Binding, gating, affinity and efficacy: the interpretation of structure-activity relationships for agonists and of the effects of mutating receptors

Propofol increases agonist efficacy at the GABA(A) receptor

Using the whole-cell patch-clamp technique, we have determined that propofol, but not midazolam, increases the efficacy of piperidine-4-sulphonic acid (P4S), a partial agonist at alpha1beta1gamma2s, GABA(A) receptors expressed in HEK 293 cells. These findings are consistent with the idea that propofol facilitates receptor gating, while midazolam increases receptor occupancy by the agonist.

Insights into ligand pharmacology using receptor-G-protein fusion proteins

Production of chimeric DNAs in which the 5' end of G-protein alpha-subunits are linked directly to the 3' tail of a G-protein-coupled receptor has recently offered an unusual strategy to explore the detailed pharmacology of receptor-G-protein interactions. Expression of such fusion proteins ensures a 1:1 stoichiometry of receptor and G-protein expression and their proximity to each other. The capacity of such fusion proteins to be regarded as agonist-activated GTPases that allow simple enzyme kinetics to be applied to issues of ligand efficacy will be considered. In addition, the effects of point mutations, in both receptors and G proteins, on ligand function are particularly amenable to the types of robust quantitative analyses that can be produced using such fusion proteins.

Transducer abstraction: a novel approach to the detection of partial agonist efficacy in radioligand binding studies

The properties of the ternary complex model (TCM) of drug action at G protein-coupled receptors (GPCRs) were examined, using theoretical computer simulations, with regard to the predicted effects of the presence of a fixed concentration of one agonist on the competition binding profile of another. Subsequently, the binding properties of the full muscarinic acetylcholine receptor (mAChR) agonists acetylcholine (ACh) and carbachol (CCh), and the partial agonists pilocarpine and McN-A-343, were investigated in competition experiments against [(3)H]N-methylscopolamine using homogenate preparations from Chinese hamster ovary cells, stably expressing the human M(1) or M(2) mAChR. At the M(2) mAChR, all agonists displayed biphasic binding curves and were readily modulated by the non-hydrolyzable GTP analogue, Gpp(NH)p, in accordance with previously established experimental observations. In contrast, agonist binding at the M(1) mAChR showed no significant change in the presence of Gpp(NH)p, even in the case of a full agonist. This phenomenon precludes using the "GTP-shift" to assess agonist efficacy at the M(1) mAChR. When the ACh competition curves were reconstructed in the presence of graded concentrations of either a full or a partial agonist, a significant redistribution of the fraction of the high-affinity state recognized by ACh was observed. However, when the procedure was repeated using the antagonist, atropine, no significant effect on the fraction of either the high or low affinity ACh binding components at the mAChR was observed. Taken together, these results indicate that changes in the profile of full agonist binding isotherms, when constructed in the presence of a partial agonist, may be more sensitive indicators of partial agonist efficacy than regular assays that directly measure partial agonist binding.

Identification of transduction elements for benzodiazepine modulation of the GABA(A) receptor: three residues are required for allosteric coupling

Modulation of GABA(A) receptors by benzodiazepines (BZDs) is believed to involve two distinct steps: a recognition step in which BZDs bind and a conformational transition step in which the affinity of the receptor for GABA changes. Previously, using gamma(2)/alpha(1) chimeric subunits (chi), we demonstrated that although the N-terminal 167 gamma(2) amino acid residues confer high-affinity BZD binding, other gamma(2) domains couple BZD binding to potentiation of the GABA-mediated Cl(-) current (I(GABA)). To determine which gamma(2) regions couple binding to potentiation, we generated chis with longer N-terminal gamma(2) segments for voltage-clamp experiments in Xenopus oocytes. Chimeras containing greater than the N-terminal 167 gamma(2) residues showed incremental gains in maximal potentiation for diazepam enhancement of I(GABA). Residues in gamma(2)199-236, gamma(2)224-236 (pre-M1), and particularly gamma(2)257-297 (M2 and surrounding loops) are important for BZD potentiation. For several positive BZD modulators tested, the same regions restored potentiation of I(GABA). In contrast, beta-carboline inverse-agonism was unaltered in chimeric receptors, suggesting that structural determinants for positive and negative BZD allosteric modulation are different. Dissection of the gamma(2)257-297 domain revealed that three residues in concert, gamma(2)T281, gamma(2)I282 (M2 channel vestibule), and gamma(2)S291 (M2-M3 loop) are necessary to impart full BZD potentiation to chimeric receptors. Thus, these residues participate in coupling distant BZD-binding events to conformational changes in the GABA(A) receptor. The location of these novel residues provides insight into the mechanisms underlying allosteric coupling for other members of the ligand-gated ion channel superfamily.

