Cocaine: mechanism of inhibition of a muscle acetylcholine receptor studied by a laser-pulse photolysis technique

In Vitro Selection of Short DNA Aptamers that Can Inhibit or Alleviate Cocaine and MK-801 Inhibition of Muscle-Type Nicotinic Acetylcholine Receptors

Ligands of high specificity and selectivity have been selected for biological molecules of interest including nicotinic acetylcholine receptor (nAChR) using combinatorial libraries of nucleic acids. The nAChR belongs to a group of structurally related proteins that regulate signal transmission between ~ 10¹² cells of the mammalian nervous system. It is inhibited by both therapeutic agents and abused drugs, including cocaine. A mechanism-based approach to alleviating noncompetitive inhibition of the mucle-type nAChR, including Torpedo, resulted in the selection of very short DNA aptamers only seven nucleotides long. By transient kinetic measurements, these DNA aptamers, which displaced cocaine from its binding site on the muscle-type nAChR, were classified into two groups based on their effects on the nAChR: Class I aptamers inhibit agonist-induced current in the muscle-type nAChR and Class II molecules alleviate inhibition by MK-801 [(+)-dizocilpine] without affecting the receptor function. The most potent Class I DNA aptamer, which inhibits the muscle-type nAChR, has an apparent dissociation constant (K_Iapt) of 5 μM, while the most efficient Class II DNA aptamer, which alleviates MK-801-induced inhibition, has an apparent dissociation constant (K_Apt) of 1.8 μM. An innovative aspect of the work is the identification of very short DNA aptamers with these properties that makes them attractive for therapeutic and diagnostic applications.

Effects of ecgonine methyl ester on cognition in scopolamine-impaired and aged rats

RATIONALE

Searches for antidotes to cocaine, and for cognition enhancers potentially applicable to Alzheimer's disease, have revealed a novel regulatory site on nicotinic acetylcholine receptors. In the presence of an agonist, inhibitors binding to this site changed the ion channel equilibrium from the open-channel form towards the closed form. Other, related, molecules could bind to the site without changing the equilibrium. These latter compounds were predicted to displace the inhibitors without affecting receptor function per se. These compounds alleviated the inhibition. One of them is ecgonine methyl ester (EME), which is generally described as inactive, but this work suggested a beneficial effect on cognition.

OBJECTIVE

This in vivo study tested for cognitive enhancement by EME in scopolamine-impaired, and aged, rats.

METHODS

Memory was the primary endpoint, but thigmotaxis became an important secondary endpoint in the light of observations made during the study. Impaired cognition was pharmacologically induced by scopolamine in young rats, and spontaneously present in aged rats. Learning ability before and after administration of EME was tested in Morris water maze protocols. Concentrations of EME in the brain and plasma were analyzed by gas chromatography-mass spectrometry.

RESULTS

A single dose of EME reversed scopolamine impairment, indicating involvement of acetylcholine receptors. Longer-term treatment improved cognition in aged rats, with enhanced rates of learning in the absence of an exogenous cognition-impairing compound. Impairment returned with a new challenge; the improvement could be re-established with continued dosing. EME also reversed thigmotaxis seen in aged rats; thigmotaxis is believed to indicate anxiety. The concentrations of EME in the brain proved adequate drug exposure.

CONCLUSIONS

Since other investigators have shown cognition impairment caused by cocaine in aged rats, this work shows that cocaine and EME have opposite effects in Morris water maze models. EME might induce cognitive enhancement and relief of anxiety in cocaine-impaired humans, and in other cognitive disorders.

Caged neurotransmitters and other caged compounds: design and application

The approaches using caged neurotransmitters described here enable transient kinetic investigations to be made with membrane-bound proteins (receptors) on a cell surface with the same time resolution as was previously possible only with proteins in solution.

