Estimating binding affinities of the nicotinic receptor for low-efficacy ligands using mixtures of agonists and two-dimensional concentration-response relationships

Agonist efficiency from concentration-response curves: Structural implications and applications

Agonists are evaluated by a concentration-response curve (CRC), with a midpoint (EC₅₀) that indicates potency, a high-concentration asymptote that indicates efficacy, and a low-concentration asymptote that indicates constitutive activity. A third agonist attribute, efficiency (η), is the fraction of binding energy that is applied to the conformational change that activates the receptor. We show that η can be calculated from EC₅₀ and the asymptotes of a CRC derived from either single-channel or whole-cell responses. For 20 agonists of skeletal muscle nicotinic receptors, the distribution of η-values is bimodal with population means at 51% (including acetylcholine, nornicotine, and dimethylphenylpiperazinium) and 40% (including epibatidine, varenicline, and cytisine). The value of η is related inversely to the size of the agonist's headgroup, with high- versus low-efficiency ligands having an average volume of 70 vs. 102 Å³. Most binding site mutations have only a small effect on acetylcholine efficiency, except for αY190A (35%), αW149A (60%), and those at αG153 (42%). If η is known, the EC₅₀ and high-concentration asymptote can be calculated from each other. Hence, an entire CRC can be estimated from the response to a single agonist concentration, and efficacy can be estimated from EC₅₀ of a CRC that has been normalized to 1. Given η, the level of constitutive activity can be estimated from a single CRC.

Orthosteric and Allosteric Activation of Human 5-HT3A Receptors

The serotonin type 3 receptor (5-HT₃) is a ligand-gated ion channel that converts the binding of the neurotransmitter serotonin (5-HT) into a transient cation current that mediates fast excitatory responses in peripheral and central nervous systems. Information regarding the activation and modulation of the human 5-HT₃ type A receptor has been based only on macroscopic current measurements because of its low ion conductance. By constructing a high-conductance human 5-HT₃A receptor, we here revealed mechanistic information regarding the orthosteric activation by 5-HT and by the partial agonist tryptamine, and the allosteric activation by the terpenoids, carvacrol, and thymol. Terpenoids potentiated macroscopic currents elicited by the orthosteric agonist and directly elicited currents with slow-rising phases and submaximal amplitudes. At the single-channel level, activation by orthosteric and allosteric agonists appeared as openings in quick succession (bursts) that showed no ligand concentration dependence. Bursts were grouped into long-duration clusters in the presence of 5-HT and even longer in the presence of terpenoids, whereas they remained isolated in the presence of tryptamine. Kinetic analysis revealed that allosteric and orthosteric activation mechanisms can be described by the same scheme that includes transitions of the agonist-bound receptor to closed intermediate states before opening (priming). Reduced priming explained the partial agonism of tryptamine; however, equilibrium constants for gating and priming were similar for 5-HT and terpenoid activation. Thus, our kinetic analysis revealed that terpenoids are efficacious agonists for 5-HT₃A receptors. These findings not only extend our knowledge about the human 5-HT₃A molecular function but also provide novel insights into the mechanisms of action of allosteric ligands, which are of increasing interest as therapeutic drugs in all the superfamily.

S. Typhimurium challenge in juvenile pigs modulates the expression and localization of enteric cholinergic proteins and correlates with mucosal injury and inflammation

The cholinergic system plays a central role in regulating critical gastrointestinal functions, including motility, secretion, barrier and immune function. In rodent models of acute, non-infectious gastrointestinal injury, the cholinergic system functions to inhibit inflammation; however, during inflammation local expression and regulation of the cholinergic system is not well known, particularly during infectious enteritis. The objective of this study was to determine the intrinsic expression of the enteric cholinergic system in pig ileum following an acute challenge with Salmonella enterica serovar Typhimurium DT104 (S. Typhimurium). At 2 d post-challenge, a three-fold reduction in ileal acetylcholine (ACh) levels was observed in challenged animals, compared with controls. Ileal acetylcholinesterase (AChE) activity was decreased (by four-fold) while choline acetyltransferase (ChAT) expression was increased in both the ileum and mesenteric lymph nodes. Elevated ChAT found to localize preferentially to mucosa overlying lymphoid follicles of the Peyers patch in challenged pigs, with more intense labeling for ChAT in S. Typhimurium challenged pigs compared to controls. Ileal mRNA gene expression of muscarinic receptor 1 and 3 was also increased in challenged pigs, while muscarinic receptor 2 and the nicotinic receptor alpha 7 subunit gene expression were unaffected. A positive correlation was observed between ChAT protein expression in the ileum, rectal temperature, and histopathological severity in challenged animals. These data show that inflammation from S. Typhimurium challenge alters enteric cholinergic expression by down-regulating acetylcholine concentration and acetylcholine degrading enzymes while increasing acetylcholine synthesis proteins and receptors. Given the known anti-inflammatory role of the cholinergic system, the divergent expression of cholinergic genes may represent an attempt to limit tissue damage by preserving cholinergic signaling in the face of low ligand availability.