Kinetics versus equilibrium: the importance of GTP in GPCR activation

Agonist-bound G-protein-coupled receptors (GPCRs) facilitate GDP-GTP exchange on their cognate G proteins. The binding properties of GPCRs are adequately described by the ternary complex model. However, in this article a more realistic (steady-state) model, which is necessary to describe the catalytic effect of agonist-bound receptors on G-protein activation, will be discussed. This model predicts that agonist potency and efficacy might vary from tissue to tissue, depending on the G-protein concentration and can be extended to explain why an agonist's ability to increase the receptor's affinity for empty G proteins (in the absence of GTP) is related to the agonist's efficacy.

Identification of transduction elements for benzodiazepine modulation of the GABA(A) receptor: three residues are required for allosteric coupling

Modulation of GABA(A) receptors by benzodiazepines (BZDs) is believed to involve two distinct steps: a recognition step in which BZDs bind and a conformational transition step in which the affinity of the receptor for GABA changes. Previously, using gamma(2)/alpha(1) chimeric subunits (chi), we demonstrated that although the N-terminal 167 gamma(2) amino acid residues confer high-affinity BZD binding, other gamma(2) domains couple BZD binding to potentiation of the GABA-mediated Cl(-) current (I(GABA)). To determine which gamma(2) regions couple binding to potentiation, we generated chis with longer N-terminal gamma(2) segments for voltage-clamp experiments in Xenopus oocytes. Chimeras containing greater than the N-terminal 167 gamma(2) residues showed incremental gains in maximal potentiation for diazepam enhancement of I(GABA). Residues in gamma(2)199-236, gamma(2)224-236 (pre-M1), and particularly gamma(2)257-297 (M2 and surrounding loops) are important for BZD potentiation. For several positive BZD modulators tested, the same regions restored potentiation of I(GABA). In contrast, beta-carboline inverse-agonism was unaltered in chimeric receptors, suggesting that structural determinants for positive and negative BZD allosteric modulation are different. Dissection of the gamma(2)257-297 domain revealed that three residues in concert, gamma(2)T281, gamma(2)I282 (M2 channel vestibule), and gamma(2)S291 (M2-M3 loop) are necessary to impart full BZD potentiation to chimeric receptors. Thus, these residues participate in coupling distant BZD-binding events to conformational changes in the GABA(A) receptor. The location of these novel residues provides insight into the mechanisms underlying allosteric coupling for other members of the ligand-gated ion channel superfamily.

Substrate turnover by transporters curtails synaptic glutamate transients

Although inhibitors of glutamate transport prolong synaptic currents at many glutamate synapses, the cause of the current prolongation is unclear. Transport inhibitors may prolong synaptic currents by simply interfering with synaptic glutamate binding to transporters, by inhibiting substrate translocation, or by promoting accumulation of ambient glutamate, which may act cooperatively at receptors with synaptic glutamate. We show that reversal of the membrane potential of astrocytes surrounding the synapse prolongs synaptic currents but does not decrease the apparent affinity of transporters or significantly alter glutamate-dependent kinetics of macroscopic transporter currents in excised membrane patches. Positive membrane potentials do not affect binding of a nontransported glutamate analog, nor do positive membrane potentials alter the number of transporters available to bind analog. We also test the hypothesis that glutamate accumulation during uptake inhibition by transporter substrates is the direct cause of synaptic current prolongations. Transporter substrates elevate ambient glutamate near synapses by fostering reverse transport of endogenous glutamate. However, increases in ambient glutamate cannot account for the prolongations of synaptic currents, because a nonsubstrate transport inhibitor does not foster reverse uptake yet it prolongs synaptic currents. Moreover, exogenous glutamate does not mimic synaptic current prolongations induced by substrate inhibitors. These results provide strong support for a major role of substrate translocation in determining the time course of the glutamate concentration transient at excitatory synapses.

A mutation in the glycine binding pocket of the N-methyl-D-aspartate receptor NR1 subunit alters agonist efficacy

Alanine 714 of the NMDA receptor NR1 subunit resides in the glycine binding pocket. The Ala714Leu mutation substantially shifts glycine affinity, but here no effect on antagonism by DCK is detected. Ala714Leu is also found to limit the efficacy of a partial agonist without altering its apparent affinity. The differential sensitivity of Ala714Leu to glycine agonists suggests that alanine 714 may be an intermediary in transducing the ligand binding signal.