Rescue of amyloid-Beta-induced inhibition of nicotinic acetylcholine receptors by a peptide homologous to the nicotine binding domain of the alpha 7 subtype

Alzheimer's disease (AD) is characterized by brain accumulation of the neurotoxic amyloid-β peptide (Aβ) and by loss of cholinergic neurons and nicotinic acetylcholine receptors (nAChRs). Recent evidence indicates that memory loss and cognitive decline in AD correlate better with the amount of soluble Aβ than with the extent of amyloid plaque deposits in affected brains. Inhibition of nAChRs by soluble Aβ40 is suggested to contribute to early cholinergic dysfunction in AD. Using phage display screening, we have previously identified a heptapeptide, termed IQ, homologous to most nAChR subtypes, binding with nanomolar affinity to soluble Aβ40 and blocking Aβ-induced inhibition of carbamylcholine-induced currents in PC12 cells expressing α7 nAChRs. Using alanine scanning mutagenesis and whole-cell current recording, we have now defined the amino acids in IQ essential for reversal of Aβ40 inhibition of carbamylcholine-induced responses in PC12 cells, mediated by α7 subtypes and other endogenously expressed nAChRs. We further investigated the effects of soluble Aβ, IQ and analogues of IQ on α3β4 nAChRs recombinantly expressed in HEK293 cells. Results show that nanomolar concentrations of soluble Aβ40 potently inhibit the function of α3β4 nAChRs, and that subsequent addition of IQ or its analogues does not reverse this effect. However, co-application of IQ makes the inhibition of α3β4 nAChRs by Aβ40 reversible. These findings indicate that Aβ40 inhibits different subtypes of nAChRs by interacting with specific receptor domains homologous to the IQ peptide, suggesting that IQ may be a lead for novel drugs to block the inhibition of cholinergic function in AD.

Identification of aptamers as specific binders and modulators of cell-surface receptor activity

In recent years, the SELEX (Systematic Evolution of Ligands by EXponential enrichment) technology has established itself as a powerful tool in basic research with promising applications in diagnostics and therapeutics. Oligonucleotides with high-affinities to their targets, denominated as aptamers, are obtained from partially random oligonucleotide pools by reiterative in vitro selection cycles and screening for binding activity. The original technique allowing the identification of aptamers binding to soluble targets, has recently been extended in order to produce aptamers binding to complex targets including receptors and ion channels embedded in the plasma membrane as well as whole cell surfaces or parasite organisms. In addition to discussing the most recent developments with focus on possible diagnostic and therapeutic application, we provide a simple protocol which has been successfully used to select for RNA aptamers as allosteric modulators of nicotinic receptor activity.

Mechanism of Inhibition of the GluA2 AMPA Receptor Channel Opening: the Role of 4-Methyl versus 4-Carbonyl Group on the Diazepine Ring of 2,3-Benzodiazepine Derivatives

2,3-Benzodiazepine derivatives are synthesized as drug candidates for a potential treatment of various neurodegenerative diseases involving the excessive activity of AMPA receptors. Here, we describe a rapid kinetic investigation of the mechanism of inhibition of the GluA2Q(flip) AMPA receptor channel opening by two 2,3-benzodiazepine derivatives, i.e. the prototypic 2,3-benzodiazepine compound GYKI 52466 [(1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine)] and 1-(4-aminophenyl)-3,5-dihydro-7,8-methylenedioxy-4H-2,3-benzodiazepin-4-one (BDZ-2). GYKI 52466 and BDZ-2 are structurally similar in that the 4-methyl group in the diazepine ring of GYKI 52466 is replaced by a carbonyl group, yielding BDZ-2. Using a laser-pulse photolysis technique with ∼60 μs time resolution, we characterize the effect of the two compounds individually on the channel-opening process of the GluA2Q(flip) receptor expressed in HEK-293 cells. We find that BDZ-2 preferentially inhibits the open-channel state, whereas GYKI 52466 is more selective for the closed-channel state of the GluA2Q(flip) receptors. Each inhibitor binds independently to its own noncompetitive site, yet the two sites do not interact allosterically. The significance of these results in the context of both the structure-activity relationship and the properties of the GluA2Q(flip) receptor channels is presented.

