Localization of azidophencyclidine-binding site on the nicotinic acetylcholine receptor alpha-subunit

Molecular mechanisms and binding site locations for noncompetitive antagonists of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors are pentameric proteins that belong to the Cys-loop receptor superfamily. Their essential mechanism of functioning is to couple neurotransmitter binding, which occurs at the extracellular domain, to the opening of the membrane-spanning cation channel. The function of these receptors can be modulated by structurally different compounds called noncompetitive antagonists. Noncompetitive antagonists may act at least by two different mechanisms: a steric and/or an allosteric mechanism. The simplest idea representing a steric mechanism is that the antagonist molecule physically blocks the ion channel. On the other hand, there exist distinct allosteric mechanisms. For example, noncompetitive antagonists may bind to the receptor and stabilize a nonconducting conformational state (e.g., resting or desensitized state), and/or increase the receptor desensitization rate. Barbiturates, dissociative anesthetics, antidepressants, and neurosteroids have been shown to inhibit nicotinic receptors by allosteric mechanisms and/or by open- and closed-channel blockade. Receptor modulation has proved to be highly complex for most noncompetitive antagonists. Noncompetitive antagonists may act by more than one mechanism and at distinct sites in the same receptor subtype. The binding site location for one particular molecule depends on the conformational state of the receptor. The mechanisms of action and binding affinities of noncompetitive antagonists differ among nicotinic receptor subtypes. Knowledge of the structure of the nicotinic acetylcholine receptor, the location of its noncompetitive antagonist binding sites, and the mechanisms of inhibition will aid the design of new and more efficacious drugs for treatment of neurological diseases.

Unique general anesthetic binding sites within distinct conformational states of the nicotinic acetylcholine receptor

General anesthesia is a complex behavioral state provoked by the pharmacological action of a broad range of structurally different hydrophobic molecules called general anesthetics (GAs) on receptor members of the genetically linked ligand-gated ion channel (LGIC) superfamily. This superfamily includes nicotinic acetylcholine (AChRs), type A and C gamma-aminobutyric acid (GABAAR and GABACR), glycine (GlyR), and type 3 5-hydroxytryptamine (5-HT3R) receptors. This review focuses on recent advances in the localization of GA binding sites on conformationally and compositionally distinct AChRs. The experimental evidence outlined in this review suggests that: 1. Several neuronal-type AChRs might be targets for the pharmacological action of distinct GAs. 2. The molecular components of a specific GA binding site on a certain receptor subtype are different from the structural determinants of the locus for the same GA on a different receptor subtype. 3. There are unique binding sites for distinct GAs in the same receptor protein. 4. A GA can activate, potentiate, or inhibit an ion channel, indicating the existence of more than one binding site for the same GA. 5. The affinity of a specific GA depends on the conformational state of the receptor. 6. GAs inhibition channels by at least two mechanisms, an open-channel-blocking and/or an allosteric mechanism. 7. Certain GAs may inhibit AChR function by competing for the agonist binding sites or by augmenting the desensitization rate.

Allosterically linked noncompetitive antagonist binding sites in the resting nicotinic acetylcholine receptor ion channel

Previous studies have established the presence of overlapping binding sites for the noncompetitive antagonists (NCAs) amobarbital, tetracaine, and 3-trifluoromethyl-3-(m-[(125)I]iodophenyl) diazirine ([(125)I]TID) within the ion channel of the Torpedo nicotinic acetylcholine receptor (AChR) in the resting state. These well-characterized NCAs and competitive radioligand binding and photolabeling experiments were employed to better characterize the interaction of the dissociative anesthetics ketamine and thienylcycloexylpiperidine (TCP) with the resting AChR. Our experiments yielded what appear to be conflicting results: (i) both ketamine and TCP potentiated [(125)I]TID photoincorporation into AChR subunits; and (ii) ketamine and TCP had very little effect on [(14)C]amobarbital binding. Nevertheless, (iii) both ketamine and TCP completely displaced [(3)H]tetracaine binding (K(i)s approximately 20.9 and 2.0 microM, respectively) by a mutually exclusive mechanism. To reconcile these results we propose that, in the resting ion channel, TCP and ketamine bind to a site that is spatially distinct from the TID and barbiturate locus, while tetracaine bridges both binding sites.

Interactions between 3-(Trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine and Tetracaine, Phencyclidine, or Histrionicotoxin in theTorpedo Species Nicotinic Acetylcholine Receptor Ion Channel

3-(Trifluoromethyl)-3-(m-[(125)I]iodophenyl)diazirine ([(125)I]TID) and [(3)H]tetracaine, an aromatic amine, are noncompetitive antagonists (NCAs) of the Torpedo species nicotinic acetylcholine receptor (nAChR), which have been shown by photoaffinity labeling to bind to a common site in the ion channel in the closed state. Although tetracaine and TID bind to the same site, the amine NCAs phencyclidine (PCP) and histrionicotoxin (HTX), which are also believed to bind within the ion channel, interact competitively with tetracaine but allosterically with TID. To better characterize drug interactions within the nAChR ion channel in the closed state, we identified the amino acids photoaffinity labeled by [(125)I]TID in the presence of tetracaine, PCP, or HTX. In the absence of other drugs, [(125)I]TID reacts with alphaLeu-251 (alphaM2-9) and alphaVal-255 (alphaM2-13) and the homologous residues in each of the other subunits. None of the NCAs shifted the sites of [(125)I]TID labeling to other residues within the ion channel. Tetracaine inhibited [(125)I]TID labeling of M2-9 and M2-13 without changing the relative(125)I incorporation at these positions, whereas PCP and HTX each altered the pattern of [(125)I]TID incorporation at M2-9 and M2-13. These results indicate that tetracaine and TID bind in a mutually exclusive manner to a common site in the closed channel that is spatially separated from the binding sites for PCP and HTX.

