Potential errors in agonist dissociation constant estimation caused by desensitization

A simple model of receptor desensitization is developed and analysed to predict the consequences of acute receptor loss on the pharmacological quantification of agonist action. The model incorporates the desensitization scheme of Katz and co-workers (1957) into the operational model of agonism (Black & Leff, 1983) and, therefore, it assumes the occupancy theory of agonist action. Pharmacological effect-time profiles are simulated which illustrate (i) the extent to which overt fade may be detectable under different conditions and (ii) the extent to which measured pharmacological effects deviate from those which would be measured in the absence of desensitization. It is shown that the resulting agonist concentration-effect curves may be displaced rightwards from their equilibrium positions and that the agonist dissociation constants estimated from them may be overestimated. Such errors are predicted to occur regardless of whether or not fade is detectable in the effect-time profiles. Estimates of agonist efficacy appears to be unaffected by desensitization. The results of this analysis are discussed with respect to their implications for receptor classification using agonist dissociation constant estimates and for the development of agonist drugs.     

Biochemical Evidence of Functional Interaction Between μ- and δ-Opioid Receptors in SK-N-BE Neuroblastoma Cell Line

Abstract: Radioligand binding assays and functional experiments revealed that the SK-N-BE neuroblastoma cell line expresses a similar ratio of μ- and δ-opioid receptors, both negatively coupled to adenylyl cyclase through pertussis toxin-sensitive G proteins. Our findings also indicate that some functional interaction occurred between the two opioid subtypes; in fact, long-term exposure to [ d -Ala²- N -methyl-Phe⁴-Gly-ol⁵]enkephalin (DAMGO), a μ-selective agonist, sensitized the functional response of the δ-selective agonist but not vice versa. It is interesting that in acute interaction experiments, we observed a shift to the right of the concentration-effect curve of either DAMGO or [ d -Pen^2,5]enkephalin (DPDPE), a δ-selective agonist, as a result of DPDPE or DAMGO administration, respectively. In addition, low doses of naloxone, an antagonist selective for μ receptors, increased the inhibitory effect of [ d -Ala², d -Met⁵]enkephalinamide (DAME), a mixed μ/δ agonist, on adenylyl cyclase activity. Taken overall, these data support the hypothesis of the existence of a cross talk between μ and δ receptors in the SK-N-BE cell line.     

Structure and activity of membrane receptors: Modeling and computational simulation of ligand recognition in a three-dimensional model of the 5-hydroxytryptamine1A receptor

A three-dimensional molecular model of the transmembrane domain of the 5-HT_1A receptor (5-HT_1AR) is presented in the context of a general strategy for modeling the macromolecular structure of a guanine nucleotide binding, regulatory protein coupled receptor (GPCR). The model of the 5-HT_1AR rests on the definition of the putative residues of the ligand-binding site guided by criteria based on specific models proposed from structure-activity studies and on published results of modifications of GPCRs using methods of molecular biology. The resulting requirements for matching recognition sites in the agonist-binding pocket define the molecular details of the interaction between the agonist 5-HT and the human 5-HT_1AR that includes: (1) the interaction between the protonated amine moiety and the conserved negative Asp-116, located in TMH 3; (2) the hydrogen bond between the hydroxyl group and Thr-199, located in TMH 5; and (3) the interaction complex between the aromatic ring portion of the ligand and the neutral form of His-192, located in TMH 5. Results from quantum mechanical calculations of the interaction between an agonist and the proposed recognition pocket of the 5-HT_1AR model suggest a trigger of the receptor activation mechanism resulting from ligand binding. The antagonist-binding pocket of the human 5-HT_1AR is inferred from the interaction sites of pindolol with the receptor model: (1) the ionic interaction between the protonated amine of the ligand and the side chain of the conserved Asp-116, located in TMH 3; and (2) the hydrogen bonds between the ether oxygen and the hydroxyl group of the ligand and Asn-385, located in TMH 7. Use of the model is proposed to facilitate the identification of the structural elements of agonists and antagonists that are key for their specific functions, in order to achieve the design of new compounds with predetermined pharmacological properties.     