Molecular recognition at cholinergic synapses: acetylcholine versus choline

KEY POINTS

Neuromuscular acetylcholine (ACh) receptors have a high affinity for the neurotransmitter ACh and a low affinity for its metabolic product choline. At each transmitter binding site three aromatic groups determine affinity, and together provide ∼50% more binding energy for ACh than for choline. Deprotonation of αY190 by a nearby lysine strengthens the interaction between this aromatic ring and both ACh and choline. H-bonds position ACh and choline differently in the aromatic cage to generate the different affinities.

ABSTRACT

Acetylcholine (ACh) released at the vertebrate nerve-muscle synapse is hydrolysed rapidly to choline (Cho), so endplate receptors (AChRs) are exposed to high concentrations of both of these structurally related ligands. To understand how these receptors distinguish ACh and Cho, we used single-channel electrophysiology to measure resting affinities (binding free energies) of these and other agonists in adult-type mouse AChRs having a mutation(s) at the transmitter-binding sites. The aromatic rings of αY190, αW149 and αY198 each provide ∼50% less binding energy for Cho compared to ACh. At αY198 a phenylalanine substitution had no effect, but at αY190 this substitution caused a large, agonist-independent loss in binding energy that depended on the presence of αK145. The results suggest that (1) αY190 is deprotonated by αK145 to strengthen the interaction between this benzene ring and the agonist's quaternary ammonium (QA) and (2) AChRs respond strongly to ACh because an H-bond positions the QA to interact optimally with the rings, and weakly to Cho because a different H-bond tethers the ligand to misalign the QA and form weaker interactions with the aromatic groups. The results suggest that the difference in ACh versus Cho binding energies is determined by different ligand positions within a fixed protein structure.

The antimalarial drug proguanil is an antagonist at 5-HT3 receptors

Proguanil is an antimalarial prodrug that is metabolized to 4-chlorophenyl-1-biguanide (CPB) and the active metabolite cycloguanil (CG). These compounds are structurally related to meta-chlorophenyl biguanide (mCPBG), a 5-hydroxytryptamine 3 (5-HT3) receptor agonist. Here we examine the effects of proguanil and its metabolites on the electrophysiology and ligand-binding properties of human 5-HT3A receptors expressed in Xenopus oocytes and human embryonic kidney 293 cells, respectively. 5-HT3 receptor responses were reversibly inhibited by proguanil, with an IC50 of 1.81 μM. Competitive antagonism was shown by a lack of voltage-dependence, Schild plot (Kb = 1.70 μM), and radioligand competition (Ki = 2.61 μM) with the 5-HT3 receptor antagonist [(3)H]granisetron. Kinetic measurements (kon = 4.0 × 10(4) M(-1) s(-1) ; koff = 0.23 s(-1)) were consistent with a simple bimolecular reaction scheme with a Kb of 4.35 μM. The metabolites CG and CPB similarly inhibited 5-HT3 receptors as assessed by IC50 (1.48 and 4.36 μM, respectively), Schild plot (Kb = 2.97 and 11.4 μM), and radioligand competition (Ki = 4.89 and 0.41 μM). At higher concentrations, CPB was a partial agonist (EC50 = 14.1 μM; I/Imax = 0.013). These results demonstrate that proguanil competitively inhibits 5-HT3 receptors, with an IC50 that exceeds whole-blood concentrations following its oral administration. They may therefore be responsible for the occasional gastrointestinal side effects, nausea, and vomiting reported following its use. Clinical development of related compounds should therefore consider effects at 5-HT3 receptors as an early indication of possible unwanted gastrointestinal side effects.

A single channel mutation alters agonist efficacy at 5-HT3A and 5-HT3AB receptors

BACKGROUND AND PURPOSE

5-HT₃ receptors are composed of 5-HT₃A subunits (homomeric receptors), or combinations of 5-HT₃A and other 5-HT₃ receptor subunits (heteromeric receptors, the best studied of which are 5-HT₃AB receptors). Here we explore the effects of partial agonists at 5-HT₃A and 5-HT₃AB receptors, and the importance of a channel-lining residue in determining the efficacy of activation.

