Cembranoid and long-chain alkanol sites on the nicotinic acetylcholine receptor and their allosteric interaction.

Long-chain alkanols are general anesthetics which can also act as uncharged noncompetitive inhibitors of the peripheral nicotinic acetylcholine receptor (AChR) by binding to one or more specific sites on the AChR. Cembranoids are naturally occurring, uncharged noncompetitive inhibitors of peripheral and neuronal AChRs, which have no demonstrable general anesthetic activity in vivo. In this study, [3H]tenocyclidine ([3H]TCP), an analogue of the cationic noncompetitive inhibitor phencyclidine (PCP), was used to characterize the cembranoid and long-chain alkanol sites on the desensitized Torpedo californica AChR and to investigate if these sites interact. These studies confirm that there is a single cembranoid site which sterically overlaps the [3H]TCP channel site. This cembranoid site probably also overlaps the sites for the cationic noncompetitive inhibitors, procaine and quinacrine. Evidence is also presented for one or more allosteric cembranoid sites which negatively modulate cembranoid affinity for the inhibitory site. In contrast, long-chain alkanols inhibit [3H]TCP binding through an allosteric mechanism involving two or more alkanol sites which display positive cooperativity toward each other. Double inhibitor studies show that the cembranoid inhibitory site and the alkanol sites are not independent of each other but interfere allosterically with each other's inhibition of [3H]TCP binding. The simplest models consistent with the observed data are presented and discussed.

Tobacco cembranoids block behavioral sensitization to nicotine and inhibit neuronal acetylcholine receptor function.

Cembranoids are cyclic diterpenoids found in tobacco and in marine invertebrates. The present study established that tobacco cembranoids inhibit behavioral sensitization to nicotine in rats and block several types of nicotine acetylcholine receptors (AChRs). 1) At the behavioral level, rat locomotor activity induced by nicotine was significantly increased after seven daily nicotine injections. This sensitization to nicotine was blocked by mecamylamine (1 mg/kg) and by the cembranoids eunicin, eupalmerin acetate (EUAC), and (4R)-2,7,11-cembratriene-4-6-diol (4R), each at 6 mg/kg. None of these compounds modified locomotor activity of nonsensitized rats. 2) In cells expressing human AChRs, cembranoids blocked carbamoylcholine-induced (86)Rb(+) flux with IC(50) in the low micromolar range. The cell lines used were the SH-EP1-halpha4beta2 cell line heterologously expressing human alpha4beta2-AChR, the SH-SY5Y neuroblastoma line naturally expressing human ganglionic alpha3beta4-AChR, and the TE671/RD cell line naturally expressing embryonic muscle alpha1beta1gammadelta-AChR. The tobacco cembranoids tested were 4R and its diastereoisomer 4S, and marine cembranoids tested were EUAC and 12,13-bisepieupalmerin. 3) At the molecular level, tobacco (4R and 4S) and marine (EUAC) cembranoids blocked binding of the noncompetitive inhibitor [(3)H]tenocyclidine to AChR from Torpedo californica electric organ. IC(50) values were in the submicromolar to low-micromolar range, with 4R displaying an order of magnitude higher potency than its diastereoisomer, 4S.

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.

Characterization of cembranoid interaction with the nicotinic acetylcholine receptor.

The class of diterpenoids with a 14-carbon cembrane ring, the cembranoids, includes both competitive and noncompetitive inhibitors of the nicotinic acetylcholine receptor (AChR). All 20 coelenterate-derived cembranoids studied in this report inhibited [piperidyl-3,4-3H]-phencyclidine ([3H]-PCP) binding to its high-affinity site on the electric organ AChR, with IC50s ranging from 0.9 microM for methylpseudoplexaurate to 372 microM for lophotoxin. Inhibition was complete with all cembranoids but lophotoxin and most Hill coefficients were close to 1. Methylpseudoplexaurate and [3H]-PCP binding was competitive. Methylpseudoplexaurate and the fourth most potent cembranoid, eunicin, competed with each other for [3H]-PCP displacement, indicating that there exist one or more cembranoid sites on the AChR. Cembranoid affinity for the AChR correlated with hydrophobicity, but was also dependent on other features. Methylpseudoplexaurate and n-octanol also competed with each other for [3H]-PCP displacement, indicating that the cembranoid site is linked to the n-octanol site on the AChR. Unlike lophotoxin, the five cembranoids tested did not inhibit [125I]Tyr54-alpha-bungarotoxin binding to the AChR agonist sites. All seven cembranoids tested on oocyte-expressed electric organ AChR reversibly blocked acetylcholine-induced currents, although the inhibitor concentration curves were shallow and the inhibition was incomplete.