Agonist activation and alpha-bungarotoxin inhibition of wild type and mutant alpha7 nicotinic acetylcholine receptors

The properties of wild type and mutant rat nicotinic alpha7 receptors expressed in Xenopus oocytes were investigated using electrophysiology and site-directed mutagenesis. When compared at individual agonist concentrations, neither the normalised nicotinic, nor acetylcholine, responses of the wild type receptors were significantly different from the corresponding responses obtained from a first extracellular domain mutant, phenylalanine(189)tyrosine (P0.05). The dissociation constants (K(D)) of the wild type (4.7 nM) and Phe(189)Tyr mutant (5.2 nM) receptors for alpha-bungarotoxin were estimated by an electrophysiological approach. The similarity of the results suggests that the mutation did not lead to a widespread disruption of structure-function relationships, although a slight change in nicotine sensitivity may have occurred. In contrast, the mutations (Tyr(190)Gln, first extracellular domain), (Glu(261)Ala, M2 region) severely compromised receptor function. An additional mutation was made in a negatively charged motif of the second extracellular domain which is conserved in homomeric nicotinic receptors. This mutation, Asp(268)Ala, also caused a loss of function. Thus the structure-function relationships in nicotinic alpha7 receptors have parallels with heteromeric nicotinic receptors, but there may also be some marked differences.

Gamma-aminobutyric acid increases the water accessibility of M3 membrane-spanning segment residues in gamma-aminobutyric acid type A receptors

Gamma-aminobutyric acid type A (GABA(A)) receptors are members of the ligand-gated ion channel gene superfamily. Using the substituted cysteine accessibility method, we investigated whether residues in the alpha(1)M3 membrane-spanning segment are water-accessible. Cysteine was substituted, one at a time, for each M3 residue from alpha(1)Ala(291) to alpha(1)Val(307). The ability of these mutants to react with the water-soluble, sulfhydryl-specific reagent pCMBS(-) was assayed electrophysiologically. Cysteines substituted for alpha(1)Ala(291) and alpha(1)Tyr(294) reacted with pCMBS(-) applied both in the presence and in the absence of GABA. Cysteines substituted for alpha(1)Phe(298), alpha(1)Ala(300), alpha(1)Leu(301), and alpha(1)Glu(303) only reacted with pCMBS(-) applied in the presence of GABA. We infer that the pCMBS(-) reactive residues are on the water-accessible surface of the protein and that GABA induces a conformational change that increases the water accessibility of the four M3 residues, possibly by inducing the formation of water-filled crevices that extend into the interior of the protein. Others have shown that mutations of alpha(1)Ala(291), a water-accessible residue, alter volatile anesthetic and ethanol potentiation of GABA-induced currents. Water-filled crevices penetrating into the interior of the membrane-spanning domain may allow anesthetics and alcohol to reach their binding sites and thus may have implications for the mechanisms of action of these agents.

Substrate turnover by transporters curtails synaptic glutamate transients

Although inhibitors of glutamate transport prolong synaptic currents at many glutamate synapses, the cause of the current prolongation is unclear. Transport inhibitors may prolong synaptic currents by simply interfering with synaptic glutamate binding to transporters, by inhibiting substrate translocation, or by promoting accumulation of ambient glutamate, which may act cooperatively at receptors with synaptic glutamate. We show that reversal of the membrane potential of astrocytes surrounding the synapse prolongs synaptic currents but does not decrease the apparent affinity of transporters or significantly alter glutamate-dependent kinetics of macroscopic transporter currents in excised membrane patches. Positive membrane potentials do not affect binding of a nontransported glutamate analog, nor do positive membrane potentials alter the number of transporters available to bind analog. We also test the hypothesis that glutamate accumulation during uptake inhibition by transporter substrates is the direct cause of synaptic current prolongations. Transporter substrates elevate ambient glutamate near synapses by fostering reverse transport of endogenous glutamate. However, increases in ambient glutamate cannot account for the prolongations of synaptic currents, because a nonsubstrate transport inhibitor does not foster reverse uptake yet it prolongs synaptic currents. Moreover, exogenous glutamate does not mimic synaptic current prolongations induced by substrate inhibitors. These results provide strong support for a major role of substrate translocation in determining the time course of the glutamate concentration transient at excitatory synapses.

Direction-independent block of bi-directional Na+/Ca2+ exchange current by KB-R7943 in guinea-pig cardiac myocytes