EXPERIMENTAL APPROACH

Wild type and mutant 5-HT₃A and 5-HT₃AB receptors were expressed in Xenopus oocytes and examined using two-electrode voltage-clamp, or expressed in HEK293 cells and examined using [³H]granisetron binding.

KEY RESULTS

Dopamine, quipazine and VUF10166 were partial agonists at wild type 5-HT₃A and 5-HT₃AB receptors, with quipazine and VUF10166 causing a long-lived (>20 min) inhibition of subsequent agonist responses. At 5-HT₃A receptors, mCPBG was a partial agonist, but was a superagonist at 5-HT₃AB receptors, as it produced a response 2.6× greater than that of 5-HT. A T6'S substitution in the 5-HT₃A subunit decreased EC₅₀ and increased R_max of dopamine and quipazine at both homomeric and heteromeric receptors. The greatest changes were seen with VUF10166 at 5-HT₃A_T6'SB receptors, where it became a full agonist (EC₅₀ = 7 nM) with an EC₅₀ 58-fold less than 5-HT (EC₅₀ = 0.4 μM) and no longer caused inhibition of subsequent agonist responses.

CONCLUSIONS AND IMPLICATIONS

These results indicate that a mutation in the pore lining domain in both 5-HT₃A and 5-HT₃AB receptors alters the relative efficacy of a series of agonists, changing some (e.g. quipazine) from apparent antagonists to potent and efficacious agonists.

Catch-and-hold activation of muscle acetylcholine receptors having transmitter binding site mutations

Agonists turn on receptors because their target sites have a higher affinity in the active versus resting conformation of the protein. We used single-channel electrophysiology to measure the lower-affinity (LA) and higher-affinity (HA) equilibrium dissociation constants for acetylcholine in adult-type muscle mouse nicotinic receptors (AChRs) having mutations of agonist binding site amino acids. For a series of agonists and for all mutations of αY93, αG147, αW149, αY190, αY198, εW55, and δW57, the change in LA binding energy was approximately half that in HA binding energy. The results were analyzed as a linear free energy relationship between LA and HA agonist binding, the slope of which (κ) gives the fraction of the overall binding chemical potential where the LA complex is established. The linear correlation between LA and HA binding energies suggests that the overall binding process is by an integrated mechanism (catch-and-hold). For the agonist and the above mutations, κ ∼ 0.5, but side-chain substitutions of two residues had a slope that was significantly higher (0.90; αG153) or lower (0.25; εP121). The results suggest that backbone rearrangements in loop B, loop C, and the non-α surface participate in both LA binding and the LA ↔ HA affinity switch. It appears that all of the intermediate steps in AChR activation comprise a single, energetically coupled process.

Energy for wild-type acetylcholine receptor channel gating from different choline derivatives

Agonists, including the neurotransmitter acetylcholine (ACh), bind at two sites in the neuromuscular ACh receptor channel (AChR) to promote a reversible, global change in protein conformation that regulates the flow of ions across the muscle cell membrane. In the synaptic cleft, ACh is hydrolyzed to acetate and choline. Replacement of the transmitter's ester acetyl group with a hydroxyl (ACh→choline) results in a + 1.8 kcal/mol reduction in the energy for gating generated by each agonist molecule from a low- to high-affinity change of the transmitter binding site (ΔG(B)). To understand the distinct actions of structurally related agonist molecules, we measured ΔG(B) for 10 related choline derivatives. Replacing the hydroxyl group of choline with different substituents, such as hydrogen, chloride, methyl, or amine, increased the energy for gating (i.e., it made ΔG(B) more negative relative to choline). Extending the ethyl hydroxide tail of choline to propyl and butyl hydroxide also increased this energy. Our findings reveal the amount of energy that is available for the AChR conformational change provided by different, structurally related agonists. We speculate that a hydrogen bond between the choline hydroxyl and the backbone carbonyl of αW149 positions this agonist's quaternary ammonium group so as to reduce the cation-π interaction between this moiety and the aromatic groups at the binding site.