Endogenous agonists may change the concentration-response curves of exogenous agonists: Source of quantitative information about the endogenous tone

The pharmacological dogma that competitive antagonists cause parallel shifts to the right with sustained maximum effect of semi-logarithmic concentration-response curves of exogenous agonists may not be true if an endogenous agonist is present in the preparation. In this case, the antagonist and the exogenous agonist interfere in a complex way with an existing circuit of regulation between the response and the endogenous agonist. In consequence, it is difficult to determine the true shift in the concentration-response curves as induced by the antagonist, since a deviation of the curves in a non-parallel manner can be observed. The extent of this deviation may be used to learn more about the variables involved.The present paper discusses this phenomenon: The regulatory circuit of the (auto)receptor modulated release of neutrotransmitters is used as an example. Paired samples of data are analysed in this example. Since the extent of the non-parallel deviation also depends on the manner in which the paired samples are mathematically linked, two different ways of data evaluation have been used. A theoretical model of the relation between receptor activation and response is proposed which allows to evaluate experimental concentration-response curves by means of non-linear regression analysis. This evaluation yields quantitative information on the parameters of the regulatory circuit: the concentration of the endogenous agonist, itsK _D value and the true shift of the concentration-response curve caused by the applied antagonist.     

Dual Gating Mechanism and Function of P2X7 Receptor Channels

The ATP-gated P2X7 receptor channel (P2X7R) operates as a cytolytic and apoptotic receptor but also controls sustained cellular responses, including cell growth and proliferation. However, it has not been clarified how the same receptor mediates such opposing effects. To address this question, we have combined electrophysiological, imaging, and mathematical studies using wild-type and mutant rat P2X7Rs. Activation of naïve (not previously stimulated) receptors by low agonist concentrations caused monophasic slow desensitizing currents and internalization of receptors without other changes in the cellular morphology, much like other P2XRs. In contrast, saturating agonist concentrations induced high-amplitude biphasic currents, reflecting pore dilation and causing rapid cell swelling and lysis. The existence of these two signaling patterns was accounted for using a revised Markov-state model that included, in addition to naïve and sensitized states, desensitized states. Occupancy of one or two ATP-binding sites of naïve receptors favored a slow transition to desensitized states, whereas occupancy of the third binding site favored a transition to sensitized/dilated states. Consistent with model predictions, nondilating P2X7R mutants always generated desensitizing currents. These results suggest that the level of saturation of the ligand binding sites determines the nature of the P2X7R gating and cellular actions.     

Dual Gating Mechanism and Function of P2X7 Receptor Channels

The ATP-gated P2X7 receptor channel (P2X7R) operates as a cytolytic and apoptotic receptor but also controls sustained cellular responses, including cell growth and proliferation. However, it has not been clarified how the same receptor mediates such opposing effects. To address this question, we have combined electrophysiological, imaging, and mathematical studies using wild-type and mutant rat P2X7Rs. Activation of naïve (not previously stimulated) receptors by low agonist concentrations caused monophasic slow desensitizing currents and internalization of receptors without other changes in the cellular morphology, much like other P2XRs. In contrast, saturating agonist concentrations induced high-amplitude biphasic currents, reflecting pore dilation and causing rapid cell swelling and lysis. The existence of these two signaling patterns was accounted for using a revised Markov-state model that included, in addition to naïve and sensitized states, desensitized states. Occupancy of one or two ATP-binding sites of naïve receptors favored a slow transition to desensitized states, whereas occupancy of the third binding site favored a transition to sensitized/dilated states. Consistent with model predictions, nondilating P2X7R mutants always generated desensitizing currents. These results suggest that the level of saturation of the ligand binding sites determines the nature of the P2X7R gating and cellular actions.     

A deterministic model for monophasic growth of batch cultures of bacteria

Experimental observations of bacterial numbers employing high resolution electrical conductance measurements of the culture provide the basis for a proposed deterministic model of monophasic growth of populations in batch culture. The model postulates that the production and growth of each bacterium is accompanied by the generation of a constant mass of toxic end-products and that specific growth rate declines in proportion to the ratio of the accumulated mass of these substances to the dry mass of the nutrient medium when the substrate is non-limiting. The theoretical relationship is found to fit extensive data for Escherichia coli (NCIB 9132) very closely and offers an analytical basis for the logistic curve frequently observed to represent the time-dependence of growth. These data incidentally provide substantial evidence that lag time and generation time are each independent of both inoculum number and concentration of the medium.     