1. We investigated the inhibitory effect of KB-R7943 on 'bi-directional' Na+/Ca2+ exchange current (iNCX) with the reversal potential of iNCX (ENCX) in the middle of the ramp voltage pulse employed. 2. Bi-directional iNCX was recorded with 'full' ramp pulses given every 10 s from the holding potential of -60 mV over the voltage range between 30 and -150 mV under the ionic conditions of 140 mM [Na]o, 20 mM [Na]i, 1 mM [Ca]o and 433 nM [Ca]i with calculated ENCX at -50 mV. 3. KB-R7943 (0.1 - 100 mirconM) concentration-dependently inhibited the current, which reversed near the calculated ENCX, indicating that the blocked current was iNCX. 4. The inhibition levels were not significantly different between outward and inward iNCX measured at 0 and -120 mV, respectively. IC50 of KB-R7943 was approximately 1 micronM for both directions of iNCX. 5. Under the bi-directional ionic conditions, only an outward or inward iNCX was induced by positive or negative 'half' ramp pulses, respectively, from the holding potential of -60 mV. KB-R7943 inhibited both direction of iNCX and the concentration-inhibition relations were superimposable to the ones obtained by 'full' ramp pulses. 6. These results indicate that KB-R7943 inhibits iNCX direction-independently under bi-directional conditions. This conclusion is different from that of our previous results obtained from iNCX under uni-directional ionic conditions, where KB-R7943 inhibited iNCX direction-dependently. The difference could be attributed to slow dissociation of the drug from the exchanger.

The assessment of antagonist potency under conditions of transient response kinetics

The muscarinic acetylcholine receptor antagonists, atropine and pirenzepine, produced an apparent insurmountable antagonism of muscarinic M(1) receptor-mediated intracellular Ca(2+) mobilization in Chinese hamster ovary (CHO) cells when tested against the agonists carbachol or xanomeline. Each antagonist caused a dextral shift of the agonist concentration-response curves with depression of the maximum response that was incomplete (i.e., saturated) and which varied with the pairs of agonist and antagonist. Equilibrium competition binding assays found no deviation from simple, reversible competitive behavior for either antagonist. The relative rates of dissociation of unlabeled atropine and pirenzepine were also assessed in radioligand kinetic studies and it was found that atropine dissociated from the receptor approximately 8-fold slower than pirenzepine. Numerical dynamic simulations suggested that the insurmountability of antagonism observed in the present study was probably a kinetic artifact related to the measurement of transient responses to a non-equilibrated agonist in the presence of a slowly dissociating antagonist. Importantly, the patterns of antagonism observed included a saturable depression of agonist maximal response, a mode of antagonism that is incompatible with the previously described phenomenon of hemi-equilibrium states. Monte Carlo simulations indicated that reasonable, semi-quantitative estimates of antagonist potency could be determined by a minor modification of standard methods, where equieffective agonist concentrations, rather than EC(50) values, are compared in the absence and presence of antagonist. Application of the latter approach to the functional data yielded estimates of antagonist potency that were in excellent agreement with those derived from the equilibrium binding assays, thus indicating that the present method can be useful for quantifying antagonist potency under non-equilibrium conditions.

Amino-terminal modifications of human parathyroid hormone (PTH) selectively alter phospholipase C signaling via the type 1 PTH receptor: implications for design of signal-specific PTH ligands

Parathyroid hormone (PTH) and PTH-related peptide (PTHrP) activate the PTH/PTHrP receptor to trigger parallel increases in adenylyl cyclase (AC) and phospholipase C (PLC). The amino (N)-terminal region of PTH-(1-34) is essential for AC activation. Ligand domains required for activation of PLC, PKC, and other effectors have been less well-defined, although some studies in rodent systems have identified a core region [hPTH-(29-32)] involved in PKC activation. To determine the critical ligand domain(s) for PLC activation, a series of truncated hPTH-(1-34) analogues were assessed using LLC-PK1 cells that stably express abundant transfected human or rat PTH/PTHrP receptors. Phospholipase C signaling and ligand-binding affinity were reduced by carboxyl (C)-terminal truncation of hPTH-(1-34) but were coordinately restored when a binding-enhancing substitution (Glu(19) --> Arg(19)) was placed within hPTH-(1-28), the shortest hPTH peptide that could fully activate both AC and PLC. Phospholipase C, but not AC, activity was reduced by substituting Gly(1) for Ser(1) in hPTH-(1-34) and was eliminated entirely by removing either residue 1 or the alpha-amino group alone. These changes did not alter binding affinity. These findings led to design of an analogue, [Gly(1),Arg(19)]hPTH-(1-28), that was markedly signal-selective, with full AC but no PLC activity. Thus, the extreme N-terminus of hPTH constitutes a critical activation domain for coupling to PLC. The C-terminal region, especially hPTH-(28-31), contributes to PLC activation through effects upon receptor binding but is not required for full PLC activation. The N-terminal determinants of AC and PLC activation in hPTH-(1-34) overlap but are not identical, as subtle modifications in this region may dissociate activation of these two effectors. The [Gly(1),Arg(19)]hPTH-(1-28) analogue, in particular, should prove useful in dissociating AC- from PLC-dependent actions of PTH.