An integrated catch-and-hold mechanism activates nicotinic acetylcholine receptors

In neuromuscular acetylcholine (ACh) receptor channels (AChRs), agonist molecules bind with a low affinity (LA) to two sites that can switch to high affinity (HA) and increase the probability of channel opening. We measured (by using single-channel kinetic analysis) the rate and equilibrium constants for LA binding and channel gating for several different agonists of adult-type mouse AChRs. Almost all of the variation in the equilibrium constants for LA binding was from differences in the association rate constants. These were consistently below the limit set by diffusion and were substantially different even though the agonists had similar sizes and the same charge. This suggests that binding to resting receptors is not by diffusion alone and, hence, that each binding site can undergo two conformational changes ("catch" and "hold") that connect three different structures (apo-, LA-bound, and HA-bound). Analyses of ACh-binding protein structures suggest that this binding site, too, may adopt three discrete structures having different degrees of loop C displacement ("capping"). For the agonists we tested, the logarithms of the equilibrium constants for LA binding and LA↔HA gating were correlated. Although agonist binding and channel gating have long been considered to be separate processes in the activation of ligand-gated ion channels, this correlation implies that the catch-and-hold conformational changes are energetically linked and together comprise an integrated process having a common structural basis. We propose that loop C capping mainly reflects agonist binding, with its two stages corresponding to the formation of the LA and HA complexes. The catch-and-hold reaction coordinate is discussed in terms of preopening states and thermodynamic cycles of activation.

Diffusion-convection effects on drug distribution at the cell membrane level in a patch-clamp setup

We present a model-based method for estimating the effective concentration of the active drug applied by a pressure pulse to an individual cell in a patch-clamp setup, which could be of practical use in the analysis of ligand-induced whole-cell currents recorded in patch-clamp experiments. Our modelling results outline several important factors which may be involved in the high variability of the electric response of the cells, and indicate that with a pressure pulse duration of 1s and diameter of the perfusion tip of 600 μm, elevated amounts of drug can accumulate locally between the pipette tip and the cell. Hence, the effective agonist concentration at the cell membrane level can be consistently higher than the initial concentration inside the perfusion tubes. We performed finite-difference and finite-element simulations to investigate the diffusion/convection effects on the agonist distribution on the cell membrane. Our model can explain the delay between the commencement of acetylcholine application and the onset of the whole-cell current that we recorded on human rhabdomyosarcoma TE671 cells, and reproduce quantitatively the decrease of signal latency with the concentration of agonist in the pipette. Results also show that not only the geometry of the bath chamber and pipette tip, but also the transport parameters of the diffusive and convective phenomena in the bath solution are determinant for the amplitude and kinetics of the recorded currents and have to be accounted for when analyzing patch-clamp data.

Bridging the gap between structural models of nicotinic receptor superfamily ion channels and their corresponding functional states

Aromatic-aromatic interactions are a prominent feature of the crystal structure of ELIC [Protein Data Bank (PDB) code 2VL0], a bacterial member of the nicotinic receptor superfamily of ion channels where five pore-facing phenylalanines come together to form a structure akin to a narrow iris that occludes the transmembrane pore. To identify the functional state of the channel that this structure represents, we engineered phenylalanines at various pore-facing positions of the muscle acetylcholine (ACh) receptor (one position at a time), including the position that aligns with the native phenylalanine 246 of ELIC, and assessed the consequences of such mutations using electrophysiological and toxin-binding assays. From our experiments, we conclude that the interaction among the side chains of pore-facing phenylalanines, rather than the accumulation of their independent effects, leads to the formation of a nonconductive conformation that is unresponsive to the application of ACh and is highly stable even in the absence of ligand. Moreover, electrophysiological recordings from a GLIC channel (another bacterial member of the superfamily) engineered to have a ring of phenylalanines at the corresponding pore-facing position suggest that this novel refractory state is distinct from the well-known desensitized state. It seems reasonable to propose then that it is in this peculiar nonconductive conformation that the ELIC channel was crystallized. It seems also reasonable to propose that, in the absence of rings of pore-facing aromatic side chains, such stable conformation may never be attained by the ACh receptor. Incidentally, we also noticed that the response of the proton-gated wild-type GLIC channel to a fast change in pH from pH 7.4 to pH 4.5 (on the extracellular side) is only transient, with the evoked current fading completely in a matter of  seconds. This raises the possibility that the crystal structures of GLIC obtained at pH 4.0 (PDB code 3EHZ) and pH 4.6 (PDB code 3EAM) correspond to the to the (well-known) desensitized state.