Potent and selective inhibition of a single alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit by an RNA aptamer

Inhibitors of AMPA-type glutamate ion channels are useful as biochemical probes for structure-function studies and as drug candidates for a number of neurological disorders and diseases. Here, we describe the identification of an RNA inhibitor or aptamer by an in vitro evolution approach and a characterization of its mechanism of inhibition on the sites of interaction by equilibrium binding and on the receptor channel opening rate by a laser-pulse photolysis technique. Our results show that the aptamer is a noncompetitive inhibitor that selectively inhibits the GluA2Q(flip) AMPA receptor subunit without any effect on other AMPA receptor subunits or kainate or NMDA receptors. On the GluA2 subunit, this aptamer preferentially inhibits the flip variant. Furthermore, the aptamer preferentially inhibits the closed-channel state of GluA2Q(flip) with a K(I) = 1.5 μM or by ∼15-fold over the open-channel state. The potency and selectivity of this aptamer rival those of small molecule inhibitors. Together, these properties make this aptamer a promising candidate for the development of water-soluble, highly potent, and GluA2 subunit-selective drugs.

Potent and selective inhibition of the open-channel conformation of AMPA receptors by an RNA aptamer

Inhibitors of AMPA receptors are useful as biochemical probes for structure-function studies and as drug candidates for a number of neurological disorders and diseases. Here we report the identification of an RNA inhibitor or aptamer by an in vitro evolution approach. Using a laser-pulse photolysis technique, we further characterized the mechanism of inhibition of this aptamer on the AMPA receptor channel-opening rate process in the microsecond-to-millisecond time domain. Our results show that the aptamer we isolated is a noncompetitive inhibitor that selectively inhibits the open-channel conformation of AMPA receptors with nanomolar affinity. The potency and the selectivity of this noncompetitive aptamer rival those of small molecule inhibitors. Our results therefore demonstrate the utility of this approach in developing water-soluble, highly potent, and conformation-selective noncompetitive inhibitors of AMPA receptors.

Minimal RNA aptamer sequences that can inhibit or alleviate noncompetitive inhibition of the muscle-type nicotinic acetylcholine receptor

Combinatorially synthesized nucleotide polymers have been used during the last decade to find ligands that bind to specific sites on biological molecules, including membrane-bound proteins such as the nicotinic acetylcholine receptors (nAChRs). The neurotransmitter receptors belong to a group of four structurally related proteins that regulate signal transmission between ~10(11) neurons of the mammalian nervous system. The nAChRs are inhibited by compounds such as the anticonvulsant MK-801 [(+)-dizocilpine] and abused drugs such as cocaine. Based on predictions arising from the mechanism of receptor inhibition by MK-801 and cocaine, we developed two classes of RNA aptamers: class I members, which inhibit the nAChR, and class II members, which alleviate inhibition of the receptor by MK-801 and cocaine. The systematic evolution of ligands by the exponential enrichment (SELEX) method was used to obtain these compounds. Here, we report that we have truncated RNA aptamers in each class to determine the minimal nucleic acid sequence that retains the characteristic function for which the aptamer was originally selected. We demonstrate that a truncated class I aptamer containing a sequence of seven nucleotides inhibits the nAChR and that a truncated class II aptamer containing a sequence of only four nucleotides can alleviate MK-801 inhibition.

Neural response to lidocaine in healthy subjects

Recent studies suggest that some of cocaine's central nervous system (CNS) effects may be mediated through its sodium channel inhibiting local anesthetic properties. Local anesthetics that lack cocaine's strong affinity for the dopamine transporter (DAT) also produce sensory and mood effects, further suggesting a role for this neural pathway. Due to an absence of affinity at the DAT, the local anesthetic lidocaine may offer the potential to assess sodium channel activity in vivo in humans. To assess the utility of lidocaine as a CNS probe, we determined regional cerebral blood flow (rCBF) with single photon emission computed tomography (SPECT) following the intravenous administration of lidocaine (0.5 mg/kg) and compared this response to procaine (0.5 mg/kg and 1.0 mg/kg), a local anesthetic with partial affinity for the DAT, and saline. Infusions were administered in nine healthy female controls over a 10-day period with at least 2 days between each scan. Increased rCBF was observed following lidocaine, relative to saline, in the insula, caudate, thalamus, and posterior cingulate. Decreased rCBF was detected in a different region of the posterior cingulate. In general, increases in rCBF were more marked following lidocaine relative to procaine. Mood and sensory changes following lidocaine were limited and significantly less than those induced by either dose of procaine. There were no significant changes in blood pressure or heart rate following either medication. These findings suggest that lidocaine can be safely used to assess sodium channel function in persons with addictive and other psychiatric disorders.