Role of the conserved lysine residue in the middle of the predicted extracellular loop between M2 and M3 in the GABA(A) receptor

In alpha1, beta2, and gamma2 subunits of the gamma-aminobutyric acid A (GABA(A)) receptor, a conserved lysine residue occupies the position in the middle of the predicted extracellular loop between the transmembrane M2 and M3 regions. In all three subunits, this residue was mutated to alanine. Whereas the mutation in alpha1 and beta2 subunits resulted each in about a sixfold shift of the concentration-response curve for GABA to higher concentrations, no significant effect by mutation in the gamma subunit was detected. The affinity for the competitive inhibitor bicuculline methiodide was not affected by the mutations in either the alpha1 subunit or the beta2 subunit. Concentration-response curves for channel activation by pentobarbital were also shifted to higher concentrations by the mutation in the alpha and beta subunits. Binding of [3H]Ro 15-1788 was unaffected by the mutation in the alpha subunit, whereas the binding of [3H]muscimol was shifted to lower affinity. Mutation of the residue in the alpha1 subunit to E, Q, or R resulted in an about eight-, 10-, or fivefold shift, respectively, to higher concentrations of the concentration-response curve for GABA. From these observations, it is concluded that the corresponding residues on the alpha1 and beta2 subunits are involved more likely in the gating of the channel by GABA than in the binding of GABA or benzodiazepines.

Comparison of rat and human parathyroid hormone 2 (PTH2) receptor activation: PTH is a low potency partial agonist at the rat PTH2 receptor

The human PTH2 receptor, expressed in tissue culture cells, is selectively activated by PTH. Detailed investigation of its anatomical and cellular distribution has been performed in the rat. It is expressed by neurons in a number of brain nuclei; by endocrine cells that include pancreatic islet somatostatin cells, thyroid parafollicular cells, and peptide secreting cells in the gastrointestinal tract; and by cells in the vasculature and heart. The physiological role of the PTH2 receptor expressed by these cells remains to be determined. All pharmacological studies performed to date have used the human receptor. We have now isolated a complementary DNA including the entire coding sequence of the rat PTH2 receptor and compared its pharmacological profile with that of the human PTH2 receptor when each is expressed in COS-7 cells. PTH-based peptides, including rat PTH(1-84), rat PTH(1-34), and human PTH(1-34), have low potency at the rat PTH2 receptor for stimulation of adenylyl cyclase (EC50 = 19-140 nM). When compared with the effect of a bovine hypothalamic extract, PTH-based peptides are partial agonists at the rat PTH2 receptor. This suggests that PTH is unlikely to be a physiologically important endogenous ligand for the PTH2 receptor. A peptide homologous to an activity detected in a bovine hypothalamic extract is a good candidate for the endogenous PTH2 receptor ligand.

Allosteric control of gating and kinetics at P2X(4) receptor channels

The CNS abundantly expresses P2X receptor channels for ATP; of these the most widespread in the brain is the P2X(4) channel. We show that ivermectin (IVM) is a specific positive allosteric effector of heterologously expressed P2X(4) and possibly of heteromeric P2X(4)/P2X(6) channels, but not of P2X(2), P2X(3), P2X(2)/P2X(3,) or P2X(7) channels. In the submicromolar range (EC(50,) approximately 250 nM) the action of IVM was rapid and reversible, resulting in increased amplitude and slowed deactivation of P2X(4) channel currents evoked by ATP. IVM also markedly increased the potency of ATP and that of the normally low-potency agonist alpha, beta-methylene-ATP in a use- and voltage-independent manner without changing the ion selectivity of P2X(4) channels. Therefore, IVM evokes a potent pharmacological gain-of-function phenotype that is specific for P2X(4) channels. We also tested whether IVM could modulate endogenously expressed P2X channels in the adult trigeminal mesencephalic nucleus and hippocampal CA1 neurons. Surprisingly, IVM produced no significant effect on the fast ATP-evoked inward currents in either type of neuron, despite the fact that IVM modulated P2X(4) channels heterologously expressed in embryonic hippocampal neurons. These results suggest that homomeric P2X(4) channels are not the primary subtype of P2X receptor in the adult trigeminal mesencephalic nucleus and in hippocampal CA1 neurons.

Modeling activation and desensitization of G-protein coupled receptors provides insight into ligand efficacy

Signaling through G-protein coupled receptors is one of the most prevalent and important methods of transmitting information to the inside of cells. Many mathematical models have been proposed to describe this type of signal transduction, and the ternary complex (ligand/receptor/G-protein) model and its derivatives are among the most widely accepted. Current versions of these equilibrium models include both active (i.e. signaling) and inactive conformations of the receptor, but do not include the dynamics of G-protein activation or receptor desensitization. Yet understanding how these dynamic events effect response behavior is crucial to determining ligand efficacy. We developed a mathematical model for G-protein coupled receptor signaling that includes G-protein activation and receptor desensitization, and used it to predict how activation and desensitization would change if either the conformational selectivity (the effect of ligand binding on the distribution of active and inactive receptor states) or the desensitization rate constant was ligand-specific. In addition, the model was used to explore the implications of measuring responses far downstream from G-protein activation. By comparing the experimental data from the beta(2)-adrenergic, micro-opioid, D(1)dopamine, and neutrophil N -formyl peptide receptors with the predictions of our model, we found that the conformational selectivity is the predominant factor in determining the amounts of activation and desensitization caused by a particular ligand.