Identification of an efficacy switch region in the ghrelin receptor responsible for interchange between agonism and inverse agonism

The carboxyamidated wFwLL peptide was used as a core ligand to probe the structural basis for agonism versus inverse agonism in the constitutively active ghrelin receptor. In the ligand, an efficacy switch could be built at the N terminus, as exemplified by AwFwLL, which functioned as a high potency agonist, whereas KwFwLL was an equally high potency inverse agonist. The wFw-containing peptides, agonists as well as inverse agonists, were affected by receptor mutations covering the whole main ligand-binding pocket with key interaction sites being an aromatic cluster in transmembrane (TM)-VI and -VII and residues on the opposing face of TM-III. Gain-of-function in respect of either increased agonist or inverse agonist potency or swap between high potency versions of these properties was obtained by substitutions at a number of positions covering a broad area of the binding pocket on TM-III, -IV, and -V. However, in particular, space-generating substitutions at position III:04 shifted the efficacy of the ligands from inverse agonism toward agonism, whereas similar substitutions at position III: 08, one helical turn below, shifted the efficacy from agonism toward inverse agonism. It is suggested that the relative position of the ligand in the binding pocket between this "efficacy shift region" on TM-III and the opposing aromatic cluster on TM-VI and TM-VII leads either to agonism, i.e. in a superficial binding mode, or it leads to inverse agonism, i.e. in a more profound binding mode. This relationship between different binding modes and opposite efficacy is in accordance with the Global Toggle Switch model for 7TM receptor activation.     

Do pharmacological methods for the quantification of agonists work when the ternary complex mechanism operates?

It is well established that many receptors couple to G-proteins in order to subserve their pharmacological or physiological effects. In those systems it is possible that a ternary complex mechanism operates in which initiation of an effect depends on the concentration of agonist-receptor-G-protein complex formed. Such systems may be considered to obey a receptor-transducer model (Black & Leff, 1983, Proc. R. Soc. B220, 141). A theoretical analysis of this model is presented which seeks to determine how the operation of the ternary complex mechanism affects the quantification of agonists by conventional pharmacological methods. Previous analyses have concluded that pharmacological models may or may not accommodate the ternary complex mechanism depending upon the relationship between the relative concentrations of receptor and transducer units, [R0], and [T0] respectively. The present study extends these in two ways. It considers the impact of the ternary complex mechanism on agonist quantification under a more complete range of conditions relating [R0] and [T0], and it does so with regard to the analysis of partial agonists (by the comparative method) as well as of full agonists (by the method of receptor inactivation). The following predictions are made: (i) reliable estimates of affinity and efficacy can be made using the comparative method under the conditions [R0] much greater than [T0] and [R0] much less than [T0] whereas the inactivation method only works under the former condition; (ii) errors occur in the estimation of affinity and efficacy by both methods when [R0] = [T0] although better estimates are produced by the comparative method; (iii) when errors occur in the absolute estimation of affinity and efficacy, the orders of affinity and efficacy determined by the comparative method will generally be correct but this is not the case for the inactivation method; (iv) in general, the comparative method for agonist quantification appears to produce more reliable information for the purposes of receptor classification and medicinal chemistry than does the receptor inactivation method.     

A molecular theory of recognition and activation at a 5-HT receptor based on a quantum chemical approach to structure activity relationships

The study of structure activity relationships (SAR) is based on the delineation of the causal relationships between the properties of molecules and the observed responses evoked by the interaction of these molecules with biological systems. The methods of theoretical and quantum chemistry describe accurately the molecular properties that are determined by molecular structure and provide a rigorous link between structure and activity. We study the molecular events in the pharmacological mechanism of drugs interacting with the receptor of 5-hydroxytryptamine (5-HT, serotonin) by defining the elements of recognition and by analyzing the changes induced in a molecular model for the receptor. These steps define the relationship between the properties of the drugs and their ability to be recognized and cause the activation of the receptors. Consequently, our quantum chemical studies of drug-receptor interactions explain the selectivity of receptors and the molecular determinants for agonism and antagonism on the 5-HT receptor.     