The 9-arginine residue of alpha-conotoxin GI is responsible for its selective high affinity for the alphagamma agonist site on the electric organ acetylcholine receptor.

The two agonist-binding domains of the electric organ nicotinic acetylcholine receptor are located at the alphagamma and alphadelta subunit interfaces. alpha-Conotoxins GI and MI are competitive antagonists of this receptor and, like d-tubocurarine, bind to the alphagamma site with much higher affinity than to the alphadelta site. In the present study, alpha-conotoxin SIA also displayed strong affinity for the alphagamma site but no measurable affinity for the alphadelta site, thus showing even greater site-selectivity. In contrast, alpha-conotoxin SI does not distinguish between the two agonist sites, although its sequence differs from that of GI at only three positions: GI, ECCNPACGRHYSC; SI, ICCNPACGPKYSC. Analogues of SI and GI modified at these three positions were studied to identify the determinants of GI's alphagamma selectivity. Substituting arginine for proline at position 9 produced peptides which displayed "GI-like" selectivity for the alphagamma site. Conversely, substituting proline for arginine at position 9 resulted in "SI-like" nonselective inhibitors. An SI analogue having alanine in place of proline 9 did not distinguish between the two agonist sites and displayed about the same affinity as SI, indicating the importance of the arginyl cation. Interchanging the residues at position 1 or at position 10 influenced the affinity for the receptor but did not measurably change peptide selectivity. Therefore, of the three sequence differences in SI and GI, the variation at position 9, proline and arginine, respectively, is sufficient to account for GI's selective high-affinity binding to the alphagamma site on the electric organ acetylcholine receptor.

Differential effects of dimethyl sulfoxide on nicotinic acetylcholine receptors from mouse muscle and Torpedo electrocytes.

The present study examined the effects of 0.1% dimethyl sulfoxide (DMSO) on nicotinic acetylcholine receptors (nAChR) from mouse muscle and Torpedo californica electrocytes. Receptors were expressed in Xenopus laevis oocytes and studied with voltage-clamp. When applied simultaneously with acetylcholine, DMSO did not inhibit current amplitude of either receptor. Preincubation with DMSO for 1 min reduced current amplitude by approximately 50% from oocytes expressing electrocyte receptor. Preincubation did not affect the muscle receptor. With electric organ membranes, 0.1% DMSO did not block either [alpha-(125)I]bungarotoxin binding to the nAChR agonist site or [3H]phencyclidine binding to its high affinity site on resting or desensitized receptor. These data suggest that DMSO might be affecting the electrocyte receptor through a second messenger system.

Gangliosides in membranes from Torpedo electric organ.