Allosteric control of gating and kinetics at P2X(4) receptor channels

The CNS abundantly expresses P2X receptor channels for ATP; of these the most widespread in the brain is the P2X(4) channel. We show that ivermectin (IVM) is a specific positive allosteric effector of heterologously expressed P2X(4) and possibly of heteromeric P2X(4)/P2X(6) channels, but not of P2X(2), P2X(3), P2X(2)/P2X(3,) or P2X(7) channels. In the submicromolar range (EC(50,) approximately 250 nM) the action of IVM was rapid and reversible, resulting in increased amplitude and slowed deactivation of P2X(4) channel currents evoked by ATP. IVM also markedly increased the potency of ATP and that of the normally low-potency agonist alpha, beta-methylene-ATP in a use- and voltage-independent manner without changing the ion selectivity of P2X(4) channels. Therefore, IVM evokes a potent pharmacological gain-of-function phenotype that is specific for P2X(4) channels. We also tested whether IVM could modulate endogenously expressed P2X channels in the adult trigeminal mesencephalic nucleus and hippocampal CA1 neurons. Surprisingly, IVM produced no significant effect on the fast ATP-evoked inward currents in either type of neuron, despite the fact that IVM modulated P2X(4) channels heterologously expressed in embryonic hippocampal neurons. These results suggest that homomeric P2X(4) channels are not the primary subtype of P2X receptor in the adult trigeminal mesencephalic nucleus and in hippocampal CA1 neurons.

Evidence for multiple rat VPAC1 receptor states with different affinities for agonists

We compare the binding properties of [125I-VIP] and [125I]-Ro 25 1553 to VPAC1 receptors, expressed in stably transfected CHO cells. [125I]-VIP labelled two VPAC1 receptor states, while [125I]-Ro 25 1553 labelled selectively a limited number of high-affinity receptors. This high-affinity state probably corresponds to an agonist-receptor-Gs ternary complex as its properties (guanyl nucleotides, EC50 values and maximal effect) were affected by cholera toxin pre-treatment. Both high- and low-affinity receptors participated in the adenylate cyclase activation. This suggested that agonists activate not only low-affinity uncoupled receptors by facilitating the ternary complex formation, but also activated the high-affinity ternary complex by accelerating the GTP binding to emptied, receptor-bound G proteins.

Stereoselective interaction of thiopentone enantiomers with the GABA(A) receptor

1. As pharmacokinetic differences between the thiopentone enantiomers seem insufficient to explain the approximately 2 fold greater potency for CNS effects of (-)-S- over (+)-R-thiopentone, this study was performed to determine any enantioselectivity of thiopentone at the GABA(A) receptor, the primary receptor for barbiturate hypnotic effects. 2. Two electrode voltage clamp recording was performed on Xenopus laevis oocytes expressing human GABA(A) receptor subtype alpha1beta2gamma2 to determine relative differences in potentiation of the GABA response by rac-, (+)-R- and (-)-S-thiopentone, and rac-pentobarbitone. Changes in the cellular environment pH and in GABA concentrations were also evaluated. 3. With 3 microM GABA, the EC50 values were (-)-S-thiopentone (mean 26.0+/-s.e.mean 3.2 microM, n=9 cells) >rac-thiopentone (35.9+/-4.2 microM, n=6, P=0.1) >(+)-R-thiopentone (52.5+/-5.0 microM, n=8, P<0.02) >rac-pentobarbitone (97.0+/-11.2 microM, n=11, P<0.01). Adjustment of environment pH to 7.0 or 8.0 did not alter the EC50 values for (+)-R- or (-)-S-thiopentone. 4 Uninjected oocytes responded to >100 microM (-)-S- and R-thiopentone. This direct response was abolished by intracellular oocyte injection of 1,2-bis(2-aminophenoxy)ethane-N, N,N1,N1-tetraacetic acid (BAPTA), a Ca2+ chelating agent. With BAPTA, the EC50 values were (-)-S-thiopentone (20.6+/-3.2 microM, n=8) <(+)-R-thiopentone (36.2+/-3.2 microM, n=9, P<0.005). 5 (-)-S-thiopentone was found to be approximately 2 fold more potent than (+)-R-thiopentone in the potentiation of GABA at GABA(A) receptors expressed on Xenopus oocytes. This is consistent with the differences in potency for CNS depressant effects found in vivo.