Agonist induction, conformational selection, and mutant receptors

Current models of receptor activation are based on either of two basic mechanisms: agonist induction or conformational selection. The importance of one pathway relative to the other is controversial. In this article, the impossibility of distinguishing between the two mechanisms under a thermodynamic approach is shown. The effect of receptor mutation on the constants governing ligand-receptor equilibria is discussed. The two-state model of agonism both in its original formulation (one cycle) and including multiple active states (multiple cycles) is used. Pharmacological equations for the double (two cycles) two-state model are derived. The simulations performed suggest that the double two-state model of agonism can be a useful model for assessing quantitatively the changes in pharmacological activity following receptor mutation.     

Pharmacological characterization of cholecystokinin receptors mediating contraction of human gallbladder and ascending colon

Cholecystokinin (CCK) produces contractions of gallbladder and colon in a number of different species. Although the effects of CCK on the human gallbladder are relatively well documented, the CCK receptors in the human colon have not been clearly characterised. Therefore, in this study, the CCK receptors in the human gallbladder and colon were compared using pharmacological techniques. Contraction of specimens of the human tissue was measured using in vitro organ bath bioassay. The effect of selective concentrations of CCK(1) and CCK(2) receptor antagonists (L-364,718 and JB93182, respectively) was determined on agonist concentration-effect (E/[A]) curves obtained by cumulative dosing with sulphated CCK. The CCK(1) antagonist L-364,718 produced a rightward shift of the CCK-8S [E/[A] curve in the human gallbladder (pA(2)=9.15 +/- 0.26) and ascending colon (pA(2)=9.20 +/- .33). In both tissues, the CCK(2) receptor antagonist, JB93182, had no effect on the CCK E/[A] curves. In addition, in the colon, pentagastrin responses were inhibited by L-364,718 but unaffected by JB93182. These data indicate that the CCK-induced contraction of the human colon and gallbladder smooth muscle is mediated solely through the CCK(1) receptor subtype, and the antagonist affinity estimates are consistent with those previously obtained in experiments on animal tissue.     

The albumin receptor effect may be due to a surface-induced conformational change in albumin

To determine whether equilibrium binding between albumin and hepatocytes involves a cell surface receptor for albumin, we incubated freshly isolated rat hepatocytes with 125I-albumin and determined the amount of albumin associated with the cells as a function of the total albumin concentration. The resulting two-phase binding curve showed the rat albumin-hepatocyte interaction to consist of a saturable binding interaction with a dissociation constant of 1.1 microM and 2 X 10(6) sites/cell in addition to a weak, nonsaturable binding interaction. However, the saturable binding of albumin to hepatocytes did not appear to result from the presence of an albumin receptor on the cell surface; the interaction was the same for different species of albumin, for chemically modified albumins, and for fragments of albumin representing mutually exclusive domains of the molecule. The saturable binding was, instead, found to involve a subpopulation of albumin with an enhanced affinity for the cell surface. We show that this subpopulation of albumin is generated upon contact with either solid surfaces or cell surfaces and can be transferred from one surface to another. We propose that the two-phase Scatchard binding curve and the "albumin receptor effect" reflect two populations of albumin that bind to the cell surface with different affinities rather than one population of albumin that binds to two classes of binding sites.     

Circuit specific functions of cannabinoid CB1 receptor in the balance of investigatory drive and exploration