The role of acetylcholine in cocaine addiction

Central nervous system cholinergic neurons arise from several discrete sources, project to multiple brain regions, and exert specific effects on reward, learning, and memory. These processes are critical for the development and persistence of addictive disorders. Although other neurotransmitters, including dopamine, glutamate, and serotonin, have been the primary focus of drug research to date, a growing preclinical literature reveals a critical role of acetylcholine (ACh) in the experience and progression of drug use. This review will present and integrate the findings regarding the role of ACh in drug dependence, with a primary focus on cocaine and the muscarinic ACh system. Mesostriatal ACh appears to mediate reinforcement through its effect on reward, satiation, and aversion, and chronic cocaine administration produces neuroadaptive changes in the striatum. ACh is further involved in the acquisition of conditional associations that underlie cocaine self-administration and context-dependent sensitization, the acquisition of associations in conditioned learning, and drug procurement through its effects on arousal and attention. Long-term cocaine use may induce neuronal alterations in the brain that affect the ACh system and impair executive function, possibly contributing to the disruptions in decision making that characterize this population. These primarily preclinical studies suggest that ACh exerts a myriad of effects on the addictive process and that persistent changes to the ACh system following chronic drug use may exacerbate the risk of relapse during recovery. Ultimately, ACh modulation may be a potential target for pharmacological treatment interventions in cocaine-addicted subjects. However, the complicated neurocircuitry of the cholinergic system, the multiple ACh receptor subtypes, the confluence of excitatory and inhibitory ACh inputs, and the unique properties of the striatal cholinergic interneurons suggest that a precise target of cholinergic manipulation will be required to impact substance use in the clinical population.

Mode of cembranoid action on embryonic muscle acetylcholine receptor

The mechanism of eupalmerin acetate (EUAC) actions on the embryonic muscle nicotinic acetylcholine receptor (nAChR) in BC3H-1 cells was studied by using whole-cell and single-channel patch-clamp current measurements. With whole-cell currents, EUAC did not act as an agonist on this receptor. Coapplication of 30 microM EUAC with 50 microM, 100 microM, or 500 microM carbamoylcholine (CCh) reversibly inhibited the current amplitude, whereas, with 20 microM CCh, current was increased above control values in the presence of EUAC. EUAC concentration curves (0.01-40 microM) obtained with 100 microM and 500 microM CCh displayed slope coefficients, n(H), significantly smaller than one, suggesting that EUAC bound to several sites with widely differing affinities on the receptor molecule. The apparent rate of receptor desensitization in the presence of EUAC and CCh was either slower than or equal to that obtained with CCh alone. The major finding from single-channel studies was that EUAC did not affect single-channel conductance or the ability of CCh to interact with the receptor. Instead, EUAC acted by increasing the channel closing rate constant. The results are not consistent with the competitive model for EUAC inhibition, with the sequential open-channel block model, or with inhibition by increased desensitization. The data are best accounted for by a model in which EUAC acts by closed-channel block at low concentrations, by positive modulation at intermediate concentrations, and by negative allosteric modulation of the open channel at high concentrations.

Selection of 2?-Fluoro-modified RNA Aptamers for Alleviation of Cocaine and MK-801Inhibition of the Nicotinic Acetylcholine Receptor

The nicotinic acetylcholine receptor (nAChR) belongs to a group of five structurally related proteins that regulate signal transmission between approximately 10(12) cells of the mammalian nervous system. Many therapeutic agents and abused drugs inhibit the nAChR, including the anti-convulsant MK-801 and the abused drug cocaine. Many attempts have been made to find compounds that prevent inhibition by cocaine. Use of transient kinetic techniques to investigate the inhibition of the receptor by MK-801 and cocaine led to an inhibition mechanism not previously proposed. The mechanism led to the development of combinatorially synthesized RNA ligands that alleviate inhibition of the receptor. However, these ligands are relatively unstable. Here we determined whether much more stable 2'-fluoro-modified RNA ligands can be prepared and used to study the alleviation of receptor inhibition. Two classes of 2'-fluoro-modified RNA ligands were obtained: One class binds with higher affinity to the cocaine-binding site on the closed-channel form and, as predicted by the mechanism, inhibits the receptor. The second class binds with equal or higher affinity to the cocaine-binding site on the open-channel form and, as predicted by the mechanism, does not inhibit the receptor, and does alleviate cocaine and MK-801 inhibition of the nAChR. The stability of these 2'-fluoro-RNAs expands the utility of these ligands.