Acetylcholine receptor channels activated by a single agonist molecule

The neuromuscular acetylcholine receptor (AChR) is an allosteric protein that alternatively adopts inactive versus active conformations (R<-->R). The R shape has a higher agonist affinity and ionic conductance than R. To understand how agonists trigger this gating isomerization, we examined single-channel currents from adult mouse muscle AChRs that isomerize normally without agonists but have only a single site able to use agonist binding energy to motivate gating. We estimated the monoliganded gating equilibrium constant E(1) and the energy change associated with the R versus R change in affinity for agonists. AChRs with only one operational binding site gave rise to a single population of currents, indicating that the two transmitter binding sites have approximately the same affinity for the transmitter ACh. The results indicated that E(1) approximately 4.3 x 10(-3) with ACh, and approximately 1.7 x 10(-4) with the partial-agonist choline. From these values and the diliganded gating equilibrium constants, we estimate that the unliganded AChR gating constant is E(0) approximately 6.5 x 10(-7). Gating changes the stability of the ligand-protein complex by approximately 5.2 kcal/mol for ACh and approximately 3.3 kcal/mol for choline.

Binding, activation and modulation of Cys-loop receptors

It is over forty years since the major neurotransmitters and their protein receptors were identified, and over twenty years since determination of the first amino-acid sequences of the Cys-loop receptors that recognize acetylcholine, serotonin, GABA and glycine. The last decade has seen the first structures of these proteins (and related bacterial and molluscan homologues) determined to atomic resolution. Hopefully over the next decade, more detailed molecular structures of entire Cys-loop receptors in drug-bound and drug-free conformations will become available. These, together with functional studies, will provide a clear picture of how these receptors participate in neurotransmission and how structural variations between receptor subtypes impart their unique characteristics. This insight should facilitate the design of novel and improved therapeutics to treat neurological disorders. This review considers our current understanding about the processes of agonist binding, receptor activation and channel opening, as well as allosteric modulation of the Cys-loop receptor family.

Decremental response to high-frequency trains of acetylcholine pulses but unaltered fractional Ca2+ currents in a panel of "slow-channel syndrome" nicotinic receptor mutants

The slow-channel congenital myasthenic syndrome (SCCMS) is a disorder of the neuromuscular junction caused by gain-of-function mutations to the muscle nicotinic acetylcholine (ACh) receptor (AChR). Although it is clear that the slower deactivation time course of the ACh-elicited currents plays a central role in the etiology of this disease, it has been suggested that other abnormal properties of these mutant receptors may also be critical in this respect. We characterized the kinetics of a panel of five SCCMS AChRs (αS269I, βV266M, εL221F, εT264P, and εL269F) at the ensemble level in rapidly perfused outside-out patches. We found that, for all of these mutants, the peak-current amplitude decreases along trains of nearly saturating ACh pulses delivered at physiologically relevant frequencies in a manner that is consistent with enhanced entry into desensitization during the prolonged deactivation phase. This suggests that the increasingly reduced availability of activatable AChRs upon repetitive stimulation may well contribute to the fatigability and weakness of skeletal muscle that characterize this disease. Also, these results emphasize the importance of explicitly accounting for entry into desensitization as one of the pathways for burst termination, if meaningful mechanistic insight is to be inferred from the study of the effect of these naturally occurring mutations on channel function. Applying a novel single-channel–based approach to estimate the contribution of Ca²⁺ to the total cation currents, we also found that none of these mutants affects the Ca²⁺-conduction properties of the AChR to an extent that seems to be of physiological importance. Our estimate of the Ca²⁺-carried component of the total (inward) conductance of wild-type and SCCMS AChRs in the presence of 150 mM Na⁺, 1.8 mM Ca²⁺, and 1.7 mM Mg²⁺ on the extracellular side of cell-attached patches turned out be in the 5.0–9.4 pS range, representing a fractional Ca²⁺ current of ∼14%, on average. Remarkably, these values are nearly identical to those we estimated for the NR1-NR2A N-methyl-d-aspartate receptor (NMDAR), which has generally been considered to be the main neurotransmitter-gated pathway of Ca²⁺ entry into the cell. Our estimate of the rat NMDAR Ca²⁺ conductance (using the same single-channel approach as for the AChR but in the nominal absence of extracellular Mg²⁺) was 7.9 pS, corresponding to a fractional Ca²⁺ current of 13%.