Amino acid volume and hydropathy of a transmembrane site determine glycine and anesthetic sensitivity of glycine receptors

Two specific amino acid residues in transmembrane segments (TM) 2 and 3 are critical for the enhancement of glycine receptor (GlyR) function by volatile anesthetics. To determine which physicochemical characteristics of these sites determine their roles in anesthetic actions, an extensive series of single amino acid mutations at amino acid residue 288 (Ala-288) in TM3 of the alpha1 GlyR subunit was tested for modulation by volatile anesthetics. The mutations changed the apparent affinities of receptors for glycine; replacements with larger volumes and less hydropathy exhibited higher affinities for glycine. Potentiation by anesthetics was reduced by specific mutations at Ala-288. The molecular volume of the substituents was negatively correlated with the extent of potentiation by isoflurane, enflurane, and 1-chloro-1,2,2-trifluorocyclobutane, whereas there was no correlation between anesthetic enhancement and polarity, hydropathy, or hydrophilicity of substituents. In contrast to anesthetics, no correlation was found between the effects of the nonanesthetics 1,2-dichlorohexafluorocyclobutane or 2, 3-dichlorooctafluorobutane and any physicochemical property of the substituent. These results suggest that the molecular volume and hydropathy of the amino acid at position 288 in TM3 regulate glycine and anesthetic sensitivity of the GlyR and that this residue might represent one determinant of an anesthetic binding site.

Selective blocking effects of tropisetron and atropine on recombinant glycine receptors

Some serotonin 5-HT3 receptor ligands of tropeine structure have been recently shown to modulate ionophore function and binding of glycine receptors. This led us to study the effects of the tropeines tropisetron and atropine on recombinant human glycine receptors transiently expressed in Xenopus oocytes by using whole-cell voltage-clamp electrophysiology. Glycine currents were inhibited by atropine in an apparently competitive manner and with considerable selectivity of the tropeines for alpha2 versus alpha1 subunits. Coexpression of beta with alpha subunits and replacement of the N-terminal region of the alpha1 subunits by the corresponding beta segment resulted in similar increases in the inhibitory potencies. Our data suggest common sites of the tropeines for inhibition on the N-terminal region of glycine receptors. The point mutations R271K and R271L of the alpha1 subunit decreased, whereas a T112A substitution increased, the inhibition constants (Ki) of the tropeines. These changes in the Ki values of the tropeines were associated with opposite changes in the EC50 of glycine. Selectivities for the tropeines versus glycine (EC50/Ki) varied within three orders of magnitude. These results, when expressed in terms of free energy changes, can be interpreted according to a two-state receptor model.

Ligand efficacy and affinity in an interacting 7TM receptor model

Quantitative understanding of the activation of G protein-coupled receptors is based mostly on some theoretical models that describe the interaction between ligand and protein partners and the activation process of the receptor. All of these models provide different definitions for observable affinity or efficacy. However, the property common to such parameters defined in the context of these models is that they are always independent of the concentration of the receptor molecule. This is based on the assumption that receptors do not interact with each other appreciably. In this article, experimental evidence for which this assumption does not seem to apply is discussed and an oligomerization model for seven-transmembrane-domain receptors that explains the relationship between receptor concentration, apparent affinity and efficacy is provided.

Mapping the agonist binding site of the GABAA receptor: evidence for a beta-strand

GABAA receptors, along with the receptors for acetylcholine, glycine, and serotonin, are members of a ligand-gated ion channel superfamily (Ortells and Lunt, 1995). Because of the paucity of crystallographic information for these ligand-gated channels, little is known about the structure of their binding sites or how agonist binding is transduced into channel gating. We used the substituted cysteine accessibility method to obtain secondary structural information about the GABA binding site and to systematically identify residues that line its surface. Each residue from alpha1 Y59 to K70 was mutated to cysteine and expressed with wild-type beta2 subunits in Xenopus oocytes or HEK 293 cells. The sulfhydryl-specific reagent N-biotinylaminoethyl methanethiosulfonate (MTSEA-Biotin) was used to covalently modify the cysteine-substituted residues. Receptors with cysteines substituted at positions alpha1 T60, D62, F64, R66, and S68 reacted with MTSEA-Biotin, and alpha1 F64C, R66C, and S68C were protected from reaction by agonist. We conclude that alpha1 F64, R66, and S68 line part of the GABA binding site. The alternating pattern of accessibility of consecutive engineered cysteines to reaction with MTSEA-Biotin indicates that the region from alpha1 Y59 to S68 is a beta-strand.