The electric organ membrane has been the subject of many studies, due principally to its rich content of nicotinic acetylcholine receptor (AChR). Knowing its lipid composition is clearly important. Although its major membrane lipids have been characterized, its ganglioside composition has not been as well-described. In this study, gangliosides were characterized in membranes prepared from two species of electric organ, Torpedo californica and T. nobiliana. The ganglioside content of total electric organ membranes and AChR-enriched membranes was similar in both species, accounting for from 0.9 to 1.5% of membrane lipid by weight. However, the AChR-enriched membranes contained significantly less ganglioside relative to AChR than did the total membrane preparations. Five major gangliosides were purified from T. californica and identified as II3NeuNAc-GgOse3 (GM2); II3(NeuNAc)2-GgOse3 (GD2), IV3NeuNAc, II3NeuNAc-GgOse4 (GD1a), IV3NeuNAc, II3(NeuNAc)2-GgOse4 (GT1b), and IV3(NeuNAc)2,II3(NeuNAc)2-GgOse4 (GQ1b). Together these five gangliosides accounted for over 90% of the total ganglioside present in the two membrane preparations from both species. The most abundant ganglioside by far was GM2, which accounted for about one-half of the ganglioside content, followed by GD2. Determination of the N-fatty acid composition was performed on gangliosides purified from T. nobiliana. The lower-order gangliosides, GM2, GD2, and GD1a, contained substantial amounts of very long chain fatty acids (> 20 carbons), including alpha-hydroxynervonic acid (15-21% of total). In contrast, unsubstituted, 14-18 carbon chains accounted for about 90% of the fatty acids on the two higher-order gangliosides, GT1b and GQ1b.

Noncompetitive inhibitors of the acetylcholine receptor help paint a picture of its ion channel.

This review describes and analyzes the evidence from studies using noncompetitive inhibitors of the nicotinic acetylcholine receptor that the receptor's ion channel is formed by the second transmembrane segment of all five receptor subunits. Inconsistencies in this generally accepted model are also presented and discussed.

The alpha-conotoxins GI and MI distinguish between the nicotinic acetylcholine receptor agonist sites while SI does not.

The alpha-conotoxins are paralytic peptide toxins from Indo-Pacific cone snails. This paper presents a detailed analysis of how alpha-conotoxins inhibit [125I]-alpha-bungarotoxin (125I-BTX) equilibrium binding to the acetylcholine receptor (AChR) from electric organ of Torpedo californica and Torpedo nobiliana. All three alpha-conotoxins studied, SI, GI, and MI, completely inhibited 125I-BTX binding with the same order of potency in both species (MI approximately GI > SI approximately d-tubocurarine). BTX-concentration curves showed that this inhibition is competitive. However, while SI appeared to bind to a homogeneous population of sites, both GI and MI displayed curare-like heterogeneous binding. Studies using partially-blocked AChR demonstrated that both GI and MI display different affinities toward the two agonist sites, much like small curariform antagonists do. The high-affinity site for these two alpha-conotoxins is also the high-affinity d-tubocurarine site, which is believed to be located at the alpha gamma-subunit interface. The high-affinity binding of MI and GI was of the same order of magnitude as that of d-tubocurarine; however, their affinity for the other agonist site was somewhat greater than that of dTC, resulting in less site selectivity. Despite being homologous to GI and MI, SI did not distinguish between the two sites. A possible molecular basis for this difference is presented.

The ion channel of muscle and electric organ acetylcholine receptors: differing affinities for noncompetitive inhibitors.

1. Muscle and electric organ acetylcholine receptors (AChR's) were expressed in Xenopus laevis oocytes and differential effects of noncompetitive blockers on each type of receptor were analyzed using a two-electrode voltage clamp. 2. The positively charged channel blockers, phencyclidine (PCP) and tetracaine, displayed a much lower potency on muscle receptor than on the electric organ receptor. The IC50 for both blockers at the electrocyte receptor was close to 1 microM at -60 mV and even lower at more hyperpolarized voltages. In contrast, with muscle receptor IC50's were 20 to 40 microM at -60 or -80 mV. 3. Eupalmerin acetate, an uncharged noncompetitive inhibitor that displaces [3H]PCP from its high-affinity binding site, inhibited both receptors with a similar potency: IC50 of 4.9 and 6.4 microM for electrocyte and muscle receptors, respectively. However, eupalmerin acetate affected the desensitization process in each receptor type differently and triggered an unusual biphasic response in the muscle receptor. 4. These results are discussed with respect to differences in the amino acid sequences of the M2 regions of the two receptors. 5. A third type of noncompetitive inhibitor, Mg2+, was also examined and it inhibited both receptors with a similar potency (IC50, 0.5-1.0 mM). However, Mg2+ appeared to act at sites other than the PCP site.