Unliganded gating of acetylcholine receptor channels

We estimated the unliganded opening and closing rate constants of neuromuscular acetylcholine receptor-channels (AChRs) having mutations that increased the gating equilibrium constant. For some mutant combinations, spontaneous openings occurred in clusters. For 25 different constructs, the unliganded gating equilibrium constant (E(0)) was correlated with the product of the predicted fold-increase in the diliganded gating equilibrium constant caused by each mutation alone. We estimate that (i) E(0) for mouse, wild-type alpha(2)beta delta epsilon AChRs is approximately 1.15 x 10(-7); (ii) unliganded AChRs open for approximately 80 micros, once every approximately 15 min; (iii) the affinity for ACh of the O(pen) conformation is approximately 10 nM, or approximately 15,600 times greater than for the C(losed) conformation; (iv) the ACh-monoliganded gating equilibrium constant is approximately 1.7 x 10(-3); (v) the C-->O isomerization reduces substantially ACh dissociation, but only slightly increases association; and (vi) ACh provides only approximately 0.9 k(B)T more binding energy per site than carbamylcholine but approximately 3.1 k(B)T more than choline, mainly because of a low O conformation affinity. Most mutations of binding site residue alphaW149 increase E(0). We estimate that the mutation alphaW149F reduces the ACh affinity of C only by 13-fold, but of O by 190-fold. Rate-equilibrium free-energy relationships for different regions of the protein show similar slopes (Phi values) for un- vs. diliganded gating, which suggests that the conformational pathway of the gating structural change is fundamentally the same with and without agonists. Agonist binding is a perturbation that (like most mutations) changes the energy, but not the mechanism, of the gating conformational change.

Use of calculated cation-pi binding energies to predict relative strengths of nicotinic acetylcholine receptor agonists

Agonists and antagonists of the nicotinic acetylcholine receptor (nAChR) are used to treat nicotine addiction, neuromuscular disorders, and neurological diseases. In designing small molecule therapeutics with the nAChR as a target, it is useful to identify chemical parameters that correlate with ability to activate the receptor. Previous studies have shown that cation-pi interactions at the transmitter binding sites of the nAChR are important for receptor activation by strong agonists such as acetylcholine. We hypothesized that a calculated estimate of cation-pi binding ability could be used to predict the efficiency for channel opening (i.e., the gating efficiency) associated with activation of the acetylcholine receptor by a series of structurally related organic cations. We demonstrate that the calculated cation-pi energy is strongly correlated with gating efficiency but only weakly correlated with closed-state binding affinity. Our results suggest that cation-pi interactions contribute significantly to the open-state affinity of these cations and that the calculated cation-pi energy will be a useful parameter for designing nAChR agonists and antagonists.

Site selectivity of competitive antagonists for the mouse adult muscle nicotinic acetylcholine receptor

The muscle-type nicotinic acetylcholine receptor has two nonidentical binding sites for ligands. The selectivity of acetylcholine and the competitive antagonists (+)-tubocurarine and metocurine for adult mouse receptors is known. Here, we examine the site selectivity for four other competitive antagonists: cisatracurium, pancuronium, vecuronium, and rocuronium. We rapidly applied acetylcholine to outside-out patches from transfected BOSC23 cells and measured macroscopic currents. We have reported the IC(50) of the antagonists individually in prior publications. Here, we determined inhibition by pairs of competitive antagonists. At least one antagonist was present at a concentration producing > or =67% receptor inhibition. Metocurine shifted the apparent IC(50) of (+)-tubocurarine in quantitative agreement with complete competitive antagonism. The same was observed for pancuronium competing with vecuronium. However, pancuronium and vecuronium each shifted the apparent IC(50) of (+)-tubocurarine less than expected for complete competition but more than expected for independent binding. The situation was similar for cisatracurium and (+)-tubocurarine or metocurine. Cisatracurium did not shift the apparent IC(50) of pancuronium or vecuronium, indicating independent binding of these two pairs. The data were fit to a two-site, two-antagonist model to determine the antagonist binding constants for each site, L(alphaepsilon) and L(alphadelta). We found L(alphaepsilon)/L(alphadelta) = 0.22 (range, 0.14-0.34), 20 (9-29), 21 (4-36), and 1.5 (0.3-2.9) for cisatracurium, pancuronium, vecuronium, and rocuronium, respectively. The wide range of L(alphaepsilon)/L(alphadelta) for some antagonists may reflect experimental uncertainties in the low affinity site, relatively poor selectivity (rocuronium), or possibly that the binding of an antagonist at one site affects the affinity of the second site.