The Importance of Brainstem Cholinergic Neurons in the Pressor Response to Cocaine

After intracisternal injection, 140 nmol (48 microg) of cocaine (but not lidocaine or procaine) evoked an increase in mean arterial pressure (MAP) of 41 mm Hg. The increase in MAP began within 1 min after injection and lasted 10 to 15 min. The pressor response to intracisternal injection of cocaine was not mediated through central alpha-adrenergic receptors, but intracisternal pretreatment with D1 or D2 dopamine receptor antagonists shortened the duration of the response. Pretreatment with intracisternal injection of hemicholinium-3 to deplete medullary acetylcholine produced a dose-dependent inhibition of the pressor and tachycardic responses to intracisternal injection of cocaine. Central pretreatment with hemicholinium-3 also inhibited the pressor response to intravenous injection of 0.5 mg/kg cocaine. Atropine pretreatment was only partly effective in blocking the pressor and tachycardic responses to intracisternal injection of cocaine. However, a single intracisternal injection of the nicotinic ganglionic receptor blocker hexamethonium inhibited the pressor response to cocaine administered intracisternally 24 h later, and on each of the following 4 days. The blocking effect of hexamethonium was not mimicked by the alpha7 selective antagonist methyllycaconitine or by the alpha4beta2 subtype-preferring antagonist dihydro-beta-erythroidine. The data suggest that the pressor response to cocaine is mediated by medullary acetylcholine release on to nicotinic receptors of the ganglionic type, enhancing the output of bulbospinal sympathetic premotor neurons. Our results provide new evidence for the prolonged inactivation of relevant central nicotinic receptors by nicotinic receptor antagonists, and suggest that such compounds might be used safely for cocaine overdose, as well as for antiabuse issues without the concern for autonomic side effects.

Rapid chemical reaction techniques developed for use in investigations of membrane-bound proteins (neurotransmitter receptors)

New techniques for investigating chemical reactions on cell surfaces in the microsecond-to-millisecond time region are described. Reactions mediated by membrane-bound neurotransmitter receptors that control signal transmission between approximately 10(12) cells of the nervous system are taken as an example. Cells with receptors on their plasma membrane are equilibrated with photolabile, biologically inactive precursors of the neurotransmitters. Photolysis of these compounds releases free neurotransmitter that interacts with the receptors, leading to the transient opening of transmembrane receptor-formed channels that are permeant to small inorganic ions. The current thus induced can be measured. The technique can be used to measure the elementary steps of the receptor-mediated reactions. To illustrate the approach it was shown that an understanding of the mechanism of inhibition of the nicotinic acetylcholine receptor by the drug cocaine was obtained and led to the first proof that compounds exist that alleviate the inhibition.

Cocaine inhibits cromakalim-activated K+ currents in follicle-enclosed Xenopus oocytes

The effect of cocaine on K+ currents activated by the KATP channel opener cromakalim was investigated in follicular cells of Xenopus oocytes. The results indicate that cocaine in the concentration range of 3-500 microM reversibly inhibits cromakalim-induced K+ currents. The IC50 value for cocaine was 96 microM. Inhibition of the cromakalim-activated K+ current by cocaine was noncompetitive and voltage independent. Pretreatment with the Ca2+ chelator BAPTA did not modify the cocaine-induced inhibition of cromakalim-induced K+ currents, suggesting that Ca2+-activated second messenger pathways are not involved in the actions of cocaine. Outward K+ currents activated by the application of 8-Br-cAMP or forskolin were also inhibited by cocaine. The EC50 and slope values for the activation of K+ currents by cromakalim were 184+/-19 microM and 1.14 in the absence of cocaine as compared to 191+/-23 microM and 1.03 in the presence of cocaine (300 microM). Cocaine also blocked K+ currents mediated through C-terminally deleted form of Kir6.2 (KirDeltaC26) in the absence of sulfonylurea receptor with an IC50 value of 87 microM, suggesting that cocaine interacts directly with the channel forming Kir6.2 subunit. Radioligand binding studies indicated that cocaine (100 microM) did not affect the binding characteristics of the KATP ligand, [3H]glibenclamide. These results demonstrate that cromakalim-activated K+ currents in follicular cells of Xenopus oocytes are modulated by cocaine.