Well balanced novelty seeking and exploration are fundamental behaviours for survival and are found to be dysfunctional in several psychiatric disorders. Recent studies suggest that the endocannabinoid (eCB) system is an important control system for investigatory drive. Pharmacological treatment of rodents with cannabinergic drugs results in altered social and object investigation. Interestingly, contradictory results have been obtained, depending on the treatment, drug concentration and experimental conditions. The cannabinoid type 1 (CB1) receptor, a central component of the eCB system, is predominantly found at the synapses of two opposing neuronal populations, i.e. on inhibitory GABAergic and excitatory glutamatergic neurons. In the present study, using different transgenic mouse lines, we aimed at investigating the impact of CB1 receptor inactivation in glutamatergic or GABAergic neurons on investigatory behaviour. We evaluated animate (interaction partner) and inanimate (object) exploratory behaviour in three different paradigms. We show that exploration was increased when CB1 receptor was deleted from cortical and striatal GABAergic neurons. No effect was observed when CB1 receptor was deleted specifically from dopamine receptor D1-expressing striatal GABAergic medium spiny neurons. In contrast, deletion of CB1 receptor from cortical glutamatergic neurons resulted in a decreased exploration. Thus, our results indicate that exploratory behaviour is accurately balanced in both, the social and non-social context, by the eCB system via CB1 receptor activation on cortical glutamatergic and GABAergic neurons. In addition, the results could explain the contradictory findings of previous pharmacological studies and could further suggest a possibility to readjust an imbalance in exploratory behaviour observed in psychiatric disorders.     

The N-terminal juxtamembrane segment of the V1a vasopressin receptor provides two independent epitopes required for high-affinity agonist binding and signaling

It is fundamentally important to define how agonist-receptor interaction differs from antagonist-receptor interaction. The V1a vasopressin receptor (V1aR) is a member of the neurohypophysial hormone subfamily of G protein-coupled receptors. Using alanine-scanning mutagenesis of the N-terminal juxtamembrane segment of the V1aR, we now establish that Glu54 (1.35) is critical for arginine vasopressin binding. The mutant [E54A]V1aR exhibited decreased arginine vasopressin affinity (1700-fold) and disrupted signaling, but antagonist binding was unaffected. Mutation of Glu54 had an almost identical pharmacological effect as mutation of Arg46, raising the possibility that agonist binding required a mutual interaction between Glu54 and Arg46. The role of these two charged residues was investigated by 1) substituting Glu54; 2) inserting additional Glu/Arg in transmembrane helix (TM) 1; 3) repositioning the Glu/Arg in TM1; and 4) characterizing the reciprocal mutant [R46E/E54R]V1aR. We conclude that 1) the positive/negative charges need to be precisely positioned in this N terminus/TM1 segment; and 2) Glu54 and Arg46 function independently, providing two discrete epitopes required for high-affinity agonist binding and signaling. This study explains why Glu and Arg, part of an -R(X3)L/V(X3)E(X3)L- motif, are conserved at these loci throughout this G protein-coupled receptor subfamily and provides molecular insight into key differences between agonist and antagonist binding requirements.     

Interactions Between Vasoactive Intestinal Peptide and Dopamine in the Rabbit Retina: Stimulation of a Common Adenylate Cyclase

Although 3,4-dihydroxyphenylethylamine (dopamine, DA) and vasoactive intestinal peptide (VIP) have been reported to stimulate adenylate cyclase activity in the rabbit retina, possible interactions between VIP-sensitive and DA-sensitive adenylate cyclase systems have not been previously investigated. To elucidate the interactions between these two putative transmitter-stimulated cyclase systems, the effects of VIP, DA, and VIP + DA on the conversion of [alpha-32P]ATP to [32P]cyclic AMP in rabbit retinal homogenates were measured. VIP stimulated adenylate cyclase activity in a biphasic manner, suggesting that two classes of VIP receptors may be involved in the induction of cyclic AMP formation. DA was less potent than VIP, and stimulated cyclase activity with a monophasic dose-response curve. When assayed together, these stimulations were partially nonadditive, implying the existence of a common adenylate cyclase pool that may be stimulated by both putative neurotransmitters. The dopaminergic antagonist (+)-butaclamol completely blocked dopaminergic stimulation, but had no significant effect on VIP-induced stimulation, indicating that VIP interacts with specific VIP receptor sites, which are distinct from the dopaminergic receptor sites. Furthermore, the specific D-2 dopaminergic receptor agonist LY141865 demonstrated no inhibitory effect on adenylate cyclase activity, suggesting that the interaction between the VIP- and DA-sensitive adenylate cyclase systems does not result from a D-2 receptor-mediated cyclase inhibition in the rabbit retina. Finally, at maximally effective concentrations, DA and VIP were less potent than fluoride or forskolin in the stimulation of cyclic AMP formation, suggesting that adenylate cyclase pools that are not sensitive to DA and VIP may also be present in this retina.(ABSTRACT TRUNCATED AT 250 WORDS)     