Modulation of nicotinic acetylcholine receptor conformational state by free fatty acids and steroids

Steroids and free fatty acids (FFA) are noncompetitive antagonists of the nicotinic acetylcholine receptor (AChR). Their site of action is purportedly located at the lipid-AChR interface, but their exact mechanism of action is still unknown. Here we studied the effect of structurally different FFA and steroids on the conformational equilibrium of the AChR in Torpedo californica receptor-rich membranes. We took advantage of the higher affinity of the fluorescent AChR open channel blocker, crystal violet, for the desensitized state than for the resting state. Increasing concentrations of steroids and FFA decreased the K(D) of crystal violet in the absence of agonist; however, only cis-unsaturated FFA caused an increase in K(D) in the presence of agonist. This latter effect was also observed with treatments that caused the opposite effects on membrane polarity, such as phospholipase A(2) treatment or temperature increase (decreasing or increasing membrane polarity, respectively). Quenching by spin-labeled fatty acids of pyrene-labeled AChR reconstituted into model membranes, with the label located at the gammaM4 transmembrane segment, disclosed the occurrence of conformational changes induced by steroids and cis-unsaturated FFA. The present work is a step forward in understanding the mechanism of action of this type of molecules, suggesting that the direct contact between exogenous lipids and the AChR transmembrane segments removes the AChR from its resting state and that membrane polarity modulates the AChR activation equilibrium by an independent mechanism.

Structural answers and persistent questions about how nicotinic receptors work

The electron diffraction structure of nicotinic acetylcholine receptor (nAChR) from Torpedo marmorata and the X-ray crystallographic structure of acetylcholine binding protein (AChBP) are providing new answers to persistent questions about how nAChRs function as biophysical machines and as participants in cellular and systems physiology. New high-resolution information about nAChR structures might come from advances in crystallography and NMR, from extracellular domain nAChRs as high fidelity models, and from prokaryotic nicotinoid proteins. At the level of biophysics, structures of different nAChRs with different pharmacological profiles and kinetics will help describe how agonists and antagonists bind to orthosteric binding sites, how allosteric modulators affect function by binding outside these sites, how nAChRs control ion flow, and how large cytoplasmic domains affect function. At the level of cellular and systems physiology, structures of nAChRs will help characterize interactions with other cellular components, including lipids and trafficking and signaling proteins, and contribute to understanding the roles of nAChRs in addiction, neurodegeneration, and mental illness. Understanding nAChRs at an atomic level will be important for designing interventions for these pathologies.

Agonist-dependent single channel current and gating in alpha4beta2delta and alpha1beta2gamma2S GABAA receptors

The family of γ-aminobutyric acid type A receptors (GABA_ARs) mediates two types of inhibition in the mammalian brain. Phasic inhibition is mediated by synaptic GABA_ARs that are mainly comprised of α₁, β₂, and γ₂ subunits, whereas tonic inhibition is mediated by extrasynaptic GABA_ARs comprised of α_4/6, β₂, and δ subunits. We investigated the activation properties of recombinant α₄β₂δ and α₁β₂γ_2S GABA_ARs in response to GABA and 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3(2H)-one (THIP) using electrophysiological recordings from outside-out membrane patches. Rapid agonist application experiments indicated that THIP produced faster opening rates at α₄β₂δ GABA_ARs (β ∼1600 s⁻¹) than at α₁β₂γ_2S GABA_ARs (β ∼ 460 s⁻¹), whereas GABA activated α₁β₂γ_2S GABA_ARs more rapidly (β ∼1800 s⁻¹) than α₄β₂δ GABA_ARs (β < 440 s⁻¹). Single channel recordings of α₁β₂γ_2S and α₄β₂δ GABA_ARs showed that both channels open to a main conductance state of ∼25 pS at −70 mV when activated by GABA and low concentrations of THIP, whereas saturating concentrations of THIP elicited ∼36 pS openings at both channels. Saturating concentrations of GABA elicited brief (<10 ms) openings with low intraburst open probability (P_O ∼ 0.3) at α₄β₂δ GABA_ARs and at least two “modes” of single channel bursting activity, lasting ∼100 ms at α₁β₂γ_2S GABA_ARs. The most prevalent bursting mode had a P_O of ∼0.7 and was described by a reaction scheme with three open and three shut states, whereas the “high” P_O mode (∼0.9) was characterized by two shut and three open states. Single channel activity elicited by THIP in α₄β₂δ and α₁β₂γ_2S GABA_ARs occurred as a single population of bursts (P_O ∼0.4–0.5) of moderate duration (∼33 ms) that could be described by schemes containing two shut and two open states for both GABA_ARs. Our data identify kinetic properties that are receptor-subtype specific and others that are agonist specific, including unitary conductance.