Diterpenoids from Caribbean gorgonians act as noncompetitive inhibitors of the nicotinic acetylcholine receptor.

1. Three cyclic diterpenoids isolated from gorgonians of the Eunicea genus and characterized as eupalmerin acetate (EUAC), 12,13-bisepieupalmerin (BEEP), and eunicin (EUNI) were found to be pharmacologically active on the nicotinic acetylcholine receptor (AChR). 2. The receptor from the BC3H-1 muscle cell line was expressed in Xenopus laevis oocytes and studied with a two-electrode voltage clamp apparatus. 3. All three compounds reversibly inhibited ACh-induced currents, with IC50's from 6 to 35 microM. ACh dose-response curves suggested that his inhibition was noncompetitive. The cembranoids also increased the rate of receptor desensitization. 4. Radioligand-binding studies using AChR-rich membranes from Torpedo electric organ indicated that all three cembranoids inhibited high-affinity [3H]phencyclidine binding, with IC50's of 0.8, 11.6, and 63.8 microM for EUNI, EUAC, and BEEP, respectively. The cembranoids at a 100 microM concentration did not inhibit [alpha-125I]bungarotoxin binding to either membrane-bound or solubilized AChR. 5. It is concluded that these compounds act as noncompetitive inhibitors of peripheral AChR.

Electric organ polyamines and their effects on the acetylcholine receptor.

1. The electric organ of Torpedo nobiliana contained putrescine (PUT), spermidine (SPD), spermine (SPM), and cadaverine (CAD). Traces of acetylated SPD and SPM were occasionaly seen. 2. Upon fractionation of the tissue by differential centrifugation, the polyamines (PA) were found predominantly in the soluble fraction. The postsynaptic membrane fraction, containing a high concentration of acetylcholine receptor (AChR), was proportionally enriched in SPM. The molar ratio of SPM to AChR was approximately two in these membranes. 3. The effect of exogeneous PA on AChR function was studied by two methods: carbamoylcholine (CCh)-dependent 86Rb+ influx into receptor-rich membrane vesicles and [alpha-125I]bungarotoxin (Bgt) binding to the AChR. 4. SPM inhibited both ion influx and the rate of Bgt binding at concentrations above 1 mM, and therefore it appears to act as a competitive antagonist of the AChR. 5. At submicromolar concentrations, and only after preincubation with the receptor-rich membrane, SPM and PUT increased the ion influx by about 20% over control values. 6. Preincubation with 100 nM SPM did not affect the equilibrium binding of iodinated toxin or the rate of toxin binding, and therefore SPM was not uncovering new receptors. 7. By measuring the initial rate of toxin binding after different periods of preincubation with 1 microM CCh, the rate of the slow phase of receptor desensitization was determined. This rate was not changed by 100 nM SPM. 8. Although these results suggest that at low concentrations SPM is a positive modulator of the AChR, the precise mechanism of action is not determined yet.

Positive modulators of muscle acetylcholine receptor.

A solution of succinic anhydride (SA) in buffer N-tris(hydroxymethyl)methyl-2-aminoethane sulfonic acid (TES), the SATES solution, potentiates the effect of carbonyl containing agonists on frog muscle (del Castillo et al., Br. J. Pharmac. 84: 275-288, 1985). Here we report that the main compound in the SATES solution is a monosuccinyl ester of TES (MST). This compound is not an agonist of the acetylcholine (ACh) receptor nor is it an inhibitor of ACh esterase, yet MST potentiates the ACh-induced tension and depolarization in frog muscle to a new maximum. It increases the amplitude of miniature endplate potentials but has no effect on the time course of miniature endplate currents. The acetylated analogue of MST (mono-acetyl-TES: MAT) had similar but more pronounced effects on frog muscle. Neither MST nor MAT affect [3H]Ach binding to receptor-rich membranes from the electric organ of Torpedo californica, or the affinity state transition induced by agonists. Radiolabeled MST does not bind to these membranes and MAT does not alter agonist-induced ion flux. Therefore, these compounds seem to act as positive modulators of muscle ACh receptor but are inactive in Torpedo vesicles.