Two active molecular phenotypes of the tachykinin NK1 receptor revealed by G-protein fusions and mutagenesis

The NK1 neurokinin receptor presents two non-ideal binding phenomena, two-component binding curves for all agonists and significant differences between agonist affinity determined by homologous versus heterologous competition binding. Whole cell binding with fusion proteins constructed between either Galpha(s) or Galpha(q) and the NK1 receptor with a truncated tail, which secured non-promiscuous G-protein interaction, demonstrated monocomponent agonist binding closely corresponding to either of the two affinity states found in the wild-type receptor. High affinity binding of both substance P and neurokinin A was observed in the tail-truncated Galpha(s) fusion construct, whereas the lower affinity component was displayed by the tail-truncated Galpha(q) fusion. The elusive difference between the affinity determined in heterologous versus homologous binding assays for substance P and especially for neurokinin A was eliminated in the G-protein fusions. An NK1 receptor mutant with a single substitution at the extracellular end of TM-III-(F111S), which totally uncoupled the receptor from Galpha(s) signaling, showed binding properties that were monocomponent and otherwise very similar to those observed in the tail-truncated Galpha(q) fusion construct. Thus, the heterogenous pharmacological phenotype displayed by the NK1 receptor is a reflection of the occurrence of two active conformations or molecular phenotypes representing complexes with the Galpha(s) and Galpha(q) species, respectively. We propose that these molecular forms do not interchange readily, conceivably because of the occurrence of microdomains or "signal-transductosomes" within the cell membrane.     

Theoretical analysis of the molecular determinants responsible for the K(+) channel blocking by aminopyridines

This work presents a theoretical analysis of the molecular determinants responsible for the pharmacological activity (K(+) channel blocking) of aminopyridines. Thus, DFT theory at the B3LYP/cc-pVDZ level is applied to a series of active compounds: 2-aminopyridine, 3-aminopyridine, 4-aminopyridine, 3,4-diaminopyridine, and 4-aminoquinoleine. The two forms present in the biological environment, neutral and cationic (protonated), are considered in vacuum as well as in aqueous solution. The results show pyramidal and planar structures for the neutral and cationic forms, respectively. An analysis of the topology of the electron density show that an increase in conjugation between the pyridine ring and the amine group is responsible for the observed planarity of the protonated forms. By computing the Laplacian of the charge density we found the pyridine nitrogen to be the preferred protonation site, as a consequence of a much higher curvature of the charge density field. Also, from three-dimensional (3D) isoLaplacian diagrams a common reactivity pattern is only found in the charged forms. This reactivity pattern implies that interaction with the biological receptor site is mediated by electrostatic interactions and hydrogen bonding. Development of a physical-mathematical model allows identification of the specific relationship of the pharmacological activity index with the affinity for the receptor and the protonation ability.     

Hormone binding and coupled response relationships in systems dependent on the generation of secondary mediators

The relationship between hormone binding and biological response curves is discussed for instances in which the response is dependent upon the generation of a secondary mediator of hormone action. In the simplest model, where the rate of mediator generation is directly proportional to the level of hormone receptor occupancy, and degradation of the secondary mediator follows first-order kinetics, the form of the biological response curve is identical to that for hormone receptor occupancy, but the curve is displaced to the left such that a half-maximal biologic response occurs at less than 50% hormone receptor occupancy (spare-receptor phenomenon). The same is shown to be the case when the model is iterated to include a sequence of coupled intermediate binding reactions intervening between initial hormone binding and the final biological response. When, in contrast, the degradation of one or more of these coupled intermediates is not strictly first order, but instead shows standard Michaelis-Menten kinetics, the response curve, while again remaining parallel to the curve for hormone binding, can now move to the right of the binding curve such that a high threshold is observed for the biological effect, and a half-maximal response may not occur until the level of hormone receptor occupancy is well over 50%. Some consequences of this model are discussed with special reference to the sensitivity and speed of reversibility of the biological response, implications for responses at pharmacological as opposed to physiological concentrations of hormone, and parallels which can be extended to coupled enzymatic reactions of the Michaelis-Menten type.