How fast does the GluR1Qflip channel open?

Opening of a ligand-gated ion channel is the step at which the binding of a neurotransmitter is transduced into the electrical signal by allowing ions to flow through the transmembrane channel, thereby altering the postsynaptic membrane potential. We report the kinetics for the opening of the GluR1Qflip channel, an alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunit of the ionotropic glutamate receptors. Using a laser-pulse photolysis technique that permits glutamate to be liberated photolytically from gamma-O-(alpha-carboxy-2-nitrobenzyl)glutamate (caged glutamate) with a time constant of approximately 30 micros, we show that, after the binding of glutamate, the channel opened with a rate constant of (2.9 +/- 0.2) x 10(4) s(-1) and closed with a rate constant of (2.1 +/- 0.1) x 10(3) s(-1). The observed shortest rise time (20-80% of the receptor current response), i.e. the fastest time by which the GluR1Qflip channel can open, was predicted to be 35 micros. This value is three times shorter than those previously reported. The minimal kinetic mechanism for channel opening consists of binding of two glutamate molecules, with the channel-opening probability being 0.93 +/- 0.10. These findings identify GluR1Qflip as one of the temporally efficient receptors that transduce the binding of chemical signals (i.e. glutamate) into an electrical impulse.

Synthesis and characterization of photolabile o-nitrobenzyl derivatives of urea

We present here the synthesis and characterization of four photolabile derivatives of urea in which alpha-substituted 2-nitrobenzyl groups are covalently attached to the urea nitrogen. These derivatives photolyze readily in aqueous solution to release free urea. The alpha-substituents of the 2-nitrobenzyl group strongly influence the rate of the photolysis reaction measured with transient absorption spectroscopy. Rates of photolysis at pH 7.5 and room temperature (approximately 22 degrees C) for N-(2-nitrobenzyl)urea, N-(alpha-methyl-2-nitrobenzyl)urea, N-(alpha-carboxymethyl-2-nitrobenzyl)urea, and N-(alpha-carboxy-2-nitrobenzyl)urea are, respectively, 1.7 x 10(4), 8.5 x 10(4), 4.0 x 10(4), and 1.1 x 10(5) s(-)(1). The quantum yields determined by measurement of free urea following irradiation by a single laser pulse at 308 nm were 0.81 for N-(2-nitrobenzyl)urea, 0.64 for N-(alpha-methyl-2-nitrobenzyl)urea, and 0.56 for N-(alpha-carboxy-2-nitrobenzyl)urea. The caged N-(alpha-carboxy-2-nitrobenzyl)urea is not a substrate of the enzyme urease, while the photolytically released urea is. Also, neither this caged urea nor its photolytic side products inhibit hydrolysis of free urea by urease. Thus, the alpha-carboxy-2-nitrobenzyl derivative of urea is suitable for mechanistic investigations of the enzyme urease.

Mechanism-based discovery of ligands that counteract inhibition of the nicotinic acetylcholine receptor by cocaine and MK-801

Nicotinic acetylcholine receptors (AChR) belong to a family of proteins that form ligand-gated transmembrane ion channels. They are involved in the fast transmission of signals between cells and the control of intercellular communication in the nervous system. A variety of therapeutic agents and abused drugs, including cocaine, inhibit the AChR and monoamine transporters and interfere with nervous system function. Here we describe a mechanism-based approach to prevent this inhibition. We had previously developed presteady-state kinetic (transient kinetic) techniques, with microsecond-to-millisecond time resolutions, for investigations of reactions on cell surfaces that allow one to determine the effects of inhibitors not only on the channel-opening probability but also on the opening and closing rates of the AChR channel. The transient kinetic measurements led to two predictions. (i) Ligands that bind to a regulatory site on the closed-channel conformation of the AChR with higher affinity than to the site on the open-channel form shift the equilibrium toward the closed-channel form, thereby inhibiting the receptor. (ii) Ligands that bind to a regulatory site with an affinity for the open conformation equal to or higher than their affinity for the closed conformations are expected not to inhibit the receptor and to displace inhibitors. The identification of such ligands in a combinatorial library of RNA ligands is reported. The implication of this approach to other protein-mediated reactions in which an inhibitor changes the equilibrium between active and inactive conformations is discussed.