Determinants of phencyclidine potency on the nicotinic acetylcholine receptors from muscle and electric organ

1. Phencyclidine (PCP) is an inhibitor of the nicotinic acetylcholine receptor (AChR) with characteristics of an open-channel blocker. The location of PCP binding site on the AChR molecule is unknown. 2. PCP inhibits the AChR from electric organ with a higher potency than muscle AChR. To find the molecular basis of this difference, we expressed the two native and six hybrid receptors, and two receptors containing mutated mouse gamma subunits in Xenopus laevis oocytes. The inhibition of ACh-induced current in these receptors by PCP was studied using whole-cell voltage-clamp. All hybrid receptors generated robust ACh-induced currents, while incomplete receptors (gamma-less or delta-less) did not. 3. PCP potency was higher on hybrids containing Torpedo beta and gamma subunits regardless of the alpha and delta subunit origin. A mouse gamma subunit containing the asparagine 6' to the serine mutation in the M2 segment conferred a high sensitivity to PCP. 4. These results support the conclusion that the amino acid residues at the position 6' of the M2 segments contribute to the PCP potency difference between Torpedo and mouse receptors. 5. Another noncompetitive inhibitor of the AChR, the cembranoid eupalmerin acetate (EUAC), also inhibited the electric organ receptor with a somewhat higher potency than muscle AChR. However, the IC50 values for EUAC inhibition of hybrid receptors did not follow the pattern observed for PCP. Therefore, these two inhibitors interact differently with the AChR molecule.

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)

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.

Binding sites for alpha-bungarotoxin and the noncompetitive inhibitor phencyclidine on a synthetic peptide comprising residues 172-227 of the alpha-subunit of the nicotinic acetylcholine receptor

The binding of the competitive antagonist alpha-bungarotoxin (alpha-Btx) and the noncompetitive inhibitor phencyclidine (PCP) to a synthetic peptide comprising residues 172-227 of the alpha-subunit of the Torpedo acetylcholine receptor has been characterized. 125I-alpha-Btx bound to the 172-227 peptide in a solid-phase assay and was competed by alpha-Btx (IC50 = 5.0 x 10(-8) M), d-tubocurarine (IC50 = 5.9 X 10(-5)M), and NaCl (IC50 = 7.9 x 10(-2)M). In the presence of 0.02% sodium dodecyl sulfate, 125I-alpha-Btx bound to the 56-residue peptide with a KD of 3.5 nM, as determined by equilibrium saturation binding studies. Because alpha-Btx binds to a peptide comprising residues 173-204 with the same affinity and does not bind to a peptide comprising residues 205-227, the competitive antagonist and hence agonist binding site lies between residues 173 and 204. After photoaffinity labeling, [3H]PCP was bound to the 172-227 peptide. [3H]PCP binding was inhibited by chlorpromazine (IC50 = 6.3 x 10(-5)M), tetracaine (IC50 = 4.2 x 10(-6)M), and dibucaine (IC50 = 2.7 x 10(-4)M). Equilibrium saturation binding studies in the presence of 0.02% sodium dodecyl sulfate showed that [3H]PCP bound at two sites, a major site of high affinity with an apparent KD of 0.4 microM and a minor low-affinity site with an apparent KD of 4.6 microM. High -affinity binding occurred at a single site on peptide 205-227 (KD = 0.27 microM) and was competed by chlorpromazine but not by alpha-Btx.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of nicotinic acetylcholine receptor function by adenine nucleotides

1. Nicotinic acetylcholine receptors (nAChR)4 from BC3H1 cells (which express a skeletal muscle-type receptor) and from Torpedo californica electric organ were expressed in Xenopus laevis oocytes and studied with a voltage-clamp technique. 2. We found that bath application of ATP in the micromolar to millimolar range increased the ACh-elicited current in both muscle and electrocyte receptors. The effect of ATP increased with successive applications. This "use-dependent" increase in potentiation was Ca2+ dependent, while the potentiation itself was not. 3. Four other nucleotides were tested on muscle nAChR: ADP, AMP, adenosine, and GTP. Of these, only ADP was a potentiator, but its effect was not use dependent. Neither ATP nor ADP affected the resting potential of the oocyte membrane. 4. ADP potentiated the response to suberyldicholine and nicotine, as well as ACh. 5. Finally, ADP reversed the phencyclidine-induced block of ACh currents in oocytes expressing muscle nAChR.