding of agonist to the opening of an intrinsic ion pore in the final step in rapid synaptic transmission. Although atomic resolution structural data have recently emerged for individual binding and pore domains, how they are linked into a functional unit remains unknown. Here we identify structural requirements for functionally coupling the two domains by combining acetylcholine (ACh)-binding protein, whose structure was determined at atomic resolution, with the pore domain from the serotonin type-3A (5-HT3A) receptor. Only when amino-acid sequences of three loops in ACh-binding protein are changed to their 5-HT3A counterparts does ACh bind with low affinity characteristic of activatable receptors, and trigger opening of the ion pore. Thus functional coupling requires structural compatibility at the interface of the binding and pore domains. Structural modelling reveals a network of interacting loops between binding and pore domains that mediates this allosteric coupling process.

Molecular Basis of the Differential Sensitivity of Nematode and Mammalian Muscle to the Anthelmintic Agent Levamisole

Levamisole is an anthelmintic agent that exerts its therapeutic effect by acting as a full agonist of the nicotinic receptor (AChR) of nematode muscle. Its action at the mammalian muscle AChR has not been elucidated to date despite its wide use as an anthelmintic in humans and cattle. By single channel and macroscopic current recordings, we investigated the interaction of levamisole with the mammalian muscle AChR. Levamisole activates mammalian AChRs. However, single channel openings are briefer than those activated by acetylcholine (ACh) and do not appear in clusters at high concentrations. The peak current induced by levamisole is about 3% that activated by ACh. Thus, the anthelmintic acts as a weak agonist of the mammalian AChR. Levamisole also produces open channel blockade of the AChR. The apparent affinity for block (190 microm at -70 mV) is similar to that of the nematode AChR, suggesting that differences in channel activation kinetics govern the different sensitivity of nematode and mammalian muscle to anthelmintics. To identify the structural basis of this different sensitivity, we performed mutagenesis targeting residues in the alpha subunit that differ between vertebrates and nematodes. The replacement of the conserved alphaGly-153 with the homologous glutamic acid of nematode AChR significantly increases the efficacy of levamisole to activate channels. Channel activity takes place in clusters having two different kinetic modes. The kinetics of the high open probability mode are almost identical when the agonist is ACh or levamisole. It is concluded that alphaGly-153 is involved in the low efficacy of levamisole to activate mammalian muscle AChRs.

Quinacrine blocks PrP (106-126)-formed channels

We investigated the action of the acridine derivative, quinacrine (QC), which has been shown to act as a noncompetitive channel inhibitor. The main effects of QC are voltage- and concentration-dependent changes in the kinetics of the prion protein fragment (PrP[106-126])-formed cation channels. The current-voltage relationships show that the maximal current (I) was not affected whereas the physiologically important mean current (I') was reduced as a result of changes in channel kinetics. These findings suggest that QC acts on the open state of the channels. The half-inhibitory concentration (IC50) for the dose-dependent effects of [QC]cis on the kinetic parameters of the PrP(106-126)-formed cation channel shows a reduction in the ratios Po(QC)/Po, Fo(QC)/Fo, and To(QC)/To, whereas Tc(QC)/Tc increases. Of these ratios, Po(QC)/Po was more sensitive than the others. The corresponding IC50 for these ratios were 51, 94, 86, and 250 microM QC, respectively. The QC-induced changes in the kinetic parameters were more apparent at positive voltages. IC50 values for Po were 95, 75, and 51 microM at +20, +80, and +140 mV, respectively. The fact that QC induced changes in the kinetics of this channel, although the conductance of the channel remained unchanged, indicates that QC may bind at the mouth of the channel via a mechanism known as fast channel block. The QC-induced changes in the kinetic parameters of this channel suggest that they are pathophysiologically significant because these channels could be the mechanisms by which amyloids induce membrane damage in vivo.

Cu2+-induced modification of the kinetics of A beta(1-42) channels

We found that the amyloid beta peptide A beta(1-42) is capable of interacting with membrane and forming heterogeneous ion channels in the absence of any added Cu2+ or biological redox agents that have been reported to mediate A beta(1-42) toxicity. The A beta(1-42)-formed cation channel was inhibited by Cu2+ in cis solution ([Cu2+]cis) in a voltage- and concentration-dependent manner between 0 and 250 microM. The [Cu2+]cis-induced channel inhibition is fully reversible at low concentrations between 50 and 100 microM [Cu2+]cis and partially reversible at 250 microM [Cu2+]cis. The inhibitory effects of [Cu2+]cis between 50 and 250 microM on the channel could not be reversed with addition of Cu2+-chelating agent clioquinol (CQ) at concentrations between 64 and 384 microM applied to the cis chamber. The effects of 200-250 microM [Cu2+]cis on the burst and intraburst kinetic parameters were not fully reversible with either wash or 128 microM [CQ]cis. The kinetic analysis of the data indicate that Cu2+-induced inhibition was mediated via both desensitization and an open channel block mechanism and that Cu2+ binds to the histidine residues located at the mouth of the channel. It is proposed that the Cu2+-binding site of the A beta(1-42)-formed channels is modulated with Cu2+ in a similar way to those of channels formed with the prion protein fragment PrP(106-126), suggesting a possible common mechanism for Cu2+ modulation of A beta and PrP channel proteins linked to neurodegenerative diseases.

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.

Quinacrine increases endothelial nitric oxide release: role of superoxide anion

The effect of acute quinacrine treatment on agonist-induced nitric oxide (NO) release was investigated in cultured human endothelial cells using electrochemical monitoring of the in situ NO concentration. Quinacrine dose-dependently increased NO release with an apparent EC50 of 0.2 microM and a maximal effect at 1 microM. Quinacrine did not modify the dependence of NO release on extracellular L-arginine. Acceleration or deceleration of O2- dismutation, which altered NO release in control cells, did not modify it in quinacrine-treated cells. Quinacrine did not modify NO amperometric signal or reaction with O2- produced by xanthine oxidation. In the presence of quinacrine, agonist-induced NO release became Mg2+ -independent and could not be attributed to an inhibition of phospholipase A2 activity. Quinacrine made NO release insensitive to Cu2+ chelation. The present study demonstrates that acute treatment by low quinacrine concentrations increases endothelial NO release, possibly through an inhibition of O2- production.