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.

In vitro selection of RNA molecules that displace cocaine from the membrane-bound nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) controls signal transmission between cells in the nervous system. Abused drugs such as cocaine inhibit this receptor. Transient kinetic investigations indicate that inhibitors decrease the channel-opening equilibrium constant [Hess, G. P. & Grewer, C. (1998) Methods Enzymol. 291, 443-473]. Can compounds be found that compete with inhibitors for their binding site but do not change the channel-opening equilibrium? The systematic evolution of RNA ligands by exponential enrichment methodology and the AChR in Torpedo californica electroplax membranes were used to find RNAs that can displace inhibitors from the receptor. The selection of RNA ligands was carried out in two consecutive steps: (i) a gel-shift selection of high-affinity ligands bound to the AChR in the electroplax membrane, and (ii) subsequent use of nitrocellulose filters to which both the membrane-bound receptor and RNAs bind strongly, but from which the desired RNA can be displaced from the receptor by a high-affinity AChR inhibitor, phencyclidine. After nine selection rounds, two classes of RNA molecules that bind to the AChR with nanomolar affinities were isolated and sequenced. Both classes of RNA molecules are displaced by phencyclidine and cocaine from their binding site on the AChR. Class I molecules are potent inhibitors of AChR activity in BC3H1 muscle cells, as determined by using the whole-cell current-recording technique. Class II molecules, although competing with AChR inhibitors, do not affect receptor activity in this assay; such compounds or derivatives may be useful for alleviating the toxicity experienced by millions of addicts.

Cocaine-like effects of intravenous procaine in cocaine addicts

Pharmacological treatments that alter dopaminergic functioning have not lessened cocaine use in addicted patients. Non-dopaminergic mechanisms may therefore be important in the chronic use of cocaine. Procaine, like cocaine, is a local anesthetic, but has only 1% of cocaine's affinity for the dopamine reuptake receptor. In order to assess the subjective effects of procaine and its similarity to cocaine, we administered procaine to nine cocaine-dependent subjects. Patients 2-3 weeks abstinent were administered placebo, low dose procaine (0.46 mg/kg), and high dose procaine (1.84 mg/kg procaine) over a single 2-hour session. Patients were assessed for craving and similarity to cocaine experience and were administered the Symptom Checklist 90 Revised (SCL90R). High dose procaine was identified as similar to cocaine and induced significant cocaine craving. High dose procaine also induced significant elevations in somatization, obsessive-compulsive symptoms, phobic anxiety, interpersonal sensitivity, anxiety, positive symptoms and global severity (from the SCL90R). Our findings suggest that procaine shares subjective effects similar to cocaine, despite a much lower affinity for the dopamine reuptake receptor. Procaine may be a useful tool to explore non-dopaminergic mechanisms of cocaine's reinforcing and addictive properties.

Rapid chemical kinetic techniques for investigations of neurotransmitter receptors expressed in Xenopus oocytes

Xenopus laevis oocytes have been used extensively during the past decade to express and study neurotransmitter receptors of various origins and subunit composition and also to express and study receptors altered by site-specific mutations. Interpretations of the effects of structural differences on receptor mechanisms were, however, hampered by a lack of rapid chemical reaction techniques suitable for use with oocytes. Here we describe flow and photolysis techniques, with 2-ms and 100-microseconds time resolution, respectively, for studying neurotransmitter receptors in giant (approximately 20-microns diameter) patches of oocyte membranes, using muscle and neuronal acetylcholine receptors as examples. With these techniques, we find that the muscle receptor in BC3H1 cells and the same receptor expressed in oocytes have comparable kinetic properties. This finding is in contrast to previous studies and raises questions regarding the interpretations of the many studies of receptors expressed in oocytes in which an insufficient time resolution was available. The results obtained indicate that the rapid reaction techniques described here, in conjunction with the oocyte expression system, will be useful in answering many outstanding questions regarding the structure and function of diverse neurotransmitter receptors.