Differential effects of serotonin and dopamine on human 5-HT3A receptor kinetics: interpretation within an allosteric kinetic model

Serotonin type 3 (5-HT3) receptors are members of the pentameric Cys-loop superfamily of receptors that modulate synaptic neurotransmission. In response to agonist binding and unbinding, members of this superfamily undergo a series of conformational transitions that define their functional properties. In this study, we report the results of electrophysiological studies using rapid solution exchange designed to characterize and compare the actions of the high-efficacy agonist serotonin and the low-efficacy agonist dopamine on human 5-HT3A receptors expressed in human embryonic kidney HEK293 cells. In the case of serotonin, receptor activation rates varied with agonist concentration, and deactivation occurred as a single-exponential process with a rate that was similar to the maximal rate of desensitization. Receptors recovered slowly from long desensitizing pulses of serotonin with a sigmoidal time course. In the case of dopamine, receptor activation rates were independent of agonist concentration, receptor deactivation occurred as a complex process that was significantly faster than the maximal rate of desensitization, and recovery from desensitization occurred more quickly than with 5-HT and its time course was not sigmoidal. We developed an allosteric kinetic model for 5-HT3A receptor activation, deactivation, desensitization, and resensitization. Interpretation of our results within the context of this model indicated that the distinct modulatory actions of serotonin versus dopamine are largely attributable to the vastly different rates with which these two agonists induce channel opening and dissociate from open and desensitized states.

Desensitization contributes to the synaptic response of gain-of-function mutants of the muscle nicotinic receptor

Although the muscle nicotinic receptor (AChR) desensitizes almost completely in the steady presence of high concentrations of acetylcholine (ACh), it is well established that AChRs do not accumulate in desensitized states under normal physiological conditions of neurotransmitter release and clearance. Quantitative considerations in the framework of plausible kinetic schemes, however, lead us to predict that mutations that speed up channel opening, slow down channel closure, and/or slow down the dissociation of neurotransmitter (i.e., gain-of-function mutations) increase the extent to which AChRs desensitize upon ACh removal. In this paper, we confirm this prediction by applying high-frequency trains of brief (∼1 ms) ACh pulses to outside-out membrane patches expressing either lab-engineered or naturally occurring (disease-causing) gain-of-function mutants. Entry into desensitization was evident in our experiments as a frequency-dependent depression in the peak value of succesive macroscopic current responses, in a manner that is remarkably consistent with the theoretical expectation. We conclude that the comparatively small depression of the macroscopic currents observed upon repetitive stimulation of the wild-type AChR is due, not to desensitization being exceedingly slow but, rather, to the particular balance between gating, entry into desensitization, and ACh dissociation rate constants. Disruption of this fine balance by, for example, mutations can lead to enhanced desensitization even if the kinetics of entry into, and recovery from, desensitization themselves are not affected. It follows that accounting for the (usually overlooked) desensitization phenomenon is essential for the correct interpretation of mutagenesis-driven structure–function relationships and for the understanding of pathological synaptic transmission at the vertebrate neuromuscular junction.

Block of muscle nicotinic receptors by choline suggests that the activation and desensitization gates act as distinct molecular entities

Ion channel block in muscle acetylcholine nicotinic receptors (AChRs) is an extensively reported phenomenon. Yet, the mechanisms underlying the interruption of ion flow or the interaction of the blocker with the channel's gates remain incompletely characterized. In this paper, we studied fast channel block by choline, a quaternary-ammonium cation that is also an endogenous weak agonist of this receptor, and a valuable tool in structure-function studies. Analysis of the single-channel current amplitude as a function of both choline concentration and voltage revealed that extracellular choline binds to the open-channel pore with millimolar apparent affinity (K(B) congruent with 12 mM in the presence of approximately 155 mM monovalent and 3.5 mM divalent, inorganic cations), and that it permeates the channel faster than acetylcholine. This, together with its relatively small size ( approximately 5.5 A along its longest axis), suggests that the pore-blocking choline binding site is the selectivity filter itself, and that current blockages simply reflect the longer-lived sojourns of choline at this site. Kinetic analysis of single-channel traces indicated that increasing occupancy of the pore-blocking site by choline (as judged from the reduction of the single-channel current amplitude) is accompanied by the lengthening of (apparent) open interval durations. Consideration of a number of possible mechanisms firmly suggests that this prolongation results from the local effect of choline interfering with the operation of the activation gate (closure of blocked receptors is slower than that of unblocked receptors by a factor of approximately 13), whereas closure of the desensitization gate remains unaffected. Thus, we suggest that these two gates act as distinct molecular entities. Also, the detailed understanding gained here on how choline distorts the observed open-time durations can be used to compensate for this artifact during activation assays. This correction is necessary if we are to understand how choline binds to and gates the AChR.