Mapping the agonist binding site of the GABAA receptor: evidence for a beta-strand

GABAA receptors, along with the receptors for acetylcholine, glycine, and serotonin, are members of a ligand-gated ion channel superfamily (Ortells and Lunt, 1995). Because of the paucity of crystallographic information for these ligand-gated channels, little is known about the structure of their binding sites or how agonist binding is transduced into channel gating. We used the substituted cysteine accessibility method to obtain secondary structural information about the GABA binding site and to systematically identify residues that line its surface. Each residue from alpha1 Y59 to K70 was mutated to cysteine and expressed with wild-type beta2 subunits in Xenopus oocytes or HEK 293 cells. The sulfhydryl-specific reagent N-biotinylaminoethyl methanethiosulfonate (MTSEA-Biotin) was used to covalently modify the cysteine-substituted residues. Receptors with cysteines substituted at positions alpha1 T60, D62, F64, R66, and S68 reacted with MTSEA-Biotin, and alpha1 F64C, R66C, and S68C were protected from reaction by agonist. We conclude that alpha1 F64, R66, and S68 line part of the GABA binding site. The alternating pattern of accessibility of consecutive engineered cysteines to reaction with MTSEA-Biotin indicates that the region from alpha1 Y59 to S68 is a beta-strand.

Comparison of glycine and GABA actions on the zebrafish homomeric glycine receptor

1. Glycine and GABA can be co-released from the same presynaptic terminals and in lower vertebrates they can activate the same glycine receptors (GlyRs). Thus we examined the effects of these two inhibitory transmitters on the homomeric GlyRs formed by the alphaZ1 subunit, of the zebrafish using two expression systems: Xenopus oocytes and the human BOSC 23 cell line. 2. The apparent affinity (EC50) of alphaZ1 for these neurotransmitters was highly variable. In Xenopus oocytes the EC50 ranged from 37 to 360 microM (mean +/- s. d. EC50 116 +/- 75 microM, n = 83) for glycine and from 8 to 120 mM (mean EC50 40 +/- 30 mM, n = 37) for GABA. 3. In BOSC cells the EC50 varied from 9 to 92 microM (mean EC50 33 +/- 17 microM, n = 19) and from 0.7 to 19.1 mM (mean EC50 4.9 +/- 4.7 mM, n = 29) for glycine and GABA, respectively. 4. GABA activated alphaZ1 GlyRs either as a weak or full agonist: its efficacy (defined as Imax,GABA/Imax,Gly) was related to EC50 by an exponential relationship. A linear relationship was observed between EC50 values for GABA and glycine. 5. In outside-out patches, GABA and glycine activated alphaZ1 with identical single-channel conductances (85-100 pS), but with different kinetics and marked effect of concentration on burst duration for glycine only. 6. In outside-out patches deactivation time constants were concentration dependent for glycine, but not for GABA. 7. Our data demonstrate that the kinetics of glycine and GABA interactions with alphaZ1 are different and that they determine the properties of these neurotransmitter actions on the GlyR.

Identification in the NK1 tachykinin receptor of a domain involved in recognition of neurokinin A and septide but not of substance P

The three mammalian tachykinins, substance P (SP), neurokinin A (NKA) and neurokinin B (NKB), exert their physiological effects through specific receptors, NK1, NK2 and NK3, respectively. However, homologous binding studies have recently demonstrated that, contrary to the generally accepted belief, NKA could bind NK1 receptor with high affinity (Hastrup and Schwartz, 1996). Using COS-7 cells expressing the human NK1 receptor, we show that two simultaneous point mutations (E193L and V195R) in a restricted five amino acid sequence (the (193-197) region), selected because of its hydropathic complementarity with the common C-terminal extremity of tachykinins, abolish both the high-affinity binding and highly potent biological activity of NKA, without affecting those of SP. In addition, the same mutations also suppressed the high functional activity of septide, a synthetic SP atypical agonist ([pGlu6-Pro9] SP 6-11). These results suggest that the (193-197) region, located at the end of the second extracellular loop of the receptor, could be part of a common high-affinity binding domain for both NKA and septide, distinct from the SP binding site.

Channel opening locks agonist onto the GABAC receptor

Determination of the activation mechanism of neurotransmitter-operated ion channels has been hindered by a limited understanding of the relationship between agonist binding and the gating of the integral ion pore. Here we describe a [3H]ligand binding assay that enables us to make repeated binding measurements from the same intact oocyte expressing recombinant human rho 1 GABAC receptors and directly correlate the binding kinetics with electrophysiological measurements. We have determined an association rate for GABA of about 10(5) M-1s-1; this is four orders of magnitude slower than diffusion, indicating GABA has restricted access to its binding site. We also demonstrate that GABA dissociates at two rates. Our data are consistent with the faster rate being the true microscopic dissociation rate of GABA, with the slower rate occurring because the opening of the pore detains agonist release.